Comparisons between macro‐ and microfabrics in a pebble‐rich, sandy till deposited by the Cordilleran Ice Sheet

• Published in: Boreas
• Main Authors: Neudorf, Christina M., Brennand, Tracy A., Lian, Olav B.
• Other Authors: Natural Sciences and Engineering Research Council (NSERC) of Canada, Geological Society of America, NSERC Discovery
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2015
• Subjects: Ice Sheet
• Online Access: http://dx.doi.org/10.1111/bor.12120; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fbor.12120; https://onlinelibrary.wiley.com/doi/pdf/10.1111/bor.12120

It is standard practice to measure particle fabrics in glacial studies to infer palaeo‐ice flow directions and processes of till formation but few studies examine the relationships between particle fabrics at different (i.e. the macro‐ and micro‐) scales. This knowledge is critical to inform the utility of the methods and limitations of the associated interpretations. Micro‐ (sand grain) and macro‐ (pebble) fabrics of pebble‐rich, sandy subglacial till (Kamloops Lake till) deposited by the Cordilleran Ice Sheet, south‐central British Columbia, were compared to assess their similarities and differences, and therefore their utility for understanding subglacial processes. Before comparisons were made, the data were tested for robustness by assessing various controls (e.g. sampling face orientation, number of particles measured, statistical variation resulting from sampling effects, particle shape, size and concentration) on particle fabrics. A new method of microfabric analysis was applied that involves the identification and delineation of distinct clusters of similarly orientated sand grains in order to compare them with macrofabrics and inferred ice‐flow directions. The results show that microfabrics, on their own, are an unreliable indicator of ice‐flow direction in Kamloops Lake till in the study area and should not be used as a substitute for macrofabric data, as they probably record late‐stage microscale strain patterns and pore‐water flow in addition to till deposition and deformation by overriding ice. We suspect that this would also be the case for coarse‐grained till elsewhere. Our findings suggest that till microfabric interpretations should always be made after assessing corresponding macrofabric data alongside sedimentological and structural observations.