SPATIAL VARIATIONS IN HILLSLOPE MORPHOLOGY RELATE TO LITHOLOGY AND BASE LEVEL

Landscape evolution depends on lithology. Here we use hillslope metrics and nondimensionalized erosion and relief to identify hillslopes in equilibrium as well as major contributing factors for morphological transience. We focus on the Navarrés Valley of the Iberian Range in southeast Spain because the southern half remains perched above lithologic knickpoints. We use a natural contrast in base level history to compare hillslope morphologies between a northern basin currently adjusting to Mediterranean base level, and a perched basin to the south. We extract slope, curvature, and flow networks, and compare contrasting lithologies (hard massive limestone and Keuper shales) across the two drainage zones. Hilltop curvature is proportional to erosion rate in soil-mantled, steady-state landscapes, and modulated by a hillslope constant. Average hillslope gradient relates to the erodibility of the substrate; harder lithologies tend to be more stable at higher gradients. Hilltops are the last part of the landscape to adjust to base level change, and hillslopes can demonstrate transience in increased gradient, for example.

Results vary by lithology and watershed zone. Highly erodible Triassic Keuper shales are exposed in the valley in both zones. Hillslope flow lengths are on average 36 m longer in the unperched northern, transient zone. Average hillslope gradients along the flow paths are higher in the unperched zone. Hilltop curvature values in the shales in the unperched zone are also more negative than in the perched zone. These spatial differences in hillslope morphology suggest that the Keuper shales in the unperched zone have been modified by base level fall, which has propagated to the hilltops. This landscape has not yet adjusted to equilibrium like in the older perched landscape. Less erodible limestone units are also exposed in each base level zone. Hillslope lengths vary from by 80 meters between the transient and perched zones, respectively. Average gradients are higher for the limestone units in the transient zone. Curvature values are more negative in the unperched zone compared to the perched zones.

Past hydrological and tectonic activity modifies today’s hillslope morphology, as does the spatial distribution of resistant caprock in high elevations. By systematically identifying transient hillslopes we are building a geomorphic model of these landscapes, and set a foundation for applying cosmogenic data to quantify erosion rates.