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.