Abstract:
The study of beach ridges plains at Laem Pho study area in Amphoe Chaiya, Changwat Surat Thani is aimed to investigate coastal geomorphology and sedimentology to analyze the history of Holocene sea level change. The methodology starts with satellite image interpretation of landform for field survey planning, then, geomorphologic map was done for locating sample collection sites. Grainsize analysis and Optically Stimulated Luminescence (OSL) dating were carried out. As a result, the study area has dominant landforms of beach ridge plains and sand spit oriented in southeast direction correspondence with southeast longshore current. Source of sediment supply to this coastal plain is possibly come from Phumriang canal locating between former and recent beach ridge plain of Lam Pho. Furthermore, the orientations of beach ridges in this area have individual characteristic and can be classified into 5 series. The innermost beach ridge series can be indicator of paleo-shoreline locating approximately 4.5 km2 far from the present shoreline. The three profiles of topography survey indicate the continuation of progradation in eastward direction. Three units of geomorphological landforms as evidences of sea level change consist of old sandy beach, young sandy beach, and old lagoon. Average grain sizes of sand from all beach ridges is coarse- to medium-grained sand. Fossils and microfossils found in beach deposit consist of bivalvia, scaphopoda, foraminiferas, and ostracods. OSL dating of the oldest sand ridge indicate the age of 7,170±460 years BP, whereas, the youngest beach forming sand spit at 110 years ago. All these results indicate that the study area including ridges and swales was formed as beach ridge plain with eastward progradation during the regression from the mid to late Holocene.
