Differing styles of stromatolite diagenesis: Implication for stromatolite origin and diagenesis of Upper Cambrian peritidal carbonates

Stromatolite morphology and the characteristics of associated sediments are commonly used to discern environments of stromatolite formation. In this study, the authors suggest that study of stromatolite diagenesis allows a better understanding of the origin of stromatolites and the diagenesis of carbonate sediments. The Upper Cambrian Maynardville Formation of the Conasauga Group in Tennessee consists of subtidal and peritidal carbonate lithologies. These contain several types of cryptalgal structures: (1) cryptalgal (microbial) laminites; (2) laterally linked hemispheroidal stromatolites; (3) stacked hemispheroidal stromatolites; (4) columnar stromatolites; (5) digitate stromatolites; and (6) thrombolites. The succession from 1 to 6 above represents increasing water turbulence from supratidal to upper subtidal depositional environments. Stromatolites form by two major mechanisms: (1) the trapping of sediment grains by microbial mats; and (2) the calcification of cyanobacterial sheaths under favorable environmental conditions. These two mechanisms of formation can be discerned in the fossil record based on stromatolite response to diagenesis. Early calcification by cyanobacteria reduces the susceptibility of stromatolites to dolomitization. Biogenic calcification was not pervasive during formation of dolomitized stromatolites due to conditions of periodic emergence and increased salinity. Under these conditions, dolomitization was an important diagenetic process that preceded early calcification. The authors interpret dolomitization tomore » be penecontemporaneous with deposition. The results of this study confirm that differing styles of stromatolite diagenesis can be used as indicators of stromatolite origin and to aid in reconstructing the diagenetic processes in peritidal carbonates.« less