COMPARATIVE TAPHONOMIC PETROGRAPHY: A CLOSER LOOK AT SHELL BEDS FROM THE CINCINNATI, OHIO AREA ORDOVICIAN (KATIAN)

Studies of shells and shell beds have advanced our understanding of the relationship between stratigraphic sequences, facies changes and shell accumulation. These studies attempt paleocommunity reconstruction, usually based on studies of shells washed or otherwise removed from surrounding sediment, or alternatively from shell/skeletal assemblages on bedding planes.

The Upper Ordovician of the Cincinnati, Ohio area includes thousands of shell beds. These beds are generally well-indurated, precluding examination of matrix-free shells. Bedding plane census counts assume that the bedding plane represents a true average of the living individuals as well as shell and skeletal debris present on the seafloor at the time of the final depositional event. However, high-energy depositional events may exhume previously-deposited material, and as these events wane, changing depositional regimes produce beds of differing fossil and sediment content during a single depositional event. Thus a shell bed is a complex of living, dead and reworked shells sorted by hydraulic processes.

We employed point-counting on large format thin-sections in succession through the entire thickness of stratigraphically distinctive units. We determined fossil, matrix, and cement content, while scoring fossil preservation. We employ four categories. Category 1 fossils show no signs of exposure on the seafloor after death, and are articulated and unbroken. Category 2 fossils are disarticulated, slightly fragmented or both. Category 3 includes fragments recognizable to broad classification types: “bryozoan” or “brachiopod”. Category 4 is “shell hash”, comminuted to the point that most diagnostic structures of the fossil are lost. A limitation of categorizing taphonomy in thin-section is the sectioned view of the shell, and the technique certainly overestimates the number of pristine shells. This method also produces volume data, not raw numerical abundance, as larger organisms are more likely to be counted multiple times than smaller ones.

While not directly comparable to studies using whole shells or bedding plane censuses, successive thin-sectioning through these beds offers both a more detailed look at the internal complexities of shell beds, as well as comparison of fossil taphonomy between facies.