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

Jason F. Thomason, Neal R. Iverson

Research output: Contribution to journalArticlepeer-review

Abstract

Deep, 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.

Original languageEnglish (US)
Pages (from-to)131-146
Number of pages16
JournalJournal of Glaciology
Volume55
Issue number189
DOIs
StatePublished - 2009

ASJC Scopus subject areas

  • Earth-Surface Processes

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