SEDIMENTATION SIGNATURES IN SMALL ROCK-CORED BASINS IMMEDIATELY FOLLOWING DEGLACIATION IN NORTHWESTERN ONTARIO, CANADA

Two lake-sediment cores are being examined to determine if early, clastic sedimentation was in a local or regional lake at deglaciation. The study area is located in northwest Ontario near Antikokan. Slim and Bay Lakes are small bedrock-sided basins approximately 25 km from each other, and on the distal side of the Steep Rock Moraine. Cores were recovered with the STRANGE, hydraulically assisted Livington corer. Core analysis includes magnetic susceptibility, loss-on-ignition (organic and inorganic carbon), particle size analysis, structure, and carbon/nitrogen ratios. An AMS 14C date from Slim Lake indicates that organic sedimentation had begun at the core site by 9535 14C yrs BP. Both cores consist of organic sediment (peat & gyttja) overlying inorganic sediment. The inorganic sediment is predominantly massive sand and silt, and rhythmically laminated silt. Little clay-sized sediment is present. A high magnetic susceptibility signal mostly corresponds with higher percentages of sand, sharply dropping off in the organic sediment. The sand consists of poorly-sorted, sub-angular grains of mostly quartz with some hornblende, feldspars and a minor amount of micas. Sand grains are moderately sorted and sub rounded in lower portions of the Slim Lake core. The dominance of sand and coarse silt comprising the sediment in the base of the cores suggests a local sediment source, thus sedimentation within a local rather than a regional lake. In addition, the 9545 14C BP age from Slim Lake corresponds in time to water levels in glacial Lake Agassiz rising to the Campbell Beach. But projections of the Campbell Beach to the field area are a minimum of 25 m below the elevation of Slim and Bay Lakes, further ruling out a regional-scaled lake. Therefore, the sedimentary signature of these lakes may be used to characterize sedimentation in small rock bounded basins in this region.