WHY THE LONG FACIES? FACTORS CONTROLLING THE EXPRESSION OF THE CAMBRIAN SPICE EVENT IN THE SOUTHERN APPALACHIANS

The SPICE (Steptoean Positive Isotopic Carbon Excursion) is a carbon isotope excursion that is found in later Cambrian marine successions worldwide. It is hypothesized to reflect a globally synchronous carbon cycle perturbation potentially caused by an expansion of deep-water anoxia. While the SPICE has been identified and studied in a variety of environments, the factors controlling the absolute carbon isotope values at individual sites and the magnitude and shape of the excursion are poorly understood. It has been suggested that factors such as carbonate platform geometry, gradients in the carbon isotope composition of seawater, changes in sea level, and diagenesis may play roles in modifying local carbon isotope records.

Here we present a high-resolution carbon isotope and sedimentological study of three new SPICE sections in the southern Appalachians, in Virginia and Maryland, USA. Using field logs and thin section petrography, we constructed a detailed sedimentological and facies analysis for each section and compared the timing of facies changes to those of the carbon isotope records. We also compared our facies trends and carbon isotope data to published records from other locations across the broader Appalachian region. Combined, these sites reflect deposition across the entire southern Laurentian margin during the later Cambrian. We found that the overall shape and magnitude of the SPICE remains consistent across localities, but that changes in local facies are coincident with second order fluctuations in carbon isotopes. This is most evident in shallower sections, where local shallowing cycles are paired with ~0.5 ‰ oscillations in carbon isotopes. Deeper sections with less facies variation have smoother carbon isotope curves, perhaps due to limited local environmental changes. Additionally, we note that despite varying degrees of both early and late burial dolomitization in the studied sections, the carbon isotope signals remain consistent and suggest a limited role of diagenesis in altering the local carbon isotope signals. These findings help clarify the environmental processes that affected individual expressions of the SPICE and provide a framework to detangle local and global processes influencing the records of other global carbon isotope excursions in the geologic record.