Abstract
Molluscan fauna from the Mita Formation in Toyama Prefecture was examined for elucidating the timing and pass ways of the warm-current influx to the Japan Sea in the middle Pliocene. The Mita Formation consists of fine- to medium-grained sandstone with three tuff beds (OT3, MT1 and MT2). The horizons between OT3 (about 4 Ma) and MT2 (2.2-2.3 Ma) yielding many molluscan fossils can be assigned to the lower to middle part of NN16 nannofossil zone. The fauna includes many characteristic species of the Plio-Pleistocene Omma-Manganji fauna as well as some Miocene relict species. Besides 51 cold-water species, 24 warm-water species were found from the every stratigraphic horizon above OT3 and to that just above MT2. The Mita fauna also includes some common species with the Plio-Pleistocene warm-water Kakegawa fauna in the southwestern Pacific side of Japan. Such species composition might be affected by the shallow warmer current than today flowing over the cold-water mass. Judging from every horizons yielding the warm-current species, the warm current always flowed since about 4 Ma to 2.2Ma. Moreover, because of more numerous common warm-water species between the Mita and Kuwae faunas than the Kuwae and the northern Tentokuji ones, and of their biogeographical setting, it can be inferred that the warm current entered through around the Tsushima Strait, not through the Tsugaru Strait.
