Fan-delta-lacustrine sedimentation and coal development in the Tertiary Ombilin Basin, W Sumatra, Indonesia

Abstract

Summary The Ombilin Basin is a Tertiary intermontane basin situated in the Barisan mountain range of W Sumatra. It covers an area of some 1500 km 2 and is known for the coal resources which have been mined in the Sawahlunto area since 1891. Seismic and borehole data indicate that the basin has been filled with approximately 4600m of middle Eocene to early Miocene sediments. The coal developed early in the basin’s history (middle Eocene). During this study the fan-delta sediments below the oldest seam, the C seam, the seam itself and the sediments immediately overlying the C seam were investigated along the western margin of the Ombilin Basin in the Sawahlunto area. The oblique convergence of the Indian Ocean plate weith the SE Asia plate created a stress field of the type that can produce a primary dextral wrench fault which in this case is named the Great Sumatran Fault Zone. The Ombilin Basin is a graben-like ‘pull-apart’ basin resulting from localized extension failure and related deformation. The failure and deformation are a function of irregular strike-slip movement along the Great Sumatran Fault Zone during the early Tertiary. Tertiary sedimentation began in the Eocene following graben formation. High basement relief adjacent to the newly formed basin resulted in debris flows and fan-delta formation around the margin of the central lake. Concurrent with the deposition of coarse-grained fan-delta sediments around the basin margins, fine-grained lake sediments accumulated in the basin centre. Reduction in relief difference between the source area and the basin was associated with an expansion of the lacustrine environment. Shallowing of the lake and a humid tropical climate allowed plant colonization to take place. Peat accumulation was influenced by the underlying fan-delta lobe geometry. In interlobe areas thick sequences of low ash peat accumulated, while central lobe areas through the same interval were sites of continued sediment influx and relatively slow peat accumulation. In these areas high ash coal or carbonaceous shale are laterally equivalent to the low ash coal. A detailed investigation of the character of the coal in the C seam indicates that, while the lateral dispersion of clastic material may play some part in the variation in the ash content of the coal from one place to another, the most significant variable is the rate of accumulation of peat.

Cenozoic ecological history of South East Asian peat mires based on the comparison of coals with present day and Late Quaternary peats

“…Extensive Oligocene coals are known only from the Sawahlunto formation (Ombilin basin), where they form part of the initial rift infill sequence (Koesoemadinata and Matasak, 1981) and from the Pematang formation of Jambi province. The Sawahlunto coal grades laterally into lake deposits (Wateley and Jordan, 1989), and is also rich in algal remains, such as Pediastrum spp. Therefore, the Sawahlunto coal most likely formed from a rheotropic, eutrophic peat.…”

Paleoenvironmental conditions in the late Paleogene, Sumatra, Indonesia

“…The basin's Cenozoic fill overlies pre-Cenozoic basement rocks and includes in ascending order; the Brani Formation, the Sangkarewang Formation, the Sawahlunto Formation, the Sawahtambang Formation, and the Ombilin Formation (Fatimah and Ward, 2009;Koesoemadinata and Matasak, 1981;Whateley and Jordan, 1989). This study focuses on a small part of the late Paleogene Sawahlunto Formation.…”

“…This study focuses on a small part of the late Paleogene Sawahlunto Formation. The oldest sediment in the Ombilin Basin, the Brani Formation, represents Eocene marginal fan deposition and consists of a sequence of breccias and polymictic pebble to cobble conglomerate in a muddy to sandy matrix that interfinger laterally and basinward with the Sangkarewang Formation (Fatimah and Ward, 2009;Whateley and Jordan, 1989). The Sangkarewang Formation is composed of dark bluish gray to black laminated shale that is commonly intercalated with greenish-gray feldspathic turbidite sandstone.…”

“…The Sawahtambang Formation overlies the Sawahlunto and is interpreted as a braided river deposit primarily consisting of cross-bedded quartz sandstone and conglomerate (Koesoemadinata and Matasak, 1981). The coarse-grained sandstone of the Sawahtambang Formation is in turn overlain unconformably by the Ombilin Formation which caps off the basin and consists of dark gray marine shale that contains limestone lenses with coral debris, shells, and plant remains (Whateley and Jordan, 1989). The overall basin fill records a progressive upward decrease in non-kaolinite clay minerals, relative to kaolinite, and an upward decrease in feldspar abundance (Fatimah and Ward, 2009).…”

Ichnological constraints on the depositional environment of the Sawahlunto Formation, Kandi, northwest Ombilin Basin, west Sumatra, Indonesia

“…1) a small intermontane basin on the western margin of the island of Sumatra. Previous studies have indicated that subsidence of the Ombilin Basin was initiated as early as the Paleocene and was dominated by continental deposition from the basin's origin through the Paleocene, Eocene and Oligocene (Koesoemadinata and Matasak, 1981;Whateley and Jordon, 1989;Fatimah and Ward, 2009). Marine influences are not inferred to have occurred until the Early Miocene, with deposition of fossiliferous limestone and calcareous shale beds of the Ombilin Formation.…”