STRATIGRAPHIC VARIATION OF THE SPICE EVENT IN UPPER CAMBRIAN CARBONATES OF SOUTHERN MISSOURI

The late Cambrian carbonate platform of southern Missouri is an excellent locality in which to document the relationship of carbon isotope excursions to stratigraphic and facies architecture. Within and adjacent to the late Cambrian intrashelf basin, extending westward from the Precambrian St. Francois Mts., carbonate rocks of the Bonneterre and Davis formations (Marjuman and Steptoean stage) are part of small- to large-scale, unconformity bounded, transgressive-regressive sequences. They record abrupt lateral and vertical variations in depositional facies, inferred water depth, and community paleoecology. New δ¹³C_carb data (n = 297) from drill cores and outcrops, representative of intrashelf basin and platform edge facies, respectively, record the Steptoean positive carbon isotope excursion (SPICE) in both limestone and dolomitized portions of the stratigraphic section. Magnitude of local δ¹³C excursions appears to be related to sedimentary facies, with maximum values of > 4‰ V-PDB in deeper-water, intrashelf basinal rocks (thin limestone in sections containing 40-60% shale) and ~ 2 to 3‰ V-PDB in shallower-water, platform edge, shaley carbonates. Coupling new data with those from previously analyzed cores (He et al., 1997; Gill et al., 2011; Schiffbauer et al., 2015), indicates that the SPICE event in southern Missouri is recorded as a time-transgressive and facies-dependent phenomenon, and is decoupled temporally from the Marjumiid-Pterocephaliid biotic crisis (Crepicephalus-Aphelaspis trilobite zone boundary). Initiation of the positive excursion coincides with an up-section shift to shale-dominated lithologies, which corresponds to the transition from Sauk II to Sauk III transgression. We interpret the covariance of carbon isotope excursion and water depth gradient recorded in southern Missouri to represent impingement of an oceanic carbon component on the local carbonate factory of the shallow shelf environment. Thus, if the timing and magnitude of the SPICE are results of transgression of deeper water bodies onto carbonate platforms, the architecture of an individual platform will control where the invading water mass impinges and where within the stratigraphy it is recorded, thereby reducing the utility of the SPICE for global correlation and as a link to biotic turnover.