The Shionofuchi-biotite (Sbi) tephra was found from the Middle Miocene Nanatani Formation at Shionofuchi, Sanjo City, Kamo District, Niigata Prefecture. The Sbi tephra is a crystal-rich coarse tuff layer (9 cm thick) intercalated between the basalt to andesite volcaniclastic rocks and light gray massive mudstone of the Nanatani Formation. This tephra contains plagioclase (oligoclase and bytownite in composition), quartz, sanidine, biotite, opaque minerals and a trace amount of zircon and allanite. Fission-track age of Sbi tephra was determined to be 13.8±0.3 Ma (1σ). This age is consistent with the biostratigraphic data. The Kbi tephra that is correlated to the Muro Pyroclastic Flow Deposit in Kii Peninsula is intercalated in the similar horizon to the Sbi tephra. Furthermore, the lithofacies of the Sbi tephra resembles that of the Kbi tephra. However, according to the inconsistency of plagioclase composition, trace heavy mineral assemblage and uranium contents in zircon, the Sbi tephra is not correlated to the Kbi tephra. Therefore, we have to pay attention to correlate these tephras.
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