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Bedrock Geologic Map of the Yucca Mountain Area, Nye County, Nevada

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Bedrock Geologic Map of the Yucca Mountain Area, Nye County, Nevada U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY BEDROCK GEOLOGIC MAP OF THE YUCCA MOUNTAIN AREA, NYE COUNTY, NEVADA By Warren C. Day,l Robert P. Dickerson,2 Christopher J. Potter,l DonaldS. Sweetkind,l Carma A. San Juan,2 Ronald M. Drake 11,2 and Christopher J. Fridrichl 1998 lU.S. Geological Survey, Denver, Colorado 2Pacific Western Technologies, Inc., Denver, Colorado Prepared in cooperation with the NEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF ENERGY Pamphlet to accompany GEOLOGIC INVESTIGATIONS SERIES I-2627 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 CONTENTS Abstract ......................................................................................................................................... 1 Introduction . 1 Notes on stratigraphic nomenclature . 4 Previous mapping . 4 l\l[ethodology . 4 Borehole designations . 5 D'ata sources . 5 Regional setting . 6 Stratigraphic setting . 6 Structural geology . 8 Description of block-bounding faults . 8 Description of relay structures . 8 Description of the prominent northwest-striking strike-slip faults ............................................. 10 Description of intra block structures . 11 Structural domains ....................................................................................................................... 12 Central Yucca Mountain domain ............................................................................................. 12 Azreal Ridge domain . 14 Yucca Wash domain ............................................................................................................... 14 Paintbrush Canyon domain . 14 Fran Ridge domain .................................................................................................................. 15 F()rtymile Wash domain .......................................................................................................... 15 Pllug Hill domain . 15 Southwest domain . 16 East Ridge domain . 16 Dune Wash domain . ...... .. .. ... .. ...... .. ... .. ... ... ... .. ... .. ...... ... ......... ........ ... ....... .. ....... ... ...... ........ 16 Discussion on the variation and timing of tectonism at Yucca Mountain . 17 References cited . 19 FIGURES 1. Index map of study area showing regional distribution of caldera structures near Yucca . 2 Mountain, Nye County, Nevada. 2. Map showing locations of prominent physiographic features in the map area .............................. 3 3. Map showing distribution of fault types in the map area .............................................................. 9 4. Map showing distribution of structural domains defined for the map area ................................... 13 CONVERSION FACTORS Multiply by To obtain millimeter (mm) 0.03937 inch (in.) centimeter (em) 0.3937 inch (in.) meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) The following abbreviation is also used in this report: Ma, millions of years before present. ii ABSTRACT extensional deformation as recorded in the amount of offset along the block-bounding faults Yucca Mountain, Nye County, Nevada, has as well as an increase in the intrablock faulting. been identified as a potential site for under­ The rocks in the map area had a protracted ground storage of high-level radioactive nuclear history of Tertiary extension. Rocks of the waste. Detailed bedrock geologic maps form an Paintbrush Group cover much of the area and integral part of the site characterization pro­ obscure evidence for older tectonism. An earlier gram by providing the fundamental framework history of Tertiary extension can be inferred, for research into the geologic hazards and hy­ however, because the Timber Mountain-Oasis drologic behavior of the mountain. This bedrock Valley caldera complex lies within and cuts an geologic map provides the geologic framework older north-trending rift (the Kawich­ and structural setting for the area in and adja­ Greenwater rift}. Evidence for deformation dur­ cent to the site of the potential repository. ing eruption of the Paintbrush Group is locally The study area comprises the northern and present as growth structures. Post-Paintbrush central parts of Yucca Mountain, located on the Group, pre-Timber Mountain Group extension southern flank of the Timber Mountain-Oasis occurred along the block-bounding faults. The Valley caldera complex, which was the source basal contact of the 11.6-Ma Rainier Mesa Tuff for many of the volcanic units in the area. The of the Timber Mountain Group provides a key Timber Mountain-Oasis Valley caldera complex is time horizon throughout the area. Other work­ part of the Miocene southwestern Nevada vol­ ers have shown that west of the study area in canic field, which is within the Walker Lane belt. northern Crater Flat the basal angular uncon­ This tectonic belt is a northwest-striking formity is as much as 20° between the Rainier megastructure lying between the more active Mesa and underlying Paintbrush Group rocks. Inyo-Mono and Basin-and-Range subsections of In the westernmost part of the study area the the southwestern Great Basin. unconformity is smaller (less than 10°), whereas Excluding Quaternary surficial deposits, the in the central and eastern parts of the map area map area is underlain by Miocene volcanic rocks, the contact is essentially conformable. In the principally ash-flow tuffs with lesser amounts of central part of the map the Rainier Mesa Tuff lava flows. These volcanic units include the laps over fault splays within the Solitario Crater Flat Group, the Calico Hills Formation, Canyon fault zone. However, displacement did the Paintbrush Group, and the Timber Mountain occur on the block-bounding faults after deposi­ Group, as well as minor basaltic dikes. The tuffs tion of the Rainier Mesa Tuff inasmuch as it is and lava flows are predominantly rhyolite with locally caught up in the hanging-wall deforma­ lesser amounts of latite and range in age from tion of the block-bounding faults. Therefore, the 13.4 to 11.6 Ma. The 10-Ma basaltic dikes in­ regional Tertiary to Recent extension was pro­ truded along a few fault traces in the north­ tracted, occurring prior to and after the eruption central part of the study area. of the tuffs exposed at Yucca Mountain. Fault types in the area can be classified as block bounding, relay structures, strike slip, and INTRODUCTION intrablock. The block-bounding faults separate the 1- to 4-km-wide, east-dipping structural This 1:24,000-scale map focuses on the blocks and exhibit hundreds of meters of dis­ area surrounding the potential high-level nuclear placement. The relay structures are northwest­ waste repository site at Yucca Mountain (figs. 1 striking normal fault zones that kinematically link and 2). Its purpose is to define the character the block-bounding faults. The strike-slip faults and extent of the dominant structural features in are steep, northwest-striking dextral faults lo­ the vicinity of and outward from the potential cated in the northern part of Yucca Mountain. repository area. As currently conceived, the po­ The intrablock faults are modest faults of limited tential repository would be a permanent under­ offset (tens of meters) and trace length (less ground facility with high-level nuclear waste than 7 km) that accommodated intrablock placed in drifts excavated in the densely welded deformation. units of the Miocene Topopah Spring Tuff. The The concept of structural domains provides repository would be built in the unsaturated zone a useful tool in delineating and describing varia­ approximately 250 m above the regional ground­ tions in structural style. Domains are defined water table. The Yucca Mountain Project cur­ across the study area on the basis of the rently is evaluating the cumulative effect of natu­ relative amount of internal faulting, style of ral geologic hazards in the site area, including deformation, and stratal dips. In general, there seismic and volcanic hazards. Hydrologic inves­ is a systematic north to south increase in tigations and computer-aided three-dimensional 1 Boundary of southwestern Nevada volcanic field I------- Nevada Test Site Timber Mountain-Oasis Valley caldera complex "'-.Amargosa Valley ' ' NEVADA ' 2~[-. "' '1(~,'1() O~'~ /V4", ' ' ' Pahrump "' 0 0 10 20 30 40 50 KILOMETERS"' 0 5 10 15 20 25 MILES Figure 1. Index map of study area showing regional distribution of caldera structures near Yucca Mountain, Nye County, Nevada. Modified from Carr and others (1986, fig. 1) and Sawyer and others (1994, fig. 1). 2 780000 770000 Fortymile Wash 760000 750000 740000 550000 560000 570000 580000 0 2 MILES 0 2 KILOMETERS EXPLANATION J Quaternary deposits ~~ '--------'~ Miocene volcanic bedrock Figure 2. Locations of prominent physiographic features in the map area. 3 geologic and hydrologic framework modeling ef­ block-bounding faults and some of the intrablock forts are ongoing to characterize the site as faults, and outlined the zoned compositional na­ fully as possible. Knowledge of the distribution ture of the tuffaceous units that underlie Yucca of geologic units as well as the stratigraphic and Mountain. After identification of Yucca Moun­ structural setting is the underpinning for such tain as a possible site for high-level nuclear investigations. This map and accompanying waste disposal by the U.S. Department of En­ text describe the location, geometry, and kine­ ergy, during the 1980's Scott and Bonk (1984) matic interplay between the various fault
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