MAGNETO-BIOSTRATIGRAPHY OF UPPER CRETACEOUS SYSTEM IN JAPAN

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A line of magnetostratigraphic studies were done focusing on the upper Cretaceous Yezo Group in central Hokkaido. Four magnetozones of normal or reversed polarity were recognized from the upper part of the Osoushinai Formation distributed along the Abeshinai River and its tributaries in Teshio-Nakagawa area. In addition, a pair of normal and reversed polarity magnetozones were recognizable from the uppermost Haborogawa Formation in Haboro area. Comparison of these magnetozones with a geomagnetic polarity time scale led to a possible magnetic polarity interpretation that the four magnetozones could be correlated from top to bottom to the late Cretaceous magnetic polarity chrons C32r.1r, C32r.1n, C32r.2r, and C33n. In terms of macro-fossil stratigraphy, the Sphenoceramus schmidti zone is correlated to polarity chron C32r.1r, the Sphenoceramus orientalis zone to C32r.1n, and the upper part of the Polyptychoceras pseudogaultinum zone to C32r.2r.Paleomagnetic studies were also conducted for the upper Cretaceous strata in Kyushu including the Goshoura Group, the Mifune Group, the Ohnogawa Group, and the upper Konmon Group. All these strata are of normal polarity correlatable to the Cretaceous long normal polarity chron, C34n.