We describe in detail the calcareous nannofossil biostratigraphy of the lower to middle Miocene formations distributed along the coast of the Japan Sea. The lowermost marine sediments overlying the “Green Tuff” volcanics in these areas are characterized by the occurrence of Helicosphaera ampliaperta, Discoaster deflundrei, Sphenolithus heteromorphus, and Cyclicargolithus floridanus. These assemblages indicate that the first marine environment invaded the coast of the Japan Sea side in NN 4 Zone of Martini's zonation (16.01Ma-18.74Ma) in Early Miocene.
The lithofacies of these formations in NN 4 to NN 5 zones (13.17Ma-18.74Ma) are composed of fossiliferous sundstone and mudstone in the Dewa Mountains area and the Sekiryo area (Ou Backbone Range) respectively. In contrast to these areas, the Akita Oil Field, trending north to south along the coast of Japan Sea on the west of the Dewa Mountains area, is characterized by the tholeiite type basalt activity in this age. Simultaneously with the end of basaltic volcanic activity at NN 5/NN 6 boundary in the Akita Oil Field, the lithofacies of sediments have changed from fossiliferous sundstone to mudstone in the Dewa Mountains area, and from mudstone to fossil bearing sandy siltstone in the Sekiryo area.
Based on these facts, we conclude that the lowermost marine sediments distributed in the Japan Sea side are correlated to Early Miocene NN 4 Zone. The Akita Oil Field on the west of the Dewa Mountains is characterized by the basaltic volcanic activity during NN 4 to NN 5 zones, and the end of its activity is related to the paleoenvironmental change from shallow to bathyal in the Dewa Mountains areas and bathyal to shallow in the Sekiryo area at the NN 5/NN 6 boundary in early Middle Miocene.