The identification of former terrestrial ice stream dynamics from geomorphic evidence and till architecture: A case study of southwestern Saskatchewan

Abstract

A multidimensional study, utilising geomorphological and sedimentological techniques, is conducted to investigate the former dynamics and regional till architecture of terrestrial ice streams during the last (Late Wisconsinan) deglaciation of the Laurentide Ice Sheet. Detailed mapping over a 57,400 km2 area of southwestern Saskatchewan reaffirmed previous proposals of a southwest trending ice stream demarcated by a corridor of megaflutes and mega-scale glacial lineations (Ice Stream 1). Extending from the Canadian shield to southwestern Saskatchewan this ice stream is cross cut by three (one previously unrecognised) southeast trending ice streams (Ice Streams 2A, B and C). Analysis of the lithologic and geophysical characteristics of 197 borehole samples within these corridors reveals a superimposed till and associated deposits comprising 17 stratigraphic units.

A 3D stratigraphic model of the 57,400 km2 swath was constructed, by extrapolating data away from boreholes using a nearest-neighbour approach. Using this model the thickness, extent and distribution of these stratigraphic units was delineated allowing the depositional history of the region to be reconstructed and thus the extent of till emplaced during ice stream operation through time and space to be inferred. Reconciling this regional till architecture with the surficial geomorphology reveals that surficial units are spatially consistent with a dynamic switch in flow direction recorded by the cross cutting corridors of Ice Streams 1, 2A, B and C. Thin tills at the centre of the trunk zone of Ice Stream 1 in many places lie unconformably over stratified sediments. This suggests widespread basal sliding may have been subordinate to subglacial sediment deformation but the general thickening of tills towards the lobate terminal margins is consistent with subglacial deformation theory. In addition, variations in till thickness are also recognised on a more localised scale. These variations are attributed to three processes; 1. down-ice thickening associated with buried valley margins; 2. upland thinning; and 3. thickening as a result of overridden glacial-marginal landforms. The significance of newly interpreted patterns of till deposition resulting from ice streaming are then considered and a model of ice stream till deposition is presented. This model provides a generalised view of the pattern of deposition resulting from fast flow over a unlithified sediment bed which may be used to infer the dynamic behaviour of other former ice sheets from their sediment imprint.