Sumatra - Geology, Resources And Tectonics

Sumatra:

Geology, Resources and Tectonic Evolution

Geological Society Memoirs Society Book Editors R. J. PANKHURST (CHIEF EDITOR) P. DOYLE F. J. GREGORY J. S. GRIFFITHS A. J. HARTLEY R. E. HOLDSWORTH J. A. H o w E P. T. LEAT A. C. MORTON N. S. ROBINS J. P. TURNER

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It is recommended that reference to all or part of this book should be made in one of the following ways: BARBER, A.J., CROW, M.J. & MmSOM, J.S. (eds) 2005. Sumatra: Geology, Resources and Tectonic Evolution. Geological Society, London, Memoirs, 31. BARBER, A.J., CROW, M.J. & DE SMET, M.J.M. 2005. Chapter 14: Tectonic evolution. In: BARBER, A.J., CROW, M.J. & MmSOM, J.S. (eds) Sumatra: Geology, Resources and Tectonic Evolution. Geological Society, London, Memoirs, 31, 234-259.

GEOLOGICAL SOCIETY MEMOIRS NO. 31

Sumatra: Geology, Resources and Tectonic Evolution EDITED

BY

A. J. BARBER Royal Holloway University of London, UK M. J. CROW Lately of the British Geological Survey, UK and J. S. MILSOM Gladestry Associates, UK

2005 Published by The Geological Society London

THE GEOLOGICAL SOCIETY

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Contents Preface

vii

Contributors

ix

Chapter 1.

Introduction and previous research

A. J. BARBER, M. J. CROW & J. S. MILSOM History of geological research in Sumatra before WWII Post-WWII research SEATAR Programme Indonesian Petroleum Association British and Indonesian Geological Surveys University of London Southeast Asian Research Group, BGS and LEMIGAS Southern Sumatra Project

Chapter 2.

Chapter 7.

Seismology and neotectonics

J. S. MILSOM

Shallow seismicity The Wadati-Benioff Zone (WBZ) Toba seismicity Relative horizontal movements GPS data, the Enggano and Simeulue earthquakes and Mentawai Fault Vertical movements

9 9 10 11

Chapter 3.

16

The gravity field

Geochemistry of the Silungkang and Palepat Formations Metavolcanics and serpentinites in the Medial Sumatra Tectonic Zone Bentong-Billiton Accretionary Complex West Sumatra Triassic Plutonic-Volcanic Arc Pahang Volcanic Belt Jurassic-Cretaceous Plutonic-Volcanic Arcs Volcanics in the Woyla Accretionary Complex Oceanic volcanic arc fragments Origins of the volcanic units and their environments of formation

13 15

J. S. MILSOM & A. S. D. WALKER

Data sources Regional gravity patterns Toba-Tawar gravity low Eastern Sumatra Gravity effects of sedimentary basins The forearc basin Seismic tomography and the long-wavelength gravity field

16 16 19 19 19 20

Chapter 4.

24 24 25 35 40

Chapter 5.

54

E. J. COBBING Isotopic ages of Sumatran granites The granite suites Conclusions

54 56 61

Chapter 6.

63

Pre-Tertiary volcanic rocks

M. J. CROW Carboniferous volcanism East Sumatra Plutonic-Volcanic Belt (Permian volcanism) West Sumatra Permian Plutonic-Volcanic Belt (Early-Mid-Permian volcanism)

79

86

91 94 95 95

Chapter 8.

98

Tertiary volcanicity

M. J. CROW Radiometric dating of volcanism and plutonism in Sumatra Tertiary volcanic stratigraphy Major and trace element geochemistry of the Tertiary volcanic rocks Volcanism, plutonism and subduction beneath Sumatra during the Tertiary: summary of Tertiary volcanism and tectonic overview

86 87 88

98 98 109

110

22

A. J. BARBER • M. J. CROW Pre-Carboniferous basement Tapanuli Group (Carboniferous - ?Early Permian) Peusanguan Group (Permo-Triassic) Woyla Group (Jurassic-Cretaceous)

Granites

68 68 71 71 71 72 77

M. E. M. DE SMET & A. J. BARBER Stratigraphic review Pre-Rift stage (Eocene) Horst and graben stage (latest Eocene-Oligocene) Transgressive stage (Late Oligocene-Mid-Miocene) Maximum transgression (Mid-Miocene) Regressive stage (Mid-Miocene-Present) Summary

Chapter 9. Pre-Tertiary stratigraphy

Tertiary stratigraphy

67

63 63 64

Quaternary volcanicity

120

M. GASPARON Quaternary volcanic arc and its relationship with main tectonic features of Sumatra Pyroclastic deposits Quaternary arc volcanoes Quaternary backarc volcanics Volcanic hazard

120 123 124 125 130

Chapter 10.

131

Fuel resources: oil and gas

J. CLURE North Sumatra Basin Central Sumatra Basin South Sumatra Basin Other Sumatran basins

131 135 137 140

Chapter 11.

142

Fuel resources: coal

L. P. THOMAS Geology and coal deposits in Sumatra Coal quality Coal resources and production

142 145 145

vi

CONTENTS

Chapter 12. Metallic mineral resources M. J. CROW & T. M. VAN LEEUWEN Sources of data Timing of metallic mineralization events in Sumatra Palaeozoic sedimentary basins (Pb-Zn) Late Triassic-Early Jurassic magmatic arc and the Tin Granites (Sn, Wo) Jurassic to Early Cretaceous magmatic arcs (Cu, Au) Woyla Group and Accretion Complex (Au-Ag, Pb-Zn) Late Cretaceous magmatic arc (Sn, A u - A g ) Palaeocene magmatic arc (Cu, Au-Ag) Late Eocene-Early Miocene magmatic arc Miocene-Pliocene magmatic arc (porphyry Cu, Mo) Neogene magmatic arc (Au-Ag) Conclusions

147

Tertiary basins in the backarc area

214

147 147 148

Chapter 14. Tectonic evolution A. J. BARBER, M. J. CROW & M. E. M. DE SMET Pulunggono & Cameron (1984) model Fontaine & Gafoer (1989) model Metcalfe (1996) model Hutchison (1994) model Revised tectonic model for Sumatra Permo-Triassic palaeogeographic reconstructions The Woyla Nappe and the Mesozoic evolution of the Sundaland margin Tertiary palaeogeography of Sumatra Recommendations for future work on Sumatran geology

234

Chapter 13. Structure and structural history A. J. BARBER • M. J. CROW The Sunda forearc The Barisan Mountains

175

149 158 159 159 159 159 159 165 174

Appendix

175 187

Radiometric age data for Sumatra

234 234 236 237 239 242 248 249 255 259

References

266

Index

282

Preface The initiative for this Memoir arose from a series of field-based geological studies in Sumatra by the Institute of Geological Sciences (later the British Geological Survey) and the University of London Group for Geological Research in Southeast Asia in collaboration with the Indonesian Ministry of Mines, through the Geological Research and Development Centre and the Directorate of Mineral Resources in Bandung, and the Research and Development Centre for Oil and Gas Technology (LEMIGAS) in Jakarta between 1975 and 1995. The Indonesian side selected Sumatra as a suitable area for this programme of scientific and technical assistance in geological, geochemical and geophysical surveys, inventories of mineral potential and the training of geoscientists in pursuance of successive five-year development plans (Pelita). The work culminated in the publication by the Geological Research and Development Centre of a series of 42 1:250 000 Geological Map Sheets with Explanatory Notes covering the whole of Sumatra. In compiling these geological maps the work of the Dutch geologists of the Netherlands Indies Geological Survey, who commenced a systematic programme of mapping in Sumatra before the Second World War, and the work of geologists working for oil companies with concessions in Sumatra, supported by the Indonesian National Oil Company (Pertamina), and published since 1971 in the Proceedings of the Indonesian Petroleum Association, were also incorporated. Map compilation, follow-up geological studies and the continuing activity of oil company and academic geologists resulted in the accumulation of a vast amount of geological information which is scatttered in diverse sources and has never been properly synthesized. A group of geologists from the BGS and the University of London, together with other collaborating scientists, agreed to synthesise and review our current knowledge of all aspects of the geology of Sumatra in the present Memoir to form the foundation on which future geological work in Sumatra may be soundly based. Credit is due to the foresight of Directors of the Indonesian Ministry of Mines (Dr John A. Katili, Director General of Geology and Mining), the Indonesian Geological Survey (Ir Johannas, Ir Salman Padmanagara), Geological Research and Development Centre (Dr H.M.S Hartono, Dr Rab Sukamto, Dr Mohamed Untung and Dr Irwan Bahar) the Directorate of Mineral Resources (Ir Salman Padmanagara, Ir Kingking A. Margawidjaja), and the Director (Dr Rachman Subroto) and Chief Geologist (Dr Bona Situmorang) of LEMIGAS, who initiated and provided administrative and logistic support for these various geological programmes and saw them through to successful conclusions. Credit is also due to the many Indonesian geologists from these various organizations who worked on the geological mapping, geophysical and mineral exploration programmes in Sumatra and acting as counterparts to BGS and University of London geologists in gathering the basic data and ensuring, frequently in challenging conditions, that expeditions in Sumatra were brought to successful and safe conclusions. On the British side, the contribution of geologists of the Institute of Geological Sciences/British Geological Survey and the provision of equipment and of scholarships was supported by the Overseas Development Administration (ODA), and later the Department of International Development, as part of a technical aid programme to Indonesia by the British Government. The technical programme was initiated by Assistant Director (IGS) Dr David Bleackley CMG, and supervised successively by Regional Geologists Dr John V. Hepworth, Dr Clive Jones OBE, Dr John Bennett and Robert Evans. The North Sumatra Project (1975-1980) was managed in Bandung by Dr Barry Page, the North Sumatra Support Project by Dr Martin Clarke, the Nortb Sumatra Mineral Exploration Project (1984-1988) by Frank Coulson, and the Southern Sumatra Geological and Mineral Exploration Project (1988-1994), by Dr Michael Crow. Sandy Macfarlane OBE managed the North Sumatra Basin Project (1985-1990) and follow-up projects (1990-1995) at LEMIGAS in Jakarta.

The University of London Group, directed by Dr A. J. Barber, became involved in Sumatra at the invitation of Dr John Hepworth (BGS) to provide training and qualifications to GRDC geologists and geophysicists who were taking part in the field mapping programmes, in return for administrative and logistical support for PhD research students and post-doctoral research assistants from the University of London. A similar arrangment was subsequently made with LEMIGAS. Financial support for British research students was provided by the Natural Environment Research Council and by a Consortium of oil companies who supported the work of the University of London Group throughout SE Asia. Indonesian geologists from GRDC, DMR and LEMIGAS studying in Britain were supported by the Overseas Development Administration, through the British Council, and by the University of London Consortium. The contributions of the many geologists both Indonesian and British who worked on projects in Sumatra under these collaborative arrangements is acknowledged in the list of references which accompanies this Memoir. The editors are indebted to Mike Atherton, Paul Burton, Nick Cameron, Chuck Caughey, Martin Clarke, John Clure, Valerie Clure, John Cobbing, Chris Elders, Derek Fairhead, Massimo Gasparon, Robert Hall, Clive Jones, David Land, Bill McCourt, Greg Moore, Tim Moore, Andrew Samuel and Steve Sparks for their reviews of chapters, or parts of chapters, in this Memoir. Some generously reviewed more than one chapter. Their expertise has resulted in the correction of errors and misunderstandings and the overall improvement of the presentation of the contributions included in the Memoir. The authors of the chapter on mineral resources are grateful for the provision of unpublished data by Terry Middleton, Rod Jones, Brian Levet and Greg Hartshorn. Discussions over many years with Andi Mangga, Mike Andrews, Nick Cameron, John Cobbing, Thamrin Cobrie, Suudi Gafoer, Agus Ganowan, Hariwidjaja, Linda Heesterman, Umi Kuntjara, Chris Johnson, Machali, Bill McCourt, Sumartono and Bob Young concerning the geology and mineral deposits of Sumatra have been invaluable in the preparation of this chapter. A. J. Barber is indebted to Elsevier Science and the International Association of Gondwana Research for permission to reproduce versions of figures which had been published previously in the Journal of Asian Earth Sciences and Gondwana Research, respectively, and to the Indonesian Petroleum Association for written permission to publish figures taken or modified from articles published in the Proceedings of their Annual Conventions. M. J. Crow and A. J. Barber are indebted to the Librarians of the Geological Society (Assistant Librarian Wendy Cawthorne) and the British Geological Survey for invaluable assistance in extensive bibliographic searches, and to Prof. Robert Hall for access to the resources of the SE Asian Library at Royal Holloway, University of London. They are also indebted to their wives Nuala and Brenda for their support and forebearance during the preparation of this Memoir. Financial support provided by ConocoPhillips Indonesia (Chief Geologist James Matthew) and of the Royal Holloway South East Asian Research Group (Director Professor Robert Hall) for the printing of coloured maps and diagrams is gratefully acknowledged. Simon Suggate of the South East Asia Research Group compiled the DEM image of Sumatra used on the cover of the volume.

A. J. Barber M. J. Crow J. S. Milsom November 2004

Dedication This Memoir is dedicated to all Earth Scientists who have contributed to our knowledge and understanding of the geology of Sumatra

Contributors A. J. Barber, Southeast Asian Research Group, Department of

Geology, Royal Holloway, University of London, Egham, Surre, TW20 OEX, UK (e-mail: [email protected]). E. J. Cobbing, Lately of British Geological Survey, Keyworth,

Nottingham NG12 5GG, UK. Present address: 25 Main Road, Radcliffe on Trent, Nottingham NG12 2BE, UK (e-mail: j. cobbing @bluecom, net) John Clure, Technical Outsourcing, Court Hill House, Letcombe

Regis, Oxon OX12 9JQ, UK (e-mail: [email protected]). M. J. Crow, Lately of the British Geological Survey, Keyworth; Present address: 28A Lenton Road, The Park, Nottingham, NG7 IDT, UK (e-mail: [email protected]). M. E. M. de Smet, Lately of the Southeast Asian Research Group, Royal Holloway, University of London; Present address: Mort-

~ ~

......

nikemuurstraat 103b, Leewarden, The Netherlands (e-mail: smet3OO.nhl.nl). Massimo Gasparon, Department of Earth Sciences, The University of Queensland, St Lucia, Qld 4072, Australia (e-mail: massimo@ earth.uq.edu.au). J. S. Milsom, The Camp, Gladestry, Kington, Herefordshire HR6

3NY, UK (e-mail: [email protected]). L. P. Thomas, Dargo Associates Ltd., Anned Bach, Michaelchurch Escley, Herefordshire, HR2 0JW, UK (e-mail: [email protected]).

Theo M. van Leeuwen, PT Rio Tinto Exploration, 28th Floor, Menara Kadin Indonesia, Jalan Rasuna Said Blok X-5 Kav 02-03, Jakarta 12950, Indonesia (e-mail: [email protected]). Adrian S. D. W. Walker, British Geological Survey, Keyworth, Nottingham, NGI2 5GG, UK (e-mail." [email protected], uk).

SOUTHEAST ASIA

5~;,

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9

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Simplified geological map of Sumatra M. J. CROW & A. J. BARBER

Rock units are separated into time bands based on palaeontological evidence of age for the sediments and radiometric dating for the intrusives and the volcanics. The main sources for the compilation of this geological map were: 1:250 000 scale quadrangle geology maps published by the Geological Research and Development Centre between 1975 and 1996; the geological map of Northern Sumatra at l:l 500 000 by Stephenson & Aspden (1982); the 1:1 000 000 geological maps of Sumatra compiled by Gafoer et al. (1992a, b, d); the 1:250000 map of Central Sumatra by Hahn & Weber (1981a) and the map of Sumatra in the geological compilation of Indonesia-West at 1:2 500 000 by M. C. G. Clarke (Land Resources 1990). Earlier sources consulted include the Netherlands East Indies Geological Survey maps (1927-1931) of southern Sumatra at 1:200 000 and the compilations of parts of Sumatra at 1: 1 000 000 by Zwierzijcki (1922a, b, 1930a). Fontaine & Gafoer (1989) presented palaeontological evidence for a medial tectonic dislocation in Sumatra, which was defined by Hutchison (1994). Outcrops of the Medial Sumatra Tectonic Zone (Barber & Crow 2003), the Kluet and Kuantan Formations, and the Bohorok and equivalent formations in the Tigapuluh Mountains, shown on the map are based on published and unpublished descriptions of the deformation, as discussed in Chapter 13 (2005, this volume). The ages of granitic intrusions for Sumatra are from Chapter 5, and for the Tin Islands from Cobbing et al. (1992). The solid geology of Bangka Island is taken from Ko (1986).

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,

106 °

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107 °

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INTRUSIVES

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SIMPLIFIED GEOLOGICAL MAP OF SUMATRA

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ang

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inas

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Modified from" Stephenson & Aspden, 1982. Simplified Geological Map of Northern Sumatra. Scale 1:1,500,000 Institute of Geological Sciences, Keyworth, U.K. Gafoer et al. 1992a. Geological Map of Indonesia, Padang Sheet. Scale 1:1,000,000. Geological Research and Development Centre, Bandung, Indonesia. Gafoer et al. 1992b. Geological map of Indonesia, Palembang Sheet. Scale 1.1,000,000. Geological Research and Development Centre, Bandung, Indonesia. 96 ° 1

97 ° I.

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Structural map of Sumatra M. J. CROW & A. J. BARBER

The main sources of data for the Structural and Tectonic Maps were: the 1: 250 000 and 1:1 000 000 series of geological maps covering Sumatra published by the Geological Research and Development Centre and the Tectonic Map of Northern Sumatra (1 : 1 500 000) by Aspden et al. (1982a) for folds, faults and lithological boundaries in the solid geology. Sub-surface structural data shown in Tertiary and younger rocks is taken mainly from publications of the Indonesian Petroleum Association. The location of the Medial Sumatra Tectonic Zone is taken from Barber & Crow (2003) and Chapter 13, and segmentation of the Sumatran Fault System and the structures within the Present Accretionary Complex follow Sieh & Natawidjaya (2000). Structures in the Forearc are taken from Izart et al. (1994) (Meulaboh Basin), Karig et al. (1980), Milsom et al. (I 995) (Nias Basin), Samuel & Harbury (1996) (Nias), Samuel et al. (1997) (the other forearc islands), Yulihanto & Wiyanto (1999), Hall et al. (1993) and Howles (1986) (Bengkulu Basin). The Mentawi Fault Zone is described by Diament et al. (1992) and Malod & Kemal (1996). Sub-surface structures in the North Sumatra Basin are taken from Davies (1984), Sosromihardjo (1988), Moulds (1989) and Tiltman (1990); in the Central Sumatra Basin from Moulds (1989) and Heidrick & Aulia (1993); in the South Sumatra Basin from De Coster (1974), Katili (1974a), Pulunggono (1986), Moulds (1989), Pulunggono et al. (1992), Rashid et al. (1998), Williams et al. (1995), Yulihanto et al. (1995); and in the the Sunda basin from Bushnell & Temansja (1986), Wight et al. (1986). The insert Tectonic Map is derived from earlier syntheses published by Van Bemmelen (1949, 1954), Westerveld (1952b), Katili (1973), Hamilton (1979), Cameron et al. (1980), Aspden et al. (1982a), Pulunggono & Cameron (1984), Wajzer et al. (1991), Hutchison (1994), McCourt et al. (1996), Metcalfe (2000), Barber & Crow (2000) and Chapter 13. The reader is referred to the main text for a more exhaustive list of the references consulted.

, 99 °

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, 102 °

STRUCTURAL MAP OF SUMATRA 0

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TERTIARY AND QUATERNARY JURASSIC AND CRETACEOUS TRIASSIC- ~ CRETACEOUS

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Volcanic Units Sedimentary Units Woyla Nappe

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Continental Deposits

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PRE-TRIASSIC BASEMENT Bentong-Belitung Accretionary Complex Sibumasu (East Sumatra) Block Medial Sumatra Tectonic Zone

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Chapter 1

Introduction and previous research A. J. BARBER, M. J. C R O W & J. S. M I L S O M

Sumatra, with an area of 473 606 km 2 is the largest island in the Indonesian archipelago and the fifth largest island in the world. The island stretches across the equator for 1760 km from NW to SE, and is up to 400 km across (Fig. 1.1). Administratively, and for the purposes of this Memoir, Sumatra includes the Mentawai islands from Simeulue to Pagai, which with Enggano form a forearc chain to the SW, and the 'Tin Islands' of Bangka and Billiton and the Riau islands to the east. The backbone of the main island is formed of the Barisan Mountains, which extend the whole length of Sumatra in a narrow belt, parallel to, and generally only a few tens of kilometres, from the SW coast. The main peaks (which are mainly Quaternary or Recent volcanoes) commonly rise 2000 m above sea level, culminating in Mt Kerinci at 3805 m. Short, steep river courses drain the Barisans towards the SW, often cuttting deep gorges, while towards the east the rivers follow long meandering courses across broad coastal plains and swamps to the Malacca Straits, which separate Sumatra from the Malay Peninsula, or to the Java Sea. Eastwards, across the Java Sea, lies the almost equally large island of Borneo (Indonesian Kalimantan), and Java lies immediately to the SE across the narrow Sunda Strait. The Malacca Strait and the Java Sea form the southern parts of the Sunda Shelf (Fig. 1.1). Across the shelf the seafloor is shallow with a depth of less than 200 m and remarkably flat. Virtually the whole of the shelf was exposed at the peak of the last glaciation. To the SW, Sumatra is separated from a linear ridge with emergent islands extending from Simeulue in the north to Enggano in the south, by marine basins more than 1000 m deep, which increase to a depth of more than 2 0 0 0 m in the south. To the SW of the ridge the seafloor slopes steeply into the Sunda Trench, 5000 m deep in the NW, deepening to > 6 0 0 0 m towards Java in the SE. The floor of the Indian Ocean, with a depth of about 5000 m, lies to the SW beyond the trench, extending all the way to to India and the east coast of Africa. Immediately to the west of Sumatra the floor of the Indian Ocean is covered by the thick sediments of the Nicobar Fan, the currently inactive eastern lobe of the Bengal Fan, composed of debris eroded from the Himalayas. The fan is separated from the main part of the Bengal Fan to the west by seamounts of the north-south trending NinetyEast Ridge (Fig. 1.2). In terms of present-day tectonics Sumatra forms the active southwestern margin of the Sunda Craton (Sundaland), the southeastern promontory of the Eurasian Plate (Fig. 1.2). The relative 7.7 cm a NNE-directed motion of the Indian Ocean results in oblique (c. 45 ~ subduction at the Sunda Trench. Seismic profiles across the landward side of the Sunda Trench imaged the removal of packages of sediment from the downgoing plate to build a forearc ridge accretionary complex (Hamilton 1979; Karig et al. 1980) (Fig. 1.3). Oblique subduction results in the northwestward movement of a 'sliver' plate (Curray 1989), decoupled both from the downgoing Indian Ocean Plate and the Sundaland Plate, along the WadatiBenioff seismic zone, which dips northeastwards at c. 30 ~ and along the vertical Sumatran Fault System. The Wadati-Benioff zone intersects the fault at a depth of some 200 km. The active Sumatran Fault System runs the whole length of the Sumatra, through the Barisan Mountains, from Banda Aceh to the Sunda Strait, and is paralleled by a line of Quaternary volcanoes, mainly quiescent, but some currently active (Fig. 1.4).

Geologically, Sumatra forms the southwestern margin of the Sunda Craton, which extends eastwards into Peninsular Malaysia and into the western part of Borneo (Fig. 1.2). A Pre-Tertiary basement is exposed extensively in the Barisan Mountains (Fig. 1.4) and in the Tin Islands of Bangka and Billiton. The oldest rocks which have been reliably dated are sediments of Carboniferous-Permian age, although Devonian rocks have been reported from a borehole in the Malacca Strait, and undated gneissic rocks in the Barisan Mountains may represent a Pre-Carboniferous continental crystalline basement. All the older rocks, which lie mainly to the NE of the Sumatran Fault System, show some degree of metamorphism, mainly to low-grade slates and phyllites, but younger Permo-Triassic sediments and volcanics are less metamorphosed. The area to the SW of the fault is composed largely of variably metamorphosed Jurassic-Cretaceous rocks. The Pre-Tertiary basement is cut by granite plutons that range in age from Permian to Late Cretaceous. Locally within the Barisans the basement is intruded by Tertiary igneous rocks and is overlain to the NE and SW by volcaniclastic and siliciclastic sediments in hydrocarbon- (oil and gas) and coal-bearing Tertiary sedimentary basins. These basins have backarc, forearc and interarc relationships to the Quaternary to Recent volcanic arc. Lavas and tufts from these young volcanoes overlie the older rocks throughout the Barisans and, in particular cover an extensive area in North Sumatra around Lake Toba (Fig. 1.4). Recent alluvial sediments occupy small grabens within the Barisan Mountains, developed along the line of the Sumatran Fault and cover lower ground throughout Sumatra. These alluvial sediments are of fluvial origin immediately adjacent to the Barisans, but pass into swamp, lacustrine and coastal deposits towards the northeastern and southwestern margins of the island.

History of geological research in Sumatra before-WWII During the late nineteenth and early twentieth centuries Sumatra was explored by geologists and engineers working for mining and petroleum companies under the auspices of the Bureau of Mines in the Dutch East Indies Colonial Administration. In 1925 a 'Palaeobotanic Expedition to Djambi (Jambi)' was undertaken to collect samples of the 'Djambi Flora'. This early work is summarized by Rutten (1927) in his 'Lectures on the Geology of the Netherlands East Indies'. Between 1927 and 1931 the Netherlands Indies Geological Survey conducted a mapping programme in South Sumatra with the production of a series of sixteen 1:200 000 Geological Map Sheets (e.g. Musper 1937), and carried out other geological studies in Central and Northern Sumatra (Musper 1929; Zwierzijcki 1922a, b, 1930a). Unfortunately, as a result of the global economic depression, this mapping programme was discontinued in 1933, before the mapping of the whole island was complete. However, the cessation of fieldwork provided an opportunity to publish the results of the 1925 Palaeobotanic expedition to Djambi (Zwierzijcki 1930a; Jongmans & Gothan 1935). Exploration by mining and petroleum companies continued throughout Sumatra, but for commercial reasons most of the reports remained confidential and unpublished. However, some of the results, notably for

2

CHAPTER 1

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work carried out in Siberut, Nias and Simeulue and the other Outer Arc Islands on behalf of the Nederlands Pacific Petroleum Maatschappij and the Geological Service of the Baatafsche Petroleum Maatschappij before WWII (Elber 1939; Den Hartog 1940a, b; Hopper 1940), were made available to van Bemmelen (1949, 1970) during the preparation of his major synthesis of 'The Geology of Indonesia'. Van Bemmelen began work on this comprehensive and masterly summary, immediately before WWII. The first manuscript version of this work was completed in Bandung between 1937 and 1941. When Java was invaded by the Japanese in 1942 van Bemmelen was taken into custody as a prisoner of war. There are reports that during the war he was permitted by the Japanese authorities to continue work on the volume. Van Bemmelen says that he entrusted his manuscript to an official of the Geological Survey,

but after the war this official refused to return it (van Bemmelen 1949, 1970). On his release from captivity van Bemmelen returned to the Netherlands, where he was commissioned to rewrite the volume by the Director of the Netherlands East Indies Bureau of Mines. Work commenced in 1946 and the first edition was published by the Government Printing Office in the Hague in 1949. A second edition was published in 1970. The volume provides a complete summary of the state of knowledge of the stratigraphy, structure, igneous history and mineral deposits of the whole of Indonesia at that time. For Sumatra, van Bemmelen (1949, 1970) developed a tectonic synthesis in which deformation proceeded as a series of waves, across the island from NE to SW, with the earliest cycle having occurred in the Malay Peninsula during the Triassic, and the most recent continuing in the outer arc islands at the present day.

INTRODUCTION

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Little geological work was possible during the years immediately after the end of WWII, but following Indonesian Independence in 1947 the Geological Survey of Indonesia (GSI) was established in the old Bureau of Mines building in Bandung. From 1969 to 1974 the Mapping Division of (GSI) commenced a systematic programme of mapping in the Padang area of West Sumatra, in collaboration with the United States Geological Survey (USGS), as part of the First Five Year Development Plan (PELITA I). Several 1:250 000 Geological Map Sheets were published as a result of this programme (Silitonga & Kastowo 1975; Rosidi e t al. 1976; Kastowo & Leo 1973). As part of this collaboration a senior geologist of the USGS, Warren Hamilton, was commissioned to prepare a series of maps and a memoir reviewing the geology of the Indonesian region in plate-tectonic terms (Hamilton 1977, 1979). Hamilton's (1979)Tectonic Map, which includes Sumatra, shows clearly present views of the tectonic setting of Sumatra.

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SEATAR Programme In 1973 a meeting was convened by the United Nations Committee for the Coordination of Joint Prospecting for Mineral Resources in Asian Off-shore waters (CCOP) in Bangkok which established the Studies in East Asian Tectonics and Resources (SEATAR) Programme. At that time a review of the current understanding of the tectonics of eastern Asia was prepared by Deryck Laming on behalf of CCOP-IOC (1974). As a result of the meeting it was proposed to concentrate research along a series of transects across the island arc systems of East and SE Asia. Subsequently A. J. Barber (University of London) and Derk Jongsma (BMR) were engaged by CCOP as Technical Consultants to prepare a report on the current state of knowledge along the lines of these transects (CCOP-IOC 1980). One of the selected transects ran from the Malay Peninsula across northern Sumatra and the forearc island of Nias to the Sunda Trench. Although the final report for this transect was never published, a great deal of important research was carried out by American researchers

4

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under the auspices of the SEATAR Programme, particularly in Nias and the surrounding seas (Curray et al. 1982; Karig et al. 1980; Moore & Karig 1980). Also in conjunction with the SEATAR Programme, Cobbing et al. (1992) made a detailed study, including isotopic dating, of the granites on the Tin Islands of Bangka and Billiton, supported by the UK Overseas Development Administration as a contribution to the work of : C O P . Since the effective termination of the SEATAR Programme, US research in Sumatra has been concentrated on neotectonics, an important part of which has been the monitoring of movement along the Sumatran Fault System, using GPS location systems (Prawirodirdjo et al. 1997).

Indonesian Petroleum Association In 1971 the Indonesian Petroleum Association (IPA) was established by petroleum companies operating in Indonesia, in association with the Indonesian national oil company, Pertamina. Since its inception the IPA has held Annual Conventions which continue to the present day. At these conventions papers on the geology of Indonesia are presented and published as the Proceedings of the Indonesian Petroleum Association. The IPA Proceedings provide an invaluable source of information on the geology of Indonesia. Most of the papers deal with Tertiary deposits and details of the stratigraphy and structure of the oil and gas fields of Indonesia, including those of Sumatra, but more general papers on geology and tectonics have also been published. The publication of the IPA Proceedings has resolved van Bemmelen's (1949) complaint of the pre-WWII situation, in which large amounts of geological data, accumulated by the oil companies, remained unpublished for commercial reasons, and were not available for the compilation of regional geological syntheses.

British and Indonesian Geological Surveys Major UK involvement in the geology of Sumatra began in 1975 when the Institute of Geological Sciences (IGS, now the British Geological Survey, BGS), in collaboration with the Geological Survey of Indonesia (GSI), commenced a five-year mapping and reconnaissance geochemical survey of northern Sumatra to the north of the equator (Northern Sumatra Project, NSP). In 1978 GSI was reorganized into a number of semi-autonomous directorates and the Directorate of Mineral Resources (DMR) became the designated Indonesian counterpart organisation in the NSP. The work of IGS in the Northern Sumatra Project, and subsequent projects by BGS in Sumatra, were funded from the Technical Assistance and Technical Cooperation budgets of the U.K. Overseas Development Administration (ODA).

The structural, stratigraphic, geochemical and tectonic results of the Northern Sumatra Project have been presented in a series of papers (Page et al. 1978, 1979; Cameron et al. 1980; Rock et al. 1982; Aldiss & Ghazali 1984) and unpublished reports. In a continuation of the NSP, geological maps and reports resulting from the project were edited by BGS personnel, and published by the Indonesian Geological Research and Development Centre (GRDC), one of the constituent directorates of GSI, as a series of 18 Geological Map Sheets at 1:250 000 scale, with accompanying Explanatory Notes. Follow-up studies of fossil localities, with the view of establishing the stratigraphical ages of the sedimentary units in Sumatra, were carried out by Metcalfe (1983, 1986, 1989a, b; Metcalfe et al. 1979) and by Fontaine and his collaborators, under the auspices of : C O P (Fontaine & Gafoer 1989). The results of the regional geochemical stream sediment sampling survey were published in a joint IGS/DMR Geochemical Atlas (Stephenson et al. 1982) and subsequently DMR published sets of single element proportional symbol distribution maps at 1:250000, for many of the quadrangles to the north of the equator. Geochemical anomalies found during the NSP were followed up by BGS and DMR in the collaborative North Sumatra Geological and Mineral Exploration Project (NSGMEP, 1985-1988). The results of a separate programme of research into the mineralization in north Sumatra, also funded by UK ODA, have been published by Bowles et al. (1984, 1985) and Beddoe-Stephens et al. (1987).

University of London Southeast Asian Research Group, BGS and LEMIGAS In 1978 members of the University of London Southeast Asian Research Group which had previously been active in Eastern Indonesia, commenced a programme of research projects in Sumatra, in collaboration with BGS, DMR and GRDC. In 1984 a joint University of London/BGS North Sumatra Basins Study Project, was set up with funding from the UK Overseas Development Administration, in collaboration with the Indonesian Research and Development Centre for Oil and Gas Technology (LEMIGAS) (Kirby et al. 1993). This project built on the major involvement by LEMIGAS in this productive basin, where one of the largest exploration blocks is operated directly by Pertamina. The overall programme was largely concerned with the stratigraphy, sedimentology and geophysics of the Tertiary basins in northern Sumatra, with the University contribution Concentrating on field studies of the relationship of the Tertiary rocks to the underlying basement, with a view to understanding the tectonic evolution, of these basins (Turner 1983; Tiltman 1987, 1990; Kallagher 1990). More recently the University of London contribution, funded by the UK Natural Environment Research Council (NERC), ODA and a number of oil companies,

INTRODUCTION

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(Wajzer et al. 1991; Barber 2000; McCarthy et al. 2001) and a study of the Sumatran Fault System throughout the island (McCarthy & Elders 1997).

Southern Sumatra Project Geological mapping, gravity surveys and geochemical programmes in Sumatra south of the equator were conducted by GRDC and DMR during PELITA II (1974-79) and in successive five year development programmes, continuing into the 1980s. In 1988 the Southern Sumatra Geological and Mineral Exploration Project (SSGMEP) was established, and BGS joined DMR and GRDC in the completion of these surveys and in research

6

CHAPTER 1

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programmes with funding from UK ODA Technical Cooporation budget. This programme was completed in 1995 with the publication by GRDC of the last of the forty three Geological Map Sheets at 1:250000 scale, covering the whole of Sumatra (Fig. 1.5) and 18 1:250000 scale Bouguer gravity anomaly maps of southern Sumatra, including Bangka and Billiton islands, but excluding the coastal swamps and the Barisan Mountains. The collaborative geochemical survey was completed in 1994 with the publication by DMR of 14 quadrangle boxed sets of 1:250 000 single element proportional symbol geochemical maps (up to 15 elements) with accompanying reports on the geochemistry, geology and mineral occurrences. Subsequently the Sumatra geochemical data was made available on CD-ROM

(Version 2 in 1999). In 1995 following a one-year 'Sustainability Phase' of the SSGMEP a Geochemical Atlas of Southern Sumatra was issued in digital form on CD-ROM (Machali et al. 1995). Publication in book form followed in 1997, with text in both Bahasa Indonesia and English (Machali et al. 1997). An evaluation of tectonic models for the Pre-Tertiary history of Sumatra based on BGS/DMR/GRDC and University of London research programmes has been published by Barber & Crow (2003).With the completion of this major phase of UK involvement in the study of the geology of the Sumatra, the time is ripe to review the vast increase in our knowledge of the geology of Sumatra since van Bemmelen's (1949, 1970) synthesis.

Chapter 2

Seismology and neotectonics JOHN MILSOM

Sumatra is an active (Andean) continental margin that would be linked by land to SE Asia if sea level fell by as little as 50 m. Present-day tectonic processes are controlled by three major fault systems, the most obvious of which is the subduction thrust which crops out in the Sunda Trench. The trench curves very little in the 800 km between Enggano and Nias, i.e. off central Sumatra (Fig. 2.1), but is markedly convex towards the Indian Ocean both further north and further south. Water depths of more than 6000 m are reached in the south but the maximum in the north may be less than 5000 m. The difference is usually, and convincingly, attributed to the presence on the Indian Ocean plate of the Nicobar Fan, consisting of sediments, derived ultimately from erosion of the Himalayas, which increase steadily in thickness towards the north (e.g. Hamilton 1979). Continuing subduction is attested by a Wadati-Benioff Zone (WBZ) that extends to depths of the order of 200 km (e.g. Newcomb & McCann 1987) and by volcanic activity in the Barisan mountains, the peaks of which generally lie within a few tens of kilometres of the coast. The change, of more than 45 ~ in the trend of the trench between 96~ and 97~ (the 'Nias Elbow') may have been initiated by subduction of the 2 km high Investigator Ridge (Investigator Fracture Zone), which trends approximately northsouth at about 98~ Sieh & Natawidjaja (2000) defined a 'Central Domain' of mainland Sumatra between the Nias Elbow and the ridge intersection as anomalous in a number of ways (notably in the differing trends of the Sumatran Fault and the volcanic line) and as distinct from more regular Northern and Southern domains on either side (Fig. 2.1). Inland, the dextral transcurrent Sumatran Fault runs the entire length of the island, from Banda Aceh to the Sunda Strait (Fig. 2.1). A variety of names have been used for both the overall fault system and parts of it, and new nomenclature developed by Sieh & Natawidjaja (2000) divided it into 19 individual segments. Even this detailed study failed to answer many fundamental questions, and estimates of total lateral displacement still vary from several hundred kilometres to as little as twenty kilometres. The 150km suggested by McCarthy & Elders (1997) seems to be about the mean of the published values. The fault trace coincides roughly with the watershed of the Barisans and with the volcanic line, although most of the volcanoes lie somewhat to the NE of the fault and only nine of the fifty youngest centres lie within 2 km of it (Sieh & Natawidjaja 2000). A more precise correlation is with the subduction thrust, since for most of its length the distance between the Sumatran Fault and the trench axis differs by no more than 30 km from the average value of 290 km. The largest deviations are a narrowing within the bight of the Nias Elbow and a broadening in the region further to the NW. The third and most enigmatic of Sumatra's major fault systems is the Mentawai Fault, at the outer margin of the forearc basin (Fig. 2.1). In many publications the name is reserved for the segment extending from the Sunda Strait to Nias (Samuel & Harbury 1996) or the Batu Islands (Diament et al. 1992), but the same disturbance zone continues at least as far as the Andaman Sea (Malod & Kemal 1996) and possibly to the Andaman and Nicobar Islands. Movement has been variously interpreted as normal, strike slip or reverse (Sieh & Natawidjaja 2000). There are considerable changes in appearance on seismic sections even within the region from Nias southwards; the structure was

described by Sieh & Natawidjaja (2000) as a homocline and by Karig et al. (1980) as a 'fault-flexure'. Magnetic anomalies in the Indian Ocean south of Sumatra trend east-west and were interpreted by Sclater & Fisher (1974) as indicating Palaeogene ages for most of the crust adjacent to the trench, with a possibility of Late Cretaceous crust in the extreme SE. Transforms such as the Investigator Fracture Zone, which may offset the anomalies by several hundred kilometres, run almost precisely north-south. With the trend of the trench varying from N40~ to N60~ and the direction of the Indian Ocean-Sumatra convergence vector being about N15~ (Fig. 2.1), Sumatra has long been recognized as a key area for studies of the partitioning of strain between thrust and transcurrent faults during oblique convergence (Fitch 1972; McCaffrey 1992, 1996; Malod & Kemal 1996). The suggestion, originally made by Fitch (1972), that the oblique motion is to a first approximation accommodated by orthogonal subduction at the trench and dextral slip along the Sumatran Fault, is now widely accepted. To the extent that this is true, the forearc region must be decoupled from both the Indian Ocean and Eurasia. The commonly used term 'sliver plate' (e.g. Curray 1989) suggests more strength and rigidity than could reasonably be expected of such a long and narrow strip of lithosphere, and any analysis of subduction beneath Sumatra must take into account the probability of independent movements of forearc fragments (e.g. McCaffrey 1991). Estimates of the movements of the Indian Ocean relative to Sumatra are shown in Figures 2.1 and 2.4. Changes in magnitude and direction from NW to SE are dictated by the East African location of the pole of rotation (Larson et al. 1997). If partitioning of orthogonal and transcurrent strain between, respectively, the trench and the Sumatran Fault were complete (and movement occurred only along these features), then sites in the forearc sliver would move parallel to the Sumatran Fault relative to SE Asia, but at right angles to the trench relative to the Indian Ocean. Trench-normal relative motion implies that the forearc sliver 'tracks' across linear features on the Indian Ocean Plate, such as the Investigator Fracture Zone, which have northsouth trends (Fig. 2.1). If the long term movement between the forearc and the Indian Ocean has actually been approximately orthogonal, the intersection point of the Investigator Fracture Zone with the trench, now near the Batu Islands, would have been north of Nias less than 10 million years ago. The relief, of more than 2 km, on the Investigator Fracture Zone might not only impede such tracking but could be responsible for cyclical uplift and subsidence in the forearc basin and ridge. Slip partitioning and subduction of Indian Ocean lithosphere produce high levels of seismicity in the Barisan Mountains, in the forearc basin and along the forearc ridge (Fig. 2.2). The potential for extremely destructive earthquakes was most recently demonstrated by the Magnitude 9 event near Simeulue in December 2004 and by the resulting tsunami, which gave rise to one of the worst natural disasters in recorded human history. However, and despite the geological evidence for a long history of subduction (e.g. Page et al. 1979), shocks deeper than 200 km are rare (Fig. 2.3). Events below 300 km are confined to the extreme SE and may be associated with north-directed subduction beneath Java rather than NE-directed subduction beneath Sumatra. The abrupt change in orientation of the active margin between these two islands must produce considerable stress in the downgoing

8

CHAPTER 2

Central Domain Northern Domain

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F i g . 2.1. Sumatra: the neotectonic setting. The figure has been oriented on the main fault direction. The India-SE Asia convergence vector changes significantly in both direction and magnitude over the length of the island, from 52 mm a-1 directed at N10~ (at 2~ 95~ to 60 mm a - l directed at N 17~ (at 6~ 102~ Convergence data (and mainland structural domains) are from Sieh & Natawidjaja (2000). Elongated rectangles in the forearc region indicate the locations of the zeros on the seismicity cross-sections in Figure 2.3. The seismic image along Line 42-43 is shown in Figure 2.7. The white stars mark the epicentres of the Enggano 2000 and Simeulue 2004 Great Earthquakes. Bathymetric contours at 200, 1000, 3000, 5000 and 6000 m are from GEBCO (1997). Shading indicates sea floor deeper than 6000 m. I.F.Z., Investigator Fracture Zone. Onshore topography derived from the Global Relief Data CD-ROM distributed by the National Geophysical Data Center, Boulder, Colorado.

slab but this is not obvious in the patterns of shallow seismicity shown in Figure 2.2 and discussed below.

Shallow seismicity As in most active continental margins, shallow ( < 6 0 km depth) earthquakes in Sumatra are distributed over wide areas of the upper plate and are not restricted to the WBZ (Fig. 2.2). Maximum shallow earthquake activity occurs within the sliver defined by the Sumatran Fault in the east and by the subduction thrust in the west and at depth, and is most intense along the line of the forearc ridge. There must be considerable forearc extension (see McCaffrey 1991 ) if the estimates of large variations in rates of transcurrent slip (more than 400 km of offset in Aceh but negligible displacements in the Sunda Strait; Curray et al. 1978) are correct (see also Bellier & Sebrier 1995). Although there have been relatively few shocks of Magnitude 6 or greater beneath the mainland, some have occurred, most notably in the vicinity of the 'equatorial bifurcation' in the Sumatran Fault identified by Prawirodirdjo et al. (2000). The insets to Figure 2.2 attempt to show separately the distributions of events within the uppermost 40 km of the crust and at depths of between 40 and 60 km. Because of the uncertainties inherent in determining the depths of shallow earthquakes (see discussion in Engdahl et al. 1998), there will be events on one map that should properly have been plotted on the other, but the overall differences between the plots are likely to be real. The 4 0 - 6 0 km events are concentrated in a narrow zone centred on the forearc basin and most are probably directly associated with the subducted oceanic lithosphere, i.e. with the WBZ. There are, however, some similarities with the patterns of shallower events, noticeably in the tendency for epicentres to be concentrated in short linear zones at right angles to the trench, presumably due to some form of forearc segmentation. The most obvious examples can be seen around Enggano and western Simeulue, i.e. close to the sites of the Great Earthquakes (defined as earthquakes with

MW magnitudes greater than about 7.8) in June 2000 and December 2004 respectively. Interestingly, the Simeulue events cluster along the crest of a basement ridge that defines the northwestern boundary of a marine and sedimentary basin (Simeulue Basin) where maximum water depths exceed 1000 m. The trend of the linear alignments changes slightly north of the Nias Elbow to partly match the change in orientation of the trench but, surprisingly, N E - S W alignments of epicentres can be seen east of the even more dramatic change between Sumatra and Java (Fig. 2.2). A second feature of the shallow seismicity is the separation of the shallowest earthquakes (Fig. 2.2; lower inset) into two divergent zones, one along the forearc ridge (with a bend or offset where the Investigator Fracture Zone enters the subduction zone near the Batu islands), the other very approximately along the west coast of Sumatra. The forearc basin itself is relatively quiet seismically at these depths. The offset at the Investigator Fracture Zone is interesting because Newcombe & McCann (1987) noted that ruptures associated with Great Earthquakes do not propagate across this region. In 1833 a Magnitude (Mw) 8.7 event faulted the plate margin for about 600 km from Enggano to the Batu Islands, while the effects of the Mw 8.4 event in 1861 were confined to a 300 km segment between the Batu and Banyak Islands.

The Wadati-Benioff Zone (WBZ) In keeping with the continental margin setting, seismicity beneath Sumatra is more diffuse than beneath a typical intra-oceanic arc. This is illustrated in Figure 2.3, which shows hypocentre distributions within three typical swathes, each 200 km wide. In the extreme NE (Fig. 2.3a) the WBZ forms the lower boundary to a seismogenic zone that extends up to the surface over a distance of approximately 300 km from the trench. The greatest concentration of events is at about I00 km from the trench and at depths of about 50 km. In the swathe immediately south of the equator, near the islands of Siberut and Sipora, there is a much

SEISMOLOGY & NEOTECTONICS

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clearer development of a linear W B Z but the scatter is still considerable (Fig. 2.3b). Sieh & Natawidjaja (2000), among others, have claimed that the depth of the W B Z beneath the volcanic line is considerably greater in this Central Domain (Fig. 2.1) than to either the N W or the SE, although the m a x i m u m depth of the seismic zone is actually smaller. The effect is not, however, obvious in Figure 2.3. The most intensely active part of the W B Z is in the extreme south, near Enggano, where there are two main event clusters, at about 40 and 70 k m (Fig. 2.3c). The seismogenic zone continues down to at least 200 km. The two deepest shocks might be associated with Java subduction but, if associated with Sumatra, indicate a pronounced steepening of the W B Z between 200 and 300 km.

seismicity

A more comprehensive picture of Sumatra seismicity than is provided by Figure 2.3 was presented by Hanus e t al. (1996), who plotted hypocentres within 50 k m wide, N E - S W swathes that together covered the whole of the island. Arguably their most interesting plot was A15, which included the northern part of the forearc island of Nias and much of the Toba caldera (Fig. 2.1). The W B Z in this region dips at an angle of a little more than 30 ~ and the deepest shocks occur between 200 and 2 5 0 k m . There is a small but noticeable gap in seismicity beneath the volcanic line at depths of about 150 to 180 k m and a corresponding region of shallow seismicity immediately beneath the volcanoes. In detail, the picture provided by Hanus e t al. (1996) is suspect because of the reliance on International Seismological Centre

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(ISC, Thatcham, UK) hypocentre locations. These, being derived from interpretations of teleseismic data based on global velocity models, are inevitably of fairly low accuracy. The significance of this limitation has been demonstrated by Fauzi e t al. (1996), who used additional data from a newly established (but now permanent) network of short-period digital seismometers to study earthquakes in the vicinity of Toba. The primary aim of the work reported, which covered the period from October 1990 to April 1993, was to investigate a hypothesized break in the d o w n g o i n g slab due to subduction of the Investigator Fracture Zone. Seismic activity was found to be unusually high in the appropriate area but no discontinuity was detected and a limit of 20 k m was placed on the magnitude of any possible displacement. There was more success with a subsidiary objective of defining the shape of the W B Z as it followed the bend in the offshore trench between Nias and Simeulue. In contrast to both the ISC and Engdahl e t al. (1998) data, hypocentres derived from the local study and plotted for narrow cross-strike swathes

10

CHAPTER 2

~.Lake

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were found to be tightly concentrated in very narrow zones that changed in dip scarcely at all around the bend (Fig. 2.4). Estimated depths also tended to be smaller than those based only on teleseismic data, especially beneath the forearc basin. A more recent seismological study of the Toba area used a temporary network comprising 30 short-period and 10 broad-band seismographs deployed for four months in the first half of 1995 (Masturyono et al. 2001). Tomographic methods were used to define velocity variations beneath the caldera. The results support the hypothesized existence of two distinct eruptive centres, one in the south-central part of the lake and the other at its northern end, which erupted at different times (Knight e t al. 1986). Low velocity zones underlying these two centres and extending down into the mantle are separated by a region with a more typical crustal velocity structure.

Relative horizontal movements The information on present-day tectonic processes in Sumatra provided by seismology is now being supplemented by geodetic data from Global Positioning System (GPS) satellites. Repeated measurements at fixed pillars provide an essential complement to earthquake studies, which record only episodic, although sometimes very large, displacements. During seismically quiet periods, GPS measurements monitor aseismic creep and can indicate

Fig. 2.4. Movements of sites in Sumatra as determined by GPS observations during the period 1989-1993 (Prawirodirdjo et al. 1997). Vectors show rates of movement relative to a stable SE Asia. They imply stress accumulation in parts of the forearc region, some of which would have been released by the June 2000 earthquake near Enggano, the December 2004 earthquake near Simeulue and the March 2005 earthquake near Nias. The locations and mechanisms of these earthquakes are indicated by the centres of the lower hemisphere projection 'beachballs', from Abercrombie et al. (2003) for Enggano and from NEIC (2005) for Simeulue and Nias. Locations of aftershocks of the Enggano earthquake for which fault-plane solutions were calculated by Abercrombie et al. (2003) are also shown. Major aftershocks to the Simeulue earthquake occurred almost entirely NW of the limits of the map. MS, Muara Siberut. S, Sinabang, PB, Pulau Babi. The pecked grey lines show the locations of barriers to propagation of ruptures from Great Earthquakes inferred by Newcomb & McCann (1987).

regions in which stress is increasing and may be released catastrophically at some time in the future. Because of the short time intervals over which observations are made (typically 3 to 5 years), GPS measurements must always be considered in the context provided by estimates of long term relative plate motions. Most of the GPS site markers in Sumatra were established by B A K O S U R T A N A L , the Indonesian mapping and geodetic survey authority, working in collaboration with various US institutes, and most are located between 2~ and 2~ (Prawirodirdjo et al. 1997; Genrich e t al. 2000). Additional measurements were made at sites near Bengkulu and Medan and on Nias and Billiton in the course of the G E O D Y S S E A study, which covered the whole of SE Asia. The G E O D Y S S E A results defined a 'Sunda' Block that includes Borneo, the Malay Peninsula and Indochina and moves east relative to Eurasia at 7 - 1 0 mm a -1 (Chamot-Rooke & Le Pichon 1999; Michel et al. 2001). Billiton Island and Medan are clearly within this block, as is much of Sumatra east of the Sumatran Fault, but motions near and to the west of the fault are much more complex. The main B A K O S U R T A N A L campaign (sites shown in Fig. 2.4) began in 1989. Detailed analyses of the data obtained to 1996 in the Central Domain (Fig. 2.1) have been provided by McCaffrey et al. (2000) and by Genrich e t al. (2000). To supplement these analyses, Prawirodirdjo et al. (2000) also considered the results of conventional triangulation surveys extending over a period of 100 years in the same area. These generally confirmed the GPS estimates of 2 0 - 3 0 m m a-1 of dextral movement

SEISMOLOGY & NEOTECTONICS

on this portion of the Sumatran Fault, but revealed very considerable differences in detail in both movement magnitudes and directions. Figure 2.4 shows the site motions relative to SE Asia as interpreted by Prawirodirdjo et al. (1997) and (also relative to SE Asia) the averaged long term Indian Ocean movement vectors (Demets et al. 1990). Strain partitioning was evidently only partially achieved, at least over the short time interval involved, nor were movements confined to the main fault systems. Sites east of the Sumatran Fault but within 50 km of it were not stationary with respect to SE Asia but recorded small but significant displacements to the north and NW. Similar patterns near other major strike-slip features have been interpreted as recording stress accumulations in wide regions of deformed rock that are ultimately released by faulting (e.g. Armijo et al. 1999). Sites in the forearc experienced much larger trench-parallel displacements, but McCaffrey et al. (2000) argued that only about two-thirds of the necessary slip was accounted for and that most of the remainder must have been accommodated oceanward of the crest of the forearc ridge. However, the situation varied considerably from place to place. On forearc islands in the Central Domain (between the Batu and Banyak Islands) the trench-normal components were small, suggesting strong partitioning of convergent and transcurrent movements, but it seems that the forearc was largely coupled to the downgoing slab everywhere to the south of the Batu Islands. The boundary between the two regimes occurs in the region where the Investigator Fracture Zone enters the trench. Prawirodirdjo et al. (1997) tentatively interpreted the northwestwards decrease in coupling as a consequence of the subduction of thick, water-rich sediments of the Nicobar Fan, resulting in high pore pressures in the forearc wedge and weakening of the upper plate by the introduction of hydrothermal fluids. The change in coupling would thus be due to the barrier to sediment flow from the NW presented by the Investigator Fracture Zone, rather than directly to its presence as

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an asperity on the lower plate. However, the magnitude of the December 2004 Simeulue earthquake suggests a 'sticky', rather than well-lubricated, fault zone. The combination of gradual change in the orientation of the Indian Ocean/SE Asia convergence vector and the change in trench orientation at the Nias Elbow implies almost orthogonal convergence across the trench in the vicinity of Simeulue and the Banyak Islands. The Sumatran Fault, however, changes direction much less noticeably, and the differences in curvature of structures on the mainland and along the forearc ridge produce a widening and deepening of the forearc basin NW of Simeulue. Rather surprisingly, the GPS motions of the two sites in the Banyak Islands were almost perfectly parallel to the trend of the Sumatran Fault, and so to the trench further south. The lack of GPS sites on Simeulue means that short-term neotectonic patterns in this critical area remain, for the moment, undefined. The data from GPS measurements and triangulation surveys can be compared with long-term slip estimates based on geologic and topographic offsets at the Sumatran Fault. Slip rates estimated from stream offsets on SPOT imagery vary from 10 mm a -1 at the Sunda Strait to 23 mm a -1 near Lake Toba (Bellier & Sebrier 1995). Much of this change occurs in the Central Domain, where the rates estimated by Sieh & Natawidjaja (2000) using geological offsets increase from 11 mm a i in the SE to 27 mm a-1 in the NW. Slip rates estimated from GPS observations vary much less, increasing by only 4 mm a -1, from 23 mm a-1 to 27 mm a-1, over the same distance (Genrich et al. 2000). Sieh & Natawidjaja (2000) suggested that the geologically indicated changes in slip rates along the fault must have developed only during the last 100 ka, because of the absence of compressional accommodation structures, but left the geological-GPS discrepancy unexplained. They also suggested that the total slip on the Sumatran Fault might be little more than the 20 km of the maximum verifiable geological offset, and that the remainder of the roughly 100 km offset required by stretching in the Sunda Strait might have been accommo-

motion

Fig. 2.5. GPS vectors and the Great Earthquake of June 2000. The upper diagram shows overall movement vectors relative to SE Asia and their trench-parallel and trench-orthogonal resolved components. The lower diagrams compare these components individually. Vector 1 is the regional convergence vector, after Demets et al. (1990). The remaining vectors are GPS vectors from the 1991-1993 campaign at sites at the bases of the arrows, after Prawirodirdjo et al. (1997). 'Beachballs' show the locations of the two subevents proposed by Abercrombie et al. (2002) for the June 2000 earthquake.

12

CHAPTER 2

dated by slip on the Mentawai Fault. Their proposed deformation history (which they emphasized was only one of a multitude of possibilities) involved arc-parallel stretching during the Pleistocene but provided no role for the segment of the Mentawai Fault north of the Nias Elbow.

Seismic reflection sections from many parts of the basin favour localized faulting in the forearc basin, since deformation of Late Neogene sediments is generally confined to the narrow zone close to the eastern coasts of the forearc islands which was named the Mentawai Fault by Diament et al. (1992). However, the now numerous published images of this feature obtained on crossings reported by Karig et al. (1980), Diament et al. (1992) (Fig. 2.6a), Malod & Kemal (1996) and Schlfiter et al. (2002) (Fig. 2.6b) and the excellent multichannel imagery obtained by the Scripps Institution of Oceanography (SIO) south of Nias (Fig. 2.7), indicate a very complex and variable structure. Considerable uncertainties remain as to its true nature. On some seismic sections (e.g. Diament et al. 1992) it appears to be a simple faulted anticline, while in other areas the zone of weakness has been exploited by shale diapirs which conceal fundamental structures (Milsom et al. 1995). The extreme linearity has been used as an argument for a fundamentally transcurrent role (Sieh & Natawidjaja 2000) but subsidence of the forearc basin and elevation of the forearc ridge imply either normal or thrust components. Where it emerges on land, in southeastern Nias, the fault was interpreted by Samuel & Harbury (1996) as an originally extensional fault that has suffered Pliocene to Recent subductionrelated inversion. Significant transcurrent movement was regarded as improbable. Interestingly, however, seismic section presented by Schltiter et al. (2002) (Fig. 2.6b) shows the disturbance as having moved away from the landward side of the forearc ridge (which is itself fragmented in this region; see Fig. 3.1) to a

GPS data, the Enggano and Simeulue earthquakes and the Mentawai Fault During the period covered by published GPS measurements, the southern forearc islands (Siberut to Enggano) were moving NW relative to Sumatra at roughly the same rate as the underlying Indian Ocean Plate (Fig. 2.4). Enggano, in particular, participated in virtually all of the motion of the Indian Ocean during the period of observation, which unfortunately in this particular case extended only from 1991 to 1993 (Fig. 2.5). Much smaller relative motions were recorded at two sites on the adjacent coast of the mainland and therefore only a small part of the trench-parallel motion required accommodation further inland, in the vicinity of the Sumatran Fault. More than half the trench-parallel motion and an even greater proportion of the trench-normal motion must have been absorbed between Enggano and the coast, either at one or more discrete faults or by distributed strain over the width of the forearc basin.

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SEISMOLOGY & NEOTECTONICS

13

Fig. 2.7. SIO Line 42-43, showingthe Mentawai Fault immediatelysouth of Nias. Section provided by Scripps Institution of Oceanography.

position within the forearc basin. This fact, and the image itself, are more compatible with transcurrent than vertical motion. Indeed, Schltiter et al. (2002) suggested that the transcurrent function of the Sumatran Fault might be in the process of shifting to the Mentawai Fault. This is an attractive hypothesis but difficult to reconcile with the suggestion by Sieh & Natawidjaja (2000) that the total offset on the Sumatran Fault is rather small, despite the abundant evidence (including occasional large earthquakes; Untung et al. 1985) for recent and continuing offsets along it. A further complication is introduced by a possible relationship between the Mentawai Fault and the Batee Fault. The latter is a dextral splay from the Sumatran Fault that trends offshore near the Banyak Islands and was interpreted by Karig et al. (1980) as displacing or terminating the Mentawai Fault near Nias (Fig. 2.1). The Mentawai Fault is often shown as either ending near Nias (e.g. Diament et al. 1992) or merging with the Batee Fault, but a very strong gravity gradient indicates a major structural discontinuity between the two westernmost islands in the Banyak group (see Fig. 3.5). This is roughly the position where a Mentawai Fault continuation would be expected if the Batee Fault were not present. Moreover, the existence of Mentawai-type structures still further north has been confirmed by Izart et al. (1994) and by Malod & Kemal (1996) using single-channel reflection data. Additional insights into the role of the Mentawai Fault in the Enggano area were provided in June 2000 by an Mw 7.9 earthquake followed by a train of strong aftershocks (Fig. 2.5). P and S wave studies of the primary event suggested that this comprised two subevents, involving strike-slip within the Indian Ocean Plate followed by thrust motion on the subduction fault (Abercrombie et al. 2003). The events were too deep, and in the wrong plate, to be due to failure on the Mentawai Fault, but they do provide important data on its relationship to the transition between the accretionary wedge and the continental margin. Matson & Moore (1992) suggested that this transition occurs near the east coast of Nias in the Central Domain and that the subduction fault originally reached the surface in this area. Its subsequent migration oceanwards was interpreted as a consequence of the development of the accretionary wedge that now forms the forearc ridge. This is consistent with the Malod & Kemal (1996) interpretation of the Mentawai Fault along its entire length as marking the transition between the wedge and a rigid backstop of pre-existing basement. On this hypothesis,

the linearity of the fault is a consequence of the linearity of the original subduction trace, which would, in turn, have been controlled by the linearity of the former passive margin. The significance of the Enggano composite earthquake to the backstop concept is that the GPS results shown in Figure 2.5 indicate that in this area, and possibly only for short periods, the accretionary wedge moves with the subducting plate and must therefore compress against the backstop, resulting in folding and reverse faulting. Potential energy stored in this folded and faulted zone can be released in large earthquakes in which the wedge moves oceanwards and deformation near the backstop is reversed. Presumably such reversals are only partial, so that deformation gradually increases. At no point in this stick-slip cycle would large earthquakes necessarily occur within the wedge, because accreted material is usually too weak to sustain large local stress. Large earthquakes will therefore be associated principally with the unsticking of the wedge from the backstop or from the downgoing slab along the main subduction thrust and with relative lateral movement between locked and unlocked segments of the forearc. Events of both types appear to have occurred in June 2000, with the movement between segments of the Indian Ocean plate increasing the stress and triggering failure along the subduction thrust (Abercrombie et al. 2003). The results of future GPS measurements in the Enggano-Bengkulu area (there have, unfortunately, been no measurements on Enggano since the earthquake) are thus likely to be very different from those obtained between 1991 and 1993. Amongst other things, they can be expected to provide insights into the highly controversial question of the extent to which trench-parallel motion is accommodated by the Mentawai Fault. It seems probable that the new vectors will resemble the vectors shown in Figure 2.4 for the islands north of Siberut, i.e. they will show almost entirely trench-parallel motion, implying a primarily transcurrent long-term function. The characteristics of both the main earthquake and the extensive aftershock sequence suggest that effects of the Enggano Great Earthquake are unlikely to be seen in the forearc north of Bengkulu (Abercrombie et al. 2003), and in fact no such effects have been observed in post-earthquake GPS studies in the Central Domain (Bock et al. 2003). If this is the case, then dangerous levels of stress must be accumulating in the region from South Pagai to Siberut. The June 2000 Enggano earthquake was completely overshadowed by the December 2004 Simeulue event, information on which was posted on the National Earthquake Information

14

CHAPTER 2

Center website within a few days (NEIC 2005). The suggested maximum displacement was 15 m, in a region where convergence is more nearly orthogonal to the trench than it is further south (see Figs 2.1 and 2.4). Bizarrely, in view of this latter fact, the results from the only GPS site NW of the change of strike, on Pulau Babi (PB on Fig. 2.4), suggest that during the 1989-1993 period the forearc moved slightly further in a direction parallel to the trench than did the Indian Ocean, the supposed driver of the forearc motion. It also seems that about half of the Indian Ocean trench-normal motion was accommodated between Pulau Babi and Sumatra, which is less than at Enggano, but much more than predicted by simple sliver-plate models. The motion of Simeulue, a few tens of kilometres to the NW, might, of course, have been different but there is no bathymetric or other evidence for the placement by NEIC (2005) of an extensional (or any other) boundary to a 'Burma Plate' immediately east of Pulau Babi. Fault-plane solutions for the Simeulue earthquake are consistent with either SW-directed thrusting dipping at about 10 ~ to the NE or NE-directed reverse faulting dipping at about 80 ~ (NEIC 2005). The first of these is much the more likely, but thrusting on a surface so nearly horizontal, when the Benioff Zone dips at about 30 ~ in the vicinity of the hypocentre, raises some questions. The Harvard Centroid Moment Tensor solution, however, places the centroid west of the forearc ridge and beneath the eastern wall of the trench (at 3.09~ 94.26~ cf. the NEIC epicentre at 3.30~ 95.96E~ Since, subject to errors introduced by faulty velocity models, hypocentres correspond to points of rupture intiation whereas centroids represent weighted average locations of moment release (Meredith Nettles pers. comm. 2005), the results can be interpreted as describing an event initiated in the vicinity of the Mentawai Fault and propagating oceanwards and also NW along the forearc. The complexity of stress patterns in the epicentral area is indicated by the multiplicity of previous smaller shocks, some of which had strike-slip solutions and others solutions similar to that of the December 2004 event (see Newcomb & McCann 1987, Fig. 2). The fact that the region around the Mentawai Fault appears to respond to stress in different ways at different places and at different times is consistent with the fault itself being the expression of a fundamental geological discontinuity rather than a simple break through an essentially homogeneous rock mass. The Simeulue event also spectacularly confirmed the extreme segmentation of the forearc. Aftershocks occurred along 1200 km of the arc, from the site of the main shock as far as the northern tip of the Andamans, but there was virtually no activity to the SE (NEIC 2005). The bathymetric high northwest of Simeulue where the epicentre was located may therefore be the surface expression of a discontinuity similar to those associated with the Banyak and Batu highs further south. The extents of Great Earthquake ruptures are strongly correlated with the extents of deep marine basins between Sumatra and the forearc ridge and, given that the NW limit of the rupture zone of the 1861 event was not at Simeulue but at the Banyak Islands (Newcomb & McCann 1987), it seems possible that stress is still building up in a 'Simeulue Basin' segment, to be catastrophically released at some time in the not too distant future.

Vertical movements It is more difficult to monitor vertical movements with GPS than horizontal movements, both because of the generally smaller displacements and because the accuracy is inherently lower. At present, more reliable estimates of rates of vertical motion are being obtained by observing short-term changes in relative sea level. Natawidjaja et al. (2000) studied the submergence and emergence of corals and deduced a pattern of progressive landward tilting of the forearc ridge, with uplift within about

115 km of the trench axis and subsidence at all greater distances. Instantaneous vertical movements of tens of centimetres associated with large earthquakes were superimposed on this pattern. Individual islands in the northern part of the forearc often record similar tilting. Islets shown on Dutch colonial maps as protecting Sinabang harbour, at the eastern end of the north coast of Simeulue (S in Fig. 2.4), are now permanently submerged, and palm trees are dying along much of the coast as salt water invades the soil around their roots. Muara Siberut, the main town on Siberut (MS on Fig. 2.4), is regularly flooded at high tide and some nearby offshore 'islands' consist entirely of mangroves with their roots submerged even at low tide. On Nias the situation is more complicated, since the west coast can be divided into two very different sectors. In the north the coastal region is flat and swampy and the beach is broad and gently sloping, but in the south there are cliffs 5 0 - 1 0 0 m high and the sea floor shelves steeply. This section of the coastline is concave seawards and appears to be a scarp created by failure of an unstable slope (see Fig. 2.1). The relatively low gravity field along the coast and offshore (see Fig. 3.5) suggests loss of mass from this region and also supports the concept of failure of a slope that has been uplifted to unsustainable elevations. On the opposite (eastern) side of the island, rivers have been incised in narrow valleys to depths of 5 - 1 0 m within a broad coastal plain east of the Mentawai Fault, suggesting recent and rapid uplift, but further north there is evidence of both uplift and subsidence. The uplift of the coastal plain on Nias could have been associated with great earthquakes. Zachariasen et al. (1999) interpreted the results of a detailed study of coral heads exposed around the Mentawai Islands of Sipora and North and South Pagai, south of Siberut, as recording aseismic subsidence followed by co-seismic uplift related to the great earthquake of 1833. In this area, and in contrast to areas further north, both aseismic and co-seismic movements appear to have involved tilting towards the trench. Deducing long-term regional displacement patterns from measurements of movements over a few years, or even over tens of years, is clearly never going to be a simple exercise.

Note added in proof The earthquake activity in the central Sumatra forearc between 26 December 2004 and the end of April 2005 is summarized in Figure 2.8. The first four plots show how the seismicity associated with the 26 December event gradually died away during the succeeding three months. It is clear that even as late as March 2005, the majority of events were part of the aftershock sequence. However, on 28 March 2005 there was a further Great Earthquake, with an epicentre just west of the Banyak Islands and an estimated magnitude of 8.6. The distribution of aftershocks to this event indicated that rupture extended throughout the whole of the region between the Banyaks and the December 26 epicentre. It was, in fact, being quite widely predicted in the first few months of 2005 that this would be where the next break would occur. However, and unexpectedly, the zone of aftershocks also extended south as far as the Batu Islands (Fig. 2.8e). It seems therefore that not only had the last remaining segment that had no historic record of Great Earthquakes failed, but that the segment that ruptured in 1861 moved with it. Fault plane solutions by both the NEIC and the Harvard group indicated a shallow thrust, at an even smaller angle of dip than had been the case the previous December. Once again, movement seems to have been initiated close to where the Mentawai Fault (assumed to be near vertical) would reach the subduction fault at depth, and once again there was a significant displacement between the calculated positions of the epicentre and the centroid. In this case, however, the centroid lay south rather than west of the

SEISMOLOGY & NEOTECTONICS

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epicentre, and was still a considerable distance from the trench. Also, and as might have been expected in view of the smaller magnitude of the shock, and hence the probable smaller width of the slip zone, the displacement between centroid and hypocentre was considerably less than in December. The greater distance of the centroid from the trench, together with the smaller magnitude, may be sufficient explanation for the much smaller associated tsunami, which was only about 3 metres high on exposed coasts of Nias and Simeulue and decreased rapidly in amplitude at more remote locations. It is also possible that submarine slides, which may have contributed to the destructive power the December wave, did not occur in March because of the absence of any remaining potentially unstable slopes. The aftershock sequence (Figures 2.8e and f) was notable for being much more tightly constrained to the region immediately beneath the forearc ridge than had been the case following the December event.

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A new train of events began still further south and just seaward of Muara Siberut in the following weeks. There were a few relatively weak shocks in this area in the period immediately after March 28 (Figure 28e), but the first major event (Mw=6.7) took place on April 10, and was followed three quarters of an hour later by another strong (Mw=6.5) shock. Once again, the Mentawai Fault appears to have controlled the location at which failure was initiated. Both events were compressional but, in contrast to the two Great Earthquakes, the slip planes were much steeper (from 30 ~ to 60~ There followed numerous weaker events in the same area but, again in contrast to the pattern associated with the Great Earthquakes, there was no significant rupture propagation (Fig. 28f). It is to be hoped that the earthquakes in this isolated cluster will prove to be the last major events in the current phase of southward-propagating unzipping of subduction west of Sumatra.

Chapter 3

The gravity field JOHN M I L S O M & A D R I A N W A L K E R

Data sources The gravity field of Sumatra and the surrounding marine areas is shown in Figure 3.1. Contours in the onshore area of Bouguer gravity, but offshore are of free-air gravity. Terrain corrections have not been applied. Although marine gravity measurements have been made in the forearc basin and elsewhere on a number of research cruises (e.g. Kieckhefer et al. 1981), the data from these generally widely spaced lines have not been used in preparing the maps because free-air gravity values obtained from inversion of satellite radar altimetry provide more systematic coverage and can resolve anomalies with widths of as little as 7 km (Sandwell & Smith 1997). The onshore and satellitederived offshore data were matched at coastlines without undue difficulty, as should be the case because both free-air and Bouguer corrections are zero at sea level. However, gradients tend to be steep at the coasts in the forearc region, partly because of the change from free-air gravity, which is strongly correlated with local bathymetry, to Bouguer gravity, which is corrected for local topography. Figure 3.2 shows the locations of the onshore stations used in preparing Figure 3.1, but not of the offshore estimates, distributed on a regular 2 minute grid. Onshore data were obtained from a variety of sources, but unfortunately the results of the many detailed gravity surveys carried out by oil companies remain confidential. The largest single available data set was assembled as part of the collaboration between the British Geological Survey (BGS) and the Geological Research and Development Centre (GRDC) during the period 1988-1995. Almost all of Sumatra south of the equator was covered at a reconnaissance level, although there are significant gaps in a few areas where access would have been especially difficult. In addition to the Sumatra mainland, measurements were made on Bangka and Billiton islands in the northeast and the Mentawai islands in the west (Fig. 3.2). GRDC have published numerous Bouguer maps at 1:250 000 scale showing contours, generally at 2 mGal intervals, and station locations. There are also two summary maps at 1 000 000 scale (Padang and Palembang sheets), contoured at 5 mGal intervals and without station positions. Terrain corrections, of up to 12 mGal, were applied in preparing the summary maps but were not used for any of the 1:250 000 detailed maps. The two versions of Bouguer gravity are therefore slightly different in the mountainous areas close to the Sumatran Fault but gradients in these areas are in any case steep, and overall patterns are very similar. Coverage north of the equator, principally by GRDC and LEMIGAS (the Indonesian Petroleum Research Institute), is less complete than in the south but is progressing rapidly. Moreover, Japanese universities working between 1977 and 1979 obtained data along many of the more important roads in the Lake Toba area (Fig. 3.2). In the northern forearc LEMIGAS collaborated with the University of London in surveys of all of the major islands (Milsom et al. 1991). Stations were mainly along the coasts, except on Nias. L E M I G A S / U o f L stations on Siberut were restricted to the southeastern corner, but the island was subsequently covered at a reconnaissance level by GRDC. In 1991 and 1992, stations were established along major roads throughout Sumatra by BAKOSURTANAL, the Indonesian

geodetic survey authority. A map showing the locations of the BAKOSURTANAL stations and Bouguer gravity contours after the application of a severe high-cut filter has been circulated on a very limited basis, but these stations are not included in Figure 3.2. An unfiltered but very small scale version of the BAKOSURTANAL Bouguer map was published by Kadir et al. (1996), and the data may also have been used by GRDC in preparing the 1:10000000 Bouguer anomaly map of Indonesia (Sobari et at. 1993). BAKOSURTANAL Bouguer values around the Toba caldera are generally 1 0 - 2 0 m G a l higher than those reported by the Japanese groups, a difference probably due to the lack of terrain corrections in the Japanese work. The onshore contours in Figure 3.1 are based on actual point gravity data where available, supplemented where necessary by values estimated at known BAKOSURTANAL station positions using the contours of Kadir et al. (1996). Accuracy is inevitably low where this has been done, and even so some significant gaps remain. The problem of making full use of good regional coverage where this exists and at the same time displaying in an acceptable way the results of interpolation across larger gaps has been addressed by overlaying the map based on a relatively fine (0.1 ~ grid, which is blank in areas of inadequate coverage, on a map produced using a much coarser grid and a greater degree of interpolation. This is obviously unsatisfactory as a quantitative method, but Figure 3.1 is intended to be used only qualitatively and the general patterns can be considered sufficiently well established to support regional interpretation. It is just possible on Figure 3.1 to identify discontinuities in the colour patterns at the edges of areas where the coarse grid has been used. Extending Figure 3.1 to include Billiton has brought western Java within the boundaries of the map. The data used were obtained in 1970 by the BGS, working in conjunction with the Geological Survey of Indonesia. The results of recent more detailed work on Java by GRDC are not shown but are generally compatible with the BGS survey.

Regional gravity patterns The most prominent features in Figure 3.1 are offshore. Gravity highs with north-south or N N E - S S W trends are associated with fracture zones and seamount chains on the Indian Ocean Plate and these control the positions of individual culminations on the broad flexural high at the outer margin of the Sumatra Trench. Two deep NW-SE-trending free-air lows, associated respectively with the trench and the forearc basin, intervene between this oceanic domain and the Sumatran mainland and are separated from each other by a high along the forearc ridge. The low over the trench exists because the mass deficit of the water column is not in local isostatic equilibrium but is balanced elastically by the offset mass of the subducting slab. Although the available gravity coverage is much less complete north of the equator than in the south, there can be no doubting the existence of fundamental differences between SE and NW Sumatra. In the south the Barisan mountains are associated with a narrow, discontinuous and rather weak Bouguer low that, where it exists, coincides quite precisely with the axis of the mountain range, but in the north the low deepens and expands to

GRAVITY FIELD

17

Fig. 3.1. The gravity field of Sumatra and the surrounding seas, based on data from sources discussed in the text. Contours are of free-air gravity offshore and Bouguer gravity onshore. The Bouguer reduction density is 2.67 Mg m -3. Faint white contours are bathymetry, at 200 m and at intervals of 500 m thereafter, from the GEBCO digital atlas prepared by the British Oceanographic Data Centre. The continuous black line running the length of Sumatra marks the approximate surface trace of the Sumatran Fault. The yellow line crossing the forearc basin near the equator marks the location of the interpreted profile of Figure 3.6. The black outlines enclosing the letters 'O' and 'B' indicate the locations of the gravity surveys of the Ombilin and Bengkulu basins shown in Figures 3.3 and 3.4 respectively. The letter B also indicates the approximate position of the town of Bengkulu. TS and T indicate, respectively, Lake Toba (including Samosir Island) and Lake Tawar. The letters 'IFZ' at about 97 ~ 30'E mark the central trough of the Investigator Fracture Zone. The inset shows the GEM-T3 long wavelength gravity field in the Sumatra region (see Lerch et al. 1994).

18

CHAPTER 3

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occupy most of the width of the island (Fig. 3.1). Values below - 6 0 mGal are associated with the Toba caldera and with an even deeper low (or, rather, a deeper culmination of the same low) that occurs farther north and extends as far as Lake Tawar (see Figs 3.1 and 3.2). The junction between the two gravity provinces (approximately along a line running N N W from Bengkulu) does not correspond to any of the terrane boundaries recognized in published accretion models of Sumatra (cf. Pulunggono & Cameron 1984) or to those identified in Chapter 14, and may reflect entirely post-amalgamation processes. It is, however, also possible that a major but hitherto unrecognized suture is being recorded by the gravity field.

Correlation of gravity patterns across major strike-slip faults can, in favourable circumstances, supplement straightforward geological matching as a means of determining total offsets. There is, however, little hope of identifying unambiguous gravity correlations across the Sumatran Fault because of the very rapid changes in gravity produced along and to the west of the fault by fault-parallel belts such as the volcanic Barisan range, the forearc basin and the forearc ridge. Detailed gravity surveys in mainland extensions of the forearc sedimentary basins and in inter-montane basins in the Barisan Highlands have revealed strong local correlations between sediment thickness and gravity field. These are, however, most

GRAVITY FIELD

precise where the basins are of only small lateral extent and are often not apparent on regional maps. The examples of the Ombilin intermontane basin and the Bengkulu forearc basin are discussed in more detail later in this chapter.

Toba-Tawar gravity low Low Bouguer gravity is to be expected in the mountainous regions of northern Sumatra because isostatic balance requires mass deficiencies at depth to support the topographic masses. Kadir et al. (1996) interpreted these low values as evidence for a structural model in which the crust is very thin and is underlain by low density mantle. The alternative, and more conventional, possibility is that the crust is in fact thicker in the vicinity of the gravity low than elsewhere and is underlain by normal mantle. Calculations based on a profile drawn across the strike of the gravity low near Lake Toba indicate that this solution is perfectly feasible and that a satisfactory crustal model can be developed on this basis without undue difficulty. The mode of compensation was discussed further by Masturyono et al. (2001), who drew attention to regions of low velocity (and hence, probably, of low density) in both the crust and uppermost mantle in two areas beneath the Toba caldera. However, they came to no firm conclusion as to the overall compensation mechanism. The Bouguer low covers an area vastly greater than the low velocity regions and the latter can therefore play only a subsidiary role in its formation. It is, however, probable that some compensation does occur within the crust and that the regional Bouguer low is due in part to the presence of a large granitic batholith that may still be in the process of formation. The T o b a - T a w a r low is almost entirely onshore. There is a weak possible extension out to sea to the north but this could be fortuitous and merely a consequence of the presence of relatively deep water and light sediments on the Mergui Shelf. A north-trending high that marks the western limit of the shelf at about 96~ is associated in part with a low-amplitude bathymetric high known as the Mergui Ridge but is probably mainly due to the transition from continental crust under the shelf to oceanic crust in the Andaman Basin. Shelf-edge free-air highs are the world-wide norm. They exist because the rapid shallowing of the Moho beneath continental slopes affects gravity fields near the edges of shelves even though the crust immediately beneath such locations is still thick and the sea is only a few hundred metres deep. The western limit of the T o b a - T a w a r low between about 96~ and 97~ is marked by a steep gradient defined by roughly north-south contours, and the northwestern tip of Sumatra is occupied by a gravity high with Bouguer values that in places exceed ,1,100 reGal. The average gradient between the base stations at Banda Aceh airport and town (Bouguer values -t-39 and +53.5 reGal respectively: see Adkins et al. 1978) is about one milligal per kilometre. The surface geology does not suggest a terrane boundary in this region and the gravitational change at the margin of the T o b a - T a w a r Low is probably largely a lateral effect of high mantle beneath the forearc basin, coupled with the effect of a change within the crust from young granitic rocks to an older and denser basement.

Eastern Sumatra Away from the Barisan Mountains, gravity fields in the vast and often swampy flatlands of eastern Sumatra are controlled by a number of competing factors. The most obvious of these is the subsurface presence in the region between the east coast o f Sumatra and the eastern margin of the South Sumatra Basin of the roughly north-south oriented Lampung Structural High (Pulunggono & Cameron 1984). The high separates the South

19

Sumatra (onshore) from the Sunda (offshore) basin, and the dense basement rocks, which almost reach the surface along its crest, produce high gravity fields. However, the magnitudes of the differences in gravity are smaller than those implied by the changes in sediment thickness and suggest some degree of crustal thinning beneath the basinal areas. A number of southwards-convex curvilinear gravity trends are superimposed on the local anomaly patterns in south and central Sumatra. These continue, and become even more prominent, offshore on the Sunda Shelf, where they are members of a set of curved anomalies that ring almost the whole of Borneo in an apparent rotational swirl. The trend lines cut across a number of Late Tertiary boundaries between basins and structural highs, including the Lampung High, and are therefore likely to be due to sources within the basement rather than to basement relief. An origin in strain accompanying the rotation of Borneo is possible, but the processes by which some of the observed gravity patterns could be generated by rotations are not clear. For example, the most prominent curved trend in the South China Sea is the shelf-edge anomaly at the western margin of the central oceanic basin (Holt 1998), and it is hard to envisage a causal link between this and Borneo rotation. An alternative explanation for the arcuate trendlines in Sumatra and on the Sunda Shelf is that these mark basement features associated with past subduction and accretion, implying that belts of former arc basement have been 'wrapped around' the core of continental SE Asia in Borneo and the Malay Peninsula. In eastern Sumatra there is some correlation between a curvilinear low sandwiched between two positive curved features and the location of the Mutus assemblage that may mark the suture between the Malacca and Mergui microplates (Pulunggono & Cameron 1984). The rotation and basement suture hypotheses can be combined by supposing that rotation of Borneo imposed curvature on sutures that were originally approximately straight.

Gravity effects of sedimentary basins The regional map of Sumatra (Fig. 3.1) is sufficient to show the broad gravity effects of most of the sedimentary basins but not the variations due to structures within them. Most of the oil-company data that might define such details in the main producing (back-arc) basins remain confidential, but there are published studies of detailed work done by LEMIGAS in the Ombilin intermontane basin (Situmorang et al. 1991) and the Bengkulu forearc basin (Yulihanto et al. 1995). The locations of these two surveys are indicated on Figure 3.1. The Ombilin Basin lies immediately to the east of the main Sumatran Fault (Fig. 3.1). It covers an area of some 1500 km 2 and in places contains more than 3000 m of Eocene to Middle Miocene sediments. It derives its economic importance from coal rather than oil or gas, and low density coals may well contribute to the gravity signature. The location suggests a genetic link to the Sumatran Fault, but Howells (1997b) interpreted the main basin as a result of wrench modification of an earlier rift rather than as a simple strike-slip pull-apart. Only the much younger Lake Singkarak rift (largely the area occupied by Lake Singkarak in Fig. 3.3) is now interpreted as having formed as a recent pullapart within the Sumatran Fault (Sieh & Natawidjaja 2000). There is good correlation between thin and thick sediments and gravity highs and lows (Fig. 3.3), both in relation to relatively small structures (Situmorang et al. 1991) and also at a basinwide scale. High Bouguer values define the main horst that separates the Ombilin Basin proper from Lake Singkarak. Low ( < - 2 0 mGal) Bouguer gravity characterizes the northern lobe of the Palaeogene basin, but these values, some 50 mGal below those on the horst block near Sulitair, are still higher than the levels (of well below - 30 mGal) in the Singkarak rift. The difference could be due to differences in sediment thickness, to more

20

CHAPTER 3

i - 0" 3 0 ' S 9

~Bohiam" o

100 ~ 45'E

IOF'E

i

i

developed isostatic compensation of the older depocentre or to the Neogene section having a significantly lower average density. The Bengkulu Basin (Fig. 3.4) is roughly the same age as the Ombilin but lies entirely west of the Sumatran Fault and at much lower elevations. A large part of it lies offshore. Traditionally, it too has been regarded as a pull-apart basin generated in a transtensional regime and this interpretation is still generally accepted (Yulihanto e t al. 1995). There are very few BGS/ GRDC gravity stations in the part of the basin lying to the SE of Manna (Nainggolan et aL 1992) and even in the west

102' 3{}'E

Fig. 3.3. Bouguer gravity and main structural controls of the Ombilin Basin, after Situmorang et al. (1991). Contour interval 10 mGal (thick contours) and 2 mGal (thin contours). Stipple indicates closed lows. Steep gradients in the west of the area are associated with the margins of the Late Neogene Singkarak pull-apart basin. Weaker, but still well defined anomalies are associated with the Palaeogene basin and testify to the complexity of the basement architecture. See Figure 3.1 for location.

103E

3 30'S

PAGARJATI 30 Bengkulu

MASMAMBANG

the regional survey provided only patchy coverage (Sobari e t al. 1992). However, very detailed onshore surveys for oil exploration (Yulihanto e t al. 1995) have confirmed the division of the main basin into two structural lows. These features (the Pagarjati Graben in the NW and the Kedurang Graben in the SE) are oriented very roughly north-south and are separated by the Masmambang High. Within these broad divisions, a series of roughly equi-dimensional highs and lows cover areas similar in size to those occupied by sub-basins within the Ombilin. A peculiarity of the Bengkulu Basin is the very high level of background gravity field, which results in strongly positive ( > + 4 0 mGal) absolute levels of Bouguer gravity even in the centres of the gravity lows. The basinal area overall appears on regional maps as a gravity high and Bouguer levels on the horst blocks may exceed -t-80 mGal (Fig. 3.4). The high fields probably reflect crustal thinning beneath both the Bengkulu sedimentary basin itself and the forearc marine basin. However, the offshore extension of the high, which is associated with a bathymetric bulge, is probably also partly due to the replacement of seawater by young sediments and to the lack of any corresponding compensatory local subsidence of the crust into the mantle. Such patterns are seen over many young deltas formed at passive continental margins, the Congo and Niger deltas being good examples (Sandwell & Smith 1997).

4S

The forearc

KEDURANG Manta

0_...........20 k,, Fig. 3.4. Bouguer gravity of the Bengkulu Basin, after Yulihanto et al. (1991). Contour interval 5 mGal (thick contours) and 1 mGal (thin contours). The overall high level of Bouguer gravity is probably largely a consequence of crustal thinning beneath the forearc basin. Local closed lows, indicated by stipple, identify the locations of separate depocentres within the basin. See Figure 3.1 for location.

basin

The northeastern margin of the deep free-air low associated with the trench west of Sumatra includes the frontal part of the forearc ridge, which is composed largely of accreted material. The crest of the forearc ridge is marked by a prominent asymmetric high, with the steeper gradients towards the forearc basin. In most cases, Bouguer gravity on the forearc islands decreases from west to east in response to increasing crustal thickness (Fig. 3.5), but on Nias there is a residual gravity high centred over the young uplifted coastal plain in the east of the island (Fig. 3.5, inset). Low free-air and Bouguer gravity characterize most of the forearc basin, with minimum values even lower than the free-air minima associated with the trench. The forearc basin low is, however, divided into two segments by a gravity high near the equator (Fig. 3.1), where a Bouguer maximum of +100 mGal has been recorded on Pini, the easternmost island in the Batu group (Fig. 3.5). Pini has an anomalous east-west orientation,

GRAVITY FIELD

A

21

S.I.O. RAMA 6 Line 58-59 Pleistocene- Recent

B a n y a k Is, Interpretation simplified after Matson & Moore (1992)

ill

-b

X\x"X\.,

/. .././/"

I~

///"

Pini

///"

///"

\ Equator

0 ~ - T 7

50kin ~ s

~ ~2~l~I~E

'~

Batu Is. 98~

lies just north of the equator and straddles the forearc basin. The high gravity is evidently not due merely to the presence of the bathymetric high, since a gravity low is associated with similar bathymetry in the Banyak group further north. A - 8 0 mGal m i n i m u m was recorded on the most easterly of the Banyak islands (Fig. 3.5). There is an obvious geographical correlation between the Pini high and high free-air gravity associated with the Investigator Fracture Zone on the Indian Ocean plate immediately to the south (IFZ; Fig. 3.1). A causal link between the two seems likely. Subduction of the fracture zone, which is a prominent bathymetric feature consisting of a deep linear trough flanked by two high standing ridges, has been suggested as a possible cause for both the change in strike of the trench and forearc north of Nias (the Nias 'elbow') and the enhanced volcanic activity in the Toba region (Fauzi et al. 1996).

Fig. 3.5. Gravity variations in the central forearc basin. In contrast to Figure 3.1, Bouguer gravity is contoured in the offshore as well as the onshore regions. Contour interval 10 mGal. Offshore data are from Scripps Institution of Oceanography (SIO) shipborne readings along the tracks shown by dashed lines. Areas of water depths in excess of 500 m in the vicinity of the Banyak and Singkel sedimentary basins indicated by light stipple. The upper inset shows SIO seismic line 58-59 across the Banyak forearc basin east of Nias, from SIO cruise Rama 6, with a simplified version of the interpretation provided by Matson & Moore (1992). Note the strong asymmetry in the basin. The lower inset shows the residual gravity anomaly over eastern Nias, obtained by subtracting a linear regional gradient parallel to the trends of Bouguer contours in the north of the island from the local values.

In part the low Bouguer and free-air values in the forearc basin reflect the presence of the water column, which is up to 1500 m thick, but there is also a significant contribution from light Neogene sediments The seismic stratigraphy of the area east of Nias was first described by Beaudry & Moore (1985), who recognized three main sequences and assigned these tentatively to the Pleistocene (Unit 4), the Pliocene and uppermost Miocene (Unit 3) and to most of the remainder of the Miocene (Unit 2). Unit 2 was further subdivided into Units 2a and 2b, separated by a generally continuous, high-amplitude seismic event. Older stratified sediments (Unit 1) can be seen in places beneath a strong regional unconformity at the base of Unit 2a, but elsewhere this region is devoid of reflectors and may comprise igneous or metamorphic basement or steeply dipping sediments. With one exception, Beaudry & Moore (1985) illustrated their discussion with oil industry seismic sections which were of

22

CHAPTER 3

rather poor quality (at least in reproduction), and the boundaries they recognised are sometimes difficult to identify on the better quality sections obtained by the Scripps Institution of Oceanography (SIO) on cruise R A M A 6. In a more detailed analysis based on the SIO profiles, Matson & Moore (1992) divided the forearc sediments into eleven sequences, of which Sequences 10 and 11 were roughly equivalent to Unit 4 of Beaudry & Moore (1985) and Sequences 8 and 9 to Unit 3. At deeper levels the correlation between the two schemes is less clear. As well as an increase in detail, Matson & Moore (1992) provided a significant new insight into the stratigraphy of the forearc basin by distinguishing between the histories of a 'Singkel' and a 'Pini' basin east of Nias. Unfortunately, their use of the term Singkel Basin differed from that of earlier authors (e.g. Karig et al. 1980), who applied it to a basin in the Singkel region of mainland Sumatra. The term Banyak Basin is used here as a preferable alternative. The Pini Basin was considered to be mainly filled with Upper Miocene sediments but the Banyak Basin (shown in the inset to Fig. 3.5) was interpreted as containing significant older section. Both sedimentary basins are associated with present-day sea floor depressions (Fig. 3.5), although the modern and palaeo-depocentres do not coincide exactly. The division between the two basins is marked by a gravity high offshore and by a residual gravity high on Nias (Fig. 3.5). On seismic sections, the most obvious feature of all the forearc sedimentary basins is their extreme asymmetry (see Karig et al. 1980; Beaudry & Moore 1985; Matson & Moore 1992; Malod & Kemal 1996). In the Banyak Basin (Fig. 3.5, inset), a Middle Miocene shelf has been tilted seawards and is now buried under younger sediments that increase in thickness up to the east coast of Nias, where sediments as old as Oligocene are exposed (Samuel & Harbury 1995). The sharp flexure at the western edge of the basins can be identified with the Mentawai Fault (see Chapter 2) and on the regional gravity map (Fig. 3.1) is associated with a steep gravity gradient that is, in fact, rather less pronounced near Nias than elsewhere. Where, SE of Enggano, the fault moves away from the flank of the forearc ridge and towards the centre of the forearc basin (Schltiter et al. 2002), this gradient largely disappears. Despite the high gravity fields, both geological mapping (Samuel & Harbury 1995) and gravity modelling (Kieckhefer et al. 1981) indicate that the material forming the forearc ridge is of generally low density (Fig. 3.6). The high fields are produced by the thin crust and the high density subducted slab, and by the large density contrast between even the lightest rocks and water. Onshore mapping and offshore seismic reflection lines all suggest that large volumes of sediments deposited in the forearc basin have been incorporated into the forearc islands. Only on Simeulue, where a small ophiolite is associated with a local gravity high (Fig. 3.2, inset) is there evidence for the presence of coherent masses of oceanic rocks beneath the ridge (Milsom et al. 1991). Gravity provides few constraints on the nature of the crust beneath the forearc basin. In one of the two alternative models of Kieckhefer et al. (1981) the basin is underlain by m61ange and in the other (reproduced here in slightly modified form as Fig. 3.6) by continental crust. In both models the forearc ridge is underlain by m61ange, and both produce acceptable fits with the gravity profile along the modelled line. As far as the Mentawai Fault is concerned, it is not the gravity data but the extreme linearity that suggests its location has been determined by the position of the former continental margin rather than by the boundary between two belts of m61ange.

Seismic tomography and the long-wavelength gravity field Despite significant recent advances in the measurement of the Earth's gravity field, the long wavelength variations are still

reGal

-5O

1

Fig. 3.6. Interpretation of a gravity profile across the forearc basin and Sunda Trench south of Nias, after Kieckhefer et al. (1981). White and black inverted triangles show the locations of controls on depth provided by, respectively, unreversed and reversed seismic refraction profiles. Densities on blocks in the model are in Mg m -3. Unlabelled blocks are sediments or m61angewith densities between 2.0 and 2.4 Mg m-3. The differences between the calculated and observed curves are too small to be apparent at the scale of the figure. Profile location shown as a yellow line on Figure 3.1.

most reliably estimated from perturbations of satellite orbits. A number of models have now been produced that integrate the results obtained by this method with results from conventional surface gravity surveys and satellite altimetry to define global gravity anomalies with half-wavelengths greater than about 400 km. The sources of these anomalies are likely to lie deep within the mantle, because the isostatic equilibrium prevailing in the Earth's outermost layers implies approximate cancellation of the gravity fields from shallower mass differences. Controversy about the origin of mass anomalies within the mantle has existed for decades. A rough correlation between geoidal highs and plate convergence zones has long been recognized (cf. Hagar 1984) but has appeared unconvincing in detail. If, however, using the same basic data, field strength (the differential of potential) is contoured rather than potential itself, the longest wavelengths are suppressed and the correlation with subduction becomes very striking (Milsom & Rocchi 1998). Major highs can be seen to the rear of almost all long-lived subduction zones, and it is reasonable to suppose that the mass excesses are associated with the subducting slabs. Since these slabs are sinking through the less dense asthenosphere, isostatic considerations do not apply. One of the most widely used of the long-wavelength (400 k m § gravity models is GEM-T3 (Lerch et al. 1994), which is complete to spherical harmonics of degree and order 50. The GEM-T3 map of the Borneo-Sumatra region (Fig. 3.1, inset) shows a distribution of long-wavelength gravity highs consistent with hypothesized patterns of past subduction. In eastern Borneo and Sulawesi, geological mapping has defined former subduction traces, marked by m~lange and ophiolites, that indicate that a

GRAVITY FIELD

part of the active margin of SE Asia lay in this area during the Late Cretaceous and Palaeogene (e.g. Wilson & Moss 1999). From southeastern Borneo the line of subduction then curved sharply to pass through western Java and on to Sumatra. Subducted lithosphere associated with this phase of convergence can be expected to have accumulated beneath Borneo and the Malacca Straits. Moreover, many theories of the evolution of Borneo require there to have been subduction beneath its northwestern margin during the Late Cretaceous and Palaeogene, leading to the complete destruction of a 'proto-South China Sea' and collision between the Borneo block and attenuated continental crust rifted from the South China margin (e.g. Milsom et al. 1997). The extent of the long-wavelength gravity high suggests that it may be recording effects from material subducted beneath Borneo from the south, east and west (Milsom & Rocchi 1998). In northwestern Sumatra, the margin of the long-wavelength high curves to an almost northerly trend and peak values decrease quite rapidly, suggesting that there is no significant deep subducted material beneath the Andaman Sea. This seems reasonable since, although the plate boundary west of the Andaman and Nicobar islands is marked by a (rather poorly defined) trench, the local convergence vector is almost parallel to the trench axis. Further light on the sources of the long wavelength gravity anomalies has been provided by the improvements in, and standardization of, seismic observatory instrumentation and the dramatic increases in speed and memory of relatively cheap computers. Thanks to these two developments it is now possible to use observations of travel times for S and P waves from remote earthquakes to model the variations of seismic wave velocities in the mantle. This seismic tomography is providing ever stronger evidence for the penetration of subducted lithosphere through the discontinuity between the upper and lower mantle at about 700 kin, below which it is not seismogenic. Because Wadati-Benioff seismic zones marking the sites of subducted lithosphere in the upper mantle are invariably associated with

23

high seismic velocities, there is a strong circumstantial case for attributing high velocity in the lower mantle to lithospheric material that has sunk to aseismic depths. The close correlation between high velocity in the lower mantle (Widiyantoro & van der Hilst 1996, 1997) and high gravity field provides additional support for this hypothesis. Tomography also provides an explanation for the absence of earthquake hypocentres at depths of more than 300 km beneath Sumatra. There is no high-velocity material at these depths (Widiyantoro & van der Hilst 1996) and hence, presumably, no subducted slab. Taken together with the interpreted presence of a large volume of dense and fast material below 700 kin, this observation supports hypotheses that involve the rupturing of slabs and the independent sinking of their detached lower portions under gravity. Even stronger support comes from farther east, north of Java, where the upper part of the detached slab protrudes above the 700 km limit and is both seismically 'fast' and seismogenic (Widiyantoro & van der Hilst 1996). The Sumatra region also conforms to the global pattern of lack of correlation between high gravity and subducted lithosphere within the seismogenic zone, i.e. at relatively shallow depths. Hagar (1984), amongst others, has used this global observation to support a model of dynamic flow that produces, at GEM-T3 wavelengths, close to perfect cancellation between the effects of positive and negative density anomalies in the upper mantle. Some doubt has, however, been thrown on this model by Wheeler & White (2002), who used oil-industry borehole data to argue that, at least in offshore SE Asia, dynamic topography amounts to no more than 300 m. Predictable improvements in data quality will undoubtedly lead to considerable refinements in interpretation and resolution of this apparent discrepancy, but it is sufficient to note that as far as the present review is concerned, the GEM T-3 gravity field provides an excellent guide to the extent of Palaeogene, but not Neogene, subduction beneath Sundaland.

Chapter 4

Pre-Tertiary stratigraphy A. J. BARBER & M. J. CROW

In the early days of mineral exploration on behalf of the Netherlands East Indies Bureau of Mines and of petroleum exploration by the oil companies it was recognized that PreTertiary rocks were extensively exposed in the Barisan Mountains in the western part of Sumatra (Fig. 1.4). These rocks are variably metamorphosed and were termed the 'Barisan-Schiefer' and the 'Old-Slates Formation' (Veerbeek 1883) in Central Sumatra, and the 'Crystalline Schists' in the Lampung area (Westerveld 1941). Locally these rocks contain fossils, and it was recognized that Carboniferous and Permian rocks occur within this PreTertiary basement. Some basement units were defined during the mapping of Sumatra by the Netherlands Indies Geological Survey between 1927 and 1931, but the definition of units according to modern stratigraphic principles began in the early 1970s, with the commencement of systematic mapping by the Indonesian Geological Survey in collaboration with the United States Geological Survey, in the Padang area of West Sumatra (Kastowo & Leo 1973--Padang; Silitonga & Kastowo 1975-Solok; Rosidi et al. 1976--Painan and Muarasiberut). Mapping and the definition of further units was continued in northern Sumatra by the Indonesian Directorate of Mineral Resources/British Geological Survey (DMR/BGS) between 1975 and 1980 as part of the Northern Sumatra Project and was extended into southern Sumatra in the 1980s and 1990s by the Indonesian Geological Research and Development Centre (GRDC), DMR amd BGS. The results of these surveys, which established the distribution of the basement units, are published by GRDC as 1:250000 Geological Map Sheets coveting the whole of Sumatra and adjacent islands (Fig.l.5). The lithologies of each stratigraphic unit are briefly described in the keys to the maps, and the units are described more fully in the accompanying Explanatory Notes. During these surveys the faunas from known fossil localities were re-examined and new localities were found. Following the survey the palaeontological evidence for the ages of stratigraphic units in Sumatra has been reviewed by Fontaine & Gafoer (1989). It has now been established that fossiliferous rock units in the PreTertiary basement of Sumatra range in age from Early Carboniferous through to mid-Cretaceous. From the occurrence of tin granites in the eastern part of Sumatra, extending into the 'Tin Islands' of Bangka and Billiton, it is supposed that the whole of Sumatra is underlain by a highly differentiated Pre-Carboniferous crystalline continental crust with ages extending back into the Precambrian. Direct evidence for a Pre-Carboniferous basement has been obtained by isotopic dating of Silurian and Lower Carboniferous granitic rocks encountered in boreholes beneath the Tertiary Basins towards the northeastern side of the island (Eubank & Makki 1981). The oldest rocks identified by their fossil content were also encountered in boreholes in eastern Sumatra. These rocks contain palynomorphs from near the Devonian-Carboniferous boundary (Eubank & Makki 1981). Older rocks, possibly ranging down into the Devonian, were reported by Adinegoro & Hartoyo (1974) from a borehole in the Malacca Strait, but no details are given in their report and a Devonian age for sediments elsewhere in Sumatra has not been confirmed during subsequent drilling or by field studies, although rocks of this age, and older ages back to the Proterozoic, occur in the Langkawi Island off NW Malaya, 300 km to the NE of Sumatra (Jones 1961).

24

It has proved very difficult to establish with certainty the stratigraphic relationships between the various rock units which make up the exposed Pre-Tertiary basement of Sumatra. This is due to the generally fault-bounded contacts between rock units and the poor biostratigraphic control on their ages; over large areas the rocks are apparently devoid of fossils. The varying metamorphic grade of the basement units makes even lithological correlations difficult. As a result, formations have generally been defined locally. When these local units have been extrapolated over broader areas they are found to include a wide variety of lithological types, so that correlation with the original units becomes more and more uncertain. The spate of new data on the geology of Sumatra generated by the systematic geological survey of the whole island has stimulated attempts at regional synthesis, e.g. Cameron et al. (1980) and Pulunggono & Cameron (1984) in northern Sumatra and McCourt et al. (1993) in southern Sumatra. These authors proposed a stratigraphic scheme which distinguished a CarboniferousPermian Tapanuli Group, a Permo-Triassic Peusangan Group and a Jurassic-Cretaceous Woyla Group (Fig. 4.1 ). This terminology is used in the present account, although it is strictly applicable only to northern Sumatra where the units were defined. In this account the basement rocks of Sumatra are described from northern, central and southern Sumatra, as far as possible in terms of their stratigraphic age, although difficulties in establishing these ages will be fully discussed. Five age units are recognized: Pre-Carboniferous basement, Carboniferous?Early Permian, M i d - L a t e Permian, Mid-Late Triassic and Jurassic -Mid-Cretaceous.

Pre-Carboniferous basement Eubank & Makki (1981 ) record shales interbedded with quartzites from the boreholes, Pusaka-l, 85 km NE of Pekanbaru, and Rupat Island, in the Malacca Strait, which yielded palynomorphs lu the Devonian-Carboniferous boundary, and used this evidence to define an Upper Palaeozoic 'Quartzite Terrain' in eastern Sumatra (Fig. 4.2). Some of these borehole records may relate to quartz sandstones in the Triassic Kualu Formation and its correlative Tembeling Sandstone of Bangka (Ko 1986). However, Eubank & Makki (1981) also obtained R b - S r ages of 426 + 41.5 Ma (Silurian) and 335 + 43 Ma (Early Carboniferous) from granites from boreholes put down into the basement beneath the Central Sumatra Basin. Turner (1983) reports gneissose rocks included as xenoliths in dykes intruding Carboniferous slates near Rao, Central Sumatra. These xenoliths were presumably derived from an underlying crystalline basement. A granitic clast from pebbly mudstone encountered in a borehole, Cucut No.l, gave an R b - S r age of 348 ___ 10 Ma, of Vis~an, Early Carboniferous age (Koning & Darmono 1984). The occurrence of intrusive granites, possibly as old as Silurian, indicates that an older basement into which these granites were intruded underlies eastern Sumatra. This is highly probable, as Proterozoic and Lower Palaeozoic rocks occur in the Malaysian Langkawi Islands only some 300 km to the NE of Sumatra along the strike (Jones 1961). Indeed, Hutchison (1994) has asserted that the buried Kluang Limestone south of Palembang,

PRE-TERTIARY STRATIGRAPHY

CENOZOIC CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

CARBONIFEROUS DEVONIAN LOWER PALAEOZOIC PRECAMBRIAN BASEMENT

Fig. 4.1. Pre-Tertiary stratigraphic units in Sumatra as proposed by the DMR/ BGS Northern SumatraProject (Cameronet al. 1980) and used on the geological maps of northern Sumatra publishedby GRDC. These units were extended to cover southern Sumatra by McCourt et al. (1993).

for which a Cretaceous age had been suggested (De Coster 1974) resembles the Silurian Kuala Lumpur Limestone in Malaya and may therefore be of Silurian age. It has also been supposed that high grade metamorphic rocks in the western part of northern Sumatra within the Alas and Kluet Formations, and the Ngaol Formation of Central Sumatra, which do not appear to be directly related to contact metamorphic aureoles around intrusions, may represent outcrops of this Pre-Carboniferous crystalline basement, but nowhere has this supposition been confirmed by fossil finds or by isotopic dating. Alternatively it has also been suggested that these high grade gneisses are due to intrusion and synkinematic deformation of granites and associated sedimentary rocks in shear zones during the formation of active magmatic arcs during Permian to Late Cretaceous times. This explanation has also been suggested for the Gunungkasih Metamorphic Complex in the Bandarlampung area of southern Sumatra (Barber 2000). The high grade metamorphic rocks of Sumatra require systematic investigation with these alternative possibilities in mind.

T a p a n u l i Group (Carboniferous- ? E a r l y Permian) Rocks in northern Sumatra considered to be of Carboniferous?Early Permian age have been classified as the Tapanuli Group (Cameron et al. 1980; Pulunggono & Cameron 1984). Three formations are recognized: the Bohorok Formation, the Kluet Formation and the Alas Formation (Figs 4.1-4.3). The Early Permian was included in the original definition of the Tapanuli Group on the supposition that the Alas Formation contained an Early Permian fauna (Cameron et al. 1980). Subsequently this fauna was shown to be of Early Carboniferous (Vis~an) age (Fontaine & Gafoer 1989). However, the Pangururan Bryozoan Bed which was mapped as part of the Kluet Formation also contains a probable Early Permian fauna (Aldiss et al. 1983), so that in this account the Tapanuli Group is considered to extend into the Early Permian.

25

The Bohorok Formation is defined from its type locality in the Bohorok River on the GRDC 1:250 000 Medan Sheet, about 65 km to the west of Medan (Cameron et al. 1982a) (Fig. 4.3). Good exposures of this formation occur for a distance of 100 m in the river section at Bukit Lawang, near the Orang Utan Sanctuary and over 50 m in the Bekail River, some 7 km to the south. No base is seen to the formation and downstream the mudstones are faulted either against the Permo-Triassic Batumilmil Limestone Formation, or the Tertiary Bruksah and Bampo Formations. The Bohorok Formation has been mapped along the eastern side of the Barisan Mountains from near Langsa in the north to Lake Toba in the south (Fig. 4.3). Even further south, comparable lithologies correlated with the Bohorok Formation, are found in the Tigapuluh Mountains, between Rengat and Jambi and are described below as the Tigapuluh Group, and similar rocks also occur in the Toboali District in the southern part of Bangka Island (Fig. 4.2). The typical lithology of the Bohorok Formation is an unbedded 'pebbly mudstone'; a poorly sorted breccia or conglomerate composed of angular to subangular rock fragments, generally 0.1-2.0 cm in size, but ranging up to 10cm and even 7 5 80 cm in east Aceh, and in the northeastern part of the Padangsidempuan Sheet (Aspden et al. 1982b). The rock fragments are enclosed in a fine-grained matrix of dark grey or dark brown siltstone or mudstone. Pebbles include vein quartz, slate, chlorite schist, phyllite, greenish calcsilicate rocks, limestone, marble, quartzose arenites, quartzite, more rarely mica-schist and granitoid, sometimes with tourmaline, rare chert and rhyolite. Single crystals of fresh microcline, forming small angular clasts, are conspicuous in thin sections (Cameron et al. 1982a). The clasts in the pebbly mudstones clearly indicate a continental provenance. In the Berkail River, pebbly mudstone near the upper part of the outcrop is interbedded with a few metres of light brown weathering, coarse to very coarse sandstone (Tiltman 1985). Cameron et al. (1982a) report that sandstone blocks found as float within the Bohorok outcrop show graded beds and slump structures. Towards the west the poorly sorted pebbly mudstone units become less common, the proportion and size of the clasts decreases, and the Bohorok Formation is represented by conglomerates, sandstones, slates and rare limestone units, becoming indistinguishable from the adjacent Kluet Formation or similar lithologies within the Alas Formation, so that the distinction between the units is arbitrary (Cameron et al. 1980). The Bohorok Formation has generally been affected by low, slate-grade, metamorphism. In the neighbourhood of igneous intrusions argillaceous rocks, including the matrix of the pebbly mudstones, are converted to schists or hornfels, often containing cordierite and tourmaline. Sediments within the Bohorok Formation are apparently devoid of fossils. The only direct evidence of age comes from the Cucut No. 1 well (Fig. 4.4) where Koning & Darmono (1984) report an Early to Mid-Carboniferous microflora from the mud matrix of a 'pebbly mudstone'. However, a granite clast in the mudstone from the same well yielded a K - A r age of 348 + 10Ma (Vis6an, Early Carboniferous) (Koning & Darmono 1984). This juxtaposition is highly improbable. It may be that both the palynomorphs and the pebble were eroded from older units and derived into the Triassic Kualu Formation which occurs in the same area, or that the K - A r age is unreliable. The pebbly mudstones of the Bohorok Formation have been interpreted as diamictites formed in a glacio-marine environment (Cameron et al. 1980). Pebbly mudstones similar to those of the Bohorok Formation have been described form the Langkawi Islands and the adjacent parts of the NW Malay Peninsula, Peninsular Thailand, Burma and southwest China. The occurrence of pebbly mudstones has been used to identify the Sibumasu (Siam, B___uurma,Malaya, Sumatra) Terrane, a crustal block which extends all the way from Sumatra to southern China (Metcalfe 1984). Bohorok Formation.

26

CHAPTER 4

I

I

102 ~

96 ~

I

104 ~

106 ~

CARBONIFEROUS Tapanuli Group

_8 ~

Bohorok Formation

/

Alas Formation

LANGKAWI~ _6 ~

;inaa

~3

ubang ~, =asu ,rmation-~"~7/._

Formation

BANDA ACEH

"k,,

[ 5:~ 61 'Quartzite Terrain'

a

_4 ~

Kluet/Kuantan Formation

%

TAPAKTUAN~t Kreung Klue

(

~

Lake

SIDIK/

.%

_2 ~

~A~

x0 \ _0 o

Member

L

q; _2 ~

Lake Sinekar NGKA

m M U A R A B- ~U: ,N. ~G, ~O# 9

.

D u a b e l a s ~:~

Q

T a r a n t a m Form;

P A L E M B A N G II

_4 ~ iG a r b a M o u n t a i n s

T ra0 Forma,ion) ~Gunungkasih LZ~,',, ~ 0

100

200

300

400

500km

KO

TA

G'--0 AGUNG ~...

Complex

~,TANJUNG ~ARANG

",,3 " ' ~ ~ 6 L z

_6 ~ 96 ~

98 ~

100 ~

102 ~

I

I

I

I

~o4o I

11o6o I/

Fig. 4.2. Distribution of Carboniferous to ?Early Permian rocks in Sumatra from GRDC geological maps. Dense tones indicate outcrops, the filled circles indicate Carboniferous rocks encountered in boreholes, paler tones indicate subcrop beneath Late Palaeozoic, Mesozoic, Tertiary and Quaternary sediments and volcanics.

PRE-TERTIARY STRATIGRAPHY

I

I

I

I

99~

98 ~

97 ~

96 ~

27

B A"N D A' A'-CEH Major Faults

Recent Volcanoes Unit

@

Permo-Triassic Intrusions

,E~::~Ujeuen tion

Sormation Tawar " ~ Formation LATE PERMIAN - LATE TRIASSIC |N (Peusangan Group) Uneun Unit, Tawar Lst Fro, Situtup Lst Fm, Sembuang Lst Fm, Ujeuen Lst Fm, Kaloi Lst Fm, Batumilmil Lst Fm (mainly limestones)

9 LANGSA

Simpang

Gnei

Kiri

Kaloi Formation

Kualu Formation (cherts & clastics) CARBONIFEROUS - ?EARLY PERMIAN (Tapanuli Group) ~. ,U.:0XkJ'~i

,:-,,C.e-.-.<-

Bohorok Formation (pebbly mudstones) Atas Formation (Vis6an) limestone member

Bohorok

Alas Formation - clastic sediments ('m'- metamorphosed) Kluet Formation (turbidites with limestone %') 9-.,....,_ =_

i i lU... =.

Ktuet Formation (metamorphosed) 9 6 <,

1

\ TA P A K T U A N

N alvvampu

Toba

tumilmil

Tufts

--.. (._~Kualu Formation

o

lOOk~

Toba Tufts

~ - ~ I~j

97 ~

I

Fig. 4.3. The distribution of Carboniferous, Permian and Triassic stratigraphic units in northern Sumatra, showing rock types and critical fossil localities, together with Late Permian to Early Triassic intrusions (after Stephenson & Aspden 1982, with additions from GRDC map sheets, Cameron et al. 1982a, b, 1983). Areas left blank are occupied by Late Mesozoic to Quaternary sediments and volcanics.

Alas Formation. The Alas Formation was defined by Cameron et al. (1982a) in the valley of the lower Alas River on the Medan Sheet (Fig. 4.3). It is distinguished by its geographical location, occupying a graben within the Sumatran Fault System, between the outcrops of the Bohorok and Kluet formations, and by a preponderance of limestones and meta-limestones. Otherwise, in the remainder of the outcrop, shales, siltstones, sandstones, sometimes calcareous, quartz wackes and conglomerates, are identical to those of the Bohorok Formation, without the pebbly mudstones, and to the Kluet Formation as well. Cameron et al. (1982a) also report the occurrence of possible green tufts. The outcrop is much dissected by faults and the rocks are intensely folded locally, intruded by granites and migmatised. Limestones in the Alas Formation are sometimes oolitic, may show cross-bedding and are locally fossiliferous with abundant productid and spiriferid brachiopods and some corals. However, the limestone is frequently metamorphosed to massive, coarsely crystalline and sometimes graphitic marble with phlogopite, and deformed to form calcareous schist. The marbles and calcschists are associated with slate, phyllite, mica schist, locally containing garnets, biotite hornfels with cordierite and/or chiastolite, quartzite and more rarely gneiss, migmatites, mylonites and cataclasites (Cameron et al. 1980). Much of this metamorphism may be attributable to the contact effects of intrusive granites, affected synchronously or subsequently by shearing, but not all

areas of metamorphic rocks are closely associated with igneous intrusions and some, particularly where the rocks are garnetiferous, may be of regional metamorphic origin and may even represent an earlier, Pre-Carboniferous, basement. The occurrence of mylonites and cataclasites suggests that some of the rocks included in the Alas Formation have undergone major shearing. A fossiliferous limestone locality within the Alas Formation at the junction of the Lau Pakam and the Sungai Alas north of Laubaleng has yielded a rich fauna (Fig. 4.4). Cameron et al. (11980) reported the coral Allotriophyllum chinense, known from the Lower Permian Chiksa Limestone of southern China, but this coral has been re-identified by Fontaine (1989) as the solitary horn-shaped rugose coral Zaphrentites, indicative of a Carboniferous age. Brachiopods, which include Cleiothyridina (?) and Marginatia, indicate a Vis6an age and Metcalfe (1983) obtained a conodont fauna from this same locality which included Gnathodus girtyi rhodesi Higgins, Gnathodus sp., Hindeodella sp., Spathognathodus campbelli Rexroad and Spathognathodus scitulus (Hinde), confirming the Vis6an age of the limestones. The form Gnathodus girtyi rhodesi, in particular, is restricted to the Bollandian Stage of the Late Vis6an, defining the age of this outcrop of the Alas Formation even more precisely (Metcalfe 1983). Kluet Formation. The Kluet Formation was defined by Cameron et al. (1982b) from outcrops along the Krueng Kluet in the

28

CHAPTER 4

\

I o L %.~.' :' ~ \ A I as..~LAk-,e,- 9 9I- ~.....T. .o. b. .a. .T. u f f s . . . . . . . . ( 98 ~ ~ % , 1 , Formation Toba,~: : : u9 L\-'.b.b,~.'~'-,_"IP"S l O / K A L ~ N G - ' ~ ' ~ } 4 ~ . " . ~ L . . . " . " . ". ". ". ? 9

/..,~ -,,-',,...z ~ - 2'>N

0

__

........

50

.'.'.'.

".

100km ...'"':",

,

L~.~:~;~:;~I BohorokFormation [~i:~,~:~1 (Pebblymudstones) ~7,~.~ Alas Formation (limestones) Kluet/Kuantan Formati Limestone Member (L _

Equator

98o I

.

"~

~ ~N, k,~ ~

~'k~--~?~-~\ ~ _ ~'~%'L~,~DANG S[ DEN P UA a

~'k ~

I 101~, Ma'or Faults

".'." '.'.'," 9

~ ~ ~ ~ _ _ ~ " 9 9~ Pangunjungan ",'~\'N~N"~E.~%8. i" i'-".N~ki~,Sibagandidg ~ " " "~--dq:~.~li~_Member - t " Pal~ka ." ".'-LimestoneMemlSer'. "~~"~%~aF~ir'.'.\:.'.'.-.#/~i!~~IRANTAUPRAPAT ~ue~-.'."~"A \ \ ' . ' . " % % " - ' . " - ~ ~ ~ ; ? " Z~ ~ -Formation_C~"~ \%-v--:7 "~k 9 9 .'~i~q./'-,~./~/ ~Y.?N. e~aru 9." . . , .'. 9 ". ". ".

LATE PERMIAN-LATE TRIASSIC (Peusangan Group) .rkualu . . r-m, . . bllungKang . . . ~-m, Telukkido Fro, Cubadak Fm Zuhur Formation CARBONIFEROUS-?EARLY PERMIAN (Tapanuli Group) ......... ,

ozoaBed.

I 10~ lc-~ '~ ~

(".,~ ~

Recent Volcanoes Permo-Triassic Imrusions

4 - ~'~'~ N\~

~ ' ~ " ~

2~

Bohorok Fm encountered in borehole

~ - ~ ' ~ ~ ~'~Mbr ~ ........ \ 'i ~,~'#~.~'~a-'~ PAffARSIBUHAN ~, kFg~,~t~_,~..n.~:~L s t ~ . \ ~ ~ ' ~ N ~ . LS~r~"-,~_ I PASIRPENGARAYAN L, \ ~',~.'~'Q"~ "~,~,"'~ _ "% ~ "~,,."~~ Pawan [ ~4"2\\ ~ . ~ \ \ ~ . "% Member ~ ' ~ a s i l ~ o n g i ~ " ~ \ a ~ - o ~ . ~ ' ~~ ",,>,..%,.x,,~,

\

~

I'%Ui::lltli:l.ll

_

~.

.

1~

Formation I

~ - - - ~

~t~-~

uhur . . ~\~.~.'~'~'~/'~0rma!!..~ r

Fig. 4.4. Distribution of Carboniferous, Permian and Triassic stratigraphic units in north central Sumatra from GRDC map sheets, showing rock types and critical fossil localities, as well as Late Permian to Triassic intrusives. Areas left blank are covered by Late Mesozoic to Quaternary sediments and volcanics.

Barisan Mountains to the north of Tapaktuan. Outcrops of the Kluet Formation on the 1:250 000 map sheets are shown lying to the southwest of the outcrops of the Bohorok and Alas formations and extend from Lake Tawar near Takengon in the north to Sibolga in the south (Figs 4.2 & 4.3). The formation consists predominantly of black slates, with phyllites, quartzose arenites and conglomeratic metagreywackes, the latter containing lithic clasts up to 40 cm in diameter. Poorly sorted volcaniclastic wackes occur along the Sibolga to Tarutung road. The size and proportion of clasts in the conglomerates decreases across the outcrop from NE to SW. Locally there are calcareous horizons and detrital limestones. More massive meta-limestones occur at Rerebe, south of Takengon (Fig. 4.3). The sandstones are generally massive and commonly devoid of sedimentary structures, although in the type area of the Krueng Kluet (Cameron et al. 1982b) and on the Sidikalang Sheet (Aldiss et al. 1983), graded beds, mud clasts, slumped units, load casts and dewatering structures, typical of deposition as turbidites are reported. Rocks of the Kluet Formation have yet to yield age-diagnostic fossils. The rocks are metamorphosed, predominantly in the slate grade, but show varying degrees of metamorphism. An extensive area of highly metamorphosed rocks of the Kluet Formation is shown occupying the southwestern side of the outcrop on the Tapaktuan Sheet, including the type area of Krueng Kluet (Cameron et al. 1982b) (Fig. 4.3). The rocks are described as coarse muscovitebiotite schists, sometimes garnetiferous, quartzo-feldspathic gneisses and calc-silicate schists. In the Blangkejeren area in

the central part of northern Sumatra metamorphic rocks include biotite-garnet-sillimanite schists, staurolite schists and biotiteandalusite hornfels, chiastolite slate, quartzite, scapolite-bearing calc-silicates, marbles and amphibolites. Some of these rocks, where they are associated with meta-limestones, are shown on the Takengon Quadrangle Sheet as part of the Alas Formation (Cameron et al. 1983a) (Fig. 4.3). The surveyors attribute the metamorphism in the Kluet Formation to contact metamorphic effects (Cameron et al. 1982a). This is clearly the case for the hornfelses and chiastolite slates, but is less certain for garnet- and staurolite-bearing schists. An obvious metamorphic aureole is developed around the Serbajadi Granite on the Langsa Sheet (Bennett et al. 1981c) where the rocks are altered to musovite-biotite hornfels and wollastonite, diopside and phlogopite marbles and skarns. As the metamorphic rocks in the Krueng Kluet are closely associated with concordant granitoids, and at Blangkejeren enclose concordant bodies of garnetiferous gneiss, interpreted as intrusions, these were also attributed to contact metamorphism. Pangururan Bryozoan Bed. On the western shore of Lake Toba

at Pangururan in the Sidikalang Quadrangle, fossiliferous, calcareous, silty mudstones and limestones, with a rich shallow water fauna are distinguished as the Pangururan Bryozoan Bed (Aldiss et al. 1983) (Fig. 4.4). The limestones contain abundant shelly debris, including brachiopods, fenestellid bryozoa and crinoid fragments and some pelecypods. Decalcified, fan-shaped fenestellids up to 10 cm long are conspicuous on weathered bedding surfaces. The

PRE-TERTIARY STRATIGRAPHY

limestones have undergone deformation with the development of alternating zones of high and low strain and the formation of pressure-solution cleavage, as illustrated by distortion of the bryozoan networks. The limestones are interbedded with sandstones and associated with slates of the Kluet Formation. Unfortunately, when they were examined at the Natural History Museum the bryozoa were found to be too decalcified, and the other fossils too fragmentary, to provide a precise age determination for this unit. The age range suggested for the fossil assemblage is from Late Carboniferous to Early Permian with the balance of opinion favouring an Early Permian age (Aldiss et al. 1983). The collection of further fossil and limestone samples from this unit are required for a more precise age determination. Kuantan Formation. As the Kluet Formation was mapped southwards towards the equator it became obvious that it was the same unit as the Kuantan Formation, previously defined on the Solok Quadrangle Sheet in West Sumatra, from outcrops along the Batang Kuantan by Silitonga & Kastowo (1975) (Fig. 4.5). On the Padangsidempuan Quadrangle Sheet to the north, the change from Kluet to Kuantan Formation was set arbitrarily where there is a break in the outcrop at 99~ longitude (Aldiss et al. 1983) (Fig. 4.4). The outcrop of the Kuantan Formation extends along the core of the Barisan Mountains from Padangsidempuan to the latitude of Padang (Figs 4.4 & 4.5). Silitonga & Kastowo (1975) distinguished a Lower Member dominated by quartzites and quartz sandstones, rarely conglomeratic, with interbedded shales, usually metamorphosed to slates or phyllites. Finer-grained sandstone units may show graded beds, small-scale cross lamination, ripples and slump structures. Subordinate components include brown chert, chloritized tufts and volcanic rocks. The lower unit was distinguished from an upper Phyllite and Shale Member in

' ~" 100{~Ex_,r~

EquatorJ

ur - i - ~ ~~ ~ ~ "<1~t .':<-"-~Tuh Formation

~,

29

which the argillaceous red brown shale and phyllite component is dominant, with intercalations of quartzite, siltstone, dark grey chert and andesitic to basaltic lava flows. No systematic sedimentological study has been carried out on the Kuantan Formation and outcrop details are not given in the Explanatory Notes for the GRDC Quadrangle sheets. Descriptions of the lithological features of the Kuantan Formation by Peter Turner (Turner 1983) from three outcrops near Rao (Fig. 4.4) are therefore particularly valuable. The first is on the Auk Mangkais to the west of the Batang Sumpur, where massive grey quartzite beds, 1-6 m are interbedded with blue-grey and black phyllites and fine siltstones 10-80 cm thick. The quartzites show both sharp tops and bases and the siltstones may show cross-lamination. Tight folds of the slaty cleavage are seen in loose blocks in the stream bed. Steeply dipping (100~176 black slates outcrop in the Sungai Nior to the east of the Batang Sumpur, showing isoclinal folds to which the cleavage has an axial plane relationship (Turner 1983). The slates are interbedded with rippled, laminated siltstones containing ribbed plant stems of C a l a m i t e s type. The siltstones are sometimes deformed by slump folds. A section in the fiver bank shows several lenses of matrix-supported conglomerate, up to 1 m thick, with bases eroded into the underlying slate. Angular to rounded clasts in the conglomerate include vein quartz, microgranite, phyllite, greywacke, quartzite and chert. Siltstone clasts show both cleavage and crenulation cleavage, indicating two earlier phases of deformation These conglomerates are interpreted as debris flows (Turner 1983). Further upstream, greywacke sandstone beds 30 cm thick are folded into upright folds, 2 - 3 m in amplitude. These rocks have been identified as distal turbidites and are distinguished by Turner (1983) as the Nior Member. Black, micaceous mudstones and slates in a small tributary of the Auk Lajang to the NE of Ciranting contain ellipsoidal

1[~2~

PAYAKUMBUH~ BUKIT O RENGAT

~~~--~~

~_~rigapuluh

raO,c,

Tabir Formation -- " ~ PERMO-TRIASSIC \

'"*" ~ ii %~ "'-~::!i!::ii!i:::

Triassic ,. ~ 2o

P e r m i a n with volcanics ~

C A R B O N I F E R O U S

\

)_~k ~

k -

?EARLYPERMIAN J

2

Mentulu Fm etc.with pebbly m u d s t Kuantan Formation

LimestoneUnits 100~'E

Certain

9 MUARABUNGO

~

Patepat Formation Formation

Duabelas S Mountains

0

L

t

101~

(~

Major Faults Recent Volcanoes Permo-Triassic Intrusions Serpentinite 2," 100km

50 '

III

. . . .

103~ I

Fig. 4.5. Distribution of Carboniferous, Permian and Triassic stratigraphic units in central Sumatra from GRDC map sheets, showing lithologies and critical localities as well as Late Permian to Early Triassic intrusives. Areas left blank are covered by Late Mesozoic to Quaternary sediments and volcanics.

30

CHAPTER 4

calcareous nodules up to 40 cm in size, around which the slaty cleavage diverges as the result of compaction. Indeterminate foraminifers were recognized in one nodule, and an insoluble residue from another yielded abundant sponge spicules. The associated mudstones contain leaf and fungal fragments. These outcrops were distinguished by Turner (1983) as the Tua Member. These records of plant fragments, foraminifers and siliceous spicules indicate that the less deformed sediments in the Kuantan Formation are very likely to yield age-diagnostic fossils to a systematic search. On the Pakanbaru Quadrangle Sheet, to the north of Solok, Clarke et al. (1982b) distinguish the Pawan and Tanjung Puah members of the Kuantan Formation (Figs 4.2 & 4.4). The Pawan Member cropping out to the east of Lubuksikaping is composed of intensely folded muscovite, tremolite, chlorite and carbonate schist. The very similar Tanjung Puah Member to the SW, also includes quartz schist. Both units show an early phase of tight isoclinal folding on vertical or steep SW-dipping axial planes and east-west or N W - S E axes, and are refolded by later upright folds on N W - S E axes. The latter are probably represented by the large-scale folds seen on aerial photographs and indicated on the Pakanbaru Quandrangle Sheet (Clarke et al. 1982b). Again, these more highly metamorphosed rocks may represent fragments of an earlier metamorphic basement, or, where rock types include tremolite and chlorite schists, may represent a hitherto unrecognized suture zone. On the Solok Sheet Silitonga & Kastowo (1975) recognized a Limestone Member within the Kuantan Formation (Fig. 4.5), composed of massive, black, white, grey or reddish limestone, locally containing irregularly-shaped chert nodules, with interbeds of quartzite and siliceous shale. Detailed petrographic studies of samples of limestone have been made by Vachard (1989a, b). He recognized algal structures, including algal mats, oolites and possible pisolites, and concluded that the limestones were deposited in an intratidal to supratidal environment. From the fossils collected during the mapping survey Silitonga & Kastowo (1975) established that the limestones in the Kuantan Formation range in age from Lower Carboniferous to Mid-Permian, although the younger limestones are better considered as a separate formation. Subsequently the fossiliferous localities were re-examined by Fontaine & Gafoer (1989). New collections were made and macro- and microfossils studied to establish the ages of these limestone occurrences more precisely. Important localities containing Carboniferous fossils occur in the Again River and the Batang Kuantan Gorge (Fig. 4.5). The limestone outcrops to the east of Lake Singkarak (Guguk Bulat) which yielded Permian fossils are considered by Fontaine & Gafoer (1989) to be best classified with the Mid-Permian Silungkang Formation, rather than, as shown on the map of the Solok Quadrangle, with the Kuantan Formation (Silitonga & Kastowo 1975). Limestone outcrops in the Again River near the bridge on the road from Bukit Tinggi to Pakanbaru yielded the alga Koninckopora and the foraminifers Palaeotextularia, Eoendothyranopsis and Archaediscus, indicating a Mid-Vis6an age. With additional samples the age range was extended from the late Early or early Mid-Vis6an to Late Vis6an (Fontaine & Gafoer 1989). A Mid-Late Vis6an age was confirmed by the discovery of conodonts, including Gnathodus girO, i rhodesi Higgins, from this locality (cf. the Alas Formation above) (Metcalfe 1983). Limestones exposed in a scenic gorge along the Kuantan River contain large colonies of the tabulate coral Syringopora, the fasciculate Tetracorallia Siphenodendron and the alga Koninckopora inflata, indicating a Late Vis~an age (Fontaine & Gafoer 1989; Vachard 1989a, b). These limestones containing the colonial coral Syringopora and intratidal algal mats, were evidently deposited in a sub-tropical to tropical, shallow, warm water environment.

Tigapuluh Group Pre-Tertiary rocks form the Tigapuluh Mountains, isolated as an inlier 70 km long and 40 km wide among the surrounding Tertiary sediments, east of the Barisan Mountains to the south of Rengat (Fig. 4.5). Three formations have been identified: the Mentulu, Pengabuhan and the Gangsal formations, interpreted as different facies of the Tigapuluh Group. The distribution of these units are shown on the Rengat and Muarabungo Quadrangle Sheets (Suwarna et al. 1991; Simandjuntak et al. 1991) (Fig. 4.6). Deformation increases in intensity from NE to SW and in the aureoles of Triassic-Jurassic granitic intrusions the sediments are converted to spotted slates or hornfels. Mentulu Formation. The Mentulu Formation, defined from outcrops in the upper part of the Mentulu River, occupies large areas in the northern and eastern parts of the Tigapuluh Mountains (Fig. 4.6). The formation is characterized by pebbly mudstones, similar to those of the Bohorok Formation of northern Sumatra. The mudstones are interbedded with greywacke sandstones and shales, the latter generally occurring as slates, or as hornfels adjacent to granite contacts. The mudstone matrix contains irregularly distributed angular to rounded clasts of granite, silicified basalt, vein quartz, slate, quartzite and feldspar. The clasts are generally of pebble size, up to a few centimetres, but may reach 30 cm in diameter. The pebbly mudstone is usually deformed, with the matrix altered to slate, and the clasts flattened and elongated within the cleavage planes. Cordierite is commonly developed where the pebbly mudstones have been converted to spotted slates or hornfels within metamorphic aureoles. The interbedded greywacke sandstones are massive, dense, grey sandstones, sometimes conglomeratic, containing folded quartz veins. The sandstones are poorly sorted and also contain irregularly distributed clasts, of the same rock types as those found in the mudstones. The conglomerates are polymict and are composed of sub-angular to rounded clasts. Finer sandstone units show parallel lamination and may be poorly graded. Shale or claystone units are well bedded and parallel laminated and contain scattered matrix-supported fragments of quartz and feldspar. Some of the sandstone units are tuffaceous and andesitic and basaltic tuf~ distinguish the Condong Member in Bukit Condong and Gunung Endalang (Fig. 4.6). The pebbly mudstones of the Mentulu Formation, like those in the Bohorok Formation in northern Sumatra are considered to be of glacio-marine origin, and the lithology of the clasts indicates a continental provenance.

Pengabuhan Formation. The Pengabuhan Formation occurs in the central part of the Tigapuluh Mountains where it is defined from outcrops in the upper part of the Pengabuhan River (Simandjuntak et al. 1991) (Fig. 4.6). The formation is composed principally of lithic greywackes or sandstones, quartzites and siltstones. These lithologies contain irregularly distributed clasts of granite, vein quartz and quartzite, similar to those seen in the Mentulu Formation. The quartzites are often feldspathic and are wellsorted, being composed of well rounded grains of quartz and feldspar. The siltstones also contain clasts of feldspar, quartz and lithic fragments. The outcrop patterns in the northern part of the Tigapuluh Mountains, as delineated by Suwarna et al. (1991) (Fig. 4.6), show the Mentulu and Pengabuhan formations interdigitating, suggesting that they are facies variants, distinguished only by the presence or absence of pebbly mudstone. Alternatively the two units may have been imbricated by thrusting. Gangsal Formation. western part of the the upper part of shown occupying

The Gangsal Formation crops out in the Tigapuluh Mountains, and was defined from the Gangsal River. The formation is also a small area between the Mentulu and

PRE-TERTIARY STRATIGRAPHY

I

I

102~

30'

TIGAPULUH MOUNTAINS

45' ,"Ut/l

I

103*00'

45'

Formation;

--I

3!

45'

l 1

Inliers of Gangsal Formation in Limau I-

Triassic-Jurassic Granites TIGAPULUH GROUP Condong (volcanic) Member ~'~ Mentulu Formation ld~:':~?:4 (pebbly mudstones) [:~i::i::iiiii::i!t Pengabuhan Formation ~.,x,%..~

[~}x...'..s ] Gangsal Formation

: Gangsal :Formation

1ooo's

.'.-.-\'.-.--.-.-...N~...2I~Mentulu .~.N[," ~ _.,-, ~ %; ~,'-.'.:,~[ "15engabuhan .... ~\...

0 15'

5

10

15

:. : . . . . . . . . . . . - 7 . - - - .

20kin

to Jambi

L

15'

30'

lO3~OO'

Fig. 4.6. Distributionof stratigraphic units in the TigapuluhHills(alter Suwama et al. 1991"Simandjuntaket al. 1991). Areas left blank are covered by Tertiaryto Recent sediments. Pengabuhan formations in the southern part of the mountains (Fig. 4.6). It is distinguished from the other Pre-Tertiary units in this area by the predominance of argillacous material, usually as dark grey or black slate, grey, white or green phyllite, by a higher degree of deformation, and in the neighbourhood of intrusions, dark hornfels. The argillacous rocks are interbedded with grey-green sandstones, composed of subangular to rounded grains of quartz with lithic fragments, dark grey quartzites and massive grey argillaceous limestones. All lithologies are extensively veined by quartz.

C o r r e l a t e d f o r m a t i o n s in s o u t h e r n S u m a t r a

An isolated outcrop of low-grade metamorphic rocks in the Duabelas Mountains to the SE of Muarabungo (Figs 4.2 & 4.5) consisting of quartzite, siltstone, claystone, marble and rare mica schist, distinguished as the Tarantam Formation, has been correlated with the Kuantan Formation (Simandjuntak et al. 1991). The Garba Mountains form an inlier of Pre-Tertiary rocks to the south of Baturaja (Fig. 4.7). Here the oldest unit, composed of low grade metamorphic rocks, is distinguished as the Tarap Formation from a type locality in the Tarap River (Gafoer et al. 1994). These metamorphic rocks crop out on both the eastern and western sides of the inlier where they are in thrust contact and imbricated with the unmetamorphosed Lower Cretaceous Garba Formation. The metamorphic rocks, which include phyllite,

schist, slate, minor quartzite and marble metamorphosed in the greenshist facies, are interpreted as the metamorphosed Palaeozoic basement of Sumatra, and are correlated lithologically with the Tarantam and Kuantan formations of Central Sumatra (Gafoer et al. 1994) and with the Gunungkasih Complex to the south near Bandarlampung (Amin et al. 1994b). Metamorphic rocks of the Gunungkasih Complex, named from a hill to the SE of Tanjungkarang, form scattered outcrops among Cretaceous granites and Quaternary volcanics in South Sumatra (Fig. 4.8). Rock types include graphitic, micaceous, sericitic, chloritic, quartzose and calcareous schist, sericitic quartzite and marble of low- to medium-grade greenschist facies, associated with migmatites, amphibolites and granitic gneisses and intruded by granites. Amin et al. (1994b) and Andi Mangga et al. (1994a) suggest that these metamorphic rocks may be correlated with the Kuantan and Kluet formations of central and northern Sumatra. The boundaries of lithological units and the foliation strike in a N W - S E direction, parallel to the Sumatran trend. Schistosity strikes in the same direction, is folded about east-west axes and is refolded by N W - S E trending upright folds and by variably oriented kink bands. K - A r ages of 125 + 5 and 115 __ 6 Ma (mid-Cretaceous) obtained from rocks of the complex are taken to indicate the age of granite intrusion and metamorphism of the metasediments. In outcrops to the NE of Kotaagung, and SW of Tanjungkarang, rocks of the Gunungkasih Complex are thrust southwestwards over unmetamorphosed sediments of the Early Cretaceous Menanga Formation.

32

CHAPTER 4

I

I

104'~00 '

Quaternary Sediments QuaternaryVolcanics Ptiocene

Qs

Qv

Late Miocene Middle Miocene Oligo-Miocene Qs ,9

...F

MARTAPURA~/-

.,

o.,

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Eocene

,J-'-:

,,,.

~ ...............

,% %

%

%,

%

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%,

%

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%,

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,,.,.

.,

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,"

%

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~

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Melange Situlanglang (chert) Member

Qs 0

5

10

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20km '

Garba (volcanic) Formation Tarap Formation (metamorphosed ?Palaeozoics)

QS ~ - " " - - - - _ _ ~ J ~ E v ' /

I

Faults

Late Cretaceous Granites Mesozoic Units (correlated with the Woyla Group)

- "'," -'," -',",,.'~

4o30 ,

....

F

~ I

Fig. 4.7. The distribution of the Pre-Tertiary units in the Garba Mountains, South Sumatra, after GRDC geological map of Baturaja (Gafoer et al. 1994). The Metamorphosed Palaeozoics are correlated with the Tapanuli Group and the Garba and Situlanglang Formations are correlated with the Jurassic-Cretaceous Woyla Group of northern Sumatra (see below).

Pemali Group, Bangka Island Carboniferous-Permian rocks of the Pemali Group occur on Bangka Island where they are imbricated with the Triassic Tempilang Sandstones (Ko 1986) (Fig. 4.2). The Pemali Group occurs in east-west trending, fault-bounded outcrops throughout the island. Rock types include isoclinally folded pyritic shales and limestones, the latter containing Permian fusulinids (De Roever 1951), volcanics and bedded cherts, with radiolaria, laminated mudstones and pebbly mudstones. According to the description by Ko (1986) the pebbly mudstones from the Toboali District in the southern part of the island resemble very closely those already described from the Bohorok and Mentulu formations, above, and contain clasts with a similar range of sizes and lithologies, although previously these same outcrops were described by De Roever (1951) as arkosic conglomerate.

Persing Complex, Singkep and the 'Quartzite Terrain' The Persing Complex of the island of Singkep consists of phyllite, slate, graphitic schists with quartz veins and bands of quartzite (Sutisna et al. 1994). The quartzites are compared lithologically with those of the Tarantam Formation in the Duablas Mountains. The Persing Complex lies along strike from the 'Quartzite Terrain' identified in oil company boreholes in the Pekanbaru area (Fig. 4.2).

Interpretation Stratigraphy. Because of poor exposure, scattered outcrops and the large numbers of faults which disrupt the sequence, it has

not yet proved possible to determine the stratigraphic relationships of the units which make up the Tapanuli Group. The Vis~an Alas Formation and Limestone Member of the Kuantan Formation are the only units for which there is direct palaeontological evidence of age. The Bohorok and Kluet/Kuantan formations have also been regarded as of Carboniferous age because of their close association with the Alas and Kuantan limestones in the field, and because all three formations contain similar lithologies, and in general show the same degree of deformation. The presence of fossils indicating an age near the Devonian-Carboniferous boundary in a borehole in the Malacca Strait (Eubank & Makki 1981), the identification of Late Carboniferous-Early Permian fossils in the Pangururan Bryozoan Bed (AIdiss et al. 1983) suggests that the Tapanuli Group may cover an age range from Late Devonian to Early Permian. The BGS/DMR surveyors, who mapped the Tapanuli Group as part of the North Sumatra Project, considered that all three units were broadly contemporaneous. They observed that pebbly mudstones, characteristic of the Bohorok Formation, are interbedded with quartz sandstones and pelitic sediments of turbidite facies. These turbiditic sediments, with variations in the proportions of the components, are the dominant lithoiogies in the Kluet and Kuantan formations and also in the Tigapuluh Group of Central Sumatra. Cameron et al. (1982a) report that, apart from the presence or absence of pebbly mudstones, the lithologies of the Bohorok and Kluet formations are so similar that the boundary between them on the Medan Sheet was drawn arbitrarily because of the difficulty in distinguishing between the two units. The outcrop of the Alas Formation is interposed between the Bohorok and Kluet formations (Figs 4.2 & 4.3). As reported above a Vis6an (Lower Carboniferous) age has been established for the Alas Formation (Fontaine 1989; Metcalfe 1983). A

PRE-TERTIARY STRATIGRAPHY

33

!

104~

~45'

105~

~

Recent'Volcanoes

Late Cretaceous Granites . ~ ". ~. .-.~" . ~ % , - - ~ .

~'<-'~

"

- 5o15 ,

Menanga Formation

R i v e rm ~--~'~_~ p u n g ~ - ,,~ ~


(mid-Cretaceous)

~

Gunungkasih Complex_ (Palaeozoic)

% "\\

~o

\

%,~% ~s

~

BANDARLAMPUNG

/~'~.-"--~,

KOTAAGUNG

atk - 5~

'\

5~45 ' -

"~'~. Strike-slip Faults "~ Thrust Faults 0 104~

'

I ........................

104~ l

'

105~

...................

......................

50km

'

................................

Vis~an age has also been established for the Limestone Member of the Kuantan Formation (Fontaine & Gafoer 1989; Metcalfe 1983; Vachard 1989a, b). The record by Turner (1983) of plant remains in the Nior member of the Kuantan Formation is compatible with this age attribution. Turbiditic sandstones and pelites, similar to those of the Kluet and Bohorok formations, occur interbedded with limestones characteristic of the Alas Formation, suggesting to the surveyors that the Alas is part of the same sedimentary sequence as the other units (Cameron et al. 1980). They therefore considered that the Bohorok, Alas and Kluet/Kuantan formations are lateral facies variants of a coherent sedimentary assemblage. Clasts in the pebbly mudstones of the Bohorok, and conglomerates in the Bohorok, Kluet and Kuantan formations and also in the Tigapuluh Group of Central Sumatra, include the same range of lithologies. Analysis of the composition of the clasts shows that all these units were derived from a low-grade metamorphic terrane composed of slates, phyllites, calc-silicate schists, marbles and quartzites which were intruded by granitic rocks. A K / A r age of 1029 Ma from a trondjemite clast from pebbly mudstones in the Langkawi Islands (Hutchison 1989, p. 16) indicates that the source area included rocks of Proterozoic age. Some argillaceous clasts show evidence from slaty cleavage and crenulation cleavages that they had already undergone multiple deformation. Locally the metamorphic grade in the source region was higher, indicated by clasts of mica schist and granitic gneiss. The granitic gneisses may have been formed by synkinematic deformation of granites intruded into an active shear zones. Rare chert clasts, may indicate the presence of oceanic rocks incorporated in a collisional suture and rhyolite clasts indicate acid volcanism. In fact, the palaeogeology of the area from which the sediments of the Tapanuli and Tigapuluh groups were derived resembles very closely the present-day geology of northern Sumatra. Cameron et al. (1980) report that, within the Bohorok Formation, pebbly mudstones die out in a southwesterly direction. With the loss of pebbly mudstones the Bohorok Formation

l

.......................

I

Fig. 4.8. The distribution of the PreTertiary units of the Bandar Lampung area, southern Sumatra after GRDC geological map sheets of Kotaagung and Tanjungkarang (Amin et al. 1994b; Andi Mangga et al. 1994a). The Gunungkasih Complex is correlated with the Palaeozoic Tapanuli Group and the Menanga Formation with the Jurassic-Cretaceous Woyla Group of northern Sumatra (see below). In areas left blank the older rocks are covered by Tertiary and Quaternary sediments and volcanics.

interdigitates with, and passes into the Kluet Formation; they regarded the latter as the lateral equivalent of the Bohorok Formation, representing a more distal turbidite facies. Similar relationships are described from Central Sumatra between the formations in the Tigapuluh Group (Fig. 4.6). Cameron et al. (1980) also observed a systematic reduction in the size and proportion of clasts towards the SW in the pebbly mudstones and in conglomerates throughout the Bohorok and Kluet formations. The inference from these observations is that the sedimentary provenance of the Tapanuli/Tigapuluh Group lay to the NE of Sumatra and that deposition occurred on a continental margin extending out into an ocean lying to the SW, in present day coordinates. As reported above, Cameron et al. (1980) suggested that the Kluet and the Bohorok formations were related facies of the same age. The erroneous identification of a fossil coral from the Alas Formation led Cameron et al. (1980) to suppose that the Alas Formation was of Early Permian age and was therefore preserved in a syncline, overlying the older Kluet and Bohorok formations. Cameron et al. (1980) proposed a stratigraphic scheme for the Tapanuli Group of northern Sumatra based on an analogy with stratigraphic relationships seen near Phuket in Peninsular Thailand (Garson et al. 1975) (Fig. 4.2). At Phuket, pebbly mudstones of the Phuket Group, similar to those of the Bohorok Formation of Sumatra, are underlain and interbedded with a thick and extensive series of turbiditic sediments. Fossils in the turbidites include the trilobite C y r t o s y m b o l e (waribole) p e r l i s e n s i s Kobayashi and Hamada (Mitchell et al. 1970) of Late Devonian to Early Carboniferous age. The same fossil occurs near the base of the pebbly mudstones and sandstones forming the Sings Group, in Langkawi, a group of islands offshore Peninsular Malaysia (Jones et al. 1966) (Fig. 4.2). In Phuket, the pebbly mudstones are overlain by thin-bedded sandstones containing a fauna of bryozoa and brachiopods and then by a 'Bryozoan Bed' considered to be of Early Permian age (Mitchell et al. 1970; Garson et al. 1975). Cameron et al. (1980) drew an analogy between the Pangururan Bryozoan Bed of northern Sumatra and

34

CHAPTER 4

the Early Permian Bryozoan Bed of Phuket. In Thailand the Phuket Group is overlain by the M i d - L a t e Permian Ratburi Limestone, which Cameron et al. (1980) correlated with the Alas Formation of Sumatra. Now that the age of the Alas Formation is firmly established as Early Carboniferous, the latter correlation is no longer valid. The present situation is, that although it is possible that Tapanuli Group and its correlatives, the Kuantan Formation and Tigapuluh Group of Central Sumatra extend down into the Devonian, the only age diagnostic fossils so far identified in Sumatra are of Lower Carboniferous, Vis6an age. No Toumaisian or Upper Carboniferous rocks have so far been recognized. The only rock unit which could possibly be of Late Carboniferous age is the Pangururan Bryozoan Bed from Lake Toba (Fig. 4.4). As already reported above, fossils collected from this locality have been identified as of Late Carboniferous to Early Permian age, with the balance of opinion in favour of the later age (Aldiss et al. 1983). This age determination confirms the correlation with the Early Permian Bryozoan Bed of Phuket proposed by Cameron et al. (1980). The Pangururan Bryozoan Bed is interbedded with, and is deformed, to the same extent as the associated sandstones and slates of the Kluet (Bohorok?) Formation, which must also therefore be partly of Early Permian age. No unconformities have so far been recognized within the Tapanuli Group so that it is probable that the group also includes rocks of Upper Carboniferous age. As has been reported above interbedded quartzites and shales were encountered beneath Tertiary sediments in boreholes to the NE of Pekanbaru, in the Malacca Strait and in the Persing Complex of Singkep Island. These occurrences were used by Eubank & Makki (1981) to define a 'Quartzite Terrain' (Fig. 4.2). Palynomorphs from the shales indicated an age near the Devonian-Carboniferous boundary. Similar rock units composed of quartz-rich sandstones with shales and mudstones described as the Kubang Pasu and Kenny Hill formations occur on the eastern side of the Malacca Strait (Fig. 4.2). The Kubang Pasu Formation outcrops in eastern Perlis and NW Kedah where it is dated by Devonian trilobite pygidia at the base and Carboniferous goniatites and brachiopods higher in the sequence, and passes upwards conformably into the Lower Permian Chuping Limestone Formation. The Kenny Hill Formation which outcrops near Kuala Lumpur contains only trace fossils and poorly preserved body fossils which do not provide a reliable indication of age. However, it is considered to be of Carboniferous age because it is younger than the adjacent Silurian Kuala Lumpur Limestone Formation, but is cut by Mesozoic granites and ore bodies (Stauffer, in Gobbett & Hutchison 1973). These quartzrich units appear to have been derived from the east and are considered to be stratigraphically equivalent to the Bohorok, Kluet and Alas formations.

submarine mass wasting on a continental slope (e.g. Mitchell et al. 1970).

In Peninsular Thailand, NW Malaysia and Baoshan in SW China (Wang et al. 2001) pebbly mudstones are interbedded with sediments containing Early Permian fossils. In Australia the occurrence of glacial deposits indicates that glaciation commenced in the Namurian, reached its peak in the Stephanian and Sakmarian and had ceased by the Artinskian (Quilty 1984). it is therefore possible that the Bohorok Formation with the diamictites ranges in age from the Late Carboniferous to the Early Permian. Palaeogeography. Cameron et al. (1980) suggest that the Tapanuli Group represents a continental margin sequence deposited on a rifted passive margin. The reduction in clast sizes in the mudstones and conglomerates of the Bohorok and Kluet formations, with a decrease in the frequency and grain size of sandstone units in a southwesterly direction, suggest that in Carboniferous times an open ocean lay in this direction. In this model turbiditic sandstones and shales were deposited in rift basins, while limestones of the Alas and Kuantan formations formed carbonate banks on horst blocks of uplifted basement, perhaps represented by the high grade metamorphic rocks associated with the Alas Formation in the field. Following Cameron et al. 1980, Fontaine & Gafoer (1989) interpreted the Carboniferous rocks in the northern part of Sumatra as a series of contemporaneous sedimentary facies formed on a continental margin (Fig. 4.9). They suggest that the Kubang Pasu and Kenny Hill formations in the western part of the Malay Peninsula, and quartzites and quartz sandstones encountered in oil company boreholes along the Malacca Straits represent littoral and shelf facies sands in the east. The pebbly

Pebbly mudstones. As noted above, pebbly mudstones similar to

those of the Bohorok Formation occur in the Langkawi Islands and in Perlis in Peninsular Malaysia and at Phuket in Peninsular Thailand. Similar deposits occur in the Mergui Series of the Shah States of Myanmar and in the Salt Ranges of Pakistan. Wherever they occur, there has been much discussion concerning the origin of these pebbly mudstones. Stauffer & Lee (1986), as part of their studies of the Singa Formation in the Langkawi islands, described 'dropstone' structures beneath clasts in laminated mudstones, which they attribute to the deposition of pebbles and boulders carried by floating ice. They conclude that the pebbly mudstones were deposited in a glacio-marine environment. Similar detailed sedimentological studies of the pebbly mudstones and their associated deposits are required in Sumatra. Following the studies of Stauffer & Lee (1986) a glacial origin for pebbly mudstones throughout the region has generally been accepted, although dissenting opinion has interpreted the pebbly mudstones, fi-om their association with turbidite deposits, as the product of debris flows, due to

0

250

I

I

500km I

Fig. 4.9. Carboniferous palaeogeography of Sumatra and the Malay Peninsula (from Fontaine & Gafoer 1989). The description of the facies and the palaeogeographic interpretation are given in the text.

PRE-TERTIARY STRATIGRAPHY

mudstones of the Bohorok Formation represent deposits from a melting floating ice-shelf or icebergs, which are interbedded with turbiditic sands and shales, passing into distal turbidites and deep water shales further offshore in the Kluet Formation. The limestones of the Alas Formation, with oolites and current bedding, as described in the foregoing account, represent shallow water carbonates deposited on a 'high' in the continental shelf environment. Fontaine & Gafoer (1989) relate the fauna and algal flora of the Visdan Alas limestones to those found elsewhere in the Sibumasu Block, in western Peninsular Malaya, Thailand and Burma. On the other hand, they relate the fauna and algal flora of the limestones in the Visdan Kuantan Formation to those of the eastern Peninsular Malaya and the Indochina Block in Thailand, Laos and Vietnam. While the Alas limestones could have been deposited in a cool environment, the fauna and flora of the Kuantan limestones clearly indicate a tropical environment of deposition. Since the Alas and Kuantan formations are contemporaneous, they must have been deposited in different environments on separate plates, and were only been brought together in Sumatra by postCarboniferous movements. This relationship is indicated on the Fontaine & Gafoer's (1989) Carboniferous palaeogeographic reconstruction of Sumatra (Fig. 4.9) by an arbitrary W N W - E S E boundary, separating the Kuantan Formation from the outcrops of the Kluet, Alas and Bohorok formations to the north. This line has no present structural expression.

Peusangan Group (Permo-Triassic) During the North Sumatra Survey, Pre-Tertiary rock units lying to the NW of the Sumatran Fault System, which were apparently less deformed than the Tapanuli Group, were classified in the Peusangan Group, named from the Peusangan River which flows northwards from Lake Tawar to the Andaman Sea. Fossil evidence showed that some of these units are of Permian and Triassic age (Cameron et al. 1980). This terminology was subsequently extended to all Permo-Triassic units throughout Sumatra (McCourt et al. 1993). Because the outcrops of the Permo-Triassic units are so scattered and correlations uncertain, each occurrence has been given a separate formation name (Fig. 4.10). Many of the units include limestones, some of which are fossiliferous so that the age may be precisely determined, but others are so recrystallized that fossils are unrecognizable. These units, with discussion of the evidence for their ages, will be described in order from north to south. Uneun Unit (Fig. 4.3). The Uneun Unit composed of slates, metamorphosed limestones and epidotized basic volcanics is named from the Kreung Uneun in the Takengon Quadrangle (Cameron et al. 1983), and extends northwards onto the adjacent Lhokseumawe Quadrangle (Keats et al. 1981). No fossils have been found in this unit. The Unuen Unit probably incorporates rock units which should more appropriately have been included in the Carboniferous Kluet Formation (slates) or the JurassicCretaceous Woyla Group (epidotized basalts). Situtup Limestone Formation (Fig. 4.3). Bedded or massive fossiliferous limestones and intermediate volcanics cropping out in Gle Situtup, a mountain 40 km to the NW of Takengon, have been designated the Situtup Limestone Formation ('Sitotop Limestone Formation' on the Takengon Quadrangle Sheet) (Cameron et al. 1983). Other limestone outcrops are shown resting on thrust planes above Tertiary sediments, or on units of the Jurassic-Cretaceous Woyla Group, which crops out extensively to the west. On the map the volcanic rocks are shown cropping out within the main limestone, and are described as

35

epidotized basaltic breccia and agglomerate, schistose locally where they have been involved in thrust zones. From this description it is possible that these volcanics belong to the Woyla Group and have been intercalated with the limestones by thrusting. Fossils have been recovered from the limestones of the Situtup Formation. They include the foraminifers, Agathammina/ Agathaminoides sp., Planinvolutina cf. mesotriassica, Involutina sp. ?sinuosa, Parafusulina sp., Pseudodoliolina sp., Neoschwagerina sp. and a coral Thecosmilia sp. (Cameron et al. 1983). Some of these fossils are of mid-Permian age (Parafusulina, Pseudodoliolina and Neoschwagerina), while others are of M i d - L a t e Triassic age (lnvolutina, Planinvolutina cf. mesotriassica and Thecosmilia) (Fontaine & Gafoer 1989). From this fossil evidence it is possible that the limestone constitutes a continuous depositional sequence extending from the mid-Permian to Late Triassic, and that the absence of Late Permian and Early Triassic fossils is due to the accident of collection. More probably, as elsewhere in Sumatra, there is an important unconformity within the outcrop, in which Upper Permian and Lower Triassic rocks are absent. Unfortunately the relationship between Permian and Triassic components of these outcrop are unknown. These relationships should be the subject of future investigation. Ujeuen Limestone Formation (Fig. 4.3). The Ujeuen Limestone Formation outcrops as massive limestones to the south of Lhokseumawe where they are relatively innaccessible and poorly known. No fossils have been reported from these outcrops (Cameron et al. 1983). Tawar Formation (Fig. 4.3). Bedded to massive limestones with minor phyllites cropping out on either side of Lake Tawar near Takengon are designated the Tawar Formation (Cameron et al. 1983). Massive limestones, identified on the Takengon Quadrangle Sheet as a Reefal Member, occur along the northern side of the lake. Phyllites and massive volcanics to the south of the lake are identified as the Toweren Member. No fossils have been found in any of these units. On the map they occur as thrust slices imbricated with the slates and phyllites of the Carboniferous-Permian Kluet Formation, the Jurassic-Cretaceous Woyla Group and Tertiary sediments. Again, it is possible that the phyllites and volcanies of the Toweren Member belong to the Woyla Group.

Sembuang Formation (Fig. 4.3). Fifty kilometres to the east of Lake Tawar is the outcrop of the Sembuang Formation composed of massive recrystallized limestones overlying metamorphosed quartz sandstones (Cameron et al. 1983). No fossils have been reported. Kaloi Limestone Formation (Fig. 4.3). The Kaloi Limestone Formation crops out 40 km to the SSW of Langsa, where it is described as massive reddish tuffaceous limestone and dolomite, pock-marked by sink holes and flanked by fossiliferous shales, limestones and sandstones (Bennett et al. 1981c). The massive limestones have yielded the trilobite Phillipsia aft. sumatraensis of Permian age (Tesch 1916). Forltaine (in Fontaine & Gafoer 1989) reports Halobia, and the shales have yielded Neoproetus indicus and Fenestella retiformis indicating a Late Triassic age. in confirmation of the age, Metcalfe (1989a) obtained a specimen of a Triassic conodont, Epigondondolella postera Kozer and Mostler, from limestones and mudstones of the Kaloi Formation in the Sungai Kaloi, 5 km upstream from Kaloi. The relationship between the Permian and Triassic components of this unit is unknown. Batumilmil Limestone Formation (Fig. 4.3). Fossiliferous 'reefal' limestones and grey calcilutites with chert lenses of the Batumilmil Limestone Formation outcrop in the eastern foothills of the Barisan Mountains to the SW of Medan. Fossils include

36

CHAPTER 4

t0 9~o 918~ 1~)0~ 14 Ch.uping. 1~2o ~ ;,I['BAN DA ACEH _^~o.-, g~ Limestone l'~ ~Uneuen LHOKSUMAWE PENANGF,_)[~~ ~ ~. ~ L[nit(NF) .'O~.. ~)~.~ ... " e ~ Ujeuen (Lst) .~k..~i Situtup(Lst) ~ 9 Formation (NF) (~rj~~176 Formation Sembuan{,st (MP,M=LT)~,Tawa~rst)~ Formation (NF) Formation e Kaloi Formation(Lst)(P LT) ' ~ ~'~ ~O LANGSA , r)~-"

-4~

"k~"

~N~" Bat~umilmil(Lst) ~ Kodiang "Nk ~ ~ ~Formation (MP,T) k,,Llmestone "~ ~ \ k ~ ~ Kualu(Cl)Formation (M-L~

1~)4o

_ 2~

Pangururan~\'h '~ Bryo~nBed'~ v -

',~..~

~ ~ [ - 0~

_

o

2~

\

~ ~ -

(EP) (NF) _60o

~'~'~k

Late Triassic

Buklt

BENGKULU'~'~~ . ~

Early Permian No age-diagnostic fossils found 300 ~

2 o_

, ~ (Ch= chert; CI= clastics)

4oo

.

nendo~o(Lst)

Middle Permian

100 6 20o

Permian (Volc)volcanic units P e r m i a n sedimentary units (CI)

Silungk.ang.(C~.~D\ Telukkido " ~ 1 ~ KUNDUR " t-ormat on (M~') . (LT-J) Formation __Cubadak(Ci)~%Formahon ,~Lrp~apan (M-LT) ,Format o6 9 LUBUK~IKAPING ] %~% LINGGA 0~ ~ (M-LTI " \ %Tuhur Formation(CI) j . f )\,~ ,~ " '~, \ (M-LT) ~ . , / q ~ " ~ (M-LT) s Silungkang(CI,Lst) Palepat(Volc). '~/ Formation (M~St)~,Palr~natlon (EP) - ~ P A D A N G ~ ,~\"~:.~,~. Barisan(CI) (' \ Tuhur(CI)""~..'r Formation ~. ~ F~ ~B~inOMUAR.ABUNG~JAMBI~. ~BtmNGKA~sandstone -,,..,_r ' ' ~ \ /~'%-.~a~epa~(vo~c) ) ~-:.:.:.:-:-.~ p u ,.~ ~ N,.,aoltCl~']~Formati0n (EP) ~ C::~r---::::::::::~ (M-LT) 2~_ ~ ~ Pemali Group(Ch Ss) I:,~ s ~ Mengkarang(cI) J,.u....~v'"~:---:.i~ (MP~ ~',,,, TnN " ~,~ " (EP) ' . . .~. .?. M ..P) ' ~ML LP) ~ , Formation -Ir !~ "~

(M-LT) Middle to

llm,-,efnntae / / e { ~ , .... ~ , ~ , ~ , , ~ ~L--,a,/

,~-%~ c-----. ~

(LT-J) Late Triassic to Jurassic - 4~ ( M P )

"~

9 9 I~:....:....:iii::lTr,asslc chert & sandstone (Ch,CI)_ 1"~..~ Permian and T r i a s s i c

- K~al~(Ch.Ssl ~.~Form~on~_LT~_ r ~ ~ ~ .......... ~.:.:.:....:.:.=

\

1~)8o

,,.,.,-,,.,I - r ' D I A O O I f etuu/n~t-~oo~u

P e u s a n g an G r o u p

'

~ , " ' o

1~6o

l~l--l")l~,/llAIkl r~-nlvll~l,~

(MP) ~.

PALEMBANG "":.Q ) o 'O 'ALEMBAI~G%

/)

)L {

4~

~.~ "-~~

OOOkm

98~

100~

102~

Fig. 4.10. Distribution of Permo-Triassic rocks in Sumatra.

fenestellids, echinoids, ?cephelapods and corals (Cameron et al. 1982a). Fontaine & Vachard (1984) report a fauna collected from the Batumilmil Limestone at Laubuluh, a village 13 km to the north of Tigabinanda with crinoids, bryozoa, productid bracbiopods and rare foraminifers Nodasaria(?), Pachiploia cukurkoyi and Multidiscus padangensis. This fauna indicates a Murghabian to Dzhulfian (mid-Late Permian) age for the Batumilmil Formation (Fontaine & Gafoer 1989). Triassic conodonts (Hindeodella triassica Muller) were found by Metcalfe (1986) in limestones of the Batumilmil Limestone Formation at Sungai Wampu (Fig 4.3). This form ranges throughout the Triassic.

Pangururan Bryozoan Bed (Fig. 4.4). The Pangururan Bryozoan Bed on Lake Toba has already been discussed in the review of the Carboniferous formations in Sumatra. The fauna was considered to range from Late Carboniferous to Early Permian, with the balance of opinion favouring an Early Permian age (Aldiss et al. 1983). No other occurrences of rocks of either of these ages have yet been found elsewhere in Sumatra. Unfortunately, this fauna was not re-examined during the review of fossil localities in Sumatra by Fontaine & Gafoer (1989).

Kualu Formation (Figs 4.3 & 4.4). The Kualu Formation crops out as small isolated exposures among Toba Tufts to the south of Medan (Cameron et al. 1982a) (Fig. 4.3) and over a much larger area to the NW of Rantauprapat and to the south of Lake Toba (Clarke et al. 1982a; Aldiss et al. 1983) (Fig. 4.4). Lithologies typical of the Kualu Formation have also been encountered in oil company boreholes to the SE of Rantauprapat, below Tertiary sediments, and have been described under the name of the 'Mutus Assemblage' (Eubank & Makki 1981). Similar rocks also occur in the island of Kundur off the coast of east Sumatra where they are called the Papan Formation (Cameron et al. 1982c) (Fig. 4.10). At the type locality in the Sungai Kualu, the lithologies are thinbedded sandstones, wackes, siltstones and mudstones. The mudstones are often carbonaceous and contain wood and plant fragments. The upper part of the succession is more arenaceous, with cross-beds, load and flute casts and slump structures in the sandstone units. The Papan Formation on Kundur is more conglomeratic. The characteristic M i d - L a t e Triassic bivalve Halobia sp. occurs at many localities, including H. tobensis and H. kwaluana. of Mid-late Carnian and H. simaimaiensis of Norian age (Fontaine & Gafoer 1989).

PRE-TERTIARY STRATIGRAPHY

A Pangunjungan Member is distinguished in the river section of the same name and is traced along the southwestern side of the main outcrop (Fig. 4.4). This unit shows the same lithological assemblage as described above, but the rocks are finer grained and include thin bedded limestones and grey to pale brown radiolarian cherts. The radiolaria from these rocks have not been identified. Irregular disharmonic folds are interpreted as sedimentary slumps (Clarke et al. 1982a). To the east and south of Lake Toba a Sibaganding Limestone Member has been distinguished (Fig. 4.4). The limestones are pale to dark grey biocalcilutites and have yielded an ammonite Alloclionites aft. timorensis (Early Norian--Ishibashi 1975), corals, brachiopods, gastropods and conodonts; the latter include the zonal form Metapolygnathus polygnatoformis (Late Carnian). At the type locality in the road section along the eastern side of Lake Toba 3 km to the north of Prapat, limestones of the Sibaganding Member with Daonella and Halobia overlie shales of the Kualu Formation (Metcalfe et al. 1979; Fontaine & Gafoer, 1989, Fig. 22). The microfauna and flora from the limestone outcrop has been identified and illustrated by Vachard (1989c) and the microfacies have described by Beauvais et al. (1989). Although the fossils include corals, calcisponges and encrusting bryozoa, and other reef-building organisms, these are scattered in a micritic matrix and do not form reef structures. The environment of deposition is interpreted as a mud mound. The rocks are moderately to tightly folded about N W - S E trending sub-horizontal axes with easterly dipping axial planes (Aldiss et al. 1983). Cubadak Formation (Fig. 4.4). The Cubadak Formation is named from the Air Cubadak on the western side of the Rao Graben to the north of Lubuksikaping (Rock et al. 1983). It is composed of dark grey, well-bedded mudstones with interbedded siltstone laminae and volcaniclastic sandstones, frequently yielding the pelecypod Halobia flattened on bedding surfaces. A section of the Cubadak Formation in the Aek (Air) Cubadak to the south of Limau Manis was described by Turner (1983). This section contains limestones which were not mentioned in the description of the formation given by Rock et al. (1983). About 100 m of blue-grey calcareous mudstones are interbedded with cm thick tuffaceous limestones, sometimes containing ooliths nucleated around mineral grains. The oolitic limestones show cross lamination. The sequence yielded Halobia sp. and several ammonites: Trachyceras sp. ind. and ?Ceratites sp. This faunal assemblage indicates that the sequence is of Ladinian age (Late Triassic). Limau Manis Formation. Turner (1983) also defined the Limau Manis Formation from outcrops in the Air Cubadak to the north of Limau Manis. These outcrops were mapped as part of the (Permian) Silungkang Formation by Rock et al. (1983). The lithologies include breccio-conglomerates with clasts of limestone and acid and basic igneous material, followed by tuffaceous mudstones, cross-bedded volcaniclastic sandstones, the cross beds indicating derivation from the NW, and bioclastic turbidites. These calciturbidites are rich in reworked fusulinids and corals of m i d - L a t e Permian age. The mudstones contain abundant ammonites Acanthinites sp., Helictites sp., ?Tibetites sp. ind. indicating a Ladinian, Carnian to Norian age (Mid-Late Triassic) (Turner 1983). Telukkido Formation (Fig. 4.4). Rock et al. (1983) defined the Telukkido Formation cropping out between Pasirpengarayan and Lubuksikaping from a stream of the same name. The rocks are dark grey quartzose sandstones and shales with minor limestones and thin coals. A Limestone Member composed of recrystallized or argillaceous limestones is also recognized. In the type locality these rocks yielded plant remains from pyritic quartzite, with leaf impressions identified as Otozamites sp. (possibly Pterophyllum) and Ptilophyllum sp. The flora is identified as of Late Triassic

37

to Early Jurassic age, most probably Jurassic. Although this unit is included in the Permo-Triassic Peusangan Group by Rock et al. (1983) they suggest that it might better be classified with the Jurassic Rawas Formation of Central Sumatra which will be discussed later. Tuhur Formation (Figs 4.4 & 4.5). Silitonga & Kastowo (1975) defined the Tuhur Formation forming extensive outcrops to the SE of Lake Singkarak in the Solok Quadrangle. This outcrop was later extended southwards into the Painan-Timurlaut Muarasiberut Quadrangle to the east of Lakes Dibawah and Diatas (Rosidi et al. 1976). A further outcrop was mapped to the NE of Payakumbuh and this outcrop was traced northwards, using aerial photographic interpretation, across the equator into the Pekanbaru Quadrangle (Clarke et al. 1982b). Silitonga & Kastowo (1975) distinguished a Slate and Shale Member, forming the greater part of the outcrop, composed of grey to dark grey slate, black shales, and brown cherts with thin greywacke sandstones, and a Limestone Member composed of poorly bedded sandy limestone and massive fossiliferous conglomeratic limestone, with thin intercalated shale and slate. Limestone pebbles in the conglomerates contain fusulinid foraminifera of Permian age. Musper (1930) suggested that this formation is of Triassic age. The Tuhur Formation may be correlated with the Kualu Formation, described above. Silungkang Formation (Figs 4.4 & 4.5). The type locality for the Silungkang Formation (Klomp6 et al. 1961) is the road and river sections around the village of Silungkang, between Solok and Sawahlunto to the SE of Lake Singkarak. The formation also crops out discontinuously along Lake Singkarak and northwestwards across the equator towards Muarasipongi. A lower Volcanic Member is composed of hornblende and augite andesites with intercalated tufts, limestones, shale and sandstone. An upper Limestone Member is also recognized, composed of massive grey limestone interbedded with shales, sandstones and tufts (Silitonga & Kastowo 1975). The rocks are commonly highly fossiliferous with large foraminifers: Doliolina lepida Schwager, Pseudofusulina padangensis, Neoschwagerina multiseptata Deprat and Fusulinella lantenoisi Deprat, at Silungkang (Katili 1969). Large fusulinacean foraminifers, Nankinella, Parafusulina and Pseudodoliolina and the porcellaneous foraminifer Hemogordius were also collected from an outcrop in the Aek Cubadak near Rao (Rock et al. 1983); these fossils indicate an Artinskian to Kazanian age for this outcrop. Waagenophyllid corals (Pavastehphyllum sp.) occur in limestones intercalated with volcanics and shales at Silungkang and in limestones at Guguk Bulat (Ipciphyllum and Wentzzelloides) where the Ombilin River flows out of Lake Singkarak; the latter indicating a Murghabian age (Fontaine 1982). The Guguk Bulat locality was classified with the Kuantan Formation by Silitonga & Kastowo (1975) but is more reasonably correlated with the Silungkang Formation (Fontaine & Gafoer 1989). Barisan Formation (Fig. 4.5). Rosidi et al. (1976) defined the Barisan Formation from outcrops of phyllite, slate, arkosic sandstone, limestone and cherts south of Solok and NE of the Sumatran Fault. The foliation in the phyllites and slates trends N N W - S S E , parallel to the fault. Rosidi et al. (1976) also defined a Limestone Member which forms linear outcrops trending in the same direction. The limestones cropping out at Bukit Cermin have yielded fusulinid foraminifers including Schwagerina sp. of Early Permian age. In the eastern part of its outcrop the Barisan Formation is equivalent to the Silungkang Formation, and Fontaine & Gafoer (1989) recommend that its designation as a separate formation should be discontinued. Palepat Formation (Fig. 4.5). Rosidi et al. (1976) defined the Palepat Formation composed of andesitic, basaltic and rhyolitic

38

CHAPTER 4

lavas and tufts interbedded with siltstones and crystalline limestones, which they considered to be a volcanic member of the Barisan Formation. It is also equivalent to the volcanic unit forming lower part of the Silungkang Formation, described above. The interbedded limestones are sometimes fossiliferous, and fragmental brachiopods and crinoids occur in the tufts. The foraminifer Fusulina sp. was identified from limestones in the Sungai Tabir. A rich brachiopod fauna and the fusulinids Veerbeekina and Sumatrina described by Meyer (1920) and Tobler (1923) from the Sugai Selajau indicates a Lower Permian age (Fontaine & Gafoer 1989).

Ngaol Formation (Fig. 4.5). The Ngaol Formation, defined by Rosidi et al. (1976) in the southeastern part of the Painan Quadrangle Sheet, includes a Limestone Member with Fusulinella, Sumatrina and Siphoneae (Tobler 1922). High-grade metamorphic gneiss, schist and marble cropping out in the same area were also inappropriately included in this unit (Rosidi et al. 1976). Fontaine & Gafoer (1989) report that limestones in the Sungai Tabir downstream of Ngaol village are rich in Middle Permian fossils, while upstream the rocks are of Jurassic age, and recommend that the recognition of the Ngaol Formation as a separate unit should be abandoned. Again, the Permian rocks in this unit may be regarded as part of the Silungkang Formation. Mengkarang Formation (Fig. 4.5). The Mengkarang Formation, famous internationally for its 'Jambi Flora', was defined by Suwarna et al. (1994) from outcrops in the Mengkarang River and adjacent river sections to the SW of Bangko. In earlier descriptions this formation was divided into the Air Kuning, Salamuku and Karing Beds (Zwierzijcki 1935), but these terms are now considered to be obsolete (Fontaine & Gafoer 1989). Rock types in the Mengkarang Formation include conglomerate, sandstone, siltstone, claystone, sometimes carbonaceous, limestone and thin coals. The sandstones are poorly sorted and clasts in conglomerates and sandstones include volcanics, quartzite and vein quartz (Simandjuntak et al. 1991). Outcrops in the banks of the Batang Tembesi at Pulau Bayer are composed of sandstone and polymict conglomerates with wood fragments and with a siliceous cement. The sandstones are folded into an anticline on an east-west axis, overturned towards the north. Thin intervening shales have not developed a slaty cleavage. These outcrops show imbrication of thin sandstone beds, indicating westward-directed thrust movements, prior to the folding. On the opposite side of the river, vertically bedded grey limestones show algae, bryozoa and gasteropods weathering out on the surface. Numerous fossil localities in the Mengkarang Formation which have yielded algae, fusulinid foraminifera, brachiopods, gastropods, crinoids and corals are indicated on maps by Fontaine & Gafoer (1989, Figs. 13 & 14). The 'Jambi Flora' was originally described by Zwierzijcki (1935), Jongmans (1937) and Marks (1956). The flora and fauna have more recently been reviewed by Asama et al. (1975), Vozenin-Serra (1989) and Fontaine & Gafoer (1989). Asama et al. (1975) concluded that the flora, which is rich in lycophytes, pteridophytes, pteridosperms, cordaites, and gymnosperms, is composed entirely of Euramerican and north Cathaysian species and includes no Gondwanan species. It is older than the typical Cathaysian Gigantopteris flora and may represent an earlier stage in its development (Asama 1976, 1984). Vozenin-Serra (1989) reported the occurrence of Cordaites and coniferous wood fragments collected by Fontaine. These wood fragments do not show annual rings, which is taken to indicate that they grew in a tropical or semi-tropical environment. After reviewing the flora, Vozenin-Serra (1989) concluded that it corresponds with the oldest horizon of the Cathaysian flora of northern China and represents the southernmost record of this flora. The plant-bearing horizons containing the Jambi Flora are interbedded with limestones containing fusulinids, tabulate and rugose corals, brachiopods and a rich tropical algal microflora

(Vachard 1989a, b). The fauna has affinities with the fauna of the Lower Permian of China and Central Europe (Fontaine & Gafoer 1989). Fusulinids indicate that the plant beds are of Upper Asselian age, possibly extending into the Sakmarian (Fontaine & Gafoer 1989, footnote on p. 55).

Bukit Pendopo (Fig. 4.10). Limestone cropping out in Bukit Pendopo in the core of a faulted anticline on the Lahat Quadrangle Sheet (Gafoer et al. 1986b) has yielded abundant Permian fossils including fusulinids, small foraminifera and algae. The fusulinids include Arminina asiatica, Cancellina praeneoschwagerinoides and Neoschwagerina simplex. These fossils indicate an Early Murghabian age for this limestone outcrop (Tien 1989).

Pemali Group (?Carboniferous-Early Permian) (Fig. 4.10) As mentioned above, rocks of Carboniferous-Permian age on the islands of Bangka and Billiton have been termed the Pemali Group. The Pemali Group in the Taboali District on the southern tip of Billiton includes 'pebbly mudstones', identical to those of the Bohorok and Mentulu formations of mainland Sumatra. Permian fusulinids were found at Air Durin on the island of Bangka by De Roever, in limestones forming part of the Pemali Group (De Neve & De Roever 1947; De Roever 1951; Ko 1986). Early Permian fusulinids have also been found offshore the north coast of the adjacent island of Billiton (Belitung) (van Overeem 1960; Strimple & Yancey 1976). Other Permian fossils recorded from Billiton include the ammonoid Agathiceras sundaicum of latest Artinskian or earliest Kungurian age, found as float in a tin placer (Archbold 1983). Archbold (1983) relates this form, and also a Permian nautiloid Neorthoceras to the Permian Bitauni fauna of Timor (Charlton et al. 2002). Strimple & Yancey (1976) report the occurrence of the crinoid Moscovicrinus from Selumar of probable Early Permian, Sakmarian age (Archbold 1983), and undescribed plant fragments of general Permian age have been ascribed to the Cathaysian floral province (van Overeem 1960).

Tempilang Sandstone (Mid-Late Triassic) (Fig. 4.10) The Middle to Upper Triassic Tempilang Sandstone crops out extensively in Bangka Island (Ko 1986). A limestone intercalated with sandstones and shales in the Lumut Tin Mine yielded Entrochus, Encrinus, Montlivaltia molukkana and Perodinella which were attributed a Norian age (De Neve & De Roever 1947). The characteristic Late Triassic thin-shelled bivalve Daonella has been reported from the island of Lingga to the north of Bangka (Bothe 1925b).

Conclusions As presently defined (Cameron et al. 1980; McCourt et al. 1993), the Peusangan Group includes units of both Permian and Triassic age. Permian rocks occur throughout the island of Sumatra from Aceh in the north to Bukit Pendopo in the south as well as in Bangka and Billiton. Triassic rocks are known only from the northern part of the main island of Sumatra, to the north of the equator, but also occur extensively in Bangka and Billiton (Fig. 4.10). The palaeontological evidence for the age of the Permo-Triassic units in Sumatra as determined by Fontaine & Gafoer (1989) is illustrated in Figure 4.11. The only possible representative of the Lower Permian in northern Sumatra is the Pangururan Bryozoan Bed whose age, on the basis of its fauna, has not been definitively established. In southern Sumatra on the other hand Lower Permian rocks

PRE-TERTIARY STRATIGRAPHY

STAGES

39

TETHYAN STAGES

RHAETIAN NORIAN CARNIAN LADINIAN ANISIAN SCYTHIAN

TATARIAN KAZANIAN UFIMIAN KUNGURIA ARTINSKIA SAKMARI3 ASSELIAN

DORASHAMIAN DZULFIAN MIDIAN MURGHABIAN KUBEGANDIAN BOLORIAN YAHTASHIAN SAKMARIAN ASSELIAN

Fig. 4.11. Palaeontological evidencc for the ages of Permo-Triassic stratigraphic units in Sumatra (data from Fontaine & Gafoer 1989). outcrop extensively in the Barisan Mountains southwards from Muarasipongi and are also found in Bangka and Billiton. Lower Permian formations in southern Sumatra include the andesitic, basaltic and rhyolitic volcanics of the Palepat Formation and the lower part of the Silungkang Formation. These volcanics are frequently interbedded with limestones and clastic sediments, and the limestones in particular, frequently contain large fusulinid foraminifera and other fossils which have allowed precise age determinations. Early Permian, Asselian to Kungurian ages, have been established for the Barisan and Palepat formations, and also for the Mengkarang Formation with its 'Jambi Flora' (Fontaine & Gafoer 1989). Cameron et al. (1980) interpreted these Lower Permian volcanics and the associated rocks as products of a Permian volcanic arc with its volcaniclastic sedimentary apron and carbonate reefs. Pulunggono & Cameron (1984) extended this interpretation into northern Sumatra on the basis of the occurrence of volcanic rocks in the Situtup Formation and volcanics of the Toweren Member of the Tawar Formation. However, no fossils have yet been found in the Tawar Formation so that its age is unknown, and fusulinids in the Situtup Formation have not been dated more precisely than mid-Permian. As noted above, it is possible that the epidotized basaltic rocks of the Situtup Formation and the Toweren Member of the Tawar Formation, should more properly be classified with the Jurassic-Cretaceous Woyla Group, cropping out in the same area, which includes similar lithologies. On the basis of the available evidence the case for the extension of the Early Permian volcanic arc into northern Sumatra is unproven. Geochemical studies and isotopic dating of the volcanic rocks are required to resolve this problem. Ages of deformation and metamorphism. During the Northern Sumatra Survey a distinction was made between the

Carboniferous-Permian Tapanuli Group, which is invariably affected by greenschist metamorphism, with the development of slates and phyllites, and the Permo-Triassic Peusangan Group, which is relatively undeformed and unmetamorphosed, except where it occurs in metamorphic aureoles (Cameron et al. 1980; Pulunggono & Cameron 1984). It was therefore proposed that the major phase of deformation occurred between the deposition of these two units. In order to establish the age of deformation and metamorphism affecting the older unit, it is essential to determine the ages of the units in the Tapanuli and Peusangan groups more precisely. The age of the Pangururan Bryozoan Bed is critical in this respect. The Bryozoan Bed is interbedded with turbiditic rocks identified as part of the Kluet (Bohorok?) Formation and is deformed with a slaty cleavage in exactly the same way as the surrounding rocks (Aldiss et al. 1983). Deformation of the Kluet/Kuantan, Alas and Bohorok formations therefore occurred after the deposition of this unit. As has been reported above, the fragmentary fauna obtained from the Pangururan Bryozoan Bed indicates a Late Carboniferous to Early Permian age, although the palaeontologists from the British Museum who made the determinations favoured the later age. If this age determination is accepted, the major deformation of the Tapanuli Group occurred after the deposition of the Bryozoan Bed, while the mid-Permian Situtup Limestone and Batumilmil Limestone formations of the Peusangan Group are undeformed. The main deformation in northern Sumatra therefore occurred in the late Early Permian or Early Middle Permian as Cameron et al. (1980) proposed. Certainly the main deformation of the Tapanuli Group in northern Sumatra occurred before the Triassic, as the Sibaganding Member of the M i d - L a t e Triassic Kualu Formation, cropping out along the shores of Lake Toba near the outcrop of the Bryozoan Bed, shows open folding, but the associated argillaceous units do not show a penetrative slaty cleavage.

40

CHAPTER 4

This conclusion can be extended throughout eastern Sumatra where the Tapanuli Group, the Malarco or Malang Formation on Kundur Island, the Persing Complex of Singkep Island and the Pemali Group of northern Bangka were all deformed prior to the mid-Triassic. However, it cannot be extended to central Sumatra. Although the Kuantan Formation in central Sumatra shows the same slaty cleavage with multiple deformation as the Kluet Formation in the same area, the Permian Barisan, the Triassic Tuhur and the Jurassic Rawas and Asai formations also show slaty cleavage and multiple deformation. Evidently in central Sumatra the major deformation event occurred after the deposition of the Jurassic sediments. Late Upper Permian and the earliest Lower Triassic deposits have not yet been recognized anywhere in Sumatra (Fig. 4.11). However, Mid-Late Triassic rocks are extensively developed in the northern part of Sumatra, from Aceh to West Sumatra and in the islands of Bangka and Billiton. The period between Late Permian and Middle Triassic was a period of regression and erosion, as reworked mid-Late Permian fusulinids are found abundantly in clasts in the mid-Late Triassic sediments of the Tuhur and Limau Manis formations (Silitonga & Kastowo 1975; Turner 1983). Therefore, the concept that the scattered outcrops of Permo-Triassic formations throughout Sumatra constitute a stratigraphic 'Group' is not valid. In future studies it would be sensible to divide these formations into Permian and Triassic groups. Triassic Correlation with West Peninsular Malaysia. A close correlation can be made between the Triassic rocks of northern Sumatra and those of Peninsular Malaysia. The Mid-Late Triassic age of part of the limestones of the Situtup Formation has been established by foraminifers (Cameron et al. 1983); the age of the Kaloi Formation, part of the Batumilmil Formation, the Sibaganding Limestone Member of the Kualu Formation by conodonts, and the Kualu Formation, the Cubadak and Limau Manis formations by ammonites and the presence of abundant Halobia. This whole assemblage of Triassic rocks in northern Sumatra can be correlated directly with the Upper Triassic Semanggol and Kodiang Limestone formations which crop out in Kedah and Perak in NW Malaya, some 200-250 km to the east across the Malacca Strait (Metcalfe 2000). The Semanggol Formation of Malaya has been divided into three members: a lower Chert Member, a Rhythmite Member and an upper Conglomerate Member (Burton 1973). The Chert Member, as its name implies, contains chert beds interbedded with shales and sandstones, the sandstones commonly showing disharmonic folding as convolutions and slumps. The Chert Member may be correlated directly with the Pangunjungan Member of the Kualu Formation of northern Sumatra. The Rhythmite Member, interpreted as a turbidite sequence with graded bedding, cross lamination slump folds and sole marks in the sandstones, and its fauna of thin-shelled bivalves, may be correlated with the thin-bedded sandstones, siltstones and mudstones of the type section of the Kualu Formation in the Sungai Kualu. The Conglomerate Member of the Semanggol Formation has not been recognized in northern Sumatra, although sandstone units become more common in the upper part of the Kualu Formation. The Conglomerate Member may be represented by the conglomeratic sandstones of the Papan Formation on Kudur Island to the south of Singapore and the Tempilang Sandstone of Bangka Island (Cameron et al. 1982c; Ko 1986). The massive Kodiang Limestone in northern Kedah, Malaya, has been identified as of M i d - L a t e Triassic age from the presence of conodonts (Ishii & Nogami 1966), and may be correlated directly with the massive limestone units in northern Sumatra described as Situtup, Kaloi, Batumilmil formations and the Sibaganding Limestone Member of the Kualu formation, which have all yielded Mid-Late Triassic conodonts (Metcalfe 1989a). Burton (1973) suggested that the lower part of the Semanggol Formation, with black carbonaceous shales and mudstones and

an abundant necktonic-planktonic fauna, was deposited in a basin of restricted circulation with anaerobic bottom conditions. He suggests that the chert beds may have resulted from the dissolution of volcanic glass in ash falls from volcanic activity at some distance from the site of deposition, as no beds of ash or pyroclastic deposits have been recognized in Malaya. However, volcaniclastic sediments and tuffs are recorded in the Cubadak and Tuhur formations of west central Sumatra (Rock et al. 1983; Turner 1983). In Malaya and in Bangka Island the increase in grain size and frequency of the sandstone units towards the east, suggest that the source area for the Semanggol sediments lay in this direction. However, there are also indications in current directions within the sandstones for derivation of sediments from local sources within the basin. The pebbles in the Conglomerate Member are composed mainly of vein quartz, quartzite and dark-coloured chert, which could have been derived from Palaeozoic rocks in the central part of the Malay Peninsula, which was evidently being uplifted in latest Triassic times. The Conglomerate Member may pass upwards into the Tembeling Formation of presumed Jurassic age (Burton 1973), which corresponds in age with the Tabir, Asai, Peneta and Rawas formations of central Sumatra (Rosidi et al. 1976; Kusnama et al. 1993b; Suwarna et al. 1994) to be described later. Mid-Late Triassic sediments in the western Malay Peninsula and northern Sumatra represent deposition on a broad continental shelf which was undergoing extension, with the formation of localized deep rift basins in which black shales and chert were deposited and into which, from time to time, turbidity cun'ents carried coarse clastic sediments. Carbonate was deposited on shallower parts of the shelf to form the massive limestone units in both northern Sumatra and western Malaya. In the basin, sandstone units increase in thickness upwards through the sequence and are replaced in Malaya by conglomerates, indicating uplift of the eastern source area. According to Metcalfe (2000) this uplift resulted from the collision between the Sibumasu (Sumatra) and Indochina blocks (East Malaya) which was taking place at this time. In his recent publications Metcalfe (2000) interprets the tectonic environment in which the Semanggol Formation was deposited as a foredeep basin, related to the collision.

Woyla Group (Jurassic-Cretaceous) Woyla G r o u p in A c e h

The Woyla Group was defined in Aceh, northern Sumatra, where the rocks are extensively exposed, but Jurassic-Cretaceous units correlated with the Woyla Group have been identified in the Barisan Mountains throughout western Sumatra (Fig. 4.12). In Aceh, areas of outcrop of the Woyla Group are shown on the GRDC Banda Aceh, Calang, Tapaktuan and Takengon 1:250 000 Quadrangle Sheets (Bennett et al. 1981a, b; Cameron et al. 1982b, 1983). The Woyla River, from which the Woyla Group was named, is on the Takengon Sheet (Fig. 4.13). The descriptions given below, except where specified, are taken largely from the reports which accompany these maps. An account of the lithological units which make up the Woyla Group and a detailed discussion of their interpretation is given by Barber (2000). During the DMR/BGS survey 13 lithostratigraphic units were distinguished in the Woyla Group in Aceh, as well as a unit of 'undifferentiated Woyla'. Many of the mapping units distinguished in the Woyla Group of Aceh during the DMR/BGS survey are made up of the same rock types, but in varying proportions. It is clear that they represent geographical, rather than genuine lithostratigraphical units. A different name was given to each distinguishable unit on each map sheet. The outcrops

PRE-TERTIARY STRATIGRAPHY

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of the actual lithologies within each formation are, on the whole, too small to be represented on the scale of the map. The stratigraphic units can be classified into three lithological assemblages: an oceanic assemblage; a basaltic-andesitic arc assemblage; and a limestone assemblage (Cameron et al. 1980). All of the units generally occur as fault-bounded lenses, distributed on both the northeastern and southwestern sides of the Sumatran Fault, and are elongated in a N W - S E direction, parallel to the Sumatran trend. The oceanic assemblage in particular is broken by a large number of minor faults and thrusts and has been interpreted as imbricated in an accretionary complex formed above a subduction zone (Barber 2000). The arc assemblage and the associated limestones are interpreted as a volcanic arc with fringing reefs (Cameron et al. 1980). The Woyla Group is affected by several large scale thrusts; the Geumpang, Takengon and Kla lines, which also affect the Miocene rocks in the area and are attributed to movements on the Sumatran Fault System. The distribution of these units and their relationships to the faults and thrusts are shown on Figure 4.13.

Oceanic assemblage. The oceanic assemblage includes serpentinites, gabbros, either massive or layered, and often altered to amphibolite, basalts, often as pillows, hyaloclastic breccias, volcaniclastic sandstones and siltstones, bedded cherts, black or purple shales and minor bedded or massive limestones.

,o,,0

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Fig. 4.12. Simplified geological map of Sumatra, showing the distribution of the Woyla Group and correlated units, with localities mentioned in the text.

Serpentinite units occur as lenses along the Sumatran Fault and along the Geumpang Line (Fig. 4.13). Several serpentinite bodies are shown on the Takengon Sheet (Cameron et al. 1983), including the largest of these lenses, the Tangse Serpentinite, which extends discontinuously for 27 km to the NW of Tangse, the Cahop Serpentinite and the Beatang Ultramafic Complex. These units are composed of massive serpentinite, representing altered harzburgite. Here and elsewhere, serpentinite is locally sheared, schistose, twisted and contorted. Sheared serpentinite may also form the matrix to m61ange, i.e. the Indrapuri Complex on the Banda Aceh Sheet (Bennett et al. 1981a). The m61ange encloses blocks of cumulate gabbro, basalt, red chert and limestones, derived from other units in the Woyla Group. Fossils collected from limestone blocks within the m61ange include: corals-Latoceandra ramosa, Stylina girodi; f o r a m i n i f e r s - - P s e u d o c y c l a m m i n a sp.; s t r o m a t o p o r o i d - - S t r o m a t o p o r a japonica, indicating a

Late Jurassic to Early Cretaceous age. In the Takengon Quadrangle large blocks of limestone enclosed in sheared serpentinite along the Geumpang Line, contain Late Miocene fossils (Cameron et al. 1983). Other units of the oceanic assemblage include the Penarum Formation, which outcrops to the northeast of the Sumatran Fault south of Takengon (Cameron et al. 1983) (Fig. 4.13), and consists of serpentinites, basalts, red cherts with radiolaria and slates. Volcanic rocks in this unit are commonly altered to greenschists. The Geumpang Formation (Banda Aceh Sheet--Bennett et al. 1981a; Tapaktuan Sheet--Cameron et al. 1982c) crops out

42

CHAPTER 4

6~

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to the SE of Banda Aceh on both sides of the Sumatran Fault. Rock types include massive or schistose basic volcanics, pillow basalts, volcaniclastic sandstones and tufts, commonly epidotized and altered to greenschists or phyllites, and thin grey or black limestones. The phyllites are usually lineated and crenulated, indicating multiple delbrmation. The rocks of the Geumpang Formation are considered to constitute the typical lithological and structural assemblage of the Woyla Group. The Geumpang Formation also includes a massive limestone member, frequently occurring as marble. The very similar Babahrot Formation cropping out to the NW of the Anu-Batee Fault towards Tapaktuan (Cameron et al. 1982c) (Fig. 4.13)includes serpentinites and talc schists, as well as metagabbroic bodies metamorphosed in the greenschist facies and highly disrupted and sheared into lenses. The Lain Minet Formation (Banda Aceh Sheet--Bennett et al. 1981a) and the similar Gume Formation (Takengon Sheet-Cameron et al. 1983) are composed of basaltic lavas, commonly epidotized, basaltic conglomerates and breccias, with volcanic and limestone clasts, but only rarely chert, graded volcaniclastic wackes, radiolarian cherts with manganese oxide veining, rhodonite, and calcareous, manganiferous and carbonaceous slates. A clast of radiolarian chert, embedded in a volcanic conglomerate with flattened clasts, was collected by Nick Cameron (pers. comm. 1999) in the Kreung Baro, Aceh, from a landslip within the outcrop of this formation. This occurrence indicates that volcanic rocks were erupted through ocean floor sediments, perhaps during the formation of a seamount. The formation also includes a recrystallized limestone member. The Jaleuem Formation cropping out 100 km to the SE of Banda Aceh on both sides of the Sumatran Fault, is composed largely of slates, but red cherts occur in float and the unit also includes a limestone member. The

Fig. 4.13. The distribution of the Woyla Group in Aceh. Modified from Stephenson & Aspden (1982), with data from Bennett et al. (1981a, b) and Cameron el aI. (1982, 1983).

Bale Formation, composed of coloured slates, with minor wackes and cherts, limestones and limestone breccias, is shown outcropping to the NW of the Sumatran Fault, and SE of Takengon. Arc assemblage. The basaltic-andesitic volcanics are interpreted as an island arc assemblage (Cameron et al. 1980) (Fig. 4.27), which is represented on the Banda Aceh Sheet (Bennett et al.

1981a) by the Bentaro Volcanic Formation, and on the Tapaktuan Sheet (Cameron et al. 1982b) by the Tapaktuan Volcanic Formation. The Bentaro Volcanic Formation is composed of porphyritic basalts and andesitic basalts with agglomerates, which are intruded by basic dykes. Basaltic vents, surrounded by breccias, tufts and volcaniclastic sediments, have been identified near Lain No and north of the Bentaro River on the Banda Aceh Sheet. A chemical analysis of a xenolithic, porphyritic basalt with pyroxene phenocrysts from this formation is given in Rock et al. (1982). The Tapaktuan Volcanic Formation occurs in fault-bounded lenses, within strands of the Anu-Batee Fault Zone, parallel to the west coast of Aceh north of Tapaktuan (Fig. 4.13). It consists of massive epidotized andesites and basalts, commonly porphyritic, and intrusive dykes of a similar composition. An analysis of hornblende microdiorite from this formation is given in Rock et al. (1982). The formation also includes agglomerates, breccias, tufts, red and purple volcaniclastic sandstones and shales, the latter often as slates, and a limestone member, composed of sparite and calcilutite, all as lenses and much disrupted by faults. Scattered outcrops of gneiss (Meukek Gneiss Complex) occur within the Tapaktuan Volcanic Formation in the Barisan Mountains to the north of Tapaktuan, between strands of the Anu-Batee Fault (Fig. 4.13). They consist of concordant leucogranodioritic gneiss, with garnet-biotite amphibolite containing

PRE-TERTIARY STRATIGRAPHY

garnets up to 8 cm in diameter, and biotite-hornblende-andesine schist (Cameron et al. 1982b). The occurrence of high-grade metamorphic rocks with garnets suggests that some of the units of the Woyla Group were deeply buried and were subsequently exhumed. These rocks warrant investigation to determine the origin of the protolith and the environment of metamorphism. Units containing a high proportion of volcaniclastic material are associated with the island arc assemblage. These include the Lho'nga Formation, which outcrops to the west of Banda Aceh, composed of grey and coloured slates and phyllites, with interbedded volcaniclastic sandstones, thin limestones and (?)radiolarian-bearing siltstones and the Lhoong Formation, which forms a large outcrop to the SW of the Sumatran Fault, and also occurs as roof pendants in the Sikuleh Batholith (Bennett et al. 1981b). The formation consists of basaltic lavas with cherts in the lower part of the sequence, followed by conglomeratic wackes with volcanic and limestone clasts, and subordinate sandstones, siltstones and limestones. Limestone units. Massive limestones, o/ten recrystallized, are also associated with the island arc assemblage and are interpreted as fringing reefs to volcanic islands. These units include the Lho'nga and Raba Limestone formations which crop out along the coast and in the Barisan Mountains to the south and west of Banda Aceh (Bennett et al. 1981a) (Fig. 4.13) and consist of massive calcarenite and calcilutite and dark thin-bedded cherty limestones and shales. The massive limestone is designated a 'Reef Member' which is closely associated in the field with the Bentaro Volcanic Formation. The Lamno Limestone Formation also crops out along the west coast of Aceh, south of Banda Aceh, and is also associated with outcrops of the Bentaro Volcanic Formation. It consists of dark limestone, with a reef-like facies, and contains volcanic clasts near the base. The limestone is commonly fossiliferous, with: corals--Actiastraea minima, S(vlosmilia corallina; algae--Clypeina sp., Permocalculus ampullacea, Lithocodium, Bacinella sp., Boueina sp., Thaumatoporella porvosiculifera; foraminifers--Pseudocyclammina lituus, indicating a Late Jurassic to Early Cretaceous age (Bennett et al. 1981a). The Teunom Limestone Formation crops out along the southwestern margin of the Sikuleh Batholith. It is composed of massive dark limestones, which are metamorphosed and recrystallized along the contact with the granite. The Sise Limestone Formation (Fig. 4.13) resembles the limestone units to the south of Banda Aceh, but anomalously crops out to the NE of the Sumatran Fault. Its present position may be due to some 200 km of dextral displacement along the fault. The unit consists of massive or bedded limestones, biocalcarenites and calcilutites with fossils: corals--Montlivaltia sp., Myriopora sp.; foraminifers--Pseudocyclammina sp. indicating a Late Jurassic to Early Cretaceous age (Cameron et al. 1983).

'Undifferentiated' Woyla (Fig. 4.13). On the geological map of the Takengon Quadrangle a large area of 'Undifferentiated' Woyla Group rocks is shown between the main strand of the Sumatran Fault and the Anu Batee Fault. This area is poorly known, but these rocks are described in the Explanatory Note as intermediate to mafic metavolcanics, slates and chert. 'Undifferentiated' Woyla is also shown in the Calang Quadrangle in the area to the south of the Sikuleh Batholith in Gunung Paling and as roof pendants within the outcrop of the batholith (Bennett et al. 1981b). These rocks are said to resemble the Kluet Formation, which crops out extensively to the NE of the Sumatran Fault, and should not be considered as part of the Woyla Group. Sikuleh Batholith. The Woyla Group in Aceh is intruded by granitoids. The largest of these is the Sikuleh Batholith shown on the Banda Aceh and Calang sheets (Bennett et al. 1981a, b). It is an elliptical body (c. 55 x 35 kin) elongated in a N W - S E direction (Fig. 4.13). Around the margins of the batholith limestones of

43

the Teunom Formation and 'undifferentiated Woyla Group rocks are altered by contact metamorphism. Lithologies resembling those of the Lhoong Formation occur as roof pendants within the batholith. The Sikuleh Batholith is a complex intrusion composed of an 'older complex' of migmatised gabbros and diorites locally gneissose and sheared and intensely veined. A 'younger complex' is more homogeneous coarser grained and unfoliated biotitehornblende granodiorite. The younger complex has been dated, from the mean of K - A r analyses of two biotites and one hornblende, as 97.7 _+ 0.7 Ma (early Late Cretaceous). Age of theWoyla Group in Aceh. Fossils from the Lamno Limestone and Sise Formations indicate that the fringing reefs around the volcanic arc were being formed during Late Jurassic to Early Cretaceous times. The K - A r ages of c. 97 Ma from the Sikuleh Batholith which intrudes the limestones and the oceanic assemblage show that the lithological units which make up the Woyla Group were in their present positions and had their present structural relationships by the early Late Cretaceous.

Woyla Group in Natal

Lithological units correlated with the Woyla Group of Aceh were mapped over an extensive area inland from Natal in North Sumatra during the Integrated Geological Survey of Northern Sumatra as part of the Lubuksikaping 1:250 000 Quadrangle Sheet (Rock et al. 1983) (Fig. 4.14). The outcrop is limited to the NE by the Sumatran Fault System and is much dissected internally by faults with a similar trend. The Woyla Group is intruded by Late Cretaceous granites and overlain unconformably by the Miocene Barus Group, by Miocene volcanic rocks, and by the products of Quaternary volcanism from the volcanoes of Sorik Merapi, Malintang and Talamau, as well as by recent alluvium. Units within the Woyla Group strike N W - S E and are very well exposed in the valley of the Batang Natal, both in the river section and in the parallel road section, which both cut across the strike (Fig. 4.15). The main outcrop of the Woyla Group is separated from a smaller outcrop in the Pasaman inlier to the south by Malintang Volcano (Fig. 4.14). In the D M R / B G S report of the Lubuksikaping Quadrangle (Rock et al. 1983) lithological units in the Batang Natal section were classified, from N E - S W , into three formations: the Muarasoma, Belok Gadang and the Sikubu formations (Fig. 4.14). Muarasoma Formation. The Muarasoma Formation outcrops in the upstream part of the Batang Natal section and in its tributary, the Aik Soma. Thicknesses of the rock units in this section were measured perpendicular to the strike for a distance of 5.5 km (Rock et al. 1983). The rock types in the measured section include cleaved argillaceous units, shale or slate, which may include calcareous concretions, laminated siltstones, and gritty sandstones showing sedimentary structures, indicating younging in a downstream direction, massive limestones, sometimes forming karstic limestone pinnacles, epidotic volcanic breccias and volcaniclastic sandstones, chloritic greenschists and muscovite-chlorite quartz schists. A 10 m 'conglomerate' (?m61ange) at the upstream end of the section, with elongated clasts of greenschist in a chloritic matrix, is probably of tectonic origin, formed in a fault or a shear zone (Rock et al. 1983). Belok Gadang Formation. The Belok Gadang Formation crops out in the central part of the Batang Natal section and is composed of sandstones, sometimes calcareous, and argillaceous rocks, often cleaved and containing bands and lenses of chert. The chert is radiolarian, but no identifiable radiolaria have so far been recovered which could be used to date the sequence. Outcrops in the

44

CHAPTER 4

I 00oe

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Fig. 4.14. The distributionof the WoylaGroup in the Natalarea, North Sumatra. ModifiedfromRock et al. (1983). KFZ,KanaikanFaultZone; SGF, SimpangGambirFault.

type locality of Belok Gadang, a tributary of the Batang Natal, show basaltic pillow lavas, with white clay interbeds and manganese-rich horizons with braunite, resembling the 'umbers', described from the Troodos Ophiolite of Cyprus (Robertson 1975). Analysis shows that the pillow basalts are spilites (Rock et al. 1982, 1983). In the type locality basalts are overlain by red, bedded cherts, but again no identifiable radiolaria have been recovered. Sikubu Formation. The Sikubu Formation, cropping out in the

lower part of the Batang Natal section, is composed of massive volcaniclastic metagreywackes, with thin shale interbeds. The sandstones show very well-developed sedimentary structures, including graded bedding, flame structures and convolutions, typical of turbidites. Massive porphyritic andesitic dykes and lava flows, with distinctive pyroxene phenocrysts, are intruded into, or interbedded with, the sediments in the lower part of the section. Fragments of porphyritic andesite, identical in composition to the dykes and lavas, occur as clasts in the sandstones 9 Woyla Group rocks in the Pasaman area include m~langes and massive and foliated peridotites (Rock et al. 1983) (Fig. 4.14). Peridotites are well exposed in the Pasaman River where they are composed mainly of harzburgite with minor dunite pods, pyroxenite dykes, disseminated chromite and rare chromite pods. Some of the peridotite is foliated, containing orthopyroxenes enclosed in augen. Coarse plagioclase-hornblende rocks, found as boulders in the float, represent metasomatised gabbro pegmatite which formed dykes in the peridotite. The peridotite is variably serpentinized, and in shear zones may be completely altered to serpentine and talc. Smaller bodies of serpentinite, with chromite pods, outcrop at the upper end of the Batang Natal section near Muarasoma (Figs 4.14 & 4.15) where they form spectacular serpentinite breccias faulted against slates and limestones of the Muarasoma Formation. Serpentinite also occurs as xenoliths in granite in the Aik Soma.

Intrusions and volcanics in the Natal area. Several large granite bodies are intruded into the rocks of the Woyla Group in the Natal area. The largest of these is the Manunggal Batholith at the northeastern end of the Batang Natal Section (Rock et al. 1983) (Fig. 4.14). This batholith is a composite body, some 2 3 0 k m 2 in extent, composed of leocogranite, granodiorite, granite and pyroxene-quartz diorite, with contaminated syenitic and monzonitic varieties, and appinites. The granitoid rocks are intruded by vogesite lamprophyre dykes. The granitoid rocks have been dated by the K - A r method at 87 Ma (Late Cretaceous) (Kanao et al. 1971, reported in Rock et al. 1983). In the Aik Soma, near Muarasoma, large granitic boulders in the river bed enclose serpentinite xenoliths, surrounded by reaction zones of amphibolite. Limestones in the same area are converted to skarns near the contact with the granite. A second granitoid, the Kanaikan is intrude into the Woyla Group in the Pasaman area (Fig. 4.14). This body is composed of coarse granodiorite and leucogranite cut by microgranitic and granophyric dykes. This intrusion lies within the Kanaikan Fault Zone, a strand of the main Sumatran Fault, and is much dissected by faults and deformed to form cataclasites along shear zones. Granitic rocks outcrop in headlands near Air Bangis along the coast to the south of Natal (Fig. 4.14). Rock et al. (1983) speculated that these rocks might be of Late Cretaceous age and analogous to the Sikuleh Batholith which intrudes the Woyla Group in Aceh. Later age dating showed that these granites were of Eocene-Oligocene age (Wajzer et al. 1991). Age constraints f o r the Woyla Group in the Natal area are provided

by a limestone sample from the Batang Kanaikan in the Pasaman inlier which yielded a colonial organism, closely resembling the samples of L o v f e n i p o r a described and illustrated by Yancey & Arif (1977) from the Indarung area, near Padang, and considered to be of Late Jurassic to Early Cretaceous age (IGS/British Museum Sample No. T C / J 1 / R l l 0 1 B - - R o c k et al. 1983).

PRE-TERTIARY STRATIGRAPHY

TH E BATA N G NATAL

BNL Jambor Baru :.~

RIVER SECTION 0

1

2

I

I

l

Formation

3km ........

-.soma

BNM

• " ' " ' • -Batu - - Nabontar

...

~, d~OMA

Limestone (BNL)

..,~: : :'.

I

BNL

Rantobi Sandstone Si Gala Gala ... :. :. :. ::: Schists ,~,~::: : :: : : :: : ::~ "'" !;fi~: i :~ ~""~~'m :i: ; ~ :.N." Batang Natal Parlampungan ~.'...... .~:.~ Megabreccia Volcanics (PV) .:::::::::: . (BNM) ,'5"iiiiiiiii:: ~ : STF

'

Panglong Melange Nabana Volcanics .~ ~,, BNL .,<,,^,,

45

'i:i:i:i:!:

~q:iiii ~ ~ i 9

!

~

. . : \ PV

....

9. : . : . . Muarasoma Turbidite

..:.:.:.:.. "" :" :" :" :

Formation (MTF)

",,',~',, 87.0Ma, z ,

Simarobu Turbidite Formation (STF) 44.8

MUARASOMA"

Ranto Sore Formation

9 . ...,..

~!!

.

Betok Gadang Siltstone

NATAL v v v v ,r "r -,

.:::::::GAMBIR~

".:::~ -...

%,* %~ %g %g %', %g %~- %,'%,, ~

~

~

-,,~

'

!

"

vvvv,~,vvvv"H""

::::::::::::::::::::::::::::::::::: . . . . . . . . . . . . . . . . . . lfi

"-:-:

%a %', ",~ o~*%p %e %'* %', %,, %', V

}~Langjsat

Tambak Baru :::::::::::::::::::::::::::::::::::::::::::::::::

~

%*~ ",P ".P %" -r

Volcanic'

"~ ~

,.,~^r . . .%,". .",d'. . V. . %" -r ~o %"

t o " "o,r %." "v" %" "v" v

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%P V %'. %'.

"~' "o,P %" %," %" ~g' ",/ %~ ",d' %," %-" Nr ",r %,- -,,e %g %,- %,- v

;4::::::~!:i:Turbidites:::::::::::::::::::::::::::::::

v , Langsat , # , v , Volcanics v v

SIMPANG

GAMBIR

~& Locationof limestoneblock ~ }'~ withLateTriassicforaminifera Locationsfor K/Ardates

_,~,O

"e" %'* %" %" -,,e '~.#" %," %,,

0

v. . . .v. . .v.+

I

:

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10

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I

%" ~ " %" ",-" %" 'Ne %*" V

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%" %" %" %," %" ",e" %r %4 %y %p %r %r

%'29.7Ma-"

v

v

v

v

",r v

v

v

v

20km

I

.i

Fig. 4.15. Geological map of the Batang Natal river section, North Sumatra. Inset shows isotopic dates, from Wajzer et al. (1991). S is serpentinite.

A minimum age for the Woyla Group is provided by the Manunggal Batholith, dated at 8 7 . 0 M a (Late Cretaceous) (Kanao et al. 1971, quoted in Rock et al. 1983), which intrudes limestones and serpentinites at the NW end of the Batang Natal section. Study by Wajzer et al. (1991). The Batang Natal section was mapped in detail by Marek Wajzer from the University of London, in a follow-up study to the Northern Sumatra Survey, in collaboration with BGS and with the assistance of Syarif Hidayat and Suharsono of GRDC (Wajzer et al. 1991). The mapping was supported by petrographic, geochemical and radiometric studies. Wajzer et al. (1991) found that each of the units recognized by Rock et al. (1983) in the Woyla Group, was composite, with the same lithologies repeated many times throughout the section, apparently in a random fashion (Fig. 4.15). Wajzer et al. (1991) distinguished 16 lithostratigraphical units in the Natal section. Correlation of these units with the mapping with those recognized by Rock et al. (1983) is shown in Table 4.1. Detailed accounts of these lithological units are given in Table 4.2. Many of the lithologies are similar to rock types described from the Woyla Group in Aceh, and by Rock et al. (1983), with the addition of several outcrops of m61ange, composed of blocks in a fine grained matrix, decribed as 'megabreccia' in Table 4.2 and Figure 4.15. One important

feature of the clastic units in the Woyla Group of the Natal area is that they are ahnost completely devoid of quartz, suggesting that they have an entirely oceanic, rather than a continental origin (Wajzer et al. 1991). The study established several additional age constraints for the Woyla Group, using fossil evidence and radiometric dating. A further specimen of L o v f e n i p o r a was obtained from a limestone block in the Simpang Gambir Megabreccia near the southwestern end of the Batang Natal section, and a Late Triassic foraminifer was found in a limestone clast in the Batang Natal Megabreccia in the central part of the section. Diorite intruded into the Jambor Baru and Batang Natal Megabreccia Formations at Batu Madingding gave a K - A t age of 84.7 4- 3.6 Ma and an andesite in the Tambak Baru Volcanic unit, interpreted as a fragment of a volcanic arc, gave 78.4 4-2.5 Ma. Both these lavas and the intrusions are of Late Cretaceous age. Andesite dykes intruded into the Si Kumbu Turbidite Formation (i.e. Sikubu Formation of Rock et al. 1983), and regarded as contemporaneous with sedimentation of this unit, gave K - A r ages of 40.1 4- 4.6 Ma and 37.6 _+ 1.3 Ma (Late Eocene) (Wajzer et al. 1991). Samples collected from the Air Bangis granites and analysed by Wajzer gave K - A r ages of 2 9 . 7 _ 1.6 and 28.2 4 - 1 . 2 M a (Late Oligocene) (Wajzer et al. 1991) showing that the Cretaceous age for these granites suggested by Rock et al. (1983) was incorrect.

46

CHAPTER 4

Table 4.1. Correlation of formations in the Woyla Group in the Natal area from Rock et al. (1983) with the lithotectonic units defined by Wajzer et al. (1991) Rocket et al. (1983)

Wajer et al. (1991)*

1. Langsat VolcanicFormation 2. Sikubu Formation

1. Langsat VolcanicFormation 2. Si Kumbu Turbidite Formation 3. Tambak Baru Volcanic Unit 4. Simpang Gambir Megabreccia Formation 5. Nabana VolcanicUnit 6. BelokGadang SiltstoneFormation 7. Panglong M61angeFormation 8. Ranto Sore Formation 9. ParlampunganVolcanic Unit

3. Belok Gadang Formation

Volcanics in both the Belok Gadang and Maurasoma Formations 4. Maurasoma Formation Schistose Member

Massive limestonesin both the Belok Gadang and Maurasoma Formations

10. Si Gala Gala Schist Formation 11. Simarobu Turbidite Formation 12. Batang Natal Megabreccia Unit 13. Rantobi Sandstone Formation 14. Jambor Baru Formation 15. Maurasoma Turbidite Formation 16. Batu Nabontar Limestone Unit

*units are listed in approximate order upstream from Langsat with no age relationship implied.

Units in central S u m a t r a c o r r e l a t e d with the W o y l a G r o u p

Outcrops of rock units with similar lithologies to those of the Woyla Group or which were formed within the same JurassicCretaceous age range have been mapped throughout western Sumatra (Fig. 4. ! 2). Many of these outcrops have been correlated by previous authors with units of the Woyla Group described from northern Sumatra. lndarung Formation. Small outcrops of the Mesozoic Indarung

Formation occur near Padang in West Sumatra. These rocks were mapped and described by Yancey & Alif (1977) and were correlated with the Woyla Group of Aceh by Cameron et al. (1980). Outcrops occur 15 km east of Padang in road, river and quarry sections near Indarung, where they are surrounded and overlain by Neogene and Quaternary volcanic and volcaniclastic rocks (Fig. 4.16). The area of outcrop is included on the Padang, Solok and Painan Quadrangle Sheets (Kastowo & Leo 1973; Silitonga & Kastowo 1975; Rosidi et al. 1976). These rocks have been mapped more recently by McCarthy et al. (2001). Yancey & Alif (1977) described rocks exposed in the Lubuk Peraku River, the Ngalau Quarry, the Karang Putib Quarry and adjacent river sections near lndarung. Rock types in these outcrops are basic volcanics, which may include pillow lavas, volcanic breccia, tuff, volcaniclastic sediments, radiolarian chert and massive or bedded limestones. The basic rocks are sometimes deformed and metamorphosed to form greenschists. On the other hand, the limestones and cherts are essentially undeformed, although disharmonic folding and small-scale thrusts in the chert and gentle folds in the limestone are seen in the quarries, and the limestones may be recrystallized (McCarthy et al. 2001). A well-exposed section of limestone and tuff occurs in the river section of the Lubuk Peraku and in the road above the river (Yancey & Alif 1977; McCarthy et al. 2001). A measured columnar section of these outcrops from McCarthy et al. (2001) is given as Figure 4.17. The lower part of the section, described as the Lubuk Peraku Limestone, is a limestone breccia, which includes volcanic clasts near the base and is interbedded with thin tuff bands near the top. The breccia is overlain by a few metres of thin-bedded limestones and shelly marls and then by

thicker bedded and more massive limestones, some oolitic. Near the top of the section a limestone conglomerate, eroded into the underlying limestone with basal scours, provides clear evidence of way-up. Above the limestone there is a break in outcrop, until further downstream and in the road section above, the Golok Tuff, a calcareous vitreous crystal tuff is exposed. Although the contact between the breccia and the tuff is not seen, this section is regarded as a n essentially continuous stratigraphic sequence McCarthy et al, (2001). In the Ngalau Quarry, near Indarung, McCarthy et al. (2001) collected samples from a 15 m section of bedded chert for radiolarian determination. In the Karang Putih Quarry, one kilometre to the south of lndarung, lenses of chert are associated with massive limestone. McCarthy et al. (2001) report that the limestone in this quarry is completely recrystallized, possibly due to the effects of a granitic intrusion which occurs a short distance to the south (Fig. 4.16). An interpretative cross section shows the cherts and limestones imbricated together along low angle thrusts (McCarthy et al. 2001). Rock units in the Indarung area are well dated from fossil and radiometric age determinations. Radiolaria from chert in the Ngalau Quarry belong to the Transhsuum hisuikoyense Zone, of Aalenian, early Mid-Jurassic age (McCarthy et al. 2001). Lithologies and fbssil content of the limestones in the Lubuk Peraku section and in the Ngalau and Karang Putih quarries were described by Yancey & Alif (1977). The limestones are biosparites, with abundant bioclasts, oolitic calcarenites and micrites. Molluscan shell fragments, pellets, calcareous algae, stromatoporoids and scleractinian corals are common components of the limestones. Among the fossils identified were the (?) stromatoporoids A c t o s t r o m a and L o v f e n i p o r a . The former is considered to be restricted to the Late Jurassic, while the latter is diagnostic of the Late Jurassic to Early Cretaceous. A K - A r age date of 105 _+ 3 Ma (Albian, mid-Cretaceous) is reported from the Golok Tuff in the Lubuk Peraku by Koning & Aulia (1985) from a Caltex Pacific Indonesia internal report. Pillow lavas and cherts of the Indarung Formation have been equated with the oceanic assemblage of the Woyla Group of Aceh and with the Belok Gadang Formation of the Natal area (Cameron et al. 1980; Rock et al. 1983). Where these rocks are imbricated, deformed and altered to greenschists they may be interpreted, as is the case in Aceh and Natal, as materials accreted from a subducted ocean floor. The recent recognition of Middle Jurassic radiolaria in the cherts (McCarthy et al. 2001) shows that part of this ocean floor was of Jurassic age. The volcanic breccias tufts and volcaniclastic sandstones of the Indarung Formation are interpreted as the products of seamount volcanism, and the massive limestone with its Late Jurassic-Early Cretaceous fossil fauna is interpreted as part of a fringing reef formed around the seamount (McCarthy et al. 2001). During subduction the seamount with its carbonate cap collided with already accreted ocean floor materials, and the whole assemblage was imbricated to form the present complex. Siguntur Formation. Mesozoic rocks of the Siguntur Formation are exposed in the Sungai Siguntur, 15 km to the south of Indarung (Fig. 4.16). The area of outcrop is shown on the Painan Quadrangle Sheet and the lithology is described in the Explanatory Note (Rosidi et al. 1976). Rock types are quartzites, siltstones and shales, the latter sometimes altered to slates, and compact limestones. The map shows that the strike of the beds is eastwest, transverse to the general Sumatran trend. In the report the rocks are described as not intensely deformed or folded, but quartzites interbedded with slates showing bedding-parallel cleavage, suggest that the rocks are more highly deformed than at first appears. The limestones are reported to contain L o v f e n i p o r a , and are therefore of a similar age to the limestones at Indarung. The 'quartzites' reported from Siguntur were taken to indicate that these rocks had a continental origin (Barber 2000) but it

PRE-TERTIARY STRATIGRAPHY

47

Table 4.2. Lithology, environmental setting, structure, metamorphic grade and age constraints for units in the Batang Natal section (in order upstream from west to east, see Fig. 4.4), from Wajzer et al. (1991 Unit*

Lithology

Environment

Structure

Metamorphism

Age constraints

Langsat Volcanic Unit

Porphyritic basic volcanics

Arc volcanics

No ductile deformation

Prehnitepumpellyite

Si Kumbu Turbidite Formation

Volcaniclastic debris flows, proximal and distal turbidites

Submarine fan--apron to volcanic arc

D2 large scale folds ( F 2 ) on WNW-ESE axes

Prehnitepumpellyite

Tambak Baru Volcanic Unit

Andesitic volcanics

Di weak foliation (Si); D,

Simpang Gambit Megabreccia Formation Nabana Volcanic Unit

Volcanic breccia with limestone megaclasts and greywacke sandstones Basic volcanics (sometimes pillowed) amygdaloidal to east keratophyres, dolerite dykes Breccias with chert, Mn sedim, limestones and volcanic clasts in chert siltstone matrix

Fragments of volcanic arc and proximal volcaniclastics Proximal sediments derived from volcanic arc, with olistostromes Ocean-floor basalts, seamount

Prehnitepumpellyite/ greenschist Prehnitepumpellyite/ greenschist Prehnitepumpellyite/ greenschist

Possibly intruded by Air Bangis Granites. K - A r 28.2 Ma, 29.7 Ma Intruded by andesite dykes K - A r 40.1 • 4.6 Ma (NR45), 37.6 • 1.3 Ma (NRI20) Andesitic lava. K - A r 78.4 _+ 2.5 Ma (BN 133)

Panglong M61ange Formation

Belok Gadang Siltstone Formation

Volcaniclastic siltstones with few fine sandstones and rare conglomerates

Ranto Sore Formation

Volcaniclastic cross-bedded and channelled sandstones and unsorted conglomerates (lahars) Porphyritic andesites

Parlumpangan Volcanic Unit Si Gala Gala Schist Unit

Banded quartz, muscovite, chlorite schists

M61ange (olistostrome) of ocean-floor materials and pelagic sediments Unconformable on Panglong M61ange; ?lower trench slope basin fill Fluviatile intra-arc deposits

?Lovfenipora sp. In limestone block (Late Jurassic-Early Cretaceous)

D~ tight to isoclinal folds (F~); Slate grade D2 open to close folds (F2) fold F~ on N W - S E axes

Older than Belok Gadang siltstone

Dipping beds with no ductile deformation

Prehnitepumpellyite

Younger than Panglong M~lange Formation

D2 open to close folds (F2) on NNW-SSE axes

Unmetamorphosed

?Younger than adjacent units

Fragments of volcanic arc

No ductile deformation

Metasediments derived from acid-intermediate volcanic arc province Ocean-floor or trench deposit

D~ schistosity (S~) and rodding (LI); D2 open to close folds (F2) on N W - S E axes

Prehnitepumpellyite/ greenschist Greenschist

Simarobu Turbidite Formation

Volcaniclastic turbidites with minor calcareous siltstones

Batang Natal Megabreccia Formation

Large clasts of limestone, rare clastic sediments and igneous rocks in slaty matrix

Melange formed as olistostrome or as mud diapirs in accretionary complex

Rantobi Sandstone Formation

Thin bedded volcaniclastic sandstones and siltstone

Forearc basin deposits

Jambor Baru Formation

Volcaniclastic conglomerate, sandstone, siltstone, limestone and tuff Thin bedded volcaniclastic turbidites with a coarser-grained member Massive recrystallized limestone, rare fossils

Shallow marine and deeper water forearc basin deposits Upper trench slope basin sediments

Muarasoma Turbidite Formation Batu Nabontar Limestone Unit

D i strong foliations ( $ 1 ) ; D? open folds and crenulations (F:) No ductile deformation

Open marine shelf limestone

Foliation (S~); D2 open to closed folds (F2); D I tight to isoclinal folds (F]) axial plane on NNE-SSE axes D~ tight to isoclinal folds (Fl); D 2 open to closed folds deform S i about NNE-SSW axes; D~ tight to isoclinal folds (Fi) with axial plane foliation (S j); D2 open to closed folds (F2) detbrm Si on NNW-SSE axes Axial plane cleavage (S~); D~ isoclinal folds (F~) with D2 closed asymmetric folds (F2) N W - S E axes D I foliation (S~); D2 closed folds (F2) on N W - S E axes Di foliation (S0; D2 folds (F2) on N W - S E axes Dl tight folds in interbedded tufts (F1), fossils show strain

Greenschist

Cut by undeformed microdiorite dyke. K - A t 49.5 +_ 2 Ma (NR 7)

Slate grade

Included limestone clasts contain Late Triassic foraminifer. Intruded by Batu Madingding Diorite. K - A r 84.7 ___ 3.6 Ma

Slate grade

Prehnitepumpellyite/ greenschist Prehnitepumpellyite/ greenschist Recrystallized

Intruded by Batu Mandingding Diorite. K - A r 84.7 + 3.6 Ma

Intruded by Batu Manunggal Batholith. K - A r 87.0 Ma

*All units are cut by numerous faults and thrusts. Vertical faults often show horizontal slickensides indicating wrench fault movements. * K - A t age of Manunggal Batholith from Kanao et al. (1971). All other K - A r ages from Wajzer et al. (1991).

48

CHAPTER 4

~.i

.~q~-~

100~ J

~

~

.

~

f

~

~ ~ 0 0 ~ 9

~ .

::::::::::::::::::::::::

PADANG l~

TALANG ~A, 2579~

0

Dibawah

C? : : : : : : : : : : : : : : : : : : : : : . . . . . . . . . :. :. : :

4 --lO15 '

o

5

~l

~

Tertiar~

:.::

_(("! i F 1U " ~ .~. "~~~/ ~ . ~ " " " ~"

~~nn/~'/~/'-'~'~.,~,~ ~ ~ ~ / D '~ 1'oJ3o'0~,,,-,.~ ~ ~

may be that they are recrystallized cherts, analogous to those at Indarung. Siulak Formation. Further outcrops of Mesozoic sedimentary and volcanic rocks occur at Siulak 150 km to the SE of Padang (Fig. 4.12), in a fault block caught between strands of the Sumatran Fault (Rosidi et al. 1976). These sediments are calcareous siltstones, calcareous shales and limestones. The shales and siltstones are carbonaceous and contain angular quartz clasts. The limestones contain Loftulisa and Hydrocorallinae of Cretaceous age (Tobler 1922, reported in Rosidi et al. 1976). The volcanic rocks are altered andesites, dacites and bedded tufts with clasts of augite, hornblende, chlorite and glass. These rocks are the product of Andean arc volcanism on the margin of Sundaland. Tabir Formation. Sixty kilometres to the east of Siulak and to the NE of the Sumatran Fault Zone, in the Batang Tabir, are outcrops of red conglomerates, sandstones and tufts of the Tabir Formation (Fig. 4.5). Clasts in the conglomerates include quartzite, and andesitic fragments derived from the adjacent Palaeozoic rocks. The presence of Ostrea is taken to indicate a Mesozoic, possibly Jurassic age (Tobler 1922, reported in Rosidi et al. 1976). Asai, Peneta and Rawas Formations. Continuous with the outcrop

of the Tabir Formation and extending southeastwards to the south of Bangko, and also lying to the NE of the Sumatran Fault shown on the GRDC Sungaipenuh and Sarolangan map sheets, are large outcrops of Mesozoic rocks of the Asai, Peneta and Rawas formations (Kusnama et al. 1993b; Suwarna et al. 1994), (Fig. 4.12). Rock types include quartz sandstones, siltstones, shales and limestones tufts. The Rawas Formation also includes andesite-basalt lava flows, tufts and volcaniclastic sandstones. Clasts in conglomeratic units in these sediments are derived

Volcano i~st~cs

Indarung Formation ~ ~

SigunturFormation Permo-Carboniferous

Fig. 4.16. Distribution of outcrops of the Indarung and Siguntur Formations in the Padang area, West Sumatra. Based on GRDC maps (Kastowo & Leo 1973; Silitonga & Kastowo 1975; Rosidi et al. 1976).

from the local Palaeozoic basement. Sandstone units show turbiditic characteristics. Argillaceous units have a slaty cleavage striking N W - S E . Fossils, including corals and ammonites, especially from the limestone members, show that these sediments range in age from Middle Jurassic to Early Cretaceous (Suwarna et al. 1994). From the presence of locally-derived clasts all these sediments, although subject to later deformation, were evidently deposited in situ on the Sundaland continental basement. Pulunggono & Cameron (1984) suggested that these units were deposited in a foreland basin, but a forearc basin, related to an Andean volcanic arc represented by the volcanics lava flows and tufts in the Rawas and Tabir Formation, is a more probable environment of deposition. The presence of basaits, dolerites and sepentinites in the Rawas and southern parts of the Peneta Formation suggests that these sediments extended out onto oceanic crust.

Units in southern Sumatra correlated with the Woyla Group

The Pre-Tertiary basement rocks are very poorly exposed in southern Sumatra, as the greater part of the area is covered by Tertiary and Quaternary sediments and volcanics. The distribution of Pre-Tertiary units correlated with the Woyla Group of northern Sumatra has been determined from the occurrence of a few scattered inliers in the Gumai Mountains, the Garba Mountains and the Gunungkasih Complex and associated sedimentary units around Bandar Lampung and from boreholes put down in the search for oil in the Central and South Sumatra Basins (Fig. 4.18). In the Gumai Mountains they are described as the Saling, Lingsing and Sepingtiang formations (Fig. 4.19), in the Garba Mountains as the Garba Formation (Fig. 4.7) and in the Bandar Lampung area as the Menanga Formation (Fig. 4.8).

PRE-TERTlARY STRATIGRAPHY

Golok Tuff Formation

49

C~slal luffs with sedimentary structures (water lain) and occasional fine to medium interbeds

(schematic)

III I

I ~ I ~ I ~ i i i I

iul

i i

Pc+~r162

i

i

I i

i

I

I

Massive limestone (biosparite) with shell and algae

+ ~+';

+++~:+:C~++:~+o<)+:++~:+,+

Pale coloured volcanics overlain by massive limestone Conglomerate with I(X)% carbonate clasts in sandy shelly carbonate matrix

No exposure

I i i i i i i i i i,,, I I l l i i i i i i' I i i I ! i i I i i i i i

i

Lubuk Peraku Formation

g

Limestone conglomerate with basal scours Massive limestone Thinly-bedded limestone with dykes Shelly oolite -heavily veined Thinly interbedded with limestones and shelly marls - boudinage~ marl flowage, veining Thin pale tuff band in limestone conglomerate

Dark marls containing blocks of dark volcanics and limestone conglomerate (?tectonic) Nearly t00% carbonate clasts

Conglomerate ? breccia. Poorly sorted, subrounded to sub-angular clasts fi'om mm to several m in size. Carbonate clasts include bedded sandy limestone with bivalves, algal fragments and solotary scleractinian corals

Minor, but significant volcanic clast component

Fig. 4.17. Colunmar section through the Lubuk Peraku Limestone and the Golok Tuff, measured in the Lubuk Peraku river section, from McCarthy et al. (2001).

part of the Gumai inlier, is composed of amygdaloidal and porphyritic andesitic and basaltic lavas, breccias and tufts, associated in the field with serpentinites and cherts. On the basis of chemical analyses and discriminant plots the lavas have been interpreted as tholeiites of oceanic affinity and have therefore been interpreted as ocean floor basalts (Gafoer et al. 1992c). However, the presence of andesites, the amygdaloidal and porphyritic textures, suggests that the Saling Formation includes fragments of a volcanic arc. The lavas are cut by diorite dykes, regarded as contemporaneous with the lavas, and dated by K - A r analysis at 116 + 3 Ma (Early Cretaceous) (Gafoer et al. 1992c). The description of the Saling Formation closely resembles that of the Bentaro Volcanic Formation of Aceh (Bennett et al. 198 la) and the Nabana Volcanic and Parlumpangan units of the Batang Natal (Wajzer et al. 1991). The Early Cretaceous age shows that the Saling Volcanic Arc was active contemporaneously with the Bentaro Arc of Aceh.

sequence of ocean floor origin, together with fragments of a volcanic arc. Although the rocks are highly deformed and folded it is not clear from the descriptions whether they are imbricated to form an accretionary complex (Gafoer et al. 1992c). The strike of bedding and cleavage in the sediments is said to be north-south. The mapped east-west contact between the Saling and the Lingsing formations is therefore presumably tectonic (Fig. 4.19). The Lingsing Formation has been interpreted as deposited in a bathyal environment (van Bemmelen 1949; Gafoer et al. 1992c). The presence of lavas interbedded with clastic deposits, suggests that the Lingsing Formation represents more distal flows, volcaniclastic sediments and clastic carbonates derived from a volcanic arc, extending out into the ocean floor environment, represented by the bedded cherts. These rocks resemble clastic units in the Lho'nga Formation of Aceh (Bennett et al. 1981a) and the Belok Gadang Siltstone and Rantobi Sandstone formations of Natal (Wajzer et al. 1991).

Lingsing Formation. The Lingsing Formation in the southern part

Sepintiang Limestone Formation. In the Gumai inlier the Saling and

of the Gumai inlier (Fig. 4.19), contains igneous rocks similar to those of the Saling Formation, interbedded with claystone, siltstone, sandstone, calcilutite and chert. The Saling and Lingsing formations are therefore considered to be contemporaneous. Since tholeiitic basalts are associated with serpentinized ultrabasic pyroxenites and cherts, this assemblage is regarded as an ophiolitic

Lingsing formations are overlain discordantly by the Sepingtiang Limestone Formation (Fig. 4.19). This is composed of massive, brecciated and bedded limestones, containing the coral Calamophylliopsis crassa (Late Jurassic), the foraminifers Pseudotexturariella, small Cuneolina (Early Cretaceous) and Orbitolina sp. (mid-Cretaceous). The contact between the Sepingtiang

Saling Formation. The Saling Formation, which forms the northern

50

CHAPTER 4

.

"o " '" ..N'.'.'.'.'.'103._.~j) .

.

.

.

.

.

, ~ ~ 104

.

"Tigapuiuh~

I

, ~ 105

106~

Mountainsl i i i i...~L~ "

PADANG -1~

,i,i-i-i-i-i,i..-->

0

50

100km

.

I

9. . . . . . . . _

9 ...........#1.o.

!

B]

.o.7.2\

(,bj, BANGKA

i ::@:;i:~,~ . :". . :". . .:.". ;,Oe?g/'}'rio0 } .

.

.

.

.

.

. . . . . . .

.Mountain ~ i~'':':''';:-:':'i'~ ~ :::'~

Formation ~%'--"".,_'",_'\"N ~.."..~.

,o \

',:":':'i:":'i'::'s

O:'''...j

au,ts Taboali

NIKN Thrusts

JURASSIC - MID-CRETACEOUS (Woyla Grot ~ Sepintiang, kingsing, Saling, Situlangang, i Garba and Menanga Formations q MID-JURASSIC - EARLY CRETACEOUS ~

Tabir, Rawas and Peneta Formations

PERMO-TRIASSIC Pemali, Tempilang, Papan, Kualu, Tuhur and Silungkang Formations EARLY PERMIAN (PEUSANGAN GROUP) [ ~ Palepat and Mengkarang Formations CARBONIFEROUS - ?EARLY PERMIAN (TAPANULI GROUP) Kuantan Formation

Mentulu (Bohorok) Formation Squares, circles and triangles indicate units encountered in boreholes

104~ I

~i~ -v\

105~ I

~

106~ I

Fig. 4.18. Distribution of the subcrop of the Pre-Tertiary stratigraphic units in southern Sumatra, including the Jurassic-Cretaceous Woyla Group. Borehole data is from De Coster (1974). Boreholes marked 'L' bottomed in the 'Kluang Limestone' regarded as Cretaceous by De Coster (1974), but considered more likely to be part of the Kuantan Formation in this account. The distribution of Permian (P) and Triassic (Tr) units on Bangka is from Ko (1986).

Limestone and the underlying units is considered to be tectonic (Gafoer et al. 1992c). The Sepingtiang Limestone may be interpreted in the same way as the limestones in Aceh, as a fringing reef surrounding a volcanic arc. Fossil evidence of the Late Jurassic to mid-Cretaceous age of the Sepintiang Limestone Formation means that it can be correlated directly with the Lamno, Teunom and Sise Limestone formations of Aceh (Bennett et al. 1981a; Cameron et al. 1983), the Batu Nabontar limestones in the Batang Natal section (Wajzer et al. 1991) and the Lubuk Peraku limestones at Indarung (Yancey & Alif 1977).

Intrusions in the Gumai Inlier. The Jurassic-Cretaceous units in

the Gumai Mountains are cut by granitic intrusions, which by analogy with similar dated granites further south in the Garba Mountains, described below, are regarded as of Late Cretaceous age (Gafoer et al. 1992c). The rocks of the inlier and the surrounding Tertiary rocks are also cut by N W - S E - t r e n d i n g faults, some showing strike-slip displacements (Fig. 4.19), and are evidently related to the Sumatran Fault System, the main strands of which lie some 25 km to the SW.

Garba Formation. The Garba Formation in the Garba Mountains is associated with metamorphic rocks of the Tarap Formation (Fig. 4.7). The Garba Formation is composed of (?)amygdaloidal and porphyritic basaltic and andesitic lavas. The volcanic rocks are associated with sheared serpentinite and lenses and intercalations of radiolarian chert. A fault-bounded sliver on the eastern side of the inlier, and a few other scattered outcrops where chert is abundant, are mapped as the Situlanglang Member (Fig. 4.7). An Insu Member is distinguished on the map, with a similar lithological assemblage, but also containing interlayered lenticular bodies of m~lange ('m' in Fig. 4.7), with boulders of basalt, andesite, radiolarian chert, claystone, siltstone, schist and massive limestone in a scaly clay matrix (Gafoer et al. 1994). The limestones found as blocks do not crop out elsewhere in the inlier, but are presumed to be derived from an unexposed component of the Garba Formation. Notably, metamorphic rocks of the Tarap Formation have not been found as blocks in the melange. The foliation in the scaly matrix and the elongation of the enclosed blocks, which are cut by tension fractures normal to their long axes, trends in a N W - S E direction (Gafoer et al. 1994). Two fold phases are recognized in the Garba Formation, an earlier phase of e a s t - w e s t folds and a later phase of N E - S W folds

PRE-TERTIARY STRATIGRAPHY

I

51

103~ '

103o00' Lm Qv

Qv

Qv

to Bengkulu 60km 9

--

3o45'

9

,

,

.

,

,

Qv Qv

9

,

9 .

.

.

.

.

.

._.,_____-.- F .

Q

Quaternary Volcanics

v

Late M i o c e n e

Lm ,

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Pliocene

PI 9

.

.

,

Tom

9, ,

Middle Miocene

.,,

.,,

-..,,

Oligo-Miocene Eocene

,.,

,,.

:: - ' - F ~ ~

~iiilil

Qv .

.

.

.

.

............

.%<..:..

Late C r e t a c e o u s G r a n i t e s Sepingtiang Limestone Formation

Qv

Lingsing ( s e d i m e n t a r y ) F o r m a t i o n Saling (volcanic) F o r m a t i o n

= |

Pyroxenite

-------

Faults

~ 0

5

.J 10

15

20km I

103o00'

103o15'

I

I

Fig. 4.19. The distribution of the Saling, Lingsing and Sepintiang Formations, correlatives of the Woyla Group, in the Gumai Mountains, South Sumatra, after GRDC map of Bengkulu (Gafoer et al. 1992c).

(Gafoer et al. 1994). Neither the cherts nor the limestones have so far yielded age-diagnostic fossils. The Garba Formation has been compared to the Woyla Group of Natal (Gafoer et al. 1994) and certainly lithological descriptions of this formation and its Insu and Situlanglang members, correspond very well with those from Aceh and the Batang Natal section. The basaltic and andesitic lavas of the Garba Formation correspond with those of the Bentaro Arc, and may similarly be interpreted as part of a volcanic arc sequence. Limestone blocks within the m61ange may represent fragments of fringing reefs or the collapsed carbonate cappings of seamounts, the latter now represented by volcanics in the Garba Formation, as has been suggested for the Natal and Indarung areas (Wajzer et al. 1991; McCarthy et al. 2001). Descriptions of the m61anges of the Insu Member of the Garba Formation (Gafoer et al. 1994) are identical to those from Natal (Wajzer et al. 1991). The interlayering of the Insu Member with lavas, chert and m61ange (Gafoer et al. 1994) suggests that these rocks are deformed and imbricated in the same way as the Woyla Group in the Batang Natal section, and similarly represent an accretionary complex formed by subduction of an ocean floor. It may be that some of the low-grade metamorphic schists mapped within the Insu Member as Tarap Formation, are part of this accretionary complex, as metamorphic rocks, up to greenschist facies, are incorporated in the accretionary complex at Natal 9 Rock units within the Garba inlier are cut and bounded by N W - S E trending faults. Although these faults are parallel to the Sumatran Fault System they do not appear to affect significantly the Tertiary rocks and must be largely of Pre-Tertiary age.

Intrusions in the Garba lnlier. Both the metamorphic Tarap and the Garba formations are intruded by the Garba Pluton (Fig. 4.7), a composite body in which an older component has been dated by the K - A t method at 115 • and 1 0 2 _ 3 M a (midCretaceous) and a younger component at 79 • 1.3 Ma and 89.3 + 1.7 Ma (Late Cretaceous) (Gafoer et al. 1994). Since the Garba Pluton (115-79 Ma) intrudes both the Tarap and the Garba formations, the accretion of the Garba Formation to the margin of Sundaland took place before the mid-Cretaceous. The age of the younger component of the Garba Pluton is comparable to that of the Sikuleh Batholith in Aceh (98 Ma) and the Manunggal Batholith (87 Ma) in Natal.

Menanga Formation. The Menanga Formation occurs in scattered outcrops between Bandar Lampung and Kotaagung to the SW of the schists and gneisses of the Gunungkasih Complex (Fig. 4.8). The Menanga Formation consists of tuffaceous and calcareous claystones, sandstones and shales with intercalated radiolarianbearing cherts, manganese nodules and coral limestones and rare porphyritic basalt. The sandstones contain clasts of glassy andesite and lithic fragments of andesite, quartz-diorite and quartzite. The cherts have not so far yielded diagnostic radiolaria, but Zwierzijcki (1932, confirmed in Andi Mangga et al. 1994a), reports the occurrence of O r b i t o l i n a sp. of Aptian-Albian (mid-Cretaceous) age fi'om limestones in the Menanga river section. The bedding strikes N W - S E with dips of 35o-60 ~ to the NE. The rocks are folded and cut by faults, with slickensides indicating reverse movement.

52

CHAPTER 4

The contact between the Gunungkasih Complex and the Menanga Formation in Gunung Kasih itself is obscured, due to rice cultivation, and in Teluk Ratai is at present inaccessible as it lies within a Naval Base (Fig. 4.8). However, the latter contact in the Menanga River was described by Zwierzijcki (1932) as occupied by a 'friction breccia'. On the GRDC maps Amin et al. (1994b) and Andi Mangga et al. (1994a) show both these contacts as thrusts (Fig. 4.8). The Menanga Formation is interpreted by Amin et al. (1994b) as a deep-water marine sequence with interbedded basalt lavas and andesitic clastic fragments, derived from a volcanic arc, and deposited in a trench or forearc environment. These sediments were deformed during accretion to the Sumatran margin, represented by the Gunungkasih Complex. K - A r radiometric ages, ranging from 125 to 108 Ma (mid-Cretaceous) from hornblende in an amphibolitic schist in the Menanga Formation, is taken as the age of accretion (Andi Mangga et al. 1994a). However, the presence of quartzite and quartz-diorite clasts suggests that the Menanga Formation was, like the Rawas and associated formations in central Sumatra, derived from an Andean arc built on a continental basement, and was deposited in a forearc environment. The Menanga Formation was overthrust by the basement at a later stage. in the Bandarlampung area. Near Bandarlampung the Gunungkasih Complex is intruded by the Sulan Pluton (Fig. 4.8). The pluton is a composite body which includes gabbro, dated by K - A r radiometric analysis at 151 + 4 M a (Late Jurassic), hornblende and biotite granites and granodiorite intruded by late aplogranite dykes. Granite from the Sulan Pluton gave an age of 113 ___ 3 Ma (mid-Cretaceous) (McCourt et al. 1996). To the north of Bandarlampung, spectacular exposures below an irrigation dam on the Sekampung River show extensive outcrops of granodioritic and dioritic gneiss, containing basic xenoliths, and cut by concordant and discordant granitic and pegmatitic veins. The granitic and granodioritic gneisses are cut by basaltic dykes, several metres thick, which contain xenoliths of gneiss. The gneiss xenoliths show evidence of melting, and towards the margins of the dykes are drawn out into streaks, which are sometimes isoclinally folded, parallel to the dyke margins. The dykes and the foliation in the gneisses both trend in a N W - S E direction. Fold structures in the dykes and the curvature of foliation in the gneisses indicate that the dyke margins have acted as strike-slip shear zones, with a sinistral sense of movement. Sub-horizontal slickensides on foliation surfaces within the gneiss indicate the same sense of movement. Diorite from the Sekumpang exposure has been dated by the K - A r method at 89 _+ 3 Ma (late mid-Cretaceous) (McCourt et al. 1996). In the same area, in the Wai Triplek, greenschist facies white mica-quartz schists are intruded by metadolerite dykes. The margins of the dykes show compositional banding which is isoclinally folded, in a similar fashion to the dykes in the Sekampung River. Further upstream the bed of the Wai Triplek exposes streaky acid and basic gneisses cut by more homogeneous basic dykes. Acid gneiss shows evidence of having been melted and recrystallized along the dyke contacts, and quartz-feldspar veins fill fractures in brecciated basic dyke material, in a process of back injection. Relics of dyke rocks occurring as basic xenoliths in gneiss, and gneiss xenoliths enclosed in basalt dykes, indicate that the intrusion of basaltic dykes and granitic bodies alternated during the development of the gneiss complex at Sekampung. Exposures in the Wai Triplek form part of the same gneiss complex, but also contain fragments of the schistose continental basement into which the igneous rocks were intruded. During or shortly after intrusion, both granitic and basic rocks were affected by sinistral shearing, which converted the granitic and Intrusions

dioritic rocks into gneisses and deformed the basic dykes. The alternation of acid and basic intrusion, with contemporaneous deformation, are characteristic features of the basal parts of a magmatic arc, where acid and basic magmas are intruded into an active strike-slip fault zone. This situation is similar to that which exists beneath Sumatra at the present day where the modern volcanic arc is built on the active Sumatran Fault Zone. However, the sense of movement along the present arc is dextral, in the opposite sense to the sinistral movement along the Cretaceous arc.

Interpretations o f the W o y l a G r o u p

On completion of the Integrated Geological Survey of Northern Sumatra the DMR/BGS mapping team published an interpretion of the Woyla Group in Aceh (Cameron et al. 1980). It was suggested than the oceanic assemblage represented an ocean floor and its overlying pelagic sediments. The arc assemblage was interpreted as a volcanic arc, and the associated limestones as the surrounding carbonate reefs. It was suggested that the volcanic arc had developed on a fragment of continental crust which had separated from the margin of the Sundaland continent along a transtensional transcurrent fault, similar to the present Sumatran Fault System. Extension led to the formation of a narrow short-lived marginal basin in a process similar to that which is forming the Andaman Sea or the Gulf of California at the present time (Cameron et al. 1980, Fig. 4a). There is no direct evidence to support the suggestion that the arc assemblage was constructed on continental crust, but a number of circumstantial arguments have been put forward in support of this interpretation: the arc assemblage is intruded by the Sikuleh Batholith, which it is suggested was derived from the underlying continental crust; quartz-rich rocks associated with the batholith and shown as 'undifferentiated Woyla Group' rocks on the Calang map sheet (Bennett et al. 1981a) are interpreted as roof pendants, uplifted from the underlying basement; and tin, recorded in stream sediment samples along the northern margin of the batholith, is normally restricted to continental crust (Stephenson et al. 1982). All of these arguments are open to objection and to alternative explanation. Unfortunately no detailed chemical analyses of the Sikuleh Batholith are available. However, it is a composite body, comprising an 'Older Complex' of variably deformed and contaminated gabbroic and dioritic rocks, into which is intruded a 'Younger Complex' of homogeneous, largely unfoliated, biotite-hornblende granodiorite, with a K - A r age of 97.7 _+ 7 Ma (Bennett et al. 1981b). The low values of stream sediment tin are associated with the outcrop of the Younger Complex, which is likely to be a mantle-derived I-type granitoid body. There is no detailed field or geochemical evidence in favour of the suggestion that roof pendants have been uplifted from an underlying basement; they could equally well have subsided from an overlying thrust sheet. It is possible that the tin in stream sediments in Aceh were derived directly by erosion and transport from the area to the east of the Sumatran Fault, or secondarily through Tertiary sediments. Although there is no direct palaeontological or isotopic evidence for the age of the Woyla oceanic crust, and the age of the volcanic arc is inferred only from the palaeontological age of the fringing reefs, in the model proposed by Cameron et al. (1980), the marginal sea is considered to have formed by extension and rifting in the Late Jurassic and Early Cretaceous. In the Late Cretaceous, compression, related to subduction on the outboard side of the Sikuleh microcontinental sliver, led to the collapse of the marginal sea to form the imbricated oceanic assemblage and the accretion of the microcontinental fragment, with its overlying volcanic arc, against the continental margin of Sundaland.

PRE-TERTIARY STRATIGRAPHY

As the D M R / B G S Survey extended southwards, the model developed in Aceh was used to interpret the Jurassic-Cretaceous rocks correlated with the Woyla Group in the Natal area (Rock et al. 1983). The Muarasoma Formation at the northeastern end of the Batang Natal section, with its turbidites and massive limestones was interpreted as shelf sediments formed on the continental margin of Sundaland. The Belok Gadang Formation, with pillow lavas manganiferous sediments and cherts, was interpreted as the imbricated floor of the marginal basin, and the Langsat Volcanics at the southwestern end of the section were interpreted as the volcanic arc overlying a continental basement. The underlying basement was inferred from the Air Bangis granites which intrude the volcanics, analogous to the situation at Sikuleh (Rock et al. 1983, Fig. 8). In the 'Tectonic Map o f Northern Sumatra' prepared by Aspden et al. (1982a) the continental fragments in Aceh and Natal were identified as the Sikuleh and Natal Microcontinental Blocks. A further block, the Bengkulu Microcontinental Block was subsequently proposed in southern Sumatra. The concept of microcontinents was taken up by Metcalfe (1996, Fig. 15) who suggested that these microcontinental fragments separated from the northern margin of Gondwana in the Late Jurassic and were accreted to the Sumatran margin in the mid-Late Cretaceous. The study by Wajzer et al. (1991) necessitated the re-interpretation of the Batang Natal section and the reassessment of the marginal sea model. It was found that the turbidites of the Muarasoma Formation were volcaniclastics, with no significant proportion of quartz, and that the massive limestones did not contain any material of continental derivation. The sediments of the

53

Muarasoma Formation are evidently of oceanic rather than of continental margin origin. The bedded cherts and manganiferous sediments in the Belok Gadang Formation were interpreted as representing the floor of an extensive ocean, rather than the floor of a restricted marginal sea. A limestone block in m61ange, interpreted as a collapsed carbonate capping to a sea mount, was found to contain a foraminifer of late Triassic age. Evidently the ocean floor accreted into the Woyla accretionary complex was already in existence in the early Mesozoic. An earlier date for the origin of the Woyla ocean floor has been confirmed by the discovery of early Middle Jurassic radiolaria from cherts in the Indarung Formation (correlated with the Woyla Group) near Padang (McCarthy et al. 2001). At the southwestern end of the Batang Natal section the Langsat Volcanics and the associated volcanoclastics were dated isotopically as of Late Eocene to Early Oligocene age (Wajzer et al. 1991). They are not, therefore, a Late Jurassic-mid-Cretaceous arc analogous to the Bentaro Volcanic arc of Aceh. The concept of microcontinental blocks accreted to the margin of Sundaland in the m i d - L a t e Cretaceous has not been proven. The arc volcanics of the Bentaro Formation and the granitoids of the Sikuleh Batholith require detailed geochemical study to determine whether they represent arc volcanics extruded through a continental basement. There is no evidence either at Natal or Bengkulu for a microcontinental block, the Langsat Volcanics and the Air Bangis granites have been shown to be part of an Eocene to Early Oligocene volcanic arc emplaced against the Natal section by late (Neogene or Quaternary?) strike-slip faulting (Barber 2000).

Chapter 5

Granites E. J. COBBING

Knowledge of the granites of Sumatra has been gathered mainly as the result of systematic mapping programmes conducted with the aim of identifying mineral resources and providing a geological data base for more detailed studies. Mapping programmes were conducted principally by Dutch and Indonesian geologists prior to the second world war, mainly in southern Sumatra and the Tin Islands. In the 1970s a combined Indonesian Directorate of Mineral Resources (DMR)/British Geological Survey (BGS) project was set up to map the geology of Sumatra to the north of the Equator. On completion of this project in the mid-1980s geological and geochemical maps for the region were published at the scale of 1:250000, together with descriptive sheet bulletins. Another useful compilation which may be refered to is the 1:2.5 million scale geological map for the whole of the Indonesian Archipelago which includes Sumatra (Clarke 1990). Subsequently BGS undertook a similar but smaller project in southern Sumatra in order to upgrade geological mapping and mineral exploration programmes which were being conducted by the Indonesian Geological Research and Development Centre (GRDC) and DMR. As part of this programme a specific effort was made to investigate the granites of this region. A combined granite workshop/regional mapping programme resulted in the identification of many granite units within batholiths such as Lassi, Bungo and Garba, as well as numerous isolated plutons. Full geochemical and isotopic analyses were provided for these granites (McCourt & Cobbing 1993; McCourt et al. 1996). Gasparon & Varne (1995) have provided further geological and geochemical information from selected granites and volcanics over the whole of Sumatra. Cobbing et al. (1986, 1992) had previously provided full geochemical and isotopic data for the granites of the Tin Islands as part of a comprehensive study of the granites of much of SE Asia. These combined studies confirmed earlier suggestions that the granites of Sumatra could be classified into a group of older, widely distributed tin-associated granites, and a group of younger, geographically restricted, volcanic-arc granites with a wide compositional range. The older tin-associated granites crop out throughout the whole of Sumatra, but are concentrated mainly to the east of the Barisan Range and also within it, but in some areas granite outcrops extend as far as the west coast. Granites of the volcanic arc suite are confined to the Barisan Range. At the present time it is difficult to provide a unified account for the granites of Sumatra, because much of the earlier work addressed different aspects of the geological, geochemical and isotopic relationships of the granites. This has resulted in difficulties in interpreting the earlier studies. Consequently the following synthesis is constrained by the different objectives and conditions under which the earlier regional work was carried out.

Isotopic ages of Sumatran granites Many of the published isotopic analyses from Sumatra are unsupported by petrographic descriptions or whole-rock chemical analyses. Moreover, in some cases isotopic ages determined for particular plutons cover such a wide range that it is impossible to establish their exact age of emplacement. In other cases the available geochemistry is sufficiently anomalous to cast doubt

54

on the reliability of the reported isotopic age. This is the case for the Ombilin Granite (Fig. 5.1), cropping out on the western shore of Lake Singkarak, for which Silitonga & Kastowa (1975) gave an R b - S r age of 256 _+ 6 Ma. This body has volcanic arctype geochemistry but is very strongly deformed, and shows highly anomalous potassium and rubidium values (McCourt & Cobbing 1993). These factors casts doubt on the reliability of the reported age, which is at least 50 Ma older than all other granites of that affinity. A further example of the difficulties in interpreting the isotopic ages of the granites of Sumatra is provided by the Sibolga Batholith in northwest Sumatra. This pluton has yielded a wide range of isotopic ages from 75 to 264 Ma. It is a very large body, and may well be composite, comprising several distinct units of different ages. In the hinterland of Sibolga the granite consists of biotite-hornblende granite and granodiorite with pink K-feldspar megacrysts, mafic enclaves and mafic dykes. These characteristics are typical of the Eastern Province Granites of Peninsular Malaysia and the Tin Islands, and distinguish these rocks from the tin-associated granites in the same areas (Cobbing et al. 1986, 1992). The position of the Sibolga Granite however, is completely anomalous, as it crops out on the far west coast of Sumatra, 300 km away from the Eastern Province Granites of Peninsular Malaysia. The isotopic age of 264 Ma (Aspden et al. 1982b) may represent the age of emplacement of the Sibolga Granite itself, but the 13 other ages recorded from this body, ranging from 75 to 264 Ma, cannot represent an emplacement age for the Sibolga Pluton, and may have been obtained from satellite plutons in the Sibolga region. Unlike the Sibolga Batholith there is no question of uncertain provenance for the Lassi Batholith (Fig. 5.1) which has yielded a much quoted Early Cretaceous age of 112 Ma (Katili 1974a). However, this is incompatible with the K - A r age of 56.3 Ma reported by Sato (1991). The five K - A r ages of 57, 55, 54, 53 and 53 Ma from different units of this batholith given in McCourt et al. (1996) and the 4~ ages of 55 and 56 Ma (Imtihanah 2000) confirm its Palaeocene age. The Lassi examples suggests that many of the isotopic ages reported from Sumatra do not reflect the age of emplacement, but it is at present impossible to distinguish these from reliable ages, unless complementary methods of isotopic dating have been used, a requirement which substantially diminishes the value of the currently available data set. For these reasons some of the isotopic ages quoted in the following acount may be subject to revision. Most of the granite ages considered in this account are those for which there is supporting isotopic and geochemical data. Until recently the U - P b zircon age of 264 Ma obtained by Liew & McCulloch (1985) from the Kuantan Granite of the Eastern Province of Peninsular Malaysia was the oldest recorded age for granites of the region. This has now been extended to 275 Ma by Schwartz & Askury (1990) who obtained K - A r biotite ages from plutons in the Kuantan-Dungun region ranging from 220 to 275 Ma. Ages from the Main Range Province in Peninsular Malaysia are generally younger, from 207 to 230 Ma (Cobbing et al. 1992). The peak of magmatism for the Main Range Granites in Peninsular Malaysia and the Tin Islands is 220 Ma, with granites ranging to older ages, especially in the Tin Islands: e.g. Belinyu 251 _ 10 and Penangas 252 _ 8 (Cobbing et al. 1992) (Fig. 5.2).

GRANITES

I

-

I

I

96OE

55

98,~

I

102~'

\ X "}

I

104"-'

i

106"

t 08~

6<,N

~

BANDA ACEIt

~ . ~""S ik u Ie h.,._.._~ , ,{JL.~ Batholith ~ \ q l o o GeuXfit'~eu~ ~ ~ _~ . . Granodiorite\'~ ;~ ?LSerbadjadl ,

Kuantan-Dungun .... i i

~\. L

4 o

Unga Diorite

MALAY PENINSULA

~....

,,[,u,on, i

2 c'

Sibolg,~ Batholith,

~' HataPang'x~-N-'~

~~ '

--

0<

(~

Muarasipongi -~Rokan

RIAU ISLANDS

L., "~ \~X "? ....Siabu Ombilill Sulit Air"[ G ran itel~.~J)te.~ ),,Sijunjung ;ingkep

- 2~ VOLCANIC ARC PROVINCE Biotite-hornblende diorites~,~ i tonalites, granodiorites and ~ ~ monzogranites of Volcanic Arc affinity, l-Types Age range 203-5 Ma MAIN RANGE PROVINCE -4'~ Biotite monzogranites of l ~,, J Post-collisional affinity S-Types l "~ J Some tin-associated Age range 247q43 Ma EASTERN PROVINCE Biotite and biotite-hornblende monzogranites of post-collisional _6 ~ [ - - ~ and crustal I-Types Age range 264-216 Ma

~anjung ~; "~lsahanU~'\"-P'---~ \ ,Gadang <,,, _North O JAMB.] I~IL Bungo Batholith ~: ~I'L~ South . BANGKA BILLITON

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0

96" .

.

.

.

.

.

.

.

1 ..................

100

200

98~ i

300

..................

400

F ranjong' Pandang'-~'---'Pluton

BENGKUI.U

~.e "--.

%

Garba LIN Batholith Padean oguru rbamba */Jatibaru Pluton ~ng~ "X>~.~'~On~;~Sulan T~

4 ~--

BANDAR LAMPUNG v,/ tti

500km

100~

l ..........................

102~ I

104~ . . . . . .

l

. . . . . . . .

Fig. 5.1. The granites of Sumatra, the Malay Peninsula and the Tin islands of Bangka and Billiton. Data from Beddoe-Stephens et al. (1987), Clarke & Beddoe-Stephens (1987), Cobbing et al. (1986, 1992), Sato (1991), McCourt & Cobbing (1993), Gasparon & Varne (1995). Broken line shows the eastern limit of the Western Province Granites in Sumatra.

In Peninsular Malaysia the Eastern Granite Province is separated by the Bentong-Raub Line from the Main Range Province, where the ages of the granites are generally younger, ranging from 207 to 230 Ma (Late Triassic) (Figs 5.1 & 5.3) although some granites, especially in the Tin Islands, have given Late Permian ages, e.g. the Belinyu and Penangas quoted above (Cobbing e t al. 1992). Although a great deal of work has been done on the granites of Sumatra only six studies have provided sufficient detail of their geological, geochemical and isotopic features for useful comparison. These are the publications of Beddoe-Stephens e t al. (1987) for the Muarasipongi Batholith, Schwartz & Surjono (1990a)

for the Tiga Puluh region, Clarke & Beddoe-Stephens (1987) for the Hatapang Granite, Sato (1991) for three I-type and S-type plutons in central Sumatra, Gasparon & Varne (1995) for selected plutons of mainly volcanic arc character from the whole island, McCourt & Cobbing (1993) who provided a complete data set of their collection for the southern half of Sumatra, and McCourt e t al. (1996) giving selected data from that data set. It is however useful to interpret the ages and affinities of other granites in Sumatra within the framework provided by these recent studies, using the field and petrographic characteristics provided by earlier studies.

56

CHAPTER 5

I

I

t08,~,

t 06~

NGAPORE

:am _La~.oi 226Ma

I~-~'~ ~ll BINTAN 5k,'~~ J East Bintan

AR~MUN ~

Eastern Province (I-Type) Granites

-% \- ~-j oBatholith X~Loban 229Ma ,~ Laut

Akat

"~

~~ %~

~

Main Range Province (S-Type) Granites

_ 0o

_

st Central

Sungai lsahan [~,~

_ Paku~-/SINGKEP "~--"~

~

-2os

P

e

n

a

n

g

a

S UM ATRA

BANGKA \,~

Belinyu Klabat Batholith 25~anjong Layang

s

~---:'~ ......)\Tanjong Batu

M ~ e ~9n u ~ n ~9a . .~l r - 200Ma 2-..---<7"z~

,. Tanjong Raya

100

200 --~ IIIIIIIIII 1 0 4 '~ I

s

213Ma \

PALEMBANG 0

Z'-

.....\

f _ j ~ % ~"

o ~

Tanjong BILLITON .......... ?f

Pluton ~ ~

n n Man

216Ma(~ g p'' 4 /)- ~ '"'~ u ~

g

~ /r., 2 0 %27nong Legau . Bukit L Toboali ~,~"-5--Batu 2 2 5 M a ~'-~ Nama Parangb~h gP Kelumpang 300km

/ (/ \

106 ~ /I

The granite suites The granites of Sumatra form two distinct groups. An older group is widely distributed as isolated plutons and batholiths over the whole island, but mainly in the area to the east of the Barisan Range. Some of these granites are tin-associated and have a narrow compositional range of SiO2 values, generally above 70%. These older granites are related to the Central (Main Range) Province of the Southeast Asian Tin Belt of Peninsular Malaysia and Thailand (Figs 5.1 & 5.3). A younger group of granites form the plutonic component of a volcanic arc suite. They are confined to the Barisan Range, where they form small batholiths and separated plutons with an extended compositional range from gabbro to monzogranite.

T h e T i n - a s s o c i a t e d suite

Tin-associated granites are of S-type affinity and are probably mostly of Triassic age. They are widely distributed in Sumatra but are poorly exposed. They are equivalent to the Main Range granites of Peninsular Malaysia and of the Indonesian Tin Islands. There is however, an almost complete lack of geochemical and isotopic data for these granites. Schwartz (1987) and Schwartz & Surjono (1990a) reported five major and trace element analyses from greisens and K-feldspar megacrystic biotite granites from the Sungei Isahan and adjacent areas in the Tiga Puluh region of South Sumatra (Fig. 5.1). Three of the analyses are of greisens and are anomalous in their composition, but two are from normal K-feldspar megacrystic monzogranites with SiO2 values of 71.7 and 71.47% which correspond closely with the geochemical signatures of granites from the Main Range Province of Peninsular Malaysia and Thailand. K - A r

1 0 8 ~;` I

Fig. 5.2. Main Range and Eastern Province granites in the Indonesian Tin Islands (after Cobbing et al. 1992). Karimun is a Tin granite, but it does have A-type affinities. Segal and Akat are both l-types. Karimunhas affinities with Dabo.

ages of 197 • 2 Ma and 193 4- 2 Ma were obtained for muscovite in greisens in the Sungei Isahan and an age of 198 4- 2 Ma from biotite in K-feldspar megacrystic granite at Bukit Kayumambang 20 km east of Sungei Isahan. The Sijunjung Batholith, which is located on the eastern flank of the Barisan Range to the northeast of Padang (Fig. 5.1), is a very large and inaccessible body, but a large sample was dated and chemically analysed by Sato (1991). The K - A r age is 247 Ma and the geochemistry, with a SiO2 value of 72.71%, is similar to that for the S-type granites of the Main Range Batholith of Peninsular Malaysia and the Tin Islands (Sato 1991). The Sungei Isahan and Sijunjung occurrences are at present the only examples of the tin-associated granites of Main Range Type in mainland Sumatra lbr which there is both geochronology and geochemical analyses. Provisionally these two occurences may be regarded as representative of the Tin-Associated Suite as a whole. Although the database for the widespread Tin-Associated Granites is small, where the writer has inspected them in the field they were found to bear a striking resemblance to granites of the Main Range (Central) Province in Peninsular Malaysia and the Tin islands. The Hatapang Granite, which is located to the south of Lake Toba (Fig. 5.1) was discovered by the investigation of a tin anomaly revealed by reconnaissance geochemical surveying. The geochemical and isotopic study by Clarke & Beddoe-Stephens (1987) established an R b - S r isochron age of 80 i 1 Ma with an initial ratio of 0.7151, which indicates an S-type affinity. They suggested on the basis of these results, that the pluton was not representative of the tin-associated granites of Triassic age, but was more likely to be one of the Western Province granites of mainly Cretaceous-Tertiary age occurring along the ThailandBurma border and the Shan Scarp region of Burma. Representatives of this suite are present at Phuket in southern Thailand north of the Ban Kram Fault Zone (Garson et al. 1975), and

GRANITES

)L;

,,,

l&o

100 ~

",,

57

1000

I

110 ~

tt

Wuntho

~

0

200

400

600

800

1000km

WPG

~i~i~'84184 ili~,'!~i:!~.~' ~':.i!~, I O0

o.

E Q. 0.

'

!~i i~~,ii i'~): :/:)~i

L

ooOO:

Z

0

=J

9 . ".--

C7

~!.?#~e ~ , "',,_

Oo

syn-COLG

9.

"~,~. ~ / "

..2../ t

. . . .

~Ol , ~lJll 9

I

I

//I I ,Ill

t0

i

i

I I ~lltl

100

1000

Log Y ppm

1000 --

b

o

=

i

~yo COLG

-=_

oo~ ~176

~illi;:~:iii _

9

2

100 9

# 9

o .J

elDOO

9"" VAG

//

go

;'.'"

/

9 ORG

o

0

i! z

o o

lg~

10

100

1000

Log Y + Nb ppm

[aub C o m p l e x

95OE

100 ~

105 ~

~

"~--.110~

Fig. 5.3. Granitic provinces of Sumatra and adjacent areas (modified after Cobbing et al. 1992 and McCourt et al. 1996). Clarke & Beddoe Stephens (1987) suggested that this suite continued southwards in central Sumatra, thus bringing stanniferous granites of younger age into an area dominated by older tin granites. The geochemical data from the Hatapang Granite suggests that it may have some alkali affinity, since it falls mostly within the 'Within Plate Granite' (WPG) field on Pearce diagrams (Fig. 5.4a, b) and in or close to, the alkali feldspar field on the QAP Le Maitre diagram (Clarke & Beddoe Stephens 1987) (Fig. 5.5). They also plot above the calc-alkali field of Kuno (1969 fig. 6.7). Such an affinity is compatible with the compositional range present in the granites of the Western Province (Cobbing et al. 1992). The Volcanic Arc Suite

It is however, the Volcanic Arc Suite (Fig. 5.1) that has provided the main focus for granite studies in Sumatra. The volcanic-arc affinity

Fig. 5.4. (a) Nb/Y and (b) Rb/(Y + Nb) discfiminant diagrams for syn-collision (syn-col), volcanic arc (VA), within plate (WP) and normal and anomalous ocean ridge (OR)granites after Pearce et al. (1984). Volcanic Arc granites, South Sumatra (filled circles; McCourt et al. 1996), the Hatapang granite (open circles; Clarke & Beddoes-Stephens 1987) and Bukit Batu (squares; Gasparon & Vame 1987).

of these granites was established by McCourt & Cobbing (1993) and McCourt e t al. (1996). Most of the currently available geochemical and isotopic data is from southern Sumatra, but BeddoeStephens e t al. (1987) published six whole-rock analyses from the Muarasipongi Batholith in northern Sumatra (Fig. 5.1), with 6 2 - 6 8 % SiO2 and an R b - S r isochron age of 158 4-23 Ma, which established its Jurassic age and volcanic arc affinity. Sato (1991) provided whole-rock geochemistry and K - A r ages for the Padangpanjang and Lassi bodies located to the northeast of Padang (Fig. 5.1). The isotopic data from these granites established a Cretaceous age of 64 Ma for Padangpanjang and 56 Ma for Lassi, and the geochemistry confirms their volcanic arc affinity. These results are similar to those of McCourt & Cobbing (1993) and McCourt et al. (1996) who provided chemical analyses and K - A t ages from 13 plutons and batholiths from southern Sumatra which, while not being a comprehensive data set, can be regarded provisionally as being representative of this group for the region as a whole. That work established an age range of 203 to 5 Ma from rocks with SiO2 values ranging from 50.83 to 76.71%. The lithological range is from gabbro to monzogranite. This range is similar to that for Volcanic Arc and Cordilleran granitoids elsewhere and all other geochemical indices confirm that affinity (McCourt & Cobbing 1993" McCourt e t al. 1996).

58

CHAPTER

5

I

I

[]

[]

~ ~ ~

//0 /

o

.,:

o

0

/

§

IO

o4

"~

e

~

0

0

.........

eS~ [fill - - - - - ~ ' - -

,-i

~ J'~ ~

O0

..~,I

i fi~

Z

8. A/

8,,/'

v

7

v

' v

+. Ov

v

9ov

O \ , 7 x , , x10 ~,

Fig. 5.5. South Sumatran Volcanic Arc Granites (filled circles), Hatapang Granite (open circles and Bukit Batu Granites (squares) plotted on the QAP modal diagram of Le Maitre (1989).

They also plot within the volcanic arc field on Pearce diagrams (McCourt & Cobbing 1993; McCourt et al. 1996) (Fig. 5.4) and in the calc-alkali field in Figs 5.6 & 5.7. At about the same time Gasparon & Varne (1995) published a study of selected granites and volcanic rocks from widely dispersed localities from the whole of Sumatra. They provided 16 analyses of granitic rocks ranging from 50 to 77% SiO2. Eleven of these analyses were from southern Sumatra and seven from northern Sumatran granites, including the Sikuleh Batholith at the northwestern tip of the Island (Fig. 5.1) from which two samples were taken, a monzogranite and a granodiorite. This is a large, complex and in part deformed and foliated batholith, for which until now only been field observations have been available. The data of Gasparon & Varne (1995) confirms the volcanic arc nature of all these granitoids. The majority of granitoids of the volcanic arc suite are undeformed, or only weakly foliated. Some however, are strongly deformed and some show clear evidence for deformation during crystallisation. During field work in 1992 five phases of synplutonic deformation were recognised from the Aroguru Pluton in southern Sumatra (Fig. 5.1). This body lies close to the present trace of the West Sumatra Fault Zone, it is however older than

FeO*

Na=O+K20

MgO

Fig. 5.6. Compositions of the Volcanic Arc granites of southern Sumatra plotted on the AFM diagram of Irvine & Baragar (1971).

0

50

t

t

60

70

............

80

SiO2(Wt%) Fig. 5.7. Compositions of the granites of southern Sumatra plotted on a total alkalies vs. SiO2 diagram, dashed lines denote the calc-alkaline field of Kuno (1979). Symbols as in Figure 5.5

the fault, which was initiated during the Miocene, and it is most likely that the deformation developed as a result of emplacement processes. Barber (2000, p. 732) has suggested that it was emplaced in an active sinistral strike-slip shear zone. Elsewhere along the West Sumatra Fault, particularly to the north of Padang, strong cataclastic deformation has been observed from plutons which were fully crystalline before the initiation of the fault. This is particularly the case for some K-feldspar megacrystic granites which are representative of the tin-associated granites.

Comparison o f recent work

Clarke & Beddoe-Stephens (1987) provided 17 chemical analyses from the Hatapang Pluton in North Sumatra (Figs 5.1, 5.5 & 5.7), ten of these were of granites and seven from greisens and veins. The pluton is an oval body of 6 x 4 km 2 located about 70 km to the SE of Lake Toba. The granite is a coarse K-feldspar megacrystic rock with a marginal zone of about 100 m width consisting of microgranites, aplites, pegmatites and greisens, grading into normal granite. The greisens are strongly mineralized with cassiterite, wolframite and other minerals, and there is a wide aureole of several hundred metres containing microgranite and pegmatite veins and dykes. Chemical analyses of the main porphyritic facies have silica values ranging from 73 to 77% SiOz. The granite has a R b - S r isochron age of 80 Ma and an initial ratio of 0.7151. The authors established an S-type affinity for the granite, and because of its age, suggested that it might be a representative of the Western Granite Province established by Beckinsale (1979). The nearest representatives of the Western Province are at Phuket in Peninsular Thailand. Gasparon & Varne (1995) have questioned this interpretation on the basis of the R b - S r initial ratio, which they intimate is too low for a Western Province granite. Cobbing et al. (1992), however, reported ages and initial ratios from the Western Province in Burma which are comparable with that for Hatapang, supporting the interpretation of Clarke & Beddoe-Stephens (1987). Beddoe-Stephens et al. (1987) studied the Muarasipongi Batholith in North Sumatra in connection with the skarn mineralization developed in contact limestones of that region, and published six chemical analyses with a silica range of 6 2 - 6 8 % SiO2 and a R b - S r isochron age of 158 +_ 3 Ma which were interpreted as

GRANITES

indicators of an I-type affinity in the scheme of Chappell & White (1974), and are similar in their composition to the Volcanic Arc Suite of South Sumatra (McCourt et al. 1996). Gasparon & Vame (1995) provided 20 chemical analyses from both northern and southern Sumatra, all from the Volcanic Arc Suite, with the possible exception of the Bukit Batu pluton (Fig. 5.1). Eleven of these are from southern Sumatra with a range of SiO2 from 49.36 to 77.23% and five from North Sumatra with a more restricted, but essentially similar range of 51.0776.81% SiO2. They also gave eight estimated Rb-Sr ages ranging from 15-t-3 to 135 +_ 7 Ma, together with estimated initial 87Sr/86Sr ratios from 0.7038 to 0.7059. The extended compositional range is similar to that from other regions of volcanic arc related plutonism and the Rb-Sr ages, although estimated, suggest an extended period of granite plutonism. This data is also in keeping with field information recorded from both northern and southern Sumatra, that the granites have a lithological range from gabbro and diorite to monzogranite, similar to that in other Volcanic Arc terrains. However, the two samples from the Bukit Batu intrusion in SE Sumatra, lying to the SW of the island of Bangka and SE of Palembang (Fig. 5.1), have highly anomalous compositions with > 10% combined soda and potash and ca. 60% SiO2. The isotopic data are also markedly different, with estimated initial SVSr/86Sr ratios of 0.71564 and 0.71477 and an estimated age of 170 i 35 Ma. On the Nb vs. Y (Fig. 5.4a) and Nb + Y (Fig. 5.4b) discriminant plots of Pearce et al. (1984) the data from both samples fall in the 'Within Plate' (WPG) field. They also have extremely high values of Ce, La and Zr, and these strange rocks seem to have an A-type affinity but are clearly quite different from the Hatapang Granite. The low silica values and high content of CaO and Na20, together with the presence of hornblende in one of the samples, suggest a possible affinity with the volcanic arc granitoids. However, the wide geographical separation between Bukit Batu and the outcrop of the Volcanic Arc Suite, restricted to the Bar• Range, does not support this interpretation. Gasparon & Varne (1995) considered these rocks to be of S-type affinity, because of their high 87Sr/86Sr estimated initial ratios and estimated age, but stated that 'they are unlike any other granitoids in Southeast Asia'. It is, however, possible that they may be of alkaline affinity. Three granites of this affinity are present in the Tin Islands Suite (Fig. 5.2), of which Karimun and Dabo are tin mineralized, and West Central Singkep is not (Cobb• et al. 1986, 1992). However, none of these granites has such an extreme composition as the Bukit Batu granite. A field and geochemical/geochronological study of Sumatra south of the equator was conducted in 1992 and reported in McCourt & Cobbing (1993) and McCourt et al. (1996). The data consists of 54 whole rock chemical analyses and 40 K - A r ages. Nineteen plutons and batholiths were investigated. Material for geochemistry and geochronology was collected from three main areas extending from the latitude of Padang to the southeastern tip of Sumatra (Fig. 5.1). The most northerly area to the east and northeast of Padang and Lake Sinkarak included the Sulit Air suite, the Lass• Batholith (Table 5.1 b) and the Lolo Pluton (Table 5.1c). To the east the large Tanjung Gadang pluton was sampled and geochemically analysed, but was not dated because of the weathered condition of the rock. Ten samples were taken from the Bungo Batholith which lies about 200 km to the SE and were geochemically analysed and six of these were dated (Table 5.1d). The Garba Batholith about 300 km further to the southeast is not well exposed, but was partially sampled and dated (Table 5.1e). The remaining plutons of Aroguru, Sulan, Padean, Jatibaru, Brant• and Waybambang are located close to the southeastern tip of Sumatra (Fig. 5.1) (Tables 5.1f-i). Most of these plutons are simple, consisting of only one granite unit, but some are more complex. Most of the plutons are characterised by primary magmatic textures, but some are foliated, sometimes strongly, and some, especially Aroguru, were affected by polyphase deformation.

59

Table 5.1. %SiO: and isotopic ages,from Sumatran Granites Pluton/Unit

Sample

Si02

Age (Ma)

Geological age

no.

(a) Sulit Air Granite Suite Guguchina SSG8

63.28

Saloga Belimbing Sulit Air

63.77 65.09 63.42

SSG l 0 SSG 12 SSG13

142 + 5Bi 149 • 5H 138 +_ 4H 183 • 4H 203 + 6Bi

Cretaceous

Trias

(b) l_ztssi Granite Batholith Guguk Sara• SSGl5 Lass• Granite SSG20 Pianggu SSG21 Lass• Granite SSG21 a Leucogranodiorite SSG23 Hornblende Diorite SSG24 Gabbro SSG25 Sungai Durian SSG26 Bukit Bais Gabbro SSG31

50.8 75.3 57.7 74.9 63.8 61.0 52.6 68.7 52.9

53 • 1.5 53 • 1.4 53 • 1.7

(c) Lolo Granite Pluton Granodiorite SSG36 Monzogranite SSG37

65.6 7 I. 14

5 • 1.2 ll + 1

(d) Bungo Granite Batholith Bungo North Bungo Granite SSG43 Rantaupandang SSG44 Rantaupandang SSG46 Muarabat SSG48 Bt Apit SSG52

76.37 60.76 60.97 73.18 75.61

129 • 4Bi 54 • 2 148 • 4

Lower Cretaceous Eocene Upper Jurassic

Bungo South Sungai Siwai Dusunburu Kalan Dusunburu Dusunburu

70.08 60.39 65.2 64.15 64.18

169 • 5Bi

Jurassic

154 + 2Bi

Jurassic

(e) Garba Granite Batholith Garba SSG70 Sungai Liki SSG72

71.46 69.46

86_+ 3 Bi 117 • 3Bi

Cretaceous

(f) Aroguru Granite Complex SSG82

65.6

89.2

Cretaceous

SSG54 SSG55 SSG58 S SG59 SSG59a

Eocene

55 • 1.6

57 • 1.5

Miocene Miocene

156 • 5H

(g) Padean Granite SSG80 SSG80a SSG80b SSG80c SSG80d SSG81

73.69 73.53 74.08 74.61 74,67 75.15

83 • 2Bi

Cretaceous

82 • 2Bi

Cretaceous

84 +_ 2

Cretaceous

SSG87

55.3

151 + 4Hb

(h) Way Sulan Gabbro

Jurassic

(i) Sulan Tonal#e, and the Jatibaru, Wayambang and Brant• granite plutons Cretaceous Sulan Tonalite SSG83 69.31 111 _+ 3Bi SSG85 69.2 113 _+ 3Bi SMO4 69.95 Jatibaru Pluton SSG88 75.6 55 • 1.5Bi Palaeocene 63 • IBi Waybambang Pluton Tcl7A 70.3 20 i 1BiHb Miocene Brant• Pluton Sm79 70.62 86 • 3Bi Cretaceous H, hornblende; Bi, biotite.

On the basis of the new data these authors introduced concepts which, while not new, had not formerly been recognized in Sumatra. These were: (1) geographical persistence of granitic source regions over lengthy periods of time; (2) occurrence of

60

CHAPTER 5

distinct plutonic episodes; (3) westward younging of the Miocene and Pliocene plutons. (1) Persistence of granitic source regions is indicated by the Sulit Air Suite which consists of three small dioritic plutons of similar lithology, located to the northeast of the Lassi Batholith. Two of these, the Guguchina and Belimbing plutons are close in age at 138 Ma and 141 Ma, but the Sulit Air Pluton gave K - A r ages of 203 ___6 and 183 + 13 Ma (Table 5.1a) and 192-193 Ma (4~ method, Imtihanah 2000). The suite was evidently emplaced over a period of 55 million years. An even more remarkable example is the Rantaupandang Unit of the Bungo Batholith which shows identical lithological and petrographic features in samples from two widely separated localities, subsequently confirmed by identical major and trace element analyses from the two samples. Biotite and hornblende K - A r geochronology provided ages of 148 ___4 Ma and 137 + 7 Ma for SSG47 and 54 ___ 2 M a for SSG44a (Table 5.1d). Duplicate analyses confirmed these results, which can only mean that the source region remained unchanged for nearly 100 million years. (2) The existence of distinct plutonic episodes is suggested by breaks in the sequence of intrusion, with durations of between 20 and 34 Ma in the ages of plutons emplaced within the same plutonic lineament. Four episodes were recognized 203-130 Ma, 117-82 Ma, 60-53 Ma and 20-11 Ma (McCourt et al. 1996). Future work may modify these results, but with the present data they appear to be real. (3) Westward younging of the plutonic arc is indicated by a distinct line of small plutons of Miocene age, extending from Lake Ranau to Padang (McCourt & Cobbing 1993). Most of the plutons sampled are characterized by primary magmatic textures but some, lbr example Sungei Durian in the Lassi Batholith and the Sulan Tonalite, are strongly foliated. In the case of the Sulan Pluton this is clearly a magmatic foliation, characterized by evenly deformed mafic enclaves and the alignment of mafic and felsic minerals. The most striking example of deformation is seen in the Aroguru Diorite in South Sumatra to the North of Bandar Lampung, where five phases of progressively weaker deformation were recorded. These phases provide a record of movement in the region during the emplacement of the pluton, which has been dated at 89 • 2 Ma (McCourt & Cobbing 1993; McCourt et al. 1996; Barber 2000). The Lassi Batholith (Table 5. l b) comprises at least nine units, five of which were dated. Most of these units are diorites and gabbros of varying lithologies and texture, but a distinctive coarse K-feldspar megacrystic granite is present in at least seven small dyke-like intrusions. The foliated and poorly exposed Sungai Durian granodiorite with an SiO2 content of 68.7% forms a large outcrop in the southern part of the body. The spread of ages from 203 to 55 Ma for the Sulit Air Suite and the Lassi Batholith is noteworthy, since their field, petrographic and geochemical characteristics are sufficiently similar for them to have been initially considered as a consanguineous group (McCourt & Cobbing 1993). The Lolo Pluton (Table 5.1c) is one of the youngest granites with a full geochemical analysis to have been dated, with an intrusion age of 15 Ma (40 At/~39 Ar method. Imtihanah 2000). It is of tonalitic composition and is a component of the belt of very young plutons close to the southwest limit of the plutonic arc (McCourt & Cobbing 1993; McCourt et al. 1996). There is little doubt that both the Lassi and the Bungo batholiths are more complex than at present appears to be the case. Most of the other granites sampled are simple plutons, consisting of one major rock type, but some plutons are zoned, having a compositional variation from diorite or tonalite to granodiorite or monzogranite. Table 5.1(c-i) show almost the whole compositional range of the South Sumatra granites and is sufficient to show their essential similarity to the data of Gasparon & Varne (1995) and, by analogy, to the entire volcanic arc suite of Sumatra.

Granitoids with volcanic arc characteristics have been recovered from oil exploration drilling programmes in NW Java (Patmosukismo & Yayha 1974). These authors report the presence of granitic rocks, described as quartz microdiorite, with a K20 content ranging from 1.29 to 4.04% and K - A t ages ranging from 94 to 56 Ma, in three exploration wells. These granitoids can be correlated provisonally with the Volcanic Arc Suite of Sumatra.

The relationship o f Sumatran granites to adjacent areas o f Sundaland

Sumatra, including the Tin Islands, the southwestern part of Kalimantan, the Malay Peninsula, Thailand and Burma constitute part of Sundaland. The tin-associated granites of Sumatra and the stanniferous and non-stanniferrous granites of the Tin Islands can be correlated with the Main Range and Eastern Granite Provinces distinguished in those areas (Hutchison 1989, 1994) (Fig. 5.3). Although there is a paucity of geochemical and isotopic data for the tin-associated granites in Sumatra, that which is available, together with their distinctive field characteristics, leaves little doubt that these granites are an expression of the same phase of plutonism as that developed in the Main Range (Central Belt) in mainland SE Asia (Mitchell 1977; Beckinsale 1979; Hutchison 1989; Cobbing et al. 1986, 1992). Similarly, the volcanic arc plutonism of the Barisan Range finds a ready analogue in the Central Valley Province of Burma, where the Wuntho Batholith and the Salingyi Complex show a range of lithologies similar to those which are developed in Sumatra, but which are restricted to the Cretaceous (Cobbing et al. 1992; McCourt et al. 1996). The Hatapang Granite of Cretaceous age is stanniferous, and Clarke & Beddoe-Stephens (1987) have suggested that it may be an outlying representative of the Western Belt, developed in Peninsular Thailand and the Shah Scarp region of Burma (Mitchell 1977; Beckinsale 1979). Most of the regional relationships of the granites of Sumatra to the geology developed during the geological evolution of Sundaland are straightforward, but some are not. Unfortunately, the most intractable problems are located in the area between Peninsular Malaysia, eastern Sumatra and the Tin Islands. These problems centre around the southward extension of the Bentong-Raub Line (Figs 5.1 & 5.3) which, in Peninsular Malaysia and Thailand, divides stanniferrous S-type granites of the Main Range (Central) Belt, from non-stanniferous and stanniferous granites of the Eastern Belt. This line is clearly marked in Peninsular Malaysia by the sporadic occurrence of ophiolites. It can also be followed northwards, across the Gulf of Thailand, as far as the border with Laos. It cannot, however, easily be followed southwards. Whereas some of the islands of the Indonesian Archipelago host stanniferous S-types, most of the granites are non-stanniferous I-types. There are also both stanniferous and non-stanniferous A-type granites (Cobbing & Mallick 1984; Cobbing et al. 1992). There is an extensive literature on this question which is summarised by Hutchison (1994) who concludes that the Raub-Bentong Line probably follows a course near the east coast of Sumatra and lies somewhere in the neighbourhood of Bangka and Billiton. Granites in most of the northern islands of the Riau Archipelago are non-stanniferous I-types, but stanniferous S-types with Main Range (Central Belt) characteristics are present on the island of Kundur and at the southwest tip of Singkep (Fig. 5.2). The prolongation of this direction leads directly towards the islands of Bangka and Billiton, and follows an arcuate form leading eastward from Sumatra towards Kalimantan. Bangka and Billiton contain a mixed population of stanniferous S-type granites and non stanniferous I-type granites (Fig. 5.2, in which the S and I type granites are mingled together and are not separated into distinctive belts). There is also a suite of intermediate character containing both

GRANITES I-type and stanniferrous S-type granites termed the Bebulu Suite (Pitfield 1987; Cobbing et al. 1992). The only logical explanation for the mixed granite population of these islands, especially of Bangka and Billiton, is that the contrasted granitic suites have different source regions. It may be that in the arcuate region to the east of Sumatra the suture was imbricated into a m61ange of deep crustal wedges derived from adjacent Gondwanan and Cathaysian blocks, providing a complex of compositionally contrasted source regions for both S and I-type granites. These compositional differences are reflected in the geochemical and isotopic characteristics of the granites derived from them (Cobbing et al. 1992). Pulunggono & Cameron (1984) proposed a similar interpretation with the Bentong-Raub Line running through Singkep and Bangka, following the southern margin of the Klabat Batholith (Fig. 5.2). They also commented that the suture zone is 'more complex than shown and is occupied by lensoid fragments of both microplates'. Similarly Gasparon & Varne (1995) considered that 'the boundary between the Central and the Eastern Granite Provinces may run through the Tin Islands'. Within the stanniferous granites of the Tin Islands, the Tanjong Pandang Pluton on the island of Billiton, is the only body in which the tin has behaved as a decoupled element, in that the tin content does not increase with magmatic differentiation (Lehman & Harmanto 1990). In this respect it corresponds to granites belonging to the Kuantan-Dungun stanniferous granites of the Eastern Province of Peninsular Malaysia, where tin contents are low and are similarly unrelated to differentiation, but increased during the hydrothermal stage (Schwartz & Askury 1990). The distribution of stanniferrous and non-stanniferous granites on these islands suggests that the Bentong-Raub Line, or perhaps a strand of that structure, runs through or close to central Bangka and northern Billiton. Moreover, the location of the Main Range type S-type granites in the northern half of Bangka and the Itypes of the Bebulu Suite in the southern half (Cobbing et al. 1992) have a distribution which is the reverse of that in Peninsular Malaysia and Thailand. This reversal of the normal pattern provides additional reason to support the concept of the nearby location of a structurally complex Bentong-Raub Line or Zone. Host rocks for granites on the islands of Bangka and Billiton include limited outcrops of pebbly mudstone facies and larger occurrences of mainly terrigenous sedimentary rocks of Carboniferous-Permian age, overlain by Triassic sandstones (Ko 1986). According to Priem et al. (1975) country rocks on both these islands are low-grade meta-sedimentary rocks of Stephanian to Norian age. These sequences are similar to those present in the Eastern province of Peninsular Malaysia. The host rocks to the tin granites of the Main Range Province in Peninsular Malaysia consist mainly of Lower Palaeozoic formations of Ordovician to Devonian age and consist mainly of pelitic rocks of low to moderate metamorphic grade with subordinate limestones. The observed sequences are essentially the cover to middle and lower crustal material present at depth. As noted above the composition of granites within the region is not confined to S- and I-types but A-types are also sporadically developed. These however, except in the Tin Islands, are not common in Sumatra (Cobbing et al. 1992). Only the Hatapang and Bukit Batu plutons can be viewed as approaching an A-type composition and these may be very highly evolved examples of S and I-type lineages, respectively. However, the isolated location of the Bukit Batu Pluton in relation to the main outcrop of the Volcanic Arc Suite at the western margin of the island does not support such an interpretation for that body. Most of the granitic rocks of Sumatra can be accommodated within the framework of granitic belts established in earlier studies, e.g. Mitchell (1977), Hutchison & Taylor (1978), Beckinsale (1979). McCourt et al. ( 1 9 9 6 ) correlated the Volcanic Arc Suite with the Central Valley Province of Burma, the Tin-Associated Suite with the Main Range Province of Peninsular Malaysia and Thailand, and the Tin islands with the Eastern

61

Province, with the granites of Bangka and Billiton being shown as of mixed affinity. Most of these correlations have been followed here, but there are some amendments, and some alternatives have been suggested. Some of the boundaries are of tectonic origin and are well defined, or at least give that impression, others are not, or appear to be 'porous' in that granites of contrasting type or age appear to be mingled together or are 'out of place'. The only known representative of the Jurassic-Tertiary Western Province on Sumatra is the Hatapang Granite (Clarke & Beddoe-Stephens 1987). While more may yet be found, all the other tin-associated granites for which there is data, are of Triassic-Jurassic age and suggest that the Main Range (Central) Province occupies virtually all of Sumatra to the east of the Barisan Range. Granites of this affinity also occur as tectonic slices within the range itself, and in the region of Sibolga, biotite granites and sedimentary rocks of the the Kluet-Kuantan Formations of Upper Palaeozoic age extend as far as the west coast of Sumatra (Clarke 1990), which suggests that the volcanic arc was built, at least in part, upon older continental crust. On the basis of the occurrence of the Hatapang granite, McCourt et al. (1996) extended the Western Belt through the whole of Sumatra as a narrow strip east of the Barisan Range. However, in the light of the available evidence this may not be the case, perhaps the Hatapang Granite is the sole representative of that belt within Sumatra. The status of the A-type Bukit Batu granitoids remains enigmatic. A-type granites have also been identified in the Tin Islands and the islands of Singkep and Karimun (Cobbing et al. 1986, 1992). The Bukit Batu granitoids are associated in the field with stream sediments containing quartz and cassiterite, but in view of their unusual composition it is highly unlikely that they are stanniferous. The sediments may be of alluvial origin, derived from the Tin Islands a short distance to the east (Katili 1974a; Pulunggono & Cameron 1984). The geochemical affinity and high estimated 86Sr/87Sr ratios of the Bukit Batu granitoids suggest correlation with the Tin Islands Suite. However, the estimated age of 163 + 50 Ma is more compatible with the Volcanic Arc Suite. If the Tin Islands affinity of these granitoids were to be confirmed this would have implications for the position of the Bentong- Raub Line.

Conclusions The granites of Sumatra have developed through two contrasting geological cycles, a Carboniferous-Permian cycle of convergence and collision followed by a younger Triassic-early Jurassic cycle in which a new subduction zone was formed along the southwestern margin of the new continent (Hutchison 1994; McCourt et al. 1996). During the first, collisional cycle, the different accreted terrains, distinguished by their stratigraphic and faunal assemblages, were host rocks to granites which, because of their contrasting geochemical and isotopic characters, seemed to mirror the lower crustal regions from which they were derived. These terrains are distinguished most clearly in Peninsular Malaysia and Thailand as contrasting belts which are additionally characterised by stanniferous S-type and generally non-stanniferous I-type granites (Beckinsale 1979). The second cycle generated granites having a wide compositional range from diorite to monzogranite, associated with the development of a late Triassic-early Jurassic volcanic arc along the southern margin of Sundaland. McCourt et al. (1996) suggested that the two cycles overlap in Sumatra. The association of the Main Range Province granites with sedimentary rocks of Gondwana affinity and the Eastern Province granites with those containing Cathaysian floras provided a further strand of evidence for the disparate geological histories of those crustal segments which eventually formed the southern borderlands of Eurasia during the Permo-Triassic (Hutchison

62

CHAPTER 5

1994). The generation of these syn- and post-collisional granites took place over an extended period from about 275 to 190 Ma, with the main peak of post-collisional plutonism from 220 to 200 Ma. It was during this period that most of the stannifeous granites of Sumatra and Peninsular Malaysia were emplaced. At about 200 Ma this phase of crustal plutonism was superseded by volcanic arc-related plutonism and vulcanism, generated as the result of the formation of a subduction zone at the southern margin of the new continent which now included Sumatra and Burma. This resulted in the production of granitic and volcanic rocks in a relatively narrow zone, with an extended compositional range from diorite to monzogranite, similar to those of other volcanic arc terrains, such as the Cordilleras of South and North America. The granites of Sumatra differ from those regions in that they do not form linear granitic batholiths of great size, but for the most part are represented by numerous isolated plutons and small batholiths, confined within a narrow belt along the southwestern margin of the island. Most of these plutons are characterised by holocrystalline plutonic textures, but some are deformed, and more rarely, some have several phases of polyphase deformation, perhaps resulting from a period of

emplacement coincident with episodic movement on major structures. There has, however been strong cataclastic deformation of earlier granites within the Sumatran Fault Zone which seems to have particularly affected the tin-associated granites. A notable feature of the Volcanic Arc granites is their extended time range, from 203 to 5 Ma. This is in marked contrast to Cordilleran batholiths which generally have a more restricted time range. There are consequently both similarities and differences between the volcanic arc granites of Sumatra and those of the western Americas. The Sumatran granites have not yet been found to have the same economic potential as similar granites in other regions, but it is uncertain whether this opinion is correct, having regard to the difficulties of the terrain. Perhaps the apparent lack of mineralization is associated with the lengthy time scale and the relatively small volume of granite produced. An additional contrast with the Cordilleran situation is that whereas the youngest and more evolved granites tend to be present towards the back arc region, the situation in Sumatra is the reverse, with younger granites located close to the coast, and hence towards the subduction zone.

Chapter 6

Pre-Tertiary volcanic rocks M. J. CROW

Volcanic activity and associated plutonism, ranging in age from the Carboniferous to the Late Cretaceous, has made an important contribution to the Pre-Tertiary geological evolution of Sumatra. This chapter summarizes the known occurrences of Pre-Tertiary volcanic rocks and their geological settings (Fig. 6.1 & Table 6.1). There has been no systematic isotopic dating

programme directed at determining the ages of the volcanic rocks, but dating of volcanic episodes in Sumatra has benefited greatly from stratigraphic palaeontological studies on the associated sedimentary units, summarized by Fontaine & Gafoer (1989). Unfortunately, little progress has been made in determining the chemistry of the volcanic rocks of Sumatra, subsequent to the

I,,I I I,' i,"iiillil ,' i i ,io5,o

96OE :,

,, I i i l l l l i ,

I. \ .

ii

6~

i 6~

i

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iS/BUMASU:.

~ I ~ T T T T I

EAST SU MATR,~,

,,\,Iiii

~-.',','.~. 9

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lllllll )lllll

".',',',

.

III

ill

.

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Sibolg

.~. r-4. i l_k4~

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Pakanbaru

Panti F m

Bentong-Raub Suture Zone Riau-Billiton Accretionary Complex

~~--:..-.N~DI

ii

" "J'l u h ( . ' "- ". -" .' " "

, ,

Silungka 'Barisan'

=~'/____~,~r

,~,.,~,,,.~,.~,.,,

~__(b~.~..~.~

Bohorok Formation (Visean)

b(

"in(kep

[.' :12,ond ong..':.." 9'. ". . ~ , M e m b e r ~ ' < . ~ : . :. -e~;,'ff, i"..".'.'.~.q.:.: ~ 2' ' ~' ' ' - ' '.-.'~..;..~..~...-...k. ~'~'~'k.'.~ "."~.J..~'-" "- ". "- ". ".~'-'-

'Kluet' Formation

~

l l~

~t~.c...",.~_- - . ' . . v _ .~lr,~-.b. . . , , ~ . ~,~.

SIBUMASU - EAST SUMATRA BLOCK Carboniferous-Early Permian

3~

ill

M4..-'> .".'t~'N.xlo1 ' ' ' ' .-//,,, ...~).....~-~-N~[TtLingg a

INDOCHINA BLOCK (and I n d o n e s i a n Islands)

...ff~..Permian&Carboniferous

I I I

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~

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Muarasipongl~ 0~

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NDOCHINA1 I l l J ! i l l llll :~BLOCK I

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Tapaktuan

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I I I

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..

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Alas Formation Quartzite Terrain and Pcrsing Complex(Singkep)

Hippogri WEST SUMATRA BLOCK Carboniferous-M id-Permian l

t"--"

Tanjung Puab & Pawan Formations (tremolite and chlorite schists) Permian Silungkang Formation (Calcareous Member) Panti, 'Barisan' & Palapat formations Kuantan Formation (Visean) 0 Kluet Formation I

6~

I

300km i

99 ~

102 ~

I

I

105 ~

6~

Fig. 6.1. Simplified Pre-Triassic geology of the West and East Sumatra Blocks and the Indochina Block of Peninsular Malaysia showing the principal Palaeozoic volcanic units and localities discussed in the text.

63

64

CHAPTER 6 Table 6.1. Pre-Tertiar3, Volcanic and Volcanic-Plutonic Belts, Arcs and occurrences in Sumatra Ma

Duration

Description

120-75

Aptian-Campanian Mid-Cretaceous Early Cretaceous Late Jurassic-Early Cretaceous Triassic onwards

169-129

Mid-Jurassic-Early Cretaceous

224-180

Late Triassic-Early Jurassic Late Triassic-Early Jurassic Triassic

Late Cretaceous Plutonic Arc* Collisionof Bentaro-Saling Oceanic Arcs with West Sumatra* Intrusions in Bentaro-Saling Oceanic Arc* Volcanism in Meso-Tethysforms Bentaro-Saling Oceanic VolcanicArcs Mid-Oceanic volcanismforms plateau in Meso-Tethysmany of which grow limestonecaps Jurassic-Cretaceous Plutonic Arc Woyla AccretionaryComplex forms behind subduction zone West Sumatra Volcanic-Plutonic Arc Pahang VolcanicBelt Medial Sumatra Tectonic Zone formed incorporatingsediments and volcanics

Mid-Permian-Mid-Triassic

Situtup Fm volcanics (in Miocenethrust zone) (West Sumatra Block)

Early-Middle Permian Early Permian Carboniferous (Vis6an)

West Sumatra Permian Plutonic-Volcanic Belt (West Sumatra Block) East Sumatra Permian Plutonic-Volcanic Belt (Sibumasu) Kuantan Formationvolcanics(West Sumatra Block)

Devonian-Late Permian

Accretionary Complex forms behind subduction zone beneath East Malaya and Riau-Billiton sections of IndochinaBlock interface with Palaeo-Tethys Ocean; accretionof volcanicsof oceanic origin

270-255 c. 270

*Associated volcanicsnot identified.

compilation of analyses reported by Rock e t al. (1982), but the initial results of a programme of detailed mapping studies by the Geological Research and Development Centre, Bandung promises an improved understanding of the geochemistry of both volcanic and plutonic rocks in the island (Suwarna et al. 2000). According to the tectonic synthesis which has been presented in this volume (Chapter 14), in the late Palaeozoic (Fig. 6.2a) the eastern half of Sumatra formed a segment of the margin of the southern Gondwana Supercontinent facing the Palaeo-Tethys ocean, off NW Australia, while Australia was undergoing glaciation. On the other hand, the western half of Sumatra lay in tropical latitudes, beyond the Greater Sula Spur of Eastern Indonesia, at the junction zone between Gondwana and the Indochina Block of the northern Cathaysian continent (Fig. 14. I1). Palaeo-Tethys was subducted beneath the Indochina Block in the Late Palaeozoic and Early Mesozoic, accumulating an accretionary complex from buoyant oceanic detritus, including ophiolitic fragments, oceanic volcanics and oceanic sediments at the margin of the Indochina Block. The deformed remains of this accretionary complex form the Bentong-Raub Suture Zone (Metcalfe 2000) and continue into the Tin Islands Archipelago. In the Early Permian Gondwana began to move southwards (Fig. 6.2b), and this movement caused extension along the Gondwana margin with Palaeo-Tethys, accompanied by volcanism and plutonism within the Sumatra blocks. The East Sumatra Block formed part of Sibumasu, a continental fragment which detached from Gondwana in the Early Permian (Sakmarian) and collided with the Indochina Block later in the Permian or in the Early Triassic (Metcalfe 2000). Following the collision of Sibumasu with the Indochina Block, the West Sumatra Block became detached from the Gondwana-Cathaysia interface in the Triassic and was translated by transcurrent faulting along the Medial Sumatra Tectonic Zone to be accreted along the outer margin of Sibumasu (East Sumatra Block) (refer to Figs 14.11 - 14.14). During the Triassic, after the collision between the Sibumasu and Indochina blocks, the orogen collapsed, into a system of horsts and grabens parallel to the orogen axis (Fig. 6.2c) and granites of the Eastern and Main Range Provinces were intruded into the collision zone. The Pahang volcanics in east Malaya represent the volcanic carapaces to Eastern Province granites, preserved along the faulted margins of the grabens. The Main

Range Granite Province with its extensive tin mineralization extends into Sibumasu, but no volcanics are reported. Between about 224 and 180 Ma (Late Triassic-Early Jurassic) the Meso-Tethys commenced subduction along the margin of the combined West Sumatra Block and Sibumasu continent and a continental margin volcano-plutonic arc was formed, a small amount of these volcanics are preserved. Accretion of oceanic materials may have been associated with the formation of this arc. Accretion between 169 and 129 Ma (Mid-Jurassic-Early Cretaceous) is better documented in the Oceanic Assemblage of the Woyla Group, composed of buoyant oceanic volcanics, sediments, oceanic crust fragments which accumulated in the Woyla accretionary complex. Accretion was associated with the formation of a Jurassic-Cretaceous continent margin plutonic arc with its associated volcanics (Fig. 6.2d). This phase of subduction/accretion was brought to a close by the arrival at the subduction zone of a large string of oceanic island arcs which had originated within the Meso-Tethys Ocean. The arrival of Bentaro and Saling Oceanic Island Arcs (Fig. 6.2e) terminated subduction, thrust the Woyla Oceanic Assemblage and Volcanic Arc over the margin of the West Sumatra Block in the Woyla Nappe, and caused deformation which penetrated deep into the Malay Peninsula. Subduction of the Meso-Tethys resumed late in the Cretaceous on the oceanward side of the Bentaro-Saling Volcanic Arcs and a new plutonic arc was formed on the Woyla Nappe and the margin of the West Sumatra Block.

Carboniferous volcanism

Gafoer & Purbo-Hadiwidjoyo (1986) used the term 'Kuantan Volcanism' for metavolcanics (Table 6.2) mapped by Silitonga & Kastowo (1975, 1995) in the Lower and Phyllite and Slate members of the Kuantan Formation in West Sumatra. The older episode, within the quartzitic Lower Member, is represented by intercalations of volcanic rock and chloritized tuff, which underlie the Limestone Member, which has been dated as Early or Midto Late Visdan (Fontaine & Gafoer 1989). The younger episode is represented by flows of andesite and basalt among the quartzites and quartz sandstones of the Phyllite and Shale Member.

PRE-TERTIARY VOLCANIC ROCKS

65

e • , • •(e)• LATE CRETACEOUS ~~A~

ona~

(d) JURASSIC-EARLY CRETACEOUS

oyla

(C) MIDDLE-LATE TRIASSIC

(b) AUSTRALIA'S POLAR WANDERING PATH

openir Meso(a) EARLY PERMIAN PALAEOGEOGRAPHY Fig. 6.2. Cartoons illustrating significant volcanic events in the geological evolution of Sumatra from its dispersal from Gondwana to the collision of the Bentaro-Saling Oceanic Volcanic Arcs. (a) Gondwana Margin Break-up Volcanicity (V, volcanic localities) at the Gondwana-Cathaysia interface after the opening of Meso-Tethys in the Early Permian. In this reconstruction the West Sumatra Block is still in position between Cathaysia and the Greater Sula Spur. Figure based on Figure 4.21 and Charlton (2001). (b) The advances and retreats of Gondwana shown by the palaeomagnetic record for Australia (after Klootwijk 1996). Gondwana reconstruction by Charlton (2001). (c) Palaeogeographic reconstruction of Sumatra and the Malay Penisula in the Mid-Late Triassic (from Fig. 4.25). The Pahang Volcanic Belt (V, volcanic localities) is shown in the Semantan Basin. (d) Sumatra in the Jurassic-Early Cretaceous showing the Plutonic Arc, the Woyla Foreland Assemblage, the Meso-Tethys and the Bentaro-Saling Arc with the Woyla Accretionary Complex. (e) In the Late Cretaceous the Bentaro-Saling Oceanic Arc has collided with and has been overthrust onto Sumarta as the Woyla Nappe. Collision was followed by the resumption of subduction in the Late Cretaceous.

66

CHAPTER 6

Table 6.2. Palaeozoic volcanic units in the West Sumatra Block

Formation

Unit with volcanies

Kluet

Kuantan

Age

Description

Reference

Probable Carboniferous-Early Permian

Green metavolcanics in phyllites in the upper Jambo Aye and green metatuffs in the upper Serbajadi river among conglomeratic metawackes, metaquartzites, metalimestones, phyllites and arenites Porphyritic matic metavolcanics associated with metasediments in the Kr. Rancah (?2936 3939). Diabase Phyllites and schistose metatuffs Flows of andesite and basalt among quartzites and quartz sandstones Intercalations of volcanic rock and chloritised tuff within quartzites, sandstones and shales

Cameron et al. (1983)

Hippogriffe rocks Local Phyllite and Shale Member

Carboniferous-Early Permian

Lower Member

Vis~an

The metatuffs mapped by Rock et al. (1983) and the diabase (Verbeek 1897) forming the Hippogriffe rocks, an islet south of P. Bangka in the Java Sea (Fig. 6.1) also may be included in this episode. The younger episode of Kuantan Volcanism has not been dated, but is post-Vis~an, and may be of Permian age (see later). The green metavolcanics noted by Cameron et al. (1982a) in the Alas Formation in the Medan Quadrangle may be of Vis6an age, like the associated limestones, while metavolcanic localities in the Kluet Formation in the north of Sumatra have not been accurately dated, but may be of Carboniferous or Permian age.

System Stage

WEST SUMATRA BLOCK

Changsingian uJ ~9 Wuchiapingian

East Sumatra Plutonic-Volcanic Belt ( P e r m i a n volcanism) The East Sumatra Plutonic-Volcanic Belt, the 'Permian magmatic arc' of Katili (1973), was defined on the basis of R b - S r age determinations on feldspars obtained from cores drilled in the concealed Setiti batholith (Setiti-4, brecciated granite, 298 + 3 0 M a and Setiti-5, sheared granite, 276 ___ 10Ma). Suwarna et al. (1991) suggest that the volcanic Condong Member of the Mentulu Formation (Table 6.3) in the nearby Tigahpuluh Mountains (Simandjuntak et al. 1991) is also of

EAST SUMATRA BLOCK

I I

I

9

l

1

I

9

I

I

!

tl I i ! I [ l l l l l l [ l l i

I

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9

~L

.......

- r -r "-i--r r I I I I I I I l i I I

Wordian

|

i

9

9

9

Guguk Bulat Bukit Pendopo

._1

Roadian Kungurian

~

<~ Sakmarian Asselian Gzhelian

Kaloi, Batumilmil Formations

?

%, ~

Calcareous Member

fi!~i~i~i~i!~i~i:!l Mengkarang Formation ? Pengabuhan Formation

ILl

ii

~

II l l l l l l l l l

f,,...., ......it VolcanicMember ~i iii~i~ii~i~ (PalepatFormation)

>5, Artinskian

8

I I

II I i 1 i I I I I I I I I l l ! I l l ! I l l

..........

O0

INDOCHINA BLOCK

Basalt

m

13_

Verbeek (1897) Rock et al. (1983) Silitonga & Kastowo (1975)

?

_J

Capitanian

Cameron et al. (1982b)

Kusimovian .........

N

Kluet Formation Basalt Condong Member 'Pebbly mudstone' Bohorok and Metulu Formations

I% k.

" % "X. "*,.

% %-. % ~'%N ", _%2"~

~.J,

Riau-Billiton Accretionary Complex (shales,siltstone basalt and serpentinite)

Gangsal Formation

9

Muscovian ....... ,

Bashkirian Serpukovian ...............

Visean Tournaisian

C

o

,9

g

Limestone Member

iL:!~-!~ !:!i

Alas Formation

Lower Member

,.t

Fig. 6.3. Simplified composite Carboniferous and Permian stratigraphies of the East and West Sumatra Blocks and the Indonesian islands in the Indochina Block.

PRE-TERTIARY VOLCANIC ROCKS

67

Table 6.3. P a l a e o z o i c volcanic units in the East Sumatra B l o c k Formation

Mentulu

Unit with volcanies

Age

Description

Reference

Probably Asselian-Artinskian Permian Stages

Tufts beneath Tertiary sediments

Eubank & Makki ( 1981)

200-250 m of metatuff, tuffaceous claystone and grey to brown, hard and porphyritic andesitic to basaltic tuff Crystal tufts and other tuffaceous rocks

Simandjuntak et al. (1991) & Suwarna et al. ( 1991) Bennett et al. (1981c)

Rhyolite clasts (unknown age)

Cameron et al. (1980)

Condong Member

?Asselian-Artinskian Permian Stages

Bohorok Pebbly Mudstone Facies

Permian age. In their reconstruction of the geology of the PreTertiary basement Eubank & Makki (1981) show an area of tufts encountered in boreholes to the NW of Pakanburu that may be related to a volcanic centre, and are similar to those of the Condong Member. In the Langsa Quadrangle Bennett et al. (1981 c) describe 'some crystal tufts and other tuffaceous rocks' belonging to an unnamed volcanic unit within the Bohorok Formation. Cameron et al. (1980) recorded rhyolite clasts within the Pebbly Mudstone Facies of the Bohorok Formation in Northern Sumatra, indicating the presence of rhyolitic volcanics in the source region from which the pebbles were derived. These rhyolites could be of any age prior to the Permian.

West Sumatra Permian Plutonic-Volcanic Belt (Early-Mid-Permian volcanism) Lower-Middle Permian volcanics and sediments and several associated granitic plutons crop out within the West Sumatra

SIBOLGA System

Stage

Ua

Block, and form a discontinuous belt, much disrupted by strike-slip movements along the Sumatra Fault Zone, parallel to the west coast of Sumatra (Fig. 6.1). In Table 6.4 these volcanic rocks are described from north to south, and their relationships to the local stratigraphy are illustrated in Figure 6.4. Katili (1969, 1973) described these plutonic-volcanic rocks as a continental margin arc, on the basis of lithology, but the non-genetic term 'belt' is used here. Two extensive but poorly exposed formations are distinguished to the south of the equator. The Silungkang Formation, named by Klomp6 et al. (1961), which lies to the SE of Lake Singkarak, consists of Volcanic and Calcareous Members. The petrology was described by Katili (1969) and the geological setting by Silitonga & Kastowo (1975). The other unit, the Palepat Formation (Rosidi et al. 1976), was previously known as the Air Kuning Beds (Zwierzijcki 1935). The main outcrop lies to the SW of Muarabungo. Earlier this formation was mapped by Tobler (1922) as the 'Oudere diabaasformatie' (Palaeodyas or Lower Permian age), overlain by the 'Porfierformatie' (Neodyas or Upper Permian age). Tobler (1917, 1922) shows outcrops of the Porfierformatie west of the main Palepat Formation outcrop in the vicinity of the

SILUNGKANG FORMATION

Aspden et al. (1982b) Katili (1969); Fontaine & Gafoer (1989) Silitonga & Kastowo (1975)

PALEPAT FORMATION

KUANTAN FORMATION

Fontaine & Gafoer (1989)

1.Silitonga & Kastowo (1975) 2. As used in present account

i UJ Chansingian

IWuchi piogian ~.

Capitanian . ~ 3_,j.,,..L~ Guguk Bulat

2.

W Wordian Shale s

uji~

Basalt Silungkang

256.1

n Kungurian

l---

259.7 Artinskian 1268.8

,~ Sakmarian U.l Asselian Gzhelian

o

Bukit Pendopo Tabir Formation

Roadian

i281.5

Sibolga Granite 264+_6

~

Ngaol Formation

Limestone

lvv vv v v v l

Volcanic Member

iv v v v v v v I Palepat Volcanics

Shale

-%~,4E~-.~--.Pawan & Tanjungpuah Formations ~'~ ~' v V[ Phyllite & shale ............... Member {:::: : ]

ii!iiiii~ii:iiii 1 Mengkarang Formation

!290

Kusimovian Muscovian Bashkirian

8

Serpukovian Visean

< O

~

~ J ~ ~JLimestoneMember Lower Member

Tournaisian

Fig. 6.4. Stratigraphy of units within the West Sumatra Permian Plutonic-Volcanic Belt.

68

CHAPTER 6

Table 6.4. Volcanic units' in the West S u m a t r a P e r m i a n P l u t o n i c - Volcanic B e l t Formation

Unit

Area

Age

Kluet

(uncertain affinity)

Sibolga

Early Permian

Panti Volcanic

Lubuksikaping

Probable Permian

Silungkang (correlation)

Lubuksikaping

Mid-Late Permian

SE Danau Singkarak (type area)

Sakmarian-Wordian stages of Permian

Silungkang

Calcareous Member

Volcanic Member

Thickness (m)

c. 1500

Outliers: Near Tanjung Gadang Lubukkarak

?Roadian- Wordian

Tabir

S. Tabir

Mid Permian

150/450

Palepat

B. Palepat

Artinskian-Wordian stages of Permian

1100

Silungkang (formerly Kuantan)

Mengkarang

Calcareous Member

B. Tabir

> 800

B. Tantan

>200

B. Mengkarang

Asselian Stage

S u m a t r a Fault Z o n e , but Z w i e r z i j c k i (1930a) s u b s e q u e n t l y attributed these outcrops to the Cretaceous, so that they are currently c o n s i d e r e d to be part o f the W o y l a Group. In the southern outcrop, the p r e d o m i n a n t l y volcanic Palepat F o r m a t i o n ( S u w a r n a e t a l . 1994) interfingers with the l o w e r parts o f the terrestrial to shallow m a r i n e M e n g k a r a n g F o r m a t i o n

?500

Description

Reference

Poorly sorted volcanic wackes in roof-pendants of the Sibolga Granite Complex Varied greenschist facies sheared metavolcanics and non-foliated volcaniclastics Meta-limestones, porphyritic metavolcanics, metatuffs, volcaniclastic sandstones and hornfelsed tufts Sandy limestone, calcareous sandstone, and clay shale with a few intercalations of agglomeratic tuff and several flows of augite andesite and basalt Hornblende andesite, augite andesite, meta-andesite and meta-dacite with thin intercalations of tuff, limestone, shale and sandstone mixed with tuffaceous material Hard, fractured, locally vesicular, dark-grey to green-grey basalt with a trachytic texture and composed of felsic and mafic minerals set in a microlilic groundmass; diabase Conglomerate and tuffaceous sandstone with intercalation of pisolitic andesite tuff Andesitic > acidic lavas and tufts; randomly distributed basalt and rhyolite. Also siltstone, shale and limestone Volcaniclastic rocks, lithic and crystalline tufts and andesitic lava, locally diabasic; local clastic sediment interbeds Andesitic-dacitic lavas, tufts, diabase, and volcanic breccias containing clasts of andesite and dacite, intercalated with shale, siltstone, sandstone, claystone and limestone; commonly altered and metamorphosed Acid-basic tuff intercalations in shallow-marineterrestial sediments

Aspden et al. (1982b)

Rock et al. (1983)

Rock et al. (1983)

Silitonga & Kastowo (1975) Katili (1969)

Silitonga & Kastowo (1975): age revised by Gafoer et al. (1992a)

Rosidi et al. (1976)

Rosidi et al. (1976)

Simandjuntak et al. (1991)

Suwama et al. (1994)

Suwarna et al. (1994)

and can be dated p a l a e o n t o l o g i c a l l y . A n d e s i t i c - d a c i t i c v o l c a n i s m c o m m e n c e d in the Asselian and p e a k e d in the Artinskian (Fontaine & G a f o e r 1989). T h e Tabir Formation, p r e v i o u s l y b e l i e v e d to be of Jurassic age ( S u w a r n a e t a l . 2000), but n o w k n o w n to be P e r m i a n , interfingers with, and overlies the Palepat and the N g a o l formations. T h e N g a o l F o r m a t i o n (obsolete term)

PRE-TERTIARY VOLCANIC ROCKS

of Rosidi et al. (1976) is of Artinskian to Wordian (Murgabian) age (Fontaine & Gafoer 1989) (see Fig. 4.11) and appears to be a facies of the Palepat Formation beneath the Tabir Formation. If so, the Tabir Formation could be younger than the Wordian. In the Painan Quadrangle (Rosidi et al. 1976) the eastern part of the Barisan Formation (obsolete term) includes discontinuous outcrops of the Palepat Formation which link up with the Silungkang Formation (Table 6.4 & Fig. 6.5a,b). The Silungkang Formation is Sakmarian-Wordian in age, although the upper age limit is not well controlled (Fontaine & Gafoer 1989). A K - A r age of 248 _+ 10 Ma obtained from a volcanic rock from the Silungkang Formation, reported by Nishimura et al. (1978), is in agreement with the stratigraphic age range. Anomalous younger K - A r ages are reported by Suwarna et al. (2000) from volcanic rocks from the Silungkang and Palepat formations, with an andesite from the Silungkang Formation dated at 140 _ 10 Ma, and an andesite from the Palepat Formation outcrop dated at 75 + 1 Ma. Evidently younger volcanic rocks have been mapped as part of the Palaeozoic outcrop, probably because similar lithologies of different ages are intermixed in discontinuous exposures. The volcanic rock from the Silungkang Formation which gave a 140 + 10 Ma age may be associated with the Lower Cretaceous andesites known to occur beneath the Tertiary sediments in the nearby Ombilin Basin, and dated at 143 + 4 M a (Koning & Aulia 1985). As mentioned above, Tobler (1922) also mapped Cretaceous volcanics as part of the Palepat Formation. Lithologies in the Silungkang Volcanic Member are similar to those in the Palepat Formation (Table 6.4). Outliers of the Calcareous Member of the Silungkang Formation intercalated with basalts 4 k i n NE of Tanjung Gadang, and diabase at Lubukkarak (Gafoer et al. 1992a), were previously mapped as part of the Phyllite and Shale Member of the Kuantan Formation

69

by Silitonga & Kastowo (1975). The results of the reappraisal of age of the limestone outliers of the Kuantan Formation (Fontaine & Gafoer 1989) suggests that a similar reappraisal is required for the volcanics in the main outcrop of the Phyllite and Shale Member. To the NW the large strike-slip duplex structure within the Sumatra Fault Zone in the Lubuksikaping Quadrangle (Rock et al. 1983) contains faulted outcrops of the Silungkang Formation and the Panti Volcanic Formation, which are deformed lithological correlatives, respectively, of the Volcanic and the Calcareous members of the Silungkang Formation in the type area. In the north of Sumatra, in the Takengon Quadrangle, the Situtup Formation, contains metavolcanics and limestones. Cameron et al. (1983) suggest that the metavolcanics are mainly of Late Permian age. Fossils from the associated limestones are of Mid-Permian (Artinskian-Capitanian) and M i d - L a t e Triassic (Ladinian-Norian) age (Fontaine & Gafoer 1989). The outcrops are allochthonous and much disrupted by Miocene thrusting. Hutchison (1994) following van Es (1919), included the Situtup volcanics within the West Sumatra Permian Plutonic-Volcanic Belt. If this correlation is correct and the Situtup Volcanic Formation extends into the Triassic, it is the youngest component of this belt. However the presence of the Situtup Volcanic Formation in the belt may be a coincidence as Barber (2000) has suggested that this formation is an allochthonous component from the Woyla Accretionary Complex.

Geochemistry of the Silungkang and Palepat Formations Chemical analyses of selected volcanics from the Silungkang and Palepat Formations are presented by Suwarna et al. (2000) who found that the compositions of the two formations are very similar. The range of SiO2 in the Silungkang Formation is

I

100o45'E

(a) SILUNGKANG FORMATIONl(b) ~

L i m e s t o n e s with intercalations of s a n d s t o n e & slate

t=

Basaltic extrusives

Undifferentiated Lower Permian

o ___O

L.~,~,.~,.,:~-,:t Tufts & a g g l o m e r a t e s I."-"."_.."z ."- ."4

l

O [i.ili![i][i[i~!ii[i[![ii!~i[iil H o r n b l e n d e andesites (tufts) with silicified shale intercalations

~

m

~__

_ _ _ .-.....-

I

m

.'.,

Augite andesites

E ~ O

NNN

Meta-andesites

::

C

__o

..-. 9 li: ,':-.S I L U N G K A N G

Meta-dacites

I Silicified shales and limestones

Plutonic Intrusions-Undifferentiated 2kin

_ 100045' --

Fig. 6.5. (a) Lithologies and members in the Silungkang Formation. (b) Geological map of the Silungkang Formation (after Katili 1969).

I

70

CHAPTER 6

(a)

BASALT

BASALTIC ANDESITE ANDESITE

100

DACITE

3 SHOSHONITIC SERIES K20 (wt%) 2

HIGH K CALC-ALKALINE

/

....~?? ............................

CALC-ALKALINE

/

57

63

(a)

I

~

La

Ce

I

o

9

I

I

Nd Sm

I

I

Eu Gd

I

I

I

Dy

I

Er

I

Yb

o ............. 9 S I L U N G K A N G FORMATION

68

Volcanics 9 Silungkang Formation FeO

I

Pr

Si20 (wt%)

(b)

/ 0 .....

10

LOW K SERIES 53

",.,.

........

/

/

45

-..@...... B .....0...

Palepat Formation

o

PALEPAT FORMATION

1000 LU ~m nra Z 0 -io o O

n-

(b)

100

ID.O__O.-O ...... O._ O

10-

,0"

1

0.1

I

I

I

I

I

Ba Rb Th K Nb

I I

I

I

I

I

I

I

La Ce Sr Nd P Sm Zr Hf

I Tb

Fig. 6.7. (a) Chondrite-normalized REE patterns for the Silungkang and Palepat Formations. (b) Chondrite-normalized spidergram for the Silungkang and Palcpat Formations. Adapted from Suwarna et ell. (2000).

Metavolcanics Sumatra Na20+K20

and

Tectonic

serpentinites

in the Medial

Zone

MgO

Fig. 6.6. (a) Potassium-silica diagram for the Silungkang and Palepat Formations. (b) AFM diagram for the Sih,ngkang and Palepat Formations. Adapted from Suwarna et al. (2000). 4 8 - 5 8 % , with a rhyolite sample at 85%, and in the Palepat Formation is 4 7 - 6 2 % . The composition of the rock samples analysed varied between basalt and andesite (Fig. 6.6a), showing both tholeiitic and calc-alkaline differentiation trends (Fig. 6.6b). K20 contents in the Palepat Formation are higher than those in the Silungkang Formation and fall in the potassic alkaline field, while K20 values in the Silungkang Formation are lower and the rocks more calc-alkaline. The magnesium number (Mg# = 100 M g / M g + Fe 2+) for the Silungkang Formation was calculated at 4 0 - 5 6 , while the range for the Palepat Formation is 31-56, indicating that the basalts were out of equilibrium with the mantle (Mg# -- 6 8 - 7 5 ) due to the fractional crystallization of olivine and pyroxene. Chondrite-normalized REE patterns (Fig. 6.7a) for two samples from each formation have moderate Eu anomalies, indicating some plagioclase fractionation. The rock/chondrite normalization diagram (spidergram) (Fig. 6.7b) shows that the range of values for the two formations overlap, but the samples from the Silungkang Formation show a greater range and fall between the normal and enriched values for MORB. Suwarna e t al. (2000) concluded that the analysed samples showed evidence for fractionation, differentiation and possibly contamination processes, and noted that the volcanics had geochemical similarities with those from an island arc setting, although a continental margin, fault-related, origin has also been proposed.

The Medial Sumatra Tectonic Zone ('Line' of Hutchison 1994) is a wide zone of deformed rocks which separates the West Sumatra Block from Sibumasu (East Sumatra Block). The zone is best known north of the equator where Rock e t al. (1983) and Clarke e t al. (1982b) described the outcrops of the intensely deformed Pawan and Tanjungpuah formations (Table 6.5). The Pawan Member consists of fine-grained chloritic metavoicanics interbedded within intensely folded muscovite, chlorite and tremolite schists, often with carbonate. The tremolite schists are deformed and metamorphosed ultrabasic rocks, and probably originated as tectonic slivers of ophiolite. To the SE, to the west of the Tigapuluh Mountains, Andi-Mangga e t al. (2000) found serpentinites within slates of the Ganggsal Formation. The Ganggsal Formation (refer to Fig. 4.6) is intensely deformed compared to the other rock units in the Tigapuluh Mountains (Simandjuntak e t al. 1991) and may be the SE continuation of the Medial Sumatra Tectonic Zone.

Bentong-Billiton

Accretionary

Complex

The 'Bentong-Billiton Accretionary Complex' is an assemblage of deformed and imbricated basic volcanics, ultrabasic rocks and sediments in Peninsular Malaya and the Tin Islands of Indonesia, occurring between the Sibumasu and the Indochina blocks (Fig. 6.8). The complex includes the Bentong-Raub Suture (Line) in Peninsular Malaysia (Metcalfe 2000). The continuation of the suture into Indonesia has been a source of speculation (see Metcalfe 1996). However, Barber & Crow (2003) suggest that the 'suture' is a broad zone of imbrication passing from the

PRE-TERTIARY VOLCANIC ROCKS

71

Table 6.5. M e t a v o l c a n i c s and meta-ultrabasic rocks in the M e d i a l Sumatra Tectonic Zone Unit with volcanics

Age

Description

Reference

Pawan Member (Kuantan Fm) Ganggsal Alas Fm

Carboniferous-Early Permian

Intensely folded muscovite, chlorite, tremolite schists (derived from ultrabasic rocks), often with carbonate. Interbedded fine-grained chloritic metavolcanics. Serpentinites within slates Rare 'possible green m e t a v o l c a n i c s ' among striped, slumped shales, siltstones, cherts, sandstones, conglomerates and wackes

Clarke et al. (1982b) Rock et al. (1983) Andi-Mangga et al. (2000) Cameron el al. (1982a)

Carboniferous-Permian Vis6an or younger

10/o E - , ~ . , ~

l ~ ~ g o ! ~

.---.._~ Malang Formation ._~_~.' ~=~", ",.. ~ i i : : ~ . . - - " - ,,"' ~ ' . , K A R I M U N d i ~ ! R A T A M i i . ~ - - ) , " ~ S [ t ;B)N.~A.N.~.. -ij ',, ,;iiiiihk BESAR _~ ~ ~'~ii:i~.~,[-{ ( ~ : ~ ~ - - - - ! ~ "~,~',~',',',,,'",'K"{ ~ . ~ _ : ! : i : i : f ~ ,...~t ~ ~ " -~*~8~

", ...,..-,...~,~ ~

~"---~~-':

", L~,ur!,P,,Y,~ ',,

"-.. .... .,

CITILIM

~ ~

,-, ','.~,,

~

k.iiiii..k,

v

c7

~. %~

I I I I I I I-b-,,. SUMATRA

centre ~ ~ ; ~

2) 0

"-~

%

s

o

o

Main Range Granites I ~ ......... (S and A-type)

TRIASSIC ~

CARBONIFEROUS[]~]

0o_

~ ~ _ ~ I ~

Volcanics

~,

~ ! ~ A ; i i ~ , ~ ~

Sediments PERMIAN

Iiiiiiiii

Eastern Province Granites (I-type)

Riau-Billiton C~(E]2~ ~ J'~ Accretionary Complex FK@d Persing Complex and

Tapanuli Group

"-"

Ill

II

9

LI INL.~L~

\

",',',', ;,\ l~iHi ,ii[i K

50km 104~ I Fig. 6.8. Simplified geological map of the Riau and Lingga Archipelagos. Granite typology after Cobbing et al. (I 992).

"":" '

SINGKEP Sl

72

CHAPTER 6

Table 6.6. Metavolcanics and meta-ultrabasic rocks in the Riau-Billiton Accretionary Complex in the Tin Islands Archipelagos

Island

Litbological description

Reference

Batam

Grey and violet sericite-schist, quartz-sericite-talc phyllite and silicified, sericitized, kaolinised metavolcanics with altered former plagioclase phenocrysts Radiolarian cherts and metavolcanics are recorded from the NW corner ?in situ Talc schist is present on Pait between Sugi and Combol lslands Narrow zone of talc schists and mica-chlorite schists south of the Klabat Batholith on both sides of Klabat Bay Serpentinites exposed in Belinyu No. 17 pit; 100 m of serpentinite encountered in a borehole at the Permali Mine Skarns at Pemali mine: idocrase-actinolite-diopside-epidote; diopside-wollastonite-calcite-quartz; hornblende-quartz-muscovite; diopside-quartz-chlorite-plagioclase; hornblende-muscovite-quartzepidote-plagioclase Permali Group: Volcanic Chert Facies with sills or stratified basic to intermediate volcanics, tufts, cherts & shales Lenticular masses of ?original fayalite in the Seloemar lode Nam Salu lithologies: metasandstone, metasiltstone, radiolarian chert, metavolcaniclastics and skarns The Schachtader lode (currently inacessible) a 2 - 3 m skarn composed of green amphibole (?actinolite), pyroxene, andradite, ilvaite, iron sulphides and cassiterite overlain by + 10 m of radiolarite beneath shales. Manganese-facies ironstone is reported in boreholes Siantu Formation: Metabasalts, agglomerates and breccias at Cape Siantu

Van der Bold & Van der Sluis (I 942)

Sugi Pait Bangka

Billiton

Malay Peninsula through the Tin Islands and beneath the Triassic graben on Bangka, rather than a discrete line as illustrated by Pulonggono & Cameron (1984) (see Fig. 14.2). The accretionary complex is well known in Malaya where it consists of severely deformed sediments, volcanics and slivers of ultrabasic rocks ranging in age between Devonian and Upper Permian (Metcalfe 2000). In the Tin Islands, where fossils are scarce, Bothe (1925a,b) distinguished Pre-Triassic (?Carboniferous-Permian) volcanics and sediments, from similar, but also deformed, Triassic volcanics and sediments, on the basis of their more intense deformation and metamorphism, their basic and ultrabasic (as opposed to acidic) composition, and the absence of associated granitic plutons. One fossil locality on Bangka yielded Permian fossils, and on Billiton, fossils spanning the Sakmarian to Kungurian stages have been identified (Fontaine & Gafoer 1989). The Permian rocks in the Tin Islands are considered to have a Cathaysian affinity (Indochina Block) on the basis of the identification by Jongmans of poorly preserved

105~

V~I

h

,I.

.~,~,~a,

TRIASSIC ~ LOWERMIDDLE ~ PERMIAN PERMIAN ~ ' ~

Penjabung

107~ Tempilang Sandstone Oceanic Facies Undifferentiated Pebbly mudstone Facies

~

(S type) ~}]q:FF~ Main Range (Stype)

:::::::::::::::::::(

t

GRANITE PROVINCE~_q /

i 9 ""'"''"'"'"'""'"'"'"'"'""

50km ,

Bebulu Batholith

~

Eastern Province (I-type) - 3~ 0

Bahruddin & Sidarto (1995)

CARBONIFEROUS-EARLY PERMIAN

iii:iiii:iii ii:iii

Thrusts

Ko (1986) See Adam (1960, Fig. 26) Schwartz and Surjono (1990b) See Adam (1960, Fig. 24)

I

lo6 ~

Cape

""~

Westerveld ( 1937); Katili (1967) Pulunggono & Cameron (1984); Suryono & Clarke (1981) Schwartz & Surjono (1991)

Gigantopteris plant fossils (van Overeem 1960; Hosking et al. 1977) and the occurrence of fusulinids (De Roever 1951; Strimple & Yancey 1976). Early geological studies in the Riau and Lingga archipelagos are summarized by van Bemmelen (1949) and the scattered occurrences of metavolcanics, ultrabasic rocks and their metamorphosed derivatives are compiled in Table 6.6 and the localities are shown in Figure 6.8. Ko (1986) identified poorly exposed pre-Triassic rocks (Fig. 6.9) on Bangka Island as facies of the Pemali Group. The Pebbly Mudstone Facies in the Toboali area in the south of the island is correlated with the glaciogenic Late CarboniferousEarly Permian Bohorok Formation of Sumatra and is included in the Sibumasu Block (Barber & Crow 2003). The other Pemali Group facies of Volcanic-Chert, Bedded Chert, Laminated Mudstone and Pyritic black shale-limestone are considered to be components of the accrelionary complex and include EarlyMid-Permian rocks (Fontaine & Gafoer 1989).

i

_ ~

Van Wessem (1942)

:'1

@ 3 ~_

106~j

TOBOALI

107~

Fig. 6.9. Simplified geological map of P. Bangka. Geology compiled from Ko (1986), Katili (1967), Osberger (1968) and Verbeek (1897). Granite typology after Cobbing et al. (1992).

PRE-TERTIARY VOLCANIC ROCKS

Ko (1986) described diabase sheets intruded into radiolarian cherts and sediments at Cape Penjabung in the NW of Bangka as part of the Volcanic-Chert facies of the Pemali Group. These diabases were previously mapped as volcanics by Zwierzijcki (1933) and Verbeek (1897), but Westerveld (1936, 1937) describes them as intrusive sills into folded rocks and suggested that they were precursors of the adjacent granite. Cobbing et al. (1992) consider that they are an early basic (dioritic) facies of the Klabat Batholith. Ko (1986) includes the lithologies described by De Roever (1951) and Schwartz & Surjono (1991) in the Pyritic Black Shale-Limestone Facies of the Pemali Group. According to Schwartz & Surjono (1991) the lithologies exposed in the open pit at the Pemali Mine are deformed hornfels and skarns derived from metasediments. However, the mineralogy (Table 6.6) and geological setting suggest that in addition to sediments, these metasomatic rocks also were derived from volcanic and ultramafic rocks described at this locality by Pulunggono & Cameron (1984) and Suryono & Clarke (1981). Similar skarns, encountered during mining, are present in the Permian rocks on Billiton (Kelapakampit Formation of Bahruddin & Sidarto 1995). Of interest are the are lenticular masses of ?original fayalite in the Seloemar lode (Adam 1960), and the presence of fayalite as a minor constituent in the tin ores at Nam Salu in the Klapa Kambit mine. Here, Schwartz & Surjono (1990b) showed that Permian metavolcanics and metasediments (Table 6.6) had been metasomatized and that tin ores had been formed in association with Triassic granite intrusions, which

R H Y O L ~ 0.1

]TRACHYANDES~ ANDESITE

U 0.01

9

j / , .-/" .... " - , , , J

r

.......... u - ' 7 7 9 " 9 ",,~

ANDESITE/BASALT ~', ' -

_ 9 9 nn -

SUB-ALKALINE BASALT o.ool O.Ol

_ ;,

A o.1

I i I

9 i 1

lO

Nb/Y Fig. 6.10. Zr/TiO2-Nb/Y discrimination diagram showing fields for volcanic rocks based on immobile elements (after Winchester & Floyd 1977). Both ratios are indices of alkalinity but only Zr/TiO2 ratio represents a differentiation index. Small squares represent element ratios in the metasomatised Nam Salu 'phyllite'. Adapted from Schwartz & Surjono (1990b).

73

were the source of the tin. The Nam Salu ore body is a layer of iron formation, corresponding to the silicate facies of Algoma Type, mixed with tuff which was metasomatized into micaceous phyllite. Schwartz & Surjono (1990b) concluded that the Nam Salu phyllite was chemically a 1:1 mixture of basalt and silicate-facies ironstone; the bulk of their analyses (Fig. 6.10) correspond to the sub-alkaline basalt field of Winchester & Floyd (1977) in a discrimination diagram using immobile elements. The mineralogy of the Schachtader lode indicates it is either a metabasalt or even a meta-serpentinite, although Schwartz & Surjono (1990b) describe it as an altered volcaniclastic rock.

West Sumatra Triassic Plutonic-Volcanic Arc Volcanic rocks associated with the West Sumatra Triassic Arc are preserved in the Cubadak Formation (Rock et al. 1983), as a sequence of dark green volcanic wackes interbedded with mudstones and siltstones containing H a l o b i a , faulted against, and possibly part of the carapace of the early Jurassic Muarasipongi Batholith, which has been dated at 197 ___ 2 Ma.

Pahang Volcanic Belt There are abundant occurrences of volcanic rocks in the Triassic of the eastern Malay Peninsula belonging to the Pahang Volcanic Series (Hutchison 1973). These volcanics are invariably associated with IS and A-type plutons of the Eastern Granite Province (Central Belt) (Cobbing, pers. comm.). This association in the Semantan Basin (Fig. 14.11) and its continuation in the Riau and Lingga archipelagoes (Fig. 6.8) is described here as the Pahang Volcanic Belt (Table 6.7). P. Karimun Besar is formed of a core of metaluminous granite of IS or A-type (Cobbing et al. 1992) which is mantled by the contact metamorphosed Malarco Formation (Cameron et al. 1982c). The presence of volcanic rocks within the graben sediments strongly suggests that the pluton was intruded into its carapace of surface volcanics in a resurgent caldera. The Karimun Besar granite has not been dated radiometrically; Cameron et al. (1982c) suggest a date of emplacement between Mid- and Late Triassic (Carnian-Norian). In the SE of Bintan the rhyolites and trachytes which abut the East Bintan batholith, intruded around 230 _+ 12 Ma ( R b - S r isochron, Cobbing et al. 1992), are likely to be relics of the volcanic carapace of this batholith. On Lingga the Lingga pluton is intruded into Triassic cherts containing D a o n e l l a and volcanic rocks which appear to be associated with this biotite-hornblende two-phase granite (Cobbing et al. 1992). The deformation noted by Bothe (1925a, b) may be due in part to later intrusion of the pluton into its own volcanic edifice.

Table 6.7. Volcanic lithologies in the Pahang Volcanic Belt in the Tin Islands Archipelagos Island

Formation

Description

Reference

Karimun Besar

Malarco

Porphyritic rhyodacites and lithic tuft's, hornfelsed shales, ?chert, ?conglomerate and limestone Rhyolites and trachytes Quartzporphyrites interfingered with Triassic sediments Rhyolites, dacites, porphyrites and accompanying tufts

Cameron et al. (1982c)

Bintan Citilim Lingga

Van Bemmelen (1949); Osberger (1968) Van Wessem (1942) Both6 (1925a,b)

CHAPTER6

74

Jurassic-Cretaceous Plutonic-Volcanic Arcs Volcanism and the associated plutonism in Sumatra has a a complicated history during the Late Mesozoic. To a large extent this is the history of the Late Jurassic-Early Cretaceous Woyla Group, as described by Cameron et al. (1980). The stratigraphy and current understanding of the geological setting of the Woyla Group are discussed by Barber (2000) and by Barber & Crow (Chapters 4 & 14). The distribution of the different assemblages

in the Woyla Group is shown in Figure 6.11 and the volcanics present are described with reference to these assemblages in Tables 6.8-6.10. In central Sumatra Late Jurassic-Early Cretaceous I-type plutons (Fig. 6.11 ) form a continental margin Andean arc related to subduction (McCourt et al. 1996). The plutons are better known than their associated volcanics. Lower Cretaceous andesites occur at Palanki in the Tertiary Ombilin Basin (143 + 4 M a , Koning & Aulia 1985) and a new date of

I

I

99~ I.~DA

I

102 ~

105 ~

WOYLA ASSEMBLAGES

ACEH

~

Jurassic-EarlyCretaceous

Oceanic Island Arc (Bentaro Arc) Accretionary Complex (ocean-floor material)

Jurassic-Early Cretaceous Foreland t:::::::t sequences: Tembesi and Rawas Fms

~

TA PAKTUAN \

Jurassic-EarlyCretaceous Plutono-VolcanicArc te Cretaceous Plutonic Arc

Parlumpah! NATA

n

I

Kanaikan

&'~,

o

0o -

Maninja Indaru

%

lanki

Lubukg~

Q

Kerinc ~ %%'"% i","-.","',."

~

"~----,,--- Thrusts Faults

0

100

200

300km

99 ~

102 ~

105~

I

I

I

Fig. 6.11. The distribution of the Woyla Group Assemblages in Sumatra.

PRE-TERTIARY VOLCANIC ROCKS

75

Table 6.8. Volcanic lithologies in the Oceanic Assemblage o f the Woyla Group. Formation

Lithological description

Reference

-t-2000 m massive, green to grey, deformed mafic to intermediate volcanics, frequently epidotized, uralized or silicified, some pyroclastics, amygdaloidal basalts, minor phyllites and pods of siticified metalimestone Calcareous, carbonaceous to manganiferous slates and meta-argillites, green volcanic wackes and chert/basalt beds; the Bengga Limestone Member is composed of metalimestones, coarse marbles and metavolcanics Basalts, red cherts, argillites, metavolcanic wackes and greenschists Partially epidotized basalt breccias & agglomerates; schistose metabasalts Includes intermediate to marie metavolcanics, cherts and slates

Bennett et al. (1981a)

Aceh Province

Geumpang

Lam Minet

Penarum Situtup Undifferentiated Woyla Group Babahrot

Bennet et al. (1981a)

Cameron el al. 1983) Cameron et al. 1983); Barber(2000) Cameron et al. 1983) 1982a)

Metavolcanics, metalimestones and serpentinites and metagabbro intrusions

Cameron et al.

Basic volcanics, including pillow lava, volcanic breccia, tufts, volcaniclastic sediments, radiolarian chert and massive or bedded limestone Quartzites, shales, siltstones, slates and volcaniclastics

Yancey & Alif (1977); McCarthy et al. (2001) Rosidi et al. (1976)

Diabases and basalts, associated with turbidites and a large limestone body Limestone, quartzite, slate, schist, tuff, igneous breccia, tuff breccia, metavolcanic, diabase and serpentinite

Suwarna et al. (1994) De Coster (1974)

Tuffaceous and calcareous claystones, sandstones and shales with intercalated radiolarian-bearing cherts, manganese nodules, coral limestones and rare porphyritic basalt. The sandstones contain clasts of glassy andesite and lithic fragments of andesite, quartz-diorite and quartzite

Barber (2000)

Padang area

Indarung Siguntur T e m b e s i - Rawas Mountains

Rawas 'Mesozoics with mafics' Lampung area

Menanga

See Table 6.9 for the Natal area.

Table 6.9. Volcanic units and volcaniclastic sediments of oceanic and continental affinity within the Woyla Group Accretionar~, Complex in the Natal area Formation

Lithological description

Environment

Ref.

Tambak Baru Volcanic Unit

Altered, purple, quite strongly sheared, porphyritic andesites and andesite agglomerates and proximal debris flows Dark green, foliated megabreccias with basic volcanic and limestone megaclasts interbedded with poorly sorted conglomerates and greywacke sandstones Vesicular basic lavas, keratophyes and dolerite dykes Breccias with basic volcanics, radiolarian cherts, limestones with Mnmineralisation Volcaniclastic siltstones, fine siltstones and rare conglomerates Volcaniclastic sandstones and unsorted conglomerates (lahars) Undeformed porphyritic andesites with amygdales and altered matrices and andesitic tufts Greenschist facies banded quartz, muscovite, chlorite schists

Volcanic centre & proximal volcaniclastics

1

Proximal sediments and olistostromes derived from volcanic centre

1

Simpang Gambir Megabreccia

Nabana Volcanic Unit Panglong Mdlange Belok Gadang Siltstone Ranto Sore Parlumpangan Volcanic Unit Si Gala Gala Schist Unit Simarobu Turbidite Batang Natal Megabreccia Rantobi Sandstone Jambor B aru Muarosoma Turbidite Mdlange Unit

Pasaman Ultramafic Complex Igneous rocks in Batu Nabontar Limestone Undifferentiated

Volcaniclastic turbidites with minor calcareous siltstones Large clasts of limestone, rare clastic sediments and igneous rocks in a slaty matrix Thin bedded volcaniclastic sandstones and siltstones Volcaniclastic conglomerate, sandstone, siltstone, limestone and tuff Thin bedded volcaniclastic turbidites with noticeable quantities of quartz clasts and less mafic and chlorite material House & room sized fragments of greenstones, greenish wackes, cleaved metatuffs, sheared fossiliferous limestones and 50% by volume cherts; the blocks are disrupted by serpentinite and invaded by dykes Variably serpentinized, massive to foliated hartzburgite, with minor dunite pods and stringers, and pyroxenite dykes Serpentinized dunites and hartzburgites intruded by thin dykes, now rodingites Banded metavolcanics, slates and limestones in north of Lubuksikaping Quadrangle

References: 1, Wajzer et al. (1991); 2, Rock et al. (1983).

Ocean-floor basalts, Seamount Mdlange ?olistostrome, of ocean-floor materials and pelagic sediments ?lower trench slope basin fill Fluviatile intra-arc deposits Volcanic arc or local volcanic centre fragments Metasediments derived from an acidintermediate arc or centre of continental type Ocean-floor or trench deposit Olistostome or mud diapirs in accretionary complex Forearc basin deposits Shallow marine and deeper water forearc basin deposits Upper trench slope basin sediments M61ange ?olistostrome of ocean-floor materials, pelagic sediments & limestone

Ocean-floor volcanics and basement slices; seamount Slices of ocean-floor basement ?Accretionary complex

76

CHAPTER 6

Table 6.10. Volcanic units in the Oceanic Volcanic Arc fragments of the Woyla Group Formation

Litbological description

Ref.

Porphyritic basalts and basalts and agglomerates with andesine, associated with mafic dykes, Basaltic vents surrounded by tufts, breccias and volcanic sediments were found near Lam No and north of the Bentaro river Volcanic wackes, subordinate sandstones and siltstones, mafic volcanics and limestones Massive, partly epidotised, frequently porphyritic andesites, subordinate basalts with feldsparphyric varieties and coeval dykes. Agglomerates, breccias and tufts are present in the southeast. Subordinate shales and slates containing volcanic debris and purple to red tuffaceous sandstones Biotite-hornblende-andesineschists & biotite amphibolites interpreted as syntectonic deformed Tapaktuan Volcanics associated with concordant gneissic leuco-granites

l

Bentaro arc

Bentaro Volcanic

Lhoong Tapaktuan Volcanic

Meukuek Gneiss Complex

l 2, 3

2

Sise

Kenyaran Volcanic

Epidotized intermediate to mafic lavas which are frequently amygdaloidal and porphyritic and agglomerates

2,3

Chloritised and prophylitised andesitic and basaltic lavas, tufts and breccias with local limestone intercalations Basalts and andesites interbedded with claystone, siltstone, calcilutite and chert (?) amygdaloidal and porphyritic lavas of basalt and andesite, crystal tufts, chert and rare serpentinite Basalt and andesite lavas with minor lenses or intercalations of chert Boulders and clasts of limestone, chert, schist and andesite similar to the andesite lava in the Garba Formation, all within a scaly matrix

4

Saling

Saling Lingsing Garba Insu Member M~lange Complex

4 5 5 5

References: l, Bennett et al. (1981a); 2, Cameron et al. (1982a); 3, Barber (2000); 4, Gafoer et al. (1992e); 5, Gafoer et al. (1994)

140-t- 10 Ma from the Silungkang Formation (Suwarna et al. 2000) indicates that Lower Cretaceous volcanic rocks are more extensive than previously thought, but were previously included with Permian volcanics. The Siulak Formation, forming a limited outcrop within the Sumatra Fault Zone near the southern margin of the Painan Quadrangle (Rosidi et al. 1976), includes dacitic lavas and tufts and a 500 m thick fossiliferous (Cretaceous) Limestone Member. It is suggested that this formation represents forearc sediments and continentally-sourced andesites trapped by strikeslip faulting within the fault zone. Continentally sourced voicaniclastic sediments which occur as fault packets in the Woyla Oceanic and Accretionary Complex in the Batang Natal section (Wajzer et al. 1991) may have been derived from erosion of the contemporaneous JurassicCretaceous Plutonic-Volcanic Arc.

Volcanics in the Woyla Accretionary Complex Volcanic lithologies occur commonly in the Woyla Group, where they are tectonically juxtaposed as fault packets within the Accretionary Complex (Tables 6.8 and 6.9). They are best known from the Batang Natal section, where Wajzer (1986) carried out detailed mapping and documented the variety and discussed the origin of oceanic and pelagic rock types (Wajzer et al. 1991). Elsewhere in Sumatra the distribution of the major lithological units within the Woyla Accretionary Complex has been established by reconnaissance mapping only.

A c e h P r o v i n c e ( r e f e r to Fig. 4 . 1 3 )

The Geumpang, Lain Minet and Penarum formations in the Banda Aceh and Takengon quadrangles include basaltic lavas, often pillowed, basaltic breccias and conglomerates, tufts and volcanic sandstones, imbricated with limestones, radiolarian chert and argillites of the Woyla Oceanic Assemblage (Bennett et al. 1981a; Cameron et al. 1983; Barber 2000). The more massive

limestones may represent the carbonate caps to seamounts constructed on oceanic crust. Serpentinite is also imbricated into these formations and sometimes occur as diapirs within the Sumatran Fault Zone. The larger bodies of serpentinite (Tangse, Cahop and Beatang Ultramafic Complexes) represent slices of oceanic upper mantle harzburgite incorporated into the accretionary complex. The volcanic rocks are often deformed and altered to greenschists, and the ultramafic rocks to talc schists. Garnetiferous amphibolites present in the Reunguet River are suggested by Barber (2000) to have been subducted and metamorphosed at high pressure before being tectonically exhumed. The large area of undifferentiated Woyla Group south of the Sumatra Fault Zone includes intermediate to mafic metavolcanics, cherts and slates, and may be considered, to be composed mainly of the Woyla Oceanic Assemblage. The Upper Permian-Triassic Situtup Formation in the Takengon Quadrangle (Cameron et al. 1983) composed mainly of limestones, also includes metavolcanics such as epidotised basalts, basaltic breccias and agglomerates and schistose metabasalts. The adjacent Toweren Member also contains massive metavolcanics. Barber (2000) points out that the descriptions of the volcanic lithologies in the Situtup and Toweren formations resemble those of the Woyla Group and suggests that Woyla volcanics may have been tectonically imbricated within the Situtup Formation.

N a t a l a r e a ( r e f e r to F i g s 4 . 1 4 a n d 6 . 1 2 )

Oceanic rocks of the Woyla Group in the Natal area were first mapped by Rock et al. (1983) as part of the Lubuksikaping Quadrangle. The rock units and their relationships were described in detail from the Batang Natal river and road sections by Wajzer (1986), with a more accessible summary in Wajzer et al. (1991). The section shows imbricated slices of massive limestone, serpentinite, volcaniclastic sandstone, sometimes turbiditic, pillow basalt, radiolarian chert and m~lange, composed of blocks of these lithologies in a clay matrix, arranged in an apparent

PRE-TERTIARY VOLCANIC ROCKS

Jambor Baru Formation indicates an oceanic environment, while absence of plutonic fragments and presence of (altered) andesitic debris indicates that the volcanic source was nearby, perhaps within the accretionary complex but Wajzer (1986) suggested the source was a oceanic island arc in process of erosion. The Jambor Baru Formation is bounded by strike-slip faults and a sliver of Parlumpangan-type volcanic rock is faulted within the outcrop. The Simarobu Turbidite Formation is composed mostly of volcaniclastic turbidites with minor calcareous siltstones, strongly deformed and metamorphosed in the greenschist facies. The calcareous siltstones may be recrystallized pelagic limestones, while in the turbidites, the sparse quartz and K-feldspar and the highly altered mafic volcanic clasts, suggest an intermediate volcanic source and a trench or ocean-floor depositional environment (Wajzer 1986). The unit is affected by thrusts and later strike-slip faults. The Parlumpangan Volcanic Unit is of extrusive origin, probably representing different levels of a volcanic pile constructed on the sea floor, which was emplaced and faulted within the accretionary complex. The Si Gala Gala Schist encloses, and is strike-slip faulted against the Parlumpangan Volcanic Unit. Wajzer et al. (1991) interpret the Parlumpangan Volcanic Unit as fragments of a non-specific volcanic arc, but stress that the associated the Si Gala Gala Schists are derived from a continentally based acid-intermediate volcanic arc. A volcanic centre within the accretionary complex broken up by faulting is a likely source of these two units.

random fashion (Fig. 6.12 & Table 6.9). Undifferentiated Pre-Tertiary banded metavolcanics, slates and limestones, on the northern margin of the Lubuksikaping Quadrangle (Rock et al. 1983 geological map) are shown as part of the Woyla Group in the geological synthesis of Stephenson & Aspden (1982) and Rock et al. (1983, fig. 4). Aspden et al. (1982b) extended subcrop of the Woyla Group up to the Sibolga Fault. These rocks are considered to belong to the Woyla Accretionary Assemblage, even though no ultrabasic rocks were described. Slivers of serpentinitized dunite and hartzburgites intruded by thin basaltic dykes, are faulted within the Batu Nabontar Limestone at the northeastern end of the Batang Natal section near Muarasoma (Wajzer 1986). These slices may be related to the Pasaman Ultramafic Complex which crops out to the SE. This complex has a length of 75 km, an area in excess of 100 km 2 (Rock et al. 1983), and is the largest ophiolite slice in Sumatra, although the thickness is not known. The complex is faulted against an extensive limestone unit, the strike equivalent of the Batu Nabontar Limestone and a mdlange unit, similar to the Batang Natal Megabreccia in the Natal section. Wajzer et al. (1991) reported a Late Triassic foraminifer from a limestone block in the Batang Natal Megabreccia, indicating that oceanic limestones, probably deposited on volcanic seamounts as old as Late Triassic, are incorporated in the accretion complex, either as an olistrosomes or as mud diapirs. The depositional environment of the Muarasoma Turbidite Formation was probably in a small basin perched on the trench slope of the accretion complex. The virtual absence of quartz in the

THE BATANG NATAL RIVER SECTION 0

1

2

3km

I

I

I

I

Jambor Baru Formation

BNL

ioma

BNM

\',,.

BNL

.:..

.~..:.:.:.:.:.: 9

:..

PV : ::~.~i ,i;i~i ii !i i,.i!i~i:.!~) i:.!.:~: " ~ ~ i "i i .Megabr . ecciaBata~.TI ng(BNM)Natal

Parlampungan Volcanics (PV)

Batu Nabontar Limestone (BNL)

SOMA

Sandstone Si Gala Gala Schists

77

9

..:.:.=. 9

.

Muarasoma Turbidite Formation (MTF)

".:.:.:.:.:.. " ' ' ' ' ' ' ' ' ' ' ' N -.'.:.:.:,-

'~': :!iii!ii!ii!ii! iiii.~:i~STF

.%..-.....,

. . . , % , % , ..., 4 /

-~L-L~z'Manung J al'x",," ,."x"x~xBatholitgh,"x~',,' ,"x"x"x 87.0MaJ'x"-,'

SimaroOu

Formation

(STF)

Panglong Melange N a b ~ a Volcanics 4 BNL -~":"L'."

~

Ranto Sore Formation

%,-,.'%..%.,

9 9. . . . :.:. .

-::: 9Belok Gadang

~ k: :

NATAL

Siltstone

i b;>

~30

%" %" " " %" %" " " " " " " " " "V" %" "," %" %" %" "," %" V %" %" ",~ %" %" V ~"

Tambak Baru Volcanics "" %'~..,..,.. %"%"%"~,~f ~i:.iiiiiiiiiiiii:.--:~i!iiii Si'l
- , - - , " --," , , " v

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for

bl

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-.., %" ~., , . - %" v

%'. . . . . } ~" ".29.7Ma"

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.-~. v %" .... v "

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0

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Fig. 6.12. Simplified geological map of the Batang Natal river section. Adapted by Barber (2000) from Wajzer

i

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~

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78

CHAPTER 6

The Nabana Volcanic Unit at the southwestern end of the Batang Natal section (Fig. 6.12) is composed of vesicular spilitic basic volcanics intruded by dolerite dykes. These vesicular pillow lavas indicate submarine extrusion at less than abyssal depths. The dolerites are metamorphosed at greenschist facies. The Nabana Volcanic Unit is interpreted as a tilted slab of oceanic crust with ocean-floor basalts and dolerite feeder dykes. Again, associated limestones may be part of a seamount carbonate capping (Wajzer et al. 1991). Two preliminary analyses of spilites from the Nabana Volcanic Unit/Belok Gadang Formation are given by Rock et al. (1983) (Table 6.11). The Tambak Baru Volcanic Unit of andesites and andesite agglomerates and the associated Simpang Gambir Megabreccia are the faulted remains respectively of a volcanic centre and associated proximal volcaniclastic erosional debris. A sample of andesite yielded a Campanian-Maastrichtian (Cretaceous) K - A r age of 78.4 + 2.5 Ma (Wajzer et al. 1991) [N.B. this date should not be given too much credence, as the rocks are affected by lowgrade metamorphism; Editor]. The unit was suggested by Wajzer et al. (1991) to represent a collided volcanic arc, but the units are not highly deformed as might be expected in a collision; a volcanic centre intruded into the accretionary complex during the Late Cretaceous is a more probable explanation.

of megabreccia composed of blocks of metasediment and serpentinite. The serpentinite body is thrust into a turbidite sequence, probably equivalent to the Rawas Formation in the Tembesi-Rawas Mountains, along strike to the SE. Tembesi-Rawas Mountains

In the Sarolangan Quadrangle the boundary of the Woyla Accretionary Complex is taken at the Rawas Thrust, marking the approximate southern boundary of the Asai Formation. Serpentinite pods are mapped along the thrust (Suwarna et al. 1994). Diabases and basalts are also present, associated with turbidites and a large limestone body in a pelagic marine sequence, which has been affected by thrusts and strike-slip faults. The generalized description of the Rawas Formation is fairly typical of the Oceanic and Accretion Complex elsewhere in Sumatra, but the detail is lacking and it is described by Suwarna et al. (1994) as interleaved within the non-volcanic, shallow marine, Peneta Formation and perhaps represents a forearc basin deposit. The Woyla Accretionary Complex is exposed in river sections where tuffaceous shales alternate with meta-limestones to the west of the Barisan Mountains, in the Sumatra Fault Zone, and to the east of Danau Kerinci (Kusnama et al. 1993b).

P a d a n g area (refer to Figs 4.16 a n d 6.13)

S u b c r o p beneath the South S u m a t r a Tertiary Basin

In the Padang Quadrangle, to the north of the Danau Maninjau volcanic centre, the northern margin of the Woyla Accretionary Complex is truncated by the Sumatra Fault Zone (Kastowo & Leo 1973). Here a zone of serpentinite pods aligned along faults has been emplaced in massive limestones, phyllites, metasandstones and metasiltstones, occasionally with mafic greenstones. Jurassic fossils were collected from the limestones at Palembanjan by Volz (19 ! 3). To the east of Padang, McCarthy et al. (2001) recognized thrusting in the volcanic-sedimentary sequence in the Indarung Formation of Yancey & Alif (1977) and identified Mid-Jurassic radiolaria in cherts, indicating that part of the accreted ocean crust was of Jurassic age. The Golok Tuff Formation composed of crystal tufts which lies above the Lubuk Peraku Limestone (Upper Jurassic-Lower Cretaceous, Yancey & Alif 1977) has been dated using the K - A r method at 105 _+ 3 Ma (Koning & Aulia 1985). McCarthy et al. (2001) interpreted the massive Lubuk Peraku Limestone as part of a fringing reef to a seamount which collided during subduction with the Accretionary Complex and was imbricated within it. The Limestone Member of the Siguntur Formation, on strike to the SE at Surian in the Painan Quadrangle, is described by Rosidi et al. (1976) as similar to the Indarung Limestone and possibly also capped a former seamount. The main outcrop of the Siguntur Formation south of Padang includes quartzites (McCarthy et al. 2001). Rosidi et al. (1976) remark on the cherty nature of quartzites, which suggests that they may have an oceanic origin. The diverse origins of sediments are typical of the Oceanic and Accretion Complex, and this poorly exposed, but extensive unit includes distal terrestrial, volcaniclastic, pelagic and chemical oceanic sediments, probably juxtaposed by thrusting and movement along strike-slip faults.

The subcrop of the Woyla Accretionary Assemblage beneath Tertiary sediments between the Gumai and Garba Mountains and Palembang has been reconstructed from oil company borehole termination records (Fig. 6.13). These were studied by Adiwidjaja & de Coster (1973) and de Coster (1974) who distinguished a belt of 'Mesozoics with mafics' south of the 'Mesozoic Metamorphics' of the Tembesi-Rawas area of the Woyla Foreland Assemblage. Mesozoics with mafics were encountered in exploration drilling of the Tertiary sediments north of Tebingtinggi beneath the headwaters of the Sungai Musi (Kikim-Teras High) and east of Baturaja (Lematang Sub-Basin). Lithologies encountered correspond with those in the Foreland, Oceanic and Accretion Complex Assemblages of the Woyla Group. The Foreland Assemblage sediments are on strike with the Peneta and Asai Formations, and the Oceanic and Accretion Complex metavolcanics beneath the Lemat Formation volcanics (Eocene), are recorded in oil-well terminations as far north as the Sungai Musi. De Coster (1974) reports a Mid-Cretaceous (?deformation) K - A r age of 121 _ 2 Ma from tuffaceous clastics at the base of the Lemat-2 well, south of the Sungai Musi.

D a n a u D i a t a s to G u n u n g Kerinci

Between Danau Diatas and Gunung Kerinci to the east of the Sumatra Fault Zone (Fig. 6.13) a 'serpentinite front' to the Woyla Oceanic and Accretion Assemblage is marked by serpentinite pods (Rosidi et al. 1976). Serpentinite and pyroxenite are also present at Galagah (McCarthy et al. 2001). North of Lubukgadang a large serpentinised hartzburgite body is associated with a lens

L a m p u n g area (refer to Fig. 4.8)

Two Pre-Tertiary units, the Menanga Formation and the Gunungkasih Complex (McCourt et al. 1993), were mapped in the Kotaagung (Amin et al. 1994b) and Tanjungkarang (Andi-Mangga et al. 1994a) Quadrangles. The Early Cretaceous Menanga Formation, which is in thrust contact with the older (Palaeozoic) Gunungkasih Complex, consists of a mixture of lithologies ranging from shales with cherts, sandstones, siltsones and claystones and rare porphyritic basalt. The claystones are tuffaceous and the sandstones include andesite, glassy andesite and quartz-diorite clasts. The sedimentary environment of the Menanga Formation is interpreted as deep marine, related to a volcanic arc, and is correlated with the Lingsing Formation of the Gumai Mountains by Amin et al. (1994b) and Andi-Mangga et al. (1994a). According to Barber (2000) the depositional environment was that of a forearc to an Andean-type volcanic arc, built on continental basement, and he interprets the sequence as part of the Foreland Assemblage of the Woyla Group. The lithological mix suggests that the Menanga Formation

PRE-TERTIARY VOLCANIC ROCKS

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CHAPTER 6

I

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GROUPASSEMBLAGES

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Oceanic Volcanic Arc -~

Lampung High

Accretionary Complex (ocean-floor material) Foreland assemblage

[

[

Palaeozoic basement 102~

103~

104~

Fig. 6.13. The distribution of the Woyla Group Assemblages in Southern Sumatra and localities mentioned in text. (Table 6.8) is a tectonic composite of oceanic and foreland lithologies.

West Java Sea To the east of Sumatra, in Java and the West Java Sea, the Woyla Group is difficult to trace, but lithologies of the Woyla Accretionary Complex have been recognized in oil well terminations in the off-shore Sunda oil field, where serpentinite and metasediments, together with Late Cretaceous granites, were encountered beneath Tertiary sediments. In the southern part of the Sunda Basin, in the East Java Sea, the Woyla Group is overlain by Late Cretaceous sediments. Ben Avraham & Emery (1973) found that the interpretation of magnetic intensity measurements in the East Java Sea was problematic, but the magnetic anomalies have large amplitudes (200-600 gamma) and the wavelengths ( 1 0 - 3 0 k i n ) are shorter than, but resemble those of oceanic crust. In the regional context these anomalies might represent the subcrop of ophiolite from the Woyla Accretionary Complex. Certain zones within the West Java Sea have the magnetic signatures of large basic or ultrabasic bodies, one example, on the SE margin of the Lampung High of SE Sumatra, has a similar magnetic signature to the Pasaman Ophiolite Complex in the Natal area.

Oceanic volcanic arc fragments Oceanic island arcs, fragments of which are incorporated within the Woyla Accretionary Complex, originated in Meso-Tethys probably in the Early Jurassic. The volcanic arcs have been suggested to have been constructed on continental basement (Cameron et al. 1980), but Hamilton (1988) and Barber (2000), with more detail, has thrown doubt on this idea, and also on the suggestion that these arcs originated as fragments of Gondwana (Metcalfe 1996). It would appear that the Woyla Oceanic Volcanic Arcs originated within Meso-Tethys, although how many island arc strings were created, and whether the strings were continuous is not certain. In Aceh there are three large arc fragments, the Bentaro, Tapaktuan and Sise (Fig. 4.13) of which the latter is possibly a different age to the other two, depending upon the nature of the undifferentiated area of Woyla Group east of the Anu-Batee Fault. In Southern Sumatra the Gumai-Garba Line (Fig. 6.13) of McCourt et al. (1993) links a string of arc fragments (Saling Arc) which appear to be of a similar age. Lithological details of the Oceanic Volcanic Arc fragments are given in Table 6.10.

Aceh Province (refer to Fig. 4.13) The Bentaro Island Arc (Barber 2000) is the largest of the oceanic island arc fragments included in the Woyla Group. The Bentaro

PRE-TERTIARY VOLCANIC ROCKS

Island Arc is faulted, thrust and intruded by Late Cretaceous and Tertiary granitoids. The component units of the Bentaro Arc are described in the Banda Aceh and Calang Quadrangles by Bennett et al. (1981a, b). Here the Bentaro Volcanic Formation is overlain by reef limestones and dark limestones (Lamno Formation) with Late Jurassic-Early Cretaceous fossils, and is faulted against and underlain by the Lhoong Formation. The Raba Limestone Formation, composed of reef limestones and thin bedded argillaceous and siliceous limestones is thrust over the Lhoong Formation. Near the Sumatra Fault Zone the Bentaro Arc is overthrust by the Geumpang Formation which belongs to the Accretionary Complex. In the Calang Quadrangle volcanics are not exposed, only reef limestones of the Teunom Formation are seen. Barber (2000) includes the Tapaktuan Volcanic Formation which crops out in the coastal plain of the Tapaktuan Quadrangle (Cameron et al. 1982b) within the Bentaro Arc. The Tapaktuan Volcanic Formation crops out as fault lozenges in the Kluet Fault Complex. In the NW of the main outcrop, the Tapaktuan

>-

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81

Formation is thrust over the oceanic Babahrot Formation, and in the Meukek River volcanics are transformed into amphibolites in the Meukek Gneiss Complex. Barber (2000) suggests these garnet amphibolites represent rocks which were subducted, metamorphosed and subsequently tectonically exhumed. Barber (2000) places the Sise Limestone Formation (reef limestones) and the Kenyaran Volcanic Formation (epidotized basalts) of the Takengon Quadrangle (Cameron et al. 1983) within the Island Arc Assemblage, from which it has been displaced by movements of the Sumatra Fault Zone.

G u m a i M o u n t a i n s (refer to Fig. 4.19)

The remote inlier of the Woyla Group in the Gumai Mountains (Musper 1937; Gafoer et al. 1992c) includes the Early Cretaceous Saling Formation (amygdaloidal and porphyritic andesite and basalt), the Sepingtiang Limestone (reef limestone) and the Lingsing Formation (andesite and basalt with interbedded sediments). Gafoer et al. (1992c) considered that these rocks constituted an oceanic assemblage, but Barber (2000) has proposed that all the units are components of the Oceanic Island Arc Assemblage, with the Lingsing Formation originally occupying a more distal location than the Saling Formation. Chemical analyses of volcanics from the Saling Formation in Table 6.11 are quoted from Gafoer et al. (1992c), but sample localities were not given. Using the discriminant plots of Floyd & Winchester (1975) the analyses indicate that the Saling Island Arc volcanics are of oceanic tholeiitic (MORB) affinity (Fig. 6.14). Faunas from the Sepingtiang fringing reef range in age from Upper Jurassic to Lower Cretaceous (Fontaine & Gafoer 1989) and diorite dykes, dated at 116 _+ 3 Ma by the K - A r method, intruding the volcanics are interpreted by Gafoer et al. (1992c) as feeders to the volcanics, indicating a younger, Aptian age for at least part of the volcanic sequence. A basic rock collected from one or other of the two large ultrabasic pods in the Lingsing Formation was dated using the K - A r method and gave an Early Cretaceous age of 122 ___ 4 Ma. Musper (1937) considered that the different facies in the Gumai Mountains were thrust together, and van Bemmelen (1949) suggested that the volcanic facies 'formed on the slope of a volcanic range or row of islands' and slid over the bathyal deposits as a result of gravitational tectogenesis. The rocks are highly deformed and folded, tectonic fabrics and banding strike e a s t west, but the sparse field data does not resolve the question of whether these units are imbricated to form part of an accretionary complex (Barber 2000).

G a r b a M o u n t a i n s (refer to Fig. 4.7)

o

100

' 200 300 Zr ppm

' 400

Fig. 6.14. Geochemical discrimination diagrams for basaltic rocks after Floyd & Winchester (1975) showing the affinityof the volcanics collected from the Saling Formation, Gumai Mountains. Diagram after Gafoer et al. (1992c).

In the Garba Mountains the Oceanic Volcanic Arc Assemblage is present in NW-SE-striking strips bounded by faults (Gafoer et al. 1994) and comprises the Garba Formation (amygdaloidal and porphyritic basaltic and andesitic lavas) and the Insu Member (m61ange). The limestone clasts are considered by Gafoer et al. (1994), to be derived from a fringing reef limestone on the continental foreland, but more likely were derived from a limestone reef fringing the island arc. A thick (500 m) chert unit (Situlanglang Member) is probably part of the oceanic assemblage.

82

CHAPTER 6

Origins of the volcanic units and their environments of formation Palaeozoic volcanism in Sumatra and the break-up o f Gondwanaland

The andesite and basalt flows in the Lower Member of the Kuantan Formation in the West Sumatra Block occur among distal turbidites and debris flows indicative of deposition in a deep-water environment, possibly in a forearc setting (Turner 1983). If these volcanics are contemporaneous with the sediments, they are Vis6an (Lower Carboniferous) in age. Volcanic rocks of this age are unusual in SE Asia and Australia (Veevers & Tewari 1995). The Kuantan Volcanism may be related to seafloor spreading in Palaeo-Tethys and be a precursor of the break-up volcanism along the margin of the Gondwana Supercontinent. Volcanics from the Gondwana Break-up Sequence are known from the dating of drill samples from the West Australian margin (Veevers & Tewari 1995) and crop out in Timor where they are stratigraphically well constrained (Charlton et al. 2002). These dated West Australian volcanics form a reference sequence for comparison with the Sumatran Permian volcanics (Fig. 6.15).

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Rhyolite clasts in the Late Carboniferous?-Early Permian Pebbly Mudstone facies of the Bohorok Formation in Sibumasu (East Sumatra Block) could be of any age, and plausibly were eroded from the same land area from which granite clasts in the mudstone also originated. A trondhjemite clast from the comparable Singa Formation on Langkawi Island, west of Peninsula Malaysia, has been dated at 1029 Ma (Hutchison 1989) suggesting a Proterozoic provenance. Volcanic rocks of the Condong Member of the upper Mentulu Formation (Bohorok Formation equivalent) and the Setiti plutons of the East Sumatra Plutonic-Volcanic Belt (c. 2 9 8 - 2 7 6 Ma) have a similar Permian Asselian-Sakmarian age, coinciding with the volcanic episode related to the breakup of the Sibumasu/Gondwana margin. The East Sumatra Plutonic-Volcanic Belt is of regional extent, being represented by volcanics in the Bohorok Formation of North Sumatra (Bennett et al. 1982c), and again by volcanic tufts which are widely distributed in the Mergui Series (comparable to the Bohorok Formation) around Mergui and Tavoy (Chhibber 1934; Pascoe 1959) and in islands offshore Peninsular Myamar. The East Sumatra Plutonic-Volcanic Belt is related in time to the fragmentation of Sibumasu from Gondwana, but a great deal more chemical and chronological data is required to amplify this suggestion.

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--

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BOHOROK & MENTULU FORMATIONS 'Pebbly Mudstones'

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(Tabir Formation) Volcanic

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Kluet volcanism ?related to sea-floor spreading in Palaeo-Tethys

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Fig. 6.15. The Permian sequence in Timor after Charltonet al. (2002) showingvolcanichorizonsrelated to the break-up of the Gondwanamargin and seafloorspreading in the Meso-TethysOcean. Sibumasu is understoodto have broken from Gondwana at the close of the Sakmarian (Metcalfe 1996) and the West Sumatra Block in the Triassic.

PRE-TERTIARY VOLCANIC ROCKS

West Sumatra Permian P l u t o n i c - V o l c a n i c Belt

It has been established that the Permian volcanics in the Mengkarang Formation in the West Sumatra Block, the Volcanic Member of the Silungkang Formation, and the Palepat Formation were erupted between the Asselian and the Artinskian and that basaltic volcanics in the Calcareous Member of the Silungkang Formation are probably Roadian. Radiometric dating suggests that some of the volcanics (the andesite-rhyolite sequence in the Volcanic Member of the Silungkang Formation and at Sibolga) are the extrusive equivalents of plutonic intrusions. The Ombilin granite is a foliated muscovite (?)S-type granite (McCourt et al. 1996) with a K - A r age of 287 • 3 Ma, corresponding to the Asselian Stage, and a younger R b - S r age of 256 _+ 6 Ma. The oldest intrusive phase in the Sibolga Granite Complex has a R b - S r isochron age of 264 • 6 Ma (Aspden et al. 1982b) and may be associated with the volcanics in the Kluet Formation. Three geological settings for the West Sumatra Permian Plutonic-Volcanic Belt have been proposed; an island arc, subduction-related continental margin arc, or continental breakup. The West Sumatra Permian Plutonic-Volcanic Belt is referred to as the 'Palepat Terrane' by McCourt et al. (1996) who discuss the suggestion by Wajzer et al. (1991) that the 'Palepat Terrane' represents an allochthonous oceanic arc which collided with Sumatra in the Late Permian or Early Triassic. This interpretation was adopted by Metcalfe (2000). The Palepat Terrane/ allochthonous oceanic island arc hypothesis is rejected by Barber (2000) on the grounds that oceanic volcanics and ophiolites have not been identified, nor is the 'Palepat Terrane' bounded along its eastern boundary by thrusts (Katili 1970), as had been supposed previously (Tobler 1922; Zwierzijcki 1930a). Cretaceous ophiolite outcrops shown within Early Permian sediments in the Solok Quadrangle by Gafoer et al. (1992a) are, according to Silitonga & Kastowo (1975), basaltic lavas interbedded within phyllites and quartzites of the Phyllite and Shale Member of the Kuantan Formation. These basalt outcrops are now considered to be an outlier of the Calcareous Member of the Silungkang Formation and are not associated with ultrabasic rocks, so their ophiolitic association is not established. Katili (1969, 1972, 1981) interpreted the Volcanic Member of the Silungkang Formation, the Palepat Formation and the associated granite suite, as relics of a continent margin magmatic arc of subduction origin. This interpretation is supported by the tholeiitic and calc-alkaline trends in these volcanics (Fig. 6.6) (Suwarna et al. 2000). The location of this magmatic arc in the palaeogeogeographic reconstruction (Fig. 14.11) would have been on the southern margin of the Cathaysian supercontinent (Fig. 6.2a), where it might have been related to a contemporary Permian magmatic arc in the Indochina Block of East Peninsular Malaysia described by Cobbing et al. (1992). A third alternative proposed by Suparaka & Sukendar (1981), is that the volcanics represent igneous activity associated with a passive continent margin. Charlton (2001), on palaeogeographic reasoning, has suggested that the West Sumatra Permian volcanics were related to the break-up at the Gondwana-Cathaysia interface. In this hypothesis the volcanism was associated with the thermal uplift of the Gondwana margin (Veevers & Tewari 1995) which coincided in the Asselian with the conclusion of the Gondwana glaciation and the start of sea-floor spreading in Meso-Tethys (Fig. 6.15). At this time the West Sumatra Block lay well to the north of the glaciated area (Fig. 14.11), so that the thermal uplift resulted in shallow-water deposition under tropical marine conditions. The geochemistry of the Silungkang and Palepat Formations as shown in the rock/chondrite normalized REE plots and the spidergrams of these volcanics (Suwarna et al. 2000) resembles similar plots for the Gondwana break-up volcanics identified in the Himalayas (Garzanti et al. 1989), and the REE pattern of the

83

dolerites and amphibolites from the Dili area of Timor (Berry & Jenner 1982). The timing and chemistry of the West Sumatra Permian Plutonic-Volcanic Belt suggest that it was linked both with subduction and continent margin faulting/seafloor spreading, but the chemical data do not discriminate which process was dominant at any particular time. This might be explained by the palaeogeographic setting of the West Sumatra Block between Cathaysia and Gondwana, where the Cathaysian margin subduction regime appears to have been affected by the break-up faulting of the Gondwana margin. This palaeogeographic setting ended when Sibumasu collided with the Indochina Block of Cathaysia in the Changsingian and Scythian (Metcalfe 2000). B e n t o n g - B i l l i t o n Accretionary Complex

The basic and ultrabasic meta-igneous and volcanic lithologies in the Riau-Billiton Permian Volcanic belt in north P. Billiton and P. Bangka, and those in west P. Batam and on P. Sugi are on the strike continuation of the Bentong-Raub collision zone. These rocks are components of an Accretionary Complex on the Palaeo-Tethys margin of the Indochina Block, derived from detached slices of the Palaeo-Tethys ocean floor, volcanic rocks, intrusions and sediments, all of which were deformed during the collision with Sibumasu. Volcanics in the Tin Islands appear to be Permian in age, but the complex as a whole contains sediments ranging in age from Late Devonian to Late Permian (Metcalfe 2000).

The Gondwana excursions and the Gondwana Margin break-up volcanicity

Charlton (2001) has a novel explanation of the Gondwana margin sequence of extension, uplift, associated magmatism, fragmentation and dispersal during the Permian, based on the study of the palaeomagnetism of Australia and its vicinity by Klootwijk (1996) (see Fig. 6.2b), Palaeomagnetic data indicate that eastern Gondwana made a northward excursion commencing in the Early Carboniferous, and reached low to moderate latitudes in the mid-Carboniferous, before moving southwards again in the later Carboniferous and Early Permian. The return phase of this excursion coincides with the rift-faulting, crustal extension, associated magmatism and fragmentation of Sibumasu from the Gondwana (Fig. 6.2a,b & 6.15). In this scenario Sibumasu did not drift away from Gondwana, as envisaged for example in the reconstructions of Metcalfe (1996), but was abandoned during the phase of crustal extension which accompanied the southward return of the Gondwana Supercontinent. The detachment of the West Sumatra Block from the area of contact between Cathaysia and Gondwana occurred later in the Triassic. By this time Sibumasu had collided with the East Malay Block resulting in the deformation of the Riau-Billiton Accretionary Complex. This event was accompanied by a second northward excursion of Gondwana in the Triassic, during which the West Sumatra Block was translated along the Medial Sumatra Tectonic Zone to arrive in its present position alongside the Sibumasu Block.

Triassic P l u t o n i c - V o l c a n i c belts in post-collision Sumatra

Extensive igneous activity took place during the Triassic in Sumatra and Peninsular Malaysia in both of which axial uplifts, resulting from successive collisions, were followed by extensional collapse (cf. Dewey 1988). This collapse led to sedimentation in faulted basins and grabens, beneath and between which, extensive granitic plutonism of the Main Range and Eastern Provinces

84

CHAPTER 6

took place in Malaya and in the Tin Islands off Sumatra. The only volcanic units related to this phase of plutonism which have survived, form the Pahang Volcanic Belt associated with the Eastern Granite Province of Peninsular Malaya. At the same time Meso-Tethys commenced subduction beneath Western Sumatra creating the continental margin West Sumatra Triassic Plutonic-Volcanic Arc. Some of these Triassic arc plutons were intruded into the (formerly) extensive limestone platform which formed at the Meso-Tethys ocean margin but few associated volcanics have been recognized (Cubadak Formation).

Jurassic-Cretaceous plutonism and volcanism

Towards the end of the Jurassic, before the accretionary margin of western Sumatra was firmly established, the Mid-Jurassic-Early Cretaceous was a time of extensive plutonism associated with volcanism of the continental margin Jurassic-Cretaceous Plutonic Arc. This magmatic pulse in Sumatra coincides with the rapid formation of the Pacific Plate (c, 1 7 5 - 1 7 0 M a , Bartolini & Larson 2001), which led to a world-wide flare-up of subduction magmatism. The rapid growth of the Pacific Plate (15 cm a -I) continued until the Oxfordian, when it reduced to 10 cm a -l. In Sumatra the Mid-Jurassic-Early Cretaceous Plutonic Arc dates from 169-129 Ma (McCourt et al. 1996) in the Meso-Tethyan Ocean and the Woyla Accretionary Complex incorporated oceanic seamounts dating from the Triassic and volcanic units derived from oceanic and continental sources (Figs. 6.16 & 14.16). Limited chemical data (Table 6.11) hints that on the basis of separation into high ( > 1%) and low (< 1%) TiO2 contents,

OCEANIC ISLAND ARC (arc assemblage) Andesitic volcanics and volcaniclastic sediments (Tambak Baru and Parlumpangan Volcanic Units)

r ' . . . , . . . . " ' ' " .

volcanic rocks from the Saling Formation of the Gumai Mountains include examples from the oceanic crust (high-Ti contents) while low-Ti samples represent volcanics of subduction origin, some high in Si and another high in Mg. Analyses of the Nabana Volcanics in the Batang Natal and from the Tapaktuan Formation are high in Ti, confirming the field identification of ocean-floor volcanics within these units (Fig. 6.14). Other volcanic units in the Woyla Accretionary Complex are suggested to be the remnants of volcanic arcs (Tambak Baru, Parlumpangan) but the absence of collision deformation suggests an alternative origin as volcanic centres intruded into the complex which were subsequently broken up by faulting. A reconstruction of the different depositional and volcanic environments within the oceanic assemblage of the Woyla Accretionary Complex is attempted in Fig. 6.16. The environments of the sedimentary units (Table 6.9) were appraised by Wajzer et al. (1991). Subsequent oblique subduction beneath the Woyla Accretionary Complex caused transcurrent faulting, which broke up and dispersed the component sediment and volcanic units as described by Wajzer et al. (1991). The large serpentinite bodies are fragments of the basal harzburgite layer of the ocean crust which have become detached from their volcanic and dyke carapaces as a result of their emplacement across the subduction complex and subsequent strike-slip faulting. The majority of serpentinite bodies in the Aceh area are of this type, but others, like the Pasaman Complex (Rock et al. 1983), and the various serpentinites in the NW corner of the Takengon Quadrangle (Cameron et al. 1983), are associated with large limestone outcrops. Such serpentinites may be the remnants of the foundations of uplifted oceanic plateaus with limestone caps (Wajzer et al. 1991) which collided with the subduction zone and were fragmented.

ACCRETIONARY COMPLEX (oceanic assemblage)

To the margin of > SUNDALAND - subducted beneath the Woyla Nappe in the mid-Cretaceous

UPPER TRENCH SLOPE BASIN greywackes LOWER TRENCH COLLAPSING (Muarasoma SLOPE BASIN SEAMOUNT Turbidite (Belok Gadang with olistostrome Formation) Siltstone (Panglong Formation) Melange TRENCH Formation) (Simarobu I Turbidite ] Forr~ation) $

9

, ' v v v

FOREARC BASIN volcaniclastic sediments and reefs (Rantobi Sandstone and Jambu Baru Formations)

/

(Triassic- mid-Cretaceous) oceanic lithosphere, ocean floor and pelagic sediments (Nabana Volcanic Unit Pasaman Ultramafic Complex)

Fig. 6.16, Cartoon reconstruction of environments of sediment and volcanic units within the Woyla Accretionary Complex of the Natal area. Sediment environments are as interpreted by Wajzer et al. (1991) and in Table 6.9, but do not represent a specific time frame.

PRE-TERTIARY VOLCANIC ROCKS

Fossil evidence indicates that the Bentaro string of island arcs began to grow within Meso-Tethys around the Jurassic Oxfordian stage. Their origin is shrouded in uncertainty, but Barber (2000) has suggested that they were generated along transform faults. Their formation may have resembled the origin of the I z u - B o n i n - M a r i a n a island arcs in the Eocene (Stern & Bloomer 1992). In this model, displacement along translbrm faults in the Pacific Plate juxtaposed oceanic crust and lithospheres of different ages, densities and thicknesses, which led to instability relieved by subduction within the ocean. Subduction led to volcanism and the growth of volcanoes, forming an oceanic island arc, which upon emergence above sea-level became surrounded by fringing reefs. The presence of at least one generation of island arcs within the Woyla Oceanic Volcanic Arc Assemblage has been deduced in NW Sumatra. Other large contemporaneous Tethyan oceanic island arcs include the Kohistan Arc of northern Pakistan (Treloar et al. 1996) which grew in the Mid-Cretaceous and the Spontang Ophiolite of the Ladakh Himalaya (Pedersen et al. 2001). The collision of the Bentaro-Saling Arcs and the associated oceanic crust carrying the Oceanic Assemblage of the Woyla Group with the West Sumatra margin of Sundaland had tectonic effects which reached into Peninsular Malaysia and beyond. However the Bentaro-Saling Arcs of Sumatra are relatively small and have not been up-ended compared to the contemporaneous giant Kohistan Arc of northern Pakistan which represents a deformed crustal section perhaps 4 0 k m thick (Hamilton 1988). The debate concerning the nature of the basement of the Bentaro Island Arc, whether continental (Cameron et al. 1980 and Pulunggono & Cameron 1984) or oceanic (Wajzer et al. 1991; Barber 2000), has already been alluded to. The Bentaro Arc was deformed and metamorphosed at low temperatures as a result of its forceful collision with the Sumatra margin. To date only a I%w localities of garnet amphibolite are known believed to be the exhumed products of subduction metamorphism (see Barber 2000 for details). The simplest explanation is that as a result of

85

the collision, the arc was detached from its oceanic basement, ramped onto the Sumatra continent margin, and so overlies thin continental lithosphere. This is demonstrated by the continent margin-type mineralogy of the Late Cretaceous (97.7 • 0.7 Ma) intrusion of the Younger Complex of the Sikuleh Batholith into the Bentaro Arc and the subsequent (Late Tertiary?) molybdenum mineralisation and drainage tin anomalies (Bennett et al. 1981b). The debate over the oceanic or continental origins of arcs is complicated by the discovery of a fragment of a continental arc within the Woyla Oceanic and Accretion Assemblage. In the Batang Natal section, severely deformed Si Gala Gala Schists represent volcanics with a more acidic (continental) source than the intermediate composition volcanics and volcanogenic sedimentary units of oceanic origin in the assemblage. The intense deformation in the Si Gala Gala Schists, compared to other units, may have been the result of a collision of a continental island arc with the accretionary margin (Wajzer 1986). Alternatively, and believed to be more likely, the Si Gala Gala Schists represent a relatively autochthonous fault-sliver of a local Sumatran volcanic centre, deformed as a result of fault movements. The intermediate composition Parlampungan Volcanic Unit is adjacent, and may be related to the Si Gala Gala Schists, but is not deformed. Wajzer (1986) suggested that it was a fault sliver transported from the continent margin Sumatra Arc by strike-slip faulting and became incorporated within the accretionary complex, but alternatively it is a variably deformed local volcanic centre with intermediate volcanics differentiated from oceanic basalts. In conclusion, the reconnaissance study of the Pre-Tertiary volcanics of Sumatra has already provided fascinating data assisting the understanding of the geological evolution of Sumatra. Further study of the volcanic rocks of Sumatra will lead to a better understanding of the history of the break-up of Gondwana, and the rearrangement of crustal blocks during collision and accretion processes throughout the Permian and the Mesozoic, with implications far outside Sumatra.

Chapter 7

Tertiary stratigraphy M. E. M. DE SMET & A. J. B A R B E R

The purpose of this account is to review the complex terminology of the Tertiary stratigraphic units in Sumatra and propose a revised and a simplified terminology based on the significance of formations for the tectono-stratigraphic development of the island. Formations are classified in terms of Pre-Rift, Horst and Graben, Transgressive, and Regressive tectono-stratigraphic stages. The island of Sumatra lies along the southwestern margin of the SE Asian continent (Sundaland) beneath which the Indian Ocean Plate is currently being subducted at a rate of about 7 cm a-1 in the Sunda Trench (Fig. 7.1). The continental margin of SE Asia is of Andean type, with active and inactive Quaternary volcanoes rising to over 3000 m above a Pre-Tertiary basement, exposed towards the west coast of the island in the Barisan Mountains. Tertiary sedimentary basins occur both to the SW and the NE of the mountains and small basins also occur within the mountain range itself. These basins are described with relationship to the present-day subduction system as forearc, backarc and intra-arc or intramontane basins (Fig. 7.1 ). The Barisan Mountains are transected by the Sumatran Fault System, a major dextral transcurrent fault zone which extends along the length of the island from the Sunda Strait to the Andaman Sea. Stratigraphic research in the Tertiary sedimentary basins commenced in the last decades of the nineteenth century when oil was discovered in the Telaga Tiga (I 883) and Telaga Said (1885) wells near Pangkalan Brandan in North Sumatra. Initially, wildcat drills were sited near oil seeps until systematic surface mapping commenced in the 1880s. Local stratigraphies in the oilfield areas were compiled from field outcrops by geologists working for the Bataafse Petroleum Maatschappij (BPM, now Shell) and the Nederlandsche Koloniale Petroleum Maatschappij (NKPM, later Stanvac) (van Bemmelen 1949). Five large and many small oilfields were discovered in Sumatra before World War II. Since the 1970s Sumatra has developed into a major oil and gas province. In the post-war period petroleum exploration has been based largely on borehole data and seismic reflection profiling. The seismo-stratigraphic units have generally been correlated with the main stratigraphic units which had been previously defined on the basis of outcrop descriptions and borehole data. A systematic compilation and correlation of the Tertiary stratigraphic units throughout Sumatra became possible through the mapping programmes of the Geological Survey of Indonesia (GSI), by the Geological Research and Development Centre (GRDC) and the Directorate of Mineral Resources (DMR), in association with the United States Geological Survey (USGS) and the British Geological Survey (BGS) carried out during the 1970s and 80s. These programmes were completed in the 1990s with the publication of forty-one geological map sheets at the scale of 1:250 000 covering the whole of Sumatra. The maps illustrate the distribution and extent of the outcrops of the Tertiary stratigraphic units and each map is accompanied by a booklet giving detailed lithological descriptions and age constraints for the units shown on the map. This account is up-dated from a study undertaken on behalf of the University of London Consortium for Geological Research in Southeast Asia (de Smet 1992).

Stratigraphic review The review of the stratigraphic terminology which has been used over the past hundred years for Tertiary sedimentary and

volcanic units in Sumatra is a formidable task. More than 200 stratigraphic groups, formations and members have been described and defined in the Tertiary of Sumatra; the majority of these names have been introduced as the result of the GSI mapping programme during the past few decades. Fortunately only about 15% of these names are in common use. Often, the regional relations of these units are not fully clear due to poor outcrop conditions and the difference in style of definitions used by the various research and exploration groups. Many of the units have been described only from localized areas and were never incorporated in the regional picture. A further problem is that names, definitions and classifications have been continually altered or revised as a result of subsequent work, and because of improvements in biostratigraphic age dating. Some of the changes in nomenclature and classification for the backarc, forearc and intra-arc basins are illustrated in Figures 7.2-7.4. Particular problems have arisen where units, which were originally described and defined from field outcrop, have been adopted by oil companies for time/rock units, defined by reflectors in seismic sections. During this process, facies variations that originally were regarded as separate formations on the basis of lithological data in the field outcrops, were incorporated within a single unit in seismostratigraphy. The ages of the earliest Tertiary sediments in Sumatra are generally poorly constrained, as the oldest units are commonly terrestrial deposits in which body fossils are exceedingly rare and palynological dating has often proved inconclusive. The earliest sediments are generally considered to be of Oligocene to earliest Miocene age, but in the absence of definitive fossil evidence an Eocene age is not precluded, and has been suggested in some areas. During the proliferation of stratigraphic terms for the Tertiary sediments of Sumatra, attempts have been made to simplify and rationalize the classification by developing hierarchical stratigraphic schemes. Oil companies use their own schemes of groups, formations and members in their concession areas, but these are rarely used consistently, and cannot be easily extended to cover broader areas. A scheme of classifying formations into groups and supergroups was developed during the GSI mapping programme and is used on the published GRDC maps. The scheme follows the recommendations of Hedberg (1976) and Whittaker et al. (1991). Groups are defined in a vertical stratigraphic sense, incorporating several successive formations, and are confined to the area of a single basin, while Supergroups link together units considered to belong to the same tectono-stratigraphic stage throughout Sumatra. In principle this may be a sound method of classification, but in practice the scheme was initially poorly applied, as the Tertiary II Supergroup covers what could be more sensibly classified as two distinct tectono-stratigraphic stages, awkwardly designated Supergroups IIa and IIb. The scheme has not proved sufficiently flexible to incorporate the flood of new data and continually revised interpretations. In the present account stratigraphic units are considered only at the formation level using the stratigraphic terminology given in Figures 7.2-7.5. Formations are described in terms of the tectono-stratigraphic stage that they represent in the history of the backarc, forearc or intra-arc basin in which they occur. Four distinct tectono-stratigraphic stages have long been recognized in the Tertiary sediments of the Sumatran backarc basins, and this scheme may readily be extended to cover the intra-arc basins within the Barisan Mountains. It may, however, only be

TERTIARY STRATIGRAPHY

I

~P

I

94 ~

~

87

I

96 ~

98 ~

ANDAMAN SEA

"~

GULF ................................................... .... / ~..... /

-6 ~

/

oF

/

[ NORTH . . . . . : ' : / SUMATRA 9 BASIN

Banda

"X

MALAY~"

N

NA,TUN A" ISLAND'S

-- 4~

\

N N

_2 ~

NA TUNA

"~

\

/

SEX

,

\ \

0

\ \

,~

Ni

A~

ISLANDS

/ _0

BATU ISLAND

o

\

LINGGA ISLANDS

\

,

~\ 2~

_

),N

t~ "5

~"

M EN TA ~W-#~ ISLANDS

k__.

Sumatran Tertiary basins outlins~, ~

4,,

N N

Sundaland continental crust

O

\~,

a,

N

Volcanoes

N

Sumatran Fault System _

~.

6 ~

0

\ "~

Subduction zone 100

200

94 =

96 ~

I

I

,

L._

INDIAN OCEAN

A

B'A\SIN >.. =

s IBERU~T

%\

_4

M ' B 1L 1N ~

<

0

300

400

\

ENGG2

\

\

500km 98 ~

I

1 00 ~

1 02 ~

I

I

\

Fig. 7.1. Structural sketch map of Sumatra showing the Tertiary backarc, forearc and intra-arc basins and localities mentioned in the text.

applied in modified form to the forearc basins, and is only applicable in the most general way to the forearc islands. The stratigraphic relations between this scheme and the most commonly recognized formations in Sumatra are shown in Figures 7.6-7.8.

Pre-Rift stage (Eocene) Sediments of the Pre-Rift stage are relatively poorly represented in Sumatra, but are more common elsewhere in Sundaland. Platform limestones that have been dated as Eocene occur unconformable on pre-Tertiary basement in Java, Sulawesi and Borneo. A comprehensive report on these limestones is presented in Wilson (2002). The units characteristically are distributed along the margin of the Sundaland pre-Tertiary basement and they clearly predate the subsequent formation of horst and graben structures.

In the earliest stages of sedimentation on Sumatra, Tertiary shallow-water continental margin sediments were deposited directly on the eroded surface of the Sundaland pre-Tertiary basement. Deposition followed a period of erosion considered to extend from the latest Cretaceous into the early Tertiary. In the backarc area these deposits, which include the Tampur and Meucampli formations (Fig. 7.2), are restricted to the North Sumatra Basin. In South Sumatra, Eocene Nummulitic limestones occur on the margins of the Bengkulu Basin (Gafoer & Purbo-Hadiwidjoyo 1986). In Central Sumatra, no formations are known from this stage, but their former presence is documented by reworked clasts of Nummulitic limestones in Early Tertiary conglomerates and melanges of the outer arc islands (van Bemmelen 1949; Budhitrisna & Andi Mangga 1990; Samuel et al. 1997). The Tampur Limestone of North Sumatra is described by van Bemmelen (1949), Cameron et al. (1980, 1982a),

88

CHAPTER 7

NORTH SUMATRA BASIN, DEVELOPMENT OF STRATIGRAPHIC TERMINOLOGY OPPENOORTH & ZWIERZYCKI 1917, VAN BEMMELEN 1949

APPROXIMATE AGE QUATERNARY

~ o

PLEISTOCENE

'

Julu Rayeu Fm

'~

Rotalia Sst Fm

Keutapang Formation

tu

e

Robulina Clay Intervening Sst

~, ~

Border Clay Peunulin Sst

MULHADIONO et al. 1978

~ J u l u

Rayeu Fm

Seureula Formation

Upper Baong Shale Middle Baong Sst I Lower Baong Shale

Baong Fm Peunulin Sst

Peunulin Sst ~

Peutu Formation

Peutu Formation

-

Belumai Formation Mica Sandstone

ILl

Seureula Formation

Keutapang Formation

Baong Formation Seumpo Sst Mb I

Black Mudstone

,,,,~

Present report, in part adapted from KIRBY et al. 1989

CAMERON et al. 1980, 1983

--

Seureula Formation

~.

z~O~

I

t,,

-

~ ~

:

Ii

F m Lignite Zone Fossiliferous Marl and Sst

6

~

Early GDRC publications

m ~,

~ ~'~ ~ _~ :~ "~ < ~=

KeutapangFormation

Z O 'r D 09 v O

Baong Formation Seumpo Sst Mb I Baong Formation Peunulin Sst Peutu " = Formation/ = % = ~~ ~ ~ ~ E'~ -=

Seureula Formation KeutapangFormation [

SecuraiShale

Baong Formation

~ ~ ~== "E ~-~

~

a. Peunu|in Sst O Peutu (.9 ~ Formation / UJ 2=~ J ~ ~o ~= .-~ 0 ~~ ; z

.~ ~ N~ ~~ = =

,,=, ~

Parapat

Parapat

Bampo Formation

Bampo Formation

Formation

Formation

Bruksah Formation

Bruksah Formation

~ Formation Meucampli ~ Formation ~

"x...F.ormation Meucampll"-~. Formation " ~

-I

I1,1

e, nl

Reefal Limestones

~ Formation Meucam.pli ~ Formation ~

~~ N D ea:

~ and Dolomite Meucamph~"'~-~ Formation

Fig. 7.2. The development of the stratigraphic terminology for the Tertiary of the North Sumatra Basin.

Bennett et al. (1981c) and Rusman Rory (1990). The formation comprises massive recrystallized limestones and dolomites with chert nodules. The unit has a basal limestone conglomerate and includes biocalcarenites and biocalcilutites. Van Bemmelen (1949) reports corals and coaly plant remains, and algal laminations may be seen in outcrops in the gorge of the Tampur River. These limestones were evidently deposited in a sub-littoral to open marine environment. Due to the absence of age-diagnostic fossils, the age of the Tampur Formation is poorly constrained, but is assumed to be of Eocene-Early Oligocene age based on its stratigraphic position and regional correlation (Bennett et al. 1981c). The Meucampli Formation crops out extensively in the northwestern parts of North Sumatra at the northern end of the Barisan Mountains, where it rests with major unconformity on the pre-Tertiary basement. The deposits are described by Bennett et al. (1981a), Cameron et al. (1980, 1983) and Keats et al. (1981). They comprise interbedded sandstones, siltstones and shales, with local intercalations of limestone and polymict and volcanic conglomerates. The sandstones show channeling, cross-beds and graded beds. The sediments were deposited in fluvial, coastal and restricted marine environments. Again, the age of the formation is poorly constrained, but is considered to be Eocene to Early Oligocene, based on its stratigraphic position. Equivalent formations are the Semelet and Kieme formations of Cameron et al. (1980), and Bennett et al. (1981c) distinguish a marine Meujeumpo Member, consisting of limestones, calcareous sandstones and shales, defined from the Meujeumpo River. From the Late Cretaceous to the Early Eocene the area of the Barisan Mountains formed part of a stable basement, extending northwards into the North Sumatra Basin and westwards into a continental shelf in the area of the present forearc basins, with the shelf margin near the present outer arc islands. Sedimentation on the margins of Sundaland in the Eocene, including in Sumatra,

is a first indication that the basement was affected by some regional change in tectonic regime after a long tectonically stable period. At this time also volcanoes were active in the Barisan Mountains, represented by the Breueh Volcanic Formation in the north (Cameron et al. 1980), and the 'Old Andesites' and Kikim Tufts of van Bemmelen (1949) in the south. Again the age of these volcanic rocks is poorly constrained.

Horst and Graben Stage (latest Eocene-Oligocene) In the late Eocene, or earliest Oligocene, continental margin sedimentation was brought to an end by the development of horst and graben structures throughout Sundaland. A similar sequence of events occurred not only in Sumatra, but also in many other areas, including the Java Sea, the Gulf of Thailand and the South China Sea (see e.g. Clure 1991 and Morley 2002b). The effect of this process on the landscape and sedimentation patterns was dramatic. The former Sundaland peneplain changed into a mountainous landscape with isolated deep, lakefilled basins in which terrestrial, fluviatile and lacustrine sediments, derived from the adjacent horsts, were deposited. Analogous landscapes at the present time include the present rift valley province in eastern Africa, as described by Morley (2002a), or the canyonlands of southeast Utah, as described by Trudgill (2002). In northern Sumatra marine influences persisted, but elsewhere the Horst and Graben Stage is represented stratigraphically by scree, alluvial fans and fluvial sediments that pass laterally into lake deposits. The sedimentation pattern was fault-controlled. Alluvial fans and fluvial deposits are sedimentologically immature and characteristically contain clasts of granite and metamorphic

TERTIARY STRATIGRAPHY

89

CENTRAL SUMATRA BASIN, DEVELOPMENT OF STRATIGRAPHIC TERMINOLOGY MUSPER 1937 VAN BEMMELEN 1949

APPROXIMATE AGE QUATER" NARY

PLEISTO" CENE i

d

J ~

I

! !

~-~J~"

(STANVAC) ~ ,

~ ~i J ~ -!-

i

e r Palembang Beds

Nilo Formation

,,,

Middle Palembang Beds

Korinci Formation

zm m m

Lower Palembang Beds

Binio Formation

w

m w

p

MERTOSONO & NAYOAN 1974 (PT CALTEX)

DE COSTER 1974

!

~

m

g

,,,

-~

Petani Formation

i

Bangko Fm (restr. marine) - ~

Minas Formation

J

Minas Formation

Petani Formation

Telisa

~ Mica Sandstone Formation

~

Minas Formation

Telisa Formation

PRAPTONO etal. 1989

CAMERON etal. 1983

Petani Formation

Telisa Formation

~ ! < ~ ~

Pematang Formation

Telisa F o r m ~ ~ I

~ (with several members)

Pematang Formation

~ ~ r

Sihapas Formation

TransitionFormation Menggala Formation

Brown Shale/ PEMATANG _Fro_ - / GROUP

Breccia

i

i I

Fig. 7.3. The development of the stratigraphic terminology for the Tertiary of the Central Sumatra Basin.

rock derived from the nearby basement. Lake sediments from this stage reach thicknesses of several kilometers, often indicating euxinic bottom conditions, and play a major role as source rocks in the Sumatran petroleum province. The age of sediments of the Horst and Graben stage is everywhere problematic as due to their terrestrial origin, age-diagnostic fossils are exceedingly rare. Palynological schemes have been used for stratigraphic correlation (e.g. Morley 1991) but due to reworking, age-dating based on palynology has often proved inconclusive. The age of the Horst and Graben sediments is constrained at a regional scale by underlying Eocene marine platform limestones and by overlying Early to Mid-Miocene marine shales. Published stratigraphic schemes show a range in age for the Horst and Graben deposits from Late Eocene to earliest Miocene. Age interpretations are rarely supported by biostratigraphic data other than by the age of the overlying marine shales. There may also be regional variation in the age of formation of the grabens but, for reasons mentioned above, this is difficult to prove. In the present account it is assumed that graben formation in Sumatra commenced in the latest Eocene and ceased in the Late Oligocene (Figs 7.6-7.8). In the North Sumatra Basin the rift sediments comprise the Bruksah and Bampo formations (Cameron et al. 1980) (Figs 7.2 & 7.6). Graben deposits from North Sumatra form an exception to the rule that most sediments from the Horst and Graben Stage are terrestrial in origin. Before the NW displacement of the forearc area along the Sumatran Fault System, commencing in the Mid-Miocene, the northern Sumatra area lay along the margin of Sundaland and subject to marine influences (see Chapter 14). The Bruksah Formation rests unconformably on the Pre-Tertiary basement and commences with thick basal breccio-conglomerates, representing alluvial fans, followed by light to dark grey, micaceous, poorly sorted quartz sandstone, siltstone and mudstone, with

local green tuffaceous quartz arenite and coarse tuff. Sandstones are commonly cross-bedded and may contain thin coal stringers and mussel bands. The Bruksah Formation varies greatly in thickness and is probably highly diachronous. It is interbedded with, and overlain by the Bampo Formation, which consists of poorly bedded, black, pyritic mudstone, locally interbedded with micaceous and carbonaceous sandstone and siltstone with a sparse fauna. Limestone nodules are locally abundant and tuffaceous intercalations also occur. Environmental conditions were ftuviatile, paralic and restricted marine. Pyritic mudstones indicate that water circulation to the open ocean was restricted by a barrier towards the west, allowing the development of euxenic conditions. In Central Sumatra rift sediments are represented by the Pematang and Kelesa formations. The Pematang Formation has sometimes been regarded as a 'Group' and subdivided into formations (e.g. Williams et al. 1985; Longley et al. 1990; Praptono et al. 1991), and as a formation it has been divided into a series of 'Members' (e.g. Lee 1982; Cameron et al. 1983). However classified, the sediments include a variety of coarse red, green grey and black breccias and conglomerates, with medium- to finegrained sandstones, claystones and shales, intercalated with coal seams. Environments of deposition are mainly continental: scree, alluvial fan, fluvial and lacustrine with locally euxenic conditions and minor marine incursions. The euxinic shales have a high organic content and include the Pematang Brown Shale, which is considered to be a good petroleum source rock. Deposition was, at least locally, interrupted by erosion, weathering and soil development, giving several internal unconformities within the succession. The Kelesa Formation was defined by De Coster (1974) and is used in Stanvac publications for the southern lateral extension of the Pematang Group. It includes a similar range of lithologies to the Pematang Formation, with the addition of tuffaceous shales, and in the Bengkalis Trough lacustrine shale with a high organic

90

CHAPTER 7

SOUTH SUMATRA BASIN, DEVELOPMENT OF STRATIGRAPHIC TERMINOLOGY MUSPER 1937

APPROXIMATE AGE PLEISTO-

QMATER-NARY CENE d

.

0

m I/A

o 8 g.

"

!

I

I

MARKS 1956

SPRUYT 1956

DE COSTER 1974 (STANVAC)

GAFOER et al. 1986 (GRDC)

III ~ -

II ] I J ~ '

Upper Palembang Beds

Palembang Mb

N ~

Middle Palembang Beds

Middle Palembang Mb

~~ ~ x~

Lower PalembangMb

~ % e.

9

Lower PalembangBeds

Kasai Tuff Formation Blue Mb

Limestone

Wood Horizon

~ r~

] ~ .~ ~ ~ ~ % ~

Brown Mb

Air Benakat Sandand Clay Formation

Upper Telisa Mb

Telisa Beds J

~

C~ ,~

~ ~

Gumai Shale Fomaation

] WelisaMb

N

Lower Telisa Mb

~ .[2-

Palembang

Kasai Formation

Middle Palembang

Muara Enim Formation

Lower Palembang

Air Benakat Formation

Telisa Formation

Gumai Formation

U

|

] Lilnestone Fm Transition Mb Gritsand Mb

~.~

Telisa

Limestone

c~

]Formation

~

~ "~ ~= ~ ~ ~

I

9 Talangakar Formation

~ ~

Talangakar Formation m<

z

2E55EEEXEEE Upper Kikim Tufts

...... ~

Lower Kikim Tufts

~"i i i .iiii

i

!iI !

II

Lemat Formation ...... ~ __ "Granite Wash"

Tuff-breccia Fomaation

Comple~ |

Lahat Formation

;i

ii ~i~

i :

i ]

I~,1 i

ijii ] 'I :I~

IKiki

:1i11: Iii

KikimTuffs

Fig. 7.4. The development of the straligraphic terminology for the Tertiary of the South Sumatra Basin.

content, containing fresh water gastropods and algae. Although lhe ages of all these sediments are poorly constrained, most publications suggest a Late Eocene to Early Oligocene age (e.g. Praptono et al. 1991; Heruyono & Villaroel 1989). In the South Sumatra Basin, rift deposition is represented by the Lahat and Lemat formations which have much in common with the Pematang Formation of Central Sumatra. The name Lahat (Series) was proposed by Musper (1937) and descriptions are given by Spruyt (1956), De Coster (1974), Hutapea (1981), Widianto & Muskin (1989), Hartanto et al. ( 1991) and Simandjuntak et al. (1991). The deposits, which outcrop in the foothills of the Tigapuluh and Duabelas mountains, include breccias, conglomerates and well-bedded greenish-grey sandstones, with volcanic intercalations along the basin margins. In the central areas of the basin, siltstones with tuffaceous shares are encountered in boreholes. The deposits rest unconformably on the basement; conglomerates contain clasts of slate, phyllite, metasandstone, marble, basalt, andesite and vein quartz derived from the basement. Environments of deposition range from scree, alluvial fan and fluviatile to fresh or brackish water lacustrine in the central parts of the basin. De Coster (1974) used the Lemat Formation as a synonym of the Lahat Formation. He distinguishes a coarse clastic member of breccias, conglomerates and sandstones, and a fine grained Benakat Member, composed of grey-brown shales, tuffaceous shakes, siltstones and sandstones with occasional thin coals, irregular carbonate bands and glauconitic units. Where beds of coarser grained material occur within finer grained units they are described as 'granite wash', the erosional product of nearby granites. They are sedimentologically so immature that outcrops of the transported product can often hardly be distinguished from the weathered in situ granite basement. Finer-grained units occur towards the central parts of the basin and in the upper part of the unit. The ages of

the Lahat and Lemat formations are given as late Mid-Eocene to Late Oligocene (NP16-NP24) by Sardjono & Sardjito (1989). For an understanding of the regional stratigraphy it is important to appreciate that at this stage the Barisan Mountains had not yet been uplifted and there was no separation between sedimentation in the backarc and forearc regions. Grabens of the Horst and Graben Stage cut across the area where the mountains now stand. The best studied example of one of these grabens is the Ombilin Basin near Solok in central Sumatra, which was subsequently uplifted and now forms an intramontane basin within the Barisans (Fig. 7.1). The Ombilin Basin, now at an elevation of 500-1100 m above sea level, has a stratigraphy which is directly comparable to that of grabens of the Central Sumatra Basin to the East. In the Early to Middle Miocene, however, this basin was still below sea level and receiving marine sediments (Ombilin Formation). In the Late Miocene marine deposition in the basin ceased, indicating that the uplift of the Barisan Mountains had commenced. Rift sediments in the Ombilin Basin are represented by the Brani and Sangkarewang formations. The Brani Formation was defined by De Haan (1942) from spectacular cliff exposures of red bmccias, conglomerates and sandstones, to the north of the main Ombilin Basin near Bukit Tinggi. A less well exposed hypo-stratotype, showing similar lithologies, was later defined by Koesoemadinata & Matasak (1981) in the Ombilin Basin. These authors distinguished two members: the Selo Member with sandstone turbidites in lacustrine shales, and a Kulampi Member, composed of upwards fining sequences. The Sangkarewang Formation was also defined by Koesoemadinata & Matasak (1981) and described as dark, grey, laminated shales, rich in plant debris, with fine- to very coarse-grained intercalations of quartz sandstone. The deposits commonly show convolute bedding and slumping on a large scale. Again the environments of deposition of the Brani and

TERTIARY STRATIGRAPHY

91

SUMATRAN FOREARC ISLANDS, STRATIGRAPHIC TERMINOLOGY BY ISLAND PAGAI AND SIPORA ISLANDS e.g. Budhitrisna & Andi Mangga 1990

APPROXIMATE AGE

QUATER-NARY PLEISTO"' CENE . dO

i'(='

.a

rI' i i' '

~ :~i = i,I

SIBERUT ISLAND e.g. Andi Mangga & Burhan, 1994

i i i ~~ i I' I J I~11 i , J i !

Simatobat Formation

! lJj

TELLO ISLAND e.g. Nas & Supanjono, 1994

II''

'i

unnamed

~

i

i ~I I I : ! I =! ~

Raparapa Formation

Kaleo Formation

Batumonga

"'

[ '~

I

Gunung Bala Formation

NIAS ISLAND e.g. Djamal et al. 1994 j ~.

. i . I.

~ ~1 l : i '

~ z

o

Saibi Formation

Maonai Formation

:

Sipika Formation

/

Dihit Sst Fm

~,~

,

I i i

I I

!1 i

lii

I'l:~

Layabaung / Sorit Fm

:4 Z

Ai Manis / Sibigo Limestone

Sigulai Formation

i

2.2 m

i="

Sinabang Formation

Hilihego Formation

Lelematua Formation Formation

II

aomo

-

,~

Ii

Gunungsitoli Formation

Formation Sagulubek / Marepan Formation

SIMEULUE ISLAND e.g. Endharto & Sukido. 1994 Situmorang et al., 1987

Pinang Conglomerate

Basal breccia?

i

tll z

l

8 w o

i Melange with Ultramafics

Tarikan Melange

Sigala Ultramafic Complex and Tanahbalah Metamorphic Complex

X Melange and Ophiolite Complex

~ 9 ~ 9 9

Baru / Umu Melange and Sibau Gabbro Group

ul LII

Fig. 7.5. Stratigraphic terminology for the Tertiary of the Sumatran Forearc Islands.

Sangkarewang Formations can be identified as scree, alluvial fan and lacustrine. Palaeogeographic models for the development of the basin were prepared by Whateley & Jordan (1989). The provenance of the sediments in the basin and its origin and structural development are discussed by Howells (1997a, b). Again, the ages of the sediments are poorly constrained, in spite of the discovery of fresh-water fishes in the Sangkarewang Formation; these proved not to be age specific. Repeated attempts to assign an age to these well-exposed and well-analysed Ombilin Basin sediments using palynology have also proved to be inconclusive. However, they are regarded as of Eocene to Oligocene age. Sediments of the latest Eocene-Oligocene rift stage are poorly represented by outcrop in the forearc region of Sumatra. Where present they are buried beneath deposits of the forearc basins, although the deeper parts of seismic sections from Meulaboh in the north (Beaudry & Moore 1985) and Bengkulu in the south (Mulhadiono & Sukendar Asikin 1989), show a faulted basement, suggesting that the forearc region was affected by the horst and graben stage of development in the same way as the rest of the basement. The deposition of the rift sediments was followed in the Late Oligocene by a change in the regional tectonic regime in which an area of predominant uplift, marked by the present Barisan Mountains, became contrasted with areas of continued sedimentation in the forearc and backarc basins. The change resulted in local inversion of graben systems with folding and thrusting of the rift sediments. Uplift and erosion resulted in a widespread unconformity when sedimentation recommenced.

Transgressive stage (Late Oligocene-Mid-Miocene) Following the change in tectonic regime in the Late Oligocene the whole region underwent regional subsidence in a sag phase, the

effects of which extended well to the east of Sumatra into Malaysia. At the same time the arc system of Sumatra started developing and the area of the Barisan Mountains became an important source of sediments for the forearc and backarc basins. The rate of subsidence was greater in the backarc area than in other areas. Initially sedimentation outpaced the rate of subsidence, with sediments transported over greater distances, so that the basins were filled with fluvial units which extended well beyond the margins of the original rift basins to rest unconformably on the basement horsts. For the first time in the Tertiary, rivers formed regionally interconnected systems that transported their sediment load to a few broad basins. Deltas extending westwards from Malaysia, and from the present Gulf of Thailand, controlled sedimentation in Central Sumatra. In North and South Sumatra and close to the present Barisan range the sources of sediments were more locally derived, although these sediments also show transport by river systems. Deltaic deposits may contain coals. Continued regional subsidence with the reduction of the size of eroding areas meant that subsidence outran sedimentation leading to marine transgression. Deposition in Sumatra subsequently changed to open marine with local deltas and characteristically with the local growth of reefs. The open marine deposits provide the oldest well age-dated units in the Tertiary of Sumatra. Their ages range from late Early to early Mid-Miocene. From the start of the transgressive stage in the latest Oligocene, the Barisan Mountains acted as a sediment source. This may not be obvious from wells drilled in the central parts of the backarc basins, which mainly show shales for this period, but is reflected in the fluvial deposits exposed in the foothills of the mountains. These deposits are sedimentologically too immature to be derived all the way fi'om Malaysia and they also contain tufts, reflecting that volcanoes were active in the range. The axis of

92

CHAPTER 7

SUMATRAN BACKARC BASINS TECTONO-STRATIGRAPHIC SCHEME AGE

REGIONAL TECTONOSTRATIGRAPHIC STAGES

NORTH SUMATRA BASIN

CENTRAL SUMATRA BASIN

SOUTH SUMATRA BASIN

Environment of deposition I lithology I comments Terrestrial: Sandstones and shales with volcanics

REG! S !merger Mount~ ~creasin

Coastal: Sandstones with coals and volcanics

Marine: Clays with major intercalations of sandstone

Ma Tran~ Marine: Clays with minor intercalations of ....--~ RANS S" bmerge untains eld ~ead~ clas Start of tnd first~ betwe= ~4ountair and ba( pRST ,~ S"

l

Deltaic

1

sandstones

I

.. Reefal

limestones

Terrestrial and deltaic: sheets of fluvial sandstones with coals

Terrestdal: Allivial fans and lake deposits tn North Sumatra Basin Area: restricted marine

Start ( PRE

In North Sumatra Basin Area: Carbonate platform and deltaic

:inal sta ci

Fig. 7.6. Generalized tectono-stratigraphy of {he Tertiary in the backarc basins of Sumatra. The diagram is highly simplified as most units interfinger and most boundaries are diachronous.

the mountain range remained an eroding area in the latest Oligocene, while the adjacent basinal areas were subsiding. It demonstrates that the structural separation between forearc basins, volcanic arc and backarc basins was in development. The influence of the Barisan Range as a sediment source area to the forearc and backarc basins was further reduced until the Mid-Miocene and remained small until the Late Miocene. This is because regional transgression initially outran the uplift of the mountain range. In the Middle Miocene only some volcanic peaks of the High Barisan were still above sea level while small deltas and reefs accumulated in the adjacent forearc and backarc areas (Figs 7.6 & 7.7). In the North Sumatra Basin, the extensive fluvial sediments from the early Transgressive Stage are represented by basal members of the Peutu Formation, in the Central Sumatra Basin by the Lower Sihapas and Menggala formations and in the South Sumatra Basin by the Talangakar Formation (Fig. 7.6). The marine sediments of the late Transgressive Stage are represented in the North Sumatra Basin by the Peutu Formation, the Belumai Formation and various reefal limestone units, in the Central Sumatra Basin by the Telisa Formation and the upper Sihapas Formation, and in the South Sumatra Basin by the Gumai Formation and Baturaja Limestones (Fig. 7.6). The Peutu Formation, comprising a wide range of lithological units of Early Miocene to earliest Middle Miocene age, was defined by Cameron et al. (1980) in the North Sumatra Basin. In the foothills of the Barisan Mountains the basal members are thick sandstone units of fluvial or shallow marine origin, while those in the upper part of the unit were deposited in a coastal to open marine environment. Cameron et al. (1980) interpreted the basal sandstones as a marginal facies to the marine members of the Peutu Formation. However, in this account the

basal units are taken to correspond to the extensive fluvial sands of latest Oligocene age which form the oldest transgressive units in the Central and South Sumatra Basins. The upper parts of the Peutu Formation are described by Cameron et al. (1980) as grey, calcareous and locally highly fossiliferous mudstones, often carbonaceous, and occasionally intercalated with thin limestones, turbiditic siltstones and fine sandstones. Several reefal limestone members are incorporated in the Peutu Formation: the Arun, Lho Sukon and Telaga Limestones. These limestones are of Early to Mid-Miocene age and contain an abundant fauna of corals and foraminifera and an algal flora. The limestones formed as reef build-ups on a series of NW-SE-trending en-echelon highs within the basin. Reef, near-reef and lagoonal facies have been described (Abdullah & Jordan 1987). These reefal limestones constitute the main gas reservoirs in northern Sumatra. Where sandstones are predominant in the Peutu Formation, Cameron et al. (1980) defined the sediments as the Belumai Formation, consisting of fine- to medium-grained sandstones, often glauconitic and sometimes carbonaceous, and shales, intercalated with reefal limestones, calcarenites and calcilutites which interfinger with the Peutu Formation and its limestone members. In the Central Sumatra Basin sediments of the Sihapas Group were originally described from outcrops in the eastern foothills of the Barisan Mountains where the group was divided into several formations (see Fig. 7.3). The lower formations consist of thick fluvial sandstones with varying amounts of intercalated shales. They include the Lakat Formation (or Lower Sihapas), which was defined by De Coster (1974) and the Menggala Formation, defined by Mertosono & Nayoan (1974). The sediments are fine- to coarse-grained sandstones with pebble conglomerates, local tuffaceous and coal horizons and subordinate

TERTIARY STRATIGRAPHY

93

BARISAN MOUNTAINS TECTONO-STRATIGRAPHIC SCHEME REGIONAL TECTONOSTRATIGRAPHIC

AGE

STAGES

HIGH BARISAN

INTRAMONTANE OMBILIN BASIN

QUATER- PLEI: PLEISTONARY CE CENE

d o

Environment of deposition I lithology / comments Major volcanism in High Barisan, Fast uplift and erosion

.a

tit

E - BARISAN FOOTHILLS

REGRESSIVE STAGE Emergenceof Baris; Mountains leads tc increasing clastic inl:

__•

Upwards increasing influx from High Barisan

Major volcanism and first extensive emergence of High Barisan

Maximum Transgression

TRANSGRESSIV STAGE Submergenceof Bari~ Mountainsand of Mala Shieldleadsto reductio clastic input

LU

,,=,

Start of regional sa( and first differentiatio between8arisan Mountainsand forear and backarc basins

Marine clays deposition in most areas. Only in the High Barisan small eroding islands remain. Slow subsidence and drowning. Local reef growth. Upwards decreasing influx from High Barisan. Volcanism and erosion in High Barisan, Fluvial sedimentation in wide basement depressions,

1

8 HORST AND GRAB STAGE

Local graben fills with terrestrial sedimentation, Erosion / non-deposition in most areas.

Start of faulting uJ

8

PRE-RIFT

Erosion / non-deposition

Final stage of stable craton

Fig. 7.7. Generalized tectono-stratigraphy of the Tertiary in the Barisan Mountains.

shales of fluvial to deltaic origin. The upper part of the Sihapas Group is dominated by marine sediments and is followed by monotonous brownish-grey and calcareous shales, thin glauconitic sandstones, siltstones and limestones of the Telisa Formation, deposited in an open marine environment, marking the maximum transgression (De Coster 1974; Cameron et al. 1983; Praptono et al. 1991). Seismic exploration in the centre of the Central Sumatra Basin later revealed that the upper Sihapas Group represented a delta and a braided river system. During this period the outlet towards the northeast was blocked by the Asahan Arch (Fig. 7.1) so that the area of the Central Sumatra Basin was occupied by the apex of a braided river system which carried sediments from the Malaysian Shield southwards across the Central Sumatra Basin into the South Sumatra Basin (Mertosono & Nayoan 1974; Wongsosantiko 1976; Heruyono & Villaroel 1989). Although these sediments have a completely different source from those of the type locality in the Barisan foothills, the stratigraphic nomenclature established in the Barisans was imposed on the remainder of the sediments of the Central Sumatra Basin. The sandstones of the Sihapas Group form the main reservoir horizons in the Central Sumatra Basin. The time equivalent Bangko Formation (Eubank & Makki 1981) is composed of marine shales. Marine shales of the Early to early Mid-Miocene Telisa Formation also overlie the Sihapas Group. This unit has a regional distribution over the entire Central Sumatra Basin and represents further marine transgression, with the reduction of the sedimentary source areas. In the South Sumatra Basin the Talangakar Formation corresponds to the Sihapas Group. Here sandstone units are thinner and finer grained, and alternate with claystones (Spruyt 1956). The rocks are described as greyish-brown channel sandstones, siltstones and shales, grading basinwards into light brown

carbonaceous shales with coal seams. The sandstones range from conglomeratic to very fine, are compact, slightly micaceous and include yellowish white tuffaceous layers. Pyrite, quantities of silicified wood and molluscs occur at some horizons. 'Granite washes' and sandstone turbidites, which provide good reservoirs for oil and gas, are particular characteristic of the Talangakar Formation. Environments of deposition range from fluvial and lacustrine to lagoonal and shallow marine. The source areas for these sediments lay in the Barisan, Tigapuluh and the Duabelas mountains. In the South Sumatra Basin the Talangakar Formation is followed by the Gumai (Tobler 1906; Spruyt 1956) and Baturaja (Musper 1937) formations. The Gumai Formation comprises a monotonous series of foraminifer-bearing grey shales and siltstones with thin intercalations of fine grained glauconitic sandstone and siltstone, and lenses of tuft'. Glauconitic sandstones and tufts become more important towards the Barisan Mountains. The Baturaja Formation is a thick and extensive platform limestone with local carbonate banks situated above basement highs. The platform limestones are glauconitic packstones and wackestones and contain thin shales. The carbonate build-ups are composed of skeletal packstones and coral-algal boundstones. These limestones extend eastwards into Java and the oilfields of the Java Sea. Distally the massive limestones pass into limestone beds intercalated with open marine shales. Within the Barisan Mountains, in the Ombilin Basin, the fluvial units are the Sawahlunto and Sawahtambang formations (Koesoemadinata & Matasak 1981). Breccio-conglomerates are developed where these units rests directly on the basement. The Sawahlunto Formation consists mainly of channeled sandstones, siltstones and shales, with interbedded coal seams up to 16 m thick. Environments of deposition range from alluvial fans, to meandering rivers with coal swamps (Koning & Aulia 1985;

94

CHAPTER

SUMATRAN REGIONAL TECTONOSTRATIGRAPHIC STAGES

AGE

QUATERNARY

PLE1STO

5

I

REGRESSIVE

a. [

]

STAGE

f -

I---I

tu ~

o

m ]~ I

I

]

I

Transgression [ T R A N S G R E S S VE

STAGE

]~ , I ~ [

Submergence of Barisan Mountains and of Malayan

,,', i

Sh,eld leads to reduction of

I

I

I

I I

[ ~. [

I , ~ ~ ". Z [ [

0

,~

I

tu o 0

t

s

7.8. G e n e r a l i z e d

~

/

,,,, ,

f'~r';.~r'~'ie"t~'~aC

it

-~'.."-Lu:_q

LI

I t'fftl l till

.I.'. / L e m a u

~

Start of regional sag

~

.

u

, ' : . ~ . ~ =..--. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . "'~':*'~'/~:':':':"Basai i

.

~

2

~ ~ "--""

Ittllllttlllllllllll:":':':..:

]

I I ltllllllllllllllt

Illtll :[I [I [ [ [t I t [I

1

I

Ii

1=:.:-:.'.'.'.'*',','.:.:,:.:,1 nasu 1 ~..,... /'.'.'.'. . . . . . Seblat ... C]astics":':':':-'~-":~:::::"-:':':" E u e n ' -':::-'":::[ :..'::':':':':t.":':'.":O:':':':

~

.

/

It111I

~

,

I

9

ultrabasic rocks

PRE RIFT

~

-

~

Final stage of stable

~ .

.

[

:::':.;" :":":"

I

t

~

~

. .

.

J .

.

_ .

.

~ .

.

[

I t:

.

.

.

.

.

.

.

.

Bathya!and shelfalse.q,ences.

cry Oeep m a n n e e n w r o n m e m s in some areas_

,., ~.,

'

:t

"." "- ,'1 "~" "-'4L2.,

"

.ro..,o,,;

.

.

.

/ r -t-

]

during transgressive stage

I Erosion / non-deposition in much of the forearc area,

nf ux of c astics from proto-Bar san MOL ntains

I

I Ememenceof } [ forearc slands9 [

'

T a m p u r Lst /

. / Slm0metl

]

I

I ~ro~ o,~on~e ~a.o~saod~ s

Metange formation in forearc islandsarea

i

9

u ~

~ ~1 ~ 1-1-r-tq-!3-~ .

I

I

v

" , :; | ~ : Nummuh.tcs Est.'.

.r ~ "

.

,.,

, non-deposition ] l l l

t~ ~ r

v

""............ "'"r-.'.,'~ "." |

~:'/':':1111

I

~ .

''"

'.,

IlllllrTPr-re_.2.ql ~ - I ['1

i

STAGE

.

,f,,

IIII

I

I

barus :':':':':1

I I FI ]hl';.~.'.'.'.'.'-'-'." 4

v~,sio./ I IIIllllll, Lros~o~, I lllll ~r nonde-p~itionllllllllll! non-depositio~: It111I ~.Seblat/t.oserXz~'~

"/i lit Iti1[ llil[[ I[[[il

theBarisanUountains

I llll

~..:::::::':::::::.:...-~.

~

.... rrt1r

' ' ' ',--' " !' ' ' , " ' I I I I I II I I I

I betweenBarisan !------~,J... ]Mountainsandforearc|

teclono-stratigraphy

,

~

~

t................ 7, "'''~-" /

, . - - . . . - - . . - - t ~ - ~ - ~ : . . - - . - - . , ~ ~ ~ l lltIItllllll = & : : . ~ ~ r, ,v~. ~ . _ . T e l i s a / G u m a i ] ]

clastic input

I and first differentiat,on

Environment of deposition 1 lithology /

'. . . . . . . . . . . . . . . . . . . . .

s

,--~----;----=----'72-- -':2;'";::';" "~.~.--,,~.--

.

Fig.

,, ,

1UJ.LL-'eL~"':~'r~,',-z=-'""='='r

'" [

wm

.';"~, "=;.=z~,.;-=.'~,.'~..-~. . . . . . . . . .

v':'sequences~::sequences~Turbidite }.}:" ShelfM :4:'7":':'5, ""~;'". . . . . . . . . . . . . . . . . ~ . . . . . . . . . ~ N-" -*,1",'-'.'.,." tJ I I I " " ~ " ~ " ' - " ~ ' " ' " .'-'.'.',','.'-',Wscouences ~.'sequellces ,'1",'.','.'." "4 I I

-

SCHEME

W - BARISAN FOOTHILLS

~ / d U I Ill :...-v::-=::...'7::..-v::.-v::7..:.:,:.:,:,:.:.:.~:::~:~:.YC~,tabakli

~

I

FOREARC BASIN OFFSHORE WEST SUMATRA ' '

~

Emer ence of Barisan

I I

TECTONO-STRATIGRAPHIC

comments

M- Jgn~tainot. . . . . . incre:sh'l- c~la~t~c~n~,ut

If'

AREA

FOREARC ISLANDS :

[ m ]

W

FOREARC

7

Ssts'

,

Carbonate platform

deposition, in forearc basin a r e a

.

o f the T e r t i a r y in the f o r e a r c a r e a o f S u m a t r a .

Whateley & Jordan 1989; Situmorang et al. 1991; Howells 1997a, b). Sections in opencast coal pits show listric growth faults, indicating that the area was undergoing extension during deposition of this unit. The overlying Sawahtambang Formation consists predominantly of thick sandstone units which are channelled and crossbedded on a large scale, with interbedded tufts and thin coal seams. The deposits extend beyond the limits of deposition of the underlying Sawahlunto Formation to rest directly on Pre-Tertiary basement. Basal breccio-conglomerates are composed of clasts of basement lithologies. Howells ( 1997a, b) recognizes local mismatches between clasts in the basal breccias and the immediately adjacent basement lithologies for the lower units of the sequence, indicating that strike-slip movement along strands of the Sumatran Fault System had occurred between the deposition of the lower and upper units. These deposits are interpreted as the products of a braided river system flowing across the area from the west (Whateley & Jordan 1989). Continued transgression of the Barisan Mountains led to further reduction of eroding areas and deposition of the monotonous open marine shales of the Ombilin Formation. The shales are dark grey, rich in foraminifers and contain thin intercalations of glauconitic sandstone. Locally a reef limestone with corals and algae, some 150 m thick, is developed over an area of several kilometres. The Ombilin Formation is dated as Early Miocene. In the western foothills of the Barisan Mountains, the area of the forearc basins and the outer arc islands, the Late Oligocene to Early Miocene transgressive phase is represented by a variety of formations composed of conglomerates and sandstones which rest unconformably either on basement or on older Tertiary deposits. These include the Loser and equivalent Sibolga formations (Cameron et al. 1980) in the north, the Seblat Formation

(Kusnama et al. 1993b) in Bengkulu to the south, the Barus Formation at Sibolga and the Kueh in the north, the 'Basal Clastic Unit' in offshore boreholes (Rose 1983) and the Pinang Conglomerate (Situmorang et al. 1987) in the outer arc island of Simeulue. In the forearc basins and the forearc islands unnamed turbidites and shelfal sequences, including several carbonate units were deposited at the time of maximum transgression on the mainland (Fig. 7.8).

Maximum transgression (Mid-Miocene) The maximum transgression of Sumatra in the Mid-Miocene is not distinguished here as a distinct tectono-stratigraphic stage, but this term is often used to indicate formations of maximum marine shale deposition and minimum clastic influx. In the maximum transgressive phase, subsidence outpaced sedimentation and the sea gained access to almost the whole area. Source areas in the Malayan shield were much reduced in size and relief and the Barisans were almost completely drowned, with the development of coral reefs in the Ombilin Basin. Eventually, even the reefal build-ups of the Arun, basal Telisa and Baturaja had been drowned and were sealed by marine shales of the Peutu, Baong, Telisa and Gumai formations. Many of these reels have become important reservoirs for oil and gas. In the North Sumatra Basin the Peutu and Belumai formations are overlain by the Baong Formation (Cameron et al. 1980; Caughey & Wahyudi 1993). The Baong Formation, of M i d Late Miocene age (N8-16), consists of a great thickness (7002500 m) of grey mudstones with thin muddy limestones, locally fossiliferous, with sandstone intercalations. Along the western margin of the basin the sands are derived from the Barisans.

TERTIARY STRATIGRAPHY

In the central part of the basin the Baong consists almost entirely of shale with one significant sandstone incursion, from the Malacca Platform to the east. This sandstone is of N 1 2 - 1 4 (Mid-Miocene) age and has been called the 'Middle Baong Sand' in this area. In seismic sections it is tbllowed by a regional unconformity. In the southern part of the North Sumatra Basin sandstone intercalations have also been called the Middle Baong Sandstones (Cameron et al. 1980). Here the sands fill incised valleys and are considered to have been derived fi'om the south (Syafrin 1995). In the subcrop of basinal areas the Baong shales are frequently overpressured, and locally, in the crests of anticlines, intrude the overlying Keutapang Formation diapirically, and erupt at the surface as mud volcanoes. Keats et al. (1981) estimated that a very rapid rate of deposition, of the order of 0.45 mm a-~, with the retention of fluids, was responsible for the development of the overpressure. The Baong shales form a seal to many of the oil and gas reservoirs in the North Sumatra Basin. In North Sumatra the transition from marine transgression to regression was originally interpreted to have occurred at a later time than in other areas of Sumatra. In the account of Cameron et al. (1980) the open marine Baong Formation was considered to represent transgression into the Late Miocene. However, Kirby et al. (1989) showed that the Middle Baong Sandstones (or Seumpo Sandstones) can seismically be correlated with the basal part of the Keutapang Formation at a more regional scale. The Lower Baong of Cameron et al. (1980) is therefore time equivalent to the upper parts of the Ombilin, Telisa and Gumai formations of Central and South Sumatra. The Middle Baong Sandstones and the Upper Baong Shale of Mulhadiono et al. (1978, 1982), together with the Securai Shale of Kirby et al. (1989) are all part of the Regressive Stage and for reasons of regional stratigraphic consistency should be considered part of the regressive Keutapang Formation. The amended stratigraphy is shown in Figures 7.2 & 7.6. This interpretation is not universally accepted, and may be appropriate only for the area studied by Kirby et al. (1989).

95

are terrestrial sands and clays with abundant volcanic debris: the Julu Rayeu Formation (Cameron et al. 1980) in the North Sumatra Basin, the Nilo (De Coster 1974) and the Minas (Cameron et al. 1980) formations in the Central Sumatra Basin and the Kasai (Spruyt 1956) in the South Sumatra Basin (Fig. 7.6). The climax of uplift and erosion of the Barisans occurred in the Late Pliocene and was accompanied by intense volcanism. This event coincided with inversion tectonics in the backarc area leading to the development of many structures which are now oil-bearing. These vertical movements were associated with small displacements along strike slip faults, parallel to the main Sumatran Fault trend and locally transecting anticlinal crests and displacing oil field structures (e.g. Minas and Petani Fields in Central Sumatra--Eubank & Makki 1981). Quaternary deposits rest unconformably on the eroded surfaces of these structures and consist of coarse conglomerates derived from the Barisan Mountains with a high proportion of volcanic debris in the neighbourhood of the Recent volcanoes, passing into fluvial deposits away from the motmtains and swamp deposits to the east along the shores of the Malacca Strait and the Java Sea. Offshore in the forearc basins, subsidence has continued to the present day, with deep sea clays and turbidites in the central parts of the basins and prograding shelfal sequences, with abundant volcanic debris, building out westwards into the basins from the Sumatran mainland (Beaudry & Moore 1985). In the outer arc islands deep water turbidite sequences, e.g. the Lelematua Formation (Djamal et al. 1994) of Nias are followed by shallow water deposits, often with carbonates, in the Late Miocene to Early Pliocene, as in the Gomo Formation of the same island (Djamal et al. 1994; Samuel et al. 1997). Deposition was followed by deformation, inversion and emergence with erosion in the Late Pliocene (Samuel et al. 1997). The Tertiary deposits as well as the uplifted we-Tertiary basement are overlain unconformably by uplifted Pleistocene coral reefs (e.g. Gunungsitoli Formation of Nias). Successive reef terraces in some parts of the outer arc islands contrast with drowned coastlines in other area (e.g. the east coast of Siberut), indicating that both uplift and subsidence are affecting the outer arc islands at the present day.

Regressive stage (Mid-Miocene-Present) in the Mid-Miocene, regional sag in Sumatra slowed down. While the forearc and backarc basins continued to subside, the Barisan Mountains emerged and became an important source of sediments. In the backarc basins from the late Mid-Miocene onwards turbiditic sandstones become an increasing component in the deep water formations. These turbiditic formations include the Seumpo, the Upper Baong and Keutapang of Cameron et al. (1980) in the North Sumatra Basin, the Binio (De Coster 1974) and Lower Petani (Mertosono & Nayoan 1974) in the Central Sumatra Basin, the Airbenakat (Spruyt 1956) in the South Sumatra Basin and unnamed turbidite sequences in the forearc area. A provenance study using heavy mineral suites by Morton et al. (1994) in the North Sumatra Basin shows that there was a major change in the source of clastic sediments in the Mid-Miocene from a granitic terrain to the east or SE in the area of the Asahan Arch and the Malay Peninsula, to the area of the Barisans to the west or SW, composed of pelitic rocks intruded by granites and volcanics, which was undergoing tropical lateritic weathering (diaspore). By the Mid-Miocene the Barisans had been uplifted and were in a position to act as a sediment source for the North Sumatra Basin. By the Late Miocene and Early Pliocene these deposits had passed upwards into shallow marine, sublittoral and deltaic sediments: the Seureula Formation (Cameron et al. 1980) in the North Sumatra Basin, the Korinci (De Coster 1974) and Upper Petani (Mertosono & Nayoan 1974) in the Central Sumatra Basin and the Muaraenim Formation (Spruyt 1956) in the South Sumatra Basin (Fig. 7.6). By Late Pliocene the dominant deposits

Summary The pre-Tertiary basement of Sundaland extends to the west across the present forearc as far as the outer arc islands to the west of Smnatra as indicated by metamorphic rocks in Tanahbala (Nas & Supandjono 1994). During the Late Cretaceous the whole of the Sumatran basement was exposed to erosion. In the Eocene at least parts of this basement was covered by shallow seas in which platform carbonates were deposited, represented by the Tampur Limestone in northern Sumatra, Nummulitic limestones near Benkulu in southern Sumatra, and clasts of these limestones in found in conglomerates in the outer arc islands. In the Late Eocene to Early Oligocene the basement, as in much of Sundaland, was subject to extension, forming a pattern of horst and graben which controlled stratigraphic development, with sedimentation in isolated rift basins derived from the erosion of the intervening horsts. These rifts extended across the area of the present Barisan Mountains (Ombilin Basin) into the forearc region (e.g. Bengkulu). This same history is evident throughout much of Southeast Asia with the development of rift basins in the Sunda Shelf, Borneo, the Malay and Gulf of Thailand Basins (Longley 1997) and extending into northern Thailand (Polachan et al. 1991). This regional extension coincided with the collision of India with the southern margin of the Asian continent and has been attributed to the extrusion and rotation of continental blocks to the southeast of the site of collision (Tapponnier et al. 1982). During the Horst and Graben Stage deposition in Sumatra was characterised by sediment transport over short distances,

96

CHAPTER 7

while subsidence in the grabens was faster than sediment input, leading to the accumulation of thick organic-rich lake deposits with sedimentologically immature sediments along the lake shorelines. In Sumatra this localized distribution of the sediments in the rift stage is reflected in a localized stratigraphic nomenclature. Although the thick euxinic lake deposits and paralic deposits in the grabens play an important role in the petroleum geology of the backarc basins, the grabens themselves preceded the origin of the basins as a whole. In the latest Oligocene there was a major change in the regional geography. Regional sediment source areas and broad depositional areas replaced the former horst and graben landscape. In addition to the source area to the north, in the Malayan Shield, the Barisans provided one of the sediment sources. The conclusion is supported by the significant amount of volcaniclastic material in the latest Oligocene sediments and by the occurrence of sedimentologically immature deposits of this age in the foothills of the Barisan Mountains. The stratigraphy reflects the development of wider basins that extended across both grabens and horsts alike, and interconnected river systems that transported sediments from larger and more distant source areas. The thick overburden of younger sediments in the backarc basins induced maturity in organic material in petroleum source rocks within the grabens, and provided the sands and limestones which constitute the main reservoir horizons for oil and gas. Again, similar environments extended throughout Southeast Asia (Longley 1997). The conclusion that the Barisan Mountains commenced their development as a major structural element in the latest Oligocene is at variance with much of the literature emanating from the petroleum industry. It is considered that the Mid-Miocene turbidite formations represent the first significant influx of sediments into the backarc basins from the Barisan Mountains, the major influx occurring during the Pliocene. There is no contradiction, however, between these two interpretations. In the Late Oligocene the Barisan Mountains were still restricted in height and extent. Following the transgression in the Early to Mid-Miocene the emergent peaks became even more restricted. The major MidMiocene to Pliocene sediment influx from the mountains into the backarc basins was due to the further growth and re-emergence of the Barisans during the regressive period, rather than to their first appearance. Transgression during the latest Oligocene and Early Miocene was the consequence of regional sag, not only in the area of Sumatra but throughout much of Sundaland (e.g. in the Gulf of Thailand). In Sumatra the forearc and backarc basins deepened and the early Barisan Mountains were almost submerged. From the Mid-Miocene onwards uplift of the Barisan Mountains and the forearc island area was faster than the continuing regional sag which caused further subsidence along the axes of the backarc and forearc basins and also in the Gulf of Thailand. These movements coincide with the inversion of basin sediments during the Miocene, and continue through the Plio-Pleistocene, with the re-activation of faults, the folding of basin sediments and the development of unconformities in the sequence. These movements may be related to variations in the angle and rate of convergence in the Sumatran subduction system, leading to extension or compression in the backarc (Cameron et al. 1980). They also coincide with activity of the Sumatran Fault System in the Miocene and continued transtensional and transpressional movements along it from then until the present day. Similar inversions in other parts of SE Asia have been attributed to the rotation of Borneo (Hall 2002) or the far field effects of collisions in Eastern Indonesia. The extent to which sedimentation in the Tertiary Basins of Sumatra has been influenced by the development of Sumatran Fault System is not fully understood. The Fault System is connected to the spreading centre in the Andaman Sea to the north, across which 460 km of displacement is considered to have taken place (Curray et al. 1979), and to pull apart structures

in the Sunda Strait in the south, along which only minor displacements of the order of 10 km have occurred (Malod et al. 1996). Direct measurement of displacement across the fault in Sumatra has proved difficult as most stratigraphic units trend parallel to the fault trace. Possible offsets of 45 km on the basis of the displacement of Permian granites (Hahn & Weber 1981a) and of up to 100 km from displacement of Tertiary basins (Beaudry & Moore 1985) have been postulated for various strands of the fault. It is probable that movement along the fault system have been taking place continuously at least since the Mid-Miocene (14-11 Ma) when spreading in the Andaman Sea is considered to have commenced (Curray et al. 1979). Presumably, movements along various parts of the fault system have continued from the time of initiation of the fault system until the present day. Recent movements are shown by displacement of Recent volcanics (Posavec et al. 1973), by the offset of stream courses (Katili & Hehuwat 1967), by continued seismic activity, by displacement of recent sediments along the fault trace (Sieh et al. 1994) and by GPS measurements (McCaffrey 1996; Sieh & Natawidjaja 2000). The difference in relative displacement at either end of the fault system shows that the forearc area was stretched over time and not displaced as a rigid block. Displacement increases progressively northwards and is considered to have occurred by cumulative strike-slip movements along a fault system oriented in a S S E - N N W direction throughout the forearc region (Curray 1989; McCaffrey 1996). In this account it is presumed that the origin of the Sumatran Fault Zone coincided with the development of Barisan Mountains and the backarc and forearc basins in the Late Oligocene. All these regional structures have a N N W - S S E trend and are overprinted over horst and graben structures that have a more north-south trend. The Barisan Mountains acted as a sediment source area from the latest Oligocene onwards and therefore it is presumed that transcurrent movements along the Sumatran Fault trend started at about the same time. A latest Oligocene age for first movements along the fault system does not conflict with a MidMiocene age of spreading in the Andaman Sea as documented by Curray et al. (1979) because extension with movement along the fault traces in that area may have occurred long before the first ocean floor spreading. The reconstruction suggests that the forearc region has extended some 460 km northwestward, relative to the rest of Sumatra, over the last 25 Ma and that the rate of extension has been at a uniform rate of about 1.8 cm a There is an obvious anomaly in North Sumatra in that during the Late Oligocene and Early Miocene the Barisans was an area of eroding terranes and shallow water facies, while deep-water marine facies prevailed in the central parts of the North Sumatra Basin. It appears that there was no landmass immediately to the SW of the North Sumatra Basin which could provide a source area. Evidently the Barisan area was only moved into its present position relative to the north Sumatra Basin to provide a sediment source after the Middle Miocene. On the other hand thick Early Miocene sandstones in the Central and South Sumatra Basins indicate that at that time the Barisan source area lay much further south. In their provenance study of the Keutapang Formation in the North Sumatra Basin Morton et al. (1994) found that the sediments were derived from the west or the SW. Evidently the Barisans were uplifted and in a position to act as a source for the North Sumatra Basin by Middle Miocene times. They also found that chrome spinel was abundant in the lower part of the Keutapang Formation, but rare in the upper Keutapang. This spinel must have been derived from an ophiolitic terrain, but there is no such terrain in a suitable position at the present time. The Pasaman ophiolite is too far south, and the northern Aceh ophiolites are too far north. Either the ophiolite which supplied spinel to the lower Keutapang Formation has been removed completely by erosion, or it has been moved northwards since the Middle Miocene by dextral movements of the order of 100 km along the Sumatran Fault System (Morton et al. 1994).

TERTIARY STRATIGRAPHY The removal of the displacement on the Sumatran Fault System gives the southwestern continental margin of Sundaland a much smoother outline in the Early Oligocene and Eocene. At that time the North Sumatra Basin and its rifted grabens lay along continental margin, rather than within the continent. With the north Sumatra basin in this position it becomes clear why this is the only backarc basin that contains Eocene shallow marine continental margin deposits, including platform limestones. Important conclusions derived from this stratigraphic analysis are: the Sundaland pre-Tertiary basement extends across the

97

area of the forearc basins to the Sumatran offshore islands; the Barisan Mountains first emerge as a structural element providing a source area for clastic sediment in the latest Oligocene, and not in the Middle Miocene as many authors have supposed. Taking into account the dextral movements along the Sumatran Fault System, replacing the displaced forearc and the southwestern segment of the Barisans, simplifies the outline of the Sundaland Margin and accounts for the occurrence of marine sediments in the early stages of the development of the North Sumatra Basin in their original positions (see Fig. 14.18a).

Chapter 8

Tertiary volcanicity M. J. CROW

The Centenary of the Netherlands Indies Geological Survey was commemorated by the publication of a synthesis of the geology of Indonesia by van Bemmelen (1949). In his account of the geology of Sumatra van Bemmelen (1949) described three distinct, but continuous, cycles of volcanic activity during the Tertiary and Quaternary: Old Neogene (Late Oligocene-MidMiocene); Young Neogene (Mid-Miocene-early in the Quaternary); and Young Quaternary. The first cycle began with the 'Old Andesites', and ended with the Mid-Miocene uplift of the Barisan Mountains. The second cycle commenced with the eruption of basic igneous products and concluded with an acidic phase which coincided with a second episode of uplift of the Barisan Mountains. Subsequently, knowledge of the Tertiary volcanic rocks in Sumatra has been refined as the result of programmes of geological mapping in the early 1970s by the Geological Survey of Indonesia and the United States Geological Survey, and between 1975 and the mid-1990s by the Geological Research and Development Centre, the Directorate of Mineral Resources and the British Geological Survey. Exploration by oil and mineral companies has also provided data concerning the distribution of Tertiary plutonic rocks in the Pre-Tertiary basement and in the Tertiary sedimentary basins, of volcanic units interbedded with sediments. Further contributions to the understanding of Tertiary volcanicity in Sumatra and its forearc islands have been made by academic researchers and post-graduate students from the Institute of Technology, Bandung and the University of London, in collaboration with the Geological Research and Development Centre, LIPI and LEMIGAS, and the British Geological Survey. Most of the Tertiary volcanic and volcaniclastic formations in Sumatra are identified on the Geological Maps published by the Geological Research and Development Centre and are described in tables in the Explanatory Notes which accompany the maps. A summary of the volcanic units in Northern Sumatra, with brief descriptions, were given by Cameron et al. (1980), while Rock et al. (1982) described their petrology and chemistry. McCourt et al. (1993) and Kusnama et al. (1993a) summarized the stratigraphy of Southern Sumatra, including the volcanic units. Rock et al. (1982) distinguished at least four climaxes of volcanism in the Tertiary of Northern Sumatra: Palaeogene (possibly Eo-Oligocene); Late Oligocene-Early Miocene; Early MidMiocene; and Mid-Late Miocene. In the present account Tertiary volcanic episodes and phases recognized in the whole of Sumatra occurred during the Palaeocene; Late Mid-Eocene; Late EoceneLate Oligocene (Late Eocene-Early Oligocene and Late OligoceneEarly Miocene phases); Late Early Miocene-Mid-Miocene (Late Early Miocene and Mid-Miocene phases); and Late MiocenePliocene. The relationship between volcanic episodes and phases and the stratigraphic succession in Sumatra is illustrated in Figure 8.1, which is based on the stratigraphy and terminology proposed by De Smet & Barber in Chapter 7.

Radiometric dating of volcanism and plutonism in Sumatra Bellon et al. (2004) report nearly 80 4~176 age dates of the volcanics and associated intrusives, for the period 6 5 - 0 Ma.

98

Their, and earlier, K - A r age determinations are listed in Table 8.1 and ages dates of plutons, dykes and volcanics are compiled in Table A.4 (Appendix). Mineral ages from fresh samples give ages younger than the time of intrusion, but give useful information on the cooling of igneous rocks through the c. 500 ~ (hornblende) and the c. 400 ~'C (biotite) isotherms. These age data are also helpful in distinguishing the effects of thermal and tectonic alteration. Macpherson & Hall (1999, 2002) have drawn attention to the problems of the interpretion of K - A r isotope data. The limitations of the K - A r dating method are due to problems of tectonic and thermal alteration and to tropical weathering, as these processes may reset the K - A r clock to yield misleading younger ages, or add potassium and 4~ to give spurious older ages (Dickin 1995). In her study of the timing of the alteration of intrusions connected to movement of the Sumatra Fault Zone in southern age dating Sumatra, Imtihanah (2000) used the 4~ method, which can identify K and Ar mobility in altered rocks.

Tertiary volcanic stratigraphy P a l a e o c e n e v o l c a n i c e p i s o d e ( T a b l e 8.2 a n d Fig. 8.2)

The informal term Kikim Volcanics (McCourt et al. 1993) is used here for the Palaeocene volcanics and volcaniclastics which occur in southern Sumatra. Previously Gafoer et al. (1992c, 1994), and the 1"1000 000 geological maps of Southern Sumatra (Gafoer et al. 1992a, b), used the term 'Kikim Formation' for all volcanic rocks of Palaeocene to Oligocene age in southern Sumatra. De Coster (1974) suggested that the Kikim Tuffs were of Upper Cretaceous to Palaeocene age, but no Cretaceous ages have been obtained from these rocks. The Kikim Tufts, comprising tuffaceous sandstones, conglomerates, breccias and clays, were encountered in boreholes at the base of the Tertiary succession in the South Sumatra Basin (Lemat-1, Lemat-2 and Tamiang-2 wells), in the Laru wells on the Musi Platform and cropping out in the Gumai Mountains. The volcanic rocks in the Tamiang-2 well were dated at 55 Ma (Palaeocene) by the K - A r method, but details of the analysis are not available. McCourt et al. (1993) report that in the Gumai Mountains Gafoer et al. (1992c) found a transition, rather than an unconformity, between the Kikim Formation and the overlying volcaniclastic Lahat Formation. This underlying unit is now considered to be part of the Lahat Formation, confirming the stratigraphic scheme in the Gumai Mountains originally proposed by Musper (1937). A K - A t age date of 63.3 + 1.9 Ma (Palaeocene) was obtained from an andesitic lava in the Kikim Volcanic Formation ( < 300 m of andesites, volcanic breccia and tuft) at Gunung Dempu in the Kotaagung Quadrangle (Amin et al. 1994b). A K - A r age of 60.3 Ma has been obtained from a basalt (location uncertain, oral communication by Pulunggono in 1985, reported in Gafoer et al. 1992c) in the Kikim Volcanics to the east of the Garba Mountains which are described by Gafoer et al. (1994) as 'often being highly tectonized'. In the Garba Mountains the Kikim Volcanics include volcanic breccias, welded tufts and andesitic to basaltic lavas with sedimentary intercalations (Gafoer et al. 1994).

TERTIARY VOLCANICITY

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100

CHAPTER 8

Table 8.1. T e r t i a r y v o l c a n i c e p i s o d e s a n d r a d i o m e t r i c a g e s f r o m v o l c a n i c r o c k s in S u m a t r a Volcanic

Type

Dating method

PALAEOCENE VOLCANIC EPISODE (65-c.50 Ma) Basalt tufT, Bentaro Volcanic Formation (LM 116A) Basalt dyke in Lhoong Formation (LM 124) Basalt flow, south-west of Banda Aceh (LM 118) Basalt dyke in Bentaro Volcanic Formation

LK MK MK

4~176 4~176 4~176 4~176

51.3 55.5 57.9 63.1

Basalt dyke, Natal area (SU 49) Andesite dyke in Woyla Group, Batang Natal (NL 41) Basalt dyke, Tambak Baru Volcanics (NL 40)

SH HK MK

4~176 4~176 4~176

52.1 • 1.2 59.6 _ 1.4 62.5 ___ 1.4

Bellon et al. (2004) Bellon et al. (2004) Bellon et al, (2004) Bellon et al. (2004), Sutanto (1997) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

Gabbro dyke in Silungkang Formation (RDC 1l) Basalt flow, Silungkang Formation (RDC 13A2) Basalt flow, Silungkang Formation (RDC 13A1)

MK LK LK

4~176 4~176 4~176

62.9 • 1.5 63.1 • 1.5 63.7 • 1,5

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

Andesite, Gunung Dempu Basalt, Garba Mountains Tuff, Tamiang 2-well

K-Ar, whole rock? K-Ar, whole rock'? K-Ar, whole rock?

63.3 • 1.9 60.3 55

Amin et al. (1994b) Gafoer et al, (1994) De Coster (1974)

LATE MIDDLE EOCENE VOLCANIC EPISODE (c.46-40 Ma) Andesite dyke, Langsat Volcanic Formation (NL 36) Basalt dyke, Indarung Calcareous Formation (RDC 20) Shoshonite dyke, Tanjungkarang area (PCE 13)

MK SH SH

4~176 4~176 4~176

41.1 • 0.9 45.8 _+ 1.1 43.5 • 1

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

LATE EOCENE-LATE OLIGOCENE VOLCANIC EPISODE (c.38-24 Ma) Late Eocene-Early Oligocene Volcanic Phase (c.38-30 Ma) Basaltic andesite dyke, Blang Pidie, Tapaktuan (TT 148) MK Basalt dyke, Langsat village, Natal area (NL 37) SH Basalt dyke in Silungkang Formation (RDC 13) LK

4~176 4~176 4~176

31.6 _+ 0.85 37.4 • 0.9 37.3 • [

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

Late Oligocene-Early Miocene Volcanic Phase (c.30-24 Ma) Basalt dyke in Woyla Group north of Tapaktuan (TT 144)

MK

4~176

26.9 • 0.72

Bellon et al. (2004)

Basalt flow, Painan Formation (PN 26)

SH

4~176

23.7 • 0.55

Bellon et al. (2004)

Andesite dyke in Painan Formation (TP 34) Dacite dyke in Painan Formation (TP 33)

MK MK

4~176 4~176

24.3 • 0.60 25.5 • 0.59

Bellon et al. (2004) Bellon et al. (2004)

4oK _ 4t~Ar 4OK_ZOAr

18.8 • 0.49 14.5 • 1.17

Bellon et al. (2004) Bellon e t a l . (2004)

21.4 • 0.59 21. l _+ 0.60 18.7 • 0.44 18.8 • 0.59 [ 8.8 • 0.45 18.3 + 0.44 17.7 + 0.7 17.5 _+ 0.42 17.1 + 0.9 16.4 • 0.6 16. I • 3.9 15.9 • 1.0 15.0 _+ 0.38

Bellon et al. (2004) Beilon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Kallagher (1990) Bellon et al. (2004) Kallagher (1990) Kallagher (1990) Kallagher (1990) Kallagher (1990) Bellon et al. (2004)

LATE EARLY MIOCENE-MIDDLE MIOCENE VOLCANIC EPISODE Late Early Miocene Volcanic Phase (c.22-14 Ma) Basalt block in Indrapuri melange, Banda Aceh (IP 113) LK Basalt dyke in Lhoong Formation (LM 126) LK

4~176 4~176 4~176 4~176 4~176 4~176 whole rock 4"K-4~ whole rock whole rock whole rock whole rock 4~176

Age (Ma)

• • • •

1.5 1.5 1.4 1.5

Reference

Basalt flow, in Calang Volcanic Formation (CL 140) Andesite dyke, Calang area (CL 135C) Andes• dyke, Calang area (GB 15) Basalt dyke in Tangla Formation (CL 135B) Basalt flow in Calang Volcanic Formation (CL 141A) Andesite dyke in Calang Volcanic Formation (CL 132) Basalt, Sayeung Volcanic Formation Andesite dyke in Tangla Formation (CL 136) Basalt, Sayeung Volcanic Formation Basalt dyke, Sayeung Volcanic Formation Basalt, Sayeung Volcanic Formation Basalt dyke, Sayeung Volcanic Formation Basaltic andesite dyke in Calang Volcanic Formation (CL 131 ) Basalt, Sayeung Volcanic Formation.

MK MK MK MK MK MK K-At, MK K-Ar, K-At, K-At, K-At, MK

K-Ar, whole rock

13.7 _+ 2.7

Kallagher (1990)

Andesite dyke in Barus Formation, Sibolga (SB 27B) Andesite flow in Angkola Volcanic Formation (SB 85)

MK MK

4~176 4~176

19.6 • 0.58 18.2 • 0.45

Bellon et al. (2004) Bellon et al. (2004)

Andesite dyke in Angkola Volcanic Formation (SB 84) Andesite dyke in Angkola Volcanic Formation (SB 83) Andesite, P. Musala

MK 4~176 MK 4~176 K-At, whole rock

16.8 • 0.47 16.8 • 0.39 17.2 • 5

Belion et al. (2004) Bellon et al. (2004) Aspden et al. (1982b)

Basalt meta-tuff, Simpang Gambir, Natal area (NL 42) Absarokite in Sikarara Volcanic Formation (NL 34)

MK SH

19.7 • 0.48 18.2 • 0.44

Bellon et al. (2004) Bellon et al. (2004)

4~176 4~176

(continued)

TERTIARY VOLCANICITY

Table 8.1

101

Continued

Volcanic

Type

Dating method

Age (Ma)

Andes• Sarik Lawas Andesite flow in Painan Formation (PN 31) Andesite flow in Painan Formation (PN 22) Basalt flow in Painan Formation (PN 24)

K-Ar, ? MK HK HK

4~176 4~176 4~176

22 19.2 19.1 19.0

Basalt lava or tuff?, well N Pekanbaru

?K-Ar

Andesite flow in Painan Formation (TP 32)

MK

Andes• flow, Bukit Sulap, Bengkulu (BSU 170) Andesite in Hulusimpang Formation (MN 116) Rhyolite dyke in Hulusimpang Formation (MN 118) Basaltic andesite dyke in Hulusimpang Formation (MN 117)

_+ 1.5 ___0.54 + 0.45 • 0.45

Reference Koning & Aulia (1985) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

17.5

Eubank & Makki ( 1981 )

4~176

14.3 _+ 0.34

Bellon et al. (2004)

MK MK MK MK

4~176 4~176 4~176 4~176

16.5 13.2 12.8 12.8

• • • _

0.38 0.43 0.31 0.38

Bellon Bellon Bellon Bellon

Rhyolite tuff in (?)Tarahan Formation (TR 33) Basalt dyke in Sulan batholith (WS 5) Andesite dyke in Hulusimpang Formation (SMK 40) Basalt dyke in Hulusimpang Formation (SMK 39) Dacite flow in Sabu Formation (PCE 9A)

MK MK MK LK HK

4~176 4~176 4~176 4~176 4~176

19.7 17.1 16.9 15.1 14.4

• • • • +

0.47 0.44 0.44 0.38 0.35

Bellon Bellon Bellon Bellon Bellon

Middle Miocene Volcanic Phase (c. 12-8 Ma) Basalt, Alem Formation Basalt, Alem Formation. Basalt dyke, Alem Formation

K-Ar, whole rock K-Ar, whole rock K-At, whole rock

11.2 -+- 0.7 10.3 • 0.4 8.74 _+ 0.82

Kallagher (1990) Kallagher (1990) Kallagher (1990)

Basalt dyke in Hulusimpang Formation (SMK 37)

MK

4~176

10.9 • 0.43

Bellon et al. (2004)

LATE MIOCENE-PLIOCENE (6-1.6 Ma) Andesite flow, Lam Teuba Volcanics (UB 110)

MK

4~176

1.76 + 0.06

Bellon et al. 2004)

(2004) (2004) et al. (2004) et al. (2004) et al.

et al.

(2004) (2004) et al. (2004) et al. (2004) et al. (2004) et al.

et al.

Diorite dyke in Bohorok Formation (PR 61) near Parapat, Lake Toba Andesite flow in Haranggoal Formation (PR 70) Andesite flow in Sibayak Complex (BR 104) Basalt dyke in Sipiso-piso lava dome (PR 101B)

HK

4~176

5.66 _+ 0.14

Bellon et al. (2004)

HK HK MK

4~176 4~176 4~176

2.88 • 0.07 2.09 • 0.29 1.89 + 0,23

Bellon et al. 2004) Bellon et al. 2004) Bellon et al. 2004)

Andesite flow in Angkola Formation, Sibolga (SB 28)

MK

4~176

5.35 +_ 0.23

Bellon et al. 2004)

Andesite, Suliki Basaltic andesite flow, Merapi volcano area (PY 82) Andesite flow, north border of Lake Maninjau (MNJ 55) Basaltic andesite flow, south of Padang (PLN 103)

K-Ar, ? MK MK HK

4~176 4~176 4~176

5.4 • 0.3 2.99 • 0.08 1.76 + 0.05 1.35 • 0.1

Koning & Aulia (1985) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

Basalt flow in Bal Formation east of Bengkulu (BN 111)

LK

4~176 4~176 4~176 4~176

6.45 5.47 5.21 4.23

Bellon et Bellon et Bellon et Bellon et

+ _ • +

0.2 0.14 0.5 0.15

(2004) (2004) al. (2004) al. (2004)

al.

al.

Basalt dyke, boulder in Gumai mountains (LH 173) Basaltic andesite flow in Pliocene volcanic Formation, northwest of Curup (CR 145) Andesite dyke in Air Benekat Formation (LH 178) Basaltic andesite dyke in Lemau Formation (BS 129) Andesite, Gunung Batu

LK MK HK MK K-Ar

4~176 4~176

2.91 • 0.09 2.41 + 0.08 4.76 • 0.32

Bellon et al. (2004) Bellon et al. (2004) Gafoer et al. (1992c)

Andesite flow in ?Lakitan Formation (PC 16)

HK

4~176

4.93 -+_ 0.13

Bellon et al. (2004)

Petrographic types: LK, = Iow-K calc-alkaline; MK, = medium-K calc-alkaline; HK, = high-K calc-alkaline; SH, = shoshonitic (see Bellon et al. 2004 for analytical details)

B e l l o n e t a l . (2004) d a t e d d y k e s b e t w e e n 62.5 a n d 52 M a in the Natal area, basalt flows a n d a d y k e s at c. 63 M a in t h e S o l o k area a n d S W o f A c e h a basaltic d y k e , flow a n d t u f f b e t w e e n 63 a n d 51 Ma. T h e K - A r a g e s o f p l u t o n s a s s o c i a t e d with the P a l a e o c e n e m a g m a t i c e p i s o d e are m o s t l y y o u n g e r t h a n the ages o f the v o l c a n i c r o c k s and m u c h o f the data relate to the c o o l i n g o f p l u t o n s . T h e Lass• b a t h o l i t h in W e s t S u m a t r a w a s e m p l a c e d c. 56 M a

( I m t i h a n a h 2000), b u t the earliest intrusion:, e x p o s e d in the G u g u k q u a r r y o n the w e s t e r n m a r g i n o f the b a t h o l i t h , is a f o l i a t e d m e g a c r y s t i c m e t a d i o r i t e , too w e a t h e r e d to date. T h e f o l i a t e d m e g a c r y s t i c m e t a d i o r i t e w a s e m p l a c e d in a shear z o n e ( p e r s o n a l o b s e r v a t i o n ) that is a c o n t i n u a t i o n o f the M u s i b a s e m e n t fault in the S o u t h S u m a t r a B a c k a r c B a s i n ( P u l u n g g o n o e t a l . 1992). By r e v e r s i n g the p o s t - M i o c e n e m o v e m e n t s a l o n g the S u m a t r a F a u l t Z o n e , the M u s i F a u l t links w i t h t h e S i p a k p a h i F a u l t ( A l d i s s

102

CHAPTER 8

et al. 1983) and the Kluet Fault (Cameron et al. 1982b) to the west of the Sumatran Fault Zone. Several plutons and volcanic outcrops are associated with the Kluet-Musi Fault (Fig. 8.2) which was active in the Early Eocene, but the amount and sense of displacement (probably dextral) is not known.

Late Mid-Late

Eocene volcanic episode

( T a b l e 8.3 a n d Fig. 8.3)

Volcanic rocks and volcaniclastic sediments have not been recognised within the Palaeogene units which occur beneath Miocene sediments in boreholes and imaged on seismic profiles in the forearc Meulaboh and Singkel basins (Karig et al. 1980). Nor have they been recognized in the 'Parallel Bedded facies' which occurs beneath the graben sequence in the Bengkulu Basin (Hall et al. 1993), or within the newly recognized Palaeogene Accretionary Wedge (Schluter et al. 2002) in the Outer Arc High to the SE of Enggano. Late M i d - L a t e Eocene volcanic rocks are found along the west coast of Sumatra, palaeogeographically reconstructed in Figure 8.3. The Breueh Volcanic Formation on Pulau Breueh to the NW of Aceh, consists of bedded subaerial pyroclastics and massive scoriaceous, feldsparphyric and epidotised basaltic lavas. Volcanic clasts at the base of the Peunasu Formation (Late Oligocene-Early Miocene), dated as Late Mid-Eocene, were derived from the Breueh Volcanic Formation (Bennett et al. 1981a). A N N E - S S W dyke swarm, which appears to emanate from the Raya Diorite and cuts both the Breueh Volcanic and the Peunasu Formations, has yielded a K - A r hornblende age of 18.9 _+ 1.2 Ma (Early Miocene). According to Rock et al. (1982), the Raya stock is a sub-volcanic intrusion and these dykes were intruded into hot plastic lavas. It is therefore probable that the Breueh Volcanic Formation also includes a Miocene volcanic unit. Volcanic rocks occur in the ?late Mid-Eocene-Early Oligocene Meucampali Formation (Bennett et al. 1981a; Cameron et al. 1983) exposed in the Barisan Mountains to the SE of Aceh. Local volcanic horizons with amygdaloidal, intermediate to mafic lavas occur within paralic-fluviatile sediments. Altered andesites occur within the Kieme and Semelit formations in the Takengon Quadrangle (Cameron et al. 1983). Cameron et al. (1980) interpreted the Kieme and Semelit formations as arc and back-arc basin sequences, associated with faulting. Porphyritic andesites in the Sitaban Formation off Tapanuli Bay also probably belong to this phase. A microdiorite within these lavas is thought to be a subvolcanic intrusion and has provided a zircon fission track age of 43 -t- 3.2 Ma (Mid-Eocene) (Aspden et al. 1982b). Bellon et al. (2004) have dated a basalt dyke in the Solok area at 46 + i Ma and an andesite dyke in the Natal area at 41 +_ i Ma.

Table 8.2. Litholo;,ies in the Kikim Volcanic Unit of'the Palaeocene volcanic

In the Natal area the bathyal Si Kumbu Turbidite Formation (Rock et al. 1983; Wajzer 1986; Wajzer et al. 1991) crops out between the Simpang Gambit Fault and the younger Langsat volcanics. The Si Kumbu Turbidite Formation is composed of volcaniclastic debris flows and proximal and distal turbidites, with negligible contents of quartz and K-feldspar. The Si Kumbu Turbidite Formation is weakly deformed by large-scale open folds and is slightly metamorphosed (prehnite-pumpellyite facies) with pervasive epidote veining in places. The Si Kumbu Formation is intruded by andesite dykes, two of which were dated using the whole-rock K - A r method, giving minimum ages of 40.1 ___ 1.6 Ma and 37.6 + 1.3 Ma, which are probably cooling ages. The andesite dykes are identical in composition to andesite clasts in the volcaniclastic breccias within the Si Kumbu Formation and are therefore considered to have been intruded contemporaneously. If the inferred syn-depositional age of the dated andesite intrusions is correct, the Si Kumbu turbidites are mostly of late Mid-Eocene age. The Si Kumbu Turbidite Formation is interpreted by Wajzer et al. (1991) to represent a fault-bounded allochthonous, and possibly rotated, submarine-fan deposit derived from the apron of an oceanic volcanic arc which lay to the west. There is no evidence of an oceanic volcanic arc to the west at this time so that the volcaniclastic debris may have been derived from a coastal volcanic centre. Rashid et al. (1998) and Netherwood (2000) consider the volcaniclastic sequence in the Gumai Mountains as 'about' Middle Eocene (47-42 Ma) in age. This is a further estimate for the age of these undated volcaniclastics which, following McCourt et al. (1993), are here correlated with the Lahat Formation (Oligocene). A shoshonite dyke in the Tanjungkarang area has been dated by Bellon et al. (2004) at 43.5 _+ 1 Ma. At Ciletuh Bay in the western part of the Java, bathyal volcanic rocks and submarine fan deposits of the Ciletuh Formation (Late Mid-Eocene-Early Oligocene) (Schiller et al. 1991) rest unconformable upon the components of an Upper Cretaceous Oceanic Accretionary Complex (Citirem Formation, the Pasir Luhur Schist and the Gunung Beas Ultrabasics) that has similar iithologies and a similar age to the Bangkaru Ophiolite Complex of the Sumatran forearc islands (Samuel et al. 1997). In the Ciletuh Formation volcanic debris is mingled with a submarine fan; turbidite deposits formed when clastic sediments of continental origin poured over a narrow continental shelf bounded by the Cimandiri Fault onto a continental slope. The volcaniclastics were deposited in half grabens and were derived from ashfalls and massive undersea pyroclastic flows. Schiller et al. (1991) suggest that some of the volcaniclastics were derived from the erosion of a nearby undersea volcano or volcanic island. The description of the Ciletuh Formation is not detailed enough to demonstrate that a subaquous caldera was present at that time, although such structures have been shown to occur elsewhere (White et al. 2004). An alternative source for the volcaniclastics is the contemporaneous Lower Old Andesites (LOA of Sukarna et al. 1993) in the Bayah area to the north.

el~isode

Location

G. Dempuj Garba Mrs2

Lahat, Lemat 1 & 2 and Tamiang 2 wells3

Lithologies

Andesite-basalt lava, tuff & volcanicbreccia; sulphides with gold. >250 m andesitic to basaltic compositionlavas, restricted to the base, welded tuff with flow structure, volcanic breccia with angularfragments of andesitebasalt material in a tuffaceous matrix, sandstone and siltstone. Tuffaceous sandstones, conglomerates,breccias and clays. K-Ar age date of 55 Ma reported from Tamiang-2 well.

References: IAminet al. (1994b), 2Gafoer et al. (1994), 3De Coster (1974).

Late Eocene-Early

Miocene

volcanic episode

Two phases are distinguished in this lengthy episode of volcanism: 1. Late Eocene-mid-Late Oligocene volcanism in Southern Sumatra. 2. Late Oligocene-Early Miocene volcanic arc in western Sumatra within the present Barisan Mountains. A complete sequence representing this volcanic episode was recorded in an offshore oil exploration well in the Bengkulu Forearc Basin (Hall et al. 1993). Elsewhere in Sumatra different

TERTIARY VOLCANICITY

k, -\,.

,\

)

\

"N~,b

~............. Offshore boreholes il

N~'6"~, .....

LaKe \xToba

PALAEOCENE

\\

/

103

\,<'

{ \i z

k

'~

\ \,

V <.N,

\

'~>' '\' \f-. '-,,

}Seukeun

...."............'x '%,,.... /N/ '~'\,, ~,\

M~

u& \

_ 6Sibubung X

\

.......:, ..-..

v2.,, \ \ ssi

"-'I

\

N . Batang Nata

~:~

~ Bungo

}

x----

;.,,,,.-.~

'

"

ULT v Tamian

'

. . . . " .........

,,~, \~"N~>,~_,~ \

/

Bukit Raja

/

v

!"-'"\~)N"-,

Lemat 1&2

(N.

Laru \.. ......

!

"-.%

Volcanic rocks Piutons

%.o4\" , ,

0

[

~"\-

Od0~\.. v)~p.

( {

V Gunung Dempu ~ Jatibaru

~ . . ,.... .....

%

......... ,.

=\.

\-,\.~

200km

"~ ""% '~ "

components of the episode can be pieced together from the volcanic formations and units identified and described during regional mapping and oil exploration. Late Eocene to mid-Late Oligocene Volcanic Phase (Table 8.4 and Fig. 8.4). In northern Sumatra, a dyke in the Calang area has been dated at 32 4-1 Ma ( K - A r method) by Bellon et al.

(2004). Extrusive volcanic rocks are well developed in the Natal area of the forearc, where Bellon et al. (2004) dated both andesite and basalt dykes between 41 and 37 Ma. Tufts, assigned to the Lahat Formation (McCourt et al. 1993), are exposed in the Tigapuluh and Gumai Mountains, where they constitute the regional Late Eocene-early Late Oligocene sedimentary formation in Southern Sumatra. De Coster (1974) placed equivalent tufts, found in boreholes drilled during exploration of the South Sumatra Basin, in the Lemat Formation. The ageequivalent Lemat and Lahat Formations are considered by De Coster (1974) to be basal Eocene to Upper Oligocene in age, revised by De Smet & Barber (see Chapter 7) to Late Eocene to early Late Oligocene. Alternatively Netherwood (2000), following Rashid et al. (1998) places the Lahat Formation in the Middle Eocene and the Lemat Formation in the Upper Eocene-Upper Oligocene. In this account these tufts are described as part of the Lahat Formation. The Langsat Volcanic Formation (Wajzer et al. 1991) at the western end of the Natal River section is composed of poorly exposed and deeply weathered porphyritic basic lavas and

.

f

1

/ /"J ~J'-~"""\L

Fig. 8.2. Distribution of volcanics and plutons associated with the Palaeocene volcanic episode. Palaeogeographic outline of Sumatra adapted from Figure 14.18a which compensates for the dextral displacement along the Sumatra Fault Zone and extension within the Forearc. Volcanic units listed in Table 8.2.

agglomerates with an elevated alkali content. The estimated age of the Langsat Volcanic Formation is between Early and Late Oligocene. The Langsat Volcanic Formation is thought to have been intruded by the Late Oligocene Air Bangis granite suite (c. 2 8 29 Ma), but due to poor outcrop and a covering of younger rocks this is not certain. Rock et al. (1983) noted sedimentary xenoliths in the Banjalarang adamellite at Air Bangis and mapped undifferentiated sediments on the shore, but no volcanic xenoliths were seen. The outcrop of the Langsat Volcanic Formation is fault-bounded, but the rocks are not internally deformed. The lavas are highly porphyritic, clinopyroxene-rich with minor plagioclase. Rock et al. (1982, 1983) noted that the Langsat Volcanic Formation differs from the other Tertiary basic lavas in Sumatra and Java in the absence of hypersthene, the rarity of plagioclase, the presence of orthoclase and sometimes of olivine at low silica percentages, and by high clinopyroxene contents, leading to elevated values of Mg, Ca, Cr, Ni and to a lesser extent of Co (Table 8.9). Rock et al. (1982) concluded that the Langsat Volcanics were abnormal mafic basaltic rocks, with affinities to basic shoshonite or absarokite (see Fig. 8.8a). Wajzer (1986) found pumpellyite in amygdales and in the groundmass of lavas in the Langsat Volcanic Formation. His chemical analyses confirmed the high K contents and the low levels of Zr, Nb, Y and depleted P and Ti values, usually high in alkali-rich basic rocks. Wajzer (1986) suggested that the initial alkali content was low, and that the high alkali levels were the result of prehnite-pumpellyite facies metamorphism.

104

CHAPTER 8

Table 8.3. Lithologies in Late Mid-Eocene-Late Eocene volcanic formations and units Volcanic F m or Unit

Breueh j

Meucampli I- 3 Kieme3

Semclit3

Sitaban4

Sibolga4 Sikumbu5

Lower Old Andesites6 Ciletuh7

Lithologies

Bedded pile of subaerial massive to scoriaceous pyroclastics, feldsparphyric, epidotized, vesicular & amygaloidal basaltic lavas which were hot and plastic at the time of intrusion by basalt, andesite and microdiorite dykes with Breueh VF clasts. Clasts of the Breueh Volcanic Formation are present in the base of the Peunasu Formation (Late OligoceneEarly Miocene). The Raya Diorite (18.9 _+ 1.2 Ma) may be a subvolcanic intrusion. Local amygdaloidal intermediate to marie volcanics within the siltstones & mudstones. Arkoses, carbonaceous & pebbly mudstones, volcanic wackes & breccio-conglomerates & sandstones; prophylitised andesites. Arkoses, carbonaceous & pebbly mudstones, volcanic wackes & breccio-conglomerates & sandstones; prophylitised andesites. Porphyritic andesites and subvolcanic microdiorites. Microdiorite dated at 43 + 3.2 Ma (fission track method). Amygdaloidal andesite interbedded with paralicfluviatile sediments near Barus. Volcaniclastic debris flows and proximal and distal turbidites with negligible contents of quartz and K-feldspar; represents a submarine fan deposit derived from the apron of a volcano. Basalts and andesitic basalts; interfingers with the Cipageur Member. Bathyal volcaniclastics banked against fault scarp derived from ashfalls and massive undersea pyroclastic flows over a narrow continental shelf. The volcaniclastics may have originated from the Lower Old Andesites and the presence of seafloor volcanoes has been suggested.

References: iBennett et al. ( 1981a), 2Keats et al. ( 1982), 3Cameron et a/. (1983), 4Aspden et al. (1982b), 5Wajzer et al. (1991), 6Sukarna et al. (1993), 7Schiller et al. ( 1991).

Wajzer e t al. (1991) considered that the Langsat Volcanic Formation represent primitive tholeiitic volcanics of island arc, or possibly mid-oceanic ridge affinity, although the results given by tectonic-setting diagrams were ambiguous. It is concluded here that in spite of the low-grade metamorphism of some samples, the Langsat Volcanic Formation are primitive submarine tholeiitic volcanics erupted in a forearc setting, and resemble the high-Ti variety shoshonites of the Eocene Kamchatka Arc of Siberia (Kepezhinskas 1995). To the NE the outcrop of the Langsat Formation is bounded by a fault parallel to the Simpang Gambir Fault (Wajzer et al. 1991) which, by reversing the post-Miocene movements of the Sumatra Fault Zone (Fig. 8.4), links the Langsat area with the contemporary fault-bounded igneous centre of the Bandan Formation. The Bandan Formation, composed of ignimbrites and tufts, is up to 500 m thick and outcrops for a distance of 26 km along the strike (Rosidi e t al. 1976; Kusnama e t al. 1993b). The pyroclastic rocks are intruded by a graphic granite and Rosidi e t al. (1976) suggested that there was evidence of fault-fissure volcanism. The Bandan volcanic centre appears to represent the eroded roots of a caldera complex, and is associated with a fault zone which extends southeastwards into the Lematang Fault (Pulunggono 1986), an important link between the graben fault troughs and highs which make up the South Sumatra

Basin (see Chapter 13). Tuffaceous horizons in the Lahat Formation in the South Sumatra Basin (Table 8.4) are distributed in a wide arc around the Bandan volcanic centre and it seems likely that the Bandan caldera structure was a major source for these tufts. The most northerly reported volcaniclastic sediments of Middle Eocene to Upper Oligocene age occur in the lacustrine and basin margin facies of the Upper Eocene Sangkarewang Formation in the intramontane Ombilin Basin (Howells 1997b). Koesoemadinta & Matasak (1981) used the term 'Brani Formation' for the basal unit of the Sangkarewang Formation in which they described minor quantities of volcanic debris within polymict conglomerates, but did not recognize any tuffs. To the east in the Central Sumatra Basin De Coster (1974) has described volcaniclastics in the basal Kelesa Formation ( O l i g o c e n e - E a r l y Miocene), now termed the Pematang Group (Upper E o c e n e - U p p e r Oligocene, see Chapter 7). The Kelesa Formation has a localised distribution, forming the initial sedimentary fill in troughs and grabens and contains tufts in the northern Tigapuluh Mountains (Simunjuntak e t al. 1991). Wain & Jackson (1995) also recognized ruffs in the Brown Shale Facies of the Pematang Group in the Kampur Uplift, NW of the Tigapuluh Mountains, near the southwestern margin of the Central Basin. The tufts and volcaniclastic sediments of the Lahat Formation are the most widely distributed Upper E o c e n e - O l i g o c e n e volcanic rocks in Southern Sumatra and Northwest Java. The Lahat Formation includes terrestial and lacustrine sediments and volcaniclastics (N.B. De Coster 1974 placed these in the Lemat Formation) deposited initially on an uneven topographic surface and later in (listric?) half grabens trending n o r t h - s o u t h and NE-SW, linked by N W - S E - t r e n d i n g transfer faults. The basal Lahat Formation is exposed on the southeastern slopes of the Tigapuluh Mountains uplift and contains tufts and volcanic debris (Suwarna e t al. 1991). In the type area of the Lahat formation in the Gumai Mountains (Musper 1937; Gafoer e t al. 1992c, McCourt e t al. 1993) finely laminated tufts occur below the Cawang Member (Lower Kikim Formation of Gafoer e t al. 1992c, pp. 6 6 - 6 7 ) , and andesitic lavas, tufts and tuffaceous claystones occur above the Cawang Member (the Upper Kikim Formation of Galber et al. 1994), which also contains volcanic debris. De Coster (1974) described the Lahat Formation resting on 'Upper Cretaceous-Palaeogene' volcaniclastics (his Kikim Tufts) below the mid-Oligocene unconformity to the east of the Gumai Mountains, in the Kikim, Lemu, Laru, Lahat and Tamiang wells. The Lahat Formation is not represented in the Garba Mountains where the volcanic breccias, welded tufts, andesitic to basaltic lavas with sedimentary intercalations were assigned to the older Kikim Volcanics by Gafoer et al. (1994). De Coster (1974) described how, towards the end of the Eocene in the South Sumatra sub-basins, the uneven topography of basement ridges and hills was deeply eroded to expose granite plutons. The granite wash derived from these plutons was buried beneath fluviatile continental sediments of the Lahat Formation and included tuff, derived partly from intermittent volcanism, but also recycled from earlier tuff deposits. In the South Palembang Sub-basin Pannetier (1994) figures volcaniclastic sediments of the basal Lahat Formation banked up against fault scarps. In the South Palembang Sub-basin, towards the top of the Lahat Formation, the Benekat Member was deposited in the Benakat Gully graben against the Lematang Fault (Pulunggono 1986), a NW-trending transfer fault that had been active during the Mesozoic (Pulunggono e t al. 1992). The lacustrine Benekat Member is composed of grey-brown shales with some beds of tuffaceous shale, siltstone, sandstone and thin coal beds. It was dated as late E o c e n e - E a r l y Oligocene on sporepollen and K - A r age dates by De Coster (1974), but is currently considered to be of Late Oligocene age.

TERTIARY VOLCANICITY

MERGUI

Me

LATE MIDDLE EOCENE

\

BASIN

" ~

,~, Semelit "-V',Kierne

( ,

Breueh~'~. ~" ~

Ts~and- ~ ~ ~ . . . .

MEULABOH~

BASIN

X

~

~

eucamZ" X \k

105

X

~~

~

X

,

"~

~

\

--

^ S/NGKELQ \ "C..,Oe," BASIN ~V~..Sibolcla p ulau~,.~.+~ Sitaban"9~ ~v'~'" Simeulue \ ."*oA \\

",%

f'-) /~ (,.

",

~\

~.~z~V Sikumt :

~-'/"

IvI

Volcanic rocks

I o

Plutons

",

~ Sungei Toboh

Too\. BENGKULU " \ BASIN

0~

[

Pulau" % Enggano ~ 200km ~

Southeast of the Garba Mountains, in the Bandar Jaya Basin, shales of the Lahat Formation, with a high volcaniclastic component ( 2 2 0 - 9 0 0 m ) , were deposited in grabens within cyclic fluvial and lacustrine environments, rich in algae (Williams et al. 1995). On the western side of Teluk Lampung the Palaeogene volcanic outcrop may not be as extensive as shown on the geological map of Tanjungkarang (Andi Mangga et al. 1994a), as according to Gasparon & Varne (1995) the volcanic rocks here belong to the Pliocene-Pleistocene Lampung Formation. West of Teluk Lampung fluvial breccias and tufts of the Sabu Formation rest unconformably on the Menanga Formation (Cretaceous). On the eastern side of Lampung Bay tufts occur in the lower part of the marine turbiditic Campang Formation. These formations, distributed around Telukbetung and Tanjungkarang, consist of tufts and breccias with tuffite intercalations deposited in a continental environment. The Sabu and Campang formations are correlated by Andi Mangga et al. ( 1 9 9 4 a ) with the Tarahan Formation, which consists of tufts and breccias with tuffite intercalations deposited in a continental environment, and is distributed around Telukbetung and Tanjungkarang.

~

Old Andes es V ~ "" "- ~ ~ I Ciletuh V,

Fig. 8.3. Distribution of volcanics and plutons associated with the Late Middle Eocene Volcanic Episode. Palaeogeographic outline of Sumatra adapted from Figure 14.18a. Volcanic units listed in Table 8.4.

On the NW coast of Java oil and gas are produced from fractured tufts in the Late Eocene-Early Oligocene Jatibarang Volcanic Formation (Arpandi & Patmosukisma 1975) which forms a basal infill in half grabens, over an iixegular topography. The greatest thickness of volcanic rocks occurs in a large offshore syn-rift graben, with a westerly dipping listric master fault (Adnan et al. 1991). This occurrence probably represents a distinct volcanic centre. In boreholes in the Bengkulu Forearc Basin (South Manna Sub-basin) an unconformity separates Palaeogene 'Parallel Bedded facies' from Upper Eocene-Upper Oligocene graben-fill sediments and volcaniclastics (Hall et al. 1993). At the bottom of the Arwana- 1 well, at the base of Megasequence I, a 60 m sequence of (?Upper Eocene-Lower Oligocene) massive volcaniclastic sediments is interbedded with tuffaceous clays and organic clays. These volcaniclastic rocks were deposited in a complex mosaic of segmented half-graben depocentres. Megasequence I is imaged on seismic profiles as a c. 2 km thick parallel-bedded sequence, deposited as a syn-rift unit within a system of NE-trending half graben, which were probably segmented by NW-trending transfer faults. The mid-Oligocene unconformity at the top of Megasequence I is

106

CHAPTER 8

Table 8.4. Lithologies in Late Eoce,e-Mid-Oligocene volcanic formations and units Volcanic F m or Unit

Lithologies

Langsat Volcanic j'2

Purple to blue-black highly porphyryritic volcanics with clinopyroxene phenocrysts, minor plagioclase, and occasional feldspar-phyric redpurple xenoliths. The groundmass is unusually potassic and consists mainly of orthoclase, but has a sodic rock composition. Chlorite pseudomorpbs are probably after olivine, and the quantity and alteration of feldspar phenocrysts is variable. The basic lavas and agglomerates show onion-skin weathering and are occasionally net-veined with quartz and/or epidote, perhaps related to small explosion vents. Tuffs are present, but are uncommon.

Sangkarawang3

Polymict conglomerates with granitic, metamorphic & minor volcanic clasts.

Kelesa4

Continental environment conglomerates, sands, shales, coals, tuffaceous material. 8 Kampar Uplift 5 Brown Shale facies: Lacustrine mudstones with clusters of lithic & crystal tufts. North Tigapuluh Mountains("7: Polymict conglomerates, gravely and pebbly tuffaceous sandstone and tuffaceous siltstone; with intercalations of fluvio-lacustrine sediments.

Bandan~,9

Monotonous sequence 400-500 m thick of acidic ignimbrites & hybrid tufts intruded by graphic granite body. Outcrop has strike of 26 km and subcrop is obscured by the Quaternary sediments associated with the D. Kerinci graben. Compacted tuff, volcanic breccia & conglomeratic tuff composed of fragments of andesite, basaltic tuff & welded tuff and of Palaeozoic and Mesozoic rocks. Prophylitised & chloritized with sulphide mineralization. Inferred fissure eruption along fault zone which is interpreted as the eroded root of a giant caldera. Gumai Mts formerly Kikim Formation (see main text), m Finely laminated tufts below (Lower Kikim Formation of Gafoer et al. 1992c), and

andesitic tufts and lavas and tuffaceous ctaystones above the Cawang Member. Lahat

Near Baturaja I I: violet, massive tuff with abundant milky plagioclase and sanidine phenocrysts and rare tiny laths of dark brown altered

mafics. South Tigapuluh Mountains: 7 Fluvio-lacustrine sediments with clasts of basalt, andesite, slate, metasediment, marble and quartz. Somewhat

tuffaceous siltstones and claystones. South Sumatra Basin: 4 Sandstones, clays, rock fragments, breccias, 'granite wash', occasional thin coal beds and tuffs. Bandar Jaya Basin ~-: basal shales have a high volcaniclastic component (220-900 m thick).

Megasequence I & 2 ]-~

Bengkuht Basin, South Manna Sub-Basin: Volcanic litharenites with clasts of ignimbrite, volcanics & vitriclasts, clay tuff & claystones.

Campang 14

Laml)tmg Basin: 1000-1500 m tuff, breccia, conglomerate, sandstone, siltstone, clay & shale.

Sabu H

Fluvial deposits of ruff, clay-tuff, conglomeratic breccia, sandslonc & claystone (c. 750 m).

Tarahan j4

Relatively massives luffs, poorly sorted breccia with clasts of andesite lava & sediments and tufliles with pymclaslic and detrital material.

Jatibarang 15

Unfossiliferous, varicoloured & mottled tuff s, porphyritic andesite, basalt and red claystone (0-1200 m).

References: JWajzer et al. ( 1991 ), 2Rock et al. ( 1982, 1983), "Koesoemadmata & Matasak ( 1981 ), 4De Coster ( 1974), 5Wain & Jackson (1995), 6Suwarna et al. ( 1991), " " ') Kusnamaetal.(1993h), ioo t J a r o c,. r ( , I a/. ( 1992c), i IGasparon&Varne(1995). leWilliamsetal.(1995), ]3Hall 7Simandjuntaketal.(1991), 8 Rosldletal.(1976), et al. (1993). HAndi Mangga et al. (1994a), 15Arpandi & Patmosukismo (1975).

interpreted as m a r k i n g a c h a n g e in the basin-forming m e c h a n i s m f r o m extension in the P a l a e o g e n e , to pull-apart, associated with oblique slip, in the N e o g e n e . Volcaniclastic rocks o c c u r in Megaseq u e n c e II in the Upper O l i g o c e n e in the A r w a n a - ! well and it appears that volcanicity was c o n t i n u o u s into the Late O l i g o c e n e Early M i o c e n e V o l c a n i c Phase.

Late O l i g o c e n e - E a r l y M i o c e n e Volcanic Phase (Table 8.5 and Fig. 8.5). T h e rise of the proto-Barisan Mountains at c. 28 Ma

marks a m a j o r tectonic event in Sumatra, causing the separation of the Forearc and B a c k a r c Basins. V o l c a n i c and volcaniclastic rocks f o r m e d during the Late O l i g o c e n e - E a r l y M i o c e n e V o l c a n i c Phase are f o u n d m o s t l y in West S u m a t r a on the rising protoBarisan land mass and along its western margins, but also in the Forearc Islands and to a lesser extent in the back arc area (Fig. 8.5). In S o u t h e r n S u m a t r a v o l c a n i s m started in the Late O l i g o c e n e , b a s e d on fossils in l i m e s t o n e intercalations in tuffaceous sandstones in the l o w e r part of the Seblat F o r m a t i o n w h i c h interfingers with the H u l u s i m p a n g F o r m a t i o n (Gafoer et al. 1992c). The ' O l d e r A n d e s i t e s ' to the SE of P a d a n g (van B e m m e l e n 1949), n o w k n o w n as the Painan F o r m a t i o n (Rosidi et al. 1976), m a r k the m a i n o u t c r o p of the Late O l i g o c e n e - E a r l y M i o c e n e V o l c a n i c Arc and their continuation to the SE is described as

the H u l u s i m p a n g Formation. T h e s e volcanic units are c o m p o s e d p r e d o m i n a n t l y o f andesite, basalt, andesitic basalt and rarer dacile lavas and pyroclastics. T h e original volcanic centres are not k n o w n , a l t h o u g h Early M i o c e n e subw)lcanic dioritic intrusions may mark the f o r m e r volcanic centres. The Painan F o r m a t i o n includes shallow water sediments, and to the S W of B e n g k u l u the Seblat F o r m a t i o n represents the remnants o f a marine volcaniclastic apron w h i c h intertingers with the lavas o f the H u l u s i m p a n g Formation. Propylitic alteration of the lavas is widespread, and chloritic alteration, sulphides and quartz veinlets are reported. T h e s e volcanics host several important Quaternary epithermal gold deposits. A basalt flow in the Padang area has been dated at 24 __+ 0.6 M a and d y k e s west o f S u n g e i p e r u h b e t w e e n 26 and 24 M a by B e l l o n et al. (2004). In the i n t r a m o n t a n e O m b i l i n Basin volcanic clasts first appear in the Rasau M e m b e r of the S a w a h l u n t o Formation, increasing in proportion u p w a r d s through the U p p e r O l i g o c e n e S a w a t a m b a n g F o r m a t i o n ( H o w e l l s 1997b); a source area in the e m e r g e n t Barisan M o u n t a i n s to the west o f the basin is probable. Further north a linear volcanic outcrop extends s o u t h w e s t w a r d s f r o m Sibolga, but individual v o l c a n i c centres have not been recognized. Pyroclastic volcanics and tufts are c o m m o n all along the western m a r g i n of the N o r t h S u m a t r a Basin. The volcanic materials o c c u r at the base o f U p p e r O l i g o c e n e - M i o c e n e sedim e n t a r y units, and are often reported to be b a n k e d against

TERTIARY VOLCANICITY

107

OLIGOCENE

\

~-~\~Bg~ii~XdanV S~ Tigapuluh" ~ X

'~

~ VGum~'B~:r:i!aSUmatra/

~ ~

-~

Volcanicand volcaniclasticrocks Pluton

V andarJay ' Basin ~~"6~' .~ Bengkul ~Basi ,. nVu ~, ~S_abu~q'Campang

200km

~

I

/ "~' . . ~ .~..

faults. In the Central Sumatra Basin volcaniclastic sandstones in the Cubadak Member of the Sihapas Formation were deposited in a deltaic environment (Rock et al. 1983). During the late Early Miocene volcanicity continued locally and reworked volcanic debris is reported in the Kompas Volcanic Member of the Loser Formation (Cameron et al. 1982a). In the Tapaktuan Quadrangle (Cameron et al. 1982b) the Rampong Formation is interbedded with the Akul Volcanic Formation, in which the eroded peaks of three volcanic centres can still be distinguished. On the west coast, adjacent to the Sikuleh Batholith, the paralic to fluviatile Tangla Formation contains localised intermediate volcanic and amygdaloidal basalts and volcaniclastics especially in the SE part of the outcrop. Bennett et al. (1981b) suggest that the volcanic rocks in the Tangla Formation, and numerous felsic and mafic dykes in the southwestern part of the Sikuleh Batholith, mark a line of former volcanoes. These volcanoes may have been the source of distal Lower Miocene tuffaceous volcaniclastic sediments found in the lbrearc islands, on Nias (Gawo Formation, N4 foram zone) and possibly also on Siberut (Samuel et al. 1997). In the Calang area, a basalt dyke has been date at 32 4- Ma by Bellon et al. (2004). In the Backarc areas volcanic rocks of this phase have been not reported within the Central Sumatra Basin. In the South Palembang Sub-Basin of the South Sumatra Backarc Basin a horizon with volcanic fragments is present in the Upper

1

Fig. 8.4. Distribution of Late Eocene-Middle Oligocene volcanic formations and units and dated plutons. Palaeogcographic outline of Sumatra after Figure 14.t8a. Volcanic units listed in Table 8.4.

JatibarangV -,..

Oligocene-Lower Miocene Talangakar Formation (Pannetier 1994), presumably representing volcanic debris washed into the basin from the volcanic arc.

Late Early M i o c e n e - M i d - M i o c e n e volcanic episode (Table 8.6 and Fig. 8.6)

A late Early Miocene Phase of volcanism is distinguished in the Meulaboh area of Northern Sumatra where Kallagher (1989, 1990) mapped volcanic rocks forming two age clusters, the first around the Lower to Middle Miocene boundary and the second around the Middle to Upper Miocene boundary. Additional age data from the Calang are for this volcanic episode are provided by Bellon et al. (2004) and summarized in Table 8.1. Kallagher (1990) states that the commencement of volcanic activity coincided with the uplift of the Barisan Mountains and the cessation of sedimentation along the margin of the Meulaboh Basin. Lower-Middle Miocene sediments show evidence of only minor contemporaneous volcanic activity, but are faulted against volcanic rocks of the same age, indicating subsequent fault movements, while Middle Miocene and younger sediments contain abundant volcanic clasts eroded from the volcanic belt. In northern Sumatra numerous volcanic formations belonging to the late Early Miocene-Mid-Miocene Volcanic Episode have been mapped. South of Lake Toba outcrops of volcanic rocks

t 08

CHAPTER 8

Table 8.5. Lithologies of the Late Oligocene-Early Miocene volcanic phase V o l c a n i c F m or Unit

Gawo i Tangla2

Smeten3,4 Sapi-3,4 Brawan3,4

Akul3,4

Kompas Volcanic Member5 Sihapas~

Sawahtambangv

Painan8

Hulusimpangg- ~5

Seblat 9- ~5

Lithologies

Tuffaceous volcanic member on Nias and ? Siberut. Volcanic facies of Tangla Formation with volcanic and conglomeratic sediments and localised intermediate volcanics & anaygdaloidal basalts. Minor intermediate volcanics in the Ligan Member. Felsic, intermediate pyroclastics. Flow banded welded tuff in Langsa quadrangle. Felsic, intermediate and marie lavas & pyroclastics; dykes. Massive hornblende andesites, agglomerates & lapilli aggregates with propylitization and subvolcanic microdiorites. Andesites, basalts, agglomerates & volcaniclastic sediments; propylitization. Interbedded with Rampong Fm. Andesites & pyroclastics; minor reworked pyroclastics; thickness 200-500 m. Part of Loser Formation. Cubadak Member contains volcaniclastic sandstones interbedded with limestones above mudstones & pebbly sandstones. Increase upwards in quantity of volcanic clasts, relative to clastic& metamorphic clasts in fluviatile conglomerates & conglomeratic sandstones. Andesitic-dacitic lavas, tufts, ignimbrites, tuff breccia, breccia, & minor sediments including arkose, bituminous shale, shaly coal, andesitic tuff, tuffaceous shale & sandstone. Andesite & basalt or andesite-basalt, rarely dacitic lavas, volcanic breccias & tufts. Often chloritised and propylitised and with sulphides and quartz veinlets (c. 700 m). Lower part lenses of conglomerate and carbonaceous sandstone. Middle part tuffaceous shale intercalated with limestone. Upper part tuffaceous siltstone & calcareous claystone & glauconitic sandstone.

References: JSamuel et al. (t997), 2Bennett et al. (1981b), 3Cameron et al. (1982a), 4Cameron et al. (1983), 5Cameron et al. (1982b), 6Rock et al. (1983), 7Howells (1997b), SRosidi et al. (1976), 9Kusnama et al. 1993b), t~ et al. (1994), IIGafoer et al. (1992c), leAmin et al. (1994a), 13Gafoer et al. (1994), J4Amin et al. (1994b), 15Andi Mangga et al. (1994a).

become more extensive, with lavas and volcaniclastics forming a discontinuous linear outcrop, mapped as a few aerially extensive formations (Table 8.6 & Fig. 8.6). Dykes and flows (Table 9.1) in the Sibologa area have been dated between 20 and 17 Ma; in the Kengkulu area between 17 and 13 Ma and between 20 and 14 Ma in the Tanjungkarang area (Bellon et al. 2004). Ashes derived from the volcanic arc occur in the forearc islands of Nias and Siberut, and probably also on Enggano, where the tuffaceous Kemiki Formation (Upper Middle M i o c e n e - P l i o c e n e ) was deposited in a terrestrial environment (Amin et al. 1994a). Acidic volcanic rocks occur in the Calang Volcanic Formation (rhyodacites) of northern Sumatra and in the extensive Bal Formation (dacites) of southern Sumatra. Otherwise the volcanic rocks are reported mostly to be andesites, with some basalts. Rock et al. (1983) describe volcanic rocks of intermediate composition from the equatorial sector. Sub-volcanic and other intrusions are observed to be associated in the field with several of the Middle Miocene volcanic fortnations (e.g. Calang and Saliguro Formations), and have been dated by Bellon et al. (2004) (Table 8.1). The Raya Diorite with a K - A r hornblende age of 18.9 _+ 1.2 Ma (average of six analyses) was emplaced within the Breueh Volcanic Formation (Late M i d d l e - L a t e Eocene) on Pulau Breueh N W of Banda Aceh. The diorite stock is associated with dykes which are described as

having been intruded into hot and plastic lavas (Bennett et al. 1981a). According to Rock et al. (1982) the Raya Stock may be the subvolcanic equivalent of the lavas, suggesting that late Early Miocene lavas are present within the Breueh Volcanic Formation, which may therefore be a composite unit. H i g h - K Series volcanism in the backarc. Eubank & Makki (1981)

described volcanic rocks encountered in seven oil exploration wells in the Central Sumatra Basin. These wells penetrated small sills, dykes, lavas and tufts of Middle Miocene age in the Coastal Plains Block along the Malacca Strait. Rock types include gabbro, micro-gabbro, olivine trachyte tuff and basalt. The extrusive rocks are crystal-lithic, vitric tufts that originated from the explosive chilling of gas-rich, partially chilled magma, The extrusives appear to have been deposited on an eroded surface, and possible pyroclastic cones were identified on seismic profiles. Uplift and erosion are known to have occurred in the Coastal Plains area during the Mid-Miocene. Submarine basalt flows encountered in the Merak-1 well are interbedded with marine sediments of N8 age ( 1 6 - 1 7 Ma) and yielded radiometric ages between 17.5-12 Ma (no analytical details are available). Some of the shallower intrusions showed contamination by sediment, but there was no significant assimilation of wall rock. The chemistry of these rocks indicates that they are K-rich shoshonites, typical of a high-K alkaline backarc association, but no chemical analyses were quoted. A seismic profile across the Buantan Intrusive Centre imaged a laccolith, about 4 km in diameter emplaced along the boundary between the Telisa and Bekasnap Formations, occupying a faulted arch in the overlying Telisa and Petani Formations (Heidrick & Aulia 1993). High-K series volcanics are present in the Natal area, where Bellon et al. (2004) have dated an absarokite flow at 18.2 + 0.4 Ma Andesitic intrusives and extrusives with radiometric ages between 18 and 14 Ma (no analytical data given), were penetrated in the Capang-1 and Abung-1 wells in the Terbanggi and Negara Batin Grabens of the Bandar Jaya Basin of Southern Sumatra (Williams et al. 1995). Late Miocene through Pliocene volcanic episode ( T a b l e 8. 7 a n d Fig. 8. 7)

Stratigraphic dating of volcanic rocks and volcaniclastic sediments indicate that the final episode of the Neogene volcanic activity continued into the Quaternary, represented in southern Sumatra by the volcaniclastic Kasai Formation. In Northern and Central Sumatra the distribution of Pliocene volcaniclastics is obscured by the extensive, younger Toba Tufts; Pliocene volcaniclastics have been recognized east of Aceh, where a flow of andesite is dated at 1.76 Ma by Bellon et al. (2004). The Haranggoal Volcanic Formation ( ? M i d d l e - U p p e r Miocene; Aldiss et al. 1982) at Lake Toba has been dated at 1.2 Ma, and now is interpreted as an early volcanic phase related to the Toba Caldera Complex (Chesner & Rose 1991). Older a ~ 1 7 6 dates for an andesite flow of 2 _ 0.3 Ma and a basalt dyke of 1.9 + 0.2 are reported by Bellon et al. (2004) from the Toba area. Pliocene volcanic centres around Lake Toba crop out as inliers within the Toba Tufts. These centres are set back slightly from the continuation of the trend of the volcanic arc in southern Sumatra. Their position and rhyolitic composition suggests a similar origin to the Toba Caldera system; a relationship to the subduction of the Investigator Fracture Zone (Fauzi et al. 1996) during the Pliocene is likely. Pliocene volcanics are recognized in equatorial Sumatra (Rock et al. 1983) and an undated linear outcrop of volcanic rocks occurs in the Painan Quadrangle (Rosidi et al. 1976), which includes volcanics of the episode (Bellon et al. 2004). In SW Sumatra volcanic centres with a rhyolite association (Pasumah and Ranau) have

TERTIARY VOLCANICITY

/

109

\

LATE OLIGOCENE

- EARLY MIOCENE

r,.Brawan X S~ 'T,Smeten

,

Lake

N N

\ O~. \

Tangla

v\\

, sj U,s "%00~ X

ihapaI

\

awatamb

?Gawo ~,,~

~~ainan

"-',~.

T

I Tufts and volcaniclastic sediments Volcanic rocks with lava flows Plutons

I

\

0

\

,

\

\ "-4

v~ ! /

Axial fault of

\ ~ 200km %

been recognized and volcanics dated between 5.5 and 2.4 Ma in the Bengkulu area by Bellon et al. (2004) (see Table 8.1). In the extreme south of Sumatra (Andi Mangga et al. 1994a) volcanic centres of andesitic lavas in the Sunda Strait at Pulau Sebuku and Gunung Durianpajung are early manifestations of the volcanicity in the Sunda Strait which climaxed during the Quaternary (see Chapter 9).

Major and trace element geochemistry of the Tertiary volcanic rocks There is more chemical data for the Neogene than for the Palaeogene volcanic rocks of Sumatra, but the majority of analyses are of major elements only; these have been discussed by Rock et al. (1982). Analyses of samples for major and minor elements from selected volcanic occurrences are given by Wajzer (1986), Kallagher (1989), Gasparon & Varne (1995) and Bellon et al. (2004). Samples from the Langsat, Lahat and Tarahan formations, forming the Late Eocene-Late Oligocene Volcanic Episode (Tables 8.8 & 8.9), are shoshonitic, and other Langsat Formation analyses fall in the medium and high-K fields (Fig. 8.8a). The bulk of the major element analyses (Tables 8.9 & 8.10) are of rocks belonging to the Mid-Late Miocene Volcanic Episode (Fig. 8.8b and see Bellon et al. 2004, fig. 3) which fall in the medium-K and high-K fields of Gill (1981).

)

simpan --~ .

Fig. 8.5. Distributionof Late Oligocene-Early Miocene volcanic units. Palaeogeographic outline of Sumatra after Figure 14.18b. Volcanic units listed in Table 8.5.

There is only sparse trace element data for Tertiary volcanic rocks from Sumatra. In Figure 8.9(a, b) selected analyses are normalized with respect to MORB, using the values given by Pearce (1982). The elements are placed in their 'CoryellMatsuda order', as recommended by McCulloch & Gamble (1991), which takes into account the low mobility of Nb and relatively high mobility of St. Coryell and Matsuda spider diagrams give similar patterns for selected analyses for the Late EoceneEarly Miocene and Mid-Miocene Volcanic Episodes. In volcanic rocks from both episodes high field strength elements (Nb, Zr, Ti, Y, Sc and Cr) are depleted relative to the large ion lithophile elements (Rb, Ba, K, Th and Sr), although in some analyses Nb, Zr and Cr show a varied behaviour, presumably due to fractionation and other magmatic effects during their passage through the crust. There is some evidence from the Sumatra dataset for the incorporation of subducted sediment in melts. In Figure 8.10, MgO is plotted against the ratio of Zr/Nb, which Macpherson & Hall (1999) consider is relatively sensitive to the recognition of sediment-derived melts that have been added to mantle wedge melts derived from N-MORB. Some volcanic rocks from the Late Eocene-Early Miocene and from the Mid-Miocene episodes have Zr/Nb ratios equal to, or greater than, that of N-MORB, which suggests that the lavas were derived from the mantle wedge beneath Sumatra, which was variably depleted with respect to N-MORB. The chemistry of the the Mid-Miocene volcanics of the Sayeung, Mirah and Calang formations of Northern

110

CHAPTER 8

Table 8.6. Lithologies of the Late Early Miocene-Mid-Miocene volcanic episode comprising the Late Early Miocene and Mid-Miocene volcanic phases

Volcanic Fro/Unit Lahomie j Salibi 2 Kemiki j6 Calang 3,4

Woyla-s Sayeung5 Tripa 5 Mirah 5 Alem5 Muereubo-s Kotabakti 5 Auran 6 Trumon 7 Pinapan 7 Toru 7 Musala s Angkolas Nabirong s Petani s Telisa9 Saligaro I~ Areas ~o Sikakara l~ Airbangisl~ Lubuksikaping area Ic~ 'Andesite' ~T Lemau 12 Balt3-17 C Sumatra Back-arc Basin Is Bandar Jaya I~

Lithology Nias, Banyak, Pini; Facies Ll. Tuff Marker Horizon 5 m. Outer neritic tufts. Siberut; tufts, claystone & siltstone. Enggano: Tuff, sandy tuff, tuffaceous sandstone & tuffaceous siltstone. Porphyritic, epidotised andesitic lavas with associated feeder dykes & subvolcanic intrusions. Subordinate basalts, microgabbroids, breccias & agglomerates. Thin sediment interbeds include coals. Unga Diorite possible subvolcanic centre, lnterbedded rhyodacites, pyroxene andesites & basalts. Some prophylitization. Eastern unit of Calang Fm. named by Kallagher (1989). Rhyolites, andesites & basalts, volcanogenic conglomerates & lithic tufts. Basalts, lahars, tufts & dykes; 14-16 Ma. Basalts, andesites, Jithic tufts, lahars and pyroclastics. Porphyritic & aphyric basalt & lahars. Basalts, 12-8 Ma. Porphyritic basalts. Base local massive tuffaceous sandstones but predominantly argillaceous and usually calcareous. Top predominantly arenaceous. Partly propylitised hornblende andesites & pyroclastics. Clasts of dacite & basalt in Agglomerates. Cut by subvolcanic intrusion dated at 12 Ma. Andesitic volcanics, agglomerates & tufts with associated hypabyssal microdiorite & microgranite. Wackes, tuffaceous wackes, mudstones & calcareous sandstones. Andesite, dacite & basaltic andesite lavas & pyroclastics also 'rhyolite' & 'trachyandesite'. Associated hypabyssal rocks include diopside vogesite dykes. Andesitic agglomerates; analysed andesite has shoshonitic affinity. Andesites, hornblende andesites, andesitic intrusives, possible subvolcanic diorites with K-Ar age date: 17.2 _+ 5 Ma. Hornblende & plagioclase phyric andesites, ?basalts, volcanic breccias & agglomerates. Volcanics often prophylitized, Intermediate volcanics, lavas, agglomerates and breccias. Sajurmatinggi Member Abundant volcanic debris in paralic mudstones, siltstones, sandstones & conglomerates. Sigama Volcanic Member Basal Telisa Formation volcanic unit composed of 300 m of tufts. Andesitic lavas and breccias with sediment intercalations of Telisa Formation. Mostly intermediate volcaniclastics, lavas & minor intrusives & sediments. Hydrothermal alteration/mineralisation in Mangani area. Aphyric, somewhat brecciated andesites and porphyritic andesites. Lithic crystal tufts, feldspar- & pyroxene phyric andesites & minor sediments. Various outcrops of varied lavas (dacites, andesites & basalts), agglomerates, breccias & tufts considered to range between Mid-Miocene-Plioccne or Pleistocene. Andesite (basaltic)microbreccia (age from Gafoer et al. 1992a). Volcaniclastic breccia, dacitic-tuffaceous sandstone, luffs & clays. Dacitic tufts unconformable on Hulusimpang Formation. T3pe area. Dacitic epiclastic breccia with sandstone intercalations & tuff. Subcrop of crystal-lithic, vitric tuff s, olivine trachyte tuff, basalt gabbro & micro-gabbro. Basalt tlows in the Merak-1 well are embedded in marine sediments of N8 age (16-17 Ma) and yielded radiometric ages between 17.5-12 Ma. Andesitic inlrusives and extrusives (14-18 Ma), in Capang-1 and Abung-1 wells.

References: ISamuel et al. (1987), -~AndiMangga et al. (1994b). ~Bennett et al. (1981a, b), 4Cameron et al. (1983), 5Kallagher (1989), r~Cameronet al. (1982a), :Aldiss et al. (1983), J IKastowo & Leo ( 1973), 12Kusnama et al. (1993b), 13Suwarna et al. (1994), HGafoer et al. (1983), 8Aspdcn eta/. (1982b), ~Cameron (1983), I~ et al. (1992c), 15Amin et al. (1994a), I(~Gafoeret al. (1994), 17Amin et al. (1994b), 18Eubank & Makki (1981), I'~Williams et al. (1995).

S u m a t r a with Z r / N b ratios l o w e r than N - M O R B , m a y reflect the incorporation o f subducted sediment. This s u b d u c t e d sediment could have been pelagic sediments riding on the o c e a n i c slab, sediments d e r i v e d from the uplift of the Barisan M o u n t a i n s and w a s h e d across the forearc into the trench, or distal turbidites d e r i v e d from erosion of the H i m a l a y a s (Curray & M o o r e 1974). Schluter et al. (2002) date the initiation of A c c r e t i o n a r y W e d g e II as M i d - M i o c e n e in Southern Sumatra, but the time o f arrival in the S u n d a T r e n c h of sediments of the N i c o b a r Fan, derived from the uplift and erosion of the H i m a l a y a s has been revised to Late M i o c e n e by C u r r a y (1994). Bellon et al. (2004) did not identify spatial or temporal geoc h e m i c a l trends within their S u m a t r a analytical data, and attributed this to the c o m p l e x i g n e o u s p e t r o g e n e s i s involving contributions f r o m the continental crust, m a n t l e w e d g e and the s u b d u c t e d slab. ' N o r m a l ' calcalkaline m a g m a types p r e d o m i n a t e , but Na-rich variants with SiO2 > 56% and very low h e a v y rare earth e l e m e n t ( H R E E ) and Y contents, k n o w n as adakites, also are present. B e l l o n et al. (2004) identified adakites within the Lassi batholith (intruded at c. 56 Ma, I m t i h a n a h 2000). E x a m p l e s of N e o g e n e plutonic adakites in Sumatra i n c l u d e the Lolo batholith (intruded at c. 15 Ma, I m t i h a n a h 2000), the W a y B a n g b a n g granite near K o t a a g u n g (intruded at c.20 Ma) and in the Anai

pluton, N E of Padang, taken from the analyses in M c C o u r t & C o b b i n g (1993). Piutonic and volcanic adakites are understood to be d e r i v e d from m a g m a s rich in residual garnet; the melting of subducted o c e a n i c meta-crust is a potential source (Juteau & M a u r y 1999), and Bellon et al. (2004) noted the potential contribution of garnetiferous m e t a m o r p h i c rocks in the crust beneath Sumatra, specifically in the T o b a area.

Volcanism, plutonism and subduction beneath Sumatra during the Tertiary: summary of Tertiary volcanism and tectonic overview T h e o r i e n t a t i o n o f S u m a t r a d u r i n g the P a l a e o g e n e a n d r o t a t i o n h i s t o r y d u r i n g the T e r t i a r y

N i n k o v i c h (1976) proposed that the long axis o f Sumatra rotated c l o c k w i s e f r o m an e a s t - w e s t orientation to N W - S E during the Tertiary, c e n t r e d on the S u n d a Strait. It is n o w confirmed by m a r i n e g e o p h y s i c a l surveys that extension in the Sunda Strait was facilitated by m o v e m e n t s b e t w e e n o v e r s t e p p i n g strike-slip faults ( H u c h o n & Le P i c h o n 1984; L e l g e m a n n et al. 2000) with

TERTIARYVOLCANICITY

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) \... LATE EARLY- MIDDLE MIOCENE

~

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no evidence for the sphenochasm proposed by Ninkovich (1976). The problem of the rotation of Sumatra during the Tertiary is discussed in detail in Chapter 14 where it is concluded that palaeomagnetic data from Borneo (Fuller et el. 1999) and Malaysia (Richter et el. 1999) demonstrate the anticlockwise rotation of the whole of the Sunda Microplate, so that Sumatra, together with Malaysia, has rotated c. 15 ~ anticlockwise since the MidMiocene. If this c. 15 ~ anticlockwise rotation is reversed, the long axis of Sumatra was oriented approximately north-south during the Palaeogene, as proposed by Davies (1984) and modelled by Hall (1996, 1998, 2002) in his reconstrucuons of Tertiary plate movement and palaeogeography of SE Asia.

Tertiary volcanism in Sumatra, extrusion tectonics and the collision of India with the Eurasian Plate In this account, following Davies (1984) and Hall (2002), it is proposed that Sumatra, forming the western margin of the Sunda Microplate, was orientated north-south during the Palaeogene, at the time when Greater India, on the western side of the Ninety East Transform Fault, passed the latitudes of Sumatra on its northwards course towards its collision with the southern margin of Eurasia (Patriat & Achache 1984) (Fig. 8.11). Previously it has been suggested by Daly et el. (1991), Hutchison (1992) and Packham (1993, 1996) among others, that the extension which formed the Sumatran backarc grabens could be explained in terms of the tectonic extrusion model of Tapponnier et al. (1986). However, backarc extension, and the associated Late Eocene-Early Oligocene phase of volcanism, occurred before the collision of Greater India with Eurasia, rather than after this event. The extrusion model, like the lithospheric thickening model of Dewey et el. (1989), assumes that Sumatra was aligned east-

"L ~

(

B~2dgr f , I

[ /

Fig.8.6. Distributionof Lower-MiddleMiocene volcanicunitsin Sumatra.Volcanicunitslistedin Table8.6.SFZ,SumatraFaultZone.

west prior to the collision of Greater india, and predicts the clockwise rotation of Sumatra in response to the impact. The subsequent anticlockwise rotation of Sumatra, together with the rest of the Sunda Microplate, cannot be due the extrusion of crustal blocks in response to the collision of India.

Subduction, volcanism and plutonism, continuous or episodic? Van Bemmelen (1949) suggested that volcanism occurred continuously in Sumatra during the Neogene. Subsequent study has established time ranges for distinct Tertiary volcanic episodes and volcanic phases. It is evident that volcanicity and the accompanying plutonism waxed and waned several times during the Tertiary. It is probable that subduction was taking place continuously beneath Sumatra during the Tertiary, but that subduction did not always lead to volcanism and plutonism. It has been suggested that volcanic activity is most intense during subduction roll-back (cf. Hamilton 1995). This was the situation in Sumatra for most of the Neogene (Macpherson & Hall 2002). The process of subduction roll-back ensures that fresh mantle material is continuously brought into contact with the subducting ocean slab, facilitating magmatism.

Palaeocene volcanic episode (Kikim Volcanics) ( 6 5 - 5 0 Me) The Kikim Volcanics and contemporaneous plutons form a magmatic arc in Southern Sumatra, the Java Sea (Hamilton 1979) and in Southern Sulawesi (Langi Volcanics of Wilson & Bosence 1996) (Fig. 8.11). Evidently a volcanic arc was active along the southern margin of the Sunda Microplate in the Palaeocene. In northern Sumatra there is evidence of a second inner arc

112

CHAPTER 8

Table 8.7.

L i t h o l o g i e s in the L a t e M i o c e n e - P l i o c e n e

volcanic episode

Formation/Centre Siap ~ Seureula 2 Takur-Takur 3 Simbolon 3,4 Surungan 5 Sihabuhabu 5 Mangani 6 Undifferentiated7'8 Rhyo_andesites9 i i Lakitan io- J4 Kasai]O.~ ~,J3-~5 Pasumah I 1,12 Ranau 12-15 Lampung ~4,15 Andesite lava 15

Lithology In part volcanic pebble to cobble conglomerates, sandstones & minor mudstones. Upwards-fining soft andesitic sandstones & conglomerates; also calcareous mudstones. Variably propylitised andesites, dacites and pyroclastic hb andesites and dykes. Local rhyolite. Andesitic to dacitic pumaceous pyroclastics and lahars. Andesitic lavas and pyroclastics, three possibly four flanking plugs of subvolcanic porphyritic hornblendic andesites. Subvolcanic intrusions of Mendem Microdiorite. Plagioclase and hornblende-phyric andesites, often agglomeritic and propylitised. More acid types present and hypabyssal equivalents noted. Acid to basic lavas including basalts and andesites, volcaniclastics and associated minor intrusives. Rhyolitic, dacitic and andesitic tuff, breccia and lava; welded, hybrid, lithic and pumiceous tuff with breccia and lava. Rhyolitic, dacitic & andesitic lavas, wclded tuff, hybrid tuff, pumiceous lithic tuff & volcanic breccia. Conglomeratic breccia alternating with tuffaceous sandstone & tuffaceous clay. Tuff & pumiceous tuff with intercalations of tuffaceous claystones & tuffaceous sandstones. M a n n a Dacitic lava (20 m) in breccia unit. Horizontally bedded welded tuffs with columnar jointing. Rhyolitic-andesitic pumiceous volcanic breccias and tuffs. Pumiceous tuff, tuffaceous sandstone locally with tuffite intercalations. Andesite lavas with sheeted jointing.

References: tBennett et al. (1981a), 2Keats et al. (1981), 3Cameron et al. (1982a), 4AIdiss et al. (1983), 5Clarke et al. (1982a), 6Rock et al. (1983), 7Kastowo & Leo (1973), SRosidi et al. (1976), 9Kusnama et al. 1993b), I~ et al. (1994), I IGafoer et al. (1992c), 12Amin et al. (1994a), ~3Gafoer et al. (1994), 14Amin et al. (1994b), 15 Andi Mangga et al. (1994a).

ANDAAC'EH

\

a- ~

\ LATE MIOCENE - PLIOCENE

~~~-~o~Ta k~.r'~aku' ~ ~Simbolon ~ i ' La'ke "'~- L "~' VSurungah~~ ~Sihabuhabu"~ |

/

~,. ~.

,, I r

I

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;

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0

-~Tuffs and volcaniclasticrocks ~ Volcaniclavas R Rhyolite # Dacite A Andesite B Basalt

,

~ 200km I

.... 9

R~aana ~

A

i

~

t

l

~

Fig. 8.7. Distribution of Upper MiocenePliocene volcanic units and dated plutons in Sumatra. Volcanic units listed in Table 8.7.

TERTIARY VOLCANICITY

113

Table 8.8. Major and trace element analyses of'Langsat Formation volcanics No.

R6029

R6030

R2785

R2786

R6028"

NR125A

NR128

NTI98

NT217

Ref.

1

2

2

2

l&2

3

3

3

3

Lithology

Pyroxenerich basalt

Pyroxene fragment in 6029

Pyroxeneplagioclase absarokitic basalt

Pyroxeneplagioclase transitional Alkali basalt

Average 3 scans ground mass R6028

Porphyritic clinopyroxene basalt

Porphyritic clinopyroxene basalt

Porphyritic basalt

Porphyritic basalt

Location

5287 0630

5287 0630

5276 0626

5263 0634

B. Natal

B. Natal

B. Natal

Batu Gajah

SiO2 TiO2 AI203 Fe203 MnO MgO CaO Na20 K20 P205 CO2 N20 Total

47.9 0.49 10.4 11.6 0.17 13.2 8.24 2.85 0.6 0.39 0.02 4.73 100.59

52.16 0.44 8.6 9.85 0.29 7.44 12.97 3.6 0.44 0.13

46.7 0.84 12.88 12.34 0.22 9.48 11.72 1.9 1.68 0.2

49.99 0.84 14.37 10.43 0.34 6.87 10.2 3.92 1.13 0.2

46.6 0.86 10.9 11.9 13.8 11.7 0.82 2.53 0.87

51.74 0.71 15.8 8.93 0.17 7.81 8.75 1.15 4.91 0.35

52.62 0.88 16.27 9.92 0.19 6.85 7.32 1.65 4.6 0.26

49.8 0.84 14.31 10.69 0.2 8.77 10.73 2.16 3.57 0.33

50.26 0.65 11.96 10.69 0.19 10.54 8.94 0.78 3.67 0.37

98.92

101.36

100.79

99.98

100.11

100.56

100.92

98.05

25 312 78 21 1

35 623 82 29 2

51 587 58 18 1

Rb Sr Zr Y Nb Th V Cr Co Ni Cu Zn

150 400 73 175 170 90

150 370 15 47 30 50

225 140 32 30 125 90

225 80 28 23 85 100

14 260 57 22 <1

150 400 73 175 170 90

References: l, Rock et al. (1983); 2, Rock et al. (1982); 3, Wajzer (1984).

beneath what later became the North Sumatra Backarc Basin. In Sumatra the majority of the plutons associated with the Palaeocene Volcanic Episode had solidified by c. 50 Ma (Table 8.2) and the youngest volcanics have been dated at c. 55 Ma (Table 8.1).

The 5 0 - 4 6

M a n o n - v o l c a n i c interval

This interval coincides in part with the Chron 24 ( 5 9 - 5 6 Ma) plate reorganization event, which led to the formation of the combined I n d i a n - A u s t r a l i a n Plate and the commmencement of spreading along the I n d i a - A n t a r c t i c Ridge. Volcanism resumed at c. 46 Ma, but Davies (1984) has questioned whether subduction was active beneath Sumatra between 55 and 44 Ma, and has suggested that at this time the continental margin of Sumatra was a transcurrent fault zone facilitating the northward passage of Greater India past Sumatra during that period (Patriat & Achache 1984). Alternatively when subduction was not operating beneath Sumatra the Ninety-East Ridge transform fault became temporally the western margin of the Sunda Microplate (A. J. Barber pers. comm.) and exerted an anticlockwise couple on the Sunda Microplate. According to Marshak & Karig (1977) during the Early to MidEocene the Wharton Spreading Axis lay in the latitude of Sumatra, forming a triple junction with the Sunda Trench (Fig. 8.12). The difficulty of subducting young, hot, buoyant ocean-ridge crust

(Cloos 1993) provides an alternative explanation for the pause in volcanism in Sumatra at this time. The Bangkaru Ophiolite Complex in the Outer Arc Islands (Samuel et al. 1997) contains igneous components formed at an ocean-spreading ridge and from oceanic fracture zones containing shear fabrics, low temperature hydrothermal metamorphism (prehnite-actinolite facies) in metagabbros and metadolerites and later brittle deformation and brecciation. Rare volcanic rocks on the Banyak Islands and in m61anges were interpreted by Samuel (1994) as being derived from oceanic islands and seamounts. The Bangkaru Ophiolite Complex represents components of Indian Ocean crust accreted into the accretionary complex at the subduction zone. It may be that the components of the Bangkaru Ophiolite Complex are the product of a short-lived 'hot accretion' episode, in which ridge crust was incorporated into the accretionary complex, because it was too hot and buoyant to be subducted, while arc volcanism was suppressed, because the subducted lithospheric mantle was not sufficiently hydrated to generate melts in the overlying mantle wedge.

Late Mid-Eocene volcanic episode

Volcanic rocks of late Mid-Eocene age are distributed in an arc parallel to the west coast of Sumatra, showing that subduction, with the generation of melts, was quickly re-established along

II 4

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TERTIARY VOLCANICITY

9

0 Shosho

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SayeungFormation Mirah Formation Alem Formation Calang Formation TanglaFormation BrawanFormation Sikaraka Formation SandudukFormation PinapanFormation / ToruFormation /~ / AngkolaFormation/.~ X

si02

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Hiah-K Hlgn-I~ ~--, V / ,__,/

Medium-K

v 50

80

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70

I

80

Fig. 8.8. Diagrams of SiO 2 (wt%) versus K20 (wt%) for low-K to shoshonitic Tertiary volcanic rocks from Sumatra. The classification scheme is by Gill (1981) and the analyses by Rock et al. (1982), Wajzer (1986), Kallagher (1989) and Gasparon & Varne (1995) are given in Tables 8.9, 8.10 & 8.1 I. (a) Upper Eocene-Lower Miocene volcanics; (b) Middle-Upper Miocene volcanic formations.

the full length of the subduction zone at this time. Volcanic rocks in the Aceh area may represent back-arc volcanism (Cameron et al. 1980). Marshak & Karig (1977) suggest that volcanic rocks in the Tapanuli area, offshore Sibolga, were due to subduction of the Wharton Spreading Centre, inactive by this stage and sufficiently hydrated to induce magmatism in the mantle wedge. Uplift of the whole of the forearc occurred in the Late Eocene producing a regional unconformity (Samuel et al. 1997). This phase of uplift coincides with the age of 40 ___ 3 Ma obtained from the Bangkuru Ophiolite Complex in Simuelue (Harbury & Kallagher 1991), which Kallagher (1990) attributes to deformation of warm oceanic crust during accretion.

Late E o c e n e - E a r l y Miocene volcanic episode Late Eocene-Early Oligocene volcanic phase (c. 37-30 Ma). Over a short period the linear volcanic arc contracted to a few centres of volcanism, the most important of which were in the Natal area of the forearc (Fig. 8.12). Contraction in the extent of volcanism was accompanied by faulting and a regional unconformity throughout the forearc. In the Natal area K-rich primitive basaltic, tholeiitic

115

and shoshonitic lavas and agglomerates of the Langsat Volcanic Formation were extruded and the Air Bangis granites were intruded (c. 3 0 - 2 7 Ma Wajzer et al. 1991). This magmatism was anomalously close to the Palaeogene trench. Reconstruction of the palaeogeography of Sumatra in the Late Eocene-Early Oligocene, by reversing the movements along the Sumatran Fault (Fig. 8.4), places the Bandan Formation caldera complex close to the outcrop of the Langsat Volcanic Formation. This caldera was an important centre of explosive acidic volcanism, and appears to be the source of the ashes which are interbedded with the sediments in the southern part of the Central Sumatra backarc basin, the Tigahpuluh Mountains and the South Sumatra Basin, an area of dispersal comparable to that of the tufts of the Toba Caldera Complex in the Quaternary (see Chapter 9). The association of uplift, volcanism and plutonism in the forearc close to the trench, and faulting and explosive volcanism inland, are features consistent with the concept of 'slab window volcanism' suggested by Thorkelsen (1996). A 'slab window' occurs where an active, or recently inactive, spreading ridge passes down a subduction zone, the crustal part of the ridge is removed by accretion at the trench while the underlying asthenosphere is subducted in direct contact with the base of the mantle wedge. In Sumatra the slab window was due to the subduction of the Wharton Spreading Centre. According to Liu et al. (1983) the Wharton Spreading Ridge was actively spreading at a rate of 30 mm a-1 shortly before it expired at c. 45.6 Ma in the late Eocene. The pattern of magnetic anomalies in the Indian Ocean crust indicate that the Wharton Spreading Ridge lay offshore Sumatra and was orientated at about 9 0 -~ to the Sumatran margin at this time (Fig. 8.12). Davies (1984) suggested that the Wharton Spreading Centre was dextrally transcurrently faulted along the continental margin of Sumatra during the Oligocene, instead of being subducted. However, Clure (1991) has suggested that a segment of the Wharton Spreading axis which lay to the east of the Investigator Fracture Zone was subducted at 5 0 - 4 5 Ma beneath the south of Sumatra, which he shows orientated east-west during this period. The concentration of Oligocene igneous activity in the Sumatran Forearc (c. 38 Ma), anomalously close to the presumed position of the subduction trench at that time, strongly suggests that the spreading axis was subducted beneath Sumatra in this period, as proposed previously by Marshak & Karig (1977). Other volcanic centres related to a linear volcanic arc are marked by outcrops of Oligocene lavas in the Gumai Mountains, possibly in the Garba Mountains and in west Java. The waning of this volcanic phase in the Early Oligocene coincided with the change in motion of the Indian-Australian Plate from northerly to north-northeasterly, which Davies (1984) suggested was responsible for the anticlockwise rotation of the Sunda Microplate relative to the Indian-Australian Plate with the formation of wrench faults parallel to the coast of Sumatra. Palaeomagnetic evidence for the Palaeogene anticlockwise rotation of the Sunda Microplate has been documented in Borneo (Fuller et al. 1999), but not yet in Sumatra, although wrench faulting has been identified during this period. A transition from extension to pull-apart and wrench modified-rifts in the Ombilin Basin was dated as mid-Oligocene by Howells (1997b), at c. 33 Ma in the Central Sumatra Basin by Packham (1993) and at 3 4 M a in the Bengkulu Basin by Hall et al. (1993). Davies (1984) related the formation of grabens and highs in the North Sumatra Back-arc Basin to zones of tension and compression between right and left stepping wrench faults (see Chapter 14). This phase of transcurrent fault movement most likely reflects the change in the direction of motion of the Indian-Australian Plate relative to the continental margin. Late Oligocene-Early Miocene volcanic phase (30-24 Ma). A late Oligocene tectonic event caused fault inversions and unconformities in all the Sumatra backarc basins between c. 28 and 26 Ma

116

CHAPTER 8

,..1 I

eo .,~ la 1 " - - ~ : 3 ~ t " ~ : 2 ~ ' ~ t " - O ' ~ - ~

r.

~

".=.~

I

y.

1o

r162 ~

TERTIARY VOLCANICITY

100,

X

Hulusimpang Formation(2) Lahatgormatior~(2)

O~

117

alang Formation Alem Formation Mirah Formation Sayeung Formation Hulusimpang Formation Lahat Formation kangsat Volcanic Formation

9

80

[] ~

60

o

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r8 1.o

40

T

E~ I (a)0lRb Ba

i K

I i Th Sr

I i i i Nb Zr Ti Y

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N-MORB

_L

76247 75246

20

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WoylaUnit, Calang Formation Alem Formation Mirah Formation SayeungFormation

i

00

I

2

I

i

4

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I

I

6

I

8

0

I

I

10

MgO (wt%)

~: O

~

{

8 rr

/

< 1.0

0.1

Fig. 8.10. Plot of MgO (wt%) against Zr/Nb for selected analyses of Upper Eocene-Middle Miocene volcanics from Sumatra. The Zr/Nb ratios higher than the range for N-MORB infer derivation from the mantle wedge, while Zr/Nb ratios lower than the range for N-MORB imply dilution of mantle wedge magma, probably by subducted sediment. The low Zr/Nb ratios coincide with the Middle Miocene Volcanic Phase but the source of the suspected subducted sediment is not certain. Range for N-MORB from Sun & McDonagh (1989).

>CUT45

<2z

I

(b) Rb Ba

I

I

K

I

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'dl

Th Sr Ce

t

Nb Zr Ti

I

',1

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Sc Cr

(see Chapter 7). This event has been attributed to the effect of collision of fragments derived from Australia with the Sunda Microplate, as marked by the accretion of ophiolite bodies in the East Arm of Sulawesi (Hall 1996). At the same time, folding of the Meureudu Group in northern Sumatra was accompanied by limited plutonism (Cameron et al. 1983). In the Sumatran Forearc sedimentation continued in the Bengkulu Basin (Hall et al. 1993), accompanied by volcanism which extended into the Late Oligocene-Early Miocene Volcanic Phase.

'~

Fig. 8.9. MORB-normalized trace elements for selected Sumatra Tertiary volcanics. Normalising factors by Pearce (1982) and trace elements plotted in the order recommended by Coryell & Matsuda (Elburg & Foden 1998). (a) Upper Eocene-Lower Miocene volcanics. Analyses by (1) Wajzer (1986) and (2) Gasparon & Varne (1995). (b) Middle Miocene volcanics. Analyses by Kallagher (1989).

//

PALAEOCENE PALAEOGEOGRAPHY

j/

EURASIAN PLATE AGj~V~ N~i~'l~G%N.

!

~/~

~ / __

/

/

PROTO-SOUTH \ CHINA SEA \ \ \ SUND~ /

/

PASSIVE MARG ~4

GREATER INDIA

/

v Volcanic rocks 9 Plutonic rocks

Fig. 8.11. Reconstruction of the Palaeocene volcanic arc along the margin of the Sunda Microplate between Sumatra, the Java Sea (Hamilton 1979) and West Sulawesi (Wilson & Bosence 1996). Adapted from Hall (1998), Clure (1991) and Figure 14.18a.

1 18

CHAPTER 8

LATE EOCENEEARLY OLIGOCEN

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Following the fault inversion event the rate of oblique subduction beneath Sumatra accelerated to 5 cm a -1, with the formation of an uplifted volcanic arc. Lavas and ashes were voluminously erupted in a linear arc parallel to the west coast of Sumatra, with tufts and volcaniclastics being deposited to the east in the backarc basins. Lavas were accompanied by sub-volcanic intrusions such as the Way Bambang Granite pluton which solidified at c. 20 Ma, and was intruded co-magmatically into volcanics of the Hulusimpang Formation (Amin et al. 1994b). This granite was intruded into into a fault zone parallel to, but predating the

v

Fig. 8.12. The subduction of the Wharton Ridge, the short-lived Natal Slab Window and other volcanic centres in southern Sumatra during the Late Eocene-Early Oligocene Volcanic Phase. Palaeogeography adapted from Figure 14.18a.

Semangko Segment of the Sumatra Fault Zone. A fault of similar age and orientation also probably occurs in the southern part of the outcrop of the Painan Formation where Rosidi et al. (1976) show several elongated granitoid intrusions. The amount of displacement along this dextral fault zone is not known. The Raya diorite and the associated dyke swarm on Pulau Breueh, off northern Sumatra, are also associated with this intrusion phase. In the mid-Oligocene uplift and erosion in the Outer Arc Islands was reversed, subsidence led to the resumption of sedimentation above an unconformity (Samuel et al. 1997). In the Sumatran

TERTIARY VOLCANICITY

backarc basins the formation of the rift grabens was followed by a Sag Phase marked by a marine transgression. In the Central Sumatra Backarc Basin sedimentation was accompanied by wrench-fault tectonism which continued until c. 21 Ma (Kelsch et al. 1998).

Late E a r l y - M i d - M i o c e n e volcanic episode

The late Early to Mid-Miocene volcanic episode is composed of two phases. A linear elevated volcanic arc was formed parallel to the west coast, and there was magmatism in the Central Sumatra Backarc Basin, where high-K and shoshonitic igneous rocks were intruded and extruded. Similar igneous activity occurred in the South Sumatra Backarc Basin between 17 and 12 Ma. Several plutons were emplaced into the volcanic a r c . 4~ ages obtained by Imtihanah (2000) from the Lolo Batholith show that the Sumatra Fault Zone was active during the latter part of the Late Early-Mid-Miocene volcanic phase (Fig. 8.6). The Lolo Granite was previously thought to be a composite intrusion (McCourt et al. 1996) within the Sumatra Fault Zone, emplaced at c. 9 M a ( K - A r on hornblende) and c. 6 M a ( K - A r on biotite). The new 4~ age data (Table 8.1) shows that the Lolo Granite was emplaced within the fault zone at c. 15 Ma, the K - A r mineral ages are considered to indicate that differential uplift occurred close to the fault zone (imtihanah 2000). The 15 Ma intrusion date for the Lolo Granite indicates that this sector of the Sumatra Fault Zone is older than previously estimated, and provides information on the rate of uplift of the Barisan Mountains. The K - A r mineral ages (van Leeuwen et al. 1987) for the Tangse stock (Table 8.2) indicate that uplift in northern Sumatra preceded that in southern Sumatra, but the time of intrusion of the Tangse stock is not known sufficiently accurately to date the fault movement. Late Miocene through Pliocene volcanic episode ( 6 - 1 . 6 Ma)

Oblique subduction of the Indian-Australian oceanic plate beneath the Sumatran arc resulted in extension and the commencement of sea-floor spreading in the Andaman Sea at c. 13 Ma. The development of transform faults from the Andaman Spreading Centre, particularly affecting northern Sumatra and the Forearc (see Chapter 13), and caused displacement along segments of the Sumatra Fault Zone in the Mid-Miocene. In northern Sumatra volcaniclastic rocks occur close to the present day coastline and were derived from buried Pliocene volcanic centres, which probably occupied a similar position to the Quaternary volcanoes. It has been suggested that the Quaternary volcanoes adjacent to the north coast of Sumatra are related to the southward subduction of Andaman Sea oceanic crust (Rock et al. 1982; Chapter 9). However, Sieh & Natiwidjaya (2000) have shown that in the northern part of the volcanic arc, the subducted Indian-Australian ocean slab has a shallow angle of dip, so that the 100 and 200 km contours are deflected eastwards beneath the volcanoes of northern Sumatra.

119

Serpentinite diapirs emplaced in strike-slip fault zones in northern Sumatra have been considered previously to have been derived from ophiolite bodies in the Woyla Group, and this may be the case (Cameron et al. 1980; Cameron et al. 1983--Takengon geological map). However, it is possible that some of these bodies represent 'push-up blocks' and slivers of serpentinised mantle wedge intruded into releasing bends in the deep crustal Sumatran strikeslip fault and thrust complex, due to disturbance of the mantle, caused by distortion of the oceanic slab (Karig 1979; Mann & Gordon 1996). Late Miocene-Pliocene volcanicity was particularly active in southern Sumatra, and the development of the volcanic arc was contemporaneous with inversion of the backarc basins c. 5 Ma which caused 'Sunda-style', N W - S E folds and associated faulting (Eubank & Makki 198 l). At the same time the Barisan Mountains reached their maximum elevation due to the combination of magmatism and tectonics. In the Forearc region the redistribution of mass in the accretionary wedge (Matson & Moore 1992) resulted in uplift of the outer arc ridge and a phase of fault inversion on the outer arc islands (Samuel et al. 1995). Intrusive m61ange diapirs, carrying blocks of the Bangkaru Ophiolite Complex, Tertiary sediments and samples of the continental crust buried beneath the Forearc, were initiated in the Pliocene and continue to the present day represented by mud volcanoes on Nias (Samuel et al. 1997). Page et al. (1979) suggested, and Fauzi et al. (1996) using seismic data have confirmed, that subduction of the Investigator Fracture Zone beneath Sumatra was the trigger for the development of the Quaternary Toba Caldera System (Chesner & Rose 1991). How far back in time volcanicity in the Toba area can be attributed to the subduction of the fracture zone is debatable. The Mid-Late Miocene Pinapan Formation contains acidic volcanics, the Toru Formation is intruded by alkaline and High-K hypabyssal bodies (Table 8.6) and the Nabirong Formation contains intermediate volcanics. These occurrences suggest that the influence of the subduction of the Investigator Fracture Zone may extend back into the Mid-Miocene. In the Backarc the Asahan Arch, which separates the North and Central Sumatra Backarc basins is parallel to the Investigator Fracture Zone and may be related to its subduction. De Smet & Barber (Chapter 7) report that the Asahan Arch formed a topographic feature from earliest Miocene times. The Investigator Fracture Zone is not the only transform fault in the ocean plate subducted beneath Sumatra. Unnamed fracture zones in the northwestern Wharton Basin to the south of Pulau Enggano (Liu et al. 1983) impact with a gentle restraining bend in the subduction trench, and project northwards beneath southern Sumatra and intersect the Sumatran Fault Zone. Shallow earthquake epicentres (Nishimura et al. 1986) and the Pliocene High-K Ranau and Pasumah Tuff fields lie along the northward projections of these fracture zones. These alignments may be coincidence; these occurrences of the rhyolitic tufts may have other explanations, related to the complex tectonics and Quaternary volcanicity in the Sunda Strait to the southeast, as discussed by Gasparon in Chapter 9.

Chapter 9 Quaternary volcanicity MASSIMO GASPARON

The Quaternary volcanoes along the Sunda and Banda arcs of Indonesia are a well-known example of subduction-related volcanism. Subduction zones are the major sites of crustal recycling on the Earth, and it is the recycling of crustal material into the mantle that contributes to the continuing chemical differentiation of the planet. Relatively primitive subduction-related magmas that might be melts of material beneath the volcanic arc, unmodified by postmelting processes, are rare, so that much attention has been dedicated in the last two decades to the study of the isotopic systematics of the most mafic volcanics as a means of identifying their source materials. These suggest that sediments--or fluids derived from the sediments--subducted along the Sunda Trench might have an effect on the composition of the Sunda-Banda arc volcanics. Gasparon & Varne (1998), however, argued that the isotopic signature of mafic volcanics in some sectors of the arc resembles that of Indian Ocean basalts, and that alongarc variations in magma types cannot be accounted for by crustal contamination in the mantle source. Indeed, Gasparon & Varne (1995, 1998) suggested that late-stage (post-melt generation) crustal contamination is the main process responsible for the wide array of volcanics in the Quaternary Sunda arc. The first detailed and comprehensive synthesis of the geology of Indonesia was published by van Bemmelen (1949), and an IAVCEI catalogue of the active volcanoes followed in 1951, compiled by Neumann van Padang (1951). This was later revised and updated by Kusumadinata (1979). These two fundamental publications, rich in information and bibliographic material, mainly describe the geology (i.e. stratigraphy and palaeontology) and, as far as the volcanoes are concerned, the morphology and type of activity of the volcanic structures. Other early works include a summary and review of the Sumatran volcanism by Westerveld (1952a), and a discussion of the relationship between tectonic setting and magmatic activity by Rittman (1953), which anticipated aspects of the ' K - h ' relationship formulated by Dickinson & Hatherton (1967). The work of van Bemmelen (1949) is essentially based on preplate tectonics ideas, and a plate-tectonic synthesis of the geodynamic evolution of the Sunda-Banda arc did not appear until the late 1970s, when Hamilton (1979) integrated the previous knowledge of the geology of Indonesia with a wealth of modern geophysical and geological data and observations, and interpreted them within the paradigm of modern plate tectonics, producing a detailed geo-tectonic map of the Indonesian region and a work that is a fundamental reference for any study of the Indonesian volcanism. Since Hamilton's work, the Quaternary volcanoes of the Banda arc and of the eastern portion of the Sunda arc (east of Sumatra) have attracted the attention of many researchers. These include Whitford et al. (1979, and references therein), Whitford & Jezek (1979), Morris & Hart (1980), Hutchison (1981), Nishimura et al. (1981), Whitford et al. (1981), Whitford & Jezek (1982), Foden & Varne (1981a, b), Foden (1983), Varne (1985), Varne & Foden (1986), Wheller et al. (1987), van Bergen et al. (1989), Varekamp et al. (1989). Volcanic centres with 'unusual' composition, such as Muriah in east Java, have been the subject of a number of works (e.g. Ferrara et al. 1981; Calanchi et al. 1983; Nicholls & Whitford 1983; Edwards et al. 1991). More recently, O, U - T h , He, Be, Sr, Nd and Pb isotope signatures have been

120

investigated to characterize mantle sources (e.g. Gerbe et al. 1992; Harmon & Gerbe 1992; Poreda & Craig 1989; Hilton et al. 1992; Gasparon et al. 1994; Poorter et al. 1991; Edwards et al. 1993; Gasparon & Varne 1998). With a few notable exceptions, the Quaternary volcanism of Sumatra has been neglected by the scientific community. Rainfall and temperature in Sumatra are higher than in the other islands of the arc, and the rate of weathering is often spectacular, even in extremely young samples. Most of the active volcanoes in Sumatra have produced only very small amounts of consolidated juvenile material in recent times, and fresh basaltic lavas are extremely rare. The other problem of Sumatra is its accessibility. The Trans-Sumatra Highway, a relatively good road, running from south to north parallel to the volcanic arc, was completed only in 1989, and air, road, and river transport to some of the most sparsely populated and remote areas, where most of the volcanic centres are situated, can still be a risky and time-consuming (albeit exciting and extremely rewarding) activity.

Quaternary volcanic arc and its relationship with main tectonic features of Sumatra The island of Sumatra, the sixth largest in the world, runs parallel to the westernmost section of the Sunda Trench, from which is separated by a well-developed forearc (Mentawai Islands) and an outer-arc basin, from about 6~S 105~E to 6~ 95~ (Fig. 9.1). The Australian-Indian Ocean Plate is currently being subducted under SE Asia at a rate of 6 to 7 cm a -~, in a N3~ direction (McCaffrey 1991). Therefore, the direction of convergence varies from about 0 ~ off Java (i.e. perpendicular to the trench), to N25"E off south Sumatra, to N31'>E off north Sumatra (Newcomb & McCann 1987). There is evidence for subduction along the SW margin of Sundaland since at least the Permian (Cameron et al. 1980), and the age of the subducted Indian Ocean crust (based on palaeomagnetic anomalies) varies from about 80 Ma in the Sunda Strait to less than 60 Ma in north Sumatra (Liu et al. 1983). An important consequence of the different angle of subduction is the difference in intensity and depth of earthquakes in Sumatra and Java. Large interplate earthquakes are common off Sumatra, and define a Benioff zone dipping at low angles. In contrast, foci of earthquakes in Java reach a maximum depth of about 650 kin, and define a much steeper Benioff zone (Hamilton 1979; Newcomb & McCann 1987). The main tectonic feature of mainland Sumatra is the Sumatra Fault System (or Semangko Fault), a strike-slip dextral fault system that extends for the whole length of the island from the Sunda Strait to the Andaman Sea, where it links with a series of transform faults which continue further north. In the southern part of Sumatra the Semangko Fault splays into a complex geometry of sections (Fig. 9.2) and pull-apart basins (Bellier & Sebrier 1994), and terminates in the Sunda Strait against a north-southtrending fracture zone (Nishimura et al. 1986) that may mark the southeastern boundary of the SIBUMASU terrane (Gasparon & Varne 1995), and the transition from a direction of subduction perpendicular to the arc in Java to oblique subduction off Sumatra (Fig. 9.3). There is general agreement in considering this fault

QUATERNARY VOLCANICITY

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Fig. 9.1. Simplified geological map of Sumatra (modified from van Bemmelen 1949), showing the main volcanic and tectonic units, and the location of identified Quaternary centres. Numbers refer to the centres listed in Table 9.1, e.g. 01 is volcano number 0601-01 (Pulau Weh).

system as a consequence of the oblique subduction, with estimates of the S E - N W offset ranging from about 100 km (Posavec et al. 1973) to up to 500 km since the Oligocene (Wajzer et al. 1991). More recently, McCarthy & Elders (1997) established an offset of 150 kin for the central part of Sumatra. This variability is related to the complexity of the forearc, as it is not clear how much of the strain is accommodated by the forearc itself. The Semangko Fault is a very important tectonic and basement boundary, as it marks the western margin of the SIBUMASU terrane (Gasparon & Varne 1995; see Fig. 9.3). North of Lake Toba the Semangko Fault separates older, mainly Tertiary, volcanic and plutonic units to the south from Quaternary volcanoes to the north. Page et al. (1979) suggested that the offset of the active volcanic arc to the north of the Semangko Fault is the result of a change of the angle of subduction corresponding with the point where the Investigator Ridge intersects the trench (Fig. 9.3). In south and central Sumatra all the Quaternary volcanic centres are situated within 50 km of the fault. As in

Java (Hamilton 1979), Tertiary volcanics lie slightly closer to the SW coast, suggesting that the Tertiary volcanic axis was closer to the Sunda Trench than the Quaternary one (Rock et al. 1982). Curray et al. (1982), however, proposed the existence of a SSE-dipping subduction zone, active since the Pleistocene, located 2 0 - 2 5 k m off the coast of north Sumatra (Aceh Province), which may be a consequence of the opening of the Andaman Sea. According to Rock et al. (1982) and Gasparon (1994), the Quaternary volcanoes situated north of Lake Toba could actually be related to this younger subduction rather than to the subduction along the Sunda Trench. The other main tectonic feature controlling the distribution of seismic and volcanic activity is the Investigator Ridge, an oceanic dextral fracture zone parallel to the 90~ Ridge and extending into the fore-arc and underneath the continental margin (Newcomb & McCann 1987). The Toba caldera is situated on the continuation of this ridge, and activity in the Toba area might be

122

CHAPTER 9

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more closely related to the Investigator Ridge than to the Sumatran Fault System, which runs west of the caldera. Also, the distribution of the Quaternary volcanic centres changes dramatically north of the intersection of the extrapolated ridge crest with the island (see Fig. 9.3), and the ridge acted as an effective barrier to the sediments from the Ganges-Brahmaputra fluvial system. The Sunda Strait marks the transition from a frontal to an oblique subduction, and is interpreted as an area of extension resulting from the northwestward motion of the forearc slivers situated between the trench and the Sumatra Fault System (Huchon & Le Pichon 1984). The area is tectonicaily and topographically very complex, and according to Ninkovich (1976) the opening of the strait is the result of 'a clockwise rotation of Sumatra of about 20 c' about an axis located in or near the Sunda Strait' since the Late Miocene. Nishimura et al. (1986) also proposed a clockwise rotation of Sumatra in relation to Java of about 5 ~~to l O Ma-Z since at least 2 Ma. More detailed recent studies (Harjono et al. 1991) supported the early conclusions by Huchon & Le Pichon (1984), and confirmed that the Sunda Strait is an area of extensional regime. An important consequence of such extensional regime may have been the eruption in recent times of large volumes of acid pyroclastic rocks and subordinate andesites and basalts within and at the margins of the strait. Nishimura et al. (1986) identified two large low gravity anomalies in south Sumatra, and an even larger one just off the coast of Java (Fig. 9.2), and suggested that these may be the sources of the thick Quaternary ignimbrites that cover large areas in south Sumatra (Lampung and Tarahan Formations, and pyroclastic deposits of the Semangko Valley) and west Java (Malingping and Banten Tufts). According to their calculations, the large low gravity anomaly off west Java

Fig. 9.2. Simplified geological map of the Sunda Strait (modified from Nishimura et al. 1986) showing the main tectonic and volcanic features and localities in the Sunda Slrail menlioned in the text.

is consistent with the existence of a caldera with a diameter of about 26 kin, that erupted over 45 km 3 of material in the last 0.1 Ma. Their estimates were based on a calculated (from gravity anomaly data) crustal density of about 2 . 4 g c m -3 (similar to the density of Tertiary sediments) in west Java, compared with a higher density of 2.6 g cm --~ (consistent with the existence of Palaeozoic gneisses and granites) in south Sumatra. Both Nishimura et al. (1986) and Harjono et al. (1991) suggested that a N35 E-trending fracture zone runs from Panaitan Island to Krakatau and on to Sebesi and Sebuku Islands, to Mt Rajabasa in mainland Sumatra, and to the Sukadana Plateau (Fig. 9.2). However, there is as yet no evidence that volcanism evolved in time and composition along this fracture, as suggested by Nishimura et al. (1986) and Harjono et al. (1991), who based their interpretation only on the location of these volcanic centres. The available age (Soeria-Atmadja et al. 1985; Nishimura et al. 1986; Simkin & Fiske 1983) and geochemical (see further discussion) data seem to suggest that these structures formed virtually at the same time, and that there are no systematic relationships between age and composition, and location along the fracture zone. The fracture zone is clearly identified by a cluster of shallow earthquakes, and is an important tectonic boundary (perhaps the southernmost margin of the SIBUMASU terrane; see Gasparon & Varne 1995) between the eastern part of the Sunda Strait (a relatively flat and shallow area filled with up to more than 3000 m of Quaternary to Upper Pliocene marine sediments and interpreted as a rapidly subsiding trough (Noujaim 1976)), and the western part, characterized by a 1800 m deep north-southtrending graben believed to be the continuation of the Sumatran Fault System (Nishimura et al. 1986; Harjono et al. 1991).

QUATERNARY VOLCANIC1TY

123

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Fig. 9.3. Synthesis of the principal Quaternary volcano-tectonic features of Sumatra. Note that the Investigator Ridge is subducted under Sumatra, and the Toba complex is situated at the intersection between the Semangko Fault and this ridge. North of the Toba complex, Quaternary volcanic centres are associated with a south-dipping subduction in the Andaman Sea rather than to the north-dipping subduction forming the Sunda arc. According to Gasparon (1994), the North Sumatra volcanics are compositionally similar to the Sunda arc volcanic (see Fig. 9.4), and it is therefore inferred that the northeastern part of Sumatra is also part of the SIBUMASU terrane. Bukit Telor and the Sukadana basalts are situated in a back-arc position along an extensional axis that probably continues into the compositionally similar Karimunjawa Islands north of eastern Java. Another structurally complex extensional area (a series of pull-apart basins related to the Semangko Fault and to the clockwise rotation of Sumatra with respect to Java) is found in South Sumatra and in the Sunda Strait. Here, the Semangko Fault terminates against a north-south-trending fracture zone that probably marks the southwestern boundary of the SIBUMASU terrane.

The widespread occurrence in Sumatra of granites and other intrusive bodies, of crystalline schists believed to be part of a pre-Mesozoic basement, and of sedimentary units as old as Carboniferous, are the basis for considering Sumatra to be mainly composed of relatively old continental crust (van Bemmelen 1949; Hamilton 1979; Clarke & Beddoe-Stephens 1987; Hutchison 1989; Gasparon & Varne 1995). Part of the Sumatran crust therefore predates the opening of the Indian Ocean, and is thus Gondwanan in its affinities. Silicic pyroclastic rocks are far more abundant than andesitic and basaltic volcanics (Westerveld 1952a; Gasparon 1994) 9

Pyroclastic deposits Compared with the other islands of the Indonesian arc, Sumatra is rich in young fragmental silicic volcanic rocks associated with major caldera-forming events, and commonly believed to have involved the melting of upper crustal material (e.g. Hamilton 1979; Gasparon & Varne 1995). Four major Pliocene to Quaternary pyroclastic deposits are known in Sumatra: the Lampung and Ranau tufts in south Sumatra, the Padang tufts in central Sumatra, and the Toba tufts

in north Sumatra (Fig. 9.1). Three of these deposits are associated with large eruptions that formed the calderas now occupied by three of the major lakes of Sumatra (Lake Ranau, Maninjau, and Toba). The location of the fourth eruption, that produced the Lampung tufts in south Sumatra, is possibly in the Sunda Strait (Nishimura et al. 1986) not far from Krakatau. The Toba tufts have been studied in some detail (see e.g. Wark 2001, and references therein), but the other major recent pyroclastic deposits have received little attention (Westerveld 1952a; Leo et al. 1980; Gasparon & Varne 1995) 9 Little is known about the Ranau and Lampung tufts. Westerveld (1952a) briefly discussed some major element analyses of tufts from several localities (including the Ranau and Lampung tufts) in his review of Sumatran volcanism, and pointed out similarities between the Sumatran Pliocene and Quaternary tufts and the Taupo ignimbrites in New Zealand. For the Lampung tufts, Nishimura (1980) and Nishimura et al. (1984, 1986) obtained a fission track age of 0.09 4-0.01 Ma, and an older age (1 4- 0.2 Ma) for an ignimbrite sampled close to Kotaagung at the southern end of the Semangko fault. Based on major and trace element evidence, they concluded that these ignimbrites are similar in composition (but not in age, nor in isotopic composition, as the new data show) to the tufts in the Lake Toba area and in central and West Java, and considered

124

CHAPTER 9

about the geochemical and P b / S r / N d isotopic composition of the Toba tufts. Gasparon & Varne (1995) noted that the Toba tufts have low NazO/K20 and high R b - S r and Nb values, typical of the S-type granites of the Central Granitoid Province of SE Asia. Chesner (1998) carried out a detailed petrological study of the Toba tuff units, and concluded that the observed compositional variation from dacitic to rhyolitic magmas resulted from extensive crystal fractionation in convecting magma bodies. Wark (2001) identified two separate magma reservoirs using Sr and Nd isotope criteria: a northern reservoir with eNd = - 1 0 . 9 a n d 87Sr/86Sr = 0.7155 a n d 87Sr/86Sr and a southern reservoir with eNd = - 1 0 . 0 values ranging from 0.7132 to 0.7140. Unlike the other Quaternary fragmental deposits, the Toba tufts are isotopically (and possibly compositionally) similar to the granitoids exposed in east Sumatra (Fig. 9.4) and peninsular Malaysia, suggesting that little or no juvenile material participated in their formation, and that they derived essentially from crustal melting (Whitford 1975; Gasparon & Varne 1995). On the other hand, the Quaternary volcanoes in the Lake Toba area show a rather variable isotopic composition, with values ranging from close to those found in the arc andesites elsewhere in Sumatra, to rather more radiogenic, suggestive of varying amounts of interaction between the same juvenile material that forms the arc andesites and the east Sumatran upper continental crust (Gasparon & Varne 1995; see Fig. 9.4).

them as the result of the remelting of the lower crust. Bellier et al. (1999) obtained a K - A r age of 0.55 Ma for feldspars separated from the Ranau tufts, and concluded that the collapse of the Ranau caldera occurred between 0.7 and 0.4 Ma. K - A r whole-rock age determinations for the andesitic centres and tufts surrounding the Maninjau caldera range from 0.83 _+ 0.42 Ma for the older, pre-caldera andesites, to 0.28 _ 0.12 Ma for the youngest rhyolitic ash-flows (Leo et al. 1980). For the same samples, 87Sr/ 86Sr values are in the range 0.7056-0.7066, and Gasparon & Varne (1995) reported an 87Sr/86Sr value of 0.70473 for a Quaternary granite in the same area. These values are slightly higher than those of most andesitic centres elsewhere in the Sunda arc and in Sumatra (Whitford 1975; Gasparon 1994), and it is suggested that they reflect the involvement of sialic crustal material. Gasparon & Varne (1995) noted that the compositions of most igneous rocks from centres in the volcanic arc and west of the Semangko fault fall within the calc-alkaline differentiation trend (Debon & Le Fort 1988), with a complete overlap between intrusive and generally more differentiated pyroclastic rocks. Their geochemical and isotopic composition is typical of volcanic arcs built on continental crust. Initial 87 Sr/ 86Sr values range from 0.7045 to 0.7065 for the fragmental deposits of Lake Maninjau, Lake Ranau and the Lampung Formation. These values are substantially lower than the lowest values observed in the granitoid provinces of SE Asia, and lower than those of the Toba tufts (Fig. 9.4). Gasparon & Varne (1995) further argued that the remarkably constant initial 87Sr/86Sr values of granitoids, fragmental deposits, and andesitic lavas along the volcanic arc suggest derivation from a common source. Whitford (1975) first suggested, on the basis of a single 87Sr/86Sr value of 0.71392, that the Toba tufts have a crustal origin. Most of the studies on the Toba caldera have dealt with the chronology and stratigraphy of the different ignimbrites (e.g. Ninkovich et aI. 1978a, b; Knight et al. 1986; Chesner et al. 1991" Chesner & Rose 1991), and relatively little is known

0.716

..........................................................................................

I'",

"

Quaternary

arc volcanoes

Volcanic rocks associated with the active volcanic arc outcrop extensively in Sumatra, and range in composition from rare basalts to abundant andesites and dacites. Active and dormant volcanoes of south and central Sumatra were visited, mapped, and described by Dutch geologists during the period 1910-1940,

;

"

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9

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............................

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0.702 0.01

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~

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(basalt to dacite)__L. . . . . 10

Fig. 9.4. Initial mSr/S6Sr v. Rb/Sr diagram for Sumatran volcanics and basement granitoids. Data from Gasparon & Varne (1995) and references therein. Note the overlap between the Toba tufts and the SIBUMASU granitoids. "Aceh Arc Volcanics' are the Quaternary centres related to the south-dipping subduction in the Andaman Sea; other units as in Figure 9.3. Note the gap in initial ~7Sr/86Sr values that separates crustal melts derived from the melting of the SIBUMASU te]xane (initial 87Sr/S6Sr higher than 0.710) from predominantly mantle-derived melts of the Sunda arc, 'Aceh' arc, and backarc basalts of Sukadana and Bukit Telor (initial mSr/86Sr lower than 0.708). The Quaternary andesitic to dacitic volcanics of the Toba area represent mixing between mantle-derived melts (Sunda arc endmember) and crustal melts (SIBUMASU endmember).

QUATERNARY VOLCANICITY and whole-rock major element analyses performed during this period were collected and published by Neumann van Padang (1951) and Westerveld (1952a). More recently, Kusumadinata (1979) reviewed the existing literature on volcanic activity in the Indonesian arc. The part of the island north of Lake Toba, the Aceh Special Province, remained virtually geologically unexplored until the mid seventies, when the North Sumatran Project was undertaken by the Indonesian and British Governments. A description of the geology of north Sumatra can be found in Page et al. (1979), Bennett et al. (1981a, b), and Cameron et al. (1983). A simplified geological map of Sumatra, with the location of identified Quaternary centres, is shown in Figure 9.1. The exact number of Quaternary volcanic centres in Sumatra is not known. Kusumadinata (1979) and Simkin (1981) reported over 180 historic eruptions from 14 different volcanic centres, with 14 more centres in the solfatara/fumarolic stage. At least 37 eruptive events from nine centres have been reported since 1980. Thirty-six Holocene centres (including Krakatau) are currently listed by the Smithsonian Institution (2002). These include centres with documented explosive activity but with no conclusive evidence of historic eruptions, and fumarole fields not associated with volcanic structures. Field evidence suggests that the number of active volcanoes is only a small portion of the total number of Quaternary centres. The majority of the historically active volcanoes are stratovolcanoes (20), with summits standing between 600 m (Pulau Weh) and 3800 m (Kerinci) above sea level. Most of these centres are structurally complex, with numerous solfatara fields and hot springs, summit craters and parasitic cones. Other structures include calderas (Toba, Ranau, Sekincau, Hulubelu and Krakatau), complex volcanoes (Peuet Sague, Talakmau, Marapi and Belirang-Beriti), fumarole fields (Helatoba-Tarutung and Gayolesten) and pyroclastic cones (Sarik-Gajah). Active maars and silicic domes have been described in the 8 x 16 km Suoh depression in south Sumatra. However, according to Bellier & Sebrier (1994) the Suoh depression is a pull-apart caldera similar to the Ranau caldera. All the centres situated north of 4~ (Pulau Weh, Seulawah Agam, Peuet Sague, Geureudong, Bur Ni Telong, and possibly the Gayolesten fumarole field) are likely to be related to the SSE-dipping subduction zone located 2 0 - 2 5 km off the coast of north Sumatra (Curray et al. 1982; Rock et al. 1982; Gasparon 1994). Therefore these centres are genetically distinct from all the other centres situated along the Sumatran arc. Volcanic rocks of the Quaternary Sumatran arc include calcalkaline basalts, andesites and dacites, typical of a volcanic arc built on continental crust. In addition to the analyses reported in Neumann van Padang (1951) and Kusumadinata (1979), geochemical data, including isotopic data, have been published by Westerveld (1952a), Whitford (1975), Leo et al. (1980), Bennett et al. (1981a, b), Rock et al. (1982), Gasparon & Varne (1995) and Bellon et al. (2004). In addition, detailed petrological studies have been carried out on Krakatau (see Smithsonian Institution 2002, for a list of references), Bukit Mapas (Della Pasqua et al. 1995) and the Sukadana basalts (see below). Helium isotope analyses of olivines and clinopyroxenes separated from lavas of seven Sumatran centres (Kerinci, Ratai, Bukit Mapas, Dempo, Bukit Telor, Krakatau and the Sukadana basalts) were reported in Gasparon et al. (1994). Relatively primitive rocks are rare, and detailed mineralogical investigations have shown that even the most primitive lavas have suffered shallow-level crustal contamination (Gasparon et al. 1994; Della Pasqua et al. 1995). Based on geochemical and Sr, Nd and Pb isotopic data reported in Gasparon (1994), Gasparon & Varne (1998) noted that the overall composition of Sumatran Quaternary arc volcanics is genetically homogeneous, and concluded that assimilation of crustal material by uprising mantle-derived magmas accounts for the overall characteristics

125

of Sumatran arc volcanics, and for their overall stronger crustal signature compared with the magmas of the other sections of the west Sunda arc.

Quaternary backarc volcanics Olivine-phyric basalts in Sumatra were first recognized by Dutch geologists in the early 1930s (cited by van Bemmelen 1949) during the geological surveys of the island. In his comprehensive work, van Bemmelen (1949) discussed the occurrence of olivinebearing basalts in Sumatra, and considered them to be 'basaltic effusions in the post-orogenic stage', related to the 'tensional stresses and major fissures along the edges of the Sunda land' caused by 'the bending of the consolidated crust due to the downwarp of marginal troughs' (van Bemmelen 1949, vol. 1, page 253). These basalts were recognized as petrographically different from the rare olivine-bearing, relatively primitive basalts found in other areas along the arc, and they clearly occupy a backarc position. According to van Bemmelen (1949), the Sukadana and the Bukit Telor basalts in Sumatra (Fig. 9.1) belong to this stage, as well as rare basalts found in other small areas in SE Asia: the Karimunjawa Islands north of central Java, Bukit Nyut in west Kalimantan and some Quaternary volcanoes in central Kalimantan, Midai Island in the Natuna Islands group, and the Isle des Cendres and Cecir de Mer (now Catwick Islands), two small islets off the southern coast of Vietnam. More recently, Westerveld (1952a) reported some analyses of basalts from the Sukadana Plateau and from Bukit Mapas, made by Dutch analysts in 1929 and 1931. In his geological sketch map of south Sumatra, the basalts from Sukadana and Bukit Mapas are described as different from (and contemporaneous with) the other basalts related with the mainly andesitic centres forming the volcanic arc, although, based on major element chemistry, they were interpreted as genetically related to the arc andesites. Gasparon (1994) and Della Pasqua et al. (1995), however, demonstrated that Bukit Mapas is genetically similar to the other magmas of the Quaternary volcanic arc. No other occurrences of this type of basalt have been confirmed in Indonesia since van Bemmelen's work. Soeria-Atmadja et al. (1985) analysed and dated some samples from the Karimunjawa Islands and the Sukadana plateau, and compared the Karimunjawa and the Sukadana basalts with the Sumatran arc andesites, pointing out some of their peculiar intra-plate and backarc characteristics. Nishimura et al. (1986) in their study of volcanism in the Sunda Strait, reported a K - A r age of 0.8 Ma and some trace element data for a sample from Sukadana. Dosso et al. (1987) and Romeur et al. (1990) described the Sukadana basalts as relatively primitive tholeiitic basalts, with 8 - 9 % MgO, 250-350 ppm Cr, and 150-200 ppm Ni, high but variable concentrations of hygromagmaphile elements, and with 87Sr/86Sr values and eNd values in the range 0.70370.7045 and + 1.6 to +6.5 respectively (Dosso et al. 1987), intermediate between MORB and OIB. With the exception of the study by Gasparon (1994), the small outcrops of Bukit Telor (also known as Bukit Ibul) have never been investigated. The Sukadana basaltic plateau is situated in SE Sumatra (Lampung Province), about 3 0 - 4 0 km NNE of the capital city of the province, Tanjungkarang. It covers an area of approximately 1000 km 2, and is made of several basaltic flows up to 2 - 3 m thick, erupted along fissures trending N W - S E , parallel to the Semangko Fault. The average height of the plateau above the surrounding area is only 30-40 m, but several hills, probably representing eruptive centres, are more than 200 m (above sea level), and although the basaltic pile might locally be up to 200 m thick, no outcrops thicker than about 10 m have been observed. Most of the area is covered by up to 2 m of lateritic soil, and outcrops are found only occasionally along river scarps, quarries and on top of the youngest,

126

CHAPTER 9

Table 9.1. Volcanic activity of Sumatra Volcano name and number (synonyms)

Type, elevation (m) and location

Status, last known eruption

Notes

Pulau Weh 0601-01

Stratovolcano, 617, 5.82~

Fumarolic, unknown (Pleistocene?)

Seulawah Agam 0601-02

Stratovolcano, 1810, 5.448"N 95.658-~'E

Historical, 1839 (possibly only hydrothermal)

Peuet Sague 0601-03

Complex volcano, 2801, 4.914~ 96.329~E

Historical, 2000 (ashfall)

Geureudong 0601-04 (Bur ni Geureudong 0601-04 and Bur ni Telong 0601-05)

Stratovolcanoes, 2624, 4.813~'N 96.82~

Historical, 1937 (explosive eruptions)

Kembar 0601-06 (Gayolesten)

Shield volcano, 2245, 3.850 N 97.664~

Fumarolic, unknown (Pleistocene'?)

Sibayak 0601-07

Stratovolcano, 2212, 3 . 2 0 N 98.52 E

Historical, 1881 (explosive eruptions)

Sinabung 0601-08

Stratovolcano, 2460, 3 . 1 7 N 98.392 E

Historical, 1881 ? (explosive eruptions)

Toba 0601-09

Caldera, 2157, 2.58~'N 98.83 E

Holocene, unknown (~70 ka)

Helatoba-Tarutung 0601 - 10

Fumarole field, 500 to 1100. 2.03N 98.93 E

Fumarolic, unknown (Pleistocene?)

Sibualbuali 0601-11

Stratovolcano, 1819, 1.556 N 99.255'E

Holocene, unknown

Lubukraya 0601-11 l

Stratovolcano, 1862, 1.478 N 99.209~ E

Holocene, unknown

Sorikmerapi 0601 - 12

Stratovolcano, 2145, 0.686 N 99.539~E

Historical, 1986 (central vent eruption, explosive eruption, phreatic explosions)

Talakmau 0601 - 13

Complex volcano, 2919, 0.079 N 99.98"~E Pyroclastic cones, unknown, 0 . 0 8 N 100.20~ Complex volcano, 2891, 0.381<S 100.473~

Holocene, unknown (uncertain central vent eruption in 1937) Holocene, unknown

Stratovolcano, 2438, 0.433'~S 100.317~

Historical, 1924 (explosive eruption and phreatic activity)

Remnant of partially collapsed older centre. Active fumaroles and hot springs. No activity reports Pleistocene-Holocene volcano built within a Pleistocene caldera. Summit crater. Flank crater with active fumarole fields. No activity reports Extremely remote volcano. Four summit peaks. Pyroclastic flows and growth of lava-dome observed in 1918-1921. Several unofficial reports prior to 1990, three activity reports since 1998. Two adjacent volcanoes. Bur ni Geureudong has a Pleistocene age and active flank solfataras and hot springs, Bur ni Telong is built on its southern flank and is historically active (explosive eruptions). No activity reports. Fumarole field on the flanks of a Pleistocene andesitic shield volcano capped by a complex of craters and cones. Numerous active fumaroles and hot springs. No activity reports. Twin-volcano complex (Sibayak and Pinto) with a compound caldera. No activity reports. Four overlapping summit craters with solfataric activity last observed in 1912. No activity reports. Earth's largest Qualernary caldera, 35 x 100 kin, lk)rmed during four major Pleistocene eruptions that produced over 2500 km ~ of ejecta. Post-eruptive activity includes lava domes and the formation of minor volcanic structures. No activity reports. Active field of over 40 sulphurous hot springs, 40 kin long, located south of Lake Toba. No activity reports. Eroded Pleistocene stratovolcano with two active solfatara fields on its eastern flank. No activity reports. Pleistocene-Holocene andcsitic stratovolcano with prominent lava dome at its southern toot. No activity reports. Stratovolcano with a summit crater lake. Several active solfatara fields and numerous phreatic eriptions recorded during the 19th and 20th centuries. Eruption in 1892 produced lahars that killed 180 people. Six activity reports since 1986. Three summit craters, the highest one filled by a lava dome. No activity reports. Two young cones, one with a rubbly lava flow. No activity reports. Sumatra's most active volcano. Broad summit with multiple overlapping summit craters constructed within a caldera. More than 50 eruptions (mostly central vent explosive eruptions) recorded since the 18th century. One fatality in 1992. Twenty activity reports since 1978. Twin volcanoes Tandikat-Singgalang, now extinct. No activity reports.

Sarik-Gajah 0601-131 Marapi 0601-14

Tandikat 0601-15

95.28~E

Historical, 2001

(continued)

QUATERNARY VOLCANICITY

Table 9.1

127

Continued

Volcano name and number (synonyms)

Type, elevation (m) and location

Status, last known eruption

Notes

Talang 0601-16

Stratovolcano, 2597, 0.978'S 100.679 E

Historical, 2001 (phreatic explosion)

Kerinci 0601-17

Stratovolcano, 3800, 1.814S 101.264E

Historical, 2002 (explosive eruption)

Hutapanjang 0601-171

Stratovolcano, 2021, 2.33"S 101.60E Stratovolcano, 2507, 2.414' S 101.728E Stratovolcano, 2151, 2.592~ S 101.63E

Holocene, unknown

Twin volcanoes Talang-Pasar Arbaa, now extinct. Two crater lakes on its flanks. All historical eruptions originated from craters on its upper NE flank. Six activity reports since 1986. Indonesia's highest volcano, and one of Sumatra's most active. Numerous moderate eruptions recorded since 1838. Eight activity reports since 1987. No activity reports.

Sumbing 0601-18 Kunyit 0601 - 19

Belirang-Beriti 0601-20

Historical, 1921 (explosive eruption) Fumarolic, unknown

Compound volcano, 1958, 2.82S 102.18 E Stratovolcano, 2467, 3.38'~S 102.37 E

Fumarolic, unknown

Kaba 0601-22

Stratovolcano, 1952, 3.52:S 102.6TE

Historical, 2000 (explosive eruption)

Dempo 0601-23

Stratovolcano, 3173, 4.03~ 103.13r

Historical, 1994 (explosive eruption)

Patah 0601-231

Unknown, 2817, 4.27~ 103.30~E

Bukit Lumut Balai 0601-24

Stratovolcano?, 2055, 4.22'~S 103.62'E

Uncertain, 1989 (new crater and fumaroles) Fumarolic, unknown

Besar 0601-25 (Marga Bajur)

Historical, 1940 (phreatic eruption)

Ranau 0601-251

Stratovolcano?, 1899, 4.43~ 103.67'E Caldera, 1881, 4.83S 103.92~

Sekincau-Belirang 0601-26

Caldera, 1719, 5.12~ 104.3TE

Fumarolic, unknown

Suoh 0601-27 (Pematang Bata)

Maars?, 1000, 5.25~'S 104.27~

Historical, 1933, possibly 1994 (phreatic eruption)

Hulubelu 0601-28

Caldera, 1040, 5.35S 104.60~E

Fumarolic, unknown

Rajabasa 0601-29

Stratovolcano, 1281, 5.78r 105.625"E Caldera, 813, 6.102~ 105.423~'E

Fumarolic, unknown

Bukit Daun 0601-21

Krakatau 0602-00

Fumarolic, unknown

Holocene?, unknown

Historical, 2001 (explosive eruption)

b e s t - p r e s e r v e d e r u p t i v e centres. T h e e x p o s e d flows m a y s h o w colun'mar j o i n t i n g , a n d w h e r e the b a s e o f the pile is visible, it o v e r l a y s Q u a t e r n a r y t u f f a c e o u s deposits o f the L a m p u n g Formation.

Several crater remnants and a crater lake. Active hot springs. No activity reports. Fumarolic activity at the youngest summit crater and on the northern flank. No activity reports. Active fumaroles in crater walls. No activity reports. Twin volcanoes Bukit Daun-Gedang. Active fumaroles in SSW flank crater. No nkown historical eruptions. No activity reports. Twin volcanoes Kaba-Hitam. Complex summit with three large, historically active craters. Two activity reports since 1979. Large structure with seven remnants of craters. Numerous hot springs. One activity report in 1999. Unconfirmed report of new crater with active fumaroles. Two activity reports in 1989. Heavily eroded volcano with three eruptive centres and active fumarole fields. No activity reports. Large solfatara field located along its north and NW flanks. No activity reports. Large caldera filled by a lake and with a postcaldera volcano-G. Seminung. Possible sub-lacustral eruptions in 19th and 20th century. No activity reports. Active fumaroles on two coalescent calderas. No activity reports. Tectonic depression with historically active maars, silicic domes, hot springs, and fumaroles. Two activity reports in 1994. Volcano-tectonic depression with postcaldera central cones and basaltic and andesitic flank volcanoes. Active solfataras, mud volcanoes, and hot springs. No activity reports. Isolated Volcano. Active fumaroles. No activity reports. Caldera with post-collapse cone (Anak Krakatau). Catastrophic eruption in 1883, second largest in Indonesia during historical times (36 000 fatalities). Frequesnt eruptions since 1927. Thirtyseven activity reports since 1972.

Samples from several localities have been dated by SoeriaA t m a d j a et al. ( 1 9 8 5 ) a n d N i s h i m u r a et al. (1986), a n d t h e i r K-Ar ages range from 1.15_ 0.17Ma to 0.44 _+ 0 . 1 3 M a f o r the o l d e s t s a m p l e s (first c y c l e o f S o e r i a - A t m a d j a et al.

128

CHAPTER 9

BUR NI TELONG 0601-05

SORIK MARAPI 0601-12

0

10kin

.0

..........5

.10kin ii

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MARAPI 060%14

LAWAS

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110kin

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Fig. 9.5. Preliminary volcanic hazard maps ('Keterangan daerah bahaya sementara') for Sumatran volcanoes, as published in Kusumadinata (1979). 'Daerah Bahaya', danger zone; 'Daerah Waspada', alert zone; 'Sungar (s.), river; 'Jalan', road. G 'gunung' (mount), D., 'danau' (lake). These maps are based on scientific and historical records, and on local knowledge. According to Kusumadinata (1979), 'they may be useful as a temporary guide for local civil authorities in taking preliminary steps-including evacuation--in the surroundings of a volcano which is expected to erupt, while waiting for the arrival of the volcanologist-in-charge'.

QUATERNARY VOLCANICITY

129

lOkm dacurup ,,,_._.

DEMPO 0601-23

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............................ Daerah Waspada" ~,,~,~,....... )

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==============================================

=========================================== ~!:!:!:!:!:!:!:!:~:i:i:i:i:i:!iY /

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,,,.I ::::::::::::::::::::::::::::::::::::::::::::

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/~"~--"

"Jalan" Roads

,"-"-,,7\~

Topographic Contours (m)

Preliminary volcanic hazard maps for Sumatran volcanoes as published in Kusumadinata (1979)

Fig. 9.5. Continued.

1985), to less than 0.01 Ma (second cycle) for the youngest ones from well-preserved flows and spatter cones. Despite its clearly backarc position, the axis of the Sukadana Plateau is situated less than 50 km away from two coeval andesitic centres (Mt Rajabasa and Mt Ratai) that are part of the Quaternary Sumatran volcanic arc, and it overlays pyroclastic products (Lampung and Tarahan Formations) emitted by centres within the volcanic arc. Bukit Telor (also known as Bukit Ibul) is an isolated hill made of basaltic material and only 38 m high, situated about 40 km NNE of Jambi (Jambi Province), more than 200 km behind the axis of the Quaternary Sumatran volcanic arc. The hill is surrounded by Holocene alluvium and swamp deposits, Pliocene to Pleistocene tuffaceous sandstones and claystones (Kasai and Muaraenim Formations), and Miocene sandstones and claystones (Airbenakat Formation). The area of the Bukit Telor outcrop is less than 4 km 2. The stratigraphic age of these basalts is clearly Quaternary, as confirmed by a K - A r age of 1.25 + 0.19 Ma (Syachrir and Kardana, Indonesian Geological Research Centre, pers. comm. 1991). All the samples from the Sukadana Plateau and Bukit Telor described in Gasparon (l 994) are basaltic lavas, and differ considerably from the arc andesites in both texture and paragenesis.

Olivine with abundant Cr-spinel inclusions is ubiquitous as a phenocryst phase, and small lherzolite xenoliths have been found in the Bukit Telor basalts. These lavas range in composition from quartz-tholeiites to slightly alkaline basalts, and show a clear within plate affinity (Westerveld 1952a; Soeria-Atmadja et al. 1985; Gasparon 1994), but with a clear trend towards compositions typical of the calc-alkaline basalts and andesites of the Sumatran volcanic arc. The basalts of Bukit Telor and their mantle xenoliths probably represent the composition of the unmetasomatized mantle wedge at some distance from the volcanic arc, and do not bear any textural nor geochemical evidence for lithospheric contamination. Their St, Nd, Pb, and He isotope signature (Gasparon 1994; Gasparon et al. 1994) is similar to that of Indian Ocean basalts enriched in a EM component (Ninetyeast Ridge), and their overall geochemical signature suggests that they might represent small degrees of partial melting of an isotopically slightly enriched Indian Ocean mantle source. The Sukadana basalts are compositionally and isotopically more complex, and their tectonic significance and genetic processes are yet to be resolved. According to Gasparon (1994) the high-Ti basalts do not need to be plume-derived, and might simply be the result of several stages of melt extraction from a depleted

130

CHAPTER 9

primitive mantle. Relatively high degrees of partial melting of the same source that produced the high-Ti basalts yielded low-Ti, less alkaline basalts, which then suffered varying degrees of crustal contamination. Mineral chemistry and Sr, Nd, and Pb isotope data indicate that contamination occurred at relatively shallow level in the crust, and not in the source, and that relatively large degrees of crustal contamination can create melts geochemically and isotopically similar to arc melts.

Volcanic hazard Indonesia has the world's largest number of volcanoes that have erupted in historic times (76), with over 1100 dated eruptions. Approximately one seventh of the recorded eruptions in the world have taken place in Indonesia, and four fifths of the historically active volcanoes have erupted in the last century. Since 1800, destructive volcanic eruptions have occurred in Indonesia every three years, causing over 140 000 casualties and destroying a large number of villages. Two of these eruptions, Tambora 1815 and Krakatau 1883, account for over 126 000 casualties. According to Kusumadinata (1979) and the Smithsonian Institution (2002) only two historic eruptions in Sumatra have directly caused loss of life: Sorik Marapi in 1892 (180 casualties) and Marapi in 1992 (1 casualty). Table 9.1 summarizes the main features of the volcanic centres listed by the Smithsonian Institution (2002). The Volcanological Survey Division of Indonesia classified as A-type volcanoes those with recorded eruptions in historic times. Primary volcanic hazards common to most Indonesian volcanoes include lava flows, bombs and nudes ardentes, with lahars common as a secondary hazard. The definition of 'danger' and 'alert' zones in hazard maps published in Kusumadinata ( ! 979) is based largely on topographic features and on known distribution of recent nudes ardentes and lahar deposits. Hazard maps published in Kusumadinata (1979) are given as Figure 9.5. There are currently 75 A-type volcanoes in Indonesia, and 12 of these are found in Sumatra (including Krakatau). Preliminary volcanic hazard maps have been prepared for nine Sumatran volcanoes: Bur Ni Telong, Sorik Marapi, Marapi, Tandikat, Talang, Kerinci, Kaba, Dempo and Krakatau (Kusumadinata 1979). No hazard maps are available for the other three A-type

Sumatran volcanoes: Peuet Sague, Seulawah Agam and Sumbing. Peuet Sague is so remote that the total extent of the danger zone is unknown, and the population living in the danger zone is considered to be nil. Overall, Sumatran A-type volcanoes have erupted at least 170 times since AD 1000, and the total number of people living in the danger and alert zones is 33 000 and 254 000, respectively. In comparison, these numbers are over 250 000 and 1030 000, respectively, for Java, and a total of over 3 000 000 for Indonesia (Kusumadinata 1979). The total area of Sumatra exposed to volcanic hazard is just over 1060 km 2, as opposed to over 2800 km 2 for Java and 16 620 for Indonesia (Kusumadinata 1979). All the historic eruptions in Sumatra have been classified as 'moderate' (Class II, up to 0.0001 km 3 of ejecta), and only Krakatau produced more powerful eruption in 1883 (Class VIII, 18 km 3 of ejecta, second largest historic eruption in Indonesia), 1963 (Class III, 0.0003 km 3 of ejecta), and 1973 (Class V, 0.012 km 3 of ejecta). In comparison, the largest eruption in historic times (Tambora 1815) produced 150km 3 of ejecta, and over 2500 km 3 of magma were emitted by the Toba complex during its life span. The 1883 eruption of Krakatau is one of the best-documented eruptions in historic times, and captured the attention of the public like no earlier eruption. The eruption and its dramatic build-up were observed by thousands of sailors, traders and villagers, and news of the eruptions was quickly telegraphed to the whole world. The giant tsunami caused by the explosion killed over 36000 people and destroyed 165 coastal villages in Sumatra and west Java, and the blast of the eruption was heard over 4 5 0 0 k m away. The passage of the air and sea waves generated by the explosion were recorded over the globe, and the large amount of volcanic dust had spectacular effects on the atmosphere and on world's climate. A detailed account of Krakatau's activity can be found in Simkin & Fiske (1983). According to the Smithsonian Institution (2002) Krakatau has erupted at least 48 times during the last 2000 years, and the devastating eruption of 1883 followed over 200 years of inactivity. Due to its location in the vicinity of densely populated areas, high tsunami hazard and historical record of volcanic activity, Krakatau should be regarded as one of the most dangerous volcanoes in Indonesia.

Chapter 10

Fuel r e s o u r c e s : oil a n d gas JOHN C L U R E

Petroleum systems are controlled by the evolution of sedimentary basins and the provenance of their sedimentary fills. As the result of a favourable combination of these factors Sumatra is rich in petroleum resources. The discovery and exploitation of commercial accumulations of oil and gas has so far been restricted to the backarc region of Sumatra, NE of the Barisan Range and the active volcanic arc, where three major sedimentary basins, the North, Central and South Sumatra Basins are distinguished. Exploration in the Sibolga, Mentawai and Bengkulu basins along the western margin of Sumatra in the forearc region has, so far, not been as successful. Commercial success has also eluded companies which have explored basins or sub-basins, such as the Ombilin Basin, which occur within the Barisan Range. Plate-tectonic mechanisms and the resultant crustal thicknesses control this distribution of the Sumatran petroleum resources. To the east of the Barisan Range, beneath the backarc basins, the crust has been stretched and thinned and thus has a high geothermal gradient, suitable for the generation of hydrocarbons. In the forearc region, to the west of the Barisan Range, the lithosphere is thicker due to the subduction of the Indian Ocean Plate beneath the Sunda Craton in Sumatra. This effective doubling of lithospheric thicMaess has resulted in lowering of the geothermal gradient, so that sediments in the forearc basinal setting have a lower thermal maturity. Also, clastic sediments in forearc basins, due to their volcanic and metamorphic provenance, tend to be poor in quartz, and are dominated by shales and clays, rather than by sandstones. Little attention has been paid to the Pre-Tertiary sediments in Sumatra until recently, as they were considered to be economic basement, despite the oil produced from fractured metaquartzite in the North Pulai Field as long ago as 1951. However, there are now numerous developed fields in Sumatra, producing from fractured Pre-Tertiary reservoirs, within the basement, both from granitic and from metamorphic rocks. According to Zeliff & Bastian (2000), Gulf has discovered eight gas fields with the primary reservoir within the basement, and has had an 80% success rate in prospects where the basement is the main objective. Gulf discoveries include the 45 km 2 Dayung Field, which has been producing since 1998. Rifting and basin formation commenced in Sumatra during the Palaeogene, at about the same time as the Indian Subcontinent collided with the Asian Plate, either due to extension tectonics resulting from the collision according to the Tapponnier model (Tapponnier et al. 1982, 1986), or to a change in the rate of convergence of the Sunda and Indian Plates (Longley 2000), which resulted in the extension, rifting and opening of the Sumatran back arc basins. Whatever their cause, the early rift systems, trending north-south and N E - S W , were critical to petroleum generation within the Sumatran basins, all major fields being adjacent to these rifts. The earliest sediments deposited in the rift valleys are volcaniclastic and the products of erosion along the margins of the rifts, forming scree slopes and alluvial fans. Eventually, lakes and marginal river systems developed within the rift valleys. Lacustrine sediments in the deeper parts of the sedimentary sequences are rarely penetrated by the drill bit, but they may well form important source rocks throughout Sumatra. Fluvio-deltaic sandstones deposited by the river systems have been widely explored and form important reservoirs in some areas in Sumatra. Swamp

vegetation, which developed on delta tops, formed coals, which may also provide an important source of hydrocarbons. Gradually a marine incursion penetrated these rift valleys resulting in the deposition of marine shales and beach sands which overlie the fluvial and lacustrine sediments. As the rifts filled with sediment, limestone build-ups and reefs were developed on basement highs in the North and South Sumatra basins, and these now form significant reservoirs. The laterally equivalent marine shales within the deeper parts of the rifts form source rocks in some areas. The rift basins were completely drowned and marine shales were deposited forming a seal over the whole sequence, thus, in Sumatra, reservoirs are found in coastal deposits, fluvial sandstones, deltaic and paralic sandstones, and limestone build-ups, all sealed by overlying marine shales. The flooding event was followed by a gradual regression with the deposition of further fluvial sequences, resulting in the formation of sandstone reservoirs. Deposition of these regressive sequences continues to present day. There are many regional variations, but this is the overall pattern of development seen in all the Sumatran back arc basins.

North Sumatra Basin Exploration in the North Sumatra Basin commenced in the 1880s. Oil seeps had been known in this area since ancient times, but in 1880 Aeilko Jans Zijlker, a tobacco farmer, exchanged lands for a plantation containing oil seeps which were being used by locals to caulk boats. Zijlker promoted the drilling of Telaga Tunggal-1 in June, 1885, which flowed oil from the MidMiocene Baong Sandstone and became the discovery well of the Telaga Said Field. The Telaga Said Field produced 8.4 million barrels of oil over the next 70 years, and very small volumes of oil are still being produced by the local people today. The company formed in 1890 to drill this well became the Royal Dutch Company. In 1907 this company merged with Shell Transport and Trading Company to form Royal Dutch Shell. It was this company, in conjunction with the colonial government, which dominated the petroleum industry in the North Sumatra Basin until the Second World War. During this period discoveries were located in anticlines, faulted anticlines or anticlines with permeability pinch outs, producing either from the Mid-Miocene Baong Sandstone or the upper Miocene-Pliocene Keutapang and Seurula Sandstones. After WWII, in the 1960s and 1970s, Pertamina and Asamera discovered further fields in the B a o n g Keutapang play, plus the Wampu and Batu Mandi Fields, which produce from the Early Miocene Belumai Sandstone. The most significant discovery in the North Sumatra Basin was made in 1971 in a completely different play, when Mobil tested gas from the giant reefal buildup at Arun. According to Situmorang et al. (1994), Arun has ultimate recoverable reserves of 14.1 TCF of gas plus 700 mmbbls of condensate, from the Arun Limestone, which lies within the lower to Mid-Miocene Peutu Shales. Since then numerous discoveries have been made in the same formation, including Lhok Sukon South A & B fields, Paseh, Alur Siwah and NSO-A offshore. Other hydrocarbon discoveries in the Arun Limestone, for example Kuala Langsa, Peusangan and Peutouw, were found to contain large percentages of carbon dioxide and have remained undeveloped. Today the

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main plays in this area include the reef developments in the Arun Limestone and clastics in the fold belt parallel to the coast of the Malacca Strait. A parallel fold belt, further inland, has not been as productive, due to breached reservoirs. Reservoirs have been found in the lower Miocene Arun Limestone, the lower Miocene Belumai sandstones, the Mid-Miocene Baong Sandstones, the upper Miocene Keutapang Sandstones and the Pliocene Seurula Sandstones. Most of the evidence indicates that the source rocks were marine shales in the Bampo, Peutu and Baong formations, although there have been suggestions of a possible lacustrine source. The various arguments in favour of the possible source rocks are discussed in the 'Source rocks and hydrocarbon type' section below. The most significant structural feature in the North Sumatra Basin is the Lhok Sukon Trough, a prominent graben system which runs north-south and acts as the main kitchen. This is the source area for gas in the region, with the traps adjacent to the trough being the essential feature of the play. Oil found within the coastal fold-belt is probably due to the remigration of the oil associated with this gas into more recently formed Plio-Quaternary structures. Any oil that has migrated beyond this first fold-belt into the westernmost fold belt is likely to have been lost, due to the breaching of reservoirs.

Tectonic elements The North Sumatra Basin has an area of about 60 000 km 2 and the Tertiary sediments are up to 5 km thick (Fig. 10.1). The Pliocene to Holocene uplift of the Barisan Mountains has masked the actual southwestern boundary of the basin. To the NE the sediments thin onto the Malacca Shelf and onto the Asahan Arch to the south, which separates the North Sumatra Basin from the Central Sumatra Basin. To the NW the North Sumatra Basin merges into the Mergui Basin in the deep waters off the north coast of Aceh. The Mergui Ridge forms the western limit of both the Mergui and North Sumatra basins. The North Sumatra Basin can be divided into two distinct parts which have different subsidence histories. Subsidence occurred faster to the west of the Rayeu Hinge, and this area also forms the southern limit of the Mergui Basin which merges into the western part of the North Sumatra Basin. This region extends northward into present deep waters of the Andaman Sea, and still lies in deep water today, with its western margin formed by the Sigli High and the Mergui Ridge. Within this subsiding trough are two horsts, which were formed during the late stages of rifting, the easternmost horst is the Arun High with the associated Arun Field. To the east of the Arun High and west of the

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OIL & GAS

Rayeu Hinge is the Lhok Sukon Deep, which is the location of part of the kitchen for the Arun Field. To the east of the Rayeu Hinge lies the Central Trough, a basinal area broken into a series of north-south-trending horsts and grabens, that include the Lhok Sukon High and the Kuala Langsa High, before the basin floor rises eastward towards the Malacca Shelf. The North Sumatran Basin was initially subject to Late Eocene rifting that formed the north-south horsts and grabens. A quiescent phase of basin sag, with widespread carbonate deposition and reef growth during the Late Oligocene and Early Miocene, followed the rifting. N W - S E wrench tectonics in the Mid-Miocene was associated with the uplift of the proto-Barisan range, and finally, S W - N E compression during the Plio-Pleistocene to Recent created the N W - S E coastal fold belts of Sumatran trend which occur throughout the basin.

generally been regarded as economic basement, although if they occur adjacent to a source kitchen and have an adequate seal, it is possible for them to act as fractured and/or vuggy reservoirs. According to Collins et al. (1995), the Tampur Formation comprises brecciated and fractured limestones and dolomites. This formation has produced gas shows from vuggy limestones in some wells and tested 6.8 MMSCF per day in Alur Siwah-8 (Barliana et al. 2000). The Rifting (horst-and-graben) Stage is the period for the development of ideal source-rock conditions. Rifting in the North Sumatra Basin was probably initiated in the Late Eocene, creating a series of rift valleys that persisted for the next 8 or 9 million years. The initial phase of rift development involves a certain amount of volcanism, due to adiabatic melting of the mantle during thinning of the lithosphere. The margins of the rift valleys were subject to sub-aerial erosion, with the development of scree slopes and alluvial fans of the Bruksah Formation; these coarse clastic lithologies do not form good reservoirs. A thick overburden in the deeper parts of the rifts has caused the loss of porosity, which is still apparent in areas that were later inverted. As elsewhere in the world the rift valley system most probably developed river systems and lakes, which provided both sourcerocks and potential reservoirs. However, these suspected lacustrine source-rocks have yet to be penetrated by the drill bit. The upper part of the Bruksah Formation basinally interfingers with and is overlain by claystones and mudstones of the Bampo Formation. Black shales of this formation form one of the potential source-rocks for the North Sumatra Basin.

Stratigraphy

The petroleum significance of the various stratigraphic units in Sumatra is described below, in terms of the tectono-stratigraphic classification used in the Tertiary section of this volume (Chapter 4) as the Cratonic Stage, the Rifting Stage, the Transgressive Stage and the Regressive Stage. In North Sumatra the Tampur and Meucampli Formations were deposited during the Cratonic Stage (Fig. 10.2). These units have

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134

CHAPTER 10

The Transgressive Stage began about 26 or 27 Ma ago (1'21-P22) due to an overall regional basinal sag and a gradual world-wide rise in sea level, which caused the rift valleys to be submerged. The deep marine black shales of the Bampo Formation were deposited as the result of this rise in sea level, and may have been oxygen deficient. The area of the Southern Mergui Basin has a different history; it subsided earlier and therefore has been dominated by marine sedimentation from very early in its history. The deep marine event was followed by reactivation of faults and a period of erosion forming an unconformity. A further marine transgression resulted in the deposition of the Peutu Shales, the Belumai Formation and the Arun Limestone. The marine 1,eutu shales represent the maximum transgression stage, although a condensed sequence in the overlying Baong Formation is a possible candidate for the maximum flooding surface. A basal 1,eutu sandstone was recognized by Cameron et al. (1980) in the Barisan Mountains. The Belumai interfingers with the Peutu in low-stand submarine fans, which could also have been charged with hydrocarbons during fault movements. During this time limestones were developed on the basement horsts. This formed the Arun Limestone, which is the main reservoir of the area (including the giant Arun gas/condensate field). Many similar, but smaller, Arun Limestone prospects have been tested; some of which have been found to contain high quantities of CO2. The reefs were eventually drowned and sealed by the Peutu or the Lower Baong shales, topped in some areas by the Baong Sandstone deposited as a lowstand fan, which forms a second reservoir formation in this area. The Regressive Stage deposited a series of interbedded sandstones and shales forming the Upper Baong and Keutapang Formations. Reservoir sands in these formations are locally sealed by the interbedded shales. Within this sequence the sediment first includes material eroded from the uplifted Barisan Mountains. Many structures involving these shallower/younger sandstones were locally breached during the Pliocene to present compressional phase, but the sandstones form reservoirs in onshore fields, such as 1,erlak (50 MMBO) (Courteney et al. 1989), Tualang (24 MMBO) and Rantau (231 MMBO) (Caughey et al. 1994). These sandstones could also have significant stratigraphic potential.

Reservoirs

Reservoirs of the North Sumatra Basin range in age from Oligocene to 1,1iocene, and include both carbonate and clastic reservoirs. The Arun Limestone has an average porosity of 16% and according to Collins et al. (1995) the pore types are variable, being dependent on the history of sub-aerial exposure and diagenesis. Microporosity is developed in the southern parts of the basin, where leaching seems to have had a lesser effect. Clastic reservoirs include the Miocene Keutapang, Baong and Belumai sandstones the Mio-1,1iocene Seurula sandstones. Percentage porosity in these reservoirs varies from the lower teens to the low thirties. S o u r c e rock a n d h y d r o c a r b o n type

All geochemical data so far indicates that the source rocks of the North Sumatra Basin are mainly marine, although Kirby et al. (1993) suggested that there was the possibility of lacustrine source rocks occurring within the rifts. According to Buck & McCulloh (1994) hydrocarbons in the basin originated from multiple source rocks, including shales in the Bampo, marls in the Peutu and shales within the Baong, all of which are marine. Buck & McCulloh (1994) reported that the Bampo was the main source of the oil in the the 1,eutu carbonate reservoirs, such as the Arun field, and stated that the Baong shales surrounding the

Arun Field have little or no generative capacity. This is due to the low organic content, and because the organic matter which is present is hydrogen poor. They report, however, that the Baong becomes richer in oil generative potential in the east and southeast of the basin, and forms part of the Baong-KeutapangSeurula petroleum system in that region. The two systems are separated by the Alur Siwah High. Buck & McCulloh (1994) state that most of the organic matter in the Bampo Formation is derived from land plants, with minor amounts of algal and amorphous kerogen. Their work indicates that the Bampo and Peutu formations have poor to moderate hydrocarbon generative capacity. They claim that the exceptionally lean organic composition of the Bampo and Peutu source rocks is partially offset by the substantial thickness of this section in the deeper part of the basin. Their maturation modeling, using in-house software, showed the deeps to be over mature at the present day, with peak generation having occurred during late Tertiary times (c. 12-4 Ma). Rapid conversion of the kerogen was brought about by substantial late Tertiary sedimentation and unusually high geothermal gradients (46.8~ km - j average for 113 wells). Hydrocarbons matured and migrated into pre-existing structures in a short period of time and thus presenting very little chance of loss. According to Courteney et al. (1989), the overpressured Baong Shale is the primary source rock of the basin with average total organic carbon (TOC) of 1.5% in the lower part of the formation. Kjellgren & Sugiharto (1989) working on the southeastern section of the North Sumatra Basin, suggested that there were three phases of oil generation. The first phase affected the Bampo, which is now over mature in the deep areas and was responsible for the oils, now biodegraded, seen in the Kemiri-I and Kambuna-I oils and the Polonia-1 condensate (it is, however, not possible to use biomarkers in biodegraded oils). These oils migrated near to the palaeo-surface and were subject to biodegradation. Structural features containing these biodegraded oils would have been present at the time when the oil was expelled from the Bampo in Lower Baong times. However, most structures have formed more recently and were formed after the migration of this oil. The second phase of oil generation oil came from the Lower Baong/Belumai Formation. The Baong is deltaic and progrades westerly to southwesterly, from proximal near shore to distal wholly marine. Light oils and condensates at Kambuna-l, Polonia-I and Glagah-1 were more proximal while black oil at Batumandi-1 was very distal. The third phase of oil generation was from a carbonate source, seen in Tonjol-1. Situmeang & Davies (1986) confirmed the Mid- Lower Baong Formation as the source rock for the light, waxy, paraffinic crudes in Gulf Resources' 'A' Block. Moreover, the Baong was found to contain a mixture of both terrestrial and marine organic matter, the predominance of one over the other being related to proximity to the source area of the sediments. Kirby et al. (1993) studied Pertamina Unit 1 area in the central part of the North Sumatra Basin and found the Baong Formation to have TOC values averaging 0.5% with an oil window between 2900 and 3300 m, only the deepest samples being within the window. They concluded that the Baong could not be a viable source rock in that area. They found that the Belumai Member had TOCs in the range of 0.2-4.8% (typically 1%) and that the organic matter was terrestrially derived. TOC analyses in the Bampo Formation ranged from 0.27% to 3.84%. The higher values came from core samples, but outcrop samples showed a great lateral variation. The hydrogen index values were low to very low, with only inert organic matter. The Bampo mudstones were classified as having only limited potential for gas generation. Since none of these units could be the source rock for the oil in the area, they analysed all the oils. Combined GCMS and isotopic analyses on a number of reservoired light oils and condensates indicated that terrestrial kerogen was the principal source for the trapped hydrocarbon. Kirby et al. (1993) therefore concluded

OIL & GAS that the most likely source rocks in the Pertamina Unit 1 area is the lacustrinal sequence of the Bruksah Formation, anticipated to occur in the basinal areas, but which has not yet been drilled. Modelling suggested that oil migration from the deepest parts of the Palaeogene sequence commenced at 11 Ma. The oil would have migrated into porous zones in existing structures in the Belumai and Bruksah formations, sealed by the Baong and Bampo shales respectively. The major phase of structuring occurred during the Plio-Pleistocene. The oil then migrated from the pre-existing structures up faults formed during this tectonic phase, through the otherwise impermeable Baong shales, into structural and stratigraphic traps in the Keutapang Formation. The deepest part of the Baong Formation would now have entered the oil window and have supplied additional oil.

Petroleum systems

According to Buck & McCulloh (1994), the petroleum system in the northern part of the North Sumatra Basin is the BampoPeutu system. Gas and condensate generated in the Bampo Shales is reservoired in the Arun Limestone, which is part of the Peutu Formation. The overlying, overpressured shales of the Baong Formation provides the seal. Buck et al. (1994) also state that overpressured shales of the Peutu Formation form a lateral seal. According to Kjellgren & Suguharto (1989), the petroleum system in the SE part of the basin is the B a o n g - B e l u m a i Keutapang system, with the Lower Baong-Belumai Formations being the source rocks for light oils and condensates. They also suggest that oil generated from the Bampo Formation, prior to the time it entered the gas window, is the source of the biodegraded oil found in Kemiri-1 and Kambuna-1 wells. Most of the Bampo is now buried deeply enough to be in the gas window. The kerogen type tends to be Type III (gas prone) or Type II/III (gas and oil prone). Fields in the basin are close to the Lhok Sukon Rift, in the region of the source kitchen. There is a good regional seal provided by the Peutu and Baong shales, with the addition of interbedded shales in the Keutapang Formation. Thus, productive petroleum systems in this basin require structural features which include the Arun Limestone, the Baong or Keutapang sandstones and proximity to the Lhok Sukon Rift to produce potential oil fields. Shallower reservoirs also require faulting to provide conduits for the migrating oil. The systems also require that subsequent inversion has not been sufficient to breach the trap.

P o t e n t i a l drilling hazards

Overpressure occurs in the Baong Shale overlying the Peutu and Belumai formations throughout the basin; this can usually be recognised on the seismic profiles by acoustic transparency. Corrosive CO2 occurs in concentrations varying from 15% at Arun to 82% at Kuala Langsa in the Peutu/Arun Limestone (Caughey & Wahyudi 1993, p. 204). The limestones also contain varying amounts of H2S. Alur Siwah, for example, contains about 1.6% H2S (Barliana et al. 2000, p. 164).

Central Sumatra Basin The lack of oil seeps discouraged exploration in the Central Sumatra Basin during the early days of Sumatran petroleum exploration. However, it has since become Indonesia's largest producing basin, with the establishment of the giant oilfields of Duri and Minas. The structural features in these oilfields are shallow, but have excellent seals. According to the IPA Oil Field Atlas,

135

the first geological survey in the basin was carried out in 1864 along the Siak, Siak Kecil and Mandau Rivers. Over half a century later two seeps were described near the village of Lubuk Bendahara. Despite this early interest, it was not until 1933 that the first exploration well was drilled by Nederlands Koloniale Petroleum Maatschappi (NKPM), and this well encountered shallow basement. The first discovery was made in 1938 with Sebang-I drilled by NKPM. This yielded gas with heavy oil. In 1939 the Lirik Field was discovered with Lirik-3 by NKPM. The giant Duri field was discovered by SOCAL in 1941. Minas-1 was about to spud when the Japanese invasion occurred and the invading forces completed the well. The wellsite geologist was Toru Oki, who many years later was to play an important role in Inpex (Indonesia's largest non-operating producer). After the war Nederlands Pacific Petroleum Maatschappij (NPPM) returned with its new Caltex Pacific name and went on to discover Pungut ( 1951 ), Kotabatak (1952) and Bekasap (1955), which have in combination, produced over half a billion barrels of oil, according to Courteney et al. (1991). Caltex put Minas on stream in 1952 and Duri, with a more viscous crude, in 1958. Caltex also established the Palaeogene oil play with their discovery at Pematang- lin 1959. NKPM returned to Sumatra as Standard Vacuum, developed the Lirik field and went on to make further discoveries in the Lirik trend, which produce from Neogene and Palaeogene sands. The North Pulai Field, which is also part of this trend, produced from fractured metaquartzite basement. The reservoirs in the Lirik trend range in age from Palaeogene Sihapas to Pleistocene Minas formation sandstones. Shales within the Minas Formation form the main regional seal, while the source rocks have traditionally been regarded as the brown shales of the Pematang Group. As in the North Sumatra Basin, the kitchens are located in the deep main grabens, the Kiri, Mandau, Bengkalis and the Central Deep. The major oil fields are all situated close to these northsouth grabens.

Tectonic elements

To the NW the Asahan Arch separates the Central Sumatra Basin from the North Sumatra Basin (Fig. 10.3), and to the SE the Tigapuluh High separates it from the South Sumatra Basin. The sediments thin to the NE onto the Malacca Shelf and the Tertiary sediments disappear beneath the Barisan Range to the SW. Rift basins were formed in the Eocene, following a north-south structural grain. The rifts include the Bengkalis Graben, the Balam, the Kiri and the Aman (Central Deep) sub-basins. The Balam Trough-Central Deep contains over 3000 in of Tertiary fill (Yarmanto et al. 1995). To the east, a region of structural highs separates the Central Graben from the Bengkalis Graben. These rift basins were later subject to compression 30 Ma ago, associated with the mid-Oligocene world-wide drop in sea-level. According to Courteney et al. (1991) this compression was caused by the commencement of subduction to the west of Sumatra. The compression also coincides with the first emergence of the Barisans as a sediment source. A second phase of compression occurred 21 Ma ago (Courteney et al. 1991), marked by an unconformity in the sequence. Reactivation of the proto-Barisans created a major unconformity 15.5 Ma ago (Courteney et al. 1991) restricting the basin even further. This period of Barisan uplift still continues. Further significant compressional periods occurred 2.8 and 1.65 Ma ago (Courteney et al. 1991), resulting in major inversions, creating the classic Sunda Folds of Eubank & Makki (1981) which formed many large traps. However, according to Courteney et al. (1991) the traps which form the giant fields of the basin, have either a long history of structural growth or were formed by drape over basement highs.

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Stratigraphy

No sediments representing the Cratonic Stage were deposited in the Central Sumatra Basin. Rifting Stage sediments were deposited directly onto the pre-Tertiary basement, which consists of greywacke in the west and quartzite in the east. According to Caughey et al. (1994), the basement provides a good seismic reflector over the structural highs, but becomes more difficult to distinguish in the troughs. The earliest Rifting Stage sediments comprise the Eocene through Early Oligocene Pematang Formation, and were deposited in the troughs (Fig. 10.4). The Pematang Formation comprises the Lower Red Beds, the Brown Shale and the Upper Red Beds. The Lower Red Beds represent an immature basin fill, of sandstones, shales and conglomerates deposited in an alluvial/fluvial environment. The Brown Shale was associated with basinal subsidence, and with the formation of permanent fresh to brackish water lakes in the Palaeogene troughs in which anoxic, saline, lacustrinal facies were deposited. These are algal-rich, dark brown to black shales, which form the main source-rock for the Central Sumatra Basin. According to Yarmanto et al. (1995), due to its high amplitude, continuous, low frequency response the Brown Shale can frequently be picked on seismic profiles. The Brown Shale and the Lower Red Beds are observed only within the troughs. The onset of a regressive phase, with the deposition of the Upper Red Beds, composed of fine to coarse sandstones, siltstones and claystones, resulted in in-filling of the lakes and a return to a fluvial/alluvial depositional environment. Palaeosols in the upper part of the red beds act as a effective seals. Seismically, the top of the Pematang is truncated by an unconformity, which provides a good seismic reflector. This unconformity was followed by the Transgressive Stage with its reservoir sandstones. These sandstones, known generally

Fig. 10.3. The structure of the Central Sumatra Basin showing the positions of horst and graben structures and the localion of oil (grey) and gas (black) fields.

as the Sihapas Group, are the main reservoirs in the basin. The various sandstones are called the Menggala, Bangko, Bekasap, Duri, Langkat and Tualang formations, with environments of deposition ranging from inner neritic to braided and meandering streams. The producing horizons of the Minas and Duri Fields are the Bekasap and Duri Sandstones, which are deltaic to tidal in origin. Overall, there was a gradual marine transgression, culminating in the deposition of the Telisa Shale. The Sihapas intercalates basinally with, and is overlain by the Telisa, which provides the main regional seal. A compressional phase resulted in a renewed development of the proto-Barisan 15.5 Ma ago, marked by the influx of sediment from the west and creating a major unconformity. This tectonic event is associated with the initiation of the Regressive Stage. The Petani Formation, the earliest formation of this stage, comprises claystones, siltstones, thin sandstones and limestones. On seismic sections this formation can be observed forming prograding wedges, derived from the west. The Plio-Pleistocene Minas Formation represents the final phase of deposition. The last major compressional phase, from 2.8 to 1.65 Ma ago brought about an inversion of the structures. Most of the major fields were formed at this time, although they are usually also associated with older pre-existing features.

Reservoirs

The Sihapas Group forms the main reservoir for this basin. It is composed of Menggala, Bangko, Bekasap, Duri, Lakat and Tualang Sandstones, varying environmentally from fluvial to inner neritic. The Upper Red Beds of the Pematang Formation can also form reservoirs, especially in the troughs; these reservoirs were formed in fluvial or alluvial sediments.

OIL & GAS

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Source rocks and hydrocarbon type

South S u m a t r a Basin

The Middle Oligocene Brown Shale, within the Pematang Formation, forms the main source-rock for the basin, with TOC (total organic carbon) averaging 5%. It is an excellent, dark brown to black, algal rich, source rock, restricted to the Palaeogene deeps and was deposited in restricted, fresh to brackish water lakes. Hydrocarbons found in the Central Sumatra Basin are predominantly oil, due to the presence of these oil-prone lacustrine source-rocks.

The South Sumatra Basin received a great deal of attention in the early days of petroleum exploration because of the numerous oil seeps in the area. According to Courteney et al. (1990), oil was first reported in the South Sumatra Basin near Muara Enim, to the east of Karangradja by Granberg in 1866. He observed three seeps from which oil was being collected and traded by the local people and suggested that this indicated the potential for larger production. Strief later described two of these seeps in 1877, but it was not until 1896 that the first discovery was made by Muara Enim Petroleum on the Kampong Minyak Anticlinorium with Kampong Minyak-1. The Kampong Minyak field is still producing over a hundred years later, having produced about 15 million barrels of oil. In the same year, according to Zeliff et al. (1985), the Royal Dutch Company, discovered the 4 million barrel Sumpal Field. However, it was a quarter of a century later before the first significant discovery was made in 1922, when 370 mmbls were discovered at Talang Akar by NKPM (later Stanvac); this is still the largest oil field discovered in the basin. The last discoveries, of greater than 100 mmbls of oil, were the Talang Jamar, which according to the IPA Oil and Gas Field Atlas had produced over 170 mmbo by 1992, and KajiSemoga, which according to Hutapea et al. (2000) contains 150 mmbo of recoverable reserves. Nearly two billion barrels have so far been discovered in the South Sumatra Basin, the largest fields being on the Pendopo-Limau Anticlinorium (Fig. 10.5).

Petroleum systems

The Pematang Sihapas System is the most prolific petroleum system, according to Howes and Tisnawijaya (1995), with an EUR (estimated untapped reserve) of 12.8 BBOE (billion barrels of oil equivalent). The low gas (about 5% of the EUR) is presumably due to oil-prone nature of the lacustrine Pematang Brown Shale source-rocks. The Menggala, Bekasap and Duri marine sandstones of the Sihapas Group comprise the reservoirs. Shale of the Telisa Formation forms the seal. The Pematang Pematang System comprises the Pematang Upper Red Beds forming the reservoir with the same Brown Shale forming the source. Palaeosol at the top of the Upper Red Beds creates the seal.

138

CHAPTER 10

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The leaky nature of the seals results in hydrocarbons migrating into reservoirs throughout the Oligo-Miocene sequence. The reservoirs include Lemat and Talang Akar Sandstones, Batu Raja Limestones, sandstones within the Gumai Formation, and sandstones throughout the Air Benakat and the Muara Enim formations (Fig. 10.6). The Late Oligocene to Early Miocene Talang Akar Sandstones are fluvial at the base and marine at the top, indicating a rise in relative sea level. The Miocene Batu Raja Limestones formed as carbonate build-ups on basement highs. Source-rocks include coals and high gamma ray shales within the Talang Akar Formation, whilst the lacustrine sediments of the Lahat contribute a distinct, high wax oil (Caughey pers. comm.). Frequently the Talang Akar Sandstones form stratigraphic traps where sandstones wedge out against basement highs. The Gumai marine shales provide the main regional seal in the basin, however, hydrocarbons do get through it, so that, as mentioned previously, the Air Benakat and Muara Enim Sandstones higher in the sequence are also significant hydrocarbon reservoirs. In the past the prime exploration target was oil, but now the emphasis has changed to gas, with recent large discoveries in deeply buried Talang Akar and fractured we-Tertiary reservoirs. Starting with discovery of Dayung in 1991, fractured basement has become a significant objective reservoir for gas. Gas is piped from the South Sumatra Basin to the Central Sumatra Basin 536 km to the north, where it is used in the Duri tertiary recovery steam flood project. Agreements were signed early in 2001 for a pipeline from South Sumatra to Singapore, and the gas will also be used locally to run small electricity generators for power generation and industrial use.

Fig. 10.5. The structure of the South Sumatra Basin showing the positions of depressions and highs and the location of oil (grey) and gas (black) lields.

Tectonic elements The Lampung High separates the South Sumatra Basin fi'om the Sunda Basin to the east and the Tigapuluh High separates it from the Central Sumatra Basin to the NW. In the NE, the basin thins towards the Bangka part of the Sunda Craton and towards the SW, like the basins to the north, it wedges beneath the Barisan Mountains (Fig. [0.5). The South Sumatra Basin formed initially during Late Eocene rifting. The basin can be divided into two distinct parts, the Palembang sub-basin to the south and the Jambi sub-basin to the north. The two sub-basins are slightly off-set from each other, and the rifts are orientated north-south in the Palembang sub-basin and N E - S W in the Jambi sub-basin. The rift valleys so formed were to become the source kitchens around which oil accumulations would later be found. Basement highs formed eroding areas providing a sediment source and were eventually submerged to form the substrate on which carbonate build-ups would form. A sag phase in the Late Oligocene to Early Miocene promoted growth of carbonate banks tbrmed on structural highs. In the Mid-Miocene wrenching occurred, and this was followed by a period of subsidence prior to a compressional phase in the Plio-Pleistocene. The end result is a pattern of north-south or N E - S W horsts and grabens with superimposed NW-SE-parallel fold trends, with associated high-angle compressional faults.

Stratigraphy Sediments representing the Cratonic Stage are absent in the South Sumatra Basin. Tertiary sediments overlie Mesozoic limestones,

OIL

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various metasediments and igneous rocks of the basement directly. The Lahat Formation represents the earliest Rifting Stage. This formation has been penetrated in the Palembang Sub-basin, but has not so far been encountered in the Jambi Sub-Basin, probably due to its greater depth in that area. The Lahat Formation is absent on basement highs, and some grabens have not been drilled below the 'overlying' Talang Akar Formation. The Lahat Formation represents the initial rift valley sediments, which overlie the Kikim Tufts, erupted as the rifts opened. Thus, the Lahat consists of alluvial fans, basal conglomerates, lacustrine and fluvial sediments. It is likely that these late Eocene lacustrine facies provide one of the sources of oil for the basin. The depositional environments of sediments of the Talang Akar Formation range from fluvio-deltaic at the base to marine at the top, and represent a transition from the last component of the Rifting Stage into the earliest component of the Transgressive Stage. The fluvio-deltaic deposits include source rocks, either as coals or high-gamma shales, between fluvial sandstones. As the sea transgressed across the basement highs, carbonate build-ups developed around them (Batu Raja Formation). These build-ups formed along a coastal shelf adjacent to the Sunda Shelf and on basement highs that protruded into the basin. The coastal shelf was widest in the Palembang Sub-Basin to the south, becoming narrower towards the north and was absent in the northern part of the Jambi Sub-basin. Shales of the Gumai Formation eventually engulfed the carbonate buildups, forming a regional seal. This seal is more effective in the Palembang Sub-basin than in the Jambi

Sub-basin, as the shales are thicker. The Gumai Formation represents the height of the transgression and is followed by the Regressive Stage Air Benakat Formation, and by the Muara Enim Formation.

Reservoirs

Pre-Tertiary basement is becoming a significant reservoir in the South Sumatra Basin, as with the development of the infrastructure, gas is becoming more significant in the economics of the area. Dayung is an example of a basement field producing gas from fractured pre-Tertiary granite wash and granite (Zeliff & Bastian 2000). Fractured metasedimentary lithologies are also reservoirs. The Talang Akar Formation contains two types of reservoir, in fluvial sandstones in the lower part of the formation and in marine sandstones in the upper part. The fluvial sandstones form thick but relatively poor quality reservoirs, being created by the coalescence of channels, while the marine sandstones tend to be thin but more porous and permeable. The basal part of the Talang Akar is sometimes conglomeratic and merges into weathered basement. The Batu Raja carbonates vary from very porous to tight. The porosity is generally secondary, with many stages of diagenesis being involved. Sometimes a dual porosity system occurs with fractures connecting the vugs. Predicting the porosity

140

CHAPTER l0

development is tricky, as with all carbonates, but there is a tendency for the limestones to have a better porosity at the top of buildups. In some areas, such as part of the Air Sedang field, the top of the limestone cannot be distinguished from the overlying shale on seismic data. This is due to the high porosity of the limestone, which brings the velocity down to that of the shale. However, the Batu Raja is usually a very clear seismic marker. Shales equivalent to the Batu Raja commonly show a velocity contrast with the overlying shales, due to their high carbonate content. The Gumai Formation frequently contains marine glauconitic sandstones which are occasionally very fine grained and tight, but may also form good reservoirs. The sandstones may also act as thief beds, downlapping onto the underlying reservoirs and allowing hydrocarbons to escape. The Air Benakat Formation contains many sandstones which may form stacked reservoirs. As this is a regressive sequence individual sandstone reservoirs vary considerably in quality and areal extent. Within the Jambi Sub-basin there are usually shows of some degree in every sand, but these sands crop out and sub-crop along the edge of the Sunda landmass where they are frequently exposed to meteoric waters. The areal extent of the sands varies and the water salinity of each sand interval varies also, this in turn has affected the extent of biodegradation of the hydrocarbons. Finally, sandstones within the Muara Enim Formation also form reservoirs in this basin.

Source rocks and hydrocarbon type Hydrocarbons in the South Sumatra Basin are both gas and oil, this is probably due to the early migration of oil from the source rocks followed by later gas migration. Source rocks are lacustrinal facies of the Lahat Formation, which may be the source of high pour point waxy oils, and the shales and coals of the Talang Akar Formation. The Talang Akar Shales have a high gamma-ray response, which is frequently associated with a high total organic carbon content. The Gumai could provide a marine source rock, but generally has low organic levels and is thermally immature in most parts of the basin.

Petroleum systems As mentioned earlier, there are several possible source rocks. Oil type analysis indicates that more than one type of oil is present, but all are derived from the Talang Akar Formation or older units. The primary system, therefore, is associated with the Talang Akar Sandstones and/or the underlying fractured basement, which form the reservoir part of the system and are usually in direct contact with the source-rock. Gas is also significant, as according to Zeliff & Bastian (2000) 14.8 TCF gas reserves have been discovered in basement reservoirs. The graben areas are the kitchens and thus plays tend to be adjacent to them. The Talang Akar sandstones are also the main conduit for hydrocarbon migration to other reservoirs, either directly or via faulting. Faulting occurred in the Mid-Miocene as well as in the Plio-Pleistocene, developing numerous pathways. Since the Talang Akar Formation wedges out on basement highs, and the Batu Raja carbonates were formed on the highs, a connection is provided between the source and the Batu Raja reservoir. The downlapping Intra-Gumai Sandstones provide a connection with either the Talang Akar Sandstones or the Batu Raja for further upward migration, while sandier parts of the Gumai and faulting produce the final contact with the Air Benakat sandstones.

Potential drilling hazards Coals in the Muara Enim Formation occasionally slough into the hole, pipe-sticking is experienced in the Gumai Formation and

circulation has been lost in both the Batu Raja Limestone and in fractured basement. In some areas the lower part of the Gumai is geo-pressured, this in combination with possible loss of circulation in the Batu Raja can lead to blow-outs. CO2 is present in varying amounts in the Batu Raja Limestone, with higher percentages in the basement and H=S has been encountered in the Batu Raja and Talang Akar formations. Zeliff & Bastian (2000) report gas columns of up to 1 km in recent highly permeable fractured basement discoveries. The well control problems that this causes have been tackled with underbalanced drilling with rotary BOPs (blow-out-preventers).

Other Sumatran basins The search for hydrocarbons in the remainder of the sedimentary basins in Sumatra has not been successful. Exploration has been limited by the perceived high risk. The remaining basins can be divided into two groups: outer-arc basins and the intramontane or intra-arc basins.

Outer-arc basins Outer-arc basins occur to the west of the Barisan Mountains and underlie the coastal region and the offshore areas between mainland Sumatra and the outer-arc islands. From north to south these basins are the Sibolga, Mentawai and Bengkulu basins. The outerarc basins, as mentioned earlier, have low geothermal gradients due to the double thickness of the plate in subduction zones, and thus a greater depth of burial is required for maturation. This may have not always been true throughout the history of the basins as there is an extinct spreading centre that intersects the outer- arc system at the Pini Arch, which separates the Sibolga Basin from the Mentawai Basin. This spreading centre, which now forms the Wharton Ridge, became inactive in the Eocene, probably due to jamming in the trench. If Sumatra was subjected to clockwise rotation caused by the collision of the Indian Plate with the Asian plate, then according to Clure (1991) the spreading centre would have been subducted beneath the Bengkulu and Mentawai Basins. The passage of the spreading centre would have resulted in a period of higher heat flow and possible oil generation in the outer arc basins. Oil shows to the west of the Barisan Mountains are found only in the Bengkulu Basin to the south of the Pini Arch, whilst to the north of the arch only gas, probably of biogenic origin, has been found. Another factor in this scenario is that volcanic and metamorphic rocks in the Barisan Mountains provided a provenance only for clays, shales and poor quality lithic sandstones, due to the limited availability of quartz. Various granite plutons provide local sources of quartz sandstone, but this type of provenance is characterized by the deficiency of coarse clastics. Prior to the uplift of the Barisan, sediments in the outer- arc basins came all the way from the Sunda Craton to the east, and the outer- arc basins formed part of the basins that became backarc basins after Barisan uplift. For example, the Bengkulu Basin is thought to have originally formed part of the South Sumatra Basin. Various attempts have been made to trace the grabens from the backarc basins into the outer-arc areas and thus explore for the rift sequences, but the success of this exercise is dependent on the estimated amount of displacement along the Sumatran Fault. If these rifts continue into the outer-arc area they are still very far from the presumed source of sediment in the exposed Sunda Craton, and therefore the clastics are likely to be finer, the coarse sediments having dropped out of the system nearer the source area of the sediments. Satellite images of Sumatra show a significant number of rivers radiating out from a point in the central part of the Barisans. Prior

OIL & GAS

to Barisan uplift, Sumatra had a regional slope from the Sunda Craton in the east, towards the sea to the SW and the drainage was in the reverse direction to the drainage at the present day. At that time the drainage pattern converged on the Mentawai Basin, thus providing the basin with a coarse clastic source. However, if reservoir quality clastic sediments are deficient, then hope lies in the carbonates; sub-commercial quantities of gas have been discovered in carbonate buildups in the Sibolga Basin. Unfortunately, these scenarios are just theoretical, and can only be proved or disproved by the drill bit. Testing in this area is greatly hampered in both the Mentawai and Bengkulu Basins by water depth, as the shelf in these areas is very narrow, and the slope quickly plunges off to many thousands of metres, stretching offshore drilling technology to its limit. This highcost, high-risk scenario has limited exploration in these basins. Exploration in the Sibolga Basin has involved a few early wells to test the carbonate play, and there has been some recent exploration by Caltex in the offshore Nias area, but like Union before them they encountered only non-commercial quantities of biogenic gas. The Mentawai Basin has not as yet attracted any drilling activity, due to the depth of the water, and it therefore remains very much a frontier zone. The Bengkulu Basin, with onshore oil seeps has, however, attracted exploration offshore, although the results to date have not been encouraging. A few companies over the years have searched for the western limits of the Talang Akar Formation of South Sumatra, or possible Baturaja

141

carbonate build-ups. Sources of oil would lie in sediments deposited in undetected lakes within rift grabens.

Intermontane basins The Intramontane or Intra-arc Basins are extensions of the Central and South Sumatra Basins and were initially part of those basins prior to the uplift of the Barisans, which isolated them from the main basin area. The earlier history in these basins is very similar to the backarc basins from which they have become disconnected. Such basins include the Mandian, Kampar Kanan, Ombilin and the Bandar Jaya basins. The Banda Jaya Basin has a reasonably complete, although thin, younger section, whilst the Ombilin Basin, due to subsequent uplift, is missing the younger section, either as the result of nondeposition, due to isolation from the main sediment source, or to erosion. Oil shows were observed in the Sinimar-1 well drilled by Caltex in the Ombilin Basin, demonstrating that generation of hydrocarbons had occurred in this area; however this is the only well to have been drilled in this basin. The Bandar Jaya Basin, which is made up of a series of smaller half grabens, has been tested by a few wells, which encountered Lahat through Air Benakat formations, but these wells were unsuccessful in finding hydrocarbons, probably due to the low maturity of sediments in this area.

Chapter 11

Fuel resources: coal L. P. THOMAS

The coal resources of Sumatra were developed rapidly during the 1980s and 1990s following the oil shocks of the 1970s. This encouraged the Indonesian Government to develop the abundant coal resources of the nation as a major source of energy, as it was appreciated that it was not sensible to rely upon any single energy source. Coal resources are now of vital importance to the indonesian economy, being used as fuel in preference to that of oil for thermoelectric generating stations, and cement works, throughout Indonesia. Coal has also been developed as one of Indonesia's major export commodities, being shipped to ASEAN countries and other countries in the Far East, such as Malaysia, Thailand, the Philippines, Taiwan, Korea and Japan, which are deficient in fuel resources, as well as further afield to Europe. Coal was first discovered under the Dutch colonial administration in the Ombilin Basin, within the Barisan Mountains, near Sawahlunto in West Sumatra in 1891 (Fig. 11.1). The area has a rugged mountainous topography and mining operations could not commence until a railway line had been constructed to transport the coal from Sawahlunto to the port of Teluk Bayur south of Padang on the west coast of Sumatra. The Ombilin area continues as a major producer of coal, mostly through open-cast mining, but much of the coal is now transported to the coast by road. in 1919, the Bukit Asam Mine in South Sumatra began production, the coal being exported again by rail transport through the ports of Kertapati near Palembang and Tarahan near Kotaagung (Fig. 11.1). Underground mining operations ceased in 1938, but mining in this area has continued through opencast mining to the present day. The Ombilin and Bukit Asam mines produced virtually all of the Indonesian coal before World War II, reaching a peak production of 2 million tonnes (Mt) in 1941. Post-war production fell to less than 0.15 Mt in 1973, due in part to the preference for oil as a cheap fuel. However in 1976, when oil prices rose dramatically coal reappeared as a major source of fuel. Currently annual coal production in Sumatra is around 12 Mt from both state and privately owned mines.

Geology and coal deposits in Sumatra Coal deposits in Sumatra, as elsewhere throughout the Indonesian Archipelago, occur almost entirely within Tertiary sequences. Traces of coal occur in the Pre-Tertiary basement in the Barisan Mountains, in rocks of Permo-Carboniferous age, but not in sufficient quantity to be of any economic importance. On the other hand, economic coal deposits of Tertiary age are abundant and distributed throughout Sumatra, ranging from the Eocene to the Pliocene (van Bemmelen 1949; Robertson Research 1974).

and igneous rocks. Following this transgression, a continental and paralic sequence of coarse clastic sediments, with interbedded coal seams and subordinate limestones, was deposited over a wide area, extending from Nias Island in the west, to the Malacca Straits in the east. This basal sequence ranges in age from Eocene and Oligocene in the north, to Lower Miocene in the SE. The continental and paralic sequence was followed by a second marine transgression with the deposition of a thick sequence of marine shales, with subordinate sandstones and limestones of Oligocene to Miocene age, represented by the Bampo Formation in the central and northern parts of the basin. The North Sumatra Basin is separated from the Central and South Sumatra Basins to the south, by the Asahan Basement High (Fig. 11.1). Coals have been described from Palaeogene sediments in the northeastern part of Nias Island to the west of Sumatra, where one or two coal seams, less than 0.5 m in thickness, dip at 2 0 - 4 0 ~ to the west. There are also Neogene coals of Miocene age on Nias (Robertson Research 1974). On the west coast of Sumatra coal occurrences are known from the Palaeogene Sibolga and Loser Formations in the Tapaktuan and Tapanuli Bay areas (Cameron et al. 1983). At Tapaktuan, the sequence dips steeply at 2 5 - 4 5 ~ and contains at least two coal seams 0.20-0.60 m thick. The coal is black, vitreous, often pyritic with a high clay content. Coals have been recorded from the rivers which flow into the northern part of Tapanuli Bay. The dips for these seams range from 8 ~ to 20' and are relatively thin, 0.2-0.5 m in thickness. A number of coals and carbonaceous horizons have also been recorded in the Neogene sequence in the Meulaboh area, in the foothills to the west of the Barisan Range. Here up to 5 coal seams have been observed, ranging from 0.4 to 3.0 m in thickness. The coals are brown and are interbedded with clay and bituminous shale. To the east of the Barisan Mountains, in the northern part of Aceh, the Palaeogene basal sequence contains a number of coal seams, usually less than 1 m in thickness. In the Bohorok district to the west of Medan, seams of 0.4-0.5 m dip at 45 ~ The coal is black, vitreous and pyritic. The thickest development of coals occurs in the Kualu River, where seams of up to 6 m have been recorded (van Bemmelen, p. 49 1949; Robertson Research 1974), but the seams decrease southwards to less than 1 m in thickness. Again the coal is black and resinous with associated pyrite. In the central area of the North Sumatra Basin, a large number of thin carbonaceous horizons occur in a late Miocene sequence, several hundreds of metres in thickness. These seams are low-rank coals, classified as brown coals.

Central and South Sumatra North Sumatra Coal deposits in north Sumatra are largely confined to rocks of Palaeogene age. The coals are black, bituminous or sub-bituminous in rank. Coal seams are only locally developed and show rapid variations in thickness. The overlying Neogene deposits contain numerous thin seams of brown coal rank. Tertiary sedimentation in the North Sumatra Basin (Fig. 11.1) commenced with a marine transgression from the NW, across an eroded surface of folded Palaeozoic and Mesozoic sedimentary

142

At the close of the Cretaceous period, Central and South Sumatra formed part of an extensive landmass with considerable topographic relief. At the beginning of the Tertiary, fault-bounded troughs formed within this landmass. The earliest Tertiary sediments were deposited in the troughs, but subsequently extended across the margins to form the Central and South Sumatra Basins. Throughout Tertiary times these basins were separated from the North Sumatra Basin by the Asahan Basement High (Fig. 11.1). These basins are asymmetric in character being

COAL

143

COAL LOCALITIES OF S U M A T R A Banda Aceh

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144

CHAPTER 11

bounded to the SW by faults and horsts of pre-Tertiary rocks along the Barisan Range, and to the NE by pre-Tertiary rocks in the Tigapuluh Hills close to the original Tertiary depositional boundary. There is evidence that the basins extended further west than the present limits of outcrop, as Tertiary sediments occur along the SW coast of Sumatra near Bengkulu, to the west of the Barisan Range (Fig. l 1.1). Both Palaeogene and Neogene sediments are present in the Central and South Sumatra basins. The Palaeogene consists of paralic and tuffaceous non-marine clastic sediments preserved in restricted grabens (the Lemat, Pematang and Kelesa Formations). Neogene sediments, consisting of marine shales, limestones and shallow water sandstones, represent a marine transgressive phase, passing upwards into non-marine shales of Middle Palembang (Muaraenim) and Korinci formations of late Miocene and Pliocene age, with widespread coal formation (de Coster 1974). Numerous records of coal exposures in Central Sumatra are listed by van Bemmelen (1949, p. 49). There are rarely more than two seams at any locality. Most coal seams are less than 1 m in thickness and many coals are of poor quality containing clay or carbonaceous shale. Significant Palaeogene coal deposits occur in the Painan District on the west coast of Sumatra south of Padang, where up to six coals are present, one reaching 2 m in thickness. The coals are interbedded with shales and the total sequence, which is 1 0 15 m thick, dips from 45 c' to vertical. These coals have been affected by volcanic intrusions of basalt and dolerite. The S u n g e i - S a p u h / S u n g e i - K e r u h District contains several coals, one of which is 2 - 4 m thick. Other occurrences of coal are in the Batang Tui area and numerous localities on the west and east coasts, all of minor importance. The most important coal development in Central Sumatra and the principal coal producer is the Ombilin Coalfield which occurs within the Eocene to ?Miocene, Sawahlunto Formation. This coalfield is situated within the Barisan Mountains 90 km inland from Padang (Fig. 11.1). The coal deposit occurs in the intermontane Ombilin Basin, the axis of which is oriented N W - S E , in line with the main structural trend of the Barisan Range. The basin is severely block-faulted in W N W - E S E , and N N E - S S W directions. The coal-bearing sediments are locally strongly folded and faulted, with both normal and reverse faults, making the correlation of individual coal seams difficult. The Ombilin Coalfield lies within the northwestern limb of the Ombilin Basin. The coalfield is subdivided geographically into the S u n g a i - D u r i a n , Tanah Hitam, Sugar, Sigalut and Parambahan coalfields. Within the Ombilin Basin the Sawahlunto Formation is made up of conglomerates, sandstones and shales. In the Tanah Hitam and Sungai Durian fields, the lower part of the sequence contains a thin coal or coaly shale layer, designated the D seam. The upper part of the formation contains three principal coal seams designated the A (average thickness 2 m), B (0.6-1.0 m) and C seams (average thickness 6 m). These seams occur in a sequence of 4 0 - 8 0 m in thickness and dip at 12 ~ towards the east (Robertson Research 1974).

Table 11.1. General coal qualities r Area or coal mine

PTBA Ombilin PTBA Bukit Asam (Steam) PTBA Bukit Asam (Anthracite) PT Allied Indo PT Bukit Sunur PT Danau Mas Hitam Cerenti area Sinamar area

To the east of Ombilin, Neogene coals have been identified in the Cerenti area near Rengat in Riau where exploration was carried out in 1988. Here the coal-bearing Mio-Pliocene Korinci Formation contains six seams ranging from 1.6 to 14.0 m in thickness. In the Sinamar coal basin, situated further south, at the border between Jambi and West Sumatra provinces, the coals are of Oligocene age, and have a thickness of 2 - 9 m. Adjoining the Sinamar area, at Mampun Pandan, coal seams 5 - 1 1 m thick are present. All of these coals are of high volatile sub-bituminous rank. The other principal economic coal deposits in central and southern Sumatra are of Neogene age. Neogene coals occur in the Korinci Basin within the Central Sumatra Basin (Fig. 11.1). These coals occur in the Muaraenim Formation of Pliocene age, are usually two or three in number and are interbedded with tuffaceous horizons, the most significant coal developments being along the Piladang River, where the total thickness of three coal seams is around 9 m. Significant coal-bearing deposits of Miocene age are found in the area of Bukit Sunur in the Bengkulu District (Fig. 11.1). The coal is being worked in three areas. The Bukit Sunur Coalfield itself is the most important of these occurrences and contains three seams, 1.6-3.5 m in thickness, with a maximum of 10 m. The coal has been affected by the intrusion of the Sunur andesite and at several localities has been altered to coke. At Susup Leman, two coals are present, with thicknesses of more than 2 - 3 m. At Bukit Puding, five or six coals are present of which one or two reach thicknesses of over 1.4 m. At Pilubang, two thick coals show evidences of alteration by the andesite resulting in a loss of volatiles. Owing to the effects of contact metamorphism, the quality of the coal in these occurrences varies considerably. In all these areas the deposits are strongly faulted. In southern Sumatra virtually all the Palaeogene coal occurrences are found within the Lahat Formation in Jambi Province. The coals are thought to be of similar age to those at Ombilin, but are generally thinner, seams not being more than 1.5 m and usually less than 0.5 m thick. The coals are present in a sequence of conglomerates, sandstones and shales similar to that at Ombilin. To date the Palaeogene coals of South Sumatra have not proved to be of economic importance. On the other hand the Neogene of Jambi Province yields numerous lignite outcrops with two or three seams, as much as 5 - 7 m thick, with low angles of dip. At Bukit Asam in South Sumatra Province, Neogene coals from the Miocene, Middle Pelambang Beds, have been exploited since 1919. Three groups of coals are present, the lower group contains the Merapi Seam ( 8 - 1 0 m thick), together with a number of thinner seams. The middle group contains the Mangus Bed which consists of coals 1 4 - 2 2 m thick including a 4 m c l a y - t u f f band. This bed is separated from the overlying Suban Bed by 15 m with no coal. The Suban Bed consists of 7 - 1 0 m of coal, containing a clay layer of 1.5 m. Some 30 m above, is the Petai Bed containing 5 - 8 m of coal. The third and uppermost group contains six or seven coal seams, the youngest of which may be as much as 30 m thick. In various parts of the Bukit Asam area,

and pro,spective Sumatran coals "air dried basis) (Soehandojo 1989) Total moisture %

Inherent moisture %

Ash %

Volatile matter %

Calorific value Kcal/kg

Total sulphur %

12 18-28 7-8 -12-16 14 ---

6 7-15 1-4 4 4-9 7-10 18

8 5-8 6-10 10 5-14 8-10 7-9 10

36 32-38 9-15 37 34-40 37-40 38 35

6900 5500-6500 7500-8000 6900 6000-6900 6300-6500 4700 5180

0.5 -0.6 0.4-0.6 1.0 max 0.5 0.8 1.0 max 0.3 1.4

17

COAL Table 11.2. Ash analysis for Air Laya Coal (yon Schwartzenberg 1989) Element

Average (%)

Range (%)

64.0 25.4 4.4 0.5 1.6 1.1 0.6 0.9 1.3 0.3

50-85 7-35 1-9 0.2 -4.0 0.2-3.5 0.3 - 3.5 0.2-2.5 0.2 -4.0 0.2-3.5 0.1-1.0

siQ A1203 Fe203 TiO2 CaO MgO K20 NazO SO3 P

145

in power station boilers. An example of ash analysis is given in Table 11.2 for the Air Laya deposit at Bukit Asam (von Schwarzenberg 1986) in which it can be seen that the chief constituents in the ash are silica and alumina. High contents of iron and/or calcium can affect the performance of the coal, by lowering the ash fusion temperature, which can cause slagging in the boiler. Similarly high amounts of reactives (K20 and Na20) are also undesirable, because they can cause fouling in the boiler.

Coal resources and production

the coals have been ameliorated by the younger andesites of the Serelo Mountains to produce locally altered coals of subbituminous, bituminous and anthracite rank. Coal-bearing sediments are found at Sukamarinda occurring immediately adjacent to Bukit Asam, where two layers of lignite, 2 and 5 m thick, have been locally altered by an igneous intrusion. In the Ajer Serillo area, a thick lignite is present, whilst in the Bunian area a lignite has been thermally altered. In the Kendin-Ringin area there are over 12 coals 5 - 1 5 m thick. All these areas have coals similar to those found at Bukit Asam. All the coals are autochthonous in nature.

Coal quality

The very large tonnages of low rank lignite found throughout Sumatra are not currently mined in any significant amounts. Consequently very little quality data has been collected for these lignites. Investigations of coal quality have centred on the coal seams of sub-bituminous, bituminous and anthracite rank. Quality determines the value and marketability of coals. Quality is primarily dependent on the rank of the coal, i.e. higher rank coals will have lower moisture content and volatile matter levels, and higher calorific values (CV) than lower rank coals. Importantly, Indonesian coals generally have low ash and sulphur levels, making them particularly attractive for use in the electricity generating industry. Table 11.1 summarizes the chief quality parameters of the principal Sumatran coal deposits. Bituminous coals from the Ombilin Basin have < 7 % inherent moisture, < 1 0 % ash, and < 1 % sulphur, with a calorific value (CV) of 6900 kcal kg -1. These coals are therefore good-quality steam coals, accounting for the long history of mining at Ombilin. Higher-rank coals are present in small amounts, due to the alteration caused by the intrusion of igneous rocks into the coal-bearing sequences. At Bukit Asam, anthracite is open-cast mined for domestic use. The surrounding unaltered coal is lower in rank, with higher moisture and lower CV levels (see Table 11.1) and is chiefly used for domestic power generation. The nature of the ash content in the coals is important, particularly in influencing the burning performance and efficiency

The total coal and lignite resources in Indonesia are estimated at 38 billion tonnes (Symon 1997, p.88). Sumatra contains 64% of the total, some 24 billion tonnes, of which 3.1 billion tonnes are reserves, defined within measured status (see Table 11.3). The bulk of these resources are in the Ombilin Basin, Central Sumatra, and in the Bukit Asam area of South Sumatra. From these figures it is clear that significant resources of coal and lignite exist in Sumatra and have yet to be exploited. The reasons for the relative lack of development of these resources is a combination of geographical inaccessibility, remoteness from markets and the general low rank and quality of the larger part of the resource. Coal production in Indonesia has risen from around 0.5 million tonnes per annum (Mtpa) in 1983, to 73 Mtpa in 1999 and to 92 Mt in 2001. Significantly, 66 Mt is exported (i.e. 72% of production). indonesia is rapidly heading to being the third largest exporter of thermal coal in the world after Australia and China (US Embassy, Jakarta statistics 2003). Coal has been mined in Indonesia since the late nineteenth century, but subsequent oil development and low oil prices saw the coal market diminish and production virtually cease. The oil crises of the 1970s radically changed this situation, reviving the interest in coal. Currently Indonesian coal is performing well in a very competitive energy industry. The growth of the Indonesian coal industry has been accelerated by the mining operations of foreign companies, which in the late 1970s and early 1980s were encouraged to invest in and to operate coal mines. The development of mining by foreign companies has been accompanied by the massive expansion of the state-owned coal mining company, PT Tambang Batubara Bukit Asam (PTBA). The Indonesian Government has authorised the state coal company PTBA to act as the main agent for coal mining, and PTBA has been able to attract private sector companies to carry out mining under product-sharing agreements. Future mining contracts will be managed by the Ministry of Mines and Energy in order to improve the regulatory framework and to allow PTBA to concentrate on its mining operations (Indonesian Mining Association 1997). The Indonesian coal industry is concentrated on mining sub-bituminous and bituminous steam coal, no coking coal is produced. Currently some 96% of coal production comes from opencast mines. In Sumatra, PTBA have underground and opencast mines at Ombilin in central Sumatra, and the Bukit Asam complex of opencast mines in South Sumatra. Private open pit mines are established in the Ombilin area (PT Allied Indo, PT Karbindo Abeysapradhi) and in West Sumatra in the

Table 11.3. Coal and lignite resources of Sumatra (Symon 1997) Region

North Central South Bengkulu Total

Measured (Mt)

Indicated (Mt)

Inferred (Mt)

Hypothetical (Mt)

Total (Mt)

% of total Indonesia reserves

-717.8 2438.8 30.9 3187.5

1272.0 2322.0 7505.5 17.0 10 920.7

2.0 105.9 2204.0 15.9 2355.9

433.0 1022.4 6891.0 -8296.5

1707.0 4169.0 18 743.5 60.0 24 759.7

4.4 10.8 48.6 0.2 64.0

146

CHAPTER 11

Table 11.4. Coal production from Sumatran mines (Directorate of Coal 1997) Company

PTBA Ombilin PTBA Bukit Asam (Steam) PTBA Bukit Asam (Anthracite) PT Allied lndo PT Bukit Sunur PT Danau Mas Hitam PT Bukit Bara Utama PT Karbindo Abesyapradhi Total

Production (Mt)

Exports (Mt)

1. l 0 8.06 0.06 0.85 0.36 0.07 0.15 0.60 11.25

0.77 1.24 -0.53 0.35 0.07 0.15 0.42 3.53

Bengkulu area (PT Bukit Sunur, PT Danau Mas Hitam and PT Bukit Bara Utama) (see Fig. 11.1 ). Production from the individual Sumatran mines is shown in Table 11.4. A total of 11.25 Mt was produced in 1997 which has since increased to 12 Mt, of which 3.8 Mt is exported. Of critical importance to the mining operations in Sumatra is the proximity of suitable port facilities to enable shipment of coal both

for domestic use, chiefly in Java, and for export into Far Eastern and European markets. The principal ports all lie on the western and southwestern coast of Sumatra (see Fig. 11.1). The port of Tarahan is operated by PTBA with a capacity for 5.5 Mtpa, accepting vessels of up to 65 000 t dwt, Teluk Bayur ships 2.0 Mtpa, in vessels up to 30 000 t dwt and Pulai Baai, with a capacity for 1.0 Mtpa in vessels up to 20 000 t dwt. The ports of Tarahan and Teluk Bayur are further supported by rail links from the mines. In the case of PTBA's Tanjung Enim mine, the rail link is 450 km to Tarahan. However, a small amount of coal from Tanjung Enim is sent 200 km by rail to the small port of Kertapati on the Musi River near Palembang. Coal is loaded onto barges for shipment from the eastern side of Sumatra to domestic markets and to nearby Malaysia. It is proposed to construct a larger terminal near Palembang to accommodate larger vessels and shipments. It is envisaged that the current production will increase in the next ten years, providing market conditions (domestic and export) that justify investment. An example of this is the expected increase in Indonesia's domestic steam coal market to satisfy the increased demand for electricity, with the proviso that there will continue to be investment in the electricity-generating sector.

Chapter 12

Metallic mineral deposits M. J. CROW & T. M. VAN LEEUWEN

This account concentrates on the the primary metallic mineral deposits and occurrences in Sumatra, in particular the recent discoveries of gold, tin and base metals. The residual and placer deposits are given less emphasis, as no significant discoveries have been made in recent years. The history of mineral exploration and discovery in Indonesia has been reviewed recently by van Leeuwen (1993, 1994), documenting the change in emphasis of mineral-based activities from western to eastern Indonesia since the World War II. These studies bring up-to-date the classic account by van Bemmelen (1949), written when the mineral deposits in western Indonesia, particularly those in Sumatra, were among the better known and prior to 1942, important contributors to the Indonesian economy. The larger mineral deposits in southern Sumatra have been described briefly by Gafoer & Purbo-Hadiwidjoyo (1986), and are referred to in the regional descriptions of the mineral deposits of SE Asia by Hutchison & Taylor (1978) and Hutchison (1996). In wider-ranging reviews the geological setting of gold and base metal deposits in indonesia have been discussed by Carlile & Mitchell (1994), while those of tin deposits in SE Asia are catalogued by Schwartz et al. (1995). Sumatra has long been known as a source of gold, the name of the island being derived from the Sanscrit word S v a r n a d v i p a , meaning 'Golden Island', dating from the importance of gold deposits to the rulers of the Hindu kingdoms that flourished in Sumatra from the seventh until the eleventh century. The estimated total production of precious metals from Sumatra to 1994 was 91 t gold and 937 t of silver (van Leeuwen 1994). Tin deposits in the Riau Archipelago, Bangka and Billiton islands ('Tin Islands') are positioned at the convergence of ancient maritime trade routes between the Middle East and India and China, and Bernal (1991) has suggested that they have been known and exploited from the earliest times, but there is no archaeological evidence for this; current exploitation of tin dates from the early eighteenth century. Between 1710 and 1942 a total of 1.5 Mt of tin was produced (van Leeuwen 1994), but currently the demand for tin is limited and the bulk of tin production in Indonesia comes from alluvial and off-shore placer deposits.

Sources of data For the purposes of this review mineral localities in Sumatra and the Tin Islands are catalogued in Tables 12.1-12.6 in terms of 'mineral clusters', the locations of which are shown in Figures 12.1 and 12.6-12.10. Mineral clusters represent concentrations of mineral occurrences, or a group of deposits formed at similar times, although a few include mineral deposits which were formed in the same area but at different times. Summaries are given of the geological setting and the history of exploitation of these deposits. Original sources should be consulted for further details. Recently discovered/investigated deposits that have not (yet) been described in the published literature are discussed in some detail in the text. Van Bemmelen (1949), Young & Johari (1980), Djaswadi (1993), Indonesian Mining Association (1995) and Crow (1995) have compiled lists and details of mineral localities in Sumatra. Summaries of this data are given in the Explanatory Notes

which accompany the 1:250 000 Geological Maps of Sumatra published by the Geological Research and Development Centre, Bandung. Additional data for southern Sumatra can be found in the Quadrangle Regional Geochemistry Atlas Series published by the Directorate of Mineral Resources and for Sumatra as a whole in the geochemical atlases of Northern Sumatra (Stephenson et al. 1982) and Southern Sumatra (Machali Muchsin et al. 1995, 1997). Historic (pre-1941) data on several precious metal deposits in North Sumatra appear in Bowles et al. (1985). Details of other mineral occurrences are given in reports of the North Sumatra Mineral Exploration Project (Directorate of Mineral Resources/British Geological Survey) and in reports and published accounts of the mineralogical and analytical studies which followed this project (e.g. Bowles et al. 1984). The Regional Physical Planning Programme for Transmigration (RePPProT Land Resources Department/Bina Programme) include a review of the mineral resources of Sumatra by Clarke (1990) in a summary of the land and natural resources of Indonesia to assist the planning of the Transmigration Programme. Government-sponsored mineral exploration activities concentrated on geological mapping and long-term regional geochemical surveys, with an emphasis on documentation but with limited follow-up. The objective of these surveys was to encourage exploration activity by the private sector. Private sector interest in investment in mineral exploration in Sumatra was stimulated by these programmes and peaked between 1985 and 1992 (Fig. 12.2). The latest cycle in exploration activity started between 1995-1997 (Fig. 12.3). Much data concerning mineralization in Sumatra has been accumulated by mineral exploration companies in their Contracts of Work (COW) areas. Relinquishment reports of COW companies are not easily found on open file, and often important information, for example on analyses and drill cores, was never reported, or was misplaced when the COW ended (van Leeuwen 1994). The most significant prospect located by the governmentsponsored regional geochemical surveys of Sumatra was the porphyry deposit at Tangse where C u - M o mineralization was outlined by preliminary geochemical surveys (Page et al. 1978) during the North Sumatra Project. This prospect was investigated by Rio Tinto Indonesia (van Leeuwen et al. 1987). Recently published descriptions of mined Sumatran mineral deposits include Lebong Tandai (Jobson et al. 1994), Mangani (Kavalieris et al. 1987) and Muara Sipongi (Beddoe-Stephens et al. 1987), and the recent discoveries include: Nam Salu (Schwartz & Surjono 1990b), Sungei Isahan in the Tigapuluh Mountains (Schwartz & Surjono 1990a), Hatapang (Clarke & Beddoe-Stephens 1987), Way Linggo (Andrews et al. 1991) and Miwah (Williamson & Fleming 1995). Descriptions of Dairi (Middleton 2003) and Martabe (Levet et al. 2003; Sutopo et al. 2003), both new discoveries, have been presented at recent conferences.

Timing of metallic mineralization events in Sumatra No comprehensive dating of mineralization events in Sumatra has been carried out. The available data are summarized in Figure 12.4, with mini-maps illustrating the trends of zones of mineralization.

147

148

CHAPTER 12

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, 980

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Fig. 12.1. Metallic mineral clusters in Sumatra and the Tin islands.

Palaeozoic sedimentary basins (Pb-Zn Table 12.1) Lead-zinc mineralization in metasediments and metavolcanics of the Bentong-Billiton Accretion Complex (Barber & Crow 2003) was found in Billiton in the 1980s in the Nam Salu open pit at the Kelapa Kampit mine, during the exploration for tin, and this deposit has been investigated by several companies during the past 30 years. It occurs as sub-parallel veins and lenses within and adjacent to the Nam Salu horizon over a strike of more than 5 km. The Nam Salu horizon consists of interbedded, iron-rich, chemically precipitated sediments and basaltic tuff, altered by metasomatic processes (Schwartz & Surjono 1990b). The total resource outlined to date is of the order of 25 Mt @ 6.5% Zn, 4.0% Pb and 60 g t -~ Ag. The style, thickness and grades of mineralization intersected in drillholes vary considerable along the strike of the mineralized zones. Three styles of mineralization have been recognized: (1) massive, fine-grained sphalerite, galena and pyrite, in places showing streaky lamination and commonly containing fragments of quartz and mudstone;

(2) brecciated quartz veins and mudstones with selvages of sulphides; (3) disseminated sphalerite and galena in sandstone (Large 1991). Several origins have been proposed for the lead-zinc mineralization: (1) sediment-hosted exhalative (first proposed in 1977 by BHP geologists); (2) possibly syngenetic/diagenetic related to volcanic exhalations with later faulting, folding and granite intrusions having variably remobilized the mineralization (van Leeuwen & Poole 1978); (3) syntectonic (?Triassic) formed from hydrothermal solutions derived from tectonically induced dewatering of the host sediments, with mineral deposition taking place in structurally dilated zones (Large 1991); and (4) veintype related to hydrothermal fluids exsolved from a crystallizing acid magma (Schwartz & Surjono 1990b). Important zinc-lead deposits, the Dairi cluster, were recently identified in northern Sumatra, in the Kluet Formation to the NW of Lake Toba by Herald Resources. The deposits include massive Pb-Zn veins that were mined on a limited scale in the early 1900s (van Bemmelen 1949). In addition to the veins,

METALLIC MINERAL DEPOSITS

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1~176176 CONTRACTS OF WORK SIGNINGS

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1967-1971

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several other styles of mineralization have been identified: sedimentary-exhalative (sedex) deposits of Mississippi Valley Type (MVT), believed to be formed by the reaction of volcanic fluids with sediments; and supergene mineralization, the latter presumably deposited recently from descending metal-rich solutions derived from the weathering of the sedex mineralisation (Middleton 2003). The sedex mineralization occurs in a dome-like structure and is traceable over a strike distance of about 5 km along the NE flank of the dome. It is hosted by carbonaceous shales and dolosiltstones and forms a single thick horizon in the SE and multiple, mostly thinner horizons in the NE. The MVT and vein -type mineralization are confined to a sequence of shelf carbonates which are in sharp contact with overlying sedex-bearing argillites (Middleton 2003). The project has reached the bankable feasibility stage. Measured and indicated resources amount to 7.1 Mt @ 16.6% Zn, 10.2% Pb and 13 g t -~ Ag. An additional 10 Mt of c. 8% Zn, 4.2% Pb and 6 g t -~ Ag has been inferred. Two extensive skarn zones at the Sarkea prospect (Hendrawan et al. 2001) located to the south of the Dairi prospect were drilltested by Rio Tinto in 2001. The skarns are related to the intrusion of a granite of the Sibolga Complex into (?calcareous) beds of the Kluet Formation. Magnetite is the dominant mineral, followed by pyrrhotite and minor sphalerite-molybdenite in a magnetitesilica-chlorite-garnet + actinolite-epidote assemblage. The skarn is locally cut by late quartz veins containing significant amounts of Ag, Cu, Pb and Zn. During a regional stream sediment sampling programme carried out in South Aceh by Rio Tinto, Zn dominant banded and laminated pyrite-pyrrhotite-sphalerite-galena mineralization, and Pb-dominant galena-sphalerite mineralization, both of apparent limited extent, were found near Beukah in an area of

~__~

6~

108 ~

I

Fig. 12.2. Contract of Work (COW) licence areas signed in Sumatra and the Tin islands between 1967 and 1992 showing deposits that have been drill-tested.

meta-argillites and subordinate meta-psammites and marbles. These are interpreted as sedex and remobilised cavity-fill deposits, respectively (Dalimunthe et al. 1996).

Late Triassic-Early Jurassic magmatic arc and the Tin Granites (Sn, Wo; Tables 12.2 and 12.3, Figs 12.5 and 12.6a, b) Mineral deposits and mineral occurrences, predominantly of tin, are associated with granitoids emplaced in the period between 220 and 195 Ma, and associated hydrothermal activity. In this period Sumatra was a part of the western margin of the SE Asia Tin Belt which extends from Myamar to Billiton Island. The majority of significant tin deposits are associated with peraluminous granites of collision origin (Mitchell 1977, 1979, 1986) that were emplaced during the Indosinian Orogeny (Hutchison 1989) (Fig. 12.5). These peraluminous granites are classified as being within the Main Range Granite Province by Cobbing et al. (1986, 1992), Cobbing (2000; see also Chapter 5) and Schwartz et al. (1995), the type area being the western part of the Malay Peninsula. Granitoids in the Eastern Granite Province in the eastern part of the Malay Peninsula are predominantly metaluminous, but some of these granitoids are also associated with tin mineralization (Fig. 12.5). Cobbing (et al. 1992 and Chapter 5) describes the occurrence of granitoids of both I- and S-types with similar age ranges, representing the two separate provinces in the north of the Riau Archipelago, overlapping south of Singkep Island and on Bangka and Billiton Islands to form a single belt. The textures, chemistry and geochronology

150

CHAPTER 12

I

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I

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DRILL-TESTED DEPOSITS C~

METAL

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of these granitoids has been described by Cobbing et al. (1992 and Chapter 5), and Schwartz et al. (1995). The foreland of the Indosinian Orogenic Belt extended from the central Malay Peninsula deep into eastern Sumatra (Sibumasu). The West Sumatra Block, when sited approximately between present day Borneo (Cathaysia) and New Guinea (Gondwana) (see also Fig. 14.11) also appears to have participated distally in this collision. In Chapter 5 Cobbing refers to the presence of Stype granites in northern Sumatra, dating from 200Ma (McCourt et al. 1996), including a suite of tin-bearing granites associated with the Medial Sumatra Tectonic Zone, and also the West Sumatra magmatic arc which is composed mainly of Volcanic Arc-type granites (as classified by Pearce et al. 1984). West S u m a t r a

McCourt et al. (1996) identified a magmatic arc in western Sumatra (219 _ 4 to 183 • 13 Ma) that overlaps the post-collision phase of the Indosinian Orogeny. Alluvial cassiterite is associated with the locally porphyritic Tantan Biotite Granite (210 + 10 Ma K - A r age, Suwarna et al. 1994). In Chapter 5 Cobbing has noted the similarity of the Sijunjung biotite granite (247 • 12 Ma K - A r age, quoted by Sato (1991) and 206 + 3 Ma by Silitonga & Kastowo 1975), with the granites of the Main Range Province, but tin is not recorded. Cobbing & Mallick (1984) include the unmineralized Payumbah Granite near Muarasipongi in the

NAPAL

I%,

Fig. 12.3. Contract of Work (COW) licence areas signed in Sumatra and the Tin Islands between 1995 and 1997 showing deposits that have been drill-tested.

Main Range Province, but found parts of the Sibolga Complex reminiscent of Eastern Province granites of Peninsular Malaysia. Minor alluvial tin is associated with the Sibolga Complex but the age and source of this tin mineralisation is uncertain (Aspden et al. 1982b). Westerveld (1937) mentions clasts of vein quartz with cassiterite in a Tertiary conglomerate 18 km to the west of Palembang in SE Sumatra. This tin appears to have been derived from the concealed Palembang Batholith, known from oil exploration (De Coster 1974). The only surface exposure of the batholith is the Bukit Batu quartz syenite pluton southeast of Palembang (van Tongeren 1936; Gasparon & Varne 1995), which is associated with quartz-cassiterite veins (Katili 1974a). The Bukit Batu syenite has geographic and chemical affinity with, and a similar SVSr/S6Sr ratio to the Main Range Province (see Chapter 5), though the end value of +5.3 (Gasparon & Varne 1995) is very different from Main Range Province ~Nd values of --8 to --10 (Cobbing et al. 1992).

M e d i a l S u m a t r a Tectonic Z o n e ( M S T Z )

The Medial Sumatra Tectonic Zone (MSTZ) (Hutchison 1994; Barber & Crow 2003) is associated with granitic plutons carrying strongly to intensely pleochroic cassiterite, similar to cassiterite associated with the Main Range Granite Province in Malaysia

M E T A L L I C M I N E R A L DEPOSITS

THE AGES OF MINERALISATION

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? SULPHIDESIN BENTONG-BELITUNG ACCRETIONARYCOMPLEX ? DIARIBASEMETALS

TRIASSIC-JURASSIC

TIN MINERALISATION

(Hosking 1977). Van Bemmelen (1949) suggested that the Medial Sumatra tin granite suite occurred within an allochthonous thrust slice sourced in the Tin Islands Archipelago. The Medial Sumatra tin zone is now considered to be related to a suite of peraluminous granitoids belonging to the Main Range Granite Province of Peninsular Malaysia (see Chapter 5). Primary and alluvial tin in the Tigahpuluh tin cluster (Table 12.2) is derived from granites emplaced in Tapanuli Group metasediments to the east of the MSTZ. Schwartz & Surjono (1990a) report K - A r ages of 197 __ 2 and 193 4- 2 Ma from muscovite in a cassiterite-bearing greisen in the cupola of the Sungei Isahan muscovite granite. The granitoids from the Tigapuluh Mountains analysed by Schwartz & Surjono (1990a) and Suwarna et al. (1991) have 'high' and 'moderate' peraluminous compositions in the scheme of Villaseca et al. (1998), suggesting reactions with peraluminous pelitic and greywacke lithologies. Upright folds with sub-horizontal plunges indicate deformational thickening of the sediment pile, facilitating hydrous fluxing and anatexis. Crustal melts were emplaced in shears within the MSTZ. The Tigapuluh Mountains have the potential for the exploitation of small deposits of alluvial cassiterite, which may be accompanied by small amounts of gold. To the NW of the Tigahpuluh cluster, sporadic elevated geochemical tin values in stream sediments (Machali et al. 1997) were probably derived from the cupolas of granites from which

Fig. 12.4. The timing of the main mineralization events and their distribution in Sumatra and the Tin Islands.

the tin has been weathered out, eroded and redistributed in Tertiary and Quaternary sediments. Alluvial tin has been won for over 50 years from the Siabu-Sungai Lipai mining area in the Rokan cluster, from which about 100 t of tin concentrate was produced up to 1982. Occasionally diamonds are found in the concentrates, which are believed to be of multi-cycle alluvial origin, originally sourced in the Tapanuli Group (Clarke et al. 1982b). The source of the tin is the Rokan-Siabu granitoid suite intruded into the Tapanuli Group on the margin of the MSTZ. Fifteen greisen, quartz vein and alluvial tin occurrences are associated with these granitoids (Clarke et al. 1982b; Rock et al. 1983). The Rokan Granite is variably cataclastically deformed and cooled to c. 400 ~ between 186 4- 2 and 189 4- 2 Ma (determinations on biotites using the K - A r method quoted by Rock et al. 1983). The roof zones of the mineralized granites were exposed to erosion by block faulting during the Neogene. The Penno-Triassic granite plutons in the Alas Valley section of the MSTZ, west of the Sumatran Fault Zone have metasomatic cupolas and, according to Cameron et al. (1982a), were emplaced during a transcurrent fault episode. The foliated muscovite-biotite granitoid plutons (Ketambe and Upper Sempali) and the Kais Intrusive Complex, which is believed to be the source of the alluvial tin in the Kais cluster (Johari 1988), from their field descriptions are similar to the anatectic granitoids which occur elsewhere in the MSTZ, but there are no chemical or isotopic data to confirm this affinity.

152

C H A P T E R 12

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154

CHAPTER 12

The I n d o s i n i a n f o r e l a n d

In Northern Sumatra, a belt of remote and poorly exposed granitoids (Fig. 12.5) north of the Medial Sumatra Tectonic Zone (MSTZ), were dated as Permo-Triassic by Cameron et al. (1980). In Chapter 5 Cobbing correlates these granitoids with the Main Range Province of the Malay Peninsula, based on the field descriptions of biotite and muscovite granites with tourmaline, reported in the Keteren, Serbajadi and Biden intrusions. The Dusun biotite granite is excluded here, as it has an early dioritic phase (Cameron et al. 1983), Mo-Cu mineralisation and is most likely associated with a Miocene intrusion (Dalimunthe et al. 1997a, b). The Serbajadi Batholith is elongated N W - S E , has a massive marginal carapace of lineated schists and gneisses (Cameron et al. 1983) and according to Bennett et al. (1981c), was emplaced during the regional slate-grade metamorphism and deformation of the Tapanuli Group. This granitoid belt coincides with a 'mid level geochemical enrichment zone' of tin identified during the North Sumatra Project stream-sediment survey (Stephenson et al. 1982), but no in situ tin mineralization has been reported. There are several islands in the Malacca Straits to the east of Sumatra composed of granite and/or greisen, with tin mineralization. The Berhala cluster occurs in the three Berhala Islands offshore Tebingtinggi. Here tin and rare-earth minerals in beach sands have been weathered from gneissic biotite granite, greisens and cordierite-sillimanite aureole hornfels (van Tongeren 1935 in Cameron et al. 1981). Van Bemmelen (1949) considered the Berhala granites to be the same age as those in the Malaysian Tin Belt. Katili (1973) reported a K - A r age of 167 Ma from an altered basalt cored during oil exploration in the area of the Berhala Islands. Pulau Perak north of the Berhala islands and SW of Langkawi Island is composed of quartz-tourmaline hornfels (Jones 1981), which is related to a concealed pluton. Several granite plutons buried beneath the Tertiary sediments of the Central Sumatra Basin were cored in the Foreland Zone during the exploration for oil. A hydrothermally altered muscovite granite pluton at the bottom of the Idris No.1 well in the Coastal Plains Block gave a K - A r muscovite age of 208 __ 7 Ma (Koning & Darmono 1982). Nearby detrital tin in the Petani Formation (Stephenson et al. 1982) appears to have been derived from another (undated) buried pluton to the north of Rengat.

The main S E A s i a n Tin Belt

The bulk of the economic tin mineralisation in the Indonesian section of the Southeast Asia Tin Belt occurs in the Riau Archipelago, Bangka and Billiton, within the Indosinian Collision Zone (Fig. 12.5 and Table 12.3). An irregular 'tin front' separates the mineralized peraluminous tin-bearing granitoids from the unmineralized metaluminous granitoids. On Bangka Island (Fig. 12.6a), the granitoids were emplaced in foreland basin sediments (Tempilang Sandstone), which unconformably overlie an accretionary complex composed of imbricated sediments and metavolcanics of the Carboniferous-Permian Pemali Group (Ko 1986; Barber & Crow 2003). On Billiton Island, (Fig. 12.6b) the accretionary complex is exposed beneath folded Triassic sediments in (former) underground mines for primary tin. Lower Palaeozoic stratigraphic units are not exposed in the Indonesian islands, unless they occur on Singkep Island among the unfossiliferous slates and graphitic schists of the Persing Complex (Sutisna et al. 1995). According to Cobbing et al. (1992 and in Chapter 5) Sn-bearing granitoids were emplaced during a post-collision peak between 220 and 200 Ma. Tin (and wolfram) mineralization is associated with late two-phase granitoid textural variants within the predominantly peraluminous megacrystic K-feldspar granitoids. The process of textural evolution from megacrystic granitoid through

heterogeneous granite porphyry to microgranite has been described by Pitfield et al. (1990). The textural changes leading to the heterogenous microgranites were attributed to sudden losses in pressure, which resulted in the quenching, fluidization and disruptive emplacement of residual melt into a partially or wholly crystalline host granitoid. The emplacement of residual melts was often accompanied by alkali metasomatism, volatilefluxing and hydrothermal alteration, culminating in replacement greisen deposits, veins and stockwork systems containing tin, wolfram and sulphides. Tin and wolfram ores (Table 12.3) occur either as massive replacement deposits with greisen, as non-massive replacements of low-grade ore, as at the Pemali Mine on Bangka, or as stockworks and simple veins. The cooling period for the granite in the Pemali Mine was between 159 and 95 Ma (Schwartz et al. 1995) based on the K - A r ages of biotites from this granite that was emplaced around 211 __ 3 Ma ( R b - S r errorchron quoted by Schwartz & Surjono 1991). The lengthy hydrothermal regime during the cooling of intrusions generated by the collision orogeny provided favourable conditions for tin mineralization on a regional scale (Lehmann 1990). Tin, and sometimes wolfram, are invariably accompanied by later sulphides, and mineralization is accompanied by tourmaline, fluorite and topaz. These replacement bodies, stockworks and vein systems, which are characterized by the absence of magnetite and paucity of basemetal and iron sulphides, formed in the cupolas of the granitoids. For example the Tikus mine of NE Billiton (Suryono & Clarke 1981" Schwartz & Surjono 1990c) was excavated in a greisen topaz-quartz pipe within the Tanjung Pandang batholith. In contrast to the other Main Range tin granites, tin was not identified in geochemical analyses of the Tanjung Pandang batholith; Lehmann & Harmanto (1990) suggested that the tin remained in solution until it was removed during the hydrothermal stage. In the southern part of Billiton, several tin deposits (e.g. Tebrong and the Senyubuk cluster) occur as stockworks and sheeted veins in metasediments, but erosion has not yet exposed the granite source. A rather unusual style of mineralization is found at the disused Kelapa Kampit mine, where complex tin-sulphide mineralization is present in both stratabound 'bedding-parallel veins' and crosscutting veins: on a mine-scale the distribution of the mineralization is stratabound. Bedding-parallel veins also are found in several other localities, including Batu Besi and Selumar. The veins are generally up to 2 km long and 3 m thick. They contain varying amounts of magnetite, sulphides, amphibole, biotite/ chlorite aggregates and quartz. Some veins are magnetite-rich, some are sulphide-rich, while others comprise both magnetiterich and sulphide-rich portions. The veins are hosted by metasediments, with the exception of the rich and thick (35 m) Nam Salu Lode (now largely mined out), which occurs in the Nam Salu horizon (mafic volcanics-ironstone). Certain characteristics of the Nam Salu Lode and bedding-parallel veins (stratabound/stratiform, sharp contacts, fine grain size and other textural features, abundance of iron minerals, and the presence of bedded barite) led several workers, including Hosking (1977), Hutchinson (1986) and van Wees & de Vente (1989) to conclude that the mineralization is of syngenetic origin. Other workers, e.g. Meyer (1979) and Schwartz & Surjono (1990b), favour an epigenetic (hydrothermal and/or pyrometasomatic) replacement origin related to granitic intrusions based on replacement textures displayed by the mineralization, chemical characteristics (of the Nam Salu horizon), and the presence of skarn-like assemblages that include amphibole, pyroxene and garnet. Recent work at Batu Besi has shown that the latter interpretation is the most likely. In this area several 'iron formations' with strike lengths of up to 6 km and up to 50 m thick, occur close to granitoids that are extensively greisenized and veined by quartz along their margins, together with felsic quartz porphyry and microgranite dykes with associated tin mineralization (Middleton 2002). They

METALLIC MINERAL DEPOSITS

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CHAPTER 12

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Granite Provinces and Granite Types

~D

SIBUMASU BLOCK Langkawi 9

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Fig. 12.5. Distribution of Late Triassic and Early Jurassic granites in Sumatra, Malaysia and the Tin Islands in the Indosinian Orogen. Granite Provinces and typology from Chapter 5.

are deeply weathered into a mixture of maghemite, goethite, hematite plus remnant magnetite. Deep drilling has revealed the primary mineralized rocks to comprise skarns of varied assemblages, which show a complex paragenesis. Early phase 'proto-skarn' is a zebrapatterned, contorted, banded lithology, with dark bands predominantly of magnetite and light bands of calcsilicate (probably mainly versuvianite) and fluorite. It resembles the so-called 'wrigglite' skarn at Moina, Tasmania (Kwak & Askins 1981). In places this early skarn phase is altered to a garnet-rich lithology, which in turn is retrogressed to carbonate-silica and clay, but the most important mineralization stage is a chlorite-biotitesulphide-fluorite assemblage, still with preserved magnetite wrigglite banding. This style commonly has > 1 % Sn grade while the 'proto-skarn' has Sn grades in the order of 0.2-0.1%.

The later stage retrograde mineralization is interpreted as associated with a late stage, volatile-rich hydrothermal fluid that also caused the greisenization of the granitoids. It is likely that the skarn was formed after a carbonate-rich protolith.

Bintan

Tin mineralization on Bintan Island is associated with metaluminous to peraluminous Volcanic Arc Granites of the East Belt (Schwartz et al. 1995) of the Eastern Province, intruded c. 230 Ma. Cobbing et al. (1992) attributed these granitoids and their mineralization to melting of the lithospheric mantle as a result of the subduction of

158

CHAPTER 12

(a)

: F-:

KLABAT BATHOLITH

~..& 106~

GRANITE PROVINCES AND GRANITE TYPE

O: I ~

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MAIN RANGE

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Tin Mining 9 Primary Deposits

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Tempilan name of mineral cluster

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/ PERMISA ~

Tem~i~ Sandstone t Pemai GrOup

Early-Mid Permian and older _

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/

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/

Fig. 12.6. (a) Bangka Island; (b) Billiton Island. Compilation maps to show distribution of granites and primary and secondary tin deposits. The structure of Bangka Island is after Ko (1986) and the granite typologyafter Cobbing et al. (1992).

(Palaeo-Tethys) oceanic crust, assimilation of continental crest lithologies and fractional crystallization processes. In Malaysia to the NE, metaluminous intrusions of the Central Belt (Schwartz et al. 1995) of the Eastern Granite Province are associated with gold, stibnite and sulphide mineralization, as exploited at Raub, but this style of mineralization is not seen in the Indonesian sector of the Indosinian orogen.

erosion and sedimentation, characterized by distinctive climatic regimes, and accompanied by a progressive rise in sea level that eventually submerged the present-day shelf platform area surrounding the tin islands (Aleva 1973, 1985; Batchelor 1979).

- ~ ,,' PARANG BULOH..-"

I II~KELUMPANG ~I~_)~~ .~" . . . .

Jurassic to Early Cretaceous magmatic arcs (Cu, Au; Table 12.4 and Fig. 12.7) P l a c e r tin

Primary tin deposits have given rise to numerous onshore and offshore placers (Fig. 12.6a, b), including Koba Tin and Cebia, from which the bulk of Indonesia's tin production has come. Most are palaeoplacers which were deposited and partly reworked from the Late Miocene to Recent times, during three major phases of

These magmatic arcs have been eroded, exposing batholiths and plutons, so that the roof structure and mineralization are rarely preserved. In Central Sumatra a few examples of intrusion-centered mineralization are known from the Mid-Jurassic-Early Cretaceous Arc. Skarn and disseminated mineralization at Muarasipongi have been described in detail by Beddoe-Stephens et al. (1987). At !

METALLIC MINERAL DEPOSITS the time of the emplacement of the batholith at 158 -+- 23 Ma precious metal and copper deposits were formed. In the Singkarak cluster copper and precious metals in the disused Timbulan quarry are associated with an altered granitoid. This intrusion is probably related to the Sulit Air suite of plutons, from which Imtihanah (2000) obtained 4~ ages of 192 + 0.4 and 193 ___4 M a for emplacement, which is the suggested time of porphyry-type mineralization. The Danau (Lake) Ranau Kelayang low-grade C u - M o mineralization in the north of the Bangko cluster occurs in altered roof rocks of the Bungo Batholith. Components of the batholith have K - A r mineral ages ranging between 169 and 129 Ma (McCourt et al. 1996).

Woyla Group and Accretion Complex (Au-Ag, Pb-Zn; Table 12.4 and Fig. 12.7) A possible example of exhalative sulphide mineralization is present within mafic lavas of the Bentaro Volcanic Formation in the Geunteut cluster. Bedded hematite-magnetite rock in the Tapaktuan Volcanic Formation is a potential, although limited, source of massive volcanic exhalative auriferous magnetite and sulphides forming the Tapaktuan and Babahrot clusters from which alluvial gold is derived (Cameron et al. 1982b). The alluvial gold in the Natal river is derived from skarn-type deposits at the contacts of Late Cretaceous intrusions and Woyla metasediments (see below). Alluvial chromite and perhaps some gold in the Pasaman cluster are derived from the Pasaman ophiolite body, which was possibly a seamount accreted within the Woyla succession. The Sungei Pagu former P b - Z n mine near Lubukgadang north of Kerinci Volcano occurs within limestones in a megabreccia, composed mostly of serpentinite boulders derived from an adjacent massive serpentinised harzburgite (Hariwidjaja & Suharsono 1990). Small diatremes and andesite and dacite dykes occur in the area. The megabreccia and ophiolite body are similar to lithologies described within the Woyla Group at Natal (Wajzer et al. 1991). The megabreccia is probably an olistostrome or a mud diapir in an accretion complex of which the massive serpentinite forms a component. Van Bemmelen (1949) suggested that the P b - Z n Mn mineralisation was of metasomatic origin, but here it is suggested to be a manganese-rich metalliferous deposit of hydrothermal type (Mitchell & Garson 1981) formed in an oceanic environment with the harzburgite representing part of a seamount, capped by limestone.

Late Cretaceous magmatic arc (Sn, Au-Ag; Table 12.4 and Fig. 12.7) Subduction beneath Sumatra was re-established in the Late Cretaceous, following the collision of the Bentaro-Saling Oceanic island Arc Complex in the Mid Cretaceous (Barber 2000). The reversal of subduction direction resulting from the collision of oceanic volcanic arcs with Sundaland in the Cretaceous was identified as potentially important for mineralization by Carlile & Mitchell (1994). In Northern Sumatra, small amounts of gold in the Sikuleh area are derived from skarns in reef limestones of the Bentaro Arc formed when the Younger Complex of the Sikuleh Batholith was emplaced at c. 98 Ma. Detrital tin, identified by stream sediment sampling during the North Sumatra Project (Stephenson et al. 1982), is probably of Tertiary age, as no tin mineralization was seen in greisens and veins at the contact of the Sikuleh Batholith with the Woyla Group, well exposed in stream sections along the northern margin of the intrusion (M.C.G. Clarke, unpublished map, pers. comm.). Precious metals and sulphides in the Natal cluster, formerly mined from magnetite bodies at the contact of the Manunggal

159

Batholith with the Woyla Group, were formed around the time of its intrusion (c. 87 Ma, Rock et al. 1983). Cassiterite and cerium-bearing monazite placers of the Garba cluster were eroded from greisens and pegmatites which formed in the cupola in a late phase of the Garba Batholith. This composite batholith was constructed during the Cretaceous, with a MidCretaceous dioritic phase (117-115 Ma, Aptian) followed by a Late Cretaceous ( 8 6 - 8 2 M a , Santonian) granitic phase with quartz-feldspar two-phase variants (McCourt & Cobbing 1993). Tin and rare earth mineralization was formed as a result of the successive fractionation of melts emplaced in a long-lived conduit and hydrothermal system developed in a favourable carapace. Alluvial tin in the Seputi cluster to the SE of the Garba Mountains is thought to be associated with a younger muscovite granite, which is a fractionated phase of the Padean Pluton (McCourt & Cobbing 1993), dated at c. 85 Ma and having low values of tin. The source of the alluvial tin was most likely the highly fractionated granite phases and greisens that have since been eroded away. The second category of Late Cretaceous tin deposits in Sumatra is associated with the Hatapang Granite, studied in detail by Clarke & Beddoe-Stephens (1987). The cassiterite and wolframite in this untested resource are derived from pegmatites and greisens developed in the carapace of the granite, emplaced at 80 i 1 Ma ( R b - S r isochron age) to the rear of the magmatic arc. The Hatapang Granite margin has a peraluminous chemistry and has chemical characters of both a within-plate A-type granite (see Chapter 5) and an S-type anatectic granite of collision origin (Clarke & Beddoe-Stephens 1987). Detrital tin weathered out of Tertiary sediments 7 0 - 8 0 km to the SE of Hatapang is possibly derived from hidden Late Cretaceous granitoids. Tin deposits formed during Late Cretaceous magmatism have two origins: (1) by fractionation and assimilation in intrusions belonging to the Late Cretaceous magmatic arc and (2) by anatexis of peraluminous metasediments caused by crustal thickening and associated mantle-derived intrusions in the backarc area.

Palaeocene magmatic arc (Cu, Au-Ag; Table 12.4) Minor sulphide mineralization in the Rawas cluster occurs within iron-rich skarns at the contact with Woyla Group metasediments and disseminated within the Bukit Rajah Granite emplaced at 54 • 2 Ma ( K - A r method, JICA 1988). Nearby is the Sungei Tuboh 1.76 Mt (estimate) skarn deposit with copper and precious metals which formed at the contact of a quartz monzonite at c. 40 • 2 Ma ( K - A r method, JICA 1988). The alluvial gold in the Rawas cluster is found in the vicinity of quartz veins, and associated with the sericitization and chloritization of Woyla Group metasediments, which also may be related to Palaeocene intrusions (Miswar & Suherman 1991).

Late Eocene-Early Miocene magmatic arc (Table 12.4) A rare example of mineralization associated with this Early Neogene volcanic arc occurs in the Breueh cluster NW of Banda Aceh. Disseminated sulphides and quartz veins are related to the intrusion of a sub-volcanic diorite body dated at 19 _+ 1 Ma (on hornblende by K - A r method) (Bennett et al. 1981a).

Miocene-Pliocene magmatic arc (porphyry Cu, Mo; Table 12.5 and Fig. 12.8) Several porphyry-type mineral occurrences (Danau Diatas, Siuluk Deras and Danau Dipatiampat) were located as the result of exploration for porphyry copper deposits in the early-1970s (van Leeuwen 1978) (Exploration Phase 2 of van Leeuwen 1994)

160

C H A P T E R 12

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METALLIC MINERAL DEPOSITS

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162

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I

I

JURASSIC - CRETACEOUS

MINERALISATION

4 ~-

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MUARASIPONGI

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~-....~ ~

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G O L D METAL BASE

~ Cr

~L

DANAU RANAU KELAYANG

U ~ I ~ eiGADA Pagt

TIN CHROMITE

'9

Late Cretaceous Plutons

IIIIII

-4~ Elm__.Mid-Jurassic to Early Cretaceous Plutons Woyla Group (Arc and Accretionary Complex)

I

I

SEPUTI

Padean

Pre-Cretaceous basement

%

-6 ~

0

4 ~_

,,,% -%\

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500km F i g . 12.7. M i n e r a l o c c u r r e n c e s a s s o c i a t e d

96~ I

98~ I

100" I

102~ I

(Fig. 12.2); others were found during regional mapping (Tangse and Dusun) and in the 1990s. The Miocene suite of equigranular and porphyritic dioritic and granitic intrusions, with C u - M o mineralization, are widely distributed in the Barisan Mountains of western Sumatra, but the mineralization is of very low grade. Porphyry-type mineralization is usually associated with arcparallel fault sets of the Sumatran Fault System, with plutons emplaced within segments and jogs of the main fault zone, or like the Lokop cluster in fault splays, although some dioritic centres, such as Tinjoen, are not associated with important faults. Van Leeuwen et al. (1987) have described the extensive investigation of the Tangse C u - M o prospect which was discovered during the geological and geochemical mapping programme of the North Sumatra Project (Young & Johari 1978). A large mineralized system is present at the Tangse prospect, but at the time of the investigation the grades were not economic. C u - M o mineralization is present between strands of the main Sumatran Fault System in altered, stockwork-fractured, multiphase (three sequential sets of intrusion were distinguished) porphyritic intrusions in the Eocene age Gle Seukeun Igneous Complex. The older group of porphyritic quartz diorites is the most extensive, with the intrusion and the alteration-mineralization having cooled between 13 and 9 Ma. A core of early chalcopyrite and biotite alteration is surrounded by a halo of chlorite and epidote. These were overprinted by two structurally controlled quartz-sericite-pyrite assemblages of which the chlorite assemblage is enriched in Cu and Mo. The mineralized system has been weathered and oxidized and there is patchy secondary Cu enrichment.

lO4~ I

106~ I

with the Mid-Jurassic-Early

Cretaceous and

t h e L a t e C r e t a c e o u s m a g m a t i c arcs.

The Tangse prospect is of particular interest as an example of dated Miocene multiphase porphyritic igneous intrusions in the Sumatran Fault System. The geochemistry and low intial Sr isotope values shown by the Tangse porphyries indicate that they represent a subduction-related, mantle-derived, normal-K calc-alkaline suite, which shows little evidence of sialic crustal contamination (van Leeuwen et al. 1987). The significance of such multiphase intrusions is the potential for successively fractionated melts with enriched metal contents to be emplaced in the same host, via the same magma conduit system, resulting in potential mineral deposits of economic value. This setting occurred in the overlaps and jogs between transcurrent faults in Sumatra during the Neogene, as at Tangse and probably also the Lolo Batholith. The mineralization at the Tangse stock (van Leeuwen et al. 1987) was completed before the intrusion of the dacite porphyry dykes, which had cooled to c. 500 ~ by 9.97 4- 0.50 Ma (magmatic hornblende by K - A r method). This suggests that the vertical and horizontal movements along the Sumatran Fault System which initially facilitated the magma conduit system at Tangse may have disrupted this system by c. 10 Ma and was followed by rapid uplift. The Tangse multiphase stock may be younger than the Lolo Batholith, which is composed of equigranular granodiorites only locally megacrystic, where the associated minor skarn mineralisation probably dates from the emplacement at c. 15 Ma (40 Ar/~39 Ar determinations by Imtihanah 2OOO). The only other porphyry copper prospects that have been drilltested to date are the Upper and Lower Tengkereng and Upper

METALLIC MINERAL DEPOSITS

163

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164

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I

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165 I

"~O-PUOCENEMINERAUSATION .

O

Mineral Locality

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Ise-Ise in the Dusun cluster. The three Dusun cluster deposits (Dalimunthe et al. 1997a, b) are associated with small (up to 550 x 3 0 0 m 2) multiple intrusive diorite-tonalite porphyry stocks. Alteration is highly telescoped with progressive overprinting of advanced argillic and phyllic alteration assemblages. Quartz stockwork veining varies from weak (1-10%) to intensive (up to 50%). The quartz stockwork is typically barren or only weakly mineralized. Sulphide mineralization consists of pyrite, covellite-chalcolite, lesser bornite and chalcopyrite and minor molybdenite. In contrast to Tangse the molybdenum content is negligible, whereas gold values are relatively high (0.170.38 g t-~ Au). It has been suggested that the general low tenor of the porphyry copper occurrences found to date in Sumatra may be due to the poor copper content of the crust that was subducted beneath the island during the Neogene (Katili 1974b) or because the process of subduction was too young to have generated suitable melts (Hutchison & Taylor 1978). Another possible explanation is that the Neogene subduction occurred (most of the time) at an even velocity, a condition which is not conducive to the generation of large, high grade deposits (Sillitoe 1997).

Neogene magmatic arc (Au-Ag; Table 12.6 and Fig. 12.9) Mining of primary deposits on the West Coast of Sumatra and in the Lebong cluster was interrupted in 1941. Subsequently mining has never reached pre-war production levels, with only Lebong

106~ I

Fig. 12.8. Mineral occurrences and prospects associated with the MiocenePliocene mineralization.

Tandai being reopened. Most of the abandoned mines were reinvestigated and drilled during the late 1980s, but extensions to the ore bodies at Mangani and Lebong Donok were not found at depth (van Leeuwen 1994). A number of new gold occurrences in Sumatra were found during the various COW investigations (1985 onwards), of which Bukit Tembang reached the mining stage while exploration is at an advanced stage at two others (Way Linggo and Martabe). The dating, quantity and source of the gold mineralization of many prospects remains poorly understood, because their perceived low economic potential has discouraged detailed study. In Table 12.6 the times of mineralization are estimates, based on the dating of host lithologies and intrusions, although the mineralization sequences are better documented. An exception is the gold mineralization at Lebong Donok for which K - A r ages between 1.2 and 1.3Ma were quoted by Henley & Etheridge (1995), which is a similar to the age to the Cirotan epithermal system in west Java, where adularia was dated by the K - A t method at 1.7 Ma (Milesi et al. 1994). This data places Neogene gold mineralization in Sumatra, at least in part, in the period after 3.5 Ma in an interval of tectonic reorganization following the collision of the Philippine Arc and the Eurasian Plate (Barley et al. 2002). Neogene epithermal precious metal deposits in Sumatra are classified following White & Hedenquist (1990), using the vein and alteration mineralogy and the form of the ore body, to infer the fluid chemistry which controlled ore formation. The high sulphidation type reflects relatively oxidised ore fluids, and the low sulphidation type reflects relatively reduced ore fluids. Examples

166

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M E T A L L I C M I N E R A L DEPOSITS

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NEOGENE " ~

MIWAH 4ON

169

GOLD

GOLD DEPOSIT TYPE O t High sulphidation ~1~

\

Low Sulphidation Sediment hosted

ABONGx

!

MINERALISATION

IAK

I:'~'~~'~':~ ~':"~:~I Jurassic-Cretaceous

4 ~ --

ii~i;iii~:iii~iiii:iii~iiil Woyla Group

2~_ ""'" ~

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F'~ i .._., ) BANGKO

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IKIT TEMBANG

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Seamount INDIAN-AUSTRALIAN O C E A N I C PLATE 6~


WAY L I N G ( ~ KOTAAGUNC

500km )n 96~ I

98~ I

100o I

1012~'~

of the latter type are commonly found in the southern half of the island, concentrated along two lineaments or axes. An Outer Neogene Gold Axis, linking the Salida and Kotaagung clusters with concentrations in the Lebong and West Coast Districts of van Bemmelen (1949), and an Inner Neogene Gold Axis, linking the Mangani and Tanjungkarang clusters can be distinguished (Machali et al. 1997) and are represented predominantly by 'classic' quartz-vein type deposits (Fig. 12.9). To date only three high-sulphidation type deposits have been found. All are located in northern Sumatra and are recent discoveries (Martabe, Miwah and Meluak). They represent fossil geothermal systems rich in magmatic volatiles. A study of the present-day hydrothermal systems in Sumatra by Hochstein & Sudarman (1993) shows that about 20% fall into this category. A third type of deposit comprises sediment-hosted mineralization found at Abong and Sihayo. The majority of Neogene epithermal gold occurrences in Sumatra are hosted in Tertiary volcanics and sediments which rest on the Woyla Group (Fig. 12.9). There are exceptions, as in the Meluak, Martabe, Mangani and Bangko clusters where the Woyla Group is not present. The spatial relationship between many of the epithermal gold deposits and the Woyla Group in Sumatra and Western Java was observed by Carlile & Mitchell (1994), who suggested that this relationship may be related indirectly to the arc reversal and emplacement of the oceanic volcanic arcs in the Woyla Group onto the Sundaland margin in the Cretaceous. The focusing of fluid flow, favourable permeability and fault structures controlling the emplacement of intrusions helps to

~PUNG

~il~ TANJUNG~ 7 ~ " KARAN G ~APAI

%. ~ .

\ 104~ l

4~_

Fig. 12.9. Mineral occurrences, prospects and former mines related to the Neogene gold mineralization.

explain the concentration of epithermal occurrences in the Neogene Gold Belt. Evidence of hydrothermal outflow in the Inner Axis is illustrated by the presence of sinters, as in the Bangko cluster. In some instances this outflow may derive from the Outer Neogene Gold Axis in the elevated Barisan Mountains. The geomorphic outflow of thermal waters from the Barisan Mountains also contributes to the low temperature reservoirs of thermal water in the Tertiary sedimentary basins in the back arc area (Hochstein & Sudarman 1993). In southern Sumatra several precious metal clusters are associated with arc-parallel master fault segments of the Sumatran Fault System (e.g. Tanjungsakti, Way Linggo and Martabe). The Mangani prospect is situated towards the termination of a major segment of the Sumatran Fault System (Kavalieris et al. 1987). The connection with arc-normal fault sets is occasionally invoked, as at Miwah and the importance of faulting in the localisation of metal occurrences is well understood. Hovig (1914) noted the fault-grid intersections controlling precious-metal mineralization in the Lebong cluster. Terpstra (1932) distinguished four groups of quartz veins at the Salida mine, based on their orientation, and Harris (1988) drew attention to the significance of fault control of mineralisation in the Lebong and Mangani clusters. As mentioned above, the majority of the low sulphidation gold deposits are located in southern Sumatra. Deposits in the Lebong cluster (Fig. 12.10) are among the better known. Jobson et al. (1994) described the Lebong Tandai deposit where underground mining recommenced in 1983 and continued into the early

170

CHAPTER 12

Fig. 12.10. The Lebong cluster of precious metal prospects, occurrences and former mines showing the 'Ketaun Zone' of eroded volcanic centres along which some the Lebong cluster mineral localities are aligned. Geology after Gafoer et al. (1992c) and Henley & Etheridge (1995).

1990s. The tabular, quartz-cemented, breccia ore bodies are localized along shears, which are related to an east-west sinistral fault system (Jobson et al. 1994 had reservations) and to a NW dextral fault system, by Jobson et al. (1994), using kinematic indicators. The mineralized zone is orientated approximately east-west over a strike of 4.3 km. It appears that no transpression or transtension was involved. The dimensions and mineralogical details of the breccia bodies are given in Table 12.6. Jobson et al. (1994) found that precious-metal mineralization was the result of hydraulic fracturing, associated with four phases of hydrothermal mineral deposition. In contrast, the precious metals at Lebong Donok are associated with quartz veins within the N W - S E Lebong Fault. Dacite dykes and andesite dykes and sheets are present. The mineralization is on the flank of an eroded andesitic volcano (Henley & Etheridge 1995) and is localized at the contact between the sediments and the volcanics. Henley & Etheridge (1995) relate the mineralization in the Lebong cluster (apart from Tambang Sawah) to the 'Ketaun Structural Trend' (Fig. 12.10), a tectonic-volcanic zone in which the individual ore bodies were emplaced at different levels, with Lebong Tandai representing the oldest mineralization and deepest structural level. Henley & Etheridge (1995) postulated that the breccia mineralization at Lebong Tandai was due to later transtensional reactivation of stepped thrusts, and that the Lebong Donok bonanza veins were formed in a dilitant transtensional setting, closely associated with the intrusion of dacite. According to Gafoer et al. (1992c) the location of the Ketaun Zone coincides with an incursion of the volcaniclastic Seblat Formation within the volcanic Hulusimpang Formation (OligoceneMiocene), and the volcanic centres are related to the Bal Formation (Middle Miocene). Postulated thrusting in the Ketaun Zone was presumably Pliocene in age, but while thrusts have not been described elsewhere in the area, they could represent the inversion of pre-existing normal faults associated with the growth of the Barisan Mountain range. It is difficult to evaluate the alternative interpretations of a clearly complex geological setting with so little information on the dating of the volcanic events and the mineralization. None the less the presence of large high-grade gold deposits at Lebong Donok and at Salida (Painan Formation volcanics on the Woyla Group), both of which are at the interface between sediments and volcanics, is significant. The settings are reminiscent of that at Hishikari in Japan where a fractured unconformity

between sediments and overlying volcanics was the focus of repeated boiling of high-temperature fluids that resulted in multiple precipitation of precious metals (Izawa et al. 1990). It is noteworthy that Lebong Donok and Hishikari show very similar mineralization characteristics (Kavalieris 1988). The ore bodies at Lebong Donok and Salida formed as a result of repeated opening of the fault zones, but in Sumatra precious metal mineralization was dispersed over larger areas, and in alignments, rather than concentrated at a single locality as at Hisikari. An unusual feature of several south Sumatra deposits, including Lebong Tandai and Lebong Donok, Mangani and Way Linggo is the occurrence of hypogene low-temperature calcium zeolites in quartz veins. Lawless et al. (1995) suggest that these deposits were formed in long lateral outflows, which facilitated extensive de-gassing of the outflowing primary hydrothermal fluid to the point where zeolites, rather than calcite, were deposited when the fluids finally boiled. In contrast, in deposits which formed in hydrothermal upflow zones, such as Bukit Tembang and Salida, CO2 contents were relatively high and consequently calcite tended to precipitate on boiling because of the de-stabilisation of bicarbonate along with adularia in response to the resulting rise in pH. Lawless et al. (1995) point out that if their model is correct, it may be a useful exploration tool for distinguishing upflow-zone from 'satellite' outflow-zone deposits which they argue is important, as the latter can be expected to have a more limited vertical extent. Turning to high sulphidation deposits, the Martabe gold system (Levet et al. 2003, Sutopo et al. 2003) was discovered in late 1997 by Normandy Anglo Asian Indonesia (subsequently taken over by Newmont), using bulk leach extraction of gold (BLEG) sampling techniques. It consists of a number of deposits over a strike length of 7 km, hosted in a series of Tertiary volcanic and sedimentary rocks (palaeontologically dated at 18-20 Ma), proximal to a fault splay of the Sumatran Fault System. Episodic fault activity has been responsible for pulses of high-level magmatism and the development of multistage phreatomagmatic breccias, dacitic flow dome complexes, hydrothermal alteration and gold mineralization in this district. The fault system consists of a major NW to NNW fault set accompanied by a conjugate set of NE extensional faults, consistent with regional dextral strike-slip tectonics. The most significant and best defined of the Martabe deposits is the Purnama deposit, which has a resource of 3.7 million ounces of gold and 46 million ounces of silver, making it the largest known gold deposit in Sumatra. It is hosted by an intrusive diatreme that has been injected along bedding planes within a sedimentary-volcanic unit. Multi-stage acid-leaching hydrothermal alteration events have produced large volumes of vuggy to massive silica with a tabular geometry. The silica zones are enveloped by silica/dickite/alunite, grading out to silica-illite and peripheral argillic alteration zones as the initial acidic vapour phase was progressively neutralized by the wall rocks and the groundwater. There is a very strong correlation between gold mineralization and silicification, as the latter has produced a vuggy permeable host that was subjected to brittle fracture during subsequent tectonic events. The mineralized zone at Purnama extends about 1.2 by 1 km. An early phase of low sulphidation silica pyrite veining and chalcedonic silica with low gold grades, associated with or immediately after the main acid sulphate alteration event, was followed by a high sulphidation phase characterized by enargite and luzonite mineralization and higher gold grades. The alteration/mineralization sequence observed at Purnama, i.e. acid-sulphide alteration-low sulphidation veining-high sulphidation veining is highly unusual for this type of deposit. The low grade Miwah prospect is found in an extensive alteration system in interbedded Pliocene sediments and andesitic volcanics, associated with arc-normal faulting and probably connected to a buried porphyry-type intrusion (Williamson & Fleming 1995). A linkage with a subduction zone beneath the North Aceh coast was proposed by Rock et al. (1982) on the basis of the chemical

METALLIC MINERAL DEPOSITS

composition of the volcanics and is favoured by Carlile & Mitchell (1994). This interpretation is considered unlikely as although seismicity is associated with this zone, oceanic crust is not involved. The deposit is related like the rest of the precious metal deposits, to the Sunda subduction zone as mapped beneath Sumatra by Sieh & Natawidjaja (2000). The geology of the Meluak area is dominated by the rift formed by the subparallel Blangkejeren-Toru and K l a - A l a s Faults that form part of the main Sumatran Fault Zone. Gold mineralization is hosted by the Quaternary Kembar volcano and is associated with hydrothermal breccias, massive and vuggy silica and c l a y pyrite alteration. The Martabe and Meluak deposits have been discovered in areas of good access without previously recorded gold occurrences. This is probably due to the very fine particle size of the gold, which has not led to obvious detrital gold signatures in the drainage, but is amenable to discovery by chemical exploration techniques such as the BLEG (bulk leach extraction of gold) method. The two known sediment-only hosted gold deposits Abong and Sihayo, are both located in northern Sumatra. The Abong prospect (Hendrawan et al. 1996) consists of a NWtrending zone, about 2300 m by 450 m, of mudstone/black shale underlain by limestone, belonging to the Bampo Formation (Upper Oligocene to Middle Miocene). Andesitic volcanics are interbedded in this unit. An irregular zone of gold-bearing stratiform jasperoid and silicified shale/siltstone with an average thickness of about 9 m is present at, or close to the hanging wall of the limestone. It shows variable development of fluid breccias grading from crackle breccia to pseudo-conglomeratic breccia. Matrix-fill material includes massive crystalline quartz, colloform quartz, cockscomb quartz and illite. Gold mineralization is accompanied by anomalous As (up to 6%), Ag (up to 680 ppm), Sb and Hg. At Sihayo (R. Jones, written comm. 2004) drilling by Aberfoyle Resources Ltd, and more recently by Oropa Limited, has outlined a well-mineralized zone with a strike length of 1 km and up to 450 m wide containing an inferred gold resource of about 600 000 ounces. It is associated with NW-SE-trending faults as well as orthogonal cross-structures that form part of a multistrand segment of the Sumatran Fault Zone. The zone is inferred to extend discontinuously over more than 4 km of strike length to adjacent prospects. Gold mineralization is hosted by regolith and silicified breccias at, and near the top of a sequence of Permian limestones, and in tuffaceous siltstone intercalations within the limestones. The tuffaceous sediments vary from wellbedded ash-rich siltstones to chaotic, slumped, clast-dominated grits. The breccias were formed by karst dissolution under phreatic conditions and subsequent collapse. Typical breccias comprise clasts of mainly limestone, dark siltstone and andesitic volcanics

171

(from intercalated beds in the limestone), and coarse calcite fragments. Dark silica alteration (jasperoid) replaces breccia matrix material (fine phreatic sediments and tuffaceous sediments). Individual jasperoid bodies can be highly irregular in shape. Sulphide content is generally less than 1 or 2%, but locally exceeds 10%. Pyrite is the dominantly sulphide phase and is invariably accompanied by arsenopyrite and stibnite. In one of the adjacent prospects late-stage epithermal white quartz with vuggy and cockade textures forms the breccia matrix and occurs extensively as veining and breccia fill. Jasperoid alteration and mineralisation postdates Oligocene sediments which disconformably overlie the Permian limestone, but is otherwise undated. Later karst processes during the ?Late Tertiary and Quaternary, have reworked the jasperoid material into new breccias, some of which are fissure fillings. Some workers distinguish two types of sediment-hosted gold mineralization, as discussed by Sillitoe (1994): one generated distally with respect to intrusion-centred districts (eg. Sillitoe & Bonham 1990); and the other the product of metamorphic dewatering of thick sedimentary sequences, as exemplified by deposits in the Carlin trend in Nevada (e.g. Seedorff 1991). Abong and Sihayo are both located in areas that contain low-grade porphyry copper deposits and may therefore belong to the former group. However Sillitoe (1994) suggests that both groups may form a single, broad genetic category. The alignments in Sumatra range in scale from the N W - S E 'Neogene Gold Axes' (Fig. 12.9) and less common east-west volcanic-tectonic alignments of mineralization as at Muaradua and the Ketaun Zone in the Lebong cluster. Posavec et al. (1983) described examples (see Fig. 13.25) of N W - S E alignments, representing the migration of older Quaternary to Recent volcanic centres in response to progressive displacement along the Sumatra Fault Zone. East-west alignments of active Quaternary volcanic centres also occur, as at Bukitinggi. At Talang, and some other active volcanoes, Posavec et al. (1983) found e a s t west aeromagnetic anomalies, thought to image large buried dioritic intrusions, but the N W - S E volcanic alignments did not show aeromagnetic signatures indicative of buried intrusions. The migration of the loci of igneous intrusion and transcurrent movement of fault blocks were both caused by the oblique subduction of the Indian-Australian Ocean crust beneath Sumatra (Fig. 12.9). The Sunda subduction zone (Sieh & Natawidjaja 2000) and the Neogene Gold Belt are deflected by subduction of the Investigator Fracture Zone. In the forearc the 'Pini Arch' has formed above the trace of the Investigator Fracture Zone, which also has been related by Page et al. (1979) to the genesis of the Quaternary Toba Caldera Complex (Chesner & Rose 1991). The Martabe deposit is situated above the projected eastern boundary

Table 12.7. Alluvial gold deposits in Sumatra Cluster name

ANU-REUNGUET

MEULABOH (WOYLA)

SINGINGI (BENGKALIS)

Orebody form

Alluvial Au derived from quartz veins & disseminated sulphides in Woyla Group River terrace sands & boulders derived from Woyla Group & epithermal min Alluvial deposits occupy broad valleys cut in Tertiary sediments

Ore elements

Time of mineralisation

Au

Quaternary

Au-Ag Cu-Hg Cr-Pt

Quaternary

Au-Ag Pt??

Source uncertain. Epithermal quartz

found in dumps. Most gold found in upper alluvial succession

Resource & notes

Reference

Cameron et al. (1983); Coulson et al. (1986) Production (est. pre- 1942) 980 kg Au/5 Mm 3 gravel. Resource: proven (1988) 11.5 M m3@ 196 mg m -3 Au

Cameron et al. (1983); Bowles et al. (1985); Van Leeuwen

in 8 areas along Kr. Woyla Resource: 17.2 Mm3 @ 149 mg/m 3 Au (1990); production to 1958:2.2 t Au; est. grade 120 mg m -3 Au; 1973-75 resource of 180 Mm3 @ 90 mg m-3 Au

(1994) Van Bemmelen (1949); Van Leeuwen (1994)

172

CHAPTER 12

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174

CHAPTER 12

of the Investigator Fracture Zone and may be an example of mineralization caused by the focussed release of volatiles into the mantle wedge as a result of post-subduction faulting of hydrated oceanic crust, which Fauzi et al. (1996) suggested might have contributed to the formation of the Toba Caldera Complex. The presence of other irregularities in the ocean crust passing through the Sunda subduction system in the past may have contributed to stalling of the subduction system, which Sillitoe (1997) has suggested creates the possibility for developing large ore deposits in a volcanic arc by steady-state, feed-back processes.

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Several of the larger alluvial gold deposits in Sumatra have been mined in the post-war period (Table 12.7), but production has never reached pre-war levels. This is because the deposits were exhausted or because the resources were insufficient for commencial exploitation, though attracting artisanal workers (as have many of the disused primary deposits). Coulson et al. (1986) suggested that the precious metals in the Quaternary Anu-Renguet alluvial goldfield in northern Sumatra were derived from quartz veins and disseminated sulphides that formed during Mid-Cretaceous deformation of the Woyla Group. Precious metals in the more economically interesting coastal plain Woyla (Meulaboh) alluvial gold field were probably sourced from skarns in the Woyla Group (Cameron et al. 1983). The source of the gold in the Singingi alluvial field south of the equator is probably weathered epithermal deposits.

Conclusions Since 1967, Sumatra and adjacent islands have seen successive phases of mineral exploration (Figs 12.2 & 12.3) for tin, bauxite and porphyry copper deposits (1967-71), gold (1985-92 and 1995-97) and since 1998 (albeit on a reduced scale) base metals, gold, and tin. These phases of exploration have led to the discovery of numerous mineral occurrences and the testing of the more important mineral deposits (Table 12.8 & Fig. 12.1 l). The Indonesian Government has encouraged foreign mineral industry private sector investment in exploration through the Contracts of Work (COW) system. Although the main focus of the exploration efforts of the private sector has been in Eastern Indonesia and Kalimantan, Sumatra with its relative accessibility and lower cost of exploration, has seen a fair amount of activity, especially during the most recent gold exploration boom in 1995-97, when large tracts of land were covered by COW applications (Fig. 12.3). Unfortunately, the boom was prematurely terminated in the wake of the Bre-X scandal in Kalimantan (Wells 1998). This scandal sapped the confidence of investors in the Indonesian mining sector, and the sector has remained

01

0.2

,

,

1 10 Resource (including past production) in millions of tonnes

160

Fig. 12.11. Gold resources, including past production and gold grades (g t l) of some Sumatran gold deposits adapted from van Leeuwen (1994).

depressed subsequently due to the 1998-99 economic crisis, the deterioration in the general investment climate, the issuance of Forestry Law 41/1999 prohibiting opencast mining in protected forest areas, which has effectively sterilized large parts of areas with mining potential (>50% in Sumatra), and the emergence of a strong anti-mining lobby. Despite these unfavourable conditions, several companies have persisted with exploration projects. Compared with other parts of Indonesia, exploration for metallic minerals in Sumatra during the past 35 years has, on the whole, produced disappointing results. In mainland Sumatra only one deposit was discovered that reached the mining stage (Bukit Tembang, a small Au deposit) and two old Dutch mines (Woyla and Lebok Tandai) were reopened for a short time. From an economic point of view none of these were very successful. The most significant discovery on the islands east of Sumatra is arguably the small, but rich Nam Salu tin deposit, which was amenable to open pit extraction. The recent discoveries of several gold and lead-zinc deposits, however, indicate that the mineral potential of Sumatra has not yet been fully tested. It is encouraging that these include styles of mineralization not previously known to exist in Sumatra. Other novel categories of mineralization may be identified in the future. The reinterpretation of the genesis of known occurrences may lead to new exploration concepts, as may a better understanding of the geological history and of the geological processes which have occurred during the evolution of Sumatra.

Chapter 13

Structure and structural history A. J. B A R B E R & M. J. C R O W

The present structure of Sumatra is dominated by the effects of the current subduction system in which the Indian Plate is being subducted northeastwards beneath the island at a rate of c. 7 cm a-~. The structure of Sumatra was described by van Bemmelen (1949) and in terms of plate tectonics by Hamilton (1979). The main structural elements of Sumatra and its surrounding region are defined with respect to the Sumatran subduction system (Fig. 13.1). (1) Forearc region, which includes the subduction trench, part of the Sunda Trench extending from Myanmar to eastern Indonesia, the developing accretionary complex, composed of ocean floor materials scraped off the Indian Plate, the forearc ridge which rises above sea level to form the forearc islands, and the forearc basins which lie between the ridge, and the volcanic arc on the mainland of Sumatra (Fig. 13.2). (2) Barisan Mountains and the Sumatran Fault System. The Barisan Mountains are composed of an uplifted basement of Upper Palaeozoic and Mesozoic sedimentary and volcanic rocks, variously metamorphosed, deformed and intruded by granites, overlain by Cenozoic sediments and volcanics, including the products of the volcanoes related to the present subduction system, which form the currently active volcanic arc. The Sumatran Fault system is a complex of dextral strike-slip faults running the whole length of the island through the centre of the Barisan Mountains from NW to SE, with zones of compression and extension, forming areas of uplift and pull apart basins which form grabens along the line of the fault system. Movement along this transcurrent fault system is attributed to the oblique subduction of the Indian Plate beneath Sumatra, which is carrying the west coast of Sumatra and the whole of the forearc region northwestwards as a 'sliver plate' (Curray 1989). (3) Backarc region, extending northeastwards from the Barisan Mountains, across the Malacca Strait to the east coast of the Malay Peninsula, occupied by Tertiary sedimentary basins, formed by Palaeogene rifting and subsidence and in filled by Neogene to present day sedimentation. The sediments are affected by folding and faulting and contain coal and the major oil and gas resources of Sumatra.

The Sunda forearc Subduction trench and accretionary complex

To the west of Sumatra and the outer arc islands, the floor of the !ndian Ocean increases in depth from 4000 m at the northern end of the island to over 5000 m in the south (Fig. 13.2). Two linear north-south submarine volcanic structures, the Ninety East Ridge and the Investigator Ridge, considered to be based on oceanic transform faults, rise several kilometres above the general level of the ocean floor (see Fig. 1.2). The basaltic crust of the Indian Ocean, which is here of Cretaceous to Eocene age (Sclater & Fisher 1974; Liu et al. 1983) (Fig. 13.2), is overlain first by Cretaceous-Eocene pelagic sediments and then by Miocene turbidites. At the northern end of Sumatra the turbidites

form part of the Nicobar Fan and are 2 km thick. The turbidites were derived from the Himalayas following their uplift during the Miocene, and formed the eastern branch of the Bengal Fan, before sediment supply was cut off by the collision of the northern end of the Ninety-East Ridge with the subduction trench in Pleistocene times. On the ocean floor the sedimentary cover decreases in thickness southwards, until at the southern end of Sumatra, the thickness of the fan sediments is reduced to less than 1 km (Fig. 13.2, inset). Sediments of the Nicobar Fan are covered by a thin veneer of Recent pelagic sediments. Seismic reflection profiles obtained by the Scripps Institution of Oceanography (SIO) around Nias in the 1970s and 1980s as a contribution to the Sumatra Transect, part of the SEATAR (Studies in East Asian Tectonics and Resources) Program (CCOP-IOC 1981), show that Indian Ocean lithosphere, and its covering of sediments, are being subducted in the Sunda Trench northeastwards beneath Sumatra (Fig. 13.3). More recently very similar seismic sections have been obtained to the south of Enggano by the R / V Sonne as part of the GINCO (Geoscientific Investigations along the active Convergence zone between the eastern Eurasian and Indo-Australian plates off Indonesia) Project (Kopp et al. 2001). The subduction trench lies about 250 km to the SW of the mainland of Sumatra and 100 km to the SW of the outer arc islands (Fig. 13.2). At the northern end of Sumatra the subduction trench is 4000 m deep, but the trench increases gradually in depth southeastwards, until at the southern end of the island it is more than 6000 m deep (Fig. 13.2, inset). A compilation of echo-sounding measurements from the floor of the trench, and seismic reflection and refraction determinations of the depth of the underlying oceanic basement shows that this increase in depth is due entirely to a decrease in the amount of sediment on the ocean floor (Moore et al. 1982) (Fig. 13.2, inset). The SIO seismic reflection profiles show sub-horizontally bedded Nicobar Fan sediments on the floor of the trench overlain by a thin wedge of more recent sediment at the foot of the inner slope. The Indian Ocean floor slopes gently northwestwards at 2 ~ towards the trench and as the trench is approached the overlying sediments and the ocean floor are broken by normal faults downthrowing towards the trench and parallel to the trench axis. At the base of the inner slope of the trench the sediments on the Indian Ocean Plate are seen in seismic sections to have been uplifted along thrust surfaces and imbricated to form an accretionary complex (Fig. 13.3a). The trenchward outer slope and normal faulting in the ocean floor are attributed to a downward flexure and a complementary bulge on the incoming plate, resulting from loading by the overlying accretionary complex. The inner slope is made up of a series of ridges and troughs parallel to the trench axis which rise steeply from the floor of the trench, and then flatten out in the outer arc ridge (Fig. 13.3a). Karig et al. (1980, fig. 4) interpret fans of recent sediment on the floor of the trench as formed by material slumping down the of the steep lower face of the accretionary complex. These fans impede the flow of sediments along the trench axis. Seismic profiling of the trench shows that the trench sediments and the underlying turbidites are uplifted along thrust faults at the toe of the accretionary complex. The ridges on the face of the accretionary complex are formed by successive anticlinal folds of ocean floor sediments, broken by faults and converted into

175

176

CHAPTER 13

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thrust slices which are underthrust and uplifted by the formation of later thrust slices as the incoming plate passes down the subduction zone (Stevens & Moore 1985) (Fig. 13.3). As is the case in other accretionary complexes (e.g. Barbados, the Makran and the Nankai Trough etc.) the steep (c. 35 ~ dip of the faults seen near the surface flatten out at depth into bedding-parallel decollement surfaces in the pelagic sediments a short distance above the oceanic basement (Moore & Curray 1980, Fig. 7). On the SIO profiles the oceanic basement of the Indian Plate can be traced landwards beneath the accretionary complex for a distance of 25 km (Moore & Curray 1980). In the R / V S o n n e profiles to the south of Enggano the surface of the downgoing slab dips at 3 ~ at the deformation front, increasing to 5 ~ beneath the outer arc ridge. The depth to the surface of the subducting plate was determined along the strike by seismic refraction at a depth

of 19 km beneath the outer arc ridge, and at 21 km beneath the forearc basin, 80 km landwards of the deformation front (Kopp et al. 2001; Schldter et al. 2002). Seismic profiles obtained by the Shell Company to the south of Java show steps in the basement which suggest that the decollement at the base of the sediments sometimes extends down into the basement, and that slices of oceanic crust have been uplifted into the base of the accretionary complex (Hamilton 1979). Karig et al. (1980) reached the same conclusion for the accretionary complex off Sumatra, as melanges on the outer arc ridge in the island of Nias contain blocks of serpentinite, pillow basalt and pelagic sediments derived from the oceanic basement. Troughs on the face of the accretionary complex become broader towards the upper, flatter part of the slope. Seismic profiles show that these troughs are slope basins filled by sediments

STRUCTURE AND STRUCTURAL HISTORY

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Fig, 13.2. Structural map of the Sumatran Forearc based on Hamilton (1979), with transform faults, magnetic anomalies and age of oceanic crust (double lines at 45 Ma mark an extinct spreading ridge) in the Indian Ocean from Sclater & Fisher (1974) and Liu et al. (1983); structures in the forearc from Izart et al. (1994), Matson & Moore (1992) and Diament et al. (1992); structures in the Nias Basin from Matson & Moore (1992), normal faults with ticks, and monoclinal flexures with triangles, indicating the downthrown sides. Onland extensions of the forearc basins are shown in white. The inset shows topographic and bathymetric profiles parallel to the arc system through the forearc islands, the forearc basins and the Sunda subduction trench after Moore et al. (1982).

178

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STRUCTURE AND STRUCTURAL HISTORY

(Moore et al. 1980a; Karig et al. 1980). While more recent sediments in these slope basins are sub-horizontal, older sediments are tilted landwards, and deeper in the basins are increasingly folded and more highly deformed and disrupted by thrusts, suggesting that the accretionary complex is under compression and that imbricate thrusts in the accretionary complex are continually re-activated to deform the sediments in the basins (Stevens & Moore 1985) (Fig. 13.3). Karig et al. (1980) suggest that the greater part of the sediments in the slope basins are derived locally by slumping of soft sediment from the face of active fault scarps, rather than from erosion on the island of Nias higher up the slope. In the seismic sections to the south of Enggano Kopp et al. (2001) make a distinction between the active accretionary complex with low seismic velocities, indicating that it is formed of unconsolidated sediments, and an older accretionary complex forming the outer arc ridge. The older complex, while still composed predominantly of sediments, is more highly consolidated and has higher velocities. They suggest that the older complex is of Palaeogene age and acted as a backstop to the younger Neogene to Quaternary complex (Kopp & Kukowski 2003).

Outer arc islands

The accretionary complex rises steeply from the floor of the trench to form an outer arc ridge, c. 120kin wide (Fig. 13.3a) which appears above sea level in a chain of islands to the west of Sumatra. In the north the ridge rises 5.5 km from the floor of the trench to the island of Simeulue, and in the south for c. 6.5 km in Enggano (Fig. 13.2). During the 1980s the geologists of the Indonesian Geological Research and Development Centre (GRDC) mapped most of the outer arc islands using aerial photographic interpretation and field traverses. The resulting maps were subsequently modified in the 1990s by interpretation of SAR (synthetic aperture radar) imagery, supplemented by additional field checking. These geological maps were published by GRDC at the scale of 1:250 000 (Simeulue and the Banyak Islands--Endharto & Sukido 1991 (Fig. 13.4); Nias--Djamal et al. 1994 (Fig. 13.4); Batu Islands--Nas & Supandjono 1994; Pagai and Sipora--Budhitrisna & Andi Mangga 1990; Siberut--Andi Mangga et al. 1994b; Enggano--Amin et al. 1994a) (Fig. 13.4). Most of the islands show restricted outcrops of melange, with blocks of serpentinite, gabbro, basalt, chert, calcilutite and rare limestones with large foraminifers, Nummulites, Discocyclina and Pellatispira of Eocene age (Douville 1912; Budhitrisna & Andi Mangga 1990), and granitic and metamorphic rocks, amphibolites, schists, phyllites and slates, together with abundant greywacke, sandstone, shales and claystone, in a sheared scaly clay matrix, in addition to the chaotic melange there are also more extensive oucrops of bedded units composed of sandstones, siltstones and clays, often tuffaceous, peats and coals, the latter indicating mangrove swamps, marls and limestones with abundant benthonic and planktonic microfossils, indicating abyssal to sublittoral environments of deposition. Microfossils show that the sediments range from Late Oligocene-Early Miocene to Pliocene in age. These older units are generally folded, faulted and thrust and are overlain unconformably by reef limestones and associated reef debris of Plio-Pleistocene age. The islands are surrounded by modem mangrove swamps and coral reefs. In many areas, particularly on the northeastern coasts of the islands, drowned mangroves indicate recent subsidence, and on southwest facing coasts raised reefs indicate recent uplift. The Karig model (Figs 13.5a, b and 13.6a). The most intensively studied of the outer arc islands is the island of Nias. Karig et al. (1980), having made a detailed study of the trench and the

179

accretionary complex from the SIO seismic reflection profiles (Fig. 13.3), completed a series of traverses across the outer arc ridge where it is exposed onland Nias, in collaboration with the Indonesian National Institute of Geology and Mining (Moore et al. 1980a; Moore & Karig 1980) as part of the Sumatran Transect of the Studies in East Asian Tectonics a n d Resources (SEATAR) Programme (CCOP-IOC 1981). The melange deposits, described as the Oyo Complex, were found to occur as linear belts several hundred metres wide and several kilometres long, parallel to the N W - S E trend of the island (Fig. 13.4). The melange alternates with belts of bedded sediment, described as the Nias Beds. The older sediments within the bedded succession are turbidites, which coarsen and thicken upwards. The oldest part of the succession lacks calcareous microfossils, interpreted as due to deposition below the CCD (carbonate compensation depth). Both the age and depth of deposition of the younger units were determined by their contained microfossils. It was found that Lower Miocene sediments were deposited at bathyal-abyssal depths > 2 0 0 0 m , Upper Miocene at depths of 2 0 0 0 - 5 0 0 m , while Pliocene deposits were accumulated on the continental shelf at < 5 0 0 m, and Pleistocene deposits were formed near sea level in a reef environment. The bedded units were folded and faulted contemporaneously with their deposition, with the older units being more highly deformed than the younger units. From their study of the offshore seismic data Karig et al. (1979) developed a model to account for the evolution of the accretionary complex, the development of the forearc ridge and the geology and structure of Nias (Fig. 13.5a, b). The Oyo m61ange was interpreted as a trench-fill deposit composed of fragments of ocean crust and turbidites that had slumped down the inner trench slope and were accreted into the base of the accretionary complex (Moore & Karig 1980). The chaotic and sheared nature of the m61ange was considered to be due to the dynamic tectonic environment within the accretionary complex, in which the original thrust surfaces were continually reactivated and new thrust planes developed, disrupting the oceanic basement and breaking it up into blocks. The oceanic basement material was continually uplifted into the accretionary complex along the developing thrusts. The age of the m61ange was not determined directly, but the youngest blocks incorporated in the melange appeared to be the Eocene limestones, so that the m41ange was considered to be of Eocene age. No stratigraphic contacts were seen between the Oyo Complex m61ange and the bedded units, but the m41ange was considered to be the oldest unit, forming a basement to the overlying bedded Nias Beds. The two belts of sediments on Nias were found to be broadly synformal but complicated by faults and small scale folds. Although no depositional contacts between the basement and the sediments were seen, Moore et al. (1980a) and Moore & Karig (1980) suggested that on the southwestern margins of the basins the sediments were deposited unconformably on the underlying m~lange, as near these contacts the m~lange is highly sheared, while the Nias Beds are only fractured. On the other hand the northeastern boundaries of the basins were found to be tectonic, with the Nias Beds sheared and mixed with the m61ange along the contact. Along strike the melange was observed to be in contact with different units of the Nias Beds. These contacts were interpreted as high angle reverse faults. Near the contact the Nias Beds are folded into tight asymmetric synclines on N E - S W axes with NE dipping axial planes and SW vergence. The fold axes plunge at low angles either to the NE or SW, and cannot be traced for more than a short distance along strike. In some examples the hinges of the folds have been sheared out along small scale reverse faults. Moore & Karig (1980) report that the older strata are more highly deformed than the younger units. Moore et al. (1980a) and Moore & Karig (1980) interpret their observations of the geology and structure of Nias in terms of

180

CHAPTER 13

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STRUCTURE AND STRUCTURAL HISTORY

181

MODELS FOR THE EVOLUTION OF THE SUMATRAN ACCRETIONARY COMPLEX AT NIAS ISLAND a. EARLY MIOCENE (Karig et al. 1980 Model) Melange formed Slope basins filled with sediment from Sumatra Ocean floor with thin by slumping into continually reactivated by thrusts on landward pelagic sediments trench and tectonic side with slumped sediment; earlier sediments deformation in toe compressed and deformed of accretionary complex J

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c. LATE MIOCENE TO RECENT (Samuel et al. 1997 Model) Accretion of Nicobar Fan sediments caused uplift and extension of the accretion complex forming rift basins in the old accretionary complex filled with sediment from the Sumatran Shelf until the Toe of Accretionary Pleistocene subsidence of the forearc basin. Complex advances Melange formed by shale diapirism with mud volcanoes. over incoming plate< NIAS Pleistocene subsidence flexure

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the model derived from the study of the seismic sections (Karig et al. 1979) (Fig. 13.5a, b). They suggest that the bedded units were deposited on top of the accretionary complex in two slope basins developed on the lower trench slope. At this stage, in the Early Miocene, the oceanic plate had only a thin sedimentary cover, so that the sediments were deposited directly on the oceanic basement. In the Late Miocene, when there was a greater thickness of Nicobar Fan sediments on the oceanic plate, the slope basins were uplifted as new material was accreted to the base of the slope. At the initiation of a slope basin, near the base of the slope, the sediments were deposited below the CCD, but as uplift continued sediments were deposited at progressively shallower depths, until the youngest deposits on Nias are uplifted coral reefs resting on the older slope basin sediments (Moore et al. 1980a).

Fig. 13.5. Comparison of models for evolution of the Sumatran Forearc base on studies on Nias Island (a) & (b) by Karig et al. (1980) and (e) by Samuel et al. (1997).

Moore & Karig (1980) predict that if the SW margins of the basins were exposed they would show the original unconformable relationships between the melange and the Nias Beds. With continual accretion the contacts and the layering in the overlying sediments were rotated to give their present steep angles of dip. On the other hand the NE margins of the basins are steep reverse faults along which the basement has been uplifted, compressing and folding the bedded sediments in the intervening sedimentary basins. The reverse faults were continually reactivated during the deposition of the sediments, so that older units are more highly deformed (Moore & Karig 1980). The Samuel model (Figs 13.5c and 13.6b). In the 1980s and early

1990s University of London Group for Geological Research in Southeast Asia, in collaboration with the Indonesian Research

182

C H A P T E R 13

CROSS SECTIONS OF THE SUMATRAN FOREARC SW

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(b) Samuel Model Fig. 13.6. Interpretative cross-sections of the Sumatran Forearc from the Indian Ocean through Nias to the volcanic arc on the mainland of Sumatra. (a) Karig model. In this model the sedimcntary basins on Nias are considered to have developed as as slope basins on the inner trench slope and to be overthrust on their northeastern sides by slices of accreted oceanic basement (after Karig et al. 1979). (b) Samuel model. In this model the sedimentary basins on Nias are considered to have originated as half graben due extension of the forearc; thrusts occurred subsequently due to inversion of the bounding normal ['aults (after Samuel & Harbury 1996) N.B.The vertical scale in (b) has been increased so that it is the same as in (a) for easier comparison of the two models.

and Development Centre for Oil and Gas Technology (LEMIGAS), studied several of the forearc islands. Reconnaissance visits were made to Simeulue and the Banyak Islands (Situmorang et al. 1987; Kallagher 1989; Harbury & Kallagher 1991) (Fig. 13.4) and the Batu Islands and Siberut (Barber et al. 1992). As part of this study the island of of Nias was remapped in detail by Samuel (1994) and Samuel et al. (1995, 1997) with tight stratigraphic control provided by microfossils. The oldest unit which can distinguished on Nias, as well as on the other forearc islands, is a basement unit, the Bangkaru Ophiolite Complex, named from one of the Banyak islands (Fig. 13.4). This consists of ocean floor material occurring either as a coherent unit, or as blocks in the melange. Rock types include serpentinised peridotites, gabbros, diorites and plagiogranites, dolerites, basalts, generally showing pillows, pillow breccias and hyaloclastites, garnet amphibolite (reported by Moore & Karig 1980), hornblende gneiss and hornblende schist, palagonite tufts, foraminiferal limestones, banded cherts, ochres, greywackes and quartz and barroisite schists (Samuel et al. 1997, Table 1). In the islands mapped or visited by the University of London Group coherent units outcrop on Bangkuru in the Banyak Islands, Simuelue, Nias and Sigata and Barogang in the Batu Islands. On Bangkuru serpentinite and gabbro outcrop in the hanging wall of a reverse fault with a strong shear fabric parallel to the fault. On Simuelue the Sibau Gabbro Group crops out on the NE coast towards the southern end of the island (Fig. 13.4). It is composed of coarse and fine metagabbro in which the

igneous minerals have been replaced by low-grade metamorphic minerals, including pumpelleyite. A high positive Bouguer gravity anomaly over the gabbro and elsewhere in Simeulue suggest that the island is underlain by a substantial slab of oceanic basement (Milsom et al. 1990). Samples of gabbro from Simeulue were analysed by Kallagher (1989, 1990) and showed an enriched MORB-type geochemistry and gave K - A t ages of 35.4 4-3.6Ma and 40.1 _+ 2.7Ma (Late Eocene-Early Oligocene). Kallagher (1989, 1990) did not consider that these ages represented the age of the original ocean floor since the presence of pumpelleyite indicates that the gabbro had been affected by a low-grade metamorphic event. She suggests that K - A r age indicates the time of metamorphism during subduction/accretion (Kallagher 1989, 1990). Scattered outcrops of coherent Bangkuru Ophiolite Complex were mapped by Samuel et al. (1995) on the SW coast of Nias, and at one location on the north coast ('B' in Fig. 13.4b). On Sigata coarse gabbro, cut by gabbro pegmatite and black dolerite dykes and veined by epidote, crops out on the southern shore of the island below the lighthouse. The lack of a positive gravity anomaly over this island (Milsom et al. 1990) suggests that the gabbro could be a large block in the m61ange. On Barogang diorite containing fine-grained dark xenoliths crops out on the northern side of the island, and the same rocks, highly brecciated, form a small offshore islet (Barber et al. 1992). Much more commonly, components of the Bangkaru Ophiolite Complex occur throughout the forearc islands as clasts in the

STRUCTURE AND STRUCTURALHISTORY mdlange, ranging in size from centimetres to more than 30 m (Samuel 1994). The ultramafic and basic rocks, serpentinites, gabbros, dolerites, pillow basalts and hyaloclastites, occurring as blocks in the m61ange, are compatible with an origin as part of an ocean floor assemblage and show the effects of low grade ocean floor metamorphism, such as occurs in the region of a spreading ridge. In central Nias blocks of serpentinite in the m61ange with foliation and linear structures show that they had been subject to ductile deformation under high temperature conditions in a mid-ocean ridge environment, possibly in a transform fault zone, prior to serpentinization. Hornblende gneiss and schist blocks may have a similar origin. However, some metasedimentary rocks, analogous to the metagreywackes of Moore & Karig (1980) and Kallagher (1989), are reported to contain prehnite, and an unusual rock composed of quartz, pyrite and an amphibole identified as barroisite (Samuel 1994; Samuel et al. 1997), suggests that ocean-floor sediments had been subducted. The pelagic limestones, bedded cherts and ochres found as blocks in the m61ange are also compatible with an ocean floor origin. A sample of pelagic chert from central Nias was found to contain foraminifers of Campanian (Late Cretaceous) age. While samples of bedded red chert yielded radiolaria of Mid-Eocene age (Samuel 1994; Samuel et al. 1997). These ocean floor sediments are compatible with the age of the ocean floor that has been subducted beneath Sumatra, as deduced from Indian Ocean spreading history indicated by magnetic anomaly patterns (Sclater & Fisher 1975) (Fig. 13.2). Samuel (1994) and Samuel et al. (1997) subdivided the Nias Beds of Moore et al. (1980a) into six units which could be correlated across the island. The oldest sediments (Oyo Formation) are thick-bedded, massive, micaceous sandstones. The unit is highly disrupted, so that coherent successions are rare, but this lithology commonly occurs as blocks in the m61ange. Early Oligocene to earliest Miocene fossils were obtained from this unit, but Samuel (1994) considers that the Early Oligocene fossils were reworked, like the limestone clast with N u m m u l i t e s of Eocene age found in a conglomerate (Douville 1912). Samuel (1994) and Samuel et al. (1997) conclude that the Oyo Formation is of Mid-Oligocene-earliest Miocene age and was deposited as turbidites in a deep marine setting below the CCD, as suggested by Moore et al. (1980a). The Oyo Formation is overlain conformably by the Gawo Formation of Early to Late Miocene age which has similar characters, but is thinner bedded and finer grained. Samuel (1994) and Samuel et al. (1997) were unable to confirm the conclusion of Moore et al. (1980a) that the 'Nias Beds' show a coarsening upwards succession. The Gawo Formation is overlain by the sandstones and mudstones of the Olodano and Lahomie formations of Early to Mid-Miocene age, which also include coral-algal limestone units formed as carbonate build-ups in a shallow marine environment. The progressive uplift with a shallowing upward sequence, from lower to upper bathyal and then sublittoral identified by Moore et al. (1980a), was confirmed by the study of the benthic foraminifera (Samuel et al. 1997). The transition from the deep water facies of the Gowa Formation to the shallow-water facies of the Olodano Formation is highly diachronous, occurring in the Early Miocene in the east, but not until late in the MidMiocene in central Nias. The conglomerates, sandstones and mudstones of the following Middle Miocene-Early Pliocene Lahomie formation indicate subsidence, with carbonate build-ups of the Olodano Formation covered by a blanket of mudstones, indicating that they had been drowned. Subsidence was followed by uplift and erosion during the Pliocene as the Late Pliocene to Recent Tetehosi (siliciclastic) and Gunungsitoli (reef limestone) Formations rest unconformably on the older units. Very recent uplift is confirmed by C 14 dating of 800 year old raised reefs of the Gunungsitoli Formation (Vita-Finzi & Situmorang 1989).

183

The origin of the mdlange. As part of his study Samuel (1994) and Samuel et al. (1997, Table 2) made a systematic study of the

mdlange and its relationships to the bedded units. They found, that in addition to the ophiolitic components, at least 50% and commonly 90% of the clasts in the mdlange were derived from the Oligocene and Lower Miocene units, while some outcrops also include clasts of the Middle Miocene to Pleistocene units. Very commonly the clasts showed the same sedimentary and structural features as seen in adjacent bedded units. It was found that the mud matrix has the same mineral composition, contains the same microfossils and also shows the same thermal history, with the same range of vitrinite values, as mudstones in the Oligocene to Lower Miocene succession. The scaly foliation which pervades the matrix is commonly vertical, but may be folded, wrapping around the clasts, and is parallel to the margins of the m61ange outcrops. Contacts between the m61ange and the bedded units are always intrusive, with the matrix penetrating along the bedding planes and fractures in the bedded units. M61ange is found cutting bedded units of all ages from Oligocene to Recent. It appears that the major period of m61ange formation occurred during the Pliocene, but m61ange formation on Nias continues to the present day, as indicated by the eruption of mud volcanoes extruding blocks and a grey mud slurry identical to the clay matrix of the m61ange (Figs 13.4 and 13.5c). As a result of this study Samuel (1994) and Samuel et al. (1995, 1997) concluded that the mdlange was the product of shale diapirism and not due to the tectonic disruption of trench fill sediments; it, therefore, does not constitute the basement upon which the bedded units were unconformably deposited, as was proposed by Karig et al. (1979). The evidence suggests that Oligocene and Lower Miocene deep-marine muds near the base of the bedded succession were periodically mobilized to intrude the Bangkaru Ophiolite Complex and the overlying bedded sediments, incorporating blocks of these units into the m61ange matrix. Mdlange mapped on the other outer arc islands also contains ophiolitic and sedimentary clasts in a scaly c l a y - m u d matrix similar to those recorded on Nias. Circular outcrops of m61ange and the active mud volcanoes mapped on Simeulue, Siberut, Sipora and Pagai (Endharto & Sukido 1994; Andi Mangga et al. 1994b; Budhitrisna & Andi Mangga 1990) suggest that the diapiric mechanism is responsible for occurrences of m~lange in all the outer arc islands. Structural evolution of the Forearc ridge. From mapping and stratigraphical study of the bedded sediments of Nias, Samuel (1994) and Samuel et al. (1997) recognized three sedimentary subbasins, including a basin in the NW of the island, the Lahewa sub-basin, in an area that had not been visited by Moore et al. (1980a) (Fig. 13.4, inset). It was found that the earliest structural features in the sediments were syn-depositional extensional faults. Samuel et al. (1995) therefore suggest that although the basins on Nias were formed on top of the accretionary complex, they developed during a phase of extension and are bounded to the NE by major extensional faults (Fig. 13.5r The increase in thickness of the bedded succession towards the northeastern margins of the basins indicates that-these margins were formed as active growth faults during the deposition of the sediments. Thrusts, sometimes bringing slices of the Bangkaru Ophiolite Complex over bedded sediments, and folds are superimposed on earlier extensional features, indicating that the basins were subsequently compressed. Localized inversion in the western part of Nias took place in Early Miocene times and was followed by the infilling of the basins, indicated by the upward shallowing of the depositional environments. Subsidence was renewed in M i d - E a r l y Pliocene times, but was followed again by widespread inversion with deformation during the Pliocene. The major bounding faults to the sedimentary basins have been reactivated as thrusts during inversion. The alternations of extension and subsidence, compression and uplift in the sedimentary basins are

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attributed to variations in the rates of convergence at the subduction zone, and the effects of transpression and transtension along transcurrent faults due to the oblique subduction. Matson & Moore (1992) proposed another model for the development of the Sumatra forearc in which the accretion of massive quantities of sediment from the Nicobar Fan in the late Mid-Miocene led to the depression of the incoming Indian Ocean Plate during the Mid-Miocene to Early Pliocene, causing the subsidence of the forearc ridge and its sedimentary basins, as recorded in the stratigraphic sequence (Samuel 1994). Clasts in the m~lange on Nias indicate that the island is underlain by upper mantle, oceanic crust and pelagic sediments derived from the Indian Ocean floor and built up into the accretionary complex. The only components of the m61ange which do not fit this model are garnet amphibolite and barroisite schist clasts reported by Moore et al. (1980a), Samuel (1994, 1997) and Samuel et al. (1997). A greater variety of clasts is reported fom the islands of Pagai and Sipora (Budhitrisna & Andi Mangga 1990). In addition to ophiolitic rocks and garnet amphibolite, clasts include garnetiferous mica schist, and granodiorite with biotite, and granitic gneiss with orthoclase and muscovite, suggesting that continental basement rocks underlie the eastern parts of some of the outer arc islands. Samuel et al. (1995) found that the Upper Palaeogene and Neogene stratigraphic sequences and lithologies in the Banyak and Batu islands, which lie within the forearc basin, and from boreholes in the forearc basin itself, resemble the stratigraphy and lithology of the same units on Nias. As will be discussed below, there is evidence that the forearc basin, which separates the outer arc islands from the mainland of Sumatra, has developed relatively recently. Samuel et al. (1995) suggest that prior to the Pleistocene, sedimentation was continuous across the present forearc basin to the outer arc islands. The common occurrence of well-rounded quartzose and metamorphic clasts in the Oligocene and Lower Miocene sandstones and conglomerates on Nias, indicate that the greater part of these sediments were derived from a mature continental provenance. Samuel et al. (1995) suggest that sediments were eroded from basement uplifts in the forearc region or were transported across the site of the present forearc basin from the mainland of Sumatra, to be deposited in extensional basins developed on top of the accretionary complex at the continental margin. Some conglomerates, however, contain locally derived ophiolite clasts, and coarse breccias, composed of large angular ophiolite and siltstone clasts, are interpreted as rock falls from active fault scarps, indicating that slices of the oceanic basement within the accretionary complex were being uplifted and eroded along the basin margins during sedimentation, as suggested also in the model of Moore et al. (1980a). Monoclinal flexure and the M e n t a w a i Fault. In the eastern part of Nias Moore & Karig (1980) mapped steeply dipping or overturned Nias Beds with westerly dipping shears and reverse faults in a zone 3 km wide along the eastern boundary of the easterly sedimentary basin. To the east this zone of steep dips is followed by Upper Pliocene and younger sediments with a low easterly dip. Pliocene sediments rest on the older rocks with an angular unconformity, but further east in the forearc basin this angular discordance disappears. Karig et al. (1979) and Moore & Karig (1980) interpreted this structure as a large 'homocline' or monoclinal flexure, between the deformed rocks of the forearc ridge and the flatlying sediments of the forearc basin. The downward displacement of the forearc basin sediments across the flexure was estimated at 3 km. They suggested that the flexure was the surface expression of a SSW-dipping back thrust at depth, on which the accretionary complex had been thrust over the forearc basement, which had acted as a back stop during the development of the complex. This flexure zone can be recognised in seismic reflection profiles (Fig. 13.3a & c), and can be traced southwards as a belt of structural disturbance to the east of the forearc islands, as far as Siberut.

Diament et al. (1992) carried out a seismic survey of the zone of disturbance. Their profiles show an uplifted block with a complex pattern of horsts and grabens, bounded on both sides by normal faults, with downthrown forearc ridge sediments on one side, and downthrown forearc basin sediments on the other. This structure was interpreted as a positive flower structure, and together with the straight trace of the fault over several hundred kilometres, led Diament et al. (1992) to suggest that the zone of disturbance was a major transcurrent fault, the Mentawai Fault, named after the outer arc archipelago (Fig. 13.2). Diament et al. (1992) went on to suggest that the Sumatran forearc was dissected into several narrow fault slivers along strike-slip faults, parallel to the main Sumatran Fault on the mainland. They suggest that these fault slivers are being displaced differentially northwards in response to the oblique subduction of the Indian Ocean Plate. They further suggest that in Nias the Mentawai Fault passed by way of the Batee Fault into the main strand of the Sumatran Fault in northern Sumatra. North of Simeulue Izart et al. (1994) found several faults in the accretionary complex to the west of the trace of the Mentawai Fault and suggests that the fault breaks up into a horsetail pattern of subsidiary faults at its northern end. Further north, opposite Banda Aceh, they found that the main trace of the fault was replaced by an easterly directed thrust fault, the West Andaman Fault (Fig. 13.2). By careful mapping and age determinations in eastern Nias, Samuel & Harbury (1996) found that the western limb of the monoclinal flexure consists of 5 km of easterly dipping Oligo-Miocene sediments. Seismic sections to the east of the flexure on eastern Nias, and in the offshore area, show that about 3 km of the Oligo-Miocene sediments seen to the west of the flexure are absent, and that to the east Upper Miocene sediments rest unconformably on the forearc basement in the Mola basement high. They, therefore, concluded that the flexure passed at depth into a major extensional fault, the boundary fault of a complex half-graben, rather than a thrust. Thrust features seen in the rocks at the surface are attributed to the effects of Late Pliocene inversion. Samuel & Harbury (1996) also studied the fold traces and lineaments seen in the SAR (synthetic aperture radar) imagery of Nias. Major anticlinal and synclinal traces and the dominant N N W - S S E lineaments run sub-parallel to the length of the island (Fig. 13.4). The dominant lineaments are faults, bounding the sedimentary basins, which Samuel & Harbury (1996) consider have been reactivated as thrusts during later inversion (see Fig. 13.4b, inset). Contrary to the suggestion of Diament et al. (1992) evidence of strike-slip movement has not been seen in outcrop in faults with this N N W - S S E orientation. However, complimentary N W - S E , and approximately north-south faults cutting across the strike of the beds do show strike-slip features in outcrop and are interpreted as conjugate shears (Fig. 13.4b, inset). Samuel & Harbury (1996) also recognize E N E - S W E lineaments, which they interpret as extensional faults, indicating that the island, and presumably the forearc as a whole, has been extended parallel to its length since the Pliocene. They consider that it is very unlikely that the Batee Fault passes into the Mentawai Fault (cf. Diament et al. 1992), it is more likely that it is represented by one of the north-south shears. Similar patterns of fold traces and fault lineaments are seen on the 1:250 000 Geological Maps of all the other forearc islands, from Enggano in the south to Simeulue in the north (Amin et al. 1994a; Budhitrisna & Andi Mangga 1990; Andi Mangga et al. 1994b; Nas & Supandjono 1994; Djamal et al. 1994; Endharto & Sukido 1994).

Forearc basins

Between the forearc islands and the mainland of Sumatra are a series of forearc basins (Fig. 13.2). At the present time the sites

STRUCTURE AND STRUCTURAL HISTORY

of the basins are depressions, with the sea floor lying at depths of up to 3000 m opposite north and south Sumatra, but rising opposite central Sumatra, where basin sediments and forearc basement are exposed in the islands of the Banyak and Batu groups, and in islands offshore Sibolga. The area of uplift coincides with a marked bend in the subduction trench, the 'Nias elbow' of Milsom (Chapter 2) (Fig. 13.1). It is probable that this area of uplift is due to the subduction of the Investigator Ridge and possibly the thermal and topographic perturbations caused by the extinct Wharton spreading ridge (Liu et al. 1983) as it passed down the subduction zone beneath this region (Malod & Kemal 1996) (Fig. 13.2). Nature of the j~rearc basement. Hamilton (1979) in his review of

the tectonics of the Indonesian region suggested that subduction complexes were developed within oceanic crust and that forearc areas are underlain by segments of remnant oceanic crust attached to the margin of the continent. Seismic refraction studies in the forearc to the east of Nias showed that the forearc basement had seismic velocities between those of oceanic and continental crust, which were compatible either with continental or thickened oceanic crust (Kieckhefer et al. 1980). The occurrence of clasts of garnetiferous mica schist, garnet amphibolite, granodiorites and granitic gneisses in m61ange on the outer arc islands of Pagai and Sipora (Budhitrisna & Andi Mangga 1990) suggest that continental crust may extend as far as the outer arc ridge. Recent seismic refraction studies during the cruise of the R / V Sonne in the forearc basin to the east of Enggano show that the basement in this area is of continental type (Kopp et al. 2001). Karig et al. (1979) concluded from the tectonic history of Sumatra, that the forearc basins were underlain by Pre-Miocene accretionary complexes which formed the continental margin against which the present complex was accreted (Fig. 13.6a). They proposed that the original, pre-present subduction phase, continental margin coincided approximately with the monoclinal flexure along the eastern side of the outer arc islands. The occurrence of ophiolitic material in the Banyak islands within the forearc suggested that this margin was irregular, with oceanic embayments (Karig et al. 1979). From the account of the PreTertiary geological development of Sumatra given in this volume (Chapters 4 & 14) the forearc basement is the western extension of the Bentaro-Saling Volcanic arc and the associated Woyla Accretionary Complex, intruded locally by Late Cretaceous and Tertiary granitoids and overlain by Palaeogene sediments and volcanics. Depositional history of the forearc basins. The forearc basins are

from north to south: the Aceh Basin; the Meulaboh (or Simeulue) Basin; the Nias (West Sumatra, Sibolga, or Singkel) Basin; and the Mentawai and Enggano (Bengkulu) Basin (Fig. 13.2). At the present day the greatest depth of the three northern basins decreases from north to south: Aceh Basin, 2710 m; Meulaboh Basin, 1150 m and the Nias Basin, 610 m; and increases again to the south: > 1000 m in the Mentawai Basin and > 2 0 0 0 m in the Enggano Basin to the south (Fig. 13.2). The basins are asymmetrical, for example in the Nias Basin the Sumatra continental shelf offshore the mainland of Sumatra deepens westwards to a shelf edge at c. 200 m, and drops down a continental slope into a deep-water basin, up to 610 m deep, further west. Sediment cores obtained from the floor of the basin are turbidites (Karig et al. 1979). The basin is cut off on its western side by a steep slope rising to Nias, coinciding with the monoclinal flexure and the Mentawai Fault. Seismic reflection surveys across the Meulaboh and Nias forearc basins calibrated by boreholes (Karig et al. 1979; Beaudry & Moore 1981, 1985; Matson & Moore 1992; Izart et al. 1994) show seismic sequences ranging in age from Palaeogene to the present day (Fig. 13.3c). The oldest dated rocks found in exploratory oil company boreholes are Upper Eocene and Lower

185

Oligocene dolomitic limestones, calcareous mudstones and pyritic shales with steep dips up to 50 ~. These rocks are poorly imaged in seismic profiles, but can be traced from the Sumatran mainland westwards beneath the continental shelf as far as the shelf edge. The sediments are at least 2 km thick, show variable dips, are cut by faults, and occupy a trough to the northwest of the Banyak Islands extending for 100 km parallel to the arc (Beaudry & Moore 1985). In the Bengkulu Basin to the south a borehole penetrated a sequence of ?Upper Eocene to Oligocene volcaniclastic sandstones interbedded with claystones which are correlated with the Lahat Formation volcaniclastics exposed onland in southern Sumatra (Hall et al. 1993). Seismic profiles indicate that these sediments occupy faulted half graben, up to 6 km deep, trending north-northeastwards and cut by NW-trending transfer faults. The trend of the graben has led to the suggestion that they may be the continuation of similar graben of the same age to the east of the Barisan Mountains in the Sumatran backarc area, displaced by c. 100 km along the Sumatran Fault (Howles 1986; Hall et al. 1993; Yulihanto et al. 1995). This correlation will be discussed in the section on the backarc area. The Palaeogene rocks are overlain with major unconformity by Lower Miocene and younger rocks (Fig. 13.3c), which on the mainland of Sumatra to the NW rest directly on the Palaeozoic and Mesozoic basement and to the west rest on the accretionary complex. In the Late Oligocene (29 Ma) the whole of the forearc area was exposed to subaerial erosion, probably with a landscape of significant relief, which supplied coarse sediment to the extensional basins which were developing on the accretionary complex which formed Nias to the west (Samuel & Harbury 1996). In the Early Miocene the forearc region underwent a marine transgression. In the shelf area sediments immediately above the unconformity are littoral sands, followed by Lower Miocene siltstones with shallow water foraminifera (Beaudry & Moore 1985). In the Early (?), M i d - L a t e Miocene carbonates were developed in the shelf area (Rose 1983). The Barisan Mountains, to the east on the mainland of Sumatra, were uplifted and eroded in the Late Miocene, supplying large quantities of terrigenous sediment to the forearc region (see Chapter 7). At the same time the forearc region itself underwent major subsidence. Prograding shallow-water clastic sediments overwhelmed the carbonate banks and, as sediment supply exceeded the rate of subsidence, built out to form a continental shelf and a continental slope towards the west. Further west, in the deeper part of the Nias Basin, deep water turbidites of Late Miocene age buried earlier Upper Miocene shallow-water carbonate mounds, which had been constructed directly above the unconformity in the early phase of subsidence. This pattern of sedimentation, with the progradation of the shelf and the deposition of pelagic turbidites in the deep basins, has continued through Late Miocene and Pliocene times to the present day. The same broad sequence of events affected all the forearc basins from the Aceh Basin in the north (Izart et al. 1994) to the Bengkulu Basin in the south (Hall et al. 1993). In the Banyak islands, between the Simuelue and Nias basins, Middle-Upper Miocene turbidites deposited in deep water are overlain directly by Pleistocene to Recent reefs, indicating that a once continuous forearc basin has been separated into two basins by recent uplift, localised in this area. This uplift has been attributed to the passage of the Investigator Ridge and an extinct Indian Ocean spreading ridge beneath the forearc (McCann & Habermann 1989; Malod & Kemal 1996; Fauzi et al. 1996). However, Matson & Moore (1992), from their study of the Banyak to Pini section of the forearc, discount the possibility that subduction of the Investigator Ridge was responsible for the uplift and subsidence in the forearc region. They suggest that due to the oblique subduction of the Indian Plate, uplift and subsidence caused by the subduction of the ridge would be expected to progress southwards along the arc with

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time, whereas throughout this segment of the arc, uplift and subsidence are synchronous (Matson & Moore 1992). Matson & Moore (1992) following the earlier work of Beaudry & Moore (1981, 1985) made a detailed study of SIO and industry seismic profiles and used stratigraphic controls provided by oil company boreholes to determine the sedimentation history and structural evolution of the Nias Basin. They found that the basin consists of two sub-basins separated by a broad area of elevation, the Singkel Basin to the northwest, limited to the west by the Batee Fault and to the east by the Singkel Fault, and the Pini Basin to the south, limited by a fault to the west and a fault or monoclinal flexure to the east. They suggest that the location of the basins is controlled by irregularities in the forearc basement which, following Karig et al. (1980), is considered to represent the position of the original continental margin. These sub-basins were found to have different subsidence histories. The rapid 2 km subsidence of the Singkel sub-basin in the Lower Miocene is attributed to a 25 km northward movement between the transcurrent Batee and Singkel faults and the movement of the basin down the inclined surface of the subducting plate (Matson & Moore 1992, Fig. 13). They attribute the even greater subsidence of the Meulaboh Basin which moved 100 km northwards along the Batee Fault to the same mechanism. Along the Mentawai Fault on the western side of the Nias Basin Miocene-Pliocene basin sediments are seen in seismic profiles to dip steeply eastwards into the basin, forming the monoclinal flexure (Fig. 13.3c). Upper Pliocene to Pleistocene deposits rest unconformably on the tilted rocks showing that the uplift of Nias to form the flexure occurred in Late Pliocene times and that the present form of the forearc basin developed only recently. The dislocation represented by the flexure occurred approximately along the original contact between the Sumatran continental margin and the accretionary complex (Karig et al. 1980; Diament et al. 1992). As already described this flexure is attributed by Samuel & Harbury (1996) to the reactivation of a basinbounding normal fault as a thrust, due to later compression. The Bengkulu Basin to the south has been studied by Mulhadiono & Sukendar Asikin (1989), Hall et al. (1993) and Yulihanto et al. (1995). They found that the Bengkulu Basin has a similar sedimentation history to the forearc basins to the north. Mulhadiono & Sukendar Asikin (1989) suggest that the graben in the Bengkulu Basin developed as pull-apart basins on strikeslip faults driven by the oblique subduction. Hall et al. (1993) and Yulihanto et al. (1995) suggest that the Bengkulu Basin originated in the Palaeogene as a series of north-south extensional graben similar to those developed in eastern Sumatran at the same time, and that during the Early Miocene transgression the Bengkulu forearc basin was connected with the South Sumatra Basin to the east, across the present site of the Barisan Mountains. They suggest that the basins later developed as pull-aparts by reactivation of earlier N W - S E transfer faults, related to the Palaeogene extensional graben.

Tectonic evolution o f the f o r e a r c region Cretaceous to Oligocene history. At the end of the Cretaceous the area of the present Sumatran forearc formed the southwestern margin of the Sundaland continent. It was composed of the Bentaro-Saling Arc and associated accretionary ocean crust which had been amalgamated with the continent in the midCretaceous, and in the Late Cretaceous was the site of a magmatic arc related to subduction of the Indian Plate beneath Sundaland (Barber 2000). in the Palaeogene the forearc area, in common with the rest of Sumatra, and SE Asia as a whole, was subject to regional extension. In Sumatra, extension led to the formation of graben structures which were occupied by lakes, with the deposition of screes and alluvial fans around their margins and

fluviatile and lacustrine sediments in the more central parts. This pattern of sedimentation probably extended into the Sumatran forearc where it is poorly imaged in seismic sections and only rarely penetrated by oil company boreholes (e.g. Beaudry & Moore 1985). At this time subduction still continued along the western margin of Sundaland represented by Palaeogene plutons and volcanic rocks which outcrop along the west coast of Sumatra, while India was moving northwards towards its collision with the southern margin of Asia. In the Oligocene the forearc area was uplifted and exposed to subaerial erosion, supplying sediment to basins developed on the accretionary complex to the west (Samuel et al. 1997). Karig et al. (1979) suggest that this period of uplift was related to compression of the forearc due to an increase in the rate of movement of the Indian Ocean Plate. Marine transgression, with the renewal of sedimentation, in the Late Oligocene and Early Miocene was due to the general world-wide rise in sea level which occurred at this time (Haq et al. 1987). An increase in the subduction rate from 5 to 6.5 cm a - l between 5 and 10 Ma may have been responsible for the Pliocene uplift and unconformity seen in Nias and the other islands (Karig et al. 1979). The accretionary wedge. In the Palaeogene the Indian Ocean floor that was being subducted or accreted into the accretionary complex consisted of Cretaceous-Eocene basaltic crust with a thin veneer of pelagic ocean floor sediments. Accretionary complexes that are formed largely of basaltic ocean crust, are able to maintain a steep inner trench slope as is the case with the accretionary complex to the south of Java at the present day. In the midMiocene the Indian Ocean floor received a vast influx of terrigenous sediment derived fi'om the collision zone of the Indian continent with the southern margin of Asia and the uplift and erosion of the Himalayas. This influx continued through the Pliocene until the Nicobar Branch of the Bengal Fan was cut off from sediment supply by the collision of the Ninety-East Ridge with the Sunda Trench adjacent to the Andaman Islands. After the mid-Miocene the great thickness of terrigenous sediments which was scraped off the Indian Plate altered the dynamics of the accretionary complex. When the accretionary complex is composed largely of incompetent sedimentary materials the inner slope of the trench will have a much lower angle of slope than one composed of basaltic ocean crust. The surface of the accretionary complex will adopt a wedge-shaped crosssectional profile with a critical taper, the angle between the topographic surface and the inclination of the downgoing plate. The critical taper will depend on factors such as the frictional resistance at the base of the wedge, the strength of the material composing the wedge, and pore fluid pressures which will also influence the strength (Davis et al. 1983). The Sumatran accretionary wedge is composed of relatively weak materials with a topographic slope of the order of 4 ~ and a 5 ~ inclination of the downgoing plate, giving a critical taper of 9". The critical taper represents an equilibrium condition. As thrust slices of sedimentary material are compressed into the toe of the wedge by the movement of the incoming plate the angle of the topographic slope will be increased and the wedge will adjust to re-establish the critical taper by moving forward across the incoming plate. This process provides a mechanism for the continual extension of the upper parts of the accretionary wedge and accounts for the formation of the half graben developed on the surface of the wedge as mapped by Samuel (1994) on Nias (Figs 13.5c and 13.6b). Matson & Moore (1992) have also pointed out that the continual increase in the volume of material incorporated into the wedge will exert a downward pressure on the incoming plate increasing the angle of inclination which will counteract the increase of the topographic slope and retard the forward movement of the toe of the wedge across the incoming plate. This in turn caused the uplift of the eastern part of the wedge. This mechanism would account for the continual uplift recorded in the Lower to Middle

STRUCTURE AND STRUCTURAL HISTORY

Miocene in the stratigraphic sequences in the basins on Nias, until in the Pliocene the upper part of the wedge and its overlying sediments emerged above sea level. They also point out that with time this mechanism will result in the westward movement of the load exerted by the wedge, causing a dislocation during the Late Pliocene along the Mentawai Fault between the uplifted forearc ridge and subsiding sedimentary basins in the western part of the forearc basin (Matson & Moore 1992). The processes and effects proposed by Matson & Moore (1992) would have operated with increasing intensity while sedimentary material was added to the surface of the Nicobar Fan and the thickness of sediment on the Indian Plate was continually increased. When the sediment supply was cut off by the collision of the Ninety-East Ridge with the Sunda Trench, the accretionary wedge was able to reach an equilibrium, and Upper Pliocene to Pleistocene sediments were deposited unconformably on the eroded surface of the older rocks. Later minor uplift and subsidence can be attributed to continual adjustments to the shape of the accretionary wedge and to the fluctuations in sea level during the Pleistocene. Continual recent uplift has been documented on Simeulue and Nias with five raised intertidal platforms to the south of Sinabang on Nias. Dating of molluscs from reef terraces on these islands gave ages ranging from c. 6000 to < 3 0 0 m m a -~ BP, with rates of uplift between 0.3 and 1.0 mm a - l (Vita-Finzi & Situmorang 1989). Drowned mangroves on the eastern side of Siberut indicate that uplift is not uniform and the islands may be tilted, and in Nias the east is uplifted while the NW is drowned. Karig et al. (1979) propose that this is due to the displacement of the crest of the outer arc ridge towards the west with the westward growth of the accretionary complex. Effects o f transcurrent faulting. An important influence on the

tectonic evolution of the Sumatran forearc is the obliquity of convergence and subduction of the Indian Ocean Plate beneath Sumatra. In models of oblique subduction the strain in the overlying plate is considered to be partitioned between compression normal to the subduction trench, which is taken up by inversion of the sedimentary basins during the Pliocene, with N N W - S S E thrusting and folding, seen in all the outer arc islands, and translation parallel to the trench along transcurrent strike-slip faults (Fitch 1972; Platt 1993; McCaffrey 1996). In the Sumatran subduction system the major component of translation is the Sumatran Fault which separates the forearc region from the Eurasian Plate as a separate Burma sliver plate (Curray 1989). There is a major difference in the amount of displacement along the Sumatran Fault System from north to south. To the north of Sumatra the displacement is represented by extension, indicated by the development of oceanic crust in the Andaman Sea, differential displacement being taken up along a series of closely spaced transform faults with a total displacement of about 460 km, the westernmost of which passes southeastwards into the Sumatran Fault System (Curray et al. 1979). On the other hand displacement of the fault system in the Sunda Strait at the southern end of Sumatra is less than 100 km (Huchon & Le Pichon 1984; Harjono et al. 1991). Some of this discrepancy may be accounted for by transcurrent movement along the Mentawai Fault (Diamant et al. 1992), and some may be taken up along splays of the Sumatran System, such as the Batee Fault which extends into the forearc region from northern Sumatra (Fig. 13.2). Minor strike-slip faults, like those described by Matson & Moore (1992) in the Singkel Basin, may be distributed throughout the Ibrearc and the occurrence of transcurrent faults within the submerged part of the accretionary complex is unknown. However, it is probable that the bulk of the differential movement, must be taken up along the large numbers of minor transcurrent faults, which form conjugate sets marked by lineaments seen in all the forearc islands, and by small scale extensional faults which bisect the obtuse angle of the conjugate shears (e.g. Nias--Samuel & Harbury 1996) (Fig. 13.4b).

187

Evidently the whole of the forearc is being deformed and has changed its shape by contraction normal to the trench and extension parallel to the trend of the arc. Prawirodirdjo et al. (1997) and Bock et al. (2003) have demonstrated from GPS measurements of the displacement of 60 sites on mainland Sumatra and on the outer arc islands, that at the present time the forearc to the south of the Batu islands is coupled to the Indian Plate and is moving parallel to the convergence direction, but at a slightly slower rate than the incoming plate (44 mm a -1 compared with 75 mm a - i ) , while to the north the forearc has an important component of northward movement parallel to the Sumatran Fault at a much slower rate (Fig. 13.7). The change in the rate and direction of movement occurs at the point where the Investigator Ridge is entering the subduction system, and it is suggested that this is due to the incorporation of water rich sediments of the Nicobar Fan into the accretion system to the north of this point. The effects of subduction of the Indian Plate extend for a few tens of kilometers to the NE of the Sumatran Fault, but the greater part of eastern Sumatra belongs to the Sunda Plate, which extends through Borneo to western Sulawesi and is moving southeastwards at 6 + 3 mm a-1 relative to the remainder of the Eurasian Plate (Bock et al. 2003) (Fig. 13.7).

The Barisan Mountains The Barisan Mountains extend for 1700 kin, from Banda Aceh in the north to Banda Lampung in the south, along the whole length of the island of Sumatra, parallel and close to the west coast. Over much of their length the mountains reach 1000 and 2000 m above sea level, locally rising above 3000 m in Aceh (Gunung (Mount) Leuser, 3381 m) and to the west of Lake Toba, and isolated volcanoes rise above the general surface in Gunung Kerinci (3805)

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,~o ~ 1t 102 ~~ ~0

/

I

104~

; t & 7~

I

106" 2

o SUNDAPE/ATEO

Siberu

INDIAN PLATE

Enggano

Fig. 13.7. Movementsin the forearcand withinSumatrarelativeto the estimated Sunda Shelf reference frame from GPS measurements 1991-2001 (after Bock et aI. 2003). The bold arrow shows the Australia/Eurasiamovement vector for the Indian Plate the finer arrows show the directions and amounts of movement measured at specific locations.The lengths of the arrows are propbrtionateto the rate of movement in mm a i. Ellipses of 95% confidencelimits have been omitted, but are in general much larger for measurement to the NE of the Sumatran Fault System, than to the SW.

188

CHAPTER 13

in the centre, and Gunung Denpo (3159) in the south. The mountain range is broadest in the north, 100 km wide, occupying almost the whole width of the island, narrowing to 50 km in the south. In the north the mountain range is formed of Pre-Tertiary rocks of Carboniferous to Cretaceous age forming the basement of Sumatra, which are overlain by Tertiary sedimentary and volcanic rocks which thicken into the basins in the forearc and backarc areas, forming low ground of less than 100 m, to the SW and NE. Locally Tertiary rocks occupy intramontane basins within the mountain range. Towards the southeastern part of the island the basement rocks are increasingly covered by Tertiary to recent sediments and volcanics, with the older rocks being exposed only in scattered inliers. At intervals along the chain basement rocks are overlain by Late Pleistocene to Recent volcanic piles, some of which are active volcanoes (Fig. 13.1).

Pre-Tertiary rocks in Sumatra Mapping the Pre-Tertiary units. Although Pre-Tertiary rocks in Sumatra form mountainous terrain, they are in general poorly exposed because of dense tropical rain forest and deep weathering. In addition, apart from areas immediately adjacent to the roads or the larger rivers, much of the area is difficult of access. During the mapping of northern Sumatra by the Indonesian Directorate of Mineral Resources (DMR) and of southern Sumatra by the Geological Research and Development Centre (GRDC) in collaboration with the the British Geological Survey (BGS), use was made of aerial photographs, Landsat and SAR (synthetic aperture radar) imagery to identify major geological structures. Major lineaments, mainly fault traces and wherever possible bedding or foliation traces were plotted to outline fold structures. These features are plotted on the geological map sheets published by the Indonesian Geological Research and Development Centre, together with lithological and structural data recorded during the fieldwork programme. Massive limestones are commonly well exposed, but form karstic terrain difficult of access. Outcrops, together with float, of other rock types occur commonly in river gorges and along rivers and stream networks and in coastal exposures. Artificial exposures occur in occasional quarries and in new road cuttings during road building programmes. Due to the high rate of tropical weathering these roadside exposures commonly last only for a few years. During the survey several areas that were found to be particularly wellexposed were subject to more detailed structural examination. Maps illustrating the distribution of all the Pre-Tertiary units in Sumatra are given in Chapter 4 of this volume, together with their definition, sedimentary features, palaeontology, stratigraphy and the palaeogeographic interpretation, while the overall tectonic evolution of Sumatra is discussed in Chapter 14. This chapter will concentrate on the structure and tectonic relationships of these units. Crustal blocks in Sundaland. The Pre-Tertiary units of Sumatra form part of Sundaland, the southeastern extension of the Eurasian tectonic plate. Sundaland is considered to have been formed by crustal blocks which were rifted from the northern margin of the Gondwana continent, were separated during the Mid-Late Palaeozoic, and amalgamated to form Sundaland in the Late Palaeozoic and Early Mesozoic. The definition of the structural blocks is based on the work of Pulunggono & Cameron (1984), Hutchison (1994), Metcalfe (1996, 2000) and Barber & Crow (2003). The pattern of crustal blocks which make up Sumatra and adjacent parts of Sundaland is illustrated in Figure 13.8 and the stratigraphic units included within blocks forming Sumatra are shown in Figure 13.9. The Indochina Block, forms the core of Sundaland and with its southern extension into East Malaya is

characterised by a Cathaysian Flora (Hutchison 1994). In the Devonian Indochina separated from Gondwana (northern Australia) with the development of Palaeotethys, and amalgamated with South China in the Late Carboniferous (Metcalfe 1996). In the Early Permian Sibumasu (SIkkim, BUrma, MAlaya, S..._U_Umatra),distinguished by glacial sediments, separated from Gondwana, and in the Late Permian or Early Triassic joined Indochina and East Malaya along the Bentong-Raub Suture and its northern extension into Thailand and China (Metcalfe 2000). Also probably in the Early Triassic the West Sumatra Block joined the previously amalgamated blocks along the Medial Sumatra Tectonic Zone, by strike slip faulting (Barber & Crow 2003). The final component of the Pre-Tertiary basement of Sumatra is the Woyla Nappe, which originated in Tethys as an oceanic island arc, and together with an accretionary complex composed of imbricated oceanic crust, was thrust over the western margin of Sundaland in the mid-Cretaceous (Barber 2O00). The Bentong-Raub Suture and the Bentong-Billiton Accretionary Complex (Figs 13.8 and 13.9). Metcalfe (2000) has given a full account and discussed the significance of the Bentong-Raub Suture Zone in Peninsular Malaysia. The zone bisects the Malay Peninsula from north to south, for a distance of over 400 kin, where its trace is marked by outcrops of serpentinite, ribbonchert, schist and melange. The suture zone is considered to mark the site of the destruction of Palaeotethys, due to the collision between the Indochina and Sibumasu blocks. There has been a long-standing controversy concerning the southward extension of the suture into Sumatra. Metcalfe (1996, Figs 1 & 10) illustrates four distinct paths which have been suggested in the literature and proposes another of his own. Hutchison (1994) recognized that the suture was the eastern margin of a much broader zone of deformation which he termed the Palaeotethys Suture Zone. His interpretation was confirmed by Metcalfe (2000) who found that radiolarian cherts of the Semanggol Formation 120 km to the west of the Bentong-Raub Suture consisted of two components, a steeply dipping sequence, Lower to Upper Permian in age, repeated either by isoclinal folds or by thrusts, and a unit of cherts, rhythmites and conglomerates of Middle to Upper Triassic age which is only gently folded. The time of collision between Sibumasu and Indochina is marked by the unconformity between the Permian and Triassic within the Semanggol Formation. Metcalfe (2000) suggested that the Permian, Devonian and Carboniferous cherts, identified in the western part of the Malay Peninsula were deposited on the floor of Palaeotethys and were subsequently incorporated into an accretionary complex. The zone of collision between Indochina and Sibumasu in Malaya is marked by a broad accretionary complex, rather than by a narrow suture. This re-interpretation means that it is no longer necessary to search in southern Sumatra for a discrete suture marking the collision between East Malaya and Sibumasu. In this account it is proposed that the accretionary complex recognised in the Malay Peninsula extends southeastwards into the islands of Bangka and Billiton. It is therefore termed the Bentong-Billiton Accretionary Complex (Fig. 13.8). An account of the lithology, stratigraphy and structure of the Carboniferous, Permian and Triassic rocks on Bangka to the SE of the mainland of Sumatra is given by Ko (1986). Although fossils are scarce, the oldest unit, the Pemali Group is considered to be of Carboniferous and Permian age, and indeed Permian fossils have been found (De Roever 1951). The bulk of the island is made up of slates and schists showing isoclinal folding and a steeply dipping N W - S E foliation, imbricated with basalts, andesites, bedded cherts, distal turbiditic sandstones, sometimes graded, mudstones, black pyritic shales and limestones. Barber & Crow (Chapter 4) have interpreted the Pemali Group as oceanic material formed on the floor of Palaeotethys and

STRUCTURE AND STRUCTURAL HISTORY

189

!....

SIBUMASU ~:: B LO C K

A(

NDOCHINA

EAST MALAYA: :~:~ B L O C K ) ~

MEDA N "%%~io 2~

Situtup Klippen

-% o, ,% ~

\ , %~ "% ~..oo

~, %o~,. ~

~.. ~176176176

BENGKULU

;T SUM, BLOCK

% % o

LAMPUNG ~

100

200

300

400

500km %

%

on

%

%

,oo

%

%

"%

" " :iii:!i:-iiiiiiiI L / 96 ~

I

98 ~

I

100 ~

1020

1040

i

I

I

%

JAVa 106 ~

I

Fig. 13.8. Crustal blocks that comprise the pre-Tertiary basement of Sumatra, based on Hutchison (1994), Metcalfe (2000), Barber & Crow (2003). Reverse arrows indicate dextral transcurrent movement on the Sumatran Fault System.

incorporated into an accretionary complex related to the Late Permian-Early Triassic collision between Sibumasu and East Malaya. Permian fossils found on Bangka and Billiton include fusulinids and a poorly preserved flora of Cathaysian affinity, indicating that the islands are related to the East Malaya Block (Van Overeem 1960). In eastern Sumatra and the offshore islands between the Malay Peninsula and Bangka the Bentong-Billiton Accretionary Complex is largely covered by Tertiary and Quaternary deposits, although rock types which may belong to the complex have

been encountered in oil company boreholes (De Coster 1974; Eubank & Makki 1981). However, at Toboali on the southern tip of Bangka, Ko (1986) describes 'pebbly mudstones', similar to those described from other areas of Sibumasu Block to the NW (Cameron et al. 1980; Stauffer & Lee 1987; Mitchell et al. 1970). The Sibumasu Block is therefore considered to extend southwards into southern Bangka. On Bangka the Pemali Group is locally intruded and hornfelsed by Late Permian-Triassic granites (see Chapter 5), constraining the age of formation of the accretionary complex and the age of

190

CHAPTER

WEST SUMATRA BLOCK (Schiefer Barisan, MSTZ Vorbarisan, Kluet and Kuantan Units)

WOYLA GROUP

13

EAST SUMATRA (SIBUMASU) BLOCK

BENTONG-BILLITON ACCRETIONARY COMPLEX

Granitic Intrusions Bintan F o r m a t i o n

Woyla Group (oceanic and volcanic arc assemblages)

Granitic Intrusions

Granitic Intrusions

(3_

o

Situtup & T u h u r Formations

Limestone blocks in melange

~ ~F

~~

~ ~~:~~~"~~"~'"~ ~ ~ ~~

-~ = E tII

Kaloi, Batumilmil and Kualu Formations

,~

-

Stutup S ungkang Palepat and Menqkaranr E'E" Format ons(tropical Jambi flora) -~ o _~-c ~~

<~ :3

Tempilang Formation

-

Kaloi and Batumilmil Formations

Pangururan Bryozoan Bec B o h o r o k (tilloids) a n d Alas C9 Formations -5 (temperate fauna z~ in l i m e s t o n e s ) O

,~

o ~ c ~ ~

~Z

N

:4 :g,:N::~ N #

~) rr O

~ Kluet and Kuantan Formations (tropical f a u n a in limestones)

,, :~o = ~ "6 ~-o ~= ~= o~

s

Pemali Group F i g . 13.9. T h e s t r a t i g r a p h i c s e q u e n c e s a n d p h a s e s o f g r a n i t i c i n t r u s i o n that c h a r a c t e r i z e the crustal b l o c k s w h i c h m a k e up the we-Tertiary basement of Sumatra. MSTZ, Medial Sumatra Tectonic Zone.

the collision between East Malaya and Sibumasu to Late to endPermian. Outcrops of the Pemali Group form east-west bands across the island and alternate with outcrops of undeformed sandstones and mudstones of the Triassic Tempilang Formation, which is folded into broad open folds. Because of their difference in degree of deformation the Tempilang Formation is considered to have been deposited unconformably on the Pemali Group, but in places later deformation has thrust rocks of the Pemali Group over the Tempilang Formation (Ko 1986).

East S u m a t r a ( S i b u m a s u ) B l o c k (Figs 13.8 a n d 13.9) Tapanuli Group. During the D M R / B G S Northern Sumatra mapping project Pre-Tertiary rocks in northern Sumatra were assigned to the Tapanuli (Carboniferous-Permian), Peusangan (Permo-Triassic) and Woyla (Jurassic-Cretaceous) groups (Cameron et al. 1980). The Tapanuli Group was further divided into three units, the Bohorok, Alas, and Kluet formations, outcropping from NE to SW, in that order (Fig. 13.10). Of these units only the Alas can be confidently ascribed palaeontologically to the Carboniferous, but the Bohorok and Kluet formations were also considered to be of Carboniferous or Early Permian age because they are associated with the Alas Formation in the field, and a proposed stratigraphic correlation with similar rocks in western Malaya and southern Thailand (Cameron et al. 1980). The Bohorok Formation, with a type locality in the Bohorok River 60 km to the west of Medan, is characterized by the occurrence of 'pebbly mudstones', interpreted as glacigenic deposits, together with massive sandstones, sometimes conglomeratic, and intervening shales interpreted as turbidites. Similar lithologies in northern Sumatra, but without the pebbly mudstones, were mapped as the Kluet Formation. It is possible that outcrops of Kluet Formation which are shown on the quadrangle sheets lying to the NE of the Medial Sumatra Tectonic Zone should more properly be assigned to the Bohorok Formation. The pebbly mudstones of the Bohorok Formation have been correlated with the Lower Permian pebbly mudstones of Phuket in southern Thailand (Cameron et al. 1980; Mitchell et al. 1970). Similar

lithologies occur in the Mentulu Formation in the Tigapuluh Hills in central Sumatra, and as has already been mentioned, pebbly mudstones also crop out at Toboali at the southern tip of the island of Bangka. This lithological association, with the presence of pebbly mudstones, is regarded as characteristic of the Sibumasu Block which therefore occupies the whole of the eastern part of Sumatra (Figs 13.8 & 13.9). Although there is no direct evidence for the age of the Bohorok Formation, support for the correlation with the Lower Permian of southern Thailand is given by an outcrop of decalcified limestone, the Pangururan Bryozoan Bed, on the western shore of Lake Toba (Aldiss et al. 1983) (Figs 13.10 & 13.11). This limestone is associated with slates and sandstones which were attributed to the Kluet Formation, presumably because it contains no pebbly mudstones, but its location NW of the Medial Sumatra Tectonic Zone of this account, suggests that it should be more correctly attributed to the Bohorok Formation. This limestone contains fenestellid bryozoans deformed in the slaty cleavage and forming ideal strain markers (Ramsay 1967). The fenestellids and the other fossils indicate a Late Carboniferous or Early Permian age and the bed has been correlated with the Lower Permian Bryozoan Bed of Peninsular Thailand (Mitchell et al. 1970; Cameron et al. 1980). This outcrop is critical to determining the age of the Tapanuli Group and also the age of its deformation and metamorphism. A brief account of the structure of the Bohorok Formation is given in the explanatory notes which accompany each of the GRDC 1:250 000 geological map sheets (Bennett et al. 1981c; Cameron et al. 1982a; Clarke et al. 1982a, b). Unfortunately, very few structural observations are recorded on the map sheets, but it is reported that general strike of bedding throughout the outcrop of the Bohorok and Mentulu formations is N W - S E , parallel to the trend of the Barisan Mountains and of Sumatra as a whole (Sumatran trend), and that the rocks are folded with steep and often vertical dips. Massive sandstones show little evidence of penetrative deformation, with only irregular jointing and quartz veining, although fracture cleavage is sometimes developed, but the intervening shales are generally tightly to isoclinally folded and converted to slates with an axial plane slaty cleavage. Crenulation cleavages, kink bands and shears are

STRUCTURE AND STRUCTURAL HISTORY

I

191

SIBUMASU (EAST SUMATRA) BLOCK Peusangan Group (Permo-Triassic)

97~

.?

LHOKSEUMAWE "~ ~Bo~J

Tapanuli Group-Bohorok Formation

,:{~,1 (Carboniferous-Early Permian) MEDIAL SUMATRA TECTONIC ZONE (MSTZ) Alas Formation (limestones etc.)

L i m e s t o e'-.. -,

~ . ~

"'" "'"

GON~

%

~

BO

~"~

AmphiboliteFacies Metamorphic Rocks >10ppm tin in stream sediment samples

;erbajadi Granite

~ Bohorok KUTACANE

C/

\

Toba Tufts Kualu (K) ,ilmil

~R

TAPAKTUAN

X

Antiform

X

Synform

" ~ M E DAN

\

LAUBALENG :~

Toba Tuffs

Graniticintrusions SIDIKALAN(

WOYLA NAPPE

Pangururan Bryozoan B

Woyla Group

P-~ (Jurassic-Cretaceous) KLUET (WEST SUMATRA) BLOCK Kluet Formation Amphibolite Facies Metamorphic Rocks 0

50

\,Granite// 264+6Ma' \ /~ SIBOLGAq

100km

I

I

97~

98~

i

I

Q ~

/

Fig. 13.10. Outcrops of pre-Tertiary units in northern Sumatra showing the distribution of formations in the Carboniferous to lower Permian Tapanuli Group and the Permo-Triassic Peusangan Group (after Stephenson & Aspden 1982, with modifications from the present study). Near Kutacane the Medial Sumatra Tectonic Zone is coincident with the outcrop of the Alas Formation and is distinguished by the juxtaposition of unmetamorphosed sediments and high-grade metamorphic rocks, syntectonic granitoid intrusions and a tin anomaly. Further north the MSTZ is traced through Takengon following outcrops of phyllite, schist and gneiss, recognised in the primary mapping, but not incorporated in the compilations. Turbiditic sediments, without pebbly mudstones to the NE of the MSTZ which were originally mapped as Kluet Formation lie on the Sibumasu Block, and are here assigned to the Bohorok Formation. Pre-Tertiary rocks are covered by Tertiary and Quaternary sediments and volcanics in areas left blank.

192

CHAPTER 13

reported indicating that the rocks have been subjected to multiple deformation (Bennett et al. 1981c; Cameron et al. 1982a; Clarke et al. 1982a, b). In a more detailed structural study of the Bohorok Formation on the Pematangsiantar Sheet, in the area to the south and SE of the Hatapang Granite, Clarke et al. (1982a) (Fig. 13.11) recognized two stages of folding, the earlier on N W - S E axes with SW-dipping axial planes, and the later with axial planes inclined at a shallow angle to the west. The intersection of the two axial surfaces defines a lineation plunging at a shallow angle (c. 15 <') to the NW. In the Pakanbaru Quadrangle to the south, dip measurements show a wide range of orientations, but Clarke et al. (1982b) report that slate units show widespread tight to isoclinal folding on axes which vary from e a s t - w e s t to N W - S E , with axial planes which are vertical or dip steeply to the SW. SAR imagery of the same area shows bedding plane traces with complex folding and fold axial plane traces trending N E - S W and N W - S E , the latter direction becomes dominant towards the SW where the Bohorok Formation is in contact with the Tanjungpuah Member of the Kuantan Formation (Fig. 13.12). The Bohorok Formation is intruded by large and small igneous bodies with the development of hornfelses, schists and gneisses in the adjacent country rocks. One of the largest is the Serbajadi Batholith to the north of Medan on the Takengon and Langsa sheets. The intrusion is separated from the surrounding slate

,,v-

~. To

ill

~

1

Prapat

"

grade rocks (shown as Kluet Formation) by a marginal zone of gneisses and schists. These metamorphic rocks were interpreted as an aureole forming a carapace brought up t?om depth together with the batholith (Bennett et al. 1981c; Cameron et al. 1983), but is here suggested to form part of the Medial Sumatra Tectonic Zone (Fig. 13.10). Cameron et al. (1982a) describe the sequence of rocks seen in metamorphic aureoles around granitoid intrusions in the Medan area as: fine-grained hornfels ~ coarse m u s c o v i t e biotite hornfels with segregations of epidote, chlorite, hornblende and tourmaline with quartzofeldspathic rims -+ schistose hornfels with flattened segregations, andalusite and cordierite ---> biotitemuscovite schists, sometimes garnetiferous--+ banded silliman i t e - b i o t i t e - m u s c o v i t e gneiss with feldspar porphyroblasts--+ coarse migmatitic gneiss with quartz-feldspar lit-par-lit layers, pods and ptygmatic veins. Flattened clasts in the hornfelsed sandstones show that the rocks had been deformed and converted to slates before they were thermally metamorphosed and before the emplacement of the igneous intrusions. To the SE in the Tigapuluh Hills in central Sumatra Pre-Tertiary rocks of the Tigapuluh Group outcrop as an inlier among Tertiary sediments (Fig. 13.1) 9 The group is composed of the Mentulu, Pangabuhan and Gangsal formations (Simandjuntak et al. 1991; Suwarna et al. 1991). The Mentulu Formation contains pebbly mudstones, similar to those of the Bohorok Formation; the other formations are turbiditic sandstones and shales, with the Gangsal

~

T~Tebingtinggi"~99~

I

i

Pleistocene Toba Tufts Tertiary sedimentsand volcanics

2~

N

Tt

Middle-LateTriassic Kualu Formation Sibaganding and Pangunjungan Limestone Members

Te

\

Oo

.....,

I-o Tebingtinggi

_4q1 Tt i Te

LAKE TOBA Tt

.....

Tt \ Tt

~'~~O451E

2~

LAKE TOBA SAMOSIR

~

ha (Tuffaceous

k

Invertedbeds CI.... g......... ddip

Zt

~d

v..... ,e,.... ,e

sed'men's

an

Te

,ntrusions, Carboniferous-Permian

,ozoan Bed

~

~

T(~*'~

Tapanuli Group

~ 4e

Tt

Ve~oalbeds

~1~nal

Haria TtPintu

'

__ 0~ Li.... iorlplLInge Beddingstrikeanddip

Bohorok Formation (pebbly mudstones) Undifferentiated ~

~

Halqbia

, Rant

~,~~ 0

5

10

15

20km

99015' Fig. 13.11. The geology of the area between Lake Toba and Rantauprapat showing the relationship between the Carboniferous-Permian Tapanuli Gl"oupand the Triassic Kualu Formation based on the GRDC Pematansiantar (Clarke et al. 1982a) and Sidikalang (Aldiss et al. ]983) Quadrangle sheets. While the Tapanuli Group is isoclinally folded with slaty cleavage and shows the effects of multiple deformation the Kualu Formation shows one set of upright fold and argillaceous units are not cleaved. Although all the contacts are faulted the Kualu Formation must have an unconformable relationship to the Tapanuli Group. The inset map shows the location of the Pangururan Bryozoan Bed (PBB) on the western shore of Lake Toba.

STRUCTURE AND STRUCTURAL HISTORY

I

lOkm I

193

Pasirpangarayan. 90km

Igneous intrusions Kuantan Formation (West Sumatra Block)

Pakanbaru 70kin .

disilan

Tanjungpuah Member (Medial Sumatra Tectonic Zone) Bohorok Formation (Sibumasu Block) 70

Strike and dip of bedding

75 ~"~20 - 0~

2,,

_

Strike and dip of cleavage Plunge of lineation

..~

Photodip

~I

Hot spring

0~

.._TJ Ban

F~angkalan-kota-baru 9 Siasam

Bukit Tinggi 50km, 0o00'

100~

.

.

.

.

101~

9 Muaraketua

Fig. 13.12. Structure across the Medial Sumatra Tectonic Zone (MSTZ) from GRDC Pakanbaru Quadrangle Sheet (Clarke et al. 1982b), central Sumatra, with the addition of bedding traces from SAR imagery. Irregular refolded folds in the Bohorok and Kluet Formations trending approximately east-west, contrast with isoclinal folds trending NW-SE within the MSTZ which incorporates the Tanjunpuah Member of the Kuantan Formation. N.B. Granitic rocks within the MSTZ show a gneissose foliation parallel to the trend of the zone. The identification of units on the map has been modified in the light of the interpretation SAR imagery. Pre-Tertiary basement rocks are overlain by Tertiary sediments in the areas left blank.

Formation being finer-grained. SAR imagery indicates that the Gangsal Formation is also more deformed than the other units, with a strong N W - S E trend. Structures in the Tigapuluh Group are similar to those reported from the Bohorok Formation, with folded bedding and steeply dipping cleavage in pelitic units, indicating tight to isoclinal upright folds (Simandjuntak et al. 1991; Suwarna et al. 1991). Although the contact between stratigraphic units and measurements of the orientation of the bedding within the group have the N W - S E Sumatran trend, cleavage measurements made in the field and shown on the Rengat and Muarabungo map sheets show a wide scatter, but trend predominantly e a s t - w e s t and dip either to north or south. The reasons for the discrepancy between the orientation of the cleavage and the bedding is not clear, and requires further study. Two phases of folding are reported, the first e a s t - w e s t and the second N W - S E , but no examples of refolded folds were identified in the field, although crenulation cleavage, indicating that the rocks were affected by a second phase of deformation, was recorded (Simandjuntak

et al. 1991; Suwarna et al. 1991). The possibility of the influence of strong Tertiary deformation in this area on the orientation of the cleavage in the basement rocks has not been clarified. Pelitic rocks of the Tigapuluh Group are altered to cordierite hornfels, biotite schist and gneiss in metamorphic aureoles around granitoids of Jurassic age (Schwartz et al. 1987). Deformation, with folding and the development of cleavage preceded the intrusion of the granitoids, as the thermal metamorphism affects rocks which were already cleaved. The Permo-Triassic Peusangan Group in the Sibumasu Block. In northern Sumatra the Permo-Triassic Peusangan Group is represented mainly by isolated limestone outcrops in the northern part of the Sibumasu Block (Fig. 13.10). Each of the isolated limestone outcrops has been given a separate formation name (for a detailed account of these formations see Chapter 4). The limestones are generally massive and recrystallized, and unlike the slates and sandstones of the Bohorok Formation, do not generally show folding or penetrative deformation. For this reason the

194

CHAPTER 13

Fig. 13.13. Map of the outcrops of the Bohorok,Alas and Kluet tbrmationsbetween Kutacaneand Laubaleng,based on the GRDC Medan Quadrangle Sheet (Cameron et al. 1982a), with the additionof beddingtraces (dashed lines) from SAR imagery;solid lines are faults. Open foldingin the Bohorokand Kluet lormationscontrasts with tighter foldingin the Alas Formation,which lies within the Medial Sumatran Tectonic Zone. In areas left blank the the we-Tertiary basement is covered by Tertiary and Quaternary sediments and volcanics,includingthe alluviumin the Kutacane Graben.

surveyors considered that the undeformed Permo-Triassic rocks rest unconformably on the deformed Bohorok Formation, although no stratigraphic contacts have been described (Cameron et al. 1980). On this basis it was suggested that the major phase of deformation seen in the Tapanuli Group occurred in the Early to Mid-Permian, before the deposition of the Peusangan Group (Cameron et al. 1980). Many of the limestone outcrops are recrystallized and apparently unlbssiliferous, but a few have yielded Permian and Triassic fossils. M i d - L a t e Permian fossils have been obtained from the Situtup Formation to the NW of Takengon, and the Kaloi and Batumilmil formations to the NW and west of Medan (Fig. 13.10). The Situtup Formation has yielded Mid-Permian fusulinids with a Cathaysian affinity, indicating that this area forms part of the West Sumatra Block. M i d - L a t e Triassic fossils have been obtained from the Situtup, Kaloi and Batumilmil formations (Fontaine & Gafoer 1989) but the relationships between the Permian and Triassic components of these outcrops, whether conformable or unconformable, have not been established. Other outcrops of Triassic rocks belonging to the Kualu Formation occur on the eastern and western shores of Lake Toba and 35 km south of Medan (Fig. 13.11). In the outcrop of the Kaloi Formation massive undeformed limestones are associated with Triassic limestones and shales

which show open folds (Bennett et al. 1981c). To the south of Medan at Parapat on Lake Toba bedded limestones and shales of the Kualu Formation are moderately to tightly folded about sub-horizontal, N W - S E axes with steep NE-dipping axial planes. The intensity of the tblding increases towards the west and the steep western limbs of the folds may be overturned, giving a westerly vergence. However, cleavage is not developed in the argillaceous interbeds in these outcrops, although highly deformed slates of the Bohorok Formation occur only a short distance away across a fault contact (Clarke et al. 1982a; Aldiss et al. 1983) suggesting that the relationships between the Kualu and the Bohorok formations are unconformable (Fig. 13.11). In the Pematangsiantar Quadrangle to the east, thin-bedded limestones and cherts of the Pangunjungan Member of the Kualu Formation show tight disharmonic folds which have been attributed to slumping (Clarke et al. 1982a). It has been argued by Barber & Crow (Chapter 4) that the observations concerning the structural and stratigraphic relationships of the Tapanuli and Peusangan groups in northern Sumatra have been misinterpreted. It is commonly observed in slate-grade metamorphic terranes that massive limestones behave as competent materials, while incompetent argillaceous materials are deformed around them. This may well be the case in northern Sumatra, where the argillaceous sediments of the Tapanuli

STRUCTURE AND STRUCTURALHISTORY Group are highly deformed, while the massive limestones of the Peusangan Group are unaffected. In this respect it is notable that massive limestones of the Alas Formation, considered to form part of the Tapanuli Group are also undeformed, with the preservation of fossils and delicate sedimentary structures, while the argillaceous rocks around them have been altered to slates and schists. Evidence for the age of deformation in the Sibumasu Block of northern Sumatra is found around Lake Toba, where on the western shore of the lake, the Pangururan Bryozoan Bed (Figs 13.8 & 13.9), a decalcified argillaceous limestone of Late Carboniferous to Early Permian age, is interbedded with slates and sandstones of the Kluet Formation (?Bohorok Formation in this account) and has been deformed to the same extent. On the other hand slaty cleavage is not developed in the folded argillaceous beds of the Middle to Upper Triassic Kualu Formation on the eastern shore of the lake. Determining the age of the Pangururan Bryozoan Bed is critical for defining the age of deformation more precisely. However, taking the evidence available, it is considered that the major deformation in northern Sumatra occurred within units classified in the Peusangan Group between the Permian and Triassic. Since no fossils representing the latest Permian or earliest Triassic have been found anywhere in northern Sumatra, it is therefore most probable that the major phase of deformation occurred during the Late Permian and Early Triassic. This is the age of the deformation seen in Peninsular Malaya, where it is regarded as marking the collision of the Sibumasu and East Malaya terranes (Metcalfe 2000).

The M e d i a l S u m a t r a T e c t o n i c Z o n e (Fig. 13.8)

The Medial Sumatra Tectonic Zone (MSTZ) forms a zone of highly deformed rocks, extending for the whole length of Sumatra, separating the Sibumasu Block from the West Sumatra Block. The MSTZ is separated into three segments by major faults. The northern segment of the zone abuts against the Samalanga Fault and must pass beneath Tertiary sediments into the Andaman Sea to the west of the fault. The central segment has been displaced southwards by c. 50 km along the LokopKutacane Fault and near Sibolga the southern segment has been displaced southeastwards for a distance of 150 km along the Sumatran Fault Zone (Fig. 13.8). The northern segment of the MSTZ is marked by a zone of phyllitic, schistose and gneissose rocks which were identified on the 1:100000 field maps and described in the initial reports prepared during the DMR/BGS Northern Sumatra Survey, but are not represented separately on the 1:250 000 Quadrangle sheets. These include the Uneuen Unit on the Lhokseumawe and Takengon Quadrangle Sheets, the Toweren Member of the Peusangan Group along Lake Tawar, and amphibolite facies schists, gneisses and marbles which were interpreted as forming the aureole of the Serbajadi Granite (Keats et al. 1981; Cameron et al. 1983) (Fig. 13.10). A calcareous bed in the Uneuen Unit yielded Triassic fossils (Cameron et al. 1978, appendix), a feature seen in the MSTZ further south. The central segment of the MSTZ corresponds with the outcrop of the Alas Formation (Figs 13.8 & 13.13) characterized by massive limestones, which locally contain a Early Carboniferous (Vis6an) fauna (Metcalfe 1983; Fontaine & Gafoer 1989), but also includes sandstones and shales, identified as turbidites, similar to those of the Bohorok Formation. The temperate Vis~an fauna identifies the Alas Formation as part of the Sibumasu Block which has been disrupted and incorporated into the MSTZ. Apart from these limestones, the Alas Formation is schistose and metamorphosed in the greenschist to amphibolite facies. Locally the limestones have been altered to coarse graphite-phlogopite marbles. In the Rikit Gaib (Fig. 13.10) area in the Takengon

195

Quadrangle near Kutapanjang, the rocks are schists and gneisses that enclose garnetiferous granites with gneissose or foliated margins (Cameron et al. 1983). The granitoids contain narrow zones of mylonite and cataclasite showing horizontal slickensides and are separated from slate-grade country rocks by a metamorphic envelope. The syntectonic granitoids are considered to have been intruded into active sub-vertical shear zones and to be responsible for the amphibolite-facies metamorphism of the adjacent rocks (Cameron et al. 1983). The MSTZ is also associated with elevated values of tin in stream-sediment samples > 10 ppm (Stephenson et al. 1982) (Fig. 13.10). The tin was derived either directly from the Pre-Tertiary basement and the associated granites (e.g. the Kais Complex, Fig. 13.10) or indirectly from Tertiary sediments. The southern segment of the Medial Sumatra Tectonic Zone is generally very poorly exposed, but crops out in a N W - S E trending belt as the Pawan and Tanjungpuah members attributed to the Kuantan Formation, between Pakanbaru and Lubuksikaping (Clarke et al. 1982b; Rock et al. 1983) (Figs 13.12 & 13.14). These rock units are composed of intensely folded muscovite, tremolite, chlorite, carbonate and quartz schists. Locally coarseto fine-grained banded marbles are interbedded with fine-grained chlorite schists derived from basic volcanics or tufts (Rock et al. 1983). Large scale folds on N W - S E axial traces have been mapped in the Tanjunpuah Member in the Pakanbaru Quadrangle using aerial photographs (Clarke et al. 1982b). On SAR imagery, irregularly oriented folds in the Bohorok Formation to the NE, and the Kuantan Formation to the SW, pass into tight to isoclinal folds with N W - S E axial traces in the Pawan and Tanjungpuah outcrops. The Pawan and Tanjungpuah units lie within a zone of intense deformation identified here as the MSTZ (Fig. 13.12). In the field Clarke et al. (1982b) recognized three phases of folding in the Pawan Member: the earliest on moderate to steeply plunging axes, the second forming tight to isoclinal folds on sub-horizontal N W - S E axes and the third as narrow zones of brittle-style refolding and kinking on NW-dipping axial planes, with the development of quartz tension gashes. The metamorphic rocks are intruded by granites with a lensoid shape, elongated parallel to the schistosity in the adjacent slates. The granites also contain a steeply dipping N W - S E internal foliation (e.g. the Pulaugadang Granite in Figs 13.12 & 13.14) that is defined by oriented mica flakes, encloses aligned felspar megacrysts, and also affects cross-cutting microgranitic veins. Adjacent to the granitic intrusions the metasediments are converted to schists with a steep schistosity. The schistosity related to the second phase of folding has been dated by the K - A t method as Early Jurassic (Clarke et al. 1982b). Southeastwards, the MSTZ may be represented by the intensely deformed Gangsal Formation on the southwestern side of the Tigapuluh Hills, but it is not exposed further to the SE, and its trace can only be inferred from schistose and basic lithologies encountered in oil company boreholes put down through Tertiary sediments (De Coster 1974; Eubank & Makki 1981) (Fig. 13.14). The zone identified here as the MSTZ has long been recognized as an important tectonic boundary in Sumatra. Van Bemmelen (1949) following the survey by Von Steiger (1922) drew the boundary between his Tectonic Zones I and III along this line, and his lensoid Zone II incorporates the Pawan and Tanjungpuah Members. Lithologies described within Zone II include quartzites, phyllites, shales, diabase-schists, limestone, radiolarian chert, conglomerates with granitic boulders and mylonitized breccia, and the zone is characterized by tin mineralization (see Figs 13.8 & 13.12). In their tectonic synthesis of Sumatra Pulunggono & Cameron (1984, Fig. 1) draw a line separating the Bohorok and Kuantan formations through central Sumatra, from the outcrop of the Alas Formation in the north to Palembang in the south. They identified a lens of material along this line, including the Pawan and Tanjungpuah members, Triassic rocks of the Kualu

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Fig. 13.14. The Medial Sumatra Tectonic Zone in central Sumatra distinguished by highly deformed metamorphic rocks, syntectonic granitic intrusions and a tin anomaly, separating the East and West Sumatra blocks.

Formation, and the Mutus Assemblage, previously described from southern Sumatra by Eubank & Makki (1981). Eubank & Makki (1981) defined the Mutus Assemblage from oil company boreholes which had penetrated the Pre-Tertiary basement. The material included 'radiolarian chert, meta-argillite, red-mauve shale, thinly-bedded limestone and deep water rhythmite sequences'. Basalt from one borehole, and the association with deep-water sediments, led them to speculate that the assemblage might include ophiolitic material; the chert and rhythmites were con-elated with the Triassic Kualu Formation of northern Sumatra. The absence of records of large bodies of ophiolite along the MSTZ suggest that the zone does not mark a major suture zone representing the collision of continents and the subduction of oceans, although tremolite schist in the Pawan and Tanjungpuah may represent slivers of deformed and metamorphosed serpentinite, and chlorite schists may represent basic igneous rocks. We interpret the MSTZ as having been initiated as a transcurrent fault along which the West Sumatra Block was emplaced against the Sibumasu Block. The subsequent uplift of high-grade metamorphic rocks, the emplacement of syntectonic granitic magmas and the flux of tin mineralizing fluids indicate that the MSTZ is a major lineament on a crustal scale. Some of the material incorporated in the zone are slivers of the adjacent Bohorok, Alas, Kluet and Kuantan formations, and their original structural patterns have been truncated and drawn out into conformity with the N W - S E trend of the shear zone (Figs 13.12 & 13.14). Local amphibolite-facies metamorphic rocks juxtaposed with unmetamorphosed sediments, including fossiliferous limestones, indicate that material has been uplifted from deeper in the basement and that material has subsided during movements

along the shear zone. Multiple phases of deformation recorded within the rocks of the shear zone indicate that movements occurred at different periods of time. The occurrence of relatively undeformed Triassic rocks of the Kualu and Tuhur formations adjacent to the shear zone suggest that initial movements occurred before the Mid-Triassic. Granites, foliated together with the associated metasediments, were emplaced syntectonically. From regional correlations these granites are considered to be of Late Triassic to Early Jurassic age. If the ages of the syntectonic granitic rocks are confirmed it suggests that movements along the MSTZ continued throughout these periods. Further age determinations on the granites would constrain the period of movement more precisely. Records of mylonitization, cataclasis, brecciation and surfaces with slickensides within the MSTZ (Cameron et al. 1982a) suggest that strike-slip movements have occurred continually within the zone. The latest movements along the MSTZ are probably associated with the movement of the Sumatran Fault Zone.

West Sumatra Block Kluet Unit (Fig. 13.8). The Kluet tectonic unit is coincident with the outcrop of the Kluet Formation, occupying the western part of the Barisan Mountains between Sibolga and Tapaktuan to the southwest of the Medial Sumatra Tectonic Zone, and is overlain to the NW and SE by the Woyla Nappe (Fig. 13.8). The Kluet Unit is considered to form the northern part of the West Sumatra Block which has become separated from the remainder of the

STRUCTURE AND STRUCTURAL H|STORY

block to the south by movements along the Sumatran Fault System. Lithologically the Kluet Formation is composed of alternating quartz-wackes, siltstones and shales, with some limestones. Over most of the outcrop the argillaceous rocks have been converted to slates, but amphibolite-grade metamorphic rocks occur on the western side of the outcrop to the east of Tapaktuan (Fig. 13.10). Cameron et al. (1982b) describe a southwestwards change from predominantly slates to pelitic schists and phyllites in the Simpali area and further southwest, in the type locality of the Kreung (River) Kluet to amphibolite facies pelitic schists, calc-schists and quartzo-feldspathic gneisses with small concordant gneissose granitoid bodies. Barber (2000) has suggested that these higher-grade metamorphic rocks mark the footprint of the overlying Woyla Nappe. These rocks have not yet been dated to test this hypothesis. Structurally the sandstones and slates of the Kluet Formation are folded on both the large and small scale. In the outcrop to the south of Sidikalang, large scale folds of the bedding, with synlbrmal and antiformal axial traces 20 km apart, have been traced on aerial photographs (Aldiss et al. 1983) (Fig. 13.15). The fold axial traces trend W N W - E S E and the folds plunge to the ESE. Variations in the general strike of bedding and cleavage from N W - S E to N E - S W recorded in some areas of the Kluet outcrop are presumed to be due to the effects of the later lbld phases (Aspden et al. 1982b; Aldiss et al. 1983). In the field pelitic rocks are often tightly to isoclinally folded, and an axial plane slaty cleavage is developed. Again the cleavage generally strikes N W - S E or W N W - E S E and dips vertically or steeply to the SW. In the same area a detailed structural study was made of the minor structures in the Kluet Formation exposed in a road cut 4 km to the south of Sidikalang (Clarke & Bagdja 1979; Aldiss et al. 1983) (Fig. 13.15). The rocks are thick, coarse, massive sandstones interbedded with finer-grained, laminated sandstones and siltstones. The fine-grained sandstones frequently show grading and occasionally small-scale current bedding. Three fold phases were recognized (Fl, F2 and F3). F1 folds are tight to isoclinal, the shape depending on lithology. The axial planes are upright to vertical and strike W N W - E S E , but may be horizontal locally where the rocks are refolded. Grading in sandstone beds in asymmetrical folds indicate that the limbs of folds are overturned towards the NE (Aspden et al. 1982b). Slaty axial plane cleavage (S0 is developed in pelitic bands and refracted through graded sandstone beds. Fracture cleavage is developed in semi-pelitic bands, and quartz tension gashes occur normal to bedding in thicker and more massive psammites. The fold axes, and a bedding-cleavage intersection lineation (L1), plunge to the SE. It is probable that this is the same phase of folding is seen on a large scale to the south on aerial photographs (Fig. 13.15). F2 folds are confined to narrow bands 100-200 m wide. These folds refold the bedding, the earlier Fl folds, which may become recumbent, and the slaty cleavage developed during F1, on near vertical axial planes. A crenulation cleavage ($2) is developed in the slates, forming a prominent sub-horizontal crenulation lineation (Lz) trending N W - S E . F3 folds are small-scale chevron folds, 1 cm to 10 m in amplitude with fold axes striking from west to SW and a lineation plunging NW at shallow to moderate angles. Directions of overturning and the predominant SW dip of the axial planes of both the first and second phase folds, show a predominant northeasterly vergence. A similar sequence of folds is seen in the road section between Pakkat and Barus some 25 km to the south of Sidikalang (Fig. 13.15). The Kluet Formation is intruded and extensively hornfelsed by the plutons of the Sibolga Granite Complex. A wide range of ages has been obtained from the main body and satellite intrusions by different dating methods. The oldest is a R b - S r whole-rock age of 264 • 6 Ma (Mid-Permian) from the main outcrop north of Sibolga, but other widely distributed granite phases in the

197

complex have given Late Triassic and Early Jurassic ages. Roof pendants of Kluet Formation in the granite, when they are not hornfelsed, show isoclinal folding and cleavage, with graded bedding indicating that some beds are inverted (Aspden et al. 1982b). It is therefore presumed that the bulk of the intrusion is post-tectonic. A detailed study of the relationships between the structures and dated intrusive phases of the Sibolga Complex may elucidate the history of deformation in the Kluet Formation.

(Fig. 13.8). The Kuantan Unit in central Sumatra is coincident with the outcrop of the Kuantan Formation. It is limited to the NE by the Medial Sumatra Tectonic Zone and to the SW across the Takung Fault by the Vorbarisan Unit (Tobler 1910, 1917). The Kuantan Formation is composed of turbiditic sandstones and shales with some limestones and resembles lithologically the Kluet Formation of northern Sumatra, and indeed the boundary between the two formations was defined arbitrarily at a break in outcrop along 99~ longitude. Massive limestones within the Kuantan Formation with a Carboniferous (Vis6an) fauna suggest a correlation with the Alas Formation of northern Sumatra. However the Kuantan fauna is of tropical type, while the Alas fauna is of temperate type, indicating that the two units cannot be correlated directly (Fontaine & Gafoer 1989). The Kuantan Block is considered to be related to the Indochina or East Malaya Block, and was emplaced in its present position by transcurrent movements along the Medial Sumatra Tectonic Zone (Hutchison 1994; Barber & Crow 2003). Structures in the turbidites of the Kuantan Formation are the same as those reported from the Kluet Formation, with steep dips of both bedding and cleavage, indicating that the rocks are affected by tight or isoclinal folding. Local variations in strike, from dominantly N W - S E to east-west, suggest that the rocks were affected by more than one phase of folding (Aspden et al. 1982b; Rock et al. 1983). In the Pakanbaru Quadrangle Clarke et al. (1982b) report refolding of the cleavage and bedding on sub-horizontal N W - S E fold axes on near vertical axial planes. Aspden et al. (1982b) suggested that in the Padangsidempuan and Sibolga Quadrangle the east-west phase of folding was the earlier. This interpretation is confirmed by the SAR imagery which shows tight folds with east-west axial traces refolded by folds with N W - S E axial plane traces (Figs 13.12 & 13.16). Around igneous intrusions the slates are converted to hornfels, schist and gneiss. Further to the south the Terantam Formation in the Duabelas Mountains, 120 km along strike to the SE, the Tarap Formation of the Garba Mountains in South Sumatra, and the Gunungkasih Complex of Lampung have all been correlated with the Kuantan Formation, and although they have not been studied in detail, from the descriptions these occurrences are very similar in lithology, structure and metamorphism (Simandjuntak et al. 1991; Gafoer et al. 1994; Amin et al. 1994b; Andi Mangga et al. 1994a). In the Gunungkasih Complex the schistosity strikes N W - S E , but is folded about east-west axes and refolded by NW-SE-trending upright folds and then by variably oriented kink-band folds (Barber 2000). The Gunungkasih Complex is intruded by gabbros and granites of the Sulan Pluton that have given K - A r ages of 151 • 4 Ma and 113 ___ 3 Ma respectively (McCourt et al. 1996). In the same area granites and basaltic dykes have been deformed by strike-slip movements to form banded gneiss. Diorites from this gneiss complex gave a K - A r age of 89 +_ 3 Ma (McCourt et al. 1996). In southern Sumatra deformation had occurred by the Late Jurassic, but continued along shear zones into the mid-Cretaceous (Barber 2000). Kuantan Unit

Vorbarisan Unit (Fig. 13.8). The 'Vorbarisan' tectonic unit was proposed by Tobler (1910, 1917, 1919) for the area occupied by Permo-Triassic rocks between the Takung and Musi faults. To the NW the Vorbarisan Unit is transected by the Sumatran

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Fig. 13.15. Detail of the GRDC Sidikalang Quadrangle sheet showing the outcrop of the Kluet Formation to the SW of Lake Toba (Aldiss et al. 1983). The highly irregular outcrop pattern is due to the infilling of valleys in a mountainous terrain by tufts from the 70 000 years Bp eruption of the Toba volcano. Bedding strikes and dips were collected in the field. Traces of bedding are from airphoto and Landsat imagery. Solid lines are faults and dashed lines are possible faults and/or pholo-lineaments plotted from the imagery.

STRUCTURE AND STRUCTURAL HISTORY

199

Fig. 13.16. Detail of the GRDC LubuksikapingQuadrangle Sheet (Rock et al. 1983) showingthe outcrop of the KuantanFormation,with the additionof bedding traces (dashed lines) from SAR imagery. Solid lines are faults. The bedding traces show folds on east-west axial traces refoldedby north-south or NW-SE folds. In the areas left blank pre-Tertiary rocks are overlain by Tertiary sediments and volcanics.

Fault system of which the Takung and Musi faults appear to be splays. Further to the NW the extension of the Vorbarisan Block lies beneath the Woyla Nappe. Permian stratigraphic units in this area have been described under the names of the Silungkang, Palepat and Mengkarang formations. The Silungkang and Palepat formations are composed of lavas and tufts, interbedded with shales, siltstones, sandstones and crystalline limestones, in the Palepat Formation passing up into an upper Limestone Member. The massive lavas and the limestones in these units are faulted and jointed and the thinner bedded units are sometimes strongly folded (Suwarna e t al. 1994). However, coal bands and plant fossils in the Mengkarang Formation are well-preserved, showing that the rocks have not been metamorphosed. In the Batang Tembesi south of Muarabungo, sandstones of the Mengkarang Formation at Pulau Bayer are folded into an anticline on an east-west axis with an overturned northern limb. In the limb of the fold thin-bedded sandstone layers are imbricated along small scale thrusts, indicating that thrust movements directed towards the west had occurred in

these beds before the rocks were folded. Similar small duplex structures have been observed in the Palepat Formation in the Batang Tantan area. Throughout the Silungkang, Palepat and Mengkarang formations finer grained units within folded beds do not show cleavage. The relatively undeformed nature of these Permian units in contrast to the Kuantan Formation to the NE and the slates and phyllites of the 'Schiefer Barisan' to the SW, together with the volcanics and the Cathaysian 'Jambi Flora' in the Mengkarang Formation, led Zwierzycki (1935) to suggest that they formed an overthrust 'Jambi Nappe'. Because of the affinities of the Permian lavas and the Cathaysian flora to those of East Malaya Zwierzycki (1935) suggested that this nappe was overthrust from the northeast over a distance of 350 km. Van Bemmelen (1949) considered that this amount of movement was too great to have occurred during a single phase of movement (in the Cretaceous Varanginian Stage according to Zwierzycki 1930a) and suggested in his 'undation hypothesis' that the nappe had been gravitationally moved westwards by successive uplifts

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cored by plutonic belts from the Triassic to the Cretaceous. The nappe hypothesis has been discounted by more recent authors (Katili 1970; Cameron & Pulonggono 1984; Hutchison 1994; Barber & Crow 2003). Barber & Crow (2003), following Hutchison (1994), have suggested that the Vorbarisan Unit and the Kuantan Unit constitute a West Sumatra Block separated from Cathaysia and emplaced in Sumatra along a major transcurrent fault, represented by the Medial Sumatran Tectonic Zone, to the SW of the Sibumasu Block. Schiefer Barisan Unit (Fig. 13.8). The Schiefer Barisan tectonic unit

lies to the SW of the Vorbarisan Unit from which it is separated by the Musi Fault; it is overlain further to the SW by the Woyla Nappe. The Permian Barisan, the Triassic Tuhur and the Jurassic and Lower Cretaceous Asai, Rawas and Peneta formations outcropping in the unit, as the name of the implies, are characterized by penetrative cleavage and foliation, in contrast to the Permian and Triassic units of the Vorbarisan Unit. The Barisan Formation shown cropping out to the south of Solok is composed of phyllite, slate, sandstones, limestones and cherts (Rosidi et al. 1976). Lower Permian fusulinids in massive limestones in the eastern part of the outcrop shown as Barisan Formation on the geological map indicate that these rocks, although more highly deformed, should be correlated with the Silungkang and Mengkarang formations of the Vorbarisan Block (Fontaine & Gafoer 1989). The foliation in the phyllites and slates strikes N N W - S S E and limestone lenses are elongated in the same direction. Later deformation is indicated by kinking of the slaty cleavage and small scale shear zones (Rosidi et al. 1976). The Triassic Tuhur Formation outcrops in the area from Lake Singkarak to Dibawah and Diatas lakes (Silitonga & Kastowo 1975; Rosidi et al. 1976). Lithologies include argillaceous sediments with brown cherts, thin turbiditic sandstones and thin limestones which resemble those described from the Kualu Formation of northern Sumatra. Silitonga & Kastowo (1976) distinguished a Slate and Shale Member and a Limestone Member. The Slate and Shale Member, in which the slaty cleavage strikes N W - S E , occupies the greater part of the outcrop. The Limestone Member includes limestone conglomerates that contain blocks of Lower to Middle Permian fusulinid limestone (Silitonga & Kastowo 1975). Similar conglomerates were described to the north of the equator by Turner (1983) from the Cubadak Formation and his Limau Manis Formation, in an area which was mapped by Rock et al. (1983) as part of the Silungkang Formation. The Cubadak and Limau Manis formations contain Halobia and ammonites indicating a M i d - L a t e Triassic age. The limestone conglomerates indicate that Permian limestones were uplifted during the formation of a horst and graben structure and were subjected to erosion between the Mid-Permian and the Mid-Triassic (Barber & Crow 2003). The Middle Jurassic to Lower Cretaceous Asai, Rawas and Peneta formations outcrop in the foothills of the Barisan Mountains to the southwest of Bangko and Sarolungan (Suwarna et al. 1994). The Asai Formation is composed of greywacke, meta-sandstone, siltstone, slate, phyllite and limestone. The slates are blue-grey or reddish in colour with a strong penetrative cleavage striking N W - S E and dipping steeply. Sandstones are cut by quartz veins and limestones by calcite veins. Fossils indicate that the Asai Formation is of Middle Jurassic age and it appears to be the oldest of the three formations (Fontaine & Gafoer 1989). The Rawas Formation consists of basalt lava flows with intrusive dolerite dykes, associated with conglomerates, greywacke sandstones and siltstones, described as a turbidite sequence (Suwarna et al. 1994). Extensive outcrops of grey slate with thin sandstone and siltstone bands and limestone lenses of the Rawas Formation are exposed in the Rawas River. Slaty cleavage strikes N W - S E and dips at 40 ~ to the SW. The bedding lamination in the siltstones is parallel to the cleavage, but the cleavage is axial planar to small isoclinal folds in limestone lenses. The slates

are folded by open asymmetrical folds overturned to the NNE, with a spaced axial plane cleavage cutting across the earlier cleavage, and an intersection lineation plunging at c. 30 ~ to the WNW. Sandstone bands up to 40 cm thick are fractured at right angles to the bedding and the fractures are filled with quartz; similarly limestones are cut by calcite veins. The Peneta Formation covers the same age range as the Rawas Formation and the description is very similar to that of the nonvolcanic, finer-grained parts of the Rawas Formation (Suwarna et al. 1994). It consists of slates, shales, siltstones, sandstones and meta-limestones. The siltstones are strongly folded with a slaty cleavage striking N W - S E , emphasized by new mica growth. The Asai, Peneta and Rawas formations are considered to have been deposited in shallow-water environments, passing into deeper water in a foreland or forearc basin on the SW margin of Sundaland (Pulunggono & Cameron 1984; Barber 2000). Deformation, with the development of folds and slaty cleavage occurred in the mid-Cretaceous, later than the youngest sediments but earlier than the intrusion of Late Cretaceous granites. Deformation of the other units of the Schiefer Barisan, the Tuhur and Barisan formations, presumably occurred at the same time. The deformation is attributed to the collision and overthrusting of the Woyla volcanic island arc and its associated accretionary complex over the margin of Sundaland in mid-Cretaceous times. The deformed low-grade metamorphic rocks of the Schiefer Barisan mark the footprint of the Woyla Nappe. Folds overturned towards the NE and SW-dipping cleavage indicate that the nappe was emplaced from the SW and overthrust towards the NE. The absence of penetrative cleavage in the Permian rocks forming the Vorbarisan Unit suggests that they were never covered by the nappe. The Takung and Musi faults which separate the Kuantan, Vorbarisan and Schieferbarisan units may be older structures, but have been reactivated in the Neogene, during movements along the Sumatran Fault.

Woyla N a p p e (Fig. 13.8)

The Woyla Group crops out discontinuously in the Barisan Mountains along the west coast of Sumatra from Banda Aceh in the north, through Natal and Padang in central Sumatra, to the Gumai and Garba Mountains and Bandar Lampung in the south. The further extent of the Woyla Group in southern Sumatra can be traced in oil company boreholes beneath the Tertiary sedimentary cover (Barber & Crow 2003). A comprehensive review of the Woyla Group in Sumatra, based on the work of Bennett et al. (198 i a, b) and Cameron et al. (1982, 1983) in northern Sumatra, Rock et al. (1983), Wajzer et al. (1991) and McCarthy et al. (2001) in central Sumatra and Gafoer et al. (1992c, 1994) in southern Sumatra, has been given by Barber (2000). Barber's (2000) review covered the areas of outcrop, the lithologies and structures, the environments of deposition of the stratigraphic units or formation of the volcanic units, palaeontological and isotopic evidence of their age, and presented a tectonic synthesis of their origin and emplacement as the Woyla Nappe on the southwestern margin of Sundaland (Fig. 13.8). Cameron et al. (1980) distinguished two lithological assemblages in the Woyla Group of northern Sumatra, an oceanic assemblage and a volcanic arc assemblage (Fig. 13.17) shown on the 'Simplified Geological Map of Northern Sumatra' (Stephenson & Aspden 1982). This distinction was extended to cover other outcrops of the Woyla Group throughout Sumatra (Barber 2000). The oceanic assemblage, which generally lies to the NE of the arc assemblage, consists of serpentinites, gabbros, mafic to intermediate volcanic rocks, commonly basaltic and showing pillow structures, hyaloclastites, volcaniclastic sandstones, red radiolarian cherts, red and purple manganiferous shales, sometimes with

STRUCTURE AND STRUCTURAL HISTORY

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manganese nodules, and rare limestones. These rock units form lens-shaped outcrops, usually separated by steep faults that show slickenside evidence of thrust, and sometimes strike-slip, movement (Wajzer et al. 1991). Interlayered melange units, composed of angular fragments of the other units in a clay or serpentinous matrix, occur within the oceanic assemblage (Fig. 13.18). The oceanic assemblage is interpreted as an ocean-floor sequence composed of serpentinized mantle peridotite, gabbroic and basaltic oceanic crust, with overlying oceanic sediments, imbricated at a subduction zone to form an accretionary complex. Blocks of massive limestone, sometimes occurring in the melanges, have been interpreted as derived from the carbonate cappings of sea-mounts. Triassic foraminifers from a massive limestone block in melange in Natal (Wajzer et al. 1991), MidJurassic radiolarian fossils from cherts at Indarung near Padang (McCarthy et al. 2001), Late Jurassic to Early Cretaceous stromatoporoids, corals and foraminifers also at lndarung (Yancey & Alif 1977) and in Aceh (Cameron et al. 1983) indicate that oceanic

crust ranging in age from Triassic to mid-Cretaceous was being subducted or imbricated to form the accretionary complex. A K - A r isotopic age of 105 + 3 Ma was reported by Koning & Aulia (I985) from a tuff at Indarung. The volcanic arc assemblage which lies along the west coast of northern Sumatra in Aceh is described as the Bentaro Volcanic Formation and consists of basaltic to andesitic volcanics, which are not pillowed, and volcaniclastic sandstones (Bennett et al. 1981a) (Fig. 13.17). The volcanics are associated with massive to bedded limestones with a variety of formation names, of which the Teunom Limestone Formation is typical (Bennett et al. 1981b). The assemblage is interpreted as representing an oceanic island arc with carbonate fringing reefs and its sedimentary apron (Cameron et al. 1980; Barber 2000). A similar assemblage of rock types crops out in the Gumai Mountains inland from Bengkulu in southern Sumatra, where they are identified as the Saling Volcanic Formation, the Lingsing Formation, composed of volcanics with interbedded sediments, and the Sepingtiang

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STRUCTURE AND STRUCTURALHISTORY Limestone Formation (Gafoer et al. 1992c). There are no isotopic ages from any of the volcanic rocks, but the limestones have yielded Late Jurassic to Early Cretaceous corals, stromatoporoids and forams (Bennett et al. 1981a, b) and an Albian (midCretaceous) foraminifer was obtained from the Sepingtiang Formation, indicating that the oceanic and arc assemblages were contemporaneous. A detailed study of the imbricated oceanic assemblage in a 2 0 k m road and river section at Natal on the west coast of Sumatra near the equator was carried out by Wajzer et al. (1991; see also Chapter 4) (Fig. 13.18, inset). Lithologies include serpentinite, pillow basalt, bedded chert, volcaniclastic sandstone, shale and melange. They found that lithological units, 1 - 2 km in width, trending N W - S E , were separated by steeply dipping or vertical faults. Also the units are disrupted internally by faults every few metres. Some fault surfaces show slickensides indicating normal or reversed movements, while others show subhorizontal slickensides indicating strike-slip movement. There is no apparent pattern in the sequence of lithological units and some units are repeated several times in the section (Wajzer et al. 1991). The grade of metamorphism varies along the section with some units being metamorphic schists and slates of prehnite-pumpellyite greenschist facies, while others are unmetamorphosed. There is no apparent pattern in the state of metamorphism, with greenschist facies schists juxtaposed against unmetamorphosed units. In Natal where foliation and slaty cleavage is developed the general trend of the strike is N W - S E with steep but variable dips. Some finer-grained units, including the clay matrix of melanges units, show isoclinal folding (Ft) with axial plane cleavage. The highest grade unit, the Si Gala Gala Schist is a quartz-muscovite-chlorite schist in which the schistosity ($1) contains a rodding lineation (Lj). In some units the foliation, schistosity or cleavage and the earlier formed folds are refolded by open to close folds (F2) on N N W - S S E or N W - S E axes, with the development of crenulation cleavage and crenulation and intersection lineations. Again there is no pattern in the amount of deformation along the section, units with simple deformation being juxtaposed against those with multiple deformation. Similar distributions of lithological units, and variations in metamorphism and deformation are reported from the oceanic assemblage of the Woyla Group in Aceh (Bennett et al. 1981a, b; Cameron et al. 1982b, 1983) with the finer-grained sedimentary and volcaniclastic rocks converted to slates and phyllites. Highergrade metamorphic rocks of the Meukek Gneiss Complex occur in a fault-bounded block in the Woyla Group to the north of Tapaktuan, including a garnet-biotite amphibolite with garnets up to 8 cm in diameter (Cameron et al. 1982b). An area of highgrade metamorphic rocks between strands of the Sumatran Fault described as 'undifferentiated', which includes coarse banded marbles, hornblende schists and mylonitized biotite-garnetstaurolite schist (Cameron et al. 1983) has probably been included in the Woyla Group erroneously and may belong to the Kluet Block or possibly the continuation of the Medial Tectonic Zone. In Aceh the general trend of the lithological units, separated by faults and thrusts, and of the strike of the schistosity, foliation and cleavage, is N W - S E , with moderate to steep dips. Isoclinal folds with an axial-plane cleavage can be seen wherever the bedding lamination can be distinguished, with a beddingcleavage intersection lineation plunging to the SE. The slaty cleavage is sometimes refolded by more open folds on subhorizontal axes and NE-dipping axial planes, with the development of secondary cleavages and lineations. Large-scale upright folds with a 7 km amplitude and subsidiary folds on the scale of 1 - 2 km are reported from the Tapaktuan Quadrangle (Cameron et al. 1982b). The massive volcanic units of the Bentaro Formation and the limestones of the Tuenom Formation and its equivalents are faulted, fractured and jointed, but are not obviously internally

203

deformed, although mylonitized limestones in which the foliation was affected by kink-band folds were observed near Banda Aceh (Barber 2000). Bedded limestones, however, are commonly folded on a large scale, as seen in the quarries in the Lho'nga Formation also near Banda Aceh (Bennett et al. 198 la) (Fig. 13.17). Two fold phases were seen, F~ isoclinal folds with vertical to steeply dipping axial planes with the intersection of bedding and cleavage plunging at a low angle to the SE. The cleavage is folded into small crenulations. Elsewhere in the quarry the first fold phase is represented by tight folds with cylindrical cores and recumbent axial planes on W N W - E S E axes and are cut by a second phase folds on N E - S W axes. Where limestones are interbedded with shales they show pinch-and-swell structures, boudinage and calcite-filled tension gashes normal to the bedding. There are variations in the structural trends in the scattered outcrops of rock units correlated with the Woyla Group in central and southern Sumatra. In the Siguntur Formation, cropping out to the south of Padang, the general strike of the bedding and slaty cleavage is east-west (Rosidi et al. 1976). In the Gumai Mountains east of Bengkulu the contact between the Saling and Lingsing formations trends east-west, and while the rocks are reported to be highly deformed and folded, the strike of the bedding and cleavage trends north-south (Gafoer et al. 1992c). A massive limestone, the Sepintiang Formation, rests discordantly across the contact of the Saling and Linsing formations. The limestone evidently represents a fragment of a fringing reef emplaced tectonically over the other formations. In the Gumai Mountains, to the SW of Baturaja, outcrops of volcanics, cherts and mrlanges are associated with metamorphosed Palaeozoic rocks and bounded by NW-SE-trending thrust faults (Gafoer et al. 1994; Barber 2000). Foliation in the scaly matrix of the m~langes also trends N W - S E ; blocks enclosed in the mrlange are elongated in the same direction and cut by tension fractures normal to their long axes. In the Garba Volcanic Formation two lbld phases are distinguished; the first, with east-west axes, is refolded by later folds on N W - S E axes (Gafoer et al. 1994). Radiolarian and foraminiferal fossils found in the oceanic assemblage of the Woyla Group show that the ocean floor of which it formed a part existed from Triassic to Early Cretaceous times. Mid-Cretaceous foraminifers from the Sepingtiang Formation and Late Jurassic to Early Cretaceous fossils that commonly occur in limestone members of the volcanic arc assemblage, together with the K - A r age of 105 + 3 Ma from a tuff at Indarung (Koning & Aulia 1985) show that the volcanic arc was constructed on the ocean crust in the Late Jurassic and remained active until mid-Cretaceous times. Granites were intruded into both the oceanic and the volcanic arc assemblages of the Woyla Group after they were accreted to the western margin of Sumatra. These include the Sikuleh Batholith in Aceh, the Manunggul and Kanaikan batholiths in Natal and the Garba Pluton in the Garba Mountains. The Manunggal Batholith has yielded a Late Cretaceous age of K - A r age of 87.0 Ma (Kanao et al. 1971, reported in Rock et al. 1983). The Woyla volcanic arc and its associated oceanic crust were evidently accreted to and thrust over the western margin of Sumatra in early Late Cretaceous times (Barber 2000).

The Sumatran Fault Zone

The Barisan Mountain Range is split along its length by the N W - S E Sumatran dextral transcurrent fault system (Fig. 13.19), a transform fault linking the Andaman Sea spreading centre in the north to an area of spreading in the Sunda Strait in the south. From the Andaman Sea to the Sunda Strait the Sumatran Fault is c. 1900 km long, and cuts through all the rock units in Sumatra, including Recent volcanic tufts and alluvial sediments. The overall shape of the fault is a lazy S, the segment to the

204

CHAPTER 13

Fig. 13.19. A simplifiedmap of the Sumatran Fault System in its tectonic setting, showing the locationof figures illustratingdetailed sections of the fault systemdiscussed in this account. Inset map shows the distribution of areas of subsidence, forming grabens, and areas of uplift along the trace of the Sumatran Fault, which Holder et al. (1994) attribute to the formation of subsiduary splays with strike-slip movement, due to transpression across the fault, with the principal compressive stress (o-I) oriented ENE-WSW.

north of the equator being concave to the SW, while the segment to the south is concave to the NE. The fault is currently active along much of its length, as indicated by frequent historic and recent earthquake shocks and measured rates of differential m o v e m e n t across the fault using GPS measurements. Splays of the main fault extend into the forearc and also into the backarc region. It is probable that prominent Pre-Neogene faults mapped in the backarc area have been reactivated in association with more recent movements along the main fault trace. This has not always been appreciated and may have led to confusion

concerning the time of initiation and the amount of displacement along the fault. In some parts of its length the active fault trace is a single discontinuous strand, with mainly right step-overs, but in other areas it bifurcates and splits into a number of strands that may rejoin to isolate fault blocks, some of which have subsided to form lakes, or have been partially or completely filled by Quaternary lacustrine and fluvial sediments. Age of the Sumatran Fault System. The time of the initiation of the fault and the amount of m o v e m e n t along the fault have been

STRUCTURE AND STRUCTURAL HISTORY

matters of continual speculation. Since the Sumatran Fault is a transform fault, clearly related to the Andaman Sea spreading system, it is most reasonable to suppose that it was initiated in the Mid-Miocene (c. 13 Ma) together with the present phase of opening of the Andaman Sea (Curray et al. 1978; Curray 1989; McCarthy & Elders 1997). The trace of the fault is commonly seen cutting Quaternary sediments and volcanics, but sometimes mylonites are exposed in outcrops along the line of the fault, indicating that the fault had an earlier history of movement (McCarthy 1997). Also Wajzer et al. (1991) report N W - S E strike-slip faults in the Woyla Group in the Natal area of North Sumatra which are cut by the Late Cretaceous Manunggal Batholith and Madingding Diorite, and Barber (2000) reports N W - S E foliated syntectonic granitic and basaltic intrusions dated by the K - A r method at 89 + 3 Ma in (mid-Late Cretaceous) in the Sekampung River near Bandarlampung in southern Sumatra, suggesting that strike-slip movements occurred along the same trend as the Sumatran Fault during the Late Mesozoic. Pulunggono et al. (1992) have interpreted a series of W N W ESE lineaments recognized in SAR imagery in the Tertiary sediments of the backarc area as the traces of successive strikeslip faults in the basement which were developed in the Sumatran continental margin during the Mesozoic. They suggest that the most northerly of these lineaments to the south of Palembang is of Triassic age and that the lineaments become progressively younger towards the SSW. Displacement along the Sumatran Fault System. During the primary

mapping of Sumatra it was appreciated that the Barisan Mountains were bisected by a series of discontinuous rift valleys, a 'longitudinal valley', which extended all the way from Aceh in the north to Semangka Bay in the south (van Es 1919). Van Bemmelen (1949) interpreted this longitudinal rift system as due to the domal uplift of the Barisans with extension and the collapse of a central 'keystone'. Durham (1940) was the first to recognize the strikeslip nature of the fault in its central section and subsequently this was recognized for other segments of the fault. The first description of the nature of the Sumatran Fault Zone in modern terms was given by Katili & Hehuwat (1967), who also presented evidence from the displacement of buildings and other structures for the amounts of strike-slip movement along segments of the fault during earthquakes, and over a longer term from stream displacements. The overall amount of movement along the fault can be deduced from its tectonic setting as a transform fault between the Andaman Sea spreading centre and the zone of extension in the Sunda Strait. Segments of continental crust on either side of the Andaman spreading ridge are now separated by c. 460 kin, with Indian Ocean side moving northwards with respect to the rest of SE Asia. To the south this movement is tranformed into the West Andaman Fault, or into strands of the Sumatran Fault System. At the southern end of the Sumatran Fault System the amount of extension in the Sunda Strait between Java and Sumatra has been estimated as 100 km since the Miocene, this extension being taken up by movements along the fault zone (Huchon & Le Pichon 1984; Harjono et al. 1991; Malod & Kemal 1996; Sieh & Natawidjaja 2000). Many suggestions for the amount of movement along the Sumatran Fault Zone in the Barisan Mountains have been proposed from the displacement of units which have been correlated across the fault. Page et al. (1979) found a displacement of at least 200 km in northern Sumatra from lithium values from stream sediment samples, which are commonly > 6 0 ppm on the NE and < 3 0 ppm on the SW side of the fault. However, this could be because the basement to the NE is largely composed of the Tapanuli Group of continental derivation, while to the SW, basement is the Woyla Group composed of rocks of volcanicarc and ocean-floor origin. In north central Sumatra the Medial Sumatra Tectonic Zone, identified during the present study, is

205

displaced by 150 km between Natal and Lake Toba by dextral movement along the Sumatran Fault (Fig. 13.8). McCarthy & Elders (1997) also suggested a displacement of 150kin in central Sumatra from a possible correlation between the midJurassic Siguntur Formation on the SW side, with the contemporaneous Asai Formation on the NE side of the fault. In central Sumatra Posavec et al. (1973) identified east-westtrending aeromagnetic anomalies that cut across the Sumatran Fault. These anomalies correspond with volcanic centres and are attributed to dioritic intrusions at depth. They found that a series increasingly deeply eroded volcanic edifices extend to the NW of the Maningjau Centre as far as Padang, and suggest that this indicates that the crust to the SW of the fault has moved northwestward with respect to the volcanic centre, indicating dextral displacement for a distance of 90 km (Fig. 13.25, inset). On the other hand volcanic edifices are displaced southeastwards by 35 km on the northeastern side of the fault, giving a total relative displacement of 125 km. This movement must have occurred during the Quaternary. Beaudry & Moore (1985), from their study of the distribution of facies in the West Aceh and West Sumatra forearc basins suggested that these two basins were contiguous in MidMiocene times. Since that time the West Aceh Basin has been displaced by some 65 km along the Batee Fault, a splay of the Sumatran Fault (Fig. 13.20). Further evidence for movement of this order of magnitude was presented by Kallagher (1989) from her study of the West Aceh Basin. Here, fine-grained clastic and calcareous sediments of Lower Miocene age are juxtaposed across the Batee Fault against coarse volcaniclastic deposits of the same age. These deposits must have been separated by tens of kilometres in Early Miocene times, before they were juxtaposed by movements along the fault. Malod & Kemal (1996) suggest that the northern part of the Sumatra Forearc constitutes an independent Aceh Plate, bounded by the Mentawai, West Andaman, Batee and the northern segment of the Sumatran Fault. In central Sumatra, Hahn & Weber (1981b) proposed 42 km of dextral displacement from the correlation of coarse- and finegrained facies in the Permo-Triassic Air Mabara and Sopan granites across the Lubuksikaping Fault (Rock et al. /983) (Fig. 13.24). Katili & Hehuwat (1967), Posavec et al. (1973) and Sieh & Natawidjaja (2000) have reported dextral offsets of 2 0 - 3 5 km from stream courses which cross the trace of the Sumatran Fault. Again these movements must have occurred during the Quaternary. Current movements along the Sumatran Fault System. In the discussion of the structural evolution of the forearc above, it was pointed out that many authors have proposed that during the oblique subduction of the Indian Ocean Plate beneath Sumatra the convergence between the two plates is partitioned between thrusting normal to the subduction trench and shearing parallel to the arc (Fitch 1972; Hamilton 1979; Jarrard 1986; Curray 1989; McCaffrey 1996). Shearing parallel to the arc is taken up along the Sumatran Fault System, including the Batee and Mentawai faults. The obliquity of convergence increases northwestwards along the arc, from zero opposite Java, where convergence is normal to the trench, to 45 ~ opposite north Sumatra. Increasing obliquity is matched by an increase in the rate of movement along the Sumatran Fault System, confirmed by measurements of actual movement along the fault by the displacement of recent sediments, from the differential movement of trigonometrical survey points over the last 100 years, and from repeated GPS surveys over the past few years (cf. Prawirodirdjo et al. 2000). Slip rates are calculated as c. 6 mm a-z at the southern end of the fault near the Sunda Strait (Bellier et al. 1991, 1999), < 1 0 m m a -1 near 5~ (Bellier et al. 1991), c. 1 0 m m a - l at the equator and 28 mm a - l near 2.2~ (Sieh et al. 1994). At the northern end of the fault, the rate of opening of the Andaman Sea is calculated

206

CHAPTER 13

Fig. 13.20. Dextral displacement of the Meulaboh and West Sumatra Forearc basins, the reef complex (brick pattern) and the shoreline along strands of the Sumatran Fault System since the Mid-Miocene (from Beaudry & Moore 1985).

as 40.4 mm a-1, averaging 37.2 mm a-J since the Mid-Miocene (Curray 1989). As has been discussed in the account of the forearc above, this differential movement implies that the forearc area is being extended, the extension being accommodated within a forearc sliver by movements along the Batee and Mentawai faults and along minor strike-slip and extension faults within the forearc islands and the accretionary complex. It has been demonstrated by GPS measurements that at the present time the forearc to the south of the Batu Islands is moving in the same direction as the Indian Plate, but at a slower rate, while the forearc to the north has a component of movement northwards, parallel to the Sumatran Fault (Prawirodirdjo et al. 1997) (Fig. 13.7). Sieh & Natawidjaja (2000) have recently prepared detailed maps of the active traces of the Sumatran Fault System, based on their geomorphic expression, using 1:50 000 topographic maps, 1:100 000 stereoscopic aerial photographs, the 1:250 000 geological maps and SAR imagery at the same scale. They divided the fault into 19 segments, named after major rivers or bays within the segment, and show the relationship of the fault traces to active volcanoes (Fig. 13.21). The Sumatran Fault System in Aceh. At the northern end of the Sumatran Fault System in Aceh the fault bifurcates into two strands, the Seulimeum and Aceh faults (Fig. 13.22). Geomorphic features show that the Seulimeum Fault has been active recently, as it cuts through Plio-Pleistocene sediments and volcanic products of the active Seulawai Again volcano; hot springs occur at its southern end. The fault transects and displaces the axial traces of east-west trending folds affecting Pliocene deposits with the SW side of the fault having been moved northwestward. Bennett et al. (1981a) suggest that this displacement amounts to 5 km, but Sieh & Natawidjaja (2000) suggest a movement of 2 0 k m , corresponding to stream offsets along the fault further south. The Aceh Fault does not show any geomorphic effects of recent movement and is considered to be currently inactive, although Soetadi & Soekarman (1964) reported that a school and other buildings were displaced by up to 0.5 m in a N W - S E direction

along the fault in the 1964 earthquake. However, Plio-Pleistocene sediments are highly deformed against the fault, showing that it was certainly more active in the recent past. A depression filled with recent alluvium and volcanic products, the Banda Aceh Embayment, lies between the Seulimeum and Aceh faults (Fig. 13.22). Towards the south rocks of the Woyla Group forming the basement, rise from beneath the younger sediments of the embayment to form a horst block. Genrich et al. (2000) estimate a rate of displacement across the two strands of the Sumatran Fault at 15 mm a -~, with the Banda Aceh Embayment being extruded towards the NW at a rate of 5 + 2 mm a -1 Pre-Tertiary and Tertiary rocks cropping out on the southwestern side of the Aceh Fault are affected by a discontinuous series of thrusts, the Geumpang Line (Fig. 13.22). The thrusts are steep against the fault, but flatten towards the SW, becoming horizontal in Gle Cuplet (Bennett et al. 1981a). The thrusts bring together Tertiary sediments and different units of the Woyla Group, which are also thrust across the Late Cretaceous Sikuleh Batholith. Serpentinites and serpentinous m~lange, presumably derived from the oceanic assemblage of the Woyla Group, sometimes outcrop along the thrusts. M~lange near Rumah Baru contains blocks of Early-Mid-Miocene fossiliferous limestone (Bennett et al. 1981a; Cameron et al. 1983), while Ni and Cr anomalies in Plio-Pleistocene sediments show that the serpentinites had been uplifted and exposed to erosion during the Neogene. Cameron et al. (1983) attribute the development of the thrusts to transpression due to the northwards movement of the forearc sliver into the constraining bend formed by the SW concavity at the northern end of the Sumatran Fault System. To the SE of Rumah Baru the Sumatran Fault bifurcates to form the Anu Batee and Blangkejeren faults, and again further south to form the K l a - A l a s Fault (Cameron et al. !983) (Fig. 13.22). The outcrop of the Blangkejeren Fault is marked by a zone of gouge; breccia, phyllonite and m~lange. Thick conglomerates in the Peutu Formation adjacent to the fault indicate that the fault was active during the early Mid-Miocene. The A n u - B a t e e Fault extends southwards into the offshore region, where it influenced the development of the Sumatran forearc basins (Beaudry & Moore 1981). From the landward part of the fault Sieh & Natawidjaja (2000) report that several of the larger

STRUCTURE AND STRUCTURAL HISTORY

207

Fig. 13.21. Active traces of the Sumatran Fault System identifiedby their geomorphicexpression, fault segments and estimated rates of dextral movement, the location of active volcanoes, lakes and extensional graben (from Sieh & Natawidjaja 2000).

river channels appear to be displaced across the fault by distances up to 1 0 k m , while smaller stream courses are unaffected, suggesting that there have been no m o v e m e n t s along this fault for the past tens of thousands of years. On the other hand Bellier & S~brier (1995) have calculated a present rate of m o v e m e n t of

12 + 5 m m a-1 from the offset on three stream courses and the estimated age of the streams, calibrated against m o v e m e n t on the main Sumatran Fault where it cuts through the c. 70 000 year Toba Tufts, and therefore the time of initiation of the stream courses is known.

208

CHAPTER 13

Fig. 13.22. Thrust structures related to the northern end of the SumatranFault Systemin Aceh. C-P, Carboniferous-Permian Tapanuli Group; P-T, Permo-Triassic Peusangan Group; ,l-K, Jurassic-Cretaceous Woyla Group; Tom; TertiaryOligo-Miocene sediments.

The east-west-trending Kla Line (thrust) (Fig. 13.22), between the Kla-Alas and Blangkejeren faults, which brings the PermoCarboniferous Kluet Formation to rest on the Jurassic-Cretaceous Woyla Group, is attributed to Late Cretaceous tectonism (Cameron et al. 1983). Near Takengon the east-west Takengon Line, a southward-directed thrust, bringing Permo-Triassic Peusangan Group over the Woyla Group and Oligocene sediments was formed prior to the deposition of the Peutu Formation which is unaffected by the thrust (Cameron et al. 1983). The outcrop of the thrust forms a marked topographic feature where Peusangan limestones rest on soft Tertiary sediments. At its western end the Takengon Line links with the dextral strike-slip Geureuggang Fault which extends to the north coast (Fig. 13.22). Movements along the Geureuggang Fault, the Takengon Line and the formation of the east-west folds in Pliocene sediments are due to north-south compression. Curray (I989, Fig. 1) suggested that a southward-directed subduction system had developed in the Andaman Sea off the north coast of Sumatra (Fig. 13.22) which could account for the compression. This postulated subduction system has also been invoked to account for the volcanoes lying to the east of the general trend of the volcanic arc in northern Sumatra (see Chapter 7). But these volcanoes are much more likely to be related to the eastward-dipping Sunda subduction system (see the contours on the Indian Plate in Seih & Natawidjaya 2000) and there is no other evidence for southward subduction, so that there is no obvious cause for the north-south compression. Kembar Volcano and the Kutacane Graben. To the SE of Aceh the K l a - A l a s and Blangkejeren faults define the SW margin of a faulted block, with the Lokop-Kutacane Fault on its NE margin, into which the active Kembar Volcanic Centre has been emplaced (Fig. 13.23). Further south the Lokop-Kutacane Fault passes into the Toru Fault which forms the NE margin of the Kutacane Graben. The Eastern and Central Barisan ranges rise

to 1000-2000 m on each side of the graben, which forms a long narrow depression (75 km long and 9 km wide at its widest part), with a floor at 180-200 m, occupied by Quaternary to Recent alluvium. The emplacement of the Kembar Volcano and the subsidence of the Kutacane Graben are attributed to transtension within a releasing bend on the concave side of the complex of faults which forms the the Sumatran Fault System in this area. The bounding faults cut the products of the Kembar Volcano and displace alluvium at the northern and southern ends of the graben, indicating that recent movement has occurred along the faults (Cameron et al. 1982a). To the south of the graben the Toru Fault cuts and displaces the 73 000 year Toba Tufts, giving an average rate of movement since their eruption of 27 mm a - j (Sieh & Natawidjaja 2000); GPS measurements indicate that the current rate of movement along the Toru Fault is 26 __ 2 mm a - i (Genrich et al. 2000). The Equatorial b~[urcation. Between l~30'N and the equator the Sumatran Fault System splits into two branches, which enclose a lens of structurally complex geology (Fig. 13.24). This structure, formed by the Barumun and Angkola fault segments, is termed the Equatorial Bifurcation by Sieh & Natawidjaja (2000) (13.21). Rock et al. (1983) suggest that the E N E - W S W trend of the lithological units within the fault block, compared with the general N W - S E trend of the rock units outside it, indicate that the lens has been rotated c. 30 ~ in an anticlockwise direction by movements along the bounding faults. In the Barumun segment, movement along the Lubuksikaping Fault, which is concave towards the SW, has formed the Rau Graben in a releasing bend at its southern end. The floor of the Rau Graben lies at 300 m with the mountains on either side rising to heights of 600-1700 m. Based on their mapping programme in central Sumatra Hahn & Weber (1981b) suggested that the Sopan Granite on the eastern side of the Lubuksikaping Fault, in which coarse- and

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209

Fig. 13.23. Kembar Volcano and the Kutacane Graben, North Sumatra, a volcanic centre and a graben filled by Quaternary alluvium in a releasing bend of the Sumatran fault System (detail from GRDC map of Medan--Cameron et al. 1982a).

fine-grained facies can be recognized, could be matched by the Air Mabara Granite on the western side of the fault, indicating a right-lateral displacement of c. 42 kin (Fig. 13.24). McCarthy & Elders (1997) visited these localities and urge caution in accepting this correlation as these granite bodies are petrologically heterogeneous. Sieh & Natawidjaya (2000) recognised 20 km of rightlateral offset on the channel of the Barumun River, but consider that this segment of the fault is relatively inactive at present. The faults in the Angkola segment bounding the lens to the SW are the Gadis and Pungkut-Barilas faults. These faults are concave towards the NE and the Panyabungan Graben has been formed in a releasing bend against the Gadis Fault. The floor of the Panyabungan Graben lies at 200 m, while the mountains on

either side rise to 1 0 0 0 - 1 7 0 0 m. The volcanic centre of Sorik Merapi has been intruded to the south of the graben, near the sharp bend between the Gadis and P u n g k u t - B a r i l a s faults (Fig. 13.24). Katili & Hehuwat (1967) found right-lateral offsets of 2 0 0 - 1 2 0 0 m on tributaries of the Angkola River at the northern end of the Panyabungan Graben, and many streams on the northeastern slopes of Sorik Merapi also show dextral offsets. The present rate of sli~ along this segment of the fault is estimated at 23 + 4 mm a - (Genrich e t al. 2000). The Gadis Fault was the site of an earthquake in 1892 in which a right-lateral displacement of 2 m was recorded trigonometrically (Mfiller 1895). The original survey data have been recalculated and have shown that the amount of dextral displacement was actually

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Fig. 13.24. Fault block bounded by segments of the Sumatran Transcurrent Fault System based on GRDC Geological Map of Lubuksikaping (Rock et al. 1983), interpreted as an extensional stepover and in terms of the strain ellipsoid. Reversed arrows indicate strike slip faults; toothed lines are reversed faults; and lines with blocks are extensional faults. The correlation of the Air Mabara and Sopan granites indicating dextral transcurrent movement of 42 km on the Lubuksikaping Fault is taken from Hahn & Weber (1981b). If the correlation of the granites is correct the fault-bounded block has rotated counter-clockwise by some 40~.

4.5 • 0.6 m (Prawirodirdjo et al. 2000). As well as topographic expression, the outcrop of the Barilas-Pungkut Fault is marked by a 20 m wide fault zone with a fault gouge, composed of sulphide-rich clays and silicified breccia with gypsum (Rock et al. 1983). In Figure 13.24 the Equatorial Bifurcation is interpreted in terms of a strain ellipse in which the Panyabungan and Rau grabens occupy the extensional segments. In this figure, apart from the bounding faults, for which there is good evidence of dextral strike-slip, movement on the other faults is inferred from their orientation with respect to the strain ellipse. The Equatorial Bifurcation is also interpreted as an extensional right-stepping step-over, developing complementary pull-apart grabens. Lake Singkarak (Fig. 13.25). Lake Singkarak, in West Sumatra to

the north of Padang, occupies a depression flanked by escarpments which rise 400 m above the lake surface. The escarpments mark the outcrop of two opposing oblique normal faults, forming a pull-apart graben structure within the Sumatran Fault System (Fig. 13.25). Tjia & Posavec (1972) report that fault traces are seen to displace lahars from recent volcanic eruptions, lake terraces and valley alluvium, and to offset stream courses for up

to a kilometre. Major historical earthquakes have occurred along this segment of the fault; the 1926 Pandangpanjang earthquake to the north of the lake, and the 1822 and 1943 earthquakes near Solok to the south. In the Pandangpanjang earthquake, buildings in the town were displaced up to 6 0 c m towards the NW by dextral fault movement (Katili & Hehuwat 1967). Genrich et al. (2000) calculate that the current rate of dextral displacement is 23 ___ 5 mm a-~ along this segment of the Sumatran Fault. Bellier & S6brier (1994) used SPOT (Satellite Propatoire d'Observation de la Terre) imagery to distinguish between active (young) and inactive (old) fault traces in their study of the fault system. They suggest that the lake formed within an extensional right step-over which developed as a graben bounded by faults to the NE and the SW. These faults have been superseded by a active major through-going fault which passes through the centre of the lake, displacing the northwest bank right laterally for a distance of 2500 m (Fig. 13.25). Holder et al. (1994) made a study of the lineament pattern in Sumatra to the south of the equator from SAR (synthetic aperature radar) imagery. They found that the Sumatran Fault was marked by series of V-shaped graben between the main fault trace and W N W - E S E splays at intervals of 5 0 - 1 0 0 kin; the apex of the

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Fig. 13.25. The pull-apart basin of Lake Singkarak, based on GRDC maps of Padang (Kastowo & Leo 1973) and Solok (Silitonga & Kastowo 1975), with modificationsto the faults from Bellier & S6brier (1994) and detail of normal fault scarps from Sieh & Natawidjaja (2000). The inset shows the progressive displacementof volcaniccentres from the Kerinci Centre towards the NW and SE by movements along the Sumatran Fault System (SFS) according to the hypothesisof Posavec et al. (1973). Vs being towards the north (Fig. 13.19, inset). Segments along the fault between the graben are areas of recent uplift, with perched river terraces and the erosion of the Tertiary and Quaternary sediments. Holder et al. (1994) suggest sinistral strike-slip movements along the splays, together with dextral movements along the main fault, induced subsidence between the main fault and the splay and uplift along the ENE sides of the splays, as crustal blocks moved along the fault. They suggest that these movements were due to oblique compression across the fault zone (Fig. 13.19, inset).

Lake Ranau and the Semanka Depression (Fig. 13.26). A 150 km long depression, filled with the products of Quaternary volcanic products and alluvium, extends from Lake Ranau to Semangka Bay in southern Sumatra (Fig. 13.26). The depression is bounded by the Ranau-Suwoh and Semangka fault segments at the southern end of the Sumatran Fault System. The fault zone

is closely associated with volcanic activity and with many hot springs. Ranau Lake at the northwestern end of the depression, occupies the caldera of a volcano that erupted in a releasing right stepover between the two fault segments. The unusual rectangular walls of the caldera represent the bounding strike-slip and normal faults of the pull-apart basin into which the volcano was emplaced (Bellier & S6brier 1994). A resurgent volcanic dome has developed on the southeastern margin of the caldera. The southwestern component of the step-over is a presently inactive fault strand (North Semangka Fault) that extends from the southern bank of the lake along the southern side of the Liwa depression. The currently active Ranau-Suwoh Fault cuts through the northeastern part of the lake replacing the stepover and the pull-apart basin and offsetting the caldera rim by 2300 i 100m (Bellier & S6brier 1994). Bellier & S6brier (1994) estimate a rate of displacement of 6 _+ 4 mm a -~. This segment of the fault was the site of the 1933 and 1994 earthquakes.

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Fig. 13.26. The pattern of faults between Lake Ranau and Semangka Bay at the southern end of the Sumatran Fault System, from S6brier et al. (1991 ) based on SPOT, Landsat and aerial photographic interpretation. Inset shows the relationship between Java, Sumatra and the Sunda deformation front in solid lines, compared to their relationships at 13 Ma (from Huchon & Le Pichon 1984) in dashed lines. The cross-hatched area indicates the area of extension and cruslal thinning in the forearc, the shaded area indicates the zone of extension in the Sunda Strait, opened up as western Sumatra moved c, lO0 km northwestwards along the Sumatran Fault.

Fifty kilometres to the SE of the lake, Quaternary alluvium fills the Suwoh Graben, occupying a releasing bend in the R a n a u Suwoh Fault. A small group of calderas, one of which erupted in 1933, occurs on the southwestern side of this graben, at the northern end of the Semangka Fault. The Semangka Fault with a significant dip-slip component downthrowing to the NE, defines the southwestern side of Semangka Bay, and a complementary fault defines its NE margin. A subsidiary fault, the Banding Fault, limits a triangular depression filled with alluvium at the head of the bay (Fig. 13.26). The Sunda Strait (Fig. 13.27). The Sunda Strait lies within the zone of transition in which normal subduction of the Indian Ocean Plate beneath Java is replaced by oblique subduction beneath Sumatra. Opposite Java there is a well-developed accretionary complex and forearc basin, while opposite the Sunda Strait the deformation front of the accretionary complex is deflected northeastwards for a distance of 40 km, the topographic expression of the accretionary complex is much reduced, and the

forearc basin is hardly developed. A well-developed accretionary complex and forearc basin is again developed further north opposite Sumatra. The curvature of the subduction trench towards the NW means that subduction becomes increasingly oblique in this direction. Seismic profiles show that these variations in the development of the accretionary prism and the forearc basin are not due to a change in the attitude of the subducting plate which has a constant rate of movement (c. 7 cm a - ~) and a constant angle of subduction (c. 7 ~, Kopp et al. 2002). Malod et al. (1995) used existing Sea Beam data and the results of a new echo-sounding survey to compile a bathymetric and tectonic map of the area of the Sunda Strait. The accretionary complex is represented by a series of small parallel basins, anticlinal ridges and large scarps, some of the latter show the characteristics of reverse faults, with a N W - S E trend, culminating in a series of rift basins and SW- and NE-facing escarpments identified as the Ujung Kulong Fault Zone (Fig. 13.27). To the north of the accretionary complex the central part of the Sunda Strait is occupied by a closed n o r t h - s o u t h depression 1800 m deep.

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Fig. 13.27. Extensional fault system in the Sunda Strait between Sumatra and Java (from Malod et al. 1995). Bathymetry in metres; toothed lines are normal faults; arrows indicate strike slip faults; triangles are active volcanoes.

The depression is bounded to the south by the accretionary complex and to the north by the northern shore of Semangka Bay, defined by the Sunda segment of the Sumatran Fault system. The tectonic map of Malod et al. (1995) shows the depression bounded to east and west by north-south escarpments representing faults downthrowing into the depression. Within the bounding faults the depression is cut by fault scarps trending N W - S E and downthrowing either to the NE or to the SW (Fig. 13.27). Seismic reflection and refraction data obtained by Lelgemann et al. (2000) confirmed the general structural pattern identified by Malod et al. (1995) and show substantial crustal thinning with the development of a horst and graben structure within the strait. Major north-south graben structures occur both to the east and west of the central depression. The graben contain up to 6 km of Neogene and Quaternary sediment. Malod et al. (1995) interpret the Sunda Strait as a north-south extensional pull-apart basin, bounded to the north by the Sunda segment of the Sumatran Fault System, and to the south by the Ujung Kulon Fault Zone. Extension evidently continues at the present day, as a north-south zone of earthquake epicentres extends through the strait, paralleled by a line of volcanoes extending northwards from Krakatoa into southern Sumatra. The opening of the Sunda Strait is interpreted as the result of oblique subduction that has thinned and extended the crust above the down-going plate, resulting in the concave form of the deformation front and the poor development of the accretionary complex. Huchon & Le Pichon (1984) suggested that forearc material to the west of the strait, including the accretionary complex, has been translated c. 100 km northwestwards along the Sumatran Fault System since the Miocene (Fig. 13.26, inset). Sieh & Natawidjaja (2000) have recently confirmed this estimate using more rigorous calculations.

The relationship between the Sumatran Fault System and the Quaternary volcanic arc. Quaternary volcanic centres and currently

active volcanoes show a close relationship to the trace of the Sumatran fault system. Posavec et al. (1973) claimed that this relationship is seen particularly in central and southern Sumatra between Lake Toba and Semangka Bay. They remark that when plotted on a small-scale map the volcanic centres lie at intervals of 7 5 - 1 0 0 km along the fault trace 'like a string of pearls'. However, from their mapping of active fault traces and of volcanic centres Sieh & Natawidjaja (2000) show that this relationship is not as close as has been supposed (Fig. 13.21). Plotting the distribution of volcanic centres relative to the line of the fault they demonstrate that the centres switch back and forth across the fault along its length, with centres occurring up to 20 km from the fault on its SW side near the equator, (Talakmau, Maninjau) and up to 50 km on the northeastern side in Aceh and in the Sunda Strait (Kapal, Krakatoa). Page et al. (1979) suggested that the eastward displacement of volcanic centres from Lake Toba northwards into Aceh is due to a fracture in the downgoing plate along the Investigator Fracture Zone which has been subducted in this area. They suggest that presence of the fracture is responsible for the extent and the intensity of the explosive eruption of Toba, and that to the north of Toba the subducted plate is passing into the mantle at a lower angle, so that the depth at which magmas are generated (c. 100 kin) is displaced towards the east (Page et al. 1979). As has already been reported, Posavec et al. (1973) found in the area of their study in central Sumatra that active volcanic centres are grouped around east-west aeromagnetic anomalies that intersect the fault zone, and which they suggest are due to granodioritic/dioritic intrusions, representing an underlying magma chamber. These east-west zones of volcanic activity at a high

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angle to the Sumatran Fault trace may be due to north-south extension related to the northward movement of the forearc sliver plate. Again, as noted above, Posavec et al. (1973) found that the present volcanic centres have given rise to trails of earlier volcanic edifices which extend towards the NW on the southwestern side of the fault, and to the SE on the northeastern side, and are increasingly eroded with distance from the volcanic centre. This displacement of the volcanic centres with time is attributed to dextral movement along the Sumatran Fault during the past few million years (Fig. 13.25, inset). In detail, as has been pointed out in the preceding account of local areas along the fault, volcanic centres are often located in stepovers and in releasing bends where they are associated with normal faulting and the formation of pull-apart sedimentary basins (Bellier & S~brier 1995) (e.g. Kembar Volcano-Fig. 13.23, Sorik Merapi--Fig. 13.24, Ranau--Fig. 13.26). The apparent close relationship between the trace of the fault zone and the distribution of the volcanic centres has led to the suggestion that there is a genetic relationship between faulting and volcanicity (e.g. Saint Blanquat et al. 1998). The suggestion is that the generation of magmas in the upper mantle and their intrusion into the upper crust has formed a weak zone of ductile material extending from the upper surface of the downgoing plate to the surface, along which the shear component of strain partitioning has been focused. In the upper crust fractures related to the fault zone provide channels for the passage of magmas to the surface to construct volcanic edifices, indeed earthquake hypocentres extend vertically below the fault zone for 100-135 kin, down to the surface of the downgoing plate as defined by the Wadati-Benioff Zone below (Seamans 1993). However, as noted above this relationship is not as close as first appears, and elsewhere in the world, in other regions of oblique subduction and strike slip faulting, volcanism does not always coincide with the active fault zone (Sieh & Natawidjaja 2000). Sieh & Natawidjaja point out that the active volcanic centres are much younger than the initiation of the active fault traces, hundreds of thousands of years as opposed to millions. They concede that the location of the fault zone may have been controlled by earlier Neogene volcanism, but conclude that the relationship seen in Sumatra at the present time between active faults and modern volcanoes is not cogenetic but coincidental.

Tertiary basins in the backarc area The backarc area of Sumatra, to the east of the Barisan Mountains and the currently active volcanic arc, is a relatively low-lying area declining in relief into the Malacca Straits, crossed by meandering rivers and passing into mangrove swamps towards the straits. Beneath the present alluvial and swamp deposits this area is underlain by Tertiary sediments which rest unconformably on the Pre-Tertiary basement and occupy a series of sedimentary basins. The basins hold major reserves of oil and gas and locally coal, and have been intensively studied by geophysical methods and by drilling by companies that hold concessions for the exploration and exploitation of oil and gas. The results of these studies have been reported mainly in the Proceedings of the Annual Conventions of the Indonesian Petroleum Association (IPA). Figures modified from these Proceedings have been used with the written permision of the IPA to illustrate the following account. The backarc region is divided by the Asahan and Tigapuluh arches into the North, Central, with its associated Ombilin Basin, and South Sumatra Basins (Fig. 13.28). The lithologies and sedimentary history of these basins has been described earlier in this volume by De Smet & Barber (Chapter 7), the environments for oil and gas by Clure (Chapter 10) and the coal deposits by Thomas (Chapter 11).

North Sumatra and N W A c e h basins

The North Sumatra Tertiary sedimentary basin and its westward extension in NW Aceh occupy the northeastern part of Sumatra between Banda Aceh and Medan, extending northwards into the Andaman Sea (Fig. 13.29). Knowledge of the geology and structure of these basins is largely due to work of the companies holding concessions in the area, including Inpex, Mobil (now ExxonMobil), Asamera (now ConocoPhillips) and Pertamina (the Indonesian National Petroleum Company). The Tertiary sediments rest unconformably on low-grade metasediments of Carboniferous-Permian age intruded by granites which are exposed in the Barisan Mountains to the south and west of the basins. Outcrops of Tertiary sediment also occur within the Barisans as fault-bounded basins or tilted caps to horst blocks. In the Malacca Strait towards the NE, Tertiary sediments thin out over the Malacca Shelf, and further east the Pre-Tertiary basement rises above sea level in Peninsular Malaya. Tertiary sediments also thin out to the SE towards the Asahan Arch, a basement high that separates the North from the Central Sumatra Basin (Fig. 13.28). There is no clear boundary to the basin towards the north where the basins pass into Thai territorial waters as the Mergui Basin (Polachan & Racey 1994). The Tertiary sediments are covered extensively by Pleistocene to Recent alluvium, swamp deposits and the products of Quaternary volcanism, including volcanic edifices, and to the south of Medan, by the Toba Tufts (Fig. 13.29). The earliest sediments in NW Aceh, and extending westwards across the Barisan Mountains into the West Aceh Basin, are conglomerates, sandstones, siltstones and shales with interbedded limestones (Meucampli and Agam formations) of Eocene to Early Oligocene age (Bennett et al. 198 la). Some of the conglomerates contain volcanic clasts suggesting that volcanicity occurred in this region at that time. Apart from the active volcanoes, it is evident that at this time the Barisan Mountains did not form a topographic feature, and that sedimentation in fluvial, coastal and restricted marine environments was continuous from the North Sumatra Basin into the West Aceh Basin. In the North Sumatra Basin the earliest Tertiary sediments are marine platform carbonates, of presumed Eocene age (Tampur Formation), extensively exposed in a karstic plateau to the west of Langsa, which rest unconformably on the eroded surface of the Pre-Tertiary basement. From a detailed study of seismic sections Situmorang & Yulihanto (1985) reconstructed sub-surface horizons and identified fault patterns in the Pertamina Block, between Pangkalan Brandan and Medan. They found that fault traces in the PreTertiary basement, which they identified as strike-slip faults, have a predominantly north-south orientation. In the Late Palaeogene to Early Miocene the platform broke-up in a 'rift phase' with the formation of extensional pull-apart basins and the development of horsts (highs) and graben or half-graben (deeps). This horst and graben structure now forms the underlying structure of the basin (Figs 13.30 & 13.31). In the Lho Sukon (Pase) Deep this basement now lies at depths of more than 3000 m (McArthur & Helm 1982). Scree deposits formed marginal to the horsts extended out into the grabens as alluvial fans, with coal swamps passing into lacustrine, estuarine and shallow marine deposits (Bruksah Formation). In the Late Oligocene to Early Miocene, the basin entered a 'sag phase', with marine conditions extending throughout the basin (Fig. 13.31). The separate graben coalesced into a regionally extensive basin, with more rapid subsidence to the west of a hinge line (Rayeu Hinge) at the margin of the Malacca Shelf (Figs 13.30 & 13.31). Subsidence outpaced sedimentation, submerging the horsts, including the Arun and the Lho Sukon highs, and the western part of the Malacca Shelf, on which carbonate build-ups developed (Peutu Formation). These build-ups host important gas fields (McArthur & Helm 1982). Continued

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Fig. 13.28. The geographical and tectonic setting of the Sumatran backarc basins. The volcanic arc follows approximately the trace of the Sumatran Fault, based on Davies (1984, Fig. l).

subsidence, coinciding with a global rise in sea level, resulted in maximum marine transgression during the Mid-Miocene (Collins et al. 1995). The reefs were submerged, source areas became restricted, and fine grained sediments (Baong Formation) were deposited throughout the basin. Carbonate reefs were buried beneath fine-grained sediments forming effective traps for oil and gas. At this stage the basin extended westwards over much of the area which now forms the Barisan Mountains. From the MidMiocene to the present time the rate of subsidence has decreased and the basin has undergone a regressive phase. This coincided with the progressive uplift of the Barisan Mountains, together with erosion and the eastward spread of fluvial deposits (Keutapang, Seureula and Julu Rayeu formations), followed by the emergence of the southern part of the basin, with continued uplift of the Barisan Mountains and the growth of the volcanic arc, while to the north beneath the Andaman Sea the basin is still submerged and deposition continues. It is estimated that the original thickness of the sediments in the central part of the basin reached over 5 km (Kingston 1988). Studies of the surface lineaments, representing fault structures in the northern part of the North Sumatra Basin using SAR (synthetic aperture radar) imagery showed that N W - S E (Sumatran) and N E - S W (antithetic) trends are dominant throughout the basin, with subordinate W N W - E S E and E N E - W S W trends (Sosromihardjo 1988). Surprisingly the north-south trend that dominates the subsurface horst and graben structure of the basin is not represented in the surface lineaments, which must reflect more recent stress systems.

Fold structures. The Tertiary sediments are folded (Figs 13.29 &

13.32). Fold structures can sometimes be recognized by outcrop patterns, bedding traces on aerial photographs and in outcrop by the dip of the bedding, especially on the margins of the Barisan Mountains and in temporary roadcuts, but many folds have been recognized only in seismic sections during the exploration for oil and gas. In the NW Aceh Basin between Banda Aceh and Lhokseumawe the fold trends are approximately east-west, parallel to the north coast. This is surprising as the underlying basement structures trend north-south (Fig. 13.30). It has been suggested that the east-west orientation of the folds is due to the incipient development of a southward-dipping subduction system in the southern Andaman Sea, offshore northern Sumatra (Bennett et al. 1981a; Curray et al. 1979), but there is no evidence of such a system in the structural syntheses prepared by the hydrocarbon industry (Nur'aini et al. 1999) (Fig. 13.30). The east-west folds affecting Plio-Pleistocene sediments are open, symmetric to slightly asymmetric, mainly synclinal folds, arranged en echelon. The corresponding anticlines are absent, or represented by interference and accommodation stuctures, especially in argillaceous units. The folding occurred after the Early Pleistocene as the present volcanic edifices have been constructed on rocks which had already been folded. An earlier east-west phase of folding is recognized in the Takengon Quadrangle to the south where folded early Tertiary rocks are overlain unconformably by Late Oligocene sediments (Cameron et al. 1983).

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Fig. 13.29. Structuralmap of the North Sumatra Basin and the distributionof Tertiary and Quaternarysedimentsin northernSumatra. The locationof the cross-sectionin Figure 13.32 is indicated.

In the North Sumatra Basin, in the area to the south of Lhokseumawe, fold traces swing round into a N N W - S S E direction, parallel to the margin of the Barisan Mountains (Fig. 13.29). The swing in strike is attributed to dextral strikeslip movement on the Lhokseumawe Fault (Bennett et al. 198 lc). Further south this fault is seen to have a major downthrow to the east and joins the Lokop-Kutacane Fault to mark the margin of the Barisan uplift. To the east of the fault zone the Simpang Kanan Monocline forms the western margin of the structural North Sumatra Basin. The Tampur Formation of (?) Eocene age, the oldest of the Tertiary units, forms a plateau on the flat limb of the monocline and is seen in aerial photographs to be intensely fractured and jointed, with a karst topography. The monocline is considered to be the surface expression of a major normal fault at depth with a 3 km downthrow to the east (Bennett et al. 1981c). The vertical limb is composed of mudstones of the Bampo Formation (Upper Oligocene-Lower Miocene), which are sheared and slickensided and cut by west-dipping reversed faults. Locally the Bampo mudstones are altered to dark slates containing deformed septarian nodules. Tight, extremely elongated anticlines and broad synclines occur in a belt to the east of the monocline in which the cores of the anticlines are formed of mudstones of the Baong Formation (Middle-Upper Miocene) and the cores of the synclines of sandstones of the Keutapang and Seureula formations (Upper Miocene-Pliocene) (Fig. 13.32). From field studies and in seismic sections it can be seen that the Baong Formation is excessively thickened over the crests of the folds (Mulhadiono

& Marinoadi 1977) and mudstones cropping out in the cores of the anticlines are often vertical, crushed, sheared and slickensided. The anticlines are commonly associated with mud volcanoes and oil, gas and warm water seepages. These features are attributed to mud-diapirism in which the rapidly deposited, water-saturated mudstone, buried beneath the sandstones, became overpressured, producing a density inversion that has caused the activated mudstones to rise diapirically towards the surface. This process is considered to have commenced in the Pliocene, but continues to the present day (Bennett et al. 1981 c). Kinking and bifurcation of the anticlinal fold traces seen in the area to the west of Aru Bay (Fig. 13.29) has suggested that the locations of the anticlines are controlled by dextral movement along strike-slip faults in the basement. Fold structures in the younger Tertiary units to the NE, become difficult to recognize on aerial photographs as they form lowamplitude domes and basins, resembling interference structures, and are covered by alluvial deposits. In a detailed study of seismic data from the Pertamina Block to the north of Aru Bay Situmorang & Yulihanto (1985) examined the orientation of faults, fractures and fold axes at different horizons in the Tertiary sequence and demonstrated that at the level of the basement the structure is extensional, with dominant northsouth-trending normal faults, while above the base of the Baong Formation the dominant structures are compressional, with fold axes and strike-slip faults trending N W - S E (Sumatran Trend). In a further study of the same area, Ryacudu et al. (1992) plotted contours of three horizons, the Belumai Formation, the Middle

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217

Fig. 13.30. Basementstruclure in the North Sumatra Basin showingthe highs and depressions which have controlledTertiary sedimentation,based on Nur'aini et al. (1999) with modificationsafter Collinset al. (1995). The whitedashed line marks the limit of thick Tertiary sediments on the Malacca Shelf, and the bold dashed line marks the position of the Early MioceneRayeu Hinge; subsidence was more rapid to the west of this hinge (after Kingston 1988). The location of the cross-sectionin Figure 13.31 is indicated.

Baong Sandstones and the Lower Keutapang Sandstone, from seismic reflection profiles. From this study they compiled a S W - N E cross-section that illustrated the structure and the structural evolution of the area (Fig. 13.32). At the base of the section the Pre-Tertiary basement is poorly imaged in the seismic data, but is overlain by the Tampur Formation which has been encountered in several boreholes. At this level extensional normal faulting is dominant, but the SW end of the section is cut by a dextral strike-slip fault parallel to the N W - S E 'Sumatran' trend. Further to the NE are several complementary NE-SW-trending 'antithetic' sinistral strike-slip faults. Following the deposition of the Belumai Formation these strike-slip faults were re-activated and inverted in a compressional tectonic regime and in the upper part of the section have developed as positive flower structures, with reverse rather than normal sense of movement, and form fold structures which increase in amplitude upwards through the section. At the SW end of the section the Keutapang Sandstone is exposed at the surface in the core of an anticline. Thickening in the Upper Baong Formation seen in this fold indicates that the structure developed by the diapiric flow of shales into the anticlinal core. The cross section is interpreted as showing that prior to the Mid-Miocene the structure of the area was developed in a transtensional tectonic regime, while after the Mid-Miocene the tectonic regime was transpressional.

Central Sumatra Basin

The Central Sumatra Basin with a width of nearly 300 km from the Malacca Straits in the NE, to the foothills of the Barisan Mountains in the SW, occupies the greater part of Sumatra from 2~ to l~ (Fig. 13.33). Faulted outliers of Tertiary deposits, such as the Ombilin Basin, suggest that before the uplift of the

Barisans the area of sedimentation was continuous with the West Sumatra Basin on the west coast of Sumatra. Exposure of the Tertiary sediments is poor, except in the Barisan foothills and to the south around the Tigapuluh Hills, where outcrops occur in river and more transient road sections. Over the greater part of the basin the Tertiary sediments are covered by Recent alluvium and swamp deposits. However, the Central Sumatra Basin is a major oil province and has been intensively investigated during oil exploration by seismic reflection profiling and by boreholes by P.T. Caltex, Pertamina and P.T. Stanvac, so that the subsurface structure (Fig. 13.34) and the sedimentation history of the basin are very well known (Fig. 13.35). The Central Sumatra Basin is separated from the North Sumatra Basin to the NW by a basement ridge, the Asahan Arch, and less sharply from the South Sumatra Basin to the SE by the Tigapuluh Arch (Fig. 13.28). Pre-Tertiary rocks have been penetrated in many boreholes during oil exploration, as the fractured basement has locally proved to be productive. It has therefore been possible to reconstruct the nature of the basement to some extent (Eubank & Makki 1981). The Pre-Tertiary basement is composed of a series of terranes with a N W - S E structural grain (see Chapter 4). In the NE beneath the Malacca Straits, boreholes encountered a 'quartzite terrain', followed to the SW by a zone of radiolarian cherts, mauve-shales, thin limestone and sandstones and shales (rhythmites) which has been termed the Mutus Assemblage and correlated with the Triassic Kualu Formation, which crops out near Medan to the north. Further to the SW is a zone of greywacke sandstones and mudstones correlated with the CarboniferousPermian Tapanuli Group of northern Sumatra. These rocks also crop out to the SE in the Tigapuluh Hills. Forming the southwestern margin of the Tigapuluh Hills and extending to the NW is a zone of highly deformed schists termed have the Medial Sumatra Tectonic Zone (MSTZ). In the Barisan foothills along the southwestern margin of the basin Tertiary sediments are

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Fig. 13.31. Diagrammatic cross-section to illustrate the tectonostratigraphic evolution of the North Sumatra Basin modified from Kingston (1988) and Collins et al. (1995). The location of the section is shown on Figure 13.30.

seen to rest u n c o n f o r m a b l y on the C a r b o n i f e r o u s K u a n t a n F o r m a t i o n , the P e r m i a n P a l e p a t and M e n g k a r a n g f o r m a t i o n s a n d the Jurassic a n d C r e t a c e o u s R a w a s , P e n e t a a n d Asai f o r m a t i o n s . T h e t o p o g r a p h y o f the b a s e m e n t is h i g h l y irregular, with ridges, h i g h s or ' u p l i f t s ' w h e r e the b a s e m e n t a p p r o a c h e s the surface,

alternating with t r o u g h s or d e e p s w h i c h are filled with Tertiary s e d i m e n t , to a d e p t h o f o v e r 5 k m in the B a r u m a n B a s i n to the n o r t h o f R a n t a u p r a p a t (Fig. 13.34). T h e ridges a n d t r o u g h s are c o n s i d e r e d to be c o n t r o l l e d by o l d e r N N W - S S E l i n e a m e n t s in the b a s e m e n t , r e p r e s e n t i n g the M S T Z and the m a r g i n s o f Triassic

Fig. 13.32. Diagrammatic cross-section of the structure in the Simpang area to the north of Aru Bay, modified from Ryacudu et al. (1992, fig. 18) based on the interpretation of seismic profiles. Normal or transtensional strike-slip faults in the lower part of the succession were inverted as transpressional faults during and after the deposition of the Middle Miocene Baong Formation. Fold structures are developed over positive flower structures related to dextral or sinistral strike-slip faults. The amplitude of the anticline at the SW end of the section has been increased by the diapiric flowage of shale into its core. The line of section is indicated on Figure 13.29.

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219

Fig. 13.33. The Central SumatraBasin,based on GRDC maps, with additionof subsurface structure from Heidrick& Aulia (1993). Tertiarysedimentsare only exposedat the surface in the southwesternpart of the basin in the foothillsof the Barisan Mountainsand also around the TigapuluhHills to the south. ElsewhereTertiarysediments are covered by Recent alluviumand swamp deposits.

graben, but also seen in the Malay Peninsula, and younger N W - S E or 'Sumatra' trend lineaments seen in the Barisans to the west. The troughs occur as two groups, a western group along the front of the Barisan Mountains, including the Baruman Basin in the north, separated by the Kubu High from the Balam and Kiri troughs to the south. A series of highs, including the Dumai High, the Rokan Uplift and the Minas and Kampar highs separate the western troughs from the Bengkalis Trough towards the Malacca Straits. In the western part of the basin, ridges and troughs trend in a N W - S E direction, but in the east the structure is dominated by the north-south Bengkalis Trough and its extensions to the south in the Genako and Bukit Susah troughs (Wain & Jackson 1995). The ridges and troughs were formed as horsts and graben by extension in the earliest phase in the structural development of the Central Sumatra Basin. The sedimentation history of the Central Sumatra Basin as illustrated by Wongsosantiko (1976) (Fig. 13.35) is similar to that of the North Sumatra Basin. The earliest sediments are breccias, conglomerates and sandstones interbedded with shales and coal seams, which were eroded from the ridges and deposited in subsided troughs or half-graben. The evironments of deposition are interpreted as scree, alluvial fan, fluvial and lacustrine with rare marine incursions (Pematang Formation). Although the age of the earliest sediments is poorly constrained they are considered to be of Late Eocene to Oligocene age. Again, as in the North Sumatra Basin, the rift phase was followed by a sag phase with

regional subsidence, so that sedimentation became more widespread, extending from the graben across the adjacent horsts. The sediments are sands and marine shales of the Menggala and Bangko formations. In the Early Miocene deltaic sediments derived from the Sunda Shelf in the region of the Asahah Arch in the NNE extended southwards into the basin, with some input from the Malay Penisula to the east (Sihapas Group). Delta front sand deposits interfinger with marine shales (Telisa Formation) towards the south. As the deltas advanced southwards marine deposits were gradually replaced by terrestrial sediments and coal seams were developed on the delta tops. Subsidence was not uniform throughout the basin, with greater subsidence in the troughs. Subsidence and rapid sedimentation was greatest in the north, so that the greatest thickness of sediments is found in the Barumen Basin (>5000 m) and the sediments thin out over the Kampur High to the south (Fig. 13.34). With continuing subsidence, but a decrease in sediment supply, a major marine transgression occurred in the Mid-Miocene, so that marine deposits of the Telisa Formation were deposited across the delta surface. At the time of maximum trangression marine sedimentation extended westwards across the present site of the Barisan Mountains to reach the Ombilin Basin (Fig. 13.33), well beyond the bounds of the Central Sumatra Basin. In the Ombilin Basin Mid-Miocene sediments include a carbonate reef (Ombilin Formation), indicating that at that time the mountains did not form a topographic feature. Uplift and erosion of the Barisan

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Fig. 13.34. Basement slructure in the Central Sumatra Basin showing highs and depressions which controlled Tertiary sedimentation, simplified after Heidrick & Aulia

(1993, Fig. 3). The lines of sections (a) and (b) in Figure 13.35 are indicated.

Mountains late in the Mid-Miocene provided a source of sediments which advanced across the basin from the west, depositing a regressive sequence of grey sandstones, siltstones and shales up to 1.5 km thick (Petani Formation) through the Pliocene and Early Pleistocene. These deposits are overlain, above an unconformity, by Pleistocene to Recent alluvial and swamp deposits of the Minas Formation (Fig. 13.35b). The Dumai and Pakanbaru Quadrangle sheets (Cameron et al. 1982d; Clarke et al. 1982b) show the stratigraphic sequence exposed in the foothills of the Barisan Mountains with local and restricted outcrops of the Pematang Formation adjacent to basement horsts, with more extensive outcrops of the Sihapas Group and the Telisa Formation forming broad N W - S E anticlines and syclines faulted into the Pre-Tertiary basement. Away from the mountain front broad anticlines with a N W - S E trend, cored by the Sihapas Group and Telisa Formation, including the folds marking the site of the prolific Minas oilfield (Fig. 13.33), occur among extensive Quaternary sands, gravels and swamp deposits. The anticlines occur above highs in the underlying Pre-Tertiary basement or mark the inversion of the sediments deposited in the troughs ('Sunda Folds', Eubank & Makki 1981). Balam Trough. The structure of the Balam and the associated

Rangau, Kiri and Aman troughs, on the western side of the Central Sumatra Basin (Central Deep on Fig. 13.34) has been studied by Williams et al. (1985) and Yarmanto et al. (1995) who describe them as a series of en echelon graben, with intervening complex basement highs or accommodation zones.

The troughs are bounded by steep normal listric growth faults on their western or southwestern margins with hinges to the east or NE broken by small normal faults. Rollover folds were developed against the major bounding faults. The troughs show dog-leg bends at the accommodation zones which are associated with N E - S W oblique faults, and the troughs terminate at faults with the same orientation. Soeryowibowo et al. (1999) have made a structural study of Tapung Half-Graben in the southern part of the Kiri Trough (Fig. 13.34). This graben is 25 km long, 8 km wide, trends N N W - S S E , and lies immediately to the SW of the Minas Field. The graben is bounded on its SW side by a series of three arcuate listric normal faults which are considered to detach at a depth of less than 6.5 km. The graben has a syn-rift section of 1500 m, the extension factor (/3-value) varies along fault fragments between 5 and 12% with a maximum extension of 2 kin. It is suggested that the graben developed as the result of extension on north-south faults in the underlying pre-Tertiary basement. Comparing the pattern of faulting in the Tapung Graben with the sandbox model studies of normal and oblique graben formation by McClay & White (/995), Soeryowibowo et al. (1999) conclude that the extension did not occur in an east-west direction, normal to the basement structures, as had been previously assumed, but obliquely in a N E - S W direction. They point out that the experiments show that only normal faults are developed during oblique extension, and that neither strike-slip nor oblique-slip faults are involved. Plio-Pleistocene compression, also in a N E - S W direction, inverted the Tapung Graben. On the

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Fig. 13.35. (a) Diagrammatic cast-west cross-section across the western part of the Central Sumatra Basin showing troughs and highs and sediment provenance (after Williams & Eubank 1995); (b) Diagrammatic north-south cross-section to illustrate the tcctonostratigraphic development of the Central Sumatra Basin (modified from Wongsosantiko 1976, fig. 3). The lines of section are shown on Figure 13.34.

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Fig. 13.36. The Bengkalis Graben. (a) Outline of the graben and controlling faults from Moulds (1989) with the addition of fault traces from Heidrick & Aulia (1993). (b) Model for the formation of the Bengkalis Graben due to extension on N W - S E and N E - S W basement fractures and the collapse of rhomboid blocks from Moulds (t989, fig. 5). (e) Cross-section showing the Bengkalis Graben as a half-graben, based on a seismic profile in the southern part of the graben, after Heidrick & Aulia (1993). Length of section is c. 60 km, vertical scale is not given. (d) Cross-section showing the Bengkalis graben as a half-graben with normal faults re-activated as thrust faults at the NNE end of the section, based on a seismic profile from the northern part of the graben after Santy (2001). Length of section is c. 30 kin. The locations of sections (b) and (c) are shown on (a). Circled 'A' and 'T' against vertical faults indicate 'away' and 'towards' on dextral strike-slip faults.

STRUCTURE AND STRUCTURAL HISTORY

other hand, in their study of the Amin Trough and related graben, Williams et al. (1995) suggest that dextral strike-slip fault movements along the main boundary faults caused compression at the dog-leg bends, with complementary sinistral strike-slip on the N E - S W faults, during both the Middle Miocene and Plio-Pleistocene detbrmation events. Bengkalis Trough. Heidrick & Aulia (1993) made an intensive study of the sub-surface structure of the 'Coastal Plains Block' covering the area to the south of Bengkalis island, including the Bengkalis Trough, on behalf of P.T. Caltex Pacific Indonesia. The 265 km long Bengkalis Trough originated as a series of extensional half grabens on north-south normal faults (Fig. 13.36). Seismic sections show steep normal faults at the surface passing into listric faults, and an inferred flat-lying decollement surface in the basement at a depth ofc. 6 km (Fig. 13.36c). At the northern end the major bounding fault is on the SW side of the trough, while in the south it is on the NE side (Fig. 13.36c, d). During the Plio-Pleistocene one of the normal faults, the Padang Fault, at the northern end of the trough, was re-activated as a reverse fault in a phase of N E - S W compression (Fig. 13.36d).The basement structure of the trough has been modelled by Moulds (1989) as due to the subsidence of the basement as rhomboidal blocks between north-south- and NNE-SSW-trending faults as the result of regional extension (Fig. 13.36b). Heidrick & Aulia (1993) recognized a complex history of structural development with two intersecting dominant structural trends, north-south and N N E - S S W , which controlled the structural development of the Central Sumatra Basin and were continually reactivated throughout its history. These structures behaved as dextral wrench faults, normal faults or reverse faults, depending on the orientation of the stress system at different stages in the structural evolution of the basin. Heidrick & Aulia (1993) calculate nearly 9 km of extension across the Bengkalis Trough and a minimum of 43 km total dextral strike-slip displacement across north-south faults. The earliest phase of deformation was rifting on north-south or N N E - S S W normal faults and reactivated W N W - E S E basement fractures during Eocene to Oligocene time. A second phase of deformation with N N E - S S W transtensional wrenching in the Early Miocene was associated with the regional sag phase and re-activated the north-south faults as dextral wrench faults, and causing counter-clockwise kinking. in the period from the Mid-Miocene to the present N N E - S S W compression has reactivated the N N W - S S E wrench faults as WSW-directed thrust faults (Fig. 13.36d). Pungut and Tandon Fields. The complex interaction between folds and faults in the structural development of anticlinal structures which form traps for oil fields is illustrated by the Pungut and Tandon fields 65 km to the NNW of Pekanbaru (Mertosono 1975; Eubank & Makki 1981) (Fig. 13.37). A N N W - S S E anticlinal and synclinal fold pair are transected and apparently displaced for some 3 km by a major dextral strike-slip fault. The Pungut Field to the north is bounded to the east by a north-south segment of the strike-slip fault. The oilfield occupies a narrow anticlinal structure developed over an upfaulted sliver of the basem e n t (Fig. 13.37). The Tandun Field to the south occupies an anticlinal fold to the east of the strike-slip fault, which here trends N N W - S S E . The strike-slip fault follows the trace of a normal fault which bounded the western margin of a half graben, filled with a thick sequence of the Upper Oligocene Pematang Formation (Fig. 13.37). The change in the orientation is significant, as this segment of the fault has been reactivated as a reverse fault. The oilfield occupies the anticlinal structure developed by the inversion of the thick sediments forming the graben fill, uplifted along the reverse fault. This is an example of the 'Sunda Folds' as described by Eubank & Makki (1981). The sequence of events which can deduced from these relationships is that the earliest stage was a period of east-west extension,

223

producing normal faulting and the formation of the half graben structure. Whether there was a component of transtension is not always possible to determine. The faults were inactive thoughout the deposition of the Sihapas Group and the Telisa Formation. Deformation with strike-slip faulting and N N E - S S W compression, causing, the reactivation and inversion of the normal fault and the formation of the fold structure in the Tandun Field, occurred during the deposition of the Petani Formation in the Late Miocene to Plio-Pleistocene. The western troughs in the Central Sumatran Basins show a similar sequence of events, as has been diagrammatically illustrated by Yarmanto et al. (1995) (Fig. 13.38). Ombilin Basin

A group of en echelon intramontane basins within the Barisan Mountains, faulted into Pre-Tertiary basement rocks, lie to the west of the Central Sumata Basin. From north to south these are the Mandian, Kampar Kanan, Payakumbuh and Ombilin basins (Fig. 13.33). The best studied of these is the Ombilin Basin in West Sumatra, some 15 km to the SW of the Barisan Mountain Front, and about 10 km to the NE of the active strand of the Sumatran Fault at Solok. The basin has been described by Koesoemadinata & Matasak (1981), Koning & Aulia (1985), Whateley & Jordan (1989), Situmorang et al. (1991), De Smet (1991) and Howells (1997a). The Tertiary rocks are preserved in a synclinal basin divided into two sub-basins, the Talawi and Sinamar sub-basins, by the north-south Tanjung-Ampolo Fault (Fig. 13.39). The basin is surrounded by Pre-Tertiary rocks of the Carboniferous Kuantan Formation to the NE and the Permo-Triassic Silungkang and Tuhur formations to the SW. To the NW the Tertiary sediments are overlain by volcanic products of the Quaternary Malintang and Merapi volcanoes. The average topographic height of the basin is c. 400 m with some peaks in the southern part of the basin reaching over 1000 m. Much of the basin is easily accessible and the Tertiary sediments are well exposed in mountainous terrain, with many river and road sections, so that conventional geological outcrop mapping is possible. In addition there are also several large open-cast coal mines in which the small-scale structures may be examined in detail. The basin has also been investigated in the search for oil and gas, so that the subsurface structure has been explored by seismic sections and boreholes. As presently exposed the basin is elongated in a N W - S E direction, the longer axis being c. 64 km, with a width of c. 25 km and a present depth of c. 4600 m (Williams & Eubank 1995). The basin is considered to have originated as a half-graben in the Late Eocene or Early Oligocene, during the same phase of extension that formed the troughs in the Central Sumatran Basin. Particular attention has been paid to the Ombilin Basin, as it is considered to be a well-exposed analogue for the early stages in development of the basins of the Central Sumatra Basin and the other basins in the Sumatran backarc area that can only be studied by seismic methods and from borehole data. The Takung Fault that bounds the northeastern margin of the basin is considered to be the major bounding fault to the half-graben, as the sediments thicken towards the fault, but the original normal fault has now been partially inverted as a thrust. The hinge zone to the SW is also broken by faults, but to the NW around the Tungkar High, and on the SW side of the basin near Kolok, the unconformity between Tertiary and Pre-Tertiary rocks is well-exposed (Fig. 13.39). Although the unconformity is wellexposed where the Tertiary rocks rest on the Tungkar Granite, its position is difficult to define precisely in the field, as weathered granite passes into arkosic sandstone without a distinct break. Sedimentation histo 9 Against Pre-Tertiary units the oldest deposits (?Late Eocene-Early Oligocene) are marginal screes

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Fig. 13.37. Structuralmap from Mertosono ( 1975, figs 7, 8) and line drawings from seismic sections of the Pungut and Tandun oilfields. Central SumatraBasin (Eubank & Makki 1981; Williams et al. 1995). 'U', upthrown sides; D, downthrown sides of faults.

and alluvial fans, passing out into braided stream sandstones and lacustrine sediments deposited in an anoxic environment in the central part of the basin (Sangkarewang Formation). The Sangkarewang Formation is equivalent to the Pematang Formation of the Central Sumatra Basin, and is estimated to be some 3000 m thick (Williams & Eubank 1995). It is followed by the (?) Oligocene Sawahlunto Formation, composed of sandstones, siltstones, mudstones and coals, deposited in meandering river and flood plain environments, 172 m thick in the Sinamar No.1 well (Fig. 13.40). Coal seams up to 10 m thick are worked in opencast pits and underground mines in the Talawi area. The Sawahlunto Formation is overlain by the (?) Upper Oligocene Sawahtambang Formation, with thick coarse, quartz-rich fluvial sandstones deposited from braided streams, with overbank and flood plain silts and coals, 1365 m thick in Sinamar No.l well. Outcrops of massive sandstones form cliffs and plateaux to the west of Sawahlunto. The Sawahtambang Formation is equivalent to the Sihapas Formation and marks the continued subsidence of the basin and the renewed influx of sediment due to the uplift of the source areas. The increase in volcanic clasts upwards in the section indicates that volcanicity had commenced in the source area, which lay to the SW of the basin in the present forearc area (Howells 1997a). Towards the top of the Sawahtambang Formation fine green sandstones are less quartz-rich and contain glauconite as well as volcanic clasts (Howells 1997a), indicating a marine incursion into the Ombilin area. The Lower Miocene Ombilin Formation, which overlies the Sawahtambang Formation conformably, is entirely marine, and consists of fine sandstones, siltstones and claystone, often carbonaceous, with local limestones, 50 m to 100 m thick, which include lenticular coral and

algal reefs. Fine sandstones with fragments of coal and amber probably represent beach sands. Howells (1997a) suggests that this marine incursion came from the backarc area to the east. Because the sediments in the lower part of the sequence were deposited in a terrestrial environment it has proved difficult to date them precisely, although fish occur in the Sangkarewang Formation and palynomorphs have been recovered from the Sawahlunto and Sawahtambang formations, these have not proved to be age-diagnostic, although a Late Eocene to Oligocene age is inferred (Bartram & Nugrahaningsih 1990; Humphreys et al. 1991). This general age is confirmed by the marine fauna in the overlying Ombilin Formation, which includes foraminifers of Early Miocene age, giving an upper age limit for the older formations (Silitonga & Kastowo 1975; Koesoemadinata & Matasak 1981; Howells 1997a). Origin o f tile Ombilin Basin. Although there is general agreement that the Ombilin Basin developed as a half-graben, there is no agreement concerning the relative importance and timing of extension, strike-slip faulting and compression in the development of the basin. Koning & Aulia (1985) and Situmorang et al. (1991), impressed by its close proximity to the Sumatran Fault System, suggested that the basin had originated as a pull-apart basin in a dextral transcurrent fault regime. Although the basin has a major controlling fault on its NE margin, this is a normal fault which has been inverted as a thrust. Strike-slip movement at some stage is indicated by mismatch between clasts in conglomerates and the adjacent basement lithologies along this margin (Howells 1997a). There is, however, no complementary major fault on the southwestern side of the basin. It is not known

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Fig. 13.38. Diagrammatic representation of the structural development of the Central Sumatra Basin, modified from Yarmanto et al. (1995, fig. 3). White circles with crosses and dots indicate 'away' and 'towards', respectively, on dextral strike-slip faults.

whether the northern margin of the basin is fault-bounded, but the sediments increase in thickness until they are covered by the volcanic products of Malintang Volcano. At the southern end of the basin the basement emerges from beneath the basin fill, with no evidence of a major basin-bounding fault. The Ombilin Basin does not show the characteristic rhomboidal shape or pattern of faulting seen in strike-slip pull-apart basins. Extensional structures. Detailed structures in the stratigraphic units in the Ombilin Basin can be studied in numerous river sections, roadcuts, quarries and in large open-cast coal pits. Evidence of extensional faulting is ubiquitous. Normal faults are common in all stratigraphic units, in particular several spectacular outcrops of extensional listric growth faults have been described from the Sawahlunto Formation. A fault in a road cut on the access road to the Parambahan open cast mine shows a NE-dipping curved surface marked with slickensides indicating normal movement (McCarthy 1997; Howells 1997a). The fault plane passes

downwards into a horizontal decollement surface, marked by a thin band of comminuted coal. On the downthrown side of the fault a wedge of sandstone thickens towards the fault trace, which has a total throw of 1.75 m. The structure is covered by a 2 m thick bed of unfaulted sandstone. A much larger version of a listric normal fault, with a throw of 4 - 5 m, is seen in the same road section. Other examples are seen in the open-cast coal pit, but are continually being removed during the excavation of the coal. Extensional faulting also occurs much higher in the succession, as a listric fault with a rollover anticline, broken by small-scale normal faults forming a crestal graben, is seen in a quarry in the Ombilin Formation, opposite the garage at Sijunjung on the Trans-Sumatra Highway. Compressional structures. As has already been mentioned, the Takung Fault which is the major bounding fault on the northeastern side of the basin is interpreted as a reverse fault from mapping and in seismic section (Koning & Aulia 1985). Small-scale reverse

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Fig. 13.39. Geologicalmap of the intramontane OmbilinBasin, Central Sumatra, from Howells (1997a) based on a compilationby de Smet (1991), from Musper (1929), Koesmadinata& Matasak (1981) and Koning& Aulia (1985).

faults are common in outcrops throughout the stratigraphic units in the Ombilin Basin. A clear example, dipping at 45', with slickensides indicating up-dip movement towards the NW, is seen along the access road to the Parambahan Mine cutting a sandstone wedge associated with a listric normal fault, giving a clear indication of relative age (McCarthy 1997). In the open-cast pit numerous reverse faults can be seen, varying in dip from steep to fiat-lying. Fault planes often have a low angle of dip where they pass through shales and become steeper when they cross sandstone beds. One reverse fault was seen to be a reactivated growth fault which has become a thrust related to the axial plane of a monoclinal fold, passing into a hanging wall anticline where the thrust runs along a bedding plane (Howells 1997a). Structurally the Ombilin Basin consists of two sub-basins. The Talawi sub-basin to the west trends N W - S E and is relatively shallow, with outcrops of the Sangkarewang Formation around the margins in contact with the Pre-Tertiary basement, and the Sawahlunto and Sawahtambang formations in the centre. This is separated from the NNW-SSE-trending Sinamar sub-basin to the east by the north-south TanjungAmpolo Fault (Fig. 13.40). As may be seen from the crosssection (Fig 13.40), the Sinamar sub-basin is a composite syncline, with subordinate anticlines and synclines on axes trending generally N N W - S S E , parallel to the trend of the basin as a whole, and broken by a series of normal faults. Figure 13.40 shows contours in seconds two-way-time on the top of the Sawahtambang Formation F o l d structures.

and closures in the crests of anticlines, isolated by cross-cutting N E - S W faults, which are potential oil-bearing structures. The Sawahlunto and Sawahtambang formations are seen to thin towards the crest of the Palangki Anticline and the Ombilin Formation onlaps the flanks of this fold showing that this anticline was a growth structure during the deposition of all the sedimentary units in the basin (Howells 1997a). Folding on the outcrop scale is seen in the Sangkarewang and Sawahlunto formations. Bedding in the laminated lacustrine shales of the Sangkarewang Formation often dip steeply or vertically, with a strike parallel to the N W - S E trend of the basin. Interbedded shales and sandstones of the Sangkarewang Formation are folded in a complex fashion. Some of these folds have been interpreted as due to sedimentary slumping, distinguished by being underlain and overlain by unfolded beds. Howells (1997a) measured the orientation of slumps in the Malakutan River near Kolok and found the general trend of the fold axes was N W - S E , parallel to the basin margin, and the vergence of the folds was to the NE. The inference is that the present basin margin is parallel to the margin of the basin during the deposition of the sediments, and that the palaeoslope was northeastwards into the basin. The sediments of the Sangkarewang and Sawahlunto formations have certainly undergone a great deal of syn-sedimentary deformation, with the formation of unusually large-scale load casts (Moss & Howells 1996), injection of sandstone dykes into the interbedded shales and, presumably at a later stage, the injection of shales into lithified sandstone beds to

STRUCTURE AND STRUCTURAL H|STORY

227

Fig. 13.40. Structural map from Koning & Aulia (1985, Fig. 7) and cross-section from Williams & Eubank (1995, based on a seismic profile from Koning & Aulia 1985) of the Ombilin Basin, Central Sumatra. The contours on the map are in ms two-way-time on the top of the Early Miocene Sawahtambang Formation, except for the closure noted in the south, which is on the top of the Oligocene Sawahlunto Formation. The maximum depth of the basin is at least 4500 m.

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form flame structures, or in extreme cases, m61ange or 'broken beds' (Howells 1997a). Evidently there was frequent earthquake activity during the formation of the rift graben and the deposition of the graben fill. Folds, clearly of tectonic origin occur particularly in the thinbedded lacustrine shales of the Sangkarewang Formation. In road-cuts up to 5 m high between Atar and Sitankai, shales and thin sandstones are folded on a large scale into chevron folds, with long limbs and tight angular hinges (45'~), overturned towards the SW on NE-dipping axial planes. The beds show slight thinning in the limbs and thickening in the hinges. Steeply dipping, alternating right-way-up and inverted beds along this section, and in a similar 8 m high section along the road between Talawi and Padang-Ganting, show that long-limbed folds with acute hinges are a common feature of the Sangkarewang Formation. In contrast to the Sangkarewang Formation, sediments in the overlying Sawahlunto, Sawahtambang and Ombilin formations show gentle dips throughout the basin, although monoclinal folds are seen in open-cast coal pits in the Sawahlunto Formation, with one steeply inclined limb and with the hinge zone broken by thrust faults along the axial plane. The/bld axes trend east-west and the folds are generally overturned towards the south (Howells 1997a). An unconformity?. The contrast between the steeply dipping and highly folded Sangkarewang Formation and the relative lack of folding in the overlying units led de Smet (1991) to suggest that there is an unconformity between the Sangkarewang and Sawahlunto formations. Indeed de Smet (1991) and Howells (1997a) report several sections where steeply dipping lacustrine beds of the Sangkarewang Formation are apparently overlain unconformably by horizontal or gently dipping, coarse, Sawahlunto sandstones, although the actual contact is not exposed. In his structural study of the Ombilin Basin, Lailey (1989) pointed out that chevron lblds, with sharp angular hinges and long limbs, seen in the laminated lacustrine shales of the Sangkarewang Formation, are characteristic of folds formed in thin-bedded highly anisotropic rock units by compression parallel to the layering. Chevron lblds are only formed where slip along the bedding planes is possible during the formation of the folds (Ramsay 1974). The geometry of the folds limits propagation of the folds for any distance through the sequence, so that individual folds die out both upwards and downwards. The folded package is bounded above and below by a decollement surface. Chevron folding, commonly associated with monoclinal folds and box folds with convergent axial planes, seen in the Sangkarewang and Sawahlunto tbrmations, is restricted to incompetent stratigraphic units with high anisotropy, and cannot be transmitted through the more competent and homogeneous sandstone units of the Sawahlunto and Sawahtambang formations. The relationships seen in the field, between steeply dipping shales and flatlying sandstones above, do not therefore necessarily indicate an unconformity. Indeed, there is no evidence of a break at the boundary between the Sankarewang and the Sawahlunto formations in the vitrinite reflectance data from the Sinamar No.1 well, although other breaks in the succession were identified (Koning & Aulia 1985).

Strike-slip faulting. Small-scale strike-slip faults, with horizontal or sub-horizontal slickensides, are common throughout all the units in the Ombilin Basin. Both Howells (1997a) and McCarthy (1997) made detailed studies of strike-slip faults from outcrops. They found that the faults are vertical or steeply dipping and fall into two sets, one sinistral, trending east-west, and the other dextral, trending N W - S E . Both sets of faults cut across dipping beds and are therefore probably later than the folding. Where the relative age could be determined the east-west set is earlier than the N W - S E set. Along the western side of the Ombilin Basin, on the road from the Trans-Sumatra Highway to

Sawahlunto an outcrop of Triassic limestone is cut by a N W - S E fault with a 0.5 m breccia zone. On one side the breccia zone is planed off along a fault surface which shows horizontal grooving and slickensides indicating dextral strike-slip movement. Evidently a phase of N E - S W extension was followed by a phase of N W - S E strike-slip faulting most probably related to movements along the Sumatran Fault System. Structural history. The structural development of the Ombilin Basin can be interpreted in terms of an initial phase of extension during which the half-graben structure was formed. As indicated by the abundant listric normal growth faults, extension continued from the ?Late Eocene, during the deposition of the Sangkarewang Formation, until the Early Miocene, during the deposition of the Ombilin Formation. The evidence for the formation of the Palangki Anticline as a growth fold indicates that there was some differential subsidence of the basement within the basin. The extent to which extension was accompanied by a component of transcurrent fault movement resulting in a pull-apart basin, as proposed by Koning & Aulia (1985) and Situmorang et al. (1991) is impossible to determine. A component of transtension in the formation of the basin is probable, as normal extension without some strike-slip component is exceedingly rare. As in the basins in the backarc area, the extensional rift phase was followed, during the deposition of the upper part of the Sawahtambang Formation and the Ombilin Formation, by a sag phase due to thermal subsidence. Volcanic clasts indicate that an active volcanic arc lay to the west of the Ombilin Basin showing that subduction of the Indian Plate was in progress at this time. From the vitrinite reflectance data and from projection of fold structures in the seismic section it is estimated that some 1800-2500 m of the Ombilin Formation has been eroded following Plio-Pleistocene uplift of the Barisan Mountains. Vitrinite reflectance data indicate that the sedimentary units in the Talawi sub-basin did not subside to the same depth as those in the Sinamar sub-basin, and that carbonaceous material in the sediments in this basin is more mature than sediments in the Central Sumatra Basin, due either to a greater depth of burial or to a higher heat flow. Deposition in the Ombilin Basin was followed by compression, causing inversion of the basin with reversal of the movement on the listric normal faults, including the Takung Fault, the major NE boundary fault which was inverted to form a thrust (Fig. 13.40 section). Compression also formed the major folds such as the Sinamar Anticline and Syncline, and intensified the Palangki Anticline. It was also responsible for the minor folding and thrusting seen in the Sangkarewang and Sawahlunto formations. The extent to which the compression was accompanied by a component of transpression is again impossible to determine, but some component of transpression is probable. The final event in the structural development of the Ombilin Basin was strike-slip faulting, dextral on a N W - S E trend and sinistral on the complementary east-west trend. Wherever the relative age of strike-slip faults, normal faults and folds could be determined, strike-slip faulting was always the youngest event. This major phase of strike-slip faulting is most probably related to movements on the Sumatran Fault Zone, which lies only 10 km to the SW of the basin. As has been discussed in an earlier section, movement on the SFZ commenced in the Middle Miocene, after the deposition of all the sediments now preserved in the Ombilin Basin. Uplift of the Barisan Mountains accompanied these movements. The Ombilin Basin then formed part of the source area for the Plio-Pleistocene sediments of the Central Sumatra Basin. South Sumatra Basin

The Central and South Sumatra basins have similar structural and sedimentary histories and once probably formed a single

STRUCTURE AND STRUCTURAL HISTORY

large basin, with a poorly defined division now marked by the exposed Pre-Tertiary basement in the Tigapuluh Hills and the Duabelas Mountains (De Coster 1974) (Fig. 13.28). To the east the South Sumatra Basin is separated from the Sunda Basin in the Java Sea by the Lampung High, and its northward extension in the islands of Bangka and Billiton; to the NE the basin deposits thin out over the Sundaland basement in the Malacca Straits; to the SW the basin is limited along the margins of the Barisan Mountains by uplifted basement, exposed in the Gumai and Garba mountains and the Gunungkasih Complex (Fig. 13.41). Within the Barisans the basin deposits are covered by Pleistocene to Recent volcanoes and their volcanic products (Kamal 2000). Internally the South Sumatra Basin is made up of the large Central Palembang Basin > 4 km deep, trending N W - S E with a northeastward extension into the N E - S W trending Jambi Trough or (Jambi Sub-Basin) (Fig. 13.42). Two further basins,

229

the Muara Enim Deep (Benakat Gulley) and the Limau Graben, occur to the SE, sometimes collectively refered to as the South Palembang Basin. Tertiary sediments reach a depth of 5 km in the Benakat Gulley (Fig. 13.43). The basins are separated and surrounded by upfaulted blocks where the Pre-Tertiary basement lies at a relatively shallow depth, such as the Tigapuluh High in the north, the Musi and Kuang platforms in the south, and the Palembang, Tamiang and Lampung highs in the east (Fig. 13.42). From a study of SAR (synthetic aperture radar) imagery and seismic data Pulunggono e t al. (1992) recognized lineaments with W N W ESE, N E - S W and north-south trends, which he considered represent structures in the Pre-Tertiary basement which were re-activated as normal faults during extension to form the highs, the basins and the troughs. Pulunggono e t al. (1992) suggest that the W N W - E S E lineaments, including the Lematang Fault, may mark Mesozoic strike-slip faults in the basement, analogous to the present Sumatran Fault Zone, which were re-activated as

Fig. 13.41. Structure of the South Sumatra Basin showing the distribution of folds and faults, based on data from GRDC map sheets, De Coster (1974), Pulunggono (1986) Pulunggono et al. (1992) and Kamal (1999). LBF, Lebak Fault; KF, Kikim Fault.

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Fig. 13.42. Basement structure of the South Sumatra Basin based on an unpublished map prepared by Pertamina/BElCIP (1985), with additional data from GRDC maps, De Coster (1974), Pulunggono (1986) Pulunggono et al. (1992) and Kamal (1999) and showing 'deeps', 'troughs', grabens and 'highs', with deplh to basement in seconds two-way-time (TWT). The Lampung High which marks the eastern boundary of the basin is about 100 km to the east of Palembang. The line of the cross-section illustrated in Figure 13.43 is indicated.

normal faults during the Palaeogene. Unlike the North and Central Sumatra basins, it has not yet been demonstrated that active strike-slip faulting has played an important part in the development of the South Sumatra Basin, although Pulunggono et al, (1992) report that the Lematang Fault is cut and displaced dextrally for 12 km by the north-south strike-slip Kikim Fault (Figs 13.41 & 13.42). S e d i m e n t a t i o n history. Apart from the greater importance of volcanic rocks, the sedimentary sequence in the South Sumatra Basin resembles those in the Central and North Sumatra basins (Fig. 13.43). The oldest deposits, the Lemat and Lahat formations (?Middle Eocene-Upper Oligocene), outcrop in the foothills of the Tigapuluh Hills and the Duabelas Mountains, and are identified in boreholes and seismic sections along the margins of the troughs and graben throughout the basin. These are volcanic and rift phase sediments, including breccias, conglomerates and 'granite wash', resting unconformably on

the Pre-Tertiary basement. Conglomerate clasts include slate, phyllite, metasandstone, marble, basalt, andesite and vein quartz derived from the underlying Tapanuli, Kuantan and Woyla groups, and from intrusive granites. Towards the central parts of the basin the conglomerates pass into bedded sandstones and siltstones with thin coals, and irregular carbonate layers and glauconitic and tuffaceous shales (De Coster 1974). Environments of deposition are interpreted as scree, alluvial fan, fluviatile and fresh to brackish water lacustrine. These deposits are followed by channel sandstones with silicified wood, alternating with siltstones and carbonaceous shales, sometimes containing molluscs, with coal seams and tuffaceous units (Talangakar Formation, Upper Oligocene-Lower Miocene), laid down in a delta plain environment, from fluvial to lacustrine, lagoonal and shallow marine, becoming euxenic in the troughs. In the troughs the Talangakar Formation follows conformably on the Lemat or Lahat Formation, but at the basin margins becomes unconformable.

STRUCTURE AND STRUCTURAL HISTORY

231

Fig. 13.43. Diagrammatic cross-section to illustrate the tectonostratigraphic development of the South Sumatra Basin modified after Kingston (1988).

Differential subsidence, with reactivation of the marginal faults continued during the deposition of the Talangakar Formation, which marks a transgressive phase, and this is followed by the fully marine Baturaja and Gumai Formations (Lower-Middle Miocene), representing the period of maximum transgression. The Baturaja Formation is a thick platform carbonate unit, sometimes including coral reefs, deposited on basement highs, passing into bedded limestones and open marine shales in the intervening depressions. The area of deposition extended eastwards across the Lampung High into the Sunda Basin. The Gumai Formation is composed of foraminiferal grey shales and siltstones, with intercalations of glauconitic and tuffaceous sandstone, which become more important westwards towards the Barisans. At this stage the Barisan Mountains had ceased to exist and the area of sedimentation extended continuously from the backarc westwards into the forearc area. Marine regression commenced with the deposition of the Airbenakat and Muaraenim formations (Upper Miocene-Lower Pliocene), which consist of sandstones and clays with coal beds and bands rich in molluscs and foraminifera. The overlying Kasai Fornaation (Pleistocene) rests with local unconformity on the Muaraenim Formation and is composed of conglomerates, tuffaceous sandstones and tuffs with lignite and silicified wood. The conglomerates contain clasts derived from the PreTertiary units and volcanic materials including pumice, marking the uplift of the Barisans and the eruption of active volcanoes. Sediment was also eroded from developing fold structures within the basin and deposited locally. From the extrapolation of the structure it is estimated that up to 1500 m of sediment has been removed from the crests of anticlinal folds (De Coster 1974). Structure. The structure of the South Sumatran Basin is dominated by outcrops of the Pre-Tertiary rocks in the Tigapuluh Hills, the Duabelas Mountains in the north and along the Barisan front to the SW. The Pre-Tertiary rocks are fringed by outcrops of the

oldest Tertiary units in the basin, the Lemat and Lahat formations, indicating later basement uplift. Fold structures, concentrated in three broad anticlinal areas (anticlinoria), the Palembang, the Pendopo and Muaraenim anticlinoria, are best developed in the central part of the basin, where the Tertiary sediments are thickest (De Coster 1974) (Fig. 13.41). The Palembang Anticlinorium extends southeastwards from the Tigapuluh Hills to Palembang. It is made up of a series of N W - S E , elongated, narrow, periclinal, asymmetrical anticlines, with intervening broader, basinal synclines. The more northerly anticlines have steeper southern limbs, while the southern folds have steeper northern limbs (Pulunggono 1986). In the Pendopo-Limau Anticlinorium SW of Palembang, the folds have a more W N W - E S E orientation (Fig. 13.41), with limbs dipping more steeply to the south; the fold axes are cut at frequent intervals by N E - S W normal faults. The anticline is considered to have formed as a drape over an uplifted basement block composed of Permian limestone and Cretaceous granite which outcrop in the core (Gafoer et al. 1986) (Fig. 13.42). The Pendopo-Limau Anticline is limited to the south by the Lematang Fault, which cuts the basement and has a throw of up to 1500 m to the south into the Benakat Gulley (Muara Enim Deep) (Pulunggono et al. 1992) (Fig. 13.42). The throw decreases eastwards and the fault dies out into a monoclinal flexure. The Muaraenim Anticline to the east of the Gumai Mountains in the southern part of the basin, is formed of a series of arcuate, asymmetrical, periclinal folds with limbs which become steeper and overturned towards the ENE, and are broken by thrusts (Pulunggono 1986) (Fig. 13.44). The folds are considered to be disharmonic, affecting Tertiary units above a detachment in the Gumai Formation (Fig. 13.44 section B - B ' ) . A gravitational origin is suggested for these folds, formed by the slumping of the Tertiary sediments towards the NE from the basement ridge which extends eastwards from the outcrop of Pre-Tertiary rocks in the Gumai Mountains (Pulunggono 1986; Holder et al. 1994) (Fig. 13.44).

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Fig. 13.44. GeologicalMap and cross-sectionsof the Gumai Mountainsand the Muaraenim Anticlinoriumbased on GRDC 1:250000 Quadrangle Sheets of Bengkulu (Gafoer et al. 1992c) and Lahat (Gafoeret al. 1986). Filled circles on the map are oil seeps, and open circles are gas seeps. The arcuate fold stuctures in the Muaraenim Anticlinorium,shown in sectionB-B', are interpretedas due to gravitationalslidingfrom the upliftedGumai Ridgeon detachmentsurfaces withinthe GumaiFormation; vertical lines in section B-B' are oil companyboreholes (after Pulunggono 1986).

Throughout the South Sumatra Basin anticlines are generally cored by outcrops of the older Tertiary units, the Talangakar, Gumai and Airbenakat formations, while the synclines are cored by the Plio-Pleistocene Kasai Formation. The folded rocks, particularly in the southern part of the basin are covered unconformably by Quaternary to Recent fluviatile and swamp deposits; the underlying structure being determined only from seismic data. The deformation of the Tertiary sediments in the South Sumatra Basin evidently occurred in the latter part of the Pleistocene. The South Sumatra Basin was formed in the Late Eocene to Early Oligocene, at the same time as the North and Central Sumatra Basins, by extension of the Pre-Tertiary basement on pre-existing faults on W N W - E S E and N E - S W trends and the subsidence of rift graben. North-south trends, dominant in the North Sumatra Basin and prominent in the Central Sumatra Basin, are less important in South Sumatra being represented only by the Kikim Fault margining the Benakat Gulley and parts of the Lembak Fault terminating the Pendopo-Limau Anticlinorium (Pulunggono 1986), although the long straight eastern coast of Sumatra, facing the Java Sea, appears to be controlled by a major north-south fault. The troughs were infilled by erosion products derived locally from basement horsts in the rift phase. The marginal faults show their greatest amount of throw in the Talangakar Formation, and die out upwards into the Structural history.

overlying Gumai Formation. Depocentres during the deposition of the Talangankar Formation were situated in areas which later became the sites of uplifted blocks (Pulunggono 1986). The rift phase was followed by thermal subsidence in a sag phase, which led to marine incursion and the deposition of fine grained marine sediments throughout the basin, with the formation of carbonate reefs on the horst blocks. Continued subsidence led to the drowning of the carbonate reefs and the deposition in deep water of the anoxic shales and marls of the Gumai Formation. Study of microfossils and strontium isotopes in the Baturaja and the Gumai formations from boreholes in the Muaraenim to Baturaja area showed that the drowning of the carbonate platform in a 'maximum flooding stage' was diachronous, and progressed from west to east (Pannetier 1994). The shallowing upwards sequence in the Gumai Formation is correlated with a cooling event and a world-wide fall in sea level due to the formation of ice sheets (Pannetier 1994). Contrary to the earlier pattern, the greatest thickness of the Gumai Formation, occurred in the areas which are now depressions (Pulunggono 1986). Pulunggono (1986) attributes this tectonic inversion to the onset of compression in the South Sumatra Basin in the early Mid-Miocene, due to the renewal of the subduction of the Indian Plate beneath west Sumatra. Marine deposition continued throughout the region in the Mid-Miocene, extending westwards into the forearc and eastwards

STRUCTURE AND STRUCTURAL HISTORY

into the Sunda basins. As the subduction system became established, volcanicity and uplift of the basement in the Barisan Mountains led to a marine regression, with the deposition of terrestrial sediments late in the Mid-Miocene, which gradually extended eastwards to cover the whole of the South Sumatra Basin by the Pleistocene. Perhaps commencing in the Late Pliocene, but completed during the Pleistocene, the basin became subject to N E - S W compression, reactivating basement faults, uplifting basement blocks and generating folds on N W - S E axes in the overlying sediments. Variations in the vergence of the fold structures, from NE to SW, are attributed to the movement of the developing folds in the Airbenakat and Muaraenim formations away from the areas of basement uplift on decollement surfaces within the underlying Gumai Formation (Pulunggono 1986). Locally, particularly on the Barisan Front along the western margin of the basin, both the basement and the overlying sediments have been affected by N E - S W dextral strike-slip faults related to movements along the Sumatran Fault System.

Origin of basins in the Sumatran backarc basins The review of the structural development of the linear belt of Tertiary sedimentary basins in eastern Sumatra given above shows that they were initiated as rift systems generated by extension and thinning of the crust. The resulting high heat flow was followed, after extension had ceased, by the development of sag basins due to thermal relaxation, enhanced by sediment loading. In the literature these basins have generally been described as 'backarc basins', as they occupy a backarc position relative to the active volcanic arc of Sumatra. The implication is that these basins developed directly as the result of the activity of the arc. For example Eubank & Makki (1981) have suggested that backarc extension was due to the establishment of convection cells and diapirism in the mantle set up by the subduction of the Indian Plate. The implication is, that if extension had continued the continental crust would have ruptured, with the generation of oceanic crust, to form a backarc marginal basin, similar to those associated with the subduction systems of the Western Pacific. One problem with this interpretation, evident from the foregoing account, is that the formation of basins in Sumatra in the early Tertiary was not restricted to the backarc region. Outlying remnants of Tertiary deposits within the Barisan Mountains indicate that these basins once extended across the site of the mountains to join with similar and contemporaneous forearc basins to the west. The term 'backarc basins' as applied to the Tertiary basins of the Sumatran backarc area is therefore a misnomer. The present position of these basins is due to the subsequent rise of the mountains and the construction of the volcanic arc that separated the basins so that they now occupy forearc and backarc positions. Morley (2002b) has suggested that the formation of basins across Sumatra was due to 'subduction rollback' caused by the sinking of the incoming Indian Plate, drawing the whole subduction system forward, causing extension in both the forearc and backarc areas. Another problem with hypotheses that relate the formation of the sedimentary basins in the Sumatran backarc area to their relationship to the present subduction system is that these basins are not restricted to Sumatra, but form part of a network of extensional rift basins, originating in the early Tertiary which formed

233

at about the same time throughout the whole SE Asian region. These basins are therefore the result of processes which affected the whole of SE Asia. The precise age of formation of these basins is difficult to determine, as the earliest deposits are usually of terrestrial origin and do not contain age-diagnostic fossils. However, it appears that during the Late Eocene to Early Oligocene extensional basins were formed across the area from the Java Sea in the south, through Sumatra and the Malay Basin and to Vietnam in the east. The regional extent of basin formation during the Palaeogene has encouraged the search for a regional rather than a local explanation. The model proposed by Tapponnier et al. (1986) for the southeastward extrusion of SE Asia following the collision of the Indian continent with the southern margin of Eurasia, commencing in the Eocene, seemed to provide such a solution. In this model SE Asia was extruded as a set of continental slivers separated by strike-slip faults, opening up pull apart basins between the continental fragments as they moved away differentially from the site of the collision. Attempts have been made to interpret the basins in the Sumatran backarc area as pull-apart basins formed during strike-slip movements (e.g. Davies 1984; Daly et al. 1987, 1991). Davies (1984) for example, attributed the formation of the Sumatran basins to pull-aparts between strike-slip faults which changed their orientation in response to the rotation of a Sunda Plate, including Sumatra, and variations in the direction and rates of subduction along the Sumatran margin. However, the consensus view is that the basins developed initially as extensional rifts, controlled by the orientation of pre-existing lineaments in the Pre-Tertiary basement. The orientation of these lineaments is different in the three basins, being north-south in the North Sumatra Basin, N W - S E and N E - S W in the Central Sumatra Basin and N N W SSE and N E - S W in the South Sumatra Basin. Strike-slip movement in the backarc area is superimposed on these earlier trends and coincides with the uplift of the Barisan Mountains, the inversion of structures in the sedimentary basins and movements along the Sumatran Fault System. Apart from specific problems with the application of the extrusion/strike-slip model to the basins in the Sumatran backarc, there are general problems in its application to SE Asia as a whole. Many of the these basins, particularly on the Sunda Shelf in the South China Sea and the Java Sea are not correctly oriented to have originated as pull-apart basins related to dextral movements on strike-slip faults (Hall & Morley 2004). The impression given by the distribution of the basins is that there was overall expansion of the whole SE Asian area during the Palaeogene. Hall & Morley (2004) point out that the area between Sumatra, including the backarc area, and eastern Borneo has a very high surface heat flow > 8 0 m W m -z, compared with the average for continental crust of 40 mW m-2. The highest surface heat flow, up to 1 8 0 m W m -2, occurs in the Central Sumatra Basin, the Malacca Strait and the adjacent part of the Malay Peninsula. Hall & Morley (2004) suggest that the high heat flow is due to high temperatures in the mantle, as indicated by low seismic velocities below the region, the result of long continued subduction. A contribution may also come from the upper crust, which in the SE Asian region contains a high proportion of radiogenic granites, insulated by a thick sedimentary pile including shales and coals. They go on to suggest that the opening of the Tertiary sedimentary basins in SE Asia was due to mantle/lower crustal flow.

Chapter 14

Tectonic Evolution A. J. BARBER, M. J. CROW & M. E. M. DE SMET

The concept that SE Asia, and indeed Asia as a whole, has been built up during the Phanerozoic by the amalgamation of allochthonous terranes derived from the northern margin of East Gondwana, is now well established in the literature (e.g. Audley-Charles 1988; Sengor et al. 1988; Metcalfe 1996, 1999 and references therein). In Early Permian time all the major continental land masses, including East and West Gondwana, were joined together in the supercontinent of Pangaea (Fig. 14.1). At this time the continental blocks of North and South China, Indochina and Simao had already separated from East Gondwana. In Metcalfe's (1999) version of the concept a series of elongated terranes separated successively from the northern Gondwana margin by the development of ocean basins behind them. These oceans are referred to as Palaeo-Tethys, Meso-Tethys and Ceno-Tethys. The Indochina Block, with East Malaya, forms the core of SE Asia and is considered to have separated from Gondwana by Late Devonian times to amalgamate with the South China Block by the Early Carboniferous. Indochina is characterized by an Upper Palaeozoic to Mesozoic fauna and flora of Cathaysian and Tethyan type, exemplified by the Gigantopteris flora of Jengka Pass (Kon'no & Asama 1970; Hutchison 1994), related to those of the North and South China blocks, but with no relationship to the flora and fauna of Gondwana. To this core was added the Shah-Thai or Sibumasu Block, which separated from Gondwana in the Permian and amalgamated with the Indochina Block in the Late Permian or Triassic (Metcalfe 1999). With the wealth of new data provided by the completion of the reconnaissance mapping of Sumatra and the follow up palaeontological studies, attempts were made in the 1980s to identify the crustal blocks that make up Sumatra, their relationship to adjacent parts of SE Asia and to determine the timing of their separation from Gondwana and their incorporation into Asia.

Pulunggono & Cameron (1984) model Following the completion of the Integrated Geological Survey of Northern Sumatra the new data were integrated with pre-existing data from the literature, and information from boreholes acquired during petroleum exploration, to compile a plate model to explain the distribution of stratigraphic units in Sumatra and the adjacent part of Malaysia (Pulunggono & Cameron 1984; Pulunggono 1985) (Fig. 14.2). In this synthesis Sumatra and the Malay Peninsula are are interpreted as composed of a series of microplates. The East Malaya Microplate to the east, characterized by Permo-Triassic magmatism, is separated from the Malacca Microplate, forming the western part of the Malay Peninsula, by the Bentong-Raub Line, marked by a zone of basic and ultrabasic rocks and m61ange, which represents the suture where the two microplates collided in the Triassic (see Metcalfe 2000). The southwards extension of this line is shown passing between the Kundur and Karimun islands, and through Singkep and the NE corner of Bangka into the Java Sea (Fig. 14.2). To the west and SW the Malacca Microplate is limited by the Kerumutan Line, interpreted as a thrust (Pulonggono & Cameron 1984), or possibly a major strike-slip fault (Eubank &

234

Makki 1981), which marks the boundary between a 'quartzite terrain', regarded as the continental margin of the Malacca Plate and the deep-water deposits of the Mutus Assemblage (Fig. 14.2). The Mutus Assemblage is characterized by radiolarian cherts, red-mauve shales and rhythmic thin-bedded sandstone and shale sequences with Late Triassic fossils. Basalts, chlorite schist, gabbro and serpentinite encountered in boreholes in the southeastern extension of this zone suggested to Pulunggono & Cameron (1984) that the Mutus Assemblage represented another suture, marking the zone of collision between the Malacca Plate and the Mergui Plate to the west. However, the characteristic rock types of the Mutus Assemblage are not restricted to this narrow zone, but are widespread, being identical to those of the MiddleUpper Triassic Kualu and Tuhur formations of Sumatra and the Semanggol Formation of Peninsular Malaysia. These rock units have been interpreted in the present account as deep water deposits laid down in rifts developed during a Triassic phase of extension. The concept of separate Malacca and Mergui Plates, as proposed by Pulunggono & Cameron (1984), is no longer tenable. The Mergui Microplate, characterized by the CarboniferousPermian 'pebbly mudstones' and a Permian arc assemblage, is shown extending across the greater part of Sumatra, including the outcrops of the Bohorok, Alas, Kluet and Kuantan formations (Fig. 14.2). The Permian volcanic arc, represented by the Palepat and Mengkarang formations with a Cathaysian flora, is shown overlying the southwestern margin of the Mergui Plate. In the northern part of Sumatra the Situtup Formation near Takengon is shown as a tectonic outlier of this arc, on the basis of the volcanics associated with limestones. Mid-Permian fusulinids Pseudodoliolina sp. and Neoschwagerina sp. (Fontaine & Gafoer, 1989), considered to be typical Cathaysian forms (Ueno Pers. Comm. 2002) have been obtained from the Situtup limestones (Cameron et al. 1983), supporting this interpretation. Oceanic and Arc assemblages of the Jurassic-Cretaceous Woyla Group are shown as the 'Woyla Terrains' along the west coast of Sumatra, thrust under (rather than over) the Permian arc and southwestern margin of the Mergui Plate (Pulunggono & Cameron 1984) (Fig. 14.2). These terranes include areas in Sikuleh, Natal and Bengkulu (not named in Fig. 14.2) identified as microcontinental blocks.

Fontaine & Gafoer (1989) model Comprehensive palaeontological studies of the CarboniferousPermian stratigraphic units in Sumatra by Fontaine, Gafoer and their colleagues (Fontaine & Gafoer 1989), prompted a reassessmerit of their age, environment of deposition and their provincial affinity (see Fig. 4.9). As has already been described, Fontaine & Gafoer (1989) interpreted the Carboniferous rocks in the northern part of Sumatra as a series of contemporaneous sedimentary facies formed on a continental margin, with littoral and shelf facies sands in the east, the glacial pebbly mudstones interbedded with turbiditic sands and shales, passing into distal turbidites and deep water shales in the Kluet Formation. The limestones of the Alas Formation represent shallow-water carbonates deposited on a 'high' in the continental shelf environment.

TECTONIC EVOLUTION

WC -West Cimmerian Blocks; QI - Qiangtang terrane; WB - West Burma Terrane

Fig. 14.2. Microplates in western Indonesia from Pulunggono (1985), after Pulunggono & Cameron (1984).

235

Fig. 14.1. Plate reconstruction and palaeogeography in the Early Permian from Metcalfe (1996).

236

CHAPTER 14

Fontaine & Gafoer (1989) relate the fauna and flora of the Vis6an Alas limestones to those found elsewhere in the Sibumasu Block, in western Peninsular Malaya, Thailand and Burma. On the other hand, they relate the fauna and algal flora of the limestones in the Vis~an Kuantan Formation to those of the eastern Peninsular Malaya and the Indochina Block in Thailand, Laos and Vietnam. In contrast, Fontaine & Gafoer (1989, p. 24) point out that the microfauna of the Kuantan Formation shows affinities not only with that of the Indochina Block, Central Asia and Western Europe, but also with the microfauna of NW Australia, where a similar assemblage has been described from well cores in the Bonaparte Basin (Mamet & Belford 1968), highlighting the provinciality of the benthic macrofauna compared with the universal distribution of planktonic micofossils throughout the world. Fontaine & Gafoer (1989) concluded that the Alas limestones were deposited on the Sibumasu Block in a cool environment, while the Kuantan limestones were deposited in a tropical environment on a separate plate related to the Indochina Block. In their interpretation the Vis6an Alas and Kuantan formations were deposited on separate plates, and were brought together in Sumatra by post-Carboniferous movements. This relationship is indicated on the Carboniferous palaeogeographic reconstruction of Sumatra (Fontaine & Gafoer 1989) (Fig. 4.9) by an arbitrary W N W - E S E boundary, which has no present structural expression, separating the Kuantan Formation from the outcrops of the Kluet, Alas and Bohorok formations to the north. As part of the study of the fauna and flora of Sumatra by Fontaine & Gafoer (1989), Vozenin-Serra (1989) reviewed the Jambi flora of West Sumatra and confirmed its Cathaysian affinity. Fontaine & Gafoer (1989), from the fusulinid fauna in the marine sediments interbedded with the plant beds, were able to date the Jambi flora very precisely as earliest Permian (Late Asselian to Sakmarian). The relative lack of deformation in the Mengkarang Formation, compared with adjacent isoclinally folded and cleaved Jurassic and Cretaceous units, led geologists of the Netherlands Indies

Geological Survey, who mapped the area in the 1920s and 1930s to suggest that these rocks had been overthrust into their present position as the 'Djambi Nappe' (Tobler 1917; Zwierzijcki 1930a) (Fig. 14.3). Zwierzijcki (1930a) suggested that the Djambi Nappe was emplaced during the Varangian Stage of the Cretaceous. Tobler (1917) proposed that the nappe had been overthrust from the SW, but the Cathaysian flora, together with Permian volcanics which he correlated with the Pahang Volcanics of East Malaya, led Zwierzijcki (1930a) to propose that the root zone lay in the Riau Islands to the east. In this interpretation unmetamorphosed Permian rocks of the nappe rest on a thrust plane above metamorphic rocks of the 'Schiefer Barisan' (Zwierzijcki 1930a) (Fig. 14.3). The concept of the Jambi Nappe has not been accepted in recent syntheses of the structure of Sumatra (Cameron et al. 1980; Pulunggono & Cameron 1984; McCourt et al. 1993; Hutchison 1994). The low-angle fault shown by Zwierzijcki (1930a) as the base of the Jambi Nappe was subsequently re-interpreted by Katili (1970) as a strike-slip fault (Fig. 4.13). Nevertheless, the Cathaysian flora and the similarities of the Permian sequence to that of the eastern part of the Malay Peninsula shows that in the Early Permian West Sumatra formed part of the Cathaysian continental block. It also shows that the affinities of eastern Sumatra to the Sibumasu Terrane and of West Sumatra to Cathaysia continued from the Carboniferous into the Mid-Permian, so that the the two blocks can only have come together after this period.

Metcalfe (1996) model Metcalfe has published many versions of his interpretation of the distribution of tectonic blocks in SE Asia, of which that published in the Geological Society's volume on the 'Tectonic Evolution of Southeast Asia' may be taken as representative (Hall & Blundell 1996). in this model, although Sumatra is not discussed in the

Fig. 14.3. The Jambi Nappe and the Lematang Line, from Pulunggono & Cameron (1984), after Zwierzijcki (1930a) and Katili (1970).

TECTONIC EVOLUTION text, the map showing the terranes and sutures in East and SE Asia shows the major part of Sumatra as part of the Sibumasu Terrane (Metcalfe 1996) (Fig. 4.14). In the Malay Peninsula the BentongRaub Line, which separates the Indochina/East Malaya from the Sibumasu Terrane bisects the peninsula from north to south. Metcalfe (2000) describes the Bentong-Raub Line as a 13 km wide zone made up of ribbon-bedded cherts, schists and elongated bodies of serpentinized mafic and ultramafic rocks. A characteristic feature is the occurrence of bodies of m~lange composed of blocks of chert, limestone, and volcanic and volcaniclastic rocks in a fine-grained mud/silt matrix. The cherts contain radiolarian faunas that range in age from Late Devonian to youngest Early Permian; the limestones contain conodonts of Early to Late Permian age (Spiller & Metcalfe 1995). No Triassic clasts have been found in the melange. The Bentong-Raub line is regarded as the suture zone marking the site of subduction of a Devonian to Late Permian ocean, Palaeotethys, which once separated the Indochina Block from the Sibumasu Terrane. The suture also marks the site of the collision of the two adjacent crustal blocks. Collision occurred following the Late Permian, the age of the youngest rocks incorporated in the suture zone, and had been completed by the Late Triassic, the age of the Malayan Main Range Granites which are intruded into the suture zone (Metcalfe 2000). There has been no consensus concerning the extension of the Bentong-Raub Line southwards into Sumatra. Several alternative positions have been proposed using different criteria. The problem is that nowhere in Sumatra is there exposed a zone that has the characteristics of the Bentong-Raub Line. Hamilton (1979) based the position of the line on the western limit of the tin granites in Malaya, but further granites have been found to the west of this line (see Chapter 5). Tjia (1989) suggested that the Bentong-Raub Line crosses the Malacca Strait, passes into the Bengkalis Graben, seen on oil company seismic data in the Central Sumatra Basin, and abuts against the Tigapuluh massif. On Metcalfe's (1996) map the Bentong-Raub Line is shown continuing into Central Sumatra, following the Bengkalis Graben, as proposed by Tjia (1989) and then turns sharply to the NW, following the boundary, proposed by Fontaine & Gafoer (1989), between the Kuantan Formation and Carboniferous rocks of the Tapanuli Group to the north. As already pointed out this line has no structural expression in Sumatra. Metcalfe's (1996) map shows the Sibumasu Terrane extending northwards from eastern Sumatra through the Langkawi Islands and Perlis, the adjacent part of Peninsular Malaya, Phuket in Peninsular Thailand, Mergui and Tenessarim on the west coast of Burma and through eastern Thailand to Southern China (Fig. 14.4). All these areas are characterized by the occurrence of the glacigenic pebbly mudstones. In Sumatra, Metcalfe's (1996) map shows a group of microcontinental blocks, the Woyla Terranes, on the southwestern margin of the Sibumasu Terrane. Metcalfe (1996, Fig. 2), follows Cameron et al. (1980), in identifying these terranes as the Sikuleb, Natal and Bengkulu terranes. This problem has been discussed by Barber (2000 and in Chapter 4) who concludes that there is no convincing evidence for microcontinental blocks in these areas.

Hutchison (1994) model The whole problem of the distribution, relationships and tectonic history of the Gondwana and Cathaysian terranes in Sumatra and the Malay Peninsula has been reviewed by Hutchison (1994). He recognizes three terranes in the Malay Peninsula and Sumatra (Fig. 14.5). The East Malaya Terrane in the east, linked to Indochina and South China, is characterized by limestones with fusulinids in the Lower Permian, Mid-Late Permian arc volcanics and an Upper Permian Cathaysian flora at Jengka Pass

237

Fig. 14.4. Accreted terranes in SE Asia after Metcalfe (1996).

and Linggiu. East Malaya is separated from the Sinoburmalaya Terrane to the west by the Medial Malaya Line ( = B e n t o n g Raub Suture of earlier literature). The use of the new terminology is due to the recognition of a 'Palaeotethys Suture Zone' shown as occupying much of the western part of the Malay Peninsula, marking the collision zone between East Malaya and the Sinoburmalaya terranes (Hutchison 1994) (Fig. 14.5). To the east, Sinoburmalaya (cf. Sibumasu of Metcalfe 1996) is characterized by quartz sandstones, occupying the western part of the Malay Peninsula and the Malacca Strait, and tilloid (pebbly mudstone)-bearing (=Singa and Bohorok) formations to the west. Hutchison (1994) (Fig. 14.5) shows the Bentong-Raub Suture following a sinuous course through southern Sumatra, following reported occurrences of basic and ultrabasic rocks. This course leaves the islands of Bangka and Billiton in the East Malayan Terrane, consistent with the presence of Permian sediments containing schwagerinid fusulinids in the northern part of the Bangka (De Roever 1951) and offshore Billiton (Strimple & Yancey 1976) and the presence of poorly preserved plant remains, tentatively identified as belonging to the Cathaysian flora (van Overeem 1960). However, as has already been reported, in the southern part of the island, where De Roever (1951) described an arkosic conglomerate, Ko (1986) identified a 'pebbly mudstone' which may be correlated with the Bohorok Formation. Hutchison (1994) acknowledges the uncertainty of the course adopted in his model by a liberal sprinkling of question marks. If Ko's (1986) identification of the pebbly mudstone is correct the Bentong-Raub Suture must pass through Bangka, where it had been placed in several earlier syntheses (Hutchison 1975, 1983; Mitchell 1977; Pulunggono & Cameron 1984). As yet, no distinct lineament marking the trace of the BentongRaub Suture has been identified in Bangka. if the pebbly mudstones in southern Bangka and the Mentulu and Bohorok formations are correctly identified as glacial deposits then the whole of the Sinoburmalaya Terrane is clearly related to Gondwana (Northern Australia). In Hutchison's (1994) synthesis, Sinoburmalaya is separated to the SW from the West Sumatra Terrane by a Medial Sumatra Line (Fig. 14.5). In identifying the West Sumatra Terrane, Hutchison (1994) follows Fontaine & Gafoer (1989) who related the

238

CHAPTER 14

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limestone fauna of the Vis6an Kuantan Formation in Central Sumatra to those of East Malaya, Laos, Vietnam and eastern Thailand. While acknowledging that the limestones of the Vis6an Alas Formation do not contain the same fauna as the Kuantan, and that during mapping the surveyors had concluded that there were sedimentary facies transitions between the Bohorok, Kluet and Alas Formation (Cameron et al. 1980), Hutchison (1994), extends the West Sumatra Terrane northwards to include the outcrops of the Kluet and Alas formations (Fig. 4.15). He suggests that the Medial Sumatra Line is a major

KluangLimestone (age unknown) Mutus Assemblage (of unknown age)

Fig. 14.5. Tectonic units which have amalgamated to make up Sumatra and the Malay Peninsula, after Hutchison (1994).

strike-slip fault, parallel to the Main Sumatran Fault, which brought the Alas and Kluet formations into juxtaposition with the Bohorok Formation (Hutchison 1994). Hutchison (1994) suggested that this strike-slip fault movement occurred during the Cenozoic, but the Middle to Upper Triassic Kualu and Tuhur Formations, with similar lithologies and faunas, occur on either side of the fault suggesting that the two plates were juxtaposed prior to the mid-Triassic. Hutchison (1994) strengthens his case for the recognition of a Cathaysian West Sumatra Terrrane by incorporating the Lower

TECTONIC EVOLUTION

Permian Jambi Series (Zwierzijcki 1930a) (cf. 'Jambi Nappe' refered to above), which includes the Menkarang Formation containing a tropical Cathaysian flora and interbedded fusulinidbearing limestones, in this terrane. The Mengkarang Formation is associated with volcanic rocks of the Palepat and Silungkang formations forming a NW-SE-trending belt along the southwestern margin of the West Sumatra Terrane (Fig. 4.15). Following Pulunggono & Cameron (1984), Hutchison (1994) identifies an outlier of this volcanic belt in the volcanics of the Situtup Formation near Takengon, where the limestones have yielded mid-Permian fusulinids of Cathaysian type. As Hutchison (1994) points out, the West Sumatra and East Malaya terranes 'have similar volcanic arc characteristics, are rich in fusulinid limestones and contain a Cathaysian flora, but all these features are of different age'. The West Sumatra Terrane is not therefore demonstratively a detached part of the East Malaya Terrane, although both were evidently once part of Cathaysia. Hutchison (1994) follows Pulunggono & Cameron (1984) in identifying the Mutus Assemblage, here shown as separating the quartzites and pebbly mudstones through Central and southern Sumatra (Fig. 14.5). Reasons have been given earlier in this volume for interpreting this assemblage as a zone of deeper water sediments occupying the site of a Triassic extensional rift. Also, in southern Sumatra Hutchison (1994) illustrates the subcrop of the Kluang Limestone identified from borehole

239

records (De Coster 1974). De Coster (1974) suggested a Cretaceous age for this massive limestone formation. Hutchison (1994) by analogy with the Kuala Lumpur Limestone in Malaya suggests a Silurian age. From the position of this occurrence, along strike to the southeast of the outcrop of the Kuantan Formation, a more reasonable correlation is with limestone units of the Carboniferous Kuantan Formation, as suggested earlier in this volume (see Fig. 4.18).

Revised tectonic model for Sumatra Barber & Crow (2003) have presented a revised plate tectonic model for the tectonic development of Sumatra, modified from earlier models in the light of the data and the discussion above, and this model is further refined in the present account. The Carboniferous, Permian and Triassic stratigraphy of the eastern part of Sumatra is illustrated diagrammatically in Figure 14.6 where it is correlated with the stratigraphy of West Malaysia and Thailand, and with NW Australia. The characteristic features of the stratigraphy of eastern Sumatra are the occurrence of Vis6an temperate floras and faunas in the limestones of the Alas Formation, the tilloids of the Bohorok and Mentulu formations and the Pangururan Bryozoan Bed in the Lower Permian. These features link eastern Sumatra firmly to the rest of Sibumasu in

Fig. 14.6. Comparison of the Carboniferous, Permian and Triassic sequences in the Sibumasu terranes of eastern Sumatra (after Cameron et sheets), West Malaysia and Thailand (after Metcalfe 2000) and the Gondwana Terrane in NW Australia (Roberts & Veevers 1973).

al.

1980 and GRDC map

240

CHAPTER 14

West Malaysia, Peninsular Thailand and areas further north in Southeast Asia (Metcalfe 1996). Sibumasu has long been considered to have been originally attached to Gondwana in the region of NW Australia and to have separated in the Late Carboniferous to Early Permian (e.g. Metcalfe 1996) (Fig. 14.1). Faunas in the Sibumasu Terrane in Thailand and western Malaysia during the Palaeozoic, from the Late Cambrian to the Early Permian from many fossil groups, show very close affinities to the Palaeozoic rocks of Western Australia, right down to the species level (e.g. Cambro-Ordovician trilobites, Ordovician nautiloids--Burrett & Stait 1985; Devonian fishes--Burrett et al. 1990 and Permian brachiopods--Shi & Waterhouse 1991). This correspondence between the faunas continues into the Early Permian but has broken down by the Late Permian. It is not surprising therefore that a correlation can be made between eastern Sumatra and the Bonaparte Gulf region of northwestern Australia in the Carboniferous and Early Permian (Roberts & Veevers 1973) (Fig. 14.6). In the Bonaparte Gulf the Lower Carboniferous, Tournaisian and Visdan sequence encountered in boreholes in the offshore area is composed of dark shales and siltstone of the Bonaparte Beds, comparable to the turbidite sequence in the Bohorok Formation of northern Sumatra. The Tamnurra Formation includes algal and oolitic limestones of late Vis6an age, comparable to the Alas Formation of Sumatra. These limestones are followed by sandstone, siltstones and crinoidal limestones of probable Namurian and Westphalian ages. No rocks of these ages have been recognised in Sumatra, but might well be hidden among the unfossiliferous sandstones and shales of the Bohorok Formation. At the top of the Bonaparte Gulf sequence the Lower Permian Kulshill and Sugarloaf formations contain glacial tillites, as well as sandstones, shales and minor coals, which may be correlated with the Bohorok and Mentulu formations of Sumatra. As has already been pointed out the Carboniferous to Early Permian Tapanuli Group in eastern Sumatra is interpreted as showing a continental margin sequence with littoral facies in the east passing into deeper water towards the west (Cameron et al. 1980; Fontaine & Gafoer 1989). The Carboniferous to Lower Permian sequence in the Bonaparte Gulf region of NW Australia has the opposite polarity, with terrestrial-littoral facies on shore, passing into deeper water facies in boreholes offshore to the north (Fig. 14.7). The Bonaparte Gulf sequence is the mirror image of the Tapanuli Group, suggesting that these sequences developed on the opposite sides of the same opening gulf. The presence of tillites in both Sibumasu and northwest Australia suggests that Sibumasu did not finally separate from the margin of Gondwana until after the Early Permian. In Figure 14.8 the Carboniferous to Early Permian stratigraphy of eastern Sumatra is contrasted with that of western Sumatra. The temperate Vis6an fauna of the Alas Limestone Formation contrasts with the tropical Vis~an fauna and flora of the limestones in the Kuantan Formation, tilloids are absent in western Sumatra and the Permian sequence contains a Cathaysian flora and voluminous volcanics. These features link western Sumatra to the East Malaya Terrane which also contains a Cathaysian fauna and voluminous volcanics (Fig. 14.8), although as Hutchison (1994) points out, these are not of exactly the same age. These similarities indicate that the West Sumatra Block formed part of the Cathaysian Block, although the differences in the stratigraphy suggest that West Sumatra was not immediately adjacent to East Malaya. West Sumatra may have separated from Gondwana with the rest of Cathaysia in the Devonian. However, in the Triassic both East and West Sumatra show similar sequences, suggesting that by mid-Triassic time they had been amalgamated and formed part of the same crustal block with their present relationship. Our interpretation of the distribution of crustal blocks in the Malay Peninsula and Sumatra is illustrated in Figure 14.9. The East Malaya Block, characterized by a Cathaysian flora and

fauna, as proposed by Hutchison (1994) and Metcalfe (1996) lies to the east, limited to the west and south by the BentongRaub Line, which separates it from Hutchison's (1994) Palaeotethys Suture and the Sibumasu Block. The BentongRaub Suture Zone has been regarded as a narrow linear belt but Metcalfe (2000) has modified this concept. He reports that bedded cherts of Permian and Triassic age that were included in the Semanggol Formation in the western part of the Malay Peninsula are divisable into two units. Cherts containing Upper Permian radiolaria are tightly folded and repeated by thrusting, while cherts of Middle Triassic age in the same area do not show this deformation. He therefore suggests that the Semanggol Formation contains a major unconformity with Upper Permian cherts forming part of an accretionary complex, overlain unconformably by undeformed Middle Triassic cherts. The implication of this discovery is that the suture, marking the zone of collision between the East Malaya and Sibumasu, is a broad zone extending we!! to the west of the traditional site of the Bentong-Raub Suture (Metcalfe 2000). This conclusion was anticipated by Hutchison (1994) by his recognition of the 'Palaeotethys Suture Zone' (Fig. 14.5). This zone is shown as an 'accretionary complex' in Figure 14.9. The Lower Palaeozoic to Carboniferous stratigraphy of the Langkawi Islands, NW Malaya and adjacent parts of Peninsular Thailand is described in Gobbett & Hutchison (1973) and has recently been reviewed by Cocks et al. (2005) and Meor & Lee (2005). Shallow-water sequences with trilobite-brachiopod faunas occur in Langkawi, Perlis and Kedah to the west, while deeper water facies with graptolites and T e n t a c u l i t e s in occur in Perak to the east. These sequences represent the shelf on the eastern margin of the Sibumasu Block passing into the oceanic deposits of the Palaeo-Tethys. If the Tapanuli Group in northern Sumatra represents the western margin, then the Sibumasu Block is only 500 km wide in Sumatra and the Malay Peninsula. A major thrust mapped in the Langkawi Islands, bringing Lower Palaeozoic rocks over the Permian, and field photographs of quarry sections in northwest Malaya in Meor & Lee (2005) show that the rocks of the shelf facies are imbricated by westerly-directed thrusts. The rocks in Langkawi and northwest Malaya are also gently folded on north-south axes, the intensity of the folding increasing eastwards, until in central Malaya the folding becomes isoclinal on easterly dipping axial planes (Gobbett & Hutchison 1973). These observations suggest that the continental margin sediments of Sibumasu were deformed into a foreland fold-and-thrust belt as the result of the collision between Sibumasu and East Malaya. The apparent random ages of the rocks in the western part of Peninsular Malaysia, ranging from Devonian through Carboniferous to Lower Permian (e.g. Metcalfe 2000, Fig. 1), where no coherent sequences have been recognized, is due to deformation in the thrust belt and the accretionary complex. The map and description of the geology of the island of Bangka given by Ko (1986) suggests that the accretionary complex recognized by Metcalfe (2000) in western Malaya extends southwards into Bangka. Here, isoclinally folded and thrust Permian rocks, including radiolarian cherts, of the Pemali Group are overlain by undeformed sandstones of the Tempilang Formation. The Bentong-Raub Suture (+Palaeo-Tethys Suture Zone) and the Bangka-Billiton accretionary complex mark the collision zone along which the East Malaya and Sibumasu blocks were amalgamated. The Sibumasu Block to the west, characterized by a temperate Vis6an fauna in the Alas Formation and the occurrence of 'pebbly mudstones' in the Bohorok and Mentulu formations, extends into southern part of the the island of Bangka to include the pebbly mudstone occurrence at Toboali described by Ko (1986). As illustrated by Hutchison (1994), the West Sumatra Block, lies to the SW of the Sibumasu Block and is separated from it by the 'Medial Sumatra Tectonic Zone' (MSTZ). This zone is

TECTONIC EVOLUTION

241

Fig. 14.7. Early Permian palaeogeography of the Bonaparte Gulf region of NW Australia (after Roberts & Veevers 1973).

buried beneath Tertiary sediments in the south, but in central Sumatra is marked by outcrops of tremolite schists and associated highly deformed rocks of the Pawan and Tanjung Puah members of the Kuantan Formation (Fig. 14.9). In northern Sumatra the zone of deformed rocks including mylonites (Cameron et al. 1983), can be traced on SAR imagery through the area of outcrop of the Alas Formation as a zone of tectonic disruption and shearing. Further north the zone has been traced through outcrops of phyllites, schists and gneisses, recognized in the primary mappping, but not incorporated in the published 1:250 000 Quadrangle Sheets, through Takengon to the Andaman Sea (Cameron et al. 1983; Keats et al. 1981) (see Fig. 13.10). In several places along the outcrop the MSTZ includes relatively undeformed Middle to Upper Triassic sediments and is interpreted as an Early Triassic shear zone along which the West Sumatra Block was emplaced against the Sibumasu Block.

The West Sumatra Block is characterized by a tropical Vis6an fauna in the Kuantan limestones, Early Permian volcanics in the Palepat and Silungkang formations, and an Early Permian Cathaysian flora (Jambi Flora) in the Mengkarang Formation. The block also includes the fossiliferous Middle Permian limestones of Silungkang, Ngaol and Pendopo. In Figure 14.9 the block is shown extending to the NW to include the Sibolga Granite, considered to form part of the Early Permian magmatic arc, the Kluet Formation and the Situtup Formation. In an earlier interpretation (Barber & Crow 2003), following Cameron et al. (1980) we considered that the Kluet and Alas Formations in northern Sumatra formed part of the Sibumasu Block, because of the temperate fauna in the Alas Formation. However, the recognition of a major structural lineament (MSTZ) passing through the outcrop of the Alas Formation has led us to reconsider this interpretation, and to include in the West Sumatra Block that part of the Kluet

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CHAPTER 14

Fig. 14.8. Comparison of the Carboniferous, Permian and Triassic sequences of the eastern Sumatra Sibumasu Terrane, after Cameron et al. (1980) and GRDC map sheets, the Indochina Terranes of West Sumatra (after GRDC map sheets) and the eastern Malay Peninsula, alter Hutchison (1994) and Metcalfe (2000).

outcrop which lies to the west of the MSTZ, and was found to be indistinguishable from the Kuantan Formation during the mapping (Aspden et al. 1982b). Because of its temperate fauna the Alas Formation itself is still considered to form part of the Sibumasu Block. The Situtup Formation with its typical Middle Permian Cathaysian fusulinids is considerd to form part of the West Sumatra Block, as Pulonggono & Cameron (1984) and Hutchison (1994) already proposed. There is no necessity to regard these outcrops as klippen overthrust on the Sibumasu Block as we previously suggested (Barber & Crow 2003). Further to the SW, and occupying the whole of the western part of Sumatra, is the volcanic island arc and imbricated ocean floor materials of the Jurassic-Cretaceous Woyla Group, thrust over the western margins of the Sibumasu and West Sumatra blocks in the 'Woyla Nappe' which will be described in the following section.

Permo-Triassic palaeogeographic reconstructions In Figure 14.10 a series of cartoons represents the major tectonic events in the development of Sumatra during the Late Carboniferous, Permian and Early Triassic. According to Seng6r et al. (1988) and Metcalfe (1996) the blocks which constituted Cathaysia, North and South China and Indochina/East Malaya separated

from the northern margin of Gondwana with the development of the Palaeo-Tethys in the Devonian. By the Early Carboniferous Cathaysia, with the West Sumatra Block forming part of its southern continental margin, lay in tropical latitudes. The continental margin sediments are represented by the Kuantan Formation with its tropical Vis~an coral-algal fauna and flora. The section in Figure 14.10a shows the situation in the Early Permian with the West Sumatra Block attached to Cathaysia. At this stage subduction of the Palaeo-Tethys had commenced beneath the southern margin of Cathaysia, generating an Andean-type magmatic arc in the West Sumatra Block. The arc is represented by intrusive granites, volcanic rocks and associated sediments with their tropical faunas and floras, of the Palepat, Mengkarang and Silungkang Formations. The geochemistry of the Early Permian granitic and volcanic rocks has not yet been studied in detail, so that it is possible that this magmatism is related to the separation of the West Sumatra Block. Subduction, with related volcanism, also commenced in the Early Permian along the section of the Cathaysian margin represented by the East Malaya, but it is unlikely that the West Sumatra Block lay immediately adjacent to East Malaya, as in East Malaya volcanism continued into the Late Permian, but in West Sumatra ceased in the mid-Permian. The section in Figure 14.10b illustrates the separation of the Sibumasu Block from Gondwana in NW Australia during Late Carboniferous and Early Permian times by extension, rifting and

TECTONIC EVOLUTION

243

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the formation of new oceanic crust on the floor of the opening rift. This new ocean crust formed part of Meso-Tethys. Volcanism related to this extension may be represented by metabasics in the Bohorok and Mentulu formations. The separation of Sibumasu occurred at a time when northern Gondwana was covered by continental glaciers and ice sheets. It is visualized that ice sheets extended as ice shelves across the opening gulf. As the ice shelves and icebergs melted they released boulders and finer grained materials to form tillite deposits on the developing continental shelves in the Bonaparte Gulf area of NW Australia and the 'pebbly mudstones' of the Bohorok Formation in Sibumasu. During the Permian Sibumasu drifted northwards into a more temperate environment as Meso-Tethys expanded (Shi & Archbold 1995). The situation described above is illustrated in a palaeogeographical map of the northern margin of northern East Gondwana and the SE Asia terranes for the Early Permian (Fig. 14.11).

Sibumasu, at high latitudes between 50 ~ and 60~ is shown beginning to separate from NW Australia and 'Argoland', a block which separated from Australia in the Late Jurassic and identified by Metcalfe (1996) as West Burma, with the development of Meso-Tethys. The opening gulf extended into the region of Timor and the Bonaparte Gulf as two rift systems forming aulacogens (Charlton 2001). To the north, Sibumasu was separated from Cathaysia (Indochina Block) by the Palaeo-Tethys which was being subducted beneath the southern and western margins of Cathaysia. The broad Palaeo-Pacific extended to the north of Cathaysia and Gondwana. In the sequence of events postulated by Seng6r et al. (1988) and Metcalfe (1996) for the separation of continental blocks from Gondwana, West Sumatra, like the other Cathaysian blocks had separated at an earlier stage and now lay to the north of PalaeoTethys and therefore to the north of Sibumasu. In Figure 14.11 West Sumatra, with its Jambi Flora, is shown linking Cathaysia

244

CHAPTER 14

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to Gondwana in the region of West Papua, the Bird's Head and the Sula islands. These were all areas of subduction-related magmatism in the Early Permian (Charlton 2001), and the occurrence of mixed Gondwana and Cathaysian floras in West Papua (Irian Jaya) (Li & Shen 1996; Rigby 1998) suggests that Cathaysia and Gondwana were linked at this point. The Cathaysia flora is considered to indicate a tropical to subtropical environment, the Jambi flora, for instance does not show annual tree rings. West Papua and West Sumatra are therefore shown at between 30 <' and 40~ latitude. Charlton (pers. comm. 2002) has suggested that warm ocean currents in the Palaeo-Pacific may also have ameliorated the climate. The problem to be addressed is: how did the West Sumatra Block arrive in its present position on the southern side of Sibumasu? The only plausible explanation is that proposed by Hutchison (1994): that West Sumatra arrived in its present position outboard of the Sibumasu Block by strike-slip faulting along the Medial Sumatra Tectonic Zone. The position of this zone is indicated in Figure 14.10(a, c). A model for the translation of continental blocks along active continental margins is provided by the history of Wrangellia, translated along the Pacific margin of North America by oblique subduction during the Late Mesozoic and Cenozoic (e.g. Coney et al. 1980).

Tethys

Fig. 14.10. (a) Cartoon to show the relationship between Palaeo-Tethys, the West Sumatra Block and Cathaysia (Indochina Block) in the Early Permian. (b) The break-up of northern Gondwana (NW Australia) and Sibumasu and the formation of the Meso-Tethys in the Late Carboniferous to Early Permian. (c) Sibumasu collides with East Malaya along the Bentong-Raub Suture and West Sumatra is emplaced against Sibumasu by strike-slip faulting along the Medial Sumatra Tectonic Zone in the period from the Late Permian to the Early Triassic.

From a study of the brachiopod faunas from southern Thailand and the Kinta Valley region of Perak, Peninsular Malaysia, Shi & Waterhouse ( 1991 ) demonstrate that there was a very rapid change in the climatic conditions in Sibumasu, in Early Permian times, from cold temperate to subtropical. This change occurred relatively abruptly over a few million years within the Sakmarian, indicating either that there was a dramatic shift in climatic zones, or that Sibumasu underwent a very rapid northwards translation in the Early Permian, or possibly both. A similar climatic change is seen in the West Australian Basins, suggesting that there was a general climatic amelioration in early Permian times, and palaeomagnetic evidence shows that Australia as whole moved northwards away from the pole in the period from the Early Permian to the Jurassic (Klootwijk 1996). In the palaeogeographic reconstruction for the Mid-Permian (Fig. 14.12) it is suggested that Sibumasu moved rapidly northwards with the expansion of the Meso-Tethys. The Meso-Tethys is also shown extending northwards, to separate West Sumatra from northern Gondwana. A connection was made between the Meso-Tethys and the Palaeo-Pacific across a transform fault. A large part of Palaeo-Tethys had now been subducted beneath Cathaysia, and the northern margin of Sibumasu was approaching the southern margin of Cathaysia.

TECTONIC EVOLUTION

Fig. 14.11. Palaeogeographic map of NE Gondwana and the SE Asian terranes in the Early Permian.

Fig. 14.12. Palaeogeographic map of NE Gondwana and the SE Asian Terranes in the Mid-Permian.

245

246

CHAPTER 14

Fig. 14.13. Palaeogeographic map of NE Gondwana and the SE Asian Terranes in the Late Permian.

Fig. 14.14. Pataeogeographic map of NE Gondwana and the SE Asian Terranes in the Early Triassic.

TECTONIC EVOLUTION In Figure 14.10c it is suggested that during the Late Permian or the very Early Triassic the final segment of Palaeo-Tethys, which lay between the Sibumasu Block and Cathaysia, was subducted beneath Cathaysia until East Sumatra and West Malaya (Sibumasu) collided with East Malaya (Indochina). This is also illustrated in the Late Permian palaeogeographic map (Fig. 14.13). The site of the collision is marked by the Bentong-Raub Suture and its western extension in the Semanggol-Bangka accretionary complex. Following collision, the site of the collision zone was invaded by granite plutonism, accompanied by tin mineralization. Hutchison (1994) suggested that translation of the West Sumatra Block into its present position, outboard of Sumatra, occurred during the Cenozoic, but the continuity of Middle to Upper Triassic sediments across the West Sumatra Block, the MSTZ, Sibumasu and East Malaya indicates that these blocks had their present relationships before Mid-Triassic times. The translation

247

of West Sumatra to its present position must therefore have occurred in very Late Permian or Early Triassic times; as pointed out earlier in this account there is no record of sediments of this age anywhere in Sumatra. Accordingly, in the palaeogeographic map for the Early Triassic (Fig. 14.14), West Sumatra is shown displaced westwards from its position at the far eastern extremity of Cathaysia, along a trancurrent strike-slip fault (MSTZ), driven by seafloor spreading in the Meso-Tethys, to arrive in its present position against East Sumatra. During the Mid- and Late Triassic, the whole of Sumatra and Peninsular Malaya were subjected to N E - S W extension, with the formation of several north-south and N W - S E graben structures, the Kualu and Tuhur basins in Sumatra, and the Semantan and Semanggol Basins in Malaya, separated by intervening horst blocks (Fig. 14.15). As the result of extension the whole area, apart from East Malaya, subsided below sea level.

Fig. 14.15. Palaeogeographicmap of Sumatra and the Malay Peninsula in the Mid- and Late Triassic.

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CHAPTER 14

Carbonates were deposited on the horst blocks, while the graben, cut off and far from sources of terrigenous sediment, accumulated bedded cherts and thin shales. The record of Middle to Upper Permian cherts in the Semanggol Formation (Sashida et al. 1995), which suggests that the Semanggol Basin originated at an earlier stage than envisaged here, has been explained by Metcalfe (2000), who reports that the Permian cherts and were deposited on the ocean floor, incorporated in the accretionary complex were deformed by the collision event, while Middle to Upper Triassic cherts in the same area, were deposited in a successor basin and show only tilting and open folding. Towards the end of the Triassic, uplift of the eastern part of the Malay Peninsula, perhaps associated with the intrusion of the granites, provided a source of terrigenous sediments. Turbiditic sands and shales were deposited in the graben, the sands becoming coarser and more conglomeratic towards the end of the Triassic in the more easterly of the graben.

intervening marginal basin (Cameron et al. 1980). Subsequently Wajzer et al. (1991) and Barber (2000) presented arguments for interpreting the Woyla Group as an intra-oceanic arc with an associated accretionary complex constructed in the Meso-Tethys to the west of Sumatra. In the original model, based on the evidence from northern Sumatra, a single subduction system beneath the arc was visualised (Barber 2000). Subsequently it was appreciated that a contemporaneous magmatic arc, marked by granitic intrusions, was present in central and southern Sumatra. Therefore a revised model for the origin and accretion of the Woyla Group to the margin of Sundaland is proposed here, with a double subduction system (Fig. 14.16a); a modern analogue would be the Molucca Sea (Hall 2002, fig. 10). Mid-Jurassic-Early Cretaceous Andean Arc

Following the strike-slip emplacement of the West Sumatra Block in the Early Triassic, a segment of the Meso-Tethys Ocean lay off the western coast of Sundaland (Fig. 14.14). This ocean had originated in the Permian by the separation of the Sibumasu continental block from the northern margin of Gondwana. In the Mid-Jurassic the Meso-Tethys began to subduct eastwards beneath the western margin of Sumatra, with the accretion of ocean floor materials against Sumatra (Fig. 14.16a). As mentioned above evidence for this phase of subduction is provided by the remnants of an Andean arc in central Sumatra, identified by a belt of Mid-Jurassic to Early Cretaceous subduction-related I-type granitoid intrusions McCourt et al. (1996) (Fig. 14.16a). These intrusions include the Sulit Air Suite (203 + 6 Ma) and the Bungo Batholith (169 + 5 Ma) (see Chapter 5). Volcanic rocks related to this Andean arc may be represented by the andesitic and basaltic lavas and volcaniclastic sediments in the Jurassic-Cretaceous Rawas, Tabir and Siulak formations of central Sumatra and the tufts in the Menanga Formation of southern Sumatra.

The Woyla Nappe and the Mesozoic evolution of the Sundaland margin The Jurassic-Cretaceous Woyla Group, composed of an 'arc assemblage' of volcanics with associated carbonates, and an 'oceanic assemblage' of imbricated ocean floor materials, occurs in the Barisan Mountains and extends all along the west coast of Sumatra. These rocks are refered to as the Woyla Terrains in Figure 14.2 (Pulunggono & Cameron 1984), but in this account, since they are considered to be thrust over the western margin of Sumatra, they are described as the Woyla Nappe (Fig. 14.9). As has been explained earlier in this volume it was originally considered that the volcanics had formed on a sliver of continental crust that had separated from the margin of Sumatra and had collided with the Sumatran margin with the collapse of the

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Fig. 14.16. Conceptual cross-sections to illustrate the origin of the Woyla Terranes and their role in the evolution of the southwestern margin of Sundaland in the Late Mesozoic.

TECTONIC EVOLUTION

Late J u r a s s i c - m i d - C r e t a c e o u s oceanic island arc

In the Late Jurassic, subduction also commenced towards the west, probably initiated along a north-south transform fault within Meso-Tethys, generating a mid-oceanic island arc constructed on oceanic crust (Fig. 14.16a). The island arc volcanics are represented by basalts and andesites of the 'arc assemblage' of the Bentaro Formation in Aceh (Bennett et al. 1981a). Ocean-floor material was imbricated into an accretionary complex against the island arc to form the 'oceanic assemblage' of the Woyla Group. The oldest material found within the Woyla Group is a limestone block containing Triassic foraminifers found in m~lange in Natal. This block is considered to be a remnant of the carbonate capping of a seamount within Meso-Tethys that has been subducted (Wajzer et al. 1991). The youngest fossils found in units correlated with the Woyla Group, are the AptianAlbian foraminifer Orbitulina, which occurs in the Menanga Formation near Bandarlampung and the Sepingtiang Limestone Formation of the Gumai Mountains. Evidently 'Woyla Ocean' lasted from the Triassic to the late mid-Cretaceous, when the last remnants were subducted. By the early Late Cretaceous, due to the combination of subduction beneath the island arc and subduction beneath Sumatra, the segment of the Meso-Tethys which lay originally between the oceanic island arc and West Sumatra had been completely subducted into the mantle. The island arc and its associated accretionary complex then collided with, and was thrust over, the margin of Sumatra to form the Woyla Nappe (Fig. 4.34b). Fragments of this mid-oceanic volcanic arc are now represented in Sumatra by the extensive Bentaro Formation of Aceh, arc volcanics in the Batang Natal section, arc volcanics at Indarung (McCarthy et al. 2001), the Saling and Garba formations in the Gumai and Garba Mountains (Gafoer et al. 1986, 1994). It is suggested that arc collision and the emplacement of the Woyla Nappe over the western margin of Sumatra produced an amphibolite facies metamorphic footprint in the Kluet Formation in the neighbourhood of Tapaktuan. It may also have been responsible for folding and the development of slaty cleavage in pelitic rocks throughout the Tapanuli Group in northern Sumatra and the Kuantan Formation and Tigapuluh Group in Central Sumatra. Folding and cleavage development in the Asai, Rawas and Peneta formations, the Jurassic-Cretaceous forearc basin deposits in central Sumatra can also be attributed to this event (cf. the 'Jambi Nappe' Fig. 14.3). The collision may also have been responsible, as a 'far-field' effect, for folding Triassic rocks in the island of Bangka and the Semanggol and Semantan basins in the Malay Peninsula (Fig. 14.16b).

Late Cretaceous continental margin

With the accretion of the island arc to the southwestern margin of Sundaland, subduction of the Meso-Tethys oceanic plate recommenced outboard of the Woyla Terrane (Fig. 14.16b). This is the situation illustrated in Figure 14.17 where the Woyla Group arc assemblage and oceanic assemblage are returned to their original position along the Sundaland margin by reversing the postMiocene dextral movements along the Sumatran Fault system. On Sundaland, the development of a Late Cretaceous magmatic arc, represented by granitoid intrusions from Sikuleh to Sekampung, provides evidence for continued subduction outboard of the accreted terranes. All these granitoid intrusions are of I-type and were intruded through continental crust (McCourt et al. 1996). This Late Cretaceous arc lies oceanward of the preceding mid-Jurassic to Early Cretaceous arc, and is largely intruded through the recently accreted arc and oceanic asemblages of the Woyla Group and its equivalents (Fig. 14.16b). In Aceh the younger element of the Sikuleh Batholith (97 Ma) is intruded into the Bentaro Arc (Bennett et al. 1981b), in Natal the

249

Manunggal (87 Ma) and Kanaikan batholiths are intruded into the oceanic assemblage, including mantle peridotite, of the Woyla Group (Rock et al. 1983). In the Gumai and Garba Mountains the accreted oceanic assemblage and the arc rocks are thrust over the West Sumatra Block and are now situated to the NE of the Sumatran Fault Zone. In Gumai granitic rocks are intruded into the Saling Formation of the arc assemblage and in the Garba Mountains the Garba Pluton (115-90 Ma) is intruded into both arc and oceanic material, and also into the Tarap Formation, regarded as metamorphosed Palaeozoics belonging to the West Sumatra Block (Gafoer et al. 1994). In Bandarlampung the Sulan Pluton ( l l 3 M a ) and the Sekampung Complex (89Ma) are intruded into the Gunungkasih Complex, again interpreted as Palaeozoic basement rocks of the West Sumatra Block (Amin et al. 1994b; Andi Mangga et al. 1994a). As has been described earlier in this account, detailed observations in the Sekampung Gneiss Complex provide evidence that granitic and basic rocks of the Late Cretaceous arc were intruded into an active shear zone, suggesting that the Late Cretaceous arc, like the present arc, was developed during a phase of oblique subduction and was intruded into an active transcurrent fault system (Fig. 14.17). Reports of 'flow foliation' in the Sikuleh Batholith (Bennett et al. 1981b) and of gneissose rocks in other Late Cretaceous plutons, may also have the same significance. Kinematic indicators at Sekampung show that this transcurrent fault system operated in a sinistral sense, in the opposite sense to the present system. This interpretation is illustrated in Figures 14.16b and 14.17.

Tertiary palaeogeography of Sumatra Following the emplacement of the Woyla Nappe in the late midCretaceous the whole of Sumatra appears to have been exposed to subaerial erosion, as no Late Cretaceous or early Palaeogene sediments have yet been recognized in situ, and the earliest Tertiary rocks rest unconformably on all the older units. However, volcanic activity occurred during this period, represented by the Kikim Volcanics from which Palaeocene ages have been obtained, cropping out in the Garba Mountains and encountered beneath Tertiary sediments in oil company boreholes in southern Sumatra (see Chapter 8). From the review of Tertiary stratigraphy earlier in this volume (Chapter 7), a series of paleogeographic maps and cross-sections have been prepared for the Tertiary of Sumatra (Figs 14.18 and 14.19). Reconstructions of Tertiary palaeogeography have previously been published by Adinegoro & Hartoyo (1974), covering the northern part of Sumatra. Company reports have sometimes included palaeogeographic reconstructions for localized areas (e.g. Koning & Aulia 1985; Whateley & Jordan 1989). The present reconstructions cover the whole of Sumatra, including the offshore islands. The maps have been refined in the light of later published work and also take into account present understanding of the effects of movements along the Sumatran Fault System which were taking place contemporaneously with the deposition of the Tertiary sediments (Curray 1989; McCaffrey 1996; McCarthy & Elders 1997).

The e m e r g e n c e o f the B a r i s a n M o u n t a i n s

From evidence of metamorphic rocks in Tanahbala (Nas & Supandjono 1994), Eocene Nummulitic limestone clasts in Nias (Douville 1912) and Oligocene conglomerates and quartz sandstones, derived from the Sumatran mainland, or from within the forearc area itself, in Nias (Moore & Karig 1976; Samuel et al. 1997), the pre-Tertiary basement of Sundaland, extends as far as the present forearc islands (Fig. 14.18a). Apart from a brief marine incursion during the Eocene in which Nummulitic

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Fig. 14.17. The structure of the southwestern margin of Sundaland in Late Cretaceous times, according to the interpretation given in the text. Data from sources quoted in the text. MSTZ: Median Sumatra Tectonic Zone. Note that the effects of post-Mid-Miocene movements along the Sumatran Fault Zone have been removed.

limestones were deposited, the basement, most probably the distal part of the Woyla Nappe, was exposed to erosion throughout the Late Cretaceous and Early Palaeogene. In the Late Eocene and Early Oligocene, the formation of horsts and graben controlled stratigraphic developments. Sedimentation occurred in isolated rift basins which developed within the basement and received sediments eroded from the local horsts. These rifts extended across the area of the present Barisan Mountains (Ombilin Basin) into the forearc region (e.g. Bengkulu). This same history is evident throughout much of Southeast Asia at this time with the development of rift basins in the Sunda Shelf, Borneo, the Malay and Gulf of Thailand Basins (Longley 1997; Hall & Morley 2004). This regional extension coincided with the collision of India with the southern margin of the Asian continent and has been attributed to the extrusion and rotation of continental blocks to the southeast of the site of collision (Tapponnier et aL 1982) (see discussion Chapter 13). During the Horst and Graben Stage (Fig. 14.18a, b) deposition in Sumatra was characterized by sediment transport over short

distances, while subsidence in the graben was faster than sediment input, leading to the accumulation of thick organic-rich lake deposits with sedimentologically immature sediments along the lake shorelines. In Sumatra this localized distribution of the sediments in the rift stage is reflected in a localized stratigraphic nomenclature. Although the thick euxinic lake deposits and paralic deposits in the graben play an important role in the petroleum geology of the backarc basins, the development of the graben preceded the formation of the present basins. In the latest Oligocene (Fig. 14.18b), there was a major change in regional geography. Regional sediment source areas and broad depositional areas replaced the former horst and graben landscape. In addition to the major source area to the NE, in the Malayan Shield, the Barisans provided one of the sediment sources. This conclusion is supported by the significant amount of volcaniclastic material in latest Oligocene sediments and by the occurrence of sedimentologically immature deposits of this age in the foothills of the Barisan Mountains. The stratigraphy reflects the development of wider basins that extended across both grabens and horsts alike,

TECTONIC EVOLUTION

251

Fig. 14.18. (a-d) Palaeogeographic maps for the Tertiary of Sumatra. The development of forearc and backarc basinal areas, separated by the Barisan Mountains occurred in the latest Oligocene to earliest Miocene. Regional sag resulted in the gradual submergence of the Barisan Mountains and the deepening of the basins in both the forearc and backarc areas. (e-h) Marine transgression continued until the Mid-Miocene when only a few isolated peaks of the Barisan Mountains still rose above sea level. The Barisan Mountains were uplifted and eroded from the Mid-Miocene onwards. Uplift was accompanied by marine regression and dextral movements on the Sumatra Fault System, until Sumatra gradually took on its present outline.

and interconnected river systems that transported sediments from larger and more distant source areas. The thick overburden of younger sediments in the backarc basins induced maturity in organic material in petroleum source rocks within the grabens, and provided sands and limestones which constitute the main reservoir horizons for oil a n d gas. Again, similar environments extended throughout the whole of SE Asia (Longley 1997). The conclusion that the Barisan Mountains commenced their development as a major structural element in the latest Oligocene is at variance with much of the literature emanating from the petroleum industry. It is considered that Middle Miocene turbidite formations represent the first significant influx of sediments into the basins in the backarc region from the Barisan Mountains, with the major influx OCCUlTing during the Pliocene. There is no contradiction, however, between these two interpretations. In the Late Oligocene the Barisan Mountains were still restricted in height and extent. Following the marine transgression in the Early to Mid-Miocene the emergent peaks became even more restricted. The major Mid-Miocene to Pliocene influx from the mountains into the sedimentary basins was due to the re-emergence and growth of the Barisans during the following period of regression, rather than to their initial appearance. The Marine transgression during the latest Oligocene and Early Miocene (Fig. 14.18b-e) was the result of a regional sag, not only in Sumatra, but throughout much of Sundaland (e.g. in the Gulf of Thailand). In Sumatra, basins in the forearc and backarc areas

deepened so that the early Barisan Mountains were almost completely submerged, indicated by the occurrence of reef limestones in the intramontane Ombilin Basin. From the Mid-Miocene onwards (Fig. 14.18e-h), the uplift of the Barisan Mountains and the forearc island area was faster than the continuing regional sag which caused further subsidence along the axes of the backarc and forearc basins and also in the Gulf of Thailand. These movements coincided with the inversion of basin sediments during the Miocene, and continued through the Plio-Pleistocene, with the re-activation of faults, the folding of basin sediments and the development of unconformities in the sequence. These movements may be related to variations in the angle and rate of convergence in the Sumatran subduction system, leading to extension or compression in the backarc (Cameron et al. 1980). They also coincide with activity of the Sumatran Fault System in the Miocene and continued transtensional and transpressional movements along it from then until the present day. Similar inversions in other parts of SE Asia have been attributed to the rotation of Borneo (Hall 2002) or the far field effects of collisions in Eastern Indonesia.

Effects of movements along the Sumatran Fault System In the palaeogeographic reconstructions movements along the Sumatran Fault System are taken into account (Fig. 14.18).

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Fig.

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14.18. ( e - h ) Continued.

The Fault System is connected to pull apart structures in the Sunda Strait (Malod et al. 1995) in the south, along which displacements of the order of 100 km have occurred, and to the spreading centre in the Andaman Sea to the north, across which 460 km of displacement is considered to have taken place (Curray et al. 1979). Direct measurement of displacement across the fault in Sumatra has proved difficult as most stratigraphic units trend parallel to the fault trace. Possible offsets of 45 km on the basis of the displacement of Permian granites (Hahn & Weber 1981a) and of up to 100 km from displacement of Tertiary basins (Beaudry & Moore 1985) and the displacement of 150 km for the Medial Sumatra Tectonic Zone (see Chapter 13) have been postulated for various strands of the fault. It is probable that movement along the fault system have been taking place continuously at least since the Mid-Miocene (14-11 Ma), when spreading in the Andaman Sea is considered to have commenced (Curray et al. 1979). Presumably, movements along various parts of the fault system have continued from the time of initiation of the fault system until the present day. Recent movements are shown by displacement of Recent volcanics (Posavec et al. 1973), by the offset of stream courses (Katili & Hehuwat 1967), by continued seismic activity, by displacement of recent sediments along the fault trace (Sieh et al. 1994) and by GPS measurements (McCaffrey 1996; Sieh & Natawidjaja 2000). The difference in relative displacement at either end of the fault system shows that the forearc area was stretched over time and not displaced as a rigid block. Displacement increases progressively northwards and is considered to have occurred by cumulative strike-slip movements along a fault

system oriented in a S S E - N N W direction throughout the forearc region (Curray 1989; McCaffrey 1996). In the present reconstructions it is presumed that the origin of the Sumatran Fault Zone coincided with the development of Barisan Mountains and sedimentary basins in the backarc and forearc areas during the Late Oligocene. All these regional structures have a N N W - S S E trend and are overprinted over horst and graben structures that have a more north-south trend. The Barisan Mountains acted as a sediment source area from the latest Oligocene onwards and it is probable that transcurrent movements along the Sumatran Fault trend started at about the same time. A latest Oligocene age for first movements along the fault system does not conflict with a Mid-Miocene age of spreading in the Andaman Sea as documented by Curray et al. (1979) because extension with movement along the fault traces in that area may have occurred long before the commencement of ocean floor spreading. The reconstruction suggests that the forearc region has extended some 450 km northwestward, relative to the rest of Sumatra, over the last 25 Ma and that the rate of extension has been at a uniform rate of about 1.8 cm a - 1 The reconstruction of the history of movement along the Sumatran Fault System explains an obvious anomaly in the sedimentary record of northern Sumatra. In the Late Oligocene and Early Miocene the Barisan Mountains were an area of eroding terranes and shallow-water facies, while to the east deep-water marine facies prevailed in the central parts of the North Sumatra Basin (Fig. 14.18b, c). The reconstructions also explain why thick Early Miocene sandstones in the Central and South Sumatra Barisans have no equivalents in the North Sumatra

TECTONIC EVOLUTION

Basin. At that time the Barisan source area lay much further south, and prior to its northward movement along the fault there was no landmass immediately to the SW of the North Sumatra Basin which could provide a source of sediments. In their provenance study of the Mid-Miocene Keutapang Formation in the North Sumatra Basin Morton et al. (1994) found that the sediments were derived from the west or the SW. Evidently the Barisans were uplifted and in a position to act as a source for the North Sumatra Basin by Mid-Miocene times. They also found that chrome spinel was abundant in the lower part of the Keutapang Formation, but rare in the upper Keutapang. This spinel must have been derived from an ophiolitic terrain, but there is no such terrain in a suitable position at the present time. The Pasaman ophiolite is too far south, and the northern Aceh ophiolites are too far north. Either the ophiolite which supplied spinel to the lower Keutapang Formation has been removed completely by erosion, or it has been moved northwards since the Mid-Miocene by dextral movements of the order of 100 km along the Sumatran Fault System (Morton et al. 1994). The Early Oligocene palaeogeographic reconstructions also provides a more convincing geography for the southwestern margin of the Sundaland continental margin at that time (Fig. 14.18a). The removal of displacement along the Sumatran Fault Zone gives the continental margin a smoother outline, with the North Sumatra Basin and its rifted grabens lying along the Sundaland continental margin, rather than forming a basin within the continent. In this position it is clear why the North Sumatra Basin is the only basin in the present backarc area that contains Eocene continental margin deposits, including platform limestones (Tampur Limestone).

Palynspastic cross-sections

Simultaneously with the palaeogeographic reconstructions, a series of SW to NE cross-sections have been prepared across Sumatra (Fig. l 4.19). Cross-sections and palaeogeographic reconstructions are based on the same stratigraphic data set, and although different interpretations are possible, the presented cross-sections and reconstructions are compatible with the stratigraphic data. The cross-sections are a model, in which an attempt is made to find the simplest conditions that meet the stratigraphic data set (sedimentation/erosion, fluvial/marine, sediment thicknesses, source areas, proximity). The following assumptions are needed to fit the data. (1) There has been a gradual and continual growth of the Sunda accretionary complex, the forearc basins, the Barisan Mountains and the backarc basins from Late Oligocene times until the Present. (2) A regional sag of about 2 km in the Late Oligocene and Early Miocene was faster than the uplift of the Barisan Mountains. As a consequence the sea transgressed across the whole area and left only the peaks of the mountains above sea level. (3) From the Mid-Miocene onwards the rate of regional sag declined, and while the central parts of forearc and backarc basins subsided further, the Barisan Mountains began to emerge, to become an increasingly important source of sediments. (4) In the model a gradual shift of the axis of maximum uplift of the mountains over about 30 km to the NE is required, to account for the emergence and erosion of the western parts of the backarc basins, while at the same time only a few kilometres of the eastern margins of the forearc basins have been exposed. Important conclusions derived from the stratigraphic analysis and the construction of the palaeogeographic maps and sections are: the Sundaland pre-Tertiary basement extends across the area of the forearc basins to the Sumatran offshore islands; the Barisan Mountains first emerge as a structural element providing a source area for clastic sediment in the latest Oligocene, and not in the Mid-Miocene as many authors suppose. Also taken

253

into account in the reconstructions are dextral movements along the Sumatran Fault System. Replacing the displaced forearc and the southwestern segment of the Barisans in their original positions simplifies the outline of the Sundaland Margin and accounts for the occurrence of marine sediments in the early stages of the development of the North Sumatra Basin.

Tertiary rotation o f Sumatra

There has been a continuous controversy concerning the direction and the extent of rotation of Sumatra during the Tertiary. Both clockwise and anticlockwise rotations of Sumatra, together with the rest of SE Asia have been proposed. Ninkovich (1976) argued for clockwise rotation. He pointed out that the Sunda Arc between the Banda Arc and Java follows a small circle, but Sumatra is set back by 20 ~ relative to the westward projection of this small circle. He suggests that the rotation of Sumatra into its present position commenced in the Oligocene due to the locking of the subduction zone, so that Sumatra was driven northeastwards together with the Malay Peninsula, along the Klong Marui and associated strike-slip faults which cut across the peninsula, by the movement of the Indian Plate. He supported his interpretation by drawing attention to the difference in the depth to which the Benioff-Wadati Zone extends beneath Java and Sumatra. Beneath Java the Benioff Zone plunges steeply to a depth of 600 km, while beneath Sumatra it dips gently to only c. 200 kin. He therefore suggested that subduction opposite Java has been continuous over a long period, but subduction beneath Sumatra commenced at a much later date. This argument has been shown subsequently to be invalid, as the downgoing slab can be traced by tomography to a greater depth in the mantle beneath Sumatra than is indicated by the Benioff Zone (Spakman & Bijwaard 1998). Ninkovich (1976) points out that in the Oligocene subductionrelated volcanic activity was restricted to Java and southern Sumatra (i.e. Lemat Formation). Volcanicity ceased during the period of maximum transgression in the Mid-Miocene, but resumed in the Late Miocene with explosive ignimbritic eruptions (Lampung Volcanics) in the region of the Sunda Straits extending progressively northwards, to Lake Toba in north Sumatra at c. 4 - 5 Ma. Ninkovich (1976) attributes the difference in the behaviour of Java and Sumatra to their different relationship to the underlying mantle: Java was formed of accreted oceanic materials only in the Cretaceous, whereas Sumatra has been a continental block since at least the Carboniferous. A clockwise rotation was also proposed by Daly et al. (1987, 1991). In this model Sumatra is visualized as lying in an approximately east-west position along the southern magin of Eurasia in the Cretaceous with Meso-Tethys being subducted northwards beneath it. The constraint on this reconstruction is the northern azimuth of the Mesozoic palaeopole determined in the Khorat Plateau, Thailand by Maranate & Vella (1986). Daly et al. (1987) accept the model of Tapponnier et al. (1986) for the eastward extrusion of crustal blocks due the collision of India with southern Asia, and the clockwise rotation of SE Asia, leading to the Palaeogene opening of the South China Sea. During this extrusion numerous extensional basins, including the basins in the Sumatran backarc area, were formed throughout SE Asia by the differential movement of small microplates. Daly et al. (1987) postulate that the Sumatran Basins originated as pull-apart basins between major strike-slip faults, one in the position of the Sumatran Fault system, and the other in the Malacca Strait. From the evidence presented earlier in the present account this model is unsatisfactory. There is no evidence that the Sumatran Fault System was active during the Palaeogene, also, as has been emphasized the basins in the Sumatran backarc area were formed contemporaneously with the forearc basins, with a

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Fig. 14.19. Palinspastic cross-sections across Sumatra from SW to NE. Letters a - h correspond to the palaeogeographic maps in Figure 14.18. The palaeogeographic conditions for the stratigraphy of Sumatra were set when the topographic distinction between the basins in the forearc and the back arc and the Barisan Mountains emerged in the Late Oligocene and continuing to the present day. This differentiation was combined by a regional sag of the order of 2 km during the period from the Late Oligocene to Mid-Miocene. An eastward shift of the axis of uplift of the Barisan Mountains of c. 30 km has occurred since the Mid-Miocene, accounting for the broader exposure of the backarc sediments in the eastern foothills, compared with those of the forearc to the west.

TECTONIC EVOLUTION

similar orientation, and for the greater part of the Tertiary deposition was continuous across both forearc and backarc areas. There is, therefore, no necessity to propose different modes of origin for the basins in the forearc and backarc areas. In addition there is no evidence for a major strike-slip fault in the Malacca Strait. An anticlockwise rotation of the Sunda region was proposed by Holcombe (1977a, b). From a detailed geometrical analysis of the faults mapped in the Malay Peninsula and extrapolated throughout Southeast Asia, he postulated that the region of the Sunda Plate (Sumatra and West Malaysia) had changed its shape since Oligocene times, by movements along a large number of closely spaced sinistral strike-slip shears. The results of palaeomagnetic studies have not so far been of much assistance in resolving the rotation problem. Results from the Malay Peninsula are confusing, with clockwise rotations of 40 ~ reported from northern Malaya and Thailand and anticlockwise rotations reported from further south (Richter et al. 1999). In Sumatra, Haile (1979) found, from a limited number of sites, that the palaeomagnetic data indicated a clockwise rotation of 40 ~ since the Triassic. Haile's (1979) conclusions were based on only one set of Triassic samples and two sets of samples of Early Tertiary age. All of these sites lie adjacent to the Sumatran Fault Zone, and Haile (1979) makes the caveat that the results may be related to local rotations within the fault zone. Palaeomagnetic studies in Borneo, which is also considered to be part of the Sunda Plate, indicate 40 ~ of anticlockwise rotation since the Early Cretaceous and 4 5 ~ ~ between 25 and 10 Ma (Fuller et al. 1999). In his animated plate tectonic model for the tectonic evolution of SE Asia, Hall (2002) adopted the conclusions of Fuller et al. (1999) from Borneo. Hall's (2002) Early Eocene reconstruction shows Sumatra with a more north-south orientation; in later reconstructions Sumatra is shown rotating anticlockwise together with the Sunda Plate to reach its present N W - S E orientation. On the other hand, recent GPS measurements suggest that the Sunda Plate, including eastern Sumatra, is slowly rotating clockwise at a rate of c. 30 mm a - ~ with respect to the rest of Eurasia (Rangin et al. 1999). The extent to which this movement could be extrapolated back into the past is unknown. There is clearly a need for more systematic palaeomagnetic studies, particularly of Tertiary sediments in Sumatra, to resolve these ambiguities concerning the direction of rotation of the Sunda Plate and perhaps throw more light on the origin of the Sumatran backarc basins. Davies (1984) from his study of the North Sumatra Basin has made the most systematic attempt to explain the structural development of Sumatran Backarc Basins in terms of regional tectonics. He suggests that Sumatra forms the SW margin of a Sunda Microplate bounded by the Sumatran Fault System, the CiletuhMeratus and North Borneo accretionary complexes, the Thai and Malay Basins and the Ranong and Khlong Marui faults in Peninsular Thailand and the Andaman Sea. He suggests, following the earlier suggestion by Holcombe (1977a, b), that this microplate has been rotating anticlockwise throughout the Tertiary, driven initially by extension in the Thai and Malay basins, and later, after the Mid-Miocene, by extension and the formation of oceanic crust in the Andaman Sea. Davies (1984) suggests that during the Eocene, when the Indian Ocean spreading system was oriented east-west, Sumatra had a north-south orientation and India was moving past the SE Asian peninsula at a rate of c. 9 cm a -~. The northwards movement of the Indian Plate generated a series of overlapping dextral transcurrent strike-slip faults along the Sumatran margin. By the Oligocene the Indian Ocean spreading ridge had assumed its present N W - S E orientation and Sumatra had rotated so that the angle of convergence of the Indian Ocean Plate with the Sunda Plate increased, and active subduction commenced along the Sumatran margin. Differential rates of movement along the transcurrent faults set up extensional stresses along the western continental margin of Sundaland opening up the backarc basins.

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These extensional faults trended in a N N E - S S W direction as at this stage Sumatra had not yet reached its present orientation. In the Early to Mid-Miocene the area was updomed, causing widespread unconformity during the initial stages of the formation of the opening of the Andaman Sea, followed by subsidence and widespread marine transgression as the opening got underway. According to Davies's (1984) model, extension and the development of oceanic crust with the opening of the Andaman Sea, from the Late Miocene to the present day, caused further anticlockwise rotation of the Sunda Microplate so that the angle of convergence with the Indian Ocean Plate gradually increased. At the same time the Indian Ocean spreading rate increased to c. 5 cm a-~. The events caused more rapid subduction, a more active volcanic arc, compression of the margin and the uplift of the Barisan Mountains, active movement along the Sumatran Fault System and initiated regressive sedimentation in the backarc area. As Sumatra rotated, the original extensional faults which defined the horst and graben structure reached their present north-south orientation. The effect of N E - S W compression in the North Sumatra Basin was to produce structural inversion, reactivate these normal faults as reverse and strikeslip faults (transpression), and to generate positive flower structures and NE-SE folds throughout the backarc area.

Recommendations for future work on Sumatran geology High-grade metamorphic complexes and the Sumatran basement

Several areas of high-grade metamorphic rocks have been identified in Sumatra. High-grade rocks adjacent to intrusive plutons have usually been interpreted as metamorphic aureoles. Where they contain cordierite and sillimanite, or include skarns from metamorphosed limestones, this explanation is most probably correct. Some occurrences of gneissose rocks, for example the Gunungkasih Complex near Bandarlampung, were regarded as part of a Precambrian basement, but gave Cretaceous ages, and have been interpreted as syntectonic granitic intrusions (McCourt et al. 1996; Barber 2000). Earlier in this chapter it is suggested that amphibolite-facies rocks that occur along the western margin of the outcrop of the Kluet Formation near Tapaktuan have been formed by burial beneath the Woyla Nappe. This hypothesis could be tested by isotopic dating to determine whether or not these rocks were metamorphosed during the Cretaceous. It possible that some of these occurrences of amphibolite-facies schists and gneisses represent the pre-Carboniferous crystalline basement of Sumatra. High-grade metamorphic rocks associated with the unmetamorphosed limestones in the Alas Formation are probably the best candidates for representatives of such a basement. According to the interpretation put forward earlier in this chapter, these gneisses and schists occur within a major shear zone (Medial Sumatra Tectonic Zone) along which the West Sumatra Block was juxtaposed with the Sibumasu Block during the Triassic. Along this shear zone rock units of different origins and derived from different crustal depths have been brought together by large scale transcurrent fault movements. A systematic programme of structural, petrographic, mineralogical, geochemical and isotopic studies would test the validity of this hypothesis and would establish whether the high-grade rocks represent a pre-Carboniferous basement. Geophysical methods could provide information concerning the deep structure beneath Sumatra, Neither deep reflection nor deep refraction seismic surveys on shore are likely to prove logistically feasible for some time to come, but the wider application of tomographic methods, using natural seismicity, could provide information on the nature of the crust and mantle and on the presence of structural discontinuities. Such information could also

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come from potential-field geophysics. Although almost the whole of Sumatra has been covered by reconnnaissance gravity surveys, there is considerable scope for more detailed work aimed at addressing specific problems. The application of better terrain corrections, perhaps based on satellite-derived 90m DEMs now available from the United States Geological Survey could considerably advance our understanding of the controls on the gravity field in mountainous areas. Aeromagnetic data could play a similarly important role, and it is to be hoped that the considerable commercially-confidential database, believed to exist, will eventually be placed in the public domain. Recent discoveries of oil in fractured crystalline rocks have stimulated an interest by petroleum exploration companies in defining more precisely the nature and structure of the basement beneath the Tertiary sedimentary basins in the Sumatran backarc area. Some indication of the nature of the crust and mantle beneath Sumatra might be determined from xenoliths or xenocrysts in volcanic rocks and minor intrusions. Isotopic studies of granitic intrusions would identify sources of magmatic material within the crust or mantle.

Further palaeontological studies

Palaeontological studies by Fontaine & Gafoer (1989) have led to major advances in the determination of the ages of the stratigraphic units, the definition of crustal blocks, palaeoclimatic conditions and plate tectonic reconstructions. However, further palaeontological studies would assist in the resolution of some of the major problems which have emerged during the present study concerning the relationships between Late Palaeozoic units in Sumatra, and the terrane amalgamations proposed for the mid-Permian to mid-Triassic interval. In their synthesis of the geology of northern Sumatra, Cameron et al. (1980) included the Bohorok, Alas and Kluet formations in the Tapanuli Group. Fossils from limestones within the Alas Formation showed that this unit was of Vis6an (Early Carboniferous) age. From the occurrence of 'pebbly mudstones', regarded as glacigenic deposits, in the Bohorok Formation this unit was presumed to be Late Carboniferous to Early Permian, as similar glacigenic deposits occur within palaeontologically-controlled stratigraphic sequences of Lower Permian age in the NW Malay Peninsula and Peninsular Thailand. These deposits define the extent of the Sibumasu Block and are correlated with the Late Carboniferous-Early Permian Gondwanan glaciation of the southern continents. The confirmation of 'pebbly mudstones' in the Tobaoli area of southern Bangka Island (Ko 1986) would extend Sibumasu to the SE of Sumatra. More direct evidence of the age of the Bohorok Formation is provided by the Pangururan Bryozoan Bed that crops out on the shores of Lake Toba (Aldiss et al. 1983). Poorly preserved and deformed fossils occur within a decalcified impure limestone. From these fossils the age of the Bryozoan Bed was established as possibly Late Carboniferous or Early Permian. During the mapping programme, from the the absence of pebbly mudstones among the adjacent sandstones and shales, the Bryozoan Bed was considered to lie within the Kluet Formation, but in the tectonic model proposed in this volume the Bryozoan Bed is considered to lie within the Bohorok Formation. Detailed mapping of the area, to determine the stratigraphic position of the Bryozoan Bed, together with an assiduous search for limestone beds or lenses where age-diagnostic fossils may be better preserved, may provide more precise age constraints for the Bryozoan Bed and for the Bohorok Formation as a whole. No fossils have so far been recorded from the Kluet Formation, but because of its association in the field with the Bohorok and Alas formations it was also presumed to be of Carboniferous age (Cameron et al. 1980). Earlier in this chapter it is proposed that the Kluet Formation forms part of the West Sumatra

(Cathaysian) Block. If this correlation is correct the Kluet Formation is most probably the same age as the Kuantan Formation of central Sumatra, where limestones have also been dated palaeontologically as Vis6an (Early Carboniferous) age. The Alas and Kuantan formations are the same age but Fontaine & Gafoer (1989) suggest that the fossils in the Alas Formation indicate that these limestones were deposited in a temperate environment, whereas those in the Kuantan Formation indicate a tropical environment; the Alas and Kuantan formations must have been deposited on different plates in different climatic zones. At present there is no direct evidence of the age of the Kluet Formation from the area near Tapaktuan in which it was originally defined. However, the geological map (Cameron et al. 1982) shows limestone lenses within the Kluet Formation which might yield age-diagnostic macrofossils or microfossils. Turner (1983b) in his detailed study of the sandstones and shales of the Kuantan Formation near Muarasipongi reported abundant fragmental plant remains with spores, and of sponge spicules in calcareous concretions, providing the possibility that further searches for spores and microfossils might yield age-diagnostic material from both the Kluet and Kuantan formations. In central Sumatra the Kuantan Formation with its tropical fauna crops out adjacent to the Early Permian Mengkarang Formation which contains the tropical Cathaysian 'Jambi Flora'. These two formations define the West Sumatra Block. The Mengkarang Formation and its flora was last studied systematically in the 1930s. Interbedded with the plant beds are limestones containing fusulinids. Further palaeontological and palaeobotanical studies to confirm the precise age and evolutionary and provincial affinities of the flora are currently in progress (Isabel van Waveren pers. comm. 2004). Permian and Triassic fossils were reported from the limestones of the Situtup, Kaloi and Batumilmil formations in northern Sumatra, which were included within the Peusangan Group (Cameron et al. 1980). Both Permian and Triassic fossils were reported from the same outcrops, but the relationship between limestones of different ages was not resolved during reconnaissance mapping. It has been suggested that important tectonic events, including the collision of Sibumasu and Indochina and the emplacement of the West Sumatra Block, occurred between the Mid-Permian and the Mid-Triassic. Detailed study may show that a major unconformity separates the Permian and Triassic components of the Peusangan Group. Very few radiolarian studies have been carried out in Sumatra. Triassic bedded cherts occur in the Kualu Formation near Medan and the Tuhur Formation near Solok, but their radiolarian fauna has never been described. The 'oceanic assemblage' of the Jurassic-Cretaceous Woyla Group, cropping out from Banda Aceh in the north to the Garba Mountains in the south, frequently includes bedded cherts. The age of these units was presumed to be of Late Jurassic to Early Cretaceous age from associated shelly faunas. Only the chert outcrop at Indarung, near Padang, has been studied for radiolaria, and unexpectedly yielded a Middle Jurassic age (McCarthy et al. 2001). A systematic study of radiolaria from other occurrences of bedded chert in the Woyla Group may extend the age of the segments of ocean floor (MesoTethys) which were subducted to form the Woyla accretionary complex.

Sedimentological studies o f pre-Tertiary sediments

No systematic sedimentological studies have been made of the Bohorok, Kluet or Kuantan formations to determine their petrography and provenance, although a mixed continental provenance was determined from clasts in the 'pebbly mudstones' (Cameron et al. 1980). It has been suggested in this chapter that the Bohorok Formation was deposited on Sibumasu, while the Kluet Kluet Kuantan Formation was deposited on the West Sumatra

TECTONIC EVOLUTION Block. A provenance study of the sandstones of the Bohorok and Kluet/Kuantan formations may confirm that these units were deposited on different continental blocks. Cameron et al. (1982a), interpreted the alternation of sandstones and shales, with slumped deposits and graded beds in the Bohorok and Kluet formations as turbidites, but this suggestion has never been examined critically, nor have current bedding and other indicators of transport directions yet been studied. As has already been mentioned, the 'pebbly mudstones' of the Bohorok Formation are interpreted as glacio-marine deposits, by analogy with the Singa Formation of the Langkawi Islands, where dropstones have been described, but comparable features have not yet been described from Sumatra. Cameron et al. (1980) proposed that the westerly decrease in the size of pebbles in the mudstones in the Bohorok Formation, and in the conglomerates in the Alas and Kluet formations, indicate that the Tapanuli Group was deposited on a continental margin facing an ocean towards the west. Since it is proposed earlier in this chapter that the Bohorok Formation was deposited on Sibumasu, and the Kluet Formation on the West Sumatra Block, this interpretation requires re-examination. Sedimentological studies are also needed on the Permian and Triassic units of northern and central Sumatra, including the Silungkang, Mengkarang, Kualu and Tuhur Formations to establish their provenance, directions of transport and environments of deposition. These studies will result in the improvement of our present palaeogeographic models for these periods. A sedimentological study of the Lower Permian Mengkarang Formation is currently in progress (Isabel van Waveren pers. comm. 2004); preliminary results have determined the palaeo-environments in which the Jambi Flora was deposited.

Structural studies Thrusts and refolded folds on vertical or steeply dipping axial planes have been reported from the Bohorok Formation and equivalent units in eastern Sumatra. These units were deposited on the Sibumasu Block but it has not been established whether the structures were formed by the Late Permian-Early Triassic collision between the Sibumasu and Indochina Blocks. It has been proposed earlier in this chapter that during the Early Triassic the West Sumatra Block was emplaced against the western margin of the Sibumasu Block along a major transcurrent shear zone (Medial Sumatra Tectonic Zone). The sense of movement and the extent to which earlier structures within the adjacent crustal blocks have been modified by strike-slip movements can be determined by a study of minor structures within the shear zone. Triassic rocks which were deposited during or shortly after the time of emplacement have been mapped within the shear zone and in adjacent areas. A study directed specifically at the structures within these Triassic rocks might better constrain the extent and the age of movements along the MSTZ. Earlier in this chapter it has been proposed that Carboniferous Permian rocks of the Pemali Group on the island of Bangka, some of which have an oceanic origin, form part of an accretionary complex due to subduction of the Palaeo-Tethys Ocean which lay between Sibumasu and Indochina, prior to their collision in the Late Permian or Early Triassic. These rocks are described as steeply dipping, highly deformed, folded and thrust, with the development of slaty cleavage in argillaceous units (Ko 1986). The Pemali Group is overlain by the more gently folded and faulted Triassic Tempilang Formation, presumably unconformably, although the unconformity has not yet been described. The rocks on Bangka are much disrupted and altered to hornfels by granitic intrusions which host tin deposits, so that relatively little attention has been paid to the structure of the country rocks. It should be straightforward to establish, by a close examin-

257

ation of the lithologies and structure, whether the Pemali Group forms part of an accretionary complex. Folding and the development of slaty cleavage in the JurassicCretaceous Rawas, Alai and Peneta formations in central Sumatra have been attributed in this account to the mid-Cretaceous collision of the West Sumatran margin with an island arc, resulting in the emplacement of the overthrust Woyla Nappe. A structural study could be directed at determining the validity of this hypothesis. In the West Sumatra Block, multiple folding and slaty cleavage are developed in the Carboniferous Kluet/Kuantan Formation. Are these structures also the result of the overthrusting by the Woyla Nappe, or is there any evidence of an earlier (?)Permian deformation in these rocks?

Geochemical analysis and isotopic dating of pre-Quaternary volcanic units and plutonic intrusions The Tin Islands of Bangka and Billiton are the only parts of Sumatra where a comprehensive geochemical and isotopic study of the igneous rocks has been carried out. Here Cobbing et al. (1992) made a thoroughly documented study of the granites (summarized in Chapter 5), providing a sound basis for future work. In compiling the geochemical database and the isotopic ages of the igneous rocks of Sumatra for this volume it was found that there are very few studies in which modern techniques of geochemical and isotopic analysis have been used. The majority of isotopic ages given in the literature are not supported by detailed information concerning location, field relationships, petrographic description or by complete geochemical analyses, and very few of the ages have been confirmed using different dating methods. In the absence of reliable modern data much of the information and many of the interpretations on the ages of volcanism, igneous intrusion, deformation and mineralization that have been given in the earlier chapters in this volume are based on inadequate geochemical and stratigraphic controls. A systematic programme of isotopic dating using the available techniques could provide precise ages for episodes of igneous intrusion, volcanic activity, deformation and mineralisation throughout Sumatra. There are large number of granitoids in northern Sumatra which are presumed to be of Permo-Triassic age, but they have never been dated. Isotopic dating of 13 samples from the Sibolga Granitoid Complex showed a wide range of ages between 264 Ma (Early Permian) and 75 Ma (Late Cretaceous). The significance of this wide age range is not understood; the oldest age is taken to be the age of emplacement, but it is not known whether the younger ages relate to alteration or to the emplacement of younger intrusions coincidently in the same area. The existing K - A r and R b - S r database requires major expansion and confirmation by the use of additional techniques, such as 39Ar/4~ U - P b and SHRIMP. For example Imtihanah (2000) using the 39Ar/4~ method, dated the emplacement of the Lolo Batholith earlier in the Miocene than the K - A r mineral ages obtained by McCourt et al. (1996), which presumably relate to the tectonic uplift of the batholith. There is scope for a systematic programme of chemical analysis to determine the tectonic environments of formation of all the plutonic intrusions and volcanic units in Sumatra to refine the interpretations that have been presented in this account. For example there is a problem concerning the environment of formation of the Jurassic-Cretaceous Bentaro Volcanics of Aceh. It was earlier suggested that this arc was built on a sliver of continental crust (Cameron et al. 1980). Earlier in this chapter it is suggested that these volcanics were formed as an intra-oceanic arc built on oceanic crust. This problem could be resolved easily by a geochemical study.

258

CHAPTER 14

Neotectonics GPS monitoring of recent crustal movements in Sumatra have so far been concentrated on the Sumatran Fault Zone and the central segment of the forearc region, defined by the Banyak Islands in the north and the Batu Islands in the south. No information has been obtained concerning the segmentation of the convergence zone, which may prove to be crucially important in asssessing the spatial distribution of hazards represented by Great Earhquakes. It is particularly frustrating that there was only one station in the segment ruptured during the 26 December 2004 earthquake (where the pillar on a very small island may have been destroyed by the tsunami) and there have been no repeat GPS measurements in the Enggano region since the Magnitude 7.9 event in June 2000. However, the spatial bias in the distribution of the stations that have been established does at least mean that a start has been made on monitoring the probable site of the next Great Earthquake to the west of Sumatra. There is clearly an urgent need, in view of the complexity of the forearc bathymetry, for predictive modelling of likely tsunami travel paths from the sites of possible future ruptures. It is especially important that such methods be applied to the very vulnerable central segment. Such an approach to hazard mitigation could well be more cost-effective, and could certainly be more quickly implemented, than the full Indian Ocean tsunami warning system now being proposed. Vertical movements are more difficult to assess than horizontal ones, but there is the intriguing possibility of obtaining significant information in the forearc region by comparing the maps and navigational charts from the Dutch colonial era with modern observations. It is known that some smaller islands have been submerged completely in the intervening period, but no systematic survey has yet been attempted. Sumatra also provides a potentially valuable, but so far underused field laboratory for studying the interactions between subduction zones and features on the downgoing plate. Sidescan sonar and swathe bathymetric studies of trench and outer forearc structures in areas such as the junction between the trench and the Investigator Fracture Zone would add significantly to our knowledge and understanding of the processes involved and the hazards that they represent.

Exploration for gold and base metal deposits Mineral exploration companies will continue the search for gold and base metal deposits in Sumatra. There are few recent descriptions of the more significant mineral deposits, and very few deposits have been dated adequately using modern techniques. Recently a P b - Z n sedex deposit of Mississippi Valley type has been found at Dairi NW of Lake Toba and a gold deposit near Sibolga, similar in size to the gold deposit in the Lebong mining area near Bengkulu, has been located by the bulk leach analysis technique. It seems that in spite of over 100 years of exploration, it is still possible to discover P b - Z n sulphide deposits of types which had not previously been known in Sumatra and to find sediment-hosted gold deposits using new exploration techniques. Novel exploration methods and the targeting of new types of mineralization may lead to further discoveries. The development of techniques for the more efficient exploitation of deposits already discovered will continue to sustain the interest of the mining companies in Sumatra, subject to the vagaries of the market, the restrictions of conservation and government regulations on mining activities.

companies to continue the search for accumulations of oil and gas in the Tertiary sedimentary basins of Sumatra and in the underlying basement. Now that Indonesia is a net importer of oil it is becoming critical that exploration goes into a new phase. The major producing basins are now mature and future reserves are dependent on small structural and stratigraphic plays. These will require advances in seismic techniques and the interpretation of seismic data, and the more efficient exploitation of known reserves, using more sophisticated recovery techniques. Success will require innovation, thinking outside the box and, occasionally, serendipity. This phase of exploration is usually taken over by independents. New independent Indonesian companies are entering the scene. This traditional petroleum area provides opportunities for innovative small companies that are not risk averse. Prospects for the expansion of the coal industry are also excellent. The demand for steam coal to supply generating stations is expected to expand in response to increasing domestic demand for electricity. The extensive reserves of coal in the Tertiary basins of Sumatra will continue to be an important source of energy for Indonesia and for export in the forseeable future. Continued exploration for energy resources in Sumatra will lead to a better understanding of the tectonic controls on the origin and development of the Tertiary sedimentary basins and conditions which led to the formation of coal and the accumulation of economic deposits of oil and gas. Fission-track studies in the basement rocks in the Barisan Mountains and in the Tertiary sedimentary basins would provide better controls on the history of uplift, erosion and sedimentation. Only one fission-track study has so far been carried out in Sumatra (Moss & Carter 1996). This study was conducted in the Ombilin Basin and on the margins of the South Sumatra Basin. As expected, this study demonstrated that the marginal sediments had never been deeply buried.

Palaeomagnetic studies to determine latitudinal movement and rotation of crustal blocks Very few palaeomagnetic studies have been carried out in Sumatra. Early studies by Sasajima et al. (1978) and Haile (1979) were very much at a preliminary reconnaissance level and the results were mainly inconclusive. No studies using modern palaeomagnetic techniques have yet been carried out. Most of the pre-Tertiary units have been metamorphosed and are unlikely to give useful palaeomagnetic results, but some of the Permo-Triassic limestones and less altered Permian volcanic rocks, and the volcanics, limestones and cherts of the Woyla Group may provide evidence of latitudinal movement of crustal blocks and provide constraints on the palaeogeographic reconstructions proposed in this chapter. One important problem that could be resolved easily by a palaeomagnetic study is to establish whether Sumatra rotated to any significant degree during the Tertiary. Both clockwise and counter-clockwise rotations of up to 40 ~ have been proposed. Tertiary limestones, siltstones and volcanic rocks, often with good stratigraphic control on their age, may yield valuable palaeomagnetic results, and are well exposed in the forearc, intramontane and backarc regions of Sumatra. By collaboration with the oil companies it should be possible to obtain oriented samples from borehole cores for a palaeomagnetic study.

Conclusion Continued search for energy resources (coal, oil and gas) Expanding demand for energy within Indonesia and worldwide, and diminishing reserves elsewhere, will encourage petroleum

This volume is the first attempt to provide a comprehensive review of all that is presently known about the geology of Sumatra since the synthesis prepared by van Bemmelen (1949,

TECTONIC EVOLUTION

1970). The authors hope that it provides a sound foundation upon which future research can be based. Many of the interpretations proposed are highly speculative and will provide ample scope for future research programmes on all aspects of the geology.

259

Hopefully, some of the suggestions put forward above will be taken up by institutions in Indonesia or elsewhere in the world, leading to a new synthesis in which some of the problems raised have been resolved.

Appendix

Radiometric age data for Sumatra

A s u m m a r y o f K - A r , R b - S r and A r - A r age data for w h i c h m e t h o d s and locations are d o c u m e n t e d . T h e s u m m a r y is u p d a t e d f r o m the c o m p i l a t i o n s b y M c C o u r t et al. (1996; S u p p l e m e n t a r y

Publication), a n d the S E A g e s D a t a b a s e (2004) h t t p : / / w w w . g l . rhul.ac.UK/seasia. Additional information kindly provided by Professors R o b e r t Hall and Herv6 Bellon.

Table AI. Radiometric age dates of volcanics and for the intrusion and cooling of plutons related to the Palaeozoic volcanism and plutonism in Sumatra Lithology

Dating method

Age (Ma)

Reference

Granite clast, Cucut No. l well (source rock not identified)

K-Ar, ?

348 • 10

Koning & Darmono ([984)

East Sumatra Plutonic Arc Kiri Well, Granite Kiri Well, Granite Set•1774 well, Granite* Idris No. 1 well Set•1775 well Granite*

Rb-Sr, Rb-Sr, Rb-Sr, Rb-Sr, Rb-Sr,

427 335 298 295 276

Eubank & Makki (1981) Eubank & Makki (1981) Katili (1973) Koning & Darmono (1984) Katili (1973)

? ? feldspar ? feldspar

• • • • •

42 43 39 3 20

West Sumatra Volcanic and Plutonic Belt VOLCANICS

Silungkang area, andesite

K-Ar, ?

248 • l0

Nishimura et aI. (1978)

K-Ar, muscovite K-Ar, ? Rb-Sr, isochron Rb-Sr, ? whole rock Rb-Sr, muscovite K-Ar, ? K-Ar, biotite

287 277 264 257 256 246 246

Hahn & Weber (1981b) Suwarna et al. (2000) Aspden et al. (1982b) Fontaine & Gafoer (1989) Silitonga & Kastowo (1975) Koning & Aulia (1985) Sato (1991)

PL UTONS

Singkarak (Ombilin) Singkarak (Ombilin) Sibolga Granite Sibolga Granite Singkarak (Ombilin) Singkarak (Ombilin) Sijunjung Granite

Granite* Granite

Granite Granite

• 3.5 • 13 _+ 6 • 24 • 6 • 7 • 12

*Suspected presence of de|brmation Locations in Figs 5. l & 6. l

Table A2. Radiometric age dates of volcanics and /br the intrusion and cooling plutons related to the Triassic-Early Jurassic" Plutonic Episode in Sumatra Lithology West Sumatra Plutonic Arc" (Eastern Province-type granites) Sibolga Granite Sibolga Granite Sibolga Granite t Sibolga Granite Sibolga Granite t Sibolga satellite Granite Sibolga satellite Granite Sumpur Granite t Sumpur Granite ~ Sumpur Granite1Tantan-Dusunbaru Granite Tantan-Dusunbaru Granite Tantan-Dusunbaru Granite Singkarak Granite SE Padangsimpuan* Sulit Air Diorite Sulit Air (98/8) no plateau Sulit Air (98/7) steps 1050- 1175~ Sulit Air Diorite Sulit Air Diorite Padang Ganting Granite (Sulit Air)

Dating method

Age (Ma)

Reference

K-At, hornblende K-Ar, biotite K-Ar. biotite K-Ar biotite K-Ar biotite K - A t biotite K-Ar biotite Rb-Sr feldspar Rb-Sr. biotite K-Ar biotite K - A t feldspar K - A t amphibole K-Ar whole rock K-Ar, biotite K-Ar, biotite K-Ar, biotite 4~ hornblende 4~ hornblende K-Ar, hornblende K-Ar, hornblende/biotite K-At, ?

219 • 4 211 • 5 211 • 3 206 _ 3 206 + 2 217 • 4 212 _+ 3 216 215 215 ___3 209 • 3 201 • 5 199 • 4 206 • 3 202 • 2 203 • 6 193 • 4 192 • 0.4 183 • 13 149 • 5 149 • 3

Hehuwat (1976) Aspden et al. (1982b) Hehuwat (1976) Fontaine & Gafoer (1989) Fontaine & Gafoer (1989) Fontaine & Gafoer (1989) Fontaine & Gafoer (1989) Hehuwat (1976) Hehuwat (1976) Hahn & Weber (1981b) Fontaine & Gafoer (1989) Fontaine & Gafoer (1989) Fontaine & Gafoer (1989) Fontaine & Gafoer (1989) Wikarno et al. (1993) McCourt & Cobbing (1993) Imtihanah (2000) hntihanah (2000) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Koning & Aulia (1985) (continued)

260

APPENDIX

Table A4.

261

Continued

Lithology Atar (Sulit Air) Granodiorite Sulit Air Diorite Sulit Air Diorite (Main Range Province type granites) Sijunjung Granite Muarasipongi Granite

Dating method

Age (Ma)

Reference

K-Ar, biotite K-Ar, hornblende/biotite K-Ar, hornblende

147 • 2 141 • 5 138 • 3

Hahn & Weber (1981b) McCourt & Cobbing (1993) McCourt & Cobbing (1993)

K-Ar, hornblende, biotite K-Ar, biotite

206 • 3 197 • 2

Silitonga & Kastowo (1975) Rock et al. (1983)

Medial Sumatra Tectonic Zone (Main Range Province granites)

Kayumambang Granite Sungai Isahan Granite-greisen Sungai Isahan Granite-greisen Rokan Granite* Rokan Granite*

K-At, K-Ar, K-Ar, K-Ar, K-Ar,

biotite muscovite muscovite biotite biotite

198 • 197 • 193 + 189 • 186+

2 2 2 2 2

Schwartz et al. (1987) Schwartz et al. (1987) Schwartz et al. (1987) Rock et al. (1983) Rock et al. (1983)

East Sumatra, Indosinian Foreland (Main Range Province granites) Idris No. 1 well, Granite K-At, Idris No. 1 well, Granite K-Ar, Idris No. 1 well K-Ar, Beruk NE No. 4 well K-Ar, Garnet-muscovite-tourmaline microgranite

muscovite albite albite ?

208 206 206 203

7 8 8 4

Koning Koning Koning Koning

• • • +

& & & &

Darmono (1984) Darmono (1984) Darmono (1984) Darmono (1984)

Indosinian Collision Zone in Riau Archipelago, Bangka and Bill•

(Main Range & Eastern Province granites) Penangas-Belinyu Granite, Bangka Belinyu Granite, Bangka East Bintang Granite, Bintan Lagoi Granite, Bintan Toboali Granite, Bangka Pading Granite, Bangka Menumbing Granite, Bangka Menumbing Granite, Bangka Tanjong Pandang Granite, Belitung Parangbuloh Granite 2sp, Belitung Parangbuloh Granite, Belitung Parangbuloh Granite, Belitung Kelapa Granite, Bangka Kelapa Granite, Bangka Kelapa Granite, Bangka Menumbing Granite, Bangka Menumbing Granite, Bangka Permisan Granite, Bangka Pemali Megacrystic Granite, Bangka Pemali Granite, Bangka Parangbuloh Granite, Belitung Tikus Granite greisen Belitung B. Pancur Granite greisen Belitung Dabo Granite, Singkep

Rb-Sr. isochron Rb-Sr. isochron Rb-Sr. isochron Rb-Sr. isochron Rb-Sr. isochron Rb-Sr. isochron Rb-Sr. biotite Rb-Sr. whole rock Rb-Sr. isochron K-Ar, biotite Rb-Sr, biotite Rb-Sr whole rock K-Ar, biotite Rb-Sr, biotite Rb-Sr whole rock K-Ar, biotite Rb-Sr isochron Rb-Sr isochron Rb-Sr errorchron K-Ar, biotites Rb-Sr, biotite K-Ar, muscovite K-Ar, muscovite Rb-Sr, 'errorchron'

252 • 8 251 • l0 229 • 7 226 • 8 225 -I- 9 223 • 16 217 • 5 217 _+ 5 216 • 3 216 • 6 216 • 6 216 • 6 216 • 6 215 • 5 215 • 5 214 • 6 200 + 4 213 • 4 2! 1 • 3 159 - 95 206 • 6 200 • 6 195 • 6 193 • 12

Cobbing et al. (1992) Cobbing et al. (1992) Cobbing et al. (1992) Cobbing et al. (1992) Cobbing et al. (1992) Cobbing et al. (1992) Priem & Bon (1982) Priem & Bon (1982) Cobbing et al. (1992) Priem & Bon (1982) Priem & Bon (1982) Priem & Bon (1982) Priem & Bon (1982) Priem & Bon (1982) Priem & Bon (1982) Priem & Bon (1982) Cobbing et al. (1992) Cobbing et al. (1992) Schwartz & Surjono (1991) Schwartz et al. (1995) Priem & Bon (1982) Jones et al. (1977) Jones et al. (1977) Cobbing et al. (1992)

*Deformation suspected. tLocation of sample point uncertain. Locations in Figs 5.1, 5.2 & in references.

Table A3. Radiometric age dates of volcanics and f o r the intrusion and cooling of plutons related to the Mesozoic Volcanic and Plutonic Episodes and Phases in Sumatra Lithology

Dating method

Age (Ma)

Reference

Mid Jurassic-Lower Cretaceous Volcanic and Plutonic Episode (180-129 Ma) VOLCANICS

Tanjung Siantu, metabasalt, Belitung Palangki, andesite Silungkang area, andesite Gumai Mts, basic volcanic Lembak AI well, andes•

K-Ar, K-Ar, K-Ar, K-Ar, K-At,

whole rock* ? ? ? ?

181 143 140 122 121

• • • + •

5 4 10 4 2

Priem et al. (1975) Koning & Aulia (1985) Suwarna et al. (2000) Gafoer et al. (1992c) Pulunggono & Cameron (1984)

PLUTONS

Kayumambang Granite Kayumabang Granite

K-Ar, whole rock K-At, biotite

180+ 7 124_ 5

Simandjuntak et al. (1991) And• Mangga et al. (2000) (continued)

262

Table A4.

APPENDIX

Continued

Lithology Kayumabang Granite Beruk NE No. 2 muscovite-tourmaline granite Lubuk Terap Granite Bungo Batholith Granite Bungo Batholith Granodiorite Bungo Batholith Granodiorite Bungo Batholith Granite Bungo Batholith Quartz diorite Bungo Batholith Quartz diorite Bungo Batholith Granite Berhala Island, gabbro S. Salai Porphyritic Granite* Tebingtinggi 1 well, Granite Duabelas Mts. Granite Muarasipongi Granite Kluang Utara-49 well Granite Way Sulan Gabbro Bungsu-1 well Granite, Beruk Tanjung Laban-1 well Granite Sibolga satellite Granite Tanjung Gadang Granite Sibolga satellite Granite S. Mentaus, Porphyritic Granite t S. Muara, Porphyritic Granite Tigapuluh Mrs. Kiri Granite* S. Manggajahan Biotite Granite Pakning No. 1 well, Granite Panyabungan Batholith

Dating method K-Ar, biotite K-At, ? K-Ar, ? K-Ar, biotite K-Ar, hornblende K-Ar, hornblende K-Ar, biotite K - A t , biotite K-Ar, hornblende K-At, biotite K-Ar, ? K-Ar, whole rock K-Ar, ? whole rock K-At, ?biotite Rb-Sr, isochron K-At, ? K-Ar, hornblende K - A t , muscovite K-Ar, ? K-Ar, biotite Rb-Sr, ? K-At, hornblende K - A t , whole rock K-Ar, whole rock K-At, ? K-Ar, whole rock K-Ar, muscovite K-Ar, biotite

Age (Ma) 123 179 175 169 156 154 153 148 131 129 167 166 160 159 158 153 151 150 149 147 145 144 144 135 134 128 122 121

Reference

-4- 3 -4- 3 -4- 6 -4- 23 -4- 5 + 4 -4- 2 -4- 4 -4- 2 -4- 4 • 2 4- 3 • 3 -4- I +_ 3 • 2 -4- 1

And• Mangga et al. (2000) Koning & Darmono (1984) Koning & Aulia (1985) McCourt & Cobb• (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobb• (1993) Katili (1973) Suwarna et at. (1991) Anon (1983) Simandjuntak et al. (1991) Beddoe-Stephens et al. (1987) Pulunggono et al. (1992) McCourt & Cobbing (1993; map) Koning & Darmono (1984) Putunggono et at. (I992) Aspden et al. (1982b) Pulungonno & Cameron (1984) Aspden et al. (1982b) Suwarna et al. ( 1991 ) Suwarna et al. (1991) Eubank & Makki (1981) J1CA (1990) Eubank & Makki (1981) Rock et al. (1983)

• 3 • 2.5 + 3 _+ I

Koning & Aulia (1985) Wajzer et ell. (1991) Suwarna et al. (2000) Suwarna et ell. (2000)

• 1 • 5 + 5 -4- 5 + 6 + 7 • 4 -4- 4 -4- 7 -4- 4

Late Cretaceous Volcanic and Plutonic Episode (120-75 Ma) VOLCANICS

Lubuk Paruku, tuff Tambak Baru Volcanic Unit Gumai area, andesite Palepat area, andes•

K-Ar, K-Ar, K-At, K-Ar,

? whole rock ? ?

105 78.4 78 75

PL UTONS

Gunung Mang Diorite, Belitung Tanjung Gadang Garba Pluton Monzodiorite Garba Pluton Monzodiorite Garba Pluton Monzograbbro* Garba Pluton Monzograbbro* Garba Pluton Garba Pluton Granite Garba Pluton Monzogranite Garba Pluton Granite Gumai Mrs Diorite Sulan Pluton Tonalite Sulan Pluton Granodiorite Lass• Granite Guntung No. 1 well Granite Sibolga satellite granite Idris No. 1 Granite Palepat Granite t Seumayam Complex granodiorite Susoh intrusion Sikuleh Granite Well 100 km NW Pakanbaru, Granite Ulai (Sontang) Granite Aroguru foliated diorite Lampung Granite Manunggal Granite Brant• Granodiorite

K-Ar, whole rock K-At, ? K-Ar, biotite K-Ar, biotite K-Ar, hornblende K-At, biotite ? ? K-Ar, biotite K-At, biotite K-At, biotite K-Ar, biotite K-At, ? K-Ar, biotite K-At, biotite Rb-Sr, biotite ? K-Ar, muscovite K-Ar, biotite K-Ar, re•177 K-At, ? K-Ar, biotite K-Ar, ? K-At, mean of 2 biotite & 1 hornblende determination Rb-Sr, ?

120 • 4 118 + 4 117 • 3 115 _+ 4 104 _+ 3 100 • 3 89 • 2 86 • 3 82 • 3 80 • 1 116 • 3 113 • 3 111 • 3 112 • 24 I 12 • 2 105 + 1 101 i- 4 100 • 1 99 • 4 98 -t- 2 98 • 1

K-At, biotite K-At, biotite Rb-Sr, 4 determinations on biotite & muscovite K-Ar, K-feldspar K-Ar, biotite

89.6 89 • 3 88

Rock et al. (1983) McCourt & Cobbing (1993) Katili (1973)

87.0 86 + 3

Kanao et al. (1971) McCourt & Cobbing (1993)

95 • 3

Priem et al. (1975) Koning & Aulia (1985) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Pulunggono et al. (1992) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Pulunggono et al. (1992) Gafoer e t a / . (1992c) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Katili (1962) Eubank & Makki (1981) Hehuwat (1976) Koning & Darmono (t984) Suwarna et al. (2000) Kallagber (1990) McCourt & Cobbing (1993, map) Bennett et al. (1981b) Eubank & Makki (1981)

(continued)

APPENDIX

Table A4.

263

Continued

Lithology Batu Madingding Diorite Padean Granite Padean Pluton Microdiorite Padean Monzogranite Padean Monzogranite Padean Monzogranite Padean Granite Senawar Quartz Diorite Hatapang Granite Sibolga satellite granite

Dating method K-Ar, whole rock K-Ar, muscovite K-Ar, muscovite (2 dets.) K-Ar, biotite K - A t , biotite K-Ar, biotite K-Ar, muscovite K-Ar, whole rock Rb-Sr, isochron K-Ar, biotite

Age (Ma)

Reference

85 ___4 84.7 + 3.6 82 + 2 82 _+ 3 82 + 2 81 + 2 79 _ 2 83.6 _+ 4.2 80 _+ 1 75 _+ 1

Wajzer et al. (1991) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) JICA (1988) Clarke & Beddoe-Stephens (1987) Hehuwat (1976)

*Deformed sample. +Location of sampling point uncertain. Locations on Fig 5.1 & in references.

Table A4. Radiometric age dates o f volcanics and Jbr the intrusion and cooling o f plutons related to the Tertiary Volcanic Episodes and Phases in Sumatra Lithology

Dating method

Age (Ma)

Reference

51.3 + 1.5 55.5 _+ 1.5 57.9 + 1.4 63.1 _ 1.5 52.1 -t- 1.2 59.6 _ 1.4 62.5 _ 1.4 62.9 _+ 1.5 63.1 + 1.5 63.7 _ 1.5 63.3 + 1.9 60.3 55

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004), Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Amin et al. (1994b) Gafoer et al. (1994) De Coster (1974)

PALAEOCENE VOLCANIC EPISODE (65-50 Ma) VOLCANICS

Basalt tuff, Bentaro Volcanic Formation (LM 116A) Basalt dyke in Lhoong Formation (LM 124) Basalt flow, SW of Banda Aceh (LM 118) Basalt dyke in Bentaro Volcanic Formation Basalt dyke, Natal area (SU 49) Andesite dyke in Woyla Group, Batang Natal (NL 41) Basalt dyke, Tambak Barn Volcanics (NL 40) Gabbro dyke in Silungkang Formation (RDC 11) Basalt flow, Silungkang Formation (RDC 13A2) Basalt flow, Silungkang Formation (RDC 13A l) Andesite, Gunung Dempu Basalt, Garba Mountains Tuff, Tamiang 2-well

4~176 4~176 4~176 4~176 4~176 4~176 4~176 4~176 4~176 4~176 K-Ar, whole rock? K-Ar, whole rock? K-At, '?whole rock

PL UTONS

Padangpanj ang Jatibaru microgranite Jatibaru microgranite Well in N Sumatra Basin, Granite Lassi Pluton gabbro Lassi Pluton biotite tonalite Lassi Pluton (98/3) Steps 1100-1250~ Lassi Pluton quartz diorite Lassi Pluton (98/2) Lassi Pluton (98/2) Steps 1100-1300~ Lassi Pluton diorite Lassi Pluton granite Lassi Pluton quartz diorite Lassi Pluton (98/4) Lassi microdiorite Lassi Pluton (98/1) 750-900~ steps Meulaboh-Meuko granodiorite Meulaboh-Meuko granodiorite Granite in well in N Sumatra Basin Bungo Batholith quartz diorite Bungo Batholith quartz diorite Nagan granodiorite Nagan granodiorite Nagan granodiorite Bukit Raja Pluton Bukit Raja Pluton Ulai (Sopan) granite Ulai (Panti) pegmatitic granodiorite Ulai granodiorite Samadua granite

K-Ar, biotite, mean 2 dets. K-Ar, biotite K-Ar, biotite Rb-Sr, ? K-Ar, hornblende K-Ar, biotite mean 2 dets 4~ hornblende K-Ar, biotite Rb-Sr, biotite 4~ biotite K-Ar, hornblende K-Ar, biotite K-Ar, biotite Rb/Sr, biotite K-Ar, ? 4~ K-feldspar K-Ar, biotite K-Ar. biotite K-Ar. biotite K-Ar. hornblende K-Ar. biotite K-Ar. biotite K-Ar. mafic K-Ar. biotite K-Ar. 9 K-Ar ? K-Ar K-Ar K-Ar, K-Ar,

biotite biotite biotite biotite

63.6 _+ 3.2 62 + 3 56 _+ 3 58 57 _+ 2 56.2 + 2.8 56.06 +_ 0.19 55 _ 2 55.02 + 0.7 54.78 _ 0.10 54 + 2 53 _+ 2 53 _+ 2 52.2 + 0.7 52 -t- 1.6 ,-~48.5 56.2 _+ 2.2 53.2 _+ 3.3 56 + 1 54 + 2 54 + 2 54.4 _+ 0.5 53.5 +_ 0.9 51.5 _ 0.7 54.1 + 2.7 51.9 _ 2.6 52.2 52.4 + 0 47.7 52 ___ 1

Sato (1991) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Wikarno et al. (1993) McCourt & Cobbing (1993) Sato (1991) Imtihanah (2000) McCourt & Cobbing (1993) Imtihanah (2000) Imtihanah (2000) McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Imtihanah (2000) Koning & Aulia (1985) Imtihanah (2000) Kallagher (1990) Kallagher (1990) Hehuwat (1976) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Kusnama et al. (1993b) Kusnama et al. (1993b) Kusnama et al. (1993b) JICA (1988) JICA (1988) Hahn & Weber (1991b) Kanao et al. (1971) Rock et al. (1983) Cameron et al. (1982b) (continued)

264

Table A4.

APPENDIX Continued

Lithology Samadua (Tapaktuan) granite Batang Natal microdiorite dyke Sibubung granite Well in N Sumatra Basin Gle Seukeun Complex granodiorite Gle Seukeun Complex granodiorite Gle Seukeun Complex hb diorite Gle Seukeun Complex Granite in well 100 km NW Pakanbaru LATE MID-EOCENE VOLCANIC EPISODE (c. 4 6 - 4 0 Ma)

Dating method K-Ar, biotite K-Ar, whole rock K-Ar, ? K-Ar, biotite K - A t , hornblende K-At, biotite K-Ar, hornblende K-Ar, mean of analyses of a hornblende and a biotite K-Ar, ?

Age (Ma) 51 49.5 50.9 50 50 47.2 47.6 42

• • + 4• • • •

1 2.5 1 1.2 1 0.7 1.0 3

45 • 1

Reference Cameron et al. (1982b) Wajzer (1986) Wikarno et al. (1993) Hehuwat (1976) Van Leeuwen et al. (1987) Van Leeuwen et al. (1987) Van Leeuwen et al. (1987) Bennett et al. (1981a) Eubank & Makki (1981)

VOLCANICS

Andesite dyke, Langsat Volcanic Formation (NL 36) Basalt dyke, Indarung Calcareous Formation (RDC 20) Shoshonite dyke, Tanjungkarang area (PCE 13)

4~176 4~176 4~176

41.1 • 0.9 45.8 _+ 1.1 43.5 • 1

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

PLUTONS

*Gabbro in ophiolite, P. Simeulue *Gabbro in ophiolite, P. Simeulue S. Tuboh Quartz Monzonite Andes• dyke in Sikumbu Fm Andesite dyke in Sikumbu Fm

K-Ar, K-Ar, K-At, K-Ar, K-At,

whole whole ? whole whole

rock rock rock rock

40.1 35.4 40.1 40.1 37.6

• • • + •

2.7 3.6 2.0 1.6 1.3

Kallagher (1990) Kallagher (1990) JICA (1988) Wajzer (1986) Wajzer (1986)

LATE EOCENE-LATE OLIGOCENE VOLCANIC EPISODE (c. 38-24 Ma)

Late Eocene-Early Oligocene phase (c. 35-30 Ma) VOLCANICS

Basaltic andesite dyke, Blang Pidie, Tapaktuan (TT 148) Basalt dyke, Langsat village, Natal area (NL 37) Basalt dyke in Silungkang Formation (RDC 13)

40K_40Ar 40K 40Ar 40K_40Ar

31.6 • 0.85 37.4 _+ 0.9 37.3 • 1

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

K-At, hornblende K-Ar, whole rock

29.7 + 28.2 •

Wajzer (1986) Wajzer (1986)

4~K_40Ar 40K_40Ar 40K_40Ar 40K_40Ar

26.9 23.7 24.3 25.5

K-Ar, biotite + hornblende duplicate Ar K-Ar, biotite + hornblende K-Ar, hornblende, mean of 6 dets.

19.8 • 0.8 20.1 _+0.7

PL UTONS

Air Bangis Granite Air Bangis Granite

1.6 1.2

Late Oligocene-Early Miocene phase VOLCANICS

Basalt dyke in Woyla Group north of Tapaktuan (TT 144) Basalt flow, Painan Formation (PN 26) Andesite dyke in Painan Formation (TP 34) Dacite dyke in Painan Formation (TP 33)

• • + +

0.72 0.55 0.60 0.59

Bellon Bellon Bellon Bellon

et et et et

al. al. al. al.

(2004) (2004) (2004) (2004)

PL UTONS

Way Bambang Granite Way Bambang Granite Way Bambang Granite Raya Diorite

18.7 • 1.9 18.9 • 1.2

McCourt & Cobbing (1993) McCourt & Cobbing (1993) McCourt & Cobbing (1993) Bennett e t a / . ( 1981 a)

18.8 14.5 21.4 21.1 18.7 18.8 18.8 18.3 17.7 17.5 17.1 16.4 16.1 15.9 15.0 13.7 19.6 18.2 16.8 16.8 17.2

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Kallagher (1990) Bellon et al. (2004) Kallagher (1990) Kallagher (1990) Kallagher (1990) Kallagher (1990) Bellon et al. (2004) Kallagher (1990) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Aspden et al. (1982b)

LATE EARLY MIOCENE-MID-MIOCENE VOLCANIC EPISODE (22-8 Ma) VOLCANICS

Late Early Miocene volcanic phase (c. 22-14 Ma) Basalt block in Indrapuri melange, Banda Aceh (IP 113) Basalt dyke in Lhoong Formation (LM 126) Basalt flow, in Calang Volcanic Formation (CL 140) Andesite dyke, Calang area (CL 135C) Andesite dyke, Calang area (GB 15) Basalt dyke in Tangla Formation (CL 135B) Basalt flow in Calang Volcanic Formation (CL 141A) Andesite dyke in Calang Volcanic Formation (CL 132) Basalt, Sayeung Volcanic Formation Andesite dyke, Calang area (CL 136) Basalt, Sayeung Volcanic Formation Basalt dyke, Sayeung Volcanic Formation Basalt, Sayeung Volcanic Formation Basalt dyke, Sayeung Volcanic Formation Basaltic andesite dyke, Calang Volcanic Formation (CL 131) Basalt, Sayeung Volcanic Formation Andesite dyke in Barus Formation, Sibolga (SB 27B) Andesite flow in Angkola Volcanic Formation (SB 85) Andesite dyke in Angkola Volcanic Formation (SB 84) Andesite dyke in Angkola Volcanic Formation (SB 83) Andesite, P. Musala

4OK_4OAr 4OK_4OAr 4oK_4oAr 4OK_4OAr 4OKr_4OAr ~oK J 0 A r 4OK_4OAr 4OK_4OAr K-Ar, whole rock

4OK_4OAr

K-Ar, whole K-Ar, whole K-Ar, whole K-Ar, whole 40K_40Ar

rock rock rock rock

K-Ar, whole rock 40K_40Ar 40K_40Ar 40K_40Ar 40K_40Ar K-Ar, whole rock

+ 0.49 • 1.17 • 0.59 • 0.60 • 0.44 + 0.59 • 0.45 + 0.44 • 0.7 • 0.42 • 0.9 • 0.6 • 3.9 + 1.0 + 0.38 + 2.7 • 0.58 • 0.45 +_ 0.47 • 0.39 + 5

(continued)

APPENDlX

Table A4.

265

Continued

Lithology

Dating method

Age (Ma) 0.48 0.44 1.5 0.54 0.45 0.45

Reference

Basalt meta-tuff, Simpang Gambir, Natal area (NL 42) Absarokite in Sikarara Volcanic Formation (NL 34) Andesite, Sarik Lawas Andesite flow in Painan Formation (PN 31) Andesite flow in Painan Formation (PN 22) Basalt flow in Painan Formation (PN 24) Basalt lava or tuff?, well N Pekanbaru Andesite flow in Painan Formation (TP 32) Andes• flow, Bukit Sulap, Bengkulu (BSU 170) Andesite in Hulusimpang Fornmtion (MN 116) Rhyolite dyke in Hulusimpang Formation (MN 118) Basaltic andesite dyke in Hulusimpang Fornmtion (MN 117) Rhyolite tuff in (?)Tarahan Formation (TR 33) Basalt dyke in Sulan batholith (WS 5) Andesite dyke in Hulusimpang Formation (SMK 40) Basalt dyke in Hulusimpang Formation (SMK 39) Dacite flow in Sabu Formation (PCE 9A)

4~176 4~176 K-Ar, ? 4~176 4~176 4~176 ?KAr 4~176 4~176 4~176 4~176 4~176 4~176 4~176 4~176 4~176 4~176

19.7 • 18.2 • 22 • 19.2 • 19.1 • 19.0 • 17.5 14.3 • 16.5 + 13.2 • 12.8 • 12.8 + 19.7 • 17.1 + 16.9 + 15.1 _ 14.4 •

0.34 0.38 0.43 0.31 0.38 0.47 0.44 0.44 0.38 0.35

Bellon et al. (2004) Bellon et al. (2004) Koning & Aulia (1985) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Eubank & Makki ( 1981 ) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004)

Middle Miocene Volcanic Phase (c. 12-8 Ma) Basalt, Alem Fm Basalt, Alem Fm Basalt dyke, Alem Fm Basalt dyke in Hulusimpang Formation (SMK 37)

K-Ar, whole rock K-Ar, whole rock K-Ar, whole rock 4~176

11.2 • 10.3 • 8.;74 • 10.9 •

0.7 0.4 0.82 0.43

KalIagher (1990) Kallagher (1990) Kallagher (1990) Bellon et al. (2004)

16 • 15.12 • 15.06 • !1 • 9.0 • 7.89 • 6.03 • 5.82 • 5.81 • 5.8 • 5.66 • 5• 4.67 • 14.3 • 13.1 • 9.97 • 13.0 • 12 • 12.1 • I1 • 10.4 • 9.77 • 9.1 • 8• 8.5 7.9 •

0.7 0.18 0.13 1 0.1 0.1 0.07 0.07 0.13 0.1 0.04 0.2 0.1 1 0.25 0.50 0.5 1 0.5 0.6 0.9 0.7 2 0,1

Wikarno et al. (1993) Imtihanab (2000) Imtihanah (2000) McCourt & Cobbing (1993) Imtihanah (2000) Imtihanah (2000) Imtihanah (2000) hntihanah (2000) Imtihanah (2000) Imtihanah (2000) Imtihanah (2000) McCourt & Cobb• (1993) Imtihanah (2000) Bennett et al. (1981a) Van Leeuwen et al. (1987) Van Leeuwen et al. (1987) Aspden et al. (1982b) Hehuwat (1976) Wikarno et al. (1993) Hebuwat (1976) Wajzer (1986) Wikarno et al. (1993) Hehuwat (1976) Hehuwat (1976) Rock et al. (1983) Hehuwat (1976)

PLUTONS

Granite, SE Padang Lolo Pluton (98/13) Lolo Pluton (98/13) Steps 900-1150'C Lolo granodiorite Lolo Pluton (98/11) Steps 1100-117P'C Lolo Pluton (98/9) Lolo Pluton (98/11) Lolo Pluton (98/10) Lolo Pluton (98/9) Steps 800-1250
K-Ar Rb-Sr, biotite 4~ biotite K-At, hornblende 4~ hornblende Rb-Sr, biotite Rb-Sr, biotite Rb-Sr, biotite 4~ biotite 4~ biotite 4~ biotite K-Ar, biotite 4~ plagioclase K-At, biotite, mean of 3 analyses K-Ar, hornblende K-At, hornblende K-Ar, hornblende K-Ar, whole rock K-At, hornblende K-Ar, whole rock K-Ar, whole rock K-Ar, biotite K-Ar, biotite K-Ar, biotite K-Ar, biotite K-Arl biotite

0.2

LATE MIOCENE-PLIOCENE VOLCANIC EPISODE (6-1.6 Ma) V O L CA N I C S

Andesite flow, Lain Teuba Volcanics (UB 110) Diorite dyke in Bohorok Formation (PR 61) near Parapat, Lake Toba Andesite flow in Haranggoal Formation (PR 70) Andesite flow in Sibayak Complex (BR 104) Basalt dyke in Sipiso-piso lava dome (PR 101B) Andes• flow in Angkola Formation, Sibolga (SB 28) Andesite, Suliki Basaltic andesite flow, Merapi volcano area (PY 82) Andesite flow, north border of Lake Maninjau (MNJ 55) Basalt flow in Bal Formation east of Bengkulu (BN 111) Basaltic andesite flow in Bal Formation (KP137) Basalt dyke, boulder in Gumai mountains (LH 173) Basaltic andesite flow in Pliocene volcanic

4OK_4OAr 40K_4OAr

1.76 • 0.06 5.66 • 0.14

Bellon et al. (2004) Bellon et al. (2004)

4OK_4OAr 4OK_4OAr 4OK_4OAr 4OK_4OAr

2.88 _ 0.07 2.09 • 0.29 1.89 _+ 0~23 5.35 • 0.23 5.4 _+ 0.3 2.99 • 0.08 1.76 • 0.05 6.45 • 0.2 5.40 • 0.14 5.47 • 0.14 5.21 • 0.5 4.23 • 0.15

Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Koning & Aulia (1985) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Bellon et al. (2004) Belion et al. (2004) Bellon et al. (2004)

K-Ar, ? 4OK_4OAr 4OK_4OAr 4OK_4OAr 4oK_4oAr 4OK_4OAr 4OK_4OAr 4OK_4OAr

(continued)

266

APPENDIX

Table AS. Continued Lithology Formation, NW of Curup (CR 145) Andesite dyke in Air Benakat Formation (LH 178) Basaltic andesite dyke in Lemau Formation (BS 129) Andesite, Gunung Batu Andesite flow in ?Lakitan Formation (PC 16)

Dating method

Age (Ma)

4~176 4~176 K-Ar 4~176

2.91 2.41 4.76 4.93

• 0.09 • 0.08 _+ 0.32 • 0.13

K-Ar, hornblende 4~ biotite K-At, ?biotite, mean 2 dets. K-Ar, plagioclase

3.48 • 0.5 ~5.5 3.5 2.5 • 1

Reference

Bellon et al. (2004) Bellon et al. (2004) Gafoer et al. (1992c) Bellon et al. (2004)

PLUTONS

Langkup Granodiorite (* ?) Sungeipenuh, no plateau Sungaipenuh granitoid* Granite in well N Sumatra Basin

Kusnama et al. (1993b) Imtihanah (2000) Kusnama et al. (1993b) Hehuwat (1976)

*Suspected deformation age. *Location of sample position uncertain. Locations on Figs 5.1, 8.4-8.7 & details in references.

Table AS. Radiometric dates of deformed and metamorphosed rocks from Sumatra

Unit

Age (Ma)

Reference

K-Ar, mica

276 • 10

Koning & Darmono (1984)

INDOSINIAN OROGENY Berembang well, phyllite Berembang well, phyllite 90 km NNW Pakanbaru, 'quartzite' Talawi, hornfels (?contact metamorphism)

K-Ar, K-Ar, K-Ar, K-Ar,

251 247 222 154

10 10 3 5

Katili (1973) Katili (1973) Eubank & Makki (1981) Koning & Aulia (1985)

BENTARO-SALING ARCS COLLISION Beruk NE No. 4 hornfelsed argillite Tanjungan amphibolite Tanjungan amphibolite Tanjungan amphibolite Beruk NE No. 3 well argillite S. Mundaran, schist

K-Ar, muscovite K-Ar, amphibole K-Ar, amphibole K-Ar, amphibole K-Ar,? K-Ar, ?

6 5 6 5 5 3

Koning & Darmono (1984) And• Mangga et al. (1994a) And• Mangga et al. (1994a) And• Mangga et al. (1994a) Koning & Darmono (1984) Koning & Aulia (1985)

Early Eocene event Beruk No. 2 well, shale

K-Ar, ?

Beruk NE No. 1 well micaceous material in shears in brecciated quartzite

Method

muscovite feldspar ? ?

• + _ _

123 _ 125 • 115 • 108 + 116• 95 •

54.5 + 0.6

Koning & Darmono (1984)

References

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Index Page numbers in italics refer to figures; page numbers in bold refer to tables. A-type granites 60, 61, 159 accretionary complex 4, 5, 13 models of evolution 179-183 seismic section 178, 179 tectonic evolution 186-187 Aceh Woyla Accretionary Complex 76 Woyla Group exposures 40-43 Aceh Fault 206, 207 Actiastraea minima 43 administrative boundaries 1, 2 Agam Formation 214 Agathammina /Agathaminoides 35 Agathiceras sundaicum 38 Ai Manis Limestone 91 Air Bangis granite 115, 265 Air Benakat Formation 90, 95, 101, 138, 139, 140, 141,231,266 Air Kuning Formation 67 Air Mabara granite 209, 210 Airbangis Volcanic Formation 110, 111 Akul Volcanic Formation 107, 108, 109 Alas Formation palaeontology 256 stratigraphic setting 25, 26, 27, 28, 32 structural setting 190, 191, 194, 195 tectonic setting 234, 236, 238, 241,242 volcanic setting 63,66, 66, 71 Alem Formation 101, 110, 114, 265 Alloclionites timorensis 37 Allotriophyllum chinese 27 alluvial gold 171, 175 Aman Basin 135, 136 Areas Formation 110, 111 Andaman Basin 19 Andaman Sea, opening 121 Angkola Fault 208 Angkola Volcanic Formation 100, 101, 110, 116, 264, 265 anthracite 145 Anu Batee Fault 206 4~ dating 260, 263, 264, 265, 266 arc volcanoes 124-125 Archaediscus 30 Arminina asiatica 38 Aroguru granite-diorite pluton 55, 58, 59, 60, 262 arsenic mineralization 160 Arun Field 132, 134

Arun High 132 Arun Limestone 89, 92, 131, 132, 133, 134, 135 Asahan Arch 135, 136, 214, 217 Asai Formation 48, 78, 200, 218, 249 Atar granodiorite 261 Auran Volcanic Formation 110, 111 Babahrot Formation 42, 75, 81 Bacinella 43 back-arc basins 214, 233 Tertiary Central Sumatra 217-223,225 North Sumatra 214-217 Ombilin Basin 94, 223-228 South Sumatra 228-233 structure 216, 217 tectonic setting 215 back-arc volcanism Quaternary 125-130 Tertiary Central Sumatra 99 North Sumatra 99 South Sumatra 99, 100 Bakasap Formation 136 Bal Formation 101, 110, 111, 108, 265 Balam Basin 135, 136 Balam Trough 219, 220-223 Bale Formation 42 Bampo Formation 88, 89, 133, 134, 142, 216, 218 Bandan Formation 105, 106 Bandar Jaya Basin 105, 141 Bangka Island 74, 158, 237, 240 Bangkaru Ophiolite Complex 113, 182 Bangko Formation 89, 93, 136, 219, 221 Banjalarang adamellite 104 Banyak Basin 22 Banyak Group 21 Banyak Islands 9, 11, 14, 177, 180, 185 Baong Formation 88, 94, 95, 215,218 Baong Sandstone 131, 132, 133, 134 Baong Shale 134, 135 Barisan Formation 29, 37, 39, 39, 69, 200 Barisan Mountains 1, 187-188 East Sumatra 190-195 emergence 249-251 gravity 16, 18 history 96 Medial Sumatra Tectonic Zone 195-196 Pre-Tertiary history 188-190

Sumatran Fault Zone 203-214 Tertiary volcanism 99, 100 West Sumatra 196-200 Woyla Nappe 200-203 Barisan Schiefer 24 Barogang Island 182 Baru M~lange 91 Baruman Basin 219 Barumun Fault 208 Barus Formation 94, 100, 264 basalts, first recognised 125 base metals exploration, future potential 258 map 148 basement 24-25, 214, 217, 218-219, 220 Batam Island 71, 72 Batang Natal Megabreccia Formation 47, 77 Batang Natal microdiorite 264 Batang-Natal Section 80 Batee Fault 13, 177, 184 Batu Group Islands 185 Batu Madingding diorite 263 Batu Mandi Field 131 Batu Nabontar Limestone Unit 47, 77, 77 Batu Raja Limestone 90, 92, 93, 138, 139, 139, 140, 231 Batumilmil Formation 27, 35-36, 39, 40, 190, 194, 239, 242, 257 Beatang Ultramafic Complex 41, 76 Bekasap Formation 89, 221 Belimbing pluton 60 Belinyu granite 54-55,261 Belirang-Beriti volcano 127 Belok Gadang Formation 43-44, 44 Belok Gadang Siltstone Formation 47, 75 Belumai Formation 88, 92, 94, 216, 217, 218 Belumai Sandstone 131, 132, 133, 134, 135 Bengal Fan 1, 3, 175, 186 Bengkalis Graben 135,222 Bengkalis Trough 136, 219, 220, 223,222 Bengkulu Basin 17, 20, 131, 140, 141, 186 Benioff Zone see Wadati Benioff Zone Bentaro-Saling arc collision 65, 266 Bentaro Volcanic Formation 42, 81,100, 159, 201,203,263 Bentong-Billiton Accretionary Complex 70-73, 148, 188, 189, 190 Bentong-Raub Line 234, 235, 237 Bentong-Raub Suture (Medial Malaya Line) 63, 64, 188, 189, 237, 238

283

284

Berhala Island gabbro 262 Beruk granite 262 Besar volcano 127 Billiton Island 72 mineralization 148, 158 Binail microdiorite 265 Binio Formation 89, 95, 137 Bintan Island 71, 73 mineralization 157-158 Blangkejeren Fault 206, 209 Bohorok Formation 256, 257 radiometric age 101, 265 stratigraphic setting 25, 26, 27, 28, 32, 33 structural setting 190-192, 190, 191, 192, 194, 195 tectonic setting 234, 235, 237, 238, 239, 240, 242, 243 volcanic setting 63, 66, 67, 67 Border Clay 88 Boueina 43 Brani Formation 90-91, 104 Branti granite pluton 55, 59, 262 Brawan Volcanic Formation 108, 109, 116 Breueh Volcanic Formation 88, 102, 104 Bruksah Formation 88, 89, 133, 135, 214, 218 Bukit Batu granite pluton 55, 57, 58, 59, 61, 150 Bukit Batu syenite 150 Bukit Daun volcano 127 Bukit Lumut Balai volcano 127 Bukit Pancur granite 261 Bukit Pendopo Formation 39, 67 Bukit Raja granite pluton 159, 263 Bukit Susah Trough 219 Bukit Telor basalt 125, 129 Bungo batholith 55, 59, 60, 159, 248, 262, 263 Bur ni Telong volcano 126 Cahop serpentinite 41, 76 Calamites 29 Calang Formation 100, 108, 110, l I l, 114, 116, 264 Campang Formation 105, 106 Cancellina praeneoschwagerinoides 38 Carboniferous history of Tapanuli Group distribution maps 26, 27, 28, 191 palaeogeography 34- 35 stratigraphy 25-29 structure 190-193 volcanism 64-66, 82 Cathaysian (Indochina Block) affinities 188, 236, 242, 243, 244, 245, 246 Central Sumatra Basin basement 220 coal 142-144, 145 petroleum exploration history 135 petroleum systems 137 reservoir rocks 136 seismicity 14, 15 source rocks 137 stratigraphy 89-90, 92-93, 136 structure 217-223, 225 tectonic setting 135 volcanism 99

INDEX

cerium mineralization 161 chemical analyses 113, 114, 116 chromium mineralization 160, 161 Ciletuh Formation 102, 104 Citilim Island 71, 73 Clay Formation 90 Cleiothyridina 27 Clyeina 43 coal resources analytical data 144 distribution map 143 first discovery 142 geographical distribution 142-145 production 145-146, 146 quality 145 reserves 145 stratigraphic age 142 coal exploration, future potential 258-259 Condong Member 66, 66 copper mineralization contract of work signings 149 Eocene-Miocene 161 Jurassic-Cretaceous 158-159, 160 Late Cretaceous 161 Miocene-Pliocene 159-165,163-164 Palaeocene 159, 161 Palaeozoic basins 152 Woyla Group 160 Cordaites 38 Cretaceous mineralization 158-159 plutonic-volcanic belt 65, 74, 84-85 radiometric dating 261-263 Woyla Group stratigraphy 40-53 Crystalline Schists 24 Cubadak Formation 28, 37, 39, 40 Dabo granite 55, 261 Daoella 37 Dayung Field 131 Dempo volcano 127 Denpo, Mt 188 Devonian sediments 24 diamictite see Bohorok Formation Doliolina lepida 37 Duabelas Mts granite 262 Dumai High 219 Duri Formation 89, 136, 137, 221 earthquakes 120 Central area (2004-2005) 14-15 Enggano (2000) 11, 12-13 Simuelue (2004)9, 11, 12, 13-14 see also seismicity East Bintang granite 261 East Malaya Microplate 234, 235 East Sumatra Block 63 East Sumatra plutonic-volcanic belt 66-67 Encrinus 38 Enggano Great Earthquake (June 2000) 11, 12-13 Entrochus 38 Eocene mineralization 159

stratigraphy 87- 88 volcanism 102, 105 radiometric dating 100, 264 tectonic relations 113 Eoendothyranopsis 30 Epigondondolella postera 35 extension events 110 extension rate 96 extrusion tectonics 110-111 fault slip rates 205-206 Fenestella retiformis 35 fission track dating 123 fold structures Ombilin Basin 226-228 Tertiary back-arc basin 215- 217 forearc basins 4, 176, 177 basement 185 depositional history 185-186 gravity 20-22 seismic section 178 setting 184-185 tectonic evolution 186-187 volcanism 99 forearc ridge and islands m61ange origin 183-184 models of evolution 179-183 role of Mentawai Fault 184 volcanism 99 fossil suites Carboniferous-Early Permian 27, 30, 38 Jurassic-Cretaceous 41, 43 Permo-Triassic 35, 36, 37 Triassic 38 fuel resources see coal; petroleum Fusulina 38 Fusulinella 38 Fusulinella lantenoisi 37 Gadang granite 55 Gadis Fault 209, 210 Ganggsal Formation 71 Gangsal Formation 30-31, 31, 71, 192-193, 195 Garba Formation 31, 50, 249 Garba granite batholith 55, 59, 159, 262 Garba inlier 50 Garba Mts basalt 263 gas see petroleum resources gas exploration, future potential 258-259 Gawo Formation 107, 108, 109, 183 Genako Trough 219 geochemistry future researches 258 granites 58-60 volcanics Permian 69-70 Tertiary 109-110, 113, 114, 116 Woyla Group 79, 81 geological maps 6 geological research, history of 1-6 Geological Survey of Indonesia (GSI) 3 Geumang Line 201 Geumpang Formation 41-42, 75, 76 Geunteut granodiorite 55, 265

INDEX Geureudong volcano 126 Geureuggand Fault 208 Gigantopteris 38, 234 Gle Seukeun complex 264 Gnathodus girtyi rhodesi 27, 30 gold mineralization 148, 258 alluvial 171, 175 contract of work signings 149 Eocene-Miocene 161 Jurassic-Cretaceous 158-159, 160 Late Cretaceous 159, 160, 161 Miocene-Pliocene 163-164 Palaeocene 159, 161 Woyla Group 160 Golok Tuff Formation 78, 262 Gomo Formation 95 Gondwana terrane 188 affinities 123,237, 239, 240, 242, 243-244 breakup 65, 82, 82, 83 palaeogeography 244, 245-248 granites distribution maps 55, 71, 72, 157 isotopic ages 54-55 recent research 58-60 Sundaland compared 60-61 tin suite 56-57 volcanic arc suite 57-58 gravity East Sumatra 19 forearc basin 20-22 long wavelength field 22-23 regional patterns 16-19, 122 sedimentary basins 19-20 Toba-Tawar low 19 gravity field 17 gravity stations 18 Guguchina pluton 60 Gumai andesite 262 Gumai Formation 90, 92, 93, 94, 138, 139, 140, 231 Gumai-Garba Line 80, 80 Gumai inlier 50 Gumai Mts basic volcanics 261 Gumai Mts diorite 262 Gume Formation 42 Gunung Batu andesite 264 Gunung Dempu andesite 263 Gunung Mang diorite 262 Gunungkasih Complex 25, 31, 78, 80 Gunungsitoli Formation 95, 183 Halobia 35, 36, 37 Haranggoal Volcanic Formation 101, 108, 265 Hatapang granite pluton 55, 56, 57, 57, 58, 58, 60, 61, 159, 263 Helatoba-Tarutung volcano 126 Hemogordius 37 Hindeodella 27 Hindeodella triassica 36 Hippogriffe rocks 63, 66, 66 hot springs 212 Hulubelu volcano 127 Hulusimpang Formation 101, 106, 108, 109, 114, 115, 265 Hutapanjang volcano 127

hydrocarbon resource see petroleum Itydrocorallinae 48 I-type granites 55, 56, 57-58, 60 Indarung Formation 46, 48, 75, 78, 100, 264 Indian Ocean, magnetic anomalies 7 Indochina Block 189 see also Cathaysian affinities Indonesian Petroleum Association (IPA) 4, 214 Indosinian orogeny, radiometric dates 266 Indrapuri Complex 41,264 Insu Member 75 Intermontane petroleum basins 141 Intervening Sandstone 88 Investigator Fracture Zone 7, 8, 10, 17 Investigator Ridge 3, 7, 121,123, 175, 185, 187 Involutina 35 lpciphyllum 37 iron mineralization 160, 161, 163-164 isotopic ages see radiometric dating Jaleuem Formation 42 Jambi Depression 138 Jambi Flora 241,257 Jambi Nappe 236, 236 Jambo Aye Group 88 Jambor Baru Formation 47, 75, 77 Jatibarang Formation 105 Jatibaru granite pluton 55, 59, 106, 263 Julu Rayeu Formation 88, 95,215, 218 Jurassic mineralization 158-159 plutonic-volcanic arc 65, 74-76, 84-85 radiometric dating 260-261 Woyla Group stratigraphy 40-53 K - A r dating 54-55, 124 problems 98 results 69, 151 100, 101, 260-266 Kaba volcano 127 Kaloi Limestone Formation 27, 35, 39, 40, 66, 190, 194, 239, 242, 257 Kampar Basin 223 Kampar High 219 Kampar Kanan Basin 141 Kanaikan batholith 249 Kanaikan granitoid 44 Kanan Basin 223 Karimun Besar Island 71 Kasai Formation 90, 108, 112, 139 Kayumabang granite 261, 262 Kayumambang granite 261 Kedurang Graben 20 Kelapa granite 261 Kelesa Formation 89, 89, 104, 106, 144 Kembar volcano 126, 208, 209 Kemiki Formation 108, 110 Kenyaran Volcanic Formation 81 Kerinci, Mt 1, 187 Kerinci volcano 127, 211 Kerumutan Line 234 Keutapang Formation 88, 95,215, 216-217, 218, 253 Keutapang Sandstone 131, 132, 133, 134, 135 Kieme Formation 88, 100, 104

285 Kikim Tufts 88, 90, 98, 104, 248 Kikim Volcanics 98, 99, 100, 111 Kiri basin 135, 136 Kiri granite 262 Kiri Trough 219 Kla-Alas Fault 206, 209 Kla Line 201 Klabat batholith 55 Kluang Limestone 24-25 Kluet Fault 100 Kluet Formation mineralization 148, 149 palaeontology 258- 259 stratigraphic setting 25, 26, 27-28, 32, 33 structural setting 190, 191, 192, 195, 196-197, 198 tectonic setting 234, 236, 238, 241 volcanic setting 63, 66, 66, 68 Kompas Volcanic Member 108, 109 Koninckopora 30 Korinci Formation 89, 137, 144 Kotabakti Volcanic Formation 110, 111 Krakatau volcano 127, 130, 213 Kuala Lansa High 132, 133 Kualu Formation palaeontology 257 stratigraphic setting 24, 28, 36-37, 39, 40 structural setting 194, 195, 196 tectonic setting 239, 242 Kuantan Formation palaeontology 256-257 stratigraphic setting 29-30 structural setting 190, 192, 193, 197, 199, 218 tectonic setting 234, 236, 238, 241 volcanic setting 64, 64, 66, 82 Kuantan granite 54, 55 Kubu High 136, 219 Kundur granite 55 Kunyit volcano 127 Kutacane Graben 208, 209 Lagoi granite 261 Lahat Formation 90, 90, 92, 103, 104, 105, 109, 114, 140, 144, 230 Lahomie Formation 110, 111, 183 Lakat Formation 89, 92 Lakitan Formation 112, 266 Lam Minet Formation 42, 75, 76 Lain Teuba Volcanics 101, 265 Lamno Limestone Formation 43, 81 Lampung, Woyla Accretionary Complex 33, 78 Lampung granite 262, 265 Lampung Formation 112 Lampung High 19, 138, 138 Lampung tufts 123 Langkat Formation 136 Langkup granodiorite 112, 266 Langsat Volcanic Formation 47, 100, 103-104, 106, 113, 115, 264 Lassi granite batholith 54, 55, 57, 59, 60, 100, 103, 262, 263 Latoceandra ramosa 41 lead mineralization Eocene-Miocene 161

286

lead mineralization (Continued) Jurassic-Cretaceous 160 Miocene-Pliocene 163-164 Palaeocene 161 Palaeozoic basins 148-149, 152 Woyla Group 159, 160 Lehat Formation 139 Lelematua Formation 91, 95 Lemat Formation 70, 90, 90, 92, 103, 104, 144, 230 Lemat Sandstone 138, 139, 139 Lematang Line 236 Lemau Formation 101, 110, i l l , 266 Leuser, Mt 187 Lho Sukon Limestone 92 Lhok Sukon Deep 132, 133 Lhok Sukon High 132, 133 Lhok Sukon Trough 132 Lhoksukon Group 88 Lho'nga Formation 43 Lhoong Formation 43, 81, 100, 263, 264 lignite 145 Limau Manis Formation 37, 39, 40 Lingga Island 71, 73, 73 Lingsing Formation 49, 51, 76, 81,201,203 Lirik Field 135 Lithocodium 43 Loftulisa 48 Lokop-Kutacane Fault 208, 209 Lolo granite pluton 55, 59, 60, 118-119, 265 Loser Formation 107, 142 Lovfenipora 45 Lubuk Paraku tuff 78, 262 Lubuk Terpa granite 262 Lubukraya volcano 126 Lubuksikaping Fault 208, 210 magnetic anomalies, Indian Ocean 7 Malacca Microplate 234, 235 Malarco Formation 73, 73 Malintang Volcano 225 Mandian Basin 141,223 Mandian Trough 220 manganese mineralization 161 Mangani Formation 112 Maninjau Lake 123 mantle xenoliths 129 Manunggal batholith 44, 249 Manunggal granite 262 Marapi volcano 126 Marginatia 27 Masmambang High 20 Maurosoma Turbidite Formation 47 Medan granite 265 Medial Malaya Line (Bentong-Raub Suture) 237, 238 Medial Sumatra Line 238 Medial Sumatra Tectonic Zone (MSTZ) 70, 71, 150-151, 191, 193, 195-196, 240-241,261 Menanga Formation 51-52, 75, 78, 249 Menggala Formation 89, 92, 136, 137, 219, 221 Mengkarang Formation stratigraphic setting 38, 39 structural setting 218

INDEX

tectonic setting 234, 236, 239, 241,242 volcanic setting 67, 68, 68 Mentawai Basin 131, 140, 141 Mentawai Fault 7, 8, 12, 13, 14, 15, 22, 177, 184 Mentulu Formation stratigraphic setting 30, 31 structural setting 190, 192 tectonic setting 237, 239, 243 volcanic setting 66, 66, 67 Menumbing granite 261 Mergui Basin 132 Mergui Microplate 234, 235 Mergui Ridge 19, 132 Mergui Shelf 19 metals see mineral deposits (metallic) metamorphic rocks dating 266 future researches 256 grade 47 Metapolygnathus polygnatoformis 37 Meucampli Formation 88, 99, 102, 104, 133, 214 Meukek Gneiss Complex, 81, 86 Meulaboh granodiorite 263 Meureudu Group 88 Minas Formation 89, 95, 136, 13Z 221 Minas High 136, 219 mineral deposits (metallic) distribution map 148 Eocene magmatic arc 159 future economic potential 172-173 history of discoveries 147 Jurassic-Cretaceous magmatic arc 158-159, 162 Late Cretaceous magmatic arc 159 Miocene- Pliocene magmatic arc 159-165 Neogene magmatic arc 165-175 Palaeocene magmatic arc 159 Palaeozoic basins 148-149, 152 timing of deposition 147, 151 Triassic-Jurassic magmatic arc 149-158 Miocene mineralization 159 palaeogeography 251, 252 stratigraphy 91-95 volcanism 102-106, 110, 111, 112, 112 radiometric dating 100, 101,264 tectonic relations 115-119 Mirah Volcanic Formation 110, 111, 114 molybdenum mineralization Jurassic-Cretaceous 159, 160 Late Cretaceous 160 Miocene-Pliocene 159, 163-164 Palaeocene 161 Montlivaltia 43 Montlivaltia molkkana 38 Moscovicrinus 38 Muara Enim Formation 90, 138, 139, 140, 144, 231 Muarasipongi granite batholith 55, 55, 57, 58, 261, 262 Muarasoma Formation 43, 44 Muarasoma Turbidite Formation 75, 77 Muereubo Volcanic Formation 110, 111 Multidiscus padangensis 36

Musala Volcanic Formation 110, 111 Musi Fault 100, 103 Mutus Assemblage 36, 196, 217, 234, 235, 238 Myriopora 43 Nabana Volcanic Unit 47, 75, 77, 78, 79 Nabirong Formation 110, 119 Nagan granodiorite 263 Nankinella 37 Natal Woyla Accretionary Complex 76-78 Woyla Group exposures 43-48 Nb + Y discriminant diagram 57, 59 eNd 124 Neoproetus indicus 35 Neoschwagerina 35,234 Neoschwagerina multiseptata 37 Neoschwagerina simplex 38 neotectonics, future researches 258 Ngaol Formation 25, 29, 38, 39, 67, 68-69 Nias Beds 91, 179, 183 Nias Elbow 7, 8, 21, 185 Nias Island 14, 180 model of evolution 179-183 Nicobar Fan l, 3, 4, 7, 175, 186 Nilo Formation 89, 137 Ninety-East Ridge 1, 3, 175, 186 Nodasaria 36 North Pulai Field 131, 135 North Sumatra Basin coal 142, 145 drilling hazards 135 petroleum exploration history 131 petroleum reserves 131 - 132 petroleum systems 135 reservoir rocks 134 source rocks 134-135 stratigraphy 88-89, 92, 133-134, 214-216 structure 132, 132, 216-217 tectonic setting 132-133 volcanism 99 oil see petroleum resources Old Andesites 98, 104 Old-Slates Formation 24 Oligocene palaeogeography 253 stratigraphy 88-91 volcanism 105-107 radiometric dating 100, 264 tectonic relations 113-115 Olodano Formation 183 Ombilin Basin 94, 107, 131, 141 coal resources 142, 145 compression 225-226 extension 225 faulting 228 folding 226-228 gravity 17, 18, 20 origin 224-225 sedimentary history 223-224 Ombilin Formation 94, 225, 227 -~ Ombilin granite 54, 55, 260 ophiolite 18, 22, 251 outer arc islands

INDEX

mdlange origin 183-184 models of evolution 179-183 role of Mentawai Fault 184 Outer Arc petroleum basins 140-141 Oyo Complex 91 Oyo Formation 183 Oyo Mdlange Complex 179 Pachiploia 36 Padang, Woyla Accretionary Complex 78 Padang Ganting granite 260 Padang tufts 123 Padangpanjang batholith 57 Padangpanjang granite 55 Padean granite 55, 59, 159, 263 Pading granite 261 Pagarjati Graben 20 Pahang Volcanic Belt 73 Painan Formation 100, 106, 108, 264, 265 Pait Island 72 Palaeobotanic Expedition to Djambi 1 Palaeocene mineralization 159 palaeogeography 117 volcanism 98-102, 103 radiometric dating 263 tectonic relations 111 palaeogeography Carboniferous 34- 35 Early Permian 65, 241, 245 Miocene 251 Palaeocene 117 Permian 241, 245, 246 Permo-Trias 242-247 Pliocene 253 Triassic 65, 250, 251 palaeomagnetism, future researches 259 palaeontology, future researches 256-257 Palaeotextularia 30 Palangki andesite 261 Palembang batholith 150 Palembang Beds 89 Palembang Depression 138 Palembang Formation 90, 144 Palembang Group 90 Palembang High 138 Palepat andesite 262 Palepat Formation stratigraphic setting 29, 37-38, 39, 39 structural setting 190, 218 tectonic setting 234, 242 volcanic setting 66, 67, 68, 69-70, 70, 71, 82 Palepat granite 262 palinspastic cross sections 253, 254 Panangas-Belinyu granite 261 Pangabuhan Formation 30, 31 Panglong M~lange Formation 47, 78 Pangururan Bryozoan Bed palaeontology 256 stratigraphic setting 25, 28-29, 32, 36, 39 structural setting 190, 191, 195 tectonic setting 239 volcanic setting 67, 68, 69-70, 70, 71, 82 Panti Formation 68, 69

Panyabungan batholith 262 Panyabungan Graben 209, 210 Papan Formation 36 Parafusulina 35, 37 Parangbuloh granite 261 Parapat Formation 88 Parlumpangan Volcanic Unit 47, 75, 77 Pasaman Ultramafic Complex 75, 77, 84 Pasumah Formation 112 Patah volcano 127 Pavastehphyllum 37 Pawan Member 71 Payakumbuh Basin 223 Payumbah granite 150 Pemali granite 261 Pemali Group 32, 38, 72, 73 188-190, 190 Pematang Formation 89, 89, 104, 136, 137, 137, 144, 221 Pematang Group 89, 104 Penangas granite 54-55 Penarum Formation 41 Peneta Formation 48, 200, 218, 249 Permian coal 142 palaeogeography 241,247- 249 plate setting 235 plutonic-volcanic belt 84, 261 East Sumatra 66-67 West Sumatra 67-69, 68, 83 stratigraphy Peusangan Group 27, 28, 35-40, 191, 193-195 Tapanuli Group 25-29, 34-35, 190-193 Permocalculus ampullacea 43 Perodinella 38 Persing Complex 32, 63 Petani Formation 89, 95, 110, I11, 136, 137, 220, 221 petroleum basins Central Sumatra Basin 135-137 intermontane 141 North Sumatra Basin 131 - 135 outer arc 140-141 South Sumatra Basin 137-140 petroleum resources 131 first discovered 86, 131 future potential 258-259 tectonic setting 131 petroleum systems Central Sumatra Basin 137 North Sumatra Basin 135 South Sumatra Basin 140 Peuet Sague volcano 126 Peunulin Sandstone 88 Peusangan Group 35-40, 257 distribution maps 27, 28, 191 structure 193-195 Peusangan High 132 Peutu Formation 88, 92, 94, 133, 134, 206, 214 Phillipsia 35 Pinang Conglomerate 91, 94 Pinapan Formation 110, 111,116, 119 Pini Basin 22

287

Pini Island, gravity 20, 21 placer tin 158 Planinvolutina 35 plate motions 1, 7, 10, 110, 187 horizontal 10-14 rotation 253-256 vertical 14 plate reconstructions 234, 235 platinum mineralization 160 Pliocene mineralization 159-165 palaeogeography 253 stratigraphy 95 volcanism 108-109, 112 radiometric dating 100, 265 tectonic relations 119 plutonism radiometric age data Mesozoic 2 6 0 - 2 6 3 Palaeozoic 260 Tertiary 2 6 3 - 2 6 6 see also granites Precambrian basement 25 Pseudocyclammina 41 Pseudocyclammina lituus 43 Pseudodoliolina 35, 37, 234 Pseudofusulina padangensis 37 Pulau Weh volcano 126 Pulaugadang granite 195 Pungkut-Barilas Fault 209, 210 Pungut Field 223,224 pyroclastics, Quaternary 123-124 Quartzite Terrain 24, 26, 32, 34, 63, 234 Quaternary volcanic events arc volcanics 124-125 back-arc volcanism 125-130 hazard analysis 128, 129, 130 history of research 120 pyroclastics 123-124 relation to Sumatra Fault System 213-214 tectonic setting 120-123 Raba Limestone Formation 43, 81 radiometric dating igneous rocks 24, 54-55 Mesozoic 260-263 Palaeozoic 260 Tertiary 98, 263-266 metamorphic rocks 266 Rajabasa volcano 127 Rampong Formation 107 Ranau, Lake 123,211,212 Ranau Formation 112 Ranau tufts 123 Ranau volcano 127 Ranau-Suwoh Fault 211,212 Ranto Sore Formation 47, 75 Rantobi Sandstone Formation 47, 75, 77 Rau Graben 208, 210 Raub-Bentong Line 55, 57, 60, 61 Rawas Formation 48, 75, 78, 200, 218, 249 Raya diorite 101, 108, 264 Raya stock 100 Rayeu Hinge 132

288

Rb-Sr dating 24, 54, 260-264 reservoir rocks Central Sumatra Basin 136 North Sumatra Basin 134 South Sumatra Basin 139-140 Riau-Billiton Accretionary Complex 83 rifting, and petroleum generation 131 Robulina Clay 88 Rokan granite 55, 151, 157, 261 Rokan Uplift 219 Rotalia Sandstone Formation 88 Rupat Island 24 S-type granites 55, 56-57, 60 S. Manggajahan granite 262 S. Mentaus granite 262 S. Muara granite 262 S. Salai granite 262 Sabu Formation 101, 105, 106, 265 Salibi Volcanic Formation 110, 111 Saligaro Volcanic Formation 110, 111 Saling Formation 49, 51, 81, 81,201,203, 248 Samadua granite 264 Sanduduk Formation 116 Sangkarewang Formation 90-91, 104, 106, 224, 228 Sapi Volcanic Formation 108, 109 Sarik-Gajah volcano 126 Sawahlunto Formation 93-94, 106, 144, 224, 227, 228 Sawahtambang Formation 93-94, 106, 108, 109, 224, 227, 228 Sayeung Volcanic Formation 100, 110, 111,114, 264 Schiefer Barisan Unit 200 Schwagerina 37 SEATAR programme 3-4, 175 Seblat Formation 94, 106, 108, 109 Securai Shale 88 sedimentary basins, gravity 19-20 seismic sections 13, 14 forearc 178 seismic tomography 22-23 seismicity 7, 8, 9 - 1 0 Central area (2004-2005) 14-15 Enggano (2000) 11, 12-13 Simuelue (2004) 9, I 1, 12, 13-14 Sekincau-Belirang volcano 127 Semanggoi Formation 188, 240 Semanka Depression 211 Semanko Fault see Sumatran Fault System Sembilan High 136 Sembuang Formation 27, 35 Semelit Formation 88, 100, 104 Senawar quartz diorite 263 Sepintiang Limestone Formation 49-50, 51, 81,201-203,248 Serbadjadi batholith 55, 154 Setiti granite batholith 66, 260 Seulawah Agam volcano 126 Seulimeum Fault 206 Seumayam Complex diorite 262 Seumpo Formation 95 Seureula Formation 88, 95, 112, 215, 216, 218

INDEX

Seurula Sandstone 131, 132, 133 Shan Thai Block see Sibumasu Block Si Gala Gala Schist Unit 47, 75, 77 Si Kumbu Turbidite Formation 47, 102, 104, 264 Siabu granite 55, 157 Sial) Formation 112 Sibaganding Formation 239, 242 Sibau Gabbro Group 91, 182 Sibayak Complex 101, 265 Sibayak volcano 126 Sibigio Limestone 91 Sibolga Basin 131, 140, 141 Sibolga Formation 104, 142 Sibolga granite batholith 54, 55, 67, 150, 260, 262, 263 Sibualbuali volcano 126 Sibumasu Block 64, 65, 189, 190, 191, 234, 237, 240, 241,242, 243, 244 Sibumasu Terrane 25, 120, 122, 123, 188, 195, 239, 242, 244 Sigala Complex 91 Sigalagala granite 265 Sigli High 132 Sigulai Formation 91 Siguntur Formation 46, 48, 75, 78, 203 Sihapas Formation 89, 107, 108, 109, 221 Sihapas Group 89, 92, 93, 136, 137, 219, 220, 221 Sijunjung granite batholith 55, 56, 150, 260, 261 Sikarara Volcanic Formation 100, 110, 111,116, 265 Sikubu Formation 44 Sikuleh granite batholith 43, 52, 55, 57, 159, 248, 249, 262 Sikumbu Formation 104, 264 Silungkang andesite 261 Silungkang Formation 28, 37, 39, 39, 66, 67, 67, 68, 69-70, 70, 83, 100, 190, 242, 263, 264 silver mineralization Jurassic-Cretaceous 160 Late Cretaceous 159, 161 Miocene-Pliocene 163-164 Palaeocene 159, 161 Palaeozoic basins 148, 149, 152 Woyla Group 158, 160 Simarobu Turbidite Formation 47, 75, 77 Simbolon Formation 112 Simeulue Basin 14 Simeulue Island 18, 22, 180, 182 seismicity 9, 11, 12, 13-14 Simpang Gambir Megabreccia Formation 47, 75 Sinabung volcano 126 Singkarak, Lake 210, 211 Singkarak Fault System 211 Singkarak (Ombilin) granite 260 Singkel Basin 22 Singkep granite 55 Singkep Island 32-35, 56, 61, 71 Sipakpahi Fault 100 Siphenodendron 30 Siphoneae 38

Sipiso-piso lava dome 101, 265 Sise Limestone Formation 43, 81 Sitaban Formation 100, 104 Situtup Formation 64, 69, 76 Situtup Limestone Formation 27, 35, 39, 39, 40, 190, 194, 234, 239, 257 Siulak Formation 48, 76, 247 skarn 69, 149, 157, 158 Smeten Volcanic Formation 108 Sontang granite 262 Sopan granite 208-209, 210, 263 Sorik Merapi Volcanic Centre 101, 126, 209 source rocks Central Sumatra Basin 137 North Sumatra Basin 134-135 South Sumatra Basin 140 South Sumatra Basin coal 142-145 drilling hazards 140 petroleum exploration history 137-138 petroleum systems 130 reservoir rocks 139-140 source rocks 140 stratigraphy 90-91, 93-95, 138-139, 228-231 structure 229, 230, 231-233 subcrop 78 tectonic setting 138 volcanism 99 Spathognatyodus campbelli 27 87Sr/86Sr ratio 124, 150 stick-slip cycle 13 Stromatopora japonica 41 structural researches, future work 257 structures, Batang Natal section 47 Stylina girodi 41 Stylosmilia corallina 43 subduction angle 121 rate 86, 120, 175 roll-back 111 Sugi Island 71, 72 Sukadana basalt 125, 129-130 Sukadana Plateau 125-129 Sulan tonalite pluton 55, 59, 60, 101, 262, 265 Sulit Air Suite 55, 59, 60, 249, 260, 261 Sumatra, name origin 147 Sumatran Fault System (Semanko Fault) 1, 3, 4, 7, 8, 120-121,123, 211,212 displacement 96 motion 252-254 Sumatran Fault Zone 203-204, 204 age 204-205 displacement 205 geographical character equatorial bifurcation 208- 210 north 206-208 Ranau section 212- 213 Singkarak section 210-211 Sunda Strait 212-213 motion 205-206, 207 relation to Quaternary volcanic arc 213-214 Sumatran Subduction System 4 Sumatrina 38

INDEX

Sumbing volcano 127 Sumpur granite 260 Sunda Craton (Sundaland) 1, 3, 4 Sunda Forearc see accretionary complex; forearc basins; forearc ridge; Sunda Trench Sunda Shelf 2, 19 Sunda Strait 122 extension 110 faults 212-213 Sunda Trench 1, 2, 4, 7, 8, 176 gravity 22 seismic section 178 subduction processes 175-176 subduction and volcanism 120, 125 Sundaland 188 evolution of 247-249, 251 granite affinities 60-61 Sungai Durian granite 60 Sungai lsahan granite 55, 151,261 Sungaipenuh granitoid 102, 266 Suoh volcano 127 Surungan Formation 112 Susoh intrusion 262 Syringopora 30 Tabir Formation 48, 67, 68, 68, 69, 248 Takengon Line 208 Takung Fault 225,226 Takur-Takur Formation 112 Talakmau volcano 126 Talang Akar Sandstone 138, 139, 139, 140 Talang volcano 126 Talangakar Formation 90, 92, 93,230, 231 Tambak Baru volcanic unit 47, 75, 77, 78, 100, 262, 263 Tampur Formation 133 Tampur Limestone Formation 87-88, 88, 214, 216 Tanahbalah Complex 91 Tandikat volcano 126 Tandun Field 223, 224 Tangla Formation 100, 107, 108, 109, 116, 264 Tangse serpentinite 41, 76 Tangse stock 102, 265 Tanjong Pandang granite pluton 55, 61, 154, 261 Tanjung Gadang granite 262 Tanjung granite 55 Tanjung Siantu metabasalt 261 Tanjungan amphibolite 266 Tantan granite 150, 153, 260 Tapaktuan Formation 42, 81 Tapaktuan granite 81,263 Tapaktuan Volcanic Formation 159 Tapanuli Group distribution maps 26, 27, 28, 191 palaeogeography 34-35 stratigraphy 25-29 structure 190-193 Tarahan Formation 101, 106, 109, 114, 265 Tarantam Formation 31 Tarap Formation 31, 197

Tarikan M41ange 91 Tawar Formation 27, 35, 39 tectonics models for evolution evaluated 234-239 revised 239-242 role in igneous events Eocene 113 Eocene-Miocene 113-118 extrusion 110-111 Miocene 118-119 Miocene-Pliocene 119 Palaeocene 111 Palaeogene rotation 110 Telaga Limestone 92 Telaga Said Field 86, 131 Telaga Tiga Field 86 Telisa Beds 90 Telisa Formation 89, 92, 93, 94, 110, 111, 136, 137, 219, 220, 221 Telisa Group 90 Telukkido Formation 28, 37, 39 Tempilang Sandstone 38, 154, 190 Tertiary see Palaeocene; Eocene; Oligocene; Miocene; Pliocene Tetehosi Formation 183 Teunom Limestone Formation 43, 81,201 Thaumatoporella porvosiculifera 43 Thecosmilia 35 Tigapuluh Arch 214, 217 Tigapuluh Group 30-31 Tigapuluh High 135, 136, 138 Tigapuluh Mts 30, 31, 151 Tikus granite 261 Timbahan granite 265 tin front 154, 158 tin islands 1 granites 54, 60-61, 147 mineralization 148, 152, 155-156 tin mineralization association with granite 149-150 contract of work signings 149 Cretaceous magmatic arc 159, 160, 161 Late Triassic-Early Jurassic arc Indosinian foreland 154 Medial Sumatra Tectonic Zone 150-153 SE belt 154-158 West Sumatra 150 Toba Caldera 8, 9-10, 18, 121 - 122, 124 Toba Lake 123 Toba tufts 108, 123, 124, 214 Toba volcano 126 Toba-Tawar gravity low 19 Tobali granite 261 Tolopulai Formation 91 topography 2 Toru Fault 206, 209 Toru Formation 110, 111,116, 119 Toweren Member 76 trace element analyses 113, 114 transcurrent faulting 187 Transition Formation 89 Triassic mineralization 149-158 palaeogeography 65, 246, 247

289

Plutonic-Volcanic Belt 73, 83-84 radiometric dating 260-261 stratigraphy 35-38 Tripa Volcanic Formation 110, 111 Trumon Volcanic Formation 110, 111 Tualang Formation 89, 136 Tuhur Formation 28, 37, 39, 190, 200 tungsten mineralization 154, 155-156, 161 U-Pb dating 54 Ujeuen Limestone Formation 27, 35 Ulai granite 111, 262, 263 Umu Mrlange 91 Uneun Unit 27, 35, 195 Unga diorite 55 Veerbeekina 38 volcanic rocks, dating of 260-265 volcanism, we-Tertiary Carboniferous 64-66, 82 Jurassic-Cretaceous 74-780, 84-85 Permian East Sumatra 66-67 West Sumatra 67-69, 83 Triassic 73, 8 3 - 8 4 volcanism, Quaternary arc volcanics 124-125 back-arc volcanism 125-130 hazard analysis 128, 129, 130 history of research 120 pyroclastics 123-124 relation to Sumatra Fault System 213-214 tectonic setting 120-123 volcanism, Tertiary Eocene 102-103, 104, 105 geochemistry 109-110, 113, 114, 115, 116, 117 history of research 98 Miocene 106-108, 110, 111 Oligocene 103-106, 108, 109 Palaeocene 98-102, 103 Pliocene 108-109, 112, 112 relation to tectonics Eocene 113 Eocene-Miocene 113-118 Miocene 118-119 Palaeocene 111 Palaeogene rotation 110 Pliocene 119 volcanoes, active 2, 5, 121, 207, 213 Vorbarisan Tectonic Unit 197- 200 Wadati Benioff Zone (WBZ) 7, 8-9, 120 Wampu Field 131 Way Bambang granite 55, 59, 115,264 Way Sulan gabbro 59, 262 Wentzzelloides 37 West Andaman Fault 177, 184 West Java Sea 80 West Sumatra Block 63 West Sumatra plutonic-volcanic belt 67-69, 73, 83 Wharton Spreading Axis 111, 115, 185 Wood Horizon 90

290

Woyla Accretionary Complex 76-80, 84 Aceh 76 Danau Diatas 76 geochemistry 79, 81 Gunung Kerinci 78 Lampung 78 Natal 76-78 ocean arc fragments 80-81 Padang 78 South Sumatra Basin subcrop 78 Tembesi-Rawas Mts 78 West Java Sea 80 Woyla Group 52-53

INDEX

arc assemblage 42-43, 200, 201-203 correlated exposures Central Sumatra 46-48 Natal 43-46 Southern Sumatra 48-52 distribution maps 41, 191 limestone assemblage 43 mineralization 159, 160 oceanic assemblage 4 l - 4 2 , 190, 200- 201,234 radiometric age 263 structure and tectonic setting 189, 200-203, 248-249

Woyla Nappe 2 0 0 - 2 0 3 , 2 4 8 - 2 4 9 Woyla Terranes 235, 237

Zaphrentites 27 zinc mineralization Eocene-Miocene 161 Jurassic-Cretaceous 160 Late Cretaceous 161 Miocene-Pliocene 163-164 Palaeocene 161 Palaeozoic basins 148-149, 152 Woyla Group 159, 160 zircon ages 54

Sumatra Geology, Resourcesand Tectonic Evolut,on Edited by A. J. Barber, M.J. Crow and J. S. Milsom This volume provides the first comprehensive account of the geology of Sumatra since the masterly synthesis of van Bemmelen (1949). Following the establishment of the Geological Survey of Indonesia, after WW II, the whole island has been mapped geologically at the reconnaissance level, with the collaboration of the geological surveys of the United States and the United Kingdom. The mapping programme, completed in the mid-1990s, together with supplementary data obtained by academic institutions and petroleum and mineral exploration companies, has resulted in a vast increase in geological information, which is summarized in this volume. The synthesis of structural controls on sedimentation and magmatism during the tectonic evolution of Sumatra since the late Palaeozoic has provided a background for the formation of economic deposits of metallic minerals, coal, oil and gas. The volume provides a sound basis for future geological research and for the exploration of the energy and mineral resources of the island.

Visit our online bookshop: http://www.geolsoc.org.uk/bookshop Geological Society web site: http:llwww.geolsoc.org.uk

Cover illustration:

Main image: topographic map of Sumatra, courtesy NASAIJPL-Caltech.

ISBN 1-86239-180-7

Top right: eruption of Merapi from Bukit Tinggi, 19 July 1993; photograph by A. J. Barber.Bottom right: oil-drilling rig in the jungle, central Sumatra; photograph by ChuckGaughey,Caltex Pacific, Indonesia.

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