Methodological advances in the use of faunal gradient analysis for regional high-resolution correlation in the type Cincinnatian Series (Upper Ordovician)

Abstract

Historically, strata of the type Cincinnatian Series have not been correlated with precision because of an assumption that individual beds are not regionally persistent. In an effort to establish a regional chronostratigraphic framework for the Cincinnatian, a technique has been developed recently that uses gradient analysis of fossil abundance data to compare vertical trends in biotic composition among five localities that span a distance of 65 km. Using faunal data to establish correlations below the meter-scale has been problematic, because variability in the lateral expression of many beds has made it difficult to sample equivalent fossiliferous strata at all localities. Here, this correlation technique is modified in an attempt to draw meter-scale correlations of the Kope and lower Fairview Formations. This modification involves grouping fossil census samples into stratigraphic intervals, such that the stratigraphic spacing of samples is identical at all localities. Samples are binned using two protocols: one that groups samples into 0.5 m thick intervals, and another that groups samples by lithologically delineated meter-scale cycles. Gradient analysis of binned samples using both protocols produces curves in which patterns in faunal composition below the meter-scale can be correlated with success only for limited intervals at select localities. The inability to consistently correlate at high resolution might indicate that correcting for regional variations in the expression of strata is not enough. Alternatively, the ability to correlate some intervals below the meter scale suggests that other problems, such as fine-scale lateral variations in biotic composition, might be impeding correlation. In addition, gradient analysis techniques are employed at a new locality to extend the lateral breadth of the existing, broader-scale chronostratigraphic framework by approximately 40 km. Although interpreted to be in an up-ramp direction from previously measured localities, this new locality does not have an overall shallower water faunal assemblage until just below the Kope/Fairview contact.