The Osaka Group

Abstract

In Kinki and Tokai districts, the eastern part of southwest Japan, there are three Plio-Pleistocene sedimentary basins, where are the Osaka, Kobiwako and Tokai Groups, respectively. The Osaka Group is composed of the cyclic sediments of lacustrine and bay in the upper part, and of lacustrine in the lower. The other Groups are composed of lacustrine sediments only. In the upper part of Osaka Group, the cyclic sedimentation caused by transgression and regression corresponding to climatic change.The stratigraphical summary of the three Groups was figured in terms of tephrochronology, paleomagnetic chronology, fission-track and K-A ages and fossil ranges. Then the climatic changes of Plio-Pleistocene age were introduced. The ages of cold climate were determined as follows.Terasho 3-3.1m.y. Saidera 1.2±m.y.Shimakumayama 2.3-2.4m.y. Gokenya 0.75-0.8m.y.Kamimura I 1.5+m.y. Manchidani 0.45±m.y.Kamimura II 1.5-m.y.In the Manchidani cold age, the coldest climatic age among them, the annual mean temperature was estimated to be about 10°C. The warmest climatic age of Pleistocene is the Shinkori warm age, 0.35m.y.. The climate of that time was mild in winter and had sufficient rainfall as in the southern part of Kyushu, being about 17°C in annual mean temperature.