PXRF SIGNATURES OF THE CAMBRIAN SPICE EVENT IN NEARSHORE DEPOSITIONAL SETTINGS OF THE WISCONSIN ARCH

The late Cambrian global SPICE (Steptoean Positive isotope Carbon Excursion) event was characterized by carbon and sulfur cycle changes, seawater temperature fluctuations, widespread ocean anoxia, trilobite extinctions, and a global sea level lowstand. In this study, we recognize shallowing-deepening trends, a subaerial unconformity (Sauk II-III boundary), and redox signatures of the SPICE event in nearshore, quartz sandstone-dominated successions of the Wonewoc Formation using portable x-ray fluorescence elemental analysis (pXRF). The studied succession includes, from oldest to youngest, the Eau Claire Formation, Wonewoc Formation, and the Tunnel City Group along two transects: 1) depositional strike of the Hollandale Embayment, western Wisconsin, and, 2) depositional dip of the Wisconsin Arch, southcentral (onshore) to southwestern (offshore) Wisconsin. The offshore strata of the Eau Claire and Tunnel City are dominated by dolomite-cemented shaly glauconitic sandstone with flat-pebble conglomerates and hummocky cross-stratification. The nearshore Wonewoc is a trough cross-stratified quartz sandstone with minor shale partings. Core and well cuttings were examined and correlated along the two transects; in general, 1 to 2 foot interval spot pXRF analyses on core exhibit more variation in elemental profiles, and higher elemental concentrations, than analysis of cuttings samples representing homogenized 5-foot intervals. Aluminum (Al), reflecting aluminum silicate abundance, shows a decreasing upward trend from the top of the Eau Claire into the lower Wonewoc (Galesville Member), with silicon (Si) showing the opposite trend, interpreted as shallowing/cleaning upward. An abrupt increase in Al and decrease in Si is interpreted as the Sauk II-III boundary and transgression at the base of the upper Wonewoc (Ironton Member). Finally, a shallowing/cleaning upward trend in the Ironton is overlain by an abrupt offset in elemental profiles at the unconformable contact with the Tunnel City. Along the relatively deeper-water strike-parallel transect, elevated sulfur concentrations (1-5%) are associated with pyritized worm burrows in the SPICE event interval, representing nearshore dysoxic conditions. These patterns in the Wonewoc are visually subtle, yet well resolved with pXRF.