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Title: Geology of the U-1a. 01 horizontal drift complex, southwestern Yucca Flat, Nevada Test Site

Abstract

The U-1a.01 complex, site of a Los Alamos National Laboratory-sponsored experiment, is located in southwestern Yucca Flat on the Nevada Test Site. The complex is comprised of the vertical U-1a shaft, two large-diameter cable access holes, and approximately 229 m (750') of horizontal drift mined within alluvium 292.6 m (960') below the surface. Geologic mapping and related work at U-1a.01 afforded a unique opportunity to observe and evaluate the characteristics of alluvium in the subsurface of Yucca Flat, and to compare the results with surface and drill hole alluvium data. The Los Alamos Support Group of the Fenix and Scisson, Inc. Geology and Hydrology Division carried out a program of mapping, sampling and photography while mining was in progress, with the dual purpose of characterizing the geology of the site and documenting the mining operation. This report details the products and the results of that effort. 12 refs., 8 figs., 25 tabs.

Authors:
; ;
Publication Date:
Research Org.:
Fenix and Scisson, Inc., Mercury, NV (USA)
OSTI Identifier:
6032322
Report Number(s):
DOE/NV/10322-37
ON: DE89015380
DOE Contract Number:
AC08-84NV10322
Resource Type:
Technical Report
Resource Relation:
Other Information: Portions of this document are illegible in microfiche products
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; 45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEFENSE; MINE SHAFTS; GEOLOGY; ALLUVIAL DEPOSITS; DRILLING; MAPPING; MINING; NEVADA TEST SITE; GEOLOGIC DEPOSITS; NATIONAL ORGANIZATIONS; SHAFT EXCAVATIONS; US DOE; US ORGANIZATIONS; 580100* - Geology & Hydrology- (-1989); 450202 - Explosions & Explosives- Nuclear- Weaponry- (-1989)

Citation Formats

Drellack, S.L. Jr., Thompson, P.H., and Rayburn, C.J.. Geology of the U-1a. 01 horizontal drift complex, southwestern Yucca Flat, Nevada Test Site. United States: N. p., 1989. Web. doi:10.2172/6032322.
Drellack, S.L. Jr., Thompson, P.H., & Rayburn, C.J.. Geology of the U-1a. 01 horizontal drift complex, southwestern Yucca Flat, Nevada Test Site. United States. doi:10.2172/6032322.
Drellack, S.L. Jr., Thompson, P.H., and Rayburn, C.J.. 1989. "Geology of the U-1a. 01 horizontal drift complex, southwestern Yucca Flat, Nevada Test Site". United States. doi:10.2172/6032322. https://www.osti.gov/servlets/purl/6032322.
@article{osti_6032322,
title = {Geology of the U-1a. 01 horizontal drift complex, southwestern Yucca Flat, Nevada Test Site},
author = {Drellack, S.L. Jr. and Thompson, P.H. and Rayburn, C.J.},
abstractNote = {The U-1a.01 complex, site of a Los Alamos National Laboratory-sponsored experiment, is located in southwestern Yucca Flat on the Nevada Test Site. The complex is comprised of the vertical U-1a shaft, two large-diameter cable access holes, and approximately 229 m (750') of horizontal drift mined within alluvium 292.6 m (960') below the surface. Geologic mapping and related work at U-1a.01 afforded a unique opportunity to observe and evaluate the characteristics of alluvium in the subsurface of Yucca Flat, and to compare the results with surface and drill hole alluvium data. The Los Alamos Support Group of the Fenix and Scisson, Inc. Geology and Hydrology Division carried out a program of mapping, sampling and photography while mining was in progress, with the dual purpose of characterizing the geology of the site and documenting the mining operation. This report details the products and the results of that effort. 12 refs., 8 figs., 25 tabs.},
doi = {10.2172/6032322},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1989,
month = 3
}

Technical Report:

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  • Geologic studies in connection with Project Discus Thrower have furnished detailed stratigraphic and structural information about northwestern Yucca Flat, Nevada Test Site. The Paleozoic rocks consist of a lower carbonate sequence, argillite of the Eleana Formation, and an upper carbonate sequence. The distribution of these rocks suggests that both top and bottom of the Eleana are structural contacts, probably thrusts or reverse faults. The overlying tuff includes several units recognized in the subsurface, such as the Fraction Tuff and tuff of Redrock Valley. Other units recognized include bedded tuff associated with the Grouse Canyon Member of Belted Range Tuff, andmore » the Rainier Mesa and Ammonia Tanks Members of the Timber Mountain Tuff. The Timber Mountain and Grouse Canyon are extensively altered to montmorillonite (a swelling clay), possibly as a result of ponding of alkaline water. The overlying alluvium locally contains at the base a clayey, tuffaceous sandstone. (auth)« less
  • In Yucca Flat, Nevada Test Site, a discrete part of Area 3 has been defined where the alluvium is largely tuff-rich sand extending to a depth of 1500 ft. Material properties (bulk density, grain density, grain size, velocity, water content, and carbon dioxide content) of the alluvial material are nearly uniform. Informally termed the Area 3 Sandpile, knowledge of its extent and nature is based on geologic and geophysical data from 42 drill sites. In order to accumulate 1500 feet of alluvium, the present alluviation pattern has existed for a considerable time and also requires a stable structural setting. Thismore » accumulation may have taken 5 m.y. or more.« less
  • The French Peak accommodation zone (FPAZ) forms an east-trending bedrock structural high in the Nevada Test Site region of southwestern Nevada that formed during Cenozoic Basin and Range extension. The zone separates areas of opposing directions of tilt and downthrow on faults in the Yucca Flat and Frenchman Flat areas. Paleomagnetic data show that rocks within the accommodation zone adjacent to Yucca Flat were not strongly affected by vertical-axis rotation and thus that the transverse strikes of fault and strata formed near their present orientation. Both normal- and oblique strike-slip faulting in the FPAZ largely occurred under a normal-fault stressmore » regime, with least principal stress oriented west-northwest. The normal and sinistral faults in the Puddle Peka segment transfers extension between the Plutonium Valley normal fault zone and the Cane Spring sinistral fault. Recognition of sinistral shear across the Puddle Peak segment allows the Frenchman Flat basin to be interpreted as an asymmetric pull-apart basin developed between the FPAZ and a zone of east-northeast-striking faults to the south that include the Rock Valley fault. The FPAZ has the potential to influence ground-water flow in the region in several ways. Fracture density and thus probably fracture conductivity is high within the FPAZ due to the abundant fault splays present. Moreover,, fractures oriented transversely to the general southward flow of ground water through Yucca Flat area are significant and have potential to laterally divert ground water. Finally, the FPAZ forms a faulted structural high whose northern and southern flanks may permit intermixing of ground waters from different aquifer levels, namely the lower carbonate, welded tuff, and alluvial aquifers. 42 refs.« less
  • This map displays interpreted structural and stratigraphic relations among the Paleozoic and older rocks of the Nevada Test Site region beneath the Miocene volcanic rocks and younger alluvium in the Yucca Flat and northern Frenchman Flat basins. These interpretations are based on a comprehensive examination and review of data for more than 77 drillholes that penetrated part of the pre-Tertiary basement beneath these post-middle Miocene structural basins. Biostratigraphic data from conodont fossils were newly obtained for 31 of these holes, and a thorough review of all prior microfossil paleontologic data is incorporated in the analysis. Subsurface relationships are interpreted inmore » light of a revised regional geologic framework synthesized from detailed geologic mapping in the ranges surrounding Yucca Flat, from comprehensive stratigraphic studies in the region, and from additional detailed field studies on and around the Nevada Test Site. All available data indicate the subsurface geology of Yucca Flat is considerably more complicated than previous interpretations have suggested. The western part of the basin, in particular, is underlain by relics of the eastward-vergent Belted Range thrust system that are folded back toward the west and thrust by local, west-vergent contractional structures of the CP thrust system. Field evidence from the ranges surrounding the north end of Yucca Flat indicate that two significant strike-slip faults track southward beneath the post-middle Miocene basin fill, but their subsurface traces cannot be closely defined from the available evidence. In contrast, the eastern part of the Yucca Flat basin is interpreted to be underlain by a fairly simple north-trending, broad syncline in the pre-Tertiary units. Far fewer data are available for the northern Frenchman Flat basin, but regional analysis indicates the pre-Tertiary structure there should also be relatively simple and not affected by thrusting. This new interpretation has implications for ground water flow through pre-Tertiary rocks beneath the Yucca Flat and northern Frenchman Flat areas, and has consequences for ground water modeling and model validation. Our data indicate that the Mississippian Chainman Shale is not laterally extensive confining unit in the western part of the basin because it is folded back onto itself by the convergent structures of the Belted Range and CP thrust systems. Early and Middle Paleozoic limestone and dolomite are present beneath most of both basins and, regardless of structural complications, are interpreted to form a laterally continuous and extensive carbonate aquifer. Structural culmination that marks the French Peak accommodation zone along the topographic divide between the two basins provides a lateral pathway through highly fractured rock between the volcanic aquifers of Yucca Flat and the regional carbonate aquifer. This pathway may accelerate the migration of ground-water contaminants introduced by underground nuclear testing toward discharge areas beyond the Nevada Test Site boundaries. Predictive three-dimensional models of hydrostratigraphic units and ground-water flow in the pre-Tertiary rocks of subsurface Yucca Flat are likely to be unrealistic due to the extreme structural complexities. The interpretation of hydrologic and geochemical data obtained from monitoring wells will be difficult to extrapolate through the flow system until more is known about the continuity of hydrostratigraphic units. 1 plate« less
  • The underground nuclear testing program of the US Department of Energy (USDOE) takes place at the Nevada Test Site (NTS), about 65 mi north-west of Las Vegas, Nevada. Underground nuclear tests at Yucca Flat, one of the USDOE test areas at NTS, have affected hydrologic conditions, including groundwater levels. The purpose of this map report, prepared in cooperation with USDOE, is to present selected water-level data collected from wells and test holes through December 1991, and to show potentiometric contours representing 1991 water-table conditions in the Yucca Flat area. The more generic term, potentiometric contours, is used herein rather thanmore » ``water-table contours`` because the hydrologic units contributing water to wells and test holes may not accurately represent the water table. The water table is that surface in an unconfined water body at which the pressure is atmospheric. It is defined by the altitude at which non- perched ground water is first found in wells and test holes. Perched ground water is defined as unconfined ground water separated from an underlying body of ground water by an unsaturated zone. This map report updates information on water levels in some wells and test holes and the resulting water-table contours in rocks of Cenozoic and Paleozoic age shown by Doty and Thordarson for 1980 conditions.« less