TAPHONOMY AND TIME-AVERAGING OF SHELL BEDS IN THE KOPE AND FAIRVIEW FORMATIONS OF THE CINCINNATI ARCH

The Upper Ordovician strata exposed in the Cincinnati Arch have long provided a “field laboratory” for the study of paleontology and sedimentary geology. These units were deposited in a shallow, subtropical continental sea during the Late Ordovician and are composed of both siliciclastic and carbonate material deposited by day-to-day seafloor processes that were frequently reworked by episodic storm events. Not all storm beds are equal in terms of time-averaging and fossil preservation and the effect of concentrated skeletal material on preservation provides the basis for this study.

By examining all of the above aspects in a total-data approach, it is possible to gain insights into the nature of preservation of fossil material, both present and absent, and its relationship to time-averaging. In the present study shell beds of the Kope and Fairview Formations (Edenian-Maysvillian age) were categorized using detailed data on taphonomy, including degree of articulation, corrasion, and fragmentation, as well as encrustation and boring. The stratigraphy of each locality was measured at a centimeter scale and each shell-bearing bed was sampled. A record of the sedimentologic characteristics of the beds was collected in field and the collected samples were scored based on their taphonomic characteristics.

Preliminary observations to be further tested with these data indicate that simple (possibly single event) beds, which concentrated and then buried skeletal material below a layer of sediment provide the best preservation of shells, especially those composed of aragonite. Skeletal accumulations that were reworked multiple times contain primarily fragmented calcitic shell material, while those insufficiently buffered by carbonate concentration and/or maintained in the taphonomically-active zone by shallow burial, would have allowed continued oxygenation and the movement of corrosive pore-water and thus enhanced dissolution. Therefore, minimally time-averaged, concentrated, but rapidly buried beds are most likely to contain well preserved material including molds and/or recrystallized molluscan shell material, which is so often absent from the Ordovician record owing to early dissolution in a calcite sea.