TEXTURAL TRENDS IN THE COCONINO SANDSTONE, CENTRAL AND NORTHERN ARIZONA, USA

Thin section preparation of rock samples from a wide range of Coconino Sandstone outcrops enable better understanding of grain size, sorting and rounding within the formation. Thin section data have been collected using a Nikon Eclipse 50i Pol microscope equipped with the Br software package making measuring and tabulating grain size statistics relatively easy. Typically the long axes of 400-600 sand grains were measured on each slide. Every grain is measured in five to ten systematically selected “fields of views” from each slide, perpendicular to bedding. The full width or length of each slide was always counted to get a representative sample of all the grains present in the thin section. Overall statistics for a particular outcrop were calculated by selecting 300 random grains from each thin section from an outcrop (i.e., a site with ten thin sections would be represented by 3,000 grains). To date, grain measurement has been accomplished on 80 slides representing 13 widespread outcrops.

Mean, mode, d₅₀, and standard deviation were calculated for each thin section and outcrop. Using Johnson’s (1994) suggestion for sorting (based on the standard deviation of the phi size using long axis measurements of grains in thin section), the sorting of each slide and the sorting of each overall outcrop was calculated (for thin sections, <0.45 Ø is very well sorted; 0.45-0.55 Ø is well sorted; 0.55-0.70 Ø is moderately sorted; 0.70-0.90 Ø is poorly sorted; and >0.90 Ø is very poorly sorted). When examining each thin section, rounding was estimated for quartz and K-feldspar using the rho scale by Folk (1955): “0” being the most angular and “6” being perfectly spherical.

Results show that the Coconino ranges from being moderately to very poorly sorted in most thin sections and at most locations throughout the width, breadth and thickness of the formation. This is contrary to what has previously been reported in the literature and what might be expected for an eolian deposit. Most sand grains in the Coconino are in the range of 2.5 to 3.5 rho (4.5 is rounded). Preliminary results show that grain size slightly increases and rounding and sorting improve toward the south, the same overall direction of transport within the formation (indicated by cross-bed dips).