Stratigraphy of the Zakimi Formation and the Ryukyu Group in Yomitan area, Okinawa-jima, Ryukyu Islands, Japan

Abstract

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 Zaki-mi 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 m thickconsists 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 m thickand consists mainly of conglomer-ate, coral limestone, and detrital limestone. The Sobe Formation unconformably overlies the Iramina Formation, reaches m in thickness , and crops out at elevations of up to 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 is most extensively exposed in this area. This unit rests unconformably on Units and at elevations greater than m and conformably overlies Unit 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 .. 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. Abstract CThe Geological Society of Japan 2005 313