Abstract

Distinctive natural landscape features of the Utah desert include weathering pits that are typically broad (meter-scale), shallow (10-50 cm) and circular in plan view. Exceptionally large pits are >30 m wide and >15 m deep. The pits commonly occur on sandstone bedrock of Permian to Jurassic age. Five different pits were examined using cosmogenic nuclide exposure dating of surfaces on the Lower Jurassic Navajo Sandstone and Middle Jurassic Entrada Sandstone to understand better the geomorphic history of weathering in the U.S. southwest desert.

Several Navajo weathering pits were sampled at the stable host rock areas surrounding the pits and the pit floor center. The pit floor of one shallow weathering pit yielded an exposure age of 20.0 (± 1.9) ka, with stable side host rock ages of 18.3 ka (± 1.8) and 25.7(± 2.4) ka. A second shallow weathering pit (~25 km away) floor gave an age of 22.7 (± 2.1) ka with stable side host rock of 62.8 (± 5.4) ka and 64.7 (± 5.6) ka. Our data suggest that pit floors erode only slightly to as much as two times faster, than the stable host rock surrounding the pits.

For very shallow pits in the Entrada Sandstone, the floor of one weathering pit yielded an age of 2.3 (± 0.2) ka with stable walls and rims of 2.5 (± 0.3) ka and 3.4 (± 0.4) ka, respectively. These data suggest that here, the weathering pit floors erode only slightly faster than stable side host rock.

There are challenges in estimating exposure ages of these geomorphic surfaces that include: a bias toward shallow pits with a minimal side/shade shielding, corrections for rainwater which may seasonally accumulate in pits and affect exposure ages, and chemical procedures necessary for quartz purification in small masses of fine-grained eolian sands. Uncertainty in ages may range up to 25% due to inherent analytical error on short exposures as well as various potential correction errors. Despite the challenges, our cosmogenic ages provide a measure of recent, variable natural weathering rates on eolian bedrock surfaces of the Colorado Plateau.