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Till Geochemistry II: Glacial Dispersal

C.A. Kaszycki
CK

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C.A. Kaszycki

Résumé du colloque

Assuming appropriate sampling and analytical control, the geochemistry of surficial sediments in glaciated terrain should reflect sample to sample variations caused by the agency of glacial dispersal. The shape, size and geochemical expression of dispersal trains, however, is a function of a number of variables including ice dynamics, patterns of subglacial and ice-marginal sedimentation, size, aspect and erodibility of the source area, rate of dilution of mineralized debris, and scale of observation. Dispersal trains can be grouped into two broad endmembers: (1) fan to patch shaped trains characterized by a systematic decrease in concentration of source material both longitudinally along the axis of the train and laterally toward the margins of the train. These trains record the integrated ice flow trends over a prolonged time period, their shape being a composite reflection of shifting ice flow throughout the region. Zonation within these trains may reflect individual ice flow events. (2) linear ribbon shaped trains characterized by uniformly large concentration of source material both longitudinally and laterally across the train. These trains represent unique dynamic conditions characterized by entrainment and transport throughout a single ice flow event with little dilution in the down ice direction, and in some instances may represent the position of former ice streams within the ice sheet. These two endmembers are not mutually exclusive, as ribbon shaped trains are often a component of larger fan shaped trains. The geochemical expression of glacial dispersal is variable. In some instances, far-travelled debris may be geochemically impoverished with respect to local bedrock. The net result being a "negative" geochemical dispersal train and a poor reflection of local mineralization. Alternatively, geochemically distinctive debris may be spread over a large area characterized by inert bedrock lithologies, leading to an incorrect assessment of mineral potential for the region. On a regional scale, the geochemical expression of glacial dispersal is independent of till facies. Where relief is subdued and drift is relatively thin, composition is uniform throughout all till facies, and sampling may be carried out without rigorous facies identification. In areas of pronounced bedrock relief, however, till composition may be facies dependent and care should be taken to correctly identify till facies when sampling. At a very detailed scale, typically used in mineral exploration, the geochemical expression of glacial dispersal may be facies dependent, and care should be taken to describe till facies as thoroughly as possible. Because transport distances are short, dispersal trains may reflect late glacial shifts and/or topographic deflection of ice flow, rather than regional ice flow trends.

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Section :
Analyse des faciès glaciaires : méthodologie
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Analyse des faciès glaciaires : méthodologie
host icon Hôte : Université du Québec à Montréal

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Thème du colloque :

Analyse des faciès glaciaires : méthodologie
Till Geochemistry II: Glacial Dispersal
C.A. Kaszycki