Subglacial sediment textures: character and evolution at Haut Glacier d'Arolla, Switzerland

Fourteen subglacial debris samples have been recovered from the margins of, or beneath, Haut Glacier d'Arolla, Switzerland. The grain-size distributions of these sediments are presented and compared with each other as bivariate plots of percentage weight against (sieve-defined) particle size and log number of particles against log particle diameter. All of the samples recovered are composed of a broad range of clast sizes and approach self-similarity over the four orders of magnitude of grain-sizes analysed. Fractal dimensions range from 2.47 to 2.77. Sample intercomparison reveals the operation of at least two processes of textural evolution: the production of fines by in-situ weathering; interpreted in terms of abrasion associated with subglacial sediment deformation, and the loss of fines, interpreted in terms of evolution by percolating subglacial meltwaters. These interpretations are supported and refined through comparison of the grain-size fractions gained (in the case of deformation) and lost (in the case of eluviation) with those fractions respectively generated in a laboratory-based simulation of sediment deformation and exiting the glacier suspended in the proglacial meltwater stream. While sediment deformation has the effect of increasing the fine fraction between 0 and 10 phi and of raising the fractal dimension of undeformed sediments from 2.47 to 2.77, eluviation removes particles between 2 and 10 phi driving the fractal dimension of deformed sediments down from 2.77 to 2.54. These fractal dimensions are generally lower than those recorded at other comparable glaciers, consistent with the relatively low rates of sediment deformation inferred from other studies at Haut Glacier d'Arolla.