POLYGONAL FRACTURE PATTERNS DEVELOPED BETWEEN CROSSBED-PARALLEL JOINTS IN THE NAVAJO SANDSTONE OF SW UTAH-- COOLING AFTER BURIAL BY A BASALTIC LAVA FLOW?

In southwestern Utah, we have found extensive, remarkably uniform, polygonal patterns of fractures in the Jurassic Navajo Sandstone. The outcrops with well- developed polygons are now laterally adjacent to and (topographically) about 10 m below a 6-24 m-thick basalt flow that erupted ~2.3 Ma, forming a cinder cone 3 km upslope. Sandstone outcrops occupying the vertical space between the polygons and the basalt are cut by closely spaced, irregular joints. Polygonal sandstone prisms are perpendicular to (and abut) joints that are sub-parallel to large-scale eolian crossbeds. In contrast, in south-central Utah, polygonal fracture patterns in Navajo outcrops are not associated with basalt flows and are common only between strongly convex sheeting joints that develop in non-stratified (structureless) sandstone (Loope & Burberry, 2018, Geosphere, 14, 1818-1836). The joints in the SW Utah rocks lie parallel to bedding and delineate tabular to lensoid sandstone slabs that range from 10 cm to 1 m thick. Eolian crossbeds in the Navajo comprise grainflow strata (> 1 cm thick, dip ~ 25°) intercalated with wind-ripple strata (thinner, lower angle). The polygonal fractures are best developed in grainflow strata. The polygonal patterns are dominated by hexagonal prisms ~50 cm in diameter, but some slabs display an orthogonal pattern of fractures of similar scale. Many of the bedding-parallel joints and some polygons abut widely spaced, through-going, vertical fractures interpreted to be of tectonic origin. Steep outcrops that face northeast (opposite direction of crossbed dip) provide cross-sectional views of the thick eolian crossbeds, and show that some polygonal and orthogonal joints developed at least 10 m below the modern land surface. Diurnal or seasonal thermal cycling cannot explain fractures at such depths. We have two working hypotheses to explain the polygonal fracture patterns: 1) They formed more than two million years ago via slow cooling of sandstone masses that were inundated by lava; 2) Tensile stresses are forming them today between sheeting joints in the shallow subsurface (they are analogous to the fractures being actively produced today within sheet-jointed granite slabs of the Sierra Nevada).