High resolution record of Quaternary explosive volcanism recorded in fluvio-lacustrine sediments of the Uwa basin, southwest Japan

摘要

The identification of widespread tephras is important, not only for understanding volcanic processes but also for age estimation and establishing stratigraphic relationships. Many widespread tephras have been erupted from six active and buried caldera volcanoes in the SW Japan-Ryukyu volcanic arcs. To reconstruct high-resolution tephrostratigraphy, we analyzed core UT from the Uwa basin on western Shikoku Island and correlated the tephras in the core with the proximal source deposits that we sampled. This 120 m-long core composed of fluvio-lacustrine deposits, spanning the middle Pleistocene to Holocene. We recognized and characterized 52 visible tephra and tephric sediment layers in the core on the basis of their stratigraphy, the petrographic features and the major-element composition of glass shards. We successfully correlated 15 tephras with the following previously identified widespread tephras, mainly from volcanoes on Kyushu: Kikai-Akahoya (7.3 ka), Aira-Tn (29 ka), Aso-4 (87 ka), Kikai-Tozurahara (95 ka), Aso-ABCD (100 ka), Ata (105-110 ka), Aso-3 (112.7 ka, calculated in this study), Aso-2 (140 ka), AtaToihama (240 ka), Aso-1 (250-270 ka), Tky-Ng1 (290-300 ka), Kakuto (330-340 ka), Oda (420-450 ka), Hiwaki (570-580 ka), and Yufugawa (600 ka). Above 50 m depth, all known widespread tephras from Kyushu younger than 350 ky were included in the sequence that has a fairly constant sedimentation rate of about 0.14 mm/yr. This high-resolution tephrostratigraphy was correlated with those of regional longcore records from the Lake Biwa in the central Japan and offshore of Shikoku that contributes to establish onshore-onshore and onshore-offshore correlations in southwest Japan. In the sediments, each of the following tephras from caldera-forming eruptions, Aso-4, Aso-ABCD, Ata, Aso-3, Aso-2 and Aso-1 occurred above multiple, compositionally similar tephra layers erupted from the same volcano over a long time period. This high-resolution record is therefore an important contribution to the correlation of regional stratigraphy and reconstruction of both the long-term eruptive history and the magma evolution of volcanoes in the SW Japan and -Ryukyu volcanic arcs. (C) 2017 The Authors. Published by Elsevier Ltd.