2006 Volume 56 Issue 11-12 Pages 411-424
Marine Pliocene deposits around the Umikawa River in Itoigawa City, Niigata Prefecture consists of the Kawazume and Nadachi Formations in ascending order. Diatom biostrati-graphy indicates that the lower part of the Nadachi Formation is assigned to the subzone NPD 7Bb (early Pliocene),while the upper part of this formation can be correlative with the zones NPD 8 and NPD 9 (late Piocene). In the molluscan faunas of the Kawazume and Nadachi Formations, the following seven associations were recognized; the upper bathyal to lower sublittoral Conchocele-Solemya, AcilaLucinoma, Conchocele, Portlandia, Macoma-Axinopsida associations, and the lower sublittoral Acila,Ophiodermella-Turritella associations. Based on the characteristics of these molluscan associations,the Umikawa area was placed in the upper bathyal to lower sublittoral zones during the Pliocene age.