Testing the bed-deformation hypothesis; applying experimental magnetic fabric studies to the geologic record; 33rd international geological congress; abstracts

Glacier movement and sediment transport by deep, pervasive bed shear was declared a new paradigm in glaciology 22 years ago, but observations have not yet confirmed this opinion. Sedimentological studies have provided clear evidence that parts of former ice sheets deformed their beds. However, was shear strain in such sediments generally large enough to account for most glacier movement (strain > 100), and was deformation sufficiently deep and widespread to dominate sediment transport? Testing the bed-deformation hypothesis requires answering these questions. Answers are not easily found in the commonly homogeneous tills that were substrates for vast areas of ice sheets. A quantitative method is needed for estimating the state of strain in these tills. Any till characteristic chosen for study requires independent calibration to strain magnitude and direction. Long ago fluvial geomorphologists faced similar problems relating characteristics of sediments to flow regimes of rivers. The problem was tackled largely through flume experiments, now a staple of that discipline. In contrast, interpretations of glacial sediments generally lack experimental grounding. To help fill this void, we conducted experiments in which fabric evolution in sheared till was studied. Our goal was not to simulate subglacial deformation but to subject tills to an idealized state of strain (simple shear) that could be used to isolate more complicated states of strain in the field. The most useful strain indicator is fabric formed by till anisotropy of magnetic susceptibility (AMS), as expressed by directions of principal susceptibility (k1, k2, k3) in intact specimens (18 mm cubes). Experiments with basal tills show that AMS is caused by alignment of small quantities (< 0.2%) of silt-sized, non-equant, magnetite grains. Orientations of k1 become tightly clustered in the direction of shear to form strong, steady-state fabrics (S1 = 0.83-0.94) at moderate shear strains (7-30). Transverse fabrics never develop, despite shear strains as high as 710. Steady-state orientations of k1 plunge up-glacier. Both k1 and k3 lie in the longitudinal flow plane, fully defining its orientation. We used these relationships to evaluate deformation of the Batestown till of the Lake Michigan lobe (Illinois): a till thought by many to have been transported in a deforming bed. Results of AMS sampling in vertical profiles indicate that deformation was different from that assumed in most models: 1) at least half of the till was not sheared to even moderate strains, including till near the tops of profiles; 2) depths of bed deformation did not exceed a few decimeters; 3) deformation commonly deviated from bed-parallel simple shear, indicating heterogeneous (patchy) deformation, and 4) bed shear was accompanied by progressive till accretion to the bed from ice. These conclusions indicate that the bed-deformation model can be a poor idealization for sediment transport by soft-bedded glaciers.