An erosional unconformity at the top of the Nyalau Formation near Bintulu, Central Sarawak (Malaysia): Its regional context and significance

Mazlan Madon and Meor Hakif Amir Hassan and John Jong (2022) An erosional unconformity at the top of the Nyalau Formation near Bintulu, Central Sarawak (Malaysia): Its regional context and significance. Bulletin of the Geological Society of Malaysia (73). pp. 35-52. ISSN 2637-109X

Abstract

This article documents an erosional unconformity at the top of the Nyalau Formation (Oligocene-Early Miocene) exposed in a road-cut northeast of Bintulu in central Sarawak. The outcrop succession essentially comprises a lower unit of thickly-bedded sandstones representing wave-dominated shoreface facies overlain by fluvio-tidal channel deposits that were deposited following a base-level lowering. This gently tilting sand-rich unit is cut by an irregular concave-up erosional surface, overlain by mudstones and thinly interbedded heterolithics that show large-scale internal stratification indicative of multiple scour-and-fill structures, progradation and/or lateral accretion. The erosional surface separating the two units not only marks a significant shift in sedimentary facies but the fact that it cuts into deformed and tilted strata (Upper Nyalau) suggests that it is an erosional unconformity caused by a tectonic deformational event. Based on regional structural synthesis and stratigraphic evidences, the unconformity probably represents the eroded top of the Nyalau Formation, overlain by a relatively undeformed succession tentatively correlated with the Early-Middle Miocene post-Nyalau formations in the Mukah-Balingian region.

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