The effect of lithology on valley width, terrace distribution, and bedload provenance in a tectonically stable catchment with flat‐lying stratigraphy

Abstract

How rock resistance or erodibility affects fluvial landforms and processes is an outstanding question in geomorphology that has recently garnered attention owing to the recognition that the erosion rates of bedrock channels largely set the pace of landscape evolution. In this work, we evaluate valley width, terrace distribution, and bedload provenance in terms of reach scale variation in lithology in the study reach and discuss the implications for landscape evolution in a catchment with relatively flat‐lying stratigraphy and very little uplift. A reach of the Buffalo National River in Arkansas was partitioned into lithologic reaches and the mechanical and chemical resistance of the main lithologies making up the catchment was measured. Valley width and the spatial distribution of terraces were compared among the different lithologic reaches. The surface grain size and provenance of coarse (2–90 mm) sediment of both modern gravel bars and older terrace deposits that make up the former bedload were measured and defined. The results demonstrate a strong impact of lithology upon valley width, terrace distribution, and bedload provenance and therefore, upon landscape evolution processes. Channel down‐cutting through different lithologies creates variable patterns of resistance across catchments and continents. Particularly in post‐tectonic and non‐tectonic landscapes, the variation in resistance that arises from the exhumation of different rocks in channel longitudinal profiles can impact local base levels, initiating responses that can be propagated through channel networks. The rate at which that response is transmitted through channels is potentially amplified and/or mitigated by differences between the resistance of channel beds and bedload sediment loads. In the study reach, variation in lithologic resistance influences the prevalence of lateral and vertical processes, thus producing a spatial pattern of terraces that reflects rock type rather than climate, regional base level change, or hydrologic variability. Copyright © 2017 John Wiley & Sons, Ltd.

Impacts of Lithologically Controlled Mechanisms on Downstream Bedrock Valley Widening

“…The implication of the power law relationship for valley width is that increasing discharge is responsible for downstream valley widening, whereas bedrock lithology controls the magnitude of valley width at any given discharge. Indeed, bedrock valleys widen more readily in weak, easily erodible lithologies compared to more resistant lithologies, where bedrock valleys tend to be narrower (Bursztyn et al, 2015;Keen-Zebert et al, 2017;Spotila et al, 2015). Additionally, it has been observed that bedrock valley walls with different lithologies are eroded by the river through distinct mechanisms, that is, plucking versus abrasion (Beer et al, 2017;Spotila et al, 2015).…”

Building a Bimodal Landscape with Varying Bed Thicknesses in Last Chance Canyon, New Mexico

“…(2) Where more thinly bedded sandstone rock is exposed, channel steepness tends to be lower. (3) Furthermore, the effect of exposed bedrock on landscape morphology is confounded by interplay with sediment input from hillslopes (Duval et al, 2004;Johnson et al, 2009;Finnegan et al, 2017, Keen-Zebert et al, 2017. Thickly bedded and steeper rock units on surrounding hillslopes contribute larger sized colluvial sediment to the channels, leading to steeper channel slopes (Thaler and Covington, 2016).…”

The role of lithology and climate on bedrock river incision and terrace development along the Buffalo National River, Arkansas

“…The Buffalo River cuts through Ordovician, Mississippian, and Pennsylvanian carbonate and clastic sedimentary rocks that are subject to both physical and chemical (karstic) weathering (Keen-Zebert et al, 2017; Fig. 2A).…”

“…The middle part of the Everton is dominated in the upper reaches (upstream of river km ∼75) by a thick sandstone sequence known as the Newton Sandstone Member, which thins significantly toward the lower reaches (Fig. 2A; Keen-Zebert et al, 2017). There is less ground water-surface water interaction in the Everton Formation than in the Boone Formation, and thus the effect of fluviokarst processes is reduced.…”