STRATIGRAPHIC AND PALEOBIOLOGIC ANALYSIS OF THROMBOLITE MEGASTRUCTURES, ST. LOUIS LIMESTONE, MONROE COUNTY, INDIANA

The St. Louis Limestone was deposited in the Illinois Basin during the Meramecian Stage of the Mississippian. In Monroe County, Indiana, the study area includes only a portion of the lower half of the formation exposed along a highway section north of Bloomington. Two meters of massive- to thinly-bedded, resistant limestones are interbedded with clayey- to silty calcareous shales to comprise a shallowing-upward depositional sequence. The lower meter of the stratigraphic section contains locally abundant clusters of Syringopora overlain by tumbled rugosans. Fragmented brachiopods, crinoids, bryozoans, and trilobites, internal molds of bellerophontid and other gastropods, and small branching burrows are also present. Beds low in the section contain asymmetrical ripples and trough cross-stratification not observed in upper beds, suggesting a decrease in water energy upsection. The upper meter of the section is characterized by thrombolite paleocommunities draped by micrite mud, overlain by laminations and clasts of algae, and in turn overlain by micritic limestone. Thrombolite megastructures occur in two levels and exhibit an increase in size and development of macrostructures from the lower to upper level. Twenty-five thrombolite macrostructures range in thickness from 15 to 33 cm and are subevenly distributed across an outcrop distance of 17.4 meters. Mesostructures can be observed in thin-sections. The paleobiologic composition of the St. Louis Limestone is important to document because of the rarity of thrombolitic occurrences reported in the literature from Upper Paleozoic strata. The fact that this outcrop is slated for destruction in the near future further emphasizes the value of this documentation. Correlations with other portions of the St. Louis Limestone south of the Bloomington area (Lawrence County) are currently under investigation.