CGRG Bibliography of Canadian Geomorphology
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Author : Gray, J.T.; Gosse, J.; and Marquette, G.
Date : 2001.
Title : Weathering zones in the Torngat Mountains, Labrador, ice sheet thickness and basal thermal regime: The contribution of 10Be and 26Al cosmogenic dating.
Publication : St. John's 2001. Geological Association of Canada - Mineralogical Association of Canada 2001 Joint Annual Meeting / l'Association géologique du Canada - l'Association minéralogique du Canada réunion annuelle conjointe. Memorial University, St. John's, Newfoundland, May 27-30 2001.
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Abstract
The recognition of three (later four) weathering zones in the Torngat Mountains has been for a long time a basic postulate in scientific discussions concerning weathering zones and glacial history on the northeastern margin of the Laurentide Ice Sheet. Our recent research at the classic Nakvak Valley location reveals that this postulate is based on questionable interpretation of the evidence. The lowest Saglek moraine limit is certainly attributable to a readvance of a tongue of the continental ice sheet, pushing eastward to the Labrador Sea. However, an incipiently frost-shattered zone, in a 150-200 m vertical interval, immediately above the Saglek moraines, described by Ives (1958) as the Koroksoak zone, is spatially very restricted, and is re-interpreted by us as a zone of meltwater washing during deglaciation. This washing zone is succeeded upwards not by a felsenmeer zone, as proposed by Ives, but by a till covered summit. Nonetheless, vertically differentiated weathering zones do exist throughout the Torngats. Our field-work and image interpretations for a large area between Saglek Fjord and Hudson Strait, have permitted the establishment of a simpler two zone model, comprised of lower glacially scoured surfaces, with a patchy till cover, and upper felsenmeer surfaces. The lack of a clearly demarcated downward sloping trend to the east of the lower felsenmeer limit, and the absence of associated lateral moraines or meltwater channels negate an interpretation as the upper vertical limit of the Laurentide Ice Sheet pushing through the Torngat Mountains from a dispersal centre to the west. An alternative model of build-up of local cold-based, thin and minimally erosive ice-caps on the individual broad summit plateaux of the Torngat Mountains is proposed by us to explain the excellent preservation of a thick felsenmeer cover, developed prior to at least the Last Glacial Maximum. As a means of testing this hypothesis, samples of extremely resistant, fine-grained gneiss, were recently obtained from large boulders and bedrock promontories, in both the felsenmeer and glacially scoured zones and are being cosmogenically dated using 10Be values for total age of exposure, and 10Be/26Al ratios for burial history beneath ice. A first exposure age of 140,000 years for a small bedrock promontory on a felsenmeer covered summit supports the concept of protection from both glacial erosion, and from sub-aerial frost shattering through an interval much longer than the Wisconsinan period.
Bibliography of Canadian Geomorphology