TWO GENERATIONS OF GEOLOGIC CROSS SECTIONS SUPPORTING HYDROLOGIC STUDIES, SOUTHERN GREAT BASIN

Comparisons between two generations of regional geologic sections (1983 by TLTG; 2002 by DSS) show the evolution of subsurface geologic interpretations for hydrogeologic investigations in the southern Great Basin. Both sets of sections were drawn to support ground-water studies of a roughly 100,000-km² region surrounding Yucca Mountain and the Nevada Test Site. In both cases, delineation of stratigraphic and structural detail was influenced by the scale of the cross sections (1:250,000) and the requirements to provide geologic and hydrologic characterization in order to evaluate potential hydrogeologic environments for isolation of high-level radioactive waste. The sections were drawn to at least 5 km depth in order to portray the combined thickness of the 4+ km-thick Paleozoic carbonate miogeoclinal sequence, the 2+ km-thick sequence of Tertiary volcanic rocks of the southwestern Nevada volcanic field, and locally thick Tertiary basin fill, each of which serve as aquifers in parts of the region. The major differences in subsurface interpretation between the two sets of sections result from: (1) availability of borehole stratigraphic control on the Nevada Test Site and new geologic mapping throughout the region; (2) integration of the newer sections with a gravity inversion model that helps define caldera shapes and estimates the elevation of the pre-Cenozoic surface; (3) consistent use in the newer sections of geometric rules for the construction of fold-thrust belts and extensional detachments; and (4) the benefits of hindsight derived from the first generation of ground-water flow models that highlighted specific areas for modification of the geologic framework where hydrologic information indicated potential geologic controls on the flow system. In contrast to other recent Great Basin studies, neither set of cross sections portrays a Paleozoic carbonate aquifer that is tectonically dismembered through extreme extension. The portrayal of a relatively continuous carbonate aquifer is based upon the prevalence of Paleozoic carbonate rocks intercepted in deep boreholes and on the thermal and geochemical signature of springs that indicate regional interbasinal ground-water flow through the carbonate aquifer.