BOULDER ARMORING OF DESERT LAND-SURFACES: EXPOSURE AGES AND EROSION RATES FOR THE NORTHWESTERN COLORADO PLATEAU

Topographic inversion is a process where valley floors become more resistant to erosion than the local bedrock and are inverted through time due to the difference in erosion rates. The "armoring" of valley floors can occur because of a variety of processes, but lava flow deposition and duricrust formation (calcrete, silcrete) are the most commonly recognized. Another process that can armor valley floors is the deposition of coarse boulder deposits (debris-flows, colluvium) predominantly comprised of rocks more resistant than the local bedrock. This type of armoring will be particularly effective in locations where the drainages being armored are small and lack discharges sufficient to remove or rework the coarse deposits.

In the Capitol Reef – Escalante area of the northwestern Colorado Plateau there are tens to hundreds of individual basaltic-andesite armored surfaces throughout the landscape. These armored surfaces are former valley floors that are now 30-200 m above the local drainages. All of these armored surfaces are capped with coarse basaltic-andesite boulder deposits derived from the high (~ 3400 m) volcanic plateaus of Boulder or Thousand Lakes Mountains. Using 3He exposure age dating we determined the exposure ages of several boulders from three of these surfaces. Based on the internal stratigraphy, maximum grain size, and closeness to the high plateau source we interpret these deposits to be ancient debris-flows and therefore assume deposition was rapid and do not include a correction for cosmogenic inheritance due to transport. We do include a correction for non-cosmogenic (nucleogenic) 3He produced in the basaltic-andesites since crystallization (~25 Ma). Maximum boulder exposure ages of these land-surfaces range from 1.2 Ma to 260 ka and represent average local erosion rates of 0.16 m/ka to 0.33 m/ka respectively. The local erosion rates we calculate appear to be related to a previously established erodibility classification of Colorado Plateau bedrock types. The range in exposure ages and common occurrence of these surfaces suggest that boulder armoring is an essential component of landscape evolution in this part of the Colorado Plateau.