Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 39 – 49
Geological Survey of Malaysia, Kuching, Sarawak.
*(Present address: Sarawak Shell Bhd, Lutong, Sarawak)
Abstract: Since Carboniferous time, west Sarawak has shown vertical sub-stability to sub-mobility, with intermittent but progressive changes in the distribution of land and sea through geologic times till the present distribution of land and sea is achieved.
The palaeogeographic development of west Sarawak since Carboniferous time is summarized in 4 maps covering 4 different geological intervals, namely Carbo-Permian and older, Triassic, Jurassic-Cretaceous, and Lower Tertiary. Highlands, and marine and mixed (fluvial-deltaic-estuarine-lacustrine-shallow marine) facies are differentiated, and the main areas of volcanic accumulations are plotted.
Present knowledge of the Carbo-Permian rocks is limited but distributions of these rocks indicate the occurrence of warm shallow and deep seas in a few scattered areas.
By Middle to Late Triassic, the older rocks appear to have been uplifted to form land which contributed detritus to the deposition of the Sadong Formation in a mixed continental-shallow marine environment. Widespread subaerial volcanism gave rise to the Serian Volcanics.
An early Jurassic orogeny probably occurred, raising the Triassic rocks to form highlands. Sediments derived from these highlands were deposited in a shallow to deep sea from late Jurassic to Cretaceous, resulting in the Kedadom and Pedawan Formations. The Bau Limestone built up in a shelf environment. Further offshore, deep-sea pelagic sedimentation resulted in the Sejingkat, Serabang and Sebangan Formations.
By Early Tertiary, the major portion of west Sarawak had been uplifted to form highlands. Sedimentation in intermontane basins gave rise to the Silantek Formation, Kayan sandstone and Plateau Sandstone.
At the end of Tertiary and during Quaternary, most of west Sarawak had been raised above sea level. Peneplanation occurred during Pliocene-Pleistocene times. Successive stages of continental and marine conditions, caused mainly by eustatic movement of the sea, resulted in the successive lowering of the sea level after each completed cycle.