Nomenclature of Formations of 

Claiborne Group, Middle Eocene 

Coastal Plain of Texas

GEOLOGICAL SURVEY BULLETIN 1251-D

Nomenclature of Formations of 

Claiborne Group, Middle Eocene 

Coastal Plain of Texas

By D. HOYE EARGLE

CONTRIBUTIONS TO GENJERAL GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1251-D

The nomenclature and correlations 

of the middle Eocene of Texas are 

updated, with special reference 

to the Rio Grande embayment

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract__ ______________________________________________________ Dl

Introduction- _____________________________________________________ 1

Formations of the Claiborne Group__________________________________ 7

Carrizo Sand and Reklaw Formation_________________--__________ 7

Bigford Formation___________________________________________ 9

Queen City Sand and Weches Formation_________________________ 10

El Pico Clay_____-____-______.______.___--_____-_-___.--__-- 11

Sparta Sand and Cook Mountain Formation_________________-____ 19

Laredo Formation-____________________________________________ 21

Yegua Formation____________________________________________ 22

Literature cited-__________________________________________________ 24

ILLUSTRATIONS

Page 

FIGURE 1. Map showing outcrop area of the Claiborne Group._________ D3

2. Geologic section A-A', extending eastward obliquely across

part of the Rio Grande embayment____________________ 6

3. Graph showing electrical characteristics of formations of the

Claiborne Group_______-___________-____-_____-_-_____ 8

4. Composite section of El Pico Clay____-_____-________-_____ 12

5. Map showing location of areas studied in the Rio Grande

region_____________ ___________________________________ 16

6. Graph showing comparison of electric log with core log_______ 20

TABLE

Page 

TABLE 1. Former and present classification of Claiborne Group in

Texas_______________________________________ D4

ITT

CONTRIBUTIONS TO GENERAL GEOLOGY

NOMENCLATURE OF FORMATIONS OF THE CLAIBORNE 

GROUP, MIDDLE EOCENE, COASTAL PLAIN OF TEXAS

By D. HOYE EARGLE

ABSTRACT

In the northeastern two-thirds of the Gulf Coastal Plain of Texas the outcropping Claiborne Group (middle Eocene) consists of vertical sequences of 

alternating marine and nonmarine or near-shore marine units. The marine formations consist mostly of glauconitic fossiliferous clay, and the nonmarine to 

shallow-water marine formations are blanket sands. Each formation is traceable 

for hundreds of miles laterally along the outcrop and for several tens of miles 

downdip. In the Rio Grande embayment of southern Texas, however, the formations, except the Carrizo Sand at the base, differ in lithologic character and 

fossil content from their equivalent formations to the northeast. Several changes 

in nomenclature are necessary to make each unit of similar lithologic character 

a separate formation and to update the stratigraphy of the Rio Grande embayment.

The name Mount Selman is here abandoned for the formation consisting, in 

ascending order, of the Reklaw, the Queen City Sand, and the Weches Greensand 

Members, and each of the three members is elevated to the rank of formation. 

South of the Sabine uplift in Louisiana the Cane River Formation is the equivalent 

of these units. In the Rio Grande embayment the equivalent of the marine clayey 

Reklaw is the Bigford Formation, formerly of member rank and consisting chiefly 

of marine sands with some beds of clay and cannel coal. The overlying beds 

equivalent to the Queen City and Weches undivided, previously unnamed and 

consisting chiefly of clay and some cannel coal, are here named the El Pico Clay. 

The beds equivalent to the Sparta Sand and the clayey Cook Mountain Formation, 

undivided in the embayment and generally called the Cook Mountain there, 

were renamed the Laredo Formation in 1938 by Julia Gardner. Although the 

name has not been widely used, if at all, since 1938, future general use of this 

appropriate name is recommended. More than half of the Laredo Formation 

consists of sands. The Yegua Formation, chiefly sand with less clay in the northeast, is chiefly clay with less sand and some banks of oyster shells in the Rio 

Grande valley.

INTRODUCTION

In its outcrop across a large part of the Gulf Coastal Plain of Texas, 

the Claiborne Group, middle Eocene, exhibits one of the best known 

examples of cyclic sedimentation; nonmarine and near-shore blanket 

sands alternate with glauconitic fossiliferous marine clays and silts. 

During Claiborne time there were at least three major and fairly

Dl
D2 CONTRIBUTIONS TO GENERAL GEOLOGY

rapid marine transgressions followed by generally less rapid regressions 

of the sea. Each principal lithologic zone, traceable for hundreds of 

miles laterally and a few tens of miles downdip (fig. 1) and marked by 

the environment in which it was deposited, is now considered a formation. Acceptance of this concept requires several changes in stratigraphic nomenclature from that previously in use.

This discussion considers chiefly the justification for the terminology 

proposed and covers only little of the history of past usage. Table 1 

shows the former and present nomenclature of the formations.

The changes in nomenclature recommended here are briefly as 

follows. The Mount Selman Formation is abandoned, and its members in ascending order in central and eastern Texas, the Reklaw, 

Queen City, and Weches are raised to formation al rank. The Queen 

City feathers out in western San Augustine County. South of the 

Sabine uplift, Weches, Queen City, and Reklaw rocks in Texas are 

equivalent to Cane River rocks in Louisiana. The name Bigford, 

once a formation but later the lower member of the Mount Selman 

in the Rio Grande embayment of southern Texas, is restored in this 

report to formational rank. The name El Pico Clay is introduced 

here for the beds once called the Mount Selman Formation in the 

Rio Grande embayment, but later called the unnamed upper member 

of the Mount Selman. The name Laredo Formation introduced 

by Gardner (1938) but never widely used is recommended for the 

beds formerly called the Cook Mountain Formation in the Rio 

Grande embayment and equivalent to the Sparta Sand and Cook 

Mountain Formation of eastern and central Texas. The Carrizo 

Sand, the basal formation, and the Yegua Formation, the uppermost 

formation of the Claiborne Group, remain unchanged. The lithologic 

character of the Yegua in the Rio Grande embayment is somewhat 

different from that to the northeast. The boundaries of the Yegua, 

having presented some problems in the past, were affirmed in eastern 

and central Texas by Stenzel (1938) and were corrected in southern 

Texas by Lonsdale and Day (1937). For south-central Texas, however, the upper boundary has been redefined, and the area has been 

remapped during the present survey.

The lateral changes within the Claiborne Group from marineshelf facies northeast of the Rio Grande embayment to fresh- or 

brackish-water facies and generally thicker units in the embayment 

are not definitely localized or sharp, but the sediments of the two 

environments interfinger complexly along the margins of the basin. 

The northwest-trending axis of the embayment, as defined for this 

report by the thickest lower Tertiary sediments, lies generally along 

the Nueces River. The Rio Grande flows along the west edge of the 

basin, nearly paralleling the strike of the rocks. The east edge of
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D3

100° 98°

EXPLANATION

Yegua Formation

Cook Mountain 

Formation and 

Sparta Sand

Laredo 

Formation

Weches Formation, Queen City 

Sand, and Reklaw 

Formation

Includes some areas 

of Sparta Sand

Bigford 

Formation

0 25 50 75 100 MILES 

I I I I I -1

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26°-

FIGURE 1. Outcrop area of the Claiborne Group in Texas. Modified from Darton, 

Stephenson, and Gardner (1937).
D4 CONTRIBUTIONS TO GENERAL GEOLOGY c

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NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D5

the basin, in the area near the Frio River where beds of differing 

characteristics interfinger and intergrade both laterally and vertically, 

is ill defined. Studies of well logs of the shallow subsurface made in 

recent years for water-resource and oil investigations (Alexander 

and others, 1964; Harris, 1965; Hargis, 1962) have resulted in a 

better understanding of the stratigraphic relations of formations of 

the Claiborne Group in southern Texas. Much fieldwork on the outcrop, however, and tracing of units by study of logs and subsurface 

materials must be done before the boundaries can be defined in 

detail.

A geologic section, prepared by W. H. Alexander and B. N. Myers 

(generalized as pi. 6 in Alexander and others, 1964), extends eastward downdip from the outcrop of the Claiborne Group in the Nueces 

River basin, then northeastward along the strike. The section shows 

the intertonguing of beds in the Rio Grande embayment, both downdip and laterally along the strike, with those of the shelf deposits to 

the northeast (fig. 2), and illustrates the necessarily arbitrary definition of limits of stratigraphic units in such instances of interfingering 

relations.

The cooperation of Dr. V. E. Barnes, Associate Director of the 

Bureau of Economic Geology, The University of Texas, who, in 

connection with the Texas Geologic Atlas Project, accompanied the 

author to the Rio Grande embayment to study outcrops, and who 

reviewed this paper, is gratefully acknowledged. Dr. P. T. Flawn, 

Director of the Bureau of Economic Geology, permitted the author 

to study the core of the San Ygnacio water well (fig. 6) and to obtain 

samples of the core for fossil determination. Mr. L. H. Henderson, 

of the International Boundary and Water Commission, furnished the 

electric log and water-quantity and water-quality data on the San 

Ygnacio well. W. H. Alexander, Jr., of the U.S. Geological Survey, 

reviewed with the author subsurface correlations in the Rio Grande 

embayment and permitted use of work sheets, prepared by him and 

B. N. Myers (also of the U.S. Geological Survey), showing the 

intertonguing of beds in the embayment (fig. 2).

The assistance of ranchers along the Rio Grande, especially Mr. 

A. F. Muller, of La Bota Ranch, and Mr. Emilio Garza, of El Pico 

Ranch, in permitting fieldwork to be done on their lands and in 

supplying geographical and historical information on the region 

northwest of Laredo, is also gratefully acknowledged.

278-25.8 65
D6 CONTRIBUTIONS TO GENERAL GEOLOGY

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NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D7

FORMATIONS OF THE CLAIBORNE GROUP 

CARRIZO SAND AND REKLAW FORMATION

The Carrizo Sand, basal formation of the Claiborne Group, is 

one of the most important aquifers of the updip Gulf Coastal Plain 

subsurface, and downdip it is a prolifically productive oil reservoir. 

The Carrizo is traceable from a featheredge north of the Sabine 

uplift for 500 miles southwestward into the Rio Grande embayment, 

where it is more than 1,000 feet thick (Alexander and others, 1964), 

and across the Rio Grande into Mexico.

The Carrizo is generally a massive to crossbedded sand with a 

few partings of carbonaceous micaceous silty clay or sand which 

probably was deposited under fluvial conditions, near shoreline 

(Todd and Folk, 1957). On the surface it unconformably overlies 

carbonaceous sands, silts, and clays of the Wilcox Group, but in 

the downdip subsurface its sands are distinguished with difficulty 

from those of the Wilcox; most geologists in the oil industry consider 

the Carrizo and Wilcox an indivisible unit.

The Reklaw Formation, which lies conformably on the Carrizo, is 

mostly marine and consists of even-bedded chocolate-colored shale 

with thin beds of glauconitic sand and of lignite (Stenzel, 1938, p. 75; 

Plummer, 1932, p. 625). Stenzel divided the outcropping formation 

into two members, the Marquez Shale at the top and the Newby Sand 

at the base. The contact of the marine Newby Sand Member with 

the underlying nonmarine Carrizo is more apparent on outcrop than 

it is in the subsurface. The Reklaw is distinguished on electric logs 

as a zone chiefly of low resistivity indicative of clay (the Marquez 

Shale Member of the outcrop). Below this zone on most logs is one 

of moderately nigh resistivity, generally with one or more low-resistivity breaks (fig. 3) foUowed in depth by a thick zone of very high 

resistivity. All geologists agree that the lower zone of very high 

resistivity is Carrizo Sand, and many call that zone the massive 

Carrizo. It is a unit of coarse massive sand containing fresh water, in 

contrast to the zones directly above it of finer sand containing water 

of higher mineral content (W. H. Alexander, Jr., oral commun., 

1966). Some consider the top of the first zone of moderately high 

resistivity to be the top of the Carrizo; others, who consider this first 

zone to be the basal sand of the Reklaw, place the top of the Carrizo 

at the top of some lower zone of moderately high resistivity. The 

writer places the top of the Carrizo at the first principal break from 

fairly low resistivity to markedly higher resistivity.

From the downdip area to the outcrop the transgressive nature of 

the Reklaw can be seen in the interfingering of its basal sands and 

clays with the sands of the Carrizo. Beds of sand near the outcrop
D8 CONTRIBUTIONS TO GENERAL GEOLOGY

Yegua Formation

Cook Mountain Formation

Sparta Sand 

Weches Formation

1000 

1500

2000

Queen City Sand

Reklaw Formation

Carrizo Sand

*Top of Carrizo Sand after Anders (1957)

FIGUBE 3. Electrical characteristics of formations of 

the Claiborne Group as demonstrated on the log of 

the Poth Water well 3. Three possible contacts of the 

Carrizo Sand with the Reklaw Formation are indicated.
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D9

grade downdip into beds of silt and clay; the stratigraphically higher 

beds grade into fine materials within a shorter distance downdip than 

do the lower beds. Thus, as one proceeds up the dip the ReklawCarrizo contact moves stratigraphically upward.

Southwestward along the strike the generally clayey nature and 

marine characteristics of the Reklaw persist on the outcrop almost to 

the Frio River. Beyond the Frio River, clay beds become much thinner 

and interfinger with sands. These sands, which contain some fossils 

and are interbedded with clays, pass laterally westward and southward into the Bigford Formation, the approximate equivalent of the 

Reklaw.

The Reklaw ranges in thickness from 30 to 130 feet in eastern 

Texas (Wendlandt and Knebel, 1929, p. 1371). It is about 100 feet 

thick in Leon County in east-central Texas (Stenzel, 1938); as much 

as 300 feet thick in Wilson County in south-central Texas (Anders, 

1957); and from 200 to 400 feet thick in southern Texas, where it 

intertongues with the Bigford Formation (Alexander and others, 1964).

n 

BIGFORD FORMATION

The Bigford Formation, the equivalent in the Rio Grande embayment, of the Reklaw, was described by Trowbridge (1923) from 

exposures along the Rio Grande at the Bigford Ranch in northwestern 

Webb County, about 45 airline miles northwest of Laredo. The type 

locality, shown as Cuatralvo Ranch on recent maps, lies north of a 

4-mile eastward-flowing reach of the Rio Grande about 20 miles 

southeast of the Maverick-Webb County line. The Bigford has been 

traced for about 35 miles in good exposures along the breaks of the 

Rio Grande from about 9 miles downstream from the MaverickWebb County line to the vicinity of the old coal mines northwest of 

Laredo. This unusual length of good exposure is due to the fact that 

the course of the river nearly parallels the strike along the west flank 

of the structural basin, the Rio Grande embayment (fig. 1). On the 

east flank of the embayment the Bigford interfingers with the Reklaw 

Formation (fig. 2; Getzendaner, 1930, p. 1436). Exact correlation has 

not been possible in the past because of poor exposures and lack of 

traceable horizons, but electric logs of wells a short distance downdip have made possible a more certain correlation and have shown 

the gradation from the predominantly clayey Reklaw northeast of 

the Rio Grande embayment to the more sandy Bigford in the embayment. (See sections in Harris, 1965.)

The descriptions of the Bigford Formation by Trowbridge (1923, 

1932) and the measured sections recorded by Lonsdale (1935) and 

Lonsdale and Day (1937) give details of the exposures in the outcrop 

area and adequately describe the lithology and fossil content. Julia
D10 CONTRIBUTIONS TO GENERAL GEOLOGY

Gardner measured many of the sections and identified the invertebrate 

fossils for both the Trowbridge and the Lonsdale and Day surveys.

Because of fossil-plant determinations by Berry (1923), Trowbridge 

considered the Bigford and the underlying Carrizo Sand to be Wilcox 

in age. Trowbridge (1932, p. 52) suggested, however, that geologists 

found that field relations and lithologic characteristics showed the Carrizo more directly related to the Claiborne than the Wilcox. Wendlandt 

and Knebel (1929), Ellisor (1929), and most geologists since that 

time show the Bigford and the Carrizo to be definitely Claiborne 

in age. Gardner, as well as most later authors, considered the Bigford 

the basal member of the Mount Selman Formation in the Rio Grande 

embayment, but she left the thicker clayey upper part unnamed.

The base of the Bigford is placed at its contact with an underlying 

massive and highly permeable sand (the Carrizo). The formation 

consists of sands, some very thin beds of fossiliferous silts and shales, 

and several coal beds. The shales are brown to gray and gypsiferous, 

and they make up about 25 percent of the formation along the Rio 

Grande (Lonsdale and Day, 1937, p. 31). Near the top of the formation 

is the San Pedro coal, one of the two cannel coals of the once commercially important Webb County coal basin.

The Bigford is reported to be a little more than 650 feet thick 

along the Rio Grande (Lonsdale and Day, 1937, p. 22) and somewhat 

thicker in the vicinity of the Nueces River where the structural basin 

of the Rio Grande embayment centers. Alexander, Myers, and Dale 

(1964, table 1) show a thickness of 400-800 feet in the Nueces River 

valley.

QUEEN CITY SAND AND WECHES FORMATION

The Queen City Sand, lying conformably on the Reklaw Formation, 

is a thick unit of sands and sandy clays that extends from eastern 

Texas southwestward to the eastern part of the Rio Grande embayment. The Queen City in eastern Texas contains beds of almost pure 

quartz sand, of sandy clay and greensand, and of lignite and bentonitic 

clay (Wendlandt and Knebel, 1929, p. 1355). A distinct and traceable 

bed of glauconite and glauconitic clay in northeastern Texas was 

called by Wendlandt and Knebel (1929, p. 1355) the Omen Member. 

In south-central Texas bright-red-weathering beds near the middle 

of the Queen City, possibly equivalent to the Omen Member, contain 

local radioactive anomalies (Moxham and Eargle, 1961) that may be 

placer concentrations of heavy-mineral sands.

South of the Sabine uplift, extending to the Sabine River from 

about 40 miles west of the river, the Queen City is marine, indistinguishable from the Weches above and the Reklaw below. This undivided section is equivilent to the Cane River Formation in Louisiana.
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS Dll

The Queen City is as much as 480 feet thick in eastern Texas 

(Wendlandt and Knebel, 1929, p. 1355) and 750 feet thick in the 

updip subsurface near Poth. Southward, it thickens further and 

gradually becomes clayey; beyond the Frio River (fig. 2) it is indistinguishable from the overlying Weches as it and the Weches interfinger 

laterally with the carbonaceous clays of the El Pico.

The Weches Formation in eastern Texas is a relatively thin marine 

formation consisting chiefly of fossiliferous glauconitic clay with some 

marl and limestone (Stenzel, 1938). It lies conformably on the Queen 

City Sand. The weathered outcrop of the Weches contains the principal iron-ore-bearing beds of eastern Texas. The marine character of 

this formation persists across most of the eastern and south-central 

part of the State, but the formation becomes gypsiferous carbonaceous 

sandy clay at the east margin of the Rio Grande embayment.

In eastern Texas the Weches is about 50 feet thick (Wendlandt and 

Knebel, 1929, p. 1371). In the updip subsurface in south-central 

Texas, where its thickness can best be determined from well logs, it 

is about 100 feet thick. Southwest of the Frio River the Weches 

becomes generally nonmarine and is indistinguishable on well logs 

from the Queen City below. There, the two formations merge laterally 

into the El Pico Clay of the Rio Grande embayment.

EL PICO CLAY

El Pico Clay is the name here proposed for the formation in the 

Claiborne Group that Trowbridge (1923) called the Mount Selman, 

of Claiborne age, in the Rio Grande region. He found it to be distinct 

from the underlying Bigford Formation, which, with the Carrizo 

Sand, he considered to be of Wilcox age. Lonsdale (1935), in his 

survey of Atascosa and Frio Counties, expanded the Mount Selman 

to include equivalents of the Bigford in south-central Texas and 

considered the Mount Selman and underlying Carrizo Sand to be 

Claiborne in age. Lonsdale and Day (1937) formally designated the 

Bigford the lower member of the Mount Selman in Webb County, as 

did Gardner (in Darton and others, 1937). Turner, Robinson, and 

White (1960) and Alexander, Myers, and Dale (1964) followed 

Lonsdale and Day in designating as the Bigford the lower of two 

members, and they have referred to the upper member, here named 

El Pico, as "post-Bigford beds." The name Mount Selman is not 

appropriate for the beds in southern Texas because of their distance 

from the Mount Selman type locality, because the character of the 

rocks is in almost no way comparable to the Mount Selman of the 

type locality, and because no purpose would be served by keeping 

the name. Thus, by describing the Bigford and the El Pico as forma-
D12

Laredo 

Formation

CONTRIBUTIONS TO GENERAL GEOLOGY

EXPLANATION

100'-

300' -=m: HH400'- =:==;

tl Pico 

Clay 600'

700'

800'

900'-

1000' -

Bigford 

Formation

Shale

-2-foot bed of lignite 

and lignitic clay

Nonmarine fossil zone 

(local)

Coal

 16-inch coal bed

 12-inch bed of clayey coal

2-inch nonmarine fossil bed 

/,4-foot sandstone with marine-fossil casts 

 16-inch coal bed~"| 

33-inch coal bed L Santo Tomas coal bed

-inch coal bed |

-inch coal bedj 

Marine-fossil zone (top of Bigford Formation)

FIGURE 4. Composite section of El Pico Clay, from sections measured 

along the Rio Grande in Webb County, Tex. Modified from Lonsdale 

and Day (1937, fig. 3 and p. 35-37).

tions, the nomenclature is simplified and opportunity is given for 

more exact definition of units in the Rio Grande embayment.

Type section. The formation is named for El Pico, a conspicuous 

mesa-butte in Webb County, about 8% airline miles northwest of 

downtown Laredo (fig. 5). El Pico is in the approximate center of a 

shoe-shaped bend of the Rio Grande (thus, the name of the ranch on
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D13

which it is located, La Bota, the boot), about three-fourths of a 

mile from the river, about a mile northwest of the mouth of Sombrerito 

Greek, and about 2 miles southwest of the bridge where Farm Road 

(FM) 1472 crosses the creek.

The El Pico Clay is exposed in many places along the breaks of the 

Rio Grande for at least 25 miles upstream from the mouth of Sombrerito Creek. Trowbridge (1923, 1932) and Lonsdale and Day 

(1937) recorded many measured sections and fossil identifications by 

Julia Gardner. A composite section (fig. 4) of the beds that make up 

this formation, compiled by Londsale and Day (1937, p, 33, 35-37), 

gives in considerable detail the characteristics of the formation along 

the Rio Grande and is designated the type section of the El Pico.

Type section of El Pico Clay

[Measured along Rio Grande for 25 miles between Bigford-Laredo contacts (fig. 1). Quoted from Lonsdale

and Day (1957, p. 35-37)]

Ft in 

Ledge of fine-grained massive yellow-brown and yellow mottled gray

micaceous sandstone__________________________-________-__-__ 4 0

Soft yellow-brown shale and massive sandstone and alternating shale- 5 0 

Yellow, gray, and brown clay and shale with iron concretions.______ 21 0

Purple, gray, and brown soft massive sandstone and alternating ironstained gypsiferous shale________________________________-_-_- 4 0

Hard silty and limy concretionary yellow-gray boulders.___________ 1 0

Gray shale and sand___________________________________________ 6 0

Deep-brown and dull-green sand and thin-bedded sandstone._______ 6

Soft brown sandstone and alternating gray shale_________________ 1 0

Gray and brown iron-stained gypsiferous sandy shale______________ 12 0

Soft rusty-brown thin-bedded sand_________________________-___- 1 0

Gray, brown, and yellow laminated iron-stained gypsiferous clay and 

sandy shale___-______________-________________________------ 8 0

Brown, gray-pink, and purplish shale and clay.___________________ 25 0

Soft fine-grained yellow mottled gray massive sandstone__________ 6 0

Fine-grained brown sandstone forming a soft ledge______________ 6 0

Soft massive yellow-brown sand______________________________-_- 7 0

Soft ledge of yellow-brown and gray bedded sandstone.____.__---_- 5 0

Soft yellow-brown massive argillaceous sand ______________________ 10 0

Gray and brown mottled shaly clay______-_______--_-_-_______- 18 0

Purplish-gray and brown gypsiferous iron-stained sandy clay_______- 19 0

Poorly bedded sandstone with a few hard boulders 6 inches in diameter_____________________________________.- 16 0

Gray, brown, and yellow mottled gypsiferous jointed clay._________ 17 0

Mostly clay covered by river alluvium, about-_______________----- 100 0

Soft massive poorly bedded gray sandstone.______________________ 4 6

Blue-gray jointed clay______________________-_______-------_- 4 0

Soft reddish, rusty-brown, and gray argillaceous sand______________ 4 0

Blue-gray jointed clay________________________________________ 6 0

Soft yellow, brown, and gray argillaceous sandstone______________ 6

Blue-gray shale and jointed clay_______________________________ 6 0

Platy hard crystalline thin blue-gray sandstone__________________ 2
D14 CONTRIBUTIONS TO GENERAL GEOLOGY

Type section of El Pico Clay Continued

Ft in

Bony black coaL______________________________________________ 7

Blue-black lignitic clay---_-________________---_-__-___-__-_-___ 1 6

Yellow-gray argillaceous sancL_______________-__-_---_---_______ 2 0

Covered.._______________________________________________ 10 0

Thick-bedded and laminated hard gray sandstone.________________ 4 0

Gray clay with some limonite and gypsum, about________________ 15 0

Soft irregularly highly crossbedded gray sandstone______--_________ 5 0

Medium-hard ripple-marked bedded gray sandstone______________ 3 0

Blue-gray joint clay______-_________________--_---_-___-______ 3 0

Brown and gray clay and partly covered, about_ _______ ___________ 70 0

Brown and gray clay with some yellow limestone concretions, about_ 30 0 

Brown iron-stained gritty irregular sandstone, with local nonmarine

fossils._____________________________________________________ 1 0

Purple, gray, and brown thin-bedded to massive soft fine-grained

sandstone________________________________________________ 13 0

Sandy gray and brown clay and shale with gypsiferous laminations. 4 0

Brown, gray, pink, and purplish mottled gypsiferous clay___________ 46 0

Hard single ledge of massive fine-grained buff and gray sandstone,

in places yellow-stained._____________________________________ 1 0

Yellow-brown, gray, pink, and purplish clay______________________ 30 0

B?own and gray clay, mostly covered____---_--____-__--___-_____ 55 0

Brown clay_________-______________________-___-______-_-_____ 5 0

Soft ledge of massive gray-brown sandstone-____-_--------_---___- 2 0

Gray and brown mottled jointed clay_______-_-___----_-_________ 15 0

Soft massive gray sandstone_____________.________-___-_________ 12 0

Blue-gray jointed clay________________________________________ 10 0

Brown clay and some yellow concretionary limestone, but mostly

covered ____________________________________________________ 120 0

Soft massive ledge of purplish-gray sandstone, stained pink and

yellow brown______________________________________________ 3 0

Blue-gray and brown gypsiferous clay and a few yellow limestone

concretions.________________________________________________ 29 0

Yellow limestone concretionary bed_______-___-___-_____-________ 6

Soft thin-bedded and crossbedded mealy gray sandstone____________ 17 0

Brown clay-______--___-_____________________________--_--___- 17 0

Clay, slight dip, aboiit___-___________________--______----_--__- 15 0

Brown gypsiferous clay_________________-_________-____________- 16 0

Shaly gray and platy buff fine-grained sandstone and some yellowgray limestone concretions__________-______-___-_-__-_-_-____- 2 0

Brown gypsiferous clay_______________________________________ 7 0

Thin-bedded yellow-brown gritty iron-stained sandstone.-..________ 3 0

Shaly gray sandstone and shale________________________________ 3 0

Brown and gray clay_________________________________________ 5 6

Very irregular rusty-brown iron-stained sandstone with worm borings. 6

Brown iron-stained gritty sandstone.____________________________ 2 0

Brown and gray clay and sandy shale_____---_--____--_----_---_- 6 6

Soft gray sand and shaly sand_________________________________ 3 0

Shaly gray and brown sand and clay_____________________________ 13 0

Soft gray sand__________________________________._____________ 3 0

Brown and gray clay, partly covered__--___--_----_____--_-_-_-_- 53 0

Coal seam, mostly clay__________________-____-________________- 4
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D15

Type section of El Pica Clay Continued

Ft in 

Brown clay___________________________________________________ 3 6

Shaly gray sandstone___-__--__--__-__---___-_---_-_-.___-_-__-- 1 0 

Ledge of mealy gray massive sandstone-_________________________ 1 0

Thin-bedded gray sandstone and gray shaly sandstone with alternate

gray and brown clay______________-___-_______-___-._______-__ 4 0

Local ledge of hard yellow and gray boulders______--___-_______-__ 1 0

Gray clay____-___--__--__--__-__---___-_---__--__--____------ 4 0

Gritty yellow mottled iron-stained fine-grained sandstone___________ 1 0

Soft ledge of fine-grained and light purplish-brown to buff-gray

bedded sandstone__-______-___-_--____-___-___--_--__--__-__- 10 0

Soft fine-grained purplish-gray and brown sand with yellow ironstained gypsum laminations_________________________________ 4 0

Blue, gray, and brown clay_________-_-_-_-______-_-____--__--_- 12 0

Clayey coal bed________________________________________________ 1 0

Gray and green-brown clay_____________________________________ 32 0

Hard ledge of buff-gray wavy laminated massive fine-grained micaceous sandstone, local-_____________________________________ 2 0

Greenish-gray and brown-mottled clay___________________________ 18 0

Medium- to coarse-grained yellow-brown crossbedded and gritty 

soft sandstone with yellow limestone concretkms_.________-__-___- 17 0

Brown gypsiferous clay____ _____________________________________ 10 0

Wavy bands of gypsum crystals, almost continuous and above clay__ 6 

Soft fine-grained and mealy gray sandstone with gypsum laminations

and a few yellow limestone concretions.________________________ 2 6

Brown and gray gypsiferous clay______-_________- _______________ 9

Thin hard slaty gray silty sandstone with dwarf fossils, chara seeds,

and nonmarine fossils-_______________________________________ 2

Soft fine-grained and gray thin-bedded yellow-mottled gray and

brown sandstone; a few fossils at the base_________-_-__-_____-_ 4 0

Greensih-gray and brown clay_________________________________ 5 6

Coal, local-___________________________________________'

Gray and brown clay__________________________________ ,

Brown shale and slaty bone coal_____-____-_________-____ 

Black cannel coal (main Santo Tomas coal)______________

_, . zone Brown clay_______-___-___-___-________________________

1 4 

8 6 

6

2 9 

7 

Brown shaly coal, local-.----.--------------------------, k 1 2

Brown gypsiferous clay with a little soft brown sandstone at the base. 13 0 

Coal, local____-__--_---___-_---_--____-______-.-__--______--__- 1 0

Hard massive yellow-stained bluish and purplish-gray sandstone___ 2 0 

Soft shaly and thin-bedded yellow-stained gray sandstone-_________ 4 6

Brown clay__--___-------------_--____-___-__--__-____________ 4 0

According to Lonsdale and Day (1937, p. 35-37), beds now called 

the El Pico Clay are 1,165 feet thick along the Rio Grande, and they 

dip generally east-northeastward 90 feet per mile. Kane and Gierhart 

(1935) show them to be 1,124 feet thick along the Laredo-Monterrey 

highway and railroad in Tamaulipas and Nuevo Leon, Mexico, along 

the west flank of the Rio Grande embayment. In the Nueces River
D16 CONTRIBUTIONS TO GENERAL GEOLOGY

100° 99 e

28'

27° -

\

Cuatralvo 

(Bigford) Ranch

EXPLANATION

Yegua Formation and younger Tertiary 

rocks undifferentiated

Laredo Formation

Bigf ord Formation and older Tertiary 

rocks undifferentiated

FIGURE 5. Map showing location of areas studied in the Rio Grande region. 

Geology modified from Lonsdale and Day (1937), Patterson (1942), and 

Harris (1965), and by reconnaissance and aerial-photograph interpretation. Rocks 

overlying Pescadito dome are, in general, older than the surrounding Yegua 

Formation.
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D17

basin the thickness of the El Pico Clay ranges from 700 (Turner and 

others, 1960) to 900 feet (Alexander and others, 1964).

The El Pico Clay is not well exposed northward from the breaks of 

the Rio Grrande; streams have not deeply dissected the land, and 

gentle slopes have developed on the clay. A few sections have been 

recorded from this region, generally measured on flanks of ridges 

held up by sandstone ledges. The formation crops out along a broad 

north-trending belt across the Nueces and Frio River valleys, then 

curves eastward to the east edge of the Rio Grande embayment; 

there, it merges with the northeast-trending outcrop of the Queen 

City and Weches.

Along the Rio Grande between Laredo and Palafox (fig. 5) the 

Santo Tomas coal occurs about 30 feet above the base of the El Pico. 

The Santo Tomas and the San Pedro (80-90 ft lower, in the Bigford 

Formation) seams are the principal seams of the once-famous Webb 

County cannel coal field. Ashley (1918, p. 251) said that this field 

was "the largest body of cannel coal in the United States if not hi the 

world." He described the Santo Tomas as a low-moisture coal almost 

as hard as anthracite and highly resistant to weathering; on distillation it yields a much larger proportion of oil (at low temperature) or 

of gas (at high temperature) than does ordinary bituminous coal. 

Several large mines operated for many years in the district, but all 

are now abandoned. No coal of commercial significance has been 

found north of the Rio Grande region, but some thin beds of coal and 

lignite and some carbonaceous clays are found in the Nueces River basin.

Section on south slope of El Pico, a small mesa-butte on A. F. Mutter's La Bota 

Ranch, 8/z l miles northwest of Laredo, Webb County, Tex.

Terrace deposit: Feet 

Gravel cemented with caliche, pebbles as much as 4 in. in diameter, 

mostly chert, some quartz, quartzite, red porphyry, and hollow 

siliceous shells that are remnants of partially replaced limestone 

pebbles ___________________________________________________ 4

Laredo Formation:

Sandstone, yellowish- to reddish-brown weathering, fine-grained, semiindurated, medium-thick-bedded, slabby; upper foot is medium 

gray, highly indurated, glauconitic, calcareous.__________________ 12

El Pico Clay:

Sand, light-gray mottled with yellow, very fine grained, soft. _____ 6

Sand, silty, shaly, medium-light-gray mottled with yellow and purple. _ 8 

Clay, chocolate to pinkish-purple. _____________ __________________ 13

Sand, clayey, yellow-brown to light-gray, soft_____________________ 17

Sandstone, dark-brown, very fine grained, very ferruginous; weathers

platy (exposed in low hills 200 yd southwest of El Pico)__________ 3

Clay, chocolate, laminated, glauconitic; upper part contains conspicuous bands of selenite as much as 1 in. thick ____________ 10

Covered slope rising above alluvium of the Rio Grande____________ 10
D18 CONTRIBUTIONS TO GENERAL GEOLOGY

The upper beds of the El Pico, and especially its contact with the 

overlying Laredo Formation, are better exposed in the bank of 

Sombrerito Creek than in the section on the slopes of El Pico.

Section in southwest bank of Sombrerito Creek, eight-tenths of a mile northwest 0} 

the confluence of the creek with the Rio Grande, on La Bota Ranch, 8 miles northwest of Laredo, Webb County, Tex.

Laredo Formation: Feet

5. Sandstone, greenish-gray, fine- to medium-grained, moderately

indurated, moderately thickly bedded, moderately glauconitic- 12 ±

4. Sand, greenish-gray (weathers light brown), fine-grained, glauconitic; middle 3 ft is gypsiferous chocolate shale____________ 15

El Pico Clay:

3. Shale, carbonaceous; below 2 ft from top grades downward into 

sand that is dark gray (weathers pale yellowish brown), fine 

grained, glauconitic, carbonaceous; sand grades downward into 

bed2________________________________________________ 6

2. Shale, pale-chocolate to pinkish-brown; upper 2 ft very dark,

shaly, easily weathered; grades downward into bed !__________ 3

1. Sand, shaly, dark- to pinkish-gray; yellow staining along bedding; 

gypsiferous, carbonaceous, highly glauconitic; grades downward into lighter, less glauconitic, clayey sand________________ 12

The beds in this exposure dip about 2° SE.

The middle part of the formation is excellently exposed on Emilio 

Garza's El Pico Ranch, 10 miles northwest of Laredo, in high bluffs 

that rise above the flood plain of the Rio Grande.

Section on El Pico Ranch of Emilio Garza, 10 miles northwest of Laredo, Webb

County, Tex. 

Terrace deposit: Feet

Gravel cemented with caliche_____-------_---_--------_-----_-_- 4

El Pico Clay:

Sand, shaly, pale-olive to pale-purple, fine-grained, medium- to thinbedded. ____________________________________________________ 14

Sandstone, gray, fine-grained, semi-indurated, thick-bedded, sparsely 

glauconitic; conspicuous sand-filled cracks % in. wide, irregularly 

distributed..______________________________________________ 4

Sand, shaly, pale-olive-gray____________________________________ 4

Sand, yellow-olive-gray, fine- to medium-grained, semi-indurated, 

irregularly crossbedded to massive; interbedded with softer beds; 

glauconitic; has conspicuous vertical sand-filled cracks or joints____ 17

Sand, yellow-olive-weathering, fine-grained, soft (a 1-ft bed near base

is semi-indurated), massive, glauconitic_______________________ 11

Shale, silty, pale-chocolate; upper part interbedded with fine sand____ 22

Sand, shaly, light-gray with yellow staining, fine- to medium-grained

ferruginous; polygonal pattern of vertical joints._________________ 4

Shale, silty and fine sandy; gray with ferruginous staining; glauconitic- 6

Sand, gray to pale-olive, fine- to medium-grained, massive to irregularly bedded; upper 5 ft contains oysters; a zone of calcareous concretions, as much as 2^ ft in diameter, 14 ft from top; holds up low 

bench in mam and tributary valleys; extends to Rio Grande flood 

plain____...__________________________________________ 28
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D19

The formation lies conformably on a conspicuous fossil-bearing 

sandstone, the uppermost bed of the Bigford Formation, that is well 

exposed in bluffs above the historic site of Palafox on the Rio Grande, 

30 miles northwest of Laredo (Lonsdale and Day, 1937, p. 20).

Logs and cores from wells drilled not far downdip from the outcrop, 

where complete sections of the El Pico have been penetrated, show 

the nature of the formation near the outcrop.

In the Laredo Water Works well 1, drilled in the west-central part 

of the city, 915 feet of mostly clayey beds (from about 215 to 1,130 

ft in depth) can be assigned to the El Pico Clay (fig. 6). Beds of sand 

from about 575 to 610 feet in depth stand out prominently on the 

log. Perhaps this sand zone is the one that can be traced for a long 

distance on the surface; it forms a ridge above the surrounding clayey 

terrain across Webb County and is conspicuous on aerial photographs. 

A similar light-gray sand is found at about 500 feet in depth near the 

middle of the El Pico Clay section in the San Ygnacio School well of 

the International Boundary and Water Commission (fig. 6).

Several exposures of the El Pico Clay have been found on the 

United States side of the Rio Grande as much as 40 miles southeast 

of Laredo, several miles south of San Ygnacio (or San Ignacio). A 

core from the International Boundary and Water Commission's San 

Ygnacio School well, stored at the Well Sample Library of the Bureau 

of Economic Geology of The University of Texas at Austin, includes 

a representative section of the El Pico (fig. 6). The core extends from 

387 to 1,749 feet in depth, and is continuous except for several short 

sections represented only by cuttings. From its top to 1,057 feet 

(top of the Bigford) the core shows that the unweathered El Pico is 

a somewhat calcareous smooth light-gray to light-greenish-gray 

claystone, in part mottled with grayish red, interbedded with gray 

sandstones that contain biotite and generally abundant plant fossils. 

A light-gray sand just below the 500-foot depth may correlate with 

both the bed of similar Lithology in the middle of the El Pico in the 

Laredo well and the bed that forms a conspicuous cuesta on the outcrop. Thin coal beds from a few inches to about a foot thick are found 

in several places in the core; the thicker ones are in the middle of the 

El Pico section.

SPARTA SAND AND COOK MOUNTAIN FORMATION

The Sparta Sand extends westward from its type locality in Louisiana as a continuous band of sandy outcrop south of the Sabine uplift. 

It also caps numerous outliers of resistant ferruginuos rocks of the 

Weches Formation north and west of the uplift. It is thicker in eastern 

Texas than it is to the southwest. In east-central Texas the Sparta 

probably ranges in thickness from 210 to 330 feet (Stenzel, 1938);
D20 CONTRIBUTIONS TO GENERAL GEOLOGY 

LAREDO WELL EXPLANATION

100

500

1000

1500

2000

SAN YGNACIO WELL

^-=-^=1?- 1000

f/v^- 500'

 - 1500'

Coal

Calcareous

Glauconite

Clay ball

Fish-scale imprint

Fossil gastropod

Fossil oyster

Fossil pelecypod 

other than oyster

Fossil plant

FIGURE 6. Comparison of the electric log of the Laredo Water Works well 1 

with the core log of the International Boundary and Water Commission's 

San Ygnacio School well, 35 miles south of Laredo.
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D21

in the Poth Water well (fig. 3) in south-central Texas it is 90 feet 

thick. On electric logs of updip subsurface wells the Sparta is one of 

the most easily recognizable units of the Claiborne. It is a relatively 

thin zone of high resistivity and self potential bounded by thick 

zones of low resistivity and self potential, the Weches below and the 

Cook Mountain above. Downdip about 30-40 miles from the outcrop 

the Sparta is a fine-grained sediment indistinguishable from the beds 

above and below.

Plummer (1932, p. 654) considered that the Sparta represents a 

period that began with the regression of the shoreline of the Weches 

sea and ended with the transgression of the shoreline of the Cook 

Mountain sea. Its middle sands, he believed, are flutiatile sediments 

spread out over a flat terrain. The movements represented 

by the Sparta Sand probably took place in a relatively short time 

span.

The Cook Mountain Formation, overlying the Sparta conformably, 

consists chiefly of marine clays with a few beds of fine sand. The 

formation has been divided by some authors into several members in 

eastern Texas, but these members have mostly local significance. In 

eastern and central Texas the formation ranges in thickness from 

125 to 450 feet (Plummer, 1932, p. 658). In south-central Texas, in 

the Poth Water well (fig. 3), it is 385 feet thick, and it thickens 

downdip. The downdip thickening of the Cook Mountain, as well as 

that of the other marine clayey formations, is due in part to the 

increasing thickness of the prism of sediments toward the Gulf of 

Mexico, and in part to the changing of facies from sand to clay 

similar to the formations above and below. Thus, the clayey formations 

increase in thickness at the expense of the sands separating them.

Southwest of the Atascosa River, sands appear hi the Cook Mountain, intertonguing with the clays, and the formation becomes less 

distinguishable from the Sparta Sand below and the Yegua Formation above. In the Nueces River valley the Sparta and Cook 

Mountain merge southward into the Laredo Formation.

LAREDO FORMATION

Gardner (1938) gave the name Laredo to the formation in the 

Rio Grande embayment described previously by Deussen (1924), 

Trowbridge (1923, 1932), and Lonsdale and Day (1937) as the Cook 

Mountain Formation, and shown by Darton, Stephenson, and Gardner (1937) with the Cook Mountain and Sparta undifferentiated. 

In describing it, Gardner pointed out that in the embayment the 

middle Eocene formations differ in lithologic composition and faunal 

assemblage from those of central and eastern Texas. More than half 

of the Laredo consists of sandstones, whereas to the northeast the
D22 CONTRIBUTIONS TO GENERAL GEOLOGY

Cook Mountain is chiefly clay. Patterson (1942) showed that along 

the Rio Grande, downstream from Laredo, the section that has 

been called Cook Mountain consists of three units thick sandstone 

members at top and bottom separated by a shale member. Kane and 

Gierhart (1935,) described in some detail the excellent exposures 

that can be found along the Rio Grande for a distance of about 80 

miles where the river, flowing nearly parallel to the strike, crosses 

and recrosses much of the formation. They stated that the formation 

constitutes the most fossiliferous part of the Eocene, and that its 

prominent cuestas of red and brown ferruginous sandstones and 

red-weathering glauconitic sands, separated by valleys of orangeyellow-weathering clays, make it one of the brightest in color and 

easiest to identify of all the Eocene formations.

Many exposures of the Laredo Formation along the Rio Grande 

were recorded by Trowbridge (1932, p. 104-129) and by Lonsdale 

and Day (1937, p. 42-55). Lonsdale and Day, especially, detailed 

the sections along the river and listed the fossils. Their composite 

section of the Laredo (Cook Mountain and Sparta equivalents) 

shows the formation to be about 620 feet thick in the Laredo area 

and to range in thickness from 600 to 700 feet in the Nueces River 

valley.

YEGUA FORMATION

The band of outcrop of the Yegua Formation traverses the entire 

Gulf Coastal Plain of Texas from the Sabine River to the vicinity 

of Falcon Dam on the Rio Grande. In about the eastern two-thirds 

of the State the Yegua is composed principally of sands, with some 

interbedded clays, thin lignites, and silts of nonmarine origin. In 

southern Texas it is chiefly clay, with minor beds of sandstone, some 

thin beds of concretionary limestone, and lenses rich in oyster shells. 

Much debate has taken place in past years on the boundaries of the 

outcropping Yegua. Stenzel (1939), in his definitive discussion of 

the "Yegua Problem/'concluded from lithologic and paleontologic 

evidence that its lower boundary in central Texas is the base of 

a thick nonmarine sand at the plane of contact with the uppermost 

marine clay of his Crockett (Cook Mountain) Formation. Most 

geologists place tbe subsurface contact in a corresponding position, 

at the top of the uppermost glauconitic marine clay. Downdip, the 

basal sands give way to clays, and the contact "moves up" in the 

section.

Along the Rio Grande, where the Yegua is best known in southern 

Texas, the basal part of the formation is chiefly clay that lies on 

highly glauconitic sands of the Laredo (Cook Mountain of Trowbridge, 1932, p. 131, and of Lonsdale and Day, 1937, p. 55). Kane 

and Gierhart (1935) place two oyster beds, each underlying a ridge-
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D23

forming sandstone near the Rio Grande both in Mexico and the United 

States, in the Yegua, but Stenzel (1940, p. 850) believes that it has 

yet to be proved conclusively that these highly fossiliferous beds do 

not belong to the underlying Laredo (Cook Mountain).

The upper boundary of the sandy Yegua Formation in eastern and 

central Texas is placed at the disconformity above which lie sediments 

reworked by the transgressing Jackson sea (Stenzel, 1940). Locally, 

where some beds of the upper part of the Yegua are of brackish-marine 

origin, it is difficult to map the contact. In south-central Texas the 

upper part of the Yegua contains red-weathering glauconitic sands 

that are probably bay or lagoonal deposits; the glauconite is of a paleamber variety, indicating a possible brackish-water origin. The overlying Caddell Formation of the Jackson Group consists of neritic-shelf 

or open-marine deposits with basal beds locally containing fossiliferous 

concretions and large abraded oyster shells and rounded boulders, 

obviously eroded by wave action. Where materials such as these can 

be found, there is little question about the contact; but generally the 

Caddell contains volcanic materials, such as ash or bentonitic clay, 

and these materials, abundant in southern Texas, are indicative of the 

Jackson.

In southern Texas, however, the location of the contact between 

the Yegua and Jackson has been disputed. Lonsdale and Day (1937, 

p. 55) thought that Trowbridge included clays of the lower part of 

the Jackson with his Yegua; they placed the contact much lower 

and limited the Yegua to a much narrower outcrop band than did 

Trowbridge. In Webb County, Lonsdale and Day placed the contact 

at the base of the lowermost bed of white volcanic ash. The ash is 

lenticular and grades laterally into ashy sandstone, sandstone, and 

clay along a slight but distinct escarpment. A silicified tuff exposed 

south of the Rio Grande and containing an oyster-and-gastropod 

fauna was placed by Kane and Gierhart (1935, p. 604, 605) in the 

upper part of the Yegua. This tuff may be the same prominent one 

that Lonsdale and Day considered the part of the basal Jackson.

A lenticular bed of similar nature, altered volcanic ash, is found 

in an identical stratigraphic position north of the Frio River in 

McMullen County. The writer extended its correlation to the Caddell 

Formation, in the basal part of the Jackson Group, in adjoining 

counties to the northeast and into the subsurface downdip. The 

assignment of this bed and its associated beds of sandstone and beritonite to the Jackson Group moves the Yegua-Jackson contact nearly 

10 miles to the northwest of the contact shown on the "Geologic 

Map of Texas" (Darton and others, 1937). The boundary shown on 

that map is, in the Frio River country, the base of the Whitsett 

Formation, near the middle of the Jackson Group. Thus, the outcrop
D24 CONTRIBUTIONS TO GENERAL GEOLOGY

band of the Yegua has been narrowed along the Frio and Atascosa 

Rivers to about the same extent as was done by Lonsdale and Day 

in the Rio Grande region.

The properties of this bed of indurated tuff north of the Frio River 

have been investigated for economic purposes by King (1940, p. 143- 

146), who considered it Yegua. Because it is dense, hard, brittle, and 

white, and has a conchoidal fracture, King called it kaolin, with 

some uncertainty; but A. D. Weeks (written commun., 1965) found 

by X-ray analysis that samples from several locations consist of more 

than 50 percent zeolite of the variety clinoptilolite, with subsidiary 

amounts of opal and montmorillonite. Further, the percentage of 

zeolite in samples increased from northeast to southwest, and the 

percentages of montmorillonite and opal decreased from northeast 

to southwest. The firmness and resistance of this bed to weathering 

make it a good marker for the base of the Jackson in this region of 

sparse outcrops.

LITERATURE CITED

Alexander, W. H., Jr., Myers, B. N., and Dale, O. C., 1964, Reconnaissance

investigation of the ground-water resources of the Guadalupe, San Antonio,

and Nueces River basins, Texas: Texas Water Comm. Bull. 6409, 106 p. 

Anders, R. B., 1957, Ground-water geology of Wilson County, Texas: Texas

Board Water Engineers Bull. 5710, 62 p. 

Ashley, G. H., 1918, The Santo Tomas cannel coal, Webb County, Texas: U.S.

Geol. Survey Bull. 691-1, p. 251-270 [1919]. 

Barnes, V. E., Proj. Director, 1966, Geologic atlas of Texas, Texarkana sheet:

Texas Univ. Bur. Econ. Geology. 

Berry, E. W., 1923, Additions to the flora of the Wilcox group: U.S. Geol. Survey

Prof. Paper 131-A, p. 1-22. 

Darton, N. H., Stephenson, L. W., and Gardner, J. A., 1937, Geologic map of

Texas: U.S. Geol. Survey. 

Deussen, Alexander, 1924, Geology of the Coastal Plain of Texas west of the

Brazos River: U.S. Geol. Survey Prof. Paper 126, 139 p. 

Ellisor, A. C., 1929, Correlation of the Claiborne of East Texas with the Claiborne

of Louisiana: Am. Assoc. Petroleum Geologists Bull., v. 13, no. 10, p. 1335-

1346. 

Gardner, J. A., 1938, Laredo, a new name for a unit of Cook Mountain age in

the Rio Grande region: Washington Acad. Sci. Jour., v. 28, no. 7, p. 297-298. 

Getzendaner, F. M., 1930, Geologic section of Rio Grande embayment, Texas,

and implied history: Am. Assoc. Petroleum Geologists Bull., v. 14, no. 11,

p. 1425-1437. 

Hargis, R. N., 1962, Stratigraphy of the Carrizo-Wilcox of a portion of south

Texas and its relationship to production, in Stapp, W. L., ed., Contributions

to the geology of south Texas: San Antonio, South Texas Geol. Soc., p.

129-145. 

Harris, H. B., 1965, Ground-water resources of La Salle and McMullen Counties,

Texas: Texas Water Comm. Bull. 6520, 96 p.
NOMENCLATURE OF CLAIBORNE GROUP, TEXAS D25

Kane, W. G., and Gierhart, G. B., 1935, Areal geology of Eocene in northeastern

Mexico: Am. Assoc. Petroleum Geologists Bull., v. 19, no. 9, p. 1357-1388;

repr. in Am. Assoc. Petroleum Geologists symposium, Gulf Coast Oil Fields,

1936, p. 588-619. 

King, P. B., 1940, Clay deposits of the San. Antonio area and Morris County,

Texas, in Clay investigations in the southern States, 1934-35: U.S. Geol.

Survey Bull. 901, p. 93-188. 

Lonsdale, J. T., 1935, Geology and ground-water resources of Atascosa and

Frio Counties, Texas: U.S. Geol. Survey Water-Supply Paper 676, 90 p. 

Lonsdale, J. T., and Day, J. R., 1937, Geology and ground-water resources of

Webb County, Texas: U.S. Geol. Survey Water-Supply Paper 778, 104 p. 

Moxham, R. M., and Eargle, D. H., 1961, Airborne radioactivity and geologic

map, Coastal Plain area, southeast Texas: U.S. Geol. Survey Geophys.

Inv. Map GP-198. 

Patterson, J. M., 1942, Stratigraphy of Eocene between Laredo and Rio Grande

City, Texas: Am. Assoc. Petroleum Geologists Bull., v. 26, no. 2, p. 256-274. 

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