越後平野北部の沖積低地における汽水湖沼の成立過程とその変遷

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タイトル別名
  • Formation and Environmental Changes of a Holocene Brackish Lake in the Northern Echigo Plain, Central Japan.
  • エチゴ ヘイヤ ホクブ ノ チュウセキ テイチ ニ オケル キスイ コショウ ノ セイリツ カテイ ト ソノ ヘンセン

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抄録

The upper Pleistocene and Holocene deposits in the northern Echigo Plain of central Japan are divided into lower, middle, and upper parts based on core data. The lower part was deposited from 40, 000 to 18, 000 yrs BP and consists of sand and gravel-supported conglomerate with alternating beds of sand and silt. The middle part (sedimentary unit I) was deposited from 10, 000 to 8, 000yrs BP and consists of sand and matrix-supported gravels. Facies analysis shows that the lower and middle parts comprise deposits of alluvial fan during the lowstand of sea level. The upper part (sedimentary units II and III) consists mostly of mud and sand that formed upward-coarsening sequences of lagoon-delta deposits. Detailed diatom analysis indicates that the history of the lagoon is divided into four stages. During the first stage (from 8, 000 to 5, 000yrs BP), a brackish lake environment with low salinity pervaded, due to flooding caused by a rapid rise in sea level of the Jomon transgression and the development of a sand dune. At the second stage (ca. 5, 000 yrs BP), the lagoon changed into a freshwater lake. During the third stage (from 5, 000 to 1, 000yrs BP), the lake accumulated a large amount of sandy sediments from the north to the south, and an eolian environment appeared. During the fourth stage (since 1, 000yrs BP), a freshwater area formed again, and has remained as the Shiotsugata Lagoon.

収録刊行物

  • 第四紀研究

    第四紀研究 41 (3), 185-197, 2002

    日本第四紀学会

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