Till fabric patterns and significance: particle response to subglacial stress

The analysis of particle arrangements within glacial sediments (fabric) has long been considered to reflect the behaviour of the ice that transported and deposited, or deformed the sediment. Particles can develop specific fabric arrangements as a response to bulk sediment strain driven by effective stress at the bed of a glacier. As such, fabric analysis has been used to try and understand the complex relationship between glacier and ice sheet dynamics and the geological evidence of former glaciation. Developments have focused on the use of stereonet patterns and statistical measures (vector and eigenvector analysis) of clast fabric to infer the process of sediment deposition/deformation, and consequent ice flow directions and dynamics. However, as this study identifies, fundamental processes of particle orientation (March, Jeffery and Taylor rotation mechanisms) would suggest more complex explanations than have been previously reported. This study reports the results of a detailed field and laboratory investigation of macro-scale (clast) and microfabrics from a till sequence in Central Scotland. The results suggest a systematic behaviour of different particle size fractions, which may be interpreted in terms of the strain response of sediment. Three fabric patterns are identified that illustrate consistent, progressively shifting and more complex particle behaviour. It is suggested these orientation patterns indicate the degree to which subglacial sediment has undergone strain, and the nature of the deforming bed (erosional or accretionary). It is concluded that this approach may yield fundamental information about the strain conditions underlying different ice masses, providing critical data linking glaciological modelling with the sedimentary evidence of glaciation. (C) 2003 Elsevier Science Ltd. All rights reserved.