STRATIGRAPHIC ANALYSIS OF AN OPEN PIT EXPOSURE: AN INVESTIGATION OF THE WESTERN MARGIN OF THE LAURENTIDE ICE SHEET DURING THE MIDDLE WISCONSIN IN THE GREAT SLAVE LAKE REGION

Known 14C age data place the western margin of the Laurentide Ice Sheet (LIS) in the Great Slave Lake region, Northwest Territories, approximately along the west margin of the Canadian Shield and Western Canada Sedimentary Basin. However, a general paucity of thick (>2 m) exposures of quaternary sediments in this part of Canada prevents further delineation of the location of the Middle Wisconsin LIS margin. A detailed investigation of a >20 m exposure, in an abandoned open pit in the past producing Pb-Zn Pine Point Mining District, NWT, was conducted to gain a better understanding of the glacial history of the area. This rare thick till exposure consists of four visually distinct till units, and based on observed sedimentology, striation measurements and clast fabrics as well as detailed grain size, geochemical, mineralogical, micromorphological, and pebble lithological analyses, a refinement for the Middle Wisconsin LIS margin is proposed. Since there is no stratigraphic and sedimentological evidence of ice-margin advance or retreat fluctuations preserved in the till exposures at Pine Point, we conclude that the Middle Wisconsin ice margin was likely north and west of the Pine Point area and probably occupied most of the Great Slave Lake basin prior to advancement of the LIS into western Canada during the Late Wisconsin.