Late Cenozoic relative sea-level highstand record from Peninsular Malaysia and Malaysian Borneo: implications for vertical crustal movements

In order to predict the manner in which global sea-level change will impact the wide variety of coastal conditions and communities in Malaysia, this study sought to gain insight into the vertical components that determine the relationship between the coastal land areas of Malaysia and changes in sea level over time periods of 106 to 102 years. This was a regional-scale, reconnaissance-level field survey of the Malaysian coasts, coastal plains, most islands and many river systems to determine the presence, absence and spatial variability of evidence for late Pliocene-Quaternary sea level(s) higher than present. In Peninsular Malaysia, no conclusive evidence was found to indicate that RSL was ever higher than the mid-Holocene maximum during the late Cenozoic. This indicates long-term (104 to 106 yrs) subsidence for at least the coastal parts of the region. Superimposed on this long-term subsidence are eustatic rise and continental levering associated with hydro-isostatic adjustment (HIA) over time scales of 102 to 103 yrs. In areas where HIA appears to be in equilibrium (e.g. NE Peninsular Malaysia), long-term subsidence and eustatic rise are the dominant factors influencing RSL change, resulting in increased flooding and coastal erosion. These detrimental effects are commonly exacerbated by anthropogenic activities. Along much of the remainder of the Peninsular Malaysia coast, uplift associated with HIA appears to continue, counterbalancing eustatic rise and resulting in little net change in RSL. This, in tandem with abundant sediment supply, has resulted in generally prograding coasts in areas minimally influenced by anthropogenic modifications. The RSL record of Malaysian Borneo is much more complex, even in western Sarawak, which is situated on supposedly stable Sundaland. Possible strandplain deposits overlying coarse alluvium in the Kuching area could reflect last interglacial highstand deposition but more likely result from uplift or eolian reworking of alluvial deposits. This alluvium, along with similar deposits associated with the Kayan and Rajang rivers, clearly represents deposition under very different environmental conditions than present. Ongoing subsidence of the peatland-dominated coastal plain, from Kuching to Bintulu, is probably mainly due to sediment loading. North of the Lupar Line, RSL histories are mainly the product of tectonism and differential movement of the upper crust. The coast and interior from Bintulu, Sarawak to Bongawan, Sabah, has undergone Quaternary uplift, probably episodic and differential, with the most recent uplift event in the Miri area during the early mid-Holocene. Relative vertical stability is suggested for mid-Holocene to present. Geomorphic indicators and lack of emergent RSL indicators along the western Sabah coast, north of Bongawan, suggest ongoing subsidence. The RSL record from eastern Sabah is exceedingly complex with, sometimes close-spaced, areas with different histories of movement over time scales of 101 to 106 yrs