A Late Cambrian positive carbon-isotope excursion in the Southern Appalachians; relation to biostratigraphy, sequence stratigraphy, environments of deposition, and diagenesis

ABSTRACT A positive carbon-isotope excursion is recorded within the Upper Cambrian sedimentary succession in the southern Appalachians that consists of the Nolichucky Shale, the Maynardville Formation, and the Copper Ridge Dolomite. The lower part of the succession contains Aphelaspis zone fauna (Early Steptoean). The extensively dolomitized and poorly fossiliferous nature of the upper part of the succession precludes any detailed biostratigraphic determinations. Correlation with similar positive carbon-isotope excursions in coeval successions elsewhere suggests that this excursion represents a perturbation in the global cycling of carbon. Comparison of excursions at different localities in North America provides a means for the application of carbon-isotope stratigraphy. In the southern Appalachians the excursion started during deposition of the upper Nolichucky Shale. Maximum 13C values (4 to 5o/oo PDB) are associated with the conformable interval at the Maynardville/Copper Ridge Dolomite transition, which has been interpreted as a correlative conformity in sequence-stratigraphic terms. The excursion ended during deposition of the lower Copper Ridge Dolomite. In western North America the excursion started at the base of the Pterocephaliid Biomere (near the base of the Aphelaspis Zone). This well-documented excursion ended prior to the end of the Pterocephaliid Biomere, with the maximum excursion at the Sauk II/Sauk III unconformity. This supports the correlation between Late Steptoean (Dresbachian/Franconian) sea-level fall and the sequence boundary at the end of Cambrian Grand Cycle deposition in the southern Appalachians. The cause of this carbon-isotope excursion remains unclear. The excursion most likely reflects the enhanced burial of organic carbon promoted by ocean stratification, a warm nonglacial climate, and a sea-level maximum during the early Late Cambrian. The onset of regression may have contributed to the maximum carbon-isotope excursion by enhancing sedimentation rates, and by increasing organic productivity because of increased nutrient availability. The removal of carbon from the ocean surface may have caused a decrease in pCO2 of the atmosphere. The resulting cooling episode could have triggered an oceanic overturn bringing 12C-enriched bottom waters to the surface, which in conjunction with oxidation of organic matter during the sea-level fall, ended the carbon-isotope excursion. Comparison of 13C and 18O values of matrix samples to the associated cement phases provides insights into the relationship between isotope variations and depositional and diagenetic environments. 13C values of peritidal dolomicrite define a rather smooth stratigraphic variation curve, whereas the values for subtidal micrite have significant scatter resulting from involvement of organic matter in diagenesis. Fibrous to bladed calcite cement from the subtidal deposits has 13C and 18O values comparable to the associated micrite, suggesting precipitation from marine water and similar diagenetic modifications. Meteoric diagenesis may be responsible for the depletion of 13C and 18O in equant calcite cement relative to the micrite. For saddle dolomite cement, the depletion of 18O and 13C values similar to those for the peritidal dolomicrite, are consistent with formation during burial at elevated temperatures in a rock-dominated system. This study demonstrates the potential of applying carbon-isotope stratigraphy, developed in highly fossiliferous successions, to stratigraphic intervals with poorly constrained biostratigraphy. Such studies require evaluation of the effects of depositional environments and diagenesis upon the preservation of marine isotope signatures.