ROUNDING OF K-FELDSPAR AND QUARTZ SAND GRAINS FROM BEACH TO DUNE ENVIRONMENTS: IMPLICATIONS FOR ANCIENT SANDSTONES

Sand samples were collected from seven different locations along the California and Oregon coastlines where beach sediments and eolian coastal dunes are in close proximity (< 0.5 km) to each other. Beach sand was collected from the swash zone and dune sand was taken from the crests or lee faces of dunes. Loose sand was impregnated with epoxy and thin sections were cut. The slides were stained for K-feldspar. About 100 quartz and 100 K-feldspar sand grains were measured and scored for rounding from each environment and from each location (14 total slides). Roundness was scored according to the roundness scale of Powers (1953) and Folk (1955) on a scale of 0-6 to the nearest 0.5. All the slides were analyzed over a two-week time period by the same worker to try and minimize bias. In order to compare rounding of similar sized sand grains from the coupled environments a size sample of 1.0 standard deviations from each side of the mean (size) was selected from the dune sample and then compared with the same size/mineral population from the beach sample (where there is a much wider range of sand grain sizes). T tests were used to see if significant rounding occurred from the beach to the dune (2 tails, unequal variance, Microsoft Excel) in this size population for each mineral.

It was found that long axes of most sand grains were in the 100-500 um range. Graphing grain size against rounding showed only a small increase of rounding (small positive slopes) regardless of mineral species. P values indicated the rounding in the same size populations of quartz grains does not change significantly over this short transport distance (0.21 mean change). K-feldspar on the other hand always showed a statistically significant change in rounding (0.66 mean change). It might be expected that further eolian transport of the sand would even show a more marked difference between the two minerals.

From these results, one might cautiously conclude that even a small amount of eolian transport can result in significant rounding of the overall population of K-feldspar sand grains compared to quartz. One might expect much better-rounded K-feldspar sand compared to quartz sand when comparing the same grain size populations in ancient eolian deposits. Angular K-feldspar may be a good indicator of an aqueous deposit. More work is encouraged to see if these results are repeatable.