Isolating lithologic controls on landscape morphology in the Guadalupe Mountains, New Mexico and Texas
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In many geomorphic studies, lithologic contrasts are often acknowledged as important for landscape form, but are otherwise ignored in attempts to infer tectonic forcing or climatic control from topography. It remains difficult to separate the effects of tectonics, climate, and lithology due to the limitations of commonly used landscape evolution models. Tectonic inactivity and relatively little spatial variability in climate make the Guadalupe Mountains of Texas and New Mexico an ideal site to isolate and investigate the effects of lithology on topography. To assess the control of lithology, I compared topographic metrics including channel steepness index, channel concavity, and topographic relief in different mapped lithologic units across the region. Topographic metrics were calculated using elevation data extracted from USGS 10m Digital Elevation Models. These metrics were grouped spatially based on 23 regionally mapped lithologic units, including abundant limestone and dolomite with some evaporites, sandstone, and shale. To better evaluate the different rock units, I used published unit descriptions to develop a simple, semi-quantitative system to estimate the relative durability rating (RDR) of each rock unit. This rating system accounts for rock type and other rock properties such as relative bed thicknesses or spatial heterogeneity. RDR values were found to be correlated with unit-averaged channel steepness for each of the 23 lithologies in the region. Channel steepness shows a moderate correlation with RDR (R² = 0.44; Kendall’s 𝜏 = 0.52), demonstrating quantifiable control on landscape form. However, concavity does not show a significant correlation (R² = 0.016; Kendall’s 𝜏 = 0.13). Stratigraphic relationships among units suggest that thick, resistant reef deposits exert the main lithologic control on overall channel forms in the region. Less resistant units stratigraphically below these reef deposits generally have higher than expected steepness given their RDR. Units at the bottom typically have high concavity values as well. The opposite is true for weaker units stratigraphically above the resistant reef formations, which have lower steepness and higher convexity. The contrasting influence of strong units above or below weaker units and their observed effects on channel form should improve our ability to infer rock properties from topography, and to predict the evolution of landscapes with lithologic variability.