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,9
-' This is to certify that the
thesis entitled
; ”A FACIES S'IUDY IN THE GULF COAST
TERTIARY OF TEXAS"
presented by
KARL A. RIGGS, JUNIOR
1
, has been accepted towards fulfillment
‘7 .T of the requirements for
idegree mm
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: Date June 13, 1952
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A FACIES STUDY IN THE GULF COAST TERTIARY

OF TEXAS

BY

KARL A. RIGGS, JR.

A THESIS

Submitted to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Ge010gy and GeOgraphy

1952

THESIS

ACKNOWLEDG ME NTS

The writer wishes to thank Dr. William A. Kelly for
directing this study, for his comments and suggestions, and
for providing samples for the study.

Many others were very helpful. Among these was Dr.
Stanard G. Bergquist, who helped locate maps and made many
helpful suggestions and comments. Dr. Bennett T. Sandefur
very willingly helped locate laboratory materials and equipment.
Dr. Lawrence M. Sommers aided the writer in locating county
maps of Texas. Dr. Andrew D. Perejda courteously set up
the pantfograph for the writer. Mrs. Jane E. Smith gave
several helpful hints which aided in making species determina-
tions. Joseph B. Long gave instructions on the use of the
LeRoy drafting equipment. Likewise, Neil S. Mallory made
numerous comments and suggestions concerning laboratory

technique which aided the writer considerably. The Texas

_Rai1road Commission was very cooperative in providing base

maps and detailed well locations. The writer would like to
take this opportunity to thank all of the individuals mentioned.

above for their kind assistance.

(7);; {f e r, 5;.

. ‘5
A-” gamitjl fi‘h‘

iii
The writer is extremely grateful to Miss Patricia A.
Hartrick, who spent many hours washing and bottling samples,
alphabetizing the faunal lists, proofreading the manuscript,
and typing the initial drafts. A humble thank you is insuffi-
cient for the time she contributed, but it is the only recogni-

tion that can be presented at this time.

TAB LE OF CONTENTS

Frio Formation .........................

Vicksburg Formation .....................

Whitsett Formation .......................

CONCLUSIONS

BIBLIOGRAPHY

10

10

13

15

19

24

29

31

Figure

Figure

Figure

Figure

LIST OF ILL US TRATIONS

Maps

Index map- - southern Texas ..........

Starr and Hidalgo Counties--shows well

locations and trend of cross section . . . .

Diagrams

Classification of middle Tertiary forma-

tions ..........................

Stratigraphic section of Lower Frio,
Vicksburg, and Whitsett ...........

Isometric projection of the Lower
Frio .............. . ....... . .

Geologic section showing continuity
of samples and formations studied

Page

pocket

pocket

pocket

PURPOSE AND INT RODUC TION

The purpose of this study is to interpret the subsurface
facies changes of the Tertiary sediments in south-central Starr
and Hidalgo Counties, Texas (see Map 1). In addition it is de-
sired to determine the depositional environment of the subsur-
face formation by the utilization of paleontologic and lithologic
data.

Many excellent papers have been written which cover the
area to a certain extent. The surface geology has been inter-
preted by such men as E. T. Dumble (1903),1 A. C. Trowbridge

Z 3
(1932), E. H. Sellards, W. S. Adkins, and F. B. Plummer (1932),

 

1
E. T. Dumble (1903), Geology of Southwestern Texas,

Trans. Amer. Inst. o_f Mining Engineers, Vol. 33.

 

Z
A. C. Trowbridge (1932), Tertiary and Quaternary

Geology of the Lower Rio Grande Region, Ii. _S_. Dept. 9_f_ Interior,
Geological Survey, Bulletin 837.

 

3
E. H. Sellards, W. S. Adkins, and F. B. Plummer (1932),
The Geology of Texas, The University g Texas, Bulletin 3232,
Vol. 1.

 

 

 

 

 

 

  

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3

W. G. Kane and G. B. Gierhard (1935),1 A. W. Weeks (1937),2
and J. M. Patterson (1942).3 Other men, such as W. G. Meyer
(1939)4 and E. D. Guzman (1949),5 have described the subsur-
face geology of the area. Many others have also written on the
Gulf Coast Province, but they have concentrated their efforts
in other localities.

Some of these workers have made detailed lithologic
and paleontologic studies of the outcrop. Others have inter-

preted the general lithology and geologic structures from

 

1
W. G. Kane and G. B. Gierhart (1935), Areal Geology

of Eocene in Northeastern Mexico, American Association o_f
Petroleum Geologists, Bulletin, Vol. 19, No. 9.

Z
A. W. Weeks (1937), Miocene, Pliocene, and Pleisto-

cene Formations in the Rio Grande Region, Starr and Hidalgo
Counties, Texas, American Association _o_f_ Petroleum Geologists,
Bulletin, Vol. 21, No. 4.

3

 

J. M. Patterson (1942), Stratigraphy of Eocene Between
Laredo and Rio Grande City, Texas, American Association 91
Petroleum Geologists, Bulletin, Vol. 26, No. 2.

4 W. G. Meyer (1939), Stratigraphy and Historical Geology
of Gulf Coastal Plain in the Vicinity of Harris County, Texas,
American Associationgf Petroleum Geolggists, Bulletin, Vol. 23, No.
2.

 

5
E. D. Guzman (1949), New Petroleum Development by

Petroleos Mexicanos, in Northeastern Mexico, American Associa-
tion 5g Petroleum Geologists, Bulletin, Vol. 33, No. 8.

4
subsurface geophysical data. These subsurface studies have been
limited to the vicinity of the present coast of the Gulf of Mexico.

It is desired to supplement the existing literature by
concentrating on the facies changes and their significance. Fur-
thermore, wells have been chosen which will disclose the strati-
graphy between the outcrop area to the west and the subsurface
stratigraphy of the coastal region to the east.

A great amount of uncertainty exists relating to the
stratigraphic nomenclature and the age of the formations in the
area. No attempt is made to solve this problem, for the in-
consistencies can only be resolved by extensive studies on a
regional basis. The development of the stratigraphic nomen-
clature is shown in graphic form in Figure 1. For the most
part, Meyer'sl classification has been used. The only changes
are the substitution of Anahuac for Middle marine Oligocene,
and Vicksburg for the Lower marine Oligocene. Both of these

names have become acceptable in recent years.

 

1
W. G. Meyer, 92. g_i_t.

2
E. D. Guzman, 2P.- git;

4
subsurface geophysical data. These subsurface studies have been
limited to the vicinity of the present coast of the Gulf of Mexico.

It is desired to supplement the existing literature by
concentrating on the facies changes and their significance. Fur—
thermore, wells have been chosen which will disclose the strati-
graphy between the outcrop area to the west and the subsurface
stratigraphy of the coastal region to the east.

A great amount of uncertainty exists relating to the
stratigraphic nomenclature and the age of the formations in the
area. No attempt is made to solve this problem, for the in-
consistencies can only be resolved by extensive studies on a
regional basis. The development of the stratigraphic nomen-
clature is shown in graphic form in Figure 1. For the most
part, Meyer'sl classification has been used. The only changes
are the substitution of Anahuac for Middle marine Oligocene,
and Vicksburg for the Lower marine Oligocene. Both of these

names have become acceptable in recent years.

 

l
W. G. Meyer, 92. gi_t_.

2
E. D. Guzman, 22- g_i_t.

 

 

 

 

 

 

 

 

 

 

 

 

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The environmental classification used is based upon the
depth of water, and is the same as that used by Lowman.1 In
case the reader is unfamiliar with these terms, the neritic zone
is defined as the zone extending from lowest tide to 100 fathoms,
the bathyal zone ranges from 100 fathoms to 1,000 fathoms, and
the abyssal zone includes all environments which exceed 1,000
fathoms in depth of water. The inner neritic zone is the sub-
division of the neritic zone which occurs nearest to the shore

line.

 

l S. W. Lowman (1949), Sedimentary Facies in Gulf Coast,

American Association 9}: Petroleum Geologists, Bulletin, Vol. 33,
No. 12, pp. 1954-55.

 

 

ME THODS AND MATERIALS

The well samples used in this study are the property of

the Department of Geology and Geography at Michigan State Col-

lege. They were originally given to the department by Dr. J. A.

Young, of the Sun Oil Company, Philadelphia, Pennsylvania.

The wells used and their locations, as shown on Map 2,

are as follows:

Roos §_t a_1. No. 1

Chapotal Land

Company No. 3

R. Flores No. l

J. W. Heard No.

D. S. DePerez
g_t_ _ai. No. l

l

elevation: 242 feet; Porcion 91
and 92, Share l-A Center of 160
acre tract, Starr County.

elevation: 198.5 feet; Porcion

99, 700 feet south of State High-
way number 4, 330 feet west of
east line of lease, Starr County.

elevation: 206.6 feet; Porcion 39,
Tract 250, 800 feet north and 330
feet west of southeast corner of
tract, Hidalgo County.

elevation: 134 feet; Los Ejidos

de Reynosa A. Viego Survey, 46'?
feet east of west line, 261 feet
from north and south lines Hidalgo
County.

elevation: 217 feet; Los Ejidos
Grant Block 225, 660 feet from
north line.

Although most of the samples are rotary cuttings, a
few core samples also were used. The interval of sampling
varies considerably. The Roos well has rotary samples with
vertical intervals varying from 8 to 24 feet. The core sam-
ples from the same well vary from 1 to 9 feet. Rotary sam-
ples were taken from the Chapotal well at intervals of approx-
imately 10 feet. In the Flores well, rotary samples were
taken every 15 feet, but the cores vary from 2 to 9 feet. Core
and rotary samples from the Heard well were taken from 1 to 4
and 30 to 49 feet, respectively. Rotary samples in the DePerez
well range from 30 to 32 feet, but the cores vary from 4 to 10
feet.

When possible, the lithologic sequence was derived from
the cores, but in their absence, the lithology was interpreted
from the rotary samples according to the method described by
LeRoy and Grain.1

After detailed examination of samples was started, it

was found that portions of the section were either sparsely

 

l
L. W. LeRoy and H. M. Crain (1949), Subsurface Geo-

logic Methods Colorado School of Mines, Golden, Colorado, pp.

_.___.___J

299-302.

 

9
represented, or altogether absent in the Frio and Catahoula for—
mations. Furthermore, many of the samples present have sam-
pling intervals exceeding 10 feet. Since R. M. Whiteside1 claims
that detailed stratigraphy can not be derived from rotary sam-
ples with intervals exceeding 10 feet and since geophysical data
was unobtainable, the original plan of attack on facies problems
had to be modified. For this reason no detailed correlation is
attempted in the Catahoula and upper Frio formations; however,
the general nature of these formations has been described. Com-
pared to the work of others, a detailed correlation has been made
in the Lower Frio, Vicksburg, and upper Jackson formations
on Figures 2 and 3; however, even these should be considered
general trends. The writer is convinced that the facies changes
are more complex, but the available material prevents the accu-
rate determination of the details.

Figure 4 shows the position of the samples upon which

this paper is based.

 

1
R. M. Whiteside (1932), Geologic Interpretation from
Rotary Cuttings, American Association 9f Petroleum Geologists,
Bulletin, Vol. 16, No. 7, p. 671.

 

 

STRATIG RAPHY
Catahoula Formation

The top of this formation is not determinable because
the highest sample in the Flores well occurs at a depth of 1,000
feet. Since the Catahoula outcrops approximately 6 miles from
the Flores well,1 and since the regional dip is not known to ex-
ceed 75 feet2 per mile in the area, it is believed that the high-
est samples are well below the top of the Catahoula.

In the Heard and DePerez wells the bottom of the Cata-
houla is placed at the top of the fossiliferous Anahuac forma-
tion.3 The bottom of the Catahoula in the Flores well is placed
at the top of a gray—green shale4 which occurs at 1,685 feet be-

low the surface. A lithologic contact between the Catahoula and

 

l
A. W. Weeks, 92. g_i_t_., p. 491.

2 E. H. Sellards, W. S. Adkins, and F. B. Plummer, QR-

git., p. 534.

W. G. Meyer, 92- 9%, p. 177.

4 E. H. Sellards, W. S. Adkins, and F. B. Plummer, gp.

g_i_t_., p. 715.

11
Frio could not be determined in the R005 and Chapotal wells
because some of the samples were missing.

Bentonitic and calcareous shales and clays are predom-
inant in the Catahoula, but lenses of sandy shales and fine sand-
stones are rather common. The beds of clays, shales, and
sandy shales are colored various shades of red, pink, gray, and
buff. Beds of mottled red and gray, and mottled red and green
clays and shales are also common. The sandstones are gen-
erally gray or white, and shards of volcanic glass can be noticed
occasionally. In the Chapotal well, a few lenses or thin beds
of lignite occur.

Charophyta and Ostracoda are very common in all the

wells, but the only Foraminifera found were:

Globigerina species A - Z specimens
Globigerinella species A - 3 specimens
Gumbelina species A - 1 specimen

The two specimens of Globigerina and one of the 3 speci-
mens of Globigerinella occur 1,000 feet below the surface in the
Flores well. The other two Globigerinellas occur at scattered
points in the Heard well. These Foraminifera are planktonic

and cannot be used to interpret the environment.

12
Judging from lithology and the presence of Charophyta
and Ostracoda, the Catahoula is predominantly deltaic. Accord—
1
ing to Twenhofel the sediments of the lower Mississippi River
area are composed of 94 per cent clays, silts, and very fine sands.
Judging from the rotary cuttings, the Catahoula has approximately
the same textural composition. Furthermore, the Charophyta are
2
characteristic of fresh or brackish water although the Ostracoda
. . . 3
are related to fresh, brackish, or marine environments. Ostra-
coda also occur in continental sediments near the shore area as
. . 4 . . . .
the result of Wind transportation. The Foraminifera may indi-
cate thin beds of marine shale, but it should not be overlooked
that the few present could have been introduced by storm waves.
At any rate, the marine shales, if present, could not be identi-

fied from the rotary samples. The lignite lenses found in the

 

1
W. H. Twenhofel (1950), Principles of Sedimentation,

New York, p. 109.
2 . .
LeRoy and Grain, 92. 9_i_t_., p. 72.
3
Ibid., p. 66.
W. A. Kell§y, personal communication.

5
W. H. Twenhofel, 92. 9_i_§., p. 105.

13
Chapotal well may have been formed on the delta or in fresh
water swamps located in the flood plain.

Although predominantly deltaic, the Catahoula may have
many lenses of aeolian deposits, swamp deposits, lake deposits
and inland fluviatile deposits.

The samples available represent only a part of the Cata-

houla; consequently, definite conclusions cannot be reached.

Anahuac Formation

Apparently the Anahuac formation pinches out in this
area, for only traces of it occur in the Heard and DePerez
wells. In the Heard well it is represented by a bed of gray
sandstone approximately 20 feet thick. This sandstone is largely
composed of gray and white quartz plus minor amounts of ac-
cessory minerals, such as glauconite and magnetite. No chem-
ical tests were used to identify these minerals. They were
merely determined on the basis of color, hardness, and form.

In the DePerez well the Anahuac is represented by ap-
proximately 30 feet of gray sandy shale which is quite calcare-

OUS .

14

The bed of gray sandstone has been correlated with the
gray sandy shale because they are definitely distinguishable as
marine in origin; whereas, both the overlying Catahoula and the
underlying Frio are predominately, if not completely, continental.

Furthermore, two species of Elphidium occur which are char-

 

acteristic of the Anahuac. The Foraminifera found in these two

wells are:

Elphidium discoidale multiloculum Cushman and Ellisor

Elphidium sagrum (d'Orbigny)

Nonion species A (l specimen)

Quinqueloculina species A (1 broken specimen)

Globigerina species A
Ostracoda and Charophyta are also abundant in these two wells.

1
According to Lowman, this foraminiferal fauna is char-

acteristic of the inner neritic zone (see page 6). Although Ostra-
coda and Charophyta also occur in the inner neritic zone, the
writer is not convinced that they should be used as environment
indicators in this case for they may not be indigenous to these

beds. Inasmuch as both occur in the overlying Catahoula, they

may be the result of caving when the hole was drilled.

 

l
S. L. Lowman, 92. _c_i_t., p. 1956.

14

The bed of gray sandstone has been correlated with the
gray sandy shale because they are definitely distinguishable as
marine in origin; whereas, both the overlying Catahoula and the
underlying Frio are predominately, if not completely, continental.
Furthermore, two species of Elphidium occur which are char-
acteristic of the Anahuac. The Foraminifera found in these two

wells are:

Elphidium discoidale multiloculum Cushman and Ellisor

Elphidium sagrum (d'Orbigny)

Nonion species A (l specimen)

Quinqueloculina species A (1 broken specimen)

Globigerina species A
Ostracoda and Charophyta are also abundant in these two wells.

1
According to Lowman, this foraminiferal fauna is char-

acteristic of the inner neritic zone (see page 6). Although Ostra-
coda and Charophyta also occur in the inner neritic zone, the
writer is not convinced that they should be used as environment
indicators in this case for they may not be indigenous to these

beds. Inasmuch as both occur in the overlying Catahoula, they

may be the result of caving when the hole was drilled.

 

1
S. L. Lowman, 92. Q9, p. 1956.

14

The bed of gray sandstone has been correlated with the
gray sandy shale because they are definitely distinguishable as
marine in origin; whereas, both the overlying Catahoula and the
underlying Frio are predominately, if not completely, continental.

Furthermore, two species of Elphidium occur which are char-

 

acteristic of the Anahuac. The Foraminifera found in these two

wells are:

Elphidium discoidale multiloculum Cushman and Ellisor

Elphidium sagrum (d'Orbigny)

Nonion species A (1 specimen)

Quinqueloculina species A (1 broken specimen)

Globigerina species A
Ostracoda and Charophyta are also abundant in these two wells.

1
According to Lowman, this foraminiferal fauna is char-

acteristic of the inner neritic zone (see page 6). Although Ostra-
coda and Charophyta also occur in the inner neritic zone, the
writer is not convinced that they should be used as environment
indicators in this case for they may not be indigenous to these

beds. Inasmuch as both occur in the overlying Catahoula, they

may be the result of caving when the hole was drilled.

 

1
S. L. Lowman, 92. 911., p. 1956.

15

1
Since the Heterostegina zone extends farthest west, the

 

inner-neritic Anahuac of this area can probably be correlated

with the Heterostegina zone in the downdip sections.

 

Frio Formation

When the Anahuac formation is present, the lower limit
of the Anahuac fauna is used as the contact between the Frio
and Anahuac formations.2 Because the samples are missing,
its top is not determinable in the R003 well. The contact be-
tween the Frio and the overlying Catahoula has already been
explained in the section on the Catahoula.

The contact of the Frio and Vicksburg formations is the
top of the Sam Fordyce sandstone of the upper Vicksburg3 (see
Figure 2).

In the R005 well, the base of the Frio is placed above

the gray sandy shale of the Vicksburg for several reasons.

 

1
A. C. Ellisor (1944), Anahuac Formation, American

Association 9f Petroleum Geolggists, Bulletin, Vol. 28, No. 9,
p. 1363.

 

 

2 E. J. Guzman, 92. 9i}, p. 1359.

3
Ibid., p. 1358.

l6

Overlying the gray sandy shale are brown shales of the conti-

nental type. Furthermore, one specimen of Eponides Vicksburg-

 

9_r_i_s.i_s_ occurs in the gray sandy shale (see Figure 2). Since this
species is unknown in the Jackson Eocene, but reported from
both Oligocene formations in the coastal region,1 this bed of
sandy shale belongs to either the Frio or Vicksburg. Finally,
since the gray sandy shale above the Sam Fordyce sandstone

in the Chapotal well (see Figure 2) has no Foraminifera, and

since the sandy shale below this sandstone has Eponides Vicks-

 

 

bigge_nsis as well as other Foraminifera, this sandy shale is
placed in the Vicksburg.

The position of the boundaries and the lithologic asso-
ciations should be clarified in Figures 2 and 4.

The principal rock types of the Frio are gray, red, red-
brown, and brown calcareous shales and clays, green sandy
shales, and gray sandy shales. In general, these shales and
clays are less bentonitic than the Catahoula. Gray, greenish-

white, buff, and brown sandstones and siltstones also are common.

 

l
A. C. Ellisor (1932), Jackson Group of Formations in
Texas With Notes on Frio and Vicksburg, American Association
91 Petroleum Geolog;i_sts, Bulletin, Vol. 17, No. 11, pp. 1322,
1326.

 

 

l7
Quartz predominates in the sandstones and siltstones, but minor
amounts of glauconite and other accessory minerals which were
not identified are present. Calcium carbonate is the predomi-
nant cementing material in the sandstones. The various colors
of the sandstone beds are due to impurities in the cement.

Also occurring in the section are thin beds of marl, thin
beds of lignite which are commonly somewhat pyritized, small
amounts of shell breccia, thin beds of green and greenish-gray
volcanic ash, small amounts of white bentonite, and a little
white tuff.

Ostracoda and Charophyta are very abundant, but Foram-
inifera are rare. In the R005 well, abraded and broken Foram-
inifera which were interpreted as representing a reworked fauna
occasionally occur. This reworked fauna is composed of:

Guttulina species A
Quinqueloculina species B
Textularia species A
Elphidium species A
In the other wells, the following Foraminifera occur:
Ammobaculites species A
Cibicides americana (Cushman)
Elphidium poeyanum (d'Orbigny)

Nonion species B
Quinqueloculina species C

18

As stated earlier, the lithology of the Frio consists of
thinly bedded multicolored shales with interbedded lenses of
sandstone, plus other lithologic types. According to Meyer,l
lenticular sand beds are characteristic of the nonmarine facies
in the Gulf Coast formations. Furthermore, the lithologic se-
quence in the Chapotal well is comparable to deposits found on
the lateral or landward margins of a delta as described by
Twenhofel2 (see Figure 3). Likewise the predominance of clays
and silts in the Frio is comparable to conditions existing in the
lower Mississippi River area.

The paleontologic data also indicate nonmarine conditions
as the dominant environment. Although a few beds in the Cha-
potal well have very prolific shore line faunas composed mainly

4
of Elphidium poeyanum, most of the strata are barren of Foram-

 

inifera. Only 5 specimens of Foraminifera occur in the Flores

and Heard wells, whereas only reworked Foraminifera occur in

 

1
W. G. Meyer, 92. 9i_t., p. 153.

2
A. C. Twenhofel, 92. in pp. 103-11.

3
Ibid., p. 109.

4 S. W. Lowman, 92. 9i_t., p. 1956.

.19
the R003 well. Although the Foraminifera in the Heard and
Flores wells may indicate a few thin beds of marine shales, it
should be recalled that Foraminifera may be carried inland by
strong winds, storm waves, or animals.1 Since the Ostracoda
occur in all environments and may even be carried inland by
winds,Z they can not be used, but the presence of abundant
Charophyta favor a fresh or brackish water origin for the Frio.
On the basis of the evidence presented above, the Frio
is interpreted as a deltaic deposit which probably has a few

flood plain and marine beds associated with it.
Vicksburg Formation

The Vicksburg formation may be unconformable below
the Frio. The elimination of the sandstone beds between the
R003 and Chapotal wells is interpreted as an unconformity on
Figure 2, but this may be a facies change which resulted when

the sea withdrew at the close of Vicksburg time. The exact

 

l
A. C. Twenhofel, 92. 931., p. 105.

2
W. A. Kelly, personal communication.

20
character of the contact cannot be determined with the data
available.

As already explained in the section on the Frio, the
Sam Fordyce sandstone is the uppermost member of the Vicks-
burg formation. This sandstone is composed of greenish-gray,
medium-textured, angular grains of white and gray quartz, cal-
cite, amphibole, and glauconite, plus a small percentage of ac—
cessory minerals. As in the other formations, these mineral
identifications are based on physical properties.

The highest occurrence of the Jackson fauna is used for
the contact between the Jackson and the overlying Vicksburg.
Although no lithologic break is apparent, the abrupt change
from Jackson fauna to Vicksburg fauna and the elimination of
several significant sand beds is convincing evidence for the
presence of one or more unconformities or diastems. This
interpretation is shown on Figure 3.

The Vicksburg is composed of marine shales, sandy-
shales, and sandstones. The calcareous shales and sandy-shales
are mainly gray, brown-gray, and green—gray, but a few beds
of mottled red and gray shales occur, as well. The sandstones

are gray and green-gray course-to-fine sandstones having

21
essentially the same mineralogic composition as the Sam For-
dyce sandstone.

Figure 3 shows the general trend of the facies changes
throughout Vicksburg time. One fact which is not emphasized
on Figure 3, but which should be considered, is the occurrence
of several sandy lenses in the upper Vicksburg shales of the
DePerez well.

These sandy shales are essentially different from both
the shales and sandy shales found in the Flores well. For the
most part they are gray shales; however, they have a few coarse
quartz grains. Because the samples occur at intervals of 30
to 32 feet in this section of the well, it is rather difficult to
estimate the thickness of the beds. However, judging from the
proportion of the sandy shale present, the writer doubts if
they attain a thickness exceeding 3 feet, and are certainly less
than 5 feet in thickness. Since they are so far from the shore
line, as implied in the Flores well, and since they lack fossils,
they probably represent reworked offshore bars.

A few specimens of Charophyta occur in the Flores sam-
ple in the 4,350-to—4,380-feet interval. Since these do not occur

in any of the other Vicksburg samples below the top of the Sam

22

Fordyce sandstone, the writer is convinced that they are indige-
nous to the original sample. Furthermore, since they are con—
sidered characteristic of brackish or fresh water, they may in-
dicate local continental conditions. Since continental conditions
imply that beds will be eroded if exposed, the writer has used
the presence of Charophyta as partial evidence for the uncon-
formities at the base of the Vicksburg.

Foraminifera and Ostracoda are relatively rare, com-
pared to the faunas reported as existing in the Vicksburg near
the coast. Although eighteen different species are listed, only
one or two specimens of many occur in the samples examined.

Cibicides americana (Cushman)

Cyclammina sp. A

Elphidium poeyanum (d'Orbigny)

Eponides vicksburgensis Cushman and Ellisor
Globigerina bulloides d'Orbigny
Globigerinella sp. A

Gumbelina sp. A

Gumbelitria sp. A

Guttulina problema (d'Orbigny)
Haplophragmoides sp. A

Nonion advenum Cushman

Quinqueloculina sp. D

Robulus limbatus (Bornemann)

Robulus vicksburgensis (Cushman)

Rotalia sp. A

Spiroplectammina mississippiensis (Cushman)
Textularia tumidula Cushman

Textularia warreni Cushman and Ellisor

23

Elphidium, Globémgrinellg, and Gumbelina are very com-
mon in the area, and especially in the sandstone beds. Accord-
ing to Lowman,l abundant numbers of Elphidium are character-
istic of the inner neritic zone. The large number of species
with few representatives is also characteristic of the inner
neritic zone.

On the basis of the paleontologic data, it is concluded
that the Vicksburg sea was rather shallow, and that the conti-
nental slope and coastal plain had a rather gentle slope. If
one compares the paleontologic data with the results of Low-
man,3 one observes that the Vicksburg sea in this area defi-
nitely did not exceed 100 feet in depth. Although a quantitative
analysis of the data was not attempted, it is believed that a
maximum depth of 40 feet is a good estimate in Starr and Hi-
dalgo Counties.

The lithologic evidence as portrayed on Figure 4 sup-

ports the theory of a shallow, gently-sloping sea floor. The

 

1
S. W. Lowman, 92. c_i£., p. 1956.

2 Ibid., p. 1952, Fig. 12.

3
Lee. 911;.

Z4
sandstones, sandy shales, and shales indicate a rather rapid
shifting of the shore line over relatively short periods of time.
Furthermore, the presence of the coarse quartz grains in some
of the shale beds in the DePerez well, as already explained
(page 24) are most easily accounted for as offshore bars formed
along a shallow-water, gently sloping continental shelf. Although
the evidence is not conclusive, this seems to be the best inter-

pretation that can be made on the basis of the present data.

Whitsett Formation

Although a complete set of samples occurs in the Whit-
sett Formation of the R003 well, the other wells have samples
only near the top of the member.

The lithology is dominantly brownish-gray and gray sandy
shales and shales, but a few beds of gray, coarse-to-fine sand-
stones also occur. Quartz is the dominant constituent of the
sands, but amphibole and glauconite are common. Minor amounts
of the accessory minerals also are present. Calcite and silica

are the predominant cementing constituents in the sandstones.

25
. 1 .
According to E. D. Guzman, field workers do not agree
on the marker horizon for the top of the Whitsett. Some prefer
to use the top of the Marginulina cocoaenensis zone, while others

prefer to use the top of the Anomalina jacksonensis zone. No

 

Maiginulina cocoaenensis occurs in these well samples and

 

therefore it cannot be used. The absence of Marginulina is

 

probably due to ecologic conditions. Since Marginulina is most

 

common in water which is 145 to 300 feet deep,2 and since the
upper part of the Jackson in this area is a shallow-water de-

posit (Figure 2), Maginulina cocoaenensis is probably very

 

rare, if it occurs at all. On the other hand, using the upper

limit of the Anomalina jacksonensis zone to determine the con—

 

tact is unsatisfactory for it occurs at a variable depth below
the top of the formation.3 The Jackson fauna differs greatly
from the Vicksburg fauna in these wells. Furthermore, the

Jackson fauna is quite abundant even in the regressive sand-

stones at the top of the formation; consequently, it offers a

 

1

E. D. Guzman, 92. git” p. 1356.
2 .

S. W. Lowman, 92. 939., p. 1961.
3

E. D. Guzman, 92. 211., p. 1356.

26
vivid contrast to the comparatively sparse fauna of the Vicks-
burg. For these reasons, the top of the Jackson fauna is used
to mark the top of the formation. For comparison, the top of

the Anomalina jacksonensis has been plotted on Figure 2.

 

The bottom of the Massilina pratti zone marks the lower

 

limit of the Whitsett Formation.l

As already mentioned, and as demonstrated in the fol-
lowing list of Foraminifera, the Jackson has a large number of
both species and individuals.

Ammobaculites agglutinans (d'Orbigny)

Anomalina bilateralis Cushman

Anomalina jacksonensis (Cushman and Applin)

Bolivina gracilis Cushman and Applin

Bolivina jacksonensis Cushman and Applin

Bolivina jacksonensis Cushman and Applin var. striatella
Cushman and Applin

Bulimina jacksonensis Cushman

Cibicides cooperensis Cushman

Cibicides sp. A

Dentalina sp. A

Elphidium eocenicum Cushman and Ellisor

Eponides pygmaea (Hantken)

Globigerina bulloides d'Orbigny

Globigerinella species A

Globulina gibba d'Orbigny

Gumbelina sp. A

Gyroidina sp. A

Hantkenina alabamensis

 

1
W. G. Meyer, 92. g_i_t_., p. 163.

Z7

Marginulina sp. A

Massilina pratti Cushman and Ellisor

Nonion inexcavatum (Cushman and Applin)
Nonion scaphum (Fichtel and Moll)

Nonionella hantkeni (Cushman and Applin)
Planularia sp. A

Pleurostomella jacksonensis Cushman and Applin
Polymorphina sp. A ,
Pseudopolymorphina dumblei (Cushman and Applin)
Robulus limbosus (Reuss)

Robulus propinquus (Hantken)

Saracenaria italica (Defrance)

Siphonina carltoni Cushman and Ellisor
Spiroplectammina alabamensis (Cushman)
Textularia hockleyensis Cushman and Applin
Textularia sp. B

Uvigerina alata Cushman and Applin

Uvigerina cocoaensis Cushman

Uvigerina dumblei Cushman and Applin
Uvigerina gardnerae Cushman

Virgulina dibollensis Cushman and Applin

The sandstones have a less abundant fauna which is dominated

by Elphidium; however, no essential difference exists between

 

the faunas of the sandy shales and the shales.

The Buliminidae and Lagenidae are the most abundant
families both in species and numbers. Comparing this faunal
assemblage with that analyzed in present-day seas leads to the
conclusion that the Jackson sea -in this area is representative

1
of the neritic zone. The predominance of the Buliminidae and

 

1
Ibid., pp. 1954-55.

28
Lagenidae indicate that the ocean was relatively deep during
most of upper Jackson time.

Although the lack of complete samples hinders definite
conclusions concerning the nature of the Whitsett sea in this
area, a few general statements can be logically interpreted. The
rapid facies changes in the lower Whitsett samples of the Roos
well (Figure 2) indicate that the shifting shore line was located
'in central Starr County at this time. Furthermore, the long
succession of sandy shales and shales in the 3,300-to-3,800-feet
interval indicate a major advance of the Jackson sea during mid—
dle Whitsett time. The sandy shales in the middle of this inter-
val probably should be interpreted as a significant retreat during
the over-all advance. The sandy shales and sandstone persisting
at the top of the Whitsett indicate a major regression of the sea
at the close of the Eocene. Such a major withdrawal must have

resulted in an unconformity, as shown on Figure 2.

CONCLUSIONS

During most of the Upper Eocene the neritic zone of the
sea occupied this area, but at the close of the Eocene the sea
withdrew, permitting the development of a major unconformity.
In Lower Oligocene time the sea gradually advanced as far west
as central Starr County. Apparently the coastal plain and con-
tinental shelf had a very low gradient at this time, for offshore
bars were develoPed. Many diastems are inferred to exist in
the Vicksburg, and the strand line seems to have fluctuated on
either side of the Starr-Hidalgo border during most of Vicksburg
time.

Another retreat of the sea is recorded at the close of the
Vicksburg, which possibly resulted in an unconformity. For the
most part, the Frio is a deltaic or continental deposit. Over-
lying it is the continental Catahoula and the marine Anahuac. The
exact westward limit of the Anahuac is not determined, but it
probably extends within two miles east of the Starr-Hidalgo
Boundary.

The exact nature of the Frio-Catahoula contact cannot be

inferred from the data which are available.

30

This study also discloses the fact that many commercial
well samples are inadequate for detailed ecologic studies. 5. W.
Lowman has made a very excellent quantitative study of the
ecolOgic relations of modern Foraminifera; however, this de-
tailed data cannot be fully applied. Rotary samples are subject
to so many sources of pollution that a. worker cannot be sure
that the Foraminifera are definitely diagnostic of the ecology of
any single bed. The Rotary samples are quite satisfactory, how-
ever, in determining the dominant environment of a formation.

Although the available samples have had a limiting effect
on the results, it is h0ped that this study has clarified the sub-

surface lith010gic and ecologic relationships in Starr and Hidalgo

Counties.

BIBLIOGRAPHY

Deussen, A., and Owen, K. D. (1939), Correlation of Surface
and Subsurface Formations in Two Typical Sections of
the Gulf Coast of Texas, American Association 9f _P__e_-
troleum Geologists, Bulletin, Vol. 23, No. 11.

 

 

Dumble, E. T. (1903), GeolOgy of Southwestern Texas, Trans-
actions o_f the American Institute 9f Mining Engineers,
Vol. 33.

 

Ellisor, A. C. (1932), Jackson Group of Formations in Texas
With Notes on Frio and Vicksburg, American Association
_o_f Petroleum GeolOgists, Bulletin, Vol. 17, No. 11, No-
vember.

 

 

(1944), Anahuac Formation, American Association 99
Petroleum Geologists, Bulletin, Vol. 28, No. 9, Septem-
ber.

 

 

Guzman, E. D. (1949), New Petroleum Development by Petroleos
Mexicanos in Northeastern Mexico, American Association
9f Petroleum Geolggists, Bulletin, Vol. 33, No. 8, August.

Kane, W. G., and Gierhart, G. B. (1935), Areal GeolOgy of
Eocene in Northeastern Mexico, American Association 9_f_
Petroleum Geologists, Bulletin, Vol. 19, No. 9, September.

 

LeRoy, L. W., and Grain, H. M. (1949), Subsurface Geologic
Methods, Colorado School of Mines, Golden, Colorado.

Lowman, S. W. (1949), Sedimentary Facies in Gulf Coast,
American Association 9f Petroleum Geologists, Bulletin,
Vol. 33, No. 12, December.

 

Meyer, W. G., (1939), Stratigraphy and Historical Ge010gy of
Gulf Coastal Plain in Vicinity of Harris County, Texas,
American Association _o_f_ Petroleum Geologists, Bulletin,
Vol. 23, No. 2, February.

 

32

Patterson, J. M. (1942), Stratigraphy of Eocene Between Laredo
and Rio Grande City, Texas, American Association 9_f_
Petroleum GeolOgists, Bulletin, Vol. 26, No. 2, February.

 

Sellards, E. H., Adkins, W. S., and Plummer, F. B. (1932), The
Geology of Texas, The University 9f Texas, Bulletin 3232,
Vol. 1.

 

 

Trowbridge, A. C. (1932), Tertiary and Quaternary Geology of
the Lower Rio Grande Region, Texas, United States De-—
partment o_f the Interior, Geological Survey, Bulletin,
837, Washington.

 

 

Twenhofel, W. H. (1950), Principles 9_f_ Sedimentation, Second
Edition, McGraw-Hill Book Company, Inc., New York.

 

 

Vaughan, T. W. (1940), Ecology of Modern Marine Organisms,
Bulletin 9f the Geological Society 9f America, Vol. 51,
No. 3, March. '

 

Weeks, A. W. (1937), Miocene, Pliocene, and Pleistocene Forma-
tions in the Rio Grande Region, Starr and Hidalgo Counties,
Texas, American Association 9f Petroleum Geologists,
Bulletin, Vol. 21, No. 4, April.

 

 

Whiteside, R. M. (1932), Geologic Interpretation From Rotary
'Cuttings, American Association 9_f_ Petroleum Geolgists,
'Bulletin, Vol. 16, No. 7, July.

 

 

 

 

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