Selective dolomitization of Cambrian microbial carbonate deposits: A key to mechanisms and environments of origin

The Upper Cambrian Maynardville Limestone (Conasauga Group) and the lower Copper Ridge Dolomite (Knox Group) of the southern Appalachians contain a variety of microbial carbonate deposits, such as stratiform (cryptalgal laminates) laterally linked hemispheroidal (LLH), vertically stacked hemispheroidal (SH), columnar, and digitate stromatolites, as well as thrombolites. Digitate stromatolites and thrombolites, in most cases, do not exhibit evidence of significant dolomitization, even though they are commonly embedded within completely dolomitized deposits. These microbial deposits 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. Extensively dolomitized microbial deposits (LLH, SH, and columnar stromatolites, and most of stratiform stromatolite laminae) formed primarily by the trapping of sediment in supratidal and intertidal environments on arid to semi-arid tidal flats. 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. Extensive dolomitization altered these peritidal carbonate sediments early in their diagenetic history (penecontemporaneous or syngenetic dolomitization). The presence of both calcitic and dolomitic Laminae is indicative of a combination of trapping and calcification. for the formation of stratiform stromatolites. The formation. of Upper Cambrian microbial deposits was primarily controlled by the conditions within the environments of deposition, but was also biotically influenced to some degree.