Stratigraphy of the Chinen Formation and the Ryukyu Group on islands off Katsuren Peninsula, Okinawa-jima, Ryukyu Islands, Japan
Oshimizu, T.; Iryu, Y.
Chishitsugaku Zasshi = Journal of the Geological Society of Japan 108(5): 318-335
2002
Document Number: 521833
The Pleistocene Ryukyu Group composed of reef complex deposits crops out on six islands off Katsuren Peninsula, Okinawa-jima, Ryukyu Islands, southwestern Japan. It conformably overlies Plio-Pleistocene sandy limestone and calcareous mudstone of the Chinen Formation containing abundant bioclasts and rests unconformably on Pliocene siltstone of the Shimajiri Group. We propose a major revision of the previous stratigraphic scheme for the Ryukyu Group and provide a formal stratigraphic description. The Ryukyu Group comprises the Yokatsu and Minatogawa Formations, in ascending order, on these islands. The Yokatsu Formation constitutes the main body of the group exceeding 30 m in thickness and is exposed at elevations up to 125 m. It is divisible into five units. Each unit comprises coral limestone and overlying rhodolith, Cycloclypeus-Operculina, and detrital limestones, showing a deepening-upward sequence. The Minatogawa Formation rests unconformably on the Yokatsu Formation, reaches 30 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 the areas at elevations less than ca. 40 m in Ikei-jima and Tsuken-jima. Geologic ages of the limestones on these islands are unknown at this time.