DOWN-SLOPE COARSENING OF EOLIAN GRAINFLOWS FROM THE NAVAJO SANDSTONE

Down-slope coarsening in avalanches has been observed on modern dunes and generated in labs, but we are unaware of previous studies that document down-slope sorting in ancient eolian grainflows. We studied the grainflow strata of the Navajo Sandstone in the southern portion of its outcrop belt from Zion National Park (west) to Buckskin Gulch (east). At each study site, thick sets of grainflow-dominated cross-strata that were deposited by large transverse dunes comprise the bulk of the Navajo Sandstone. We collected and analyzed 179 samples from grainflow tongues within 35 sets of cross-strata. We sought to collect samples along a line parallel to the dip of cross-strata when possible, so as to increase the chances of obtaining samples from just one grainflow tongue. At some sites, however, samples had to be obtained at a slight angle to the dip direction and thus those samples may include sediment from different avalanche events that had become amalgamated along strike. We measured and sampled three stratigraphic sections composed almost exclusively of eolian cross-strata—two near Buckskin Gulch, and one at the East Entrance to Zion National Park. Samples were obtained by gently scraping grains from the individual, friable, grainflow strata, and were analyzed using a Malvern Mastersizer 2000 laser unit. The median grain size of grainflow samples ranges from fine sand (127 um) to coarse sand (617 um). The grainflows near Buckskin Gulch are generally coarser than the Zion samples. Using Folk and Ward criteria, all samples are well-sorted to moderately-well-sorted. For both sections near Buckskin Gulch, within 25 sets of cross-strata with two or more samples, the median grain size is greatest at the base and becomes finer toward the top of the set. For the Zion section, however, down-slope coarsening is apparent in only one of the three sampled sets.