EVALUATING THEORETICAL MODELS FOR THE SPATIAL DISTRIBUTION OF SUBGLACIAL EROSION IN THE SAWATCH RANGE OF COLORADO

Due to the inaccessibility of the basal transport zone, the spatial distribution of subglacial erosion remains poorly understood. West (1968) predicted that the intensity of erosion decreased down flowline based on observations of sediments and landforms. Flint (1971) predicted that the same patterns could be produced by the varying erodibility of bedrock and active frost wedging ahead of an advancing glacier. Boulton (1974) suggested based on process mechanics that the intensity of erosion would decrease in response to ice thickness. In 1996, Boulton presented a different model based on the deforming bed theory that predicted four erosion-depositional zones. These predicted distributions represent hypotheses to be tested against the geologic record.

Although the theoretical models were developed for ice sheets, they were tested in valley glaciers where the flow geometry and history have better controls and where an entire flowline could be reasonably sampled. Subglacial sediment was collected along the flowlines of two valleys in the Sawatch Range of Colorado. Dispersal curves were constructed for each valley by plotting the concentrations of pebble lithologies against the sample distance from the cirque headwall. A simple numeric model was used to translate the predicted distributions from the theoretical models into testable dispersal curves. The modeled and observed curves were compared using the standard deviation of differences for paired observations. This statistic was used to empirically identify the best-fit theoretical model to the geologic data.

Flint's (1971) model was found to have the highest average standard deviation of differences for both valleys (16.7% and 23.7%). The geologic evidence does not support this model; therefore, the process of frost wedging loosening material in front of the glacier is unlikely to be occurring. Boulton's (1974) model was found to have the lowest average standard deviation of differences (10.6% and 11.8%) for both valleys of the Sawatch Range. This suggests that the distribution of subglacial erosion in these valleys is primarily controlled by ice thickness. The question still remains whether Boulton's (1974) model represents the best solution or if untested models could better predict the dispersal patterns preserved in the geologic record.