Abstract
Study of calcareous nannofossils from Furutsu 1 well, about 2 kilometers west of the Niitsu oil field, Niigata Prefecture, revealed that the Pleistocene index fossils, Gephyrocapsa oceanica KAMPTNER and G. caribbeanica BOUDREAUX and HAY appear in the lower Nishiyama formation below the no. 2 Globorotalia inflata (D'ORBIGNY) bed.
Also an extinction level of Helicopontosphaera sellii BUKRY and BRAMLETTE suggesting early Pleistocene in age comes above the first occurrences of two Gephyrocapsa species above mentioned.
This coexistence zone corresponds to NN 19 (MARTINI, 1971), or Cyclococcolithina macintyrei Zone -Helicopontosphaera sellii Zone (GARTNER, 1977).
From this fact it is concluded that the Pliocene-Pleistocene boundary is drawn in the middle lower Nishiyama formation.
