GLACIAL AND PERIGLACIAL MORPHOGENETIC ENVIRONMENTS AROUND THE HIDAKA RANGE IN THE WÜRM GLACIAL AGE

Abstract

In the former paper (Hirakawa & Ono, 1974), the authors described the landform evolution of the Tokachi Plain. I t was revealed that the landform evolution of the Tokachi Plain had been controlled by the sea level changes in the lowest reaches of the Tokachi River and by the climatic changes in the alluvial fan areas or the upper reaches. The conspicuous valley filling and expansion of alluvial fans occurred in the Tokachi Plain, when the glacial extent reached its maximum in the Hidaka Range (Poroshiri Stadial). Whereas neither deposition nor downcutting was remarkable inthe alluvial fan areas and the upper reaches, when the glaciation was much less extensive in the Hidaka Range (Tottabetsu Stadial). In order to clarify the interrelationship between the glaciation and the landform evolution in the plain, it is necessary to reconstruct the morphogenetic environments of the whole region. In the present paper, the authors intend the reconstruction of the morphogenetic envi-ronments around the Hidaka Range during the Wurm Glacial age, through the study of glacial or periglacial landforms, fossil periglacial phenomena, and the analysis of terrace gravels and slope deposits, and also by pollen analysis. Without these syntheses, it is impossible to relate many fragmental data, and to ex-plain them in accordance. Through the study of glacial topography, two stadials of gla-ciation are established, contrary to the generally held view that the older glacial topogra-phy is correlated to the Riss Glacial in age. The climatic conditions suggested by the glaciation of these two stadials are in good agreement with those which are suggested by the analysis of terrace gravels and slope deposits at the same time. During each Stadial, glacial extent, slope development, fluvial process and periglacial phenomena of this region were quite different, as described below: 1) Poroshiri Stadial This Stadial is characterized by the largest extension of the glacier, the abundant supply of muddy slope deposits in the Hidaka. Range and the construction of filltop terraces/allu-vial fans in the plain. As the snow line depression estimated from the glacial topography is 1200- 1500 m, the whole area of the range should have been beyond the timber line. The lowering of the timber line caused an increase in debris supply, especially from the valley walls. The filltop terrace gravels are smaller in diameter and poorer in roundness compared with the gravels of the present river bed (Fig. 12). Though there are few fossil periglacial phenomena in the earlier part of this stadial, ice-wedge cast and many involu-tions are observed in the later part (Fig. 16). 2) Tottabetsu Stadial Glaciation in the Hidaka Range was much more limited in extension than that of the Poroshiri Stadial. Glaciers occupied only a part of the large cirques excavated in the Poro-shiri Stadial (Fig. 2). The supply of slope deposits decreased. And there are no evidences of remarkable deposition nor downcutting in the area of alluvial fans. A number of dunes were distributed all over the Tokachi Plain, and the structures such as ice-wedge cast, involutions and other periglacial phenomena were developed. These facts allow the authors to deduce the morphogenetic environments of each stadial as follows. The climate of the Poroshiri Stadial was favorable to the extension of the glacier ; much snow in winter and cool in summer. On the other hand, the climate of the Tottabetsu Stadial was dry and cold especially in winter (Fig. 16).