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

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

A.J. BarberSE 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 JurassicCretaceous 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 oceanicassemblage. Rocks of the arc assemblage are considered to be underlain by a continental basement because of the occurrence of the intrusivegranite and of tin anomalies identified in stream sediments. Quartzose sediments associated with the granite have been correlated with unitsin the Palaeozoic basement of Sumatra. From these relationships a model has been proposed in which a continental sliver was separated fromthe margin of Sundaland in the Late Jurassic to Early Cretaceous in an extensional strike-slip faulting regime, producing a short-livedmarginal basin. The separated continental fragments have been designated the Sikuleh and Natal microcontinents. In the mid-Cretaceous theextensional regime was succeeded by compression, crushing the continental fragments back against the Sundaland margin, with thedestruction of the marginal basin, now represented only by the imbricated oceanic assemblage. Modifications of this scenario are requiredby subsequent studies. Age-dating of the volcanic assemblage and intrusive granites in the Natal area showed that they formed part of anEoceneOligocene volcanic arc and are not relevant to the model. Thick-bedded radiolarian chert and palaeontological studies in the oceanicWoyla Group rocks of the Natal and Padang areas showed that they formed part of a more extensive and long-lived ocean basin which lastedfrom at least Triassic until mid-Cretaceous. This raised the possibility that the Sikuleh microcontinent might be allochthonous to Sumatra andencouraged plate tectonic reconstructions in which the Sikuleh microcontinent originated on the northern margin of Gondwanaland andmigrated northwards across Tethys before colliding with Sundaland. Since these models were proposed, the whole of Sumatra has beenmapped and units correlated with the Woyla Group have been recognised throughout western Sumatra. These units are reviewed and thevalidity of their correlation with the Woyla Group of northern Sumatra is assessed. From this review a revised synthesis for the LateMesozoic tectonic evolution of the southwestern margin of Sundaland is proposed. q 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

The Indonesian island of Sumatra forms the southwesternmargin of Sundaland, the Southeast Asian promontory ofthe Eurasian continental tectonic plate. At the present timethe IndianAustralian plate is being obliquely subductedbeneath this margin at a rate of ,7 cm/a21. Geologically,Sumatra consists of a Pre-Tertiary continental basement,overlain by a number of oil and gas-bearing Tertiary sedi-mentary basins (Fig. 1). Towards the southwestern coast ofthe island the basement is uplifted, exposing Pre-Tertiaryrocks in the Barisan Mountains. The Quaternary volcanicarc, with currently active volcanoes, has been constructedon top of this uplifted basement. All the rock units in Suma-tra, Pre-Tertiary basement, Tertiary sediments and Recentvolcanics, are cut by the dextral Sumatran strike-slip faultsystem which extends from NWSE along the whole length

of the island (Fig. 1). Earthquake activity indicates thatmovement along the fault, continues to the present day.The Sumatran Fault System is interpreted as the result ofoblique subduction of the Indian Ocean Plate beneath Suma-tra (Fitch, 1972).

The concept of the Woyla terranes, was developed fromthe interpretation of the mapping results of the IntegratedGeological Survey of Northern Sumatra, conducted jointlyby the Indonesian Directorate of Mineral Resources (DMR)and the British Geological Survey (BGS) between 1975 and1980. This survey comprised geological and geochemicalreconnaissance mapping of the whole of Sumatra to thenorth of the equator (Page et al., 1978). Among the resultsof the survey, a series of 16 geological map sheets at thescale of 1:250,000 with accompanying descriptive booklets,were published by the Indonesian Geological Research andDevelopment Centre (GRDC), Bandung. A summary of thegeology and the stratigraphy, with an interpretation of thetectonic development of Sumatra, based on the results of

Journal of Asian Earth Sciences 18 (2000) 713738

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E-mail address: [email protected] (A.J. Barber).

this survey, were presented at the 9th Annual Convention ofthe Indonesian Petroleum Association (Cameron et al.,1980). Geological mapping of the southern part of Sumatraby the Indonesian Geological Survey in collaboration withthe United States Geological Survey commenced in the1970s and was completed by 1995 with the publication ofthe last of 43 geological map sheets covering the whole ofSumatra.

The DMR/BGS mapping programme commenced inAceh in northern Sumatra where three tectonostratigraphicunits forming the Pre-Tertiary basement were recognised:the Permo-Carboniferous Tapanuli Group; the Permo-Trias-sic Peusangan Group; and the JurassicCretaceous WoylaGroup. As mapping proceded southwards similar units,covering the same age ranges, were recognised and corre-lated with those of northern Sumatra. Subsequently, unitsresembling the Woyla Group and with the similar ages havebeen identified throughout the whole of the western part ofSumatra (Fig. 1). Rock units correlated with the WoylaGroup are described from Natal in North Sumatra, at Indar-ung and Siguntur near Padang in West Sumatra, in the

Gumai and Garba Mountains of South Sumatra, and furthersouth near Bandarlampung along the Sunda Strait.

In the north, the Woyla Group and its correlatives lielargely to the southwest of the Sumatran Fault Zone,while to the south, they are only exposed to the northeastof the fault zone. In this contribution, the extent to whichthese units have been correctly correlated with the WoylaGroup, the interpretations which have been proposed for theorigin of the Woyla Group and its correlatives, and thevalidity of the concept of the Woyla terranes, will bediscussed and re-examined in the context of the tectonicevolution of the southwestern margin of Sundaland duringthe Late Mesozoic.

2. Woyla Group in Aceh

The Woyla Group was defined in Aceh, northern Suma-tra, where these rock units are extensively exposed in theBarisan Mountains (Fig. 2). Areas of outcrop of the WoylaGroup are shown on the GRDC Banda Aceh, Calang,

A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713738714

Fig. 1. Simplified geological map of Sumatra, showing the distribution of the Woyla Group and correlated units, with localities mentioned in the text.

Tapaktuan and Takengon 1:250,000 Quadrangle Sheets(Bennett et al., 1981a, b; Cameron et al., 1982, 1983). Thedescriptions given below, except where specified, are takenlargely from the reports which accompany these maps. TheWoyla River, from which the Woyla Group was named, ison the Takengon Sheet (Fig. 2).

During the DMR/BGS survey 13 lithostratigraphic unitswere distinguished in the Woyla Group, as well as a unit ofundifferentiated Woyla. These units generally occur asfault-bounded lenses, distributed on both the northeasternand southwestern sides of the Sumatran Fault, and are elon-gated in a NWSE direction, parallel to the Sumatran trend.The Woyla Group is also affected by several thrusts; theGeumpang, Takengon and Kla lines. The distribution ofthese units and the outcrops of the thrusts are shownon Fig. 2.

2.1. Lithological units in the Woyla Group

2.1.1. Tangse Serpentinite (Cahop Serpentinite, BeatangUltramafic ComplexTakengon Sheet)

Massive serpentinite, representing altered harzburgite,

occurs as lenses along the Sumatran Fault and along theGeumpang Line. The largest of these lenses extends discon-tinuously for 27 km to the northwest of Tangse. Chromiteoccurs in float in streams near this locality. Here and else-where, serpentinite is locally sheared, schistose, twisted andcontorted. Sheared serpentinite may also form the matrix tomelange (Indrapuri Complex). The melange encloses blocksof cumulate gabbro, basalt, red chert and limestones,derived from other members of the Woyla Group. Fossilscollected from limestone blocks within the melange include:corals: Latoceandra ramosa, Stylina girodi; foraminifera:Pseudocyclamina sp.; stromatoporoid: Stromatopora japo-nica, indicating a Late Jurassic to Early Cretaceous age. Inthe Takengon Quadrangle large blocks of limestoneenclosed in sheared serpentinite along the GeumpangLine, contain Late Miocene fossils (Cameron et al., 1983).

2.1.2. Penarum FormationOutcrops to the northeast of the Sumatran Fault, south of

Takengon, and consists of serpentinites, basalts, red chertswith radiolaria and slates. Volcanic rocks are commonlyaltered to greenschists.

A.J. Barber / Journal of Asian Earth Sciences 18 (2000) 713738 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).

2.1.3. Geumpang Formation (Banda Aceh Sheet); BabahrotFormation (Tapaktuan Sheet)

Outcrops occur, on both sides of the Sumatran Fault to thesoutheast of Banda Aceh and to the northwest of the Anu-Batee Fault towards Tapaktuan (Fig. 2). Rock types includemassive or schistose basic volcanics, pillow basalts, volca-niclastic sandstones and tuffs, commonly epidotised andaltered to greenschists or phyllites, and thin grey or blacklimestones. The phyllites are usually lineated and crenu-lated, indicating multiple deformation. The rocks of theGeumpang Formation are considered to constitute the typi-cal lithological and structural assemblage of the WoylaGroup. The Geumpang Formation also includes a massivelimestone member, frequently occurring as marble.

The Babahrot Formation, north of Tapaktuan includesserpentinites and talc schists, as well as metagabbroicbodies metamorphosed in the greenschist facies, highlydisrupted and sheared into lenses.

2.1.4. Lam Minet Formation (Banda Aceh Sheet); GumeFormation (Takengon Sheet)

Composed of basaltic lavas, commonly epidotised, basal-tic conglomerates and breccias, with volcanic and limestoneclasts, but only rarely chert, graded volcaniclastic wackes,radiolarian cherts with manganese oxide veining, rhodonite,and calcareous, manganiferous and carbonaceous slates. Aclast of radiolarian chert, embedded in a volcanic conglom-erate with flattened clasts, was collected by Nick Cameron(personal communication, 1999) in the Kreung Baro, Aceh,from a landslip within the outcrop of this formation. Thisoccurrence indicates that volcanic rocks were eruptedthrough ocean floor sediments, perhaps during the formationof a seamount. The formation also includes a recrystallisedlimestone member.

2.1.5. Jaleuem FormationOutcrops occur 100 km to the southeast of Banda Aceh,

on both sides of the Sumatran Fault. Composed largely ofslates, but red cherts occur in float and the unit also includesa limestone member. Rb/Sr dating on 11 samples of slategave 99 ^ 30 Ma (late Early Cretaceous).

2.1.6. Bale FormationShown outcropping to the northwest of the Sumatran

Fault, and southeast of Takengon. Composed of colouredslates, with minor wackes and cherts, limestones and lime-stone breccias.

2.1.7. Sise Limestone FormationConsists of massive or bedded limestones, biocalcarenites

and calcilutites with fossils: corals: Montlivaltia sp., Myrio-pora sp.; foraminifera: Pseudocyclamina sp. indicating aLate Jurassic to Early Cretaceous age.

2.1.8. Bentaro Volcanic Formation (Banda Aceh Sheet);Tapaktuan Volcanic Formation (Tapaktuan Sheet)

Porphyritic basalts and andesitic basalts with agglomer-ates, intruded by basic dykes. Basaltic vents surrounded bybreccias, tuffs and volcaniclastic sediments have been iden-tified near Lam No and north of the Bentaro River on theBanda Aceh Sheet. Chemical analysis of a xenolithic,porphyritic basalt with pyroxene phenocrysts is given inRock et al. (1982).

The Tapaktuan Volcanic Formation occurs in fault-bounded lenses, within strands of the Anu-Batee FaultZone, parallel to the west coast of Aceh north of Tapaktuan.It consists of massive epidotised andesites and basalts,commonly porphyritic, and intrusive dykes of a similarcomposition. An analysis of hornblende microdiorite fromthis formation is given in Rock et al. (1982). The formationalso includes agglomerates, breccias, tuffs, red and purplevolcaniclastic sandstones and shales, the latter often asslates, and a limestone member, composed of sparite andcalcilutite, all as lenses and much disrupted by faults.

Scattered outcrops of gneiss (Meukuk Gneiss Complex)occur within the Tapaktuan Volcanic Formation in the Bari-san Mountains to the north of Tapaktuan, between strands ofthe Anu-Batee Fault (Fig. 2). They consist of concordantleucogranodioritic gneiss, with garnet-biotite amphibolitecontaining garnets up to 8 cm in diameter, and biotite-horn-blende-andesine schist.

2.1.9. Lhonga FormationOutcropping to the west of Banda Aceh and composed of

grey and coloured slates and phyllites, with interbeddedvolcaniclastic sandstones, thin limestones and (?)radiolar-ian-bearing siltstones.

2.1.10. Lhoong FormationForms a large outcrop to the southwest of the Sumatran

Fault and also occurs as roof pendants in the Sikuleh Bath-olith (Bennett et al., 1981b). It consists of basaltic lavas withcherts in the lower part of the sequence, followed byconglomeratic wackes with volcanic and limestone clasts,and subordinate sandstones, siltstones and limestones.

2.1.11. Raba Limestone FormationOutcrops along the coast and in the Barisan Mountains to

the south of Banda Aceh and consists of massive calcareniteand calcilutite and dark thin-bedded cherty limestones andshales. The massive limestone constitutes a reef memberwhich is closely associated in the field with the BentaroVolcanic Formation.

2.1.12. Lamno Limestone FormationOutcrops along the west coast of Aceh, south of Banda

Aceh, where it is associated with outcrops of the BentaroVolcanic Formation. Consists of dark limestone, whichincludes a reef-like facies, and contains volcanic clastsnear the base. The limestone is commonly fossiliferous,


with: corals: Actiastraea minima, Stylosmilia corallina;algae: Clypeina sp., Permocalculus ampullacea, Litho-codiumBacinella sp. Boueina sp., Thaumatoporella porvo-vesiculifera; foraminifera: Pseudocyclammina lituus,indicating a Late Jurassic to Early Cretaceous age.

2.1.13. Teunom Limestone FormationOutcropping along the southwestern margin of the Siku-

leh Batholith. Composed of massive dark limestones, meta-morphosed and recrystallised along the contact with thegranite.

Undifferentiated Woyla Group rocks are shown on theCalang Quadrangle Sheet in the area to the south of theSikuleh Batholith in Gunung Paling and as roof pendantswithin the outcrop of the batholith (Bennett et al., 1981b). Inthe Explanatory Note to this sheet these rocks are said toresemble the Kluet Formation, low-grade Carboniferousmetasediments of the Sumatran basement, which outcropextensively to the northeast of the Sumatran Fault. Theimplication here is that these rocks are composed of slateswith interbedded quartzose sandstones. They are alsoreported to include quartzite (Bennett et al., 1981b). Quart-zose rocks are otherwise unknown in the Woyla Group.

Many of the mapping units distinguished in the WoylaGroup of Aceh during the DMR/BGS survey are made up ofthe same rock types but in varying proportions. It is clearthat they represent geographical, rather than genuine lithos-tratigraphical units. A different name was given to eachdistinguishable unit on each map sheet. The actual litholo-gical units within each formation are, on the whole, toosmall to be represented on the scale of the map.

2.2. Folds and related structures

The rocks of the Woyla Group in Aceh commonly showthe effects of intense deformation. The finer grained sedi-mentary and volcaniclastic rock types have been altered toslates and phyllites. Isoclinal folds can be seen whereverbedding lamination can be distinguished, with axial planecleavage parallel to bedding, and a bedding/cleavage inter-section lineation plunging to the SE. Slaty cleavage may berefolded, into more open folds on sub-horizontal axes andNE-dipping axial planes, with the development of second-ary cleavages, lineations and kink band folds. Larger kilo-meter scale upright folds, up to ,7 km, on NWSE trendingaxes and steep axial planes are reported from the TapaktuanVolcanic Formation, with secondary folds on the scale of,12 km (Cameron et al., 1982).

Massive limestone units in the Woyla Group are lessobviously internally deformed, although mylonitised andkink-banded limestones are seen near Banda Aceh. Beddedlimestones are sometimes folded on a large scale, as seen inquarries in the Lhonga Formation between Banda Aceh andLhonga (Bennett et al., 1981a). Also in the same area,limestones interbedded with shales show pinch and swell

structures, boudinage and the development of tensiongashes normal to the bedding.

On the map scale all the units in the Woyla Group arefault-bounded and form large scale lenses elongated in aNWSE direction parallel to the Sumatran trend. Internallythe units are disrupted by large numbers of faults. Some ofthese faults are thrusts, along which rock units have beenimbricated; others show slickensided surfaces indicatingstrike-slip movement along faults parallel to, and probablyrelated to the Sumatran Fault System.

2.3. Sikuleh Batholith

The Woyla Group is intruded by granitoids, the largest ofwhich is the Sikuleh Batholith, illustrated on the BandaAceh and Calang sheets (Bennett et al., 1981a, b). This isan approximately elliptical body (,55 35 km) elongatedin a NWSE direction (Fig. 2). Around the margins of thebatholith, undifferentiated Woyla Group rocks and lime-stones of the Teunom Formation, have been altered bycontact metamorphism. Roof pendants of metamorphosedWoyla Group rocks, shown as undifferentiated on the map,but compared to the Lhoong formation, occur within thebatholith (Bennett et al., 1981b). Rocks of the GeumpangFormation are thrust over the northeastern margin of thebatholith along the Geumpang Line.

Although no attempt was made during the mapping todelineate the various components of the batholith, descrip-tions show that it is a complex and highly variable body(Bennett et al., 1981b). An older complex is distinguishedin the northern part of the complex, composed of dark andxenolithic gabbroic and dioritic bodies, migmatitic andintensely veined. The older complex is locally gneissoseor sheared. Where foliation is developed, it is parallel tothe foliation in the adjacent Geumpang Formation rocks.The younger complex, which has an intrusive contactagainst the earlier complex, is a more homogeneous, coarsergrained and largely unfoliated, biotite-hornblende grano-diorite, with minor amounts of diorite and pink granite.The granodiorite contains mafic xenoliths and becomesmore mafic towards its margins, where it shows flow folia-tion. The granodiorite and is cut by pegmatites and aplites,the latter being more common adjacent to the contact withthe older complex. The batholith shows porphyry-typemolybdenum mineralisation and tin anomalies of 1080ppm have been identified from stream sediment samplingalong the contact between the older and younger complex,although no primary tin mineralisation has been recognised(Stephenson et al., 1982). The younger complex has beendated from the mean of K/Ar analyses on two biotites andone hornblende, as 97.7 ^ 0.7 Ma (early Late Cretaceous)(Bennett et al., 1981b). The complex is cut by later basicdykes.

2.4. Thrusts

Bennett et al. (1981a, b) and Cameron et al. (1980, 1983)


recognised three major thrust zones in northern Sumatra(Fig. 2):

2.4.1. Takengon LineThis thrust has an EW outcrop to the south of Lake

Tawar, where it brings Permo-Triassic rocks of the Peusan-gan Group on top of the Penarum Formation of the WoylaGroup along a flat-lying, southward-directed thrust plane.To the west of Takengon, along the continuation of this line,Miocene rocks are thrust over the Woyla Group.

2.4.2. Kla LineA northwards-directed thrust plane, outcropping to the

south of Takengon between the main Sumatran Fault andthe Anu-Batee Fault, brings Permo-Carboniferous TapanuliGroup rocks onto undifferentiated Woyla Group. Mylo-nites are developed along the thrust plane.

2.4.3. Geumpang LineIn the area to the southeast of Banda Aceh, units of the

Woyla Group are separated by an easterly dipping thrust, theGeumpang Line, which outcrops parallel to the SumatranFault, and has the same NWSE trend. Here members of theWoyla Group have been thrust southwestwards over LateMiocene sediments, the northeastern margin of the SikulehBatholith and other members of the Woyla Group. Serpen-tinite melange, containing a variety of Woyla Group rocks,including blocks of Late JurassicEarly Cretaceous andLate Miocene limestone, occurs along the fault plane.

Although these thrusts were active during the Tertiary, andaffect rocks up to Late Miocene in age, Bennett et al. (1981a, b)and Cameron et al. (1983) consider that they represent morefundamental thrust surfaces in the Pre-Tertiary basementwhich were reactivated during Late Tertiary or Quaternarymovements along the Sumatran Fault Zone. They suggestthat serpentinite melanges, commonly found along the faultand thust planes, were derived from mantle peridotite thrustbeneath the margin of Sundaland, which was hydrothermallymobilised and rose diapirically along fault and thrust planesduring subsequent fault movements.

2.5. Interpretation of the Woyla Group in AcehCameron et al. (1980) distinguished two lithological assem-

blages in the Woyla Group in Aceh: an oceanic (ophiolitic) andan (volcanic) arc assemblage. The distribution of theseassemblages is represented on the Simplified GeologicalMap of Northern Sumatra (Stephenson and Aspden, 1982):

2.5.1. Oceanic assemblageThis includes serpentinised harzburgite, metagabbro,

mafic to intermediate volcanics, often basaltic and showingpillow structures, volcanic breccias, volcaniclastic sand-stones, red and purple manganiferous slates, red radiolariancherts. These rock types also occur as blocks in breccio-conglomerates or melanges.

Units of the oceanic assemblage include the Tangse andCahop serpentinites, the Beatang Ultramafic Complex andthe Penarum, Geumpang, Babahrot, Lam Minet, Jaleueumand Bale formations. All these units lie to the northeast ofthe Geumpang Line, and almost entirely to the northeast ofthe Sumatran Fault Zone (Fig. 2). Unfortunately, no age-diagnostic radiolaria have yet been obtained from the manyoccurrences of bedded (?radiolarian) chert which occuramong these units, so that their age(s) has not been estab-lished directly. Limestone blocks in serpentinous melangealong the Geumpang Line, which yielded Late Jurassic toEarly Cretaceous fossils, were ascribed to the GeumpangFormation (Cameron et al., 1983), but are more reasonablyregarded as derived tectonically from underlying limestoneunits during the thrust movements, in the same way as LateMiocene blocks which are also included in the melange.

The oceanic assemblage of the Woyla Group in Acehshows an intimate mixture of ocean floor materials fromdifferent structural levels, from mantle to abyssal sediments,variously internally deformed, separated by faults, oftenidentified as thrusts and arranged in a random order.These are the characteristic features of an accretionarycomplex, where ocean floor materials are imbricated againstthe hanging wall of a subduction zone. Garnetiferousamphibolites of the Meukuk Gneiss in the Tapaktuan Volca-nic Formation suggest that gabbroic rocks of the oceanicassemblage were subducted into the mantle, before beingreturned tectonically to the surface.

2.5.2. Arc assemblageThis is typified by the Bentaro and Tapaktuan Volcanic

Formations, and is composed of porphyritic andesite, occur-ring as vents near Bentaro, basalt lavas, intrusive dykes,agglomerates, breccias, volcaniclastic sandstones and shales.

These igneous rocks are closely associated in the fieldwith massive or bedded limestones, which commonlycontain volcanic clasts or are interbedded with volcaniclas-tic units. These limestone units include the Lhonga, Raba,Lamno, Lhoong, and Teunom Limestone formations, aswell as the limestone lenses which occur within the Tapak-tuan Volcanic Formation. Most of the units belonging to thearc assemblage lie to the southwest of the Geumpang Lineand the Sumatran Fault. The only exception to this distribu-tion, according to the geological map of Stephenson andAspden (1982), is the Sise Limestone Formation, outcrop-ping to northeast of the Sumatran Fault, to the the south ofTakengon (Fig. 2). This anomaly can be removed by rever-sing displacement along the Sumatran Fault. The Lamnoand Sise limestones and the Geumpang limestone blockswithin the Indrapuri Complex all contain fossils which indi-cate a Late JurassicEarly Cretaceous age.

Cameron et al. (1980) suggest that the intrusive andvolcanic rocks represent the remnants of a Late JurassicEarly Cretaceous volcanic arc. This arc emerged above sealevel, where it was eroded to form breccias and volcaniclas-tic sediments. The arc was surrounded and overlain by a


fringing reefs represented by the massive limestones. In thismodel, massive limestones were eroded to form limestonebreccias, and detritus was transported into deeper water toform bedded limestones. Cameron et al. (1980) put forwardevidence which indicated that the volcanic arc was built upon a sliver of continental crust. The observation of a volca-nic breccia containing radiolarian chert clasts in the LamMinet Formation indicates that volcanic rocks were some-times extruded through ocean floor sediments.

2.6. The marginal sea model

Cameron et al. (1980), noting the absence of a mid-Cretaceous magmatic arc in the eastern part of northernSumatra, rejected the interpretation that the oceanic assem-blage of the Woyla Group represents the accreted remnantsof older segments of the Indian Ocean in an earlier stage ofthe subduction process which continues today offshoresouthwest Sumatra. Instead, they interpreted the oceanicassemblage as the remnants of a short-lived marginal ocea-nic basin, developed along the southwestern margin ofSundaland in a phase of extreme oblique subduction andactive strike-slip faulting, with the formation of a pull-apart basin and then oceanic crust.

The marginal basin was formed in Late JurassicEarlyCretaceous times by the separation of a continental sliverfrom the western margin of Sundaland, in the same way asthe Andaman Sea or the Gulf of California are being forming atthe present time (Cameron et al., 1980, their Fig. 4a). Theseparated continental sliver is now represented by the BentaroVolcanic Arc and the Sikuleh Batholith. Also associated withthe batholith are undifferentiated Woyla metasedimentswhich resemble the Carboniferous Kluet Formation of theSumatran basement, and tin anomalies. All these features aretaken as evidence that the arc volcanics overlie continentalcrust. On the Tectonic Map of Northern Sumatra the basementunderlying the arc assemblage is described as the SikulehContinental Fragment (Aspden et al., 1982).

Subsequently, as a result of continuing subductionoutboard of the continental fragment and transpressionalstrike-slip movement, this small marginal ocean basin wascompressed, crushing both the floor of the marginal basinand the detached continental fragment against the Sunda-land margin. The Takengon and Kra lines represent thethrust surfaces along which the materials of the marginalbasin were thrust beneath the continental margin of Sunda-land. The suture, representing the site of this disappearedmarginal basin, coincides approximately with the line of thepresent day Sumatran Fault. Also, as the result of compres-sion, the continental fragment and its overlying arc werethrust beneath the oceanic assemblage along the GeumpangLine (Cameron et al., 1980).

Although there is no direct evidence for the age of theocean floor materials, and the age of the volcanic arc isestablished only from the palaeontological dating of itsfringing reefs, all these events are considered to have

occurred between the Late Jurassic and Late Cretaceous.It is suggested that rifting of the continental margin andthe formation of the volcanic arc occurred in the Late Juras-sic and the opening of the marginal basin took place in theEarly Cretaceous. The intrusion of the Sikuleh Batholithinto both the oceanic and the arc assemblages shows thatthe closure of the basin and the compression of the arcagainst the continental margin was completed by LateCretaceous times (Cameron et al., 1980).

Woyla Group rocks which took part in these events arenow sliced into lenticular slivers by more recent movementsalong the Sumatran Fault (Cameron et al., 1980). In the fieldit is difficult to distinguish between thrust faults which wereformed by the imbrication of the units during subduction,and later strike-slip faults related to the Sumatran FaultSystem unless slickensides are present. It is impossible todistinguish between strike-slip faults formed as the result ofoblique subduction and those due to later fault movements(Wajzer et al., 1991).

As has already been suggested, the anomalous position ofthe massive limestones of the Sise Limestone Formation,which now lie on the northeastern side of the main strandof the Sumatran Fault, may be the result of movementsalong the fault. The Sise Limestone Formation and theLamno Limestone Formation near Banda Aceh, containsimilar fossil assemblages and are of the same age. Rever-sing the dextral movement along the fault for a distance of,200 km would bring the Sise and Lamno formations intojuxtaposition. This order of movement along the SumatranFault is compatible with the evidence from regional consid-erations, with a 460 km movement in the Andaman Seareducing southwards to a displacement of only a few kilo-metres in the Sunda Strait (Curray, 1989; McCaffrey, 1991).Strike-slip fault movements along the Sumatran FaultSystem are also considered to have reactivated fundamentalthrusts in the basement along the Takengon, Kla and Geum-pang lines, as Neogene rocks are affected by this movement,in transpressional zones along strands of the fault system(Cameron et al., 1980).

3. Woyla Group in Natal

Rocks correlated with the Woyla Group of Aceh weremapped over an extensive area inland from Natal in NorthSumatra during the Integrated Geological Survey of North-ern Sumatra as part of the Lubuksikaping 1:250,000 Quad-rangle Sheet (Rock et al., 1983) (Fig. 3). The outcrop islimited to the northeast by the Sumatran Fault System andis much dissected internally by faults with a similar trend.The Woyla Group is intruded by Late Cretaceous granitesand overlain unconformably by the Miocene Barus Group,by Miocene volcanic rocks, and by the products of Quatern-ary volcanism from the volcanoes of Sorik Merapi, Malin-tang and Talamau, as well as by recent alluvium. Unitswithin the Woyla Group strike NWSE and are very well


exposed in the valley of the Batang Natal, both in the riversection and in the parallel road section, which both cutacross the strike (Fig. 4). The main outcrop of the WoylaGroup is separated from a smaller outcrop in the Pasamaninlier to the south by Malintang Volcano (Fig. 3).

3.1. DMR/BGS report

In the DMR/BGS report of the Lubuksikaping Quadran-gle (Rock et al., 1983) lithological units in the Batang Natalsection were classified, from NESW, into three forma-tions: the Maurosoma, Belok Gadang and the Sikubu forma-tions (Fig. 3).

The Maurasoma Formation outcrops in the upstream partof the section and in its tributary the Aik Soma. Thicknessesof the rock units in this section were measured perpendicu-lar to the strike for a distance of 5.5 km (Rock et al., 1983).The rock types include cleaved argillaceous units, shale orslate, which may include calcareous concretions, laminatedsiltstones, and gritty sandstones showing sedimentary struc-tures, indicating younging in a downstream direction,massive limestones, sometimes forming limestone pinna-cles, epidotic volcanic breccias and volcaniclastic sand-stones, chloritic greenschists and muscovite-chloritequartz schists. A 10 m conglomerate (? melange) at theupstream end of the section, with elongated clasts of greens-chist in a chloritic matrix, is probably of tectonic origin,formed in a fault or a shear zone (Rock et al., 1983).

The Belok Gadang Formation outcrops in the central partof the section and is composed of sandstones, sometimescalcareous, and argillaceous rocks, often cleaved andcontaining bands and lenses of chert. The chert is radiolar-ian, but no identifiable radiolaria have so far been recoveredwhich could be used to date the sequence. Outcrops in thetype locality of Belok Gadang, a tributary of the BatangNatal, show basaltic pillow lavas, with white clay interbedsand manganese-rich horizons with braunite, resembling theumbers, described from the Troodos Ophiolite of Cyprus(Robertson, 1975). Analysis shows that the pillow basaltsare spilites (Rock et al., 1982, 1983). In the type localitybasalts are overlain by red, bedded cherts, but again noidentifiable radiolaria have been recovered.

The Sikubu Formation, outcropping in the lower part ofthe Batang Natal section, is composed of massive volcani-clastic metagreywackes, with thin shale interbeds. Thesandstones show very well-developed sedimentary struc-tures, including graded bedding, flame structures and convo-lutions, typical of turbidites. Massive porphyritic andesiticdykes and lava flows, with distinctive pyroxene pheno-crysts, are intruded into or interbedded with the sedimentsin the lower part of the section. Fragments of porphyriticandesite, identical in composition to the dykes and lavas,occur as clasts in the sandstones.

Woyla Group rocks in the Pasaman area includemelanges and massive and foliated peridotites (Rock etal., 1983) (Fig. 3). Peridotites are well exposed in the


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 SimpangGambir Fault.

Pasaman River where they are composed mainly of harzbur-gite with minor dunite pods, pyroxenite dykes, disseminatedchromite and rare chromite pods. Some of the peridotite isfoliated, containing orthopyroxenes enclosed in augen.Coarse plagioclase-hornblende rocks, found as boulders inthe float, represent metasomatised gabbro pegmatite whichformed dykes in the peridotite. The peridotite is variablyserpentinised, and in shear zones may be completely alteredto serpentine and talc. Smaller bodies of serpentinite, withchromite pods, outcrop at the upper end of the Batang Natalsection near Maurasoma (Figs. 3 and 4) where they formspectacular serpentinite breccias faulted against slates andlimestones of the Maurasoma Formation. Serpentinite alsooccurs as xenoliths in granite in the Aik Soma.

3.2. Langsat Volcanic Formation

Porphyritic, net-veined and amygdaloidal basic lavas ofthe Langsat Formation, associated with agglomerates,outcrop at the southwestern end of the Batang Natal section(Rock et al., 1983) (Fig. 3). The lavas are melanocratic butcontain leucocratic xenoliths. The basic rocks are highlyporphyritic, with abundant clinopyroxene phenocrysts, butshow very little plagioclase in thin section; the dominantfeldspar in the ground mass being orthoclase. Analyses

show that these lavas have a highly unusual chemicalcomposition with low SiO2 and high MgO contents (Rocket al., 1982, 1983). Exceptionally high contents of Cr, Niand Co are correlated with the abundance of augite. Theabsence of hypersthene, the lack of plagioclase and thepresence of orthoclase, together with the overall chemicalcomposition, shows that these lavas are absarokites (basicshoshonites) (Rock et al., 1983).

3.3. Late Mesozoic intrusions

Several large granitoid bodies are intruded into the rocksof the Woyla Group in the Natal area. The largest is theManunggal Batholith at the northeastern end of the BatangNatal section (Rock et al., 1983) (Fig. 3). This batholith is acomposite body, some 230 km2 in extent, composed ofleucogranodiorite, granodiorite, granite, pyroxene-quartzdiorite with contaminated syenitic and monzonitic varieties,and appinites. The granitoid rocks are intruded by vogesitelamprophyre dykes. This granitoid has been dated by the K/Ar method at 87 Ma (Late Cretaceous) (Kanao et al., 1971,reported in Rock et al., 1983). In the Aik Soma, near Maur-asoma, large granitic boulders in the river bed encloseserpentinite xenoliths, surrounded by reaction zones of


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

amphibolite. Limestones in the same area are converted toskarns near the contact with the granite.

A second granitoid, the Kanaikan intrusion, is intrudedinto the Woyla Group in the Pasaman area (Fig. 3). Thisbody is composed of coarse granodiorite and leucogranitecut by microgranitic and granophyric dykes. The intrusionlies within the Kanaikan Fault Zone, a strand of the mainSumatran Fault, and is much dissected by faults anddeformed to cataclasite along shear zones.

3.4. Palaeogene intrusions

Granitic rocks form scattered outcrops in headlands nearAir Bangis, along the coast to the south of Natal (Fig. 3). Alarge area inland of these outcrops on the LubuksikapingSheet is also shown as occupied by granitoid. This inlandoutcrop is largely conjectural, as the interpretation is basedon aerial photographic interpretation, and much of the areais covered by cloud on the photographs used in the inter-pretation (Rock et al., 1983). The coastal outcrops arecomposed of hornblende-biotite adamellites, leucogranitesand granites, cut by microgranitic and microdioritic dykes.These intrusions are shown on the Lubuksikaping Sheet asof EoceneOligocene age. However, Rock et al. (1983) intheir report, speculate that these intrusions might be of LateCretaceous age, analogous to the Sikuleh intrusion whichocupies a similar position with respect to the Woyla Groupin Aceh.

3.5. Age constraints in the Natal area

Age constraints for the Woyla Group in the Natal area areprovided by a limestone sample from the Batang Kanaikanin the Pasaman inlier which yielded a colonial organism,closely resembling the samples of Lovcenipora describedand illustrated by Yancey and Alif (1977) from the Indarungarea, near Padang, and considered to be of Late Jurassic toEarly Cretaceous age (IGS/British Museum Sample No.TC/J1/R1101B- Rock et al., 1983). A minimum age for theWoyla Group is provided by the Manunggal Batholith,dated at 87.0 Ma (Late Cretaceous) (Kanao et al., 1971,quoted in Rock et al., 1983), which intrudes limestonesand serpentinites at the northwest end of the Batang Natalsection.

3.6. Interpretation the marginal sea model

The model developed by the DMR/BGS mapping team toexplain the origin and tectonic environment of the WoylaGroup in Aceh was extended southwards, with minor modi-fications, to account for the distribution of similar rockassemblages of Jurassic to Cretaceous age outcropping inthe Natal area of North Sumatra (Rock et al., 1983).

Rock et al. (1983) interpreted the Maurosoma Formationat the northeastern end of the section, which includes turbi-dites and massive limestones, as shelf sediments of theSundaland continental margin. The Belok Gadang Forma-

tion, with pillow basalts and cherts, was interpreted as thedeformed floor of the marginal basin, and the SikubuFormation in the southwest as the erosion products of avolcanic arc. The arc itself is represented by the the LangsatVolcanics which outcrop at the southwest end of the BatangNatal section (Rock et al., 1983) (Figs. 3 and 4). As in Aceh,the arc was considered to overlie continental basement andconstitutes a microcontinental block, identified in theTectonic Map of Northern Sumatra as the Natal Continen-tal Fragment (Aspden et al., 1982). The Air Bangis grani-toids, considered to be intrusive into the Langsat Volcanics,although this relationship is not actually observed, areregarded as evidence that the arc is underlain by continentalcrust. Again, using the age constraints provided by theoccurrence of Lovcenipora in Woyla Group rocks in thePasaman inlier and the age of intrusion of the ManunggalBatholith, Rock et al. (1983, their Fig. 8) postulate thatbetween Late Jurassic and Early Cretaceous time, a pull-apart rift developed into a short-lived marginal basin alongthe margin of Sundaland, which was then compressed anddestroyed by the Late Cretaceous. Rock et al. (1983)acknowledged that if the Cretaceous age ascribed to theAir Bangis granitoids was not correct, then the marginalsea model was invalid and the arc and its underlying conti-nental basement might constitute a terrane allochthonous toSumatra. In the key to the Lubuksikaping Map Sheet the AirBangis granitoids are shown as of Late Cretaceous/Palaeo-gene age.

3.7. Wajzer studyThe Batang Natal section was mapped in detail by Marek

Wajzer from the University of London, in a follow-up studyto the Northern Sumatra Survey, in collaboration with BGSand with the assistance of Syarif Hidayat and Suharsono ofGRDC (Wajzer et al., 1991). The mapping was supported bypetrographic, geochemical and radiometric studies. Wajzeret al. (1991) found that the Woyla Group was not neatlydivisible into three distinct lithological units, but that thesame recognisable lithologies were repeated many timesthroughout the section, apparently in a random fashion(Fig. 4). Wajzer et al. (1991) distinguished 16 lithostratigra-phical units in the Natal section. Correlation with themapping units recognised by Rock et al. (1983) is shownin Table 1. Detailed accounts of the lithological units aregiven in Table 2. Many of the lithologies are similar to rocktypes described from the Woyla Group in Aceh, and byRock et al. (1983), with the addition of several outcropsof melange, composed of blocks in a fine grained matrix,decribed as megabreccia in Table 2 and Fig. 4. One impor-tant feature of the clastic units in the Woyla Group of theNatal area is that they are almost completely devoid ofquartz, suggesting that they have an entirely oceanic, ratherthan a continental origin (Wajzer et al., 1991).

The study established several additional age constraintsfor the Woyla Group, using fossil evidence and radiometric


dating. A further specimen of Lovcenipora was obtainedfrom a limestone block in the Simpang Gambir Megabreccianear the southwestern end of the Batang Natal section, and aLate Triassic foraminifera was found in a limestone clast inthe Batang Natal Megabreccia in the central part of thesection. Diorite intruded into the Jambor Baru and BatangNatal Megabreccia Formations at Batu Madingding gave aK/Ar age of 84.7 ^ 3.6 Ma and an andesite in the TambakBaru Volcanic unit, interpreted as a fragment of a volcanicarc, gave 78.4 ^ 2.5 Ma. Both these lavas and the intrusionsare of Late Cretaceous age. Andesite dykes intruded into theSi Kumbu Turbidite Formation (i.e. Sikubu Formation ofRock et al., 1983), and regarded as contemporaneous withsedimentation of this unit, gave K/Ar ages of 40.1 ^ 4.6 and37.6 ^ 1.3 Ma (Late Eocene) (Wajzer et al., 1991). Samplescollected from the Air Bangis granitoids and analysed byWajzer gave K/Ar ages of 29.7 ^ 1.6 and 28.2 ^ 1.2 Ma(Late Oligocene) (Wajzer et al., 1991) raising the possibilitythat the Cretaceous age for these granitoids suggested in theDMR/BGS report (Rock et al., 1983) was incorrect.

Wajzer et al. (1991) also studied the structure in detail(Table 2). Most units in the Natal River section show somedegree of internal deformation. Deformation ranges frombroad open folding of the bedding in the Si Kumbu andRante Sore formations, to isoclinal folding and penetrativeslaty cleavage in fine grained units and mylonitic foliation inthe Si Gala Gala Schists. Clasts in breccia formations(melanges) are flattened and elongated parallel to the clea-vage in the matrix. Several units are affected by multiple

deformation, with the earlier isoclinal folds and schistosityfolded by more open folds, and the development of second-ary cleavages and lineations. Some units such as the RanteSore Formation are completely unmetamorphosed, whilemetabasites show pumpelleyite and actinolite, and schistscontain coarse white micas, indicating metamorphism up togreenschist facies. There is no systematic progression eitherin intensity of deformation or in grade of metamorphismthrough the section, the units appearing to be randomlyarranged (Wajzer et al., 1991).

Each of the units distinguished in the Woyla Group in theNatal River section is separated from adjacent units by stee-ply dipping or vertical faults. In addition units are frequentlydisrupted internally by faults, often every few metres. Someof the fault planes show vertical slickensides, indicatingnormal or reverse movement, while others show horizontalslickensides, indicating strike-slip movement (Wajzer et al.,1991).

3.10. Interpretation the allochthonous terrane model

The Batang Natal section was reinterpreted by Wajzer etal. (1991). They pointed to the absence of any significantamount of quartz in the volcaniclastic sediments or anycontinental clastic material in the massive limestones ofthe Maurosoma Formation, and suggested that these depos-its were of oceanic, rather than of continental margin origin.They concluded, from the extensive deposits of bedded radi-olarian chert and associated manganiferous argillites, thatthe oceanic deposits represented accreted fragments of thefloor of a major ocean basin, rather than the floor of arestricted marginal sea. As in Aceh, in the absence radio-metric ages for the pillow basalts, or of diagnostic radiolariain the chert, there is no direct evidence of the age of forma-tion of the ocean floor. Wajzer et al. (1991) suggest that thevolcanic units represent seamounts or fragments of volcanicarcs, and that limestone blocks in the melanges representfragments of the collapsed carbonate cappings to theseseamounts, or fringing reefs to the arcs. Foraminifera in alimestone block in one of these melanges suggest that aseamount was constructed on ocean floor which was atleast as old as Triassic, and that therefore the ocean basinwas already in existence in the early Mesozoic.

Wajzer et al. (1991), on the basis of interbedded lavaflows and the occurrence of fragments of andesite in theconglomerates, recognisably derived from the LangsatVolcanics, accepted the interpretation of Rock et al.(1983) that the Si Kumbu Turbidite Formation (their SikubuFormation) was contemporaneous with the Langsat Volca-nics, and that the volcanics and volcaniclastic turbiditesrepresent a volcanic arc. Andesite dykes intruded into theSi Kumbu Formation, of similar composition to the lavas,gave radiometric dates of 37.6 and 40.1 Ma (Late Eocene).As has already been reported, Rock et al. (1983), followingthe marginal sea model from Aceh, suggested that the AirBangis granitoids, which are presumed to be intruded into


Table 1Correlation of formations in the Woyla Group of the Natal area from Rocket al. (1983) with lithotectonic units defined by Wajzer et al. (1991)a

Rock et al. (1983) Wajzer et al. (1991)

1. Langsat Volcanic Formation 1. Langsat Volcanic Formation2. Sikubu Formation 2. Si Kumbu Turbidite Formation

3. Tambak Baru Volcanic Unit4. Simpang Gambir Megabreccia

Formation3. Belok Gadang Formation 5. Nabana Volcanic Unit

6. Belok Gadang Siltstone Formation7. Panglong Melange Formation8. Ranto Sore Formation

Volcanics in both the BelokGadang and MaurasomaFormations

9. Parlampungan Volcanic Unit

4. Maurasoma Formation 10. Si Gala Gala Schist FormationSchistose Member 11. Simarobu Turbidite Formation

12. Batang Natal Megabreccia Unit13. Rantobi Sandstone Formation14. Jambor Baru Formation15. Maurasoma Turbidite Formation

Massive limestones in both theBelok Gadang and MaurasomaFormations

16. Batu Nabontar Limestone Unit

a Units are listed in approximate order upstream from Langsat (see Fig. 4)with no age relationship implied.

A.J.Barber/JournalofAsian

EarthSciences

18(2000)713738

724Table 2Lithology, environmental setting, structure, metamorphic grade and age constraints for units in the Batang Natal section (in order upstream from W to E, see Fig. 4) (from Wajzer et al., 1991)a

Unit Lithology Environment Structure Metamorphism Age constraints

Langsat Volcanic Unit Porphyritic basic volcanics Arc volcanics No ductile deformation Prehnite-pumpellyite Possibly intruded by Air BangisGranites. K/Ar 28.2 Ma, 29.7 Ma

Si Kumbu TurbiditeFormation

Volcaniclastic debris flows, proximaland distal turbidites

Submarine fan apron tovolcanic arc

D2 large scale folds (F2) on WNWESE axes Prehnite-pumpellyite Intruded by andesite dykes K/Ar40.1 ^ 4.6 Ma (NR45),37.6 ^ 1.3 Ma (NR120)

Tambak Baru VolcanicUnit

Andesitic volcanics Fragments of volcanic arc andproximal volcaniclastics

D1 weak foliation (S1).D2? Prehnite-pumpellyite/greenschist

Andesitic lava. K/Ar 78.4 ^ 2.5Ma (BN133)

Simpang GambirMegabreccia Formation

Volcanic breccia with limestonemegaclasts and greywacke sandstones

Proximal sediments derivedfrom volcanic arc, witholistostromes

D1 strong foliations (S1).D2 open folds andcrenulations (F2)

Prehnite-pumpellyite/greenschist

?Lovcenipora sp. In limestoneblock (Late JurassicEarlyCretaceous)

Nabana Volcanic Unit Basic volcanics (sometimes pillowed)amygdaloidal to E, keratophyres,dolerite dykes

Ocean-floor basalts, seamount No ductile deformation Prehnite-pumpellyite/greenschist

Panglong MelangeFormation

Breccias with chert, Mn sedim.limestones and volcanic clasts in chertsiltstone matrix

Melange (?olistostrome) ofocean-floor materials andpelagic sediments

D1 tight to isoclinal folds (F1). D2 open toclose folds (F2) fold F1 on NWSE axes

Slate grade Older than Belok Gadangsiltstone

Belok Gadang SiltstoneFormation

Volcaniclastic siltstones with few finesandstones and rare conglomerates

Unconformable on PanglongMelange. ?lower trench slopebasin fill

Dipping beds with no ductile deformation Prehnite-pumpellyite Younger than Panglong MelangeFormation

Ranto Sore Formation Volcaniclastic cross-bedded andchannelled sandstones and unsortedconglomerates (lahars)

Fluviatile intra-arc deposits D2 open to close folds (F2) on NNWSSEaxes

Unmetamorphosed ?Younger than adjacent units

Parlumpangan VolcanicUnit

Porphyritic andesites Fragments of volcanic arc No ductile deformation Prehnite-pumpellyite/greenschist

Si Gala Gala Schist Unit Banded quartz, muscovite, chloriteschists

Metasediments derived fromacid-intermediate volcanic arcprovince

D1 schistosity (S1) and rodding (L1). D2 opento close folds (F2) on NWSE axes

Greenschist

Simarobu TurbiditeFormation

Volcaniclastic turbidites with minorcalcareous siltstones

Ocean-floor or trench deposit Foliation (S1). D2 open to close folds (F2). D1tight to isoclinal folds (F1) axial plane onNNESSE axes

Greenschist Cut by undeformed micro-diorite dyke. K/Ar 49.5 ^ 2 Ma(NR 7)

Batang NatalMegabreccia Formation

Large clasts of limestone, rare clasticsediments and igneous rocks in slatymatrix

Melange formed asolistostrome or as mud diapirsin accretionary complex

D1 tight to isoclinal folds (F1). D2 open toclose folds deform S1 about NNESSW axes.D1 tight to isoclinal folds (F1) with axial planefoliation (S1). D2 open to close folds (F2)deform S1 on NNWSSE axes.

Slate grade Included limestone clastscontain Late Triassicforaminifera. Intruded by BatuMadingding Diorite. K/Ar84.7 ^ 3.6 Ma

Rantobi SandstoneFormation

Thin bedded volcaniclastic sandstonesand siltstone

Forearc basin deposits Axial plane cleavage (S1). D1 isoclinal folds(F1) with D2 close asymmetric folds (F2)

Slate grade

Jambor Baru Formation Volcaniclastic conglomerate,sandstone, siltstone, limestone and tuff

Shallow marine and deeperwater forearc basin deposits

D1 foliation (S1). D2 close folds (F2) on NWSE axes


Intruded by Batu MadingdingDiorite. K/Ar 84.7 ^ 3.6 Ma

Maurosoma TurbiditeFormation

Thin bedded volcaniclastic turbiditeswith a coarser grained member

Upper trench slope basinsediments

D1 foliation (S1). D2 folds (F2) on NWSEaxes


Batu Nabontar LimestoneUnit

Massive recrystallised limestone, rarefossils

Open marine shelf limestone D1 tight folds in interbedded tuffs (F1), fossilsshow strain

Recrystallised Intruded by Batu ManunggalBatholith. K/Ar 87.0 Ma

a All units are cut by numerous faults and thrusts. Vertical faults often show horizontal slickensides indicating wrench fault movements. *K/Ar age of Manunggal Batholith from Kanao et al. (1971). Allother K/Ar ages from Wajzer et al. (1991).

the Langsat Volcanics, were of Late Cretaceous age, andtherefore analogous to the Sikuleh Batholith. However, asalready pointed out, they acknowledged that if this correla-tion was incorrect then the Aceh marginal /sea model couldnot be applied to the Natal area. In the event, the Air Bangisgranitoids gave ages of 28 and 29 Ma, indicating that theLangsat Arc was formed during Late EoceneEarly Oligo-cene times (Wajzer et al., 1991).

Wajzer et al. (1991) were impressed by the evidence ofstrike slip faulting throughout the Natal section, and in parti-cular by the major Simpang Gambir Fault, which separatesthe Si Kumbu Turbidites (Sikubu Formation of Rock et al.,1983) and the Langsat Volcanics from the Belok Gadangocean floor assemblage (Fig. 4). They therefore suggestedthat the Natal Continental Fragment was an allochthonousterrane, which had reached its present position by strike-slipmovement from a location further to the south along theSumatran continental margin. The possibility of anallochthonous origin for the Natal Continental Fragmentwas also considered by Rock et al. (1983), but rejected infavour of the interpretation previously reached for thetectonic evolution of the Woyla Group in Aceh (Cameronet al., 1980). Although as already pointed out they acknowl-edged that if the Late Cretaceous age for the Air Bangisgranitoids was incorrect, then an allochthonous origin wasprobable. Wajzer et al. (1991) reversing the correlation,suggested that if the Natal Microcontinental Fragment isan allochthonous terrane, then it is possible that the SikulehMicrocontinental Fragment although of a different age, isalso allochthonous.

4. Other units in Sumatra correlated with the WoylaGroup

Outcrops of rock units with similar lithologies to those ofthe Woyla Group or which were formed within the sameJurassicCretaceous age range have been mapped through-out western Sumatra (Fig. 1). Many of these outcrops havebeen correlated with the Woyla Group of northern Sumatraby previous authors.

4.1. Indarung formationSmall outcrops of the Mesozoic Indarung Formation

occur near Padang in West Sumatra. These rocks weremapped and described by Yancey and Alif (1977) andwere correlated with the Woyla Group of Aceh by Cameronet al. (1980). Outcrops occur 15 km east of Padang in road,river and quarry sections near Indarung, where they aresurrounded and overlain by Neogene and Quaternary volca-nic and volcaniclastic rocks (Fig. 5). The area of outcrop isincluded on the Padang, Solok and Painan QuadrangleSheets (Kastowo and Leo, 1973; Silitonga and Kastowo,1975; Rosidi et al., 1976). These rocks have been mappedmore recently by McCarthy et al. (2001).

Yancey and Alif (1977) described rocks exposed in the

Lubuk Peraku River, the Ngalau Quarry, the Karang PutihQuarry and adjacent river sections near Indarung. Rocktypes in these outcrops are basic volcanics, which mayinclude pillow lava, volcanic breccia, tuff, volcaniclasticsediments, radiolarian chert and massive or bedded lime-stones. The basic rocks are sometimes deformed and meta-morphosed to form greenschists. On the other hand, thelimestones and cherts are essentially undeformed, althoughdisharmonic folding and small scale thrusts in the chert andgentle folds in the limestone are seen in the quarries, and thelimestones may be recrystallised (McCarthy et al., 2001).

A well exposed section of limestone and tuff occurs in theriver section of the Lubuk Peraku and in the road above theriver (Yancey and Alif, 1977; McCarthy et al., 2001).Bedding in the limestones has a steep, but variable dip tothe southwest, and the section youngs in the same direction.A measured columnar section of these outcrops fromMcCarthy et al. (2001) is given as Fig. 6. The lower partof the section, described as the Lubuk Peraku Limestone, isa limestone breccia, which includes volcanic clasts near thebase and is interbedded with thin tuff bands near the top.The breccia is overlain by a few metres of thin-bedded lime-stones and shelly marls and then by thicker bedded and moremassive limestones, some oolitic. Near the top of the sectiona limestone conglomerate, eroded into the underlying lime-stone with basal scours, provides clear evidence of way-up.Above the breccia there is a break in the outcrop, untiloutcrops further downstream and in the road section aboveexpose a calcareous vitreous crystal tuff, the Golok Tuff.Although the contact between the breccia and the tuff isnot exposed, McCarthy et al. (2001) regard this section asan essentially continuous stratigraphic sequence, as tufflayers occur within the limestone breccia and tuffs withinthe overlying Golok Tuff unit are calcareous.

In the Ngalau Quarry, near Indarung, McCarthy et al.(2001) collected samples from a 15 m section of beddedchert for radiolarian determination. In the Karang PutihQuarry, 1 km to the south of Indarung, excavated in anisolated hill for the manufacture of cement, lenses of chertare associated with massive limestone. McCarthy et al.(2001) report that the limestone in this quarry is completelyrecrystallised, possibly due to the effects of a granitic intru-sion which occurs a short distance to the south (Fig. 5). Theysuggest that relationships between limestone and chert inthis quarry are tectonic. In an interpretative cross sectionthey show the cherts and limestones imbricated togetheralong low angle thrusts (McCarthy et al., 2001).

Rock units in the Indarung area are well dated from fossiland radiometric age determinations. Radiolaria from chertin the Ngalau Quarry belong to the Transhsuum hisui-koyense Zone, of Aalenian, early Middle Jurassic age(McCarthy et al., 2001). Lithologies and fossil content ofthe limestones in the Lubuk Peraku section and in theNgalau and Karang Putih quarries were described byYancey and Alif (1977). The limestones are biosparites,with abundant bioclasts, oolitic calcarenites and micrites.


Molluscan shell fragments, pellets, calcareous algae,stromatoporoids and scleractinian corals are commoncomponents of the limestones. Among the fossils identifiedwere the (?) stromatoporoids Actostroma and Lovcenipora.The former is considered to be restricted to the Late Juras-sic, while the latter is diagnostic of the Late Jurassic to EarlyCretaceous. A K/Ar age date of 105 ^ 3 Ma (Albian, mid-Cretaceous) is reported from the Golok Tuff in the LubukPeraku by Koning and Aulia (1985, from a Caltex PacificIndonesia internal report).

Pillow lavas and cherts of the Indarung Formation havebeen equated with the oceanic assemblage of the WoylaGroup of Aceh and with the Belok Gadang Formation ofthe Natal area (Cameron et al., 1980; Rock et al., 1983).Where these rocks are imbricated, deformed and altered togreenschists they may be interpreted, as is the case in Acehand Natal, as materials accreted from a subducted oceanfloor. The recent recognition of Middle Jurassic radiolariain the cherts (McCarthy et al., 2001) shows that part of thisocean floor was of Jurassic age. The volcanic breccias tuffsand volcaniclastic sandstones of the Indarung Formation areinterpreted as the products of seamount volcanism, and the

massive limestone with its Late JurassicEarly Cretaceousfossil fauna is interpreted as part of a fringing reef formedaround the seamount (McCarthy et al., 2001). Duringsubduction the seamount with its carbonate cap collidedwith already accreted ocean floor materials, and the wholeassemblage was imbricated to form the present complex.

4.2. Siguntur formationMesozoic rocks of the Siguntur Formation are exposed in

the Sungai Siguntur, 15 km to the south of Indarung (Fig. 5).The area of outcrop is shown on the Painan QuadrangleSheet and the lithology is described in the ExplanatoryNote (Rosidi et al., 1976). Rock types are quartzites, silt-stones and shales, the latter sometimes altered to slates, andcompact limestones. The map shows that the strike of thebeds is EW, transverse to the general Sumatran trend. Inthe report the rocks are described as not intensely deformedor folded, but sandstones interbedded with slates showingbedding-parallel cleavage, suggest that the rocks are morehighly deformed than first appears. The limestones arereported to contain Lovcenipora, and are therefore of a


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).

similar age to the limestones at Indarung. However, abun-dant terrigenous quartz in the clastic sediments, notablyabsent from the Woyla Group in Natal and the rock unitsat Indarung, suggests that the sediments of the SigunturFormation had a continental provenance and were depositedin a continental margin environment.

Although these rocks are deformed, with the develop-ment of slaty cleavage, there is no suggestion in thedescriptions that these rocks have been imbricated or canbe interpreted as part of an accretionary complex (Rosidi etal., 1976). In the Siguntur Formation we are dealing for thefirst time in this account with continentally-derived sedi-

ments which were deposited on the Sundaland continentalmargin.

4.3. Suilak, Tabir, Asai, Peneta and Rawas formations incentral Sumatra

Further outcrops of Mesozoic sedimentary and volcanicrocks occur at Suilak 150 km to the southeast of Padang(Fig. 1), in a fault block caught between strands of theSumatran Fault (Rosidi et al., 1976). These sediments arecalcareous siltstones, calcareous shales and limestones. Theshales and siltstones are carbonaceous and contain angular


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).

quartz clasts. The limestones contain Loftulisa and Hydro-corallinae of Cretaceous age (Tobler, 1922, reported inRosidi et al., 1976). The volcanic rocks are altered ande-sites, dacites and bedded tuffs with clasts of augite, horn-blende, chlorite and glass. These rocks are the product ofAndean arc volcanism on the margin of Sundaland.

Sixty kilometers to the east of Siulak and to the northeastof the Sumatran Fault Zone, in the Batang Tabir, areoutcrops of red conglomerates, sandstones and tuffs of theTabir Formation. Clasts in the conglomerates include quart-zite, and andesitic fragments derived from the adjacentPalaeozoic rocks. The presence of Ostrea is taken to indi-cate a Mesozoic, possibly Jurassic age (Tobler, 1922,reported in Rosidi et al., 1976). Continuous with theseoutcrops and extending southeastwards to the south ofBangko, and also to the northeast of the Sumatran Fault,are large outcrops of Mesozoic rocks of the Asai, Penetaand Rawas formations (Kusnama et al., 1993; Suwarna etal., 1994), (Fig. 1). Rock types include quartz sandstones,siltstones, shales, limestones and tuffs. The Rawas Forma-tion also includes andesite-basalt lava flows, tuffs and volca-niclastic sandstones. Clasts in conglomeratic units in thesesediments are derived from the local Palaeozoic basement.Sandstone units show turbiditic characteristics. Argillac-eous units have a slaty cleavage striking NWSE. Fossils,

including corals and ammonites, especially from the lime-stone members, show that these sediments range in age fromMiddle Jurassic to Early Cretaceous (Suwarna et al., 1994).

All these sediments, although subject to later deforma-tion, were evidently deposited in situ on the Sundalandcontinental basement. Pulunggono and Cameron (1984)suggested that these units were deposited in a forelandbasin, but a forearc environment, related to an Andeanvolcanic arc represented by the volcanics lava flows andtuffs in the Rawas and Tabir Formation is more probable.

4.4. Gumai Mountains

Pre-Tertiary rocks form an inlier to the northeast of theSumatran Fault Zone in the Gumai Mountains of SouthSumatra, where they occupy the core of an anticline,surrounded by Tertiary sediments and volcanics (Fig. 7).These Pre-Tertiary rocks are described as the Saling, Lings-ing and Sepingtiang formations (Gafoer et al., 1992).

The Saling Formation, which forms the northern part ofthe inlier, is composed of amygdaloidal and porphyriticandesitic and basaltic lavas, breccias and tuffs. The lavasare cut by diorite dykes, regarded as contemporaneous withthe lavas, and dated at 116 ^ 3 Ma (Early Cretaceous). Onthe basis of chemical analyses and discriminant plots the


Fig. 7. The distribution of the Saling, Lingsing and Sepintiang formations, correlatives of the Woyla Group, in the Gumai Mountains, South Sumatra (afterGRDC map of Bengkulu, Gafoer et al., 1992).

lavas are interpreted as tholeiites of oceanic affinity (Gafoeret al., 1992). The Lingsing Formation in the southern part ofthe inlier, contains similar igneous rocks, interbedded withclaystone, siltstone, sandstone, calcilutite and chert. TheSaling and Lingsing formations are therefore consideredto be contemporaneous. Since the tholeiitic basalts are asso-ciated with serpentinised ultrabasic pyroxenites and cherts,the whole assemblage is regarded as ophiolitic sequence ofocean floor origin. Although the rocks are highly deformedand folded it is not clear from the descriptions whether theyare imbricated to form an accretionary complex (Gafoer etal., 1992). The strike of bedding and cleavage in the sedi-ments is said to be NS. The the mapped EW contactbetween the Saling and the Lingsing formations is thereforepresumably tectonic (Fig. 7).

The Saling and Lingsing formations are overlain discor-dantly by the Sepingtiang Limestone Formation (Fig. 7).This is composed of massive, brecciated and bedded lime-stones, containing the coral Calamophylliopsis crassa (LateJurassic), the foraminifera Pseudotexturariella, smallCuneolina (Early Cretaceous) and Orbitolina sp. (mid-Cretaceous). The contact between the Sepingtiang Lime-stone and the underlying units is considered to be tectonic(Gafoer et al., 1992).

All of these units are cut by granitic intrusions, which byanalogy with similar dated granitoids further south in theGarba Mountains, are regarded as of Late Cretaceous age(Gafoer et al., 1992). The rocks of the inlier and thesurrounding Tertiary rocks are also cut by NWSE trendingfaults, some showing strike-slip displacements (Fig. 7), andare evidently related to the Sumatran Fault System, the mainstrands of which lie some 25 km to the southwest.

As already noted, Gafoer et al. (1992) interpreted theigneous rocks of the Saling Formation as ocean floor basalts,on the basis of their tholeiitic composition and their associa-tion in the field with serpentinites and cherts. However, thepresence of andesites, the amygdaloidal and porphyritictextures, together with association of the volcanics withthe massive Sepintiang Limestone, suggest that the SalingFormation also includes part of a volcanic arc. The descrip-tion of the Saling Formation closely resembles that of theBentaro Volcanic Formation of Aceh (Bennett et al., 1981a)and the Nabana Volcanic and Parlumpangan units of theBatang Natal (Wajzer et al., 1991). The Early Cretaceousage shows that the Saling Volcanic Arc was active contem-poraneously with the Bentaro Arc of Aceh.

The Sepingtiang Limestone may be interpreted in thesame way as the limestones in Aceh, as a fringing reefsurrounding a volcanic arc. Fossil evidence of the LateJurassic to mid-Cretaceous age of the Sepintiang LimestoneFormation means that it can be correlated directly with theLamno, Teunom and Sise Limestone formations of Aceh(Bennett et al., 1981a; Cameron et al., 1983), the BatuNabontar limestones in the Batang Natal section (Wajzeret al., 1991) and the Lubuk Peraku limestones at Indarung(Yancey and Alif, 1977).

The Lingsing Formation, composed of siltstones, shalesand cherts, has been interpreted as deposited in a bathyalenvironment (van Bemmelen, 1949; Gafoer et al., 1992).The presence of lavas interbedded with clastic deposits,suggests that the Lingsing Formation represents more distalflows, volcaniclastic sediments and clastic carbonatesderived from the arc extending out onto an ocean floor,represented by the bedded cherts. These rocks resembleclastic units in the Lhonga Formation of Aceh (Bennett etal., 1981a) and the Belok Gadang Siltstone and RantobiSandstone formations of Natal (Wajzer et al., 1991).

4.5. Garba Mountains

The Garba Mountains form an inlier of pre-Tertiaryrocks, about 100 km to the southeast of the Gumai Moun-tains and also northeast of the Sumatran Fault Zone (Fig. 1).The inlier is shown on the GRDC Baturaja QuadrangleSheet (Gafoer et al., 1994) (Fig. 8). The oldest unit, whichoutcrops on both eastern and western sides of the inlier, isthe Tarap Formation, composed of phyllite, schist, slate,minor quartzite and marble, metamorphosed in the greens-chist facies. The age of the Tarap Formation is unknown, butit is presumed to be older than the adjacent, but unmetamor-phosed Garba Formation, and is interpreted as the metamor-phosed Palaeozoic basement of Sumatra (Gafoer et al.,1994). Outcrops of the Tarap Formation occur within theGarba Formation and in large areas to the east and west. Therelationships between the metamorphic Tarap Formationand the Garba Formation, as shown on the map, are notclear (Gafoer et al., 1994) (Fig. 8). The eastern contactsare faulted and possibly thrust. If the Tarap is part of themetamorphosed Palaeozoic basement of Sumatra, it mayrepresent the western margin of the Sundaland continent,thrust westwards over accreted ocean floor materials repre-sented by the other units in the inlier. The present distribu-tion of these metamorphic rocks could be due to thedisruption of the thrust sheet by faulting and erosion.

The Garba Formation consists of (?)amygdaloidal andporphyritic basaltic and andesitic lavas, associated withsheared serpentinite and lenses and intercalations of radi-olarian chert. An Insu Member is distinguished on the map,with a similar lithological assemblage, but also containinginterlayered lenticular bodies of melange, with boulders ofbasalt, andesite, radiolarian chert, claystone, siltstone, schistand massive limestone in a scaly clay matrix (Gafoer et al.,1994). The limestones found as blocks do not outcrop else-where in the inlier, but are presumed to be derived from anunexposed component of the Garba Formation. Blocks ofthe Tarap metamorphics are notably absent from themelange. The foliation in the scaly matrix and the elonga-tion of the enclosed blocks, which are cut by tension frac-tures normal to their long axes, trends in a NWSE direction(Gafoer et al., 1994). A fault-bounded sliver on the easternside of the inlier, and a few other scattered outcrops wherechert is abundant, are mapped as the Situlanglang Member


(Fig. 8). Two fold phases are recognised in the GarbaFormation, an earlier phase of EW folds and a laterphase of NESW folds (Gafoer et al., 1994). Neither thecherts nor the limestones have so far yielded age-diagnosticfossils.

Both the Tarap and the Garba formations are intruded bythe Garba Pluton, a composite body in which an oldercomponent has been dated at 115 ^ 4 and 102 ^ 3 Ma(mid-Cretaceous) and a younger component at 79 ^ 1.3and 89.3 ^ 1.7 Ma (Late Cretaceous). Rock units withinthe inlier are cut and bounded by NWSE trending faults.Although these faults are parallel to the Sumatran FaultSystem they do not appear to affect significantly the Tertiaryrocks and must be largely of Pre-Tertiary age.

The Garba Formation has been compared to the WoylaGroup of Natal (Gafoer et al., 1994) and certainly litholo-gical descriptions of this formation and its Insu and Situlan-glang members, correspond very well with those from Acehand the Batang Natal section. The basaltic and andesiticlavas of the Garba Formation correspond with those of theBentaro Arc, and may similarly be interpreted as part of avolcanic arc sequence. Limestone blocks within themelange may represent fragments of fringing reefs or thecollapsed carbonate cappings of seamounts, the latter now

represented by volcanics in the Garba Formation, as hasbeen suggested for the Natal and Indarung areas (Wajzeret al. 1991; McCarthy et al., 2001). Descriptions of themelanges of the Insu Member of the Garba Formation(Gafoer et al., 1994) are identical to those from Natal(Wajzer et al., 1991). The interlayering of the Insu Memberwith lavas, chert and melange (Gafoer et al., 1994)suggests that these rocks are deformed and imbricated inthe same way as the Woyla Group in the Batang Natalsection, and similarly represent an accretionary complexformed by subduction of an ocean floor. It may be thatsome of the low grade metamorphic schists mapped withinthe Insu Member as Tarap Formation, are part of thisaccretionary complex, as metamorphic rocks, up to greens-chist facies, are incorporated in the accretionary complexat Natal.

The mid- to Late Cretaceous Garba Pluton (11579 Ma)intrudes both the Tarap and the Garba formations, so that theaccretion of the Garba Formation to the margin of Sunda-land, if the Tarap is correctly identified as Palaeozoic base-ment, took place before the mid-Cretaceous. The age of theyounger component of the Garba Pluton is comparable tothat of the Sikuleh Batholith in Aceh (98 Ma) and theManunggal Batholith (87 Ma) in Natal.


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

4.6. Bandarlampung area

Pre-Tertiary rocks occur as scattered outcrops amongTertiary and Quaternary volcanic and volcaniclastic sedi-ments in the area between Kotaagung and Bandarlampungin Lampung Province, southern Sumatra (Amin et al., 1994;Andi Mangga et al., 1994) (Fig. 9). Outcrops towards thenortheast of the area consist of pelitic, calcareous, quartzoseor graphitic schists and sericitic quartzites, with intercalatedgneiss, marble and amphibolites, described as the Gunung-kasih Complex. Within the complex there is a tendency foroutcrops to the northeast to be composed of orthogneiss,while those to the southwest are composed of metasedi-ments. The strike of the foliation in the schists trends gener-ally NWSE but the schistosity is folded about EW axesand refolded by NWSE trending upright folds and then byvariably oriented kink band folds. These rocks are suggestedto be equivalent to the Tarap Formation of Garba and meta-morphic equivalents of the Palaeozoic Tapanuli Group inAceh and the Kuantan Formation in the Padang area (Aminet al., 1994; Andi Mangga et al., 1994).

Outcrops of unmetamorphosed rocks, identified as theMenanga Formation, lie largely to the the southwest of

the Gunungkasih Complex (Fig. 9). These consist of tuffac-eous and calcareous claystones, sandstones and shales withintercalated radiolarian-bearing cherts, manganese nodulesand coral limestones and rare porphyritic basalt. The sand-stones contain clasts of glassy andesite and lithic fragmentsof andesite, quartz-diorite and quartzite. The cherts have notso far yielded diagnostic radiolaria, but Zwierzycki (1932,confirmed in Andi Mangga et al., 1994), reports the occur-rence of Orbitolina sp. of Aptian-Albian (mid-Cretaceous)age from limestones in the Menanga river section. Thebedding strikes NWSE with dips of 358608 to the north-east. The rocks are folded and cut by faults, with slicken-sides indicating reverse movement.

The contact between the Gunungkasih Complex and theMenanga Formation in Gunung Kasih is obscured, due torice cultivation, and in Teluk Ratai is at present inaccessibleas it lies within a Naval Base (Fig. 9). However, the lattercontact in the Menanga River was described by Zwierzycki(1932) as occupied by a friction breccia. On the GRDCmaps Amin et al. (1994) and Andi Mangga et al. (1994)show both these contacts as thrusts (Fig. 9).

The Menanga Formation is interpreted by Amin et al.(1994) as a deep water marine sequence with interbedded


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

basalt lavas and andesitic clastic fragments, derived from avolcanic arc, and deposited in a trench or forearc environ-ment. These sediments were deformed during accretion tothe Sumatran margin, represented by the GunungkasihComplex. K/Ar radiometric ages, ranging from 125108Ma (mid-Cretaceous) from hornblende in an amphiboliticschist in the Menanga Formation, is taken as the age ofaccretion (Andi Mangga et al., 1994). However, thepresence of quartzite and quartz-diorite clasts suggeststhat the Menanga Formation was, like the Rawas and asso-ciated formations in central Sumatra, derived from anAndean arc built on a continental basement, and was depos-ited in a forearc environment. The Menanga Formation wasoverthrust by the basement at a later stage.

Near Bandarlampung the Gunungkasih Complex isintruded by the Sulan Pluton (Fig. 9). The pluton is acomposite body which includes gabbro, dated by K/Arradiometric analysis at 151 ^ 4 Ma (Late Jurassic), horn-blende and biotite granites and granodiorite intruded by lateaplogranite dykes. Granite from the Sulan Pluton gave anage of 113 ^ 3 Ma (mid-Cretaceous) (McCourt et al., 1996).

To the north of Bandarlampung, spectacular exposuresbelow an irrigation dam on the Sekampung River showextensive outcrops of granodioritic and dioritic gneiss,containing basic xenoliths, and cut by concordant anddiscordant granitic and pegmatitic veins. The granitic andgranodioritic gneisses are cut by basaltic dykes, severalmetres thick, which contain xenoliths of gneiss. The gneissxenoliths show evidence of melting, and towards themargins of the dykes are drawn out into streaks, which aresometimes isoclinally folded, parallel to the dyke margins.The dykes and the foliation in the gneisses both trend in aNWSE direction. Fold structures in the dykes and thecurvature of foliation in the gneisses indicate that the dykemargins have acted as strike-slip shear zones, with a sinistralsense of movement. Sub-horizontal slickensides on foliationsurfaces within the gneiss indicate the same sense of move-ment. Diorite from the Sekumpang exposure has been datedby the K/Ar method at 89 ^ 3 Ma (late mid-Cretaceous)(McCourt et al., 1996).

In the same area, in the Wai Triplek, greenschist facieswhite mica-quartz schists are intruded by metadoleritedykes. The margins of the dykes show compositional band-ing which is isoclinally folded, in a similar fashion to thedykes in the Sekampung River. Further upstream the bed ofthe Wai Triplek exposes streaky acid and basic gneisses cutby more homogeneous basic dykes. Acid gneiss showsevidence of having been melted and recrystallised alongthe dyke contacts, and quartz-feldspar veins fill fracturesin brecciated basic dyke material, in a process of backinjection.

Relics of dyke rocks occurring as basic xenoliths ingneiss, and gneiss xenoliths enclosed in in basalt dykes,indicate that the intrusion of basaltic dykes and graniticbodies alternated during the development of the gneisscomplex at Sekampung. Exposures in the Wai Triplek

form part of the same gneiss complex, but also containfragments of the schistose continental basement intowhich the igneous rocks were intruded. During or shortlyafter intrusion, both granitic and basic rocks were affectedby sinistral shearing, which converted the granitic and diori-tic rocks into gneisses and deformed the basic dykes. Thealternation of acid and basic intrusion, with contempora-neous deformation, are characteristic features of the basalparts of a magmatic arc, where acid and basic magmas areintruded into an active strike-slip fault zone. This situationis similar to that which exists beneath Sumatra at the presentday where the modern volcanic arc is built on the activeSumatran Fault Zone. However, the sense of movementalong the present arc is dextral, in the opposite sense tothe sinistral movement along the Cretaceous arc.

5. Tectonic evolution of the Sundaland margin

5.1. Pre-Cretaceous Sundaland Continent

The concept that Southeast Asia, and indeed Asia as awhole, has been built up during the Phanerozoic by theamalgamation of allochthonous terranes derived from thenorthern margin of East Gondwanaland, is now well estab-lished in the literature (e.g. Metcalfe, 1996 and referencestherein). In Metcalfes (1996) version of the concept a seriesof elongated terranes separated successively from the NorthGondwanaland margin by the development of ocean basinsbehind them. These oceans are referred to as Palaeo-Tethys,Meso-Tethys and Ceno-Tethys. The Indochina Block whichforms the core of Southeast Asia had separated from Gond-wanaland by Late Devonian times and amalgamated withthe South China Block by the Early Carboniferous. To thiscore was added the Shan-Thai or Sibumasu Block, whichseparated from Gondwanaland in the Permian and amalga-mated with the Indochina Block in the Late Permian orTriassic (Metcalfe, 1996).

Following the completion of the Integrated GeologicalSurvey of Northern Sumatra, these concepts were extendedby Aspden et al. (1982) and Pulunggono and Cameron(1984) to interpret the structural evolution of the southwes-tern margin of Sundaland in Sumatra. The model was laterextended to cover southern Sumatra (McCourt et al., 1993).The interpretation of the structure of Sumatra illustrated inFig. 10 is adapted from the Tectonic Map of NorthernSumatra (Aspden et al., 1982). In this map Sumatra isshown with its present NWSE orientation, but severalreconstructions, including those by Metcalfe (1996), showSumatra with an EW orientation during the Mesozoic. TheIndochina Block is represented by the East Malaya Micro-plate, occupying the eastern part of the Malay Peninsula andsutured to the Malacca Microplate along the Raub-BentongLine. To the west the Malacca Microplate is joined by theMergui Microplate along a Triassic suture known as theMutus Assemblage, identified only from the occurrence of


mauve shales, cherts, volcanic rocks, gabbros and serpenti-nites in oil company boreholes (Eubank and Makki, 1981;Pulunggono, 1983).

In northern Sumatra the Mergui Microplate is made up ofthe Permo-Carboniferous Tapanuli Group, including theBohorok Formation, composed of pebbly mudstones, inter-preted as glacio-marine sediments, and correlated with theSingha Group in the Langkawi Islands and the PhuketGroup of western Thailand (Cameron et al., 1980). South-wards, these rocks are recognised in the Tigapuluh hills ofcentral Sumatra (Simandjuntak et al., 1991; Suwarna et al.,1991). It is also suggested that the Gunungkasih Complex ofthe Bandarlampung area may represent the extension of themetamorphosed representatives of the Tapanuli Group intosouthern Sumatra (McCourt et al., 1993; Amin et al., 1994;Andi Mangga et al., 1994). A Permian volcanic arc was addedto the southwestern margin of these amalgamated terranes inthe Triassic (McCourt et al., 1996).

5.2. The Late Mesozoic Continental Margin

In their review of Mesozoic to Cenozoic plutonism in

Sumatra, McCourt et al. (1996) identify a belt of MiddleJurassic to Early Cretaceous intrusions, including the SulitAir suite (203 ^ 6 Ma) and the Bungo Batholith (169 ^ 5Ma), adjacent to the suture between the Mergui Microplateand the Permian Volcanic arc. These plutons are subduc-tion-related, I-type granites formed by the northeasterlysubduction of an oceanic plate which lay off the continentalmargin of Sundaland, to the southwest (Fig. 11A).Remnants of the volcanic rocks related to this phase ofmagmatism may be represented by andesitic and basalticvolcanic rocks in the Rawas, Tabir and Siulak formationsof central Sumatra and by tuffs in the Menanga Formation ofsouthern Sumatra. However, there is no evidence that thisarc extended into northern Sumatra.

This magmatic arc assemblage is termed the Rawas Arcin the reconstruction illustrated in Fig. 11A. Associateddeep-water turbiditic sandstones, shales and the shallow-water fossiliferous limestones of the Rawas, Asai, Penetaand Menanga formations, well-dated by fossil evidence andranging in age from mid-Jurassic to Early Cretaceous, werecontemporaneous with the magmatic arc, and can be inter-preted as forearc basin deposits (Fig. 11A). Further to the


Fig. 10. The structure of the southwestern margin of Sundaland (with its present orientation) in Late Cretaceous times, according to the interpretation given inthe 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.

northwest, clastic sediments of the Tabir, Siulak and Sigun-tur formations which contain both volcanic and continen-tally-derived clasts, may also have been deposited in thissame forearc basin.

Oceanic crustal material and slices of mantle, remnants ofthe oceanic plate which was being subducted off the south-western margin of Sundaland at this time and incorporatedinto an accretionary complex, now form the oceanic assem-blage of the Woyla Group. As described in this account,units composed of ocean floor components, and correlatedwith the Woyla Group, have been recognised all along thewest coast of Sumatra, from Banda Aceh to the GarbaMountains. The continental margin against which accretiontook place can be recognised in Aceh, where rocks of theTapanuli and Peusangan groups, representing the MerguiMicroplate and the Permian Volcanic

Documents

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