The Origin of the Woyla Terranes in Sumatra and the Late Mesozoic Evolution of the Sundaland Margin

Journal of Asian Earth Sciences 18 (2000) 713–738 www.elsevier.nl/locate/jseaes

The origin of the Woyla Terranes in Sumatra and the Late Mesozoic evolution of the Sundaland margin

A.J. Barber

SE Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK Received 5 June 1999; accepted 29 April 2000

Abstract The Jurassic–Cretaceous Woyla Group of northern Sumatra includes fragments of volcanic arcs and an imbricated oceanic assemblage. The arc rocks are intruded by a granitic batholith and are separated from the original continental margin of Sundaland by the oceanic assemblage. Rocks of the arc assemblage are considered to be underlain by a continental basement because of the occurrence of the intrusive granite and of tin anomalies identiﬁed in stream sediments. Quartzose sediments associated with the granite have been correlated with units in the Palaeozoic basement of Sumatra. From these relationships a model has been proposed in which a continental sliver was separated from the margin of Sundaland in the Late Jurassic to Early Cretaceous in an extensional strike-slip faulting regime, producing a short-lived marginal basin. The separated continental fragments have been designated the Sikuleh and Natal microcontinents. In the mid-Cretaceous the extensional regime was succeeded by compression, crushing the continental fragments back against the Sundaland margin, with the destruction of the marginal basin, now represented only by the imbricated oceanic assemblage. Modiﬁcations of this scenario are required by subsequent studies. Age-dating of the volcanic assemblage and intrusive granites in the Natal area showed that they formed part of an Eocene–Oligocene volcanic arc and are not relevant to the model. Thick-bedded radiolarian chert and palaeontological studies in the oceanic Woyla Group rocks of the Natal and Padang areas showed that they formed part of a more extensive and long-lived ocean basin which lasted from at least Triassic until mid-Cretaceous. This raised the possibility that the Sikuleh microcontinent might be allochthonous to Sumatra and encouraged plate tectonic reconstructions in which the Sikuleh microcontinent originated on the northern margin of Gondwanaland and migrated northwards across Tethys before colliding with Sundaland. Since these models were proposed, the whole of Sumatra has been mapped and units correlated with the Woyla Group have been recognised throughout western Sumatra. These units are reviewed and the validity of their correlation with the Woyla Group of northern Sumatra is assessed. From this review a revised synthesis for the Late Mesozoic tectonic evolution of the southwestern margin of Sundaland is proposed. ᭧ 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction of the island (Fig. 1). Earthquake activity indicates that movement along the fault, continues to the present day. The Indonesian island of Sumatra forms the southwestern The Sumatran Fault System is interpreted as the result of margin of Sundaland, the Southeast Asian promontory of oblique subduction of the Indian Ocean Plate beneath Suma- the Eurasian continental tectonic plate. At the present time tra (Fitch, 1972). the Indian–Australian plate is being obliquely subducted The concept of the Woyla terranes, was developed from beneath this margin at a rate of ϳ7 cm/aϪ1. Geologically, the interpretation of the mapping results of the ‘Integrated Sumatra consists of a Pre-Tertiary continental basement, Geological Survey of Northern Sumatra’, conducted jointly overlain by a number of oil and gas-bearing Tertiary sedi- by the Indonesian Directorate of Mineral Resources (DMR) mentary basins (Fig. 1). Towards the southwestern coast of and the British Geological Survey (BGS) between 1975 and the island the basement is uplifted, exposing Pre-Tertiary 1980. This survey comprised geological and geochemical rocks in the Barisan Mountains. The Quaternary volcanic reconnaissance mapping of the whole of Sumatra to the arc, with currently active volcanoes, has been constructed north of the equator (Page et al., 1978). Among the results on top of this uplifted basement. All the rock units in Suma- of the survey, a series of 16 geological map sheets at the tra, Pre-Tertiary basement, Tertiary sediments and Recent scale of 1:250,000 with accompanying descriptive booklets, volcanics, are cut by the dextral Sumatran strike-slip fault were published by the Indonesian Geological Research and system which extends from NW–SE along the whole length Development Centre (GRDC), Bandung. A summary of the geology and the stratigraphy, with an interpretation of the E-mail address: [email protected] (A.J. Barber). tectonic development of Sumatra, based on the results of

Fig. 1. Simplified geological map of Sumatra, showing the distribution of the Woyla Group and correlated units, with localities mentioned in the text. this survey, were presented at the 9th Annual Convention of Gumai and Garba Mountains of South Sumatra, and further the Indonesian Petroleum Association (Cameron et al., south near Bandarlampung along the Sunda Strait. 1980). Geological mapping of the southern part of Sumatra In the north, the Woyla Group and its correlatives lie by the Indonesian Geological Survey in collaboration with largely to the southwest of the Sumatran Fault Zone, the United States Geological Survey commenced in the while to the south, they are only exposed to the northeast 1970s and was completed by 1995 with the publication of of the fault zone. In this contribution, the extent to which the last of 43 geological map sheets covering the whole of these units have been correctly correlated with the Woyla Sumatra. Group, the interpretations which have been proposed for the The DMR/BGS mapping programme commenced in origin of the Woyla Group and its correlatives, and the Aceh in northern Sumatra where three tectonostratigraphic validity of the concept of the Woyla terranes, will be units forming the Pre-Tertiary basement were recognised: discussed and re-examined in the context of the tectonic the Permo-Carboniferous Tapanuli Group; the Permo-Trias- evolution of the southwestern margin of Sundaland during sic Peusangan Group; and the Jurassic–Cretaceous Woyla the Late Mesozoic. Group. As mapping proceded southwards similar units, covering the same age ranges, were recognised and correlated with those of northern Sumatra. Subsequently, units 2. Woyla Group in Aceh resembling the Woyla Group and with the similar ages have been identified throughout the whole of the western part of The Woyla Group was defined in Aceh, northern Suma- Sumatra (Fig. 1). Rock units correlated with the Woyla tra, where these rock units are extensively exposed in the Group are described from Natal in North Sumatra, at Indar- Barisan Mountains (Fig. 2). Areas of outcrop of the Woyla ung and Siguntur near Padang in West Sumatra, in the Group are shown on the GRDC Banda Aceh, Calang, A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 715

Fig. 2. The distribution of the Woyla Group in Aceh. For explanation see text. Modiﬁed from Stephenson and Aspden (1982), with data from Bennett et al. (1981a, b) and Cameron et al. (1982, 1983).

Tapaktuan and Takengon 1:250,000 Quadrangle Sheets occurs as lenses along the Sumatran Fault and along the (Bennett et al., 1981a, b; Cameron et al., 1982, 1983). The Geumpang Line. The largest of these lenses extends discon- descriptions given below, except where speciﬁed, are taken tinuously for 27 km to the northwest of Tangse. Chromite largely from the reports which accompany these maps. The occurs in ﬂoat in streams near this locality. Here and else- Woyla River, from which the Woyla Group was named, is where, serpentinite is locally sheared, schistose, twisted and on the Takengon Sheet (Fig. 2). contorted. Sheared serpentinite may also form the matrix to During the DMR/BGS survey 13 lithostratigraphic units me´lange (Indrapuri Complex). The me´lange encloses blocks were distinguished in the Woyla Group, as well as a unit of of cumulate gabbro, basalt, red chert and limestones, ‘undifferentiated Woyla’. These units generally occur as derived from other members of the Woyla Group. Fossils fault-bounded lenses, distributed on both the northeastern collected from limestone blocks within the me´lange include: and southwestern sides of the Sumatran Fault, and are elon- corals: Latoceandra ramosa, Stylina girodi; foraminifera: gated in a NW–SE direction, parallel to the Sumatran trend. Pseudocyclamina sp.; stromatoporoid: Stromatopora japo- The Woyla Group is also affected by several thrusts; the nica, indicating a Late Jurassic to Early Cretaceous age. In Geumpang, Takengon and Kla lines. The distribution of the Takengon Quadrangle large blocks of limestone these units and the outcrops of the thrusts are shown enclosed in sheared serpentinite along the Geumpang on Fig. 2. Line, contain Late Miocene fossils (Cameron et al., 1983).

2.1. Lithological units in the Woyla Group 2.1.2. Penarum Formation Outcrops to the northeast of the Sumatran Fault, south of 2.1.1. Tangse Serpentinite (Cahop Serpentinite, Beatang Takengon, and consists of serpentinites, basalts, red cherts Ultramafic Complex–Takengon Sheet) with radiolaria and slates. Volcanic rocks are commonly Massive serpentinite, representing altered harzburgite, altered to greenschists. 716 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 2.1.3. Geumpang Formation (Banda Aceh Sheet); Babahrot 2.1.8. Bentaro Volcanic Formation (Banda Aceh Sheet); Formation (Tapaktuan Sheet) Tapaktuan Volcanic Formation (Tapaktuan Sheet) Outcrops occur, on both sides of the Sumatran Fault to the Porphyritic basalts and andesitic basalts with agglomer- southeast of Banda Aceh and to the northwest of the Anu- ates, intruded by basic dykes. Basaltic vents surrounded by Batee Fault towards Tapaktuan (Fig. 2). Rock types include breccias, tuffs and volcaniclastic sediments have been iden- massive or schistose basic volcanics, pillow basalts, volca- tified near Lam No and north of the Bentaro River on the niclastic sandstones and tuffs, commonly epidotised and Banda Aceh Sheet. Chemical analysis of a xenolithic, altered to greenschists or phyllites, and thin grey or black porphyritic basalt with pyroxene phenocrysts is given in limestones. The phyllites are usually lineated and crenu- Rock et al. (1982). lated, indicating multiple deformation. The rocks of the The Tapaktuan Volcanic Formation occurs in fault- Geumpang Formation are considered to constitute the typi- bounded lenses, within strands of the Anu-Batee Fault cal lithological and structural assemblage of the Woyla Zone, parallel to the west coast of Aceh north of Tapaktuan. Group. The Geumpang Formation also includes a massive It consists of massive epidotised andesites and basalts, limestone member, frequently occurring as marble. commonly porphyritic, and intrusive dykes of a similar The Babahrot Formation, north of Tapaktuan includes composition. An analysis of hornblende microdiorite from serpentinites and talc schists, as well as metagabbroic this formation is given in Rock et al. (1982). The formation bodies metamorphosed in the greenschist facies, highly also includes agglomerates, breccias, tuffs, red and purple disrupted and sheared into lenses. 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. 2.1.4. Lam Minet Formation (Banda Aceh Sheet); Gume Scattered outcrops of gneiss (Meukuk Gneiss Complex) Formation (Takengon Sheet) occur within the Tapaktuan Volcanic Formation in the Bari- Composed of basaltic lavas, commonly epidotised, basal- san Mountains to the north of Tapaktuan, between strands of tic conglomerates and breccias, with volcanic and limestone the Anu-Batee Fault (Fig. 2). They consist of concordant clasts, but only rarely chert, graded volcaniclastic wackes, leucogranodioritic gneiss, with garnet-biotite amphibolite radiolarian cherts with manganese oxide veining, rhodonite, containing garnets up to 8 cm in diameter, and biotite-horn- and calcareous, manganiferous and carbonaceous slates. A blende-andesine schist. clast of radiolarian chert, embedded in a volcanic conglomerate with flattened clasts, was collected by Nick Cameron 2.1.9. Lho’nga Formation (personal communication, 1999) in the Kreung Baro, Aceh, Outcropping to the west of Banda Aceh and composed of from a landslip within the outcrop of this formation. This grey and coloured slates and phyllites, with interbedded occurrence indicates that volcanic rocks were erupted volcaniclastic sandstones, thin limestones and (?)radiolar- through ocean floor sediments, perhaps during the formation ian-bearing siltstones. of a seamount. The formation also includes a recrystallised limestone member. 2.1.10. Lhoong Formation Forms a large outcrop to the southwest of the Sumatran Fault and also occurs as roof pendants in the Sikuleh Bath- 2.1.5. Jaleuem Formation olith (Bennett et al., 1981b). It consists of basaltic lavas with Outcrops occur 100 km to the southeast of Banda Aceh, cherts in the lower part of the sequence, followed by on both sides of the Sumatran Fault. Composed largely of conglomeratic wackes with volcanic and limestone clasts, slates, but red cherts occur in float and the unit also includes and subordinate sandstones, siltstones and limestones. a limestone member. Rb/Sr dating on 11 samples of slate gave 99 ^ 30 Ma (late Early Cretaceous). 2.1.11. Raba Limestone Formation Outcrops along the coast and in the Barisan Mountains to 2.1.6. Bale Formation the south of Banda Aceh and consists of massive calcarenite Shown outcropping to the northwest of the Sumatran and calcilutite and dark thin-bedded cherty limestones and Fault, and southeast of Takengon. Composed of coloured shales. The massive limestone constitutes a ‘reef member’ slates, with minor wackes and cherts, limestones and lime- which is closely associated in the field with the Bentaro stone breccias. Volcanic Formation.

2.1.12. Lamno Limestone Formation 2.1.7. Sise Limestone Formation Outcrops along the west coast of Aceh, south of Banda Consists of massive or bedded limestones, biocalcarenites Aceh, where it is associated with outcrops of the Bentaro and calcilutites with fossils: corals: Montlivaltia sp., Myrio- Volcanic Formation. Consists of dark limestone, which pora sp.; foraminifera: Pseudocyclamina sp. indicating a includes a reef-like facies, and contains volcanic clasts Late Jurassic to Early Cretaceous age. near the base. The limestone is commonly fossiliferous, A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 717 with: corals: Actiastraea minima, Stylosmilia corallina; structures, boudinage and the development of tension algae: Clypeina sp., Permocalculus ampullacea, Litho- gashes normal to the bedding. codium–Bacinella sp. Boueina sp., Thaumatoporella porvo- On the map scale all the units in the Woyla Group are vesiculifera; foraminifera: Pseudocyclammina lituus, fault-bounded and form large scale lenses elongated in a indicating a Late Jurassic to Early Cretaceous age. NW–SE direction parallel to the Sumatran trend. Internally the units are disrupted by large numbers of faults. Some of these faults are thrusts, along which rock units have been 2.1.13. Teunom Limestone Formation imbricated; others show slickensided surfaces indicating Outcropping along the southwestern margin of the Siku- strike-slip movement along faults parallel to, and probably leh Batholith. Composed of massive dark limestones, meta- related to the Sumatran Fault System. morphosed and recrystallised along the contact with the granite. 2.3. Sikuleh Batholith ‘Undifferentiated’ Woyla Group rocks are shown on the Calang Quadrangle Sheet in the area to the south of the The Woyla Group is intruded by granitoids, the largest of Sikuleh Batholith in Gunung Paling and as roof pendants which is the Sikuleh Batholith, illustrated on the Banda within the outcrop of the batholith (Bennett et al., 1981b). In Aceh and Calang sheets (Bennett et al., 1981a, b). This is the Explanatory Note to this sheet these rocks are said to an approximately elliptical body (ϳ55 × 35 km) elongated resemble the Kluet Formation, low-grade Carboniferous in a NW–SE direction (Fig. 2). Around the margins of the metasediments of the Sumatran basement, which outcrop batholith, ‘undifferentiated’ Woyla Group rocks and lime- extensively to the northeast of the Sumatran Fault. The stones of the Teunom Formation, have been altered by implication here is that these rocks are composed of slates contact metamorphism. Roof pendants of metamorphosed with interbedded quartzose sandstones. They are also Woyla Group rocks, shown as ‘undifferentiated’ on the map, reported to include quartzite (Bennett et al., 1981b). Quart- but compared to the Lhoong formation, occur within the zose rocks are otherwise unknown in the Woyla Group. batholith (Bennett et al., 1981b). Rocks of the Geumpang Many of the mapping units distinguished in the Woyla Formation are thrust over the northeastern margin of the Group of Aceh during the DMR/BGS survey are made up of batholith along the Geumpang Line. the same rock types but in varying proportions. It is clear Although no attempt was made during the mapping to that they represent geographical, rather than genuine lithos- delineate the various components of the batholith, descrip- tratigraphical units. A different name was given to each tions show that it is a complex and highly variable body distinguishable unit on each map sheet. The actual litholo- (Bennett et al., 1981b). An ‘older complex’ is distinguished gical units within each formation are, on the whole, too in the northern part of the complex, composed of dark and small to be represented on the scale of the map. xenolithic gabbroic and dioritic bodies, migmatitic and intensely veined. The older complex is locally gneissose or sheared. Where foliation is developed, it is parallel to 2.2. Folds and related structures the foliation in the adjacent Geumpang Formation rocks. The ‘younger complex’, which has an intrusive contact The rocks of the Woyla Group in Aceh commonly show against the earlier complex, is a more homogeneous, coarser the effects of intense deformation. The finer grained sedi- grained and largely unfoliated, biotite-hornblende grano- mentary and volcaniclastic rock types have been altered to diorite, with minor amounts of diorite and pink granite. slates and phyllites. Isoclinal folds can be seen wherever The granodiorite contains mafic xenoliths and becomes bedding lamination can be distinguished, with axial plane more mafic towards its margins, where it shows ‘flow folia- cleavage parallel to bedding, and a bedding/cleavage inter- tion’. The granodiorite and is cut by pegmatites and aplites, section lineation plunging to the SE. Slaty cleavage may be the latter being more common adjacent to the contact with refolded, into more open folds on sub-horizontal axes and the older complex. The batholith shows ‘porphyry-type’ NE-dipping axial planes, with the development of second- molybdenum mineralisation and tin anomalies of 10–80 ary cleavages, lineations and kink band folds. Larger kilo- ppm have been identified from stream sediment sampling meter scale upright folds, up to ϳ7 km, on NW–SE trending along the contact between the older and younger complex, axes and steep axial planes are reported from the Tapaktuan although no primary tin mineralisation has been recognised Volcanic Formation, with secondary folds on the scale of (Stephenson et al., 1982). The younger complex has been ϳ1–2 km (Cameron et al., 1982). dated from the mean of K/Ar analyses on two biotites and Massive limestone units in the Woyla Group are less one hornblende, as 97.7 ^ 0.7 Ma (early Late Cretaceous) obviously internally deformed, although mylonitised and (Bennett et al., 1981b). The complex is cut by later basic kink-banded limestones are seen near Banda Aceh. Bedded dykes. limestones are sometimes folded on a large scale, as seen in quarries in the Lho’nga Formation between Banda Aceh and 2.4. Thrusts Lho’nga (Bennett et al., 1981a). Also in the same area, limestones interbedded with shales show pinch and swell Bennett et al. (1981a, b) and Cameron et al. (1980, 1983) 718 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 recognised three major thrust zones in northern Sumatra Units of the oceanic assemblage include the Tangse and (Fig. 2): Cahop serpentinites, the Beatang Ultramafic Complex and the Penarum, Geumpang, Babahrot, Lam Minet, Jaleueum 2.4.1. Takengon Line and Bale formations. All these units lie to the northeast of This thrust has an E–W outcrop to the south of Lake the Geumpang Line, and almost entirely to the northeast of Tawar, where it brings Permo-Triassic rocks of the Peusan- the Sumatran Fault Zone (Fig. 2). Unfortunately, no age- gan Group on top of the Penarum Formation of the Woyla diagnostic radiolaria have yet been obtained from the many Group along a flat-lying, southward-directed thrust plane. occurrences of bedded (?radiolarian) chert which occur To the west of Takengon, along the continuation of this line, among these units, so that their age(s) has not been estab- Miocene rocks are thrust over the Woyla Group. lished directly. Limestone blocks in serpentinous me´lange along the Geumpang Line, which yielded Late Jurassic to 2.4.2. Kla Line Early Cretaceous fossils, were ascribed to the Geumpang A northwards-directed thrust plane, outcropping to the Formation (Cameron et al., 1983), but are more reasonably south of Takengon between the main Sumatran Fault and regarded as derived tectonically from underlying limestone the Anu-Batee Fault, brings Permo-Carboniferous Tapanuli units during the thrust movements, in the same way as Late Group rocks onto ‘undifferentiated’ Woyla Group. Mylo- Miocene blocks which are also included in the me´lange. nites are developed along the thrust plane. The oceanic assemblage of the Woyla Group in Aceh shows an intimate mixture of ocean floor materials from 2.4.3. Geumpang Line different structural levels, from mantle to abyssal sediments, In the area to the southeast of Banda Aceh, units of the variously internally deformed, separated by faults, often Woyla Group are separated by an easterly dipping thrust, the identified as thrusts and arranged in a random order. Geumpang Line, which outcrops parallel to the Sumatran These are the characteristic features of an accretionary Fault, and has the same NW–SE trend. Here members of the complex, where ocean floor materials are imbricated against Woyla Group have been thrust southwestwards over Late the hanging wall of a subduction zone. Garnetiferous Miocene sediments, the northeastern margin of the Sikuleh amphibolites of the Meukuk Gneiss in the Tapaktuan Volca- Batholith and other members of the Woyla Group. Serpen- nic Formation suggest that gabbroic rocks of the oceanic tinite me´lange, containing a variety of Woyla Group rocks, assemblage were subducted into the mantle, before being including blocks of Late Jurassic–Early Cretaceous and returned tectonically to the surface. Late Miocene limestone, occurs along the fault plane. Although these thrusts were active during the Tertiary, and 2.5.2. Arc assemblage affect rocks up to Late Miocene in age, Bennett et al. (1981a, b) This is typified by the Bentaro and Tapaktuan Volcanic and Cameron et al. (1983) consider that they represent more Formations, and is composed of porphyritic andesite, occur- fundamental thrust surfaces in the Pre-Tertiary basement ring as vents near Bentaro, basalt lavas, intrusive dykes, which were reactivated during Late Tertiary or Quaternary agglomerates, breccias, volcaniclastic sandstones and shales. movements along the Sumatran Fault Zone. They suggest These igneous rocks are closely associated in the field that serpentinite me´langes, commonly found along the fault with massive or bedded limestones, which commonly and thust planes, were derived from mantle peridotite thrust contain volcanic clasts or are interbedded with volcaniclas- beneath the margin of Sundaland, which was hydrothermally tic units. These limestone units include the Lho’nga, Raba, mobilised and rose diapirically along fault and thrust planes Lamno, Lhoong, and Teunom Limestone formations, as during subsequent fault movements. well as the limestone lenses which occur within the Tapak- tuan Volcanic Formation. Most of the units belonging to the 2.5. Interpretation of the Woyla Group in Aceh arc assemblage lie to the southwest of the Geumpang Line and the Sumatran Fault. The only exception to this distribu- Cameron et al. (1980) distinguished two lithological assem- tion, according to the geological map of Stephenson and blagesin the Woyla Group in Aceh: an oceanic (ophiolitic) and Aspden (1982), is the Sise Limestone Formation, outcrop- an (volcanic) arc assemblage. The distribution of these ping to northeast of the Sumatran Fault, to the the south of assemblages is represented on the ‘Simplified Geological Takengon (Fig. 2). This anomaly can be removed by rever- Map of Northern Sumatra’ (Stephenson and Aspden, 1982): sing displacement along the Sumatran Fault. The Lamno and Sise limestones and the ‘Geumpang’ limestone blocks 2.5.1. Oceanic assemblage within the Indrapuri Complex all contain fossils which indi- This includes serpentinised harzburgite, metagabbro, cate a Late Jurassic–Early Cretaceous age. mafic to intermediate volcanics, often basaltic and showing Cameron et al. (1980) suggest that the intrusive and pillow structures, volcanic breccias, volcaniclastic sand- volcanic rocks represent the remnants of a Late Jurassic– stones, red and purple manganiferous slates, red radiolarian Early Cretaceous volcanic arc. This arc emerged above sea cherts. These rock types also occur as blocks in breccio- level, where it was eroded to form breccias and volcaniclas- conglomerates or me´langes. tic sediments. The arc was surrounded and overlain by a A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 719 fringing reefs represented by the massive limestones. In this occurred between the Late Jurassic and Late Cretaceous. model, massive limestones were eroded to form limestone It is suggested that rifting of the continental margin and breccias, and detritus was transported into deeper water to the formation of the volcanic arc occurred in the Late Juras- form bedded limestones. Cameron et al. (1980) put forward sic and the opening of the marginal basin took place in the evidence which indicated that the volcanic arc was built up Early Cretaceous. The intrusion of the Sikuleh Batholith on a sliver of continental crust. The observation of a volca- into both the oceanic and the arc assemblages shows that nic breccia containing radiolarian chert clasts in the Lam the closure of the basin and the compression of the arc Minet Formation indicates that volcanic rocks were some- against the continental margin was completed by Late times extruded through ocean floor sediments. Cretaceous times (Cameron et al., 1980). Woyla Group rocks which took part in these events are 2.6. The marginal sea model now sliced into lenticular slivers by more recent movements along the Sumatran Fault (Cameron et al., 1980). In the field Cameron et al. (1980), noting the absence of a mid- it is difficult to distinguish between thrust faults which were Cretaceous magmatic arc in the eastern part of northern formed by the imbrication of the units during subduction, Sumatra, rejected the interpretation that the oceanic assem- and later strike-slip faults related to the Sumatran Fault blage of the Woyla Group represents the accreted remnants System unless slickensides are present. It is impossible to of older segments of the ‘Indian Ocean’ in an earlier stage of distinguish between strike-slip faults formed as the result of the subduction process which continues today offshore oblique subduction and those due to later fault movements southwest Sumatra. Instead, they interpreted the oceanic (Wajzer et al., 1991). assemblage as the remnants of a short-lived marginal ocea- As has already been suggested, the anomalous position of nic basin, developed along the southwestern margin of the massive limestones of the Sise Limestone Formation, Sundaland in a phase of extreme oblique subduction and which now lie on the northeastern side of the main strand active strike-slip faulting, with the formation of a pull- of the Sumatran Fault, may be the result of movements apart basin and then oceanic crust. along the fault. The Sise Limestone Formation and the The marginal basin was formed in Late Jurassic–Early Lamno Limestone Formation near Banda Aceh, contain Cretaceous times by the separation of a continental sliver similar fossil assemblages and are of the same age. Rever- from the western margin of Sundaland, in the same way as sing the dextral movement along the fault for a distance of the Andaman Sea or the Gulf of California are being forming at ϳ200 km would bring the Sise and Lamno formations into the present time (Cameron et al., 1980, their Fig. 4a). The juxtaposition. This order of movement along the Sumatran separated continental sliver is now represented by the Bentaro Fault is compatible with the evidence from regional consid- Volcanic Arc and the Sikuleh Batholith. Also associated with erations, with a 460 km movement in the Andaman Sea the batholith are ‘undifferentiated’ Woyla metasediments reducing southwards to a displacement of only a few kilo- which resemble the Carboniferous Kluet Formation of the metres in the Sunda Strait (Curray, 1989; McCaffrey, 1991). Sumatran basement, and tin anomalies. All these features are Strike-slip fault movements along the Sumatran Fault taken as evidence that the arc volcanics overlie continental System are also considered to have reactivated fundamental crust. On the Tectonic Map of Northern Sumatra the basement thrusts in the basement along the Takengon, Kla and Geum- underlying the arc assemblage is described as the Sikuleh pang lines, as Neogene rocks are affected by this movement, Continental Fragment (Aspden et al., 1982). in transpressional zones along strands of the fault system Subsequently, as a result of continuing subduction (Cameron et al., 1980). outboard of the continental fragment and transpressional strike-slip movement, this small marginal ocean basin was compressed, crushing both the floor of the marginal basin 3. Woyla Group in Natal and the detached continental fragment against the Sunda- land margin. The Takengon and Kra lines represent the Rocks correlated with the Woyla Group of Aceh were thrust surfaces along which the materials of the marginal mapped over an extensive area inland from Natal in North basin were thrust beneath the continental margin of Sunda- Sumatra during the Integrated Geological Survey of North- land. The suture, representing the site of this disappeared ern Sumatra as part of the Lubuksikaping 1:250,000 Quad- marginal basin, coincides approximately with the line of the rangle Sheet (Rock et al., 1983) (Fig. 3). The outcrop is present day Sumatran Fault. Also, as the result of compres- limited to the northeast by the Sumatran Fault System and sion, the continental fragment and its overlying arc were is much dissected internally by faults with a similar trend. thrust beneath the oceanic assemblage along the Geumpang The Woyla Group is intruded by Late Cretaceous granites Line (Cameron et al., 1980). and overlain unconformably by the Miocene Barus Group, Although there is no direct evidence for the age of the by Miocene volcanic rocks, and by the products of Quatern- ocean floor materials, and the age of the volcanic arc is ary volcanism from the volcanoes of Sorik Merapi, Malin- established only from the palaeontological dating of its tang and Talamau, as well as by recent alluvium. Units fringing reefs, all these events are considered to have within the Woyla Group strike NW–SE and are very well 720 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738

Fig. 3. The distribution of the Woyla Group in the Natal area, North Sumatra. Modified from Rock et al. (1983). KFZ Kanaikan fault zone; SGF Simpang Gambir Fault. exposed in the valley of the Batang Natal, both in the river The Belok Gadang Formation outcrops in the central part section and in the parallel road section, which both cut of the section and is composed of sandstones, sometimes across the strike (Fig. 4). The main outcrop of the Woyla calcareous, and argillaceous rocks, often cleaved and Group is separated from a smaller outcrop in the Pasaman containing bands and lenses of chert. The chert is radiolar- inlier to the south by Malintang Volcano (Fig. 3). ian, but no identifiable radiolaria have so far been recovered which could be used to date the sequence. Outcrops in the 3.1. DMR/BGS report type locality of Belok Gadang, a tributary of the Batang Natal, show basaltic pillow lavas, with white clay interbeds In the DMR/BGS report of the Lubuksikaping Quadran- and manganese-rich horizons with braunite, resembling the gle (Rock et al., 1983) lithological units in the Batang Natal ‘umbers’, described from the Troodos Ophiolite of Cyprus section were classified, from NE–SW, into three forma- (Robertson, 1975). Analysis shows that the pillow basalts tions: the Maurosoma, Belok Gadang and the Sikubu forma- are spilites (Rock et al., 1982, 1983). In the type locality tions (Fig. 3). basalts are overlain by red, bedded cherts, but again no The Maurasoma Formation outcrops in the upstream part identifiable radiolaria have been recovered. of the section and in its tributary the Aik Soma. Thicknesses The Sikubu Formation, outcropping in the lower part of of the rock units in this section were measured perpendicu- the Batang Natal section, is composed of massive volcani- lar to the strike for a distance of 5.5 km (Rock et al., 1983). clastic metagreywackes, with thin shale interbeds. The The rock types include cleaved argillaceous units, shale or sandstones show very well-developed sedimentary struc- slate, which may include calcareous concretions, laminated tures, including graded bedding, flame structures and convo- siltstones, and gritty sandstones showing sedimentary struc- lutions, typical of turbidites. Massive porphyritic andesitic tures, indicating younging in a downstream direction, dykes and lava flows, with distinctive pyroxene pheno- massive limestones, sometimes forming limestone pinna- crysts, are intruded into or interbedded with the sediments cles, epidotic volcanic breccias and volcaniclastic sand- in the lower part of the section. Fragments of porphyritic stones, chloritic greenschists and muscovite-chlorite andesite, identical in composition to the dykes and lavas, quartz schists. A 10 m ‘conglomerate’ (? me´lange) at the occur as clasts in the sandstones. upstream end of the section, with elongated clasts of greens- Woyla Group rocks in the Pasaman area include chist in a chloritic matrix, is probably of tectonic origin, me´langes and massive and foliated peridotites (Rock et formed in a fault or a shear zone (Rock et al., 1983). al., 1983) (Fig. 3). Peridotites are well exposed in the A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 721

Fig. 4. Geological map of the Batang Natal river section, North Sumatra. Inset shows isotopic dates (from Wajzer et al., 1991). S serpentinite.

Pasaman River where they are composed mainly of harzbur- show that these lavas have a highly unusual chemical gite with minor dunite pods, pyroxenite dykes, disseminated composition with low SiO2 and high MgO contents (Rock chromite and rare chromite pods. Some of the peridotite is et al., 1982, 1983). Exceptionally high contents of Cr, Ni foliated, containing orthopyroxenes enclosed in augen. and Co are correlated with the abundance of augite. The Coarse plagioclase-hornblende rocks, found as boulders in absence of hypersthene, the lack of plagioclase and the the float, represent metasomatised gabbro pegmatite which presence of orthoclase, together with the overall chemical formed dykes in the peridotite. The peridotite is variably composition, shows that these lavas are absarokites (basic serpentinised, and in shear zones may be completely altered shoshonites) (Rock et al., 1983). to serpentine and talc. Smaller bodies of serpentinite, with chromite pods, outcrop at the upper end of the Batang Natal section near Maurasoma (Figs. 3 and 4) where they form 3.3. Late Mesozoic intrusions spectacular serpentinite breccias faulted against slates and limestones of the Maurasoma Formation. Serpentinite also Several large granitoid bodies are intruded into the rocks occurs as xenoliths in granite in the Aik Soma. of the Woyla Group in the Natal area. The largest is the Manunggal Batholith at the northeastern end of the Batang 3.2. Langsat Volcanic Formation Natal section (Rock et al., 1983) (Fig. 3). This batholith is a composite body, some 230 km2 in extent, composed of Porphyritic, net-veined and amygdaloidal basic lavas of leucogranodiorite, granodiorite, granite, pyroxene-quartz the Langsat Formation, associated with agglomerates, diorite with contaminated syenitic and monzonitic varieties, outcrop at the southwestern end of the Batang Natal section and appinites. The granitoid rocks are intruded by vogesite (Rock et al., 1983) (Fig. 3). The lavas are melanocratic but lamprophyre dykes. This granitoid has been dated by the K/ contain leucocratic xenoliths. The basic rocks are highly Ar method at 87 Ma (Late Cretaceous) (Kanao et al., 1971, porphyritic, with abundant clinopyroxene phenocrysts, but reported in Rock et al., 1983). In the Aik Soma, near Maur- show very little plagioclase in thin section; the dominant asoma, large granitic boulders in the river bed enclose feldspar in the ground mass being orthoclase. Analyses serpentinite xenoliths, surrounded by reaction zones of 722 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 amphibolite. Limestones in the same area are converted to tion, with pillow basalts and cherts, was interpreted as the skarns near the contact with the granite. deformed floor of the marginal basin, and the Sikubu A second granitoid, the Kanaikan intrusion, is intruded Formation in the southwest as the erosion products of a into the Woyla Group in the Pasaman area (Fig. 3). This volcanic arc. The arc itself is represented by the the Langsat body is composed of coarse granodiorite and leucogranite Volcanics which outcrop at the southwest end of the Batang cut by microgranitic and granophyric dykes. The intrusion Natal section (Rock et al., 1983) (Figs. 3 and 4). As in Aceh, lies within the Kanaikan Fault Zone, a strand of the main the arc was considered to overlie continental basement and Sumatran Fault, and is much dissected by faults and constitutes a microcontinental block, identified in the deformed to cataclasite along shear zones. ‘Tectonic Map of Northern Sumatra’ as the Natal Continen- tal Fragment (Aspden et al., 1982). The Air Bangis grani- 3.4. Palaeogene intrusions toids, considered to be intrusive into the Langsat Volcanics, although this relationship is not actually observed, are Granitic rocks form scattered outcrops in headlands near regarded as evidence that the arc is underlain by continental Air Bangis, along the coast to the south of Natal (Fig. 3). A crust. Again, using the age constraints provided by the large area inland of these outcrops on the Lubuksikaping occurrence of Lovçenipora in Woyla Group rocks in the Sheet is also shown as occupied by granitoid. This inland Pasaman inlier and the age of intrusion of the Manunggal outcrop is largely conjectural, as the interpretation is based Batholith, Rock et al. (1983, their Fig. 8) postulate that on aerial photographic interpretation, and much of the area between Late Jurassic and Early Cretaceous time, a pull- is covered by cloud on the photographs used in the inter- apart rift developed into a short-lived marginal basin along pretation (Rock et al., 1983). The coastal outcrops are the margin of Sundaland, which was then compressed and composed of hornblende-biotite adamellites, leucogranites destroyed by the Late Cretaceous. Rock et al. (1983) and granites, cut by microgranitic and microdioritic dykes. acknowledged that if the Cretaceous age ascribed to the These intrusions are shown on the Lubuksikaping Sheet as Air Bangis granitoids was not correct, then the marginal of Eocene–Oligocene age. However, Rock et al. (1983) in sea model was invalid and the arc and its underlying conti- their report, speculate that these intrusions might be of Late nental basement might constitute a terrane allochthonous to Cretaceous age, analogous to the Sikuleh intrusion which Sumatra. In the key to the Lubuksikaping Map Sheet the Air ocupies a similar position with respect to the Woyla Group Bangis granitoids are shown as of Late Cretaceous/Palaeo- in Aceh. gene age. 3.5. Age constraints in the Natal area 3.7. Wajzer study Age constraints for the Woyla Group in the Natal area are provided by a limestone sample from the Batang Kanaikan The Batang Natal section was mapped in detail by Marek in the Pasaman inlier which yielded a colonial organism, Wajzer from the University of London, in a follow-up study closely resembling the samples of Lovçenipora described to the Northern Sumatra Survey, in collaboration with BGS and illustrated by Yancey and Alif (1977) from the Indarung and with the assistance of Syarif Hidayat and Suharsono of area, near Padang, and considered to be of Late Jurassic to GRDC (Wajzer et al., 1991). The mapping was supported by Early Cretaceous age (IGS/British Museum Sample No.TC/ petrographic, geochemical and radiometric studies. Wajzer J1/R1101B- Rock et al., 1983). A minimum age for the et al. (1991) found that the Woyla Group was not neatly Woyla Group is provided by the Manunggal Batholith, divisible into three distinct lithological units, but that the dated at 87.0 Ma (Late Cretaceous) (Kanao et al., 1971, same recognisable lithologies were repeated many times quoted in Rock et al., 1983), which intrudes limestones throughout the section, apparently in a random fashion and serpentinites at the northwest end of the Batang Natal (Fig. 4). Wajzer et al. (1991) distinguished 16 lithostratigra- section. phical units in the Natal section. Correlation with the mapping units recognised by Rock et al. (1983) is shown 3.6. Interpretation — the marginal sea model in Table 1. Detailed accounts of the lithological units are given in Table 2. Many of the lithologies are similar to rock The model developed by the DMR/BGS mapping team to types described from the Woyla Group in Aceh, and by explain the origin and tectonic environment of the Woyla Rock et al. (1983), with the addition of several outcrops Group in Aceh was extended southwards, with minor modi- of me´lange, composed of blocks in a fine grained matrix, fications, to account for the distribution of similar rock decribed as ‘megabreccia’ in Table 2 and Fig. 4. One impor- assemblages of Jurassic to Cretaceous age outcropping in tant feature of the clastic units in the Woyla Group of the the Natal area of North Sumatra (Rock et al., 1983). Natal area is that they are almost completely devoid of Rock et al. (1983) interpreted the Maurosoma Formation quartz, suggesting that they have an entirely oceanic, rather at the northeastern end of the section, which includes turbi- than a continental origin (Wajzer et al., 1991). dites and massive limestones, as shelf sediments of the The study established several additional age constraints Sundaland continental margin. The Belok Gadang Forma- for the Woyla Group, using fossil evidence and radiometric A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 723

Table 1 deformation, with the earlier isoclinal folds and schistosity Correlation of formations in the Woyla Group of the Natal area from Rock a folded by more open folds, and the development of second- et al. (1983) with lithotectonic units defined by Wajzer et al. (1991) ary cleavages and lineations. Some units such as the Rante Rock et al. (1983) Wajzer et al. (1991) Sore Formation are completely unmetamorphosed, while metabasites show pumpelleyite and actinolite, and schists 1. Langsat Volcanic Formation 1. Langsat Volcanic Formation contain coarse white micas, indicating metamorphism up to 2. Sikubu Formation 2. Si Kumbu Turbidite Formation 3. Tambak Baru Volcanic Unit greenschist facies. There is no systematic progression either 4. Simpang Gambir Megabreccia in intensity of deformation or in grade of metamorphism Formation through the section, the units appearing to be randomly 3. Belok Gadang Formation 5. Nabana Volcanic Unit arranged (Wajzer et al., 1991). 6. Belok Gadang Siltstone Formation Each of the units distinguished in the Woyla Group in the 7. Panglong Me´lange Formation 8. Ranto Sore Formation Natal River section is separated from adjacent units by stee- Volcanics in both the Belok 9. Parlampungan Volcanic Unit ply dipping or vertical faults. In addition units are frequently Gadang and Maurasoma disrupted internally by faults, often every few metres. Some Formations of the fault planes show vertical slickensides, indicating 4. Maurasoma Formation 10. Si Gala Gala Schist Formation normal or reverse movement, while others show horizontal Schistose Member 11. Simarobu Turbidite Formation 12. Batang Natal Megabreccia Unit slickensides, indicating strike-slip movement (Wajzer et al., 13. Rantobi Sandstone Formation 1991). 14. Jambor Baru Formation 15. Maurasoma Turbidite Formation Massive limestones in both the 16. Batu Nabontar Limestone Unit 3.10. Interpretation — the allochthonous terrane model Belok Gadang and Maurasoma Formations The Batang Natal section was reinterpreted by Wajzer et al. (1991). They pointed to the absence of any significant a Units are listed in approximate order upstream from Langsat (see Fig. 4) amount of quartz in the volcaniclastic sediments or any with no age relationship implied. continental clastic material in the massive limestones of the Maurosoma Formation, and suggested that these deposits were of oceanic, rather than of continental margin origin. dating. A further specimen of Lovçenipora was obtained They concluded, from the extensive deposits of bedded radi- from a limestone block in the Simpang Gambir Megabreccia olarian chert and associated manganiferous argillites, that near the southwestern end of the Batang Natal section, and a the oceanic deposits represented accreted fragments of the Late Triassic foraminifera was found in a limestone clast in floor of a major ocean basin, rather than the floor of a the Batang Natal Megabreccia in the central part of the restricted marginal sea. As in Aceh, in the absence radio- section. Diorite intruded into the Jambor Baru and Batang metric ages for the pillow basalts, or of diagnostic radiolaria Natal Megabreccia Formations at Batu Madingding gave a in the chert, there is no direct evidence of the age of forma- K/Ar age of 84.7 ^ 3.6 Ma and an andesite in the Tambak tion of the ocean floor. Wajzer et al. (1991) suggest that the Baru Volcanic unit, interpreted as a fragment of a volcanic volcanic units represent seamounts or fragments of volcanic arc, gave 78.4 ^ 2.5 Ma. Both these lavas and the intrusions arcs, and that limestone blocks in the me´langes represent are of Late Cretaceous age. Andesite dykes intruded into the fragments of the collapsed carbonate cappings to these Si Kumbu Turbidite Formation (i.e. Sikubu Formation of seamounts, or fringing reefs to the arcs. Foraminifera in a Rock et al., 1983), and regarded as contemporaneous with limestone block in one of these me´langes suggest that a sedimentation of this unit, gave K/Ar ages of 40.1 ^ 4.6 and seamount was constructed on ocean floor which was at 37.6 ^ 1.3 Ma (Late Eocene) (Wajzer et al., 1991). Samples least as old as Triassic, and that therefore the ocean basin collected from the Air Bangis granitoids and analysed by was already in existence in the early Mesozoic. Wajzer gave K/Ar ages of 29.7 ^ 1.6 and 28.2 ^ 1.2 Ma Wajzer et al. (1991), on the basis of interbedded lava (Late Oligocene) (Wajzer et al., 1991) raising the possibility flows and the occurrence of fragments of andesite in the that the Cretaceous age for these granitoids suggested in the conglomerates, recognisably derived from the Langsat DMR/BGS report (Rock et al., 1983) was incorrect. Volcanics, accepted the interpretation of Rock et al. Wajzer et al. (1991) also studied the structure in detail (1983) that the Si Kumbu Turbidite Formation (their Sikubu (Table 2). Most units in the Natal River section show some Formation) was contemporaneous with the Langsat Volca- degree of internal deformation. Deformation ranges from nics, and that the volcanics and volcaniclastic turbidites broad open folding of the bedding in the Si Kumbu and represent a volcanic arc. Andesite dykes intruded into the Rante Sore formations, to isoclinal folding and penetrative Si Kumbu Formation, of similar composition to the lavas, slaty cleavage in fine grained units and mylonitic foliation in gave radiometric dates of 37.6 and 40.1 Ma (Late Eocene). the Si Gala Gala Schists. Clasts in breccia formations As has already been reported, Rock et al. (1983), following (me´langes) are flattened and elongated parallel to the clea- the marginal sea model from Aceh, suggested that the Air vage in the matrix. Several units are affected by multiple Bangis granitoids, which are presumed to be intruded into 724 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 2.5 lange 2Ma ^ ´ ^ 3.6 Ma a ^ sp. In limestone 4.6 Ma (NR45), 1.3 Ma (NR120) 3.6 Ma ¸enipora ^ ^ ^ Lovc – – – Granites. K/Ar 28.2 Ma, 29.7 Ma 40.1 37.6 Andesitic lava. K/Ar 78.4 Ma (BN133) block (Late Jurassic–Early Cretaceous) ? siltstone Formation diorite dyke. K/Ar 49.5 (NR 7) contain Late Triassic foraminifera. Intruded by Batu Madingding Diorite. K/Ar 84.7 Intruded by Batu Madingding Diorite. K/Ar 84.7 Batholith. K/Ar 87.0 Ma Manunggal Batholith from Kanao et al. (1971). All o E, see Fig. 4) (from Wajzer et al., 1991) greenschist greenschist Prehnite-pumpellyite/ greenschist Slate grade Older than Belok Gadang Unmetamorphosed ?Younger than adjacent units greenschist Greenschist – Greenschist Cut by undeformed micro- Slate grade Included limestone clasts Slate grade – Prehnite-pumpellyite/ greenschist Prehnite-pumpellyite/ greenschist Recrystallised Intruded by Batu Manunggal D2 large scale folds (F2) on WNW–ESE axes Prehnite-pumpellyite Intruded by andesite dykes K/Ar D1 weak foliation (S1).D2? Prehnite-pumpellyite/ D1 strong foliations (S1).D2 opencrenulations folds (F2) and D1 tight to isoclinal foldsclose (F1). folds D2 (F2) open fold to F1 on NW–SE axes Dipping beds with no ductile deformation Prehnite-pumpellyiteaxes Younger than Panglong Me D1 schistosity (S1) and roddingto (L1). close D2 open folds (F2) on NW–SE axes tight to isoclinal folds (F1)NNE–SSE axial axes plane on D1 tight to isoclinal foldsclose (F1). folds D2 deform open S1 to aboutD1 tight NNE–SSW to axes. isoclinal folds (F1)foliation with (S1). axial D2 plane open todeform close S1 folds on (F2) NNW–SSE axes. (F1) with D2 close asymmetric folds (F2) D1 foliation (S1). D2 close foldsSE (F2) axes on NW– D1 foliation (S1). D2 foldsaxes (F2) on NW–SE show strain lange (?olistostrome) of lange. ?lower trench slope lange formed as ´ ´ ´ Submarine fan — apronvolcanic to arc proximal volcaniclastics Proximal sediments derived from volcanic arc, with olistostromes Ocean-floor basalts, seamount No ductile deformationMe ocean-floor materials and pelagic sediments Prehnite-pumpellyite/ Unconformable on Panglong Me basin fill Fluviatile intra-arc deposits D2 open to close folds (F2) on NNW–SSE Metasediments derived from acid-intermediate volcanic arc province Ocean-floor or trench deposit Foliation (S1). D2 open to close folds (F2). D1 Me olistostrome or as mud diapirs in accretionary complex Forearc basin deposits Axial plane cleavage (S1). D1 isoclinal folds Shallow marine and deeper water forearc basin deposits Upper trench slope basin sediments Open marine shelf limestone D1 tight folds in interbedded tuffs (F1), fossils Volcaniclastic debris flows, proximal and distal turbidites Andesitic volcanics Fragments of volcanic arc and Volcanic breccia with limestone megaclasts and greywacke sandstones Breccias with chert, Mn sedim. limestones and volcanic clasts insiltstone chert matrix amygdaloidal to E, keratophyres, dolerite dykes Volcaniclastic siltstones with few fine sandstones and rare conglomerates channelled sandstones and unsorted conglomerates (lahars) Porphyritic andesitesschists Fragments of volcanic arc No ductile deformation Prehnite-pumpellyite/ Volcaniclastic turbidites with minor calcareous siltstones Large clasts of limestone, raresediments clastic and igneous rocks inmatrix slaty Thin bedded volcaniclastic sandstones and siltstone sandstone, siltstone, limestone and tuff Thin bedded volcaniclastic turbidites with a coarser grained member Massive recrystallised limestone, rare fossils lange ´ All units are cut by numerous faults and thrusts. Vertical faults often show horizontal slickensides indicating wrench fault movements. *K/Ar age of a Table 2 Lithology, environmental setting, structure, metamorphic grade and ageUnit constraints for units in the Batang Natal sectionLangsat (in Volcanic order Unit upstream from W t Porphyritic basic volcanics Lithology Arc volcanics No ductile deformation Environment Prehnite-pumpellyite Structure Possibly intruded by Air Bangis Metamorphism Age constraints Si Kumbu Turbidite Formation other K/Ar ages from Wajzer et al. (1991). Tambak Baru Volcanic Unit Simpang Gambir Megabreccia Formation Formation Nabana Volcanic Unit Basic volcanics (sometimes pillowed) Panglong Me Belok Gadang Siltstone Formation Ranto Sore Formation Volcaniclastic cross-bedded and Parlumpangan Volcanic Unit Si Gala Gala Schist Unit Banded quartz, muscovite, chlorite Simarobu Turbidite Formation Batang Natal Megabreccia Formation Rantobi Sandstone Formation Jambor Baru Formation Volcaniclastic conglomerate, Maurosoma Turbidite Formation Batu Nabontar Limestone Unit A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 725 the Langsat Volcanics, were of Late Cretaceous age, and Lubuk Peraku River, the Ngalau Quarry, the Karang Putih therefore analogous to the Sikuleh Batholith. However, as Quarry and adjacent river sections near Indarung. Rock already pointed out, they acknowledged that if this correla- types in these outcrops are basic volcanics, which may tion was incorrect then the Aceh marginal /sea model could include pillow lava, volcanic breccia, tuff, volcaniclastic not be applied to the Natal area. In the event, the Air Bangis sediments, radiolarian chert and massive or bedded lime- granitoids gave ages of 28 and 29 Ma, indicating that the stones. The basic rocks are sometimes deformed and meta- Langsat Arc was formed during Late Eocene–Early Oligo- morphosed to form greenschists. On the other hand, the cene times (Wajzer et al., 1991). limestones and cherts are essentially undeformed, although Wajzer et al. (1991) were impressed by the evidence of disharmonic folding and small scale thrusts in the chert and strike slip faulting throughout the Natal section, and in parti- gentle folds in the limestone are seen in the quarries, and the cular by the major Simpang Gambir Fault, which separates limestones may be recrystallised (McCarthy et al., 2001). the Si Kumbu Turbidites (Sikubu Formation of Rock et al., A well exposed section of limestone and tuff occurs in the 1983) and the Langsat Volcanics from the Belok Gadang river section of the Lubuk Peraku and in the road above the ocean floor assemblage (Fig. 4). They therefore suggested river (Yancey and Alif, 1977; McCarthy et al., 2001). that the Natal Continental Fragment was an allochthonous Bedding in the limestones has a steep, but variable dip to terrane, which had reached its present position by strike-slip the southwest, and the section youngs in the same direction. movement from a location further to the south along the A measured columnar section of these outcrops from Sumatran continental margin. The possibility of an McCarthy et al. (2001) is given as Fig. 6. The lower part allochthonous origin for the Natal Continental Fragment of the section, described as the Lubuk Peraku Limestone, is was also considered by Rock et al. (1983), but rejected in a limestone breccia, which includes volcanic clasts near the favour of the interpretation previously reached for the base and is interbedded with thin tuff bands near the top. tectonic evolution of the Woyla Group in Aceh (Cameron The breccia is overlain by a few metres of thin-bedded lime- et al., 1980). Although as already pointed out they acknowl- stones and shelly marls and then by thicker bedded and more edged that if the Late Cretaceous age for the Air Bangis massive limestones, some oolitic. Near the top of the section granitoids was incorrect, then an allochthonous origin was a limestone conglomerate, eroded into the underlying lime- probable. Wajzer et al. (1991) reversing the correlation, stone with basal scours, provides clear evidence of way-up. suggested that if the Natal Microcontinental Fragment is Above the breccia there is a break in the outcrop, until an allochthonous terrane, then it is possible that the Sikuleh outcrops further downstream and in the road section above Microcontinental Fragment although of a different age, is expose a calcareous vitreous crystal tuff, the Golok Tuff. also allochthonous. Although the contact between the breccia and the tuff is not exposed, McCarthy et al. (2001) regard this section as an essentially continuous stratigraphic sequence, as tuff 4. Other units in Sumatra correlated with the Woyla layers occur within the limestone breccia and tuffs within Group the overlying Golok Tuff unit are calcareous. In the Ngalau Quarry, near Indarung, McCarthy et al. Outcrops of rock units with similar lithologies to those of (2001) collected samples from a 15 m section of bedded the Woyla Group or which were formed within the same chert for radiolarian determination. In the Karang Putih Jurassic–Cretaceous age range have been mapped through- Quarry, 1 km to the south of Indarung, excavated in an out western Sumatra (Fig. 1). Many of these outcrops have isolated hill for the manufacture of cement, lenses of chert been correlated with the Woyla Group of northern Sumatra are associated with massive limestone. McCarthy et al. by previous authors. (2001) report that the limestone in this quarry is completely 4.1. Indarung formation recrystallised, possibly due to the effects of a granitic intrusion which occurs a short distance to the south (Fig. 5). They Small outcrops of the Mesozoic Indarung Formation suggest that relationships between limestone and chert in occur near Padang in West Sumatra. These rocks were this quarry are tectonic. In an interpretative cross section mapped and described by Yancey and Alif (1977) and they show the cherts and limestones imbricated together were correlated with the Woyla Group of Aceh by Cameron along low angle thrusts (McCarthy et al., 2001). et al. (1980). Outcrops occur 15 km east of Padang in road, Rock units in the Indarung area are well dated from fossil river and quarry sections near Indarung, where they are and radiometric age determinations. Radiolaria from chert surrounded and overlain by Neogene and Quaternary volca- in the Ngalau Quarry belong to the Transhsuum hisui- nic and volcaniclastic rocks (Fig. 5). The area of outcrop is koyense Zone, of Aalenian, early Middle Jurassic age included on the Padang, Solok and Painan Quadrangle (McCarthy et al., 2001). Lithologies and fossil content of Sheets (Kastowo and Leo, 1973; Silitonga and Kastowo, the limestones in the Lubuk Peraku section and in the 1975; Rosidi et al., 1976). These rocks have been mapped Ngalau and Karang Putih quarries were described by more recently by McCarthy et al. (2001). Yancey and Alif (1977). The limestones are biosparites, Yancey and Alif (1977) described rocks exposed in the with abundant bioclasts, oolitic calcarenites and micrites. 726 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738

Fig. 5. Distribution of outcrops of the Indarung and Siguntur formations in the Padang area, West Sumatra. Based on GRDC maps (Kastowo and Leo, 1973; Silitonga and Kastowo, 1975; Rosidi et al., 1976).

Molluscan shell fragments, pellets, calcareous algae, massive limestone with its Late Jurassic–Early Cretaceous stromatoporoids and scleractinian corals are common fossil fauna is interpreted as part of a fringing reef formed components of the limestones. Among the fossils identified around the seamount (McCarthy et al., 2001). During were the (?) stromatoporoids Actostroma and Lovçenipora. subduction the seamount with its carbonate cap collided The former is considered to be restricted to the Late Juras- with already accreted ocean floor materials, and the whole sic, while the latter is diagnostic of the Late Jurassic to Early assemblage was imbricated to form the present complex. Cretaceous. A K/Ar age date of 105 ^ 3 Ma (Albian, mid- Cretaceous) is reported from the Golok Tuff in the Lubuk 4.2. Siguntur formation Peraku by Koning and Aulia (1985, from a Caltex Pacific Indonesia internal report). Mesozoic rocks of the Siguntur Formation are exposed in Pillow lavas and cherts of the Indarung Formation have the Sungai Siguntur, 15 km to the south of Indarung (Fig. 5). been equated with the oceanic assemblage of the Woyla The area of outcrop is shown on the Painan Quadrangle Group of Aceh and with the Belok Gadang Formation of Sheet and the lithology is described in the Explanatory the Natal area (Cameron et al., 1980; Rock et al., 1983). Note (Rosidi et al., 1976). Rock types are quartzites, silt- Where these rocks are imbricated, deformed and altered to stones and shales, the latter sometimes altered to slates, and greenschists they may be interpreted, as is the case in Aceh compact limestones. The map shows that the strike of the and Natal, as materials accreted from a subducted ocean beds is E–W, transverse to the general Sumatran trend. In floor. The recent recognition of Middle Jurassic radiolaria the report the rocks are described as not intensely deformed in the cherts (McCarthy et al., 2001) shows that part of this or folded, but sandstones interbedded with slates showing ocean floor was of Jurassic age. The volcanic breccias tuffs bedding-parallel cleavage, suggest that the rocks are more and volcaniclastic sandstones of the Indarung Formation are highly deformed than first appears. The limestones are interpreted as the products of seamount volcanism, and the reported to contain Lovçenipora, and are therefore of a A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 727

Fig. 6. Columnar section through the Lubuk Peraku Limestone and the Golok Tuff, measured in the Lubuk Peraku river section (from McCarthy et al., 2001). similar age to the limestones at Indarung. However, abun- ments which were deposited on the Sundaland continental dant terrigenous quartz in the clastic sediments, notably margin. absent from the Woyla Group in Natal and the rock units at Indarung, suggests that the sediments of the Siguntur 4.3. Suilak, Tabir, Asai, Peneta and Rawas formations in Formation had a continental provenance and were deposited central Sumatra in a continental margin environment. Although these rocks are deformed, with the develop- Further outcrops of Mesozoic sedimentary and volcanic ment of slaty cleavage, there is no suggestion in the rocks occur at Suilak 150 km to the southeast of Padang descriptions that these rocks have been imbricated or can (Fig. 1), in a fault block caught between strands of the be interpreted as part of an accretionary complex (Rosidi et Sumatran Fault (Rosidi et al., 1976). These sediments are al., 1976). In the Siguntur Formation we are dealing for the calcareous siltstones, calcareous shales and limestones. The ﬁrst time in this account with continentally-derived sedi- shales and siltstones are carbonaceous and contain angular 728 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738

Fig. 7. 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., 1992). quartz clasts. The limestones contain Loftulisa and Hydro- including corals and ammonites, especially from the lime- corallinae of Cretaceous age (Tobler, 1922, reported in stone members, show that these sediments range in age from Rosidi et al., 1976). The volcanic rocks are altered ande- Middle Jurassic to Early Cretaceous (Suwarna et al., 1994). sites, dacites and bedded tuffs with clasts of augite, horn- All these sediments, although subject to later deforma- blende, chlorite and glass. These rocks are the product of tion, were evidently deposited in situ on the Sundaland Andean arc volcanism on the margin of Sundaland. continental basement. Pulunggono and Cameron (1984) Sixty kilometers to the east of Siulak and to the northeast suggested that these units were deposited in a foreland of the Sumatran Fault Zone, in the Batang Tabir, are basin, but a forearc environment, related to an Andean outcrops of red conglomerates, sandstones and tuffs of the volcanic arc represented by the volcanics lava flows and Tabir Formation. Clasts in the conglomerates include quart- tuffs in the Rawas and Tabir Formation is more probable. zite, and andesitic fragments derived from the adjacent Palaeozoic rocks. The presence of Ostrea is taken to indi- 4.4. Gumai Mountains cate a Mesozoic, possibly Jurassic age (Tobler, 1922, reported in Rosidi et al., 1976). Continuous with these Pre-Tertiary rocks form an inlier to the northeast of the outcrops and extending southeastwards to the south of Sumatran Fault Zone in the Gumai Mountains of South Bangko, and also to the northeast of the Sumatran Fault, Sumatra, where they occupy the core of an anticline, are large outcrops of Mesozoic rocks of the Asai, Peneta surrounded by Tertiary sediments and volcanics (Fig. 7). and Rawas formations (Kusnama et al., 1993; Suwarna et These Pre-Tertiary rocks are described as the Saling, Lings- al., 1994), (Fig. 1). Rock types include quartz sandstones, ing and Sepingtiang formations (Gafoer et al., 1992). siltstones, shales, limestones and tuffs. The Rawas Forma- The Saling Formation, which forms the northern part of tion also includes andesite-basalt lava flows, tuffs and volca- the inlier, is composed of amygdaloidal and porphyritic niclastic sandstones. Clasts in conglomeratic units in these andesitic and basaltic lavas, breccias and tuffs. The lavas sediments are derived from the local Palaeozoic basement. are cut by diorite dykes, regarded as contemporaneous with Sandstone units show turbiditic characteristics. Argillac- the lavas, and dated at 116 ^ 3 Ma (Early Cretaceous). On eous units have a slaty cleavage striking NW–SE. Fossils, the basis of chemical analyses and discriminant plots the A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 729 lavas are interpreted as tholeiites of oceanic affinity (Gafoer The Lingsing Formation, composed of siltstones, shales et al., 1992). The Lingsing Formation in the southern part of and cherts, has been interpreted as deposited in a bathyal the inlier, contains similar igneous rocks, interbedded with environment (van Bemmelen, 1949; Gafoer et al., 1992). claystone, siltstone, sandstone, calcilutite and chert. The The presence of lavas interbedded with clastic deposits, Saling and Lingsing formations are therefore considered suggests that the Lingsing Formation represents more distal to be contemporaneous. Since the tholeiitic basalts are asso- flows, volcaniclastic sediments and clastic carbonates ciated with serpentinised ultrabasic pyroxenites and cherts, derived from the arc extending out onto an ocean floor, the whole assemblage is regarded as ophiolitic sequence of represented by the bedded cherts. These rocks resemble ocean floor origin. Although the rocks are highly deformed clastic units in the Lho’nga Formation of Aceh (Bennett et and folded it is not clear from the descriptions whether they al., 1981a) and the Belok Gadang Siltstone and Rantobi are imbricated to form an accretionary complex (Gafoer et Sandstone formations of Natal (Wajzer et al., 1991). al., 1992). The strike of bedding and cleavage in the sediments is said to be N–S. The the mapped E–W contact 4.5. Garba Mountains between the Saling and the Lingsing formations is therefore presumably tectonic (Fig. 7). The Garba Mountains form an inlier of pre-Tertiary The Saling and Lingsing formations are overlain discor- rocks, about 100 km to the southeast of the Gumai Moun- dantly by the Sepingtiang Limestone Formation (Fig. 7). tains and also northeast of the Sumatran Fault Zone (Fig. 1). This is composed of massive, brecciated and bedded lime- The inlier is shown on the GRDC Baturaja Quadrangle stones, containing the coral Calamophylliopsis crassa (Late Sheet (Gafoer et al., 1994) (Fig. 8). The oldest unit, which Jurassic), the foraminifera Pseudotexturariella, small outcrops on both eastern and western sides of the inlier, is Cuneolina (Early Cretaceous) and Orbitolina sp. (mid- the Tarap Formation, composed of phyllite, schist, slate, Cretaceous). The contact between the Sepingtiang Lime- minor quartzite and marble, metamorphosed in the greens- stone and the underlying units is considered to be tectonic chist facies. The age of the Tarap Formation is unknown, but (Gafoer et al., 1992). it is presumed to be older than the adjacent, but unmetamor- All of these units are cut by granitic intrusions, which by phosed Garba Formation, and is interpreted as the metamor- analogy with similar dated granitoids further south in the phosed Palaeozoic basement of Sumatra (Gafoer et al., Garba Mountains, are regarded as of Late Cretaceous age 1994). Outcrops of the Tarap Formation occur within the (Gafoer et al., 1992). The rocks of the inlier and the Garba Formation and in large areas to the east and west. The surrounding Tertiary rocks are also cut by NW–SE trending relationships between the metamorphic Tarap Formation faults, some showing strike-slip displacements (Fig. 7), and and the Garba Formation, as shown on the map, are not are evidently related to the Sumatran Fault System, the main clear (Gafoer et al., 1994) (Fig. 8). The eastern contacts strands of which lie some 25 km to the southwest. are faulted and possibly thrust. If the Tarap is part of the As already noted, Gafoer et al. (1992) interpreted the metamorphosed Palaeozoic basement of Sumatra, it may igneous rocks of the Saling Formation as ocean floor basalts, represent the western margin of the Sundaland continent, on the basis of their tholeiitic composition and their associa- thrust westwards over accreted ocean floor materials repre- tion in the field with serpentinites and cherts. However, the sented by the other units in the inlier. The present distribu- presence of andesites, the amygdaloidal and porphyritic tion of these metamorphic rocks could be due to the textures, together with association of the volcanics with disruption of the thrust sheet by faulting and erosion. the massive Sepintiang Limestone, suggest that the Saling The Garba Formation consists of (?)amygdaloidal and Formation also includes part of a volcanic arc. The descrip- porphyritic basaltic and andesitic lavas, associated with tion of the Saling Formation closely resembles that of the sheared serpentinite and lenses and intercalations of radi- Bentaro Volcanic Formation of Aceh (Bennett et al., 1981a) olarian chert. An Insu Member is distinguished on the map, and the Nabana Volcanic and Parlumpangan units of the with a similar lithological assemblage, but also containing Batang Natal (Wajzer et al., 1991). The Early Cretaceous interlayered lenticular bodies of me´lange, with boulders of age shows that the Saling Volcanic Arc was active contem- basalt, andesite, radiolarian chert, claystone, siltstone, schist poraneously with the Bentaro Arc of Aceh. and massive limestone in a scaly clay matrix (Gafoer et al., The Sepingtiang Limestone may be interpreted in the 1994). The limestones found as blocks do not outcrop else- same way as the limestones in Aceh, as a fringing reef where in the inlier, but are presumed to be derived from an surrounding a volcanic arc. Fossil evidence of the Late unexposed component of the Garba Formation. Blocks of Jurassic to mid-Cretaceous age of the Sepintiang Limestone the Tarap metamorphics are notably absent from the Formation means that it can be correlated directly with the me´lange. The foliation in the scaly matrix and the elonga- Lamno, Teunom and Sise Limestone formations of Aceh tion of the enclosed blocks, which are cut by tension frac- (Bennett et al., 1981a; Cameron et al., 1983), the Batu tures normal to their long axes, trends in a NW–SE direction Nabontar limestones in the Batang Natal section (Wajzer (Gafoer et al., 1994). A fault-bounded sliver on the eastern et al., 1991) and the Lubuk Peraku limestones at Indarung side of the inlier, and a few other scattered outcrops where (Yancey and Alif, 1977). chert is abundant, are mapped as the Situlanglang Member 730 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738

Fig. 8. The distribution of the Garba and Situlanglang formations, correlatives of the Woyla Group, in the Garba Mountains, South Sumatra (after GRDC geological map of Baturaja, Gafoer et al., 1994).

(Fig. 8). Two fold phases are recognised in the Garba represented by volcanics in the Garba Formation, as has Formation, an earlier phase of E–W folds and a later been suggested for the Natal and Indarung areas (Wajzer phase of NE–SW folds (Gafoer et al., 1994). Neither the et al. 1991; McCarthy et al., 2001). Descriptions of the cherts nor the limestones have so far yielded age-diagnostic me´langes of the Insu Member of the Garba Formation fossils. (Gafoer et al., 1994) are identical to those from Natal Both the Tarap and the Garba formations are intruded by (Wajzer et al., 1991). The interlayering of the Insu Member the Garba Pluton, a composite body in which an older with lavas, chert and me´lange (Gafoer et al., 1994) component has been dated at 115 ^ 4 and 102 ^ 3Ma suggests that these rocks are deformed and imbricated in (mid-Cretaceous) and a younger component at 79 ^ 1.3 the same way as the Woyla Group in the Batang Natal and 89.3 ^ 1.7 Ma (Late Cretaceous). Rock units within section, and similarly represent an accretionary complex the inlier are cut and bounded by NW–SE trending faults. formed by subduction of an ocean floor. It may be that Although these faults are parallel to the Sumatran Fault some of the low grade metamorphic schists mapped within System they do not appear to affect significantly the Tertiary the Insu Member as Tarap Formation, are part of this rocks and must be largely of Pre-Tertiary age. accretionary complex, as metamorphic rocks, up to greens- The Garba Formation has been compared to the Woyla chist facies, are incorporated in the accretionary complex Group of Natal (Gafoer et al., 1994) and certainly litholo- at Natal. gical descriptions of this formation and its Insu and Situlan- The mid- to Late Cretaceous Garba Pluton (115–79 Ma) glang members, correspond very well with those from Aceh intrudes both the Tarap and the Garba formations, so that the and the Batang Natal section. The basaltic and andesitic accretion of the Garba Formation to the margin of Sunda- lavas of the Garba Formation correspond with those of the land, if the Tarap is correctly identified as Palaeozoic base- Bentaro Arc, and may similarly be interpreted as part of a ment, took place before the mid-Cretaceous. The age of the volcanic arc sequence. Limestone blocks within the younger component of the Garba Pluton is comparable to me´lange may represent fragments of fringing reefs or the that of the Sikuleh Batholith in Aceh (98 Ma) and the collapsed carbonate cappings of seamounts, the latter now Manunggal Batholith (87 Ma) in Natal. A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 731

Fig. 9. The distribution of the Menanga Formation, correlative of the Woyla Group, in Lampung (after GRDC geological map sheets of Kotaagung and Tanjungkarang, Amin et al., 1994; Andi Mangga et al., 1994). In areas left blank the older rocks are covered by Tertiary and Quaternary sediments and volcanics.

4.6. Bandarlampung area the Gunungkasih Complex (Fig. 9). These consist of tuffac- eous and calcareous claystones, sandstones and shales with Pre-Tertiary rocks occur as scattered outcrops among intercalated radiolarian-bearing cherts, manganese nodules Tertiary and Quaternary volcanic and volcaniclastic sedi- and coral limestones and rare porphyritic basalt. The sand- ments in the area between Kotaagung and Bandarlampung stones contain clasts of glassy andesite and lithic fragments in Lampung Province, southern Sumatra (Amin et al., 1994; of andesite, quartz-diorite and quartzite. The cherts have not Andi Mangga et al., 1994) (Fig. 9). Outcrops towards the so far yielded diagnostic radiolaria, but Zwierzycki (1932, northeast of the area consist of pelitic, calcareous, quartzose confirmed in Andi Mangga et al., 1994), reports the occur- or graphitic schists and sericitic quartzites, with intercalated rence of Orbitolina sp. of Aptian-Albian (mid-Cretaceous) gneiss, marble and amphibolites, described as the Gunung- age from limestones in the Menanga river section. The kasih Complex. Within the complex there is a tendency for bedding strikes NW–SE with dips of 35Њ–60Њ to the north- outcrops to the northeast to be composed of orthogneiss, east. The rocks are folded and cut by faults, with slicken- while those to the southwest are composed of metasedi- sides indicating reverse movement. ments. The strike of the foliation in the schists trends gener- The contact between the Gunungkasih Complex and the ally NW–SE but the schistosity is folded about E–W axes Menanga Formation in Gunung Kasih is obscured, due to and refolded by NW–SE trending upright folds and then by rice cultivation, and in Teluk Ratai is at present inaccessible variably oriented kink band folds. These rocks are suggested as it lies within a Naval Base (Fig. 9). However, the latter to be equivalent to the Tarap Formation of Garba and meta- contact in the Menanga River was described by Zwierzycki morphic equivalents of the Palaeozoic Tapanuli Group in (1932) as occupied by a ‘friction breccia’. On the GRDC Aceh and the Kuantan Formation in the Padang area (Amin maps Amin et al. (1994) and Andi Mangga et al. (1994) et al., 1994; Andi Mangga et al., 1994). show both these contacts as thrusts (Fig. 9). Outcrops of unmetamorphosed rocks, identified as the The Menanga Formation is interpreted by Amin et al. Menanga Formation, lie largely to the the southwest of (1994) as a deep water marine sequence with interbedded 732 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 basalt lavas and andesitic clastic fragments, derived from a form part of the same gneiss complex, but also contain volcanic arc, and deposited in a trench or forearc environ- fragments of the schistose continental basement into ment. These sediments were deformed during accretion to which the igneous rocks were intruded. During or shortly the Sumatran margin, represented by the Gunungkasih after intrusion, both granitic and basic rocks were affected Complex. K/Ar radiometric ages, ranging from 125–108 by sinistral shearing, which converted the granitic and diori- Ma (mid-Cretaceous) from hornblende in an amphibolitic tic rocks into gneisses and deformed the basic dykes. The schist in the Menanga Formation, is taken as the age of alternation of acid and basic intrusion, with contempora- accretion (Andi Mangga et al., 1994). However, the neous deformation, are characteristic features of the basal presence of quartzite and quartz-diorite clasts suggests parts of a magmatic arc, where acid and basic magmas are that the Menanga Formation was, like the Rawas and asso- intruded into an active strike-slip fault zone. This situation ciated formations in central Sumatra, derived from an is similar to that which exists beneath Sumatra at the present Andean arc built on a continental basement, and was depos- day where the modern volcanic arc is built on the active ited in a forearc environment. The Menanga Formation was Sumatran Fault Zone. However, the sense of movement overthrust by the basement at a later stage. along the present arc is dextral, in the opposite sense to Near Bandarlampung the Gunungkasih Complex is the sinistral movement along the Cretaceous arc. intruded by the Sulan Pluton (Fig. 9). The pluton is a composite body which includes gabbro, dated by K/Ar radiometric analysis at 151 ^ 4 Ma (Late Jurassic), horn- 5. Tectonic evolution of the Sundaland margin blende and biotite granites and granodiorite intruded by late aplogranite dykes. Granite from the Sulan Pluton gave an 5.1. Pre-Cretaceous Sundaland Continent age of 113 ^ 3 Ma (mid-Cretaceous) (McCourt et al., 1996). To the north of Bandarlampung, spectacular exposures The concept that Southeast Asia, and indeed Asia as a below an irrigation dam on the Sekampung River show whole, has been built up during the Phanerozoic by the extensive outcrops of granodioritic and dioritic gneiss, amalgamation of allochthonous terranes derived from the containing basic xenoliths, and cut by concordant and northern margin of East Gondwanaland, is now well estab- discordant granitic and pegmatitic veins. The granitic and lished in the literature (e.g. Metcalfe, 1996 and references granodioritic gneisses are cut by basaltic dykes, several therein). In Metcalfe’s (1996) version of the concept a series metres thick, which contain xenoliths of gneiss. The gneiss of elongated terranes separated successively from the North xenoliths show evidence of melting, and towards the Gondwanaland margin by the development of ocean basins margins of the dykes are drawn out into streaks, which are behind them. These oceans are referred to as Palaeo-Tethys, sometimes isoclinally folded, parallel to the dyke margins. Meso-Tethys and Ceno-Tethys. The Indochina Block which The dykes and the foliation in the gneisses both trend in a forms the core of Southeast Asia had separated from Gond- NW–SE direction. Fold structures in the dykes and the wanaland by Late Devonian times and amalgamated with curvature of foliation in the gneisses indicate that the dyke the South China Block by the Early Carboniferous. To this margins have acted as strike-slip shear zones, with a sinistral core was added the Shan-Thai or Sibumasu Block, which sense of movement. Sub-horizontal slickensides on foliation separated from Gondwanaland in the Permian and amalga- surfaces within the gneiss indicate the same sense of move- mated with the Indochina Block in the Late Permian or ment. Diorite from the Sekumpang exposure has been dated Triassic (Metcalfe, 1996). by the K/Ar method at 89 ^ 3 Ma (late mid-Cretaceous) Following the completion of the Integrated Geological (McCourt et al., 1996). Survey of Northern Sumatra, these concepts were extended In the same area, in the Wai Triplek, greenschist facies by Aspden et al. (1982) and Pulunggono and Cameron white mica-quartz schists are intruded by metadolerite (1984) to interpret the structural evolution of the southwes- dykes. The margins of the dykes show compositional band- tern margin of Sundaland in Sumatra. The model was later ing which is isoclinally folded, in a similar fashion to the extended to cover southern Sumatra (McCourt et al., 1993). dykes in the Sekampung River. Further upstream the bed of The interpretation of the structure of Sumatra illustrated in the Wai Triplek exposes streaky acid and basic gneisses cut Fig. 10 is adapted from the Tectonic Map of Northern by more homogeneous basic dykes. Acid gneiss shows Sumatra (Aspden et al., 1982). In this map Sumatra is evidence of having been melted and recrystallised along shown with its present NW–SE orientation, but several the dyke contacts, and quartz-feldspar veins fill fractures reconstructions, including those by Metcalfe (1996), show in brecciated basic dyke material, in a process of back Sumatra with an E–W orientation during the Mesozoic. The injection. Indochina Block is represented by the East Malaya Micro- Relics of dyke rocks occurring as basic xenoliths in plate, occupying the eastern part of the Malay Peninsula and gneiss, and gneiss xenoliths enclosed in in basalt dykes, sutured to the Malacca Microplate along the Raub-Bentong indicate that the intrusion of basaltic dykes and granitic Line. To the west the Malacca Microplate is joined by the bodies alternated during the development of the gneiss Mergui Microplate along a Triassic suture known as the complex at Sekampung. Exposures in the Wai Triplek Mutus Assemblage, identified only from the occurrence of A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 733

Fig. 10. The structure of the southwestern margin of Sundaland (with its present orientation) in Late Cretaceous times, according to the interpretation given in the paper. Note that the effects of post-Middle Miocene movements along the Sumatran Fault Zone have been removed. Data from sources quoted in the text. mauve shales, cherts, volcanic rocks, gabbros and serpenti- Sumatra, McCourt et al. (1996) identify a belt of Middle nites in oil company boreholes (Eubank and Makki, 1981; Jurassic to Early Cretaceous intrusions, including the Sulit Pulunggono, 1983). Air suite (203 ^ 6 Ma) and the Bungo Batholith (169 ^ 5 In northern Sumatra the Mergui Microplate is made up of Ma), adjacent to the suture between the Mergui Microplate the Permo-Carboniferous Tapanuli Group, including the and the Permian Volcanic arc. These plutons are subduc- Bohorok Formation, composed of ‘pebbly mudstones’, inter- tion-related, I-type granites formed by the northeasterly preted as glacio-marine sediments, and correlated with the subduction of an oceanic plate which lay off the continental Singha Group in the Langkawi Islands and the Phuket margin of Sundaland, to the southwest (Fig. 11A). Group of western Thailand (Cameron et al., 1980). South- Remnants of the volcanic rocks related to this phase of wards, these rocks are recognised in the Tigapuluh hills of magmatism may be represented by andesitic and basaltic central Sumatra (Simandjuntak et al., 1991; Suwarna et al., volcanic rocks in the Rawas, Tabir and Siulak formations 1991). It is also suggested that the Gunungkasih Complex of of central Sumatra and by tuffs in the Menanga Formation of the Bandarlampung area may represent the extension of the southern Sumatra. However, there is no evidence that this metamorphosed representatives of the Tapanuli Group into arc extended into northern Sumatra. southern Sumatra (McCourt et al., 1993; Amin et al., 1994; This magmatic arc assemblage is termed the Rawas Arc Andi Mangga et al., 1994). A Permian volcanic arc was added in the reconstruction illustrated in Fig. 11A. Associated to the southwestern margin of these amalgamated terranes in deep-water turbiditic sandstones, shales and the shallow- theTriassic(McCourtetal.,1996). water fossiliferous limestones of the Rawas, Asai, Peneta and Menanga formations, well-dated by fossil evidence and 5.2. The Late Mesozoic Continental Margin ranging in age from mid-Jurassic to Early Cretaceous, were contemporaneous with the magmatic arc, and can be inter- In their review of Mesozoic to Cenozoic plutonism in preted as forearc basin deposits (Fig. 11A). Further to the 734 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738

Fig. 11. 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. The sections are drawn to approximate true scale. The length of each section is about 800 km. northwest, clastic sediments of the Tabir, Siulak and Sigun- with the Woyla Group, are the Aptian-Albian foram Orbi- tur formations which contain both volcanic and continen- tulina which occurs in the Menanga Formation near Bandar- tally-derived clasts, may also have been deposited in this lampung and the Sepingtiang Limestone Formation of the same forearc basin. Gumai Mountains. Evidently the last remnants of the Woyla Oceanic crustal material and slices of mantle, remnants of Ocean were subducted late in mid-Cretaceous times. This the oceanic plate which was being subducted off the south- segment of ocean floor can be identified as part of the Meso- western margin of Sundaland at this time and incorporated Tethys, which lay to the north of India before that continent into an accretionary complex, now form the ‘oceanic assem- separated from Gondwanaland (Metcalfe, 1996). blage’ of the Woyla Group. As described in this account, units composed of ocean floor components, and correlated 5.3. Bentaro-Saling Volcanic Arc with the Woyla Group, have been recognised all along the west coast of Sumatra, from Banda Aceh to the Garba Basaltic and andesitic volcanic rocks, agglomerates, tuffs Mountains. The continental margin against which accretion and volcaniclastic sediments correlated with the Woyla took place can be recognised in Aceh, where rocks of the Group occur throughout the western part of Sumatra, from Tapanuli and Peusangan groups, representing the Mergui Banda Aceh to the Gumai Mountains. These units include Microplate and the Permian Volcanic arc, are thrust over the Bentaro and Tapaktuan Volcanic formations in Aceh, the Woyla Group along the Kla and Takengon lines. the Tambak Baru and Parlumpungan units in Natal, the The age of the ocean floor which was being subducted is Saling Formation in the Gumai Mountains and the Garba indicated only by fragmentary evidence, but a Late Triassic Formation in the Garba Mountains. At Bentaro near Banda age is indicated by foraminifera in a limestone block in the Aceh these rocks occur as vents surrounded by breccias, Batang Natal section, interpreted as a seamount capping tuffs and volcaniclastics and intruded by basic dykes. The (Wajzer et al., 1991) and by mid-Jurassic radiolaria in lavas are often amygdaloidal and porphyritic and are closely chert from Indarung (McCarthy et al. 2001). Abundant associated in the field with massive or bedded limestones. fossil evidence from Late Jurassic–Early Cretaceous faunas Limestone units include the Lho’nga, Raba, Lamno, in limestones correlated with the Woyla Group, from Aceh Lhoong and Teunom Limestone formations of Aceh, the to the Gumai Mountains, show that the ocean with sea- Batu Nabontar Limestone Unit of Natal, the Lubuk Peraku mount caps or fringing reefs continued to exist throughout and Karang Putih limestone formations of Indarung and the these periods. The youngest fossils, found in units correlated Sepingtiang Limestone Formation of the Gumai Mountains. A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 735 The limestones commonly contain volcanic clasts, espe- blage, including mantle peridotite, of the Woyla Group. In cially in basal units, and may be interbedded with volcani- the Gumai and Garba Mountains the accreted oceanic clastic deposits. No direct age for the volcanism has yet assemblage and the arc rocks are now situated to the north- been obtained from the volcanic rocks themselves but, as east of the Sumatran Fault Zone. In Gumai granitic rocks are detailed in this account, the limestone units have sometimes intruded into the Saling Formation of the arc assemblage proved abundantly fossiliferous and contain a wide range of and in the Garba Mountains the Garba Pluton (115–90 Ma) fossil groups, indicating ages ranging from Late Jurassic to is intruded into both arc and oceanic material and also into Early Cretaceous. Cameron et al. (1980) interpreted the the Tarap Formation, regarded as metamorphosed Palaeo- intrusive and extrusive volcanic rocks in Aceh as the zoics of the Mergui Plate (Gafoer et al., 1994). In Bandar- remnants of a Late Jurassic to Early Cretaceous volcanic lampung the Sulan Pluton (113 Ma) and the Sekampung arc, emergent above sea level where it was eroded to form Complex (89 Ma) are intruded into the Gunungkasih breccias and volcaniclastic sediments. The arc was Complex, again interpreted as Palaeozoic basement rocks surrounded and overlain by fringing reefs represented by of the Mergui Plate (Amin et al., 1994; Andi Mangga et al., the massive limestones. In this model, limestones were 1994). eroded to form breccias, passing into bedded detrital lime- As has been described in this account, detailed observa- stones in deeper water. This interpretation may be extended tions in the Sekampung Gneiss Complex provide evidence to all the other occurrences of these rock units in Sumatra that granitic and basic rocks of the Late Cretaceous arc were which have been correlated with the Woyla Group (Fig. 11). intruded into an active shear zone, suggesting that the Late The Bentaro-Saling Volcanic Arc collided with the Cretacous arc, like the present arc, was developed during a margin of Sundaland when the last fragment of the subduct- phase of oblique subduction and was intruded into an active ing ‘Woyla’ (Meso-Tethys) ocean was incorporated into the transcurrent fault system. Reports of ‘flow foliation’ in the accretionary complex in the late mid-Cretaceous. The site of Sikuleh Batholith (Bennett et al., 1981b) and of gneissose the collision is marked in Aceh by the Geumpang Line rocks in other Late Cretaceous plutons, may also have the where the ‘oceanic assemblage’, forming an accretionary same significance. Kinematic indicators at Sekampung complex, is thrust over the rocks of the arc and its fringing show that this transcurrent fault system operated in a sinis- reefs (Figs. 10, 11B). The collision had repercussions tral sense, in the opposite sense to the present system. This further back on the continental margin where Palaeozoic interpretation is shown on Figs. 10 and 11B. basement rocks of the Megui Microplate were thrust over the eastern margin of the Asai-Rawas-Peneta Forearc Basin, 6. Conclusions accounting for the deformation with the development of folds and slaty cleavage in the agillaceous units which is The concept of the Woyla terranes can now be reconsid- seen in these formations, and also in the Siguntur Formation ered in the light of the evidence which has been reviewed in near Padang (Fig. 11B). this account, and in the context of the tectonic model for the evolution of the southwestern continental margin of Sunda- 5.4. Late Cretaceous continental margin land during the Late Mesozoic which has been derived from this evidence and presented above. With the accretion of the Bentaro-Saling Arc to the southwestern margin of Sundaland, subduction of the oceanic 6.1. Natal terrane plate continued outboard of the new margin. This situation is illustrated in Fig. 10 in which the arc terrane and the The Natal terrane was identified Rock et al. (1983) and Woyla accretionary complex are returned to their original interpreted by analogy with the Sikuleh terrane of Aceh position along the Sundaland margin by reversing the post- (Cameron et al., 1980). In Rock et al.’s (1983) interpretation Miocene dextral movements along the Sumatran Fault the Natal terrane was composed of the Langsat Volcanics, of system. On Sundaland, the development of a Late Cretac- unusual chemical composition (K-rich absarokites), and the eous magmatic arc, represented by granitoid intrusions from contemporaneous volcaniclastic turbidites of the Sikubu Aceh to Bandarlampung, provides evidence for continued Formation at the southwestern end of the Natal river section. subduction outboard of the accreted terranes. All these gran- Rock et al. (1983) argued that, since the volcanics are itoid intrusions are of I-type and were intruded through intruded by the Air Bangis granites, they must be underlain continental crust (McCourt et al., 1996). The Late Cretac- by continental crust and constitute a microcontinental block. eous arc lies oceanward of the preceding mid-Jurassic to However, age-dating of the Langsat Volcanics (Wajzer et Early Cretaceous arc, and is largely intruded through the al., 1991) showed that the volcanics were of Late Eocene recently accreted oceanic material and the newly accreted age, and therefore unrelated to the Triassic–Early Cretac- volcanic arc of the Woyla Group and its equivalents (Fig. eous Woyla Group, which forms the remainder of the 11B). In Aceh the Sikuleh Batholith (97 Ma) is intruded into section, and therefore also quite unrelated to the Late Juras- the Bentaro Arc, in Natal the Manunggal (87 Ma) and sic–Early Cretaceous Bentaro Volcanics of Aceh. Kanaikan batholiths are intruded into the oceanic assem- The Langsat Formation is separated from the Woyla 736 A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 Group by the Simpang Gambir Fault (Figs. 3 and 4) which is west of the present arc. In a continuously subducting parallel to, and presumably part of the main Sumatran Fault subduction system it would be expected that younger arcs System. It is probable that these volcanic rocks have been would lie progressively oceanward (southwestward in the moved into their present position by movements along the Sumatran arcs) of older arcs. This is clearly not the case in fault, from a position further south along the west Sumatran Sumatra. One possible explanation is that extensive areas of margin. In this respect they constitute an allochthonous the forearc have been carried away along strike-slip faults, terrane. parallel to the Sumatran Fault System as the result of obli- Although the Langsat Volcanics have a unique composi- que subduction. The present position of these slices of fore- tion, not identified elsewhere in Sumatra, they may have arc should now be sought further to the northwest, in the moved no great distance. Oligo-Miocene volcanic units of Andaman Sea or in the Arakan Ranges of Burma. However, basaltic to andesitic composition are found all along the there is no basis for suggesting that the Langsat Volcanics west coast of Sumatra. In Breueh Island, to the north of are entirely allochthonous to western Sumatra. Banda Aceh, Oligocene volcanic rocks of basaltic to andesitic composition, with subaerial pyroclastics, have shown 6.2. Sikuleh terrane by palaeomagnetic measurements that they were extruded at 4Њ of latitude, ϳ400 km to the south of their present position The concept of the Sikuleh terrane and the marginal sea (i.e. near Sibolga), and must have been translated along the model for its origin were developed by Cameron et al. Sumatran Fault System to their present position since Oligo- (1980) to account for the occurrence of the imbricated cene times (Haile, 1979). This interpretation is compatible Woyla oceanic assemblage between the Bentaro Volcanic with regional considerations, which indicate that move- Arc, regarded as overlying a fragment of Sumatran conti- ments along the Sumatran Fault, related to the spreading nental crust, and the remainder of Sumatra. Evidence for the ridge in the Andaman Sea, reduce southwards, from presence of a continental fragment beneath the arc includes: 460 km in the north, to virtually nothing in the Sunda Strait the intrusive Sikuleh Batholith; components of the Suma- to the south (Curray, 1989; McCaffrey, 1991). In this model tran basement among arc rocks; and tin anomalies asso- the Langsat Volcanics are likely to have originated some ciated with the granite. All of these observations are open 200 km to the south of their present position. to question. Volcanic rocks of Eocene–Oligocene age extend all The presence of the Sikuleh Batholith, by itself, does not along the west coast of Sumatra to the south of Natal. constitute conclusive evidence that the Bentaro volcanic arc These were designated the ‘Old-Andesites’ by van Bemme- is underlain by continental crust. The batholith is a compo- len (1949) and include the Oligo-Miocene Painan Volcanic site body, including gabbroic, dioritic and granodioritic Formation of andesitic to dacitic composition, with ignim- components, with little or no true granite, in which later brites and acid tuffs, outcropping along the coast to the south intrusions cut earlier, and the older components are gneis- of Padang; the (?) Eocene Bandan Volcanic Formation, sose. These are the characteristics of a subduction-related, composed of ignimbrites, andesites and tuffs, which overlies mantle-derived, I-type granitoid body, forming part of a the Mesozoic inlier at Siulak; and the (?) Palaeocene– magmatic arc, which was intruded into an active transcur- Eocene Kikim Volcanic Formation, composed of andesitic rent fault zone. The detailed petrography and geochemistry breccias and welded tuffs interbedded with quartzose sedi- of the batholith need to be the subject of future studies to ments, overlying Mesozoic rocks in the Gumai and Garba determine the origin of the components of the igneous body mountains of South Sumatra. Eocene–Oligocene ‘Old- and to determine whether it was intruded through continen- Andesites’ also occur along the south coast of Java (van tal crust. Bemmelen, 1949). The argument for a separated sliver of continental base- None of these other volcanic units resemble the Langsat ment to the Bentaro Volcanic Arc is considerably strength- Volcanic Formation, either petrographically or geochemi- ened by the occurrence of the ‘undifferentiated’ Woyla cally, but this may be because the latter shows only limited Group rocks along the southern contact and as roof pendants exposure. Rock et al. (1983) emphasised that both the within the Sikuleh Batholith. These rocks are described as composition and the position of the Langsat Volcanics are resembling metasediments of the Carboniferous Kluet anomalous. K-rich absarokites, are characteristic of a back- Formation which constitutes part of the Palaeozoic base- arc tectonic environment. Presumably, the volcanic arc to ment of Sumatra. In the Explanatory Note to the Calang which these rocks are related is developed much more Quadrangle these rocks are described as constituting an extensively to the west, but is now buried beneath younger ‘inlier’ within the Bentaro Arc rocks (Bennett et al., Tertiary deposits in the forearc basin. Rock et al. (1983) 1981b), the implication here is that the Kluet underlies the point out that the position of the Langsat Volcanics on the arc rocks, and has been uplifted and exposed by erosion. To west coast of Sumatra, well to the west of the present day the northeast of the Sumatran Fault, however, Palaeozoic volcanic arc in the Barisan Mountains, is also anomalous. basement rocks are thrust over the Woyla Group along the As also is the position of the younger Miocene volcanic arc, Geumpang, Takengon and Kla lines. It has been suggested which lies on the western side of the Barisans, again to the in the model presented in this account, following Cameron A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713–738 737 et al. (1980), that thrusting occurred during the accretion of East Gondwanaland off northern Australia, like the other the Woyla Group rocks to the Sumatran margin, and prior to components of Southeast Asia (Metcalfe, 1996). The the intrusion of the Sikuleh Batholith. A possible alternative Eocene–Oligocene age-dating of the Natal terrane excludes explanation to a basement origin for the Kluet rocks is that it from this scenario. In his synthesis Metcalfe (1996, his they formed part of a thrust sheet overlying the Woyla Fig. 13), following earlier authors, suggests that a linear Group, and subsided into the batholith during its intrusion. continental sliver, which included Sibumasu (Mergui These possibilities can only be resolved by careful study of Microplate in Fig. 10), separated from the northern margin the field relationships of these rock units. of Gondwanaland in the Early Permian with the formation A further line of evidence pointing to the presence of of Meso-Tethys, and was accreted to Southest Asia by Late continental basement rocks beneath the Bentaro Volcanic Triassic times. He further suggests that a second linear Arc are the tin anomalies associated with the Sikuleh Bath- sliver, including West Burma, West Sulawesi and the Siku- olith. Tin ores are commonly associated with highly pera- leh Microcontinent, separated from Gondwanaland in the luminous granitic rocks concentrated by a process of Late Jurassic with the formation of Ceno-Tethys (Metcalfe, magmatic differentiation and generally restricted to conti- 1996, his Fig. 15). This second sliver had collided with the nental crust. However, there are peculiarities in the occur- southwestern margin of Sundaland by the Late Cretaceous. rence of the tin in Aceh. No primary ore deposits were Three hypotheses can be proposed for the origin of the identified and there are no records of tourmaline or fluorite, Bentaro Volcanic Arc and the Sikuleh terrane: the arc devel- associated with tin deposits elsewhere in the world. Nor is oped on a fragment separated from the Sumatran continental there any record of muscovite granites, although both basement as an Andean arc as suggested by Cameron et al. pegmatites and aplites occur in Sikuleh (Bennett et al., (1980); the arc developed on a continental fragment 1981b). separated from the northern margin of Gondwanaland Tin anomalies are not restricted to the granite outcrop, but during its passage across Tethys and before its collision extend southeastwards parallel to the Anu-Batee Fault, with Sumatra (cf. Metcalfe, 1996); the arc developed as across the outcrop of the oceanic assemblage of the an intra-oceanic arc built up on the oceanic crust of Meso- Woyla Group (Stephenson et al., 1982). This cannot be a Tethys before colliding with Sumatra (Fig. 11). Further primary relationship, as tin ores have never been found in an studies are required to distinguish between these alternative oceanic environment. In Sikuleh tin anomalies were identi- hypotheses. fied by the analysis of stream sediment samples. Similar anomalies, associated with peraluminous granites, occur Acknowledgements extensively throughout the eastern part of Sumatra to the east of the Sumatran Fault, where they form part of the Thai- The author is deeply indebted to the geologists of the Malay tin belt. It is possible that tin, found in stream sedi- Indonesian Directorate of Mineral Resources, the Geologi- ments in Sikuleh and on the Woyla outcrop, was derived cal Research and Development Centre, Bandung and the from the area to the east of the Sumatran Fault by the British Geological Survey who gathered the greater part erosion of primary tin deposits, or secondarily from basal of the data presented in this paper. He is especially grateful Tertiary sediments, and transported across the fault. to geologists from these organisations who guided him in As detailed above the presence of Kluet metasediments the field in Sumatra, most recently Nana Suwana and Aang and tin anomalies provide a good case for regarding the Achdan of GRDC. He has benefited from administrative and Bentaro Volcanic arc as underlain by a detached fragment logistic support arranged over many years by successive of the Sumatran continental basement, as proposed by Directors of DMR, GRDC and by Dr Bona Situmorang, Cameron et al. (1980). On the other hand, the marginal previously Head of the Geology Division of Lemigas, Jakarta. sea hypothesis (Cameron et al., 1980), in which the oceanic Financial support for fieldwork in Sumatra has been provided assemblage of the Woyla Group is interpreted as the product to the University of London Research Group, by the UK. of a narrow and short-lived ocean basin has been shown to Natural Environment Research Council (NERC) and a be unlikely. Evidence has been presented in this account Consortium of Oil Companies. The author is also indebted which indicates that the Woyla Group represents the to Ian Metcalfe for inviting this contribution and Nick remnants of a major ocean basin, lasting from at least Trias- Cameron for a challenging review of the original draft. sic times until the mid-Cretaceous, a time span of more than one hundred million years. This time span coincides with the development of the Meso-Tethys in the plate reconstruc- References tions of Metcalfe (1996). 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