十勝平野の地形発達史

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タイトル別名
  • THE LANDFORM EVOLUTION OF THE TOKACHI PLAIN
  • トカチ ヘイヤ ノ チケイ ハッタツシ

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The Tokachi Plain is located on the eastern side of the Hidaka Range, the back-bone of Hokkaido, where a number of glacial landforms are well preserved (Fig.1). It has long been pointed out that the Tokachi Plain is one of the most important region for the Quaternary research in Japan. Widely developed alluvial fans at the foot of the Hidaka Range, an alluvial plain in the lowest reach of the Tokachi River and marine terraces along the Pacific coast are no less stimulative for geomorphologists than the glacial topographies in the Range.<br> In this paper, the authors discussed the landform evolution of the Tokachi Plain in order to clarify (1) the relation between the construction of alluvial fans in the Plain and the glaciatin in the Range, and (2) the relation between fluvial terraces and marine terraces indicating sea level changes in the late Quaternary. The surfaces of these terraces are covered with tephra layers, to which the authors refer as the Tokachi Loam (Fig.2). Key beds of characteristic pumice or scoria layers in the Tokachi Loam and continuity of terrace surfaces lead to a classification of the fluvial and marine terra ces (Table 1, Fig.3) and their chronological arrangement (Fig. 8).<br> Of the marine terraces, the O II surface consisting of thick marine deposits seems to have been built under the heighest sea level in the Eemian Interglacial age. The 0 IV surface shows a slight transgression at about 60, 000 years P. P.. Consequently, the fluvial surfaces younger than the 0 IV surface are included in the Warm Glacial and Postglacial ages. From the view point of the landform evolution, at least three phases are recognized in these ages as described below.<br> (1) 60, 000 30, 000 Tears B. P.<br> Valley filling and expansion of alluvial fans were conspicuous in the Plain; the Ko I filltop terrace surface was formed just before the fall of Spfa (Sh.ikotsu Pumice Fall Deposits).<br> (2) 30, 000.10, 000 years B. P.<br> Neither deposition nor downcutting were remarkable in the area of alluvial fans. The alluvial fans of the Ko II and Ko gj surfaces were constructed as strath terrace surfaces in rather limited areas compared to the former Ko I surface. Whereas in the lowest reach of the Tokachi River, deep incision, which was caused by a regression reaching about 1.20 m below sea level at the maximum, resulted in a steeper gradient of the longitudinal profile of the Ko II surface (Fig.7).<br> (3) 10, 000 years B. P.-Present<br> In the area of alluvial fans vigorous incision reaching 20 to 50 m in depth took place ; alluvial fans and river terraces of the Ko I, II and f surfaces were cut down to form many strath terraces of the Ko IV surfaces. While in the lowest reach, of the Tokachi River, the Postglacial transgression caused deposition of thick alluvium to bury the surfaces of Ko I-Ko II (Fig.7).<br> The glacial landforms in the Hidaka Range, consisting of cirques and moraines, are either younger and fresh or older and dissected. The distribution of the older ones is much extensive than the younger. Contrary to the generally held view that the older landforms are correlated to the Kiss Glacial in age, the authors are of opinion that they are rather correlated to the early stadial of Wurm Glacial age. The moraines and out-wash deposits of the older stadial (Poroshiri Stadial) are probably correlated to the fill gravels of the Ko I surface. And those of the younger stadial (Tottabetsu Stadial) are probably correlated to the deposits of Ko II surface.<br> It can be concluded that the la.ndform evolution of the Tokachi Plain has been controlled by the sea level changes in the lowest reach of the Tokachi River and by the climatic changes in the alluvial fan areas or the upper reaches.

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  • 地理学評論

    地理学評論 47 (10), 607-632, 1974

    公益社団法人 日本地理学会

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