MORPHOLOGICAL ANALYSIS OF “GRAINFLOW” CROSS-STRATA IN THE COCONINO SANDSTONE (PERMIAN), ARIZONA

Massive cross-beds in the Coconino Sandstone (Permian) of northern and central Arizona are conventionally interpreted as eolian grainflow deposits, but detailed studies on these beds have not been published. A description of massive cross-bed morphology, including lateral width, dip angle (and down-dip variation in dip angle), and the nature of bedding near the bases of cross-bed sets, may prompt a new interpretation for these beds.

Bedding plane measurements, strikes and dips, and photographs were taken at outcrops near the towns of Ash Fork, Holbrook, and Seligman, Arizona. Massive cross-beds generally appeared laterally-extensive along-strike: while some measured bedding planes were only exposed for a short distance, others extended up to 10-20 meters or more (n=394). Complete narrow “tongue-shaped” beds were not observed. The massive beds (average: 20.1°, range: 11-29°, n=87) dipped at slightly steeper angles than the laminated beds (average: 18.3°, range: 3-25°, n=35) overall. While both massive and laminated beds dipped at angles below the angle of repose (30-34°), the steepest dips were associated with massive beds near the tops of some cross-bed sets. This may imply an overall down-dip decrease in dip-angle in those sets. We did not observe sharp pinch-outs at the lower ends of massive beds; where exposed, they appeared to fade into lamination near the set bases.

The massive cross-beds observed in these study areas display characteristics that appear incompatible with the eolian grainflow interpretation, since modern eolian grainflows typically exhibit a narrow “tongue-like” form, dip at angles near the angle of repose along their lengths, and pinch out abruptly at their bases. This study of cross-bed morphology, in conjunction with textural data, should enable us to move toward a refined depositional model for the cross-bedding in the Coconino Sandstone.