HIGH RESOLUTION GPR INVESTIGATION OF A LAKE MANLY SHORELINE DEPOSIT, DEATH VALLEY, CALIFORNIA

Pluvial Lake Manly inundated Death Valley, California, during the Pleistocene. At its maximum extent, it was approximately 185 km long and 180 m deep. Fluctuating lake levels have been linked to regional climate changes with the various lake stands collectively referred to as Lake Manly. Today, evidence of the former lake and its lake level fluctuations are recorded in paleo-shoreline deposits and/or erosional scarps throughout Death Valley. The purpose of our ground penetrating radar (GPR) study was to investigate a Lake Manly coastal depositional feature located at one of the higher lake stands (50 m asl). The barrier bar deposit which is cut by Beatty Junction Road within Death Valley National Park is approximately 500 m long, 50 - 100 m wide, and 5 - 6 m in height. Prior geophysical studies on the barrier suggested that higher frequency GPR datasets be collected to provide higher resolution stratigraphic images of the barrier's interior. We utilized a pulseEKKO 1000 GPR system with an automated odometer along two shore parallel and seven cross barrier transects. GPR data sets were topographically corrected with laser leveling equipment and georeferenced with a Trimble ProXR GPS unit. To show the general framework of the shoreline deposit, 225 MHz data were collected along all nine transects while higher frequency antennae (450 and 900 MHz) were used along selected lines to provide higher resolution images. The lines varied in length from 41 m to 266 m with traces collected every 0.1 m to 0.03 m depending on antennae frequency. A common midpoint survey provided a near surface velocity of 0.142 m/ns. Based on this velocity, the depth of penetration was 1.8 m for the 225 MHz antennae, 0.9 m for the 450 MHz, and 0.45 m for 900 MHz. Using radar stratigraphic analysis, the GPR transects show continuous to semi-continuous, horizontal reflection patterns. Along selected locations on cross barrier profiles dipping reflection patterns can be observed with dip angles ranging from 6.1 to 26.6 degrees. Exposed stratigraphy where the road-cut bisects the barrier bar deposit correlates well with the interpreted GPR data.