COMPLEX NEOPROTEROZOIC MICROBIAL REEFAL FRAMEWORKS IN THE CALLISON LAKE FORMATION OF YUKON

The Neoproterozoic Era represents a critical transition in the nature of reef-building organisms, as it featured the first appearance of reef platforms with significant topographic relief built by diverse, depth-zoned, and enigmatic microbialites. Detailed documentation of reef fabrics in the Tonian and Cryogenian periods is required to better understand this transition and how it may relate to coeval environmental shifts, including the changing chemistry and potential oxygenation of the oceans, and Snowball Earth events. Here, we present new, exceptionally preserved microbial textures from the Tonian (ca. 745 Ma) Callison Lake Formation in Yukon, Canada. High-resolution logs of a carbonate ramp setting reveal three new textural facies, which are distinct from other Neoproterozoic microbialites at both meso- and micro-scale: a three-dimensional shrubby microbial framework, with large cavities filled with primary marine cement and internal sediment; complex fenestral cavities within micrite, with possible microbial growth downwards from the roof of the cavities; and cuspate chambered structures. The studied microbialites are found in close association with ooids, oncoids, and a range of stromatolite forms. We also present petrographic and mineralogical evidence for syn-sedimentary dolomite precipitation and authigenic phosphate precipitation, supporting models for high concentrations of seawater magnesium and phosphorus at this time. Further sedimentological and geochemical studies of these reef systems will continue to inform our understanding of the co-evolution of life and the marine environment during the Neoproterozoic.