NEW INSIGHTS ON CROSS-BED FACIES IN THE PERMIAN COCONINO SANDSTONE (ARIZONA, USA) FROM HIGH-RESOLUTION SCANS AND SCANNING ELECTRON MICROSCOPY

Interpreting cross-bed depositional processes in the Coconino Sandstone (Permian) is a challenging task, since dune stratification types – grainflow, grainfall, and ripple lamination – are typically indistinguishable at the outcrop scale. High-resolution scans of thin sections were used to describe facies variation within cross-bed sets. While we previously reported a lack of textural lamination in the Coconino Sandstone (Maithel et al., 2016), laminae were more visible at this scale than under the petrographic microscope.

We identified massive, laminated, and indistinctly laminated textures in our outcrops, with “lamination” defined by vertical changes in framework grain size. Some of the laminae appeared to be graded, but the grading was not consistently normal or reverse. Stylolite seams, which were associated with clay, muscovite, and quartz grain dissolution, were also prevalent and mimicked textural lamination in many slides. We observed thick, repeating stylolites to be more common in laminated samples.

Some samples contained pores that were much larger (average ~0.3 ɸ; from long axis measurements) than surrounding framework grains (average 3.2 ɸ). These pores did not appear to be associated with grain residue or dust rims, but dissolution pits (observed in adjacent quartz under the scanning electron microscope), as well as nearby intergranular calcite and siderite, may suggest that carbonate minerals once occupied the voids and were dissolved during diagenesis. The large pores were most abundant in massive and indistinctly laminated samples.

These facies may provide critical information for interpreting cross-bed depositional processes. Distinct laminae might indicate traction or suspension deposition, and dissolved grains could affect characterization of porosity and textural trends. Additional work must be done to classify the lamination and pore space and understand their relationship to depositional processes in the Coconino Sandstone.