RECOGNITION OF SPICE AND OTHER CARBON ISOTOPE EXCURSIONS IN THE UPPER CAMBRIAN OF SOUTHERN MISSOURI: INTERACTION OF LOCAL AND GLOBAL CARBON FACTORIES

Upper Cambrian (Marjuman and Steptoean stages) carbonate rocks of the Bonneterre and Davis fms. in southern Missouri are part of small- to large-scale, unconformity bounded, transgressive-regressive sequences, which are characterized by distinct facies in linear belts that developed on and adjacent to basement highs. New carbon isotope data (n = 270) from five drill cores and a reinterpretion of data from He et al. (1997) indicate the presence of a distinct positive δ¹³C excursion within the Steptoean carbonate rocks, as well as smaller, negative δ¹³C excursions within the underlying Marjuman/Steptoean rocks. The positive and negative excursions are preserved in both limestone and dolomitized portions of the stratigraphic section.

The magnitude of the positive δ¹³C excursion in the Steptoean rocks appears to be related to sedimentary facies, with shifts of ~ 6‰ in the intrashelf basinal facies (thin limestone beds in rocks with 40 to 60% shale), ~ 3‰ in shallow platform limey mudstone/shale, and ~ 2.5‰ in the backreef facies (microbial laminate/mudstone). Initiation of the positive excursion appears to coincide with the beginning of large-scale relative sea-level rise near the Sauk II/III boundary and the maximum positive excursion occurs during the Sauk III transgression. We interpret the variation of the C isotope signal across the depth gradient to result from impingement of an oceanic carbon component on the local carbonate factory of the shallow shelf environment.

Negative δ¹³C excursions found in subjacent rocks, especially near the Marjuman/Steptoean boundary, are similar in magnitude to those shown to be related to trilobite extinction events in carbonate sections in South Dakota (Perfetta et al., 1999). The magnitudes of the negative δ¹³C excursions in Marjuman rocks are larger for shallower platform rocks (~ 1.5‰) than for those of the deeper, intrashelf-basin (< 1‰).

We are expanding our studies to include rocks representative of the deep ramp facies near the Reelfoot Rift and the marginal platform facies on the Lebanon Arch to further test the effects of facies architecture on δ¹³C values. Our goal is to deconvolve the global and local contributions to the C isotope signal preserved in the carbonate rocks in order to determine what effect, if any, the processes leading to C isotope variability had on biota.