Lithostratigraphy of the Late Triassic Huai Hin Lat Formation at Amphoe Nam Nao, Changwat Phetchabun

Abstract

The Huai Hin Lat Formation is the pre-Khorat Group strata (Racey, 1996) distributed at the western edge of the Khorat plateau. It can be divided into 5 members (Chonglakmani and Sattayarak, 1978) which almost all consist of grey to black shale, mudstone, siltstone, sandstone, conglomerate, limestone, volcanic clastic rock and volcanic rock. In the study area, Amphoe Nam Nao, Changwat Phetchabun, the distribution of Huai Hin Lat Formation is from the northwestern side to the southeastern side of the area. The aim of this report is to study in detail the lithostratigraphy of Huai Hin Lat Formation in the area of Amphoe Nam Nao, Changwat Phetchabun. It is found that rock units in the study area can be classified into 4 units from ascending order as follows; unit A consists of black calcareous shale, well bedded and likely to be deposited in deep lacustrine environment. Unit B consists of grey calcareous shale, mudstone, siltstone, argillaceous limestone and coal seam which were deposited in the lacustrine margin and swamp environment. Unit C consists of grey calcareous shale, mudstone, siltstone and sandstone which was deposited in the transitional zone from lacustrine to fluvial system. Unit D consists of purplish red sandstone and conglomerate which is not classified into Huai Hin Lat Formation.