EFFECTS OF STRATIGRAPHIC VARIATION IN ROCK STRENGTH ON EROSION RATE PATTERNS IN LANDSCAPE EVOLUTION FROM NUMERICAL MODELS AND COSMOGENIC SAMPLING IN GRAND CANYON AND GRAND STAIRCASE

Rock strength variation can affect topography and erosion rate response to baselevel fall. LithoCHILD simulations imply diagnostic erosion rate differences between 4 scenarios, where the first two are nearly identical: 1) weak-over-strong rock with steady baselevel fall rate and, 2) weak-over-strong rock where baselevel fall rate increases once stepwise. In both scenarios, encountering the strong layer produces upstream migrating knickpoints. Until knickpoints reach channel heads, the initial baselevel fall rate is maintained in the headwaters. Thus the first scenario yields erosion rates in headwaters that equal erosion rates in the canyons due to constant baselevel fall. The second scenario yields erosion rate in the headwaters that match initial baselevel fall rate but the canyon erosion rates match the new, higher baselevel fall rate. In simulation 3) strong-over-weak rock with steady baselevel fall produces a scarp along the contact caused by efficient erosion of the weak rock which effectively undermines the strong rock. The average erosion rates in 3 are higher than baselevel fall rate. 4) Repeated weak/strong couplets of rock are eroded, resulting in average erosion rates that track baselevel fall. Simulations also include ¹⁰Be production in hillslopes and tracked to channel sediment.

Incision rates calculated from dated Quaternary-age river terrace remnants and catchment-averaged, cosmogenic erosion rates are roughly similar in the Grand Canyon (GC, ~150 m/Ma). These rates exceed incision rates in the plateau north of the canyon, called the Grand Staircase (GSc, Utah, USA, ~75 m/Ma), like 2. ¹⁰Be erosion rates from 8 catchments in the GSc sample a strong unit exposed as a scarp, with apparent erosion rates >200 m/Ma but a baselevel fall rate of 75 m/Ma. This disparity is like case 3 above, where the upper, stronger rock unit is effectively undermined. The GC cosmogenic erosion rates, which sample numerous rock layers, match incision rates as expected from simulation 4. Thus GC and GSc are therefore interpreted as co-evolved landscapes that result from an increase in baselevel fall rate in the last few million years that is acting on stratigraphy with varied rock strength, providing complex erosion rate patterns that are predictable patterns based on combined landscape evolution and ¹⁰Be inventory modeling.