ipa 1981_ stratigraphy and sedimentation ombilin basin central sumatra (west sumatra province)
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PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Tenth Annual Convention, May 1981
STRATIGRAPHY AND SEDIMENTATION OMBILIN BASIN CENTRAL SUMATRA (WEST SUMATRA PROVINCE)
RP. Koesaemadinata *) Th. Matasak *)
The Ombilin basin is a Tertiary sedimentary and structural basin located on the crest of the Bukit Barisan Range in Central Sumatra (West Sumatra Province). The basin is well know for its Tertiary coal resources. The southern part of the basin is well exposed, while the northern part is covered by Quaternary tuffs. The exposed part of the basin is 27 km wide and 60 km long, trending in the direction of the Sumatra Trend.
Present tectonics show the basin to be asymmetric, being rather gentle on its western side, presumably bounded by normal base- ment faults, and fairly steeply folded in the eastern part, with pre-Tertiary rocks overriding the Tertiary strata along a reverse fault; the Takung Fault. Sets of normal and strike-slip faults also dominate the basin. The Pre-Ter- tiary consisb of Carboniferous Limestones (Kuantan Formation), Permian volcanics (Si- hngkang Formation), and Triassic sedimentary strata (Tuhur Formation) which were intruded by granites and granodiorites.
Detailed geologic mapping of the Tertiary strata based strictly on lithofacies reveals interesting lateral facies relationships. A re- vised stratigraphic nomenclature is proposed, based on detailed measured sections to serve as stato-types. The lateral lithofacies relation- ships shed an interesting light on the sedimen- tary processes within the basin.
The oldest Tertiary unit, the Sangkarewang Formation, presumably Paleocene in age (based on palynology), consists of lacustrine shales, where fish fossils have been found. Interfingering with these lacustrine shales are
*) Dept. of Geology, Institute of Technology Bandung.
the conglomerates of the Brani Formation, which are shown to be alluvial fan deposits. In the northwestern part of the basin these units are overlain by the Sawahlunto Formation, which is the coal bearing measure. It consists of a sequence of shales, coals and sandstones. The sandstones show indication of lacustrine deltaic deposits. In certain areas the sandstone shows a typical pointbar sedimentary struc- tures sequence. Palynological data suggest an Eocene age for this sequences. The Sawah- lunto Formation is overlain by and pre- sumably interfingers with the Sawahlunto For- mation, which is Oligocene in age. This for- mation is composed of cross-bedded quartz sandstones and conglomerates, and is inter- preted as having been deposited by braided river streams. The lower part (Rasau Member) still shows a typical point bar sandstone sedimentary structures sequence. In the south and eastern part of the basin the Sawahtambang Formation lies directly over the Brani For- mation with a transitional sequence (Kulampi Member of the Brani Formation). The Sawah- tambang Formation also shows a few coal Seams on the upper part (Poro Member).
The whole sequence, about 2600 m thick, is conformably (with local erosional surface) overlain by the marine globigerina clay-mark of the Ombilin Formation (Lower Miocene), which is in turn unconformably overlain by the Quaternary tuffs of the Ranau Formation.
The Paleogene cycle of sedimentation re- presents the initial terrestrial phase of the Tertiary sequence. I t was presumably deposit- ed in an intermontane basin, developed in the
IPA, 2006 - 10th Annual Convention Proceedings, 1981
beginning of the Tertiary, when the pre-Ter- tiary landmass was blockfaulted into grabens. This graben-like basin was filled from all sides by alluvial fans, while several lakes existed in the middle. As the topographic relief decreas- ed, the basin became an alluvial valley with meandering rivers and later on braided river systems, before it was folded and uplifted in Lower Miocene Time.
The Ombilin basin may serve as a sedimenta- tion model for the Lower Tertiary of Sumatra.
The Ombilin basin is a paleogene sedimen- tary basin well know for its coal seams in Sumatra. It is located in Central Sumatra, in the so called Padang Highland, southwest of the famous Minas oil field. Administration- wise it is located in the Province of West Sumatra. Geologically it is located in a graben- like depression in the Bukit Barisan Range, which is largely compesed of pre-Tertiary rocks, and therefore, it is called an intra- montane basin by van Bemmelen (1949). As it will be discussed in this paper, from a sedi- mentation point of view it was truly an intra- montane basin during the Early Tertiary times.
The area is well cultivated, with numerous villages, and crisscrossed by good country roads. The nortwest-southeast trending basin is cros- sed by a modem highway, which is a part of the projected Trans Sumatra Highway. Im- portant towns are Sijunjung (capital of the district) and Sawahlunto, a coal mining town which is the headquarters for the state-owned Coal Mining Company (PN Batubara), which operates an underground coal mine north of the town.
The scope of this paper is to describe the sedimentation pattern of the Lower Tertiary.
In his discussion of the geologic history of the South and Central Sumatra basinal areas, deCoster (1976) described the facies of the Lower Tertiary as mainly non marine, consist- ing of a complex lacustrine, deltaic and fluviatile sedimentary environment. The Om- bilin basin offers a rare opportunity to study
the Early Tertiary of Sumatra, with its ex- cellent exposures and accessibility. The re- cently constructed TransSumatra Highway has exposed fresh road-cuts where sedimentary structures are available for detailed study. Underground as well as surface mining ac- tivities also offers a wealth of data, augmented by numerous core drillings.
In the past five years the authors super- vised mapping activities of final year geology students of ITB working for their thesis, and had the opportunity to study the excellent section at critical places themselves. Mapping was directed toward the study of the coal deposits for the State mining coal company, financed by Center for Mineral Technology Development Center. In this mapping program the stratigraphy has been approached strictly from a lithofacies point of view. Some of the results have been partially published elsewhere (Koesoemadinata and Hardjono, 1978; Koe- soemadinata, Hardjono, Sumardiry, Usna, 1977; Matasak, Hardjono and Ruslan, 1979). The mapping program is still to be continued this year, and a comprehensive monograph of the whole basin is envisioned as a doctoral dissertation by the junior author.
An earlier work on this basin is by Musper (1929) who described Tertiary fresh-water fish fossils. Possavec et.al. (1973) discussed the fault tectonic features of the area. Work on Pre-Tertiary formations of this area was publish- ed by Klompe et. al. (1957), while Katili (1962) discussed the Triassic granites. Another recent work covering this area is by Tjia (1972). The area is also covered by Quadrangle geologic map of Kastowo and Silitonga (1973) of the Geological Survey of Indonesia.
The sedimentary pattern of this basin could serve as model for understanding the depositional environments of the Early Ter- tiary of Sumatra.
Regional Setting Tobler (1971) and van Bemmelen (1949)
subdivided the Central Sumatra area into7 distinct physio-tectonic units. 1. The Alluvid
Plains of Eastern Sumatra, 2 . The Tertiary Basin of Central Sumatra, 3. The Vor-Barisan (The Barisan friont range), 4. The Median Depression of Barisan Range, 5. The Eastern Barisan Range or the Schiefer Barisan Range, 6. The High Barisan Range with its volcanic crowns and 7. The Alluvial Plain of West Sumatra.
From the point of view of plate-tectonics, Central Sumatra can be subdivided into (from east to west); (1) the continental shelf area belonging to the Sunda Platform, (2) the retro-arc basin (foreland basin or backdeep basin), (3) the magmatic arc (the inner volcanic arc, or geanticline of Sumatra of Van Bemme- len, 1949), (4) the interarc basin (or the inter- deep basin), (5) the front arc (the outer non- volcanic arc) and (6) the foredeep basin or subduction zone.
The Ombilin basin is then located on the magmatic arc. This magmatic arc (or the geanticline or backbone of Sumatra) is domi- nated also by the Semangko Rift Zone or the Trans Sumatra Strike-slip Fault Zone of Katili and Hehuwat (1967), which, according to Possavec et al. (1973), is associated with secondorder thrust faulting and tensional faults which formed graben like structures; e.g. the Sinkarak Lake, subdividing the magmatic arc into a western Barisan Mountain Range with volcanoes, the Eastern Schiefer Barisan (now volcanic), the Median Depressions and the Barisan Front Range, which are non-volcanic in nature. The Eastern Barisan Range (called the Schiefer Barisan) and the Barisan Front Range (the Vor-Barisan of Tobler or the Suligi-Lipat Kain Zone of Van Bemmelen, 1949) consists of pre-Tertiary metasediments (slates, crystalline limestones, volcanics) and granitic intrusives, while the Median Depres- sions are fiiled by Early Tertiary Sediments.
The Ombilin Basin is such a depression filled with Early Tertiary sedimentary strata, flanked by the pre-Tertiary rocks of the Barisan Front Range in the East, and the pre-Tertiary rocks of the Eastern ("Schiefer") Barisan Range in the west. The Early Tertiary strata, however, are not confined to this basin only, there are several other localities in the median depressions, such as Sinamar Basin to-
ward the south east. The Early Tertiary strata are also found west of the High Barisan Range in the west, in the eastern flank of the inter- arc basin, e.i. the Painan basin