THE ICHNOLOGY OF THE LATE PENNSYLVANIAN AMES LIMESTONE IN SOUTHEASTERN OHIO

The Late Pennsylvanian Ames Limestone in southeastern Ohio was deposited under normal marine conditions in a low accommodation, ramp setting during the maximum transgression of the Ames Sea into the Appalachian Basin. The effect of low accommodation coupled with deltaic influence led to limestone deposition under highly variable paleoenvironmental conditions. To recognize the small-scale vertical and lateral changes in this paleoenvironment, the ichnology of the Ames was examined. In this study, blocks of the Ames Limestone were collected, cut into slabs, and polished surfaces were examined. Biogenic structures were defined as structures with a distinct morphology or a change in sediment texture. Evidence of bioturbation was recognized throughout the Ames and 9 types of ichnofossils were identified: 1) general bioturbation; 2) burrows with defined morphology; 3) elliptical burrows with disrupted fill; 4) Planolites; 5) Diplocraterion; a 6) U-shaped burrow with disrupted fill; a 7) V-shaped structure; 8) chevron structures with a central shaft; and a 9) backfilled burrow. In general, ichnofossils were small, simple, deposit-feeding structures, and there was low ichnodiversity. There was little difference in the amount of bioturbation within a single unit, and therefore little vertical variability, but there was noticeable lateral variability in terms of complexity of ichnoassemblages. Performing a study on the ichnofossils of the Ames Limestone provided data on the conditions within the substrate during deposition. Previous analyses using body fossils to examine the depositional environment of the Ames presented data related to the conditions above the sediment-water interface. Since the Ames was deposited in normal, open-circulation marine waters, as indicated by the presence of brachiopods and crinoids, sediment consistency was likely the main limiting factor of ichnodiversity. To account for a lateral difference in total bioturbation, nutrient availability was likely the main limiting factor since the substrate consistency was similar throughout the study area. This ichnological analysis of the Ames, when combined with other sedimentologic and paleontologic data, presents a more detailed analysis of the paleoenvironment and paleoecology of low accommodation, shallow marine basins.