Yo Naruse

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

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.

On the sedimentary Environment of the Toyofusa Formation, Boso Peninsula, Chiba Prefecture, Japan

The Toyofusa formation in the southernmost part of the BOSO Peninsula is characterized mainly by mixed fossil biocoenoses and variable lithologic facies with tuffaceous sediments and intraformational conglomerates., These were examined genetically in relation to diastrophism and the sedimentary environment of the formation., Conclusions are as follows: 1 The Toyofusa formation accumulated in a part of a geosynclinal trough., Subsidence of the floor continued in the south and compensatory upheaval in the north of this part of the geosyncline during the epoch., 2., Clastic material for the formation was supplied from the north and west of this area., 3., A warm current comparable to the Kuroshio of today, ran near the rocky shore., There was also an under current of low temperature comparable to the present Oyashio under current., 4., Silt and sand rhythmically accumulated on the deep sea floor of the trough., Occasionally shell-bearing tuffaceous coarse sands with siltstone blocks of various sizes slid down the slope and were intercalated in the above-mentioned alternation., 5., These conditions and mode of sedimentation continued during almost the whole period of accumulation of the formation., 6., The sea in which the Toyofusa formation was deposited finally disappeared during the Plio-Pleistocene tectonic activity of the Tanzawa orogensis.,