Diffusive mixing between shearing granular layers: constraints on bed deformation from till contacts

Shearing of subglacial till has bem invoked widely as a mechanism of glacie r motion and sediment transport, but standard indicators for determining sh ear strain from the geologic record are not adequate for estimating the ver y high strains required of the bed-deformation model. Here we describe a la boratory study of mixing between shearing granular layers that allows an up per 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 characterize d with a single mixing coefficient, D. Ring-shear experiments with equigran ular beads and lithologically distinct tills provide the value of D, althou gh in experiments with till D decreases systematically with strain to a min imum value of 0.0045 mm(2). Kinetic gas theory provides an estimate of the dimensionless mixing coefficient which is within an order of magnitude of l aboratory values. Knowing the minimum value of D, the distribution of index lithologies measured across till contacts in the geologic record can be us ed to estimate the maximum shear strain that has occurred across till conta cts. Application of this technique to the contact between the Des Moines an d Superior Lobe tills in east-central Minnesota, U.S.A., indicates that she ar strain did not exceed 15 000 at the depth of the contact.