TEXTURAL ANALYSIS OF THE COCONINO SANDSTONE, CHINO POINT, ARIZONA

The Permian Coconino Sandstone is a cross-bedded layer of sandstone in the western United States. Samples used in this study were collected through the 130 m outcrop on the west side of Chino Point, near Seligman, Arizona. Thin sections were cut perpendicular to bedding and 14 slides were selected for study. At least 400 grains were measured per slide. The longest axis of every grain from 5 to 10 preselected areas on each slide was measured. In addition to these grain size measurements, the angularity of the grains was also estimated. Statistical analysis of this data provided graphs of grain size distribution and sorting. Standard deviation was also calculated.

According to Johnson (1994), the standard deviation of the grain sizes in thin section can indicate how well-sorted the sand is: 0.00-0.44 very well-sorted; 0.45-0.54 well-sorted; 0.55-0.69 moderately sorted; 0.70-0.89 poorly sorted; and ≥ 0.90 very poorly sorted. Of the fourteen slides studied, one was very poorly sorted, one was poorly sorted, five were moderately sorted, and seven were well-sorted. Selecting 300 random grains from each of the 14 slides and recalculating the statistics showed that overall, the grains at this site were moderately sorted. The rounding data indicated that the grains were sub-rounded. The mean grain size of all fourteen samples was 3.20 Ø, or very fine sand.

Additionally, K-feldspar grains were present in the samples; a small amount was found in the lower part of the outcrop, but there was significantly more towards the top. Some of these grains were rounded and some were relatively angular. Trace amounts of mica were also found, as well as some heavy minerals and organic material. The distribution of these varied throughout the section.

The Coconino sandstone is usually recognized as an eolian dune deposit. Eolian deposits today are typically characterized by rounded and well-sorted sand, although exceptions can occur. Softer minerals, like K-feldspar and especially mica, are generally not present because they are likely more quickly destroyed by the wind abrasion. The results of this study are therefore surprising given the character of modern eolian deposits. I thank NCSF, Calgary Rock and Materials Services Inc. and Cedarville University for support and funding.