EVIDENCE FOR SULFIDIC SHOREFACE SETTINGS DURING THE LATE CAMBRIAN SPICE EVENT

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 eustatic changes. Geochemical studies of the SPICE Event have focused mostly on shelfal carbonate and deeper shale successions. In contrast, few geochemical studies investigate nearshore settings such as quartz and glauconitic sandstone of the Upper Mississippi Valley (UMV) Hollandale Embayment. In the UMV, primary carbonate cements are uncommon and sandstone from both outcrop and subsurface is ubiquitously colored orange-brown by iron oxide cements in the form of quartz grain coatings, intergranular cements, burrow-fillings, and liesegang banding, the latter cross-cutting sedimentary structures and indicating post-depositional diagenesis. For this reason, previous UMV SPICE geochemical studies focused primarily on carbon isotopic analysis of chitinophosphatic brachiopod material.

We present geochemical data from a pristine UMV core, in which SPICE-interval strata are gray-colored and in places sulfide-cemented. Sulfides occur as pyritic burrow linings, coatings on inarticulate brachiopods, and intergranular cements. Pyrite δ³⁴S values record a ~30‰ positive excursion associated with the SPICE Event, from 8.2 to 38.7‰. This excursion culminates in trough cross-stratified quartz sandstone with Skolithos ichnofacies of the uppermost Wonewoc Formation, below a sequence boundary with the overlying glauconitic Lone Rock Formation. Although the uppermost Wonewoc records the shallowest facies in the study interval it contains the maximum abundance of pyrite observed, suggesting deposition in sulfidic waters. Subsequent glauconite deposition in the Lone Rock indicates oscillating redox conditions. Previous studies (Gill et al., 2011; Leroy and Gill, 2019) have proposed that sulfidic waters expanded locally, if not globally, onto shallow shelf settings during the SPICE Event leading to trilobite faunal turnover. This study suggests that sulfidic conditions extended incredibly far into the Hollandale Embayment during the SPICE Event, resulting in sulfidic shoreface settings.