SIGNIFICANCE OF PARABOLIC RECUMBENT FOLDS IN PERMIAN ROCKS, SEDONA, ARIZONA

According to Allen and Banks (1972), subaqueously deposited cross-beds can penecontemporaneously deform via liquefaction to produce two major types of deformation: First are parabolic recumbent folds (PRFs) which appear like a series of parabolas lying on their sides; second are contorted cross-beds, where deformation is particularly present near the top of the set. In both types of deformation, bedding usually becomes faint or disappears near the top of the set. Although the exact mechanism of PRF formation is still debated, papers describing PRFs agree that strong water currents combined with liquefaction play major roles in overturning the top of a cross-bed set during deposition to form the fold types. PRFs are well documented in the literature from both fluvial and marine settings, modern and ancient. Published laboratory experiments have only produced the folds in subaqueous settings and attempts have failed to produce them in dry or wet subaerial sands.

The cross-bedded portions of the Schnebly Hill, Coconino Sandstone and Toroweap Formations (Arizona, USA) are considered by most to be primarily eolian deposits. However, we have recently found multiple PRFs in all three of these formations. Deformed cross-bed sets occur over a wide area (>375 km²) at many different locations and horizons, particularly in the Sedona area. Some PRFs in single cross-bed sets can be traced for 400 m along ridge tops. Field evidence shows the folding was penecontemporaneous. The folds in these rocks are quite specific and are identical in scale and form to PRFs produced experimentally in subaqueous sand and PRFs observed in many deposits of known subaqueous origin. There are specific features that distinguish PRFs from deformation structures in slumping eolian dunes. Some of the PRFs we have discovered are large-scale, ranging in size up to 5 m high, while slumping eolian dunes produce only small-scale folds and faults, usually at cm scales. There does not appear to be any evidence that the deformed beds are fluvial deposits within an eolian sand sea or that the deformation occurred by post-depositional groundwater movement or by seismic activity, as has been documented in other sandstones. These features are distinctly different. Where these folds occur, it suggests these formations were deposited by strong underwater currents.