Laboratory studies of till deformation with implications for the motion and sediment transport of the Lake Michigan Lobe

Date
2006-01-01
Authors
Thomason, Jason
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Neal R. Iverson
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Geological and Atmospheric Sciences
Abstract

Glacier motion and sediment transport are attributed commonly to pervasive shearing of subglacial till to high strains (the bed-deformation hypothesis). However, the processes that control bed deformation are difficult to observe beneath modern glaciers, and observations of the geologic record have not been sufficient to determine whether tills have been sheared to the high strains required by this hypothesis;The shear strength of the ice-till interface controls whether a glacier slips over its bed or shears it pervasively over much of its thickness. When clasts gripped by basal ice plow through the bed, the ice-till interface is weakened. Results of experiments using a ring-shear device in which hemispheres were pushed through water-saturated till indicate a progressive decrease in resistive force on hemispheres with increasing plowing speed. This effect is due to the generation of high pore-water pressures in front of the hemispheres at fast plowing speeds (e.g. 380 m y-1), which weakens till and reduces resistive forces. These results indicate that increases in glacier sliding speed will reduce resistive forces exerted by till on plowing clasts, which will promote decoupling of the ice from the bed and inhibit pervasive bed deformation. In addition, this positive pore-pressure feedback may constitute a flow instability that promotes fast glacier flow;The ring-shear device was also used to develop criteria at microscopic scales for identifying highly sheared tills. Results of experiments indicate that the alignment (fabric) of elongate, sand-sized particles becomes strong at high strains. Furthermore, microshears become more abundant and align progressively with the shearing direction as strain increases. Magnetic fabrics calibrated to strain in an ancillary ring-shear study and sand-particle fabrics were used to test the till-deformation model as applied to the Batestown advance of the Lake Michigan Lobe. Results indicate that magnetic and sand-particle fabrics are not sufficiently strong to support the bed-deformation model. Furthermore, consistent changes in fabric direction with depth and fabric patterns around cobbles and small boulders indicate that the till likely sheared only in thin zones near the glacier sole during till accretion to the bed from ice. Bed shearing likely contributed only minimally to sediment transport.

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