海峡地形に記された海水準変動の記録

Abstract

In this paper, some problems of the late Quaternary geohistory of the Japanese Islands were discussed, basing upon the following results of marine investigation with topographical and sedimentological data on the straits of the Japanese Islands.1. Charts show that wherever sizeable inland sea are separated from the ocean by narrow straits, deep hole (sea caldron) exists either in the narrows or directly adjacent them. The Korean strait (Fig. 5) is a part of the shelf, although there is a relatively deep hole (to -220m) along the west side of Tsushima Island. The depth of this deep hole attains 100m below the surrounding relict wave cut terrace (-120 to -130m). In this case, the depth of the relict wave cut terrace may reflect the sea level (about -110m) at the time of the Korean strait formation.2. From the detailed survey of the submarine topography in the Tsugaru strait (Fig. 6), six submarine terraces were found on the sill. It is noteworthy that the depth of those terrace on both areas of the Cape Shirakami and Tappi is nearly the same (Fig. 9). This fact suggests that there has been no distinct crustal movement after the building of those terraces on the sill of the strait.Among the submarine valleys developed on the continental shelf, one of which continued to a valley on land can be detected on the eastern part of the Cape Shirakami (Fig. 7). From these circumstances of submerged terraces and valley about 80m depression of sea-level at the maximum Wurm is deduced.3. The breadth to depth ratio of the straits around the Japanese Islands (Fig. 10) seems to reflect each still standing sea-level stage, such as; -100±10, -80±5 and -45±5m, after the formation of these straits.4. According to radiocarbon ages of shallow-water shells and peat obtained from the continental shelf and coastal plain around the Japanese islands (Fig. 12), sea-level at the maximum Wurm (17, 000 to 20, 000 y.B.P) was about 80m below the present level. Therefore, those results agree with the evidence from the topography of the straits around the Japanese Islands. Fig. 11 shows the coastal geography of the Japanese Islands during the low sealevel time of 80m below the present level.5. In most of Pleistocence, the Japanese islands was connected with the Korean Peninsula and present major islands themselves were tied to each other. It is sure that large mammals such as elephants migrated into the Japanese islands through land bridges. In the early Shimosueyoshi transgression when the sea-level was about -100m (Riss -Wurm interglacial period), the Japan Sea connected with the Pacific through narrow passages located in the Korean and Tsugaru straits. At the time of maximum Wurm when the sea-level was depressed to about -80m, the land bridges between Honshu and other lands were never formed. It is now believed by us that it might have been perhaps 12, 000 years ago when the sea-level rose to about -45m. This was the final stage of the land bridge in the Soya strait between Sakhalin and Hokkaido.