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

我分析了日本本州中部太平洋一侧出露的上新世第一群上新统大一组贝壳密集层，并将该层序解释为滨面沉积。利用一个43 cm高、40 cm宽、30 cm深的块体，研究了该层的四个层位的物种组成、贝壳取向模式、双壳类的凸下比和贝壳大小比。对块内25种腹足类1966号标本和18种双壳类1131号标本进行了鉴定，认为它们是生活在砂质海底、泥质砂质海底和浅水硬质海底的物种的混合组合，贝壳床沉积的条件是由西北向东南的高速高密度流瞬间堆积贝壳和贝壳碎片。在海面环境中，可能会假设一股地转流和一场大风暴的巨浪的反冲的瞬时组合流。它在沉积物上方产生非常强的剪应力，可能很容易形成密度流，它的能量很快就会减少，可能会导致密度流瞬间积累贝壳。

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.