Early Jurassic radiolarian faunae were extracted from mudstone samples collected at three localities (Se1, Ik1, and Jo2) in an accretionary complex within the Ashio Terrane in the Kambara Mountains, Niigata Prefecture, Japan. The faunae of both Se1 and Ik1 are dominated by Canoptum species, in association with two characteristic species belonging to Gorgansium (G. sp. A and G. sp. B) for Se1, and species belonging to Parahsuum, Lantus, and Helvetocapsa for Ik1. The fauna in Jo2 consists mainly of the species of Parvicingula, Zhamoidellum, and gen. et sp. indet. B.
Based on a comparison of these radiolarians with those from North America, Europe, and Southwest Japan, the ages of the faunae from Se1, Ik1, and Jo2 are Hettangian, Late Pliensbachian, and Middle to Late Toarcian/Aalenian?, respectively.
This is the first report of radiolarian fossils from terrigenous clastic rocks (mudstone) in the Kambara Mountains; thus, this result contributes to reconstructions of oceanic plate stratigraphy in the region and to correlating the area with other accretionary complexes in the Ashio Terrane.