“RIPPLE MARKS,” “SLUMP FEATURES,” AND “RAINPRINTS” IN THE COCONINO SANDSTONE NEAR ASH FORK, ARIZONA

Low-relief sedimentary structures were observed on Coconino Sandstone (Permian) bedding planes near the town of Ash Fork, Arizona. Structures were described and photographed from a single cross-bed set with beds measuring at least 51 m long and dipping at an average of 22.6° (n = 33). Both massive and laminated textures were observed within the set. Each type of structure was distributed vertically throughout the cross-beds, across various dip angles, and on both massive and laminated bedding. Similar structures have also been noted in Coconino Sandstone outcrops near the town of Seligman (~40 km west of Ash Fork). McKee (1945) interpreted these structures as eolian ripple marks, slump features, and rainprints.

Ripple marks were oriented with crests (spaced ~9-16 cm apart) generally parallel to the dip of the cross-beds. Slump features varied in morphology, with several examples exhibiting the “step-like” form that McKee (1945) described. Rainprints were scattered randomly across bedding planes or arranged in linear bands parallel to the cross-bed dip direction. Different structures were commonly associated on a given plane; for example, both the “rainprint bands” and slump features were often observed with ripple marks.

In eolian sandstones, laminated cross-beds are typically interpreted as wind ripple or grainfall deposits, while massive beds are thought to be avalanche deposits. Slumping is an angle of repose (30-34°) process associated with avalanching, but these slump features occur on beds dipping at angles in the low to mid-20s and as low as 18°. Furthermore, the preservation of true “rainprints” in linear, parallel bands has not been documented. More work must be done to explain these anomalies that appear difficult to reconcile with typical eolian processes.