CHARACTERIZING VARIATION AMONG PRINTS INTERPRETED AS “RAINDROP IMPRESSIONS” IN THE PERMIAN COCONINO SANDSTONE (NORTHERN ARIZONA, USA)

Shallow, concave prints, which have previously been interpreted as “raindrop impressions,” are abundant on some cross-bedding planes in the Permian Coconino Sandstone. While the best examples of these prints occur in retired quarries north of Ash Fork, Arizona, they have also been observed near the towns of Seligman and Holbrook, and in loose slabs near Matterhorn Mountain.

These “rainprints” are typically up to ~ 1 cm in diameter and exhibit a range of morphologies, from simple concave prints with or without raised rims, to mottled concave prints, to large (up to ~ 3+ cm), irregular “raised rims” with or without a central depression. Convex prints of various forms were also observed, with most occurring in loose slabs and likely representing the underside of concave prints. However, a few possible examples of true convex-up prints were documented. In some cases, print morphology and distribution changes abruptly on a given bed. While they are commonly scattered across a bedding plane, examples of prints in parallel bands have also been observed.

Variation among the “rainprints” might be attributed to different sediment or process parameters. To characterize the sediment, grain size and composition data were collected from samples containing prints. Our initial data indicate no clear correlations between particle size/composition and print morphology. While we might expect prints with raised rims to have a higher clay content, we found that samples with raised-rim prints are coarser and better sorted than many others containing prints without raised rims. Preliminary thin-section data from one sample with two print types show no distinct differences in packing or porosity, but additional petrographic study is needed.

Though these prints have been interpreted as “raindrop impressions,” the observed variation indicates that other parameters might have controlled their formation and preservation. Refining models for these structures may enable us to better interpret larger-scale processes in the Coconino Sandstone.