Growth dynamics, framework composition, and microstructure of giant early Neoproterozoic calcimicrobial reefs, Little Dal Group, Mackenzie Mountains, N.W.T

Large (ca. 500 m thick; kilometres in diameter), early Neoproterozoic calcimicrobial reefs from the Little Dal Group, Mackenzie Mountains, represent an ecological intermediate between two fundamental reef archetypes: comparatively simple stromatolite reefs typical of the Proterozoic, and biotically and sedimentologically complex reefs of the Phanerozoic. They provide a virtually unique opportunity to investigate the early evolution of one of Earth's most important and complex ecosystems.;Four shale to limestone cycles in off-reef strata record fluctuating paleoenvironmental conditions. Shale-dominated strata reflect relative sea-level rise, and are associated with reef nucleation, aggradation, and contraction; carbonate-dominated strata reflect episodes of probable basin restriction (relative sea-level lowstand or stillstand), and are geometrically related to reef progradation and talus-shedding. These patterns are comparable to similar phenomena associated with Phanerozoic reefs.;Reef framework consists predominantly of calcimicrobial fabric that is compositionally uniform at the petrographic scale. Nonetheless, mesoscopic framework possesses a wide range of morphotypes. Steeply encrusting layers, anastomosing digits and nodules, and well ordered, erect columns are members of this spectrum, each prevailing at different stages of reef growth. Thus, although the microbial community in general had a wide range of environmental tolerance, specific environmental conditions, specifically water turbulence and light levels, led to the development of a depth-related zonation of reef framework elements.;The Little Dal reefs are the oldest reefs composed primarily of calcimicrobes. Variations in timing of post-mortem calcification resulted in variable preservation of a single progenitor community of filamentous cyanobacteria, resulting in a range of calcimicrobial, thrombolitic, and stromatolitic microstructures.;On the basis of ecological and sedimentological attributes more characteristic of Phanerozoic buildups than Proterozoic ones, the Little Dal reef system is interpreted as the earliest known example of 'modern'-style reef growth, that served as a template for the Phanerozoic skeletal reefs that emerged in the Early Cambrian.