Evidence of Cordilleran Late Wisconsinan glaciers in the ‘ice-free corridor’

Abstract

Glaciers flowing down the Athabasca River valley during the last glacial period were fed both by local montane glaciers and by Cordilleran ice moving easterly across the Continental Divide from British Columbia, resulting in a major eastward surge into the ‘ice- free corridor’. The timing of this event has been disputed for many years and has important implications for the study of paleoenvironmental conditions and the north-south migration of biota in the corridor. Chronologic and lithostratigraphic data from the Jasper region suggest that this event took place in the Late Wisconsinan. Subtill radiocarbon dates on wood in the Athabasca River valley indicate that non-glacial alluvial sediments began accumulating there sometime before 48 ka. A finite date of 29,100 ± 560 BP (GSC 3792) on wood under till near Jasper provides a maximum age for the onset of the last glaciation in the region, although ice from west of the Rocky Mountain Trench probably did not cross the Continental Divide until after 21.5 ka. The occurrence of supraglacial metamorphic erratics as well as the distribution and nature of mapped glacial deposits and the trend of subglacial paleoflow indicators in the region east of the Rockies suggests that Late Wisconsinan glaciers flowing from the mountains coalesced with Laurentide glaciers and were deflected southeasterly along the mountain front. An even more extensive glacial event may have effected the region in pre-Late Wisconsinan times. The model of Late Wisconsinan glaciation presented here is in agreement with all known radiocarbon data from the area and is especially attractive when supratill dates on non-woody lacustrine organics are viewed in light of possible contamination by old carbon.