A COMPARISON OF TILL MACROFABRIC AND MICROFABRIC USING SEQUENTIAL IMAGING AND AUTOMATED FEATURE EXTRACTION

The apparent orientation of sand-sized grains observed in thin section (microfabric) of glacial sediments has been used as a measure of till response to applied stress for more than forty years. Modern image analysis software minimizes the tedious and laborious task of determining the orientation of individual grains thereby making the potential utility of the technique relatively attractive. Similarly, the alignment of large clasts residing in glacial sediments (macrofabric) is commonly used as an indicator of applied stress and, in some cases, the type of strain experienced by a till. This study presents the results of a comparison between microfabric and macrofabric data gathered from the same sediment sample. Little correlation is observed although significant realignment of sand-sized grains by large clasts is noted at several locations. Analysis of the interaction among large rotating elements with a known three-dimensional geometry and their surrounding grains allows for evaluation of clast rotation mechanisms. March rotation appears to dominate for large clasts in the sediment used for this study. This finding is in agreement with independent experimental results on tills and supports the notion that large clasts rotate as passive markers in response to simple shear.