Science and Mathematics Faculty Presentations

Along-dip Textural Trends in the Permian Coconino Sandstone from Thin Section and Laser Diffraction Particle Analysis

Document Type

Poster Session

Event Date



The Geological Society of America 129th Annual Meeting


Seattle, WA


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.