SHORT RESEARCH Dip, Layer Spacing, and Incision Rate Controls on the Formation of Strike Valleys, Cuestas, and Cliffbands In
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SHORT RESEARCH Dip, layer spacing, and incision rate controls on the formation of strike valleys, cuestas, and cliffbands in heterogeneous stratigraphy Dylan J. Ward DEPARTMENT OF GEOLOGY, UNIVERSITY OF CINCINNATI, CINCINNATI, OHIO 45221, USA ABSTRACT Landscapes developed over heterogeneous stratigraphy exhibit a spectrum of landforms from dramatic cliffbands to hogbacks, depend- ing on the dip and spacing of the layers. In deeply incised landscapes, a single cliffband may consist of multiple resistant layers, whereas similar stratigraphy elsewhere is separated by strike valleys into individual cuesta benches or hogbacks. This paper presents a geometric analysis, informed by a numerical landscape model, to explain the conditions for development of a strike valley floored by erodible rocks. The results define a threshold incision rate below which strike valleys are more likely to form; this threshold incision rate is proportional to the stratigraphic spacing of cliff-forming layers and a trigonometric function of dip angle. The analysis also yields a time scale for the adjustment of structural landforms to changes in regional incision rate, which is a function of dip angle and the coupling between cliff retreat rate and escarpment height. In example landscapes of the Colorado Plateau, this time scale is likely much longer than that of documented variations of incision rates due to late Quaternary climate and land-use changes. The transitional state of escarpments in layered rock may therefore contain information about regional downcutting rates over time scales different from those recorded by the fluvial network. The utility of such features will require better understanding of the coupling between incision of a foot slope and the retreat rate of the cliff above in different kinds of rocks. LITHOSPHERE; v. 11; no. 5; p. 697–707; GSA Data Repository Item 2019305 | Published online 2 August 2019 https://doi.org/10.1130/L1056.1 INTRODUCTION on landform is therefore complicated by multiple sources of variability: changes to dip, stratigraphic thickness, erodibility, and internal sources of Motivation transience in the downcutting rates (Darling and Whipple, 2015). Nonfluvial landforms that commonly develop on variably erodible In landscapes with uniform rocks and a steady rate of rock uplift or stratigraphy include cliffbands, cuestas, homoclinal ridges, hogbacks, base-level fall, topography evolves to a quasi-steady form that equalizes and flatirons. These “structurally controlled” landforms are typically clas- erosion rates spatially across the landscape (Gilbert, 1877; Tucker and sified based on somewhat arbitrary ranges of dip angles of the resistant Bras, 1998). Even where rocks differ spatially in erodibility, a quasi-steady rock capping the feature (Twidale and Campbell, 2005). There is not, topography can form under conditions of uniform base-level fall, where however, any unifying theory that relates stratigraphic and lithologic landforms on more resistant rocks evolve to have steeper slopes to attain properties under various rates of base-level fall to specific structurally the same erosion rates as those on weaker rocks (Gilbert, 1877; Hack, controlled landforms. It is my purpose here to explain a key morphologi- 1960). This relationship implies that, given appropriate information about cal distinction along this spectrum of landforms, which is the presence material and erosion processes, uplift rates and gradients thereof may be or absence of a strike valley floored by more erodible rocks. The result deduced from topography (Wobus et al., 2006). lends insight into the steady and transient states of landscapes developed In contrast, recent work has highlighted how the heterogeneity in rock on heterogeneous stratigraphy. strength can lead to the indefinite persistence of transient topography and to varying local erosion rates in landscapes experiencing steady base-level Colorado Plateau fall. For example, where fluvial networks cross dipping stratigraphy, litho- logic knickpoints that form on harder units will migrate upstream, which Structurally controlled landscapes occur on sedimentary rocks world- may be updip or downdip. This process results in a local, internal base- wide. Prominent examples include the Valley and Ridge region of eastern level control that behaves differently from the external base level (Cook et North America; the Atlas Mountains of northern Africa; the Jura Moun- al., 2009; Berlin and Anderson, 2009; Forte et al., 2016; Perne et al., 2017). tains of Europe; the Arabian Desert and Negev Desert; the Flinders and Interpretation of erosion rates, or tectonics, in these landscapes based Macdonnell Ranges of Australia; and the Qinling and Dalou Mountains of China. Some of the most iconic landscapes of this type are found in Dylan Ward http://orcid.org /0000 -0002 -5741 -7830 the Colorado Plateau region of the United States. Geological© 2019 The SocietyAuthors. of Gold America Open |Access: LITHOSPHERE This paper | Volume is published 11 | underNumber the 5 terms| www.gsapubs.org of the CC-BY-NC license. 697 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/11/5/697/4830210/697.pdf by guest on 24 September 2021 WARD | Controls on the formation of strike valleys in layered rocks SHORT RESEARCH In all of these areas, the landforms are clearly controlled by the strati- the trunk streams due to drainage integration through the Grand Canyon. graphic spacing of resistant units. For example, resistant layers spaced This transience resulted in several sets of major (~200-m-relief) knick closely together commonly form a compound cliffband, wherein the foot zones along the river system, some of which may reflect a combination of slope below an upper cliff drains directly over the lower cliff (Fig. 1). base-level and lithological effects (Cook et al., 2009; Darling et al., 2012; Where resistant layers are spaced more widely, broad benches may form Bursztyn et al., 2015) or differential tectonic uplift rates (e.g., Crow et al., on the weaker rock between them, separating the cliffs and routing drain- 2014). Superimposed on these major, long-term incision events, there are age parallel to strike. Dip is another primary control: Steeply dipping 102 to 104 yr cut-and-fill cycles that are typically of 1 to 10 m in magnitude layers form homoclinal ridges or hogbacks separated by strike valleys, (Harvey et al., 2011; Pederson et al., 2013b; Sheehan and Ward, 2018). whereas more horizontal layers of the same rock may form compound These episodes of incision of different magnitudes and durations imply cliffs. The Laramide monoclines of the Colorado Plateau provide clas- that variability in the surrounding landforms may be due to their current sic examples of this stratigraphic and dip control on the topography of states of response to the variability in downcutting, with their specific escarpments (Fig. 1). morphology conditioned by the bedrock template. Because the field-measurable terms of interlayer spacing and structural dip contribute to the variability of landforms, the relationship between Reference Case: Coal Cliffs Cuesta, Utah, USA landform and base-level fall is not as straightforward as in uniform sub- strates (Forte et al., 2016). In the example of the Colorado Plateau, the As noted already, steeply dipping cap rocks that form hogbacks are cliff and cuesta landforms developed during a time of significant docu- commonly separated by strike valleys, but it is less common for strike val- mented spatial and temporal variability in downcutting rates (Darling et leys to form among subhorizontal rocks. An example of this less common al., 2012; Jochems and Pederson, 2015). The most prominent transience case is the Coal Cliffs cuesta (Fig. 2), located on the western flank of the in the fluvial networks is related to deep, late Cenozoic entrenchment of San Rafael Swell in central Utah. The San Rafael Swell is a Laramide-aged mesa A compound cliff strike valley hogbacks backscarp strike valley (bench) compound cliffs cuesta cuesta Dip: steeper subhorizontal B compound cliffs transverse stream valleys compoun d strike cliff s & hogback strike valley N38.32º, W111.04º View to the north Figure 1. (A) Schematic cross section of an upwarp such as the San Rafael Swell, illustrating dip and stratigraphic control on structural landforms. (B) Strike valley on the Colorado Plateau between Hanksville, Utah, and Capitol Reef National Park. The near-horizontal stratigraphy of North Caineville Mesa in the background is ringed by compound cliffs containing several cliff-forming layers. Closely spaced sandstone units form a compound cliffband on the east side of the strike valley, which is developed in a thicker shale unit with a moderate dip. As the dip steepens to the west (left side of the image), more closely spaced resistant strata form hogbacks separated by narrow strike valleys. The strike valleys drain to a tributary of the Fremont River, which acts here as a transverse stream, crossing the layers perpendicular to strike. Geological Society of America | LITHOSPHERE | Volume 11 | Number 5 | www.gsapubs.org 698 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/11/5/697/4830210/697.pdf by guest on 24 September 2021 WARD | Controls on the formation of strike valleys in layered rocks SHORT RESEARCH A B C Figure 2. (A) Location and topography of the Coal Cliffs cuesta on the western, shallowly dipping flank of the San Rafael Swell, Utah (UT). R— River. (B) Oblique air photo