Geology and age model of the Lower Pleistocene Nojima,Ofuna, and Koshiba Formations of the middle Kazusa Group,a forearc basin‐fill sequence on the Miura Peninsula,the Pacific side of central Japan |
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| Authors: | Atsushi Nozaki Ryuichi Majima Koji Kameo Saburo Sakai Atsuro Kouda Shungo Kawagata Hideki Wada Hiroshi Kitazato |
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| Institution: | 1. Graduate School of Environment and Information Science, Yokohama National University, , Yokohama, 240‐8501 Japan;2. Faculty of Environment and Information Science, Yokohama National University, , Yokohama, 240‐8501 Japan;3. Department of Earth Sciences, Chiba University, , Chiba, 263‐8522 Japan;4. Japan Agency for Marine‐Earth Science and Technology, , Yokosuka, 237‐0061 Japan;5. INPEX Corporation, , Tokyo, 107‐6332 Japan;6. Faculty of Education and Human Sciences, Yokohama National University, , Yokohama, 240‐8501 Japan;7. Faculty of Science, Shizuoka University, , Shizuoka, 422‐8529 Japan |
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| Abstract: | We present field and core observations, nannofossil biostratigraphy, and stable oxygen isotope fluctuations in foraminiferal tests to describe the geology and to construct an age model of the Lower Pleistocene Nojima, Ofuna, and Koshiba Formations (in ascending order) of the middle Kazusa Group, a forearc basin‐fill succession, exposed on the northern Miura Peninsula on the Pacific side of central Japan. In the study area, the Nojima Formation is composed of sandy mudstone and alternating sandy mudstone and mudstone, the Ofuna Formation of massive mudstone, and the Koshiba Formation of sandy mudstone, muddy sandstone, and sandstone. The Kazusa Group contains many tuff beds that are characteristic of forearc deposits. Thirty‐six of those tuff beds have characteristic lithologies and stratigraphic positions that allow them to be traced over considerable distances. Examination of calcareous nannofossils revealed three nannofossil datum planes in the sequences: datum 10 (first appearance of large Gephyrocapsa), datum 11 (first appearance of Gephyrocapsa oceanica), and datum 12 (first appearance of Gephyrocapsa caribbeanica). Stable oxygen isotope data from the tests of the planktonic foraminifer Globorotalia inflata extracted from cores were measured to identify the stratigraphic fluctuations of oxygen isotope ratios that are controlled by glacial–interglacial cycles. The observed fluctuations were assigned to marine isotope stages (MISs) 49–61 on the basis of correlations of the fluctuations with nannofossil datum planes. Using the age model obtained, we estimated the ages of 24 tuff beds. Among these, the SKT‐11 and SKT‐12 tuff beds have been correlated with the Kd25 and Kd24 tuff beds, respectively, of the Kiwada Formation on the Boso Peninsula. The Kd25 and Kd24 tuff beds are widely recognized in Pleistocene strata in Japan. We used our age model to date SKT‐11 at 1573 ka and SKT‐12 at 1543 ka. |
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| Keywords: | age model forearc basin Globorotalia inflata Kazusa Group Kd24 Kd25 Lower Pleistocene MIS Miura Peninsula nannofossils |
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