Sedimentary facies, diatom and foraminifer assemblages in a late Pleistocene-Holocene incised-valley sequence from the Mekong River Delta, Bentre Province, Southern Vietnam: the BT2 core

A detailed description of sediment facies, diatom, mollusca and foraminifer assemblages, and C-14 ages of the Mekong River Delta, Southern Vietnam, is presented in this paper. A 71 m long core, recovered in 1997 at the Bentre province in the eastern p art of the Mekong River Delta, is divided into eight units and nine sedimen t facies on the basis of sedimentary properties, diatom and foraminifer ass emblages, and C-14 ages. These units show facies changes from transgression to regression in relation to the late Pleistocene-Holocene sea-level chang es. The Postglacial transgression caused by a sea-level rise led to infilling o f the incised valley and formation of estuarine sediments and open bay mudd y sediments in ascending order. Estuarine sediments consist of slightly oxi dized yellowish-grey silty sand and intercalated greenish-grey and stiffly brownish-grey sandy silt and silty clay. The muddy facies, 13.5 m thick, is characterized by abundant marine plank-tonic diatoms and open-sea foramini fers. This facies indicates the maximum Holocene marine influence was aroun d 5300 cal yr BP at the BT2 site. The regressive succession is composed of deltaic sediments from prodelta, d elta front, sub- to inter-tidal flat and beach ridge in ascending order. C- 14 ages indicate that the delta front passed the core site at approximately 4000-3000 cat yr BP. The coastal progradation rate was 17-18 in yr(-1) dur ing the 5300-3500 cal yr and decreased to 13-14 m yr(-1) during the last 35 00 cat yr. (C) 2001 Elsevier Science Ltd. All rights reserved.