Reconsideration on Alluvium stratigraphy in the central Echigo Plain, northern Japan

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  • 越後平野中央部における沖積層層序の再検討
  • エチゴ ヘイヤ チュウオウブ ニ オケル チュウセキソウ ソウジョ ノ サイケントウ

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Abstract

Ten drilling cores that reached the basement of Alluvium in the central Echigo Plain were examined lithologically and chronologically. The Alluvium of which time-planes were drawn up according to many 14C dates was divided into nine members. As a result, some new knowledges of sedimentary process were obtained as follows. 1) The formational process of the Echigo plain can be divided into four sedimentary stages as follows. Stage I: The beginnings of transgression; late glacial stage (ca. 15,000 to 10,000 yrs BP). Stage II: The expansion of drowned valley; early Holocene (ca. 10,000 to 8,000 yrs BP). Stage III: The appearance of barrier system; transgressive and high standing stage of the Jomon Transgression (ca. 8,000 to 5,000 yrs BP). Stage IV: The development of strand plain and disappearance of lagoon; after high standing stage of the Jomon Transgression (ca. 5,000 yrs BP to present). 2) The deposition of Alluvium in the Echigo plain began at the late glacial stage. 3) Any unconformity is not found in the Alluvium of the Echigo Plain. The uppermost Pleistocene to Holocene was deposited continuously. 4) Assuming that 10,000 yrs BP is Holocene basement horizon, the depth of position is -100m in the coastal area of the Plain and -70m in the inland area respectively. 5) A subsurface sand bank with NE-SW elongation is located on the central part of the Echigo Plain. It probably continues to the New Sand Dune I. The appearance of a large lagoon in the Shirone area was probably caused by this sand bank.

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