MICROCLIMATES AND THERMAL PROFILES OF THE JURASSIC NAVAJO SANDSTONE: IMPLICATIONS FOR WEATHERING CRACKS AND SURFACE CRUSTS

A well exposed 3-dimensional knob near Boulder, Utah exhibits characteristic weathering cracks and surface hardening crusts common to massive facies of the Jurassic Navajo Sandstone across much of southern Utah. To test the potential effects of microclimates on the weathering phenomena, temperature probes were emplaced in 2” diameter cored holes within the sandstone on each aspect (north, south, east, and west faces) of the knob along a consistent stratigraphic horizon (6600’ +/- 5’ asl). Three probes on each aspect recorded temperatures at 10cm, 20cm, and 30cm depths, running from August 2011 to August 2012. This rock temperature data was correlated and compared with maximum and minimum air temperatures collected through the Western Regional Climate Center at Boulder.

Analysis of the rock temperatures on different aspects suggests that freeze-thaw cycles contribute to differential weathering features around the Navajo outcrop. During winter months, the temperature probes on the north aspect recorded multiple days of freeze/thaw at all 3 depths during the winter, while the west was significantly warmer, falling below 32°F on only 2 days at 30cm. Temperature within the sandstone was significantly higher on south and west aspects, up to 10°F warmer than the north aspect at 10cm depths. Measured large weathering crack sizes around the knob on north, east, south, and west facing aspects average 431 cm, 357 cm, 232 cm, and 116 cm, respectively. The best developed (i.e., deep and large) polygonal weathering cracks occur on north-facing slopes and show nested crack patterns (smaller polygons within larger ones). A cohesive, yet thin, 1-2 mm surface crust occurs in many of the weathered Navajo Sandstone exposures where lichen and endolithic bacteria can retain moisture. Moisture availability, porosity, and low winter to low early spring insolation likely play a significant role in the weathering crack development as well as the development of thin, microbial surface crusts.