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Permian radiolarian fossils were discovered from red siliceous mudstone of the North Kitakami Terrane, in the Rikuchu-Seki District, Iwate Prefecture, Japan. This is the first report of Paleozoic radiolarians from the North Kitakami Terrane. Preservation of the radiolarian fossils are bad, because the accretionary complex in the district has been affected by contact metamorphism of Cretaceous granitic rocks. The radiolarian fauna, composed of Pseudoalbaillella sp. cf. Ps. elegans, Ps. sp. cf. Ps. simplex, Ps. sp. cf. Ps. sakmarensis and others, indicates early Early Permian (Asselian -early Sakmarian) in age. It is considered that this radiolaria-bearing red siliceous mudstone was originally overlain by chert and underlain by basaltic rocks in the reconstructed oceanic plate stratigraphy of the Jurassic accretionary complex in Japan.

Abstract:The Nanjō Mountains located in the central region of Fukui Prefecture, Southwest Japan, are chiefly underlain by the sedimentary complex of various rock-types such as basalt, limestone, chert, mudstone and sandstone. Among these rocks within the mountains, twenty-seven chert outcrops (localities) were explored for their radiolarian content. Twenty-six rock samples from seventeen localities in the studied Kanmuri Yama district yielded moderately-to poorly-preserved radiolarian remains as a result. Most of the samples contain Triassic to Middle Jurassic species, with Spumellaria and Entactinaria dominant among the Triassic faunas and with Nassellaria dominant among the Jurassic faunas. The description and faunal analysis of these radiolarians revealed that the cherts in the Kanmuri Yama district span a wide range of age from the Late Olenekian to Middle Bathonian stages.