Bulletin of the Geological Society of Malaysia, Volume 33, Nov. 1993, pp. 163-180
Geological Research and Development Centre, Bandung, Indonesia
Abstract: The geological setting of the Bayah area in the Paleogene is indicated by the presence of alternating sedimentary and volcanic rocks. The sedimentary rocks consist of fine to coarse clastic sediments, which suggest regressive deposition in a shallow basin. In the Neogene time, sediments with open marine characteristics and contemporaneous volcanic rocks were deposited in the transition area at the southern border of the basin. The clasts of Neogene sedimentary rocks were derived from a southern source area. The boundary between the Paleogene and the Neogene sequences is characterized by an unconformity, indicated in the field by an erosion surface. Three further tectonic phases can be recognized in this area. They occurred in the Late Paleogene, late Middle Miocene and latest Miocene age.
The Tertiary volcanic rocks can be divided into a lower sequence of Late Eocene to Early Oligocene age (LOA) and upper sequence of Oligocene to Miocene age (UOA). All the rocks, except for one sample from the LOA, are rocks typical of a calc-alkaline island arc. The LOA consists of basalts and basaltic andesites with characteristics that suggest a more primitive magma than those the UOA. The LOA rocks probably correspond to fractional crystallization and contamination with a component of low-Th concentration of the intermediate lower crust. The LOA was generated in the volcanic front and appears to be related to a period of slower subduction. The UOA, which ranges from basalt to rhyolite composition, correspond to an increase of differentiation. They experienced contamination of the parental magma terrigenous sediments and occur in high crustals levels.