Open-File Report 01-335
Potentiometric Surface, Carbonate-Rock Province, Southern Nevada and Southeastern California, 1998-2000
Within the carbonate-rock province, carbonate rocks were deposited from about 280 million to about 570 million years ago along the continental shelf of what was the west coast of North America during the Early Paleozoic Era (Stewart, 1980). These rocks consist of up to 30,000 feet of ancient marine sediment, with less significant interfingering layers of shale, quartzite, chert, and siltstone. These rocks are underlain by Precambrian metamorphic and granitic rocks and by Late Precambrian to Middle Cambrian clastic sedimentary rocks, and are overlain by Late Paleozoic to Mesozoic clastic sedimentary rocks, Cenozoic volcanic rocks, and Cenozoic basin-fill deposits. Intrusions of Late Mesozoic to Cenozoic igneous rocks are also common in the carbonate-rock province.
The physiography of the study area is characterized by generally parallel, north- to northeast-trending mountain ranges separated by broad alluvial desert basins. These ranges typically rise 1,000 to 7,000 ft above the intervening valleys. The elongated ranges and valleys generally are 5 to 15 mi wide and 40 to 80 mi long (Prudic and others, 1995). The Basin and Range topography that typifies the carbonate-rock province consists of extensional (normally) faulted terrains forming complex, heterogeneous geologic settings, each with unique local and regional characteristics. Strike-slip faults from the Early Jurassic to the Late Tertiary associated with compressive and extensional faulting add to the structural complexity of the study area (Stewart, 1980).
The carbonate rocks in the province form complex aquifers whose shapes and sizes are largely unknown. These aquifers are interconnected with aquifers of other rock types. Where deformed and fractured, these saturated ancient sedimentary rocks have the potential to transmit ground water. Within the carbonate-rock province, the carbonate rocks are believed to be the principal water-bearing zones because of their brittleness and capacity to chemically dissolve in flowing water, and because of the fractures and joints that have been widened by solution to varying degrees (Winograd and Thordarson, 1975, p. 19). Although many hydrogeologic properties (including transmissivity, storativity, hydraulic conductivity, and porosity) of the different lithologies probably influence flow, the broadest and simplest hydrogeologic influences in these areas may be variations in the thickness and continuity of carbonate-rock aquifers that are associated with extended terrains (Dettinger and Schaefer, 1995). Dettinger and Schaefer (1996) analyzed gravity and aeromagnetic profiles and indicated identifiable patterns of extension in the eastern Great Basin. These large-scale extensional features correlate with the regional ground-water flow system and can be used to define directions of ground-water flow (Dettinger and Schaefer, 1996). Regionally, in areas of extreme extension, most of the carbonate rocks that would connect and integrate flow in basin-fill and volcanic-rock aquifers have been thinned and removed. Where there has been only slight extension, carbonate rocks form thick layers, resulting in continuous aquifers that exhibit broadly integrated flow. Dettinger (1989) described a thick, laterally continuous, corridor of Paleozoic carbonate rock resulting from 25 million years of extension (pl. 1, green-shaded area). Dettinger and Schaefer (1996) noted that the bounds of these terrains commonly mark divides between regionally deep and locally shallow ground-water flow (pl. 1). Furthermore, Dettinger and Schaefer (1996) and Laczniak and others (1996) previously defined the southernmost extent of this terrain where it crosses into southeastern California. This report, however, does not depict the boundary of thick continuous carbonate rock past the Nevada-California border.
Recharge to the carbonate aquifers originates in high mountain ranges where snowpack accumulations release water over a sustained period (Dettinger, 1989, p. 7). Discharge from carbonate aquifers consists of direct evaporation from the soil, evapotranspiration from plants, and the flow of springs.
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