Geology of part of the 

Horseshoe Atoll in 

Borden and Howard 

Counties, Texas

GEOLOGICAL SURVEY PROFESSIONAL PAPER 315-B

Prepared in cooperation with the 

Bureau of Economic Geology of 

The University of Texas
Geology of part of the 

Horseshoe Atoll in 

Borden and Howard 

Counties, Texas

By R. J. BURNSIDE

PENNSYLVANIAN AND LOWER PERMIAN ROCKS OF PARTS 

OF WEST AND CENTRAL TEXAS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 315-B

Prepared in cooperation with the 

Bureau of Economic Geolovy of 

The University of Texas

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D. C.
CONTENTS

Abstract________________________________________

Introduction._____________________________________

Acknowledgments________________________________

Stratigraphy _______________________________________

Regional stratigraphic relations.__________________

Stratigraphy of the reef complex._________________

Relation of reef limestone to surrounding shale. ____

Characteristics of the reef rock_______________________

Lithologic components_________________________

Distribution of lithologic components__________

Chemical composition.________________________

Porosity and permeability__________________

Fauna and flora__-________________________________

Reef history..____________________________________

Factors affecting the concepts of reef development __ 

An hypothesis of reef development.--.-___________

Page 

21 

21 

21 

23 

23 

23

23

24

24

25

26

26

27

27

27

28

Reef History Continued

Effects of eustatic changes of sea level on the Horse- P x *e

shoe atoll_________________________ 29

Hypotheses of reef growth_________ ____ 29

Ecologic controls.___ _ 30

History of the reef during Pennsylvanian time___ 31

History of the reef during Wolf camp time_______ 31

Cementation and cyclical reef growth _________ _ __ 32

Oil and gas_.__________________________ 33

Oil fields..........._________________ 33

Discoveries and production.____________________ 33

Oil accumulation.__________________________ 33

Completion practices___________________ 34

References cited________________________________ 34

Index_______________________________ 35

ILLUSTRATIONS

I All plates except plate 11 are in pocket]

PLATE 10. Structure-contour map of the top of the reef limestone in the southern part of the Horseshoe atoll.

11. Cores containing: A, Calcilutite; B, calcarenite; C, calcirudite; D, calcirudite (bioclastic) _ _ ________________ 

12. Cross sections showing porosity and fusilinid zones. A A' and B-B', the Good field. C-C" and D-D', the East 

Vealmoor field.

13. Possible correlation between unconformities in the Horseshoe atoll and outcropping rocks in central Texas.

14. Cross section showing stratigraphic relationships of the rocks of pre-Leonard and post-Devonian age penetrated by

wells in Borden and Howard Counties. 

FIGURE 6. Index map of part of west Texas showing the area of this report _ ______________ _______________ ____

7. Microlog and lithologic log of Skelly Oil Co.'s No. 5 Gunn well __ _______________________________________

8. Microlog and lithologic log of Pan-American Producing Co.'s and Seaboard Oil Co. of Delaware's No. 24 Good well.

24

22

24

25

TABLES

TABLE 1. Insoluble residue of a core from Seaboard Oil Co. of Delaware and Pan-American Producing Co.'s No. 24 Good 

well in the Good field_____________________-__________________-__-_-___--_--__-------_------------_

2. Insoluble residue of a core from Skelly Oil Co.'s No. 5 Gunn well in the East Vealmoor field._________________

3. Discovery wells in the Vealmoor and other fields in the area of this report.-- ________________________ 

HI

26

26

33

PENNSYLVANIAN AND LOWER PERMIAN ROCKS OF PARTS OF WEST AND CENTRAL TEXAS

GEOLOGY OF PART OF THE HORSESHOE ATOLL INBORDEN AND HOWARD COUNTIES, TEXAS

By K. J. BUENSIDE

ABSTRACT

The Horseshoe atoll is an arcuate mass of deeply buried fossiliferous limestone of Pennsylvanian and early Permian age in the 

Midland basin of west Texas. The southern part of the atoll is in 

southern Borden and northern Howard Counties. From early 

Strawn until early Wolf camp time this atoll, which had some of 

the characteristics of a reef, was a prominent feature on the floor 

of the Midland basin. The crest of the atoll is a series of buried 

hills and saddles. In the area of this report, 10 oil fields have 

been found in the hills along the crest, 2 on spurs on the flank of 

the atoll, 2 in the distant seaward area, and 2 on the seaward 

flank occupied by the central lagoon.

Calcilutite, calcarenite, and calcirudite comprise most of the 

reef rock, but there are a few thin beds of shale. The complex 

facies relations and stratigraphy of this reef are best explained 

as effects of oscillation of sea level during the time of reef growth. 

The cementation of the reef and subsequent development of 

porosity can be explained by the same oscillations of sea level, 

plus leaching by ground water during the periods of emergence. 

Reef growth was terminated in early Wolfcamp time when large 

volumes of fine-grained terrigenous sediments were deposited in 

the area.

The oil probably accumulated during Wolfcamp time shortly 

after the atoll was covered by black shale. Some peculiarities in 

the observed distribution of oil in the atoll may be explained by 

the apparent stratification of porosity.

INTRODUCTION

The Horseshoe atoll, as identified by Adams and 

others (1951), is a large curvilinear subsurface accumulation of limestone in the northern part of the Midland 

basin. The northern side of the atoll is open, as shown 

in figure 6. The east-west diameter is about 90 miles 

and the north-south diameter about 70 miles. This 

study covers the southern part of the atoll in southern 

Borden and northern Howard Counties, where it curves 

from a north-northeasterly trend on the east to a northwesterly trend on the west. The southeastern part of 

the Horseshoe atoll, which is known as the "Scurry 

Reef", has been described by Rothrock and others 

(1953). It is a northeastern extension of the area included in the present report.

Fossiliferous limestone of Pennsylvanian and early 

Permian age forms the Horseshoe atoll. This limestone is surrounded and overlain by shale of early 

Permian age. The crest of the limestone mass, which 

ranges from about 4,200 feet to more than 6,000 feet 

below sea level in the mapped area, is a series of irregular 

buried hills and saddles (pi. 10). The reservoirs of oil 

fields hi the atoll are on the hills; oil and gas are not 

produced from the saddles. Ridges and valleys on the 

flanks of the atoll merge with the hills and saddles on 

the crest.

The terms "atoll" and "reef" have generally been 

applied to this subsurface accumulation of limestone; 

however, as noted by Heck and others (1952, p. 5), the 

limestone does not contain large amounts of recognizable f rame-building reef organisms. Probably this limestone mass had some of the characteristics of a r^ef 

during the time of its growth, however, and therefore 

the terms "atoll" and "reef" will be used in this report. 

Because of uncertainty as to whether the Horseshoe 

atoll ever contained a rigid actively growing organic 

core, regarded by most geologists as an essential cl aracteristic of a reef, this atoll might be called a "bibhern" 

in accordance with the definition proposed by Cloud 

(1952, p. 2128), who suggested that "ancient reef-like 

masses of uncertain potential or doubtfully wave-resistant nature may be termed bioherms."

Data for cross sections prepared to : accompany this 

report were obtained from studies of. micrologs, electrical logs, and radioactivity logs. The contour map of 

the reef (pi. 10) is based on interpretations of electrical 

and radioactivity logs., On these logs the top of the 

reef is indicated by strpng increases in resistivity md 

usually in self-potential.' At the top of the reef, gammaray curves show a marked lessening in intensity of 

radioactivity whereas neutron curves show a sharp increase. Cores from 12 wells were studied for tl sir 

lithologic character and fossil content.

ACKNOWLEDGMENTS

This investigation was aided by the generous cooperation of many organizations. Cores were contributed by

21
22 PENNSYLVANIA^ AND LOWER PERMIAN ROCKS, WEST AND CENTRAL TEXAS

103° 102° 100°

103' 102° 100*

10 20 80 40 Miles

Area covered by this report Area covered by U. S- Geological

Survey Professional Paper 315 A

FIGURE 6. Index map of part of west Texas showing the area of this report and the location of main geologic feature".

Area covered by U. S. Geological 

Survey OM 143
HORSESHOE ATOLL IN BOBDEN AND HOWARD COUNTIES, TEXAS 23

Seaboard Oil Co. of Delaware (Seaboard's and PanAmerican Producing Go's. No. 24 T. J. Good well, Good 

field); Amerada Oil Co. (Amerada's No. 6 Weathers 

weU, Von Boeder field); SkeUy Oil Co. (Skelly's No. 5 

Gunn well, East Vealmoor field); and L. H. Armer 

(Armer's No. 2 McAdams well, Vealmoor field). Also 

Seaboard Oil Co. permitted the Geological Survey to 

sample cores from the T. J. Good lease in the Good field, 

wells 1, 12, 23, 24, 27, and 28; from the North Vealmoor 

field, the No. 1 Porter Hanks well; and from the Vealmoor field, No. 1 H. W. Zant well. Richard V. Hollingsworth, of the Paleontological Laboratory at Midland, 

Tex., furnished fusulinid determinations and other data 

from wells in this area from which cores were not available. Data from other wells outside the area were 

also made available by the Paleontological Laboratory; 

this information revealed many details of the history 

of the "atoll". The Bureau of Economic Geology of 

The University of Texas, John T. Lonsdale, Director, 

has provided funds in support of this work.

Age determinations of many of the reef rocks are 

based on identifications of fusulinids from cores by 

Keith A. Yenne and Donald A. Myers, of the Geological 

Survey.

STRATIGRAPHY

REGIONAL STRATIGRAPHIC RELATIONS

The subsurface Midland basin, the northern part of 

which is largely occupied by the Horseshoe atoll and 

its central lagoonal area, is bounded on the north by 

the Matador Arch and on the west by the central basin 

platform (fig. 6). The area east of the northern Midland basin is generally known by the term "eastern 

platform", and although no structural platform actually exists comparable to the central basin platform, 

nevertheless the term "eastern platform" will be used 

for that area in this report. The eastern platform area 

has a relatively thick section of Pennsylvanian rocks, 

including considerable amounts of limestone, shale, and 

sandstone; the Midland basin, except for the Horseshoe atoll, has a much thinner section of Pennsylvanian 

rocks, consisting mainly of dark-colored, unfossiliferous 

shale and siltstone (Adams and others, 1951). The 

thickness of Pennsylvanian rocks, mostly fossiliferous 

limestones, is much greater in the atoll than elsewhere 

in the basin.

The Permian system in the Midland basin includes 

shale, siltstone, dolomite, and limestone and is thicker 

than on the adjacent platforms. The lower part of this 

sequence of rocks, mostly dark-colored shale, is equivalent in age to some of the limestone in the upper part of 

the atoll.

During the subsidence of the Midland basin, limestone accumulated in the area of the atoll; as much as

1,500 feet are present in Scurry County in the eastern 

part of the area covered by this report, and about 3,000 

feet in Dawson County on the west side of the area. 

A maximum of about 2,000 feet of limestone is present 

in the East Vealmoor oil field, about in the center of 

the area described (pi. 10).

STRATIGRAPHY OF THE BEEF COMPLEX

The limestone accumulations of the Horseshoe atoll 

range in age from early Strawn (Pennsylvanian) 

through early Wolf camp (Permian). Ages of the different parts of the complex have been determined by 

the study of the fusulinids, but inasmuch as fossils from 

older parts of the limestone mass have been reworked 

and incorporated into younger parts of the reef, the age 

relations are complex and difficult to work out in detr.il.

The general distribution of rocks of Canyon, Cisco, 

and Wolfcamp ages in the Horseshoe atoll is shown in 

the cross sections on plate 12. In many parts of the 

reef, older rocks are at topographically higher positions 

than younger rocks. For example, just west of the 

East Vealmoor field, about 90 feet of limestone of 

Wolf camp age was penetrated in the well (marked Cf;V'7" 

in pi. 10) far down on the flank of the reef and more 

than 300 feet below the top of the rocks of early Cisco 

age in the center of the reef.

Except for relatively thin discontinuous beds of shale, 

the rocks in the reef complex show little or no stratification. Where these beds of shale are present, they 

seem to separate rocks containing significantly different 

faunas and thus to mark the contacts between rocks of 

Wolfcamp and Cisco, Canyon and Cisco, and early and 

late Cisco ages. Zones of different porosities, which have 

been observed in the reef, may indicate obscure stratification. The significance of these zones is discussed on 

page 26.

RELATION OF BEEF LIMESTONE TO STJRROUNDITG

SHALE

Intertonguing of limestone and shale at the top of 

the Scurry County part of the atoll has led some 

authors to consider that the atoll there is contemporaneous with the black shale enclosing the reef 

(Rothrock and others, 1953). Intertonguing of liir 3- 

stone and shale has not been observed in the southevn 

part of the atoll and is found elsewhere only near the 

top of the reef. Almost all black shale is of Wolfcamp 

age. Brinkerhoff Drilling Co.'s No. 1 Jones well (in 

the extreme western part of Howard County) peretrated about 200 feet of limestone of Wolfcamp age 

overlying a limestone containing mixed lower Canyon 

and upper Strawn fusulinids. A similar relation has 

been found in Phillips Petroleum Co.'s No. 1 Dennis 

well in northern Borden County, where black shrle
24 PENNSYLVANIA^ AND LOWER PERMIAN ROCKS, WEST AND CENTRAL TEXAS

adjacent to the reef overlies limestone of "Wolfcamp age. 

Therefore, the black shale can be no older than "Wolfcamp.

CHARACTERISTICS OF THE REEF ROCK 

LITHOLOGIC COMPONENTS

The reef rock in the southern part of the Horseshoe 

atoll consists mainly of fossiliferous clastic limestone 

but includes small amounts of dolomitic limestone, 

chert, and thin beds of black shale. Black bituminous 

clay is concentrated along stylolites, which are abundant 

in the reef. Some limestone breccia lies in a black 

clay matrix. Lithologic constituents of parts of the 

reef complex are shown in figures 7 and 8.

The reef limestone consists of calcilutite (limestone 

composed mainly of clay- and silt-sized particles), 

calcarenite (limestone composed mainly of sand-sized 

particles), and calcirudite (limestone composed mainly 

of fragments larger than sand size). The characteristics of these constituents in the Scurry Reef have 

been described by Bergenback and Terriere (1953); the 

reef limestones in the southern part of the atoll are 

generally similar. Typical specimens of calcilutite, 

calcarenite, and calcirudite are shown in plate 11.

Although the relative amounts of these limestones 

are different in different cores in general, the proportions of calcilutite, calcarenite, and calcirudite in the 

reef limestone seem to be roughly 2:2:1.

Calcilutite is generally understood to be a clastic 

rock, and no doubt many of the reef limestones of this 

type are clastic. However, this description does not 

apply to all, and possibly not even to the greater part, 

of these rocks, although the actual amount of limestone 

of detrital origin is uncertain. In the absence of a 

better term for this rock the writer uses calcilutite in a 

textural sense without regard to origin.

Usual petrographic techniques fail to show definite 

evidence of the nature and origin of much of the material in the calcilutite. J. Harlan Johnson examined 

some of the cores and expressed the opinion that some 

of the calcilutite was probably precipitated by algae, 

possibly by primitive blue-green algae, which left few 

structures. How much of the calcilutite was precipitated in this way remains uncertain. Some portion of 

the calcilutite should be ascribed to the reworking of 

coarser sediment by bottom-dwelling scavengers.

The calcarenite is composed of grains that seem to be 

mostly fragments of organisms. Ordinarily the sorting is so poor that it is difficult to determine the precise 

size classification of the rock. The sorting seems to 

improve, however, with the degree of rounding of the 

grains.

Depth of well

in feet below

ground surface

Spontaneous potential Resistivity

Calcilrtite 

Limestone composed 

of particles of clay 

size

Calcarenite

Limestone composed 

of particles of sand 

size

Limestone composed of 

particles of greatersize than eand

7. Microlog and lithologic log of Skelly Oil Co.'s 

No. 5 Gunn well.
GEOLOGICAL SUKVEY PROFESSIONAL PAPER 315 PLATE 1]

A. CALCILUTITE. B. CALCARENITE.

C. CALCIRUDITE. D. BIOCLASTIC CALCIRUDITE.

CORES FROM THE SOUTHERN PART OF THE HORSESHOE ATOLL, ILLUSTRATING ROCK TYPES

HORSESHOE ATOLL IN BORDEN AND HOWARD COUNTIES, TEXAS 25

Depth of well

in feet below

ground surface

Spontaneous potential Resistivity

8000-

8050-

8100-

8150-

Set casing 

Run 2

EXPLANATION

Not cored or no 

core recovery

Calcarenite

Limestone composed 

of particles of sand

Limestone composed of 

particles of greater 

size than sand

FIGURE 8. Microlog and lithologic log of Pan-American Producing Co.'s and Seaboard Oil Co. of Delaware's No. 24 

Good well.

Two types of calcirudite are present in the reef. 

The first is breccia made up of broken unsorted limestone fragments in a matrix of calcilutite, or in some 

places of black clay. A few cores of calcirudite show 

structures that probably indicate submarine slumping. 

The second type is bioclastic breccia composed of fragments of crinoid columnals, fusulinids, brachiopods, 

bryozoans, and other organisms. The writer defines a 

bioclastic rock as a rock composed mainly of fossils or

484724 59 2

readily recognizable fossil fragments. Thus, a calcilutite or a fine-grained calcarenite may be bioclastic, but 

unless it is composed of such very small organisms as 

Foraminifera, it may not be readily recognized as such.

Both kinds of breccia are commonly cemented ly 

calcilutite, but in some places they have a matrix of 

black clay. Both types of calcirudite with black-clay 

matrix are present at the top of the reef. Calcirudites 

with a black-clay matrix are always stratigraphically 

either at positions occupied elsewhere by thin beds of 

shale, or at the top of the reef. Mostly, calcirudite ar d 

calcarenite have matrices of calcilutite. Nonbioclastic 

calcirudite commonly has stylolitic contacts between 

the larger fragments. These stylolites do not everywhere extend into the matrix.

Sorting in the clastic limestones is almost everywhere 

very poor to nonexistent, although thin zones in a few 

cores are well sorted. Coquinas consisting essentially 

of fusulinids and calcite cement have been found, ard 

the fusulinids are large enough to justify classification 

of these rocks as calcirudites. This is the only rock 

type, coarser than fine-grained calcarenite, that shoves 

uniformity in size distribution of constituent particles.

DISTBIBUTION OF LITHOLOGIC COMPONENTS

If Horseshoe atoll had grown normally without interruptions, ordinary facies and stratigraphic relations 

would be expected. On the seaward side of the rea,f 

mainly calcirudite and calcarenite would be formed. 

The reef core would be made up of organic limestone 

with many organisms in or near their position of 

growth. In the reef core considerable calcarenite 

might be present but little nonbioclastic calcirudite 

would be expected. The backreef (lagoonal) facies 

would consist largely of calcarenite and perhaps of a 

small amount of calcirudite, grading laterally into 

calcilutite.

Forereef, backreef, and reef-core facies, as these 

terms are commonly understood (Cumings and Shrock, 

1928; King, 1948; Lowenstam, 1948; and Adams ard 

Frenzel, 1950), apparently do not exist in any part of 

the Horseshoe atoll. Calcirudite is common in what 

would normally be considered the core of the reef, and 

calcarenite and calcilutite are abundant throughout tl e 

entire reef complex. No organism was observed in its 

position of life with the possible exception of the rather 

indefinite algal structures.

In the southern part of the Horseshoe atoll, nonbioclastic calcirudites commonly overlie the thin beds 

of shale and are confined to such positions. The bioclastic calcirudites are a common constituent of the 

entire reef, as is indicated by cores. Oalcarenites ard 

calcilutites have equally widespread distribution. No 

stratigraphic significance has been found in the distri-
26 PENNSYLVANIAN AND LOWER PERMIAN ROCKS, WEST AND CENTRAL TEXAS

bution of calcarenites and calcilutites, but the nonbioclastic calcirudites are generally associated with the 

thin beds of black shale.

Attempts to correlate cores of adjacent wells by 

lithologic means have been unsuccessful. Occasionally 

in this study, certain zones of calcarenite or calcilutite 

seemed to be laterally equivalent, but such equivalence 

was suspect because of the general lack of correlation.

CHEMICAL COMPOSITION

Chemical analyses of representative cores show that 

the limestone is almost pure calcium carbonate. The 

average insoluble residue amounts to slightly more 

than 1 percent of the total rock, by weight. Analyses 

of limestone cores from two wells whose columnar sections and micrologs are illustrated in figures 7 and 8 

were made by the U. S. Geological Survey (tables 1 

and 2).

TABLE 1. Insoluble residue of a core from Seaboard Oil Co. of 

Delaware and Pan-American Producing Co.'s No. 24 Good well 

in the Good field

Sample depth 

(feet below surface)

8088.3.. .____.__._.

8093.5 ________ .

8094.5 ________ .

8096.7. _____ ....

8101.1 ________ .

8102.4.. _______ .

8106.0 ________ .

8108.0 ________ .

8112.0 ________ .

8113.1 ________ .

8119.5... _ ... _ .

8142.0... _ .......

Insoluble 

residue 

(percent)

__ 0. 39

._ .91

__ .76

._ 1. 34

.. .25

.. .55

 .49

._ 1.27

__ . 36

 .37

__ .35

 .57

Sample depth 

(feet below surface)

8148.0.. __ _ ___

8153.0 ____________

8160.3 ____________

8167.1. __ .... ____

8174.0. ____ . ____

8178.2. ___________

8181.6 ____________

8186.0. ___________

Average of

analyses _

Insoluble 

residue 

(percent)

... 0. 87

... .55

__ .51

_ .41

_ . 43

_ . 49

... . 57

... . 64

20

_ 0. 60

TABLE 2. Insoluble residue of a core from Skelly Oil Co.'s No. 5 

Gunn well in the East Vealmoor field

Insoluble

Sample depth residue 

(feet below surface) (percent)

7364.0 __________ 2.90

7367.7... ________ 2.50

7373.4______.__ ...... 1.90

7380.7-. _________ 1.40

7386.7. ..._.___..._.. .81

7395.8 __ . _______ .53

7402.0 __________ 1.00

Insoluble

Sample depth residue 

(feet below surface) (percent)

7170.3 __ _ _______ 0.38 

7173.7_____. .__.._.__ .31 

7220.0. _________ .34 

7225.0. __________ .80 

7230.0 ___ .

7242.0

. ______ .74 

	.38

7247.0. __ ... ____ 2.40

7253.2. __...___....._ .20

7257.0... _..._._.._._ 3.00

7261.0 __________ .45

7287.3 ___ . ______ 1. 50

7292.0. __________ 2.20

7300.8.___. _._..._... 7.30

7305.4 __________ 5.70

7309.0 ____ . _____ 1.60

7334.0 __________ .56

7338.0. _________ .69

7344.4.... _ __ __ _. 7.00

7351.8 ____ - ____ 5.60

7407.3 ._._._-..- .66

._______ .28

._._._._ .24

._..__._... .29

..._.. _. .58

._ __.... .29

7491.0-__________ 3.00

7494.6....__.__...__. .52

7423.5.

7431.0.

7439.0.

7468.0.

Average of 34

analyses__ .. 1. 70

POROSITY AND PERMEABILITY

The reef, as originally deposited, was probably a 

very porous mass because of the intergranular pore 

spaces in the clastic limestones. These pore spaces, 

however, were subsequently filled by calcium carbonate 

cement. Later, the entire mass was leached and secondary pores were developed. These pores are interconnected and penetrate both the initially deposited 

organic debris and the cement. They range hi diameter 

from submicroscopic to as much as 2 inches. This 

secondary porosity was then reduced by partial filling 

of pores (especially the larger cavities) with drusy 

calcite, and, locally, with authigenic quartz. Reports 

from commercial laboratories indicate that the present 

pore space is about 6 percent of the reef rock.

Permeability is low except for open fractures; it 

ordinarily ranges from 0 to 85 millidarcys (measured 

horizontally to air). The average vertical permeabilities are considerably lower than horizontal one*. This 

seems to indicate stratification of the porosity.

Cross sections A-A', B-B', C-C', and D-D' (plate 

12) show interpreted correlations from well to well of 

porosity data taken from micrologs. The porosity of 

the porous limestone shown on the cross sections may 

be 4.5 percent or greater, but that hi the les^1 porous 

limestone may be less than 5 percent. This overlap 

of porosities is inherent in microlog interpretation. 

The microlog measures not porosity but the resistivity 

of the zone invaded by the drilling mud. This is a 

function of the concentration and penetration of the 

mud into the sides of the bore hole, the resistivity of 

the mud, and the resistivity of the host rock.

Stafford (1955) compared nearly 5,000 analyses of 

cores from the eastern part of the Horsesboe atoll 

made by commercial laboratories with microlc? porosity ("permeability") classes. He found that in 90 

percent of the comparisons the microlog shows good 

porosity where more than 4.5 percent effective porosity 

exists. He also found that in about 90 percent of the 

observations the microlog shows very poor porosity 

where porosity is 5 percent or less. The statistical 

overlap is 0.5 percent.

Comparison of micrologs with lithologic logs shows 

that there is little correlation between lithologic character and porosity. However, comparison of the 

cross section with fusulinid determinations shows a 

relation between the porosity and faunal zones. 

The top of each faunal zone almost invariably shows 

good porosity whereas the base of each zone shows poor 

porosity and is associated with thin discontinuous 

beds of shale.

Because the porosity seems to be stratified, the 

microlog can be an effective aid in preparing cross
HORSESHOE ATOLL IN BORDEN AND HOWARD COUNTIES, TEXAS 27

sections. The cross sections themselves seem to 

present a reasonable picture of reef growth and a 

definite relation between the porosity and fauna! 

boundaries. There is, however, wide latitude for 

interpretation where paleontological control is meager 

and traceable units such as thin beds of shale are 

absent.

Porosity resulting from fractures has been observed 

in several cores. Some fractures were filled or partly 

filled with calcite. The open fractures markedly 

influence the permeability measurements of these 

cores. Whether a system of jointing has been established so that one set of fractures was filled with calcite 

while another set was not, or whether all fractures were 

locally filled, is at present undetermined. Further 

evidence is discussed in the section on Oil accumulation (p. 33).

FAUNA AND FLORA

Crinoids make up the largest part of the fauna in 

the southern part of the Horseshoe atoll. Bryozoans 

and brachiopods are next in abundance. Corals are 

rare only two were found in the cores examined from 

this area. Small Foraminifera also are present. Calcitornellid Foraminifera are common, as well as the 

textularian genus Climacammma. Other genera are 

inconspicuous. None of the more common members 

of this fauna are capable of erecting a reef framework 

that would be wave resistant.

Elliott and Kirn (1952) identified algae of the families Porostromata and Codiaceae in the core of a well 

in Terry County on the western side of the atoll. 

Algae were by far the most abundant of all the organisms recognized by them in this core. The calcilutites 

as weh1 as some of the calcarenites in the southern part 

of the atoll commonly contain many structures that 

apparently are organic but of uncertain taxonomic 

position. Some of these may well be members of the 

Cyanophyta (blue-green algae). A few structures 

resembling Girvanella, have also been noted. For the 

most part, however, the structures referable to algae 

have no diagnostic features, so that they cannot 

definitely be placed in their proper biologic position. 

It is difficult to identify the types of algae here involved. 

Every core examined, however, contained large amounts 

of material that could be interpreted as algae.

It is worth noting that the algae that are reef framebuilders today (primarily Lithothamnion) are the Khodophyta (red algae), whereas the other algae (primarily 

Halimeda) are found in abundance only in the quiet 

backreef waters. In the Paleozoic, the only family that 

has been referred to the Khodophyta are the Solenoporaceae. No organisms referrable to this family 

have been found or reported from the Horseshoe atoll.

In addition to the marine biota, Donald A. Myers 

has identified remains of fossil plants resembling Catamites in cores from three wells in the reef in Scurry 

County. In each well these fossils were in shale just 

above the base of the limestone of Cisco age.

REEF HISTORY

FACTORS AFFECTING THE CONCEPTS OF B-EIF 

DEVELOPMENT

From the foregoing discussion it is obvious that the 

Horseshoe atoll is considerably different from r^efs 

forming at the present tune, and from many fossil 

reefs described in the literature. The question may well 

be raised as to whether this structure is a true reef as, 

for example, defined by Lowenstam (1950, p. 443), who 

wrote:

a reef, in terms of ecologic principles, is the product of the 

actively building and sediment-binding biotic constituents, which 

because of their potential wave resistance have the abilit7 to 

erect rigid wave-resistant topographic structures.

It has been shown that the shale surrounding and 

overlying the atoll is of Wolf camp age and therefore 

could not interfinger with the parts of the reef wlich 

are of Strawn, Canyon, and Cisco age. Comparison of 

limestone thicknesses above the base of the atoll in the 

area of the East Vealmoor field with thicknesses in the 

distant forereef area reveals that at the site of the 

East Vealmoor oil field the atoll stood as much as 

1,800 feet above the surrounding bottom during G^co 

tune. Thick masses of calcirudite, composed of fragments of prelithified limestone, suggest that this structure was within the zone of strong wave action and was 

wave resistant.

Few of the organisms in the biota, however, seeir to 

have been reef framework builders or sediment binders. 

The holdfasts of crinoids, where great colonies were 

present, could, to some extent, have bound together 

the detrital sediments; it is doubtful, however, that 

crinoids could have lived in a strong surf.

Most of the fossils, other than Foraminifera, are 

present as broken fragments. The rare shells of brachiopods or "algal" structures found intact are clorely 

associated with fragmented shells. The implication is 

that the unbroken shell was only fortuitously transported whole from its place of growth to its place of 

entombment, while most of the neighboring sheUs were 

broken. The angularity of the fragments indicates that 

these organic remains were not strongly abraded or 

transported far from their places of growth. The 

unsorted nature of the deposits and the angularity suggests that the agencies of erosion and transportation 

were strong but very local. Coarse and fine fragments 

alike were dropped at the point of deposition ahrost 

immediately after being torn from their placep of origin.
28 PENNSYLVANIAN AND LOWER PERMIAN ROCKS, WEST AND CENTRAL TEXAS

The fauna of the reef limestone seems to indicate 

growth in quiet water. Unsorted, angular fragments, 

however, seem to contradict this evidence. These 

contradictions may perhaps be explained by the 

occurrence of occasional storms that attack the atoll. 

Thus, a relatively fragile biota, flourishing in the 

normally quiet waters around the atoll, could have been 

torn from the bottom during storms and cast into the 

more protected waters beneath wave base or behind 

the atoll. Some fragments may have been deposited 

above sea level on the beaches of the carbonate islands.1

Thus, the evidence suggests that the Horseshoe 

atoll was not a reef but a giant shell bank that was not 

wave resistant enough to meet Lowenstam's definition 

of a reef.

Identifiable reef core could not be found in the 

Horseshoe atoll, as pointed out elsewhere in the text. 

MacNeil (1954a), however, has strongly urged that the 

lack of a reef core does not prove that none ever existed. 

Ladd (1950, p. 204), who was of like mind, wrote:

In recent years having examined some of the larger recent 

reefs and having done some diving and dredging in their lagoons 

and on their outer slopes the writer has come to realize that 

though the rigid framework is a very essential part of a reef 

like the walls and rim of a pail that holds water it may quantitatively be very unimportant and only in rare elevated reefs 

is it preserved and satisfactorily exposed.

Ladd and Tracey (1949) wrote:

Since 1945 Hoffmeister and Ladd have studied both existing 

and elevated reefs in various parts of Micronesia and now feel 

that they were too restrictive in their identifications of "reef 

limestone." They failed to recognize that "reef structure" 

(imbricating colonies of flat corals in positions of growth such 

as characterized the marginal zones of existing reefs) forms a 

very small percentage of the entire reef. It is an important 

part, to be sure as important as the sides of a pail that holds 

water but it may make up only 5 to 10 percent of the reef 

mass, and, furthermore, may be the first part to be destroyed 

when the reef is elevated and eroded. By thus recognizing the 

quantitative unimportance of reef structure, the occurrence of 

scattered reef corals that are not [sic] in position of growth 

assumes greater significance. Such occurrences may indeed 

suggest a talus slope or deposition on a submarine bank, or>ly 

parts of which projected into the zone of reef growth, but it is 

perhaps more likely that such scattered corals were deposited 

on the side reef flat behind the marginal zone or in the shallow 

waters of a lagoon.

A second factor overlooked by Ladd and Hoffmeister was the 

significance of texture in sediments. Fine sediments do not accumulate on unprotected banks, and, therefore, the occurrence 

of thick sections of such materials is evidence of the existence, of 

a protecting rim at the time of accumulation, even though such 

a rim may no longer exist.

MacNeil (1954a, p. 392) suggests that solution might 

play an even more important role than mechanical erosion in the destruction of reef cores. If this is true, no 

evidence of the core would remain, and it would render

1 For an excellent account of the building and shaping of reefs and "coral islands" 

by winds and currents, see Fairbridge (1950, p. 356-362).

interpretation of the remaining deposits extremely hazardous.

The organisms which have been found in the Horseshoe atoll certainly do no violence to the concept that 

they represent a backreef population. Green algae, 

echinoids, certain kinds of Foraminifera, and F^yozoa 

have all been found to be members of modern backreef 

biota. The fragmentation of organisms and poor sorting of the sediments could perhaps be assigned to the 

work of the scavengers which constantly work the bottom through their digestive tracts. Furthermore, the 

relatively abundant calcilutite is better explained as a 

backreef deposit than as a deposit in the main body of 

an organic bank. The "scattering of corals" not in positions of growth, which suggested to Ladd and Tracey 

a possibility of backreef deposition, is also present in 

these sediments.

MacNeil (1954a) considered that large backreef-type 

deposits were so conclusive of the former existence of a 

reef core that "The burden of proof would rest with the 

one seeking to show that no bioherm ever bounded 

them." The writer is not quite so certain that backreef-type sediments and organisms could not be produced by other ecologic conditions, and he favors the 

possibility that the Horseshoe atoll was an extremely 

large shell bank.

AN HYPOTHESIS OF BEEF DEVELOPMENT

A reef tends to maintain its top at or near se^. level; 

thus, the upper surface of a reef is an excellent datum. 

The thin beds of black shale within the reef coirplex of 

the Horseshoe atoll seem to overlie erosional unconformities having several hundred feet of relief. The 

unconformities strongly suggest oscillations of sea level 

with respect to the top of the reef. Relative shift of 

sea level may be due either to tectonic uplift of the Midland basin or to eustatic lowering of sea level.

The presence of about 2,000 feet of reef limestone in 

the southern part of the Horseshoe atoll indicates that a 

similar amount of subsidence must have taken place 

during the growth of the reef. If the indicated shifts 

of sea level were due to tectonic movements, a basin is 

indicated that is subsiding most of the time b\it that 

now and again rises several hundred feet before starting 

downward again. This explanation poses proWems.

Deep drilling in most of the well explored sedimentary 

basins has shown that most unconformities found at 

the margins of these basins are represented in the deeper 

parts of the basin by a complete section of sedimentary 

rocks. Where an unconformity extends completely 

across a basin it usually marks an interval of strong 

folding and faulting. The history of most basin? seems 

to follow a single pattern of down warping accompanying 

sedimentation with minor unconformities at the mar-
HORSESHOE ATOLL IN BOEDEN AND HOWARD COUNTIES, TEXAS 29

gins. The downwarping may eventually have been 

followed by strong folding and faulting, but before 

that the deeper part of the basin seems to have been 

an area of almost continuous down warp. These considerations lead one to consider some alternate explanation for the unconformities.

The second explanation lowering and rising of sea 

level seems more reasonable. The mechanism for 

these postulated fluctuations is not known.

If it is assumed that the unconformities within the 

reef are related to eustatic shifts of sea level, then 

several consequences may be postulated. Probably the 

most important is the lowering of the base level of 

erosion which should cause more rapid erosion in areas 

that had formerly undergone only slight erosion. In 

areas that formerly had been receiving sediments and 

were near sea level, streams should cut deep channels. 

The sediments exposed in north-central Texas were 

apparently deposited in such an area. The limestones 

interbedded with continental shales and sometimes coals 

seem to have been deposited in an area that alternated 

between shallow sea and continental lowland. The 

more striking channels, now filled with sandstones, 

conglomerates and shales may be related to the unconformities in the reef. Plate 13 illustrates these correlations and seems to bear out the hypothesis of eustatic 

changes of sea level.

Wanless (1950) and others (Wanless and Shepard, 

1936; Wanless and Patterson, 1951) have considered 

that the cyclic type of sedimentation so common to 

rocks of Pennsylvanian and Wolfcamp age was caused 

by eustatic changes of sea level. The evidence presented here seems to support this view but with possibly fewer changes of sea level than these authors have 

envisaged.

EFFECTS OF EUSTATIC CHANGES OF SEA LEVEL 

ON THE HORSESHOE ATOLL

Lowering of sea level in a reef area would produce 

islands of increasing areal extent and relief above sea 

level. The area and relief of these islands would be 

proportional to the original area of the reef, the amount 

that sea level is lowered, and the relief of the reef 

relative to the surrounding sea floor. During the time 

of lowered sea level the islands would be attacked by 

the waves, and wave-cut benches and other surface 

features resulting from subaerial erosion would be 

formed. The agencies of erosion would eventually 

plane off the islands to the new wave-base level. If 

sea level rose again before this planation were complete, 

such fringing reefs as may have been growing during 

the time of lowered sea level would advance up the 

flanks of the islands with the rise of the sea and eventually develop into the new main reef.

F. Stearns MacNeil (1954b, p. 402-426) has recently 

discussed the effects of emergence and subaerial eroHon 

on reefs and limestone banks. In general, Macl Teil 

believes that the emergence of a table reef or limestone 

bank will result in the solution of the central area to a 

greater degree than the rim, and that this will produce 

a rimmed foundation ideally suited for the growth of 

an atoll if the island again becomes submerged. Hhe 

Horseshoe atoll was never a table reef, but the rim of the 

old atoll did become the site of a renewed growth following each emergence. MacNeil considered such a situation and pointed out that such foundations will cause 

reef growth to follow the topography developed by 

subaerial erosion. The topography of the southern 

part of the Horseshoe atoll resembles that described by 

MacNeil.

During the time of lowering of sea level, the tJme 

during which the sea is at the lower level, and also during the rise of sea level, large amounts of clastic material derived from erosion of the reef would be deposited 

on wave-cut benches and shore platforms, as well as 

on the flanks of the reef. The resulting complex facies 

distribution would then be preserved as the younger 

reef grows over the older detritus with the rise of sea 

level. The abnormal abundance of detritus wo^ild 

cause the normally steep reef slopes to become much less 

steep. This detritus would contain a fauna older tl an 

the sea in which it was deposited and faunas of two 

different ages would therefore be mixed. This mixing 

of faunas has contributed to some confusion about the 

age of parts of the Horseshoe atoll.

HYPOTHESES OF BEEF GROWTH

If the cyclic deposition of the Pennsylvanian end 

early Permian in the Midcontinent region is a result of 

periodic fluctuations of sea level, then the growth of the 

Horseshoe atoll would also have been cyclic, in a manner 

somewhat similar to the growth of the Pleistocene reefs 

in response to changes in sea level related to periods of 

glaciation, as visualized by Daly (1910, 1915, 1934). 

The complex textural and faunal relations, which hr.ve 

been observed within the limestone mass, represent, in 

part, facies conditions that may be expected in a rief 

complex whose development is modified and interrupted by oscillation of sea level.

Two hypotheses may be advanced to relate the 

growth of the Horseshoe atoll more directly to the 

postulated changes in sea level in the Midland basin. 

In the first, normal reef growth is proposed, together 

with several lowerings of sea level to account for the 

reworked faunas and superabundance of clastic limestone; it is also proposed that sea level was lowered only 

after subsidence of the basin was sufficiently rapid to 

cause the reef to restrict itself to small areas of growth
30 PENNSYLVANIA!* AND LOWER PERMIAN ROCKS, WEST AND CENTRAL TEXAS

such as the Good, East Vealmoor, and Reinecke hills. 

These hills were then exposed to erosion for a very short 

interval and sea level rose again. Marine organic 

growth over the old "highs" recommenced. This 

process was repeated several times; the erosion intervals 

are marked by the thin beds of shale. Thus the reef 

hills are considered to be essentially a natural-reef configuration due to rapid subsidence, slightly modified by 

erosion. 

Several objections can be raised to this theory.

1. If the sea level were lowered less than the height of 

the hills above the main body of the reef, then 

these hills would be truncated or partly so and the 

wave-cut bench would be well above the surrounding, but topographically lower, main body of the 

reef. The unconformity of middle Cisco age in 

the Good field seems to have the proper configuration for a lowering of sea level less than the height 

of the restricted reef knob. Under this assumption, however, no explanation can be found for the 

limestone of late Cisco age that lies 90 feet or more 

down the flanks from the unconformity, below the 

probable depth at which the Codiacae can flourish 

when sea level is at the position of the unconformity. (See Cloud, 1952, p. 2134.) Some of 

this limestone could be attributed to reef detritus, 

but the distribution of limestone of Cisco age, as 

shown in plate 10, is too widespread to attribute 

it all to scree.

2. The rapid subsidence, which caused the restriction 

of reef growth to the scattered hills on the crest of 

the Horseshoe atoll, must be stopped at precisely 

the right time. If sea level were to commence 

rising again before the subsidence has stopped or 

slowed, then surely the rate of sea-level rise and 

rapid subsidence combined would drown the reef, 

and its top would be placed below the zone of 

biotic potential.

3. Comparison of reef thicknesses shows that the 

thinnest part of the reef is in the northeast. In 

the Salt Creek field (Galbraith and Barker, 1952), 

in the northeastern part of the atoll in Kent 

County, the reef has a maximum thickness of 

about 1,100 feet. In the East Vealmoor field the 

reef has a maximum thickness of about 2,000 feet. 

The limestones in these different parts of the atoll 

were deposited during the same interval of time; 

and inasmuch as the biota probably had a rather 

limited depth at which it could flourish, the subsidence of each area was probably about equivalent 

to the thickness of the reef. The rate of subsidence would, therefore, have been a function of 

the thickness. If subsidence is required to be so 

rapid that growth is restricted to small patches

in the Salt Creek area, the East Vealmoor field 

area would surely have drowned, inasmuch as the 

thicknesses indicate the rate of subsidence in the 

East Vealmoor area to be twice that in the Salt 

Creek area.

Because of these objections, the above hypothesis is 

rejected in favor of an hypothesis which postulates that 

the internal "highs" are erosional remnants of an older, 

thicker, and broader structure, rather than configurations of restricted growth resulting from rapid subsidence. These "highs" certainly caused the reef to 

accommodate its growth to the older topography, but 

before the existence of these highs the top of the reef 

was probably rather smooth and continuous.

ECOLOQIC CONTROLS 

The direction of the prevailing winds and currents 

has a strong effect upon reefs and organic banks. If 

current and wind direction very nearly coincide for the 

greater part of the year, the strongest growth will be 

on the windward side. Horseshoe-shaped atolls and 

carbonate cays develop where winds have a fairly constant direction during the entire year (Kuenen, 1950, 

p. 441-442; Fairbridge, 1950, p. 361). From this we 

may assume that the wind and currents in the northern 

Midland basin had a fairly constant direction coming 

from slightly west of south.

By analogy with modern carbonate environments 

the environment of this area was probably wr.rm and 

tropical. Similarly, at the very start of reef growth in 

early Strawn time a very shallow epeiric sea probably 

covered the area. The sea bottom was nearly flat, 

but it had a regional slope toward the south and west 

where reefs of pre-Strawn age are known (Imbt and 

McCollum, 1950, p. 246).

From the shape of the atoll it may be inferred that 

a fairly constant wind and current came from the south 

or slightly west of south. Almost no terrigenous 

elastics were entering this area and we may therefore 

assume that the water was clear. Normal salinity is 

suggested by the abundance of varied typer of organisms.

This environment prevailed from early f \it not 

earliest) Strawn time, through Canyon, Cis-x>, and 

earliest Wolfcamp times, but not without several interruptions. The thin beds of shale shown in the cross 

sections probably indicate increased turbidity, and 

mark unconformities that in some places show several 

hundred feet of relief.

The black shales, which accumulated on the floor of 

the Midland basin during the time of reef growth, indicate that a nonaerated toxic environment probably was 

present. Thus, two environments were probably superimposed: stagnant, toxic water on the sea floor; and 

warm, clear, oxygenated water at the surface. Newell
HORSESHOE ATOLL IN BORDEN AND HOWARD COUNTIES, TEXAS

and others (1953, p. 12-14) have pointed out similar 

relations of several fossil reefs. The Capitan reef of 

Trans-Pecos Texas, the Mississippian reefs of the Central Province Basin north of England, the Permian 

reefs of eastern Greenland, and the Upper Triassic 

reefs of the Italian Tyrol all are associated with stagnant basins. Newell (Newell and others, 1953, p. 14) 

is of the opinion that the deep stagnant waters may 

provide "an unusually rich source of up welling nutrient 

salts for lime-secreting reef plants."

HISTORY OF THE BEEF DURING PENNSYLVANIAN

TIME

Rocks of Strawn age have been recognized throughout 

the area covered by the atoll. The lowest 50 to 200 

feet are limestones containing thin shales that can be 

traced in the subsurface far from the atoll. This part 

of the section is not considered a part of the reef rock. 

From this foundation the reef growth began. More 

than 750 feet of reef rock of Strawn age accumulated 

above the earlier stratified limestone of Strawn age. 

Thin beds of shale within the reef of Strawn age probably mark unconformities much as those found at the 

top of rocks of Strawn age and within rocks of Canyon, 

Cisco, and Wolfcamp age.

Sea level was lowered in early Canyon time and the 

atoll was then truncated. Large quantities of limestone, removed from the crest and deposited in both the 

forereef and lagoonal areas, were spread for tens of 

miles over the sea floor. The top of the rocks of 

Strawn age has little relief in the area of the crest of 

the atoll, but the tremendous quantities of limestone 

that must have come from this truncation suggest 

that this period of erosion probably lasted longer and 

contributed more clastic limestone to the surrounding 

area than any previous or later erosion interval. A 

rise of sea level or possibly subsidence of the basin 

then placed the crest of the atoll beneath the effective 

action of the waves, and the thin beds of shale at the 

base of the reef of Canyon age were deposited over the 

truncated reef of Strawn age. The source of the 

argillaceous material may have been land areas that 

emerged or extended closer to the Midland basin as a 

result of the fall of sea level. Some of this material, 

however, may have been derived from the atoll itself 

as insoluble residues concentrated on the surface of 

the atoll. Chemical analysis and lithologic appearance show that the clay concentrations on the stylolites 

and these thin shales are very similar. This surface 

was the site of renewed reef growth during Canyon 

time.

Similar cycles of reef development, growth during 

high sea level combined with basin subsidence, and 

later erosion during low sea level, were probably

repeated four more times during the Pennsylvania 

period, twice during Canyon time, and twice during 

Cisco time; but with the possible exception of tl Asucceeding Canyon erosion cycle, which occurred in 

middle or late Canyon time, the atoll was never again 

so completely truncated as at the beginning of Canyon 

time, and never again was so much limestone detritus 

contributed to the environs of the Horseshoe ato1!. 

Plate 12 illustrates the partial planation that took 

place during middle Cisco time.

Four types of limestone of Wolfcamp age are believed 

to be present in the southern part of the Horseshoe 

atoll. They will be discussed in the order of their 

deposition.

The first type is normal reef limestone that was 

deposited on the top of the East Vealmoor, Vealmoor, 

and Good fields, and that is essentially free of clastic 

material derived from erosion of the old reef.

The second type probably is a fringing deposit 

containing scree derived from the subaerial topographic 

"highs" and deposited around the margins of the hills 

having large amounts of relief. This took place after 

sea level had been lowered in early Wolfcamp time. 

This type is represented by limestone of Wolf can: p 

age found in the dry hole immediately west of the Ea^t 

Vealmoor field. (Marked with a "W" on pi. 10.)

The third "type is limestone of Wolfcamp age fourd 

in a dry hole southwest of the Vealmoor field in extreme 

western Howard County. (Also marked with a "W" 

on pi. 10.) The base of the Wolfcamp in this well 

(Brinkerhoff Drilling Co.'s No. 1 Jones well) is at 

6,693 feet below sea level, some 1,600 feet lower than 

the highest part of the Vealmoor field and about 800 

to 1,000 feet lower than the lowest Wolfcamp known 

from the reef-crest area. (See pi. 14.) Newell ard 

others (1953) have described tongues of limestone, 

apparently derived from the Capitan reef, transported 

by submarine landslides, and deposited several miles 

from the reef crest on the floor of the Delaware basin. 

It is very difficult to identify deposits of this kind in 

the subsurface, but the extremely low position (with 

reference to the reef crest) of this type of limestone, 

which seems to have been brought into a black-shale 

environment, suggests that it was transported ly 

submarine landsliding from a higher position on tl~e 

reef. The presence of this limestone indicates that the 

reef had more than 1,800 feet of relief in early Wolfcamp 

time, and that nearly all the shale surrounding arid 

overlying the reef is of Wolfcamp age. The secord 

and third types of limestone are probably for the most 

part contemporaneous.
32 PENNSYLVANIA AND LOWER PERMIAN ROCKS, WEST AND CENTRAL TEXAS

The fourth type of limestone of Wolfcamp age is 

represented by tongues extending into the surrounding 

black shale near the top of the reef (Kothrock and 

others, 1953). These limestone tongues probably are 

erosional products derived from a dead reef.

Every hill west of the Hobo field has reef limestone 

of Wolfcamp age on its crest. All reef hills east and 

northeast of the Hobo field, as well as those in the Hobo 

field itself, have reef limestone of Cisco age on their 

crests, but limestone of Wolfcamp age is present low 

on the flanks of these hills. (See pi. 10, and Heck, 

Yenne, and Henbest, 1952.) The limestone of Wolfcamp age low on the flanks of the eastern reef hills 

may represent either detrital limestone from a topographically higher source, a fringing reef developed 

during a low stand of the sea, or a combination of both. 

Examination of cores from this limestone (Wilshire Oil 

Co.'s No. 8 Lunsford well and Pan-American Producing 

Co.'s No. 1 Glass) by geologists of the Geological 

Survey has shown that hi large part it is of detrital 

origin.

From the Hobo field westward, successively thicker 

and younger deposits of limestone of Wolfcamp age are 

present on the crest of the Horseshoe atoll. This 

relation and the presence of what look like foreset beds 

hi the terrigenous Wolfcamp rocks suggest that reef 

growth was stopped by the encroachment of deltas 

onto the atoll from the east and north in Wolfcamp 

tune. The southwestern part of the reef continued to 

grow for a considerable part of Wolfcamp time after 

the clastic sediments were spread as aprons over the 

eastern and northern parts of the atoll. The increased 

turbidity probably even more strongly limited the 

depths at which the reef could grow in the southern part 

of the atoll, and restricted growth largely to the highest 

parts of the old hills developed during the erosion 

interval during early Wolfcamp time.

During this tune vast quantities of calcareous clay 

and silt were deposited in the northern part of the 

Midland basin. The mud eventually covered the reef. 

As the mud filled the basin the fourth type of limestone 

was deposited as lenses of carbonate detritus in muds of 

Wolfcamp age, which then surrounded the dead reef.

CEMENTATION AND CYCLICAL BEEF GROWTH

In the borings of the Cenozoic reefs of the Pacific 

Fairbridge (1950) and Ladd and others (1953) have 

noted that, except for their surfaces, these reefs are 

generally not well cemented. In those areas which are 

alternately wetted and dried, cementation is very rapid. 

Cans and bottles have been incorporated into the socalled "beach rock" by this rapid process of cementation. The length of time involved hi the postulated 

fluctuations of sea level should have been more than

adequate to accomplish the same process of cementation by wetting and drying in the Horseshoe atoll. As 

sea level was lowered, successive parts of the reef that 

came into the tidal range became cemented. Thus, by 

the time a particular part of the reef was undergoing 

subaerial erosion it was already a well-cemented rock 

because of the previous wetting and drying and consequent loss of CO2 and precipitation of CaCO3 

inherent in any such lowering of sea level.

The secondary porosity was probably caused by leaching of the exposed reef by meteoric water after the 

cementation had taken place. The leaching vas probably facilitated by the presence of humic acids. It 

seems likely that the exposed reef would develop a 

tropical type of vegetation. In this respect "Czlamites" 

remains found at the base of the Cisco probably are 

significant. With the rise of sea level such vegetation 

that may have developed on the exposed island would 

have been drowned; fossilized plants may be expected 

to be found at the unconformity the positior at which 

these fossils were found in three separate bore holes.

The sequence of growth and cementation in each reef 

cycle seems to have been as follows:

1. Normal reef growth with subsidence of the basin 

amounting to several hundred feet.

2. Lowering of sea level which probably is never completely stationary and emergence and erosion 

of the upper part of the reef. As sea level was 

lowered, the alternate wetting and drykg by the 

tides caused loss of the CO2 in the water trapped in 

the pore spaces in the reef, with consequent precipitation of CaCO3, which cemented the reef.

3. Wave attack of the islands formed by the lowering 

of sea level and formation of wave-cut benches 

and platforms. Solution by ground wate~ abetted 

by humic acids caused secondary porosity and 

other solution features in the islands. Streams on 

the surface left their imprint in the form of deep 

gullies. Considerably greater amounts of claysized particles entered the basin as a result of the 

much closer shoreline. At the same time, new 

sources of material for erosion were made available 

by the lowering of sea level.

4. Kise of sea level, so that the wave-cut benches and 

platforms were placed beneath the zone of wave 

activity and received such clay particle? as were 

still entering the area, plus the insoluble residue 

concentrate resulting from the solution ard weathering of the reef itself. Thin clay deposits covered 

the older wave-cut benches, which were formerly 

kept clear by their proximity to the surface and by 

the action of the waves. Deposition of mud may 

have been more rapid in the lee of the islands than 

on the seaward side. Fringing reefs grew upward
HORSESHOE ATOLL IN BORDEN AND HOWARD COUNTIES, TEXAS

with the rise of sea level (principally on the seaward side) and as the top of the old island was 

covered over, they became the new reef proper.

OIL AND GAS

OIL FIELDS

Seventeen fields producing oil from the reef have been 

discovered (October 15, 1954) in the area shown on the 

accompanying map (pi. 10). From west to east, the 

following larger fields are producing oil from the reef: 

Good, Oceanic, Vealmoor, North Luther, East Vealmoor, Hobo, Keinecke, Von Boeder, and Scurry. In 

the area between the Vealmoor and East Vealmoor fields 

is a Stanolind Oil Co. well named the Central Vealmoor 

field. The Von Boeder field in the northeast is a composite field of three small structures. The Vincent, 

Vealmoor-Kead, and Sara Mag fields are on the seaward flank of the atoll and entirely within Howard 

County, while the Pancho-Mag and Pancho-Ma^ 

Southwest fields are in the lagoonal area of the atoll. 

The very large Scurry field extends northeastward from 

the mapped area. The Luck Pot field, northwest of 

the Good field, seems to be on the crest of the ree*, 

but more drilling will be necessary before it can le 

placed in the class with the other reef-crest fields.

DISCOVERIES AND PRODUCTION

The first company to find oil in the Horseshoe ato1 ! 

in the area mapped in this report was the Seaboard 

Oil Co. of Delaware. This discovery was made in 

the Vealmoor field in January 1948. The discovery 

well was structurally low and is now shut in, because 

of excessive production of salt water.

The discovery wells for this and other fields in the 

area of this report are shown in table 3.

TABLE 3. Discovery wells in the Vealmoor and other fields in the area of this report

Discovery date

Jan. 31, 1948

Mar. 21, 1949

Apr. 14, 1949

Dec. 21, 1949

Feb. 21, 1950

Mar. 2, 1950

Aug. 10, 1950

Nov. 11, 1950

Dec. 20, 1950

July 14, 1951

Nov. 21, 1952

Apr. 27, 1953

Aug. 3, 1953

Jan. 27, 1954

Mar. 16, 1954

Mar. 26, 1954

June 8, 1954

Aug. 2, 1954

Field name

Vealmoor _ __________ ______

East Vealmoor_______________

Good. __---______._____. __

Von Roeder __ _____________

Reinecke. _--__-_-___________

North Vealmoor..,, _ __ ______

O' Daniel (Canyon age) ________

Bond (Canyon age) ___________

Vincent.. _ _________________

Hobo __ _ _ ______ ___ _ _

North Luther. _______________

Central Vealmoor ____________

Oceanic (Pennsylvanian) _____

Pancho-Mag_ ________________

Luck Pot_________________ __

Sara-Mag___._ _____________

Vealmoor-Read_ _____________

Pancho-Mag Southwest. ______

Company

Producing Co. 

Amerada Petroleum Corp.. _______________________

George P. Livermore, Inc _________________________

Shell Oil Co _____ ___ ___ .... ___ __. __ _ _

Stanolind Oil & Gas Co ___ ________ __ ___

Fred M. Manning, Inc __ _______________________

LarioOil&Gas Co __ ___________________________

Stanolind Oil & Gas Co ___ _ ______________

Oceanic Oil Co., Green & McSpadden_ _____________

Falcon-Seaboard Drilling Co. and Green & McSpadden. 

Russell Maguire. ._ ____________ _________ _____

Trans- Texas Drilling Co. _-__-_-_-___----_--------

Well No. and lease

1-B Caldwell.

1 Wilson.

1 Good.

1 Von Roeder.

1 Reinecke.

1 Porter Hanks.

1 O'Daniel.

1 Burton.

1 Chester L. Jones.

1 T. L. Griffin.

1 Pauline Hamlin.

1 Minnie Smith.

1 Lou Winans.

1 Beal.

1 Clayton and Johnson.

1 Chandler.

1 Read Ranch.

1-A Beal.

All the oil fields shown on plate 10 produce oil which 

ranges from 43° gravity A. P. I. to 47° gravity A. P. I. 

The following information on cumulative production 

from the reef in the southern part of the Horseshoe atoll, 

to January 1, 1954, was taken from the files of the Oil 

and Gas Division of the Texas Kailroad Commission.

Oil produced 

Fields (bbls)

Bond (Canyon)_____________________ 47,980

Central Vealmoor____________________________ 3, 708

East Vealmoor____________________.__ 8, 587, 612

Good ________________________ 7, 210, 054

Hobo___________________________ 1, 888, 574

North Luther..____________________ 134, 416

North Vealmoor____________________________ 43, 981

Oceanic (Pennsylvanian)___________________ 92,788

O'Daniel (Canyon)___________________ 21,749

Reinecke_____ _________________ 9, 920, 695

Vealmoor_________________________ 7, 447, 432

Vincent_______________.__________ 60, 413

Von Roeder_____________ __________ 3, 956, 056

Total_______________________ 39, 415, 458

OIL ACCUMULATION

A few wells, such as Seaboard and Pan-American's 

Nos. 19 and 27 T. J. Good wells, which are structurally 

high enough to be commercial producers, nevertheless 

were dry or very short-lived producers. Porosity 

studies show that the rocks penetrated by these wells 

either lacked sufficient porosity, or the porous zone was 

not connected with the main reservoir. Commercir.l 

amounts of oil did not accumulate in these disconnected 

porous zones during the time of oil migration from source 

rock to reservoir rock, apparently because the permeability of rocks surrounding the porous zones was 

too low to permit the passage of oil, and fractures in 

the reef did not allow unrestricted migration of oil to all 

parts of the reef.

Structural closure for those fields on the crest of the 

Horseshoe atoll is the result of eroded reef topography, 

but it involves regional tilting to the southwest at 

rate of about 30 feet per mile.
34 PENNSYLVANIAN AND LOWER PERMIAN ROCKS, WEST AND CENTRAL TEXAS

COMPLETION PRACTICES

Some operators drill only 10 or 20 feet into the reef 

before placing a well into production. They probably 

consider that vertical migration of the oil will give their 

wells adequate drainage of the reservoir. Inspection 

of the porosity zones on the cross sections shows that 

the vertical movement of oil in these reservoirs is impeded or impossible. Wells drilled into oil reservoirs 

in the Horseshoe atoll should therefore be carried to 

depths just short of the oil-water interface to obtain 

adequate reservoir drainage.

REFERENCES CITED

Adams, J. E., and Frenzel, H. N., 1950, Capitan barrier reef, 

Texas and New Mexico: Jour. Geology, v. 58, no. 4, p. 

289-312.

Adams, J. E., Frenzel, H. N., Rhodes, M. L., and Johnson, 

D. P., 1951, Starved Pennsylvanian Midland basin: Am. 

Assoc. Petroleum Geologists Bull., v. 35, no. 12, p. 2600- 

2607.

Anderson, K. C., 1953, Wellman field, Terry County, Texas: 

Am. Assoc, Petroleum Geologists Bull., v. 37, no. 3, p. 

509-521.

Bergenback, R. E., and Terriere, R. T., 1953, The petrography 

and petrology of the Scurry reef, Scurry County, Texas: 

Am. Assoc. Petroleum Geologists Bull., v. 37, no. 5, p. 

1014-1029.

Burnside, R. J., Stafford, P. T., and Myers, D. A., 1954, Geological Survey investigations of the Scurry reef and "Horseshoe atoll" in western Texas: Science, v. 119, no. 3906, 

p. 617-618.

Cheney, M. G., and Eargle, D. H., 1951, Geologic map of 

Brown County, Texas: Texas Univ. Bur. Econ. Geology 

map.

Cloud, P. E., Jr., 1952, Facies relationships of organic reefs: 

Am. Assoc. Petroleum Geologists Bull., v. 36, no. 11, p. 

2125-2149.

Cumings, E. R., and Shrock, R. R., 1928, Niagaran coral reefs 

of Indiana and adjacent States and their stratigraphic relations: Geol. Soc. America Bull., v. 39, no. 2, p. 579-620.

Daly, R. A., 1910, Pleistocene glaciation and the coral reef 

problem: Am. Jour. Sci., 4th ser., v. 30, p. 297-308.

 1915, Glacial-control theory of coral reefs: Am. Acad. 

Arts and Sci. Proc., v. 51, p. 155-251.

 1934, The changing world of the ice age: New Haven,

Conn., Yale Univ. Press, chap. 11. 

Elliott, R. H. J., and Bam, O. J., 1952, Pennsylvanian reef

limestone, Terry County, Texas: Colorado School of Mines

Quart., v. 47, no. 2, p. 71-94. 

Fairbridge, R. W., 1950, Recent and Pleistocene coral reefs

of Australia: Jour. Geology, v. 58, no. 4, p. 330-401. 

Galbraith, G. S., and Barker, J. C., 1952, The Salt Creek field,

Kent County, Texas: Abilene Geol. Soc., Geol. Contr.

(Monroe Cheney memorial volume), p. 72-73. 

Heck, W. A., Yenne, K. A., and Henbest, L. G., 1952, Boundary

of the Pennsylvanian and Permian (?) in the subsurface

Scurry reef, Scurry County, Texas: Texas Univ. Pept. Inv. 

13 [1953].

Imbt, R. F., and McCollum, S. V., 1950, Todd I ?ep field, 

Crockett County, Texas: Am. Assoc. Petroleum Geologists 

Bull.* v. 34, no. 2, p. 239-262.

King, P. B., 1948, Geology of the southern Guadalupe Mountains: U. S. Geol. Survey Prof. Paper 215.

Kuenen, P. H., 1950, Marine geology: New York, John Wiley 

& Sons, Inc., 568 p.

Ladd, H. S., 1950, Recent reefs: Am. Assoc. Petroleum Geologists 

Bull., v. 34, p. 203-214.

Ladd, H. S., and Tracey, J. I., Jr., 1949, The problem of coral 

reefs: Sci. Monthly, v. 69, p. 296-305.

Ladd, H. S., Ingerson, Earl, Townsend, R. C., Russel\ Martin, 

and Stephenson, H. K., 1953, Drilling on Eniw?tok atoll, 

Marshall Islands: Am. Assoc. Petroleum Geologists Bull., 

v. 37, no. 10, p. 2257-2280.

Lee, Wallace, Nickell, C. O., Williams, J. S., and Henbest, 

L. G., 1938, Stratigraphic and paleontologic studies of 

the Pennsylvanian and Permian rocks in noHh-central 

Texas: Texas Univ. Bull. 3801.

Lowenstam, H. A., 1948, Marine pool, Madison County, Illinois, 

Silurian reef producer, in Structure of typical American 

oil fields, v. Ill (Alexander Watts McCoy memorial volume): Tulsa, Okla., Am. Assoc. Petroleum Geologists, 

p. 153-188.

 1950, Niagaran reefs of the Great Lakes area: Jour.

Geology, v. 38, no. 4, p. 430-487. 

MacNeil, F. S., 1954a, Organic reefs and banks and associated

detrital sediments: Am. Jour. Sci., v. 252, no. 7, p. 385-401.

 1954b, The shale of atolls: an inheritance from subaerial 

erosion forms: Am. Jour. Sci., v. 252, no. 7, p. 402-427.

Moore, R. C., 1949, Rocks of Permian(?) age in the Colorado 

River valley, north-central Texas: U. S. Geol. Survey Oil 

and Gas Inv. Prelim. Map 80.

Newell, N. D., Rigby, J. K., Fischer, A. G., Whiteman, A. J., 

Hickox, J. E., and Bradley, J. S., 1953, The Permian reef 

complex of the Guadalupe Mountains region, Texas and 

New Mexico: San Francisco, Calif., W. H. Freeman and 

Co., 226 p.

Rothrock, H. E., Bergenback, R. E., Myers, D. A., Stafford, 

P. T., and Terriere, R. T., 1953, Preliminary report on 

the geology of the Scurry reef in Scurry County, Texas: 

U. S. Geol. Survey Oil and Gas Inv. Map OM 143.

Stafford, P. T., 1955, The zonation of the late Paleozoic Horseshoe atoll in Scurry and southern Kent Counties, Texas: 

U. S. Geol. Survey Oil and Gas Inv. Chart OC 53.

 1957, Scurry field, p. 295-302, in Herald, F. A., editor,

Occurrence of oil and gas in west Texas: Texas Univ. Pub.

5716, 442 p. 

Wanless, H. R., 1950, Late Paleozoic cycles of sed'mentation

in the United States: Internat. Geol. Cong., 18th, Great

Britain, Rept., pt. 4, 1948, p. 17-28. 

Wanless, H. R., and Patterson, J., 1951, Cyclical sed'mentation

in the marine Pennsylvanian of the southwestern United

States: 3e Congres de Stratigraphie et de Geologic du

Carbonifere Heerlen, p. 655-664. 

Wanless, H. R., and Shepard, F. P., 1936, Sea level and climatic

changes related to late Paleozoic cycles: Geol. So"- America

Bull., v. 47, no. 8, p. 1177-1206.
INDEX

Page 

Abstract__ _____________________________________ 21

Amerada Oil Co., cores contributed_______________________ 23 

Armer, L. H., cores contributed_________________________ 23 

Authigenic quartz___ __ _______________________ 26

Backreef facies________ 

Black clay. ________ 

Black shale...._________ 

Brachiopods__________ 

Breccia, biodastic..______ 

Brinkerhoff Drilling Co.'s well~

...__.....____________.___ 25

._______________________ 25

..._..........__________.___ 23

 . 25

........................................ 25

 .. 23 

Bryozoans ______________.____________________ 25,27,28

Catamites...................___.____________.____________ 27,32

Calcarenite . _. _______________________ 24,25 

Calcnutite________.________________________.___24,25 

Calcirudite____________________________________ 24,25 

Canyon age, rocks________________________________ 23,31 

Carbonate islands, faunal fragments deposited on beaches__________ 28 

Central basin platform______________________________ 22,23 

Cisco age, rocks________________________________ 23,31,32 

Clastic limestone, intergranular pores in____________________ 26 

Climacammina................................................................ 27

Codiaceae . ... _____.. .. 27 

Coquinas.-____________________________________ 25 

Cores, chemical composition.__________________________ 26 

Crinoids . . _______________________ 25,27 

Cyanophyta .. . 27

Dolomitic limestone___.___________________________ 23 

Drusy calcite_______.___________________________ 26

East Vealmoor oil field _____________________________ 23 

Eastern platform_________________________________ 22,23 

Echinoids____________________,___.______.____ 28 

Ecologic controls... ___________________ 30-31 

Erosion, subaerial____-___-_______________________ 29

Fauna and flora of the area____________________________ 27 

Foraminifera... . . ___.__. __.... .. _. 27,28 

Forereeffaeies..__________________._______________ 25 

Fusilinid determinations, by E. V. Hollingsworth__._____ .. 23 

Fusilinids .._.._..... ......__.........................._...... 25

GirvaneUa... 

Green algae.

Halimeda.....___.___________________.....____.......... 27

Hollingsworth, E. V., fusilinid determinations._.. .__.__..... 23

Horseshoe atoll, cyclic growth... f°-30 

during Pennsylvanian time_ 31 

during Wolfcamp time.... . . ....... ....... .... 31-32

Johnson, J. H., core examination__ . 24

Ladd, H. 8. and Tracy, J. I., quoted. . . 28 

Lagoonal facies... 25 

Lithothamnion.. 27

Location of the area.. 22,23

MacNeil, F. S., on effects of emergence and subaerial erosion on reefs. 29 

quoted.... 28

Matador Arch. .. . . .. ..... 22,23

Meteoric water, leaching of reefs 32 

Microlog measurement of resistivity. 26 

Midlandbasin..... . 22,23,28

Myers, D. A., fossil plants identified__. - - 27

Oil, accumulation__ . - - - ---. - 33,34 

Oil and gas, discoveries and production. . 33 

Oil fields listed as producers______ ___ 33

Phillips Petroleum Co.'s well.... . .... - - 23 

Porostromata_ : 27

Eeef, defined by H. A. Lowenstam. 27

Eeef-core fades... 25

Eeef development, cementation and cyclical growth.. 32-33

factors affecting... ___ 27-28

hypotheses..____-_______ ____ - - 28-29,30

Reef rock, chemical composition 26

lithologic components .. 24-26

porosity and permeability.. 26-27

Reefs, porosity___ 32

Ehodophyta . - 27

Scavengers, sediments reworked by__. . 24

Seaboard Oil Co. of Delaware, cores contributed_ 23

Sea-level oscillations, with respect to the reef 28,29

Skelly Oil Co., cores contributed...... . -- 23

Solenoporaceae... 27

Sorting, in the clastic limestones.. . 25

Strawn age, rocks. ... .. 23,31

Stylolites...... -.. - -. "- 25

Table reef, not applied to Horseshoe atoll. 29

Unconformities, erosional.

Wolfeampage.rocks... .. - . 23,31,32

85

O