Abstract
Molluscan fauna of the Kuwae Formation in Niigata Prefecture was reexamined for elucidating the paleoceanographic conditions in the central part of Japan Sea Borderland. The Kuwae Formation is lithologically subdivided into the lower, middle and upper parts. The upper middle to upper parts of the formation yielding many molluscan fossils can be assigned to the lower to middle part of NN16 nannofossil zone (middle Pliocene). The fauna includes many characteristic species of the Plio-Pleistocene Omma-Manganji fauna. Besides many cold-water species, 35 warm-water ones were found. It is noted that much more warm-water species are recognized than the middle Pliocene Tentokuji fauna in Akita Prefecture, reflecting the geographical thermocline.Moreover, the Kuwae fauna includes some common species with the Plio-Pleistocene Kakegawa fauna in the southwestern Pacific side of Japan as well as some warm-water species which do not inhabit the Recent Japan Sea. This is believed to be the result of the shallow current which was warmer than today flowing over the cold-water mass.
