Sheeting joints and polygonal patterns in the Navajo Sandstone, southern Utah: Controlled by rock fabric, tectonic joints, buckling, and gullying

Abstract

Sheeting joints are ubiquitous in outcrops of the Navajo Sandstone on the west-central Colorado Plateau, USA. As in granitic terrains, these are opening-mode fractures and form parallel to land surfaces. In our study areas in south-central Utah, liquefaction during Jurassic seismic events destroyed stratification in large volumes of eolian sediment, and first-order sheeting joints are now preferentially forming in these structureless (isotropic) sandstones. Vertical cross-joints abut the land-surface-parallel sheeting joints, segmenting broad (tens of meters) rock sheets into equant, polygonal slabs ~5 m wide and 0.25 m thick. On steeper slopes, exposed polygonal slabs have domed surfaces; eroded slabs reveal an onion-like internal structure formed by 5-m-wide, second-order sheeting joints that terminate against the crossjoints, and may themselves be broken into polygons. In many structureless sandstone bodies, however, the lateral extent of first-order sheeting joints is severely limited by pre-existing, vertical tectonic joints. In this scenario, non-conjoined sheeting joints form extensive agglomerations of laterally contiguous, polygonal domes 3-6 m wide, exposing exhumed sheeting joints. These laterally confined sheeting joints are, in turn, segmented by short vertical cross-joints into numerous small (~0.5 m) polygonal rock masses. We hypothesize that the sheeting joints in the Navajo Sandstone form via contemporaneous, land-surface-parallel compressive stresses, and that vertical cross-joints that delineate polygonal masses (both large and small) form during compression-driven buckling of thin, convex-up rock slabs. Abrasion of friable sandstone during runoff events widens vertical tectonic joints into gullies, enhancing land-surface convexity. Polygonal rock slabs described here provide a potential model for interpretation of similar-appearing patterns developed on the surface of Mars.

Surficial fractures in the Navajo sandstone, south‐western USA: the roles of thermal cycles, rainstorms, granular disintegration, and iterative cracking

“…Although Loope and Burberry (2018) tied polygonal fractures in the Navajo sandstone to extensional stresses generated during buckling of sandstone slabs during formation of underlying, convex sheeting fractures, here we report evidence that requires rejection of that hypothesis. On broad, planar exposures in south-western Utah, we have found extensive, polygonal fracture patterns that are not closely associated with sheeting fractures.…”

“…Given their distribution, the rock temperature data, and the low rock strength, the simplest and best interpretation of the shallow fractures on Navajo sandstone outcrops (including the sheeting fractures as well as the polygonal fracture patterns) is that they were triggered by thermal stress. Shallow sheeting fractures (Bradley, 1963;Loope and Burberry, 2018) in the Navajo sandstone (including those that are interlaced with polygons) were triggered by rock expansion; the polygonal fractures perpendicular to the land surface were formed via rock contraction. (Figure 1), 22 km northwest of the St George, UT weather station.…”

“…The diverse and aesthetically pleasing landforms developed on eolian sandstones of the Colorado Plateau, USA have long been heralded by writers, photographers, hikers, and geologists (Abbey and Hyde, 1971;Crampton, 1973;Hamilton et al, 1984;Young et al, 2009). The sheeting fractures that form on the Plateau's naked rock surfaces (Bradley, 1963;Howard and Kochel, 1988;Loope and Burberry, 2018) drive the evolution of the landscape by accelerating mass wasting of the sandstone, but their origins and those of associated polygonal fracture patterns have received little attention. Studies of exposed granite using thermal and acoustic monitoring (Eppes et al, 2016;Collins et al, 2018) have demonstrated that diurnal insolation cycles and wetting events can drive subcritical fracturing that weakens outcropping boulders and bedrock, leading to the initiation and propagation of macroscopic fractures, and culminating in rock failure.…”

“…Chan et al (2008) showed that polygonal patterns similar to those exposed on the surface of Mars are widespread on outcrops of Navajo sandstone on the Colorado Plateau (Figure 1) and hypothesized that they may have originated via thermal stresses. Polygons are especially well developed in sheet-jointed exposures, where the two fracture types are interlaced (Loope and Burberry, 2018). Hexagonal fracture patterns (polygonal patterns that are dominated by "Y" vertices rather than the "T" vertices of orthogonal patterns) have been described from sandstones around the world (Johnston, 1927;Netoff, 1971;Branagan, 1983;Williams and Robinson, 1989;Robinson and Williams, 1992;Thomas et al, 2005;Young et al, 2009).…”

“…Hexagonal fracture patterns (polygonal patterns that are dominated by "Y" vertices rather than the "T" vertices of orthogonal patterns) have been described from sandstones around the world (Johnston, 1927;Netoff, 1971;Branagan, 1983;Williams and Robinson, 1989;Robinson and Williams, 1992;Thomas et al, 2005;Young et al, 2009). They are common in massive (unstratified) outcrops of the Navajo sandstone (Chan et al, 2008;Loope and Burberry, 2018). However, the most uniform, hexagon-dominated fracture systems that we describe here formed instead on multiple surfaces parallel to bedding (Figure 2).…”