USING TEXTURAL TRENDS TO INTERPRET CROSS-BED DEPOSITIONAL PROCESSES IN THE COCONINO SANDSTONE (PERMIAN), ARIZONA

Cross-beds are deposited by sedimentation processes that occur on dune foresets. These processes, which commonly include grainflow, grainfall/suspension, and ripple migration, produce distinct textural trends that could be identified in cross-bedded sandstones.

Down-dip changes in grain size were characterized in two cross-bed sets of Coconino Sandstone (Permian) near Ash Fork and Seligman, Arizona. Samples were collected at various down-dip positions along the cross-beds: seventeen from the Ash Fork outcrop and eight from the outcrop near Seligman. Long axes of grains were measured in thin section photos of each sample with ImageJ software, and the 75^th percentile grain size for each cross-bed set was calculated. We could then describe the distribution of the upper quartile (coarsest) grains in the cross-bed sets.

Two distinct trends were visible in each outcrop: a group of samples with more coarse (fourth quartile) grains near the top of the cross-beds, and a group of samples with fewer coarse grains further down-dip. In the coarser group, we observed an increasing number of coarse grains with distance down-dip. Overall, the samples from these outcrops appeared to lack distinct, repeating laminae defined by variation in framework grain size, which likely suggests that they were not deposited by migrating ripples. Rather, the beds with more coarse grains (that also coarsen down-dip) may represent grainflow deposits, and those with fewer coarse grains may have been deposited from suspension.

While measuring grains in thin section may allow us to interpret cross-bed depositional processes, the method is time-consuming and not practical for analyzing many samples. Other methods should be explored to interpret processes in sandstones that, like the Coconino Sandstone, are hard to disaggregate. With more efficient methods, additional work could be done to develop depositional models for these cross-beds and to interpret processes in other Coconino Sandstone outcrops.