This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Monday, July 1, 2019

Checkerboard Mesa, Utah: An example of ??

Checkerboard Mesa, Utah and companion mesa showing fracture pattern. Photo by SWK
One of the most distinctive and popular geologic features in Zion National Park, Utah, is Checkerboard Mesa near the east entrance of the Park. [Often overlooked is that it has a companion feature (at the right in the photo) showing similar features.] The distinctive features of these mesas are the sub-vertical and sub-horizontal cracks. The features, most prominent here, are actually found in a few places in the park and always on the North facing sides of slopes.

Detail of fractures. Photo by G. Lopez
 The cracks are in the Navajo Sandstone, a formation prominent in the spectacular cliffs of Zion. The Sandstone is over 2000 feet thick in the Park and is comprised of ancient desert sand dunes. The sub-horizontal lines are layers, called cross-bedding, within individual dunes. The dunes were compressed as material, now eroded, was deposited on top of them. Individual grains of sand were glued together by calcite (CaCO3) and iron oxides (which gives them the red color) to form the sandstone.
      Some details of these features are shown in the second figure. The pattern of horizontal and vertical cracks have been called a subset of  "polygonal cracks" in bedrock due to weathering (Chan et al., https://doi.org/10.1016/j.icarus.2007.09.026). As pointed out by Chan et al. , the sub-vertical cracks are, in detail, perpendicular to the layers in the wind-blown dune deposits of the Navajo Sandstone. They  change orientation when they encounter the bounding surfaces of the aeolian layers of the dunes, as shown in the detail of fractures in the second photo here. At the high elevations of these features in the Park, summers are hot and winters are cold. The north-facing orientation suggests a relation to freeze-thaw cycles that cause expansion and contraction of the rocks and cracks, a process referred to as freeze-thaw cycles.  Erosion is also enhanced by runoff from rain and water from melting snow. The Chan et al. interpretation is that the patterns are the products of tensile weathering stresses caused by temperature and moisture fluctuations (see also Loope and Burberry summary in Geosphere [14(4), 1818-1836, 2018]. These fluctuations cause expansion and contraction of the rocks, leading to the formation of the fractures through tensile stress development. Erosion is also enhanced by runoff from rain and water from melting snow.
Rectangular cracks perpendicular and parallel to bedding on Mars
     The pattern is particularly common is particularly common in porous massive sandstones found in semi-arid climates characterized by large temperature and moisture fluctuations, and has been observed in outcrops on Mars (see third figure, which is Figure 1F from Chan et al.) Cracks form perpendicular to outcrop surfaces and are thin and limited in their penetration into the host rock. Rectangular cracks form perpendicular and parallel to cross beds where the host rock is anisotropic, but where the sandstone is massive (i.e., isotropic), 5- and 6-sided polygonal forms develop. Differential erosion along the cracks compared to the polygon surfaces gives the pattern a domal relief on a microscale.


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