An evaluation of evidence pertaining to the origin of vein deposits exposed in Trench 14, Nevada Test Site, Nevada

Large vein-like deposits of calcite and opaline silica that infill the Bow Ridge fault are exposed by Trench 14 at the Nevada Test Site. The origin of the deposits has been the center of considerable controversy because the deposits occur on the edge of Yucca Mountain, which is being characterized geologically as a possible site for the nation`s first high level nuclear waste repository, and the various proposed modes of origin have differing implications for the performance of a geologic repository. Isotopic data for oxygen, carbon, strontium, and uranium in the carbonates preclude deposition by upwelling waters by any mechanism from either of the regionally extensive aquifers known to exist beneath Yucca Mountain. Data from the adjacent Ash Meadows flow system further suggest that the isotopic compositions of ground water in southern Nevada have not chanted markedly during the last 300 to 600 ky, and that therefore, conclusions based on present-day water compositions are probably valid for at least the last 600 ky. Geologic and paleontologic data are inconsistent with shallow a perched water spring origin for the veins, but are consistent with a pedogenic origin. Mineralogic and isotopic data match well with those for pedogenic deposits with perhaps minor modification from entrained or reacted wall rock. Taken as a whole, the data show that the carbonate and opaline silica deposits exposed in Trench 14 must have formed by a pedogenic process. Preliminary results suggest that veins in the sand ramps west of Busted Butte formed by the same mechanism. 26 refs., 11 figs

Yucca Mountain, Nevada - A Proposed Geologic Repository for High-Level Radioactive Waste (Volume 1) Introduction

Yucca Mountain in Nevada represents the proposed solution to what has been a lengthy national effort to dispose of high-level radioactive waste, waste which must be isolated from the biosphere for tens of thousands of years. This chapter reviews the background of that national effort and includes some discussion of international work in order to provide a more complete framework for the problem of waste disposal. Other chapters provide the regional geologic setting, the geology of the Yucca Mountain site, the tectonics, and climate (past, present, and future). These last two chapters are integral to prediction of long-term waste isolation

Geology of the Yucca Mountain Region, Chapter in Stuckless, J.S., ED., Yucca Mountain, Nevada - A Proposed Geologic Repository for High-Level Radioactive Waste

Yucca Mountain has been proposed as the site for the Nation's first geologic repository for high-level radioactive waste. This chapter provides the geologic framework for the Yucca Mountain region. The regional geologic units range in age from late Precambrian through Holocene, and these are described briefly. Yucca Mountain is composed dominantly of pyroclastic units that range in age from 11.4 to 15.2 Ma. The proposed repository would be constructed within the Topopah Spring Tuff, which is the lower of two major zoned and welded ash-flow tuffs within the Paintbrush Group. The two welded tuffs are separated by the partly to nonwelded Pah Canyon Tuff and Yucca Mountain Tuff, which together figure prominently in the hydrology of the unsaturated zone. The Quaternary deposits are primarily alluvial sediments with minor basaltic cinder cones and flows. Both have been studied extensively because of their importance in predicting the long-term performance of the proposed repository. Basaltic volcanism began about 10 Ma and continued as recently as about 80 ka with the eruption of cones and flows at Lathrop Wells, approximately 10 km south-southwest of Yucca Mountain. Geologic structure in the Yucca Mountain region is complex. During the latest Paleozoic and Mesozoic, strong compressional forces caused tight folding and thrust faulting. The present regional setting is one of extension, and normal faulting has been active from the Miocene through to the present. There are three major local tectonic domains: (1) Basin and Range, (2) Walker Lane, and (3) Inyo-Mono. Each domain has an effect on the stability of Yucca Mountain

Strontium isotopes in carbonate deposits at Crater Flat, Nevada

Strontium isotope studies of carbonates from soils, veins, eolian dust and Paleozoic basement samples near Crater Flat, southwest of Yucca Mountain, provide evidence for the origins of these materials. Vein and soil carbonates have nearly identical ranges of {sup 87}Sr/{sup 86}Sr ratios at the lower end of the pedogenic range. The average {sup 87}Sr/{sup 86}Sr of Paleozoic basement from Black Marble Hill is similar to the {sup 87}Sr/{sup 86}Sr in the eolian dust, perhaps indicating a local source for this material. Possible spring deposits have generally higher {sup 87}Sr/{sup 86}Sr than the other carbonates. These data are compared with similar data from areas east of Yucca Mountain. 7 refs., 5 figs