237 Geophysics/Tectonophysics/Seismology

Wednesday, 19 October 2005
1:30 PM - 5:30 PM

THE ROLE OF FAULTING IN EVAPORITE KARST DEVELOPMENT: SOUTHERN VIRGIN MOUNTAINS, NEVADA

Paper number: 237-10
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

Karst development is commonly perceived as being linked to areas typically underlain by carbonates, but also occurs where gypsum or halite is present. This study documents evaporite karst in southern Nevada that has been observed where gypsum is exposed at the surface and is thought to underlie alluvial fan deposits. The study area is at the southern edge of a pull-apart basin bounded by northeast-southwest trending strands of the left-lateral strike-slip Lake Mead Fault System (LMFS). We hypothesize that portions of this fault system provide pathways for fluids that infiltrate gypsum beds and facilitate dissolution and development of karst features, even in those areas where gypsum is in the subsurface. The karst features are primarily sinkholes, with depths up to 50 m and widths as great as 100 m. Although the occurrence of these features may appear to be random, the active fault history of this area plays a role in the formation of many of these sinkholes. The role of faults in the development of karst was determined through an interdisciplinary approach utilizing geologic mapping and geophysical methods such as ground-penetrating radar (GPR), seismic reflection/refraction profiling, and ground magnetic surveys.

GPR acquisition across a larger sink and across two smaller sinks showed inward dipping reflectors that terminated at a highly attenuated clay-rich layer. Therefore, seismic refraction/reflection profiling was conducted across the same features using a hammer source to penetrate below the clay layer. The reflection data showed continuously dipping reflectors similar to the GPR survey. In addition, a bright reflector at about 20 m depth could be interpreted as either an impermeable layer, below which imaging is difficult, or fluids ponding at that interval. To test whether faulting plays a role in the formation of the sinks, a 4.9 sq km magnetic survey at 100 m station spacing was conducted over a large sink. The magnetic survey shows that previously mapped surficial faults can now be extended and track the trend of existing sinkholes. This suggests that faulting is a mechanism for the flow of dissolving fluids along these pathways. In addition, this may now lead into understanding the timing and overall development of the evaporite karst system in southern Nevada.