[SVC46-P03] Validity of thickness of Aso-4 tephra fall deposit of the Uwa Basin, western Shikoku
Keywords:Aso-4, tephra fall deposit, thickness, Uwa Basin
The high-quality thickness data of the Aso-4 tephra fall deposit in core UT from the Uwa Basin on western Shikoku was reported to be a 31-cm-thick by Tsuji et al. (2018). The Uwa Basin is in a relatively short distance from Kyushu Island volcanoes. Because tephras of eruption events of various scales are likely to be recorded, many tephra layers are reported in the thick sediments which continuously deposited for 0.7 Myr sediments from the Uwa Basin (e.g., Tsuji et al., 2018).
It is desirable to evaluate the thickness data of the Aso-4 fall deposit with multiple cores from the Uwa Basin. In this study, we obtained the thickness of the Aso-4 fall deposit in the core UT-iwk which sampled at about 500 m north of the core UT to examine the validity of the Aso-4 fall deposit thickness from the Uwa Basin. The thickness of the Aso-4 fall deposit of the core UT-iwk was 20–30 cm, slightly thinner than the core UT. The determination methods of the thickness of primary fall deposits are (1) observation of sedimentary structures by utilizing half cores and surface peel specimens, (2) existence of accidental clasts, and (3) measurement of grain size change of volcanic glass shards. Overall, the Aso-4 fall deposit in the core UT-iwk doesn’t exhibit lamination and includes a minor amount of accidental clasts (<5%). The bottom layer is characterized by a concentration of large pumices; the maximum pumice size of 3 mm. The lower part consists mostly of coarse phenocrysts and pumices. The upper part is weakly sorted; the average size of volcanic glasses decreases slightly towards the top. These detailed descriptions allow the getting of the thickness of primary tephra layers.
Additionally, the core B1 was sampled at a point about 1 km northwest of the core UT. The thickness of the Aso-4 fall deposit in it was about 30 cm (Soyama et al., 2012, Soyama et al., 2013). The bottom altitudes of the Aso-4 fall deposit in the core UT, the core B1, and the core UT-iwk is 206.00 m, 206.93 m, and 206.17 m, respectively. The thicknesses of the Aso-4 fall deposit in all cores have a range of 20–30cm-thick in three sites with about the same altitude. These data suggest the thickness of the Aso-4 fall deposit in western Shikoku is estimated to be 30 cm. The thickness data is useful in calculating the volume of the Aso-4 fall deposit.
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