Subglacial clast behaviour and its implication for till fabric development: new results derived from wireless subglacial probe experiments

Abstract

This study has investigated the three-dimensional movement of clasts within deformation till, using embedded wireless probes. These probes were part of an environmental sensor network, which measured subglacial properties (temperature, water pressure, resistivity, case strain and tilt) six times a day, and relayed that data via radio to the glacier surface, where they were forwarded and broadcast on-line. The system was installed at Briksdalsbreen, Norway and operated from August 2004 until August 2006. Approximately 2000 probe days worth of data were collected, with an increase in performance (41% more readings) during the second year. The probes showed similar patterns of water pressure rises throughout the two years, but with slightly different magnitudes and timings. These changes in water pressure could be related to clast behaviour. The probes decreased their dip over the year, and the rate of change was related to an increase in glacier velocity. After initial changes in dip, the probes experienced changes in orientation, followed by rotation about the a-axis. This continuous rotation was similar to the motion suggested by Jeffery [1922. The motion of ellipsoidal particles immersed in a viscous fluid. Proceedings of the Royal Society of London, Series A 102, 161–179] for the behaviour of clasts within a viscous material. In addition, some probes also showed short, frequent dip oscillations in spring and autumn, which were interpreted to reflect stick-slip events, similar to lodging; and demonstrated how local conditions can interrupt the predicted rotation pattern. Overall, it is demonstrated that when water pressures were high, decoupling occurred associated with basal sliding and dip oscillations; and when water pressures fell, the ice and sediment were coupled and till deformation occurred. These events happened during summer and autumn. It is this combination of “lodgement” and deformation that builds up both a complex (but predictable) fabric and a resultant composite till sedimentology.