GLACIAL LAKE NISLING AND THE PLEISTOCENE IN THE UPPER BASIN OF THE NISLING RIVER IN YUKON

Glacial Lake Nisling and the Pleistocene in the upper basin of the Nisling River in Yukon. The study area is the Nisling Valley between its tributaries Mackintosh and Stevens, in southwestern Yukon. Sedimentary facies analyses at seven sites, together with comparisons of granulometry and mineralogy of samples from each of these sites, allows a tentative reconstruction of Pleistocene events in and around the gorge of the Nisling Valley. We identify a clayey subglacial till corresponding to a pre-Reid glaciation, followed by a melting event represented by a silt unit. The Reid Glaciation appears as a prograding delta in a short-lived pro-glacial lake. During this time, the gorge was dammed by an ice tongue, and the lake covered the upper basin of Nisling Valley. The delta was then overriden by the glacier, which left an ablation till. Upvalley from the gorge, these deposits are cut by a glaciofluvial outwash terrace; this is attributed to the McConnell Glaciation. Finally, the most recent downcutting and alluvial deposition occur during the Holocene. This study also shows the mineralogical similarity of the pre-Reid clayey subglacial till with the glacial and fluvio-glacial McConnell sediments in the Aishihik basin. Thus during both glaciations, the ice tongues came from the same source-region, the St. Elias Mountains.