Stratigraphy and paleontology of marine triassic rocks in Amphoe Mae sot-Phop phra, Changwat Tak, Thailand

Abstract

The aims of this study are to define lithostratigraphy of marine Triassic sedimentary rocks, identify radiolarians and clarify the geological age of these rocks in Amphoe Mae Sot-Phop Phra, Changwat Tak, northwestern part of Thailand. Marine Triassic rocks in the study area can be subdivided into 3 facies: pelagic limestone, turbidite sequence and chert sequence. The pelagic limestone is composed of well bedded gray and dark gray limestone with abundant radiolarian faunas and bivalve fragments, intercalated with thin black calcareous shale containing Halobia. Turbidite sequence consists of fine- to coarse-grained and thin- to thick-bedded sandstone interbedded with thin calcareous shale. Halobia has been observed in calcareous shales. Bedded chert is composed mainly of 3-10 cm thick, well bedded chert intercalated with thin shale containing abundant radiolarias. Radiolarias observed in bedded chert belong to 26 gerera and 43 species: Oertlispongus sp., Pseudostylosphaera japonica, Pseudostylosphaera spinulosa, Pseudostylosphaera timorensis, Pseudostylosphaera sp., Eptingium manfridi manfridi, Eptingium cf. manfridi manfridi, Eptingium sp., Pentaspongodicus symmetricus, Muellertortis cochleata cochleata, Triassocampe cf. deweveri, Triassocampe sp., Spongostephanidium japonicum, Spongostephanidium sp., Baumgartneria bifurcata, Falcispongus falciformis, Falcispongus sp., Staurolonche trispinosa, Canoptum rhaeticum, Canoptum laxum, Canoptum cf. levis, Canoptum sp., Vinassaspongus sp., Capnuchosphaera triassica, Capnuchosphaera cf. triassica, capnuchosphaera cf. deweveri, Capnuchosphaera sp., Paronaella sp., Paleososaturnalis sp., Orbiculiforma sp., Hagiastrum augustum. Pentaspongodiscus sp., Dumitricasphaera sp., Poulpus sp., Castrum peronatum, Xiphotheca longa, Xiphotheca sp., Annulotriassocampe sulovensis, Zhamojdasphaera latispinosa, Kahlerosphaera sp., Ferresium sp., Canesium sp. Based on the evidences of paleontology and petrology, the age of the rocks in the study area should be assigned as Anisian to Norian (Middle to Late Triassic) and they show that the rocks were accumulated on continental shelf to slope.