2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 229-31
Presentation Time: 9:00 AM-6:30 PM


MAITHEL, Sarah A.1, BRAND, Leonard R.1 and WHITMORE, John H.2, (1)Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, CA 92350, (2)Department of Science and Mathematics, Cedarville University, 251 N. Main St, Cedarville, OH 45314, smaithel@llu.edu

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.