Evidence of Holocene sea level regression from Chumphon coast of the Gulf of Thailand

Abstract

Sets of beach ridge plains located as far as ten kilometres inland at the Chumphon estuary area, southern peninsular Thailand, provide evidence of an ancient sea-level change. Relict coastal landforms including former beach ridge plains, old lagoons, swales, and former tidal flats between beach ridges are found inland at elevations of 1-5 m above the present sea level. Ancient beach ridges here comprise three sets and all are inferred to have been deposited during the Holocene. The orientation of the inner beach ridge plains confirms their deposition by south to north longshore currents, whereas the middle and outer ridges were probably formed by north-to-south currents. Grain size analysis shows that all ancient beach ridge deposits are similar and are composed of fine- to medium-grained sand with high sphericity. Likewise, the compositions are similar, with quartz as a major component and feldspar and ferromagnesian minerals as minor components, reflecting the same sediment provenance. Marine fossils found in the former tidal deposit indicate intertidal and mangrove environments. Optical stimulating luminescence dating of three sets of beach ridge plains indicate that deposition occurred 8900-5600, 5900-2700, and 3800-1600 years ago for inner, middle, and outer beach ridge plains, respectively.