Taphonomic study of shell-concentrated bed: case studies in Kakegawa, Tohnohama, and Miyazaki Groups.

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I analyzed a shell-concentrated bed in the upper Pliocene Dainichi Formation of the Kakegawa Group, exposed at the Pacific side of central Honshu, Japan, and interpreted the sequence including the bed as a shoreface deposit.I examined the species composition, the shell-orientation patterns, the convex-down ratio of bivalves, and the shell-size ratios, of the four horizons of the bed, using a 43cm- high, 40cm-wide, and 30cm-depth block. I identified the 1966 specimens of 25 gastropod species and the 1131 specimens of 18 bivalve species in the block, and interpreted them to be a mixed assemblage of the species that live in a sandy bottom, a muddy sand bottom, and a hard bottom in shallow waters.I interpreted the shell bed to have deposited under a condition that a high-speed and a high-density current that flowed from northwest to southeast, accumulated instantaneously the shells and the shell fragments. In a shoreface environment, it is possible to assume an instantaneous combined current of a geostrophic flow and a backwash of a huge wave of a major storm. It gives a very strong shear stress just above the sediments, probably easily making a density current, and it's energy is reduced very quickly, probably resulting instantaneous accumulation of the shells from the density current.