Bulletin of the Geological Society of Malaysia, Volume 8, Dec, 1977, pp. 61 – 74
1Dept. of Geology, Idaho State University, Pocatello, Idaho 83209
2Conzinc Riotinto, Indonesia
(Submitted 27 February 1974; revised version accepted 19 August 1974)
Abstract: Upper mesozoic strata are exposed beneath the Cenozoic volcanic cover near Indarung, a few kilometres east of the city of Padang, on the west coast of central Sumatra. These strata are separated from the overlying Cenozoic volcanic rocks by a major unconformity, and were deformed and moderately folded before deposition of the volcanics. The pre-Tertiary strata at Indarung are all of upper Jurassic-lower Cretaceous age. The carbonates contain a distinctive biota of coralline fossils whose most important member is the coral-like Lovcenipora, which is age restricted to the upper Jurassic-lower Cretaceous, and is a useful guide fossil for regional correlations.
The upper Mesozoic strata belong in the newly described Indarung Formation. This formation consists of interbedded calcareous, terrigenous and siliceous strata, and contains two distinctive subunits, the Ngalau Member of bedded cherts near the top of the formation, and the Karang Putih Member of massive limestone near the base of the formation.
The Indarung Formation section has been used in cross-fault comparisons to determine offset along the Sumatra Fault. Dating the section as upper Mesozoic invalidates previous comparisons which gave estimates of 250-270 kilometres, and revised correlations suggest a maximum of 200 kilometres separation.