Stratigraphic correlation between the Plio-Pleistocene Yabuta and Junicho Formations using volcanic ash beds, and diatom and calcareous nannofossil biostratigrahy of lower part of the Junicho Formation in northwestern Toyama Prefecture, Central Japan

The Yabuta Formation consists of the early to late Pliocene marine sediments distributed along the Nadaura coast, and the Junicho Formation includes late Pliocene to early Pleistocene marine sediments near Asahiyama hill, in northwestern part of Toyama Prefecture.We examined volcanic ash beds in the Yabuta and Junicho Formations by petrographical method and measuring chemical component of volcanic glass. Two pairs of the volcanic ash beds (MT2-T1, UN-T3) can be correlated between the Yabuta and Junicho Formations, and the middle-upper part of the Yabuta Formation can be correlated to the lower part of the Junicho Formation. Fossil diatoms from the lower part of the Junicho Formation showed that the boundary between the Neodenticula koizumii-Neodenticula kamtschatica Zone and the N. koizumii Zone is around the T3 volcanic ash bed that is widely traced in the Hokuriku Region. Calcareous nannofossil biostratigraphy of the lower part of the Junicho Formation indicates that Datum A (2.75 Ma) is demarcated about 4 meter below the T3 volcanic ash bed. A widespread tephra of the Habutaki I-MT2 ash layer is situated about 14 m below the Datum A in the Junicho Formation.