Stratigraphy and leaf fossils at Doi Ton, Mae Kasa sub-district, Mae Sot district, Tak province: implications for the paleoenvironment

Abstract

Doi Ton is a small hill in Mae Kasa Sub-district, Mae Sot District, Tak Province, northern Mae Sot Basin. This area is outstanding for its leaf fossil deposit in sandstone of which the geologic age is still controversial. This research focuses on sedimentology, stratigraphy, petrography, mineralogy, geochemistry of the rock, and leaf fossils in order to interpret the paleoenvironment, paleoclimate and compare the geologic age of the study area. The stratigraphy of Doi Ton Formation can be divided into two members, the lower Doi Ton member and the upper Doi Ton member with conformable contact. The beds are orientated about N45 E and 10 dipping in the NW direction. The thickness of this formation is probably greater than 85 meters. The lower Doi Ton member consists of greenish gray mudstone interbedded with thin to very thin, well bedded blackish gray sandstone with carbonate cement, and contains a lot of plant debris (branch, bark, leaf with cluster deposit), gastropod and bivalve fossils. Outcrop of the lower Doi Ton member is very rare in the study area; however, the valuable information was taken from a small agricultural pond at the 200 meters contour line only. The upper Doi Ton member is cropped out at the Doi Ton Temple and consists of conglomerate, sandstone, and siltstone, containing plant debris, leaf fossils, gastropod, bivalve fossils, and some unclear fossil that similar to echinoderm or fruit imprints. This study was focused on the upper Doi Ton member. The facies analysis of the upper Doi Ton member can be divided into five facies, Sm, Sg, Sr, Sp, and Gm (Gm and Gmg). Furthermore, these facies make up two facies associations, which are (1) channel deposit (Gm, Gmg, Sg, Sr, Sp, and Sm), and (2) sandy bar (Sg, Sr, and Sp). These facies associations indicate the braided river depositional environment. Moreover, the lower member suggests the lacustrine paleoenvironment. The results of petrographic study classified 21 sandstone samples from the upper Doi Ton member, as quartzarenite and sublitharenite; and 9 siltstone samples as sandy siltstone and mudstone. Shape of detrital grains are subangular to subrounded, and low sphericity and they are poorly sorted. Rock fragments consist of mudstone, silicified rock, sandstone, metamorphic rocks, and plutonic igneous rock. The highest and lowest amount of the mineral composition by XRD analysis are quartz followed by kaolinite and mica or illite, respectively. The major oxide compositions were carried out by XRF analysis. The upper Doi Ton member shows highest SiO2 followed by Al2O3 and Fe2O3, MgO, CaO, Na2O, K2O, TiO2, and P2O are low to non-detected content, while there’s small amount to non-detected of MnO in almost every samples. The leaf fossil study can classify the leaves to be 23 morphotypes of dicot leaves and two morphotypes of unknown leaves. The petrographic results suggested the provenance settings as recycled orogeny and interior craton. Besides, the geochemistry shows the intermediate to felsic composition of source rock or quartzose sedimentary rock that are deposited in the basin on a passive margin. The paleoclimate during the sedimentation is suggested to be hot and humid conditions with high intensity of weathering process by the petrographic and geochemistry studies. The source rock can be related to the rocks along with the western mountain range of Mae Sot Basin, which is Precambrian to Cretaceous in age. The dicot leaves suggest that the paleovegetation is semi-evergreen forest in low land and lower-montane areas. The paleoclimate parameters interpreted by dicot leaves and suggest that this area has mean annual temperature around 21.2-27.58 C and mean annual precipitation around 135.48-154.9 centimeters. This vegetation and climate interpretations greatly conformed with the climatic conditions of the area in present-day Mae Sot and are similar to the Eocene of south China.