Tephrochronology and Paleogeography of the Plio-Pleistocene in the Eastern Setouchi Geologic Province, Southwest Japan

Abstract

The correctly identified pyroclastic layers are the best keys for the regional geologic chronology. Japan is a good field for Neogenic tephrochronology, since the Neogene systems of this country have a relatively high frequency of intercalations. In Part I of this paper, the tephrochronology of the Plio-Pleistocene series in the eastern Setouchi geologic province, Southwest Japan, is summarized, together with the descriptions of the stratigraphy related to the tephra, lithofacies and horizons of fossils. The individual volcanic ash layer should be correctly discriminated. In order to identify the volcanic ash layers exactly, the writer observed the following features of the volcanic ash layers; namely, horizon, thickness, color, grain-size, internal succession, heavy mineral composition, refractive indices of glasses, shape of glass flakes and paleomagnetic polarity. Characters of the volcanic ash layers in the Plio-Pleistocene series of Central Japan such as the Osaka, Kobiwako and Tokai groups are shown in Tables 1-5 and Fig. 3. There are several separate sedimentary basins in the east Setouchi Geologic Province. One of them is of marine facies in part but the others are lacustrine deposits. The correlation was very difficult, but it might be easy where volcanic ash layers are discovered in common in every basin. The three groups, Osaka, Kobiwako and Tokai in the eastern part of the Setouchi Province, have several common volcanic seams, such as Sakura, Azuki, Pumice, Masugi and Yubune. Hence, the correlation of these three groups was established by means of the tephra. In Part II, the paleogeographical history of the Central Japan in the PlioPleistocene Age is given. It was conceived mainly from the paleocurrent directions deduced from the cross-beddings and the geographic changes in lithology. The lithologic variations were made clear by regarding the volcanic ash layers as time surfaces. The generalized results are shown in Figs. 17-21 The paleocurrent directions in the Setouchi Province changed evidently from the south in late Pliocene to the north in the earliest Pleistocene. This fact indicates that the southern zone of the Setouchi Province began to upheave at this time and that the northern zone began to subside successively. The subsided zone of this period is named "Biwako-Osaka subsidence zone".