Multiple shallow sediment cores were obtained from Pilot Valley playa, a sub-basin located in the northwestern Bonneville basin. Analysis of stratigraphy, ostracodes, mineralogy, chemistry, total inorganic carbon (TIC), total organic carbon (TOC), and stable isotopes were performed to better place these sediments into proper context with respect to the Lake Bonneville cycle. Results showed Pilot Valley playa contains a nearly full sequence of Lake Bonneville deep-water marl in addition to sediments deposited before and after the Lake Bonneville cycle. Within the marl is a sequence of organic rich algal laminated marl correlated with the Stansbury oscillation. Four 14C ages ranging from ~22.4 k 14C years to ~15.8 k 14C years from preserved algae filaments in this sequence place it well within the time frame of the Stansbury oscillation. Oolitic sand found below this sequence indicates the existence of a shallow (<~5 m), saline lake in Pilot Valley prior to the transgression of Lake Bonneville. Analysis of sediments deposited during the late regressive phase of Lake Bonneville indicates the lake may have fallen to levels below that of Pilot Valley prior to transgressing to the Gilbert level.

Pilot Valley, located in the eastern Basin and Range, north of Wendover, UT, contains numerous shorelines and depositional remnants of late Pleistocene Lake Bonneville. These remnants present classic ground penetrating radar (GPR) targets due to their coherent stratification, low clay, low salinity, and low moisture content. Three-dimensional (3D) GPR imaging can resolve fine-scale stratigraphy of these deposits down to a few centimeters. While lake levels fluctuated due to flooding events, climatic changes were the dominant factor in controlling lake levels. In Pilot Valley, the paleowind entered from the northwest, with storms coming from the south, and circulated clockwise around the basin, forming offshore sand bars. On the western side of the valley, a uniquely well-preserved interpreted regressive phase beach deposit, dated late Pleistocene, is hypothesized to have been a point bar shortly after the Provo Shoreline period. 3D GPR data, measured stratigraphic sections, cores, mineralogical analysis, and the collection of gastropod samples for radiocarbon dating constrain a reconstruction of the deposit's depositional environment and local paleoclimate for Lake Bonneville. The GPR images, visualized with state-of-the-art petroleum industry tools, reveal fine-scale stratigraphic detail that can be analyzed using seismic stratigraphy concepts. Our study provides a comprehensive model for ancient pluvial lake-shore depositional environments in a Basin and Range setting using an integration of geological and geophysical data.

A vast area of the northeastern Great Basin of the western USA was inundated by a succession of Plio-Pleistocene lakes, including Lake Bonneville (28 ka to 12 ka). The Pilot Valley playa, located just east of the Utah-Nevada border near Wendover, Utah, within the eastern Basin and Range Province, represents an 8 to 16 km wide and ~50 km long remnant of these lakes. The playa corresponds to the upper surface of a closed basin that is delimited by two mountain ranges, which are mantled by recent alluvial fans over which the playa sediments have prograded. In order to investigate the interaction of Plio- Pleistocene lake sedimentation and alluvial fan development, high-resolution seismic reflection profiles have been acquired near the base of both the west-bounding and the east-bounding ranges. On the western side of the basin, the seismic profiles provide images of sub-horizontal playa sediments prograding over the inclined alluvial fans. Theboundary between the playa and fan sediments is marked by a prominent angular unconformity. Seismic images from the eastern side of the basin reveal a markedly different structural and stratigraphic style with down-to-the-basin normal faulting of relatively shallow Paleozoic bedrock overlain by alluvial fan deposits, which are in turn on-lapped by a thin veneer of playa sediments. The results contained herein reveal for the first time the stratigraphic relationships between Quaternary pluvial sediments as a shoreline depositional facies and the adjacent bounding fan deposits. Post-stack reprocessing of lower-resolution but deeper penetration seismic data located in an analogous basin to the southwest, provides a likely context for the Pilot Valley seismic data. The new geophysical images, when integrated with available geologic mapping and limited well control, aid in constraining how deep aquifers are locally recharged from an adjacent range. The results also clearly demonstrate the strong structural asymmetry of the range and playa system, which is consistent with a classic half-graben structure. Lastly, this study demonstrates the utility of the shallow seismic reflection method as a tool to provide high-resolution sub-surface images in the geophysically challenging environment of Basin and Range geology.

Direct mountain front recharge (MFR), water table recharge at the base of the mountain front, was evaluated on the arid (<250 mm/yr precipitation) Silver Island Mountains by comparing mountain precipitation to groundwater response. Direct MFR contributions were assessed on two catchments, one bedrock (i.e., mountain block) dominated and the other alluvial fan (i.e., mountain front) dominated. Catchment precipitation and shallow groundwater levels at each catchment outlet were measured for a 24 month period beginning October 2005. This time period captured one complete hydrologic cycle (December 2005-February 2007) for which annual and seasonal direct MFR rates were calculated. Annual direct MFR was calculated using a modified version of the water table fluctuation (WTF) method as 0.015-0.016% of precipitation on both catchments, with seasonal variations of 0% in summer up to 0.023% in winter, spring and fall. Seasonal direct MFR contributions are similar on the bedrock and the alluvial fan dominated catchments, with a notable exception during fall 2006 when direct MFR was twice as effective on the bedrock dominated system than on the alluvial fan dominated system (0.022% and 0.011% of precipitation, respectively). Darcy's law calculations show similarly low annual direct MFR contributions (0.013-0.032% of precipitation) as those calculated by the WTF method. Calculated direct MFR is 10% or less than typical calculated combined MFR (near surface recharge and deep underflow from the mountain block) for similar terrains and climates, and is only 3.5% of the combined MFR for the Silver Island Mountains as calculated by the Maxey-Eakin model. However, based on total recharge to the adjacent playa, it is apparent that the Maxey-Eakin model overestimates combined MFR, and the small calculated direct MFR is at least 50% of combined MFR. Despite some uncertainty in the numerical results, several patterns are evident in the data. The data show that direct MFR occurs in response to small rainfall events throughout much of the year, and that snowmelt is not necessary to produce direct MFR. The data also show that direct MFR responds more quickly and flushes through the system faster on the alluvial fan catchment than on the bedrock catchment.