Late Permian radiolarians from the Ajima Formation of the Ultra-Tamba Terrane in the Sasayama area, southwest Japan

The Ajima Formation of the Ultra-Tamba Terrane has been regarded as Mesozoic fore-arc sediments and is mainly distributed in the western part of the Sasayama area, southwest Japan. The formation consists mainly of clastic rocks with a small amount of basalt and hydrothermal chert, and tectonically overlies the Middle Triassic Kamitaki Formation. The Ajima Formation is previously considered as Mesozoic or Late Jurassic, the newly discovered radiolarian fauna from the siliceous mudstone intercalated with sandstone and siltstone of the formation, however, corresponds to the upper part of the Neoalbaillella ornithoformis assemblage zone and the Albaillella levis abundance zone (early Changhsingian). Although the Ajima Formation and Ultra-Tamba Terrane in the Hokusetsu area are regarded as Late Jurassic fore-arc sediments (Inagawa Group of Ishiga, 1990a), unconformably overlying the Permian subduction-related accretionary complex, this study suggests that these strata tectonically overly the Triassic and Jurassic subduction-related accretionary complex of the Tamba Terrane or Middle Triassic formation, as Permian strata. Then the radiolarian fauna from the Ajima Formation implies that the highly abundant fauna of A. levis thrived not only in the central Panthalassa but also in the western margin of the Panthalassa.