6:15 PM - 7:30 PM
[HQR23-P06] Basin fill sediments and late Quaternary tephras under the eastern part of Aizu basin, Northeast Japan
Keywords:Aizu basin, Boring, Tephra, Underground geology, Late Quaternary
GS-SOK-1 core comprises alternate layers of gravel beds and fine sediment beds in total. From surface to 26 m depth silt and organic silt beds are dominant, although thick gravel beds deposit from 26 m to 60 m. Two tephra layer are detected at 1.6~1.8 m (pumice layer) and 81.1~81.7 m (volcanic ash layer). It is suggested that the former is Nm-NM (5.4 ka ) and the latter is a tephra derived from the Sunagohara volcano. Gravel beds composed of subrounded and subrubbled pebbles in size 20-50 mm with cobbles. Andesite cobble and tuff are included in total. Three radiocarbon ages of woods obtained from the core are as follows: 17310-17710 cal yrBP (6.26 m depth), 17330-17730 cal yrBP (6.67 m), and 29530-30360 cal yrBP (7.64 m).
Suzuki et al. (2013) detected three tephra layer in AB-12-2 core; Nm-NM (4.1 m depth), AT (17.1 m), and Sn-MT (88.3 m). If volcanic ash at 81.1~81.7 m depth is compared with Sn-MT, average accumulation rate of sediments since 0.2 Ma in eastern part of the Aizu Basin is comparable with one in western part. On the other hand, GS-SOK-1 core consists of alternate layers of gravel and fine sediment, although AB-12-2 core dominantly comprises fine sediment in total. Boring site of GS-SOK-1 is near fans of eastern margin of the basin and volcanic-fan of Mt. Nekoma, suggesting that coarse sediments have been supplied several time in GS-SOK-1 site.