Latest Pleistocene to Holocene stratigraphic record and evolution of the Paleo-Mekong incised valley, Vietnam

Abstract

Recent 200+ km progradation of the Mekong River delta over the last 6000 years has provided a sequence of incised-valley fills with insights into the sedimentary response of a large river system to sea-level changes since the Last Glacial Maximum (LGM). This study provides an analysis of comprehensive synthesis of the incised-valley and successive progradational deposits of the paleo-Mekong River across a regional area spanning from the Cambodian lowland to the Vietnamese shelf. The study is based on the analysis of the depositional facies, diatom and foraminiferal assemblages, and radiocarbon ages of three new cores and previously published cores. The depositional succession observed in these cores is divided into the lower and upper units representing Pleistocene weathered sediment and incised valley fills and delta sediment since the LGM, respectively. The upper unit shows an upward-deepening succession from the fluvial channel to estuary and subtidal deposits, followed by an upward-shallowing succession consisting of progradational deltaic deposits. This unit can be divided into four phases: the fluvial channel sedimentation (before ~13.4 ka); retrogradational and aggradational estuarine system (~13 to ca. 8 ka), aggradational and progradational delta system (ca. 8 to 6 ka), progradational delta system (6 ka to present). Tides were dominant from 13 to ~3.5 ka, followed by wave- and tide-dominated environments until present at its river-mouth and coast.The data from three new cores, along with existing data, enable paleogeography reconstruction of the incised valley and subsequent delta progradation in four stages at 10, 8, 6, and 4 ka. The results indicate the Mekong River incised valley was ‐−35 to −40.0 m deep and 25 km wide in comparison with surrounding interfluve areas at 6 ka. Additionally, it was filled mostly during transgression from 13 to 8 ka and early highstand periods including its shelf valley beneath the present delta plain, which was filled completely by 4 ka. In the upper delta plain in Vietnam, the succession formed during 8–6 ka appears to have been removed and then deposited by subsequent fluvial channel/tidal river facies dated 4.4 to 2.2 ka.