Stratigraphic and structural characteristics of volcanic rocks in core hole USW G-4, Yucca Mountain, Nye County, Nevada

Abstract

Detailed strati graphic and structural studies, performed in connection with the Nevada Nuclear Waste Storage Investigations program, have been in progress since 1978. The purpose of these studies is to characterize volcanic rocks underlying Yucca Mountain a volcanic highland situated along the western boundary of the Nevada Test Site in southern Nye county, Nevada. Core hole USW G-4 was cored from 41 ft (13 m) to a depth of 3,001 ft (915 m) at a location approximately 300 ft (91 m) southwest of the proposed site of an exploratory shaft that will be used for in situ studies of the geotechnical, geologic, and hydrologic characteristics of rock in the unsaturated zone to aid in evaluating the suitability of Yucca Mountain for storage of high-level nuclear wa'ste. The primary objectives of this study were to (1) verify that geologic conditions are similiar to those identified in nearby boreholes and (2) determine geologic and geophysical characteristics for use in the design and construction phases of the exploratory shaft. Strati graphic section in core hole USW G-4 is composed entirely of thick sequences of ash-flow tuff that are separated by fine-to coarse-grained ashfall tuff and tuffaceous sediments. All rocks are of Tertiary age and vary in composition from rhyolitic to quartz latitic. Major stratigraphic units include the Paintbrush Tuff, tuffaceous beds of Calico Hills, and Crater Flat Tuff. All four members of the Paintbrush Tuff were identified in USW G-4. In descending order, the members are the: Tiva Canyon, Yucca Mountain, Pah Canyon, and Topopah Spring. The Tiva Canyon and Topopah Spring Members are dominantly densely welded and devitrified, except for the basal part of the Tiva Canyon and upper and lower parts of the Topopah Spring where the rock is non-to partially welded and vitric. In contrast, the Yucca Mountain and Pah Canyon Members are entirely non-to partially welded and vitric. They represent the distal edges of ash-flow tuffs that thicken to the north and northwest. Plots of poles of fractures indicate that conspicuous fracture sets have orientations of (1) N. 22° E., 65° N.W. in the Tiva Canyon Member, (2) N. 12° W., 89-90° N.E. and S.W. in the Topopah Spring Member, and (3) N. 23° E., 45° N.W. and N. 50° E., 55° S.E. in the Crater Flat Tuff. Oriented fracture data obtained from television camera observations show no preferential strike of fractures in the Tiva Canyon Member and a wide range in the strike of fractures in the Topopah Spring Member, most of which strike between N. 30° W. and N. 60° E. Rock quality characteristics, as defined by the core index, indicate that greater amounts of broken core are associated with densely welded zones in the Tiva Canyon and Topopah Spring Members, and in zones that contain concentrations of shear fractures. Greater amounts of lost core and excessive hole enlargement are associated with vitric, non-to partially welded tuff. Geophysical logs from USW G-4 correlate well with logs from other drill holes at Yucca Mountain. Anomalous differences between t...

Hydration swelling effects on time‐dependent deformation of zeolitized tuff

“…Test samples were representative of the massive, nonwelded ash flow tuffs of Calico Hills that are pervasively zeolitized within portions of the Yucca Mountain block. All test samples contain approximately 75 % clinoptilolite, 15 % cristobalite, and lesser amounts of quartz, smectite, and alkali feldspar [Spengler and Chornack, 1984;Bish and Chipera, 1989]. The clinoptilolites in the test samples are of a mixed Na-K-Ca type with the alkalic ions dominating the exchangeable cation content (Table 1).…”

Physical properties of ash flow tuff from Yucca Mountain, Nevada

“…Borehole U SW G-4 (hereafter referred to as G-4), located in the north central part of Yucca Mountain, was drilled to a depth of 915 m during 1982, passing through the same section penetrated by borehole G-3 and terminating in the upper part of the Tram Member of the Crater Flat Tuff [Spengler and Chornack, 1984]. Thin intercepts of the Yucca Mountain and Pah Canyon members of the Paintbrush Tuff were also encountered.…”

“…It is rare that a macroscopically observed geological feature such as degree of welding correlates so well with a physical measurement. Because it has such a fundamental control on porosity, the degree of welding also influences other geological parameters such as alteration and fracturing [Scott and Castellanos, 1984;Spengler and Chornack, 1984]. Zeolitic alteration occurs more frequently in the nonwelded and poorly welded samples.…”

Impact of fracture coatings on fracture/matrix flow interactions in unsaturated, porous media

“…Fractures at Yucca Mountain are generally coated or filled to some extent at all depths, down to and below the water table [Sykes et al, !979;Spengler et al, 1984;Carlos, 1985; Crowe and Vaniman, 1985; Carlos, 1987]. The mineralogy, thickness, and amount and uniformity of coverage of fracture coatings are highly variable and uncertain.…”

“…Whereas some fractures appear to have no coating, others are completely coated or filled. Typical coatings include manganese oxides, manganates, iron oxides, iron hydroxides, silica, zeolites, smectites, or carbonates [Spengler et al, 1984;Carlos, 1985]. If the hydrologic properties of these mineral-ized coatings are much different than the matrix rock, the coatings could have a significant impact on the capillary uptake of water by the matrix from the fracture.…”