Miocene diatoms of the upper part of the Arakawa Group distributed in the Karasuyama area, Tochigi Prefecture, central Japan (part 1): Diatom biostratigraphy

摘要

Late Miocene marine diatom biostratigraphy is established for the upper Arakawa Group distributed in eastern Tochigi Prefecture, central Japan. The upper Ogane Formation and the Tanokura Formation are assigned to the upper Thalassiosira yabei Zone (NPD 5C) of the North Pacific diatom zonation (Yanagisawa and Akiba, 1998). The overlying Irieno Formation is correlated to the Denticulopsis dimorpha Zone (NPD 5D). The age of the upper Arakawa Group is estimated to be between 10.5 Ma and 9.2 Ma by combining data from diatom, radiolarian and nannofossil biostrati graphy, and a potassium-argon dating.