Stacks and Cracks and Blocks, Oh My! Simulating Staircase Landscape Evolution in Heterolithic Stratigraphy

Landscapes composed of heterolithic stratigraphy are globally widespread. The impact of these lithologic differences on landscape morphology is well understood. Yet, detailed attention is still needed to understand the dynamics of cliff formation and development seen in some staircase landscapes. Controls on cliff and slope morphology reasonably include the variability in lithological properties and stratigraphic organization. The relative role and importance of these factors in developing cliffs and staircase landscapes has not yet been investigated. However, Glade et al. (2018) found that for individual cliffs and hogbacks formed of one hard rock layer over softer rock, large rocky debris is extremely important in the preservation of such features through time. We build upon the modeling work of Glade et al. (2018) by extending their model to include multiple sets of hard cliff-forming layers. We then explore how variation in hard layer thickness, relative ease of weathering of hard layers, and properties of large slope blocks control hillslope morphology. A suite of model runs was compared via geomorphic metrics including sediment export as well as visual inspection of the final time step. The original models ability to generate large blocks that move downslope was preserved and used to test if slope armoring plays a role in stairstep morphology genesis and sediment export variability. Results suggest that landscapes with multiple stairsteps may be harder to develop than expected. We find that often, hard rock layers that were assumed to form cliffs or slope breaks were instead buried by soil being transported across the hard rock layer via diffusion. This occurred over a wide range of hard layer thickness and hardness. We propose that the presence of multiple bare rock surfaces in a stair step landscape requires that hard rock units must have a certain amount of permeability to allow sediment bypass to prevent burial of hard lithology. This permeability would take the form of fractures and joints of a rock unit that acts as a conduit for soil routing. We show some first field explorations based on these simulations that suggest that the idea has merit. Further field investigation would have to be performed to constrain this process further.