SUBSURFACE EVIDENCE FOR LAKE BONNEVILLE FLUCTUATIONS NEAR LITTLE GRANITE MOUNTAIN, DUGWAY PROVING GROUND, UT

Soil materials at a site in the east-central area of Dugway Proving Ground (DPG) were sampled and described during October 2010. The study centered on two pits 280 m apart excavated to a depth of > 6m. Project schedule and safety issues limited field operations to a total of four days, after which the pits were backfilled. The team collected minimally disturbed samples at 20-cm intervals in density-drive cylinders. Field activities also included stratigraphic description, measurement of magnetic susceptibility, and determination of in situ moisture and density. Sediments observed in the study area were interpreted as fine-grained lacustrine and shoreline deposits representing a shallow arm of Lake Bonneville, as well as related alluvial-fan and windblown deposits. Sediment sources were the mountains surrounding the study area: Davis and Little Davis Mountain to the south and southeast; Cedar Mountains to the north, northeast, and northwest; and Little Granite Mountain to the West. Field observations and subsequent laboratory analyses – including grain-size analyses, density, porosity, and saturated hydraulic conductivity – revealed evidence of at least two periods of lacustrine deposition punctuated by subaerial exposure and soil formation. Geophysical surveys, conducted prior to excavation using two electromagnetic-resistivity instruments, provided additional subsurface data. Field and laboratory data contributed to a 3-dimensional model of the site, built using a geologic-conceptual-model option within the Department of Defense Groundwater Modeling System software.