The Nonacho Basin has classically been considered a large intracontinental basin formed in a regime of sinistral strike-slip in response to the collision of Rae and Slave cratons at around 1.9 Ga. Strata of the Nonacho Group are sandstone-dominated and have hitherto been related to deposition in a variety of continental settings, including scree slope and alluvial fan to fluvial and lacustrine. Previously published data on provenance is limited to U-Pb dating of detrital zircon from three individual samples; the occurrence therein of an age peak at 1969 Ma corroborates a geodynamic link between generation of the Nonacho Basin and convergent tectonics along the borders of the Rae Craton. However, poly-phasic folding and associated schistosity, (sub-)greenschist-facies metamorphism, and local development of shear zones have thus far hampered the establishment of a regional provenance framework supported by paleocurrent analysis. In other words, it currently remains unknown whether the detritus deposited in the Nonacho Basin was derived from dismantling of the Thelon-Taltston torographic range or elsewhere. In a broader sense, the lack of paleogeographic constraints on such a large and centrally located basin within the core of the Laurentian plate hampers our full understanding of Paleoproterozoic geodynamics.
To fill such a gap is one of the goals of the Nonacho Bedrock Mapping Project – a multi-disciplinary geoscience initiative led by the Northwest Territories Geological Survey in collaboration with a number of Canadian institutions including Laurentian University. In the summer of 2019, preliminary sedimentologic and stratigraphic investigations were undertaken in the area surrounding Nonacho and MacInnis lakes. Selected units were also sampled for U-Pb dating of detrital zircons. We recognized the originally proposed stratigraphic units of, from the bottom, the Hjalmar, Tronka Chua, Chief Nataway, Newshethdezza, Thekulthili, and Taltson formations. Central to our fieldwork was the detailed bed-by-bed stratigraphic logging and facies analysis of the Tronka Chua and Chief Nataway formations, each of which revealed an abundance of sedimentologic indicators pointing to nearshore-marine – rather than fluvial-lacustrine – deposition, e.g., pervasive wave-ripple cross-lamination, abundant hummocky-cross stratification, and occasional tidal indicators such as bundles and herring-bone cross-stratification. Ongoing research is currently focused on: refinement of the facies analysis; investigation of potential surfaces of stratigraphic unconformity within the Nonacho Basin; the U-Pb age distribution of detrital zircons from the all the formations in the basin; and the analysis of a dataset of paleocurrent indicators derived from cross-beds in the Tronka Chua, Newshethdezza, Thekulthili, and Taltson formations.