Approaches to the study of glacier bed deformation

Investigation of glacier bed conditions and subglacial processes is an obligatory requirement for the modelling and understanding of contemporary glacier behaviour, past glacier behaviour. glacier response to both spatial and temporal variations in bed material, and glacier response to external forcing factors such as climate change. The recognition of the importance of coupling between the glacier and its bed resulted in a 'paradigm shift' in the subject and drew attention to the critical importance of subglacial deformation as opposed to simply subglacial deposition and sliding. A number of methods have been adopted to measure glacier bed deformation, ranging from direct observation and instrumentation, the interpretation of macroscale and microscale sedimentary structures. to the interpretation of glacier bedforms and glacier/bed modelling. This paper is concerned to describe the methods used by the two authors and co-workers, and attention is given to field methods, the type and arrangement of sedimentary structures. thin-section micromorphology and microfabric. and clast microfabric and glacier bedform morphology as evidence of subglacial deformation. Sedimentary structures (macro- and mesoscale) are used to identify constructional deformation where deformational units are preserved one on top of another due to the formation of a thin deforming layer by low basal stresses, and excavational deformation. in which the deformational style is superimposed on pre-existing styles due to increasing thicknesses of glacier deformation caused by increasing basal shear stresses. Thin section micromorphology has been used to define a range of glacial processes of which subglacial deformation is just one, but has drawn attention to critical criteria, such as discrete microscale shears, orientated matrix, kink structures and fractured grains, as well as deformed intraclasts, that can be used to recognise deformational processes. More recently, individual aggregates taking the form of rounded 'pebbles' within till matrix, have been shown to reflect rotational processes during glacial deformation and provide an additional mechanism to discrete shear within subglacial deformation and an explanation for the transport paths of subglacial materials. Finally, measurements of sediment flow directions using clast macrofabric characteristics, coupled with measurement of quantities of sediment transfered and detailed palaeoglacial properties have been used to determine the rates and scales of subglacial deformation associated with the formation of small-scale glacier bedforms. (C) 2001 Elsevier Science Ltd and INQUA. All rights reserved.