Causes and consequences of Oligocene-Miocene relative sea level change in Asia: An example from the Labuan Island (Sabah), Malaysia

Abstract

Belait Formation: (A) Trough cross-bedded sandstone (fluvial) (B) Interbedded sandstone-mudstone (sandstone dominated) facies overlying mudstone facies (marine). • The Miocene Belait Formation in the Labuan Island formed due to forced regression. • The upper part of the Belait Formation indicates transgression and marine flooding. • The depositional environment changed during the Oligocene-Miocene in Sabah and Sarawak. • This was linked to regional tectonics and/or global relative sea level. • No record of paleoclimate shift like central Asia is inferred in this study. Labuan Island represents the emergent part of a north-eastward plunging anticline. The stratigraphic succession at Labuan is made up of four formations: the Crocker Formation, the Temburong Formation, the Setap shale Formation and the Belait Formation. The sedimentological aspects of the Labuan stratigraphic succession have been studied by several authors but the stratigraphic relationships between the formations is still a matter of conjecture. In this paper we have undertaken an outcrop based (1) sedimentary facies analysis of the Middle Miocene Belait Formation and (2) inferred the mode of stratigraphic sequence building of the Oligocene-Miocene successions. In significant contrast to the tide-storm interactive shallow-marine Belait Formation of Brunei and Sarawak, the Belait Formation of the Labuan island is made up of a lower fluvial and an upper shallow marginal marine to shelf facies association. The sedimentary succession from the Crocker, Temburong, Setap Shale and Belait formations represents a marine regression either due to a relative sea-level fall (forced regression) or to sediment supply exceeding the rate of sea-level rise (normal regression). The ~17 million years unconformity separating the fluvial facies association of the Miocene Belait Formation from the underlying shelfal sedimentary units (Oligocene Temburang and/or Setap shale formations) is a subaerial unconformity that is a consequence of rapid uplift and associated forced regression, and thus indicates the influence of regional tectonics on the mode of stratigraphic sequence building The change in paleocurrent pattern recorded by earlier researchers across the Oligocene-Miocene boundary also indicates tectonic tilt, and a change in sediment dispersal direction because of upliftment. The upper marginal marine facies association of the Belait Formation indicates transgression and the thick shelfal mudstone unit that overlies the marginal marine deposit indicates subsequent maximum flooding. Our sedimentological and stratigraphic analyses reveal that the shift in depositional environment during the Oligocene-Miocene transition is linked to regional tectonics and consequent sea level change and unrelated to paleoclimatic shift.