The best section containing the Eocene–Oligocene boundary in Japan is a composite section exposed along the course of Kumanosawa and Yubari River in the Yubari area, central Hokkaido. The Kumanosawa section represents the upper portion of the Poronai Formation whereas the Yubari River section comprises the top level of the Poronai Formation and the conformably overlying Momijiyama Formation. The sequence is generally homoclinal with variable dips of between 12° and 60° and seems to represent a shallowing facies in a somewhat restricted inland basin of middle bathyal to outer neritic environments. This sequence is rich in molluscs, crustaceans, and benthic foraminifers whereas planktonic microfossils are rather scarce.
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