Tertiary Coals in South Texas: Anomalous

 Cannel-Like Coals of Webb County (Claiborne Group, Eocene)

 and Lignites of Atascosa County (Jackson Group, Eocene) -

 Geologic Setting, Character, Source-rock and 

Coal-bed Methane Potential

 The 1999 AAPG Annual Convention

 Energy Minerals Division 

 Field Trip # 15

Field Trip Guidebook

April 14-15, 1999

FIELD TRIP GUIDEBOOK

U.S. Geological Survey Open-File Report 99-301

Edited by P.D. Warwick, C.E. Aubourg, and J.C. Willett

Field Trip Leaders: 

Peter D. Warwick, U.S. Geological Survey, Reston, VA

Robert W. Hook, Consultant, Austin, TX

John R. SanFilipo, U.S. Geological Survey, Reston, VA 

Tertiary Coals in South Texas: Anomalous

Cannel-Like Coals of Webb County (Claiborne Group, Eocene)

and Lignites of Atascosa County (Jackson Group, Eocene) --

Geologic Setting, Character, Source-rock and 

Coal-bed Methane Potential

Field Trip Guidebook

The 1999 AAPG Annual Convention

Energy Minerals Division 

Field Trip # 15

April 14-15, 1999

Leaders: Peter D. Warwick, U.S. Geological Survey, Reston, VA, 

 Robert W. Hook, Consultant, Austin, TX, 

 and John R. SanFilipo, U.S. Geological Survey, Reston, VA

Edited by P.D. Warwick, C.E. Aubourg, and J.C. Willett

U.S. Department of Interior

U.S. Geological Survey Open-File Report 99-301

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey

editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive

purposes only and does not imply endorsement by the U.S.G.S.

1999
Copies of this Field Guide are available from: 

USGS Information Services 

Box 25286 Denver Federal Center 

Denver CO 80225-0046 

Tel: 303-202-4700; 

Fax 303-202-4188

or

http://energy.er.usgs.gov
iii

CONTENTS

Introduction Tertiary coals in South Texas: Anomalous cannel-like coals of Webb 

County (Claiborne Group, Eocene) and lignites of Atascosa County 

(Jackson Group, Eocene)

 Peter D. Warwick, Robert W. Hook, John R. SanFilipo........................................ 1

Itinerary.............................................................................................. 2

Other Information.............................................................................. 4

Chapter 1 Regional geologic setting of South Texas and the Santo Tomas coal 

deposits

Thomas E. Ewing................................................................................................. 5

Introduction.......................................................................................... 5

General Overview of the Field Trip Route............................................. 5

 Structural and Stratigraphic Setting of the Santo Tomas District............. 8

 References Cited...................................................................................11

Chapter 2 The San Pedro and Santo Tomas Coal Beds (Claiborne Group, Eocene) 

of Webb County, Texas

 Robert W. Hook and Peter D. Warwick............................................................... 13

Introduction...................................................................................... 13

Geologic Setting and Stratigraphy..................................................... 13

Mining History.................................................................................. 16

Depositional Setting.......................................................................... 17

Coal Characteristics........................................................................... 18

Summary..................................................................... ..................... 20

Acknowledgments............................................... ............................. 21

References Cited.................................................. ............................. 21

Chapter 3 The San Miguel lignite deposit, Jackson Group (Eocene), South Texas

Peter D. Warwick, Sharon S. Crowley, Leslie F. Ruppert, and James

Pontolillo........................................................................................................... 23

Introduction....................................................................................... 23

Geologic Setting................................................................................. 23

Lignite Resources and Mining............................................................. 24

Character of the San Miguel Lignite.................................................... 26

Megascopic characteristics....................................................... 26

Proximate analyses................................................................... 28

Major, Minor, and Trace element data...................................... 29

Mineralogy of San Miguel Partings........................................... 31

 Palynology and Organic Petrology............................................ 31
iv

 Relationship Between Ash yield, and Petrographic and 

Geochemical Characteristics of the San Miguel 

Lignite Deposit................................................................................... 33

Summary............................................................................................ 34

Acknowledgments.............................................................................. 34

References Cited.................................................................................. 34

Chapter 4 Some speculations on coal rank anomalies of the South Texas 

Gulf Province and adjacent areas of Mexico and their impact on 

coal bed methane and source rock potential

 John R. SanFilipo............................................................................................... 37

Introduction....................................................... ............................... 37

Cretaceous bituminous coals............................................................... 37

Tertiary bituminous and related coals................................................... 37

 Extent of the coal rank anomaly and its possible impact on 

hydrocarbon generation...................................................................... 42

Conclusions........................................................................................ 45

Acknowledgments.............................................................................. 46

References Cited................................................................................. 46

FIGURES

Introduction

Figure 1. Map showing outcrop of coal-bearing formations in the U.S. 

Gulf of Mexico coastal area.................................................................... 2

Figure 2. Stratigraphy of coal-bearing intervals in the U.S. Gulf of 

Mexico Coastal Plain.............................................................................. 3

Chapter 1

Figure 1. Route of the 1999 EMD field trip, with major tectonic features 

and outcrop belts shown......................................................................... 6

Figure 2. General northwest-southeast stratigraphic section of Mesozoic 

 and Paleogene strata along the Rio Grande, from Eagle Pass 

to Freer.................................................................................................. 7

Figure 3. Subsurface structure, Top Middle Wilcox marker, Santo 

Tomas area............................................................................................ 9

Figure 4. Electric log, Hughes & Hughes #1 Palafox Exploration “B”,

 showing the correlation from surface mines to subsurface 

stratigraphy.......................................................................................... 10
v

Chapter 2

Figure 1. Location of study area, undergroun mine entries and prospects............. 14

Figure 2. Representative corehole records from Vaughn (1900) that

 include both coal zones........................................................................ 15

Figure 3. Bed sections and sample intervals of the San Pedro, Santo 

Tomas, and Santo Tomas rider coal beds.............................................. 16

Figure 4. Selected proximate and ultimate data................................................... 19

Figure 5. Schematic block diagram of depositional environments for

 the San Pedro and Santo Tomas coal zones in the middle 

 part of the Claiborne Group................................................................. 20

Chapter 3

Figure 1. Index map of the San Miguel Mine area showing towns, roads, 

and sample sites..................................................................................... 24

Figure 2. Generalized stratigraphy and stratigraphic sections of the 

coal-bearing interval at the san Miguel Mine........................................... 25

Figure 3. Representative photographs and photomicrographs of outcrops 

 and samples from the San Miguel lignite interval.................................... 26

Figure 4. Bar graphs shoing concentrations of ash yield, total sulfur, and 

caloric values......................................................................................... 27

Figure 5. Bar graphs showing the distribution of trace element concentrations for

 twelve potentially environmentally 

 sensitive elements identified by the 1990 U.S. Clean Air Act 

amendments...................................................................................... 28-29

Figure 6. Bar graphs showing the distribution of trace element concentrations for

those elements inferred to be derived 

from volcanic ash.................................................................................... 30

Figure 7. Bar graphs of the major petrographic constituents for the channel sample

sets......................................................................................................... 32

Chapter 4

Figure 1. Generalized columnar section for South Texas coal areas and 

adjacent areas of Mexico......................................................................... 38

Figure 2. Regional geology and location of bituminous coal deposits of 

South Texas and adjacent areas of Mexico.............................................. 39

Figure 3. Standard analysis and elemental ratios for selected samples from

 the Santo Tomas coal field and Sandow Mine......................................... 41

Figure 4. Selected rank parameters for selected samples from the Santo 

Tomas coal field and Sandow Mine......................................................... 43

Figure 5. Van Krevelen-type diagram for various coal types of South Texas 

and vicinity............................................................................................. 45
vi
U.S. Geological Survey Open-File Report 99-301

1

Introduction

Tertiary coals in South Texas: Anomalous

cannel-like coals of Webb County (Claiborne Group, Eocene)

and lignites of Atascosa County (Jackson Group, Eocene)

(AAPG - EMD Field Trip, April 14-15, 1999)

By Peter D. Warwick, Robert W. Hook*, and John R. SanFilipo

U.S. Geological Survey, 956 National Center, Reston, VA 20192

*Consultant, 1301 Constant Springs Drive, Austin, TX 78746

The coal-bearing Gulf of Mexico

Coastal Plain of North America contains a

variety of depositional settings and coal types.

The coal-bearing region extends westward

from Alabama and Mississippi, across

Louisiana to the northern part of the

Mississippi Embayment, and then southward

to eastern Arkansas, Texas and northern

Mexico (fig. 1). Most of the coal currently

mined in Texas is lignite from the upper part of

the Wilcox Group (Paleocene-Eocene) and, in

Louisiana, lignite is mined from the lower part

of the Wilcox (fig. 2). Gulf Coast coal is used

primarily as fuel for mine-mouth electric

plants. On this field trip we will visit the only

two non-Wilcox coal mining intervals in the

Texas-Louisiana Coastal Plain; these include

the San Pedro - Santo Tomas bituminous

cannel-like coal zone of the Eocene Claiborne

Group, and the San Miguel lignite coal zone of

the Eocene Jackson Group (fig. 2). Other

coal-mining areas in northern Mexico are

currently producing bituminous coal from the

Cretaceous Olmos Formation of the Navaro

Group (fig. 2).

The Claiborne-Jackson coal-bearing

intervals of south Texas are very different from

the Wilcox coal-bearing intervals found in

central and northeastern Texas and

northwestern Louisiana. The cannel-like coal

deposits of south Texas are unique and are one

of the few mined cannel-like coal deposits in

the world. This field guidebook contains

papers that focus on the regional geologic

setting of south Texas (Chapter 1), and papers

on the Santo Tomas and San Pedro coal

deposits and San Miguel lignite deposits

(Chapters 2 and 3). Finally, there is an

overview paper on the anomalous coal-rank

changes of south Texas, and a discussion of

the source-rock and coal-bed methane

potential for the south Texas coal fields

(Chapter 4).

The field trip will visit two active mines

in South Texas and will provide the

participants an opportunity to observe the

mining operations and coal geology of the

surface mines. The older Claiborne Group

coals that occur near the U.S.-Mexico border

(fig. 1), are long-known for their cannel-like

character and will be seen in workings of the

Farco Mining Company. These unusual

nonbanded coals have high calorific values

(3800 to 6000 kcal/kg or 6800 to 10,800 btu,

dry basis) and substantial oil and gas yields

upon distillation. Recent petrographic and

geochemical investigations have shown that

these coals are bituminous in rank, hydrogen
U.S. Geological Survey Open-File Report 99-301

2

Mining areas

Jackson Group

Claiborne Group

Wilcox Group

Naheola Formation

Olmos Formation

Geologic Units

0 300 600 miles

Paleocene

Eocene

U. Cretaceous

USA

Mexico

Figure 1. Map showing outcrop of coal-bearing formations in the U.S. Gulf of Mexico coastal area.

Active, recent and proposed mining areas are shown in red.

rich, contain abundant green algae remains,

and are potential sources of hydrocarbon. The

field trip leaders will discuss the depositional

setting, petrology, geochemistry, and regional

geology of these coal deposits.

The lignites of the San Miguel deposit

(Jackson Group, Eocene), differ greatly from

the coals of the border area. Four principle

lignite beds comprise the San Miguel deposit.

The beds are separated by reworked volcanic

ash partings (generally less than 0.25 m or

0.75 ft thick). Drill records and in-mine

studies indicate that these lignites were

deposited in environments that were partially

marine influenced. The geochemistry and

depositional setting for these lignites, and

regional South Texas/Northeastern Mexico

coal geology will be discussed. The field trip

will spend the first night in Laredo, a U.S.-

Mexico border town, and will visit the two

surface mines the next day. 

ITINERARY 

This is a tentative schedule based upon

ideal weather and traffic conditions. 

Wednesday, 14 April

4:30 PM - depart H.B. Gonzalez Convention
U.S. Geological Survey Open-File Report 99-301

3

Eocene Paleocene Upper Cret.

Claiborne Group

Navarro Group

Jackson Group

Wilcox Group

Midway Group

Figure 2. Simplified stratigraphy of coalbearing intervals in the U.S. Gulf of Mexico

Coastal Plain. Arrows indicate stratigraphic

position of major coal-bearing intervals.

Center in San Antonio for Laredo, a nonstop

drive of approximately 2.5 hours.

7:00 PM - check-in at La Posada Hotel,

downtown Laredo.

7:30 PM - short (less than 30 min.) meeting at

La Posada. We will provide an overview of

South Texas coal geology and give

suggestions for dinner.

Thursday, 15 April

7:00 AM - depart La Posada for Palafox, an

approximately 30 minute drive. 

8:00 AM - arrive at Farco Mining's Palafox

Mine Office. David Wadsack and Gustav

Holm, hosts. Safety training in office. 

Stop 1: Trevino Mine. Observe San Pedro

Coal (Bigford Formation) and overlying tidal

deposits. Discuss unique mining methods.

Stop 2: Palafox Mine. Observe San Pedro

Coal, overlying sandstone and mudstone

deposits, and thin coal seam.

Stop 3: Coal yard. Discuss coal preparation

and utilization. 

10:30 AM - return to Farco office for

convenience stop; depart for San Miguel Mine

at 10:45 AM.

Optional Stop A: FM 1472 roadcut east of

Santo Tomas. Observe coastal-barrier facies

in El Pico Clay (10 minutes).

12:15 PM - lunch stop (30 minutes), Picnic

Area south of Cotulla at Mile Marker 59 on

I-35 North. This is a no-toilet rest area.

1:00 PM - convenience and gas stop (15

minutes) at Mobil station in Cotulla at junction

of Business I-35 and Texas Route 97.

2:15 PM - arrive North American Coal's San

Miguel Mine Office. Roger Fish, Engineering

Manager, host. Safety certification and mine

overview at office.

Stop 4: E area final pit. Observe completed

highwall exposure of Jackson Group from

spoil bank. Discuss lithofacies overlying coal,

ash disposal, and reclamation methods.
U.S. Geological Survey Open-File Report 99-301

4

Stop 5: E area, active pit. Observe coal

seams and splits, discuss unique mining

methods.

Optional Stop B: B area. 

4:30 PM - depart San Miguel Mine for San

Antonio.

OTHER INFORMATION

Both Farco Mining and North American Coal

require hard hats, safety glasses, and steel-toe

boots. 

Exercise extreme caution at all times when on

mine property. Do not approach highwall

exposures without the consent of company

personnel.

Coal sampling and photography are permitted

by both operators. Later publication of

coal-quality data or mine photographs requires

company consent and acknowledgment.

We will stop at a mandatory U.S. Border

Inspection station north of Laredo on

Thursday morning. No illegal substances or

firearms should be on your person or in your

belongings. Non-U.S. citizens should have the

appropriate travel papers available for

presentation.
U.S. Geological Survey Open-File Report 99-301

5

CHAPTER 1

Regional geologic setting of South Texas and the Santo Thomas coal deposits

By Thomas E. Ewing

Venus Exploration, Inc., 1250 Loop 410, Suite 1000, San Antonio, TX 78209

INTRODUCTION

This paper supplements the work that

the U.S. Geological Survey has recently

conducted in South Texas on the coal geology

of the Eocene Claiborne Group of the Laredo

area (Warwick and Hook, 1995) and the

Jackson Group of the San Miguel lignite mine

area (Warwick and others, 1996). I will first

present a general overview of the trip route in

relationship to the major structural and

stratigraphic features of South Texas. Then I

will present subsurface well log data to show

the correlation of the coal zones into the

subsurface, and place them in a regional

context.

GENERAL OVERVIEW OF THE 

FIELD TRIP ROUTE

When we leave San Antonio, we will

proceed southwest along Interstate 35 towards

the city of Laredo. San Antonio is an oasis

city, built around springs and deep wells

drawing water from the Lower Cretaceous

Edwards Aquifer along the (Miocene-age)

Balcones fault zone. The city is built on rocks

ranging in age from Albian (Edwards

Limestone) to Paleocene (lower 

Wilcox Group), with younger rocks generally

to the southeast. 

Tectonically, the San Antonio area lies

on the southwest flank of the San Marcos

Arch (fig. 1), a broad southeast-trending area

of lesser subsidence than surrounding areas

during Mesozoic and Cenozoic time. During

the Early Cretaceous, the area was repeatedly

occupied by extensive carbonate platforms (the

Sligo Limestone of Aptian age and the

Edwards Group carbonates of Albian age; fig.

2). As regional sedimentation shifted from

carbonates to siliciclastic material in the Late

Cretaceous (Campanian - Maastrichtian), the

San Antonio area lay east of the major San

Miguel and Olmos wave-dominated deltas of

South Texas, and mainly marine shales and

thin sandstones were deposited. In the

Paleogene, Wilcox, Carrizo and younger

sandstones were deposited in the San Antonio

area, which lay in the transition between the

largely strike-oriented strandplain, wavedominated delta and bar - lagoon systems of

South Texas and the fluvial-deltaic deposits of

East Texas.

As we drive southwest toward Laredo,

we cross the Rio Grande Embayment, a broad

synclinal area between the San Marcos Arch of

Central Texas and the Laramide fold systems

of Mexico (fig. 1). The present Rio Grande

flows along the southwest side of this

embayment. The road slowly climbs the

stratigraphic section, reaching the late middle

Eocene ("Laredo Formation," Sparta and

Cook Mountain equivalent; fig. 2) at Laredo.

South of Cotulla (fig. 1), the road crosses the

buried Stuart City Reef margin of the Edwards

Group carbonate platform. The reef margin is

gas-productive, and also controls faulting and
U.S. Geological Survey Open-File Report 99-301

6

Figure 1. Route of the 1999 EMD field trip, with major tectonic features and outcrop belts

shown. Open boxes represent locations of Upper Cretaceous coal deposits; “X” represents

igneous rocks; Kig, Tig, QTig = Cretaceous, Tertiary, and Quaternary igneous rocks.

shallower marine sandstone development in

the Olmos Formation. At Laredo, the position

of the buried Sligo reefal shelf margin is

reached. This reef contains a major gas field

just across the river in the western part of

Nuevo Laredo (Totonaca field) at depths of

some 13,000 ft. 

Jose de Escandon founded Laredo, the

chief port of entry from Mexico into Texas, in

the 1750s as the northernmost city of the new

province of Nuevo Santander (the lower Rio

Grande valley). The early trade routes from

Monclova to San Antonio passed north of the

Laredo area, but the location of Laredo on the

shortest route from Monterrey to San Antonio,

coupled with the main rail links to both cities

and to the port of Corpus Christi, ensured

Laredo's economic growth. 

Laredo, and the Santo Tomas district

to its northwest, lie on the southwest flank of

the Rio Grande Embayment. Dips are

generally one degree or higher to the east in

outcrops near the river. Southwest into

Mexico, the dips continue until the base of the
U.S. Geological Survey Open-File Report 99-301

7

Figure 2. General northwest-southeast stratigraphic section of Mesozoic and Paleogene strata

along the Rio Grande, from Eagle Pass to Freer (fig. 1). The stratigraphic location of significant

coal and lignite deposits are shown.
U.S. Geological Survey Open-File Report 99-301

8

Tertiary outcrops about 12.5 mi into the

interior. Several anticlinal axes, collectively

referred to as the "Salado arch" or the "BurroPeyotes arch", trend southeastward, with

intermittent exposures of Austin limestone on

anticlines. The east to northeast dip off the

flanks of the arch towards the Rio Grande is

easily visible on satellite photography.

Secondary folds are seen in some areas.

After visiting the Santo Tomas area,

we will travel back to the north to Cotulla,

turning east to Tilden, then north to the San

Miguel lignite mine (fig. 1). The trip east from

Cotulla to Tilden proceeds over younger rocks

than any other part of the trip. Sandy Yegua

Formation deposits are overlain by the lignitebearing Jackson Group. The hills east of

Tilden are underlain by the largely

volcaniclastic Catahoula (or Gueydan)

Formation of Oligocene age. Zeolites have

been mined from the basal Jackson strata just

west of Tilden, and used as kitty litter. North

of Tilden, we cross San Miguel Creek (with

natural lignite exposures in its banks) and

proceed to the San Miguel mine. After this

classic Gulf Coast lignite deposit is viewed, we

will travel back to San Antonio across Eocene

and Upper Cretaceous rocks.

STRUCTURAL AND STRATIGRAPHIC

SETTING OF THE SANTO 

TOMAS DISTRICT

The coal beds of the Santo Tomas

district lie within the Bigford Formation - El

Pico Clay succession of the Rio Grande

Embayment. These two formations correlate

with the Queen City cycle of the Claiborne

Group, and are early Middle Eocene in age

(fig. 2). In South Texas, the Queen City

sandstones were deposited in a mixed wavedominated deltaic and barrier bar environment,

with a strike-oriented sandstone maximum in

eastern Webb County about 30 miles east of

Santo Tomas (Guevara and Garcia, 1972).

Northwest of the sand maximum, the upper

part of the cycle contains a low percentage of

sandstone and much lagoonal shale; this has

been called the El Pico Clay by Eargle (1968).

The lower part of the cycle contains high

percentages of sandstone (Bigford Formation

as redefined by Eargle from Trowbridge,

1923), and rests directly on the Carrizo

Formation and upper Wilcox sandstones with

very little break. The downdip marine Reklaw

formation passes westward into the Bigford

shoreline sandstones.

The contact between Bigford

Formation and El Pico Clay is gradational.

Sandstones do occur in the El Pico "Clay", and

may become more abundant northward away

from the Rio Grande. The transitional beds

between Bigford and El Pico are intimately

associated with the San Pedro and Santo

Tomas coal zones, as we will see.

To better understand the location of

the Santo Tomas coal deposits within the

stratigraphic section, a pilot study was

undertaken of well logs in the area around and

east of the mines. The area has been

extensively drilled for gas reserves in the

Wilcox Group sandstones (Palafox field) and

in the Olmos Formation (Las Tiendas, Booth

Ranch, and La Cruz fields); gas has also been

found in reefal Edwards carbonates in Galan

field, just south of the Rachal Mine (fig. 3).

Many of these well logs have been released,

and most were logged to within a few hundred

feet of the surface. A regional shale which

correlates to the Middle Wilcox can be

identified on all logs, and forms a good

structural datum. The structure map (fig. 3)

shows structural dip of 1.0-1.4 degrees to the
U.S. Geological Survey Open-File Report 99-301

9

Figure 3. Subsurface structure, Top Middle Wilcox marker, Santo Tomas area. The contoured data

on the Santo Tomas and San Pedro coal beds are from Warwick and Hook (1995). Gas field names

are also shown.

east or east-northeast on the Middle Wilcox

marker. This corresponds well to the 0.9-1.2

degrees of dip seen in the Trevino and Rachal

mine areas (data from Warwick and Hook,

1995). From comparison of the Middle

Wilcox data and the mine elevations, the Santo

Tomas bed at the Rachal Mine lies 1900±20 ft

above the Middle Wilcox marker. Less

certainly, the San Pedro seam at the Trevino

lease lies about 1600-1650 ft above the

marker, or 250-300 ft below the Santo Tomas.

Elsewhere, however, the San Pedro is only 90

ft below the Santo Tomas. This suggests that

the San Pedro beds are no t
U.S. Geological Survey Open-File Report 99-301

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Figure 4. Electric log, Hughes & Hughes #1 Palafox Exploration "B", showing the correlation from

surface mines to subsurface stratigraphy. Location of well shown on figure 3. Vertical log scale in

ft.
U.S. Geological Survey Open-File Report 99-301

11

wholly correlative, although there may be

other solutions.

There is slight thickening of some

stratigraphic units to the northeast, perhaps

indicating structural growth on the Salado

folds during Eocene time. However, the total

amount of growth is not more than 100-150 ft.

The coal beds in the Rachal area can be

confidently projected into the Copano #A-1

Palafox Exploration well, an Edwards test

drilled in 1962 about 3 miles SSE of the

Rachal mine (fig. 3). The correlation indicates

that the coal beds correlate to a thin-bedded

interval lying above a 60-ft thick sandstone

and below a 100-ft thick shale unit. This shale

("Pink" Marker) can be observed on logs

throughout the area mapped, and continues

downdip at least to the Laredo airport (fig. 3).

The sequence can be correlated to the east into

the Hughes and Hughes #1-B Palafox

Exploration well, where the overlying clay-rich

sequence is also logged (fig. 4). The Pink

Marker and the overlying sandstone can also

be correlated to lithologic logs presented by

Lonsdale and Day (1937) and Warwick and

Hook (1995). Although the individual coal

beds cannot be definitely located on the well

logs (the coal may have some SP response?),

the interval is reliably picked, and can be

carried throughout the area. Downdip past the

old workings towards Laredo, the interval

becomes sand-rich and loses its thin-bedded,

resistive character.

Due to the density of drilling in the Las

Tiendas area of northwest Webb County, it

should be possible, with detailed study of the

subsurface logs, to more closely define the

depositional geometry of the algal coals of the

Santo Tomas district and estimate their

resource potential.

As an added point of interest in the

area, Kyle (1994, p. 41-46) has described a

significant surface occurrence of barite in the

Pinto Creek drainage, just northeast of the

Rachal mine. The barite occurs in clays that

lie 80-250 ft above the Santo Tomas seam.

REFERENCES CITED

Eargle, D.H., 1968, Nomenclature of

formations of Claiborne Group,

Middle Eocene coastal plain of Texas:

U.S. Geological Survey Bulletin 1251-

D, 25 p. 

Guevara, E.H., and Garcia, R., 1972,

Depositional systems and oil - gas

reservoirs in the Queen City Formation

(Eocene), Texas: BEG Geological

Circular 72-4, 22 p.

Kyle, J.R., 1994, The barite industry and

resources of Texas: Bureau of

Economic Geology, The University of

Texas at Austin, Mineral Resources

Circular 85, 86 p.

Lonsdale, J.T., and Day, J.R., 1937, Geology

and ground-water resources of Webb

County, Texas: U.S. Geological

Survey Water-Supply Paper 778, p.1-

104.

Trowbridge, A.C., 1923, A geologic

reconnaissance in the Gulf Coastal

Plain of Texas, near the Rio Grande:

U.S. Geological Survey Professional

Paper 131-D, p. 85-107.
U.S. Geological Survey Open-File Report 99-301

12

Warwick, P.D., and Hook, R.W., 1995,

Petrography, geochemistry and

depositional setting of the San Pedro

and Santo Tomas coal zones:

anomalous algae-rich coals in the

middle part of the Claiborne Group

(Eocene) of Webb County, Texas:

International Journal of Coal Geology,

v.28, p. 303-342.

Warwick, P.D., Crowley, S.S., Ruppert, L.F.,

and Pontilillo, James, 1996,

Petrography and geochemistry of the

San Miguel lignite, Jackson Group

(Eocene), South Texas: Organic

Geochemistry, v. 24, no. 2 p. 197-217.
U.S. Geological Survey Open-File Report 99-301

13

CHAPTER 2

The San Pedro and Santo Tomas Coal Beds 

(Claiborne Group, Eocene) of Webb County, Texas

By Robert W. Hook and Peter D. Warwick*

Consultant, 1301 Constant Springs Drive, Austin, TX 78746

*U.S. Geological Survey, MS 956, 12201 Sunrise Valley Drive, Reston, VA 20192

INTRODUCTION

The San Pedro and Santo Tomas coal

zones of Webb County, Texas, are unique

within the realm of Gulf Coast coal geology.

Coal beds within these zones are distinguished

by calorific yields and vitrinite-reflectance

values that indicate a rank as high as

high-volatile bituminous C, which is

considerably greater than other Tertiary coals

within the Gulf Coastal Province. In the past,

the San Pedro and Santo Tomas were

identified as cannel coal because of their

blocky, nonbanded character, and conchoidal

fracture (Vaughan, 1900; Ashley, 1919;

Lonsdale and Day, 1937). Unlike true cannel

coal known from the Upper Carboniferous of

the Appalachians and Europe, the Webb

County deposits are not particularly rich in

spores or cuticular material and are not

restricted to local channel fills. Rather, the

San Pedro and Santo Tomas coals contain

abundant green-algae fructifications and occur

as multiple tabular coal beds within zones that

extend for tens of miles along strike (Warwick

and Hook, 1995).

Recent work has addressed the

depositional origin, petrography, and

geochemistry of the San Pedro and Santo

Tomas coal beds, but several basic questions

remain. The full extent of these deposits has

not been determined. In adjacent parts of

Mexico, numerous cannel-like beds occur

within the Claiborne Group (SPP, 1989), but

no comparable deposits are known elsewhere

in Texas outside of Webb County. The

question of rank also cannot be resolved at this

time. Previously, we attributed the high rank

of the San Pedro and Santo Tomas to

geothermal influences (Warwick and Hook,

1995). Although the suggestion of geothermal

influence is consistent with data that indicate

an elevated rank of Wilcox coal deposits in

adjacent counties, additional study is required

to confirm the origin and delineate the extent

of such alteration. Likewise, the significance

of abundant green-algae fructifications in these

coals, identified only from petrographic study

of polished blocks, cannot be judged fully at

this time because comparable polished block

petrographic data do not exist for the great

majority of other Tertiary coals. 

GEOLOGIC SETTING AND 

STRATIGRAPHY 

The Farco mine complex of Webb

County lies within the outcrop of the

southwestern flank of the Rio Grande

Embayment, which extends southwesterly

through Coahuila, Nuevo León, and
U.S. Geological Survey Open-File Report 99-301

14

Cannel Coal Co. no. 2

Santo Tomas shaft

Dolores shaft

Darwin shaft

Minera

 drift mines

Colombia

Palafox mine

Cannel Coal

 Co. no. 6

Villa 

 Hildago Rio Grande

Coahuila

Nuevo Leon

Tamaulipas

Texas

FM1472

0 5 km

SP Channel 94

SP Channel 93

ST Channel 94

27 37'30' o

99 37'30" o

100 o

28 o

Texas

Study

 area

Trevino Lease Laredo

Old mine prospect

Webb County

Outline of

Rio Grande

Embayment

Palafox

'

Rio Grande

Rachal Mine

Approximate

outcrop of San 

Pedro and 

Santo Tomas 

coal zones

Mexico

Figure 1. Location of study area, underground mine entries and prospects, Cannel Coal Company

coreholes 2 and 6, surface mines, and sample sites. Inset shows location of study area, Webb

County, and Rio Grande Embayment in South Texas. From Warwick and Hook (1995).

Tamaulipas in Mexico (fig. 1). Within the Rio

Grande Embayment, the lower to middle part

of the Claiborne Group consists of marine

mudstones (Reklaw Formation) in the east and

northeast, and sandstones and mudstones

(Bigford Formation) in the south and

southwest (fig. 2). The marine mudstones

coarsen upward into the fluvialdeltaic Queen

City Sand, which prograded gulfward across

eastern and central Texas. To the west and

southwest, the interval overlying the Bigford

Formation becomes less sandy and claystones

of the El Pico Clay Formation predominate.

The San Pedro and Santo Tomas coal zones

occur within the fining-upward transition from

sandstone-dominated rocks to mudstone-rich

deposits. The coal beds dip approximately 2E

to the northeast, toward the synclinal axis of

the embayment. 

The San Pedro coal zone ranges up to

10 m (33 ft) in thickness and contains as many

as five organic-rich beds that vary from

carbonaceous mudstone to nonbanded coal in

composition. The thinner Santo Tomas coal

zone occurs approximately 25 to 35 m (82 to
U.S. Geological Survey Open-File Report 99-301

15

Santo Tomas

coal zone

Fossil bed

San Pedro 

coal zone

Sandstone

Claystone and 

 mudstone

Cannel Coal Company

 no. 2 corehole

Cannel Coal Company

 no. 6 corehole

San Pedro 

coal zone

Carbonaceous

 shale and lignite

Unnamed

coal bed

15 m

Limestone

Santo Tomas 

coal zone

Unnamed

coal bed

Paleocene Eocene

Jackson Group

Wilcox Group

Midway Group

Claiborne Group

Carrizo Sand

El Pico 

Clay

Bigford 

Fm.

Queen City Sand

Weches

Formation

Sparta Sand

Cook Mountain Fm.

Laredo 

Fm.

Reklaw

Formation

Eastern and 

central Texas

South

Texas

Yegua Formation

29 km

Figure 2. Representative corehole records from Vaughan (1900) that include both coal zones.

Approximate locations shown on figure 1. Regional stratigraphy after Eargle (1968) and Guevara and

Garcia (1972). From Warwick and Hook (1995).
U.S. Geological Survey Open-File Report 99-301

16

50 cm

Siltstone, rooted, becoming

 sandy upwards, 193 cm

Coal, nonbanded, less 

 blocky than below,

 36 cm (ST94-2)

Coal, nonbanded, cleated,

 33 cm (ST94-1)

Mudstone, rooted,

 <4.8 m

Coal, nonbanded, blocky,

 20 cm (STR94-1)

Sandstone, flat-

 bedded, 100 cm+

Mudstone

Claystone, 

 burrowed, 10 cm

Coal, nonbanded, 

 cleated, 42 cm 

 (SP93-A1)

Coal, massive, 7 cm

 (SP93-A2)

Coal, impure, 7 cm

 (SP93-A3)

Mudstone, rooted

Sandstone, scoured

 base, 100 cm+

Coal, nonbanded, 

 cleated, 34 cm

 (SP94-2)

Coal, impure, 

 massive, 26 cm 

 (SP94-1)

Claystone, rooted

Rachal Mine 

Palafox Mine

Coal, impure, <90 cm

 (core data, bench not 

 exposed in mine)

 

Santo Tomas coal Rider

coal

San Pedro coal

San Pedro coal

Lat. 27 56'32"N

Long. 99 o 51'17"W

Lat. 27 o 50'02"N

Long. 99 o 51'26"W

Lat. 27o

 49'01"N

Long. 99 o 50'19"W

o

Figure 3. Bed sections and sample intervals of the San Pedro, Santo Tomas, and Santo Tomas rider

coal beds. Sample numbers are in parentheses. Approximate locations are shown in Figure 1. From

Warwick and Hook (1995).

115 ft) above the San Pedro zone, averages

approximately 2.4 m (7.9 ft) in thickness, and

consists of a lower main bed and an overlying

rider, both of which may occur as nonbanded

coal, impure coal, or carbonaceous mudstone

(fig. 3). Within the Farco property and leases,

coal beds of the San Pedro zone are mined at

the Palafox and Trevino mines, and the Santo

Tomas zone is developed at the Rachal mine

(fig. 1). 

MINING HISTORY

The Santo Tomas coal zone of Webb

County was first mined commercially by the

Rio Grande Coal and Irrigation Company

between 1881 and 1914 in modest drift

openings in the vicinity of Minera (fig. 1). By

1895, the Cannel Coal Company had

completed a shaft and had driven mains in both

the Santo Tomas and San Pedro zones at

Darwin (fig. 1). The company, which

employed several hundred miners, was

directed by David Darwin Davis, a mining
U.S. Geological Survey Open-File Report 99-301

17

engineer from Wales (Hopson, 1975). A

second shaft of the Cannel Coal Company at

Dolores also mined both coal zones. The

Santo Tomas Coal Company opened a shaft to

the Santo Tomas coal bed between 1910 and

1912 near the village of Santo Tomas. This

operation ceased in 1918 after a mine fire.

The Darwin mines were abandoned in 1921,

and the Dolores mines closed in 1939.

In 1979, Farco Mining Company of

Texas, Inc., initiated surface mining at Palafox

(fig. 1). The Rachal and Trevino mines began

production in 1983 and 1996, respectively.

Cumulative production from 1979 through

1997 totaled approximately 6 million short

tons. The coal is washed and sized at a

cleaning plant at Palafox. A significant

amount of the coal is exported to the Republic

of Ireland as lump coal for domestic use, and

other production is consumed by local cement

plants. Kelmac, Inc., a subsidiary of Chevron

Corporation, recently acquired the Webb

County mines of Farco Mining Company of

Texas and operates currently under the name

Farco Mining, Inc. 

DEPOSITIONAL SETTING

The coal-bearing portion of the lower

to middle Claiborne Group of Webb County is

a fining-upward sequence situated above a

thick, sandstone-dominated section that

contains poorly preserved marine

invertebrates, and below a mudstone-rich

section that yields mainly nonmarine fossils.

Corehole records and natural exposures show

that the San Pedro coal zone overlies a barren

interval dominated by thick (7.6-10.7 m or 25-

35 ft), predominantly flat-bedded, sandstone

deposits.

At Palafox, the San Pedro coal zone

consists of a thick (< 0.85 m or 2.9 ft),

nonbanded upper bed and a thin, impure bed

approximately 3 m (9.8 ft) below; additional

thin coal beds and carbonaceous mudstones

occur in the lower part of the coal zone but are

not mined (fig. 3). The upper coal bed is

underlain by a dark gray, root-penetrated, silty

claystone, and the uppermost portion of this

coal bed contains mud-filled invertebrate

burrows and trails. As seen in-mine, the

contact between this coal and overlying

mudstone or sandstone generally is sharp. 

The interval immediately above the San

Pedro coal zone consists mainly of sandstone,

a slightly smaller proportion of mudstone, a

minor intraformational conglomerate, local

fine-grained limestone nodules or bands, and

thin (< 0.45 m or 1.5 ft), discontinuous coal

beds that range from nonbanded coal to clay

or silt-rich impure coal. The sandstones are

calcareous and are composed chiefly of fine to

very fine, subangular to rounded quartz grains;

individual beds are massive, cross bedded,

streaked or interbedded with mudstone,

rippled, or rooted or otherwise biotrubated.

An en echelon pattern characterizes sandstone

bodies above the San Pedro coal zone in the

Palafox Mine. These sandstone bodies are flat

botttomed, flat to ripple bedded, and exhibit

characteristics of mouth-bar to crevasse-splay

sands deposited in shallow water. Between

the main coal bed and these sandstone

deposits, coarsening-upward, rhythmically

interbedded mudstones and sandstones contain

flaser bedding suggestive of tidal influences. A

few zones of poorly preserved marine

invertebrate fossils (pelecypods, gastropods)

occur above the San Pedro coal zone

(Lonsdale and Day, 1937).

At the Rachal Mine (fig. 1), the Santo

Tomas coal zone consists of a thick (0.69 m)

nonbanded main seam, a clastic interval that

ranges from 0.91 to 1.98 m (3 to 6.5 ft) in
U.S. Geological Survey Open-File Report 99-301

18

thickness, and a thin (0.20 m or 0.6 ft) rider

coal bed (fig. 3). Both beds are underlain by

paleosols, and the top of the main bed contains

invertebrate burrows and trails. The interval

between these coal beds consists of

flat-bedded siltstone and fine to very fine

grained, noncalcareous sandstone that fine

laterally into coarsening-upward mudstone

deposits. These mudstones lack the flaserbedded sandstone interbeds that occur above

the San Pedro coal bed at Palafox. Channelform deposits within this interval are small

(0.25 m or 0.8 ft thick, 1 m or 3.3 ft wide),

few in number and limited to the top of thick

sandstone deposits. Overall, the interval

between the main Santo Tomas coal bed and

rider bed records the drowning of an initial

mire, infilling of the resulting floodbasin by

minor crevasse splays, and re-establishment of

mire conditions. 

The presence of several thin coal and

carbonaceous shale beds a short distance

above the Santo Tomas rider bed indicates that

clastic-rich mires persisted locally. Overlying

coarsening-upward sequences represent the

terminal flooding and clastic infilling of the

floodbasin. Lonsdale and Day (1937) reported

thin sandstone beds in the area of Darwin that

contained poorly preserved nonmarine

molluscs and abundant green algae

fructifications. Although they interpreted

these fossil occurrences as a unique bed of

considerable persistence, Gardner (1945), who

measured and collected many of the

fossiliferous sections reported by Trowbridge

(1923, 1932) and Lonsdale and Day (1937),

stated clearly that several such beds occur.

The presence of these locally rich beds of

nonmarine aquatic fossils above the Santo

Tomas coal zone indicates that lacustrine

settings may have been common within the

floodbasin. 

COAL CHARACTERISTICS

The nonbanded coals that are mined in

Webb County are extremely hard, fracture

conchoidally, and have cleats developed at a

scale similar to that of bituminous nonbanded

coals. The lower part of the San Pedro coal

bed at Palafox is very massive and breaks into

blocks that average 0.5 m (1.6 ft) in width; to

a lesser extent, the main Santo Tomas coal bed

becomes more massive and less fractured

downward. 

Physical and geochemical analyses (fig.

4) reported by us previously (Warwick and

Hook, 1995) are comparable to earlier

analyses of these coal beds (Evans, 1974;

Mukhopadhyay, 1989). All the San Pedro and

Santo Tomas samples have relatively high ash

yields (9.5-18% dry basis) at their base. Total

sulfur content (0.8-2.0%, dry basis) generally

increases upward, and organic sulfur is

predominant. Calorific values increase upward

within each coal bed, with a maximum of

7,434 kcal/kg (13,371 Btu, dry basis) found in

the top part of the main San Pedro bed at

Palafox. Hydrogen content follows calorific

values, ranging from 5.4 to 6% (dry basis).

Additional physical and geochemical data,

including trace-element analyses, are reported

in Warwick and Hook (1995).

Petrographic study of particle pellets

and unetched polished blocks from San Pedro

and Santo Tomas coal samples (Warwick and

Hook, 1995) indicate that a highly degraded

groundmass composed of eugelinite is the

main organic component (approximately 71%,

mineral-matter-free basis). An enriched

liptinite fraction (approximately 23%)

accounts for a portion of the high calorific
U.S. Geological Survey Open-File Report 99-301

19

0 5 

10 15 

20 

50 cm

0 4 8 

12 16 

0 4 8 

12 

16 

0 

0.2 

0.4 0.6 0.8 

1 

1.2 

0 

0.2 0.4 0.6 0.8 

1 

1.2 

0 

2000 6000 

10000 

14000 

0 

6000 

10000 

14000 

0 

2 4 6 

0 2 4 6 

Santo Tomas and 

 rider coal beds 0 0.5 1 1.5 2 2.5 

0 

2000 6000 

10000 14000 

0 

0.5 

1 

1.5 

0 

2 

20 

40 

60 

San Pedro 93 

channel sample

San Pedro 94 

channel sample

2000 

0 

2000 6000 

10000 

14000 

0 

2 

4 

6 

ASH (db) S (db) BTU (db) H (db)

ASH (db) S (db) BTU (db) H (db)

ASH (db) S (db) BTU (db)

ASH (db) S (db) BTU (db) H (db)

Figure 4. Selected proximate and ultimate data. Sample descriptions and symbols are the same as

those used in figure 3. All values as percentages except Btu. Sample locations are shown in

Figure 1. Complete sample data are given in Warwick and Hook, 1995. From Warwick and

Hook (1995).
U.S. Geological Survey Open-File Report 99-301

20

Sandstone

Mudstone/claystone

Coal

Carbonaceous mudstone

Algae-rich

 lake

San Pedro

coal bed

Santo Tomas

coal bed

Distributary channel

and crevasse splay

Clastic

swamp

Crevasse 

splay Santo Tomas

rider bed

Mouth bar

sandstone

Algae-rich

 lake

Algae-rich

 lake

Alluvial 

floodplain

Figure 5. Schematic block diagram of depositional environments for the San Pedro and Santo Tomas

coal zones in the middle part of the Claiborne Group. No scale implied. The geometric distribution

of lithologies on the panels is inferred from borehole and mine highwall data. From Warwick and

Hook (1995).

 values that distinguish these coals. Most of

this enrichment represents fusiform

green-algae fructifications, which because of

their size (up to 2 mm or 0.1 in. in length),

cannot be identified or quantified readily by

conventional coal petrography techniques or

palynologic preparations that involve

acidification. There is a negligible proportion

of inertinite macerals in these coals.

Average maximum vitrinite reflectance

(Romax) values for San Pedro and Santo Tomas

coal samples reported previously by us was

0.53. This was comparable to the Romax

reported by Mukhopadhyay (1989) for one

San Pedro sample. Compared to standard

rank parameters, the San Pedro and San

Tomas coal deposits fall within the range of

subbituminous A to high-volatile bituminous C

coal. 

SUMMARY

The unusual coals of the San Pedro

and Santo Tomas coal zones of the Claiborne

Group in Webb County, Texas, are

distinguished by physical, petrographic, and
U.S. Geological Survey Open-File Report 99-301

21

geochemical characteristics unlike those of

other Gulf Coast Tertiary coal deposits.

Whereas the San Pedro deposits accumulated

in marine-influenced lower delta plain settings,

the overlying Santo Tomas interval represents

a nonmarine, mudstone-dominated sequence

(fig. 5). Mineable coal beds within both coal

zones contain an abundance of green algae

fructifications, which account in part for the

anomalously high calorific value. Data from

Mexico (SPP, 1989), however, record

Quaternary igneous intrusive rocks less than

100 km (62 mi) west of the Rio Grande area,

and coal-quality data from coal beds in the

underlying Wilcox Group of Texas counties

bordering Webb County indicate increased

rank. Additional work is required to

determine the postdepositional history of the

coal deposits of South Texas.

ACKNOWLEDGMENTS

This summary is based mainly upon a

1995 paper published in the International

Journal of Coal Geology. We thank Elsevier

Science B.V. for allowing us to reproduce

portions of our previous work. Our original

study was made possible through the

assistance of Farco Mining Company of Texas,

Inc., and the Railroad Commission of Texas.

We also thank those at the U.S. Geological

Survey and the U.S. National Museum who

contributed to our earlier work. 

REFERENCES CITED 

Ashley, G.H., 1919, The Santo Tomas Cannel

Coal, Webb County, Texas: U.S.

Geological Survey Bulletin 691-I, p.

251-270.

Eargle, D.H., 1968. Nomenclature of

Formations of Claiborne Group,

Middle Eocene Coastal Plain of Texas:

U.S. Geological Survey Bulletin1251-

D, 25 p.

Evans, T.J., 1974, Bituminous Coal in Texas:

Bureau of Economic Geology, The

University of Texas at Austin,

Handbook 4, 65 p.

Gardner, J.A., 1945, Mollusca of the Tertiary

Formations of Northeastern Mexico:

Geological Society of America

Memoir 11, 332 p.

Guevara, E.H., and Garcia, Roberto, 1972,

Depositional systems and oil-gas

reservoirs in the Queen City Formation

(Eocene), Texas: The University of

Texas at Austin Bureau of Economic

Geology Geologic Circular 72-4, 22 p.

Hopson, F.S., 1975, Resume of cannel coal

development, in, Triple Energy Field

Trip, Duval, Webb, and Zapata

Counties, Texas: Corpus Christi

Geological Society Guide Book, 14 p.

Lonsdale, J.T., and Day, J.R., 1937, Geology

and Ground-Water Resources of Webb

County, Texas: U.S. Geological

Survey Water-Supply Paper 778, 104

p.
U.S. Geological Survey Open-File Report 99-301

22

Mukhopadhyay, P.K., 1989, Organic

petrology and organic geochemistry of

Texas Tertiary coals in relation to

depositional environment and

hydrocarbon generation: Bureau of

Economic Geology, The University of

Texas at Austin, Report of

Investigations188, 118 p.

SPP [Secretaría de Programación y

Presupuesto], 1989, Carta Geologica

[Monterrey]: Dirección General de

Geografía del Territorio Nacional, San

Antonio Abad, Mexico, scale

1:1,000,000.

Trowbridge, A.C., 1923, A geologic

reconnaissance in the Gulf Coastal

Plain of Texas, near the Rio Grande:

U.S. Geological Survey Professional

Paper 131-D, p. 85-107.

Trowbridge, A.C., 1932, Tertiary and

Quaternary Geology of the Lower Rio

Grande Region, Texas: U.S.

Geological Survey Bulletin 837, 260 p.

Vaughan, T.W., 1900, Reconnaissance in the

Rio Grande Coal Fields of Texas: U.S.

Geological Survey Bulletin 164, 100 p.

Warwick, P.D., and Hook, R.W., 1995,

Petrography, geochemistry, and

depositional setting of the San Pedro

and Santo Tomas coal zones:

anomalous algae-rich coals in the

middle part of the Claiborne Group

(Eocene) of Webb County, Texas:

International Journal of Coal Geology,

v. 28, p. 303-342.
U.S. Geological Survey Open-File Report 99-301

23

CHAPTER 3

The San Miguel lignite deposit, Jackson Group (Eocene), South Texas

By Peter D. Warwick, Sharon S. Crowley+

, Leslie F. Ruppert,

and James Pontolillo*

U.S. Geological Survey, MS 956 (MS 910+, MS 432*), 12201 Sunrise Valley Drive, Reston, VA 20192

INTRODUCTION

The San Miguel lignite deposit (late

Eocene, lower Jackson Group) of South Texas

is mined by the North American Coal

Corporation, at the San Miguel Mine near

Jourdanton, Texas (fig. 1). The deposit

consists of four or more thin (generally < 1 m

or 3.3 ft thick) lignite benches that are

separated by claystone and mudstone partings

(fig. 2). The partings are composed of altered

volcanic air-fall ash that has been reworked by

tidal or channel processes associated with a

back-barrier depositional environment

(Snedden and Kersey, 1981; Gowan, 1985;

Ayers, 1986; Senkayi and others, 1987; and

Warwick and others, 1996). The field trip

stop at the San Miguel Mine will include a trip

into the working opencast pit. We will be able

to observe mining methods, and the rocks

associated with the coal-bearing interval. This

summary of the geology of the San Miguel

deposit is based largely on Warwick and others

(1996), which focuses on the petrology and

geochemistry of the lignite deposit.

GEOLOGIC SETTING

The San Miguel lignite deposit is in the

undivided lower Jackson Group of South

Texas (fig. 2). In the mine area, the Jackson

strata dip to the southeast about 17 m/km (90

ft/mi) (Ayers, 1986). The mine is located

updip of a strike-elongated sand complex that

has been interpreted as a barrier-bar/strand

plain complex by Kaiser and others (1980) and

Gowan (1985). Gowan (1985) and Ayers

(1986) suggest that the mud-dominated

sediments the comprise the floor, partings, and

roof of the San Miguel lignite deposit

originated in lagoonal mires landward (west)

of the sandstone-dominated barrier island

complex. The lignite-bearing interval is

laterally persistent along strike for more than

40 kilometers (25 mi) (Ayers, 1986, 1989;

Tewalt and others 1983). More detailed

descriptions of the geological setting of the

area can be found in McNulty (1978), Snedden

(1979), Snedden and Kersey (1981), Tewalt

and others (1983), Gowan (1985), and Ayers

(1986, 1989). 

The San Miguel lignite interval consists

of four (or more) thin (generally <1 m or 3.3 ft

thick) lignite benches that are separated by

claystone and mudstone partings. The upper

bed is designated as the A bed and is underlain

by the B, C, and D beds (figs. 2, 3a) (Gowan,

1985). Tewalt and others (1983) referred to

this interval as South Jackson seam no. 10.

The rocks above the lignite beds are

dominated by fine siltstone and claystone with

occasional thin layers of gastropod and bivalve

shells. 

Kaiser (1982), Gowan (1985), and

Ayers (1986, 1989) suggested that the rock

partings within the San Miguel lignite deposit
U.S. Geological Survey Open-File Report 99-301

24

McMullen Co Atascosa Co

Live Oak Co

3387

140

791

ALT

281

281

Campbellton

Whitset

Crowther

Approximate

boundary of 

San Miguel Mine

Permit

791

99

98 25' o

28 45' o

28 35' o

98 15' o 98 35' o

0 5 km

Texas

SM-1

SM-2

99

16

To Jourdanton

Figure 1. Index map of the San Miguel Mine area showing towns, roads, and sample sites. Mine

outline from the San Miguel Lignite Mine Permit Application, on file at the Texas Railroad

Commission in Austin, Texas. From Warwick and others (1996).

are composed of volcanic air-fall ash that has

been reworked by possibly tidally influenced

channel processes. Senkayi and others (1987)

examined the partings in detail and found that

most contained altered kaolinite and mixed

detrital and volcanic quartz grains, thereby

supporting the interpretation of a volcanic

origin and subsequent detrital mixing of the

sediment that forms these layers. These

findings are consistent with the conclusions of

Warwick and others (1996) (see below).

LIGNITE RESOURCES AND MINING

Jackson Group lignite resources for the

South Texas area have been estimated to be

about 6 billion short tons (5.4 metric tons)

with 750 million tons (680 metric tons)

occurring near surface and the remainder

classified as deep-basin deposits (Kaiser and

others, 1980). Surface mining operations at

the San Miguel Mine began in 1980 and the

1997 annual production was about 3.5 million

tons (3.2 metric tons) (data from the U.S.

Department of Energy, Energy Information

Agency and the Railroad Commission of

Texas). Since 1997, the San Miguel Mine has

been operated by the North American Coal

Corporation; prior mining was carried out by

the Atascosa Mining Company. The

Keystone Coal Industry Manual (PRIMEDIA

Intertec, 1999) reports the total reserves for

the North American Coal Corporation in the

San Miguel Mine area to be about 42 million

tons with 35 million tons recoverable (38.2

and 32 million metric tons respectively).

Average overburden is about 30 m (98 ft),

with a stripping ratio of 7:1. Overburden is

removed by dragline methods. The coal is

removed by Easi-Miners (a modified asphalt

stripper) and front-end loaders. The lignite is
U.S. Geological Survey Open-File Report 99-301

25

Paleocene Eocene

Jackson Group

Wilcox Group

Midway Group Midwayan

Sabinian Claibornian

Jacksonian

Danian

Selandian

Ypresian Lutetian

Bartonian

Priabonian

Series

European

stage N. American

stage

Group

Claiborne

 Group

Mudstone roof 0.15cm (RFT)

Mudstone 13cm (P1.1)

Lignite 17cm (B1)

Lignite 31cm (C1)

Lignite 35cm (C2)

Claystone 9cm (P3.1)

Lignite 36cm (D1)

Lignite 33cm (D2)

SM-1 SM-2

Mudstone roof

Carbonaceous shale 

 20cm (A.1)

Lignite 33cm (A.2)

Mudstone 14cm 

Claystone 16cm

Lignite 23cm (B.1)

Lignite 13cm (B.2)

Lignite 15cm (B.3)

Claystone 23cm

Lignite 31cm (C.1)

Lignite 37cm (C.2)

Claystone 8cm

Lignite 60cm (D.1)

Mudstone floor 33cm 

Green claystone

San Miguel Mine

Claystone 9cm (P3.3)

Carbonaceous shale 18cm

(RF2)

Lignite 22cm (A1)

Lignite 8 cm (A2)

Mudstone 15cm (P1.2)

Claystone 8cm (P1.3)

Lignite 32cm (B2)

Claystone 29cm (P2)

Claystone 9cm (P3.2)

Claystone floor (GRAB)

1 m

Mudstone 12cm 

Mudstone 10cm 

Claystone 21cm 

Lignite 40cm (D.2)

Claystone 11cm (P3.4)

A

B

C

D

* *

* *

*

*

Mudstone roof 0.15cm (RFB)

 300 m

Figure 2. Generalized stratigraphy and stratigraphic sections of the coal-bearing interval at the San

Miguel Mine. Sample numbers are in parentheses, and channel sample sites are shown on figure 1.

The approximate positions of the volcanic ash beds identified by Senkayi and others (1987) are

indicated by an asterisk. Stratigraphy is after Breyer (1991). From Warwick and others (1996).

moved by truck and used as steam coal in the

mine-mouth 409 megawatt San Miguel Power

Plant operated by the San Miguel Electric

Cooperative (PRIMEDIA Intertec, 1999).

Some mining problems that are

encountered in the San Miguel Mine include

floor heaves, mudstone dikes, and channels

within the coal interval. Some of these

features may be observed during our field trip.

In some areas, a greenish mudstone located 1-

2 meters (3.3-6.6 ft) below the D bed (fig. 2)

intrudes into the floor of the open pit causing

problems with pit traffic and general mining

operations (fig. 3a). Ayers (1986) described

clastic dike intrusions that originate primarily

from the partings between beds C and D. The

intrusions are generally no more than a few

meters in length and are less than 0.5 m (1.6

ft) wide. Minor faulting is usually associated

with these dikes. Ayers (1986) suggested that

emplacement of these dikes may be related to

loading and dewatering processes. Another

interesting feature that is often observable in

the San Miguel Mine are small interseam

channels that usually scour into lignite beds C

or B and are filled with inter-seam parting

material. The channels are usually no more

than a few meters (several ft) wide and are

confined to a single clastic parting zone within

the A-D coal bed complex. Ayers (1996)

described these channels in detail and suggests

that were formed by marsh drainages that

occupied the area between periods of peat

accumulations.
U.S. Geological Survey Open-File Report 99-301

26

B

A

C

D

A B

C D

15 cm

scale

Floor heave

EL

EG

Figure 3. Representative photographs and photomicrographs of outcrops and samples from the San

Miguel lignite interval. (a) View of a highwall with A (upper), B, C, and D beds and the partings.

Note that the debris on the mine floor seen on the left of the photograph is due to floor heaves. (b)

Clay-filled burrows are common in the San Miguel lignite. Scale = 15 cm. (c) Arrow points to a

compressed log in the D bed. (d) Typical photomicrograph of an etched and unetched surface of a

polished pellet of San Miguel lignite. EG = eugelinite, EL = etch line. Scale bar = 25μm. From

Warwick and others (1996)

CHARACTER OF THE SAN MIGUEL 

LIGNITE

Megascopic characteristics

As described by Gowan (1985), the

San Miguel Lignite interval has been

extensively reworked by burrowing

organisms. This is apparent from the

lithologic descriptions of the channel sample

sites described by Warwick and others (1996)

(fig. 3b). Some of the burrows near the top of

the A lignite bed were identified as Gyrolithes

by Gowan (1985) and probably were formed

by organisms that lived in the water that

encroached over the top of the peat deposit.

The burrows, usually filled with a mixture of

organic material and kaolinitic claystone, is

one source of ash in the lignite beds (fig. 3b).

Individual lignite beds generally exhibit a dull,

somewhat massive texture near their base and

grade upward into attrital-rich lignite with

increasing xylinitic bands (3-4 cm or 1.2-1.6 in

thick) towards the top of the individual bed.

Xylinitic bands can compose up to 30% of

individual lignite beds. Occasionally,

compressed logs are exposed in
U.S. Geological Survey Open-File Report 99-301

27

SM-1 0 10 

20 30 40 

50 

60 70 

0 

0.5 

1 

1.5 

2 

2.5 

3 

3.5 

0 

2000 4000 

6000 

8000 

10000 

1 m

SM-2 

0 

10 

20 

30 

40 

0 

0.5 

1 

1.5 

2 

2.5 

3 

3.5 

0 

2000 4000 6000 8000 

10000 

12000 

 Sulfur

 CV

mass %

mass %

Btu/lb

0 

10

20

30

Mj/kg

0 

10

20

30

Mj/kg

Ash Yield

SM-1 SM-2 

SM-1 SM-2 

Figure 4. Bar graphs showing concentrations of ash yield, total sulfur, and calorific value (CV, dry

basis) for the sample sets SM-1 and SM-2. Location of samples are shown on figure 1 and lithologic

symbols are the same as used on figure 2. Distance between sample sites is about 300 m. From

Warwick and others (1996).
U.S. Geological Survey Open-File Report 99-301

28

0 

1 

2 

0 

20 

40 

60 

80 

100 

0 

4 8 

12 

16 

Cr 

Co 

Hg 

As

0 

1 2 

3 

Cd0 

20 

0 4 

8 

12 

0 

0.2 

0.4 

SM-1

SM-2

Be 

40 

SM-1 SM-1

SM-1 SM-1 SM-2 SM-2

SM-2

SM-2 1 m 0 0.2 0.4 0.6 

SM-2

0 

20 

40 

60 

80 

100 

SM-1

0 

5 

10 

15 

20 

25 

0 

20 40 

60 80 

100

0 

5 

10 

15 

Figure 5. Bar graphs showing the distribution of trace element concentrations (dry USGS ash basis)

for twelve potentially environmentally sensitive elements identified by the 1990 U.S. Clean Air Act

Amendments. Location of samples are shown on figure 1 and lithologic symbols are the same as used

on figure 2. All values are parts per million (ppm). From Warwick and others (1996).

cross section on the exposed face of the lignite

bench (fig. 3c). The middle lignite beds (B and

C) appear to contain the greatest percentage of

xylinitic banding when compared to the upper

and lower beds (A and D).

Proximate analyses

Chemical and physical data obtained

from San Miguel lignite samples have been

reported on an individual bed basis (5 samples;

Mukhopadhyay, 1989) and on a combined bed

basis (23 samples; Tewalt, 1986). From these

data, the dry ash yields of samples range from

25 to 55% (avg 38%; Tewalt, 1986) with the

A bed having the greatest average ash yield

(39%, Mukhopadhyay, 1989). Equilibrium

moisture values range from 27 to 33%

(Tewalt, 1986). Total sulfur contents (on a

dry basis) range from 1.5 to 3.1% (Tewalt,

1986). Organic sulfur is the dominant sulfur

type. Calorific values (on a moisture-free

basis) for the San Miguel samples range from

5384 to 8690 Btu/lb (12.5-20.2 Mj/kg)

(Tewalt, 1986). Tewalt (1986) and

Mukhopadhyay (1989) report that the rank of

the Jackson Group and the San Miguel lignites

is lignite A.

Warwick and others (1996) reported

proximate analyses for 17 lignite samples

collected from the San Miguel deposit (fig. 4).

The data indicate that dry ash yields average

30.16%; total dry sulfur content averages

2.76% (dominated by organic sulfur, 2.04%);
U.S. Geological Survey Open-File Report 99-301

29

0 

20 

40 

60 

80 

100 

0 

10 

20 

25 

0 

0 

10 20 30 

Pb 

0 

Mn

100 

300 500 

0 

10 20 

NI

0 

20 

40 

0 2 4 

6 8 

10 

Sb

0 

5 

Se

0 

10 

20 

U

SM-2

SM-1

0 

5 

10 

15 

20 

0 

100 

300 

500 

700 

SM-1

SM-2 SM-1 SM-2 2 

4 

6 

SM-1 SM-2

SM-1 SM-2

SM-1 SM-2 1 m

Figure 5. Continued - Bar graphs showing the distribution of trace element concentrations (dry

USGS ash basis) for twelve potentially environmentally sensitive elements identified by the 1990 U.S.

Clean Air Act Amendments. Location of samples are shown on figure 1 and lithologic symbols are

the same as used on figure 2. All values are parts per million (ppm). From Warwick and others

(1996).

and dry calorific values average 8611 Btu/lb

(20 Mj/kg) (fig. 4). As-received moisture

content averages 33.4% for the SM-2 channel

sample set. At both sample sites, ash yields

were greatest from the upper and lower lignite

benches. Total sulfur content is fairly uniform

both within and between each sample site (fig.

4).

Major, Minor, and Trace element data 

Tewalt (1986) reported major oxide

and trace-element data for the San Miguel

lignite interval. She found that the San Miguel

lignite interval contains greater amounts of

Na2O (avg 3.67%; N=66) as compared to

other lignite deposits of Texas (range of

averages: 0.41-1.22%), and suggested that this

enrichment was either the result of sodium

sorption from saline waters during and after

deposition, or of sodium cation exchange with

present-day ground water. Tewalt (1986)

reported trace-element concentrations for

fifteen elements (As, B, Be, Cd, Cr, Cu, Hg,

Mn, Mo, Ni, Pb, Se, U, V, Zn) that were

obtained from two samples from the San

Miguel deposit. Based on the results from

other Jackson Group lignites of East Texas,

Tewalt suggested that the Jackson lignites

contain greater amounts of B, Be, Mn, and Zn
U.S. Geological Survey Open-File Report 99-301

30

SM-1

SM-2

1 m

0 

20 40 

60 80 

Nb

100 120 

0 

20 40 

60 80 

100 

0 

200 400 600 800 

Zr

1000 

0 

200 400 600 800 

1000 

0 

100 200 

Ce

0 

100 200 300 

0 

10 20 30 40 50 60 

Th

0 

10 20 30 40 50 60 

0 2 4 6 8 

10 

0 2 4 6 

Ta

SM-1

SM-2

SM-1

SM-2 SM-2

SM-1

SM-1

SM-2

Figure 6. Bar graphs showing the distribution of trace element concentrations (dry USGS ash basis)

for those elements inferred to be derived from volcanic ash. Location of samples are shown on figure

1 and lithologic symbols are the same as used on figure 2. All values are parts per million (ppm).

From Warwick and others (1996).

than the older Wilcox lignites of eastern Texas.

Warwick and others (1996) reported

trace element data obtained from two channel

samples taken from the San Miguel mine (figs.

1, 2). Of particular interest for this study was

the distribution of 12 environmentally sensitive

elements that were described by the 1990 U.S.

Clean Air Act Amendments. A summary of

the trace element data (on a dry, ash basis) for

both sample sites is plotted on figure 5. Visual

analysis of the SM-2 bar graphs shows that

some of the elements are concentrated in the

organic-rich (or lignite) samples. Linear

correlations between ash yield and the

concentrations of the environmentally sensitive

elements on an ash basis for those samples

containing less than 50% ash (i.e. what is

normally mined) showed that only Mn 

has a strong negative (r <-0.5) relationship to

sample ash yield. On a whole-coal basis (for

those samples containing less than 50% ash) a

strong positive correlation (r >0.5) was found

between ash and Pb, Se, and Na2O.

Warwick and others (1996) also

plotted bar graphs for both San Miguel rock

and lignite samples to show the vertical

distribution of elements that may be associated

with volcanic ash partings (fig. 6). Mean

concentrations are: Ce, 117 ppm: Nb, 36 ppm;

Ta, 2.4 ppm; Th, 29 ppm; and Zr, 477 ppm (all

concentrations in coal on an ash basis) (fig. 6).

Concentrations of these elements are generally
U.S. Geological Survey Open-File Report 99-301

31

enriched in the B and C lignite beds. Previous

work has indicated that concentrations of these

elements in coal can be enriched by

incorporation of volcanic ash in peat or by

leaching from volcanic ash beds (Zielinski,

1985; Crowley and others, 1989). In coal

samples from intervals adjacent to altered

volcanic ash partings in the Cretaceous C coal

bed of Utah, Crowley and others (1989)

reported mean concentrations (on an ash basis)

of elements having a volcanic origin as

follows: Zr (752 ppm), Nb (57 ppm), Th (60

ppm), and Ce (246 ppm). Although the mean

concentrations of these elements in the San

Miguel samples do not reach the reported

averages for the Utah C coal bed, specific

samples from SM-1 and SM-2 exceed the

Utah averages for Zr, Nb, and Ce.

Mineralogy of San Miguel partings

Warwick and others (1996) reported

results from preliminary examination of San

Miguel rock parting samples based upon

scanning electron microscopy (SEM) and

energy-dispersive X-ray analyses (EDXA).

These analyses indicate that the upper part of

the mudstone parting below the A bed (SM-2

P1.1, fig. 2) is composed primarily of kaolinite

with some mixed-layer clay present.

Vermicular kaolinite is present throughout the

sample, as are small (<50μm) K-feldspars.

Isolated plagioclase, quartz, barite, framboidal

pyrite, anatase, and a single Al-phosphate were

also observed. The mudstone is very organic

rich. Although individual organic stringers are

rare, irregular pieces of coaly material are

found throughout the sample.

The matrix of the claystone below the

B bed (sample SM-2 P2, fig. 2) is composed

primarily of mixed-layer clay and abundant

organic material. Both alkali and plagioclase

feldspars were observed throughout the

sample, but they are rare. Quartz, pyrite, and

Ca-Al phosphate (crandalite group?) are

present in trace amounts.

Examination of samples from the

claystone parting between the C and D beds

(P3.1, P3.2, P3.3, and P3.4, fig. 2), which was

described as a major ash bed by Senkayi and

others (1987), shows that the matrices are

composed primarily of mixed-layer clays.

Kaolinite is present as cell-fillings, vermicular

clasts, and rarely as matrix material.

Accessory grains are uncommon. The

dominant accessory minerals are subhedral and

often etched alkali feldspars. Plagioclase is

also present but not as abundant as the

alkalies. Barite and pyrite, and rare

subrounded quartz and euhedral zircons were

observed in each of the examined samples.

Palynology and Organic Petrography

Gennett (1985) and Gennett and

Raymond (1986) examined incremental bench

samples from a core of the San Miguel lignite

and found that the lower D bed had pollen

derived from marsh ferns and palms which

suggested a brackish-water influence on the

deposition of the D bed paleo-mire. Pollen

types found in the overlying beds showed

fresh-water associations as indicated by a

vertical transition from Nyssa (tupelo) to

Engelhardia (walnut family) and Fagaceous

(oak family) types. Mukhopadhyay (1989)

reported that beds B and D contain marsh

pollen types and suggested that the San Miguel

lignites are dominated by

Engelhardia/Juglandaceae and herbaceous(?)

monocots.

Mukhopadhyay (1986, 1989)

examined the petrographic characteristics of

channel and lithotype samples from each of

 the four major lignite beds exposed at the San
U.S. Geological Survey Open-File Report 99-301

32

0 

20 40 60 

80 

SM-1

100 

1 m

SM-2 

1 m

Major 

Huminites

 (%)

Major 

Inertinites 

(%)

0 

1 m

Major 

Liptinites 

(%)

EG HM DG EG HM DG

LPD ALG RSD LPD ALG RSD

MAC SCL FUS 0 20 40 60 80 

100 

0 5 

10 

15 

20 

25 

30 

1 

2 

0 

5 

10 

0 1 2 3 4 5 6 

MAC SCL FUS

Figure 7. Bar graphs of the major petrographic constituents for the channel-sample sets SM-1 and

SM-2. Location of samples are shown on figure 1 and lithologic symbols refer to figure 2.

Abbreviations are as follows: EG = eugelinite, HM = humotelinite, DG = detrogelinite, LPD =

liptodetrinite, ALG = alginite, RSD = resinite in attritus, MAC = macrinite, SCL = sclerotinite, FUS

= fusinite. From Warwick and others (1996)
U.S. Geological Survey Open-File Report 99-301

33

Miguel mine. Mukhopadhyay's channel

sample data indicate that bed A (fig. 2)

contains greater amounts of liptinite macerals

(32%, primarily sporinite and liptodetrinite)

than the huminite-rich middle lignite beds.

Bed C contains the greatest amount of

huminitic material (85%, primarily ulminite

and attrinite/densinite) when compared to the

other beds. Inertinite content for the beds

range from 1 to 4% and is primarily in the

form of sclerotinite and fusinite. The D bed

contained a few marine dinoflagellates (algal

cysts).

Warwick and others (1996) reported

that the San Miguel lignite deposit is

dominated by the huminite maceral group

(89%) followed by the liptinite (10%), and

inertinite (1%) maceral groups (figs. 3d and 7).

The three dominant huminite maceral

subgroups in decreasing order of abundance

are: eugelinite (73%), humotelinite (11%), and

detrogelinite (3%). The liptinite maceral

group is dominated by liptodetrinite (4%),

followed by alginite (2%) and resinite in

attritus (1%). The inertinite maceral group is

composed of macrinite, sclerotinite, and

fusinite. 

Vertical and lateral variations in

maceral composition can be observed in the

petrographic data (fig. 7). Channel-sample set

SM-1 contains similar amounts of huminite

macerals as found in SM-2 samples, but SM-1

samples have markedly less liptinite and

inertinite components. Vertical trends in the

petrographic data include slightly increased

liptodetrinites in samples from the top and

bottom of the lignite interval and a sharp

increase in inertinite (primarily macrinite and

fusinite, fig. 7) in the lignite samples directly

below the parting between the A and B beds.

Humotelinite content increases, although

slightly, in the upper part of each bed. This

finding is in agreement with megascopic

descriptions of the coal in the field.

RELATIONSHIP BETWEEN ASH

YIELD, AND PETROGRAPHIC AND

GEOCHEMICAL CHARACTERISTICS

OF THE SAN MIGUEL LIGNITE

DEPOSIT

Warwick and others (1996) examined

the relationship between the ash yield and the

petrographic and geochemical characteristics

of the San Miguel lignite as mined. The major

conclusions of their study are as follows: (1)

The distribution of Mn is inversely related to

the ash yield of the lignite samples. This

indicates an organic affinity, or an association

with finely disseminated minerals in the lignite

that contain this element. (2) On a whole-coal

basis, the concentration of the environmentally

sensitive element Pb is positively related to ash

yield in lignite samples. This indicates an

inorganic affinity for Pb. (3) Average

whole-coal concentrations of As, Be, Sb, and

U in the San Miguel samples are greater than

published averages for these elements in other

U.S. lignites. (4) The upper and lower lignite

benches of the San Miguel deposit are both

ash- and algal-rich, indicating that these

intervals were probably deposited in wetter

conditions than those in which the middle

intervals formed. (5) The dominance of the

eugelinite maceral subgroup over the huminite

subgroup indicates that the San Miguel lignites

were subjected to peat-forming conditions

(either biogenic or chemical) that enabled

degradation of wood cellular material into

matrix gels, or that the plants that formed

these lignite benches were less woody and

more prone to formation of matrix gels. (6)

An inertinite-rich layer (top of the B bed)

might have formed from widespread oxidation
U.S. Geological Survey Open-File Report 99-301

34

of the San Miguel peat as a result of a volcanic

ash fall that was subsequently reworked.

SUMMARY

There have been numerous studies

conducted through the years detailing the

geology of the San Miguel lignite deposit. The

depositional setting of the lignite deposit is

interpreted to have originated in back-barrier

lagoonal mires (Snedden and Kersey, 1981;

Kaiser and others,1980; Gowan,1985; Ayers,

1986). The deposit consists of four or more

thin (generally < 1 m or 3.3 ft thick) lignite

benches that are separated by claystone and

mudstone partings (fig. 2). The partings are

composed of altered volcanic air-fall ash that

has been reworked by tidal or channel

processes associated with a back-barrier

depositional environment (Snedden and

Kersey, 1981; Gowan, 1985; Ayers, 1986;

Senkayi and others, 1987; and Warwick and

others, 1996).

Individual lignite beds generally exhibit

a dull, somewhat massive texture near their

base, which grades upwards into attrital-rich

lignite with increasing xylinitic bands (3-4 cm

or 1.2-1.6 in thick) towards the top of the

individual bed. Proximate data indicate that

dry ash yields from the San Miguel deposit

average 30.16%; total dry sulfur content

averages 2.76% (dominated by organic sulfur,

2.04%); and dry calorific values average 8611

Btu/lb (20 Mj/kg) (Warwick and others 1996).

Petrographically, the San Miguel lignite

deposit is dominated by the huminite maceral

group (89%) followed by the liptinite (10%),

and inertinite (1%) maceral groups (Warwick

and others, 1996). Average whole-coal

concentrations of As, Be, Sb, and U in the San

Miguel samples are greater than published

averages for these elements in other U.S.

lignites (Warwick and others, 1996). 

ACKNOWLEDGMENTS

This summary is based mainly upon a

1996 paper published in Organic

Geochemistry. We thank Elsevier Science

Ltd. for allowing us to reproduce parts of our

previous work. We wish to thank John Palfy,

General Manager, and Roger C. Fish,

Engineering Manager, and North American

Coal Corporation for facilitating our visit to

the San Miguel Mine. 

REFERENCES CITED

Ayers, W. B., Jr., 1986, Geology of the San

Miguel lignite mine, Jackson Group,

South Texas, in Finkelman R. B. and

Casagrande D. J., eds, Geology of

Gulf Coast Lignites: Geological .

Society of America, Coal Geology

Division Field Trip Guidebook, p.

54-67, Environmental and Coal

Associates, Houston, Texas.

Ayers, W. B., Jr., 1989, Depositional setting

of the San Miguel Lignite Mine,

Jackson Group (Eocene), South

Texas, in Ayers W. B. Jr., Breyer J. A.

and Finkelman R. B, eds., Depositional

settings of Texas Lignites, 28th

International Geological Congress:

Field Trip Guidebook T173, p. 33-37.

Breyer J. A., 1991, Tertiary coals of the Gulf

Coast, in Gluskoter, H. J., Rice D. D.

and Taylor, R. B., eds., Economic

Geology U. S.: Geological Society of

America, The Geology of North

America P-2, p. 573-582.
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35

Crowley, S. S., Stanton, R. W., and Ryer, T.

A., 1989, The effects of volcanic ash

on the maceral and chemical

composition of the C coal bed, Emery

Coal Field, Utah: Organic

Geochemistry, v. 14, p. 315-331.

Gennett, J. A., 1985, A core from the Eocene

San Miguel lignites, South Texas:

palynomorphs and depositional

environments [abs.]: American

Association Stratigraphic

Palynolologists. Program and

Abstracts, 18th Annual Meeting. p. 12.

Gennett, J. A. and Raymond, A. L., 1986,

Changes in floral composition with

depositional environment in Texas

Eocene Manning Formation lignites

[abs.]: American Association of

Petroleum Geologists Bulletin, v. 70,

p. 1181-1182.

Gowan, S. W., 1985, Depositional

environment of the San Miguel lignite

deposit in Atascosa and McMullen

Counties, Texas: College Station,

Texas A&M University, Ph.D.

dissertation, 199 p.

Kaiser, W. R., 1982, Lignite depositional

models, Texas Eocene: a regional

approach to coal geology, in Shobert

H. H., ed., Proceedings of the Basic

Coal Science Workshop, Houston,

Texas, 1981: U. S. Department of

Energy, Grand Forks Energy

Technology Center, p. 10-67.

Kaiser, W. R., Ayers, W. B., Jr., and La Brie,

L. W., 1980, Lignite resources in

Texas: The University of Texas at

Austin Bureau of Economic Geology

Report of Investigations 104, 52 p.

McNulty, J. E., Jr.,1978, Geology of the San

Miguel lignite deposit (Atascosa and

McMullen Counties, Texas, in Kaiser

W. R., ed., Proceedings, Gulf Coast

lignite conference: geology, utilization,

and environmental aspects: The

University of Texas at Austin, Bureau

of Economic Geology Report of

Investigations 90, p. 79-83.

Mukhopadhyay P. K., 1986, Petrography of

selected Wilcox and Jackson Group

lignites from the Tertiary of Texas: in

Finkelman R. B. and Casagrande D. J.,

eds., Geology of Gulf Coast Lignites:

Geological Society of America Coal

Geology Division Field Trip

Guidebook, Environmental and Coal

Associates, Houston, Texas, p. 126-

145.

Mukhopadhyay, P. K., 1989, Organic

petrology and organic geochemistry of

Texas Tertiary coals in relation to

depositional environment and

hydrocarbon generation: The

University of Texas at Austin, Bureau

of Economic Geology, Report of

Investigations 188, 118 p.

PRIMEDIA Intertec, 1999, Keystone coal

industry manual:Chicago, Illinois,

PRIMEDIA Intertec, 807 p.

Senkayi, A. L., Ming, D. W., Dixon, J. B., and

Hossner, L. R., 1987, Kaolinite,

opal-CT, and clinoptilolite in altered

tuffs interbedded with lignite in the

Jackson Group, Texas: Clays & Clay

Minerals, v. 35, p. 281-290.
U.S. Geological Survey Open-File Report 99-301

36

Snedden, J. W., 1979, Stratigraphy and

environment of deposition of the San

Miguel lignite deposit, northern

McMullen and southeastern Atascosa

counties, Texas: College Station,

Texas A&M University, M.S. thesis,

161 p.

Snedden, J. W., and Kersey, D. G., 1981,

Origin of San Miguel lignite deposit

and associated lithofacies, Jackson

Group, South Texas: American

Association of Petroleum Geologists

Bulletin, v. 65, p. 1099-1109.

Tewalt, S. J., 1986, Chemical characterization

of Texas lignite: University of Texas at

Austin, Bureau of Economic Geology

Geological Circular 86-1, 54 p.

Tewalt, S. J., Bauer, M. A., Mathew, D.,

Roberts, M. P., Ayers, W. B., Jr,

Barnes, J. W., and Kaiser, W. R.,

1983, Estimation of coal resources in

Texas Gulf Coast, Ohio Northern

Appalachian, and Wyoming Powder

River Basins: a comparison of

statistical approaches: The University

of Texas at Austin, Bureau of

Economic Geology Report of

Investigations 136, 137 p.

Warwick, P.D., Crowley, S.S., Ruppert, L.F.,

and Pontilillo, James, 1996,

Petrography and geochemistry of the

San Miguel lignite, Jackson Group

(Eocene), South Texas: Organic

Geochemistry, v. 24, no. 2 pp. 197-

217.

Zielinski, R. A., 1985, Element mobility

during alteration of silicic ash to

kaolinite--a study of tonstein:

Sedimentology, v. 32, p. 567-579.
U.S. Geological Survey Open-File Report 99-301

37

CHAPTER 4

Some speculations on coal-rank anomalies 

of the South Texas Gulf Province and adjacent

areas of Mexico and their impact on coal-bed 

methane and source rock potential

By John R. SanFilipo

U.S. Geological Survey, 956 National Center, Reston, VA 20192

INTRODUCTION

There are two known occurrences of

bituminous coal within the generally lignitic

Gulf of Mexico Coal Province. One occurs in

the Olmos Formation (Maastrictian) and the

other occurs in the lower-middle part of the

Claiborne Group (middle Eocene, fig. 1).

Both occur within the Rio Grande vicinity of

south Texas and Mexico, and each can be

considered somewhat anomalous in rank in

comparison with typical U.S. coals of similar

age, which are generally less mature.

CRETACEOUS BITUMINOUS COALS

The Olmos Formation coals, which will

not be visited on the field trip, occur in the

Eagle Pass area of Maverick County, Texas,

where a permit to mine is pending (Dos

Republicas permit, fig. 2). This deposit

extends into the Sabinas Basin of Mexico,

where high-volatile C bituminous coal is being

mined for electrical power generation by the

Minera Carbonifera Rio Escondido (MICARE)

for the Comission Federal de Electicidad

(CFE) Carbon I and II projects (fig. 2). Core

samples of these high-ash coals from the Eagle

Pass area are typically about 7500 Btu/lb on an

as-received (AR) basis and about 12,000

Btu/lb on a moist, mineral matter-free

(MMmF) basis. Samples from Carbon I and II

are reported to have an average (Rm?) vitrinite

reflectance of 0.58% (Verdigo and Arciaga,

1991). These and other rank parameters for

Sabinas Basin coals are somewhat higher than

those for most shallow occurrences of Upper

Cretaceous coals of the southern Rocky

Mountain Province, for example the southern

part of the San Juan Basin of New Mexico, but

they are consistent with rank-elevated coals

towards basin interiors or near igneous

provinces on basin margins. The rank

elevation of the Olmos Formation coals can

probably be attributed to residual heat flow

from slightly younger shallow intrusive activity

of the Balcones igneous province, or much

younger igneous activity within Mexico (fig.

2). 

TERTIARY BITUMINOUS AND

 RELATED COALS

The second documented occurrence of

bituminous coals in the Gulf Province is the

Santo Tomas coal field in Webb County,

Texas, which will be visited on the field trip.

Here the Farco Mining Company (fig. 2) is

mining high-volatile A bituminous coal from

the lower-middle part of the Claiborne Group
U.S. Geological Survey Open-File Report 99-301

38

 SOUTH TEXAS

(RIO GRAND EMABAYMENT)

EAST-CENTRAL TEXAS

(HOUSTON EMBAYMENT)

JACKSON GROUP CLAIBORNE GROUP WILCOX GROUP MIDWAY GROUP NAVARRO GROUP

CAMPANIAN

TAYLOR

MAASTRICHTIAN LOWER MIDDLE UPPER LOWER UPPER

PALEOCENE EOCENE

CRETACEOUS TERTIARY

(south Texas)

upper

lower

Whitsett

Manning

Wellborn

Caddell

Yegua Yegua

Laredo

Sparta

Weches

Queen City

El Pico

Bigford

Carrizo Carrizo

Reklaw

Indio

Calvert Bluff

Simsboro

Hooper

Kincaid

upper

lower

Escondido Marlbrook

 Marl

Olmos

Anacacho

Limestone

San Miguel

Cook

 Mtn.

Tj

Ty

Tl

Tep

Tb

Tc

Tr

Tin

Tk Tm

Kes

Ko

Ksm

Kac

Tcm

Ts

Tw

Tqc

Twi

STRAT COLUMN SYMBOLS

Fluvial Carbonate

Fluvial-deltaic

Deltaic (fluvial -

dominated)

Delataic (wavedominated)

Strandplain/barrier-bar

Lagoonal

Marine/prodelta shale

Marl

Wilcox and above modified 

from Ayers and Kaiser, 1987

Bituminous coal

Lignite

sand

mud

Symbols (e.g. Ksm) where shown for formation

 names refer to geologic map (fig. 2)

EXPLANATION

Ki

OTHER MAP UNITS

San Miguel Mine

Santo Tomas Field

 (Farco Mines)

Uvalde area

Eagle Pass / MICARE

Kl Lower Cretaceous undivided

Upper Cretaceous undivided

 (includes Olmos in places) Ku

Intrusive rock of the

Balcones Province Ki

Ti Tertiary intrusive rocks

Qb Quaternary basalt

Figure 1. Generalized columnar section for south Texas coal areas and adjacent areas of Mexico.
U.S. Geological Survey Open-File Report 99-301

39

Kac

Kac

Ksm

Kol

Kes

Kes

Tm

Tk

Twi

Tin

Tin

Tc

Tc

Tr

Tb

Tb

Tqc

Tep

Tl

Tw Ts

Tcm

Ty

Tj

Tep

Ku

Ku

Tk

Kol

MEXICO

U.S.

101 100

28

27

Laredo

Geology modified from published maps of the Texas Bureau of Economic Geology and the Govt. of Mexico.

Ki

Ki

Ki

Ki

Balcones Igneous 

 Province

Ki

Qb

Ti

Ti

99

29

Ksm

Kl

Kl

Kl

 CFE 

Power

 Plant

MICARE

 Mine

 Blocks

 Dos

Republicas

 Permit

FARCO

MINING

 CO.

Eagle Pass

Uvalde

Qb

Qb

0 50 km

30 mi

Ku

Ki

INDEX MAP

TX

N

BITUMINOUS

SUBBITUMINOUS

LIGNITE

Figure 2. Regional geology and location of bituminous coal deposits of south Texas and adjacent

areas of Mexico. See figure 1 for map units. Also shown are speculative boundaries of coal-rank

changes.
U.S. Geological Survey Open-File Report 99-301

40

for use in local cement plants and for export as

lump coal for use in the Republic of Ireland.

The coal occurs in two primary zones, the San

Pedro and Santo Tomas zones, near the

ill-defined contact of the Bigford and overlying

El Pico Formations, about 3,000 ft

stratigraphically higher than the Olmos

Formation (fig. 1). Speculations on the cause

of the relatively high rank of these coals are

more problematic than those for the Olmos

coals. Because these coals occur higher in the

section than Wilcox Group coals from the

same general area, which have traditionally

been described as lignite (Maxwell, 1962,

Breyer and McCabe, 1986), depth of burial

would presumably not account for the rank

elevation. Numerous lignite beds have also

been described as intercalated within the San

Pedro – Santo Tomas canneloid coals

themselves (Lonsdale and Day, 1937), which

would also presumably rule out depth of burial

or local thermal gradients as the cause of rank

elevation. 

Based on these considerations, it

would be logical to assume that the highly

sapropelic nature and enriched liptinite content

(reported to be 23% in Warwick and Hook,

1995) of these coals could account for their

anomalously high heating value (~12,000

Btu/lb AR, 14,000 MMmF, figs. 3 and 4),

without an added thermal factor. A

re-examination of nearby Wilcox coals

suggests, however, that a much wider area and

stratigraphic range of South Texas has been

subjected to elevation in rank, probably by

local heat flow, than previously thought.

Recalculating the Wilcox coal quality

parameters for the Uvalde area (fig. 2) from

Maxwell (1962, tables 30, 31, and 32) gives a

range of MMmF Btu/lb from 10,494 to

11,893, (i.e. subbituminous A to high-volatile

C bituminous). Note that sample no. 1-A from

Maxwell's table 30, which is lignite on an

MMmF basis, is clearly weathered, and some

other samples from his tables appear to have

lost moisture; normalizing to 15% moisture

consistent with table 32 results in an apparent

rank of subbituminous B to high-volatile C

bituminous. Proprietary analytical data

recently made available to the USGS for the

National Coal Resource Assessment supports

these determinations. Together they indicate

that on the basis of heating value, the Wilcox

coals range from subbituminous B just east of

Uvalde to high-volatile bituminous A towards

the west, although even the most mature of

these coals is generally described as lignite in

company drilling records. An average

reflectance value of 0.37% (Rm; range

0.30-0.45%) reported by Breyer and McCabe

(1986) from an area just east of Uvalde is

somewhat equivocal, but consistent with

subbituminous coals of the Powder River

Basin of Wyoming (R.W. Stanton, USGS,

personal communication). These values can be

compared with some previously unpublished

rank parameters and standard analysis of the

Santo Tomas and San Pedro coal zones and

typical Wilcox lignites from the Sandow mine

near Austin, approximately 250 mi northeast

of the Santo Tomas field, in figures 3 and 4.

To the author's knowledge, no

published analysis of the intercalated "lignites"

of the Bigford-El Pico interval exist. One

highly weathered sample of a banded,

somewhat splintery, stray bed that occurs

within the San Pedro interval about 40 ft

above the main bed at the Farco Trevino Mine

has been analyzed by the USGS with equivocal

results (sample SP-UN-96-1, figs. 3 and 4).

On a Btu basis, this coal would seem to be

classified as lignite; however, an average Rmax

of 0.74% and extremely high
U.S. Geological Survey Open-File Report 99-301

41

Canneloid

 coal

Lignite Carb 

shale Siltstone Claystone

Dirty coal

(> 25% ash,

 d.b.)

AL-H2-6-1

AL-H2-P

AL-H2-8-1

NA (pollen sample)

SANDOW MINE (lower Calvert Bluff Fm)

?

10.5 ft

ST-UN-96-1

STR2-96-1

SP-UN-96-1

4.7 ft

ST2-96-3

ST2-96-2

ST2-96-1

SP-3-96-2

SP-3-96-1

NA (pollen sample)

 USGS

SAMPLE NO.

% Sulfur 

(as-rec'd)

 BTU 

(as-rec'd)

TREVINO MINE (San Pedro Zone) RACHAL MINE (Santo Tomas Zone)

 2,000 ft

∼

40 ft ∼

40 ft ∼

 M. CLAIBORNE WILCOX

6.58 77.98 12.34

1.45 1.65

PROXIMATE ANALYSIS

 (as-rec'd)

ULTIMATE ANALYSIS

 (dry, ash-free)

%Moist %Ash %VM %FC

0

100

101520253035404550

5

556065707580859095

0

2,0004,0006,0008,000

10,00012,00014,000

0

0.51.0

1.52.02.5

101520253035404550

5

556065707580859095

0

100

2.90

42.82 32.88 21.40

6.97

27.72 37.81 27.50

5.07 11.33

48.42 35.18

10.38 51.73 22.50 15.39

26.56 32.62 26.51 14.31

2.22 13.32 51.61 32.85 3.19

19.70 45.09 32.02

34.35 14.38 27.12 24.15

33.87 9.90 29.60 26.63

7739 0.90

8622 2.28

12151 1.02

4678 0.55

3449 1.37

12730 0.86

11499 0.99

6497 1.53

7284 1.14

6.32 72.03

1.21 3.49

6.49 79.71 1.38

1.22

11.20

6.47 70.37

1.11 1.45

20.60

3.20 61.38

2.18 3.35

29.89

6.69 80.36

1.64 1.02

10.29

6.66 80.14

1.79 1.28

10.13

5.41 73.46

1.43

2.98 16.72

5.54 75.13

1.41

2.03 15.89

16.95

ATOMIC RATIOS

 (dry basis)

H/C O/C C/N C/S 

1.00 0.12 63 126

1.04 0.18 69 55

0.97 0.11 67 174

1.09 0.22 74 130

0.62 0.37 33 49

0.99 0.10 57 210

0.99 0.09 52 167

0.88 0.17 60 66

0.88 0.16 62 99

%H %C %N %S %O

0

1 ft

0.5

Figure 3. Standard analysis and elemental ratios for selected samples from the Santo Tomas coal

field (middle Claiborne) and Sandow Mine (Wilcox).
U.S. Geological Survey Open-File Report 99-301

42

volatile-matter content imply a rank more

similar to the canneloid coals. Other

maturation parameters (apparent specific

gravity, residual moisture, agglomerating

character, etc.) are more consistent with

Wilcox lignite, although the weathered nature

of the sample is readily apparent form the high

oxygen content, and none of these results can

be considered definitive. The higher

reflectance value for the stray bed versus the

main San Pedro – Santo Tomas beds as shown

in figure 4 may be attributable to

weathering-related diminution of the vitrinite

suppression typical in canneloid coals. This

sample fluoresces when examined under blue

light and shows algal cell structures similar to

the San Pedro and Santo Tomas canneloid

coals (P.D. Warwick, USGS, personal

communication).

EXTENT OF THE COAL-RANK

ANOMALY AND ITS POSSIBLE

IMPACT ON HYDROCARBON

GENERATION

As previously stated, it now appears

that a much wider area of South Texas has

been subjected to coal-rank elevation than has

previously been recognized. Early versions of

the "Coal Fields of the United States" map

(e.g. Campbell, 1908, through Trumball, 1960)

showed small pods of bituminous coal within

the overall lignite trend, and no intervening

subbituminous halos. The area of

hypothesized rank elevation was slightly

expanded for a preliminary revision to this map

(Tully, 1996), without the benefit of most of

the new data or rank recalculations cited

herein. Based on the information currently

available, additional revisions to the map that

will show a much wider and more graduated

area of rank elevation in South Texas are being

proposed, as shown in figure 2. Any such

revisions are speculative, however, without

analysis of fresh samples of the intercalated

stray beds in the El Pico – Bigford interval and

additional reflectance values and analyses from

the Wilcox, Jackson and Yegua intervals (figs.

1 and 2). 

Coal-bed methane (CBM) shows have

been recorded in some gas wells in Maverick

and Dimmit counties, and a number of gas

fields occurring in the general area produce

from Olmos and Wilcox sandstones (see

Ewing this volume). The extent of the

coal-rank variation within South Texas can

therefore be considered particularly important

from the consideration of the CBM and

conventional source-rock potential. The H/C

and O/C ratios from figures 3 and 4 are plotted

on a Van Krevelen diagram in figure 5, along

with data for Olmos Formation coals derived

from various sources (Evans,1974, table 5;

Miller and others, 1998; and unpublished

USGS files). A few examples of Jackson

Group coals, which are generally not discussed

in this paper, but will be seen on the field trip,

are also shown for completeness; these are

from unpublished USGS sampling of the San

Miguel Mine. As to be expected, the Wilcox,

Olmos, and Jackson coals plot within the type

III kerogen path, and the less humic Santo

Tomas and San Pedro coal zones plot closer to

the type II field. The Santo Tomas coals,

which appear slightly more oil prone on the

diagram, yielded 52 gal/ton oil and 5672 SCF

per ton gas from low-temperature distillation

done by the U.S. Bureau of Mines (Ashley,

1919). Rock-Eval pyrolysis for one Santo

Tomas sample by Mukhopadhyay (1989, fig.

28a), which yielded 66% total organic carbon

and very high and low hydrogen and oxygen

indices respectively, placed these coals directly

on the type II kerogen field.
U.S. Geological Survey Open-File Report 99-301

43

AL-H2-6-1

AL-H2-P

AL-H2-8-1

NA (pollen sample)

SANDOW MINE (lower Calvert Bluff Fm)

?

1.29

1.32

52.65 47.35

52.89 47.11 1.35

0.33***

0.39***

10.5 ft

ST-UN-96-1

STR2-96-1

SP-UN-96-1

4.7 ft

ST2-96-3

ST2-96-2

ST2-96-1

SP-3-96-2

SP-3-96-1

NA (pollen sample)

 USGS

SAMPLE NO.

200040006000

8000

10,000

12,00014,000

16,000

% Volatile 

 matter

505254565860

62

64

66

60.57

% Fixed 

 carbon

Apparent Sp. Gr.*

(as-rec'd)

57.90

57.91

59.38

64.93

% Residual 

 moisture

1.381.0

2.03.04.05.06.07.08.09.0

0

BTU (MMmF)

TREVINO MINE (San Pedro Zone) RACHAL MINE (Santo Tomas Zone)

1.54

1.43

1.24

1.68

1.01.1

1.21.31.41.51.61.71.8

1.13

1.201.16 1.12

1.63

1.77

3335

37

394143454749

39.43

5,281

14,446

0.53**

**approx. interval, sampled in 1994

***modified from Rm in similar interval,

 adjacent pit, Mukhopadhyay, 1989

12,365

13,876

10,611

7,677

8,149

40.62

42.09

42.10 2.13

2.14

2.06

9.26

 2,000 ft ∼

∼40 ft

∼ 40 ft

8.84

35.07 0.74

8.51

0

14,901

14,645

61.11

58.47

38.89

41.53

1.12

1.22

1.26

1.39

1.32

0.51**

1.11

 M. CLAIBORNE WILCOX

Canneloid coal Lignite Carb shale Siltstone Claystone

Dirty coal

(> 25% ash, 

 d.b.)

 Reflectance,

Avg. Rmax (oil)

*calculated from as-rec'd values using 

 an assumed ash density of 3.0 g/cc

.22

.30.38

.46.54.62.70.78.86

Free-swelling

 Index

1.0

0.5

1.0

0

0

1.0

0

0

0

1 ft

0.5

except as noted, all are dry, ash-free

 basis

Figure 4. Selected rank parameters for selected samples from the Santo Tomas coal field (middle

Claiborne) and Sandow Mine (Wilcox).
U.S. Geological Survey Open-File Report 99-301

44

The remarkable ease of ignition that enables

these coals to be exported in considerable

quantities for domestic use is further evidence

of their oil-generating potential. Most of the

Olmos data shown on figure 5 approaches the

lower limits of hydrocarbon generating

maturity. In addition to being somewhat

oil-prone in composition, these coals may lack

the permeability to adsorb gas or produce

CBM commercially.

The Olmos Formation data shown on

figure 5 is generally consistent with that of a

somewhat more mature humic coal than the

other samples. Available coal-quality data for

the Olmos Formation is, however, scarce and

somewhat questionable. The data shown on

figure 5 from the MICARE mine samples

(Miller and others, 1998) shows consistently

lower oxygen values than the other Olmos

samples, and thus more apparent maturation,

placing them well beyond fluid hydrocarbon

generation thresholds. While this conceivably

could reflect increasing rank towards local

igneous activity, it seems more probable that

this is an analytical bias. Other data from the

MICARE mines (Verdigo and Arciaga, 1991,

table 2) show higher average oxygen values,

but the reporting basis is not clear. The Dos

Republicas sample analysis appears to be

reliable, but these and other recent analyses

from the Eagle Pass area show a very wide

range of ash, and corresponding elemental

values where available, that may be

attributable to sampling techniques. The "old

Eagle Pass" samples on figure 5 came from a

variety of mine sources sampled before 1912

(see Evans, 1974), including unwashed and

washed samples, with some apparent drying,

and possibly weathered. The available data on

heating value for the MICARE coals is also

inconclusive in this regard. The Olmos data

shown on figure 5 represent an attempt to

select "typical" samples with relatively similar

ash values from the various data sets, with the

exception of the "old" samples, where the

widest range is shown. The two San Miguel

samples selected for the diagram are probably

slightly atypical, in that they are at the higher

end of the recorded heating values for USGS

samples from that mine. These and the

Sandow samples shown on the diagram fall in

the relatively less mature end of the humic

pathway as expected. All of the oxygen values

used for figure 5 are by difference rather than

direct measurement, with the possible but

unlikely exception of the "old Eagle Pass"

samples. 

No elemental data is currently available

to the USGS for Wilcox, Yegua, or Jackson

coals from the higher rank areas shown in

figure 2. Based on their location, at least some

of the reported CBM shows in the area are

without question from Olmos Formation coals.

It appears likely that a considerable area of

Wilcox coals may also have reached thermalgas-generating maturity in the South Texas

area. Although highly speculative, based on

the extent of Tertiary volcanism in nearby

areas of Mexico, it appears that Yegua and

Jackson coals could have been similarly

elevated in rank as far south as Falcon

Reservoir (about 90 mi southeast of Laredo,

fig. 2). In a detailed study of the petrology

and organic geochemistry of Texas coals,

Mukhopadhyay (1989) suggested that many

Texas coals are enriched in liptinite relative to

other coals of similar rank and may have

generated liquid hydrocarbons earlier on the

maturation pathways than is normally

expected. Although very few of his samples

are from South Texas, where the maturation

potential would seem highest, he suggests (p.

102) that some of the oil produced from the
U.S. Geological Survey Open-File Report 99-301

45

1.5

1.0

0.5

0 0.1 0.2 0.3 0.4

Jackson (SanMiguel)

Claiborne (Santo Tomas)

Olmos (Dos Republicas)

Olmos (MICARE)

Olmos (old Eagle Pass)

Wilcox (Sandow)

Type III

Type II

Type I Kerogen

Atomic O/C

Atomic H/C

Arrows represent mean maturation paths

Figure 5. Van Krevelen-type diagram for various coal types of South Texas and vicinity.

Wilcox and Claiborne intervals of South and

East Texas may be sourced in the Wilcox. 

One potential constraint on commercial

CBM viability in South Texas may be the

thickness of the coal beds, which for individual

beds is generally less than ten feet thick.

There are multiple beds in each of the

coal-bearing intervals, however, and in general

coal thicknesses beyond shallow (i.e. mineable)

depths have not been thoroughly investigated.

It should be noted that nothing in this paper

addresses the potential for early or late

biogenic gas occurrences in Texas coals, which

should be considerable based on recent activity

in the Powder River Basin of Wyoming. It is

also worth noting that although the

coal-bearing facies of the Olmos Formation is

mapped with an eastward pinch-out into

marine rocks a few miles southwest of Uvalde

(fig. 2), anecdotal evidence presented in

Maxwell (1962, p. 90) suggests that

Cretaceous coal-bearing rocks may be present

somewhat farther east and north. 

CONCLUSIONS

Available information based on limited

and poorly controlled sampling and analysis

suggests that the coal rank anomalies

recognized in the Olmos Formation and middle

Claiborne Group of South Texas and adjacent

areas of Mexico may extend to a considerably
U.S. Geological Survey Open-File Report 99-301

46

wider area and stratigraphic range. Additional

sampling and analysis of the entire interval

from the Upper Cretaceous Olmos Formation

through the upper Eocene Jackson Group

would be useful in refining the known extent

of rank variation. Coal-bed desorption and

drill-stem or more sophisticated down-hole

testing would help determine the methaneproducing potential of these coals. Biomarker

and stable isotope studies of hydrocarbons

produced from more conventional reservoirs in

the area may also provide useful information

on the source-rock potential of these coals. 

ACKNOWLEDGMENTS

The author wishes to thank the staff of

the Farco Mining Company and the Alcoa

Sandow Mine for their permission to visit and

sample their mines, and the USGS staff who

collectively participated with me in the

sampling used for this report: Peter Warwick,

Robert Hook, and Sharon Crowley.

REFERENCES CITED

Ashley, G. H., 1919, The Santo Tomas cannel

coal, Webb County, Texas: U.S.

Geological Survey Bulletin 659, p.

1-127.

Ayers, W. B., Jr., and Kaiser, W.R., 1987,

Regional depositional setting o,

resources, and quality of lignite in the

Wilcox Group of east Texas and the

Jackson Group of east and South

Texas, in Finkelman, R.B.,

Casagrande, D.J., and Benson, S.A.,

eds., Gulf Coast lignite geology,

Prepared for the fourteenth biennial

lignite symposium on the technology

and utilization of low-rank coals:

Environmental and Coal Associates,

Reston, Va., p. 83 – 128. 

Breyer, J.A., and McCabe, P.J., 1986, Coals

associated with tidal sediments in the

Wilcox Group (Paleogene), South

Texas: Journal of Sedimentary

Petrology, v. 56, no. 4, p. 510-519.

Cambell, M. R., 1908, Coal fields of the

United States: U.S. Geological Survey,

scale1:7,000,000.

Evans, T.J., 1974, Bituminous Coal in Texas:

Bureau of Economic Geology,

University of Texas at Austin,

Handbook 4, 65 p., 1 pl..

Lonsdale, J.T., and Day, J.R., 1937, Geology

and groundwater of Webb County,

Texas: U.S. Geological Survey Water

Supply Paper 778, p. 1-104.

Maxwell, R. A., 1962, Mineral resources of

South Texas, region served through

the Port of Corpus Christi: Bureau of

Economic Geology, University of

Texas at Austin, Report of Inv. No.

43, 140 p., 5 pl., 44 tables. 
U.S. Geological Survey Open-File Report 99-301

47

Miller, J.D., Parga, J.R., Drelich, Jaroslaw,

and Lin, C.L., [1998?], Coal cleaning

opportunities for SO2 emission

reduction in the border region, Final

Report to the U.S. Southwest Center

for Environmental Research and

Policy: University of Utah, Salt Lake,

and Institute of Technology, Saltillo,

Mexico, 37 p.

Mukhopadhyay, P.K., 1989, Organic

petrology and organic geochemistry of

Texas Tertiary coals in relation to

depositional environment and

hydrocarbon generation: The

University of Texas at Austin, Bureau

of Economic Geology Report of

Investigations 188, 118 p.

Trumball, James, 1960, Coal fields of the

United States: U.S. Geological Survey,

sheet 1 of 2, scale1:5,000,000.

Tully, John, digital compiler, 1996, Coal fields

of the conterminous United States:

U.S. Geological Survey Open-File

Report 96-92, 1:5,000,000.

Verdigo, F.D., and Arciaga, M., 1991, The

Fuentes-Rio Escondido coal basin,

Coahuila, in Salas, G. P., ed.,

Economic Geology, Mexico: Boulder,

The Geological Society of America

(Decade of North American Geology),

Geology of North America, V.P-3, p.

107-128.

Warwick, P.D., and Hook, R.W., 1995,

Petrography, geochemistry, and

depositional setting of the San Pedro

and Santo Tomas coal zones:

anomalous algae-rich coals in the

middle part of the Claiborne Group

(Eocene) of Webb County, Texas:

International Journal of Coal Geology

28, p. 303-342.

.