Stratigraphic and structural relations of volcanic rocks in drill holes USW GU-3 and USW G-3, Yucca Mountain, Nye County, Nevada
Major silicic ash-flow tuffs extend through Yucca Mountain with only minor variations in thickness, but the tuffs vary somewhat in degree of welding, physical properties, and extent of alteration. Two major faults sets cut Yucca Mountain at the surface. One set consists of Basin and Range style normal faults. These faults strike north-northwest to north-northeast, and in oriented core from USW GU-3, have a dominant north-northwest strike. Although most movement on the faults occurred between 13 and 11.5 m.y. ago, minor activity has persisted into the Pleistocene. The other fault set consists of northwest-striking strike-slip faults that appear to have right-lateral offset with activity restricted to between 13 and 11.5 m.y. ago. In age, attitude, sense of offset, and geographic location these faults seem to be related to the Walker Lane-Las Vegas Valley shear zones. These faults are largely restricted to the northern part of Yucca Mountain and are not observed in core from drill holes USW GU-3 and USW G-3. Fractures in drill holes USW GU-3 and USW G-3 have a dominant north-northwest strike and a steep southwest dip parallel to fractures observed on the surface. In the upper kilometer of rock, fracture densities in densely welded tuff are between 20 and 40 fractures per unit cubic meter and between less than 1 and about 3 fractures per unit cubic meter in nonwelded tuff. Below depths of 1 kilometer, the density of fractures is roughly an order of magnitude lower, from 3.5 fractures per unit cubic meter in moderately welded tuff to 0.3 fractures per unit cubic meter in nonwelded tuff. Because fractures provide the major conduits for movement of both vadose and ground water in these tuffs, the correlation between density of fractures and degree of welding of these tuffs becomes important for characterization of this potential nuclear waste respository. 39 references, 31 figures, 13 tables.
- Research Organization:
- Geological Survey, Denver, CO (United States); Fenix and Scisson, Inc., Mercury, NV (United States)
- DOE Contract Number:
- AI08-78ET44802
- OSTI ID:
- 59670
- Report Number(s):
- USGS-OFR--84-491; ON: DE85001367
- Country of Publication:
- United States
- Language:
- English
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