The Chinen Formation in southern Okinawa-jima, southwestern Japan, is composed of calcareous sandstone and sandy limestone correlative with the transitional lithofacies between massive siltstone of the Upper Miocene to Pliocene Shimajiri Group and detrital to reefal limestone of the Pleistocene Ryukyu Group. The "Reddish Limestone" is the lowest lithostratigraphic unit in the Ryukyu Group in southern Okinawa-jima and consists mainly of highly altered detrital and rhodolith limestones. The calcareous nannofossil biostratigraphy of the Chinen Formation and the "Reddish Limestone" in two boring cores from Komesu and Giza areas were studied. Abundant specimens of the calcareous nannofossil,
Gephyrocapsa spp. (small), whose base occurred at 2.09 Ma, were recognized in the uppermost Shimajiri Group to large part of the Chinen Formations in the two cores. Although the latest Pliocene datum plane, the last occurrence of
Discoaster brouweri. (1.97 Ma), was recovered from the lower Chinen Formation in Komesu core, it was not detected in Giza core due to relatively rare and sporadic occurrence of this genus. It was likely that two early Pleistocene datum planes, the first occurrence of
Gephyrocapsa caribbeanica (1.73 Ma) and the first occurrence of
G. oceanica (1.65 Ma), were concentrated at the base the "Reddish Limestone". The early Pleistocene datum plane, the first occurrence of
Gephyrocapsa spp. (large) (1.45 Ma), is situated in the "Reddish Limestone" in Giza core and possibly at the top of the Limestone in Komesu core. These indicate that the Chinen Formation is correlated with the latest Pliocene (>1.73 Ma) and that the "Reddish Limestone" formed in Early Pleistocene time between 1.65 Ma and 1.21 Ma.
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