Sedimentary Facies of Borehole Cores from the Mekong River Floodplain in Cambodia

Abstract

This study analyzes three borehole cores to examine the sedimentary facies of the extensive floodplain of the Mekong River in Cambodia and discusses the relationship between sedimentary facies and surface topography. Sedimentary facies vary according to surface topographic features on the floodplain. Core PA, located near the end of an abandoned channel, records the history of the formation and subsequent filling of the channel during the Holocene. The channel was filled with interlaminated sand and mud overlain by grayish brown to dull reddish brown stiff mud during the last 4500 years. Core CK was located on the southwestern edge of an area of scroll bars and swales between the Mekong and Bassac rivers. In this core, muddy overbank deposits overlie sandy deposits that probably represent a point bar. The accumulation of the overbank deposits may have started around 2000 cal yr BP. Core TA was obtained from the floodplain along the west bank of the Tonle Sap River. Mottled stiff mud containing granules occurs about 1m below the surface and overlies subangular granules. Muddy overbank deposits corresponding to those of the other two boreholes are not present at the site despite frequent flooding of the Tonle Sap River.