Abstract
The molluscan faunas from the Shitoka Formation and the lowest part of the Uonuma Group were examined along the Kamakurazawa River, central Niigata Prefecture, to assess the influence of a warm-water current on the northern Fossa Magna Region during the Pliocene. The Shitoka Formation consists of pebbly mudstone and siltstone, and unconformably overlies the Nishitajiri Formation, which is assigned an upper Miocene age based on diatom fossils, representing the lower to middle part of the NN16 nannofossil zone. The lowest part of the Uonuma Group is composed mainly of conglomerate and sandstone, and conformably overlies the Shitoka Formation. The Kamakurazawa fauna within both formations includes seven characteristic species of the Plio-Pleistocene Omma-Manganji fauna as well as one Miocene relict species. In addition to 14 cold-water species, 14 warm-water species were found in almost all sample horizons. During the late Pliocene in the northern Fossa Magna Region, the proportion of warm-water species decreased from the northernmost Shibata area (18.3%) to the southernmost Nagano area (2.0%). The studied area has a proportion of warm-water species intermediate between these values, and lies halfway between the Shibata and Nagano areas. During the Pliocene, a large northeast-facing bay developed at the north Fossa Magna, and a shallow warm current flowed northeastward from around the Tsushima Strait. The Shibata area, facing the open sea, was directly influenced by the warm-water current, whereas the Nagano area, located in the innermost part of the bay, was not.
