Abstract
The Chinen Formation in southern Okinawa-jima, southwestern Japan, is composed mainly of calcareous sandstone and siltstone, sandy limestone, and detrital limestone that represent the transitional lithofacies between siltstones of the upper Miocene to Pliocene Shimajiri Group and detrital to reefal limestones of the Pleistocene Ryukyu Group. The calcareous nannofossil biostratigraphy of the Chinen Formation and the Ryukyu Group in two drillcores from the Katsuren Peninsula were studied. Abundant specimens of the calcareous nannofossils, Gephyrocapsa caribbeanica and G. oceanica, whose biostratigraphic bases occurred at 1.73 Ma and 1.65 Ma, were recognized in the Chinen Formation and the Ryukyu Group in the two cores. The early Calabrian datum plane, the first occurrence of large specimens of Gephyrocapsa spp. (>6μm; 1.45 Ma), is situated in the Chinen Formation. Biostratigraphic correlation of the Shimajiri and Chinen Formations and the Ryukyu Group, compiled from selected boreholes and surface outcrops, indicates that the Chinen Formation on the Katsuren Peninsula and its environs is correlated with the late Gelasian to early Calabrian between >1.73 Ma and 1.21 Ma and that the deposition of the Ryukyu Group initiated in early Calabrian time between 1.45 Ma and 1.21 Ma.
