A biostratigraphic study of fossil diatoms revealed that the most of the Kumi Formation in Dogo, Oki Islands, is correlative with the Crucidenticula kanayae Zone to Thalassiosira yabei Zone, and that the Tsuma Formation with the Rouxia californica Zone to lower Neodenticula kamtschatica Zone, respectively. On the other hand, the lowermost of the Kumi Formation yielded a Lower Miocene freshwater planktonic diatom assemblage. The assemblage mainly consists of various forms of the genus Auracoseira associated with lacustrine Actinocyclus species. It is overlain by the marine Kumi Formation correlated to, or lower than, the Lower Miocene Crucidenticula kanayae Zone (16.6-17.0 Ma) which evidences that the geologic section recorded the first marine water invasion related to the Japan Sea opening in the area. This abrupt change of the diatom assemblage from freshwater to marine planktons suggests the rapid transgression in the area where wide continental shelf did not develop. In addition, this study shows the first discovery of the biostratigraphic index diatom fossil N. kamtschatica from the Tsuma Formation which is the westernmost occurrence from the Japan Sea side.
