Principles of Glacial Dispersal and Sedimentation

Glacial dispersal is a term describing the processes through which debris is created and picked up by glaciers, transported in, on or under them, and ultimately released from them some distance from its origin. This size distribution of mineral or rock grains in glacial sediments and, therefore, the geochemical characteristics of different size fractions is dictated by the physical properties of the various components, properties which govern how effectively the components will be comminuted by the crushing and abrasive processes that accompany glacial erosion and transport. Because each pass of a glacier erodes unweathered bedrock and adds it to the lightly weathered debris produced during earlier glaciations, glacial sediment tends to be rich in labile components (sulphides, carbonates, etc.), which are particularly susceptible to weathering in the near-surface environment. To be able to interpret the patterns of geochemical dispersal effectively in glaciated terrain, it is imperative that the geochemist/glacial geologist recognize the possible geochemical effects of chemical partitioning by grain size and the effects of weathering on labile components. Sample populations and analytical techniques must be appropriate for depicting glacial dispersal so that they do not produce results that merely represent artifacts of sample-to-sample inconsistency in textural variation or weathering status. Along with the precautions mentioned above, samples must ensure that sediment facies are properly identified and that the stratigraphic position of the unit sampled is understood. If the above criteria can be fulfilled satisfactorily, areal patterns of geochemical dispersal from specified source areas will be seen to have characteristic ribbon or fan shapes with the apices at or near the sources. Geochemical concentration profiles in drift within dispersal trains are ideally exponential, high concentrations of a component near its source (head of dispersal) declining exponentially to concentrations just above background (tail of dispersal) for some distance down ice. Recently, large-scale dispersal trains with uniformly high, non-declining concentrations of debris from easily eroded source areas, thought to be related to ice streaming within continental ice sheets, have been observed to obscure local geochemical signatures in drift over significant areas. Local drift at the base of these exotic sediments, which often can be sampled only by drilling, generally has provenance characteristics that reflect dispersal from local sources. Finally, the composition of any sample may be the integrated geochemical signal of overlapping or superimposed dispersal trains at a variety of scales from continental (lOOs of km in length) to regional (10s of km in length) to local