2019 年 106 巻 p. 19-34
Chemosynthetic, cold-seep-dependent fossil assemblages occur at five fossil localities in the riverside cliffs along the Sagami River, Sagamihara City, central Japan, where the upper part (2.5 ‒ 2.0 Ma) of the marine Nakatsu Group is exposed. The upper Nakatsu Group is marked by coarsening-upward and subsequent fining-upward sequences as follows (in ascending order): mudstone (Lithofacies A), mudstone and sandy mudstone (B), sandy mudstone (C), alternations of sandy mudstone and muddy sandstone (D), sandy mudstone (E), and mudstone (F). The fossil assemblages are dominated by the bivalve molluscs Lucinoma and/or Conchocele that show high rates (79.2 % ‒ 100 %) of valve articulation, occur in clusters, and are frequently preserved in life positions. The fossils are commonly associated with authigenic carbonate concretions consisting of high-magnesian calcite and/or dolomite that are greatly depleted in 13C (δ13C = -33.31 ‰ to -22.60 ‰ VPDB), suggesting the influence of anaerobic oxidation of methane. These observations indicate that the fossil assemblages are chemosynthetic and cold-seep-dependent. The bathymetric ranges of extant molluscan species and the water temperature inferred from the stable oxygen isotope ratios of the authigenic carbonates suggest that lithofacies E was deposited in water depths of 150 ‒ 240 m.