LATE PLEISTOCENE EROSION AND RETREAT OF A COLORADO PLATEAU CUESTA: INSIGHTS USING 36CL

The retreat of cliffbands acts as a mechanism for maintaining high-relief landforms on the Colorado Plateau. Many cliffs, particularly cuesta escarpments, serve an important role in landscape evolution by acting as local or even regional drainage divides. As cliff retreat moves an escarpment position over time, local drainage divides migrate as well, allowing one catchment to gain drainage area from another. Studying processes of cliff retreat provides insight into mechanics of the evolving Colorado River Basin, including landscape adjustment to lowered baselevels, variable erosion rates in different lithologies, and a regional transition from low to high-relief topography.

Below the Coal Cliffs of central Utah, colluvial diamicts have been topographically inverted by post-depositional erosion. Remnants of the diamicts are preserved on surfaces 10-50 meters above the modern zone of colluvium deposition. Extrapolation of these surfaces suggests a continuous, colluvial apron formed below the cliffs and was later incised by ephemeral streams. Also present below the Coal Cliffs are smaller bedrock escarpments striking perpendicular to the main cliffband. These features appear to be local migrating drainage divides transiently adjusting the landscape to a new baselevel. Field observations suggest they are a mechanism by which high relief is originally created and subsequently maintained along many Colorado Plateau landforms.

We present a ³⁶Cl surface exposure chronology with 32 ages from boulders on the abandoned colluvial apron. Measured ages consistently constrain apron deposition to the late Pleistocene (30–20 ka) and abandonment prior to the Holocene. Erosion rates average 0.5–2.5 mm/yr since boulder deposition, on par with rates measured elsewhere on the Colorado Plateau. Results suggest that cliff morphology is strongly controlled by baselevels of local streams. Increased discharge in response to changing hydraulic conditions up-catchment likely accelerated erosion below the Coal Cliffs at the end of the last glacial, implying possible connections between escarpment evolution and regional climate variability.