Fabric Pattern in a Basal Till Succession and Its Significance for Reconstructing Subglacial Processes: Reply

Abstract

ABSTRACT Fabric data from a 2.5-m-thick basal till succession in Kurzetnik, Poland, consisting of three structurally distinct till units including a deformation till were examined to test whether different processes of deposition and deformation as indicated by till structure are reflected in the till fabric pattern. Surprisingly, throughout the succession the till fabric is unidirectional and very highly clustered (mean S1 = 0.876) and standard fabric interpretation methods including binary and triangular diagrams failed to differentiate between the three units and constrain the origin of the till facies. It is shown that the a-axis length as well as the ratio between a/b and a/c axes has no effect on the fabric strength, and there is no correlation between the fabric strength and clast size within the 0.7-5.6 cm grain-size range. The bootstrapping method applied to test the statistical significance of variance shows that (1) some variations in the fabric strength within the same till facies are due to the sampling effect, while other variations can be interpreted as true effects of strain, and (2) fabric strength may vary significantly over very short distances within the same till facies. Very high fabric strength in the deformation till is inconsistent with a model of weak to moderate fabric strength resulting from penetrative deformation, supporting the theory that subglacial deforming tills behave as Coulomb plastic rather than viscous materials.