Bulletin of the Geological Society of Malaysia, Volume 73, May 2022, pp. 35 – 52
An erosional unconformity at the top of the Nyalau Formation near Bintulu, Central Sarawak (Malaysia): Its regional context and significance
Mazlan Madon1,*, Meor Hakif Amir Hassan2, John Jong3
1 Malaysian Continental Shelf Project, National Security Council, c/o 11th Floor, Wisma JUPEM, Jalan Semarak, 50578 Kuala Lumpur, Malaysia
2 Department of Geology, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
3 JX Nippon Oil and Gas Exploration (Malaysia) Ltd, Level 17, Menara Prestige, No. 1, Jalan Pinang, 50450 Kuala Lumpur, Malaysia
* Corresponding author email address: firstname.lastname@example.org
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.
Keywords: Nyalau Formation, Sarawak Cycles, unconformity, erosional surface, facies
0126-6187; 2637-109X / Published by the Geological Society of Malaysia.
© 2022 by the Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution (CC-BY) License 4.0.