ALONG-DIP TEXTURAL TRENDS IN THE PERMIAN COCONINO SANDSTONE FROM THIN SECTION AND LASER DIFFRACTION PARTICLE ANALYSIS

Small-scale processes, which include grainflow, grainfall, and ripple migration, transport and deposit sediment on modern dune foresets. In sandstone, the cross-stratification produced by these processes can be distinguished by textures and sedimentary structures.

Coconino Sandstone cross-bed sets were sampled at various along-dip positions (some exceeding 30-40m) in outcrops near the towns of Ash Fork, Seligman, and Holbrook, Arizona, and on the Hermit Trail in Grand Canyon. The coarsest grains in each sample (long axis ≥ 150μm) were counted on printed thin section photos using a light table and transparent overlay. While previously reported thin section data revealed down-dip fining trends in two outcrops (Maithel et al., 2016), several additional cross-bed sets were found to contain more coarse grains near their bases. This down-dip coarsening may provide evidence for grainflow depositional processes in these outcrops.

Because the Coconino Sandstone is generally well-lithified by quartz overgrowth cement, we had previously assumed disaggregation and whole particle analysis to be impossible. However, new methods utilizing a quarter-inch microtip sonicator probe have successfully disaggregated small pieces of even strongly-cemented samples. The loose sand from these samples was then analyzed with a laser diffraction particle analyzer. While average grain sizes from laser analysis were generally higher than those from thin sections (as expected), along-dip trends from the laser analyzer appeared to compare favorably with thin section results in at least several outcrops. Our disaggregation methods may therefore provide a viable and more efficient alternative to petrography for characterizing textures in the Coconino Sandstone and similar quartz-cemented units.