Biostratigraphic evidence for Oligocene-Miocene and Oligocene-Pliocene unconformities, onshore Sarawak, Malaysia and their regional implications

Abstract

This study presents biostratigraphic and sedimentological insights from six newly documented and two legacy described outcrops from the Tinjar Province of Central Sarawak, Malaysia. Palynological analysis identified four local palynomorph assemblage zones: FT-I (Late Oligocene), FT-III (Early Miocene), SP-I (Late Miocene–Pliocene) and SP-II (Late Pliocene). The Nyalau Formation contains the FT-I and FT-III zones, while strata containing SP-I and SP-II zones are part of an unnamed unit that is age-equivalent to the Liang Formation. Two unconformities were identified in the studied sections: (i) an angular unconformity separating upper Oligocene from Lower Miocene strata in the Nyalau Formation with an approximately 2 Myr age gap correlated with the Base Miocene Unconformity of the offshore Sarawak Basin; and (ii) an unconformity between the upper Oligocene Nyalau Formation and the Pliocene Liang Formation-equivalent strata; this unconformity represents a hiatus of approximately 19 Myr correlated with the offshore Base Cycle VI Unconformity and is linked to a widespread uplift event in NW Borneo during the Miocene. The sediments were deposited in the supratidal to intertidal zone of a marginal marine setting, in proximity to fluvio-deltaic sources and influenced by periodic marine transgressive events. Sedimentological records revealed four facies successions: (i) tide-dominated delta; (ii) estuarine bay; (iii) wave-dominated shoreface; and (iv) fluvio-tidal channel. Palynological data suggest a predominantly warm-wet, humid climate with intermittent seasonal conditions for the Late Oligocene, Early Miocene and Late Miocene–Pliocene.