GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 188-15
Presentation Time: 9:00 AM-6:30 PM


GOLDSTEIN, Harland L., SPRINGER, Kathleen B., PIGATI, Jeffrey S. and SKIPP, Gary L., U.S. Geological Survey, Denver Federal Center, Box 25046, MS 980, Denver, CO 80225

The Las Vegas Formation (LVF) is a middle Pleistocene to early Holocene sequence of fine-grained groundwater discharge deposits that represent desert wetlands that occupied the Las Vegas Valley, Nevada between approximately 570 and 8.5 ka. These sediments accumulated in various spring hydrologic settings including outflow streams, spring pools, marshes and wet meadows. Desert wetlands are known to be sensitive to groundwater fluctuations, and studies of LVF deposits in Tule Springs Fossil Beds National Monument (TUSK) have demonstrated that wetland expansion and contraction occurred in direct response to abrupt climate oscillations in the recent geologic past. The stratigraphy, chronology, and hydrology of the LVF deposits have been studied in detail but quantifying the physical and chemical components of the sediments and determining the processes by which they accumulate are lacking. In this study, we tested the hypothesis that there is an aeolian component to paleowetland deposits by evaluating properties of LVF sediments from TUSK and comparing them to the local bedrock, which is composed almost entirely of calcareous lithologies. Particle sizes of the LVF sediments range from silt to sandy-silt with a median of about 35 micrometers; consistent with wind-derived sediments and classified as sandy- to clayey-loess. Primary and secondary calcium and magnesium carbonates dominate wetland sediment mineralogy and are similar to the local bedrock, but other components of the wetland sediments contrast strongly with bedrock parameters. For example, the presence of quartz, plagioclase, and potassium feldspars in the wetland deposits suggests sediment input from non-local sources, as do magnetic minerals, which are present in the wetland sediments at higher concentrations than could be explained by contributions solely from local bedrock. Overall, the physical and mineralogical data do not correspond to a purely local source but are instead consistent with a combination of local contributions and aeolian input from non-local sources. Additional chemical and petrographic studies are ongoing to further understand the contribution of wind-derived sediments to paleowetland deposits.