Sedimentology and Geochemistry of the 2930 Ma Red Lake–Wallace Lake Carbonate Platform, Western Superior Province, Canada

Abstract

AbstractChemical sedimentary rocks in the Red Lake–Wallace Lake area of the Canadian Shield form the Earth's oldest known carbonate platform and, as such, provide a unique opportunity to explore the floor of a 2930 Myr old warm, shallow sea. Peritidal depositional features dominate the platform top, ranging from colloform crusts, teepee structures, and evaporate pseudomorphs in the supratidal; pseudomorph crystal fans and laterally linked domal stromatolites, associated with stromatactis‐like structures, sheet cracks and liminoid fenestral fabrics extending from the lower supratidal through the intertidal; and herring‐bone cross‐stratification, isolated domal stromatolites and herring‐bone calcite cement in tidal channels. These lithofacies indicate a low energy, restricted evaporitic environment. Limited subtidal platform top deposits are characterized by laterally linked domal stromatolites and pseudomorph crystal fans on a larger scale than those found in the intertidal areas. A transitional lithofacies to deeper water were deposited further offshore. It consists of ribbon rock (mixed laminae and beds of carbonate, slate and iron oxide sediments), slump structures composed of intraclastic carbonate lithoclasts in a marl matrix and carbonate‐associated iron formation. Basinal deposits consist of chert and chert‐oxide facies iron formation. Peritidal lithofacies are composed of ferroan dolomite, whereas deep subtidal to upper slope lithofacies are composed of calcite. The dolomites have O and Sr isotopic ratios which were probably reset during dolomitization, whereas only O is altered in the limestones. The δ13C values for all the carbonate samples were 0 ± 1·1‰V‐PDB, with samples formed in deeper water having lighter δ13C values, suggesting the deeper open ocean had a lighter δ13C budget than water in the platform interior. Post Archean Australian Shale normalized rare earth element patterns for the carbonate samples have positive La and Eu anomalies, suprachondritic Y/Ho ratios and slight heavy rare earth element enrichment in most samples. Basin lithofacies are characterized by heavy rare earth element enrichment and positive Eu anomalies. One sample had a significant negative Ce anomaly. These data probably indicate restricted circulation in the supra and intertidal areas and possibly the development of spatially limited areas where oxygen production could move the redox boundary out into the water column.