Diagenetic variation in the Oregon Coast Range: Implications for rock strength, soil production, hillslope form, and landscape evolution

Abstract

The mechanisms by which lithology modulates geomorphic processes are poorly known. In the Oregon Coast Range (OCR), rhythmically bedded sandstones of the Eocene Tyee Formation underlie steep, soil-mantled hillslopes, with relatively uniform ridge-valley spacing. These characteristic landforms are perturbed where diagenetic variations manifest as resistant cliffs. Here we use petrology, rock mechanics, and lidar to characterize grain-scale variations in rock properties and their influence on rock strength, hillslope processes, and landscape morphology in two adjacent watersheds. Petrographic analyses suggest that a suite of diagenetic products in the "resistant" bedrock account for a 2.5 times increase in tensile strength relative to "typical" Tyee bedrock. Our reference catchment exhibits negligible resistant outcrops, and consistent hillslope gradients and longitudinal valley profiles. By contrast, the adjacent catchment teems with resistant, 1 to 10 m thick, noncontiguous sandstone beds that form hanging valleys with gentle upstream hillslopes and anomalously narrow valleys. Mechanical and topographic analyses suggest that the low fracture density characteristic of these resistant beds may render them relatively impervious to comminution by tree root activity, the dominant OCR soil production mechanism. Based on both hillslope gradient-and hilltop curvature-erosion models, we estimate that hillslopes perched above resistant beds erode at approximately half the pace of hillslopes unencumbered by downstream knickpoints. The diagenetic variations likely influence relief at the watershed scale. Depositional position and diagenetic processes appear to control the occurrence of resistant beds, providing a framework to quantify how seemingly subtle variations in rock properties can impose first-order controls on landscape form and evolution.

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…Under mobile soils that are thin, the patterns of rock fracturing and weathering may be an important limit on the rate of detachment of sub-soil material, and on the size of detached fragments incorporated up into the mobile soil. In such cases, trees affect the efficiency of mobile soil production (Jackson and Sheldon, 1949;Marshall and Roering, 2014). This contrasts with settings with thick regolith , whereby climate or slow erosion rates diminish the role of trees in the production of mobile soil thickness to the point that roots do not penetrate deeper than h (see Table 1 and Fig.…”

“…However, few studies have systematically addressed lithological controls on root penetration into unweathered or weathered rock (e.g., Zwieniecki and Newton, 1994;Marshall and Roering, 2014). Although such close coupling has been used to argue that root growth can fracture rock, this may not be the case.…”

“…As biotic engines, trees thus strongly impact the energy, water, and element cycles in forested and savannah ecosystems, shaping and sculpting landscapes and soils over long timescales (Reneau and Dietrich, 1991;van Breemen et al, 2000;Balogh-Brunstad et al, 2008a;Pawlik et al, 2016). Soils and landscapes in turn affect plant species composition and size as well as above-and below-ground productivity and rooting depth (Bennie, 1991;Clark et al, 2003;Hahm et al, 2014;Marshall and Roering, 2014). Only by studying the entire CZ using concepts from hydrology, soil science, geomorphology, geochemistry, and ecology will a synthetic view of tree-soil-landscape co-evolution emerge.…”

How does grid-resolution modulate the topographic expression of geomorphic processes?

“…However, more recent work has demonstrated the spatial variability of many topographic measurements in this landscape (Marshall and Roering, 2014;Grieve et al, 2016b), and as such it provides a more challenging test case for our analyses. Furthermore, these sites were selected as they each have high-resolution lidar data covering a large spatial area and have been the subject of many previous studies (Reneau and Dietrich, 1991;Roering et al, 1999Roering et al, , 2001Montgomery, 2001;Pinter and Vestal, 2005;Roering et al, 2007;Perron et al, 2009;Perroy et al, 2010Perroy et al, , 2012Marshall and Roering, 2014;Grieve et al, 2016a, b), which should provide a good basis for the evaluation of the results of this study in a wider geomorphic context.…”

“…2d), USA, is a densely vegetated upland landscape, dominated by coniferous and hardwood forests (Schmidt et al, 2001), with a humid climate (Roering et al, 1999). Qualitative observations of the landscape suggest that the valleys are regularly spaced, with a particular uniformity found in the dimensions of firstorder drainage basins (Roering et al, 1999(Roering et al, , 2007Marshall and Roering, 2014). Such observations have been supported by measurements of hillslope length across the landscape (Grieve et al, 2016a).…”