Stratigraphy, age and depositional processes of Yosasagawa Debris Avalanche deposit, North Tochigi Prefecture, Japan

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  • Kikuchi Akihiko
    Department of Earth Sciences, College of Science, Ibaraki University Asia Air Survey Co., Ltd.
  • Hasegawa Takeshi
    Department of Earth Sciences, College of Science, Ibaraki University

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  • 栃木県北部，余笹川岩屑なだれ堆積物の層序・年代と運搬過程
  • トチギケン ホクブ,ヨ ササガワ ガンセツナダレ タイセキブツ ノ ソウ ジョ ・ ネンダイ ト ウンパン カテイ

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<p>The Yosasagawa Debris Avalanche (YDA) is the oldest and largest debris avalanche associated with sector collapse of the Nasu Volcanic Group, North Tochigi Prefecture, Japan, during the middle Pleistocene. The deposit is widely distributed along the rivers that cut the Nasu Volcanic Group, such as the Yosasagwa and Naka rivers. Here, we report an investigation of the relative age, distal correlation, and depositional processes associated with the YDA. The YDA is characterized by the presence of matrix and block facies containing quartz-bearing pyroxene andesite to dacite (SiO2 = 58-63 wt.%) lava blocks and clasts that occur stratigraphically between widespread biotite-bearing tephras, such as KMT (Kaishio Kamitakara Tephra, 0.62 Ma) and APm (Ohamachi APm tephra, 0.36-0.33 Ma), in the Chubu region (Central Japan). Although previous research has reported the age of the YDA as 0.3 Ma, the tephra-based stratigraphy suggests that the YDA occurred before 0.36-0.33 Ma. Using lithofacies (debris avalanche-like), petrography, and the whole-rock geochemistry of lava blocks in the deposit, the YDA can be correlated with the Awakawa Pumice Bed in northern Ibaraki Prefecture. Based on this correlation, the runout distance can be estimated to be more than 100 km from the source, Sanbonyaridake Volcano, Nasu Volcanic Group. Although the lithofacies of the upper part of the YDA are uniquely massive and contain mega-blocks at all outcrops, the lithofacies in the lower part of the YDA show remarkable lateral variation; i.e., an increasingly proportion of lava blocks with clast-supported structure is observed downstream. This indicates that the YDA was water-saturated and fluidized by river transport and thus able to travel large distances as lahar.</p>

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