対馬海峡大陸だなの地形発達 対馬陸橋に関連して
Geomorphological Evolution of Continental Shelves of Tsushima Strait

Akio MOGI
1981 The Quaternary Research (Daiyonki-Kenkyu)  
Akio MOGT2) The Tsushima Strait are consists of broad continental shelves, connecting the Japanese Islands with the Asian Continent. Systematic sea bottom surveys of this area with two-mile spacing soundings were carried out by Hydrographic Department of Japan from 1973 to 1976. From the seismic profiling, it was revealed that the center of broad continental shelves are surfaced with thick Quaternary sediments where longitudinal depressions lie, trending in the NE-SW direction. In 1977 and
more » ... n. In 1977 and 1978, with 0.5-mile-spacing soundings more detailed surveys were conducted by Hydrographic Department in the areas from the southern coast of Tsushima Island to the north of Iki Island and near Shiro-se off the west coast of the Goto Islands, respectively. The distinct depositional terraces were detected at depths of -90m to -110m and -110m to -130m in the Shiro-se area. Along these terrace edges there are sand bars, suggesting the ancient shore lines, and inward shallow depressions. It is noticeable that the basement of the longitudinal depression is buried by sediments which form the depositional terrace at the depths of -110m to -130m. This fact means that the longitudinal depression represents an uncovered portion of the basin reclaimed by shelf sediments. The present topography of the longitudinal depression is modified, however, by currents indicated by a pair of caldrons excavated from the bottom of the depression on the west of Korai-sone. Can the other hand, a lot of submarine sand ridges were found in the Tsushima East Channel. These submarine sand ridges are classified into two types: one is a U-type, developing in the narrowest part of the channel between the southern coast of Tsushima Island and Iki Island, and the other is a longitudinal type with a NE trend, developing abundantly in the vast area behind the U-type sand ridge area. From the relations between regular distribution patterns of the sand ridges, and distributions of current velocity and grain size of sediments, the author concluded that these sand ridges are not growing at the present with the Tsushima Current, but have been made by tidal currents in the past when sea level lowered to a depth of -80m. These sand ridges cover the submarine terraces at depths of -90m to -120m. Accordingly, the abundant submarine sand ridges are formed at a certain stage of the sea level rising after the Wurm glacial stage when the -110m to -120m terrace was presumably cut. A more detailed survey on a scale of 1/10000 was carried out by Hydrographic De-
doi:10.4116/jaqua.20.243 fatcat:bxv5lypmvze4dgg6slpig5tlya