北上川上流域における後期更新世の周氷河現象と火山灰層序

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The fossil periglacial phenomena, involution, mass-movement, and gentle slope (cryopediment) formation, are often observed in the upper Kitakami River basin, northeastern Japan. The tephrochronological studies of late Pleistocene periglacial phenomena are carried out and the Quaternary history in this area is also discussed.At the foot of Mt. Iwate (alt. about 200-300m), involution is dominantly observed on FSc, ISc (about 33, 000y.B.P.), WSc-1, WSc-2, S1P, and S2P, which are the marker beds of the Shibutami and Nishine Volcanic Ash formations erupted from the Nishi-Iwate Volcano. KP (about 13, 500-16, 300y.B.P.) erupted from the Akita-Komagatake Volcano is also disturbed on a small scale. However, OD (about 34, 000y.B.P.) and KwP, which are respectively erupted from the Nishi-Iwate and Akita-Yakeyama Volcanoes, are scarcely disturbed. Involution observed in these marker beds is tephrochronologically estimated to have been formed about 16, 300 to 12, 400 years ago for KP, about 34, 000 to 16, 3000 years ago for FSc and ISc, on the first half of the Würm glacial age for WSc, and on the period of cold climate prior to that for S1P and S2P.At the foot of Mt. Himekami (alt. about 400-700m), the solifluction deposits consisting of granodiorite boulder and volcanic ash are put between VP (or FSc) and OD, and are also observed in the layer directly overlain by OD or in the Nishine Volcanic Ash formation. The mass-movement (the Block Stream) at Mt. Himekami probably took place about 34, 000 to 16, 300 years ago and in the first half of the Würm glacial age and/or on the period of cold climate prior to that.The gentle slope developed in the area of the Sotoyama Plateau (alt, about 700-900m) is covered with AK-h, AK-g, and Holocene tephras. Solifluction deposits rich in an angular gravel are extensively observed in the Shibutami Volcanic Ash formation on the Sotoyama and Hayasaka Plateaus, whichis underla in by the red weathering crust. On the basis of the stratigraphical relationships, they are probably formed about 34, 000 to 16, 300 years ago and may be related to the formation of gentle slope developed in the Kitakami Mountains.According to the pollen analysis, it seems to have been cool to cold in Upper Pleistocene. Therefore, the periglacial environment in Upper Pleistocene would have a great influence upon the landform development in this region.
日本第四紀学会の論文

北上川上流域における後期更新世の周氷河現象と火山灰層序

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