Campus Units

Geological and Atmospheric Sciences

Document Type

Article

Publication Version

Published Version

Publication Date

12-2000

Journal or Book Title

Journal of Glaciology

Volume

46

Issue

155

First Page

641

Last Page

651

DOI

10.3189/172756500781832666

Abstract

Shearing of subglacial till has been invoked widely as a mechanism of glacier motion and sediment transport, but standard indicators for determining shear strain from the geologic record are not adequate for estimating the very high strains required of the bed-deformation model. Here we describe a laboratory study of mixing between shearing granular layers that allows an upper limit to be placed on bed shear strain in the vicinity of till contacts. Owing to random vertical motions of particles induced by shearing, mixing can be modeled as a linearly diffusive process, and so can be characterized with a single mixing coefficient, D. Ring-shear experiments with equigranular beads and lithologically distinct tills provide the value of D, although in experiments with till D decreases systematically with strain to a minimum value of 0.0045 mm2. Kinetic gas theory provides an estimate of the dimensionless mixing coefficient which is within an order of magnitude of laboratory values. Knowing the minimum value of D, the distribution of index lithologies measured across till contacts in the geologic record can be used to estimate the maximum shear strain that has occurred across till contacts. Application of this technique to the contact between the Des Moines and Superior Lobe tills in east-central Minnesota, U. S. A., indicates that shear strain did not exceed 15 000 at the depth of the contact.

Comments

This article is from Journal of Glaciology 46 (2000): 641, doi:10.3189/172756500781832666. Posted with permission.

Rights

Creative Commons Attribution license.

Copyright Owner

International Glaciological Society

Language

en

File Format

application/pdf

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