PALAEO-ICE STREAMS OF THE SOUTHWEST LAURENTIDE ICE SHEET

Former ice streams of the SW Laurentide Ice Sheet have been identified using regional glacial geomorphology and sedimentology. During ice sheet recession from western Canada some ice streams appear to have terminated in surge lobes, as documented by the recognition of a surging glacier landsystem. During the LGM, the most prominent ice streams produced a 1,000km long swath of subglacial lineations from the Athabasca basin in the north to the Milk River Ridge in the south. Landforms within the ice stream trunk display high levels of spatial coherency suggesting that they represent the bed at at an instant in time ('rubber stamped'), whereas at the margin, ice stream recession is recorded by large transverse ridge complexes (end moraines) in the Red Deer and Bow river drainage basins. The moraines comprise multiple ridge crests, often comparable in form to glacitectonic thrust ridges. A glacitectonic origin is supported by exposures through thrust and deformed stacks of Quaternary sediments and bedrock. Additionally, spectacular subglacial meltwater channels may have contributed large volumes of water for localized ice stream decoupling.

The lack of thick sediment sequences on the Milk River Ridge also appears to have dictated the nature of ice stream marginal deposition. Here, minor flutings fan out to numerous small push moraines, similar to the landform assemblages produced by contemporary actively receding temperate glacier snouts. Overridden thrust moraines are evident in the Pakowki basin in SE Alberta, confirming a spatial relationship between large scale thrusting and thick, pre-existing sediment sequences. Additionally, thick sequences of tills, glacitectonites and associated bedrock mega rafts crop-out on the down-ice sides of buried valley systems. Ice stream beds can contain palimpsests of their initial advance phase, and late stage streaming moulds and redistributes pre-existing depo-centres/moraines so that the occurrence and pattern of bed deformation and sliding is dictated by the localized “continuity” of subglacial materials.