Abstract
Three‐dimensional surface visualization models derived from high‐resolution LiDAR data provide new information about the type and scale of erosional processes below Late Wisconsin palaeo‐ice streams traversing the boundary between Canadian Shield crystalline rocks with offlapping Palaeozoic limestones in central Ontario. The hard bed is directly analogous to that found below ice streams in East Antarctica and East Greenland and provides insight into the effects of abrupt changes in substrate type on subglacial processes. Erosion of hard crystalline Canadian Shield rock was largely ineffectual consisting of areal abrasion of rounded whalebacks and local lee side plucking. In contrast, fast flow over the strike of gently dipping well‐bedded and jointed Palaeozoic limestones cut large flow‐parallel grooves and ridges akin to mega‐scale glacial lineations reflecting intense abrasion below narrow streams of subglacial debris dominated by hard crystalline Shield clasts (erodents). Regionally extensive plucking of structurally weak, well‐jointed and bedded limestone produced large volumes of rubbly carbonate debris leaving a 25‐km‐wide belt of uncontrolled hummocky rubble terrain (long known as the Dummer Moraine in Southern Ontario) some 350 km long and locally as much as 10 m thick. Subglacial plucking and abrasion under fast flowing ice were highly effective in stripping limestone cover rocks from Precambrian basement, and over many glacial cycles, may have played a role in the location and excavation of numerous large and deep lake basins around the Shield–Palaeozoic boundary zone in North America.
Highlights
Three-dimensional surface visualization models derived from high-resolution LiDAR data provide new information about the type and scale of erosional processes below Late Wisconsin palaeo-ice streams traversing the boundary between Canadian Shield crystalline rocks with offlapping Palaeozoic limestones in central Ontario
In their review of modern and ancient hard bedded Antarctic ice streams Livingstone et al (2012: p. 98) stated that it is ‘surprising’ that so few studies have taken advantage of exposed palaeo-ice stream beds for comparison with modern ice streams whose beds cannot be directly observed (e.g. Schroeder et al 2014; Brisbourne et al 2017). This paper addresses this gap in knowledge of subglacial processes by mapping the well-exposed hard bed of Late Wisconsin palaeo-ice streams in Southern Ontario in central Canada composed of Precambrian crystalline and Palaeozoic sedimentary strata, using recently acquired high-resolution (0.5 m) LiDAR data (Ontario Ministry of Natural Resources and Forestry Ontario, 2018; Fig. 1A)
High-resolution topographic images generated by processing of newly available LiDAR data reveal new details of the hard crystalline and sedimentary rock bed of fast flowing ice within the Late Wisconsin Laurentide Ice Sheet in central Ontario
Summary
Schroeder et al 2014; Brisbourne et al 2017) This paper addresses this gap in knowledge of subglacial processes by mapping the well-exposed hard bed of Late Wisconsin palaeo-ice streams in Southern Ontario in central Canada composed of Precambrian crystalline and Palaeozoic sedimentary strata, using recently acquired high-resolution (0.5 m) LiDAR data (Ontario Ministry of Natural Resources and Forestry Ontario, 2018; Fig. 1A). The study area (4000 km2) was covered by the Laurentide Ice Sheet (LIS) during the Late Wisconsin glacial maximum and extends some 350 km along the southern periphery of the Canadian Shield and the northern limit of overlying Palaeozoic platformal sedimentary strata This sharp boundary extends from southern Georgian Bay in the west to upper New York State east of Lake Ontario and the St. Lawrence Valley (Fig. 1).
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