2018 Volume 127 Issue 2 Pages 247-271
Proximal pyroclastic deposits of the 46 ka caldera-forming eruption of Shikotsu volcano are investigated at new outcrops along the Opoppu river, south of the volcano. The deposits can be divided into 6 units, from A to F in ascending order, according to lithofacies, components, and time intervals. Based on the stratigraphy of the deposits, the sequence of the 46 ka Shikotsu eruption is revealed. Activity started with phreatomagmatic and phreatoplinian eruptions (Phase 1: Unit A). The eruption style changed to magmatic without a time interval before the plinian eruption (Phase 2: Unit B). The eruption column was intermittently unstable, producing pyroclastic surge and flow deposits during the latter period of Phase 2. The lithic breccia content also increased in the latter period. After a possible erosional interval, explosive eruptions occurred, producing voluminous pyroclastic flows and ended with the effusion of lag breccia (Phase 3: Unit C). After a dormant period, pyroclastic flows effused intermittently (Phase 4: Units D and E). At the final stage, small plinian eruption occurred, associated with a pyroclastic surge (Phase 5: Unit F). Juvenile materials of the 46 ka Shikotsu eruption are mainly crystal-poor (CP type) rhyolite pumice from Phases 1 to 3. Small amounts of crystal-rich (CR type) dacite and andesite pumice occurred from the final stage of the Phase 3. Two types of silicic material coexisted during Phases 4 and 5. According to temporal changes of lithic contents, the caldera collapse was almost completed at the end of Phase 3 with the formation of lag breccia. Therefore, it could be considered that activity in Phases 4 and 5 was either the terminal phase of caldera-forming activity or the initial activity of post-Shikotsu caldera volcanoes.
