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Cet article est diffusé et préservé par Érudit.Érudit est un consortium interuniversitaire sans but lucratif composé de l'Université de Montréal, l'Université Laval et l'Université du Québec à Montréal. Il a pour mission la promotion et la valorisation de la recherche. www.erudit.org Tous droits réservés © Les Presses de l'Université de Montréal, 1992 Géographie physique et Quaternaire, 1992, vol. 46, n° 3, p. 295-309, 11 ABSTRACT Information from a wide range of sources is integrated in a basin analysis of the Wabigoon Basin, a Quaternary clay basin located on the Canadian Shield in northwestern Ontario. The basin sediments were deposited between 10.9 ka and 9.5 ka, along the margin of the Rainy Lobe of the Laurentide Ice Sheet, which formed the northern boundary of proglacial Lake Agassiz. The basin architecture is dominated by four major elements: end moraines, eskers, kames and a clay plain, all of which overlie irregular bedrock topography. End moraines, eskers and kames are composed mainly of a fining upward sequence of gravels and sands. The geometry of these sedimentary units, and their sedimentary structures indicates they were deposited mainly by high and low-density turbidity currents, on ice-marginal subaqueous outwash fans. Eskers contain a core of coarse gravel and sand deposited within subglacial meltwater conduits, overlain by subaqueous fan sediments deposited at the conduit mouth. Esker ridges were formed during conduit filling events and flanking deposits were formed when a conduit remained in use during icemarginal retreat. Where conduits were shortlived, isolated subaqueous fans (kames) were formed. A depositional model is proposed which relates moraine formation to catastrophic releases of subglacial meltwater and sediment simultaneously along the entire margin of the Rainy Lobe. The clay plain forms a broad blanket of fine-grained, rhythmically-bedded sediment which obscures bedrock topography, and often buries esker and kame deposits. Seismic profiles and overburden drilling reveal deep (50-70 m) bedrock lows beneath the clay plain. These lows, oriented sub-parallel to the ice margin, acted as sediment traps, and were infilled by the deposits of underflows generated at the ice margin.

Glacial geological mapping and drift prospecting were conducted in the Lake of the Woods area to aid exploration for gold and base metals. Glacial advances from the northeast and northwest created large areas of erosion and deposits of thin and discontinuous (1-3 m) glacial sediment over granite and greenstone bedrock. Northward retreat of ice was within glacial Lake Agassiz. Thick glaciolacustrine deposits (>50 m), sand and gravel underlying clay, fill topographic basins such as that near Lake Wabigoon. Three long, arcuate, end moraines (Lac Seul, Hartman, and Eagle-Finlayson) contain stratified drift deposited rapidly on subaqueous fans in Lake Agassiz. Rhythmic sediments (varves) provide a 1000 year chronology of basin history and ice wastage. A modified history of events in glacial Lake Agassiz results. Subaqueous and retreat origins for the major moraines indicate the continuous presence of high lake stages in the area. Red varves found north of Hartman Moraine suggests an age of ~10 440 BP rather than 10 000 BP. Low water outlets were apparently not present in the area north of Fort Frances until the Nipigon Lake phase ~ 9500 years ago. Drift sampling was at reconnaissance levels, 219 samples over the 25 500 square kilometre study area. Drift composition indicates that glacial dispersal is minor and composition closely reflects underlying bedrock. Geochemical anomalies thus reflect element distribution in major rock types. Data points are widely spaced and as a result major geochemical trends in tills are not necessarily detected. Summary anomaly maps show the distribution of gold and related elements and base metals. The geochemical maps indicate that more closely spaced sampling might yield results in places, for example near Vermilion Bay or Manitou Lakes. Conventional bedrock mapping and prospecting combined with till geochemistry is recommended in most areas where drift cover is thin. Heavy mineral work on glaciofluvial sediments may also yield further results because these sediments are common and glacial dispersal is low. Basin analysis in thick drift areas provides data on key environmental issues, water supply, waste disposal, and aggregate resources.

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