米代川支流,小猿部川における最終氷期後半以降の河成段丘形成

抄録

Formation of a wide strath (partially fillstrath) terrace during the downcutting stage which follows the valleyfilling terminated around 20, 000y. B. P, is demonstrated in the previous works in the Ou Backbone Range. The climatic change in the latest Würm Stage seems to be a possible cause of the above river terrace development. The hypothesis leads to investigate river terraces in the other mountains which has different relief and structural conditions and similar climatic condition. This paper presents terraces of the Osarube river in the Dewa Mountains situated to the west of the Ou Backbone Range.
The river terraces of the study area are classified into the Shinarui, the Sendoishi, the Nakahata, and the lower terraces in descending order. Stratigraphic evidences show that the formation of the Shinarui terrace precedes the Takaichi pyroclastic flow deposit older than 30, 000 y. B. P. The lower terraces are below the Nakahata terrace, and each of them is a kind of non-cyclic terrace.
The Sendoishi terrace is underlain by a valley-fill. The change from valley-filling to downcutting is inferred to have occurred in the latest Würm Stage from both the facts of that the Sendoishi terrace is overlain by the Torigoe pyroclastic flow deposits of 14, 000-12, 000y. B. P, and that the reconstructed longitudinal profile of the Sendoishi terrace corresponds to a lower sealevel prior to the Postglacial transgression.
The Nakahata terrace, which is the conspicuous terrace, has a veneer of gravel laid down after the valley-filling and shows gentler sloping longitudinal profile than that of the filltop Sendoishi terrace. In a cross-section the outer part of the Nakahata terrace is overlain by the Torigoe pyroclastic flow deposit, which does not exist in the inner part of the same terrace near the main channelway. The alluvial cone deposit which covers a part of the Nakahata terrace is dated as 18, 550 y. B. P. The above facts show that the formation of the Nakahata terrace, that is, the sedimentation of a veneer of gravel occurred prior to about 18, 000y. B. P. and terminated posterior to about 12, 000y. B. P.
Thus it was confirmed that the sequence of river terrace development in the latest Würm Stage was common to rivers in both the Ou Backbone. Range and the Dewa Mountains.