5万分の1地質図幅「加茂」地域の足尾帯珪質泥岩から見出された後期三畳紀放散虫化石

概要

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Late Triassic radiolarian fossils were extracted from siliceous mudstone in an accretionary complex of the Ashio Terrane in the Kamo district (Quadrangle series 1:50,000 of the Geological Survey of Japan), Niigata Prefecture, Japan. The siliceous mudstone occurs as a block within a muddy matrix. The siliceous mudstone also belongs to the upper unit of the Senmi Complex, which is distributed widely in the Kanbara Mountains within the Kamo district. The fauna in the siliceous mudstone consists mainly of species of the genera Canoptum, Betraccium, Cantalum and Fontinella, including Canoptum sp. cf. C. rhaeticum, Fontinella sp. cf. F. clara, Fontinella sp. cf. F. primitiva etc. Based on radiolarian biostratigraphy of these taxa previously documented elsewhere, the radiolarian fauna studied was dated to the Late Triassic (Rhaetian). This is a first report of Triassic radiolarian fossils from clastic rocks of the Ashio Terrane in the Kanbara Mountains; thus, this find contributes to reconstructions of oceanic plate stratigraphy of the Ashio Terrane in this region.