POLYGONAL CRACKS ON BOUNDING SURFACES IN THE PERMIAN COCONINO SANDSTONE, ARIZONA

The Permian Coconino Sandstone in northern Arizona is a cross-bedded fine sandstone, and most workers consider it to be an eolian deposit, from an extensive desert.

Polygonal cracks are common on bounding surfaces in the Coconino Sandstone. Superficially they look like desiccation cracks, but their detailed features are incompatible with that interpretation. The clay content in the areas with these cracks is too low to support shrinkage into desiccation cracks. Also, when seen in cross-section it is evident they were never open cracks. Each sandstone laminae is depressed in the polygonal pattern, with these depressed patterns continuing for at least several cm down from the surface.

The polygonal cracks occur only associated with bounding surfaces, and continue about 10-15 cm above and below the bounding surface, measured at right angles to the bounding surface. The polygonal patterns are persistently continuous 3-dimensional features, continuing from the bounding surface down the face of each cross-bed, and up into the bottomset beds above the bounding surface. Beyond 10-15 cm from the bounding surface the cracks fade out and disappear.

Since the cracks occur on a bounding surface and in the cross-beds both above and below the bounding surface, they must have occurred as one event, after all the beds involved at that bounding surface were present. They could not be from desiccation on the bounding surface, since the patterns are continuous from the bounding surface up into the bottomset beds above it.

Polygonal cracks have been found in several places at the Grand Canyon and are abundant in flagstone quarries near Seligman and Ash Fork, Arizona. They have not been found farther east and south at Holbrook or Strawberry, Arizona. Their distribution is very patchy. They are stratigraphically widespread in the Coconino Sandstone, but not on every bounding surface. On a particular stratigraphic level they are on some bounding surfaces but not on others. They also occur on parts of a particular bounding surface, but not on other parts of the same surface.

I suggest that some stress, perhaps seismic stress, has caused more instability at the depositional discontinuity associated with bounding surfaces, resulting in settling of the sand into these polygonal crack patterns.