Holocene evolution of Phra Thong's beach-ridge plain (Thailand) — Chronology, processes and driving factors

Beach-ridge plains are a common morphological element of coastal areas all over the world. While morphological depressions of beach-ridge plains (swales) may function as archives for deposits of extreme wave events, the ridges are not only used as an indicator for coastal evolution but also to reconstruct the processes that are responsible for their formation. In this study, the morphological, sedimentary and chronological structure of the beach-ridge plain on Phra Thong Island was used as an archive for the temporally and spatially complex pattern of shoreline changes during the Holocene. The presence of three spatio-temporal ridge-plain units separated by episodes of shoreline erosion and characterised by varying directions and rates of shoreline progradation, reveals six distinct phases of island evolution within the last 6000 years. Unit I was deposited during phase 1 (5500–4000 years ago) with high progradation rates of 2.1–2.5 m/year. Subsequent to a short episode of erosion during phase 2 (4000–3800 years ago), fast shoreline progradation at a rate of 2.7 m/year (unit IIa) occurred once again in phase 3 (3800–3300 years ago). After 3300 years ago, the sedimentation rate decreased to less than 1 m/year, resulting in the formation of unit IIb during phase 4 (3300–1500 years ago). In phase 5, between 1500 and 800 years ago, ridge-plain formation was once more interrupted by significant shoreline erosion. Finally, over most of the last 800 years, during phase 6, slow shoreline progradation led to the formation of unit III. This evolution is best explained by the interaction of long-term sea-level change, wave climate, episodic events (tsunamis and storms) and local sediment supply. However, the linkage of successive ridge and swale formation, changing progradation rates and switch-over between phases of erosion and deposition with its trigger mechanisms remains speculative, since (i) the processes involved vary locally and are poorly understood for Phra Thong Island, and (ii) even our comparably large dataset cannot reconstruct the complex spatial and temporal pattern of shoreline evolution without significant uncertainties. This implies that interpretations of ridge plains based on small datasets might be too generalised to capture the real complexity of the system and, as a consequence, might lead to incorrect conclusions about palaeoenvironmental conditions.