Stratigraphy of the Ryukyu Group in Maeda-misaki area, Okinawa jima, Ryukyu Islands, Japan

Abstract

The Pleistocene Ryukyu Group consisting mainly of reef-complex deposits is exposed in Maeda-misaki area, central part of Okinawa-jima, Ryukyu Islands, southwestern Japan. The Pleistocene sequence, unconformably overlying pre-Tertiary phyllite of the Nago Formation, comprises the Quaternary Ryukyu Group and Holocene beach and alluvial deposits in this area. The Ryukyu Group consists of the Sobe Formation which is unconformably overlain by younger limestones. The Sobe Formation, the main body of the group, reaches 35m in thickness and crops out at elevations of up to 90m. Five lithologic units have been identified in the carbonate sequence of the Sobe Formation in the neighboring area Yomitan and are numbered sequentially from the base upwards (Units 1-5). Of these, four units (Units 2-5) extend in the study area. Unit 2 is composed chiefly of conglomerate and sandy to gravelly limestone. Each of Units 3 and 4 consists of shallow-water coral limestone grading upward into deep-water deposits (rhodolith, Cycloclypeus-Operculina, and detrital limestones). Unit 5 is composed exclusively of coral limestone. The younger limestones are less than 4m in thickness, rest unconformably on the Sobe Formation, and include detrital and coral limestones. These limestones are found at 3 sites at elevations of <-30m on the coast of the study area. The stratigraphic relationship between the detrital and coral limestones remains unknown due to their limited occurrences. Nannofossil biostratigraphy indicates the sandy limestone of Unit 2 of the Sobe Formation ranges in age from 0.41 to 1.65Ma.