Upwarpimg Movement during the Quaternary in the Atsumi Peninsula, Central Japan

The Atsumi Peninsula is situated in the southern part of Aichi Prefecture, Central Japan. Geologically, this peninsula is mainly composed of the relatively thick marine Pleistocene sediments about 100m in thickness which is called the Atsumi formation, exclusive of the inlier Paleozoic mountainland in the west. In this paper, the authors intend to induce the mode of crustal movement during the Quaternary in this peninsula on the basis of the distribution, character and altitude of the marine terraces. The results are summarized as follows: 1) In the Atsumi Peninsula, there are five groups of the marine terraces which indicate the existence of the former high sea level. They are the higher terraces, Tempakuhara terrace, Fukue terrace, the lower terrace and the alluvial plain in the descending order and their distribution is shown in Fig. 1. The higher terraces are the fragmentary abraded rocky surfaces and subdivided into two levels, both of them represent the high sea level during the early Quaternary before the deposition of the Atsumi formation. Tempakuhara terrace can be considered originally, as the accumulation terrace of the Atsumi formation, as pointed out by Kuroda, Kaizuka and Tsuchi's previous studies. At present, this surface shows the undulated hilly topography, dissected densely by many small rivers. Fukue terrace is developed as the typical marine terrace which has the distinct back scarps and flat terrace plains composed of marine beach gravels overlying unconformably the Atsumi formation. This terrace can be thought to correlate with Shimosueyoshi terrace in Kanto district, according to the resemblance of geological and morphological characteristics in the both terraces. Consequently, it is probable that Fukue terrace has been formed during the period of the high sea level at the last interglacial stage. The lower terrace is generally fluvial one along the rivers, but some places such as near the mouth of the River Shio, it has the characters of marine origin. The alluvial plain, which is several meters in height above the present sea level at its inner edge, seems to show the postglacial transgression. 2) The height of former shoreline of each level varies considerably with locality as shown in Fg. 1. For example, that of Fukue terrace varies from 8m at the southwestern part to 42m at the northeastern. To know the general tendency of the altitude distribution, Fg. 2 is drawn in which the heights of former shorelines higher than Fukue terra