Mud Flow Deposits Derived from the Activity of Asama Volcano and Their Impacts on the Geomorphic Development of the Northwestern Part of the Kanto Plain, Central Japan

The sediment supply caused by the erosion processes peculiar to Quaternary volcanoes often plays an important role in the landform development of the downstream part of drainage basins. In particular, the sediment derived from the large-scale destruction of volcanic cones has great potential to modify the landform of drainage basins. Therefore the author attempted to elucidate the geomorphic development of the Maebashi and Takasaki area located in the middle reach of the Tone River drainage basin which had Quaternary volcanoes in the upstream area focusing on the relationship between large-scale volcanic events and the river process.<br> This study found that the debris avalanche called the Maebashi Mud flow derived from the Asama volcano and the newer volcanic Inokawa Mud flow strongly influenced the geomorphic development of the study area. The results can be summarized as follows:<br>1) The Tone River formed a vast alluvial fan in this region around 25, 000 years ago.<br>2) Around 24, 000 years ago, the Maebashi Mud flow was deposited over the entire area and changed the landform dramatically as represented by the decrease in gradient and the elevation of ground level. The prototype of the Maebashi surface was formed at this time.<br>3) After deposition of the Maebashi Mud flow, the Tone River and the Karasu River started incising the depositional surface markedly. The former was fixed to the west and the latter was fixed slightly to the north compared with the present river courses. On the Maebashi Mud flow depositional surface, the Maebashi Peat layer began to be formed simultaneously with the decrease in gradient.<br>4) From about 16, 000 years ago, the Tone River shifted to the east and incised the Maebashi Mud flow depositional surface again. As a result, the Hirosegawa Gravel layer composing the Hirose surface was formed.<br>5) About 11, 000 years ago, Inokawa Mud flow flowed down the Karasu River channel. The Takasaki area was covered with this deposit, resulting in the formation of the Takasaki surface. The main channel of the Karasu River shifted to the present position.<br>6) As the Ino River slightly dissected the eastern part of the Inokawa Mud flow depositional surface, the Ino surface was formed as an erosional one.<br> Based on the above, the author concluded that the geomorphic development of the study area was strongly related to the sediment supply by volcanic events in the upstream area and that it is necessary to reevaluate the Quaternary volcanoes as the sediment source involved in the landform development of the downstream part of the drainage basin.