EVOLUTION OF A NORTH AMERICAN MID-CONTINENTAL PALEO-ICE STREAM SYSTEM

New observations combined with re-evaluation of past work on deglacial dynamics across the North American mid-continent indicates that whereas past research has tended to attribute varying tills to varying ice dome dominance, observations now seem more consistent with formation and evolution of multiple paleo-ice streams having played a major role in the stability and decay, and thus sedimentation, of the Laurentide Ice Sheet. In Saskatchewan, new field work, analysis, and compilation of data on till provenance and sediment-landform assemblages of Laurentide Ice Sheet deposits were found to provide evidence for the presence of at least two large paleo-ice stream systems that were active after the Last Glacial Maximum (LGM) (Adams, 2009, Ross et al., 2009). Key aspects of the analysis included till fabric, carbonate content, airborne radiometric measurements (Th/K) and ⁴⁰Ar/³⁹Ar dating of hornblende grains as a provenance indicator, which collectively reveal dispersal patterns consistent with cross-cutting streamlined glacial landforms, thus indicating that the same ice flow was responsible for both the sediment transport and formation of landforms. These geomorphic and compositional determinations allowed three distinct phases of ice flow to be inferred, of which two were interpreted as paleo-ice streams (Adams, 2009). Similar reinterpretations can be made to the east. For example, in southeastern Manitoba, previous research by Teller and Fenton (1980) described five lithologic formations (tills) whose textural and mineralogical variability was attributed to a shift in ice dome dominance. In Minnesota, however, Lusardi et al. (2011) showed that the complex sequence of distinct Des Moines lobe tills had subtly different provenances that now seem most attributable to the evolution and varying dominance of nearby and competing ice stream catchment areas during Laurentide Ice Sheet drawdown, and in at least one instance, more than one ice stream simultaneously contributed to the lobe. It therefore can now be concluded that post-LGM mid-continental Laurentide glaciation was dominated by major glacial dynamic shifts as ice streaming underwent ongoing reorganization.