2018 Volume 69 Issue 3 Pages 141-151
Lithostratigraphy and diatom biostratigraphy are established for the Pliocene marine strata in the northwestern area of the Nishikubiki Mountains, Niigata Prefecture, central Japan. Pliocene to lower Pleistocene marine deposits are divided into the Nechi and Nadachi formations, in ascending order. The Nechi Formation is composed of massive sandy mudstone, and correlated to the diatom fossil subzone NPD7Bb. The Nadachi Formation is dominantly composed of massive sandy mudstone with thin sandstone beds and tuffaceous sandstone, and is assigned to the diatom fossil zones NPD8 and NPD9. The Nadachi Formation of this area conformably covers the Nechi Formation. The boundary between the Nechi and Nadachi formations is coincident with the boundary between diatom zones NPD7Bb and NPD8. The Nadachi Formation in this area is correlative with the Kawazume and Nadachi formations in the middle to eastern Nishikubiki Mountains. Our result indicates that the unconformity previously inferred at the base of the Nadachi Formation is absent.