Anin situVesicomyid-dominated Cold-seep Assemblage from the Lowermost Pleistocene Urago Formation, Kazusa Group, Forearc Basin Fill on the Northern Miura Peninsula, Pacific Side of Central Japan

Abstract

Abstract. 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 wit...