Sedimentology of the Semantan Formation (Middle - Upper Triassic) along the Karak-Kuantan Highway, central Pahang

Abstract

Outcrops of the Semantan Formation (Middle to Upper Triassic) along the Karak-Kuantan Highway at km 114.7, km 115, km 140 and km 149.3 were studied. Some beds were found to contain bivalves (Entolium subdemisum Muenster, Neoschizodus sp., Costatoria pahangensis(Kobayashi and Tamura) and Costatoria chegarpahangensis (Kobayashi and Tamura)) and gastropod (?Nerita sp.), which support the Middle to Upper Triassic age for the Semantan Formation. Sedimentary facies and facies associations were examined to gain a better understanding of deep-marine sedimentation processes in relation to submarine fan models. The main facies recognized in the field include conglomerate, pebbly sandstone, thick-bedded sandstone, interbedded sandstone-shale, contorted sandstone-shale, and shale-dominated heterolithics. The facies associations include fining- and coarsening-upward fan-lobe parasequences, slump deposits, and outer fan/basin plain shales. The sediments therefore represent a range of subenvironments from slope to outer fan. Both debris flow and turbidity current deposits are recognized as the main depositional processes in Semantan Formation. Features, such as disorganized clasts in conglomerate, floating mudclasts, and scour-and-fill structures indicates debris flow processes whereas normal graded bedding (fining upward) and thin waning-flow sandy layers in shale indicates turbidity current processes.