The Plio-Pleistocene Tokai Group and the Tectonic Development Around Ise Bay of Central Japan since Pliocene

Abstract

The Tokai Group, one of the representative non-marine Plio-Pleistocene sequences in Japan, is distributed around Ise Bay in the southern part of central Japan. In this paper, the litho- and tephrostratigraphy of the Group in central and northern area (Hokuse area) on the west coast of Ise Bay are first described precisely. From biostratigraphy (plant remains and fossil mammals), paleomagnetism and fission-track ages, those sediments are estimated to range from about 3.0 Ma to about 1.2 Ma, in other words, from Kaena event of the Gauss Normal Polarity Epoch to early Matuyama Reversed Polarity Epoch. The correlation was made among sediments of Tokai Group distributed in various localities like as Hokuse, Nanse (southern area on the west coast of Ise Bay), Nagoya and Chita Peninsula. As a conclusion, a geohistory of the sedimentary basin of Lake Tokai is divided into two stages, Stage I (ca. 6.0 Ma to 3.0 Ma) and Stage II (ca. 3.0 Ma to 1.2 Ma). In connection with this, paleogeography of Lake Tokai and its mode of transition are explained as follows. After the time of wide alluvial plains distribution in ca. 6.0 Ma, a large water body appeared in southern area of the basin in Stage I. At ca. 3.0 Ma, the northwestward shifting of the basin came to induce an appearance of water body in northern area (the beginning of Stage II). In the last of Stage II, a large amount of gravel was supplied from the Suzuka Mountain area, and those materials filled up whole of the sedimentary basin nearly completely. In such a way, Lake Tokai became to extinct at last at ca. 1.2 Ma. In relation to basin transition, mode of tectonism may be interpreted as the results of interaction between upheaving of southern area under the tectonic stress in N-S direction and tilting of the Chubu (central) Block influenced by the stress state in E-W direction. Comparative study of geohistory was made for Tokai, Kobiwako and Osaka Groups which are all allied sequences in the Second Setouchi Sedimentary Basin. Consequently, it becomes clear that the division between Stage I and Stage II of Lake Tokai (ca. 3.0 Ma) is fairly coincidental with that between Older I and Older II stages of Paleo-lake Biwa. Moreover, one of them is with the initial stage of Paleo-lake Osaka. Therefore, this 3.0 Ma phase seems to play an important role in geohistory of the Second Setouchi Inland Depression. It is also important that the extinction of Lake Tokai at about 1.2 Ma is able to be correlated with the time confined between Older and Actual stages of Paleo-lake Biwa, and with the first marine transgression in the area around Osaka Bay. Furthermore, similar pattern of basin migration was recognized in both Lake Tokai and Paleo-lake Biwa, but it should be noted that northwestward migration of Paleo-lake Biwa on a large scale occurred at a later date to compare with that of Lake Tokai. The causes of migration of two sedimentary basins were commonly explained as mutual interaction between upheaving of southern area and tilting of eastern area.