Abstract
We examined fossil diatoms in the middle Miocene Oidawara Formation distributed in Mizunami City, Gifu Prefecture, central Japan, to determine the precise sedimentation age. Well preserved fossil marine diatoms with high diversity have been found in the diatomaceous mudstone of the Oidawara Formation. The diatom assemblage of the formation can be assigned to the short interval between diatom biohorizons D41 (the last occurrence of Denticulopsis praelauta, 15.8 Ma) and the first occurrence of Actinocyclus ingens f. nodus (15.7 Ma)in the diatom zone NPD 4A (Denticulopsis lauta Zone) of the Neogene North Pacific diatom zonation. As a result, the sedimentation age of the Oidawara Formation was determined to be between 15.8 and 15.7 Ma. Reversed magnetic polarity reported from the Oidawara Formation corresponds to chron C5Br.
