Tephrochronology, Paleogeography and Tectonic Development of Plio-Pleistocene in Kinki and Tokai Districts, Japan

Abstract

In Kinki and Tokai districts, the eastern part of southwest Japan, three basins are developed, separated by mountain ranges of N-S trend. They are Osaka Bay, Lake Biwa and Ise Bay from west to east. The foothill-lands (50-300m. high) surrouding those basins are composed of Plio-Pleistocene sediments, wich are called the Osaka, Kobiwako and Tokai Groups respectively. The mountains consist of Cretaceous granitic rocks and Paleozoic formations. It was difficult to correlate accurately three groups because of the isolated distribution. But they contain many volcanic ashes by which the interbasinal correlation is possible.This paper mainly treats with tephrochronology and correlation by detailed examination of volcanic ashes. In addition, some problems of paleontology, paleogeography and tectonic development are discussed.1) The Age, Tokoname and Seto groups, which are used locally, are synthesized into the Tokai Group, because these groups have been deposited a single sedimentary basin, the Tokai Lake. The Karayama Formation corresponds to the upper part of the Tokai Group.2) Table 1 shows the thickness, occurrence, type locality, heavy mineral composition and glass-index of volcanic ashes. Pumice Volcanic Ash-Layer is found through all three groups. The ash-layers found in neighbouring two groups are Nuka, Ichiuno, Dacite, Sagami, Kaigake, Komazuki, and Sakura. The tephrochronology of the three groups are established by the stratigraphical relations of those volcanic ash-layers. (Fig. 2)3) Stratigraphic horizons of main fossils of mammal, mollusc and plant are shown in Table 2, along with climatic oscilation curve obtained by pollen analysis.4) The depression of the Plio-Pleistocene age in Kinki and Tokai districts had the NE-SW axis and moved toward west. (Fig. 4, 5)