Chapter H

Summary of Cretaceous Stratigraphy and

Coal Distribution, Black Mesa Basin,

Arizona

By J. Dale Nations,1 Robert L. Swift,1 and Henry W. Haven, Jr.1

U.S. Geological Survey Professional Paper 1625–B*

U.S. Department of the Interior

U.S. Geological Survey

National Coal

Resource

Assessment

Click here to return to Disc 1 

Volume Table of Contents

Chapter H of

Geologic Assessment of Coal in the Colorado Plateau:

Arizona, Colorado, New Mexico, and Utah

Edited by M.A. Kirschbaum, L.N.R. Roberts, and L.R.H. Biewick

1

 Northern Arizona University, Flagstaff, Arizona

* This report, although in the USGS Professional Paper series,

is available only on CD-ROM and is not available separately
Contents

Introduction .................................................................................................................................................H1

Acknowledgments........................................................................................................................................ 1

Location, Physiography, and Structure of Black Mesa Basin............................................................... 1

Previous Mining Activity.............................................................................................................................. 4

Cretaceous Stratigraphy ............................................................................................................................. 6

Previous Geologic Studies ................................................................................................................. 6

Dakota Formation............................................................................................................................... 10

Mancos Shale .................................................................................................................................... 11

Mesaverde Group.............................................................................................................................. 13

Toreva Formation ............................................................................................................................... 13

Wepo Formation................................................................................................................................. 14

Yale Point Sandstone ........................................................................................................................ 15

Geophysical Type Log ....................................................................................................................... 15

Distribution of Coal in Black Mesa Basin ............................................................................................... 15

Coal Distribution and Thickness in the Dakota Formation................................................................... 16

Coal Distribution and Thickness in the Toreva Formation.................................................................... 17

Coal Distribution and Thickness in the Wepo Formation ..................................................................... 18

Coal Quality in Black Mesa Basin............................................................................................................ 21

Coal Sampling and Analysis............................................................................................................. 21

Analytical Data on Coal Quality....................................................................................................... 22

Coal Quality in the Dakota Formation .................................................................................... 22

Coal Quality in the Toreva Formation..................................................................................... 22

Coal Quality in the Wepo Formation ...................................................................................... 22

Comparison of Coal Quality with Peabody Group Lease Area................................................... 23

Coal Resources in Black Mesa Basin ..................................................................................................... 27

Coal Resources in the Wepo Formation......................................................................................... 27

References Cited ........................................................................................................................................ 27

Figures

 1. (Frontispiece) Photograph of eastern escarpment of Black Mesa in the 

Rough Rock area showing upper Mancos Shale and Mesaverde Group ......................... iv

 2. Photograph of CBM Project field crew planning day’s activity ..........................................H2

 3. Map of Four Corners area showing maximum western extent of 

Late Cretaceous marine transgression and location and extent of coal fields ................. 2

 4. Map showing locations of measured sections and subsurface information 

in Black Mesa, Arizona................................................................................................................ 3

 5. Map showing area of study, Black Mesa Basin, northern Arizona ..................................... 4
 6. Map showing large-scale Laramide folds in Cretaceous rocks 

of northeastern Arizona............................................................................................................... 5

 7. Map showing Laramide folds and erosional limits of Cretaceous rocks, 

northeastern Arizona ................................................................................................................... 6

 8. Geologic map of Black Mesa ..................................................................................................... 7

 9. Chart showing annual coal production, Black Mesa Basin, Arizona .................................. 8

 10. Map of Peabody Group lease area and locations of Kayenta and 

Black Mesa mines ........................................................................................................................ 8

 11. Photograph of Black Mesa mine with active pit and backfilled pit ..................................... 9

 12. Diagram showing stratigraphic sequence and ages of Upper Cretaceous 

formations in the Black Mesa Basin ......................................................................................... 9

 13. Oblique aerial photograph of the northeastern escarpment in the 

Rough Rock area of Black Mesa ............................................................................................. 10

 14. Diagrammatic NW.-SE. stratigraphic cross section A-A’ of Cretaceous rocks, 

Black Mesa Basin, Arizona....................................................................................................... 11

 15. Diagrammatic SW.-NE. stratigraphic cross section B-B’ of Cretaceous rocks, 

Black Mesa Basin, Arizona....................................................................................................... 12

 16. Generalized schematic diagram showing cross sectional distribution of 

Cretaceous units and the contact with the Jurassic Entrada and Morrison 

Formations from southwest to northeast across Black Mesa............................................ 12

 17. Isopach map of the Dakota Formation, Black Mesa Basin, Arizona ................................. 13

 18. Isopach map of the Mancos Shale, Black Mesa Basin, Arizona ....................................... 13

 19. Isopach map of the Toreva Formation, Black Mesa Basin, Arizona .................................. 14

 20. Isopach map of the Wepo Formation, Black Mesa Basin, Arizona.................................... 14

 21. N.-SE. stratigraphic cross section C-C’ of the Wepo Formation ........................................ 15

 22. Photograph of Henry Haven measuring and describing a section of 

the Yale Point Sandstone .......................................................................................................... 16

 23. Typical Black Mesa subsurface log, Skelly Hopi A No. 1 .................................................... 16

 24. Cumulative net-coal thickness in Dakota, Toreva, and Wepo Formations, 

Black Mesa, Arizona .................................................................................................................. 17

 25. Isopach map of net-coal thickness in the Dakota Formation, 

Black Mesa, Arizona .................................................................................................................. 18

 26. Isopach map of net-coal thickness in the Toreva Formation, 

Black Mesa, Arizona .................................................................................................................. 19

 27. Isopach map of net-coal thickness in the Wepo Formation, 

Black Mesa, Arizona .................................................................................................................. 20

 28. Photograph of coal and carbonaceous shale in Dakota Formation 

at Coalmine Canyon, Black Mesa, Arizona ............................................................................ 21

 29. Aerial overview of the Kayenta mine, Black Mesa, Arizona............................................... 21

 30. Photograph of working face in Kayenta mine, Black Mesa, Arizona ................................ 22

 31. Isopach map of net-coal thickness of the Wepo Formation in the 

Rough Rock area......................................................................................................................... 23

 32. E.-W. stratigraphic cross section of sedimentary facies in the upper 

carbonaceous member of the Wepo Formation, Rough Rock area................................... 24
Figure 1 (Frontispiece). Eastern escarpment of Black Mesa in the Rough Rock area showing upper 

Mancos Shale and Mesaverde Group. The Rough Rock road is visible in the distance.

Tables

 1. Coal analyses from the Dakota Formation, Black Mesa, Arizona ....................................H24

 2. Coal Analyses from the Toreva Formation.............................................................................. 25

 3. Coal Analyses from the Wepo Formation ............................................................................... 25

 4. Seven analyses of channel samples or grab samples from the 

Upper Cretaceous Wepo Formation........................................................................................ 26

 5. Representative coal analyses from Peabody Group Coal Company ................................. 26

 6. Summary of Wepo coal quality data from CBM Project surface samples ....................... 26
Introduction

As a part of the U.S. Geological Survey’s National Coal 

Resource Assessment Project, this report provides a brief summary and guide to the Cretaceous geology and coal deposits 

of the Black Mesa Basin in northeastern Arizona, which is in 

the Colorado Plateau region. Information in these reports has 

been acquired during a National Science Foundation-funded 

research project (CBM Project) that has been ongoing since 

1993 (fi g. 2).

The purpose of this research has been to determine the 

distribution and estimate the volume of coal in Cretaceous 

rocks of Black Mesa, an erosional remnant in the center of 

the Black Mesa structural basin (fi g. 3). Previous basin-wide 

estimates of coal volume were made more than 25 years ago 

and were based mostly on regional stratigraphic information. 

The data resulting from the work herein described will make 

possible an improved assessment of the resource.

The report summarizes stratigraphic information from 

more than 230 locations, from which a regional stratigraphic 

framework has been developed (fi g. 4). The precision of stratigraphic correlation is at the formation level. Coal data are 

limited (for most of the data points) to total thickness, or even 

simply presence or absence of coal, at a particular site. These 

data have allowed only generalized cross sections and isopach 

maps depicting gross trends of coal occurrence and thickness 

in the Dakota, Toreva, and Wepo coal-bearing formations in 

Black Mesa.

A second objective of this work has been to evaluate 

the potential for coal-bed methane gas production. Due to 

depth of burial and the rarity of thick coal beds in the coalbearing strata, only a small portion of Black Mesa coal can be 

exploited through mining techniques, but the coal-bed methane 

that may be contained within more deeply buried coal could be 

produced by standard well-drilling techniques. If present, and 

if exploration work is allowed, such production might provide 

a valuable source of energy for local use, and possibly for 

commercial production.

Field work on the Navajo Nation was conducted under a 

permit from the Navajo Nation Minerals Department. Any persons wishing to conduct geologic investigations on the Navajo 

Nation must fi rst apply for, and receive, a permit from the 

Navajo Nation Minerals Department, P.O. Box 1910, Window 

Rock, Arizona 86515.

Field work on the Hopi Tribal Land was conducted under 

a permit from the Chairman of the Hopi Tribe. Any persons 

wishing to conduct geologic investigations on the Hopi Reservation must receive permission by applying to the Offi ce of 

Mining and Mineral Resources, The Hopi Tribe, P.O. Box 123, 

Kykotsmovi, Arizona 86039.

Acknowledgments

The research for this paper was supported since August 

1993 by four grants from the National Science Foundation and 

one from the Mobil Corporation. Continuation of the work 

was funded in 1997 by the U.S. Geological Survey under 

the National Coal Resources Data System Program (NCRDS). 

The primary grant was entitled RUI: “Student-Based Facies 

Analysis and Determination of Coalbed Methane Potential of 

Cretaceous Coal-Bearing Rocks, Navajo and Hopi Reservations” (CBM Project). Drafting help was provided by Marin 

Popov and Chris French.

Location, Physiography, and 

Structure of Black Mesa Basin

Black Mesa is located in the southwestern part of the Coloorado Plateau Region and occupies parts of Navajo, Apache, 

and Coconino Counties of northeastern Arizona (fi g. 5). The 

mesa lies entirely within the Navajo and Hopi Reservations, 

between the towns of Chinle, Kayenta, Tuba City, and Winslow, Arizona. Black Mesa is a physiographic mesa in the 

center of the Black Mesa structural basin, which is bounded 

by the Kaibab uplift to the west, the Defi ance uplift to the 

Summary of Cretaceous Stratigraphy and 

Coal Distribution, Black Mesa Basin, 

Arizona

By J. Dale Nations, Robert L. Swift, and Henry W. Haven, Jr.

H1
Figure 2. CBM Project fi eld crew planning day’s activity.

Arizona

New Mexico

Utah

Colorado

Western extent of Late Cretaceous

 (early Turonian) marine transgression

Coal field or oil and gas basin 

City or town

Explanation

114 ̊ 113 ̊ 112 ̊ 111 ̊ 110 ̊ 109 ̊ 108 ̊ 107 ̊ 106 ̊

38 ̊

37 ̊

36 ̊

35 ̊

34 ̊

33 ̊

32 ̊

100 miles

Kaiparowits

Basin

San Juan

Basin

Pinedale

coal field

Kayenta

Kingman

St. Johns

Kanab Page

Show Low 

Flagstaff

Deer

Creek

coal

field

Durango

Henry Gunnison

Basin

Black

Mesa

Basin

Phoenix

Tucson

Figure 3. Map of Four Corners area showing 

maximum western extent of Late Cretaceous marine 

transgression (dashed line; modifi ed from Cobban 

and Hook, 1984) and location and extent of coal 

fi elds (Arizona portion modifi ed from Peirce and 

Wilt, 1970). Outlines of coal fi elds are approximately 

drawn.

H2 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
Outcrop limit of Dakota Formation

W Measured section by Williams

CBM Measured section by Coalbed Methane Project

C Measured section by Carr

N, J Peabody core holes

10 miles

111 00' 110 00' 110 30'

36 00'

36 30'

B

A B' C

C'

A'

W11

CBM Coalmine

Wash Gap

CBM Photo

Point

CBM Rock Gap

W10 N9-3 Chilchinbeto

W5 

W12

CBM Cow

 Springs Mine

W18

W17

W16

W 15

W 9

Kirkland

 Lohali Point

Steamboat

 Canyon

Coalmine 

 Canyon

Blue Gap

area 

CBMWRV

CBM Rough C28 Rock

C32 C35

N4-1

N6-2

J21-1A

J21-10 Navajo #8

Skelly Hopi A No. 1

Figure 4. Locations of measured sections in Black Mesa Arizona from Carr (1987), Williams (1951), and the Coalbed Methane Project, 

and subsurface information including oil and gas, coal, and water well drill holes. Line of cross sections A-A’, B-B’, and C-C’ also shown. 

Locations cited in text or included in cross sections are labeled.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H3
east, the Monument uplift to the north, and the Mogollon slope 

to the south (fi g. 6). The Black Mesa Basin is a Laramide 

structure that has a depositional and tectonic history similar to 

that of the San Juan Basin in northwestern New Mexico and 

the Kaiparowits Basin in southern Utah (fi g. 3). The basin is 

asymmetrical with steep dip on the eastern fl ank and gentler 

dip on the western margin, and it is crossed by numerous 

small-scale folds (fi g. 7). 

Along its north and east sides, Black Mesa is defi ned 

by prominent escarpments that result from erosion of cliffforming strata, including the Late Jurassic Morrison Formation and Cow Springs Sandstone. It is capped by resistant 

sandstone strata of the Late Cretaceous Yale Point, Wepo, and 

Toreva Formations (fi g. 8). Black Mesa is roughly circular, 

approximately 65 mi in diameter, and covers an area of 3,300 

mi2. Elevations range from about 6,000 ft above sea level 

in the southwestern part to 8,000 ft above sea level along 

the northeastern escarpment. It is a dissected mesa that rises 

as much as 2,000 ft above the surrounding terrain along its 

eastern margin, and slopes gently to the southwest, where the 

cliffs are 200–300 ft high. The top of the mesa slopes gently 

to the southwest, tending to expose younger strata in higher 

areas to the north and northeast and gradually older strata to 

the southwest (fi g. 8).

Previous Mining Activity

Based on evidence from coal ash in kivas, primitive stone 

stoves, and pottery fi ring pits that date back at least to the 

year 1300 A.D., Black Mesa coal was fi rst used by prehistoric 

peoples (Brew and Hack, 1939). Prior to 1926, small amounts 

of coal were mined to supply local fuel requirements. During 

1926–34, 1942, and 1944–46, recorded coal production was 

88,730 short tons valued at $358,800 (Wilson and Roseveare, 

1949). From 1943 to 1960, less than 10,000 short tons was 

produced annually, most of which was mined for local use 

40

Utah

Arizona

Colorado

New Mexico

Limit of 

Flagstaff

Winslow

Page

Tuba City 

Kayenta

Chinle

Window

Limit of

Limit of

Navajo Reservation 

Hopi

Reservation

Peabody Coal

Co. Lease 

Joint Use Area

Cretaceous Rock

Cretaceous

COCONINO

COUNTY NAVAJO COUNTY APACHE 

COUNTY

COCONINO

COUNTY

NAVAJO COUNTY

APACHE 

COUNTY 50 miles 

36°00'

111°00' 110°00'

Figure 5. Area of study, Black Mesa Basin, northeastern Arizona. The dark-green irregular outcrop pattern of 

the Dakota Formation marks the limit of Cretaceous rocks.

H4 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
at schools on the reservations and for limited shipment to Holbrook, Winslow, and Flagstaff (Averitt and O’Sullivan, 1969). 

After natural gas pipelines were extended through northern 

Arizona about 1960, coal production decreased to less than 

1,000 short tons annually (U.S. Bureau of Mines, 1960–67) 

until the Peabody Group mines began production. 

The Black Mesa and Kayenta coal mines, operated by 

the Peabody Group, are located on Black Mesa near Kayenta 

and produce coal from a large reserve leased from the Navajo 

Nation and the Hopi Tribe (fi g. 5). Royalties and other payments generated from the mining operations provide annual 

revenues for tribal operations. In addition, over 700 tribal 

members work at the Arizona mines and support facilities, 

placing Peabody among the Nation’s largest private employers 

of American Indians. Peabody is the world’s largest coal producer (Phillips and others, 1997).

Since 1975, coal production from the Black Mesa and 

Kayenta mines has averaged about 12 million short tons per 

year (Haven, 1997) (fi g. 9). In 1996, coal production was 

13,192,000 short tons, having an estimated value of $300 million.

High-quality coal is strip-mined from the Kayenta and 

Black Mesa mines. The coal is subbituminous with an average 

quality of 11,000 Btu/lb, 0.5 percent sulfur, and 10 percent ash. 

Both mines are now using both 150-ton- and 230-ton-capacity 

tractor trailer bottom-dump trucks to transport coal from the 

mine to the conveyors and pipeline feed plants.

The Black Mesa mine (fi g.10) was opened in 1970 

and produces approximately 4.5 million short tons of steam 

coal annually using draglines in two mining areas. The mine 

employs about 265 people and sells coal under terms of a 

35-year contract signed in 1970. The coal is crushed, mixed 

with water, and then transported through the underground 

Black Mesa pipeline 273 mi to Southern California Edison’s 

Mohave Generating Station near Laughlin, Nevada. 

The Kayenta mine (fi g. 10) is adjacent to the Black 

Mesa mine and began operating in 1973. This mine produces 

approximately 7.5 million short tons of steam coal annually 

using three draglines in three mining areas. The mine employs 

more than 400 people and sells coal under a 35-year contract 

with the Salt River Project. The coal is crushed, then carried by 

conveyor belt 17 mi to storage silos, where it is loaded on an 

electric train and transported 83 mi to the Navajo Generating 

Station near Page, Arizona.

Peabody’s operations at Black Mesa are model reclamation programs. Mining and reclamation proceed at the same 

rate of approximately 500 acres annually. As an area is mined, 

Syncline, dashed where approximate

Anticline, dashed where approximate Thrust fault

100 miles

EXPLANATION

| | | | | | ||||| | | | |

| | |

| | | | | | | | | | | | | | | | | | | | | | | |

Sev

ei r T

hrust Belt

| | | | | | | | | |

| |

| | | | | | | |

Echo Cliffs Uplift

Monument

Uplift

Kaibab

Uplift

Circle

Cliffs

Uplift Kaiparowits

Basin

Zuni Uplift

Defiance Uplift

UT CO

AZ NM

Mogollon Rim

Mogollon Slope

Black

Mesa

Basin

115° 114° 113° 112° 111° 110° 109° 108°

36°

37°

38°

39°

35°

Figure 6. Large-scale Laramide folds in Cretaceous rocks of northeastern Arizona (modifi ed 

from Kelley, 1958; Chapin and Cather, 1981; Nations and others, 1985; Davis and Kiven, 1975).

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H5
the topsoil is removed and stored. After mining is completed, 

the topsoil is returned and the surface is contoured (fi gs. 11 

and 29). The resultant reclaimed land, used for grazing, is 

more productive than the original land (Phillips and others, 

1997). 

Cretaceous Stratigraphy

Previous Geologic Studies

The fi rst published reference to Cretaceous rocks of Black 

Mesa Basin was made by Newberry (1861). Campbell and 

Gregory (1911) recognized the Dakota Sandstone, the Mancos 

Shale, and the Mesaverde Formation in the Black Mesa area. 

Reeside and Baker (1929) correlated the Mesaverde Formation 

of Black Mesa Basin into southern Utah and they also recognized the Dakota Sandstone south of the Navajo Reservation in 

the Show Low area (fi g. 3).

Williams (1951) described the Mancos Shale and raised 

the rank of the Mesaverde Formation to Group. The stratigraphy of the eastern part of Black Mesa was described by 

Merrin (1954). Repenning and Page (1956) recognized three 

formations within the Mesaverde Group, which they named, 

in ascending order, the Toreva Formation, the Wepo Formation, and the Yale Point Sandstone (fi g. 12). These authors 

established the basic biostratigraphic framework of Cretaceous 

rocks in the area and correlated it with the faunal zones of 

the Cretaceous Western Interior. They further subdivided the 

Toreva Formation into three informal members. Young (1960) 

reviewed the stratigraphy of the Dakota Group of the Colorado 

Plateau, and Lessentine (1965) compared the Cretaceous stratigraphy and geologic history of the Kaiparowits Basin of Utah 

with that of the Black Mesa Basin. Peterson (1969) related 

the Cretaceous stratigraphic history in the Kaiparowits Basin 

to tectonic activity and interpreted the effect of local tectonic 

movement on sedimentation within the area. Peterson and 

Kirk (1977) established the chronologic similarity of major 

Defianc e Anticline

Howell Mesa Syncline

Echo Cliffs Monocline

Cow

Springs Anticline

Black

Mesa

Syn cline

Cow Springs M onoc l ine

Comb Monocline

Tuba

City Syncline

Mount Beautiful Anticline

Organ

Rock

M

Monocline

aloney

Syn cline

0 50

0 50

miles

kilometers

Cretaceous rocks

Anticline, dashed

 where approximate

Syncline, dashed

 where approximate

N

111°00' 110 110°30' °00'

36°00'

36°30'

35°30'

35°00'

111°30' 109°30'

Figure 7. Laramide folds and erosional limits of Cretaceous rocks, northeastern Arizona (modifi ed from Kelly, 

1958; Davis and Kiven, 1975; and Reynolds, 1988).

H6 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
111° 00' 110°30' 110°00'

36°00'

36°30'

EXPLANATION

Alluvium Yale Point

Sandstone

Wepo

Formation

Toreva

Formation

Mancos

Shale

Dakota

Formation

Quaternary Upper Cretaceous

10 miles

N

MOENKOPI

WASH

ORAIBI

WASH

DINNEBITO

WASH

WEPO

WASH

JEDDITO

WASH

POLACCA

WASH

Figure 8. Geologic map of Black Mesa (modifi ed from Cooley and others, 1969). See fi gure 12 for stratigraphic sequence.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H7
YEAR

1965197019751980198519901995

15,000,000

10,000,000

5,000,000

0

SHORT TONS

Figure 9. Annual coal production, Black Mesa Basin, Arizona (Navajo Nation Minerals Department, 1995).

Scale (feet)

0 15,000

Lease 8580

effective 1964

24,858 Acres

Kayenta Mine 8580

Lease 8580 Minerals

100% Navajo

Navajo

Black Mesa

Mine 8580

Black Mesa Mine

9910/5743 former

Joint Use Area

Kayenta Mine 9910/5743

Leases 9910/5743

effective 1966

40,000 Acres

Hopi Partition Lands Navajo Partition lands

North

Leases 9910/5743

Minerals

50% Navajo (9910)

50% Hopi (5743)

Figure 10. Peabody Group lease area and locations of Kayenta and Black Mesa mines 

(in green) (Haven, 1997; U.S. Offi ce of Surface Mining., 1990). The Joint Use Area is 

shared equally between the Navajo Nation and the Hopi Tribe.

H8 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
depositional events between the Kaiparowits, Henry, San Juan, 

and Black Mesa basins. Franczyk (1983, 1987) revised the 

stratigraphic nomenclature of the Toreva Formation in northeastern Black Mesa by excluding the upper marine sandstone 

and shale of Repenning and Page (1956) and naming them 

the Rough Rock Sandstone and the Wind Rock Tongue of the 

Mancos Shale (fi g. 13). Eaton and others (1987) revised the 

age assignment of the Rough Rock Sandstone and suggested 

that the name Mesaverde Group not be used for Cretaceous 

rocks in Black Mesa because they are considerably older than 

the type section of the Mesaverde Formation in southwestern 

Colorado.

Very little detailed information on the lithologic composition of the Mesaverde Group in Black Mesa has been 

published. Peirce and Wilt (1970) summarized the published 

information that was available at that time. Franczyk (1983, 

1987) described the Toreva Formation, the lower carbonaceous 

member of the Wepo Formation, and the Rough Rock Sandstone in 10 measured sections along the southern half of the 

eastern escarpment. Carr (1987, 1991) described and correlated the upper carbonaceous member of the Wepo Formation 

in 41 measured sections in a small area along the Rough 

Rock road near the eastern edge of the mesa (fi g. 4). Haven 

(1997) incorporated new lithologic descriptions, stratigraphic 

sections and maps of Cretaceous strata of Black Mesa that 

were prepared during this study (fi gs. 14 and 15). He also 

developed additional lithologic data and stratigraphic cross 

Figure 11. Black Mesa mine with active pit and backfi lled pit.

Late Cretaceous

Formation

Mesaverde Group

Yale

Point Ss 

Wepo Fm

Rough Rock Ss 

Toreva Fm

Toreva Fm

Mancos

Shale

Dakota Fm

0- 

328 

328-

820

0-

66

98-

312 

558-

689 

49-

115 

BLACK MESA 

Series Stage

Campanian

Santonian

Coniacian

Turonian

Cenomanian

ft

Figure 12. Stratigraphic sequence and ages of Upper Cretaceous 

formations in the Black Mesa Basin (modifi ed from Eaton and others, 

1987). White, vertically lined areas indicate hiatuses.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H9
sections of the Wepo Formation and revised estimates of coal 

resources in the Wepo Formation.

Cretaceous rocks of Black Mesa were divided into the 

Dakota Sandstone, Mancos Shale, and Mesaverde Formation 

by Gregory (1917). Subsequently, several other authors have 

refi ned the interpretations of the stratigraphy, age, and depositional environments of these units, which are summarized 

below. 

Dakota Formation

The Upper Cretaceous Dakota Formation unconformably 

overlies Jurassic strata throughout the Black Mesa region and 

northwestward into southern Utah. Although there is no apparent angular unconformity at any one exposure, the Dakota 

Formation unconformably overlies progressively older deposits toward the west and southwest (Williams, 1951). An erosional surface was cut into successively older rocks toward 

the Mogollon Highlands to the south and the Sevier orogenic 

belt to the west, and this resulted in a beveled surface on 

which the Dakota Formation was deposited. Along the north 

and northeast sides of Black Mesa, the Dakota Formation overlies the Upper Jurassic Cow Springs Member of the Entrada 

Sandstone and the Morrison Formation (Peterson, 1988). In 

southern Black Mesa, the Dakota rests on the middle part of 

the Entrada Sandstone (fi g. 16). Near Show Low (fi g. 3), about 

70 mi south of the Black Mesa area, the Dakota overlies the 

Upper Triassic Chinle Formation, and farther south at McNary, 

Arizona, it rests on upper Paleozoic strata (Harshbarger and 

others, 1957).

The Dakota Formation is divided into three members; 

lower sandstone member, middle carbonaceous member, and 

upper sandstone member (fi g. 16). Lithologic units within the 

Dakota are highly lenticular and thin and thicken laterally. The 

thickness of the Dakota Formation varies from 50 to 350 ft 

with no apparent trend to this thickening and thinning (fi g. 

17).

The lower sandstone member of the Dakota Formation in 

Arizona has yielded no age-diagnostic fossils, but is believed 

to be of Cenomanian age (Repenning and Page, 1956; Cobban 

and Hook, 1984). Palynological data from the middle carbonaceous member has been interpreted as indicating a Cenomanian age (Agasie, 1969; Carter, 1975; am Ende, 1986). 

The upper sandstone member is late Cenomanian in northern 

Arizona, based on bivalve and ammonite zonation (Cobban 

and Hook, 1984; Kirkland and Cobban, 1986). 

The Dakota Formation in the Black Mesa area generally 

Figure 13. Oblique aerial photograph of the northeastern escarpment in the Rough Rock area of Black Mesa. Ky, Yale 

Point Sandstone; Kwu, upper carbonaceous member of the Wepo Formation; Kr, Rough Rock Sandstone; Kmw, Wind 

Rock Tongue of the Mancos Shale; Kwl, lower carbonaceous member of the Wepo Formation; Kt, Toreva Formation; Km, 

Mancos Shale; the Dakota Formation is out of view below the bottom of photo.

H10 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
grades upward through fl uvial, paludal, brackish, and nearshore marine deposits, although considerable lateral variation 

occurs (Repenning and Page, 1956; Molenaar, 1983; Kirkland, 

1983, 1991).

Mancos Shale

The Mancos Shale overlies the upper sandstone member 

of the Dakota Formation and is exposed in steep to gentle 

slopes around the periphery of Black Mesa, and in areas of 

deep erosion on top of the mesa, especially where regional 

drainages cross anticlinal axes (fi gs. 7 and 8). The Mancos 

Shale is predominantly dark gray, bluish-weathering, siltstone 

and claystone. It is about 700 ft thick on the north side of 

Black Mesa and thins to about 475 ft in the southernmost 

complete exposures at Blue Point on Padilla Mesa (Kirkland, 

1983) (fi g. 18). It continues to thin southward toward its pinchout near Show Low (Repenning and Page, 1956).

The Mancos shale was divided by Kirkland (1983) into 

four informal members, which were later designated the: (1) 

lower fossiliferous calcareous shale member; (2) middle welllaminated calcareous shale member, (3) Hopi sandy member, 

and (4) upper noncalcareous claystone member (Eaton and 

others, 1987). Franczyk (1987) named the Wind Rock Tongue 

of the Mancos Shale that occurs in the lower part of the Wepo 

Formation (fi gs. 14 and 16).

The age of the Mancos Shale at Black Mesa is late 

Cenomanian to middle Turonian, based on ammonite biostratigraphic zonation (Cobban and Hook, 1984; Kirkland and 

Cobban, 1986). The lower Mancos fauna refl ects the maximum 

transgression and depth of the Western Interior Seaway near 

the Cenomanian-Turonian boundary (Kauffman, 1977, 1984; 

Kirkland, 1983). However, Olesen (1987, 1991) demonstrated 

that maximum depth in the southern part of Black Mesa did not 

occur until early middle Turonian time (base of Collignoniceras 

woolgari woolgari zone), based on the occurrence of abundant 

planktonic forams in that part of the lower Mancos Shale.

The upper contact of the Mancos Shale is gradational and 

intertongues with the overlying Toreva Formation. The contact 

is placed where sandstone begins to dominate the sequence 

(Kirkland, 1983).

Dakota

UPPER CRETACEOUS

NW 

A A'

FmMancos Shale

Toreva

FmWepo Fm Yale Point Ss 

Williams

M.S.

11

Coal Mine

Wash Gap

M.S.

Rock

Gap

M.S. Rough Rock

Composite

M.S. Wind Rock

Kirkland

Lohali Point

M.S.

Valley

M.S.

Photo

Point

M.S.

Yale Point Ss

upper Wepo Fm

lower Wepo Fm

Rough Rock Ss

Windrock Tongue

U

M

L

DATUM

lower Toreva

Tongue of

Mancos Shale

Hopi Ss

Mancos Shale

Dakota Fm

SE 

8.0 mi. 6 mi. 17.8 mi. 5.7 mi. 12.4 mi.

Feet

200

100

0

Figure 14. Diagrammatic NW.-SE. stratigraphic cross section A-A’ of Cretaceous rocks, Black Mesa Basin, 

Arizona. Location of cross section and measured sections (M.S.) are shown on fi gure 4.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H11
200

100

0

Feet

W

9

W

15

W

16

W

18

W

17

W

12

CBM Cowsprings

 Mine

M.S.

Navajo

#8

W

5

W

10

W

11

Present Erosion Surface

Wepo Fm 

Upper Toreva Fm

Middle Toreva Fm

lower Toreva Fm

DATUM

Mancos Shale

Dakota Fm

Yale

Point

Ss

1.5 mi3.1 mi

4.0 mi

1.2 mi

9.3 mi 17.8 mi 4.1 mi 4.0 mi 4.6 mi

B B'

SW NE

Figure 15. Diagrammatic SW.-NE. stratigraphic 

cross section B-B’ of Cretaceous rocks, Black 

Mesa Basin, Arizona. Location of cross section 

and measured sections (M.S.) are shown on 

fi gure 4.

unconformity

SW

Kwu

Kwl

Dakota

Fm

Wepo Fm

NE

Kr

Kmw

Ky

upper sandstone member

present erosion surface

upper shale member

middle shale member

lower shale member

Hopi sandy member

upper sandstone member

Cow Springs member

Entrada Fm

Morrison Fm

upper sandstone member

Blue Point tongue, Toreva Formation

lower sandstone member

lower sandstone member

middle carbonaceous member

middle carbonaceous member

Toreva Fm

Mancos

Shale

Figure 16. Generalized schematic diagram showing cross sectional distribution of Cretaceous units and the contact with the Jurassic 

Entrada and Morrison Formations from southwest to northeast across Black Mesa (modifi ed from Eaton and others, 1987). Ky, Yale Point 

Sandstone; Kwu, upper carbonaceous member of the Wepo Formation; Kr, Rough Rock Sandstone; Kmw, Wind Rock Tongue of the 

Mancos Shale; and Kwl, lower carbonaceous member of the Wepo Formation.

H12 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
Mesaverde Group

On Black Mesa, the term “Mesaverde Group” refers collectively to units that overlie the Mancos Shale. The three 

formations on Black Mesa as defi ned by Repenning and Page 

(1956) are, in ascending order, the Toreva Formation, the Wepo 

Formation, and the Yale Point Sandstone (fi gs. 14 and 15). 

These formations were deposited in a variety of environments 

along coastal plains and strandlines that record the last infl uence of marine environments in northern Arizona (Molenaar, 

1983).

Toreva Formation

The Toreva Formation in the southern part of Black Mesa 

was subdivided into three members by Repenning and Page 

(1956): a lower sandstone member, a middle carbonaceous 

member, and an upper sandstone member (fi g. 16). The Toreva 

Formation crops out over an area of about 743 mi2 and varies 

from about 200 to 470 ft in thickness (fi gs. 8 and 19).

The lower sandstone member is about 100 ft thick and 

forms a prominent cliff throughout Black Mesa. It is composed 

of fi ne- to medium-grained, quartzose sandstone with crossbedding that is indicative of offshore to beach depositional 

environments and is gradational with the underlying Mancos 

Shale (Repenning and Page, 1956). 

The middle carbonaceous member consists of carbonaceous siltstones, mudstones, and sandstones that are exposed 

around the perimeter and at the surface in the southern half 

of Black Mesa but is absent from the northern part (Peirce 

and Wilt, 1970). It forms a slope approximately 100 ft high in 

southern Black Mesa and thins rapidly to the north apparently 

from erosion of its upper surface. It is composed of lenticular 

shale, coal, and sandstone, refl ecting delta-plain deposition 

(Franczyk, 1983, 1987). The fl at and thinly bedded dark mudstones, varicolored siltstones, coal, and thin yellowish-gray 

sandstones of the middle carbonaceous member were probably 

deposited in marshy lagoons and swampy areas behind the 

beach as the sea retreated farther eastward.

The upper sandstone member consists dominantly of 

medium- to coarse-grained, sheetlike sandstone bodies that 

grade upward into tabular and lenticular sandstones and fi negrained deposits. It is commonly conglomeratic, and sedimentary structures indicate deposition by streams fl owing northeastward across a broad coastal plain. The upper sandstone 

member generally rests on a sharp scour contact with the 

middle carbonaceous member, but locally rests directly on the 

lower sandstone member (Peirce and Wilt, 1970). It varies 

111°00' 110°30' 110°00'

36°00'

36°30'

150

100

50

100

150

50

100

50

100

50

150

100

150

150

100

50

100

100 Thickness, in feet Contour Interval = 50 ft

Dakota Outcrop

Figure 17. Isopach map of the Dakota Formation, Black Mesa Basin, 

Arizona. 111°00' 110°30' 110°00'

36°00'

36°30'

100 Thickness, in feet Contour Interval = 100 ft

Dakota Outcrop

500

600

700

700

600

800

500

Figure 18. Isopach map of the Mancos Shale, Black Mesa Basin, 

Arizona.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H13
from 25 to 120 ft in thickness and forms a prominent cliff. The 

upper sandstone member was deposited in a continental fl uvial 

environment (Franczyk, 1983, 1987) and overlies an erosional 

unconformity that represents a late Turonian diastem (Peterson 

and Kirk, 1977; Eaton and others, 1987). 

The Toreva Formation in northern Black Mesa was redefi ned by Franczyk (1983, 1987) to exclude three lithologic 

units from the Toreva as described by Repenning and Page 

(1956). Franczyk (1983, 1987) renamed these units, respectively, the lower carbonaceous member of the Wepo Formation, the Wind Rock Tongue of the Mancos Shale, and the 

Rough Rock Sandstone (fi gs. 12 and 16). 

Wepo Formation

The Wepo Formation is exposed at the surface across 

much of Black Mesa but has been eroded from the southern 

and western margins (fi gs. 8 and 20). Along the northeastern 

margin, it is capped by the massive, yellowish-gray Yale Point 

Sandstone (fi gs. 8, 14, and 15). About 1,270 mi2 of the Wepo 

Formation is exposed at the surface of Black Mesa (Peirce and 

Wilt, 1970).

The Wepo Formation varies in thickness from 400 ft in 

the southwestern portions of Black Mesa to 200 ft in the 

northeastern portions (fi g. 20). Thinning of the Wepo to the 

northeast is due to intertonguing with the underlying Toreva 

Formation and the overlying Yale Point Sandstone (Molenaar, 

1983) (fi gs. 15 and 16). It is 600 ft thick east of Cow Springs, 

more than 400 ft thick in the central part of its exposure, and 

to the east it thins to 318 ft at Rough Rock (fi g. 20). Because 

the top of Black Mesa is an erosional surface, the remaining 

Wepo thickness in a particular area depends upon its structural 

position and the extent of downcutting by streams (Peirce and 

Wilt, 1970). 

The Wepo Formation is composed dominantly of interbedded shale, siltstone, sandstone, and coal that erode to form 

steep slopes. Locally interbedded lenticular, trough-crossbedded, well-cemented sandstones form cliffs as much as 40 ft 

high. The fl at-lying shales, siltstones, and coal commonly contain a well-preserved fl ora, and a brackish-water fauna is preserved in some of the shales. Siderite concretions and gypsum 

are common through much of the formation.

The Wepo Formation is undifferentiated throughout most 

of its extent, but was subdivided along the northeastern margin 

of Black Mesa into the lower and upper carbonaceous members where they are separated by the Wind Rock Tongue of 

the Mancos Shale and the Rough Rock Sandstone (Franczyk, 

1983, 1987) (fi gs. 14 and 16). The Rough Rock Sandstone 

contains sedimentary structures and trace fossils that indicate 

Contour Interval = 100 ft

Thickness, in feet 100 10 miles

111°00' 110 110°30' °00'

36°00'

36°30'

400

100

300

200

300

200

200

200

100

100

100

400

400

200

300

300

300

300

400

300

Dakota Outcrop

Figure 19. Isopach map of the Toreva Formation, Black Mesa Basin, 

Arizona.

Contour Interval = 200 ft

Areal distribution of the Wepo Formation

Thickness, in feet

10 miles

100

Dakota Outcrop

200

200 200

200

400

400

400 400

400

400

600

200

111°00' 110 110°30' °00'

36°00'

36°30'

Figure 20. Isopach map of the Wepo Formation, Black Mesa Basin, 

Arizona. 

H14 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
a regressive beach deposit (Molenaar, 1983) and inoceramids 

that indicate a Coniacian age (fi g. 12) (Eaton and others, 

1987). Both members of the Wepo Formation consist of coal, 

carbonaceous siltstone, and mudstone, and tabular and lenticular sandstone bodies with scour bases. The lower carbonaceous 

member is gradational with the upper sandstone member of 

the Toreva Formation, and its upper contact with the Wind 

Rock Tongue or the Rough Rock Sandstone is erosional. Carr 

(1987, 1991) measured and described 41 sections of the upper 

carbonaceous member of the Wepo Formation in the Rough 

Rock area and interpreted the environment of deposition as a 

delta plain (fi gs. 4 and 21). 

Yale Point Sandstone

The Yale Point Sandstone is present only along the northeastern edge of Black Mesa, where it forms a prominent paleorange cliff approximately 200 ft high, and extends about 6 

mi west of the escarpment. (fi gs. 8, 15, 16, 21, and 22). The 

limited lateral extent is a result of intertonguing with the Wepo 

Formation and extensive beveling of Black Mesa to the south 

and west by Cenozoic erosion (Repenning and Page, 1956). 

The thick sandstone deposits of the Yale Point Sandstone represent stacked beach and nearshore marine sands, shoreward 

(to the west) of which, thick coal deposits of the Wepo Formation accumulated on a delta plain. No Cretaceous rocks 

younger than the Santonian Yale Point Sandstone are present 

in northeastern Arizona (fi g. 12). Cenozoic erosion may have 

removed as much as 4,500 ft of Cretaceous rocks from the 

Black Mesa, based on the thickness of younger Cretaceous 

rocks in the San Juan Basin (Molenaar, 1983; Nations, 1989; 

Cook and Bally, 1975). This depth of burial would have caused 

a higher degree of thermal maturation and methane gas generation than might be expected for the relatively shallow presentday burial depths of the coals in Black Mesa.

Geophysical Type Log

Even though there are only a few deep drill holes in Black 

Mesa, they provide excellent subsurface information. We have 

selected one of these (fi g. 4), the Skelly Hopi A No.1 well, 

as the geophysical type log for the area, from which to pick 

the contacts of the Wepo, Toreva, Mancos, and Dakota Formations. The log was used to differentiate the sand and shale 

intervals using the gamma-ray and neutron curves (fi g. 23).

Distribution of Coal in 

Black Mesa Basin

Coal seams occur in the carbonaceous members of the 

Dakota, Toreva, and Wepo formations. The Yale Point Sandstone contains only a minor seam or two in one small area 

and is therefore not considered to be of economic interest 

(Peirce and Wilt, 1970). The areas of outcrop of various formations are shown on the geologic map of Black Mesa (fi g. 8). 

Upper Cretaceous 

Yale Point Ss Wepo Formation undifferentiated Toreva Fm

Peabody Coal Company

core holes

Present Erosion

Surface

Datum upper Toreva Formation

lower Wepo Formation

Wind Rock Tongue

of Mancos Shale

Rough Rock Ss

Yale Point Ss

C C' N SE

upper Wepo

Formation

Williams M.S. 11 

N9-3

N4-1 N6-2

J21-1A

J21-10

Carr 

M.S.

28

Carr 

M.S.

32

Carr 

M.S.

35

CBM Rough Rock M.S.

7 mi 3.5 mi 1.8 mi 6.5 mi 1.8 mi 18.9 mi 1.8 mi 1.8 mi 2.7 mi

200

100

0

Feet

Figure 21. N.-SE. stratigraphic cross section C-C’ of the Wepo Formation (modifi ed from Haven, 1997). Measured sections (M.S.) are from 

Williams (1951), Carr (1987), and the Coalbed Methane Project. Location of cross section shown on fi gure 2.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H15
These formations were deposited in a variety of coal-forming 

environments along coastal plains and strandlines that record 

the last infl uence of marine environments in northern Arizona 

(Molenaar, 1983). Coal thickness in the Black Mesa Basin 

is interpreted from isopach maps of the cumulative coal thickness for all formations (fi g. 24), coal thickness in the Dakota 

Formation (fi g. 25), coal thickness in the Toreva Formation 

(fi g. 26), and coal thickness in the Wepo Formation (fi g. 27). 

The black outline of each of these maps indicates the erosional 

limit of the Dakota Formation.

Coal Distribution and Thickness 

in the Dakota Formation

The middle carbonaceous member of the Dakota Formation contains yellowish-gray to black carbonaceous siltstone, 

shale, coal, and thin sandstones. The unit weathers into a 

smooth slope with minor ledges, varies irregularly in thickness 

between 20 and 80 ft, and is thickest on the eastern side 

of Black Mesa where the other Dakota members are absent 

(Pickens, 1974). The better quality coal seams are in the 

upper siltstone-claystone beds of the middle member near the 

upper sandstone member. Most seams average 2 ft in thickness 

except in the southeastern and southwestern portions of the 

mesa (fi g. 25) where they are 7–9 ft thick, (Kiersch, 1955; 

Figure 22. Henry Haven measuring and describing a section of the 

Yale Point Sandstone.

gamma ray neutron 100 200 300 400 500 600 700 800 900 1000 CRETACEOUS

JURASSIC

Cow Springs Sandstone 997'

Dakota Formation 918'

Mancos Shale 400'

Lower Toreva 265'

Middle Toreva

Carbonaceous Member 160'

Upper Toreva 115'

Wepo

SKELLY HOPI A #1

SECTION 35, T30N, R17E

UTM 546500E 3980500 N 

NAVAJO CO., AZ.

Kelly Bushing Elevation 6119'

Figure 23. Typical Black Mesa subsurface log, Skelly Hopi A No. 1 

(from Haven, 1997). Location shown on fi gure 4. The gamma-ray curve 

counts increase to the right indicating shaley intervals, whereas a 

cleaner sandstone yields a lower count and the curve is defl ected to 

left. The neutron curve is defl ected to the right in sandy intervals and 

to the left in shaley intervals.

H16 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
Williams, 1951; Merrin, 1954). Some coal seams are lenticular 

and vary in thickness from several feet to several inches 

within a few hundred feet laterally as a result of deposition 

within local depressions or by being cut off by other channels 

(Williams, 1951; Kiersch, 1955). Although the carbonaceous 

member occurs nearly everywhere in Black Mesa, the thicker 

and more extensive coal deposits are in the southwestern part 

where the upper sandstone member is frequently absent. Three 

mines have obtained coal from this formation, the Tuba City 

No. 3 mine, the Chinle No. 1 mine, and the Montezuma mine 

(fi gs. 25 and 28).

Coal occurrence and (or) thickness has been determined 

from 92 measured sections and other data points, and its thickness in the basin may be interpreted from the coal isopach 

map (fi g. 25). 

Coal Distribution and Thickness 

in the Toreva Formation

Coal beds or carbonaceous siltstones occur in the Toreva 

Formation in all measured sections throughout Black Mesa, 

Contour Interval = 10 and 20 ft

Thickness, in feet

10 miles

20

10

20

30

40

50

60

Outcrop limit of

Dakota Fm

1. Maloney mine and Kayenta Number 1 mine

2. Cow Springs mine

3. Tuba City mine

4. Keams Canyon mines

5. Chinle Number 1 mine

6. Chinle Number 2 mine

7. Montezuma mine

8. Oraibi mine

9. Steamboat mine

 COAL MINES

111°00' 110°30' 110°00'

36°00'

36°30'

10

10 20

40

60

10

20

10

20

10

20

0

10

20

10

20

10

0 0

10

20

20

40

60

60

40

2

1

3

4

8

9

5

6

7

Figure 24. Cumulative net-coal thickness in Dakota, Toreva, and Wepo Formations, Black Mesa, Arizona. 

Historic coal mines are shown. See fi gure 27 for area of active mining.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H17
although individual beds may not be continuous from place 

to place. The thickest and most extensive coal in the Toreva 

Formation has been mapped in the north-central part of Black 

Mesa. Three mines have obtained coal from the Toreva Formation, including the Keams Canyon mine, Chinle #2 mine, and 

the Oraibi mine, all in the southern portion of the mesa (Peirce 

and Wilt, 1970).

The thickness of coal in the Toreva Formation was determined from 21 measured sections, and its thickness in the 

basin may be interpreted from the coal isopach map (fi g. 26).

Coal Distribution and Thickness 

in the Wepo Formation

The Wepo Formation contains the highest rank and highest quality coal on Black Mesa as well as the largest minable 

reserves. The coal seams are thicker, more numerous, more 

widespread, and more accessible for strip mining than are 

those in the Toreva or Dakota Formations. Coal occurs within 

an alternating sequence of dark olive-gray to brown siltstones 

and mudstones and yellowish-gray sandstones.

Contour Interval = 5 and 10 ft

Thickness, in feet

10 miles

50

111°00' 110 110°30' °00'

36°00'

36°30'

10

20

5

0

10

10

5

0

5

10

0

0

5

5

5

0

10

20

5

9

3

7

Coal Mines in Dakota Formation

3. Tuba City No. 3 mine

5. Chinle No. 1 mine

7. Montezuma mine

9. Steamboat mine

Outcrop limit of

Dakota Fm

Figure 25. Isopach map of net-coal thickness in the Dakota Formation, Black Mesa, Arizona. Historic coal 

mines are shown.

H18 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
Greater thicknesses of Wepo strata, and therefore potentially more coal, are preserved in synclines such as the Maloney and the Black Mesa synclines (fi gs. 7 and 27). The principal coal reserves that are being mined by the Peabody Group 

Coal Company (fi g. 29) are associated with the Maloney syncline (fi g. 7).

The Wepo contains at least 10 coal beds thicker than 3 ft 

in the area examined by Williams (1951) along the northwestern margin of Black Mesa in the Cow Springs area (fi g. 27). 

The coal seams average 4 to 8 ft thick although individual 

seams may be from 12 to 20 ft thick (Kiersch, 1955). Individual coal seams persist for hundreds to thousands of feet 

but invariably thin laterally to seams a few inches to a foot 

thick. Because some of the coal near the surface has either 

been burned out, cut out locally by erosion, or covered, only a 

detailed drilling program can indicate the presence, thickness, 

and depth of coal, and provide fresh samples for testing (Peirce 

and Wilt, 1970). 

The Wepo coal in the Black Mesa Basin occurs primarily 

in three areas of the northern half of Black Mesa, the Peabody 

Contour Interval = 5 ft

Areal distribution of Toreva Formation

Thickness, in feet

10 miles

50

6

4

111°00' 110 110°30' °00'

36°00'

36°30'

5

5

15

5

15

20

5 40

50

20

15

20

10

10

Outcrop limit of 

Dakota Fm

Coal Mines in Toreva Formation

4. Keams Canyon mines

6. Chinle No. 2 mine

8. Oraibi mine

8

Figure 26. Isopach map of net-coal thickness in the Toreva Formation, Black Mesa, Arizona. Historic coal 

mines are shown. 

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H19
lease area, the Cow Springs area, and the Rough Rock area 

(fi g. 27). The coal thickness as shown in fi gure 27 was mapped 

as net feet of coal including intervals down to less than a foot. 

Data sources include sample logs from drill holes, measured 

sections from the CBM project, tribal water wells, and unpublished theses (fi g. 4). 

The largest known coal deposit occurs in the north-central 

part of Black Mesa and is currently being mined by the Peabody Group Coal Company (fi gs. 5 and 10). The coal deposits 

trend northwest to southeast across the lease area and appear to 

extend southward into the Rough Rock area (fi g. 27). The coal 

deposit in the lease area occurs in the undifferentiated Wepo, 

primarily as three discontinuous deposits that total as much as 

70 ft in net-coal thickness (fi gs. 21 and 30). The coal occurs in 

individual seams as thick as 20 ft to less than a foot. The coal 

is considered economically feasible to mine down to depths 

of 250 ft (R. Willson, 1995, Peabody Group Coal Company, 

oral commun.). The coal deposits on the Peabody Group lease 

thicken in the Maloney syncline that trends north-south in the 

northern part of the basin (fi g. 7). The thicker coal appears 

to be preserved in the structurally low areas and to thin over 

Areal distribution of Wepo Formation 

Thickness, in feet Contour Interval = 20 ft

10 miles

50

Outcrop limit of

Dakota Outcrop

Cow Springs area

Peabody lease area

60

40

20

20

20

40

20

Rough Rock area

Thickness varies widely

(see figure 31 )

1

2

Coal mines in Wepo Formation

1. Maloney mine and Kayenta #1 mine

2. Cow Springs mine

Peabody Kayenta and Black Mesa mines

111°00' 110 110°30' °00'

36°00'

36°30'

Figure 27. Isopach map of net-coal thickness in the Wepo Formation, Black Mesa, Arizona. Historic and 

active coal mines are shown.

H20 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
structurally high areas due to erosion. The average net thickness of coal is 40 ft over the Peabody Group lease, and it thins 

to approximately 20 ft near Rough Rock.

The Rough Rock coal deposit appears to be on the same 

structural and stratigraphic trends as the Peabody Group lease 

to the north. The area between the lease and the Rough Rock 

coal deposit to the southeast could be an area of future coal 

development. The coal deposit in the Rough Rock area covers 

approximately 10 mi2 and net coal is approximately 20 ft 

thick (fi gs. 27 and 31). Figure 32 is a cross section through 

20 measured sections that illustrates the facies relationships, 

including coal, in the upper carbonaceous member of the Wepo 

Formation. 

The Cow Springs coal deposit averages 20 ft in thickness, 

but may be considerably thicker in places. Williams (1951) 

reported 70 net ft of coal in his measured section 14, which is 

approximately 6 mi to the east of the old Cow Springs Trading 

Post. This thickness was not confi rmed during this study, but 

we measured a partial section of the upper Wepo at the old 

Tuba City No. 4 mine and observed 7 ft of net coal. The Cow 

Springs area in the northwestern portion of the mesa has not 

been mapped in detail, but its location on the Black Mesa 

syncline suggests that considerable coal may be preserved 

there (fi gs. 7 and 27).

Coal Quality in Black Mesa Basin

Coal Sampling and Analysis

Peirce and Wilt (1970) list 30 coal analyses found in the 

published literature, of which 11 were from the Wepo FormaFigure 28. Coal and carbonaceous shale in Dakota Formation at Coalmine Canyon, Black 

Mesa, Arizona.

Figure 29. Aerial overview of the Kayenta mine, Black Mesa, Arizona 

with top of escarpment capped by the Yale Point Sandstone on 

skyline. The formation being mined is the Wepo Formation.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H21
tion, 8 from the Toreva Formation, and 11 from the Dakota 

Formation. Moore and Swanson (1977) published analyses of 

26 samples of coal from the Wepo Formation that were collected from the Peabody Group mines on Black Mesa. These 

data were used by Haven (1997) to determine the coal quality, 

coal density, and remaining coal resources in the Wepo Formation.

Fourteen new coal samples were collected during the 

course of this study, from the Dakota, Toreva, and Wepo Formations in measured sections and in road cuts around the 

mesa. The sample locations were determined by Geographic 

Positioning System. Peabody Group Coal Company conducted 

chemical analyses of these samples on a cooperative basis and 

provided information on the rank, ash yield, sulfur content, 

and calorifi c value. Because these analyses are on outcrop 

samples, their reliability may be questionable as compared to 

published mine-sample analyses; however, they are reported 

here in that they provide a measure of minimum coal quality 

outside the Peabody Group-leased coal areas.

Analytical Data on Coal Quality

Coal Quality in the Dakota Formation

The rank and quality of coal in the Dakota Formation in 

Black Mesa is known from only 15 sample analyses (table 1). 

Moisture, volatile matter, fi xed carbon, ash, sulfur and Btu/lb 

have been published for 10 samples from the Tuba City No. 

3 mine (Campbell and Gregory, 1911; Cooper and others, 

1947; Williams, 1951; Kiersch, 1955), and one sample from 

the Montezuma mine (Kiersch, 1955). Four additional analyses 

were determined by Peabody Group on the samples collected 

by the CBM project.

Coal Quality in the Toreva Formation

The rank and quality of coal in the Toreva Formation in 

Black Mesa is known from only 10 sample analyses (table 

2). Moisture, volatile matter, fi xed carbon, ash, sulfur, and 

calorifi c value have been published for four samples from the 

Keams Canyon No. 4 mine (Cooper and others, 1947; Kiersch, 

1955), three from the Chinle No. 2 mine (Kiersch, 1955), and 

one from the Oraibi mine (Campbell and Gregory, 1911). Two 

additional samples were analyzed by Peabody Group Coal 

Company on the samples collected by the CBM Project (table 

2).

Coal Quality in the Wepo Formation

Analyses of six samples from the Wepo Formation 

collected from the Tuba City No. 4 mine: and fi ve samples 

from the Kayenta No. 2, or Maloney, mine as summarized by 

Peirce and Wilt (1970) are shown in table 3. Seven analyses 

of channel and grab samples from the upper Cretaceous Wepo 

Formation, Black Mesa coal fi eld, Navajo County, Arizona, 

are shown in table 4, modifi ed from Moore and Swanson 

(1977). The information contained in table 4 is also available 

from larger published databases (Bragg and others, 1998; 

Affolter, chap. G, this CD-ROM). These databases contain a 

complete proximate/ultimate analysis and forms of sulfur and 

detailed trace element analyses.

The coal in the Peabody Group coal mines, located in 

the north-central part of the Black Mesa coal fi eld, has been 

ranked as bituminous (Peirce, 1975) to subbituminous (Bragg 

and others, 1998). It has an average ash yield of 5.2 percent, 

average 12,382 Btu per pound, and average sulfur content 

of 0.58 percent (Peirce, 1975). The rank and quality of coal 

in the Wepo Formation is much better known from samples 

taken within the Peabody Group lease. Analytical coal quality 

Figure 30. Working face in Kayenta mine, Black Mesa, Arizona 

exposing three 10-ft-thick coal beds in the Wepo Formation.

H22 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
Contour Interval = 10 ft

Thickness, in feet

20

3

30

3

18 12

12

7

9

8

3

1

5

5

5

20

22

15

22

22

25

16

13

19 11

26 25

29

21

25

1914

9 11

15

15

5

30

23

37

9

26

31

10

20

20

30

10

30

20

10

0

580 000 590 000 600 000

4 040 000

4 030 000

4 020 000

data from the Wepo Formation at the Peabody Group Kayenta 

and Black Mesa mines were made available to the project. 

Four representative analyses of samples from north to south 

across the north-central part of the lease are shown in table 5.

Eight additional Wepo coal samples were collected from 

surface outcrops by the CBM project in 1994 and were 

analyzed by Peabody Group Coal Company for moisture, ash, 

calorifi c value, sulfur, and rank (table 6). The samples were 

collected from the Photo Point section (fi g. 4; northeastern 

Black Mesa—approximately 5 mi northwest of Chilchinbito), 

the Blue Gap area (fi g. 4; located approximately 6 mi 

northeast of Blue Gap Chapter house), and the Cow Springs 

mine (fi g. 4; approximately 5 mi east of the old Cow Springs 

Trading Post).

Comparison of Coal Quality with 

Peabody Group Lease Area

The ash and the sulfur content of coal from the Photo Point 

area (fi g. 4) of the mesa were higher and the calorifi c value was 

lower than that of coal from the Peabody lease area. Samples 

from the old Cow Springs mine are higher in ash content, lower 

in calorifi c value, but the sulfur content appears to be the same 

as samples from the Peabody lease area. The better quality coal 

(high calorifi c value, low sulfur and ash) deposits occur in the 

north-central and southeastern portions of Black Mesa. The coal 

samples from the Blue Gap area appear to match the calorifi c 

value of Peabody Group’s as-received analyses, but appear to be 

higher in ash and sulfur.

Figure 31. Isopach map of net-coal thickness of the Wepo Formation in the Rough Rock area shown with UTM 

grid for reference. Location of area within the Black Mesa Basin is shown on fi gure 27.

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H23
1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20

upper carbonaceous member, Wepo Fm

50

Ft

0 Mi 1

Coal

Clinker

10 Measured section

Sandstone

Heterolithic

sandstone/mudrock Heavily burrowed

Organic-rich

mudrock

Rough Rock Sandstone

Yale Point Sandstone

Figure 32. E.-W. stratigraphic cross section of sedimentary facies in the upper carbonaceous member of the Wepo Formation in 

the Rough Rock area (from Carr, 1987).

Table 1. Coal analyses from the Dakota Formation, Black Mesa, Arizona. 

[Data from sources 1–4 are as compiled by Peirce and Wilt, 1970. Moisture content and volatile matter reported on an as-received basis] 

 Moisture Volatile Fixed Ash Sulfur **Original 

 Mine Location content matter carbon content content Btu/lb source 

 (%) (%) (%) (%) (%) 

 Tuba City Coal Canyon 12.78 32.36 24.12 30.74 0.81 5,119 (1) 

 No. 3 16 mi SE. of 13.62 43.93 16.67 25.78 0.89 5,592 (1) 

 Tuba City 9.15 43.59 37.23 10.03 1.70 8,837 (1) 

 10.01 40.09 39.90 10.00 1.57 8,914 (1) 

 11.72 41.68 33.96 12.64 2.29 7,683 (1) 

 11.8 36.5 40.7 11.0 2.0 10,410 (2) 

 11.8 35.8 41.1 11.3 1.8 10,270 (2) 

 9.1 33.3 43.9 13.7 1.28 10,490 (3) 

 9.9 31.4 44.5 14.2 - - (4) 

 10.3 33.8 42.3 13.6 - 10,550 (4) 

 Montezuma Montezuma’s Chair 7.3 33.1 43.4 11.2 0.7 10,510 (1) 

 Outcrop Steamboat Canyon 12.08 20.92 0.48 8,591 (5) 

 Outcrop Chilchinbeto area 15.5 6.21 0.48 9,785 (5) 

 Outcrop Coalmine Canyon 10.63 18.01 0.92 9,175 (5) 

 Outcrop Coalmine Canyon 13.54 13.54 1.58 9,151 (5) 

**Sources: (1) Kiersch (1955), p. 52, 53. (2) Cooper and others (1947), p. 32–34. (3) Campbell and Gregory (1911), p. 237. (4) 

Williams (1951), p. 88, 188. (5) CBM Project (1994). 

H24 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
Table 2. Coal Analyses from the Toreva Formation. 

[Data from sources 1 and 2 are as compiled by Peirce and Wilt, 1970. Moisture content and volatile matter reported on an as-received 

basis. M.S., measured section] 

 Moisture Volatile Fixed Ash Sulfur **Original 

 Mine Location content matter carbon content content Btu/lb source 

 (%) (%) (%) (%) (%) 

 Keams Keams 3.4 29.8 16.0 50.8 0.6 5,430 (1) 

 Canyon Canyon 5.3 36.8 38.9 19.0 1.1 10,270 (1) 

 No. 4 11.7 32.3 47.2 8.8 1.0 11,200 (2) 

 11.4 32.5 45.0 11.1 0.9 10,650 (2) 

 Chinle 6 mi S. of 5.4 37.6 42.3 14.7 1.2 10,650 (1) 

 No. 2 Salina 5.1 37.0 43.5 14.4 1.0 10,800 (1) 

 8.62 34.31 38.87 18.20 1.30 9,807 (1) 

 Oraibi 4 mi E. of 

 mine Oraibi 9.9 32.6 46.9 10.62 1.12 10,800 (1) 

Outcrop Coalmine Wash 

 Gap M.S. 8.8 26.56 0.42 8,091 (3) 

Outcrop Blue Gap 22.3 13.39 0.62 9,858 (3) 

**Sources: (1) Kiersch (1955), p. 52, 53. (2) Cooper and others (1947), p. 32–34. (3) CBM Project (1994). 

Table 3. Coal Analyses from the Wepo Formation. 

[Data from sources are as compiled by Peirce and Wilt, 1970. Moisture content and volatile matter reported on an as-received basis] 

 Ref. Moisture Volatile Fixed Ash Sulfur **Original 

 Mine Location no. content matter carbon content content Btu/lb source 

 (%) (%) (%) (%) (%) 

 Tuba City 7 mi E. of 20 8.0 40.2 43.1 8.7 0.5 11,540 (1) 

 No. 4 Cow Springs 21 8.4 39.7 45.2 6.7 0.4 11,830 (1) 

 22 7.01 40.52 47.05 5.42 0.49 11,985 (1) 

 23 10.4 37.3 45.8 6.5 0.4 11,590 (2) 

 24 11.7 36.8 45.7 5.8 0.6 11,410 (2) 

 25 17.4 37.0 41.6 4.0 - 10,450 (4) 

 Kayenta 30 mi S. of 26 8.2 42.4 45.5 3.9 0.5 12,060 (1) 

 No. 2 or Kayenta 27 8.6 38.8 48.3 4.3 0.7 11,930 (1) 

 Maloney 28 11.6 40.2 44.8 3.4 0.7 11,690 (2) 

 mine 29 11.5 37.5 46.9 4.1 0.9 11,660 (2) 

 30 11.0 37.7 47.1 4.2 - 11,640 (3) 

**Sources: (1) Kiersch (1955), p. 52, 53. (2) Cooper and others (1947), p. 32–34. (3) Williams (1951), p. 88, 188. 

Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H25
Table 4. Seven analyses of channel samples or grab samples from the upper Cretaceous Wepo Formation, Black Mesa 

Coal Field, Navajo County, Arizona. 

[Modified from Moore and Swanson, 1977, their table 3. Bed designations (red, green, blue) are as used by Peabody Group Coal 

Company. Moisture content and volatile matter reported on an as-received basis] 

Sample Seam Moisture 

(%) 

Volatile 

matter (%) 

Fixed carbon 

(%) 

Ash 

(%) 

Sulfur 

(%) 

Btu/lb 

D176225 Green 10.6 38.3 42.0 9.1 0.5 10,770 

D176226 Green 9.3 40.8 41.4 8.5 0.5 11,100 

D176227 Blue 9.3 40.1 45.3 5.3 0.4 11,560 

D176231 Red 10.2 41.2 43.9 4.7 0.3 11,470 

D176235 Red 8.6 40.0 42.3 9.1 0.5 10,910 

D176239 Composite 10.9 37.5 44.5 7.1 0.4 10,930 

D176241 Composite 21.9 31.4 39.6 7.1 0.3 9,490 

Table 5. Representative coal analyses from Peabody Group Coal Company. 

[R. Willson, 1994, oral commun., Peabody Group Coal Company] 

 Dry basis As-received 

 Ash Sulfur Btu/lb Ash Sulfur Btu/lb 

 (%) (%) (%) (%) 

 6.93 0.56 12,688 6.10 0.49 11,166 

 8.56 0.75 12,477 7.53 0.66 10,980 

 8.87 0.47 1,2367 7.81 0.42 10,883 

 9.78 0.52 12,014 8.61 0.46 10,572 

Avg. 8.54 0.58 12,387 7.51 0.51 10,900 

Table 6. Summary of Wepo coal quality data from CBM Project surface samples. 

 Moisture Ash Btu/lb Sulfur Rank 

 (%) (%) (%) 

Samples as-received from CBM Photo Point measured section— 

northeastern part of mesa 

 8.24 13.48 9,723 0.95 subbituminous 

 9.59 21.03 8299 0.87 subbituminous 

Avg. 8.91 17.25 9011 0.91 

Samples as-received from CBM Cow Springs measured section— 

northwestern part of mesa 

 13.28 8.61 9709 0.52 subbituminous 

 15.23 9.31 9794 0.60 subbituminous 

 8.84 21.94 8502 0.65 subbituminous 

 15.34 13.52 9357 0.46 subbituminous 

Avg. 13.17 13.35 9341 0.56 

Samples as-received from Blue Gap area— 

east-central part of mesa 

 14.80 12.08 9665 0.62 subbituminous 

 5.21 11.05 10838 1.75 bituminous 

Avg. 10.00 11.56 10252 1.19 

H26 Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah
Coal Resources in Black Mesa Basin

In 1909, the U.S. Geological Survey gathered data on 

the Black Mesa coal fi eld and estimated that it contained 8 

billion short tons of recoverable coal (Gregory and Campbell, 

1911). Kiersch (1955) estimated the “minable” reserves of 

Black Mesa as 2 billion short tons. Averitt (1969) estimated 

4 billion short tons for the inferred coal resources of Black 

Mesa. The Arizona Bureau of Mines estimated that as much 

as 21 billion short tons of coal lay beneath Black Mesa, including the Wepo coal resource as estimated at 5.65 billion short 

tons, the Toreva at 6.0 billion short tons, and the Dakota at 

9.6 billion short tons (Peirce and Wilt, 1970). Because of the 

limitations of our data on coal thickness in the Dakota and 

Toreva Formations, this study attempted to reevaluate only 

the Wepo coal resources. However, improved coal volume 

estimates may be calculated from the coal isopach maps of the 

Toreva and Dakota Formations.

Coal Resources in the Wepo Formation

Haven (1997) calculated the original coal resources in the 

Wepo Formation of Black Mesa at 4 billion short tons, which 

is considerably less than the 5.6 billion short tons estimated 

by Peirce and Wilt (1970). The Wepo coal thickness was 

determined by adding all net-coal intervals in measured sections and subsurface logs, constructing an isopach map, and 

determining the in-place volume (Haven, 1997). A volume of 

2,200,000 acre-ft was calculated from the isopach map using a 

program called GeoView. To determine the resources, density 

needed to be calculated. Haven (1997) calculated a density of 

1.34 g/cm3 based on an average ash content of 7 percent for 

the coal (table 4). The weight of the coal was determined to 

be 1,818 short tons per acre-ft. As of 1996, about 265,000,000 

short tons of coal have been produced from Black Mesa resulting in a remaining resource of about 3.7 billion short tons 

of coal. The maximum recovery depths for the Wepo coal 

range from 150 ft to 250 ft on the Peabody Group lease (U.S. 

Offi ce of Surface Mining, 1990). No overburden maps were 

constructed because of limitations of the data, but since most 

of the upper Wepo has been removed by erosion, much of the 

coal is near the surface.

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Williams, G.A., 1951, The coal deposits and Cretaceous stratigraphy of 

the western part of Black Mesa, Arizona: Tucson, Ariz., University 

of Arizona Ph.D. dissertation, 274 p.

Wilson, E.D., and Roseveare, G.H., 1949, Arizona nonmetallics, a summary of past production and present operations: Arizona Bureau of 

Mines Bulletin 155, 60 p. 

Young, R. G., 1960, Dakota Group of Colorado Plateau: American Association of Petroleum Geologists Bulletin, v. 44, p. 156–194.

National Coal

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Summary of Cretaceous Stratigraphy and Coal Distribution, Black Mesa Basin, Arizona H29