Slumping at the late Miocene shelf-edge offshore West Sabah: a view of a turbidite basin margin

Levell, B. K. and Awang Kasumajaya. (1985) Slumping at the late Miocene shelf-edge offshore West Sabah: a view of a turbidite basin margin. Bulletin of the Geological Society of Malaysia, 18. pp. 1-29. ISSN 2637-109X

Abstract

A coalescing series of elongate spoon or scoop-shaped unconformities can be mapped along approximately 150 km of the 250 km long late Miocene shelf-edge offshore West Sabah. In many cases the unconformities both truncate and are overlain by marine sediments and they are interpreted as being due to retrogressive submarine slumping. Subsequent modification by erosional turbidity currents may have occurred, but most unconformities retain a smooth slump scar morphology. Two areas with well developed slump scars are described: (1) Samarang Area: Slump scars have been cut into a 500 m thick sequence of seismically-foresetted slope clays and overlying shallow-water topsets on the upthrown side of a major normal fault. Earthquake activity and slope instability along a submarine fault scarp are thought to have been responsible for the slumping. Failure occurred in either one single event or a series of events closely spaced in time. (2) St. Joseph Area: Slump scars are cut into slope and shallow water sediments deposited on a flank which was rotating down towards the basin, resulting in slope oversteepening. Slumping occurred repeatedly during the deposition of a 1.5-2 km thick section over a relatively long time period. The proximity of the slump scars to the palaeo-coastline resulted in extensive deposition of sandy turbidites further offshore. In both areas the dimensions of the slump scars are remarkably similar. Typically 1-5 cubic kilometres of sediments were re-deposited down-slope during slumping and subsequent erosion. Four exploration wells have penetrated the slump scar unconformities and their fills. With one exception, the fill consists of a monotonous deep water claystone succession. On seismic sections the fill is normally poorly reflective and shows weak seismic foresetting indicative of slope progradation. The slump scars have a two-fold significance for hydrocarbon exploration: Firstly the relief created between neighbouring slump scars, overlain by slope clays provides potential for stratigraphic trapping. Secondly the unconformities allow identification of the stratigraphic units which have been redeposited basinward. Hence the sand- proneness of a turbidite basin can be indirectly assessed. Offshore West Sabah there is a clear relationship between the destructive slope and a major sandbearing turbidite basin in the northern part of the area. The widespread occurrence of slump scars in the late Miocene of offshore West Sabah is attributed to active linear basement fault zones which acted as basin margins, and rapid progradation of a thick clastic sequence, possibly aided by an early late Miocene (global) fall in sea level.

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