Deformation of the Batestown till of the Lake Michigan lobe, Laurentide ice sheet

Jason F Thomason,Neal R Iverson

doi:10.3189/002214309788608877

Abstract

AbstractDeep, pervasive shear deformation of the bed to high strains (>100) may have been primarily responsible for flow and sediment transport of the Lake Michigan lobe of the Laurentide ice sheet. To test this hypothesis, we sampled at 0.2 m increments a basal till from one advance of the lobe (Batestown till) along vertical profiles and measured fabrics due to both anisotropy of magnetic susceptibility and sand-grain preferred orientation. Unlike past fabric studies, interpretations were guided by results of laboratory experiments in which this till was deformed in simple shear to high strains. Fabric strengths indicate that more than half of the till sampled has a <5% probability of having been sheared to moderate strains (7–30). Secular changes in fabric azimuth over the thickness of the till, probably due to changing ice-flow direction as the lobe receded, indicate that the bed accreted with time and that the depth of deformation of the bed did not exceed a few decimeters. Orientations of principal magnetic susceptibilities show that the state of strain was commonly complex, deviating from bed-parallel simple shear. Deformation is inferred to have been focused in shallow, temporally variable patches during till deposition from ice.

Highlights

Numerous authors have suggested that soft-bedded glaciers can move primarily by widespread shearing of their beds over thicknesses exceeding several decimeters (e.g. Alley and others, 1987; MacAyeal, 1992; Jenson and others 1995; Boulton, 1996b; Clark and Pollard, 1998; Licciardi and others, 1998)
The goal of this study was to evaluate whether the Batestown till, a basal till deposited by a late-Wisconsin advance of the Lake Michigan lobe of the Laurentide ice sheet (Fig. 1), deformed in a manner consistent with the beddeformation model
Friday 3 k1 and sand-grain fabrics Laboratory experiments indicate that both k1 and sand-grain fabrics strengthen in the flow direction with shear deformation, so fabric strengths help indicate deformation magnitude

Summary

Introduction

Numerous authors have suggested that soft-bedded glaciers can move primarily by widespread shearing of their beds over thicknesses exceeding several decimeters (e.g. Alley and others, 1987; MacAyeal, 1992; Jenson and others 1995; Boulton, 1996b; Clark and Pollard, 1998; Licciardi and others, 1998). There is widespread but not universal agreement that high cumulative strains in till result in strong pebble fabrics parallel to the direction of shear (Benn, 1995; Benn and Evans, 1996; Piotrowski and others, 2001; Carlson and others, 2004; Evans and others, 2006) This relationship is supported by experiments in which till containing pebblesized inclusions was sheared to known strains and fabric development was measured (Hooyer and Iverson, 2000a). Clark, 1997; Lian and others, 2003; Hart, 2006) Such transverse or weak fabrics at high strains are expected from the model of Jeffery (1922), but slip of matrix grains across the surfaces of pebbles in till clearly violates the no-slip condition of the Jeffery theory. This slip at the surfaces of pebbles, as indicated by their commonly striated surfaces (e.g. Benn, 2002), keeps them aligned in the direction of shear (Hooyer and Iverson, 2000a; Iverson and others, 2008)

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Discussion

Conclusion