ORIGIN OF IRON OXIDE CEMENTS IN THE CAMBRIAN WONEWOC FORMATION SANDSTONE OF WISCONSIN

Upper Mississippi Valley Cambrian quartz sandstones are commonly colored orange-brown by iron oxide cements. In outcroppings and most near-surface cores of the Wonewoc Formation, iron oxide is observed as quartz grain coatings, intergranular cements, and liesegang banding. Recent bedrock coring through a major ridge in west-central Wisconsin has recovered an atypical core containing sulfide-cemented Wonewoc quartz sandstone at depths >348’ below land surface. In this core, pyrite occurs as Skolithos burrow linings, coatings on inarticulate brachiopods, and as intergranular cements. Nearby cores where the Wonewoc is closer to land surface and the water table are more typical: Skolithos burrows consist of goethite, and other iron oxide cements abound.

Mineralogical, preliminary sulfur isotopic, and trace metal data suggest that sulfide mineralization in the Wonewoc may have formed as early diagenetic cements sourced from Cambrian seawater. Burrow-lining pyrite has a δ³⁴S composition of 32.5‰, consistent with published values for the SPICE event and enriched relative to MVT sulfides (<22.0‰). Furthermore, Pb and Zn concentrations of <10 and 2 ppm, respectively, in this pyrite suggest it is not MVT sourced. In contrast, vug-filling pyrite up section in this core has Zn and Pb concentrations upwards of 195.5 and 54.5 ppm, respectively. The association of Wonewoc pyrite with burrow linings and brachiopods suggests that some pyrite formation occurred in the Cambrian during organic matter oxidation under low-oxygen conditions by sulfur-reducing bacteria.

The association of the Skolithos-burrow sulfides with trough cross stratification suggests that nearshore settings may have experienced dysoxic conditions during the late Cambrian SPICE event. Additionally, iron oxide cements within the Wonewoc Formation along depositional strike are a result of post-depositional oxidation of early diagenetic and MVT-related sulfides. We envision these sulfides are easily oxidized close to the land surface and/or water table, which explains both regional iron oxide-cementation and local groundwater trace metal contamination. This oxidation appears to be a geologically recent/current process; it is unclear what role Driftless area geologic history and glacial meltwater have had in oxidation extent and onset.