CGRG Bibliography of Canadian Geomorphology
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Author : Broster, B.E.; and Pronk, A.G.
Date : 2003.
Title : Influence of subglacial topography on the spatial variability of till composition.
Publication : Joint Annual Meeting of the Canadian Quaternary Association and the Canadian Geomorphology Research Group. Halifax, Nova Scotia, June 8-12, 2003.
Issue :
Page(s) :
Abstract
First-order till geochemical, mineral and clast dispersal patterns vary in length and shape as a function of variables mostly related to the volume of material eroded directly from bedrock, its preservation, and it's mixing with other material entrained by the glacier. For example, outcrop size and orientation affects the amount of rock exposed for erosion. Competent minerals within source lithologies may be highly resistant to erosion and comminution to terminal grade, while units composed of low-strength minerals are often preferentially eroded and thus create large dispersal patterns that can mask dispersion of exotic or resistant components. Increased erosion with distance of travel is considered to be the main cause of comminution of debris to matrix-sized granules. However, sub-glacial topography affects glacial dynamics causing changes in ice velocity, flow regime and the location and amount of stress applied at the ice-rock interface. Till studies in New Brunswick demonstrate that topography significantly affects the length and intensity of dispersal patterns for till components and consequently, the mineral and geochemical composition. Commonly the clay-sized fraction in till is increased in some localities due to the glacier overriding of low-lying areas and especially near coasts or in areas that represent former lakes or valleys. In these localities till geochemical dispersal patterns are somewhat diffuse, producing subtle or indistinct dispersal patterns from known mineralized sources. In contrast, distinctive mineral dispersal trains and increased sand content in some localities occur at and down-ice of, units that lie at topographic high points. Relative difference of the topographic high and lower elevations can be as little as a few hundred metres. In part this is due to the increased stress on the substrate and net upward shearing accompanying compressional ice flow at the stoss side of topographic obstacles, but also because longer dispersal patterns are achieved from transport of debris at higher elevations within the glacier. These observations have implications to conventional theories relating to drift prospecting and till maturity. (1) The length of the debris train may reflect the effect of compressional flow and erosion from topographic highs rather than merely the size of the source outcrop. (2) The abundance of minerals at terminal grade-size will be increased in tills located down ice of topographic highs or in areas of topographic undulation were a glacier has alternated between extending and compressive flow regimes. (3) The volume of minerals at terminal grades in a till matrix should not be used as the only indicator of the maturity of tills or distance of travel as these concentrations can be biased due to extensive shearing during incorporation.
Bibliography of Canadian Geomorphology