ALLUVIAL FAN SURFACE AGES, SOIL AND SURFACE FEATURES, AND SURFACE STABILITY MODELING FOR CULTURAL SITE PRESERVATION AT STONEWALL FLAT, NELLIS AFB, NEVADA

Surface age data, soil, and surface characteristics were recorded from alluvial fans in Stonewall Flat on Nellis Air Force Base, southwestern Nevada, in order to assess surface stability for cultural site preservation. Cosmogenic radionuclide surface ages of <1 Ka, 36 Ka, and 40 Ka, and varnish microlamination (VML) surface ages of 13.15 Ka, 25.55 Ka, 35.75 Ka, 65.15 Ka, and 86.75 Ka were aquired. In order of increasing age, the fan soils displayed increasing thickness, increasing clay content, and increasing vesicularity of the AV horizon, a thickening and reddening of the B horizon, and locally increased carbonate nodule development. Younger fan surfaces were characterized by bar-and-swale topography, diverse sediments, abundant coppice deposits, common bioturbation, and robust plant communities of diverse composition. Older fan surfaces were characterized by planar surfaces, sparse coppice deposits, sparse vegetation of restricted composition, and stone pavements that increased in gravel content and rock varnish development with increasing age.

Increased rock varnish and decreased vegetation with increased age suggests a correlation of lighter-colored, younger fan surfaces with light-colored areas on satellite photos, and of older, darker fans with darker areas on photos. This correlation was supported when the dated fan surfaces were plotted on photos of Stonewall Valley. Additionally, surface features and sediment observed on the younger fans suggest surface environments dominated by the higher energy flash-flood and sheet wash deposition. Surface features and sediment on older fans indicate lower energy eolian processes dominate. Since higher energy processes are more disruptive of the surface than lower energy processes, we assume that lighter-colored fans on photos are less likely to preserve cultural features than darker fans. A preliminary fan surface stability map was developed for Stonewall Valley and spot checked at locations not previously visited. Lighter areas typically contained bar-and-swale topography and more eolian deposits, and darker areas contained pavements and sparse vegetation; anecdotal field observations revealed more cultural artifacts casually observed on fans with pavements than fans with bar-and swale topography and eolian deposits.