CHARACTERIZATION OF SAND IN THE NEBRASKA SANDHILLS

The Nebraska Sandhills is one of the largest eolian deposits in the western hemisphere. Bulk samples of these dunes were collected around Merritt Reservoir and along US Highway 83 between North Platte and Valentine, Nebraska. Eleven bulk samples were collected including one from an area interpreted as interdunal. In characterizing the sandhills, three criteria were evaluated: grain size, rounding, and mineral composition. Petrographic thin section slides were prepared from a small portion of each bulk sample. Bulk samples (~150 gm) were dried and then sifted through ½ phi size sieves. Standard equations for grain size analysis were used to calculate the grain size statistics. Mean results were plotted (with 2 standard deviation error-bar whiskers). Mean phi sizes fell between 1.42 and 2.62 phi. While many of the samples included grains of every size (-1.0 to > 4.0 phi); samples mainly consisted of coarse, medium, and fine grain sand particles (0.5 to 3.0 phi). By category, the very coarse, coarse, and medium sand grains were mostly rounded and subrounded. The fine and very fine sand grains were mostly subrounded and subangular. The single sample from the interdunal area had the widest range of grain sizes (-1.0 to > 4.0 phi). Thin section analysis of grain shape revealed mostly equant grains, but also the surprising occurrence of many prolate or rod shaped grains. The sand throughout the sandhills consisted of approximately 50% quartz. The remaining amount was comprised of varying amounts of chert, K-feldspar and plagioclase. Trace amounts of volcanic rock fragments, pyrite, and zircons were also present. No micas were found. The wide variety of grain types indicates igneous, metamorphic and sedimentary sources for the sand. Quartz overgrowths and dust rims provide evidence of some previous sedimentary sources. Statistical analysis of the sample means did not show a clear north to south or east to west pattern (i.e., there was no significant difference between the samples, except for the interdunal deposit).