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A cold-seep assemblage is present in slope to outer-shelf strata of the Upper Pliocene to Lower Pleistocene Ofuna Formation and the Lower Pleistocene Koshiba Formation of the Kazusa Group in Yokohama City, on the Pacific side of central Japan. Four cores were taken in order to learn the three-dimensional extent of the assemblage: one core was parallel to bedding and three were normal to bedding. Study of the cores and outcrops shows that: (1) the assemblage is at least 37 m thick stratigraphically and extends across the boundary between the Koshiba Formation and the underlying Ofuna Formation; (2) parallel to bedding, the assemblage is at least 16 m wide in a north-south direction and 30 m long in an east-west direction; (3) the assemblage consists mostly of large (up to 10 cm in maximum diameter), articulated bivalves of the genera Lucinoma, Conchocele, and Acharax, in aggregations or as sporadic shells; (4) authigenic carbonate cement in the matrix enclosing the assemblage ranges from dense to sparse and is greatly depleted in 13C (δ13C = −47.99‰ to −55.06‰), indicating that the carbonate resulted from the microbial oxidation of methane; (5) there is a subsurface horizon about 10 m thick that is characterized by: (a) scoria beds in which the matrix consists entirely of pure, white carbonate, (b) abundant brecciated clasts of the host sediment occurring along with extremely abundant bivalve fragments, many of which are very small, and (c) lithologic boundaries greatly discordant with the overall dip within the study area. Observations 5a–c together suggest very active seepage and/or an explosive effusion of subsurface material, and the presence of “pockmarks” in the subsurface of the study area. Twenty tuff beds are observable and accurately correlatable among outcrops where no cold-seep assemblage occurs. However, these tuff beds cannot be correlated with those in the cores. This is probably due to highly active bioturbation and by severe seepage and/or the explosive effusion of seep materials. We infer that these activities and events dispersed, disturbed, brecciated and/or reconcentrated the originally deposited tuff beds.

A slope to outer-shelf cold-seep assemblage in the Plio-Pleistocene Kazusa Group, Pacific side of central Japan