ANALYSIS OF ANCIENT STORM DEPOSITS IN CHANGWAT PRACHUAP KHIRI KHAN

Abstract

The analysis of ancient storm deposits in Changwat Prachuap Khiri Khan along the Gulf of Thailand is aimed to analyze the sedimentological characteristics and to define depositional age of ancient storm. Ancient storm sediments were found within a distance of 350 m landward from the present shoreline. Thickness of ancient storm deposits varies from 0.5 to 50 cm. The numbers of ancient storm layers possibly indicate the numbers of storm occurrences through time. In this research, ancient storm layers were found up to twenty-seven layers. Grain size of ancient storm sediments ranges from coarse- to very fine-grained sand. Sedimentary structures of ancient storm deposits contain sharp upper and lower contact, normal and reverse grading, parallel lamination and mud rip-up clasts. The thicknesses and grain size of deposit are thinner and finer landward. Furthermore, ancient storm layers contain several microfossils (foraminifera and ostracod) and macrofossils (gastropod and bivalve). The ancient storm sediments contain mostly quartz, heavy minerals and shell fragments. The results of Optically Stimulated Luminescence ages from beach ridges and the correlation of AMS radiocarbon age from a wood fragment from the adjacent area indicate that there are four ancient storm layers occurred in the late-Holocene to the mid-Holocene. There are twenty-three ancient storm layers older than the mid-Holocene. The variability in number of ancient storm deposit during two periods may due to several controlling factors including potential preservation of the area, storm characteristic, the intensity of storm, wind speed, surge height, and climatic condition in the past.