Abstract
The calcareous nannofossil biostratigraphy of the upper Pliocene to lowermost Pleistocene Tentokuji and Sasaoka Formations distributed in the Minehama, Tokiwa, and Masayamazawa areas located in the northern part of Akita Prefecture, Honshu, Japan are described in detail. The assemblages of the Tentokuji Formation in the Minehama and Masayamazawa areas, and of the Sasaoka and Tentokuji Formations in the Tokiwa area, are characterized by abundant Reticulofenestra spp.(small form) and Dictyococcites spp.(small form). The Sasaoka Formation in the Minehama and Masayamazawa areas is characterized by the abundant occurrence of the cold water nannofossil, Coccolithus pelagicus. Gephyrocapsa oceanica and G. caribbeanica, which indicate Pleistocene age, are found only in the uppermost Sasaoka Formation in the Minehama area. The Tentokuji and Sasaoka Formations, except the uppermost Sasaoka Formation in the Minehama area, correlate with the late Pliocene, between 3.85 and 1.73Ma. Datum plane A (2.75Ma) which is correlated with the start of heavy glaciation in the high northern latitudes in the early late Pliocene, is traceable to between the Sasaoka and Tentokuji Formations in the Minehama and Masayamazawa areas. We reconstructed the paleoenvironmental change during the late Pliocene to early Pleistocene in the Akita area based on the correlation among the uppermost Cenozoic formations distributed in the Akita areas. Calcareous nannofossil assemblages that are found in the upper Pliocene on the Japan Sea side are very similar to those in the high latitudes of the north Pacific and Arctic Ocean. The assemblages contrast with those on the Pacific side of Japan that are characterized by occurrences of warm water flora. These data indicate that the drastic increase of the ice sheet in the Arctic Ocean at 2.75Ma strongly influenced the nannofossil and Omma-Manganji molluscan fauna on the Japan Sea side.
