Some Considerations conerning the Plio-Pleistocene Boundary in South Kanto Region

Abstract

1. At Isone, Osawa-Machi, Chiba Prefecture, an unconformity between the Nagahama formation and the underlying Nakazeki formation (≈Iwasaka formation) has been newly recognized (Fig.1). The relation between the two was formerly regarded conformable. Thus it is proved that stratigraphic column from the upper part of the Otadai formation through the Kakinokidai formation (≈Akimoto group) in the east coast of the Boso Peninsula is lacking in the west coast (Fig. 2).2. Several rock fragments dredged at the Nakanose Bank off Yokohama were examined, and the submarine topography was studied. The results suggest that the rocks of the Nakanose Bank belong to the Miura group.3. Based on the above-mentioned facts and comparison of lithic character and fossil-coenosis, the writers have correlated the late Cenozoic strata on both sides of the Tokyo Bay as follows:4. Two different opinions have been expressed concerning the stratigraphic position of the Plio-Pleistocene boundary. One draws it at the base of the Tsurumai group, and another at the base of the Akimoto group in the Boso Peninsula. In either case, however, the above-mentioned boundary is represented by the base of the Nagahama formation in the Boso Peninsula, and by the base of the Koshiba formation in the Miura Peninsula.