Facies analysis of the Plateau Sandstones (Eocene to early Miocene?), Bako National Park, Sarawak, Malaysia

Abstract

Sandstones, located in the Kuching area, western Sarawak, are known as the ‘Plateau Sandstones’ (of possible Eocene to early Miocene age). However, based on a number of factors, including: (i) anomalous kerogene compositions; (ii) proximity of the on-lap surface; and (iii) palaeocurrent direction (generally to the NNE), it is thought that the sands exposed on the Bako Peninsula are unrelated to the Plateau Formation (located to the south of the Bako Penisula) and therefore a new name is coined; the Bako Sandstones, which form a subgroup of the Bako Sandstone Group. The Bako Sandstones form the Bako Peninsula, a flat-topped cliffed plateau which extends into the South China Sea at a latitude of 1°30′N. The plateau has a gently dipping surface, sloping northwards from a height of about 300 to 150 m. The sandstones form a succession of very thick bedded sandstones (up to 6 m thick), with lenses of conglomerates and subordinate sandy mudstones. The sandstones consist of pebbly coarse-medium grained sands, interbedded with polymictic pebble conglomerates. The sandstones are mainly lithic arenite, poorly to moderately sorted and consist of subangular to subrounded grains. Isolated pebbles are common throughout the sandstones. The most common structure in both sandstones and conglomerates is cross-bedding; planar cross-bedding and trough cross-bedding, together with thick sequences of climbing ripples. These structures suggest extensive tractional transport, forming both ripple and dune structures along the base of the channel. The geometry of the sands is either (i) lensoidal, or (ii) tabular, with the channel-fill interpreted as scour-fill channels or migrating dunes, respectively. Both types are commonly stacked vertically or amalgamate laterally to form thick interconnected units. The conglomeratic lenses, scour-fill features and rip-up shale clasts are related to higher energy erosional events, whilst the mud-draped ripples, ripple rejuvenation surfaces and two-tiered channel margins indicate a lower energy and stasis period. Slope instabilities at the channel margin are inferred from the slump structures present and shale clast slurries. The sandstones at Bako are thought to have formed within a braided channel environment (subject to exposure, from the presence of mud cracks within the formation).