An Erosional Unconformity At The Top Of The Nyalau Formation Near Bintulu, Central Sarawak (Malaysia): Its Regional Context And Significance

• Published in: Buletin Persatuan Geologi Malaysia Vol. 73; no. 1; pp. 35 - 52
• Main Authors: Madon, Mazlan, Amir Hassan, Meor Hakif, Jong, John
• Format: Journal Article
• Language: English
• Published: 31.05.2022
• ISSN: 0126-6187; 2637-109X
• DOI: 10.7186/bgsm73202204

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.