Jc RS Japanese Corral Reef S ocuq Galaxea, JCRS, 3 : 13 - 24 (2001) 沖縄県久米島の琉球層群 Pleistocene coral reef deposits (the Ryukyu Group) on Kume jima, Okinawa Prefecture, Japan 江 原 由樹1,2・井 龍 康文1・ 中 森 亨1・ 小 田 原 啓1 Y. Ehara 1 , 2, Y. Iryu 1, T. Nakamori 1 and K. Odawara 1 1東北大学大学院理学研究科地学専攻地圏進化学講座 980 -8578 仙台市青葉区青葉山 2現住 所 国際協力事業団鉱工業開発調査部資源開発調査課 151 -8558 東京都渋谷区 代 々木2丁 目1番1号 新宿マインズタワー 1 Institute of Geology and Paleontology , Graduate School of Science, Tohoku University, Aobayama, Sendai 980-8578, Japan 2 Current address: Mining and Industrial Development Study Division , Mining and Industrial Development Study Department, Japan Interna- tional Cooperation Agency, Shinjuku Maynds Tower, Yoyogi 2-chome 1-1, Shibuya-ku, Tokyo 151-8558, Japan Abstract: The Ryukyu Group, composed of Pleistocene reef-complex deposits that pass laterally into terrigenous sediments, crops out on Kume-jima and its adjacent islet, Oha- jima, Ryukyu Islands, southwestern Japan. We propose a major revision of the previous stratigraphic scheme for the Ryukyu Group, based on new investigations, and provide a formal stratigraphic description. These Pleistocene deposits comprise the Nakandakari, Kumejima, and Torishima Formations, in ascending order, on Kume-jima. The lowest of these, the Nakandakari Formation, consists of pumice-bearing detrital limestone (<20 m thick) ; its surface exposure is confined to the type locality, which is on the coast to the northwest of Nakandakari. The unconformably overlying Kumejima Formation exceeds 30 m in thickness and is exposed in the northwestern part of Kume-jima at elevations less than ca. 50 m. It is divisible into at least three units, each comprising coral lime- stone and overlying rhodolith, Cycloclypeus-Operculina, and detrital limestones. The Torishima Formation rests unconf ormably on the Kumejima Formation, and is charac- teristically thin (<15 m thick) and composed of well-sorted detrital and coral limestones that are thought to have been deposited in a shallow lagoon (moat). The Oha Limestone is limited in its distribution to Oha-jima and consists of diagenetically altered, reddish to brownish, coral limestone. The stratigraphic relationship between limestones on Kume-jima and those on Oha-jima remains unknown, as does the geological age of these limestones. It is evident from the stratigraphic succession and configuration of lithof acies that the reefs grew in response to at least three repeated cycles of sea-level change with amplitudes of up to 80 m during deposition of the Kume-jima Formation. Subsequently, a relatively small reef now assigned to the Torishima Formation grew to fringe the older reefs. Key words: Pleistocene, Ryukyu Islands, Kume-jima, Ryukyu Group, limestone