Building a bimodal landscape: bedrock lithology and bed thickness controls on the morphology of Last Chance Canyon, New Mexico, USA

Abstract

Abstract. We explore how rock properties and channel morphology vary with rock type in Last Chance Canyon, Guadalupe Mountains, New Mexico, USA. The rocks here are composed of horizontally to near-horizontally interbedded carbonate and sandstone. This study focuses on first- and second-order channel sections, where the streams have a lower channel steepness index (ksn) upstream and transition to higher ksn values downstream. We hypothesize that differences in bed thickness and rock strength influence ksn values, both locally by influencing bulk bedrock strength and also nonlocally through the production of coarse sediment. We collected discontinuity intensity data (the length of bedding planes and fractures per unit area), Schmidt hammer rebound measurements, and measured the largest boulder at every 12.2 m elevation contour to test this hypothesis. Bedrock and boulder mineralogy were determined using a lab-based carbonate dissolution method. High-resolution orthomosaics and digital surface models (DSMs) were generated from drone and ground-based photogrammetry. The orthomosaics were used to map channel sections with exposed bedrock. The United States Geological Survey (USGS) 10 m digital elevation models (DEMs) were used to measure channel slope and hillslope relief. We find that discontinuity intensity is negatively correlated with Schmidt hammer rebound values in sandstone bedrock. Channel steepness tends to be higher where reaches are primarily incising through more thickly bedded carbonate bedrock and lower where more thinly bedded sandstone is exposed. Bedrock properties also influence channel morphology indirectly, through coarse sediment input from adjacent hillslopes. Thickly bedded rock layers on hillslopes erode to contribute larger colluvial sediment to adjacent channels, and these reaches have higher ksn values. Larger and more competent carbonate sediment armors both the carbonate and the more erodible sandstone and reduces steepness contrasts across rock types. We interpret that in the relatively steep, high-level ksn downstream channel sections, the slope is primarily controlled by the coarse alluvial cover. We further posit that the upstream low-level ksn reaches have a base level that is fixed by the steep downstream reaches, resulting in a stable configuration, where channel slopes have adjusted to lithologic differences and/or sediment armor.

Sediment cover modulates landscape erosion patterns and channel steepness in layered rocks: Insights from the SPACE Model

“…Sediment grain size also plays an important role in influencing the degree of cover effect. Field studies in a range of settings have shown that coarse sediment and boulders in particular from resistant lithologies significantly impact channel steepness (e.g., Anderson et al, 2023;Johnson et al, 2009;Shobe et al, 2020;Thaler & Covington, 2016). For example, Anderson et al (2023) found that the steepest channel reaches in their study area were those armored with boulders from thick, resistant limestone beds.…”

“…Field studies in a range of settings have shown that coarse sediment and boulders in particular from resistant lithologies significantly impact channel steepness (e.g., Anderson et al, 2023;Johnson et al, 2009;Shobe et al, 2020;Thaler & Covington, 2016). For example, Anderson et al (2023) found that the steepest channel reaches in their study area were those armored with boulders from thick, resistant limestone beds. Thaler and Covington (2016) similarly found that channel steepness in their study was correlated with boulder size and percent boulder coverage rather than underlying bedrock lithology.…”

“…For example, a highly fractured but otherwise strong rock type may produce relatively immobile coarse grains, while a softer lithology such as shale may produce much finer sediment. A recent field study highlighted how more resistant carbonate units produce larger boulders than the less resistant sandstones in the Guadalupe Mountains of Texas and New Mexico (Anderson et al, 2023). These carbonate boulders in turn were responsible for increasing steepness in downstream reaches of sandstone.…”

“…It encompasses a combination of rock strength, the degree of fracturing, weathering, and climate (Whipple & Tucker, 1999). Lithology also influences the size and quantity of sediment delivered to channels as a result of differences in both fracture spacing, strength, and susceptibility to weathering (Anderson et al, 2023;Neely & DiBiase, 2020;Scott & Wohl, 2019;Sklar et al, 2017). Coarse sediment can aid or inhibit channel incision, by acting as both tools that abrade the bed and by armoring it from grain impacts and other erosion processes (referred to as the cover effect) (Sklar & Dietrich, 2004).…”

Sediment Cover Modulates Landscape Erosion Patterns and Channel Steepness in Layered Rocks: Insights From the SPACE Model

“…It encompasses a combination of rock strength, mechanisms of incision, the degree of fracturing and weathering, climate, basin hydrology, and hydraulic geometry and roughness (Barnhart et al., 2020; Han et al., 2014; Lague, 2014; Whipple, 2004; Whipple & Tucker, 1999). Lithology also influences the size and quantity of sediment delivered to channels as a result of differences in both fracture spacing, strength, and susceptibility to weathering (Anderson et al., 2023; Carr et al., 2023; Neely & DiBiase, 2020; Scott & Wohl, 2019; Sklar et al., 2017) Coarse sediment can aid or inhibit channel incision, by acting as both tools that abrade the bed (referred to as the tools effect ) and by armoring it from grain impacts and other erosion processes (referred to as the cover effect ) (Sklar & Dietrich, 2004). Several studies have shown that channels within the same lithology that are mantled with coarse sediment tend to be steeper (DiBiase et al., 2018; Johnson et al., 2009; Shobe et al., 2020; Sklar et al., 2017; Thaler & Covington, 2016).…”