Holocene sea‐level changes and the influence of storms on beach ridge formation in the Lower Gulf of Thailand.

Beach ridges are depositional landforms that provide information related to coastal evolution, storm activity and sea‐level variations. However, beach ridges are sometimes modified by aeolian processes, storm washover and/or human activity. Therefore, systematic investigations are required to use beach ridges as an archive of palaeoenvironmental conditions and sea‐level changes. In this study, a multidisciplinary approach involving ground‐penetrating radar, sedimentological analysis and optically stimulated luminescence as well as radiocarbon dating was used to reconstruct sedimentary processes and past sea‐level changes that formed beach ridges in the coastal zone of Nakhon Si Thammarat province, Lower Gulf of Thailand. Ground‐penetrating radar data reveal evidence of beach progradation coupled with the presence of beach scarps and washover deposits. This implies that the formation of the beach ridges occurred under swash processes on the beachface and was later punctuated by erosion during storm surges, leading to the deposition of washover sediments. During a storm, elevated seawater transported moderately to poorly sorted medium‐to‐coarse sand onto a higher position along the beach profile, resulting in an elevation of the beach ridge of up to 4.6 m above mean sea‐level. In addition, aeolian processes contributed to vertical accretion by depositing well‐sorted fine sand on the surface of the beach ridges. The dating results indicate that the formation of the beach ridges occurred approximately between 8.6 ka and 6.1 ka and can be attributed to an upper sea‐level limit of 1.8 to 2.3 m above present‐day mean sea level. Both allogenic (e.g. sea‐level and climate variability) and autogenic (e.g. sediment supply and wave action) factors play crucial roles in the formation and evolution of beach ridges. Therefore, the multidisciplinary approach of this study enhances the understanding of these composite depositional processes and improves palaeoenvironmental reconstructions derived from beach ridges. [ABSTRACT FROM AUTHOR]