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Origin of Polygonal Fractures in Sand, Uppermost Navajo and Page Sandstones, Page, Arizona
Gary Kocurek, Ralph E. Hunter
Polygonal fractures in eolian sandstones occur along the upper surface of the Navajo Sandstone and along four additional surfaces within the Page Sandstone, near Page, Arizona. The polygons are largely rectangular and range from 1 to 6 m across. Most of the fractures are less than 1.5 m deep, and fracture widths range from "hairlines" to tens of centimeters or more. Fracture fill most commonly shows vertical layering, and, in some cases, a series of layers is evident symmetrical about the midline of the fracture. Other fractures are filled with contorted or structureless deposits, subhorizontal laminae, and displaced or brecciated host strata. In some cases, surfaces marked by polygonal fractures can be shown to have been topographically low at the time of deposition, and the fracture sandstone below these low areas contains calcite or siliceous nodules and siliceous cement.
Interpreted as contractional in origin, the polygonal fractures formed on an exposed evaporite-encrusted surface in a manner similar to ice wedges. Irregularities in these fractures are attributed to erosion by wind or water. Evaporites, evidenced by anhydrite inclusions, pore shapes, megaquartz replacement, and pseudomorphs of evaporitic minerals, provided the cohesion and contraction that caused the tensional stresses necessary for polygonal fracturing. Stresses were the result of thermal contraction. Such surfaces are thought to represent hiatuses in eolian sand-sea development, and probably represent deflation to near the water table during net deflationary conditions.
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