The Sunda outer-arc ridge, North Sumatra, Indonesia

Journal of Southeast Asian Earth Sciences, Vol. 6, No. 3/4, pp. 463-476, 1991 Printed in Great Britain

0743-9547/91 $3.00 + 0.00 Pergamon Press Ltd

The Sunda Outer-Arc Ridge, North Sumatra, Indonesia N. A. HARBURY* and H. J. KALLAGHER t *Department of Geology, Birkbeck College London, Malet Street, London WC1E 7HX, U.K. and tDepartment of Geology, Royal Holloway and Bedford New College, University of London, Egham, Surrey TW20 0EX, U.K. (Received 29 August 1990; accepted for pubfication 5 May 1991) Akstract--Forearc islands such as Nias and Simeulue provide unique opportunities to study the active accretion of sediment in the outer part of a forearc. These islands are located along the active margin of the Sunda forearc basin and separate it from the trench slope. Recent fieldwork has revealed that the sedimentological and structural evolution of the northern Sumatran forearc islands can be related to the rate of plate convergence, sea-level variations and uplift of the magmatic arc on Sumatra. A revised stratigraphy for the outer arc part of the forearc is presented and the Tertiary evolution described. During Oligocene and Eocene times an increase in the subduction rate led to basin inversion and uplift of the outer arc ridge. Deposits include melanges (? Eocene-Oligocene) and Neogene sedimentary successions initially (Early Miocene) deposited in relatively deep water. Stable convergence rates persisted through Mid-Miocene times and deposition was dominated by relatively shallow water clastic and carbonate sediments deposited on a well-developed shelf and shelf-break. In the Late Miocene, outer shelf limestones correlate with a low stand of sea level. Plio-Pleistocene sediments are composed of clastic sediments containing fine grained volcanic detritus derived from the rapidly eroding volcanic arc on Sumatra.

sedimentological and structural data will be published elsewhere.

INTRODUCTION THE ISLANDS of Nias and Simeulue form part of the emergent, non-volcanic outer-arc ridge and mark the western margin of the Sunda Forearc Basin of northwestern Sumatra (Fig. 1). This chain of islands and sea-floor rises, between 100 and 150 km off the coast of West Sumatra, forms a structurally controlled topographic ridge nearly 200 km wide (Karig et al. 1979), that extends from the Andaman Sea to the southeast of Java. Although little studied because of their remote location, the islands contain good exposures in road and river sections where the southwestern margin of the forearc basin can be examined. The islands of the Sunda Forearc have been widely used as models for other forearc terrains (Byrne 1982, Leggett et al. 1982). The island of Nias, in particular, has been frequently cited as a classic model of an accretionary complex, although comparatively few studies have been carried out on the other non-volcanic islands of the Sunda Arc chain. This paper presents new data resulting from reconnaissance field mapping on the outer-arc islands and presents a revised lithostratigraphy for these islands (Figs 2 and 3). The aim of our ongoing research programme is to establish the structural and stratigraphic evolution of the Sunda Forearc, and to determine the influence of factors such as the development of the accretionary complex, uplift of continental basement along the western margin of Sumatra, the growth of the volcanic arc and changes in sea level on the history of sedimentation in the region. These sedimentation patterns have been further complicated by dextral movements along the major transcurrent Sumatra Fault System. A detailed description of the "t Present address: Esso Exploration and Production, U.K. Ltd, Esso House, Ermyn Way, Leatherhead, Surrey KT22 8UY, U.K.

REGIONAL TECTONIC SETTING AND STRATIGRAPHY Sumatra lies on the western edge of Sundaland, a southern extension of the Eurasian Continental Plate, now interpreted to be constructed by collision and suturing of discrete microcontinents in late Pre-Tertiary times (Pulunggono and Cameron 1984, Barber 1985). At the present-day, the Indian Ocean Plate is being subducted beneath the Eurasian Continental Plate in a N20°E direction at a rate of between 6 and 7 cm/yr. This zone of oblique convergence is marked by the active Sunda Arc-Trench system which extends for more than 5000 km, from Burma in the north to where the Australian Plate is in collision with Eastern Indonesia in the south (Hamilton 1979). The complex processes resulting in basin formation and inversion during the evolution of Indonesia owe their origin to the interplay between the Indian, Eurasian, Australian and Pacific lithospheric plates (Daly et al. 1987, 1991). The relative Tertiary motions between these plates is relatively well constrained by ocean floor magnetic anomalies and palaeomagnetic data. However, older ocean floor has been subducted and earlier plate motions must therefore be inferred. The islands of Nias and Simeulue share a broadly comparable geology of melange overlain by interbedded sandstone and siltstone sequences with limestone intervals. Early authors (Elber 1939, Hopper 1940) mapped units primarily on the basis of palaeontological evidence. However, the poor fossil assemblages recovered from these units makes such stratigraphy unreliable (Burrough and Neilson 1970). This paper presents a revised lithostratigraphy for the outer arc islands of NW

463

464

N.A. HARBURYand H. J. KALLAGHER

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density of curved, polished shear planes (M. Jones, pers. commun. 1990). The age of the Oyo Complex remains unresolved by palaeontological analyses but is considered to be older than the oldest Nias Beds of early Miocene age (Moore and Karig 1980). Interpretation. The Oyo Complex melange is composed of a wide variety of lithologies with different provenances. Coarse clastic sediments are interpreted as derived from mainland Sumatra to the east (Moore 1979), whereas finer-grained micaceous sandstones are considered to be derived from the Himalayas and transported southwards along the Bengal and Nicobar fans. These deformed trench-fill turbidites together with deep sea pelagites and oceanic crust were subsequently accreted to the base of the inner trench slope (Moore and Karig 1980). Tectonic, olistostromal and diapiric processes are considered possible mechanisms for the formation of the melange. A more detailed discussion of the provenance of blocks within the Oyo Complex is beyond the scope of this paper but will be presented elsewhere. Nias Beds

The Nias Beds of Early Miocene-Pliocene age, rest with a probable unconformable contact, on the Oyo Complex. The Nias Beds are composed of a wide variety of carbonate and siliciclastic facies, including mudstone, marl, siltstone, sandstone and conglomerate with intervals of bioclastic limestones. The lower part of the Nias Beds is considered to be Lower Miocene in age (Moore et al. 1980a). These beds NIAS are exposed in the central part of the island, where they Union Oil geologists, Burrough and Power (1968), are dominated by interbedded fine-grained sandstone proposed the first lithostratigraphy (supported by (with some tuffaceous sandstones) and siltstone that palaeontological evidence) for Nias. Nias lithologies contain both glauconite and carbonate lithoclasts. The were divided into two principal units, the Oyo Complex upper parts (Mid-Upper Miocene) are well exposed in and the Nias Beds by Moore and Karig (1980; see their the SE of the island (Idano Gawo and Gido Rivers). Fig. 2). The contact between the Oyo Complex and the These successions are composed of alternating siliciclastic and carbonate intervals. The siliciclastic lithologies Nias Beds has not been observed in the field. include mudstones (some very montmorillonite-rich), immature sandstones and conglomerate. The finerOyo Complex Melange grained lithologies are variably carbonaceous and conThe lowest stratigraphic unit on Nias, the Oyo tain a well-preserved shelf microfauna. Previous workers Complex, is Pre-Miocene in age, and is described by have identified benthic foraminifera indicative of deeper Moore and Karig (1980) as a "tectonic melange", in water environments (Moore et al. 1980a). Our studies of which a wide variety of block types are immersed in a these Mid-Upper Miocene sediments have not yielded similar benthic assemblages. The carbonate facies insheared and disrupted matrix. The complex is composed of sedimentary blocks, clude relatively clean calcirudites, coarse calcarenites including conglomerates, sandstones and siltstones, with and calcilutites containing a shallow water microfauna subordinate mafic plutonic rocks, pillow basalts and and mixed carbonate-siliciclastic breccias containing cherts. Sandstone blocks form the dominant clast type in abundant reef detritus. The Upper Miocene and Lower the SW part of the island (Fig. 4), while pillow basalts Pliocene sediments are represented by a greater proand gabbros form some of the largest blocks (up to portion of fine-grained sandstones, siltstones and mud200 m in diameter) on the west coast of the island. The stones with only minor calcarenites. Finally, a variety of dominance of certain lithologies in different parts of the bioclastic limestones including some reefal deposits of island suggests that the melange may be stratified or Pliocene age are observed in NW Nias; these represent locally sourced. Although outcrops of Oyo Complex the youngest part of the Nias Beds. Interpretation. The Nias Beds have been previously where the matrix could be studied are only rarely observed in the field, in central Nias (Moi River) and SW interpreted as uplifted slope basin deposits, which in turn Nias the matrix forms a typical scaly clay, with a high are overlain by Pliocene shelf deposits and capped by

Sumatra based upon recent fieldwork and subsequent detailed sedimentological, palaeontological and geochemical analyses. A discussion and interpretation of the major lithological units from the outer arc islands is presented below.

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throughout the Neogene, our studies of the lithologies and microfaunas suggest that from Mid-Miocene times the Nias Beds were deposited on a shelf on which there were many local carbonate buildups. The environment of deposition for the lowermost part of the Nias Beds is not well understood, although the absence of in situ shallow marine faunal communities and the presence of deep water foraminifera (Moore et al. 1980a) suggest that at least some of the older (Early Miocene) Nias Bed strata, were deposited in a deep water environment. The absence of any deep water faunal assemblages in our Mid- to Upper Miocene samples suggests that the younger parts of the Nias Beds were deposited on a relatively shallow shelf. We consider that this conflicting evidence might indicate that the upper parts of the Nias Beds were deposited in variable water depths. If the sediments that we have examined were redeposited in deeper water, we would expect to see some bathyal fauna. In addition, we recognise no sedimentological features indicative of redeposition in these deposits. Similar lithologies have also been recovered from cores drilled in the offshore areas by Union Oil (Beaudry and Moore 1985, Karig et al. 1979) and are directly comparable with sequences exposed on mainland Sumatra (e.g. Cut Formation, Kallagher 1990). The alternation between horizons dominated by shallow water bioclastic limestones and fine grained siliciclastic deposits are interpreted as representing minor shallowing and deepening events on the outer shelf which continued into the Early Pliocene.

The Sunda Outer-Arc Ridge, North Sumatra, Indonesia In summary, Moore and Karig (1980) suggest that by Mid-Miocene times there was continued deposition of the Nias Beds in discrete trench slope basins. Our evidence from eastern Nias indicates that the Nias Beds were deposited on a shelf margin intimately associated with shallow water limestone deposits. This evidence implies, that in addition to the deeper water setting for Nias Bed sediments described by earlier workers, some relatively shallowwater sediments were deposited at this time. Our conclusion therefore, is that the depositional environment of the Nias Beds was locally variable during the Mid-Upper Miocene. These environments of different palaeobathymetries persisted into the Pliocene.

SIMEULUE Simeulue lies slightly off-strike and to the northwest of Nias, but to date has been little studied. Recent work on this, the northernmost outer-arc island by the University of London/LEMIGAS (Situmorang et al. 1987, Kallagher 1990) established a new stratigraphy for the island; this paper presents an amended version of their nomenclatures (Fig. 2). With the exception of some aspects described below, Simeulue shares a broadly comparable geology with Nias, of melange overlain by interbedded sandstone and siltstone sequences, with parts of the succession dominated by bioclastic limestones. Although lithological variations do exist, the most notable differences between the two islands is one of structural style. Sibau Gabbro Group

The oldest rocks exposed on the island are represented by the Sibau Gabbro Group (Situmorang et al. 1987) named from their type locality at Mt Sibau in the north central part of Simeulue (Fig. 5). The Sibau Gabbro

467

Group is composed mainly of meta-igneous lithologies with predominantly transitional contacts. The ophiolite correlates closely with a partially defined gravity high in this area indicating that these basic igneous rocks form a major body, extending to a depth of several kilometres (J. Milsom, pers. commun. 1990). Lithologies identified within the group include gabbros, meta-dolerite and meta-volcanics, all with abundant chlorite and pumpellyite suggesting that these rocks are ~.'.1 low-grade metamorphics. Maximum pressures of 2-3 kb (< 10km), and temperatures <200°C are thus inferred. Pumpellyite is rare or absent in products of ocean-floor metamorphism (Turner 1968) and the rocks of the Sibau Gabbro Group are therefore associated with a metamorphic event other than ocean-floor processes. All lithologies are extensively cross-cut by calcite veins. Geochemically these rocks plot within the transitional low K tholeiites and calc-alkaline field with rare earth element patterns that are characteristic of enriched Mid Ocean Ridge Basalt (KaUagher 1990). Whole rock potassium-argon (K-Ar) dating of several samples suggest that the Sibau Gabbro Group and Baru Melange Formation were metamorphosed between 40.1 ___2.7 Ma (Late Eocene) and 35.4 + 3.6 Ma (Early Oligocene). Baru Melange Formation

The Baru Melange Formation outcrops in the Kuala Makmur river section immediately to the west of Mt Sibau in the central part of Simeulue (Fig. 5). Situmorang et al. (1987) describe the formation as being in structural (thrust) contact with basalts at the top of the Sibau Gabbro Group. Blocks within the melange include fine-grained, micaceous sandstone some of which are fractured; very well-consolidated, weakly sheared, micaceous mudstone, poorly-sorted meta-greywacke; iron-rich meta-dolerite; brecciated meta-basalt; metavolcanics and calcite-rich, lithic and crystal tufts. Blocks

Fig. 5. Geologicalobservations from Simeulue. After Situmorang et al. (1987) and Kallagher (1990).

468

N.A. HARBURYand H. J. KALLAGHER

within the melange may be in excess of 10 m in diameter and may occur as isolated boulders within the river or as river bank exposures. Smaller blocks are commonly enclosed within a sticky blue/grey clay matrix containing organic material, or within a cleaved mudstone matrix. Clasts within this latter category range in size from <1 cm to 50cm, but are more typically between 5-10 cm. No bedding or other sedimentological characteristics, within the blocks of the melange or the clay matrix, can be used to determine the stratigraphical base or top of the Baru Melange Formation in R. Kuala Makmur. The apparent random distribution of blocks of different lithology within the outcrop area suggests that the melange is unsorted. It is estimated that sedimentary blocks within the melange account for up to 35% of the total (sandstone 25%, mudstone 15%), and that igneous and metaigneous lithologies make up the remaining 65%; of these, meta-basalt is volumetrically most important. Analysis of the clay matrix for calcareous nannofossils was undertaken by L. Gallagher (UCL), but all samples were found to be barren. The thickness of the formation is estimated from exposure in the Kuala Makmur river section to be approximately 200 m. Interpretation. The mode of formation of the Baru Melange Formation is considered to be similar to that of the Oyo Complex although the provenance of blocks within the melange is partly different. In addition to Himalayan-derived micaceous sandstones and a minor tuffaceous component, interpreted as derived from Sumatra, a significant proportion of blocks were derived locally from the Sibau Gabbro Group. All the blocks were incorporated in a fine-grained matrix which was subsequently tectonised. Ai Manis Limestone Formation

The Ai Manis Limestone Formation outcrops in the Ai Manis river section and forms a NW-SE orientated ridge close to Mt Sibau in the east central part of Simeulue (Fig. 5). The formation is approximately 260-350 m thick and consists of both biostromal, biohermal (composed of in situ corals) and bioclastic limestones. The major part of the formation consists of bioclastic packstones composed of skeletal bioclasts, large benthic foraminifera and quartz grains. At the base of the formation a coarse-grained sequence (the Pinang Conglomerate Member) is locally observed resting on the Sibau Gabbro Group. A Late Oligocene to Early Pliocene age is suggested for this formation on the basis of palaeontological evidence (Situmorang et al. 1987). The Pinang Conglomerate Member is between 0.5 and 5 m thick and is exposed in the Ai Manis region (Fig. 5), where it rests with an angular unconformity on the Sibau Gabbro Group. The conglomerate is poorly-sorted and consists of clasts (mm-50cm in diameter) of metaigneous rock fragments, including meta-basalt and metagabbro, and quartz, in a medium-grained calcarenite matrix. Polycrystalline quartz and sand-sized grains of monocrystalline quartz account for some 15% of the

total conglomerate clast composition. A shallow water benthonic foraminiferal assemblage indicating a Late Oligocene to Early Miocene age was recovered from the conglomerate (Situmorang et al. 1987). Interpretation. The Ai Manis Limestone Formation was deposited in a shallow water, shelf-forereef region. The majority of the clasts and matrix in the Pinang Conglomerate were derived locally: the meta-igneous clasts were derived from the Sibau Gabbro Group on which the unit rests, whilst the calcarenite matrix was derived from coeval shallow water accumulations. The quartz grains indicate that there was either some detrital input from outside the subduction zone, such as from the Sunda shelf, or, more probably, from the reworking of sandstones from blocks within the Baru Melange Formation. Passing upwards, the formation becomes finergrained and suggests deposition in a shelf region adjacent to a variety of carbonate buildups, including bioherms, from which reworked bioclasts and skeletal sands were derived. Dihit Formation

The Dihit Formation (equivalent to the Dihit Sandstone Formation of Situmorang et al. 1987) is widely exposed in most parts of Simeulue, along river sections (R. Dihit), road sections and tracks (Fig. 5). The maximum thickness of the formation is estimated from the Dihit section, to be between 800 and I000 m. The Dihit Formation contains no foraminifera or calcareous nannofossils, nor is there independent palaeontological or stratigraphical control on the age of the formation. Den Hartog (1940) considers the formation to be of Palaeogene age, whereas, on the basis of lithological similarities between the Dihit Formation and the Nias Beds, the formation is considered by Situmorang et al. (1987) to be of Late Miocene to Early Pliocene age. Micropalaeontological work (including extensive palynological studies) is in progress to resolve the age of this formation. The Dihit Formation is composed of grey, predominantly fine-grained sandstone usually interbedded with siltstone or shale. The sandstone is well-sorted, moderately well-consolidated, and unlike the Nias Beds, is micaceous. Bed thickness varies from 4 cm to 15 m in the most massive beds, but more characteristically is between 50 and 100cm. Parallel laminations are rarely developed in the sandstone, but where present are very fine (< 1 mm), and are laterally continuous through the outcrop. Organic matter, where present, occurs as small disseminated lignitic woody fragments and as very fine, disseminated carbonaceous material; calcareous concretions are rarely observed. Sandstone, where interbedded with shale or mudstone, is usually the dominant lithology, with sandstone:shale ratios between 2:1 and 30:1. The shale/mudstone interbeds are between 4 and 140cm, although beds are more characteristically in the order of tens ofcm (20-50 cm); basal and top surfaces are planar. The sandstone is fine-grained, well-sorted and predomi-

The Sunda Outer-Arc Ridge, North Sumatra, Indonesia nantly matrix-supported. Grains are composed of angular to subrounded monocrystalline quartz with both uniform (70-80%) and undulose extinction (20--30%). Muscovite mica is present in all samples (trace-3%). Massive sandstone, sandstone/siltstone and laminated sandstone/mudstone lithofacies can be recognised from the Dihit Formation sediments. Interpretation. The absence of foraminifera and calcareous nannofossils from the Dihit Formation is unexpected given their lithological similarities with the Neogene sediments on Nias. The general absence of sedimentary structures and fauna throughout the Dihit Formation hinders interpretation of its environment of deposition. The sandstone beds with sharp upper and lower contacts and parallel laminations were probably deposited by turbidity currents, although stormgenerated deposition cannot be ruled out. The massive sandstone beds were probably deposited as grainflows. The age and tectono-sedimentary history of this formation remains problematic and is discussed in more detail below.

OTHER ISLANDS Small islands such as Sarangbaung and the Banyak Islands which lie between the outer arc ridge and mainland Sumatra also expose Tertiary sediments. The correlation between these sequences, those exposed on Nias and Simeulue and also with comparable sequences exposed on mainland Sumatra, will enhance the interpretation of the geological evolution for the northern Sumatran Forearc.

Banyak Islands The Banyaks are a small group of islands between 30 and 120 km off the coast of W. Sumatra (Fig. 6). In spite ~

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of their important geological location, they have been little studied in the past. The nomenclature used here is that of Burrough and Neilson (1970; Fig. 2). The oldest rocks are strongly jointed, olivine basalt/diabase, outcropping on the west coast of P. Bangkaru; they are thought to be Pre-Tertiary in age by Burrough and Neilson (1970). Siltstone, considered to be the equivalent of the Bangkaru Beds, is poorly exposed along river sections on P. Tuangku. The sediment is barren of calcareous nannofossils but contains rare, non-age diagnostic, shallow water foraminifera. Burrough and Neilson (1970) describe the siltstone as being of Eocene and Oligocene age, based upon their lithological similarities with the Singkel Beds on mainland Sumatra. The Tuangku Beds named from P. Tuangku where the coralline and algal limestone outcrops over much of the island are particularly well-exposed on Bukit Tusa, a prominent limestone hill (318 m) on the northern part of the island. Palaeontological analyses undertaken during the present study were unable to determine the age of the limestone; Burrough and Neilson (1970) describe the Tuangku Beds as being of ?Miocene to Mid-Pliocene age. The youngest sediments are uplifted ?Quaternary reefs, exposed around the coast of many of the islands; most islands, at the present day, are also surrounded, at least in part, by living coral reefs, Interpretation. The basic igneous rocks probably represent ocean-floor material, sub-aerially exposed by movements along the Anu-Batee Fault, a major splay of the SFS that cuts through the West Aceh Basin on mainland Sumatra (Kallagher 1990). The fault can be traced on seismic reflection profiles offshore, across the Banyak Shelf until it intersects the Pliocene flexure on Nias (Karig et al. 1979, Beaudry and Moore 1985). Movements along the fault on mainland Sumatra are thought to have climaxed in Late Miocene to Pliocene times, related to renewed spreading in the 3"

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]

KEY CoralReef(Quaternary)

]

TuangkuBeds(?Miocene)

]

Bangkaru Beds(?EoceneOligocene) BasicIgneousRocks (Pre-Teniary)

] ~ 0 I

i

469

Reef 5 i km

10 I

i

Fig. 6. Geologicalmap of reconnaissancestudieson the BanyakIslands.InsetshowsP. Bangkaru, 14km westof the main island group.

470

N.A. HARBURYand H. J. KALLAGHER

Andaman Sea (Cameron et al. 1980). Siltstones of the Bangkaru Beds were probably deposited on the Sunda Shelf during Eocene-Oligocene times; this area became much shallower in Miocene times (possibly associated with localised uplift), and limestone of the Tuangku Limestone Beds was deposited on the Banyak Shelf. Uplift of this area in Quaternary times, is demonstrated by the abundance of Quaternary coral terraces that are commonly found around islands both large and small. Sarangbaung Island

Sarangbaung is a small island ca. 1 km in diameter, some 20 km north of the northernmost tip of Nias. A Lower Miocene succession is well exposed on the west coast. The succession consists of approximately 200 m of clastic lithologies including (in order of importance) sandstones (lithic arenites, calclithites, and rare quartz arenites), shales and poorly sorted breccias. Many of the sandstones have a high carbonaceous content and locally are intensely bioturbated. Erosive channels and intraformational truncation surfaces are observed locally within the succession. Interpretation. The variety of lithologies, high carbonaceous content and amount of bioturbation observed in the deposits on Sarangbaung Island suggest deposition on a shelf dominated by siliciclastic deposition. The erosive channels and intraformational truncation surfaces indicate that deposition took place on a slope or shelf-break.

and small wavelength folds (2-10m) interpreted as slumps (M. Jones, pers. commun. 1990). Steeply dipping strata (70 ° to the NE) with numerous normal faults were observed in the east part of the island in the Idano Gawo region. In contrast to the Nias Beds, the Dihit Formation on Simeulue is characterised by a NW-SE strike and by steep dips. A number of upright tight anticlines and synclines with average wavelengths of between 10 and 200 m were mapped, although fold closures are rarely observed in the field. Sediments are not intensely faulted but where faults are seen to cut the strata they are extensional. The timing of deformation of the Neogene formations is not well constrained. The tilting of Pliocene strata indicates that at least some of the deformation postdates this period. The timing of deformation of the Dihit Formation remains speculative as the age of the sediments has yet to be determined. A comparison of the style of deformation within the Neogene successions with that of the older units is critical to the understanding of the relationship between the Nias Beds and the Oyo complex melange. If the Neogene successions have been directly involved in thrusting, considered from seismic reflection profiles to occur within the accretionary complex (Moore et al. 1980b, Karig et al. 1980a, b, Kieckhefer et al. 1980), the Nias Bed sediments might be expected to be more intensely deformed than the gently folded nature of the strata observed in the field. Our field observations suggest that the thrusts in the accretionary complex either do not penetrate the overlying Neogene strata, or disrupt only the base of the Nias Beds as suggested by Moore and Karig (1980).

STRUCTURE This discussion is not intended as a thorough review of the structural geology of the outer-arc islands, but as a summary of the important structural elements that are observed which enhance our understanding of the evolution of the forearc region. Structural data from Nias and Simeulue is largely confined to the Neogene deposits as their well bedded nature allows observation of structural offsets. Strata on both islands are characterised by a NW-SE strike and by horizontal to steep dips. Fold closures are more commonly observed on Simeulue than on Nias. Sediments are not intensely faulted; faults observed in the field are all extensional, normal faults, usually on a small-scale with displacement of 2 m or less. We recognise no major thrust faults; minor thrusts, with displacements of only metres are rarely observed. Moore and Karig (1980) identified several thrusts on the basis of overturned beds and age inversions determined from detailed microfossil study. Over the central and SW part of Nias, the Nias Beds are characterised by coherent, horizontal to subhorizontal strata, that are gently folded in parts of the R. Gomo area, SW Nias. Detailed examination of the Nias Beds in the central part of the island indicates that the Nias Beds have been subjected to normal faulting

TERTIARY HISTORY OF THE OUTER ARC RIDGE The Pre-Tertiary history of the Sumatran Outer arc ridge is poorly understood so this paper therefore concentrates on the Tertiary evolution. Arc-related lithologies along the length of Sumatra suggest that the island has been at or near an active convergent plate boundary, at least intermittently, since Late Permian times (Karig et al. 1979, Katili 1973, 1975, Hutchison 1973). In Cretaceous times (70 Ma), India separated from Africa and was converging on Eurasia in a NNW direction at a velocity of between 15 and 20cm/yr (Patriat and Achache 1984). A north-south spreading regime established in the Indian Ocean during Late Cretaceous times, resulted in a system of oblique subduction of the IndoAustralian Plate beneath the Eurasian Continental Plate at the Sunda Margin beneath Sumatra, even allowing for the inferred clockwise rotation of Sundaland (Tapponnier et al. 1982). No sedimentary record exists on mainland North Sumatra during the Late Cretaceous. The oldest dated rocks exposed in this part of the Sunda Forearc are Mid-Cretaceous granodiorite plutons outcropping in the West Aceh Basin on mainland Sumatra (Kallagher 1990).

The Sunda Outer-Arc Ridge, North Sumatra, Indonesia Palaeocene (65 M a ) to Eocene (36 M a )

Bengal Fan to the north. The Oyo Complex is also composed of coarse clastic material probably derived from Sumatra to the east (Moore 1979), and the Baru Melange Formation contains blocks of meta-igneous lithologies derived from the underlying Sibau Gabbro Group. The coarse clastic material from Sumatra is further evidence for an elevated source area in this region and the transport of clastics across the forearc from the Sumatra shelf, indicative of a subdued trench slope break (Karig et al. 1979). An Oligocene unconformity underlies most of the Sunda Shelf. The unconformity is undeformed in most areas except in proximity to major faults of apparent large-scale displacement (Karig et al. 1979). Structural inversion of the forearc basins in Sumatra and Java occurred during Early Oligocene times; this marks a period when stable convergence rates (3-4 cm/yr) were established. As a result, the extensional force along the subducting margin changed to one of oblique compression (Daly et al. 1991). In the Late Oligocene (Fig. 7) the convergence rate of the Indian Plate along the SE Asian margin, had stabilised at ca. 5cm. Northwest Sumatra was uplifted (?associated with renewed magma emplacement) and the shelf subaerially exposed. Fine, clastic sediments were transported along the trench from the Bengal and Nicobar fans in the north. The early accretionary complex may have acted as a restricting sill (following emplacement of the Sibau Gabbro Group on Simeulue), pending restricted basin sediments in the deepest part of the forearc. The Late Oligocene was a global lowstand of sea level (Haq et al. 1987). Changes of this magnitude would accentuate erosion due to uplift and therefore increase the amount of sediments supplied to the forearc. On Simeulue, the presence of the Pinang Conglomerate Member (?Oligocene-Early Miocene), a basal conglomerate composed of clasts derived from the underlying Sibau Gabbro Group in a calcarenite matrix containing shallow-water foraminifera, is further evidence that the western margin of the forearc was uplifted (?was uplifting) during this time. The geometry of the

This period is marked by the continuing rapid convergence of India on Eurasia and the eastward movement of Australia. No Palaeocene record exists for any part of the Sunda Forearc, whilst Eocene paralic sediments have been recorded from offshore boreholes (Beaudry and Moore 1985, Karig et al. 1979). The Eocene collision between India and Eurasia at ca. 50 Ma (Patriat and Achache 1984) caused the convergence rate to decrease from ca. 15 cm/yr either to a cessation (Graham et al. 1975, Hamilton 1977) or to a slowed subduction rate of ca. 3 cm/yr (Molnar and Tapponnier 1975, Karig et al. 1979). This decrease in convergence rate during Eocene times created extension in the arc environment and was responsible for the development of many of the basin systems in Southeast Asia (Daly et al. 1991). The timing of both forearc and backarc basin formation, coincident with India/Eurasia collision, strongly suggests a genetic relationship between the two. Late Eocene (40 M a ) to Early Oligocene (30 M a )

In Late Eocene-Early Oligocene times, a tectonic event occurred on Simeulue with the accretion or emplacement of the Sibau Gabbro Group. This tectonism at the outer-arc ridge may be related to renewed spreading in the Indian Ocean which caused an increase in the subduction rate at Sumatra from ca. 3cm/yr to 5-6 cm/yr. This change in the tectonic regime placed the Sunda Forearc under contractional stress which resulted in forearc basin inversion throughout much of the region during Oligocene times. Oligocene (36-25 M a ) (Fig. 7)

On Nias and Simeulue, the Oligocene is marked by deposition/accretion of the Oyo Complex (?Eocene-earliest Miocene) and the Baru Melange Formation (Oligocene), respectively. Both melange formations are composed of fine-grained, micaceous sandstone blocks considered to be derived from the

Late

Oligocene (30-25 Ma)

Outerarc r,d e ossib~ of sufficient religeEftPo ( act Yas restrictin sil to baslna g ~ sequence tn deepest part i

Sediments derived from Bengal & ~ ~ Nicobar Fans transported along Irench

471

f

i

3

j

J/

Northwest Sumatra above sea-level & shelf area subaeriallv exposed: ~"=most rive s drain eastwards ~

-

i)'~z -

"

Fig. 7. Schematicsection across the Sunda Forearc region m Late Oligocenetimes.

472

N . A . HARBUItY and H. J. KALLAGHER

Early to Middle Miocene (25-11 Ma) Stable tectonic ¢ondffio~ls exilt a c r ~

~_,.,,,~--.,.~C~u'ponate platforms, palch reefs & ~..~..--'~. ~ .~-~ fine-grained shallow mar~e . "" ~ - . - ~ s e d l m l m l s deposhed on shelf

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Km Fig. 8. Schematic section across the Sunda Forearc region in Early to Middle Miocene times.

strata on Nias indicates that uplift was in the order of several km (Karig et al. 1979).

transgression, which began in latest Oligocene times, culminated in the Mid-Miocene (Cameron et al. 1980). The presence of Neogene reefal limestones outcropping in NW and SE Nias, indicates that Nias was uplifted during Neogene times; the timing of uplift cannot be more precisely constrained due to the absence of age-diagnostic fauna within the limestone. It is possible that during Neogene times, the limestone acted as a "barrier", ponding coeval forearc basin sediments landward of the trench (Karig et al. 1979, Moore et al. 1980b). Stable, low energy conditions were established on the Sunda Shelf during Early to Mid-Miocene times and several kilometres of sediment were deposited.

Early to Middle Miocene (25-11 M a ) (Fig. 8)

Shallow marine carbonate sedimentation prevailed on Simeulue (Ai Manis Limestone Formation) and in parts of the forearc basin. On Nias, the earliest Miocene marks a change in sedimentation patterns from deposition/ accretion of melange to siliciclastic and bioclastic limestone deposits (Nias Beds) closely associated with shallow water carbonate buildups. Lower to Middle Miocene volcanics were extruded in the West Aceh Basin, but little volcaniclastic material was transported across the forearc basin to the outer arc ridge at this time. The Neogene record throughout the forearc area is more complete. The convergence rate was ca. 5-6 cm/yr and stable tectonic conditions and subsidence prevailed across the forearc region. Active volcanism occurred in NW Sumatra (Cameron et al. 1980, Kallagher 1990). Relatively low energy, shallow marine conditions were established on the western margin of the Sunda Shelf and parts of the outer-arc ridge must have been at or close to sea level during this time. A major marine Late Miocene (11-5 Ma)

Late Miocene (11-5 M a ) (Fig. 9)

The sedimentary successions exposed on the islands indicate that the outer-arc ridge had subsided, possibly resulting from flexural loading, with Mid-Miocene limestone deposition giving way to predominantly siliciclastic deposits. On Nias, deposition of the Nias Beds continued, with broadly comparable (though not well dated) sequences observed on Simeulue (the Dihit Formation). At this time both formations were probably deposited on the outer shelf margin or shelf break. The Movements along SFS juxtaposes 'anomalous' lithologies

Sporadic volcanicity; no , ~ sedimentary record preserved ~ . ~ ' ~ in Norlhweet Sumalra

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Fig. 9. Schematic section across the Sunda Forearc region in Late Miocene times.

The Sunda Outer-Arc Ridge, North Sumatra, Indonesia absence of significant proportions of sediment clearly deposited in shallow shelf environments suggests active subsidence of the forearc region. As the Late Miocene was a period of regression and lowstand of sea level (Haq et al. 1987) a marine transgression cannot be invoked to explain the deeper marine environment of deposition. These changes may be explained in terms of an increase in the rate of subduction from 5 cm/yr to 6.5 cm/yr (Molnar and Tapponier 1975). During Late Miocene times (10 Ma), the northward movement of India caused continued strike-slip movements along the Sumatra Fault System, and structural inversion of the backarc strike-slip basins of Sumatra, thought to be due to rotation inherent in a strike-slip fault system (Daly et al. 1991). Uplift of Sumatra began in late Mid-Miocene times. The mechanisms for uplift of the Bar|san Mountains are not well understood but are thought to be associated with renewed magma input related to an increased subduction rate from 3-4 cm/yr to between 5 and 7 cm/yr. Uplift probably climaxed at the Mio-Pliocene boundary, and has continued intermittently until the present-day (Cameron et al. 1980). Plio-Pleistocene (5-0.01 M a ) (Fig. 10)

The Plio-Pleistocene is marked by an increase in the convergence rate between the Indian and Eurasian plates to ca. 7 cm/yr, and uplift and subsequent erosion of the Bar|san Mountains on Sumatra. Coarse clastic sediments were deposited offshore in the forearc basin in westward-draining, fluviatile, deltaic and shallow marine systems which prograded away from the Bar|san Mountains into the forearc basin. This significant change in the drainage pattern of Sumatra from predominantly eastwards directed drainage prior to Pliocene times to westward directed is considered to be related to major movements along the Sumatran Fault Zone and to uplift of the Bar|san Mountains (Cameron et al. 1980). This pattern continues at the present day. Little sediment was transported axially along the trench due to the impingement of Ninetyeast Ridge on the Bengal and Nicobar fans (Moore et al. 1980b). Pliocene sediments on Nias

P |o-Pleistocene (5-2 Ma)

r

a

o ~ -

n

s

p

Holocene (0.01 M a to p r e s e n t - d a y ) (Fig. 11)

From the Holocene to the present day the convergence rate of the Indian Plate along the SE Asian margin is between 5 and 6 cm/yr. Recent movements along the Sumatran Fault System (SFS) and continued uplift of the Bar|san Mountains are indicated by the presence of warped alluvial river terraces and river offsets. On the outer-arc ridge, the late Quaternary is characterised by coral reef terraces, some of which are presently up to 50 m above sea level. These reefs are observed today forming around Nias, Simeulue and the Banyak islands. Uplifted Quaternary coral reef complexes found around the coast of Simeulue have been used by other workers to infer rates of uplift and subsidence in Quaternary times (Vita-Finzi and Situmorang 1989). On the south coast of Simeulue, Quaternary coral reef dipping ca. 4°N, together with submerged reef on the north coast of the island, is indicative of tectonic movements at the present day (Situmorang et al. 1987). Uplift and erosion of the outer-arc ridge is recorded by small slumps seen in seismic reflection profiles adjacent to Nias (Beaudry and Moore 1985). DISCUSSION The presence of pumpellyite in all the low-grade metamorphics of the Sibau Gabbro Group implies that a

o

r

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include montmorillonite-rich clays which probably reflect erosion of a volcanically active terrain on Sumatra during this period. Sedimentation continued on Simeulue until latest Pliocene times, with the deposition of reworked shelf sediments of the Dihit Formation. Shallow structures mapped across the forearc basin margin suggest that the subduction complex was reactivated and uplifted in latest Pliocene times (Karig et al. 1979), contemporaneous with renewed strike-slip fault movements along the west coast of Sumatra. Uplift of the outer-arc ridge at this time is indicated by Quaternary coral terraces deposited unconformably on Neogone shelf sediments.

Culmination of

Uplift of paris of outerarc

473

'

~

~

~Z,~'-~',~

/ |

I Fig. I0. Schematicsection across the Sunda Forearc region in Plio-Pleistocenetimes.

474

N.A. HARBURYand H. J. KALLAGHER Present-Day Movements continue along the SFS

~

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Fig. 11. Schematicsection across the Sunda Forearc region at the present day.

metamorphic event (?tectonic, such as ?accretion/ obduction) must be invoked to explain its occurrence. The timing of this event is poorly constrained. It is improbable that the Eocene age recorded from the Simeulue ophiolite represents the age of formation of the ocean floor. The age of oceanic crust presently being subducted along the Sunda Trench off North Sumatra is inferred to be Eocene from ocean-floor magnetic data (Sclater and Fisher 1974) and this would suggest that it was accreted very recently. This is not consistent with the stratigraphic evidence whereby sediments of Oligocene age contain clasts derived from the Sibau Gabbro Group. It is therefore more probable that the ophiolite is older than the K-Ar age, and that this age represents the age of metamorphism. In the Andaman sector, Karig et al. (1979) suggest "ophiolitic scraps" were accreted when thinly sedimented oceanic crust was subducted during Late Cretaceous to Early Tertiary times. The ophiolite was subsequently deformed during the Mid-Oligocene, associated with renewed spreading in the Indian Ocean which caused an increase in the subduction rate to approximately 5-6 cm/yr at Sumatra (Sclater and Fisher 1974). It is possible that the Simeulue ophiolite had a comparable history; pumpellyite in the Sibau Gabbro Group indicates that the ophiolite has been buried to depths of up to 10kin and has subsequently been uplifted. Deformation and/or metamorphism in Eocene times may therefore be related either to an increase in the subduction rate, or alternatively may be associated with a change from strike-slip motion to subduction in that sector of the Sunda Arc (D. Aldiss, BGS, pars. commun. 1989). One problem remaining is the stratigraphy and sedimentary history of the Dihit Formation which outcrops extensively throughout Simeulue. The age of the strata and their depositional setting remains speculative given the absence of fauna. A comparable depositional environment with the Nias Beds, i.e. slope basins on the accretionary complex or outer shelf margin, is considered most probable, although the Nias Beds are quite rich in both shallow marine bethonic foraminifera and

calcareous nannofossils. An important difference between the Dihit Formation and the Nias Beds lies in their differing structural styles; the Nias Beds are characterised by sub-horizontal or gently-dipping strata, whereas the Dihit Formation is typified by quite strongly deformed, although coherent, sediments. One explanation for these structural differences might be in terms of the angle of subduction of the Indian Ocean Plate beneath the Eurasian Plate, which at the present day is less oblique beneath Simeulue than beneath Nias. Changes in shape along the Sunda Margin through time are not documented; only general convergence rates for example for the "Sumatran part" of the Sunda Forearc, are presented in the literature (e.g. Sclater and Fisher 1974). The age and tectono-sedimentary history of the Dihit Formation remains problematic and two features of this formation in particular require explanation: the absence of calcareous fauna from these sediments and the more deformed nature of these sediments compared with their supposed equivalents, the Nias Beds. If the Dihit Formation is of ?Miocene-Pliocene age and was deposited below the carbonate compensation depth (CCD), thought to have been at about 4000 m in the Early Miocene (Berger and Winterer 1974, van Andel 1975), rapid uplift must therefore have occurred. This is not supported by sedimentological evidence from the Sunda shelf and mainland Sumatra which indicate the forearc was tectonically stable at this time. Alternatively the relatively barren nature of the Dihit Formation may be explained by the development of an oxygen minimum zone, possibly on the Sunda Shelf margin. However, the sediments are not rich in either skeletal debris or phosphate and they do not contain abundant organic matter. A more plausible, but as yet unsubstantiated explanation may be that differences between the Nias Beds and the Dihit Formation occur because the Dihit Formation sediments are Palaeogene rather than Neogene in age. In this case they would therefore be stratigraphic equivalents of the Salul Beds (Member 1), and Members 2 and 3 of the pre-Miocene Complex (Den Hartog 1940). As the contact between the Dihit Formation and other rocks outcropping on Simeulue was not observed in the

The Sunda Outer-Arc Ridge, North Sumatra, Indonesia field, this explanation remains tentative, and perhaps implausible in the light of great thicknesses of Neogene sediments deposited elsewhere in the forearc region. However, a Palaeogene age for the Dihit Formation might explain the greater amount of deformation suffered by these sediments, having been involved in the compressional events associated with the accretion of the Sibau G a b b r o Group. Until age-diagnostic fauna can be recovered from these sediments the precise age of the Dihit Formation, together with the most plausible environment of deposition, cannot be reliably established. It is interesting to note that given the accepted compressional model for the formation of these islands in the literature (e.g. Karig e t al. 1979, Moore and Karig 1980) features associated with compression might be expected to predominate in the Neogene successions. However, it is extensional, normal faults which predominate in the Nias Beds and thrust and reverse faults, although observed, are not common.

CONCLUSIONS Our studies of the structure and stratigraphy of Nias, Simeulue and other islands of the Sunda outer-arc chain document the development of the outer-arc ridge, and record the evolution of the Sunda Forearc Basin with greater certainty. Sequences outcropping on Simeulue are broadly comparable with those of Nias, yet we see significant differences in stratigraphy and structural style between the successions on these islands, separated by less than 150 km, which add to our understanding of the evolution of the forearc basin through the Tertiary, The pre-Miocene melanges and the Neogene successions on the islands of the Sunda outer-arc chain are composed of lithologies derived from the Indian Ocean floor, the Bengal-Nicobar Fan, Sumatra and in situ carbonate production. Subaerially exposed ocean-floor material outcrops on Simeulue and the Banyaks; on both Nias and Simeulue pre-Miocene melange is present, although component blocks are composed, in part, of different lithologies. Neogene limestone, and interbedded sandstone and siltstone sequences, are exposed on Nias and Simeulue. The history of the outer-arc ridge can be inferred from Eocene-Oligocene times, when accretion of ocean-floor material and formation of melange by Sumatran-derived coarse clastics, and Himalayan-derived, finer-grained detritus is considered to be occurring. During early Neogene times, the Nias Bed strata were deposited in both relatively deep, marine environments ("trenchslope basins"), and shelf margin settings. From Mid- to Late-Miocene times parts of Nias were uplifted, and approximately 3 km of shelf sediments accumulated including several well developed carbonate buildups deposited over large regions of the forearc. A possible Pliocene subsidence resulted in the deposition of clays, followed by uplift allowing Quaternary coral terraces to form in parts of Nias and Simeulue. The islands have

475

been uplifted in Recent times, exposing the Quaternary coral as linear limestone ridges up to 50 m high along the east coast. Changes in the convergence rate are seen to be the most important parameter controlling the evolution of the Sunda forearc basin. Relatively slow convergence rates can be equated with extension and basin evolution, whilst rapid subduction may be equated with compression and basin inversion. Although not demonstrated from the field data, seismic studies in the offshore forearc basin have shown that superimposed on the tectonic regime is a eustatic control, which can enhance or subdue the tectonic effect (Karig et al. 1979, Matson and Moore 1991). The geological evolution of Nias and Simeulue is complex and several critical problems are still unresolved including: (1) the mode of formation of the pre-Miocene melange units; (2) the relationship of these pre-Miocene melanges with the overlying Neogene formations; and (3) the detailed stratigraphy and depositional setting of the Neogene formations. In addition, further studies of the structural style of the deposits exposed on the outer-arc islands are required to better our understanding of accretionary complex formation. Acknowledgements--In Indonesia we thank Bona Situmorang and M.

Husen from LEMIGAS, Sandy Macfarlane (BGS), and Kate Bartram, Jakarta. We thank Mike Audley-Charles, Tony Barber, Gary Nichols, Michael De Smet, Mervyn Jones and John Milsom for helpful discussions. Liam Gallagher and Fred Banner for microfossilidentification and Chris Rundle at NIGC, for K-Ar dating. Financial support from the Natural Environmental Research Council (H.K.), the Royal Society and the Consortium for GeologicalResearch in Southeast Asia is gratefully acknowledged.

REFERENCES Andel, T. H. van 1975. Mesozoic/Cenozoiccalcitecompensation depth and the global distribution of calcareous sediments. Earth Planet Sci. Lett. 26, 187-194. Barber, A. J. 1985. The relationship between the tectonic evolution of Southeast Asia and hydrocarbon occurrences. In: Tectonostratigraphic Terranes of the Circum-Pacific Region (Edited by Howell, D. G.). Circum Pacific Council for Energy and Mineral Resources, Earth Science Series 1, 523--528. Beaudry, D. and Moore, G. F. 1985. Seismic stratigraphy and Cenozoic evolution of West Sumatra forearc basin. Am. Assoc. Petrol. Geol. Bull. 69, 742-759. Berger, W. H. and Winterer, E. L. 1974. Plate stratigraphy and the fluctuating carbonate line. In: Pelagic Sediments: on Land and under the Sea (Edited by Hsu, K. J. and Jenkyns, H. C.). Spec. Publ. Int. Ass. Sed. l 1-48. Burrough, H. C. and Neilson, R. J. 1970. 1969 field survey northern part of North West Sumatra contract area. Union Oil Company of Indonesia, report no. RGE 47 (unpubl.). Burrough, H. C. and Power, P. E. 1968. Field survey southern part of North West Sumatra contract area. Union Oil Company of Indonesia, report no. RGE 43 (unpubl.). Byrne, T. 1982. Structural evolution of coherent terranes in the Ghost Rocks Formation, Kodiak Island, Alaska. In: Trench-Forearc Geology (Edited by Leggett, J. K.). Spec. Publ. Geol. Soc. Lond. 10, 229-244. Cameron, N. R., Clarke, M. C. G., Aldiss, D. T., Aspden, J. A. and Djunuddin, A. 1980. The geologicalevolution of Northern Sumatra. Proc. Indonesian Petrol. Assoc. 9, 149-188. Daly, M. C., Hooper, B. G. D. and Smith, D. G. 1987. Tertiary plate tectonics and basin evolution in Indonesia. Proc. Indonesian Petrol. Assoc. 16, 399-428.

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