ASSESSING SINKHOLE DEVELOPMENT ON ALLUVIAL FANS AROUND THE DEVIL’S THROAT, SOUTHERN NEVADA

The population of southern Nevada is rapidly growing and land management is becoming increasingly important. As a result, resources within areas of critical environmental concern are being assessed. One of these regions contains a large sinkhole on an alluvial fan south of the Virgin Mountains, NV. This unique geologic feature, Devil's Throat, is approximately 30 m in diameter, 25 m deep and its formation is poorly understood. In addition, previous and current workers have identified at least seven features that may represent early stages of sinkhole formation. The goal of this study was to document the characteristics of these features, determine if they are sinkholes, and ultimately determine a mechanism of formation. An initial airphoto analysis revealed circular features that are 30 – 50 m diameter with the largest over 100 m. Field inspection showed that they are different from the Devil's Throat in that they are not well-developed, deep features with vertical walls. Rather, they are circular depressions with a higher elevation rim and a flat floor where the relief can range from 4 – 9 m and contain high proportions of silt and clay, relative to the gravel-rich margins.

In an effort to delineate these features, we acquired ground penetrating radar (GPR) profiles across three of the depressions. The GPR transects support the initial field observations that the floor is silt and clay dominated with gravelly margins. The reflections in the gravels show stratification that dips toward the center of the depressions. The floors exhibit more continuous, horizontal reflectors that undulate, suggesting slumping gravels may be interbedded with silt and clay. Rapid attenuation of the GPR signal prevented imaging of depths below 3 m in the gravel-rich areas and 1 m in the depression floor. Even with limited penetration, the dipping reflectors may indicate subsidence and an early stage of development. Causes for the development of these features may be attributed to dissolution in the subsurface or erosion by groundwater flow (piping). Future work will entail the acquisition of seismic reflection profiles to image geometry and stratification in more detail. The subsidence of these features could be destructive in urban areas; thus, understanding the formation of these features can improve land-use suitability assessments.