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.
Deep-marine rocks of the Belaga Formation (Cretaceous-Eocene) in the Sibu-Tatau area, Sarawak, show a variety of facies types, which are characterized by grain fabric, bed thickness, and sedimentary structures. The main facies types are (A) thick-bedded sandstone, (B) thinly-bedded heterolithic sandstone-mudstone interbeds, and (C) mudstone facies. These facies may be interpreted in relation to a submarine fan model, in which facies A represents a proximal position (near to source area) while facies B and C represent deposition in the middle to distal parts of the system, respectively. Within this general fan model, a detailed characterization of the facies can be made to understand the depositional processes operating in the deep-marine environment. Facies A, for instance, comprises massive and graded sand beds that appear to be linked genetically; the massive bed, often with floating mudclasts at the top, probably represent a debris flow deposit laid down over a pre-cursor turbidity flow deposit, which is commonly preserved as a thin graded bed at the base of the sandbody. Such linked debrite-turbidite facies association seems to be a common feature in the Belaga Formation, similar to many other deep-marine depositional systems, including the West Crocker in Sabah.
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