GLACIAL GEOMORPHOLOGY AND SEDIMENTOLOGY OF TERRESTRIAL-TERMINATING FAST FLOW LOBES/ICE STREAM MARGINS IN THE SOUTHWEST LAURENTIDE ICE SHEET

Large arcuate assemblages of moraines and thick complex sequences of tills and associated glacigenic sediments in southern Alberta demarcate the lobate termini of corridors of fast ice flow trunk zones and thereby represent the terrestrial equivalents of the trough-mouth fans of the marine palaeo-ice stream record. Thick units or sequences of diamicton are explained as the product of two, often co-existing, depositional scenarios based upon their sedimentological characteristics, structural features and clast macrofabrics: 1) glacier-marginal till thickening and the stacking of till wedges in areas where the ice stream margin was stationary for short periods of time; 2) proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons associated with the advection of sub-marginal till into preglacial bedrock depressions. Additionally, the glacial overriding and deformation of valley infills and the dislocation and entrainment of bedrock rafts at the margins of bedrock valley walls has given rise to the widespread development of glacitectonites and megablocks in the ice-stream marginal depositional record. Such deposits are at their thickest over preglacial valley thalwegs and may play an important role in the siphoning of groundwater away from subglacial deforming layers and thereby promoting sticky spots on interfluves located down flow. Stacks of multiple subglacial traction tills potentially represent records of annual sub-marginal incremental thickening. This is consistent with the geomorphological evidence of densely spaced recessional push moraines that document the early retreat history of the palaeo ice streams in southern Alberta and which have been equated with seasonal climatic forcing and effective subglacial meltwater drainage. Thick sequences of stratified diamicton and glacilacustrine sediments relate to ice stream margin oscillations and debris influx into discontinuous proglacial lakes, a depositional scenario symptomatic of preglacial valley sediment sinks for materials that would otherwise have been used to construct sub-marginal till wedges and proglacial moraine ridges.