Biostratigraphy and paleoenvironment of marine jurassic rocks in The Mae Sot-Phop Phra Areas, Changwat Tak

Abstract

The aims of this study are to define biostratigraphy and paleoenvironment of marine Jurassic sedimentary rocks (Hua Fai Group) based on their lithologies, bivalves and ammonoids in Amphoe Mae Sot-Phop Phra, Changwat Tak, northwestern part of Thailand. In the Mae Sot area, the lithostratigraphic units have been established in ascending order: Khun Huai, Doi Yot and Pha De Formations of the Hua Fai Group. Mudstone, siltstone, sandstone, limestone and marl have been observed and faunal assemblges are dominated by bivalves and ammonoids. Lithostratigraphically, seven sections in the study area from north to south (Ban Mae Kut Luang, Tak-Mae Sot Road, Khun Huai Mae Sot, Padaeng-Tak mine, Ban Pu Toe, Doi Huai Mot and Huai Wale) are studied in detail and samples were collected. Marine Jurassic rocks are composed of mudstone, siltstone, sandstone, marl, limestone and oolitic limestone. Fossils are abundant containing bivalves and ammonoids, at least 17 genera of bivalves found together with ammonoids assemblage. Bivalves consist of Astarte sp., Actenostreon sp., Bositra ornati, Ceratomya sp., Camptonectes sp., Entolium sp., Gervillia sp., Goniomya sp., Grammatodon sp., Homomya? sp., Lima? sp., Parvamussism donaiense, Pholadomya sp., Pinna sp. Protocardia sp., Thracia sp., Trigonia sp., and a rudist bivalve. Additional fossils contain rhynchonellid brachiopod, nerineid gastropod, coral, trace fossils and plant remains. Bositra zone has been established. This zone is defined by the first and last occurrences of Bositra ornati, representing middle Toarcian-Aalenian. Based on lithological and paleontological aspects along studied sections, it can be interpreted that the depositional environment was in shallow marine continental shelf, intertidalsubtidal, inner to outer continental shelf (neritic), shoreface, fan-deltas, protected lagoon, intertidal, subtidal, and inner to outer ramp environments with occasional carbonate platform and reef flat. The age of rocks based on the bivalves and ammonoids in the study area is assigned as Toarcian-Aalenian (Lower Jurassic)