Sedimentology and post-glacial history of Lake Manitoba

Thumbnail Image
Last, William.
Journal Title
Journal ISSN
Volume Title
Lake Manitoba is a large (4700 km2), shallow (z = 4.0 m), brackish water lake located in south-central Manitoba. Sediment cores up to 13'5 m in length were taken at 50 offshore sites in the main South Basin of the lake. These cores were analyzed at varying intensities for over 40 physical, chemical, and mineralogical sediment parameters. The modern offshore sediments of the South Basin are mainly silts and clayey silts, and are composed of detrital and authigenic carbonates (20-60%), clay minerals (20-50%), organic matter (15-30%), quartz (15-30%), and feldspar (2-10%). Quartz, feldspar, and dolomite are in greatest abundance around the margin of the offshore basin, whereas organic matter, calcite, and clay minerals are concentrated in the central portion of the basin. The authigenic calcite, precipitated in response to high levels of supersaturation brought about by evaporative concentration and extensive phytoplankton productivity during the ice-free season, contains an average of about 7 mole percent MgCO3, in the crystal lattice. The post-glacial sediments in the South Basin of Lake Manitoba are composed dominantly of silty clay and clayey silt except toward the extreme southern end of the basin where less clayey and more sandy sediments are common. The clay content, quartz, feldspar, and dolomite as well as K, Na,Fe,and Mg show a general increase with depth in the sediment cores,' while silt, moisture content, organic matter, calcite, and Ca decrease with depth. On the basis of variations in these and other sediment parameters investigated, six lithostratigraphic units are recognized in the post-glacial record of the lake (Units A, B, C, D, E, and F from bottom to top)...