千曲川上流地方の第四紀地質(その3) : とくに上小(じょうしょう)湖成層について
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概要
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After the disappearance of paleo-Komoro lake which was filled up by the Pliocene Komoro group, a new lake basin was formed, and the older Josho lake sediment was deposited in it. After an interval of erosin stage, the younger Josho lake sediment was settled in the same basin. These Pleistocene formations are named the Josho lake-deposit as a whole. The relation between the older and the younger Josho formations is an unconformity. Their rock facies are different between the southern part and the eastern part. Especially the strata in the eastern part are rich in conglomerate, whereas those of the southern part consist mainly of mudstone. Judging from the fact that the sandy pumice in the younger volcanic ash formation overlies conformably on them everywhere, it is suppose that the Josho lake-deposit is contemporaneous in both areas irrespective of the difference in rock facies. The geologic age of the older Josho formation is correlated with the early Tama age, while that of the upper part of the younger Josho formation corresponds with the early Musashino age, though its main part is correlative with the age of Shimosueyoshi loam. The younger Josho formation is also supposed to have been formed during Wurm glacial age through the examination of mammal fossils in it. The center of these older and younger Josho formations lies in the Shioda basin in the southern part of the whole lake basin. They are not suppossed to be the lakes which were formed through the dam-up effect of the pyroclastic flow. The uplift of the surrounding mountains and the corresponding subsidence of the lower lands may be one of the main factors of the formation of these lakes.
- 地学団体研究会の論文
- 1969-03-25
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