SAND-WEDGE AND VENTIFACT PALEOENVIRONMENTAL INDICATORS IN NORTH-WEST SASKATCHEWAN, CANADA, 11 KA TO 9.9 KA BP

Exhumed sand wedges and ventifacts in the upper Churchill watershed of north-western Saskatchewan indicate that aeolian and periglacial proc esses were dominant from c. 11 to c 10.5 ka sp. Nineteen sand wedges w ere identified that display vertical foliation, form polygonal nets, a nd penetrate either till, glaciolacustrine sediment or glaciofluvial d iapiric sand. The sand wedges, and sand lenses within the till and gla ciolacustrine sediments, indicate permafrost aggradation after glaciat ion and lake drainage. Ventifacts that overlie the sand wedges have pr eferred keel orientations that correlate with palaeo-katabatic wind di rections interpreted from stabilized parabolic dune orientations. Rat- tail lineaments on faceted surfaces of coarse-grained, granitic ventif acts also indicate palaeo-wind flow direction. Strong and weakly devel oped lineaments on the same einkanter are explained by opposing flow f rom prevailing winds, katabatic winds and anticyclonic air flow from t he adjacent Laurentide Ice Sheet, or by flow separation at the ventifa ct keel under unidirectional air flow. Two geomorphic surfaces are ide ntified: (1) an older surface with sand wedges and weakly developed ve ntifacts indicative of permafrost and brief aeolian activity; and (2) a lower and younger segment of the former Lake Agassiz basin, without permafrost indicators but with well-developed ventifacts. The upper su rface, developed in association with the Beaver River Moraine and glac ial Lake Meadow-McMurray, is 11-10.5 ka sp in age, synchronous with th e Younger Dryas cold period. The younger, lower surface was last modif ied beginning at 9.9 ka BP, when the north-west outlet of Lake Agassiz opened, was susceptible to greater aeolian reworking than the upper s urface, and postdates the Younger Dryas.