GROUND PENETRATING RADAR INVESTIGATION OF PLEISTOCENE SHORELINES OF PLUVIAL LAKE CLOVER, ELKO COUNTY, NEVADA

Approximately 1500 m of ground penetrating radar (GPR) transects were collected across five beach ridges in Elko County, Nevada constructed by pluvial Lake Clover during the Late Pleistocene. The primary objective was to document the internal architecture of these shorelines and to evaluate whether they were constructed during episodes of lake rise or fall. To constrain the morphology of the ridges, detailed topographic surveys were collected with a Topcon GTS-235W total station. Position measurements were referenced to a second class vertical benchmark located ~400 m from the center of the study area that was established in 1934 by the Coast and Geodetic Survey. GPR data were collected with a ground-coupled 400-Mhz antenna and a SIR-3000 controller. Transects crossed the beach ridge built by Lake Clover at its highstand 19-17 ka BP (5665 ft), as well as four lower ridges at 5662, 5658, 5650, and 5648 ft (elevations given in feet to correspond with the 7.5’ topographic map of the study area). In addition, GPR data were collected at the crests of three ridges where excavations revealed prominent stratigraphic discontinuities. Matching these discontinuities and their measured depths to the GPR data yielded an average dielectric constant of 5.0, typical for dry sand and gravel. This value was applied for estimating depths and stratigraphic thicknesses from the GPR transects. GPR processing included a time-zero correction to eliminate the direct wave, a full pass background removal, distance and surface normalization (using points measured with the total station), and horizontal stacking. Consistent radar signals were returned to a depth of ~2 m; stratigraphic resolution was generally better in layered sandy gravels of beach ridges, whereas fewer features were apparent in the finer-grained sediments occupying swales. Shorelines are resolvable as 90 to 160-cm thick stratified packages overlying a prominent lakeward-dipping reflector. The gradient of this reflector is equal to the landscape slope above the highstand beach ridge, indicating that this is the original pre-lake land surface upon which the beach ridges were constructed. This reflector is offset by a scarp, possibly erosional, upon which the second-highest (5662 ft) ridge is superposed. The lower four ridges all exhibit offlapping internal stratigraphy, consistent with construction during lake regression.