GRAIN-SIZE DISTRIBUTION IN DEFORMING SUBGLACIAL TILLS - ROLE OF GRAINFRACTURE

New ways of looking at grain-size distributions may yield insights int o 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 alignment s support most of the applied shear stress. If deformation is due to f ailure of such bridges by fracture, rather than by, say, dilation, the grain-size distribution will tend toward one that provides the maximu m support for the grains. This size distribution is fractal and has a fractal dimension of 2.6. We analyzed the grain-size distribution of t hree deforming tills collected from beneath modern glaciers. The size distributions are fractal, and the mean fractal dimension is similar t o 2.9, suggesting an excess of fines. For comparison, grain-size distr ibutions of samples from some other common sedimentary environments we re 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.