Stratigraphy of the Mae Sariang Group of Northwestern Thailand: Implication for Paleoenvironments and Tectonic Setting

Abstract

Marine Triassic rocks of the Mae Sariang Group in the Mae Hong Son-Mae Sariang area of north-western Thailand have been investigated in detail for lithostratigraphic subdivision on the bases of field mapping, petrographic, and remote-sensing information. The Mae Sariang Group occurs in a long and narrow north-south facing belt with somewhat folded and faulted structures. The Group conformably overlies the deformed Permian marine clastic rocks and unconformably underlies the subhorizontal, much less deformed, Jurassic clastic rocks of the Huai Pong Group. The Triassic rocks with an overall thickness of 900 meters can be grouped into the following three units: (i) the Kong Sum, (ii) the Pratru Muang and (iii) Mae Laeb units, in ascending order. The Kong Sum unit, with a thickness of 150 to 250 meters, consists mainly of 2 lithofacies; conglomerate and overlying lithic sandstone. Halobia and Daonella bivalves are found in the lithic sandstone and we assign the age of the Kong Som unit as Middle (to Early?) Triassic. The Pratru Muang unit, with a total thickness varying from 200 to 770 meters, comprises of four lithofacies which are from bottom to top; mudstone and sandstone, chert with interbedded mudstone, conglomerate with interbedded sandstone, and sandstone and shale. Abundant radiolarian faunas are reported from the chert and a few Daonella bivalves occur in the sandstone and we consider the age of the Pratru Muang unit to be Triassic. The 80-120 m-thick Mae Laeb unit consists predominantly of three lithofacies; calcareous mudstone and sandstone, siliceous shale with interbedded mudstone, and sandstone with shale in ascending order. Radiolarian faunas in the siliceous shale and abundant Halobia and Posidonia bivalves in the calcareous mudstone suggest the age as being Late Triassic. Thus, based upon stratigraphic and paleontological points of view, the age of the Mae Sariang Group is considered as Middle to Late Triassic. The overall lithology, stratigraphy, sedimentary structure, geometry and fossil assemblages reflect a deep-water submarine fan environment in a subduction-related tectonic setting. Such Triassic deep-water marine facies can be found in several places in northern and southern Thailand.