Reconstruction of a Paleosurface Across the Southern Rocky Mountains and High Plains of North America, and Implications for Neogene Landscape Evolution

The modern surface of the High Plains of the western United States shows extensive fluvial dissection at the foot of the Southern Rocky Mountains. Previous work has argued for both tectonic uplift and climate change as mechanisms for driving the observed incision; here we combine paleotopographic reconstruction with landscape evolution modeling in order to quantify the role that bedrock erodibility has played in shaping this transition zone.

Evidence from high-level fluvial gravel deposits within the mountains suggests that, during the Miocene period, sedimentary systems graded smoothly from high elevations in the west to low elevations in the east. This paleosurface, representing regional topography at ca. 5 Ma, was reconstructed by interpolating between sparse, montane, Late Tertiary gravel patches and the Miocene Ogallala Formation of the Great Plains. Reconstruction of the paleosurface enabled quantification of both the magnitude and spatial distribution of incision that has occurred across the High Plains since Late Miocene time. The amount of material removed from the plains is on the order of 10⁴ km³ across the study area, and 50% of material has been removed within 100 km of the mountain front. These metrics, along with channel steepness indices taken from modern topography, form the benchmarks for evaluating the performance of a landscape evolution model across various scenarios of broad-wavelength, down-to-the-east tilting. This work improves our understanding of the magnitude and spatial patterns of incision that form as a result of contrasting substrate erodibilities when landscape response to long-term climatic and/or lithospheric variation is superimposed on rocks with strongly contrasting resistance to erosion.