Please do not destroy or throw away this publication. If you have no further use 

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UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. C. Mendenhall, Director

Professional Paper 175—D

ORIGIN OF THE ANHYDRITE GAP ROCK 

OF AMERICAN SALT DOMES

BY

MARCUS I. GOLDMAN

Shorter contributions to general geology, 1932-33

(Pages 83-114)

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1933

For sale by the Superintendent of Documents, Washington, D. C. ------ Price 15 cents
CONTENTS

Abstract-_ ________________________________________

Introduction ______________________________________

Terminology __________________________________

Two hypotheses of origin_______________________

The process of origin of cap rock by residual accumulation___ _________________________________________

Considerations and evidence in support of the hypothesis 

of origin by residual accumulation _________________

Evidence from the breccia fragments_____________

Sedimentary origin of the breccia fragments. __ 

Primary katatectic cap rock _________________

Three hypotheses of brecciation._____________

An intraformational sedimentary breccia __

A brecciated overlying bed or a residual

breccia _____________________________

The parallel bands (katatectic bands)__-__-___-__ 

Examples of katatectic banding..____________

Hypotheses of origin of katatectic banding.___

Page 

83 

83 

83 

83

86

88

88

88

89

89

89

89

89

Considerations and evidence in support of the hypothesis 

of origin by residual accumulation—Continued.

The parallel bands (katatectic bands)—Con. Page 

Considerations and evidence in support of the 

origin of katatectic banding and of the anhydrite cap rock in general by residual accumulation. . ____________________________ 90

Probability of periodicity in residual accumulation. _________________________ 90

Differences from shear surfaces._________ 91

The salt table. ________________________ 92

The contact of salt and anhydrite______._ 92

Probable relation of an overlying bed of

anhydrite to salt after flowage of the salt _ 92 

Variations in thickness of cap rock_______ 93

Presence or absence of cap rock _-__-__-_----.. ._-.--_., 93

Acknowledgments.... _________ _____________________ 94

ILLUSTRATIONS

Page

PLATE 24. Figure 1. General view of a band of impure 

salt, carrying disseminated anhydrite crystals, from one of the Five Islands salt 

domes, Louisiana. Figure 2. Polished face 

of same specimen enlarged_____________ 96

25. Figure 3. Anhydrite crystals residual from 

the solution of salt from shaft on Hockley 

salt dome, Texas. Figure 4. Anhydrite 

crystals residual from the solution of the 

band of impure salt illustrated in figures 

1 and 2___________________________ ___ 97

26. Figures 5 and 6. General views of a specimen 

of salt with lenses of disseminated anhydrite 

from Brenham salt dome, Texas. Figure 7. 

Detail of polished face of same specimen __ 98

27. Figure 8. Contact of banded anhydrite cap 

rock and salt from Hockley salt dome, 

Harris County, Tex____________________ 99

28. Figure 9. General view of thin section 5 from 

preceding specimen, showing the contact 

of the lowest anhydrite band with the one 

above it. Figure 10. General view of thin 

section 2 of same specimen, showing the 

contact of two higher bands.____________ 100

29. Figure 11. Detail of preceding thin section 2 

showing the irregular distribution of impurities along the contact of two bands. __ 101

30. Figures 12 and 13. Detail of thin section 5 

showing slight strain and crushing along 

contact of bands_.._____________________ 102

31. Figures 14 and 15. Detail of figure 10, showing 

straining, but without appreciable crushing, 

within a katatectic band________________ 103

PLATE 32. Figures 16 and 17. Detail of thin section 2 

showing a crushed patch not along the 

contact of two bands.__________________

33. Figures 18 and 19. Detail of figure 11 showing carbonate and crushing along the 

contact of two bands.__________________

34. Figure 20. Solid primary sedimentary anhydrite rock with no recognizable stratification seams. Figure 21. A large fragment 

of primary sedimentary anhydrite with unusually clear and pronounced but widely 

spaced stratification seams showing distortion. Figure 22. A core consisting apparently almost entirely of primary sedimentary anhydrite rock but very much 

brecciated. Figure 23. Largest single 

piece of well-stratified primary sedimentary 

anhydrite rock encountered in diamonddrill core ______________------_-----_--

Page

104

105

106

35. Figure 24. An unusually long core showing 

primary katatectic anhydrite cap rock 

with unusually long intervals showing 

little or no katatectic banding adjacent 

to well-defined katatectic bands. Figure 

25. Specimen showing considerable thicknesses of primary katatectic anhydrite 

cap rock, with little or no recognizable 

banding, in association with pronounced 

katatectic banding____________________ 107

36. Figures 26 and 27. Specimen from contact of 

salt and anhydrite cap rock on Bay St. 

Elaine salt dome, Terrebonne Parish, La__ 108
ILLUSTRATIONS III

37. Figure 28. Specimen, from diamond-drill 

core, that shows the best example of a 

breccia fragment of stratified sedimentary 

anhydrite cap rock. Figure 29. Another 

example of a breccia fragment showing 

distinct stratification but with the stratification surfaces more widely spaced than 

in figure 28. Figure 30. Photomicrograph 

of a thin section of specimen illustrated 

in figure 28, showing the appearance of the 

impurities along the stratification surfaces 

and something of the relation of the breccia 

fragment to its matrix_________________ 109

38. Figure 31. Detail of figure 32, at contact of 

matrix and clast. Figure 32. Photomicrograph of thin section 4, figure 29, illustrating the relation of breccia fragment 

and matrix_____-__-_-___-__________-_- 110

39. Figure 33. Photomicrograph of thin section 1, 

Figure 27, showing a particularly clearly 

stratified breccia fragment of primary sedimentary anhydrite cap rock and its matrix. 

Figure 34. Detail of preceding at contact 

of clast and matrix_____________________ 111

40. Figures 35 and 36. Examples of ordinary 

katatectic anhydrite cap rock showing less 

sharply defined breccia fragments. Figure 

37. Specimen from diamond-drill core most

Page

typically illustrating katatectic banding, the 

banding close and sharp and without recognizable breccia fragments in the bands. 

Figures 38 and 39. Two specimens from 

diamond-drill core showing pronounced 

katatectic banding but the banding unusually converging and irregular_____--_-___ 112

41. Figure 40. A rather unusual example of 

schistosity in the cap rock of the Sulphur 

salt dome. Figure 41. An example of curving schistosity parallel to a curving surface 

of parting unusual in the Sulphur cap rock. 

Figure 42. Diagram showing gypsum-anhydrite cap rock overlying complex folds, 

including an anhydrite bed, in a German 

salt stock. Figure 43. Diagram showing 

a more complex relation between gypsumanhydrite cap rock and the anhydrite of 

the salt stock in a German salt dome. 

Figure 44. Diagram showing a bed of 

anhydrite projecting above the salt table 

into the overlying gypsum-anhydrite cap 

rock on a German salt dome____________ 113

42. Figures 45 and 46. Core from depth of about 

950 feet on Hockley salt dome, Harris 

County, Tex., showing a breccia of calcareous rock fragments in sandy clay_-_---- 114

ORIGIN OF THE ANHYDRITE CAP ROCK OF AMERICAN SALT DOMES

By MARCUS I. GOLDMAN

ABSTRACT

The thesis of this paper is that the anhydrite cap rock of salt 

domes originated by the residual accumulation and consolidation, on top of a salt stock, of sedimentary anhydrite freed from 

the salt by solution of the top of the stock. This hypothesis is 

compared with that of origin from a bed of sedimentary anhydrite supposed to have overlain the salt of the salt stock in 

depth and to have been forced up on top of the stock as it rose. 

The strongest basis for the hypothesis of residual accumulation 

is the presence, between the anhydrite cap rock and the top of 

the salt stock on many salt domes, of a flat solution surface, the 

"salt table," decapitating anhydrite-bearing folds in the salt. 

The paper considers other general geologic evidence and internal petrographic evidence with a view to ascertaining to 

what extent they confirm or at least fit this interpretation. 

The cap rock contains breccia fragments of bedded sedimentary 

anhydrite. These cannot be of intraformational sedimentary 

origin, because similar bedded anhydrite is nowhere found in 

normal interbedded relations in the cap rock. Therefore, if the 

anhydrite cap rock is derived from a sedimentary bed which 

overlay the salt, that bed has been brecciated and recemented 

since the intrusion of the salt stock. Anhydrite cap rock is also 

characterized by roughly flat and parallel bands lying horizontal or at low dips, here named katatectic bands. If the 

breccia fragments, which lie within these bands, were produced 

by brecciation after sedimentation, the bands themselves can 

not be of primary sedimentary origin. The hypotheses of their 

origin by shearing or by periodicity in the process of residual 

accumulation are compared.

Considerations favoring the origin of the katatectic banding 

by residual accumulation are (a) the probability of periodicity 

in residual accumulation; (b) the difference of the katatectic 

surfaces from shear surfaces in their lack of convergence, their 

wide spacing, and their occurrence directly above the flat salt 

table; and (c) the absence of appreciable crushing along them 

in this and most other positions.

General considerations favoring the residual origin of the 

anhydrite cap rock are the presence of a layer of brine and 

anhydrite sand at the contact of salt and anhydrite on several 

salt stocks; the common occurrence of the relatively flat salt 

table on top of salt stocks; the consideration that if during the 

formation of a stock the pressure of a bed of salt shattered an 

overlying bed of anhydrite, it would probably engulf the fragments of that bed, and so a cap rock derived from such an overlying bed of anhydrite would, in the end, also be formed by 

residual accumulation; and the variations in the thickness of 

the cap rock, which are more readily accounted for by residual 

accumulation of anhydrite interbedded in the salt stock than by 

derivation from an overlying sedimentary bed. The absence of 

cap rock on certain salt stocks is not accounted for, but the 

suggestion is made that it may be due to gypsification at lesser 

depths, accompanied by too active circulation of water, with 

consequent solution of calcium sulphate as well as of salt, 

favored, on some stocks, by the absence of beds of sedimentary 

anhydrite rock in the salt stock; or that a cap rock may not yet 

have had time to form.

INTRODUCTION

TERMINOLOGY

To avoid confusion in the nomenclature of salt 

domes it is suggested that the word "dome" be restricted to the surface uplift marking the presence of a 

salt intrusion, and that "stock" be used for the 

intruded body of salt, which may not have affected the 

surface at all. The word "core" has been used for this 

intruded body, but as the salt is often cored in drilling 

on salt domes the use of that word opens the way for 

confusion. This use of the word "stock" conforms 

to its general use in geology.

As gypsification of the anhydrite cap is common it 

will be necessary, where its existence as anhydrite is 

not specifically in question, to refer to it as the gypsumanhydrite cap.

TWO HYPOTHESES OF ORIGIN

The two hypotheses of the origin of anhydrite cap 

rock to be considered in this paper are (1) that the cap 

rock is derived from an original bed of sedimentary 

anhydrite which overlay the salt of the salt stock in 

depth and was pushed up on top of the stock during its 

intrusion into the overlying beds, and that during its 

upward progress the bed of anhydrite was brecciated, 

and the breccia, recemented, makes up the cap rock; 1 

(2) that it is formed by the cementation and consolidation, on top of the salt stock, of grains of anhydrite 

and fragments of anhydrite rock freed from the salt 

stock by solution of its upper end.2

Hereinafter the terms "sedimentary hypothesis," 

"sedimentary origin," etc., will be used in reference to 

a cap rock formed according to the first of these hypotheses; "residual hypothesis," "residual origin," etc., 

in reference to one formed according to the second 

hypothesis. The residual hypothesis is the one 

advocated in this paper.

The mere presence of breccia fragments of beds of 

sedimentary anhydrite, which led to the acceptance of 

the sedimentary hypothesis in my earlier paper (1925), 

seems, in the absence of more detailed analysis of the

1 Of. Goldman, M. I., Petrography of salt-domp cap rock: Am. Assoc. Petroleum 

Geologists Bull., vol. 9, No. 1, pp. 42-78, 1925; also in DeGolyer, E. L., and others, 

Geology of salt-dome oil fields, pp. 50-86,1926. Brown, L. S., Cap-rock petrography: 

Am. Assoc. Petroleum Geologists Bull., vol. 15, No. 5, pp. 509-529, 1931.

2 Goldman, M. I., Features of gypsum-anhydiite salt-dome cap rock [abstract]: 

Geol. Soc. America Bull., vol. 40, No. 1, pp. 99-100,1929; discussion of Brown, L. S., 

Cap-rock petrography: Am. Assoc. Petroleum Geologists Bull., vol. 15, No. 5, pp. 

525-527, 1931; Bearing of cap rock on subsidence on Clay Creek salt dome: Idem, 

vol. 15, No. 9, pp. 1105-1113, 1931.

83
84 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1932-33

problem, as favorable to one as to the other of these 

hypotheses. The strongest petrographic evidence in 

favor of the residual hypothesis is supplied by the 

roughly parallel bands, generally horizontal or dipping 

at low angles, which occur throughout the cap rock 

and which were discussed but not explained in the 

paper of 1925.

As a substitute for the long phrase, "roughly parallel 

bands, generally horizontal or dipping at low angles" 

the term "katatectic bands" (from Kara, down, and 

TeKTeiv, to build) is used in this paper. The surfaces 

bounding these bands are called "katatectic surfaces."

It will not do to call the structure merely "banding" 

because the important consideration is to differentiate 

it from the banding of the included breccia fragments 

and to a certain extent from flow or schistosity banding. It is, to be sure, a type of stratification, but as 

such it must be differentiated from two other types of 

stratification that come into question in the interpretations of the origin of cap rock. One of these is the 

banding, which I regard as stratification, in the breccia 

fragments. The other is normal sedimentary stratification of the cap rock as a whole, which these katatectic 

bands are by some supposed to represent.

The outstanding and most weighty general geologic 

fact pointing to the residual interpretation is the 

presence, on top of the salt stock and under the 

gypsum-anhydrite cap rock of many or most American 

salt domes, of a flat solution surface or salt table such 

as is found also on most German domes. Much evidence for this is contained in the volume on salt-dome 

oil fields. 3

It is obvious in cross sections of German salt domes, 

such as those reproduced in figures 42 and 43 (pi. 41), 

that the salt table decapitates folded salt including 

beds of anhydrite. No plausible process other than 

solution of the salt suggests itself, and, as far as I 

know, no other has been suggested for this decapitation. 

The fact that the salt stocks of American salt domes 

have been extruded from depth makes it highly 

probable that they also have been complexly folded. 

Where penetrated by mine workings, as in the Five 

Islands,4 such folding is found. Beds of salt dipping 

70° were also encountered in the salt stock of the 

Grand Saline salt dome, Van Zandt County, Tex.5

Theoretical considerations and observation therefore indicate that on American salt stocks, as on the 

German, complexly folded beds are decapitated by the 

salt table.

In all the mine workings just mentioned and in those 

on the Hockley salt stock, Harris County, Tex.,5 the 

bedding, represented by the so-called "annual layers" 

in the salt, is defined by dark layers carrying dissem3 DeGolyer, E. L., and others, op. cit.

4 Vaughan, F. E., The Five Islands, in DeGolyer, E. L., and others, op. cit., 

pp. 356-397, especially fig. 5, p. 369, 1926.

8 Letter to the writer from L. P. Teas, of the Humble Oil & Refining Co., May 

16, 1931.

inated anhydrite crystals. (See figs. 1 to 4, pis. 24, 25.)

From what follows it will appear that, if the residual 

theory is accepted, the occurrence of breccia fragments of solid beds of sedimentary anhydrite rock in 

the cap rock proves that anhydrite in this form also is 

present in the salt stocks of American salt domes. 

But adequately verified records of the finding of such 

beds in these salt stocks seem to be lacking. Howe 

and Moresi 6 have gathered records of supposed occurrences of anhydrite rock in Texas and Louisiana salt 

stocks, but none of these can be regarded as well 

authenticated or conclusive.

With the help of Dr. Donald C. Barton, of Houston, 

and through the generosity of Mr. Fitzsimmons I 

obtained the specimen of interbanded salt and anhydrite-bearing salt from a depth of 2,172 feet in the 

Fitzsimmons No. 1 well, on the Brenham dome, 

Washington and Austin Counties, Tex., to which 

Howe and Moresi refer. It is illustrated in figures 5, 

6, and 7 (pi. 26). This specimen, as sketched from 

memory, was illustrated by Doctor Barton.7 Although 

he recognized in his description the gradation between 

the bands, his sketch makes them appear straighter 

and more regular and, therefore, more like true beds 

than they are. The purer salt was evidently more 

extensively dissolved in coring than the more anhydritic salt, and the lenticular relations of the two 

types of material are thereby brought out, as shown 

in figures 5 and 6. Figure 7 shows the termination, in 

the anhydritic salt of the polished face, of a lens of 

pure salt. This figure also shows that none of the 

anhydrite is aggregated into solid rock but that all of 

it is disseminated through the salt and that it is of the 

type of that which occurs in the annual layers and 

which forms the matrix of the anhydrite cap rock 

(cf. figs. 2 to 4, pis. 24 and 25, and figs. 9 to 19, pis. 

28 to 33), not the fine-grained, equant type of that 

which makes up the clasts (cf. figs. 30 to 34, pis. 37 to 

39).

It seems probable that the lenticular relations of the 

two types of material are due to flowage in the salt 

stock, involving bands of disseminated anhydrite in the 

salt.

As the specimen in question occurs in an interval, 

extending from 2,100 to 2,200 feet, reported as "interbedded salt and anhydrite", 8 it may be assumed that 

the entire interval contained the same sort of material, 

not true anhydrite rock.

The accumulation of such a thickness of this material 

may be due to original deposition of an unusually large 

proportion of layers of disseminated anhydrite; or it 

may be that the anhydritic salt reacted as a less plastic 

rock and accumulated, by squeezing out of the inter6 Howe, H. V., and Moresi, C. K., Geology of Iberia Parish: Louisiana Dept. 

Conservation Geol. Bull. 1, p. 118, 1931.

i Barton, D. C., Salt domes of south Texas, in DeGolyer, E. L., and others, op. cit., 

fig. 8, p. 755, 1926; Am. Assoc. Petroleum Geologists Bull., vol, 9, No, 3, p. 573, 1925.

s Barton, D. C., op. cit., 1926, p. 754; 1925, p. 572.
ORIGIN OP ANHYDRITE CAP ROCK OF AMERICAN SALT DOMES 85

vening more plastic salt 9 ; or it may have accumulated 

within the stock by solution of the salt.

Howe and Moresi base their record of the occurrence of anhydrite rock in the salt of the Hockley salt 

stock on the statement by Chapman 10 that "specimens of the rock salt encountered show some anhydrite 

in the upper part of the deposit." This is, I believe, 

more properly interpreted as indicating anhydrite 

grains rather densely disseminated in salt than solid 

beds of anhydrite. The shaft recently sunk into the 

Hockley salt stock has encountered salt with unusually 

large amounts of disseminated anhydrite crystals.

It is from a specimen of such impure salt kindly 

supplied by Dr. Marcus A. Hanna, of Houston, Tex., 

that the crystals illustrated in figure 3 were obtained. 

As I have seen it, this material has even less the 

character of anhydrite rock than the specimen of 

figures 5, 6, and 7 from the Fitzsimmons well. In 

reply to a specific inquiry regarding the occurrence of 

anhydrite rock in the Hockley salt, Dr. Hanna writes 

on April 16, 1932:

I went to Hoekley day before yesterday in order to learn as 

much as possible about the recent development in the shaft. 

The shaft is now at a depth of slightly below 1,450 feet, or 

approximately 450 feet below the base of the massive anhydrite [cap]. Within this 450 feet of salt nothing in the way of 

anhydrite other than more or less homogeneously distributed 

anhydrite crystals has been found. In other words, to the 

present depth of the shaft no bedded anhydrite is present. 

Further than that, some core drilling has been done from the 

bottom of the shaft, and in this core drilling no solid anhydrite 

has been found. One core drill hole was drilled vertically from 

the bottom of the shaft to somewhat below 2,000 feet. This 

gives a known section of salt from the base of the anhydrite 

[cap] at 1,010 feet of slightly over 1,000 feet. This material 

has been very carefully examined, and in none of it has any 

solid anhydrite appeared. From this and the other core holes 

we may conclude that with the rather extensive exploration 

so far carried out at the Hockley dome, nothing but disseminated 

anhydrite is present.

In the log of the Knapp No. 1 well on the Belle Isle 

salt dome, 11 to which Howe and Moresi refer, anhydrite 

is reported at several levels. No samples of this material appear any longer to be available, so that the 

determination cannot be verified. Certain inconsistencies between the verbal log as given by Lucas on 

page 1037 and a graphic log as given in his figure 2 

on page 1045 might raise doubts as to the accuracy of 

the record. The association of anhydrite and sulphur 

at a depth of 2,606 to 2,628 feet also raises doubts, in 

case a considerable proportion of sulphur is implied. 

So far as I have observed, abundant sulphur in cap 

rock is invariably associated with calcium carbonate. 

Aside from possible uncertainties in the record, also, 

both the paper by Lucas and the discussion of Belle

» Cf. Seidl, Erich, Die permische Salzlagerstatte im Graf Moltke Schacht: 

Archiv fur Lagerstattenforschung, Heft 10, Berlin, K. preuss. geol. Landesanstalt, 

1914.

"Chapman, L. C., The Hockley salt dome: Am. Assoc. Petroleum Geologists 

Bull., vol. 7, No. 3, p. 298, 1923.

11 See Lucas, A. P., A review of explorations at Belle Isle, la.: Am. Inst. Min. 

Eng. Trans., vol. 17, pp. 1034-1049, 1917.

Isle by Vaughan 12 indicate complex conditions which 

suggest engulfing of surrounding sediments by the 

salt, so that evidence as to the original condition of the 

salt is by that much more uncertain. However, it 

may be concluded that, although the observations 

cannot be checked, there is considerable evidence for 

the occurrence of anhydrite rock in the salt of Belle Isle.

I have been unable to check the reported occurrence 

of gypsum at a depth of 2,260 feet—1,360 feet below 

the top of the salt—in Drake's salt dome. 13 It is 

improbable, however, that gypsum was encountered 

2,260 feet below the surface, as the prevailing evidence 

indicates that anhydrite is the stable form of calcium 

sulphate at such depths.

There is much confusion in the differentiation of 

calcite rock, anhydrite, and gypsum by drillers on the 

salt domes of Texas and Louisiana. The records of the 

presence of beds of anhydrite rock in American salt 

stocks must, therefore, be regarded as inconclusive. 

That such beds have not been encountered or more 

frequently encountered there may be due to the fact 

that the beds, though present, are thinner than those 

in the salt of German salt stocks (cf. fig. 44, pi. 41) and 

so have been more fractured and dispersed by flowage 

of the salt; or that the salt has flowed more intensely 

than in the German stocks, with the same result. The 

resulting fragments should be encountered in the salt 

but might be overlooked more readily than a continuous 

bed of anhydrite rock.

As pointed out above, however, the presence of beds 

or fragments of beds of anhydrite rock in American 

salt stocks is established by the indirect evidence of the 

cap rock itself, if the theory of the residual origin of 

the cap is accepted.

According to that theory the solution which produces the salt table would be likely to leave any 

anhydrite contained in the salt as a residue. The 

gypsum-anhydrite cap rock lying on the salt table unconformably across the beds of the underlying stock, and 

containing fragments of what I interpret as primary 

sedimentary anhydrite rock, is taken to be this residually accumulated anhydrite. In fact, it is highly 

improbable that a gypsum-anhydrite cap would be 

present in this position and not be derived, at least in 

part, from anhydrite freed by solution of the top of the 

salt stock. Figures 42, 43, and 44 (pi. 41) furnish 

examples of the relation of the gypsum-anhydrite cap 

rock to the salt stock and its contained anhydrite beds 

on German domes.

Figure 42 is significant as showing the close and 

complex folding and the large amount of decapitation. 

The gypsum-anhydrite cap rock must, however, have 

been derived from beds higher in the series than the 

anhydrite bed shown in the cross section, as very 

little of that has been decapitated. But in the sec-

!2 Vaughan, F. E., The Five Islands, La., in De Golyer, E. L., and others, op. cit., 

pp. 383-392; Am. Assoc. Petroleum Geologists Bull., vol. 9, No. 4, pp. 783-792, 1925.

w Spooner, W. C., Interior salt domes of Louisiana, in De Golyer, E. L., and others, 

op. cit., p. 327; Am. Assoc. Petroleum Geologists Bull., vol. 10, No. 3, p. 275, 1926.
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1932-3

tion shown in figure 43 the distribution of the gypsumanhydrite cap can, as pointed out in the description 

of that figure, to a certain extent be directly correlated with its distribution in the underlying stock. 

In figure 44 the intimate association of gypsumanhydrite cap rock and an anhydrite bed contained 

in the salt is strikingly brought out, apparently on 

the basis of the concrete evidence afforded by a 

shaft, by the bed of sedimentary anhydrite projecting 

from the salt into the cap rock. This relationship 

seems to prove pretty definitely that the gypsumanhydrite cap rock is not an unfragmented bed of 

sedimentary anhydrite which overlies the salt in 

depth. It also seems to make almost inescapable, 

when considered in connection with figures 42 and 43, 

the conclusion that it consists largely, if not entirely, 

of anhydrite residual from the solution of the salt 

stock. For if gypsum or anhydrite has survived or 

accumulated here to form a cap, that residual from 

solution of the salt stock must be a large part of such 

a cap and may well be all of it.

Specimens from the contact of salt and cap rock 

received since the earlier paper was written have also 

clarified and tended to confirm the interpretation, 

arrived at from these general geologic relations and 

from the katatectic bands in the cap rock, of origin 

by residual accumulation. According to that interpretation this cap rock at the contact with the salt 

would be the most recently formed and would therefore be more likely than higher parts to give clear 

and trustworthy evidence about its origin. The 

following pages are given to the consideration of the 

extent to which other general geologic facts and 

internal petrographic evidence confirm or fit the 

residual theory of the origin of the cap rock. Before 

the evidence is discussed a fuller presentation of my 

conception of the process of residual accumulation is 

desirable.

THE PROCESS OF ORIGIN OF CAP ROCK BY RESIDUAL 

ACCUMULATION

The formation of the cap rock begins with the 

solution of the unprotected top of the salt core by 

water from some bed that it penetrated or at the 

surface of the earth. Once a protective cap has 

been formed by such a process, it is assumed that in 

some way u water from the same or a different source 

finds its way along the contact between salt and 

anhydrite cap and that the salt continues to be dissolved and removed in solution. In the process that 

forms the katatectic bands and surfaces, various 

factors tend to preserve for a time an open space 

between the cap rock and the top of the salt stock as 

this solution goes on. The closing of the space may 

be due to descent of the cap rock, to rise of the salt

14 See Goldman, M. I., Bearing of cap rock on subsidence on Clay Creek salt 

dome: Am. Assoc. Petroleum Geologists Bull., vol. 15, No. 9, pp. 1105-1113, 1931.

stock, or to both. If it is due to descent of the cap 

rock, friction and cohesion might delay this descent. 

It is believed, for instance, that most anhydrite caps 

extend not only across the top of the salt stock but 

also a certain distance down its flanks, though it is 

surprising to discover how few definite examples of 

this relationship have been recorded. In seeking 

evidence on this, as on many other features of salt 

domes, it is necessary to distinguish carefully between facts founded on definite observations on the 

particular salt dome in question, and extrapolations 

on the basis of assumptions acquired from other 

occurrences.

Thus, while the Geology of Salt-Dome Oil Fields 

contains several illustrations of cap rock extending 

down the flanks of the salt stock, only three were noted 

that showed definite evidence of such a relationship. 

These were figure 4 of Barton's paper on Pine Prairie 

(p. 428); figure 3 of Aplin's paper on Stratton Ridge 

(p. 654); and figure 4 of Barton's paper on salt domes 

of south Texas (p. 726). However, where flank cap 

rock is present there would be a certain amount of 

frictional resistance along its outside surface—that is 

to say, between flank cap rock and surrounding sediments—tending to hold up the cap. On the inside of 

the flanking cap rock, on the other hand, the space 

separating it from the salt stock would, if there were 

water on the top of the core, presumably also be 

occupied by water, so that the inside surface would 

offer little resistance to movement. Cohesion between 

the top of the cap rock and the overlying beds would, 

however, also tend to hold up the cap rock. Further 

support to the cap rock would be given by the thicker 

of the folded beds of solid anhydrite in the salt, which 

would be left, by solution of the salt, projecting up 

into the brine-filled space between salt and cap rock. 

At the bottom of this layer of brine, on the salt, lie 

the crystals of anhydrite that were disseminated 

through the salt, mainly along the so-called "annual 

layers." (See figs. 1 to 4.) When the weight of the cap 

rock finally became too great for the forces resisting 

its movement, it would settle down, brecciating and 

grinding the projecting beds of solid anhydrite and 

compacting the whole mass of residual anhydrite into 

a breccia consisting of a matrix of anhydrite sand— 

partly crystals originally disseminated on the "annual 

layers", partly debris from the crushing of bedded 

anhydrite—in which the breccia fragments would be 

scattered and the interstices of which would be filled 

with brine. Within this brine the usual process of 

recrystallization by solution and deposition of the 

inclosing solid material—the anhydrite—would tend 

to take place, cementing the grains into a coherent 

rock. This periodic settling followed by cementation 

is assumed to be the cause of the katatectic banding.

Upward movement of the salt to meet the cap rock 

would have the same effect, but it seems to me less
ORIGIN OF ANHYDRITE CAP ROCK OF AMERICAN SALT DOMES 87

likely that in the plastic salt, under the conditions 

attending the occurrence of salt stocks, this movement 

would take place discontinuously at short intervals.

If the process came to a stop at this point, a porous 

anhydrite rock with salt in the oores, like the bottom 

layer (n-n, figs. 8 and 9, pis. 27 and 28) of the specimen 

from the Hockley salt dome, Harris County, Tex., 

would result. If it continued, it must be assumed that 

enough time would elapse before a fresh layer of water 

would work its way in along the contact, to permit 

cementation of the preceding layer, and thus its differentiation from the next layer. Where the boundary is 

sharp and relatively flat there was presumably time 

for this cementation. Where the boundary is very 

irregular, as at the top of the Hockley specimen 

(c-c-c, fig. 8), and probably where the boundary is not 

sharp, the overlying layer was presumably not yet 

well consolidated when compaction of the layer below 

it took place. Some of the sharper boundaries are 

marked by impurities. In the Hockley specimen, for 

instance, the boundary b-b (figs. 8,10, and 11, pis. 27-29) 

has some fine-grained mineral, probably largely calcite, 

distributed along and adjacent to it. But the irregularity of the distribution of these grains, both longitudinally and laterally, indicates epigenetic distribution 

by waters penetrating along the katatectic boundary 

as a boundary of weakness, rather than syngenetic 

deposition. But whether of syngenetic or epigenetic 

origin in relation to the katatectic bands, this concentration of impurities on and adjacent to the katatectic surfaces indicates the presence of a definite bounding 

surface.

In the above presentation of the katatectic origin 

of the cap rock I have ascribed brecciation of anhydrite beds, freed from the salt, to crushing between 

cap rock and salt stock. It is quite possible that at 

least some of this brecciation may have taken place 

while the anhydrite bed was still in the salt stock, as 

the result of folding and flowage in the stock. 15 In 

either case, consideration of such disturbances and 

changes as I have observed in anhydrite cap rock lead 

me to believe that most of the more extreme distortion seen in breccia fragments of primary anhydrite 

rock, such as is illustrated in figures 21, 22, 23, 25, 

26, and 27 (pis. 34-36), is due either to crushing by 

the descending cap rock at the time the layer containtaining the distorted fragments was formed, or to 

crushing within the salt stock; and not to flowage 

within the cap rock after its formation. That pronounced distortion can take place within the salt stock 

is indicated by figure 12 on page 62 of the paper by 

Schauberger just cited, where a U bend has been 

produced in a fragment of an anhydrite bed in the salt.

15 Cf. Seidl, Erich, Die permische SalzlagerstatteimGrafMoltkeSchacht: Archiv 

fiir Lagerstattenforschung, Heft 10, pp. 52-54, Berlin, K. preuss. geol. Landesanstalt, 

1914; Die Salzstocke des deutschen (germanischen) und des Alpen-Permsalz Gebietes: Kali, 21. Jahrgang, especially pp. 305-306 and fig. 58, p. 304, 1927. Schauberger, O., Die Fliessstrukturen im Hallstatter Salzlager: Berg. u. Hattenm. Jahrb., 

vol. 79, especially pp. 57-68, Vienna, 1931.

165472-

From the random, scattered, and rather uniform 

distribution of groups of strained and distorted or 

crushed anhydrite crystals, like those in the Hockley 

cap rock, illustrated in figures 12, 13, 14, 15, 16, 17, 

18, and 19, it seems very likely that they originated 

as a result of settling of the cap rock while the residual 

anhydrite was still unconsolidated.

In the above presentation I have also spoken of the 

cap rock as if it were a rigid disk that settled onto the 

underlying salt only as a whole, without bending or 

breaking beforehand. The following simple calculations made with the help of P. G. Nutting, physicist of 

the United States Geological Survey, show that it 

would, however, bend and possibly break of its own 

weight before it settled as a whole.

The yield of a disk under uniform load is expressed 

roughly by the formula 16

T=% (1-\)B

in which

T= horizontal unit stress.

X = lateral contraction.

d = thickness of disk, in feet.

r = clear radius of disk, in feet.

R = pressure, in pounds to the square inch. 

T may be taken as 900 pounds to the square inch 

at rupture, by estimation from the value of that 

quantity for various other related rocks; X as K, as a 

rough approximation; d as 800, as for the Sulphur 

salt dome; r as 1,100, as for the Sulphur salt dome. 17 

Then

900 =

/1,100\ 

V 800 /

R = roughly 450.

With T equal to the unit stress at rupture, R becomes equal to the pressure at rupture.

Rough determinations on cores from the Sulphur 

salt dome give 1.31 pounds as the weight of a column 

of anhydrite 1 foot long and 1 square inch in cross 

section, and 1.118 pounds as the weight of a like 

column of gypsum. If 1.25 pounds is taken as the 

average weight of such a column anywhere in the cap 

rock the pressure of the 800-foot thickness of cap rock 

on its own base is 1,000 pounds to the square inch, 

which is greater than the value obtained for R, the 

pressure required to produce rupture, and the cap 

rock of the Sulphur salt dome would therefore be 

likely to break if unsupported on its entire base. 

Faults or evidences of faults have been encountered 

at several levels in the continuous diamond-drill core 

from this cap rock, and some of these may be due to 

breaking of the cap rock under its own weight. As 

many anhydrite caps have a greater diameter than that 

of the Sulphur salt dome they would be still more

!6 Merriam, Mansfield, Mechanics of materials, llth ed., p. 410, New York, 

John Wiley & Sons, 1910.

" See Kelley, P. K., The Sulphur salt dome, Louisiana, in DeGolyer, E. L., and 

others, op. cit., fig. 3, facing p. 456.
88 SHOKTEK CONTRIBUTIONS TO GENERAL GEOLOGY, 1932-33

likely to break. In any case, however, before breaking, 

the cap rock would yield by bending and would 

probably come to rest on the salt near the center of 

the stock and perhaps at other places. Whether the 

result of bending or of breaking, a few such contacts, 

by decreasing the length of the unsupported portions, 

would be likely to leave these able to support themselves. That such contacts are not common is indicated by the absence, as explained on page 91, of 

tangencies or unconformities of the katatectic bands 

in all the specimens of anhydrite cap rock I have 

studied.

The pockety occurrence of anhydrite sand on top 

of the salt on the Winnfield dome, mentioned on page 

92, may illustrate the results of local settling of the 

anhydrite cap rock by bending or breaking.

CONSIDERATIONS AND EVIDENCE IN SUPPORT OF 

THE HYPOTHESIS OF ORIGIN BY RESIDUAL ACCUMULATION

EVIDENCE FROM THE BRECCIA FRAGMENTS

SEDIMENTARY ORIGIN OF THE BRECCIA FRAGMENTS

Figure 28 (pi. 37) illustrates an excellent example of 

the clasts of dense white fine-grained anhydrite with 

thin parallel dark bands, which I regard as fragments 

of an original bed of sedimentary anhydrite reworked 

into the cap rock. The thin section in figure 30 

(pi. 37) adds nothing essential to the macroscopic 

evidence. It shows nondescript dark granular impurities in rather irregular lines. The exact location of 

this thin section, unfortunately, is not known, but the 

coarser anhydrite in figure 30 may undoubtedly be 

taken to be matrix or else recrystallized clast. If that 

is eliminated, there are no clear textural differences 

between bands. A slightly different type of banded 

breccia fragment is illustrated in figures 29 (pi. 37), 

31, and 32 (pi. 38). Still more distinct sedimentary 

stratification in a breccia fragment is illustrated in 

figures 26 and 27 (pi. 36). Here the banding, regarded 

as stratification, in fragments like a and e, is thin, 

sharp, and parallel and involves distinct color differences Here, also, the boundaries are angular and 

sharp and leave little room to doubt that these are 

distinct clasts, not merely portions of the rock differentiated from their matrix by recrystallization of that 

matrix. The thin section (figs. 33 and 34, pi. 39) 

again adds little to the macroscopic evidence, except 

to bring out the distinct difference in grain between 

clast and matrix.

In the earlier paper 18 I used the distribution and 

character of carbonates in a fine-grained breccia 

fragment, in a specimen of anhydrite rock of unknown 

origin, to support the interpretation of these clasts as 

of primary sedimentary origin. That evidence has, 

I still think, a good deal of validity, although from 

what I have since seen of the penetration of impurities

is Goldman, M. I., op. cit., 1925, pp. 56-64; 1926, pp. 67-72, figs. 20, 21.

along any boundary of weakness, such as a bedding 

surface would afford, and of the tendency of waters 

entering along surfaces of weakness to penetrate the 

adjacent rock unsymmetrically, in general probably 

upward rather than downward, I have become a 

little less sure of it. Moreover, I think that the 

identification of the carbonates in the clasts as dolomite rather than calcite should be accepted with 

reservation, inasmuch as it was based on their rhombic 

form, which is a fairly good indication but not conclusive proof. I would rather rest the case for the 

sedimentary origin of the fine-grained clasts on the 

macroscopic characters, which seem to me quite 

adequate in themselves.

PRIMARY KATATECTIC CAP ROCK

Once it is clearly recognized from specimens like 

those illustrated in figures 28 and 29 (pi. 37) and 26 

and 27 (pi. 36), in which the character and differentiation of the two main constituents are sharply 

defined, that anhydrite cap rock consists essentially of 

breccia fragments of sedimentary anhydrite, generally 

fine-grained, dense, and light-colored, many of them 

distorted, in a matrix of coarser-grained, darker 

anhydrite, the same two constituents can be recognized 

in other specimens in which their character and differentiation is less distinct. Such specimens are illustrated in figures 22 (pi. 34), 24 and 25 (pi. 35), and 35 

and 36 (pi. 40).

This, according to the interpretation I am offering, 

is the original cap rock as it is formed by residual 

accumulation—the "primary" cap rock. This cap 

rock as it is first formed undergoes various modifications—such as gypsification—which are undoubtedly 

secondary, and various departures from this type, 

also believed to be secondary, appear within the cap 

rock. Even though conclusive differentiation of 

primary and secondary characters of the cap rock 

may not in all cases be possible, it is, nevertheless, 

very desirable in discussion to differentiate this primary type from various deviations from it that are 

encountered in studying cap rock. But this "primary" cap rock contains the breccia fragments of 

sedimentary anhydrite, which in another sense are also 

"primary", and there is, therefore, the necessity of 

differentiating between two "primary" rocks. To 

bring out clearly and to emphasize this difference, and 

to differentiate between sedimentary and residual cap 

rock, I propose to call the breccia fragments "primary sedimentary breccia fragments or clasts" and 

the type of cap rock just defined "primary katatectic 

cap rock".

THREE HYPOTHESES OF BRECCIATION

Three possible interpretations of these breccia 

fragments have been considered in their bearing on 

the origin of the cap rock and will be discussed here.
ORIGIN OF ANHYDRITE CAP ROCK OF AMERICAN SALT DOMES 89

All of them are based on the acceptance of the hypothesis of the primary sedimentary origin of the clasts.

An intraformational sedimentary breccia.—One interpretation is that the breccia originated as an intraformational conglomerate or breccia, or what the 

British, in describing certain limestones, call penecontemporaneous breccia—that is to say, that the 

clasts were formed by brecciation of a bed of anhydrite 

before deposition of the next overlying bed. But if 

the breccia fragments were formed in that way there 

should be present in the cap rock, and rather abundantly one would expect, conformable included layers 

of this dense white anhydrite rock, in part at least 

with the closely spaced thin dark partings. The fact 

that I have found such layers nowhere in cap rock, 

and that, as far as I can judge, all the cap rock where 

unaltered is of the coarse-grained dark type, generally 

with breccia fragments—the type I have called primary katatectic cap rock—practically eliminates that 

interpretation of the origin of the breccia, and, with 

it, the interpretation of the cap rock, in its present 

condition, as an original bed of sedimentary anhydrite.

A brecciated overlying bed or a residual breccia.— 

The other two interpretations of the origin of the 

breccia that I have considered are that it resulted 

from brecciation, during upward movement of the 

salt stock, of a bed of sedimentary anhydrite overlying 

the salt in its original bedded position; or that it is 

the result either of brecciation by flowage—within the 

salt stock—of included beds of anhydrite, or of 

crushing—between the top of the salt stock and the 

overlying cap rock—of anhydrite residual from the 

solution of the salt stock.

I shall try to show, in discussing the katatectic 

surfaces, that they afford a means of differentiating 

between these two interpretations; but in the breccia 

itself there is little evidence on which such a differentiation can be based.

Only two characteristics of the breccia itself seem to 

fit the residual origin of the cap rock better than its 

sedimentary origin. One of these is the size of the 

breccia fragments. It seems likely that brecciation of 

an overlying bed of anhydrite as a whole by the 

upward push of a salt stock a mile or two in diameter 

through the midst of tightly enclosing sediments would 

yield at least a great many large fragments, some the 

full thickness of the bed and many feet in length. If 

that were the case, coring of the cap rock should yield 

long continuous sections of well-bedded, primary sedimentary anhydrite. Such material I have never seen. 

Some of the largest fragments of apparently primary 

anhydrite that I have noted are illustrated in figures 

20, 21, and 23 (pi. 34). Their dimensions are not within the range suggested above. The objection might be 

raised that the assumption of large fragments implies 

only a single brecciation, but that if there were one, 

there is no obvious reason for not expecting several. 

That brings me to the second consideration which

somewhat favors the residual origin. If there were 

several such periods of brecciation, one would expect 

to find not merely clasts of primary sedimentary anhydrite in dark matrix anhydrite but breccia fragments 

of breccia—that is, primary clasts and matrix, very 

probably showing katatectic banding—in a later matrix 

from which they could be differentiated. I have seen 

nothing in which I can recognize a trace of that 

structure. It might be suggested that recrystallization, of which there has undoubtedly been a great deal 

in the cap rock, would weaken the evidences of repeated 

brecciation; but in that case it could hardly leave such 

distinct proof of one brecciation and none of any others.

THE PARALLEL BANDS (KATATECTIC BANDS)

EXAMPLES OF KATATECTIC BANDING

Two of the best examples of katatectic banding are 

illustrated in figures 37 (pi. 40) and 8 (pi. 27), the latter 

from the contact of anhydrite cap and salt in the shaft 

at Hockley. Figures 24 and 25 (pi. 35) are good 

examples of more average occurrences and show 

more of the general relations. I have not had the 

opportunity to make a continuous study of the section 

of any cap rock. The diamond-drill core through the 

gypsum-anhydrite cap rock in well 194 of the Union 

Sulphur Co., wliich has formed the principal material 

for my study of gypsum-anhydrite cap rock, had to be 

discarded for lack of space in which to store it, after 

a range of specimens assumed to represent it adequately had been selected. It has therefore not been 

possible to follow up various problems that were developed by the study of the pieces of core selected and 

that required the study of intervening portions of the 

core. But the common occurrence of katatectic banding in specimens of anhydrite cap rock from various 

other salt domes, as well as from the Sulphur dome, 

leads at least to the conclusion that the cause, whatever 

it may be, is widespread and characteristic.

HYPOTHESES OP ORIGIN OF KATATECTIC BANDING

Three alternative hypotheses to account for the 

origin of the katatectic bands have been considered. 

One is that they are original strata of a bed of sedimentary anhydrite which overlay the salt. Another 

is that they originated as a result of periodicity in the 

process of accumulation of the cap rock as a residue 

from the solution of the top of the salt core—the 

process on which the term "katatectic" is based. 

The third hypothesis is that they are the result of 

shearing in the cap rock.

The hypothesis of their origin by some process of 

diffusion as suggested in my earlier paper 19 seems 

untenable in view of the fact that the dividing surfaces 

mark zones of mechanical weakness along which water 

probably circulated more freely and along wliich slight 

movements have taken place. Besides, to produce 

such straight and parallel bands, diffusion would, it

19 Goldman, M. I., op. cit., 1925, pp. 56, 59; 1926, pp. 64, 66.
90 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1932-33

seems probable, have to act in a homogeneous medium. 

The cap rock, with its inclusions of fine-grained primary anhydrite rock clasts in a coarser matrix and 

with other differences of texture and grain, is far from 

being such a homogeneous medium. Finally, if the 

bands were formed by diffusion, some of them should 

cut through the clasts. This I have not seen in any 

specimen examined.

The hypothesis that they are strata in a bed of sedimentary anhydrite requires the assumption that the 

breccia of dense white primary anhydrite fragments 

is intraformational. I have presented above (p. 89) 

the evidence against that hypothesis.

The differentiation between the other two methods 

of origin is of fundamental importance, for unless the 

origin of the katatectic banding by residual accumulation can be demonstrated there is no conclusive evidence in the cap rock itself of its residual origin. If the 

surfaces called katatectic are due to shearing, the 

breccia, as far as conclusive internal petrographic 

evidence goes, might have originated by the brecciation 

of a bed of anhydrite overlying the salt of the stock 

as well as by residual accumulation, and the only 

evidence in support of residual origin is that of the 

general geologic relations.

CONSIDERATIONS AND EVIDENCE IN SUPPORT OF THE 

ORIGIN OF KATATECTIC BANDING, AND OF THE ANHYDRITE CAP ROCK IN GENERAL, BY RESIDUAL ACCUMULATION

PROBABILITY OF PERIODICITY IN RESIDUAL ACCUMULATION

I am not prepared to go into a general philosophic 

discussion of periodic processes in nature. It is, 

however, probably safe to say that periodicity in 

natural processes is the rule rather than the exception.

Movements along faults are now generally believed 

to take place spasmodically, and the settling of the 

cap rock would be a somewhat similar process. On 

the other hand, sediments showing no recognizable 

results of periodicity in their formation are certainly 

known. There would thus be no basis for asserting 

that all geologic processes are recognizably periodic 

and that, therefore, periodicity in the katatectic 

formation of cap rocks, such as would result in the 

observed banding, is to be expected—that the katatectic surfaces are due to periodicity in the residual 

accumulation of the cap rock and are therefore evidence of its origin by that process. It is safe to say, 

however, that periodicity and banding in the residual 

process are rather likely and that the occurrence of the 

katatectic bands is therefore not inconsistent with this 

hypothesis of the origin of the cap rock.

There is a specific consideration that favors the 

conclusion that there must have been periodicity in 

the formation of cap rocks containing breccia fragments of primary sedimentary anhydrite. As explained above (p. 84), it is assumed that the salt 

table cuts across folded beds in the salt stock in

which steeply dipping beds of anhydrite are included, 

as illustrated in figures 42 to 44 (pi. 41). As the salt 

table is lowered by solution the upper ends of these 

beds are left projecting above it. At the same time 

anhydrite disseminated along the "annual layers" in 

the salt stock is freed by solution and gathers as a 

layer on the salt table. The overlying cap rock in 

settling is supposed to crush the projecting ends of the 

anhydrite beds, whose fragments mingle with the 

anhydrite sand. It is assumed that these are the 

breccia fragments found in the cap rock formed by 

consolidation of the sand. It is the size of these 

breccia fragments that is taken as evidence of a 

certain periodicity in the formation of the cap rock. 

If the cap rock descended continuously as the salt 

table was lowered, the projecting ends of the beds 

of anhydrite would be crushed as fast as they were 

freed by solution, and only a fine powder would result. 

To yield the fragments, some of them a few inches in 

diameter, that are encountered in the cap rock, it 

seems necessary that descent of the cap rock should 

halt at intervals, during which larger portions of the 

anhydrite beds would come to project above the 

descending salt table, so that when they were finally 

crushed they would yield these larger fragments.

Considerable thicknesses of anhydrite cap rock with 

little or no recognizable katatectic banding might be 

due to variations in the length of periods rather than 

to the absence of periodicity. Such variations could, 

for instance, be caused by differences in the thickness 

and strength of beds of anhydrite at different levels 

of the salt core. These differences might be syngenetic in the anhydrite bed or might be induced 

by the folding of the salt core.20 A thick solid bed 

of anhydrite would give the cap rock more support 

and thus cause the formation of a thicker katatectic 

band. The fact that considerable thicknesses of 

anhydrite cap rock with little or no recognizable 

katatectic banding contain unusually abundant clasts 

of primary anhydrite rock (as illustrated, for instance, 

in the upper two-thirds of figs. 24, pi. 35; in parts of 

fig. 25, pi. 35; and in fig. 35 and 36, pi. 40) may be 

regarded as supporting this interpretation.

On the other hand, katatectic surfaces undoubtedly 

favor the penetration of water and as a consequence 

the recrystallization of the cap rock above them, so 

that the greater scarcity of primary anhydrite clasts 

above the katatectic surfaces in more closely banded 

cap rock, as in the more closely banded parts of the 

specimens illustrated in figures 24, 25, and 36, may be a 

secondary character.

If the difference in the abundance of clasts of primary 

sedimentary anhydrite in banded and unbanded cap 

rock is mainly secondary and due to the disappearance

20 Cf. Seidl, Erich, Die permische Salzlagerstatte im Qraf Moltke Schacht: Archiv 

fiir Lagerstattenforschung, Heft 10, Preuss. geol. Landesanstalt, 1914, especially pp. 

52-54 and the colored plates at the back of the volume, showing variations in the 

thickness of an anhydrite bed as a result of folding.
ORIGIN OF ANHYDRITE CAP ROCK OF AMERICAN SALT DOMES 91

of the clasts by recrystallization, the wide spacing of 

katatectic surfaces in specimens like those illustrated 

in figures 24, 25, and 36 may be due to the fact that for 

lack of time or for some other reason cementation did 

not take place between the settling movements of the 

cap rock. This view is supported by the fact that 

vague traces of banding can be seen even in the upper 

part of the specimen shown in figure 24.

DIFFERENCES FROM SHEAR SURFACES

There is no doubt that there has been movement 

along many of the katatectic surfaces, but the amount 

of movement indicated by crushing along them is very 

slight and little in excess of the various other internal 

movements in the rock adjacent to them. As these 

surfaces are boundaries of weakness, the crushing 

along them cannot be taken as evidence of their origin 

by shearing.

On the other hand, it must be realized that in view 

of the undoubtedly large amount of recrystallization 

in the cap rock the present grain of the rock adjacent 

to the katatectic surfaces is not conclusive evidence 

as to changes that may have taken place along these 

surfaces in the past. But the fact that intense mylonitization has occurred throughout the cap rock and 

that I have found only the slightest evidence of mylonitization along the katatectic surfaces makes it 

improbable that more intense mylonitization ever took 

place along them. On that question also the evidence 

from the basal cap rock in the Hockley shaft (figs. 8 to 

19, pis. 27 to 33) is significant. Some of the characters 

of this specimen may, therefore, first be pointed out 

in somewhat more detail.

The katatectic surfaces seen in the polished face 

(fig. 8, pi. 27) are for the most part roughly flat, 

roughly horizontal, and roughly parallel, but they are 

not entirely so. They also exhibit a great range in 

sharpness of definition, some being sharper than 

others, and even different parts of the same boundary 

differing in sharpness. The lowest band is an intergrowth of salt and anhydrite. (See fig. 9, pi. 28.) 

Its upper boundary also varies in sharpness of definition. The uppermost katatectic surface included in 

the specimen (c-c-c, fig. 8) is both vague and unusually irregular.

Under the microscope the anhydrite crystals are 

seen to be on the whole fairly uniform in size, but 

scattered among the dominant size, as illustrated in 

figures 12 and 13 (pi. 30), 16 and 17 (pi. 32), and 18 

and 19 (pi. 33), are small patches of finer material. 

Strain is common in the larger crystals. The material 

has undoubtedly been subjected to some stress, and 

the fine portions are the result of crushing. On the 

katatectic surfaces, however, as illustrated in figures 

12 and 13 and figures 18 and 19, the amount of excess 

crushing is trivial.

In my earlier paper I presented evidence against 

the assumption that relatively flat, parallel surfaces,

spaced like these are the result of shearing. Further 

observation, especially of specimens from the continuous diamond-drill core from the gypsum-anhydrite cap 

rock of the Sulphur salt dome, has tended to confirm 

this conclusion. In the earlier paper considerations 

were presented for expecting shearing to produce lenticular structure—that is, converging surfaces. Such 

structure is illustrated by a specimen like that shown 

in figure 40 (pi. 41), in which, in spite of pronounced 

parallelism, there is a distinct lenticular structure. 

The specimen illustrated in figure 41 (pi. 41) shows, 

parallel to a vague curving, flowlike, lenticular structure, a curved surface of parting, presumably produced 

by a shearing stress. The katatectic surfaces, on the 

other hand, are dominantly flat and parallel. Such 

departures from flatness as they show are irregular 

and unsystematic. As pointed out above (p. 88), 

local sagging of the cap rock is likely to result in some 

converging katatectic surfaces, and possible irregularities in the salt table—the top surface of the salt stock 

would tend to have the same effect; but in very few 

of the several hundred specimens studied was even 

a well-defined tendency to convergence of katatectic 

surfaces noted. Three of these are illustrated in figures 37, 38, and 39 (pi. 40). In the specimen shown 

in figure 37 the thinness of the bands is the main 

cause of the appearance of greater convergence in their 

boundaries, but even there no two katatectic surfaces 

can be seen definitely to meet. The characters of 

specimens adjacent to the specimen shown in figure 

39 indicate that it is very near a fault or some similar 

disturbance which has undoubtedly affected the lower 

part of the specimen, including the katatectic surface 

in it. The specimen shown in figure 38 is also within 

a few feet of a disturbed zone.

The usually wide spacing of the katatectic surfaces, 

running around 1 to 2 centimeters, also does not seem 

to be a character that would result from shearing.

I have pointed out that in general the lack of evidence of pronounced crushing along the katatectic 

surfaces could not be accepted as disproof of their 

origin by shearing, because recrystallization might have 

effaced such evidence. This qualification does not, 

however, seem to me to have much force with reference to the specimen from the contact with the salt 

at Hockley (fig. 8, pi. 27). There might have been 

recrystallization along the upper katatectic surfaces in 

that specimen, but it is not likely that recrystallization could have taken place along the lowest of those 

surfaces (a-a, figs. 8, 9, 12, and 13) and yet have left 

the lowermost band (n-n, figs. 8 and 9; cf. fig. 12) the 

intergrowth of salt and anhydrite that it is at present. 

It would, furthermore, be difficult to conceive of 

any force connected with a salt dome that would be 

able to produce such horizontal surfaces in this position by shearing. Horizontal shear surfaces can be 

produced by lateral yielding of a horizontal mass to
92 SHOETEE CONTEIBUTIONS TO GENERAL GEOLOGY, 1932-33

vertical pressure,21 but they are not likely to be widely 

but regularly spaced and parallel, like those illustrated 

in figure 8 (pi. 27), nor so near the surface along which 

the pressure was applied.

Finally, the irregularity of some of the katatectic 

surfaces, such as the topmost one (c-c-c) in the Hockley specimen shown in figure 8, could not conceivably 

result from shearing.

THE SALT TABLE

The fundamental significance of the salt table in 

relation to the origin of anhydrite cap rock has been 

discussed in the introduction.

THE CONTACT OF SALT AND ANHYDRITE

I am indebted to Mr. L. P. Teas,22 of the Humble 

Oil & Refining Co., of Houston, Tex., for detailed 

information about the contact of anhydrite cap and 

salt in shafts on the Grand Saline salt dome, Van 

Zandt County, Tex., and the Winnfield salt dome, 

Winn Parish, La.

On the Grand Saline salt dome the contact was at 

a depth of about 130 feet, and 4 or 5 inches of anhydrite sand with included brine are reported. The 

brine was under considerable pressure.

On the Winnfield salt dome, likewise, 3 to 4 inches 

of anhydrite sand are reported as present at the contact, which was at a depth of 438 feet. Here, also, 

there was water under considerable pressure (220 

pounds to the square inch, corresponding to the weight 

of a column of water about 508 feet high, 70 feet more 

than the distance to the surface). At this contact the 

anhydrite is said apparently to have been in pockets, 

for it was found only over an area 4 feet square. The 

possibility of the localization of solution and residual 

accumulation in this way on the top of a salt dome as 

the result of control by fissures in the overlying rock 

has been considered in my paper on Clay Creek.23 

Irregular sagging of the cap rock as discussed above 

(pp. 87-88) or irregularities in the top surface of the salt 

might also cause such localization. The area to which 

residual anhydrite sand is said to be restricted on this 

stock, however, seems rather small. In view of the 

difficulty of making satisfactory observations in the 

bottom of a shaft in the presence of water under considerable pressure, it seems best, for the present, to 

accept this record with reservations.

It is worth noting that the flow of water under 

pressure at both these contacts fits well with the 

conclusion reached in my paper on Clay Creek,24 that 

the formation of the flat salt table is probably to be 

ascribed to some more permeable bed, a "water sand".

These occurrences of anhydrite sand and water at 

the contact of salt and anhydrite cap rock, if they are

21 Cf. Cloos, Hans, Bau und Bewegung der Gebirge, fig. 6, p. 260, Berlin, Gebriider 

Borntraeger, 1928.

22 Letter of June 18, 1931.

23 Goldman, M. I., op. cit. (1931), p. 1112.

24 Idem, p. 1112.

thought of in terms not of a static condition but of 

geologic processes in geologic time, seem to point 

pretty clearly to residual accumulation of anhydrite 

cap rock. Even if there is no circulation of water, 

solution by diffusion is almost certain. The residual 

anhydrite is there in the form of an anhydrite sand, 

as is to be expected, and, if allowance is made for compaction by settling of the cap rock, even the thickness 

of the space and of the sand layer conforms to the 

average thickness of katatectic bands.

This layer of brine between anhydrite cap rock and 

salt has not been encountered on all salt domes. I 

have discussed elsewhere 25 the problem of its presence 

or absence.

In view of the foregoing observations, the salt table 

establishes a strong probability in favor of residual 

accumulation of anhydrite cap rock. As stated above 

(p. 85), even if a bed of anhydrite overlying the salt 

in depth were brought up on top of the salt stock, the 

probability that anhydrite would be added to it by 

residual accumulation beneath it, on those salt stocks 

which have a salt table, would make improbable the 

presence on them of a cap rock derived purely from 

such an overlying bed.

PROBABLE RELATION OF AN OVERLYING BED OF ANHYDRITE TO SALT 

AFTER FLOWAGE OF THE SALT

If the steps in the upward progress of a bed of salt 

changing to a salt stock are visualized, it is difficult to 

conceive of the formation of an anhydrite cap containing breccia fragments, even from an overlying bed of 

anhydrite, without the intervention of solution. For, 

if a substance as plastic as salt were to brecciate overlying or surrounding beds, it is hard to believe that it 

would not penetrate and engulf the breccia. According to Seidl 26 something of that kind has taken place 

in the Alps and under the old miners' name "Hazelgebirge" (hazel-nut formation) is described and illustrated by him. Schauberger,27 however, does not share 

this interpretation; he considers the "Hazelgebirge " 

to be the product of flowage in a bed of salt which 

originally contained as thin interbedded layers the material of the breccia now found in it. In SeidPs figure 

58 the inclusion of fragments of an overlying anhydrite 

bed in salt that has broken through it is photographically illustrated. But the conditions in that occurrence 

are not quite like those considered in this paper, because there the anhydrite bed was also overlain by salt. 

It should be noted in passing that a corollary of this 

assumption seems to be that if there was not a bed of 

anhydrite directly overlying the salt, then whatever 

beds did overlie the salt, especially the more brittle 

among them, should, before the formation of cap rock 

by solution, have yielded a breccia to be included in

25 Idem, p. 1111.

26 Seidl, Erich, Die Salzstocke des deutschen (germanischen) und des AlpenPermsalz-Gebietes: Kali, Jahrgang 21, especially pp. 305-306, 1927.

27 Schauberger, O., Die Fliessstrukturen im Hallstatter Salzlager: Berg- u. hiittenm. Jahrb., vol. 79, pp. 27-38, 57-78, 82-89, 1931.
ORIGIN OF ANHYDRITE CAP ROCK OF AMERICAN SALT DOMES 93

the outer parts of the salt, and that when solution of 

the salt did begin, these breccia fragments should have 

accumulated as an outermost layer, cemented or uncemented according to circumstances, above the anhydrite cap on top and outside it on the flanks. Is there 

any evidence of such an occurrence known from American salt domes? In my earlier paper 28 I suggested 

that some of the sedimentary material of the calcite 

cap (more correctly, probably, the false cap 29 ) might 

be such material brought up from depth. The suggestion still seems valid, though I would no longer regard 

the occurrence of this sedimentary material as proof of 

the primary sedimentary origin of the anhydrite cap. 

Some breccias above salt, such as that from Hockley 

illustrated in figures 45 and 46 (pi. 42), might also 

represent this material.

If this argument is well founded, it confirms the 

conclusion arrived at above that the anhydrite cap 

rock can not be an original bed of sedimentary 

anhydrite. It seems, also, that in that case there 

would be no essential difference, in such characteristics as have hitherto been recognized in it, whether 

it had been derived from an overlying bed of anhydrite or from included anhydrite beds and disseminated crystals. It might be that there would be 

differences in such features as the size, shape, and 

distortion of the breccia fragments, but, so far as I 

know, the basis for discriminating such differences is 

not now available.

VARIATIONS IN THICKNESS OF CAP ROCK

There is evidence that the cap rock above the salt 

table varies in thickness and is most commonly 

thickest toward the center, but, as mentioned on 

page 86, the information about cap rock is so often 

extrapolated that it becomes difficult to find satisfactory samples. There is perhaps none published 

about American salt domes more satisfactory than 

that contained in figure 3 of Applin's paper on Stratton Ridge.30 In addition to the variation in thickness 

above the salt table there is the pronounced thinning on 

the flanks, which may be regarded as merely an extension of the thinning toward the edges above the salt 

table. Irregular variations in thickness in general 

may, it seems to me, be much more readily accounted 

for by the hypothesis of residual accumulation than 

by that of an original sedimentary bed, because anhydrite would be likely to be irregularly distributed in 

the folds of a salt stock (see figs. 42 and 43, pi. 41, 

and pp. 85-86), and the decapitation of such a stock 

by solution would therefore leave various thicknesses 

of residual anhydrite in different parts. Thickening 

toward the center fits in well with the residual theory, 

because, in the upward progress of the salt stock,

28 Goldman, M. L, op. cit., 1925, pp. 72-74; 1926, pp. 80-8?.

29 Brown, L. S., Cap-rock petrography: Am. Assoc. Petroieum Geologists Bull., 

vol. 15, No. 5, pp. 510 (false cap), 511 (calcite cap), 1931. Goldman, M. I., op. cit., 

1925, pp. 68-74; 1926, pp. 76-82.

30 Applin, P. L., in DeGolyer, B. L., and others, op. cit., p. 654,

friction of the surrounding sediments would presumably retard the outer parts, so that (a) flow would tend 

to be more rapid toward the center, (6) in the formation 

of a flat salt table the greatest amount of solution 

would therefore be present there, and hence, (c) other 

things being equal, the thickest accumulation of residual 

anhydrite would occur toward the center.

PRESENCE OR ABSENCE OF CAP ROCK

No hypothesis of the origin of gypsum-anhydrite cap 

rock is satisfactory which cannot be made to account 

for the absence of this cap rock on certain domes. 

Various possible explanations of this condition, on the 

basis of the hypothesis of sedimentary origin, were 

offered in my earlier paper. 31 It seems easier to 

explain under that hypothesis than under the hypothesis of residual origin. Information on this topic 

is not readily enough available to permit a satisfactory analysis. One of the suggestions offered in the 

earlier paper, however, seems still to have a good 

deal in its favor. That is, that where cap rock is 

absent, solution may have been so active that calcium 

sulphate as well as salt may have been dissolved.

As the following figures 32 show anhydrite is, after 

all, rather soluble and gypsum much more so.

At a temperature of 10° C., 0.19 gram of gypsum is 

dissolved per 100 cubic centimeters of water; at 40° C., 

0.21 gram; at 100° C., 0.16 gram. The solubility of 

gypsum thus reaches a maximum around 35° to 40° C. 

At 100° C., 0.0626 gram of anhydrite is dissolved per 

100 cubic centimeters of water. No other figures for 

anhydrite were found.

In salt solution the solubility of gypsum is much 

greater, and this is of course significant, as residual 

separation of anhydrite or gypsum on top of a salt 

stock would take place in water saturated or nearly 

saturated with salt. The solubility of calcium sulphate 

(presumably gypsum) in a brine containing 32.05 

grams of NaCl per 100 cubic centimeters at 26° C. (a 

saturated solution of NaCl at 25° C. contains about 36 

grams per 100 grams of water)33 is 0.572 gram of calcium 

sulphate per 100 cubic centimeters of brine. 34

In the paper on Clay Creek 35 I have referred to 

evidence indicating more active solution above a depth 

of 2,000 feet than below that depth. Gypsum is the 

stable form of calcium sulphate in the presence of 

water down to depths of a few hundred feet. Newland 36 says that anhydrite is probably the stable form 

of calcium sulphate below 200 to 300 feet, and Udden 37 

reports gypsum to a depth of 850 feet.

si Goldman, M. I., op. cit., 1925, pp. 67-68; 1926, pp. 75-76.

s2 Seidell, A., Solubilities of inorganic and organic compounds, 2d ed., p. 214, 

New York, Van Nostrand, 1919.

33 Idem, p. 640.

3* Idem, p. 218.

35 Goldman, M. I., op. cit. (1931), pp. 1105-1113.

35 Newland, D. H., Geology of gypsum and anhydrite: Econ. Geology, voL 16, p. 

394, 1921.

3? Udden, J. A., Laminated anhydrite in Texas: Geol. Soc. America Bull., vol. 35, 

p. 348, 1924.
94 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1932-33

In the earlier paper 3S I referred to the effect of salt 

in solution in lowering the temperature at which the 

transformation of anhydrite to gypsum is possible. 

Under geologic conditions the dehydration of gypsum 

and its conversion back to anhydrite is probably slow, 

so that the mere presence of gypsum at a certain depth 

is not proof that it is stable at that depth. A large 

amount of evidence, carefully controlled geologically 

and petrographically, will be needed to permit more 

exact determination of the depth at which gypsum is 

stable under given conditions. But, in any case, at 

the lesser depths at which there is reason for believing 

that more water is available for dissolving the salt of 

salt stocks, gypsum—the more soluble form of calcium 

sulphate—is also more likely to occur. We might, 

therefore, expect to find cap rock absent more frequently on shallow salt stocks than on deeper ones. 

The scarcity or absence of gypsum-anhydrite cap 

rock on the Five Islands salt stocks,39 except Belle 

Isle, where it is very incomplete, fits this suggestion. 

On Jefferson Island, Vaughan reports 18 to 24 inches 

of "limestone" [calcite] cap rock 40 in only a few of the 

holes drilled to the salt. On C6te Blanche he records 

"only small lentils of cap rock from a few inches up to 

4 feet in thickness" 41 and does not state whether they 

are calcium carbonate or calcium sulphate. At Belle 

He "590 feet of gypsum, anhydrite, and sulphur" 42 

are reported immediately above the salt in one hole, 

but other drill holes encountered little or none. 

Vaughan's figure 12 43 gives cross sections showing 

the very irregular distribution of the cap rock on this 

dome. On the Weeks and Avery Island salt stocks no 

cap rock is reported.

As I have seen them, the salt stocks of the Five 

Islands have another character which might help to 

explain the absence of cap rock on most of them— 

namely, that they do not appear to contain beds of 

sedimentary anhydrite rock. It may be that where

ss Goldman, M. I., op. cit., 1925, p. 77; 1926, p. 85.

s' Of. Vaughan, P. E., The Five Islands, Louisiana, in DeGolyer, E. L., and 

others, op. cit., pp. 356-397. 

«Jdem, p. 360. 

« Idem, p. 383. 

« Idem, p. 388. 

« Idem, p. 391.

such beds are absent the disseminated anhydrite of 

the "annual layers" is more likely to be dissolved as 

fast as it is freed from the salt stock by solution of the 

salt.

On very young salt stocks cap rock might also be 

lacking, because it had not yet had time to form.

ACKNOWLEDGMENTS

I will not try to name all the individuals to whom I 

am indebted for material, information, or discussion 

that has helped me in preparing this paper. Those 

who have contributed specimens used as illustrations 

are named in connection with those illustrations. I 

wish here to thank all who have supplied these specimens and those who have helped in other ways. 

Special thanks are due to Mr. L. P. Teas, of the 

Humble Oil & Refining Co., for specimens (one of 

which furnished important evidence for this paper) 

from the shaft on Hockley salt dome, and for information and discussion about the conditions encountered there and elsewhere; to Dr. Marcus A. Hanna, of 

the Gulf Production Co., who supplied valuable material and information for the study of the anhydrite 

occurring in the salt of salt stocks and very generously 

yielded the right to use material on which he was himself preparing to publish; to Mr. Albert G. Wolf, of the 

Texas Gulf Sulphur Co., whose keen interest in the 

scientific problems of salt domes has, I feel, been the 

basis of the loyal manner in which he has at all times 

kept my investigations in mind and sent me or directed 

me to especially significant material and information; 

to the late Dr. Sidney Powers, of the Amerada Petroleum Corporation, who gave to my investigations, as 

to so many others, the enthusiastic interest and 

cooperation which was characteristic of him, and who 

on many occasions supplied me with information and 

specimens; and finally to Dr. Donald C. Barton, of 

Houston, who, although he has not hitherto accepted 

the theory of residual origin of anhydrite cap rock on 

American salt domes, has been a stanch supporter 

of my investigations with material, information, and 

discussion.
PLATES 24-42

165472—33——3 95
PLATE 24

FIGURE 1. General view of salt from one of the Five Islands salt domes, Louisiana, containing a band of impure salt carrying disseminated anhydrite crystals. On account of the transparency and loose texture, especially of the matrix salt, and the consequent 

large amount of internal reflection, the photograph does not give a very correct idea of the appearance of the band in relation 

to the matrix. Though the boundaries of the band are not sharp, they are more straight, regular, and parallel to each other 

than they appear in the illustration. The specimen was selected for the sharpness and thickness of the impure band. The 

crystals of the matrix are very transparent and sharply defined and are so loosely attached to each other that the matrix is 

extremely friable. In the dark impure band the crystals are somewhat smaller and the band has much more cohesion—is, in 

fact, very tough. U.S.N.M. 97788 A. Natural size. Cf. figures 2 and 4. (See pp. 84, 86.)

FIGURE 2. Polished facs of the band of impure salt illustrated in figure 1, with specular illumination. The illustration gives some 

indication of the very irregular and random distribution of the anhydrite crystals, which are the small bright grains projecting 

above the polished surface. As this salt is of sedimentary origin and has been subjected to intense flowage, it probably was 

not originally in such coarse crystals as it now is. It may have recrystallized, however, without affecting the distribution of 

the anhydrite crystals, which was probably originally also random. The anhydrite grains have been retouched to bring them 

out more clearly. U.S.N.M. 97788 G. (See pp. 84, 86.)

96
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 24
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 25
PLATE 25

FIGURE 3. Anhydrite crystals residual from the solution, by water, of a sample of the impure salt about 300 feet below the top 

of the salt stock in the mine of the Houston Salt Co., Hockley salt dome, Harris County, Tex. The fragment a at the center 

is made up of many small crystalline grains of anhydrite; hence its white opaque appearance. Grains of this kind, some of 

them as much as 1 centimeter in diameter and many of them containing small clear grains of sulphur, are common in the water 

insoluble residue of this salt. Comparison with figure 4 will show that these anhydrite crystals from Hockley are somewhat 

more rounded than those from the Five Islands salt. This may be due to more attrition during the flowage of the salt or 

to solution during its recrystallization. These also appear less transparent than those from the Five Islands salt. This is 

due to small fracture surfaces in the crystals from Hockley, an additional indication that they have been subjected to more 

stress. The crystals from Hockley also vary less in size, and there are fewer fine crystals among them. The illustration fails 

to bring out the prevalence of tabular prismatic crystals in this sample; b is probably one of these on edge and c and d, some 

lying on their bases. It is from material like this that, it is assumed in this paper, the cap rock illustrated in figures 8 to 19 

is derived. The crystals were obtained from a specimen received from Dr. Marcus A. Hanna, of the Gulf Production Co., 

Houston, Tex. U.S.N.M. 97789. (See pp. 84-86.)

FIGURE 4. Anhydrite crystals residual from the solution, by water, of the band of impure salt from one of the Five Islands salt 

domes, Louisiana, illustrated in figures 1 and 2. It is material like this that, it is assumed in this paper, forms the matrix of 

anhydrite cap rock. U.S.N.M. 97788 B. (See pp. 84, 86.)

97
PLATE 26

FIGURES 5 and 6. General views of a specimen of salt with lenses of disseminated anhydrite from depth of 2,172 feet in Fitzsimmons 

no. 1 well on Brenham salt dome, Washington and Austin Counties, Tex. Lenses of salt full of disseminated anhydrite crystals 

grade rather sharply into lenses with little or no disseminated anhydrite. The purer salt is gray; the anhydritic salt more 

yellowish white. Figure 5 shows the natter face, which is probably the one Barton had in mind when making his sketch. 

(See p. 84.) The flat surface in what is shown as the upper part of the specimen has been ground since Barton saw it. 

Figure 6, taken at an angle from the left side of the specimen as seen in figure 5, brings out the much greater solvent action 

of the water on the purer salt than on the more anhydritic salt of the specimen. Specimen obtained from Mr. Fitzsimmons, 

who drilled the well. U.S.N.M. 97790. Natural size. (See pp. 84, 85.)

FIGURE 7. Detail of the polished face of the specimen illustrated in figures 5 and 6, with oblique illumination. The area shown is 

just to the right of the V-shaped reentrant that penetrates, from the left, the polished face in the upper part of the specimen, 

as illustrated in figure 5. The ragged and irregular left edge of the picture is the edge of the polished face here. As indicated 

by the presence of the V-shaped depression, a lens of purer salt tapers off, toward the apex of the V, into the more anhydritic 

salt of the area of the polished face. In figure 7 the end of this lens of salt appears as a larger area of salt in the center and 

upper left part of the picture and passes out into the more anhydritic salt surrounding it. On account of the oblique lighting, 

which was needed in order to obtain this picture, the surface of the salt in some of the bordering parts of the area represented, 

where the anhydrite crystals are more crowded, cannot be recognized. But the strong shadows cast by the anhydrite crystals 

there indicate that these crystals, like those in the more open and central parts of the area, project above the softer salt which 

incloses them. In general the orientation of the anhydrite crystals is random. But a magnified surface, larger than could be 

reproduced photographically, shows a moderate prevalence of elongation of the crystals in the direction of the elongation of 

the lenses of purer and of more anhydritic salt, indicating "flowage." By their size and shape and the integrity of their 

crystal outlines, these anhydrite crystals indicate that they are of the disseminated type which forms the matrix of cap rock, 

and not products of the crushing of primary sedimentary anhydrite rock. (See pp. 84, 85.) 

98
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 26
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 27
PLATE 27

FIGURE 8. Contact (x-x) of banded anhydrite cap rock and salt; from the shaft of the Houston Salt Co. on the Hockley salt dome, 

Harris County, Tex. The band n-n directly above the salt is an intergrowth of salt and anhydrite in varying proportions along 

different parts of the band, a-a, b-b, c-c mark katatectic surfaces, intervening bands being katatectic bands. Note the 

irregularity of the boundary c-c-c. This boundary has been slightly emphasized in the illustration. U.S.N.M. 97791. Natural 

size. Specimen received from Mr. L. P. Teas, of the Humble Oil & Refining Co., Houston, Tex. (See pp. 87, 89, 91, 92.)

165472—33——-4 99
PLATE 28

FIGURE 9. General view of thin section 5 of the specimen of figure 8, showing the contact of the lowest anhydrite band n-n with the 

one above it. Before grinding the thin section, the chip to be used was boiled in water to remove salt. The pores thus created 

were filled with bakelite, which appears dark in the illustration. The approximate position of the ends of the contact of the 

two bands is marked a-a, as in figure 8. The boundary does not appear much more distinct under the microscope than in the 

illustration. (See pp. 84, 87, 91.)

FIGURE 10. General view of thin section 2 of the specimen of figure 8, showing the contact (b-b) of the two bands. The contact 

appears somewhat more distinct under the microscope than in the illustration. (See pp. 84, 87, 91.) 

100
U. S. GEOLOGICAL SURVEY PEOFESSIONAL PAPER 175 PLATE 28

^^^-M^' m^A^^y^ 1 ^-^^^ • - • :---^-^

f"^i:^im^»:T-'.- .-•-£ ;•••.' •--"?" "-A •.
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 29
PLATE 29

FIGURE 11. Detail of thin section 2, figure 10, to show the irregular distribution of impurities along the contact of two bands. The 

stringers and patches of dark material distributed along an irregular line between the points b-b are the impurities. Quartz 

crystals are numerous. The rest of the impurities are too fine for certain identification but appear to be fine-grained carbonate, 

sulphides in more or less spherular forms, and some sulphur. The position of the area illustrated in this figure is shown in the 

more general view of this thin section in figure 10. (See pp. 84, 87, 91.)

101
PLATE 30

FIGURES 12 and 13. Detail of thin section 5, figure 9, to show crushing and straining along the surface a-a (12, plain light; 13, crossed 

nicols). The crushing and straining of the crystal o and of the crystals around it are clearly shown, and crushing and straining 

can also be plainly seen around p. The area of figure 12 is indicated in figure 9. (See pp. 84, 87, 91.) 

102
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 30

13
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPEE 175 PLATE 31
PLATE 31

FIGURES 14 and 15. Detail of thin section 2, figure 10, to show straining, but without appreciable crushing, within a katatectic band 

(14, plain light; 15, crossed nicols). The straining, brought out between crossed nicols, is mainly in the row* of crystals along o-o. 

The approximate boundaries of individual crystals in the strained group have been emphasized in the figures, in order to make 

the straining more readily recognizable. The position of the area of these figures is shown in figure 10. (See pp. 84, 87, 91.)

103
PLATE 32

FIGURES 16 and 17. Detail of thin section 2, figure 8, showing a crushed patch not along the contact of two bands (16, plain light; 

17, crossed nicols). Here the crushing, which is quite distinct between crossed nicols under the microscope, is difficult to 

recognize in the photographs. Strain can be seen in the large crystals around the edge of the area of the figure at o, o, and o. 

Under the microscope general crushing is most conspicuous in the area around p and in the figures is indicated by the poor 

definition of many of the crystals. (See pp. 84, 87, 91.) 

104
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 32

17
tT. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 175 PLATE 33

19
PLATE 33

FIGUEES 18 and 19. Detail of figure 11, showing impurities and crushing along the contact of two bands (18, plain light; 19, 

crossed nicols). The dark material along an irregular line between the points b and b consists of the impurities. The crushing 

can be most readily recognized between crossed nicols in the general area between the heavy band of impurities, b'-b', and the 

crystal n, especially just below the crystal n. Note especially the comblike arrangement of fragments between the points p and p. 

o is a crystal of a carbonate, probably dolomite. The area of this thin section is shown in figure 11, and the area of figure 

11 is shown in figure 10. (See pp. 84, 87, 91.)

105
PLATE 34

FIGURE 20. Polished face of specimen La. 1, 1014-1023, 107-110}!,l a solid primary sedimentary anlrvdrite rock showing no 

recognizable stratification seams. There are two slightly darker bands running across the specimen at a-a and b-b, but these 

are even more distinct in the illustration than in the specimen. These seem too faint and indefinite to be stratification bands. The 

darker spots along the bottom between c and c represent gypsified portions. U.S.N.M. 97792. Natural size. (See p. 89.)

FIGURE 21. Polished face of specimen La. 1, 1023-1033, lOS^i-lOG. A large fragment of primary sedimentary anhydrite with 

unusually clear and pronounced but widely separated stratification seams, n-n, n'-n', n"-n"', showing distortion. The series of 

dark spots across the stratification along the line a-a is due to gypsification. U.S.N.M. 97792. Natural size. (See pp. 87, 89.)

FIGURE 22. Polished face of specimen La. 1, 1286-1296, 76-78^4. A core consisting apparently almost entirely of primary sedimentary anhydrite rock but very much brecciated. Along the line a-a a stratified fragment can be recognized, gradually passing 

into confused breccia. Dark distorted seams throughout the specimen, such as that at b, probably represent remnants of 

stratification banding similar to that in the fragment a-a. U.S.N.M. 97792. Natural size. (See pp. 87, 88.)

FIGURE 23. Polished face of specimen La. 1, 1248-1258, 39^-45^4. Largest single piece of well-stratified primary sedimentary 

anhydrite rock encountered in this diamond-drill core; the stratification is unusually thin and pronounced but much distorted. 

Matrix anhydrite can be seen at the top and bottom of the specimen; that at the bottom has pronounced katatectic banding. 

The dark spots along the bands at the bottom are due to gypsification. U.S.N.M. 97792. Natural size. (See pp. 87, 89.)

1 All specimens marked La. 1, which corresponds to U.S.N.M. 97792, are parts of a diamond-drill core from well 194 of the Union Sulphur Co. on the Sulphur salt dome, 

Louisiana. The entire core was given to the Geological Survey by Mr. W. R. Keever, of that company. The first pair of numbers gives the depth to the top and bottom 

of the interval represented by the section of core in the box from which the specimen was taken. The second pair of numbers represents roughly the distance in inches 

from the top of the series of core fragments in the box to the top and bottom of the specimen.

106
PROFESSIONAL PAPER 175 PLATE 34

S. GEOLOGICAL SURVEY
U. S. GEOLOGICAL SURVEY

PROFESSIONAL PAPER 175 PLATE 35
PLATE 35

FIGURE 24. Polished face of specimen La. 1, 1450-1458, 35J4-48J>4. An unusually long core showing primary katatectic anhydrite 

cap rock, with unusually long intervals showing little or no katatectic banding, adjacent to well-defined katatectic bands. 

TJ.S.N.M. 97792. About natural size. (See pp. 88-91.)

FIGURE 25. Polished face of specimen La. 1, 1228-1238, 63J/2-75, show'ng considerable thicknesses of primary katatectic anhydrite 

cap rock, with little or no recognizable banding, in association with pronounced katatectic bands. Many breccia fragments, 

as at a-a and b, show pronounced distortion. U.S.N.M. 97792. About natural size. (See pp. 87-91.)

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FIGURES 26 and 27. Two polished faces of specimen La. 46, from depth of 2,354 feet, at the contact of anhydrite cap rock and salt, 

in the Texas Co.'s State Bay St. Elaine No. 3 on the Bay St. Elaine salt dome, Terrebonne Parish, La. The surface illustrated 

in figure 27 is about a quarter of an inch above that illustrated in figure 26. The specimen shows unusually clearly and thinly 

stratified, unusually well defined breccia fragments of primary sedimentary anhydrite, some of them distorted. In the breccia 

fragment a there is a striking duplication of strata up and down, respectively, from the middle zone n-n, which in figure 26 is dark. 

The duplication suggests that a long breccia fragment may have been broken about the middle and doubled over on itself on 

a hinge line whose intersection with the plane of the polished face would be at the point marked c. The way the area of convexity at the right-hand end of the fragment below this point seems to fit into the concavity at the right-hand end above it 

supports this suggestion. In figure 27 the assumed surface of contact, n-n, of the two halves is scarcely recognizable, and in 

thin section 1, illustrated in figure 33, which was made from the area indicated in figure 27, it is consequently also not pronounced. Somewhat coarser anhydrite can, however, be recognized along this line n-n in figure 33. If this is a surface of 

contact between two fragments, the lack of separating material shown in figures 27 and 33 may be due to squeezing out of 

matrix anhydrite. The development of the zone n-n in figure 26 may, however, be due merely to recrystallization, along a 

more pervious stratification boundary, and the apparent duplication above and below it may be merely accidental. Distinct 

schistosity can be seen in both the anhydrite and the underlying salt—in figure 26 clearly recognizable in the anhydrite and 

somewhat recognizable in the salt. At the bottom of the anhydrite as seen in figure 26 there is an intergrowth with salt similar 

to that in the specimen illustrated in figure 8, from the contact of anhydrite and salt on the Hockley salt dome. The specimen 

is said to be from the flank of the salt stock, and the slope of the contact and the schistosity of the specimen may be due to that 

fact. Specimen received from Mr. R. C. Stewart, of the Texas Co., Shreveport, La. U.S.N.M. 97793. About natural 

size. (See pp. 87-88.)

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FIGURE 28. Polished face of specimen La. 1, 1193-1203, 50%-54. The specimen from this diamond-drill core that shows the best 

example of a breccia fragment of stratified sedimentary anhydrite rock; somewhat distorted. U.S.N.M. 97792. Natural 

size. (See p. 88.)

FIGURE 29. Polished face of specimen La. 1, 1238-1248, 99%-102X2 . Another example, from this core, of a breccia fragment 

showing distinct stratification but with the stratification surfaces more widely spaced than in figure 28. U.S.N.M. 97792. 

Natural size. (See p. 88.)

FIGURE 30. Photomicrograph of a thin section of the specimen illustrated in figure 28, showing the appearance of the impurities 

along the stratification surfaces and something of the relation of the breccia fragment to its matrix. The fine-grained portion 

in the upper part is the breccia fragment. The coarser-grained material in the lower part is the matrix. On account of 

recrystallization, as can be seen in figure 28, the boundary is here not sharp. (See pp. 84, 88.)

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FIGURE 31. Detail of figure 32, to show more clearly the relative size and shape of grain and the sharpness of the contact of the 

clast and the matrix. (See pp. 84, 88.)

FIGURE 32. Photomicrograph of thin section 4 of the specimen shown in figure 29, illustrating the relation of the fine grain of the 

breccia fragment to the coarse grain of the matrix, the sharpness of the contact, and the very slight indications, in this section, 

of the stratification surface. This surface may perhaps be indicated by a very vague zone of parallel orientation of the anhydrite 

crystals recognizable on close study of the thin section. The approximate position of the thin section is indicated in figure 29. 

(See pp. 84, 88.)

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FIGURE 33. Photomicrograph of thin section 1, figure 27, showing a particularly clearly stratified breccia fragment of primary 

sedimentary anhydrite rock and its matrix. It is worth noting that the stratification does not appear as pronounced as would 

be expected from the polished face illustrated in figures 26 and 27. In view of this, it is not so surprising that stratification, 

such as that illustrated in figure 29, should be scarcely recognizable in the thin section of the same material illustrated in 

figure 32, and that it should be poorly defined in figure 30. The possible significance of the line n-n, along which some coarser 

anhydrite can be seen, as a line of contact between two fragments of the same breccia fragment, is more fully discussed in 

connection with figures 26 and 27. (See pp. 84, 88.)

FIGURE 34. A detail of the area indicated in figure 33, showing the sharp contact between breccia fragment and matrix and the 

relative size of grain of the two. This enlargement of the breccia fragment shows, in greater detail, the internal structure of 

such a fragment of what is taken to be primary sedimentary anhydrite. As may be seen, the grains are generally small but 

differ widely in size, are very irregular in outline, and show no regular orientation in spite of the pronounced banding of the 

breccia fragment as a whole. (See pp. 84, 88.)

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FIGURE 35. Polished race of specimen La. 1, 1338-1348, 86^4-89^2. An example of ordinary katatectic anhydrite cap rock showing 

stratified breccia fragments and matrix, but the breccia fragments less sharply denned than in the specimens illustrated in 

figures 26, 27, 28, and 29. U.S.N.M. 97792. Natural size. (See pp. 88, 90.)

FIGURE 36. Polished face of specimen La. 1, 1434-1438, 37}<i-48%. Illustrates the same thing as figure 35. The black lines around 

and in so many of the breccia fragments in this specimen are striking and unusual. Comparison with figure 22 suggests that 

these black lines are perhaps distorted stratification seams and that the stratified sedimentary anhydrite rock from which 

breccia fragments of this specimen were derived was particularly clearly and perhaps closely stratified. U.S.N.M. 97792. 

About natural size. (See pp. 88, 90, 91.)

FIGURE 37. Polished face of specimen La. 1, 1438-1442, 18}4-22}2 . The specimen from this diamond-drill core most typically 

illustrating katatectic banding; the banding close and sharp and without recognizable breccia fragments in the bands. The 

white patch in the lower left part is due mainly to fine-grained carbonate. In the rest of the specimen the whiter parts are in 

general finer grained, probably because they are less recrystallized than the darker parts. U.S.N.M. 97792. About natural 

size. (See pp. 89, 91)

FIGURE 38. Polished face of specimen La. 1, 1348-1358, 78-81}i Another specimen from this diamond-drill core showing pronounced katatectic banding, but the banding unusually converging and irregular. The convergence and irregularity are 

believed to be due to proximity to a fault. U.S.N.M. 97792. Natural size. (See p. 91.)

FIGURE 39. Polished face of specimen La. 1, 1442-1450, 42}^-45}i Illustrates another one of the few examples of convergence of 

katatectic surfaces in this core. The specimen is very near a fault or some similar disturbance, to which the generally disturbed 

appearance and the convergence of the katatectic surfaces in the lower part are ascribed. U.S.N.M.. 97792. Natural size. 

(See p. 91.)

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FIGURE 40. Polished face of specimen La. 1, 958-968, 55J4-58. A rather unusual example of schistosity in the cap rock from this 

diamond-drill core. U.S.N.M. 97792. Natural size. (See p. 91.)

FIGURE 41. Polished face of specimen La. 1, 1450-1458, 68^-70. An example of curving schistosity parallel to a curving surface of 

parting unusual in the cap rock from this diamond-drill core. The specimen came from the vicinity of a fault. U.S.N.M. 

97792. Natural size. (See p. 91.)

FIGURE 42. Diagram from Seidl,2 showing complex folds, including an anhydrite bed in a German salt stock, and the truncation of 

these folds by the salt table, which is overlain by gypsum-anhydrite cap rock. The conspicuous, not quite solidly black bed 

in the stock is anhydrite, and the similar material across the top of the stock is the gypsum-anhydrite cap rock. Immediately 

or almost immediately below the anhydrite bed is a bed of potash salt indicated by solid black, hard to differentiate in the 

illustration as here reproduced. Scale about 1:57,580. (See pp. 84-86, 90, 93.)

FIGURE 43. Another example to illustrate the same thing as figure 42 3 but showing a more complex relation between gypsumanhydrite cap rock and the anhydrite of the salt stock. Anhydrite and cap rock are indicated in not quite solid black, as in 

figure 42. Note that the absence of cap rock in drill hole 2 and the greater thickness in the shaft to the left of it are definitely 

established by these exploitations. The absence of cap rock in drill hole 2 seems at first surprising, because a pronounced 

anticlinal fold in the salt, which should have carried a large portion of the anhydrite bed well above the present salt table, 

has been decapitated by the salt table. The phenomenon is probably accounted for by the squeeezing out of anhydrite on 

anticlines and its accumulation in synclines, as explained in Seidl's paper on the Graf Moltke shaft (see footnote 9, p. 85) and 

illustrated in the small folds in the beds of anhydrite and of potash salt in figure 42. (See pp. 84-86, 90, 93.) Scale about 

1:59,100.

FIGURE 44. Another diagram from the same source as figures 42 and 43,4 showing a bed of anhydrite projecting above the salt table 

into the overlying gypsum-anhydrite cap rock. As this occurrence is penetrated by a shaft, it has apparently been definitely 

observed, and it shows clearly the intimate relation between anhydrite in the salt stock and the gypsum-anhydrite cap. According to the illustration, there is a transition from the anhydrite bed to the cap rock. The indicated thickness of this bed, some 

35 meters, as well as the relatively slight overburden (the distance from the surface to the salt table is about 165 meters, or 

560 feet) probably accounts for the distance, some 35 meters, that the bed projects into the cap rock. According to the 

interpretation presented in this paper, the cap rock between the level of the upper end of this bed and the salt table would 

constitute a single katatectic layer. The thickness of such a layer would be several hundred times the prevailing thickness 

of the katatectic layers I have seen in the anhydrite cap rock of American salt domes. The difference conforms to the evidence, 

discussed on pp. 84-85, that thick beds of anhydrite are not common in American salt stocks. Scale about 1:5,310. (See 

pp. 85, 86, 90.)

2 Seidl, Erich, Schurfen, Belegen und Schachtabteufen an deutschen Zechstein-Salzhorsten: Archiv fur Lagerstattenforschung, Heft 26, fig. 45, p. 129, Preuss. geol. 

Landesanstalt, 1921.

3 Idem, fig. 46, p. 131. 

«Idem, fig. 58, p. 165.

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FIGURES 45 and 46. Core from a depth of about 950 feet in the Texas Exploration Co.'s Warren No. 1 well on the Hockley salt 

dome, Harris County, Tex., showing a breccia of calcareous rock fragments in sandy clay (45, outside surface; 46, polished 

face). This breccia might be made up of sedimentary beds that overlay, in depth, the bedded salt from which the salt of the 

salt stock is derived and that were brecciated when the salt broke through them to form the stock. U.S.N.M. 97794. About 

natural size. Specimen probably received from Mr. Statti, of the Texas Exploration Co., Houston, Tex. (See p. 93.) 

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