Abstract
Middle Miocene fossil diatom assemblages were reported from the Nanatani and Minamiimogawa Formations in the Neogene Niigata sedimentary basin, central Japan. The diatoms in calcareous concretions from the upper part of the Nanatani Formation are completely pyritized and correlated to the upper part of the early Middle Miocene Denticulopsis hyalina Zone (NPD 4B) of Akiba (1986). The diatom flora in calcareous concretions from the upper part of the Minamiimogawa Formation are correlative with the latest Middle Miocene Thalassiosira yabei Zone (NPD 5C), and indicates that this formation is equivalent to the Teradomari Formation, one of the standard Middle Miocene stratigraphic units in the Niigata basin. The diatom floras from the two formations are composed of mixtures of abundant cold water species and rare warm water species, suggesting that the paleoceanographic environment of the Nanatani and Minamiimogawa Formations was influenced by cold water current, and to a lesser extent by warm water current.
