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A study of the subsurface geology was undertaken in association with the Nevada Nuclear Waste Storage Investigations to identify any near-surface structural features that may be present beneath one of four linear northwest-trending washes that transect the northeastern part of Yucca Mountain at the Nevada Test Site. Four drill holes were continuously cored to depths of about 500 feet. In descending order, the drill holes penetrated the Tiva Canyon, Yucca Mountain, Pah Canyon, and Topopah Spring Members of the Paintbrush Tuff of Tertiary age. These units consist almost entirely of nonwelded to densely welded rhyolitic ash-flow tuffs, separated by thin beds of air-fall and reworked tuff.The Yucca Mountain and Pah Canyon Members are of particular interest because their total thicknesses were penetrated in each of the four holes. A comparison of thickness variations of both units indicate progressive thinning from northwest to southeast, which appears consistent with that of typical thinning away from source areas located to the northwest.An analysis of oriented core samples taken within the densely welded Topopah Spring Member in each drill hole indicates two complementary sets of high-angle fractures striking N. 12° W. and N. 37° E. Compilations of fracture frequencies show that core from drill holes located close to the wash margins is more fractured than core from drill holes near the center of the wash. The northeasterly margin of the wash shows evidence of minor faulting inferred from the presence of a zone of en echelon shear fractures found in core from hole UE25a-4. These shear fractures are high angle, show evidence of brecciation, and most strike parallel to the wash lineation. Identification of structure along the southwest edge of the wash was independently obtained through the analysis of eutaxitic structure within core of the Topopah Spring Member, and confirmed by paleomagnetic studies within the same member. Measurements of foliation attitudes in drill holes UE25a-4, -5, and -7 indicate a mean strike direction of about N. 10° W. within the wash. In marked contrast, the mean foliation attitude in UE25a-6, located outside the wash trend, strikes N. 23° E. This 33° change in strike occurs abruptly over a distance of about 1,000 feet. The difference between these orientations, as well as the absence of vertical offset along shear fractures in UE25a-4 and between drill holes seem best explained by tectonic rotation that occurred about a near-vertical axis as a result of minor strike-slip or oblique-slip movement within the wash. As no abrupt changes in thickness were apparent in the Yucca Mountain and Pah Canyon Members, the magnitude of displacement along the structure is considered to be of small scale. Additional subsurface information, particularly in the area underlying ridges northeast of the wash, is needed not only to confirm the existence of the structure but also to aid in determining the amount of lateral movement.

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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...

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