HYDROGEOLOGIC FRAMEWORK OF THE SOUTHERN DEATH VALLEY FLOW SYSTEM, CALIFORNIA AND NEVADA

The southern Death Valley flow system extends from the Spring Mountains ≈100 km southwestward to southern Death Valley. The major springs in the lower part of this system—in Tecopa basin—feed the southern Amargosa River, which provides habitat for numerous endangered species. The major aquifer is the regional Paleozoic carbonate aquifer; lesser ones include Cenozoic alluvium and strongly fractured, mainly siliciclastic Proterozoic rocks. Recharge occurs mainly in the Spring Mountains. Most of the discharge issues from several springs and >10,000 wells in Pahrump Valley, a large closed basin immediately to the southwest. Hydrologists have long proposed that a significant fraction of the flow under Pahrump Valley exits this basin by interbasin flow because the downgradient springs in Tecopa basin have very light O- and H-isotopic ratios, and the Spring Mountains are the only range in the region high enough in elevation to be the source.

Based on new fieldwork, the interbasin flow pathway from Pahrump basin is formed by Paleozoic carbonate rocks that are folded into a broad footwall syncline beneath the Chicago Pass thrust. This geologic feature extends southwestward from Pahrump basin, under two ranges and one intervening basin, and across the Tecopa basin to its southern margin, where it is truncated by the Sheephead fault. The numerous strands of this fault control the locations of the major springs. The chemistry of the Tecopa basin springs indicates a Paleozoic carbonate aquifer source, but also reflects interaction with Proterozoic siliciclastic rocks, increasing to the south. The high temperatures (≤48°C/118°F) of the springs indicate that the carbonates lie at a depth of ≥1-to-3 km under most of Tecopa basin. To reach the surface, the spring water must rise through ≥1-to-3 km of brecciated siliciclastic rocks along fault zones that cut the upper plate of the Chicago Pass thrust.

Based on this framework, the only significant threat to flow in the southern part of Amargosa River is from groundwater overdrafting in Pahrump Valley. This threat can be monitored by the drilling of a 100-170-m well where the footwall syncline intersects the southwest margin of Pahrump basin. Moreover, the flow regime in this groundwater system could be better understood if O- and H-isotopic ratios were determined for all of the major springs.