A NEW MECHANISM OF ESCARPMENT RETREAT: QUANTIFYING THE PROCESSES OF LATERALLY-PROPAGATING “WAVES OF EROSION”

Escarpments beveled into horizontal, layered sedimentary rocks are primary geomorphic features of continental interiors. Long-term changes in escarpment position can profoundly affect certain Earth systems (i.e. regional geothermal gradients, crustal isostasy, drainage basin geometry, etc.) and their associated feedbacks, compelling a detailed understanding of escarpment retreat and its underlying mechanisms. Here, we recognize a unique erosional process (which we call a “wave of erosion (WOE)”) that may accommodate substantial amounts of escarpment retreat under specific lithologic and hydraulic conditions.

In central Utah, rivers traverse >100 m sections of mechanically weak shales stratigraphically bounded by more competent sandstones. Rheological differences between these two units cause them to exhibit contrasting erosional responses to a common forcing. Below several escarpments composed of shales capped and underlain by massive sandstones, we observe smaller escarpments (WOEs) trending perpendicular to their larger counterparts. Each WOE acts as a local drainage divide which propagates laterally along the base of its parent escarpment, gradually increasing the parent’s relief and, thereby, rate of retreat. The timescales over which WOEs are created and later propagate along various, staircase-like escarpments may provide a first-order control both on local escarpment morphologies and regional spacing between escarpments.

We seek to constrain modern WOE propagation rates below several escarpments in central Utah. Field observations suggest propagation is controlled by two factors: knickpoint generation in channels below the WOE and the subsequent response to those knickpoints by regolith-mantled hillslopes in the channel headwaters. Here, we begin by comparing the morphology of several different WOEs and obtaining in-situ erosion measurements. High-resolution (<10 cm) digital elevation models are constructed using structure-from-motion photogrammetry. Erosion pins are placed along WOE hillslopes and in channels to directly measure hillslope erosion, regolith flux, and knickpoint propagation rates over several years. Future work will use this data to calibrate numerical model simulations of knickpoint propagation and hillslope adjustments to baselevel lowering.