THE GEOCHEMICAL AND STRATIGRAPHIC CONTEXT OF EDIACARAN FOSSILS IN NORTHWESTERN CANADA

Ediacaran strata in NW Canada contain Ediacaran type fossils, abundant trace fossils, large carbon isotope anomalies, and significant shifts in Fe speciation data that have been previously attributed to a rise in oxygen coincident with the first appearance of Ediacaran type fossils. Here we report new geological mapping, stratigraphic sections, Fe speciation, trace element, and C_carb, C_org and S isotope data from Ediacaran exposures across northwestern Canada. These data demonstrate that strata containing the Ediacaran fossils, such as Aspidella, preserved predominantly in Bouma C beds in the relatively distal Sekwi Brook sections, occur above an unconformity and are not correlative with the basal Ediacaran Sheepbed Formation at more proximal sections near Shale Lake, as had been previously argued based on its stratigraphic position beneath the Gametrail and Blueflower formations. The lower Ediacaran bearing units at Sekwi Brook, referred to as the informal June beds, are also isotopically distinct from both the Sheepbed Formation and the type Gametrail Formation. These data imply that the reported link between Fe speciation data and the first appearance of Ediacaran-type fossils in the Mackenzie Mountains reflects a miscorrelation. Instead, a large negative carbon isotope anomaly, potentially correlative with the Shuram anomaly, occurs in the transgressive sequence of the lowermost Gametrail Formation, directly below the first well-developed trace fossil assemblages, but above the FAD of Ediacaran fossils. Extremely depleted carbon isotope values also occur in the Blueflower Formation, but these carbonates are authigenic in origin. Together, these data lead to a more refined picture of oxidation and evolution in the terminal Ediacaran Period.