[H-QR04] Quaternary, Diachronic dynamics of human-environment interactions

[HQR04-P06] Identification of reworked volcaniclastic deposits composing the upper Alluvium along the Moto-arakawa River, east part of the Saitama Prefecture, Central Japan

*Masanori Murata¹, Takayuki Takahashi², Kaori Aoki¹, Fumikatsu NISHIZAWA³, Makoto Kobayashi⁴, Takehiko Suzuki^1,5 (1.Research Center for Volcanic Hazards and Their Mitigation, Tokyo Metropolitan University, 2.International Research Institute of Disaster Science, Tohoku University, 3.Kanagawa Prefectural Museum of Natural History, 4.Mt. Fuji World Heritage Centre, Shizuoka, 5.Department of Geography, Tokyo Metropolitan University)

Keywords:alluvial lowland, volcanic hazard, Saitama Prefecture

In order to study the impacts of the reworking of volcanic products of volcanoes in the northern Kanto region on the Nakagawa and Tokyo Lowlands, we investigated the secondary volcanic products in the alluvial deposit along the Moto-arakawa River. According to the "Borehole map using WebGIS" published by Center for Environmental Science in Saitama, many pumice clasts have been found in the sediments at a depth of less than 5 m from the ground surface. along the Moto-arakawa River. These pumice clasts are considered to have been transported by the Tone River, because the lowland along the Moto-arakawa River was the former channel of the Tone River for some time from the beginning of the 8th century. We carried out borehole investigations at two locations, Iwatsuki Bunka Park, Saitama City (Iwatsuki Bunka Park core) and Koshigaya, Koshigaya City (Koshigaya 2236 core). Based on the analysis of the volcanic products in the two cores, it was found that the upper Alluvium at the depths of 4-5 m in Iwatsuki Bunka Park, and 2-5 m in Koshigaya, Koshigaya City contains secondary volcanic products originated from Hr-FP and Asama tephras. This suggests that the reworking of volcanic products from the upper reaches of the Tone River had a significant impact on the development of the alluvial lowland in the central part of the Kanto Plain during the late Holocene. In the future, we plan to reconstruct the ages and paleoenvironments of the sediments using radiocarbon dating, and to assess the spatial and temporal impacts of the reworking of volcanic products in the Nakagawa and Tokyo Lowlands.