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

Abstract

A detailed description of sediment facies, diatom, mollusca and foraminifer assemblages, and 14C 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 part of the Mekong River Delta, is divided into eight units and nine sediment facies on the basis of sedimentary properties, diatom and foraminifer assemblages, and 14C ages. These units show facies changes from transgression to regression in relation to the late Pleistocene–Holocene sea-level changes. The Postglacial transgression caused by a sea-level rise led to infilling of the incised valley and formation of estuarine sediments and open bay muddy sediments in ascending order. Estuarine sediments consist of slightly oxidized 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 planktonic diatoms and open-sea foraminifers. This facies indicates the maximum Holocene marine influence was around 5300 cal yr BP at the BT2 site. The regressive succession is composed of deltaic sediments from prodelta, delta front, sub- to inter-tidal flat and beach ridge in ascending order. 14C ages indicate that the delta front passed the core site at approximately 4000–3000 cal yr BP. The coastal progradation rate was 17–18 m yr −1 during the 5300–3500 cal yr and decreased to 13–14 m yr −1 during the last 3500 cal yr.