Abstract
The Kuri Formation is a constituent formation of the Miocene standard stratigraphic section in the San'in district, southwest Japan. The upper part of the formation exposed along the type section contains the benthic foraminiferal datum plane “Foram. Sharp Line” (FSL), which is correlated with the boundary between the planktonic foraminiferal zones N.9 and N.10. Using a new disaggregation method for hard mud rocks, we extracted abundant planktonic foraminiferal fossils. Calcareous nannofossils were also found in one sample. The fossil associations collectively confirm that the type section correlates to planktonic foraminiferal Zone N.8 and calcareous nannofossil Zone CN3. The overlapping ages of these zones is at least 0.5 million years older than that of the FSL. According to these microfossil ages, the studied section can be correlated with strata below the FSL in the upper part of the Bihoku Group. This result implies that most of the Chugoku district containing the San'in and backbone districts was strongly influenced by open-ocean waters just prior to the age of formation of the FSL.
