THE EFFECTS OF FAUNAL PATCHINESS ON THE USE OF GRADIENT ANALYSIS FOR REGIONAL, HIGH-RESOLUTION CORRELATION IN THE TYPE CINCINNATIAN SERIES (UPPER ORDOVICIAN)

High-resolution faunal gradient analysis in the type Cincinnatian Series has been effective in regional correlation at vertical scales of a few meters by numerically comparing stratigraphic trends in fossil composition among localities. Correlating finer-scale trends has been hampered not only by a lack of lateral persistence of individual strata from outcrop to outcrop, but also by significant vertical variation in faunal composition at scales of less than a meter. The source of this finer-scale stratigraphic variation in biotic composition is unknown: it might reflect small-scale environmental differences among localities, or it might indicate random fluctuations in biotic composition. Here, fine-scale patchiness in the distribution of fossils is assessed as a potential cause of fine-scale stratigraphic variation in biotic composition, and the effects this spatial variability has on the use of fossil data for regional correlation are examined.

Previous studies in the Cincinnatian recognize spatial heterogeneity in the distribution of faunal assemblages. Sampling from a single point along a particular bed potentially will capture only a subset of the entire suite of fossils from that bed, such that stratigraphic patterns of biotic transitions in the same interval may differ from one sampling site to another at one locality. If spatial variation is greater than stratigraphic variation, then patchiness is likely to mask the true stratigraphic signal of biotic composition at a fine scale, and minimizing the effects of patchiness may be necessary for regional high-resolution correlation.

In the present study, the degree to which patchiness affects fine-scale trends in faunal composition is assessed by using gradient analysis to quantify faunal composition along multiple stratigraphically limited vertical transects at a single locality. Spatial variation is evaluated by comparing the numerical faunal composition values among transects. Results indicate that spatial variation in faunal composition is significant enough to affect stratigraphic trends, often with certain taxa absent from single transects across multiple successive beds. Despite this, similar fine-scale stratigraphic patterns can be detected in all transects, suggesting that patchiness does not completely obscure the biotic signal.