Bulletin of the Geological Society of Malaysia, Volume 15, December 1982, pp. 19 – 30
Department of Geology, University of Malaya
Abstract: Middle Triassic (Ladinian) sediments (dated by bivalves and ammonites) in a recently exposed section of the Semantan Formation near Lanchang consist of interbedded dominant dark grey mudstone and subordinate coarse and fine quartz-bearing tuff.
Normal graded bedding in tuffs, some of which exceed 1 m in thickness, and the presence of outsize tabular mudstone clasts in some tuffs which show erosive bases, indicate deposition by turbidity currents and high energy mass flows. Small-scale cross bedding, overturned load structures and flame structures in silty mudstones indicate an original westward or southwestward down-flow direction. The dark mudstones, which contain transported neritic bivalves, indicate deposition in a ‘black shale‘ environment at a moderate or great water depth. All the coarse and much of the fine materials in the sediments are of volcanic origin, and the site of deposition is interpreted as being located on the western or southwestern flank of a Triassic volcanic island or island chain.