A LATE CAMBRIAN POSITIVE CARBON-ISOTOPE EXCURSION IN THE SOUTHERN APPALACHIANS - RELATION TO BIOSTRATIGRAPHY, SEQUENCE STRATIGRAPHY, ENVIRONMENTS OF DEPOSITION, AND DIAGENESIS

A positive carbon-isotope excursion is recorded within the Upper Cambr ian sedimentary succession in the southern Appalachians that consists of the Nolichucky Shale, the Maynardville Formation, and the Copper Ri dge Dolomite, The lower part of the succession contains Aphelaspis zon e fauna (Early Steptoean), The extensively dolomitized and poorly foss iliferous nature of the upper part of the succession precludes any det ailed biostratigraphic determinations. Correlation with similar positi ve 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 Ame rica provides a means for the application of carbon-isotope stratigrap hy, In the southern Appalachians the excursion started during depositi on of the upper Nolichucky Shale, Maximum delta(13)C values (4 to 5 pa rts per thousand PDB) are associated with the conformable interval at the Maynardville/Copper Ridge Dolomite transition, which has been inte rpreted as a correlative conformity in sequence-stratigraphic terms. T he excursion ended during deposition of the lower Copper Ridge Dolomit e, In western North America the excursion started at the base of the P terocephaliid Biomere (near the base of the Aphelaspis Zone). This wel l-documented excursion ended prior to the end of the Pterocephaliid Bi omere, with the maximum excursion at the Sauk II/Sauk III unconformity . This supports the correlation between Late Steptoean (Dresbachian/Fr anconian) sea-level fall and the sequence boundary at the end of Cambr ian Grand Cycle deposition in the southern Appalachians. The cause of this carbon-isotope excursion remains unclear. The excursion most like ly reflects the enhanced burial of organic carbon promoted by ocean st ratification, a warm nonglacial climate, and a sealevel maximum during the early Late Cambrian, The onset of regression may have contributed to the maximum carbon-isotope excursion by enhancing sedimentation ra tes, and by increasing organic productivity because of increased nutri ent availability. The removal of carbon from the ocean surface may hav e caused a decrease in p(CO2) of the atmosphere, The resulting cooling episode could have triggered an oceanic overturn bringing C-12-enrich ed bottom waters to the surface, which in conjunction with oxidation o f organic matter during the sealevel fall, ended the carbon-isotope ex cursion. Comparison of delta(13)C and delta(18)O values of matrix samp les to the associated cement phases provides insights into the relatio nship between isotope variations and depositional and diagenetic envir onments, delta(13)C values of peritidal dolomicrite define a rather sm ooth stratigraphic variation curve, whereas the values for subtidal mi crite have significant scatter resulting from involvement of organic m atter in diagenesis. Fibrous to bladed calcite cement from the subtida l deposits has delta(13)C and delta(18)O values comparable to the asso ciated micrite, suggesting precipitation from marine water and similar diagenetic modifications. Meteoric diagenesis may be responsible for the depletion of C-13 and O-18 in equant calcite cement relative to th e micrite, For saddle dolomite cement, the depletion of O-18 and delta (13)C values similar to those far the peritidal dolomicrite, are consi stent with formation during burial at elevated temperatures in a rock- dominated system. This study demonstrates the potential of applying ca rbon-isotope stratigraphy, developed in highly fossiliferous successio ns, to stratigraphic intervals with poorly constrained biostratigraphy . Such studies require evaluation of the effects of depositional envir onments and diagenesis upon the preservation of marine isotope signatu res.