SAND GRAIN SIZE AND CRYSTALIZATION, SOURCES AND TRANSPORT OF GYPSUM SAND IN THE THE WHITE SANDS DUNE FIELD

Studies of eolian dune fields indicate there is no consensus on what shapes grain sizes in dunes. Some studies indicate upward coarsening and others upward fining to the crests of dunes. In most active dune fields the variation within and between dunes is small, because wind forms a very efficient sorting mechanism. The White Sands dune field is an ideal place to study the sources and transport of sediment because the re-crystallization of gypsum creates a side variety of grain sizes in the dune fields. Transects along a 3 mile trend allow us to see how grain size is modified down wind. Detailed 1-5 m²-spaced grid sampling of ten individual dunes and their associated interdunes illustrates how grain size varies within individual dunes. White sands dunes show three separate sources that originate with distinctly different grain sizes. 0.1 to 0.3 mm needle-shaped crystals are formed by crystallization in vegetated interdune areas. 0.5 to 3 mm blade-shaped grains are eroded from recrystallized sand in erosional interdunes and the lee sides of dunes. Equant frosted grains are derived from more distant areas of the dune field. The three populations mix to create the “grain size measured on individual dunes”. While several recent studies have attempted to model grain size within the dune field as transport and abrasion from a source along the upwind edge of the dune field, detailed analysis indicates that the sources of grains are much more and the sources of grain size more complex.