Submarine Sand Ridges in the East Channel of the Tsushima Strait

Abstract

A lot of submarine sand ridges were found in the East Channel of the Tsushima Strait by the survey which is one of the project of the Hydrographic Department, Maritime Safety Agency, Japan, named “the Basic Map of the Sea in Coastal Waters”.These submarine sand ridges are classified into two types; one of which is U-type and the other is longitudinal type. The U-type sand ridges develop in the narrowest part of the channel between southernmost of Tsushima Island and Iki Island. The longitudinal type sand ridges extending northeastward develop abundantly in the vast area behind the U-type sand ridge area. From the relations between that regular arrangement of sand ridges and distributions of current velocity and grain size of sediments, the authors concluded that at the present these sand ridges are not in growing stage by the Tsushima Warm current, but have been made by tidal currents at a past age in which sea level lowered at the depth of -80 meters. It's inferred that in that stage, the narrowest channel was 40km broadening to 80km in the northeast; and moreover the pression about 40m-depth ran through the center over which 2-3 knot-tidal currents flowed. In these circumstances like the present Malacca Strait, both the narrower spacing sand ridges in the northeast shallow bottom and the broader spacing ones in the central depression were built up.These sand ridges covered the submarine terraces at the depth of -90m to -120m. Accordingly, abundant submarine sand ridges are formed on the way of the rising sea level after the Würm glacial stage when -110m to -120m terrace cut presumably.