The Silent Canyon Caldera Complex—

A Three-Dimensional Model Based on DrillHole Stratigraphy and Gravity Inversion

by Edwin H. McKee1, Thomas G. Hildenbrand1, Megan L. Anderson1,

Peter D. Rowley2, and David A. Sawyer3

Open-File Report 99-555

Version 1.0

http://geopubs.wr.usgs.gov/open-file/of99-555/

Prepared in cooperation with the

NEVADA OPERATIONS OFFICE

U.S. DEPARTMENT OF ENERGY

(Interagency Agreement DE-AI08-96NV11967)

1999

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or

with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only

and does not imply endorsement by the U.S. Government.

Manuscript approved November 23, 1999

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

1Menlo Park, California

2Las Vegas, Nevada

3Denver, Colorado
CONTENTS

Abstract ..................................................................................................................... 1

Introduction ............................................................................................................... 2

Geologic Setting ........................................................................................................ 4

Caldera Concept ........................................................................................................ 8

Geologic and Geophysical Character of the SWNVF ............................................ 11

Silent Canyon Caldera Complex ............................................................................. 13

Volcanic Stratigraphy ............................................................................................. 17

Hydrostratigraphy ................................................................................................... 20

Timber Mountain composite unit ................................................................ 21

Topopah Spring tuff aquifer ........................................................................ 22

Calico Hills composite unit ......................................................................... 22

Crater Flat composite unit ........................................................................... 31

Bullfrog Tuff confining unit ....................................................................... 31

Belted Range aquifer ................................................................................... 40

Pre-Belted Range composite unit................................................................. 40

Basement ..................................................................................................... 49

Structure................................................................................................................... 49

Basement Surface .................................................................................................... 56

Density-depth functions............................................................................... 58

Basin geometry ............................................................................................ 62

Three-Dimensional Model ...................................................................................... 63

Acknowledgements.................................................................................................. 73

References ............................................................................................................... 73

List of Tables

Table 1. Major stratigraphic units in the Silent Canyon complex .......................... 19

Table 2. Density-depth functions used to determine thickness of basin-filling

deposits and calculated depths near drill hole Ue20f............................................... 60

List of Figures

Figure 1. Map of the western part of the Nevada Test Site and areas to the west.

Figure 2. Diagram showing part of the three-dimensional surface of the Silent Canyon

caldera fault created in “EarthVision.”
Figure 3. Diagram of collapse caldera models showing some variations observed in the

field.

Figure 4. Diagrams showing isostatic residual gravity anomaly of the central part of the

SW Nevada volcanic field (SWNVF) and 3D view of basement surface beneath the

central part of the SWNVF.

Figure 5. Diagram showing the development of the Silent Canyon caldera complex.

Figure 6a. Distribution and thickness map of the Topopah Spring tuff aquifer.

Figure 6b. Three-dimensional view of the Topopah Spring tuff aquifer looking northeast.

Figure 6c. Three-dimensional view of the Topopah Spring tuff aquifer looking east.

Figure 6d. Three-dimensional view of the Topopah Spring tuff aquifer looking north.

Figure 7a. Distribution and thickness map of the Calico Hills composite unit.

Figure 7b. Three-dimensional view of the Calico Hills composite unit looking northeast.

Figure 7c. Three-dimensional view of the Calico Hills composite unit looking east.

Figure 7d. Three-dimensional view of the Calico Hills composite unit north.

Figure 8a. Distribution and thickness map of the Crater Flat composite unit.

Figure 8b. Three-dimensional view of the Crater Flat composite unit looking northeast.

Figure 8c. Three-dimensional view of the Crater Flat composite unit looking east.

Figure 8d. Three-dimensional view of the Crater Flat composite unit looking north.

Figure 9a. Distribution and thickness map of the BullfrogTuff confining unit.

Figure 9b. Three-dimensional view of the BullfrogTuff confining unit looking northeast.

Figure 9c. Three-dimensional view of the BullfrogTuff confining unit looking east.

Figure 9d. Three-dimensional view of the BullfrogTuff confining unit looking north.

Figure 10a. Distribution and thickness map of the Belted Range aquifer.

Figure 10b. Three-dimensional view of the Belted Range aquifer looking northeast.

Figure 10c. Three-dimensional view of the Belted Range aquifer looking east.

Figure 10d. Three-dimensional view of the Belted Range aquifer looking north.

Figure 11a. Distribution and thickness map of the Pre-Belted Range composite unit.

Figure 11b. Three-dimensional view of the Pre-Belted Range composite unit looking

northeast.
Figure 11c. Three-dimensional view of the Pre-Belted Range composite unit looking east.

Figure 11d. Three-dimensional view of the Pre-Belted Range composite unit looking

north.

Figure 12a. Diagram showing the Silent Canyon caldera complex in plan view.

Figure 12b. Three-dimensional view of Silent Canyon ring fault looking north.

Figure 12c. Three-dimensional view of Silent Canyon ring fault looking northeast.

Figure 13. Three-dimensional diagrams showing two views of the Timber Mountain

caldera margin cutting off the Silent Canyon caldera margin.

Figure 14. Map showing the major faults mapped at the surface of Pahute Mesa.

Figure 15. Diagrams showing two views of the East Boxcar, West Greeley, and

Almendro faults within the Silent Canyon caldera complex.

Figure 16. Diagram showing basement surface beneath the region of the Silent Canyon

caldera complex based on the inversion of gravity data.

Figure 17. Three-dimensional diagrams showing two views of the Silent Canyon caldera

complex with the upper 2000 feet of rock removed down to water-table level.

Figure 18. Cross sections across the buried Silent Canyon caldera complex drawn from

the “EarthVision” three-dimensional model.

Figure 19. Three-dimensional diagram of Pahute Mesa sliced along A – A’ shown in

figure 18a.

Figure 20. Three-dimensional diagram of Pahute Mesa sliced along B – B’ shown in

figure 18a.

Figure 21. Three-dimensional diagram of Pahute Mesa sliced along C – C’ shown in

figure 18a.
1

The Silent Canyon Caldera Complex—A Three-dimensional Model

Based On Drill-Hole Stratigraphy And Gravity Inversion

E.H. McKee, T.G. Hildenbrand, M.L. Anderson,

 P.D. Rowley and David A. Sawyer

U.S. Geological Survey, Menlo Park, Calif.,

 Las Vegas, Nev., and Denver, Colo.

ABSTRACT

 The structural framework of Pahute Mesa, Nevada, is dominated by the

Silent Canyon caldera complex, a buried, multiple collapse caldera complex.

Using the boundary surface between low density Tertiary volcanogenic rocks

and denser granitic and weakly metamorphosed sedimentary rocks

(basement) as the outer fault surfaces for the modeled collapse caldera

complex, it is postulated that the caldera complex collapsed on steeplydipping arcuate faults two, possibly three, times following eruption of at least

two major ash-flow tuffs. The caldera and most of its eruptive products are

now deeply buried below the surface of Pahute Mesa. Relatively low-density

rocks in the caldera complex produce one of the largest gravity lows in the

western conterminous United States. Gravity modeling defines a steep sided,

cup-shaped depression as much as 6,000 meters (19,800 feet) deep that is

surrounded and floored by denser rocks. The steeply dipping surface located

between the low-density basin fill and the higher density external rocks is

considered to be the surface of the ring faults of the multiple calderas.

Extrapolation of this surface upward to the outer, or topographic rim, of the

Silent Canyon caldera complex defines the upper part of the caldera collapse

structure. Rock units within and outside the Silent Canyon caldera complex

are combined into seven hydrostratigraphic units based on their predominant

hydrologic characteristics. The caldera structures and other faults on Pahute

Mesa are used with the seven hydrostratigraphic units to make a three-
2

dimensional geologic model of Pahute Mesa using the "EarthVision"

(Dynamic Graphics, Inc.) modeling computer program. This method allows

graphic representation of the geometry of the rocks and produces computer

generated cross sections, isopach maps, and three-dimensional oriented

diagrams. These products have been created to aid in visualizing and

modeling the ground-water flow system beneath Pahute Mesa.

INTRODUCTION

 The testing of underground nuclear devices at the Nevada Test Site (NTS)

on Pahute Mesa (fig. 1) between 1966 and 1992 introduced radionuclides into

ground-water that is flowing southwesterly beneath Pahute Mesa

(Blankennagel and Weir, 1973; Winograd and Thordarson, 1975; Laczniak

and others, 1996). The present water table of this ground-water system is

about 600 meters (2,000 feet) below the surface of Pahute Mesa and is in the

upper and middle part of a thick sequence of volcanic rocks that are the

eruptive products of extinct calderas beneath, as well as outside of, Pahute

Mesa. The calderas underlying Pahute Mesa on the NTS are part of what is

called the Silent Canyon caldera complex (SCCC). All parts of the complex

are buried beneath younger ash-flows that erupted from younger calderas

outside the SCCC.

 The objective of this investigation is to develop a three-dimensional

geological and hydrogeological model of the SCCC that may be used for

ground-water flow studies of Pahute Mesa in the northwestern part of the

Nevada Test Site (NTS). The conceptual view of the buried SCCC is based,

in general, on the arcuate collapse fault systems of mapped calderas, both

near the SCCC and in other parts of the world. Models for such calderas

derive from geologic mapping and concepts developed by Williams (1941),
95

95

TOLICHA

PEAK BLACK MTN

CALDERA

SILENT CANYON

CALDERA COMPLEX

YUCCA FLAT

BEATTY

WASH

SHOSHONE

MOUNTAIN

CALICO

HILLS

JACKASS

FLATS

NEVADA TEST SITE BOUNDARY

CLAIM CANYON

CALDERA

Wahmonie

volcano

Mercury

BULLFROG

HILLS

TIMBER MOUNTAIN

CALDERA COMPLEX

TIMBER MTN

OASIS

VALLEY

Beatty YUCCA

MTN

BARE

MTN

CRATER

FLAT

PAHUTE MESA

RAINIER

MESA

37°15'N

37°00'

36°45'

caldera topographic

walls

caldera structural

margin or inferred

caldera margin

0

0

15 km

10 ml

Figure 1. Map of the western part of the Nevada Test Site (NTS) and areas to the west. S

 represents mostly Miocene volcanic rocks of the southwestern Nevada volcanic field (SW

The three- dimensional model developed here includes all of the Silent Canyon caldera c
4

Smith and Bailey (1968), and numerous other geologists (see the section on

the caldera model). Geologic maps of Pahute Mesa give some surface control

on the geology, but because the SCCC is buried by volcanic rocks from

nearby younger calderas, our geologic knowledge of the subsurface is based

mostly on evaluation of drill hole and geophysical information. The geologic

model presented here is uses two different types of data. The caldera structure

is based on the basement surface beneath Pahute Mesa as defined by gravity

and projected upward to the caldera complexe’s topographic rim (fig. 2). The

stratigraphy is based on about 52 drill sites scattered across the mesa and on

geologic maps. Drill hole analysis of the petrology and petrochemistry of

samples based on a compilation and rigorous review of data from numerous

scientists over the past 30 years compiled by Warren and others (1999; see

references in Warren and others, 1999 for individual contributions) was used

to determine the stratigraphy in the wells.

GEOLOGIC SETTING

 The rocks that crop out on and near Pahute Mesa are mostly middle

Miocene silicic volcanic rocks that are part of the southwestern Nevada

volcanic field (SWNVF, fig. 1). Most of these rocks are the products of

volcanoes that formed collapse calderas during and after eruption of large

volume ash-flow sheets. At least six major calderas have been identified in the

SWNVF (Sawyer and others, 1994; Wahl and others, 1997). The certainty of

their recognition is inversely related to their relative age because the older

ones were progressively collapsed into and buried by the younger calderas

and related intrusions. Like calderas elsewhere in the world these calderas are

the sites of thick lava accumulations, and breccias, both tectonic and volcanic,

are commonly associated with such calderas. Most of the volcanic rocks are

pyroclastic types (ash-flow and air-fall tuffs) that are made up of variable
AB

AB

Figure 2. Part of the three-dimensional surface 

of the Silent Canyon caldera fault created in 

"EarthVision". Hatured area is the fault surface 

calculated from gravity. Solid lines are 

boundaries of the clipped polygon used to limit 

the upward and downward continuation of the 

fault surface. (A) The upper boundary is 

defined by the topographic rim of the SCCC 

taken from a geologic map (Wahl and others, 

1997) and clipped at the elevation at which the 

Silent Canyon caldera fault dies out before 

reaching the land surface. (B) The lower 

boundary is the elevation at which the gridded 

fault surface (the surface between low and high 

density rocks defined by gravity inversion) cuts 

downward into the basement. The colored 

portion of the fault between the polygon 

boundaries is used in the three-dimensional 

model.
6

amounts of ash, pumice, mineral crystals, and xenolithic debris. They range

from welded to nonwelded, poorly to tightly compacted, and unaltered to

completely altered to a variety of secondary minerals. The lava flows form

lenticular bodies with complex interfingering relationships. The ash-flow tuffs

develop a sheet or layered aspect outside the calderas but are often massive

where they accumulate to greater thickness inside the caldera.

 The volcanic rocks known to underlie Pahute Mesa span nearly the entire

period of volcanic activity of the SWNVF. Units that crop out at the surface

are ash-flow sheets from the Timber Mountain caldera to the south or the

Black Mountain caldera to the west (fig. 1). The youngest rocks are about 9.4

Ma (Sawyer and others, 1994). Rocks as old as about 15.0 Ma are

encountered in the deepest drill holes about two-thirds of the way through the

volcanic sequence based on the thickness of low density rocks determined by

gravity inversion calculations. Rocks beneath the drill holes, lower in the

sequence, include rocks as old as the oldest volcanic rocks in the SWNVF,

about 15.5 Ma and they are most likely different units in different parts of the

caldera complex. Units in the SCCC whose eruption caused caldera collapse

include the 13.7 Ma Grouse Canyon Tuff and the 13.3 Ma Bullfrog Tuff

(Sawyer and others, 1994). Some rocks older than the Grouse Canyon Tuff

in the volcanic pile may also have erupted from the Silent Canyon complex

and may have caused calderas or partial calderas that form the deepest parts

of the structural basin beneath Pahute Mesa. The basement beneath the

Miocene volcanic rocks is inferred to be mostly granitic rock and represents

the upper part of the magma chamber within a batholithic complex from

which the volcanic units of the SWNVF originated. Precambrian and

Paleozoic siliceous and carbonate strata may be included as wallrock, inliers,

stoped blocks, and xenoliths in this granitic basement.
7

 All the volcanic rocks, particularly the ash-flow sheets, are types typically

associated with collapse calderas. The basement configuration delineated by the

gravity data is consistant with this caldera tectonic model (Healey, 1968; Healey

and others, 1987; Hildenbrand and others, 1999). Although the Silent Canyon

caldera complex has been recognized for several decades, the complexity of

this large multi-eruptive volcanic area has led to a variety of geologic scenarios.

These include: (1) a single, large, circular, collapse caldera system as shown in

figure 1 (Noble and others, 1968; Orkild and others, 1969), (2) a nested,

double-arcuate, caldera complex (Sawyer and Sargent, 1989; Minor and others,

1993; Sawyer and others, 1994) and (3) a group of rectilinear, fault-block

basins formed by caldera collapse on preexisting linear, high-angle, normal

faults (Ferguson and others, 1994). We adopt a model by which a large

ellipsoid complex caldera system developed around a series of proximate,

arcuate, presumed partial to whole calderas. The collapse features formed

during and after eruption of some of the many voluminous ash-flow units

underlying the area. Younger, high-angle faults displace rocks locally within the

caldera complex. This model has aspects of all the previous models. Like Noble

and others (1968) and Orkild and others (1969) most of the caldera collapse is

essentially around a large circular fault zone, and like Sawyer and Sargent

(1989) and Minor and others (1993) our model envisions several major periods

of collapse and caldera formation. It is similar to the Ferguson and others

(1994) model in that it is characterized by an exceptionally deep tectonicallyformed basin (>5 km). The geologic complexities shown in the Ferguson and

others (1994) model (e.g., numerous local fault-block grabens), however, do not

appear in our model, because the geophysical data are insufficient to detect the

high-angle fault block basins. Nor do the geologic data from drill holes in the

upper 900 meters (3,000 feet) provide solid evidence for of deeper rectilinear
8

basins. Our conceptually simple model is supported by the geological and

geophysical data and provides a framework for future additional data defining

caldera tectonics.

CALDERA CONCEPT

 Ash-flow tuffs, among the most voluminous of volcanic products, result from

explosive eruptions that form widespread units emplaced in mere instants of

geologic time. These eruptions cause virtually instantaneous evacuation of their

source-underlying shallow magma chambers, causing the roof over the magma

chamber to collapse. The collapse feature has been called a caldera, and this

explanation became widely recognized following the landmark reviews of caldera

formation by Williams (1941), ash-flow processes by Smith (1960), and resurgent

calderas by Smith and Bailey (1968). Detailed mapping of many areas, especially

the most widespread volcanic terranes of the U.S. such as the Great Basin, Snake

River Plain, Cascades, San Juan Mountains, Rio Grande rift, as well as the Andes

of Peru, disclose scores of calderas. Calderas have been found in nearly all other

volcanic regions throughout the world where pyroclastic rocks occur. In

particular, the study of the many larger and complex calderas in the San Juan

Mountains of Colorado (Lipman, 1976 a, b, 1984; 1997; and Steven and

Lipman, 1976) clarify details of caldera evolution. Work on the well-exposed

Black Mountain caldera and the Timber Mountain caldera complex near Pahute

Mesa by Christiansen and Noble (1965), Christiansen and others (1977), Byers

and others (1968; 1976 a, b), Smith and Bailey (1968), Noble and Christiansen

(1974), and Kane and others (1981) has revealed much about caldera genesis

that is directly applicable to the Silent Canyon caldera complex.

 Much of what is known about calderas is reviewed by Lipman (1997). He

notes that although many variations exist, most well- exposed calderas are

broadly similar to each other (fig. 3). Most calderas form above shallow
PISTON

Single, large volume eruption

TRAP-DOOR

asymmetrical pluton

DOWNSAG

small volume, deep pluton

small, deep pluton

PIECEMEAL

multicyclic

(a)

(b)

(c)

(d)

(e)

Figure 3. Collapse caldera models showing some of the variations that have been observed in 

the field. The Silent Canyon caldera complex is probably a combination of all of these variations. 

Figure is from Lipman, 1997, figure 2.
10

cupolas that rise (intrude) above the tops of a batholithic complex. When the

cupola intrudes near the surface, a violent eruption occurs. Because most

cupolas are circular in plan, most calderas are also circular. This original shape

for roof failure is defined by arcuate ring faults that dip steeply into the

underlying cupola. These ring faults define what is called the structural margin

of the caldera. When a ring fault extends to the earth's surface, rocks inside

the ring fault subside to create a basin. The basin floor generally subsides as a

single piston-like plate, although piecemeal subsidence of parts of the floor has

been noted in a number of calderas (fig. 3b). The scarp that results from

collapse of the caldera floor has a slope above the angle of repose; therefore

breccia formed by landslide debris, avalanches and rockfalls break away from

the scarp, all eroding the structural margin until the scarp reaches its angle of

repose and attains stability. The retreat of the caldera edge as a result of these

processes makes the caldera larger. The outer edge of this zone of mass

wasting is called the topographic margin of the caldera. Products of the

landslides are spread into the interior of the caldera, where they intertongue

with the products of continued volcanic eruption. The sum total of the

deposits, known as intracaldera deposits, consists mostly of ash-flow tuff and

interlayered collapse breccia. Continued eruption results in further subsidence

of the caldera floor. Intracaldera deposits also include generally post-collapse

subordinate volumes of lava flows that erupt from vents and small domes

commonly located along the ring fracture faults. In the waning stages of

caldera eruption, magma from the original magma chamber commonly rises

and intrudes the intracaldera fill and may uplift by resurgence the caldera fill

along the ring faults thereby destroying the caldera floor and resulting in a

structural dome.
11

 Variations in caldera geometries summarized by Lipman (1997) and shown

here in figure 3 include the trap door caldera, in which only one side subsides

along a partial, arc-shaped, ring fault (fig. 3c), a downsag caldera, in which the

magma chamber is so deep that no through-going fault develops (fig. 3d), and

a small funnel-shaped caldera, in which the floor was so disrupted that the

caldera is more similar to a breccia pipe and was more likely excavated

explosively rather than formed by subsidence (fig. 3e).

GEOLOGIC AND GEOPHYSICAL CHARACTER OF THE SWNVF 

 Starting in the 1950's, the Nevada Test Site (NTS) and adjacent areas of

southern Nevada were geologically mapped at 1:24,000 and 1:48,000 scale.

These maps were the basis for the adoption of the concept of calderas as the

source for most of the volcanic rocks in the region -- rocks that comprise the

SWNVF. Later updated summary maps and reports endorsed and expanded

on the caldera model in the SWNVF although the location of some segments

of the caldera margins and the interpretations about other aspects of the

geology were modified as new information was added with each new

generation of maps. The map pattern of calderas on the recent summary

geologic map of the NTS (Wahl and others, 1997) evolved from maps of

Minor and others (1993), Sawyer and others (1995), Carr and others (1996),

and the report of Sawyer and others (1994). The most recent geologic map of

the NTS (Slate and others, 1999) does not change the caldera model.

 Geophysical mapping has been a major component of the geologic

investigation of the NTS. The early geophysical studies (e.g., Healey, 1968;

Kane and others, 1981) interpreted a caldera origin for the gravity anomaly

beneath Pahute Mesa. Kane and others (1981) noted that the geophysical data

near Timber Mountain suggested that some deformation caused by volcanic

events has been accommodated along straight-line faults that are aligned in
12

such a fashion as to give a curvilinear appearance to the regional structure.

He suggested that some of the curvilinear geometry was caused by erosion

and landsliding of caldera walls. Recent geophysical investigations (Ferguson

and others, 1994; Grauch and others, 1997; McCafferty and Grauch, 1997;

Hildenbrand and others, 1999; Mankinen and others, 1999; and Schenkel and

others, 1999), which synthesize much new data use more refined geophysical

modeling methods of gathering and reducing data, likewise support a caldera

model for the structure beneath Pahute Mesa. Where caldera outlines are well

expressed by geologic parameters, gravity maps usually reveal the structural

margins of the caldera as the boundary between lower density caldera fill and

higher density basement or country rock. Where calderas are buried and

geologic mapping cannot define them, geophysical data provide a means to

define calderas.
13

SILENT CANYON CALDERA COMPLEX

 Pahute Mesa is underlain by a circular, cup-shaped gravity anomaly that is

one of the most prominent gravity lows in the western conterminous United

States. The SCCC was first recognized on the basis of this large negative

gravity depression (Healey, 1968). This deep, circular basin defined by the

inversion of gravity data (Hildenbrand and others, 1999 and fig. 4) is

interpreted to be a volcanic basin filled with low density, locally- derived lavas

and tuffs and covered by younger welded ash-flow tuffs. The Silent Canyon

caldera, named after Silent Canyon in the north-central part of Pahute Mesa,

was applied to this buried volcano-tectonic feature by Healey (1968), Noble

and others (1968) and Orkild and others (1968, 1969). These geologists

recognized that some of the regional ash-flow sheets originated at Pahute

Mesa and they postulated that these ash-flow eruptions caused subsidence that

created the depression. The main collapse of the Silent Canyon caldera was

attributed to eruption of the Tub Spring and Grouse Canyon Members of the

Belted Range Tuff (Noble and others, 1968). The Tub Spring Tuff is now

separated from the Belted Range Group and its source is uncertain. These

tuffs are encountered at depths of 1,200 m (4,000 ft) or more below the

surface in drill holes on Pahute Mesa. A second collapse of the SCCC, named

the Area 20 caldera, was equated with eruption of the Bullfrog Tuff (Sawyer

and Sargent, 1989). This caldera is shown diagrammatically in maps by

Sawyer and others (1994, 1995), Ferguson and others (1994) and Wahl and

others (1997). We, however, do not model the Area 20 caldera because the

gravity surface beneath the SCCC defines a ridge across the trace of the

projected down-dropped Area 20 caldera fault indicating that a caldera

subsidence related to the Area 20 caldera displacing the basement cannot be

distinguished in this area. Both caldera forming tuffs, the Grouse Canyon and
(b) Basement

 surface

Distance, km

N

E

20 30 40 50 60

0

-5

Depth, km 

BMC

TMCC

SCCC

-64

-56

-48

-40

-32

-24

-16

 -8

mGal

116¡40« 116¡30« 116¡20«

37¡

37¡10«

37¡20«

TMCC

BMC

SCCC

Nevada Test Site

0 10 20 KM

Figure 4. (a) Isostatic residual gravity anomaly of the central part of the SW Neva

volcanic field (SWNVF) from Hildenbrand and others (1999). Anomalies express 

first order the average density of the middle and upper crust. Small crosses repre

locations of gravity stations. Letters denote: TMCC—Timber Mountain caldera 

complex, SCCC—Silent Canyon caldera complex, and the BMC—Black Mountain 

(b) 3D view of the basement surface beneath the central part of the SNVF (from 

Hildenbrand and others, 1999). Note the deep ovoid basin beneath the Silent Can

caldera complex (highlighted by the white arrow). No vertical exaggeration. 
15

the Bullfrog, are thicker within the SCCC than in regions surrounding the

complex, have substantial amounts of related post-caldera lavas, and both are

comprised of facies that reflect near-source, intracaldera accumulation. In

addition, roughly the upper one-third of the SCCC is comprised of younger,

regional ash-flow sheets whose sources are from the south or west of Pahute

Mesa. These sheets become progressively thinner and constant in thickness as

they successively fill, and ultimately blanket, the depression produced by the

earlier caldera collapses. The combined maximum thickness of all the units,

including the caldera-forming Grouse Canyon and Bullfrog tuffs, related lavas

and overlying basin-filling tuffs, however, accounts for only about one-half of

the total volcanic fill in many parts the volcanic depression (see figs.19, 20,

and 21). Below the Grouse Canyon Tuff there is estimated to be as much as

3,000 m (9,900 ft) of rock in the deepest parts of the depression. Because

only the upper part of this basal 3,000 m (9,900 ft) of rock has been

penetrated by drilling, little is known about it except from regional gravity

measurements. Gravity analysis indicates that the basin fill is less dense than

the basement. It is most likely silicic volcanic rock similar to the known lavas

and tuffs in the upper part of the depression and in surrounding regions. The

fact that these presumed volcanic rocks fill a circular deep, depression in the

lower part of the SCCC suggests that they are also caldera-related eruptive

rocks. We envision, therefore, that there was significant caldera collapse prior

to those reasonably established Grouse Canyon caldera-forming eruptions that

occur about midway in the stratigraphic sequence. The only evidence of this

old caldera system is the deep circular basin defined by inversion of the

gravity data (Healey, 1968; Ferguson and others, 1994; Hildenbrand and

others, 1999) which makes up the deepest part of the regional gravity low. It

is deeply buried and, except for its shape, mostly obscured by later collapse
16

calderas. It is analogous in many respects to the large tuff-filled hole or

depression in pre-Tertiary rocks beneath Crater Flat about 20 miles south of

Pahute Mesa (Snyder and Carr, 1984). This feature, postulated to be a caldera

complex by Snyder and Carr (1984) was outlined by gravity inversion

modeling in the same manner as the SCCC is modeled here.

In summary, the SCCC is a multiple collapse system that developed by

a combination of piston-like, trap door, and downsag calderas (fig. 3) formed

repeatedly during eruption of large volume ash-flow tuffs from beneath what

is now the central part of Pahute Mesa. Collapse was essentially along various

parts of this ring fracture system. The caldera forming phases of this volcanic

center lasted a little more than 2 million years, from at least 15.5 to 13.3 Ma.

The remnants of these caldera basins filled with volcanic rocks during the

following 4 million years from volcanoes south and west of the SCCC. The

oldest of these caldera-burying tuffs show considerable thickening at Pahute

Mesa, but successively younger tuffs are more uniform in thickness (sheetlike), reflecting the gradual filling of the depression. As much as 6,000 m

(19,800 ft) of low density volcanic or sedimentary rocks fill the SCCC at

some places. Beneath the two major caldera-forming eruptive tuffs (Grouse

Canyon and Bullfrog Tuffs), the sequence probably contains at least one

caldera forming tuff and a variable thickness of Miocene lava flows and

intrusive rocks that were the first volcanic rocks of the SWNVF. These

volcanic rocks were deposited on weakly metamorphosed Precambrian and

Paleozoic sandstone, shale, and limestone intruded by a granitic batholithic

complex that was the magma source of the subsequent volcanic eruptions.

Development of the SCCC is shown schematically in figure 5.
17

VOLCANIC STRATIGRAPHY

 As much as 6,000 m (19,800 ft) of Tertiary rocks underlie Pahute Mesa in

the SCCC based on gravity inversion data. The deepest drill hole on Pahute

Mesa (UE 20f, fig.18) penetrates 4,147 m (13,686 ft) of Tertiary rock in a

location above the basement near the western margin of the caldera complex.

Other deep drill holes bottom in volcanic rocks at other places in the caldera;

most are about one-half way down in the sequence of moderately low-density

rocks that fill the depression. The rocks encountered in the more than 85 drillholes on Pahute Mesa are mostly silicic volcanic types including ash-flow and

air-fall tuffs, lava flows, and other types of pyroclastic rocks. This sequence

contains units named and mapped in areas surrounding Pahute Mesa, and

serves as a guide to the stratigraphy of the buried section at Pahute Mesa.

The ash-flow tuffs are especially useful in identifying the units of the SCCC

because each is unique in its phenocryst content and whole rock and mineral

chemistry. Relative thickness, degree of welding, number and type of

xenolithic inclusions, and amount of alteration are an indication of proximity

to the source but are much less important in correlating units.

 The names of the rock units used here (Table 1) follow the terminology of

Sawyer and others (1994), Warren and others (1999), and Drellack and

Prothro (1997) and the hydrostratigraphic units are a modification of the

classification devised by Drellack and Prothro, (1997) and the hydrogeologic

units outlined by Warren (1994). The unit identification from the drill holes is

based on petrology and rock chemistry compiled by Warren and others

(1999). The units vary significantly in thickness and geometry in this volcanocaldera setting. The lava flows tend to be tabular, steep sided bodies that have

complex interfingering relationships. The pyroclastic rocks, which reflect

more closely the surface on which they were deposited, show pronounced
c

a

d

g

e f

b

a

d

c

b

a

a

b

I

II

III

IV

V

Figure 5. Development of the Silent Canyon caldera complex. Hydrostratigraphic units outlined omn table 1. The sequence of events, from bottom to top,

 includes three caldera forming ash-flow eruptions (a, b, c) and four caldera filling and overlapping units (d, e, f, g). (a) Pre- Belted Range composite unit, 

 (b) Belted Range aquifer, (c) Bullfrog Tuff confining unit,(d) Crater Flat composite unit, (e) Calico Hills composite unit, (f) Topopah Spring tuff aquifer,

 (g) Timber Mountain aquifer.
19

thickening in depressions and are laterally more continuous. The major ashflow tuffs outlined in table 1 have a layered, sheet-like aspect in many

localities.

Table 1. Summary of major stratigraphic units in the Silent Canyon complex, volcanic source and

hydrostratigraphic unit. The classification is based on data from Warren and others (1999) and is a

modification of the classification of Drellack and Prothro (1997).

 Name Volcanic source Hydrostratigraphic unit

 --------------------------------------

 Thirsty Canyon Group Black Mountain caldera

 Gold Flat Tuff

 Trail Ridge Tuff

 Pahute Mesa Tuff

 Rocket Wash Tuff

 volcanics of Fortymile Canyon

 Beatty Wash Formation

Timber Mountain Group Timber Mountain caldera complex

 Ammonia Tanks Tuff Timber Mountain

 Rainier Mesa Tuff composite unit

 rhyolite of Fluorspar Canyon (mostly above water table)

 tuff of Holmes Road

 rhyolite of Windy Wash

Paintbrush Group

 rhyolite of Benham

 rhyolite of Scrugham Peak

 Tiva Canyon Tuff Claim Canyon caldera Tiva Canyon Tuff is locally saturated

 rhyolite of Delirium Canyon

 rhyolite of Echo Peak

 rhyolite of Silent Canyon -----------------------------------------

 Topopah Spring Tuff Claim Canyon caldera ? Topopah Spring tuff aquifer

 -----------------------------------------

Calico Hills Formation Calico Hills composite unit

 -----------------------------------------

Crater Flat Group Silent Canyon caldera complex ? (filling)

 tuff of Pool

 rhyolite of Inlet

 Prow Pass Tuff Crater Flat composite unit

 tuff of Jorum

 rhyolite of Sled

 rhyolite of Kearsarge

 andesite of Grimy Gulch ---------------------------------------

 Bullfrog Tuff Silent Canyon caldera complex Bullfrog Tuff confining unit

 ---------------------------------------

Tram Tuff

Belted Range Group

 Dead Horse Flat Formation Silent Canyon caldera complex (filling)

 Belted Range aquifer

 Grouse Canyon Tuff Silent Canyon caldera complex

 ---------------------------------------
20

 comendite of Split Ridge

 comendite of Quartet Dome

 Tram Ridge Group Pre Belted Range composite unit

 Lithic Ridge Tuff

 rhyolite of Picture Rock

 Tunnel Formation

volcanics of Quartz Mountain

 rhyolite of Quartz Mountain

 tuff of Tolicha Peak

 dacite of Mount Helen

 volcanics of Big Dome

 Tub Spring Tuff Silent Canyon caldera complex ?

 volcanics of Oak Spring Butte Pre-Belted Range composite unit

 tuff of Yucca Flat

 Redrock Valley Tuff Silent Canyon caldera complex ?

 Tuff of Twin Peaks

 tuff of Argillite Wash

 older Tertiary volcanic rocks

HYDROSTRATIGRAPHY

 The hydrogeology of Pahute Mesa was summarized by Blankennagel and

Weir (1973). Later evaluation of the ground-water flow in a much larger

region that includes Pahute Mesa was made by Winograd and Thordarson

(1975), Borg and others (1976) and Laczniak and others (1996). These

studies show the general direction and depth of the ground-water flow and

indicate the principal factors that influence it. In order to understand the

details of the flow, it is especially important to understand the structures of

the SCCC, not only as they relate to the thickness and lithologic character of

the stratigraphic units, but as permeable or impermeable channels.

 The rocks in and near Pahute Mesa are classified hydrologically on the

basis of their lithologic properties. These properties include the amount of

compaction and welding, amount of alteration, and the type of secondary

mineralization. Compaction and welding influence the amount of fracturing

which, in turn, have a strong control on permeability. A comprehensive

evaluation of the hydrogeologic characteristics of the volcanic units in the

SCCC and a hydrostratigraphic classification of these rocks is in Laczniak and

others (1996) and Drellack and Prothro (1997). For this study, the 41

stratigraphic units (Table 1) are combined or split according to their
21

hydrologic properties into three categories-- aquifers, confining units, and

composite units. The composite unit, a name first used by Drellack and

Prothro, (1997), contains both aquifers and confining units. The composite

units defined here are similar but are not the same as those used by Drellack

and Prothro, (1997). In this investigation some units are classified as

composite units because they change from aquifer to confining unit by

compaction, welding, alteration or other hydrologic altering process across

relatively short distances.

 Table 1 lists the seven hydrostratigraphic units used in the model developed

here and the stratigraphic units from which they are derived. These groups are

comprised mostly of rocks with similar hydrologic characteristics and are

typically a certain lithologic type such as welded tuff or altered non-welded tuff.

The thickness and distribution of a rock unit was also a factor in developing the

hydrostratigraphic groups. Thin or local units are difficult to show at the scale

of 1:48,000 used for this three-dimensional model. At this scale, in an area the

size of Pahute Mesa, stratigraphic units of about 400 ft (120 m) thick are the

minimum that can be shown in cross section (less then 0.1 in or about 2 mm

thick). Because the hydrostratigraphic units in this study and the

hydrostratigraphic units developed for western and central Pahute Mesa by

Drellack and Prothro (1997) are both based on drill-hole petrology of Warren

and others (1999) there is a strong similarity between the two

hydrostratigraphic classifications. The Drellack and Prothro (1997) model

recognizes 21 hydrostratigraphic units, we have combined many of these into

seven units.

Timber Mountain composite unit Because most of the Timber Mountain

composite unit lies above the water table at Pahute Mesa, only the lower part

is hydrologically significant in this area. The upper part, which includes tuffs

of the Thirsty Canyon Group and the Ammonia Tanks Tuff of the Timber

Mountain Group, is everywhere above the water table at Pahute Mesa. These

welded tuffs are readily fractured and have the hydrologic properties of an

aquifer. The Rainier Mesa Tuff underlies the Ammonia Tanks Tuff and, in the
22

southwest part of Pahute Mesa (south-central part of Area 20), is below the

water table and is an aquifer. Beneath the Rainier Mesa Tuff, and locally

beneath the water table, is a sequence of relatively thin lava flows and

nonwelded tuff. This sequence includes the rhyolite of Flourspar Canyon, the

tuff of Holmes Road, the rhyolite of Windy Wash, the rhyolite of Benham, the

rhyolite of Scrugham Peak, the Tiva Canyon Tuff (a significant aquifer in the

southwestern part of Pahute Mesa), the rhyolite of Delirium Canyon, the

rhyolite of Echo Peak, and the rhyolite of Silent Canyon (Table 1). Where

these units lie below the water table, the lava flows and welded tuffs are

aquifers, whereas the nonwelded tuffs and areas of alteration and secondary

mineralization in the lavas and welded tuffs are mostly confining units.

Because neither aquifer nor confining unit is the dominant hydrologic type,

this interval of rocks is classified as a composite unit and is part of the Timber

Mountain composite unit.

Topopah Spring Tuff aquifer

 The Topopah Spring Tuff, mostly a thin unit in the Pahute Mesa area,

thickens in the southwestern part of the SCCC (fig. 6) where it lies mostly

below the water table. It is a fractured, moderately welded tuff that acts as an

aquifer.

Calico Hills composite unit

 The Calico Hills Formation is a caldera-filling group of lava flows and tuffs.

It is found across most of the SCCC (fig. 7), with the thickest deposits in

western Area 20. The upper units of the Calico Hills composite unit, found in

the eastern part of the SCCC, are vitric, weakly-to non-welded tuffs that act

as an aquifer. Where these tuffs are altered (mostly to zeolites), they restrict

the flow of water. The formation contains lenticular lava flows that are

interlayered with tuffaceous rocks that are locally altered to clay minerals and

zeolites. The lava flows and unaltered tuffs tend to be permeable, whereas the

altered tuffs are impermeable. This thick and complex group of intertonguing

aquifers and confining units is important in the western part of the SCCC; it
Contour Interval = 100 ft.

Figure 6a. Topopah Spring tuff aquifer. Distribution and thickness map (thickness in feet). Fault 

traces are grey where the orientation of the fault is not vertical and a thickness calculation is not 

possible. Drill holes that penetrate this unit or units stratigraphically beneath it are labelled by 

number; control points used to constrain the surface in EarthVision are dots. No thickness is 

shown south of the Timber Mountain caldera fault because of lack of drill hole control. 

116 30' 116 20'

 37 10'

 37 20'

116 30' 116 20'

 37 10'

 37 20'

U20BD

U20AF

U20D

UE20AF

U20AX

U20C

U20Y

PM3

PM2 UE20P

U20M

UE20E1

Timber Mountain

fault

West Greeley

fault

East Boxcar

fault

N

thickness

in feet

Link to oversize version
East Boxcar

fault

West Greeley

fault

0 0

5 5

10

10

15

15

20

20

25

-8000

0

8000

elevation (ft)

km km

Figure 6b. Topopah Spring tuff aquifer. No vertical exaggeration. Three-dimensional view 

looking NE. Note the trace and offset on the East Boxcar and West Greeley north-south faults. 

Timber Mountain fault block is omitted because of lack of drill hole control. 

TimberMountain

caldera fault N

E
0

5

10

15

20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 6c. Topopah Spring tuff aquifer. No vertical exaggeration. Three-dimensional view 

looking east, shown in place over lower units. Timber Mountain fault block is omitted because 

of lack of drill hole control. 

East Boxcar

fault

Almendro

fault

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 6d. Topopah Spring tuff 

aquifer. No vertical exaggeration. 

Three-dimensional view looking 

north, shown in place over lower 

units. 

East Boxcar

fault Almendro

fault

25

Silent Canyon

caldera fault

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
Contour Interval = 300 ft.

Figure 7a. Calico Hills composite unit. Distribution and thickness map (thickness in feet). Fault 

traces are grey where the orientation of the fault is not vertical and a thickness calculation is not 

possible. Drill holes that penetrate this unit or units stratigraphically beneath it are labelled by 

number; control points used to constrain the surface in EarthVision are dots. No thickness is 

shown south of the Timber Mountain caldera fault because of lack of drill hole control. 

thickness

in feet

116 30' 116 20'

 37 10'

 37 20'

116 30' 116 20'

 37 10'

 37 20'

PM3

UE20F

U20D

U20C U20AX

UE20H

U20BD

U20AP

PM1

U20A2WW

U20AZ

ER20-2-1

U19Q

U19BG1

U19AU1

U19AF

UE19F\S

U19V

UE19W1

U19AB

UE19C

U19AC

UE19Z

U19AN

UE19B

U19L

U19D2 UE19H

U20G

U20E1

U20AN U20I

UE20P

U20M

Silent Canyon

caldera fault

Almendro

fault

Silent Canyon

caldera fault

Timber Mountain

caldera fault

East Boxcar

fault

West Greeley

fault

N

Link to oversize version
East Boxcar

fault

West Greeley

fault

0 0

5 5

10 10

15 15

20 20

25

-8000

0

8000

elevation (ft)

km

km

Figure 7b. Calico Hills composite unit. No vertical exaggeration. Three-dimensional view 

looking NE. Note the trace and offset of the north edge of the Timber Mountain caldera 

margin, and the East Boxcar, West Greeley, and Almendro north-south faults. Timber 

Mountain fault block is omitted because of lack of drill hole control. 

Almendro

fault

N

E

TimberMountain

caldera fault

Silent Canyon

caldera fault
0

5

10

15

20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

East Boxcar

fault

Almendro

fault

Figure 7c. Calico Hills composite unit. No vertical exaggeration. Three-dimensional view 

looking east, shown in place over lower units. Timber Mountain fault block is omitted because 

of lack of drill hole control. 

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 7d. Calico Hills composite unit. No vertical 

exaggeration. Three-dimensional view looking 

north, shown in place over lower units. 

East Boxcar

fault Almendro

fault

25

Silent Canyon

caldera fault

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
31

thins abruptly north, west and south of the caldera margin. Most of the Calico

Hills composite unit is saturated.

Crater Flat composite unit

 Stratigraphic units above the Bullfrog Tuff in the Crater Flat Group are

caldera-filling lava flows and tuffs of various types that make up the Crater

Flat composite hydrostratigraphic unit. Most of these rocks are found in the

western part of the SCCC (Area 20, fig. 8), where collapse associated with

Bullfrog tuff eruption was apparently greatest. Distinctive lava or tuff units

from the upper part of the Crater Flat Group include, in decending order, the

tuff of Pool, the rhyolite of Inlet, the tuff of Jorum, the rhyolite of Sled, and

the rhyolite of Kearsarge. These units are only locally present and are mostly

aquifers except for zeolitized parts of the tuff of Jorum and the rhyolite of

Sled. Intertonguing, non-welded tuffs, which are zeolitized in many places, are

impermeable and form lenticular flow barriers. The three-dimensional

distribution of the permeable and impermeable zones in the Crater Flat

composite unit and the Calico Hills composite unit above it create a

complicated ground-water flowpath in the western part of the SCCC. In the

eastern part of the SCCC, in Area 19, the Crater Flat composite unit is

comprised of the rhyolite of Kearsarge, a lava aquifer and a thin sequence of

non-welded devitrified and altered tuffs that may act as flow barriers.

Bullfrog Tuff confining unit

 The Bullfrog Tuff is considered to be a major caldera-forming, eruptive

unit of the SCCC (Sawyer and Sargent, 1989; Ferguson and others, 1994;

Sawyer and others, 1994). Its eruption is cited as the cause of a nested caldera

within the SCCC, called the Area 20 caldera by these authors. The tuff occurs

across the entire caldera complex but is especially thick in the west central

part of the structure (fig. 9). Outside and east of the buried caldera complex

rim the Bullfrog Tuff is relatively thin. Within the SCCC, where the Bullfrog

Tuff is thickest, it is non-welded and lithic-rich and has been extensively

zeolitized, making it a confining unit. The Bullfrog Tuff confining unit is the

most significant confining unit in the SCCC. It is widespread and relatively
Contour Interval = 250 ft.

Figure 8a. Crater flat composite unit. Distribution and thickness map (thickness in feet). Fault 

traces are grey where the orientation of the fault is not vertical and a thickness calculation is not 

possible. Drill holes that penetrate this unit or units stratigraphically beneath it are labelled by 

number; control points used to constrain the surface in EarthVision are dots. No thickness is 

shown south of the Timber Mountain caldera fault because of lack of drill hole control. 

thickness

in feet

116 30' 116 20'

 37 10'

 37 20'

116 30' 116 20'

 37 10'

 37 20'

PM3

UE20F UE20H PM1

U20Z

U19Q

UE19F\S

U19V

UE19W1

U19AB

U19C

U19AC

UE19H

U19BA U19E

UE19Z

U19AJ U20G U19AN

U20I

U19G

UE20E1

UE20P

U20M

Silent Canyon

caldera fault

Timber Mountain caldera fault

East Boxcar

fault

West Greeley

fault

Silent Canyon

caldera fault

Almendro

fault

N

Link to oversize version
East Boxcar

fault

West Greeley

fault

Silent Canyon

caldera fault

0 0

5 5

10 10

15 15

20 20

25

-8000

0

8000

elevation (ft)

km km

Figure 8b. Crater Flat composite unit. No vertical exaggeration. Three-dimensional view 

looking NE. Note the trace and offset on the north edge of the Timber Mountain caldera 

margin, and the East Boxcar, West Greeley and Almendro north-south faults. 

Almendro

fault

N E
0

5

10

15

20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 8c. Crater Flat composite unit. No vertical exaggeration. Three-dimensional view 

looking east, shown in place over lower units. Timber Mountain fault block is omitted because 

of lack of drill hole control. 

East Boxcar

fault

Almendro

fault

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 8d. Crater Flat composite unit. No 

vertical exaggeration. Three-dimensional 

view looking north, shown in place over lower 

units. 

East Boxcar

fault

Almendro

fault

25

Silent Canyon

caldera fault

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
116 30' 116 20'

 37 10'

 37 20'

116 30' 116 20'

 37 10'

 37 20'

Contour Interval = 500 ft.

Figure 9a. Bullfrog Tuff confinig unit. Distribution and thickness map (thickness in feet). Fault 

traces are grey where the orientation of the fault is not vertical and a thickness calculation is not 

possible. Drill holes that penetrate this unit or units stratigraphically beneath it are labelled by 

number; control points used to constrain the surface in EarthVision are dots. No thickness is 

shown south of the Timber Mountain caldera fault because of lack of drill hole control. 

PM3

UE20F

UE19F\S

U19V

U19A\S

UE19W1

U19AB3

UE19C

U19D2

UE19H

UE19E

U19G

U19Q

PM1

U20G

UE20E1

U20P

U20M

thickness

in feet

Silent Canyon

caldera fault

Timber Mountain caldera fault

East Boxcar

fault

West Greeley

fault

Silent Canyon

caldera fault

Almendro

fault

N

Link to oversize version
Silent Canyon

caldera fault East Boxcar

fault

West Greeley

fault

Silent Canyon

caldera fault

0 0

5 5

10 10

15 15

20 20

25

-8000

0

8000 elevation (ft)

km km

Figure 9b. Bullfrog Tuff confining unit. No vertical exaggeration. Three-dimensional view 

looking NE. Note the trace and offset on the north edge of the Timber Mountain caldera 

margin, the East Boxcar, West Greeley and Almendro north south faults, and the eastern edge 

of the Silent Canyon caldera margin. Timber Mountain fault block is omitted because of lack of 

drill hole control. 

Almendro

fault

TimberMountain

caldera fault N E
0

5

10

15

20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 9c. Bullfrog Tuff confining unit. No vertical exaggeration. Three-dimensional view 

looking east, shown in place over lower units. Timber Mountain fault block is omitted because 

of lack of drill hole control. 

East Boxcar

fault

Almendro

fault

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 9d. Bullfrog Tuff confining unit. No vertical exaggeration. 

Three-dimensional view looking north, shown in place over lower 

units. 

East Boxcar

fault Almendro

fault

25

Silent Canyon

caldera fault

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
40

thick and, in the southwestern two thirds of the caldera complex, is below the

water table at depths where it should have a great effect on ground-water

flow.

Belted Range aquifer

 The Belted Range aquifer is composed of lava flows and tuffs of the Dead

Horse Flat Formation underlain by the Grouse Canyon Tuff. Comendite lava

flows of Split Ridge and Quartet Dome are considered to be precursor lavas

to the Grouse Canyon Tuff but not caldera causing eruptions (Sawyer and

Sargent, 1989; Sawyer and others, 1994).These units comprise the Belted

Range Group (table 1). Lava flows of the Dead Horse Flat Formation and

Grouse Canyon Tuff are the principal caldera-filling rocks of a caldera that

developed in the NE half of the SCCC during and after eruption of the

Grouse Canyon Tuff (Noble and others, 1968; Sawyer and others, 1994;

Ferguson and others,1994). The Belted Range aquifer underlies most of

Pahute Mesa, and has been identified in drill holes across all of the SCCC

(fig.10). It is particularly thick (greater than 1,800 m (6,000 ft)), in the

northeastern part of the caldera complex near the down-faulted eastern

margin. The Grouse Canyon tuff and lava flows of the Belted Range Group

thin at the caldera’s margin and crop out as thin out- flow (extracaldera) units

around and outside most of the caldera complex (fig. 10 a,b,c,d). Within the

SCCC, the Belted Range aquifer is saturated; outside the caldera it is above

the water table.

Pre-Belted Range composite unit

 The lower half or more of the depression filling rocks of the SCCC

beneath Pahute Mesa comprise the pre-Belted Range composite unit (fig. 11).

The deepest drill-holes, and drill holes SE and W of the SCCC on Pahute

Mesa have penetrated the upper part of this hydrostratigraphic unit but as

much as 3,000 m (9,900 ft) of presumed volcanic rock exists between the

deepest holes and the basement. These volcanic rocks are most likely lava

flows and welded and nonwelded tuffs. Rocks in the Tertiary sequence below

the Belted Range Group in areas adjacent to Pahute Mesa include the rhyolite
PM3

UE20F

UE19F\S

UE19I

UE19C

UE19W1

ER19-1

U19D2

UE19E

U19G

UE19H

PM1

UE20E1

U20M

U20P PM2

116 30' 116 20'

 37 10'

 37 20'

116 30' 116 20'

 37 10'

 37 20'

Contour Interval = 500 ft.

thickness

in feet

Figure 10a. Belted Range aquifer. Distribution and thickness map (thickness in feet). Fault traces 

are grey where the orientation of the fault is not vertical and a thickness calculation is not possible. 

Drill holes that penetrate this unit are labelled by number; control points used to constrain the 

surface in EarthVision are dots. No thickness is shown south of the Timber Mountain caldera fault 

because of lack of drill hole control. 

Silent Canyon

caldera fault

Timber Mountain caldera fault

East Boxcar

fault

West Greeley

fault

Silent Canyon

caldera fault

Almendro

fault

N

Link to oversize version
0 0

5 5

10 10

15 15

20 20

25

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault East Boxcar

fault

West Greeley

fault

Silent Canyon

caldera fault

km km

N E

Figure 10b. Belted Range aquifer. No vertical exaggeration. Three-dimensional view looking 

NE. Note the trace and offset on the east and west parts of the Silent Canyon cadlera margin 

and the East Boxcar and West Greeley north-south faults. 
0

5

10

15

20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 10c. Belted Range aquifer. No vertical exaggeration. Three-dimensional view looking 

east, shown in place over lower units. Timber Mountain fault block is omitted because of lack 

of drill hole control. 

East Boxcar

fault

Almendro

fault

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 10d. Belted Range aquifer. No vertical exaggeration. 

Three-dimensional view looking north, shown in place over lower 

units. 

East Boxcar

fault

Almendro

fault

25

Silent Canyon

caldera fault

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
PM3

U20M

UE20F PM1

UE19W1

U19D2

U20P

PM2

116 30' 116 20'

 37 10'

 37 20'

116 30' 116 20'

 37 10'

 37 20'

Silent Canyon

caldera fault

Timber Mountain caldera fault

East Boxcar

fault

West Greeley

fault

Figure 11a. Pre-Belted Range composite unit. Distribution and thickness map (thickness in feet). 

Fault traces are grey where the orientation of the fault is not vertical and a thickness calculation is 

not possible. Drill holes that penetrate this unit are labelled by number; control points used to 

constrain the surface in EarthVision are dots. No thickness is shown south of the Timber Mountain 

caldera fault because of lack of drill hole control. 

thickness

in feet

N Contour Interval = 1000 ft.

Link to oversize version
Silent Canyon

caldera fault

East Boxcar

fault

West Greeley

fault

0 0

5 5

10 10

15 15

20 20

25

-8000

0

8000

elevation (ft)

km km

Figure 11b. Pre-Belted Range composite unit. No vertical exaggeration. Three-dimensional 

view looking NE. Note the trace and offset of the Silent Canyon caldera margin and the East 

Boxcar and West Greeley north-south faults. 

N

E

Silent Canyon

caldera fault
0

5

10

15

20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 11c. Pre-Belted Range composite unit. No vertical exaggeration. Three-dimensional 

view looking east, shown in place over lower units. Timber Mountain fault block is omitted 

because of lack of drill hole control. 

East Boxcar

fault

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault

West Greeley

fault

km

N

E

Figure 11d. Pre-Belted Range composite unit. 

No vertical exaggeration. Three-dimensional 

view looking north, shown in place over lower 

units. 

East Boxcar

fault

25

Silent Canyon

caldera fault

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
49

of Quartet Dome, the Lithic Ridge Tuff, the tuff of Tolicha Peak, the dacite of

Mount Helen, the Tub Spring Tuff, the Redrock Valley Tuff, and other units

(Table 1). The lava flows and welded tuffs in this sequence generally form

fracture-flow aquifers. The nonwelded tuffs are almost always devitrified and

zeolitized and are confining units. Rocks in the lower parts of the pre-Belted

Range composite unit most likely act as a confining unit due to compaction

and alteration of these rocks which are buried to great depths.

Basement

 Nowhere in Pahute Mesa on the NTS do basement rocks crop out or are

they penetrated by any drill hole. Gravity modeling indicates that the

basement is generally more than 1,200 m (4,000 ft), and in places as much as

6,000 m (19,800 ft), below the surface (see Basement gravity section). It is

postulated here that the basement of Pahute Mesa is granitic rock of the

upper part of the SCCC magma system that intruded slightly metamorphosed

quartzite, siltstone, and limestone of Precambrian and Paleozoic age. In the

pre-Tertiary rocks, carbonates frequently act as aquifers and siliceous rocks,

including granitoids, are confining units. All of the basement rocks, however,

are much faulted which create possible flow paths unless the faults are healed

and thus impede flow. In general, the basement probably does not act as a

barrier to ground-water flow although it is at such great depth beneath the

SCCC that it is assumed to be below the principal zone of ground-water flow.

STRUCTURE

 The structural framework of Pahute Mesa is that of a buried multiple

collapse caldera complex-- the Silent Canyon caldera complex (SCCC). The

elements of the caldera complex are not visible at the surface, being concealed

by hundreds of feet of nearly flat-lying welded tuff from sources south and

west of Pahute Mesa. The only evidence of the buried calderas at the surface

are inward dips of the Timber Mountain Tuff (Orkild and others, 1969)

possibly caused by ramping against, and over, the topographic margin of the

partially buried caldera complex. The youngest tuffs, the Trail Ridge, Pahute

Mesa, and Rocket Wash Tuffs from the Black Mountain volcanic center to the
50

west of Pahute Mesa spread across the western edge of the completely buried

caldera topographic rim.

 The location and approximate geometry of the SCCC caldera margin can

be determined only by indirect means, although the general area of

subsidence is well documented by the many drill holes that penetrate calderafilling rocks within the structural basin underlying Pahute Mesa. Gravity

analyses (see Basement surface section) estimate the location of the edge,

slope, and depth of the basin beneath the nearly horizontal upper welded

tuffs. Extrapolation of this surface upward to the outer, or topographic, edge

of the SCCC as shown by Wahl and others (1997) and Laczniak and others

(1996) defines the upper part of the caldera collapse structure. Contrary to

the commonly held conceptual view of a caldera fault, and described here in

the section “Caldera Concept”, the extrapolated upper part of the structure in

this model of the SCCC is more steeply dipping than the deeper gravitydefined part of the surface. The reason for this departure from the conceptual

caldera view is probably due to the imprecise location of the buried

topographic rim and the simplification of effects of gravity inversion to

smooth curves at depth. A caldera topographic margin is not shown because

more empirical data is needed to build it into the model. The crenulated,

steeply dipping, oval shaped surface of the basin is assumed to be the

youngest ring fault at the margin of the caldera complex (fig.12a, b, c). Older

caldera faults within the SCCC that are partially or completely destroyed are

not detected by gravity contrasts in the low density rocks of the SCCC. The

deeper gravity defined surface beneath the SCCC may be a series of steeply

dipping step-like faults. Because of the great depth of this surface (>3,000 m;

(10,000 ft)), the inverted gravity data will likely be unable to resolve the

individual fault surfaces. As a result of this the caldera boundary surface is

smooth and gently dipping. Boundaries of younger calderas within the SCCC

are also not seen by gravity contrasts but geologic evidence from drill hole

data, for the Area 20 caldera is compelling. Eruption of the Bullfrog Tuff is

presumed to be responsible for this caldera (Sawyer and Sargent, 1989;
Timber Mountain

caldera fault (topographic rim)

P A H U T E M E S A

Area 20 Area 19

37°24'

37°09' 37°09'

116°36' 116°15'

116°36' 116°15'

37°24'

NTS BOUNDARY

Figure 12a. The Silent Canyon caldera complex margin. (a) Plan view, with arrows showing dip direction of the buried ring fault. Inner edge is deeper

 than the outer edge, with a dip 45o or more in most places. (b) three-dimensional view of the Silent Canyon ring fault looking N (same as plan

view) and (c) looking NE. The southwestern portion is cut off by the Timber Mountain caldera margin (not shown).

(a)
N

E

0

5

10

15

20

-8000

0

8000

elevation (ft)

km

0 5 10 15 20 25

-8000

0

8000

N

W

(b)

(c)

Figure 12 b, c. The Silent Canyon caldera fault surface from the "EarthVision" model. No 

vertical exaggeration. (b) View in the same orientation as figure 12a. (c) View looking 

northeast.

elevation (ft)

km
53

Sawyer and others, 1994) and the thick accumulation of this tuff (see Figure

9) outlines the general margin of the Area 20 caldera as envisioned by

Sawyer and others (1994). Because our model bases the caldera margins on a

density contrast the Area 20 caldera fault does not appear in our model.

 A second, arcuate, south-dipping surface which is a segment of the Timber

Mountain caldera complex caldera margin truncates the southwestern part of

the SCCC (fig.13). The faults around the southern part of the SCCC and rocks

within this caldera complex are down-dropped into the northern part of the

younger Timber Mountain caldera complex. Lack of drill hole data makes

geologic models for this area very speculative.

 Faults younger than the SCCC cut the youngest post-caldera Tuffs of the

Thirsty Canyon and Timber Mountain Groups at many places on Pahute Mesa.

These faults are all north-striking, high-angle, normal faults; on most of them, the

western side is down dropped. Few of them can be traced north or south of Pahute

Mesa and those that can be, extend only a short distance. Vertical offset on the

faults is small, generally less than a few hundred feet. The most important of these

faults are shown on figure 14; only those with enough offset to be judged

significant on the 1:48,000-scale geologic cross sections are used in this study --

they are highlighted in red on figure 14. Even the three faults considered most

significant do not have much offset at the surface: the East Boxcar fault has 76 to

121 m (250 to 400 ft), the West Greeley has 76 to 152 m (250 to 500 ft) and the

Almendro has 33 to 66 m (100 to 200 ft) (Orkild and others, 1969; Blankennagel

and Weir, 1973). The eposodic nature of the faults on Pahute Mesa, however, has

been addressed by Warren and others (1985) who point out the differences in their

offsets at depth. Drill hole data have been interpreted to show progressively greater

offset at depth so that there is almost 600 m (1,980 ft) of displacement of the Calico

Hills Formation to about 1,500 m (4,950 ft) below the surface. Ferguson and

others (1994) suggested that the fault was active during emplacement of the Calico

Hills Formation in the SCCC and acted as an ancillary fault to the circular caldera

ring fault of the Area 20 caldera. The East Boxcar, West Greeley, and Almendro
0

5

10

15

20

-8000

0

8000

elevation (ft)

km

E

S

0

0

5

5

10

10

15

15

20

20

-8000

0

8000

elevation (ft)

km

km

25

N

W

Figure 13. Two views of the north edge of the Timber Mountain caldera margin (purple) 

cutting off the Silent Canyon caldera margin (pink). The Timber Mountain caldera curves 

around to the south outside the limits of the block-diagram. No vertical exaggeration.
Area 20 Area 19

37°24'

37°09' 37°09'

116°36' 116°15'

116°36' 116°15'

37°24'

NTS BOUNDARY

Purse fault

fault

fault

fault

fault fault

fault

fault

fault

fault

West

West

East

East

Chartreuse

East

Buteo

Greeley

Greeley

Almendro

Scrugham

Peak

Boxcar

Boxcar

Estuary

Figure 14. The major faults mapped at the surface of Pahute Mesa. These faults are mostly within the surface trace of the buried Silent Canyon caldera complex (see fig.

 12a and 15). The three faults colored red offset units within the caldera complex enough to be significant at the scale of 1:48,000 used to compile and evaluate the data

 for this 3-dimensional model. The West Greeley fault has increasing offset with depth (see cross section A-A', figure 18).
56

faults all lie within the SCCC; their geometric relationship to the SCCC caldera and

the north edge of the Timber Mountain caldera complex are shown in figure 15.

BASEMENT SURFACE

 The isostatic residual gravity field of the region shown in Figure 4a is based

on about 4,000 gravity measurements (Mankinen and others, 1999). The overall

distribution of gravity stations is roughly one station within 0.7 km2

 on the

average, although denser coverage exists within the area of the SCCC. Mankinen

and others (1999) described the data reduction process to obtain the Bouguer

gravity anomaly. The isostatic residual anomaly was calculated using a reduction

density of 2,670 kg/m3

, crustal thickness of 30 km, and a mantle-crust density

contrast of 350 kg/m3

 (see Simpson and others, 1986). All data were gridded at

a spacing of 1 km using a minimum curvature algorithm of Webring (1981).

 To the first order, the isostatic residual gravity field reflects the pronounced

contrast between dense basement rocks (about 2,670 kg/m3

) and significantly less

dense (generally < 2,500 kg/m3

) younger volcanic and sedimentary rocks. The

prominent gravity lows (fig. 4a) over the SCCC and parts of the Timber

Mountain caldera complex mostly delineate the thick, anomalously low-density

volcanic caldera fill (Healey, 1968; Ferguson and others, 1994; Grauch and

others, 1997; Hildenbrand and others, 1999). The gravity low over Pahute Mesa

is one of the most prominent lows in western U.S., suggesting an unusually thick

volcanic pile. In contrast granitic resurgent domes (density about 2,670 kg/m3

)

under Timber Mountain and Black Mountain appear to produce gravity highs

(Kane and other, 1981; Grauch and others, 1997).

 Using the gravity inversion method derived by Jachens and Moring (1990;

modified to include drill-hole data), we separated the isostatic residual

anomaly into a basin field and a basement field. The basin gravity field reflects

variations in the thickness and density of low-density Tertiary volcanic and

sedimentary rocks. The basement gravity field reflects changes in density

related to lithologic variations within the denser pre-Tertiary rocks or dense

Tertiary granitic intrusions. In the inversion process, the density of basement
0

5

10

15

20

km

25

-8000

0

8000

elevation (ft)

S

E

0

0 5

5

10

10

15

15

20

20

-8000

0

8000

elevation (ft)

km

km

25

N

W

(a)

(b)

Figure 15. (a, b) Two views of the East Boxcar (yellow), West Greeley (blue), and Almendro (green) 

north-south faults within the Silent Canyon caldera complex (pink). Note the Timber Mountain 

caldera margin (purple) which cuts off the south edge of the SCCC. No vertical exaggeration. (c) 

Fault tree showing the hierarchy of faults in this model. Those faults higher up in the tree will take 

precedence over those lower in the tree where the faults intersect.

Timber Mountain

caldera fault

Silent Canyon

caldera fault

East Boxcar

fault

West Greeley

fault

Almendro

fault

(c)
58

varies horizontally but the density of basin-filling deposits is fixed using a

representative density versus depth relationship. Knowledge of the thickness

of low density rocks (outside the SCCC) and of basement rock types and

densities based on drill hole information constrains the calculations. Refer to

Hildenbrand and others (1999) for details on the generation and constraints of

the basin thickness data shown in figure 4b.

 In general, basement is shallower outside the caldera along the western and

southern parts of the study area. Basement descends to great depths beneath

the SCCC. What are most likely the resurgent granitic domes of the Black

Mountain caldera and the Timber Mountain caldera complex are clearly

expressed as elliptically shaped basement highs.

Density-depth functions

 The accuracy of the thickness calculation is dependent on the assumed

density-depth relation of the Tertiary rocks and the initial density assigned to

basement rocks. The Tertiary rocks are almost entirely of volcanic origin

(although thin alluvial deposits are found throughout the study area).

Complicating this task of selecting representative densities at a particular

depth is the significant variation in density related to the degree of welding

and alteration of the ash-flow tuffs, to structure (e.g., landslides and shallow,

dense volcanic domes), and to water saturation. Because density generally

increases with depth due to compaction, a layered density model is assumed

for the Cenozoic deposits, although the lenticular nature of the volcanic rocks

is acknowledged.

 Based on deep-borehole gravimeter studies and borehole density logs from

moderately deep and deep wells in the region of the SWNVF, Hildenbrand

and others (1999) used a density-depth relation of the Cenozoic basin fill and

basement shown as model A (table 2) to derive the basement depths in figure

4b. Although this relation may be a reasonable approximation for the large

region shown in figure 4, it may lead to significant errors in more local areas.

For example, anomalously low-density tuff bodies are observed beneath

Pahute Mesa. Borehole gravimeter measurements in the SCCC (Ferguson
59

and others, 1994; Warren, personal communication, 1999) indicate densities of

2,100 kg/m3

 extending in many areas to depths of over 1,000 m (3,300 ft). In

such situations calculated basement depths in figure 4b will be overestimated.

Thus for the present study a different density-depth relation for the SCCC area

was necessary. Several models were considered and are shown in table 2.
60

Table 2. Density-depth functions used to determine the thickness of basinfilling deposits and calculated depths near and at drill hole Ue20f.

Depth Range Density (kg/m3) for 5 models

A B C D E

0.0–0.2 km 1,900 1,900 1,900 1,900 1,900

0.2–0.6 km 2,100 2,100 2,100 2,100 2,100

0.6–1.2 km 2,300 2,100 2,100 2,100 2,100

1.2–1.5 km 2,450 2,300 2,100 2,100 2,100

1.5–2.0 km 2,450 2,300 2,100 2,100 2,300

 >2.0 km 2,450 2,450 2,450 2,300 2,450

Max. Depth (km) 8.9 6.6 5.4 3.8 6.2

Ue20f depth (km) 6.8 4.5 3.3 2.7 4.3

 In table 2, for all five models densities are the same above 600 m (1,980 ft)

but vary considerably below 600 m. A careful analysis of deep drill hole data

outside the SCCC (Hildenbrand and others, 1999) and of a large drill hole data

base of density values from Warren (1999, personal communication) for the

SCCC area leads us to the same density-depth relationship to depths of 600 m

(1,980 ft) for all models. Below 600 m the basin fill within the SCCC is likely

greater than or equal to 2,100 kg/m3, although less dense than the rocks outside

the caldera complex (assumed to be represented by model A). Models B through

E are considered representative of the possible range of general density

distributions within the SCCC. To understand the sensitivity of the basement

depths to changes in the depth-density function, maximum depth to basement

just east (about 3.2 km or 2 mi) of drill hole Ue20f (figure 16) and basement
-12000 -9600 -7200 -4800 -2400 0 2400 Feet

37 ̊10 ́

37 ̊20 ́

0 10 KM

 Silent Canyon 

Caldera Complex

PM2

U19d2

Ue20f

PM1

U19w1

Nevada Test Site

Figure 16. Basement surface beneath the region of the Silent Canyon caldera complex based 

on the inversion of gravity data. Inside the inner dashed black line, a density-depth relation

is used with densities lighter than those in the density-depth relation in the region outside the 

outer dashed line. Basement elevations in the region between the dashed lines were derived

by interpolation using a minimum curvature algorithm. The Nevada Test Site boundary and 

eight drill holes (black circles) provide points of reference.

0 10 MI

PM3
62

depth at Ue20f were determined for all models and are shown in table 2. At a

depth of about 4.2 km (13,743 ft) Ue20f bottomed in volcanic rocks.

 General comments on the depth results shown in table 2 are (1) basement

depths are sensitive to the densities assumed below 0.6 km where our

knowledge is limited, (2) models B and E provide reasonable basement depths

at Ue20f, which bottomed in old volcanic rock at 4.2 km and (3) a viable

density-depth function can only be derived intuitively by our knowledge of

rock types (and thus their densities) inside and outside the caldera that are

expected to be present at depth within the SCCC.

 Our knowledge of rock densities at depth (> 2 km) is based on limited data

from deep drill holes (e.g., Hildenbrand and others, 1999; Warren,1999,

personal communication). Based on these data and due to expected high

pressures and temperatures at depth, one would not expect substantial

volumes of low density volcanic rocks (< 2450 kg/m3) at depths greater than

2 km. A density-depth function depicting volcanic rock of density of about

2100 kg/m3 extending to depths of 2 km is also unlikely (simply because such

a large volume of these unusually low density rocks has not been observed in

any drill hole in southern Nevada). Thus models C and D are considered

unrealistic. Moreover, models C and D lead to basement depths at Ue20f that

are much shallower than the known depth of volcanic rocks. Both models B

and E lead to similar, reasonable solutions and possess densities that are

defensible based on drill hole density data. For our study we chose models A

and E to represent the density distributions outside and within the SCCC,

respectively.

Basin geometry

 The basement surface in the region of the SCCC (figure 16) was derived

using the two density-depth models (models A and E, table 2). To

successfully merge the two depth data it was necessary to define a zone

dividing regions outside and inside the caldera complexes where no depth

values were assigned (a buffer region between the dashed lines in figure 16).

In this zone depth values were calculated using a minimum curvature
63

algorithm that utilized the depths based on the low-density model E inside the

pair of dashed lines and depth values from the high-density model A outside

these lines.

 The resulting basement surface in figure 16 indicates that the estimated average

thickness of the basin fill is about 4.5 km (14,850 ft.) and that the maximum thickness

may exceed 6 km (19,800 ft.). A north-north-west-striking gravity high and resulting

basement ridge cuts the SCCC and lies beneath mapped surface faults with small

displacements. Because the density data of Warren (1999, personal communication) do

not suggest a shallow dense zone, the source of this gravity high probably lies at or

near the basement surface. The source of the NNW-striking gravity high is uncertain

although it is subparallel to the surface trace of the West Greeley fault and may be

related in some way to that fault. The associated basement ridge may be caused by

granitic intrusions related to a resurgent dome at depth or it may be related to a preTertiary structural high. Another basement ridge occurs at the southern edge of

Pahute Mesa (the SW edge of figure 16) and is probably within the structural margin

of the SCCC.

THREE-DIMENSIONAL MODEL

 The Silent Canyon caldera complex is modeled in three-dimensions using

the "EarthVision"(Dynamic Graphics Inc.) geological computer program.

EarthVision produces a three-dimensional model of the geology of a defined

area using structural and stratigraphic data. The fault data and cross- cutting

hierarchy defines a fault tree or framework in which the stratigraphic units fit

(fig. 15). The faults and stratigraphic units are modeled using a minimum

curvature function based on Briggs (1974) controlled by drill hole depths. The

units are offset the appropriate amount in the established fault framework.

After a structural model has been developed, data can be added that describe

the properties of the stratified units, and surfaces can be created that model

the properties inside of each stratigraphic unit. The Pahute Mesa subsurface
64

model was created using the Universal Transverse Mercator coordinate

system with x and y units in meters and z units in feet. The scale of 1:48,000

was selected for the map and cross section evaluation because it produced

maps of a manageable size with enough detail to model ground-water flow

paths beneath Pahute Mesa. No vertical exaggeration was used in the

resulting block diagrams and cross sections. The fault data were collected

from gravity inversion calculations (see Basement Surface section), geologic

map surface traces, and geologic data from drill holes with inferred geology

between drill holes. Data for the stratigraphic units were compiled from drill

hole data (52 sites were used) and the gravity measurements for the basement

surface. The surface terrain was produced from the 30-m digital elevation

model for Pahute Mesa. The EarthVision minimum curvature function (called

the "minimum tension") was used for modeling the faults and stratigraphic

units in this model, because the density of data points is appropriate.

 The surfaces produced in the computer model are only as good as the

quality and quantity of the data that control them. Surfaces defined by many

drill hole depth intercepts will produce a mathematically sound model, which

can be checked for geologic validity. The model can be manipulated in many

ways to give perspective views or can be used to create a variety of products

such as cross sections, isopach maps, and block diagrams (see figs. 7, 8, 9, 10,

11, 17, 18, 19, 20, and 21). The computer- generated, three-dimensional

model was checked for geologic validity by comparing cross sections made

from it (fig.18 b,c,d) with conventionally drawn cross sections. This model is

limited to the three-dimensional geologic structure and hydrostratigraphy of

Pahute Mesa and does not include stratigraphic unit property data. Block

diagrams (figs.19, 20, and 21) cut along the same vertical planes as the cross

sections shown in figure 18 b, c, and d give a three-dimensional aspect which
0

5

10

15

20

-8000

0

8000

elevation (ft)

km

N E

water

flow

0 5 10 15 20 25

-8000

0

8000

elevation (ft)

N

E

km

Figure 17. Two views of the Silent Canyon caldera complex with 

the upper 2000 feet of rock removed down to the water table 

level. This shows the hydrostratigraphic units that the south 

flowing water would encounter. Timber Mountain fault block is 

omitted because of lack of drill hole control. No vertical 

exaggeration. 

Silent Canyon

caldera fault

East Boxcar 

fault

West Greeley

fault

Almendro

fault

water

flow

Timber Mountain composite unit

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
P A H U T E M E S A

Area 20 Area 19

37°09' 37°09'

116°36' 116°15'

NTS BOUNDARY

A

A'

C

C'

B

B'

PM2

UE20J

U20M

UE20E1

UE20F

PM3

UE20H

PM1

UE19GS

UE19E

UE19I

UE19FS UE19WI

U19D2

Figure 18a. Cross sections across the buried Silent Canyon caldera complex, drawn from "EarthVision" 3-dimensional

 model. Black dots are wells, red dots are the deepest wells and are identified by letter and number. Pink ribbon is the

 buried Silent Canyon caldera complex fault, purple line is the north edge of the Timber Mountain caldera fault.
km

feet

PM2

UE20E1

PM1

UE19W1

A A’

Figure 18b. Profile A-A’ as shown in figure 18a. 

Dotted line is the approximate elevation of the water 

table. No vertical exaggeration.

Silent Canyoncaldera fault

East Boxcar fault

West Greeley faultAlmendro fault

Silent Canyoncaldera fault

bend in section

Timber Mountain composite unit

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
feet

km

PM1

U19D2

Figure 18c. Profile B-B’ as shown in figure 18a. Dotted line is 

the approximate elevation of the water table. No vertical 

exaggeration. The Silent Canyon caldera fault at the northeast 

end of this section is interpreted as two smaller faults. 

Splitting of the Silent Canyon caldera fault at this small of a 

resolution was impossible in the 3 dimensional model because 

of lack of data coverage. 

B B’ Timber Mountain caldera fault East Boxcar fault West Greeley fault Silent Canyon caldera fault

Timber Mountain composite unit

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
PM1

km

feet

UE20F

UE20H

UE19E

C C’ Silent Canyon caldera fault East Boxcar fault West Greeley faul

Silent Canyon caldera fault

Figure 18d. Profile C-C’ as shown in figure 18a. Dotted line 

is the approximate elevation of the water table. No vertical 

exaggeration.

Timber Mountain composite unit

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20

-8000

0

8000

elevation (ft)

Silent Canyon

caldera fault West Greeley

fault

km

N

E

East Boxcar

fault

Almendro

fault

25

Figure 19. Three dimensional diagram of 

Pahute Mesa sliced along section A-A’ in 

figure 18a, the same section as figure 18b. 

No vertical exaggeration.

Silent Canyon

caldera fault

A

A’

bend in

section

Timber Mountain composite unit

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0

5

10

15

20

25

-8000

0

8000

elevation (ft)

km

N

W

Figure 20. Three-dimensional diagram of 

Pahute Mesa sliced along section B-B’ in 

figure 18a, the same section as figure 18c. 

No vertical exaggeration.

B

B’

Timber Mountain

caldera fault

East Boxcar

fault

West Greeley

fault Silent Canyon

caldera fault

Timber Mountain composite unit

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
0 5 10 15 20 25

-8000

0

8000

elevation (ft)

km

N

E

Silent Canyon

caldera fault

C C’

East Boxcar

fault

West Greeley

fault

Almendro

fault

Silent Canyon

caldera fault

Figure 21. Three-dimensional diagram of 

Pahute Mesa sliced along section C-C’ in 

figure 18a, the same section as figure 18d. 

No vertical exaggeration.

Timber Mountain composite unit

Topopah Spring tuff aquifer

Calico Hills composite unit

Crater Flat composite unit

Bullfrog Tuff confining unit

Belted Ranger aquifer

Pre-Belted Range composite unit

Basement

Hydrostratigraphic Units
73

allows better understanding of the geologic relationships of the SCCC. The

block diagrams are especially useful in visualizing structural and stratigraphic

relationships. Figure 17 illustrates the visual impact, and usefulness for

ground-water flow analysis, of a three-dimensional model of Pahute Mesa

viewed at the water table surface. From these diagrams one can see that

ground-water at the top of the water table will flow above the Bullfrog Tuff

confining unit through the Crater Flat composite unit, the Calico Hills

composite unit, the Topopah Spring tuff aquifer, and the bottom of the

Timber Mountain composite unit in the western part of the SCCC. It will flow

beneath the Bullfrog Tuff confining unit through the Belted Range aquifer in

the eastern part of the SCCC. Other depth slices and other view angles can be

used to visualize the components of the ground-water system from almost

any imaginable perspective.

ACKNOWLEDGEMENTS

 This investigation was performed as part of an interagency effort between

the USGS and DOE under Interagency Agreement DE-AI08-96NV11967.

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