U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany Scientific Investigations Map 3142

2011

Prepared in cooperation with the National Park Service

Geologic Map of

Big Bend National Park, Texas
Front Cover Photographs (left to right):

Ocotillo bloom (Daniel P. Miggins)

Yucca bloom (Daniel P. Miggins)

Burro Mesa near pouroff (Robert B. Scott)

Santa Elena Canyon (NPS Big Bend Visitor Photo Gallery)

Volcanic landscape near Castolon (NPS Big Bend Visitor Photo Gallery)

Mule Ear Peaks (NPS Big Bend Visitor Photo Gallery)
Prepared in cooperation with the National Park Service

Scientific Investigations Map 3142

U.S. Department of the Interior

U.S. Geological Survey

By Kenzie J. Turner, Margaret E. Berry,William R. Page,Thomas 

M. Lehman,Robert G. Bohannon,Robert B. Scott,Daniel P. Miggins,

James R. Budahn, Roger W. Cooper, Benjamin J. Drenth,

Eric D. Anderson,and Van S. Williams

Geologic Map of Big Bend National Park, Texas
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Turner, K.J., Berry, M.E., Page, W.R., Lehman, T.M., Bohannon, R.G., Scott, R.B., Miggins, D.P., Budahn, J.R., Cooper, 

R.W., Drenth, B.J., Anderson, E.D., and Williams, V.S., 2011, Geologic map of Big Bend National Park, Texas: U.S. 

Geological Survey Scientific Investigations Map 3142, scale 1:75,000, pamphlet 84 p.
iii

Contents

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

Methods...........................................................................................................................................................7

Description of Map Units..............................................................................................................................9

Surficial Deposits..................................................................................................................................9

Eolian Deposits.............................................................................................................................9

Hillslope and Mass-Movement Deposits.................................................................................9

Spring Deposits............................................................................................................................9

Alluvial and Basin-Fill Deposits.................................................................................................9

Tertiary Mass-Movement Deposits ........................................................................................12

Undivided Surficial Deposits....................................................................................................12

Middle Tertiary Volcanic and Intrusive Rocks ...............................................................................13

Lower Tertiary to Lower Cretaceous Sedimentary Rocks ...........................................................17

Paleozoic Rocks..................................................................................................................................20

Stratigraphy ..................................................................................................................................................21

Paleozoic Rocks..................................................................................................................................21

Lower Cretaceous to Lower Tertiary Sedimentary Rocks ...........................................................22

Middle Tertiary Volcanic Rocks .......................................................................................................30

Chisos Formation........................................................................................................................30

Undivided Older and Younger Parts of the Chisos Formation ...................................31

Lava and Ash-Flow Members of the Younger Part of the Chisos Formation ..........34

Revision of South Rim Formation and Definition of Burro Mesa Formation ....................37

South Rim Formation ................................................................................................................39

Trachytic Lava, Undivided ........................................................................................................42

Burro Mesa Formation..............................................................................................................44

Alluvial Deposits..................................................................................................................................45

Late Tertiary to Quaternary Basin Fill .....................................................................................45

Rio Grande Integration History................................................................................................46

Pleistocene and Holocene Landscape Development..........................................................48

Structure........................................................................................................................................................50

Proterozoic Rifting .............................................................................................................................50

Paleozoic Marathon Orogeny ..........................................................................................................50

Triassic to Cretaceous Rifting...........................................................................................................52

Late Cretaceous–Early Tertiary Laramide Orogeny ......................................................................52

Middle Tertiary Volcanic and Intrusive Features .........................................................................58

Pine Canyon Caldera.................................................................................................................58

Sierra Quemada .........................................................................................................................60

Dominguez Mountain ................................................................................................................60

Major Laccoliths and Sills........................................................................................................60

Tectonic Setting for BBNP Magmatism.................................................................................62

Late Tertiary Basin-and-Range Faulting .........................................................................................62

Quaternary Landslides.......................................................................................................................66
iv

Summary........................................................................................................................................................68

Acknowledgments.......................................................................................................................................70

References Cited..........................................................................................................................................72

Appendix........................................................................................................................................................79

Figures

1. Index map of BBNP showing geographic location of features ..................................................2

2. Generalized map of major structures and geographic features in BBNP..................................3

3. Index of geologic mapping used in map compilation.....................................................................6

4. Diagrammatic relationships between the older and younger parts of the

Chisos Formation, Canoe Formation, and underlying Tertiary–Cretaceous

sedimentary rocks ......................................................................................................................31

5. Total alkali-silica diagram for the Ash Spring Basalt and Alamo Creek

Basalt Members of the Chisos Formation ..............................................................................35

6. Total alkali-silica diagram for lavas including the Bee Mountain Basalt and

Tule Mountain Trachyandesite Members of the Chisos Formation,

trachytic lava that is divided into classifications, and possibly related dikes

of the Dominguez dike swarm ..................................................................................................35

7. High Field Strength Element (HFSE) ratio plot for the Bee Mountain Basalt and

Tule Mountain Trachyandesite Members of the Chisos Formation,

trachytic lava that is divided into classifications, and dikes related to

the Dominguez Mountain dike swarm ....................................................................................36

8. Examples of pediment sequences in BBNP ..................................................................................48

9. Map showing transform faults and lineaments related to Proterozoic rifting

along the southern North American continental margin.....................................................51

10. Map showing location of Paleozoic Marathon orogenic belt between the

North and South American plates ...........................................................................................51

11. Map showing location of the Diablo platform and the Chihuahua trough................................52

12. Generalized map of the major structures that formed in the BBNP area

during the Laramide orogeny....................................................................................................53

13. Reduced-to-pole magnetic anomaly map of northern BBNP......................................................61

Tables

1. Stratigraphic table comparing units of the Chisos Formation in this report

with units of Maxwell and others (1967) in BBNP, and Henry and Davis (1996)

in adjacent Big Bend Ranch State Park..................................................................................30

2. Stratigraphic table comparing units of the South Rim and Burro Mesa Formations

in this report with the South Rim Formation of Maxwell and others (1967) and

Barker and others (1986) ...........................................................................................................37
By Kenzie J. Turner,1

 Margaret E. Berry,1 William R. Page,1

Thomas M. Lehman,2

 Robert G. Bohannon,1 Robert B. Scott,3

Daniel P. Miggins,1

 James R. Budahn,1 Roger W. Cooper,4

Benjamin J. Drenth,1

 Eric D. Anderson,1 and Van S. Williams1

Introduction

The purpose of this map is to provide the National Park 

Service and the public with an updated digital geologic map 

of Big Bend National Park (BBNP). The geologic map report 

of Maxwell and others (1967) provides a fully comprehensive 

account of the important volcanic, structural, geomorphological, and paleontological features that define BBNP. 

However, the map is on a geographically distorted planimetric 

base and lacks topography, which has caused difficulty in 

conducting GIS-based data analyses and georeferencing the 

many geologic features investigated and depicted on the 

map. In addition, the map is outdated, excluding significant 

data from numerous studies that have been carried out since 

its publication more than 40 years ago. 

This report includes a modern digital geologic map that 

can be utilized with standard GIS applications to aid BBNP 

researchers in geologic data analysis, natural resource and 

ecosystem management, monitoring, assessment, inventory 

activities, and educational and recreational uses. The digital 

map incorporates new data, many revisions, and greater detail 

than the original map of Maxwell and others (1967). Locations 

for features discussed in the text are shown in figures 1 and 2. 

Although some geologic issues remain unresolved for BBNP, the 

updated map serves as a foundation for addressing those issues.

Funding for the Big Bend National Park geologic map 

was provided by the United States Geological Survey (USGS) 

National Cooperative Geologic Mapping Program and the 

National Park Service. The Big Bend mapping project was 

administered by staff in the USGS Geology and Environmental 

Change Science Center, Denver, Colo. Members of the USGS 

Mineral and Environmental Resources Science Center completed investigations in parallel with the geologic mapping 

project. Results of these investigations (Gray and Page, 2008) 

addressed some significant current issues in BBNP and the 

U.S.-Mexico border region, including contaminants and human 

health (Gray and others, 2008), ecosystems (Shanks and others, 

2008a), and water resources (Shanks and others, 2008b). Funding for the high-resolution aeromagnetic survey in BBNP, and 

associated data analyses and interpretation, was from the USGS 

Crustal Geophysics and Geochemistry Science Center. Mapping 

contributed from university professors and students was mostly 

funded by independent sources, including academic institutions, 

private industry, and other agencies.

1

U.S Geological Survey, Denver, Colorado

2

Texas Tech University, Lubbock, Texas

3

U.S. Geological Survey, Corvallis, Oregon

4

Lamar University, Beaumont, Texas

Geologic Map of

Big Bend National Park, Texas
MEXICO MEXICO

TEXAS

TEXAS

BS DC

DF

CM

TE

FB SP

PG

LC

CR SM

DM

LM

MM BA

AS ES PB PA KB

BU VB

CG TU NM EB

EP JC

RA BP SR

BO

BQ

BR

SA PM

CH KM

GM

BE

SE

TC

HC CC

RM TA

CA

PS

RC

CO TS

SC

GL

TM

GS

BM

TO

DS

HH

PE

GO

Alamo Creek

Smoky Creek

Fresno Creek

Tornillo Creek

Blue Creek

Javelina Creek

Terlingua

Creek

Dawson

Creek

Rio Grande

Rio Grande

Mariscal Mountain

Rosillos Mountains

Telephone Canyon

Tornillo Flat

0 5 10 15 20 KILOMETERS

103°30’ 103°00’

29°00’

29°30’

Geographic location of features

Paved roads

Unimproved roads

EXPLANATION

CM

Ash Spring, AS

Bantu Shut-In, BA

Bee Mountain, BE

Black Mesa, BM

Blue Creek Ranch, BR

Bone Spring, BS

Boquillas Crossing, BQ

Boquillas Canyon, BO

Burro Mesa, BU

Burro Mesa Pouroff, BP

Casa Grande Peak, CG

Castolon, CA

Cerro Castellan, CC

Chilicotal Mountain, CH

Christmas Mountains, CM

Cow Heaven Mountain, CO

Croton Peak, CR

Dagger Flat, DF

De la Ho Spring, DS

Dog Canyon, DC

Dogie Mountain, DM

Emory Peak, EP

Ernst Basin, EB

Estufa Spring, ES

Fossil Bone Exhibit, FB

Glenn Spring, GL

Goat Mountain, GM

Grapevine Spring, GS

Government Spring, GO

Hannold Hill, HH

Horseshoe Canyon, HC

Juniper Canyon, JC

K-Bar Park housing, KB

Kit Mountain, KM

Little Christmas Mountain, LC

Lone Mountain, LM

Maverick Mountain, MM

Nugent Mountain, NM

Paint Gap Hills, PG

Panther Spring, PA

Pena Mountain, PM

Persimmon Gap, PE

Pulliam Bluff, PB

Punta de la Sierra, PS

Rattlesnake Mountain, RA

Round Mountain, RM

Reed Camp, RC

Santa Elena Canyon, SE

Sierra Aguja, SA

Sierra de Chino, SC

Slickrock Mountain, SM

South Rim, SR

Sue Peaks, SP

Talley Mountain, TA

Terlingua, TE

Tortuga Mountain, TO

Trap Mountain, TM

Triangulation Station Mountain, TS

Tuff Canyon, TC

Tule Mountain, TU

Vernon Bailey Peak, VB

Figure 1. Index map of BBNP showing geographic location of features (keyed to red abbreviations) discussed in the text: 

2 Geologic Map of Big Bend National Park, Texas
DB

SB

DM

GH

MH

EB

SR

MM

WM TB

PP

PC

MA

CD

SD

SD

SM

RM

PALEOZOIC

ROCKS

BURRO MESA

 FAULT

CHALK DRAW FAULT

DUGOUT WELLS FAULT 

MEXICO MEXICO

TEXAS

SQ

TERLINGUA

FAULT

TEXAS

Rio Grande

Rio Grande

0 5 10 15 20 KILOMETERS

Early Quaternary–Late Tertiary basin fill

Tertiary intrusive bodies, undivided

Tertiary volcanic rocks, undivided

Tertiary–Cretaceous clastic sedimentary rocks, undivided

Cretaceous carbonate sedimentary rocks, undivided

Paleozoic rocks, undivided

Contact

Fault—Bar and ball on downthrown side

Thrust fault

Anticline

Boundary of Chisos Mountains

Pine Canyon caldera

EXPLANATION

29°00’

29°30’

103°30’ 103°00’

Chalk Draw basin, CD

Delaho bolson, DB

Dominguez Mountain, DM

Estufa bolson, EB

Grapevine Hills, GH

Mariscal Mountain, MM

Mesa de Anguila, MA

McKinney Hills, MH

Pine Canyon caldera, PC

Figure 2. Generalized map of major structures and geographic features in BBNP. Features labeled on map include:

Pulliam Peak, PP

Rosillos Mountains, RM

Santiago Mountains, SM

Sierra del Carmen, SD

Sierra Quemada, SQ

Solis basin, SB

South Rim, SR

The Basin, TB

Ward Mountain, WM 

Introduction 3
4 Geologic Map of Big Bend National Park, Texas

 

Prominent Researchers 

in the History of Big Bend Geology

Robert T. Hill

Member of the 1899 USGS expedition with 

boat in Santa Helena (Elena) Canyon, Big 

Bend National Park. The expedition was led 

by Robert T. Hill, USGS geologist.

Robert T. Hill, U.S. Geological Survey geologist and father of Texas Geology, was one of the 

first to explore the Big Bend region in the 1899 survey of the canyons of the Rio Grande. Hill led a 

six man expedition from Presidio to Langtry, Texas, and published an article describing the survey 

in one of the first issues of Century Illustrated Magazine in 1901, titled “Running the cañons of the 

Rio Grande.” Hill was hired by John Wesley Powell (first Director of the U.S. Geological Survey) in 

1885. His expertise was stratigraphy of the Cretaceous rocks in Texas, and he discovered and defined 

the Comanchian series of the Lower Cretaceous system as a result of his Texas studies. In his career, 

he went on to author more than 100 publications on the geology of Texas and adjacent states, mineral 

deposits in Mexico, and the geology of Puerto Rico, Cuba, and Jamaica. 

Introduction 5

Ross Maxwell, first superintendent of Big Bend National Park, shown here in the1980s.

Ross A. Maxwell

Ross A. Maxwell became the first superintendent of Big Bend National Park in 1944. 

As an effective administrator, Maxwell not only ran park operations, including the building of 

roads, but he also began mapping the park’s geology. When he began mapping, the park had no 

paved roads, no electricity, and the nearest telephone was 100 miles away. Because no topographic maps existed then, the geologic information was superimposed on vintage planimetric 

maps that showed only approximate locations of roads, drainage patterns, and mountain peaks. 

By the late 1960s, Maxwell and his colleagues had published a remarkable geologic map and 

supporting text (Maxwell and others, 1967), and a geologic and cultural history of the park 

for the public (Maxwell, 1968). We recognize Ross A. Maxwell and his co-workers for their 

invaluable research contributions and map of Big Bend National Park.
EXPLANATION

2

3

4

4

5

5

6

6

6

6

7 7

7

8

9

9

10

13 19

11

11

14 15 16

17

5

5

6

6

6

1*

18

12

7

 1* M.E. Berry and V.S. Williams, new mapping this report; 

 surficial deposits throughout park with the exception of 

 reference 16 where primary mapping was integrated 

 by M.E. Berry

 2 R.B. Scott, W.R. Page, and L. Snee, new mapping this report

 3 Poth (1979)

 4 Moustafa (1988)

 5 R.W. Cooper and D.A. Cooper, new mapping this report

 6 T. Lehman, new mapping this report

 7 R.B. Scott and L. Snee, new mapping this report

 8 Henry and others (1989)

 9 K. Wache, new mapping this report

 

10 M.S. Stevens and J.B. Stevens, unpublished mapping

11 Bohannon (2011)

12 Ogley (1978)

13 DeCamp (1985)

14 Stevens (1969)

15 R.B. Scott, L. Snee, K.J. Turner, and D.P. Miggins, 

 new mapping this report

16 Collins and others (2008)

17 Maler (1987)

18 Adams (2004)

19 R.B. Scott, new mapping this project

103°30’ 103°00’

29°00’

29°30’

Figure 3. Index of geologic mapping used in map compilation.

6 Geologic Map of Big Bend National Park, Texas
Methods 7

Methods

The geologic map of BBNP (pl.) was compiled by 

combining geologic map data of many different authors from 

academia, Texas Bureau of Economic Geology, and USGS 

(fig. 3). Once existing mapping was digitally compiled, we 

evaluated the quality and consistency of each existing map 

by comparing it with digital orthophotographs, aerial photographs, and satellite imagery. Problem areas were identified and field checked, and unit contacts and structures were 

adjusted where necessary. In addition, as mapping was compiled into a comprehensive database, data gaps were identified 

and new mapping was completed in those areas. 

The map includes incorporation of new geologic mapping of Upper Cretaceous to Eocene sedimentary rocks 

throughout BBNP (Lehman, 2002, 2004, 2007). New mapping 

was completed in the high Chisos Mountains that sheds new 

light on Sierra Quemada (Scott and others, 2007), and on the 

character of other volcanic features in this highly inaccessible, 

and rugged terrain (Bohannon, 2011). Other areas of new 

mapping include the recent northern addition to BBNP (the 

Harte Ranch section acquired in 1987), which includes parts 

of the Twin Peaks, Butterbowl, Persimmon Gap, and Bone 

Spring 7.5-minute quadrangles (R.B. Scott, W.R. Page, and 

L. Snee, unpub. mapping), and where little detailed mapping 

had been completed. Most of the new mapping for BBNP 

was carried out using traditional geologic mapping methods, 

which include extensive fieldwork and recording data using 

1:40,000-scale color-infrared and black-and-white aerial photographs, and GPS technology. In most cases, mapping data 

were then transferred and georeferenced onto 1:24,000-scale 

topographic maps, and then converted to digital datasets.

The lack of detailed mapping of surficial deposits 

required extensive new mapping, which was completed for 

the entire BBNP with a combination of field and remote 

sensing techniques (Berry and Williams, 2004, 2007, 2008). 

Surficial geologic units were interpreted from stereoscopic 

pairs of 1:40,000-scale (color-infrared and black-and-white) 

aerial photographs, satellite imagery, and topographic data, 

and digitally mapped on color-infrared orthophotograph 

images with 1-meter ground resolution using VRone® mapping software; for some areas, a computerized PG-2 instrument in the Denver photogrammetry laboratory was used 

with 1:80,000-scale aerial photograph diapositive film to aid 

mapping. Surface morphology, tone, relative height above 

modern stream channel, and map pattern were used to interpret surficial geology from the imagery. Maps were printed 

on a topographic base and field checked, at which time soil, 

weathering, and pavement characteristics were used to refine 

interpretation of mapping units. 

Material suitable for radiocarbon dating is scarce and 

Quaternary volcanic ashes have not been found in BBNP, so 

relative-age criteria were used to interpret age relations of the 

surficial deposits. Criteria used to evaluate 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. Stages of soil-carbonate morphology follow criteria outlined in Birkeland (1999), modified from Gile and others (1966, 1981) and Machette (1985). Age estimates for time 

divisions within the late Tertiary and Quaternary are based on 

the 2009 Geological Society of America Geologic Time Scale 

(Walker and Geissman, 2009), and supplemental information 

from Head and others (2008)5, which place the base of the 

Quaternary at 2.6 Ma. This new mapping provides data critical 

in resource and ecosystem management activities in the park, 

including assessment, monitoring, inventory, groundwater 

studies, and in understanding the history of the Rio Grande.

Geologic mapping of volcanic and intrusive rocks was 

supported by 40Ar/39Ar and U-Pb isotopic age determinations 

and major and trace element analyses, and this report presents 

new age determinations in the appendix for 86 samples. To 

quickly access sample data in the appendix for discussion of 

the new isotopic age determinations in the report, we cross 

referenced the Map sample number listed in the first column of 

the table in the appendix. The new isotopic and geochemical 

data were critical in understanding and refining the complex 

volcanic stratigraphy and structure of BBNP, and will serve 

as a foundation for further geologic investigations. A more 

complete and comprehensive review of the new geochronologic and geochemical data for BBNP will be published at a 

later time.

5

Pliocene, 5.3–2.6 Ma; early Pleistocene, 2.6 Ma–781,000 yr; middle Pleistocene, 781,000–126,000 yr; late Pleistocene, 126,000–11,700 yr; Holocene, 

11,700–0 yr.
Oldest intermediate axial river deposits (Qiw3) of the Rio Grande preserved in isolated terrace 37 m above the Rio Grande floodplain; 

Sierra del Carmen in background. (Photograph by Margaret E. Berry)

8 Geologic Map of Big Bend National Park, Texas
Description of Map Units 9

Description of Map Units

Surficial Deposits

Eolian Deposits

Qe Eolian sand (Holocene)—Unconsolidated, moderately sorted, silty fine to medium sand, 

locally intermixed with small amounts of fine gravel likely introduced by surface wash 

or bioturbation. Very pale brown. Commonly forms small coppice dunes (nabkhas) 

around base of brushy vegetation; individual dunes too small to map separately. Thickness generally less than 1 m

Hillslope and Mass-Movement Deposits

Qrf Rock fall deposits (Holocene and late Pleistocene)—Generally unsorted, angular, cobbleto boulder-sized rock fragments forming talus on steep slopes. Thickness variable from 

about 2 m to more than 6 m 

Qc Colluvium and colluvial-fan deposits (Holocene and Pleistocene)—Unconsolidated to 

moderately consolidated, unsorted to poorly sorted, unbedded to weakly bedded mixture 

of gravel, sand, silt, and clay, forming thin mantle or thicker fan-shaped accumulations 

of debris on the flanks of slopes. Cobble- to boulder-size clasts common. Clasts angular 

to subangular. Locally cemented by carbonate. Commonly ribbed and fluted due to various combinations of mass movement, rain splash, and wash processes active on slopes. 

Locally includes bedrock outcrops too small to map separately. Thickness variable from 

about 1 m to more than 6 m

Qls Landslide deposits (Holocene and Pleistocene)—Slumps, flows, and slides of bedrock, 

colluvium, and alluvium. Terrain typically hummocky or irregular with closed depressions and spires. Large areas of landslide deposits, such as at Chilicotal and Talley 

Mountains (Collins and others, 2007, 2008), represent multiple landslide events. Most 

likely Pleistocene in age. Thickness as much as 50 to 100 m

Spring Deposits

Qs Spring deposits (Holocene and Pleistocene)—Powdery to well-cemented, white, very palebrown, light brownish-gray, or gray, fine-grained, spring-generated calcium carbonate 

mixed with fine sand and silt; locally nodular or laminated. Mapped mostly near Grapevine Spring and northwest of Kit Mountain. Thickness about 5 m near Grapevine Spring

Alluvial and Basin-Fill Deposits

Qaw Active tributary wash and river deposits (latest Holocene)—Unconsolidated gravel, 

sand, and silt deposited in active channels and flood plains of larger creeks and the Rio 

Grande. Light gray, light brownish gray, grayish brown, pale brown or very pale brown. 

In washes, deposits typically poorly to moderately sorted and poorly to moderately bedded. Cobble- and small boulder-size clasts common. Clasts subangular to subrounded, 

locally rounded. Braided channels and bar-and-swale topography common. Along the 

Rio Grande, deposits moderately to well sorted and moderately to well bedded, with 

subrounded to well-rounded clasts. Flood-plain deposits made up mostly of fine-grained 

sediment. Meander scroll topography common. Near Cottonwood Campground (west of 

Castolon), flood-plain deposits dating to within the last 17 years are at least 3.5 m thick 

(Dean and Schmidt, 2008)

Qyw1 Younger of the young axial river deposits (Holocene)—Differentiated from other types of 

alluvial deposits only along the Rio Grande. Forms low-level terraces generally within 3 

m of active flood plain. Meander scroll topography commonly well preserved. Typically 

fine-grained deposits of unconsolidated to weakly consolidated sand and silt with pebble 

lenses and stringers. Near Boquillas Canyon, 14C date on organic material from buried A 

horizon suggests that upper 2 m of alluvium at that locale was deposited within the last 

3,000 yr (Mandel, 2002). Surfaces deflated locally by eolian processes, and covered in 

places by small dunes. Little or no pavement, varnish, or soil-carbonate development. 

Thickness 3 to 4 m or more
10 Geologic Map of Big Bend National Park, Texas

Qyw2 Older of the young axial river deposits (Holocene to late Pleistocene)—Differentiated 

from other types of alluvial deposits only along the Rio Grande. Unconsolidated to 

weakly consolidated, moderately to well-sorted, moderately to well-bedded gravel, 

sand, and silt forming low-level terraces generally 4–9 m above active flood plain. Limited preservation of meander scroll topography. Cobble- and small boulder-size clasts 

common; clasts typically subrounded to well rounded. Pavement moderately developed 

to densely packed and uniform with strong-brown, yellowish-red, dark reddish-brown, 

dark reddish-gray, and black varnish colors. Development of soil carbonate generally 

stage I+ or less6. Thickness less than 1 m to about 4 m

Qya Young alluvial deposits, undivided (Holocene to late Pleistocene)—Pediment, fan, 

stream, and sheet-flow deposits. Unconsolidated to weakly consolidated, poorly to moderately sorted, poorly to moderately bedded gravel, sand, and silt. Cobble- and small 

boulder-size clasts common. Locally mostly silty sand or sandy silt with interspersed 

fine gravel. Clasts typically subangular to subrounded, locally angular or well rounded. 

Typically has low relative height where alluvial surfaces are incised. Elsewhere may 

veneer older deposits. Includes Qaw too narrow to map separately. Unit subdivided 

into Qya1 and Qya2 where divisions are evident and large enough to map separately. 

Thickness variable from less than 1 m to 8 m or more

Qya1 Younger of the young alluvial deposits (Holocene)—On or within several meters of valley floor. Occasionally flooded and (or) modified by sheet flow. Bar-and-swale topography and braided-channel morphology commonly well preserved. Surfaces deflated 

locally by eolian processes, and covered in places by dunes too small to map separately. 

Little or no pavement, varnish, or soil-carbonate development. Thickness variable from 

less than 1 m to 8 m or more

Qya2 Older of the young alluvial deposits (Holocene to late Pleistocene)—Typically elevated 

3 m or more above valley floor. Limited preservation of bar-and-swale topography and 

braided-channel morphology. Surfaces commonly partly incised. Pavement weakly 

developed to densely packed and uniform. Pavement clasts lightly to well varnished 

with strong brown, yellowish-red, dark reddish-brown, and black varnish colors. Soilcarbonate development generally stage I or less. Thickness variable from less than 1 m 

to 8 m or more

Qiw Intermediate axial river deposits, undivided (late and middle Pleistocene)—Differentiated from other types of alluvial deposits only along the Rio Grande and upper Tornillo 

Creek. Weakly to moderately well-consolidated, moderately to well-sorted, moderately 

to well-bedded gravel, sand, and silt; sand locally crossbedded. In Rio Grande deposits, 

cobble- and small boulder-size clasts common to abundant. Clasts subrounded to well 

rounded. Some surface and subsurface clasts weathered. Unit subdivided into Qiw1, 

Qiw2, and Qiw3 where divisions are evident and large enough to map separately. 

Thickness commonly 3 to 4 m, but ranges from less than 1 m to more than 15 m

Qiw1 Youngest intermediate axial river deposits (late Pleistocene)—Terrace surfaces generally 12–18 m above Rio Grande flood plain. Surfaces typically broad and flat with a 

stage II soil-carbonate horizon, and a moderately to densely packed, uniform pavement. 

Pavement clasts moderately to well varnished; varnish colors typically strong brown, 

yellowish red, dark reddish brown, and black. Thickness commonly 3 to 4 m

Qiw2 Older intermediate axial river deposits (late to middle Pleistocene)—Terrace surfaces 

generally 18–25 m above Rio Grande flood plain. Surfaces typically broad and flat with 

a stage II to weakly cemented stage III soil-carbonate horizon 1 m or more thick, and a 

moderately to densely packed, uniform pavement. Pavement clasts moderately to well 

varnished; varnish colors typically strong brown, yellowish red and dark reddish brown. 

Thickness commonly 3 to 4 m

6

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. For further discussion, see Birkeland (1999). 
Description of Map Units 11

Qiw3 Oldest intermediate axial river deposits (middle Pleistocene)—Terrace surfaces generally 30–45 m above Rio Grande flood plain and about 40 m above upper Tornillo Creek 

flood plain. Surfaces vary from broad and flat to gently undulatory or irregular, with a 

stage I+ to moderately-cemented stage IV soil-carbonate horizon, and a weak to wellformed uniform pavement. Where pavement well preserved, pavement clasts moderately 

to well varnished; varnish colors typically yellowish red and dark reddish brown. Thickness commonly 3 to 4 m

Qia Intermediate alluvial deposits, undivided (late and middle Pleistocene)—Pediment, fan, 

and stream deposits. Weakly to moderately well consolidated, poorly to moderately 

sorted, poorly to moderately bedded gravel, sand, and silt. Cobble- and small bouldersize clasts common to abundant. Clasts typically subangular to subrounded, locally 

angular or rounded. Typically has intermediate relative height where alluvial surfaces 

are incised. Surfaces typically uniform due to loss of bar-and-swale topography and 

braided channel morphology (smoothed by post-depositional surface processes); partly 

incised, and slightly rounded at the edges. Pavement weakly developed to densely 

packed and uniform. Varnish on pavement clasts ranges from almost none where pavement contains mostly limestone clasts, to moderately varnished with strong-brown, 

yellowish-red, and dark reddish-brown colors. Locally includes Qaw and Qya too 

narrow or small to map separately. Unit subdivided into Qia1 and Qia2 where divisions are evident and large enough to map separately. Seismic data indicate that on the 

northeast side of the Chisos Mountains, stacked alluvial sequences that include Qia and 

buried older alluvium (Qoa) reach total thicknesses in excess of 50 m (Monti, 1984). 

Thickness of Qia variable from less than 1 m to about 10 m

Qia1 Younger of the intermediate alluvial deposits (late to middle Pleistocene)—Typically 

has stage I–II soil-carbonate horizon 1–1.5 m thick. Less commonly has weak to moderately cemented stage III carbonate horizon 30–50 cm thick, especially where deposit 

contains mostly limestone clasts. Some surface clasts weathered and (or) split. Pediment 

surfaces generally 6–24 m above valley floor. Along Rio Grande, intertongues with, and 

grades into or overlaps Qiw1 and Qiw2 terraces. Thickness variable from less than 1 m 

to about 10 m

Qia2 Older of the intermediate alluvial deposits (middle Pleistocene)—Typically has weakly 

to moderately cemented stage III–IV soil-carbonate horizon 1 m or more thick. Weathered and (or) split surface and subsurface clasts common at some locales. Has higher 

relative height than Qia1 where alluvial surfaces are incised; pediment surfaces generally 12–40 m above valley floor. Along upper Tornillo Creek, grades to Qiw3 terrace. 

Thickness variable from less than 1 m to about 10 m

Qow Old axial river deposits (middle to early? Pleistocene)—Differentiated from other types 

of alluvial deposits primarily along the Rio Grande; mostly preserved as isolated terrace 

remnants 55 to 60 m above flood plain. Weakly to well-consolidated, poorly to moderately well sorted, moderately bedded gravel, sand, and silt; sand commonly crossbedded. Pebble-, cobble-, and boulder-size clasts common to abundant. Clasts mostly 

subrounded to well rounded. Where preserved, soil has weakly cemented stage II+–III 

soil-carbonate horizon; pavement moderately packed and uniform with strong-brown, 

yellowish-red, and dark reddish-brown varnish colors. Thickness typically 2 to 6 m

Qoa Old alluvial deposits, undivided (middle to early? Pleistocene)—Pediment, fan, and 

stream deposits. Typically moderately to well-consolidated, poorly to moderately 

sorted, poorly to moderately bedded gravel, sand, and silt. Cobble- and small bouldersize clasts common to abundant. Clasts typically subangular to subrounded, locally 

angular or rounded. Has higher relative height than Qia where alluvial surfaces are 

incised. Surfaces dissected into ridge and ravine (ballena) topography where gravel 

deposits are thick (in fans), or preserved as planar to gently undulatory remnants 30–50 

m above valley floor where gravel deposits are thin (on pediments). Where preserved, 

soils have weakly to moderately cemented stage IV carbonate horizon 1–1.5 m or more 

thick. Weathered surface and subsurface clasts common. Pavement ranges from weakly 

developed to moderately packed and uniform. Varnish on pavement clasts ranges from 

none where pavement contains mostly limestone clasts, to moderately varnished with 
12 Geologic Map of Big Bend National Park, Texas

yellowish-red and dark reddish-brown colors. Locally includes Qaw and Qya too narrow or small to map separately. Thickness variable from less than 1 m to about 15 m, 

but based on seismic data, may be as much as 30 m thick on the north side of the Chisos 

Mountains (Monti, 1984)

QTa Very old alluvium (early Pleistocene and Pliocene)—Mostly moderately to well-consolidated 

or indurated, poorly bedded, poorly to moderately sorted, alluvial-fan deposits of cobble 

gravel, sand and silt; clasts mostly subangular to subrounded. Also includes lesser 

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

moderately bedded, commonly crossbedded, gravelly stream deposits with subrounded 

to rounded clasts, and moderately consolidated, fine-grained deposits of possible 

playa-lake origin. Beds tilted near faults. Deposits eroded and deeply dissected to form 

ridge and ravine (ballena) topography. Surface soils, where preserved, have cemented 

stage III–IV carbonate morphology in horizons as much as 2 m thick. Includes informal Fingers formation (Burro Mesa area; Stevens, 1969, 1988; Stevens and Stevens, 

1989), considered upper basin fill of the Delaho bolson (Lehman and Busbey, 2007), 

and informal Estufa Canyon formation (Stevens and Stevens, 1989, 2003; also called 

Estufa member by Thurwachter, 1984), comprising upper basin fill of Estufa bolson 

(lower Tornillo Creek area). In Sierra del Carmen, where very old alluvium has not been 

studied in detail, unit may also include some undifferentiated lower basin-fill deposits 

(Ta). Thickness 18 to 208 m for informal Fingers formation (Stevens, 1988), and about 

284 m for informal Estufa Canyon formation (Stevens and Stevens, 1989)

Ta Basin-fill deposits (Miocene)—Alluvial-fan, stream, and playa deposits in fault-bounded 

basins (bolsons). Weakly to well-consolidated or indurated, poorly to moderately bedded, poorly to moderately sorted gravel, sand, silt, and clay. Formerly combined with 

Quaternary deposits by Maxwell and others (1967). Deeply dissected. Beds tilted by 

faulting; in Cerro Castellan area, dips as steep as 20°–35° reported (Maxwell and others, 

1967). Clasts subangular to rounded. Near Castolon, deposits formally named Delaho 

Formation by Stevens and others (1969). Delaho Formation consists of pinkish- to 

brownish-buff siltstone, sandstone, mudstone, and gray conglomerate deposited in 

Delaho bolson (Stevens and Stevens, 1985, 1989). Lower (unnamed) member of Delaho 

Formation contains earliest Miocene vertebrate fossils of Castolon local fauna (Stevens 

and others, 1969; Stevens, 1977, 1991), and near De la Ho Spring, is interbedded near 

its base with thin basaltic flow dated by K-Ar method at 23.3 ± 0.6 Ma (Stevens, 1988; 

Stevens and Stevens, 1989). Upper Smoky Creek Member of Delaho Formation disconformably overlies lower member and contains few fossils but is proposed as late early 

Miocene (Stevens and Stevens, 1989) or middle? Miocene (Stevens, 1988) in age. In 

lower Tornillo Creek area, deposits named informal Banta Shut-in formation by Stevens 

and Stevens (1989, 2003; also called informal La Noria member by Thurwachter, 1984). 

Informal Banta Shut-in formation consists of pinkish- to reddish-buff sandstone and 

siltstone, red mudstone, and pinkish-gray conglomerate, deposited as lower basin fill in 

Estufa bolson. Contains late Miocene vertebrate fossils of the Screw Bean local fauna. 

Thickness about 670 m for Delaho Formation (Stevens, 1988), and 300 to 360 m for 

Banta Shut-in formation (Stevens and Stevens, 2003)

Tertiary Mass-Movement Deposits

Tgs Gravity slide blocks (Miocene?)—Blocks of Cretaceous limestone interpreted as gravity 

slide blocks (Maxwell and others, 1967; Poth, 1979) on southwest flank of the southern 

Santiago Mountains; age and origin unknown, but likely formed from uplift of Santiago 

Mountains related to Neogene basin-and-range faulting. Blocks consist mostly of Glen 

Rose Limestone, Boquillas Formation, and Buda Limestone, interpreted to have slid 

chiefly over Pen and Aguja Formations; stratigraphic order in blocks mostly preserved; 

5 to 20 m thick

Undivided Surficial Deposits

QTu Quaternary to Tertiary deposits, undivided (Holocene to Miocene)—Shown in cross 

section only
Description of Map Units 13

Middle Tertiary Volcanic and Intrusive Rocks

Tv Volcanic rocks, undivided (Oligocene)—Volcanic rocks of unknown composition overlying units of South Rim Formation at northern end of Burro Mesa; about 60 m thick

Tirc Intrusive complex at Rattlesnake Mountain (Oligocene)—Black to greenish-black 

analcime-bearing monzonite sill; contains bodies of syenite related to in-place differentiation of monzonite liquid (Carmen and others, 1975). Monzonite comprises about 

90 percent of intrusion; composed of 65–70 percent plagioclase and alkali feldspar, and 

ophitic augite, biotite, olivine, Fe-Ti oxides, analcime, and secondary minerals. Grain 

size varies from 0.5 mm at the chilled margin to 2–5 mm in the central zone. Syenite is 

lighter colored, medium to coarse hypoautomorphic to pegmatitic texture, composed of 

mostly alkali feldspar with lesser plagioclase, ophitic to subophitic pyroxene, amphibole, and accessory minerals (Carmen and others, 1975). K-Ar ages 28.6 ± 0.4 Ma and 

27.9 ± 0.4 Ma (Henry and McDowell, 1986); sill about 80 m thick

Ti Intrusive rocks, undivided (Oligocene to Eocene)—Scattered intrusive bodies and dikes 

throughout park of unknown composition and includes intrusions of multiple compositions such as Pena Mountain (28.77 ± 0.08 Ma; appendix; 75; and 28.7 ± 0.4 Ma; appendix; 74), which is compositionally similar to intrusive complex at Rattlesnake Mountain 

(Tirc). Also shown in east end of cross section B-B' as undivided intrusive rocks 

Tir Rhyolitic and other felsic composition intrusive rocks, undivided (Oligocene to 

Eocene)—Sills, dikes, and other irregular bodies with rhyolitic and other felsic compositions as defined below. Where chemical analysis available, includes rocks with SiO2

greater than 63 percent (volatile-free), and where chemical analysis unavailable, composition based on determinations of Maxwell and others (1967) to include quartz syenite, 

quartz diorite, and granite. Unit mostly exposed southeast and northwest of Chisos 

Mountains. Isotopic age determinations range from 41.6 ± 0.4 Ma (Maverick Mountain; 

appendix; 85) to 28.1 ± 0.3 Ma (Tortuga Mountain; appendix; 72). Also includes intrusions at Chilicotal and Talley Mountains, Government Spring (33.0 ± 0.2 Ma; appendix; 

84), Slickrock Mountain (31.7 ± 0.6 Ma; appendix; 80), Glenn Spring (30.22 ± 0.09 

Ma; appendix; 77), and intrusion encompassing Paint Gap Hills and Croton Peak

Tia Andesitic and other intermediate composition intrusive rocks, undivided (Oligocene 

to Eocene)—Sills, dikes, and other tabular bodies with andesitic and other intermediate compositions as defined below. Where chemical analysis available, includes 

rocks with SiO2 between 52 and 63 percent (volatile-free); where chemical analysis 

unavailable, composition based on determinations of Maxwell and others (1967) to 

include syenite, syenodiorite, diorite, and alkali syenite. Exposed north of Rosillos 

Mountains, north and west of Nugent Mountain (31.45 ± 0.18 Ma; appendix; 79), and 

southern end of Little Christmas Mountain 

Tib Basaltic and other mafic composition intrusive rocks, undivided (Oligocene to 

Eocene)—Sills, dikes, and other tabular bodies with mafic compositions as defined 

below. Where chemical analysis available, includes rocks with SiO2 less than 52 

percent (volatile-free); where chemical analysis unavailable, composition based on 

determinations of Maxwell and others (1967) to include basalt, diabase, gabbro, 

syenogabbro, and trachydolerite. Includes intrusive bodies at Bone Spring, Dagger 

Flat, Mariscal Mountain, Cow Heaven Mountain, Mesa de Anguila, and north of 

Rosillos Mountains

Tfb Basaltic flow (Oligocene)—Lava flows overlying Burro Mesa Formation at Burro Mesa. 

Consists of several basaltic trachyandesite aa flows (Henry and others, 1989); microphenocrysts of highly altered olivine, biotite, minor amphibole; groundmass hypocrystalline with feldspar laths and altered olivine and opaques; dated at 29.53 ± 0.33 Ma and 

29.10 ± 0.05 Ma (appendix; 1) 

Burro Mesa Formation (Oligocene)—Includes rhyolite and Wasp Spring members, and related intrusive 

rocks, erupted from local vents in western BBNP (Henry and others, 1989; Holt, 

1998; Adams, 2004). Descriptions from Becker (1976), Holt (1998), Adams (2004), 

and Benker (2005)
14 Geologic Map of Big Bend National Park, Texas

Tbr Rhyolite member—Blue-gray, rhyolitic ash-flow tuff and lava; contains anorthoclase 

and quartz phenocrysts and accessory minerals, including fayalite, arfvedsonite, acmite, 

and aenigmatite; upper part contains about 20 percent phenocrysts of anorthoclase 

with minor quartz and mafic inclusions (only present at Burro Mesa); rocks in lower 

part contain about 2 percent phenocrysts predominantly of alkali feldspar; lava often 

flow-banded and has ramp structures formed during emplacement of extrusive volcanic 

domes; dates range from 29.45 ± 0.08 Ma (appendix; 6) to 29.25 ± 0.07 Ma (appendix; 

2); exposed at Burro Mesa, Goat and Kit Mountains, and in scattered outcrops between 

Castolon and Chisos Mountains; about 120 to 150 m thick at Burro Mesa

Tbw Wasp Spring member—Gray, yellowish-gray, and brownish-gray surge deposits, 

debris-flow breccia, and ash-flow tuff; unit has tuffaceous matrix with abundant lithic 

fragments, fiamme, and volcanic blocks and bombs; dates range from 29.48 ± 0.16 Ma 

(appendix; 10) to 29.27 ± 0.07 Ma (appendix; 7); thickest exposures in southern Burro 

Mesa area along Blue Creek; about 30 to 100 m thick 

Tbi Intrusive rocks, undivided—Light-gray to blue-gray rhyolite, associated with plugs, 

domes, and dikes; mostly holocrystalline, fine- to medium-grained groundmass, and 

variably porphyritic with up to 15 percent phenocrysts of quartz and alkali feldspar commonly as glomerocrysts; interstitial arfvedsonite within groundmass; dike north of Blue 

Creek Ranch dated at 29.09 ± 0.07 Ma (appendix; 12), and intrusion west of Burro Mesa 

Pouroff dated at 29.03 ± 0.12 Ma (appendix; 11)

Tt Trachytic lava, undivided (Oligocene)—Black and dark-brown porphyritic trachyte, trachyandesite, and basaltic trachyandesite lava; basaltic trachyandesite groundmass has trachytic 

texture composed of plagioclase laths, opaque minerals, and altered pyroxene; phenocrysts 

include plagioclase and alkali-feldspar glomerocrysts, and altered pyroxene; trachyte 

and trachyandesite groundmass are intergranular to intersertal and composed of feldspar, 

clinopyroxene, and opaque minerals; phenocrysts include glomerocrysts of alkali feldspar 

with minor plagioclase and clinopyroxene. Trachyandesite likely stratigraphically below 

trachyte, relative stratigraphy of basaltic trachyandesite unknown; trachyandesite from Tuff 

Canyon dated at 30.42 ± 0.06 Ma (appendix; 15); trachyte from Goat Mountain dated at 

30.29 ± 0.05 Ma (appendix; 14) and from Blue Creek Ranch area dated at 30.23 ± 0.11 Ma 

(appendix; 13). Present in southwestern part of BBNP north and west of Punta de la Sierra 

southward to Sierra de Chino where previously mapped as Tule Mountain Trachyandesite 

Member of Chisos Formation by Maxwell and others (1967); about 100 to 250 m thick

Sierra Quemada related rocks (Oligocene)—Intrusive complex in southern part of Chisos Mountains 

composed primarily of rhyolitic intrusive rocks (Duex and Tucker, 1989; Duex and others, 1994; Duex, 2007; Scott and others, 2007)

Tqd Sierra Quemada ring dike—Pink, mildly altered, porphyritic rhyolite with 20 to 30 

percent phenocrysts and glomerocrysts of altered alkali feldspar and quartz; fine- to 

medium-grained groundmass with micrographic to granophyric texture; U-Pb zircon age 

of 29.93 ± 0.40 Ma (appendix; 21) 

Tqi Sierra Quemada intrusive rocks, undivided—Pink rhyolite, dark-gray gabbro, and 

medium-gray syenite intrusive bodies; rhyolite typically in dike-like intrusions and similar in composition to ring dikes (Tqd); gabbro intrusions contain plagioclase, pyroxene, 

and alteration minerals, including epidote, sulfide, and possibly fibrous arfvedsonite; 

syenite is porphyritic and arfvedsonite-bearing and has glomerocrystic feldspar with 

resorbed edges; U-Pb zircon ages range from 31.09 ± 0.48 Ma (appendix; 23) to 29.06 ± 

0.91 Ma (appendix; 19)

Tqv Sierra Quemada vent breccia—Light-pink, extremely lithic-rich volcanic breccia with 

minor rhyolitic ash matrix; lithic clasts consist of volcanic rocks, Cretaceous limestone, 

and Paleozoic rocks, and range in size from microscopic to Cretaceous limestone blocks 

200 m in length; greater than 150 m thick in central part of Sierra Quemada 

Dominguez Mountain related rocks (Oligocene)—Mafic to intermediate intrusive and volcanic complex 

to east of Punta de la Sierra, in southern Chisos Mountains (Maxwell and others, 

1967; Bohannon, 2011). Extensive dike swarm associated with complex is best 

exposed on west side where dikes appear to merge upward with trachytic lavas (Tt) 

(Bohannon, 2011)
Description of Map Units 15

Tdm Dominguez Mountain mafic lava flows—Unstudied dark-gray to black lava flows and 

mafic tuff in core of Dominguez Mountain; flows are of basaltic to andesitic composition; in places, metamorphosed to dense hornfels

Tdd Dominguez Mountain dike swarm—Dikes consist of wide range of compositions, 

including granite, rhyolite, syenite, trachyte, gabbro, and basalt. Intermediate dikes are 

abundant, particularly southwest of volcanic center where they pass upward into trachyte 

flows at Punta de la Sierra. Most rocks are porphyritic; mapped as a discrete unit where 

dike-on-dike intrusion forms a wide body; maximum dike thickness about 10 m 

Tdi Dominguez Mountain intrusive rocks, undivided—Light-gray to pale yellowish-brown 

rhyolite to dacite porphyry, but also includes more mafic compositions; rhyolite is finegrained, vuggy, and contains sparse white subhedral feldspar phenocrysts. Intermediate 

to mafic composition intrusions are coarse-grained and porphyritic olivine syenodiorite 

and syenogabbro (Maxwell and others, 1967) 

South Rim Formation (Oligocene)—Includes Emory Peak rhyolite, Boot Rock, and Pine Canyon rhyolite 

informal members and related intrusive rocks. Descriptions from Maxwell and others 

(1967), Barker and others (1986), Benker (2005), and White and others (2006) 

Tse Emory Peak rhyolite member—Gray, welded, devitrified, rhyolite ash-flow tuff, lava, 

and vitrophyre erupted from intracaldera and extracaldera vents; lava is holocrystalline 

and with minor granophyric texture, and contains up to 12 percent euhedral anorthoclase 

and 12 percent quartz; ash-flow tuff and vitrophyre are eutaxitic with up to 2 percent 

phenocrysts of alkali feldspar; accessory minerals include biotite, magnetite, monazite, 

and zircon. Exposed at Emory Peak and in Blue Creek Ranch area. Dates range from 

32.25 ± 0.07 Ma (appendix; 27) to 31.93 ± 0.13 Ma (appendix; 24); about 100 m thick

Tsb Boot Rock member—Gray to pinkish-gray quartz trachyte to rhyolite ash-flow tuff, 

lava flows, and maar surge breccia, erupted from intracaldera and extracaldera vents 

of the Pine Canyon caldera (Barker and others, 1986; White and others, 2006); lava 

has trachytic texture and ash-flow tuff is pilotaxitic to eutaxitic; contains less than 10 

percent euhedral to subhedral anorthoclase and quartz phenocrysts, and trace amounts 

of clinopyroxene, ilmenite, and magnetite (Benker, 2005). Intracaldera units contain 

subangular quartz trachyte and rhyolite lithic fragments; overlies Pine Canyon rhyolite 

member (Tsp) in Pine Canyon caldera area. Extracaldera units poorly to non-welded, 

and consist mostly of breccia and ash-flow tuff; exposed in South Rim area and northern 

Burro Mesa. Dated at 32.33 ± 0.07 Ma (appendix; 29) and 32.17 ± 0.09 Ma (appendix; 

28); about 100 to 120 m thick

Tsp Pine Canyon rhyolite member—Brown to light brownish-gray, densely welded, intracaldera rhyolitic tuff with pilotaxitic to eutaxitic texture; contains 2 to 7 percent euhedral 

to subhedral anorthoclase phenocrysts, and minor fayalite, apatite, hedenbergite, and 

ilmenite (Barker and others, 1986; Benker, 2005; White and others, 2006); exposed 

within Pine Canyon caldera; dated at 32.33 ± 0.07 Ma (appendix; 31) and 32.11 ± 0.23 

Ma (appendix; 30); thickness from 300 to greater than 500 m

Tsr Outflow deposits, undivided—Includes unstudied outflow deposits, which may include 

Boot Rock and (or) Emory Peak rhyolite members; exposed west of Pulliam Peak and 

near Blue Creek Ranch

Tsd Ring dike—Reddish-brown to tan rhyolite with granophyric groundmass; contains 5 to 7 

percent phenocrysts of quartz and sanidine and glomerocrysts of highly altered sanidine. 

Unit includes Hayes Ridge ring dike with a U-Pb zircon age of 32.6 ± 0.3 Ma, and an 

40Ar/39Ar age on K-feldspar of 31.99 ± 0.32 Ma (appendix; 32); about 20 to 400 m thick

Tsi Intrusive rocks, undivided—Consists of pre- and post-caldera intrusive rocks; 1 to 7 

percent phenocrysts of highly to mildly altered alkali feldspar, unaltered alkali feldspar, quartz with resorbed edges, and clinopyroxene, orthopyroxene, and euhedral 

arfvedsonite; groundmass varies from trachytic texture, with interstitial poikilitic alkali 

feldspar, to granular and micrographic texture; interstitial arfvedsonite common. Unit 

includes intrusions at Ward Mountain and Pulliam Peak (32.4 ± 0.4 Ma; appendix; 37), 

K-bar area (32.4 ± 0.3 Ma; appendix; 36), Nugent Mountain (31.5 ± 0.3 Ma, appendix; 

33), Lone Mountain, Panther Spring, and other smaller intrusions in The Basin area
16 Geologic Map of Big Bend National Park, Texas

Tigh Fayalite syenite of Grapevine Hills (Oligocene)—Gray, fine- to medium-grained syenite; 

consists of euhedral phenocrysts of alkali feldspar, quartz, fayalite, and pyroxene; minor 

granophyric and porphyritic texture with phenocrysts of carlsbad-twinned alkali feldspar. 

U-Pb zircon age of 31.9 ± 0.2 Ma, and an 40Ar/39Ar age on K-feldspar at 31.62 ± 0.30 Ma 

(appendix; 81); about 200 m thick

Timh Fayalite syenite of McKinney Hills (Oligocene)—Blue to blue-green, fine-grained syenite; 

consists of subhedral phenocrysts of alkali feldspar, quartz, fayalite, augite, plagioclase, and 

hornblende. U-Pb zircon age of 32.2 ± 0.3 Ma (appendix; 83); individual bodies range from 

about 200 to 400 m thick

Tirm Syenite of Rosillos Mountains (Oligocene)—Blue to blue-green, fine- to medium-grained 

syenite; consists of euhedral to subhedral phenocrysts of alkali feldspar, augite, fayalite, and 

quartz. U-Pb zircon age of 32.1 ± 0.2 Ma (appendix; 82); about 200 to 600 m thick

Chisos Formation (Oligocene and Eocene)—Includes younger and older parts (table 1) described below. 

Rocks of younger part of Chisos Formation (Tcy) are widely exposed in western BBNP, 

but they also are exposed in the northeastern Chisos Mountains where they flank the Pine 

Canyon caldera, and unconformably overlie rocks of the older part of the Chisos Formation. Rocks of older part are restricted to the Chisos Mountains, east of Sierra Quemada and 

Burro Mesa

Tcy Younger part, undivided (Oligocene and Eocene)—Gray, white, red, pink, and brown tuffaceous clay, mudstone, sandstone, siltstone, tuff, conglomerate, and some lacustrine limestone. In western BBNP, base of unit defined by Alamo Creek Basalt, and variably includes 

lava and ash-flow members described below (Tcac, Tcas, Tcbm, Tcme, and Tctm). In 

Chisos Mountains, consists mostly of tuffaceous sedimentary rocks and most lava and ashflow members present in western BBNP are absent, but base of unit is locally defined by 

Ash Spring Basalt Member north of Pulliam Bluff and west of Panther Spring, and between 

Ward Mountain and Burro Mesa; in most places in Chisos Mountains, however, tuffaceous 

sedimentary rocks of unit unconformably overlie sedimentary rocks of older part of Chisos 

Formation. Contains Eocene mammal and turtle bone fragments, and fresh water gastropods (Maxwell and others, 1967). 40Ar/39Ar ages on sanidine range from 42.31 ± 0.10 Ma 

(appendix; 62) to 32.96 ± 0.06 Ma (appendix; 38), and U-Pb zircon ages at 45.3 ± 0.7 Ma 

(appendix; 63) and 42.6 ± 0.4 Ma (appendix; 62). About 400 to 600 m thick

Tctm Tule Mountain Trachyandesite Member (Oligocene)—Brownish-gray porphyritic 

trachyte to trachyandesite lava; phenocrysts and glomerocrysts consist of skeletal 

plagioclase, alkali feldspar, clinopyroxene, and orthopyroxene. Exposed at Burro 

Mesa, Tule Mountain, and Sierra Aguja. 40Ar/39Ar ages on groundmass concentrate 

range from 33.88 ± 0.54 Ma (appendix; 40) to 33.18 ± 0.10 Ma (appendix; 39). About 

20 to 60 m thick 

Tcbm Bee Mountain Basalt Member (Oligocene)—Dark-gray, fine- to medium-grained, 

vesicular to nonvesicular, basalt to basaltic trachyandesite lava; includes phenocrysts 

of plagioclase, olivine, and clinopyroxene. Vesicles often filled with secondary minerals of calcite, zeolite, and microcrystalline quartz. In southwestern BBNP, consists of 

several flows above and below Mule Ear Spring Tuff Member. Dates range from 34.03 

± 0.17 Ma (appendix; 52) to 33.07 ± 0.15 Ma (appendix; 41); about 90 m thick in Blue 

Creek Ranch area, and 160 m at Bee Mountain and south of Cerro Castellan (Maxwell 

and others, 1967)

Tcme Mule Ear Spring Tuff Member (Oligocene)—Gray, brown, and red, densely to poorly 

welded, silicified ash-flow tuff; phenocrysts of alkali feldspar, biotite, and clinopyroxene. Generally overlain and underlain by tuffaceous sedimentary units, but in places in 

contact with Bee Mountain Basalt Member (Tcbm) above or below. Dates range from 

33.67 ± 0.09 Ma (appendix; 47) to 33.64 ± 0.08 Ma (appendix; 43); about 2 to 3 m thick

Tcl Undifferentiated lava flows (Eocene)—Lava flows east of Burro Mesa mapped by Maxwell and others (1967) as undifferentiated lava; lava is dark gray, porphyritic with finegrained groundmass; phenocrysts include large plagioclase laths up to 2 cm in length, 

and completely altered pyroxene and olivine. Dated at 38.87 ± 0.09 Ma (appendix; 53). 

About 20 m thick
Description of Map Units 17

Tcas Ash Spring Basalt Member (Eocene)—Dark-gray, porphyritic, fine-grained basaltic 

to trachyandesitic lava, with large white plagioclase laths from 1 to 2 cm long, and equant 

plagioclase grains and glomerocrysts; also contains olivine and pyroxene. 40Ar/39Ar age on 

groundmass concentrate of 40.92 ± 0.07 Ma (appendix; 58). Exposed in northern and western Chisos Mountains, and in Tule Mountain and Kit Mountain areas. About 30 m thick

Tcac Alamo Creek Basalt Member (Eocene)—Dark-gray, dense, fine-grained basalt to trachybasalt, and basaltic trachyandesite to trachyandesite; scoriaceous in places. Consists of several flows; locally, some of the flows are porphyritic and contain plagioclase phenocrysts 

and glomerocrysts, but in other places, such as the Alamo Creek area, lacks visible phenocrysts. Dates range from 47.09 ± 0.33 Ma (appendix; 71) to 44.41 ± 0.22 Ma (appendix; 

64). Defines base of Chisos Formation in western BBNP; about 30 to 60 m thick 

Older part, undivided (Eocene)—Includes 3 informal units of Bohannon (2011) described below. Restricted 

to Chisos Mountains, east of Sierra Quemada and Burro Mesa. Unconformably overlies 

Aguja Formation in southern Chisos Mountains, and Black Peaks Formation on eastern 

flank of Chisos Mountains 

Tco Older part, undivided—Shown in cross section only and includes units described below

Tcstr Sandstone, tuff, and rhyolite unit—Brown sandstone with accessory tuffaceous sandstone and rhyolite. Bedding is mostly parallel with even bedding surfaces and uniform 

bed thickness maintained over large distances. Many beds cross-stratified internally, 

others are more massive. Some beds have a conglomeratic base with clasts typically of 

limestone. Sand grains are mostly quartz; fragments of tuff and glass shards are abundant in places. Present in Fresno Creek area, south of Hayes Ridge, and around perimeter of Pine Canyon caldera; 690 m thick

Tcks Siltstone unit—Greenish, purplish, and gray siltstone, probably derived from Upper 

Cretaceous Javelina Formation (Kj), and white to brown quartz-rich beds that are probably altered and silicified air-fall tuff beds; unit altered to hornfels near Juniper Canyon. 

Intertongues with map unit Tcstr; thickness is variable with a maximum of 175 m near 

Juniper Canyon on the east flank of Chisos Mountains

Tcrt Rhyolite tuff unit—Light-pink to light-gray rhyolite tuff; fine-grained to very fine 

grained; in discontinuous, irregular beds a few centimeters to a few tens of centimeters 

thick; characteristically contains small (1–2 cm) medium-gray globular concretions. Most 

glass shards altered to clay. Also includes silt and clay with parallel, continuous, and even 

bedding, and interbedded layers of air-fall tuff and siltstone; tuff and siltstone beds are 

parallel to one another and are 3–50 cm thick, and bedding surfaces are even and beds 

are uniform in thickness. Beds of each lithology are internally laminated. Sandy lenses as 

thick as 1–2 m occur locally. Disconformably overlies metamorphosed Aguja Formation; 

present between Fresno Creek and Dominguez Mountain. About 65 m thick

Tx Christmas Mountains related volcanic rocks (Eocene)—Orangish-gray air-fall and ash-flow 

tuff, lava, and debris flows associated with initial eruptive events of the Christmas Mountains caldera complex; tuff is rhyolitic with up to 30 percent pumice, alkali feldspar phenocrysts, and sparse biotite; lava is porphyritic quartz trachyte with up to 20 percent alkali 

feldspar phenocrysts and altered pyroxene; groundmass has trachytic texture composed of 

alkali feldspar and interstitial to poikilitic quartz. K-Ar dates range from 42.8 ± 0.9 to 41.2 

± 0.9 Ma (Henry and others, 1989). About 246 m thick at Little Christmas Mountain, and 

about 40 m thick at Dogie Mountain

Lower Tertiary to Lower Cretaceous Sedimentary Rocks

Tc Canoe Formation (Eocene)—Light-gray, reddish-gray, brownish-gray, and yellowish-gray 

sandstone and conglomeratic sandstone, purple and gray tuffaceous mudstone and claystone, gray to whitish-gray nodular tuff, basaltic lava, and minor limestone at top of unit. 

Tuffaceous sediments and tuff dominate upper part of formation; base includes 10-m-thick 

Big Yellow Sandstone Member (Maxwell and others, 1967), consisting of fine- to coarsegrained sandstone; conglomeratic at base. Vertebrate fossils (Maxwell and others, 1967; 

Runkel, 1988; Busbey and Lehman, 1989; Lehman and Busbey, 2007), including a wide 

variety of mammals, some crocodile and turtle bones, and petrified wood. Unit mostly 

exposed in Tornillo Flat, but includes several small exposures in Estufa Spring and Dawson 

Creek areas. About 350 m thick
18 Geologic Map of Big Bend National Park, Texas

Thh Hannold Hill Formation (Eocene)—Mostly gray and red mudstone, and gray to reddishbrown conglomeratic sandstone; also contains some lignite beds and calcareous nodules. 

Unit subdivided into following units (Beatty, 1992), from top to base: upper mudstone, 

upper conglomeratic sandstone, lower mudstone, and Exhibit Sandstone Member, a basal 

conglomeratic sandstone. Exhibit Sandstone Member consists of conglomeratic sandstone 

with clasts of Cretaceous limestone, chert, sandstone, oyster shell fragments, and petrified wood. Unit contains abundant vertebrate fossils (Maxwell and others, 1967; Hartnell, 

1980; Busbey and Lehman, 1989; Lehman and Busbey, 2007). Unit exposed in Tornillo 

Flat area where it is unconformable with overlying Canoe Formation, and as much as 70 m 

thick 

TKbp Black Peaks Formation (Paleocene to Upper Cretaceous)—Mostly variegated gray, black, 

red, and white mudstone, and interbedded white, light-gray, grayish-brown, and yellowishgray, medium- to coarse-grained, partly conglomeratic sandstone; also includes freshwater 

limestone about 65 m above base containing gastropods and pelecypods. Sandstone contains reddish-brown “cannonball” concretions. Upper contact with Hannold Hill Formation 

disconformable; base of formation is marked by conglomeratic, crossbedded sandstone. 

Formation contains vertebrate fossils, including dinosaur, turtle, and crocodile bones, fish 

scales and bones, Paleocene mammals, and petrified logs and wood fragments (Maxwell 

and others, 1967; Schiebout, 1974; Schiebout and others, 1987; Busbey and Lehman, 

1989; Lehman and Busbey, 2007). About 300 to 400 m thick

Kj Javelina Formation (Upper Cretaceous)—Variegated, gray, brown, red, and purple calcareous mudstone, and yellowish-gray, yellowish-brown, and dark-brown, lenticular, crossbedded, partly conglomeratic sandstone. Mudstone contains calcareous nodules associated 

with paleosol zones; unit contains vertebrate fossils (Maxwell and others, 1967; Busbey 

and Lehman, 1989; Lehman and Busbey, 2007) and petrified wood; tuff bed in middle part 

of formation dated at 69.0 ± 0.9 Ma (Lehman and others, 2006); formation conformable 

with overlying Black Peaks Formation, and is about 100 to 200 m thick 

Ka Aguja Formation (Upper Cretaceous)—Consists of six informal members described below 

in descending order (Lehman, 1985): (1) upper shale member is light- to dark-gray and 

reddish shale and mudstone, and interbedded light-brown to reddish-brown sandstone near 

top; mudstone is carbonaceous and lignitic, and contains some petrified tree stumps; (2) 

Terlingua Creek sandstone is light-gray, calcareous, fine- to medium-grained sandstone, 

and some light-gray shale and lignite interbeds; (3) middle shale member is light-gray to 

dark-gray, calcareous, carbonaceous, and lignitic shale; (4) Rattlesnake Mountain sandstone member is mostly light-gray, fine-grained, calcareous sandstone; sandstone is ripple 

marked and trough crossbedded, and contains oysters and Ophiomorpha burrows; (5) 

lower shale member is light-gray to dark-gray, laminated, silty, carbonaceous shale, and 

some brown and gray sandstone and siltstone; contains some coal and lignite, and reddishbrown concretions and oysters; and (6) basal sandstone member consists of yellowish-gray 

to dark reddish-brown, fine-grained, calcareous sandstone; contains some conglomeratic 

beds near base with light-gray claystone clasts derived from underlying Pen Formation. 

Formation contains vertebrate fossils and petrified tree stumps (Maxwell and others, 1967; 

Busbey and Lehman, 1989; Lehman and Busbey, 2007). Mafic pyroclastic deposits in 

uppermost part of formation dated at 76.9 ± 1.2 Ma (Breyer and others, 2007; Befus and 

others, 2008). Upper contact with Javelina Formation is gradational and placed at the top 

of sandstone beds in the Aguja, which are overlain by predominantly variegated mudstone 

of the Javelina. Combined maximum thickness about 100 to 280 m thick

Kp Pen Formation (Upper Cretaceous)—Mostly light-gray, yellowish-gray, and yellowishbrown claystone, and some sandstone and chalk; claystone is calcareous and sandy. 

Unit contains yellowish-gray to dark-brown concretions, up to 1 m in diameter, some 

which form layers, and others that are isolated; concretions are sandy and fossiliferous. Formation includes sandstone beds, up to 1.5 m thick, which increase in number 

and thickness upward; sandstone beds near top of formation contain gastropods and 

bivalves. Upper contact with Aguja Formation is gradational and intertonguing with 

Aguja sandstone units. Formation contains ammonites, pelecypods, and gastropods. 

Forms slope and about 100 to 230 m thick
Description of Map Units 19

Kb Boquillas Formation, undivided (Upper Cretaceous)—Thin-bedded, medium-gray, lightgray, yellowish-gray, and brownish-gray limestone, shale, claystone, marl, chalk, 

and very minor siltstone and sandstone. Contact with overlying Pen Formation 

gradational (Lehman, 1985); contact with underlying Buda Limestone is sharp and 

disconformable, and marked by a distinctive color change from gray tones in Buda 

Limestone to light-brown tones in the lowermost Boquillas Formation. Formation 

contains abundant inoceramids, as well as ammonites, echinoids, cephalopods, foraminifera, and fish bones. Unit subdivided into San Vicente and Ernst Members in 

areas along the western flank of Sierra del Carmen, and near Sierra San Vicente and 

Mariscal Mountain, but in other areas of BBNP, unit mapped undivided. About 220 

to 245 m thick 

Kbs San Vicente Member—Medium-gray, finely crystalline, thin-bedded limestone, and 

brownish-gray and yellowish- to light-gray claystone, calcareous shale, marl, and 

chalk. Limestone is argillaceous and chalky; claystone is calcareous and contains 

some clay minerals including kaolinite, montmorillonite, and illite; about 145 m thick 

Kbe Ernst Member—Mostly medium-gray, finely crystalline, thin-bedded silty limestone, 

and brownish-gray to light-brown calcareous shale, and some siltstone, claystone, 

and marl. Top of member marked by brown-weathering limestone and shale beds that 

contain Allocrioceras hazzardi Zone (Cooper, 2000; Cooper and others, 2007, 2008); 

about 75 to 100 m thick

Kbd Buda Limestone and Del Rio Clay, undivided (Upper Cretaceous)—Unit shown in 

cross section only

Kbu Buda Limestone (Upper Cretaceous)—Consists of an upper cliff-forming limestone, 

a middle slope-forming marly limestone, and a lower cliff-forming basal limestone; 

upper limestone thicker than lower limestone. Upper and basal limestones are white 

to light-gray, finely crystalline, partly burrowed, massive bedded, and have distinctive 

conchoidal fractures; middle marly limestone is light gray, nodular, and dense. Formation contains gastropods and pelecypods. Contact with overlying Boquillas Formation is unconformable. Exposed in southern Santiago Mountains, Sierra del Carmen, 

Mariscal Mountain, and Mesa de Anguila; about 20 to 30 m thick 

Kdr Del Rio Clay (Upper Cretaceous)—Pale-red, yellowish-gray, and light-gray claystone, 

interbedded dark yellowish-orange to moderate yellowish-brown, thin-bedded to 

laminated limestone, and some yellowish-brown, friable sandstone; formation is limonitic. Limestone contains abundant benthic foraminifera (Haplostiche texana most 

common) and ammonite shell fragments. Contact with overlying Buda Limestone is 

sharp. About 36 m thick in Dog Canyon area, 30 m thick in the Mesa de Anguila area, 

and only about 1.5 m thick in Sierra Del Carmen near the Rio Grande, where basal 

parts of formation are absent (Maxwell and others, 1967) 

Kse Santa Elena Limestone (Lower Cretaceous)—Light-gray, thin- to thick-bedded, finely 

crystalline limestone and some yellowish-gray marly limestone, and brown chert nodules; contains abundant silicified rudistids and some gastropods. Forms massive cliffs 

and exposed in Sierra del Carmen, Mariscal Mountain, and Mesa de Anguila; about 

225 m thick 

Ksp Sue Peaks Formation (Lower Cretaceous)—Upper part is light-gray nodular, thinbedded limestone and interbedded yellowish-gray shale. Middle part is a 6-m-thick 

massive gray limestone ledge. Lower part is yellowish-gray marly shale and lesser 

amounts of thin-bedded, nodular limestone. Unit contains gastropods, echinoids, 

and ammonites. Unit forms slope between the more resistant Santa Elena Limestone 

above and the Del Carmen Limestone below. About 76 m thick in Sierra del Carmen 

and 80 m thick in Santa Elena Canyon

Kdc Del Carmen Limestone (Lower Cretaceous)—Massive cliff-forming, light-gray to 

yellowish-gray, finely crystalline limestone and brown chert layers and nodules. Unit 

contains rudistids and ammonites. Sharp contact with underlying Telephone Canyon 

Formation. About 100 m thick in Sierra del Carmen and Santiago Mountains, and 

144 m thick in Mesa de Anguila area
20 Geologic Map of Big Bend National Park, Texas

Ktm Telephone Canyon Formation and Maxon Sandstone, undivided (Lower Cretaceous)—Telephone Canyon Formation is alternating yellowish- to brownish-gray 

nodular limestone and yellowish-gray marl; contact with overlying Del Carmen Limestone sharp and disconformable. Maxon Sandstone is reddish-weathering, fine-grained, 

calcareous, silty sandstone; lower and upper contacts gradational with underlying Glen 

Rose Limestone and overlying Telephone Canyon Formation (Poth, 1979); Maxon reported 

to be about 3 m thick in BBNP (Maxwell and others, 1967). Combined unit forms slope 

between more resistant Glen Rose Limestone below and Del Carmen Limestone above; 

exposed in Santiago Mountains, Sierra del Carmen, and Santa Elena Canyon area; about 20 

to 45 m thick

Kgr Glen Rose Limestone (Lower Cretaceous)—Most of unit consists of alternating resistant 

limestone and less resistant marl beds. Limestone is medium to light gray, finely to 

medium crystalline, and mostly thick bedded, and includes interbeds of light-brown 

marl; contains pelecypods, gastropods, and echinoids. Basal 20 m of unit consists of 

pale-red pebble to cobble conglomerate and interbedded gray and red sandstone, and 

pale-red claystone. Conglomerate is crudely bedded with some clast imbrication and 

graded bedding; clasts are subangular to subrounded and consist mostly of gray sandstone, siltstone, and mudstone derived from underlying Tesnus Formation. A 10-m-thick 

nodular limestone unit above basal conglomerate and clastic beds contains a distinctive 

coquina of the oyster Exogyra quitmanensis. Unit exposed in Santiago Mountains where 

it unconformably overlies Pennsylvanian–Mississippian Tesnus Formation, and in Sierra 

del Carmen; about 100 to 150 m thick 

Paleozoic Rocks

Pzu Paleozoic rocks, undivided (Lower Pennsylvanian through Ordovician)—Mapped 

where individual units could not be subdivided. Includes parts of units described below, 

but some exposures may include parts of older units including the Lower Ordovician 

Marathon Limestone and Alsate Shale (Maxwell and others, 1967); exposed in southern 

Santiago Mountains and southeast of Dog Canyon. Unit shown in cross section as Paleozoic rocks, undivided

*Mt Tesnus Formation (Lower Pennsylvanian and Upper Mississippian)—Shale, sandstone, 

and siltstone. Shale is black, brown, and gray, and sandstone and siltstone is light gray 

to light brown. Pebble conglomerate at base of unit about 0.3 m thick and consists of 

subangular chert clasts derived from Caballos Novaculite below; unconformable with 

underlying Caballos Novaculite. Unit exposed in Persimmon Gap area; about 150–200 

m thick; incomplete thickness

MOu Mississipian to Ordovician rocks, undivided—Includes the Caballos Novaculite (Mississippian to Silurian) and Maravillas Formation (Upper and Middle Ordovician). Caballos 

Novaculite is novaculite, chert, and some shale. Upper part is black to brown thin-bedded 

to laminated chert with interbeds of brown silicious shale. Lower part is light-gray to 

white novaculite that forms resistant ledge; Caballos Novaculite unconformable with 

underlying Maravillas Formation and about 20 m thick. Maravillas Formation includes 

limestone and alternating chert, and interbedded shale. Upper 10 to 12 m includes Persimmon Gap Shale of Wilson (1954), which consists of interbedded reddish-weathering 

shale and black chert of Late Ordovician age. Below Persimmon Gap Shale is alternating limestone and interbedded chert. Limestone is medium gray, finely to medium crystalline, and thin bedded; some beds contain silicified brachiopod, crinoid, and bryozoan 

fragments; limestone beds contain Late Ordovician conodonts including Amorphognathus ordovicicus, Belodina, Oistodus venustus, Panderodus gracilis, Panderodus unicostatus, and Phragmodus undatus (Page and Harris, 2007). Chert is black and thin bedded 

to laminated. Exposed in Persimmon Gap area of Santiago Mountains; about 60 to 80 m 

thick; base not exposed
Stratigraphy

Paleozoic Rocks

Paleozoic rocks in the park are exposed near Persimmon Gap (fig. 1), and include the 

upper part of the Middle to Upper Ordovician Maravillas Formation, the Lower Silurian to 

lowermost Mississippian Caballos Novaculite, and part of the Upper Mississippian-Lower 

Pennsylvanian Tesnus Formation. Maxwell and others (1967) reported some exposures of 

flaggy limestone and shale in the Santiago Mountains southeast of Persimmon Gap that 

they tentatively identified as parts of Lower Ordovician Marathon Limestone and Alsate 

Shale. Because these rocks lack positive identification, and they may include parts of the 

units listed below, they were mapped on the plate as Paleozoic rocks, undivided (Pzu). 

Paleozoic rocks are well exposed in the Marathon Basin north of the map area (Barnes, 

1979; Anderson and others, 1982), and like the Persimmon Gap section, they are deformed 

by northwest directed thrust faults related to the Paleozoic Ouachita-Marathon-Sonora 

orogeny (Maxwell and others, 1967; Poole and others, 2005).

The Middle and Upper Ordovician Maravillas Formation consists of chert, limestone, and some shale and interbedded chert at the top (Persimmon Gap Shale of Wilson, 

1954). The Persimmon Gap Shale was originally mapped with the Caballos Novaculite 

by Maxwell and others (1967), but because of its Late Ordovician age, it probably shares 

greater affinity to the Maravillas Formation. The limestone beds in the Maravillas contain 

silicified brachiopods, crinoids, and bryozoans, and conodonts, including representatives of Amorphognathus ordovicicus Branson & Mehl, Belodina sp. (of Late Ordovician morphotype), Oistodus venustus Stauffer, Panderodus gracilis Branson & Mehl, 

Panderodus unicostatus Branson & Mehl, Periodon aculeatus Hadding (likely reworked), 

Phragmodus undatus Branson & Mehl, and Protopanderodus insculptus Branson & Mehl 

(Page and Harris, 2007). These taxa are known from many collections of Late Ordovician 

age in North America from the eastern Appalachians westward through the Great Basin. 

Alternating limestone and chert beds in the Maravillas Formation at Persimmon Gap. (Photograph 

by William R. Page)
22 Geologic Map of Big Bend National Park, Texas

With the exception of the panderodids, most of the conodonts 

are incomplete to fragmentary and include a mix of shallow- to 

deep-water taxa that indicate relatively high-energy post-mortem 

transport basinward. The Maravillas Formation was likely deposited in a deep-water, basinal environment based on the fragmented and reworked nature of the fauna in combination with 

the occurrence of both shallow and deep water forms (McBride, 

1989; Poole and others, 2005; Page and Harris, 2007). 

The Caballos Novaculite consists of novaculite and some 

shale, and is Silurian to Mississippian in age based primarily 

on radiolarians (Noble, 1994; Poole and others, 2005). Because 

the Caballos Novaculite is only 20 m thick in the map area, 

we combined it with the Maravillas Formation on the plate 

(MOu). The formation forms a resistant ledge in the Persimmon Gap area, and like the Maravillas Formation, was deposited in a deep-water environment (Poole and others, 2005).

The Mississippian–Pennsylvanian Tesnus Formation 

consists of shale, siltstone, and sandstone, and it locally 

includes a pebble conglomerate (consisting mainly of Caballos 

Novaculite clasts) at the base. The age of the Tesnus Formation is based mainly on conodonts identified from the formation in the Marathon Basin (Ellison and Powell, 1989); fossil 

plants of Pennsylvanian age were also reported from the 

formation in the Marathon Basin (Maxwell and others, 1967). 

The Tesnus Formation is synorogenic flysch deposited in front 

of the advancing Marathon orogenic belt (Poole and others, 

2005). The top of the formation is not exposed in BBNP.

Lower Cretaceous to Lower Tertiary 

Sedimentary Rocks

The Glen Rose Limestone (Kgr) consists of a basal clastic 

sequence of conglomerate, sandstone, and claystone, and an 

overlying sequence of alternating limestone and marl beds 

that make up a majority of the formation. A distinctive oyster 

coquina (Exogyra quitmanensis) is present about 5 m above the 

top of the clastic sequence near the Persimmon Gap area; the 

lower part of the formation is not exposed in the Santa Elena 

Canyon area, so the extent of this coquina marker horizon is 

unknown southward in BBNP. The formation also contains 

other pelecypods, gastropods, and echinoids. The carbonate 

rocks of the Glen Rose were deposited in a shallow, openshelf environment (Smith, 1981), and the lower clastic units 

were interpreted as nearshore facies resulting from northward 

retrogradation of the coastal environments (Maxwell and others, 1967). The Glen Rose Limestone unconformably overlies 

Paleozoic rocks in the region, and is gradationally overlain by 

the Maxon Sandstone and (or) Telephone Canyon Formation. 

The Maxon Sandstone and Telephone Canyon Formation 

were mapped as a combined unit (Ktm), and they generally 

form a reddish-colored slope separating the cliff-forming Glen 

Rose (Kgr) and Del Carmen (Kdc) Limestones. The Maxon 

Sandstone is not recognized everywhere in the park likely due 

to a facies change from calcareous sandstone to marl or limestone, southward across BBNP (Maxwell and others, 1967). 

In the Santiago Mountains southeast of Persimmon Gap, the 

Maxon Sandstone is calcareous, silty sandstone that grades 

upward into marl and nodular limestone of the Telephone 

Canyon Formation (Poth, 1979). The echinoid Enallaster sp. 

and snail Turritella sp. are some of the more common fossils 

in the Telephone Canyon Formation (Maxwell and others, 

1967). The Maxon Sandstone and Telephone Canyon Formation interrupted carbonate shelf deposition of the Glen Rose 

Limestone, and represented an influx of terriginous clastic 

sediments derived from uplift in the Marathon region of Texas 

(Smith, 1981).

The Del Carmen Limestone (Kdc) was named by 

Maxwell and others (1967) for prominent exposures in the 

Sierra del Carmen of BBNP, but it is also exposed at Mesa de 

Anguila. The formation is characterized by massive cliffs of 

limestone that contain discontinuous layers and nodules of 

chert. The formation resulted from transgression across the 

carbonate-shelf interior (Smith, 1981), and contains abundant 

rudistids that formed in reefs along the shelf. The formation also contains gastropods, echinoids, and foraminifera. 

Maxwell and others (1967) noted a complete section of the 

formation in Telephone Canyon, Sierra del Carmen, where 

upper and lower cliff-forming, rudistid-bearing limestones are 

separated by a middle, less-resistant marly limestone. The Del 

Carmen Limestone is similar in appearance to the Santa Elena 

Limestone (Kse), and Poth (1979) described criteria to differentiate the units; he reported that the Santa Elena generally 

has a finer-grained texture and lighter color tones than the Del 

Carmen, and fossils and rudistid bioherms in the formation are 

better preserved than the Del Carmen.

The Sue Peaks Formation (Ksp) was named for exposures in the Sue Peaks area of the Sierra del Carmen (Maxwell 

and others, 1967), and the unit forms a slope between the 

cliff-forming Del Carmen Limestone below and the Santa 

Elena Limestone above. Maxwell and others (1967) observed 

a three-part subdivision for the formation in the Sierra del 

Carmen, consisting of a lower marly shale, a middle massive 

limestone ledge, and an upper thin-bedded limestone. In the 

Caves and other dissolution features forming in the Del Carmen 

Limestone, Boquillas Canyon area. (Photograph by Kenzie J. Turner)
Stratigraphy 23

Santa Elena Canyon area, the base of the unit is covered, but 

the upper part differs from the Sierra del Carmen sections 

because it contains more marl than limestone. The formation is 

abundantly fossiliferous, and contains gastropods, echinoids, 

and ammonites.

The Santa Elena Limestone (Kse) was named by Maxwell and others (1967) for the prominent cliffs on the northwest side of Mesa de Anguila in the Santa Elena Canyon area 

of BBNP. The formation is also exposed in Sierra del Carmen, 

Mariscal Mountain, and north of the Rosillos Mountains. Like 

the Del Carmen Limestone (Kdc), the formation contains 

silicified rudistids, gastropods, and foraminifera, and nodules 

and layers of chert, and was deposited in a carbonate shelf 

environment. The formation contains marl beds mostly in the 

upper part, which form less resistant intervals between the 

more resistant, dense limestone beds. 

The Del Rio Clay (Kdr) consists of claystone, laminated 

limestone, and some sandstone, and generally forms yellowish-, 

orange-, and reddish-covered slopes. Deposition of the Del 

Rio Clay represented another influx of clastic sediment from 

areas to the north. The formation is observed to thin southeastward across BBNP, from about 20 to 30 m thick in southern 

Santiago Mountains, northern Sierra del Carmen, and Mesa 

de Anguila, to less than 1 m thick in the southern Sierra del 

Carmen. Maxwell and others (1967) attributed the thickness 

variation to absence of basal parts of the formation indicated 

by missing faunal zones, due to a possible combination of 

non-deposition and (or) post-depositional erosion.

The Buda Limestone (Kbu) consists of a cliff-forming, 

massive-bedded limestone at the top and base, and a slopeforming nodular marly limestone unit in the middle. Thickness 

of the unit decreases southward across BBNP, where part of the 

upper member of the unit was subject to post-depositional erosion. The unit is distinctive due to its whitish-colored, conchoidally fractured limestone, and represents a transgressional lime 

mudstone, deposited in a deep shelf environment above mixed 

clastic and carbonate sediments of the Del Rio Clay (Kdr). 

Santa Elena Limestone in Boquillas Canyon area. (Photograph by Kenzie J. Turner)
Stratigraphy 25

The Boquillas Formation (Kb) includes the lower Ernst (Kbe) and upper San 

Vicente (Kbs) Members (Maxwell and others, 1967), and consists mainly of thin-bedded 

limestone, calcarenite, and interbedded shale, claystone, marl, siltstone, and very minor 

sandstone. Maxwell and others (1967) reported some bentonitic clay and minor tuff 

beds mostly in the San Vicente Member (Maxwell and others, 1967), but in general, 

there are very few indicators of volcanic activity as compared with equivalent units of 

the Cretaceous Western Interior Seaway. The presence of thin layers of pure kaolinite 

in the San Vicente Member is enigmatic (Cooper and others, 2004). There is a general 

upward increase in marl and shale beds in the formation. Cooper and others (2007, 2008) 

reported the formation was deposited during a global sea level rise, and they investigated 

and correlated important ammonite and inoceramid zones with established zones in other 

parts of the Cretaceous Western Interior Seaway (Cobban and others, 2008; Cooper and 

others, 2008). The identified invertebrate assemblages range from the upper lower Cenomanian near the base of the Ernst Member to at least middle Santonian near the top of the 

San Vicente Member. Maxwell and others (1967) originally defined the contact between 

the Ernst and San Vicente Members at the top of beds containing the ammonite Coilopoceras sp., but Cooper and others (2005, 2007, and 2008) redefined the top of the Ernst 

at the top of the Allocriceras hazzardi zone (Cooper, 2000), about 50 m below the previously defined contact, based on the presence of distinctive brown-weathering limestone, 

more easily identified for mapping purposes. 

The Pen Formation (Kp), named by Maxwell and others (1967), is a distinctive 

slope-forming claystone unit that has poor expression due to weathering. Sandstone units 

increase upward in the formation. The unit is entirely marine and facies indicate shallowingupward, progradational deposition in a shallow carbonate shelf environment (Lehman, 

1985). The formation contains ammonites, pelecypods, and gastropods, and it also 

contains sandy, fossiliferous concretions. The contact with the overlying Aguja Formation 

(Ka) is gradational and placed at the base of the first substantial sandstone above marine 

claystone of the Pen Formation (Lehman, 1985). The Pen Formation intertongues with 

the Aguja Formation as represented by the McKinney Springs Tongue (Lehman, 1985), 

a wedge of marine claystone within the Aguja in the McKinney Hills and in the Mariscal 

and Cow Heaven Mountains area, which is separated from the main part of the Pen by 

basal sandstone and the Rattlesnake Mountain sandstone member of the Aguja. 

Distinctive alternating layers of limestone, claystone, and shale in the Boquillas Formation, Hot 

Springs area. (Photograph by William R. Page)

Yellow slope-forming claystone of the Pen Formation near the west entrance of Big Bend National Park; Maverick Mountain in 

background. (Photograph by Daniel P. Miggins)
26 Geologic Map of Big Bend National Park, Texas

The Aguja Formation (Ka) records the beginning of a shift 

from marine to continental deposition, and includes deposits 

of meandering stream channels, flood plains, swamps, lakes, 

and deltas in a coastal plain environment, and open to restricted 

marine environments (Lehman, 1985). The formation consists 

of sandstone and interbedded carbonaceous and calcareous 

shale, and lignite. Lehman (1985) defined six informal members 

of the Aguja Formation (see Description of Map Units); the 

individual members are not mapped separately across BBNP 

because of their limited extent, complex facies changes, and 

intertonguing relations with adjacent units in the region. For 

example, the basal sandstone member is thickest and best developed in the eastern part of BBNP, compared to exposures in 

western BBNP where the member consists of thinner sandstone 

beds separated by shale beds; the McKinney Springs tongue of 

the Pen Formation (Kp) is better developed in the eastern part 

than the western part of the park where it is partly equivalent to 

the lower and middle shale members of the Aguja Formation; 

and the Rattlesnake Mountain sandstone member is present in western BBNP but absent in eastern BBNP. The Aguja 

Formation is widely exposed in BBNP; it thins northeastward 

across the park and is gradational with both the underlying Pen 

Formation and the overlying Javelina Formation (Kj). Continental deposits of the Aguja contain abundant vertebrate fossils 

including dinosaur, turtle, and crocodile remains; marine units 

contain ammonites, oysters, and fish scales and bones (Lehman 

and Busbey, 2007). Breyer and others (2007) reported an age 

of 72.6 ± 1.5 Ma (U-Pb) for a mafic pyroclastic deposit in the 

uppermost part of the formation along the eastern flank of the 

Rosillos Mountains, and Befus and others (2008) reported an 

age of 76.9 ± 1.2 Ma for a similar deposit at about the same 

stratigraphic interval near Pena Mountain.

Sandy concretions in the Aguja Formation southwest of Pena Mountain. (Photograph by Daniel P. Miggins)
Stratigraphy 27

Paleontology

 in the History of Big Bend National Park

Photograph of Work Projects Administration (WPA) field crew at dinosaur bone excavation site in Aguja Formation (1938). Significant 

and diverse fossil collections are an important part of the natural history of Big Bend National Park, including numerous Cretaceous 

dinosaur and crocodile remains, Paleocene–Oligocene mammals, and unique assemblages of Miocene vertebrate fauna. Dinosaur 

bones were first discovered in the Big Bend area in the early 1900s, and fossil collections in the park were made from quarries in the 

1930s by the WPA, the American Museum of Natural History, and the Texas Memorial Museum. Important Tertiary mammal fossils 

were discovered in the park in the 1950s, and paleontologic investigations continue in the park to this day. 

The fossilized remains of gigantic crocodiles have been 

discovered in the Aguja Formation in the south-central 

part of the Big Bend National Park. These are among the 

largest crocodiles ever known. With lengths of 40–50 

feet and jaws studded with 6-inch teeth, these powerful 

predators were extraordinarily equipped to feed upon a 

variety of dinosaurs. Just like modern day crocodilians, 

Deinosuchus riograndensis probably hunted by ambush...

lying submerged near shore, and violently seizing large 

dinosaurs as they foraged amid the vegetation of Big 

Bend’s ancient swamps (National Park Service).

A “Super-Croc”
28 Geologic Map of Big Bend National Park, Texas

Maxwell and others (1967) named the Javelina Formation 

(Kj) for exposures in the Javelina Creek area of Tornillo Flat. 

The formation consists of variegated mudstone and lenticular 

sandstone. The mudstone represents overbank and lacustrine 

deposits, and the sandstone was deposited in meandering 

channels in a fluvial flood-plain environment. The mudstone 

contains calcareous nodules and carbonaceous layers reflecting 

paleosol development (Lehman, 1985). The Javelina contains 

abundant vertebrate fossils and petrified wood, and lacustrine 

deposits in the upper part of the formation are known for their 

pterosaur assemblage, discovered in northern Tornillo Flat 

(Pterodactyl ridge; Lehman and Busbey, 2007). The Javelina is 

gradationally overlain by the Black Peaks Formation. Lehman 

and others (2006) reported an age of 69.0 ± 0.9 Ma (U-Pb) for 

a tuff bed in the middle part of the Javelina Formation.

The Black Peaks Formation (TKbp) was named by Maxwell and others (1967) for exposures at Hannold Draw in Tornillo Flat, near McKinney Hills. The formation was originally 

interpreted as entirely Paleocene (Maxwell and others, 1967), 

but subsequent studies indicated that the Cretaceous-Tertiary 

boundary is likely in the lower part of the formation (Lehman 

and Busbey, 2007). The Black Peaks contains fluvial sandstone 

and flood-plain deposits of mudstone, and is lithologically and 

depositionally similar to the Javelina Formation (Kj) (Lehman, 

1985). Subtle differences help to distinguish between the two 

formations (Lehman, 1985; Lehman and Busbey, 2007); the 

Black Peaks Formation contains distinctive black mudstone 

color bands absent in the Javelina Formation, and a regionally 

extensive marker bed with abundant petrified logs, informally 

known as “the log jam sandstone,” is present in the Black Peaks 

throughout BBNP (Lehman and Busbey, 2007). The Black 

Peaks Formation was originally mapped only in the Tornillo 

Flat area of BBNP (Maxwell and others, 1967), but new mapping by Lehman (unpub. mapping; this report) shows a much 

wider distribution south and west of the Chisos Mountains (pl.) 

where it was formerly mapped mostly as Javelina Formation. 

Variegated mudstone and sandstone of the Black Peaks Formation, capped by the Alamo Creek Basalt Member of the Chisos Formation 

in the Alamo Creek area. (Photograph by Kenzie J. Turner)
Stratigraphy 29

The Hannold Hill Formation (Thh) was named by Maxwell and others (1967) for 

exposures in Tornillo Flat. Maxwell and others (1967) mapped the formation in the area 

between Pulliam Peak and Grapevine Hills, but Lehman (unpub. mapping; this report) 

identified exposures in those areas as Black Peaks Formation (TKbp), and noted that the 

distribution of the formation is restricted to Tornillo Flat. The formation consists of four 

units recognized by Beatty (1992); they include, from base to top, a basal conglomerate 

(Exhibit Sandstone Member), a lower mudstone, an upper conglomeratic sandstone, and 

an upper mudstone. Maxwell and others (1967) originally mapped the base of the formation at about 100 m below the base of the Exhibit Sandstone Member. Beatty (1992) 

later placed the contact at the base of the Exhibit Sandstone because it is a more practical 

contact for mapping purposes. The revised lower contact was utilized by Lehman (unpub. 

mapping; this report) as it was not possible to identify the basal contact as defined by 

Maxwell and others (1967), in any exposures other than the type section. The Fossil Bone 

Exhibit in northern BBNP is in the Exhibit Sandstone Member, and contains a unique 

assemblage of vertebrate fossil mammals, especially Coryphydon. 

The Canoe Formation (Tc) was named by Maxwell and others (1967) for exposures in Tornillo Flat. The formation contains the basal Big Yellow Sandstone Member, 

a distinctive, 10-m-thick, yellowish-weathering fluvial sandstone, which unconformably 

overlies variegated mudstone of the Hannold Hill Formation (Thh). Maxwell and others 

(1967) noted that the Canoe Formation thins northwestward across Tornillo Flat. Most of 

the formation consists of tuffaceous mudstone, claystone, sandstone, siltstone, tuff beds, 

and basalt flows, indicating initiation of widespread volcanism in BBNP (Runkel, 1990; 

Lehman, 1991). Wilson and Runkel (1989) correlated the Canoe Formation with part of 

the Chisos Formation based on mammalian fauna. Maxwell and others (1967) suggested 

that the lava flows in the upper Canoe Formation may be time equivalent with the Alamo 

Creek Basalt Member (Tcac) of the Chisos Formation. Runkel (1990) further identified 

basalt flows at two horizons in the formation in Tornillo Flat; the lower flow was referred 

to as basalt A (fig. 4), which he interpreted to be time equivalent to the Alamo Creek 

Basalt, and the upper flow, referred to as basalt B, he interpreted to be time equivalent 

with the Ash Spring Basalt. Geochronological studies on the Canoe basalts are needed to 

confirm these correlations. 

Big Yellow Sandstone Member of the Canoe Formation in Tornillo Flat. (Photograph by William 

R. Page)
EOCENE OLIGOCENE

Younger part

Western facies

Eastern facies

Older part

Sandstone, tuff, 

and rhyolite unit 

Alamo Creek

Basalt Member

Ash Spring

Basalt Member

Bee Mountain

Basalt Member

Mule Ear Spring

Tuff Member

Bee Mountain

Basalt Member

Tule Mountain

Trachyandesite Mbr.

Alamo Creek

Basalt Member

Ash Spring

Basalt Member

Bee Mountain

Basalt Member

Mule Ear Spring

Tuff Member

Undifferentiated

lavas

Tule Mountain

Trachyandesite Mbr.

Siltstone unit 

Rhyolite tuff unit 

This report Henry and Davis (1996)

(Big Bend Ranch State Park)

Maxwell and others (1967)

(Big Bend National Park)

Alamo Creek

Basalt

Bee Mountain

Basalt

Mule Ear Spring

Tuff

Bee Mountain

Basalt

Bee Mountain

Basalt

Tule Mountain

Trachyandesite

Ash Spring

Basalt Member

Bee Mountain

Basalt Member

Chisos Formation

Chisos Formation

Chisos Group

30 Geologic Map of Big Bend National Park, Texas

Middle Tertiary Volcanic Rocks 

Chisos Formation

The Chisos Formation was originally named Chisos beds by 

Udden (1907) from exposures in the Chisos Mountains to include 

a thick section of tuffaceous sedimentary rocks, tuff, and lava. 

Maxwell and others (1967) subsequently raised the rank of these 

rocks to the Chisos Formation, and they described a western and 

eastern facies for the formation that varied significantly in thickness and lithology. The eastern and western facies were interpreted as interfingering, age-equivalent strata characterized by 

dramatic facies and thickness changes from western BBNP to the 

Chisos Mountains. The geographic boundary between the eastern 

and western facies of the Chisos Formation extends from Dominguez Mountain, northward to the east side of Sierra Quemada, 

and between Burro Mesa and the northwest flank of the high 

Chisos Mountains. Maxwell and others (1967) defined the western facies rocks to contain undifferentiated tuffaceous sedimentary units with mappable lava flows; these flows were designated 

as members of the formation and included (in ascending order), 

the Alamo Creek Basalt (Tcac), Ash Spring Basalt (Tcas), Bee 

Mountain Basalt (Tcbm), Mule Ear Spring Tuff (Tcme), and 

the Tule Mountain Trachyandesite Members (Tctm). The base of 

their western facies rocks is defined by the Alamo Creek Basalt 

Member, which generally overlies eroded Black Peaks Formation 

(TKbp). Maxwell and others (1967) described the eastern facies 

rocks as much thicker than the western facies, and characterized 

these rocks as mostly tuffaceous sedimentary units that lacked 

many of the interbedded lava flows of the western facies. 

Henry and Davis (1996) noted that the tuffaceous sedimentary rocks and volcanic flows that Maxwell and others (1967) 

defined as the western facies of the Chisos Formation were 

widespread, mappable units in the Big Bend Ranch State Park 

area (Henry and Davis, 1996), west of BBNP, and they raised the 

rank of the formation to group status, and raised the rank of the 

members of Maxwell and others (1967) to formations with the 

exception of the Ash Spring Basalt (Tcas), which was not present. They defined the age of the group to be constrained between 

about 47 and 33 Ma, and that the 32.7-Ma Mitchell Mesa Rhyolite (Henry and Davis, 1996; Henry and others, 1998), similar in 

age to the South Rim Formation in BBNP, overlies the group.

We retained formational rank for the Chisos because of 

unresolved stratigraphic issues and lack of consensual definition 

for the unit across the BBNP region. Table 1 compares units of 

the Chisos Formation used in this report with units of the Chisos 

defined by Maxwell and others (1967) for the BBNP area, and 

Henry and Davis (1996) and Henry and others (1998) in the 

adjacent Big Bend Ranch State Park.

Table 1. Stratigraphic table comparing units of the Chisos Formation in this report with units of Maxwell and 

others (1967) in BBNP, and Henry and Davis (1996) in adjacent Big Bend Ranch State Park. Gray shading represents 

undivided tuffaceous sediments of Chisos Formation; note the older part of the Chisos Formation in this report is older 

than the Chisos Formation of Maxwell and others (1967), and the Chisos Group of Henry and Davis (1996).
SW NE

Western

Big Bend National Park

Chisos Mountains Tornillo Flat

Alamo Creek Basalt

Younger part of 

Chisos Formation

Older part of 

Chisos Formation

Black Peaks Formation

Javelina Formation

Hannold Hill Formation

Canoe Formation Basalt A

Big Yellow Sandstone

Aguja Formation

? ?

Stratigraphy 31

Figure 4. Diagrammatic relationships between the older and younger parts of the Chisos Formation, Canoe Formation, and 

underlying Tertiary–Cretaceous sedimentary rocks, from southwest to northeast across BBNP. Rocks of the older part of the 

Chisos Formation were deposited into a local basin in the Chisos Mountains area, which pre-dated deposition of the younger part

of the Chisos Formation. Major unconformities are shown in blue. Basalt A in the Canoe Formation is time equivalent with the Alamo 

Creek Basalt Member of the Chisos Formation (Runkel, 1990). 

Undivided Older and Younger Parts 

of the Chisos Formation

In this report, we subdivided the Chisos Formation into 

older and younger parts (table 1). Rocks of the older part are 

restricted to the Chisos Mountains, east of Sierra Quemada 

and Burro Mesa, and are equivalent to the Fresno Creek facies 

of the Chisos Formation of Bohannon (2011). The older part 

of the Chisos is lithologically equivalent to the lower part 

of the eastern facies of Maxwell and others (1967), but they 

interpreted their eastern and western facies as the same age, 

in contrast to our interpretation of the older part as pre-dating 

their eastern and western facies (table 1; fig. 4). The older part 

also pre-dates the Chisos Group of Henry and Davis (1996), as 

they did not recognize any rocks in the Chisos older than the 

Alamo Creek Basalt (Tcac).

Rocks of the older part of the Chisos Formation were 

further subdivided into a basal rhyolite tuff unit (Tcrt), and an 

upper sandstone, tuff, and rhyolite unit (Tcstr) of Bohannon 

(2011). Bohannon (2011) also mapped a siltstone unit (Tcks) 

which intertongues with map unit Tcstr, and is interpreted 

to pinch out westward, and thicken eastward to the southeast 

flank of the Chisos Mountains near Juniper Creek. The basal 

unit (Tcrt) is exposed in the Tortuga Mountain area, mostly 

between Dominguez Mountain and Fresno Creek, and consists 

of tuff and tuffaceous mudstone with some sandstone lenses 

and concretions. Although the unit unconformably overlies 

the Cretaceous Aguja Formation (Ka), it is folded in the same 

manner as the underlying Cretaceous units (Bohannon, 2011). 

The next youngest unit (Tcstr) contains sandstone, tuffaceous 

sandstone, and rhyolitic tuff beds. Some of the sandstone beds 

are conglomeratic at the base, and the rock is mottled. The unit 

does not appear to be affected by Laramide folding as much as 

the basal unit.

Lithologic, stratigraphic, and structural data indicate 

rocks of the older part were deposited into a local basin in the 

Chisos Mountains area (fig. 4), prior to deposition of rocks of 

the younger part of the formation (including the Alamo Creek 

Basalt, Tcac), and during a transitional period from near the 

end of Laramide deformation to the beginning of widespread 

volcanism in BBNP. The older part of the Chisos is correlative with the lower part of the Canoe Formation (Tc, fig. 4) 

(Runkel, 1990) below Canoe basalt A of Runkel (1990), which 

he interpreted as time equivalent with the Alamo Creek Basalt 

(fig. 4). Rocks of the older part of the Chisos are much thicker 

(about 900 m thick in the Chisos Mountains) than equivalent 

strata of the lower Canoe Formation (about 70 to 80 m thick in 

Tornillo Flat), suggesting that the high Chisos Mountains were 

the basin depocenter for these rocks, while thinner equivalent 

strata of the Canoe in the Tornillo Flat area were likely deposited near the northeastern basin margin. Rocks of the older part 

of the Chisos are absent west of Sierra Quemada and the Burro 

Mesa fault, suggesting these areas represent the southwestern 

basin margin.
32 Geologic Map of Big Bend National Park, Texas Stratigraphy 33

Rocks of the younger part of the Chisos Formation 

(Tcy) are widely exposed in western BBNP, but they also are 

exposed in the Chisos Mountains (pl.). Rocks of the younger 

part are equivalent to the Smoky Creek facies of the Chisos 

Formation of Bohannon (2011), and to the western facies of 

the Chisos of Maxwell and others (1967) in western BBNP, 

and to the upper part of their eastern facies in the Chisos 

Mountains. The Chisos Group of Henry and Davis (1996) 

mostly corresponds to our younger part of the Chisos in western BBNP, except their sequence lacks the Ash Spring Basalt 

(Tcas), and has Bee Mountain Basalt (Tcbm) flows above the 

Tule Mountain Trachyandesite (Tctm) (table 1). 

In western BBNP, tuffaceous sedimentary rocks of 

the younger part are variably interbedded with the lava and 

ash-flow members of the Chisos, and the base of the younger 

part is defined by the Alamo Creek Basalt (Tcac, fig. 4). In 

most areas of the Chisos Mountains, however, tuffaceous 

sedimentary rocks of the younger part unconformably overlie 

tuffaceous sedimentary rocks of the older part of the Chisos 

Formation (fig. 4), and lack the lava flow members present in 

the western part of the park. However, the base of the younger 

part is locally defined by the Ash Spring Basalt Member 

(Tcas) between Ward Mountain and Burro Mesa, and north 

of Pulliam Bluff and west of Panther Spring (pl.). The occurrence of the Ash Spring Basalt at the base of the younger part 

of the Chisos in this area likely resulted from non-deposition 

and pinching out of the Alamo Creek Basalt and undivided 

sedimentary rocks below Ash Spring Basalt. Alternatively, 

these rocks may have been removed along some pre-Ash 

Spring Basalt unconformity. Rocks of the younger part of 

the Chisos are thicker in western BBNP (more than 600 m 

thick) compared to the Chisos Mountains (about 400 m thick) 

(cross section B–B' ); reduced thicknesses for these rocks in 

the Chisos Mountains is partly due to erosion during doming 

associated with early development of the Pine Canyon caldera 

(Barker and others, 1986). 

Rocks of the younger part of the Chisos Formation (Tcy) 

are conspicuously much lighter in color than those in the 

older part, primarily owing to the abundance of white to very 

light-gray tuff. The younger part contains rocks of diverse 

lithologies including tuffaceous sandstone, mudstone and 

claystone, conglomerate, tuff, and some lava and limestone. 

Most of these rocks in western BBNP are above the Alamo 

Creek Basalt Member (Tcac) and below the Bee Mountain 

Basalt Member (Tcbm), but they are also present at different 

stratigraphic horizons above the Bee Mountain Basalt. 

(Facing page top). Gray and red tuffaceous mudstone and 

sandstone, tuff, conglomerate, and dark-gray lava flows in the 

upper part of the Chisos Formation west of Round Mountain and 

Mule Ear Peaks. (Photograph by Daniel P. Miggins)

(Facing page bottom). Light-gray tuff, tuffaceous mudstone 

and sandstone of the upper Chisos Formation with local 

discontinuous conglomerate beds; same area as previous 

photograph. (Photograph by Daniel P. Miggins)
34 Geologic Map of Big Bend National Park, Texas

Lava and Ash-Flow Members of the 

Younger Part of the Chisos Formation

The Alamo Creek Basalt Member (Tcac) is a composite 

of multiple flows in western BBNP, and extends westward 

into the Big Bend Ranch State Park area and southward into 

Mexico. The unit is well exposed in the Trap Mountain, Round 

Mountain, and Black Mesa areas (pl.). In most areas of BBNP, 

the Alamo Creek forms the base of the Chisos Formation and 

unconformably overlies the Black Peaks Formation (TKbp), 

but tuff beds of the Chisos have been identified below the 

Alamo Creek locally in the Dogie Mountain area (T. Runkel 

and C. Henry, written commun., 2010). Stewart (1984) and 

Carmen and others (2003) reported the unit to consist of as 

many as nine chemically and petrographically distinct lava 

flows that originated from multiple sources in Mexico and the 

Big Bend region, including some within BBNP. Major element 

composition of the Alamo Creek indicates two common classifications, basalt to trachybasalt, and basaltic trachyandesite to 

trachyandesite (fig. 5). Henry and McDowell (1986) reported 

K-Ar ages for the Alamo Creek ranging from 46.9 to 39.7 

Ma, and Schucker and Nelson (1988) reported a K-Ar age of 

46.5 Ma. Most new 40Ar/39Ar ages for the Alamo Creek Basalt 

range between 47 and 46 Ma (appendix; 71–64). 

The Ash Spring Basalt (Tcas) consists of two or more 

flows in BBNP, and was named by Maxwell and others (1967) 

for exposures in the Ash Spring area, north of Pulliam Peak in 

the Chisos Mountains, and between Ward Mountain and Burro 

Mesa. Scattered outcrops of the unit are also exposed west of 

Burro Mesa, north of Kit and Goat Mountains, westward to the 

Tule Mountain area, and Maxwell and others (1967) identified 

an unmapped outcrop at Casa Grande. Identification of the Ash 

Spring is difficult due to similar appearance and composition 

with the Alamo Creek Basalt Member (Tcac). Major element 

classification overlaps the Alamo Creek with most Ash Spring 

Basalt classified as basaltic trachyandesite to trachyandesite 

(fig. 5). Furthermore, trace element data do not differentiate 

the two units. Henry and McDowell (1986) reported a K-Ar 

age from the type locality of 34.5 ± 1.7 Ma, and Schucker and 

Nelson (1988) obtained a K-Ar age of 44.5 ± 1.8 Ma from the 

unit north of Goat Mountain. New 40Ar/39Ar ages determined 

on groundmass concentrate from the Ash Spring Basalt are 

40.92 ± 0.07 and 40.38 ± 0.35 Ma (appendix; 58), but these 

ages are younger than constraining tuff ages elsewhere. Single 

crystal sanidine 40Ar/39Ar ages of 42.31 ± 0.10 Ma (appendix; 

62) from tuff below the Ash Spring Basalt southwest of the 

Burro Mesa Pouroff (fig. 1), and 41.41 ± 0.07 Ma (appendix; 

59) and 41.75 ± 0.11 Ma (appendix; 60) from tuff that overlies 

the Ash Spring on the northeast side of Burro Mesa, and north 

of Tule Mountain (fig. 1), respectively, suggest an age around 

41 to 42 Ma. 

The Bee Mountain Basalt Member (Tcbm) is composed 

of multiple lava flows ranging from basalt to basaltic trachyandesite (fig. 6). These lava flows extend from the Chisos 

Mountains westward to the Bofecillos Mountains in Big Bend 

Ranch State Park (Maxwell and Dietrich, 1970; McKnight, 

1970; Henry and others, 1998), and southward into Mexico 

(Maxwell and others, 1967). The thickest sections of the unit 

in BBNP are at Bee Mountain and southeast of Castolon. 

Maxwell and others (1967) originally restricted the unit to 

below the Mule Ear Spring Tuff Member (Tcme) in BBNP; 

however, Bee Mountain lava flows are present above the Mule 

Ear Spring Tuff in places within BBNP (Turner and others, 

2008). Henry and Davis (1996) also noted Bee Mountain lava 

flows at several horizons above the Mule Ear Spring west of 

BBNP. In BBNP, most Bee Mountain lava flows above the 

Mule Ear Spring are more evolved than the flows below, with 

higher silica and total alkalis (fig. 6), lower Hf/Ta, and higher 

La/Yb ratios (fig. 7). However, some of the stratigraphically 

highest Bee Mountain flows above the Mule Ear Spring appear 

less evolved with lower silica and total alkalis and higher Hf/

Ta ratios (figs. 6 and 7) than other flows above the Mule Ear 

Spring Tuff; parental magma of these uppermost flows may 

have originated from a source of similar composition and (or) 

depth as the source for flows below the Mule Ear Spring Tuff. 

Henry and others (1986) reported a K/Ar age of 34.5 ± 1.8 Ma 

for the unit. 40Ar/39Ar ages determined for this report range 

between 34.03 ± 0.17 and 33.37 ± 0.48 Ma (appendix; 52–49) 

for the lower flows (below the Mule Ear Spring); dates on the 

upper flows (above the Mule Ear Spring) are 33.68 ± 0.09 Ma 

(appendix; 42) and 33.07 ± 0.15 Ma (appendix; 41). 

Bee Mountain Basalt Member of Chisos Formation overlying white 

tuffaceous sediments of the Chisos Formation at Kit Mountain; 

uppermost pinkish-weathering bed is Mule Ear Spring Tuff Member 

of the Chisos Formation. (Photograph by Daniel P. Miggins)
35 40 45 50 55 60 65 70 75

0

2

4

6

8

10

12

14

SiO2

Basalt

PicroBasalt

Basaltic

andesite

Andesite

Dacite

Trachyte

Rhyolite

Trachyandesite

Basaltic

trachyTrachy- andesite

basalt

Tephrite or

Basanite

PhonoTephrite

Tephriphonolite

Ash Spring Basalt Phonolite

Alamo Creek Basalt

Na O+K 2 O2

EXPLANATION

35 40 45 50 55 60 65 70 75

0

2

4

6

8

10

12

14

SiO2

Basalt

PicroBasalt

Basaltic

andesite

Andesite

Dacite

Trachyte

Rhyolite Trachyandesite

Basaltic

trachyandesite

Trachybasalt

Tephrite or

Basanite

PhonoTephrite

Tephriphonolite

Phonolite

Na O+K 2 O2

EXPLANATION

Trachyandesite lava flows

Basaltic trachyandesite lava flows

Trachyte lava flows

Dikes related to Dominguez Mountain

Tule Mountain Trachyandesite

Upper Bee Mountain Basalt

Lower Bee Mountain Basalt

Stratigraphy 35

Figure 6. Total alkali-silica diagram of Le Maitre and others (1989) for lavas including the 

Bee Mountain Basalt and Tule Mountain Trachyandesite Members of the Chisos Formation 

trachytic lava that is divided into classifications, and possibly related dikes of the Dominguez 

Mountain dike swarm. Analyses normalized to volatile-free and total iron as FeO derived by 

multiplying total iron (FeTO33) by 0.8998.

Figure 5. Total alkali-silica diagram of Le Maitre and others (1989) for Ash Spring Basalt and 

Alamo Creek Basalt Members of the Chisos Formation. Analyses normalized to volatile-free 

and total iron as FeO derived by multiplying total iron (FeTO3) by 0.8998.
EXPLANATION

Trachyandesite lava flows

Basaltic trachyandesite lava flows

Trachyte lava flows

Dikes related to Dominguez Mountain

Tule Mountain Trachyandesite

Upper Bee Mountain Basalt

Low silica upper Bee Mountain Basalt

Lower Bee Mountain Basalt

0 5 10 15 20

2

1

0

3

4

5

6

7

Hf/Ta

La/Yb

36 Geologic Map of Big Bend National Park, Texas

The Mule Ear Spring Tuff Member (Tcme) is a distinctive ash-flow tuff exposed in western BBNP, and it extends 

westward to the Bofecillos Mountains in Big Bend Ranch 

State Park (McKnight, 1970; Henry and others, 1998), and 

southward into Mexico. The unit is well exposed in the Goat, 

Kit, and Trap Mountain areas, and north and south of Cerro 

Castellan. Gregory (1981) correlated a tuff in the El Matadero 

Formation in Chihuahua, Mexico, with the Mule Ear Spring, 

and subsequently, the unit has been recognized and mapped 

in other areas of Chihuahua and parts of Coahuila, Mexico 

(Chuchla, 1981; Gunderson and others, 1986; Ritter and 

Cepeda, 1991). Henry and others (1998) reported an 40Ar/39Ar 

age of 33.767 ± 0.19 Ma for the Mule Ear Spring in BBNP. 

Five new laser-fusion ages (appendix 47–43); from BBNP 

indicate an average age of 33.65 Ma, which agrees with the age 

determination from Henry and others (1998). Previous studies 

have suggested Sierra Quemada was the source of the Mule 

Ear Spring Tuff (Ogley, 1978; Henry and Price, 1986; Duex 

and Tucker, 1989; Duex and others, 1994), but Scott and others 

(2007) demonstrated that intrusive rocks at Sierra Quemada are 

about 2.5 m.y. younger than the Mule Ear Spring Tuff. Furthermore, compositional comparison shows the Mule Ear Spring 

Tuff has a smaller negative Eu anomaly and is depleted in rare 

earth elements and high field strength elements (Zr, Hf, and Ta) 

relative to Sierra Quemada ring dikes and other intrusions. 

The Tule Mountain Trachyandesite Member (Tctm) is 

the uppermost lava flow member of the Chisos Formation in 

western BBNP (Maxwell and others, 1967). Turner and others 

(2008) reported rocks previously mapped by Maxwell and others (1967) as the Tule Mountain Trachyandesite Member in a 

large area of BBNP, extending from Kit Mountain and Punta de 

la Sierra southward to Sierra de Chino, are much younger (30.4 

to 30.2 Ma; appendix; 15–13), and have different major and 

trace element composition than the Tule Mountain Trachyandesite Member (figs. 6 and 7). These newly recognized rocks are 

not part of the Chisos Formation, and are mapped in this report 

as trachytic lava, undivided (Tt). Confirmed exposures of the 

Tule Mountain Trachyandesite Member in BBNP are at Burro 

Mesa, where it is overlain by a tuff of the Boot Rock member 

(Tsb) of the South Rim Formation (32.33 Ma; appendix; 29); 

at Tule Mountain, where it overlies the Mule Ear Spring Tuff 

(33.67 Ma); and at Sierra Aguja. Rocks from these localities are 

trachyandesite to trachyte (fig. 6) and compositionally similar to 

the Tule Mountain Trachyandesite in the Fresno Canyon area of 

Big Bend Ranch State Park (Henry and Davis, 1996; Henry and 

others, 1998). New 40Ar/39Ar dates for this report show age variance for the unit in BBNP, but the most stratigraphically compatible ages are 33.18 ± 0.10 Ma (appendix; 39) for a sample 

collected at Sierra Aguja, and 32.71 ± 0.24 Ma (appendix; 40) 

for a sample from Tule Mountain. 

Figure 7. High Field Strength 

Element (HFSE) ratio plot for 

the Bee Mountain Basalt and 

Tule Mountain Trachyandesite 

Members of the Chisos Formation 

trachytic lava that is divided into 

classifications as in figure 6, and 

dikes related to the Dominguez 

Mountain dike swarm. The general 

progression is from mafic and less 

evolved in the upper left to silicic 

and more evolved in the lower right. 

7

 Age is corrected to a neutron fluence monitor age for Fish Canyon 

Tuff sanidine of 28.02 Ma (Renne and others, 1998) to allow comparison 

with newly derived ages in appendix; correction of age following methods 

described at www.earth-time.org.
Burro Mesa 

Riebeckite

Rhyolite 

Burro Mesa 

Riebeckite

Rhyolite 

Burro Mesa 

Rhyolite member 

absent absent 

Burro Mesa 

Rhyolite

member 

Wasp Spring 

Flow Breccia 

Lost Mine

Rhyolite

Wasp Spring 

Flow Breccia 

Wasp Spring 

Flow Breccia 

Brown Rhyolite 

Lost Mine

Member

Boot Rock

member

Pine Canyon 

Rhyolite** 

Rhyolite member 

Wasp Spring member 

Emory Peak rhyolite member 

Boot Rock member 

Pine Canyon rhyolite member 

* Modified stratigraphy from Urbanczyk and White (2000) and Benker (2005) 

** Ogley (1978) first renamed Brown Rhyolite to Pine Canyon Rhyolite South Rim Formation

South Rim Formation

South Rim Formation

Burro Mesa

Formation

This report*

Barker and others (1986) Maxwell and others (1967)

OLIGOCENE

Western BBNP Chisos Mountains Western BBNP Chisos Mountains

Stratigraphy 37

Revision of South Rim Formation and 

Definition of Burro Mesa Formation

We revised the South Rim Formation in this report based 

on results of petrologic, geochemical, and petrographic studies 

subsequent to the original work of Maxwell and others (1967), 

and new geochronologic analyses completed for this report. 

Table 2 compares units of the South Rim and Burro Mesa 

Formations in this report with units of the South Rim defined 

by Barker and others (1986) and originally by Maxwell and 

others (1967). The most significant revisions are removal of 

rocks equivalent to the Burro Mesa Riebeckite Rhyolite and 

Wasp Spring Flow Breccia from the South Rim Formation of 

Maxwell and others (1967), and definition of the Burro Mesa 

Formation as a separate unit from the South Rim Formation 

(table 2). These revisions were originally recommended by 

Benker (2005) and White and others (2006) based on more 

comprehensive and updated geochemical and petrographic 

data, and on new geochronologic data presented by Miggins 

and others (2004, 2007, and 2008) (appendix). We further 

designate two informal members of the Burro Mesa Formation, a lower and more locally restricted Wasp Spring member 

(Tbw) (formerly Wasp Spring Flow Breccia of the South Rim 

Formation of Maxwell and others, 1967; Wasp Spring member 

of South Rim Formation of Barker and others, 1986; Wasp 

Springs member of the Burro Mesa Formation of Benker, 

2005), and an upper more widespread rhyolite member (Tbr) 

(formerly Burro Mesa Riebeckite Rhyolite of the South Rim 

Formation of Maxwell and others, 1967; Burro Mesa rhyolite 

member of South Rim Formation of Barker and others, 1986; 

and Burro Mesa rhyolite member of the Burro Mesa Formation of Benker, 2005). The Wasp Spring and rhyolite members 

are lithologically equivalent to the Wasp Spring Flow Breccia 

and Burro Mesa Riebeckite Rhyolite, respectively, as originally 

defined and mapped in their type areas in western BBNP by 

Maxwell and others (1967).

Fundamental reasons for exclusion of the Burro Mesa 

units from the South Rim Formation are the 3-Ma age difference between these units and the South Rim Formation, and 

restriction of these units to western BBNP. Miggins and others 

(2004, 2007, and 2008) conducted new geochronologic analyses for this report indicating the Burro Mesa units range from 

29.48 to 29.25 Ma (appendix; 10-2), compared to rocks of the 

South Rim Formation, which range from 32.33 to 31.93 Ma 

(appendix; 31-24). This indicates that Burro Mesa units are not 

related to the South Rim Formation and Pine Canyon caldera, 

as originally proposed by Maxwell and others (1967).

The Burro Mesa Formation rocks were originally 

included in the South Rim Formation by Maxwell and others 

(1967) because of compositional and petrographic similarities. 

However, Benker (2005) identified differences in trace element composition and subtle petrographic differences between 

the formations. Burro Mesa units generally show light rare 

earth element enrichment and depletion of Ba, Sr, Ti, and K 

relative to South Rim Formation units (Benker, 2005). Petrographic comparison indicates the Burro Mesa units contain 

an abundance of feldspar compared to South Rim Formation 

units (Benker, 2005).

Table 2. Stratigraphic table comparing units of the South Rim and Burro Mesa Formations in this report with the 

South Rim Formation of Maxwell and others (1967) and Barker and others (1986).
38 Geologic Map of Big Bend National Park, Texas

Units of the South Rim Formation are here restricted to 

eruptive rocks of the Pine Canyon caldera magmatic system 

(Ogley, 1978; Barker and others, 1986, Benker, 2005; White 

and others, 2006), and units of the Burro Mesa Formation were 

erupted from multiple vents in western BBNP (Holt, 1998; 

Parker, 2002; Adams, 2004). We revised the South Rim Formation to include the following informal members (from base 

to top), the Pine Canyon rhyolite member (Tsp), Boot Rock 

member (Tsb), and Emory Peak rhyolite member (Tse). The 

term Pine Canyon rhyolite was introduced by Ogley (1978) to 

replace the Brown rhyolite of Maxwell and others (1967), and 

to signify it as the main caldera-filling unit of the Pine Canyon 

caldera. The term was subsequently used by Barker and others 

(1986) (table 2), Benker (2005), and White and others (2006), 

therefore we propose the name for consistency and to indicate 

a genetic link to the caldera. The name Boot Rock member was 

proposed as an informal member of the South Rim Formation 

by Barker and others (1986) to replace units formerly mapped 

as the Wasp Spring Flow Breccia by Maxwell and others (1967) 

in the South Rim, The Basin, and Pine Canyon areas of the high 

Chisos Mountains (table 2). We combined the Lost Mine Rhyolite (originally named by Maxwell and others, 1967) with the 

Boot Rock member, based on recent studies by Benker (2005). 

He reported that trachytic and rhyolitic rocks of the Lost Mine 

Rhyolite show no petrographic or geochemical difference from 

the quartz trachytes to rhyolites of the Boot Rock member, and 

field evidence indicated no clear distinction between the two 

units. In addition, Ogley (1978) and Barker and others (1986) 

did not identify the Lost Mine Rhyolite at the type locality as 

defined by Maxwell and others (1967). The Emory Peak rhyolite is the upper member of the South Rim Formation, and the 

name was introduced by Urbanczyk and White (2000) for exposures in their type area of Emory Peak to replace units formerly 

mapped as Burro Mesa Riebeckite Rhyolite at Emory Peak by 

Maxwell and others (1967) and Barker and others (1986).

Aerial view of Emory Peak (high peak on left) and Toll Mountain (flat-topped mountain on right), exposing volcanic units of the South Rim 

Formation. (Photograph by Don Corrick)

(Facing page) Aerial view of South Rim, comprised of thick outflow 

units of the South Rim Formation. (Photograph by Don Corrick)
Stratigraphy 39

South Rim Formation 

The Pine Canyon rhyolite member (Tsp) is the primary 

fill of the Pine Canyon caldera (Ogley, 1978; Barker and others, 1986), and its known distribution is limited to within the 

caldera boundary (pl.; cross section B–B'). The unit is a peralkaline rhyolite composed of multiple cooling units of densely 

welded vitrophyre to welded ash-flow tuff. Aeromagnetic data 

indicate the Pine Canyon rhyolite is reversely magnetized 

(Drenth and Finn, 2007), and these data help to estimate its 

thickness within the caldera. Thickness estimates based on 

geophysical modeling for the unit range from about 400 m 

to over a kilometer (Drenth and Finn, 2007); Ogley (1978) 

reported a minimum thickness of 300 m for the unit. 

The Boot Rock member (Tsb) consists of quartz trachyte 

and rhyolite lava, tuff, and maar-type surge deposits erupted 

from vents around the caldera following collapse (Barker and 

others, 1986; Urbanczyk and White, 2000). The Boot Rock 

member is widely exposed in the Pine Canyon caldera, and in 

the South Rim area; many of these exposures were originally 

mapped as the Wasp Spring Flow Breccia and Lost Mine 

Rhyolite by Maxwell and others (1967). Distal outflow tuff 

of the Boot Rock member is exposed as far west as Burro 

Mesa (pl.). The tuff at Burro Mesa has similar age (appendix; 

29), rare earth element concentration, magnitude of negative 

Eu anomaly, and high field strength element concentration as 

samples from the South Rim and Pine Canyon caldera areas 

reported by Benker (2005) and White and others (2006). The 

Pine Canyon rhyolite and the Boot Rock members share a 

common magma source and are interpreted to have evolved 

primarily through crystal fractionation of a mafic melt with 

alkali basalt composition (Benker, 2005; White and others, 

2006). Multiple intrusive bodies external to the caldera, such 

as Nugent and Lone Mountains, have similar composition to 

the Boot Rock and Pine Canyon rhyolite members, and are 

likely related to the caldera’s parental magma. 

The Emory Peak rhyolite member (Tse) consists of lava, 

rheomorphic tuff, and vitrophyre erupted from intracaldera 

and extracaldera vents. The unit is exposed at Emory Peak and 

in the Blue Creek Ranch area. The Emory Peak rhyolite in the 

Blue Creek Ranch area is the same age (appendix; 25) as rocks 

at Emory Peak, and has similar peralkalinity, REE concentrations, and a large negative Eu anomaly consistent with analyses reported by Benker (2005) and White and others (2006). 

Numerous intrusions, including the Hayes Ridge ring dike, 

and Ward Mountain and Pulliam Peak intrusion, among others, 

are compositionally similar to the Emory Peak rhyolite (White 

and others, 2006). Parental magma of the Emory Peak rhyolite 

is interpreted to not be cogenetic with magma that produced 

the Pine Canyon rhyolite and Boot Rock members, but likely 

developed contemporaneously from partial melting of a lower 

crustal source (Benker, 2005; White and others, 2006). 
View from Lost Mine trail along the rim of the Pine Canyon 

caldera, looking southward toward Elephant Tusk. (Photograph 

by Daniel P. Miggins)
42 Geologic Map of Big Bend National Park, Texas

Trachytic Lava, Undivided

Lava flows mapped as trachytic lava, undivided (Tt), 

formerly mapped by Maxwell and others (1967) in areas of 

BBNP as Tule Mountain Trachyandesite Member (Tctm) of 

the Chisos Formation, are mostly exposed from Kit Mountain 

to Punta de la Sierra, and southward to Sierra de Chino. The 

lava flows consist of three compositional groups: basaltic 

trachyandesite, trachyandesite, and trachyte (fig. 6). Although 

scatter is present in the high field strength elements, likely due 

to crustal contamination, observed similarities in trace-element 

chemistry suggest a genetic relationship between the groups 

(fig. 7). Geochronology indicates the trachyandesite is 30.42 

Ma (appendix; 15), slightly older than the trachyte dated at 

30.29 Ma (appendix; 14) and 30.23 Ma (appendix; 13), and at 

Triangulation Station Mountain the trachyte overlies a 30.40 

Ma tuff (appendix; 18). Henry and others (1986) reported a 

K-Ar age of 29.3 ± 1.5 Ma from Smoky Creek near the Rio 

Grande, which is within analytical error of dates on the trachyte. The trachyte group is exposed in the Blue Creek Ranch, 

Goat Mountain, Round Mountain, and Sierra de Chino areas, 

and probably at Punta de la Sierra. The trachyandesite group is 

exposed in a belt west of the trachyte group, on the southwest 

flank of Burro Mesa, Bee Mountain, Tuff Canyon, and southeast of Cerro Castellan. The basaltic trachyandesite group is 

only identified between Cerro Castellan and Sierra de Chino.

A possible source for the undivided trachytic lava may be 

the volcanic complex at Dominguez Mountain. Two dikes in 

the Sierra de Chino area, which are interpreted to be associated with the Dominguez Mountain dike swarm (Tdd), are 

compositionally similar to the trachyandesite and trachyte 

groups (figs. 6 and 7). Furthermore, some dikes of the 

Dominguez Mountain dike swarm appear to merge upward 

into the trachytic flows capping the eastern cliffs of Punta de 

la Sierra (Bohannon, 2011). The flows at Punta de la Sierra 

appear to be in the same stratigraphic position as elsewhere, 

and are underlain by two tuffs that are around 30 Ma (appendix; 16, 17). Further investigations of volcanic and intrusive 

rocks at Dominguez Mountain, the dike swarm, and flows 

capping Punta de la Sierra are needed to confirm Dominguez 

Mountain as the source for the trachytic lava.

Chisos Formation

Trachytic lava

(Below) Southeastern point of Punta de la Sierra showing 

trachyitic lava (dark-gray rock capping mountain peak), dike 

swarm (light-brown, sub-vertical dikes in central part of mountain) 

cross-cutting sediments of the Chisos Formation, and Alamo Creek 

Basalt Member of the Chisos Formation (dark-gray basalt flow 

near base of photograph). (Photograph by Robert G. Bohannon)

(Facing page) Tuff Canyon after rainstorm; dark-gray rock in the 

lower part of the photograph is trachyitic lava (Tt), and overlying 

light-gray to tan rocks forming rim are outflow units of the Burro 

Mesa Formation. (Photograph by Blake Trester)

44 Geologic Map of Big Bend National Park, Texas

Burro Mesa Formation

Maxwell and others (1967) originally included the 

Burro Mesa Riebeckite Rhyolite and Wasp Spring Flow 

Breccia as members of the South Rim Formation (table 2). 

Although they defined and mapped these units in type sections in western BBNP, they also mapped these units in the 

Chisos Mountains. Barker and others (1986) restricted the 

Wasp Spring Flow Breccia and Burro Mesa rhyolite to areas 

south and west of the Chisos Mountains (table 2), with the 

exception of one outcrop of Burro Mesa at Emory Peak, 

mapped in this report as the Emory Peak rhyolite (Tse) 

(Urbanczyk and White (2000). Becker (1976), Henry and 

others (1989), and Benker (2005) noted compositional differences, and Miggins (2004, 2008) documented age differences 

between Burro Mesa rocks in their type area and in the Chisos Mountains. Barker and others (1986) and Henry and others (1989) reported the distribution of the Wasp Spring Flow 

Breccia and Burro Mesa Riebeckite Rhyolite south and west 

of the Chisos Mountains could not have erupted from a single vent, and multiple source vents for those units have been 

identified (Holt, 1998; Parker, 2002; Adams, 2004). In this 

report, the newly defined Burro Mesa Formation includes a 

lower more restricted Wasp Spring member (Tbw), and an 

upper more widespread rhyolite member (Tbr). Numerous 

new 40Ar/39Ar ages for the Burro Mesa Formation range from 

29.48 to 29.25 Ma (appendix; 10–2). 

Rocks of the Burro Mesa Formation are exposed at 

Burro Mesa, Goat and Kit Mountains, and in scattered 

outcrops between Castolon and the Chisos Mountains. The 

Wasp Spring member (Tbw) consists of surge deposits, and 

air-fall and ash-flow tuff, and most beds contain abundant 

lithic fragments and fiamme. The member is one of the 

more recognizable volcanic units in BBNP because it forms 

brightly colored orange to yellow cliffs near Burro Mesa 

Goat Mountain along Ross Maxwell Drive; rocks of the Burro Mesa Formation form the steep cliffs in the upper part of the mountain. 

(Photograph by Daniel P. Miggins)

Burro Mesa from pouroff parking lot, showing light-brown, orange, 

and yellow-weathering deposits of the lower Wasp Spring member 

of the Burro Mesa Formation, overlain by thick, dark-brown flows of 

the upper rhyolite member. (Photograph by Robert B. Scott)

Pouroff and at Cerro Castellan. The rhyolite member (Tbr) 

consists of lava flows and ash-flow tuff, which were erupted 

from local maar-type rhyolite lava domes (Holt, 1998; 

Parker, 2002; Adams, 2004). Extrusive rocks of the formation were erupted from vents at Burro Mesa, Kit and Goat 

Mountains, Cerro Castellan, and Horseshoe Canyon to name 

a few (Holt, 1998; Adams, 2004). Rhyolitic intrusive bodies 

in southwestern BBNP are compositionally identical and 

time equivalent (appendix; 11, 12) to Burro Mesa extrusive 

rocks; these rocks are mapped on the plate as map unit Tbi.
Stratigraphy 45

Alluvial Deposits

Late Tertiary to Quaternary Basin Fill

During the Miocene, Pliocene, and early part of the 

Pleistocene, alluvial-fan, stream, and playa deposits accumulated in fault-bounded basins (bolsons), formed by basin-andrange extensional faulting. Two main basins in the park are the 

Delaho bolson, on the west side near Castolon, and the Estufa 

bolson on the east side in the lower Tornillo Creek area (fig. 2). 

Basin fill deposited in the Delaho bolson has been deeply 

eroded and dissected such that only disconnected remnants 

remain. The older part of the fill, mapped as Ta in this report, 

is made up of debris-flow, fluvial-channel, and sheet-flow 

deposits of the Delaho Formation (Stevens and others, 1969; 

Stevens and Stevens, 1989). The unnamed lower member 

of the Delaho Formation contains a diverse vertebrate fauna 

of large and small mammals, tortoises, and lizards of earliest Miocene age (Castolon local fauna; Stevens and others, 

1969; Stevens, 1977, 1991), and is interbedded near its base 

with a thin basaltic flow yielding a K-Ar age of 23.3 ± 0.6 Ma 

(Stevens, 1988; Stevens and Stevens, 1989), indicating that 

deposition in the Delaho bolson began in the earliest Miocene. 

The Smoky Creek Member of the Delaho Formation disconformably overlies the lower member and represents a later 

stage of sediment deposition in the bolson. Because the Smoky 

Creek Member contains few fossils, timing of its deposition is 

poorly constrained, but estimates of late early Miocene (Stevens and Stevens, 1989) and middle? Miocene (Stevens, 1988) 

have been proposed. The youngest sediments attributed to the 

Delaho bolson are gravelly alluvial-fan deposits in the Burro 

Mesa area (Lehman and Busbey, 2007), named the informal 

Fingers formation by Stevens (1969, 1988) and mapped as 

QTa in this report. No fossils have been found in these coarsegrained deposits, and their age is unknown, but Stevens and 

Stevens (1989), who have done most of the work on bolson 

deposits in the park, consider them possibly Blancan (North 

American mammal age), which they associate with a PliocenePleistocene geologic age (Stevens and Stevens, 1989, 2003). 

Basin fill in the Estufa bolson is less deeply eroded and 

dissected than in the Delaho bolson, and in places along the 

western edge of the Estufa bolson, the fill sequence is largely 

intact. The older part of the fill, named the informal Banta 

Shut-in formation by Stevens and Stevens (1989, 2003) and 

mapped as Ta in this report, is made up of fine-grained, distal 

alluvial-fan and playa deposits interbedded with coarsergrained, mid-fan deposits. The formation contains large and 

small mammals, tortoises, lizards, snakes, and toads of late 

Miocene age (Screw Bean local fauna; Stevens and Stevens, 

1989, 2003), indicating that deposition began much later in the 

Estufa bolson than in the Delaho bolson. The upper Miocene 

section is overlain unconformably by a younger sequence of 

fill made up of gravelly alluvial-fan deposits, referred to as 

the informal Estufa Canyon formation by Stevens and Stevens 

(1989, 2003) and mapped as QTa in this report. The informal Estufa Canyon formation is considered correlative to the 

informal Fingers formation of the Delaho bolson (Stevens and 

Stevens, 1989); as with the informal Fingers formation, its age 

is unknown, but is considered possibly Pliocene–Pleistocene 

(Maxwell and others, 1967; Stevens and Stevens, 1989, 2003). 

Based on lithologic characteristics, gravels of the informal 

Estufa Canyon formation previously were correlated to an isolated outcrop of calcareous silt near Grapevine Spring (Maxwell and others, 1967); elephant teeth found in the calcareous 

silt suggest a Pleistocene age for that deposit (Lonsdale and 

others, 1955). However, we interpret the calcareous silt as part 

of a Pleistocene spring deposit that post-dates deposition of 

the upper bolson fill. Some beds within the gravelly alluvialfan deposits have been tilted near faults, or are offset, indicating that basin-bounding faults were still active at the time 

upper bolson fill was being deposited. 

(Top) Informal Estufa Canyon (QTa) and (Bottom) Banta Shut-in (Ta) 

Formations of Estufa bolson. (Photographs by Margaret E. Berry)
Rio Grande Integration History

Long-term aggradation represented by the bolson deposits was followed by degradation that resulted in the erosion of large volumes of rock and sediment from the Big Bend 

landscape during the Quaternary. This shift from aggradation to degradation was probably 

due to declining rates of fault activity and the establishment of the Rio Grande as an integrated drainage network. Since its establishment, the fluvial system in the BBNP region 

has been dominated by long-term incision, punctuated by episodes of aggradation that 

were likely related to fluctuations in climate and river hydrology.

The timing of river integration is still debated. Although some workers think that integration of the Rio Grande through the Big Bend region occurred relatively recently, around 

700 ka (middle Pleistocene; Pazzaglia and Hawley, 2004), others suggest that the Rio 

Grande was through-flowing by the end of the Pliocene or beginning of the Pleistocene 

(Gustavson, 1991; Dickerson and Muehlberger, 1994), an interpretation more consistent 

with the surficial geology of BBNP. The ancestral Rio Grande is thought to have expanded 

its drainage southward by aggrading basins along its course; as basins filled with sediment, 

drainages breached divides and spilled into the next lower basin downstream (Henry, 

1998; Connell and others, 2005). This southward growth of the river system is thought 

to have been facilitated by slowing rates of regional extension in the southeastern Basinand-Range, such that rates of deposition exceeded rates of tectonic subsidence (Connell 

and others, 2005). Work by Gustavson (1991) suggests that around 2.25 Ma, the ancestral 

Rio Grande breached the drainage divide at the southern end of the Hueco bolson, in 

western Texas (northwest of BBNP). Based on the extent to which Hueco bolson fill was 

subsequently eroded, breaching of this divide may have integrated the northern segment of 

the Rio Grande with an ancestral southern segment of the river, such that the Rio Grande 

became through-flowing to the Gulf of Mexico at that time (Gustavson, 1991; Mack and 

others, 2006). 

Extensive erosion of the Big Bend landscape inverted the topography of both the 

informal Fingers formation of the Delaho bolson (Burro Mesa) and the informal Estufa 

Canyon formation of Estufa bolson (for example, Hannold Hill area), which now stand in 

relatively high relief even though they were deposited in basins. The oldest alluvial deposits that post-date erosional inversion of the bolson deposits have strong calcic soils with 

cemented, stage IV carbonate horizons that probably took several hundred thousand years 

or longer to form (Machette, 1985), indicating that the gravel deposits on which the soils 

formed were deposited in the middle (or early?) Pleistocene (Berry and Williams, 2008). 

Given the amount of erosion required to invert the bolson topography prior to deposition 

of these gravels, in an environment where rates of erosion were probably relatively low, it 

is likely that long-term degradation of the Big Bend landscape associated with integration 

of the fluvial system began well prior to 700 ka (middle Pleistocene), and could be consistent with timing around 2.25 Ma (early Pleistocene).

Stratigraphy 47

Endangered Rio Grande Silvery Minnow

 in Big Bend National Park

Scientists with the USGS Texas Water 

Science Center collaborate with scientists 

at the National Park Service in Big Bend 

National Park, U.S. Fish and Wildlife Service, 

and the Mexican Maderas del Carmen and 

Cañon de Santa Elena protected areas to 

evaluate available habitat of the federally 

endangered Rio Grande silvery minnow. 

USGS scientists conduct field mapping 

studies of available habitat over a range of 

river flow critcial to spawning and dispersal 

of various life stages of this species.

—J. Bruce Moring, USGS Biologist, Texas 

Water Science Center (Austin)

(Above) The Rio Grande, its flood plain (densely vegetated), 

and late Pleistocene terrace (Qiw1, elevated surface to left 

of flood plain) upstream of Boquillas Canyon (carved in Santa 

Elena Limestone, background). U.S. is on the left side of river; 

Mexico is on the right side. Foreground is a remnant of the 

Qiw2 terrace. (Photograph by Margaret E. Berry) 

The Rio Grande silvery minnow.
48 Geologic Map of Big Bend National Park, Texas

Pleistocene and Holocene 

Landscape Development

Evidence that extensive erosion in BBNP continued 

throughout much of the Quaternary is provided by the multiple 

levels of pediment surfaces prominent in the park (fig. 8). The 

highest pediment level, considered middle (or early?) Pleistocene in age based on its relative age characteristics, is generally 30–50 m above the valley floor (Qoa). Below the highest 

level are more extensively preserved, intermediate pediment 

surfaces, generally 12–40 m (Qia2) and 6–24 m (Qia1) above 

the valley floor, that probably were formed during the middle 

and late Pleistocene. Still younger surfaces, generally 3 m or 

more above the valley floor, are probably late Pleistocene to 

Holocene in age (Qya2). 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 Chilicotal and Talley Mountains (Collins and others, 

2007, 2008) and the northwest mountain front of the Chisos.

Periodic changes in climate and river hydrology during the 

Quaternary interrupted long-term incision of the river system 

with episodes of aggradation (Dethier, 2001; Connell and others, 2005), represented in BBNP by multiple levels of coarsegravel terrace deposits along the Rio Grande and its tributary 

streams. The highest level is preserved in isolated remnants 

roughly 55–60 m above the Rio Grande flood plain (Qow), 

and is likely middle (or early?) Pleistocene in age based on soil 

development and geomorphic relations (Berry and Williams, 

2008). Three intermediate levels present along the Rio Grande, 

roughly 30–45 m (Qiw3), 18–25 m (Qiw2), and 12–18 m 

(Qiw1) above the Rio Grande flood plain are probably middle 

and late Pleistocene in age. Whereas remnants of the two higher 

intermediate levels are fairly local in their extent, the lowest 

intermediate terrace forms a broad, flat remnant surface that can 

be traced all along the stretch of Rio Grande that flows through 

the park. The lowest terrace levels, about 4–9 m (Qyw2) and 

3 m or less (Qyw1) above the active flood plain, are likely 

late Pleistocene to Holocene, and Holocene in age. In contrast 

to the coarse gravels that characterize the Pleistocene and late 

Pleistocene to Holocene terrace deposits, fine-grained sand and 

silt predominate the Holocene deposits.

In recent times (last 100 years), the Rio Grande has 

changed dramatically from a broad sandy riverbed to a narrow 

channel with steep banks and flood plain heavily vegetated by 

invasive plant species (Purchase, 2002; Dean and Schmidt, 

2008). Irrigation diversions and the construction of dams 

upstream have reduced flow and the occurrence of floods 

to the point where the river has been unable to transport its 

sediment load (Purchase, 2002). These factors, combined with 

increased bank roughness and decreased channel capacity, 

caused by the invasion of non-native vegetation, have resulted 

in significant aggradation of the flood plain and narrowing of 

the active channel (Dean and Schmidt, 2008). 

Figure 8 (Facing page). Examples of pediment sequences in 

BBNP. Labels mark surfaces of thin, gravel-rich, Quaternary-age 

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 

alluvial deposits; Qia1, younger of the intermediate alluvial 

deposits; Qya2, older of the young alluvial deposits; Qya1, 

younger of the young alluvial deposits; Qaw, active tributary wash 

and river deposits. 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 (toward 

the Chisos Mountains) in the Reed Camp area. (Photographs by 

Margaret E. Berry)

(Above) Tamarisk (salt cedar) in flood plain of the Rio Grande 

near Santa Elena Canyon. Tamarisk is an invasive plant species 

that chokes out native plants, and reduces the diversity of native 

plant and animal species in the riparian habitat. (Photograph by 

Daniel P. Miggins)
Qoa

Qia2 Qia1

Qya2

Qia2

Qya1

SANTA ELENA CANYON

Qia2

Qia2

Qia1

Qya2

Qia1

Qya1

Qaw

A

B

CHISOS MOUNTAINS

PUNTA DE LA SIERRA

Stratigraphy 49
50 Geologic Map of Big Bend National Park, Texas

Structure

Proterozoic Rifting 

Although the oldest recorded tectonism in BBNP is 

related to the late Paleozoic Marathon orogeny, Proterozoic 

rifting and protocontinental plate collision events likely had 

a profound influence in shaping BBNP’s landscape (Muehlberger, 1980; Dickerson, 1980; Page and others, 2008). These 

older events have been interpreted to have established the 

northwest trend of major structural features that characterize 

the park today (fig. 2).

The Texas lineament (Albritton and Smith, 1957; Muehlberger, 1980; Dickerson, 1980) is a zone of proposed 

northwest-striking basement faults extending from Presidio, 

Tex., southward into the Big Bend area, and into Mexico 

(fig. 9). The zone is deeply buried and Dickerson (1980) and 

Muehlberger (1980) speculated that faults of the lineament 

formed from rifting events as long ago as 1,500–1,000 Ma, 

and they emphasized that faults in the zone have been reactivated during subsequent tectonic events, including the late 

Paleozoic Marathon orogeny, Late Cretaceous–early Tertiary 

Laramide orogeny, and late Tertiary basin-and-range faulting. 

Rifting between the North American and South American plates was proposed to have occurred about 800–550 

Ma along the southern edge of the North American continent (Thomas, 1991; Poole and others, 2005), and produced 

northwest-striking transform faults interpreted to have laterally 

offset the southern continental margin (fig. 9). In the southern 

Texas part of the continental margin, the synrift transform 

faults were reported to be episodically active from about 600 

to 570 Ma, and apparently established a tectonic fabric in the 

continental crust that persisted at least through Paleozoic time 

(Poole and others, 2005), and probably well into the Cenozoic 

in the BBNP region as indicated by the prominent northwesttrending structural grain. The postulated position of the rift 

zone and offsetting transform faults shown in figure 9 is based 

on data points (Poole and others, 2005) indicating the location 

of stable North American continental basement rocks relative 

to South American volcanic-plutonic arc rocks. The relationship between the Neoproterozoic transform faults and faults 

of the Mesoproterozoic Texas lineament is unclear; although, 

they both are subparallel (fig. 9), postulated to have formed 

during rifting events at the southern continental margin, and 

they separate stable North American plate continental basement rocks from less stable, rifted South American plate 

volcanic and plutonic rocks.

Paleozoic Marathon Orogeny 

During the Paleozoic, BBNP was near the southern edge 

of the North American continent and was the site of shallow 

marine shelf deposition and complex interactions between the 

North American and South American tectonic plates (fig. 10). 

The oldest structures in the park are thrust faults associated 

with the Middle Mississippian to Early Permian Marathon 

orogeny (Poole and others, 2005). The Marathon orogenic 

belt (fig. 10) is one segment of the larger Ouachita-MarathonSonora orogenic belt (Poole and others, 2005) that extends 

along the Paleozoic continental margin from Mississippi and 

Arkansas (Ouachita segment), through Texas (Marathon segment), and into Chihuahua and Sonora, Mexico (Sonora 

segment). A regional synthesis of the Paleozoic tectonic 

history of BBNP and Trans-Pecos, Texas, is summarized in 

Muehlberger and Dickerson (1989).

In west Texas, deep-water ocean basin rocks originally 

deposited south of the Big Bend area were thrust northwestward onto the North American continent during the convergence between the North and South America plates 

(fig. 10). Paleozoic rocks of the Marathon belt are well 

exposed to the north of BBNP in the Marathon Basin, where 

they are deformed by northeast-striking, northwest-vergent 

thrust faults and folds (Anderson and others, 1982; Barnes, 

1979). Remnants of the Marathon orogenic belt are exposed 

in the Persimmon Gap area in northern BBNP (fig. 1), but 

elsewhere in the park, the record of the orogenic events is 

overprinted by later structures and buried by younger deposits. These remnants are characterized by small outcrop belts 

containing Paleozoic rocks (fig. 2) and northwest-directed 

thrust faults (Maxwell and others, 1967). Paleozoic rocks 

of the Marathon orogenic belt include, from base to top, the 

Middle and Upper Ordovician Maravillas Formation, Silurian 

to Mississippian Caballos Novaculite, and Mississippian to 

Pennsylvanian Tesnus Formation. The Maravillas Formation 

and Caballos Novaculite are pre-Marathon orogenic, allochthonous, deep-water basin rocks, and the Tesnus Formation 

is synorogenic flysch deposited into the Pedregosa foreland 

basin, in front of the advancing Marathon orogenic belt. 

Several northwest-vergent thrust faults duplicate the Paleozoic section and juxtapose rocks of the Maravillas Formation 

above the Tesnus Formation (Maxwell and others, 1967). The 

Paleozoic thrust block at Persimmon Gap was subsequently 

deformed during the Late Cretaceous–early Tertiary Laramide 

orogeny, which juxtaposed the Paleozoic thrust block with 

Cretaceous sedimentary rocks above and below. Preserved 

northwest vergent thrust faults of the Paleozoic Marathon 

orogeny and their relationship with later southwest vergent 

Laramide faults are shown in detailed geologic maps of the 

Persimmon Gap area (Maxwell and others, 1967; Poth, 1979). 
NEW MEXICO

TEXAS MEXICO

UNITED

STATES

NORTH AMERICAN PLATE

SOUTH AMERICAN PLATE

Presidio

Modified from Poole

and others (2005)

BIG BEND 

NATIONAL PARK

105° 100°

30°

0 100 200 KILOMETERS

Leading edge of

Marathon

orogenic belt

NEW MEXICO

TEXAS MEXICO

UNITED

STATES

NORTH AMERICAN PLATE

SOUTH AMERICAN PLATE

Modified from Poole

and others (2005)

BIG BEND 

NATIONAL

PARK

105° 100°

30°

0 100 200 KILOMETERS

NW SE

NORTH AMERICAN

PLATE

SOUTH

AMERICAN

PLATE

Ocean basin rocks

BIG BEND 

NATIONAL PARK

Structure 51

Following Paleozoic deformation, the Marathon orogenic highlands eroded during Triassic and Jurassic time. The 

absence of Triassic- and Jurassic-age rocks in BBNP reflects 

this period of erosion, which is characterized locally in the 

park, in the Persimmon Gap area, by a major unconformity 

separating rocks of the Lower Cretaceous Glen Rose Limestone above from rocks of the Mississippian–Pennsylvanian 

Tesnus Formation below. 

Figure 9. Map showing transform faults and lineaments related to 

Proterozoic rifting along the southern North American continental 

margin. Solid black lines are Neoproterozoic transform faults 

(Thomas, 1991; Poole and others, 2005); arrows show relative 

motion of continental plate offset; gray double-banded line is 

interpreted rift zone between North and South American plates; 

solid red lines are Texas lineament (Muehlberger, 1980); dark 

green area is BBNP.

Figure 10. Map showing location of Paleozoic Marathon orogenic 

belt (gray), formed from convergence between the North and 

South American plates. Dotted black lines are traces of Proterozoic 

transform faults shown in figure 9; solid red line is approximate line 

of section for inset figure; dark green area is BBNP.
BIG BEND

NATIONAL PARK

Diablo platform

Chihuahua trough

SW NE

Chihuahua trough

Diablo platform

NEW MEXICO

TEXAS

MEXICO

UNITED STATES

BIG BEND 

NATIONAL

PARK

105°

30°

0 100 200 KILOMETERS

Modified from

Goldhammer (1999)

52 Geologic Map of Big Bend National Park, Texas

Triassic to Cretaceous Rifting

From the Late Triassic into the Late Cretaceous (about 200–

85 Ma), the BBNP area experienced subtle effects 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. 11) (Henry, 1998; Goldhammer, 1999), where sediments 

of Lower and Upper Cretaceous limestone and shale units were 

deposited in a shallow, open-marine shelf environment. These 

units include, from base to top, the Glen Rose Limestone, Maxon 

Sandstone, Telephone Canyon Formation, Del Carmen Limestone, Sue Peaks Formation, Santa Elena Limestone, Del Rio 

Clay, Buda Limestone, Boquillas Formation, and Pen Formation. 

Southwest of the platform, an ocean basin known as the 

Chihuahua trough formed in association with the opening of the 

Gulf of Mexico (fig. 11) (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 (fig. 11) controlled 

subsidence of the trough from the Jurassic to Late Cretaceous 

time (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–early Tertiary Laramide orogeny 

was a period of contractional deformation that produced 

monoclinal uplifts, basins, and faults and folds from northeastsouthwest compression broadly related to subduction of the 

Farallon plate at the western edge of North America (fig. 12). 

The plate convergence formed the Cordilleran overthrust belt, 

which extends from Canada to Mexico in the intermountain 

region of western North America, and the Laramide foreland 

east of the overthrust belt (fig. 12). The two provinces are temporally and structurally distinct, and thin-skinned Cretaceous 

deformation of the overthrust belt generally pre-dated thickskinned, basement-involved deformation of the Laramide foreland (Dickinson and others, 1988). The Chihuahua tectonic 

belt, southwest of BBNP (fig. 12), represents the frontal edge 

of the overthrust belt in the region. Structures in BBNP likely 

formed in the Laramide foreland from vertical uplift along 

basement-involved transpressional faults, and broad folding, 

similar to other structures in the foreland of Wyoming, Colorado, New Mexico, and Arizona (Muehlberger, 1980). 

The major Laramide structures in BBNP include the Mesa 

de Anguila monocline, an uplifted monocline on the southwest 

margin of the park; the Sierra del Carmen-Santiago Mountains 

monocline, an uplifted and thrust-faulted monocline bounding 

the eastern part of the park; and the Tornillo basin (Lehman, 

1991), which developed between the uplifted monoclines 

(fig. 12). Major Laramide folds in BBNP include the Mariscal 

Mountain and Cow Heaven anticlines (fig. 12). Thrust faults in 

BBNP are mostly southwest-vergent and include the Santiago 

thrust (Poth, 1979; Cobb, 1980; Cobb and Poth, 1980) in the 

southern Santiago Mountains (cross section A–A' ), and smallscale thrusts at Mariscal Mountain (Maxwell and others, 1967) 

and in the southern Sierra del Carmen (Maler, 1987). 

Figure 11. 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.
MEXICO

MEXICO

TEXAS

TEXAS

Rio Grande

Rio Grande

Sierra del Carmen-Santiago Mountains monocline

Mesa de Anguila 

monocline Cow Heaven anticline

Marathon

uplift

TORNILLO

BASIN

Santiago

thrust

Mariscal Mountain anticline

Terlingua uplift

Terlingua monocline

0 5 10 15 20 KILOMETERS

103°30’ 103°00’

29°00’

29°30’

Modified from Lehman

and Busby (2007)

Subduction zone

Chihuahua

tectonic

belt

?

?

?

?

?

NORTH

AMERICAN

PLATE Laramide

foreland

FARALLON

PLATE

Cordilleran overthrust belt

BIG BEND 

NATIONAL

PARK

0 500 1,000 KILOMETERS

Structure 53

Figure 12. Generalized map of the major structures that formed in the BBNP area during the Laramide orogeny. Inset map 

shows location of North American and Farallon plates, subduction zone along the western North American continental margin, 

Cordilleran overthrust belt, and Laramide foreland in western North America, BBNP in dark green, and Chihuahua tectonic belt 

southwest of BBNP.
Northwest view of the southern Santiago Mountains, extending from Dog Canyon area to Persimmon Gap. Shown in foreground are 

outcrops of Buda Limestone (ledge) and overlying Boquillas Formation (slope), which are in the lower plate of the Santiago thrust fault. 

The main ridge of southern Santiago Mountains is composed mostly of the Lower Cretaceous Glen Rose Limestone, which forms the 

upper plate of the Santiago thrust fault. Shown along main ridge is scar from 1987 rock slide. (Photograph by Kenzie J. Turner)
Kgr

Kp SANTIAGO THRUST FAULT

56 Geologic Map of Big Bend National Park, Texas

The Mesa de Anguila monocline was described by 

DeCamp (1981, 1985) as a northeast-directed monoclinal flexure that formed from northeast-directed Laramide compression of the Chihuahua tectonic belt against the stable Diablo 

platform in the BBNP area. DeCamp (1981, 1985) discussed 

evidence for left-lateral strike-slip faults at Mesa de Anguila 

and speculated that these faults largely 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 uplift (fig. 12), west of BBNP 

(Erdlac, 1990). The age of the Terlingua uplift is reported to 

be between 68 and 50 Ma, and it was interpreted as a pushup block, or transfer zone, which primarily accommodated 

left-lateral shearing related to northeast-directed Laramide 

compression (DeCamp, 1981, 1985; Muehlberger, 1989; 

Muehlberger and Dickerson, 1989; Erdlac, 1990).

The Sierra del Carmen-Santiago Mountains monocline 

(cross sections B–B' and C–C') forms the southwestern margin 

of the much broader Marathon uplift (fig. 12) (Lehman, 1991; 

Ewing, 1991), a Laramide dome, which uplifted and exposed 

deformed Paleozoic strata northeast of BBNP. The monocline 

is thrust-faulted in the Santiago Mountains by the Santiago 

thrust fault (Cobb, 1980; Cobb and Poth, 1980), a southwestdirected thrust which juxtaposes mainly Lower Cretaceous 

rocks in the upper plate above the Upper Cretaceous Aguja 

and Pen Formations in the lower plate (cross section A–A' ). 

The thrust extends southward into the Dagger Flat area, where 

it dies out into a series of range blocks containing several 

monoclines formed during the Laramide (Moustafa, 1988), 

that were later domed by middle Tertiary intrusions, and 

faulted by late Tertiary normal faults. Maler (1987) mapped 

some small-scale, southwest-directed thrust faults in the southern Sierra del Carmen, northwest of Boquillas Canyon (pl.), 

and like the Santiago thrust, they are southwest vergent and 

juxtapose Lower Cretaceous rocks in the upper plate above 

Upper Cretaceous rocks in the lower plate. 

Dickerson (1980) interpreted the Sierra del CarmenSantiago Mountains monocline as a raised basement ridge 

that was uplifted during the Laramide along reactivated 

basement-cored faults. Muehlberger (1980) proposed the 

Sierra del Carmen-Santiago Mountains monocline to have 

formed by vertical uplift from upthrusting and drape folding, 

and described it as a typical Laramide basement-involved 

uplift, similar to other basement-cored uplifts in the Laramide 

foreland. Lehman and Busbey (2007) also noted similarities 

between the uplift with other basement-cored uplifts in the 

Laramide foreland. Although the origin of basement-cored 

uplifts remains debatable in BBNP and other parts of the 

Laramide foreland, and whether they resulted from vertical 

or horizontal tectonism, or a combination of both, the Sierra 

del Carmen-Santiago Mountains monocline was attributed to 

horizontal compression and left-lateral shearing along major 

transpressional faults (Cobb and Poth, 1980; Muehlberger, 

1980, 1989; Moustafa, 1988; Muehlberger and Dickerson, 

1989; Maler, 1990), similar to the Terlingua uplift described 

by Erdlac (1990), west of BBNP. Other models of transpressional deformation associated with basement-cored uplifts 

in Laramide foreland of Colorado and New Mexico were 

described by Chapin and Cather (1983).

View near Persimmon Gap showing the approximate trace of the Santiago thrust fault (thick dashed line; 

arrow shows direction of movement on fault), and southwest-vergent, overturned anticline in the Lower 

Cretaceous Glen Rose Limestone (Kgr) in the upper plate of the thrust, above the Upper Cretaceous Pen 

Formation (Kp) in the lower plate. (Photograph by Kenzie J. Turner)
Structure 57

The Tornillo basin of Lehman (1991) formed between the 

Mesa de Anguila and Sierra del Carmen-Santiago Mountains 

monoclines (fig. 12). The basin is asymmetric with the deepest 

part adjacent to the western flank of the Sierra del CarmenSantiago Mountains monocline. The 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. Based on sedimentological and structural data, 

Lehman (1991) reported that Laramide deformation in the 

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. 

The Canoe Formation unconformably 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 include the 

northwest-trending Mariscal Mountain and Cow Heaven 

anticlines (fig. 12). The Mariscal Mountain anticline is an 

asymmetric fold that is deformed by small-displacement 

thrust faults (pl.; cross section C–C'). Maxwell and Dietrich 

(1965), Maxwell and others (1967), and Bumgardner (1976) 

interpreted mafic sills, partly concordant with bedding planes 

in the anticline, were emplaced prior to Laramide folding 

based on field relations and preliminary K-Ar geochronology. 

Henry and McDowell (1986) reinterpreted the mafic sills were 

emplaced after Laramide folding based on field observations 

and on K-Ar age determinations for sill rocks at about 38 Ma 

(Henry and others, 1986), and paleomagnetic and geochronologic investigations by Harlan and others (1995) further 

supported the sills were emplaced at about 37 Ma, following 

Laramide folding. Our age determinations for the mafic sills 

at Mariscal Mountain were inconsistent, and we obtained a 

U-Pb zircon age as old as 46.5 ± 0.3 Ma, and an 40Ar/39Ar age 

as young as 36.11 ± 0.19 Ma (appendix; 86). All these ages for 

the mafic sills are compatible with widespread mafic magmatism which post-dated major episodes of Laramide folding 

throughout the BBNP region (Henry and others, 1986). The 

Cow Heaven anticline has mafic sills like Mariscal Mountain, 

and although these rocks were not dated, they probably were 

emplaced in a similar manner and time period as the sills at 

Mariscal Mountain. The Cow Heaven anticline is an asymmetric fold, with steep easterly dips (as great as 80°) along the 

east flank of the anticline (cross section C–C'). 

North-northwest-trending folds in Cretaceous rocks are 

present in other parts of the park (pl.), such as those east of 

Terlingua Creek, near Slickrock Mountain, and southeast 

of McKinney Hills, and many of these are likely Laramide 

structures; however, some of the folded Cretaceous rocks 

are adjacent to Oligocene intrusions; therefore, it is difficult 

to determine if folds are Laramide, or are Laramide folds 

reactivated during Oligocene magmatism. Maxwell and others 

(1967) discussed probable Laramide folds in Cretaceous rocks 

in BBNP that they interpreted as refolded during Oligocene 

magmatism. Other folds deform volcanic and intrusive rocks 

in the Chisos Mountains, and formed primarily during Oligocene magmatism. Some examples include the Hayes Ridge 

anticline, attributed to collapse of the Pine Canyon caldera 

(Ogley, 1978), local folds formed from doming at Sierra 

Quemada (Scott and others, 2007), and folds adjacent to other 

domes, plutons, and intrusions. 

Mafic sill (above thin brownish-weathering bed) in the Pen Formation (below brownish-weathering bed) at 

Mariscal Mountain. (Photograph by Daniel P. Miggins)
Middle Tertiary Volcanic and Intrusive Features 

Pine Canyon Caldera

At about 32 Ma, the Pine Canyon caldera complex (fig. 2; 

pl.; cross section B–B') erupted rocks of the South Rim Formation, which are mostly preserved in the high Chisos Mountains. 

Pre-eruptive development of the caldera began with doming of 

the country rock as a result of rising magma, as indicated by 

thinning of the Chisos Formation, and an angular unconformity between the Chisos and overlying South Rim 

Formation units around the perimeter of the caldera 

(Maxwell and others, 1967; Ogley, 1978; Barker and 

others, 1986). A down-sag style of collapse was 

interpreted for the caldera based on 

inward dips in the Chisos Formation and the apparent lack of 

ring faults (Ogley, 1978; Barker and others, 1986). Initial caldera subsidence accompanied eruption of the Pine Canyon rhyolite, causing the tuff to pond within the caldera (cross section 

B–B'). Following initial caldera development, rocks of the Boot 

Rock member erupted from multiple vents around the periphery 

of the caldera, and were deposited within the caldera, in the 

South Rim area, and as far west as Burro Mesa. Ponding of the 

Boot Rock member within the caldera suggests that collapse 

may have continued after eruption of the Pine Canyon rhyolite, 

or the caldera simply remained as a topographic low (Barker 

and others, 1986). Volcanic activity related to the Pine Canyon 

caldera concluded with eruption of the Emory Peak rhyolite 

from vents along the caldera periphery (Ogley, 1978; Barker 

and others, 1986; Benker, 2005; White and others, 2006). 

South-dipping units of the South Rim 

Formation along the southeast flank of the 

Pine Canyon caldera, near the mouth of 

Pine Canyon. (Photograph by Don Corrick)

Intrusive activity related to the Pine Canyon caldera 

magmatic system mainly occurred after caldera development 

(Ogley, 1978; Barker and others, 1986), and included emplacement of the Hayes Ridge ring dike (Ogley, 1978) and Ward 

Mountain-Pulliam Peak pluton (fig.2; pl.). Emplacement of 

these intrusions likely followed the main pulse of caldera-forming 

extrusive activity, based on deformed South Rim outflow 

deposits adjacent to the intrusion northwest of Pulliam Peak and 

on the west side of the South Rim (Maxwell and others, 1967). 

The Ward Mountain-Pulliam Peak intrusion uplifted Cretaceous 

strata, which are exposed adjacent to the intrusion in The Basin. 

Recent analysis of aeromagnetic and gravity data revealed 

new details of the structure, igneous geology, and temporal evolution of the Pine Canyon caldera (Drenth and Finn, 2007). The 

primary intracaldera fill and oldest unit of the South Rim Formation, the Pine Canyon rhyolite, is reversely magnetized, and 

aeromagnetic data were used to estimate caldera fill thickness. 

Modeling of gravity and aeromagnetic data indicates that Pine 

Canyon rhyolite is probably thickest in the northeastern part of 

the caldera, where it may be greater than 1 km thick (Drenth 

and Finn, 2007). Lineaments interpreted from high-resolution 

aeromagnetic data (Drenth and Finn, 2007) suggest the presence 

of buried faults beneath the caldera fill (cross section B–B' ), 

which may have led to increased subsidence in the northeast 

part of the caldera, allowing for thicker sections of caldera fill to 

accumulate there.
60 Geologic Map of Big Bend National Park, Texas

Sierra Quemada

The Sierra Quemada dome and intrusive complex (fig. 2; 

cross section B–B') is a strikingly circular feature in the southwest part of the Chisos Mountains (pl.). Previous workers 

interpreted Sierra Quemada as a caldera formed by eruption 

of the Mule Ear Spring Tuff Member of the Chisos Formation 

at 33.65 Ma (Ogley, 1978; Henry and Price, 1984; Duex and 

Tucker, 1989; Duex, 2007). However, Scott and others (2007) 

concluded that Sierra Quemada was most likely not a caldera, 

but was formed as rising magma domed the overlying country 

rock (cross section B–B' ), followed by intrusion of a nearly 

continuous, circular rhyolitic ring dike (pl.) between about 

31–30 Ma. No evidence for collapse or faults connecting gaps 

in the ring dikes was found. Intrusive activity was accompanied, or possibly preceded, by minor explosive venting, which 

is preserved as several irregularly shaped bodies of lithic-rich 

volcanic breccia. Lithic clasts consist of Chisos Formation volcanic rocks, some Paleozoic rocks, and blocks of Cretaceous 

limestone as large as 200 m in length. No outflow units were 

observed suggesting only a small volume of gaseous material 

erupted before venting ceased (Scott and others, 2007). 

Dominguez Mountain

A small volcano formed at Dominguez Mountain with 

numerous vertical dikes radiating into the surrounding country 

rock (pl.), and the mountain still preserves the original cone 

shape of this volcano. The volcano has not been studied in 

detail, but good descriptions exist for the rocks that compose 

the volcano and dikes (Maxwell and others, 1967). Small 

granitic, intrusive bodies surround the volcano at Dominguez 

Mountain, and its interior is a mixture of layered flows and 

dense dike swarms. The layered rocks dip gently outward from 

the volcanic center and likely originated as lava flows forming 

the flanks of the volcanic buildup (Bohannon, 2011). The dike 

swarm cuts the layered rocks vertically and forms large bodies 

of rock in which dikes intrude other dikes to the extent that 

country rock is nearly absent. The dikes include both mafic and 

intermediate rock types, with intermediate types being more 

prevalent. Most dikes radiate outward from the volcanic center 

to the southwest but dikes radiate to the east and north as well. 

The most conspicuous dikes to the southwest of the center (pl.) 

merge upward into a series of 30-Ma trachytic flows (Tt) at the 

top of Punta de la Sierra (Bohannon, 2011). The source of the 

30-Ma flows is unknown, but their spatial proximity and compositional similarity to the dike swarm suggests that Dominguez Mountain should be considered as a possibility. 

Major Laccoliths and Sills

Many of the larger intrusive bodies in BBNP were 

emplaced at about the same time as the Pine Canyon caldera 

(about 32 Ma). The laccoliths at the Rosillos Mountains, 

McKinney Hills, and Grapevine Hills (fig. 2) were emplaced 

between 33 and 32 Ma (appendix; Miggins and others, 2007), 

and may be related based on similar geochemical and petrologic 

characteristics (Maxwell and others, 1967). Although the timing 

of emplacement for these intrusions is similar to the Pine Canyon caldera, it is unlikely that they are directly related. However, it is clear there was a significant influx of magma rising 

into the park at the time of caldera eruption. 

Integrated detailed geologic mapping and analysis of highresolution aeromagnetic data indicate the major laccoliths and 

sill complexes in BBNP show a variety of geometries ranging 

from single to stacked, mostly concordant intrusive bodies (Scott 

and others, 2004a and b; Anderson, 2007). The surficial exposure 

of these intrusive bodies ranges from nearly completely exposed 

(Rosillos Mountains), to partly exposed (McKinney Hills, 

Grapevine Hills, Dagger Flat, and Bone Spring) (fig. 13).

The laccolith at Rosillos Mountains (Scott and others, 

2004b; Anderson, 2007) is one of the largest in BBNP, and is 

semicircular in shape (10.5 km by 7.5 km) with its long axis 

striking north-northwest (fig. 13). Geophysical modeling shows 

the laccolith at Rosillos Mountains is one intrusive body greater 

than 600 m thick at the northern end, and less than 200 m thick 

at the southern end. The laccolith at McKinney Hills is a more 

complex feature consisting of several stacked, quasi-concordant intrusive bodies (cross section B–B' ). The upper body is 

exposed in the eastern part where it overlies the Pen Formation, 

and the lower body is in the western part where it is mostly concealed, and underlies the Pen Formation (cross section B–B' ). 

These relations led Maxwell and others (1967) to interpret “pine 

tree” geometry for the laccolith, with different bodies extending laterally at different stratigraphic levels. Aeromagnetic 

data indicate that the laccolith at Dagger Flat is adjacent to the 

laccolith at McKinney Hills, and although it has limited surface 

expression, its subsurface distribution is larger than the laccolith 

at McKinney Hills (fig. 13) (Scott and others, 2004a; Morgan 

and Shanks, 2008). A positive magnetic anomaly extends about 

5 km east of the laccoliths at McKinney Hills and Dagger Flat 

(fig 13; intrusive 1); modeling of the anomaly indicates an 

intrusive body about 300 to 500 m thick with its top about 300 

to 1,000 m depth (east end of cross section B–B'). It is unknown 

whether this is a separate body from the two laccoliths or an 

extension of one of the two. The laccolith at Grapevine Hills 

consists of a single intrusive body about 3.5 km wide and about 

200 m thick. The laccolith at Bone Spring is about 50 to 100 m 

thick (cross section A–A'), and about 6 km by 3 km wide, with 

the long axis trending north-northeast (fig. 13). Aeromagnetic 

data show a laccolith similar in size to the laccolith at Bone 

Spring on the downthrown side of the Chalk Draw fault (intrusive 2), north of the Rosillos Mountains (fig. 13); the laccolith 

has limited surface expression, and is characterized by several 

small, scattered outcrops. 
Tirm

Timh

Tigh

Tib

Tib

Tib

Tib

Tib

Tib

Tib

DUGOUT WELLS

FAULT

CHALK DRAW

FAULT

Ridge crest 

anomalies

BIG BEND

NATIONAL PARK

BOUNDARY

Persimmon 

Gap

?

?

?

RM

DF

DF

MH

GH

BS

1

2

103°22’30” 103°00’00”

29°37’30”

29°22’30”

Normal fault—Bar and ball on 

 downthrown side

Approximate intrusion boundary 

EXPLANATION

0 5 KILOMETERS

683 688 693 698 703 708 713 718 724 729 734 739 744 749 754 759 765 770 775

nT

Structure 61

Figure 13. Reduced-to-pole magnetic anomaly map of northern BBNP. Map shows approximate intrusion boundaries (thick 

black-dashed lines) compared to intrusion surface exposure (solid white lines labeled by map unit). Map also shows linear 

magnetic anomalies correlated with Chalk Draw fault and northern end of the Dugout Wells fault. Laccoliths at Bone Spring, BS; 

Dagger Flat, DF; Grapevine Hills, GH; McKinney Hills, MH; Rosillos Mountains, RM; unnamed intrusion 1, 1; unnamed intrusion 

2, 2; basaltic and other mafic composition intrusive rocks, Tib; fayalite syenite of McKinney Hills, Timh; syenite of Rosillos 

Mountains, Tirm; and fayalite syenite of Grapevine Hills, Tigh.
62 Geologic Map of Big Bend National Park, Texas

Tectonic Setting for BBNP Magmatism

Magmatic activity in the Big Bend region occurred from 

about 47 Ma to at least 17 Ma, extending from shortly after the 

Laramide orogeny to well after the onset of basin-and-range 

rifting; the opposing stress regimes induced by the two tectonic 

events influenced magma source, generation, and compostion. 

The Laramide orogeny corresponds to shallow subduction of 

the Farallon plate beneath the western margin of the North 

American plate (fig. 12) that resulted in an eastward-migrating 

continental arc in a compressional stress regime. A compressional regime during the Laramide orogeny was succeeded by 

an extensional regime associated with basin-and-range rifting 

after 31 Ma, but no later than 28 Ma (Henry and others, 1991). 

Basalts with a distinct rift-related composition common to 

basin-and-range rifting were first erupted around 24 Ma, and 

persisted until 17 Ma (Henry and others, 1986), and possibly 

longer, but none of these basalts have been identified in BBNP.

Barker (1977) proposed all Trans-Pecos magmatism 

resulted from rifting based on the abundance of alkalic rocks 

and the assumption that extension-related faulting and magmatism were contemporaneous. However, later studies have 

shown a compressional regime persisted until about 31 Ma, 

with an extensional regime starting no earlier than 31 Ma, and 

rift-related normal faulting began about 25 Ma (Stevens and 

Stevens, 1985; Henry and Price, 1986; Stevens, 1988; Henry 

and others, 1991). Price and others (1987) and James and Henry 

(1991) identified a compositional shift in mafic rocks throughout Trans-Pecos Texas corresponding to the stress regime 

change around 31 Ma. 

White and others (2006) speculated that rifting began 

at around 36 Ma, based on the assumption that generation of 

peralkaline magma associated with the Pine Canyon caldera, 

required extension. They applied the early continental rift model 

of Lawton and McMillan (1999), developed for the Rio Grande 

rift in New Mexico, that suggested foundering of the subducting 

slab caused uplift, rifting, and peralkaline and bimodal volcanism. Parker and McMillan (2007) suggested that peak magmatism in Trans-Pecos Texas between 38–35 Ma was an indication 

of early rifting based on the assumption that peralkaline magma 

generation is restricted to extensional environments. However, 

structural data supporting the onset of rifting prior to 31 Ma are 

lacking (Henry and others, 1991). 

Late Tertiary Basin-and-Range Faulting

The last major tectonic episode in BBNP was basin-andrange faulting which occurred in the park region from about 

25 to 2 Ma (Henry, 1998). Basin-and-range faulting resulted 

from rifting across the Basin-and-Range Province of western North America. The rifting caused stretching or pulling 

apart of the western North American continental crust to form 

uplifted ranges and intervening down-dropped basins. Big 

Bend National Park is part of the Rio Grande rift, an eastern 

subprovince of the Basin-and-Range, which extends northward from Texas, through New Mexico, and into southern 

Colorado. Most basin-and-range faults in the BBNP area are 

north-northwest-striking, high-angle normal faults (fig. 2), 

which formed perpendicular to east-northeast directed extension in the region (Henry, 1998; Henry and Price, 1986).

Some of the basin-and-range faults formed local depositional basins, such as the Delaho bolson, which extends 

beyond the boundaries of BBNP, and Estufa bolson on the 

eastern flank of the Chisos Mountains (Stevens and Stevens, 

1989). The Delaho bolson is characterized by numerous 

northwest-striking faults, and the Estufa bolson is bound on 

the west side by the Dugout Wells fault (fig. 2). The bolsons 

contain thick accumulations of Miocene to Pleistocene sediments (Ta and QTa, pl.). Smaller depositional basins in BBNP 

include the Solis basin (Maxwell and other, 1967), east of 

Mariscal Mountain (fig. 2), and several local basins in the 

Sierra del Carmen (pl.). Most of these basins also contain units 

as old as Pliocene to early Pleistocene age (QTa).

Southwest of the Chisos Mountains, northwest-striking 

faults in the Delaho bolson are steep (greater than 70o dips) and 

mostly down to the southwest (fig. 2; pl.). These faults repeat 

sections of the Chisos Formation and overlying Tertiary basinfill sediments (Ta) from Goat and Kit Mountains, southward 

to Sierra de Chino (pl.; cross section B–B' ). The fault with the 

greatest amount of offset juxtaposes Eocene and Oligocene 

sedimentary units of the Chisos Formation in the footwall 

against Miocene alluvium in the hanging wall, with about 

500 m of throw. Dickerson and Muehlberger (1994) interpreted 

the Delaho bolson as a deep, rift basin based on the presence 

of syn-rift basaltic dikes with mantle-derived xenoliths in the 

northwestern part of the bolson near Terlingua (fig. 1).
Sierra Ponce

(MEXICO)

Mesa de Anguila

(USA)

TERLINGUA FAULT

Structure 63

Stevens (1969) and Muehlberger (1989) interpreted the 

northeast margin of the Delaho bolson to be defined by the 

Burro Mesa fault, and the southwest margin by the Terlingua 

fault (fig. 2), indicating the bolson extends over a distance 

of about 25 km in western BBNP. The Burro Mesa fault is a 

down-to-the-west normal fault exposed on the east flank of 

Burro Mesa. The northern segment of the fault is reported to 

have about 900 m of offset (Maxwell and others, 1967), and 

it downdrops rocks of the Chisos Formation against the Aguja 

and Pen Formations. The southern segment of the fault downdrops rocks of the Burro Mesa Formation and units as young 

Pliocene–Pleistocene basin-fill sediments (QTa), against units 

of the Chisos Formation, including the Bee Mountain Basalt 

Member (pl.). The southernmost part of the fault dies out in the 

undifferentiated units of the younger part of the Chisos Formation, west of Sierra Quemada (Maxwell and others, 1967). 

The Terlingua fault (fig. 2) forms steep limestone cliffs on 

the northeast side of Mesa de Anguila near the mouth of Santa 

Elena Canyon, and kinematic data indicate mostly normal 

displacement (DeCamp, 1981). At the mouth of Santa Elena 

Canyon (fig. 1), the fault has about 850 to 900 m of displacement (Maxwell and others, 1967; DeCamp, 1981; Dickerson 

and Muehlberger, 1994; Lehman and Busbey, 2007), and juxtaposes the Aguja Formation in the hanging wall against Glen 

Rose Limestone in the foot wall. The fault zone extends 30 km 

southward into Mexico where throw increases to more than 

1,300 m, and it defines the northeast edge of Sierra Ponce, the 

southeastern extension of Mesa de Anguila (DeCamp, 1981). 

Throw on the fault decreases northwest of Santa Elena Canyon.

Southwest view of Santa Elena Canyon showing the approximate trace of the Terlingua fault (thick dashed line; bar and ball on 

downthrown side), which forms the steep escarpment on the northeast side of Mesa de Anguila (Texas) and Sierra Ponce (Mexico). At 

the mouth of the canyon on the U.S. side, the fault juxtaposes Lower Cretaceous strata (Glen Rose Limestone) in the foot wall, against 

Upper Cretaceous strata (Aguja Formation) in the hanging wall. (Photograph by Daniel P. Miggins)
Calcite-lined fracture in Pliocene–Pleistocene alluvial-fan gravels (QTa) in the Dugout Wells fault 

zone near Estufa Spring; steepened dip of gravels caused by faulting. (Photograph by Van S. Williams) 
DUGOUT

WELLS

FAULT

Structure 65

The Estufa bolson (Stevens and Stevens, 1989) is about 

25 km long and 10 km wide, and is bound on the west by the 

Dugout Wells fault (fig. 2; pl.; cross section B–B'). The Dugout Wells fault is covered along most of its projected trace, but 

extends northward to the Grapevine Hills area (figs. 2, 13). 

Southward, the fault loses throw and likely dies out about 1 

to 2 km south of its intersection with cross section C–C'. The 

fault was interpreted to be active, based on offset caliche horizons and unlithified sediments, and disrupted stream drainages 

(Stevens and Stevens, 1989), but new mapping of surficial 

deposits in BBNP indicated no clear evidence for active faulting. The fault offsets rocks as young as Pliocene–Pleistocene 

basin-fill sediments (QTa) in the Estufa Spring area, and near 

its southern end. 

Numerous northwest-striking basin-and-range faults 

in Sierra del Carmen (fig. 2; and pl.) originated from eastnortheast-directed extension across the BBNP region (Henry, 

1998). The major faults in the Sierra del Carmen are down to 

the east (cross sections B–B' and C–C'), and they formed a 

series of uplifted blocks and intervening grabens that characterize the range. The most prominent uplifted blocks in 

the Sierra del Carmen form steep limestone escarpments of 

Sierra del Caballo Muerto and Cuesta Carlota (pl.). Maxwell 

and others (1967) referred to the Sierra del Caballo Muerto as 

the backbone ridge of Sierra del Carmen, and it contains Sue 

Peaks, one of the highest points (1,784 m) in the entire range 

(east end of cross section B–B'; pl.). Normal faults bounding 

Sierra del Caballo Muerto have estimated offsets of about 500 

m (Maxwell and others, 1967). The most prominent graben in 

Sierra del Carmen is Ernst basin (Ernst Valley of Maxwell and 

others, 1967), on the east side of Cuesta de Carlota (pl.). 

East view toward Dugout Wells fault near Estufa Spring. Approximate location of fault is shown by dashed line, bar and ball on 

downthrown side. Low hills on downthrown side are Pliocene-Pleistocene basin-fill sediments (QTa) of Estufa bolson. Rocks on the 

upthrown side are gray and red units of the Canoe Formation. (Photograph by Van S. Williams)
66 Geologic Map of Big Bend National Park, Texas

The Rosillos Mountains, Chalk Draw basin, and Bone 

Spring areas of northern BBNP contain numerous northweststriking basin-and-range faults, and a majority of these faults 

are down to the northeast (pl.; cross section A–A'). The most 

prominent is the Chalk Draw fault, which forms the escarpment on the west side of Chalk Draw basin (pl.; fig. 2). The 

fault strikes northwest, and juxtaposes rocks as old as the 

Santa Elena Limestone in the footwall, against mainly rocks of 

the Aguja Formation in the hanging wall, and offset is estimated to be about 700 m (pl.; cross section A–A' ). The fault 

forms a linear anomaly in the reduced-to-pole magnetic data 

(fig. 13) (Anderson, 2004); the lineation in the magnetic data 

is defined by a series of aligned ridge-crest anomalies (fig. 13), 

which reflect offset of shallow, magnetic intrusive rocks in 

the subsurface. In the Chalk Draw fault area, these rocks are 

probably Tertiary mafic sills like those that have intruded parts 

of the Boquillas Formation on the mesa to the west (pl.). The 

fault extends southward across the northeastern edge of the 

Rosillos Mountains (pl.) where offset decreases from about 

600 m along the northwestern part of the range, to about 200 

m along the southeastern part. Numerous other northweststriking faults offset the laccolith at Rosillos Mountains; geologic and geophysical models of the laccolith indicate these 

faults do not have significant vertical offset or any horizontal 

displacement (Anderson, 2007; Finn and Anderson, 2008). 

New mapping revealed a series of northwest-striking 

faults that offset the laccolith at Bone Spring and Cretaceous 

sedimentary rocks of the Aguja Formation (cross section 

A–A' ; pl.). These faults are steep (dips of greater than 70°),

and kinematic data indicate mostly dip-slip movement (W.R. 

Page, R.B. Scott, and L. Snee, unpub. mapping). A down-tothe-northeast fault at the southwestern edge of the laccolith 

was originally interpreted by Page and others (2004) as an 

active fault, based on apparent offset of Quaternary terraces. 

Upon further investigation, however, the Quaternary terraces 

were observed not to be offset, and were mapped as terraces of 

different age, indicating the fault is likely a Tertiary basin-andrange fault-line scarp. 

Basin-and-range faults predated integration of late Tertiary basins with the Rio Grande, which began about 2 Ma, 

and they were the last structures to shape BBNP’s presentday landscape. The Quaternary Period (2.6 Ma to present) in 

BBNP was characterized by extensive erosion, downcutting, 

and some aggradation related to integration of BBNP basins 

with the Rio Grande. 

Quaternary Landslides

The BBNP area contains several large landslide complexes (Qls) in the Chilicotal and Talley Mountains area 

(Collins and others, 2007, 2008), on the northwest flank of the 

Chisos Mountains (R.B. Scott, unpub. mapping; Bohannon, 

2011), and on the northeast flank of the Rosillos Mountains 

(R.B. Scott and W.R. Page, unpub. mapping). The age of the 

landslides in BBNP is Quaternary (Collins and others, 2007; 

Berry and Williams, 2008); some Tertiary basin-and-range 

faults are concealed by the slide masses, and younger surfical units are shown to overlap parts of the landslides. The 

landslides are recognized by their hummocky, irregular upper 

surfaces, disrupted strata, and they characteristically occur 

adjacent to areas of high relief, where more competent rock 

units overlie, or are adjacent to, less competent Cretaceous 

mudstone, claystone, shale, and sandstone units.

The landslides at Chilicotal and Talley Mountain areas (pl.) 

were mapped and described by Collins and others (2007, 2008), 

and consist of a mixture of Tertiary syenite sill material which 

slid over incompetent mudstone and sandstone units of the 

Cretaceous–Tertiary Javelina and Black Peaks Formations. 

Detailed mapping indicates these are composite landslides 

resulting from multiple events since middle Pleistocene time 

when the climate was wetter in the region. Numerous pressure 

ridges were noted over the fan-shaped landslide surfaces, and 

younger surficial deposits overlap parts of the landslides in places. 

One of the largest landslides in BBNP is on the northwest 

flank of the Chisos Mountains, north of Vernon Bailey and 

Pulliam Peaks (pl.) (R.B. Scott, unpub. mapping; Bohannon, 

2011). The landslide is complex, having formed during several 

different events. The rocks within it are variably fractured, but 

in the higher parts of the slide, large bodies of rock are still 

intact, and the original stratigraphy is somewhat preserved. 

Near the toe of the slide, the rocks are nearly pulverized and 

different rock types are mixed together. Most of the rocks 

involved in this slide are the Chisos Formation, including the 

Ash Spring Basalt. All these units slid northwestward over 

Cretaceous–Tertiary mudstone. 

The landslides on the north flank of the Rosillos Mountains 

(pl.) contain mostly Cretaceous sedimentary rocks of the Pen 

and Aguja Formations, and form rubbly zones downslope of 

the higher relief intrusive rocks. The landslides conceal Tertiary 

basin-and-range faults and are confined to zones where uplifted 

Cretaceous units flank the steep rim of the laccolith. 

Cobb (1980) mapped landslide deposits in the Santiago 

Mountains, on the west side of Persimmon Gap. The landsides 

cover an area of a few square kilometers, and consist mostly of 

rocks of the Glen Rose Limestone that slid over incompetent 

mudstone and sandstone of the Pen and Aguja Formations. 

Cobb (1980) reported the age of the slides to be Tertiary, and he 

interpreted them to have formed during basin-and-range faulting, 

but characteristics of the slides indicate they are likely Quaternary, similar to the age of other major landslides in BBNP. 
Approximate extent

of landslide

THE BASIN

Chisos Mountain

Lodge

Structure 67

View to the southeast of the large landslide on the north flank of the Chisos Mountains. The image is derived from Google EarthTM

(see http://earth.google.com/).

Ground view of the landslide in middle 

of photo (above yellowish-brown 

terrace), showing hummocky, irregular 

slide surface with disrupted strata. 

(Photograph by Van S. Williams)
68 Geologic Map of Big Bend National Park, Texas

Summary

The purpose of this map is to provide the National Park 

Service (NPS) and the public with an updated digital geologic 

map of BBNP. The geologic map of Maxwell and others (1967) 

is a comprehensive source of detailed geologic information for 

BBNP, and will remain an important data source for decades. 

However, the map is outdated and lacks a topographic base. Our 

objectives were to provide a new digital geologic map that can 

be utilized for standard GIS applications by NPS and the public, 

and to incorporate updated geologic information collected since 

the 1967 map was published, including the results of new studies 

carried out for this report. 

New studies carried out for this project resulted in collaboration between the USGS, university professors and students, 

the Texas Bureau of Economic Geology, and the NPS. The map 

incorporates new detailed geologic mapping of surficial deposits 

over the entire BBNP, and provides data critical in carrying out 

GIS-based natural resource and ecosystem management activities. 

Surficial mapping also forms the foundation for understanding 

Quaternary and modern active processes in the park as they relate 

to the geologic history of the Rio Grande. New geologic mapping of Cretaceous–Eocene sedimentary units in the entire BBNP 

contributed to a highly refined understanding of the stratigraphy 

and structure of the park. The map presents new geologic mapping in the high Chisos Mountains, sheds light on features such 

as Sierra Quemada and Dominguez Mountain, and presents new 

findings on the Chisos Formation. New detailed mapping of the 

recently acquired Harte Ranch section presents new findings on 

the geometry and structure of the laccoliths at Rosillos Mountains 

and Bone Spring. An extremely important contribution in terms 

of mapping the volcanic and intrusive units in BBNP includes 

new isotopic age determinations and trace element geochemical 

analyses, which has helped to better define and constrain the age 

and composition of these rocks in BBNP. The map also identifies the distribution of major Quaternary landslides not shown in 

Maxwell and others (1967).

The oldest rocks in the park are Paleozoic units, which 

include the Ordovician Maravillas Formation, SilurianMississippian Caballos Novaculite, and the MississippianPennsylvanian Tesnus Formation. Conodont samples from the 

Maravillas Formation collected for this project indicate the part 

of the formation exposed in the park is Late Ordovician in age. 

The Maravillas Formation and Caballos Novaculite were deposited in deep-water, basinal environments, and the Tesnus Formation is synorogenic flysch deposited in front of the advancing 

Marathon orogenic belt. The oldest recorded tectonic episode 

in BBNP is mountain building associated with the Marathon 

orogeny from Mississippian to Permian time (about 330–285 

Ma). During this tectonism, deep ocean basin rocks originally 

deposited south of BBNP were thrust northwestward onto the 

North American continent by convergence between the North 

and South American plates. Only remnants of the Marathon 

orogeny can be seen in the present-day BBNP landscape; these 

remnants include small outcrop belts in the Persimmon Gap area 

containing Paleozoic rocks and northwest-directed thrust faults.

The absence of Triassic and Jurassic rocks in BBNP reflects 

a major period of erosion, which is locally characterized by 

an unconformity separating the Lower Cretaceous Glen Rose 

Limestone above, from rocks of the Mississippian–Pennsylvanian Tesnus Formation below. From the Late Triassic to the Late 

Cretaceous (about 200–85 Ma), the BBNP area experienced 

subtle effects of rifting, or seafloor spreading, between North and 

South America; this rifting controlled the opening of the Gulf of 

Mexico. During rifting, BBNP was part of the Diablo platform, 

where sediments of Lower and Upper Cretaceous limestone and 

shale units were deposited. Lower Cretaceous rocks in BBNP, 

including the Glen Rose Limestone, Del Carmen Limestone, and 

Santa Elena Limestone, consist of limestone and marl deposited 

in shallow, carbonate shelf environments of the Diablo platform. Lower Cretaceous rocks, including the Maxon Sandstone, 

Telephone Canyon Formation, and Sue Peaks Formation, are 

mixed clastic and carbonate rocks, which repeatedly interrupted 

carbonate deposition on the Diablo platform, and represented 

cyclic influx of terriginous material from tectonic uplift in source 

areas in the Marathon region, north of BBNP. 

Upper Cretaceous sedimentary rocks in BBNP represented 

continued marine deposition, but there was a shift to mainly 

continental deposition reflected by the Javelina Formation, 

which was deposited mostly in fluvial, flood-plain environments. 

Continental deposition continued through the early Tertiary, and 

units of the Black Peaks, Hannold Hill, and Canoe Formations 

contain a variety of unique vertebrate fossils including dinosaurs, 

mammals, turtles, and crocodiles. The Late Cretaceous to early 

Tertiary Laramide orogeny (about 70–50 Ma) was a period of 

contractional deformation which produced uplifts, basins, and 

faults and folds broadly related to convergence of the North 

American continent and the Farallon oceanic plate at the western 

edge of the continent. Major Laramide structures in BBNP 

include the Mesa de Anguila monocline on the southwest margin 

of the park, the Sierra del Carmen-Santiago Mountains monocline, a thrust-faulted monocline bounding eastern BBNP, the 

Tornillo basin which developed between the uplifted monoclines, 

and the Mariscal Mountain and Cow Heaven anticlines.
Summary 69

BBNP is known for its volcanic landscape, and major 

volcanism in the park began about 47 Ma, with deposition of the 

Chisos Formation, and ended about 29 Ma, with eruption of the 

Burro Mesa Formation. Principal features formed during this 

period include volcanic flows extruded from a complex of vents 

and lava domes in western BBNP and Mexico, and the 32-Ma 

Pine Canyon caldera complex in the high Chisos Mountains, perhaps the most prominent geographic feature in the entire BBNP. 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. 

The last major tectonic episode to affect BBNP was 

basin-and-range faulting from about 25 to 2 Ma. Features of 

this time period include high-angle normal and oblique-slip 

faults that formed during periods of continental extension and 

rifting. Movement on some of the major faults formed local 

fault-controlled, depositional basins which flank the high Chisos 

Mountains, and include the Delaho and Estufa bolsons. Basinand-range faults were the last structures to significantly modify 

the BBNP landscape into its present-day configuration.

The Quaternary Period (2.6 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 geologic and climatic history 

in BBNP is recorded in multiple levels of pediment-capping 

gravel, alluvium, and alluvial fan deposits that cover much of 

the park. Based on relative-age indicators, the oldest deposits 

are middle (or early?) Pleistocene, and the youngest deposits are 

Holocene. The extensive erosion of the BBNP landscape during 

the Quaternary contributed to destabilization of bedrock slopes, 

which resulted in development of large, multi-event landslides in 

BBNP, including those at Chilicotal and Talley Mountains, and 

on the northwest flanks of the Chisos and Rosillos Mountains. 
70 Geologic Map of Big Bend National Park, Texas Acknowledgments 71

Acknowledgments

We are greatly appreciative to the National Park Service and USGS National Cooperative 

Geologic Mapping Program for funding the Big Bend geologic map project, and we especially 

thank Bruce Heise (NPS, Denver) for supporting the project by serving as NPS liaison with the 

USGS in developing and carrying out the project. We thank NPS BBNP staff for their support 

and collaboration on the project, including Don Corrick, geologist, Jeff Bennett, hydrologist, and 

Bill Wellman, Big Bend National Park Superintendent. Big Bend National Park staff kindly 

facilitated our studies by arranging permits, housing, and other important logistical activities 

required to do the work. We also appreciate the helpfulness, gratitude, and information-filled 

discussion with staff members including Betty and Tom Alex, Vidal Davila, and many others. 

We thank the many professors and students from numerous universities, and the Texas Bureau 

of Economic Geology, who contributed to the project by communicating results of their past 

and present studies, and by presenting talks at professional meetings and Big Bend symposia. 

Those contributors include: William R. Muelhberger, Emeritus, University of Texas (Austin); 

Patricia W. Dickerson, Visiting Research Fellow, American Geological Institute and University 

of Texas Jackson School of Geoscience (Austin); Eddie Collins, Bureau of Economic Geology 

(Austin); Dan Barker, University of Texas (Austin); Jim and Margaret Stevens, formerly 

of Lamar University and currently residing in Terlingua, Tex.; Fred McDowell, University of 

Texas (Austin); Dee Ann Cooper, Research Fellow, Texas Natural Science Center, University 

of Texas (Austin); John White and Christian Benker, Eastern Kentucky University; Art Busbey 

and students, Texas Christian University; Richard Hanson and students, Texas Christian 

University; Timothy Duex and students, University of Louisiana (Lafayette); Don Parker 

and students, Baylor University; Joseph Satterfield and students, San Angelo State University; 

Kevin Urbanzcyk and students, Sul Ross University; and Minghua Ren, University of Texas 

(El Paso). Anita G. Harris (USGS) processed, analyzed, and identified conodont faunas from 

the Maravillas Formation. William Cobban (USGS) identified some Late Cretaceous fossils 

from BBNP. Discussions with USGS scientists Carol Finn, John Gray, Larry Snee, Lisa 

Morgan, and Pat Shanks, on their studies at BBNP also helped improve the report. USGS geologists 

Ren Thompson and Chris Fridrich also provided valuable input to improve the report. The 

New Mexico Bureau of Mines and Geology geochronology research labs (under the direction of 

William C. McIntosch) analyzed a subset of our samples for argon geochronology. SGS Canada 

analyzed samples for major element geochemical analyses in 2007. We appreciate the comprehensive 

review of the report by Christopher D. Henry of the Nevada Bureau of Mines and Geology, and 

Jeremiah B. Workman of the USGS; their comments vastly improved the report. 
72 Geologic Map of Big Bend National Park, Texas

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and Standhardt, B.R., 1987, Stratigraphy of CretaceousTertiary and Paleocene-Eocene transition rocks of Big Bend 

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dates of mafic rocks within Big Bend National Park, Texas: 

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geochemical, and geophysical studies by the U.S. Geological Survey in Big Bend National Park, Texas: U.S. Geological Survey Circular 1327, p. 77–84.

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78 Geologic Map of Big Bend National Park, Texas

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southwestern Brewster County, Trans-Pecos Texas, in

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deformation and depositional timing, Trans-Pecos Texas,

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and tectonics of Trans-Pecos Texas: West Texas Geological 

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early Miocene Formation and vertebrate local fauna, Big 

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country, Brewster County, Texas: University Of Texas Bulletin 93, 101 P.

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Appendix 79

Appendix

Cerro Castellan, a distinctive geologic landform near Castolon. 

Shown in lower part of photograph are white and red undifferentiated 

tuffaceous sediments and dark gray basaltic lava (Bee Mountain 

Basalt Member) of the Chisos Formation, overlain by light-gray and 

reddish-brown rocks of the Burro Mesa Formation. New isotopic 

age determinations in the appendix were critical in refining the 

stratigraphy and the age of the Chisos and South Rim Formations, 

and in defining and constraining the age of the Burro Mesa 

Formation. (Photograph by William R. Page)
Appendix. Summary of 40Ar/39Ar and U-Pb zircon ages for Big Bend National Park.—Continued

Map

sample

number

Field

sample

number1

Map

unit

name

Map

unit

symbol

UTM

Easting

UTM

Northing

Age

(Ma)

Material 

analyzed2 Technique Comment1

Basaltic flow

1 12DM-4-30-03 Basaltic flow Tfb 656339 3235035 29.10±0.05 G.C. 40Ar/39ArA Plateau age (steps 4–5) comprising 

56.0% of gas; lava, near Burro 

Mesa pouroff

29.53±0.33 Biotite 40Ar/39ArA Plateau age (steps 6-10) comprising 

67.0% of gas

Burro Mesa Formation

2 13DM-4-30-03 Rhyolite 

member

Tbr 657049 3234726 29.25±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; porphyritic lava, east of Burro 

Mesa pouroff

3 60DM-3-20-04 Rhyolite 

member

Tbr 652719 3227747 29.33±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; lava, Trap Mountain

4 10DM-4-30-03 Rhyolite 

member

Tbr 655175 3235400 29.35±0.10 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; lava, Burro Mesa pouroff

5 22DM-5-2-03 Rhyolite 

member

Tbr 654659 3230725 29.35±0.09 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 7 sanidine crystals; lava, north face of Goat Mountain

6 7DM-4-30-03 Rhyolite 

member

Tbr 646319 3224846 29.45±0.08 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 9 sanidine crystals; lava, Cerro Castellan

7 3aBS26.2.03 Wasp Spring 

member

Tbw 646054 3224792 29.27±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 9 sanidine crystals; basal tuff, Cerro Castellan

8 165aDM-3-22-05 Wasp Spring 

member

Tbw 645362 3229090 29.37±0.05 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; 

ash-flow tuff, west of Bee Mountain

9 81DM-3-22-04 Wasp Spring 

member

Tbw 648044 3224841 29.39±0.12 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; pumice in nonwelded tuff, east of 

Tuff Canyon

10 10BS26.2.03 Wasp Spring 

member

Tbw 655195 3235320 29.48±0.16 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 4 sanidine crystals; ash-flow tuff, Burro Mesa pouroff

11 48DM-3-17-04 Intrusive rocks, 

undivided

Tbi 653283 3235005 29.03±0.12 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; 

intrusion, west of Burro Mesa pouroff

12 14DM-4-30-03 Intrusive rocks, 

undivided

Tbi 657734 3234322 29.09±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of multiple sanidine 

crystals; dike, east of Burro Mesa 

Pouroff and northwest of Blue 

Creek Ranch

Trachytic lava

13 23DM-5-2-03 Trachytic lava, 

undivided

Tt 658961 3233003 30.23±0.11 Plagioclase

40Ar/39ArA Plateau age (steps 2–7) comprising 

86.4% of gas; trachyte lava, north of 

Blue Creek Ranch

14 8BS26.2.03 Trachytic lava, 

undivided

Tt 654249 3230460 30.29±0.05 G.C. 40Ar/39ArA Plateau age (steps 4–6) comprising 

66.1% of gas; trachyte lava, west face 

of Goat Mountain

30.33±0.13 Plagioclase 40Ar/39ArA Plateau age (steps 5–6) comprising 

55.9% of gas

15 1aDM-4-29-03 Trachytic lava, 

undivided

Tt 647190 3225460 30.42±0.06 G.C. 40Ar/39ArA Plateau age (steps 4–5) comprising 

69.4% of gas; basaltic trachyandesite 

lava, Tuff Canyon

Tuffs of unknown origin

16 336KT3.4.05 not mapped 

separately

663301 3226539 28.1±1.0 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 5 sanidine 

crystals; ash-flow tuff, south of 

Sierra Quemada

17 212DM-12-2-06 not mapped 

separately

660614 3221885 30.34±0.11 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 15 sanidine crystals; ash-flow tuff, south face Punta 

de la Sierra, block originating from 

horizon below base of trachytic lava

18 11:48KT2-3-08 not mapped 

separately

653367 3219450 30.40±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 15 sanidine 

crystals; tuff, Triangulation 

Station Mountain

80 Geologic Map of Big Bend National Park, Texas
Appendix 81

Appendix. Summary of 40Ar/39Ar and U-Pb zircon ages for Big Bend National Park.—Continued

Map

sample

number

Field

sample

number1

Map

unit

name

Map

unit

symbol

UTM

Easting

UTM

Northing

Age

(Ma)

Material 

analyzed2 Technique Comment1

Sierra Quemada related rocks

19 5BS4.3.05 Intrusive rocks, 

undivided

Tqi 662294 3228308 29.06±0.91 Zircon U-PbD Weighted mean age of 18 spots; syenite 

intrusion, southeast Sierra Quemada

20 7BS6.3.05 Intrusive rocks, 

undivided

Tqi 661298 3230542 29.45±0.48 Zircon U-PbD Weighted mean age of 36 spots; rhyolite 

intrusion, northwest Sierra Quemada

21 6BS17.1.05 Ring dike Tqd 661067 3228522 29.93±0.40 Zircon U-PbD Weighted mean age of 32 spots; ring 

dike, southwest Sierra Quemada

22 5BS3.3.05 Intrusive rocks, 

undivided

Tqi 661161 3229029 30.42±0.44 Zircon U-PbD Weighted mean age of 30 spots; rhyolite 

intrusion, southwest Sierra Quemada

23 9BS6.3.05 Intrusive rocks, 

undivided

Tqi 661397 3231037 31.09 ± 0.48 Zircon U-Pb D Weighted mean age of 32 spots; rhyolite 

dike, northwest Sierra Quemada

South Rim Formation

24 TCP-407a Emory Peak 

rhyolite mbr.

Tse 664723 3235774 31.93 ± 0.13 Sanidine 40Ar/39Ar A Plateau age (steps 3–9) comprising 

79.4% of gas; vitrophyre, Emory Peak

25 144DM-3-12-05 Emory Peak 

rhyolite mbr.

Tse 658904 3233673 32.1 ± 0.3 Sanidine 40Ar/39Ar 

laser B

Weighted mean age of 10 sanidine crystals; 

tuff, north of Blue Creek Ranch

26 16DM-5-1-03 Emory Peak 

rhyolite mbr.

Tse 664764 3236232 32.25 ± 0.05 Sanidine 40Ar/39Ar 

laser B

Weighted mean age of 10 sanidine crystals; 

ash-flow? tuff, Emory Peak

27 19DM-5-1-03 Emory Peak 

rhyolite mbr.

Tse 665275 3236438 32.25 ± 0.07 Sanidine 40Ar/39Ar 

laser B

Weighted mean age of 10 sanidine crystals; 

rhyolite, east side Emory Peak

28 TPC-325a Boot Rock mbr. Tsb 665209 3235781 32.17 ± 0.09 Sanidine 40Ar/39Ar A Plateau age (steps 2–4) comprising 

69.4% of gas; ash-flow tuff, Pinnacle 

Pass in The Basin

29 BB089KWb Boot Rock mbr. Tsb 656140 3241284 32.33 ± 0.07 Sanidine 40Ar/39Ar 

laser B

Weighted mean age of 9 sanidine crystals; ash-flow tuff, east Burro Mesa

30 BJD-PCR Pine Canyon 

rhyolite mbr.

Tsp 669400 3238130 32.11 ± 0.23 Sanidine 40Ar/39Ar A Plateau age (steps 2–10) comprising 

94.1% of gas; vitrophyre, Pine 

Canyon pouroff

31 131DM-2-11-05 Pine Canyon 

rhyolite mbr.

Tsp 668896 3239465 32.33 ± 0.07 Sanidine 40Ar/39Ar 

laser B

Weighted mean age of 10 sanidine crystals; tuff, west side Lost Mine Peak

32 33DM-5-4-03 Ring dike Tsd 675590 3237184 32.6 ± 0.3 Zircon U-Pb C Weighted mean age of 10 spots; dike, 

Hayes Ridge, mouth of Pine Canyon

31.99 ± 0.32 K-Feldspar 40Ar/39Ar A Weighted mean age (steps 5–11) comprising 47.5% of gas

33 128DM-2-10-05 Intrusive rocks, 

undivided

Tsi 676919 3237517 31.5 ± 0.3 Zircon U-Pb C Weighted mean age of 5 spots; sill, 

Nugent Mountain

34 TPC-329a Intrusive rocks, 

undivided

Tsi 666303 3238568 32.03±0.25 Sanidine 40Ar/39ArA Plateau age (steps 3–9) comprising 

74.4% of gas; feeder dike of Casa 

Grande dome

35 132DM-2-11-05 Intrusive rocks, 

undivided

Tsi 668944 3239435 32.21±0.05 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; dike, east side Lost Mine Peak

36 94DM-3-25-04 Intrusive rocks, 

undivided

Tsi 676957 3243033 32.4±0.3 Zircon U-PbC Weighted mean age of 8 spots; sill, near 

K-bar park housing

31.69±0.19 K-Feldspar 40Ar/39ArA Weighted mean age (steps 2–7) comprising 84.8% of gas

32.32±0.22 Amphibole 40Ar/39ArA Plateau age (steps 3–4) comprising 

72.7% of gas

37 139DM-2-12-05 Intrusive rocks, 

undivided

Tsi 662378 3240130 32.4±0.4 Zircon U-PbC Weighted mean age of 10 spots; rhyolite 

intrusion, Windows Trail south of 

Vernon Bailey Peak

Chisos Formation, younger part

38 198DM-11-28-06 Younger part, 

undivided

Tcy 647467 3222486 32.96±0.06 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 15 sanidine crystals; 

possibly reworked pumaceous tuff, 

southeast of Cerro Castellan

39 168KT-2-7-07 Tule Mountain 

Trachyandesite 

Mbr.

Tctm 633209 3232371 33.18±0.10 G.C. 40Ar/39ArB Plateau age (steps 3–9) comprising 

90.6% of gas; lava flow Sierra Aguja
Appendix. Summary of 40Ar/39Ar and U-Pb zircon ages for Big Bend National Park.—Continued

Map

sample

number

Field

sample

number1

Map

unit

name

Map

unit

symbol

UTM

Easting

UTM

Northing

Age

(Ma)

Material 

analyzed2 Technique Comment1

Chisos Formation, younger part—Continued

40 45DM-3-16-04 Tule Mountain 

Trachyandesite 

Mbr. 

Tctm 648354 3237603 33.84±0.05 G.C. 40Ar/39ArA Average age (steps 5–7) comprising 

43.7% of gas; lava flow, top of 

Tule Mountain

32.71±0.24 Plagioclase 40Ar/39ArA Average age (steps 3–7) comprising 

71.9% of gas

33.88±0.54 G.C. 40Ar/39ArB Isochron age (steps 1–8)

41 11:40KT2-3-08 Bee Mountain 

Basalt Mbr.

Tcbm 653332 3219378 33.07±0.15 G.C. 40Ar/39ArA Plateau age (steps 3–10) comprising 89.6% 

of gas; lava flow above Mule Ear Spring 

Tuff, Triangulation Station Mountain

42 167bDM-3-24-05 Bee Mountain 

Basalt Mbr.

Tcbm 647307 3222725 33.68±0.09 G.C. 40Ar/39ArA Weighted mean age (steps 3–5) comprising 

65.0% of gas; lava flow above Mule 

Ear Spring Tuff, southeast of Cerro 

Castellan

43 83DM-3-22-04 Mule Ear 

Spring Tuff 

Mbr.

Tcme 652928 3229535 33.64±0.08 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine 

crystals; ash-flow tuff, southeast side 

Kit Mountain

44 53DM-3-18-04 Mule Ear 

Spring Tuff 

Mbr.

Tcme 654142 3230075 33.64±0.09 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine 

crystals; ash-flow tuff, west side Goat 

Mountain

45 118DM-2-6-05 Mule Ear 

Spring Tuff 

Mbr.

Tcme 654779 3226838 33.65±0.06 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; ash-flow tuff, Mule Ear Spring

46 124DM-2-9-05 Mule Ear 

Spring Tuff 

Mbr.

Tcme 646239 3227255 33.67±0.06 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 9 sanidine 

crystals; ash-flow tuff, northwest 

of Tuff Canyon

47 8DM-4-30-03 Mule Ear 

Spring Tuff 

Mbr.

Tcme 646430 3224157 33.67±0.09 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 9 sanidine 

crystals; ash-flow tuff, southeast of 

Cerro Castellan

48 68DM-3-21-04 Younger part, 

undivided

Tcy 654102 3224373 33.84±0.06 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; tuff, above Bee Mountain Basalt 

flows at Round Mountain

49 2BS26.2.03 Bee Mountain 

Basalt Mbr.

Tcbm 645906 3224908 33.37±0.48 G.C. 40Ar/39ArA Isochron age (steps 3–8); lava flow at 

Cerro Castellan

50 BJD-BEM Bee Mountain 

Basalt Mbr.

Tcbm 669612 3242290 33.70±0.05 G.C. 40Ar/39ArA Average age (steps 3–4) comprising 

29.1% of gas; lava flow, along road 

into The Basin

51 153DM-3-18-05 Bee Mountain 

Basalt Mbr.

Tcbm 659130 3231741 33.76±0.12 G.C. 40Ar/39ArA Preferred age (steps 5–7) comprising 

52.8% of gas; lowest lava flow, off 

Dodson Trail just south of Blue Creek

52 163DM-3-22-05 Bee Mountain 

Basalt Mbr.

Tcbm 647497 3229499 34.03±0.17 G.C. 40Ar/39ArA Weighted mean age (steps 4-8) 

comprising 75.2% of gas; lava flow, 

Bee Mountain

53 5DM-4-29-03 Undifferentiated

lava flow

Tcl 658262 3236538 38.87±0.09 G.C. 40Ar/39ArA Total gas age (steps 1-8) comprising 100.0% of gas; lava flow, north 

of Blue Creek Ranch, along Ross 

Maxwell Scenic Drive

54 71DM-3-21-04 Younger part, 

undivided

Tcy 653343 3223772 39.72±0.19 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 7 sanidine crystals; 

tuff, west of Round Mountain

55 72DM-3-21-04 Younger part, 

undivided

Tcy 653206 3223428 39.86±0.06 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 17 sanidine crystals; 

tuff, west of Round Mountain

56 5BS26.2.03 Younger part, 

undivided

Tcy 645572 3225308 40.55±0.18 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 8 sanidine crystals; 

tuff, base of Cerro Castellan

57 73DM-3-21-04 Younger part, 

undivided

Tcy 652922 3223079 40.83±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 20 sanidine crystals; 

tuff, west of Round Mountain

58 2BS27.2.03 Ash Spring 

Basalt Mbr.

Tcas 658187 3236904 40.92±0.07 G.C. 40Ar/39ArA Total gas age (steps 1–8) comprising 

100.0% of gas; lava flow, north 

of Blue Creek Ranch, along Ross 

Maxwell Scenic Drive

40.38±0.35 G.C. 40Ar/39ArB Integrated age (steps 1–10) comprising 

100% of gas

43.39±0.19 Plagioclase 40Ar/39ArA Weighted mean age (steps 4–7) comprising 

69.5% of gas

82 Geologic Map of Big Bend National Park, Texas
Appendix 83

Appendix. Summary of 40Ar/39Ar and U-Pb zircon ages for Big Bend National Park.—Continued

Map

sample

number

Field

sample

number1

Map

unit

name

Map

unit

symbol

UTM

Easting

UTM

Northing

Age

(Ma)

Material 

analyzed2 Technique Comment1

Chisos Formation, younger part—Continued

59 BB012KWb Younger part, 

undivided

Tcy 656700 3242021 41.41±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 8 sanidine crystals; 

ash-flow tuff, east side Burro Mesa

60 88DM-3-25-04 Younger part, 

undivided

Tcy 646644 3237469 41.75±0.11 Sanidine 40Ar/39Ar 

laserB

Weighted mean of 10 sanidine crystals; 

ash-flow tuff, northwest side Tule 

Mountain, possibly related to 

Christmas Mountains caldera complex

61 BB078KWb Younger part, 

undivided

Tcy 654825 3244105 42.06±0.07 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine 

crystals; ash-flow tuff, northeast 

side Burro Mesa, possibly related to 

Christmas Mountains caldera complex

62 51DM-3-17-04 Younger part, 

undivided

Tcy 652884 3234415 42.6±0.4 Zircon U-PbC Weighted mean age of 8 spots; tuff, west 

of Burro Mesa pouroff

42.31±0.10 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 8 sanidine crystals

63 75DM-3-21-04 Younger part, 

undivided

Tcy 652154 3222503 45.3 ± 0.7 Zircon U-PbC Weighted mean age of 5 spots; tuff, west 

of Round Mountain

64 135DM-2-11-05 Alamo Creek 

Basalt Mbr.

Tcac 643279 3237789 44.41±0.22 G.C. 40Ar/39ArB Integrated age (steps 1–10) comprising 

100% of gas; lava flow, Alamo Creek

65 213DM-12-2-06 Alamo Creek 

Basalt Mbr.

Tcac 660269 3221192 45.54±0.52 G.C. 40Ar/39ArB Integrated age (steps 1–10) comprising 

100% of gas; lava flow, northwest of 

Punta de la Sierra

66 1BS26.2.03 Alamo Creek 

Basalt Mbr.

Tcac 645234 3224700 46.11±0.21 G.C. 40Ar/39ArB Plateau age (steps 2–10) comprising 

99.9% of gas; lava flow, northeast 

of Castolon

67 76DM-3-21-04 Alamo Creek 

Basalt Mbr.

Tcac 651743 3223673 46.42±0.07 G.C. 40Ar/39ArA Plateau age (steps 3–5) comprising 

61.8% of gas; lava flow, southwest 

of Round Mountain

68 150DM-3-16-05 Alamo Creek 

Basalt Mbr.

Tcac 651196 3226913 46.61±0.10 G.C. 40Ar/39ArA Plateau age (steps 3–5) comprising 

71.0% of gas; lava flow, southwest 

of Trap Mountain

69 145DM-3-12-05 Alamo Creek 

Basalt Mbr.

Tcac 636582 3231113 47.04±0.17 G.C. 40Ar/39Ar A Plateau age (steps 4–5) comprising 

61.5% of gas; lava flow, southeast 

of Sierra Aguja

70 106DM-2-3-05 Alamo Creek 

Basalt Mbr.

Tcac 647443 3232280 47.04±0.25 G.C. 40Ar/39ArA Plateau age (steps 4–6) comprising 

57.0% of gas; lava flow, Black Mesa

71 163KT2.7.07 Alamo Creek 

Basalt Mbr.

Tcac 633879 3232404 47.09±0.33 G.C. 40Ar/39ArB Plateau age (steps 5–10) comprising 

85.4% of gas; lowest lava flow, 

Sierra Aguja

Miscellaneous intrusions

72 04LS08c Rhyolitic 

and felsic 

intrusive rocks, 

undivided

Tir 667903 3229922 28.1±0.3 Zircon U-PbC Weighted mean age of 4 spots; central 

rhyolite plug, Tortuga Mountain

73 04LS09c Basaltic 

and mafic 

intrusive rocks, 

undivided

Tib 668255 3229480 28.63±0.15 G.C. 40Ar/39ArA Total gas age (steps 1–8) comprising 

100% of gas; basalt sill surrounding 

Tortuga Mountain

74 148DM-3-13-05 Intrusive rocks, 

undivided

Ti 642190 3232087 28.7±0.4 Zircon U-PbC Weighted mean age of 7 spots; Pena 

Mountain

75 147DM-3-13-05 Intrusive rocks, 

undivided

Ti 642190 3232087 28.77±0.08 Sanidine 40Ar/39Ar 

laserB

Weighted mean age of 10 sanidine crystals; 

Pena Mountain

76 H86-35d Basaltic 

and mafic 

intrusive rocks, 

undivided

Tib 656251 3252499 29.03±0.08 G.C. 40Ar/39ArA Plateau age (steps 3–6) comprising 

67.9% of gas; basalt sill, north of 

Slickrock Mountain

77 G-11e Rhyolitic 

and felsic 

intrusive rocks, 

undivided

Tir 678363 3229779 30.22±0.09 K-Feldspar 40Ar/39ArA Weighted mean age (steps 8–12) 

comprising 44.0% of gas; laccolith at 

Glenn Spring

78 04LS010c Basaltic 

and mafic 

intrusive rocks, 

undivided

Tib 669150 3229283 30.68±0.27 G.C. 40Ar/39ArA Total gas age (steps 1–8) comprising 

100% of gas; basalt sill, east of 

Tortuga Mountain
Prepared by the Denver Publishing Service Center, Denver, Colorado

Manuscript approved for publication November 30, 2010

Edited by Gayle M. Dumonceaux

Layout and graphic design by Carol A. Quesenberry

Appendix. Summary of 40Ar/39Ar and U-Pb zircon ages for Big Bend National Park.—Continued

Map

sample

number

Field

sample

number1

Map

unit

name

Map

unit

symbol

UTM

Easting

UTM

Northing

Age

(Ma)

Material 

analyzed2 Technique Comment1

Miscellaneous intrusions—Continued

79 127DM-2-10-05 Andesitic and 

intermediate 

intrusive rocks, 

undivided

Tia 674618 3237746 31.45±0.18 G.C. 40Ar/39ArA Total gas age (steps 1–8) comprising 

100.0% of gas; sill, near mouth of 

Pine Canyon

80 38DM-3-14-04 Rhyolitic 

and felsic 

intrusive rocks, 

undivided

Tir 657588 3249797 31.7±0.6 Zircon U-PbC Weighted mean age of 5 spots; rhyolite 

laccolith, Slickrock Mountain

81 27DM-5-2-03 Fayalite

Syenite of

Grapevine Hills

Tigh 674236 3254439 31.9±0.2 Zircon U-PbC Weighted mean age of 10 spots

31.62±0.30 K-Feldspar 40Ar/39ArA Plateau age (steps 4–9) comprising 

59.1% of gas

82 04LS14c Syenite of 

Rosillos

Mountain

Tirm 668367 3268309 32.1±0.2 Zircon U-PbC Weighted mean age of 12 spots

83 04LS323-1c Fayalite 

Syenite of

McKinney Hills

Timh 688581 3249524 32.2±0.3 Zircon U-PbC Weighted mean age of 8 spots

84 6323f Rhyolitic

and felsic 

intrusive rocks, 

undivided

Tir 668411 3246888 33.0±0.2 Zircon U-PbC Weighted mean age of 8 spots; fayalite 

syenite of Government Spring

85 175DM-11-21-06 Rhyolitic

and felsic 

intrusive rocks, 

undivided

Tir 645203 3243462 41.6±0.4 Zircon U-PbC Weighted mean age of 9 spots; rhyolite 

sill, Maverick Mountain

86 34bDM-5-4-03 Basaltic 

and mafic 

intrusive rocks, 

undivided

Tib 677182 3221686 46.5±0.3 Zircon U-PbC Weighted mean age of 5 spots; basalt sill, 

Mariscal Mountain

36.11±0.19 K-Feldspar 40Ar/39ArA Average age (steps 9–11) comprising 

55.9% of gas

1

Additional geochronological information on the sample will be published in a later report.

2

G.C. Groundmass Concentrate.

a

Sample submitted by Fred McDowell, Bureau of Economic Geology, University of Texas at Austin, Austin, Tex.

b

Sample submitted by Kay Wache, Martin-Luther-Universitaet Halle-Wittenberg, Department of Geological Science, Halle, Germany.

c

Sample submitted by Larry Snee, U.S. Geological Survey, Denver, Colo.

d

Sample submitted by Christopher Henry, Nevada Bureau of Mines and Geology, Reno, Nev.

e

Sample submitted by Patricia Dickerson and Bill Muehlberger, University of Texas at Austin, Austin, Tex.

f

Sample submitted by Minghua Ren, University of Texas at El Paso, El Paso, Tex.

AAnalysis obtained using furnace heating method at Denver Argon Laboratory, U.S. Geological Survey, Denver, Colo.

BAnalysis obtained by laser fusion (indicated) and/or furnace heating methods at New Mexico Geochronology Research Laboratory, Socorro, N. Mex.

CAnalysis obtained using SHRIMP-RG at Stanford USGS MicroAnalysis Center (SUMAC), Stanford University, Stanford, Calif.

DAnalysis obtained by LA-ICPMS at Arizona Laserchron Center, University of Arizona, Tucson, Ariz.

84 Geologic Map of Big Bend National Park, Texas
Back Cover Photographs (left to right):

Yellow rock nettle (Daniel P. Miggins)

Texas Banded gecko (Daniel P. Miggins)
Turner and others—Geologic Map of Big Bend National Park, Texas—Scientific Investigations Map 3142