A Middle Miocene widespread tephra in the Nanatani Formation of the Niigata sedimentary basin, northeast Japan: Correlation between the Kbi tephra and the Muro Pyroclastic Flow Deposit in Kii Peninsula

Abstract

The Komadegawa-biotite (Kbi) tephra is a crystal-rich coarse tuff layer (9 cm thick) intercalated in the upper part of the Middle Miocene Nanatani Formation in the eastern area of the Niigata sedimentary basin, northeast Japan. The Kbi tephra contains plagioclase (oligoclase to labradorite in composition), quartz, sanidine, biotite, trace amounts of garnet, and reddish and colorless zircons. The Kbi tephra is intercalated in the N.9 planktonic foraminiferal zone and the CN4 calcareous nannofossil zone, corresponding to an age of 14.2–14.7 Ma, which is consistent with a newly determined fission track age of 14.6±0.3 Ma (1σ). On the basis of the petrography, fission track age and biostratigraphy, the Kbi tephra is correlated to the Muro Pyroclastic Flow Deposit in Kii Peninsula and the Kn-1 tephra in Boso Peninsula. These tephras are considered as a widespread tephra layer associated with the formation of the Kumano Acidic Rocks in Kii Peninsula. This tephra is expected to be found across large parts of Japan and provides a stratigraphic marker in various sedimentary basins.