field trip guidebook - usgs · (chapter 4). the field trip will visit two active mines in south...

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

Tertiary Coals in South Texas: AnomalousCannel-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 ConventionEnergy Minerals Division

Field Trip # 15April 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 InteriorU.S. Geological Survey Open-File Report 99-301

This report is preliminary and has not been reviewed for conformity with U.S. Geological Surveyeditorial 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........................................ 1Itinerary.............................................................................................. 2Other Information.............................................................................. 4

Chapter 1 Regional geologic setting of South Texas and the Santo Tomas coal depositsThomas E. Ewing................................................................................................. 5

Introduction.......................................................................................... 5General 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............................................................... 13Introduction...................................................................................... 13Geologic Setting and Stratigraphy..................................................... 13Mining History.................................................................................. 16Depositional Setting.......................................................................... 17Coal Characteristics........................................................................... 18Summary..................................................................... ..................... 20Acknowledgments............................................... ............................. 21References Cited.................................................. ............................. 21

Chapter 3 The San Miguel lignite deposit, Jackson Group (Eocene), South TexasPeter D. Warwick, Sharon S. Crowley, Leslie F. Ruppert, and JamesPontolillo........................................................................................................... 23

Introduction....................................................................................... 23Geologic Setting................................................................................. 23Lignite Resources and Mining............................................................. 24Character of the San Miguel Lignite.................................................... 26

Megascopic characteristics....................................................... 26Proximate analyses................................................................... 28Major, Minor, and Trace element data...................................... 29Mineralogy 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................................................................................... 33Summary............................................................................................ 34Acknowledgments.............................................................................. 34References 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............................................................................................... 37Introduction....................................................... ............................... 37Cretaceous bituminous coals............................................................... 37Tertiary bituminous and related coals................................................... 37 Extent of the coal rank anomaly and its possible impact on hydrocarbon generation...................................................................... 42Conclusions........................................................................................ 45Acknowledgments.............................................................................. 46References 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.................................................................................................. 7Figure 3. Subsurface structure, Top Middle Wilcox marker, Santo

Tomas area............................................................................................ 9Figure 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............. 14Figure 2. Representative corehole records from Vaughn (1900) that

include both coal zones........................................................................ 15Figure 3. Bed sections and sample intervals of the San Pedro, Santo

Tomas, and Santo Tomas rider coal beds.............................................. 16Figure 4. Selected proximate and ultimate data................................................... 19Figure 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 forthose elements inferred to be derived from volcanic ash.................................................................................... 30

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


1

Introduction

Tertiary coals in South Texas: Anomalouscannel-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 MexicoCoastal Plain of North America contains avariety of depositional settings and coal types.The coal-bearing region extends westwardfrom Alabama and Mississippi, acrossLouisiana to the northern part of theMississippi Embayment, and then southwardto eastern Arkansas, Texas and northernMexico (fig. 1). Most of the coal currentlymined in Texas is lignite from the upper part ofthe Wilcox Group (Paleocene-Eocene) and, inLouisiana, lignite is mined from the lower partof the Wilcox (fig. 2). Gulf Coast coal is usedprimarily as fuel for mine-mouth electricplants. On this field trip we will visit the onlytwo non-Wilcox coal mining intervals in theTexas-Louisiana Coastal Plain; these includethe San Pedro - Santo Tomas bituminouscannel-like coal zone of the Eocene ClaiborneGroup, and the San Miguel lignite coal zone ofthe Eocene Jackson Group (fig. 2). Othercoal-mining areas in northern Mexico arecurrently producing bituminous coal from theCretaceous Olmos Formation of the NavaroGroup (fig. 2).

The Claiborne-Jackson coal-bearingintervals of south Texas are very different fromthe Wilcox coal-bearing intervals found incentral and northeastern Texas and

northwestern Louisiana. The cannel-like coaldeposits of south Texas are unique and are oneof the few mined cannel-like coal deposits inthe world. This field guidebook containspapers that focus on the regional geologicsetting of south Texas (Chapter 1), and paperson the Santo Tomas and San Pedro coaldeposits and San Miguel lignite deposits(Chapters 2 and 3). Finally, there is anoverview paper on the anomalous coal-rankchanges of south Texas, and a discussion ofthe source-rock and coal-bed methanepotential for the south Texas coal fields(Chapter 4).

The field trip will visit two active minesin South Texas and will provide theparticipants an opportunity to observe themining operations and coal geology of thesurface mines. The older Claiborne Groupcoals that occur near the U.S.-Mexico border(fig. 1), are long-known for their cannel-likecharacter and will be seen in workings of theFarco Mining Company. These unusualnonbanded coals have high calorific values(3800 to 6000 kcal/kg or 6800 to 10,800 btu,dry basis) and substantial oil and gas yieldsupon distillation. Recent petrographic andgeochemical investigations have shown thatthese coals are bituminous in rank, hydrogen


2

Mining areas

Jackson GroupClaiborne GroupWilcox GroupNaheola Formation

Olmos Formation

Geologic Units

600 miles3000

Paleocene

Eocene

U. Cretaceous

USAMexico

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. Thefield trip leaders will discuss the depositionalsetting, petrology, geochemistry, and regionalgeology of these coal deposits.

The lignites of the San Miguel deposit(Jackson Group, Eocene), differ greatly fromthe coals of the border area. Four principlelignite beds comprise the San Miguel deposit.The beds are separated by reworked volcanicash partings (generally less than 0.25 m or0.75 ft thick). Drill records and in-minestudies indicate that these lignites weredeposited in environments that were partiallymarine influenced. The geochemistry and

depositional setting for these lignites, andregional South Texas/Northeastern Mexicocoal geology will be discussed. The field tripwill spend the first night in Laredo, a U.S.-Mexico border town, and will visit the twosurface mines the next day.

ITINERARY

This is a tentative schedule based uponideal weather and traffic conditions.

Wednesday, 14 April

4:30 PM - depart H.B. Gonzalez Convention


3

Eo

cen

eP

aleo

cen

eU

pp

erC

ret.

Claiborne Group

Navarro Group

Jackson Group

Wilcox Group

Midway Group

Figure 2. Simplified stratigraphy of coal-bearing intervals in the U.S. Gulf of MexicoCoastal Plain. Arrows indicate stratigraphicposition of major coal-bearing intervals.

Center in San Antonio for Laredo, a nonstopdrive 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 atLa Posada. We will provide an overview ofSouth Texas coal geology and givesuggestions 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 PalafoxMine Office. David Wadsack and GustavHolm, hosts. Safety training in office.

Stop 1: Trevino Mine. Observe San PedroCoal (Bigford Formation) and overlying tidaldeposits. Discuss unique mining methods.

Stop 2: Palafox Mine. Observe San PedroCoal, overlying sandstone and mudstonedeposits, and thin coal seam.

Stop 3: Coal yard. Discuss coal preparationand utilization.

10:30 AM - return to Farco office forconvenience stop; depart for San Miguel Mineat 10:45 AM.

Optional Stop A: FM 1472 roadcut east ofSanto Tomas. Observe coastal-barrier faciesin El Pico Clay (10 minutes).

12:15 PM - lunch stop (30 minutes), PicnicArea south of Cotulla at Mile Marker 59 onI-35 North. This is a no-toilet rest area.

1:00 PM - convenience and gas stop (15minutes) at Mobil station in Cotulla at junctionof Business I-35 and Texas Route 97.

2:15 PM - arrive North American Coal's SanMiguel Mine Office. Roger Fish, EngineeringManager, host. Safety certification and mineoverview at office.

Stop 4: E area final pit. Observe completedhighwall exposure of Jackson Group fromspoil bank. Discuss lithofacies overlying coal,ash disposal, and reclamation methods.


4

Stop 5: E area, active pit. Observe coalseams and splits, discuss unique miningmethods.

Optional Stop B: B area.

4:30 PM - depart San Miguel Mine for SanAntonio.

OTHER INFORMATION

Both Farco Mining and North American Coalrequire hard hats, safety glasses, and steel-toeboots.

Exercise extreme caution at all times when onmine property. Do not approach highwallexposures without the consent of companypersonnel.

Coal sampling and photography are permittedby both operators. Later publication ofcoal-quality data or mine photographs requirescompany consent and acknowledgment.

We will stop at a mandatory U.S. BorderInspection station north of Laredo onThursday morning. No illegal substances orfirearms should be on your person or in yourbelongings. Non-U.S. citizens should have theappropriate travel papers available forpresentation.


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 thatthe U.S. Geological Survey has recentlyconducted in South Texas on the coal geologyof the Eocene Claiborne Group of the Laredoarea (Warwick and Hook, 1995) and theJackson Group of the San Miguel lignite minearea (Warwick and others, 1996). I will firstpresent a general overview of the trip route inrelationship to the major structural andstratigraphic features of South Texas. Then Iwill present subsurface well log data to showthe correlation of the coal zones into thesubsurface, and place them in a regionalcontext.

GENERAL OVERVIEW OF THE FIELD TRIP ROUTE

When we leave San Antonio, we willproceed southwest along Interstate 35 towardsthe city of Laredo. San Antonio is an oasiscity, built around springs and deep wellsdrawing water from the Lower CretaceousEdwards Aquifer along the (Miocene-age)Balcones fault zone. The city is built on rocksranging in age from Albian (EdwardsLimestone) to Paleocene (lower Wilcox Group), with younger rocks generallyto the southeast.

Tectonically, the San Antonio area lieson the southwest flank of the San MarcosArch (fig. 1), a broad southeast-trending area

of lesser subsidence than surrounding areasduring Mesozoic and Cenozoic time. Duringthe Early Cretaceous, the area was repeatedlyoccupied by extensive carbonate platforms (theSligo Limestone of Aptian age and theEdwards Group carbonates of Albian age; fig.2). As regional sedimentation shifted fromcarbonates to siliciclastic material in the LateCretaceous (Campanian - Maastrichtian), theSan Antonio area lay east of the major SanMiguel and Olmos wave-dominated deltas ofSouth Texas, and mainly marine shales andthin sandstones were deposited. In thePaleogene, Wilcox, Carrizo and youngersandstones were deposited in the San Antonioarea, which lay in the transition between thelargely strike-oriented strandplain, wave-dominated delta and bar - lagoon systems ofSouth Texas and the fluvial-deltaic deposits ofEast Texas.

As we drive southwest toward Laredo,we cross the Rio Grande Embayment, a broadsynclinal area between the San Marcos Arch ofCentral Texas and the Laramide fold systemsof Mexico (fig. 1). The present Rio Grandeflows along the southwest side of thisembayment. The road slowly climbs thestratigraphic section, reaching the late middleEocene ("Laredo Formation," Sparta andCook Mountain equivalent; fig. 2) at Laredo.South of Cotulla (fig. 1), the road crosses theburied Stuart City Reef margin of the EdwardsGroup carbonate platform. The reef margin isgas-productive, and also controls faulting and


6

Figure 1. Route of the 1999 EMD field trip, with major tectonic features and outcrop beltsshown. Open boxes represent locations of Upper Cretaceous coal deposits; “X” representsigneous rocks; Kig, Tig, QTig = Cretaceous, Tertiary, and Quaternary igneous rocks.

shallower marine sandstone development inthe Olmos Formation. At Laredo, the positionof the buried Sligo reefal shelf margin isreached. This reef contains a major gas fieldjust across the river in the western part ofNuevo Laredo (Totonaca field) at depths ofsome 13,000 ft.

Jose de Escandon founded Laredo, thechief port of entry from Mexico into Texas, inthe 1750s as the northernmost city of the newprovince of Nuevo Santander (the lower RioGrande valley). The early trade routes from

Monclova to San Antonio passed north of theLaredo area, but the location of Laredo on theshortest route from Monterrey to San Antonio,coupled with the main rail links to both citiesand to the port of Corpus Christi, ensuredLaredo's economic growth.

Laredo, and the Santo Tomas districtto its northwest, lie on the southwest flank ofthe Rio Grande Embayment. Dips aregenerally one degree or higher to the east inoutcrops near the river. Southwest intoMexico, the dips continue until the base of the


7

Figure 2. General northwest-southeast stratigraphic section of Mesozoic and Paleogene strataalong the Rio Grande, from Eagle Pass to Freer (fig. 1). The stratigraphic location of significantcoal and lignite deposits are shown.


8

Tertiary outcrops about 12.5 mi into theinterior. Several anticlinal axes, collectivelyreferred to as the "Salado arch" or the "Burro-Peyotes arch", trend southeastward, withintermittent exposures of Austin limestone onanticlines. The east to northeast dip off theflanks of the arch towards the Rio Grande iseasily 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 SanMiguel lignite mine (fig. 1). The trip east fromCotulla to Tilden proceeds over younger rocksthan any other part of the trip. Sandy YeguaFormation deposits are overlain by the lignite-bearing Jackson Group. The hills east ofTilden are underlain by the largelyvolcaniclastic Catahoula (or Gueydan)Formation of Oligocene age. Zeolites havebeen mined from the basal Jackson strata justwest of Tilden, and used as kitty litter. Northof Tilden, we cross San Miguel Creek (withnatural lignite exposures in its banks) andproceed to the San Miguel mine. After thisclassic Gulf Coast lignite deposit is viewed, wewill travel back to San Antonio across Eoceneand Upper Cretaceous rocks.

STRUCTURAL AND STRATIGRAPHICSETTING OF THE SANTO

TOMAS DISTRICT

The coal beds of the Santo Tomasdistrict lie within the Bigford Formation - ElPico Clay succession of the Rio GrandeEmbayment. These two formations correlatewith the Queen City cycle of the ClaiborneGroup, and are early Middle Eocene in age(fig. 2). In South Texas, the Queen Citysandstones were deposited in a mixed wave-dominated deltaic and barrier bar environment,

with a strike-oriented sandstone maximum ineastern Webb County about 30 miles east ofSanto Tomas (Guevara and Garcia, 1972).Northwest of the sand maximum, the upperpart of the cycle contains a low percentage ofsandstone and much lagoonal shale; this hasbeen called the El Pico Clay by Eargle (1968).The lower part of the cycle contains highpercentages of sandstone (Bigford Formationas redefined by Eargle from Trowbridge,1923), and rests directly on the CarrizoFormation and upper Wilcox sandstones withvery little break. The downdip marine Reklawformation passes westward into the Bigfordshoreline sandstones.

The contact between BigfordFormation and El Pico Clay is gradational.Sandstones do occur in the El Pico "Clay", andmay become more abundant northward awayfrom the Rio Grande. The transitional bedsbetween Bigford and El Pico are intimatelyassociated with the San Pedro and SantoTomas coal zones, as we will see.

To better understand the location ofthe Santo Tomas coal deposits within thestratigraphic section, a pilot study wasundertaken of well logs in the area around andeast of the mines. The area has beenextensively drilled for gas reserves in theWilcox Group sandstones (Palafox field) andin the Olmos Formation (Las Tiendas, BoothRanch, and La Cruz fields); gas has also beenfound in reefal Edwards carbonates in Galanfield, just south of the Rachal Mine (fig. 3).Many of these well logs have been released,and most were logged to within a few hundredfeet of the surface. A regional shale whichcorrelates to the Middle Wilcox can beidentified on all logs, and forms a goodstructural datum. The structure map (fig. 3)shows structural dip of 1.0-1.4 degrees to the


9

Figure 3. Subsurface structure, Top Middle Wilcox marker, Santo Tomas area. The contoured dataon the Santo Tomas and San Pedro coal beds are from Warwick and Hook (1995). Gas field namesare also shown.

east or east-northeast on the Middle Wilcoxmarker. This corresponds well to the 0.9-1.2degrees of dip seen in the Trevino and Rachalmine areas (data from Warwick and Hook,1995). From comparison of the MiddleWilcox data and the mine elevations, the SantoTomas bed at the Rachal Mine lies 1900±20 ftabove the Middle Wilcox marker. Less

certainly, the San Pedro seam at the Trevinolease lies about 1600-1650 ft above themarker, or 250-300 ft below the Santo Tomas.Elsewhere, however, the San Pedro is only 90ft below the Santo Tomas. This suggests thatt h e S a n P e d r o b e d s a r e n o t


10

Figure 4. Electric log, Hughes & Hughes #1 Palafox Exploration "B", showing the correlation fromsurface mines to subsurface stratigraphy. Location of well shown on figure 3. Vertical log scale inft.


11

wholly correlative, although there may beother solutions.

There is slight thickening of somestratigraphic units to the northeast, perhapsindicating structural growth on the Saladofolds during Eocene time. However, the totalamount of growth is not more than 100-150 ft.

The coal beds in the Rachal area can beconfidently projected into the Copano #A-1Palafox Exploration well, an Edwards testdrilled in 1962 about 3 miles SSE of theRachal mine (fig. 3). The correlation indicatesthat the coal beds correlate to a thin-beddedinterval lying above a 60-ft thick sandstoneand below a 100-ft thick shale unit. This shale("Pink" Marker) can be observed on logsthroughout the area mapped, and continuesdowndip at least to the Laredo airport (fig. 3).The sequence can be correlated to the east intothe Hughes and Hughes #1-B PalafoxExploration well, where the overlying clay-richsequence is also logged (fig. 4). The PinkMarker and the overlying sandstone can alsobe correlated to lithologic logs presented byLonsdale and Day (1937) and Warwick andHook (1995). Although the individual coalbeds cannot be definitely located on the welllogs (the coal may have some SP response?),the interval is reliably picked, and can becarried throughout the area. Downdip past theold workings towards Laredo, the intervalbecomes sand-rich and loses its thin-bedded,resistive character.

Due to the density of drilling in the LasTiendas area of northwest Webb County, itshould be possible, with detailed study of thesubsurface logs, to more closely define thedepositional geometry of the algal coals of theSanto Tomas district and estimate theirresource potential.

As an added point of interest in thearea, Kyle (1994, p. 41-46) has described a

significant surface occurrence of barite in thePinto Creek drainage, just northeast of theRachal mine. The barite occurs in clays thatlie 80-250 ft above the Santo Tomas seam.

REFERENCES CITED

Eargle, D.H., 1968, Nomenclature offormations 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 - gasreservoirs in the Queen City Formation(Eocene), Texas: BEG GeologicalCircular 72-4, 22 p.

Kyle, J.R., 1994, The barite industry andresources of Texas: Bureau ofEconomic Geology, The University ofTexas at Austin, Mineral ResourcesCircular 85, 86 p.

Lonsdale, J.T., and Day, J.R., 1937, Geologyand ground-water resources of WebbCounty, Texas: U.S. GeologicalSurvey Water-Supply Paper 778, p.1-104.

Trowbridge, A.C., 1923, A geologicreconnaissance in the Gulf CoastalPlain of Texas, near the Rio Grande:U.S. Geological Survey ProfessionalPaper 131-D, p. 85-107.


12

Warwick, P.D., and Hook, R.W., 1995,Petrography, geochemistry anddepositional setting of the San Pedroand Santo Tomas coal zones:anomalous algae-rich coals in themiddle 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 theSan Miguel lignite, Jackson Group(Eocene), South Texas: OrganicGeochemistry, v. 24, no. 2 p. 197-217.


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 coalzones of Webb County, Texas, are uniquewithin the realm of Gulf Coast coal geology.Coal beds within these zones are distinguishedby calorific yields and vitrinite-reflectancevalues that indicate a rank as high ashigh-volatile bituminous C, which isconsiderably greater than other Tertiary coalswithin the Gulf Coastal Province. In the past,the San Pedro and Santo Tomas wereidentified as cannel coal because of theirblocky, nonbanded character, and conchoidalfracture (Vaughan, 1900; Ashley, 1919;Lonsdale and Day, 1937). Unlike true cannelcoal known from the Upper Carboniferous ofthe Appalachians and Europe, the WebbCounty deposits are not particularly rich inspores or cuticular material and are notrestricted to local channel fills. Rather, theSan Pedro and Santo Tomas coals containabundant green-algae fructifications and occuras multiple tabular coal beds within zones thatextend for tens of miles along strike (Warwickand Hook, 1995).

Recent work has addressed thedepositional origin, petrography, andgeochemistry of the San Pedro and SantoTomas coal beds, but several basic questionsremain. The full extent of these deposits has

not been determined. In adjacent parts ofMexico, numerous cannel-like beds occurwithin the Claiborne Group (SPP, 1989), butno comparable deposits are known elsewherein Texas outside of Webb County. Thequestion of rank also cannot be resolved at thistime. Previously, we attributed the high rankof the San Pedro and Santo Tomas togeothermal influences (Warwick and Hook,1995). Although the suggestion of geothermalinfluence is consistent with data that indicatean elevated rank of Wilcox coal deposits inadjacent counties, additional study is requiredto confirm the origin and delineate the extentof such alteration. Likewise, the significanceof abundant green-algae fructifications in thesecoals, identified only from petrographic studyof polished blocks, cannot be judged fully atthis time because comparable polished blockpetrographic data do not exist for the greatmajority of other Tertiary coals.

GEOLOGIC SETTING AND STRATIGRAPHY

The Farco mine complex of WebbCounty lies within the outcrop of thesouthwestern flank of the Rio GrandeEmbayment, which extends southwesterlythrough Coahuila, Nuevo León, and


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 94SP Channel 93

ST Channel 94

27 37'30'o

99 37'30"o

100 o

28 o

Texas

Study area

LaredoTrevino Lease

Old mine prospect

Webb County

Outline ofRio GrandeEmbayment

Palafox

'

Rio Grande

Rachal Mine

Approximateoutcrop of San Pedro and Santo Tomas coal zones

Mexico

Figure 1. Location of study area, underground mine entries and prospects, Cannel Coal Companycoreholes 2 and 6, surface mines, and sample sites. Inset shows location of study area, WebbCounty, and Rio Grande Embayment in South Texas. From Warwick and Hook (1995).

Tamaulipas in Mexico (fig. 1). Within the RioGrande Embayment, the lower to middle partof the Claiborne Group consists of marinemudstones (Reklaw Formation) in the east andnortheast, and sandstones and mudstones(Bigford Formation) in the south andsouthwest (fig. 2). The marine mudstonescoarsen upward into the fluvialdeltaic QueenCity Sand, which prograded gulfward acrosseastern and central Texas. To the west andsouthwest, the interval overlying the BigfordFormation becomes less sandy and claystonesof the El Pico Clay Formation predominate.

The San Pedro and Santo Tomas coal zonesoccur within the fining-upward transition fromsandstone-dominated rocks to mudstone-richdeposits. The coal beds dip approximately 2Eto the northeast, toward the synclinal axis ofthe embayment.

The San Pedro coal zone ranges up to10 m (33 ft) in thickness and contains as manyas five organic-rich beds that vary fromcarbonaceous mudstone to nonbanded coal incomposition. The thinner Santo Tomas coalzone occurs approximately 25 to 35 m (82 to


15

Santo Tomascoal 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

Unnamedcoal bed

15 m

Limestone

Santo Tomas coal zone

Unnamedcoal bed

Pale

ocen

eE

ocen

e

Jackson Group

Wilcox Group

Midway Group

Cla

ibor

ne

Gro

up

Carrizo Sand

El Pico Clay

Bigford Fm.

Queen City Sand

WechesFormation

Sparta Sand

Cook Mountain Fm.Laredo Fm.

ReklawFormation

Eastern and central Texas

SouthTexas

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 andGarcia (1972). From Warwick and Hook (1995).


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)

Sant

o T

omas

coa

lR

ider

coal

San

Pedr

o co

al

San

Pedr

o co

alLat. 27 56'32"NLong. 99 o 51'17"W

Lat. 27 o 50'02"NLong. 99 o 51'26"W

Lat. 27 o 49'01"NLong. 99 o 50'19"W

o

Figure 3. Bed sections and sample intervals of the San Pedro, Santo Tomas, and Santo Tomas ridercoal beds. Sample numbers are in parentheses. Approximate locations are shown in Figure 1. FromWarwick and Hook (1995).

115 ft) above the San Pedro zone, averagesapproximately 2.4 m (7.9 ft) in thickness, andconsists of a lower main bed and an overlyingrider, both of which may occur as nonbandedcoal, impure coal, or carbonaceous mudstone(fig. 3). Within the Farco property and leases,coal beds of the San Pedro zone are mined atthe Palafox and Trevino mines, and the SantoTomas zone is developed at the Rachal mine(fig. 1).

MINING HISTORY

The Santo Tomas coal zone of WebbCounty was first mined commercially by theRio Grande Coal and Irrigation Companybetween 1881 and 1914 in modest driftopenings in the vicinity of Minera (fig. 1). By1895, the Cannel Coal Company hadcompleted a shaft and had driven mains in boththe Santo Tomas and San Pedro zones atDarwin (fig. 1). The company, whichemployed several hundred miners, wasdirected by David Darwin Davis, a mining


17

engineer from Wales (Hopson, 1975). Asecond shaft of the Cannel Coal Company atDolores also mined both coal zones. TheSanto Tomas Coal Company opened a shaft tothe Santo Tomas coal bed between 1910 and1912 near the village of Santo Tomas. Thisoperation 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 ofTexas, Inc., initiated surface mining at Palafox(fig. 1). The Rachal and Trevino mines beganproduction in 1983 and 1996, respectively.Cumulative production from 1979 through1997 totaled approximately 6 million shorttons. The coal is washed and sized at acleaning plant at Palafox. A significantamount of the coal is exported to the Republicof Ireland as lump coal for domestic use, andother production is consumed by local cementplants. Kelmac, Inc., a subsidiary of ChevronCorporation, recently acquired the WebbCounty mines of Farco Mining Company ofTexas and operates currently under the nameFarco Mining, Inc.

DEPOSITIONAL SETTING

The coal-bearing portion of the lowerto middle Claiborne Group of Webb County isa fining-upward sequence situated above athick, sandstone-dominated section thatcontains poorly preserved marineinvertebrates, and below a mudstone-richsection that yields mainly nonmarine fossils.Corehole records and natural exposures showthat the San Pedro coal zone overlies a barreninterval dominated by thick (7.6-10.7 m or 25-35 ft), predominantly flat-bedded, sandstonedeposits.

At Palafox, the San Pedro coal zoneconsists of a thick (< 0.85 m or 2.9 ft),

nonbanded upper bed and a thin, impure bedapproximately 3 m (9.8 ft) below; additionalthin coal beds and carbonaceous mudstonesoccur in the lower part of the coal zone but arenot mined (fig. 3). The upper coal bed isunderlain by a dark gray, root-penetrated, siltyclaystone, and the uppermost portion of thiscoal bed contains mud-filled invertebrateburrows and trails. As seen in-mine, thecontact between this coal and overlyingmudstone or sandstone generally is sharp.

The interval immediately above the SanPedro coal zone consists mainly of sandstone,a slightly smaller proportion of mudstone, aminor intraformational conglomerate, localfine-grained limestone nodules or bands, andthin (< 0.45 m or 1.5 ft), discontinuous coalbeds that range from nonbanded coal to clayor silt-rich impure coal. The sandstones arecalcareous and are composed chiefly of fine tovery 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 sandstonebodies above the San Pedro coal zone in thePalafox Mine. These sandstone bodies are flatbotttomed, flat to ripple bedded, and exhibitcharacteristics of mouth-bar to crevasse-splaysands deposited in shallow water. Betweenthe main coal bed and these sandstonedeposits, coarsening-upward, rhythmicallyinterbedded mudstones and sandstones containflaser bedding suggestive of tidal influences. Afew zones of poorly preserved marineinvertebrate fossils (pelecypods, gastropods)occur above the San Pedro coal zone(Lonsdale and Day, 1937).

At the Rachal Mine (fig. 1), the SantoTomas coal zone consists of a thick (0.69 m)nonbanded main seam, a clastic interval thatranges from 0.91 to 1.98 m (3 to 6.5 ft) in


18

thickness, and a thin (0.20 m or 0.6 ft) ridercoal bed (fig. 3). Both beds are underlain bypaleosols, and the top of the main bed containsinvertebrate burrows and trails. The intervalbetween these coal beds consists offlat-bedded siltstone and fine to very finegrained, noncalcareous sandstone that finelaterally into coarsening-upward mudstonedeposits. These mudstones lack the flaser-bedded sandstone interbeds that occur abovethe San Pedro coal bed at Palafox. Channel-form 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 thicksandstone deposits. Overall, the intervalbetween the main Santo Tomas coal bed andrider bed records the drowning of an initialmire, infilling of the resulting floodbasin byminor crevasse splays, and re-establishment ofmire conditions.

The presence of several thin coal andcarbonaceous shale beds a short distanceabove the Santo Tomas rider bed indicates thatclastic-rich mires persisted locally. Overlyingcoarsening-upward sequences represent theterminal flooding and clastic infilling of thefloodbasin. Lonsdale and Day (1937) reportedthin sandstone beds in the area of Darwin thatcontained poorly preserved nonmarinemolluscs and abundant green algaefructifications. Although they interpretedthese fossil occurrences as a unique bed ofconsiderable persistence, Gardner (1945), whomeasured and collected many of thefossiliferous 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 ofnonmarine aquatic fossils above the SantoTomas coal zone indicates that lacustrinesettings may have been common within thefloodbasin.

COAL CHARACTERISTICS

The nonbanded coals that are mined inWebb County are extremely hard, fractureconchoidally, and have cleats developed at ascale similar to that of bituminous nonbandedcoals. The lower part of the San Pedro coalbed at Palafox is very massive and breaks intoblocks that average 0.5 m (1.6 ft) in width; toa lesser extent, the main Santo Tomas coal bedbecomes more massive and less fractureddownward.

Physical and geochemical analyses (fig.4) reported by us previously (Warwick andHook, 1995) are comparable to earlieranalyses of these coal beds (Evans, 1974;Mukhopadhyay, 1989). All the San Pedro andSanto Tomas samples have relatively high ashyields (9.5-18% dry basis) at their base. Totalsulfur content (0.8-2.0%, dry basis) generallyincreases upward, and organic sulfur ispredominant. Calorific values increase upwardwithin each coal bed, with a maximum of7,434 kcal/kg (13,371 Btu, dry basis) found inthe top part of the main San Pedro bed atPalafox. Hydrogen content follows calorificvalues, ranging from 5.4 to 6% (dry basis).Additional physical and geochemical data,including trace-element analyses, are reportedin Warwick and Hook (1995).

Petrographic study of particle pelletsand unetched polished blocks from San Pedroand Santo Tomas coal samples (Warwick andHook, 1995) indicate that a highly degradedgroundmass composed of eugelinite is themain organic component (approximately 71%,mineral-matter-free basis). An enrichedliptinite fraction (approximately 23%)accounts for a portion of the high calorific


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

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0 2 4 6

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0 0.5 1 1.5 2 2.5

0 2000

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40

60

San Pedro 93 channel sample

San Pedro 94 channel sample

2000

0 2000

6000

10000

14000 2 4 6

0

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


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


Figure 4. Selected proximate and ultimate data. Sample descriptions and symbols are the same asthose used in figure 3. All values as percentages except Btu. Sample locations are shown inFigure 1. Complete sample data are given in Warwick and Hook, 1995. From Warwick andHook (1995).


20

SandstoneMudstone/claystoneCoalCarbonaceous mudstone

Algae-rich lake

San Pedrocoal bed

Santo Tomascoal bed

Distributary channeland crevasse splay

Clasticswamp

Crevasse splay Santo Tomas

rider bed

Mouth barsandstone

Algae-rich lake

Algae-rich lake

Alluvial floodplain

Figure 5. Schematic block diagram of depositional environments for the San Pedro and Santo Tomascoal zones in the middle part of the Claiborne Group. No scale implied. The geometric distributionof lithologies on the panels is inferred from borehole and mine highwall data. From Warwick andHook (1995).

values that distinguish these coals. Most ofthis enrichment represents fusiformgreen-algae fructifications, which because oftheir size (up to 2 mm or 0.1 in. in length),cannot be identified or quantified readily byconventional coal petrography techniques orpalynologic preparations that involveacidification. There is a negligible proportionof inertinite macerals in these coals.

Average maximum vitrinite reflectance(Romax) values for San Pedro and Santo Tomascoal samples reported previously by us was0.53. This was comparable to the Romax

reported by Mukhopadhyay (1989) for oneSan Pedro sample. Compared to standardrank parameters, the San Pedro and SanTomas coal deposits fall within the range ofsubbituminous A to high-volatile bituminous Ccoal.

SUMMARY

The unusual coals of the San Pedroand Santo Tomas coal zones of the ClaiborneGroup in Webb County, Texas, aredistinguished by physical, petrographic, and


21

geochemical characteristics unlike those ofother Gulf Coast Tertiary coal deposits.Whereas the San Pedro deposits accumulatedin marine-influenced lower delta plain settings,the overlying Santo Tomas interval representsa nonmarine, mudstone-dominated sequence(fig. 5). Mineable coal beds within both coalzones contain an abundance of green algaefructifications, which account in part for theanomalously high calorific value. Data fromMexico (SPP, 1989), however, recordQuaternary igneous intrusive rocks less than100 km (62 mi) west of the Rio Grande area,and coal-quality data from coal beds in theunderlying Wilcox Group of Texas countiesbordering Webb County indicate increasedrank. Additional work is required todetermine the postdepositional history of thecoal deposits of South Texas.

ACKNOWLEDGMENTS

This summary is based mainly upon a1995 paper published in the InternationalJournal of Coal Geology. We thank ElsevierScience B.V. for allowing us to reproduceportions of our previous work. Our originalstudy was made possible through theassistance of Farco Mining Company of Texas,Inc., and the Railroad Commission of Texas.We also thank those at the U.S. GeologicalSurvey and the U.S. National Museum whocontributed to our earlier work.

REFERENCES CITED

Ashley, G.H., 1919, The Santo Tomas CannelCoal, Webb County, Texas: U.S.Geological Survey Bulletin 691-I, p.251-270.

Eargle, D.H., 1968. Nomenclature ofFormations 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, TheUniversity of Texas at Austin,Handbook 4, 65 p.

Gardner, J.A., 1945, Mollusca of the TertiaryFormations of Northeastern Mexico:Geological Society of AmericaMemoir 11, 332 p.

Guevara, E.H., and Garcia, Roberto, 1972,Depositional systems and oil-gasreservoirs in the Queen City Formation(Eocene), Texas: The University ofTexas at Austin Bureau of EconomicGeology Geologic Circular 72-4, 22 p.

Hopson, F.S., 1975, Resume of cannel coaldevelopment, in, Triple Energy FieldTrip, Duval, Webb, and ZapataCounties, Texas: Corpus ChristiGeological Society Guide Book, 14 p.

Lonsdale, J.T., and Day, J.R., 1937, Geologyand Ground-Water Resources of WebbCounty, Texas: U.S. GeologicalSurvey Water-Supply Paper 778, 104p.


22

Mukhopadhyay, P.K., 1989, Organicpetrology and organic geochemistry ofTexas Tertiary coals in relation todepositional environment andhydrocarbon generation: Bureau ofEconomic Geology, The University ofTexas at Austin, Report ofInvestigations188, 118 p.

SPP [Secretaría de Programación yPresupuesto], 1989, Carta Geologica[Monterrey]: Dirección General deGeografía del Territorio Nacional, SanAntonio Abad, Mexico, scale1:1,000,000.

Trowbridge, A.C., 1923, A geologicreconnaissance in the Gulf CoastalPlain of Texas, near the Rio Grande:U.S. Geological Survey ProfessionalPaper 131-D, p. 85-107.

Trowbridge, A.C., 1932, Tertiary andQuaternary Geology of the Lower RioGrande Region, Texas: U.S.Geological Survey Bulletin 837, 260 p.

Vaughan, T.W., 1900, Reconnaissance in theRio Grande Coal Fields of Texas: U.S.Geological Survey Bulletin 164, 100 p.

Warwick, P.D., and Hook, R.W., 1995,Petrography, geochemistry, anddepositional setting of the San Pedroand Santo Tomas coal zones:anomalous algae-rich coals in themiddle part of the Claiborne Group(Eocene) of Webb County, Texas:International Journal of Coal Geology,v. 28, p. 303-342.


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 (lateEocene, lower Jackson Group) of South Texasis mined by the North American CoalCorporation, at the San Miguel Mine nearJourdanton, Texas (fig. 1). The depositconsists of four or more thin (generally < 1 mor 3.3 ft thick) lignite benches that areseparated by claystone and mudstone partings(fig. 2). The partings are composed of alteredvolcanic air-fall ash that has been reworked bytidal or channel processes associated with aback-barrier depositional environment(Snedden and Kersey, 1981; Gowan, 1985;Ayers, 1986; Senkayi and others, 1987; andWarwick and others, 1996). The field tripstop at the San Miguel Mine will include a tripinto the working opencast pit. We will be ableto observe mining methods, and the rocksassociated with the coal-bearing interval. Thissummary of the geology of the San Migueldeposit is based largely on Warwick and others(1996), which focuses on the petrology andgeochemistry of the lignite deposit.

GEOLOGIC SETTING

The San Miguel lignite deposit is in theundivided lower Jackson Group of SouthTexas (fig. 2). In the mine area, the Jacksonstrata dip to the southeast about 17 m/km (90ft/mi) (Ayers, 1986). The mine is located

updip of a strike-elongated sand complex thathas been interpreted as a barrier-bar/strandplain complex by Kaiser and others (1980) andGowan (1985). Gowan (1985) and Ayers(1986) suggest that the mud-dominatedsediments the comprise the floor, partings, androof of the San Miguel lignite depositoriginated in lagoonal mires landward (west)of the sandstone-dominated barrier islandcomplex. The lignite-bearing interval islaterally persistent along strike for more than40 kilometers (25 mi) (Ayers, 1986, 1989;Tewalt and others 1983). More detaileddescriptions of the geological setting of thearea can be found in McNulty (1978), Snedden(1979), Snedden and Kersey (1981), Tewaltand others (1983), Gowan (1985), and Ayers(1986, 1989).

The San Miguel lignite interval consistsof four (or more) thin (generally <1 m or 3.3 ftthick) lignite benches that are separated byclaystone and mudstone partings. The upperbed is designated as the A bed and is underlainby the B, C, and D beds (figs. 2, 3a) (Gowan,1985). Tewalt and others (1983) referred tothis interval as South Jackson seam no. 10.The rocks above the lignite beds aredominated by fine siltstone and claystone withoccasional thin layers of gastropod and bivalveshells.

Kaiser (1982), Gowan (1985), andAyers (1986, 1989) suggested that the rockpartings within the San Miguel lignite deposit


24

McMullen Co

Atasc

osa

Co

Live

Oak C

o

3387

140

791

ALT281

281

Campbellton

Whitset

Crowther

Approximateboundary of San Miguel MinePermit

791

99

98 25'o

28 45'o

28 35'o

98 15'o98 35'o

0 5 km

TexasSM-1

SM-2

99

16

To Jourdanton

Figure 1. Index map of the San Miguel Mine area showing towns, roads, and sample sites. Mineoutline from the San Miguel Lignite Mine Permit Application, on file at the Texas RailroadCommission in Austin, Texas. From Warwick and others (1996).

are composed of volcanic air-fall ash that hasbeen reworked by possibly tidally influencedchannel processes. Senkayi and others (1987)examined the partings in detail and found thatmost contained altered kaolinite and mixeddetrital and volcanic quartz grains, therebysupporting the interpretation of a volcanicorigin and subsequent detrital mixing of thesediment that forms these layers. Thesefindings are consistent with the conclusions ofWarwick and others (1996) (see below).

LIGNITE RESOURCES AND MINING

Jackson Group lignite resources for theSouth Texas area have been estimated to beabout 6 billion short tons (5.4 metric tons)with 750 million tons (680 metric tons)occurring near surface and the remainderclassified as deep-basin deposits (Kaiser andothers, 1980). Surface mining operations at

the San Miguel Mine began in 1980 and the1997 annual production was about 3.5 milliontons (3.2 metric tons) (data from the U.S.Department of Energy, Energy InformationAgency and the Railroad Commission ofTexas). Since 1997, the San Miguel Mine hasbeen operated by the North American CoalCorporation; prior mining was carried out bythe Atascosa Mining Company. TheKeystone Coal Industry Manual (PRIMEDIAIntertec, 1999) reports the total reserves forthe North American Coal Corporation in theSan Miguel Mine area to be about 42 milliontons with 35 million tons recoverable (38.2and 32 million metric tons respectively).Average overburden is about 30 m (98 ft),with a stripping ratio of 7:1. Overburden isremoved by dragline methods. The coal isremoved by Easi-Miners (a modified asphaltstripper) and front-end loaders. The lignite is


25

Pal

eoce

neE

ocen

e

Jackson Group

Wilcox Group

Midway Group

Midway

an

Sabi

nian

Cla

ibor

nian

Jack

sonia

n

Danian

Selandian

Ypres

ianLu

tetia

n

Bartonian

Priabonian

SeriesEuropean

stage N. A

merican

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-2SM-1

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


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 floor (GRAB)

1 m

Mudstone 12cm

Mudstone 10cm

Claystone 21cm

Lignite 40cm (D.2)


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

moved by truck and used as steam coal in themine-mouth 409 megawatt San Miguel PowerPlant operated by the San Miguel ElectricCooperative (PRIMEDIA Intertec, 1999).

Some mining problems that areencountered in the San Miguel Mine includefloor heaves, mudstone dikes, and channelswithin the coal interval. Some of thesefeatures 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 causingproblems with pit traffic and general miningoperations (fig. 3a). Ayers (1986) describedclastic dike intrusions that originate primarilyfrom the partings between beds C and D. Theintrusions are generally no more than a few

meters in length and are less than 0.5 m (1.6ft) wide. Minor faulting is usually associatedwith these dikes. Ayers (1986) suggested thatemplacement of these dikes may be related toloading and dewatering processes. Anotherinteresting feature that is often observable inthe San Miguel Mine are small interseamchannels that usually scour into lignite beds Cor B and are filled with inter-seam partingmaterial. The channels are usually no morethan a few meters (several ft) wide and areconfined to a single clastic parting zone withinthe A-D coal bed complex. Ayers (1996)described these channels in detail and suggeststhat were formed by marsh drainages thatoccupied the area between periods of peataccumulations.


26

BA

C

DBA

C D

15 c

m

scale

Floor heave

EL

EG

Figure 3. Representative photographs and photomicrographs of outcrops and samples from the SanMiguel 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 acompressed log in the D bed. (d) Typical photomicrograph of an etched and unetched surface of apolished pellet of San Miguel lignite. EG = eugelinite, EL = etch line. Scale bar = 25µm. FromWarwick and others (1996)

CHARACTER OF THE SAN MIGUEL LIGNITE

Megascopic characteristicsAs described by Gowan (1985), the

San Miguel Lignite interval has beenextensively reworked by burrowingorganisms. This is apparent from thelithologic descriptions of the channel samplesites described by Warwick and others (1996)(fig. 3b). Some of the burrows near the top ofthe A lignite bed were identified as Gyrolithesby Gowan (1985) and probably were formed

by organisms that lived in the water thatencroached over the top of the peat deposit.The burrows, usually filled with a mixture oforganic material and kaolinitic claystone, isone source of ash in the lignite beds (fig. 3b).Individual lignite beds generally exhibit a dull,somewhat massive texture near their base andgrade upward into attrital-rich lignite withincreasing xylinitic bands (3-4 cm or 1.2-1.6 inthick) towards the top of the individual bed.Xylinitic bands can compose up to 30% ofindividual lignite beds. Occasionally,compressed logs are exposed in


27

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mass %

Btu/lb

0

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10 30Mj/kg

0

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10 30

Mj/kg

Ash Yield

SM-2 SM-1

SM-2 SM-1

Figure 4. Bar graphs showing concentrations of ash yield, total sulfur, and calorific value (CV, drybasis) for the sample sets SM-1 and SM-2. Location of samples are shown on figure 1 and lithologicsymbols are the same as used on figure 2. Distance between sample sites is about 300 m. FromWarwick and others (1996).


28

0 1

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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 ActAmendments. Location of samples are shown on figure 1 and lithologic symbols are the same as usedon figure 2. All values are parts per million (ppm). From Warwick and others (1996).

cross section on the exposed face of the lignitebench (fig. 3c). The middle lignite beds (B andC) appear to contain the greatest percentage ofxylinitic banding when compared to the upperand lower beds (A and D).

Proximate analysesChemical and physical data obtained

from San Miguel lignite samples have beenreported on an individual bed basis (5 samples;Mukhopadhyay, 1989) and on a combined bedbasis (23 samples; Tewalt, 1986). From thesedata, the dry ash yields of samples range from25 to 55% (avg 38%; Tewalt, 1986) with theA bed having the greatest average ash yield(39%, Mukhopadhyay, 1989). Equilibriummoisture values range from 27 to 33%

(Tewalt, 1986). Total sulfur contents (on adry basis) range from 1.5 to 3.1% (Tewalt,1986). Organic sulfur is the dominant sulfurtype. Calorific values (on a moisture-freebasis) for the San Miguel samples range from5384 to 8690 Btu/lb (12.5-20.2 Mj/kg)(Tewalt, 1986). Tewalt (1986) andMukhopadhyay (1989) report that the rank ofthe Jackson Group and the San Miguel lignitesis lignite A.

Warwick and others (1996) reportedproximate analyses for 17 lignite samplescollected from the San Miguel deposit (fig. 4).The data indicate that dry ash yields average30.16%; total dry sulfur content averages2.76% (dominated by organic sulfur, 2.04%);


29

0

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U

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100

300

500

700

SM-1

SM-2SM-1SM-2

2 4

6

SM-2SM-1

SM-2SM-1

SM-1 SM-21 m

Figure 5. Continued - Bar graphs showing the distribution of trace element concentrations (dryUSGS 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 arethe 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 moisturecontent averages 33.4% for the SM-2 channelsample set. At both sample sites, ash yieldswere greatest from the upper and lower lignitebenches. Total sulfur content is fairly uniformboth 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 Miguellignite interval. She found that the San Miguellignite interval contains greater amounts ofNa2O (avg 3.67%; N=66) as compared to

other lignite deposits of Texas (range ofaverages: 0.41-1.22%), and suggested that thisenrichment was either the result of sodiumsorption from saline waters during and afterdeposition, or of sodium cation exchange withpresent-day ground water. Tewalt (1986)reported trace-element concentrations forfifteen elements (As, B, Be, Cd, Cr, Cu, Hg,Mn, Mo, Ni, Pb, Se, U, V, Zn) that wereobtained from two samples from the SanMiguel deposit. Based on the results fromother Jackson Group lignites of East Texas,Tewalt suggested that the Jackson lignitescontain greater amounts of B, Be, Mn, and Zn


30

SM-1SM-2

1 m

0

20

40

60

80

100

120

Nb

0

20

40

60

80

100

0

200

400

600

800

1000

Zr

0

200

400

600

800

1000 0

100

200

Ce

0

100

200

300

0

10

20

30

40

50

60

Th0

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 figure1 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 channelsamples taken from the San Miguel mine (figs.1, 2). Of particular interest for this study wasthe distribution of 12 environmentally sensitiveelements that were described by the 1990 U.S.Clean Air Act Amendments. A summary ofthe trace element data (on a dry, ash basis) forboth sample sites is plotted on figure 5. Visualanalysis of the SM-2 bar graphs shows thatsome of the elements are concentrated in theorganic-rich (or lignite) samples. Linearcorrelations between ash yield and theconcentrations of the environmentally sensitiveelements on an ash basis for those samples

containing less than 50% ash (i.e. what isnormally mined) showed that only Mn has a strong negative (r <-0.5) relationship tosample ash yield. On a whole-coal basis (forthose samples containing less than 50% ash) astrong positive correlation (r >0.5) was foundbetween ash and Pb, Se, and Na2O.

Warwick and others (1996) alsoplotted bar graphs for both San Miguel rockand lignite samples to show the verticaldistribution of elements that may be associatedwith volcanic ash partings (fig. 6). Meanconcentrations are: Ce, 117 ppm: Nb, 36 ppm;Ta, 2.4 ppm; Th, 29 ppm; and Zr, 477 ppm (allconcentrations in coal on an ash basis) (fig. 6).Concentrations of these elements are generally


31

enriched in the B and C lignite beds. Previouswork has indicated that concentrations of theseelements in coal can be enriched byincorporation of volcanic ash in peat or byleaching from volcanic ash beds (Zielinski,1985; Crowley and others, 1989). In coalsamples from intervals adjacent to alteredvolcanic ash partings in the Cretaceous C coalbed of Utah, Crowley and others (1989)reported mean concentrations (on an ash basis)of elements having a volcanic origin asfollows: Zr (752 ppm), Nb (57 ppm), Th (60ppm), and Ce (246 ppm). Although the meanconcentrations of these elements in the SanMiguel samples do not reach the reportedaverages for the Utah C coal bed, specificsamples from SM-1 and SM-2 exceed theUtah averages for Zr, Nb, and Ce.

Mineralogy of San Miguel partingsWarwick and others (1996) reported

results from preliminary examination of SanMiguel rock parting samples based uponscanning electron microscopy (SEM) andenergy-dispersive X-ray analyses (EDXA).These analyses indicate that the upper part ofthe mudstone parting below the A bed (SM-2P1.1, fig. 2) is composed primarily of kaolinitewith some mixed-layer clay present.Vermicular kaolinite is present throughout thesample, as are small (<50µm) K-feldspars.Isolated plagioclase, quartz, barite, framboidalpyrite, anatase, and a single Al-phosphate werealso observed. The mudstone is very organicrich. Although individual organic stringers arerare, irregular pieces of coaly material arefound throughout the sample.

The matrix of the claystone below theB bed (sample SM-2 P2, fig. 2) is composedprimarily of mixed-layer clay and abundantorganic material. Both alkali and plagioclasefeldspars were observed throughout the

sample, but they are rare. Quartz, pyrite, andCa-Al phosphate (crandalite group?) arepresent in trace amounts.

Examination of samples from theclaystone parting between the C and D beds(P3.1, P3.2, P3.3, and P3.4, fig. 2), which wasdescribed as a major ash bed by Senkayi andothers (1987), shows that the matrices arecomposed primarily of mixed-layer clays.Kaolinite is present as cell-fillings, vermicularclasts, and rarely as matrix material.Accessory grains are uncommon. Thedominant accessory minerals are subhedral andoften etched alkali feldspars. Plagioclase isalso present but not as abundant as thealkalies. Barite and pyrite, and raresubrounded quartz and euhedral zircons wereobserved in each of the examined samples.

Palynology and Organic PetrographyGennett (1985) and Gennett and

Raymond (1986) examined incremental benchsamples from a core of the San Miguel ligniteand found that the lower D bed had pollenderived from marsh ferns and palms whichsuggested a brackish-water influence on thedeposition of the D bed paleo-mire. Pollentypes found in the overlying beds showedfresh-water associations as indicated by avertical transition from Nyssa (tupelo) toEngelhardia (walnut family) and Fagaceous(oak family) types. Mukhopadhyay (1989)reported that beds B and D contain marshpollen types and suggested that the San Miguell i g n i t e s a r e d o m i n a t e d b yEngelhardia/Juglandaceae and herbaceous(?)monocots.

Mukhopadhyay (1986, 1989)examined the petrographic characteristics ofchannel and lithotype samples from each of the four major lignite beds exposed at the San


32

0

20

40

60

80

100

SM-1

1 m

SM-2

1 m

Major Huminites (%)

Major Inertinites (%)

0

1 m

Major Liptinites (%)

DGHMEG DGHMEG

LPD ALG RSD LPD ALG RSD

MAC SCL FUS

0

20

40

60

80

100

0 5

10

15 20

25

30

1 2

5 0

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 andSM-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)


33

Miguel mine. Mukhopadhyay's channelsample 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 ofhuminitic material (85%, primarily ulminiteand attrinite/densinite) when compared to theother beds. Inertinite content for the bedsrange from 1 to 4% and is primarily in theform of sclerotinite and fusinite. The D bedcontained a few marine dinoflagellates (algalcysts).

Warwick and others (1996) reportedthat the San Miguel lignite deposit isdominated by the huminite maceral group(89%) followed by the liptinite (10%), andinertinite (1%) maceral groups (figs. 3d and 7).The three dominant huminite maceralsubgroups in decreasing order of abundanceare: eugelinite (73%), humotelinite (11%), anddetrogelinite (3%). The liptinite maceralgroup is dominated by liptodetrinite (4%),followed by alginite (2%) and resinite inattritus (1%). The inertinite maceral group iscomposed of macrinite, sclerotinite, andfusinite.

Vertical and lateral variations inmaceral composition can be observed in thepetrographic data (fig. 7). Channel-sample setSM-1 contains similar amounts of huminitemacerals as found in SM-2 samples, but SM-1samples have markedly less liptinite andinertinite components. Vertical trends in thepetrographic data include slightly increasedliptodetrinites in samples from the top andbottom of the lignite interval and a sharpincrease in inertinite (primarily macrinite andfusinite, fig. 7) in the lignite samples directlybelow the parting between the A and B beds.Humotelinite content increases, althoughslightly, in the upper part of each bed. This

finding is in agreement with megascopicdescriptions of the coal in the field.

RELATIONSHIP BETWEEN ASHYIELD, AND PETROGRAPHIC AND

GEOCHEMICAL CHARACTERISTICSOF THE SAN MIGUEL LIGNITE

DEPOSIT

Warwick and others (1996) examinedthe relationship between the ash yield and thepetrographic and geochemical characteristicsof the San Miguel lignite as mined. The majorconclusions of their study are as follows: (1)The distribution of Mn is inversely related tothe ash yield of the lignite samples. Thisindicates an organic affinity, or an associationwith finely disseminated minerals in the lignitethat contain this element. (2) On a whole-coalbasis, the concentration of the environmentallysensitive element Pb is positively related to ashyield in lignite samples. This indicates aninorganic affinity for Pb. (3) Averagewhole-coal concentrations of As, Be, Sb, andU in the San Miguel samples are greater thanpublished averages for these elements in otherU.S. lignites. (4) The upper and lower lignitebenches of the San Miguel deposit are bothash- and algal-rich, indicating that theseintervals were probably deposited in wetterconditions than those in which the middleintervals formed. (5) The dominance of theeugelinite maceral subgroup over the huminitesubgroup indicates that the San Miguel ligniteswere subjected to peat-forming conditions(either biogenic or chemical) that enableddegradation of wood cellular material intomatrix gels, or that the plants that formedthese lignite benches were less woody andmore prone to formation of matrix gels. (6)An inertinite-rich layer (top of the B bed)might have formed from widespread oxidation


34

of the San Miguel peat as a result of a volcanicash fall that was subsequently reworked.

SUMMARY

There have been numerous studiesconducted through the years detailing thegeology of the San Miguel lignite deposit. Thedepositional setting of the lignite deposit isinterpreted to have originated in back-barrierlagoonal mires (Snedden and Kersey, 1981;Kaiser and others,1980; Gowan,1985; Ayers,1986). The deposit consists of four or morethin (generally < 1 m or 3.3 ft thick) lignitebenches that are separated by claystone andmudstone partings (fig. 2). The partings arecomposed of altered volcanic air-fall ash thathas been reworked by tidal or channelprocesses associated with a back-barrierdepositional environment (Snedden andKersey, 1981; Gowan, 1985; Ayers, 1986;Senkayi and others, 1987; and Warwick andothers, 1996).

Individual lignite beds generally exhibita dull, somewhat massive texture near theirbase, which grades upwards into attrital-richlignite with increasing xylinitic bands (3-4 cmor 1.2-1.6 in thick) towards the top of theindividual bed. Proximate data indicate thatdry ash yields from the San Miguel depositaverage 30.16%; total dry sulfur contentaverages 2.76% (dominated by organic sulfur,2.04%); and dry calorific values average 8611Btu/lb (20 Mj/kg) (Warwick and others 1996).Petrographically, the San Miguel lignitedeposit is dominated by the huminite maceralgroup (89%) followed by the liptinite (10%),and inertinite (1%) maceral groups (Warwickand others, 1996). Average whole-coalconcentrations of As, Be, Sb, and U in the SanMiguel samples are greater than publishedaverages for these elements in other U.S.

lignites (Warwick and others, 1996).

ACKNOWLEDGMENTS

This summary is based mainly upon a1996 paper published in OrganicGeochemistry. We thank Elsevier ScienceLtd. for allowing us to reproduce parts of ourprevious work. We wish to thank John Palfy,General Manager, and Roger C. Fish,Engineering Manager, and North AmericanCoal Corporation for facilitating our visit tothe San Miguel Mine.

REFERENCES CITED

Ayers, W. B., Jr., 1986, Geology of the SanMiguel lignite mine, Jackson Group,South Texas, in Finkelman R. B. andCasagrande D. J., eds, Geology ofGulf Coast Lignites: Geological .Society of America, Coal GeologyDivision Field Trip Guidebook, p.54-67, Environmental and CoalAssociates, Houston, Texas.

Ayers, W. B., Jr., 1989, Depositional settingof the San Miguel Lignite Mine,Jackson Group (Eocene), SouthTexas, in Ayers W. B. Jr., Breyer J. A.and Finkelman R. B, eds., Depositionalsettings of Texas Lignites, 28thInternational Geological Congress:Field Trip Guidebook T173, p. 33-37.

Breyer J. A., 1991, Tertiary coals of the GulfCoast, in Gluskoter, H. J., Rice D. D.and Taylor, R. B., eds., EconomicGeology U. S.: Geological Society ofAmerica, The Geology of NorthAmerica P-2, p. 573-582.


35

Crowley, S. S., Stanton, R. W., and Ryer, T.A., 1989, The effects of volcanic ashon the maceral and chemicalcomposition of the C coal bed, EmeryCoal Field, Utah: OrganicGeochemistry, v. 14, p. 315-331.

Gennett, J. A., 1985, A core from the EoceneSan Miguel lignites, South Texas:palynomorphs and depositionalenvironments [abs.]: AmericanA s s o c i a t io n S t r a t i g r a p h i cPalynolologists. Program andAbstracts, 18th Annual Meeting. p. 12.

Gennett, J. A. and Raymond, A. L., 1986,Changes in floral composition withdepositional environment in TexasEocene Manning Formation lignites[abs.]: American Association ofPetroleum Geologists Bulletin, v. 70,p. 1181-1182.

Gowan, S. W., 1985, Depositionalenvironment of the San Miguel lignitedeposit in Atascosa and McMullenCounties, Texas: College Station,Texas A&M University, Ph.D.dissertation, 199 p.

Kaiser, W. R., 1982, Lignite depositionalmodels, Texas Eocene: a regionalapproach to coal geology, in ShobertH. H., ed., Proceedings of the BasicCoal Science Workshop, Houston,Texas, 1981: U. S. Department ofEnergy, Grand Forks EnergyTechnology Center, p. 10-67.

Kaiser, W. R., Ayers, W. B., Jr., and La Brie,L. W., 1980, Lignite resources inTexas: The University of Texas atAustin Bureau of Economic GeologyReport of Investigations 104, 52 p.

McNulty, J. E., Jr.,1978, Geology of the SanMiguel lignite deposit (Atascosa andMcMullen Counties, Texas, in KaiserW. R., ed., Proceedings, Gulf Coastlignite conference: geology, utilization,and environmental aspects: TheUniversity of Texas at Austin, Bureauof Economic Geology Report ofInvestigations 90, p. 79-83.

Mukhopadhyay P. K., 1986, Petrography ofselected Wilcox and Jackson Grouplignites from the Tertiary of Texas: inFinkelman R. B. and Casagrande D. J.,eds., Geology of Gulf Coast Lignites:Geological Society of America CoalGeology Division Field TripGuidebook, Environmental and CoalAssociates, Houston, Texas, p. 126-145.

Mukhopadhyay, P. K., 1989, Organicpetrology and organic geochemistry ofTexas Tertiary coals in relation todepositional environment andhydrocarbon generat ion: TheUniversity of Texas at Austin, Bureauof Economic Geology, Report ofInvestigations 188, 118 p.

PRIMEDIA Intertec, 1999, Keystone coalindustry manual:Chicago, Illinois,PRIMEDIA Intertec, 807 p.

Senkayi, A. L., Ming, D. W., Dixon, J. B., andHossner, L. R., 1987, Kaolinite,opal-CT, and clinoptilolite in alteredtuffs interbedded with lignite in theJackson Group, Texas: Clays & ClayMinerals, v. 35, p. 281-290.


36

Snedden, J. W., 1979, Stratigraphy andenvironment of deposition of the SanMiguel lignite deposit, northernMcMullen and southeastern Atascosacounties, 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 depositand associated lithofacies, JacksonGroup, South Texas: AmericanAssociation of Petroleum GeologistsBulletin, v. 65, p. 1099-1109.

Tewalt, S. J., 1986, Chemical characterizationof Texas lignite: University of Texas atAustin, Bureau of Economic GeologyGeological 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 inTexas Gulf Coast, Ohio NorthernAppalachian, and Wyoming PowderRiver Basins: a comparison ofstatistical approaches: The Universityof Texas at Austin, Bureau ofEconomic Geology Report ofInvestigations 136, 137 p.

Warwick, P.D., Crowley, S.S., Ruppert, L.F.,and Pont ilillo, James, 1996,Petrography and geochemistry of theSan Miguel lignite, Jackson Group(Eocene), South Texas: OrganicGeochemistry, v. 24, no. 2 pp. 197-217.

Zielinski, R. A., 1985, Element mobilityduring alteration of silicic ash tokaolinite--a study of tonstein:Sedimentology, v. 32, p. 567-579.


37

CHAPTER 4

Some speculations on coal-rank anomalies of the South Texas Gulf Province and adjacentareas 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 ofbituminous coal within the generally ligniticGulf of Mexico Coal Province. One occurs inthe Olmos Formation (Maastrictian) and theother occurs in the lower-middle part of theClaiborne Group (middle Eocene, fig. 1).Both occur within the Rio Grande vicinity ofsouth Texas and Mexico, and each can beconsidered somewhat anomalous in rank incomparison with typical U.S. coals of similarage, which are generally less mature.

CRETACEOUS BITUMINOUS COALS

The Olmos Formation coals, which willnot be visited on the field trip, occur in theEagle Pass area of Maverick County, Texas,where a permit to mine is pending (DosRepublicas permit, fig. 2). This depositextends into the Sabinas Basin of Mexico,where high-volatile C bituminous coal is beingmined for electrical power generation by theMinera Carbonifera Rio Escondido (MICARE)for the Comission Federal de Electicidad(CFE) Carbon I and II projects (fig. 2). Coresamples of these high-ash coals from the EaglePass area are typically about 7500 Btu/lb on anas-received (AR) basis and about 12,000

Btu/lb on a moist, mineral matter-free(MMmF) basis. Samples from Carbon I and IIare reported to have an average (Rm?) vitrinitereflectance of 0.58% (Verdigo and Arciaga,1991). These and other rank parameters forSabinas Basin coals are somewhat higher thanthose for most shallow occurrences of UpperCretaceous coals of the southern RockyMountain Province, for example the southernpart of the San Juan Basin of New Mexico, butthey are consistent with rank-elevated coalstowards basin interiors or near igneousprovinces on basin margins. The rankelevation of the Olmos Formation coals canprobably be attributed to residual heat flowfrom slightly younger shallow intrusive activityof the Balcones igneous province, or muchyounger igneous activity within Mexico (fig.2).

TERTIARY BITUMINOUS AND RELATED COALS

The second documented occurrence ofbituminous coals in the Gulf Province is theSanto Tomas coal field in Webb County,Texas, which will be visited on the field trip.Here the Farco Mining Company (fig. 2) ismining high-volatile A bituminous coal fromthe lower-middle part of the Claiborne Group


38

SOUTH TEXAS(RIO GRAND EMABAYMENT)

EAST-CENTRAL TEXAS(HOUSTON EMBAYMENT)

JAC

KS

ON

GR

OU

PC

LAIB

OR

NE

GR

OU

PW

ILC

OX

GR

OU

PM

IDW

AY

GR

OU

PN

AV

AR

RO

GR

OU

PT

AY

LOR

CA

MP

AN

IAN

MA

AS

TR

ICH

TIA

NLO

WE

RM

IDD

LEU

PP

ER

LOW

ER

UP

PE

R

PA

LEO

CE

NE

EO

CE

NE

CR

ET

AC

EO

US

TE

RT

IAR

Y

(sou

th T

exas

)

upper

lower

Whitsett

Manning

WellbornCaddell

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

AnacachoLimestone

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

CarbonateFluvial

Fluvial-deltaic

Deltaic (fluvial -dominated)

Delataic (wave-dominated)

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

Lower Cretaceous undividedKl

Upper Cretaceous undivided (includes Olmos in places)

Ku

Intrusive rock of theBalcones ProvinceKi

Tertiary intrusive rocksTi

Quaternary basaltQb

Figure 1. Generalized columnar section for south Texas coal areas and adjacent areas of Mexico.


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.

100101

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

DosRepublicas Permit

FARCOMINING CO.

Eagle Pass

Uvalde

Qb

Qb

0 50 km

30 mi

KuKi

INDEX MAP

TX

N

BITU

MIN

OU

SSU

BB

ITU

MIN

OU

SL

IGN

ITE

Figure 2. Regional geology and location of bituminous coal deposits of south Texas and adjacentareas of Mexico. See figure 1 for map units. Also shown are speculative boundaries of coal-rankchanges.


40

for use in local cement plants and for export aslump coal for use in the Republic of Ireland.The coal occurs in two primary zones, the SanPedro and Santo Tomas zones, near theill-defined contact of the Bigford and overlyingEl Pico Formations, about 3,000 ftstratigraphically higher than the OlmosFormation (fig. 1). Speculations on the causeof the relatively high rank of these coals aremore problematic than those for the Olmoscoals. Because these coals occur higher in thesection than Wilcox Group coals from thesame general area, which have traditionallybeen described as lignite (Maxwell, 1962,Breyer and McCabe, 1986), depth of burialwould presumably not account for the rankelevation. Numerous lignite beds have alsobeen described as intercalated within the SanPedro – Santo Tomas canneloid coalsthemselves (Lonsdale and Day, 1937), whichwould also presumably rule out depth of burialor local thermal gradients as the cause of rankelevation.

Based on these considerations, itwould be logical to assume that the highlysapropelic nature and enriched liptinite content(reported to be 23% in Warwick and Hook,1995) of these coals could account for theiranomalously high heating value (~12,000Btu/lb AR, 14,000 MMmF, figs. 3 and 4),without an added thermal factor. Are-examination of nearby Wilcox coalssuggests, however, that a much wider area andstratigraphic range of South Texas has beensubjected to elevation in rank, probably bylocal heat flow, than previously thought.Recalculating the Wilcox coal qualityparameters for the Uvalde area (fig. 2) fromMaxwell (1962, tables 30, 31, and 32) gives arange of MMmF Btu/lb from 10,494 to11,893, (i.e. subbituminous A to high-volatileC bituminous). Note that sample no. 1-A from

Maxwell's table 30, which is lignite on anMMmF basis, is clearly weathered, and someother samples from his tables appear to havelost moisture; normalizing to 15% moistureconsistent with table 32 results in an apparentrank of subbituminous B to high-volatile Cbituminous. Proprietary analytical datarecently made available to the USGS for theNational Coal Resource Assessment supportsthese determinations. Together they indicatethat on the basis of heating value, the Wilcoxcoals range from subbituminous B just east ofUvalde to high-volatile bituminous A towardsthe west, although even the most mature ofthese coals is generally described as lignite incompany drilling records. An averagereflectance value of 0.37% (Rm; range0.30-0.45%) reported by Breyer and McCabe(1986) from an area just east of Uvalde issomewhat equivocal, but consistent withsubbituminous coals of the Powder RiverBasin of Wyoming (R.W. Stanton, USGS,personal communication). These values can becompared with some previously unpublishedrank parameters and standard analysis of theSanto Tomas and San Pedro coal zones andtypical Wilcox lignites from the Sandow minenear Austin, approximately 250 mi northeastof the Santo Tomas field, in figures 3 and 4.

To the author's knowledge, nopublished analysis of the intercalated "lignites"of the Bigford-El Pico interval exist. Onehighly weathered sample of a banded,somewhat splintery, stray bed that occurswithin the San Pedro interval about 40 ftabove the main bed at the Farco Trevino Minehas been analyzed by the USGS with equivocalresults (sample SP-UN-96-1, figs. 3 and 4).On a Btu basis, this coal would seem to beclassified as lignite; however, an average Rmaxof 0 .74% and ex t r eme ly h igh


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)

SA

ND

OW

MIN

E (

low

er C

alve

rt B

luff

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)

USGSSAMPLE NO.

% Sulfur (as-rec'd)

BTU (as-rec'd)

TR

EV

INO

MIN

E (

San

Ped

ro Z

one)

RA

CH

AL

MIN

E (

San

to T

omas

Zon

e)

2,000 ft∼∼

40 ft∼

40 ft∼

M. C

LAIB

OR

NE

W

ILC

OX

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

10010 20 30 40 5015 25 35 455

55 60 65 70 75 80 85 90 95 02,000

4,000

6,000

8,00010,000

12,00014,000 0

0.5

1.0

1.52.0

2.5

10 20 30 40 5015 25 35 455 55 60 65 70 75 80 85 90 950

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 70.371.11 1.45

20.60

3.20 61.38

2.183.35

29.89

6.69 80.36

1.64 1.02

10.29

6.66 80.14

1.791.2810.13

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

%O%S%C%H %N

0

1 ft

0.5

Figure 3. Standard analysis and elemental ratios for selected samples from the Santo Tomas coalfield (middle Claiborne) and Sandow Mine (Wilcox).


42

volatile-matter content imply a rank moresimilar to the canneloid coals. Othermaturation parameters (apparent specificgravity, residual moisture, agglomeratingcharacter, etc.) are more consistent withWilcox lignite, although the weathered natureof the sample is readily apparent form the highoxygen content, and none of these results canbe considered definitive. The higherreflectance value for the stray bed versus themain San Pedro – Santo Tomas beds as shownin figure 4 may be attributable toweathering-related diminution of the vitrinitesuppression typical in canneloid coals. Thissample fluoresces when examined under bluelight and shows algal cell structures similar tothe San Pedro and Santo Tomas canneloidcoals (P.D. Warwick, USGS, personalcommunication).

EXTENT OF THE COAL-RANKANOMALY AND ITS POSSIBLEIMPACT ON HYDROCARBON

GENERATION

As previously stated, it now appearsthat a much wider area of South Texas hasbeen subjected to coal-rank elevation than haspreviously been recognized. Early versions ofthe "Coal Fields of the United States" map(e.g. Campbell, 1908, through Trumball, 1960)showed small pods of bituminous coal withinthe overall lignite trend, and no interveningsubbituminous halos. The area ofhypothesized rank elevation was slightlyexpanded for a preliminary revision to this map(Tully, 1996), without the benefit of most ofthe new data or rank recalculations citedherein. Based on the information currentlyavailable, additional revisions to the map thatwill show a much wider and more graduatedarea of rank elevation in South Texas are being

proposed, as shown in figure 2. Any suchrevisions are speculative, however, withoutanalysis of fresh samples of the intercalatedstray beds in the El Pico – Bigford interval andadditional reflectance values and analyses fromthe Wilcox, Jackson and Yegua intervals (figs.1 and 2).

Coal-bed methane (CBM) shows havebeen recorded in some gas wells in Maverickand Dimmit counties, and a number of gasfields occurring in the general area producefrom Olmos and Wilcox sandstones (seeEwing this volume). The extent of thecoal-rank variation within South Texas cantherefore be considered particularly importantfrom the consideration of the CBM andconventional source-rock potential. The H/Cand O/C ratios from figures 3 and 4 are plottedon a Van Krevelen diagram in figure 5, alongwith data for Olmos Formation coals derivedfrom various sources (Evans,1974, table 5;Miller and others, 1998; and unpublishedUSGS files). A few examples of JacksonGroup coals, which are generally not discussedin this paper, but will be seen on the field trip,are also shown for completeness; these arefrom unpublished USGS sampling of the SanMiguel Mine. As to be expected, the Wilcox,Olmos, and Jackson coals plot within the typeIII kerogen path, and the less humic SantoTomas and San Pedro coal zones plot closer tothe type II field. The Santo Tomas coals,which appear slightly more oil prone on thediagram, yielded 52 gal/ton oil and 5672 SCFper ton gas from low-temperature distillationdone by the U.S. Bureau of Mines (Ashley,1919). Rock-Eval pyrolysis for one SantoTomas sample by Mukhopadhyay (1989, fig.28a), which yielded 66% total organic carbonand very high and low hydrogen and oxygenindices respectively, placed these coals directlyon the type I I kerogen f ie ld .


43

AL-H2-6-1

AL-H2-P

AL-H2-8-1

NA (pollen sample)

SA

ND

OW

MIN

E (

low

er C

alve

rt B

luff

Fm

)

?

1.29

1.32

52.65 47.35

1.3552.89 47.11

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.

20004000

6000

800010,000

14,00016,000

12,000

% Volatile matter

50 52 54 56 58 60 62

64 66

60.57

% Fixed carbon

Apparent Sp. Gr.*

(as-rec'd)

57.90

57.91

59.38

64.93

% Residual moisture

1.38

1.0

2.03.0

4.05.0

6.0

7.0

8.09.00

BTU (MMmF)

TR

EV

INO

MIN

E (

San

Ped

ro Z

one)

RA

CH

AL

MIN

E (

San

to T

omas

Zon

e) 1.54

1.43

1.24

1.68

1.4

1.2

1.6

1.8

1.3

1.5

1.7

1.01.1

1.13

1.20

1.161.12

1.77

1.63

35 373933 41 43 45 47 49

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

0.7435.07

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. C

LAIB

OR

NE

W

ILC

OX

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

0.5except as noted, all are dry, ash-free basis

Figure 4. Selected rank parameters for selected samples from the Santo Tomas coal field (middleClaiborne) and Sandow Mine (Wilcox).


44

The remarkable ease of ignition that enablesthese coals to be exported in considerablequantities for domestic use is further evidenceof their oil-generating potential. Most of theOlmos data shown on figure 5 approaches thelower limits of hydrocarbon generatingmaturity. In addition to being somewhatoil-prone in composition, these coals may lackthe permeability to adsorb gas or produceCBM commercially.

The Olmos Formation data shown onfigure 5 is generally consistent with that of asomewhat more mature humic coal than theother samples. Available coal-quality data forthe Olmos Formation is, however, scarce andsomewhat questionable. The data shown onfigure 5 from the MICARE mine samples(Miller and others, 1998) shows consistentlylower oxygen values than the other Olmossamples, and thus more apparent maturation,placing them well beyond fluid hydrocarbongeneration thresholds. While this conceivablycould reflect increasing rank towards localigneous activity, it seems more probable thatthis is an analytical bias. Other data from theMICARE mines (Verdigo and Arciaga, 1991,table 2) show higher average oxygen values,but the reporting basis is not clear. The DosRepublicas sample analysis appears to bereliable, but these and other recent analysesfrom the Eagle Pass area show a very widerange of ash, and corresponding elementalvalues where available, that may beattributable to sampling techniques. The "oldEagle Pass" samples on figure 5 came from avariety of mine sources sampled before 1912(see Evans, 1974), including unwashed andwashed samples, with some apparent drying,and possibly weathered. The available data onheating value for the MICARE coals is alsoinconclusive in this regard. The Olmos datashown on figure 5 represent an attempt to

select "typical" samples with relatively similarash values from the various data sets, with theexception of the "old" samples, where thewidest range is shown. The two San Miguelsamples selected for the diagram are probablyslightly atypical, in that they are at the higherend of the recorded heating values for USGSsamples from that mine. These and theSandow samples shown on the diagram fall inthe relatively less mature end of the humicpathway as expected. All of the oxygen valuesused for figure 5 are by difference rather thandirect measurement, with the possible butunlikely exception of the "old Eagle Pass"samples.

No elemental data is currently availableto the USGS for Wilcox, Yegua, or Jacksoncoals from the higher rank areas shown infigure 2. Based on their location, at least someof the reported CBM shows in the area arewithout question from Olmos Formation coals.It appears likely that a considerable area ofWilcox coals may also have reached thermal-gas-generating maturity in the South Texasarea. Although highly speculative, based onthe extent of Tertiary volcanism in nearbyareas of Mexico, it appears that Yegua andJackson coals could have been similarlyelevated in rank as far south as FalconReservoir (about 90 mi southeast of Laredo,fig. 2). In a detailed study of the petrologyand organic geochemistry of Texas coals,Mukhopadhyay (1989) suggested that manyTexas coals are enriched in liptinite relative toother coals of similar rank and may havegenerated liquid hydrocarbons earlier on thematuration pathways than is normallyexpected. Although very few of his samplesare from South Texas, where the maturationpotential would seem highest, he suggests (p.102) that some of the oil produced from the


45

1.5

1.0

0.5

0 0.1 0.2 0.40.3

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

Ato

mic

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 andEast Texas may be sourced in the Wilcox.

One potential constraint on commercialCBM viability in South Texas may be thethickness of the coal beds, which for individualbeds is generally less than ten feet thick.There are multiple beds in each of thecoal-bearing intervals, however, and in generalcoal thicknesses beyond shallow (i.e. mineable)depths have not been thoroughly investigated.It should be noted that nothing in this paperaddresses the potential for early or latebiogenic gas occurrences in Texas coals, whichshould be considerable based on recent activityin the Powder River Basin of Wyoming. It isalso worth noting that although the

coal-bearing facies of the Olmos Formation ismapped with an eastward pinch-out intomarine rocks a few miles southwest of Uvalde(fig. 2), anecdotal evidence presented inMaxwell (1962, p. 90) suggests thatCretaceous coal-bearing rocks may be presentsomewhat farther east and north.

CONCLUSIONS

Available information based on limitedand poorly controlled sampling and analysissuggests that the coal rank anomaliesrecognized in the Olmos Formation and middleClaiborne Group of South Texas and adjacentareas of Mexico may extend to a considerably


46

wider area and stratigraphic range. Additionalsampling and analysis of the entire intervalfrom the Upper Cretaceous Olmos Formationthrough the upper Eocene Jackson Groupwould be useful in refining the known extentof rank variation. Coal-bed desorption anddrill-stem or more sophisticated down-holetesting would help determine the methane-producing potential of these coals. Biomarkerand stable isotope studies of hydrocarbonsproduced from more conventional reservoirs inthe area may also provide useful informationon the source-rock potential of these coals.

ACKNOWLEDGMENTS

The author wishes to thank the staff ofthe Farco Mining Company and the AlcoaSandow Mine for their permission to visit andsample their mines, and the USGS staff whocollectively participated with me in thesampling used for this report: Peter Warwick,Robert Hook, and Sharon Crowley.

REFERENCES CITED

Ashley, G. H., 1919, The Santo Tomas cannelcoal, 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 theWilcox Group of east Texas and theJackson Group of east and SouthTexas, in Finkelman, R.B.,Casagrande, D.J., and Benson, S.A.,eds., Gulf Coast lignite geology,Prepared for the fourteenth bienniallignite symposium on the technologyand utilization of low-rank coals:Environmental and Coal Associates,Reston, Va., p. 83 – 128.

Breyer, J.A., and McCabe, P.J., 1986, Coalsassociated with tidal sediments in theWilcox Group (Paleogene), SouthTexas: Journal of SedimentaryPetrology, v. 56, no. 4, p. 510-519.

Cambell, M. R., 1908, Coal fields of theUnited 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, Geologyand groundwater of Webb County,Texas: U.S. Geological Survey WaterSupply Paper 778, p. 1-104.

Maxwell, R. A., 1962, Mineral resources ofSouth Texas, region served throughthe Port of Corpus Christi: Bureau ofEconomic Geology, University ofTexas at Austin, Report of Inv. No.43, 140 p., 5 pl., 44 tables.


47

Miller, J.D., Parga, J.R., Drelich, Jaroslaw,and Lin, C.L., [1998?], Coal cleaningopportunities for SO2 emissionreduction in the border region, FinalReport to the U.S. Southwest Centerfor Environmental Research andPolicy: University of Utah, Salt Lake,and Institute of Technology, Saltillo,Mexico, 37 p.

Mukhopadhyay, P.K., 1989, Organicpetrology and organic geochemistry ofTexas Tertiary coals in relation todepositional environment andhydrocarbon generation: TheUniversity of Texas at Austin, Bureauof Economic Geology Report ofInvestigations 188, 118 p.

Trumball, James, 1960, Coal fields of theUnited States: U.S. Geological Survey,sheet 1 of 2, scale1:5,000,000.

Tully, John, digital compiler, 1996, Coal fieldsof the conterminous United States:U.S. Geological Survey Open-FileReport 96-92, 1:5,000,000.

Verdigo, F.D., and Arciaga, M., 1991, TheFuentes-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, anddepositional setting of the San Pedroand Santo Tomas coal zones:anomalous algae-rich coals in themiddle part of the Claiborne Group(Eocene) of Webb County, Texas:International Journal of Coal Geology28, p. 303-342.

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