Rock strength along a fluvial transect of the Colorado Plateau – quantifying a fundamental control on geomorphology

Abstract

Bedrock strength is a key parameter that influences slope stability, landscape erosion, and fluvial incision. Yet, it is often ignored or indirectly constrained in studies of landscape evolution, as with the K erodibility parameter in stream-power models. Empirical datasets of rock strength suited to address geomorphic questions are rare, in part because of the difficulty in measuring those rocks at Earth's surface that are heterolithic, weak, or poorly exposed. Here we present a large dataset of measured bedrock strength organized by rock units exposed along the length of the trunk Green–Colorado River through the iconic Colorado Plateau of the western U.S. Measurements include field compressive tests, fracture spacing, and Selby Rock Mass Strength at 168 localities, as well as 672 individual tensile-strength tests in the laboratory. Tensile strength results are compared to geomorphic metrics of unit stream power, river gradient, and channel and valley-bottom width through the arid Colorado Plateau, where the influence of bedrock is intuitive but unquantified.Our dataset reveals logical trends between tensile and compressive strength as well as between strength, rock type and age. In bedrock reaches of the fluvial transect, there is a positive rank-correlation and a strong power-law correlation between reach-averaged rock strength and unit stream power, as well as a linear relation between tensile strength and river gradient. Expected relations between fracture spacing and topography are masked partly by the massive yet weak sandstones in the dataset. To constrain values for weak rock types such as shale, we utilize the inverse power-law scaling between tensile strength and valley-bottom width to estimate their “effective” tensile strength. Results suggest that tensile strength varies to at least an order-of-magnitude smaller values than evident with directly testable rocks in this landscape, and values for erodibility (K) in numerical simulations may be informed by this dataset. In terms of landscape evolution, these results support the finding that equilibrium adjustment to bedrock strength, not differential uplift or transient incision, is the first-order control on large-scale fluvial geomorphology in the Colorado Plateau. This has broad implications for the interpretation of topography in terms of tectonic drivers.