MORPHOLOGY OF AVALANCHE BEDS IN THE COCONINO SANDSTONE AT CHINO WASH, SELIGMAN, ARIZONA

Avalanching is the primary depositional process on the foresets of large dunes, occurring as sand accumulates beyond its angle of repose (~30-34°) and slides down the slipface. The large cross-beds in the Permian Coconino Sandstone are commonly interpreted as the preserved foresets of eolian dunes, and massive (non-laminated) cross-beds are usually thought to be avalanche deposits.

While avalanches occur on both eolian and subaqueous dunes, the resulting deposits tend to differ in their morphology. In dry sand, avalanche deposits are narrow “tongues” that are lenticular in cross-section, whereas subaqueous avalanche beds may appear as very wide tongues or tabular sheets. Avalanche deposits in eolian and some low-velocity subaqueous environments tend to dip at high angles, near the angle of repose, but beds deposited by continuous subaqueous avalanching may dip at lower angles.

Cross-beds exposed on a 25.5 m² bounding surface in a large outcrop in Chino Wash, near Seligman, AZ were photographed and described. The beds were observed on the nearly horizontal (13-14° dip) bounding surface and in the underlying cliff. Dip angles for massive beds throughout two outcrops in Chino Wash ranged from 10-24° and averaged 18.1° (n=51).

Both massive and laminated beds were visible on the bounding surface, though most beds were massive. While some of the massive beds pinched out in the photographed area, many were laterally extensive and could be traced at least 4.5 m. Most of these beds were partially covered and were likely more extensive since they did not pinch out at the edges of their exposures.

The wide nature of the massive beds observed in Chino Wash is surprising if they were deposited by the avalanching of dry sand, suggesting that another mechanism may be needed to explain the width of the cross-beds in the Coconino Sandstone.