Bulletin of the Geological Society of Malaysia, Volume 37, July 1995, pp. 471 – 478
Geophysical Program, Department of Geology, Bandung Institute of Technology, Indonesia
Abstract: Neogene sediments in Siberuang area consists of Sihapas Formation, Telisa Formation and Petani Formation. Sihapas Formation is a friable, white to brown sandstone which intercalated by claystone and some coal layers. It was deposited in the channel or bar of a braided-stream system during the N.4-N.8 period. Telisa Formation which lies unconformably above Sihapas Formation is a calcareous claystone intercalated by glauconitic sandstones and some limestone lenses. It was deposited under middle to outer neritic environment during the N.8-N.l0 period. Petani Formation is a claystone containing land detrital fragments and in the upper section some coal lenses and tuffs. It was deposited in outer neritic environment on a regressive sequence. The Neogene sediments lie unconformably above the Paleogene Pematang Formation, which is mainly composed of brown conglomerate and conglomeratic sandstone. The Pematang Formation lies unconformably above the pre-Tertiary rocks. A gravity line had been run in northeast direction, crossing the structural trend of the region. The Bouguer density used is 1.9 g/cm3. The Bouguer map shows that two anomaly wavelengths exist in the area, i.e. 10 km and 15 km. Moving average filtering is applied to separate the regional and residual anomaly. Gravity map analysis and modelling results show a graben type structure and that the Neogene sediment thickness increase to the southwest ranging from 600 m in the northeast to 2,800 m in the southwest and being thickest in the center of the graben. The faults in the southwest area are mostly dipping northeast while those in the northeast area are mostly dipping southwest.