DEPRESSIONS ON THE SURFACE OF MORMON MESA, NV: EVIDENCE AND ARCHIVES OF SURFACE PROCESSES SINCE THE PLEISTOCENE?

Mormon Mesa, located between Overton and Mesquite, NV, is capped by a ~3-4 m set of petrocalcic horizons that create a resistant caprock atop less-indurated Miocene Muddy Creek Fm. sediments. Closed circular to elliptical depressions along with eolian sand sheets and faulted sections of the mesa reveal a dynamic geomorphic surface. A wetter environment during the Pleistocene is hypothesized to be a factor in the depressions initial development. If these features have existed as depressions on the surface of the mesa since the Pleistocene, then they may hold a record of surficial dynamics due to climatic changes and faulting over time.

Two perpendicular ground penetrating radar (GPR) transects that roughly crossed the center of a depression were collected using a MALA Geosciences X3M pushcart system with a 500 MHz antenna. RadExplorer software was used to process the data. Line 1 is oriented north-south at 819 m in length and Line 2 is oriented east-west at 701 m in length. A ~50 cm hole was dug in the center of the depression to evaluate sediment texture and materials present. Another ~50 cm deep hole was dug in the center of an adjacent depression and texture analyses were performed using a modified hydrometer method. Scanning Electron Microscope with electron dispersive spectroscopy was used to characterize the sediments from the sampled test pit.

Results from the GPR transects showed distinct changes across each transect. Reflectors appear to highlight differences in subsurface materials indicating the possible thinning of petrocalcic materials from the outer rim to center of the depression. Changes in the GPR data appear to coincide with noted changes on the surface. A significant observed change occurs in the center of the depression where no petrocalcic material appears to be present. The test hole dug at the center also revealed no signs of petrocalcic materials at depth. Abundant angular grains such as micas with few well-rounded quartz grains were identified in the test hole sample. At 20 cm depth, the material was 41% sand, 38% silt, and 21% clay.

Our hypothesis is preliminarily supported with the data we have collected. However, additional data collected from coring, geophysical work, and detailed mapping will provide more information on their formation, and will help to determine if they hold a record of landscape change due to climate and tectonic activity.