Complexities of landscape evolution during incision through layered stratigraphy with contrasts in rock strength

Abstract

Variation in the erodibility of rock units has long been recognized as an important determinant of landscape evolution but has been little studied in landscape evolution models. We use a modified version of the Channel-Hillslope Integrated Landscape Development (CHILD) model, which explicitly allows for variations in rock strength, to reveal and explore the remarkably rich, complex behavior induced by rock erodibility variations in even very simple geologic settings with invariant climate and tectonics. We study the importance of relative contrasts in erodibility between just two units, the order of these units (whether hard rocks overlie soft or soft rocks overlie hard) and the orientation of the contact between the two units. We emphasize the spatial and temporal evolution of erosion rates, which have important implications for basin analysis, detrital mineral records, and the interpretation of cosmogenic isotope concentrations in detrital samples. Results of the landscape evolution modeling indicate that the stratigraphic order of units in terms of erodibility, the gross orientation of the contact (i.e. dipping away or toward the outlet of the landscape) and the contact dip angle all have measurable effects on landscape evolution, including significant spatial and temporal variations in erosion rates. Steady-state denudation conditions are unlikely to develop in landscapes with significant contrasts in rock strength in horizontal to moderately tilted rock layers, at least at the scale of the entire landscape. Additionally, our results demonstrate that there is no general relation between rock erodibility and erosion rates in natural settings. Although rock erodibility directly controls the erosion rate constant in our models, it is not uncommon for higher erosion rates to occur in the harder, less erodible rock. Indeed erosion rates may be either greater or less than the rock uplift rate (invariant in time and space in our models) in both hard and soft rocks, depending on the local geology, topography, and the pattern of landscape evolution.

Resistant rock layers amplify cosmogenically‐determined erosion rates

“…For S ct ≥ S t , the topography follows bedding planes (a dip-slope). Similar to cliffs on threshold hillslopes, over-steepened knickzones form because of a tendency for undermining at strong-over-weak contacts (Forte et al, 2016). Erosion rate amplification in the channel network As Perne et al (2017) demonstrated, the formation of oversteepened knickzones in channels where weak rocks underlie strong rocks is analogous to the threshold hillslope problem discussed earlier.…”

“…This contrast dictates that the rate of horizontal retreat below the contact (celerity of the weak layer, C ew ) will tend to be faster than above the contact (celerity of the strong layer, C es ) and thus undermining is expected. As a result, a near-vertical waterfall forms and grows steadily in height (m) during knickzone retreat, eventually occupying the full extent of each strong-weak couplet (Forte et al, 2016;Perne et al, 2017). Where n ≠ 1, the rate of horizontal retreat depends on stream gradient (Equation (4)).…”

“…This happens when a reduction of local base-level fall rate above the underlying weak-over-strong contact (Darling and Whipple, 2015;Forte et al, 2016) propagates through the full thickness of the weak layer (thicker weak layers are required to satisfy the assumption throughout simulations in the case of upstream bedding dip). The simulation is for the stream power incision model (n = 2) like our 1D solutions.…”

“…Gilbert, 1877;Davis, 1899;Ritter et al, 1995). In both Forte et al (2016) and Perne et al (2017), the erosion rates in retreating cliff bands, which in nature generally form in the stronger rock layers, are significantly higher than the rate of base-level fall. For example, Forte et al (2016) and Perne et al (2017) show that landscapes produced in simulations with a stream-power river incision model never achieve steady state, with markedly non-uniform erosion rates persisting even where the rate of base-level fall is steady over long timescales (e.g.…”

Quantification of rock control in geomorphology

“…The stratigraphic order of rock units, their erodibility, and their orientations and dips are fundamental controls of landscape development (Forte et al, 2016). Alberti et al (2013) described a method to distinguish river terrace levels based on rock clast hardness and degree of weathering measured with an Equotip.…”