札幌・苫小牧低地帯およびその周辺山地の形成過程

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The Sapporo-Tomakomai Depression is one of the significant depression belts in Hokkaido, Japan. It extends in a north-south direction from Ishikari side of the Japan Sea to the Tomakomai side of the Pacific Ocean. This depression belt lies between highlands in the eastern, the western and the northern area that are composed of sedimentary and volcanic materials formed in Pleistocene. The belt developed as basement from many formations of the Miocene series to the Pliocene. The western highlands have the characteristic deposits of the green tuff region. The eastern and northern highlands are composed of non-green tuff deposits. The western highlands have mainly monoclinic structure with a small amount of small folds and older than the eastern formations in age. The prominent structural feature of the basements is characterized by block structure, made by two systems of faults, that is, three or four faults running in a NNW-SSE direction and many faults running in a NE-SW direction. These faults were many formed from Miocene to Pliocene in age. Only the eastern most fault in the western highlands, however, limited the sedimentary area of the Nopporo formation of the lowest Pleistocene series and moved again during the middle Pleistocene period. As a result it has completed the above mentioned block structure in the western area. The northern and the eastern highlands are characterized by folding structures in the N-S direction formed in latest Pliocene. A eastern synclinal zone among them had been successively retained as a area of deposition in Quaternary. The lowest Pleistocene formations distribute in the both areas of east and west. They are called the Nopporo formation in the western area and the Nitapporo formation in the eastern. It is a remarkable characteristic that the deposits of them are composed chiefly of the materials from the eastern highlands. After the deposition of the Nopporo formation, in a few stage of erosion, the faults were covered, mostly by andesite gravels, from the western side of the fault. This is the present Wattsu formation, which distributs in the wider area than that of the Nopporo formation. Subsequently, the whole land masses rose so that the above mentioned highlands were raised above sea level, and only the central area of Wattsu formation remained below sea level. The Nishinosato formation deposited precisely in this area. In the Wurm glacial stage, the sea level regressed, causing wide areas to erode under the cool and dry conditions. During the same period the volcanic activities relating to the Shikotsu caldera had been extensively done and the pumice falls or the pyroclastic flows covered over the lower land in the mid-area. Thus the Sapporo-Tomakomai Depression divided into the northern and the southern areas. Then the sea level rose again and alluvial sediments deposited and formed the present Ishikari Plain in the northern area.
日本地質学会の論文
1972-12-25

札幌・苫小牧低地帯およびその周辺山地の形成過程

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