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Abstract

Quaternary deposits in Thailand crop out widely in five physiogra-phical regions: (1) the highlands of Northern Thailand; (2) NE Thailand; (3) the Central Plain; (4) the Upper and Lower Gulf of Thailand coast; and (5) the Andaman Sea coast (Fig. 12.1). Classification of the Quaternary depositional environments in Thailand has tended to be largely dependent on the relationship between landforms and the chronological evidence. The Pleistocene deposits are mainly found to be related to changing river courses, alluvial and braided systems and the degree of weathering of the rock basements. Instances where deposits are related to neotectonics are rare in the literature. Holocene deposits were much influenced by changes in climatic conditions and the sea level.

The distribution of Quaternary deposits (Fig. 12.2) reflects a variety of physiographic settings which in turn determine the stratigraphy from one region to another. In the highlands of the north of the country, the river systems are constrained by the complex basement geology and fluvial deposits reflect this. They are present along the four major river courses (the Ping, the Wang, the Yom and the Nan). They were laid down within restricted intermontane basins, generally as terraces within a limited avulsion plain. Some deposits derived from basaltic rocks are locally lithified. In NE Thailand where Mesozoic sandstones form the basement rocks of the Khorat Plateau, relict terrace gravel beds and aeolian sandy soil (possibly loess) are the dominant Quaternary deposits. Large areas of Quaternary deposits occupy the low-lying Central

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