Publications

Ombilin Basin is currently located in the middle of Sumatra magmatic Arc. Koesoemadinata and Matasak (1981), based on outcrop study, believed that Ombilin Basin was an intermountain basin (Figure 1). This conclusion in certain degree was influenced by its present day position in the intra arc setting. Situmorang (1991) based on gravity data, described Ombilin Basin as a transtensional basin, resulted from dextral strike slip movement. Koning (1985), based on subsurface data, tended to support Situmorangs model, saying that Ombilin Basin was a pull a part basin. Seismic interpretation by Koning showed 4 reflector groups, Sangkarewang, Sawahlunto, Sawahtambang, and Ombilin. Seismic of Sangkarewang showed folded sediment, strong reflector and transparent. Geometrically, the Sangkarewang reflector group was not clear. The study by Koesoemadinata and Matasak (1981) indicated that Sangkarewang consisted of shale, sandstone, and conglomerate, interpreted as lacustrine deposit. Seismic of Sawahlunto was indicating sediment thickening southwestward, parallel to divergent. Based on Koesoemadinata and Matasak (1981), Sawahlunto Formation was characterized by shale, quartz sandstone and coal. The formation was described as meandering river deposit. Seismically Sawahtambang was generally characterized by strong reflector, sediment thickening northeastward, parallel to divergent. Koesoemadinata and Matasak (1981), described that Sawahtambang Formation consisted of conglomerate, sandstone, shale and coal in the upper part. The whole section of the interval was interpreted as meandering to braided river deposit. Seismic characters of Ombilin reflector are generally transparent and are partly strong indicated mainly composed of shaly facies. Geometrically, the interval cannot be interpreted since the upper part is mostly eroded. Based on seismic character described previously, the evolution of the basin is reinterpreted using tectonostratigraphy framework and rift basin development concept. In this regards, the Sangkarewang is interpreted as Syn-rift deposit while the Sawahlunto Formation is attributed to late Syn-rift deposition and Sawahtambang Formation as Post-rift interval.