Polygonal linear depressions in the Coconino Sandstone (Permian) of Arizona, and their relevance for interpreting paleoenvironment


 Polygonal “cracks” are common in the Coconino Sandstone in Arizona. They have been called desiccation cracks, but several features indicate they are not desiccation cracks. They were never open cracks, but are merely linear depressions, linked to form polygons. They occur only on bounding surfaces, containing almost no clay, and the cracks extend 10 to 15 cm above and below the bounding surfaces. The polygonal patterns continue down from one sandstone lamina to another, for several centimeters. They are persistently continuous across all surfaces within their 20–30 cm vertical range, from the bottomset beds, onto the bounding surface, and continuing into individual cross-beds below the bounding surface. The cracks occur at the Grand Canyon, and are especially numerous and visible in flagstone quarries in the Seligman and Ash Fork area. They occur on some bounding surfaces but not on others, and in some quarries but not in others. The polygonal cracks have been mentioned in passing, but this is the first reported research on these cracks in the Coconino Sandstone. Polygonal cracks have been reported in the Navajo, Page, and Entrada Sandstones, but there are significant differences between these and the Coconino Sandstone cracks, which may indicate differences in their origin.