SEDIMENT FACIES AND LATE HOLOCENE EVOLUTION OF THE MEKONG RIVER DELTA IN BENTRE PROVINCE

Abstract

Three cores (BT1, BT2, and BT3) were taken from Bentre Province in the lower delta plain, Mekong River Delta, with the objective to investigate the sediment facies and evolution of the Mekong River Delta during late Holocene. These cores were examined by analyses of grain size, molluscan shells, foraminifers, diatoms, sedimentary structure by X-radiographs and l4C ages by accelerator mass spectrometry (AMS). The incised valley that formed during last glacial maximum was located around the BT2 site. Transgressive incised-valley fill sediments are composed of estuarine channel/tidal river sandy silt, muddy tidal flat/salt marsh, estuarine marine sand, transitional sandy silt, and finally open bay mud facies in ascending order and dated from 13 to 5.3 ky BP. The coarsening-upward succession of regressive deltaic facies after 5.3 ky BP., with the thickness ranging from 10 to 20 m, are prodelta mud, delta front sandy silt, subtidal to intertidal flat sandy silt, and subaerial delta plain facies. Progradation rate was decreased from 17-18 m y-1 during 5.3 to 3.5 cal ky BP to 13–14 m y-1 during the last 3.5 cal ky. The reduced progradation rates are reflected by southward sediment dispersal by longshore currents caused by monsoon generated waves.