Sedimentology and Geochemistry of Regressive and Transgressive Surfaces in the Gunflint Formation, Northwestern, Ontario

Abstract

The 1.878 Ga Gunflint and equivalent iron formations of the Animikie Basin were deposited during the period after the rise in atmospheric oxygen during the Great Oxidation Event, which began at approximately 2.4 Ga. The atmospheric composition, and especially oxygen content, during this period after the rise in atmospheric oxygen, is not well known. The Gunflint and correlative formations, having been deposited in a shallow-marine setting, might provide information on oxygen levels present in the atmosphere-ocean system at this time. This is further made possible as the rocks that make up these formations are excellent records of transgression and regression of the sea. This record is reflected in up-section changes in lithology of the rocks that make up these formations. Further, the strata show evidence of exposure indicative of maximum regression. This exposure would have allowed the sediments and rocks to interact chemically with the atmosphere and meteoric water at the time. Stromatolites developed in the near-shore deposits and the rocks upon which the stromatolites grew show evidence of exposure during the repeated relative sea level low stands. These rocks have an alteration pattern that depends on their lithology. The rocks that comprise the Archean crystalline basement below the Gunflint show an intense alteration pattern that includes the formation of large corestones as well as increases in Fe and Mn content. Their geochemistry is largely the result of alteration by basin derived fluids. The exposure surfaces in the grainstones that make up the Gunflint and equivalent formations exhibit cementation and brecciation of the grainstones directly beneath the stromatolites. Geochemical analysis performed on these rocks targeted the concentration of redox sensitive oxides and elements; Fe2O3, MnO, V, Cr, Mo and U. Also considered were the Ce and Eu anomalies present. These oxides and elements react differently in oxidizing vs. reducing fluids. Subsequently, this analysis showed a shift in the Ce anomaly throughout the stratigraphy. The change from negative Ce anomalies in the basal samples to the positive Ce anomalies in the middle and upper lowstand layers indicate a shifting redox boundary, where the lowest stromatolite layers had produced enough oxygen to balance the flux of Fe2+- rich seawater in a position away from them, whereas the middle and upper stromatolite areas had redox boundary at the surface of the stromatolite themselves. Enrichment in Cr and V, as well as depleted Eu anomalies in strata associated with lowstands, points to the mixing of oxic freshwater, probably a combination of surface runoff from the craton and diffusion of groundwaters, and the anoxic seawaters filling the basin.