Impact of "24 ka Nakanojo Mud Flow" Event on the Fluvial Landform Development in Nakanojo Basin, Central Japan

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In order to understand the long-term geomorphic development of drainage basins that include some Quaternary stratovolcanoes in the upstream area, as occur in Japan, it is necessary to evaluate not only the usual river processes associated with glacial-interglacial alternations but also the influence of sediment supply derived from large-scale volcanic events-for example, the sector collapse of volcanic edifices peculiar to Quaternary volcanoes. From this point of view, this paper discusses the depositional process of "Nakanojo Mud flow (NMf)" and its impact on the fluvial landform development in Nakanojo basin, located in the drainage basin of the Agatsuma river, central Japan.In Nakanojo basin, a terrace landform is well developed. However, the relation of NMf and long-term geomorphic development has not been discussed in detail by previous works. NMf has been considered to be the debris avalanche deposit associated with the sector collapse of Asama Volcano around 24, 000 years ago.The main part of Nakanojo Surface is the depositional surface of NMf sediments running onto the existing fluvial terraces composed of the Nakanojo gravel layer formed during the Younger stage of the Last Glacial period. Along with the morphological features of the Nakanojo Surface, such factors as the direction of the long axis of the flow mounds and the 3-D distribution of the mud flow deposit suggest the flow direction and relative magnitude of NMf; within Nakanojo basin, NMf maintained the property of a debris avalanche more than 40m thick, and flowed onto the existing terraces mainly on the attacked slope side 20-30m above the river bed at that time.In Nakanojo basin, the deposition of NMf strongly disturbed the valley cut and fill sequences which are normally controlled by climatic conditions. It is supposed that the river bed fluctuations had been regulated by this event until the emergence of Isemachi I Surface with its relatively coarse gravels. This period may correspond to the termination of the Last Glacial, which is often characterized by the increase of discharge related to tractive force.
日本第四紀学会の論文

Impact of "24 ka Nakanojo Mud Flow" Event on the Fluvial Landform Development in Nakanojo Basin, Central Japan

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