Regional geological correlation of Paleogene sedimentary rocks between Sabah and Sarawak, Malaysia

Abstract

Field observations, aided by SAR and satellite images interpretation, have enabled the recognition of at least three groups of Paleogene sedimentary rock units that can be correlated regionally between Sabah and Sarawak. The three informal groups of rock units have been mapped based on similarities in their ages, lithological characteristics and degree of structural deformation. The first unit has been interpreted to range in age from Lower Paleocene to early Eocene. A large part of this unit has been metamorphosed to slate, sub-phyllite and metasandstone. The remaining part of the unit consists mostly of sandstone and shale with minor occurrence of conglomerate and lenses oflimestone. Structurally, this unit has suffered intense deformation, characterised by the presence of folded cleavage, quartz veins and refolded folds. The second unit has been interpreted to range in age from late Lower Eocene to early Upper Eocene. This unit is mainly made up of a thick sequence of sandstone interbedded with shale. Structurally this unit is also characterised by intense deformation, where most of its bed dip steeply due to tight folding and thrust faulting. The thrust faults generally show transport direction to the north and northwest. The development of cleavage is restricted to the core of folds. The minor occurrence of cleavage, lack of refolded fold and lack of quartz veins differentiate this unit from the older unit above. The third unit is interpreted to range in age from early Upper Eocene to late Upper Oligocene. The unit is composed mainly of sandstone and shale, with local occurrence of conglomerate, limestone and marl. A large part of the unit also shows intense deformation, characterised by tight upright folds and thrust faults, similar to Unit 2. The thrust faults also generally show a transport direction to the north or northwest. Regional geological interpretations suggest that the Paleogene sediments were deposited in a large elongated basin along NW Borneo. The deposition and deformation of sediments occurred in successive stages-younger sediments were deposited on top or in front of older accreted sediments, in response to northwest-southeast closure of the elongate basin.