SELECTIVE DOLOMITIZATION OF CAMBRIAN MICROBIAL CARBONATE DEPOSITS - AKEY TO MECHANISMS AND ENVIRONMENTS OF ORIGIN

The Upper Cambrian Maynardville Limestone (Conasauga Group) and the lo wer Copper Ridge Dolomite (Knox Group) of the southern Appalachians co ntain a variety of microbial carbonate deposits, such as stratiform (c ryptalgal laminates) laterally linked hemispheroidal (LLH), vertically stacked hemispheroidal (SH), columnar, and digitate stromatolites, as well as thrombolites. Digitate stromatolites and thrombolites, in mos t cases, do not exhibit evidence of significant dolomitization, even t hough they are commonly embedded within completely dolomitized deposit s. These microbial deposits formed primarily by calcification of cyano bacteria in lower intertidal and upper subtidal environments, which we re not primary sites for dolomitization. Early diagenetic calcificatio n of cyanobacteria additionally reduced the susceptibility of these de posits to dolomitization. Extensively dolomitized microbial deposits ( LLH, SH, and columnar stromatolites, and most of stratiform stromatoli te laminae) formed primarily by the trapping of sediment in supratidal and intertidal environments on arid to semi-arid tidal flats. Pervasi ve syndepositional calcification of cyanobacteria may have been preclu ded by conditions of periodic emergence, hypersalinity, and the presen ce of fine-grained sediment serving as competing sites for carbonate m ineral nucleation. Extensive dolomitization altered these peritidal ca rbonate sediments early in their diagenetic history (penecontemporaneo us or syngenetic dolomitization). The presence of both calcitic and do lomitic Laminae is indicative of a combination of trapping and calcifi cation. for the formation of stratiform stromatolites. The formation. of Upper Cambrian microbial deposits was primarily controlled by the c onditions within the environments of deposition, but was also biotical ly influenced to some degree.