SEDIMENTARY PROCESSES ASSOCIATED WITH DEGLACIATION NEAR SIOUX POND LOOKOUT, NORTHWEST ONTARIO

Lacustrine sediments at Sioux Pond Lookout, northwest Ontario record the depositional events during the abrupt transition from the end of the Younger Dryas to the early Holocene. Published radiocarbon ages at Sioux Pond average 9740 ± 100 BP (Svante Bjorck, 1984). A core thrust taken from 650-750 cm depth at Sioux Pond includes 25 cm of gray clay-silt rhythmites overlain by a 20 cm gray massive silt unit overlain, in turn, by 50 cm of gyttya deposits. Grain size analysis in the gray silt region and rhythmite layers indicates a strong overprint of eolian processes. SEM images of sub-angular quartz grains support the partial influence of a wind blown sediment source. Total Gamma (API) spectral intensities are high in the rhythmite section, reflecting a high potassium content in the clay minerals. Gamma intensities are low in the gray massive silt due to a higher quartz and lower illite content, and at moderate levels in the gyttya. Total carbon is high in the gyttya, as expected, and moderate in the massive gray silt and rythmite sections. X-ray powder diffraction analysis of both bulk and <2?m size fractions show clay minerals, quartz, feldspar and carbonate in varying proportions throughout the section. Variable levels of illite and chlorite found throughout the core probably reflect sediment sources in the Canadian Shield. The massive gray silt region and the rythmite section both show significant levels of illite/smectite (I/S). We hypothesize that the I/S also reflects an eolian source, most likely from westerly winds transporting soil clays from smectite-rich Cretaceous shales in the Western Interior. A general decrease in detrital illite and chlorite was detected from the bottom of the core up through the massive gray silt. This trend is consistent with grain size data showing that the size of non-clay particles decreases upwards as eolian supply became more dominant. The influx of organics which increase upwards into the gyttya deposits suggests that the sediment source changed from eolian-dominated to fluvial-dominated. Relatively articulated pennate Diatoms (250-300um) were discovered in the middle gray silt layer below the gyttya deposits, reflecting the onset of an episode of climatic warming.