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

贝壳集中层的埋藏学研究：挂川组、远滨组和宫崎组的案例研究。

• 批准号: 03640654
• 负责人: MAJIMA Ryuichi
• 金额: $ 1.09万
• 依托单位: Yokohama National University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for General Scientific Research (C)
• 财政年份: 1991
• 资助国家: 日本
• 起止时间: 1991 至 1992
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-03640654/
• 关键词: Taphonomy; Shell-concentrated bed; Kakegawa Group; Tohnohama Group; Pliocene; Molluscan fossil; 貝殼集積層; タホノミ-; 宮崎層群; 古生物学

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.

我分析了位于日本本州中部太平洋一侧的挂川群上新世大日组的一个贝壳密集层，并将该层解释为海岸沉积物。我检查了物种组成、贝壳-使用高 43 厘米、宽 40 厘米、 30厘米深的块。我鉴定了该区块内1966个腹足类25种标本和18个双壳类1131个标本，并将其解释为生活在浅水区沙底、泥沙底和硬底物种的混合组合。我认为贝壳床的沉积是在一股由西北向东南流动的高速、高密度水流瞬间堆积贝壳和贝壳碎片。在岸面环境中，可以假设地转流和大风暴巨浪反冲洗的瞬时组合流。它在沉积物上方产生非常强的剪切应力，可能很容易产生密度流，并且它的能量很快减少，可能导致密度流中贝壳的瞬时积累。