Table 1. Principal stratigraphic and associated hydrogeologic units of Nevada Test Site and vicinity

[Based on information from Blankennagel and Weir, 1973, table 1; Winograd and Thordarson, 1975, table 1; Byers and others, 1976; Cole and others, 1990; Sawyer and others, 1990 Sawyer and others, 1994; and R.G. Warren, Los Alamos National Laboratory, written commun., 1991]

Stratigraphic unit1  Principal lithology2  Hydrogeologic unit3  Known or inferred 
hydrologic significance4 
Valley-fill deposits 
[Qa, QTa, Qp, Qe, Qc] 
Miocene to Quaternary		 Gravel and sand (eolian sand and lakebed deposits)  Valley-fill aquifer  Generally unsaturated except in deepest structural basins at Yucca Flat, Frenchman Flat, Emigrant Valley, and Amargosa Desert. Saturated thickness highly variable and can exceed 1,000 ft locally; local source of recharge to regional carbonate-rock aquifer. Fine-grained lakebed deposits present in Ash Meadows area may inhibit regional flow and divert ground water to surface [AM, OV, AFFCR] 
Basalt 
[Qb, QTb, Tb] 
Miocene to Quaternary 		 Basalt in thin flows, dikes, and cinder cones n/c Generally unsaturated; dikes and local flows may influence local ground-water movement [OV, AFFCR, AM] 
Thirsty Canyon Group 
[Tt] 
9.4 Ma		 Variable welded peralkaline 
rhyolite ash-flow tuff 
(trachyte lava) 		 Welded-tuff aquifer  Generally unsaturated; may contain local perched water in Pahute Mesa and beneath Oasis Valley [OV, AFFCR] 
Timber Mountain Group 
[Tm] 
12.5-11.5 Ma 
Ammonia Tanks Tuff  Variable welded ash-flow tuff 
(bedded tuff) 		 Welded-tuff aquifer  Generally unsaturated except in Oasis Valley and possibly Jackass Flats [OV, AFFCR?] 
Rhyolite of 
Tannenbaum Hill		 Rhyolite lava  Lava-flow aquifer  Local aquifer in northwestern part of Timber Mountain caldera [AFFCR, OV] 
Rainier Mesa Tuff  Variably welded ash-flow tuff (nonwelded tuff; rhyolite lava) Welded-tuff aquifer  Major aquifer in deeper parts of Yucca Flat, CP Basin, Frenchman Flat, and Jackass Flats, and in western Pahute Mesa and Oasis Valley [AM, AFFCR, OV]
Pre-Rainier Mesa rhyolites Rhyolite lava (nonwelded tuff) Lava-flow aquifer Local aquifers in Pahute Mesa and at Yucca Mountain [AFFCR]
Paintbrush Group 
[Tp] 
12.8-12.7 Ma 
Post-Tiva Canyon rhyolites Rhyolite lava (nonwelded tuff)  Lava-flow aquifer Generally unsaturated; may form local aquifers in fault blocks in western and central Pahute Mesa; contains perched water elsewhere [OV, AFFCR] 
Tiva Canyon Tuff  Variably welded ash-flow tuff 
(nonwelded tuff; locally zeolitized)		 Welded-tuff aquifer 
(tuff confining unit)		 Major aquifer in Pahute Mesa and Jackass Flats; confining unit where nonwelded in Yucca Flat [AFFCR, AM] 
Middle Paintbrush Group rhyolites (includes rhyolites of Echo Peak and Silent Canyon)  Rhyolite lava (welded ash-flow tuff; nonwelded tuff, locally zeolitized) Lava-flow aquifer (welded-tuff aquifer; tuff confining unit) Local aquifers in Pahute Mesa; minor confining unit at Yucca Mountain [AFFCR]
Topopah Spring Tuff  Variably welded ash-flow tuff (nonwelded tuff, locally zeolitized) Welded-tuff aquifer (tuff confining unit)  Major aquifer in western Pahute Mesa, Jackass Flats, southern Yucca Flat, CP Basin, and Frenchman Flat [OV, AFFCR, AM] 
Calico Hills Formation 
[Ta] 
12.9 Ma
Volcanics of Area 20 Rhyolite lava and nonwelded tuff, commonly zeolitized (welded tuff)  Lava-flow aquifer, tuff confining unit (welded-tuff aquifer) Major aquifer in central and western Pahute Mesa; major confining unit in eastern Pahute Mesa, Yucca Mountain, Jackass Flats, and Yucca Flat [AFFCR, OV?] 
Rhyolite of Inlet Rhyolite lava  Lava-flow aquifer  Major aquifer in eastern and central Pahute Mesa [AFFCR, OV?]
Wahmonie Formation 
[Tw] 
13.0 Ma 		 Andesite and dacite volcanic breccia (lava; nonwelded tuff)  Tuff confining unit (lava-flow aquifer or confining unit?) Confining unit west of Frenchman Flat and in Jackass Flats; contains perched water near Cane Springs and Pavits Spring; thin confining unit in Yucca Flat [AM, AFFCR] 
Crater Flat Group 
[Tc] 
13.25-13.1Ma
Prow Pass Tuff  Variably welded ash-flow tuff (nonwelded tuff, commonly zeolitized) Welded-tuff aquifer (tuff confining unit) Major aquifer at Yucca Mountain; confining unit where nonwelded in Yucca Flat and Rainier Mesa [AFFCR, AM] 
Middle units Rhyolite lava (latite lava; nonwelded tuff)  Lava-flow aquifer  Local aquifers in Pahute Mesa; confining units where nonwelded and zeolitized [AFFCR, AM] 
Bullfrog Tuff  Variably welded ash-flow tuff (nonwelded tuff, commonly zeolitized) Welded-tuff aquifer (tuff confining unit) Major aquifer at Yucca Mountain; major confining unit in Pahute Mesa [AFFCR, AM] 
Tram Tuff Variably welded ash-flow tuff Welded-tuff aquifer Major aquifer at Yucca Mountain [AFFCR, AM] 
Belted Range Group 
[Tb] 
13.85-13.6 Ma
Deadhorse Flat Formation  Rhyolite lava and variably welded ash-flow tuff (nonwelded tuff, commonly zeolitized)  Lava-flow aquifer; welded-tuff aquifer Major aquifers in eastern Pahute Mesa [AFFCR, OV?]
Grouse Canyon Tuff  Peralkaline welded ash-flow tuff (nonwelded tuff, commonly zeolitized) Welded-tuff aquifer (tuff confining unit)  Local aquifer in Yucca Flat and Pahute Mesa [AFFCR, AM, OV]
Tunnel Formation   Nonwelded tuff, commonly zeolitized  Tuff confining unit Major confining unit in Rainier Mesa and Yucca Flat; contains perched water at about 14 Ma Rainier Mesa [AFFCR, AM] 
Volcanics of Big Dome 
[Tu] 
about 15 Ma
Rhyolite of Quartet Dome  Peralkaline rhyolite lava Lava-flow aquifer  Local aquifer in eastern Pahute Mesa [AFFCR] 
Tub Spring Tuff  Variably welded ash-flow tuff  Welded-tuff aquifer  Local aquifer in northern Yucca Flat and in parts of Pahute Mesa and Rainier Mesa [AFFCR, AM] 
Older volcanics 
[To] 
16-15 Ma 
Tunnel bed 2 and Tunnel bed 1 Nonwelded tuff, commonly zeolitized Tuff confining unit  Confining unit in Rainier Mesa and Yucca Flat; contains perched water at Rainier Mesa [AFFCR, AM] 
Red Rock Valley Tuff, Fraction Tuff, and pre-fraction tuffs  Variably welded ash-flow tuff; nonwelded tuff, commonly zeolitized  Tuff confining unit (welded-tuff aquifer)  Major confining unit where nonwelded in southern Yucca Flat and Frenchman Flat; local aquifers in northern Yucca Flat, southern Rainier Mesa, and parts of Pahute Mesa. Unit locally includes ash- flow tuffs of older mid-Tertiary volcanic centers [AFFCR, AM] 
Pavits Spring Formation 
[Ts] 
Miocene		 Tuffaceous sandstone; local conglomerate and siltstone (ash-fall tuff; lacustrine limestone) n/c Significance unknown; mapped as an informal unit east, south, and west of Frenchman Flat; partly coeval with some other volcanic units [AM]
Horse Spring Formation 
[Ts] 
mid-Tertiary		 Conglomerate and tuffaceous sandstone (lacustrine limestone; ash-fall tuff) n/c Significance unknown; mapped as an informal unit south and west of Frenchman Flat along Rock Valley fault system [AM]
Paleocolluvium 
[Tx] 
Tertiary		 Sedimentary breccia n/c Locally very porous; thickness variable along basal contact of Tertiary section; only known from Yucca Flat and Rainier Mesa areas [AM, AFFCR] 
Granite 
[Kg] 
Cretaceous 		 Porphyritic monzogranite n/c Relatively impermeable; forms local cylindrical stocks north of Rainier Mesa and Yucca Flat; locally yields water from fracture zones [AFFCR, AM] 
Tippipah Limestone 
[PPT] 
Chiefly Pennsylvanian		 Thin-bedded limestone (pebble conglomerate) Upper carbonate-rock aquifer Local aquifer in western Yucca Flat; limited lateral extent [AM]
Eleana Formation 
[MDe] 
and Chainman Shale 
[Mch] 
Chiefly Mississippian		 Siliceous siltstone; chert sandstone; cobble conglomerate (quartzite; bioclastic limestone) Eleana confining unit Major confining unit along boundary between AFFCR and AM subbasins in western Yucca Flat and northern Jackass Flats; locally yields water from fracture zones in quartzite and limestone [AFFCR, AM]
Guilmette Formation 
[Dg] 
Devonian 		 Limestone, locally sandy and dolomitic Lower carbonate-rock aquifer Regional carbonate-rock aquifer; conveys most ground water in Yucca Flat-Frenchman Flat area and along Rock Valley fault system; significant aquifer in southern part of Yucca Mountain and south into AFFCR subbasin [AM, AFFCR]

Due to displacement on low-angle faults, carbonate rocks of these stratigraphic units occur locally out of stratigraphic order above Eleana Formation; in such situations, structural blocks of these units do not form regional carbonate-rock aquifer, but rather act similar to upper carbonate-rock aquifer
Simonson Dolomite 
[Ds] 
Devonian		 Dolomite, locally silty or cherty
Sevy, Laketown, and Lone Mountain Dolomite; 
Roberts Mountain Formation; 
dolomite of Spotted Range 
[various symbols] 
Chiefly Silurian and Devonian		 Dolomite, locally silty or cherty
Ely Springs Dolomite 
[Oes] 
Ordovician		 Dolomite, cherty dolomite
Eureka Quartzite 
[Oe] 
Ordovician		 Siliceous orthoquartzite
Pogonip Group 
[Op] 
Ordovician		 Limestone and silty limestone (dolomite)
Nopah Formation 
[ Cambrian		 Limestone and dolomite, locally cherty and silty; Dunderberg Shale at base
Bonanza King Formation 
[ Cambrian		 Dolomite and limestone (silty dolomite; chert)
Carrara Formation 
[c] 
Cambrian 		 [upper part] Limestone and silty limestone
Carrara Formation 
[c] 
Cambrian		 [lower part] Shale and siltstone (limestone and quartzite) Quartzite confining unit Widespread confining unit that forms hydrologic basement in southern Nevada; forms major barrier to lateral ground-water flow in northeastern Yucca Flat, south of Oasis Valley near Beatty, and south of Ash Meadows; locally yields substantial water from fractured quartzite or fault zones in northern Yucca Flat [OV, AFFCR, AM]
Zabriskie Quartzite 
[z] 
Cambrian 		 Siliceous orthoquartzite
Wood Canyon Formation 
[Zw] 
Cambrian and Eocambrian		 Micaceous siltstone and orthoquartzite (dolomite; pebbly quartzite)
Stirling Quartzite 
[Zs] 
Eocambrian		 Siliceous orthoquartzite and micaceous siltstone (quartzite conglomerate)
Johnnie Formation 
[Zj] 
Eocambrian 		 Siliceous orthoquartzite and micaceous siltstone (silty limestone)
Noonday (?) Dolomite 
[Zn?] 
Eocambrian 		 Dolomite n/c Known only from bottom of deep drillhole UE-15d in northern Yucca Flat; stratigraphic identify uncertain (Barnes, 1962) 

 
1 Map symbol shown in brackets. Approximate age given in millions of years ago (Ma) or indicated by geologic system.
2 Rock type listed in order of decreasing prevalence; minor but significant rock types listed in parentheses.
3 Hydrogeologic units in order of decreasing prevalence. Stratigraphic classification of Tertiary volcanic rocks is based on age and composition. Lithologic and hydrologic properties may be extremely variable from place to place. n/c, stratigraphic unit has not been characterized as a hydrogeologic unit, generally because its presence in the unsaturated zone is minor.
4 Code in brackets at end of entry gives ground-water subbasin or subbasin in which stratigraphic unit is prevalent. AFFCR, Alkali Flat-Furnace Creek Ranch; AM, Ash Meadows; and OV, Oasis Valley. Query indicates uncertainty regarding presence or significance of units in subbasin.


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