Sedimentological characteristics of storm-induced washover deposits along the coastal zone of Changwat Prachuap Khiri Khan to Nakhon Sri Thammarat, Thailand

Abstract

Modern and ancient storm surge washover deposits from Thailand generated by several storm surge events along the southern coast of the Gulf of Thailand side were analyzed aiming to improve the understanding in the typical sedimentary features within the washover deposit. The studies sites were focused where there have been affected by modern storm surges at Prachuap Khiri Khan, Chumphon, Surat Thani, and Nakhon Sri Thammarat Provinces. Washover storm deposits were found in a swale behind beach, tidal floodplain, and sand spit. Washover deposits were characterized into three types including, perched fan, washover terrace, and sheetwash lineations. These depositional patterns were found within a distance 100 m and 400 m far inland from shoreline for modern and ancient deposits, respectively. The investigations in sedimentary structures were performed in both lateral and horizontal variations. As a result, the modern washover sediments were differentiated into two types based on sedimentary characteristics, including (i) a thick-bedded sand of multiple reverse grading layers and (ii) a medium-bedded sand of multiple normal grading layers. Washover deposits show landward dipping of horizontal to subhorizontal layers with the bottom sharp contact to the underlying buried surface. The thickness ranges from 1-80 cm and commonly decreases inland. Multiple grading layers of normal and reverse graded sand (up to 12 layers) were identified and can be used as typical feature of washover storm deposit for this study. At the lower part of each storm depositional layer, laminae of medium to coarse sand and shells were observed. Foreset bedding and lamination are common at the distal part of washover margin. Inclination of bedding was recognized only in unidirectional side from seaward to landward. Post modifications of the deposit by bioturbation from root plants, aeolian process, and erosion by the latter storm surge were also recognized. Dark organic layer of burial vegetation and small grain sand layer reworked by aeolian process between sand successions is one characteristic that can be used as sedimentological clue to distinguish the two different storm depositional events.