THE ST. LAWRENCE ICE STREAM LANDFORM ASSEMBLAGE – NEW INSIGHT BASED ON LIDAR SURVEYS

In the lower St. Lawrence valley, between Québec City and Rivière-du-Loup, a large system of glacially streamlined terrain is herein recognized as part of the St. Lawrence Ice Stream (SLIS) footprint; a paleo ice stream that operated from about 17 to 14 ka cal BP. Below the limit of post-glacial marine submergence (155 mASL in Rivière-du-Loup), most glacial lineations have either been obliterated or extensively reworked by waves. However, above the marine limit, ice stream indicators and other ice flow features are generally well preserved and are ubiquitous at all scales. Thousands of bedform features have been mapped and interpreted using several regional airborne LiDAR elevation datasets, as well as land elevation data from NASA SRTM (1-Arc Second Global). Ice stream features include patterns of parallel and elongated partly eroded bedrock ridges or knobs followed by long lee-side tails of glacial sediments , mega-scale glacial lineations (MSGLs), glacially lineated hard beds, shear margin moraines, sticky spots, mega-grooves and over-deepened furrows. The recorded length of MSGLs varies between 20 m and 10 km.

This assemblage of ice flow features unequivocally shows that the last ice flow features recorded in the lower St. Lawrence valley converged into tracts of parallel lineations, defining the footprint of a 40 km-wide and over 200 km long ice stream flowing towards the north-east. Onset zones and tributary systems can be observed on both shores of the St. Lawrence River, most notably in the areas of the Charlevoix astroblem, Quebec City, Lotbinière, and as far west as Mauricie. While topographic steering seemingly controlled ice flow patterns at small scales, the topography alone cannot explain the ice stream position and direction, as uphill ice flow features can be observed in various parts of the valley. The precise timing of events, particularly with respect to the Laurentian Channel ice stream in the Gulf of St. Lawrence, remain elusive, as datable material has proven hard to find. However, the highly variable ice flow directions inferred from glacial features found in the Charlevoix and Lotbinière areas suggest that ice flow within the streaming zone was very dynamic. Analysis of this new data set suggests that the scale and regional significance of the SLIS phenomenon have been greatly underestimated in the past.