Lithostratigraphy of some triassic clastic rocks in southern part of Amphoe Muang Mae Hong Son, Changwat Mae Hong Son, Northwestern Thailand

The marine Triassic rocks in the Mae Hong Son-Mae Sariang area in northwestern Thailand have long been known as Mae Sariang Group. The study area of approximately 2,240 square kilometers is located in Mae Hong Son Provinces. In this study, the Mae Sariang Group is proposed and lithostratigraphically subdivided into 3 formations, namely, Kong Sum, Pra Trumuang, and Mae Leab formations, respectively, in ascending order. The Mae Sariang Group overlies the marine Permian rocks. The total thickness above 900 meters. The Kong Sum formation consists of 2 lithofacies; the lower conglomerate and the lithic sandstone. The total thickness varies 150 to 250 meters. The Pra Trumuang formation consists mainly of 4 lithofacies; the dark gray mudstone and sandstone, the chert interbedded mudstone, the conglomerate interbedded sandstone, and the sandstone and shale, respectively. The total thickness varies from 200 to 770 meters. The Mae Leab Pong formation predominantly consists of the calcareous mudstone and sandstone, the siliceous shale interbedded mudstone, and the medium sandstone in ascending order. The calcareous mudstone with abundant Halobia sp. indicates in the Triassic age. The total thickness varies from 80 to 120 meters. The lithology, sedimentary structures, geometry, and fossil assemblages reflect deep-water submarine fan environment. Stratigraphically and paleontologically, the age of the Mae Sariang Group should be assigned as Middle to Upper Triassic. Detrital chromian spinels probably indicate the provenance from ultramafic and basaltic volcanics for the Mae Sariang Group spinels. Evidences from geochemical as well as petrographical investigations reveal that the detrital chromian spinels of the Mae Sariang Group occurred in response to mid-ocean ridge and intraplate basalt.