Late Pleistocene sea-level changes and the formation and fill of bent valleys incised into the shelf of the western South China Sea

During times of lowered sea level, Mekong River and Red River incised valleys into the ancient coastal plains of the exposed shelves of the western South China Sea. The deglacial fill history of the incised valley were investigated by seismic surveys and sediment cores. The channels mainly exhibit a low-sinuosity course, but some channel segments are bent. The oldest part of the channel-bend fill exhibits shingled reflectors in Parasound seismic records documenting lateral channel migration typical of meandering rivers. Above, vertically stacked reflectors, which extend from the inner-bend side onto the cut-bank side document that the river-mouth channel turned to a mainly depositional mode. Vertical aggradation started when sea level was ~ 1-2 m below river water-level. During this phase, about two-third of the channel depth was filled by "fluvial-to- estuarine" mud having negative log(Ti/Ca) values as ewvidenced in XRF core scan data. Typically estuarine conditions developed when river water-level was approximate to sea level. Today channel bends form in estuaries within the zone of bedload convergence. Therefore, it is suggested that the studied channel bends represent antecedent, inherited features that formed during phases of prolonged phases of lowered, but relatively stable sea level to allow bends morphologically to develop. In fact, the bends occur not only in the western South China Sea but also in other areas of the world within a depth range that corresponds to times of retarded sea-level fall during MIS 5b, 5d, 4, and 3.