Middle-Late Devonian radiolarians from Klaeng District, Rayong Province, southeastern Thailand: Geotectonic significance of the Rayong area as a continental margin of the Sibumasu Block

A fine clastics-siliceous succession exposed at Laem Krabang Phet, approximately 20 km southwest of Klaeng, Rayong area, southeastern Thailand was examined based on lithology, radiolarian biostratigraphy, and geological correlation. In the study section, thinly bedded chert layers are intercalated with black carbonaceous shale, light-colored tuffaceous shale, brown glassy tuff, and quartz-rich sandstone layers. A horizon of the chert in the section yielded poorly preserved radiolarians, however, several forms including Palaeoscenidium cladophorum Deflandre, Stigmosphaerostylus cf. pusilla (Hinde), and Entactiniidae gen. et spp. indet. were identified. This fauna is probably referable to the Middle-Late Devonian. The study section had been assigned to various stratigraphic units, such as Silurian-Devonian or Carboniferous-Permian, due to a poor fossil occurrence. However, the occurrence of the Devonian radiolarians in this study provides concrete evidence for the sedimentary age of the succession, and supports assignment to the Silurian-Devonian. Although the study succession exposed at Laem Krabang Phet is coeval with the Devonian part of the Fang chert, northern Thailand, the lithological assemblage is dissimilar, and the post-Devonian radiolarian chert is absent. These lithological, chronological, and stratigraphic results indicate that the depositional environment of the study section was close to the continental margin, such as a continental slope or rise.