Fossil spring deposits in the southern Great Basin and their implications for changes in water-table levels near Yucca Mountain, Nevada, during Quaternary time

Fossil spring deposits are common in the southern Great Basin, and their distribution provides important constraints on the hydrologic response of the regional water table to climate change. This information is crucial, because the proposed high-level nuclear waste repository at Yucca Mountain will be located ∼200–400 m above the modern water table. Water tables will rise in response to a future return to glacial climates, but the magnitude of the change—and the consequences for radionuclide travel times and overall repository integrity—are key uncertainties. Increased recharge during past pluvial periods in the Spring Mountains and Sheep Range caused water tables to rise and ground water to discharge over broad expanses of the Las Vegas Valley system, and in nearby Pahrump, Sandy, and Coyote Springs Valleys. In contrast, other valleys in the region contain only small areas of Pleistocene discharge resulting from local damming of ground water by faults that cut valley alluvium. In these instances, which include the Valley Wells area, and Piute and northern Coyote Springs Valleys, smaller ranges such as the Clark and New York Mountains supplied the moisture. The change in water-table levels since the last full glacial period varies between and within valleys, from as little as 10 m in several areas to 95 m in the Coyote Springs Valley. At Yucca Mountain, the water table has probably changed by ≤115 m in response to climate change.

The spring deposits and the mollusk faunas found with them, often misinterpreted as lacustrine in origin, share many essential features with active spring systems in northeast Nevada. Deposits associated with discharge mainly consist of pale brown silt and sand that is entrapped by dense stands of phreatophytes covering valley bottoms when water tables are high. Pale green mud, containing a mix of aquatic, semiaquatic, and moist terrestrial mollusks, accumulates in wet meadows and marshes associated directly with spring discharge.

The record in subbasins of the Las Vegas Valley system is dominated by late Wisconsin–age sediments, although pond sediments and alluvium belonging to at least one older (pre-Wisconsin?) pluvial period are also locally exposed. Deposits from two even earlier episodes of spring discharge, both of which also occurred during Rancholabrean time (10 to <450 ka), are exposed in the Pahrump, Chicago, Piute, and Coyote Springs Valleys, and in the Valley Wells. The records from the Pahrump and Coyote Springs Valleys are especially well exposed and likely extend back to at least early Pleistocene time.