北上低地帯における河成段丘面の編年および後期更新世における岩屑供給
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The Ou Range, the backbone of Northeast Japan which is less than 1, 000m high except for sporadic Quaternary volcanoes and some peaks, has shed debris eastward into the Kitakami Lowland during the Quaternary. This paper, after establishing the classification and correlation of the fluvial terrace surfaces in the Kitakami Lowland in stratigraphic relation with a number of marker tephras, discusses fluctuations of debris supply during the Late Pleistocene in eastern Japan.The fluvial surfaces in the Kitakami Lowland are divided into the T1-T3, H1-H2, M1-M2 and L1-L2 surfaces, in descending order. T1-T3 surfaces are accumulation surfaces that had developed during the Middle Pleistocene. The H1-H2 surfaces are correlated to the penultimate glacial in age. The M1 surface is a distinct depositional surface, and was formed in the period of stage 5 of oxygen isotope record before 90(80)ka. The M2 and L1 surfaces were built up in the early stadial (approx. 60-40ka) and the later stadial (around 20ka) of the Last Glacial, respectively. The L1 surface is an accumulation surface. However, there is no evidence of remarkable deposition associated with the M2 surface. The L2 surface is dated at several ka.Two characteristic features on debris-supply fluctuations during the late Pleistocene can be pointed out as follows: 1) a distinct accumulation period in stage 5 and in the later stadial, 2) very small debris accumulation in the early stadial. A swell in supplied debris indicate that the forest-line of the Ou Range dropped and that expansion of debrissupply area in the range occurred. Thus, the biggest drop in forest-line altitude occurred in stage 5 and/or in the later stadial. The area above the forest-line had scarcely developed around the Ou range in the early stadial.A comparison of landform evolution around the Ou Range to that aroud a mountainous area that has been high in altitude or in latitude, such as the Hidaka Range (Hokkaido) or the Japan Alps (Central Japan), enables us to infer the morphogenetic environments during the late Pleistocene in eastern Japan as follows: 1) in stage 5 and the later stadial, the vegetation zone had so lowered that a large amount of debris was supplied during each period in almost all the mountains in eastern Japan; 2) in the early stadial, a comparatively large area had been beyond the forest-line in the Hidaka Range and the Japan Alps, but a very small area had been beyond it in the other mountains; 3) because the early stadial was a long cold period, a large amount of debris could be accumulated in the high mountains, in spite of a rather low rate of supply.A vertical change of vegetation zone to a lower altitude, resulting in an abundant supply of debris, is closely related to cooling. From the point of view of temperature, it is concluded that it had been warmer during the early stadial than during the later stadial. The accumulation period in stage 5 should correspond to cold substages (substage 5d and/or 5b). Although it might have been very cold during substage 5d and/or 5b, the sea level had still been high. Thus, it seems very likely that these periods in stage 5 were characterized by heavy snowfall and the largest glacial extension in the Japanese mountains.
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