Neogene molluscan biostratigraphy and paleoecology of the Hokuriku district

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Neogene molluscan fossils distributed in the Hokuriku district have been studied from the view points of biostratigraphy and paleozoogeography in refer to the new microbiochronology, in particular the Diatom biochronology.The Neogene molluscan faunas of the Hokuriku district can be divided into four as 1). the Kurosedani fauna (late Early Miocene; 16.5 - 15.5Ma), 2) the Nanao fauna (early Middle Miocene; 15.5 - 14Ma), 3) the Otogawa fauna (Late Miocene; ca. 9ma), and 4) the Omma fauna (Late Pliocene to Early Pleistocene; 2-1Ma), respectively.These faunas can be assumed that the faunas had been fourished under the tropical to subtropical (16,5-15,5Ma), warm-temperate (15.5-14Ma), mild to cool-temperate (ca. 9Ma) and mixed of subboreal to cool-temperate and warm-temperate, respectively. Among them, the Otogawa fauna can be assigned to the Japanese endemic one which might be established during Miocene sea-water cooling. Although the Otogawa fauna has been considered to the cold-water type, some molluscan elements which are survivals of the warm-water origin suggest that the fauna is mild to cool-temperate ones. The first appearence of the subboreal marine environment which is comparable to those of the Recent Oyashio (Oyashio Current) might be in the Pliocene time. Consequently, faunal changes of the Miocene to the Early Pleistocene of the Hokuriku district well corresponded to worl-wide cooling that was probably taking place gradually through the late Miocene to early Pleistocene.