A very good exposure of outcrops along the road from Gunung Tua to Padangsidempuan, North Sumatra Province, has been the subject of this research to reveal the tectonics and sedimentation history of the most northwestern part of Central Sumatra Basin. The basement rock exposed in this area are karstic limestone and marbles of Permian age along with phyllite, slate, and quartzite. They are characteristic as continental recycled orogen terrain of Mergui micro continent. The lack of Pematang Facies and age equivalent section in the area indicating a possible high area during synrift. A major thrust fault dissecting the Sihapas separates hilly Barisan morphology with low lying undulated terrain of predominantly Telisa and Upper Sihapas folds. A strike slip fault at the boundary of Sihapas with Telisa suggests that the folds observed are of flower structure types. Sedimentation models of Sihapas Formation are mostly associated with transition and tidally influenced environments with sometimes influx of pronounced fluvial processes. At the contact with Telisa Formation (Siholbung Glauconitic Member) condition of high wave energy was inferred possibly related to the connection of the area with open ocean to the west (paleo Indian Ocean) during the start of Telisa transgression. Feldspatic evolution with time in Sihapas petrography indicating tectonic evolution toward more stable provenance. The Telisa outcrops benthic fossils indicate middle neritic to upper bathyal environment. This slope related bathymetri may be suggesting a foreland basin setting rather than normal back arc rifting. Telisa also shows some reservoir qualities in their interbedded tidally influenced shelf sands. They might become reservoir analogues for Telisa plays, especially developed at the Middle to Upper Telisa.
Author : A. Bachtiar, M. Rozalli, F. I. Barus, K. Simanjuntak, H. Gultaf, I. Ansari,H. R. Melsa