Ice-flow history of the Laurentide Ice Sheet in the southwest Northwest Territories: a Shield to Cordillera transect

Abstract

Fieldwork conducted since 2010 by the Geological Survey of Canada under the GEM1 and GEM2 programs has revealed a more complex glacial history for the southern Great Slave Lake region of the Northwest Territories than was previously reported. New reconstructions of the Laurentide Ice Sheet paleo-ice flow history have been established from field observations of erosional and/or depositional ice-flow indicators (e.g., striae, bedrock grooves, till clast fabrics, and streamlined landforms), new geochronological constraints, and interpretations of glacial stratigraphy. Three distinct ice-flow phases are consistently observed in areas proximal to the western margin of the Canadian Shield between the Slave River near Fort Smith and Hay River further west. These phases are 1) an oldest southwest flow; 2) a long-term sustained ice flow to the northwest ; and, 3) a youngest west-southwest flow during Late Wisconsin deglaciation, which includes extensions of the Great Slave Lake and Hay River ice streams further east than previous mapped. At Hay River approaching the eastern limit of soft Cretaceous bedrock of the Western Canada Sedimentary Basin, the ice flow pattern no longer shows the aforementioned consistent chronology. From Hay River to the Liard River, near the zone where the Laurentide and Cordilleran ice sheet coalesced, a thinning ice profile, topographic highlands such as the Cameron Hills and Horn Plateau, and the deep basin that Great Slave Lake currently occupies, played a significant role on the dynamics of the Laurentide Ice Sheet during early ice advance, retreat during Marine Isotope Stage 3, Late Wisconsin advance and deglaciation. Other factors, such as increased sediment supply and clay content from Cretaceous shale bedrock were also significant in influencing ice-sheet behaviour. The role of elevated porewater pressures over subglacial clay-rich sediments controlled the extent and dynamics of several discordant ice streams in upland and lowland regions within the study area.