Abstract Giant reefs of the early Neoproterozoic Little Dal Group, Mackenzie Mountains, N.W.T., Canada, differ from most previously described Proterozoic buildups in containing a calcimicrobial and thrombolitic framework. Systematic vertical changes in composition permit the identification of five framework stages. Each stage contains a persistent community of calcimicrobes, yet the expression of element morphologies throughout the reefs is exceedingly varied, indicating that environment exerted the predominant control over framework attributes. Framework development is correlated with extrinsic paleoenvironmental controls, namely change in relative sea level. Deepest-water intervals are characterized by accretion of dense, layered crusts (Stage IV), intermediate water depths are reflected by intricately anastomosing, morphologically diverse framework elements (Stages I and 111), and shallowing on reef tops is expressed as thin successions of erect, well-ordered, columnar microbialites (Stages II and V). Reef growth occurred in low- to moderate-energy regimes, within the photic zone, on hard substrates, and in the absence of significant settling of carbonate or terrigenous mud. The growth window is interpreted to have been delimited by the base of the photic zone at depth, and by excessive fragmentation near the water surface. Optimal growth occurred in moderate water depths, between fair-weather wave base and a limit determined by light attenuation at depth. The Little Dal reefs record a major inflection point in the development of reefa) ecosystems: although they display a combination of attributes from both Proterozoic and Paleozoic reef ecosystems, there is a preponderance of Phanerozoic-style features, including mineralized reef-building organisms, complex framework complete with growth cavities containing internal sediment and synsedimentary cement, vertical and lateral framework zonation, and large-scale accretion style that varies with relative-sea-level change. They are therefore the earliest known representatives of “modern”-style reef growth.