The Avalanche Lake rock avalanche, Mackenzie Mountains, Northwest Territories, Canada: description, dating, and dynamics

• Published in: Canadian Geotechnical Journal
• Main Authors: Evans, S.G., Hungr, O., Enegren, E.G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1994
• Subjects: Avalanche Lake; Canada; Northwest Territories; glacier*; Mackenzie mountains
• Online Access: http://dx.doi.org/10.1139/t94-086; http://www.nrcresearchpress.com/doi/pdf/10.1139/t94-086

At Avalanche Lake, located in the Backbone Ranges of the Mackenzie Mountains, about 200 × 10 6 m 3 of massive Devonian carbonate rock slid down remarkably planar bedding surfaces dipping at 30° and created a spectacular runup on the opposite valley side onto a topographic feature called the Shelf. The interpretation of events at Avalanche Lake has recently been subject to controversy. It has been argued by other workers that the rock avalanche could not have run onto the Shelf without glacier ice partially filling the valley, thus reducing the magnitude of the actual runup, and implying that the rock avalanche took place at the end of the Pleistocene. Evidence is presented indicating that the rock avalanche occurred in an ice-free environment. It consists of the nature of the detachment surface, the morphology and location of the rock avalanche debris, the presence of levees in the debris and isolated patches of debris on valley-side slopes, and the entrainment of alluvial deposits and conifer fragments from the valley floor in the Shelf Lobe debris. In addition, radiocarbon ages obtained from entrained wood in the debris, converted to calendric years, indicate that the landslide took place in this millennium, with a 95% probability of it having occurred no earlier than 1440 A.D. No glacier ice then existed in the valley. Based on this evidence the behaviour of the rock avalanche is reconstructed. It is characterized by dramatic mobility in which the rock avalanche split into two parts. The west part smashed into the opposite valley side and about 5 × 10 6 m 3 rode up onto the Shelf. The remainder (155 × 10 6 m 3 ) fell back into the valley, partially running back up the detachment surface to an elevation 360 m above the valley, and then, reversing direction again, ran back into the valley bottom where it was deposited. The east part, the South Lobe (40 × 10 6 m 3 ), ran down a valley reentrant opposite the detachment surface. The maximum vertical drop in the path is 1220 m, and the maximum runup is 640 m. The ...