STRATIGRAPHIC EVIDENCE FOR EDIACARAN AND EARLY CAMBRIAN EXTENSIONAL FAULTING, MACKENZIE MOUNTAINS, NORTHWESTERN CANADA

The Mackenzie Mountains of northwestern Canada preserve several kilometres of well-exposed Cryogenian to Cambrian strata. However, tectonostratigraphic interpretation of these strata is hampered by changes in stratigraphic succession between adjacent structural panels. One influential model for the evolution of these strata emphasized the role of extensional faulting during early Windermere Supergroup deposition, relating it to an inferred rifting event. The same model downplayed the role of faulting at the Ediacaran-Cambrian transition, emphasizing erosional bevelling beneath a profound sub-Cambrian unconformity or unconformities. As a result, Ediacaran-Cambrian evolution of northwestern Canada has been viewed as different from the history recorded in the southern Canadian Cordillera, where evidence for early Cambrian extensional faulting is well documented. Recent bedrock mapping and stratigraphic studies indicate that the Neoproterozoic-Cambrian evolution of the Mackenzie Mountains should be reconsidered. Lithostratigraphic and biostratigraphic correlations reveal several cases of formations or biozones appearing abruptly, or increasing radically in thickness, from east to west between adjacent structural panels. Between the hanging wall of the Plateau Fault and the Sekwi Brook panel (about 30 km), a marine submember is added at the base of the upper member of the Backbone Ranges Formation (Terreneuvian) beneath the fluvial sandstone typical of this unit, and the Sekwi Formation (Cambrian Series 2) thickens radically by addition of a lower member and a four-fold increase in thickness of the upper member. Similarly, between the Sekwi Brook and June Lake panels (about 10 km), there is a three-fold increase in thickness of the Blueflower Formation (Ediacaran), coincident with the appearance of an additional unit (Ingta Formation) at the base of the Cambrian succession. These stratigraphic transitions may record the influence of ancient extensional structures that partly controlled the position of Laramide faults. The Ediacaran-Cambrian transition in the Mackenzie Mountains can be interpreted as the product of interplay between unconformity development and extensional faulting, a scenario more reminiscent of the southern Canadian Cordillera than has previously been appreciated.