Biofacies replacement in a sequence stratigraphic framework: Middle and Upper Ordovician of the Nashville Dome, Tennessee, USA

Quantitative analysis of fossil assemblages from ten depositional sequences identified the spatial and temporal distribution of shallow and deep subtidal biofacies in Middle and Upper Ordovician strata of the Nashville Dome. Articulate brachiopods and bryozoans dominate most biofacies, although corals, sponges, gastropods, and bivalves are also important taxa that dominate biofacies locally. Temporal patterns in both lithofacies and biofacies indicate a close link between environmental shifts and faunal changes that took place over a period of approximately 13 million years. The primary cause of the environmental and biotic changes was Taconic orogenesis, which increased the flux of fine-grained siliciclastics and nutrients to the epeiric seaway and affected circulation patterns by changing basin topography. Biofacies replacement was accomplished not only by taxonomic turnover, but also by wholesale changes in the dominance structure of local assemblages. Changes in dominance also occurred at the guild level, with dominance by reclining epifaunal suspension feeders (RESF) shifting to pedunculate epifaunal suspension feeders and back to RESF in concert with environmental changes. Although all major macrofaunal groups involved in biofacies replacement were affected by the changes in paleoceanographic conditions, local extinction and immigration was not synchronous among all genera, but occurred at different times for different genera. This suggests that these Middle and Upper Ordovician biofacies are composed of communities that were not tightly integrated in their structure, but rather were fluid with species and genera moving in and out of communities in response to local changes in environmental conditions.