Geochemical Preservation of the Steptoean Positive Carbon Isotope Excursion Event in Dolomites of the Furongian Franconia Formation in the Illinois Basin

The Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE) is a well-documented global event that marked geochemical excursions in the world’s oceans and terrestrial environments. A core from the Franconia Formation, which lies on the southwestern edge of the Illinois Basin, appears to have captured the carbon-isotope excursion associated with the SPICE. In this section of the Illinois Basin, the SPICE is contained within the Sauk III sequence. Petrographic analysis revealed that the Franconia Formation transitioned from a shallow marine high-energy environment in the lower sections of the core toward a deltaic-influenced environment during a marine regression sequence as sea level lowered in the Furongian Epoch. Although extensive dolomitization has occurred, the δ¹³C values of the carbonate do not appear to have been significantly altered. The δ¹³C marine carbonate isotope values were able to track the SPICE closely, revealing a peak enrichment value of 4.9‰. Carbonate-associated sulfate δ³⁴S values showed characteristic enrichment in ³⁴S during the Furongian, but these values did not exhibit a notable trend. This study emphasizes the potential for isotope geochemical preservation in carbonates despite extensive dolomitization and identifies the timing and local environmental conditions capturing the SPICE.

Circular 600, Geochemical Preservation of the Steptoean Positive Carbon Isotope Excursion (SPICE) Event in Dolomites of the Furongian Franconia Formation in the Illinois Basin by Dana M. Labotka and Jared T. Freiburg can be downloaded from the University of Illinois IDEALS repository. To order a print-on-demand copy of the circular, visit our secure online shop or call the Illinois State Geological Survey Information Office at 1-217-244-2414 Monday–Thursday from 8 a.m. to 1 p.m. Central time.