The Bawang Member, a unique unit of Belaga Formation in the Miri Zone, Central Sarawak, NW Borneo Malaysia: Revised stratigraphic and sedimentological characteristics

Abstract

Bawang Member (Unit 4) of Late Eocene age is the fifth and the youngest member of Belaga Formation. In general, the Late Cretaceous to Late Eocene turbidite sequence of Rajang Group in the lower Rajang area (Central Sarawak) is characterized by Belaga Formation. This member is unique and is only exposed in the vicinity of the Tatau area (Arip valley and Tatau High region) with limited exposures. In this research we present new evidence, to support its position on the stratigraphic column, based on field observations (stratigraphic and sedimentological characteristics) and lab analysis (TOC). Numerous recently exposed sections were visited in the study area. Four litho-facies were identified in the Bawang Member based on sedimentological observations; (1) Massive sandstone facies (SF1), (2) Medium to thick-bedded sandstone facies (SF2), (3) Thin bedded heterolithic facies (SF3), and (4) Mudstone or Shale facies (SF4). Reddish maroon and green shales exposed in Tatau High region are only exposed in another member of Belaga Formation i.e. Kapit Member. The Bawang Member sediments differ from the Tatau High region sediments in absence of cleavage and internal sedimentary features (rip-up clasts), the occurrence of bioturbation, and with lesser exposures of mud facies. On the contrary, the Tatau High sediments show higher ratios of mud facies, presence of cleavage in shales and even in few sandstones, and absence of major bioturbation of strata. The presence of internal sedimentary features (rip-up clasts) in Tatau High and in Kapit Member supports the idea that these are the parts of the same rock unit. Furthermore, contrasting TOC values between these two units (Bawang and Tatau High), and the similarity of TOC among Tatau High and Kapit Member are also in support of the same idea. Based on the differences of facies (rock) types, the effect of cleavage, absence of rip-up clasts, the occurrence of bioturbations and different TOC values, we confirm that the Tatau High rocks must be older than most of Bawang Member exposures and are actually the faulted blocks of Kapit Member.