DEVELOPMENT OF CENOZOIC BASINS IN THAILAND

There are over 60 Cenozoic basins distributed in various parts of Thailand, both onshore and offshore. These basins are mainly N-S trending half grabens or grabens and most were initiated in the Late Oligocene. The structural framework of the basins is generally governed by N-S trending extensional faults which are spatially related to the movement of the NW-SE and NE-SW trending strike-slip faults that have been active since the Oligocene. The NW-SE faults are the principle dextral strike-slip faults whereas the NE-SW faults are the conjugate sinistral strike-slip faults. The collision of the India Craton with Southern Asia since 40-50 Ma caused clockwise rotation of Southeast Asia and movements on the strike-slip faults with the associated development of Cenozoic basins in this region. Four major Cenozoic basins, Mergui, Pattani, Malay and Phitsanulok, contain up to 8000 m of sediments. Generally, the Cenozoic fills are continental sediments, except in the Mergui Basin in the Andaman Sea where the sedimentary fill is mainly marine sediments. Fluviatile with occasional lacustrine deposits were prominent in the Gulf of Thailand and onshore basins during the Oligocene to Early Miocene. From Early to Middle Miocene, widespread perennial lake conditions were well developed in the onshore basins and in the Western Graben Area of the Gulf of Thailand. A change in tectonic and/or climatic conditions took place in the Middle to Late Miocene times resulting in the cessation of perennial lake conditions and development of a regional unconformity.