Subglacial Lake McGregor, south-central Alberta, Canada

It is proposed that a lake, here named “Subglacial Lake McGregor”, existed beneath the Laurentide Ice Sheet at, or near, the last glacial maximum. The lake resided in the ancient buried McGregor and Tee Pee preglacial valleys, which are now mostly filled with glacigenic deposits. The greatest thickness of sediment in the valleys is in the form of chaotically deposited lake beds that were laid down in a subaqueous environment by a number of process: gravity flow, water transport, and suspension settling. Topographic, sedimentary, and stratigraphic evidence point to a subglacial, not a proglacial, origin for the beds. During the early stages of lake existence, ice movement was significant as there are numerous sets of shear planes in the sedimentary beds. This indicates that the lake filled (lake sedimentation) and drained (shearing of the beds by overlying ice when ice contacted the bed) often. Thus, early in its history, the lake(s) was/were ephemeral. During the later stages of lake existence, the lake was relatively stable with no rapid draining or influx of sediment. Gradual drainage of the lake resulted in lowering of the ice onto the lake beds resulting in subglacial till deposition. Drainage was not a single continuous event. Rather it was characterized by multiple phases of near total drainage (till deposition), followed by water accumulation (lake sedimentation). Water accumulation events became successively less significant reflected by thinning of lake beds and thickening of till beds higher in the stratigraphic sequence. Since subglacial lake sedimentation appears to be restricted to the subglacial valleys, it is suggested that the valleys acted as a large-scale interconnected cavity system that both stored and transported water. It is also suggested that these acted as the main routes of water flow beneath the Laurentide Ice Sheet.