DELTA INITIATION AND HOLOCENE SEA-LEVEL CHANGE: EXAMPLE FROM THE SONG HONG (RED RIVER) DELTA, VIETNAM

Delta initiation and subsequent delta growth were investigated by using radiocarbon-dated borehole cores from the Song Hong (Red River) delta, Vietnam. The sediments underlying the present delta plain record three depositional systems since the last glacial maximum (LGM): fluvial, estuarine, and deltaic, in ascending order. The landward part of the initial delta was characterized by an active tide-influenced accumulation of sandy sediments. The seaward part of the initial delta is composed of muddy prodelta and seafloor sediments that accumulated slowly. Radiocarbon ages of molluscan shells and organic materials suggest that the transition from estuary to delta occurred at approximately 8500 cal yr BP. The transition was induced by deceleration of the postglacial sea-level rise, which was coeval with the delta initiation. The stacking pattern of the delta system is very different from that of the estuary system. The sediments of the estuary that developed during the rapid rise in sea level between 11000 and 8500 cal yr BP display a retrogradational stacking pattern, forming a thick transgressive systems tract. On the other hand, aggradational as well as progradational stacking with clinoform architecture characterized the delta system between 8500 and 6500 cal yr BP as the sea level continued its slow rise. Progradation has been dominant during the last 6500 yr because the cessation of sea-level rise resulted in little accommodation being added. The results of this study are similar to those for other major delta systems, such as the Changjiang and the Ganges-Brahmaputra deltas, despite large differences in sediment discharge, tectonics, basin size, and coastal environment. This implies that the postglacial sea-level rise and the change in the rise rate markedly influenced the evolution of depositional systems at river mouths.