Preliminary results of gravity investigations at Yucca Mountain and vicinity, southern Nye County, Nevada

Exploration for a high-level-nuclear-waste-repository site in the Yucca Mountain area, Nev., resulted in the addition of 423 new gravity stations during the past 2 years to the 934 existing stations to form the data base of this study. About 100 surface-rock samples, three borehole gamma-gamma logs, and one borehole gravity study provide excellent density control. A linear increase in density of 0.26 g/cm3 per km is indicated in the thick tuff sequences. This steady increase of density within the tuff sequences makes the density contrast across the basal contact of the tuff the only strong source of gravity fluctuations. Isostatic and 2.0-g/cm3 Bouguer corrections were applied to the observed gravity values to remove deep-crust-related regional gradients and topographic effects, respectively. The resulting residual-gravity plot shows significant gravity anomalies that correlate closely with the structures inferred from drill-hole and surface geologic studies.

Gravity highs over the three Paleozoic rock outcrops within the study area--Bare Mountain, the Calico Hills, and the Striped Hills--served as reference points for the gravity models. The Bare Mountain gravity high, which peaks at 48 mGal, connects with a larger gravity high over the Funeral Mountains to the south; together, these highs may result from a continuous block of mildly metamorphosed Precambrian and Paleozoic rocks stretching from the east edge of Death Valley to Bare Mountain. The Calico Hills gravity high appears more likely to originate from a northeast-trending remanent topographic high of buried Paleozoic rocks rather than from an isolated structural dome. This buried ridge may extend southwestward beneath Busted Butte, 5 km southeast of the proposed repository, where 2 1/2- and 3dimensional modeling indicates that the pre-Cenozoic rocks may be less than 1,000 m deep.

At least 3,000 m of tuff fills a large steep-sided depression in the prevolcanic rocks beneath Yucca Mountain and Crater Flat. The gravity low and thick tuff section probably lie within a large collapse area comprising the Crater Flat-Timber Mountain-Silent Canyon caldera complexes. Gravity lows in Crater Flat itself are thought to coincide with the source areas of the Prow Pass Member, the Bullfrog Member, and the unnamed member of the Crater Flat Tuff. Southward extension of the broad gravity low associated with Crater Flat into the Amargosa Desert is evidence for sector graben-type collapse segments related to Timber Mountain caldera and superimposed on the other structures within Crater Flat.