Stratigraphy and palaeoenvironment of the Bangkok Clay (Holocene) from Samut Sakhon Province, Central Thailand

Abstract. This study investigates the lithostratigraphy and palaeoenvironmental conditions of Holocene sediments from five boreholes in the Lower Central Plain of Thailand, with a focus on ostracod assemblages and sedimentary facies. Lithostratigraphic analyses identified five lithologic units belonging to two distinct facies representing tidal–intertidal and prodelta environments. Ostracod analysis identified 15 species, representing genera found in the Indo-Pacific and South China regions, including Neocyprideis, Sinocytheridea, Propontocypris, Hemicytheridea, Keijella, Neomonoceratina, Aglaiocypris, Lankacythere, Cytherella, and Stigmatocythere. Ostracods recovered from tidal–intertidal facies suggest transportation from shallow marine environments to tidal channels, while the greater diversity in prodelta facies indicates a more stable and favourable environment. Neocyprideis agilis was found abundantly in the samples, marking the oldest known record of this species in the South China Sea during the Late Holocene. The facies succession is characterised by a fining-upward trend, reflecting a shift from Lower–Middle Holocene tidal channels and intertidal flats to a Late Holocene prodelta system. These findings clarify the depositional history of the Lower Central Plain, demonstrating how tidal and marine processes shaped a dynamic, tide-dominated palaeoenvironment throughout the Holocene.