USING PALEOECOLOGICAL GRADIENT ANALYSIS TO ENHANCE STRATIGRAPHIC INTERPRETATION: A PENNSYLVANIAN-PERMIAN EXAMPLE

The Pennsylvanian-Permian succession of the North American Midcontinent (upper Wabaunsee, Admire, and lower Council Grove Groups) records the depositional history of a broad, flat epeiric platform deposited during a time of global ice-house climate. Facies interpretation of this interval is greatly enhanced by gradient analysis of the benthic fauna, which was a more sensitive recorder of environmental conditions than lithology. The interval is dominated by fifty-one 5th-order (104-105 years) carbonate-siliciclastic couplets stacked into five 4th-order (105-106 years) composite depositional sequences. This stratigraphic hierarchy is interpreted to record high-amplitude, high-frequency changes in glacioeustasy and glacioclimate analogous to those of the Quaternary. The extreme degree of facies change within the stratigraphic cycles ? from subaerial exposure through open-marine conditions back to subaerial conditions within 10-15 m of section ? and the large number of stratigraphic discontinuities in the succession make it very difficult to identify larger scale depositional trends based on lithology alone. Correspondence analysis of 474 presence/absence collections of bivalves and brachiopods, however, reveals two environmentally sensitive biofacies, one dominated by each group, as well as previously unidentified gradients within each biofacies. The gradient within the bivalve biofacies records the transition from restricted, lagoonal/paralic to better-circulated, open-marine conditions. The gradient within the brachiopod biofacies records the transition from open-marine to dysoxic conditions. The taxonomic composition of each collection allows it to be placed along these indirectly derived paleoecological gradients. Changes in the faunal composition of collections indicate that each of the five composite sequences represents successively greater marine inundation of the platform with a strong basinward shift near the top of the interval. The application of indirect gradient analysis permits recognition of previously unknown environmental trends in this interval of patchwork facies and subtle stratigraphic discontinuities.