上総層群浦郷層と野島層(三浦半島北部:鮮新〜更新統)の新化石産地から産出した貝化石による古水深の再検討

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We reported molluscan fossils from two new localities of the Urago and the overlying Nojima Formations, the Upper Pliocene to Lower Pleistocene Kazusa Group, a forearc basin fill deposit, Miura Peninsula, central Japan. These molluscan fossils are characterized by deep-water species, such as Ginebis japonicus, Phanerolepida transenna, Profundinassa babylonica, Limopsis tajimae and Halicardia nipponensis. This is the first report of the occurrence of Halicardia nipponensis from the Kazusa Group of the Miura Peninsula. Some of these fossils occur sporadically in the bioturbated muddy sandstones, which suggests that they are autochthonous. We estimated paleobathymetries using the bathymetric ranges of extant species of the autochthonous molluscan fossils: the Urago and the Nojima Formations were deposited at the water depth of 400m to 600m and 400m to 500m, respectively. The bathymetric estimation of the Urago Formation is deeper than the previous estimation based on the benthic foraminiferal assemblage(30m to 200m), and indicates that there is no significant paleobathymetric difference between the Urago and the Nojima Formations.
2012-03-30