The intrusion of S-type granite commencing in Late Cretaceous time along the western half of Thailand produced tensional fault systems which gave rise to isolated intermontane and intercratonic basins throughout Thailand. During the Tertiary period,coal,oil, and oil shale-bearing formations accumulated in these basins. The associated facies, floral, faunal, and microfossil contents indicate that most deposits accumulated in freshwater environments with occasional marine incursion in northern Thailand, while littoral environments dominated in southern Thailand.
In northern Thailand, Cenozoic deposition may be divided into three periods, vis: fault-graben development ; organic sediment accumulation; and diagenetic periods. The fault-graben development period is represented by the Nong Bua Formation which comprises a fluvial fining-upward sedimentary sequence. In the organic sediment accumulation period, the following nine Formations are recognised: Lower Mae Tun Formation of intramontane forest swamp environment; Li, Upper
Mae Tun, Pratu Tao and Mae Long Formations of fluvial-forest/fluviolacustrine environment; Lan Krabu, Lower Mae Sod, Upper Mae Sod and Mae Moh Formations of lacustrine environment. In the. diagenetic period, six fluvial and fluviolacustrine Formations are recognised . They are Mae Fang, Huai Luang, Yom, Ping, Moei River and Mae Taeng Formations.