新潟平野の形成過程
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概要
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Being located in the central part of Japan proper and facing to the Japan Sea, the so called Niigata lowland is an alluvial plain stretching along the Shinano River. The geology in Niigata lowland were clarified through such investigation of theirs as exploratory drillings, penetration test, micropalaeontological analysis, absolute chronology by the radiocarbon method, and soil mechanical method. In this connection, pollen analysis proved to be very useful for the correlation of each formation and diatom analysis for knowing the sedimental environment. The late Quaternary deposits can be divided into five layers, I, II, III, IV, V. Geological characteristics of each of the abovementioned five layers are as follow. Layer I; Deposits in humid plains like the swampy land in regression stage of the Latest Holocene with a climate as mild as the present. Its general depth is less than 20 meters below the present sea levels. Its consists of sand, silt, and clay containing fossil wood, and is very lowly conpacted in general. Layer II; Lagoon or inland bay sediments under a stable water level of Holocene with a milb climate, the temperature being a little lower than that of the present. Its general depth is from 20 to 40 meters below the sea level. The said layers consists of very soft clay at Shirone, and of sand or silt at Kurotori which is situated around the boundary line between the inland and the seaside areas. The extent of the clay bed coinsides with the area where a considerable subsidence is observed in the inland. Layer III; Lagoon or inland bay deposits in the process of gradual stabilization of the sea level with a mild climate. Its general depth is from 40 to 80 meters below the sea level. The inland parts of this layer chiefly consists of clay, with the seaside one consisting of silt. Layer IV; Lagoon or lacustrine deposits in the process of radical changing water level with a mild climate. Its general depth is between 80 and 100 meters below the sea level. Layer V; Lacustrine sediments deposited less than 20,000 years ago under a stable water level with a cool climate. This layer is generally distributed between 100 and 150 meters below the sea level. Underlying Layer V there are terrace-like deposits called "Nishikanbara Group" which were formed more than 25,000 years ago. The distribution of the mean N-value and the sand gravel ratio in Layer II coincides with that of land subsidence in the inland, indicating regional difference of lithofacies. From those findings, it is to be surmised that Niigata lowland as a whole has grown in a lagoon like condition since of 20,000 y.B.P., thougth temporarily it may have been severaly affected by sea water or may have transformed into an inland bay or delta.
- 日本地質学会の論文
- 1972-12-25