石狩海岸平野における埋没地形と上部更新〜完新統について

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The Ishikari coastal plain is situated in the northwestern end of the so-called Sapporo-Tomakomai Lowland in Hokkaido, and bears such topographic features as coastal sand dunes, ridged beach plain, the older dunes and back marshes.The present report intends to clarify sea levels, stratigraphy and sedimentary process of deposits after the Würmian glacial maximum (20, 000-18, 000 years B.P.).At first buried landforms of the area in the Würmian glacial maximum are considered on the basis of many boring data. Consequently the shape and the depth of four buried terraces deposits (Bt1-Bt4), palaeo-delta and buried valleys have been discriminated. Among them Bt1 was formed before the eruption of Shikotsu volcano (caldera). Bt2-Bt4 and palaeo-delta deposits were formed after the Paudorf interglacial epoch untill the Würmian glacial maximum. Whereas gravel beds of buried valleys were flood plain deposits of old rivers in the Würmian glacial maximum. Estimation of sea level at that time of glacial age is difficult owing to that the Ishikari coastal plain was a land area then. However it is almost certain that the sea level was at least 65 meters below the present one.Then, the sea gradually invaded into the land, following transgression after the Würmian glacial maximum, and palaeotopographic relief was completely buried by later deposits, of which the upper part is characterized by the presence of black, soft clay. Later temporary regression caused a sand bar linking mountains southwest to the Ishikari hills in the northeast. Finally it developed to a sand-gravel barrier at the place of Momijiyama sand dune, and it created a big change in sedimentary environments of the area. Namely thereafter, swampy deposits were developed in the inland parts of the sand-gravel barrier, and rows of beach ridges were developed in seaward.The Bannaguro lowland formed by the development of continuous sedimentary processes when the sea level was 4-4.5 meters higher than the present one. From radiocarbon dating of marine shells contained in the deposits of beach ridges, it was clarified that the age of high sea level (4-4.5 meters) continued from 5, 800 to 2, 000 years B.P.Then followed a sudden lowering of sea level down to the present one and we see the present topography.
日本第四紀学会の論文

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松下勝秀
北海道立地下資源調査所

石狩海岸平野における埋没地形と上部更新〜完新統について

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