High-resolution faunal gradient analysis and an assessment of the causes of meter-scale cyclicity in the type cincinnatian series (Upper Ordovician)

The origin of meter-scale cyclicity in the type Cincinnatian Series has long been debated. Some models hypothesize that changes in water depth driven by sea-level fluctuations were responsible for producing meter-scale alternations: of shale-rich and limestone-rich intervals. Other models link meter-scale cyclicity to changes in storm intensity and frequency, with no change in water depth. Previous interpretations have relied upon lithological variations, which have proven to be ambiguous with respect to meter-scale cyclicity. Here, the role of water depth in producing meter-scale lithologic patterns is assessed using gradient analysis of high-resolution fossil abundance data from the Kope and lower Fairview Formations. Studies have demonstrated that the distribution of biota in this interval is controlled by environmental variables correlated to water depth. Therefore, a direct comparison of stratigraphic variations in faunal composition to meter-scale lithologic alternations is an appropriate test of the influence of water depth on meter-scale cyclicity. In the present analyses, ordination scores generated from faunal abundance data are grouped into bins that correspond to the upper proximal and lower distal parts of each meter-scale cycle, using three different binning protocols. For each cycle, ordination scores from the lower bin are compared to those from the upper bin; consistent differences between the two would suggest a water depth control on meter-scale biotic patterns, and, thus, cyclicity. However, results indicate no consistent correspondence of faunal patterns to meter-scale lithologic patterns, suggesting that water depth does not play a significant role in the formation of meter-scale cycles. While the different binning protocols did affect analytical outcomes in various ways, the lack of a consistent difference between upper and lower bins within each cycle was robust to all protocols. A model invoking oscillations of storm intensity and frequency appears to provide the most parsimonious explanation for the origin of Cincinnatian meter-scale cyclicity.