<専報>対馬海峡の地形と底質についての予察的研究

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As part of the first year survey of the Second Natural History Research Program of the Japanese Islands by our Museum, the writers studied the topography and bottom sediments of the Tsushima Straits, and obtained the general features as reported preliminarily in this paper. These and other features, however, would be elucidated in more detail by many other works, including those of the writers themselves, which are under way and will be reported in the next issue of the Memoirs. As shown in Fig. 2,the bottom relief of the part east of the Tsushima Islands, i.e. Tsushima Straits (s.s.), is rather even except for some weak rises and grooves trending in a NNE-SSW direction, whereas a prominent trough, Tsushima Trough, exists just west of and along the whole length of the Tsushima Islands in the Korea Straits. The eastern wall of the trough is rather straight and so steep that it seems to be a fault scarp or fault-line scarp, in contrast with the western wall which rises gently up to a shallow bench surrounding the southern coast of Korea. The Tsushima Islands are located near the middle of the Tsushima Straits (s.l.) and its topography and geology seem to offer an important key for understanding the nature of the strait. General topographic features of the islands reflect very well the difference of hardness of the constituent rocks as exemplified in a summit-level map (Fig. 3), which shows a plain, about 250 to 200m above the sea-level, covering almost throughout the islands. At the present it is difficult to decide whether the plain was formed by peneplanation or by wave-abrasion. If the peneplanation was existent, its evidence should be found in Kyushu, but no paper has ever reported it. Besides the summit-level, the geologic structure accompanied by folding and many faults with a general trend of NNE-SSW may account for the general shape of the islands with steep and staight cliffs running in the NNW-SSW or N-S direction in many places. Nearly none of the cliffs, however, represents fault scarp differing from the previous opinions (e.g. SATO, 1954; ISHII, 1964). These steep and straight cliffs, no matter whether they are of fault origin or not, may have been formed by uprushing waves and sweep of the Tsushima Current flowing from SSE to NNE in general. As shown in Fig. 2,the islands are completely encircled by a wave-cut bench about 1 to 3km wide and 60m deep at the edge. The sweeping by current, moreover, may have prevented the accumulation of detritus which is expected to occur along the outer side of the bench. The prominent cliffs constituting the major part of the western coast may be defined as fault-line scarps which retreated from the presumable fault or fault-line scarp on the eastern border of the Tsushima Trough. Drowned valleys, another characteristic topography of the islands, can be seen in many places as already pointed out by TAYAMA (1954), though some of them are obscure as their down-stream parts were eroded away. The abrasion seems to advance into submarine valleys or buried valleys, which are supposed to be continuous with the drowned valleys. These drowned valleys cannot be ascribed to a subsidence of land but mainly to a rise of sealevel, because the typical rias coast is observed along the south and west coasts of Korea and the northwestern coast of Kyushu. The maximum invasion of the sea occurred in the prehistoric Jyomon Period of post-glacial age; since then some of the drowned valleys have been buried up as a typical instance was reported from northern Kyushu by YAMAZAKI (1955). It seems that the Tsushima Straits (s.l.), connecting the Sea of Japan with the Pacific, was formed during or a little before the Jyomon Period when the relative subsidence of the region culminated, after the uplifted plain had been remarkably denuded as indicated by the summit-level. According to the charts published by the Hydrographic Office, sand or shell sand covers almost the whole region of the strait, even in the dee
国立科学博物館の論文
1969-09-13

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