関東山地橋立層群から産出したジュラ紀放散虫

概要

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Early Jurassic radiolarians were newly found from pale greenish grey mudstone of the Hashidate Group, distributed around Mt. Buko in the central part of the Kanto mountains, Japan. They are Canoptum sp., Canoptum (?) sp., Hsuum (?) sp., Podobursa (?) sp., Tricolocapsa sp., Parvicingula sp., Parvicingula (?) sp., and some indeterminate nassellarians. The presence of two Canoptum species indicates involvement of Early Jurassic sediments in the Hashidate Group. The Hashidate Group is biostratigraphically correlated with such geotectonic units in the northern part of the Chichibu belt, as the Kazahaya-toge, Kashiwagi, Manba and Kamiyoshida units, which have been ascribed to an Early to Late Jurassic subduction-accretion complex. Among them, the Kashiwagi unit is lithologically similar to the Hashidate Group. Our discovery of the Early Jurassic radiolarian assemblage in the Hashidate Group strongly suggests that the formation of the subduction-accretion complex of the Chichibu belt commenced in the Early Jurassic.
国立科学博物館の論文
1995-12-22