ABSTRACT: Retrogressive thaw slumps: an open window into the Quaternary of the Richardson Mountains, NWT.

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Abstract

Retrogressive thaw slumps open a window into the Quaternary of Willow River, NWT. In many places throughout the valley they expose two units, a clayey till (~10m) underlying a diamicton (~5m). These units are the result of Pleistoceneglaciation and the events that followed during the Holocene. Located in the Richardson Mountains 30 km west of Aklavik, the valley was partially invaded by a glacial tongue of the Laurentide ice sheet. Progressing against topography, the tongue blocked the drainage and formed a glacial lake. The glaciolacustrine sediments were overridden by progressing ice and reworked into a clayey till rich in organics. Since the retreat, two elements have profoundly modified the landscape: (1) Ongoing incision of Willow River to 300 m during the Holocene; (2) Periods of retrogressive thaw slump activity. Five 14C dates were obtained from the Willow River cut which established a regional chronology. Ten 14C dates were obtained from the slump sites in the overlying diamicton which indicate periods of important thermokarst activity around 9.5 ka BP and a modern reactivation. Units similar to the Willow River units have been observed elsewhere in polycyclic retrogressive thaw slumps which helps us to understand the genesis of the overlying diamicton. It is interpreted as being an infilled fossil thaw slump, which is exposed by an active slump. Judging by the extensive distribution of the two units and by the many slump scars, we can infer that the retrogressive thaw slumps have been, and still are, a dominant geomorphological agent in Willow River landscape.

Bibliography of Canadian Geomorphology