PRIMARY PRODUCER DOMINATED ENVIRONMENTS AND A LATE EDIACARAN ‘DIVERSITY CRISIS’ IN THE MACKENZIE MOUNTAINS, NORTHWEST CANADA (Invited Presentation)

The late Ediacaran to early Cambrian (E-C) transition records a seeming decline in diversity, in contrast to rich communities of macroscopic Ediacara-type biota preserved in older units worldwide. Our understanding of this biotic change and its environmental drivers suffers from stratigraphic incompleteness, as mid-upper Ediacaran and Cambrian strata are rarely exposed in a continuous sequence in the same area and often represent different depositional settings. An almost continuous succession of Cryogenian to mid-Cambrian strata in the Mackenzie Mountains, Northwest Territories records a broadly uniform depositional setting and allows us to assess biotic and environmental changes through this interval. We have investigated the carbonates and siliciclastics through the Blueflower, Risky, Ingta, Backbone, and Vampire formations, and integrated palaeontological and geochemical data (carbon and nitrogen isotope chemostratigraphy and palaeoredox proxies) to produce a high resolution biostratigraphic and chemostratigraphic profile for the E-C boundary interval. The Blueflower Formation hosts Ediacara-type macrofossils, including rangeomorphs. The E-C boundary occurs in the overlying Ingta Formation, and is estimated by sporadic occurrences of treptichnid traces. This unit lacks metazoan macrofossils and contain s simple traces, but is rich in cyanobacterial organic-walled microfossils and uniserial macroalgae similar to kelp. Complex traces and carbonaceous animal remains appear and diversify much higher in the stratigraphy, in the Vampire Formation. The absence of organically preserved animal remains in a setting otherwise conducive to organic preservation implies a true metazoan absence. These strata are further marked by oscillating palaeoredox conditions, dominated by anoxia. This integrative bio- and chemostratigraphic study of the E-C boundary interval in the Mackenzie Mountains shows an environmentally-driven diversity decline of complex fossils, and corresponds to the ‘Kotlinian crisis’ recognised on Baltica.