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

Abstract: The rift basins of Thailand exhibit remarkable diversity of fault displacement patterns, fault length–displacement characteristics and mapped fault patterns during late rift, and post-rift, stages. These patterns reflect influences by: (1) zones of strength anisotropy in the pre-rift basement; (2) syn-rift fault patterns on post-rift faults; (3) spatial stress deflection, commonly related to irregularities in major fault profiles, and the basement–sediment interface; (4) temporal stress rotation, usually related to changes in the regional plate setting; and (5) varying strength properties (strain hardening or softening) of fault zones during their life. These influences created strongly segmented boundary faults, and long, low-displacement post-rift fault trends. The former are commonly strongly over-displaced, while the latter can be strongly under-displaced with respect to their length compared with typical length:displacement distributions. Seismic interpretation of multi-rift fault patterns requires 3D data to identify the complexities, otherwise the linkage pattern between deeper and shallower faults, and the changing fault strike-directions with depth, may be incorrectly mapped. Incorrect identification of fault patterns as breached relay structures may also arise. Oblique extension, the influence of pre-existing trends and stress rotation in multi-phase rifts provides a more comprehensive explanation for the observed features than the strike-slip interpretation of previous studies.

Abstract Cretaceous sedimentary rocks in Thailand are entirely of non-marine facies. They are most widespread on the Khorat Plateau in NE Thailand (Fig. 8.1 ) where they are well exposed, rela tively simply deformed and fossiliferous. For those reasons the Cretaceous there has received more attention than elsewhere in the country (e.g. Ward & Bunnag 1964 ; Mouret et al. 1993 ; Racey et al. 1996 ) and much of what follows in this account relates to the stratigraphy of the Khorat Plateau region. However, the Cretaceous occurs also in Southeast Thailand and on the Peninsula. Brown et al. (1951) introduced the term Korat Series (sic) for the largely red-bed sequence of the Khorat Plateau and extended its use across the whole of Thailand. Although they considered its age to be Triassic-Jurassic they used the term as a synonym for the Mesozoic. That broad definition of the Khorat Group (note the change of spelling), extending up from the base of the Triassic Huai Hin Lat Formation to the top of the Phu Thok Formation, has been followed by most geologists in Thailand and adopted by the Department of Mineral Resources (DMR) in their 1:1 million scale geological map published in 1999. Others have adopted a more restricted definition of the Khorat Group, notably Racey et al. (1994) who recognized an unconformity at the base of the Phu Kradung Formation (also spelt Phu Kadung) and adopted the term Khorat Group for the five formations