Late Triassic (Late Early to Early Middle Norian) and Late Triassic or Early Jurassic Radiolarians from Limestone in the Tha Sao Area, Kanchanaburi Province, Western Thailand: Low-Latitude Fauna in the Eastern Tethys

Abstract. Moderately-preserved Late Triassic (late early to early middle Norian) and Late Triassic or Early Jurassic radiolarians have been identified from two sections of limestone and dolomitic limestone, respectively in the Tha Sao area, Kanchanaburi Province in western Thailand. Previously these limestones have been assigned as the Permian because of the similarity of lithology without any fossil evidence. Section 1 is a succession of about 18 m in thickness, consisting of calcareous mudstone, micritic limestone, and dolomitic limestone, in ascending order. Section 2 is about 30 m in thickness and consisting of dolomitic limestone. Middle micritic limestone of about 10 m thick of Section 1 yielded diversified radiolarians. Dolomitic limestone of Section 2 yielded rare radiolarians from two levels. We identified radiolarians from five levels of micritic limestone and two levels of dolomitic limestone and totally identified 53 species belonging to 39 genera including two species of which generic position is uncertain and one new species Colum tekini Sashida and Ito sp. nov. At the Late Triassic time, the Tethys Ocean between Sibumasu and Indochina blocks almost closed due to the collision of these two continental blocks. Upper Triassic radiolarian-bearing limestones have been deposited in the eastern to western Tethyan areas within 35° of both hemispheres. The present radiolarian fauna is representative of low latitude Tethyan fauna. The faunal similarity between the eastern and western Tethys indicates the uniformity of the establishment of the low latitude fauna by the Late Triassic.