Abstract
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 Pliocene Shinzato Formation of the Shimajiri Group and detrital to reefal limestone of the Pleistocene Ryukyu Group. The calcareous nannofossil and planktonic forminiferal biostratigraphy of the Chinen Formation in the Urizun and Golf Links sections, both located near the type locality, were studied. Abundant specimens of the calcareous nannofossils, Gephyrocapsa spp. (small), whose base occured at 2.09 Ma, was recognized throughout the uppermost Shinzato to Chinen Formations in the Urizun section. Three latest Pliocene to early Pleistocene datum planes, the last occurrence of Discoaster spp. (1.97 Ma), the first occurrence of Gephyrocapsa caribbeanica (1.73 Ma) and the first occurrence of G. oceanica (1.65 Ma), were concentrated in the lower part of the Chinen Formation. The latest Pliocene planktonic foraminifera datum planes, the first occurrence of Globorotalia (Globorotalia) truncatulinoides and the coiling change of Pulleniatina spp. from dextral to sinistral, were also recognized in the same horizon in the lower part of the Chinen Formation. These indicate that the Chinen Formation correlates with the latest Pliocene to earliest Pleistocene between 2.09 Ma and 1.65 Ma. The Pliocene/Pleistocene boundary, indicated by the first occurrence of G. caribbeanica (1.73 Ma), is situated in the lower part of the Chinen Formation in which a chronostratigraphic gap of 0.3 m.y. is present.
Conversely, the stratigraphic distribution of calcareous nannofossils and planktonic foraminifera in the Golf Links section, shows that the disconformity recognized in the lower part of the Chinen Formation at the Urizun section, is correlated with the boundary between the Shinzato and Chinen Formations. The lithostratigraphic boundary beween the Shinzato and Chinen Formations in these adjacent sections, does not correspond with the chronostratigraphy.
