青森県弘前市南方の付加体泥岩から産出した前期ジュラ紀放散虫化石

概要

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Radiolarian assemblages including Canoptum spp. in part with Natoba minuta or ?Parahsuum spp. were extracted from terrigenous mudstone within an accretionary complex located south of Hirosaki City, near the western margin of the North Kitakami Belt. These assemblages suggest sedimentation during the Early Jurassic, older than any known fossil ages (Middle Jurassic to latest Jurassic-earliest Cretaceous) obtained for mudstone in the North Kitakami Belt. This result indicates the occurrence of near-continuous accretionary tectonics in NE Japan from the Early Jurassic to earliest Cretaceous, comparable with age data obtained for SW Japan. Although the accretionary complex located south of Hirosaki occurs as an isolated inlier among younger cover sediments, its age and location suggest that it corresponds to one of the structurally uppermost units in the North Kitakami Belt.