COLLABORATIVE RESEARCH: Subglacial Deforming Beds as Erosive and Sedimentary Agents: An Experimental Study of Particle Comminution and Rock Erosion

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9819346TulaczykSubglacial sediment deformation is becoming increasingly accepted as the new, distinct, and perhaps even predominant mechanism of fast ice flow and till generation. By this mechanism, extensive layers of till may be generated, transported at fast rates, and deposited entirely subglacially. At present, two fundamental assumptions are made in the deforming-bed models of ice flow and till generation: (1) that these rapidly deforming beds erode underlying bedrock efficiently enough to sustain the required high sediment fluxes; and (2) that they can produce the evidence of debris comminution which is commonly observed in tills. Results of observational and experimental investigations of the till from beneath Ice Stream B. West Antarctica suggest that these assumptions changes significantly the presently existing understanding of deforming beds. However, it is not clear whether the observations from Ice Stream B till can be extrapolated to other subglacial environments. A series of experiments is proposed here to study the rates of particle comminution and bedrock erosion by shearing till. In these experiments, an existing ring shear device will be used to shear six different types of till to large strains (~100,000) under effective normal stresses which are applicable to the deforming-bed model (6 kPa N 100 kPa). Comparison of pre-shear and post-shear granulometry, composition of clay-size fraction, as well as sand grain shape and micromorphology will determine the extent of particle comminution. The rate of rock erosion will be measured on rock plates of common lithologies mounted in the ring shear device. The results will be used to test the general validity of the assumptions (1) and (2). The collected data will provide an experimental basis for quantitative formulations of erosion and comminution that can be used in future modeling of deforming beds. Finally, these experiments have the potential to yield solid sedimentological criteria for testing whether a given till may or many not have been generated fully by subglacial deformation.