The Mud-flow Deposits and the Geomorphic Development in the Kaminoyama Basin, Yamagata Prefecture

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  • 上山盆地における泥流堆積と盆地の埋積過程
  • カミノヤマ ボンチ ニ オケル デイリュウ タイセキ ト ボンチ ノ マイセキ カテイ

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The geomorphic development of the Kaminoyama Basin to the south of the Yamagata Basin, was under the control of the deposition of mud-flows, derived from the western slopes of Zao Volcanic Group. The author investigated the relationships between the deposition of mud-flows and the geomorphic development in the Basin, based on analysis of landforms and underground geology by columnar profiles of many boring wells.<br>It is summarised as follows:<br>1) The base rocks lie 155 meter below s. l. in the central part of the Kaminoyama Basin 175 meter or more below s. l. in the Kanagame Gorge and about 200 meter below s. l. in the southern part of the Yamagata Basin. Therefore, the basin bottom of the Kaminoyama Basin is continuous to that of the Yamagata Basin.<br>2) The basin deposits consist of four layers of gravels (the I, II, III, IV-layers), making alternate stratum with bluish cleyey layers. On the other hand, the mud-flow deposit in the Kanagame Gorge are classified into five layers (the A, B, C, D, E-layers). Each of the lowest depth of mud-flow layers corresponds with the lowest one of the bluish clayey layers in the Basin. Therefore, the bluish clayey layers are lacustrine deposits in the dammed lakes, formed by the deposition of repeated mud-flows.<br>3) The mud-flow landforms in the Kanagame Gorge are divided into three—the Sukawa, Ashinokuchi and Kanagame mud-flows. Correlating those mud-flow landforms to the mud-flow deposits above-mentioned and the lake terraces in the northern part of the Basin, the Sukawa mud-flow coincides with the B-layer and the middle lake terrace, the Ashinokuchi or Kanagame mud-flow coincides with the A-layer and the lower terrace.

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