[S-VC35] Integrated Program for Next Generation Volcano Research and Human Resource Development

[SVC35-P12] Differences among three major pyroclastic flow deposits (Handa, Shimosakata, and Miyagi) at the southern foot of Kuju Volcano and stratigraphy of "Shimosakata pyroclastic flow deposit"

*Masahide Okada¹, Tomohiro Tsuji¹ (1.Yamaguchi University)

Keywords:Active Volcano, Volcanic geology, Eruption history

[Introduction] Kuju volcano is an active volcano located in the Beppu-Shimabara rift on the volcanic front in central Kyushu (Kawanabe et al., 2015). Ono et al. (1977) found three hornblende dacite unconsolidated pumice flow deposits of Kuju volcanic origin, the Miyagi pyroclastic flow deposit (pfd) is between Aso-3/2, the Shimosakata pfd is between Aso-4/3, and the Handa pfd is upper Aso-4. These are also considered to be large scale pfds in the Kuju volcano (Kawanabe et al., 2015). However, the Miyagi, Shimosakata, and Handa pfds are similar in lithology and can be distinguished only by their stratigraphic relationship with ejecta from the Aso Volcano (Ono et al., 1977). The Shimosakata pfd is distributed between Aso-3/4 at the Imamura area of Kuju town, and it is distributed above the Aso-3 pfd, although the relationship with the Aso-4 pfd is unknown at Kamisakata-Shimosakata area (Ono et al., 1977). Thus, pfd of Kuju volcanic origin are not possible to determine which is the source of the Aso pfd. Therefore, the purpose of this study is to pfd of the Kuju volcano up to the Handa pfd by directly identifying the Miyagi, Shimosakata, and Handa pfds. As an effect of this study, the reexamination of the eruption volume of the Miyagi, Shimosakata, and Handa pyroclastic flows are expected to revise the Cumulative erupted volume diagram of Kuju volcanoes reported by Nagaoka and Okuno (2015) and Yamasaki et al. (2016). In order to achieve our objectives, we conducted ground reconnaissance, grain size analysis, and particle composition of the Miyagi, Shimosakata, Handa, and Aso-4 unconsolidated pfds.
[Results] The Miyagi pfd contains reddish-brown oxidized volcanic rock fragments container than anoxidized volcanic rock fragments. But, Shimosakata and Handa pfds don't contain so many oxidized rock fragments. At the Fukuhara area at the northern part of the Kamisakata-Shimosakata area, pfd of Kuju volcanic origin (Kuju X pfd) are founded above the Aso-4 unconsolidated pfd. The distribution locality is close to the "Shimosakata pfd" in the Kamisakata-Shimosakata area, and the elevation is almost the same, and the grain size analysis and particle composition results are similar. The pfd below the Aso-4 unconsolidated pfd was founded in this study at the Imamura area that is Ono et al. (1977) reported the Shimosakata pfd between Aso-4/3.
[Discussion] The Miyagi pfd is distinguished from the Shimosakata and Handa pfds by the presence of many oxidized rock fragments. "Shimosakata pfd" in the Kamisakata-Shimosakata area and the Kuju X pfd can be compared to the Handa pfd because of upper Aso-4 pfd. Therefore, the layer corresponding to the Shimosakata pfd, Ono et al. (1977) reported to found between Aso-4/3 in the Imamura area, is considered to be a pfd that has not been reported to date because the surface geological survey results show it to be lower than the Aso-4 unconsolidated pfd. In addition, this pfd is not founded in the Shimosakata area, a different name other than "Shimosakata" is considered necessary.
[References] Kawanabe et al. (2015) Geological Map of Kuju Volcano, Geological Survey of Japan. Nagaoka and Okuno (2015), Geomorphology, 36, 141-158; Ono, K. et al. (1977), Geological map at 1:50,000, Geology of the Taketa area, Geological Survey of Japan. Yamasaki et al. (2016), Volcanoes, 61, 519-531.