Lithostratigraphy and petrography of marine Jurassic rocks in the Mae Sot area, Tak Province, western Thailand: Implications for depositional environment and tectonics

摘要

Geology, lithostratigraphy, petrography, sedimentary structures and depositional environment of marine Jurassic rocks of the Hua Fai Group (HFG) from the Mae Sot area of Tak Province, western Thailand have been studied in detail. Based mainly on five measured sections in the Mae Sot Basin, the group can be divided into three formations: Khun Huai; Doi Yot; and Pha De, in ascending order. Seventeen sedimentary rock units have been established with a total thickness varying from 200 to 832 m. According to the fossil assemblages, the HFG was deposited during Toarcian–Bajocian. Analysis of lithofacies associations in the sedimentary sequences of the group discerns shoreface, fan-delta, protected lagoon, intertidal, subtidal and inner to outer ramp environments with occasional carbonate platforms and reef flats. Toarcian rocks are represented by transgressive–regressive (T–R) cycles that in the Aalenian gradually changed to highest sea level and water depth. During late Aalenian to early Bajocian, sea level was still changing to a transgressive phase, then after early Bajocian, sea level retreated again. The eustatic curves for the Toarcian–early Bajocian in Thailand correspond to the global situation, but differ significantly in the Late Jurassic–Cretaceous, when T–R phases were out of step with the global pattern and instead probably responded to local tectonic movements.