Abstract

Early Neoproterozoic reefs of the Little Dal Group, Northwest Territories, are built by stromatolites and thrombolites containing calcified filamentous cyanobacteria and interstitial cement. Micritic and microcrystalline carbonate grew in or on extracellular cyanobacterial sheaths, preserving filaments when mineralization was early relative to sheath degradation, or grumeaux when mineralization was later. Filamentous microstructure is volumetrically predominant in the reefs; less common are micritic and grumelous microstructures already known from late Proterozoic stromatolites and Phanerozoic thrombolites. Textural intergradation of filamentous-calcimicrobial microstructure with these non-filamentous microstructures reflects microstructural variation developed through differential preservation at the scale of individual 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 be derived from a single progenitor community. This suggests that taphonomic variables may be as important in the development of microbialite microstructure as the biology of the microbial mat community. It also challenges recent suggestions that the Neoproterozoic increase in thromboids was related to the rise of multicellular organisms. These conclusions have broad implications for the interpretation of fossil microbialites, many of which might have been more closely related in origin than hitherto suspected.

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