The Geology of Indonesia/Sumatra

Contributors: A. Pulunggono, B. Situmorang, H. DarmanSumatra Island is the northwest oriented physiographicexpression, lied on the western edge of Sundaland, asouthern extension of the Eruasian Continental Plate(Fig. 2.1). The Sumatra Island has an area of about435,000 km2, measuring 1650 km from Banda Aceh inthe north to Tanjungkarang in the south. Its width isabout 100-200 km in the northern part and about 350 kmin the southern part. The main geographical trendlinesof the island are rather simple. Its backbone is formedby the Barisan Range which runs along the western side.This region divides the west and the east coast. The slopetowards the Indian Ocean is generally steep, consequentlythe west belt is mostly mountainous, with the exceptionof two lowland embayments in north Sumatra which areabout 20 km wide. The eastern belt of the island is cov-ered by broad, hilly tracts of Tertiary formations and al-luvium lowlands. At Diamond Point, in Aceh, this loweastern belt has a width of about 30 km; its width in-creases to 150-200 km in central and south Sumatra. TheSumatra island is interpreted to be constructed by colli-sion and suturing of discrete micrcontinents in late Pre-Tertiary times (Pulunggono and Cameron 1984, Barber1985). At the present-day, the Indian Ocean Plate is be-ing subducted beneath the Eurasian Continental Plate ina N20oE direction at a rate of between 6 and 7 cm/yr(Fig. 2.2). This zone of oblique convergence is markedby the active Sunda Arc-Trench system which extendsfor more than 5000 km, from Burma in the north towhere the Australian Plate is in collision with Eastern In-donesia in the south (Hamilton 1979). The basinal con-guration of Sumatra is directly related to the presenceof the subduction-induced non-volcanic forearc and thevolcano-plutonic backarc, the morpho-structural back-bone of the Island.In general the region can be divided into 6 regions (Fig.2.1): 1. Sunda outer-arc ridge, located along the ac-tive margin of the Sunda forearc basin and separate itfrom the trench slope. 2. Sunda forearc basin, lyingbetween the accreting non-volcanic outer-arc ridge withsubmerged segments, and the volcanic back arc of Suma-tra. 3. Sumatra back arc basins including North, Cen-tral and South Sumatra basin. The system developed asdistinct depressions at the foot of the Barisan range. 4.Barisan mountain range, occupies the axial part of the is-land and is composed mainly of Permo-Carboniferous toMesozoic rocks. 5. Sumatra intra-arc or intermontanebasin, separated by subsequent uplift and erosion fromthis former depositional area, thus with similar litholo-

Tectonic setting map of Sumatra

gies to the fore-and backarc basins.

1

2 1 2.1. SUNDA OUTER-ARC RIDGE

1 2.1. SUNDA OUTER-ARCRIDGE

The Sunda non-volcanic outer-arc ridge marks the west-ern margin of the Sunda Forearc Basin of West Sumatra.This chain of islands and sea-oor rises, between 100 and150 km o the coast of West Sumatra, forms a struc-turally controlled topographic ridge nearly 200 km wide(Karig et al., 1979), that extends from the Andaman Seato the southeast of Java. Nias, Simeulue, and Banyak Is-land lithologies represent the stratigraphy of the Sundaourter-arc ridge in genereal. The geology of the Sundaouter-arc ridge is represented by Nias and Simeulue Is-land in this chapter.

2.1.1. NIAS

Geology sketch map of Nias Island

Nias Island is located approximately 125 km o the westcoast of Sumatra (Fig. 2.1) and it has been frequentlycited as a classic model of an accretionary complex (Fig.2.3). Nias lithologies were divided into two principalunits, the Oyo complex and the Nias Beds (Fig. 2.4).The contact between the two units has not been observedin the eld.

Modied Nias-Sumatra cross section after Karig et al, 1979

Fig. 2.7. Hypothetical shallow structure across the Sundaarc in the Nias area. Tectonic positions of subsequentproles are indicated beneath the section (Karig et al.,1979)2.1.1.1. OYO COMPLEX MELANGE

3The Oyo Complex is described by Moore and Karig(1980) as a tectonic melange. On Nias, outcrops of OyoComplex are seen as isolated blocks and boulders in riversections, along road sections and coastal exposures. TheComplex is composed of sedimentary blocks, includingconglomerates, sandstones and siltstones, with subordi-nate mac plutonic rocks, pillow basalts and cherts (Har-bury et. al., 1990). Sandstone blocks form the dominantclast type in the SWpart of the island, while pillow basaltsand gabbros form some largest blocks (up to 200 m diam-eter) cropping out mostly along the west coast of the NiasIsland (Fig. 5). Texturally, the sediment boulders aresub to mature clastic with mainly subangular to roundedand well sorted sediments, and are either grain supportedor matrix supported. In the area where the melange ispresent, landslips are common to occur and the fresh ma-trix of the Oyo Complex can be observed. Good outcropof melange is exposed in central Nias (Moi River) andSW Nias. The matrix forms a typical scaly clay, with ahigh density of curved, polished shear planes. The age ofthe Oyo Complex remains unresolved by paIeontologicalanalysis.2.1.1.2. NIAS BEDSOverlying the Oyo Complex, with probable uncon-formable contact, are a series of clastic sediments of shal-low to deep marine deposits of Nias Beds which are wellexposed along the eastern part of the island (Fig 2.4 &2.5). It consists of coarse to ne sandstone, conglomer-ate, mudstone, shale and limestone. The age of the NiasBeds has been interpreted by previous authors as EarlyMiocene-Pliocene. On the contrary, Situmorang & Yuli-hanto (1992) eldwork indicates that the lower part of theNias Beds is Upper Oligocene in age.2.1.2. SIMEULUESimeulue lies slightly o-strike and to the northwest ofNias (Fig. 2.1). This island shares a broadly compara-ble geology with Nias, of melange overlain by interbed-ded sandstone and siltstone sequences, with parts of thesuccession dominated by bioclastic limestones. Althoughlithological variations do exist, the most notable dier-ences between the two islands is one of structural style.2.1.2.1. SIBAU GABBRO GROUPThe oldest rocks exposed on the island are representedby the Sibau Gabbro Group (Situmorang et al. 1987;Fig. 2.4)). The Sibau Gabbro Group is composed mainlyof meta-igneous lithologies with predominantly transi-tional contacts. The ophiolite correlates closely with apartially dened gravity high in this area indicating thatthe basic igneous rocks form a major body, extendingto a depth of several kilometres (J. Milsom, pers. com-mun. 1990). Lithologies identied within the group in-clude gabbros, meta-dolerite and meta-volcanics, all withabundant chlorite and pumpellyite suggesting that theserocks are all low-grade metamorphics. Rock dating sug-gest that the Sibau Gabbro Group and Baru Melange For-mation were metamorphosed between Late Eocene and

Early Oligocene (Harbury & Kallagher, 1991).2.1.2.2 BARU MELANGE FORMATIONSitumorang et al. (1987) describe the Baru Melangeformation as being in structural (thrust) contact withbasalts at the top of the Sibau Gabbro Group (Fig.2.4). Blocks within the melange include ne-grained,micaceous sandstone some of which are fractured; verywell-consolidated, weakly sheared, micaceous mudstone,poorly-sorted meta-greywacke; iron-rich meta-dolerite;brecciated meta-basalt; meta- volcanics and calcite-rich,lithic and crystal tufts. Blocks within the melange maybe in excess of 10 m in diameter. Smaller blocks of5 10 cm in diameter are commonly enclosed withina sticky blue/grey clay matrix containing organic mate-rial, or within a cleaved mudstone matrix. No bedding orother sedimentological characteristics, within the blocksof the melange or the clay matrix, can be used to deter-mine the stratigraphical base or top of the Baru MelangeFormation. The apparent random distribution of blocksof dierent lithology within the outcrop area suggests thatthe melange is unsorted. The thickness of the formationis estimated to be approximately 200 m.2.1.2.3 AI MANIS LIMESTONE FORMATIONThe Ai Manis Limestone Formation forms a NW SEorientated ridge in the east central part of Simeulue. Theformation is approximately 260 350 m thick and con-sists of both biostromal, biohermal (composed of in situcorals) and bioclastic limestones. The major part of theformation consists of bioclastic packstones composed ofskeletal bioclasts, large benthic foraminifera and quartzgrains. At the base of the formation a coarse-grainedsequence (the Pinang Conglomerate Member) is locallyobserved resting on the Sibau Gabbro Group. A LateOligocene to Early Pliocene age is suggested for this for-mation on the basis of palaeontological evidence (Situ-morang et al. 1987; Fig. 2.4). The Pinang ConglomerateMember is between 0.5 and 5 m thick and is exposed inthe Ai Manis region, where it rests with an angular un-conformity on the Sibau Gabbro Group. The conglom-erate is poorly-sorted and consists of clasts (mm 50cmin diameter) of metaigneous rock fragments, includingmeta-basalt and meta-gabbro, and quartz, in a medium-grained calcarenite matrix. A shallow water benthonicforaminiferal assemblage indicating a Late Oligocene toEarly Miocene age was recovered from the conglomerate(Situmorang et al. 1987).2.1.2.4 DIHIT FORMATIONThe Dihit Formation is widely exposed in most parts ofSimeulue). The maximum thickness of the formationis estimated from the Dihit section, to be between 800and 1000 m. The Dihit Formation contains no strati-graphical control on the age of the formation. Base onlithological similarities between the Dihit Formation andthe Nias Beds, the formation is considered to be of LateMiocene to Early Pliocene age (Situmorang et al., 1987;Fig. 2.4). The Dihit Formation is composed of grey,

4 2 2.2. SUNDA FORE ARC BASINS

predominantly ne-grained sandstone usually interbed-ded 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 min the most massive beds, but more characteristically isbetween 50 and 100 cm. Parallel laminations are rarelydeveloped in the sandstone, but where present are veryne (

5of reef limestones with some calcirudite and calcaren-ite lenses, and the Saling Formation mostly containing ofvolcanic materials such as lavas, breccias, and tus.2.2.2.1.2. Tertiary SuccessionSurface geological studies exhibits that Tertiary sedi-ments cropout in this onshore area is represented by Hu-lusimpang, Seblat, Lemau, Simpangaur, and BintunanFormations (Fig. 2.8). The Hulusimpang Formation iscomposed of andesitic and basaltic lavas, volcanic brec-cias and tu with sandstones intercalation. This forma-tion is well exposed in the northern and eastern marginof the basin, toward the Barisan Mountain. In general,the Hulusimpang Formation is known as Early Oligocenesediments which deposited in uviatile up to shallow ma-rine. The aproximate thickness is 700 m. The upper partof the Hulusimpang Formation has interngered with thelower part of the Seblat Formation.The Seblat Formation composes of sandstones, siltstones,claystones, conglomerates with limestones intercalation.They are mostly shallow - deep marine turbidite sedi-ments of Late 0ligocene - Early Miocene age. The ap-proximate thickness maesured in Tanjung Sakti area is +298m.The Middle to Late Miocene stratigraphy is representedby the Lemau Formation. It consists of claystones,calcareous siltstones and sandstones, breccias, and thincoal seams and limestones intercalation, containing abun-dance of small foram and mollusc which was depositedin shallow marine up to transitional zone. This Forma-tion is we11 exposed in the southern area such as Ta-lang Beringin, Air Keruh, Rantau Panjang, Lubuk Tapi,Batang Rikibesar and Tebing Kekalangan areas. Thethickness recorded is+785 m.The Late Miocene - Pliocene sediment is representedby the Simpangaur Formation. It consists of tuaceoussandstones, tu, tuaceous siltstones, with intercalationof lignites, and also typied by abundance of foram andmollusc fragments.. The total thickness is about 785 mthick.The youngest stratigraphic unit cropout in this area is thePlio-Pleistocene Bintunan Formation which laying un-conformably upon the older units. It composes of sand-stones and tuaceous claystones with pumice clast, con-glomerates, breccias, limestones with lignite, and carbonintercalation. Lithologically, compare to the SimpangaurFormation, the Bintunan Formation in general is coarserthan Simpangaur and often containing silicied wood andpumice clasts. This formation was deposited in shallowmarine and uvial environment, and it ranges of about200 m thick.

3 2.3. SUMATRA BACK ARCBASINS

2.3.1. NORTH SUMATRA BASINIt is important to emphasize that the present southwestgeographical limit of the North Sumatra Basin at thenortheast foot of the Barisan Range does not correspondto the depositional limit of the Tertiary sediments (Fig.2.1). The original limit of this deposition extended muchfurther to the southwest than the more recently upliftedBarisan Range. This observation is supported by evi-dence of Baong shale outcrops in the midst of the moun-tains and also their presence in the Southwest SumatraInterdeep. The eastern and southeastern limits of thebasin are formed by the Asahan Arch (or TebingtinggiPlatform; Fig. 2.9), which separated it, in Tertiary timefrom the more extensive basin developed in Central andSouth Sumatra. At basement level this limit is marked bya north-south exure, immediately east of Medan. East-ward from the Medan Flexure structural deformation isminimal on the platform. The present southwest struc-tural limit of the basin runs along the Barisan Range, fromwhich it is separated by one or more compressional faults.In the narrow wedge between the Medan Flexure and thefront of the Barisan Range, the structural trends at base-ment level are oriented north-south. In this area, a exuremay be present between Telaga and Basilam, as indicatedby the greater depth (3.0 seconds TWT on seismic sec-tions) of the Belumai Formation in the western than in theeastern block (approximately 2.5 seconds TWT). Thereis no evidence that this exure also exists at basementlevel, because the basement conguration become vaguewherever the 2-way seismic time interval between top ofBelumai and top of basement is less than 0.2 seconds.The possible presence of a exure could be reected inthe right-lateral movement deduced from the virgation offolds and faults, changing from northwest in the south-eastern block, to north where the exure would be locatedif present.2.3.1.1. STRATIGRAPHY2.3.1.1.1. BasementThe basement (Fig. 2.10) consists of sandstone, lime-stones or dolomites; they are azoic, generally dense andfracture, with steep dips up to 45o, but they are not meta-morphically altered. In some plugs or cores, in the ab-sence of dating, these sediments are not easily recognis-able as basement. On the other hand, the high resistiv-ities and velocities generally constitute a good contrastwith those of the overlying beds. Thus the top of thissection is readily identied with the deepest, continuousseismic marker and conveniently been called economicbasement (Beicip, 1977).2.3.1.1.2.Tampur FormationTampur Formation (Fig. 2.10) comprises massive, partlybiocalcarenites and biocalcilutites. Chert nodules are

6 3 2.3. SUMATRA BACK ARC BASINS

found in this formation, whereas the dolorites are com-mon. The formation also consists of basal conglomer-atic and dolomitic limestones. This formation was de-posited in the sublittoral - open marine condition dur-ing Late Eocene to Early Oligocene, formed as trans-gressive formation overlain by both Bruksah and BampoFormation. Source of basal limestone clasts is still un-known but it assumed widely extended in the subsurface.The Eocene Tampur limestone generally only occurred inMalacca shelf (Rjacudu & Sjahbuddin, 1994). The restof the Tertiary history of the North Sumatra Basin canbe divided into three phases: 1)Syn-rift; 2) Transitional(Early Foreland); and 3) Compressional (Late Foreland;Fig. 2.10). The stratigraphy of the basin is closely relatedto these evolutionary phases.2.3.1.1.3. Early syn-Rift Phase: Bruksah and BampoFormationsThe initial syn-rift phase began in the middle Paleogene(Eocene?) and continued until early Miocene, duringwhich time the N-S and NE-SW trending horsts, grabensand half-grabens developed. This was also a time of ma-jor marine transgression (dened as a relative rise in sealevel within the basin, probably as a results of back-arcsubsidence). Initial graben-ll consisted of continentalsandstones and conglomerates. As the grabens deepenedand transgression progressed, areas of sand deposition de-creased and shale deposition dominated. The later sandsaccumulated mainly in coastal plain to marine environ-ments. The shales are typically dark grey to black in colorand deposited in deepmarine environment (bathyal). Thesands were mainly derived from the Malacca Platformand the Asahan Arch, augmented by local contributionsfrom the horst blocks, most of which remained exposedduring this time. The conglomerates and sandstones de-posited during this phase comprise the Bruksah Forma-tion (Fig. 2.10), dened by Cameron and others (1983)from eld mapping in the Barisan Mountains. Litholo-gies include limestone conglomerates and breccias, mi-caceous quartzose sandstones, and silty mudstones. TheBruksah is overlain the Bampo Formation, a locally thicksequence (500 to perhaps 2400 m) of marine black shale,siltstone, and muddy ne grained. Stratigraphic relation-ships indicate that the upper part of the Bruksah is at leastpartly equivalent in age to the Bampo Formation.2.3.1.1.4. Late Syn-Rift to Transitional Phase: Belumaiand Peutu FormationsThe transitional phase of basin evolution occurred duringthe early Miocene to early Middle Miocene and repre-sents a period of relative tectonic activities. Movementon the N-S trending faults ceased, although back-arc sub-sidence probably continued. This stage was character-ized mainly by forced regression (sea level constant orrising but sediment inux sucient to cause regression)and basin lling. As the central grabens lled and becameshallower, calcareous marine sands and siltstones alongwith argillaceous and sandy limestones accumulated in

the lows while the highs remained at least intermittentlyexposed. These basin-ll deposits comprise the BelumaiFormation (Fig. 2.10).The Belumai is lithologically diverse, both vertically andlaterally. Sandstones and siltstones are generally quartz-rich and tend to be very calcareous (up to 40-50% carbon-ate). Quartz content decreases southwest to only 10-30%,presumably as a result of increasing distance from sandsources on the Malacca Platform. Source areas are thesame for the Belumai and for the older and less calcare-ous Bruksah clastics. A possible explanation for morecalcium carbonate in the Belumai is that this unit accu-mulated after widespread shallow seas rst covered pre-Tertiary topography, much of which consists of carbon-ates. These oceans might have been nearly saturated (oreven super saturated) with calcium carbonate, and theymight have maintained equilibrium by dissolving carbon-ate bedrock while precipitating calcite cements. Rapidsedimentation would protect the calcite in the sandstonesfrom re-dissolution. In some areas, the original calcitehas been replaced by dolomite.In late early Miocene time, a major marine transgressionoccurred, probably resulting from continued subsidencecoupled with a eustatic sea level rise. The Malacca Plat-form and the central horsts were ooded and became thesites of shallow marine limestone deposition, includingreefs, that comprise the Peutu Formation (Kamili et al.,1976) and a signicant thickness of shale that might tbetter in the overlying Baong Formation. Sedimentationof basinal Belumai deposits (calcareous sand, shale, andargillaceous limestone) continued during accumulation ofPeutu skeletal limestones and reefs on adjacent platforms.This results in age equivalence between the Peutu and atleast the upper part of the Belumai Formation.In the deepest parts of theNorth Sumatra Basin, Belumai-equivalent deposits consist of dark gray to black marinemudstones and calcareous shales that are dicult to dis-tinguish from the overlying Baong. Middle and upperBaong shales are greenish gray to brown in color, but thecolor of lower Baong shales is dark gray to black. Forpractical purposes, the contact between Peutu or Belumaiwith the overlying Baong is determined by an abrupt de-crease in calcium carbonate.The contact between the Baong and underlying Peutu orBelumai varies from gradational to abrupt. Some high-standing Peutu buildups (Arun, South Lho Sukon, AlurSiwah) are overlain by middle Baong, with the lowerBaong section (N8-N12) missing. The entire Baong sec-tion is preserved in other areas. At Kuala Langsa, for ex-ample, a massive buildup of coralline limestone is over-lain by lower Baong shale without a noticeable gap in pa-leoenvironments (inner neritic to middle neritic) or lithol-ogy (limestone to calcareous shale to shale). Paleonto-logic evidence does not unequivocally indicate a gap inage, but seismic proles show onlap of basal Baong re-ectors.

72.3.1.1.5. Early Foreland Basin Fill: Baong FormationA major transgression accompanied sedimentation of thePeutu/upper Belumai interval. The onset of this increasein relative sea level may relate to an eustatic rise at about15.5 m.y. (N8- N9), but the change from paralic tobathyal environments reects a reordering of basinal ar-chitecture as well. Changes in the tectonic regime areevident from reactivation and inversion of the old horst-graben fault systems, initial development of major tran-scurrent faulting, and local compressional folding. Re-gional subsidence accompanying these changes formeda deep, extensive foreland basin. The Baong Formationlled the basin with a thick (750-2500 m) section dom-inated by monotonous gray or brown mudrocks. TheBaong varies in age from Lower to Middle Miocene (N8-N16; Fig. 2.10). Early workers subdivided this formationvertically into upper, middle, and lower units. Distribu-tion of Lower Baong shales indicates widespread bathyalconditions. A ood of Globigerinid foraminifera withinthe Lower Baong marks a maximum ooding surface atabout the N8/N9 faunal zone. Mudrocks dominate thelower Baong section, but turbidite sands also occur in ar-eas along the basin margins. In the Middle Baong (N13-N14), the inux of detrital sand and silt increased fromboth sides of the basin. This was accompanied by a gen-eral shoaling in paleoenvironments from bathyal to outeror middle neritic water depths. Sands attributed to botheastern and western sources are similar in composition.They vary from lithic arenites to lithic arkoses, with sed-imentary and metamorphic lithic clasts. This contrastssharply with the overlying Keutapang, which containsmore volcanic detritus. Middle Baong sands do not reachthe central basin area, but the interval can still be rec-ognized from increased silt and ne sand content of themudrocks, brown color (in contrast to dark gray to blackin the lower Baong), and shallower water fauna.Middle Baong sedimentation ended with a period oftectonic quiescence. Pre-existing structural highs wereeroded, resulting in a widely recognized seismic uncon-formity of N-14 age. Except for local reworked sandsabove the unconformity, overlying Upper Baong sedi-ments consist of clay-rich mudrocks. Paleoenvironmentsdeepened again to bathyal water depths, followed by grad-ual shoaling upward topped by paralic sands of the over-lying Keutapang Formation. The uppermost Baong thusconsists largely of basin-lling prodelta and slope depositsassociated with progradation of Keutapang deltas (Fig.2.11).2.3.1.1.6. Late Foreland Basin: Keutapang and YoungerFormationsThe Late Foreland phase completed initial tilling of thebasin. Transpressional tectonics continued, but sedimentinux kept pace with basin subsicience. Paralic to alluvialenvironments were thus maintained from Late Mioceneonward. Sedimentation occurred as a series of deltaicpulses, which were likely driven by changes in relative sea

level and sediment supply.The Keutapang Formation marks the rst major event ofdeltaic sedimentation. The unit is dominated by beds ofresistant sandstone, which crop out as a band of ridgeswith up to 200 m of relief. This precipitous terrain standsout in sharp contrast to gently rolling topography of re-cessive Baong shales, and surface relief appears to haveguided early mapping. Actual lithologic contacts are gra-dational and much less obvious. The Keutapang varies inthickness from about 700-1500 m in East Aceh. Plank-tonic foraminifera for this unit span zones N15/16 toN19, or Late Miocene to Early Pliocene (Fig. 2.10).The unit consists of gray to gray brown or bluish graysandstones interbedded with subordinate shales and rare,thin limestones. Sandstone grains vary in size from veryne grained sand to pebble conglomerates. Sandstonesare commonly glauconitic and/or fossiliferous, contain-ing gastropod and pelecypod fragments and foraminifera.Coally plant fragments are common, and interbeddedshales are gray, blocky, and highly bioturbated.Keutapang sandstones are classied as lithic arenites, but,unlike the Baong, lithic clasts include common to abun-dant volcanic rock fragments. Sandstone isopachs indi-cate derivation from Barisan source terrain to the southand southwest. Keutapang sands are interpreted to be de-posits of sand-rich delta systems that prograded north-eastward. Uplift of the Barisan provided sucient detri-tus to extend the shelf platform in this manner and ll theonshore part of the North Sumatra Basin.The upper contact of the Keutapang is poorly dened inboth outcrop and subsurface, and this boundary appearsto be both gradational and diachronous. Overlying sedi-ments of the Seurula Formation contain more shale andweather recessively, forming low, rounded hills. It is earlyPliocene in age (N18- N19), and varies in thickness fromabout 700-900 m.The Seureula consists of bluish gray shale and subordi-nate ne to medium and locally coarse or conglomeraticsandstones. Both sands and shales are fossiliferous andcontain coaly plant fragments. Volcanic clasts are abun-dant in the sandstones, and shales are described as rarelytuaceous (Bennett and others, 1981). Although stud-ied far less than the subjacent Keutapang, the Seureulaconsists of volcanic-rich detritus apparently derived fromBarisan sources to the west. These accumulated in gen-erally mud rich delta margin and deltaic environments.The Late Pliocene Julu Rayeu Formation (Fig. 2.10) con-sists largely of coarse clastics. Thin lignites commonlyoccur in shales interbedded with the sandstones, and pa-leoenvironments vary from alluvial to paralic. Uncon-formably overlying the Julu Rayeu are geomorphicallydistinct but poorly exposed Pleistocene terrace depositsof gravel, sand and mud. These comprise the Idi Forma-tion, described by Bennett and others (1981) as 50 m ofsemi-consolidated gravel, sands and mudstone.Holocene sedimentation has extended the coastal plain

8 3 2.3. SUMATRA BACK ARC BASINS

2 to 25 km. north and east of the high- standing Pleis-tocene terrace. These recent sediments include lobate toarcuate deltas of the Jambo Aye, Arakunda, Peureulak,and Tamiang rivers plus intervening chenier plain andtidal estuarine deposits. The at, low-lying coastal plainis heavily populated and supports extensive developmentof shrimp ponds in coastal marshes and rice cultivationfarther inland.2.3.2. CENTRAL SUMATRA BASINFor a complete discussion regarding regional setting ofthe Central Sumatra Basin we refer the readers to pa-pers by Mertosono and Nayoan ( 1974), Wongsosantiko( 1976), and Eubank and Makki ( 1981), Williams, et.al.,1985. Figure 12 is a summary of the stratigraphy inthis basin. The Central Sumatra Basin was formed dur-ing the Early Tertiary (Eocene-Oligocene) as a series ofhalf grabens arid horst blocks developed in response toan East-West direction of extensional regime (Eubank &Makki, 1981). A divergent transform boundary (non-coupling) between the Sunda Microplate and the IndianOceanic Plate during Paleogene gave rise to extensionalregime and crustal stretching of the western part of theSunda Land resulting in the formation of Pematang typegrabens (Davies, 1984). Pematang Graben Developmentcan be divided in 3 stages: 1. Pregraben Stage, minorblock rotation along pre-existing zone of weakness, be-ginning of the Lower Redbeds deposition; 2. GrabenStage, rapid block rotation/subsidence, development ofa deep anoxic lake with slow deposition of the BrownShale Formation associated with lateral facies variationsuch as alluvial fan along graben and lake margins; 3.Post Graben Stage, slower rate of subsidence coupledwith a major sea-level drop in Upper Oligocene causedworn-down of the graben rim and the lake was dried up.Subsequently, the lake was ll with coarser clastic de-posits of the Upper Red Beds Formation. A mild tec-tonic event occurred during Late Oligocene marked bya major unconformity relationship with the overlying Si-hapas Group. Lower Miocene marine sediments of Siha-pas were mainly derived from the Malacca Land direc-tion, while older section is thought to be locally derived.Biostratigraphy and seismic data indicate an importantnon-depositional break separating the Telisa and PetaniFormations. This break probably corresponds to an im-portant tectonic pulse at the initial time of the Barisanuplift coincident with a major low-stand event duringMiddle Miocene. It reects the reversal of sedimenta-tion from the Malaysian Shield (Lower Miocene) to theBarisan source (since Middle Miocene) and is consideredto be N7 to N12 in age. Structuring in the Central Suma-tra Basin is related to the rst order NW-SE trendingright lateral strike-slip fault (the Sumatra Fault System),in response to an oblique northward low angle subduc-tion of the Indian Ocean Plate beneath the Asian Platewhich gave rise to a transpressional stress system. Neo-gene structures within the basin are dominantly WNWto NW trending folds and high angle reverse faults and

NNW to N trending right lateral strike-slip faults. Theseare all second order structural features in relation to theprimary NW trending of the Sumatra Fault Zone. Minorstructures within the Basin are second order NE trend-ing normal faults and NNE trending third order right lat-eral strike slip faults (Verral, 1982). An earlier, Paleo-gene east-west extensional deformation aected the Pre-Neogene section, producing large NS trending grabenlled with Pematang Formation. Dierential compactionand recurrent movement of this earlier system has a tec-tonic overprint on the Neogene structural system.2.3.3. SOUTH SUMATRA BASINThe South Sumatra Basin is located to the east of theBarisan mountains and extends into the oshore areasto the northeast and is regarded as a foreland (back-arc)basin bounded by the Barisanmountains to the southwest,and the pre-Tertiary of the Sunda Shelf to the northeast(de Coster, 1974). The South Sumatra Basin was formedduring east-west extension at the end of the pre-Tertiaryto the beginning of Tertiary times (Daly et d., 1987).Orogenic activity during the Late Cretaceous-Eocene cutthe basin into four sub-basins. The following details areafter van Gorsel (1988).The structural features present in the basin are the resultof the three main tectonic events (de Coster, 1974). Theyare Middle-Mesozoic orogeny, Late Cretaceous-Eocenetectonism and Plio-Pleistocene orogeny. The rst twoevents provided the basement conguration including theformation of half grabens, horsts and fault blocks (Adi-widjaja and de Coster, 1973; de Coster, 1974; Pulung-gono et al., 1992). The last event, the Plio-Pleistoceneorogeny, resulted in formation of the present northwest-southeast structural features and the depression to thenortheast (de Coster, 1974).In the South Sumatra Basin the best surface sections arefound around the Gumai Mountain anticline. From oldto young the following lithostratigraphic units were de-scribed:2.3.3.1. STRATIGRAPHY2.3.3.1.1. CretaceousThe complexly folded Pre-Tertiary in the Gumai Moun-tains contains two dierent units, the relations of whichare unclear : - Saling Formation: Mainly poorly-beddedvolcanic breccias, tus and basaltic-andesitic lava ows,hydrothermally altered to greenstones,. Three interca-lations of dark gray reefal limestone occur, with Meso-zoic fossils like the coral Lovcenipora and the gastro-pod Nerinea. The Saling Formation rocks may be a LateJurassic-Early Cretaceous volcanic island arc associationwith fringing reefs.-- Lingsing Formation: Mainly grey-black, thin-beddedshales or slates, with minor interbeds of green andesitic-basaltic rock, radiolarian-bearing chert and one severaltens of meters thick limestone bed rich in the Early Cre-taceous foraminifer Orbitolina, but without corals. The

9Lingsing Formation rocks suggest an Early Cretaceousdeep water facies. Whether it is a deep water equiva-lent of the Saling Formation or whether it is younger orolder is not clear. Both formations were intruded by LateCretaceous or Early Tertiary granodiorites. Pulunggonoand Cameron (1983) regarded the GumaiMountains Pre-Tertiary as part of their Woyla basement terrane, and in-terpreted it as a possible Cretaceous subduction complex.2.3.3.1.2. Paleogene- Lahat Formation (Musper, 1937)Unconformably overlying the Pre-Tertiary, but con-formable under Talang Akar and Baturaja sediments isa thick (up to 3350m) series of andesitic volcanic brec-cias, tus, lahar deposits and lava ows, with a remark-able quartz-sandstone horizon in the middle. Except forsome silicied wood, fossils are absent and exact age isuncertain. The formation is possibly an equivalent of thewidespread Old Andesites of Sumatra and Java. OnJava these are dated as Oligocene, overlying marine Mid-dle and Late Eocene beds. Three members are distin-guished, from old to young:1. Lower Kikim Tu Member: Andesitic tus, brecciasand some lava beds. Lava beds seem to decrease in north-ern direction. Thickness is variable (0-800m). 2. Quartz-sandstone Member: This member is conformable, orwith a minor unconormity over the Lower Kikim tus,or may directly overlie Pre-tertiary rocks. It could bemapped all around the Gumai anticline. The base is a.5 to 3m thick conglomerate, followed by ner conglom-erates and sandstones. Cross-bedding is common. Al-most all grains are quartz (polycrystalline; probably de-rived from granitic rock), but dark cryptocrystalline vol-canic rock fragments were found, too. Thickness variesbetween 75 and 200m.3. Upper Kikim Tu Member Conformable over, thequartz sandstone, and with a gradual transition, is anotherseries of greenish andesitic volcanics. Overall grain size isner than that of the lower member. Fine-grained, well-bedded tus and tuaceous claystones are interbeddedwith coarse-grained, lahar-like deposits. Lava ows areextremely rare; most material appears to be redepositedvolcanics. Thickness decreases to the NW from 2500 to309o, suggesting an eruption center somewhere to the SE(Musper, 1937). The Lahat Formation underlies the Ta-lang Akar Formation and consists of uvial or alluvialfan sands, lacustrine and uvial clays and coals and it isquestionable whether these are the same as the Lahat vol-canics.2.3.3.1.3. Pre-Baturaja ClasticsIn the South Sumatra basin a highly variable complex ofclastic sediments is found between the Lahat volcanicsand the Early Miocene marine Baturaja or Telisa Forma-tions. Thick series are found in predominantly N-S trend-ing grabens (Benakat gully, Lematang trough), whichformed in the Oligocene, perhaps also somewhat earlier.

The basal part with volcanoclastic sediments and lacus-trine clays is called Lemat Formation, and is either a distalfacies of the Lahat Formation or, more likely, a youngerunit rich in debris from the Lahat Formation. The upperpart of the graben-ll series is the uvial and deltaic Ta-lang Akar Formation, which is mainly Late Oligocene inage. Thickness in the oileld areas is up to 800-1000 m.Neither the Lemat, nor the Talang Akar Formation havebeen properly dened and no type sections were desig-nated.No good outcrops of these graben ll sediments areknown. In surface sections around the Gumai Mountainsclastic sediments between the Lahat Volcanics and Batu-raja Formations are very thin or absent.Musper (1937) called the thin clastic interval below theBaturaja the Wood-horizon, because large silicied treetrunks are common at the base of the unit. Thicknessis about 20-30m. In the Cawang Saling section it is atransgressive series, with at the base a few meters ofpoorly sorted conglomerates with pebbles of quartz, vol-canic rock and silicied wood, and cross-bedded sand-stone (uvial or alluvial fan deposits). These are overlainby 2 m of lenticular-bedded sand and clay, overall ning-upward (intertidal), followed by l m of calcareous sand-stone with common shallow marine larger foraminifera(Early Miocene; marine transgressive sand).2.3.3.1.4. Baturaja FormationLimestones found in various places near the base of theTelisa Formation are usually attributed to the BaturajaFormation. It is locally developed shallow water facies ofthe lower Telisa shales and should probably be regardedas a member of this formation. Surface outcrops of Bat-uraja limestone are found at several places around theGumai Mountains. Maximum thickness is about 200m,but is usually less. Both massive reefal facies and deeperwater ne-grained well-bedded limestone with thin marlintercalations are present. In the subsurface, Baturajalimestones are found only on paleohighs and along thebasin margin. It is absent over low areas with thickgraben-ll, where amarine shale facies with a typical, richforaminifera assemblage is found (Vaginulina zone; basalTelisa). Age of this formation is within the early part ofthe Early Miocene (Upper Te larger foram assemblages,equivalent of planktonic foram zones N5-N6).2.3.3.1.5. Telisa Formation (Tobler 1910) / Gumai For-mation (Tobler 1906)The thick series of Early (and locally also early Middle)Miocene deep marine shales and marls in South and Cen-tral Sumatra was described under two dierent names.The Gumai Formation is based on sections along the Gu-mai Mountains, while the Telisa Formation is named af-ter the Telisa river near Surolangun, Jambi. The forma-tion is characterized by a thick series of dark grey clays,usually with common planktonic foraminifera that mayform thin white laminae. Whitish tus and brown tur-biditic layers composed of andesitic tuaceous material

10 3 2.3. SUMATRA BACK ARC BASINS

are locally common. Layers with brown, lenticular cal-careous nodules up to 2 m in diameter are most commonin the upper part of the formation.Thickness of the Telisa Formation is highly variable(from a few hundred to 3000m or more). This is mostlycontrolled by dierential subsidence; but it probably alsoreects the fact that in the thick, basinal areas the Telisamay include marine lateral equivalents of the upper Ta-lang Akar, Baturaja and Lower Palembang formations.Towards the top the open marine Globigerina marls gradeinto brownish prodelta clays with fewer planktonics, butuntil more carbonaceous material and common rotalidforaminifera. Where sands become frequent (whetherdeltaic, shallowmarine or turbiditic) the overlying Palem-bang Formation is reached, but since the transition is usu-ally gradual there is a great element of subjectivity inpicking the boundary.Age of the formation varies. Where no Baturaja lime-stone is developed the basal Telisa beds have zone N4planktonic foraminifera (earliest Miocene). Where Bat-uraja is thick the oldest Telisa beds have zone N6 orN7 faunas (within Early Miocene). The top also varies,from within zone N8 (latest Early Miocene) to zone N10(within Middle Miocene), depending on position in thebasin and where the formation boundary is picked.2.3.3.1.6. Palembang Formation (Air Benakat, MuaraEnim andKasai Formation) This formation is the regres-sive stage of the South Sumatra basin ll. Facies showan overall shallowing-upward trend from predominantlyshallow marine at the base; through coastal deposits touvial beds in the top member. In detail the formationis composed of numerous thin transgressive-regressivepara-sequences. Three members are distinguished:- Lower Palembang Member (Air Benakat Fm.) Thelower boundary is where signicant, continuous sandbeds are found and where the clays have few or no plank-tonic foraminifera. The upper boundary is at the baseof the lowest coal beds. Sands are usually glauconitic.Clays contain glauconite, carbonaceous material, shallowmarine molluscs and foraminifera. The basal sands mayeither be coastal facies (beach, tidal at, deltaic) or, insome areas, deeper water turbidites. Thickness of the for-mation is ranging from 100 m to 1000 m. Outcrops arepoor due to softness of the beds. Age is Middle Miocene,possibly ranging up into the Late Miocene.- Middle Palembang Member (Muara Enim Fm.) Topand bottom of this unit are dened by the upper and loweroccurrence of laterally continuous coal beds. Thicknessin the area around Muara Enim and Lahat is around 500-700m, about 15% of which is coal. Where the memberis thin, coal beds become very thin or are absent; sug-gesting subsidence rates played an important role in coaldeposition and preservation. Where studied in detail, theformation consists of stacked shallowing-upward parase-quences, typically l0m-30m thick, with shallowmarine orbay clays at the base, and shoreline and delta plain facies

(sand, clay, coal) at the top. Sands may be glauconitic andcontain volcanic debris. Especially the upper part of themember clear bipyramidal quartz and light-colored acidtus are common. In most of the basin, the coals are low-grade lignites. Only around young andesite intrusions,like Bukit Asam, the lignites were altered to high-gradecoal. In this area coal occur in three groups: an upper(with 6-7 seams), a middle, and a lower group (Merapiseam; 8-l0 m). The roofs of coalbeds may be silicied,especially where overlain by tu beds (volcanic ash falls).At their base root horizons and in situ true trunks maybe found, suggesting most coals are autochtonous. Treespecies identied from the coal point to upland forest con-ditions, no elements of mangrove swamp vegetation havebeen reported (Musper, 1933). Age of the member hasnever been determined accurately, but must be within theLate Miocene - Early Pliocene.- Upper Palembang Member (Kasai Fm.) Most surfacesediments in the South Sumatra basin are of this unit, butdue to its soft rocks exposures tend to be poor and farapart. The lower 250-350m are characterized by com-mon ne-grained, rhyolitic tephra (acid air-transportedvolcanics), i.e. yellow-white pumice tus (often withclear bipyramidal quartz crystals and black hexagonal bi-otite akes and tuaceous sandstones. Coals are absent.Conglomeratic sandstones and plant material are rare.The upper part of the member (300-500m thick) stillhas common quartz-rich pumice tus, but also containscommon cross-bedded coarse sandstone and pumice-richconglomerate beds. For the rst time erosional prod-ucts from older formations (Telisa, Lahat, Saling, etc.)are found, suggesting uplift and signicant erosion ofthe Gurnai Mountains within this period. Much of theupper Palembang may be regarded as synorogenic de-posits, developed mainly in synclines. Depositional fa-cies are uvial and alluvial fan with frequent ashfalls (non-andesitic:). Fossils are rare, only some fresh-water mol-luscs and plant fragments have been reported (Musper1933, 1937). Most likely age is Late Pliocene to Pleis-tocene.2.3.3.1.7. Quaternary The youngest beds in the region,that are not aected by the"Plio-Pleistocene folding,were grouped under the term Quaternary. They may un-conformably overlie Palembang or older formations, andcan usually be distinguished from Palembang beds by thepresence of dark-coloured andesitic and basaltic volcanicrocks. Quaternary andesitic volcanism was particularlyabundant in the Barisan Mountains, but also between theLematang and Enim rivers, where numerous intrusionsand extrusive products now make up the Bukit Asam,Serelo and Djelapang groups of hills. Other rocks in-cluded: in the Quaternary are the liparites (ignimbrites)lling valleys in the Pasumah region south of the GumaiMountains, the andesitic tus and lahars in the Pasumahregion derived from Barisan volcanoes like Dempo, andterrace deposits along the major rivers.

11

4 2.4. BARISAN MOUNTAINRANGE (after Nishimura, 1980)

2.4.1. ACEH AREA The most prominent topographicelement of the island is the Barisan Range, 1650 km longand about 100 km wide. This range skirts the southernend of the Andaman Basin. In this area, the stratigraphyand tectonic structure of the Barisan Range correspondsmore with to the northern part of the Sunda mountainsystem more than to that of the Sumatran section. TheSumatran trendIines, paralleling those of the MalayanPeninsula, begin with the N-S trending van Daalen Rangewhich meets the main body of the Barisan Range at rightangles. Here occurs an intersection of Pre-Tertiary trend-lines which belong to two dierent centres of orogenicactivity, that of Mergui and that of the Sunda Area. Thefoothills, formed by truncated Tertiary anticlines skirt,the central Pre-Tertiarymountains of northern Aceh. ThePuncak Lemby (2,983m) is a central knot fromwhich thevan Daalen Range extends northward, the Central GajoRange westward, and the Wilhelmina Range southeast-ward. In southern Aceh, south of Blangkedjeren, a NW-SE trend of the Barisan System prevails.2.4.2. TOBA AREA (NORTH SUMATRA) Betweenthe Wampu and the Barumun Rivers, the Barisan Rangedisplay a typical oblong culmination (NW-SE acis of 275km length and 150 km width). This culmination hasbeen called by van Bemmelen the Batak Tumor. In thisBatak Tumor, which is about 2,000 m high (Sibuatan,2,457 m), lies the great Toba area with Lake Toba.2.4.3. CENTRAL SUMATRA The Barisan system ofcentral Sumatra consists of a number of NW-SE trend-ing block mountains. The system is narrowest at itstransition into the Batak Timor near Padangsidem-puan from which point it gradually widens south- east-ward to 175 km in the Padang section. These blockmountain ranges are highest on the southwestern side ofthe Barisan System, which they attain altitudes of over2,000 m. They descend towards the east Sumatran low-lands. The Pre-Tertiary core of the Suligi-Lipat KainRange can be traced, via some anticlinal ridges of Ter-tiary formations to the northwestern corner of the Tiga-puluh Mts., which are situated in the middle of the Ter-tiary basin of east Sumatra. The Lisun-Kwantan-LaloRange plunges southeastward, disappearing under a 50kmwide basin, called the Sub-Barisan Depression, whichseparates the Tigapuluh Mts. from the main Barisan Sys-tem. The fore-Barisan begins in the Ombilin area, east ofLake Singkarak, where it wedges out between the Lisun-Kwantan-Lalo Range and the Schiefer Barisan; southeast-ward it disappears under the Tertiary deposits of the eastSumatra basin. The schiefer Barisan can be traced alongthe entire length of the island. The High-Barisan is par-ticularly well developed in the southern half, south ofPadang. In the northern half of the island no distinctioncan be made between the Schiefer-Barisan and the High-

Barisan, because Pre-Tertiary rocks are exposed over theentire area, capped by more or less isolated young volca-noes.2.4.4. SEMANGKO ZONE (SOUTH SUMATRA) Onefeature which characterizes the Barisan geanticline alongits entire length is a median depression zone on its top,called the Semangko zone named after a prototypicalsection in the Semangko valley of south Sumatra. ThisSemangko zone begins in the Semangko Bay of SouthSumatra and can be traced from there to the junction ofthe Aceh Valley with Banda Aceh at the northern end ofthe island. Some sections have been silled and capped byyoung volcanoes.Total view of the main structural Trendlines of SumatraBased upon the above descriptions, the main structuraltrendlines of Sumatra may be outlined as follows: Thewest ank of the Barisan Range, extending west fromthe Semangko Zone, is rather regularly formed in thesouthern half of the range, south of Padang. This south-ern part of the west ank was formed by a long crustalblock, which tilted toward the Indian Ocean, while theelevated northeastern edge breaks down along the Se-mangko Zone. This tilted block, called the BengkuluBlock, is similar to the southern mountains of Jawa. Theescarpment along the Semangko Zone general forms thedivide between the east and the west coast of Sumatra.This is the High-Barisan. The west coast rivers are short,having a steep grade towards Indian Ocean. The riversdescending eastward are much longer, owing throughan erosional plain, which truncates the anticlines of theNeogene Basin, and then owing through a wide alluviallowlands until they empty into the Sunda Shelf sea andthe Strait of Bangka. The southern end of the Barisanin the Lampung district is nearly 150 km wide. Hereone may distinguish between the west ank, or BengkuluBlock, the top part of the Lampung Block, and the eastank, or Sekampung Block. North of Lake Ranau therange narrows to less than 100 km because the Sekam-pung Block disappears under the Neogene South Sumatrabasin and the Lampong Block becomes covered by Neo-gene strata. The Pre-Tertiary besement complex of thelatter reappears in the culminations of the Garba, Gumai-and Tambesi-Rawas Mts., which belong to the SchieferBarisan, while the edge of the Bengkulu Block, capped bya series of young volcanic cones, forms the High-Barisan.Between Padang and Padangsidimpuan the structure ofthe Barisan Range is less distinct. It is cut into a numberof longitudinal block-mountains both in the east ank andin the west ank. The latter are exemplied by the BatangGadis after it has left the Batang Angkola trough of theSemangko Zone. The Batak tumor part of the BarisanRange is a great dome, traversed by an arcuate section ofthe Semangko-rift zone. The northern part of the Barisanrange, of the Batak Tumor, is the most complicated por-tion of the range. It is into a number of block mountainstructures. The Leuser Block and the western mountainsoccupy a position in the South of the Bengkulu Block.

12 5 2.5. SUMATRA INTRA-ARC BASIN

The Barisan Range forms a section of the volcanic innerarc of the Sunda Mountain System. It is separated fromthe old Sunda landmass by the Sumatra back-arc basinsThis downwrap of the Pre-Tertiary basement complex abackdeep, is lled with Neogene sediments which werefolded in Plio-Pleistocene time. During or after the mainphase of folding, a dome was elevated in the center ofthis backdeep which now forms the Tigapuluh Mts. Inother places the basement complex is exposed in the coresof Tertiary anticlines. These anticlines have eroded totheir basement levels during their folding so that a pri-mary peneplain of subaerial erosion truncates the Ter-tiary anticlines. The Pre-Tertiary basement complex ofthe Sunda area crops out at some places in the alluvialmarshes along the east coast. These are, in fact, formerislands in the Sunda Shelf Sea which have been connectedwith the main land of Sumatra by depositions in subre-cent time. Physiographically, the backdeep of the SundaMountain System now forms a lowland in the Sumatrasection, while in other sections, with less sedimentationin Neogene time, the backdeep forms sea basins such asthe Andaman Basin of the Mergui section in north Suma-tra. West of the Barisan Range stretches the interdeep ofthe Sunda Mountain System which forms the sea basinbetween Sumatra and the island festoon to the west. Thisisland chain is part of the non-volcanic outer arc of theSunda Mountain System.

5 2.5. SUMATRA INTRA-ARCBASIN

In terms of overall geomorphology of Sumatra, the Om-bilin Basin is a median graben which is situated betweenthe East and West Barisan mountain range (Fig. 2.1).This median graben extends from south of Solok andtrends northwest past Payakumbuh, a distance of approx-imately 120 km. Towards the northern end of the basinthe median graben is covered by Quaternary and recentvolcanic products of the Malintang, Merapi, Singgalang,and Maninjau volcanoes. Despite the relatively smallsize of the basin, 1500 sq km, (25 x 60 km, Figure 2),the basin ll is very thick. Up to 4,600 meters of Ter-tiary sediments, ranging in age from Eocene to early mid-dle Miocene is preserved in the Ombilin Basin (Koning,1985). Major river drainage of the Ombilin Basin is pro-vided by the Ombilin, Sinamar and Palangki Rivers alongwith their many tributaries. Mean elevation of the cen-tral basin is approximately 400 meters. However, in thenorthern portion of the Ombilin Basin, Merapi and Mal-intang volcanoes reach elevations of 2891 and 2262 me-ters respectively.2.5.1. TECTONIC SETTING The Ombilin Basin isa northwest-southeast trending, elongate, sedimentarybasin. The basin is located within the Barisan Moun-tain range of West and Central Sumatra. The area isunique since it is one of the few intermontane basins

in Indonesia which exposes early to middle Tertiary la-custrine sediments, thick sequences of stacked braidedstream deposits, and marginal alluvial debris fans. Thepresence of economically important coal bearing strata inthe Sawahlunto Formation has generated much geologicinterest in the area. TheOmbilin Basin has a complex his-tory of reverse, wrench and extensional tectonism. Initialbasin conguration and quantity of sediment in the Om-bilin Basin is due to a north- south compression whichcreated a graben dog leg or pull apart style basin in theOmbilin and Payakumbuh region. This compression wasintroduced by the subduction of the Indian- Australianplate beneath the Sunda Craton (Figure 4). Subductionstarted in the early middle Eocene (Daly 1990) and cre-ated an extensional tectonic regime which formed numer-ous grabens in a back arc extensional tectonic setting. TheBengkalis trough, Aman, Kiri, Jambi and Palembang de-pressions are examples of this type of basin development.The Ombilin Basin is believed to be similar in evolutionto these grabens and portray an early example of one ofthese features.2.5.2. STRATIGRAPHYMany authors proposed dier-ent stratigraphic nomenclatures of this basin. The follow-ing stratigraphic description is after Kosoemadinata &Matasak (1981), Kastowo & Silitonga (1975), and sum-marized by Fletcher & Yarmanto (1993).2.5.2.1. PRE-TERTIARY STRATIGRAPHY The pre-Tertiary framework of Sumatra consists of a mosaic ofcontinental and oceanic microplates accreted in the lateTriassic when the Mergui, Malacca, and East Malaya mi-croplates were joined together to form the Sunda Craton.Further accretion followed during late Mesozoic timesinvolving the Woyla Terrains (Pulunggono & Cameron,1984). The Ombilin Basin is largely oored by meta-volcanics and meta-sediments of the Mergui accretionaryterrain. These consist of limestones and marbles from theCarboniferous Kuantan Formation and meta-volcanicsfrom the Permian Silungkang Formation. West of theOmbilin Basin fenesters of the Woyla oceanic accre-tionary terrain sporadically outcrop between Quaternaryvolcanic deposits. The sequence consists predominantlyof limestones from the Permian Silungkang and TriassicTuhur Formations. Pre-Tertiary sedimentary rocks of theMergui andWoyla accretionary terrains were intruded bygranites, granodiorites, quartz diorites, and quartz por-phyries of various ages. Radio- metric dating indicatesan Upper Jurassic to Cretaceous age for most outcrops(Koning, 1985). However, samples have been dated fromPermian to Quaternary (Figure 8).2.5.2.2. TERTIARY STRATIGRAPHY PALEOGENEThe coarse grained Brani Formation consists of fanglom-erates and debris ow sediments deposited along ac-tive basin bounding faults from late Paleogene to middleEocene (Fletcher & Yarmanto. 1993). They are predom-inantly reddish brown to purple with mottling indicatingthe presence of rootlets or burrows. Style of sedimen-tation indicates these deposits are fanglomerates and de-

13

bris ows are a result of rapid uplift along the anks ofnewly formed grabens (Whateley & Jordan, 1987). Dur-ing the early evolution of the Ombilin Basin in Eocenetimes, organic rich lacustrine sediments of Sangakare-wang Formation was deposited in the central portion ofthe basin. These sediments rapidly thinned towards thebasin margins where they coalesced with alluvial fan anddebris ow sediments which contributed conglomeraticand breccia material from up-thrown fault blocks wherebasement was exposed. Concurrently, the surroundingmargins of the basin were the site of coarse grained, allu-vial fan sedimentation. These fan sediments were sourcedfrom up thrown fault blocks around the margin of thebasin (Figure 11). Sawahlunto Formation is late Eoceneto early Oligocene in age and unconformably overliesSangkarewang, Brani and basement. This formation isthemost economically important unit in the area due to itslarge coal reserves, outcrops extensively along the west-ern margins of the Ombilin. It is a ning upward se-quence deposited in a ood plain/mire type depositionalenvironment (Whateley and Jordan, 1987). The base ofthe sequence consists of grey, ne to medium grained,well sorted sandstones. Sands commonly have an ero-sional base and are interbedded with ner grained, clays,and coals. This sandstone rich basal sequence is overlainby ripple laminated, carbonaceous, si1tstones and shales.The entire sequence is capped by a series of interbed-ded grey mudstones, coal, and organic rich shales. TheRasau Member of the Sawahtambang Formation is re-ported to be locally developed along the western portionof the Ombilin Basin and represents a transition betweenthe meandering stream sediments of the Sawahlunto For-mation and braided stream sediments of the Sawahtam-bang Formation. It is included in Koesoemadinata andMatasaks classication as a basal member of the Sawah-tambang Formation and is dated as lower to late earlyOligocene. The Rasau Member is characterized by in-terbedded coarse grained sandstones and argillaceous silt-stones During Oligocene times, the basin became domi-nated by parasequence sets of continental sediments de-posited in a ood plain or meandering river depositionalenvironment of Sawahtambang Formation. These de-posits consist of interbedded siltstones, claystones andne to coarse-grained sandstones commonly represent-ing alluvial channel lls (DeSmet, 1991). Locally, coalsup to 18 meters thick were deposited in interlobe, mire-type depositional environments along the western mar-gin of the basin (Whateley & Jordan, 1989). In the lateOligocene the Ombilin Basin became increasingly u-vial, dominated by braided stream deposits of Sawahtam-bang Formation. The areal extent of these formations in-creased during this phase of deposition and reached itsmaximum during late Oligocene to early Miocene (Situ-morang, 1991). Thick sequences of ne to coarse grainedchannel sandstones are commonly stacked several tens upto 100s of meters thick (Plate 3).NEOGENE Conformably overlying the braided streamsediments of late Oligocene age are Ombilin Formation

calcareous shales and marls representing a major marineincursion which inundated the Ombilin Basin area as-well-as much of Sumatra. Increased tectonic couplingbetween the Sunda Craton and Indian-Australian platein the late Miocene-Pliocene marked the culmination ofthe Barisan orogeny creating the complex wrench tec-tonic framework we presently observe in West Sumatra.The Ombilin Formation consists of grey, silty to slightlysandy, moderately calcareous mudstones with commoncarbonaceous material. Interbedded with mudstones areo-white to white, very ne to ne grained, calcareous,glauconitic sandstones and soft, o-white, calcareous silt-stones. Thickness of Ombilin Formation varies dramat-ically in dierent portions of the basin. In the northernarm of the basin seismic interpretation show up to 4000meter of marine shales have accumulated (Per. comm.VardNelson, 1993 in Fletcher&Yarmanto, 1993). How-ever, in Sinamar-1 well only 692 meters were encoun-tered. Volcanic activity in the area reached its peakduring Late Pleistocene-Holocene time and the volcanicproducts are grouped as Ranau Formation. Compositionof the deposits varies but generally consists of andesite tobasalt lava ows, lahar deposits and tus. Provenance forthe Ranau Formation is from a combination of the Man-injau, Merapi, Malintang, and Singallang volcanoes. Thevolcanoes are situated both along and at right angles to theSumatra Fault zone. The northwest-southeast volcanictrend is easily explained by formation along a weakercrustal zones created by strike slip rnovement along theSumatra Fault Zone. However, the east-west trend ismore dicult to explain and is postulated to be a responseto crustal weakening around releasing bends between theOmbilin Basin and Payakumbuh Subbasin.

6 2.6. REGIONAL STRUC-TURES

Along the Java-Sumatran trench system the Indo-Australian plate is subducting under the Eruasian platewith a convergence rate of 75 mm/yr (Minster and Jorda,1978; DeMets et al., 1990). Analysis of slip vectors de-ducted from earthquake focal mechanisms suggests an ap-proximately N-tending convergence between these twoplates (Jarrard, 1986; McCarey, 1991). O Java, wherethe average trench azimuth is approximately N100oE,the convegence is nearly normal to the Java Trench andis essentially accomodated by the subduction process.Conversely, because the azimuth of the Sumatra Trench,West of the Sunda Strati, is N140oE, the convegenceisoblique. Mechanically, this convergence obliquity has tobe accomodated both by subduction (aconvegence com-ponent normal to the trench) and strike-slip deforma-tion (a convergence component parallel to the trench).The strike-slip deformation is interpreted as being locatedalong the Great Sumatran Fault System (Fitch, 1972;Beck, 1983; Jarrard, 1986b). This NW-trending fault

14 7 2.7. SOURCES

zone is a major, 1650-km-long structure of, right-lateralstrike-slip fault segments that follows the Sumatra mag-matic arc and parallesl the trench, from north to south,from the Andaman Sea back-arc basin to the Sunda Straitextensional fault aone. The slip rate of the Great Suma-tran Fault has been indirectly estimated, from global platemotions and the opening rate of nearby basins, and di-rectly calculated from measurements of osets along itstrace. Assuming that the Great Sumatran Fault zone isaccomodating all the trench-parallel component of theconvergence between the Indo-Australian and Eurasianplates. The slip rate of the Sumatra Fault System shouldrange between 30 and 50 mm/yr (Jarrard, 1986). Thishigh slip rate on the Sumatra Fault System appears highwhen compared to the relatively moderate activity of thecrustal seismicity and the slip rate estimated in south-ern Sumatra (Pramudmijoyo, 1991; Pramumijoyo et al.,1991). High resolution SPOT image analyses of theGreat Sumatran Fault trace have conrmed its right lat-eral strike-slip style. These images show right lateral o-sets of geomorphologic surface features such as streams,calderas and lineaments. Precise oset measurementsperformed along the Sumatra Fault System have shownthat its dextral slip rate increases to the northwest (Bel-lier et al., 1993), from 6+4 mm/yr in southern Suma-tra (at about 5oS) (Bellier et al., 1991) to 28 mm/yr(Shieh et al., 1991) in norther Sumatra near Lake Toba(at about 2o10N). However, the northern Sumatra Faultslip rate is still too low to accommodate the whole trench-parallel compnenet of the convergence. This suggests thata combination of two models should accommodate the 30mm/yr slip rate dierence between northern and south-ern Sumatra; that is, slip transfer to the Mentawai FaultZone (Diament et al., 1991, 1992; Malod et al., 1993)along the Batee Fault link and northwestward stretchingof the fore-arc platelet (McCarey, 1991), to explain thealong-strike variation in slip rate south of the Batee Fault.

7 2.7. SOURCESBona Situmorang: research on North Sumatra DannyHilman: PhD on Sumatra Fault

Structural framework map of Sibolga Basin and North Sumatra

15

north sumatra basin and tectonic map

16 8 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

8 Text and image sources, contributors, and licenses8.1 Text

The Geology of Indonesia/Sumatra Source: https://en.wikibooks.org/wiki/The_Geology_of_Indonesia/Sumatra?oldid=2961362 Con-tributors: Mike.lifeguard, Herman Darman, Adrignola, Aldnonymous, Herman darman and Anonymous: 3

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cense: CC BY-SA 4.0 Contributors: Own work Original artist: Herman darman File:Sibolga-basin.jpg Source: https://upload.wikimedia.org/wikipedia/commons/1/1e/Sibolga-basin.jpg License: CC BY-SA 4.0 Con-

tributors: Own work Original artist: Herman darman File:Sumatra-subduction.jpg Source: https://upload.wikimedia.org/wikipedia/commons/b/b8/Sumatra-subduction.jpg License: CC

BY-SA 4.0 Contributors: Own work Original artist: Herman darman File:Tectonic_setting_map_of_Sumatra.jpg Source: https://upload.wikimedia.org/wikipedia/commons/7/7f/Tectonic_setting_map_

of_Sumatra.jpg License: CC BY-SA 4.0 Contributors: Own work Original artist: Herman darman

8.3 Content license Creative Commons Attribution-Share Alike 3.0

2.1. SUNDA OUTER-ARC RIDGE 2.2. SUNDA FORE ARC BASINS 2.3. SUMATRA BACK ARC BASINS 2.4. BARISAN MOUNTAIN RANGE (after Nishimura, 1980)2.5. SUMATRA INTRA-ARC BASIN 2.6. REGIONAL STRUCTURES 2.7. SOURCES Text and image sources, contributors, and licensesTextImagesContent license