Chapter 7 Tertiary stratigraphy

Abstract

The purpose of this account is to review the complex terminology of the Tertiary stratigraphic units in Sumatra and propose a revised and a simplified terminology based on the significance of formations for the tectono-stratigraphic development of the island. Formations are classified in terms of Pre-Rift, Horst and Graben, Transgressive, and Regressive tectono-stratigraphic stages.The island of Sumatra lies along the southwestern margin of the SE Asian continent (Sundaland) beneath which the Indian Ocean Plate is currently being subducted at a rate of about 7 cm a-1 in the Sunda Trench (Fig. 7.1). The continental margin of SE Asia is of Andean type, with active and inactive Quaternary volcanoes rising to over 3000 m above a Pre-Tertiary basement, exposed towards the west coast of the island in the Barisan Mountains. Tertiary sedimentary basins occur both to the SW and the NE of the mountains and small basins also occur within the mountain range itself. These basins are described with relationship to the present-day subduction system as forearc, backarc and intra-arc or intramontane basins (Fig. 7.1). The Barisan Mountains are transected by the Sumatran Fault System, a major dextral transcurrent fault zone which extends along the length of the island from the Sunda Strait to the Andaman Sea.Stratigraphic research in the Tertiary sedimentary basins commenced in the last decades of the nineteenth century when oil was discovered in the Telaga Tiga (1883) and Telaga Said (1885) wells near Pangkalan Brandan in North Sumatra. Initially, wildcat drills were sited near oil