由布川火砕流の噴出年代

概要

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The Yufugawa pyroclastic flow is a large-scale ignimbrite which occurs along Oita River in central Kyushu and is a useful key marker for volcanic stratigraphy among isolated volcanic edifices in the eastern part of Hohi volcanic zone. Eruption age of the Yufugawa flow has been in controversy owing to insufficient stratigraphic and radiometric data for volcanic rocks in this area. We determined both K-Ar ages and paleomagnetism of the volcanic rocks which overlie and underlie the Yufugawa flow: The Yufugawa overlies the Imaichi pyroclastic flow (reversed polarity; 0-8-0.9 Ma) and some other lava flows dated at 0.5-1.30 Ma. The Yufugawa underlies the lava flows dated 0.2-0.58 Ma. Accordingly the Yufugawa is interpreted to have erupted at about 0.6 Ma in middle Pleistocene. Stratigraphy, geochronology, Bouguer gravity anomaly, and level of pyroclastic plateau suggest that the Yufugawa pyroclaslic flow vented either at the south of Mizukuchiyama or at Yufuin Basin.
特定非営利活動法人日本火山学会の論文
1991-12-25