The Paleogene Iojima Group is exposed in the Takashima coalfield of northwestern Kyushu, southwest Japan. The group is divided into four formations: the Okinoshima, Magome, Funazu, and Daimyoji formations (in ascending stratigraphic order). Among these strata, mudstone and muddy sandstone of the Okinoshima and Funazu formations contain a number of common age-diagnostic calcareous microfossils, nannofossils, and planktic foraminifers.

Our biostratigraphic study of the Paleogene Iojima Group indicates that the Okinoshima Formation corresponds to calcareous nannofossil Zone CP14 and planktic foraminiferal Zones E10-E13; the Funazu Formation is assigned to Zones CP14-CP16. The Okinoshima and Funazu formations are assigned to the late middle Eocene and late middle Eocene to early Oligocene, respectively, suggesting that 1) the Okinoshiman molluscan Stage correlates with the middle Eocene, as proposed in previous studies; and 2) the Magome Formation, which includes coal-bearing strata, correlates with the Onga marine regression.