Abstract
The Chinen Formation, consisting mainly of calcareous sandstone and sandy limestone, is exposed in the middle-eastern and southern parts of Okinawa-jima, southwestern Japan. This formation represents a transitional lithofacies between siliciclastic deposits (the Shimajiri Group) and coral-reef carbonates (the Ryukyu Group). Here we report the occurrence of the Chinen Formation from a submarine core (about 19 m below sea level) drilled off Naha Port, west of Okinawa-jima, where the formation has not been reported previously. We examined the lithology, calcareous nannofossil biostratigraphy, and large benthic foraminiferal assemblages in seven sediment samples collected from the core interval between 28 and 36 m below sea floor, which is overlain by limestone of the Ryukyu Group. These sediments consist of unconsolidated calcareous silt and sand, consistent with the lthology of the Chinen Formation. Calcareous nannofossil assemblages indicate that the core interval corresponds to Quaternary nannofossil datum planes 11 and 10 (1.706–1.392 Ma). The benthic foraminiferal assemblage is dominated by Pseudorotalia papillosa, accompanied by Operculina spp. and Reophax spp., indicating that the sediments were deposited on a muddy sand bottom of the upper shelf slope (water depth of 100–200 m). These results indicate that calcareous sandstone and siltstone were deposited along the shelf slope at the eastern and western margins of Okinawa-jima during the deposition of the Itoman Formation (shallow-water limestone).
