The Pleistocene Ryukyu Group composed of reef complex deposits crops out in Yomitan area, Okinawa-jima, Ryukyu Islands, southwestern Japan. The group rests unconformably on the upper Cenozoic Zakimi Formation and the pre-Tertiary Nago Formation. We propose a major revision of the previous stratigraphic scheme for the Zakimi Formation and the Ryukyu Group, based on new investigations, and provide a formal stratigraphic description. The Zakimi Formation (>40 m thick) consists of conglomerate comprising poorly sorted, up to cobble-sized, subrounded gravel derived from the Nago Formation, displaying a clast-supported texture. The Ryukyu Group comprises the Iramina and Sobe Formations in study area. The Iramina Formation, observed in drill cores, is thin (>11 m thick) and consists mainly of conglomerate, coral limestone, and detrital limestone. The Sobe Formation unconformably overlies the Iramina Formation, reaches 70 m in thickness, and crops out at elevations of up to 80 m. It is divisible into five units. Each unit is composed of coral limestone overlain by rhodolith, Cycloclypeus-Operculina, and detrital limestones and shows a deepening-upward sequence. Of these units, Units 4 is most extensively exposed in this area. This unit rests unconformably on Units 2 and 3 at elevations greater than 20 m and conformably overlies Unit 3 below the elevation. It is equivalent to the Yontan Limestone in previous stratigraphic literatures. Calcareous nannofossil biostratigraphy indicates that the lower part of the Sobe Formation was deposited in early Pleistocene time, ranging from 0.41-0.85 Ma. Stratigraphic position and age-diagnostic nannofossils indicate that the Sobe Formation may be partly correlative to the main body of the Ryukyu Group on Toku-no-shima, Okierabu-jima, Yoron-jima, southern Okinawa-jima, and Miyako-jima.
View full abstract