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 Nis ling 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 granulom etry and mineralogy of samples from each of these sites, allows a tent ative 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 sh ort-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 t ill. Upvalley from the gorge, these deposits are cut by a glaciofluvia l outwash terrace; this is attributed to the McConnell Glaciation. Fin ally, the most recent downcutting and alluvial deposition occur during the Holocene. This study also shows the mineralogical similarity of t he pre-Reid clayey subglacial till with the glacial and fluvio-glacial McConnell sediments in the Aishihik basin. Thus during both glaciatio ns, the ice tongues came from the same source-region, the St. Elias Mo untains.