日本周縁の陸棚に関する二三の考察
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概要
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The continental shelves around the Japanese Islands are composed of three terraces, one shallower than 40 meters, one 70 to 100 meters and one 100 to 160 meters in depth. Terraces, shallower than 40 meters, have been built by deposition or abrasion along the coasts since the beginning of the Holocene. Terraces, 70 to 100 meters in depth, develop along the San-in coast, the coast of the Japan Sea in southwestern Honshu, and continue to the bottom of the Yellow Sea. On these terraces, sand and mud bottoms predominate although elevations composed of rock are found in some localities. Terraces, 100 to 160 meters in depth, have slightly steeper declivities and are not always flat. In many localities the rock foundations of these terraces are exposed. It is significant that knick points may be found at the depth of about 150 meters in the profiles of shelves where terraces 100 to 160 meters in depth can not be found. In some localities, the relation of the shelves to subaerial land forms are as follows : 1) At the mouth of Suruga Bay, an inlet of the Pacific Ocean in the central part of Japan, the shelf extends southeastwards from Cape Omae. The outer portion of this shelf, deeper than 60 meters, has some relief and is concordant with the southeastward extension of the subaerially denuded lowland, overlaid by the upper Pleistocene gravel beds in Makino-hara and Omae-saki Uplands, Shizuoka Pref. 2) The shelf along the Pacific coast of Kwanto lowland is shallower than 30 meters. On the other hand, the shelves are composed of two terraces, about 30 and 100 meters in depth, along the coasts of Joban and the southern part of Boso Peninsula where coastal lowlands are narrow and mountains composed of Tertiary or older formations rise close to the coasts. Therefore, it seems that terraces of about 100 meters in depth are overlaid by the upper Pleistocene deposits which cover the Kwanto diluvial uplands. Similar cases are expected in other localities. 3) In Seto-naikai, the inland sea of southwestern Japan, the bottom is very shallow and flat, but in many narrow straits remarkably deep caldrons are found, some of which are deeper than 200 meters. These caldrons appear to be submerged valley or divides, a little abraded and not buried by violent action of tidal currents. Although submerged topography is not detected, it seems that, in the western part of the Akashi Strait, near Kobe, a subaerially denuded surface overlaid by the upper Pleistocene gravel bed submerged and was buried by sedimentation. 4) Along the Chinese coast, the continental shelf is very wide. Off the mouth of the Yangtze Kiang, there is a remarkable bank, called the Great Yangtze Bank, 40 to 50 meters in depth, which seems to be a submerged delta. The outer portion of the shelf, 75 to 150 meters in depth, has a slightly steeper declivity and irregularities. On its outer margin, there stands a small Japanese island group, Danjo-gunto, separated from Kyushu by deep sea, deeper than 200 meters. The foundation of this shelf may be an extension of low-lying peneplains in eastern China and western Korea which had been denuded since the late Miocene and in the early Pleistocene epoch were considerably subdued. The continental shelves around the Japanese Islands have similar topographical features in many regions. It seems that the origin of the continental shelves around the Japanese Islands may not be due to present action of subaqueous agencies, but to the submergence of subaerially denuded lowlands in the late Tertiary or early Pleistocene, which may be not related to glacial eustasy in the Pleistocene Ice Age. However, the Great Yangtze Bank may be related to the rise of sea level in the post-Glacial Age, which is estimated to be about 50 meters. This amount of change of sea level is less by about 50 meters than that estimated in the Sunda Shelf by Umbgrove. Therefore, it appears difficult to estimate the amount of world-wide change of sea level in the Pleistocene only by such submarine topography, as the Great Yangtze Bank and the drowned valley system in the Sunda Shelf, because submergence of those topographies may be partly due to differential earth movements. The depth of the outer margin of the continental shelves is not uniform, but varies to some extent, in many instances at 130±50 meters. Even if such variation of depth of shelf margin is neglected and uniformity of depth of shelves is assumed, such uniformity may not be due to the present action of subaqueous agencies or the glacial eustasy in the Pleistocene, but to uniformity of the amount of earth movement in the Pleistocene.
著者
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吉川 虎雄
Department of Geography, Faculty of Literature and Education, Ochanomizu University
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吉川 虎雄
Department Of Geography Faculty Of Literature And Education Ochanomizu University