Abstract: The Mekong River Delta in southern Vietnam is a typical mixed tide- and wave-dominated delta with a wide delta plain formed during the last 6 to 8 ky. The late Pleistocene-Holocene sequence in this region since the last sea-level lowstand is composed of an incised- valley fill, comprising late Pleistocene to early Holocene fluvial to estuarine sediments, and Holocene deltaic sediments. The deltaic sediments are composed of early Holocene aggradational sediments and progradational sediments of middle to late Holocene age. The progradational delta has evolved from a tide-dominated delta to the present mixed wave- and tide-dominated delta during the last 3 ky and shows a sudden regime shift of coastal environment. The tide-dominated delta was characterized by a well-developed mangrove forest on the subaerial delta plain, cross-shore sediment dispersal, and tide-influenced sedimentary facies. The tide- and wave-dominated delta exhibits a beach-ridge system in the subaerial delta plain, longshore sediment dispersal, and a steep delta-front topography, in which the sediments of the proximal river-mouth area are coarse-grained and those more distal are fine-grained. As river-mouth coastal systems evolve from estuaries to deltas in response to sea-level changes, the formation of a tide-dominated delta marks the initiation of the highstand systems tract.

Abstract The Holocene Mekong Delta is a good example of a delta that has evolved from a tide-dominated to a tide–wave mixed-type delta in its shape, sediment facies, and progradation rate during the regression after the maximum flooding during the Holocene. This evolution indicates that tide-dominated deltas developed well in the early phase of the highstand systems tract and that they changed into more wave-dominated deltas in response to subsequent regression. The Mekong Delta, which is located at the southeast tip of the Indochina Peninsula, is one of the largest deltas in Southeast Asia, with a large delta plain ranked third largest in the world. The present Mekong Delta is classified as a tide-dominated delta, but close to a wave-dominated delta, in the triangular deltaic depositional classification system. Two rotary-drilled cores, VL1 and TV1, were obtained from the flood plain and strand plain, respectively, of the delta plain. Detailed core descriptions, high-resolution 14 C dating by accelerator mass spectrometry (AMS), and topographical analysis of the delta plain indicate that the Mekong Delta evolved from a tide-dominated delta from 6.5 to 2.5 ka to a tide–wave mixed delta from 2.5 ka to the present. The present delta is characterized by beach ridges on the strand plain, dominantly wave-influenced sediments, and a low progradation rate due to longshore sediment dispersal by waves. As the sea level rises, marine flooding causes coastal inundation and the formation of embayments, which amplifies tides and shelters deltas from waves, so that they are more tide dominated. However, as a delta progrades and its shoreline migrates seaward, the resultant smoothed coastline, narrow shelf, and increased wave energy create a more wave-dominated coastal setting. This change will become more pronounced as the delta continues to prograde towards shelf margins.