長野県長野市西部の中新統小川層産貝化石群
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概要
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長野県長野市西部の上部中新統小川層論地泥岩部層の6産地より48種の貝化石を採集した。貝化石群中には中新世塩原（型）動物群の特徴種に加え，鮮新世～更新世前期の大桑・万願寺動物群の特徴種が認められた。また，群集解析により，この化石群は種多様性の高い水深10～30m付近の寒流系種の卓越した群集であることが明らかになった。The Ogawa Formation in the western part of Nagano City, Nagano Prefecture, is subdivided into the Susobana Tuff and the Ronji Mudstone Members in ascending order. The Ronji Member consists of mudstone, alternation of sandstone and mudstone, and fine -grained sandstone. It rests conformably on the Susobana Member and is conformably overlain by the Pliocene Shigarami Formation. The age of the Ronji Member can be assigned to Late Miocene by K-Ar and F. T. dating methods. Forty-eight species of molluscan fossils were obtained from fine- to medium-grained sandstone of the lower part of the Ronji Member. Among these species, some characteristic Miocene species such as Phos iwakianus and Neogenella hokkaidoensis are included. It is noteworthy that this fauna also contains a few characteristic species of Plio-Pleistocene Omma-Manganji fauna which is distributed in the Japan Sea borderland. In this fauna, Lucinoma-Mercenaria Association can be recognized. From the distribution of moderen species, the Lucinoma-Mercenaria is considered as an association which mostly consists of cold-water species and lived in about 10 to 30m of depth. It is interesting to note that this association also shows a high species diversity like as that of the Mercenaria-Clinocardium Association of the Middle Miocene Togeshita fauna in Hokkaido whose generic composition is similar to this association.
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