男女群島周辺の海底地質

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A submarine geological survey was conducted around the Danzyo Islands, situated at about 70 km southwest of Fukue Island, Nagasaki Prefecture, during August 19～22, 1973 aboard the R/V WAKASHIO of the Fuyo Ocean Development and Engineering CO. Ltd., under a project of the Nagasaki Prefectural Government. The two main submarine topographic units recognized around the Danzyo Islands are the steep slope nearshore and the wide flat plain offshore. The depth of the boundaries of these two units are about 100 meters at the eastern side and about 140 meters at the western side of the Islands. The larger part of the steep slope is bare of sediments, thus the massive compact rock body which is regarded as the Danzyo Welded Tuff is exposed on the sea bottom. The offshore flat plain has two terraces, one is 100 to 200 meters in depth and the other is deeper than 140 meters. These two are covered with a thin layer of sediments. Although the area deeper than 150 meters was not surveyed, the flat plain is situated nearly at the edge of the continental shelf of the East China Sea and extends down to the continental slope which is the western side of the Okinawa Trough. The sediments around the Danzyo Islands are composed of terrigenous minerals nd rock fragments mingled with calcareous shells or debris of marine organisms, such as molluscs, brachiopods, corals, echinoids, bryozoans and Foraminifera (benthic and planktonic). The calcium carbonate content in the sediments ranges from 41 to 77 per cent. The high temperature and transparency of the surface water and high percentage of calcium carbonate in the sediments around the Danzyo Island suggests the existence of a warm current flowing around the Islands. The northward-flowing warm, saline, clear water of the Tsushima Current, a branch of the Kuroshio Current, may favor the deposition of the calcareous shells and debris. The roundness of the beach gravels of the Meshima shows that the larger particles are well rounded and smaller particles less rounded or more angular. These features may be explained that the original rod-shaped angular blocks derived from the Danzyo Welded Tuff rapidly become rounded ellipsoidal or spheroidal gravels by the action of strong waves, while the abundant angular small particles remained in the pore spaces between the rounded beach gravels. Subsequently, the angular finerfractions may be transported to the offshore to become mixed with the bottom sediments.
長崎大学教育学部の論文
1975-03-00

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鎌田泰彦
長崎大教育

男女群島周辺の海底地質

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