Depositional facies and radiocarbon ages of a drill core from the Mekong River lowland near Phnom Penh, Cambodia: Evidence for tidal sedimentation at the time of Holocene maximum flooding

The depositional facies and radiocarbon ages of a long Holocene sediment core (KS) from Cambodia are reported here to clarify the sedimentary environments of the Mekong River delta system at the time of Holocene maximum flooding by the sea. The 30.7-m-long KS core, from the upper Mekong River lowland about 20 km southeast of Phnom Penh, penetrated five depositional facies, A to E in ascending order. Facies A is cross-laminated fluvial sand. Facies B is laminated, very fine sand with mud drapes. Facies C to E are salt marsh, flood-plain and natural-levee deposits, characterized by a succession of peat, organic clay, and reddish-brown silt. Facies B dates to 9.0–7.5 ka and is interpreted as aggradational tidal deposits, 16.5 m thick, deposited during a rapid rise of sea level; these deposits became overlain by an accumulation of salt marsh and flood-plain deposits during the subsequent period of slowly rising and then falling sea level. The maximum flooding surface lies in the Facies B interval. The presence of tidal deposits implies that the core site, about 230 km inland from the present river mouth, was near the shoreline and experienced strong tidal influences during the early Holocene.