USING STATISTICAL ANALYSIS GRAIN SIZE DISTRIBUTIONS, GRAIN SHAPES TO DISTINGUISH POPULATIONS OF GYPSUM SAND, WHITE SANDS NATIONAL MONUMENT

Unlike silicate sands, the gypsum sands of the White sands of New Mexico can be enlarged, or crystallized in the subsurface. Initial studies of white sands show very fine grained crystals, (?? microns) forming in discrete layers in the subsurface along with the more distinct populations of rounded, transported eolian grains, and larger, angular grains enlarged by recrystallization. Rounded and transported grains vary locally in size. Enlarged grains are found in the upwind interdunes and alkali flat, where erosion is common. Discrete layers of finer grained crystals are 1-5 cm thick and are largely found at least 10 cm above the water table. The are associated with surface crusts and brownish horizons, from which sulfate reducing bacteria have been recovered. Fine grained crystals also occur in white nodular masses, found near the water table.

More than 250 samples derived from the walls of borings in interdunes in White Sands National Monument and were quantitatively analysed for both size distributions and shape. The samples were analysed using end member mixing analysis along with similar factor analyses. End Member mixing discriminated four populations. Image analysis of photographs of the samples allows quantitative analysis of size and shape of grain populations. Resulting populations include finer grained needle like crystals, rounded transported grains. Cemented aggregates of smaller crystals, and angular, enlarged grains formed by overgrowths on transported grains. End member mixing analysis discriminated 4 end members based on size distributions. The end members based on size contained a proportion of one shape class, but included varying proportions of the more common transported grains.