FLUID CHEMISTRY AND MINERALOGY OF SURFICIAL SEDIMENTS IN THE NORTHERN BONNEVILLE BASIN, WESTERN UTAH

The Bonneville or Great Salt Lake Desert of western Utah is a hydrological closed basin underlain by a shallow saline aquifer system developed in playa and lacustrine clays, carbonates and gypsiferous evaporite. The salts and sediments are a remnant of Pleistocene Lake Bonneville, which once occupied the entire basin to considerable depth. Our study has concentrated on the part of the area flooded by pump-over of Great Salt Lake waters during the high stands of the 80s; this is west of the Newfoundland Mountains and north of the Interstate 80 corridor. Samples of sediment, pore-fluid, and open-borehole brine were collected along right angle transects which intersect close to the lowest point in the basin. This lowest point contains the thickest evaporite (gypsum + halite) section and most concentrated Mg, KCl brine. To the south and west there is evidence of decreasing salinity and fluid concentrations of Mg & K, plus increased evidence of surficial waters depleted in bitterns, probably as a result of rainfall re-solution.

Pore fluids were obtained by high-speed centrifugation, or draining of coarsely crystalline salt layers. All pore fluid samples extracted from greater than 5 foot depth have Mg & K concentrations below 0.5%, except at sites near the topographic low (0.9 to 1.3%). At similar depth sulfate ranges from 0.5% to 2% in peripheral areas, but up to 4.5% near the basin low. In the upper reaches of the most saline profiles the sulfate contents peak between 2 and 4 feet depth, rather than near the surface, and resemble diffusion profiles. In contrast, the upper pore fluid profiles for sites distant from the topographic low peak very close to the surface.

Trends in semi-quantitative x-ray diffraction mineralogy correlate well with pore fluid compositions along each transect, both laterally and with depth. Examination of over 100 samples from more than 15 sites revealed an expected negative association between carbonate and silicates. Aragonite appears to indicate a lacustrine environment, whereas gypsum (and sparse dolomite) is characteristic of playa or mudflat conditions. In peripheral areas calcite is associated with increased quartz, suggesting alluvial input. Thus the north Bonneville Desert illustrates the zoned “evaporation dish” pattern originally described by C. Hunt for the Death Valley salt pan.