[SVC50-P14] Reexamination of late Pleistocene tephras of Shikotsu-Toya Volcanic Field
Keywords:tephras, late Pleistocene, Shikotsu-Toya Volcanic Field, glass composition
In this study, geological survey has been done mainly in southern and eastern part of lake Shikotsu (< 65 km). Pyroclastic flow deposits of Shikotsu caldera-forming eruption widely and thickly covered around the lake, we also carried out boring explorations in the proximal area (10 km and 25 km east from the lake center), and observed two cores of Japan Meteorological Agency (10 km south) and National Research Institute for Earth Science and Disaster Prevention (25 km SSE). We used four thick tephra layers as key beds in STVF; Spfa-1 & Spfl, Kt-1, Ssfa & Ssfl and Toya. To correlate tephra layers, we firstly investigate stratigraphic relationships with key tephra layers and then compare petrological characteristics with proximal samples. Then, we distinguished at least 27 tephra layers in STVF, and discovered six new tephra layers in this study. As a results, the beginning of eruptive activity dates back to 120 ka for Shiribetsu volcano and ca. 80 ka for Yotei and Shikotsu volcanoes.
Together with K-Ar ages of volcanic rocks around this area, eruptive history of STVF are summarized as follows. Andesite volcanism had occurred until middle Pleistocene and had terminated around 0.6-0.5 Ma. After a long dormancy (ca. 400 ky), STVF started its eruptive activity 130 ka at Toya, and 120 ka at Yotei volcanoes. A catastrophic caldera-forming eruption occurred ca. 110 ka at Toya volcano. Then, the activity has propagated toward the east. Kuttara and Shikotsu volcanoes started their activity almost simultaneously ca. 90 and 85 ka, respectively. Subsequently, a large stratovolcano; Yotei has been also constructed since ca. 75 ka at the back-arc side of the STVF. The explosive eruptions of VEI=5-6 had repeated at Kuttara and Shikotsu volcanoes, and VEI=6 eruption occurred at Kuttara, and the largest caldera-forming eruption in the STVF occurred at Shikotsu volcano (VEI=7) ca. 45 ka. Since then, Yotei and post-caldera volcanoes in STVF have continued their eruptive activity until now. It should be noted that there exist three active periods during 130-110 ka, 95-75 ka and 60-45 ka in STVF.