The Topographic Features of the Base of Recent Alluvium near the Nogoya Port

Abstract

The result of our investigation by means of more than one hundred borings together with cone penetration tests shows that there are three erosional surfaces under the recent alluvium in the coastal zone of the Nobi alluvial plain. (1) The upper surface, at the place of 0-10 meters below the present sea-level, can be seen in the southeastern part of the plain. This surface seems to have been an abrasion bench which was formed on and after the middle stage of the Recent. (2) The medial surface, at the depth of 20-30 meters below the present surface, could have been formed due to river erosion during the late Pleistocene when the sea-level was approximately 40 meters lower than that of the present. (3) The lower surface can be seen in the western part of the plain, at the place of 40-55 meters below the present sea-level. This surface is the floor of the Kiso pre-Recent entrenched valley system, during the lowest sea-level period in the last glacial stage. The buried topographic features found in the Nobi Plain, which are in accord with those found in both Tokyo and Yokohama area and Hamamatsu area, are also very much alike to those in the lower Mississippi alluvial valley which was investigated by H. N. Fisk. Similarities in topographic features in all those areas suggest that those topographic features were controlled by oscillation in sea-level during or after the late Pleistocene. The existence of those topographic feature shows that the sea-level in the late Pleistocene changed by the following process: (1) After the Pleistocene high sea-level period, the sea-level was lowered to the depth of 30-40 meters below the present sea-level (A Period), (2) then, the sea-level was lowered again to the lowest level in the late Pleistocene (B. Period.). Since rejuvenation did not extend to the upper part of the buried tributaries, it is clear that the duration of the deeply entrenched valley was limited to a relatively short period of time. A Period is believed to have been shorter than B Period.