Diatom zonal key species are identified in the samples from the Morozaki (the Toyohama, Yamami and Utsumi Formations), Iwamura (the Toyama Formation) and Tomikusa (the Awano and Yonekawa Formations) Groups. These assemblages are characterized by the abundant occurrences of Kisseleviella carina and Stephanopyxis spp. The presence of Crucidenticula sawamurae, C. ikebei, Delphineis kamenooensis and D. miocenica, and the absence of Crucidenticula kanayae suggest that all of those samples from above groups are placed within the middle to upper part of C. sawamurae Zone (late Early Miocene).
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