Abstract

New ways of looking at grain-size distributions may yield insights into sedimentary processes or environments. For example, during shearing of a granular material, alignments of grains, or bridges, develop with orientations such that compressive forces parallel to these alignments support most of the applied shear stress. If deformation is due to failure of such bridges by fracture, rather than by, say, dilation, the grain-size distribution will tend toward one that provides the maximum support for the grains. This size distribution is fractal and has a fractal dimension of 2.6. We analyzed the grain-size distribution of three deforming tills collected from beneath modern glaciers. The size distributions are fractal, and the mean fractal dimension is ∼2.9, suggesting an excess of fines. For comparison, grain-size distributions of samples from some other common sedimentary environments were also analyzed. Samples of dune sand and of glacial outwash were not fractal, but a debris-flow sample was, and had a fractal dimension of 2.8.

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