THERMAL CONTRACTION CRACKS IN UTAH SLICKROCK

Fracture patterns dominated by Y-type intersections and 5-, 6-, and 7-sided polygons are well-developed on broad, gently sloping land surfaces underlain by Navajo Sandstone near St. George, Utah. As rock slopes steepen, these patterns smoothly change to slope-parallel, orthogonal patterns involving bedding planes and fractures. We interpret these fracture patterns as products of stresses that developed during annual thermal cycling. Fractures terminate against sheeting joints and some bedding planes, and likely penetrate no farther than 1 m below the land surface. Along the land surface, the fractures are seen to end at the edges of alluvial and eolian deposits, apparently because loose sediment shields underlying bedrock from the steepest thermal gradient. Polygonal fractures abut scattered sheeting joints from both above and below. Continuity of polygonal fracture patterns along the modern topographic surface shows that stresses forming sheeting joints were independent of those that generated polygons. Our interpretation largely supports the work of others on fractured sandstone (southern Utah), and granite in the Sierra Nevada. On steep outcrops, fractures that abut sheeting joints give the illusion that the fracture networks grew vertically in the subsurface. A better interpretation is that lateral propagation of surficial fractures led to this relationship.

Cyclic air temperature fluctuations alter temperature gradients in the shallow subsurface. Cooling of the land surface in the coldest month steepens the subsurface temperature gradient, generating tensile stress at the land surface. At St. George, the difference between mean annual air temperature (63.9° F; 290.9° K) and average air temperature for the coldest month (December; 41.3° F; 278.3° K) is 12.6° K. Assuming a Young’s Modulus of 20 GPa and a specific density of 0.47, this temperature regime generates sufficient tensile stress (~7.1 MPa) to fracture Navajo Sandstone (tensile strength <3 MPa) at the land surface. Counterintuitively, in the conterminous United States (especially in the intermountain west), sites with the greatest difference between these two means lie at low elevations. These sites get much hotter in summer than high elevation sites, which, because they are cool or cold year-round, have lower annual means.