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The sedimentary record of drifting ice (early Wisconsin Sunnybrook deposit) in an ancestral ice-dammed Lake Ontario, Canada |
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| Authors: | Nicholas Eyles Carolyn H Eyles Todd A Randall |
| Institution: | a Environmental Earth Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough ON Canada M1C 1A4 b School of Geography and Geology, McMaster University, Hamilton ON Canada L8S 4K1 c Guigne International Ltd., 685 St. Thomas Line, Paradise NFLD Canada A1L 1C1 d Department of Civil Engineering, McMaster University, Hamilton ON Canada L8S 4K1 |
| Abstract: | Outcrops of pebbly mud (diamict) at Scarborough in Southern Ontario, Canada (the so-called Sunnybrook ‘Till’) are associated with the earliest incursion of the Laurentide Ice Sheet (LIS) into mid-continent North America some 45,000 years ago. The Sunnybrook is a blanket-like deposit containing deepwater ostracodes and occurs conformably within a thick (100 m) succession of deltaic and glaciolacustrine facies that record water depth changes in a large proglacial lake. Contextual evidence (associated facies, sedimentary structures, deposit geometry and landforms) indicates a low energy depositional setting in an ice-dammed ancestral Lake Ontario in which scouring by floating ice masses was an important process. U-shaped, iceberg-cut scours (with lateral berms) up to 7 m deep, occur on the upper surface of the Sunnybrook and are underlain by ‘sub-scour’ structures that extend several meters below the scour base. Ice-rafted concentrations of clasts (‘clast layers’), grooved surfaces formed by floating ice glissading over a muddy lake floor (‘soft sediment striations’) and melanges of sand and mud mixed by grounding ice keels (‘ice keel turbates’) are present and are all well known from modern cold environments. The wider significance of this depositional model is that the LIS margin lay east of Scarborough and did not overrun Southern Ontario. This finding is in agreement with recent data from the Erie Basin of Canada, Ohio, and Indiana where deposits formerly correlated with the Sunnybrook (and thus implying an extensive early Wisconsin ice sheet) are now regarded as Illinoian. A speculative hypothesis is proposed that relates deposition of the Sunnybrook and two younger deposits of similar sedimentology, to surge-like instabilities of the southern LIS margin. |
| Keywords: | Sunnybrook Till Ice-keel turbation Early Laurentide Ice Sheet |
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