SUBMARINE TOPOGRAPHY AND SEDIMENTS ON THE CONTINENTAL SHELVES ALONG THE COASTS OF JOBAN AND KASHIMANADA

Abstract

As the results of the survey of the continental shelves along the coasts of Kashimanada and Joban. Pacific, the authors have found that these shelves are composed of four terraces, one less than 20 meters in depth, one 25 to 50 meters, one 40 to 60 meters and one 110 to 140 meters in depth. The terraces, less than 25 meters in depth, have been built by the deposition or abrasion along the coasts concerning the present sea level. The terraces, 25 to 50 meters in depth, extend about 10 kilometers in width along these coasts. On these terraces, rock, gravel and coarse sand bottoms predominate, and surfaces of the terraces are cut down by several submarine valleys. The terraces, 40 to 60 meters in depth, are narrow in width and located on the offshore part of submarine valleys. The terraces, 110 to 140 meters in depth, have flat profiles. On the southern part of Kashimanada, these terraces become narrower in width and steeper in inclination. The topographic development of these continental shelves is as follows. In the latest Pleistocene, the sea-level lowered about 100 meters below the present sea level, and the valley now buried under the R. Kuji was cut down to about 100 meters below the present sea level. In the begining of Holocene, the sea level rose about 80 meters and abrasion platforms were built at the depth of 25 to 50 meters. Then the sea level lowered again to the depth of about 50 meters below the present sea level. During the lowering of the sea level, valleys were again cut down to about 50 meters below the present sea level and narrow abrasion platforms were built at a depth of 40 to 60 meters. Then, the coastal plains were built by the regression succeeding the transgression up 4 to 5 meters above the present sea level which extensively affected the coasts of the Japanese Islands.