新潟県柏崎平野における上部更新統の層序と古環境の復元

DOI Web Site 被引用文献2件 参考文献19件

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タイトル別名
  • The Stratigraphy and Paleoenvironmental Reconstruction of the Late Pleistocene Deposits in Kashiwazaki Plain, Niigata Prefecture.
  • ニイガタケン カシワザキ ヘイヤ ニ オケル ジョウブ コウシントウ ノ ソウ

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抄録

The middle-late Pleistocene deposits composed of the Yasuda Formation, the Ominato sand bed, and the Banjin sand bed were formed at Kashiwazaki Plain in Niigata prefecture. Clarifying their stratigraphy and sedimentary environment is very important for revealing the paleoenvironment during the late Pleistocene in the Kashiwazaki Plain.<br>The Yasuda Formation is divided into an upper and a lower part. Both parts were formed with the character of valley fills because of a marin transgression under fresh or brackish water conditions in a bay which was separated from the open sea. The terrace surface of the upper part of the Yasuda Formation is correlative with the Shimosueyoshi surface in the south Kanto region.<br>The Ominato sand bed, newly named in this paper, comprises littoral or neritic sands, which cover the lower part of the Yasuda Formation conformably. The Nakago pumice layer (ca. 150-130ka) is intercalated near the boundary between the uppermost part of the Ominato sand bed and the Banjin sand bed, composed of eolian sands which cover the Ominato sand bed comformably. From to this it is inferred that the surface of the Ominato sand bed below emerged with the Yasuda terrace at the Shimosueyoshi stage; in addinon, the Ominato sand bed and upper part of the Yasuda Formation are heterotopic facies. Under the influence of the paleoenvironment, the bar which consisted the Ominato sand bed was formed in front of Kashiwazaki Plain and the upper part of the Yasuda Formation was deposited in the lagoon behind the bar at the Shimosueyoshi transgression stage. This suggests that a barrier system was present at that stage.<br>There existed a bar at the beginning of the regression. Because of the regression, the lagoon dried up and formed the Yasuda surface. After the Yasuda Formation and the Ominato sand bed emerged, the Banjin sand bed which was composed of eolian sands was deposited at the Shimosueyoshi stage; the Obaradai stage and the other regression stages emerged later. The Old Dune sand has remained for almost fifty thousand years.

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