Isotope chemostratigraphy of the lower Mississippian St. Joe Group in southwestern Missouri and northwestern Arkansas

Abstract

The petrography and isotope geochemistry of the St. Joe Group in southwestern Missouri and northwestern Arkansas were analyzed to determine (1) if the positive excursion in the global isotopic curve for delta13C and delta18O during the Tournaisian (Kinderhookian) is evident in the St. Joe Group, and (2) if isotopic evidence can be used to interpret the completeness of the section and the nature of contacts within the group. The results indicate that the robust positive excursion for delta13C and delta18O in the Tournaisian is not evident in profiles of the examined sections. A weak positive excursion is evident in delta13C profiles for the thicker sections and delta13C and delta18O are not covariant. In addition, there is no evidence to support significant intervals of missing section. The St. Joe Group is determined to be conformable in the study area and there is no evidence that diagenetic changes induced by subaerial exposure overprinted the original isotope values. Delta18O values were likely modified by meteoric water and diagenetic fluids. The weak to absent positive excursion in delta13C values in Tournaisian (Kinderhookian) samples is problematic but could be the result of meteoric processes and/or isolation of the area from the Tournaisian sea. Integrated geochemistry, lithology and existing conodont biostratigraphy support the contention that the respective contacts between the Compton Limestone, Northview Formation and Pierson Formation are relatively conformable in the outcrops in this study area and these formations have isotopic profiles that at best, weakly correlate to regional and global Kinderhookian isotope profiles.