Chapter 1 Introduction and previous research A. J. BARBER, M. J. CROW & J. S. MILSOM , 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 Mentawaiislands from Simeulue to Pagai, which with Enggano form aforearc chain to the SW, and the 'Tin Islands' of Bangka and Billiton and the islands to the east. The backbone of the main island is formed of the Barisan Mountains, which extendthe whole length of Sumatra in a narrow belt, parallel to, andgenerally 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 MtKerinci 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 2000m 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 >6000 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 Ninety- East Ridge (Fig. 1.2). In terms of present-day tectonics Sumatra forms the active southwestern margin of the Sunda Craton (Sundaland), the southeasternpromontory 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 Wadati-Benioff 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 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 around (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 ()' 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 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 ~AN.~D AACEH elo 1 O0 ~ 0 % 1MEDAN % Batu Indian Ocean \ \ % t

(b) LATE CARBONIFEROUS - EARLY PERMIAN Separation of Sibumasu from Gondwana Transient Ice Shelf Continental Prograding Continental Ocean-floor Transient Ice Shelf C ontinental, Glaciers and Shelf Sediments Spreading Prograding Continental Ice Sheets Bonaparte, Tanamurra Opening Gulf Shelf Deposits u aclers an(] and tillites with icebergs Bohorok (titlites), Alas Ice Sheets MESO-TETHYS (c) END PERMIAN - EARLY TRIASSIC Collision of West Sumatra and Sibumasu with East Malaya (Indochina) Block Magmatic Arc No sedimentary record BRS Volcanics I ! i l L I ~ .~4~.Subducted ! "~.~L segment of Medial Sumatra "~Palaeo- Tectonic Zone Tethys BRS - Bentong-Raub Suture 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. 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). 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 245 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. 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 247 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 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 NE-SW extension, with the formation of several north-south and NW-SE 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. Palaeogeographic map of Sumatra and the Malay Peninsula in the Mid- and Late Triassic. 248 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. 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 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. krn 50 WOYLA ARC SEGMENT OF MESO-TETHYS MARGIN OF SUNDALAND Late Jurassic- Triassic - Mid-Cretaceous Ocean Middle Jurassic- Early Cretaceous Early Cretaceous Oceanic Arc Seamount Magmatic Arc _~ Accretionary with carbonate Accretionary Asai-Rawas-Peneta sea, ._c7 ~ reefs Complex can Complex Forearc Basin /b.. (a) MID-CRETACEOUS km 50 ACCRETED WOYLA TERRANE SUNDALAND CONTINENT Renewed Oceanic island arc Late Cretaceous Inverted Triassic Basins Subduction overthrust onto Magmaf itic Arc Woy_la9 (b) LATE CRETACEOUS 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 249 Late Jurassic-mid-Cretaceous 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 Aptian- Albian 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 post- Miocene 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 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 (ll3Ma) 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 mid- Cretaceous 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 emergence of the Barisan Mountains 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 250 CHAPTER 14 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 and 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). 252 CHAPTER 14 Fig. 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 SSE-NNW 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 NNW-SSE 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 -1 been at a uniform rate of about 1.8 cm a 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 253 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 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 of 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 254 CHAPTER 14 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 255 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 45~ ~ 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 NW-SE 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 Ciletuh- Meratus 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 NW-SE 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. These extensional faults trended in a NNE-SSW 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 NE-SW 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 256 CHAPTER 14 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 (Meso- Tethys) which were subducted to form the Woyla accretionary complex. Sedimentological studies of 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 257 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 examination of the lithologies and structure, whether the Pemali Group forms part of an accretionary complex. Folding and the development of slaty cleavage in the Jurassic- Cretaceous 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-Ar and Rb-Sr database requires major expansion and confirmation by the use of additional techniques, such as 39Ar/4~ U-Pb 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-Ar 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 Pb-Zn 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 Pb-Zn 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. Continued search for energy resources (coal, oil and gas) Expanding demand for energy within Indonesia and worldwide, and diminishing reserves elsewhere, will encourage petroleum 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 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 259 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. 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 summary of K-Ar, Rb-Sr and Ar-Ar age data for which methods and locations are documented. The summary is updated from the compilations by McCourt et al. (1996; Supplementary Publication), and the SEAges Database (2004) http://www.gl. rhul.ac.UK/seasia. Additional information kindly provided by Professors Robert Hall and Herv6 Bellon.