Moderately well-preserved Late Permian to Middle Triassic radiolarians are identified in chert beds that occur in the Shan-Thai Block of northern Thailand. These radiolarians are identical to the faunas of the Late Permian Neoalbaillella ornithoformis and N. optima Assemblage Zones and the Triassic Parentactinia nakatsugawaensis and Triassocampe coronata Assemblage Zones reported in chert sequences of Japan. We discovered the radiolarian faunas, apparently indicating Late Permian and Early Triassic ages, in almost continuous sequences of chert and shale exposed in the north of Chiang Mai. The occurrence of these radiolarian faunas provides important data to solve the P/T (Permian/Triassic) boundary in pelagic sequences. Our present discovery also furnishes significant data to reconstruct the paleobiogeography of Mainland Thailand during Late Permian to Middle Triassic times. Fifty species belonging to 35 genera, including three unidentified genera, are investigated taxonomically. Four new species Pseudospongoprunum? chiangdaoensis, Cenosphaera igoi, Cenosphaera? rugosa, and Tlecerina? apsornae are described.

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Two types of chert are defined in Thailand based on lithology, faunal content, and stratigraphy. 'Pelagic chert' consists of densely packed radiolarian tests in a microcrystalline quartz matrix with no terrigenous material and is found as blocks embedded within sheared matrix. 'Hemipelagic chert' also has a microcrystalline quartz matrix, and contains not only scattered radiolarian tests, but also calcareous organisms such as foraminifers. The pelagic cherts range in age from Devonian to Middle Triassic, whereas hemipelagic chert is only from the Early to the Late Triassic. Lithological and stratigraphic characteristics indicate that the pelagic chert originated in the Paleo-Tethys, whereas the hemipelagic chert accumulated on the eastern margin of the Sibumasu Block. The hemipelagic and pelagic chert are exposed in two north-trending belt-like zones. The western zone includes the hemipelagic chert, as well as glaciomarine and other Paleozoic to Mesozoic successions, overlying a Precambrian basement that consists exclusively of Sibumasu elements. The eastern zone contains pelagic chert and limestone and should be correlated to the Inthanon Zone. The Inthanon Zone is characterized by the presence not only of Paleo-Tethyan sedimentary rocks, but also of Sibumasu Block elements that structurally underlie the Paleo-Tethyan rocks. The boundary between the Sibumasu and Paleo-Tethys zones is a north-trending, low-angle thrust that resulted from the collision of the Sibumasu and Indochina blocks.

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