Trace fossils and sedimentologic interpretation—McGregor Member of Platteville Formation (Ordovician) of Wisconsin

The McGregor Member is a dominantly micritic carbonate unit, approximately 10 m thick in southwestern Wisconsin, which displays considerable uniformity laterally in terms of sedimentary structures, fossils, and trace fossils. Two trace fossil genera are prominent and help to interpret the unit's depositional history. The base of the Member is a sharp contact exhibiting Trypanites and is here interpreted as a submarine hardground. Within the Member Chondrites is present in profusion. Burrows are outlined by dark pyritic rims on fresh rock surfaces; weathering oxidizes the pyrite to limonite, causing the burrows to lose definition and eventually to disappear into a diffuse limonitic stain. On weathered outcrops the pervasive Chondrites bioturbation is cryptic. In areas where the McGregor is partially dolomitic, the dolomitization mostly affected burrows and the sediment immediately surrounding them, suggesting control of the diagenetic fluids by the permeability variations of the bioturbate zones. Chondrites show varying amounts of compression relative to the nodular bedding planes of the McGregor: burrows are circular to elliptical within micritic lenses but flattened within the shaly partings between lenses. The compression demonstrates that the bedding is secondary, probably produced by differential compaction of a thoroughly bioturbate, quiet-water mud; this interpretation is at odds with previous workers who ascribed the wavy bedding to ripples and inferred that the depositional environment was shallow and agitated. The presence of a basal hardground across the entire study area (110 km) and the dominance of Chondrites everywhere within the unit suggests that no significant environmental gradient, such as water depth or intensity of physical reworking, was present, even across the Wisconsin Arch.