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

Abstract

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 to 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.

Peat formation concentrates arsenic within sediment deposits of the Mekong Delta

“…The peat at 6 m depth in our site, aged 5.675 ± 0.020 ka, which directly underlies gray sediments representative of a flood plain depositional environment, likely corresponds to the "organic mud" of similar depth in sediment cores collected proximal to our field site (Tamura et al, 2009). Thus, the peat developed in a saline environment that likely continued even as the coastline receded southeast to present-day Vietnam, as saltwater intrusion may have occurred up to tens of kilometers along the Mekong during high tide in the dry season -the extent of present-day seawater intrusion (Hori, 2000;Tamura et al, 2007). Seawater likely assisted in supplying the necessary sulfur (sulfate) for arsenian pyrite formation.…”

Monsoon-influenced variations in morphology and sediment of a mesotidal beach on the Mekong River delta coast

“…1; Hori, 2000). The delta has grown since 8 ka (Nguyen et al, 2000;Ta et al, 2005;Tamura et al, 2007Tamura et al, , 2009, and its apex lies just north of Phnom Penh, Cambodia, more than 200 km from the river mouth. From its apex, the delta fans out to the southeast and, along with the Camau Peninsula, separates the South China Sea from the Gulf of Thailand.…”

“…The delta coast has prograded more than 200 km seaward since 8 ka (Tamura et al, 2007(Tamura et al, , 2009) as a result of the voluminous supply of sediment from the Mekong River. The rate of progradation over the last 3 ka has been estimated to be about 10-20 m y − 1 (Ta et al, 2005).…”

Luminescence dating of beach ridges for characterizing multi-decadal to centennial deltaic shoreline changes during Late Holocene, Mekong River delta

“…The huge sediment supply to the coast has resulted in an extensive delta plain with an area of 5.0 × 10 4 km 2 (Hori, 2000). The delta apex lies just north of Phnom Penh, Cambodia, more than 200 km inland from the present river mouth (Tamura et al, 2007(Tamura et al, , 2009, from which the delta fans out to the southeast (Fig. 1A).…”

“…The Mekong River delta, an excellent example of a mixed tide-and wave-dominated delta (Galloway, 1975), has prograded more than 200 km seawards since c. 8000 years (Tamura et al, 2007(Tamura et al, , 2009. Tamura et al (2012), based on 31 OSL ages of beach ridges on several delta plains of the Mekong delta coupled with sediment drill cores and their radiocarbon dates (Ta et al, 2005), clarified the multi-centennial to millennial-scale, three dimensional evolution of the delta.…”