Nihon Tennengas, Japan

We describe in detail the validity of the latest Cenozoic calcareous nannofossil datums recognized in the North Atlantic Ocean during DSDP Leg 94 based on the relationship among the nannofossil datums, magnetostratigraphy and oxygen isotope stages. The relations found in the uppermost sequence of Hole 607A located in the North Atlantic Ocean, are same as those in the Boso Peninsula and Choshi area located in the Pacific side of Japan. This indicates that the Quaternary nannofossil datums are useful for detailed age determination such as hundred kilo-years order. On the basis of nannofossil datum planes, we correlate the Quaternary sequences in both Japan Sea and Pacific sides, and also clarify the relationship among nannofossil datums, tephrostratigraphy and the oxygen isotope stages. We also discuss some problems on the correlation among the Pliocene sequences around Japan based on nannofossils. The nannofossil assemblages of the Pliocene sequences in Japan Sea side are composed of low species diversity with abundant occurrences of cold water nannofossils, and are correlative to those in the Arctic Ocean. On the contrary, the assemblages in the Pacific side, are characterized by high species diversity including a lot of warm water nannofossils, and are same as those found in low-latitude regions. This indicates that two different paleobiogeograhical provinces were present around Japan.