SHEAR AND CLAY DRAPES BETWEEN CROSS-BEDS 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.

Where cross-beds have been eroded to form a bounding surface, it could be expected that such bounding surfaces will be a continuous, flat surface. In the Coconino Sandstone this is often not the case. Some bounding surfaces are indeed a flat surface, with no indication of any movement of individual cross-beds after the formation of the bounding surface. However, on some bounding surfaces it is evident that there has been shearing movement between individual cross-beds, after the formation of the bounding surface. This has produced up to several centimeters of vertical offset between individual cross-beds, producing an uneven bounding surface.

Dental molding material was used to document the nature of bounding surfaces, where there were and where there were not offsets. Then cores were taken through the sandstone, adjacent to the documented surfaces. Thin sections of the cores were examined to compare the nature of contacts between cross-beds at these intervals. At the contacts showing shear and offsets, there was frequently a distinct clay drape, primarily illite, between the individual cross-beds. These clay drapes were not present at contacts with no offset at the bounding surface. There is also a small amount of clay scattered through the Coconino Sandstone, mostly kaolinite. This scattered clay is not more abundant at bounding surfaces with offset contacts.

It appears that the clay drapes have facilitated shearing movement at these contacts, allowing the offsets at the bounding surface. The presence of the clay drapes at specific intervals implies the presence of water at these intervals, with changes in water energy facilitating the deposition of the clay drapes.

It may be that these clay drapes are the key to the commercial success of the flagstone industry in the Coconino Sandstone, causing the sandstone to break cleanly along these clay layers.