| Abstract |
The Soda Lake geothermal area is located in the Carson Desert in the western part of the Carson Sink basin in west-central Nevada. The field is centered ~3.5 km NNE of the Soda Lake maar, from which it takes its name. The Soda Lake field was identified serendipitously during the drilling of an irrigation survey well in the early 20th century, and was explored for electricity production potential by Chevron and Phillips in the 1970s. It has been producing power for more than 2 decades and is currently operating at 16 MW. Dense drilling at the Soda Lake field permits good correlation of stratigraphy in the subsurface below the feature-poor Carson Desert. Unconsolidated sediments extend more than 1000 m below surface in this part of the basin. The upper few hundred meters are composed of non-marine fluvial and lacustrine sediments derived from the surrounding highlands. The basin fill becomes increasingly uniform and volcanic-sourced with depth. In many wells this deeper volcaniclastic fill may grade into a lapilli lithic tuff of similar composition. At variable distances below the tuff, the unconsolidated fill is intruded by a plug-like trachytic basalt body. The basalt overlies fine-grained, organic-rich lacustrine sediments and a thick package of altered basalts. The Soda Lake geothermal field falls within a NNE--SSW surface trend defined by the Soda Lake maar, the Upsal Hogback cinder cones, and hot ground and silicified sands that overlie the field itself. Drilling and seismic data have confirmed that the field lies within a narrow, NNE-trending graben. The faults that define this structure likely control both upwelling hot fluids and these small eruptive centers – both the surface-breaching volcanic centers and the previously unrecognized buried basaltic plug at the geothermal field. A more complete stratigraphic column, based on analysis of cuttings, is in progress for the immediate Soda Lake area. A stratigraphy based on correlation between lithologic logging and borehole geophysics will allow us to 1) determine local throw on the primary NNE-striking Soda Lake faults, 2) aid in locating the subsidiary faults and/or step-overs, and thereby 3) allow correlation of the prevailing structures with regional-scale trends observed at the boundaries of the Carson Sink basin. |