Grain size distribution with geomorphology on gypsum dunes in the White Sands Erg, White Sands National Monument, New Mexico.

摘要

In order to examine grain size distribution over dunes at the White Sands dune field, 1214 geolocated surficial samples were collected and analyzed by laser diffraction. Local variations in dunes such as neighboring interdune types and the presence of vegetation, deflation features, and granule ripples on the dunes were noted. This study indicates that recrystallization of gypsum and the generation of fresh sediments is occurring in significant quantities within interdunes, providing both high and low anomalies in grain size distribution through much of the central portion of the dune field. Other local variations as previously mentioned such as scour surfaces on dunes and local topographic alteration of air movement all overprint more field-scale effects. Coarse sands are found in granule ripples in troughs and swales and in avalanche deposits on the slipface apron. Saltating sand samples selected by microscopy from the stoss face of dunes showed a general coarsening trend with relative elevation. Samples collected to the lee of dune crests were generally finer-grained than those on the stoss side, and interdune samples collected within 60m of the slipface aprons may indicate a lee sheltering influence extending well across the interdune. Patches of finer-grained sediments were found on dunes downwind of vegetated interdunes, and where it was present on dunes, vegetation was associated with deposition of comparatively finer grains in its immediate vicinity. To effectively model transport, these variations will all have to be taken into account.