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Printed on recycled paper

Scientific Investigations Map 3140

2011

U.S. Department of the Interior

U.S. Geological Survey
Geologic Map of the Chisos Mountains, 

Big Bend National Park, Texas

By Robert G. Bohannon

Pamphlet to accompany Scientific Investigations Map 3140

U.S. Department of the Interior

U.S. Geological Survey
Suggested citation: Bohannon, R.G., 2011, Geologic

map of the Chisos Mountains, Big Bend National Park,

Texas: U.S. Geological Survey Scientific Investigations

Map 3140, 1 sheet, scale 1:50,000, pamphlet, 37 p.
Contents

Introduction.....................................................................................................................................................1

Acknowledgments, Mapping Sources, and Methods .............................................................................1

Geologic Setting.............................................................................................................................................1

Chisos Mountains Geology...........................................................................................................................4

Seaway Deposits ..................................................................................................................................4

Laramide Orogeny.................................................................................................................................7

Chisos Episode—Volcanic and Sedimentary Influx........................................................................7

Fresno Creek-Facies Group .......................................................................................................9

Smoky Creek-Facies Group........................................................................................................9

Eruptive and Intrusive Episodes.......................................................................................................12

Basin-and-Range Extension .............................................................................................................15

Denudational Episode ........................................................................................................................16

Description of Map Units............................................................................................................................20

References....................................................................................................................................................36

Figures

1. Map showing tectonic and volcanic features in Big Bend National Park....................................2

2. Map showing location of Big Bend National Park............................................................................3

3. Map showing topography of Chisos Mountains area ......................................................................5

4. Stratigraphic column for the Chisos Mountains................................................................................6

5. Diagrammatic view of the Chisos Formation and eruptive-phase rocks ......................................8

6. Geologic map of the Chisos Mountains ............................................................................................10

7. Photograph of Fresno Creek-facies group .......................................................................................11

8. Map showing distribution of rocks of the eruptive episode..........................................................12

Photographs of:

 9. Dominguez Mountains ....................................................................................................................14

 10. Punta de la Sierra ............................................................................................................................15

 11. The Basin area in the Chisos Mountains.....................................................................................16

 12. Cryptocrystalline granite pluton near Dodson Spring ...............................................................17

 13. Landslide in the vicinity of Ash Spring .........................................................................................19

 14. Rhyolite flows of Emory Peak.........................................................................................................23

 15. Types of dikes in the Dominguez Mountain dike complex ........................................................25

 16. Bee Mountain Basalt in Blue Creek Canyon...............................................................................27

 17. Chisos Formation..............................................................................................................................28

 18. Chisos Formation in Blue Creek Canyon......................................................................................29

 19. Ash Spring Basalt Member of Chisos Formation near Ash Spring .........................................30

 20. Chisos Formation near Tortuga Mountain ...................................................................................31

 21. Nodular rhyolite facies of the Chisos Formation ........................................................................31

 22. Lower siltstone facies of the Chisos Formation..........................................................................32
iv

 23. Lacustrine facies of the Chisos Formation .............................................................................33

 24. Rhyolite and tuff facies of the Chisos Formation...................................................................34

 25. Mudstone of the Aguja Formation near Tortuga Mountain .................................................36

Table

 1. Radiometric age determinations of select rocks in the Chisos Mountains .........................18
Geologic Map of the Chisos Mountains, 

Big Bend National Park, Texas

By Robert G. Bohannon

Introduction

This report describes the geology of the Chisos Mountains in the central part of Big Bend National Park, Brewster 

County, Texas (fig. 1). The compilation summarizes and 

integrates new geologic mapping and stratigraphic studies with 

various topical studies and geologic maps that were published 

between the late 1960’s and 2006 (for example, Maxwell and 

others, 1967; Maxwell, 1968; Muehlberger, 1989; Muehlberger and Dickerson, 1989; Dickerson and Muehlberger, 

1994; Barker and others, 1986; and White and others, 2006). 

The purpose of the work is to summarize the geology 

of the Chisos Mountains, one of the most highly visited 

areas in Big Bend National Park. Although a geologic map 

of an earlier vintage exists (Maxwell, 1968), the new map 

included in this report displays the geology in far greater 

detail at 1:50,000 scale, and it is plotted on a scale-stable, 

geo-registered base keyed to a modern grid based on the 

WGS-84, UTM-zone-13 coordinate system. This map and 

report are also designed to serve as detailed extensions of the 

contemporary regional geologic map of the entire Big Bend 

National Park at 1:75,000 scale that is in production by the 

U.S. Geological Survey (USGS) as of this writing. 

Acknowledgments, Mapping Sources, 

and Methods

This map was produced under the partial auspices of the 

United States National Park Service (USNPS). Cartography 

for roads and trails was provided by GIS specialists at the 

USNPS Park Headquarters in Big Bend. The geologic data 

come from a variety of sources and the map represents the 

work of several geologists from the U.S. Geological Survey 

(USGS), University of Texas at Austin (UTA), and Texas 

Tech University (TTU). All of the geology as presented is 

primarily based on the author’s interpretation of ortho-rectified 

USGS color-infrared photogrammetry, which facilitated a 

uniform horizontal accuracy of geologic features to within 

2 m throughout the mapped area. The small map included 

on the plate shows the areas where the author extensively 

field-checked the geology during the fall, winter, and spring 

months of 2003–2007. Large parts of the map also benefited 

from field studies conducted by several other USGS geologists 

including Robert Scott, Lawrence Snee, and Kenzie Turner 

(see plate). Tom Lehman (UTA and TTU) and Ric Page 

(USGS) conducted regional studies of the Cretaceous and 

lower Paleogene deposits throughout Big Bend National Park, 

and all deposits of that age that are shown on this map follow 

their designations. The surficial deposits (rocks of Quaternary 

age) of Big Bend has been studied to various degrees by 

Margaret Berry and Van Williams (Berry and Williams, 2008) 

and this map reflects their work. Dan Miggins (USGS and 

University of Texas at El Paso) provided age and petrographic 

data for specific sites over much of the area. 

Geologic Setting

The Chisos Mountains (fig. 2) form some of the highest 

ground in Texas, second only to Guadalupe Peak near the New 

Mexico border. The northern half of the range is mostly above 

5,500 feet with Emory Peak the high point at 7,825 feet. The 

mountains are centrally located in Big Bend National Park 

between Panther Junction and Punta de la Sierra (fig. 2). 

Big Bend National Park lies near the diffuse border 

between the Great Plains Province to the northeast and the 

Sonoran section of the Basin-and-Range structural province 

to the west and southwest (fig. 1). These geologically 

unique regions are distinguished from one another by large 

differences in their landscape and by the amount and style of 

internal structural deformation. The Great Plains Province 

is characterized by flat-lying or gently dipping sedimentary 

strata, low topographic relief, shallow stream valleys, and 

by a general lack of faulting. Very little active deposition 

is occurring on the plains, except in the bottoms of active 

stream valleys. In southwestern Texas the plains stand at 

average elevations of 2,000 to 3,300 feet and slope gently east 

toward the Mississippi River and the Gulf of Mexico. The 
C A L I F O R N I A

G U L F

PACIFIC

OCEAN

O F

A

R I Z

O

N

A

S

O

N

O

R

A

C

H

I

H

U

A

H

U

A

N

E

W

M

E

X I C

O

T

B A J A C A L I F O R N I A

S U R

N O R T E

B A J A C A L I F O R N I A 

Guadalupe

Peak

Ri o S o u t h 

M o g o l l o g o l lon

D a D a t il

v o v o l c a n i oo l l c a n i icc

f i e l d s

Laramide

deformation

C o l orad o Pl a t ea us

B 

asi n an 

d

Ra nge 

Oc c i d en t a l

c

a t n

O c c 

Se rr a 

Ma dd r e 

O

a M S e r r a M a d r e O

l

G r e 

P l a i 

114° 112°

26°28°

30°

32°

34°

110° 108° 106° 104° 

00 150 KILOMETERS

25 125 MILES

50

50

100

75 100
Geologic Setting 3

Figure 1 (facing page). Generalized map showing the location 

of Big Bend National Park (darker green area) relative to main 

tectonic and volcanic features of the southwestern United States 

and northwestern Mexico. Largest volcanic fields are shown with 

black stippled pattern. Trans-Pecos fields are widespread in west 

Texas and cross the Rio Grande into Mexico. Purple regions are 

affected by Laramide compressional deformation. Basin-andRange structural province extends throughout southwest Arizona, 

southern New Mexico, Sonora, and Chihuahua. It includes the 

Rio Grande rift. Topography is color-graded from lowlands in pale 

green to highlands in pale brown.

Area of report

Terlingua

Ghostown

Lajitas

Study Butte

THE

BASIN

Castalon

Santa

Elena

Panther Junction

Boquillas

Del Carmen

Rio

Grande

Village

SIERRA

PONCE

SANTA

ROSILLOS

Chilicotal Mountain

Burro

Mesa

Bee

Mountain

Tule

Mountain

Delaho

bolson

Terlingua

fault

Estufa

bolson

Talley Mountain

Elephant Tusk

Cow Heaven

Mountain

CHRISTMAS

S I E R R A

D E L 

C A R M E N 

Black

Mesa

Agua Fria

Mountain

BLUE

RANGE

Nine

Point

Mesa

MOUNTAINS

MOUNTAINS

C

ELENA ANYON

C H I S O S

M O U N T A I N S

PUNTA DE LA SIERRA

MARISCAL MOUNTAIN

Santiago

Mountains

UNITED ST

ME

ATES

XICO

UNITED STATES

MEXICO

RIO

RIO

GRANDE

GRANDE

103.5° 103.0°

29.5°

29.0°

0 5

0 5 10 KILOMETERS

10 MILES

Figure 2 (below). Generalized map of Big Bend National Park, 

shown by black dotted border. Some of the larger faults that 

developed during the basin-and-range extensional period are 

shown with gray lines. The deepest parts of Estufa and Delaho 

bolsons (where some remnants of the original graben fill are preserved) are shown as yellow-brown areas. Topography is colorgraded from lowlands in green to highlands in dark brown, giving 

a good sense of the height and extent of the Chisos Mountains. 

Paved highways are shown in red.
4 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Great Plains have remained relatively unchanged for the last 

65 million years, except that they have been uplifted to their 

present height from lower elevations probably in the last 5 

million years. The Basin-and-Range province is characterized 

by linear parallel mountain ranges, deep sediment-filled 

valleys, and high structural and topographic relief. The 

eastern part of the province is at a slightly higher average 

elevation than the plains (fig. 1). The province is known for 

its complex patterns of Cenozoic faulting. Today it bears little 

resemblance to the way it was during the Paleocene when the 

entire Trans-Pecos region was a simple lowland that was near 

or slightly below sea level.

The geologic history of Big Bend National Park has 

been punctuated by several distinct episodes of deformation, 

sediment aggradation, volcanic eruption and intrusion, and 

denudation. 

1) There were numerous deformational episodes between 

the Neoproterozoic and the end of the Permian. Evidence for 

some of these events is present in the Paleozoic strata exposed 

in the northern part of Big Bend National Park (Muehlberger 

and Dickerson, 1989), but all the rocks old enough to be 

deformed by these events lie buried by younger deposits in 

most other places, including the Chisos Mountains and they 

are not discussed further in this report. 

2) The Paleocene and Cretaceous was a time when thick 

shallow-water marine and coastal sediments were deposited 

within and on the edges of an interior seaway system that 

was widespread in many parts of southwest Texas, northern 

Mexico, central New Mexico, and the Rocky Mountains 

(RMAG, 1972, Lehman, 1991). The seaway deposits are the 

oldest rocks exposed in the Chisos Mountains.

3) The Cretaceous and Paleocene rocks were folded and 

faulted during the Laramide orogeny throughout Trans-Pecos 

Texas (Muehlberger, 1989; Muehlberger and Dickerson, 1989; 

Lehman, 1991). The Laramide ended at about 46–47 Ma in 

the trans-Pecos region (Harlan and others, 1995). Lehman 

(1991) described the local deposits associated with the 

Laramide orogeny, but none of these are present in the Chisos 

Mountains. 

4) The end of Laramide folding corresponded to the 

beginning of a prolonged influx of volcaniclastic sediment 

and lava flows in the Chisos Mountain region that continued 

through the remainder of the Eocene into the early Oligocene. 

I call this the Chisos episode, named after the Chisos Formation of Maxwell and others (1967). The source of the volcanic 

material has not been identified.

5) A relatively short, but intense episode of local volcanism began at 32–33 Ma in the Chisos Mountains and lasted 

a few million years at the longest. Several large intrusive and 

eruptive centers as well as widespread flows are preserved in 

the range. I refer to this simply as the eruptive and intrusive 

episode. 

6) Big Bend National Park encompasses at least two large 

grabens, known as the Estufa and Delaho bolsons that formed 

during the Miocene Basin-and-Range extension. Although 

no deposits are preserved in the Chisos Mountains, remnants 

of the graben fill have been described from either side of the 

range (Dickerson and Muehlberger, 1994). 

7) Big Bend National Park has been the site of regional 

denudation since the introduction of the Rio Grande around 

the end of the Miocene. Short, local events of sediment 

aggradation occurred amid considerable regional downcutting 

and erosion resulting in a broad spectrum of different surficial 

deposits preserved throughout the Chisos Mountains. This 

was the denudational episode.

Chisos Mountains Geology

Most geographic locations mentioned in the following 

discussion can be found on figure 3. Figure 4 is a diagrammatic summary of the most widespread stratigraphic units 

present in the Chisos Mountains.

Seaway Deposits

The deposits of the seaway and associated coastal 

plains that are exposed in the parts of the Chisos Mountains 

described in this report include marine limestone of the 

Cenomanian to lower Coniacian Boquillas Formation (Kb); 

marine shale of the late Coniacian and Santonian Pen Formation (Kp); marine shale and fluvial sand of the Campanian and 

Maastrichtian Aguja Formation (Kag); fluvial, lacustrine, and 

overbank mudstone and sandstone of the Maastrichtian Javelina Formation (Kj); and fluvial sandstone of the Maastrichtian 

and Paleocene Black Peaks Formation (TKbp). Cretaceous 

and Paleocene beds are mostly exposed around the periphery 

of the Chisos Mountains south of Punta de la Sierra, and north 

and west of Ash Spring and Ward Mountain. Some are also 

exposed high in the range near The Basin and in the core of a 

small structural dome near Tortuga Mountain.

Seaway and related deposits once covered the entire area 

in a thick sheet as the Big Bend region slowly subsided at 

about the same rate that sediment accumulated, allowing the 

land surface to remain near sea level for a long time period. 

At times the area fluctuated between being just below or 

slightly above sea level, but overall the area went from shallow 

marine to lowland nonmarine with time. This offers evidence 

that the influx of sediment eventually overwhelmed and filled 

the slowly sinking basin. 

The best example of marine deposits left in the Chisos 

Mountains are the thin-bedded limestone, chalk, and marl 

of the Boquillas Formation (Kb) in The Basin and Laguna 

Meadow areas. Nautilus, ammonites, fish (including sharks), 

and marine reptiles once thrived in the waters while these beds 

were forming (Maxwell and others, 1967). 

Remnants of claystone and fine-grained sandstone of 

the Pen Formation (Kp) overlie the marine limestone west 

of Emory Peak and in the core of the anticline near Cow 

Heaven Mountain (see fig. 2 for location). Most likely a large 

river system, draining a wide area of land nearby, dumped its 
Chisos Mountains Geology 5

29°07’30’’

29°15’00’’

103°15’00’’

0

0 5 KILOMETERS

1 2 3 4 5 MILES

1 2 3 4

Panther Junction

Basin Junction

K-Bar

Park Headquarters

Wilson

Ranch

Map edge (plate)

BLUE

FRESNO

SMOKY

CREEK

CREEK

CANYON

CANYON

JUNIPER

PINE

CREEK

Glenn

Spring

Dominguez

Spring

Fresno

Spring

Waterworks

area

Rough

Spring

Ash

Spring

Croton

Spring

Ward

Spring

Dodson

Spring

Smokey

Spring

Dominguez Spring

Trail

Elephant Tusk Trail

Mule Ears Viewpoint Trail

Dodson Trail

Dodson Trail

Juniper

Canyon

Trail

S o u

t

h

R i m

Emory Peak

Tortuga

Mountain

Talley

Mountain

C h i l c o t a l

M o u n t a i n

Elephant

Tusk

Backbone

Ridge

Dominguez

Mountain

S i e r r a

Q u e m a d a

Casa Grande

Toll

Mountain

Ward Mountain

Pulliam Bluff

Lost Mine Peak

Crown Mountain

Pummel Peak

Panther Peak

Nugent Mountain

The

Window

The

Basin

Laguna

Meadow H a y e s R i d g e

P u n t a d e

l a S i e r r a

Figure 3. Generalized map of the Chisos Mountains. Topography is color-graded from lowlands in pale green to highlands in dark brown. 

Paved highways are shown in red, well-traveled gravel and dirt roads in gray, and trails in black dotted lines. 
6 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Figure 4. A simple stratigraphic column for the Chisos Mountains. Stratigraphic names shown for Cretaceous 

and Paleocene rocks are widely accepted. The Chisos Formation as shown is restricted to tuffaceous rocks 

and intercalated lava flows beneath the ignimbrites of the South Rim Formation, a convention not followed 

by all workers. South Rim Formation units as shown are not universally agreed upon, but they correspond to 

those shown on the geologic map (see plate). 

Pen

Boquillas

Aguja

Javelina

Black Peaks

Chisos

Fresno-Creekfacies group

Smoky-Creek-facies

group 

Eocene Oligocene Paleocene

CRETACEOUS

Upper Cretaceous

TERTIAR

Y

QUATERNARY

South Rim

and equivalent

rocks

ERUPTIVE AND

INTRUSIVE EPISODE

SEDIMENTARY AND VOLCANIC INFLUX

(CHISOS EPISODE)

CRETACEOUS AND PALEOGENE SEAWAY GREAT

DENUDATION

Surficial

(no formal names)

Pine Canyon

Rhyolite

Member

Wasp Spring/

Boot Rock mbr.

(informal)

Lost Mine

Member

Alamo Creek Basalt Member

Ash Spring Basalt Member

Bee Mountain Basalt Member

Unconformity

(no deposition)

Trachyandesite

of Punta de la

Sierra

FORMATION THICKNESS

(meters)

450

800

250

250

400

180

Not fully exposed

SYSTEM

Series

EPISODES
Chisos Mountains Geology 7

debris into the seaway during Pen time. Fossil ammonites and 

pelecypods attest to marine conditions prevailing over most of 

this period (Maxwell and others, 1967).

An influx of silt and sand overwhelmed the region, and 

the remains of the resultant deposits of the Aguja Formation 

(Kag) are now exposed in the anticline near Cow Heaven 

Mountain, along the main highway northwest of the Chisos 

Mountains, and at several locations in The Basin. Marine 

fossils are present in the oldest of the sand beds, but dinosaur 

bones, turtle remains, and fossil wood in the younger beds 

show that the unit became nonmarine with time (Maxwell and 

others, 1967). Thus, it appears to have evolved from a delta 

into a river flood plain as marine waters regressed from the 

area. This unit is evidence that the seaway simply filled slowly 

with clastic material that eventually displaced the seawater. 

The Chisos Mountains region probably remained above 

sea level to the present day once the Cretaceous seaway waters 

finally receded. Deposits of claystone with minor amounts 

of sandstone of the Javelina Formation (Kj) cover the delta 

and flood-plain sands in a thick sheet. Although not actually 

preserved in the Chisos Mountains, these beds are known from 

many other places in Big Bend. Volcanic ash from volcanoes 

far away provided most of the material that made up the clay 

that accumulated in widespread mudflats. The sand probably 

originated in the channels of small rivers that coursed over the 

mudflats. Dinosaurs wandered the forests that covered these 

flats (Maxwell and others, 1967). 

River sand and tuffaceous clay of the Black Peaks Formation (TKbp) are present in small outcrops to the northeast and 

northwest of the Chisos Mountains and probably once covered 

the entire area as well. Fossils of various types of mammals 

are described from the river floodplain deposits of this unit 

(Maxwell and others, 1967). 

Laramide Orogeny

The slow subsidence and relatively continuous deposition 

in the marine seaway was interrupted at some time in the late 

Paleocene or early Eocene by the events of folding and uplift 

associated with the Laramide compressional tectonism. Strata 

of early Eocene to middle Eocene age, the Hannold Hill and 

Canoe Formations have been described elsewhere in Big Bend 

as Laramide-related deposits (Lehman, 1991). Neither of these 

units is preserved in the Chisos region. The Laramide deformation has been described as being Late Cretaceous to middle 

Eocene in age in parts of Trans-Pecos Texas (Muehlberger 

and Dickerson, 1989; Lehman, 1991), but the nearly continuous subsidence and seaway deposition suggests that it did not 

affect the Big Bend area until late in this time period. The end 

of Canoe Formation deposition has been reported to coincide 

with the end of the Laramide orogeny, locally at about 50 Ma 

(Lehman and Busby, 2007).

One Laramide fold, a large anticline, occurs on the 

geologic map between Elephant Tusk and Cow Heaven 

Mountain and there is a similar one at Mariscal Mountain just 

south of the map. The folding began in the Late Cretaceous 

(Lehman, 1991) and was almost over by the time large sills 

were intruded at about 47 Ma (Harlan and others, 1995). The 

folds in Big Bend all have long axes that are oriented northnorthwest. The folds created a landscape dominated by low 

northwest-oriented ridges and valleys and this landscape still 

existed when the first volcanic rocks erupted in the region.

Chisos Episode—Volcanic and Sedimentary 

Influx

Volcanism was widespread in the southwestern United 

States and northwestern Mexico, and the stippled pattern on 

figure 1 shows how extensive it was. Big Bend lies within 

a large volcanic field known as the Trans-Pecos volcanic 

belt (Henry and others, 1991). The Trans-Pecos belt is the 

easternmost of several large contemporaneous volcanic fields 

that include the Mogollon Datil field, the Sierra MadreOccidental fields of Sonora and Chihuahua, and numerous 

others scattered throughout the Sonoran section of the Basinand-Range (fig. 1). It has been widely acknowledged by 

volcanologists for a number of years that all this volcanism 

is unusual for two reasons: it is abnormally widespread and 

much of it occurred far inland of the active continental edge at 

the Pacific Ocean. By comparison the Andean arc is only half 

as wide as the volcanic fields of Mexico and the southwest 

United States and the most inland of the volcanoes in South 

America are less than half as far from the Pacific as the TransPecos fields are today. Possibly not all of the volcanism was 

arc-related. Henry and others (1991) speculated that post-31 

Ma volcanism resulted from intraplate extension as opposed 

to continental-arc volcanism, which dominated prior to then. 

The big problem with an extensional origin of volcanism 

in the Chisos Mountains is the tiny volume of post-31 Ma 

volcanic material (Dan Miggins and others, 2008) and the lack 

of evidence for any pre-Miocene extension (Dickerson and 

Muehlberger, 1994). Thus, the volcanism is probably arcrelated but its present distribution is probably influenced by 

the later extension. 

The aforementioned mafic sills and the oldest mafic lava 

flows in the area indicate that volcanism had begun in Big 

Bend by 46–47 million years ago (Dan Miggins, USGS and 

others, 2008). Voluminous tuffs and tuffaceous sedimentary 

rocks of the Chisos Formation indicate significant regional 

siliceous volcanic activity during the Eocene, but local sources 

for the volcanic material have not been identified. The Chisos 

episode is clearly part of the first wave of volcanism in TransPecos Texas. 

The volcanogenic sandstone and tuff of the Chisos 

Formation have for a long time been thought of as being fairly 

uniform and nondescript in character. Figure 5, which is a 

diagrammatic representation of the Chisos facies mapped in 

this report, suggests that this is far from true. Two groups of 

facies are evident on the geologic map and on figure 5. The 

older and more regionally restricted group comprises all those 
Folded Cretaceous and Paleocene rocks Ward-Mountain/

Dominguez Line

Ward-Mountain/

Dominguez Line

Folded Cretaceous

and Paleocene rocks Fold

and 

Sierra Quemada

dikes and plutons

Gabbro

sill

Tcl Tcnr

Tcstr

Tcstr

Tcnr

Tcks

Tcwt

Teslm

Tcksu

Tcwt

Tfacb

Sill?

Tfbmb

Tetps

Tcyot

Tfra

Tfbu

Tortuga dome

Tcrt

Granite

Granite

Sierra

Quemada

Punta

de la

Sierra

Fresno

Creek

South

Rim

Figure 5. Complex facies relations within the Chisos Formation and eruptive-phase rocks. This diagrammatic view is meant to convey ships only and it does not represent a rigorously constructed cross-section line through any one particular area. There is also a consivertical exaggeration. Unit designations are from the geologic map as: Tcrt, rhyolite and tuff facies; Tcnr, nodular rhyolite facies; Tclsiltstone facies; Tcstr, sandstone, tuff, and rhyolite facies; Tfacb, Alamo Creek Basalt Member; Tcwt, white tuff facies; Tcyot, yellowrhyolite ash-flow facies; Tcksu, upper siltstone facies; Tfbmb, Bee Mountain Basalt Member; Tetps, trachyte of Punta de la Sierra; TLost Mine Member of South Rim Formation. The fill and outflow from Pine Canyon are listed as Tesbr, lower fill and outflow (Boot Rocupper fill and outflow (Pine Canyon Member).
Chisos Mountains Geology 9

facies beneath the white tuff facies (Tcwt) and the Alamo 

Creek Basalt Member of the Chisos Formation (Tfacb). This 

group, which was called the eastern facies by Maxwell and 

others (1967), only occurs in the Fresno Creek drainage and 

the eastern Pine Mountain area and it is called the Fresno 

Creek group in this report. The younger and more widespread 

group, which was called the western facies by Maxwell and 

others (1967), is known from many other parts of Big Bend 

in addition to the Chisos Mountains and it is also present in 

the northeastern part of the range. It is the thickest and best 

exposed in the vicinity of Smoky Creek west of Sierra Quemada and it called Smoky Creek group in this report. The map 

distribution of these two groups is shown on figure 6.

Fresno Creek-Facies Group

Two members of the Fresno Creek-facies group (Tcrt, 

and Tcl on fig. 5) are restricted to a band of exposure that 

extends from the flanks of the dome at Tortuga Mountain 

to the northeast flank of Dominguez Mountain and they are 

not known from anywhere else, so they are referred to as the 

localized part of the Fresno Creek-facies group (fig. 6). Their 

age is poorly constrained at best. They unconformably overlie 

folded Cretaceous units, but they too are folded by almost 

as much as the older rocks, so they must predate some of 

the folding and are probably a little older than 47 Ma. They 

underlie a sequence of lava flows (Tfbu), but those flows 

are highly altered and remain undated. These facies may be 

equivalent in age to the aforementioned Canoe Formation that 

was deposited elsewhere in the Park during the latter stages of 

Laramide folding (Lehman, 1991), but no positive correlation 

has been established. 

Rhyolitic tuff and 50 m of very well bedded tuff and 

mudstone comprise the two localized facies. The rhyolitic tuff 

indicates there was active volcanism during the early stages 

of Chisos deposition, but the source of this tuff is not known. 

Slight angular unconformities are present within and above 

this section indicating that deformation was occurring while 

these rocks were being deposited. The unconformities result 

in thinning of each facies in proximity to Tortuga dome (see 

fig. 5). The dome, which is cored by thermally metamorphosed Cretaceous rocks, appears to have been a small, but 

growing topographic high at the time the older Chisos facies 

were laid down and it acted as a barrier that restricted the 

distribution of tuffs and flows. The map relations suggest that 

the two localized facies accumulated discontinuously in low 

areas associated with folds in the underlying Cretaceous strata, 

probably as fine unconsolidated air-fall tuff washed into the 

low areas from the surrounding countryside. The fine-bedded 

tuff may have accumulated in a small lake or closed depression that formed near the rising dome. 

The more widespread members of the Fresno Creekfacies group (Tcnr, Tcstr, and Tcks) are known from the 

Fresno Creek, Pine Canyon, and northwestern Ward Mountain areas, but nowhere else (figs. 5 and 6). They surround 

the periphery of the Chisos Mountains, but they do not occur 

within the high parts of the range. Northeast of Pine Canyon 

these beds exhibit radial dips inward toward the center of the 

high mountain mass. Near Fresno Creek they once completely 

covered the Tortuga dome, but they exhibit slight arching coincident with the old dome, showing that folding was not quite 

finished as these units accumulated. The pronounced thinning 

in proximity to the dome, evident in the two localized facies, is 

not conspicuous in these overlying units however. Thus active 

Laramide-fold growth must have been greatly diminished, but 

not over, by the time they were deposited. The radial inward 

dips around Pine Canyon caldera are also present in younger 

rock units, so they are assumed to be caused by post-Laramide 

sagging associated with caldera collapse. 

Various rhyolite flows, ash falls, and volcanogenic sandstone beds comprise the widespread members of the Fresno 

Creek-facies group throughout the Fresno Creek drainage, 

and surrounding the north and east sides of the Pine Canyon 

region. West of Juniper Canyon thick local mudstone beds 

are also present. Mostly these lithologies reflect a volcanic 

origin, but not the mudstone. Cretaceous mudstone is widespread southeast of Juniper Canyon, and it probably provided 

the source for similar beds in the Chisos. Some of the Chisos 

mudstone is thermally hardened, making it easy to distinguish 

from its source rock, but where unaltered it is nearly identical. 

The mudstone facies is in fault contact with its Cretaceous 

source, offering a likely cause for the uplift and erosion of the 

source area. Much of the rhyolite most likely originated as 

air-fall tuff or possibly ash flows, but it is extensively recrystallized, making it hard to determine the nature of the original 

deposit. As it accumulated, it was instrumental in filling in and 

smoothing out the remnants of topography left over from the 

earlier folding events.

The Fresno Creek-facies group extends no farther west 

than Dominguez Mountain or eastern Sierra Quemada (fig. 

6). It clearly locally underlies a basalt sill and the facies of 

the Smoky Creek-facies group in the Fresno Creek area, and 

it is beneath the Smoky Creek-facies group everywhere east 

of there (fig. 7 and geologic map, plate). West of Dominguez 

Mountain, the Smoky Creek-facies group and the Alamo 

Creek Basalt Member (Tfacb) rest unconformably on Cretaceous beds at Punta de la Sierra. Thus, the entire Fresno 

Creek-facies group is obviously missing at Punta de la Sierra, 

due either to erosion or non-deposition (see figs. 5 and 6). 

Younger intrusive rocks and sediments obscure any details 

regarding this poorly resolved transition, but it is apparent 

everywhere to the west of a crude north-south line that extends 

from southern Ward Mountain to the west side of Dominguez 

Mountain (fig. 6); referred to as the Ward/Dominguez line. 

Smoky Creek-Facies Group

The Ward/Dominguez line also delineates important differences in the Smoky Creek-facies group. West of the line 

the Smoky Creek-facies group is thick, and it includes several 

important lava flows, but east of the line it is thin and the lava 

flows are either not present or very thin and discontinuous. 
10 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Figure 6. Simplified geologic map of the Chisos Mountains that shows the distribution of the Fresno Creek-facies group of the 

Chisos Formation (divided into localized and widespread components) relative to the Smoky Creek-facies group. (Note that the 

Smoky Creek-facies group extends well beyond the Chisos Mountains in the southwest direction). 

103°22'30'' 103°15'00''

29°15'00''

29°07'30''

0

0 5 KILOMETERS

1 3 MILES

1 2 3 4

2

Surficial deposits

AS Ash Spring

DM Dominguez Mountain

PC Pine Canyon

PS Punta de la Sierra

SQ Sierra Quemada

TB The Basin

TM Tortuga Mountain

WM Ward Mountain

Intrusive rocks (all ages)

Eruptive cycle flows

Smoky Creek

 facies group

Bee Mtn. Basalt

 Member flows

Ash Spring Basalt

 Member flows

Widespead Fresno

 Creek facies group

Basalt sill 

Locallized Fresno Creek

 facies group

Paleocene and Cretaceous substrate

Chisos Formation

Ward/Dominguez Line between

 Fresno-Creek and Smoky-Creek

 facies groups

Alamo Creek

 Basalt Member

DM flows

PS

TM

Fresno Creek

Blue Canyon

WM

PC

AS

TB

SQ

EXPLANATION
Chisos Mountains Geology 11

These differences were apparent to Maxwell and others (1967) 

who attributed them to the influence of a buried anticline in 

the Cretaceous rocks that they thought extended from The 

Basin area south to Dominguez Mountain. The evidence for 

or against an anticline is slim (pl.), but the major stratigraphic 

differences in the Chisos are real. In addition, numerous faults, 

the west edge of Dominguez Mountain, the east edge of Sierra 

Quemada, and the east edge of the Ward Mountain pluton, all 

lie along this same line (fig. 6). However poorly understood, it 

is doubtless an important feature. 

White tuffaceous sandstone (Tcwt) makes up most of 

the Chisos Formation southwest of the Ward/Dominguez line. 

This facies is also present east of the line, where it is much 

thinner and it interfingers in a complex way with mudstone 

and brown sandstone (fig. 5). The white tuffaceous sandstone 

is highly visible from many roads, especially the road to 

Castalon, so it is the facies that is most commonly thought of 

as being synonymous with the Chisos Formation. A distinct 

marker horizon of yellow-orange tuff (Tcyot) is interbedded 

with the upper part of the white tuffaceous sandstone facies. 

This tuff sequence is well exposed in Blue Creek Canyon 

along the Blue Canyon trail. It forms a more-or-less continuous band of outcrop to the east of there to an eventual pinchout beneath the eastern South Rim (pl.). It is also exposed in 

the northeast wall of Juniper Canyon and in a small outcrop 

north of Pine Canyon. The tuffs show that volcanic activity 

was ongoing somewhere nearby, but the source of the volcanic 

material is not known. Much of the material may have come 

from centers outside of the mapped area, although even that 

is not clear. More work is needed in order to unambiguously 

define source areas.

The Bee Mountain Basalt Member (Tfbmb) occurs in 

the upper part of the Chisos Formation, but it is not present 

everywhere the upper Chisos is found. It is also highly variable 

Smoky-Creek-facies group

(Tcwt) Fresno-Creek-facies group

(Tcstr)

Basalt sill (Tsbu)

Basalt sill (Tsbu)

Figure 7. Photograph to the northwest from Fresno Creek showing the Fresno Creek-facies group underlying a basalt sill (Tsbu) 

that has been dated at about 30 Ma (level dark band of rock crossing mountain front in background) that may have originated in 

the Dominguez volcano. The sill is overlain by white tuffs of the Fresno Creek-facies group (Tcwt) and underlain by brown-colored 

sandstone, tuff, and rhyolite (Tcstr) of the Fresno Creek-facies group.
12 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

in thickness. The thickest and best-exposed Bee Mountain 

Basalt Member in the Chisos Mountains occurs in Blue Creek 

Canyon area (fig. 6) and it gets thinner southward to where 

it locally pinches out near Punta de la Sierra. Similar basalt 

flows are present in the high Chisos Mountains, but these are 

thin and discontinuously present. In some places, notably the 

Juniper Canyon area, two thin flows occur that are separated 

by tuffaceous beds. 

Figure 8. Simplified map showing the distribution of the rocks of the eruptive episode. Dotted lines outline 

some of the larger intrusive and eruptive centers. 

ERUPTIVE EPISODE

in the

Chisos Mountains

showing the

main volcanic centers,

erupted rocks (green),

intruded rocks (red), and

older and younger rocks (yellow)

SIERRA

QUEMADA

DOMINGUEZ

MOUNTAIN

WARD

MOUNTAIN

Tortuga

Mountain

Dodson

Spring

Elephant

Tusk

Backbone

Ridge

Punta de la Sierra

Trachyandesite flows

Boot Rock

Hayes Ridge

Lost Mine

Peak

Pullman

Bluffs

Panther

Junction

K-Bar

Nugent

Mountain

Juniper

Draw

Rough

Spring

Chisos

Basin

South Rim

PINE

CANYON

103°22'30'' 103°15'00''

29°15'00''

29°07'30''

0

0 5 KILOMETERS

1 3 MILES

1

2

2 3 4
Chisos Mountains Geology 13

Eruptive and Intrusive Episodes

Calderas, failed calderas, and volcanoes have been 

described in the Pine Canyon area, at Sierra Quemada, and 

at Dominguez Mountain, respectively (fig. 8). Numerous 

eruptions resulted in widespread lava flow and ash flow sheets, 

particularly at South Rim and Punta de la Sierra. The violent 

ejection of magma from deep beneath Pine Canyon caused the 

collapse of a near-circular caldera, which filled with a thick 

sequence of hot pyroclastic debris (Barker and others, 1986). 

Large intrusive bodies, such as the one at Ward Mountain, 

and many smaller ones formed then. Some wide dikes outline 

the remnants of a failed caldera at Sierra Quemada where a 

complex ring of wide dikes (Tdsq) is preserved (Scott and 

others, 2007). A thick sequence of lava flows of trachyte 

accumulated at Punta de la Sierra. A small volcano, defined 

by flows, dikes, and ring-shaped intrusions, at Dominguez 

Mountain may have been the source of the trachyte flows, but 

this has yet to be demonstrated. All of these features and more 

are shown on figure 8. 

The largest eruptive center was in the Pine Canyon area 

where a nearly circular collapsed caldera 6–7 km in diameter 

formed as a result of an explosive eruption from a deep 

magma source (fig. 8). The outflow and caldera fill associated 

with this eruption are collectively referred to as the South Rim 

Formation in this report. The collapsed area in the core of the 

caldera is revealed by the 10°–40° inward dips of Chisos beds 

surrounding the feature and by the thick local accumulation 

of ignimbrites overlying the Chisos. Collapse apparently 

occurred by sagging of the Chisos in the core area, not by 

faulting, since faults are not in evidence (Barker and others, 

1986). The bowl-shaped sag is evidence for the evacuation of 

a considerable amount of magma from beneath the core area 

during several violent eruptions. The collapsed center filled 

with its own eruptive debris, so the net result of much of the 

activity was a simple transfer of most of the deep magma to 

the surface where it filled its own collapse depression. The 

caldera fill is of two different types. The oldest fill is rhyolitic 

ash-flow tuff that is at least 300 m thick in Pine Canyon 

proper and it is mapped as a member of the South Rim, the 

Pine Canyon Rhyolite Member (Tespc), in this report. The 

youngest is a thick sequence of flow breccia with a densely 

welded ignimbrite matrix that is well exposed in the Lost Mine 

Peak area and is mapped as the informal Boot Rock member 

(Tesbr) in this report. 

Some outflow, primarily from the younger eruptive 

episodes, managed to escape the caldera confines to flow 

southwest to the South Rim area. Outflow from Pine Canyon 

caldera covered the upper Chisos Formation with a thin 

deposit of non-welded ash flow, tuff, and breccia at least as far 

to the southwest as the modern edge of South Rim. The latter 

flows are included in the Boot Rock member (Tesbr)in this 

report. Eruptions continued after the caldera had completely 

filled and these resulted in a thick ash-flow sequence, mostly 

of quartz trachyte, that now covers most of the South Rim 

plateau. The trachyte, which is referred to as another member 

of the South Rim Formation, the Lost Mine Member (Teslm), 

in this report, forms welded-and-nonwelded ash-flow tuffs and 

local lava flows that overlie the early outflows. The tuffs and 

flows are mostly unstratified and they resist erosion, forming 

the steep cliffs that define modern edge of South Rim. 

A small volcanic center at Sierra Quemada (fig. 8) began 

to develop much the same way Pine Canyon probably did, 

but there is no evidence that it ever exploded, collapsed, and 

filled (Scott and others, 2007). All that remains of the crudely 

oblong center is a collection of dikes, stocks, and flows intruding the Chisos. The older Chisos beds that now occupy the 

interior of Sierra Quemada would have been blown away had 

this center suffered a major explosive, caldera-forming eruption accompanied by a rapid evacuation of magma. Here there 

is no pool of young erupted flows in the volcanic center like 

at Pine Canyon, prompting Scott and others (2007) to describe 

Sierra Quemada as a failed caldera. Since there is no evidence 

of a large-scale eruption at Sierra Quemada it is not considered 

to be the source of any of the large flows in the region.

A small volcano formed at Dominguez Mountain (fig. 9) 

and numerous vertical dikes radiate into the country rock to its 

southwest. Dominguez Mountain still preserves the original 

cone shape of this volcano. The volcano at Dominguez Mountain has not been studied in any detail, but good descriptions 

exist for the rocks that make it up (Maxwell and others, 1967). 

Small granitic intrusive bodies surround the Dominguez 

volcano, and its interior is a mix of layered rocks and dense 

dike swarms (fig. 9). The layers dip gently outward from the 

volcanic center and the dikes cut them vertically. The layered 

rocks in the volcanic core have not been studied. They might 

be related one of the Chisos facies (for example, Maxwell 

and others, 1967), but they are much darker gray in color than 

any of the known Chisos facies and field examination of float 

at the mountain base did not reveal any Chisos-like samples. 

Most likely the layered rocks originated as lava flows forming 

the flanks of the volcanic buildup. The dike swarms (Tdds) 

form large bodies of rock in which dikes intrude other dikes to 

the extent that country rock is nearly absent. The dikes include 

both mafic and intermediate rock types, with intermediate 

types being more prevalent. Most dikes radiate outward from 

the volcanic center. Several intrude the east half of the volcano 

and a small number of dark-colored dikes extend into the 

country rock to the north. The most conspicuous dikes intrude 

all the rocks to the southwest of the center and they merge 

upward into flows of trachyte at the top of Punta de la Sierra 

(fig. 10). 

Trachyte flows (Tetps) form a thick and widespread 

field of volcanic rock due west of Dominguez Mountain (fig. 

8). This field forms high, mountainous ground that extends 

northwest from Punta de la Sierra to beyond the western edge 

of the map (pl.). The source of these flows is not known, but 

their relation to the Dominguez dikes suggests that Dominguez 

Mountain should be considered as a possibility. 

A voluminous intrusive center formed at Ward Mountain 

and this intrusion now partially encircles The Basin to the 

north and west. There are also numerous small plutons, stocks, 
14 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

sills and dikes spread throughout the Chisos Mountains that 

relate to the various volcanic centers in poorly understood 

ways. The most prominent of these occur at Backbone Ridge, 

Elephant Tusk, Tortuga Mountain, Dodson Spring, Rough 

Spring, Boot Rock, Hayes Ridge, southwest of Panther Junction, in the Nugent Mountain/K-bar area, and near Juniper 

Draw (fig. 8). 

The pluton at Ward Mountain (Tiwm) is a large 

C-shaped body (fig. 8) that is a complex mix of flow-banded 

rhyolite and fine-grained granitic rock types. These rocks are 

very resistant to erosion and they uphold all the steep cliffs 

and mountainous areas around Ward Mountain west of The 

Basin and in the Pulliam Bluffs area to the north (fig. 11). 

Other small intrusions are scattered from Backbone Ridge 

in the south to Rough Spring in the north, wrapping around 

the east side of the range (fig. 8). Small stocks at Backbone 

Ridge and Elephant Tusk are probably eroded volcanic necks 

that once were the roots of small volcanoes. Small laccoliths 

occur at Rough Spring, Dodson Spring, southwest of Juniper 

Draw, and southwest of Panther Junction (fig. 8). The one at 

Dodson Spring shows evidence that the injection took place 

by the intrusion of one dike after another in the eastern half of 

the body and by one sill after another in the west (fig. 12). The 

dome at Tortuga Mountain is associated with a thick granodiorite sill and a small intrusion in the core of the dome that may 

be the top of a buried laccolith. Some of the doming there may 

be the result of the intrusion uplifting the local country rocks, 

including the sill. Near Juniper Draw at the east-central border 

of the map there is also a thick sill of coarse-grained feldsparrich granitic rock. The numerous small plutons northeast of 

the range were not examined in any detail. 

The eruptive and intrusive episodes took place in the 

early part of the Oligocene (roughly 29–34 Ma) and new Ar/

Ar age data suggest that most of the activity was confined 

to the 32–32.5 Ma timeframe in the Chisos Mountains (Dan 

Miggins, USGS, written commun., 2008; and table 1). The 

new dates from the stratigraphically oldest to the youngest 

outflows at the base of the South Rim to the top of Emory 

Peak almost all fall in the 32- to 32.5-Ma range. The largest 

plutonic body and the older fill of the Pine Canyon caldera fall 

within this range as well, except one sample from the pluton 

that might be 0.2 million years older. Outside of the high 

Flows forming interior

of Dominguez Mountain

volcanic center (Tfdmt)

(possibly eqivalent to

an undetermined

Chisos Formation facies)

Interior dike swarm

of volcanic neck

at Dominguez Mountain

(Tdds)

Dominguez Mountain

ring dike

(Tdds)

Figure 9. Photograph of Dominguez Mountain taken from the southwest flank. 
Chisos Mountains Geology 15

Chisos, a small body of explosion breccia (Teeb) from Sierra 

Quemada dated in the 32.0- to 32.5-Ma range, but another 

similar sample of it came out at 29.5 Ma. Most of the isolated 

small plutons are in this range, except Nugent Mountain 

(31.5 Ma), a dike at Sierra Quemada (29 Ma), and a sill near 

K-bar that is far out of range at 41 Ma. Trachyte flows in the 

southern parts of the Chisos Mountains have not been dated, 

but similar flows near Castalon are between 30.0 and 30.5 

million years old. 

The precision of the age data is not yet good enough 

for us to sort out the sequence of events with any degree of 

confidence within this short time period. Outflow stratigraphy 

remains our best tool, but that does not help much if we wish 

to relate plutons to flows or if we want to describe isolated 

volcanic centers relative to one another. Earlier workers 

thought the pluton of Ward Mountain (Tiwm) post-dated all 

the flows in the high Chisos, but at least two ages (32.4 and 

32.7 Ma) indicate it may be slightly older than all of them. 

Sierra Quemada may be the youngest local center if the 29 Ma 

ages there prove to be accurate (at least one other age from 

that center falls in the 32.0–32.5 Ma range). Possibly Dominguez Mountain and the trachyte flows in the south formed 

between the two time periods. More dates are necessary if any 

of this is to become well established. 

Basin-and-Range Extension

The rifting in the Sonoran section of the Basin-andRange is mostly Miocene in age, beginning about 23 Ma 

(Dickerson and Muehlberger, 1994), or roughly 5 m.y. after 

the last major volcanic eruptions in the Big Bend area. There 

can be no denying the spatial coincidence of extension and 

volcanism in this part of North America, but any temporal 

connection is dubious. Extension was probably facilitated by 

heat from the earlier volcanism rather than being causal to it. 

Figure 1 shows that Big Bend National Park is within the 

eastern extent of the Basin-and-Range near the southern terminus of the Rio Grande rift. Two large grabens in the park form 

the lower country on either side of the Chisos Mountains. To 

the northeast is the Estufa bolson, between Panther Junction 

and Sierra del Carmen, and to the southwest is the Dehlaho 

bolson between Punta de la Sierra and Sierra Ponce (fig. 2). 

Both of these grabens filled with sediment during Miocene 

time. The oldest sediment in Estufa bolson, which is largely 

Trachyte flows of

Punta de la Sierra (Tetps)

Dikes emanating from

Dominguez Mountain

volcanic center (Tdds)

Chisos

Formation

(Tcwt)

Alamo Creek Basalt

Member of Chisos Formation (Tfacb)

Figure 10. Photograph of the southeast point of Punta de la Sierra taken from the southeast. The large near-vertical dikes are part of 

the complex, southwest-trending swarm of dikes that emanate from the Dominguez Mountain volcanic center. Many of these dikes 

merge upward into the trachyte flows that form most of the high mass of Punta de la Sierra. Flows and dikes both probably formed 

during eruptive episodes at Dominguez Mountain. Trachyte flows are about 30 Ma to the south of Punta de la Sierra (Dan Miggins, 

USGS, written commun., 2005)
16 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

conglomerate, has been dated at about 10 Ma and the youngest 

fill is Pliocene (Dickerson and Muehlberger, 1994). Some of 

the faults that bound the graben apparently were active in the 

Quaternary and the hot springs near Rio Grande Village are 

probably related. The Delaho bolson began in the Miocene as 

early as 23 Ma. Only remnants of the oldest graben fill remain 

in Big Bend National Park, owing to later erosion related to 

the incision of the Rio Grande in Quaternary time. The original fill was at least 340 m thick at one time (Dickerson and 

Muehlberger, 1994). 

The Sierra del Carmen, which flanks the Estufa bolson 

to the east, is a large range consisting of tilted fault blocks 

primarily formed of Lower Cretaceous limestone that was 

originally deposited in the Cretaceous seaway. One of the most 

notable normal-fault escarpments in the area flanks the southwest side of this range near Boquillas del Carmen, Mexico. 

The amount of extension represented by this faulting is very 

small compared to other places in the Basin-and-Range, but 

the range front is topographically impressive.

The southwest edge of Dehlaho bolson occurs at Sierra 

Ponce, a large southwest tilted block of Lower Cretaceous 

limestone. The block is tilted gently to the southwest and 

steep, prominent cliffs mark its northeast edge along the 

Terlingua fault (fig. 2) upon which 1,000 m of displacement 

has occurred. 

Denudational Episode

It is not clear whether or when Basin-and-Range deformation ended in the park, as some faults show evidence of 

Quaternary activity and might still be active. However, by the 

Chisos

Mountain

Lodge

Chisos Basin

campground

Laguna Meadow

trail

Laguna Meadow

trail

Pinnacles

trail

Pluton of Ward

Mountain

Pluton of Ward

Mountain

Boquillas

Formation

Aguja

Formation

Intrusive contact

Figure 11. Photograph to the north of The Basin area in the high part of the Chisos Mountains. Several important geologic features are 

labeled in yellow. Important cultural features are labeled in white. The prominent gap in the mountains, known locally as the window, 

is to the left (west) of the view as shown. 
Chisos Mountains Geology 17

end of the Miocene Epoch about 5 million years ago rates of 

deformation had slowed and most of the fault activity was 

over. The grabens were filled with sediment by that time based 

on the meandering course followed by the Rio Grande even 

where it cuts the rocky highlands of Sierra Ponce, Mariscal 

Mountain, and Sierra del Carmen. Regional isostatic uplift 

and the consequent erosion associated with the incision of the 

river removed most of the nearly unconsolidated basin fill in 

fits and starts. Today a complex assemblage of surficial clastic 

deposits remains as the only record of the denudation. The 

surficial deposits occur at many different topographic levels 

and tell a complex story of accumulation and degradation in 

response to factors like climate changes and irregular stream 

erosion. These deposits take the form of rock falls, colluvium, 

wash deposits, alluvial deposits of various ages, landslides, 

slumps, and debris-avalanches, none of which have been 

formally named.

One especially notable mass-movement deposit is found 

on the northwest flank of the mountains just west of Pulliam 

Bluff. This landslide is big enough to be viewed easily from 

space (fig. 13), and it is very complex probably having formed 

in several different events. The rocks within it are almost all 

fractured to one extent or another, but in the higher parts of 

the slide large bodies of rock are still intact enough to map 

as if they were bedrock. Near the downhill end of the slide, 

the rocks are nearly pulverized and different rock types are 

mixed up with one another. An attempt was made on the 

geologic map (pl.) to delineate these areas. Also the primary 

source rocks are identified in places where the amount of 

mixing is not too great. The rocks involved in this slide are 

the Chisos Formation (Alamo Creek Basalt Member (Tfacb) 

and younger) and several units of the eruptive and intrusive 

cycle. All these units slid to the northwest over mudstones of 

Cretaceous age beneath them. The sliding probably occurred 

Subhorizontal

base of pluton

Chisos Formation

sandstone, tuff, and rhyolite facies

(Tcstr) 

Steeply dipping dikes

of cryptocrystalline granite

Steeply dipping dikes

of cryptocrystalline granite

Subhorizontal sills of

cryptocrystalline granite

Figure 12. Photograph to the north of a small pluton of cryptocrystalline granite to the east of Dodson Spring. The pluton is formed 

of dikes that intrude dikes, and sills that intrude sills. The eastern dikes are steep to vertical and the western sills are subhorizontal, 

parallel to the base of the pluton.
18 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Number Unit Age (Ma) Method Sample MapX MapY

1 Terf 32.11 ± 0.11 Ar/Ar 17DM-5-1-03 664884 3236530

2 Tdsrp 32.12 ± 0.10 Ar/Ar 16DM-5-1-03 669644 3236517

3 Tesbr 32.11 ± 0.23 Ar/Ar 32DM-5-4-03 669318 3238420

4 Tesbr 32.23 ± 0.07 Ar/Ar 131DM-2-11-05 668923 3239649

5 Tesbr 32.21 ±0.05 Ar/Ar 132DM-2-11-05 668945 3239603

6 Tesbr 31.93 ± 0.13 Ar/Ar TPC-407 664672 3235973

7 Tesbr 32.43 ± 0.29 Ar/Ar TPC-414 664708 3236872

8 Terf 32.03 ± 0.10 Ar/Ar TPC-315 666268 3238853

9 Teeb 29.5 ± 0.07 Ar/Ar 4BS11.3.05 662742 3230020

10 Tdsq 29 Ar/Ar 2BS22.1.05 662419 3232023

11 Tsbu 31 Ar/Ar 4BS22.1.05 663815 3231737

12 Tetc 32.1 ± 0.3 Ar/Ar 144DM-3-12-05 658849 3233872

13 Tdsrp 41.6 ± 0.4 SHRMP 75DM 677043 3243161

14 Tsf 32.3 ± 0.3 SHRMP EC43 (G-11) 671934 3231401

15 Kags 81.7 ± 1.0 SHRMP 04LS07 667265 3230394

16 Kags 76.8 ± 0.7 SHRMP 04LS06a 667008 3231129

17 Tcyot 41.4 ± 0.5 SHRMP 6DM 659903 3233103

18 Terf 32.3 ± 0.4 SHRMP 16DM-5-1-03 664713 3236435

19 Tdsrp 32.6 ± 0.6 SHRMP 33DM-5-4-30 675552 3237371

20 Tinm 31.5 ± 0.3 SHRMP 128DM 676885 3237714

21 Tiwm 32.4 ± 0.3 SHRMP 138DM 662955 3239942

22 Tiwm 32.7 ± 0.3 SHRMP 139DM 662388 3240360

23 Teeb 32.3 ± 0.7 SHRMP 3BS 662689 3229967

24 Tfvt 32.1 ± 0.3 Ar/Ar 144DM-3-12-05 658880 3233689

25 Tsgb 26-30 SHRMP 04LS09 668244 3229679

26 Tfasb 40.38-43.39 Ar/Ar 2BS27.203 658352 3236702

27 Tfbmb 33.74 ± 0.12 Ar/Ar 153DM-3-18-05 659168 3231740

Table 1. Radiometric age determinations of select rocks in the Chisos Mountains.

[Ar/Ar refers to 40Ar/39Ar method of isotopic dating and SHRMP refers to senstive high resolution ion microprobe 

technique for uranium-lead dating of zircon. MapX is Universal Transverse Mercator NAD 27 horizontal position in 

meters, and MapY is vertical position. Data supplied by Dan Miggins, USGS written commun., 2008.]
Figure 13. View to the southeast of the large landslide in the vicinity of Ash Spring. This rendition, derived from images in Google EarthTM com/), gives an excellent view of the undulating and lobate land surface that characterizes the slide. 

Approximate extentof great landslide

THE BASIN

Chisos Mountain

Lodge
20 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

during a prolonged wet period when the underlying mudstone, charged with groundwater, 

was weakened and gave way under the weight of the overlying rock column. 

Description of Map Units

(Map descriptions of bedrock units are summarized from field observations and from 

descriptions found in various published literature sources. Descriptions of surficial units 

were provided by Margret Berry (written commun., 2006). Color terms are descriptive of 

fresh rock surfaces, unless otherwise noted.) 

Surficial Deposits

Deposits of the denudational episode. Stages of soil-carbonate morphology follow 

criteria outlined in Birkeland (1999), modified from Gile and others (1966, 1981) and 

Machette (1985).

Sedimentary Rocks

Qya Alluvium, undivided (Holocene to latest Pleistocene)—Alluvial fan, pediment, and stream deposits. Unconsolidated to weakly consolidated, 

poorly to moderately well-sorted, poorly to moderately well-bedded 

gravel, sand, and silt. Cobble- and small boulder-size clasts common. 

Clasts typically subangular to subrounded, locally angular or well 

rounded. Bar and swale topography and braided channel morphology 

commonly well preserved. Typically has low relative height, but buries 

older alluvial deposits at some mountain front and up-valley locales. 

Thickness variable from less than 1 m to at least 8 m

Where divisions within Qya are evident and large enough to map separately, unit is 

subdivided into Qya1 and Qya2 as follows: 

Qya1 Younger alluvium (Holocene)—On or within several meters of valley floor. 

Occasionally flooded. Little or no pavement, varnish, or soil-carbonate 

development

Qya2 Older alluvium (latest Pleistocene)—Generally elevated 3 m or more above 

the valley floor. Surfaces generally planar and partly incised. Pavement weakly developed to densely packed and uniform. Clasts with 

pavement are lightly to well varnished; varnish colors typically strong 

brown, yellowish red, dark reddish brown, and black 

Qrf Rock fall deposits (Holocene)—Generally unsorted, angular, cobble- to 

boulder-sized rock fragments forming talus on steep slopes. Thickness 

is 0–3 m

Qc Colluvium and colluvial fans (Holocene)—Unconsolidated to moderately 

consolidated, unsorted to poorly sorted, nonbedded to weakly bedded 

mixture of gravel, sand, silt, and clay. Locally cemented by carbonate. 

Cobble- to boulder-size clasts common. Clasts angular to subangular. 

Forms thin mantle of debris or thicker, fan-shaped colluvial accumulations on flanks of steep slopes. Commonly ribbed or fluted due to 

various combinations of rainwash, sheetwash, creep, and mass-wasting 

processes active on steep slopes. Locally includes bedrock outcrops 

too small to map separately. Thickness variable from about 1 m to over 

6 m

Qe Eolian sand (Holocene)—Unconsolidated, moderately to well-sorted, silty 

fine- to medium-grained sand, locally intermixed with small amounts 

of fine gravel. Very pale brown. Commonly forms small dunes around 

the base of brushy vegetation. Several small occurrences in northwest 
 

Description of Map Units 21

part of map north of Chisos Mountain front. Thickness generally less 

than 1 m

Qs Spring deposits (Pleistocene)—Powdery to well-cemented, white, very pale 

brown, light brownish-gray, or gray, fine-grained, spring-generated 

calcium carbonate mixed with fine sand and silt; locally nodular or 

laminated. One small occurrence southeast of Elephant Tusk. Thickness is not known 

Qia Intermediate age alluvial deposits, undivided (late and middle 

Pleistocene)—Alluvial fan, pediment, and stream deposits. Locally 

includes Qya in deposits too narrow or small to map separately. 

Weakly to moderately well-consolidated, poorly to moderately 

well-sorted, poorly to moderately well-bedded gravel, sand, and 

silt. Cobble- and small boulder-size clasts common to abundant. 

Clasts typically subangular to subrounded, locally rounded. Surfaces 

generally planar, partly incised and slightly rounded at the edges. 

Pavement weakly developed to densely packed and uniform. Varnish 

on pavement clasts ranges from almost none where pavement 

contains mostly limestone clasts, to moderately varnished with strong 

brown, yellowish-red, and dark reddish-brown colors. Typically has 

intermediate relative height, but buries older alluvial deposits at some 

mountain-front locales. Seismic data indicate that on the north side of 

the Chisos Mountains, stacked alluvial sequences that include Qia and 

older alluvium reach total thicknesses in excess of 50 m. Thickness of 

Qia typically variable from less than 1 m to about 10 m

Where divisions within Qia are evident and large enough to map separately, unit is 

subdivided into Qia1 and Qia2 as follows:

Qia1 Younger intermediate age alluvial deposits (late Pleistocene)—Typically has 

a stage-I to -II soil-carbonate horizon 1 to 1.5 m thick. Less commonly 

soil profile includes a 30 to 50 cm thick weakly cemented stage III 

carbonate horizon. Some surface clasts weathered and (or) split 

Qia2 Older intermediate age alluvial deposits (middle Pleistocene)—Typically 

has a weakly to moderately cemented, stage-III to -IV soil-carbonate 

horizon 1 m or more thick. Weathered and (or) split surface and subsurface clasts common at some locales. Forms higher surface relative to 

Qia1

Landslide deposits (Pleistocene)—Slumps, flows, and slides of bedrock, colluvium, and alluvium. Terrain typically hummocky or irregular with 

closed depressions and spires. Thickness is at least 60 m, possibly as 

great as 100 m. These deposits typically came from single source areas 

and are subdivided, based on primary lithology that makes them up as 

follows:

Qlb Basalt source—Derived from Alamo Creek Basalt Member of Chisos 

Formation (Tfacb) south of Punta de la Sierra, and Ash Spring Basalt 

Member of Chisos Formation (Tfasb) north of The Basin

Qlc Chisos Formation source—Mostly derived from white tuffaceous facies of 

Chisos Formation (Tcwt)

Qlg Granitic source—Mostly derived from nearby felsic granite sill (Tsf)

Qlj Javalina Formation source—Mostly derived from nearby claystone of 

Javalina Formation (Kj)

Qlu Undifferentiated source—Derived from a variety of source rocks

Qls Sandstone source—Mostly sandstone of unknown derivation

Qlt Trachytic source—Mostly trachyte derived from trachyte flows of Punta de la 

Sierra (Tetps)

Qlp Pine Mountain source—Mostly derived from Pine Canyon Rhyolite Member 

of South Rim Formation (Tespc)
22 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Qoa Old alluvium (middle to early Pleistocene)—Alluvial fan, pediment, and 

stream deposits. Locally includes Qya where those units are too 

narrow or small to map separately. Typically moderately to wellconsolidated, poorly to moderately well-sorted, poorly to moderately 

well-bedded gravel, sand, and silt. Cobble- and small boulder-size 

clasts common to abundant. Clasts typically subangular to subrounded, 

locally angular or rounded. Surfaces typically eroded and dissected 

into broad, rounded ballenas, especially where gravel deposits are 

thick, but may also be gently undulatory, or form generally planar 

remnants 30 to 50 m above valley floor. Soils typically have a 

weakly to moderately cemented carbonate horizon 1 m or more thick. 

Weathered surface and subsurface clasts common. Pavement ranges 

from weakly developed to moderately packed and uniform. Varnish 

on pavement clasts ranges from none where pavement contains mostly 

limestone clasts, to moderately varnished with yellowish-red and dark 

reddish-brown colors. Typically has higher relative height than Qia, 

but is buried by younger alluvial deposits at some mountain front and 

up-valley locales. Thickness is typically 4 m to 10 m

QTa Extremely old alluvium (early Pleistocene and Pliocene)—Moderately 

to well-consolidated, poorly to moderately well-sorted, poorly to 

moderately well-bedded gravel, sand, and silt predominantly of alluvial 

fan origin; exposed along the west map edge and in northeast map 

corner. Cobble- and boulder-size clasts common. Clasts angular 

to subrounded. Surfaces eroded and deeply dissected into ballena 

morphology. Where preserved, soils have carbonate horizon as much 

as 2 m thick. Total thickness is variable with a maximum of 35 m 

within mapped area, possibly greater elsewhere

Flows and Intrusions of the Eruptive and Intrusive Episodes 

(Oligocene)

Tsgb Gabbro sill—Gabbro sill intruding metamorphosed Cretaceous rocks in waterworks area along Fresno Creek. Described by Lonsdale and Maxwell 

(1967) as dark gray and medium grained with labradorite crystals as 

large as 8 mm and augite as large as 4 mm. Texture is seriate and 

partly ophitic. A poorly resolved U/Pb SHRMP age of 26–30 Ma 

(table 1) suggests that this may be one of the youngest intrusive bodies 

in Chisos region. Thickness approximately 15 m

Tebr Breccia—Volcaniclastic breccia exposed at tops of several small hills near 

central-west map border south of Blue Creek Canyon. Overlies tuff 

and conglomerate unit (Tetc). Thickness approximately 25 m 

Terf Rhyolite flow—Young rhyolite flows exposed at 1) top of Emory Peak, 2) top 

of Casa Grande, 3) northwest part of Sierra Quemada, and 4) extreme 

west map boundary. At Emory Peak and Casa Grande, these flows have 

been included by most workers with Burro Mesa Rhyolite Member of 

the South Rim Formation, but new dates indicate that flows at Emory 

Peak and Casa Grande are too old to be Burro Mesa, which is 29 Ma. 

At Emory Peak, rhyolite is medium-grained, crystal-rich tuff (fig. 14); 

rhyolite forms a localized lava dome at Casa Grande. At the westcentral map area there is an incomplete section of medium-grained, 

gray rhyolite ash-flow tuff that is siliceous, highly welded, and has 

quartz phenochrysts in a rebeckite matrix. This ash flow is not dated 

in mapped area, but elsewhere numerous ages on it have a consistent 

average of 29 Ma. It is typically mapped as a member of the South 

Rim Formation, but is younger and probably had a different source. 
Therefore it is not considered to be part of South Rim. Flows at Sierra 

Quemada are of local derivation. Thus, these different outcrops are 

not mapped as Burro Mesa Rhyolite Member herein and they are not 

included with South Rim Formation, as has been done in the past. Age 

determinations # 1 = 32.11 ± 0.11 Ma (Ar/Ar), # 8 = 32.03 ± 0.1 Ma 

(Ar/Ar), and # 18 = 32.3 ± 0.4 Ma (SHRMP) (table 1) on flows at top 

of Emory Peak and Casa Grande; other locations have not been dated 

within mapped area. Thickness about 100 m at Emory Peak

Teeb Explosion Breccia of Sierra Quemada—Extremely lithic-rich, rhyolitic 

explosion breccia scattered within perimeter of ring dikes of central 

Sierra Quemada. Lithic fragments within explosion breccia (mm-size 

to 200 m long) consist of vertically standing, highly elongate, Tertiary, 

Cretaceous, and Paleozoic rocks. Age determinations # 9 = 29.5 ± 0.07 

Ma (Ar/Ar) and # 23 = 32.3 ± 0.07 Ma (SHRMP) (table 1). Thickness 

not known

Tetps Trachyte flows of Punta de la Sierra—Trachytic flows, resistant, brown, 

porphyritic trachyte that has a gray to brownish-gray groundmass and 

feldspar phenocrysts as large as one-half inch across (1–2 cm). The 

groundmass is darker than phenocrysts, producing a spotted pattern, 

Description of Map Units 23

Figure 14. Photograph of crystal-rich tuff that forms the ryhyolite flows near the top of Emory Peak (Terf). White pad shows two scales: 

3 in. on the left and 9 cm on the right.
24 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

not unlike that in more mafic dikes at Dominguez Mountain (Maxwell 

and others, 1967). Phenocrysts include both alkalai feldspar and 

plagioclase with latter being more common and groundmass includes 

microphenocrysts of clinopyroxene. Mostly occurs in Punta de la Sierra 

and in mountainous region to north. Discontinuous exposures also 

present on northwest flank of Chisos Mountains and in Homer Wilson 

Ranch area. Source unknown. Thickness about 300 m at Punta de la 

Sierra. Similar flows nearby to south dated at approximately 30 Ma 

(Dan Miggins, written commun., 2008). Originally mapped as Tule 

Mountain Trachyandesite (Maxwell and others, 1967), but that unit is 

known to be 3 million years older than this sequence of flows

South Rim Formation (Oligocene)

Widely exposed on South Rim and in Pine Canyon Region. Divided into several 

members as follows:

Teslm Lost Mine Member—Description based on Barker and others (1986). Quartz 

trachyte ash-flow tuffs and local lava flows; densely welded, lacking 

lithic clasts; contains sparse phenocrysts of subhedral anorthoclase and 

clinopyroxene. Forms prominent cliffs along South Rim. One K/Ar age 

determination reported by Barker and others (1986) of 33.3 ± 0.7 Ma. 

Thickness about 120 m at South Rim

Tesbr Boot Rock member (informal)—Description based on Barker and others 

(1986). Composed of two ash-flow tuff facies, caldera fill, and outflow. 

Caldera fill is multi-colored breccia with densely welded glassy matrix; 

contains up to 75 percent by volume of subangular lithic fragments of 

quartz trachyte and rhyolite. Rhyolite clasts are mostly from underlying 

Pine Canyon Rhyolite Member of South Rim Formation. Caldera fill 

is about 100 m thick. Outflow facies is poorly welded or not welded, 

contains a variety of breccias and ash flows, and is 10 to 20 m thick. 

Age determinations #4 = 32.23 ± 0.07 Ma, # 5 = 32.21 ± 0.05 Ma, # 

6 =31.93 ± 0.13 Ma, and # 7 =32.43 ± 0.29 Ma, (all Ar/Ar) (table 1). 

Thickness highly variable, about 275 m at Casa Grande

Tespc Pine Canyon Rhyolite Member—Description based on Barker and others (1986). Consists of multiple cooling units of peralkaline rhyolitic 

ash-flow tuff, light brown-gray, densely welded, contains euhedral 

to subhedral anorthoclase phenocrysts. Rounded fragments of quartz 

trachyte are abundant in upper part of unit. Eruption led to collapse of 

Pine Canyon caldera and Pine Canyon Rhyolite Member is mostly confined to collapsed central part of caldera; at least 300-m thick within 

collapsed area. Age determination # 3 = 32.11 ± 0.23 Ma (Ar/Ar) (table 

1). Thickness greater than 580 m in Pine Canyon area

Tesre Rocks equivalent to South Rim Formation (Oligocene)—Other rhyolite 

flows and ignimbrites equivalent to South Rim flows or trachyte flows 

of Punta de La Sierra (Tetps), but not continuous with any of those 

units; exposed in Blue Creek Canyon area. Thickness not known

Tiwma Altered parts of Ward Mountain pluton (Oligocene)—Altered zone at top of 

Ward Mountain pluton; not studied

Tii Intermediate to mafic intrusion (Oligocene)—Chiefly diorite, possibly gabbro. In several small intrusive bodies south of Tortuga dome in Fresno 

Creek

Tdsrp Rhyolite porphyry dikes and sills (Oligocene)—Various porphyritic dikes 

and sills, mostly rhyolite or rhyodacite in composition, includes 

“ring-dike complex” of Pine Canyon caldera; also exposed in numerous 

localities in eastern part of map. Ring-dike complex at Hayes Ridge 
Description of Map Units 25

(fig. 3) was described as light-gray sodic granophyre with chalky 

microperthite and a microcrystalline groundmass (Lonsdale and 

Maxwell, 1967). Age determinations # 2 = 32.12 ± 0.10 Ma (Ar/Ar) 

and # 13 = 41.6 ± 0.4 Ma (SHRMP) (table 1)

Tdds Dominguez Mountain dike swarms (Oligocene?)—Mostly light-colored 

rhyolite to dacite porphyry, mapped as a discrete unit in Dominguez 

Mountain area where dike-on-dike intrusion forms a wide body of dike 

rock. Rhyolite dikes are abundant, particularly southwest of volcanic center where they pass upward into trachyte flows at Punta de la 

Sierra. These are pale yellow brown and fine grained, most contain 

sparse white subhedral feldspar phenocrysts, and are slightly vuggy 

(fig. 15A). In rare cases, interior parts of widest dikes are medium to 

coarse grained. In these cases, feldspar grains are dense enough to be in 

contact with one another, and matrix is red brown and medium grained. 

A B

Figure 15. Photographs of types of dikes in the Dominguez Mountain dike complex (Tdds). (A) Rhyolite and (B) porphyritic syenodiorite. 

Pencil for scale.

Rare mafic dikes are also present (fig. 15B) and these coarse-grained 

and porphyritic dikes are probably olivine syenodiorite or syenogabbro 

according to Maxwell and others (1967). Not dated, but presumably 

equivalent in age to 30 Ma trachyte flows of Punta de la Sierra

Tdsq Sierra Quemada dike complex (Oligocene)—Thick dike-like intrusions 

that form outer rim of volcanic center. Lonsdale and Maxwell (1967) 

described them as augite-microgranite porphyry, ganophyre, and 

porphyritic rhyolite. Age determination # 10 = 29 Ma (Ar/Ar) (table 1)

Tdr Rhyolite dikes of Sierra Quemada (Oligocene)—Rhyolite dikes in Sierra 

Quemada; not dated
26 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Tsf Felsic sill (Oligocene)—Gray feldspar-rich, porphyritic riebeckite microgranite with weathered alkalai-feldspar phenocrysts set in a groundmass 

of weathered laths of alkalai feldspar and anhedral quartz (Lonsdale 

and Maxwell, 1967) near east edge of map, 3.5 km due east of Tortuga 

Mountain. Age determination # 14 = 32.3 ± 0.3 Ma (SHRMP) (table 

1). Thickness 140 m

Tiwm Ward Mountain pluton (Oligocene)—Rhyolite pluton, mostly flow-banded 

rhyolite and fine-grained granite and quartz syenite, contains finegrained sparsely quartzo-phyric texture with poikolitic amphibole; 

pluton surrounds west and north sides of The Basin. Age determinations # 20 = 31.5 ± 0.3 Ma, # 21 = 32.4 ± 0.3 Ma, and # 22 = 32.7 ± 0.3 

Ma (all SHRMP) (table 1)

Tigr Granite plutons and stocks (Oligocene)—Various granite to monzogranite 

plutons and stocks scattered throughout map; includes a large laccolith 

in region of Dodson Spring that shows signs of being emplaced as 

dikes in east and as sills in west (fig. 12). The laccolith is a light gray 

microgranite comprised of 83 percent feldspar, 8 percent quartz, and 

5 percent pyroxene with xenomorphic granular texture according 

to Lonsdale and Maxwell (1967). Other notable intrusions include 

Elephant Peak stock, several small intrusions surrounding Dominguez 

Mountain, two small plutons in Pine Canyon area, and small stocks 

along Hayes Ridge; not yet dated

Tim Mafic intrusions of Pine Canyon caldera and Dominguez Mountain 

(Oligocene)—Several small mafic stocks in central part of Pine 

Canyon caldera near Pummel Peak; also a large ring-shaped intrusive 

body surrounding interior pluton of Dominguez Mountain. Near 

Pummel Peak Lonsdale and Maxwell (1967) described dark greenishgray diabase with fine-grained subophitic texture. At Dominguez 

Mountain dikes are comprised of gabbro that is very porphyritic, 

containing large euhedral to subhedral grains of white feldspar, with 

a dark-colored, fine-grained matrix. Not yet dated, but they intrude 

youngest Pine Canyon fill in Pine Canyon area

Tinm Intrusions of Nugent Mountain (Oligocene)—Several small intrusions of 

porphyritic alkalic rhyolite or microgranite and alkalic granophyre 

at and near Nugent Mountain in northeastern part of map. Lonsdale 

and Maxwell (1967) described alkalic microgranite as comprised of 

sanidine-anorthoclase and augite phenocrysts in a green groundmass of 

feldspar laths and alkali feldspar. Dated at 31.5 ± 0.3 Ma on west side 

of Nugent Mountain

Tipl Panther laccolith (Oligocene)—Large laccolith of porphyritic sodic rhyolite 

north of Pine Canyon caldera. Described as a greenish-gray dense aggregate alkali fieldspar, green clinopyroxene, and 10 percent quartz by 

Lonsdale and Maxwell (1967). Not dated 

Tirs Rough Spring laccolith (Oligocene)—Light-gray to dark-gray quartz microsyenite that is also referred to as Government Spring laccolith. 

Generally hypidiomorphic granular in texture, medium grained, with a 

salt-and-pepper appearance in hand specimen. Comprised of sanidineanorthoclase, oligoclase, and quartz. Not dated

Tir Rhyolite intrusions of Sierra Quemada (Oligocene)—Small rhyolite stocks 

in central part of volcanic center at Sierra Quemada

Tetc Tuff and conglomerate (Oligocene)—Air-fall tuff and conglomerate at base 

of eruptive sequence in a very local exposure north of Blue Creek 

Canyon near west map border. Dated at 32.1 Ma (Ar/Ar) (table 1)

Tsbu Undifferentiated basalt sills (Oligocene)—Two exposures of possibly same 

sill are included in this unit. In both places sill probably originated in 

Dominguez Mountain volcanic center. Each is a dark-colored basalt 
Description of Map Units 27

that intrudes middle of Chisos Formation in central part of map area 

along contact between Smoky Creek (above) and Fresno Creek (below) 

facies groups. Rock is characterized by abundant feldspar laths, which 

is also typical of many other rocks associated with Dominguez volcano. 

Determined to be 31 Ma (Ar/Ar; sample # 11) (table 1)

Tdsm Mafic dikes and sills (Oligocene?)—Dark colored, probably basaltic in 

composition, intrudes Chisos Formation east of Pine Canyon caldera 

at about same stratigraphic interval that Ash Spring Basalt Member 

(Tfasb) occurs

Tfdmt Dominguez Mountain core flows (Oligocene?)—Dark-colored bedded 

flows in core of Dominguez Mountain (fig. 9). Maxwell and others 

(1967) described these as layered pyroclastic rocks and lavas that have 

been thermally metamorphosed to a dense hornfels-like rock. They 

are probably andesitic to basaltic in composition. Not dated, but are 

probably Oligocene

Tfdb Dominguez Mountain basalt(?) flows (Oligocene?)—Black lava flows on 

lower east flank of Dominguez Mountain. Identified by unique reflectance characteristics on color air photography. Age and composition 

not known, probably Oligocene

Chisos Formation and Equivalent Volcanic Rocks

(Oligocene and Eocene)

Volcanic-sedimentary facies and temporally equivalent flows of Chisos episode of 

volcanic buildup.

Tcbr Breccia facies (Oligocene)—Breccia and conglomerate in upper part of Chisos 

Formation just above basalt flow (maybe upper flow unit of Tfbmb) in 

Wilson Ranch area (fig. 16). Thickness 38 m north of Blue Canyon

Tcwtu Upper white tuff facies (Oligocene)—Tuff, sandstone, and rhyolite forming highest facies of Chisos Formation. Generally above basalt flows 

mapped as Bee Mountain Basalt Member (Tfbmb) in Blue Creek 

Canyon area

Bee Mountain Basalt

Member of Chisos Formation (Tfbmb)

Wilson Ranch House

Upper Chisos Formation

(Tcwtu)

Chisos Formation breccia (Tcbr)

Rocks equivalent to

South Rim Formation (Tesre)

Figure 16. Photograph of small hill east of Wilson Ranch house in Blue Creek Canyon. Bee Mountain Basalt Member of Chisos 

Formation (Tfbmb) is in lower part of hill below and including small cliff-forming flow.
28 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Tfvt Vitric tuff (Oligocene)—Thin vitric tuff northwest of Blue Canyon Ranch just 

above Bee Mountain Basalt Member (Tfbmb) in Blue Creek Canyon 

area

Tfbmb Bee Mountain Basalt Member and equivalent flows (Oligocene)—Darkcolored basalt flows that are conspicuously scoriaceous. The basalt 

is mostly fine to medium grained and dark gray in color. Secondary 

minerals fill many of vesicles, giving rock a mottled appearance. In 

western part of Chisos Mountains, uppermost flow is distinctly more 

resistant than those below it, forming a small cliff (fig. 16). Present 

high in Chisos Formation or just at base of trachyte flows (Tetps), and 

South Rim Formation rocks and their equivalents. Thickness is variable 

regionally; 90 m in Blue Creek Canyon area

Tcwt White tuff facies (Eocene)—Tuff and tuffaceous sandstone, generally white. 

Consists of sandstone and air-fall tuff in parallel, continuous beds that 

are 1–10 m thick. Bedding is much more conspicuous than it is in older 

facies to east, and this facies is much lighter in color, primarily owing 

to abundance of tuff, which is commonly white or very light gray (fig. 

17). Sandstone and tuff beds have low-angle, internal cross-stratification that suggests that flowing water was an important factor in their 

depositional history. This unit most likely had a mostly volcanic origin, 

but reworking in a fluvial environment is likely. Widespread in western 

part of map south of Ward Mountain, in Sierra Quemada region and 

south of Punta de la Sierra where it forms lowest unit of Chisos Formation and overlies Alamo Creek Basalt Member (Tfacb). Thickness is 

greater than 900 m around and west of Sierra Quemada and is about 

150 m south of South Rim

Tcksu Upper siltstone facies (Eocene)—Greenish, purplish, and gray siltstone, probably derived from shales of Upper Cretaceous Javalina Formation (Kj). 

Thickness is approximately 200 m south of South Rim

Tfra Rhyolite ash flow (Eocene)—Thin, discontinuous rhyolite ash flow interbedded with white tuff facies of Chisos Formation (Tcwt) just west of 

Juniper Canyon. Thickness is 22 m

Tcyot Yellow-orange tuffaceous sandstone facies (Eocene)—Tuffaceous(?) sandstone, distinct yellow-orange color, thick and resistant. Chiefly well 

bedded. Beds vary between 3–50 cm thick and some are cross-stratified 

(fig. 18). Most of beds were probably once air-fall tuff, but original 

grains have probably been reworked by wind and water since their 

initial deposition. It makes a unique marker horizon in upper part of 

Chisos Formation at same stratigraphic horizon as a ryholite ash flow 

Figure 17. Photograph of white tuffaceous facies of the Chisos Formation (Tcwt) in broad sycline to the east of Castalon highway near 

west edge of map at about 29° 15’ North. 
Description of Map Units 29

(Tfra) just west of Juniper Canyon; age determination # 17 = 41.4 ± 

0.5 Ma (SHRMP) (table 1). Thickness is 55 to 85 m

Tckss Tuffaceous sandstone facies (Eocene)—A small exposure of tuffaceous 

sandstone; present along west map edge southwest of Punta de la Sierra 

escarpment. Thickness not known

Tcss Undifferentiated sandstone facies (Eocene)—Thin, discontinuous sandstone 

layers intercalated with white tuff facies (Tcwt) near west and south 

sides of Sierra Quemada volcanic center. Thickness not known

Tfasb Ash Spring Basalt Member (Eocene)—Dark-colored, fine-grained basalt near 

base of white tuffaceous facies of Chisos Formation (Tcwt) north and 

west of Chisos Mountain front. Basalt is conspicuously porphyritic in 

many places with large, white plagioclase laths 1–2 cm in length (fig. 

19). Age determination # 26 = 40.38–43.39 Ma (table 1); sampled near 

highway west of Ward Spring at west map edge. Thickness is 30 to 31 

m along northern Chisos Mountain flank

Tfacb Alamo Creek Basalt Member and equivalent flows (Eocene)—Dark-colored 

basalt present beneath Chisos Formation in southwestern part of map. 

Basal flows are typically scoriaceous, but most of upper flows are 

dense. Locally some flows are porphyritic and are rich in plagioclase 

phenocrysts that are as large as 0.5 cm across. Although not dated in 

Figure 18. Photograph of typical yellow-orange tuffaceous facies of Chisos Formation (Tcyot) in Blue Creek Canyon.
30 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

mapped area, elsewhere is about 45.5 to 47 Ma. Thickness 30 to 45 m 

south of Punta de la Sierra

Tcstr Sandstone, tuff, and rhyolite facies (Eocene)—Thick unit of well-bedded 

brown-colored sandstone with accessory tuffaceous sandstone (fig. 20). 

Bedding is mostly parallel with even bedding surfaces and uniform 

bed thickness maintained over large distances. Many beds are crossstratified internally, others are more massive. Some beds have a conglomeratic base with clasts typically of limestone. Mostly sand grains 

are quartz, making it hard to determine where original source was, but 

in some cases fragments of tuff and glass shards are still in evidence, 

suggesting rivers were probably eroding a nearby volcanic terrain 

composed mostly of tuff. Present in Fresno Creek area, south of Hayes 

Ridge, and around circumference of Pine Canyon caldera. Thickness is 

690 m 

Tcnr Nodular rhyolite facies (Eocene)—White rhyolite flows with nodular concretions of quartz ubiquitous, very poorly bedded, and characterized by 

presence of abundant nodules and irregularly shaped blobs of chalcedony or low-grade agate (fig. 21). Present in Fresno Creek drainage and 

east and southeast of Pine Canyon area. Thickness is 210 m

Figure 19. Photograph of numerous plagioclase crystals in poprphyritic part of Ash Spring Basalt Member of Chisos Formation (Tfasb) 

near Ash Spring on the northwest flank of the Chisos Mountains. Hammer for scale.
Description of Map Units 31

Dike (Tdsrp)

Chisos Formation sandstone, tuff,

and rhyolite facies

(Tcstr)

Dike (Tdsrp)

Figure 20. Photograph of a well-exposed section of Chisos Formation sandstone, tuff, and ryholite facies (Tcstr) just to the east of 

Tortuga Mountain. A large dike of porphyitic rhyolite (Tdsrp) can also be seen.

Figure 21. Photograph of typical exposure of nodular rhyolite facies of the Chisos Formation (Tcnr). White pad in top center shows two 

scales: 3 in. on the left and 9 cm on the right.
32 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

Tcks Lower siltstone facies (Eocene)—Interbedded greenish, purplish, and gray 

siltstone, probably derived from shale of Upper Cretaceous Javalina 

Formation (Kj); and white to brown quartz-rich beds that are probably 

altered and silicified air-fall tuff beds (fig. 22). Shows signs of hornfels 

texture and thermal metamorphism in eastern part of map area. Thickness is variable with a maximum of 175 m near eastern map edge

Tfbu Basalt flows, undifferentiated (Eocene)—Basaltic lavas undifferentiated, 

occur at various stratigraphic horizons within Chisos Formation. One 

prominent flow sequence occurs in lower parts of Fresno Creek-facies 

group between siltstone rhyolite tuff (Tcrt) and nodular tuff (Tcnr) 

facies. This sequence forms a thick and very lenticular body of flows 

that are dark green-gray to purple-gray basaltic andesite in layers 1–2 

m thick. Also contains pods, pockets, and lenses of epidote-rich rock. 

Several thin sills cut layered sequence at low angles. A new partially 

analyzed sample age suggests that sills are about 30–31 Ma, but that 

age probably postdates flows by a considerable amount. Other thin 

flows are also present in upper parts of Chisos Formation. Thickness 

variable

Tcl Lacustrine facies (Eocene)—Silt and clay, parallel, continuous, and even 

bedding; consists of interbedded layers of air-fall tuff and siltstone (fig. 

23). Tuff and siltstone beds are parallel to one another and are 3–50 

cm in thickness. Bedding surfaces are even and beds are very uniform 

in thickness. Beds of each lithology are internally laminated. Occurs 

Figure 22. Photograph of one of the better exposures of the lower siltstone facies of the Chisos Formation (Tcks), which was 

derived from Cretaceous siltstone exposed nearby.
Description of Map Units 33

beneath undifferentiated basaltic andesite sill (Tsbu) in southeastern 

part of map near Fresno Creek. Thickness is about 50 m

Tcrt Rhyolite tuff facies (Eocene)—Rhyolite and tuff in basal unit of Chisos Formation. Chiefly light pink to light gray rhyolite tuff, characterized by 

presence of small (1–2 cm) medium-gray globular concretions (fig. 24). 

Fine-grained to very fine grained, in irregular beds a few centimeters 

to a few tens of centimeters thick. Most original glass shards are now 

altered to clay throughout unit. Individual beds are discontinuous, but 

sub-parallel to one another. Rare sandy lenses as thick as 1–2 m occur 

locally. Paraconformably overlies metamorphosed Aguja Formation 

Figure 23. Photograph of the lacustrine facies of the Chisos Formation (Tcl) showing the 

thin varve-like laminations that are typical of this facies. Hammer for scale.
34 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

units (Kags); present between Fresno Creek and Dominguez Mountain. Thickness about 65 m

Older Sills (Eocene)

Tsm Mafic sills—Dark-colored sills that intrude Cretaceous rocks in southern and 

northwestern parts of area; might be equivalent to Mariscal Mountain 

gabbro sill dated at 46.7 ± 0.6 Ma. Thickness is about 60 m

Tsi Intermediate sill—Dark-colored basaltic(?) sill intruding Cretaceous rocks in 

southern part of area. Might be equivalent to Mariscal Mountain gabbro 

sill dated at 46.7 ± 0.6 Ma. Thickness not known

Marine Seaway Deposits (Paleocene and Cretaceous)

TKbp Black Peaks Formation (lower Paleocene and Upper Cretaceous)—Chiefly 

cross-bedded sandstone, includes some interbedded claystone, similar 

to upper part of Aguja Formation. Includes a tuffaceous component to 

claystone more like that of Javelina Formation. Mammal remains and 

freshwater mollusks are present indicating Paleocene age. Not widely 

exposed near Chisos Mountains, but is present in small outcrops to 

Figure 24. Photograph of the rhyolite and tuff facies of the Chisos Formation (Tcrt) with its characteristic dark-colored nodules. 

Camera lens cover for scale.
Description of Map Units 35

northeast and northwest of them. Thickness is 85–265 m regionally 

(Maxwell and others, 1967)

Kj Javelina Formation (Upper Cretaceous)—Claystone containing thin 

lens-shaped interbeds of buff cross-bedded sandstone. Clay beds are 

varicolored including brown, green, maroon, and dull gray and are 

mostly bentonitic, suggesting source of airborne volcanic ash. Bentonite weathers to puffy-textured low ground and dull gray landscape of 

low hills cut by canyons. Sandstone interbeds are typically cross bedded, ripple marked, and commonly conglomeratic, indicating that they 

were originally deposited in river channels. Dinosaur bones and fossil 

wood fragments are common, especially in sandstone, establishing 

nonmarine nature of formation. Javelina is widely exposed south and 

northwest of Chisos Mountains. Thickness is 162–285 m regionally 

(Maxwell and others, 1967)

Kag Aguja Formation (Upper Cretaceous)—Claystone that includes thick layers 

of buff-colored sandstone that are 200–300 m thick in some places. 

Sandstone is crossbedded and contains ripple marks. Light-gray claystone with concretions, that is similar to claystone in formations above 

and below, is interbedded with sandstone layers. Lower parts of formation are marine and upper parts are nonmarine. Nonmarine fossils 

include dinosaur bones, turtle remains, and silicified wood (Maxwell 

and others, 1967). Aguja is exposed in anticline near Cow Heaven 

Mountain, along main highway northwest of Chisos Mountains, and 

at several locations in The Basin. Probable Aguja Formation is also 

exposed in core of a structural dome near Tortuga Mountain. Nautilus 

fossils have been identified from Aguja indicating that it is marine in 

origin. Thickness is 232 to 354 m regionally (Maxwell and others, 

1967)

Kags Metamorphosed sandstone or quartzite facies near Tortuga dome—Arkosic metasandstone and quartzite. Detrital sand grains of zircon have 

been dated and youngest of these is 76.8 Ma, which is a minimum age 

Kaghf Hornfels facies—In Tortuga dome, Aguja Formation has been slightly 

metamorphosed, causing it to have a different appearance than it does 

elsewhere (fig. 25). Metamorphism is thermal in nature, caused by heat 

from a thick sill of granodiorite and possibly a small granite pluton that 

intruded formation locally. Light-gray claystone has been changed into 

dark red-brown hornfels and sandstone into quartzite by this metamorphic process. Hornfels is hard and resistant, unlike its protolith (parent 

rock), and it forms steep cliffs 

Kp Pen Formation (Upper Cretaceous)—Primarily claystone that includes 

thin parallel interbeds of fine-grained sandstone. Maxwell and others 

(1967) described it as weathering light yellow-gray in most places. 

Concretions are common and most of them are cemented by calcite, 

others by iron. Pen is marine in origin, and fossil ammonites and 

pelecypods are abundant. The claystone is not resistant, so unit 

commonly forms lowlands with smooth, soft-looking slopes on 

hillsides. A small exposure of Pen occurs west of Emory Peak, and it 

is exposed in core of anticline near Cow Heaven Mountain, otherwise 

it is not present in Chisos Mountains. Thickness is 140 m regionally 

(Maxwell and others, 1967)

Kb Boquillas Formation (Upper Cretaceous)—Thin-bedded limestone, chalk, 

and marl; resistant limestone beds commonly weather into thin 

flagstone sheets. There is typically more chalk and marl than flagstone, 

and chalk beds form resistant ledges. Although Maxwell and others 

(1967) subdivided Boquillas into members, no attempt has been made 

to differentiate these in Chisos Mountains. Their upper member, San 
36 Geologic Map of the Chisos Mountains, Big Bend National Park, Texas

 References

Barker, D.S., Henry, D.H., and McDowell, F.W., 1986, Pine 

Canyon caldera, Big Bend National Park: a mildly peralkaline magmatic system, in Price, J.G.; Henry, C.D., Parker, 

D.F., and Barker, D.S., eds., Igneous geology of Trans-Pecos 

Texas—Field trip guide and research articles: University of 

Texas, Austin, v. 23, p. 266–285.

Berry, M.E., and Williams, V.S., 2008, Surficial deposits of 

Big Bend National Park, in Gray, J.W., and Page, W.R., eds., 

Geological, geochemical and geophysical studies by the 

U.S. Geological Survey in Big Bend National Park, Texas: 

U. S. Geological Survey, Circular 1327, p. 15–28.

Birkeland, P.W., 1999, Soils and geomorphology (3d ed.): 

New York, Oxford University Press, 430 p.

Dickerson, P.W., and Muehlberger, W.R., 1994, Basins in the 

Big Bend segment of the Rio Grande rift, Trans-Pecos, 

Texas, in Keller, G.R., and Cather, S.M., eds, Basins of the 

Rio Grande Rift: Structure, stratigraphy and tectonic setting: 

Boulder, Colo., Geological Society of America Special 

Paper 291, p. 283–297.

Gile, L.H., Hawley, J.W., and Grossman, R.B., 1981, Soils 

and geomorphology in the Basin and Range area of Southern New Mexico—Guidebook to the Desert Project: New 

Mexico Bureau of Mines and Mineral Resources, Memoir 

39, 222 p.

Vincente Member, is probably only one exposed in Range. Boquillas 

is chiefly a marine unit that formed in Cretaceous seaway. Numerous 

species of nautilus and ammonite, as well as fish bones, shark teeth, 

and mosasaur remains have been reported from it (Maxwell and others, 

1967). Only places formation is exposed in Chisos Mountains are in 

The Basin and Laguna Meadow areas. Thickness of upper Boquillas 

(San Vincente Member) is about 120 m regionally (Maxwell and 

others, 1967)

Figure 25. Photograph of mudstone of the Aguja Formation (Kaghf) that has undergone a slight thermal metamorphism to hornfels in 

Fresno Creek near Tortuga Mountain.
References 37

Gile, L.H., Peterson, F.F., and Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulation in 

desert soils: Soil Science, v. 101, no. 5, p. 347–360.

Harlan, S.S., Geissman, J.W., Henry, C.D., and Onstott, T.C, 

1995, Paleomagnetism and 40Ar/39Ar geochronology of 

gabbro sills at Mariscal Mountain anticline, southern Big 

Bend National Park, Texas: Implications for the timing of 

Laramide tectonism and vertical axis rotations in the southern Cordilleran orogenic belt: Tectonics, v. 14, p. 307–321.

Henry, C.D., Price, J.G., and James, E.W., 1991, MidCenozoic stress evolution and magmatism in the southern 

Cordillera, Texas and Mexico: Transition from continental 

arc to intraplate extension: Journal of Geophysical 

Research, v. 96, p. 13545–13560.

Lehman, T.M., 1991, Sedimentation and tectonism in the 

Laramide Tornillo Basin of west Texas: Sedimentary 

Geology, v. 75, p. 9–281.

Lehman, T.M., and Busbey, A.B., 2007, Society of Vertebrate 

Paleontology Fall 2007 field trip field guide: University 

of Texas, Austin, Society of Vertebrate Paleontology Field 

Guide, 67 p.

Lonsdale, J.T., and Maxwell, R.A., 1967, Geology of intrusive 

igneous rocks, in Maxwell, R.A., Lonsdale, J.T., Hazzard, 

R.T., and Wilson, J.A., eds., Geology of Big Bend National 

Park, Brewster County, Texas: Bureau of Economic 

Geology, University of Texas, Austin, p. 170–270.

Machette, M.N., 1985, Calcic soils of the southwestern United 

States, in Weide, D.L., ed., Soils and Quaternary geology 

of the southwestern United States: Geological Society of 

America Special Paper 203, p. 1–21.

Maxwell, R.A., 1968, The Big Bend of the Rio Grande: A 

guide to the rocks, landscape, geologic history and settings 

of the area of Big Bend National Park: Bureau of Economic 

Geology, University of Texas, Austin, Guidebook 7, scale 

1:62,500.

Maxwell, R.A., Lonsdale, J.T., Hazzard, R.T., and Wilson, 

J.A., 1967, Geology of Big Bend National Park, Brewster 

County, Texas: Bureau of Economic Geology, University of 

Texas, Austin, 320 p.

Miggins, D.P., Ren, M., and Anthony, E.Y., 2008, Volcanic 

geology of several prominent outcrops in the western part 

of Big Bend National Park, in Gray, J.W., and Page, W.R., 

eds., Geological, geochemical, and geophysical studies by 

the U.S. Geological Survey in Big Bend National Park, 

Texas: U.S. Geological Survey Circular 1327, p. 29–42.

Muehlberger, W.R., 1989, Summary of structural geology of 

Big Bend National Park and vicinity, in Muehlberger, W.R., 

and Dickerson, P.W., eds., Sedimentation and tectonics of 

western North America; v. 3, Structure and stratigraphy of 

Trans-Pecos Texas: 28th International Geological Congress 

Fieldtrip Guidebook T317, p. 179–197.

Muehlberger, W.R., and Dickerson, P.W, 1989, A tectonic 

history of Trans-Pecos, Texas, in Muehlberger, W.R., and 

Dickerson, P.W., eds., Sedimentation and tectonics of 

western North America; v. 3, Structure and stratigraphy of 

Trans-Pecos Texas: 28th International Geological Congress 

Field Trip Guidebook T317, p. 35–54.

Rocky Mountain Association of Geologists (RMAG), 1972, 

Geologic atlas of the Rocky Mountain region: Rocky 

Mountain Association of Geologists, Denver, Colo., 

331 p.

Scott, R.B., Turner, K.J., Budahn, J.R., and Miggins, D.P., 

2007, Sierra Quemada–a failed caldera, Big Bend National 

Park (Abs): Geological Society of America Abstracts with 

Programs, v. 39, no. 6, p. 512.

White, J.S., Benker, S.C., Ren, M., Urbanczyk, K.M., and 

Corrick, D.W., 2006, Petrogenesis and tectonic setting of 

the peralkaline Pine Canyon caldera, Trans-Pecos Texas, 

USA: Lithos, v. 91, p. 74–94.