Evolution and significance of an overfilled alluvial foreland basin: Burnside Formation (1.9 Ga), Kilohigok Basin, N.W.T., Canada

The Burnside Formation records the transition from marine shelf to largely alluvial conditions in the palaeo-Proterozoic (about 1.9 Ga) Kilohigok foreland basin in the Slave Province of the Canadian Shield. The Burnside Formation forms a thick (up to 3.5 km), N W-tapering siliciclastic wedge, representing a braided alluvial system that prograded transversely across the Slave Province. The stratigraphic architecture of this unfossiliferous, predominantly alluvial succession documents the location and distribution of unconformities caused by lithospheric flexure in a Proterozoic foreland basin. The locations of these unconformities demonstrate the changing locations of tectonic and sedimentary loads on both sides of the Silave craton. Associations of sedimentary facies and palaeocurrents indicate that initially a braid delta system drained longitudinally, and was restricted to the proximal part of the basin between the orogenic hinterland (the Thelon Orogen) and a syn-sedimentary flexural arch. Palaeocurrents for the lowermost braid delta facies in the proximal foreland were parallel to palaeocurrents in underlying foreland basin turbidites, parallel to the flexural arch closest to the Thelon Orogen, parallel to the strike of the orogen, and perpendicular to the transport direction of thrust-nappes directed out of the orogenic belt. An abrupt shift to a transverse palaeocurrent mode accompanied the onset of higher-gradient braided stream conditions across the entire foreland basin, showing that from this time, sediment supply exceeded subsidence in the proximal foreland. The long-lived, laterally extensive transverse dispersal and the presence of sedimentary facies lacking interbedded mud or sedimentary structures indicative of desiccation suggest that fluvial discharge was probably perennial. The sharp superposition of medial braid plain facies directly on marine shelf and deltaic facies in the distal foreland indicates that the transition to non-marine conditions marks an erosional unconformity over two coeval flexural arches. The eastern flexural arch relates to covergence on the eastern margin of the Slave Province and to thrust loading in the Thelon Orogen that created the Kilohigok foreland basin. The evolution of a passive margin on the western side of the Slave Province caused the other flexural arch. In the centre of Kilohigok Basin a profound unconformity exists at the top of the Burnside alluvial succession. This unconformity suggests that a single flexural arch existed at the end of alluvial sedimentation that may signify the migration and yoking together of the two previously independent flexural arches. This migration may have been caused by the combined mass of the thick alluvial sedimentary wedge over the eastern edge of the Slave craton, coupled with initial thrust loading and foreland basin development in Wopmay Orogen. The transition within a foreland basin from largely marine to largely alluvial deposition reflects changing subsidence patterns across the basin due to changes in rates of convergence and changing subsidence and sedimentation patterns due to erosional mass redistribution from the orogenic hinterland into the foreland basin. This transition appears to separate two distinct phases in the evolution of the Kilohigok foreland basin. In the first phase, active thrust loading and asymmetric flexural subsidence restricted alluvial facies near the orog;en. In the second phase, convergence slowed or ceased, and erosional unloading and uplift of the hinterland caused subsidence patterns to become more uniform across the foreland basin, promoting rapid progradation of alluvial facies across the basin. Palaeomagnetic and sedimentological data indicate that the basin lay in the trade wind belt and that humid or monsoonal climatic conditions existed at the time. This suggests that moist trade winds blew onto the Thelon Orogen on the side closest to the Kilohigok Basin. Such an orographic effect would have caused rapid erosion, localized the position of the Thelon orogenic front, increased the flux of material through the orogenic belt, and caused more extensive progradation of the alluvial system.