MESOPROTEROZOIC MICROBIAL MATS FROM THE ANGMAAT FORMATION AS CONSTRAINTS FOR SILICIFICATION

Early diagenetic chert is common in Meso- and Neoproterozoic peritidal successions and informs much of what we know about life of the Proterozoic; yet its origin is not well constrained. Previous models for the origin of early diagenetic chert have focused on highly evaporative environments, where enhanced silica saturation can overcome kinetic inhibitions to precipitation, and costal mixing zones, where pH changes are favorable to both carbonate dissolution and silica precipitation. Here we focus on microfossiliferous early diagenetic chert from the Mesoproterozoic Bylot Supergroup, northern Baffin Island. Silicified microbial mats preserve exceptional microfossil morphology and taphonomy. Petrographic examination of silicified microbial, carbonate, and evaporite phases suggest that silicification occurred at the sediment-water interface, penecontemporaneously with mat growth. In many cases, silica is the primary precipitated mineral phase, while in other cases, microfabrics indicate primary precipitation of both silica and carbonate phases. Mimetic replacement of carbonate, gypsum, and halite mineral phases is also present. These observations require that early diagenetic chert formed in a range of chemical environments capable of precipitation of primary carbonate phases, dissolution and replacement of carbonate phases with silica, and replacement of gypsum and halite.

Building on a previous mixing model, preserved fabrics within the Angmaat Formation chert were used to further constrain the geochemical requirements of silicification for early diagenetic chert. Hypothetical seawater solutions, seawater-derived brines, and the solutions produced by interaction with freshwater suggest that silicification is capable of occurring under a wide range of fluid chemistries. This model demonstrates the breadth of fluids capable of precipitating silica and mineral assemblages observed in early diagenetic chert.