Late Permian and Early to Middle Triassic radiolarians from the Hat Yai area, southern peninsular Thailand: Implications for the tectonic setting of the eastern margin of the Sibumasu Continental Block and closure timing of the Paleo-Tethys

The fine clastic and siliceous sedimentary successions distributed in the Hat Yai area, southeastern peninsular Thailand were examined using radiolarian biostratigraphy, lithology, and stratigraphy. The studied succession was essentially divided into two kinds of fine-grained sedimentary rock units: the lower shale unit and the upper chert unit. The lower shale unit and the upper chert unit yielded late Middle to early Late Permian and Early to Middle Triassic radiolarians, respectively. We clarified that the sedimentary rocks distributed in the study area are not entirely Carboniferous, but contain some sediments from the Permian and Triassic. This age determination suggests that the clastic–chert succession distributed in the study area should be distinguished from the Carboniferous Yaha Formation and correlated with the Permian to Triassic Semanggol Formation. Lithological change with increasing siliceous composition was observed in the uppermost part of the lower shale unit, and the geological age based on the radiolarians indicates a nearly conformable stratigraphic relationship for the lower shale unit and the upper chert unit. The stratigraphy and lithology of the Permian to Triassic succession in the study area, together with geological correlation around southernmost Thailand and northern Malaysia, suggest that the Triassic chert should be interpreted as continental slope sediments overlying Permian clastic and/or calcareous facies, rather than typical pelagic deep-water sediments formed on an abyssal plain. The depositional environment of the chert in this area was likely restricted to the vicinity of a continental slope. Considering with the wide distribution of Triassic platform carbonates over southeastern peninsular Thailand, the continental margin of the Sibumasu along the Paleo-Tethys was represented by a stable passive margin during the Middle–early Late Triassic time. On the basis of the sedimentary setting and stratigraphy in the Permian and Triassic, it is suggested that the closure of the Paleo-Tethys between the Sibumasu and Indochina continental blocks took place at least after the Middle Triassic in southeastern peninsular Thailand.