Taphonomic control on microstructure in early neoproterozoic reefal stromatolites and thrombolites

Early Neoproterozoic reefs of the Little Dal Group, Northwest Territories,a re built by stromatolites and thrombolites containing calcified filamentous cyanobacteria and interstitial cement. Micritic and microcrystalline carbo nate grew in or on extracellular cyanobacterial sheaths, preserving filamen ts when mineralization was early relative to sheath degradation, or grumeau x when mineralization was later. Filamentous microstructure is volumetrical ly predominant in the reefs; less common are micritic and grumelous microst ructures already known from late Proterozoic stromatolites and Phanerozoic thrombolites. Textural intergradation of filamentous-calcimicrobial microst ructure with these non-filamentous microstructures reflects microstructural variation developed through differential preservation at the scale of indi vidual filaments and laminae. Textural gradients from filaments to grumeaux , and from calcimicrobial to stromatolitic and thrombolitic microstructure types, imply that a wide variety of microbialite microstructure types can b e derived from a single progenitor community. This suggests that taphonomic variables may be as important in the development of microbialite microstru cture as the biology of the microbial mat community. It also challenges rec ent suggestions that the Neoproterozoic increase in thromboids was related to the rise of multicellular organisms. These conclusions have broad implic ations for the interpretation of fossil microbialites, many of which might have been more closely related in, origin than hitherto suspected.