Relative Changes of Sea-Level and Late-Quaternary Formations in Japan

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概要

• 論文の詳細を見る

• In Japan, the changes of the sea-level seem to have occurred during the late-Quaternary, as are shown in the following table.sea-level atLast Glaciation about -80m.Earliest Jomon stage (about 4000 B. C.) -10∼15m.Early or middle Jomon stage (3500-2500B. C.) +3∼5m.Late Jomon stage (about 1000 B. C.) 2∼3m.Late Yayoi stage (about A. D. 200) ±0m.These are indicated by correlations between the late-Quaternary formations and the topographical features, and the discovery of the prehistoric remains.The bedrock floors of large river valleys are filled with tha late-Quaternary deposits usually at 60-80 meters below the present sea-level. Ordinarily, they are covered with the fluviatile gravel beds of about 10 meters thick. These are supposedly formed during the time of the low sea-level of the latest glaciation. Near the mouth of large rivers, the gravel beds generally are found at 50-70 meters deep, with some exceptional instances, such as Setouchi region and Echigo plain. In some districts, the seaward extension of the gravel beds is connected with the abrasion platforms off-shore at a depth 70-100 meters below the present sea-level. This shows that the sea-level, before the Holocene marine transgression, was about 70-80 meters lower than the present.There are some fresh-water shell middens formed in the earliest Jomon stage, in some parts of the islets of the Seto Inland Sea. This shows that the sea-level then was about 15 meters lower than the present. But in Kanto district, it has been known that the level was at about -10 meters.Judging from the distribution of the middens in Kanto district, the highest sea-level was supposed to have taken palce in the early Jomon stage. However in Nobi plain, it was found to have occurred in the middle Jomon stage. The marine deposits are now discovered at a level few meters high above the present sea-level, showing that the sea-level then was higher than today.The sea-level dropped soon after its maximum rise, and, consequently, the new coastal plains were eroded by the extended rivers. But the river beds are now found at about 3 meters below the present sea-level, showing the sea-level to be 2 or more meters lower than the present.During the later Jomon stage and the Yayoi stage, the sea-level finally rose to its present height, since the channels were filled up with marsh deposits. Paddy-field rice plant was introduced into Japan about that time and it was cultivated at the first stage in marshy land without irrigation.Fluvial deposits covering the surface of the marshy plains and the greater part of the marine deposits at less than 5 meters above sea-level began to form after the middle Yayoi stage. These deposits are less than few meters thick.

• 日本第四紀学会の論文

著者

• 井関 弘太郎

  名古屋大学

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