Abstract

Situated within the continental crust and along the Sumatra Fault System (SFS), the Ombilin Basin exhibits a complex tectonic development since the Paleogene time. Recent detailed geological mapping indicates that the complex tectonics is due to the overprinting of older structures which were formed by transtension mechanism along the SFS by the younger structures produced by transgressive processes along the same system.

Transtensional phase took place in the Paleocene time with the formation of the first asymmetrical graben in the northeastern portion of the basin. This phase was subsequently followed by further dextral motions along the N-S trending Kolok and Tigotumpuk Faults which resulted in the formation of the second and third grabens all of which appear to be migrated southeastward. Those grabens were filled up by the late Eocene-Oligocene deposits of the Sangkarewang Formation. At the same time the fan deposits of the Brani conglomerate formation was deposited along the major NW-SE boundary faults.

The main transgressive episode occurred in Oligo-Miocene times. Extensive slump deposits characterize the Sangkarewang Formation; a major unconformity was formed on top of this formation upon which meandering and braided river deposits of Sawahlunto and Sawahtambang Formations respectively were deposited with no sign of intensive deformation. Within the Sangkarewang Formation, a minor erosional surface is recognized from the outcrops and the drilling of Sinamar-1 well. Upthrusting of the Silungkang Formation onto the lacustrine deposits of the Sangkarewang Formation is also associated with this transgressive episode.

The final structural development was characterized by transtensional phase during the Early Miocene time; the fourth graben was formed to the southeast of the earlier depocentres bounded by the N-S trending Tanjungampalu Fault. The Neogene shallow to deep marine sediments of the Ombilin Formation was deposited in the graben which formed the deepest portion of the Ombilin Basin. Those tectonic episodes have produced complicated structures which were then affected by Plio-Pleistocene orogeney.