UNEXPECTED DISCOVERIES IN THE COCONINO SANDSTONE (PERMIAN, ARIZONA)

Edwin McKee published the first comprehensive study of the Coconino Sandstone in 1934, and since then, little original research has appeared on this prominent Grand Canyon formation. Over the past 20 years, we visited numerous outcrops covering the thickness and areal extent of the formation and studied 400 thin sections with the goal of better understanding the nature of this well-known but little-studied formation. The following is a summary of our published findings.

Regarding petrology, the sand has an average grain size of about 3.0 ϕ, is moderately sorted, subrounded to subangular and increases slightly in grain size and sorting from north to south (in the same direction as cross-bed dip). Dolomite occurs widely as occasional beds, ooids, clasts, cement and rhombs. Detrital muscovite occurs in nearly every thin section; sometimes the flakes are bigger than the quartz grains that surround them. Detrital K-feldspar comprises about 10% of most samples and is often angular or subangular. Thin sections show little indication of significant compaction, most having 10-15% porosity.

Regarding sedimentology, our large data set of cross-bed dips average about 20º and are similar to Reiche’s data (1938). Narrow grainflow tongues, which commonly occur on eolian dunes, were not observed; instead, parallel laminae can be traced for many meters along strike on bounding surfaces. Patterns resembling primary current lineation occur on most foreset surfaces and trend down-dip. Especially in western outcrops of Coconino, linear bands of circular pits can be found, also trending down dip. Along Tanner Trail, the Coconino interfingers with the underlying Hermit Formation. Our published data suggest that tabular sand bodies extending downward into the Hermit in Grand Canyon are sand injectites and not desiccation cracks due to their orientation, internal structure, association with the Bright Angel Fault, and the sparsity of clay (< 3%) in the Hermit. In Sedona, multiple examples of large-scale soft sediment deformation features, confined between sets of planar- and cross-beds, extend continuously for 50-400 m in dip direction. The deformation matches the characteristics of subaqueous parabolic recumbent folds. Considering all these data, it is time for more work in order to refine depositional models for the Coconino Sandstone.