Webber, Andrew J.1
(1) Miami University, Hamilton, Hamilton, OH
ABSTRACT: The Effects of Patchiness and Time-Averaging on the Stratigraphic Signal of Biotic Composition in the Type Cincinnatian Series (Upper Ordovician)
In the type Cincinnatian Series, gradient analysis has been effective in quantifying subtle, water depth-related variations in the stratigraphic and geographic distributions of marine assemblages. This information presents the potential to evaluate, among other things, how marine organisms respond in the face of climatically-induced environmental change on a variety of scales. Local variations, or patchiness, in the distribution of fossils can influence finer scale paleoecological analyses by artificially inflating compositional differences among localities. Here, the effect of spatial patchiness on the signal of biotic change in the type Cincinnatian Series was examined by quantifying the amount of lateral variation in composition per level at a single outcrop of the Kope Formation.
Local spatial patchiness affects the fine-scale stratigraphic pattern of biological variation, such that compositional differences among localities depend on the composition of the patch sampled for gradient analysis. These differences are typically produced by the addition or subtraction of just a few taxa. Furthermore, comparisons of samples that are quantified using gradient analysis reveal a consistent variability among strata in the degree of patchiness related to lithology. In general, limestone-rich strata display less variation in quantified composition than do mudstone-rich strata. Mudstone-rich strata are less consistent: some show high compositional variability whereas others are low. This indicates that the biotic composition of any given limestone-rich strata is relatively similar, whereas any given mudstone-rich strata potentially contain either similar or disparate fauna. This difference may reflect the tendency of fossil assemblages occurring in mudstones to exhibit less time-averaging and within-habitat transport of skeletal remains by storms relative to limestone-rich strata. Although storm reworking and condensation may serve to homogenize lateral faunal differences in limestone beds relative to mudstone beds, this time-averaging is not enough to remove the signal of patchiness entirely.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.