DIFFERENTIATING THE UNDIFFERENTIATABLE? A CLASSIFICATION OF HARD-TO-DISTINGUISH DUNE STRATIFICATION TYPES IN THE PERMIAN COCONINO SANDSTONE

The Permian Coconino Sandstone (northern and central Arizona, USA) is widely known for its distinctive, large-scale cross-bedding, but little has been published on the fine-scale processes that may have deposited its cross-beds. Furthermore, interpreting such processes in this formation is a challenging task, since individual dune stratification types – grainfall, grainflow, and ripple lamination – are hard to distinguish at the outcrop scale.

We used a suite of textures and sedimentary structures to interpret cross-bed depositional processes in the Coconino Sandstone. Data were collected via a combination of field measurements and laboratory analyses to characterize cross-bedding at multiple scales. We found that most beds are laterally extensive along strike and dip at angles ranging from the mid-teens to mid-twenties. Grain sizes become coarser near the base of several cross-bed sets, but vertical changes in texture vary within a single set. High-resolution scans of thin sections reveal massive and laminated microfacies, and while some inversely graded lamination was observed, laminae contacts are commonly diffuse and grading is difficult to define. Stylolite seams are also prevalent and resemble textural lamination in some slides. Samples from one outcrop contain pores that are significantly larger than adjacent framework grains, and these may indicate dissolution of carbonate clasts or cement.

Based on the observed bedding geometry, textural trends, and microfacies, we suggest that the cross-beds are best interpreted as undifferentiatable grainfall and grainflow deposits, which may explain why individual stratification types are hard to distinguish in outcrop and thin section. This characterization of fine-scale processes will serve as a critical part of the endeavor to develop and refine larger-scale depositional models for the Coconino Sandstone.