Occurrence of Calyptogena sp. (Bivalvia

Fossils of the seep-related bivalve Calyptogena sp. occur in alternating beds of turbiditic sandstone and siltstone in the Pliocene Horinouchi Formation of the Sagara-Kakegawa area, central Japan. The Horinouchi Formation consists of submarine fan deposits which filled the Plio-Pleistocene fore-arc basin formed by oblique subduction of the Philippine Sea Plate beneath the southwestern Honshu Arc. The Horinouchi Formation near the fossil locality interfingers with the Abina Formation, which consists of disorganized, angular pebble- to boulder-conglomerate distributed along the landward marginal fault of the basin.Calyptogena shells representing various growth stages occur at many horizons in a Horinouchi Formation sequence about 20 m thick consisting of alternating sandstone and siltstone. Most of the Calyptogena shells are disarticulated and buried in a convex-up position in the basal part of the sandstone beds, but some are articulated and present in the siltstone beds. These occurrences suggest that the Calyptogena habitat was continuously present on the landward slope during deposition of the 20-m-thick turbidite. This habitat may be related to the tectono-sedimentary settings characteristic of the landward margin of a fore-arc basin, such as marginal fault activity associated with basin subsidence, or related to high input of nutrient-rich organic matter by turbidity currents.On the basis of hinge characters, Calyptogena sp. from the Horinouchi Formation is distinct from the Calyptogena (s.l.) species reported from the Tamari and the Hijikata Formations, both of which consist of massive Pliocene siltstone present in the center of the same basin. The occurrences of different Calyptogena (s.l.) species reflect variations in habitat conditions for chemoautotrophic bivalves in a fore-arc basin.