Microbialite Diversity and Ocean Redox Geochemistry of the Late Tonian Callison Lake Formation.

Microbialites provide a unique insight into ancient microbial processes and environments, but trends in the diversity of unlaminated microbialites remain poorly understood. The ca. 745 Ma Callison Lake Formation in the Yukon (northwest Canada) features a range of microbialites which are diverse at both the mesoscale and microscale. Four microbialite facies are recognised from the Ramp member of the Callison Lake Formation, all of which include framework structures with syn-depositional cavities. The thrombolite facies of the Callison Lake Formation is typified by exceptional preservation, with isopachous primary marine cements and distinct microclots that we suggest formed through early, mimetic carbonate precipitation. However, based on fine-scale textural relationships within the rock, we infer that the other microbialite textures were affected by diagenetic crystal growth, which reaffirms the importance of considering paragenetic history when describing and comparing ancient microbialites. The trace and rare earth element geochemistry of primary marine cements in the Callison Lake Formation microbialites suggests that the unit was deposited in a stratified, dominantly anoxic basin, with oxygenation above a very shallow chemocline. The relative depletion of chalcophile elements implies a euxinic depositional setting. This case study highlights the complexity of unlaminated microbialites and emphasises the need for detailed documentation of microbialites at multiple scales.