SOFT-SEDIMENT DEFORMATION FEATURES IN THE PERMIAN COCONINO SANDSTONE, ARIZONA, POSSIBLY PRODUCED BY SEISMIC ACTIVITY

The Permian Coconino Sandstone is a cross-bedded, fine sandstone which most workers consider to be an eolian deposit. Soft-sediment deformation features (SSD) are common in the Coconino Sandstone. Some of these structures were observed at the Grand Canyon, but they were most readily studied in retired flagstone quarries near Ash Fork, Arizona. Such features were found in six quarries where extensive foreset surfaces were exposed.

Most of these deformation structures consisted of narrow ridges and folds on foreset surfaces, ranging from 2-10 cm wide and from a few cm to 10 m long. The features that were prevalent in quarries near Ash Fork varied from approximately straight to zigzag in shape. In some locations, displaced pieces of sandstone were observed within individual cross-beds. Where exposed in cross-section, the deformation appeared to be fairly shallow: sandstone laminations 2 to 3 cm (sometimes more) below the structures were undeformed. These features occurred on foreset slopes with dips of 18-26^o. In one quarry, a cross-bed surface had a significantly along-strike curve, with dip direction values ranging from 8^o on one side of the curve to 80^o on the other side. SSDs were abundant on the 8^o surface, but nonexistent on the 80^o surface.

If the structures were produced by sediment loading, we might expect them to be oriented along the strike of the cross-bedding. However, they are seldom oriented that way. Short folds, less than 2.5 m in length, were oriented randomly, while longer folds (3.5-10 meters in length) were oriented consistently along the dip of the cross-beds (approximately north-south).

We are evaluating the hypothesis that these SSDs may have been produced by seismic events.