伊勢湾と周辺地域の埋没地形と第四系 : "沖積層"細分と伊勢湾の新しい沈降盆地化の問題

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The geologic structure of shallow parts of the sea bottom in Ise and Mikawa-bay was investigated by using the continuous seismic profiling, which the the Geological Survey of Japan carried out. In this paper, combining these results with geological data from borings and field survey on the coastal regions of the bay area, some problems of the latest Quaternary geohistory in Ise and Mikawa-bay area, were discussed. In the bay area, river valleys and terraces were fomed during the last ice age at which time sea level was lowering, as in the coastal regions, and his erosional topography was drowned in consequence of the post glacial rise of sea level and was buried by the younger sediments. The buried major valley bottom in those inland bay areas was dissected to the depth of 70m below the present sea level at the main Wurm phase. The buried terraces were formed along the valleys by the river erosion during the degradation stages, which can be classified concerning with levels and their relative situation into three groups: ET (-50m〜-60m), LT-1 (-40m〜-50m), and LT-2 (-30m〜-40m). Latter two groups, LT-1 and LT-2, are correlated with each two distinct lower level terraces on the coastal region, and the age of the higher one is deduced to be about 27,000 years B.P. from the radio carbon dating. Soft sediments covering those buried topography are divided into three beds, as A, B, and C bed, in descending order. The upper-most layer, A bed, covers most of the sea bottom in the bay area and may be a deposit in the Holocene. B bed underlying A overlies C bed filling up the main valley, and covers up to the LT-2 terrace surface. The upper most parts of the B bed may reach to about 20m below present sea level, eroded out, however, in most case to a -40m level in the area along the main valleys during the regressional phase following the deposition of B. This bed is correlated with Noobi formation in the coastal region the latest Pleistocene sediments in Allerod stage. "Rill channels" are grooved on the top plain of C bed, and it is being close to the level of LT-1 terrace, modifies the terrace surface to a little lower one. From these informations, the sea level change after the younger Wurm stage is discussed. Around Ise-bay, many active faults and block movements during the latest Quaternary are detected from the study of the Quaternary System in the coastal region. In the bay area also, active faults are revealed owing to the displacement of the younger Pleistocene sediment. Major fault, running through along the NW-SE trend in Ise-bay. Ise Bay fault, and its vertical displacement may be assumed to be more than 50m during the late Quaternary or post "Atsuta phase". Subsiding the south-western side of the Ise Bay fault, main part of Ise-bay area has been turning to newerly formed sedimentary basin in the lafe Pleistocene.
日本地質学会の論文
1972-12-25

伊勢湾と周辺地域の埋没地形と第四系 : "沖積層"細分と伊勢湾の新しい沈降盆地化の問題

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