Colloque

The Geochemical Signature of Surficial Sediment in Glaciated Terrain

W.W. Shilts

Membre a labase

W.W. Shilts

Résumé du colloque

The geochemical signature of surficial sediment in glaciated terrain is a complex function of provenance glacial and sedimentological processes, physical and mechanical properties of rocks and their derived sediments, and secondary weathering or chemical processes. Geochemical anomalies may be produced by one or any combination of these variables, and it is only with well designed sampling programs and analytical strategies that geochemical patterns related solely to provenance can be recognized. Because glacial sediments are formed largely by the abrasion of unweathered bedrock, the geochemical reflection of mineralization is also a function of the physical and mechanical properties of source bedrock minerals. The residence site, or optimal grain size of mineralized debris within glacial sediments is dependent upon the size to which the host mineral can be reduced by the forces available from glacial abrasion. The mineralogical partitioning, resulting from the physical process of abrasion, results in a geochemical partitioning throughout various grain size fractions. Appropriate laboratory and analytical strategies must be chosen to optimize the geochemical signature of mineralization. Secondary chemical processes, primarily the result of surface weathering and soil formation, also significantly influence the geochemistry of surficial sediments. The geochemical characteristics of the fine (<250µm) fraction of till are radically different than those of ice-contact gravels. This is because the fines in till are primarily rock flour dominated by primary bedrock minerals, such as well-crystallized phyllosilicates. Fines in sorted, sandy, gravelly sediments are largely secondary minerals such as iron-manganese oxides and hydroxides, and poorly crystallized phyllosilicates, dominated by mixed layer clays. Samples with a large concentration of secondary minerals tend to have a high cation exchange capacity and thus fix cations concentrated from groundwater solutions. The net result is enhanced geochemical background values within these sediments compared to adjacent till. In addition, slight changes in degree of oxidation can significantly influence till geochemistry. Even slightly oxidized tills will be depleted in labile minerals, such as sulphides and carbonates, relative to unoxidized tills. This can make it impossible to compare the geochemistry of heavy minerals of oxidized till with that of unoxidized till or clastic sediments generated by post-depositional fluvial erosion. Assuming that these technical, sedimentological, and diagenetic difficulties can be overcome, a compositional data set should represent the pattern of glacial dispersal.

Contexte

Section :

Analyse des faciès glaciaires : méthodologie

Thème du colloque :

Analyse des faciès glaciaires : méthodologie

Hôte : Université du Québec à Montréal

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Analyse des faciès glaciaires : méthodologie

The Geochemical Signature of Surficial Sediment in Glaciated Terrain

W.W. Shilts