SURFACE MORPHOLOGY AND TEXTURAL VARIATION WITHIN A PARIALLY VEGETATED INTERDUNE

The White Sands of Southern New Mexico is the largest gypsum dune field in the world. It offers unique opportunities to study local sources and transport of sand grains because gypsum grains are enlarged by overgrowths during burial within and between dunes. Our previous research indicates that grains on the dunes at White Sands National Monument are composed of a combination of grains recycled from within the dune and grains derived from the interdunes immediately upwind. Mixing of these two populations result in distinctive populations and grain size means and standard deviations that vary from dune to dune. Furthermore, although most of our original sampling was from dunes, the samples from interdunes suggested that each type of interdune surface exhibited its own distinctive populations of grains resulting from recrystallization of the sand and generation of new crystals at and just below the surface.

White Sands has several types of interdune surfaces. Evaporite surfaces, erosional surfaces, vegetated surfaces and dry interdune surfaces composed of mobile sand. This is a detailed study of an individual interdune that contains patches of all four surfaces. 124 samples were collected for grain size analysis. Grain mounts of the samples were analyzed microscopically. Rippled sands are finer-grained and equant. Sands from evaporite surfaces sands are more poorly sorted and exhibit fine silt sized particles and coagulated flakes and blades that probably crystallized near the surface. Vegetated interdunes contain contains fine-sand sized needles. Erosional interdunes are the coarsest grained and contained blades and polycrystalline grains re-cemented in the dunes.

The different interdune surfaces exhibited significantly different grain size distributions that correlate with the different surface types. Wind rippled sands are well sorted and medium-coarse grained. Samples from near plants are bimodal with a coarse fraction and a dominant very-fine-grained and silt fraction. Erosional interdunes are also bimodal, with a dominant coarse fraction and evaporites are similar, but finer-grained. Maps of grain size reflect different sources in the interdune rather than rather than transport and very significantly over 1-meter distances.