The Geology of Indonesia/Sumatra1 The Geology of Indonesia/Sumatra Sumatra Island is the northwest oriented physiographic expression, lied on the western edge of Sundaland, a southern extension of the Eruasian Continental Plate (Fig. 2.1). The Sumatra Island has an area of about 435,000 km2, measuring 1650 km from Banda Aceh in the north to Tanjungkarang in the south. Its width is about 100-200 km in the northern part and about 350 km in the southern part. The main geographical trendlines of the island are rather simple. Its backbone is formed by the Barisan Range which runs along the western side. This region divides the west and the east coast. The slope towards the Indian Ocean is generally steep, consequently the west belt is mostly mountainous, with the exception of two lowland embayments in north Sumatra which are about 20 km wide. The eastern belt of the island is covered by broad, hilly tracts of Tertiary formations and alluvium lowlands. At Diamond Point, in Aceh, this low eastern belt has a width of about 30 km; its width increases to 150-200 km in central and south Sumatra. The Sumatra island is interpreted to be constructed by collision and suturing of discrete micrcontinents 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 N20oE direction at a rate of between 6 and 7 cm/yr (Fig. 2.2). 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 basinal configuration of Sumatra is directly related to the presence of the subduction-induced non-volcanic forearc and the volcano-plutonic backarc, the morpho-structural backbone of the Island. In general the region can be divided into 6 regions (Fig. 2.1): 1. Sunda outer-arc ridge, located along the active margin of the Sunda forearc basin and separate it from the trench slope. 2. Sunda forearc basin, lying between the accreting non-volcanic outer-arc ridge with submerged segments, and the volcanic back arc of Sumatra. 3. Sumatra back arc basins including North, Central and South Sumatra basin. The system developed as distinct depressions at the foot of the Barisan range. 4. Barisan mountain range, occupies the axial part of the island and is composed mainly of Permo-Carboniferous to Mesozoic rocks. 5. Sumatra intra-arc or intermontane basin, separated by .1. 2.1. This chain of islands and sea-floor rises.2. On Nias. mudstone. NIAS Nias Island is located approximately 125 km off the west coast of Sumatra (Fig.1. OYO COMPLEX MELANGE The Oyo Complex is described by Moore and Karig (1980) as a tectonic melange. landslips are common to occur and the fresh matrix of the Oyo Complex can be observed. Situmorang & Yulihanto (1992) fieldwork indicates that the lower part of the Nias Beds is Upper Oligocene in age. NIAS BEDS Overlying the Oyo Complex.1. 2 2. al..1) and it has been frequently cited as a classic model of an accretionary complex (Fig. between 100 and 150 km off the coast of West Sumatra. 5). Nias. The contact between the two units has not been observed in the field. and Banyak Island lithologies represent the stratigraphy of the Sunda ourter-arc ridge in genereal. Nias lithologies were divided into two principal units.5). 1990). are a series of clastic sediments of shallow to deep marine deposits of Nias Beds which are well exposed along the eastern part of the island (Fig 2. 2. 2. including conglomerates. The age of the Nias Beds has been interpreted by previous authors as Early Miocene-Pliocene.1. pillow basalts and cherts (Harbury et. and are either grain supported or matrix supported. It consists of coarse to fine sandstone. conglomerate. Good outcrop of melange is exposed in central Nias (Moi River) and SW Nias. the Oyo complex and the Nias Beds (Fig.4 & 2.The Geology of Indonesia/Sumatra subsequent uplift and erosion from this former depositional area. the sediment boulders are sub to mature clastic with mainly subangular to rounded and well sorted sediments. The matrix forms a typical scaly clay. Texturally.1. polished shear planes. 1979). The age of the Oyo Complex remains unresolved by paIeontological analysis.1. 2.1. Simeulue.1. The geology of the Sunda outer-arc ridge is represented by Nias and Simeulue Island in this chapter. SIMEULUE . 2. In the area where the melange is present. The Complex is composed of sedimentary blocks. SUNDA OUTER-ARC RIDGE The Sunda non-volcanic outer-arc ridge marks the western margin of the Sunda Forearc Basin of West Sumatra. forms a structurally controlled topographic ridge nearly 200 km wide (Karig et al. outcrops of Oyo Complex are seen as isolated blocks and boulders in river sections. with probable unconformable contact. 2. with subordinate mafic plutonic rocks. On the contrary. sandstones and siltstones. that extends from the Andaman Sea to the southeast of Java. shale and limestone. Sandstone blocks form the dominant clast type in the SW part of the island.4). while pillow basalts and gabbros form some largest blocks (up to 200 m diameter) cropping out mostly along the west coast of the Nias Island (Fig.2. along road sections and coastal exposures. with a high density of curved. 2.3). thus with similar lithologies to the fore-and backarc basins. 4). micaceous mudstone. can be used to determine the stratigraphical base or top of the Baru Melange Formation. 1987. of melange overlain by interbedded sandstone and siltstone sequences. the most notable differences between the two islands is one of structural style. 1990).1. 1987). 2. or within a cleaved mudstone matrix. 2. At the base of the formation a coarse-grained sequence (the Pinang Conglomerate Member) is locally observed resting on the Sibau Gabbro Group. within the blocks of the melange or the clay matrix.1. 1987. The major part of the formation consists of bioclastic packstones composed of skeletal bioclasts. extending to a depth of several kilometres (J. 2. Although lithological variations do exist. the formation is considered to be of Late Miocene to Early Pliocene age (Situmorang et al.1. Smaller blocks of 5 – 10 cm in diameter are commonly enclosed within a sticky blue/grey clay matrix containing organic material.2. 2. 2. Bed thickness varies from 4 cm to 15 m in the most massive beds.4 DIHIT FORMATION The Dihit Formation is widely exposed in most parts of Simeulue). and quartz.4)). 1987. meta. weakly sheared. poorly-sorted meta-greywacke. Fig. Rock dating suggest that the Sibau Gabbro Group and Baru Melange Formation were metamorphosed between Late Eocene and Early Oligocene (Harbury & Kallagher. moderately well-consolidated. pers. meta-dolerite and meta-volcanics.2. very well-consolidated.1.2 BARU MELANGE FORMATION Situmorang et al. including meta-basalt and meta-gabbro. The sandstone is well-sorted. 2.3 AI MANIS LIMESTONE FORMATION The Ai Manis Limestone Formation forms a NW – SE orientated ridge in the east central part of Simeulue. to be between 800 and 1000 m. A Late Oligocene to Early Pliocene age is suggested for this formation on the basis of palaeontological evidence (Situmorang et al. iron-rich meta-dolerite. predominantly fine-grained sandstone usually interbedded with siltstone or shale. and unlike the Nias Beds. biohermal (composed of in situ corals) and bioclastic limestones. Parallel laminations are rarely developed in the sandstone. The Pinang Conglomerate Member is between 0. Fig. commun. A shallow water benthonic foraminiferal assemblage indicating a Late Oligocene to Early Miocene age was recovered from the conglomerate (Situmorang et al.4). 1991). and are laterally 3 . 2. micaceous sandstone some of which are fractured. all with abundant chlorite and pumpellyite suggesting that these rocks are all low-grade metamorphics. with parts of the succession dominated by bioclastic limestones.The Geology of Indonesia/Sumatra Simeulue lies slightly off-strike and to the northwest of Nias (Fig. Fig. No bedding or other sedimentological characteristics. The formation is approximately 260 – 350 m thick and consists of both biostromal. but where present are very fine (<1 mm). Blocks within the melange may be in excess of 10 m in diameter. Lithologies identified within the group include gabbros. lithic and crystal tuft’s. The conglomerate is poorly-sorted and consists of clasts (mm – 50cm in diameter) of metaigneous rock fragments. The Sibau Gabbro Group is composed mainly of meta-igneous lithologies with predominantly transitional contacts. SIBAU GABBRO GROUP The oldest rocks exposed on the island are represented by the Sibau Gabbro Group (Situmorang et al. The Dihit Formation contains no stratigraphical control on the age of the formation. in a medium-grained calcarenite matrix.5 and 5 m thick and is exposed in the Ai Manis region. The maximum thickness of the formation is estimated from the Dihit section. 2. (1987) describe the Baru Melange formation as being in structural (thrust) contact with basalts at the top of the Sibau Gabbro Group (Fig. brecciated meta-basalt.1. The apparent random distribution of blocks of different lithology within the outcrop area suggests that the melange is unsorted.4). The thickness of the formation is estimated to be approximately 200 m.volcanics and calcite-rich. is micaceous.2. The ophiolite correlates closely with a partially defined gravity high in this area indicating that the basic igneous rocks form a major body.1).. large benthic foraminifera and quartz grains. Blocks within the melange include fine-grained. 2. The Dihit Formation is composed of grey. but more characteristically is between 50 and 100 cm. Milsom.2. This island shares a broadly comparable geology with Nias. Base on lithological similarities between the Dihit Formation and the Nias Beds. where it rests with an angular unconformity on the Sibau Gabbro Group. BENGKULU BASIN (after Yulihanto et al. 2. where interbedded with shale or mudstone. siltstone and coal. calcareous to non-calcareous. is usually the dominant lithology. occurs as small disseminated lignitic woody fragments and as very fine. calcareous concretions are rarely observed. The Sibolga Basin is asymetrical to the southwest with upwards of 6100 m of Neogene sediments adjacent to the outer-arc ridge. Recrystallized belemnites have been reported in cores indicating possible Mesozoic rocks unless the fossils are reworked. 2. 2. disseminated carbonaceous material. In spite of these faults.l).1. The mudstone and siltstone are dark in color. Sandstone.2.The Geology of Indonesia/Sumatra continuous through the outcrop. Pre-Neogene dacite tuff-lava was penetrated in the south of the basin with total thickness of 31 m. 2. The thickness of the Paleogene interval ranges from less than 30 m up to 350 m. The environment of deposition of this interval is assumed to be shelf. moderately soft at the top but becoming more indurated with depth. The Upper Eocene to Lower Oligocene interval is dominated by mudstone with minor interbeds of shale. there are two Sunda fore arc basins in west Sumatra. Pre-Neogene The pre-Neogene sedimentary section is separated from Neogene rocks by an angular unconformity.6).1. The onshore part of the basin can be divided into two sub -basin i. Massive sandstone. siltstone and sandstone. well-sorted and predominantly matrix-supported.1. sandstone. called Sibolga Basin in the nortwest of Sumatra and Bengkulu Basin in the southwest (Fig.1). SUNDA FORE ARC BASINS In general. To the north and northeast lies Barisan Mountain range.1. 2. Muscovite mica is present in all samples (trace – 3%). 2. The northern end terminates against the northwest extension of the Sumatra Fault System at about latitude 6o30’ N. The sandstone is fine-grained. Organic matter.SE.e Pagarjati sub-basin in the nort and Kedurang sub-basin in the south which separated by north south trending Masmambang High. where present. coal and minor limestone. averages 110 km wide and is approximately 800 km long (Fig.2.2.2.1. it is trending NW . fine to medium-grained. The mudstone is dark grey to black. Basal Miocene clastics Directly overlying the Paleogene angular unconformity is a sequence of sandstone. Fossil recovery was poor in this interval leading to a tentative age of Mid Miocene up to Lower Miocene. parallel to Sumatra Island with about 600 kms length and 150 .1. STRATIGRAPHY 2.1. the majority of the Neogene sedimentary rocks in the basin are undeformed. 2. The basin trends northwest-southeast.200 kms wide. while in the south and southwest is bounded by islands or slope break of the Sunda Arc Trench System (Fig. 2. Coal beds are about 1 m thick and are interbedded with mudstone.1.1). firm and commonly interbedded. 4 2. In general. quartzose with common vari-colored rock grains and slightly calcareous. A high-angle fault zone forms the western margin of the basin and created associated drag structures as did strike-slip faults that cut diagonally through the basin in the vicinity of Nias-Banyak islands. The sandstone is gray. In the Singkel area a correlative clastic sequence is dated Upper Miocene in the vicinity of the well control but interpretation of seismic records basinward suggests possible Mid Miocene rocks also. 2.2.1 SIBOLGA BASIN (after Rose 1983) The Sibolga Basin lies between the island of Sumatra and the adjacent outer-arc ridge to the west and is considered a fore-arc (outer-arc) basin (Fig.2. In the Meulaboh area the clastic sequence consists of nearshore marine and non-marine mudstone. The southern end of the Sibolga basin was arbitrarily placed in the vicinity of Pini and Batu Islands where a broad.2. 2.2.2.7). sandstone/siltstone and laminated sandstone/mudstone lithofacies can be recognised from the Dihit Formation sediments. shale.between 2:1 and 30:1. STRATIGRAPHY . with sandstone:shale ratios. Seismic interpretation indicates several hundred meters of folded sedimentary rocks beneath the unconformity in the Meulaboh-Teunom area.2. southwest trending low-lying arch separates it from the Bengkulu basin to the southeast. 1996) The Bengkulu Basin is located in the southeast part of the Sumatra Island covering both onshore and offshore (Fig. 2.deep marine turbidite sediments of Late 0ligocene . compare to the Simpangaur Formation. Tertiary Succession Surface geological studies exhibits that Tertiary sediments cropout in this onshore area is represented by Hulusimpang.1). NORTH SUMATRA BASIN It is important to emphasize that the present southwest geographical limit of the North Sumatra Basin at the northeast foot of the Barisan Range does not correspond to the depositional limit of the Tertiary sediments (Fig.2. and tuffs.. The Middle to Late Miocene stratigraphy is represented by the Lemau Formation. Rantau Panjang.3. tuffaceous siltstones.3. tuff.2. The Lingsing Formation consists of claystones.1. It consists of tuffaceous sandstones. siltstones and calcilutite with sandstones and chert intercalation of Late Jurassic . and Bintunan Formations (Fig..Early Miocene age. breccias. conglomerates with limestones intercalation. This Formation is we11 exposed in the southern area such as Talang Beringin. Based on few seismics sections and wells drilled in the Bengkulu offshore area known that the sediment thickness is about 4000m ( 1. containing abundance of small foram and mollusc which was deposited in shallow marine up to transitional zone. siltstones. claystones. In general. and carbon intercalation. Sepitiang and Saling Formations.1. The youngest stratigraphic unit cropout in this area is the Plio-Pleistocene Bintunan Formation which laying unconformably upon the older units. Recent onshore gravity work done by Lemigas has indicated two sub-basin with low bouguer anomaly. They are mostly shallow . Lithologically. and it ranges of about 200 m thick. The Late Miocene . calcareous siltstones and sandstones. 2. This formation is well exposed in the northern and eastern margin of the basin.8). conglomerates. the Hulusimpang Formation is known as Early Oligocene sediments which deposited in fluviatile up to shallow marine.000 feet). The upper part of the Hulusimpang Formation has interfingered with the lower part of the Seblat Formation.2. The original limit of this deposition extended much further to the southwest than the more recently uplifted Barisan Range. This observation is supported by evidence of Baong shale outcrops in the midst of the mountains and also their presence in the Southwest Sumatra Interdeep. volcanic breccias and tuff with sandstones intercalation.1. It consists of claystones.1. The Sepitiang Formation composes of reef limestones with some calcirudite and calcarenite lenses. 2. Seblat. breccias. The Hulusimpang Formation is composed of andesitic and basaltic lavas. limestones with lignite. The approximate thickness maesured in Tanjung Sakti area is + 298m. The Seblat Formation composes of sandstones. Fig. This Lingsing series has interfinger relationship with Sepitiang and Saling formations. Lubuk Tapi.2. The thickness recorded is+785 m. Air Keruh. 5 2. 2. and also typified by abundance of foram and mollusc fragments. and the Saling Formation mostly containing of volcanic materials such as lavas.9).Early Cretaceous age. in Tertiary time from the more extensive basin . This formation was deposited in shallow marine and fluvial environment.Pliocene sediment is represented by the Simpangaur Formation. It composes of sandstones and tuffaceous claystones with pumice clast. The eastern and southeastern limits of the basin are formed by the Asahan Arch (or Tebingtinggi Platform.The Geology of Indonesia/Sumatra The stratigraphy of the onshore Bengkulu Basin composes of a series of Oligo-Miocene up to Pliocene sediments overlaying unconformably the Pretertiary basements complex (Fig. toward the Barisan Mountain. Batang Rikibesar and Tebing Kekalangan areas. Pre-Tertiary Rocks The Pre-Tertiary basement complex is represented by metasediments of Lingsing. The aproximate thickness is 700 m.8). the Bintunan Formation in general is coarser than Simpangaur and often containing silicified wood and pumice clasts.2. SUMATRA BACK ARC BASINS 2. and thin coal seams and limestones intercalation. The total thickness is about 785 m thick. which separated it. 2. Lemau. Simpangaur. breccias. The detail descriptions of the stratigraphy as follow: 2. with intercalation of lignites. 10) consists of sandstone.1. formed as transgressive formation overlain by both Bruksah and Bampo Formation. whereas the dolorites are common. because the basement configuration become vague wherever the 2-way seismic time interval between top of Belumai and top of basement is less than 0. The stratigraphy of the basin is closely related to these evolutionary phases. to north where the flexure would be located if present. 2. 2. partly biocalcarenites and biocalcilutites. Late Syn-Rift to Transitional Phase: Belumai and Peutu Formations The transitional phase of basin evolution occurred during the early Miocene to early Middle Miocene and represents a period of relative tectonic activities.1. In the narrow wedge between the Medan Flexure and the front of the Barisan Range. a flexure may be present between Telaga and Basilam. Movement on the N-S trending faults ceased. Lithologies include limestone conglomerates and breccias. The sands were mainly derived from the Malacca Platform and the Asahan Arch. The rest of the Tertiary history of the North Sumatra Basin can be divided into three phases: 1)Syn-rift. defined by Cameron and others (1983) from field mapping in the Barisan Mountains. Basement The basement (Fig. a locally thick sequence (500 to perhaps 2400 m) of marine black shale. The present southwest structural limit of the basin runs along the Barisan Range.1. with steep dips up to 45o. The formation also consists of basal conglomeratic and dolomitic limestones. 2. they are azoic.0 seconds TWT on seismic sections) of the Belumai Formation in the western than in the eastern block (approximately 2.5 seconds TWT). although back-arc subsidence 6 . 1977).3.1.1.3. STRATIGRAPHY 2. This formation was deposited in the sublittoral .1. Early syn-Rift Phase: Bruksah and Bampo Formations The initial syn-rift phase began in the middle Paleogene (Eocene?) and continued until early Miocene. but they are not metamorphically altered. and muddy fine grained. Thus the top of this section is readily identified with the deepest. in the absence of dating. The Bruksah is overlain the Bampo Formation. during which time the N-S and NE-SW trending horsts. probably as a results of back-arc subsidence).1. 2. 1994). as indicated by the greater depth (3.Tampur Formation Tampur Formation (Fig. 2.10). This was also a time of major marine transgression (defined as a relative rise in sea level within the basin.3. limestones or dolomites. The conglomerates and sandstones deposited during this phase comprise the Bruksah Formation (Fig. most of which remained exposed during this time. continuous seismic marker and conveniently been called “economic basement” (Beicip. The Eocene Tampur limestone generally only occurred in Malacca shelf (Rjacudu & Sjahbuddin. and silty mudstones. 2. In some plugs or cores. In this area. Eastward from the Medan Flexure structural deformation is minimal on the platform. 2.3. the high resistivities and velocities generally constitute a good contrast with those of the overlying beds.2 seconds.open marine condition during Late Eocene to Early Oligocene. grabens and half-grabens developed. micaceous quartzose sandstones.The Geology of Indonesia/Sumatra developed in Central and South Sumatra.10) comprises massive.2. As the grabens deepened and transgression progressed.1. immediately east of Medan. and 3) Compressional (Late Foreland. Initial graben-fill consisted of continental sandstones and conglomerates.1. The shales are typically dark grey to black in color and deposited in deep marine environment (bathyal). 2) Transitional (Early Foreland). Chert nodules are found in this formation. Source of basal limestone clasts is still unknown but it assumed widely extended in the subsurface. the structural trends at basement level are oriented north-south. Fig. There is no evidence that this flexure also exists at basement level. these sediments are not easily recognisable as basement.4. augmented by local contributions from the horst blocks. At basement level this limit is marked by a north-south flexure.10). generally dense and fracture. siltstone.3. changing from northwest in the southeastern block. areas of sand deposition decreased and shale deposition dominated.1. On the other hand.1. Stratigraphic relationships indicate that the upper part of the Bruksah is at least partly equivalent in age to the Bampo Formation.3. 2. The later sands accumulated mainly in coastal plain to marine environments. from which it is separated by one or more compressional faults. The possible presence of a flexure could be reflected in the right-lateral movement deduced from the virgation of folds and faults. and they might have maintained equilibrium by dissolving carbonate bedrock while precipitating calcite cements. calcareous marine sands and siltstones along with argillaceous and sandy limestones accumulated in the lows while the highs remained at least intermittently exposed. South Lho Sukon. This results in age equivalence between the Peutu and at least the upper part of the Belumai Formation. The onset of this increase in relative sea level may relate to an eustatic rise at about 15. The Belumai is lithologically diverse. At Kuala Langsa. Early workers subdivided this formation vertically into upper. A possible explanation for more calcium carbonate in the Belumai is that this unit accumulated after widespread shallow seas first covered pre-Tertiary topography.10). including reefs. Fig. In the deepest parts of the North Sumatra Basin. A flood of Globigerinid foraminifera within the Lower Baong marks a maximum flooding surface at about the N8/N9 faunal zone. The entire Baong section is preserved in other areas.. The contact between the Baong and underlying Peutu or Belumai varies from gradational to abrupt. This was accompanied by a general shoaling in 7 . This stage was characterized mainly by forced regression (sea level constant or rising but sediment influx sufficient to cause regression) and basin filling. Middle and upper Baong shales are greenish gray to brown in color. Sedimentation of basinal Belumai deposits (calcareous sand.1. but seismic profiles show onlap of basal Baong reflectors. initial development of major transcurrent faulting. for example. Distribution of Lower Baong shales indicates widespread bathyal conditions. the original calcite has been replaced by dolomite. Rapid sedimentation would protect the calcite in the sandstones from re-dissolution. For practical purposes. with the lower Baong section (N8-N12) missing. Regional subsidence accompanying these changes formed a deep. The Baong Formation filled the basin with a thick (750-2500 m) section dominated by monotonous gray or brown mudrocks. In some areas. In the Middle Baong (N13-N14). extensive foreland basin.1.10). shale. These oceans might have been nearly saturated (or even super saturated) with calcium carbonate. much of which consists of carbonates. Quartz content decreases southwest to only 10-30%. but the change from paralic to bathyal environments reflects a reordering of basinal architecture as well. (N8. The Malacca Platform and the central horsts were flooded and became the sites of shallow marine limestone deposition. As the central grabens filled and became shallower.5.N9). Alur Siwah) are overlain by middle Baong. but the color of lower Baong shales is dark gray to black. the influx of detrital sand and silt increased from both sides of the basin.The Geology of Indonesia/Sumatra probably continued. Changes in the tectonic regime are evident from reactivation and inversion of the old horst-graben fault systems. and local compressional folding. middle. These basin-fill deposits comprise the Belumai Formation (Fig.3. presumably as a result of increasing distance from sand sources on the Malacca Platform. a major marine transgression occurred. that comprise the Peutu Formation (Kamili et al. and argillaceous limestone) continued during accumulation of Peutu skeletal limestones and reefs on adjacent platforms. Early Foreland Basin Fill: Baong Formation A major transgression accompanied sedimentation of the Peutu/upper Belumai interval. In late early Miocene time. 2. Paleontologic evidence does not unequivocally indicate a gap in age. 1976) and a significant thickness of shale that might fit better in the overlying Baong Formation. 2. a massive buildup of coralline limestone is overlain by lower Baong shale without a noticeable gap in paleoenvironments (inner neritic to middle neritic) or lithology (limestone to calcareous shale to shale). the contact between Peutu or Belumai with the overlying Baong is determined by an abrupt decrease in calcium carbonate. both vertically and laterally.5 m. Some high-standing Peutu buildups (Arun. Belumai-equivalent deposits consist of dark gray to black marine mudstones and calcareous shales that are difficult to distinguish from the overlying Baong. The Baong varies in age from Lower to Middle Miocene (N8-N16. but turbidite sands also occur in areas along the basin margins.y. and lower units. Source areas are the same for the Belumai and for the older and less calcareous Bruksah clastics. Sandstones and siltstones are generally quartz-rich and tend to be very calcareous (up to 40-50% carbonate). probably resulting from continued subsidence coupled with a eustatic sea level rise. 2. Mudrocks dominate the lower Baong section. Keutapang sandstones are classified as lithic arenites. with sedimentary and metamorphic lithic clasts. overlying Upper Baong sediments consist of clay-rich mudrocks. Coally plant fragments are common. Sandstone grains vary in size from very fine grained sand to pebble conglomerates. and shallower water fauna. lithic clasts include common to abundant volcanic rock fragments.10) consists largely of coarse clastics. Except for local reworked sands above the unconformity. Sandstone isopachs indicate derivation from Barisan source terrain to the south and southwest. Transpressional tectonics continued. but. Late Foreland Basin: Keutapang and Younger Formations The Late Foreland phase completed initial tilling of the basin. which contains more volcanic detritus. Sedimentation occurred as a series of deltaic pulses. Although studied far less than the subjacent Keutapang. Paleoenvironments deepened again to bathyal water depths. The Seureula consists of bluish gray shale and subordinate fine to medium and locally coarse or conglomeratic sandstones. Keutapang sands are interpreted to be deposits of sand-rich delta systems that prograded northeastward. and varies in thickness from about 700-900 m. Uplift of the Barisan provided sufficient detritus to extend the shelf platform in this manner and fill the onshore part of the North Sumatra Basin. and shales are described as rarely tuffaceous (Bennett and others. rounded hills. These accumulated in generally mud rich delta margin and deltaic environments. This contrasts sharply with the overlying Keutapang. resulting in a widely recognized seismic unconformity of N-14 age. and this boundary appears to be both gradational and diachronous. The Keutapang varies in thickness from about 700-1500 m in East Aceh. 2. unlike the Baong.6. but the interval can still be recognized from increased silt and fine sand content of the mudrocks.10). Planktonic foraminifera for this unit span zones N15/16 to N19. The unit is dominated by beds of resistant sandstone. Middle Baong sedimentation ended with a period of tectonic quiescence. and paleoenvironments vary from alluvial to paralic. Middle Baong sands do not reach the central basin area. 2. The upper contact of the Keutapang is poorly defined in both outcrop and subsurface. Unconformably overlying the Julu Rayeu are geomorphically distinct but poorly exposed Pleistocene terrace deposits of gravel. Volcanic clasts are abundant in the sandstones. Thin lignites commonly occur in shales interbedded with the sandstones. It is early Pliocene in age (N18. This precipitous terrain stands out in sharp contrast to gently rolling topography of recessive Baong shales. These comprise the Idi Formation. Actual lithologic contacts are gradational and much less obvious. The uppermost Baong thus consists largely of basin-filling prodelta and slope deposits associated with progradation of Keutapang deltas (Fig. containing gastropod and pelecypod fragments and foraminifera. Both sands and shales are fossiliferous and contain coaly plant fragments. Sandstones are commonly glauconitic and/or fossiliferous. The Late Pliocene Julu Rayeu Formation (Fig.N19). which crop out as a band of ridges with up to 200 m of relief. and highly bioturbated. described by Bennett and others (1981) as 50 m of semi-consolidated gravel. 1981).1. thin limestones.3. but sediment influx kept pace with basin subsicience. The unit consists of gray to gray brown or bluish gray sandstones interbedded with subordinate shales and rare. sand and mud. followed by gradual shoaling upward topped by paralic sands of the overlying Keutapang Formation. Sands attributed to both eastern and western sources are similar in composition. forming low. Pre-existing structural highs were eroded.1. The Keutapang Formation marks the first major event of deltaic sedimentation.11).The Geology of Indonesia/Sumatra paleoenvironments from bathyal to outer or middle neritic water depths. 8 . blocky. 2. and surface relief appears to have guided early mapping. Overlying sediments of the Seurula Formation contain more shale and weather recessively. sands and mudstone. 2. the Seureula consists of volcanic-rich detritus apparently derived from Barisan sources to the west. and interbedded shales are gray. which were likely driven by changes in relative sea level and sediment supply. Paralic to alluvial environments were thus maintained from Late Miocene onward. brown color (in contrast to dark gray to black in the lower Baong). They vary from lithic arenites to lithic arkoses. or Late Miocene to Early Pliocene (Fig. al.. Paleogene east-west extensional deformation affected the Pre-Neogene section. 1974). producing large NS trending graben filled with Pematang Formation. development of a deep anoxic lake with slow deposition of the Brown Shale Formation associated with lateral facies variation such as alluvial fan along graben and lake margins. Neogene structures within the basin are dominantly WNW to NW trending folds and high angle reverse faults and NNW to N trending right lateral strike-slip faults. and Eubank and Makki ( 1981). 2. the Plio-Pleistocene orogeny.. The Central Sumatra Basin was formed during the Early Tertiary (Eocene-Oligocene) as a series of half grabens arid horst blocks developed in response to an East-West direction of extensional regime (Eubank & Makki. The South Sumatra Basin was formed during east-west extension at the end of the pre-Tertiary to the beginning of Tertiary times (Daly et d. The structural features present in the basin are the result of the three main tectonic events (de Coster.3. Differential compaction and recurrent movement of this earlier system has a tectonic overprint on the Neogene structural system. 1987). 1982). These recent sediments include lobate to arcuate deltas of the Jambo Aye. 1974). 2. 1992). rapid block rotation/subsidence. minor block rotation along pre-existing zone of weakness. Late Cretaceous-Eocene tectonism and Plio-Pleistocene orogeny. The first two events provided the basement configuration including the formation of half grabens. horsts and fault blocks (Adiwidjaja and de Coster. in response to an oblique northward low angle subduction of the Indian Ocean Plate beneath the Asian Plate which gave rise to a transpressional stress system. Subsequently. resulted in formation of the present northwest-southeast structural features and the depression to the northeast (de Coster. Pematang Graben Development can be divided in 3 stages: 1. CENTRAL SUMATRA BASIN For a complete discussion regarding regional setting of the Central Sumatra Basin we refer the readers to papers by Mertosono and Nayoan ( 1974). beginning of the Lower Redbeds deposition. Williams. A divergent transform boundary (non-coupling) between the Sunda Microplate and the Indian Oceanic Plate during Paleogene gave rise to extensional regime and crustal stretching of the western part of the Sunda Land resulting in the formation of Pematang type grabens (Davies. The flat. low-lying coastal plain is heavily populated and supports extensive development of shrimp ponds in coastal marshes and rice cultivation farther inland. 2. et.1985. while older section is thought to be locally derived. This break probably corresponds to an important tectonic pulse at the initial time of the Barisan uplift coincident with a major low-stand event during Middle Miocene. The last event. Minor structures within the Basin are second order NE trending normal faults and NNE trending third order right lateral strike slip faults (Verral. Biostratigraphy and seismic data indicate an important non-depositional break separating the Telisa and Petani Formations. Figure 12 is a summary of the stratigraphy in this basin. 1973. Pregraben Stage. Pulunggono et al. 1984). the lake was fill with coarser clastic deposits of the Upper Red Beds Formation. Peureulak. The following details are after van Gorsel (1988). 3.The Geology of Indonesia/Sumatra Holocene sedimentation has extended the coastal plain 2 to 25 km.3. They are Middle-Mesozoic orogeny. slower rate of subsidence coupled with a major sea-level drop in Upper Oligocene caused worn-down of the graben rim and the lake was dried up. These are all second order structural features in relation to the primary NW trending of the Sumatra Fault Zone. Arakunda. 1974.3. de Coster. 9 . It reflects the reversal of sedimentation from the Malaysian Shield (Lower Miocene) to the Barisan source (since Middle Miocene) and is considered to be N7 to N12 in age. and Tamiang rivers plus intervening chenier plain and tidal estuarine deposits. Lower Miocene marine sediments of Sihapas were mainly derived from the Malacca Land direction. and the pre-Tertiary of the Sunda Shelf to the northeast (de Coster. Structuring in the Central Sumatra Basin is related to the first order NW-SE trending right lateral strike-slip fault (the Sumatra Fault System). 1981).2. Graben Stage.standing Pleistocene terrace. Wongsosantiko ( 1976). A mild tectonic event occurred during Late Oligocene marked by a major unconformity relationship with the overlying Sihapas Group. An earlier.. 1974). north and east of the high. Orogenic activity during the Late Cretaceous-Eocene cut the basin into four sub-basins. Post Graben Stage. SOUTH SUMATRA BASIN The South Sumatra Basin is located to the east of the Barisan mountains and extends into the offshore areas to the northeast and is regarded as a foreland (back-arc) basin bounded by the Barisan mountains to the southwest. and with a gradual transition.Lingsing Formation: Mainly grey-black.3. followed by finer conglomerates and sandstones. which is mainly Late Oligocene in age.The Geology of Indonesia/Sumatra In the South Sumatra Basin the best surface sections are found around the Gumai Mountain anticline. The formation is possibly an equivalent of the widespread "Old Andesites" of Sumatra and Java.3. -. Thickness varies between 75 and 200m. It could be mapped all around the Gumai anticline. overlying marine Middle and Late Eocene beds. 2. with a remarkable quartz-sandstone horizon in the middle.3. On Java these are dated as Oligocene.1. Thick series are found in predominantly N-S trending grabens (Benakat gully. radiolarian-bearing chert and one several tens of meters thick limestone bed rich in the Early Cretaceous foraminifer Orbitolina. or may directly overlie Pre-tertiary rocks.1.Saling Formation: Mainly poorly-bedded volcanic breccias.3. Cross-bedding is common. suggesting an eruption center somewhere to the SE (Musper. with Mesozoic fossils like the coral Lovcenipora and the gastropod Nerinea. The Saling Formation rocks may be a Late Jurassic-Early Cretaceous volcanic island arc association with fringing reefs. which formed in the Oligocene. but conformable under "Talang Akar" and Baturaja sediments is a thick (up to 3350m) series of andesitic volcanic breccias. and interpreted it as a possible Cretaceous subduction complex. STRATIGRAPHY 2. Lematang trough). too. the relations of which are unclear : . Paleogene .3. 2.3. Three members are distinguished. Lava flows are extremely rare.5 to 3m thick conglomerate. Both formations were intruded by Late Cretaceous or Early Tertiary granodiorites. The upper part of the graben-fill series is the fluvial and deltaic Talang Akar Formation. thin-bedded shales or slates.1. Neither the Lemat. Three intercalations of dark gray reefal limestone occur. The Lingsing Formation rocks suggest an Early Cretaceous deep water facies. The basal part with volcanoclastic sediments and lacustrine clays is called Lemat Formation. Pulunggono and Cameron (1983) regarded the Gumai Mountains Pre-Tertiary as part of their Woyla basement terrane. Pre-Baturaja Clastics In the South Sumatra basin a highly variable complex of clastic sediments is found between the Lahat volcanics and the Early Miocene marine Baturaja or Telisa Formations. Cretaceous The complexly folded Pre-Tertiary in the Gumai Mountains contains two different units. Overall grain size is finer than that of the lower member. Fine-grained. Upper Kikim Tuff Member Conformable over. Except for some silicified wood. Lower Kikim Tuff Member: Andesitic tuffs. Thickness is variable (0-800m). lacustrine and fluvial clays and coals and it is questionable whether these are the same as the Lahat volcanics. lahar deposits and lava flows. Whether it is a deep water equivalent of the Saling Formation or whether it is younger or older is not clear. From old to young the following lithostratigraphic units were described: 2.3. 2. and is either a distal facies of the Lahat Formation or. with minor interbeds of green andesitic-basaltic rock. Thickness decreases to the NW from 2500 to 309o. nor the Talang Akar Formation have been properly 10 . tuffs. 3. 1937) Unconformably overlying the Pre-Tertiary. but dark cryptocrystalline volcanic rock fragments were found. probably derived from granitic rock). Quartz-sandstone Member: This member is conformable. a younger unit rich in debris from the Lahat Formation. 1937). from old to young: 1.1. The Lahat Formation underlies the Talang Akar Formation and consists of fluvial or alluvial fan sands. breccias and some lava beds.3. lahar-like deposits. tuffs and basaltic-andesitic lava flows. hydrothermally altered to greenstones. more likely.Lahat Formation (Musper. Thickness in the oilfield areas is up to 800-1000 m. the quartz sandstone.3.2. but without corals.1.. perhaps also somewhat earlier. is another series of greenish andesitic volcanics. The base is a . most material appears to be redeposited volcanics. Lava beds seem to decrease in northern direction. or with a minor unconfformity over the Lower Kikim tuffs. Almost all grains are quartz (polycrystalline. well-bedded tuffs and tuffaceous claystones are interbedded with coarse-grained. fossils are absent and exact age is uncertain. basinal areas the Telisa may include marine lateral equivalents of the upper Talang Akar. shallow marine or turbiditic) the overlying Palembang Formation is reached. In the subsurface. Layers with brown. The Gumai Formation is based on sections along the Gumai Mountains. Three members are distinguished: .The Geology of Indonesia/Sumatra defined and no type sections were designated. where a marine shale facies with a typical.5. The formation is characterized by a thick series of dark grey clays. but since the transition is usually gradual there is a great element of subjectivity in picking the boundary.Lower Palembang Member (Air Benakat Fm.6.3.1. In surface sections around the Gumai Mountains clastic sediments between the Lahat Volcanics and Baturaja Formations are very thin or absent. Telisa Formation (Tobler 1910) / Gumai Formation (Tobler 1906) The thick series of Early (and locally also early Middle) Miocene deep marine shales and marls in South and Central Sumatra was described under two different names. In detail the formation is composed of numerous thin transgressive-regressive para-sequences.1. Sands are usually glauconitic.3. Thickness is about 20-30m.3. Surface outcrops of Baturaja limestone are found at several places around the Gumai Mountains. basal Telisa). carbonaceous material. shallow marine molluscs 11 . It is absent over low areas with thick graben-fill.3. while the Telisa Formation is named after the Telisa river near Surolangun. followed by l m of calcareous sandstone with common shallow marine larger foraminifera (Early Miocene. This is mostly controlled by differential subsidence. Baturaja Formation Limestones found in various places near the base of the Telisa Formation are usually attributed to the Baturaja Formation. overall fining-upward (intertidal). lenticular calcareous nodules up to 2 m in diameter are most common in the upper part of the formation.3.3. through coastal deposits to fluvial beds in the top member. rich foraminifera assemblage is found (Vaginulina zone.) The lower boundary is where significant. marine transgressive sand). from within zone N8 (latest Early Miocene) to zone N10 (within Middle Miocene). No good outcrops of these graben fill sediments are known. Facies show an overall shallowing-upward trend from predominantly shallow marine at the base. Where Baturaja is thick the oldest Telisa beds have zone N6 or N7 faunas (within Early Miocene). Jambi. Where sands become frequent (whether deltaic. Where no Baturaja limestone is developed the basal Telisa beds have zone N4 planktonic foraminifera (earliest Miocene). depending on position in the basin and where the formation boundary is picked. continuous sand beds are found and where the clays have few or no planktonic foraminifera. Maximum thickness is about 200m. Age of this formation is within the early part of the Early Miocene (Upper Te larger foram assemblages. and cross-bedded sandstone (fluvial or alluvial fan deposits). The upper boundary is at the base of the lowest coal beds. but until more carbonaceous material and common rotalid foraminifera. Towards the top the open marine Globigerina marls grade into brownish prodelta clays with fewer planktonics. 2. The top also varies. It is locally developed shallow water facies of the lower Telisa shales and should probably be regarded as a member of this formation. 2. Age of the formation varies. Clays contain glauconite. with at the base a few meters of poorly sorted conglomerates with pebbles of quartz. but is usually less. Musper (1937) called the thin clastic interval below the Baturaja the "Wood-horizon". Thickness of the Telisa Formation is highly variable (from a few hundred to 3000m or more). Baturaja and Lower Palembang formations. These are overlain by 2 m of lenticular-bedded sand and clay. but it probably also reflects the fact that in the thick. Whitish tuffs and brown turbiditic layers composed of andesitic tuffaceous material are locally common. usually with common planktonic foraminifera that may form thin white laminae. Both massive reefal facies and deeper water fine-grained well-bedded limestone with thin marl intercalations are present. because large silicified tree trunks are common at the base of the unit. 2.4. Palembang Formation (Air Benakat. equivalent of planktonic foram zones N5-N6). Muara Enim and Kasai Formation) This formation is the "regressive" stage of the South Sumatra basin fill.1. In the Cawang Saling section it is a transgressive series. volcanic rock and silicified wood. Baturaja limestones are found only on paleohighs and along the basin margin. where numerous intrusions and extrusive products now make up the Bukit Asam. Age is Middle Miocene.) Most surface sediments in the South Sumatra basin are of this unit. The Sumatran trendIines. suggesting most coals are autochtonous.Upper Palembang Member (Kasai Fm. Saling. Age of the member has never been determined accurately. and terrace deposits along the major rivers. that of Mergui and that of the Sunda Area. . the coals are low-grade lignites. Serelo and Djelapang groups of hills. but due to its soft rocks exposures tend to be poor and far apart.1. but also between the Lematang and Enim rivers.4. The upper part of the member (300-500m thick) still has common quartz-rich pumice tuffs. in some areas. Outcrops are poor due to softness of the beds.3. possibly ranging up into the Late Miocene. In this area. Coals are absent. This range skirts the southern end of the Andaman Basin. suggesting uplift and significant erosion of the Gurnai Mountains within this period. 1933). The foothills. Much of the upper Palembang may be regarded as synorogenic deposits. that are not affected by the"Plio-Pleistocene" folding. In most of the basin. Especially the upper part of the member clear bipyramidal quartz and light-colored acid tuffs are common. The roofs of coalbeds may be silicified. the lignites were altered to high-grade coal. no elements of mangrove swamp vegetation have been reported (Musper. suggesting subsidence rates played an important role in coal deposition and preservation. For the first time erosional products from older formations (Telisa. 2. Fossils are rare. clay. Depositional facies are fluvial and alluvial fan with frequent ashfalls (non-andesitic:). formed by truncated Tertiary anticlines skirt.The Geology of Indonesia/Sumatra and foraminifera.) are found. and a lower group (Merapi seam. coal beds become very thin or are absent. Thickness in the area around Muara Enim and Lahat is around 500-700m. 12 2. At their base root horizons and in situ true trunks may be found. Sands may be glauconitic and contain volcanic debris. 1650 km long and about 100 km wide. BARISAN MOUNTAIN RANGE (after Nishimura. Where studied in detail. but also contains common cross-bedded coarse sandstone and pumice-rich conglomerate beds. the central Pre-Tertiary mountains of northern Aceh. The basal sands may either be coastal facies (beach. The lower 250-350m are characterized by common fine-grained. like Bukit Asam. with shallow marine or bay clays at the base. especially where overlain by tuff beds (volcanic ash falls). In this area coal occur in three groups: an upper (with 6-7 seams).7. the andesitic tuffs and lahars in the Pasumah region derived from Barisan volcanoes like Dempo. Quaternary andesitic volcanism was particularly abundant in the Barisan Mountains. developed mainly in synclines. paralleling those of the Malayan Peninsula. rhyolitic tephra (acid air-transported volcanics). 1937). the formation consists of stacked shallowing-upward parasequences. begin with the N-S trending van Daalen Range which meets the main body of the Barisan Range at right angles. etc. and can usually be distinguished from Palembang beds by the presence of dark-coloured andesitic and basaltic volcanic rocks.Middle Palembang Member (Muara Enim Fm. but must be within the Late Miocene . Tree species identified from the coal point to upland forest conditions. Lahat.983 m) is a central knot from which the van Daalen . Conglomeratic sandstones and plant material are rare. deeper water turbidites. 1980) 2. Here occurs an intersection of Pre-Tertiary trendlines which belong to two different centres of orogenic activity. about 15% of which is coal. Most likely age is Late Pliocene to Pleistocene. coal) at the top.4. 8-l0 m). Quaternary The youngest beds in the region.1.Early Pliocene. were grouped under the term Quaternary.) Top and bottom of this unit are defined by the upper and lower occurrence of laterally continuous coal beds. They may unconformably overlie Palembang or older formations. deltaic) or. . ACEH AREA The most prominent topographic element of the island is the Barisan Range. yellow-white pumice tuffs (often with clear bipyramidal quartz crystals and black hexagonal biotite flakes and tuffaceous sandstones. Other rocks included: in the Quaternary are the "liparites" (ignimbrites) filling valleys in the Pasumah region south of the Gumai Mountains. tidal flat. only some fresh-water molluscs and plant fragments have been reported (Musper 1933. a middle.3.e. i. Where the member is thin. typically l0m-30m thick. and shoreline and delta plain facies (sand. Thickness of the formation is ranging from 100 m to 1000 m. The Puncak Lemby (2. the stratigraphy and tectonic structure of the Barisan Range corresponds more with to the northern part of the Sunda mountain system more than to that of the Sumatran section. Only around young andesite intrusions. 000 m. SEMANGKO ZONE (SOUTH SUMATRA) One feature which characterizes the Barisan geanticline along its entire length is a median depression zone on its top. The rivers descending eastward are much longer. This Semangko zone begins in the Semangko Bay of South Sumatra and can be traced from there to the junction of the Aceh Valley with Banda Aceh at the northern end of the island.4. called the Bengkulu Block. or Bengkulu Block. Total view of the main structural Trendlines of Sumatra Based upon the above descriptions. The latter are exemplified by the Batang Gadis after it has left the Batang Angkola trough of the Semangko Zone. south of Padang. southeastward it disappears under the Tertiary deposits of the east Sumatra basin. south of Padang. 2. which are situated in the middle of the Tertiary basin of east Sumatra. In the northern half of the island no distinction can be made between the Schiefer-Barisan and the High-Barisan. This is the High-Barisan. which is about 2. the main structural trendlines of Sumatra may be outlined as follows: The west flank of the Barisan Range.000 m high (Sibuatan. traversed by an arcuate section of the Semangko-rift zone. The southern end of the Barisan in the Lampung district is nearly 150 km wide. forms the High-Barisan. is the most complicated portion of the range. and then flowing through a wide alluvial lowlands until they empty into the Sunda Shelf sea and the Strait of Bangka. is similar to the southern mountains of Jawa. 2. east of Lake Singkarak. The Batak tumor part of the Barisan Range is a great dome. which separates the Tigapuluh Mts. and the east flank. The system is narrowest at its transition into the ”Batak Timor” near Padangsidempuan from which point it gradually widens south. It is cut into a number of longitudinal block-mountains both in the east flank and in the west flank. Here one may distinguish between the west flank. In this “Batak Tumor”. The schiefer Barisan can be traced along the entire length of the island. The escarpment along the Semangko Zone general forms the divide between the east and the west coast of Sumatra. TOBA AREA (NORTH SUMATRA) Between the Wampu and the Barumun Rivers. 2. They descend towards the east Sumatran lowlands. capped by a series of young volcanic cones. This culmination has been called by van Bemmelen the “Batak Tumor”. from the main Barisan System. which tilted toward the Indian Ocean. Gumai. The west coast rivers are short. called the Sub-Barisan Depression.and Tambesi-Rawas Mts.The Geology of Indonesia/Sumatra Range extends northward. which belong to the Schiefer Barisan. The High-Barisan is particularly well developed in the southern half. south of Blangkedjeren. because Pre-Tertiary rocks are exposed over the entire area. The northern part of the Barisan range. a NW-SE trend of the Barisan System prevails. and the Wilhelmina Range southeastward. It is into a number of block mountain structures. of the Batak Tumor.3. the top part of the Lampung Block. capped by more or less isolated young volcanoes. lies the great Toba area with Lake Toba. The Pre-Tertiary besement complex of the latter reappears in the culminations of the Garba. having a steep grade towards Indian Ocean..4. In southern Aceh. where it wedges out between the Lisun-Kwantan-Lalo Range and the Schiefer Barisan.4. flowing through an erosional plain. disappearing under a 50 km wide basin. the Central Gajo Range westward. which they attain altitudes of over 2. called the Semangko zone named after a prototypical section in the Semangko valley of south Sumatra. or Sekampung Block. which truncates the anticlines of the Neogene Basin. This tilted block. while the edge of the Bengkulu Block. North of Lake Ranau the range narrows to less than 100 km because the Sekampung Block disappears under the Neogene South Sumatra basin and the Lampong Block becomes covered by Neogene strata. 2. These block mountain ranges are highest on the southwestern side of the Barisan System. the Barisan Range display a typical oblong culmination (NW-SE acis of 275 km length and 150 km width). The Leuser Block and the western 13 . extending west from the Semangko Zone.. Some sections have been silled and capped by young volcanoes. The Lisun-Kwantan-Lalo Range plunges southeastward. is rather regularly formed in the southern half of the range.2.457 m). This southern part of the west flank was formed by a long crustal block.eastward to 175 km in the Padang section. The fore-Barisan begins in the Ombilin area. via some anticlinal ridges of Tertiary formations to the northwestern corner of the Tigapuluh Mts. CENTRAL SUMATRA The Barisan system of central Sumatra consists of a number of NW-SE trending block mountains. while the elevated northeastern edge breaks down along the Semangko Zone.4. Between Padang and Padangsidimpuan the structure of the Barisan Range is less distinct. The Pre-Tertiary core of the Suligi-Lipat Kain Range can be traced. The basin is located within the Barisan Mountain range of West and Central Sumatra. These are. The Barisan Range forms a section of the volcanic inner arc of the Sunda Mountain System. This compression was introduced by the subduction of the Indian.south compression which created a graben dog leg or pull apart style basin in the Ombilin and Payakumbuh region. TECTONIC SETTING The Ombilin Basin is a northwest-southeast trending. wrench and extensional tectonism. Despite the relatively small size of the basin. Malacca. The Pre-Tertiary basement complex of the Sunda area crops out at some places in the alluvial marshes along the east coast. the backdeep forms sea basins such as the Andaman Basin of the Mergui section in north Sumatra.5. thick sequences of stacked braided stream deposits.600 meters of Tertiary sediments. Singgalang. is filled with Neogene sediments which were folded in Plio-Pleistocene time.5.2. the backdeep of the Sunda Mountain System now forms a lowland in the Sumatra section. sedimentary basin. Jambi and Palembang depressions are examples of this type of basin development. Major river drainage of the Ombilin Basin is provided by the Ombilin. Merapi. 1985). elongate. (25 x 60 km. Kastowo & Silitonga (1975).5. and Maninjau volcanoes.5. During or after the main phase of folding. and summarized by Fletcher & Yarmanto (1993). while in other sections. These anticlines have eroded to their basement levels during their folding so that a primary peneplain of subaerial erosion truncates the Tertiary anticlines. with less sedimentation in Neogene time. Physiographically. Up to 4. The Ombilin Basin has a complex history of reverse. 2. 2. ranging in age from Eocene to early middle Miocene is preserved in the Ombilin Basin (Koning. STRATIGRAPHY Many authors proposed different stratigraphic nomenclatures of this basin. It is separated from the old Sunda landmass by the Sumatra back-arc basins This downwrap of the Pre-Tertiary basement complex a backdeep. The Ombilin Basin is believed to be similar in evolution to these grabens and portray an early example of one of these features. 1984). The Ombilin Basin is largely floored by . 1500 sq km.The Geology of Indonesia/Sumatra mountains occupy a position in the South of the Bengkulu Block. SUMATRA INTRA-ARC BASIN In terms of overall geomorphology of Sumatra. Subduction started in the early middle Eocene (Daly 1990) and created an extensional tectonic regime which formed numerous grabens in a back arc extensional tectonic setting. the Ombilin Basin is a median graben which is situated between the East and West Barisan mountain range (Fig.1. Aman. a distance of approximately 120 km. In other places the basement complex is exposed in the cores of Tertiary anticlines. and marginal alluvial debris fans. 14 2. in fact. a dome was elevated in the center of this backdeep which now forms the Tigapuluh Mts. Initial basin configuration and quantity of sediment in the Ombilin Basin is due to a north. former islands in the Sunda Shelf Sea which have been connected with the main land of Sumatra by depositions in subrecent time. The Bengkalis trough. The following stratigraphic description is after Kosoemadinata & Matasak (1981). Sinamar and Palangki Rivers along with their many tributaries. The area is unique since it is one of the few intermontane basins in Indonesia which exposes early to middle Tertiary lacustrine sediments. This island chain is part of the non-volcanic outer arc of the Sunda Mountain System. However.1). Merapi and Malintang volcanoes reach elevations of 2891 and 2262 meters respectively. the basin fill is very thick.1. Figure 2). 2. and East Malaya microplates were joined together to form the Sunda Craton. The presence of economically important coal bearing strata in the Sawahlunto Formation has generated much geologic interest in the area. Further accretion followed during late Mesozoic times involving the Woyla Terrains (Pulunggono & Cameron.Australian plate beneath the Sunda Craton (Figure 4). in the northern portion of the Ombilin Basin. Towards the northern end of the basin the median graben is covered by Quaternary and recent volcanic products of the Malintang. West of the Barisan Range stretches the interdeep of the Sunda Mountain System which forms the sea basin between Sumatra and the island festoon to the west. Mean elevation of the central basin is approximately 400 meters. PRE-TERTIARY STRATIGRAPHY The pre-Tertiary framework of Sumatra consists of a mosaic of continental and oceanic microplates accreted in the late Triassic when the Mergui. This median graben extends from south of Solok and trends northwest past Payakumbuh. 2. Kiri.2. Interbedded with mudstones are off-white to white. moderately calcareous mudstones with common carbonaceous material. West of the Ombilin Basin fenesters of the Woyla oceanic accretionary terrain sporadically outcrop between Quaternary volcanic deposits. coals up to 18 meters thick were deposited in interlobe. It is a fining upward sequence deposited in a flood plain/mire type depositional environment (Whateley and Jordan.2. samples have been dated from Permian to Quaternary (Figure 8). The areal extent of these formations increased during this phase of deposition and reached its maximum during late Oligocene to early Miocene (Situmorang. Concurrently. The sequence consists predominantly of limestones from the Permian Silungkang and Triassic Tuhur Formations. the surrounding margins of the basin were the site of coarse grained.The Geology of Indonesia/Sumatra meta-volcanics and meta-sediments of the Mergui accretionary terrain. calcareous siltstones. silty to slightly sandy. 1985). Sands commonly have an erosional base and are interbedded with finer grained. Thickness of Ombilin Formation varies dramatically in different portions of the basin. Thick sequences of fine to coarse grained channel sandstones are commonly stacked several tens up to 100’s of meters thick (Plate 3). calcareous. However. well sorted sandstones. 1989). These consist of limestones and marbles from the Carboniferous Kuantan Formation and meta-volcanics from the Permian Silungkang Formation. claystones and fine to coarse-grained sandstones commonly representing alluvial channel fills (DeSmet. 1991). In the northern arm of the 15 . granodiorites. 1987). outcrops extensively along the western margins of the Ombilin. alluvial fan sedimentation. and quartz porphyries of various ages. Sawahlunto Formation is late Eocene to early Oligocene in age and unconformably overlies Sangkarewang. The Ombilin Formation consists of grey. NEOGENE Conformably overlying the braided stream sediments of late Oligocene age are Ombilin Formation calcareous shales and marls representing a major marine incursion which inundated the Ombilin Basin area as-well-as much of Sumatra. clays. 1993). 2. Brani and basement. These deposits consist of interbedded siltstones. carbonaceous. coal. Locally. The base of the sequence consists of grey. Increased tectonic coupling between the Sunda Craton and Indian-Australian plate in the late Miocene-Pliocene marked the culmination of the Barisan orogeny creating the complex wrench tectonic framework we presently observe in West Sumatra. very fine to fine grained. organic rich lacustrine sediments of Sangakarewang Formation was deposited in the central portion of the basin. They are predominantly reddish brown to purple with mottling indicating the presence of rootlets or burrows. the basin became dominated by parasequence sets of continental sediments deposited in a flood plain or meandering river depositional environment of Sawahtambang Formation. Style of sedimentation indicates these deposits are fanglomerates and debris flows are a result of rapid uplift along the flanks of newly formed grabens (Whateley & Jordan.5. dominated by braided stream deposits of Sawahtambang Formation. This sandstone rich basal sequence is overlain by ripple laminated.metric dating indicates an Upper Jurassic to Cretaceous age for most outcrops (Koning.2. These sediments rapidly thinned towards the basin margins where they coalesced with alluvial fan and debris flow sediments which contributed conglomeratic and breccia material from up-thrown fault blocks where basement was exposed. quartz diorites. Pre-Tertiary sedimentary rocks of the Mergui and Woyla accretionary terrains were intruded by granites. ”mire-type” depositional environments along the western margin of the basin (Whateley & Jordan. and coals. fine to medium grained. These fan sediments were sourced from up thrown fault blocks around the margin of the basin (Figure 11). glauconitic sandstones and soft. Radio. The Rasau Member is characterized by interbedded coarse grained sandstones and argillaceous siltstones During Oligocene times. si1tstones and shales. and organic rich shales. It is included in Koesoemadinata and Matasak’s classification as a basal member of the Sawahtambang Formation and is dated as lower to late early Oligocene. TERTIARY STRATIGRAPHY PALEOGENE The coarse grained Brani Formation consists of fanglomerates and debris flow sediments deposited along active basin bounding faults from late Paleogene to middle Eocene (Fletcher & Yarmanto. 1987). 1991). In the late Oligocene the Ombilin Basin became increasingly fluvial. off-white. The Rasau Member of the Sawahtambang Formation is reported to be locally developed along the western portion of the Ombilin Basin and represents a transition between the meandering stream sediments of the Sawahlunto Formation and braided stream sediments of the Sawahtambang Formation. During the early evolution of the Ombilin Basin in Eocene times. The entire sequence is capped by a series of interbedded grey mudstones. This formation is the most economically important unit in the area due to its large coal reserves. . Mechanically. McCaffrey. This NW-trending fault zone is a major. 1993). The slip rate of the Sumatra Fault System should range between 30 and 50 mm/yr (Jarrard. Composition of the deposits varies but generally consists of andesite to basalt lava flows. DeMets et al. However. from north to south. to explain the along-strike variation in slip rate south of the Batee Fault. 1991) to 28 mm/yr (Shieh et al.. This high slip rate on the Sumatra Fault System appears high when compared to the relatively moderate activity of the crustal seismicity and the slip rate estimated in southern Sumatra (Pramudmijoyo. 1991. from 6+4 mm/yr in southern Sumatra (at about 5oS) (Bellier et al. this convergence obliquity has to be accomodated both by subduction (aconvegence component normal to the trench) and strike-slip deformation (a convergence component parallel to the trench). High resolution SPOT image analyses of the Great Sumatran Fault trace have confirmed its right lateral strike-slip style. 1993 in Fletcher & Yarmanto. Off Java. Pramumijoyo et al. Precise offset measurements performed along the Sumatra Fault System have shown that its dextral slip rate increases to the northwest (Bellier et al. Assuming that the Great Sumatran Fault zone is accomodating all the trench-parallel component of the convergence between the Indo-Australian and Eurasian plates. 1972. The northwest-southeast volcanic trend is easily explained by formation along a weaker crustal zones created by strike slip rnovement along the Sumatra Fault Zone. Provenance for the Ranau Formation is from a combination of the Maninjau. Conversely.The Geology of Indonesia/Sumatra basin seismic interpretation show up to 4000 meter of marine shales have accumulated (Per.. the northern Sumatra Fault slip rate is still too low to accommodate the whole trench-parallel compnenet of the convergence. and Singallang volcanoes.. slip transfer to the Mentawai Fault Zone (Diament et al. Vard Nelson. that is. 1991). Jarrard. . in Sinamar-1 well only 692 meters were encountered. 1986. right-lateral strike-slip fault segments that follows the Sumatra magmatic arc and parallesl the trench. 1993). This suggests that a combination of two models should accommodate the 30 mm/yr slip rate difference between northern and southern Sumatra. The slip rate of the Great Sumatran Fault has been indirectly estimated. 1983. Merapi. the east-west trend is more difficult to explain and is postulated to be a response to crustal weakening around releasing bends between the Ombilin Basin and Payakumbuh Subbasin. Volcanic activity in the area reached its peak during Late Pleistocene-Holocene time and the volcanic products are grouped as Ranau Formation. 1992. Analysis of slip vectors deducted from earthquake focal mechanisms suggests an approximately N-tending convergence between these two plates (Jarrard. 1986b). Malintang. 16 2. because the azimuth of the Sumatra Trench. West of the Sunda Strati. is N140oE... and directly calculated from measurements of offsets along its trace. REGIONAL STRUCTURES Along the Java-Sumatran trench system the Indo-Australian plate is subducting under the Eruasian plate with a convergence rate of 75 mm/yr (Minster and Jorda.. where the average trench azimuth is approximately N100oE. lahar deposits and tuffs. Beck. the convegenceis oblique. These images show right lateral offsets of geomorphologic surface features such as streams. 1991). The volcanoes are situated both along and at right angles to the Sumatra Fault zone. Malod et al. The strike-slip deformation is interpreted as being located along the Great Sumatran Fault System (Fitch. from global plate motions and the opening rate of nearby basins. 1978. the convegence is nearly normal to the Java Trench and is essentially accomodated by the subduction process. However. 1991). 1650-km-long structure of. calderas and lineaments. comm. However. 1991) in norther Sumatra near Lake Toba (at about 2o10’N). 1993) along the Batee Fault link and northwestward stretching of the fore-arc platelet (McCaffrey. 1991. 1990). 1986). from the Andaman Sea back-arc basin to the Sunda Strait extensional fault aone.6. The Geology of Indonesia/Sumatra 17 2.7. SOURCES Bona Situmorang: research on North Sumatra Danny Hilman: PhD on Sumatra Fault . wikibooks.lifeguard. Licenses and Contributors Image:Sumatra map. Herman Darman Image:Nias cross.php?title=File:Sumatra_map.wikibooks.Article Sources and Contributors 18 Article Sources and Contributors The Geology of Indonesia/Sumatra Source: http://en.org/w/index.jpg License: Public Domain Contributors: Adrignola.jpg Source: http://en. Herman Darman License Creative Commons Attribution-Share Alike 3. 0/ .org/w/index. Herman Darman. Mike. 4 anonymous edits Image Sources.org/w/index.php?title=File:Nias_cross.jpg License: Public Domain Contributors: Adrignola.php?oldid=1682932 Contributors: Adrignola.0 Unported http:/ / creativecommons.jpg Source: http://en. org/ licenses/ by-sa/ 3.wikibooks.