WEATHERING PATTERNS ON THE JURASSIC NAVAJO SANDSTONE IN GRAND STAIRCASE ESCALANTE AND VERMILION CLIFFS NATIONAL MONUMENTS: IMPLICATIONS FOR MICROCLIMATES

The Jurassic Navajo Sandstone of the Colorado Plateau has distinctive surficial weathering patterns. Two localities compared in this study include knobs of Navajo Sandstone near Boulder, Utah just inside the Grand Staircase Escalante National Monument (GSENM) and White Pocket within the Vermilion Cliffs National Monument (VCNM) in northern Arizona. Mapping of polygonal (5 and 6-sided) and rectilinear weathering patterns show consistent results with the best developed (i.e. deepest) cracks on north-facing slopes. Some of these resemble “pillows” and can show nested scales of polygonal crack patterns (smaller polygonal cracks within larger polygons). East and west-facing slope aspects show moderate development of the weathering patterns, with east facing slopes better developed than west facing slopes, and the south-facing slope aspects have the weakest weathering crack development. The implications of the pattern occurrences on different slope aspects likely reflect host rock properties and microclimates, such as exposure to direct sunlight, that control the weathering crack patterns and where they are developed. The slope angle of Navajo rock face exposures is also a significant contributing factor to the weathering patterns. Slopes greater than 60 degrees commonly do not display weathering cracks. The “pillowed” type polygonal cracks are typically on slopes that range from 15-40 degrees. Recently exposed rock faces from fresh rock falls or spalling typically lack the weathering cracks, indicating that they are too young to have developed cracks. Down cutting of the Colorado River and its tributaries place a maximum age constraint that is likely on the order of several millions years, although the true age is difficult to determine. The fine-grained size of the sandstone presents difficulties in attempting cosmogenic exposure dating.

Although the two study areas are approximately 100 km apart, the similarities of the weathering patterns suggests that the north-facing slope aspects are influenced by the lack of direct sunlight and the corresponding increased moisture that the porous sandstone can retain. These polygonal cracks comprise a distinctive weathering pattern that records the microclimate history in the desert landscape of the Colorado Plateau.