VOLCANIC BOULDER ARMORED SURFACES IN THE ESCALANTE CANYON SECTION OF THE GRAND STAIRCASE ESCALANTE NATIONAL MONUMENT

In the Escalante Canyon section of Grand Staircase Escalante National Monument there are tens to hundreds of individual volcanic boulder armored surfaces throughout the landscape. These armored surfaces are former valley floors that now stand 10–200 m above the modern drainages. All of these armored surfaces are capped with coarse basaltic-andesite boulder gravels derived from the high (>3200 m) volcanic plateaus of Boulder Mountain and Awapa Plateau. Soils forming in the highest (oldest) of these deposits have strong carbonate horizons with Stage II-III carbonate development. Using 3He exposure age dating we determined the exposure ages of multiple boulders from four of the highest and oldest surfaces in the area. We interpret these deposits to be from proximal debris-flows, and therefore assume deposition was rapid and do not include a correction for cosmogenic inheritance during exhumation and transport. We do include a correction for non-cosmogenic (nucleogenic) 3He produced in the basaltic-andesites since crystallization (~25 Myr). 3He exposure ages of boulders on the New Home Bench surface range from 380±15 to 656±18 ka while boulders on Durffey Mesa range from 349±8 to 759±17 ka. Boulders from the Deer Creek Bench range from 308±11 to 1,222±47 ka and boulders from the Black Mesa Bench yielded ages ranging from 490±13 to 683±27 ka. There is a strong correlation between individual exposure ages and boulder heights above the present land surface, suggesting that boulder surface erosion and/or deposit exhumation have affected the exposure ages. We therefore interpret the maximum boulder age from each surface to be a minimum limiting age for the deposit capping each surface. Using the minimum limiting ages we estimate bedrock incision rates (maximum rates) of 160 to 320 m/Myr for northern tributaries of the Escalante River. Hypotheses to explain the abandonment of the highest and most prominent of these armored surfaces include the un-roofing of the Navajo Sandstone and the incorporation of that major aquifer into the surface hydrologic cycle, a fundamental shift in the Earth's climate state and variability since the Early Pleistocene, and changes in the rate of retreat of the southern edge of the Boulder Mountain plateau.