[S-GL21] Geochronology and Isotope Geology

[SGL21-P03] The bural process of Ubuka Basin in the Abu monogenic volcanoes, SW Japan

Mao Oshita¹, *Mitsuru Okuno¹, Ku Nishikawa¹, Toshiyuki Fujiki², Toshimichi Nakanishi³, Akira Hayashida⁴, Masayuki Torii⁵, Wan Hong⁶ (1.Osaka Metropolitan University, 2.Okayama University of Science, 3.Museum of Natural and Environmental History, Shizuoka, 4.Doshisha University, 5.Kumamoto University, 6.Korea Institute of Geoscience and Mineral Resources)

Keywords:Abu monogenic volcanoes, Ubuka Basin, bural process, Kikai-Akahoya (K-Ah) tephra, Nabeyama scoria cone

In the Abu monogenic volcanes, lava flows/domes and the scoria cones are scattered at about 400 km², and started activities from about 2 Ma in the K-Ar age (Kakubuchi et al., 2000). The latest eruption is a TL age (Nagao, 2002) of about 9 ka of Kasayama scoria cone in the western part, and was selected as an active volcano. The Ukaga Basin is surrounded by Gongenyama (ca. 0.31 Ma) on the south side, Takamureyama (ca. 0.18 Ma) on the north side , Nishidai and Higashidai (ca. 0.04 Ma) on the east side, and Nabeyama scoria cone, which is too young for the K-Ar age measurement. It is known that burial sediments cover long range from the last glacial to postglacial periods, but tephra, such as Kikai-Akahoya (K-Ah), is not detected (Hatanaka and Miyoshi, 1980). Many buried trees (5130-3676 cal BP) are occurred in the depth of 50 to 60 cm from the surface (Hakozaki et al., 2012). In this study, to order to reveal the burial process of the basin, and estimate an eruption age of Nabeyama scoria cone (with Horikoshi lava) the latest one in these volcanoes, a total of 10 core samples (up to about 7 m in depth) were collected in eight sites on the east and west of the basin. First, the facies of core sample was observed, and then X-ray imaging, digital color measurement, initial magnetization measurement, and AMS¹⁴C dating on wood pieces and plant fragments were also performed. The core samples are gradually changed from the lowest silt and clay layer to the top of the plant fragments rich peaty layers, and it is same outline shown by Hatanaka and Miyoshi (1980). A peat layer (almost a plant fragments) was found up to a depth of 1.5 m at only one site in the center, and a bubble wall-type volcanic glass that could be correlated with the K-Ah tephra was detected. In the other seven sites, K-Ah cannot be detected and it is considered that there was sedimentation hiatus before and after K-Ah. After 5130 cal BP, it is thought that there is some relationship with the environment where the cedar can enter the area around the basin.