There has been renewed debate over the mechanisms of uplift and erosion in the Colorado Plateau, and in order to understand the patterns of topography and process in this landscape a third factor of bedrock properties must be considered. Our goal is to compile a dataset of bedrock strength and explore it in the context of topographic metrics. To do so, a methodology problem must be addressed. Traditional rock-strength measures such as Schmidt hammer rebound, Selby rock-mass strength, or tensile strength ignore weak rock types. We hope to develop an indirect, topography-based method that reliably estimates the strength of mud-rocks that are too incompetent to test directly.
We will complete measurements of the characteristics of rock formations along the Colorado and Green rivers to allow erosional resistance to be quantified. The drainage in the Colorado Plateau can be broken up into 48 reaches underlain by a variety of bedrock compositions including quartzite, sandstone, shale, limestone, evaporites, and crystalline basement. Compressive strength data from most of the outcropping rock formations have been collected from all reaches, however tensile strength data for these rocks are incomplete, and there are no data from any incompetent rocks. Our existing data indicate there are strong relations between rock strength and channel/valley width and gradient and stream power. Documentation will be done to complete modified Selby rock mass strength evaluations, noting thickness and proportion of beds that are too weak to be tested. From these new data, we aim to back-calculate strength of “weak” beds using functional relations between measured channel widths and known rock strengths, similar to previous numerical modeling in the region.
Bursztyn, Natalie and Pederson, Joel, "A Study of Bedrock Strength Controls on the Erosion of the Colorado Plateau" (2012). Graduate Student Posters. Paper 6.