Bulletin of the Geological Society of Malaysia, Volume 13, Dec. 1980, pp. 27 – 42
Department of Geological Sciences, Chiang Mai University, Chiang Mai, Thailand
Abstract: Cenozoic deposits occur in many small intermontane basins, especially in northern Thailand; in the Chao Phraya basin; and offshore in the Gulf of Thailand. Basins are mainly fault-bounded graben and half graben, formed by reactivation of basement structures. Dips of strata are low but variable, related to local fault movements. Older Cenozoic strata in onshore basins are less than 3000 m thick, and have been dated as ?U. Eocene to L.-M. Pliocene. Strata in offshore basins have been drilled to more than 4000 m depth, with some wells reaching the Oligocene near T.D. A regional upwards facies change from mudstone and coal below to sandstone and mudstone above, locally unconformable, occurs in several northern basins. Pleistocene to Holocene sandstone and conglomerate lie unconformably above the Tertiary. Coal is abundant in the Tertiary, occurring as seams up to 35 m thick interbedded with massive mudstone (shallow lake or swamp deposits); minor carbonate, diatomite, gypsum and oil shale occur locally. Oil shale is common, associated with mudstone-carbonate and sandstone beds (moderate-depth lake deposits). Woody organic fragments occur rarely in coarse terrigenous sequences (fluvial deposits). Rank of coal (68 samples) ranges from lignite to high volatile C bituminous (4523-15,309 Btu/lb). Moisture averages 17.73%, volatile matter 53.81% (dry, mineral-free basis), and sulphur 3.70% (high). All samples are non-agglomerating. Heating value is apparently higher and sulphur content lower in more westerly basins, probably because of local differences in tectonic and thermal history.