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

A coalescing series of elongate spoon or scoop-shaped unconformities can be mapped along approximately 150 Ian 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 weU developed slump scars are described: (I) Samarang Area: Slump scars have been cut into a 500 m thick sequence of seismicaUy foresetted slope clays and overlying shaUow-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) SI. 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 ofthe 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 fiU is normally poorly reflective and shows weak seismic foresetting indicative of slope progradation.