U.S. Department of the Interior

U.S. Geological Survey

Circular 1327

Geological, Geochemical, and Geophysical Studies by the 

U.S. Geological Survey in Big Bend National Park, Texas
Cover photographs. Front upper: View looking west at Cerro Castellan located northeast of the village of Castolon in Big Bend National Park. This exposure is a 

series of Tertiary age volcanic units. In the foreground are white outcrops of Tertiary age tuffaceous sedimentary deposits and an ocotillo cactus. Photo by David 

L. Fey. Front lower: View looking east toward Mule Ear Peaks, which are eroded rhyolitic units of Tertiary age. Photo by John E. Gray. Back cover: View looking 

west through “The Window” located in The Basin of Big Bend National Park. The Window is a narrow opening between intrusive rocks of Ward Mountain and 

Pulliam Peak. Runoff and drainage from The Basin flows through The Window and downstream into Oak Creek. Photo by John E. Gray.
Geological, Geochemical, and Geophysical 

Studies by the U.S. Geological Survey in 

Big Bend National Park, Texas

Edited by J.E. Gray and W.R. Page

Circular 1327

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Gray, J.E., and Page, W.R., eds., 2008, Geological, geochemical, and geophysical studies by the U.S. Geological

Survey in Big Bend National Park, Texas: U.S. Geological Survey Circular 1327, 93 p.
iii

Foreword

A key goal of the work of the U.S. Geological Survey (USGS) is to enhance and protect the 

quality of life by describing and understanding the Earth around us. This Circular describes 

work that is part of ongoing collaborative studies—involving the USGS, National Park Service, 

the Texas Bureau of Economic Geology, and several universities—of the composition, history, 

and age of the rocks that underlie Big Bend National Park in southwest Texas. 

Big Bend National Park, located along the Rio Grande at the United States border with Mexico, 

is situated in the Chihuahuan Desert. The fragile desert ecosystem displays enormous beauty, 

includes mountain and basin environments, hosts an extraordinary diversity of native plants and 

animals, and is the site of significant mineral deposits and historic mercury mines. The studies reported in this Circular were largely funded by the USGS Mineral Resources and National 

Cooperative Geologic Mapping Programs. The scientists who conduct the work of these 

programs provide new information for uses ranging from understanding sources and effects of 

nonfuel mineral deposits to improving management of Federal lands.

The eight technical papers included in this Circular present a range of studies and results. Taken 

as a whole, the new geologic mapping and geophysical data, together with analyses of surface 

and subsurface waters, abandoned mine sites, and food sources of large mammals, provide a 

fundamental and objective scientific foundation for land-use, water-use, and resource-use decision makers in and near Big Bend National Park. In addition, geochemical studies of miningrelated mercury contamination in and around the park address a specific local challenge by 

demonstrating that, although mercury concentrations are elevated in mine waste, concentrations 

are generally low in the downstream ecosystems, primarily because of hot and dry desert climate conditions of the region. These diverse scientific studies conducted in Big Bend National 

Park not only inform decisions there, but also provide a basis for understanding similar environments elsewhere in the United States and around the world.

Kathleen M. Johnson

Mineral Resources Program Coordinator
iv

Contents

Foreword, by K.M. Johnson ........................................................................................................................iii

Introduction, by J.E. Gray and W.R. Page..................................................................................................1

1. Tectonic History of Big Bend National Park, by W.R. Page, K.J. Turner, and

R.G. Bohannon ..................................................................................................................................3

2. Surficial Deposits of Big Bend National Park, by M.E. Berry and V.S. Williams........................15

3. Volcanic Geology of Several Prominent Outcrops in the Western Part of Big Bend

National Park, by D.P. Miggins, M. Ren, and E.Y. Anthony......................................................29

4. Where Magma Meets Limestone: Dagger Flats, an Example of Skarn Deposits in Big

Bend National Park, by L.A. Morgan and W.C. Shanks ...........................................................43

5. Evaluation of Mercury Contamination at Inactive Mercury Mines in and around Big

Bend National Park, by J.E. Gray, P.M. Theodorakos, and D.P. Krabbenhoft.......................57

6. The Waters of Big Bend: Geochemical Variations, Ages, and Sources, by W.C. Shanks,

L.A. Morgan, J.E. Gray, A.H. Manning, and P.A. Gemery-Hill..................................................65

7. Stable Isotope and Trace Element Studies of Black Bear Hair, Big Bend Ecosystem,

Texas and Mexico, by W.C. Shanks, E.C. Hellgren, C. Stricker, P.A. Gemery-Hill, and

D.P. Onorato.....................................................................................................................................77

8. Aeromagnetic and Radioelement Analyses Identify Igneous Rocks in the Big Bend

National Park Region, by C.A. Finn and E. Anderson ...............................................................85

Figures

1–1. Map of BBNP showing late Tertiary Basin and Range faults, middle Tertiary volcanic 

and plutonic features, and present-day distribution of generalized rock units .................5

1–2. Index map showing transform faults and lineaments related to Proterozoic rifting

along the southern edge of North America..............................................................................6

1–3. Map showing location of Marathon orogenic belt and subduction zone formed from 

collision between the North and South American plates......................................................6

1–4. Stratigraphic column showing geologic rock units and major tectonic episodes

in Big Bend National Park ...........................................................................................................8

1–5. Map showing location of the Diablo Platform and the Chihuahua trough..........................9

1–6. Generalized map of the major structures that formed in BBNP during the

Laramide orogeny.......................................................................................................................11

2–1. Map showing locations within Big Bend National Park that are referred to in

the text..........................................................................................................................................16

2–2. Photograph taken near Estufa Spring, looking east (downstream) towards the

boundary fault and sediments of Estufa bolson ....................................................................19

2–3. Example of the strong calcic soil typical of Qoa deposits in Big Bend National Park....20

2–4. Examples of pediment sequences in Big Bend National Park............................................22

2–5. Rio Grande terraces near Santa Elena Canyon Overlook, southwest Big Bend

National Park...............................................................................................................................23

2–6. Banded vegetation and surface characteristics of deposits on which the banded

vegetation forms .........................................................................................................................24

3–1. Landsat 7 image showing Big Bend National Park and surrounding regions..................30

3–2. Location map and stratigraphic composite section for the Chisos Group and

South Rim Formations ................................................................................................................32
v

3–3. Total alkali versus silica diagram showing the major rock types discussed

in the text......................................................................................................................................33

3–4. Landsat 7 image of the western half of Big Bend National Park indicating some

of the major features and geologic units discussed in the text..........................................34

3–5. Various photos and composite stratigraphic section for the Blue Creek Ranch area ...36

3–6. Photo of Tule Mountain indicating the major volcanic rocks exposed in the area .........37

3–7. Photos showing distant and close up views for the stratigraphy of the Round

Mountain area located south of the Mule Ears Viewpoint..................................................38

3–8. Location map and photos showing the stratigraphy of Cerro Castellan...........................39

3–9. Various photos showing the Bee Mountain Basalt above the Mule Ear Spring Tuff......40

4–1. Index maps and photo of study area in Big Bend National Park and at Dagger Flats....44

4–2. Geologic and magnetic intensity maps of the Dagger Flats vicinity ..................................45

4–3. Series of photographs of composite sills exposed in the Dagger Flats area and

their range in composition ........................................................................................................47

4–4. Four diagrams from various sites at Dagger Flats where schematic stratigraphic

sections are compared with variation in content of SiO2, CaO, and Al2O3 and with 

changes in depth in the capping mineralized limestone units above the sill. Also

compared are alteration mineralogies for the skarn and sills ............................................48

4–5. Photographs of features exposed in skarns at Big Bend National Park...........................51

4–6. Simplified schematic diagrams illustrating a model for the intrusion of sills at

Dagger Flats into the Cretaceous Boquillas Formation and formation of the skarn .......52

5–1. Location of the study area and mercury mines studied.......................................................58

5–2. Photographs of Study Butte and Mariscal mercury mines and mine waste....................60

5–3. Concentration of mercury in mine waste and sediment collected in and around

Big Bend National Park .............................................................................................................61

5–4. Concentration of mercury versus methylmercury in water collected in the

Big Bend area..............................................................................................................................62

5–5. Mercury concentrations in air and soil gas collected from mercury mines in the

Big Bend area and from uncontaminated baseline sites in the park.................................63

6–1. Location of Big Bend water samples from springs, streams, and drinking

water wells ..................................................................................................................................66

6–2. Oxygen and hydrogen composition of Big Bend waters......................................................68

6–3. Chemical variation diagrams for selected constituents that illustrate fundamental

differences between waters from different areas................................................................69

6–4. Geologic setting of drinking water production sites in Big Bend National Park .............72

6–5. Photograph of vertical fractures (joints) in the Ward Mountain rhyolitic intrusive

rocks east of Ward Spring.........................................................................................................75

7–1. Black bear in Big Bend National Park.....................................................................................78

7–2. Map showing general location of the Big Bend ecosystem where black bear hair

samples were collected for this study ....................................................................................79

7–3. Plots of nitrogen isotopes versus carbon and sulfur isotope values for Big Bend

black bear hair ............................................................................................................................81

8–1. Magnetic anomaly and magnetic sources from the Big Bend National Park region .....86

8–2. Magnetic data superimposed on an aerial photo draped on a DEM along the south 

central portion of the Chalk Draw Fault..................................................................................87

8–3. Mapped faults superimposed on the maxima of the horizontal gradient of the

reduced to the pole magnetic anomaly ..................................................................................89

8–4. Color composite of potassium, thorium, and uranium superimposed on a grey shade 

map of the topography, with outlines of selected geologic units.......................................90
Tables

2–1. Age estimates for time divisions within the Tertiary and QuaternaryPeriods .................17

2–2. Surficial deposits in Big Bend National Park.........................................................................17

2–3. Rio Grande terraces and their general correlation to other alluvial deposits in

the park.........................................................................................................................................21

6–1. Helium-tritium and rare gas results.........................................................................................70

7–1. Stable isotope data for bear hair in the Big Bend National Park area..............................81

7–2. Average composition and standard deviation of selected elements in Big Bend 

National Park bear hair..............................................................................................................82

vi
1

Introduction

By J.E. Gray and W.R. Page

Big Bend National Park (BBNP), Tex., covers 801,163 acres (3,242 km2) and was established in 1944 through a transfer of land from the State of Texas to the United States. The park 

is located along a 118-mile (190-km) stretch of the Rio Grande at the United States–Mexico 

border. The park is in the Chihuahuan Desert, an ecosystem with high mountain ranges and 

basin environments containing a wide variety of native plants and animals, including more 

than 1,200 species of plants, more than 450 species of birds, 56 species of reptiles, and 75 

species of mammals. In addition, the geology of BBNP, which varies widely from high 

mountains to broad open lowland basins, also enhances the beauty of the park. For example, 

the park contains the Chisos Mountains, which are dominantly composed of thick outcrops of 

Tertiary extrusive and intrusive igneous rocks that reach an altitude of 7,832 ft (2,387 m) and 

are considered the southernmost mountain range in the United States. Geologic features in 

BBNP provide opportunities to study the formation of mineral deposits and their environmental 

effects; the origin and formation of sedimentary and igneous rocks; Paleozoic, Mesozoic, and 

Cenozoic fossils; and surface and ground water resources. Mineral deposits in and around 

BBNP contain commodities such as mercury (Hg), uranium (U), and fluorine (F), but of these, 

the only significant mining has been for Hg. Because of the biological and geological diversity 

of BBNP, more than 350,000 tourists visit the park each year. 

The U.S. Geological Survey (USGS) has been investigating a number of broad and diverse 

geologic, geochemical, and geophysical topics in BBNP to provide fundamental information 

needed by the National Park Service (NPS) to address resource management goals in this park. 

Scientists from the USGS Mineral Resources and National Cooperative Geologic Mapping 

Programs have been working cooperatively with the NPS and several universities on several 

research studies within BBNP. Because the last geologic map of the entire BBNP was published in the 1960s, one of the primary goals of the USGS is to provide a new geologic map of 

BBNP at a scale 1:100,000; this work is ongoing among the USGS, NPS, the Texas Bureau of 

Economic Geology, and university scientists. This USGS Circular summarizes eight studies 

funded and primarily carried out by the USGS, but it is not intended to be a comprehensive 

reference of work conducted in BBNP. This Circular describes topical research of the recently 

completed interdisciplinary USGS project, which has provided information leading to a more 

complete understanding of the following topics in BBNP:

· Tectonic and geologic history (Chapters 1, 2, and 3),

· Age and formation processes of a skarn mineral deposit (Chapter 4),

· Geoenvironmental effects of abandoned mercury mines (Chapter 5),

· Age, source, and geochemistry of surface and subsurface water resources (Chapter 6),

· Isotopic tracing of food sources of bears (Chapter 7), and

· Geophysical characteristics of surface and subsurface geology (Chapter 8).

Additional information and the geochemical and geophysical data of the USGS studies in 

BBNP are available on line at http://minerals.cr.usgs.gov/projects/big_bend/index.html.

Abstract

Geologic features in Big Bend National Park (BBNP) 

formed from repetitive cycles of rifting, mountain building, basin 

development, faulting and folding, and volcanism and plutonism. 

The oldest tectonic episode recorded in the park is mountain 

building associated with the Marathon orogeny, which deformed 

the rocks in the park region from Middle Mississippian to Early 

Permian time (about 330–285 Ma). During this episode, deep 

ocean basin rocks originally deposited south of BBNP were 

thrust northwestward onto the edge of the North American continent by collision between the North and South American plates.

From the Late Triassic into the Late Cretaceous (about 

200–85 Ma), the BBNP area experienced subtle effects of 

rifting between the North and South American continents; this 

rifting largely controlled the opening of the Gulf of Mexico. 

During rifting, BBNP was part of the Diablo Platform, a 

carbonate platform where the sediments that would become 

Lower and Upper Cretaceous limestone and shale units were 

deposited in shallow, open-marine water.

The Late Cretaceous to early Tertiary Laramide orogeny 

(about 70–50 Ma) had significant influence on the tectonic 

evolution of BBNP. This period of contractional deformation produced major uplifts, basins, and faults and folds that 

formed from northeast-directed compression related to collision between the North American continent and the Pacific/

Farallon plate at the western edge of the continent. Major Laramide features in BBNP include the northeast-facing Mesa de 

Anguila, an uplifted monocline on the southwest margin of the 

park, and the southwest-facing Sierra del Carmen–Santiago 

Mountains, an uplifted and thrusted monocline bounding the 

east part of the park. The Tornillo Basin developed between 

these uplifted basin margins.

Major episodes of volcanism and plutonism occurred 

in the park area from about 46 to 28 Ma. Principal features 

formed in this time period include numerous volcanic flows 

extruded from a complex system of volcanic vents and lava 

domes in the western part of the park and adjacent areas of 

Mexico, and the Pine Canyon caldera complex, which constitutes the high Chisos Mountains. It was during this time that 

major laccolithic complexes developed and intruded Cretaceous 

and Tertiary sedimentary rocks to form the Rosillos Mountains, 

McKinney Hills, and Grapevine Hills. These volcanic features, 

especially The Basin area of the Chisos Mountains, are some of 

the most visited and scenic parts of the park.

The last major tectonic episode to affect BBNP was Basin 

and Range faulting from about 25 to 2 Ma. Features formed 

in BBNP during this time period include mostly high-angle 

normal faults that cut the entire stratigraphic sequence through 

Pliocene basin-fill deposits. Movement on some of the major 

faults has formed local depositional basins, such as the Estufa 

and Delaho bolsons on the flanks of the high Chisos Mountains. Basin and Range faults predated integration of the late 

Tertiary basins with the Rio Grande, which began about 2 Ma, 

and they were the last structures to modify the landscape into 

its present-day configuration.

Introduction

This report is a brief overview of the salient aspects of 

the tectonic history of BBNP. A more thorough review of the 

state of knowledge of tectonism for the BBNP area is beyond 

the scope of this report. The report does provide the reader 

with key references that address the tectonic history of BBNP 

and adjacent areas in greater detail. Muehlberger (1980, 1989) 

and Muehlberger and Dickerson (1989) reviewed the tectonic history of the Trans-Pecos Texas region, including the 

BBNP area, related to the pervasive influence of preexisting 

structural grain on the development of later tectonic features. 

Dickerson (1980) summarized the tectonic history of the 

Trans Pecos region in terms of major structural zones, which 

were reactivated by structures of different regional stress 

regimes throughout time. Lehman (1991) and Lehman and 

Busbey (2007) provided summaries of Laramide and Neogene 

tectonism in BBNP based on detailed sedimentation studies. 

The early work of Maxwell and others (1967) provided an 

impressive, detailed account of the tectonic history and major 

structural features specific to BBNP. These reports, in addition 

to those referenced below, serve as a foundation for understanding the complex tectonic history of the BBNP region.

Proterozoic Rifting

Although the oldest recorded tectonism in BBNP is 

related to the late Paleozoic Marathon orogeny, Proterozoic 

rifting and protocontinental plate collisional events likely had 

1. Tectonic History of Big Bend National Park

By William R. Page, Kenzie J. Turner, and Robert G. Bohannon
4 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

a profound influence in shaping BBNP’s landscape. These 

older events are thought to have established the northwest 

trend of major structural and geographic features that characterize the park today (fig. 1–1) (Muehlberger, 1980).

Neoproterozoic rifting (about 800–550 Ma) along the 

southern edge of the North American continent produced 

northwest-striking continental transform faults that laterally 

offset the southern continental margin (fig. 1–2) (Thomas, 

1991; Poole and others, 2005). Poole and others (2005) 

reported that these faults controlled the geometry and configuration of the Ouachita-Marathon-Sonora orogenic belt; figure 

1–3 illustrates how these older faults define major northwestto-southwest bends in the Marathon belt that mimic transform 

fault offset patterns.

The transform faults are subparallel with the Texas 

lineament (Albritton and Smith, 1957; Muehlberger, 1980), a 

northwest-trending zone of proposed wrench faulting extending from Presidio, Tex., southward into the Big Bend area 

(fig. 1–2). Muehlberger (1980) and Dickerson (1980) suggested that faults related to the deeply buried Texas lineament may have formed during rifting events as long ago as 

1,500–1,000 Ma, and the fault zone was reactivated during 

subsequent tectonic episodes, including the late Paleozoic 

Marathon orogeny, Late Cretaceous–early Tertiary Laramide 

orogeny, and late Tertiary Basin and Range faulting. Some 

of the major northwest-trending features in BBNP include 

the Sierra del Carmen–Santiago Mountains and the Mesa de 

Anguila monoclinal uplifts, and numerous northwest striking 

faults and folds (fig. 1–1).

Paleozoic

During its early history (Paleozoic), BBNP was near the 

southern edge of the North American continent and was the 

site of marine deposition and complex tectonic interactions 

between the North and South American plates. The oldest tectonic episode recorded in the park is mountain building associated with the Marathon orogeny, which deformed rocks in the 

BBNP region from Middle Mississippian to Early Permian 

time (about 330–285 Ma; fig. 1–3) (Poole and others, 2005). 

The Marathon orogenic belt is one segment of the much larger 

Ouachita-Marathon-Sonora orogenic belt of Poole and others 

(2005), which extends along the Paleozoic continental margin 

from Mississippi and Arkansas (Ouachita segment), through 

Texas (Marathon segment), and into Chihuahua and Sonora, 

Mexico (Sonora segment).

In the west Texas area, deep-water ocean basin rocks 

originally deposited south of the Big Bend area were thrust 

northwestward onto the North American continent above a 

subduction zone created by the collision between the North 

and South American plates (fig. 1–3). Along this zone, the 

subducting slab of North American continental crust plunged 

southeast below the overriding South American plate. Paleozoic rocks of the Marathon belt are well exposed to the north 

of BBNP in the Marathon Basin (Wilson, 1954; Muehlberger 

and Tauvers, 1989) where they are deformed by northeaststriking, northwest-vergent thrust faults and folds.

Only remnants of the Marathon orogeny can be seen in 

BBNP’s present-day landscape; these remnants are characterized by small outcrop belts (fig. 1–1) containing Paleozoic 

rocks and northwest-directed thrust faults. Paleozoic rocks 

of the Marathon orogeny are best exposed in the Persimmon 

Gap area of the park (fig. 1–3) (Maxwell and others, 1967) 

and include, from base to top, the Middle Ordovician Maravillas Formation, Silurian to Mississippian Caballos Novaculite, and the Mississippian to Pennsylvanian Tesnus Formation (fig. 1–4) (Poole and others, 2005). Northwest-vergent 

thrust faults duplicate the Paleozoic section and juxtapose 

rocks of the Maravillas Formation above the Tesnus Formation (Maxwell and others, 1967; Poth, 1979). The Paleozoic 

thrust block at Persimmon Gap was subsequently deformed 

during the Late Cretaceous–early Tertiary Laramide orogeny 

(see below) by southwest-vergent thrust faults, which juxtaposed the Paleozoic thrust block with Cretaceous sedimentary rocks above and below (Maxwell and others, 1967; Poth, 

1979).

The Maravillas Formation and Caballos Novaculite 

are interpreted as pre-orogenic, deep-water basinal rocks 

(McBride, 1989; Noble, 1994; Page and Harris, 2007), and 

the Tesnus Formation is interpreted as synorogenic flysch 

deposited into foreland basins in front of the advancing Marathon thrust sheet (Poole and others, 2005). The Marathon 

orogenic highlands slowly eroded during Triassic and Jurassic 

Periods. The absence of Triassic- and Jurassic-age rocks in 

BBNP reflects this period of erosion, which is characterized 

locally in the park by a major unconformity separating rocks 

of the Lower Cretaceous Glen Rose Limestone above from 

rocks of the Mississippian-Pennsylvanian Tesnus Formation 

below (fig. 1–4).

Mesozoic-Cenozoic

Triassic-Cretaceous Rifting

From the Late Triassic into the Late Cretaceous (about 

200–85 Ma), the BBNP area experienced subtle affects of 

rifting, or seafloor spreading, between North and South 

America; this rifting controlled the opening of the Gulf of 

Mexico (Muehlberger, 1980; Muehlberger and Dickerson, 

1989; Lehman and Busbey, 2007). During rifting, BBNP was 

part of the Diablo Platform (fig. 1–5) (Henry, 1998; Goldhammer and Johnson, 1999), where sediments of Lower and Upper 

Cretaceous limestone and shale units were deposited in shallow open-marine water. These units include, from base to top, 

the Glen Rose Limestone, Del Carmen Limestone, Sue Peaks 

Formation, Santa Elena Limestone, Del Rio Clay, Buda Limestone, and Boquillas Formation (fig. 1–4), and are preserved 

today mostly in the Sierra del Carmen–Santiago Mountains, 

Nine Point Mesa, Mariscal Mountain, and Mesa de Anguila 

areas of the park (fig. 1–1).
Mesozoic-Cenozoic 5

0 10 20 5 Kilometers

1:500,000

Tertiary intrusive bodies, undivided

Upper Tertiary basin-fill

Tertiary volcanic rocks, undivided

Tertiary-Cretaceous clastic sedimentary rocks, undivided

Cretaceous carbonate sedimentary rocks, undivided

Paleozoic rocks, undivided

103 ̊30” 103 ̊00”

29 ̊00”

29 ̊30”

DB

SQ

GH

MH

EB

SR

MM

WM B

PP PC

MA

CD

SD

SD

SM

RM

NP

Paleozoic rocks

Terlingua fault zone

Burro Mesa

 fault

Chalk Draw fault

Dugout Wells fault zone

MEXICO

MEXICO

Texas

Texas

Figure 1–1. Map of BBNP showing late Tertiary Basin and Range faults (last major tectonic episode in BBNP), middle Tertiary volcanic 

and plutonic features, and present-day distribution of generalized rock units (colors). Geologic features are designated by upper case 

letters and include: SM, Santiago Mountains; CD, Chalk Draw graben; NP, Nine Point Mesa; RM, Rosillos Mountains; SD, Sierra del 

Carmen; GH, Grapevine Hills; MH, McKinney Hills; EB, Estufa bolson; MM, Mariscal Mountain; PC, Pine Canyon caldera; B, The Basin; 

PP; Pulliam Peak; WM, Ward Mountain; SR, South Rim; SQ, Sierra Quemada; DB, Delaho bolson; and MA, Mesa de Anguila. Thick black 

dashed line in central part of figure is outline of Chisos Mountains; green dashed line is Pine Canyon caldera. Solid black lines with bar 

and ball are late Tertiary Basin and Range faults; bar and ball is on downthrown side.
6 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

0 100 200 Kilometers

New Mexico

Texas

MEXICO

USA

NORTH AMERICAN PLATE

SOUTH AMERICAN PLATE

Presidio

modified from Poole

and others (2005)

NW SE

North American

plate

South American

plate

Ocean basin rocks

Big Bend 

National Park

0 100 200 Kilometers

105°0'0"W 100°0'0"W

30°0'0"N

Marathon orogenic belt

New Mexico

Texas

MEXICO

USA

NORTH AMERICAN

PLATE

SOUTH AMERICAN PLATE

PG

modified from Poole 

and others (2005)

Figure 1–2. Index map showing 

transform faults and lineaments 

related to Proterozoic rifting along 

the southern edge of North America. 

Solid black lines are Neoproterozoic 

continental transform faults (Poole 

and others, 2005; Thomas, 1991); 

arrows show relative motion of 

continental plate offset; gray 

double-banded line is main rift zone 

between North and South American 

plates; solid red lines are Texas 

lineament (Albritton and Smith, 1957; 

Muehlberger, 1980); and dark green 

area is BBNP.

Figure 1–3. Map showing location 

of Marathon orogenic belt and 

subduction zone (inset figure) 

formed from collision between the 

North and South American plates. 

Dotted black lines are traces of 

transform faults shown in figure 

1–2; short solid red line represents 

approximate line of section for inset 

figure; dark green area is BBNP; PG, 

Persimmon Gap.
Mesozoic-Cenozoic 7

Southwest of the platform, an ocean basin known as the 

Chihuahua trough formed in association with the opening of the 

Gulf of Mexico (fig. 1–5) (Muehlberger, 1980; Muehlberger and 

Dickerson, 1989; and Lehman and Busby, 2007). Development 

of the trough resulted from Late Triassic rifting, and faulting 

along its margin with the Diablo Platform controlled subsidence 

of the trough from the Jurassic to Late Cretaceous time (fig. 

1–5) (Lehman and Busbey, 2007). Cretaceous rocks in the Chihuahua trough are much thicker than time-equivalent rocks of 

the Diablo Platform. Lehman and Busbey (2007) reported that 

trough rocks of the Ojinaga Formation (found outside of BBNP) 

are about six times thicker than equivalent platform rocks of the 

Boquillas Formation in BBNP.

Deposition of the Upper Cretaceous Aguja and Javelina 

Formations in BBNP indicate a significant shift from marine 

to continental deposition. This change in depositional regime, 

coupled with thickness similarities between equivalent trough 

and platform rocks, indicate the trough had filled and major 

subsidence had ended by Late Cretaceous time (Lehman and 

Busbey, 2007).

Late Cretaceous–Early Tertiary Laramide 

Orogeny

The Late Cretaceous to early Tertiary Laramide orogeny 

(about 70–50 Ma) had significant influence on the tectonic evolution of BBNP; it was one of the most tectonically active times 

in the park’s history. This period of contractional deformation 

produced major uplifts, basins, and faults and folds that formed 

from northeast-directed compression related to subduction of the 

Pacific/Farallon plate beneath the North American continent at 

the western edge of the continent (fig. 1–6). The plate collision 

had a major influence on the development of the Laramide fold 

and thrust belt, which extends from Canada to Mexico; the Chihuahua tectonic belt, south of BBNP, represents the frontal edge 

of the Laramide fold and thrust belt in the region.

Features of the Laramide orogeny in BBNP (fig. 1–6) 

include the northeast-facing Mesa de Anguila, an uplifted 

monocline on the southwest margin of the park, and the 

southwest-facing Sierra del Carmen–Santiago Mountains, an 

uplifted and thrust-faulted monocline bounding the east part of 

the park. The Tornillo Basin developed between these uplifted 

monoclines (Lehman, 1991; Lehman and Busbey, 2007).

The Mesa de Anguila monocline in BBNP was described 

by DeCamp (1985) as a northeast-directed monoclinal flexure 

that formed from northeast-directed compression of the 

Chihuahua tectonic belt against the Diablo Platform. DeCamp 

(1985) discussed evidence for left-lateral, strike-slip faults at 

Mesa de Anguila that accommodated uplift of the monocline 

during the Laramide. The monocline and strike-slip faults 

were interpreted as an eastern extension of the much broader 

Terlingua-Solitario uplift, west of BBNP (Erdlac, 1990). Erdlac reported the age of the uplift at between 68 and 50 Ma and 

interpreted it as a Laramide push-up block, or transfer zone, 

which primarily accommodated left-lateral shearing related to 

northeast-directed Laramide compression.

The Sierra del Carmen–Santiago Mountains monocline is 

characterized by a southwest-directed flexure containing southwest-directed thrust, or reverse faults (fig. 1–6, Santiago thrust) 

(Poth, 1979; Cobb, 1980; Cobb and Poth, 1980). The feature 

represents the southwestern margin of a broader feature known 

as the Marathon uplift (Muehlberger and Dickerson, 1989; Lehman, 1991) or dome (Ewing, 1991). Dickerson (1980) interpreted the feature as a raised basement ridge that was uplifted 

along basement-cored faults on the Sierra del Carmen trend, 

and Lehman and Busbey (2007) noted similarities between the 

uplift and other basement-cored uplifts in the Rocky Mountain 

Cordilleran foreland belt. The uplift has also been attributed to 

left-lateral shearing along the faults at the uplift margin (Cobb 

and Poth, 1980; Muelberger, 1980; Muehlberger and Dickerson, 

1989) and was interpreted as part of a broad transfer zone similar to the Terlingua-Solitario uplift (Erdlac, 1990).

The Tornillo Basin of Lehman (1991) formed between 

the Mesa de Anguila and Sierra del Carmen–Santiago Mountains monoclines (fig. 1–6). Synorogenic rocks that compose 

the basin fill are mostly fluvial deposits and include, from base 

to top, the Javelina, Black Peaks, and Hannold Hill Formations (fig. 1–4). Based on sedimentological and structural 

data, Lehman (1991) reported that Laramide deformation in 

the Tornillo Basin began with deposition of sediments of the 

Javelina Formation (70–65 Ma) and ended after the middle 

Eocene, with deposition of sediments of the Canoe Formation, 

about 50 Ma (fig. 1–4). The Canoe Formation overlaps folds 

as well as thrust and reverse faults in the Hannold Hill, Black 

Peaks, Javelina, and Aguja Formations (Lehman and Busbey, 

2007). Major Laramide folds in the Tornillo Basin of BBNP 

include the Cow Heaven, Mariscal Mountain, and San Vicente 

anticlines (fig. 1–6).

Middle Tertiary Volcanism and Plutonism

Major episodes of volcanism and plutonism occurred in 

BBNP from about 46 to 28 Ma (Maxwell and others, 1967; 

Barker and others, 1986; Henry and McDowell, 1986). Volcanism and plutonism resulted from shallow subduction of the 

Farallon plate on the western margin of the North American 

continent. As the subducted plate descended, it increased in 

temperature and melting caused magma to form. The magma 

rose through the continental crust either to erupt at the surface 

to form volcanic rocks, or to be injected at depth where it 

solidified to form plutonic rocks. Volcanoes and volcanic 

rocks in the Big Bend area formed as part of a continental arc 

that extends from Mexico into Canada.

The oldest volcanic rocks in BBNP are lavas and tuffs 

of the Chisos Group (fig. 1–4) (Maxwell and others, 1967; 

Henry and McDowell, 1986; Henry and Davis, 1996). These 

rocks include mafic lava flows, silicic ash-fall, and ash-flow 

tuffs that were erupted between about 46 and 33 Ma from 

poorly known sources in southwest BBNP and adjacent areas 

in Mexico (Henry and Davis, 1996). These rocks are south and 

west of the Pine Canyon caldera complex and extend west and 

south into Mexico (fig. 1–1).
8 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas PALEOZOIC MESOZOIC Ordovician Silurian Devonian Mississippian Pennsylvanian Lower Cretaceous Maravillas Fm. Caballos Novaculite Tesnus Fm. Glen Rose Ls. Del Carmen Ls. Sue Peaks Fm. Santa Elena Ls. Del Rio Clay Buda Ls. Boquillas Fm. Upper Cretaceous MESOZOIC Upper Cretaceous Pen Fm. Aguja Fm. Javelina Fm. Paleocene Black Peaks Fm. Eocene

Canoe

Fm. Chisos Group

Oligocene

South

Rim Fm.

Burro Mesa

Fm.

Miocene

&

Pliocene

rocks

CENOZOIC

Quaternary

Tertiary

Hannold

Hill Fm.

Middle

Tertiary

volcanism

and

plutonism

(~48–28 Ma)

Late Cretaceous–

early Tertiary

Laramide

orogeny

(~70–50 Ma)

Late Triassic–

Late Cretaceous

rifting

(~200–85 Ma)

TriassicJurassic

erosion

Late

Paleozoic

Marathon

orogeny

(~330–285 Ma)

Late Tertiary Basin and

Range faulting

(~25–2 Ma)

Figure 1–4. Stratigraphic column showing geologic rock units and major tectonic episodes in Big Bend National Park.
Mesozoic-Cenozoic 9

Big Bend

National Park

Diablo Platform

Chihuahua trough

SW NE

Chihuahua trough

Diablo Platform

105°0'0"W 100°0'0"W

30°0'0"N

0 100 200 Kilometers

New Mexico

Texas MEXICO

USA

modified from 

Goldhammer and Johnson (1999)

Figure 1–5. Map showing location of the Diablo Platform and the Chihuahua trough, features that 

formed in the Big Bend area during Late Triassic through Late Cretaceous rifting between North and 

South America. Solid red line is approximate line of section for inset figure; dark green area is BBNP.

At about 32 Ma, the Pine Canyon caldera complex (fig. 

1–1) (Ogley, 1978; Barker and others, 1986; Benker, 2005; 

White and others, 2006) erupted rocks of the South Rim 

Formation (Barker and others, 1986; Benker, 2005; White and 

others, 2006). Rocks of the South Rim Formation are mostly 

preserved in the high Chisos Mountains, particularly the South 

Rim area (fig. 1–1) where the formation’s name originates 

(Maxwell and others, 1967). The Pine Canyon caldera complex developed originally as a large volcano prior to caldera 

formation, which elevated the topography throughout the 

Chisos Mountains (Ogley, 1978; Barker and others, 1986). The 

massive intrusion surrounding The Basin (Ward Mountain–

Pulliam Peak) is likely related to the Pine Canyon caldera 

magma system, based on similar chemistry (White and others, 

2006), and is probably slightly older than the caldera and the 

South Rim Formation.

Many of the larger intrusive bodies in BBNP were 

emplaced at about the same time as the Pine Canyon caldera. 

The laccoliths at the Rosillos Mountains, McKinney Hills, 

and Grapevine Hills (fig. 1–1) were emplaced between 33 and 

32 Ma (Miggins and others, 2007). Although the timing of 

emplacement for these intrusions is similar to the Pine Canyon 

caldera, it is unlikely that they are related. However, it is clear 

there was a significant influx of magma rising into the park at 

the time of caldera eruption.

At approximately 30 Ma, the Sierra Quemada dome (fig. 

1–1) formed to the south of The Basin (fig. 1–1). Sierra Quemada has previously been interpreted as a caldera that formed 

at 33.6 Ma (Duex and Tucker, 1989; Duex, 2007), but recent 

studies indicate the feature likely formed about 31–30 Ma by 

doming rather than caldera collapse (Scott and others, 2007). 

Magma injection caused the existing country rock to dome; 

the domed rocks were eventually cut by discontinuous ring 

dikes and smaller sills and dikes, and only a small quantity of 

extremely lithic-rich material erupted.

The youngest major volcanic and intrusive episode in 

BBNP is related to the 29.5-Ma Burro Mesa Formation (Miggins and others, 2004; Benker, 2005; White and others, 2006). 

The Burro Mesa Formation is composed of ash-flow tuffs 

erupted from small localized vents and lava domes in the southwest part of the park (Holt, 1998; and Parker, 2002). These 

eruptive rocks are seen as thick capping units above rocks of 

the Chisos Group at Kit Mountain, Goat Mountain, and Cerro 

Castellan (Miggins and others, this volume). Volcanism and 

intrusive events of the Burro Mesa Formation represent the last 

significant phase of igneous activity in BBNP.
10 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Late Tertiary Basin and Range Faulting

The last major tectonic episode in the park was Basin 

and Range faulting, which occurred in the park from about 

25 to 2 Ma. Basin and Range normal faulting began at about 

25 Ma in Trans-Pecos Texas (Henry, 1998) and resulted from 

rifting across the Basin and Range Province of western North 

America. The rifting caused stretching or pulling apart of 

the western North America crust to form uplifted ranges and 

intervening down-dropped basins. Big Bend National Park 

is part of the Rio Grande rift, a subprovince of the Basin 

and Range Province, which extends northward through New 

Mexico and into Colorado. Dickerson (1980), Muehlberger 

(1980), and DeCamp (1985) reported that some Basin and 

Range faults in the BBNP area of the Rio Grande rift (Mesa 

de Anguila area) also exhibited a component of right-lateral 

shear, reflecting northwestward movement of Mexico relative 

to Texas.

Major Basin and Range faults in BBNP are shown in 

figure 1–1. Some of these faults formed local depositional 

basins, such as the Estufa and Dehalo bolsons (Stevens and 

Stevens, 1989) on the flanks of the high Chisos Mountains 

(fig. 1–1). The bolsons contain thick accumulations of Miocene-Pliocene sediments that were deposited into half-grabens dipping away from the Chisos Mountains (Stevens and 

Stevens, 1989; Lehman and Busbey, 2007). The Delaho bolson is characterized by three or four major northwest-striking 

faults, while the Estufa bolson is bound on its west side by 

the northwest-striking Dugout Wells fault zone (Stevens and 

Stevens, 1989).

Other notable Basin and Range faults in BBNP include 

the Terlingua and Sierra del Carmen fault zones and the Chalk 

Draw and Burro Mesa faults (fig. 1–1). The Terlingua fault 

zone forms the steep limestone cliffs on the northeast side 

of Mesa de Anguila near the mouth of Santa Elena Canyon 

(DeCamp, 1985; Lehman and Busbey, 2007). The Sierra del 

Carmen fault zone consists of numerous northwest-striking 

normal faults that flank the Sierra del Carmen. The Terlingua 

and Sierra del Carmen fault zones are interpreted to have 

formed by reactivation of the older Laramide Mesa de Anguila 

and Sierra del Carmen–Santiago Mountains monoclinal uplifts 

(Poth, 1979; Cobb, 1980; Cobb and Poth, 1980; DeCamp, 

1985). In northern BBNP, the northwest-striking Chalk Draw 

fault bounds the Chalk Draw graben and was interpreted 

as a late Paleozoic structure that was reactivated during the 

Laramide orogeny and again during Basin and Range faulting (Dickerson, 1980; Muehlberger and Dickerson, 1989). 

The Burro Mesa fault is a down-to-the-southwest normal fault 

exposed on the northeast flank of Burro Mesa, where it drops 

rocks of the Chisos Group down against the Aguja Formation (Maxwell and others, 1967). The fault forms the eastern 

boundary of the Castolon graben as defined by Stevens (1969) 

and Muehlberger (1989).

Basin and Range faults predated integration of the late 

Tertiary basins with the Rio Grande, which began about 2 

Ma (Henry, 1998), and they were the last structures to shape 

BBNP’s present-day landscape. The Quaternary Period (1.8 

Ma to present) in BBNP was characterized by extensive erosion, down-cutting, and some aggradation related to integration of BBNP basins with the Rio Grande. The Quaternary 

geology of BBNP is summarized in Berry and Williams (this 

volume), Berry (2008), and Lehman and Busbey (2007).

Summary

The oldest tectonic episode recorded in the park is mountain building associated with the Paleozoic Marathon orogeny, 

which affected the rocks in the park region from Middle Mississippian to Early Permian time (about 330–285 Ma). Only 

remnants of this oldest episode remain in BBNP’s present-day 

landscape.

From the Late Triassic into the Late Cretaceous (about 

200–85 Ma), the BBNP area experienced subtle affects of 

rifting, or seafloor spreading, between the North and South 

American continents; this rifting largely controlled the opening of the Gulf of Mexico. During rifting, BBNP was part of 

the Diablo Platform, a carbonate platform where the sediments 

that would become Lower and Upper Cretaceous limestone 

and shale units were deposited in shallow, open-marine water.

The Late Cretaceous to early Tertiary Laramide orogeny 

(about 70–50 Ma) had significant influence on the tectonic 

history of BBNP. This period of contractional deformation produced major uplifts, basins, and faults and folds that 

formed from northeast-directed compression related to collision between the North American continent and the Pacific/

Farallon plate at the western edge of the continent. Major Laramide features in the park include the northeast-facing Mesa 

de Anguila uplifted monocline on the southwest margin of the 

park and the southwest-facing Sierra del Carmen–Santiago 

Mountains uplifted and thrust-faulted monocline bounding the 

east part of the park. The Tornillo Basin developed between 

the uplifted basin margins.

Major episodes of volcanism and plutonism occurred 

in the park area from about 46 to 28 Ma. Principal features 

formed in this time period include numerous volcanic flows 

extruded from a complex system of volcanic vents and lava 

domes in the western part of the park and adjacent areas of 

Mexico, and the Pine Canyon caldera complex in the high Chisos Mountains. Laccolithic complexes developed and intruded 

Cretaceous and Tertiary sedimentary rocks to form the Rosillos Mountains, McKinney Hills, and Grapevine Hills.

The last major tectonic episode to affect BBNP was 

Basin and Range faulting from about 25 to 2 Ma. Movements on some of the major faults have formed local depositional basins, such as the Estufa and Delaho bolsons on 

the flanks of the high Chisos Mountains. Basin and Range 

faults predated integration of the late Tertiary basins with 

the Rio Grande, which began about 2 Ma, and they were the 

last structures to modify the landscape into its present-day 

configuration.
Summary 11

Sierra del Carmen–Santiago

M

o

u

ntains m

o

n

o

clin

e

Mesa de Anguila 

monocline Mariscal Mountain anticline

Cow Heaven anticline

San Vicente

anticline

Terlingua monocline

Tornillo

Basin

Santiago

thrust

0 10 20 5 Kilometers

1:500,000

103 ̊30 ̋ 103 ̊00”

29 ̊00”

29 ̊30”

MEXICO

MEXICO

Texas

Fa r a l l o n p l a t e

Laramid

e fold an

d thrust belt

Subduction zone

Chihuahua

tectonic

belt

N

o r t h A

m

e r i c a n

p l a t e

0 500 1,000 250 Kilometers

modified from Henry (1998)

modified from Lehman 

and Busby (2007)

Figure 1–6. Generalized map of the major structures that formed in BBNP during the Laramide orogeny. Inset map shows location of 

Laramide fold and thrust belt in western North America and Chihuahua tectonic belt in the BBNP area.
12 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

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Texas: Austin, Tex., University of Texas, M.A. thesis, 55 p.

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structural-tectonic events along and near the Rio Grande, 

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p. 635.

Abstract

The Quaternary geologic and climatic history of Big 

Bend National Park is recorded in multiple levels of pedimentcapping gravel deposits, stream and river alluvium, and 

alluvial-fan deposits that cover much of the park. Based on 

relative-age indicators (soil development and surface morphology) on these surfaces, the oldest ones probably are middle 

(or possibly early) Pleistocene, the intermediate ones middle 

and late Pleistocene, and the youngest ones late Pleistocene to 

Holocene and Holocene in age. Eolian landforms in this arid 

region are relatively minor in extent. Alluvial valley floors 

have been deflated locally by wind erosion and in places are 

covered by small coppice dunes, but large areas of eolian sand 

deposition are generally lacking. In the northernmost part of 

the park, patterns that resemble eolian deposits on aerial photographs are formed by bands of relatively dense vegetation 

separated by more sparsely vegetated interband areas. This 

type of banded vegetation develops in desert environments like 

that of Big Bend National Park to maximize water available 

for plant growth.

Introduction

Surficial deposits in Big Bend National Park, west 

Texas, record the long and complex Quaternary geologic and 

climatic history of this border region. Since the Rio Grande 

became established, probably in late Pliocene to early 

Pleistocene time (fig. 2–1, table 2–1) (Dickerson and Muehlberger, 1994), the fluvial system has been dominated by 

long-term incision (the downward erosion of a stream, deepening the stream channel by removal of rock and sediment) 

punctuated by episodes of aggradation (the building-up of 

a stream valley by deposition of sediment) that were likely 

related to fluctuations in climate (Dethier, 2001; Connell 

and others, 2005). The Big Bend area was far removed 

from alpine glaciations that occurred during the Pleistocene 

in the mountainous headwaters of the Rio Grande, located 

in northern New Mexico and southern Colorado (Connell 

and others, 2005). Data from fossil packrat middens (the 

piles of plant and animal remains collected by packrats and 

preserved by crystallized packrat urine) found in and around 

Big Bend National Park indicate that during the last major 

glaciation (around 20,000 years ago) Big Bend’s climate was 

characterized by milder winters, cooler summers, and more 

precipitation than in postglacial times; they also show that 

conifer woodlands extended from the mountains out onto the 

lowlands (Elias, 1997). Since the transition to a postglacial 

environment 10,000–11,000 years ago, the climate in the Big 

Bend area has become hotter and drier with less effective 

moisture, and for the last 4,200 years or more the lowlands 

have supported desert vegetation similar to that found in the 

region today (Elias, 1997).

Quaternary Alluvial Deposits

Thin gravel deposits capping pediments (broad, gentlysloping, laterally-planed erosion surfaces), alluvial-fan 

deposits, and stream or river alluvium cover a large part of 

the park. Material suitable for radiocarbon dating of the 

deposits is scarce, and volcanic ashes, useful in constraining 

ages of Quaternary surficial deposits elsewhere in the western 

and southwestern United States, have not been found as far 

south as Big Bend. Therefore, relative-age criteria are used 

to interpret age relationships of the geomorphic surfaces of 

the deposits (for example, Bull, 1991). Criteria used to assess 

relative age include relative height above the valley floor, 

which is higher for older surfaces where streams have incised; 

preservation state of depositional morphology, which is subdued through time by post-depositional processes; and amount 

of surface dissection, characteristics of desert pavement, soil 

development, and weathering, all of which tend to increase or 

change predictably through time.

The surficial deposits are grouped based on their genesis 

and relative age (table 2–2). Groups Qya, Qyw, Qia, and Qiw

can be further subdivided by relative age in many places in 

the park. River deposits are differentiated from other types of 

alluvium mainly along the Rio Grande, where morphological 

distinctions are pronounced. Terraces along this portion of the 

Rio Grande typically are underlain by coarse-gravel deposits 

with subrounded to well-rounded clasts. Soil carbonate accumulation on the terraces tends to be somewhat greater than 

that in corresponding fan and pediment alluvium, likely due 

to subtle textural differences that may influence percolation 

through the soil.

2. Surficial Deposits of Big Bend National Park

By Margaret E. Berry and Van S. Williams
118 Texas Santa Elena Canyon Chisos Mtns Rio 

Grande

Big Bend National Park

Tornillo Creek

Chalk Draw

Estufa

Canyon

Santiago Draw

Reed Camp

Area

Persimmon Gap

118

Texas

385

US

Tornillo  Flat

United States Mexico N 10 km 10 mi 0 5 0 5

Alamo Creek

Castolon

Estufa Spring

.

.

.Overlook

Chilicotal  Mtn

Talley  Mtn

Figure 2–1. Map showing locations within Big Bend National Park that are referred to in the text.
Quaternary Alluvial Deposits 17

Table 2–2. Surficial deposits in Big Bend National Park.

Holocene

Qaw

Qe Qya Qyw

Qrf Qea

Qc Qls Qs

Pleistocene

late

Qia Qiw

middle

Qoa Qow

early

QTa Pliocene

Definition of unit symbols and estimated relative age ranges:

Qaw: Active tributary wash and river deposits; latest Holocene

Qe: Eolian sand; Holocene

Qya: Young alluvial deposits (pediment, fan, and stream), undivided; Holocene to late Pleistocene

Qyw: Young axial river deposits; Holocene to late Pleistocene

Qrf: Rock-fall deposits; Holocene and late Pleistocene

Qea: Mixed eolian and alluvial deposits; Holocene and late Pleistocene

Qc: Slope deposits consisting of colluvial aprons and colluvial-fan deposits; Holocene to early? Pleistocene

Qls: Landslide deposits; Holocene to early? Pleistocene

Qs: Spring deposits; Holocene to early? Pleistocene

Qia: Intermediate alluvial deposits (pediment, fan, and stream), undivided; late and middle Pleistocene

Qiw: Intermediate axial river deposits; late and middle Pleistocene

Qoa: Old alluvial deposits (pediment, fan, and stream), undivided; middle to early? Pleistocene

Qow: Old axial river deposits; middle to early? Pleistocene

QTa: Very old alluvium; early Pleistocene and Pliocene

Table 2–1. Age estimates for time divisions within the Tertiary 

and Quaternary Periods. Based on USGS “Divisions of Geologic 

Time” (U.S. Geological Survey Geologic Names Committee, 

2007) and the ICS time scale (Ogg, 2004).

Geologic time 

period Time divisions Age estimate

Holocene

Pleistocene

Quaternary (Q)

11,500–0 yr

126,000–11,500 yr

781,000–126,000 yr

1.8 Ma–781,000 yr

5.3–1.8 Ma

23–5.3 Ma

33.9–23 Ma

55.8–33.9 Ma

65.5–55.8 Ma

late

middle

early

Pliocene

(T)

Miocene

ertiary 

Oligocene

T

Eocene

Paleocene
18 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

General characteristics of the young alluvial deposits 

(Qya and Qyw, Holocene to late Pleistocene) include limited preservation of original surface morphology (channel 

topography and bar-and-swale or meander-scroll topography) and low relative height above active stream channels 

or valley floors. Deposits are generally unconsolidated to 

weakly consolidated. Development of desert pavement, 

varnish, and soil carbonate range from little or none on the 

younger of the Qya and Qyw deposits to densely packed 

and uniform pavement with well-varnished clasts and stage 

I1 soil-carbonate morphology on the older of the Qya and

Qyw deposits. For further discussion of soil-carbonate 

morphology and the formation of soils in arid environments, 

see Birkeland (1999). 

Intermediate-age alluvial deposits (Qia and Qiw, late 

and middle Pleistocene), at intermediate relative height, typically have surfaces that are uniform (due to the obliteration of 

original, depositional morphology), partly incised, and slightly 

rounded at the edges. Deposits are generally weakly to 

moderately well consolidated. Pavement ranges from weakly 

developed to densely packed and uniform with well-varnished clasts. Soils typically have stage I–II (locally weakly 

cemented stage III) carbonate morphology on the younger of 

the Qia and Qiw deposits and weakly to moderately cemented 

stage III–IV carbonate morphology on the older of the Qia 

and Qiw deposits. 

Old alluvial deposits (Qoa and Qow, middle to early? 

Pleistocene), at high relative height, are commonly moderately to well consolidated. Surfaces have been dissected into 

ridge and ravine (ballena) topography where gravel deposits 

are relatively thick (as in the fan deposits), or are preserved 

as generally planar or gently undulatory remnants high above 

adjacent valley floors where gravel deposits are thin (as on the 

pediments). Surface pavements range from weakly developed 

to moderately packed and uniform with moderately varnished 

clasts; soils typically have weakly to moderately cemented 

stage IV carbonate morphology. 

Very old alluvium (QTa, early Pleistocene and Pliocene), typically well consolidated, thick deposits primarily of 

alluvial fan origin, are eroded and deeply dissected into ridge 

and ravine (ballena) topography. Near faults, beds have been 

titled by drag (the folding of beds on either side of a fault due 

to shear created by movement on the fault). Where preserved, 

soils have stage III to IV carbonate morphology in horizons as 

much as 2 m thick. 

1

 In gravelly deposits, the progressive development of soil-carbonate morphology is described briefly as follows: Stage I—thin, discontinuous coatings 

of CaCO3 are present on the bottom side of clasts. Stage II—CaCO3 coatings 

are continuous around clasts; the soil matrix is mostly loose, but several clasts 

may be cemented together locally. Stage III—CaCO3 is disseminated throughout the soil matrix, cementing the horizon and giving it a whitish color. Stage 

IV—thin laminar layers of almost pure CaCO3 are present in the upper part of 

the horizon; the rest of the horizon is plugged with accumulated CaCO3.

Bolson Fill

Thick accumulations of Tertiary- and early Quaternaryage sediment, interpreted as bolson fill,2 are preserved in 

several places in the park (Stevens and Stevens, 1989, 1990; 

Dickerson and Muehlberger, 1994). On the previous map 

of the park (Maxwell and others, 1967), the bolson fill was 

combined with Pleistocene pediment, fan, and terrace deposits and referred to as QTog. Fossils found in the deposits 

indicate that much of the bolson fill is Miocene in age—for 

example, the Delaho Formation in the Castolon area (Stevens 

and others, 1969; Stevens, 1977) and the informal Banta 

Shut-in formation in the lower Tornillo Creek area (Stevens 

and Stevens, 1989, 2003). In the Estufa bolson, northeast of 

the Chisos Mountains in the lower Tornillo Creek area (fig. 

2–1), an upper Miocene section of sediment is overlain by 

very old, fault-bounded alluvial-fan deposits (QTa) containing clasts thought to be derived from the Chisos Mountains 

(informal Estufa member of Thurwachter, 1984, and informal 

Estufa Canyon formation of Stevens and Stevens, 1989, 2003). 

The age of these alluvial-fan deposits is unknown, but has 

been interpreted as Pliocene-Pleistocene (Stevens and Stevens, 

1989, 2003), and possibly correlative with very old alluvialfan deposits in the Burro Mesa area (informal Fingers formation of Stevens, 1969, 1988; Stevens and Stevens, 1989). 

Landscape Development

Long-term aggradation represented by the thick bolson fills 

was followed by degradation that resulted in the erosion of large 

volumes of rock and sediment from the Big Bend landscape 

during the Quaternary period; this shift from aggradation to 

degradation was likely associated with establishment of the Rio 

Grande and declining rates of fault activity. The Estufa bolson 

provides a striking example of the amount of erosion that has 

occurred. The inferred Pliocene-Pleistocene alluvial-fan sediments of the Estufa bolson (QTa) were deposited in a graben 

(fault-bounded basin), but now stand in high relief, 46 to 73 

m above younger Quaternary sediments subsequently deposited on the upthrown side of the fault (fig. 2–2) (Thurwachter, 

1984). The oldest alluvial deposits that postdate erosion of 

Pliocene-Pleistocene sediments and bedrock from the upthrown 

side of the fault have strong calcic soils with cemented stage 

IV horizons 1 m thick or more (Qoa; fig. 2–3). This degree of 

soil development likely took several hundred thousand years to 

form (Machette, 1985) and indicates that the following sequence 

of events—(1) erosion producing the inverted topography of 

Estufa bolson, followed by (2) deposition of Qoa gravels, and 

(3) stabilization of the Qoa surface that allowed a soil to start 

2

 The term bolson, a Spanish word for large purse, is used in desert regions 

of the southwestern United States for a broad, mostly internally drained, 

interior basin (Neuendorf and others, 2005) commonly formed by faulting. 

Sediment deposited in these closed or semi-closed basins is referred to as 

bolson fill.
Landscape Development 19

QTa

Qia Qya2

Qya1

Qaw

Qya2

Figure 2–2. Photograph taken near Estufa Spring, looking east (downstream) towards the boundary fault (approximate location 

shown by dashed line) and sediments of Estufa bolson. Labels mark surfaces of deposits: QTa, inferred Pliocene-Pleistocene 

alluvial-fan sediments of Estufa bolson, originally deposited in a graben but now standing in high relief above younger Quaternary 

deposits; Qia, intermediate-age alluvial deposits; Qya2, older of the young alluvial deposits; Qya1, younger of the young alluvial 

deposits; Qaw, alluvium deposited in the active tributary wash. Nearby Qoa alluvial deposits are just out of view to the south 

(right) and north (left).
20 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

laminar layer cemented

stage IV

horizon

Figure 2–3. Example of the strong calcic soil typical of Qoa deposits in Big Bend National Park. The cemented stage IV horizon (with 

laminar layers at the top) likely took several hundred thousand years to form (Machette, 1985), suggesting that the Qoa deposits are 

middle Pleistocene or older. For scale, the larger clasts visible in the photograph are 10 to 15 cm in diameter.
Eolian Landforms 21

forming—was probably complete by several hundred thousand 

years ago, if not earlier. Given the amount of erosion required 

in an environment where rates of erosion were probably relatively low, it is likely that the onset of long-term degradation 

occurred sometime prior to the middle Pleistocene (in the early 

Pleistocene or late Pliocene). 

That extensive erosion of the landscape continued during the middle and late Pleistocene as well is shown by the 

multiple levels of gravel-capped pediments that are prominent throughout the park (fig. 2–4). Surfaces that make up 

the highest and therefore oldest relative-age group (Qoa) are 

thought to be middle to early? Pleistocene, based on the presence of strong calcic soils (cemented stage IV horizons) developed in the gravel deposits capping the pediment surfaces. 

These old pediment surfaces are typically 30–50 m above 

the valley floor. Below the highest level are more extensively preserved, intermediate pediment surfaces (Qia2 and

Qia1, generally 12–40 m and 6–24 m above the valley floor, 

respectively) that probably formed during the middle and late 

Pleistocene, based on their relative age characteristics. Still 

younger pediment surfaces (Qya2), generally 3 m or more 

above the valley floor, are probably late Pleistocene to Holocene in age. This extensive erosion of the Big Bend landscape 

during the Quaternary probably contributed to the destabilization of bedrock slopes, which resulted in large, multi-event 

landslides in several areas of the park, such as those at the 

Chilicotal and Talley Mountains (Collins and others, 2007) 

and the northwest mountain-front of the Chisos.

Rio Grande Terraces

Multiple levels of coarse-gravel terrace deposits are 

preserved along the Rio Grande in Big Bend National Park. 

The fluvial deposits range in thickness from less than 1 m to 

over 15 m, but commonly they are at least 3–4 m thick and 

overlie bedrock. These fluvial deposits probably reflect times 

during the Quaternary when, in response to periodic changes 

in climate and river hydrology, long-term incision of the river 

system was disrupted by episodes of aggradation (Dethier, 

2001; Connell and others, 2005). 

The highest level (Qow), represented by a few small isolated 

remnant deposits, is about 55–60 m above the Rio Grande floodplain (table 2–3, fig. 2–5). The relative age of Qow is equivocal, 

but based on its height above the next lower level (Qiw3) and 

the strength of calcic soils found in places on that lower level 

(moderately cemented stage III to IV horizons), Qow is likely 

middle Pleistocene or older. Three intermediate levels of terraces 

(Qiw3, Qiw2, and Qiw1; table 2–3), perhaps best observed in the 

area between Santa Elena Canyon Overlook and Castolon (figs. 

2–1 and 2–5), are probably middle and late Pleistocene in age. 

Whereas remnants of Qiw3 and Qiw2 are fairly local in their 

extent, Qiw1 forms broad, flat remnant surfaces that can be traced 

all along the stretch of river that flows through the park. The lowest terraces along the river (table 2–3) are likely late Pleistocene 

to Holocene (Qyw2) and Holocene (Qyw1) in age.

Eolian Landforms

Eolian landforms in this arid region are relatively minor 

in extent. In areas such as Tornillo Flat, alluvial valley floors 

have been deflated locally by wind erosion and in places are 

covered by small coppice dunes (also referred to as nabkhas, 

dunes formed when wind-blown sand accumulates around 

and on the lee side of shrubs or vegetation clumps), but large 

areas of eolian sand deposition are generally lacking. Patterns seen on aerial photographs of the northernmost part 

of the park, particularly near Chalk Draw, Santiago Draw, 

and northern Tornillo Flat, resemble eolian features but are 

actually banded vegetation (figs. 2–1 and 2–6). Best seen 

in aerial view, banded vegetation, also known as tiger bush 

or banded mosaic, is made up of bands of relatively dense 

vegetation separated by more sparsely vegetated interband 

areas (Aguiar and Sala, 1999; Wakelin-King, 1999; Saco 

and others, 2006). The banded pattern develops in arid and 

semiarid environments where water available for plant growth 

is limited. The bands align perpendicular to slope and form 

where surface gradients are low such that runoff remains 

unchanneled. During rainfall events, sheet flow generated in 

the sparsely vegetated interband areas flows downslope into 

the dense vegetation bands where it is trapped and infiltrates 

into the soil. This process maximizes the amount of water 

available for plant growth in an environment with limited 

rainfall (Aguiar and Sala, 1999; Wakelin-King, 1999; Saco 

and others, 2006). Interband areas in Big Bend commonly 

have a moderately to well-developed pavement or surface 

crusts that help promote sheet flow (fig. 2–6B, C).

Table 2–3. Rio Grande terraces and their general correlation to 

other alluvial deposits in the park. 

Terrace

height (m)1 Terrace Correlation2

≤3 Qyw1 Qya1

4–9 Qyw2 Qya2

12–18 Qiw1

Qia1

18–25 Qiw2

30–45 Qiw3 Qia2

55–60 Qow Qoa?

1

 General height above Rio Grande floodplain.

2

 Based on field relations and relative-age data.
22 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Qia2

Qoa

Qia2 Qia1 Qya2

Qia2

Qya1

Santa Elena Canyon

Qia2

Qia2

Qia1

Qya2

Qia1

Qya1 Qaw

Chisos Mountains

Punta de la Sierra

A

B

Figure 2–4. Examples of pediment sequences in Big Bend National Park. Labels mark surfaces of thin, gravel-rich, Quaternaryage alluvial deposits that cap Cretaceous-age bedrock made up of yellowish-brown sandstone and yellowish-brown and maroon 

clay: Qoa, old alluvial deposits; Qia2, older of the intermediate-age alluvial deposits; Qia1, younger of the intermediate-age 

alluvial deposits; Qya2, older of the young alluvial deposits; Qya1, younger of the young alluvial deposits; Qaw, alluvium deposited 

in the active tributary wash. A, View looking southwest across Alamo Creek towards Santa Elena Canyon, a deep gorge cut by 

the Rio Grande through Cretaceous-age limestone. B, View looking north (towards the Chisos Mountains) in the Reed Camp area.
Eolian Landforms 23

Qiw1

Qiw3 Qiw2 Qow

Qiw1

Figure 2–5. Rio Grande terraces near Santa Elena Canyon Overlook, southwest Big Bend National Park 

(the overlook is just out of view to the right, on the Qiw1 terrace). Three levels of terraces thought to range 

from middle to late Pleistocene in age (Qiw3, Qiw2, and Qiw1) are visible in this photograph. An older, 

higher terrace (Qow) is barely visible far in the distance, near the center of the photograph.
Figure 2–6 (left and facing page). Banded 

vegetation and surface characteristics of 

deposits on which the banded vegetation 

forms. A, Portion of a color-infrared 

digital orthophoto image of the southwest 

quarter of Persimmon Gap quadrangle, in 

the northernmost part of the park. Image 

shows aerial view of banded vegetation 

(dark bands) separated by lighter colored 

interband areas where vegetation cover is 

relatively sparse. B, Mud cracked surface 

crust on a young alluvial surface (Qya1) 

in an interband area near Dog Canyon. 

Crust has been partly eroded by surface 

wash. Notebook (14 cm wide) for scale. C, 

Interband area adjacent to the vegetation 

band shown in figure 2–6D, formed on an 

intermediate-age alluvial surface (Qia). 

Pavement is moderately packed and 

uniform with moderately varnished clasts. 

D, Relatively thick vegetation growing in a 

vegetation band adjacent to the area shown 

in figure 2–6C. Ground surface within the 

vegetation band is relatively rough and 

lacks desert pavement.

0 0.5 km

A

B

24 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas
C

D

Eolian Landforms 25
26 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Summary

The long and complex Quaternary geologic and climatic 

history of Big Bend National Park is reflected in the multiple 

levels of pediment-capping gravel deposits, alluvial-fan deposits, and stream and river alluvium that cover much of the park. 

Erosion of the landscape has been extensive during the Quaternary period. The Rio Grande fluvial system has been dominated by long-term incision since it was established, probably 

in the late Pliocene to early Pleistocene. Periodically, incision 

was disrupted by episodes of aggradation, evidence of which 

is reflected in the multiple levels of coarse-gravel terrace 

deposits preserved along the Rio Grande. These fluctuations 

in the depositional regime likely were associated with periodic 

changes in climate and river hydrology.

Acknowledgments

We thank Michael Machette and Marith Reheis for their 

thoughtful and constructive reviews of this manuscript, and 

our coworkers on the Big Bend project for their support.

References Cited

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New York, Oxford University Press, 430 p.

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New York, Oxford University Press, 326 p. 

Collins, E.W., Muehlberger, W.R., and Dickerson, P.W., 2007, 

Geologic map of the Glenn Spring Quadrangle, Big Bend 

National Park, Texas: University of Texas at Austin, Bureau 

of Economic Geology Open-file map, scale 1:24,000, 2 

sheets.

Connell, S.D., Hawley, J.W., and Love, D.W., 2005, Late 

Cenozoic drainage development in the southwestern Basin 

and Range of New Mexico, southeasternmost Arizona, and 

western Texas, in Lucas, S.G., Morgan, G.S., and Zeigler, 

K.E., eds., New Mexico’s ice ages: Albuquerque, N. Mex., 

New Mexico Museum of Natural History and Science Bulletin 28, p. 125–150.

Dethier, D.P., 2001, Pleistocene incision rates in the western 

United States calibrated using Lava Creek B tephra: Geology, v. 29, no. 9, p. 783–786.

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: Geological Society of America Special Paper 291, 

p. 283–297.

Elias, S.A., 1997, The ice-age history of southwestern 

National Parks: Washington, D.C., Smithsonian Institution 

Press, 200 p.

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., Lonsdale, J.T., Hazzard, R.T., and Wilson, 

J.A., 1967, Geology of Big Bend National Park, Brewster 

County, Texas: University of Texas at Austin, Bureau of 

Economic Geology Publication 6711, 320 p.

Neuendorf, K.K.E, Mehl, J.P., Jr., and Jackson, J.A., 2005, 

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Geological Institute.

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sections and points (GSSPs): International Commission on 

Stratigraphy, http://www.stratigraphy.org/gssp.htm.

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Stevens, J.B., 1969, Geology of the Castolon area, Big Bend 

National Park, Brewster County, Texas: Austin, Tex., University of Texas, Ph.D. dissertation, 129 p.

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Sotol Vista to Cerro Castellan, Big Bend National Park, 

southwestern Brewster County, Trans-Pecos Texas, in

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structural-tectonic events along and near the Rio Grande, 

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Mexico, in Dickerson, P.W., Stevens, M.S., and Stevens, 

J.B., eds., Geology of the Big Bend, and Trans-Pecos, 

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[for the American Association of Petroleum Geologists 

annual meeting, San Antonio, Tex., 1989], p. 73–116.

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(early Miocene), Big Bend National Park, Texas: Texas 

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deposits and vertebrate faunas, Trans-Pecos Texas, in

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References Cited 27

Abstract

Big Bend National Park (BBNP) contains a diverse suite 

of volcanic and plutonic rocks. Ongoing geologic mapping, 

geochronology, and geochemical studies have focused on the 

timing and geological relationships associated with these volcanic and plutonic rocks. New geochemical data and 40Ar/39Ar 

age determinations aid in our understanding of the volcanic 

stratigraphy in the western part of BBNP. This study focused 

on volcanic rocks in four areas in western BBNP: (1) Blue 

Creek Ranch, (2) Tule Mountain, (3) Mule Ear Peaks, and (4) 

Cerro Castellan. Geochronological data indicate that volcanism within the western part of the park occurred from 46.5 to 

29 Ma. The oldest volcanic rocks in the western part of BBNP 

are the Alamo Creek lavas dated at 46.5 Ma. Samples from the 

Ash Spring Basalt yielded 40Ar/39Ar ages of about 42 Ma. The 

Alamo Creek and Ash Spring Basalts are difficult to distinguish from one another and are similar both petrographically 

and geochemically. The Bee Mountain Basalt is dated at 33.7 

Ma and has a variable thickness in western BBNP. We identified lava flows above the Mule Ear Spring Tuff and below 

the Tule Mountain Trachyandesite; they are likely correlative 

with the Bee Mountain Basalt, indicating the eruption of the 

basalt was interrupted by the Mule Ear Spring Tuff. The Mule 

Ear Spring Tuff, a regional ash-flow tuff dated at 33.6 Ma, is 

a significant marker unit within the Chisos Group. The Tule 

Mountain Trachyandesite is dated at approximately 33 Ma at 

Tule Mountain, but we have obtained 40Ar/39Ar ages of 30.3 

Ma for a widespread unit previously mapped as Tule Mountain 

Trachyandesite south and west of Tule Mountain. The Wasp 

Spring and Burro Mesa members of the Burro Mesa Formation are dated at 29.4 Ma and represent the last major volcanic 

eruptions in BBNP west of the Chisos Mountains.

Introduction

The park contains a diverse suite of volcanic and plutonic 

rocks representing the easternmost extension of the Trans-Pecos magmatic province (fig. 3–1) (Barker, 1977). A number of 

geological studies have been conducted within and surrounding BBNP to understand the volcanic and plutonic rocks with 

respect to their stratigraphic and tectonic setting (Maxwell 

and others, 1967; Barker, 1977, 1987; Henry and Price, 1984; 

Barker and others, 1986; Henry and others, 1991; White and 

others, 2006). These studies provide a solid foundation for 

further geochemical and geochronological studies.

Limited 40Ar/39Ar and numerous K/Ar ages poorly 

constrained the volcanic and plutonic history (Maxwell and 

others, 1967; Laughlin and others, 1982; Henry and McDowell, 1986). New geologic mapping, 40Ar/39Ar geochronology, 

and geochemical data are helping us better understand the 

volcanic stratigraphy and timing of the numerous intrusive 

rocks throughout BBNP as they relate to the park’s geologic 

history. Volcanic stratigraphic units in BBNP include the 

Chisos Group (Henry and Davis, 1996), the South Rim Formation (White and others, 2006), and the Burro Mesa Formation 

(Benker, 2005; White and others, 2006).

This paper discusses some prominent outcrops in BBNP, 

mainly along Ross Maxwell Scenic Drive, and presents new 

geochronological and geochemical data of selected volcanic 

samples from the Chisos Group and Burro Mesa Formation.

Stratigraphy

This report focuses on the volcanic rocks in the western 

part of BBNP, which includes the Eocene-Oligocene Chisos 

Group of Henry and Davis (1996), and Oligocene Burro Mesa 

Formation of Benker (2005) and White and others (2006). 

The rocks of the South Rim are not discussed here and we 

generally follow Benker (2005) and White and others (2006) 

in restricting these rocks to sources in the Pine Canyon caldera complex of the high Chisos Mountains (fig. 3–1).

3. Volcanic Geology of Several Prominent Outcrops in the 

Western Part of Big Bend National Park

By Daniel P. Miggins, Minghua Ren, and Elizabeth Y. Anthony
30 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Figure 3–1. Landsat 7 image showing Big Bend National Park 

and surrounding regions. See figure 3–4 for detail of boxed region. 

TPMP, Trans-Pecos magmatic province; BBNP, Big Bend National 

Park; R, Roy’s Peak; PM, Peña Mountain; BCR, Blue Creek Ranch; 

MG, Mariscal Mountain gabbro; SQ, Sierra Quemada; P, Punta de 

la Sierra; ME, Mule Ear Peaks.

Rosillos

Mtns

Mariscal

Mtn

Grapevine

Hills

Chisos

Mtns

Study

Butte

Cerro 

Castellan

New Mexico

Texas

El Paso

BCR

BBNP

R

Landsat 7 image of Big Bend National Park and surrounding region

MG

Sierra del Chino

TPMP

Rio Grand e

PM

10 km

N

P ME

SQ
Stratigraphy 31

The Chisos Formation was originally defined by Maxwell 

and others (1967) to include (in ascending order), the Alamo 

Creek Basalt, Ash Spring Basalt, Bee Mountain Basalt, Mule 

Ear Spring Tuff, and Tule Mountain Trachyandesite. These 

members are separated by undifferentiated, tuffaceous sedimentary rocks of the Chisos Formation including conglomerate, sandstone, mudstone, tuff, tuffaceous sandstone and clay, 

and minor lacustrine limestone. Henry and Davis (1996) noted 

that many of the volcanic flows that Maxwell and others (1967) 

designated as members of the Chisos Formation were widespread, prominent, identifiable units in the Trans-Pecos region, 

and thus, they subsequently renamed this formation as the 

Chisos Group; in addition, most of its members (Alamo Creek 

Basalt, Bee Mountain Basalt, Mule Ear Spring Tuff, and Tule 

Mountain Trachyandesite) were renamed formations. The Ash 

Spring Basalt of Maxwell and others (1967) was not recognized outside of BBNP, and therefore, was not included as a 

formation in the Chisos Group by Henry and Davis (1996). The 

volcanic rocks within the Chisos Group are mafic and felsic 

rocks that are exposed mostly in the western part of BBNP.

Henry and Davis (1996) reported that the Alamo Creek 

Basalt contained multiple flows of about 47 Ma and of highly 

variable composition ranging from basalt, through trachybasalt and basaltic trachyandesite, to trachyandesite. Several 

samples of Alamo Creek Basalt collected within the park are 

geochemically classified as trachyandesites (figs. 3–2 and 

3–3), but these flows have been reported to be variable in 

composition (Maxwell and others, 1967, Carman and others, 

2003). Exposures of the Alamo Creek Basalt are limited to the 

western half of BBNP, where they generally unconformably 

overlie prevolcanic Upper Cretaceous and Lower Tertiary sedimentary rocks. The Alamo Creek Basalt has been reported to 

contain up to six individual flows (Carman and others, 2003). 

The thickest sections of Alamo Creek Basalt are exposed 

west of the Mule Ear Peaks and Punta de la Sierra (fig. 3–1). 

We collected and analyzed several samples of the Alamo 

Creek Basalt from locations in western BBNP. These samples 

yielded 40Ar/39Ar ages of 46.5 Ma, which is similar to ages of 

this unit reported by Henry and Davis (1996). 

The next unit above the Alamo Creek Basalt consists of 

lavas mapped by Maxwell and others (1967) as the Ash Spring 

Basalt (fig. 3–2B). The Ash Spring Basalt is petrographically 

and geochemically similar to the Alamo Creek Basalt; these two 

units can be difficult to distinguish in outcrop. Maxwell and others (1967) described the Ash Spring Basalt as two or more flows 

of porphyritic basalt with abundant plagioclase phenocrysts. 

We collected a sample of the Ash Spring Basalt along the Ross 

Maxwell Scenic Drive that yielded a 40Ar/39Ar age of 42.5 Ma. 

The thickest mafic lavas of the Chisos Group include 

flows of Bee Mountain Basalt. Henry and Davis (1996) 

defined the Bee Mountain Basalt west of BBNP to include at 

least five flows below the Mule Ear Spring Tuff and three or 

four flows above the Mule Ear Spring Tuff that range in age 

from 34 to 33 Ma with a composition ranging from trachybasalt to basalt. Exposures of the Bee Mountain Basalt generally 

form steep cliffs, and the flows contain abundant plagioclase. 

Several samples that we collected from flows below the Mule 

Ear Spring Tuff have similar compositions to those described 

by Henry and Davis (1996), and our 40Ar/39Ar ages of 33.7 Ma 

are similar to theirs. A series of lavas in BBNP above the Mule 

Ear Spring Tuff, but below the Tule Mountain Trachyandesite, 

are likely correlative to the Bee Mountain lavas based on trace 

element analyses by Turner and others (in press). These upper 

Bee Mountain lavas are in the same stratigraphic position outside of BBNP as those described by Henry and Davis (1996).

The Mule Ear Spring Tuff was defined by Henry and Davis 

(1996) to include a single rhyolite flow with an age of about 33 

Ma. Exposures of this tuff range from non-welded to moderatelywelded with varying amounts of lithic clasts and pumice. Unlike 

most tuffs exposed in BBNP, the Mule Ear Spring Tuff has a 

nearly uniform thickness ranging from 6–8 m. Several samples of 

this tuff yield consistent ages of 33.6 Ma, similar to those reported 

by Henry and Davis (1996) and Henry and others (1998).

The Tule Mountain Trachyandesite was defined by Henry 

and Davis (1996) in areas west of BBNP as a single flow of 

intermediate to moderately silicic composition lava flow dated 

at about 33 Ma. Benker (2005) indicated that rocks mapped as 

Tule Mountain Trachyandesite by Maxwell and others (1967) 

consist of lava flows, dikes, and small volcanic plugs (fig. 

3–2B). However, our study and findings by Turner and others 

(in press) indicate some of these rocks may be misidentified. 

Rocks from Burro Mesa, Tule Mountain, and Sierra Aguja have 

similar trace element and whole rock compositions (Turner 

and others, in press) and have been dated at around approximately 33 Ma. In contrast, rocks southwest of Tule Mountain 

to Sierra del Chino (fig. 3–4) are compositionally distinct from 

the approximately 33 Ma rocks (Turner and others, in press), 

and 40Ar/39Ar ages indicate they are much younger at 30.3 Ma. 

Further studies are needed to fully comprehend the relationship 

these two rocks share with other Chisos Group rocks. 

The undifferentiated tuffaceous sedimentary rocks represent a significant part of the Chisos Group stratigraphy (fig. 

3–2B). Exposures of these tuffs range in thickness from a few to 

hundreds of meters and were deposited between the major lava 

flows found throughout the western half of BBNP. Deposits 

of the Chisos tuffs contain one or more of the following in any 

given outcrop: air-fall deposits, tuffaceous sediments, conglomerates, and reworked and ponded tuffs, such as those exposed at 

Cerro Castellan and west of Mule Ear Peaks (fig. 3–1).

The Burro Mesa Formation of Benker (2005) and White 

and others (2006) (fig. 3–2A) includes the Wasp Spring member and Burro Mesa Rhyolite member (fig. 3–2B). The Wasp 

Spring member is one of the more easily recognizable volcanic 

rocks. Typical exposures of this rock vary in color from bright 

orange, to gray, to sometimes white, containing 5–60 percent 

lithic clasts. The Wasp Spring member reaches thicknesses of 

200 m or more, with the thickest exposures at Goat Mountain 

(fig. 3–4A). The Burro Mesa Rhyolite member consists of lava 

flows and associated vents. Typically, lavas of the Burro Mesa 

are light to dark gray and contain variable amounts of sanidine 

and riebeckite. Most exposures of the Burro Mesa Rhyolite 

show rheomorphic folding and exposed flows can be greater 

than 50 m thick; 40Ar/39Ar ages for the Wasp Spring and Burro 

Mesa Rhyolite members are about 29.4 Ma.
32 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

 basalt

 correlative

to Burro Mesa 

Burro Mesa Rhyolite member Formation

Chisos tuffs

Chisos tuffs

Chisos tuffs

Mule Ear 

Spring Tuff

Bee Mtn Basalt

Chisos tuffs

Alamo Creek Basalt

Ash Spring 

Basalt

Ages (Ma)

29.4

30.3

33.6

33.7

46.6

Wasp Spring member

32.5 Ma

Rocks previously mapped as 

Tule Mountain Trachyandesite

29.4

Cretaceous sediments

Burro Mesa

Formation

South Rim

Formation

Chisos

Group

29.0

42.5

~33

Chisos tuffs

Chisos tuffs

Chisos tuffs

Tule Mountain Trachyandesite Bee Mountain Basalt

above Mule Ear 

Spring

Tuff

B

A

South Rim

Fm

Chisos Group

 & Burro Mesa

Fm

688

906

1142

meters

10,000 10,000

os Mo

 M

Figure 3–2. Location map and 

stratigraphic composite section for the 

Chisos Group and South Rim Formations. A, 

False color relief map of Big Bend National 

Park. Generalized geographic locations 

for the Chisos, Burro Mesa, and South Rim 

Formations; modified from Drenth (2005). 

B, Generalized volcanic stratigraphy of 

studied area. The 40Ar/39Ar ages are from 

this study.
Stratigraphy 33

35 40 45 50 55 60 65 70 75

0

2

4

6

8

10

12

14

16

Na2O+K 2

O

SiO

Picrobasalt

Basalt

Basaltic

andesite

Andesite

Dacite

Rhyolite

Trachyte

Trachydacite

Trachyandesite Basaltic trachyandesite

Trachy- basalt

Tephrite

Basanite

PhonoTephrite

Tephriphonolite

Phonolite

Foidite

Burro Mesa Rhyolite 29.4 

Rocks previously mapped as 30.3

 Tule Mountain Trachyandesite 

Tule Mountain Trachyandesite ~33

Bee Mountain Basalt 33.6 

 above Mule Ear Spring Tu 

Bee Mountain Basalt 33.7

 below Mule Ear Spring Tu 

Ash Spring Basalt 42.5 

Alamo Creek Basalt 46.6

Rock type Age (Ma)

2

EXPLANATION

Lower lava flow off

Maverick Road

LBCR

Lava unit from SCC

Lava unit from top

of Tule Mountain

Upper lava unit off

Maverick Road

Lava from top 

of mesa at BCR

Lava unit

at TC

Lava unit east

of Tule Mountain

Lava unit from

RMSD

Figure 3–3. Total alkali versus silica diagram showing the major rock types discussed 

in the text. This diagram illustrates the diversity of rock types in the western half of Big 

Bend National Park, as well as the variation among rocks of similar age. The explanation 

indicates the type of unit and age or age range for each sample. The Alamo Creek, Tule 

Mountain, and unknown units (may be Alamo Creek, Ash Spring, or Bee Mountain lavas) 

show the greatest geochemical differences. For example, the Tule Mountain units have 

compositions ranging from basaltic-trachyandesite to trachydacite to trachyte. BCR, Blue 

Creek Ranch; RMSD, Ross Maxwell Scenic Drive; TC, Tuff Canyon; LBCR, lower Blue Creek 

Ranch lavas; SCC, Southern Cerro Castellan. (From LeBas and others, 1986.)
R i o G r a n d e Chisos Mo

RM BM BCR BMV CC ME Sierra de Chino T L SB Tr CMaverick Drive Ross Maxwell Scenic Drive Highway 118

4 km

GM

RDM

Figure 3–4. Landsat 7 image of the western half of Big Bend National Park indicating some of the major features and geologic units discussed in the red and light colored rocks are felsic in composition and darker rocks are mafic in composition. See figure 3–8C for detail of boxed region. SB, Study BRM, Rattlesnake Mountain; BM, Burro Mesa; T, Tule Mountain; BCR, Blue Creek Ranch; C, Castolon; CC, Cerro Castellan; BMV, Burro Mesa vent area; Round Mountain; GM, Goat Mountain.
Volcanic Stratigraphy

Blue Creek Ranch Overlook

A set of geologic outcrops that we have studied includes 

exposures near the Blue Creek Ranch Overlook on the Ross 

Maxwell Scenic Drive (figs. 3–1 and 3–4A). Maxwell and others (1967) mapped the outcrops in the Blue Creek Ranch area, 

from lower to upper units, as Bee Mountain Basalt, Mule Ear 

Spring Tuff, and Tule Mountain Trachyandesite. 

The lowermost exposed unit at Blue Creek Ranch is the 

Bee Mountain Basalt. Above the Bee Mountain Basalt are 

thick deposits of lahars and conglomerates (fig. 3–5). The 

conglomerate contains clasts of a variety of volcanic rock 

types that range from a few millimeters to 50 cm in diameter. Unconformably overlying the conglomerates and lahar 

deposits are thick sequences of ash-flow tuffs (figs. 3–5A, B). 

A sequence of three ash-flow tuffs is exposed on the north 

side of Blue Creek. The middle tuff yielded a 40Ar/39Ar age 

from sanidine of 32.1 Ma. The limited aerial extent of these 

tuffs suggests that they were erupted from a local source, and 

the age (32.1 Ma) is similar to volcanic and intrusive rocks 

associated with the South Rim Formation. We suggest that the 

rhyolite tuffs exposed in the Blue Creek area may represent 

outflow tuffs related to the South Rim Formation, but further 

geochemical analyses and detailed geologic mapping are 

needed to correlate these rocks.

The uppermost volcanic rocks were previously mapped 

by Maxwell and others (1967) as the Tule Mountain Trachyandesite (fig. 3–5D) but are dated at 30.3 Ma and are 

chemically similar to other rocks discussed earlier (Turner and 

others, in press). A rhyolite dike exposed on the north side of 

Blue Creek yielded a 40Ar/39Ar age of 29.0 Ma, cross-cuts this 

section, and represents some of the youngest intrusive activity 

found in BBNP (fig. 3–5C).

Tule Mountain

The volcanic rocks exposed at Tule Mountain in stratigraphic order are Chisos tuffs, Ash Spring Basalt, Chisos 

tuffs and conglomerates, Mule Ear Spring Tuff, and the Tule 

Mountain Trachyandesite (fig. 3–6). Maxwell and others 

(1967) mapped flows at the base of Tule Mountain as Ash 

Spring Basalt (figs. 3–2 and 3–6). In this section, the Ash 

Spring Basalt consists of one to three lava flows. Above the 

Ash Spring Basalt is a 2-m-thick ash-flow tuff, which yielded 

a 40Ar/39Ar age of 42.1 Ma, indicating that the underlying 

Ash Spring Basalt is at least 42.1 m.y. old. Above the ashflow tuff is a thick sequence of Chisos tuffs and conglomerates (fig. 3–6). The conglomerates tend to be thicker towards 

the base of the overlying Mule Ear Spring Tuff. The Mule 

Ear Spring Tuff is intermittently exposed along the upper 

flanks of Tule Mountain (fig. 3–6). The uppermost unit at 

Tule Mountain is the Tule Mountain Trachyandesite dated at 

approximately 33 Ma.

Mule Ear Peaks Viewpoint and

Round Mountain Section

The Mule Ear Peaks are visible just off Ross Maxwell 

Scenic Drive (Mule Ears Viewpoint) and are one of the more 

prominent geologic features in BBNP. Field observations 

suggest that the Mule Ear Peaks are dominantly Burro Mesa 

Rhyolite member of the Burro Mesa Formation and represent 

the last major eruptive rhyolite found throughout southwestern 

BBNP (fig. 3–3) with an age of 29.4 Ma (Miggins and others, 2004, 2006). Although not apparent from the Mule Ears 

Viewpoint, there is a thick stratigraphic section of the Chisos 

Group and overlying Burro Mesa Formation exposed at Round 

Mountain, several kilometers south of the viewpoint (fig. 

3–7B). This section is significant because the Alamo Creek 

Basalt is about 100 m thick and represents one of the thickest 

sections of the unit in the entire park. This thick accumulation of lava suggests this area was a paleo-topographic valley, 

drainage, or depression during eruption of the Alamo Creek 

Basalt. Tuffaceous, sedimentary deposits of the Chisos Group 

above the Alamo Creek Basalt are also thick, indicating this 

area remained a lowland during their deposition. These deposits consist of about 500 m of air-fall tuff and ponded, reworked 

units comprising massive tuff beds intercalated with conglomerate and tuffaceous sandstone (fig. 3–7).

Above the Chisos Group tuffaceous and sedimentary 

rocks is a massive section of Bee Mountain Basalt (fig. 3–7). 

Stratigraphically above the Bee Mountain Basalt are a series 

of trachyandesite lava flows (fig. 3–7). It is presently unclear 

if these trachyandesites belong to the approximately 33 Ma 

Tule Mountain Trachyandesite as defined by Henry and Davis 

(1996), or are part of the 30.3 Ma trachyandesite previously 

mapped as the Tule Mountain Trachyandesite by Maxwell 

and others (1967). The Burro Mesa Formation caps the top of 

Round Mountain (fig. 3–7).

Cerro Castellan

Cerro Castellan, near Castolon (figs. 3–5 and 3–8A), is 

another popular geologic outcrop in BBNP. Cerro Castellan 

includes units of the Chisos Group and Burro Mesa Formation. The base of the Cerro Castellan section includes tuffaceous sedimentary units of the Chisos Group. These rocks 

are overlain by the Bee Mountain Basalt, which is in turn 

overlain by undifferentiated Chisos tuffs (Parker, 2002). The 

Wasp Spring member of the Burro Mesa Formation unconformably overlies Chisos tuffs (fig. 3–9A). The Wasp Spring 

member is an ignimbrite that yielded a 40Ar/39Ar date of 29.4 

Ma, representing activity before the eruption of the Burro 

Mesa Rhyolite member, which also yielded a date of 29.4 Ma. 

At Cerro Castellan, it is possible to see various Wasp Spring 

flows ranging from non-welded to welded ignimbrite. The 

Burro Mesa Rhyolite member caps the top of Cerro Castellan. 

Multiple vents for the Burro Mesa Rhyolite member have been 

identified in this area. The most spectacular exposures of this 

Volcanic Stratigraphy 35
36 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Unkown ash-flow

tuffs

Bee Mtn Basalt

Chisos tuffs??

30.3

33.7?

Blue Creek Ranch composite section

32.1

Chisos tuffs??

Conglomerate

Lahars

Rhyolite dike

29.0

40Ar/39Ar Age (Ma)

Rocks previously mapped as 

Tule MountainTrachyandesite

D

Chisos tuffs?

View to SE C

Rhyolite dike

Ross Maxwell Scenic Driv e

Unit 1

Unit 2

Unit 3

B View looking east

Ridge north

of Blue Creek

Bee Mountain Basalt

Ash-flow tuff

Chisos tuff

Chisos tuff??

Lahars, breccias, and mudflows

Conglomerate

Rocks previously mapped as

Tule MountainTrachyandesite 

A View looking east

Figure 3–5. Various 

photos and composite 

stratigraphic section for 

the Blue Creek Ranch area. 

A, Photo showing geologic 

section on the south side 

of Blue Creek Ranch 

Overlook. B, Photo showing 

the three distinct ash-flow 

tuffs on the north side of 

Blue Creek Ranch. C, Photo 

of rhyolite dike exposed 

along the Ross Maxwell 

Scenic Drive. D, Volcanic 

stratigraphy at Blue Creek 

Ranch.
Volcanic Stratigraphy 37

Tule Mtn Trachyandesite

Mule Ear Spring Tuff

View looking east

Ash Spring Basalt Chisos tuffs & conglomerates

Pink ash-flow tuff

Figure 3–6. Photo of Tule Mountain indicating the major volcanic rocks exposed in the area, which include from base to top: a 

trachyandesite mapped by Maxwell and others (1967) as the Ash Spring Basalt; a thick section of Chisos Group tuffs, undivided; the Mule 

Ear Spring Tuff; and the Tule Mountain Trachyandesite. 

member are on the north side of Cerro Castellan (fig. 3–8D) 

where fissures and tubes of the Burro Mesa Rhyolite member 

are exposed, cutting through Chisos tuffs (fig. 3–9B).

To the south of Cerro Castellan, along Cerro Castellan 

ridge, are exposures of the Mule Ear Spring Tuff and Bee 

Mountain lava flows (fig. 3–9A). Basaltic lavas stratigraphically above the Mule Ear Spring Tuff and below the Tule 

Mountain Trachyandesite were mapped by Maxwell and others (1967) as Tertiary unnamed lavas. Geochemical analyses 

suggest these lavas correlate with similar lavas described by 

Henry and Davis (1996) further west as Bee Mountain Basalt 

flows between the Mule Ear Spring Tuff and Tule Mountain 

Trachyandesite (Turner and others, in press). In this respect, 

Bee Mountain lava flows were interrupted by the Mule Ear 

Spring Tuff (Henry and Davis, 1996). The Bee Mountain 

lavas above the Mule Ear Spring Tuff are diagnostic in BBNP 

because they often contain pea- to boulder-size xenoliths of 

quartzite up to 1 m in diameter (fig. 3–9C). These lavas are 

easily recognized because they are generally plagioclase poor 

and contain quartzite xenoliths, whereas the Bee Mountain 

flows below the Mule Ear Spring Tuff have abundant plagioclase. Further dating and geochemical analyses of these rocks 

are in progress to test the correlation of these rocks with the 

Bee Mountain Basalt.
38 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Rocks previously

mapped as

Tule Mountain

Trachyandesite

Alamo Creek lavas

Mule Ear Peaks

Burro Mesa Formation

Chisos tuffs

Bee Mountain lavas

View looking NE Nearly complete stratigraphy of the Chisos Group

and Burro Mesa Formation 

B

Round Mountain

Mule Ear Peaks

A View looking east

Figure 3–7. Photos showing 

distant and close up views for the 

stratigraphy of the Round Mountain 

area located south of the Mule 

Ears Viewpoint. A, Photo showing 

Mule Ear Peaks taken from the 

Mule Ears Viewpoint. B, Photo 

showing the volcanic stratigraphy 

taken approximately 2 km south of 

the Mule Ears Viewpoint. Some of 

the thickest deposits of the Alamo 

Creek lavas and Chisos tuff are 

exposed in this section.
Volcanic Stratigraphy 39

Bee Mtn Basalt

Burro Mesa Rhyolite

member

29.4 Ma

Wasp Spring member

 29.4 Ma

Chisos tuffs

Cerro Castellan section View looking east

Densely-welded

Moderately-welded

Non-welded

Talus

Chisos tuffs

A

Chisos tuff

Vents of Burro

Mesa type rhyolite 

Chisos tuffs

B

C

C

CC

TC

ME

Sierra de Chino

Rio Grande

SCC

Figure 3–8. Location map and photos showing the 

stratigraphy of Cerro Castellan. A, Photo of Cerro Castellan 

showing the various volcanic rocks exposed. Outcrops of 

the Wasp Spring member of the Burro Mesa Formation 

represent one of the few areas showing the non-welded 

Chisos tuffs and the moderately-welded and welded nature 

of the Wasp Spring. B, Photo showing several vents of the 

Burro Mesa Rhyolite exposed in Chisos tuffs. Vents are 

one of many Burro Mesa vents exposed north of Cerro 

Castellan. C, Landsat 7 image showing some of the major 

geographic locations around the Cerro Castellan area. 

ME, Mule Ear Peaks; SCC, Southern Cerro Castellan; C, 

Castolon; CC, Cerro Castellan; TC, Tuff Canyon.
40 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Figure 3–9. Various photos showing the Bee 

Mountain Basalt above and below the Mule Ear 

Spring Tuff. A, Photo of exposed volcanic rocks 

located south of Cerro Castellan. Major units 

exposed are Bee Mountain lavas, Mule Ear Spring 

Tuff, and undivided Chisos tuffs. B, Close up view 

of the Cerro Castellan lavas exposed above the 

Mule Ear Spring Tuff. There are at least three 

distinct flows in this section. C, Photo showing one 

of the many quartzite xenoliths in Cerro Castellan 

lava exposed between two distinct flows.

Mule Ear Spring Tuff

33.6 Ma

Bee Mtn Basalt

A View looking SW

Bee Mountain Basalt above

Mule Ear Spring Tuff

View looking West

Bee Mountain Basalt

Chisos tuffs

Mule Ear Spring Tuff

B

Quartzite

Xenolith

C

Upper flow

Lower flow
References Cited 41

Conclusions

Big Bend National Park contains a diverse suite of 

volcanic and plutonic rocks that are exposed primarily along 

the western half of the park. These volcanic rocks consist 

of the Chisos Group, and the South Rim and Burro Mesa 

Formations. Numerous geologic studies on the volcanic rocks 

within and surrounding BBNP have been conducted in order 

to determine the timing of these rocks and their formation processes. Understanding the formation of these volcanic rocks is 

critical in determining how this igneous activity is associated 

with volcanic rocks throughout the western United States, 

in particular the Trans-Pecos magmatic province. The park 

makes up the southeastern portion of this magmatic province, 

where igneous activity occurred from 46.5 to 17 Ma (Henry 

and McDowell, 1986).

Ages and geochemical data obtained from numerous 

rock units have helped to constrain the complex geologic 

history that has occurred in the western half of BBNP. Based 

on geological mapping, detailed field sampling and interpretations, geochronology, and geochemistry, we have been 

able to contribute to a better understanding of some of the 

most prominent geologic features in the western half of the 

park, especially lava flows located near the west entrance of 

BBNP.

Acknowledgments

We wish to thank William R. Page, Kenzie Turner, and 

John E. Gray for their detailed reviews that greatly improved 

the organization and clarity of this report. We thank Ross Yeoman and Rae Ann Orkild for helping with the argon analyses 

and Jim Budahn for the geochemical analyses. We also thank 

Rae Ann Orkild, Nathan Thompson, and Kay Wache for their 

invaluable help with the mineral separations and Paul Wigton 

and Kay Wache for their help in the field by assisting with the 

collection of many needed rock samples over several kilometers of rugged landscape. Without their help this study would 

not be possible. We thank Big Bend National Park for allowing us to conduct this work over the years.

References Cited

Barker, D.S., 1977, Northern Trans-Pecos magmatic province—

Introduction and comparison with the Kenya rift: Geological 

Society of America Bulletin, v. 88, p. 1,421–1,427.

Barker, D.S., 1987, Tertiary alkaline magmatism in Trans-Pecos Texas: Geological Society of London Special Publication, v. 30, p. 415–431.

Barker, D.S., Henry, C.D., 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 at Austin, Bureau of Economic Geology 

Guidebook 23, p. 266–285.

Benker, S.C., 2005, The petrology of the South Rim Formation, Big Bend National Park, Texas: Richmond, Ky., Eastern Kentucky University, M.S. thesis, 104 p.

Carman, M.F., Stewart, R.M., Jr., and Casey, J.F., 2003, 

Characterization of the Alamo Creek Basalt, Big Bend 

National Park, Texas: Texas Journal of Science, v. 55, no. 2, 

p. 99–128.

Drenth, B.J., 2005, Interpretation of geophysical anomalies 

over the Pine Canyon Caldera, Big Bend National Park, 

Texas: El Paso, Tex., University of Texas at El Paso, M.S. 

thesis, 78 p.

Henry, C.D., and Davis, L.L., 1996, Tertiary volcanic, volcaniclastic, and intrusive rocks adjacent to the Solitario, in

Henry, C.D., and Muehlberger, W.R., eds., Geology of the 

Solitario Dome, Trans-Pecos Texas—Paleozoic, Mesozoic, 

and Cenozoic sedimentation, tectonism, and magmatism: 

University of Texas at Austin, Bureau of Economic Geology 

Reports of Investigations 240, p. 81–108.

Henry, C.D., Davis, L.L., Kunk, M.J. and McIntosh, W.C., 

1998, Tertiary volcanism of the Bofecillos Mountains and 

Big Bend Ranch State Park—Revised stratigraphy and 

40Ar/39Ar geochronology: University of Texas at Austin, 

Bureau of Economic Geology Report of Investigations 253, 

76 p.

Henry, C.D., and McDowell, F.W., 1986, Geochronology 

and magmatism in the Tertiary volcanic field, TransPecos Texas, 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 at Austin, Bureau of Economic Geology Guidebook 23, p. 99–122.

Henry, C.D., and Price, J.G., 1984, Variations in caldera 

development in the Tertiary volcanic field of Trans-Pecos 

Texas: Journal of Geophysical Research, v. 89, no. B10, p. 

8,765–8,786.
Henry, C.D., Price, J.G., and James, E.W., 1991, Mid-Cenozoic stress evolution and magmatism in the southern Cordillera, Texas and Mexico—Transition from continental arc to 

intraplate extension: Journal of Geophysical Research, v. 

96, no. B8, p. 13,545–13,560.

Laughlin, A.W., Kress, V.C., and Aldrich, M.J., 1982, K-Ar 

ages of dike rocks, Big Bend National Park, Texas: Isochron 

West, v. 35, p. 17–18.

LeBas, M.J., LeMaitre, R.W., Streckeisen, A., and Zanettin, 

B., 1986, A chemical classification of volcanic rocks based 

on the total alkali silica diagram: Journal of Petrology, v. 27, 

p. 745–750.

Maxwell, R.A., Lonsdale, J.T., Hazzard, R.T., and Wilson, 

J.A., 1967, Geology of Big Bend National Park, Brewster 

County, Texas: University of Texas at Austin, Bureau of 

Economic Geology Publication 6711, 320 p.

Miggins, D.P., Ren, M., Anthony, E., Iriondo, A., and Wooden, 

J.L., 2006, New constraints on the timing of volcanism 

and associated intrusive rocks in Big Bend National Park 

using 40Ar/39Ar and U-Pb geochronology [abs.]: Geological 

Society of America Abstracts with Programs, v. 38, no. 7, 

p. 156.

Miggins, D., Scott, R.B., Snee, L.W., 2004, New 40Ar/39Ar 

ages from Big Bend National Park [abs.]: Geological 

Society of America Abstracts with Programs, v. 36, no. 5, 

p. 128.

Parker, D.F., 2002, Horseshoe Canyon volcanic dome, in

White, J.C., ed., The Geology of Big Bend National Park: 

What Have We Learned Since Maxwell and others (1967)?: 

Geological Society of America South-Central meeting, 

Alpine, Tex., Field trip guide, p. 24–32.

Turner, K.J., Budahn, J.R., and Miggins, D.P., in press, A 

reinterpretation of some correlations of Chisos Group mafic 

lavas with new geochemistry and geochronology, Big Bend 

National Park, Texas [abs.]: Geological Society of America 

Abstracts with Programs.

White, J.C., Benker, C.S., 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.

42 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas
Abstract

Igneous rocks of Big Bend National Park make up greater 

than half of all rock types in the park. An igneous style dominant in the northeastern part of the park is manifest by a series 

of intrusive units forming tabular bodies that were emplaced 

into the Earth’s upper crust as sills. At Dagger Flats, the intrusion of predominantly alkaline mafic magmas as sills into the 

Cretaceous Boquillas Formation, a sequence of near-shore 

marine carbonate units, resulted in the formation of skarn 

deposits. These deposits have formed by contact metamorphism resulting from intrusion of hot magma into the upper 

crust. Reaction with host limestone units forms an assemblage 

of alteration minerals that include calcsilicates such as wollastonite, grossular garnet, prehnite, and zeolites, as well as clays 

and recrystalized calcite and quartz. At Dagger Flats, detailed 

sampling through several stratigraphic sections from the sill 

upward through the top of the capping skarn indicates that the 

porosity and permeability of various carbonate rocks within 

the Boquillas Formation controlled hydrothermal fluid infiltration where skarn-forming reactions influenced the distribution 

of mineralized rock.

Introduction

The igneous landscape dominating much of Big Bend 

National Park (BBNP) includes, but is not limited to, tectonically controlled dike and sill systems, shallow intrusive bodies, 

lava flows, and large ignimbrite-producing calderas. These 

igneous forms have been variably influenced or controlled 

by regional tectonism (Henry and Price, 1984). Many are 

associated with fossil hydrothermal systems that altered and 

mineralized the surrounding host rocks to varying degrees. 

One example of the interaction among magma, host rock, and 

hydrothermal mineralizing fluids can be found at Dagger Flats 

in the northeast corner of BBNP (fig. 4–1A). Lower Oligocene mafic and subordinate felsic sills intruded Cretaceous 

limestone rocks and produced skarn deposits. The focus of 

this paper is to describe one of several igneous landscapes in 

BBNP and document this type of environment where skarn 

deposits are associated with shallow intrusions.

Skarn deposits are characterized by calc-silicate minerals in limestone or limestone-bearing rocks that have formed 

by contact metamorphism resulting from the intrusion of hot 

magma into cooler country rock. At the contact between the 

intrusive body and the host rock, heat, circulating hydrothermal fluids, and magmatic gases interact and react with the 

limestone host to form minerals such as wollastonite, CaSiO3

, 

and diopside, CaMg(Si2O6). The composition and physical 

characteristics of the intrusive body, in addition to the physical 

and chemical nature of the carbonate host, control the mineralogy of skarn deposits (Lentz, 1998). In turn, the porosity and 

permeability in the varying depositional near-shore environments of the carbonate rocks control hydrothermal fluid 

infiltration where the skarn formed.

Dagger Flats

The Dagger Flats area is found along the southwestern 

base of the Sierra del Carmen, a series of long, northwesttrending ridges composed of massive Lower Cretaceous 

limestone (fig. 4–1A) (Maxwell, 1968). The landscape of Dagger Flats is represented by a series of low northwest-trending 

fault-bounded hills (fig. 4–1) cored with thick sills (tabular, 

conformable igneous bodies) that intruded into the Cretaceous 

Boquillas Formation (fig. 4–2A). Skarn deposits are exposed 

in the capping carbonate strata directly above the sills.

Sills

At Dagger Flats, sills are exposed as broad resistant 

ledges (fig. 4–1C) conformably capped by relatively thin (up 

to 20–25 m thick) erosional remnants of a sequence of carbonaceous marls, clays, mudstones, and siltstones of the Cretaceous Boquillas Formation. The sills are composite bodies 

with exposures 10 to 30 m thick that gently dip approx. 5–10o

S-SW.

4. Where Magma Meets Limestone: Dagger Flats, an 

Example of Skarn Deposits in Big Bend National Park

By Lisa A. Morgan and W.C. Pat Shanks III 
B. Cretaceous Boquillas Fm (skarn deposits) series of Oligocene sills (mostly basanite)

Cretaceous Boquillas Fm

Dagger Mountain

McKinney Hills

RosillosMountains

ChisosMountains

Sierra del Carmen

United States Mexico Rio GrandeWild and

Scenic River

Big Bend National Park

TEXAS

Big Bend National Park

DaggerFlats

Grapevine Hills

0

0

5

5

10 kilometers

10 miles

l l l l l l

!.

!.

!.

!.

!.

!.

!.

!.

!.

261

248

285

286

283

282

281

247

284

263

264

103°5'0"W 103°4'0"W

29°29'30"N

29°29'0"N

0 0.25 0.5 km

Qal--Quaternary alluvium

Tig--Tertiary (Oligocene) sills

Kbo--Cretaceous Boquillas FormationKbu--Cretaceous Buda Limestone

Qal

Kbo

Tig

Kbu Kbu

Tig

Tig

Kbo

QTog--Quaternary Tertiary older gravels

Tcf--Tertiary Canoe FormationTbp--Tertiary Black Peaks Formation

Kjf-- Cretaceous Javalina FormationKag--Cretaceous Aguja FormationKpf--Cretaceous Pen FormationKag--Cretaceous Sana Elena Formation

skarn deposits

skarn deposits

A B

C
103°8' 103°6' 103°4' 103°2' 103°0'

29°30'

29°28'29°26'

29°24'

29°22'

29°20'

29°18'

0 2,500 5,000 METERS

DaggerFlats

McKinney Hills McKinney Hills sill

DaggerFlats sill

A B C

103°8' 103°6' 103°4' 103°2' 103°0' 103°8' 103°6' 10

29°30'

29°28'29°26'

29°24'

29°22'

29°20'

29°18'

29°30'

29°28'29°26'

29°24'

29°22'

29°20'

29°18'

0 2,500 5,000 METERS 0 2,500 

Figure 4–2. Maps of the Dagger Flats vicinity. A, Geologic map (from Maxwell, 1968) of Dagger Flats (shown in white outline) and McKinney Hills to t

shaded-relief topographic map. Units are the same as shown in figure 4–1B. B, High-resolution, reduced-to-pole magnetic intensity map of same area 

Finn, written commun., 2004). Magnetic intensities range from 1060 nT to 930 nT; higher values are shown in warm colors and lower values are in coollimited extent of the Tertiary sills exposed at Dagger Flats as shown on the geologic map in figure 4–2A with the extent of the large positive magnetic body associated with the sills at Dagger Flats has a much larger extent than the exposed sills. Faults from the geologic map have been draped on the and correspond well with breaks in intensity values in the large sill body, measured at approx. 5 km northeast by 12–22 km northwest. Other similar bofaults can be seen to the south at McKinney Hills, where the exposure of sills is prominent (seen here) and to the west in the Rosillos Mountains. C, Cthe strong correlation between the geologic map of Dagger Flats and McKinney Hills and the reduced-to-pole magnetic intensity map. The geologic mshaded-relief topographic map (fig. 4–2A), which, in turn, is draped by the magnetic intensity map (fig. 4–2B).
46 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

High-resolution aeromagnetic mapping in BBNP (C.A. 

Finn, written commun., 2004; Anderson and others, 2004) 

shows that the Dagger Flats area is underlain by a positive 

magnetic anomaly that is roughly oval-shaped in plan view 

and has a larger extent than the area commonly known as 

Dagger Flats (fig. 4–2B). The oval body, interpreted as a 

shallow-level sill, has moderate positive values of magnetic 

intensity and is a northwest-striking structure about 15 to 22 

km long and about 5 km wide. The Dagger Flats sill is of 

comparable size to or larger than adjacent sills and intrusive 

bodies, including those in the McKinney Hills (fig. 4–2B) and, 

to the west, in the Rosillos Mountains and Grapevine Hills 

(fig. 4–1A). Assuming a typical maximum thickness of 100 m 

documented seismically for many sills (Chevalier and Woodford, 1999; Smallwood and Maresh, 2002; Thomson, 2004; 

Thomson and Hutton, 2004), the sills at Dagger Flats give an 

estimated maximum volume of the sill body at approximately 

11 km3

. If we assume a thinner body of 30 m (which we see 

exposed at the surface), the minimum estimated volume for 

the Dagger Flats sill would be about 3.3 km3, about eight times 

the volume of total erupted magma in the multiple eruptions 

during 1980 at Mount Saint Helens, Wash.

The majority of the sills at Dagger Flats are a series of 

dark brown, coarse-grained, pyroxene-plagioclase-bearing 

silica-undersaturated alkaline intrusive rocks, referred to as 

alkali gabbros; silica content ranges from 41 to 44 wt. % 

(weight percent) (fig. 4–3B). Sill lenses are 3 to 10 m thick, 

vertically jointed, and weather as large spheroidal blocks. 

Individual pyroxene phenocrysts, up to 7 cm in length, reflect 

a relatively long cooling period for the intrusions and indicate 

emplacement at some depth where the surrounding rocks are 

warmer due to temperature increase along the geothermal 

gradient of the earth.

In various exposures at Dagger Flats, a few smaller (<1 

m thick) monzonite sills intrude the coarser alkali gabbroic sill 

sequence (fig. 4–3) and are common features in at least 10 sill 

and laccolith localities in the Big Bend area (Carman, 1994). 

The monzonite sills have sharp contacts with the main mafic 

body and are light gray, fine- to medium-grained, subhorizontal thin sheets rich in alkali feldspar and plagioclase. Chemically, these rocks plot in the monzonite field on the TAS (Total 

Alkali Silica) diagram (fig. 4–3B) (Middlemost, 1994). Silica 

content for these later-intruded sills ranges between 52 to 56 

wt. % SiO2

; total water for the monzonite sills range from 3.1 

to 4.4 wt. %. Along the contact with the alkali gabbroic sill, 

thermal cracks perpendicular to the orientation of the monzonite sill sheet (fig. 4–3C) indicate that it intruded into the 

cooled mafic sill and was quenched. The thickness of the monzonite is somewhat variable and ranges from a few centimeters 

to about 1 m thick. A high-precision 40Ar/39Ar radiometric 

analysis of fine-grained groundmass from the monzonite 

(BB-05-248.1) yields a date of 32.47+0.41 Ma.

Hydrothermal Alteration of the Sills

Hydrothermal alteration is present in both the alkali gabbroic sill and to a greater extent in the monzonite sill, based on 

alteration mineralogy and oxygen isotope values. Alteration 

mineralogy associated with the alkali gabbroic sill is primarily 

zeolitic (analcime, thomsonite, tetranatrolite) with clinochlore, 

phlogopite, and kaolinite (fig. 4–4). Analcime, kaolinite, 

montmorillonite, and chlorite were identified as alteration 

minerals associated with the smaller monzonitic sill.

Samples from the alkali gabbro sill have whole rock 

oxygen isotope values (δ18Owr) of 6.9 to 10.1. Most δ18Owr

values for unaltered igneous rocks range between 6 and 8. 

One coarse-grained alkali gabbro sill sample yields a δ18Owr

value of 6.9; however, three other alkali gabbro sill samples 

yield δ18Owr values ranging from 9.6 to 10.3. Samples from the 

volumetrically subordinate monzonite sills have higher δ18Owr

values ranging from 12.6 to 13.1, indicating that hydrothermal 

alteration was more prevalent in these sills than the alkali gabbro host sill. The oxygen isotopic values and silicate mineralogy of the altered sills indicate late-stage lower temperature 

alteration, especially in late monzonite intrusions. These inferences, based on oxygen isotopes, are consistent with the stability range for some zeolite minerals, which is about 150–250o

C 

(Bird and others, 1984).

Figure 4–3 (facing page). Series of photographs of composite 

sills exposed in the Dagger Flats area and their range in 

composition. A, Exposure showing a composite sill exposed in 

hillside at Dagger Flats flanked below and above by the Boquillas 

Formation. Thin (<1 m), light-colored, monzonite sill is exposed 

between thicker (>5 m) and darker alkali gabbroic sills. Skarn 

is capping hill. B, Total Alkali Silica (TAS) diagram showing the 

compositions of the dominant alkali gabbroic sill and volumetrically 

minor monzonite sill (from Middlemost, 1994). C, View of monzonite 

sill intruded into the alkali gabbroic sill showing chilled margins of 

the smaller monzonite sill. Thickness of monzonite sill is approx. 

60 cm. D, Photograph of composite sill. Lighter gray unit is the 

monzonite sill; thickness here is approx. 4 cm.
BB-04-247.4

BB-04-247.3

35 45 50 55 60 

02468

10121416

Na 2O+K2O

SiO

Foidite

DaggerGrape Vine HillsPaint GapMcKinney HillsRosillos

FoidSyenite

FoidMonzoSyenite

FoidMonzoDiorite Alkali

Gabbro

MonzoGabbro

Monzo-diorite

Monzonite 

Dior

Gabbro

Gabbro Diorite Peridito

Gabbro

40

249

262.13 

2

281.1 

284.1 

285.1A 

285.2 

287.1 

287.2 

427.1 

B

D

alkali gabbrosill

monzonitesill

chilled margin

of smaller

monzonite

sill in alkali

gabbro sill

BoquillasFormation (covered)

alkaligabbrosill

A

C

2
48 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

A

CT, (grs, di, preh,aug,or, ab)

CT (grs, di, preh, aug)

CT, grs, and r, (aug, di, wo, preh, ph)

CT, GRS, aug, di, anal, qtz (preh)

CT, grs, (di, aug, preh)

CT (grs, di, aug, preh)

CT (grs, di, aug, preh)

CT, wo, grs, (anal, or)

calcite 

veins

ANAL, AN, AB, aug, tetranatrolite, 

th, ph, (musc, maricopiate, kao)

anal, aug, an, ab, (clincl, ph)

BB-05-284.5

38.4 wt. % SiO2

l l

l

l

l l l

l l l l l

l l l l

% CaO

v

v

v

v

v

v

v

v

x

x

x

x

x

x

x

o

o

o

o

o

o

o

o

x

284.2

282.3

281.3a

281.3b

282.2

281.2

284.3

284.4

talus

2 m

2 m

l 2 m

3 m

calcite crystals

l

284.1

281.1 o v x

o v x

l l

% SiO2

% Al2O3



18O = 10.1



18O = 6.9

mudcracks

leopard-spotted

weathered surface

massive

alkali gabbro sill

~9.0 m total thickness (skarn)

B

281−284 (closer to interior)

50403020100 60

286.10

100 20 30 40 50 60

286.1

286.2

286.3

286.4

286.5

286.6

286.7

x

x

x

x

x

x

x

x

x

o

o

o

o

o

o

o

o

o

CT, qtz (ab)

CT, PREH, di, aug 

CT, GRS (aug, qtz)

CT, qtz (ab)

CT, qtz (ab)

CT, qtz (ab)

CT, qtz (ab)

CT, wo (aug, ab, or)

CT, grs, (aug, preh, qtz, wo)

v

v

v

v

v

v

v

v

v

285.2

AB, OR, MCL, anal, mont

ANAL, AUG, LAB, kao, ph, clincl, th

x

x

x

o

o

o

v

v

v

monzonite sill

l l l l l l l l l l l

ANAL, AN, AUG, th, clincl, (ph)

massive

nodule 

rich

deformed, slightly 

folded laminae

increasing alteration and deformation

l

talus

talus

site 286 (at edge)

alkali gabbro sill

laminated

~20.0 m total thickness (skarn)

alkali gabbro sill

% CaO

lllllll

285.1

% SiO2

% Al

O2 3



18O = 13.1



18O = 9.6



18O = 10.3

l l

287.2

286.8

Figure 4–4 (left and facing page). Four 

diagrams from various sites at Dagger Flats 

where schematic stratigraphic sections are 

compared with variation in content of SiO2

(marked by X), CaO (marked by O), and Al2

O3

(marked by V) and with changes in depth in the 

capping mineralized limestone units above the 

sill. Also compared are alteration mineralogies 

for the skarn and sills. Values in δ18O, shown 

in red, are for samples collected from the sills. 

Minerals identified and represented in these 

stratigraphic sections include: CT (calcite), 

QTZ (quartz), AB (albite), OR (orthoclase), AUG 

(augite), WO (wollastonite), GRS (grossular 

garnet), PREH (prehnite), DI (diopside), ANAL 

(analcime), LAB (labrodorite), KAO (kaolinite), 

ph (phlogopite), MCL (microcline), mont 

(montmorillonite), clincl (clinochlore), th 

(thomsonite), musc (muscovite), AN (anorthite), 

vermic (vermiculite), sm (smectite), andr 

(anhydrite), and chl (chlorite). Major minerals 

are those in upper case letters, minor minerals 

are in lower case letters; trace minerals are in 

lower case letters in parentheses. A, Site 286 

(fig. 4–1B) showing characteristics of the A

CT, (grs, di, preh,aug,or, ab)

CT (grs, di, preh, aug)

CT, grs, and r, (aug, di, wo, preh, ph)

CT, GRS, aug, di, anal, qtz (preh)

CT, grs, (di, aug, preh)

CT (grs, di, aug, preh)

CT (grs, di, aug, preh)

CT, wo, grs, (anal, or)

calcite 

veins

ANAL, AN, AB, aug, tetranatrolite, 

th, ph, (musc, maricopiate, kao)

anal, aug, an, ab, (clincl, ph)

BB-05-284.5

38.4 wt. % SiO2

l l

l

l

l l l

l l l l l

l l l l

% CaO

v

v

v

v

v

v

v

v

x

x

x

x

x

x

x

o

o

o

o

o

o

o

o

x

284.2

282.3

281.3a

281.3b

282.2

281.2

284.3

284.4

talus

2 m

2 m

l 2 m

3 m

calcite crystals

l

284.1

281.1 o v x

o v x

l l

% SiO2

% Al2O3



18O = 10.1



18O = 6.9

mudcracks

leopard-spotted

weathered surface

massive

alkali gabbro sill

~9.0 m total thickness (skarn)

B

site 281−284 (closer to interior)

50403020100 60

286.10

100 20 30 40 50 60

286.1

286.2

286.3

286.4

286.5

286.6

286.7

x

x

x

x

x

x

x

x

x

o

o

o

o

o

o

o

o

o

CT, qtz (ab)

CT, PREH, di, aug 

CT, GRS (aug, qtz)

CT, qtz (ab)

CT, qtz (ab)

CT, qtz (ab)

CT, qtz (ab)

CT, wo (aug, ab, or)

CT, grs, (aug, preh, qtz, wo)

v

v

v

v

v

v

v

v

v

285.2

AB, OR, MCL, anal, mont

ANAL, AUG, LAB, kao, ph, clincl, th

x

x

x

o

o

o

v

v

v

monzonite sill

l l l l l l l l l l l

ANAL, AN, AUG, th, clincl, (ph)

massive

nodule 

rich

deformed, slightly 

folded laminae

increasing alteration and deformation

l

talus

talus

site 286 (at edge)

alkali gabbro sill

laminated

~20.0 m total thickness (skarn)

alkali gabbro sill

% CaO

lllllll

285.1

% SiO2

% Al

O2 3



18O = 13.1



18O = 9.6



18O = 10.3

l l

287.2

286.8
Dagger Flats 49

lllll

l l l l ll

“leopard-spot

features”

v

v

v

v

o

o

o

o

o

o

x

x

x

x

x

x

340o

40o

laminated

siltstones

mudcracks

calcite veins

dark gray jointed

mudstone

60

CT, mesolite, preh, (aug, grs)

CT, wo, qtz, grs

CT, GRS, wo, preh

CT, WO, GRS, preh, anal 

(annite, ph,vermic, sm)

alkali gabbro sill

monzonite sill

alkali gabbro sill

BB-04-248.2.3

laminated

siltstones

mudcracks

laminated

siltstones

laminated

siltstones

 jointed gray mudstone concretions

2 sets of massive joints

talus

BB-05-247.5

BB-04-247.2

BB-04-247.4

BB-04-247.3

v

v

% CaO

BB-04-247.1

% SiO

% Al

2

2O3

l l

~15.0 m total thickness (skarn)

D

site 247

50403020100

C 100 20 60504030

site 261 (with contributions from 262, 248, 287)

talus

0.5 m

0.2 m

0.45 m

0.45 m

0.57 m

0.3 m

1.5 m

0.24 m

0.2 m

0.5

talus

talus

~11.5 m total thickness (skarn)

40o 340o

l l l ll l ll l l l l l

x

x

x

x

x

x

x

x

x

x

x

x

x

o o

o

o

o

o

o

o

o

o

o

o

o

v

v

v

v

v

v

v

v

v

v

v

v

v

talus

talus

0.8 m

0.5 m

1.7 m

0.45 m

2.5 m

0.3 m marl

0.3 m

261.13

261.12

261.11

261.10

261.9

261.8

261.4

261.5

261.6

261.7

261.1

261.2

261.3

0.5 m

monzonite sill l l

o v x

o v x

alkali gabbro sill l

o v x

alkali gabbro sill

% SiO2

% Al2O3

% CaO

lllll l

l l l llllllllll llll l l



18

248.1 O = 12.6

262.13 

CT, grs, wo, preh

287.1 

ANAL, AB, mcl,

sm, (kao, chl)

lllll

l l l l ll

“leopard-spot

features”

v

v

v

v

o

o

o

o

o

o

x

x

x

x

x

x

340o

40o

laminated

siltstones

mudcracks

calcite veins

dark gray jointed

mudstone

60

CT, mesolite, preh, (aug, grs)

CT, wo, qtz, grs

CT, GRS, wo, preh

CT, WO, GRS, preh, anal 

(annite, ph,vermic, sm)

alkali gabbro sill

monzonite sill

alkali gabbro sill

BB-04-248.2.3

laminated

siltstones

mudcracks

laminated

siltstones

laminated

siltstones

 jointed gray mudstone concretions

2 sets of massive joints

talus

BB-05-247.5

BB-04-247.2

BB-04-247.4

BB-04-247.3

v

v

% CaO

BB-04-247.1

% SiO

% Al

2

2O3

l l

~15.0 m total thickness (skarn)

D

site 247

50403020100

C 100 20 60504030

site 261 (with contributions from 262, 248, 287)

talus

0.5 m

0.2 m

0.45 m

0.45 m

0.57 m

0.3 m

1.5 m

0.24 m

0.2 m

0.5

talus

talus

~11.5 m total thickness (skarn)

40o 340o

l l l ll l ll l l l l l

x

x

x

x

x

x

x

x

x

x

x

x

x

o o

o

o

o

o

o

o

o

o

o

o

o

v

v

v

v

v

v

v

v

v

v

v

v

v

talus

talus

0.8 m

0.5 m

1.7 m

0.45 m

2.5 m

0.3 m marl

0.3 m

261.13

261.12

261.11

261.10

261.9

261.8

261.4

261.5

261.6

261.7

261.1

261.2

261.3

0.5 m

monzonite sill l l

o v x

o v x

alkali gabbro sill l

o v x

alkali gabbro sill

% SiO2

% Al2O3

% CaO

lllll l

l l l llllllllll llll l l



18

248.1 O = 12.6

262.13 

CT, grs, wo, preh

287.1 

ANAL, AB, mcl,

sm, (kao, chl)

Figure 4–4 (continued). bedrock 

(Cretaceous Boquillas Formation) in 

which the Dagger Flats skarn formed. 

The Boquillas Formation consists 

of a series of alternating massive, 

marly mudstones and laminated 

silty carbonates that have been 

variably mineralized. Thickness of 

the capping skarn is approx. 4 m. The 

skarn overlies a composite sill. Site 

286 is located along the east side 

of a major northwest-trending fault 

and is located near the edge of the 

large sill body at depth. B, Schematic 

composite sketch of exposures at 

sites 281, 282, 283, and 284 (fig. 4–1B). 

Inset is a schematic sketch of a 

banded carbonate with tiny thrust 

fault; this sample is enriched with 

respect to silica (38.4 wt. % SiO2). 

C, Schematic sketch of site 261 with 

contributions from sites 262, 248, and 

287 (fig. 4–1B). D, Schematic sketch of 

site 247 (fig. 4–1B).
50 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Capping Skarns

The skarn deposits at Dagger Flats are present in exposures capping the sill complex (fig. 4–5) in the Cretaceous 

Boquillas Formation. Thicknesses of the skarn deposits stratigraphically measured range from <4 m to as thick as 20 m; the 

complex extends for up to 10 km along its northwest strike. A 

series of alternating calcareous mudstones and fine-grained, 

well-sorted sandstones and siltstones, reflective of deposition 

in a near-shore marine environment, have been hydrothermally 

altered by heat, gases, and advecting fluids driven by emplacement of the underlying sills. Mineralogy of the skarn includes 

recrystallized calcite and quartz, wollastonite, grossular, 

prehnite, zeolites (mesolite, analcime), and clays (smectite). 

The carbonate host rock is strongly jointed and alteration is 

concentrated vertically along joints and fractured surfaces 

and laterally in the more porous fine-grained sandstones 

and siltstones (figs. 4–4 and 4–5). Alteration also appears 

zoned, with more intense alteration near the contact with the 

underlying sill complex (fig. 4–4). Fluid flow and heat were 

preferentially focused along well-defined vertical joints and 

the more porous, laminated, well-sorted siltstone units. Two 

sets of joints, oriented at 340o

 with a subordinate set at 40o, 

are similar in orientation to the prevailing northwest-trending 

faults cutting the sill complex at Dagger Flats and controlling 

the orientation of the low-lying hills (figs. 4–1B and 4–2).

Mineralogy and Geochemistry

Results of X-ray diffraction mineralogical studies, confirmed by examination of thin sections, indicate that the calcsilicate skarn deposits at Dagger Flats predominantly contain 

recrystallized calcite and quartz, wollastonite (CaSiO3), grossular garnet (Ca3

Al2(SiO4)

3

), prehnite (Ca2Al2Si3O10(OH)2), 

and augite ((Ca,Mg,Fe)2Si2O6) and (or) some other pyroxene.

This mineralogy is typical of skarn deposits. Wollastonite 

forms by the classic contact metasomatic reaction:

CaCO3 (limestone) + SiO2 (quartz or hydrothermal 

fluid) = CaSiO3 + CO2

The CO2 may be removed from the system by advecting 

hydrothermal fluids. Grossular garnet, prehnite, and pyroxene (usually diopside) are typical skarn minerals and form by 

reaction of chemical components in the original limestones 

with introduced hydrothermal SiO2 (silica), Na (sodium), Ca 

(calcium), and other cations (Fe (iron), Mg (magnesium)).

Lower temperature, probably late-stage minerals include 

the zeolites mesolite (Na2

Ca2Al6Si9O3+8H2O) and analcime 

(NaAlSi2O6+H2O). The presence of the zeolite group mineral 

mesolite suggests low temperature conditions during waning 

stages of hydrothermal mineralization.

Chemical data for the skarn samples substantiate the 

mineralogical analyses and allow some quantification of 

mineral content. For example, assuming all carbonate carbon 

is in calcite, we calculate CaCO3

 contents ranging from 10 to 

80 wt. %. This assumption is justified by the generally low Mg 

contents (<2 wt. %) of these rocks and the lack of dolomite 

or other carbonate minerals. The large variations in CaCO3

contents also confirm field and thin section observations that 

the original limestone units are diverse in lithologic characteristics and porosity ranging from calcitic siltstones to relatively 

pure limestone units. The monzonite and alkali gabbro sills, 

in contrast, contain little carbonate, and thus did not experience metasomatic introduction of CO2 in the samples we have 

examined to date. The presence of smectite, kaolinite, and 

chlorite in the monzonite sill (fig. 4–4) and water contents as 

high as 4.1 wt. % provides strong evidence, along with the 

oxygen isotope analysis, that the sills have experienced hydrothermal alteration.

Minor element geochemistry of the contact metasomatic 

skarn deposits indicates that these samples contain low contents of base metals (copper, lead, and zinc), and molybdenum. 

Concentrations of copper (Cu) range from 51 to 150 μg/g in 

the alkali gabbro sills and from 8.0 to 25.0 μg/g in the monzonite sill. In the capping skarn deposit, concentrations of Cu 

range from 1.2 to 18.0 μg/g; elevated Cu is present in the skarn 

at the contact with the sill where concentrations of Cu range 

from 29 to 44 μg/g. Concentrations of lead (Pb) range from 

2.0 to 4.0 μg/g in the alkali gabbro sill and 7.0 to 9.4 μg/g in 

the monzonite sill. In the skarn, Pb concentrations range from 

1.2 to 5.0 μg/g with higher Pb concentrations present at the sill 

contact ranging from 6.7 to 14.0 μg/g. Concentrations of zinc 

(Zn) range from 78 to 100 μg/g in the alkali gabbro sill and 

from 81 to 94 μg/g in the monzonite sill. In the skarn deposit, 

Zn concentrations range from 16 to 31 μg/g with elevated Zn 

concentrations in the laminated siltstones (61 to 78 μg/g) and 

at the sill contact ranging from 120 to 150 μg/g. With respect 

to concentrations of molybdenum (Mo), both the alkali gabbro 

and monzonite sills are comparable and range from 1.2 to 6.2 

μg/g. In the capping skarn deposit, concentrations of Mo are 

low (0.79 to 5.3 μg/g) for the mineralized mudstones; however, in the skarn deposit, concentrations of Mo are somewhat 

elevated at the contact between the skarn and sill (27 to 110 

μg/g), in faulted sections (46 μg/g in sample BB-05-284.5), 

and in the mineralized laminated siltstone units (56 to 98 

μg/g). Total iron (Fe) content ranges up to 3.2 wt. % as Fe2

O3

and total sulfur (S) ranges from <0.05 to 0.43 wt. %, which 

suggests most Fe is in the silicates, and most of the sulfur 

occurs as pyrite or pyrrhotite. Low Fe content in the unaltered 

Cretaceous limestone units suggests that Fe was probably 

introduced during metasomatism.

Geochemical and mineralogical data show how the 

extent of mineralization and recrystallization was affected 

by the porosity of the host rock (fig. 4–5). Alternating grey 

massive limey mudstones and orange-stained laminated 

siltstones are present in many skarn exposures in the Cretaceous Boquillas Formation (fig. 4–4A). Sample BB-04-247.5 

is from a laminated silty limestone unit full of secondary 

concretions and corresponds to an enriched silica (SiO2

) 

content of 39.8 wt. % (fig. 4–4D). Correspondingly, the major 
A

BB-04-248.

BB-04-24

site 247 BB-04-247.2

D EB C

Figure 4–5. Photographs of features exposed in skarns at Big Bend National Park. A, Exposure of skarn in the Cretaceous Boquillas Formation 

total thickness of skarn here is about 12 m. B, Photograph of mineralized carbonate mudstone (BB-04-247.2) with mudcracks. C, Photograph of m(BB-04-248.2.3) showing “leopard spot” replacement. D, Photograph of skarn showing mudstone in contact with mineralized laminated silty carbof mineralized, laminated silty carbonate (BB-04-247.1).
magma at depth magma at depth magma at depth 

stratified sedimentary units developing faults A

E

B C D

magma at depth

GRS

WO

T=~1100oC

T=~550oC

T=~420oC

site 286

sites 281-284

recrystallizedcalcite

zeolitesand clays

PREH

recrystallizedquartz

*

*

T=~250-350oC
Dagger Flats 53

alteration minerals in this sample are grossular garnet and 

recrystallized calcite, with minor wollastonite and prehnite. 

A lower unit in the section (BB-04-247.2) is a dense marly 

limestone containing well-developed mudcracks (fig. 4–5B). 

In contrast to BB-04-247.5, sample BB-04-247.2 has significantly lower total silica content (9.29 wt. % SiO2

, fig. 4–4D), 

suggesting this carbonate unit was less permeable and not 

as influenced by the infiltration of reactants and heat. Major 

alteration mineralogy of BB-04-247.2 is calcite, most likely 

recrystallized, with minor wollastonite, quartz, and grossular 

garnet. Sample BB-04-247.1 (fig. 4–4D) is at the base of the 

skarn in contact with the underlying composite sill. This sample is a laminated, fine-grained, limey siltstone (fig. 4–5E) 

and is full of mineralized concretions up to 4 mm in diameter. 

Like BB-04-247.5, sample BB-04-247.1 contains enriched 

silica (33.2 wt. % SiO2, fig. 4–4D), and its major alteration 

mineralogy consists of wollastonite, grossular garnet, and 

calcite with minor alteration mineralogy of prehnite and analcime. Lower temperature zeolite and clay alteration formed in 

later stages where meteoric water infiltrated and reacted with 

the cooling sill. Sample BB-04-247.1 was subjected to both 

early high-temperature and later low-temperature alteration. 

The relationship between varying silica content and the extent 

and type of alteration with respect to the physical and chemical characteristics of the host rock is seen repeatedly in the 

skarn exposures at Dagger Flats (fig. 4–4).

Faulting also influenced the infiltration of mineralizing 

fluids and heat into the carbonate host and is an important 

component aiding in the transport of fluids. Faulting also may 

have played a significant role in transporting mineralizing 

fluids. Sample BB-05-284.5 is associated with well developed 

slickensides with recrystallized calcite and quartz along its 

fault plane (fig. 4–4B) and is enriched with respect to Mo, Zn, 

and Cu.

Figure 4–6 (facing page). Simplified schematic diagrams 

illustrating a model for the intrusion of sills at Dagger Flats into 

the Cretaceous Boquillas Formation and formation of the skarn. 

A, Magma chamber at depth below sequence of sedimentary 

rocks. B, Fractures in sedimentary rocks form in conjunction 

with extensional tectonism, allowing magma at depth to 

flow toward space created by fractures. Dike propagation 

is focused on fractures where magma fills voids created by 

extension. C, Dike propagation continues along extensional 

fractures. Magmatic temperatures for alkali gabbros are 

about 1100o

C. As the alkali gabbroic dike propagates along the 

fracture, the magma heats the wall rock. An increase of heat 

to the wall rock results in facilitating the ascent of magma. D, 

At some level higher in the crust (in this case, the level of the 

Cretaceous Boquillas Formation), the presence of lower density 

strata may result in a change to the overall stress field. The 

upward ascent of magma in the propagating dike, controlled by 

location of the fractures and axis of least compressive stress 

(at depth most likely horizontal resulting in dike propagation 

in the vertical plane) is affected by a change in stress field. At 

shallower depths in the crust, the least compressive stress field 

changes from a horizontal plane to a vertical plane affecting 

the upward ascent of dike propagation to flow where the axis 

of least compressive stress becomes dominant and magmas 

flows horizontally. E. Close-up diagram of figure 4–6D showing 

schematically the thermal aureole developed in the host rock 

(in this case, the Cretaceous Boquillas Formation) in response 

to the emplacement of hot, mostly mafic magma, associated 

magmatic gases rich in H2

O, CO2, and H2S, and percolating 

hydrothermal fluids. Minerals in the thermal aureole and 

their associated temperatures of formation are depicted 

schematically with the higher temperature minerals such as 

wollastonite (WO) forming in the limestone unit at the contact 

with the sill and lower temperature minerals found farther away 

from the intrusion. Recrystalized quartz and calcite are found 

at the perimeter of the thermal aureole. A range of alteration 

mineral minerals are present in the sills and represent a broad 

range of temperatures of formation associated with hotter 

magmatic gases and lower temperature hydrothermal fluids. 

PREH, prehnite; GRS, grossular garnet.
54 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Discussion

About 32 Ma, north-northeast-trending faults formed in 

and around BBNP and were associated with south-southwest 

extension. This tectonic activity coincided with magmatic 

activity, which included intrusion of large oval-shaped sill 

complexes in the northeast quadrant of BBNP. Composition 

of the alkaline tabular intrusions at Dagger Flats is predominantly alkali gabbro with subordinate monzonite intrusions. 

Monzonite bodies within alkali gabbroic sills and laccoliths 

are common features in the Big Bend region and most likely 

formed as in-situ differentiates involving crystal fractionation 

(fig. 4–3B) (Carman, 1994).

Timing of emplacement of the Dagger Flats sills is 

closely contemporaneous with eruption of large-volume rhyolitic ignimbrites and intrusion of later felsic dikes less than 10 

km to the south at the Pine Canyon caldera. Intrusive activity at Dagger Flats occurred about 32.47+0.41 Ma, while the 

predominantly rhyolitic volcanic activity at the Pine Canyon 

caldera ranged from 32.22+0.12 Ma to 30.86+0.41 Ma (Morgan and others, 2006).

Emplacement of the alkali gabbroic sills at Dagger Flats 

coincides with early tectonic extension in the area. From field 

relationships, such as a well-preserved chilled margin in the 

monzonite bodies within the alkali gabbroic sills, it is clear the 

monzonite sheets were quenched in the slightly younger but 

already cooled alkali gabbro. The monzonite sills, which make 

up only a small volumetric fraction of the total sill (probably 

<2 percent total volume).

As shown on the magnetic intensity map (fig. 4–2B), an 

oval, northwest-trending body with positive magnetic intensities is present below Dagger Flats extending to the south and 

southeast and may be related to other similar structures in 

the vicinity, such as those that underlie the McKinney Hills 

(fig. 4–2B), and the Rosillos Mountains and Grapevine Hills 

(fig. 4–1). These features are large (>10 km2

), oval intrusive 

bodies. Samples taken from each of these areas indicate that 

the Dagger Flats sills and those in the Rosillos Mountains 

to the west are dominantly alkali gabbroic in composition, 

whereas compositions of samples collected from exposures 

in the McKinney Hills and the Grapevine Hills range from 

syenitic to quartz monzonitic to granitic (figs. 4–1 and 4–2). 

Additional studies at McKinney Hills and the Grapevine 

Hills may be warranted to determine the full range of compositions for their intrusions. The similar magnetic intensities of these bodies compared to those at Dagger Flats and 

the Rosillos Mountains indicate that they are probably also 

alkali gabbroic in composition. The more felsic-rich silllike subhorizontal sheets at McKinney Hills and laccoliths 

at Grapevine Hills that are now exposed may represent only 

a small fraction of the total intrusions. The total magnetic 

intensity values of these sill and laccolith bodies indicate that 

the larger, parental part of the intrusion is still buried and is 

alkali gabbroic or more mafic in composition than what is 

exposed.

Comparison of mineralogic, chemical, and oxygen 

isotope data for the Dagger Flats skarn deposits with other 

similar occurrences suggests that main stage skarn formation 

in the limestone rocks of the contact zone at Dagger Flats 

resulted from high-temperature reaction with magmatic fluids 

(Bowman, 1998). Skarn mineralogy adjacent to the sills is 

determined by the composition and physical characteristics 

of the intrusive body and the physical and chemical nature of 

the carbonate host rock. At Dagger Flats, emplacement of the 

alkali gabbroic sills occurred at temperatures that exceeded 

1100o

C and resulted in intense heating and degassing of magmatic gases (H2

O, CO2, and H2S). These gases escaped into 

the adjacent carbonate strata and promoted reactions, typically 

at temperatures of 250–550oC, which produced skarn minerals such as wollastonite, garnet, pyroxene, and prehnite (fig. 

4–6). Additionally, intrusion of the sills into carbonate strata 

promoted recrystallization of the calcite and quartz that are 

present in the host rock. Following the main stage of skarn 

mineralization at Dagger Flats, meteoric waters convected in 

the skarn and the sills resulted in lower temperature (100–

350o

C) alteration of the host rocks and produced additional 

alteration including the formation of chlorite, zeolites, and 

clays (fig. 4–6), as well as other sheet silicates, such as mica.

References Cited

Anderson, E.D., Finn, C.A., Scott, R.B., and Snee, L., 2004, 

Mapping and visualizing faults using aeromagnetic data 

and GIS in northern Big Bend National Park, Texas [abs.]: 

Geological Society of America Abstracts with Programs, v. 

36, no. 5, p. 127.

Bird, D.K., Schiffman, P., Elder, W.A., Williams, A.E., and 

McDowell, S.D., 1984, Ca Flc-silicate mineralization in 

active geothermal systems: Economic Geology, v. 79, p. 

671–695.

Bowman, J.R., 1998, Stable-isotope systematics of skarns, 

in Lentz, D.R., ed., Mineralized intrusion-related skarn 

systems: Mineralogical Association of Canada Short Course 

Series, v. 26, p. 99–146.

Carman, M.F., 1994, Mechanisms of differentiation in shallow 

mafic alkaline intrusions, as illustrated in the Big Bend area, 

western Texas: Journal of Volcanology and Geothermal 

Research, v. 61, p. 1–44.

Chevalier, L., and Woodford, A., 1999, Morph-tectonics and 

mechanism of emplacement of the dolerite rings and sills of 

the western Karoo, South Africa: South African Journal of 

Geology, v. 102, p. 43–54.

Henry, C.D., and Price, J.G., 1984, Variations in caldera 

development in the Tertiary volcanic field of Trans-Pecos 

Texas: Journal of Geophysical Research, v. 89, no. B10, p. 

8,765–8,786.
References Cited 55

Lentz, D.R., 1998, Mineralized intrusion-related skarn 

systems, in Lentz, D.R., ed., Mineralized intrusion-related 

skarn systems: Mineralogical Association of Canada Short 

Course Series, v. 26, preface.

Maxwell, R.A., 1968, The Big Bend of the Rio Grande—A 

guide to the rocks, landscape, geologic history, and settlers 

of the area of Big Bend National Park: Bureau of Economic 

Geology Guidebook 7, 138 p.

Middlemost, E.A.K., 1994, Naming materials in the magma/

igneous rock system: Earth-Science Reviews, v. 37, 

p. 215–224.

Morgan, L.A., Shanks, W.C.P., and McIntosh, W.C., 2006, 

Late Oligocene igneous activity in Big Bend National Park, 

Texas—Contemporaneous activity at Dagger Flats sill complex and Pine Canyon caldera [abs.]: Geological Society of 

America Abstracts with Programs, v. 38, no. 7, p. 34.

Smallwood, J.R., and Maresh, J., 2002, The properties, 

morphology and distribution of igneous sills—Modeling, 

borehole data and 3D seismic data from the Faeroe-Shetland 

area, in Jolley, D.W., and Bell, B.R., eds., The North Atlantic igneous province—Stratigraphy, tectonic, volcanic, and 

magmatic processes: Geological Society of London, Special 

Publication 197, p. 271–306.

Thomson, K, 2004, Sill complex geometry and internal architecture—A 3D seismic perspective, in Breitkreuz, C., and 

Petford, N., eds., Physical geology of high-level magmatic 

systems: Geological Society of London, Special Publication 

234, p. 229–232.

Thomson, K., and Hutton, D., 2004, Geometry and growth of 

sill complexes—Insights using 3D seismic from the North 

Rockall Trough: Bulletin of Volcanology, v. 66,

p. 364–375.

Abstract

The concentration of mercury in mine waste, stream 

sediment, water, soil gas, and air was determined in and 

around inactive mercury mines in the Big Bend region of 

southwest Texas to evaluate possible contamination and 

transport of mercury into local ecosystems that eventually flow into the Rio Grande. In addition, concentrations 

of methylmercury were measured in mine waste, stream 

sediment, and water to evaluate conversion from inorganic 

mercury to highly toxic organic mercury compounds, such as 

methylmercury. Mine waste contains mercury and methylmercury concentrations as high as 19,000 μg/g and 1,500 

ng/g, respectively, which are highly elevated but similar to 

those reported from other mercury mines worldwide. Mercury concentrations in stream sediment samples were generally below the probable effect concentration of 1.06 μg/g (the 

concentration above which harmful effects are likely to be 

observed in sediment dwelling organisms), but mercury concentrations in mine waste were found to exceed this concentration (however, this standard is a sediment quality guideline 

and not directly applicable to mine waste). Mercury concentrations in water samples collected vary from 1.2 to 14 ng/L 

in mine runoff, 2.7 to 13 ng/L in the Rio Grande, 0.68 to 9.7 

ng/L in baseline streams, and 0.049 to 10 ng/L in wells and 

springs. Mercury concentrations found in all water sampled 

in this study were significantly below the 2,000 ng/L drinking water standard for mercury and were also generally 

below the 12 ng/L aquatic wildlife standard for mercury, 

which are recommended by the U.S. Environmental Protection Agency (EPA). Although mercury and methylmercury 

concentrations are locally high in some mine waste samples, 

little mercury appears to be exported from these mercury 

mines through stream sediment and water primarily because 

of the arid climate, lack of precipitation, and resulting lack 

of mine runoff in this region. Mercury emission data for 

the mines studied suggest that, although concentrations of 

mercury in soil gas in mine waste are elevated (as much as 

21,000 ng/m3

 at Mariscal mine), persistent wind in southwest 

Texas disperses mercury in air (2.5–59 ng/m3) within a few 

meters of the ground surface to concentrations similar to or 

slightly higher than those found in air at regional baseline 

sites (1.2–17 ng/m3

). In addition, baseline mercury measurements in air and soil gas in Big Bend National Park further 

suggest little regional influence of mercury emissions from 

these mines.

Introduction

Mining of mercury was prevalent in the Big Bend area 

beginning about 1888 with intermittent operations continuing until about 1973 (Sharp, 1980; Avery and others, 1996). 

There are no mercury mines presently operating in the United 

States, primarily because of environmental concerns related 

to mercury toxicity and the significant recycling of mercury 

that reduces the demand for new resources (Sznopek and 

Goonan, 2000). Several inactive mercury mines constitute the 

Terlingua district, located in and around Big Bend National 

Park (BBNP). Total production from this region exceeded 

5,000 t of mercury (Sharp, 1980). Consequently, the Terlingua 

district ranks as the third largest mercury-producing region in 

the United States; only mercury mines in the California Coast 

Ranges (100,000 t of mercury) and McDermitt, Nev., (10,000 t 

of mercury) are larger (Rytuba, 2000; Gray and others, 2002). 

Mariscal, one of the smaller mercury mines of the Terlinugua 

district, located in the southern part of BBNP, produced about 

50 t of mercury during mining between about 1900–1943 and 

became part of BBNP when the park was established in 1944. 

Access to these mines is generally unrestricted, especially in 

BBNP, and as a result, tourists often visit these sites. Runoff 

and downstream transport of waste containing mercury from 

the mines potentially contaminates streams and rivers in the 

area, which are located in or flow through BBNP and eventually flow into the Rio Grande (fig. 5–1). Because the Mariscal 

mine is the only mercury mine in the park, it was the dominant 

focus of U.S. Geological Survey (USGS) mercury contaminant 

research in BBNP.

5. Evaluation of Mercury Contamination at Inactive 

Mercury Mines in and around Big Bend National Park

By John E. Gray, Peter M. Theodorakos, and David P. Krabbenhoft
58 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

The most significant environmental concerns associated 

with mercury mines include mercury contamination in mine 

waste, sediment, and soil in and around the mines as well as 

evasion of mercury gas from these mined areas. Evaluation of 

downstream transport of mercury and chemical and microbial 

transformation of mercury to bioavailable organic compounds 

such as methylmercury, the most toxic of the mercury compounds, is especially important in areas of past mercury mining. 

Microbial activity is primarily responsible for the conversion 

of inorganic mercury to organic mercury compounds; although 

abiotic, chemical mercury methylation is also a known conversion process (Ullrich and others, 2001). Methylmercury is particularly problematic because it is water soluble and is readily 

transferred from the sediment column to the water column, then 

to aquatic wildlife. Methylmercury bioaccumulates in living 

organisms; its concentration increases with increasing trophic 

position in the food chain (biomagnification), and it is a human 

neurotoxin. Conversion and transfer of methylmercury from 

active and inactive mercury mines to surrounding ecosystems 

is a potential concern worldwide (Gray and others, 2004). As 

part of the USGS project in BBNP, concentrations of mercury 

and (or) methylmercury were measured in mine waste, stream 

sediment, soil, water, and air around several mines in order to 

evaluate mercury contamination at these sites. 

0

0

10 miles

20 kilometers

5

10

Rio Grande

Rio Grande

Mariposa

 mine

Study

Butte

mine

Terlingua

 mine

Mariscal mine

X

X

X

X

Rough Run Creek

Terlingua

Long Draw

Creek

Mariscal Mountain

Chisos

Mountains

Big Bend National

 Park boundary

Fresno Creek

Tornillo Creek

29°00'

103°45'

29°30'

TEXAS

 Big Bend

National Park

X Mines studied

EXPLANATION

103°00'

Figure 5–1. Location of the study area and mercury mines studied.

Mine Waste and Sediment

The Terlingua district includes more than 30 separate 

mines (Ross, 1941), but the USGS mercury contaminant studies summarized in this report focused mostly on the Study 

Butte, Mariposa, Terlingua (also known as the Chisos mine), 

and Mariscal mines (fig. 5–1). Similar to most mercury mines 

worldwide, mercury ore at the studied mines is dominantly 

cinnabar (hexagonal HgS), but minor metacinnabar (isometric 

HgS), elemental mercury (Hgo

), and oxychlorides such as terlinguaite (Hg2

ClO) and eglestonite (Hg2Cl2O) were identified 

at some mines (Ross, 1941; Sharp, 1980).

Extraction of mercury during mining is generally carried 

out in a furnace where mercury ore is heated to temperatures 

of about 600–700o C (a process termed retorting), which easily converts the ore (mostly cinnabar) to elemental mercury, 

the final mercury product that is used for commercial trade 

(Bailey and Phoenix, 1944). This ease of conversion combined with the unique properties of elemental mercury is the 

reason why mercury has been mined for such a long time, 

more than 2,000 years. However, the retorting of mercury ore 

is known to be an inefficient and incomplete process, and as 

a result, mine waste found at most mercury mines contains 

unconverted cinnabar, elemental mercury, and other mercury 

compounds formed during processing (Gray and others, 

2006). Even after many years of inactivity, areas affected 
Water 59

by mercury mining continue to be characterized by highly 

elevated mercury concentrations as a result of the inefficient 

and incomplete mercury extraction process (Gray, 2003). 

One consequence of mercury mining is that considerable 

mine waste is generated during ore processing; this waste is 

typically discarded at the site of mining, and much of this 

waste contains toxic mercury compounds. There are greater 

than 2,000,000 m3

 of mine waste remaining in the Terlingua 

district and approximately 30,000 m3

 at the Mariscal mine 

(fig. 5–2) (Gray and others, 2006). This mine waste contains 

elevated mercury concentrations as a result of incomplete 

mercury recovery and generation of mercury byproduct 

compounds (such as calomel, Hg2

Cl2) during retorting, some 

of which are water soluble (Kim and others, 2003; Gray and 

others, 2004). Thus, these mine wastes contain significant 

amounts of mercury potentially available for downstream 

transport, leaching, and biogeochemical transformation to 

methylmercury. Runoff from mines in the Terlingua district 

potentially transports various mercury compounds downstream into tributaries that flow into Terlingua Creek, which 

flows through the western side of BBNP and eventually into 

the Rio Grande (fig. 5–1). Similarly, runoff from the Mariscal 

mine is transported downstream to Fresno Creek, which flows 

into the Rio Grande.

Mercury concentrations in mine waste samples collected 

from the Terlingua district vary widely from 4.1 to 19,000 

μg/g (fig. 5–3) and methylmercury concentrations vary from 

0.051 to 1,500 ng/g. Mine waste collected from the Mariscal 

mine also contains elevated mercury concentrations (6.9–150 

μg/g) and methylmercury concentrations (0.55–1.5 ng/g), 

although these concentrations are generally lower than those 

found at other mines in the Terlingua district. Lower mercury concentrations at Mariscal are probably related to more 

efficient recovery of mercury at this mine during ore retorting 

(Gray and others, 2006). Mercury and methylmercury concentrations in mine waste collected from the Terlingua district are 

highly elevated relative to those in stream sediment samples 

collected from uncontaminated regional baseline sites in this 

study. Baseline samples contain mercury ranging from less 

than 0.010 to 0.028 μg/g (fig. 5–3) and methylmercury ranging 

from less than 0.010 to 0.025 ng/g. Mercury concentrations 

in most mine waste samples also exceed the probable effect 

concentration (PEC) of 1.06 μg/g in sediment; harmful effects 

are likely to be observed in sediment dwelling organisms when 

mercury concentrations exceed this level (MacDonald and others, 2000). (It should be noted, however, that this concentration is a mercury sediment quality guideline and is not directly 

applicable to mine waste.) Highly elevated and variable mercury concentrations in waste from the mines studied relates to 

the presence of cinnabar and other mercury compounds. Due 

to the highly toxic nature of methylmercury, concentrations of 

this compound are most noteworthy. At present, it is unclear 

whether the methylmercury concentrations in mine waste 

reported here would be harmful to human health if such mine 

waste were inhaled because health guidelines for methylmercury in sediment have not been established.

Stream sediment samples were collected along Fresno 

Creek downstream from the Mariscal mine to evaluate transport of mercury and methylmercury to the Rio Grande, about 

10 km downstream from the mine. These stream sediment 

samples show significant dilution of mercury and methylmercury concentration with increasing distance from the mine. 

No structures have been built at the Mariscal mine to prevent 

erosion and downslope transport of mine waste, so some mine 

waste material (although minor) is present in stream beds 

within a few meters of the Mariscal mine. Concentrations 

of mercury (7.9 μg/g) and methylmercury (0.62 ng/g) are 

elevated in the stream sediment sample collected nearest to the 

Mariscal mine (0.5 km downstream); however, within a few 

kilometers downstream, mercury and methylmercury concentrations are similar to those found in the regional baseline 

stream sediment sites. In addition, only the stream sediment 

sample collected nearest the Mariscal mine contains mercury 

that exceeded the PEC of 1.06 μg/g in sediment. Mariscal is 

the closest mercury mine to the Rio Grande, but geochemical results from the Mariscal mine area suggest that processes 

favoring microbial mercury methylation are limited and that 

the transference of significant mercury and methylmercury 

from mine waste to downstream sediment then to the Rio 

Grande is unlikely.

Water

To evaluate potential water-borne mercury contamination, 

water samples were collected from five sources: (1) near mercury mines, (2) the Rio Grande, (3) uncontaminated baseline 

streams, (4) hot and cold springs, and (5) wells in the Big Bend 

region. Because the study area is in the Chihuahuan Desert, 

there is little precipitation (<25 cm/year) and little mine runoff; 

therefore, mine water runoff was collected downstream from 

the Study Butte and Terlingua mines, the only mine sites where 

water was found flowing during fieldwork from 2003–2007. 

Mercury concentrations in seven samples of mine water runoff 

varied from 1.2 to 14 ng/L and methylmercury ranged from 

0.034 to 1.3 ng/L, whereas water samples collected from nine 

uncontaminated baseline stream sites in BBNP were found 

to contain mercury ranging from 0.68 to 9.7 ng/L and methylmercury ranging from 0.026 to 0.61 ng/L (fig. 5–4). Water 

samples collected from 10 sites along the Rio Grande contained 

mercury ranging from 2.7 to 13 ng/L and methylmercury ranging from 0.22 to 2.0 ng/L, and 20 water samples collected from 

wells and springs in BBNP contained mercury concentrations 

ranging from 0.049 to 10 ng/L and methylmercury ranging 

from <0.020 to 0.099 ng/L.

Average mercury concentrations measured in collected 

water samples are 8.5 ng/L in mine runoff, 5.1 ng/L in the Rio 

Grande, 4.2 ng/L in baseline streams, and 1.7 ng/L in wells 

and springs. These data indicate that wells and springs in 

BBNP contain the lowest mercury concentrations of the water 

samples collected, and thus, ground water collected in BBNP 

has lower mercury concentrations than those found in surface 
60 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Study Butte mercury mine, Terlingua, Texas

Mine waste

Mariscal mercury mine, Big Bend National Park

Inset of old retort

Mine waste

Retort bricks containing mercury

Air measurement

equipment

Retort

A

B

C

Figure 5–2. Photographs 

of Study Butte and Mariscal 

mercury mines and mine 

waste. A, View of the Study 

Butte mine showing large 

volume mine wastes in the 

foreground. B, View of the 

Mariscal mercury mine in 

Big Bend National Park. 

Mariscal was one of the 

smaller mines studied, was 

mined from 1900–1943, and 

became part of Big Bend 

National Park when the park 

was designated in 1944. C, 

View of a dilapidated brick 

retort at the Mariscal mine. 

Bricks from this retort still 

contain significant mercury 

even though the mine has not 

operated since the 1940s.
Soil Gas and Air 61

0.001

0.1

10

1,000

100,000

Mine waste Mine runoff sediment Baseline sediment Rio Grande

Probable effect concentration

Mercury Concentration (μg/g)

Stream sediment collected 0.5 km

downstream from the Mariscal mine

Figure 5–3. Concentration of mercury in mine waste and sediment collected in and around Big Bend National Park. The probable 

effect concentration of 1.06 μg/g in sediment is shown for reference and is the mercury concentration above which harmful effects 

are likely to be observed in sediment dwelling organisms (MacDonald and others, 2000). Only one stream sediment sample collected 

nearest the Mariscal mine contains mercury exceeding this probable effect concentration for mercury in sediment.

water samples collected in this study. Mercury concentrations found in all water samples collected in this study are 

significantly below the 2,000 ng/L drinking water standard 

for mercury used in the United States (U.S. Environmental 

Protection Agency, 2003). Mercury concentrations in these 

water samples are also generally below the 12 ng/L mercury 

standard recommended by the EPA to protect against chronic 

effects to aquatic wildlife (U.S. Environmental Protection 

Agency, 1992). Only two water samples collected exceed 

the EPA aquatic life standard: (1) mine water runoff from the 

Terlingua mine that contained 14 ng/L mercury, and (2) Rio 

Grande water collected just outside of the western boundary of 

BBNP that contained 13 ng/L. There are no water standards 

established for methylmercury concentrations in water; however, methylmercury concentrations in well and spring water, 

which are potential drinking water sources in BBNP, are similar to those reported for uncontaminated worldwide baselines 

(fig. 5–4) (Gray and others, 2004). With respect to mercury 

concentration, water quality in the samples collected in the Big 

Bend region is generally good where mercury concentrations 

are below acceptable EPA standards.

Soil Gas and Air

Mercury concentrations were measured in air and soil gas 

at the Mariscal mine in BBNP, as well as the Terlingua and 

Study Butte mines just outside of BBNP, to evaluate mercury 

emissions from mines in the Big Bend region. Concentrations of mercury emitted from mine waste at the Mariscal 

mine ranged from 690 to 1,400 ng/m3, and mercury emitted 

from mine waste at the Terlingua and Study Butte mine ranged 

from 720 to 1,600 ng/m3. However, concentrations of mercury 

emitted near an old brick Mariscal mine retort were higher and 

varied from 18,000 to 21,000 ng/m3 (figs. 5–2C and 5–5). Such 

mercury emissions reflect the presence of elemental mercury 

in mine waste and retort bricks. Mercury mine emission data 

indicate that the average flux of mercury emitted from the 

mines studied may exceed 2,000 ng/m2/hr. Such mercury concentrations and mercury flux rates are high but are within the 

range of concentrations measured from other mercury mines 

worldwide (Ferrara and others, 1991; Gustin and others, 1996; 

Gray, 2003). In contrast, mercury concentrations in ambient air 

measured 2 m above the ground surface at these mercury mines 

ranged from 2.5 to 59 ng/m3, suggesting rapid atmospheric 

dispersion of mercury gas. Ambient air measured at 2 m 
62 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

above the surface at five baseline sites within BBNP contained 

mercury concentrations ranging from 1.2 to 17 ng/m3, similar 

to mercury found in the atmosphere worldwide, ranging from 

about 1 to 4 ng/m3 (Porcella, 1994; Lamborg and others, 2002). 

Soil at these baseline sites emitted mercury concentrations 

ranging from 2.6 to 71 ng/m3.

There are no mercury standards directly comparable to 

air and soil gas mercury data shown here, although the allowable mercury inhalation ceiling for the occupational health 

standard for workers in the United States is 100,000 ng/m3

(U.S. Occupational Safety and Health Administration, 1976); 

this mercury concentration cannot be exceeded at any time 

during the work day. Mercury concentrations measured in soil 

gas near the dilapidated brick retort at the Mariscal mine (fig. 

5–2C) were the highest observed in this study (21,000 ng/m3). 

These concentrations do not exceed the OSHA occupational 

standard, however, because these concentrations were measured in soil gas at the Mariscal mine retort area and are not 

0.1

1

10

100

0.001 0.01 0.1 1 10 100

1,000

10,000

Mine water Wells and springs Baseline streams

Methylmercury concentration (ng/L)

Mercury concentration (ng/L)

EPA aquatic life standard

EPA drinking water standard

Worldwide uncontaminated baselines

Rio Grande

Figure 5–4. Concentration of mercury versus methylmercury in water collected in the Big Bend area. The EPA 

drinking water standard and the mercury standard to protect against adverse chronic effects to aquatic life are shown 

for reference. The range of data for worldwide uncontaminated baselines is also shown for reference.

directly analogous to the OSHA standard set for mercury 

concentrations inside of buildings. These results suggest that 

humans should not be exposed to mercury concentrations 

such as those found around the Mariscal mine retort for long 

periods of time, especially in closed spaces. People do not 

live near the Mariscal mine and there are no buildings located 

there for permanent or temporary occupation. The mercury 

emission data for the mines studied suggest that although concentrations of mercury in soil gas in mine waste are elevated, 

persistent wind in southwest Texas disperses mercury in air 

within a few meters of the ground surface. It is unlikely that 

tourists visiting mercury mines in the Big Bend region inhale 

enough mercury to make its presence a health concern. Furthermore, mercury concentrations found in air and soil gas at 

several baseline sites in BBNP suggest little regional influence 

of mercury emissions from these mines because the mercury 

concentrations from these baseline sites are similar to ambient 

air worldwide.
Conclusions 63

Conclusions

Mercury was mined in the Big Bend region from about 

1888 to 1973 and constituted an important mercury resource 

in the United States. Production exceeded 5,000 t of mercury, 

mostly from mines in and around the town of Terlingua, Tex.; 

one smaller mine, Mariscal, is located in BBNP. Although 

mercury and methylmercury concentrations in mine waste are 

locally elevated, there is only minor transference of mercury to 

surrounding ecosystems based on the mercury data for stream 

sediment and water samples collected downstream from the 

mines studied. Concentrations of mercury and methylmercury 

are generally low in stream sediment and water samples collected downstream from the mines studied, exhibiting significant mercury dispersion downstream. Low methylmercury 

1

10

100

1,000

10,000

100,000

Baseline air

Baseline soil gas

Mariscal mine waste

Mariscal mine air

Mariscal mine retort

Other Terlingua mines waste

Other Terlingua mines air

Ambient air

worldwide

Occupational health standard

Mercury in air and soil gas (ng/m3)

Baselines

Mariscal mine

Othe

Te

r

rlingua mines

Reference Points

Figure 5–5. Mercury concentrations in air and soil gas collected from mercury mines in the Big Bend area and from 

uncontaminated baseline sites in the park. Concentrations of mercury in ambient air worldwide and the occupational health 

standard used in the United States are shown for reference.

concentrations in stream sediment and water indicate that the 

arid climate and lack of precipitation in the Chihuahuan Desert 

inhibit mercury methylation downstream, probably because 

methylating bacteria are dormant under such conditions. The 

Rio Grande is the most important ecosystem in the region, 

but mines in this region are distant from the Rio Grande (>10 

km), and again, due to the lack of precipitation, mine runoff is 

minimal; when runoff is present, mercury is transported and 

significantly diluted before it reaches the Rio Grande. This 

lack of mining-related mercury influence in the Rio Grande is 

corroborated by the fact that mercury and methylmercury concentrations in Rio Grande sediment are similar to those found 

in regional baseline sediment samples (Gray and others, 2006).
64 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

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1996, Preliminary assessment and site inspection, the 

Mariscal mine and mill site, Big Bend National Park, 

Brewster County, Texas: U.S. Bureau of Mines, Site Characterization Section special report. 

Bailey, E.H., and Phoenix D.A., 1944, Quicksilver deposits 

in Nevada: Nevada Bureau of Mines and Geology Mining 

Series, v. 41, 206 p.

Ferrara, R., Maserti, B.E., Breder, R., 1991, Mercury in abiotic 

and biotic compartments of an area affected by a geochemical anomaly (Mt. Amiata, Italy): Water, Air, and Soil Pollution, v. 56, p. 219–233.

Gray, J.E., ed., 2003, Geologic studies of mercury by the U.S. 

Geological Survey: U.S. Geological Survey Circular 1248, 

41 p.

Gray, J.E., Crock, J.G., Fey, D.L., 2002, Environmental 

geochemistry of abandoned mercury mines in West-central 

Nevada, USA: Applied Geochemistry, v. 17, p. 1,069–1,079.

Gray, J.E., Hines, M.E., and Biester, H., 2006, Mercury 

methylation influenced by areas of past mercury mining 

in the Terlingua district, southwest Texas, USA: Applied 

Geochemistry, v. 21, no. 11, p. 1,940–1,954.

Gray, J.E., Hines, M.E., Higueras, P.L., Adatto, I., and 

Lasorsa, B.K., 2004, Mercury speciation and microbial 

transformations in mine wastes, stream sediments, and surface waters at the Almadén mining district, Spain: Environmental Science and Technology, v. 38, p. 4,285–4,292.

Gustin, M.S., Taylor, G.E., Jr., Leonard, T.L., and Keislar, 

R.E., 1996, Atmospheric mercury concentrations associated 

with geologically and anthropogenically enriched sites in 

central western Nevada: Environmental Science and Technology, v. 30, no. 8, p. 2,572–2,579.

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2003, Mercury speciation by X-ray absorption fine structure spectroscopy and sequential chemical extractions—A 

comparison of speciation methods: Environmental Science 

and Technology, v. 37, p. 5,102–5,108.

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of global-scale mercury biogeochemistry with interhemispheric atmospheric gradients: Geochimica et Cosmochimica Acta, v. 66, p. 1,105–1,118.

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quality guidelines for freshwater ecosystems: Archives of 

Environmental Contamination and Toxicology, v. 39, p. 

20–31.

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pollution, integration and synthesis: Boca Raton, Fla., CRC 

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64, 32 p.

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of mercury in the economies of the United States and the 

world: U.S. Geological Survey Circular 1197, 28 p.

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standards; establishment of numeric criteria for priority 

toxic pollutants; states’ compliance; final rule: Federal Register, Title 40, Part 131, 57/246, p. 60,847–60,916.

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accessed 11 March 2008.
Abstract

Geochemical studies show a variety of different types of 

water in Big Bend National Park. Rainfall recharges ground 

water aquifers that supply wells and springs that constitute 

significant drinking water supplies at The Basin, the Headquarters area, Rio Grande Village, and the Castolon area. 

Panther Junction, K-Bar, and Oak Spring produce water that is 

dilute relative to surface water and hot spring water. Heliumtritium studies show these are mixed waters with old (>50 yr) 

and young components. Noble gas concentrations indicate that 

Oak Spring water is recharged above 1,600 m altitude, probably on Ward Mountain.

Hot springs in the Rio Grande Village (RGV) area 

are mostly old water that has much higher concentrations 

of dissolved solids resulting from subsurface reaction with 

limestone at moderately high temperatures (130–180o

C). Hot 

spring water is notably enriched in arsenic and lithium relative 

to other water in the park. Noble gas concentrations indicate 

that such water is recharged at an altitude of at least 900 m. 

Likely recharge areas are 5 km north of RGV in Cretaceous 

Santa Elena Limestone or 15 km to the south in similar rocks 

in Mexico. Fluid flow is along fault zones and deep circulation 

allows heating along the geothermal gradient.

Water in the Castolon area is produced from wells near 

Cottonwood campground and generally has the highest dissolved solids of any ground water in the Park. Such water is 

young (0–5 yr) and has chemical similarities to, but is more 

concentrated than, modern Rio Grande water. Most likely, 

Cottonwood water has dissolved soluble sodium, calcium, 

chloride, and sulfate salts that are stored in river alluvium from 

past evaporation events. Enriched concentrations of arsenic and 

lithium in the Cottonwood water may also come from soluble 

salts or may be a result of mixing with cooled geothermal water.

Introduction

Big Bend National Park (BBNP) encompasses 800,000 

acres with altitudes from approximately 1,800 ft (550 m) along 

the Rio Grande to 7,800 ft (2,380 m) in the Chisos Mountains. 

This includes environments ranging from extremely arid 

Chihuahuan low desert with mesquite-lechuguilla-creosotecactus-sotol habitats to montane islands of pinyon-juniper-oak 

woodland, and cypress-pine-oak associations (Hellgren, 1993). 

Annual precipitation averages 30.5 cm, with most falling during thunderstorms in July, August, and September. Rainfall 

accumulations generally increase with elevation in the park, 

as indicated by average annual precipitation of about 25 cm at 

RGV and about 50 cm in The Basin.

In the arid climate of BBNP, water quality and water 

supply are persistent issues. Geochemical data reported here 

are designed to evaluate sources, transport, residence time, and 

dispersion of metals and other constituents in surface water, 

ground water, and active hot springs. Determination of chemical and isotopic compositions of water provides baselines 

against which future trends related to climate change, drought, 

and other regional events can be compared. Water samples 

were collected from a broad range of geologic and geographic 

settings in BBNP including: hot springs near RGV, nonthermal 

springs throughout the park, water supply wells, and water 

from the Rio Grande and other streams (fig. 6–1).

Chemical compositions (major elements, trace elements, 

and noble gases), stable isotopes (hydrogen and oxygen), and 

radiogenic isotopic compositions (helium and tritium) have 

been analyzed using established geochemical procedures at 

the U.S. Geological Survey (USGS) laboratories in Denver, 

Colo. (Epstein and Mayeda, 1953; Kendall and Coplen, 1985; 

Aeschbach-Hertig and others, 1999; Solomon and Cook, 2000; 

Hageman, 2002; Lamothe and others, 2002; Theodorakos, 

2002).

The geologic setting of BBNP (fig. 6–1) controls ground 

water flow paths through permeable units or along fault zones. 

Subsurface reactions with rocks and sediments and soluble 

salts determine water composition. In simplest terms, geologic 

units in BBNP include stream and alluvial fan deposits of 

mud, sand, and gravel of Holocene to Miocene age; felsic to 

mafic igneous rocks, including dikes, sills, laccoliths, stocks, 

lava flows, and pyroclastic flow and fall deposits; and Cretaceous limestone, sandstone, and shale units (fig. 6–1). Recent 

stream deposits, fractured igneous rocks, permeable sandstone 

layers, and fractured and faulted limestone units are the most 

important aquifers in BBNP (Baker and Buszka, 1993).

6. The Waters of Big Bend: Geochemical Variations,

Ages, and Sources

By W.C. Pat Shanks III, Lisa A. Morgan, John E. Gray, Andrew H. Manning, and Pamela A. Gemery-Hill
66 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Rio Grande 

Village

Chisos

Mountains

Castolon

Panther 

Junction

Terlingua

Cottonwood

campground

The 

Basin

Study

Butte

Terlingua Cr.

Tornillo Cr.

Long 

Draw 

Rough

Run 

Creek

Terlingua

Creek

Tributary

Tornillo Cr.

fig. 6-4A

fig. 6-4C fig. 6-4D

fig. 6-4B

BBNP boundary

m

Tt

Figure 6–1. Location of Big Bend water samples from springs, streams, and drinking water wells. Surface and 

spring water samples (yellow squares) and well waters (yellow circles) were collected in and around Big Bend 

National Park. Geology from Maxwell and others (1967). Locations of detailed maps shown in figure 6–4 are shown 

by open boxes.
General Characteristics of Big Bend 

Water

Oxygen and hydrogen stable isotope data indicate that all 

water samples from Big Bend are of meteoric origin (have participated in an atmospheric cycle) and generally enter the surface and subsurface water cycle as rainfall. The isotopes, δ18O 

and δD, of stream water samples (fig. 6–2) follow the Global 

Meteoric Water Line (Craig, 1961). Thermal water and spring 

water samples are shifted to slightly higher values because of 

water-rock reactions in the subsurface or evaporation at the surface. Many spring water samples, collected in winter or spring, 

are from pools in alluvial drainages that show little evidence of 

flow and thus are subject to surface evaporation.

Chemical data shows that ground water generally has the 

lowest total dissolved solids in BBNP, consistent with fresh 

rainfall reacting at low temperatures with alluvium, limestones, 

sandstones, and igneous rocks. Hot spring water and Rio 

Grande water samples contain significantly more dissolved 

solids, and each has characteristic compositional ranges.

Variation plots (fig. 6–3) of major constituents (sodium, 

potassium, calcium, magnesium, sulfate, and chloride) and 

selected trace elements (arsenic, lithium, and strontium) show 

that there are significant differences among water types in 

BBNP. Stream water samples (fig. 6–3A) have the largest 

range of chemical composition; factors that affect this range 

are surface evaporation, dissolution of salt crusts, and agricultural irrigation runoff. Water samples from the Rio Grande and 

hot spring water samples (fig. 6–3B) are distinctly higher in 

total dissolved concentrations than Oak Spring and The Basin 

area waters (fig. 6–3C) or waters from the Panther Junction 

and K-Bar wells (fig. 6–3D).

In addition to chemical data and stable isotope data, radiogenic isotopes and noble gases have been analyzed to provide 

information about the age and conditions of recharge of ground 

water systems. Tritium (3

H) is a radioactive isotope of hydrogen produced during atmospheric nuclear bomb testing in the 

1950s and 1960s. Tritium enters ground water by infiltration of 

precipitation and then decays with time into another isotope, 

helium-3 (3He). Because the rate of radioactive decay is known 

(tritium half-life = 12.43 yr), tritium and helium-3 can be used 

to date ground water younger than about 50 yr, as well as to 

determine the approximate fractions of old water (>50 yr old) 

and young water (<50 yr old) in mixed age samples.

Helium-tritium data for Big Bend water (table 6–1) 

indicate that most ground water utilized as drinking water is a 

mixture of young water and an older, more deeply circulating 

water. In particular, K-Bar well water is composed mainly of 

old water (>50 yr) and Oak Spring, RGV, and Panther Junction 

(fig. 6–1) are mixed waters, with important components of both 

old and young water. Cottonwood well water, which supplies 

Castolon and Cottonwood campground (fig. 6–1), is mainly 

young water. Noble gas data indicate that aquifers that supply 

Oak Spring and RGV wells are recharged at elevations significantly higher than the elevation where the water is collected.

River and Stream Water

The water of the northern Rio Grande is derived from 

runoff in the mountainous regions of southern Colorado and 

northern New Mexico, but most of the water is lost to irrigation use, evaporation, and infiltration by the time the river 

reaches Texas. South of El Paso, the river is dry during many 

years, and more than 75 percent of the water in the Rio Grande 

at BBNP is derived from the Rio Conchos, which drains 

77,000 km2 in northern Mexico and joins the Rio Grande at 

Ojinaga, just north of Presidio. The Rio Conchos carries significant dissolved solids resulting from evaporation, irrigation 

runoff, fertilizers and pesticides, and wastewater (Gutierrez 

and Borrego, 1999).

Most other streams in BBNP are ephemeral, but Terlingua 

Creek and Tornillo Creek have significant stretches of perennial 

flow. Chemical data (fig. 6–3A) show that perennial streams 

have lower total dissolved solids. Terlingua Creek waters are 

similar to typical ground waters (fig. 6–3C, D). Ephemeral 

streams (Long Draw, Rough Run, and a northern tributary of 

Terlingua Creek) can have very high concentrations attributed 

to evaporation in drainages or to dissolution of previously 

deposited salts like calcium sulfate. The Rio Grande dissolved 

solids generally are higher than perennial streams like Terlingua Creek or Big Bend ground waters (fig. 6–3A).

Hot Spring Waters

Hot springs in BBNP are located along the Rio Grande river 

valley, especially near RGV (figs. 6–1 and 6–4A). Hot springs 

have temperatures up to 40.1oC and are enriched in major constituents (sodium, potassium, calcium, magnesium, sulfate, and 

chloride) and some trace elements (arsenic, lithium, rubidium, 

strontium, thallium, uranium, and tungsten) relative to nonthermal springs. Potentially toxic trace metals in thermal waters have 

concentration ranges as follows: arsenic (6.2–13.8 μg/L), mercury (1.3–10 μg/L), thallium (0.13–0.68 μg/L), tungsten (to 7.0 

μg/L), and uranium (7.0–8.4 μg/L). All of these concentrations 

are below drinking water limits (where established), except for 

arsenic in Langford hot spring, which exceeds the Environmental Protection Agency limit for drinking water of 10 μg/L (U.S. 

Environmental Protection Agency, 2007). Langford hot spring 

water, however, is not used for drinking, and water samples analyzed in this study from wells at RGV do not exceed the limit.

Helium-tritium and noble gas data for hot spring water 

samples collected in a production well that supplies drinking 

water for RGV (fig. 6–4A) indicate a large component of old 

water and relatively warm recharge temperatures (table 6–1). 

They are geothermally heated during deep circulation along 

major fault zones. Chemical geothermometer calculations 

indicate they probably reach temperatures of 130–180oC along 

their flow path.

Noble gas data indicate that recharge to the aquifer carrying RGV thermal water probably occurrs at altitudes greater 

than 900 m (table 6–1). The closest area with this altitude, and 

Hot Spring Waters 67
68 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Figure 6–3 (facing page). Chemical variation diagrams for selected constituents that illustrate fundamental differences between 

waters from different areas. A, Stream water samples from the Big Bend area. The Rio Grande, Tornillo Creek, and Terlingua Creek are 

perennial streams. Long Draw, Rough Run, and the tributary of Terlingua Creek (fig. 6–1) are ephemeral streams, and their compositions 

are strongly affected by evaporation or dissolution of evaporative salts. Terlingua Creek is compositionally similar to many ground 

water samples from springs and wells. Inset: Photo of ephemeral stream bed with Ward Mountain in the background. B, Rio Grande, 

hot spring, and Cottonwood campground water samples are significantly more enriched in most constituents (see text for discussion). 

Note that the composition of Na in Cottonwood water samples is estimated because they exceeded the calibrated range during ICP-MS 

analyses. Inset: Photograph of Langford hot spring along the bank of the Rio Grande, located a few km west of Rio Grande Village. C, 

Water samples from La Paloma spring in The Basin, The Window spring, and Oak Spring are relatively dilute and are very similar in 

composition, except for significant variation in Ca and SO4

. La Paloma water flows into pools along an incised bedrock drainage where 

evaporation during dry periods would cause deposition of calcite and gypsum. Gypsum may redissolve during wetter periods. Dilution 

occurs along the flow path to Oak Spring (indicated by yellow arrows in figure 6–4C). Inset: Photograph of The Window. D, Water from 

Panther Junction and K-Bar wells are dilute Na-Cl-Ca-SO4

, like many ground water samples in the park.

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

-10.0 -8.0 -6.0 -4.0 -2.0 0.0 2.0



18O (per m il)

D (per m il)

T h erm al waters [R G V area]

N o n -th erm al sp rin g s

G lo b al M eteo ric W ater Lin e

T erlin g u a an d L o n g D raw

R io G ran d e

T o rn illo C reek

E vap o ratio n

W ater-ro ck reactio n s

Figure 6–2. Oxygen and hydrogen composition of Big Bend waters. All waters are meteoric (rainfall 

or snow derived). Shifts off the Global Meteoric Water Line (Craig, 1961) are due to evaporation or 

water-rock reaction. RGV, Rio Grande Valley.
River and Stream Water 69

0

20

40

60

80

100

N a

(m g/L)

K * 10

(m g/L)

M g

(m g/L)

C a

(m g/L)

C l 

(m g/L)

SO4

(m g/L)

As * 10

(g/L)

 Li 

(g/L)

Sr * 10

(m g/L)

K -B ar well head

P anther Junction well

D

0

50

100

150

200

N a

(m g/L)

K * 10

(m g/L)

M g

(m g/L)

C a

(m g/L)

C l 

(m g/L)

SO4

(m g/L)

As * 10

(g/L)

 Li 

(g/L)

Sr * 10

(m g/L)

La P alom a S pring

T he W indow

O ak S pring (2004) 

O ak S pring (2006)

C The Window

40.1°C Langford

hot spring

Concentration

0

100

200

300

400

500

600

N a

(m g/L)

K * 10

(m g/L)

M g

(m g/L)

C a

(m g/L)

 C l 

(m g/L)

SO4

(m g/L)

As * 10

(g/L)

 Li 

(g/L)

Sr * 10

(m g/L)

Langford hot spring

R io G rande V illage hot spring av erage

C ottonwood av erage

R io G rande riv er av erage

?

B

Ward Mountain

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

N a

(m g/L)

K * 10

(m g/L)

M g

(m g/L)

C a

(m g/L)

C l

(m g/L)

SO4

(m g/L)

As * 10

(g/L)

 Li 

 (g/L)

Sr * 10

(m g/L)

R io G rande riv er av erage

T erlingua C reek av erage

T ornillo C reek av erage

Long D raw C reek

R ough R un C reek

T ributary of T erlingua C reek

A
Table 6–1. Helium-tritium and rare gas results.

Sample site Site type Apparent 

age1

 (yr)

Tritium 

(TU)2

Initial 

tritium 

(TU)

Interpretation of results

K-Bar well head 

(BB-06-370)

production 

water well 0.0 0.66 0.66 Mostly pre-nuclear bomb testing. >80% over 50 yr old. Age of modern component uncertain.

Cottonwood well 

#1

(BB-06-380)

production 

water well 5.2 3.43 4.60 Mostly modern. Modern component 0–5 yr old. Cool recharge temperature consistent with Rio Grande water.

Rio Grande Village 

water system 

well head

(BB-06-383)

production 

water well >50 0.11 4.11

Pre-bomb. Requires recharge elevation >900 m (<10% 

probability of recharge elevation <900 m) and deep water table (warm recharge temperature). High 3

He values 

indicate a mantle component.

Panther Junction 

water system 

from building

(BB-06-384)

production 

water well 27.8 2.77 13.24 Mixed water. Apparent age of young component could be 0–28 yr.

Oak Spring

(BB-06-400) mixing box 0.0 1.30 1.30

Mixed water. >60% over 50 yr old. Age of modern component uncertain. Requires recharge elevation >1,900 

m (<10% probability of recharge elevation <1,900 m) 

and shallow water table (cool recharge temperature).

1

For mixed waters, the apparent age is the approximate age of the modern water component.

2

TU is a tritium unit, defined as one tritium-bearing water molecule in 1,018 H2O molecules.

70 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas
Castolon and Cottonwood Campground Water 71

Figure 6–4 (following 2 pages). Geologic setting of drinking water production sites in Big Bend National Park. See figure 6–1 for 

locations of these subarea maps. Turquoise square symbols indicate water samples. Geologic units are: Qal, Quaternary alluvium; Qco, 

Quaternary colluvium; QTog, Quaternary to Tertiary older gravels; Tacb, Tertiary Alamo Creek Basalt; Tasb, Tertiary Ash Springs Basalt; 

Tig, Tertiary igneous rocks; Tch, Tertiary Chisos Formation; Tcf, Tertiary Canoe Formation; Tbmb, Tertiary Bee Mountain Basalt; Tbr, 

Tertiary Brown Rhyolite; Twsf, Tertiary Wasp Spring Flow Breccia; Kag, Cretaceous Aguja Formation; Kp, Cretaceous Pen Formation; 

Kbo, Cretaceous Boquillas Formation; Kbu, Cretaceous Buda Formation; Kse, Cretaceous Santa Elena Formation; Kdr, Cretaceous Del 

Rio Formation; Kdc, Cretaceous del Carmen Formation; Ksp, Cretaceous Sue Peak Formation; Kjf, Cretaceous Javelina Formation. Big 

Bend National Park geology from Maxwell and others (1967). Mexican geologic mapping from Servicio Geológico Mexicano (2003). A, 

Geologic and shaded relief map of the Rio Grande Village (RGV) area. Note location of Langford hot spring, west of RGV, the hottest 

thermal water spring in the Park at 40.1°C. Water samples at Rio Grande Village are all thermal waters similar in composition, but 

somewhat more diluted than Langford waters. Hot spring water is probably recharged about 5 km north of RGV in higher ground of the 

Cretaceous Santa Elena limestones and flows along N-S fault zones (yellow arrows) to the hot spring sites. Possible recharge sites to 

the south in Mexico also are shown. Mine symbols (crossed picks) indicate fluorite mines. B, Castolon area geology with location of 

water samples near Cottonwood campground. Castolon water is young water from Rio Grande alluvium. C, Geology and topography of 

The Basin area. The Basin water supply is at Oak Spring, which is a mixed water with young components recharged near Oak Spring 

and older (>50 yrs) waters sourced in the high Chisos Mountains, probably near Ward Mountain and LaPaloma spring (see text for 

discussion). Yellow arrows indicate probable flow path of waters that recharge Oak Spring. D, Geology and shaded relief map of the 

Panther Junction and K-Bar areas where important water production wells produce water from tuffaceous sandstones of the Tertiary 

Chisos Formation.

thus the most probable recharge area, is about 5 km north of 

RGV in the Cretaceous Santa Elena Limestone. A potential 

ground-water flow path from this recharge area is along N-S 

trending faults (fig. 6–4A). Another potential recharge area 

with elevations >900 m is in Mexico about 15 km south of 

the hot springs. The geology to the south is also dominated 

by Cretaceous limestone that is similarly faulted, although 

mapping there shows no major structure directly south of the 

Langford hot spring site (fig. 6–4A).

Castolon and Cottonwood Campground 

Water

Helium-tritium analyses for waters from the Cottonwood 

water-supply well show that the well produces young water 

(0–5 yr) with cool recharge temperatures. This result is reasonable for a geologic setting in alluvial deposits adjacent to the 

Rio Grande (fig. 6–4B) and suggests that modern Rio Grande 

water is the dominant component in the Castolon and Cottonwood water supply.

Cottonwood water samples (fig. 6–3B) have substantially higher concentrations of all elements than the typical 

ground water samples collected from Panther Junction, K-Bar, 

and Oak Spring. Moreover, chemical data (fig. 6–3B) show 

that Cottonwood water samples are significantly enriched in 

sodium, calcium, chloride, sulfate, arsenic, lithium, and strontium relative to Rio Grande water.

The potassium, lithium, and arsenic concentrations in 

Cottonwood water samples are intermediate between Rio 

Grande water and hot spring waters at RGV (fig. 6–3B). It is 

possible that Cottonwood waters have mixed with a cooled 

component of thermal water to produce the unusual enrichments in these elements. Thermal springs are not known to 

be in the Castolon area, but they are found to the north in the 

Presidio area and, of course, to the south in the RGV area. A 

thermal water component, however, cannot explain the enrichments in sodium, calcium, chloride, and sulfate (fig. 6–3B), so 

an additional or different source is required.

A likely source of sodium, calcium, chloride, and sulfate 

for Cottonwood water is the dissolution of salts deposited by 

evaporation after flood events or during periods of drought 

when river levels are especially low. Howari (2002) found 

three types of salt crusts in soil along the Rio Grande: hydrated 

calcium sulfate, sodium sulfate, and sodium chloride. These 

salt crusts form through evaporation of Rio Grande water and 

are redissolved during flood events. Over time, soluble salts 

would be buried in Rio Grande alluvium and might be redissolved by flowing ground water like that pumped from the 

Cottonwood wells.

Dissolution of soluble salts into modern Rio Grande 

water is the best explanation for the major element chemistry 

of Cottonwood water, but it is not clear if this process can also 

provide the high arsenic and lithium. Further investigation 

of the trace metal composition of soluble salts is needed to 

resolve this issue.
72 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

"/

"/"/

"/"/

103°2'W 102°58'W 102°54' W

29°14' N

29°10' N

 A

Kse

Kbo

Kag

Kp

Kag

Kbu

Kse

Kbo

Kse

QTog

Qal

Qal

Qal

Qal

QTog

Kag

Qal

Kp

Kp

QTog

Langford 

hot

spring

Kdc

KcossLu − Cz

− Cz

.

.

. .

.

.

.

.

. .

.

Qhoal .

KcetLu − Cz

. KceCz

Qhoco

Qhoal

KaCz

ToGr − Si

KceCz

Qco

KceLu − Cz

KcetLu − Cz

Qhoal

KceCz

.

KceLu − Cz

KceLu − Cz

Qhoal

KaCz

KceCz

Kdc

Ksp

KcossCz − Lu

KcossLu − Cz

KcetLu − Cz

KceCz

KceLu − Cz

750

750

SAN VICENTE

ZONA MINERALIZADA

EL SALTO

SUBIRÁN

5°

88°

75°

80°

8°

12°

84°

7° 5°

6°

11°

5° 12° 35°

10° 9°

11°

12°

35°

7°

6°

11°

−

−

possible 

recharge

area 

Kbo Kbu

Kdr

Kse

Kbu

Kdr

Kse

Qal

Qal

Qal

Qal

Qco

QTog

QTog

Qco

Kse

Kp San Vicente

Rio Grande

Rio Grande

Boquillas

Rio Grande

Rio Grande

Ksp

B

Rio 

Grande

Village

"/

"/

Tacb

Tch

Castolon

Rio 

29o8’N

29o

9’N

103o

33’W 103o 103 31’W o

32’W

Kag

Kag

Qal

Kjf

Tig

Kjf

Grande 

River

Qal

QTog

Qal

Cottonwood

samples

0 0.5 1 Kilometers

0 1.25 2.5 5 Kilometers
Castolon and Cottonwood Campground Water 73

C

D

"

"/

"/

Oak Spring

The Basin

Ward

Mountain

Vernon Bailey

Peak

The 

Window

La Paloma

spring

Qal

Tig

Tch

Tasb

Kbo

Kp

Kag

Kag

Kag

Kbo

Qal

Qal

0 0.. 5 1 0 25 Kilometers

29o17’N

29o16’N

103o20’W 103o18’W

Qal

"/!.

"/

"/

Panther Junction

well

K-Bar

well

QTog

Park

 Headquarters

Tch

Tig

Tig Tig

Tig

Tig

Qal

Tcf

Tbr

Twsf

Tbmb

Tch

Qal Kag

29o18’N

29o20’N

103o13’W 103o11’W

0 1 2 Kilometers

Tcf

Tcf Tch
Source of the Basin Water Supply

The water supply for The Basin is at Oak Spring near 

the base of the Chisos Mountains, directly below The Window (figs. 6–1 and 6–4C). Helium-tritium data indicate that 

Oak Spring has mixed waters that contain approximately 60 

percent old water (that is, water greater than 50 yr old). Noble 

gas data indicate that water in Oak Spring is recharged at an 

altitude above 1,900 m, which means it recharges in the high 

Chisos Mountains, most likely on Ward Mountain. Large 

areas of Ward Mountain are above 1,900 m in elevation and 

have drainage gradients toward La Paloma spring (fig. 6–4C), 

which is on the main drainage to The Window. Baker and 

Buszka (1993) used test drilling and seismic reflection data 

to show that the subsurface aquifer that locally supplies Oak 

Spring is a confined 1.5-m-thick sand bed. They concluded 

that most recharge occurs below The Window within approximately 500 m of Oak Spring; however, they also discussed 

the possibility of some of the recharge coming from the 

mountains above through fractures in the igneous rocks that 

underlie The Window.

Our new data suggest that a substantial portion of the 

recharge comes from high elevations in the Chisos Mountains, 

probably from the Ward Mountain area, with subsequent flow 

along the drainage occupied by La Paloma spring (fig. 6–4C) 

and into the subsurface along fractures (fig. 6–5) to the topographically lower Oak Spring. Flow pathways to Oak Spring 

could vary with time, meaning that the primary recharge area 

for Oak Spring water may vary seasonally.

The Basin area is underlain by rhyolitic intrusive bodies 

and limestone units, yet the water is similar to Panther Junction and K-Bar well water (fig. 6–3D). The main difference is 

a somewhat higher calcium and sulfate content in The Basin/

Oak Spring water. Geochemical similarities between water 

samples from LaPaloma spring on the flank of Ward Mountain 

and water samples from The Window spring downstream and 

Oak Spring below The Window pouroff (fig. 6–3C) indicate 

that such water is probably related and that the main compositional changes are due to salt dissolution followed by dilution 

along the flow path. Both The Window spring and La Paloma 

spring are in bedrock drainages where water may evaporate 

and precipitate minerals during dry periods. Geochemical calculations of evaporative removal of water indicate that calcium 

carbonate (calcite) deposition predominates, but calcium 

sulfate (gypsum) is deposited during dry periods. Later, during 

wet periods, these minerals may redissolve. Gypsum is much 

more soluble than calcite and hence the Ca and SO4

 content 

of the water is strongly affected by salt dissolution. Because 

of salt dissolution, our La Paloma sample has higher initial 

values of these constituents, and dilution along the flow path 

leads to less concentrated water at Oak Spring.

This interpretation is consistent with the helium-tritium 

and noble gas solubility data that indicate young and old water 

components. The older component is recharged in the Chisos 

Mountains and the younger component is recharged in alluvium or in sedimentary deposits near Oak Spring.

Panther Junction and K-Bar Drinking 

Water Wells

Water from production wells at Panther Junction and 

K-Bar (fig. 6–4D) is produced from subsurface aquifers 

in tuffaceous sandstone of the Tertiary Chisos Formation 

(Abbott, 1983; Gibson, 1983); this water has reacted mainly 

with rhyolitic ash. Helium-tritium data indicate that K-Bar 

water is mostly older than 50 yrs and Panther Junction water 

is mixed water with roughly equal proportions of old water 

and young water. Panther Junction and K-Bar waters have 

some of the lowest dissolved solids in BBNP and are sources 

of good quality water.

Conclusion

Water samples collected from BBNP show a large range 

of chemical compositions and complex histories of subsurface 

flow and reaction. Helium-tritium data show that most drinking water sources in BBNP are mixed waters with both young 

and older components. Exceptions are K-Bar water, which 

predates nuclear bomb testing (>50 yr), and Cottonwood well 

water, which is mostly young (0–5 yr). Noble gas solubility 

calculations show that RGV well water and Oak Spring water 

are recharged at higher elevations in surrounding mountains. 

Hot spring water found at RGV has a relatively long residence time during subsurface flow and is geothermally heated 

along the flow path, which probably follows N-S trending 

fault zones.

Water samples collected from BBNP are meteoric 

(derived from rainfall) and gain chemicals through subsurface reaction with rocks and sediments. Dissolved chemicals 

are further concentrated by evaporation in some springs and 

streams, or by dissolution of soluble salts deposited when 

water evaporates. Water from hot springs reacts more intensely 

with rocks at higher temperatures and has higher contents of 

certain elements, especially arsenic and lithium. Rio Grande 

water is dominated by water from the Rio Conchos and carries 

more dissolved solids than most waters in BBNP. Cottonwood 

well water, which supplies the village of Castolon, contains 

high concentrations of sodium, calcium, chloride, sulfate, arsenic, and lithium from the dissolution of soluble salts in river 

alluvium and possibly from mixing with spent thermal water.

74 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas
Conclusion 75

Figure 6–5. Photograph of vertical fractures (joints) in the Ward Mountain rhyolitic intrusive rocks east of Ward Spring. 

Such fractures, with spacing from centimeter to meter scales, are the likely conduits for subsurface fluid flow from The 

Basin through intrusive rocks in The Window area to Oak Spring below The Window pouroff.
76 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Acknowledgments

The authors thank Vidal Davila, Jeffrey Bennett, Betty 

Alex, Raymond Skiles, Tim Lafever, and Don Corrick of the 

National Park Service, Big Bend National Park, for assistance with field work and many helpful discussions. We 

thank Roland Thurston, Michael Anthony, Monique Adams, 

David Fey, and Casey Gulbransen of the U.S. Geological 

Survey for assistance with analyses and field work. The 

manuscript was reviewed by Rich Wanty and Phil Verplanck 

of the USGS. Finally we thank Louise Hamer of Wildhorse 

Station for good friendship and excellent accommodations 

during field work.

References Cited

Abbott, C.L., 1983, Bedrock aquifer geometry in the Panther Junction area of Big Bend National Park, Texas: College Station, Tex., Texas A&M University, M.S. thesis, 

113 p.

Aeschbach-Hertig, W., Peeters, F., Beyerle, U., and Kipfer, 

R., 1999, Interpretation of dissolved atmospheric noble 

gases in natural waters: Water Resources Research, v. 35, 

p. 2,779–2,792.

Baker, E.T., Jr., and Buszka, P.M., 1993, Hydrogeology, 

geochemistry, and quality of water of The Basin and Oak 

Spring Areas of the Chisos Mountains, Big Bend National 

Park, Texas: U.S. Geological Survey Water-Resources 

Investigations Report 93-4112, 76 p.

Craig, H., 1961, Isotopic variations in meteoric waters: Science, 

v. 133, p. 1,702–1,703.

Epstein, S., and Mayeda, T., 1953, Variations of O18 contents 

of waters from natural sources: Geochimica et Cosmochimica Acta, v. 4, p. 213–224.

Gibson, J.L., 1983, Ground-water hydrology of the Panther 

Junction area of Big Bend National Park, Texas: College 

Station, Tex., Texas A&M University, M.S. thesis, 110 p.

Gutierrez, M., and Borrego, P., 1999, Water quality assessment of the Rio Conchos, Chihuahua, Mexico: Environment 

International, v. 25, p. 573–583.

Hageman, P.L., 2002, Mercury in water by flow injection-cold 

vapor-atomic fluorescence spectrometry, in Taggart, J.E., Jr., 

ed., Analytical methods for chemical analysis of geologic 

and other materials: U.S. Geological Survey Open-File 

Report 02-223, chap. N, 5 p. 

Hellgren, E.C., 1993, Status, distribution, and summer food 

habits of black bears in Big Bend National Park: The Southwestern Naturalist, v. 38, p. 77–80. 

Howari, F.M., 2002, Sources, types, storage, and release of 

salts in the soil-water system of the Rio Grande, Texas 

[abs.]: Geological Society of America Abstracts with Programs, v. 34, no. 6, p. 262.

Kendall, C., and Coplen, T.B., 1985, Multisample conversion 

of water to hydrogen by zinc for stable isotope determination: Analytical Chemistry, v. 57, p. 1,437–1,440.

Lamothe, P.J., Meier, A.L., and Wilson, S.A., 2002, The 

determination of forty four elements in aqueous samples by 

inductively coupled plasma-mass spectrometry, in Taggart, 

J.E., Jr., ed., Analytical methods for chemical analysis of 

geologic and other materials: U.S. Geological Survey OpenFile Report 02-223, chap. H, 11 p.

Maxwell, R.A., Lonsdale, J.T., Hazzard, R.T., and Wilson, 

J.A., 1967, Geology of Big Bend National Park, Brewster 

County, Texas: University of Texas at Austin, Bureau of 

Economic Geology Report 6711, 320 p. 

Servicio Geológico Mexicano, 2003, Manuel Benavides 

H13-9, Chihuahua y Coahuila: Servicio Geológico Mexicano Carta Geológico-Minera, 1 sheet, scale 1:250,000.

Solomon, D.K., and Cook, P.G., 2000, 3

H and 3He, in Cook, 

P.G., and Herczeg, A.L., eds., Environmental tracers in 

subsurface hydrology: Boston, Kluwer Academic Publishers, p. 397–424.

Theodorakos, P.M., 2002, Fluoride, chloride, nitrate, and sulfate in aqueous solution utilizing auto-suppression chemically suppressed ion chromatography, in Taggart, J.E., Jr., 

ed., Analytical methods for chemical analysis of geologic 

and other materials: U.S. Geological Survey Open-File 

Report 02-223, chap. V, 7 p.

U.S. Environmental Protection Agency, 2007, Arsenic in 

drinking water—Basic information: U.S. Environmental 

Protection Agency, http://www.epa.gov/safewater/arsenic/

basicinformation.html.
Abstract

Hair from black bears (Ursus americanus), collected 

from four areas in the Big Bend ecosystem, has been analyzed 

for stable isotopes of carbon, nitrogen, and sulfur to determine major food sources and for trace metals to infer possible 

effects of environmental contaminants. Results indicate that 

black bears are largely vegetarian, feeding on desert plants, 

nuts, and berries. Mercury concentrations in bear hair are 

below safe level standards (<2 μg/g), and other trace metal 

concentrations are generally below levels that have been associated with adverse affects in other mammals. These results 

indicate low potential for negative health affects on bears due 

to trace element contaminants in the Big Bend ecosystem.

Introduction

Naturalist Vernon Bailey described bears as common in 

the upper canyons of the Chisos Mountains in 1901 (Bailey, 

1905). When Big Bend National Park (BBNP) was established 

in 1944, there were virtually no resident bears due to extirpation by Federal predator control and unregulated hunting 

(Onorato and Hellgren, 2001). Black bears reappeared in 

BBNP in the late 1980s. Sightings (fig. 7–1) increased dramatically after 1988 and included evidence of residency and 

breeding pairs (Hellgren and others, 2005). These observations allowed estimation of up to 6 resident adult females and 

30 total individuals in the Park in 2000, with a decline to 2–3 

females in 2001–2002 after an exodus to Mexico in the fall of 

2000 (Hellgren and others, 2005). 

Natural recolonization by bears is extremely rare and 

is of interest for predicting similar events in other parts of 

the Trans-Pecos region. Migrations into the BBNP area are 

believed to have come from the larger Sierra del Carmen and 

Serranias del Burro mountain ranges in Mexico (fig. 7–2). 

Bears in the region concentrate in montane islands like the 

Chisos Mountains, showing a preference for moderate to 

high elevations with pinyon-oak-juniper habitat, where they 

feed on acorns, juniper berries, pinyon nuts, and berries of 

Texas madrone and sumac. They also may feed on sotol, 

yucca, lechuguilla, and on prickly pear and persimmon fruits. 

Hellgren (1993) used scat analysis to conclude that black bear 

diet in the Chisos Mountains consists of 77–86 percent plant 

material with an emphasis on succulents, fruits, and nuts, and 

only 11–14 percent animal material, roughly half of which was 

black bear hair (from grooming) and half insects.

Past mining activities in the Big Bend area, including 

mercury, fluorite, and silver-lead-zinc mines, justify assessment of important possible impacts of mining-related contaminants on the ecosystem. For example, Mora and others (2002) 

noted that Peregrine falcons (Falco peregrinus anatum) have 

low reproductive rates in BBNP, which might be caused by 

metal contamination or organic toxins such as DDE (a metabolite of DDT). They examined metals and organic contaminants 

in Peregrine falcon prey species including Black phoebe

(Sayornis nigricans), Say’s phoebe (Sayornis saya), Cliff 

swallow (Petrochlidon pyrrhonota), Northern rough-winged 

swallow (Stelgidopteryx serripennis), and Mexican free-tailed 

bat (Tadarida brasiliensis); they found mercury up to 2.5 μg/g 

dw (micrograms per gram dry weight) and selenium up to 15 

μg/g dw. Many of the measured mercury concentrations are 

above the threshold of 0.4 μg/g dw accepted for protection 

of bird health (Eisler, 1987). Mora and others (2002) noted 

cases of selenium concentrations of 14.5–16.5 μg/g dw in the 

diet that produced embryo deformities and oxidative stress in 

nestlings, so some of the prey species concentrations are in the 

range where adverse effects might be expected. All of the prey 

species analyzed in BBNP are insectivorous and Mora and 

others (2002) suggested that mercury in the insects originated 

in former mine sites, whereas selenium may be related to agricultural fertilization and irrigation practices.

The purpose of this study was to use stable isotope 

(carbon, C; nitrogen, N; and sulfur, S) and trace element data 

derived from bear guard hairs to assess dietary habits and evaluate possible trace metal accumulation resulting from mine 

waste or agricultural practices within the ecosystem. Hair 

samples analyzed in the present study were collected from 

bears in the Chisos Mountains, Black Gap Wildlife Management Area, and from the Sierra del Carmen and Serranias del 

Burro ranges in Mexico (fig. 7–2) (Onorato and others, 2004).

7. Stable Isotope and Trace Element Studies of Black Bear 

Hair, Big Bend Ecosystem, Texas and Mexico

By W.C. Pat Shanks III, Eric C. Hellgren, Craig Stricker, Pamela A. Gemery-Hill, and David P. Onorato
78 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Stable Isotopes and Bear Diet

Many elements have more than one stable (non-radioactive) isotope. Isotopes of an element have a different number 

of neutrons in the nucleus, but because they are the same 

element, have almost identical chemical and physical properties. However, some natural processes produce small changes, 

called fractionation, in the ratio of stable isotopes in a substance. These small changes can be measured with a mass 

spectrometer and provide information on processes that have 

affected the substance. In the case of stable isotopes in animals, the classic studies of DeNiro and Epstein (1978, 1981) 

have shown that “you are what you eat.” Modern techniques 

allow automated, high-precision analysis of stable isotopes in 

plant and animal materials (Fry and others, 1992; Kester and 

others, 2001).

Hilderbrand and others (1996) studied stable isotopes 

(δ15N and δ13C) in diets of captive bears and demonstrated a 

strong correlation between isotope values in plasma and foods. 

They also showed that hair, bone collagen, and red blood cells 

have the same isotopic values as plasma and foods when animals are on a consistent diet. Thus, stable isotopic analysis of 

BBNP black bear hair provides a tool for assessing diet. 

Results (table 7–1) of δ13C studies of bear hair from the 

four areas sampled indicate that bear diet in the Big Bend 

area comprises mainly plants that thrive in desert environments. Plants in arid environments use one of two different 

metabolic pathways in photosynthesis (either C4 or CAM, 

crassulacean acid metabolism, an elaborate carbon fixation 

pathway in some photosynthetic plants usually found in 

plants living in arid conditions). These pathways contrast with 

most woody plants, which utilize the C3 metabolic pathway. 

Plants that use C4 metabolism include sugar cane, maize, 

sorghum, finger millet, amaranth, and switchgrass. Plants that 

use CAM metabolism include succulents, cacti, and flowering 

plants.

Park and Epstein (1961) were the first to show that different types of plants fractionate carbon isotopes differently 

during photosynthesis of atmospheric carbon dioxide (CO2

). 

As a result, C4 plants have higher δ13C ranges (-11 to -15) 

than the much more abundant C3 plants (-20 to -35), and 

CAM plants have intermediate values (-15 to -19) (Dawson 

and others, 2002). Therefore, differences in δ13C values of 

plants relate to the photosynthetic pathways for fixation of 

Figure 7–1. Black bear in Big Bend National Park.
Stable Isotopes and Bear Diet 79

0 500 Kilometers

N

W E

S

Mexico

U.S.A.

Burros

Carmens

Black Gap

Big Bend NP

Trans-Pecos

Figure 7–2. Map showing general location of the Big Bend ecosystem where black bear hair samples were collected for this study. 

Shaded areas show black bear range. From Onorato and others (2004).
80 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

CO2 (Farquhar and others, 1982). The δ13C values of Big 

Bend bear hair (-16.9 to -19.8; table 7–1) and the feeding habits deduced from scat analysis suggest a significant fraction of 

CAM and possibly C4 plants in the diet.

Fellicetti and others (2003) studied stable isotopes in 

grizzly bear (Ursus arctos horribilis) hair and possible bear 

foods in Yellowstone National Park and showed that nitrogen 

and sulfur isotopes also are quite useful for estimating assimilated diets. Plots of nitrogen isotopes versus carbon and sulfur 

isotope values for Big Bend black bear hair (fig. 7–3) show 

fairly confined ranges with some possible differences between 

different geographic locations. The less-depleted δ13C values 

in the Chisos and the Black Gap samples may not be significant statistically from the two Mexican ranges, but their lower 

elevation and more arid environment is consistent with more 

C4 and CAM plants in the diet.

Ranges for δ13C and δ34S in Yellowstone National Park 

(YNP) grizzly bear hair (Felicetti and others, 2003; Chaffee 

and others, 2007) are also outlined on figure 7–3 and show 

quite a different range than Big Bend black bear hair. Felicetti 

and others (2003) and Chaffee and others (2007) also collected and analyzed potential food sources consumed by YNP 

grizzly bears; the ranges of isotope values are indicated in 

figure 7–3. The ranges of values for individual food sources 

demonstrate that pine nuts, fish, and ungulates are critically 

important food resources for Yellowstone grizzly bears, 

while moths and vegetation are also part of their diet, and the 

diet varies temporally during the year and from year to year 

depending on the size of mast and spawning runs. Similar 

detailed analysis of Big Bend bear diet and potential foods 

are in progress. Some of the differences in isotope values 

(fig. 7–3) among bears from Chisos, Black Gap, and Burro 

Mountains/Sierra del Carmen may be related to differences in 

aridity, foods, or geological environment.

Trace Elements in Bear Hair

Trace element studies of BBNP bear hair (table 7–2) 

indicate detectable concentrations as follows, with average and 

range of concentrations in μg/g for all samples in parentheses: 

barium (Ba: 13; 2–43), cadmium (Cd: 1.2; 0.04–9.9), copper 

(Cu: 15; 7–49), iron (Fe: 319; <50–1,120), mercury (Hg: 0.1; 

<0.02–0.2), manganese (Mn: 17; 1–51), molybdenum (Mo: 

0.2; <0.05–0.2), nickel (Ni: 5.5; 1.4–14), lead (Pb: 3.1; 0.7–

30.0), selenium (Se: 1.2; <1–2), and zinc (Zn: 129; 94–151). 

Except for mercury, few comparative data are available in the 

literature. Concentrations of Hg are low (0.1 μg/g) compared 

to grizzly bear from Yellowstone National Park (17–2,600 

μg/g) that feed on spawning cutthroat trout. Concentrations of 

Ba, Cd, Cu, Mn, Ni, and Pb are generally higher in Big Bend 

bear hair than in YNP hair, whereas Zn is lower. 

Standards for metal concentrations in mammal hair have 

not been set for many elements, but there are some studies 

that can be compared. Mora and others (2000) analyzed the 

hair of ocelots from the lower Rio Grande valley. They found 

the following average concentrations—Hg (0.7 μg/g), Se (5.9 

μg/g), and Pb (0.7 μg/g)—and observed no adverse effects on 

the animals. These ocelot Hg and Se concentrations are well 

above those for BBNP bears, but Pb is about 4–7 times higher 

in the BBNP bears. Burger and others (1994) studied hair from 

healthy opossum in a national park in Puerto Rico, mainly to 

establish concentrations so these abundant omnivorous marsupials can be used as bio-monitors for future contaminants as 

agricultural development proceeds. They found the following 

concentrations (average and standard error, μg/g dw): Cd (36 

± 4), Hg (228 ± 30), Mn (34,119 ± 7,867), and Pb (319 ± 46). 

These concentrations are all significantly higher than those 

measured in the Big Bend bear hair. Thus, the concentrations 

measured for bear hair from the Big Bend ecosystem (table 

7–2) are generally below concentrations found in other mammal hair from apparently healthy ecosystems. 

Table 7–1. Stable isotope data for bear hair in the Big Bend National Park area.

[Stable isotope values are per mil deviations from the carbon isotope standard VPDB (Vienna Peedee Belemnite), nitrogen isotope standard AIR (atmospheric 

nitrogen composition), and the sulfur isotope standard VCDT (Vienna Canyon Diablo Trolite); ± values are 1 standard deviation]

δ13CVPDB δ15NAIR δ34SVCDT

Chisos Mountains, Tex. -16.9±1.6 4.5±1.0 5.2±0.5 (n=20)

Black Gap Wildlife Area, Tex. -17.1±2.2 5.1±0.3 7.7±0.1 (n=2)

Sierra del Carmen mountains, Mexico -18.5±0.1 5.1±0.04 3.2±0.3 (n=2)

Serranias del Burro mountains, Mexico -19.8±0.7 4.1±0.5 7.3±0.5 (n=9)
Trace Elements in Bear Hair 81

-5.0

0.0

5.0

10.0

15.0

-35.0 -30.0 -25.0 -20.0 -15.0 -10.0

Chisos Mtns-Big Bend

Burro Mtns

Black Gap

Sierra del Carmen

-5.0

0.0

5.0

10.0

15.0

-5.0 0.0 5.0 10.0 15.0

Chisos Mtns-Big Bend

Burro Mtns

Black Gap

Sierra del Carmen

Yellowstone 

bear foods

Yellowstone 

bear foods

YNP

grizzly

hair

YNP

grizzly

hair

fish

nuts

meat

fish

nuts

meat

plants

plants

15

N



13C 

34S

15

N

Figure 7–3. Plots of nitrogen isotopes versus carbon and sulfur isotope values for Big Bend black bear hair. Ranges for Yellowstone 

National Park (YNP) grizzly bear hair and potential YNP grizzly bear foods are shown for comparison (from Felicetti and others, 2003; 

Chaffee and others, 2007).
82 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Table 7–2. Average composition and standard deviation of selected elements in Big Bend National Park bear hair (all concentrations 

in μg/g, dry weight).

[All samples had concentrations less than the following detection limits in μg/g: Ag <2, As <1, Tl <0.08, Gd, Ho, Ta <0.02, Tm <0.006. All elemental analyses 

by Inductively Coupled Plasma-Mass Spectrometry following acid decomposition, except Hg analyzed by Cold Vapor Atomic Absorption Spectrometry. Ba, 

Barium; Cd, Cadmium; Cu, Copper; Fe, Iron; Hg, Mercury; Mn, Manganese; Mo, Molybdenum; Ni, Nickel; Pb, Lead; Se, Selenium; and Zn, Zinc]

Location Ba Cd Cu Fe Hg Mn Mo Ni Pb Se Zn

Chisos Mountains, 

Big Bend, Tex. 5.4 0.1 10 249 0.04 21 0.1 7.2 3.2 1.0 127

Std Dev (n=20) 2.7 0.08 3 237 0.01 18 0.1 5.4 3.9 15

Serranias del Burro 

mtns, Coahuila, 

Mexico

6.9 0.2 10 98 0.14 8 0.09 6.9 5.0 1.4 133

Std Dev (n=9) 7.7 0.2 2 57 10 0.02 3.8 9.5 0.3 8

Black Gap Wilderness Area, Tex. 21 0.4 10 437 0.11 15 0.45 8.0 3.0 < 1 124

Std Dev (n=3) 18 0.1 2 593 0.06 8 0.65 5.8 1.5 25

Sierra del Carmen 

mtns, Coahuila, 

Mexico

28 5.3 30 662 --- 39 0.17 3.4 2.5 < 1 134

Std Dev (n=2) 24 6.5 28 564 18 0.10 1 0.6 12.0

YNP grizzly bear 

hair average 

(n=4)

6.71 0.1 12 400 0.62 13 0.18 1.4 0.8 0.9 252
References Cited 83

Eisler (1987) proposed that mercury concentrations in 

mammal hair at levels less than 2 μg/g constitute a safe level 

standard for animal health. Mercury analyses for BBNP bear 

hair are all at least a factor of ten below this standard. Additional information on mercury comes from feeding trials on 

captive bears (Felliceti and others, 2004) that demonstrated a 

direct correlation between mercury in a diet of fish from Yellowstone Lake and mercury in blood, plasma, and hair of the 

bears. They also sampled 42 Yellowstone grizzly bears with 

hair traps along cutthroat trout spawning streams and found 

mercury concentrations in hair that averaged about 360 μg/g 

and ranged from 17 to 2,600 μg/g.

Fellicetti and others (2004) did not conduct controlled 

studies to examine the effects of mercury intake on reproduction by grizzly bears. However, they note that captive adult 

grizzly bears fed large amounts of salmon containing an 

average of 240 μg/g Hg over multiple years had hair mercury 

concentrations averaging 4,778 μg/g (9–35 times higher than 

YNP bears) and still showed healthy reproduction. Thus, it is 

unlikely that Big Bend black bears, with average Hg concentrations in hair of 0.1 μg/g, have adverse health effects related 

to mercury in the diet. 

One big distinction between BBNP and YNP bear populations is that the latter have a higher proportion of meat and 

fish in the diet. Plants bio-accumulate far less mercury than 

animals, and the BBNP bears are feeding at a relatively low 

trophic level on a food web that appears to be rather simple 

(plants and bugs). Thus, despite a habitat with potentially high 

metal contamination in some areas, mercury does not appear 

to have propagated into higher consumers, such as bears. 

Conclusions

Stable isotope analysis of carbon, nitrogen, and sulfur in 

black bear hair is a useful tool for understanding the diet of 

bears. Future studies of bear foods will allow more detailed 

conclusions, but data presented here indicate that bear from 

the Big Bend area are largely vegetarians that feed on C4 and 

CAM plants, which flourish in the desert environment. Trace 

metal studies of bear hair indicate low concentrations of Hg, 

suggesting that black bear are not being adversely affected by 

mercury in the environment around BBNP.

References Cited

Bailey, V., 1905, Biological survey of Texas: U.S. Department 

of Agriculture, Bureau of the Biological Survey North 

American Fauna Number 25, 216 p.

Burger, J., Marquez, M., and Gochfeld, M., 1994, Heavy 

metals in the hair of opossum from Palo Verde, Costa Rica: 

Archives of Environmental Contamination and Toxicology, 

v. 27, p. 472–476.

Chaffee, M.A., Shanks, W.C., III, Rye, R.O., Schwartz, 

C.C., Carlson, R.R., Crock, J.G., Adams, M., Gemery, 

P.A., Gunther, K.A., Kester, C.L., King, H.D., and Podruzny, S.R., 2007, Applications of trace-element and 

stable-isotope geochemistry to wildlife issues, Yellowstone 

National Park and vicinity, in Morgan, L.A., ed., Integrated 

geoscience studies in the greater Yellowstone area—Volcanic, hydrothermal and tectonic processes in the Yellowstone geoecosystem: U.S. Geological Survey Professional 

Paper 1717, p. 299–334.

Dawson, T.E., Mambelli, S., Plamboeck, A.H., Templer, P.H., 

and Tu, K.P., 2002, Stable isotopes in plant ecology: Annual 

Review of Ecological Systems, v. 33, p. 507–559.

DeNiro, M.J., and Epstein, S., 1978, Influence of diet on the 

distribution of carbon isotopes in animals: Geochimica et 

Cosmochimica Acta, v. 42, p. 495–506.

DeNiro, M.J., and Epstein, S., 1981, Influence of diet on the 

distribution of nitrogen isotopes in animals: Geochimica et 

Cosmochimica Acta, v. 45, p. 341–351.

Eisler, R. 1987, Mercury hazards to fish, wildlife, and invertebrates—A synoptic review: U.S. Fish and Wildlife Service 

Biological Report 85(1.10), 90 p.

Farquhar, G.D., O’Leary, M.H., and Berry, J.A., 1982, On the 

relationship between carbon isotope discrimination and the 

intercellular carbon dioxide concentration in leaves: Australian Journal of Plant Physiology, v. 9, p. 121–137.

Felicetti, L.A., Schwartz, C.C., Rye, R.O., Gunther, K.A., 

Crock, J.G., Haroldson, M.A., Waits, L., and Robbins, C.T., 

2004, Use of naturally occurring mercury to determine the 

importance of cutthroat trout to Yellowstone grizzly bears: 

Canadian Journal of Zoology, v. 82, p. 493–501.

Felicetti, L.A., Schwartz, C.C., Rye, R.O., Haroldson, M.A., 

Gunther, K.A., Phillips, D.L., and Robbins, C.T., 2003, 

Use of sulfur and nitrogen stable isotopes to determine the 

importance of whitebark pine nuts to Yellowstone grizzly 

bears: Canadian Journal of Zoology, v. 81, p. 763–770.

Fry, B., Brand, W., Mersch, F.J., Tholke, K., and Garritt, R., 

1992, Automated analysis system for coupled δ13C and 

δ15N measurements: Analytical Chemistry, v. 64,

p. 288–291.
Hellgren, E.C., 1993, Status, distribution, and summer food 

habits of black bears in Big Bend National Park: The Southwestern Naturalist, v. 38, p. 77–80.

Hellgren, E.C., Onorato, D.P, and Skiles, J.R., Jr., 2005, 

Dynamics of a black bear population within a desert metapopulation: Biological Conservation, v. 122, p. 131–140.

Hilderbrand, G.V., Farley, S.D., Robbins, C.T., Hanley, T.A., 

Titus, K., and Servheen, C., 1996, Use of stable isotopes to 

determine diets of living and extinct bears: Canadian Journal of Zoology, v. 74, p. 2,080–2,088.

Kester, C.L., Rye, R.O., Johnson, C.A., Schwartz, C.C., and 

Holmes, C.W., 2001, On-line sulfur isotope analysis of 

organic material by direct combustion—Preliminary results 

and potential applications: Isotopes Environmental Health 

Studies, v. 37, p. 53–65.

Mora, M.A., Laack, L.L., Lee, M.C., Sericano, J., Presley, R., 

Gardinali, P.R., Gamble, L.R., Robertson, S., and Frank, 

D., 2000, Environmental contaminants in blood, hair, and 

tissues of ocelots from the lower Rio Grande Valley, Texas, 

1986–1997: Environmental Monitoring and Assessment, v. 

64, p. 477–492.

Mora, M., Skiles, R., McKinney, B., Paredes, M., Buckler, D., 

Papoulias, D., and Klein, D., 2002, Environmental contaminants in prey and tissues of the peregrine falcon in the Big 

Bend Region, Texas, USA: Environmental Pollution, v. 116, 

p. 169–176.

Onorato, D.P., and Hellgren, E.C., 2001, Black bear at the 

border—Natural recolonization of the Trans-Pecos, in

Maehr, D.S., Noss, R., and Larkin, J., eds., Large mammal 

restoration—Ecological and sociological challenges in the 

21st century: Washington, D.C., Island Press, p. 245–259.

Onorato, D.P., Hellgren, E.C., Van Den Bussche, R.A., and 

Skiles, J.R., Jr., 2004, Paternity and relatedness of American 

black bears recolonizing a desert montane island: Canadian 

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and C13 in plants: Plant Physiology, v. 36, p. 133–138.

84 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas
Abstract

Airborne magnetic and radioelement surveys were flown 

over the Big Bend National Park region in order to aid in 

surficial and subsurface geologic mapping. The data were 

collected along northeast-southwest lines spaced 400 m apart 

at a nominal flight height of approximately 400 m above the 

ground (Drenth and Finn, 2007). Aeromagnetic data facilitate 

geologic mapping of igneous rocks and faults in the inaccessible terrane of the Big Bend National Park region by mapping the distribution of magnetic minerals at and beneath the 

surface. Variations in magnetization that reflect differences in 

concentrations of magnetic minerals produce observed magnetic anomalies. The radioelement data are used to estimate 

the concentrations of potassium, uranium, and thorium in the 

near-surface soils and rocks (most of the gamma-rays detected 

originate in the top 30 cm of material). These estimated concentrations can in turn provide clues regarding the lithologic 

characteristics of the materials. In this paper, we provide a 

broad overview of how to use aeromagnetic data to map both 

exposed and buried igneous rocks and a brief description of 

radioelement mapping of the surficial igneous geology.

Identifying Igneous Rocks with 

Aeromagnetic Data

The dramatic topography of Big Bend National Park 

is largely formed or influenced by igneous processes. The 

aeromagnetic data provide a critical tool for imaging significant intrusions and thick sections of volcanic rocks. In order 

to facilitate geologic interpretation, reduction to pole transformation (a standard geophysical technique to center anomalies 

over their sources) was applied to the total field magnetic 

data (fig. 8–1A) (Baranov and Naudy, 1964; Blakely, 1995). 

Magnetic highs (red colors) centered over their exposed and 

buried Tertiary igneous sources dominate the reduced to the 

pole magnetic map of the region. The buried rocks are identified as continuations of anomalies related to similar magnetic 

signatures from exposed rocks. Most of the topographic features in the park are influenced by intrusions at varying scales. 

Many of the broad, high amplitude anomalies correlate with 

topography produced by laccoliths and other intrusive bodies 

such as the Rosillos Mountains granitic laccolith (fig. 8–1B) 

(Anderson, 2007) and the Chisos Mountains, which are underlain by a large buried intrusion that presumably caused the 

uplift of the mountains (fig. 8–1C) (Drenth and Finn, 2007). 

Other small buried intrusions often cause local doming of beds 

(fig. 8–2) that, in places, is superimposed on uplift related to 

the Laramide orogeny (for example, east of Javalina Creek, 

fig. 8–1A). Not all intrusions produce magnetic anomalies 

(compare outcrop with anomalies, fig. 8–1A), either because 

they are too thin or their magnetization does not significantly 

differ from surrounding rocks, or both. Many of the mapped 

geologic units in the Big Bend region, particularly the sedimentary and volcaniclastic rocks, lack spatial correlation with 

magnetic anomalies, leading to the interpretation that these 

units are not anomaly sources.

Other sources of magnetic anomalies include normally 

and reversely magnetized Tertiary volcanic rocks, which 

produce magnetic highs and lows, respectively. Reversely 

magnetized volcanic rocks fill calderas in the northeastern 

and southwestern edges of the Chisos Mountains and in the 

Christmas Mountains (fig. 8–1A). Other small circular, highamplitude magnetic lows may relate to old volcanic centers. 

Mapped lava flows in the northeastern portion of the map 

produce generally northwest trending high amplitude magnetic highs and lows. Northwest trending magnetic highs and 

lows over the rest of this region may indicate similar buried 

or unmapped igneous sources, such as those over mafic 

lavas exposed southwest of the Chisos Mountains. The high 

amplitude, northeast trending magnetic highs in the western 

portion of the region (fig. 8–1A) delineate buried pre-Tertiary 

basement rocks—perhaps Precambrian in age (Drenth and 

Finn, 2007).

The magnetic data can be used to estimate the depths 

to buried sources and, where constrained by geologic mapping and rock property measurements, to determine the 

source geometry. Depth estimates show that most sources 

in the region lie in the upper 1,600 m of the surface. Three 

dimensional models (fig. 8–1B), show that an 800-m-thick, 

12-km-diameter granitic laccolith underlies the Rosillos 

Mountains (Anderson, 2007). Other laccoliths in the region, 

8. Aeromagnetic and Radioelement Analyses Identify 

Igneous Rocks in the Big Bend National Park Region

By Carol A. Finn and Eric D. Anderson
86 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Tertiary volcanic rocks 5,000 0 5,000 15,000 meters

Tertiary intrusive rocks

Rosillos Mountains Intrusion Chisos Mountains Intrusion

Caldera boundary

N

N

-103o 45' -103o 30' -103o 15' -103o -102o 45'

29o 45' 29o 30' 29o 15' 29o

-103o 45' -103o 30' -103o 15' -103o -102o 45'

29o 45' 29o 30' 29o 15' 29o

location of g. 8-2

B C

A

Magnetic Anomaly (nanotesla)

Park boundary

McKinney

Hills

Javalina

Creek

Rosillos Mountains

Chisos

Mountains

PRE-TERTIARY

BASEMENT

Christmas

Mountains
Identifying Igneous Rocks with Aeromagnetic Data 87

Figure 8–1 (facing page). Magnetic anomaly and magnetic 

sources from the Big Bend National Park region. A, Color 

shaded relief image of reduced to pole magnetic anomaly map 

with outlines of selected geologic units (Barnes, 1979). Blue box 

north of the Rosillos Mountains shows the location of figure 8–2. 

B, Magnetic model of the Rosillos laccolith (Anderson, 2007). 

The topography is shown as a blue triangulated mesh; beige 

body represents the Rosillos laccolith; green and blue bodies 

represent Tertiary reversely magnetized intrusions; yellow body 

is a thin, normally magnetized intrusive body. View is to the 

northwest. C, Magnetic model of the Chisos Mountains intrusion 

(from Drenth and Finn, 2007). Gray shaded surface is the digital 

elevation model; black lines outline the Pine Canyon (NE) and 

Sierra Quemada (SW) calderas; and the orange body represents 

the intrusion with a susceptibility of 0.03 SI. View is from the 

southwest.

706 708 710 712 714 717 719 721 723 725 727 729 731

nT

~100 m Chalk Draw Fault

Jack Eden Mesa

White

Hills

N

~200 m

Figure 8–2. Magnetic data (colors) superimposed on an aerial photo draped on a DEM along the south central portion of the 

Chalk Draw Fault. Red dotted line indicates approx. 10 nT high related to a buried intrusion. View from the northeast. Vertical 

exaggeration is 2.

such as the McKinney Hills and Javalina Creek, have similar 

amplitude and wavelength anomalies (fig. 8–1A), suggesting 

comparable thickness and geometry. The largest intrusion in 

the region underlies the Chisos Mountains (fig. 8–1C) and 

is approximately 1–4 km thick and 28–34 km in diameter 

(Drenth and Finn, 2007). 
88 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Identifying Faults with Aeromagnetic 

Data

Faults, fractures, and lithologic contacts can be delineated 

by the magnitude of the horizontal gradients of the reduced to 

pole aeromagnetic anomalies (Cordell and Grauch, 1985; Phillips, 2000). Local maxima that form linear and curved ridges 

mark magnetic property contrasts in the subsurface. Closed 

circular peaks suggest magnetic sources such as laccoliths, 

plugs, caldera boundaries, and cinder cones. Long, narrow, linear peaks typically indicate faults or dikes. In some locations 

circular peaks interrupt linear trends (for example, in the south 

central region of the park, fig. 8–3), suggesting that igneous 

bodies intrude fault planes, which indicates whether igneous 

activity is either syn- or post-tectonic. 

The mapped faults almost exclusively trend NNW with a 

few NE trending faults (fig. 8–3). The NNW trend dominates 

the local extrema from the maximum horizontal gradient data, 

but the NE trend is also prominent. Many mapped faults correspond to maxima of the horizontal gradient, but many do not. 

Some mapped faults are parallel to nearby gradient maxima, 

which could indicate that the faults offset magnetite-poor 

rocks at the surface and more magnetic ones at depth and dip 

in the direction of the gradient. The faults that do not correspond to magnetic gradient maxima typically offset magnetitepoor rocks at the surface, but not deeper magnetic sources 

(such as in the northwest portion of the region, fig. 8–3). This 

suggests that the faults have limited vertical extent. Although 

the NNW trend parallels Cretaceous structures, most of the 

faults cut Tertiary intrusions, including the Rosillos laccolith 

(fig. 8–1B), indicating that they formed in the Late Tertiary 

during northwest-directed Basin and Range Extension. The 

magnetic model from the Rosillos laccolith indicates that the 

faults do not have significant vertical offset (300–600 m) and 

have no demonstrable horizontal offset (fig. 8–1B).

Identifying Surficial Geologic Units 

with Radioelement Data

Radioelement data can be used to help map surficial rock 

types. Here, we use the data to further the understanding of 

the distribution and composition of surficial igneous rocks. 

In order to maximize the utility of the radioelement data, we 

added gridded potassium (red), thorium (green), and uranium 

(blue) data together to produce a ternary radioelement map 

(fig. 8–4). White regions contain high concentrations of all 

three elements, whereas dark areas contain low concentrations.

Tertiary intrusions in the region are enriched in potassium, uranium, and thorium, as are most of the volcanic 

rocks (fig. 8–4). Intrusions slightly enriched in thorium 

(yellow color), such as the Rosillos laccolith and a large 

intrusion to the northwest, indicate slightly more mafic 

compositions than those in the Chisos Mountains region. 

Brownish regions reflecting low uranium content characterize mafic lava flows in the northeast and gabbro near the 

Mariscal Mountains (fig. 8–4). White regions in the Chisos 

Mountains region not associated with igneous rocks relate 

primarily to sediments shed from igneous rocks, as do the 

pink-orange regions around the Rosillos laccolith (Anderson, 

2007). Other rock types can be identified with radioelement 

data through various classification methods (Duval, 2005).
Edges of magnetic bodies 5,000 0 5,000 15,000 meters (faults or contacts) Mapped faults Park boundary

Chalk Draw Fault

-103o 45' -103o 30' -103o 15' -103o -102o 45'

-103o 45' -103o 30' -103o 15' -103o -102o 45'

29

o

 45' 29o 30' 29o 15' 29o

Figure 8–3. Mapped faults (Barnes, 1979) superimposed on the maxima of the horizontal gradient of the reduced to the pole maganomaly (fig. 8–1).
5,000 0 5,000 15,000 meters

Tertiary volcanic rocks Tertiary intrusions Park boundary

-103o 45' -103o 30' -103o 15' -103o -102o 45'

-103o 45' -103o 30' -103o 15' -103o -102o 45'

29

o

 45' 29o 30' 29o 15' 29o Chisos Mountains Mariscal Mountain

RosillosMountains

Figure 8–4. Color composite of potassium (red), thorium (green), and uranium (blue) superimposed on a grey shade map of the towith outlines of selected geologic units (Barnes, 1979).
References Cited

Anderson, E.D., 2007, Integrating 2D geologic data with 

geophysics to generate a 3D geologic map of the Rosillos 

Mountains in Big Bend National Park, TX [abs.]: Geological 

Society of America Abstracts with Programs, v. 39, no. 6, 

p. 634.

Baranov, V., and Naudy, H., 1964, Numerical calculation of 

the formula of reduction to the magnetic pole: Geophysics, 

v. 29, p. 67–79.

Barnes, V.E., 1979, Emory Peak-Presidio sheet, Geologic atlas 

of Texas: University of Texas at Austin, Bureau of Economic Geology, 13 p.

Blakely, R.J., 1995, Potential theory in gravity and magnetic 

applications: Cambridge, U.K., University Press, 441 p.

Cordell, L., and Grauch, V.J.S., 1985, Mapping basement 

magnetization zones from aeromagnetic data in the San 

Juan basin, New Mexico, in Hinze, W. J., ed., The utility 

of regional gravity and magnetic anomaly maps: Society of 

Exploration Geophysicists, p. 181–197.

Drenth, B.J., and Finn, C.A., 2007, Aeromagnetic mapping of 

the structure of Pine Canyon caldera and Chisos Mountains 

intrusion, Big Bend National Park, Texas: Geological Society of America Bulletin, v. 119, p. 1,521–1,534.

Duval, J.S., 2005, Aerial gamma-ray, Landsat TM, and digital 

elevation data, Big Bend Area, Texas: U.S. Geological 

Survey Open-File Report 2005-1371, http://pubs.usgs.gov/

of/2005/1371/.

Phillips, J.D., 2000, Locating magnetic contacts—A comparison of the horizontal gradient, analytic signal, and local 

wavenumber methods: Society of Exploration Geophysicists 

Technical Program Expanded Abstracts, v. 21, p. 402–405.

References Cited 91
92 Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas

Contributing Authors

John E. Gray

editor, main point of contact

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 973

Denver, CO 80225

jgray@usgs.gov

William R. Page

co-editor

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 980

Denver, CO 80225

rpage@usgs.gov

Eric D. Anderson

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 964

Denver, CO 80225

ericanderson@usgs.gov

Elizabeth Y. Anthony

Department of Geological Sciences

University of Texas at El Paso

500 University Ave.

El Paso, TX 79902

eanthony@geo.utep.edu

Margaret E. Berry

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 980

Denver, CO 80225

meberry@usgs.gov

Robert G. Bohannon

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 980

Denver, CO 80225

bbohannon@usgs.gov

Carol A. Finn

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 964

Denver, CO 80225

cfinn@usgs.gov

Pamela A. Gemery-Hill*

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 973

Denver, CO 80225

Eric C. Hellgren

Cooperative Wildlife Research Laboratory

Southern Illinois University at Carbondale

Carbondale, IL 62901

hellgren@siu.edu

David P. Krabbenhoft

U.S. Geological Survey

8505 Research Way

Middleton, WI 53562

dpkrabbe@usgs.gov

Andrew H. Manning

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 964

Denver, CO 80225

amanning@usgs.gov

Daniel P. Miggins

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 974

Denver, CO 80225

dmiggins@usgs.gov

Lisa A. Morgan

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 973

Denver, CO 80225

lmorgan@usgs.gov

David P. Onorato

Fish and Wildlife Research Institute

Florida Fish and Wildlife Conservation Commission

Naples, FL 34104

dave.onorato@myFWC.com

Minghua Ren

Department of Geological Sciences

University of Texas at El Paso

500 University Ave.

El Paso, TX 79902

ren@geo.utep.edu
Contributing Authors 93

W.C. Pat Shanks III

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 973

Denver, CO 80225

pshanks@usgs.gov

Craig A. Stricker

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 963

Denver, CO 80225

cstricker@usgs.gov

Peter M. Theodorakos

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 973

Denver, CO 80225

ptheodor@usgs.gov

Kenzie J. Turner

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 980

Denver, CO 80225

kturner@usgs.gov

Van S. Williams

U.S. Geological Survey

Denver Federal Center

Box 25046, Mail Stop 980

Denver, CO 80225

vwilliam@usgs.gov

* At time of publication, Pamela A. Gemery-Hill is no longer 

affiliated with the U.S. Geological Survey. Author can be 

contacted at pagemeryhill@aol.com

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Team Chief Scientist, USGS Central Mineral Resources

Box 25046, Mail Stop 973

Denver, CO 80225

(303)236-1562

Or visit the Central Mineral Resources Team Web site at:

http://minerals.cr.usgs.gov/
Gray and Page, eds.—Geological, Geochemical, and Geophysical Studies by the USGS in Big Bend National Park, Texas—Circular 1327

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