Talk
Mountains graphic with white diamonds on itGeoscience and Exploration

Reconsidering the Backbone Ranges Formation, a Problematic Ediacaran-Cambrian Unit in the Mackenzie Mountains, NW Canada

Thursday, November 21, 2019 - 8:40am to 9:00am Theatre One

Author(s)

R.B. MacNaughton (Presenting)
Geological Survey of Canada
K.M. Fallas
Geological Survey of Canada

The Backbone Ranges Formation is widely distributed in the Mackenzie Mountains of northwestern Canada. From its type area in NTS 95L (Glacier Lake map area) it has been mapped north and west along strike through six 1:250 000 map areas, to the easternmost Yukon (eastern NTS 106C). In its type area, the Backbone Ranges Formation is more than 1 km thick, lies unconformably upon the Ediacaran-aged Sheepbed Formation, and consists of a basal siliciclastic member, a middle carbonate member, and an upper member of cliff-forming quartz arenite. None of the members has yielded fossils in the type area. Early workers treated the Backbone Ranges Formation as the oldest Cambrian unit above the sub-Cambrian unconformity. Later workers recognized a karstic unconformity atop the middle member, and suggested that the lower two members were Ediacaran and the upper member Cambrian. Detailed correlations with more distally deposited Ediacaran units (Windermere Supergroup) and pre-trilobite Cambrian formations remained uncertain. 

In Bonnet Plume Lake map area (NTS 106B), the lower and middle members of the Backbone Ranges Formation resemble their manifestations in the type area, but the upper member contains previously unrecognized internal subdivisions. In ascending order, these are: a semi-resistant unit of maroon, tan, and grey sandstone and siltstone; a recessive package dominated by brown siltstone; a resistant carbonate marker that is a few metres thick in the study area but thickens markedly to the south and west; a recessive unit of brown siltstone and sandstone; and a unit dominated by cliff-forming quartzite. New occurrences of Ediacaran megafossils and Cambrian trace fossils suggest that the Ediacaran-Cambrian boundary is at the top of the carbonate marker, which likely correlates with the Risky Formation. Regionally, the top of the Risky Formation is a karstic, unconformable contact that delineates both the top of the Windermere Supergroup and the sub-Cambrian unconformity.

The new work emphasizes several issues with the Backbone Ranges Formation. First, the formation contains two unconformities (top of middle member; top of carbonate marker in upper member). Second, because the new units just described are subdivisions of the upper member, they must at present to treated as submembers, despite being regionally mappable and as thick as many formations in the Mackenzie Mountains. Third, it is undesirable to have the top of a supergroup be correlative with the top of a submember in the same region, as is implied by correlation of the sub-Cambrian unconformity. There is a long-standing need for revision, and probably abandonment of the Backbone Ranges Formation. On updated GSC maps for northern NTS 106B (in progress) the lower and middle members will be assigned new formation names, as will the subdivisions of the upper member. The new units will permit more detailed mapping of geological relationships, while contributing to more precise Ediacaran-Cambrian correlations and tectonic models in the Mackenzie Mountains and regionally.