INFLUENCE OF MICROBIAL PROCESSES ON DIAGENESIS OF CARBONATE DEPOSITS: UPPER CAMBRIAN OF THE SOUTHERN APPALACHIANS

The Upper Cambrian Maynardville Limestone (Conasauga Group) and the lower Copper Ridge Dolomite (Knox Group) from the southern Appalachians contain a variety of carbonate microbialites. These include thrombolites and stromatolites of stratiform, domal, columnar and digitate morphologies. Selective dolomitization and isotopic compositions of these deposits provide useful information on their origin and diagenetic history.

Digitate stromatolites and thrombolites, in many cases remain calcitic even though they are commonly embedded within extensively dolomitized deposits. These microbialites formed primarily by calcification of cyanobacteria in lower intertidal and upper subtidal environments, which were not primary sites for dolomitization. Early diagenetic calcification of cyanobacteria additionally reduced the susceptibility of these deposits to dolomitization. This early diagenetic calcification, however, promoted the formation of growth cavities and borings, which served as conduits for fluids during later diagenesis of calcimicrobial deposits. This resulted in more pronounced diagenetic alterations of these deposits relative to the less porous, non-microbial deposits, as indicated by: (1) more negative oxygen isotope values of microbial calcimicrite in comparison with associated non-microbial calcimicrite; and (2) overlapping oxygen isotope values of microbial calcimicrite and non-marine carbonate cements.

Stratiform, domal and columnar stromatolites, by comparison, are commonly extensively dolomitized. These deposits formed by the trapping or agglutination of sediment in supratidal and intertidal environments on an arid to semi-arid tidal flat. Pervasive syndepositional calcification of cyanobacteria may have been precluded by conditions of periodic emergence, hypersalinity, and the presence of fine-grained sediment serving as competing sites for carbonate mineral nucleation. Therefore, these deposits were affected early in their diagenetic history by extensive penecontemporaneous or syngenetic dolomitization. These deposits do not differ substantially in their primary porosity from associated non-microbial dolomicrite and, consequently, they experienced a similar diagenetic history as suggested by their overlapping isotopic compositions.