UNUSUAL BEDDING STYLES FOR THE COCONINO SANDSTONE (PERMIAN), ARIZONA

The Coconino Sandstone typically contains large-scale, planar-tabular cross-stratification, often interpreted as eolian dunes. We report one-meter-thick and up to forty-meter-long massive beds in the Grand Canyon area at the base of parts of the formation along Grandview Trail, Hance Trail, South Fork of Rock Canyon and near the Navajo Bridge. These beds sometimes contain stratified clasts of sandstone and are intimately associated with sand injectites penetrating into the Hermit Formation below. Higher up in the formation, several flat-bedded horizons (up to 2 m thick, laminated and often contain carbonate cement) have been found along Hance and Hermit Trails and the Navajo Bridge. Similar zones have been found in the Sedona and Holbrook areas. The Sedona flat beds are laterally extensive and cover at least tens of square kilometers. Preliminary petrographic study seems to indicate the flat-bedded zones do not differ much in grain size and sorting from the cross-bedded portions of the sandstone; with the exception of some medium sand in the flat-bedded portion of the Navajo Bridge section. In Sedona, one- to four-meter-thick sequences of contorted and convolute bedding (contemporaneous with deposition) have been found sandwiched between flat-bedded horizons. The convolute layers are laterally extensive and may also be continuous for many square kilometers. We have previously reported thin, flat dolomite beds within the Coconino in the Andrus Point area.

The massive beds and associated injectites are indicative of seismic activity in water-saturated and unlithified sand that occurred after deposition. The flat beds and the dolomite beds are indicative of aqueous deposition for at least parts of the formation. It is possible for flat-bedded horizons to form in eolian settings such as in between dunes, in interdunal ponds, or at the perimeters of a dune field. When flat-bedded sediments occur in eolian settings they are notoriously poorly sorted, often quite localized, characterized by discontinuous laminae and lag deposits. The flat-bedded horizons described here do not have these characteristics, suggesting they were formed by subaqueous processes. We are not sure of the origin of the convolute beds; eolian dune slumping is problematic because there is no slope for sliding (the convolute beds lie on flat beds).