Les glissements rétrogressifs de fonte de la rivière Willow, Territoires du Nord-Ouest, Canada: Caractéristiques sédimentologiques, distribution spatiale et temporelle

Abstract

The Willow River (Richardson Mountains, N.W.T.) drainage basin is located at the western limit reached by the Laurentide ice-sheet during the Last Glacial Maximum (LGM). The icesheet covered the eastern fringe of the Mountains and deposited a lodgment till. The textural maturity and the lithology of the erratics indicate a distant origin to the till. During deglaciation, the retreating ice-sheet provided a supply of melt water to the aggrading permafrost. The resulting ice rich permafrost has been the host to two periods of thermokarst activity. The first, synchronous with the early Holocene warm period, led to an increase in active layer thickness and slope instability. The second, more recent, dates to the Little Ice Age and may have resulted from an increase in either fire frequency or intensity, or in modifications to the vegetation cover. Today, numerous active and inactive retrogressive thaw slumps can be seen throughout the drainage basin, but their distribution is restricted to the LGM. The slumps are polycyclic in nature and their headwall typically expose 2 units: the lodgment till (unit 1) overlain by a diamicton (unit 2) separated by a thaw unconformity. The latter is not associated to the paleo-active layer observed elsewhere in the Canadian northwest. Air photographs show an increase in thermokarst activity during the two last decades.