Stratigraphy of the Ryukyu Group in southern Okinawa-jima, Ryukyu Islands, Japan

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  • Sagae Kenichi
    JX Nippon Oil and Gas Exploration Corporation
  • Humblet Marc
    Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, University of Tokyo Present address: Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
  • Odawara Kei
    Hot Springs Research Institute of Kanagawa Prefecture
  • Chiyonobu Shun
    Institute of Geology and Paleontology, Graduate School of Science, Tohoku University Research Institute of Innovative Technology for the Earth, CO<sub>2</sub> Storage Research Group
  • Sato Tokiyuki
    Institute of Applied Earth Sciences, Faculty of Engineering and Resource Science, Akita University
  • Kabamoto Junichi
    Agriculture, Forestry and Fisheries Department, Okinawa General Bureau, Cabinet Office, Government of Japan Farmland Conservation Office of Kochi Sanbagawa Belt, Chugoku-Shikoku Regional Agricultural Administration Office, Ministry of Agriculture, Forestry and Fisheries
  • Takayanagi Hideko
    Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
  • Iryu Yasufumi
    Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University

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Other Title
  • 沖縄本島南部に分布する琉球層群の層序
  • オキナワ ホントウ ナンブ ニ ブンプ スル リュウキュウソウグン ノ ソウ ジョ
  • Stratigraphy of the Ryukyu Group in southern Okinawa-jima, Ryukyu Islands, Japan (in Japanese)

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Abstract

The Pleistocene Ryukyu Group, composed of reef-complex deposits, crops out in southern Okinawa-jima, Ryukyu Islands, southwestern Japan. Here, based on new investigations, we revise the previous stratigraphic scheme for the Ryukyu Group and provide a formal stratigraphic description. In the study area, the Ryukyu Group comprises the Itoman, Naha, and Minatogawa formations. The Itoman Formation, composed mainly of coralline algal limestone, is thin (>2 m thick) and sporadically distributed. The Naha Formation rests unconformably on the Itoman Formation, reaches 50 m in thickness, and is exposed at elevations of up to ca. 170 m. The Naha Formation is divisible into four units; each unit is composed of coral limestone overlain by rhodolith, Cycloclypeus—Operculina, and detrital limestones, showing a deepening-upward sequence. The contact between the upper two units (units 3 and 4) is unconformable; this unconformity is present in other areas in Okinawa-jima, such as Yomitan, the Motobu Peninsula, and Ie-jima. Calcareous nannofossil assemblages indicate that deposition of the Naha Formation began at 1.392—1.706 Ma, and continued until after 0.853 Ma. The Itoman Formation and the lowest interval of the Naha Formation are coeval, and they represent, respectively, reef and off-reef facies of reef-complex deposits. The Minatogawa Formation rests unconformably on the Naha Formation, reaches 20 m in thickness, and consists of well-sorted detrital and coral limestones that are thought to have been deposited in a shallow lagoon (moat); its surface exposure is confined to elevations of less than 50 m. The geologic age of the Minatogawa Formation is unknown at this time.

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