INSIGHTS INTO CLOSED-BASIN DEPOSITION FROM THE QUATERNARY DEPOSITIONAL HISTORY OF THE BONNEVILLE SALT FLATS, UTAH, USA

The Bonneville Salt Flats (BSF) in northwestern Utah is a large lens-shaped saline pan deposit of up to 2 m of bedded halite with gypsum sand. This saline pan has long been considered a remnant of Pleistocene Lake Bonneville, but the information presented here conflicts with that interpretation. We investigated and dated depositional records from multiple cores from the saline pan and underlying sediments, contrasting them with nearby sedimentary records to understand how the underfilled basin stage may affect the depositional record. We identified multiple cycles of shallow saline lake deposition and deflation from ~30 to ~45-50 ka, the period before Lake Bonneville. Lake Bonneville sediments did not occur in our cores, indicating one or more deflationary periods occurred between Lake Bonneville’s desiccation at 13 ka and the onset of saline pan deposition preserved as gypsum sand at ~8 ka. The deepest halite layer was deposited between 5 to ~3.5 ka. BSF saline pan sediments reflect regional changes in moisture with gypsum layers corresponding to drier periods and bedded halite layers corresponding to moister periods. Depositional records strongly differ between BSF and nearby basin-floor sediments. Lake Bonneville deposits persist to the east and south of this site. However, in contrast to BSF, Holocene deposition in these areas is eolian or absent. The erosional pattern observed at BSF with deflation focused on a basin low is similar to the Lake Lucero deflation playa at White Sands National Park. These results indicate that under sustained underfilled conditions, which predominate in the Quaternary Bonneville basin, basinal lows and hydrologically isolated subbasins are more likely to become deflation surfaces with more incomplete depositional records than higher-lying areas or springs and lakes with sustained moisture input. Furthermore, the depositional record from modern basins can be spatially heterogenous with saline pans and the surrounding areas preserving different depositional periods.