Abstract
Pliocene molluscacn fauna and its paleoenvironmental setting of Matsunoyama-machi, Niigata Prefecture, are important for considering succession from Miocene to Plio-Pleistocene shallow sea faunas in Japan Sea borderland as well as considering paleoenvironmental change in the northern Fossa Magna Region.
Pliocene deposits in the southeastern flank of Matsunoyama Anticline consist of the Kurokura, Higashigawa, Ikenosawagawa, Arikurayama formations from lower to upper. In the northwestern flank, the Higashigawa formation is conformably covered with the Naradate. Every formations is in conform able relation involving partial development of a contemporaneous heterotopic facies relation.
Many cold-water species are included among Pliocene molluscan fauna in this area. Only a few horizons yielding warm-water species are recognized in the Higashigawa and the Ikenosawagawa formations.
Seven associations are discriminated in the molluscan faunas of this area. Based on the faunal analyses, it has become clear that this area was in an upper bathyal zone in the Early Pliocene, and the sea became shallower subsequently. In the middle Pliocene, the area in the southeastern flank of Matsunoyama Anticline was in an embayment-tidal zone, whereas that in the northwestern one was in an upper sublittoral zone.
