対馬海峡大陸棚の地形発達

対馬陸橋に関連して

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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 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.
On 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 Würm 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 Department of Japan in the area off the west coast of Tsushima Island. From the survery, distinct five submarine terraces and fifteen buried shelf channels were discovered. The second terrace, -20m to -35m deep, is a depositional plain covered by Alluvium of 5m to 20m thickness. On the other hand, the fourth terrace at -60m to -70m depths is an erosional plain with a veneer of coarse sediments. The buried shelf channels start from the land river mouths and continue to a depth of about 100m. It is noticeable that the course of the buried shelf channels are interrupted at the fourth terrace, and arc crossed by the second terrace.
As a result, the author summarized evolution of the continental shelves as follows. During the Würm glacial stage, the coast line regressed to the present depth of about -100m, and the marine terrace was formed by wave abrasion at that level. Many rivers extended to that coast and cut new dried land. And then sea level rose toward the present sea level.
At a certain stage of the sea level rising the sea level stayed at the present depth of about -80m, and many submarine sand ridges were formed in the ancient Tsushima East Channel as in the case of the present Malacca Strait. After the rising of sea level from -80m, the sea level stayed again at the present depth of about -60m. At that time, wave abrasion made the fourth terrace and scratched the submarine valleys out on it.