Stratigraphy and paleoenvironmental changes of the Upper Pleistocene in the eastern end area of the Ishikari Low Land, Hokkaido, Japan

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  • 北海道中央部石狩低地帯東縁地域の上部更新統層序と古環境変遷
  • ホッカイドウ チュウオウブ イシカリ テイチタイ トウエン チイキ ノ ジョウブ コウシントウソウ ジョ ト コ カンキョウ ヘンセン

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The Upper Pleistocene deposits, the Higashi-Chitose, Kakuta, Moseushi Formations in ascending order, are distributed in the Yuni-Abira Low Land, eastern end area of the Ishikari Low Land. We redefined the Kakuta and Moseushi Formations with designation of each lectostratotipes, and examined the sedimentation period of those formations on the basis of marker tephras. As a result, the sedimentation period of the Kakuta and Moseushi Formations understood about 120 to 45Ka and 42-40Ka respectively. Paleoenvironmental changes in this area are estimated as follows; The Higashi-Chitose Formation Period (about 130 to 120Ka): Alluvial fan is formed by "Paleo-Yubari-gawa River" which flowed into the Pacific Ocean. The Kakuta Formation Period (about 120 to 45Ka): The moor spread with warming by degrees in this period. Then, the moor changed into land by degrees from the south area, and the whole area become land until about 45ka. The Moseushi Formation Period (42-40Ka): Because a huge pumice flow, Spfl, happened in 42-40Ka, "Paleo-Ishikari-gawa River", which flowed into the Pacific Ocean through Ebetsu to Tomakomai in the Ishikari Low Land by that time, was dammed up by the Spfl in north side of Spfl flowed area. As a result, in the Moseushi Low Land, water level rose to 60m above the sea level, and the mud with pumice, which fitted around the Low Land, were deposited. This "Paleo-Ishikari-gawa dam lake" and the back marsh spread in Nopporo. Then a channel was cut open toward Sapporo. The dam disappeared, and fluvial deposits with a lot of pumice pebble derived from Spfal were transported in a moment for the Sapporo area. Thereafter the Ishikari-gawa River has begun to flow into the Japan Sea.

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