WEATHERING OF JURASSIC SANDSTONES: COSMOGENIC NUCLIDE DATING OF GEOMORPHIC SURFACES IN SOUTHERN UTAH

Distinctive natural landscape features of the Utah desert include various “potholes” that are shallow pans (centimeters deep) to carved pools (up to 10 m deep) which collect rainwater and are enhanced by wind abrasion. Flat surfaces of homogeneous eolian sandstone are prime areas for potholes to form. Several potholes were examined using cosmogenic nuclide exposure ages from surfaces on the Lower Jurassic Navajo Sandstone and Middle Jurassic Entrada Sandstone.

Two shallow Navajo potholes with a simple geometry were sampled at the stable sides and at the pothole center floor. The center floor for one pothole showed a 11.8 kyr age, with stable sides ages of 19.8 kyr and 14.9 kyr. For a second pothole (~25 km away) the center floor showed 17.4 kyr with stable sides of 37.9 kyr and 38.6 kyr. Data suggest that pothole centers erode ~ 2 times faster than stable sides.

For very shallow Entrada potholes, the center floor of one pothole showed 2.1 ka with stable sides of 2.4 kyr and 3.2 kyr. A second pothole center floor showed 5.7 kyr with stable sides of 6.6 kyr. An adjacent third pothole center floor showed a 6.0 kyr age, and the stable sides showed 6.3 kyr. These data suggest that here the pothole center erodes only slightly faster than stable sides.

There are a number of challenges in estimating ages of these geomorphic surfaces that include a bias toward shallow potholes with a minimal amount of side/shade shielding, corrections for rainwater which may seasonally accumulate in potholes, and chemistry procedures necessary for quartz purification in small masses of fine 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, these cosmogenic nuclide ages provide a measure of recent natural weathering in a desert regime and suggest variable rates of weathering on eolian surfaces on the Colorado Plateau.