MICROFABRIC INVESTIGATION OF SUBGLACIAL TILLS AS A KEY TO UNDERSTANDING THE ROLE OF SUBGLACIAL DEFORMATION IN THE DYNAMICS OF FORMER GLACIERS AND ICE SHEETS

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), and these suggest more complex explanations than have been previously reported.

The results of a detailed field and laboratory investigation of macro-scale (clast) and microfabrics from a till sequence in central Scotland, and a recently deposited till at Langjokull, Iceland are presented. The data highlights the systematic behaviour of different particles according to their size fractions, which orient either parallel or transverse to the stress field direction. This data has been interpreted in terms of the strain response of sediment to the applied total stress. Three fabric patterns are identified, that illustrate consistent (Type A) and progressively shifting (Types B1 and B2) subglacial conditions. More complex particle behaviour (Type C) is interpreted as amalgamations of type A and B patterns occurring during rapidly changing subglacial conditions. 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, thus providing critical data for linking glaciological modelling with the sedimentary evidence of glaciation. In particular this may be used to assess the significance of subglacial deformation in the evolution of Late Quaternary mid-latitude ice sheets.