Field trip guide book: Rift Basin Petroleum System of Ombilin Basin
Instructor: Dr. Dardji N.
Years: 2014
Instructor: Dr. Dardji N.
Years: 2014
REGIONAL GEOLOGY
Ombilin basin has been studied by geologists since the 1870's. For more than a century ago various authors have been conducting research and making the regional aspects of the theory and the evolution of the Basin Ombilin. In general, SE-trending basin Ombilin Northwest, and an elongated sedimentary basins. This basin is located between Bukit Barisan Mountains in the west and the mountains of Central Sumatra. This area is one of the unique regions of slight rift basins in Indonesia, which was outcropped from the beginning – mid Tertiary lacustrine sediments, thick sequences of stacked braided stream sediment and stream sediment marginal alluvial fans.Initial configuration and the number of Ombilin Basin sediments is due to a period of extensional half graben that forms the rift basin in the region and PayakumbuhOmbilin. This configuration is controlled by the presence of a tectonic subduction zone Indo Australian plate beneath the Sunda Shelf and the configuration of the old structure.
STRATIGRAPHY
(de Smet, 1991 in Yarmanto et al. 1984)
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Stratigraphycally, based on a summary of the earlier research (Koesoemadinata and Matasak, 1981; Koning, 1985; Situmorang, et al., 1991; Yarmanto and Fletcher, 1993; Barber, et al., 2005) has Ombilin Basin rocks with Pre-Tertiary age (Perm and Triassic) to Quaternary age rocks.
They introduced new formation names to member classifications.
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Pebbly mudstone, grey, various fragments with silt to fine grain matrix, fragments
consist of phyllite, quartzite and granite, intensively fractures and quartz vein, glacial environment (Gondwana land) amalgamated to western margin of Sunda Land. |
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Greywacke, sandstone, conglomerate (monomict), breccia (monomict), commonly grey in colour, sulphide alteration (pyrite), quartz veins, carbonaceous clay, and fault and
fractures intensity. |
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Outcrop of quartz sandstones and shale of Tanjungpauh Member, Kuantan Fm with fractures. Silitonga & Kastowo (1975) distinguished a Lower Member dominated by quartzites and quartz sandstones, rarely conglomeratic, with interbedded shales, usually metamorphosed to slates or phyllites.
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Conglomeratic Sandstone, red, fine grain to gravel, very tight (compact), fragment
consist of quartzite, rounded – sub rounded, close fabric, bad sorting, there are channel filled by conglomerate, alluvial fan environment, very intensively fractures and faults. |
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Consist of Permian granite: white-grey, brown-yellow, euhedral-subhedral,
Phaneric, minerals quartz, orthoclas, felsic, miccas, intensivelly wheatered |
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Cretaceous granite: white-grey, euhedral-subhedral, Porphiritic, Folliation, minerals
quartz, orthoclas, felsic, miccas, intensivelly wheatered, ductile shear structure. |
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Interbedded sandstone: grey, browny cross bedding, medium sand – coarse sand, thick bedding sandstone, finning upward, conglomeratic, non calcareous. Paleosoil / hardground contact with Shale: dark grey, friable, bioturbation, burrowing, calcareous
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Ngarai Sianok is evidence of active tectonic at Bukit Barisan Ridge. Lithology of Ngarai Sianok is containing of pyroclastic from Mount Maninjau. At this location, we observed two kind of pyroclastic unit; those are fall pyroclastic and flow pyroclastic.
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Outcrop of andesite lava with columnar joint at Lembah Anai. Andesite: blackish grey in colour, porphiro-aphanitic, contain of plagioclase, quartz, hornblende, biotite as phenocrists and aphanitic mineral as groundmasses, has a columnar joint structure as result of magma cooling from Mount Tandikat.
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Outcrop of crystalline limestone, white – grey in colour, saccaroidal weathered, there was an intrusion contact. Intrusion: dark grey in color, coarse grain, contain of quartz, plagioclase, and biotite.
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