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 subsequent uplift and erosion from this former depositional area, thus with similar lithologies to the fore-and backarc basins.
2.1. SUNDA OUTER-ARC RIDGE
The Sunda non-volcanic outer-arc ridge marks the western margin of the Sunda Forearc Basin of West Sumatra. This chain of islands and sea-floor rises, between 100 and 150 km off the coast of West Sumatra, forms a structurally controlled topographic ridge nearly 200 km wide (Karig et al., 1979), that extends from the Andaman Sea to the southeast of Java. Nias, Simeulue, and Banyak Island lithologies represent the stratigraphy of the Sunda ourter-arc ridge in genereal. The geology of the Sunda outer-arc ridge is represented by Nias and Simeulue Island in this chapter.
Nias Island is located approximately 125 km off the west coast of Sumatra (Fig. 2.1) and it has been frequently cited as a classic model of an accretionary complex (Fig. 2.3). Nias lithologies were divided into two principal units, the Oyo complex and the Nias Beds (Fig. 2.4). The contact between the two units has not been observed in the field.
Fig. 2.7. Hypothetical shallow structure across the Sunda arc in the Nias area. Tectonic positions of subsequent profiles are indicated beneath the section (Karig et al., 1979)
220.127.116.11. OYO COMPLEX MELANGE
The Oyo Complex is described by Moore and Karig (1980) as a tectonic melange. On Nias, outcrops of Oyo Complex are seen as isolated blocks and boulders in river sections, along road sections and coastal exposures. The Complex is composed of sedimentary blocks, including conglomerates, sandstones and siltstones, with subordinate mafic plutonic rocks, pillow basalts and cherts (Harbury et. al., 1990). Sandstone blocks form the dominant clast type in the SW part of the island, 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. 5). Texturally, the sediment boulders are sub to mature clastic with mainly subangular to rounded and well sorted sediments, and are either grain supported or matrix supported. In the area where the melange is present, landslips are common to occur and the fresh matrix of the Oyo Complex can be observed. Good outcrop of melange is exposed in central Nias (Moi River) and SW Nias. The matrix forms a typical scaly clay, with a high density of curved, polished shear planes. The age of the Oyo Complex remains unresolved by paIeontological analysis.
18.104.22.168. NIAS BEDS
Overlying the Oyo Complex, with probable unconformable contact, 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 2.4 & 2.5). It consists of coarse to fine sandstone, conglomerate, mudstone, shale and limestone. The age of the Nias Beds has been interpreted by previous authors as Early Miocene-Pliocene. On the contrary, Situmorang & Yulihanto (1992) fieldwork indicates that the lower part of the Nias Beds is Upper Oligocene in age.
Simeulue lies slightly off-strike and to the northwest of Nias (Fig. 2.1). This island shares a broadly comparable geology with Nias, of melange overlain by interbedded sandstone and siltstone sequences, with parts of the succession dominated by bioclastic limestones. Although lithological variations do exist, the most notable differences between the two islands is one of structural style.
22.214.171.124. SIBAU GABBRO GROUP
The oldest rocks exposed on the island are represented by the Sibau Gabbro Group (Situmorang et al. 1987; Fig. 2.4)). The Sibau Gabbro Group is composed mainly of meta-igneous lithologies with predominantly transitional contacts. The ophiolite correlates closely with a partially defined gravity high in this area indicating that the basic igneous rocks form a major body, extending to a depth of several kilometres (J. Milsom, pers. commun. 1990). Lithologies identified within the group include gabbros, meta-dolerite and meta-volcanics, all with abundant chlorite and pumpellyite suggesting that these rocks are all low-grade metamorphics. Rock dating suggest that the Sibau Gabbro Group and Baru Melange Formation were metamorphosed between Late Eocene and Early Oligocene (Harbury & Kallagher, 1991).
126.96.36.199 BARU MELANGE FORMATION
Situmorang et al. (1987) describe the Baru Melange formation as being in structural (thrust) contact with basalts at the top of the Sibau Gabbro Group (Fig. 2.4). Blocks within the melange include fine-grained, micaceous sandstone some of which are fractured; very well-consolidated, weakly sheared, micaceous mudstone, poorly-sorted meta-greywacke; iron-rich meta-dolerite; brecciated meta-basalt; meta- volcanics and calcite-rich, lithic and crystal tuft’s. Blocks within the melange may be in excess of 10 m in diameter. Smaller blocks of 5 – 10 cm in diameter are commonly enclosed within a sticky blue/grey clay matrix containing organic material, or within a cleaved mudstone matrix. No bedding or other sedimentological characteristics, within the blocks of the melange or the clay matrix, can be used to determine the stratigraphical base or top of the Baru Melange Formation. The apparent random distribution of blocks of different lithology within the outcrop area suggests that the melange is unsorted. The thickness of the formation is estimated to be approximately 200 m.
188.8.131.52 AI MANIS LIMESTONE FORMATION
The Ai Manis Limestone Formation forms a NW – SE orientated ridge in the east central part of Simeulue. The formation is approximately 260 – 350 m thick and consists of both biostromal, biohermal (composed of in situ corals) and bioclastic limestones. The major part of the formation consists of bioclastic packstones composed of skeletal bioclasts, large benthic foraminifera and quartz grains. At the base of the formation a coarse-grained sequence (the Pinang Conglomerate Member) is locally observed resting on the Sibau Gabbro Group. A Late Oligocene to Early Pliocene age is suggested for this formation on the basis of palaeontological evidence (Situmorang et al. 1987; Fig. 2.4). The Pinang Conglomerate Member is between 0.5 and 5 m thick and is exposed in the Ai Manis region, where it rests with an angular unconformity on the Sibau Gabbro Group. The conglomerate is poorly-sorted and consists of clasts (mm – 50cm in diameter) of metaigneous rock fragments, including meta-basalt and meta-gabbro, and quartz, in a medium-grained calcarenite matrix. A shallow water benthonic foraminiferal assemblage indicating a Late Oligocene to Early Miocene age was recovered from the conglomerate (Situmorang et al. 1987).
184.108.40.206 DIHIT FORMATION
The Dihit Formation is widely exposed in most parts of Simeulue). The maximum thickness of the formation is estimated from the Dihit section, to be between 800 and 1000 m. The Dihit Formation contains no stratigraphical control on the age of the formation. Base on lithological similarities between the Dihit Formation and the Nias Beds, the formation is considered to be of Late Miocene to Early Pliocene age (Situmorang et al., 1987; Fig. 2.4). The Dihit Formation is composed of grey, predominantly fine-grained sandstone usually interbedded with siltstone or shale. The sandstone is well-sorted, moderately well-consolidated, and unlike the Nias Beds, is micaceous. Bed thickness varies from 4 cm to 15 m in the most massive beds, but more characteristically is between 50 and 100 cm. Parallel laminations are rarely developed in the sandstone, but where present are very fine (<1 mm), and are laterally continuous through the outcrop. Organic matter, where present, occurs as small disseminated lignitic woody fragments and as very fine, disseminated carbonaceous material; calcareous concretions are rarely observed. Sandstone, where interbedded with shale or mudstone, is usually the dominant lithology, with sandstone:shale ratios.between 2:1 and 30:1. The sandstone is fine-grained, well-sorted and predominantly matrix-supported. Muscovite mica is present in all samples (trace – 3%). Massive sandstone, sandstone/siltstone and laminated sandstone/mudstone lithofacies can be recognised from the Dihit Formation sediments.
2.2. SUNDA FORE ARC BASINS
In general, there are two Sunda fore arc basins in west Sumatra, called Sibolga Basin in the nortwest of Sumatra and Bengkulu Basin in the southwest (Fig. 2.1).
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. 2.1). The basin trends northwest-southeast, averages 110 km wide and is approximately 800 km long (Fig. 2.6). The northern end terminates against the northwest extension of the Sumatra Fault System at about latitude 6o30’ N. The southern end of the Sibolga basin was arbitrarily placed in the vicinity of Pini and Batu Islands where a broad, southwest trending low-lying arch separates it from the Bengkulu basin to the southeast. The Sibolga Basin is asymetrical to the southwest with upwards of 6100 m of Neogene sediments adjacent to the outer-arc ridge. 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. In spite of these faults, the majority of the Neogene sedimentary rocks in the basin are undeformed.
220.127.116.11. STRATIGRAPHY 18.104.22.168.1. Pre-Neogene
The pre-Neogene sedimentary section is separated from Neogene rocks by an angular unconformity. Seismic interpretation indicates several hundred meters of folded sedimentary rocks beneath the unconformity in the Meulaboh-Teunom area. Recrystallized belemnites have been reported in cores indicating possible Mesozoic rocks unless the fossils are reworked. The Upper Eocene to Lower Oligocene interval is dominated by mudstone with minor interbeds of shale, siltstone and sandstone. The mudstone is dark grey to black, moderately soft at the top but becoming more indurated with depth. The environment of deposition of this interval is assumed to be shelf. The thickness of the Paleogene interval ranges from less than 30 m up to 350 m. Pre-Neogene dacite tuff-lava was penetrated in the south of the basin with total thickness of 31 m.
22.214.171.124.2. Basal Miocene clastics Directly overlying the Paleogene angular unconformity is a sequence of sandstone, shale, coal and minor limestone. In the Meulaboh area the clastic sequence consists of nearshore marine and non-marine mudstone, sandstone, siltstone and coal. Fossil recovery was poor in this interval leading to a tentative age of Mid Miocene up to Lower Miocene. The mudstone and siltstone are dark in color, calcareous to non-calcareous, firm and commonly interbedded. The sandstone is gray, fine to medium-grained, quartzose with common vari-colored rock grains and slightly calcareous. Coal beds are about 1 m thick and are interbedded with mudstone. 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.
2.2.2. BENGKULU BASIN (after Yulihanto et al, 1996)
The Bengkulu Basin is located in the southeast part of the Sumatra Island covering both onshore and offshore (Fig. 2.l). In general, it is trending NW - SE, parallel to Sumatra Island with about 600 kms length and 150 - 200 kms wide. To the north and northeast lies Barisan Mountain range, while in the south and southwest is bounded by islands or slope break of the Sunda Arc Trench System (Fig. 2.7). The onshore part of the basin can be divided into two sub -basin i.e Pagarjati sub-basin in the nort and Kedurang sub-basin in the south which separated by north - south trending Masmambang High.
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. 2.8). Based on few seismics sections and wells drilled in the Bengkulu offshore area known that the sediment thickness is about 4000m ( 1,.000 feet). Recent onshore gravity work done by Lemigas has indicated two sub-basin with low bouguer anomaly. The detail descriptions of the stratigraphy as follow:
126.96.36.199.1. Pre-Tertiary Rocks
The Pre-Tertiary basement complex is represented by metasediments of Lingsing, Sepitiang and Saling Formations. The Lingsing Formation consists of claystones, siltstones and calcilutite with sandstones and chert intercalation of Late Jurassic - Early Cretaceous age. This Lingsing series has interfinger relationship with Sepitiang and Saling formations. The Sepitiang Formation composes of reef limestones with some calcirudite and calcarenite lenses, and the Saling Formation mostly containing of volcanic materials such as lavas, breccias, and tuffs.
188.8.131.52.2. Tertiary Succession
Surface geological studies exhibits that Tertiary sediments cropout in this onshore area is represented by Hulusimpang, Seblat, Lemau, Simpangaur, and Bintunan Formations (Fig. 2.8). The Hulusimpang Formation is composed of andesitic and basaltic lavas, volcanic breccias and tuff with sandstones intercalation. This formation is well exposed in the northern and eastern margin of the basin, toward the Barisan Mountain. In general, the Hulusimpang Formation is known as Early Oligocene sediments which deposited in fluviatile up to shallow marine. The aproximate thickness is 700 m. The upper part of the Hulusimpang Formation has interfingered with the lower part of the Seblat Formation.
The Seblat Formation composes of sandstones, siltstones, claystones, conglomerates with limestones intercalation. They are mostly shallow - deep marine turbidite sediments of Late 0ligocene - Early Miocene age. The approximate thickness maesured in Tanjung Sakti area is + 298m.
The Middle to Late Miocene stratigraphy is represented by the Lemau Formation. It consists of claystones, calcareous siltstones and sandstones, breccias, and thin coal seams and limestones intercalation, containing abundance of small foram and mollusc which was deposited in shallow marine up to transitional zone. This Formation is we11 exposed in the southern area such as Talang Beringin, Air Keruh, Rantau Panjang, Lubuk Tapi, Batang Rikibesar and Tebing Kekalangan areas. The thickness recorded is+785 m.
The Late Miocene - Pliocene sediment is represented by the Simpangaur Formation. It consists of tuffaceous sandstones, tuff, tuffaceous siltstones, with intercalation of lignites, and also typified by abundance of foram and mollusc fragments.. The total thickness is about 785 m thick.
The youngest stratigraphic unit cropout in this area is the Plio-Pleistocene Bintunan Formation which laying unconformably upon the older units. It composes of sandstones and tuffaceous claystones with pumice clast, conglomerates, breccias, limestones with lignite, and carbon intercalation. Lithologically, compare to the Simpangaur Formation, the Bintunan Formation in general is coarser than Simpangaur and often containing silicified wood and pumice clasts. This formation was deposited in shallow marine and fluvial environment, and it ranges of about 200 m thick.
2.3. SUMATRA BACK ARC BASINS
2.3.1. 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. 2.1). 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. The eastern and southeastern limits of the basin are formed by the Asahan Arch (or Tebingtinggi Platform; Fig. 2.9), which separated it, in Tertiary time from the more extensive basin developed in Central and South Sumatra. At basement level this limit is marked by a north-south flexure, immediately east of Medan. Eastward from the Medan Flexure structural deformation is minimal on the platform. The present southwest structural limit of the basin runs along the Barisan Range, from which it is separated by one or more compressional faults. In the narrow wedge between the Medan Flexure and the front of the Barisan Range, the structural trends at basement level are oriented north-south. In this area, a flexure may be present between Telaga and Basilam, as indicated by the greater depth (3.0 seconds TWT on seismic sections) of the Belumai Formation in the western than in the eastern block (approximately 2.5 seconds TWT). There is no evidence that this flexure also exists at basement level, because the basement configuration become vague wherever the 2-way seismic time interval between top of Belumai and top of basement is less than 0.2 seconds. The possible presence of a flexure could be reflected in the right-lateral movement deduced from the virgation of folds and faults, changing from northwest in the southeastern block, to north where the flexure would be located if present.
The basement (Fig. 2.10) consists of sandstone, limestones or dolomites; they are azoic, generally dense and fracture, with steep dips up to 45o, but they are not metamorphically altered. In some plugs or cores, in the absence of dating, these sediments are not easily recognisable as basement. On the other hand, the high resistivities and velocities generally constitute a good contrast with those of the overlying beds. Thus the top of this section is readily identified with the deepest, continuous seismic marker and conveniently been called “economic basement” (Beicip, 1977).
Tampur Formation (Fig. 2.10) comprises massive, partly biocalcarenites and biocalcilutites. Chert nodules are found in this formation, whereas the dolorites are common. The formation also consists of basal conglomeratic and dolomitic limestones. This formation was deposited in the sublittoral - open marine condition during Late Eocene to Early Oligocene, formed as transgressive formation overlain by both Bruksah and Bampo Formation. Source of basal limestone clasts is still unknown but it assumed widely extended in the subsurface. The Eocene Tampur limestone generally only occurred in Malacca shelf (Rjacudu & Sjahbuddin, 1994). The rest of the Tertiary history of the North Sumatra Basin can be 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 related to these evolutionary phases.
184.108.40.206.3. Early syn-Rift Phase: Bruksah and Bampo Formations
The initial syn-rift phase began in the middle Paleogene (Eocene?) and continued until early Miocene, during which time the N-S and NE-SW trending horsts, grabens and half-grabens developed. This was also a time of major marine transgression (defined as a relative rise in sea level within the basin, probably as a results of back-arc subsidence). Initial graben-fill consisted of continental sandstones and conglomerates. As the grabens deepened and transgression progressed, areas of sand deposition decreased and shale deposition dominated. The later sands accumulated mainly in coastal plain to marine environments. The shales are typically dark grey to black in color and deposited in deep marine environment (bathyal). The sands were mainly derived from the Malacca Platform and the Asahan Arch, augmented by local contributions from the horst blocks, most of which remained exposed during this time. The conglomerates and sandstones deposited during this phase comprise the Bruksah Formation (Fig. 2.10), defined by Cameron and others (1983) from field mapping in the Barisan Mountains. Lithologies include limestone conglomerates and breccias, micaceous quartzose sandstones, and silty mudstones. The Bruksah is overlain the Bampo Formation, a locally thick sequence (500 to perhaps 2400 m) of marine black shale, siltstone, and muddy fine grained. Stratigraphic relationships indicate that the upper part of the Bruksah is at least partly equivalent in age to the Bampo Formation.
220.127.116.11.4. 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. Movement on the N-S trending faults ceased, although back-arc subsidence probably continued. This stage was characterized mainly by forced regression (sea level constant or rising but sediment influx sufficient to cause regression) and basin filling. As the central grabens filled and became shallower, calcareous marine sands and siltstones along with argillaceous and sandy limestones accumulated in the lows while the highs remained at least intermittently exposed. These basin-fill deposits comprise the Belumai Formation (Fig. 2.10).
The Belumai is lithologically diverse, both vertically and laterally. Sandstones and siltstones are generally quartz-rich and tend to be very calcareous (up to 40-50% carbonate). Quartz content decreases southwest to only 10-30%, presumably as a result of increasing distance from sand sources on the Malacca Platform. Source areas are the same for the Belumai and for the older and less calcareous Bruksah clastics. A possible explanation for more calcium carbonate in the Belumai is that this unit accumulated after widespread shallow seas first covered pre-Tertiary topography, much of which consists of carbonates. These oceans might have been nearly saturated (or even super saturated) with calcium carbonate, and they might have maintained equilibrium by dissolving carbonate bedrock while precipitating calcite cements. Rapid sedimentation would protect the calcite in the sandstones from re-dissolution. In some areas, the original calcite has been replaced by dolomite.
In late early Miocene time, a major marine transgression occurred, probably resulting from continued subsidence coupled with a eustatic sea level rise. The Malacca Platform and the central horsts were flooded and became the sites of shallow marine limestone deposition, including reefs, that comprise the Peutu Formation (Kamili et al., 1976) and a significant thickness of shale that might fit better in the overlying Baong Formation. Sedimentation of basinal Belumai deposits (calcareous sand, shale, and argillaceous limestone) continued during accumulation of Peutu skeletal limestones and reefs on adjacent platforms. This results in age equivalence between the Peutu and at least the upper part of the Belumai Formation.
In the deepest parts of the North Sumatra Basin, Belumai-equivalent deposits consist of dark gray to black marine mudstones and calcareous shales that are difficult to distinguish from the overlying Baong. Middle and upper Baong shales are greenish gray to brown in color, but the color of lower Baong shales is dark gray to black. For practical purposes, the contact between Peutu or Belumai with the overlying Baong is determined by an abrupt decrease in calcium carbonate.
The contact between the Baong and underlying Peutu or Belumai varies from gradational to abrupt. Some high-standing Peutu buildups (Arun, South Lho Sukon, Alur Siwah) are overlain by middle Baong, with the lower Baong section (N8-N12) missing. The entire Baong section is preserved in other areas. At Kuala Langsa, for example, 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). Paleontologic evidence does not unequivocally indicate a gap in age, but seismic profiles show onlap of basal Baong reflectors.
18.104.22.168.5. Early Foreland Basin Fill: Baong Formation
A major transgression accompanied sedimentation of the Peutu/upper Belumai interval. The onset of this increase in relative sea level may relate to an eustatic rise at about 15.5 m.y. (N8- N9), but the change from paralic to bathyal environments reflects a reordering of basinal architecture as well. Changes in the tectonic regime are evident from reactivation and inversion of the old horst-graben fault systems, initial development of major transcurrent faulting, and local compressional folding. Regional subsidence accompanying these changes formed a deep, extensive foreland basin. The Baong Formation filled the basin with a thick (750-2500 m) section dominated by monotonous gray or brown mudrocks. The Baong varies in age from Lower to Middle Miocene (N8-N16; Fig. 2.10). Early workers subdivided this formation vertically into upper, middle, and lower units. Distribution of Lower Baong shales indicates widespread bathyal conditions. A flood of Globigerinid foraminifera within the Lower Baong marks a maximum flooding surface at about the N8/N9 faunal zone. Mudrocks dominate the lower Baong section, but turbidite sands also occur in areas along the basin margins. In the Middle Baong (N13-N14), the influx of detrital sand and silt increased from both sides of the basin. This was accompanied by a general shoaling in paleoenvironments from bathyal to outer or middle neritic water depths. Sands attributed to both eastern and western sources are similar in composition. They vary from lithic arenites to lithic arkoses, with sedimentary and metamorphic lithic clasts. This contrasts sharply with the overlying Keutapang, which contains more volcanic detritus. Middle Baong sands do not reach the central basin area, but the interval can still be recognized from increased silt and fine sand content of the mudrocks, brown color (in contrast to dark gray to black in the lower Baong), and shallower water fauna.
Middle Baong sedimentation ended with a period of tectonic quiescence. Pre-existing structural highs were eroded, resulting in a widely recognized seismic unconformity of N-14 age. Except for local reworked sands above the unconformity, overlying Upper Baong sediments consist of clay-rich mudrocks. Paleoenvironments deepened again to bathyal water depths, followed by gradual shoaling upward topped by paralic sands of the overlying Keutapang Formation. The uppermost Baong thus consists largely of basin-filling prodelta and slope deposits associated with progradation of Keutapang deltas (Fig. 2.11).
22.214.171.124.6. Late Foreland Basin: Keutapang and Younger Formations
The Late Foreland phase completed initial tilling of the basin. Transpressional tectonics continued, but sediment influx kept pace with basin subsicience. Paralic to alluvial environments were thus maintained from Late Miocene onward. Sedimentation occurred as a series of deltaic pulses, which were likely driven by changes in relative sea level and sediment supply.
The Keutapang Formation marks the first major event of deltaic sedimentation. The unit is dominated by beds of resistant sandstone, which crop out as a band of ridges with up to 200 m of relief. This precipitous terrain stands out in sharp contrast to gently rolling topography of recessive Baong shales, and surface relief appears to have guided early mapping. Actual lithologic contacts are gradational and much less obvious. The Keutapang varies in thickness from about 700-1500 m in East Aceh. Planktonic foraminifera for this unit span zones N15/16 to N19, or Late Miocene to Early Pliocene (Fig. 2.10). The unit consists of gray to gray brown or bluish gray sandstones interbedded with subordinate shales and rare, thin limestones. Sandstone grains vary in size from very fine grained sand to pebble conglomerates. Sandstones are commonly glauconitic and/or fossiliferous, containing gastropod and pelecypod fragments and foraminifera. Coally plant fragments are common, and interbedded shales are gray, blocky, and highly bioturbated.
Keutapang sandstones are classified as lithic arenites, but, unlike the Baong, lithic clasts include common to abundant volcanic rock fragments. Sandstone isopachs indicate derivation from Barisan source terrain to the south and southwest. Keutapang sands are interpreted to be deposits of sand-rich delta systems that prograded northeastward. 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.
The upper contact of the Keutapang is poorly defined in both outcrop and subsurface, and this boundary appears to be both gradational and diachronous. Overlying sediments of the Seurula Formation contain more shale and weather recessively, forming low, rounded hills. It is early Pliocene in age (N18- N19), and varies in thickness from about 700-900 m.
The Seureula consists of bluish gray shale and subordinate fine to medium and locally coarse or conglomeratic sandstones. Both sands and shales are fossiliferous and contain coaly plant fragments. Volcanic clasts are abundant in the sandstones, and shales are described as rarely tuffaceous (Bennett and others, 1981). Although studied far less than the subjacent Keutapang, the Seureula consists of volcanic-rich detritus apparently derived from Barisan sources to the west. These accumulated in generally mud rich delta margin and deltaic environments. The Late Pliocene Julu Rayeu Formation (Fig. 2.10) consists largely of coarse clastics. Thin lignites commonly occur in shales interbedded with the sandstones, and paleoenvironments vary from alluvial to paralic. Unconformably overlying the Julu Rayeu are geomorphically distinct but poorly exposed Pleistocene terrace deposits of gravel, sand and mud. These comprise the Idi Formation, described by Bennett and others (1981) as 50 m of semi-consolidated gravel, sands and mudstone.
Holocene sedimentation has extended the coastal plain 2 to 25 km. north and east of the high- standing Pleistocene terrace. These recent sediments include lobate to arcuate deltas of the Jambo Aye, Arakunda, Peureulak, and Tamiang rivers plus intervening chenier plain and tidal estuarine deposits. The flat, low-lying coastal plain is heavily populated and supports extensive development of shrimp ponds in coastal marshes and rice cultivation farther inland.
2.3.2. 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), Wongsosantiko ( 1976), and Eubank and Makki ( 1981), Williams, et. al.,1985. Figure 12 is a summary of the stratigraphy in this basin. 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, 1981). 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, 1984). Pematang Graben Development can be divided in 3 stages: 1. Pregraben Stage, minor block rotation along pre-existing zone of weakness, beginning of the Lower Redbeds deposition; 2. Graben Stage, rapid block rotation/subsidence, 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; 3. Post Graben Stage, 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. Subsequently, the lake was fill with coarser clastic deposits of the Upper Red Beds Formation. A mild tectonic event occurred during Late Oligocene marked by a major unconformity relationship with the overlying Sihapas Group. Lower Miocene marine sediments of Sihapas were mainly derived from the Malacca Land direction, while older section is thought to be locally derived. Biostratigraphy and seismic data indicate an important non-depositional break separating the Telisa and Petani Formations. 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. 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. Structuring in the Central Sumatra Basin is related to the first order NW-SE trending right lateral strike-slip fault (the Sumatra Fault System), 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. 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. These are all second order structural features in relation to the primary NW trending of the Sumatra Fault Zone. Minor structures within the Basin are second order NE trending normal faults and NNE trending third order right lateral strike slip faults (Verral, 1982). An earlier, Paleogene east-west extensional deformation affected the Pre-Neogene section, producing large NS trending graben filled with Pematang Formation. Differential compaction and recurrent movement of this earlier system has a tectonic overprint on the Neogene structural system.
2.3.3. 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 the pre-Tertiary of the Sunda Shelf to the northeast (de Coster, 1974). 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., 1987). Orogenic activity during the Late Cretaceous-Eocene cut the basin into four sub-basins. The following details are after van Gorsel (1988).
The structural features present in the basin are the result of the three main tectonic events (de Coster, 1974). They are Middle-Mesozoic orogeny, 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, 1973; de Coster, 1974; Pulunggono et al., 1992). The last event, the Plio-Pleistocene orogeny, resulted in formation of the present northwest-southeast structural features and the depression to the northeast (de Coster, 1974).
In the South Sumatra Basin the best surface sections are found around the Gumai Mountain anticline. From old to young the following lithostratigraphic units were described:
The complexly folded Pre-Tertiary in the Gumai Mountains contains two different units, the relations of which are unclear : - Saling Formation: Mainly poorly-bedded volcanic breccias, tuffs and basaltic-andesitic lava flows, hydrothermally altered to greenstones,. Three intercalations of dark gray reefal limestone occur, 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.
-- Lingsing Formation: Mainly grey-black, thin-bedded shales or slates, with minor interbeds of green andesitic-basaltic rock, radiolarian-bearing chert and one several tens of meters thick limestone bed rich in the Early Cretaceous foraminifer Orbitolina, but without corals. The Lingsing Formation rocks suggest an Early Cretaceous deep water facies. Whether it is a deep water equivalent of the Saling Formation or whether it is younger or older is not clear. Both formations were intruded by Late Cretaceous or Early Tertiary granodiorites. Pulunggono and Cameron (1983) regarded the Gumai Mountains Pre-Tertiary as part of their Woyla basement terrane, and interpreted it as a possible Cretaceous subduction complex.
- Lahat Formation (Musper, 1937)
Unconformably overlying the Pre-Tertiary, but conformable under "Talang Akar" and Baturaja sediments is a thick (up to 3350m) series of andesitic volcanic breccias, tuffs, lahar deposits and lava flows, with a remarkable quartz-sandstone horizon in the middle. Except for some silicified wood, fossils are absent and exact age is uncertain. The formation is possibly an equivalent of the widespread "Old Andesites" of Sumatra and Java. On Java these are dated as Oligocene, overlying marine Middle and Late Eocene beds. Three members are distinguished, from old to young:
1. Lower Kikim Tuff Member: Andesitic tuffs, breccias and some lava beds. Lava beds seem to decrease in northern direction. Thickness is variable (0-800m). 2. Quartz-sandstone Member: This member is conformable, or with a minor unconfformity over the Lower Kikim tuffs, or may directly overlie Pre-tertiary rocks. It could be mapped all around the Gumai anticline. The base is a .5 to 3m thick conglomerate, followed by finer conglomerates and sandstones. Cross-bedding is common. Almost all grains are quartz (polycrystalline; probably derived from granitic rock), but dark cryptocrystalline volcanic rock fragments were found, too. Thickness varies between 75 and 200m.
3. Upper Kikim Tuff Member Conformable over, the quartz sandstone, and with a gradual transition, is another series of greenish andesitic volcanics. Overall grain size is finer than that of the lower member. Fine-grained, well-bedded tuffs and tuffaceous claystones are interbedded with coarse-grained, lahar-like deposits. Lava flows are extremely rare; most material appears to be redeposited volcanics. Thickness decreases to the NW from 2500 to 309o, suggesting an eruption center somewhere to the SE (Musper, 1937). The Lahat Formation underlies the Talang Akar Formation and consists of fluvial or alluvial fan sands, lacustrine and fluvial clays and coals and it is questionable whether these are the same as the Lahat volcanics.
126.96.36.199.3. 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. Thick series are found in predominantly N-S trending grabens (Benakat gully, Lematang trough), which formed in the Oligocene, perhaps also somewhat earlier. The basal part with volcanoclastic sediments and lacustrine clays is called Lemat Formation, and is either a distal facies of the Lahat Formation or, more likely, a younger unit rich in debris from the Lahat Formation. The upper part of the graben-fill series is the fluvial and deltaic Talang Akar Formation, which is mainly Late Oligocene in age. Thickness in the oilfield areas is up to 800-1000 m. Neither the Lemat, nor the Talang Akar Formation have been properly defined and no type sections were designated.
No good outcrops of these graben fill sediments are known. In surface sections around the Gumai Mountains clastic sediments between the Lahat Volcanics and Baturaja Formations are very thin or absent.
Musper (1937) called the thin clastic interval below the Baturaja the "Wood-horizon", because large silicified tree trunks are common at the base of the unit. Thickness is about 20-30m. In the Cawang Saling section it is a transgressive series, with at the base a few meters of poorly sorted conglomerates with pebbles of quartz, volcanic rock and silicified wood, and cross-bedded sandstone (fluvial or alluvial fan deposits). These are overlain by 2 m of lenticular-bedded sand and clay, overall fining-upward (intertidal), followed by l m of calcareous sandstone with common shallow marine larger foraminifera (Early Miocene; marine transgressive sand).
188.8.131.52.4. Baturaja Formation
Limestones found in various places near the base of the Telisa Formation are usually attributed to the Baturaja Formation. It is locally developed shallow water facies of the lower Telisa shales and should probably be regarded as a member of this formation. Surface outcrops of Baturaja limestone are found at several places around the Gumai Mountains. Maximum thickness is about 200m, but is usually less. Both massive reefal facies and deeper water fine-grained well-bedded limestone with thin marl intercalations are present. In the subsurface, Baturaja limestones are found only on paleohighs and along the basin margin. It is absent over low areas with thick graben-fill, where a marine shale facies with a typical, rich foraminifera assemblage is found (Vaginulina zone; basal Telisa). Age of this formation is within the early part of the Early Miocene (Upper Te larger foram assemblages, equivalent of planktonic foram zones N5-N6).
184.108.40.206.5. 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. The Gumai Formation is based on sections along the Gumai Mountains, while the Telisa Formation is named after the Telisa river near Surolangun, Jambi. The formation is characterized by a thick series of dark grey clays, usually with common planktonic foraminifera that may form thin white laminae. Whitish tuffs and brown turbiditic layers composed of andesitic tuffaceous material are locally common. Layers with brown, lenticular calcareous nodules up to 2 m in diameter are most common in the upper part of the formation.
Thickness of the Telisa Formation is highly variable (from a few hundred to 3000m or more). This is mostly controlled by differential subsidence; but it probably also reflects the fact that in the thick, basinal areas the Telisa may include marine lateral equivalents of the upper Talang Akar, Baturaja and Lower Palembang formations.
Towards the top the open marine Globigerina marls grade into brownish prodelta clays with fewer planktonics, but until more carbonaceous material and common rotalid foraminifera. Where sands become frequent (whether deltaic, shallow marine or turbiditic) the overlying Palembang Formation is reached, but since the transition is usually gradual there is a great element of subjectivity in picking the boundary.
Age of the formation varies. Where no Baturaja limestone is developed the basal Telisa beds have zone N4 planktonic foraminifera (earliest Miocene). Where Baturaja is thick the oldest Telisa beds have zone N6 or N7 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 the basin and where the formation boundary is picked.
220.127.116.11.6. Palembang Formation (Air Benakat, Muara Enim and Kasai Formation) This formation is the "regressive" stage of the South Sumatra basin fill. Facies show an overall shallowing-upward trend from predominantly shallow marine at the base; through coastal deposits to fluvial beds in the top member. In detail the formation is composed of numerous thin transgressive-regressive para-sequences. Three members are distinguished:
- Lower Palembang Member (Air Benakat Fm.) The lower boundary is where significant, continuous sand beds are found and where the clays have few or no planktonic foraminifera. The upper boundary is at the base of the lowest coal beds. Sands are usually glauconitic. Clays contain glauconite, carbonaceous material, shallow marine molluscs and foraminifera. The basal sands may either be coastal facies (beach, tidal flat, deltaic) or, in some areas, deeper water turbidites. Thickness of the formation is ranging from 100 m to 1000 m. Outcrops are poor due to softness of the beds. Age is Middle Miocene, possibly ranging up into the Late Miocene.
- Middle Palembang Member (Muara Enim Fm.) Top and bottom of this unit are defined by the upper and lower occurrence of laterally continuous coal beds. Thickness in the area around Muara Enim and Lahat is around 500-700m, about 15% of which is coal. Where the member is thin, coal beds become very thin or are absent; suggesting subsidence rates played an important role in coal deposition and preservation. Where studied in detail, the formation consists of stacked shallowing-upward parasequences, typically l0m-30m thick, with shallow marine or bay clays at the base, and shoreline and delta plain facies (sand, clay, coal) at the top. Sands may be glauconitic and contain volcanic debris. Especially the upper part of the member clear bipyramidal quartz and light-colored acid tuffs 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-grade coal. In this area coal occur in three groups: an upper (with 6-7 seams), a middle, and a lower group (Merapi seam; 8-l0 m). The roofs of coalbeds may be silicified, especially where overlain by tuff beds (volcanic ash falls). At their base root horizons and in situ true trunks may be found, suggesting most coals are autochtonous. Tree species identified from the coal point to upland forest conditions, no elements of mangrove swamp vegetation have been reported (Musper, 1933). Age of the member has never been determined accurately, but must be within the Late Miocene - Early Pliocene.
- Upper Palembang Member (Kasai Fm.) Most surface sediments in the South Sumatra basin are of this unit, but due to its soft rocks exposures tend to be poor and far apart. The lower 250-350m are characterized by common fine-grained, rhyolitic tephra (acid air-transported volcanics), i.e. yellow-white pumice tuffs (often with clear bipyramidal quartz crystals and black hexagonal biotite flakes and tuffaceous sandstones. Coals are absent. Conglomeratic sandstones and plant material are rare. The upper part of the member (300-500m thick) still has common quartz-rich pumice tuffs, but also contains common cross-bedded coarse sandstone and pumice-rich conglomerate beds. For the first time erosional products from older formations (Telisa, Lahat, Saling, etc.) are found, suggesting uplift and significant erosion of the Gurnai Mountains within this period. Much of the upper Palembang may be regarded as synorogenic deposits, developed mainly in synclines. Depositional facies are fluvial and alluvial fan with frequent ashfalls (non-andesitic:). Fossils are rare, only some fresh-water molluscs and plant fragments have been reported (Musper 1933, 1937). Most likely age is Late Pliocene to Pleistocene.
18.104.22.168.7. Quaternary The youngest beds in the region, that are not affected by the"Plio-Pleistocene" folding, were grouped under the term Quaternary. They may unconformably overlie Palembang or older formations, and can usually be distinguished from Palembang beds by the presence of dark-coloured andesitic and basaltic volcanic rocks. Quaternary andesitic volcanism was particularly abundant in the Barisan Mountains, but also between the Lematang and Enim rivers, where numerous intrusions and extrusive products now make up the Bukit Asam, Serelo and Djelapang groups of hills. Other rocks included: in the Quaternary are the "liparites" (ignimbrites) filling valleys in the Pasumah region south of the Gumai Mountains, the andesitic tuffs and lahars in the Pasumah region derived from Barisan volcanoes like Dempo, and terrace deposits along the major rivers.
2.4. BARISAN MOUNTAIN RANGE (after Nishimura, 1980)
2.4.1. ACEH AREA The most prominent topographic element of the island is the Barisan Range, 1650 km long and about 100 km wide. This range skirts the southern end of the Andaman Basin. In this area, 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. The Sumatran trendIines, paralleling those of the Malayan Peninsula, begin with the N-S trending van Daalen Range which meets the main body of the Barisan Range at right angles. Here occurs an intersection of Pre-Tertiary trendlines which belong to two different centres of orogenic activity, that of Mergui and that of the Sunda Area. The foothills, formed by truncated Tertiary anticlines skirt, the central Pre-Tertiary mountains of northern Aceh. The Puncak Lemby (2,983 m) is a central knot from which the van Daalen Range extends northward, the Central Gajo Range westward, and the Wilhelmina Range southeastward. In southern Aceh, south of Blangkedjeren, a NW-SE trend of the Barisan System prevails.
2.4.2. TOBA AREA (NORTH SUMATRA) Between the Wampu and the Barumun Rivers, the Barisan Range display a typical oblong culmination (NW-SE acis of 275 km length and 150 km width). This culmination has been called by van Bemmelen the “Batak Tumor”. In this “Batak 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 of central Sumatra consists of a number of NW-SE trending block mountains. The system is narrowest at its transition into the ”Batak Timor” near Padangsidempuan from which point it gradually widens south- eastward to 175 km in the Padang section. These block mountain ranges are highest on the southwestern side of the Barisan System, which they attain altitudes of over 2,000 m. They descend towards the east Sumatran lowlands. The Pre-Tertiary core of the Suligi-Lipat Kain Range can be traced, via some anticlinal ridges of Tertiary formations to the northwestern corner of the Tigapuluh Mts., which are situated in the middle of the Tertiary basin of east Sumatra. The Lisun-Kwantan-Lalo Range plunges southeastward, disappearing under a 50 km wide basin, called the Sub-Barisan Depression, which separates the Tigapuluh Mts. from the main Barisan System. The fore-Barisan begins in the Ombilin area, east of Lake Singkarak, where it wedges out between the Lisun-Kwantan-Lalo Range and the Schiefer Barisan; southeastward it disappears under the Tertiary deposits of the east Sumatra basin. The schiefer Barisan can be traced along the entire length of the island. The High-Barisan is particularly well developed in the southern half, south of Padang. In the northern half of the island no distinction can be made between the Schiefer-Barisan and the High-Barisan, because Pre-Tertiary rocks are exposed over the entire area, capped by more or less isolated young volcanoes.
2.4.4. SEMANGKO ZONE (SOUTH SUMATRA) One feature which characterizes the Barisan geanticline along its entire length is a median depression zone on its top, called the Semangko zone named after a prototypical section in the Semangko valley of south Sumatra. 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. Some sections have been silled and capped by young volcanoes.
Total view of the main structural Trendlines of Sumatra Based upon the above descriptions, the main structural trendlines of Sumatra may be outlined as follows: The west flank of the Barisan Range, extending west from the Semangko Zone, is rather regularly formed in the southern half of the range, south of Padang. This southern part of the west flank was formed by a long crustal block, which tilted toward the Indian Ocean, while the elevated northeastern edge breaks down along the Semangko Zone. This tilted block, called the Bengkulu Block, is similar to the southern mountains of Jawa. The escarpment along the Semangko Zone general forms the divide 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 rivers descending eastward are much longer, flowing through an erosional plain, which truncates the anticlines of the Neogene Basin, and then flowing through a wide alluvial lowlands until they empty into the Sunda Shelf sea and the Strait of Bangka. The southern end of the Barisan in the Lampung district is nearly 150 km wide. Here one may distinguish between the west flank, or Bengkulu Block, the top part of the Lampung Block, and the east flank, or Sekampung 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. The Pre-Tertiary besement complex of the latter reappears in the culminations of the Garba, Gumai- and Tambesi-Rawas Mts., which belong to the Schiefer Barisan, while the edge of the Bengkulu Block, capped by a series of young volcanic cones, forms the High-Barisan. Between Padang and Padangsidimpuan the structure of the Barisan Range is less distinct. It is cut into a number of longitudinal block-mountains both in the east flank and in the west flank. The latter are exemplified by the Batang Gadis after it has left the Batang Angkola trough of the Semangko Zone. The Batak tumor part of the Barisan Range is a great dome, traversed by an arcuate section of the Semangko-rift zone. The northern part of the Barisan range, of the Batak Tumor, is the most complicated portion of the range. It is into a number of block mountain structures. The Leuser Block and the western mountains occupy a position in the South of the Bengkulu Block. The Barisan Range forms a section of the volcanic inner arc of the Sunda Mountain System. It is separated from the old Sunda landmass by the Sumatra back-arc basins This downwrap of the Pre-Tertiary basement complex a backdeep, is filled with Neogene sediments which were folded in Plio-Pleistocene time. During or after the main phase of folding, a dome was elevated in the center of this backdeep which now forms the Tigapuluh Mts. In other places the basement complex is exposed in the cores of Tertiary anticlines. These anticlines have eroded to their basement levels during their folding so that a primary peneplain of subaerial erosion truncates the Tertiary anticlines. The Pre-Tertiary basement complex of the Sunda area crops out at some places in the alluvial marshes along the east coast. These are, in fact, former islands in the Sunda Shelf Sea which have been connected with the main land of Sumatra by depositions in subrecent time. Physiographically, the backdeep of the Sunda Mountain System now forms a lowland in the Sumatra section, while in other sections, with less sedimentation in Neogene time, the backdeep forms sea basins such as the Andaman Basin of the Mergui section in north Sumatra. 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. This island chain is part of the non-volcanic outer arc of the Sunda Mountain System.
2.5. SUMATRA INTRA-ARC BASIN
In terms of overall geomorphology of Sumatra, the Ombilin Basin is a median graben which is situated between the East and West Barisan mountain range (Fig. 2.1). This median graben extends from south of Solok and trends northwest past Payakumbuh, a distance of approximately 120 km. Towards the northern end of the basin the median graben is covered by Quaternary and recent volcanic products of the Malintang, Merapi, Singgalang, and Maninjau volcanoes. Despite the relatively small size of the basin, 1500 sq km, (25 x 60 km, Figure 2), the basin fill is very thick. Up to 4,600 meters of Tertiary sediments, ranging in age from Eocene to early middle Miocene is preserved in the Ombilin Basin (Koning, 1985). Major river drainage of the Ombilin Basin is provided by the Ombilin, Sinamar and Palangki Rivers along with their many tributaries. Mean elevation of the central basin is approximately 400 meters. However, in the northern portion of the Ombilin Basin, Merapi and Malintang volcanoes reach elevations of 2891 and 2262 meters respectively.
2.5.1. TECTONIC SETTING The Ombilin Basin is a northwest-southeast trending, elongate, sedimentary basin. The basin is located within the Barisan Mountain range of West and Central Sumatra. The area is unique since it is one of the few intermontane basins in Indonesia which exposes early to middle Tertiary lacustrine sediments, thick sequences of stacked braided stream deposits, and marginal alluvial debris fans. The presence of economically important coal bearing strata in the Sawahlunto Formation has generated much geologic interest in the area. The Ombilin Basin has a complex history of reverse, wrench and extensional tectonism. Initial basin configuration and quantity of sediment in the Ombilin Basin is due to a north- south compression which created a graben dog leg or pull apart style basin in the Ombilin and Payakumbuh region. This compression was introduced by the subduction of the Indian- Australian plate beneath the Sunda Craton (Figure 4). 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 Bengkalis trough, Aman, Kiri, Jambi and Palembang depressions are examples of this type of basin development. The Ombilin Basin is believed to be similar in evolution to these grabens and portray an early example of one of these features.
2.5.2. STRATIGRAPHY Many authors proposed different stratigraphic nomenclatures of this basin. The following stratigraphic description is after Kosoemadinata & Matasak (1981), Kastowo & Silitonga (1975), and summarized by Fletcher & Yarmanto (1993).
22.214.171.124. 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, Malacca, and East Malaya microplates were joined together to form the Sunda Craton. Further accretion followed during late Mesozoic times involving the Woyla Terrains (Pulunggono & Cameron, 1984). The Ombilin Basin is largely floored by meta-volcanics and meta-sediments of the Mergui accretionary terrain. These consist of limestones and marbles from the Carboniferous Kuantan Formation and meta-volcanics from the Permian Silungkang Formation. West of the Ombilin Basin fenesters of the Woyla oceanic accretionary terrain sporadically outcrop between Quaternary volcanic deposits. The sequence consists predominantly of limestones from the Permian Silungkang and Triassic Tuhur Formations. Pre-Tertiary sedimentary rocks of the Mergui and Woyla accretionary terrains were intruded by granites, granodiorites, quartz diorites, and quartz porphyries of various ages. Radio- metric dating indicates an Upper Jurassic to Cretaceous age for most outcrops (Koning, 1985). However, samples have been dated from Permian to Quaternary (Figure 8).
126.96.36.199. 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. 1993). They are predominantly reddish brown to purple with mottling indicating the presence of rootlets or burrows. 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, 1987). During the early evolution of the Ombilin Basin in Eocene times, organic rich lacustrine sediments of Sangakarewang Formation was deposited in the central portion of the basin. 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. Concurrently, the surrounding margins of the basin were the site of coarse grained, alluvial fan sedimentation. These fan sediments were sourced from up thrown fault blocks around the margin of the basin (Figure 11). Sawahlunto Formation is late Eocene to early Oligocene in age and unconformably overlies Sangkarewang, Brani and basement. This formation is the most economically important unit in the area due to its large coal reserves, outcrops extensively along the western margins of the Ombilin. It is a fining upward sequence deposited in a flood plain/mire type depositional environment (Whateley and Jordan, 1987). The base of the sequence consists of grey, fine to medium grained, well sorted sandstones. Sands commonly have an erosional base and are interbedded with finer grained, clays, and coals. This sandstone rich basal sequence is overlain by ripple laminated, carbonaceous, si1tstones and shales. The entire sequence is capped by a series of interbedded grey mudstones, coal, and organic rich shales. 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. 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. The Rasau Member is characterized by interbedded coarse grained sandstones and argillaceous siltstones During Oligocene times, the basin became dominated by parasequence sets of continental sediments deposited in a flood plain or meandering river depositional environment of Sawahtambang Formation. These deposits consist of interbedded siltstones, claystones and fine to coarse-grained sandstones commonly representing alluvial channel fills (DeSmet, 1991). Locally, coals up to 18 meters thick were deposited in interlobe, ”mire-type” depositional environments along the western margin of the basin (Whateley & Jordan, 1989). In the late Oligocene the Ombilin Basin became increasingly fluvial, dominated by braided stream deposits of Sawahtambang Formation. The areal extent of these formations increased during this phase of deposition and reached its maximum during late Oligocene to early Miocene (Situmorang, 1991). Thick sequences of fine to coarse grained channel sandstones are commonly stacked several tens up to 100’s of meters thick (Plate 3).
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. 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. The Ombilin Formation consists of grey, silty to slightly sandy, moderately calcareous mudstones with common carbonaceous material. Interbedded with mudstones are off-white to white, very fine to fine grained, calcareous, glauconitic sandstones and soft, off-white, calcareous siltstones. Thickness of Ombilin Formation varies dramatically in different portions of the basin. In the northern arm of the basin seismic interpretation show up to 4000 meter of marine shales have accumulated (Per. comm. Vard Nelson, 1993 in Fletcher & Yarmanto, 1993). However, in Sinamar-1 well only 692 meters were encountered. Volcanic activity in the area reached its peak during Late Pleistocene-Holocene time and the volcanic products are grouped as Ranau Formation. Composition of the deposits varies but generally consists of andesite to basalt lava flows, lahar deposits and tuffs. Provenance for the Ranau Formation is from a combination of the Maninjau, Merapi, Malintang, and Singallang volcanoes. The volcanoes are situated both along and at right angles to the Sumatra Fault zone. 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. However, 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.
2.6. 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, 1978; DeMets et al., 1990). Analysis of slip vectors deducted from earthquake focal mechanisms suggests an approximately N-tending convergence between these two plates (Jarrard, 1986; McCaffrey, 1991). Off Java, where the average trench azimuth is approximately N100oE, the convegence is nearly normal to the Java Trench and is essentially accomodated by the subduction process. Conversely, because the azimuth of the Sumatra Trench, West of the Sunda Strati, is N140oE, the convegenceis oblique. Mechanically, 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). The strike-slip deformation is interpreted as being located along the Great Sumatran Fault System (Fitch, 1972; Beck, 1983; Jarrard, 1986b). This NW-trending fault zone is a major, 1650-km-long structure of, right-lateral strike-slip fault segments that follows the Sumatra magmatic arc and parallesl the trench, from north to south, from the Andaman Sea back-arc basin to the Sunda Strait extensional fault aone. The slip rate of the Great Sumatran Fault has been indirectly estimated, from global plate motions and the opening rate of nearby basins, and directly calculated from measurements of offsets along its trace. Assuming that the Great Sumatran Fault zone is accomodating all the trench-parallel component of the convergence between the Indo-Australian and Eurasian plates. The slip rate of the Sumatra Fault System should range between 30 and 50 mm/yr (Jarrard, 1986). 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; Pramumijoyo et al., 1991). High resolution SPOT image analyses of the Great Sumatran Fault trace have confirmed its right lateral strike-slip style. These images show right lateral offsets of geomorphologic surface features such as streams, calderas and lineaments. Precise offset measurements performed along the Sumatra Fault System have shown that its dextral slip rate increases to the northwest (Bellier et al., 1993), from 6+4 mm/yr in southern Sumatra (at about 5oS) (Bellier et al., 1991) to 28 mm/yr (Shieh et al., 1991) in norther Sumatra near Lake Toba (at about 2o10’N). However, the northern Sumatra Fault slip rate is still too low to accommodate the whole trench-parallel compnenet of the convergence. This suggests that a combination of two models should accommodate the 30 mm/yr slip rate difference between northern and southern Sumatra; that is, slip transfer to the Mentawai Fault Zone (Diament et al., 1991, 1992; Malod et al., 1993) along the Batee Fault link and northwestward stretching of the fore-arc platelet (McCaffrey, 1991), to explain the along-strike variation in slip rate south of the Batee Fault.
Bona Situmorang: research on North Sumatra Danny Hilman: PhD on Sumatra Fault