U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3078

September 2007

Integrated Geologic, Geochemical, and Geophysical Studies 

of Big Bend National Park, Texas

Printed on recycled paper

Introduction

Big Bend National Park (BBNP), Texas, 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-mi (190-km) stretch of the Rio Grande at 

the United States border with Mexico. The park is situated in the 

Chihuahuan Desert, and this desert ecosystem displays enormous 

beauty including mountain and basin environments with an 

extraordinary biological diversity 

of native plants and animals. The 

park is home to a variety of flora 

and fauna including more than 

1,200 species of plants, more than 

450 species of birds, 56 species 

of reptiles, and 75 species of 

mammals. Geologic features in 

BBNP provide opportunities to 

study the formation processes of 

sedimentary and igneous rocks, as 

well as opportunities to investigate 

Cretaceous and Tertiary fossils. 

Mineral deposits in and around 

BBNP contain commodities such 

as mercury, uranium, and fluorite, 

but of these, the only significant 

mining has been for mercury. 

The Chisos Mountains reach an 

altitude of 7,832 ft (2,387 m) and 

are considered the southernmost 

mountain range in the United 

States. More than 350,000 tourists 

visit BBNP each year. 

The U.S. Geological Survey 

(USGS) began a 5-year project 

in 2003 with the objective of 

studying a number of broad and 

diverse geologic, geochemical, 

and geophysical topics in BBNP. 

Working cooperatively with the 

National Park Service (NPS), USGS scientists from the Mineral 

Resources and National Cooperative Geologic Mapping Programs 

have carried out (1) studies of mercury contamination at sites of 

past mercury mining in and adjacent to BBNP; (2) research of 

volcanic stratigraphy and geochronology (dating) of intrusive and 

extrusive igneous rocks; (3) evaluations of surface and subsurface 

water quality and resources, as well as processes related to rock 

alteration in the park; (4) characterization studies of geophysical 

properties of the surface and subsurface geology in BBNP; and (5) 

mapping of the surface geology in order to produce a new geologic 

map of the park. This fact sheet describes results of some of the 

research by USGS scientists working in BBNP.

Contaminant Studies of Inactive Mercury Mines

One of the primary objectives of research in BBNP involves 

studies to understand potential contamination from mercury mines 

in and around the park (fig. 1). Mercury has no known biological 

function and is toxic to all organisms, including humans. Presently, no mines are operating in the area; however, total mercury 

production ranks the Terlingua district as the third largest mercury 

mining region in the United States. The effect of runoff from mines 

Recently completed studies of Big Bend National Park 

provide new information on the following topics:

• Effect of historical mercury mining in and 

 near the park

• Origin and composition of metals in ground 

 and surface water

• History of volcanic and intrusive activity

• Complex geologic history

x Terlingua

district mines

Park Boundary

Rio Grande

Glenn

Spring

x Mariscal mine

103°W

103°W

103°30'W

103°30'W

29°30'N

29°30'N

29°N

29°N

0 10 20 KILOMETERS

The Basin Chisos

Mountains

Hot Spring

Rio Grande

Village

Croton Spring

Oak Spring

Figure 1. Shaded relief map of the Big Bend area showing the outline (in red) of Big Bend National

Park and locations of areas studied by the USGS. The Terlingua district mines shown here produced

more than 5,000 t of mercury during the 1890s to 1970s; they were the subject of recent USGS studies.
located inside and just outside the park boundary is a concern for 

both wildlife and humans because mercury is highly concentrated 

around the abandoned mines, and water draining the mines enters 

streams that are part of local ecosystems. The USGS has been measuring mercury concentrations in mine waste, sediment, water, and 

air throughout BBNP to address potential mercury contamination 

around these mercury mines.

The Mariscal mine, the only mine that produced mercury in 

BBNP, has been the subject of much of the work by the USGS on 

this project. One of the smaller mines in the Terlingua mercury 

district and active from about 1900 to 1943, the Mariscal mine 

was included in BBNP when the park was established in 1944. 

Because Mariscal is located inside BBNP, tourists often visit this 

site. The volume of mine waste at the Mariscal mine is estimated 

at about 39,200 yd3 (30,000 m3), and the concentration of mercury 

in samples of mine waste is as high as 150 μg/g (micrograms 

per gram), several orders of magnitude higher than that typically 

found elsewhere in uncontaminated soil in BBNP. Minor mine 

waste was observed in a stream channel (Fresno Creek) draining 

the mine, but the concentration of mercury in stream sediment 

collected greater than 0.6 mi (1 km) from Mariscal (fig. 2) is 

similar to that in uncontaminated regional baseline sediment (Gray 

and others, 2006). No mine-water runoff was observed draining 

Mariscal during studies from 2003–2007. Although mercury and 

methylmercury concentrations are locally elevated in some mine 

waste samples, little mercury appears to be exported from these 

mercury mines in stream sediment or water primarily due to the 

arid climate and lack of precipitation and mine runoff in this 

region. These studies indicate that conversion of inorganic mercury 

to highly toxic organic mercury forms (methylmercury) is also low 

in this arid climate setting. 

Studies of Igneous Rocks

Extrusive and intrusive igneous rocks, known to be the source 

of some mineral occurrences, were the subject of detailed studies in 

BBNP. A broad spectrum of igneous rocks—including structurally 

controlled dikes (cross-cutting bedding), shallow intrusive bodies, and large ash-flow tuffs, which have produced calderas (large, 

generally circular volcanic depressions formed by extrusion of 

magma and subsequent collapse of surrounding ground)—dominate 

the landscape. Dagger Flats and Pine Canyon caldera were identified as two sites for detailed studies to understand how the igneous 

rocks were formed because they are associated to some degree with 

extensive fossil hydrothermal systems (figs. 3 and 4).

Low northwest-trending hills in the Dagger Flats area are 

bounded by northwest-trending faults cored with thick sills that 

were intruded along bedding and are capped by Cretaceous 

sedimentary rocks. Sill and associated magmatic hydrothermal 

solutions that intruded into marine sedimentary rock resulted in 

formation of skarns at Dagger Flats. Sills at Dagger Flats form a 

well-defined northwest-trending body that is about 9–14 mi (15–22 

km) long by 3 mi (5 km) wide, which is comparable to, or larger 

than, adjacent intrusive bodies in the McKinney Hills and the 

Rosillos Mountains. The Dagger Flats intrusive complex is slightly 

larger than the intrusive bodies associated with the Pine Canyon 

caldera. High-precision radiometric dates (using isotopes of argon, 

40Ar/39Ar) of the Dagger Flats sills indicate that these bodies were 

intruded from 32.47±0.41 to 30.95±0.61 Ma during a 1.5-millionyear interval beginning at about 31 million years ago (Ma), which 

is contemporaneous with peak intrusive and volcanic activity 

less than 6 mi (10 km) to the south at the Pine Canyon caldera. 

Radiometric dates in the Pine Canyon area indicate that igneous 

activity resulted in formation of a caldera that ranged in age from 

32.22±0.12 to 30.86±0.41 Ma. Field observations coupled with 

dating of rocks at Dagger Flats and Pine Canyon contributed to the 

understanding of the history of igneous rocks in BBNP.

Surface and Subsurface Water Quality and Resources

In the desert climate of BBNP, water quality and water supply 

are persistent issues. Geochemical studies were designed to evaluate the sources, transport, and dispersion of metals in surface water, 

ground water, and active hot springs. Monitoring of major-element, 

TOTAL HG AND METHYL-HG (μg/g)

DISTANCE FROM MARISCAL MINE (in km)

0.00001

0.001

0.1

10

1,000

0 5 10 15 20 25 30

Hg Fresno Creek

Methyl-Hg Fresno Creek

Probable effect concentration (1.06 μg Hg/g)

Regional uncontaminated baseline sediment

Mine waste

Figure 2. A. Stream sediment collected downstream from the Mariscal mine, Texas, shows rapid dispersion of total mercury (Hg) and

methylmercury (methyl-Hg). Results show that total mercury concentrations decrease in sediment collected more than 1 km from the mine and

are below the probable effect concentration for mercury, the concentration where harmful effects are likely in sediment-dwelling organisms.

Methylmercury concentrations in stream sediment samples were generally below that found in regional uncontaminated baselines. Although

mercury concentrations were elevated in mine waste, the concentrations of methylmercury were generally low in the ecosystems downstream

because conditions for transformation to methylmercury are unfavorable in this hot and dry desert climate. B. View of the Mariscal mercury

mine located in Big Bend National Park.

mine waste

retorts

A. B.
29 °1 0 '0 "N

29 °2 0 '0 "N

Tex t

McKinney

Hills

Dagger

Flats

Pine

Canyon

caldera

Modified from Maxwell (1968)

CHISOS MOUNTAINSRio Grande

Rosillos Mountains

KILOMETERS

Holocene alluviumPliocene-Miocene alluvial depositsDelaho Formation (Pliocene and  Miocene)—Sandstones and  conglomeratesSouth Rim Formation (Oligocene) Burro Mesa Rhyolite Member Lost Mine Member Boot Rock member (informal) Pine Canyon Rhyolite MemberStocks, laccoliths, sills, dikes of  basalt, trachyte, rhyolite (Tertiary  and Cretaceous)Chisos Group (Oligocene and Eocene) Tule Mountain Trachyandesite Mule Ear Spring Tuff Bee Mountain BasaltCretaceous rocks (various greens) Javelina, Aguja, Pen, Boquillas, Buda, Del Rio, Santa Elena,  Telephone Canyon, and Glen Rose Formations

102°50’W 103°W 103°10’W 103°20’W 103°30’W 103°40’W

102°50’W 103°W 103°10’W 103°20’W 103°30’W 103°40’W

29°30’N

29°30’N29°20’N29°10’N29°N

Big Bend National Park

0 30 15

Figure 3. General geologic map of Big Bend National Park (modified from Maxwell, 1968) showing the location of Dagger Flats, McKinney Hills, Pine Canyon caldera,and the Rosillos Mountains where detailed igneous rock and geophysical studies were conducted.
mudstone

mudstone

mafic sill

skarn

laminated siltstones

Figure 4. At Dagger Flats, sedimentary rocks were

intruded by sills that produced associated mineralized rocks

forming a low-grade skarn deposit containing quartz, pyrite,

wollastonite, prehnite, and grossular garnet. The skarn

deposits have formed in the caprock directly above the sills,

which are composed primarily of mafic rocks (containing

magnesium and iron-bearing minerals). The movement of

magmatic hydrothermal solutions into marine sedimentary

rock resulted in the skarn formation.

trace-element, and isotopic compositions in water helps to establish 

base lines against which future trends related to climate change, 

drought, and other regional events can be measured. Water samples 

were collected from a broad range of geologic 

and geographic settings in BBNP—including the 

Rio Grande, several hot and cold springs, and 

water wells—to characterize the water chemistry, 

and to more thoroughly understand the age, flow 

path, and host lithology of surface and ground 

water.

Potentially toxic trace metals in various 

spring-water samples range in concentration as 

follows: copper (<0.5–23 μg/L, micrograms per 

liter), lead (<0.05–3.2 μg/L), cadmium (<0.02–

3.5 μg/L), thallium (<0.05–0.68 μg/L), selenium 

(0.81–11 μg/L), and uranium (0.14–29 μg/L); 

these data provide assurance that these trace 

metals are all below drinking water maximum 

contaminant levels (MCLs) established by the 

U.S. Environmental Protection Agency. Water 

sources throughout BBNP are also generally 

below the MCL for arsenic (10 μg/L) and fluoride 

(4.0 μg/L). However, potential water quality 

problems were found in water collected from 

Glenn Spring containing 16 μg/L arsenic, the hot 

spring west of Rio Grande Village with 16 μg/L 

arsenic, and Croton Spring containing 19 μg/L 

fluoride, but these are not used as drinking water 

sources in BBNP. Drinking water produced from wells 

at Rio Grande Village contains a significant component 

of hot spring water, but does not exceed the MCL 

for arsenic or other elements. Additional studies are 

underway to more thoroughly address the source of such 

contaminants, but data suggest that meteoric water (of 

recent atmospheric origin) may leach some potentially toxic metals 

during circulation and interaction with surrounding altered and 

mineralized limestone and volcanic rocks. Using stable isotopic 

tracer techniques, it is possible to understand unique interactions 

of meteoric water with surrounding rocks. Stable isotopes indicate 

that all BBNP water is meteoric in origin, and evaporation produces 

a significant range of oxygen and hydrogen isotope values (fig. 5). 

Hot and cold spring water samples are shifted from the global 

meteoric water line to slightly higher oxygen and hydrogen isotopic 

compositions due to water-rock reactions in the subsurface and (or) 

minor evaporation at the surface.

Evidence of nuclear bomb tests in the 1950s and 1960s can 

be used to date various types of water. Tritium is a radioactive 

isotope of hydrogen produced in abundance during nuclear bomb 

testing. Tritium enters ground water as infiltrating precipitation, 

then decays with time to another isotope, helium-3. Because the 

rate of radioactive decay is known, tritium and helium-3 can be 

used to date ground water younger than about 50 years, as well as 

to determine the approximate fractions of old water (>50 years old) 

and young water (<50 years old) in mixed age samples. In addition, 

noble gas concentrations (helium, neon, argon, krypton, xenon, and 

radon) can be used to calculate the temperature and elevation of 

ground-water recharge. Water collected from the hot spring west 

of Rio Grande Village has circulated deeply, probably along fault 

zones, and the water was geothermally heated during this deep 

circulation. Oak Spring water, which is the water supply for The 

Basin (see fig. 1), originates in The Basin and probably flows down 

along fractures and faults to the topographically lower Oak Spring. 

These new studies indicate that most ground water utilized as 

drinking water in BBNP is a mixture of young water, and an older, 

more deeply circulating water component.

0

-80

-70

-60

-50

-40

-30

-20

-10

10

20

-10.0 -8.0 -6.0 -4.0 -2.0 0.0 2.0

δ18

O (PER MIL)

δD (PER MIL)

Thermal waters [RGV area]

Non-thermal springs

Global meteoricwater line

Terlingua, and LongDraw

Rio Grande

Tornillo Creek

Evaporation

Water-rock reactions

Croton

Spring

Cottonwood Campground

water

Figure 5. Hydrogen (δD) versus oxygen isotope (δ18O) values showing

domination by meteoric waters. δ is conventionally defined as the ratio of

isotopes of an element compared against a standard, such as standard mean

ocean water. Many hot and cold spring waters (which commonly occur in

pools at the surface) are shifted off the meteoric water line, probably due to

evaporation. Rio Grande water is generally less evaporated than springs. Croton

Spring is highly evaporated water.
Geophysical Studies

Geophysical studies in BBNP provided key information in the 

interpretation of the volcanic stratigraphy and geology of the park. 

The Pine Canyon Rhyolite Member of the South Rim Formation, 

the main unit in the Pine Canyon caldera, is shown by direct and 

indirect observations and modeling to be reversely magnetized 

(Drenth and Finn, in press). This conclusion is supported by a new 

40Ar/39Ar age of 32.03±0.08 Ma, which corresponds to a time when 

the magnetic field of the Earth was reversed. Modeling of gravity 

and magnetic anomalies indicate that the Pine Canyon Rhyolite 

Member has lower magnetization intensities and densities in the 

southwestern portion of the caldera, possibly due to the effects of 

hydrothermal alteration (fig. 6). A geophysical magnetic high was 

found in this area, and it is interpreted as a broad laccolith (a planoconvex concordant igneous body with a flat bottom) that overlies 

the Pine Canyon caldera and surrounding Chisos Mountains region. 

This feature represents the largest intrusion (17–21 mi (28–34 km) 

diameter, 0.6–2.5 mi (1–4 km) thick, 168–720 mi3 (700–3,000 

km3) in volume) in an area where relatively small laccoliths are 

found. The intrusion most likely represents a long-lived reservoir 

replenished by small batches of magma of varying composition as 

reflected in the variation of volcanic rocks in the Chisos Mountains. 

The intrusion may represent the easternmost occurrence of voluminous Tertiary magmatism in the southwestern United States.

Another geophysical study undertaken in collaboration with 

geologists mapping in BBNP was conducted around the Rosillos 

Mountains. This particular study integrated and interpreted aeromagnetic, radiometric, gravity, Landsat Enhanced Thematic 

Pine Canyon Rhyolite Member

Boot Rock 

member

Boot Rock 

member

Pine Canyon Rhyolite Member

caldera margin

METERS

Figure 6.  Surface magnetization map of the

Pine Canyon caldera showing strong negative

magnetizations (blue and green, indicating

reversed polarity) assigned to the Pine

Canyon Rhyolite Member of the South Rim

Formation and weaker positive magnetizations

(yellow and orange) assigned to the Boot

Rock member, an informal member of the

South Rim Formation. Geophysical studies

were key to unraveling the geology of the Pine

Canyon area. A/m = ampere/meter.

Mapper (ETM), and geologic mapping to produce a three-dimensional geologic map of the region and refine the surficial geologic 

mapping. Data sets were processed, and the geologic information 

was used to prepare a representation of the geology in three dimensions. A number of different data sets (Landsat ETM, aeromagnetic, and radiometric data) were used to distinguish surficial 

igneous units, provide clues to relative ages through magnetic 

reversals within the igneous rocks, and help define the geochemical 

composition of the igneous rocks. The aeromagnetic and gravity 

data were modeled to determine volumes of the intrusive rocks, 

as well as to investigate the structural offset of the main laccolith 

within the Rosillos Mountains. This modeling shows a variation in 

offset along a fault in the northern end of the Rosillos Mountains, 

which offsets the main intrusive body by nearly 500 m; the offset 

decreases toward the south to only 200 m.

Geologic Mapping

Current geologic mapping in BBNP is part of a large project designed to provide a new 1:100,000-scale map of the park 

because BBNP has not been mapped since the 1960s (Maxwell, 

1968). USGS geologists cooperated with colleagues from academia 

and the NPS to identify and compile existing mapping, as well as 

provide new mapping where required. A portion of the new BBNP 

map compilation (fig. 7) illustrates the level of detail that will be 

portrayed on the new map. In addition to mapping BBNP, USGS 

geologists are also studying a number of research topics related to 

structural geology, tectonics, volcanology, petrology, stratigraphy, 

paleontology, and geomorphology of surficial deposits.
103°15 ́

29°30 ́

103°15 ́

29°22 ́30 ̋

103°07 ́30 ̋

103°07 ́30 ̋

LIST OF MAP UNITS

0 1 2 3 4 KILOMETERS

Young alluvium (Holocene to Pleistocene)

Active wash (Holocene)

Younger

Older

Spring deposits (Holocene and Pleistocene)

Colluvium (Holocene and Pleistocene)

Rock fall (Holocene and Pleistocene)

Landslide (Holocene and Pleistocene)

Intermediate alluvium (Pleistocene)

Younger

Older

Intermediate river deposits (Pleistocene)

Old alluvial deposits (Pleistocene)

Extremely old alluvial deposits (Pleistocene

 and Pliocene)

Igneous intrusive rocks, undivided (Tertiary)

Canoe Formation (Tertiary)

Hannold Hill Formation (Tertiary)

Black Peaks Formation (Tertiary and Cretaceous) 

Javelina Formation (Cretaceous)

Aguja Formation (Cretaceous)

Pen Formation (Cretaceous)

Boquillas Formation (Cretaceous)

Buda Limestone (Cretaceous)

Figure 7. Preliminary geologic map compilation of parts of the Grapevine Hills and McKinney Springs quadrangles. Map

represents compilation of new and pre-existing bedrock mapping and new surficial mapping for the new 1:100,000-scale

Big Bend National Park geologic map.

References Cited

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.

Drenth, B.J., and Finn, C.A., in press, Aeromagnetic mapping of the structure of Pine 

Canyon caldera and Chisos Mountains intrusion, Big Bend National Park, Texas: 

Geological Society of America Bulletin.

Maxwell, R.A., 1968, The Big Bend of the Rio Grande: Texas Bureau of Economic 

Geology Guidebook 7, 138 p. 

By John E. Gray, Carol A. Finn, Lisa A. Morgan, William R. Page, and Wayne C. Shank

For more information contact:

John E. Gray

U.S. Geological Survey

P.O. Box 25046, MS 973

Denver, CO 80225

email: jgray@usgs.gov

Geochemical and geophysical data available on line at: http://minerals.cr.usgs.gov/

projects/big_bend/index.html