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2026-08-26

The Tonian Period (1000–720 Ma) likely represents a critical transition in microbial reef construction, given the emergence of reefs built by thrombolites and other cavity-dense microbialites. However, there are few detailed studies on Tonian reef development to constrain this ecologically significant transformation. Here, we build on previous documentation of reefal buildups in the ca. 850780 Ma Fifteenmile Group in the Coal Creek inlier (Ogilvie Mountains) and contribute seven new detailed stratigraphic sections featuring microbial accumulations in the Reefal assemblage. We document the stratigraphic context for these Tonian reefs and distinguish between predominantly laminated and unlaminated microbial textures. In addition, we identify rare instances of exceptional preservation of unlaminated microbialite, suggesting that reef-building microbialites of the Reefal assemblage have similar framework morphologies to those of the approximately coeval Little Dal Group in the Mackenzie Mountains.

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