2005 Volume 111 Issue 9 Pages 527-546
The Pleistocene Ryukyu Group, composed of reef complex deposits and carbonate-siliciclastic sediments, crops out extensively on northern Motobu Peninsula, Okinawa-jima, Ryukyu Islands, southwestern Japan. We propose a major revision of the previous stratigraphic scheme for the Group and provide a formal stratigraphic description. The Group comprises the Nakoshi and Kourijima Formations and younger limestones. The Nakoshi Formation is composed of sandstone, siltstone, sandy to silty limestone, and conglomerate, passing laterally into the lowest unit (Unit 1) of the Kourijima Formation. The Kourijima Formation constitutes the main body of the Group reaching 80 m in thickness and is exposed at elevations up to 107 m. It is divisible into three units (Unit 1, 2, and 3). Unit 1 consists of deep-water deposits represented by detrital limestone associated with rhodolith and Cycloclypeus-Operculina limestones. Unit 2 begins with shallow-water coral limestone that grades upward into the deep-water deposits. Unit 3 is composed exclusively of coral limestone. Coral limestones which are exposed sporadically on the pre-Tertiary basement cannot be assigned to any certain unit due to their isolated occurrence.
The younger limestones include coral, Cycloclypeus-Operculina, and well sorted detrital limestones, all of which are limited in their distribution and rest unconformably on the Kourijima Formation and pre-Tertiary basement. Calcareous nannofossil biostratigraphy indicates that deposition of the Ryukyu Group started in earliest Quaternary time (1.45-1.65 Ma) and continued for more than 0.6 million years (< 0.85 Ma).