Ice stream catchments and till production records in core regions of the Laurentide Ice Sheet

English Abstract:

Evidence of crosscutting flowsets in core regions of the Laurentide Ice Sheet (LIS) has lead to the idea that ice divide migration is one important factor of landscape evolution and till production in these areas, with deglacial records being possibly over-represented. Here we report on evidence suggesting that ice stream catchments extending far inside ice sheets is another important mechanism to consider for certain flowsets and to explain thick till successions in core regions of the LIS. In ‘ice divide’ areas, such as central mainland Nunavut on the Canadian Shield, geophysics, drillcores, and river sections show that till successions 10s of meters thick occur in places and appear to extend laterally across the till plain. In most cases, the thickest till sheet has a lithological provenance and fabric signature consistent with the orientation of a prominent landform imprint that is partially overprinted by the deglacial record. These well-preserved, but overprinted flowsets fit the criteria for paleo-ice streams such as landform elongation, converging patterns, and overall dimension/geometry. When pieced together they appear to form large systems that can be associated to an ice flow phase also responsible for widespread till production. Hence, these preserved sediments and landforms are interpreted as the record of a large and thick LIS. The partially overprinting sediment-landsystems are confidently assigned to the deglaciation. In one study area in central Nunavut, the associated surficial retreat phase till is thin (0-5 m) relative to the underlying till, which reaches up to 12 m. Ice stream catchments extending far inside an ice sheet may thus contribute in a major way to the evolution of glacial landscapes and to the stratigraphy of core regions of ice sheets. Ice stream dynamics at times of ice sheet maxima is not yet well understood and represent one important knowledge gap towards understanding ice sheets.