Depositional sequences of the Mekong river delta and adjacent shelf over the past 140 kyr, southern Vietnam.

• Depositional sequences and sedimentary facies/units were established on core records and seismic profiles. • Depositional sequences were primarily constrained by the sea-level change over the past 140 kyr. • Regressive systems tract is dominant compared to transgressive systems tract in a complete sequence. Two 40 m-long sediment cores BL01 and SC02 were drilled in the lower Mekong River Delta plain and compared to high-resolution seismic profiles from the adjacent shelf documenting the presence of two depositional sequences (SQ1, SQ2) for the past 140 kyr. SQ2 is a fourth-order sequence and covers a complete sea-level cycle of 120 kyr. SQ1 is a fifth-order sequence comprising a sea-level half cycle in the past 20 kyr. These two sequences are bounded by the sequence boundaries (SB1, SB2) and the topographic surface. The transgressive systems tract (TST2) of SQ2 was probably formed during the sea-level rise from the Marine Isotope Stage (MIS) 6 lowstand to the MIS 5e highstand, indicated by riverine-estuarine and shallow-marine sediments. The regressive systems tract (RST2) of SQ2 corresponds to the sea-level lowering from MIS 5e to 3 with deltaic and alluvial plain sediments prevailing. These sediments were then gradually exposed to tropical subaerial processes to generate the formation of patchy reddish-orange stiff sandy silt and clay. The overlaying sequence SQ1 contains the lowstand systems tract (LST1), TST1, and RST1. LST1 consists of deltaic sediments at the shelf edge deposited during the MIS 2 sea-level lowstand. TST1 includes retrograding-fluvial, estuarine, and shallow-marine sediments deposited during the deglacial sea-level rise 19–8 cal kyr BP and the maximum flooding period 8–6 cal kyr BP before the advance of the modern Mekong River Delta. RST1 comprises the modern delta-facies association formed during the past 8 kyr and is restricted to the water depth of 25 m. [ABSTRACT FROM AUTHOR]