Pleistocene coral reef deposits (the Ryukyu Group) on Kume-jima, Okinawa Prefecture, Japan

Abstract

The Ryukyu Group, composed of Pleistocene reef-complex deposits that pass laterally into terrigenous sediments, crops out on Kume-jima and its adjacent islet, Ohajima, Ryukyu Islands, southwestern Japan. We propose a major revision of the previous stratigraphic scheme for the Ryukyu Group, based on new investigations, and provide a formal stratigraphic description. These Pleistocene deposits comprise the Nakandakari, Kumejima, and Torishima Formations, in ascending order, on Kume-jima. The lowest of these, the Nakandakari Formation, consists of pumice-bearing detrital limestone (<20m thick); its surface exposure is confined to the type locality, which is on the coast to the northwest of Nakandakari. The unconformably overlying Kumejima Formation exceeds 30m in thickness and is exposed in the northwestern part of Kume-jima at elevations less than ca. 50m. It is divisible into at least three units, each comprising coral limestone and overlying rhodolith, Cycloclypeus-Operculina, and detrital limestones. The Torishima Formation rests unconformably on the Kumejima Formation, and is characteristically thin (<15m thick) and composed of well-sorted detrital and coral limestones that are thought to have been deposited in a shallow lagoon (moat). The Oha Limestone is limited in its distribution to Oha-jima and consists of diagenetically altered, reddish to brownish, coral limestone. The stratigraphic relationship between limestones on Kume-jima and those on Oha-jima remains unknown, as does the geological age of these limestones. It is evident from the stratigraphic succession and configuration of lithofacies that the reefs grew in response to at least three repeated cycles of sea-level change with amplitudes of up to 80m during deposition of the Kume-jima Formation. Subsequently, a relatively small reef now assigned to the Torishima Formation grew to fringe the older reefs.