We describe the modes of occurrence of a vesicomyid-dominated fossil assemblage in the lowermost Pleistocene Urago Formation, a forearc basin fill on the Miura Peninsula, Pacific side of central Japan. The assemblage consists mostly of Calyptogena (Archivesica) kawamurai (Kuroda) with a minor amount of Conchocele bisecta (Conrad). The shells occur in cross-bedded and massive sandstones of an outcrop that is approximately 25 m wide and 10 m high. The sandstones are interpreted to have been formed by migration of dunes under northward- to eastward-directed bottom currents, judging from the dips of their foreset laminae. Many of the shells are disarticulated and show evidence of reworking by bottom currents, as indicated by their convex-up positions with their commissure planes dipping southwestward (up-current direction) or concordant with the cross laminae. In the massive sandstones, some articulated vesicomyids are preserved in their life position, that is, perpendicular to the bedding plane with their anterior parts oriented downward. Massive authigenic carbonates, which are developed in nearly all horizons of the vesicomyid-bearing sandstones, consist exclusively of Ca-rich dolomites (δ13C, -37.78‰ to -24.16‰ VPDB; δ180, 0.69‰ to 4.35‰ VPDB). Biogenic carbonates have entirely dissolved, and some of the resulting molds are filled with dolomite-cemented fine clastics.
We consider this fossil assemblage to be an in situ cold-seep dependent assemblage because of its association with 13C-depleted authigenic carbonates, which clearly reflect the influence of the anaerobic oxidation of methane. We infer that the vesicomyids lived on sand dunes during time intervals between intermittent sanddeposition events under the unidirectional bottom currents.