CLIFF RETREAT DYNAMICS MODULATE SIGNALS OF BASE-LEVEL CHANGE IN LAYERED ROCKS, COLORADO PLATEAU, USA (Invited Presentation)

The retreat of cliffs is an important mode of erosion in layered rocks of variable strength. For example, the iconic Colorado Plateau landscapes of Grand Canyon, Canyonlands, and Monument Valley owe their unique forms to this process. Interactions between stochastic rockfall and first-order channels draining a cliff complicate the cliff retreat response to a base-level fall. The broad-scale response to incision in these landscapes therefore depends upon the local dynamics of cliff retreat.

The amount of talus covering the footslopes of cliffs is commonly assumed to be modulated by climate. These systems also respond to the arrival of “waves of erosion” propagated as knickpoints from downstream. These knickpoints may be climatic or tectonic in origin, but might not reflect the local climate conditions at the time of their arrival. These hypotheses have different implications for the long-term patterns of cliff retreat and its role in exhuming the modern Colorado Plateau.

We present ongoing work at a natural experiment site in Utah. The site has a sandstone-over-shale stratigraphy with an ongoing transient response to base level fall. Our work integrates high-resolution DEMs, cosmogenic dating, and a 2D numerical model of cliff erosion. The model simulates fluvial and hillslope erosion, and rockfall from resistant units, including talus redistribution. Our results show that, in escarpments with multiple cliff-forming layers, cliff retreat response to baselevel change is complex. The base level signal reaches the upper cliffs when knickpoints propagate along strike between the resistant layers from transverse drainages. In this sense, resistant layers act as waveguides for erosion pulses. As a result of these effects: 1) the separation between the upper and lower cliffs expands and contracts through time; 2) incision waves reach the upper cliff after a significant lag and work to increase cliff height progressively along the cliff; 3) a single baselevel drop may become several separate waves of incision before it is felt by the upper cliff; and 4) rockfall debris then buffers the backwearing response of the cliff itself to the arriving incision pulse.