Cold-seep carbonate mounds containing Vesicomya (Calyptogena) kawamurai paleocommunity occur in massive siltstones of the Pliocene Tamari and Hijikata Formations, both deposited on the upper to middle slope of a forearc basin in the Sagara-Kakegawa area, central Japan. The shell-rich carbonate mounds vary from lenticular (2 m in diameter and 0.5 m in maximum thickness) to barrel-shaped (0.8 m in diameter and 1.5 m in length). One lenticular shelly concretion consisted of densely crowded vesicomyid shells lying parallel to bedding, and overlies a brecciated siltstone with dolomicritic cement. A barrel-shaped concretion contained abundant articulated valves of V. (C.) kawamurai in life orientation, and preserves a three-dimensional view of a chemosymbiotic habitat maintained over several generations of these large clams. In addition, the carbonate mounds contain various void spaces (open burrows and dissolved-shell molds) which are fringed by authigenic carbonates (splayed fibrous aragonites and dolosparites). The aragonite bur-row-linings in the carbonate mounds and calcite/Mg-calcite micronodules in the siltstone matrix have very 13C-depleted isotopic signatures (δ13C = −43.59 ‰ to −54.54‰ PDB).
The taphonomic, petrographic, and stable isotopic evidence confirms that the vesicomyid paleocommunities formed due to biogenic methane seepage. Brecciation of the siltstone shows an explosive collapse of the sediment fabric, possibly triggered by decomposition of gas hydrates in near-surface sediments. The subsequent biological architecture of burrows and large, dead-shell cavities acted as efficient conduits that facilitated continued seepage. The chemosymbiotic bivalves were able to colonize slope mudstone during the methane-seepage associated with gas hydrate decomposition, and their biological activities altered the muddy substrate to promote the methane seepage that sustained several generations of clams.
