USING GRADIENT ANALYSIS TO CHARACTERIZE LITHOLOGICALLY-RELATED VARIABILITY IN THE EXTENT OF TIME-AVERAGING AMONG FOSSIL ASSEMBLAGES

Preservational biases in the geological record have the potential to influence perceived patterns in the distribution of marine fossil assemblages. In the type Cincinnatian Series, gradient analysis has been used to characterize fine-scale lateral trends in biotic composition at a single outcrop of the Kope Formation. Comparisons of the amount of lateral variation among local assemblages (i.e., faunal patchiness) have revealed consistent differences between assemblages occurring in limestone-rich bedsets and those in mudstone-rich bedsets. Limestone-rich bedsets generally display less lateral variation in quantified composition than do mudstone-rich bedsets. Mudstone-rich bedsets are less consistent: some show high compositional variability whereas others are low. This indicates that the local biotic composition of any given limestone-rich bedset is relatively similar, whereas any given mudstone-rich bedset potentially contains either similar or disparate fauna. This pattern likely is caused by differences in the amount of time-averaging related to lithology.

Fauna from the less variable, limestone-rich bedsets may reflect the tendency of these fossil assemblages to exhibit greater time-averaging and within-habitat transport of skeletal remains by storms relative to mudstone-rich bedsets. This is supported by evidence of intense reworking and condensation of shelly material within limestone strata in the Kope, signifying that multiple storm events affect the deposition of even individual limestone beds. Conversely, mudstone strata tend to show features consistent with episodic sedimentation. Mudstone-rich bedsets from the Kope have an overall lower density of skeletal remains, and generally contain better preserved specimens, including many smothered faunas. Furthermore, limestone-rich bedsets in the study interval have an overall higher biotic diversity relative to mudstone-rich bedsets. This most likely reflects the greater probability that habitat shifting would add to the diversity of preserved organisms in increasingly time-averaged deposits. Despite variable degrees of time-averaging among bedsets of different lithology, larger-scale trends in biotic composition acquired through gradient analysis are robust.