Geology and Geochemistry 

of Uranium 

in Marine Black Shales 

A Review

GEOLOGICAL SURVEY PROFESSIONAL PAPER 356-C

Prepared on behalf of the 

U.S. Atomic Energy Commission 

and published with the permission 

of the Commission
Geology and Geochemistry 

of Uranium 

in Marine Black Shales 

A Review

By VERNON E. SWANSON

URANIUM IN CARBONACEOUS ROCKS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 356-C

Prepared on behalf of the 

U.S. Atomic Energy Commission 

and published with the permission 

of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON s 1961
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C.
CONTENTS

Abstract _ ________________________________________

Introduction. ____________________ _ _______________

Acknowledgments __ ___________ _ _________________

What is "black shale"? ____________________ 

What is "uraniferous black shale"? __ _______________

Historical review of investigations for uraniferous black 

shales. ______________________________ _ _________

Investigations, 1944-47 _ _______________________

Investigations, 1947-52 _ ______ _ __________ _ _

Investigations, 1952-57 _ _______________________

Summary.- ._ __________________________________

Sources of uranium and mechanisms of its enrichment. __ 

Examples of uranium-bearing shale and mud_______

Sources _ _____________________________________

Sources and chemical form of uranium in sea 

water. __________________________________

Uranium content of modern sea water_________

Uranium content of water in ancient seas______

Influence of source rock on uranium content of 

water in bordering seas______-_---____-____

Mechanisms of enrichment _ ________________ _ __

Organic matter.... _________________________

Amounts and types in uranium-bearing 

shale and mud_ ____ ______ _

Solid organic matter________________ __

Humic acids. __________________________

Uranium incorporated by living plants or 

animals __ _________________________

Hydrogen sulfide __________________ ______

Precipitation of uranium __ _____________

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Page 

Sources of uranium Continued

Mechanisms of enrichment Continued 

Hydrogen sulfide Continued

Uranium precipitated by hydrogen sulfide in

Chattanooga shale____ __-_ 90

Relation of pyrite to uranium.___________ 91

Bottom water of Norwegian fjords and

Baltic sea_._ _ 92

Possible extrapolations and interpretations. 93

Phosphate. ___________-_____________ 95

Relation to uranium..._____________ __ 95

Geochemical conditions for deposition....- 96

Cycle of deposition__ ______ __ 97

Other chemical variables__. _______ 98

Acidity_______________ ..__ 98

Oxidation-reduction potential.___ _ 99

Salinity____ ___ ._ 99

Diagenetic changes in black muds__ 100

Summary___________-_ __ . _____ 101

Factors controlling uranium distribution in black

shale__________________________ 101 

Known and theoretical relations.___ ____ _____ 102

Source of uranium.----.--.-- _. ,__ 102

Organic matter 103 

Hydrogen sulfide..-____ _ ___. 103 

Phosphate-_________ ___ ___- 104 

pH, Eh, and salinity. 104 

Conclusion_______ _________________________ 104

Literature cited__________________________________ 105

Index___-_--______.____________________ 111

ILLUSTRATIONS

Page 

FIQUKE 23. Cross section showing hypothetical distribution of uranium derived from eroded granite and deposited in geosynclinal and platform sediments_______________________________________________________________ 85

24. Map indicating amounts of dissolved organic matter entering the Baltic Sea area ___________________________ 88

25. Cross section of fjord water charged with hydrogen sulfide, with calculations on precipitated uranium.________ 94

26. Columnar section of upper part of Chattanooga shale showing distribution of uranium and phosphate.__-___ 97

27. Graph showing estimated distribution of uranium in shales and muds_______________ ______________ 105

TABLES

TABLE 1. Summary of chemical information bearing on concentrations of uranium in marine black shales and muds.

2. Laboratory experiments in precipitating and in concentrating uranium ___________ _ .__ _ ____ __ _..

3. Chemical data on bottom sediment and water in Norwegian fjords and in the Baltic Sea near Gotland...

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rrr

URANIUM IN CARBONACEOUS ROCKS

GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES, A REVIEW

By VERNON E. SWANSON

ABSTRACT

More than 200 formations in the United States containing 

marine black shale units, ranging from Precambrian to Tertiary in age, were examined during the period 1944-1957 for 

black shale that might be exploited as a source of uranium. 

Black shale beds were checked for their radioactivity at thousands of outcrops, in thousands of feet of well core, in thousands of samples of drill cuttings, and on many hundreds of 

gamma-ray logs, and an estimated 8,000 samples were chemically analyzed for uranium. Of all the shale beds tested, 

none was found that could be mined and processed economically for uranium. The Chattanooga shale of the southeastern United States and the phosphatic black shales of Pennsylvanian age in Kansas and Oklahoma were found to be the 

most extensive uraniferous shales in the United States. Their 

uranium content is relatively high, generally between 0.005 

and 0.010 percent.

The exact chemical and physical forms and associations of 

most of the uranium in marine black shales cannot be determined by existing chemical, microscopic, or other methods. 

Thus the form, the associations with other shale constituents, 

and the mechanisms of emplacement of the uranium have to 

be interpreted from interrelations of other constituents in the 

sediment, from chemical processes active in the environment 

of black mud deposition, and from the properties of uranium 

and the processes resulting in its concentration.

The geologic controls and geochemical processes involved in 

the concentration of uranium in marine black shales as presented here are based on (a) geologic studies of uraniferous 

marine black shales, as represented by the upper part of the 

Chattanooga shale, which contains about 0.008 percent uranium; (b) sedimentologic studies of modern uranium-bearing 

black muds of the Norwegian fjords and of the Baltic Sea, 

which contain maxima of 0.006 and 0.001 percent uranium, 

respectively; (c) hydrologic studies of the waters in which 

these muds are deposited; and (d) laboratory experiments in 

uranium precipitation related to conditions observed in the first 

three categories.

The immediate source of the uranium concentrated in marine black shales was sea water. Interpretations that quantitatively relate the uranium content of shale to the uranium 

content of the sea water in which the shale was deposited, 

however, have little basis in fact; it is concluded that ancient 

sea water did not contain significantly more or less uranium 

than modern sea water, about 3 parts per billion of uranium. 

The second-order interpretation that granites or other types of

rock yielded abnormally large amounts of uranium to blackshale seas is even more tenuous and should not be invoked 

to explain high uranium content of black shales.

The uranium in marine black shales and muds probably was 

concentrated in several forms and by several processes, but 

organic matter is directly or indirectly responsible for concentrating most of the uranium. Marine shale without organic matter generally contains 0.0001 to 0.0004 percent uranium, most of which is held in the small fraction of resistate 

minerals such as zircon and sphene, and some of which is 

held by ion exchange in clay minerals. If the shale contains 

2 or more percent organic carbon, it is here termed "black 

shale," and its uranium content generally is from a few to as 

many as 250 times greater than that of noncarbonaceous marine shale.

Living plants and animals accumulate only very small 

amounts of uranium in their tissues, and, therefore, do not 

enrich a sediment in uranium by being buried and preserved 

with it. Some decomposition products of plant organisms can 

sorb uranium from solution so that the resulting material 

contains from 0.01 to as much as several percent uranium. 

This applies to solid substances such as partially decomposed 

fragments in peat and, more particularly, to the soluble organic substances commonly termed humic acids. A large part 

of the uranium in many marine black shales may have been 

sorbed from river and sea water by these substances before 

deposition.

Most of the organic matter deposited on a sea bottom is 

either directly reprocessed by organisms as food or is oxidized and destroyed. Where organic matter is deposited in 

quiet water, the oxidation process is soon terminated, decomposition is greatly slowed and largely limited to that by 

anaerobic bacteria, the water becomes increasingly acid and 

is fouled with hydrogen sulfide to become reducing (Eh of 

 0.0 to 0.4), and much of the organic matter is preserved. 

As much as 199 cu cm of hydrogen sulfide per liter of water 

is present in the bottom water of some Norwegian fjords; the 

hydrogen sulfide probably reduces the uranium dissolved in 

this water from the hexivalent to the tetravalent state, whereupon the uranium is precipitated onto the organic-rich sediment, probably as submicroscopic grains of uraninite. The 

original uranium content of the sea water, which is generally 

only a few parts per billion (O.OOOOOOX percent), and the 

height of the water column invaded by hydrogen sulfide 

sharply limit the amount of uranium so precipitated. Intermittent circulation and consequent renewal of uranium-bear67
68 URANIUM IN CARBONACEOUS ROCKS

ing water, alternating with periods of water stagnation and 

accumulation of hydrogen sulfide, would be necessary to explain sizeable additions of uranium to mud by this method. 

More than 50 percent of the uranium in some shales is believed to have accumulated by direct precipitation from sea 

water by the action of hydrogen sulfide.

Phosphate, generally in the form of carbonate-fiuorapatite 

nodules, is a common and distinctive component of many 

uraniferous black shales; in some black shales, phosphatic 

nodules and layers with 0.02 to 0.1 percent uranium contain 

much of the uranium in the shale, but in other uraniferous 

black shales the nodules contain only 0.001 to 0.003 percent 

uranium. The biogenic phosphate that settles into a relatively acid environment (pH of 6.5 to 7.0) wherein black mud 

is accumulating is taken into solution and becomes concentrated to as much as 700 mg per cu m of water. A subsequent rise in pH toward 7.5 permits some phosphate precipitation, but if the water is largely depleted in uranium by 

hydrogen sulfide precipitation, the resulting phosphate contains very little uranium. Where the water has not been depleted in uranium or is undergoing some circulation and 

where phosphate precipitation takes place slowly, the phosphate deposited with the black mud may contain as much as 

0.1 percent uranium. The amount of phosphate and the 

amounts of uranium in the phosphate and in the organicrich mud are dependent on a sensitive interplay of changes 

in the pH and Eh and on the availability of uranium and 

phosphate in solution in the water.

Based on the known and theoretical relations of uranium 

with other constituents, nine mechanisms of emplacement of 

uranium and the amounts attributable to each are given for 

a unit of the Chattanooga shale (0.0079 percent uranium), 

the black mud of a Norwegian fjord (0.0060 percent), an 

organic-rich mud from the Gotland deep of the Baltic Sea 

(0.0010 percent), and a phosphatic black shale of Pennsylvanian age (0.0050 percent). Most of the uranium in the first 

two sediments probably was concentrated by direct precipitation by the action of hydrogen sulfide and by the sorption of 

uranium onto solid and soluble organic matter of the humic 

type. In the Baltic Sea mud, the uranium sorbed by soluble 

organic matter derived from land and by settling of partly 

decomposed plankton probably constitutes about one-half of 

the uranium in the mud; the other half is distributed fairly 

evenly among the clastic minerals and the phosphate, with 

only very minor amounts attributable to hydrogen sulfide 

precipitation. Most of the uranium in the phosphatic black 

shales of Pennsylvanian age is in the phosphate, with the 

solid and soluble organic matter of the humic type containing 

much of the remainder.

INTRODUCTION

Sources in the United States for a large supply of 

uranium were unknown when the need for this metal 

for nuclear weapons became apparent in 1943. This 

need instigated an intensive and extensive 10-year 

search for a domestic supply and resulted in meeting 

military requirements and the anticipated needs for 

uranium for peaceful uses of atomic energy.

Early in the search, black shale was investigated as 

one of the possibilities for a source of large quantities of uranium. Concentrations of uranium in marine black shales in excess of 0.1 percent had been 

known for half a century (Nordenskiold, 1893) from 

descriptions and analyses of kolm lenses in the alum 

shales of southern Sweden (see also Larsson, 1919; 

Munthe, Westergard, and Lundquist, 1928). For many 

years, the only lead-uranium age determination made 

on a sedimentary rock was one made on kolm samples from these shales of Late Cambrian age (Holmes, 

1931). Abnormally high radioactivity of another marine black shale, the Dictyonema, shale of Ordovician 

age in Estonia and Russia, was also known from papers by Orlov and Kurbatov (1934-36) and by Glebov 

(1941). The generally greater abundance of uranium 

in marine carbonaceous rocks, as compared to other 

sedimentary rock types, was established as a result of 

radioactivity determinations on hundreds of rock samples from the United States by Beers and Goodman 

(1944) and by Russell (1944).

Numerous marine black shales of tremendous tonnages and wide distribution throughout the United 

States were known. One of the logical steps to meet 

the urgent demand for uranium during the latter part 

of "World "War II was to search for black shale from 

which a large amount of uranium could be produced. 

This search was in large part a reconnaissance that 

involved field checking of outcrops of black shale by 

means of Geiger-Miiller counters and, later, scintillation counters. No body of shale from which uranium can be economically recovered was located during these investigations nor during subsequent detailed 

studies of the more uraniferous shales. Discoveries 

in the period of 1949-55 of large uranium deposits in 

sandstones, conglomerates, and veins, which contain 

10 to 100 times more uranium per unit of rock than 

is known in black shales, practically eliminated interest in black shale as a source of uranium.

The purpose of this report is twofold: (a) To review briefly the geologic and geochemical investigations conducted during the period 1944-57 on the 

amounts and distribution of uranium in the black 

shales in the United States; (b) to assemble and 

evaluate the geologic and geochemical information 

that directly or indirectly pertains to the origin and 

localization of uranium in marine black shale.

The origin of uranium in shale is a complex subject 

that is far from being completed understood; thus, 

this report is a progress report on the status of current knowledge. In general, the stratigraphic distribution and lithologic associations of uranium in black 

shales are known, and the possible and probable processes and factors involved in the concentration of uranium in black shale have been deduced. But many 

additional studies, particularly of the geochemical
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 69

type, are needed to determine precisely the quantitative values and limits applicable in the chemical processes known to concentrate uranium in the environments of deposition of marine black shale.

ACKNOWLEDGMENTS

This report was prepared as a part of a general 

study by Geological Survey personnel on geologic 

problems involving the genesis and distribution of 

uranium that was sponsored by the Division of Eaw 

Materials of the U.S. Atomic Energy Commission. 

The author wishes to acknowledge especially the help 

and guidance of William S. Twenhofel, Curt Teichert, 

Louis C. Conant, and James D. Vine in the development of ideas and in the preparation of this paper.

WHAT IS "BLACK SHALE"?

No precise definition of the term "black shale" 

would satisfy the requirements as might be set forth 

by each of the several thousands of geologists who frequently use the term. "Black shale" is similar to the 

term "sandstone," in that both refer to rocks of widely 

differing origins, chemical compositions, mineralogical 

suites, grain sizes, colors, and other descriptive characteristics. Defined in its most general sense, "black 

shale" is simply a dark-colored fine-grained clastic 

sedimentary rock. Most geologists would also include 

as additional parts of this definition the characteristic 

of abundant organic matter and the limitation of 

grain size to silt and clay sizes; inclusion of such 

additional restrictive items as the fissility of the rock, 

the specific color, the ratio of quartz to clay minerals, 

the minimum amount of organic matter, and the type 

of organic matter would be unacceptable to many geologists because it would exclude many rocks that have 

long been known as black shales.

As a lithologic term, "black shale" must be defined 

as that large class of sedimentary rocks composed 

chiefly of mineral grains of clay and silt size and 

containing sufficient organic matter, iron sulfide, or 

manganese oxide to give the rock an overall darkgray to black color. Within this class of rocks, several tens of types of black shales have been differentiated in a general or a precise fashion, depending on 

the particular constituent or constituents of primary 

interest. Thus, terms such as "humic shale," "oil 

shale," "graptolitic shale," "alum shale," "euxenic 

shale," "sapropelite," "gyttja," and many others are 

commonly applied to particular shales.

Because most black shale owes its color and distinctiveness to finely disseminated organic matter, 

"carbonaceous shale" for all practical purposes may 

be generally taken to be synonymous with "black

shale." The general term "bituminous shale," though 

still commonly used to indicate shale that will yield 

oil when heated to high temperatures, does not conform to the now generally accepted and more precise 

usage of the terms "bitumen" and "bituminous" as 

defined by Abraham (1945, p. 47-52); bituminous 

substances are largely the hydrocarbons soluble in 

carbon disulfide, including petroleums and native mineral waxes and asphaltites, and are only sparsely 

present and rarely recognized in carbonaceous shales.

Strictly speaking, the adjective "black" in the term 

"black shale" should mean that the rock is devoid of 

color or incapable of reflecting light. However, few 

if any black shales have the Nl or black value of the 

National Eesearch Committee Eock-Color Chart 

(1948). Most black shale is actually dark gray (N3), 

or grayish black (N2), and dark hues of brown (SYR 

2/1) and olive (5 Y 3/2-2/1) are not uncommon. Shale 

lighter in color than the values given above should be 

excluded from the general class of black shale. The 

dark color of most shale is imparted largely by the disseminated carbonaceous matter, and, in order to comply 

with the color restriction just given, only carbonaceous 

shale containing 2 or more percent organic carbon by 

weight should be classed as black shale.

The lithologic term "shale" must also be used in a 

loose sense because of its common, widespread, and 

long-term usage, particularly in studies of well cuttings, to indicate any fine-grained unmetamorphosed 

sedimentary rock. The connotation of fracture habit, 

or fissility, whether this feature is explained by mass 

grain orientation, minute lamination, or secondary 

structural causes, can no longer be considered definitive and distinctive. Thus "shale" is synonymous with 

"mudstone," and, therefore, includes siltstone and claystone; the term is not used, of course, to describe 

those fine-grained rocks that are composed largely of 

carbonate minerals.

Black shales of more than 200 formations were investigated in the United States from 1944 to 1957 during the search for uranium; a review of the stratigraphic relations of each of the black shales in these 

formations only emphasizes the diversity of occurrences and origins of the rocks included under the 

general term "black shale." This diversity is exemplified by comparing (a) a black shale parting less 

than 1 inch thick in the Salem limestone of Mississippian age in Missouri, (b) the black shale hundreds 

of feet thick in the Utica shale of Ordovician age in 

New York, (c) the black shale about 1 foot thick 

overlying the uppermost coal bed in the Cherokee 

shale of Pennsylvanian age in Kansas, and (d) the
70 URANIUM IN CARBONACEOUS ROCKS

black shale ranging in thickness from a few inches to 

25 feet that is associated with salt and gypsum of the 

Paradox formation of Pennsylvanian age in Utah. 

The term "black shale" becomes even more indefinite 

when applied in a general sense to a stratigraphic 

unit, as illustrated by a single black shale of large 

areal extent, the Chattanooga shale and many of its 

correlatives of Late Devonian age in the eastern and 

central United States. This black shale ranges from 

a fraction of an inch to several hundred feet in thickness; ranges in color through many shades of gray, 

olive gray, and brownish black, but rarely, if ever, 

is actually black; has a fracture pattern that is described in different places as massive, conchoidal, 

splintery, fissile, and paper thin; in different areas is 

interbedded with limestone, sandstone, phosphate, or 

chert; and has a content of organic matter ranging 

from less than 1 to more than 25 percent. These examples indicate only a few of the many stratigraphic, 

compositional, and genetic variations included under 

the term "black shale."

Similarly, modern accumulations of black mud, the 

progenitor of black shale, originate in a variety of 

environments such as swamps, lakes, tidal flats, coastal 

lagoons, fjords, the Black Sea, the Baltic Sea, and 

deep seas. The range in environments of deposition 

of black muds and the interpreted environments of 

deposition of black shales is large, but therein lies the 

currently used and most useful and practical basis for 

differentiating among black shales. The most easily 

discernible contrast among black shales is their 

marine or nonmarine origin. Marine black shales are 

characterized by uniform thickness and lithology over 

wide areas and by containing marine fossils or by being interbedded with and closely related to other 

lithologies having marine fossils. Nonmarine black 

shales are erratic in thickness and distribution, lack 

marine fossils, and generally are interbedded with or 

are lateral facies of sandstones and coals having primary structures and plant remains clearly identifiable 

as originating in a nonmarine environment. Subdivision of these two major types of black shales into 

more specific environmental types, such as eugeosynclinal, near-shore marine, estuarine, paludal, and lacustrine, are made on more explicit lithologic, paleontologic, and paleogeographic criteria.

WHAT IS "URANIFEROUS BLACK SHALE"? 

Shale contains an average of a few parts per million (0.0003 or 0.0004 percent) uranium. Black shale 

contains more uranium than the average for all shale; 

it probably has an average of 8 ppm (parts per million), and a general range of 3 to 250 ppm. Black 

shales of marine origin have only a slightly higher 

average than do all black shales; but, if the thick 

marine black shales deposited in geosynclinal areas 

are excluded, the average uranium content of marine 

black shales is about 20 ppm, or about 0.0020 percent.

The marine black shales hundreds to thousands of 

feet thick that accumulated in and marginally to the 

early Paleozoic eugeosynclinal and miogeosynclinal 

areas of the Eastern United States and the Mesozoic 

eugeosynclinal and miogeosynclinal areas of the Western United States rarely contain more than 0.002 percent uranium. The black shales that commonly contain more than 0.002 percent uranium are those shales 

only one to a few tens of feet thick of Paleozoic age 

that were deposited in epicontinental seas on the more 

stable cratonic areas of the Central and East Central 

United States. It is this latter kind of black shale 

that is referred to and discussed in this paper.

The uranium in black shale is a minor or trace element. The adjectives "uraniferous" and "uraniumbearing" have been used to describe shales containing 

relatively large amounts of uranium, much the same 

as "cupriferous," "chromiferous," and "vanadiferous" 

have been used to indicate comparable concentrations 

of other metals. As used in this report, "uraniferous" 

and "uranium-bearing" are applied only to those black 

shales generally containing in excess of 20 ppm, or 

0.0020 percent, uranium.

HISTORICAL REVIEW OF INVESTIGATIONS FOR 

URANIFEROUS BLACK SHALES

The primary objective of all investigations reviewed 

here was to find black shale that could be economically 

utilized as a source of uranium. The investigations 

were generally of two types: reconnaissance studies, 

which involved a rapid field check of outcrops and 

some sample collecting to determine the radioactivity 

and uranium content of as many black shales as possible in the shortest length of time; and detailed geologic studies, to determine the stratigraphic, lithologic, and geochemical factors controlling uranium 

distribution in shale.

INVESTIGATIONS, 1944-47

During the period from early 1944 to 1947, the Geological Survey was commissioned by the U.S. Army's 

Manhattan Engineer District, the forerunner of the 

Atomic Energy Commission, to conduct extensive 

reconnaissance studies to find uranium-bearing black 

shales. The geologic literature on black shales in the 

United States was reviewed, and the described out-
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 71

crops of scores of marine black shales were checked 

in the field for their radioactivity by means of GeigerMuller counters. The names of the individuals who 

accomplished this early field reconnaissance, and who 

submitted informal progress reports which guided 

subsequent investigations, are here recorded for their 

rarely acknowledged part in the search for uraniferous black shales: K. G. Brill, Jr., A. P. Butler, Jr., 

C. W. Chesterman, S. E. Clabaugh, W. H. Hass, P. F. 

Narten, J. M. Nelson, W. W. Kubey, A. L. Slaughter, 

and E. V. Stratton.

No black shale was found during these early reconnaissance studies that equals in uranium content the 

Swedish shale, which contains from 0.020 to 0.030 percent uranium; of all the shales tested and otherwise 

evaluated, the Chattanooga shale of the Southeastern 

United States and the black shales of Pennsylvanian 

age in Kansas and Oklahoma have the highest uranium content of United States shales, generally between 0.005 and 0.010 percent. Many other shales in 

the United States were tested after 1947, but, as potential sources of uranium, none have been found that have 

the combined high uranium content and mining practicability of the black shale beds in the Chattanooga and 

those of Pennsylvanian age. The following important 

conclusions were formulated during these early studies 

to locate shale of high uranium content: (a) Uranium 

content is generally greater in marine than in nonmarine black shales; (b) uranium content increases 

with increasing organic matter; (c) phosphatic nodules in black shales have a high uranium content; 

(d) uranium content is greater in shales deposited 

slowly.

INVESTIGATIONS, 1947-52

A second phase of investigations for uraniferous 

black shale in the United States encompasses the 

period 1947-52, when the U.S. Geological Survey conducted detailed field studies of the Chattanooga shale 

and made additional reconnaissance investigations for 

uraniferous shale, mainly in the Western States and 

Alaska. These and later studies by the Geologcial 

Survey were made on behalf of the Division of Kaw 

Materials and of the Division of Kesearch of the U.S. 

Atomic Energy Commission.

During this second period, a large Geological Survey field party, under the general direction of L. C. 

Conant, conducted detailed stratigraphic studies and 

a widespread, systematic sampling program on the 

Chattanooga shale, mainly in central Tennessee but 

also in southern Kentucky and northern Alabama. 

More than 400 outcrops of Chattanooga shale were 

studied during the course of these investigations, and

590640 61 2

about 3,000 samples were collected for uranium determinations. The objectives of this work for the 

Atomic Energy Commission were to study the shale 

geologically, to evaluate it as a potential source of 

uranium, and, consequently, to designate the area or 

areas where the shale contains the most uranium which 

could be mined most economically; an area near Center Hill Keservoir in DeKalb and White Counties, 

Tenn., was so designated. The following geologists, 

all of the Geological Survey, contributed materially to 

this phase of work: Andrew Brown, L. C. Conant, 

W. H. Hass, W. A. Heck, T. M. Kehn, K .A. Laurence, 

W. H. Monroe, K. C. Kobeck, K. E. Smith, and V. E. 

Swanson The following papers are an outcome of 

this work: A summary (Swanson, 1953) and a detailed description (Brown, 1956) of some of the mineralogic and stratigraphic associations of uranium in 

the Chattanooga shale; an interpretation (Conant, 

1956) of the environment of deposition of the shale 

as related to uranium concentration; a review of the 

stratigraphy of the Chattanooga shale and overlying 

Maury formation, and of the age relations of the units 

in these formations as based on conodonts (Hass, 

1956); and a detailed description of the distribution, 

lithology, and interpreted sedimentologic and paleogeographic conditions of the Chattanooga shale in the 

southeastern United States (Conant and Swanson, in 

press).

Chemical studies of the Chattanooga shale, aimed 

at developing uranium-extraction methods, were conducted by the Battelle Memorial Institute at Columbus, Ohio, and by the Oak Kidge National Laboratory 

of the U.S. Atomic Energy Commission; the results 

of these studies have not been made publicly available.

Concurrent with this study of the Chattanooga 

shale, a widespread reconnaissance search for uraniferous shale was conducted by the Geological Survey 

in the Western United States, under the general direction of N. M. Denson. Eighty formations containing 

black shale were investigated in 10 Western States, 

and about 380 samples were collected for radioactivity 

and uranium determinations. The results of this reconnaissance were reported by Duncan (1953); Duncan's report is based on the work of the following 

geologists: G. O. Bachman, N. M. Denson, D. C. 

Duncan, J. K. Gill, W. J. Hail, J. D. Love, G. W. 

Moore, C. B. Read, J. I. Simmons, and J. D. Vine. 

Excluding the phosphorites in the Phosphoria formation and 2 hydrothermally enriched shale units in 

Colorado, 12 formations have black shale units con-
72 URANIUM IN CARBONACEOUS ROCKS

taining a maximum of 0.003 to 0.005 percent uranium; 

outcrop samples of shale from the Heath formation 

of Mississippian age in Montana contain a maximum 

of 0.006 percent uranium; and a few beds, each less 

than 1 foot thick, in the Hartville formation of 

Permian, Pennsylvanian, and Mississippian (?) age in 

the subsurface of southeastern Wyoming and western 

Nebraska, contain 0.010 to 0.019 percent uranium.

Similar reconnaissance in Alaska (White, 1952; 

Wedow, 1954) revealed several Precambrian and Paleozoic black shales containing 0.001 to 0.003 percent 

uranium, but only the phosphatic black shale near the 

base of the Calico Bluff formation of Mississippian 

age was found to contain more, generally between 

0.003 and 0.006 percent uranium.

During this same period, Gott and Hill (1953) investigated the radioactivity in oil fields of southeastern Kansas and reported on the radioactivity of the 

Paleozoic black shales as indicated by the logs and 

samples of the wells studied.

Papers by Bain (1950, p. 291-292) and by McKelvey and Nelson (1950) summarize the general characteristics of uranium-bearing marine black shales 

known at the time of publication. The review by 

McKelvey and Nelson of general associations and 

probable origins of uranium in black shale served as 

an important guide in planning and executing subsequent black shale studies.

During the period 1947-52, the American Petroleum Institute sponsored a study at the Massachusetts 

Institute of Technology on the possible role of radioactivity in the transformation of organic materials 

into petroleum hydrocarbons. As a result of this 

study, the observed associations and the probable 

genetic relations of uranium to organic material and 

phosphate were summarized by Whitehead (1954) for 

several black shales, including the Antrim shale of 

Michigan, the Cherokee shale of Pennsylvanian age 

in Oklahoma, and the "Miocene nodular shale" of 

California.

INVESTIGATIONS, 1953-57

The third phase of studies of uraniferous black 

shale, extending from 1952 to 1957, included a variety 

of more specialized geologic studies by Federal organizations, State geological surveys, and several university groups. Early in this period, existence was 

established of ample supplies of uranium ore within 

the United States, especially in sandstone ore of the 

Colorado Plateau that contains 0.1 or more percent 

uranium. The extensive reconnaissance studies of

black shale had fairly certainly established that the 

United States lacks black shale that has a uranium 

content exceeding 0.01 percent and that can be mined 

cheaply. Studies of black shale during the period 

1952-57 were thus largely devoted to analysis of existing information and to special field and laboratory 

studies designed to investigate the geologic factors 

affecting the distribution of uranium in shale.

The Chattanooga shale field investigations were 

continued by the Geological Survey until 1955; these 

included a core-drilling program, centered mainly in 

DeKalb County, Tenn. (Kehn, 1955), to determine 

exact thicknesses of overburden and of the shale and 

to provide data for calculating reserves in areas designated as possible mining sites. Some additional study 

on the stratigraphic distribution of uranium in the 

Chattanooga shale in eastern Tennessee, northwestern 

Georgia, and northeastern Alabama was made by 

Glover (1959). Laboratory studies on the lithologic 

and chemical associations of uranium in the Chattanooga shale (table 2) were made during this period 

(Deul and Breger, 1954; Breger and Schopf, 1955; 

Deul, 1957). The U.S. Bureau of Mines (1954) conducted tests in cooperation with the U.S. Atomic 

Energy Commission to determine the percentage and 

characteristics of oils retorted from the Chattanooga 

shale.

Contracts to conduct research on the Chattanooga 

shale were given by the U.S. Atpmic Energy Commission to: (a) Pennsylvania State University, to determine the mineralogic and petrographic associations 

of uranium in shale (work directed by T. F. Bates; 

see Bates and others, 1956; Bates and Strahl, 1957) 

and to determine the chemical nature of the organic 

matter in shale and its association with uranium (work 

directed by C. R. Kinney; see Kinney and Schwartz, 

1957); (b) the University of Tennessee, for a detailed 

study of the stratigraphic distribution of uranium 

(work directed by P. B. Stockdale and H. J. Klepser; 

see Stockdale, 1955; Klepser, 1957); and (c) Columbia University, for metallurgical studies of uranium 

extraction from shale (work directed by M. D. Hassialis; see Pollara and others, 1958).

During the period 1952-57, the U.S. Geological Survey completed special studies on the distribution of 

uranium in the following shales: (a) The Sharon 

Springs member of the Pierre shale of Late Cretaceous 

age in South Dakota, Nebraska, Colorado, and Kansas 

(Dunham, 1954, p. 157-158; Tourtelot, 1956; Kepferle, 

1959; Landis, 1959); (b) the Eagle Ford shale of Late
GEOLOGY AND GEOCHEMISTRY OP URANIUM IN MARINE BLACK SHALES 73

Cretaceous age in Texas (Eargle and Hollingsworth, 

1958); (c) the several thin black shale layers in the 

cyclothems of Pennsylvanian age in southeastern Kansas and northeastern Oklahoma (Danilchik and Hyden, 

1956); (d) the Chattanooga and the Wbodford shales 

and the black shale in the Arkansas novaculite, all of 

Late Devonian and Early Mississippian age, in Oklahoma, Arkansas, and Kansas (Landis, 1958); (e) the 

black shales of Pennsylvanian age in southern Illinois 

(Patterson, 1955), southwestern Indiana (Snider, 

1954a), Ohio (Snider, 1954b), eastern Kentucky 

(Welch, 1953a), southern West Virginia and Virginia 

(Snider, 1953), northern West Virginia (Patterson, 

1954a), and Pennsylvania (Welch, 1953b; Patterson, 

1954b; and Ferm, 1955). During this same period, 

Mapel (1956a, b) reported on the uranium content of 

about 150 samples from 80 outcrops of black shale 

representing 22 formations in 5 Western States.

Several summaries were published by the Geological 

Survey during this period. Summaries of the general 

characteristics and of ideas on the genesis of uraniumbearing shales were presented (McKelvey, 1955, p. 14- 

16; McKelvey, Everhart, and Garrels, 1955, p. 514- 

520). The uranium contents of the marine black 

shales in the United States were reviewed by Swanson 

(1956), who later described the relation of the oil 

yields to the uranium contents of some of these shales 

(Swanson, 1960). An annotated bibliography of papers on the geology of uranium-bearing marine black 

shales in the United States was compiled by Fix 

(1958).

The State Geological Survey of Illinois made radioactivity and some uranium determinations on 175 

samples of Paleozoic black shales, most of them of 

Pennsylvanian age, taken from outcrops in Illinois 

(Ostrom and others, 1955). A statistical analysis of 

the areal distribution of radioactivity in a black shale 

unit overlying the "No. 6 coal" of Pennsylvaman age 

in western and southern Illinois was made by Krumbein and Slack (1956). The uranium contents of the 

phosphatic nodules present in some of these Pennsylvanian black shales were reported for shales in Kansas 

(Runnels, Schleicher, and Van Nortwick, 1953) and 

in Illinois (Ostrom and others, 1955).

This review does not include the results of the Geological Survey's extensive studies of the phosphorite 

and black shale in the Phosphoria formation of 

Permian age in southeastern Idaho and adjacent 

States. Reports resulting from these studies, which

were under the general direction of V. E. McKelvey, 

and reports on other uranium-bearing phosphorites, 

were listed and annotated by Curtis (1957). Sheldon 

(1959) presented interpretations of the geochemical 

conditions prevailing during deposition of the Phosphoria formation.

SUMMAEY OF INVESTIGATIONS, 1944-57

More than 200 formations in the United States made 

up entirely or partly of black shale units, ranging 

from Precambrian to Tertiary in age, were checked 

for their radioactivity during the period 1944-57. 

Eight to ten thousand samples of black shale were 

collected from about 100 of these formations and 

fluorometrically analyzed for their uranium content. 

Additional chemical determinations (for example, for 

phosphate, organic carbon, and sulfur) were made on 

several hundred of these samples. The oil yields, 

based on destructive distillation tests, were determined 

for more than 500 black shale samples; semiquantitative spectrochemical analyses provided data on about 

30 elements for each of more than 400 shale samples.

Thousands of outcrops, thousands of feet of well 

core, thousands of samples of drill cuttings, and many 

hundred gamma-ray logs were examined to find black 

shale that might be exploited as a source of uranium. 

Such a black shale should meet the following requirements: (a) It should contain more than 0.01 percent 

uraniumj (b) it should be several feet thick and extend over an area of tens of square miles; (c) it should 

be situated so that it could be mined economically, 

preferably by open-pit methods. The vast exploratory program and all the analytical data reviewed 

above failed to reveal such a black shale in the United 

States. Some shales, for example the black shales in 

the Hartville formation of Permian, Pennsylvanian, 

and Mississippian (?) age in southeastern Wyoming 

and western Nebraska (J. D. Love, written communication, 1951; Duncan, 1953, p. 82-85) and possibly 

the Heath shale of Mississippian age of Montana 

(Mapel, 1956b, p. 233) and the Woodford shale of 

south-central Texas, contain more than 0.01 percent 

uranium but are located thousands of feet below the 

surface so as to make economical mining impossible. 

Similarly, selected samples of the Chattanooga shale 

in Tennessee and of the partial equivalent of the Chattanooga shale in northern Arkansas contained as much 

as 0.035 and 0.71 percent uranium, respectively, but 

the mass of shale from which these samples were 

taken contains about 0.005 percent uranium.
74 URANIUM IN CARBONACEOUS ROCKS

The net result of the entire exploration program for 

uranium-bearing black shale was the determination 

that the Chattanooga shale of central Tennessee most 

nearly meets the requirements set forth above. Here, 

a unit of black shale 15 to 18 feet thick contains between 0.006 and 0.007 percent uranium and is present 

within a few hundred feet of the surface over many 

tens of square miles (Kehn, 1955). Certainly, neither 

the Chattanooga shale nor any other black shale in 

the United States will be mined within the near future for uranium, considering the large reserves of 

higher grade ores now known in the United States 

and other countries.

A major result of the search for uranium-bearing 

black shale was the great increase in knowledge of 

the geology of many formations. The thickness, distribution, lithofacies, and age of many formations 

containing black shale are now known in great detail; 

and many enigmatic problems on the paleontologic, 

mineralogic, and geochemical relations within black 

shales were solved or at least clarified.

SOURCES OF URANIUM AND MECHANISMS OF ITS 

ENRICHMENT

The exact chemical and physical forms and mineralogic associations of most of the uranium in marine 

black shale are not known. The amount of uranium 

in black shale can be determined precisely by fluorometric analysis and the uranium can be extracted by 

several chemical methods, but the chemical bonds, if 

any, of uranium with other elements in the shale are 

undetermined, and no uranium minerals have been 

separated or identified. Physical and chemical isolation of specific constituents of the shale, such as the 

organic or mineral matter, and chemical analyses of 

the isolates have not revealed any consistent relation 

between uranium and other constituents in the shale. 

By noting the sources of radioactive emanations on 

thin-section autoradiographs, it has been determined 

that much of the uranium is disseminated as submicroscopic specks through the shale, but, again, these 

point-sources of radiation have no consistent relation 

to other identifiable constituents. The failure to relate precisely the uranium with a uranium-bearing 

mineral or a specific shale component has resulted in 

several interpretations, some conflicting, on the manner of emplacement and factors controlling the distribution of uranium in shale.

Many kinds of observations have been made on the 

amount and distribution of uranium in shale. By 

using beta- and gamma-ray counters in the field and 

then relating laboratory uranium determinations on 

rock samples to field observations, geologists have 

shown that the amount of uranium in some shales can 

be related to measurable characteristics of the strata, 

such as color, thickness of beds, grain size, and 

amounts of pyrite, organic matter, and carbonate 

minerals. From these and other features of the shales, 

they have concluded that the amount of uranium is 

related to rate of sediment deposition, chemistry of 

the water in. which the sediment was deposited, and 

to several paleogeographic factors, such as the distance from and the uranium content of source rocks 

and the depth and rate of circulation of sea water. 

All these general relations have proved of some use 

in searching for uraniferous shale and in understanding the origin of uranium in shale. However, without 

exception, each generalization is of indefinite value 

because of numerous and major inconsistencies, either 

when applied to many shale formations or when applied quantitatively to a single black shale unit in a 

restricted area.

The problems of determining and quantitatively 

evaluating the sources and mechanisms of enrichment 

of uranium in shale are complex and largely unsolved. 

The complexities probably are the result of failure to 

recognize a combination of several sources, several 

processes of enrichment, and several forms of uranium 

in shale. The scope of the subject makes it impracticable to consider all problems connected with each 

of the many uranif erous shale formations; rather, representative examples and data are selected and discussed to provide a framework of knowledge pertaining to the origin of uranium in black shale and mud.

The following discussion is primarily a description 

of selected examples and data, accompanied by previously proposed interpretations and generalizations on 

uranium concentration. Each of the latter are evaluated critically in light of the information known on 

the examples given, with emphasis on the chemical 

environment of uranium deposition, and a conclusion 

is reached on the possible or probable quantitative importance of each in the origin of uranium in shale. 

The net result is not a series of final and conclusive 

answers, but is a summation of what is known; it can 

serve as basis for designing studies that may yield 

answers to many problems still remaining.
GEOLOGY AND GEOCHEMISTRY OP URANIUM IN MARINE BLACK SHALES 75

EXAMPLES OF URANIUM-BEARING SHALE AND HUD

The origin of uranium in black shale probably can 

best be understood by studying modern sediments accumulating under conditions similar to those under 

which the uraniferous black shale accumulated. A 

second instructive approach involves laboratory experiments designed to duplicate as nearly as possible 

one or more of the probable conditions wherein uranium might have been concentrated in black shale. A 

third approach, which is the one most commonly used 

by the geologist and geochemist, is the detailed study 

of the shale itself, where the origin of the uranium is 

interpreted from the observed and determinable physical and chemical properties of the rock. Comparison 

and integration of the results of all three types of investigation are the basis for this report.

Tables 1 and 2 present in a brief form known chemical data believed to pertain to the concentration of 

uranium in marine black shales and muds. The general purpose of the tables is to allow ready comparison and evaluation of chemical factors that have been 

determined from four main categories of studies: (a) 

uraniferous marine black shale; (b) modern marine 

black muds whose uranium content is known; (c) the 

characteristics of the sea water in which these muds 

were deposited; and (d) relevant laboratory experiments on possible mechanisms of uranium concentration. The chemical constituents and properties given 

in the tables, and which will be discussed sequentially 

in the following pages, are: uranium, organic matter, 

hydrogen sulfide, phosphate, pH (acidity), Eh (oxidation-reduction potential), and salinity.
76 URANIUM IN CARBONACEOUS ROCKS

TABLE 1. Summary of chemical constituents and properties

Uranium (U) in 

Sediments 

(percent)

Waters 

(parts per billion)

Organic matter in 

sediments 

(percent)

Sulfides (H2S, FeS2) in 

Sediments 

(percent)

Waters 

(cu cm per liter)

URANIFEROUS MARINE BLACK SHALE 

Chattanooga shale, upper unit of Gassaway member; central Tennessee; Late Devonian; about 5 ft thick

0.0030-0.0120, over 

1,000 sq mi; 0.0065- 

0.0120, over 100 sq 

mi; 0.0079, average 

over 100 sq mi (modified from Kehn, 1956).

10-28, general range; 

20, general average 

(modified from Bates 

and others, 1956, 

p. 38-40).

Pyrite(FeS2): 7-22, 

general range; 12, 

general average 

(modified from Bates 

and others, 1956, 

p. 38-40).

MODERN MARINE MUDS 

Black muds; Norwegian fjords; 2 in.-2 ft thick

0.0013-0.0060, 10 

samples from 9 fjords; 

0.0036, average 10 

samples (table 3).

1-2, estimate for lower 

saline waters; 0.5-1, 

estimate for upper 

fresh waters (p. 81-82).

6.4 to 39.7, samples 

from 9 fjords (calculated, using 1.7 times 

percent organic carbon) ; 18, estimated 

average (Str0m, 1936, 

p. 61-63).

All black mud samples 

have strong H2S odor; 

>5 percent FeS2 estimate on basis of 

Valnesf jordvatn sample (Str0m, 1936, 

p. 60; 1937, p. 11).

H2S present in bottom 

water of many fjords 

(free oxygen absent) , 

commonly tens of feet 

above bottom; maximum 199.5; 5-10 common (Str0m, 1936, 

p. 52; 1948).

Black and gray muds; Baltic Sea, vicinity of Gotland; 10-16 in. thick

}.00022-0.00103, 10

samples from 3 cores;

0.00048, average 10

samples (table 3) .

1.2-5.9, lower waters;

3.2, estimated average, lower waters.

0.8-2.0, upper waters;

1.4, estimated average, upper waters.

(Koczy, Tomic, and

Hecht, 1957, p. 90.)

8-10, maximum, deep

east of Gotland

(Gripenberg, 1955,

p. 305) ; the "content

of organic matter [in

the samples] is very

high" (Koczy, Tomic,

and Hecht, 1957,

p. 91).

"[The samples] have a

high content of iron

sulfide, are black, and

distinctly smell, often

strongly, of hydrogen

sulfide" (Koczy,

Tomic, and Hecht,

1957, p. 91).

H2S rarely present in

bottom waters (Granquist, 1932; Str0m,

1955, p. 366). Bottom

waters where samples

collected contain 1.03

to 1.50 cu cm oxygen

per liter, which is about

20 to 25 percent of

oxygen in normal sea

water (Koczy, Tomic,

and Hecht, 1957,

table 2).

MODERN OCEANS

0.0001-0.0003, «wh--, 

sediments (Koczy, 

1956, p. 95).

1-3.5, general range; 3, 

average value used in 

this report (p. 81).

<1.0, estimate (Sverdrup, Johnson, and 

Fleming, 1942, 

p. 1008).

H2S very rarely present; 

sediments generally 

oxygenated.

General range of oxygen, 

though highly variable 

and dependent on 

depth, latitude, and 

seasons, is between 4.0 

and 7.0 cu cm per liter.
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 

controlling the concentration of uranium in marine black shales and muds

77

Phosphate (P205) in 

Sediments 

(percent)

Waters 

(mg per cu m)

Acidity 

(pH)

Oxidation-reduction 

potential 

(Eh)

Salinity 

(parts per thousand)

URANIFEROUS MARINE BLACK SHALE-Continued 

Chattanooga shale, upper unit of Gassaway member; central Tennessee; Late Devonian; about 5 ft thick Continued

0.1-0.8, general range; 

0.3, general average 

(Brown, 1956; written 

communication, 

1958).

MODERN MARINE MUDS Continued 

Black muds; Norwegian fjords; 2 in.-2 ft thick Continued

0.14-0.36, 9 black mud 

samples from 7 fjords ; 

0.25, estimated average (Str0m, 1936, 

p. 61-63).

700, maximum in bottom waters; about 

300, average in bottom waters of 9 fjords 

(table 3) (Str0m, 

1936).

6.89-7.48 in bottom 

waters of 9 fjords 

(table 3) ; 7.0, estimate average, 0-10 

feet above black 

muds.

 0.1 0.4, in bottom 

waters, estimate as 

indicated by H2S 

concentration (Str0m, 

1936).

24.63-34.81, in bottom 

waters; 34.40, average 

of bottom waters 

(table 3) (Str0m, 1936)

Black and gray muds; Baltic Sea, vicinity of Gotland; 10-16 in. thick Continued

Not known ___ _ __ 225, maximum in bottom waters of Gotland

deep (Buch, 1932) ;

about 100, estimated

average of bottom

waters.

About 7.0 in deep

waters of Baltic; 8.0

in surface waters

(Segerstrale, 1957,

p. 766).

0.0-+0.1 in bottom

waters, estimate as

indicated by oxygen

concentration (Koczyt

Tomic, and Hecht,

1957, p. 90).

10.65-13.69, in bottom

waters; 11.95, average.

in bottom waters

(table 3).

MODERN OCEANS Continued

Probably <0.2 (Sverdrup, Johnson, and 

Fleming, 1942, 

p. 992).

100, mean for ocean 

water (Smirnov, 1958, 

fig. 1) ; surface waters 

rarely contain more 

than 50. Maximum 

values generally at 

500-1,000 m depth, 

where general range 

is 100-300 (Barnes, 

1957).

About 8.0, in open sea, 

with general range 

7.8-8.4.

+0.1-+0.3, generally 

positive.

34.73, mean surface 

salinity; salinities of 

deep waters slightly less 

(Fleming, 1957, p. 89, 

105).
78 URANIUM IN CARBONACEOUS ROCKS

TABLE 2. laboratory experiments in precipitating and in 

concentrating uranium

Uranium in waters

Concentrations in experiment solutions, usually as uranyl sulfate or uranyl nitrate, generally >200,000 ppb. Garrels 

(1955) and Miller (1958) reviewed the chemistry of uranium 

in natural waters.

Organic matter in sediments

Uranium in solution is sorbed and firmly fixed by organic 

substances.

1. By solid organic matter:

a. Peat and lignite, after being placed in uranyl solution, contained 1-10 percent uranium (Szalay, 

1954).

b. Wood, peat, coal, and other carbonaceous substances 

extracted different amounts of uranium from solution; peat, lignite, and subbituminous coal extracted 98.0-99.9 percent of available uranium 

(Moore, 1954).

c. Subbituminous coal, after being placed In uranyl 

solution, contained 31.8 percent uranium with recognizable uraninite; similar results obtained and 

with degraded spruce wood (I. A. Breger and 

B. T. Moore, written communication, 1955).

d. Samples of carbonaceous shales are capable of irreversibly sorbing uranium from uranium solutions (Tolmachev, 1943).

e. Lignin or humic fractions of peat and lignitic wood 

firmly fix uranium extracted from solution (B. S. 

Barghoorn, written communication, 1956).

f. Lignite sorbed as much as 11.8 percent uranium dioxide from uranyl snlfate solution; maximum 

sorption in solution of pH 5-6 (Rozhkova, and 

others, 1958).

g. Uranium is sorbed from saline uranyl solutions by 

dead micro-organisms (Ewing, Drynan, and 

Gloyna, 1955).

2. By soluble organic substances:

a. Humic acids quickly extract large amounts of uranium from solution by ion exchange at pH 3-7 

(Szalay, 1954, 1958).

b. Humic and fulvic acids readily take up uranium 

from uranyl solutions; amount depends on pH; 

forms organo-metallic complex (Manskaya, Drozdova, and Emelyanova, 1956).

c. Uranium is reduced and precipitated from uranyl 

solutions by organic reagents, for example, ether 

(Gruner, 1952).

d. Soluble humic acid extract from peat withdraws 

uranium from a solution containing only 60 ppb 

uranium (Vine, Swanson, and Bell, 1958).

e. Sorption capacity of humic acid is less than that of 

lignite; but humic acid sorbed 1-2 percent uranium dioxide from uranyl sulfate solution, with 

optimum pH between 5 and 6 (Rozhkova and 

others, 1958).

Sulfides <HSS. FeS2)

Uranium is reduced and precipitated as uraninite or amorphous 

pitchblende from acid uranyl solutions by introducing H2S 

or iron sulfide minerals into the solution. 

1. Pitchblende was dissolved with HNOs and reprecipitated with BUS at temperature of 30° C (Kessler, 

1857).

2. H2S was bubbled through uranyl sulfate solutions to 

saturation and, with ferrous iron as catalyst and 

pH 0.89-2.8, pitchblende precipitated (Gruner, 1952).

3. Pitchblende was precipitated from uranyl solutions 

saturated with H2S; no catalyst necessary (L. J. Miller 

and P. F. Kerr, written communication, 1954).

4. H2S in natural gas is the reducing agent for precipitating insoluble tetravalent uranium compounds 

from uranyl solutions (Colorado School of Mines 

Besearch Foundation, 1957).

5. Pitchblende precipitated from several kinds of uranyl 

solutions by BUS, with varying temperature, pressure, uranium concentration, anion concentration, 

pH, and time; precipitation shown to be likely within 

range of natural conditions (Miller, 1958).

6. Beduction and precipitation of uranium from uranyl 

sulfate and uranyl carbonate solutions by HaS, ferrous iron, and sulfide minerals at different concentrations and under different temperature and pH 

conditions (Bafalsky, 1958).

7. Black pitchblende is precipitated from uranyl solutions 

as crust on surfaces of pyrite and marcasite crystals (Gruner, 1952, p. 17).

8. Most of uranium in uranyl solution reduced and precipitated, probably as hydroxide, in presence of melnikovite; uranium hydroxide subsequently converted 

to uraninite, and melnikovite to its more stable 

form of pyrite (Gruner, Bosenzweig, and Smith, 

1953, p. 9).

Phosphate (PsOs)

Uranium substitutes for calcium in carbonate fluorapatite. 

Experiments resulting in coprecipitation of phosphate and 

uranium in combined form are not known.

1. Phosphate rock immersed in uranyl sulfate solution 

extracted 63 percent of uranium in solution (Moore, 

1954).

2. "Carbonapatite," stirred in sea water enriched in uranium, extracted variable amounts of uranium, in 

some cases more than 50 percent, dependent on 

amount of "carbonapatite," amount of uranium in 

solution, and length of experiment (Bain, 1956, p. 

54-58).

3. Sedimentary phosphorites sorb about 10 times more 

uranium from uranyl solutions than apatite crystals; 

the former contain as much as 3.0 and the latter 

0.28 percent uranium dioxide (Rozhkova and others, 

1958).

Acidity (pH)

Change in pH alone will not effect reduction or precipitation of uranium. Most solutions in experiments cited had 

ph <2.5.

1. Optimum pH for sorption and concentration of uranium by humic acids is between 5 and 7 (Manskaya, 

Drozdova, and Emelyanova, 1956).

2. Uranium is reduced and precipitated by HaS in pH 

range 2-8 (Miller, 1958, p. 530, 537).

3. Maximum uranium sorption by organic substances in 

solutions having pH between 5 and 6 (Bozhkova and 

others, 1958).

Oxidation-reduction potential (Eh)

Oxidizing conditions (positive Eh) probably existed initially 

in the solutions of all experiments cited to left; reducing 

conditions (negative Eh) created in some by addition of
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 79

TABLE 2. Laboratory experiments in precipitating and in concentrating uranium Continued

H2S, iron sulfide, or organic substances to the solution, 

whereupon the uranyl (VI) ions were reduced and precipitated in the uranous (IV) form.

1. See "Sulfides (H2S,FeSs)"above.

2. Reduction and precipitation of uranium in reducing 

environment created in uranyl solution by woody 

material (R. M. Garrels and A. M. Pommer, written communication, 1956).

Salinity

No experiments known where salinity conditions were duplicated to note effect on uranium sorption and precipitation. 

Roshkova and others (1958), p. 426) determined, however, 

that addition of great amounts of sodium, magnesium, and 

calcium ions to uranyl solutions does not change the uranium sorption capacities of organic substances, phosphorites, 

and other materials.

The Chattanooga shale was chosen as an example 

of a uraniferous shale because more data, both chemical and geologic, are available for this shale than for 

any other in the United States. The data are limited 

to that on the upper part of the Gassaway member of 

the Chattanooga shale because the thickness of the 

unit and the chemical data on it are similar to the 

available data on the modern sediments described.

The chemical composition of this part of the Chattanooga shale is:

[Based on analyses of two balk samples from central Tennessee, as determined by 

the Battelle Memorial Institute of Columbus, Ohio, for the U.S. Atomic Energy 

Commission]

Constituent Percent

U_______________ 0.0079

-_--____-_-__ 48.60

_---_-_--._ 10.35

Fe2O3 - 

Ti02 - 

CaO_. 

MgO. 

K2O_. 

Na20.

9.71

.65

.97

1.37

3.68

.36

Constituent Percent

P2O6________.__ 0.38

S (sulfate)____... .23

S (sulfide)_______ 6. 1

S (organic)_______ .87

CO2 _______________ .39

H_______________ 1.6

C (organic) _____ 13. 85

Loss on ignition_ __ 23. 0

Much information is available on the chemistry of 

the sea water of modern basins of deposition. To the 

writer's knowledge, however, the amounts of uranium 

and other chemical constituents of the deposited black 

muds have been determined and the chemical regimen 

of the overlying waters is fairly well known for only 

two areas the fjords of southern Norway and the 

Baltic Sea (table 3). Some data on the Chesapeake 

Bay (Jaffe and Hughes, 1953), on the Atlantic Ocean

TABLE 3. Chemical data on bottom sediment and water in Norwegian fjords and in the Baltic Sea near Gotland

Norwegian fjords and channel

Locality

Indre Topdalsfjord ____________________________

Isefjoerfjord ________________________________ Hellefjord ............................................................

Framvaren ________________________________

Drammensfjord ......................................................

Indre Oksefjord _____________________________

Sediment (Strtfm, 1948)

Depth from 

which sample 

collected 

(meters)

65 

18 

26 

70 

/ 160 

\ 160 

53 

34 

117 

81

Uranium 

(percent)

0.0060 

.0050 

.0050 

.0040 

.0040 

.0035 

.0035 

.0020 

.0015 

.0013

Bottom waters (Strtfm, 1936)

Depth 

(meters)

66 

15 

20 

70

} 160

50 

35 

100 

80 

500

HiS 

(cu cm per 1)

i about 1 

5.42 

7.92 

40.72

1199.5

9.90 

U.6

4.79

8

PiO$

(mg per cu m)

60 

120 

362.5 

700

>217

530 

123 

515 

73 

30

PH

7.48 

6.96 

7.05 

6.89

6.90

7.04 

7.34

7.08 

7.46 

7.97

Salinity 

(parts per 

thousand)

34.40 

33.08 

31.15 

33.26

24.63

32.43 

34.20 

29.87 

34.25 

35.21

Baltic Sea

[Compiled from Koczy, Tomic, and Hecht (1957)]

Locality

Lat 5S°35' N., long 18°13' ^......... ..................... .... ... 

Lat 57°46' N., long 17°31' E... ..-.-....-....-.................

Lat 57°20' N., long 19°64' E..._._ ___ .. __ . _ ..... _ ... ____ .

Sediment

Depth from 

which sample 

collected 

(meters)

445

135

215

Sample 

interval, from 

sediment 

surface 

(centimeters)

0-1 

1-3

8-10 

38-40 0-2 

8-10 

24-26 0-2 

8-10 

24-26

Uranium 

(percent)

0.00051 

.00060 

.00038 

.00022 

.00046 

.00036 

.00026 

.00044 

.00103 

.00056

Bottom waters

Depth

(meters)

/430 

\455

127 

200

Uranium 

(parts per 

billion)

1.3

4.0

1.2 

5.9

Oxygen 

(ml per 1)

1.21 

1.50

1.03 

1.43

Salinity 

(parts per 

thousand)

11.40 

11.67

10.65 

13.69

i Str0m (1948).

* Contains 3.2 cu cm oxygen per liter (Strtfm, 1948).

» Contains 7.04 cu cm oxygen per liter (Strtfm, 1948).

596549 61 3
80 URANIUM IN CARBONACEOUS ROCKS

(for example, Urry and Piggot, 1941, and Holland 

and Kulp, 1954), and on some seas and lakes in and 

near the U.S.S.K. (for example, Kurbatov, 1936, and 

Kurbatov and Ermolaev, 1937) relate radioactivity to 

types of bottom sediment, but these studies are not 

strictly applicable for comparative purposes, either 

because they do not concern black muds or because of 

the paucity of specific information. The many reports 

on conditions of deposition of modern black muds are 

not reviewed here because no reference is made in them 

to the uranium content of the muds.

Str0m (1936) described excellently and in detail 

the bottom muds and waters of some thirty fjords on 

the coast of Norway. In a later note (1948) he gave 

the uranium content of the black mud and the amount 

of hydrogen sulfide in the overlying waters of nine 

of these fjords. In preparing the material for tables 

1 and 3, it was necessary to combine some not entirely 

analogous descriptions and chemical data from the 

two papers (table 3). For example, the samples for 

which the uranium data are given are not the same 

samples for which analyses for organic matter and 

phosphate are given. For this reason, properties and 

constituents of the muds and waters are compared and 

related only in a semiquantitative manner.

The descriptive and analytical data on the sediment 

and water of the Baltic Sea (tables 1, 3) are primarily from a report by Koczy, Tomic, and Hecht (1957). 

Some of the data are from other reports, but because 

these data represent samples from different localities, 

they are used to indicate only in a general way the 

characteristics of the sediment and water in the southcentral part of the Baltic Sea in the vicinity of the 

island of Gotland.

The chemical characteristics of modern ocean water and sediment are shown in table 1 to enable the 

reader to contrast some of the abnormal characteristics of the bottom sediment and water of the 

fjords and the Baltic Sea with those of common ventilated sea water. The chemical composition of sea 

water and the chemical reactions postulated as taking 

place in sea water are grossly oversimplified in this 

report; but, as they affect the distribution and deposition of uranium, they are believed consistent with 

what is now known. Hutchinson's (1957) review of 

the complex interrelations among the chemical variables such as hydrogen sulfide, phosphate, pH, and 

Eh, concerns lake waters, but is applicable to many 

parts of the following discussions, and it has been 

used as a general reference.

Table 2 is a compilation of reported results of experiments on precipitating or concentrating uranium 

in the laboratory as they may be relevant to each of

the several chemical constituents and properties listed. 

The aqueous solutions in nearly all the laboratory 

experiments cited had uranium contents considerably 

greater than that of "average" sea water and of the 

water of the Baltic Sea and Norwegian fjords. The 

feasibility of uranium precipitation or sorption from 

natural waters by the mechanisms indicated in subsequent pages is, therefore, somewhat conjectural; but, 

until proved otherwise, these mechanisms are assumed 

to be effective in concentrating uranium from extremely dilute uranium solutions such as sea water, 

given enough time and an ample supply of water.

SOURCES OF URANIUM

The marine muds and black shales discussed in this 

report contain from 0.0002 to 0.0120 percent uranium.

The chemical and mineralogical form of the uranium in modern muds and in black shales is not 

known except that most of it is in a reduced or tetravalent state. Detailed studies of the Chattanooga 

shale were made by several laboratory groups in an 

attempt to determine the chemical and mineralogic 

associations of the uranium in the shale. Bates and 

Strahl (1957, p. 1311), after studies extending over 

several years and utilizing numerous chemical and 

mineralogic techniques, stated: "No evidence of a uranium mineral, even of 'electron microscopic' dimensions, has been observed, and the manner in which the 

uranium atoms are tied to other components in the 

rock has not been determined." This statement, in 

essence, summarizes present knowledge on the chemical and mineralogic state of uranium in the shale.

Microautoradiographs show that the uranium in marine black shales is thoroughly disseminated through 

the rock. The regional distribution of the uranium 

also is remarkably uniform. Uranium content of 

about 0.0090 percent was consistently found in 47 

samples collected over a distance of 1 mile in central 

Tennessee from a bed of Chattanooga shale less than 1 

inch thick. The lateral changes in uranium content 

are very small over hundreds, even thousands, of 

square miles of Chattanooga shale units several feet 

thick. This uniformity in uranium content and the 

high radioactivity as compared to other sedimentary 

rocks are the reasons why the Chattanooga and most 

other black shales are used as "marker beds" in interpreting subsurface stratigraphy over large areas by 

gamma-ray logs of wells. These observations support 

the now-unquestioned conclusions that the uranium in 

marine black shales was deposited at, or nearly at, the 

same time as the enclosing sediment and that the only 

logical source of this uranium was the overlying blanket of sea water. There is little doubt that most of
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 81

the uranium in the muds of the Norwegian fjords and 

the Baltic Sea also is derived from the overlying 

water.

The conclusion that uranium in shale was derived 

from sea water has been the basis for a large variety 

of interpretations to explain the origin of uranium 

and the differences in uranium contents of black 

shales. Evidence supporting these interpretations 

ranges from specific and definitive to very indirect and 

ambiguous. The remainder of this report is devoted 

to an evaluation of these interpretations and the evidence for each. The general starting point for this 

evaluation is our knowledge concerning uranium in 

sea water.

SOURCES AND CHEMICAL FORM OF URANIUM IN SEA

WATER

The major sources of uranium in sea water are the 

igneous, metamorphic, and sedimentary rocks that release their uranium to surface and ground waters as 

they are decomposed and disintegrated by weathering 

processes. Much of the uranium is dissolved in the 

ground and surface waters and is then transported to 

the sea in streams and rivers. An average content of 

these streams, both in the past and at present, may be 

assumed to be less than 1 ppb (part per billion), or 

less than 0.0000001 percent uranium, on the basis of 

studies of modern streams by Fix (1956, p. 669), though 

Koczy (1956, p. 101) gives an average value of 1±0.5 

ppb uranium in river waters.

Some uranium is transported to the sea as a part of 

solid mineral particles such as zircon and sphene, 

which are highly resistant to chemical and mechanical breakdown. These heavy minerals generally are 

concentrated in the coarse clastic material of fluviatile 

and near-shore marine environments. They make up 

only a small fraction of 1 percent of most marine 

shales, and because they generally contain 0.05 percent 

or less uranium, these heavy minerals have less than 

1 ppm or less than 0.0001 percent uranium in the 

total volume of the Chattanooga shale; they probably 

contribute a similar small amount in the modern muds. 

The main mass of clastic minerals transported as solid 

particles to the sea, exclusive of the aforementioned 

resistates, generally contains so little uranium (1 or 2 

ppm) as to act as the major diluent in any uraniferous 

black mud accumulation.

The uranium dissolved in sea water may be present 

in several forms, depending mainly on the pH and 

Eh of the water (Garrels, 1955). In the normal 

slightly alkaline and oxidizing environment of sea 

water (pH about 8.0, Eh about +0.1), practically all 

the uranium is in the hexavalent, or uranyl, form and is 

combined as carbonate, sulfate, and several other complex ions. These ions are highly soluble and consequently are widely distributed in sea water.

URANIUM CONTENT OF MODERN SEA WATER

The range in uranium content of modern ocean water is small, from about 0.1 to 5.9 ppb (Karlik, 1939; 

Bona and Urry, 1952; Stewart and Bentley, 1954; 

Koczy, 1956; Eona, Gilpatrick, and Je 'rey, 1956; 

Koczy, Tomic, and Hecht, 1957); the C( intent is in 

general dependent on depth, salinity, aid position 

relative to the mouths of rivers. The average uranium content of sea water of normal salnity in the 

oceans today is 1.3 ppb according to Koczy (1954,

p. 122); 2 ppb (Baranov, Bonov, and £unashova,

1957, p. 3); 2±1 ppb as more recently proposed by 

Koczy (1956, p. 95); 2.8 ppb (Holland and Kulp, 

1954, p. 198); and 3.3 ppb as determine by Eona, 

Gilpatrick, and Jeffrey (1956, p. 700). FDr purposes 

of discussion in this paper, 3 ppb of iranium, or 

0.0000003 percent, will be used for the average amount 

of uranium in sea water. 

The uranium content of samples of water from 7

stations in the Baltic Sea ranges from 0.7 

(Koczy, Tomic, and Hecht, 1957, p. 86); 

content is between 1 and 2 ppb uranium.

of high salinity flowing into the Baltic froi i the North

Sea contains 1.8 ppb uranium, while the 

saline water that flows out of the Baltic 

ppb. 

The uranium content of water in the

fjords is not known. Estimates of the uranium content of this water can be made, however, 

data on samples from Gullmarfjord on the 

Sweden (Hernegger and Karlik, 1935, p. 

1939, p. 11), from the Skagerrak (Karlik, 

Koczy, Tomic, and Hecht, 1957, table 2),' 

body of marine water separating southe;

way, southwestern Sweden, and Denmark 

rivers emptying into the Skagerrak and 

Sea (Koczy, Tomic, and Hecht, 1957, tab 

94).

The hydrography and sediments of G illmarfjord 

(lat 58°19' N.; long 11°33' E.) probably are similar 

to the cited Norwegian fjords for, as shown by available maps (Charts 72 and 73, 1:50,000 scale, Hydroand Naviis compaarea, and

other geographic features. It has a maxinum depth 

of 119 meters (390 feet), but if a threshold is present 

at the mouth of Gullmarfjord, it is ill

graphic Dept., Eoyal Board of Shipping 

gation, Stockholm, Sweden), Gullmarfjon 

rable in general to other fjords in shape

exceeds 100 feet in depth in places. The w 

fjord contain 0.36 ppb uranium at the surf

to 5.9 ppb 

he average 

The water

upper, less 

ontains 0.9

Norwegian

y utilizing

est coast of

13; Karlik,

1939, p. 11;

hich is the

stern Norand from 

the Baltic 

e 1, p. 86,

efined and 

ters in this 

ce, 1.1 ppb
82 URANIUM IN CARBONACEOUS ROCKS

at % depth of 60 meters (196.8 feet), and 1.5 ppb at a 

depth of 100 meters (328 feet).

The Skagerrak water, which is the sea water entering and making up the saline bottom water of 

Gullmarfjord and most of the Norwegian fjords discussed here, contains only about 0.45 ppb uranium at a 

depth of 5 meters (16.4 feet) according to Karlik 

(1939); the water contains 1.5 ppb at a depth of 15 

meters (49.2 feet) and 1.2 ppb at a depth of 120 meters 

(393.6 feet) according to Koczy, Tomic, and Hecht 

(1957); and 1.2 and 1.5 ppb of uranium for depths of 

400 meters (1,312 feet) and 500 meters (1,640 feet), 

respectively, according to Karlik (1939). From these 

analyses it is reasonable to assume that the uranium 

content of the saline water entering the Norwegian 

fjords is between 1 and 2 ppb.

The uranium content of rivers flowing into and 

making up much of the surface water in the Norwegian fjords may be comparable to the uranium content of nine rivers draining regions of granitic and 

gneissic rocks and emptying into the Baltic Sea and 

the Skagerrak. These rivers contain from 0.2 to 0.7 

ppb uranium, and average 0.5 ppb (Koczy, Tomic, and 

Hecht, 1957, table 1, p. 86, 94). The Orekils Kiver, 

which empties into Gullmarfjord, contains 0.5 ppb 

uranium and is responsible for the low uranium content of the surface water (see above) of Gullmarfjord. 

Most of the rivers flowing into the Norwegian fjords 

drain similar terranes (Str0m, 1936, p. 17-20) and 

hence may be assumed to contain a similar amount of 

uranium, about 0.5 ppb.

On the basis of the data and comparisons presented 

above, the waters of the Norwegian fjords are estimated to contain between 0.5 and 2 ppb of uranium, 

the smaller value being more typical of the upper 

water, and the larger value typical of the saline water entering and occupying the zone of deeper water 

in the fjords.

Uranium is measured as only a fraction of one to a 

few parts per billion in sea water and a few tens to a 

few hundreds of parts per million in uraniferous mud 

and shale. The overall problem to be evaluated and 

solved is how this trace amount of uranium in sea 

water is withdrawn and concentrated in sediment to 

about 10,000 to 100,000 times the amount in sea water.

URANIUM CONTENT OF WATER IN ANCIENT SEAS

No method has been developed for measuring directly the amount of uranium that existed in the sea 

water in which ancient sediments were deposited. Consequently, interpretations of the uranium content of 

this water must be based on a general evaluation of 

changes in sea water composition in the geologic past,

with particular reference to the measurable chemical 

and geologic characteristics of the sediments themselves.

One line of reasoning that has been used (for example, Breger and Deul, 1956, p. 508; Brown, 1956, 

p. 461-462) in attempting to explain differences in 

uranium content among marine sediments is that the 

amounts of uranium available for withdrawal from 

sea water differed at the times the different sedimentary units were deposited. Though not referred to in 

any previous reports, an example of this reasoning 

would be a comparison of the alum shale of southern 

Sweden, which contains 0.024 percent uranium, and 

the Chattanooga shale of the Southeastern United 

States, which contains 0.006 percent uranium; the inference would be that, other factors being equal, the 

Late Cambrian sea of Sweden contained four times 

more uranium than the Late Devonian sea of the 

United States.

The point of reference in attempting to determine 

the uranium content of ancient seas is the uranium 

content of modern sea water, here given as 3 ppb. 

The first question that might be asked is, did "average" 

sea water in the geologic past contain more than 3 

ppb uranium? Holland and Kulp (1954, p. 204), in 

their analysis of the history of uranium and other 

radioactive elements from source to final deposition, 

concluded that the concentration of uranium in ocean 

water has not changed greatly since Paleozoic time. 

This conclusion is in line with those of Rubey (1951, 

p. 1114), Vinogradov (1953, p. 569), and Redfield 

(1958, p. 220) who all believed that the overall composition of sea water has varied very little since early 

geologic time.

Koczy (1954, p. 121; 1956, p. 98) thought that the 

uranium content of sea water may have varied in the 

geologic past because the chemical and biologic processes favoring uranium deposition in certain ocean 

areas probably changed considerably from one geologic period to the next; he (1956, p. 98) speculated 

that the concentration of uranium in sea water may 

have varied by a factor of 3. The objection to this 

interpretation is that the relative volumes of water 

that may have been wholly or partly depleted of their 

uranium were very small as compared to the total 

volume of sea water. Even if all the uranium was 

withdrawn from the water in a widespread epicontinental sea spanning most of a continent, the net loss 

of uranium in the total volume of the ocean would be 

insignificant. Add to this hypothesis the factor of 

time, because it presumably took millions of years for 

abnormally uraniferous sediments to accumulate, and
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES

the net change in uranium content of the ocean is seen 

to be probably negligible.

An argument that could be used to support the interpretation that ancient sea water probably contained 

more uranium than modern sea water is that, of the 

many marine shales that have been tested for uranium, the most uraniferous shales are of pre-Mesozoic age (Russell, 1945, p. 1486; Bell, 1954, p. 108; 

Nininger, 1954, p. 75; and others). The present author submits, however, that data are too few and too 

varied to permit a valid conclusion on the relation of 

geologic age to uranium content of black shale.

The results of the comprehensive geochemical study 

of more than 10,000 samples of the sedimentary rocks 

of the Russian platform, conducted by the Vernadskii 

Institute in Russia, show that the distribution of uranium is markedly uniform in clay and carbonate rocks 

of Cambrian to Recent age (Baranov, Ronov, and 

Kunashova, 1957, table 1). An objective of a chemical 

study on Precambrian sedimentary rocks of Canada 

(1958, p. 83) was to determine if there are any variations in trace element content with age; on comparing 

the content of seven trace elements, including vanadium, nickel, copper, and manganese, in these rocks 

with published analyses of many younger marine argillaceous rocks, he concluded that no significant variations can be established in the amount of trace elements in sediments with relation to the age of the 

sediment. The explanation for the greater abundance 

of uraniferous shale in formations of pre-Mesozoic 

age more likely rests with specific circumstances of 

tectonics, rates of sediment deposition, chemical environments, and probably the proportions and types 

of organic material rather than with a hypothetical 

decrease of uranium content of sea water with younger 

geologic age.

From this very brief review, it can be seen that any 

statement made to the effect that "average" water in 

ancient seas contained more or less uranium than 

present sea water must be considered only as a speculation which at present is insupportable and unwarranted. It seems best to assume, therefore, that "average" sea water in the geologic past probably contained 

about the same amount of uranium as sea water today, 

3 ppb.

A second question that should be considered is 

whether certain types or parts of seas might have 

contained more or less uranium than "average" sea 

water. An arm or embayment of an epicontinental 

sea theoretically could receive river water containing 

abnormally large amounts of uranium, and, because 

of partly restricted circulation with the open ocean, 

might have water of high uranium content. Seas with

are is

restricted circulation are favorable for 

accumulation (Str0m, 1955, p. 366) and 

rich in uranium might be expected to ac 

this type of sea.

The Baltic Sea is a modern example 

restricted epicontinental sea having 

mud deposition, and uranium analyses o 

and mud data are available. Koczy, Tomk 

(1957, p. 88) reported analyses of the 

rivers entering the Baltic Sea; the waters 

but one contain less than 1.5 ppb uranium, 

of nine contain about 0.5 ppb uranium, 

uranium content of the water in the Bal 

is between 1 and 2 ppb, so this epicontine] 

the water flowing into it actually contain 

than "average" sea water.

The uranium contents of other modern 

tal seas wherein organically rich muds p 

being deposited, such as the Yellow Sea ! 

Bay, are not known, so the validity of th 

ancient seas with restricted circulation 

more or less uranium than "average" sea 

be judged solely on the Baltic Sea data, 

the waters of the Adriatic Sea (Karlik, 

and of the Gulf of Mexico (Rona, 

Jeffrey, 1956, p. 699) indicate that the: 

water, which are largely surrounded by 

well-circulated water, have about the 

content as the open ocean.

83

black mud 

black mud 

cumulate in

INFLUENCE OF SOURCE BOCK ON URANIUM CONTENT 

OF WATER, IN BORDERING SEAS

The type of rock making up the drain 

a marine basin of deposition has been 

be of importance in determining the 

nium delivered to a sea and subsequent] 

in the marine muds and shales. As a resul 

of the uranium content of black muds in 

fjords, Str0m (1948) concluded: "There 

definite connection between drainage 

areas and uranium content in the black 

being most conspicuous with the record 

per ton [0.0060 percent] in Topdalsfjord, 

empties a large river draining extended gr 

Brown (1956, p. 461^62) suggested that 

landmass of Appalachia could have 

than average amounts of uranium to the 

the Chattanooga shale was deposited. GL 

considered the source of the uranium in 

nema shales of Russia and Estonia to be 

marginal to the sea in which these sha 

lated; and Joseph Eklund (cited in 

hart, and Garrels, 1955, p. 517) subscribed 

granitic-terrane theory for the source of

of a partly 

of black 

the water 

and Hecht 

v raters of 11 

of all rivers 

and those 

The average 

ic probably 

tal sea and 

less uranium

picontinenrobably are 

nd Hudson 

concept of 

t lat contain 

water must 

Analyses of 

939, p. 11) 

Giliatrick, and 

bodies of 

la id but have 

sane uranium

ige area of 

ccnsidered to 

amount of uradeposited 

, of a study 

Norwegian 

seems to be a 

fr 3m granite 

muds, this 

value of 60 g 

into which 

nite areas."

he granitic 

proviled greater 

sua in which 

bov (1941) 

;he Dictyothe granite 

es accumuMcKelvey, Everx> a similar 

uranium in
URANIUM IN CARBONACEOUS ROCKS

the Cambrian alum shales of Sweden. McKelvey 

(1955, p. 15), in a list of distinguishing characteristics 

of uraniferous shales, included bordering granitic 

terranes.

Granitic terranes may well be the source of the 

sediments for the black muds and shales mentioned, 

but the implication that granitic terranes provided an 

abnormally large supply of uranium to the seas and 

that the subsequent concentration of uranium in the 

black muds is thus explained, is certainly open to 

question. The basis for the implied relation is that, 

of the common rock types, granite contains the highest average uranium content, 0.0004 percent (Holland 

and Kulp, 1954, p. 203).

The sequence of reasoning in the interpretation that 

the high uranium content of a shale is due to the 

relatively high uranium content of the granite which 

provided the sediment for this shale is apparently as 

follows: (a) If the uranium content of the black shale 

is high, the uranium content of the sea water in which 

the shale was deposited was high; (b) if the uranium 

content of the sea water was high, the uranium content of the streams emptying into the sea was high;

(c) if the streams had a high uranium content, the 

rocks being eroded had a high uranium content;

(d) of common rock types, granite has the highest 

average uranium content. In order to ascribe any 

validity to the interpretation that the uranium content of a shale is related to the uranium content of 

the source rock, each step in this sequence should be 

logical in itself and supported by some evidence.

The logic and evidence for relatively high uranium 

content in the water of ancient seas have already been 

shown to be highly dubious and implausible. From 

the thousands of analyses of surface waters, collected 

from all types of drainage areas in the United States, 

no relation between type of terrane and amount of 

uranium in the river waters has been noted (P. F. Fix, 

oral communication, 1957). Waters draining granite 

areas have no more uranium than the average of all 

stream waters (less than 1 ppb); because of the low 

permeability of most granites (and particularly if 

mechanical disintegration is the main weathering 

process), these waters actually may carry less uranium. 

The results of a study of the uranium contents of 

12 rivers emptying into the Baltic Sea and Skagerrak 

(Koczy, Tomic, and Hecht, 1957, p. 86) show that the 

rivers draining areas of igneous rock have an average 

of 0.5 ppb uranium, whereas rivers draining regions 

of sedimentary rock contain 2 or 3 times this amount. 

Hecht, Kupper, and Petrascheck (1958), in their study 

of creeks and rivers in Austria, reported similar observations.

If it were to be interpreted that eroded granite 

provided most of the sediment and the uranium in a 

black shale, it should not be assumed that all the granite's uranium was transplanted directly to the shale. 

It is known that upon weathering granite releases 

only a part of its total uranium to solution, because 

much of the uranium remains stable and fixed in 

highly resistant mineral grains such as zircon, sphene, 

and monazite. Most of these resistant grains are deposited in near-shore sandstones, with only a very 

small fraction reaching areas of black mud deposition. 

Most uraniferous shale contains less than 0.1 percent 

of these minerals, and the minerals in turn generally 

contain less than 0.1 percent uranium, so their contribution to the total uranium content of the uraniferous 

shale is almost negligible. Thus, nearly all the uranium in shale is that which was readily leached from 

granite and taken into solution.

The percent of the total uranium in granite that 

goes into solution upon weathering and that ultimately 

reaches the sea is not known, but laboratory studies 

by G. J. Neuerburg (oral communication, 1957) suggest that it is less, probably considerably less, than 

50 percent. Pulverized samples of several granites 

from the United States yielded an average of about 

20 percent of their total radioactive material when 

leached with dilute nitric acid (L. T. Silver and 

E. L. Kowalkowski, written communication, 1957); 

similar experiments on rhyolitic rocks from northern 

New Mexico resulted in leaching of only 1 to 13 percent of the uranium (E. S. Cannon, Jr., oral communication, 1957). On the other hand, data obtained 

by Larsen and others (1956, p. 67-68) and by Pliler 

and Adams (1959) indicate that about 60 percent of 

the uranium in granitic rocks is leached by acid solutions. Some loss of soluble uranium being inevitable 

between the place of granite erosion and the site of 

shale deposition, it is very doubtful that more than 

25 percent of the total uranium in the granite is incorporated in the shale.

If an abnormally rich source of uranium is sought 

to explain the abnormally high uranium content of 

seas from which the black muds and shales drew their 

uranium, present information suggests that volcanic 

tuffs and other effusive rocks are the rock types that 

have a relatively high uranium content and that readily yield their uranium to surface and ground waters 

on weathering. In some areas these rocks contain an 

average of 0.0015 to 0.0035 percent uranium (Denson, 

Zeller, and Stephens, 1956, p. 680; Baranov, Ronov, 

and Kunashova, 1957, p. 3), and waters draining the 

area may contain several tens of parts per billion of
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES

uranium (Denson, Zeller, and Stephens, 1956, p. 673); 

if such waters drained into a sea, they might notably 

increase the uranium content of that sea.

The presence of a bentonite bed in the Chattanooga 

shale in Tennessee (Brown, 1956, p. 462) and of bentonite beds associated with the Sharon Springs member of the Pierre shale in Nebraska and South Dakota 

(Tourtelot, 1956, p. 78) has been cited as indicating 

the uranium content of the sea waters in which these 

beds were deposited was increased by volcanic ash 

falling directly into the waters. Whether volcanicash source rocks existed during times of black shale 

deposition or whether the volcanic ash that fell into 

black shale seas originally contained large amounts of 

uranium is unknown; these questions must be held as 

points of speculation at present.

From this evaluation, it is concluded that no evidence exists to support the oft-cited idea that granite, 

as a source of sediment, is also an abnormally rich 

source of uranium that can be dissolved and transported to the sea. On the other hand, studies on the

Pfatform

stratigraphy and paleogeography of the 

shales and muds indicate that these sedinn 

were derived from granitic rocks, and it 

the uranium in them was also derived from 

ite. The granitic rocks probably were not 

rich sources of soluble uranium; but, when 

proper perspective, they provide an ampl 

the uranium in the beds derived from theia 

the black shales. Figure 23 illustrates a 

and highly generalized, but nevertheless 

rangement of uranium distribution in tie 

derived from a granitic terrane. As visua 

restored section of a body of rocks depo 

a limited period of time, the greater part

85

cited black 

ntary rocks 

follows that 

the granabnormally 

viewed in 

supply for 

, including 

ypothetical 

Dossible, arsediment 

ized in this 

ited during 

of the uranium in the geosynclinal sedimentary b ids is that

originally retained in the resistant mineral grains, 

such as zircon and monazite, which were transported 

as suspended particles in the water. The major part 

of the uranium in the sediment deposited Dn the platform was derived from sea water by precipitation or 

sorption of the uranium in solution.

Geosyncfine Source area

Granite eroded, N i >ris, 

^<>',0;0004 percent

Uraniferous black 

shale, 2 percent 

of N tons. 0.0050 

percent U

Predominantly carbonaceous 

shale, 8 percent of /Vtons, 

0.0008 percent U

Sea water

Predominantly shale and 

siltstone, noncarbonaceous, 55 percent of N 

tons, 0.0003 percent U

100 50 

I _I

Predominantly 

sandstone, 35 

percent of N 

tons, 0.0002 

percent U

200 MILES

FIGURE 23. Cross section showing hypothetical relation of volume (shown graphically) and weight (N tons) of granite removed from 

source area to volume and weight of sediment deposited; the uranium content of the original rock and the possible resulting balance 

In uranium content of the sediment are Indicated. The average uranium contents shown are only approximate, based in part on data 

from Holland and Kulp (1954, table 3).

SUMMARY OF URANIUM SOURCES

Sea water is recognized as the major source of uranium in marine black mud and shale. But the importance of differences in the uranium content of ancient 

sea water in determining the uranium content of black 

shale has received undue emphasis. No reliable evidence or interpretations can be cited to support the 

belief that ancient seas contained significantly more 

uranium than the 3 ppb that modern seas have; nor 

can it be substantiated that eroded granite supplied 

abnormally large amounts of soluble uranium to seas,

as maintained by some geologists to 

uranium content of some black shale and 

The emphasis on the concentration of 

ancient seas has been misplaced. Whethe 

contained 1 or 10 ppb, they still were ex 

solutions of uranium. Of far greater 

the fact that, at either concentration, sea 

and is a vast and ample reservoir of 

that sea water did and does serve as a 

medium for rapidly transporting and 

tributing uranium.

higlly

-15,000

-10,000

-5000

- SEA LEVEL

-5000

-10,000

explain the high 

mud.

iranium in 

these seas 

tremely dilute 

importance is 

water was 

and

effective 

widely disurimum,
86 URANIUM IN CARBONACEOUS ROCKS

MECHANISMS OF ENRICHMENT 

ORGANIC MATTER

Organic matter, either directly or indirectly, is the 

most important constituent in the geochemical processes of concentrating uranium from sea water in 

black mud and shale. If uranium is present in abnormally large amounts in these deposits, the organic 

matter is directly or indirectly responsible; on the 

other hand, the presence of organic matter in either 

modern or ancient sediments does not indicate necessarily that uranium is present in large concentrations.

The shale and the modern sediments cited in table 1 

all have more than 5 percent organic matter. The 

data on these deposits do not permit an estimation of 

the minimum amount of organic matter necessary to 

bring about a concentration of uranium; nor do they 

in themselves provide firm basis for postulation of a 

quantitative relation of organic matter to the abundance of uranium that would lead to a clear understanding of the genetic significance of organic matter. 

Neither does the available information quantitatively 

differentiate the kinds and types of organic matter 

present in the shale and mud. For these reasons, the 

relation of uranium to organic matter in these deposits necessarily must be generalized.

Ample evidence is available from the laboratory 

experiments cited in table 2, however, to indicate that 

there can be a close and, in some cases, a direct tie 

between uranium and several kinds of organic matter.

AMOUNTS AND TYPES IN URANIUM-BEARING SHALE AND MTTD

Most of the organic matter in the Chattanooga shale 

is paleontologically unidentifiable; woody tissues of 

plants, now coalified (Breger and Schopf, 1955), and 

several types of spores and algae have been identified. 

The uppermost unit of the Chattanooga shale, which 

is used as the shale example in this discussion, contains 20 percent organic matter. Kinney and Schwartz 

(1957, p. 1125, table 1) stated that this organic matter is similar to low-rank coals, the chemically separated organic matter having the composition on a 

percent-by-weight ash-free basis of:

Carbon _________________________ 64.4 

Hydrogen _______________________ 5.3 

Oxygen (by difference, and 

including sulfur and nitrogen) _________ 30.3

According to Deul (1957, p. 215-216), the organic 

matter in the Chattanooga shale has a composition 

close to that of a subbituminous coal; on a percentby-weight ash-free basis, calculated from analysis of 

the shale itself, it has this composition:

Carbon _________________________ 78.3 

Hydrogen ________________________ 6.9

Sulfur __________________________ 2.9

Nitrogen _________________________ 2.3

Oxygen ____________________ 9.7

One of the major conclusions from the studies of 

the Chattanooga shale at Pennsylvania State University (Bates and Strahl, 1956, 1958; Bates and others, 

1956) is that of all the constituents of the shale that 

were considered, such as carbon, pyrite, silicates, and 

numerous trace metals, only the carbon shows a constant, significantly positive coefficient of correlation 

(about 0.7) with the uranium. The genetic and precise chemical relations of the uranium to the organic 

matter, which this carbon represents, are not obvious 

from these studies.

Swanson (1960, p. 28) concluded that organic matter of the woody or humic type and the soluble organic 

substance derived therefrom are the mechanisms of 

concentration of a major fraction of the uranium in 

the Chattanooga shale. This conclusion was based on 

a study of the relation of the oil yield to the uranium 

content of the Chattanooga shale and on a comparison 

of the uranium contents of samples of identified algae 

(Foerstia sp.) and of coalified wood (Calliwylon sp.), 

as determined by J. M. Schopf (written communication, 1953) and Breger and Schopf (1955). In this 

study by Swanson, two main types of organic matter 

were distinguished the sapropelic type, derived from 

algae, spores, pollens, resins, cuticles, and chemically 

analogous plant and animal remains; and the humic 

type, derived from cellulose, lignin, and analogous 

woody parts of plants. Both types are present in 

most black shale. The sapropelic type of organic 

matter yields 4 to 5 times more oil on distillation than 

does the humic, whereas the humic type contains, or 

indirectly induces the precipitation of, most of the 

uranium in black shale.

The amounts of organic matter in the samples of 

the Norwegian fjord muds that were analyzed for 

uranium (Str0m, 1948) are not known; however, the 

total organic matter in these samples is probably between 10 and 40 percent by dry weight, based on 

analyses of "black, flocculent" muds from 8 fjords 

cited by Str0m (1936, p. 61-62). Wood fibers, saw 

dust, pine pollen, and fragments of sphagnum moss 

are the types of plant material in the black mud 

accumulating in the Norwegian fjords (Str0m, 1936, 

p. 60-65). The amounts of each type in the samples 

are not given, but woody material, or the humic type 

of organic matter, is certainly present; and, as suggested by the abundance of the decay product hydrogen sulfide, the relatively low pH, and the reducing 

conditions in the overlying waters, the humic type of 

organic matter probably predominates.
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 87

The amounts and types of organic matter in the 

Baltic Sea samples described by Koczy, Tomic, and 

Hecht (1957) are not known, but these authors concluded (p. 86) that the uranium is complexed by the 

organic materials in the water to form insoluble compounds which settle to the bottom. Samples of mud 

collected in 1930 (Gripenberg, 1934, p. 119; 1955) 

from the Gotland deep, only a few miles from the 

place where the most uraniferous sediment reported 

by Koczy, Tomic, and Hecht (1957, table 3) was collected, contain the most organic matter (6.7 to 9.9 

percent of the dry mud) of any of the samples from 

more than 60 stations in the Baltic. Most of the 

modern muds in the Baltic contain organic matter in 

the 3 to 4 percent range (Gripenberg, 1934, p. 137). 

Gripenberg (1934, p. 22, 144, 159) repeatedly stated 

that the organic matter in the Baltic sediments is 

largely "land humus," or plant material of terrestrial 

origin; she further described most of this organic 

matter as colloidal and as having been precipitated 

from the sea water to form a gelatinous-appearing 

black mud (p. 16^165, 217). Segerstrale (1957, p. 

766) described the abundant "humus compounds" 

transported into the northern Baltic Sea from the 

large peat bog districts, mainly in Finland, which in 

places give the sea a brown color. These general descriptions suggest that humic substances, possibly 

flocculated humic acids in part, may form some of the 

organic matter in the Baltic Sea sediments; but the 

types of organic matter and the relations of each 

to uranium in the Baltic Sea muds are still to be 

investigated.

The applicability of any of the cited experimental 

results (table 2) in defining the relation of the organic matter to the uranium in the modern black 

muds or in the shales must await the careful separation and the accurate identification and classification 

of the types of organic matter, with uranium determinations made on each type. Any discussion of the 

possible relation of uranium to types of organic matter in sediments, as indicated by the experiments, 

therefore must be considered at present as largely 

subjective and interpretative.

SOLID ORGANIC MATTER

The fact that uranium can be sorbed from a uranium 

solution and irreversibly fixed by solid organic matter, 

particularly wood, peat, and coal, has been demonstrated many times in the laboratory (for example. 

Moore, 1954; Szalay, 1954; Kozhkova and others, 

1958; see also table 2). Natural concentrations of 

uranium in decaying plant substances from surface 

water having a high uranium content have been recorded from alpine meadows (Swanson and Vine,

69654S 61 4

1958). The manner of chemical combination of organic matter and uranium is not known, but probably is by ion exchange or formation of a metalorganic compound. Fragments of coalified drift wood, 

(7atti%ylony in the Chattanooga shale are the most 

uraniferous constituents (with as much as 0.035 percent uranium) of this rock, On the basis of this experimental and geologic evidence, it is suspected that 

some of the woody fibers and sawdust particles in the 

fjord mud contain more uranium than the sediment 

as a whole and may account for a significant fraction 

of the uranium in this mud.

HUMIC ACIDS

The soluble organic substances that are extracted 

by slightly alkaline waters from decaying humic plant 

matter such as peat generally are referred to as humic 

acids. These colloidal or soluble humic substances constitute most of the organic matter in the water of 

many lakes (Hutchinson, 1957, p. 901-902), and, 

though not so well studied, at least some of the organic 

matter in marine sediments (Waksman, 1936, p. 295- 

301). For example, Waksman (1933, p. 136), using a 

dilute alkaline solution, extracted 70 percent of the 

total organic matter from a sample of marine mud 

from Buzzards Bay, in southeastern Massachusetts.

Humic acids have been shown experimentally to be 

effective in withdrawing and firmly attaching uranium 

from solution (Szalay, 1954; Manskaya, Drozdova, 

and Emeliyanova, 1956); Szalay (1958) concluded 

that the process is one of ion exchange, where uranium 

ions displace the hydrogen ions of the organic substance. The uranium-bearing humic acids are flocculated as a dark-brown gel when the pH of the solution 

is lowered to the acid side or when divalent cations 

such as calcium are added. These flocculated humic 

acids may contain several percent uranium, dry weight 

(Szalay, 1954 and 1958), and their counterpart in 

nature may account for a large part of the uranium 

in some black mud and shale deposits (Beers, 1945, 

p. 15; Vine, Swanson, and Bell, 1958, p. 190). The 

importance of this mechanism of uranium concentration could easily be demonstrated by extracting the 

alkaline-soluble organic substance, or humic acids, 

from modern uraniferous black mud such as that in 

the Norwegian fjords, then recoagulating, drying, and 

determining the uranium content of this substance.

The dark-brown to black structureless material 

which constitutes much of the organic matter in marine black shale, as seen in thin sections, may be flocculated humic acids or their salts, which are here 

termed "humates." Just as in coals, these solidified 

humic acids or humates have lost their initial solu-
88 URANIUM IN CARBONACEOUS ROCKS

Humic acids, 

Chattanooga shale 

(percent)

64. 1

2. 5

2.8

2.2

28.4

Humic acids, 

Pittsburgh coal 

(percent)

65.6

2.3

1.0

1.4

29.7

bility characteristics during diagenesis, and they can 

be dissolved in dilute alkaline solutions and extracted 

only after being partially oxidized; the humic acids 

dissolved and extracted from shale in this manner have 

a composition and structure which are probably very 

similar to the original material. The studies on the 

Chattanooga shale by Kinney and Schwartz (1957, 

table 2) show that more than 50 percent of the carbon 

in the shale is in the "regenerated" humic acids extracted from the partially oxidized shale. The ashfree composition of these humic acids from the Chattanooga shale and of the humic acids similarly derived 

from coal are very much alike, as shown by the 

analyses below from Kinney and Schwartz (1957, 

table 6):

Carbon____________

Hydrogen_ _ _ _____

Sulfur___________

Nitrogen___ _ _____

Oxygen__ __________

The humic acids extracted from the Chattanooga 

shale retain a part of the uranium initially in the 

shale (Kinney and Schwartz, 1957, p. 1126), but their 

original uranium content is not known; when the 

abundant pyrite of the shale is oxidized and the soluble 

products are freed, sulfuric acid forms and undoubtedly takes much of the uranium into ionic solution. 

It has been suggested (Swanson, 1960, p. 24) that the 

scattered lenses of kolm in the Upper Cambrian alum 

shale of Sweden are humates because of their high 

uranium content (0.1 to 0.7 percent), their relatively 

low oil yield (a few percent), and their similar appearance to natural and artificial humates.

Dissolved organic matter in marine waters may be 

derived from two major sources. The first is the plant 

and animal life within the sea itself, mainly planktonic organisms such as algae and Protozoa but also 

other planktonic invertebrates. Scavengers or processes of aerobic decomposition eliminate the carbohydrates, proteins, and fatty acids that make up the 

larger part of the dead organisms, but some humic 

acids may form during decomposition, particularly 

from some algae. Humic acids are generally resistant 

to additional bacterial and chemical attack and are 

eventually precipitated, deposited, and incorporated as 

organic matter in the bottom sediment.

The second source is the decomposing plant material on land. The humic acids in shallow continental 

seas derived from decomposed land plants may be 

quantitatively more significant than the humic acids 

from organisms living in the sea, especially in tropical and subtropical climates. The land-derived humic

acids are flushed from peat and soil and are transported by streams into the sea in a colloidal state, 

where they ultimately are precipitated and deposited. 

Waksman (1933, p. 135) stated that commonly 70 to 

75 percent of the total organic matter in soil can be 

removed by dilute alkaline solutions. Clarke (1924, 

p. 110) reported that the percent of organic matter 

in the dissolved solids of 18 rivers in the world ranges 

from 3.25 to 59.90 percent and averages about 20 percent. Using Clarke's data, Corbett (1955, p. 1609) 

estimated that more than 100 million tons of soluble 

organic matter, or humic acid, is brought to the sea 

every year.

Gripenberg (1934, p. 163-164, table 22) estimated the 

amounts of dissolved "humus stuffs" transported annually by rivers into the Baltic Sea and presumably 

deposited there (fig. 24). She estimated that the mean 

organic content of river water, exclusive of suspended 

organic matter, entering the Baltic is 30 grams per 

cubic meter of water, and the mean annual addition 

to the bottom sediment of the Baltic, if this material 

is precipitated and preserved, is 19 grams per square

31 g per m3

18 g per m3

30 g per m

60 g per m3

EXPLANATION

36 g per m3

Mean content of dissolved 

organic matter in river 

waters, in grams per cubic 

meter of water

28 g per m*

Calculated mean annual addition of precipitated organic matter to sea bottom 

sediment, in grams per 

square meter of sea bottom

66.600 km*

Area of sea to which associated figures apply, in 

square kilometers of sea 

surface 

200 MILES

200 KILOMETERS

FIGURE 24. Map indicating the amounts of dissolved organic matter In the river water entering the Baltic Sea area, and the estimated amounts precipitated and deposited annually with the 

bottom sediment. Data from Gripenberg (1934, table 22).
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 89

meter. Evaluation of all the data for the entire Baltic 

Sea area, including the Gulf of Finland and the Gulf 

of Bothnia, gives the estimated mean annual addition 

of precipitated humates of about 33 grams per square 

meter. (For purposes of comparison, Emery and Rittenberg, 1952, p. 776, estimated that between 31 and 

52 g of total organic matter per sq m reach the bottom 

water of the basins off southern California; see also 

Trask, 1955). The actual amount of humates in the 

bottom sediment obviously is dependent on the distribution of these humic acids, on the rates of their biologic and chemical destruction, and on the local rates 

of deposition of clastic sediment; but, at least theoretically, humic material makes up much of the organic matter accumulating on the bottom of large 

parts of the Baltic Sea area.

A logical criticism of the thesis that a large part 

of the organic substances in marine sediment was once 

in solutions derived from land is that the distribution 

of this material would be limited and would be localized near the land. When humic acids in fresh water 

enter the sea, a part may be precipitated as humates 

by such cations as calcium, magnesium, or aluminum, 

though this manner of precipitation has been questioned by Alekin and Moricheva (1958). Some of the 

precipitated humates may be concentrated near the 

mouths of rivers, but much of the humic acids will 

be carried far out to sea by the fresh water that 

remains at the surface and causes the water to be 

stratified over large areas (Schuchert, 1915, p. 266). 

The lighter fresh water becomes mixed with the lower 

saline water only when they are intermittently circulated by storms, or when an inversion takes place due 

to the cooling of the surface water by seasonal changes 

in atmospheric temperatures.

When the humic acids are precipitated or flocculated 

they may be widely dispersed before they are deposited. In still water, the flocculated humates settle to 

the bottom very slowly as small, filmy, gelatinous 

particles; minor agitations or currents tend to keep 

them in suspension, and an even wider distribution 

results.

If the origin, the large amount, and the wide distribution of humic acids in shallow seas are accepted 

as likely, and if this material does adsorb or complex 

uranium with which it comes into contact, humic acids 

may be an important mechanism for implanting uranium within marine black shale. The retention of 

some uranium by the humic acids chemically extracted 

from the Chattanooga shale, and the similarities between the highly uraniferous Swedish kolm and 

humates are strongly suggestive evidence for this

postulated mechanism of uranium concentration in 

shale.

URANIUM INCORPORATED BY LIVING PLANTS AND ANIMALS

One possible relation between organic matter and 

uranium not indicated in table 1 is that the uranium 

may have been extracted from sea water and incorporated by living plants and animals. The meager 

data on the uranium content of living marine organisms do not lend support to this possible origin of the 

uranium in black shales.

D. R. Norton (written communication, 1955) found 

that plankton in six bulk samples collected from 17 

to 27 meters below the surface of the northwestern 

Atlantic Ocean contain 0.08, 0.66, 4.8, 1.5, 1.7, and 

0.99 ppm of uranium in the dried material, whose 

water-soluble salts had been removed. Norton also 

recorded the uranium content of a dried, washed sample of the alga Saryassum as being only 0.6 ppm, or 

0.00006 percent. A. P. Pierce (written communication, 1958) reported that marine algae from off the 

coast of Japan contained uranium in the ash, as 

follows: PorpJiyra sp. (purple alga), 1.7 ppm; Larrdnaria sp. (calcareous alga), 0.8 ppm; and Undaria sp. 

(brown alga), 3.0 ppm. As indicated by these data, 

most of the marine plankton settling to the bottom of 

a sea probably contains only 1 to 2 ppm uranium 

(0.0001 to 0.0002 percent), or only 5 ppm at the most.

It might be argued that after the death and decomposition of these organisms as they settled to the 

bottom, the percent of uranium would be greater in 

the remaining organic residue which is deposited. It 

is believed, however, that this deposited residue generally would contain 10 ppm or less uranium, which 

is still far too low to explain the several tens of parts 

per million in the gross shale and mud samples cited 

in table 1. This decomposing organic substance might 

sorb additional uranium from sea water during the 

time taken to settle to the bottom of the sea; Koczy, 

Tomic, and Hecht (1957, p. 86, 98-99) suggested this 

mechanism of uranium withdrawal to explain the 

lower uranium content of the upper water of the 

Baltic Sea.

Two samples of the Devonian alga, Foerstia sp., 

one from the Ohio shale (J. M. Schopf, written communication, 1953) and the other collected from the 

New Albany shale in Indiana by R. C. Robeck, contained 0.0016 and 0.003 percent uranium, respectively. 

These uranium contents are much higher than that 

of modern marine algae or plankton, but the difference may be explained either by the contamination of 

the sample with other black shale material as indicated by 55.8 percent ash in the sample from the Ohio
90 URANIUM IN CARBONACEOUS ROCKS

shale or by sorption of uranium by the algal matter 

after death, rather than by an originally high content of uranium in the living plant. Two samples, 

collected by the author, of what appeared to be the 

carbonized remains of fish from the Chattanooga shale 

contained 0.002 and 0.003 percent uranium, but this 

amount of uranium probably is also best explained by 

either contamination or sorption after death of the 

animal. It might also be mentioned here that uranium content of fish bones, linguloid brachiopod shells, 

and conodonts, which are all phosphatic fossils present in the Chattanooga shale, is less than 0.005 percent; inasmuch as these fossils make up less than 1 

percent of the rock, their contribution to the total 

uranium in the shale is insignificant. This limited 

evidence suggests, therefore, that uranium now found 

in black mud and shale was not concentrated by the 

living marine organisms before they died and were 

buried in these sediments.

HYDROGEN STJLFIDE

PRECIPITATION OF URANIUM BY HYDROGEN SULFIDE

Hydrogen sulfide is generated when sulfur-bearing 

organic substance is decomposed by anaerobic bacteria. 

It is also formed when marine anaerobic bacteria attack the sulfate present in water to obtain the oxygen 

in the sulfate ion for their metabolic processes. Hydrogen sulfide dissolved in water increases the reducing capacity of the water and tends to make the water slightly acid, and it may react with metallic ions 

to form sulfide minerals, or when circulated with oxygenated waters it may be reoxidized to form sulfate 

ions.

The hydrogen sulfide produced in the black 

mud environment may be critical in the precipitation 

of uranium in and on organic-rich sediment. The 

precipitation of uranium from solution by hydrogen 

sulfide was accomplished in the laboratory more than 

a century ago (Kessler, 1857), and hydrogen sulfide 

has been cited as a probable mechanism causing the 

precipitation of some of the primary uranium ores in 

sandstones of the Colorado Plateau region of the 

United States (Gruner, 1952; Jensen, 1958).

Str0m (1948) suggested that the uranium in the 

muds of the Norwegian fjords was precipitated as the 

upper ventilated brackish water came into contact 

with the lower salt water containing hydrogen sulfide. 

Goldschmidt (1954, p. 566), McKelvey, Everhart, and 

Garrels (1955, p. 518), and Breger and Deul (1956, p. 

507) also suggested that uranium precipitated by hydrogen sulfide may be the explanation for some or 

most of the uranium in black shale.

The effectiveness of hydrogen sulfide as a reducing 

agent capable of precipitating uranium from solutions in the laboratory is well established (table 2). 

However, no experiments (for example, using large 

amounts of water containing 5 ppb uranium, and 10 

to 200 cu cm of H2S per liter of water) have been 

conducted to determine if uranium can be precipitated 

and concentrated by hydrogen sulfide within the range 

of values present in natural marine environments. Carroll (1958, p. 12, 20) showed experimentally that introduction of 40 cu cm of hydrogen sulfide per liter 

to sea water caused the pH to drop from 7.95 to 7.0 

and the water to change from an oxidizing medium to 

a reducing one with a drop in Eh from +0.380 to 

-0.025. Rafalsky (1958, p. 439) showed by a series of 

experiments, mainly at elevated temperatures and with 

rich uranium solutions, that even where hydrogen sulfide is present in highly dilute concentrations the reduction and precipitation of uranium still takes place. 

Finally, Miller (1958) showed that the complex uranyl 

ions which probably are present in natural water will 

react with hydrogen sulfide, at pH, temperature, pressure, and uranium-concentration ranges comparable to 

those that exist in nature, to precipitate very small 

crystallites of pitchblende.

The mechanism of uranium precipitation and concentration from sea water by the action of hydrogen 

sulfide certainly is indicated as an important one, on the 

basis of the abundance of pyrite in the uraniferous 

shales, the abnormal amounts of hydrogen sulfide in 

waters of some of the Norwegian fjords, and the highly 

suggestive experimental evidence.

URANIUM PRECIPITATED BY HYDROGEN SULFIDE IN 

CHATTANOOGA SHALE

In a shale such as the Chattanooga, the presence of 

hydrogen sulfide at some stage in the history of the 

sediment is indicated by the presence of sulfide minerals. The most common sulfide mineral is pyrite 

(FeSg); there is some marcasite (FeS2), and, though 

rare, minute crystals of galena (PbS) also have been 

observed. The genesis of pyrite in sedimentary rocks 

will not be reviewed here (see Galliher, 1933; Yoshimura and Wada, 1938; Emery and Rittenberg, 1952; 

Debyser, 1957; Huber, 1958), but there is no reason 

to doubt that shale containing 7 to 22 percent by 

weight of pyrite (or 3 to 10 percent pyrite sulfur) 

strongly indicates mud in which abundant hydrogen 

sulfide was produced and trapped.

Studies of the Chattanooga shale at Pennsylvania 

State University (Bates and others, 1956, p. 62-63) 

show a positive, but not significant, coefficient of correlation (about 0.4) between the amounts of pyrite 

and uranium, but the pyrite and uranium are not 

chemically combined (Bates and Strahl, 1957, p. 

1310). An even higher coefficient of correlation (about
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 91

0.65) is shown between the amount of uranium and 

the amount of carbon and pyrite combined; Bates and 

Strahl interpreted this correlation to mean that the 

environment conducive to carbon accumulation is also 

conducive to the growth of pyrite and the precipitation of uranium.

A similar conclusion was reached by C. R. Kinney 

and others (written communication, 1958), another 

group at Pennsylvania State University that attempted to determine the exact chemical associations 

between uranium and the organic matter in the Chattanooga shale. After many experiments designed to 

isolate and fractionate the organic matter and after 

subsequent analysis of the fractions, their results indicate "that the uranium is deposited more or less 

evenly throughout the shale * * *. Although uranium is probably deposited in the shale because of 

the presence of organic matter, it is not associated 

with either the mineral matter or the organic matter 

exclusively." This conclusion is compatible with and 

supports the interpretation that the organic matter 

was responsible for the production of hydrogen sulfide, which in turn precipitated the available uranium.

Deul (1955; 1957) subjected samples of Chattanooga shale to several kinds of mechanical separation 

procedures and found that uranium was not concentrated in either a dominantly mineral or organic-matter fraction, but was concentrated in a colloidal-size 

fraction which contained as much as 0.08 percent uranium. Insufficient material of this colloidal phase 

existed to determine the chemical form or associations 

of the uranium, but Deul (1957, p. 218) concluded 

that the uranium was not combined with the organic 

matter but was derived from sea water by reduction 

of the uranyl ion to uranium dioxide.

A few logical questions should be considered. Why 

is there always such a close correlation between uranium and organic matter in the Chattanooga shale? 

If uranium was precipitated as a solid, why are minerals of uranium oxide not identifiable and separable 

from the shale? Because hydrogen sulfide may be a 

direct product of decomposition of organic matter, 

any uranium precipitated from overlying water by 

the rising hydrogen sulfide probably would fall on or 

very near the organic matter. Further, uranium so 

precipitated from a solution as dilute in uranium as 

sea water probably would be in the form of ultramicroscopic, but relatively heavy (sp gr 8 to 10), particles disseminated on and through the organic sediment, which results in an admixture that with 

existing methods is largely physically inseparable. It 

is also possible that the uranium so precipitated became physically or chemically bound in some way to

adjacent particles in the sediment during diagenesis 

and that it no longer exists as discrete identifiable 

uraninite particles.

RELATION OF PTBITE TO UBAN1UM

The significance so far given to the iron sulfide minerals, particularly pyrite, in either black mud or black 

shale is that they are positive evidence of the former 

existence of hydrogen sulfide in these sediments. Most, 

if not all the iron sulfide minerals are formed by combination of hydrogen sulfide and ferrous ions within 

bottom water or within bottom sediment and are deposited as a syngenetic, or penesyngenetic, amorphous 

substance with the black mud.

Two general hypothetical circumstances should be 

considered as to the relations among hydrogen sulfide, 

pyrite, and uranium in the black mud environment: 

whether the hydrogen sulfide is confined within the 

sediment, or whether the hydrogen sulfide rises from 

the sediment into the overlying water. In the first 

circumstance, oxidizing or only very slightly reducing 

conditions, with little or no hydrogen sulfide, generally 

exist in the uppermost few inches of sediment (Zobell and Feltham, 1942; Emery and Rittenberg, 1952, 

p. 773; Harvey, 1955, p. 150), and oxygen is present 

and hydrogen sulfide absent in the immediately overlying water. In this case, iron sulfide minerals form 

below the oxidizing zone, and any reduction and precipitation of uranium by hydrogen sulfide must be 

from water within the sediment. This interstitial 

water probably contains no more uranium than sea 

water, and because the volume of this water is very 

small in comparison to the mass of overlying sea water, the concentration of uranium by hydrogen sulfide 

is negligible. These conditions undoubtedly obtain in 

the Baltic Sea muds, and they may in part explain 

the rather small amounts of uranium in these sediments. Debyser (1957, fig. 3) showed that the pyrite 

in the Recent organic-rich muds from many parts of 

the Baltic generally forms 20 to 250 cm below the 

surface of deposition.

In the second circumstance, where the hydrogen 

sulfide rises from the sediment into the overlying water, the reduction and precipitation of uranium from 

the water could proceed readily until the uranium 

from the hydrogen-sulfide-charged water is exhausted. 

Iron sulfide minerals, of course, would form in and 

on the mud in this example.

It should be obvious that the formation of abundant pyrite does not indicate necessarily that conditions favoring the precipitation and concentration 

of uranium existed simultaneously. The absence or 

small amount of uranium in many pyritiferous black
92 URANIUM IN CARBONACEOUS ROCKS

and gray shales suggests either that the hydrogen sulfide was confined within the sediment or that uranium 

was not available in the overlying water. On the 

other hand, it seems reasonable that shale with abundant pyrite and more than 0.001 percent syngenetic 

uranium probably formed where hydrogen sulfide was 

diffused upward from the decomposing organic sediment into the overlying water.

An explanation similar to that of the pyrite-uranium relation probably applies to a large suite of 

metals commonly concentrated in a black mud environment lead, zinc, copper, molybdenum, cobalt, 

and probably other elements. For example, Korolev 

(1958, p. 461) showed experimentally, using hydrogen 

sulfide, that 70 to 96 percent of the molybdenum is 

coprecipitated with iron sulfide from solutions whose 

compositions resemble natural ones in their pH and 

salinity.

BOTTOM WATER OF NORWEGIAN FJORDS AMD BALTIC SEA

As graphically shown by Str0m (1936) and indicated in table 3, hydrogen sulfide is present in the 

stagnant bottom water of many of the fjords. The 

amount of free oxygen in the water gradually decreases with depth until oxygen is absent; at about 

this zero-oxygen depth, traces of hydrogen sulfide are 

recorded, and the amount of hydrogen sulfide in the 

water than increases with greater depth to the bottom of the fjords. The maximum amount of hydrogen sulfide is 199.5 cu cm per liter of water, with 

amounts in the range 1 to 10 cu cm per liter being 

the most common. The zone of water containing 

hydrogen sulfide in the Framvaren fjord in 1933 was 

about 450 feet thick. Uranium is very likely precipitated within this hydrogen sulfide zone and falls on 

the bottom black mud.

The importance of precipitation of uranium by hydrogen sulfide is difficult to evaluate, but its possible 

importance can be suggested on the basis of the data 

in table 3. Some of the fjord muds contain much 

more uranium than do the Baltic Sea muds, and one 

of the obvious differences between the two areas is 

that the bottom waters of the fjords are characterized by hydrogen sulfide whereas most of the bottom 

water of the Baltic is oxygenated. No positive correlation is apparent between uranium content of the 

sediment samples and amount of hydrogen sulfide in 

the bottom waters of the fjords, but it is considered 

very possible that such a relation could be shown if 

other factors were evaluated.

If it is assumed that sedimentation in a fjord is 

uniform and continuous and that the lower waters 

are stagnated, the available uranium in the mass of

water invaded by hydrogen sulfide is seen to be a 

finite amount. Once the hydrogen sulfide and oxygen 

boundary reaches a static upper level and the water 

below this boundary is depleted of its uranium by 

hydrogen sulfide precipitation, continued stagnation 

for long periods of time will not result in the addition of uranium to the sediment. It is thus conceivable that the mud containing abundant organic matter which is deposited late during a period of bottomwater stagnation might have little or no uranium, 

though this mud may be contiguous with and otherwise physically identical with the underlying uraniferous mud. This latter condition might serve to 

explain the variable correlation coefficient of uranium 

to organic matter, to carbon (for example, Bates and 

others, 1936, p. 54r-57), or, for that matter, to any other 

constituent in black shale.

Black mud from the Indre Topdalsfjord contains 

the most uranium (table 3), but hydrogen sulfide in 

the bottom water was barely detectable. This example does not negate a possible genetic relation of uranium to hydrogen sulfide, but rather may be used to 

verify the relation and to shed light on the importance of the availability of uranium in the water. 

From information given by Str0m (1936, p. 50, fig. 

36) the bottom water of Indre Topdalsfjord is ventilated and partly flushed out by oxygenated sea water 

almost yearly, and the process of stagnation and of 

formation of hydrogen sulfide sets in immediately 

after this renewal of water. A hydrogen sulfide zone 

did exist in the bottom water of this fjord through 

1926 and early 1927, in early 1932, in early 1933, and 

probably in 1935; the measurement by Str0m (1948) 

in 1947 presumably was made either at the beginning 

or end of another period of bottom-water stagnation. 

This periodic or intermittent influx of saline water 

might explain a replenishment of uranium which could 

be precipitated to result in exceptionally uraniferous 

mud.

The two fjords that had the most hydrogen sulfide 

in their bottom waters are Framvaren (199.5 cu cm 

per liter) and Hellefjord (40.7 cu cm per liter). 

Str0m (1936, p. 46) stated that "there is reason to believe that the [Framvaren] bottom waters have not 

been ventilated for a very long time." The observed 

renewals of water in the 1850's and in 1905 indicate 

that Hellefjord "is only ventilated at intervals of 

some 50 years" (p. 57). Except for Vestrhusfjord, 

which was ventilated in 1923 and 1933 (p. 54), no 

additional information is given on the times of water 

ventilation of the fjords for which uranium data on 

sediment samples are available. It is postulated here 

that the water containing uranium in solution, small
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 93

as the uranium content of this water may be, must be 

renewed continually or periodically to deposit muds 

having a high uranium content. In the case of Framvaren and Hellefjord, access to more uranium-bearing waters could be attained only if the upper boundary of the hydrogen-sulfide zone rises; this rise 

apparently had been terminated when the two fjords 

were studied in 1933, as the upper limits of hydrogen 

sulfide in both were at depths of 50 to 65 feet, the 

approximate maximum depths of surface-water disturbance (Str0m, 1936, p. 58).

In the Baltic Sea the amount of hydrogen sulfide, 

the periodicity of its expulsion from the sediment into 

the overlying water, and the volume of bottom water 

affected by hydrogen sulfide are unknown. In 1931, 

Granquist (1932, p. 24) recorded hydrogen sulfide in 

the bottom water at only 1 of more than 70 stations 

occupied in the Baltic that year; in 1932, he (1933a, 

p. 12) noted only that no oxygen was present in the 

lower water at the same station; and in 1933 (1933b, 

p. 12), he recorded 1.40 cu cm of oxygen per liter. 

Gripenberg (1934, p. 128) showed that the bottom water at this same station was slightly oxygenated in 

1926 and 1930. This station, No. F81 (lat 57°22' N.; 

long 19°57' E.), was in the Gotland deep, very near 

locality 6 (lat 57°20' N.; long 19°54' E.) of 

Koczy, Tomic, and Hecht (1957), which in 1953 had 

1.43 cu cm of oxygen per liter in its bottom water. 

As recorded in table 3, one of the samples of sediment collected at the latter locality contained the most 

uranium, 0.00103 percent, of any of the Baltic Sea 

samples. It should also be noted that in 1931 the 

bottom water (200-220 meters deep) of station F81, 

as reported by Granquist, had a pH of 6.94 to 

7.09 and contained ">200" mg P205 per cu m, minimum pH and maximum P2O5 values for all the Baltic 

stations; thus, the presence of hydrogen sulfide, the 

pH, and the amount of phosphate make this Baltic 

water comparable, at least occasionally, to the bottom 

waters of some of the Norwegian fjords. The fact 

that the Baltic mud contains considerably less uranium and, as indicated by Granquist's data, that the 

bottom water of the Baltic is rarely charged with 

hydrogen sulfide, does lend support to the postulated 

genetic and geochemical relation of the uranium and 

the hydrogen sulfide.

POSSIBLE EXTRAPOLATIONS AND INTERPRETATIONS

From the preceding discussion, which has touched 

on only some of the more obvious relations of hydrogen sulfide, pyrite, and uranium in the deposition of 

organic-rich mud, the interdependency of several factors that might affect concentration of uranium by

hydrogen sulfide in black marine muds can be interpreted and summarized.

Hydrogen sulfide generated from decomposing organic matter in the bottom muds of some fjords rises 

into and accumulates in large amounts (generally 

about 10 cu cm per liter, but as much as 199.5 cu cm 

per liter) in the overlying water. This hydrogen sulfide acts as a reducing agent, and the uranium in the 

water probably is precipitated as colloidal-size uranium oxide which falls onto the bottom sediment.

Iron sulfide minerals, common to and abundant in 

many black shales and muds, clearly indicate the presence of a hydrogen sulfide environment in which uranium could have been precipitated at the time the 

sediment was deposited. A positive correlation between the amounts of iron sulfide and of uranium 

might thus be expected, but in actuality such a quantitative relation would be fortuitous only. If it is 

assumed that the formation of abundant iron sulfide 

minerals in the sediment is at a constant rate, the 

amount of uranium precipitated by hydrogen sulfide 

can be shown to differ radically, depending on whether 

the hydrogen sulfide is confined below the sedimentwater interface or is diffused into the overlying water; if the latter circumstance were existent, the 

amount of precipitated uranium would vary with the 

uranium content of the overlying water and the volume of the water column invaded by hydrogen sulfide.

The amount of uranium that can be precipitated by 

hydrogen sulfide is limited to the amount of uranium 

in the water reached by hydrogen sulfide. Estimates 

can be made of the mass of water that would have to 

be charged with hydrogen sulfide to explain the uranium in the bottom muds of the Norwegian fjords. 

As indicated in figure 25, the bottom waters of the 

fjords would have to be renewed several times in a 

10-year period to explain the uranium in the bottom 

muds.

For example, if the bottom sediment of fjord X 

contains 0.0060 percent uranium (&, fig. 25) and the 

uranium content due to precipitation by hydrogen 

sulfide is 0.0050 percent (5); if the fjord water is 100 

m deep, and the lower 50 m of water intermittently 

becomes stagnant, charged with hydrogen sulfide, and 

depleted of all its uranium by precipitation; if the 

original renewing water contains 2 ppb of uranium 

(c); and if the rate of accumulation of the bottom 

mud is 1 cm every 10 years (d) (approximated from 

data on Drammensfjord sediment, Str0m, 1936, p. 65 

and 68), and the weight of the partially compacted 

mud is 100 Ib per cu ft or about 1,600 kg per cu m; 

then this bottom water would have to be replaced 8
94 URANIUM IN CARBONACEOUS ROCKS

Black mud. 1 cm thick, accumulating at rate of 1 cm per 10 years

FIGURE 25. (Diagrammatic cross section of the water in a fjord showing different thicknesses of water charged with hydrogen 

sulfide, with corresponding calculations on the number of times this water must be changed to explain the uranium precipitated on and within a layer of bottom mud 1 cm thick:

Factor

Symbol

a

b

c 

d

e

Description

Total uranium content of bottom sediment __ . _____ percentUranium content of sediment due to precipitation by H2S._ percentOriginal uranium content of stangat water... _______ . .. ..ppb.. 

Rate of accumulation of bottom sediment (partly compacted basis)

cm per 10-yr period..

Number of times water must be replaced to explain the uranium in 1 cm of sediment

per 10-yr period 

100 m (328 

ft)

0.0060

.0050

2

1

4

50 m (164 

ft)

0.0060

.0050

2

1

8

25 m (82 

ft)

0.0060

.0050

2

1

16

10 m (32.8 

ft)

0. 0060

.0050

2

1

40

5 m (16.4 

ft)

0.0060

.0050

2

1

80

Vertical thickness above mud of stagnant water charged 

with HaS

times every 10 years (e) to account for the uranium 

in the muds of fjord X.

Unfortunately, the desired uranium data on one of 

the classic examples of black mud deposition, the 

Black Sea, are not available, but enough is known to 

provide basis for speculation on the uranium content 

of the black muds of the Black Sea.

Most of the known and postulated chemical factors 

favorable for uranium concentration are identified in 

what is known of the chemistry of the stagnant bottom water and, in places, the organic- and sulfide-rich

black muds of the Black Sea (Androussow, 1897; 

Archanguelsky, 1927; Pirogova, 1953; and Chilingar, 

1956). At depths of 1,500 to 2,000 meters, the water 

contains 5 to 6 ml of hydrogen sulfide per liter (Chilingar, 1956, p. 2766; see also Gaspers, 1957, p. 809, 

821) and has an Eh of about 0.2 (Skopintsev, 

1956); traces of hydrogen sulfide are intermittently 

present at a depth of 115 to 225 meters, depending on 

the season, position in the sea and, presumably, the 

maximum depth of convection currents (Gololobov, 

1953).
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 95

The postulated process of uranium concentration by 

hydrogen sulfide precipitation might well apply, then, 

except for the previously emphasized necessity of periodic renewal of waters. Androussow (1897) concluded that it takes 1,700 years for the inflowing saline bottom water entering the Black Sea through the 

Strait of Bosporus to accomplish renewal of the entire salt-water content of the Black Sea; Merz and 

Moller, as reported by Gaspers (1957, p. 820), calculated that a complete change of the water below the 

30-meter surface zone takes 2,500 years. Both rates 

of renewal are far too slow to provide a source of 

soluble uranium for hydrogen sulfide precipitation 

sufficiently large to result in highly uraniferous sediment. If the original saline waters contained 2 ppb 

uranium, all of which was released from a column of 

water 2,000 meters thick, the uranium content of the 

mud would be increased by less than 10 ppm or 0.001 

percent uranium, using the rate of sedimentation for 

the deepwater gray clay of the Black Sea of 0.2 mm 

per year, as determined by Archanguelsky (1927, p. 

276) and recorded by Gaspers (1957, p. 829). The 

older black clay in the Black Sea probably accumulated more slowly and, conceivably, might contain 

more than 0.001 percent uranium.

To the critical observer, any one or all of the factors 

on which these calculations on uranium in the Norwegian fjords and the Black Sea are based are variables, 

but one very important point stands out: If hydrogen sulfide precipitation of uranium from sea water 

was an important process in the concentration of uranium in black shales, periodic or intermittent renewal 

of the water in which they were deposited was essential, followed by periods of stagnation and production 

of hydrogen sulfide. During deposition of the Chattanooga shale, which probably was extremely slow in 

a relatively shallow epicontinental sea (probably several tens of feet to a few hundred feet deep) that 

contained only a few parts per billion of uranium, 

circulation of the waters was certainly necessary; but 

this circulation probably was spasmodic rather than 

continuous, perhaps owing to storms, to thermal inversions, or to invasions, such as have been described 

by Woolnough (1937, p. 1129, 1145), of marine waters into a large semibarred basin at infrequent intervals. The minute ripple marks, discontinuity of microscopic layers, megascopic cross lamination, and 

other primary structural features of the shale represent incidents in the depositional history of the shale 

that support this interpretation.

PHOSPHATE

RELATION TO URANIUM

Phosphate (PaOg), generally in the form of nodules, 

is a common and distinctive lithologic component of 

many uraniferous black shales. The uranium content of these phosphatic nodules ranges from 0.001 

to nearly 0.1 percent, and the P2Og content from 10 

to 35 percent. In some black shale units such as the 

thin layers of Pennsylvanian age in the Midcontinent area (Danilchik and Hyden, 1956, p. 260; written 

communication, 1958), the phospate contains most of 

the uranium in the unit.

Phosphatic nodules 1 to 3 inches in greatest diameter are distributed sparingly through the uppermost 

part of the upper unit of the Gassaway member of 

the Chattanooga shale in parts of central Tennessee. 

These nodules contain 0.0030 percent or less uranium, 

which is less than that of the enclosing shale; of further interest is the fact that this nodule-bearing shale 

contains 0.0020 to 0.0030 percent less uranium than 

the underlying shale which has no nodules. That 

shale that is devoid of visible nodules and that contains the greater amount of uranium has an average 

content of 0.3 percent P2Og, and little or no uranium 

appears to be associated with the phosphate of this 

shale.

No data are available on the phosphate content of 

sediments from the Baltic Sea. The phosphate analyses of sediment in the Norwegian fjords (Strom, 1936, 

p. 61-63) was not made on the same samples that have 

been analysed for uranium but because the black muds 

of the fjords represented by both sets of samples are 

described in almost identical fashion, the amounts of 

phosphate in both sets of samples are probably very 

similar. The PgOs contents range from 0.14 to 0.36 

percent; the samples from the Gassaway member of 

the Chattanooga shale have a similar range in phosphate content. The phosphate in both the black fjord 

muds and the Chattanooga shale probably is present 

as an inherent constituent of the organic matter, as 

phosphatic shells or skeletons, and as detrital apatite.

The amounts of phosphate in the waters of the 

Baltic Sea were determined by Buch (1932), and his 

data show that the maximum phosphate values are 

always in the bottom waters. These bottom-water 

values generally range from 5 to 50 mg PaOg per cu 

m of water, but have an average (Buch, 1932, p. 15) 

of 110 mg P2O5 per cu m in the Baltic Sea area from 

which the samples of Koczy, Tomic, and Hecht (1957, 

fig. 1) were collected. The maximum values reported 

by Buch (1932, p. 14) are for the bottom water of 

station F81, previously described on page 93 as very 

near the locality in the Gotland deep where Koczy,
96 URANIUM IN CARBONACEOUS ROCKS

Tomic, and Hecht (1957, loc. 6) collected their most 

uraniferous samples. In 1928, 1929, and 1930, this 

water from depths of 200 to 220 meters contained 

200, 225, and 220 mg P2Os per cu m, respectively.

One of the most significant observations to come 

out of Str0m's (1936) studies of the fjord waters is 

that the amount of phosphate in a body of water generally increases with increasing degree of water stagnation, but that there is practically no deposition of 

phosphate in the black muds. The fact that phosphate is not deposited, that the waters are too poisonous to support an abundant fauna and flora which 

ordinarily deplete water of phosphate, and that sinking phosphatic organic remains such as shells are dissolved in these waters, probably explains the nearrecord amounts (as much as 700 mg PgOs per cu m) 

of phosphate in solution in the bottom waters of the 

fjords.

As pointed out by Kazakov (1937) and by Krumbein and Garrels (1952, p. 9) the precipitation of 

phosphate is strongly dependent on the pH of the 

water, the solubility of the phosphate increasing with 

decreasing pH. In the fjords, where no phosphate 

precipitation is known but where the amount in solution is exceedingly high, the pH of the bottom waters 

is on the acid side, or nearly neutral, ranging from 

6.9 to 7.5. Because uranium is being concentrated in 

the black muds of the fjords and the phosphate is 

retained in solution, it can be firmly stated that the 

uranium in marine black shale can be, and in many 

shale units undoubtedly is, geochemically and mineralogically independent of any phosphate present in 

the shale. It also should be pointed out that if the 

pH of the water of the fjord were to rise to 7.5 or 

higher and phosphate were to be precipitated accordingly, a lesser degree of stagnation would be indicated, 

and the chemical system most favorable for uranium 

concentration by hydrogen sulfide would no longer 

exist. In other words, phosphate deposition may be 

indicative of a lesser rate of uranium accumulation, 

and probably is with the nodule-bearing Chattanooga 

shale unit described above.

GEOCHEMICAL CONDITIONS FOB PHOSPHATE DEPOSITION

The preceding discussion has been devoted primarily 

to those muds and shales that are almost devoid of 

phosphate; but, as previously stated, phosphate is the 

component in some shales that contains the major part 

of the uranium. The intimate genetic, geochemical, 

and stratigraphic associations of uraniferous phosphorites and black shales have been cited by many 

authors (for example, McKelvey and Nelson, 1950, 

p. 40-41; Whitehead, 1952, p. 196-198; McKelvey,

Everhart, and Garrels, 1955, p. 514; and Chentsov, 

1956). The geochemistry of uranium in phosphorites 

is the subject of an exhaustive study by Altschuler, 

Clarke, and Young (1958); and Sheldon (1959) described and interpreted in detail the geochemical conditions relating to the concentration of uranium in the 

phosphorite and black shale of the Phosphoria formation of the Northwestern United States.

The presence or absence of phosphate in marine 

black shale provides a basis for developing a logical 

sequence of changing chemical conditions in the bottom waters of the seas where black muds are deposited, 

here described as involving four stages. The distribution of phosphate in the upper part of the Chattanooga shale and in the overlying Maury formation 

and Fort Payne chert of Mississippian age is used to 

illustrate this sequence, as shown in figure 26. In 

general, marine black shale devoid of phosphate, as 

in the lower part of the section in figure 26, typifies 

stage 1; and black shale containing scattered phosphatic nodules, as in the uppermost part of the Chattanooga shale, represents stage 2. Stage 3 is represented by rock composed chiefly of phosphate, only 

very minor amounts of carbonaceous matter, and, 

commonly, abundant glauconite, as present in the 

lower part of the Maury formation. Stage 4 in this 

theoretical cycle and transition of geochemical conditions is represented by limestone, commonly containing 

a large amount of chert, with only a very minor 

amount of phosphate and little or no carbonaceous 

matter. The Fort Payne chert is typical of stage 4 

in the figured example. In application of this sequence of stages to other stratigraphic sections, the 

cycle may be reversed, to go from stage 4 to stage 1, 

or, in beginning with stage 4, it may be visualized 

as being arrested for long periods in stages 3 or 2, 

before returning to stage 4.

Chemically, the water conditions controlling the 

retention of phosphate in solution or the precipitation and deposition of phosphate are a result of the 

change from a stagnant anaerobic environment 

(stage 1), caused by the slow decomposition of the 

organic matter and poor water circulation on the bottom of the sea, to an oxygenated and more alkaline 

environment due primarily to water circulation wherein calcium carbonate could be deposited (stage 4). 

The key chemical change probably is that of pH 

(Kazakov, 1937; Krumbein and Garrels, 1952, p. 9; 

Smirnov, 1958), phosphate in solution being available and other factors being equal. At low pH, about 

6.5 to 7.0, the phosphate remains in solution, the phosphatic materials that settle into this water are dis-
GEOLOGY AND GEOCHEMISTRY OP URANIUM IN MARINE BLACK SHALES 97

Chert, bedded, some limestone; uranium content 0.0005 percent or less; 

estimated P2 O5 content less than 1.0 percent

Phosphatic nodules, scattered, small, typically ball-shaped, in light 

greenish-gray claystone; uranium content of nodules and claystone 

about 0.0020 percent

Phosphatic nodules, tightly packed, commonly platy, in gray siltstone; 

glauconite common; uranium content of nodules about 0.0030 percent, 

of siltstone, about 0.002 percent; P2 O5 content of nodules 25 to 30 

percent

Phosphatic nodules, sparsely scattered, embedded in black shale; 

uranium content of nodules about 0.0030 percent, of the shale, about 

0.0050 percent; R, Og content of nodules about 28 percent

Black shale, devoid of phosphate except in few fossils and as few scattered grains; uranium content of shale about 0.0080 percent; P2 Og 

content about 0.2 percent

FEET rO

-0.5

-i.o

-1.5

FIQUBH 26. Generalized stratigraphic section of the uppermost Devonian and lowermost Mississippian rocks In central 

Tennessee showing the uranium content of the different units as related to distribution of phosphate, and the interpretated pH and Eh that existed at or just above the water-sediment contact at time of deposition.

solved, and the phosphate content of the water is 

increased (stage 1). It is probable that as the general 

pH in this lower water rises toward about 7.5 as a 

result of limited water circulation, very minute and 

scattered centers of slightly higher pH are formed in 

the water or at the sediment-water interface, primarily 

around fragments of calcareous or phosphatic shells 

or other hard parts of pelagic animals that constitute 

the "seeds" for phosphate precipitation, and that phosphatic pellets or nodules start to form (stage 2). As 

the pH rises above 7.5, this precipitation of phosphate 

takes place more readily, and if the rise in pH is 

rapid, the precipitation of phosphate is accelerated 

accordingly and results in a layer of chiefly phosphatic sediment in the form of oolites, pellets, nodules, 

and even discontinuous laminae (stage 3). With 

continued rise in pH to about 8.0, the originally large 

supply of phosphate in solution is exhausted, and 

phosphate becomes only a very minor constituent in 

the sediment accumulating in this normal sea water 

environment (stage 4). This theoretical sequence of

phosphate deposition from sea water is in general 

compatible with the results of experiments by Smirnov 

(1958, figs. 1-3) to determine the physicochemical factors governing phosphate precipitation in sea water.

CYCLE OF PHOSPHATE DEPOSITION

In the initial stage (stage 1) no phosphate is deposited, and uranium is concentrated in the sediment 

by organic matter or by the action of hydrogen sulfide. During the next stage (stage 2), when only a 

few, very scattered nodules form and are deposited, 

the chemical combination of uranium in carbonate 

fluorapatite by ionic substitution for calcium (Altschuler, Clarke, and Young, 1958, p. 69) is in large 

part dependent on the availability of uranium in the 

waters where precipitation of phosphate takes place; 

if, as is considered possible in interpreting the low 

uranium content of the nodules in the upper part of 

the Chattanooga shale, the uranium in these waters 

had already been depleted by direct hydrogen sulfide 

precipitation, the resulting phosphate would have 

little or no uranium. On the other hand, if hydrogen
98 URANIUM IN CARBONACEOUS ROCKS

sulfide had not reached the sphere of phosphate precipitation and uranium was available in the water, 

the scattered phosphatic nodules deposited during 

stage 2 might contain as much as several hundredths 

percent uranium.

During stage 3, when theoretically the major phosphate deposition takes place, sufficient water circulation is involved to result in phosphate containing 0.01 

to 0.10 percent uranium. In the final stage (stage 4) 

where only a very minor amount of phosphate is precipitated, the phosphatic particles may have a relatively high uranium content, but because they are 

disseminated sparingly through nonuraniferous carbonate or siliceous sediment, the rock in which they 

reside rarely has more than a few parts per million 

uranium (O.OOOX percent).

Carbonate fluorapatite, or "calcium phosphate," has 

the capacity to take up uranium and fluorine any 

time after it has been deposited (Altschuler, Clarke, 

and Young, 1958, p. 83). This property explains the 

variable uranium contents of marine phosphorite, 

apatite, and phosphatic fossils (for example, shells 

and bones). Much of the uranium in the phosphate 

formed in the described cycle is undoubtedly syngenetic, but subsequent uptake of uranium may take 

place diagenetically or by later exposure of the phosphate to uranium-bearing ground water. Postsyngenetic uptake of uranium is probably insignificant in 

the phosphate of black shales because of the general 

impermeability of these shales until they are greatly 

weathered.

The relation of the uranium to the phosphate in 

a black shale and the interpreted chemical conditions 

prevailing in the overlying water at time of deposition are illustrated in figure 26. Here, the four stages 

of the cycle are illustrated, though the absence of 

highly uraniferous phosphatic materials in stage 3 

must be explained by the lack of uranium in the 

waters where the phosphate was precipitated, by the 

abundance of calcium ions which precluded abundant 

ionic substitution, or by other reasons unknown.

The thin phosphatic marine black shale beds of 

Pennsylvanian age in the midcontinent area of the 

United States are believed to illustrate best stage 2 

of phosphate precipitation, where uranium is associated chiefly with the phosphatic nodules and layers 

in the shale. It is postulated that, at the time these 

shale beds were deposited, the hydrogen sulfide-reducing environment was confined to the sediment, and 

that the uranium available in the overlying waters 

was fixed by the precipitating phosphate. The uraniferous phosphorite in the Phosphoria formation of

southwestern Montana, southeastern Idaho, and western Wyoming probably best illustrate stage 3 in the 

cycle, though all 4 stages are represented within the 

vertical and lateral facies of this formation.

OTHER CHEMICAL VARIABLES

The pH, Eh, and salinity values are measurements 

of important chemical characteristics of Water that 

are critical in understanding and determining the 

probable and possible chemical reactions that can take 

place in a complex chemical environment. To these 

three characteristics may be added those of temperature, pressure, density, and others, but the significance 

of these in the sorption or precipitation of uranium 

from sea water is unknown.

ACIDITY

The bottom water of both the Norwegian fjords and 

parts of the Baltic Sea are only very slightly acidic, 

about pH 7.0 or slightly less. This lowering of the 

pH from about 8.0 in normal sea water to about 7.0 

is primarily due to carbon dioxide produced by anaerobic bacteria attacking the organic matter in the 

bottom mud, though the soluble organic acids and the 

hydrogen sulfide which may be similarly produced 

are also weak acids. Presumably the water a few 

inches above the organic-rich mud may have a pH as 

low as 6.5, and the water in the mud may have a pH 

in the 5.5 to 6.5 range.

The significance of the pH of the sea water on the 

concentration of uranium in marine black mud and 

shale can be evaluated by noting three factors: (a) The 

direct effect that pH has in the ability and capacity 

of certain substances to sorb uranium; (b) the direct 

effect that pH has in the precipitation of uraniumbearing substances; (c) the indirect relations of pH 

to the uranium content of the sediment.

Rozhkova and others (1958, p. 427, 430-431) 

showed by a series of experiments that maximum 

amounts of uranium are sorbed by coaly substances 

and phosphorites from solutions having a pH of 5 to 6; 

as the pH increases to 8 or decreases to 3, the sorptive 

capacity of these materials, which include lignite and 

humic acids, rapidly decreases. Manskaya, Drozdova, 

and Emilyanova (1956) experimented with the precipitation of uranyl humates, the uranium-bearing 

humic acids, as a function of pH. Using an alkaline 

extract from peat, they found that the optimum production of uranyl humates, the precipitate that contains the most uranium, is at a pH close to 5. In 

experiments using the humic acids separated from 

the peat extract and washed to remove the chloride 

ion, the pH optimum for precipitation of uranium in 

the humates is within the 5.5 to 7.0 range. These
GEOLOGY AND GEOCHEMISTRY OF UKANIUM IN MARINE BLACK SHALES 99

empirical observations indicate that the pH of the 

waters associated with black mud deposits approaches 

the range of pH wherein maximum uranium sorption 

can take place, because these waters have a pH of 6.5 

to 7.5. It is probable, however, that the materials in 

these muds rarely sorb uranium to their full capacity, 

because the amount of uranium in sea water is too 

low or the length of time in which they are exposed 

to sea water having this pH is too short.

The direct effect of pH of sea water in the precipitation of uranium-bearing substances, particularly 

humic acids and phosphate, has been indicated in 

preceding sections. Manskaya, Drozdova, and Emelyanova (1956) showed that as the pH of alkaline water 

which contains dissolved humic acids is lowered, very 

minor coagulation of humates takes place at pH of 8 

and production of coagulated humates increases as the 

pH is lowered to 5, when the water is almost depleted 

of its humic acids. Undoubtedly, the major fraction 

of the uranium-bearing humic acids transported by 

sea water is flocculated and deposited when it meets 

the bottom water of black-mud seas having a pH in 

the 6.5 to 7.5 range. The precipitation of uraniumbearing phosphate has been described as being dependent on the increase of pH of this bottom water from 

6.5 to 8.0.

Other, but indirect, relations of pH to the uranium 

content of marine muds and shales should be noted. 

Sea water having pH of 7.2 or less is probably indicative of water wherein organic matter is being decomposed and hydrogen sulfide is being produced, and 

where uranium is being precipitated directly from the 

water. It is mainly the Eh level that controls the 

reduction and precipitation of uranium as uraninite( ?) 

crystallites, but commonly both Eh and pH of the 

water are lowered simultaneously as organic matter 

decomposes, so pH may be an indirect measure of this 

precipitation action. Some exceptions to the parallel 

rise and fall of pH and Eh were pointed out, however, by Emery and Rittenberg (1952, p. 770-771). 

Carbonate sediments will not form in waters having 

a pH less than about 7.8 (Krumbein and Garrels, 

1952, fig. 8); this fact explains the general inverse 

relation of uranium to calcium carbonate in marine 

black shales, noted by McKelvey and Nelson (1950, 

p. 39), Swanson (1956, p. 453), and others.

OXIDATION-REDUCTION POTENTIAL

The Eh of the bottom water and of the mud in both 

the Norwegian fjords and the Baltic Sea can be decribed only in general terms because no direct measurements are known. A possible index of the Eh of 

the water, however, is the relation of oxygen to hydrogen sulfide in the water; in general, the water that 

has only minute traces of both free oxygen and hydrogen sulfide may be considered as having an Eh of 0.0 

volt. Because reducing conditions are most pertinent 

here in the discussion of processes of uranium concentration, the decrease of Eh from 0.0 to -0.4 volt may 

be crudely indicated by the proportional increase of 

hydrogen sulfide from a small fraction of 1 to 200 cu 

cm per liter.

Biologic processes are the fundamental cause of reducing conditions in the marine environments discussed here. Aerobic bacteria consume free oxygen 

until it is depleted, whereupon anaerobic bacteria 

become established and obtain their oxygen either 

from the bound oxygen of the organic matter or from 

the sulfate ions in the saline water, with consequent 

release of hydrogen sulfide. The extent and "strength" 

of the resulting reducing conditions, or the negative 

Eh produced, is a function of the rate and length of 

time of anaerobic decomposition in stagnant water. 

Given abundant organic matter in the bottom sediment, and still water above, reducing conditions will 

be established in the bottom water fairly rapidly; it 

is thus conceivable that water which is circulated intermittently or infrequently actually may be reducing 

in character for cumulatively long periods of time; the 

probability of this being true for large epeiric blackshale seas in the past as well as in the fjords today 

was described in detail by Str0m (1955).

The Eh of the black mud sediment is always low 

and generally is of negative potential (Zobell and 

Feltham, 1942; Emery and Rittenberg, 1952, p. 775), 

except for the uppermost inch or two of sediment 

during times of ventilation. That a negative Eh existed in the sediment through diagenesis and until 

Recent weathering set in is indicated by the abundant 

organic matter and iron-sulfide minerals preserved in 

black shales.

SALINITY

A general review of uranium geochemistry indicates 

that it is doubtful that salinity, which is a quantitative measure of the total dissolved solids in water, 

can in itself be an important factor in the geochemical 

processes of uranium precipitation and concentration 

in sediments. Salinity measurements can be used, 

however, to indicate the amount and distribution of 

uranium in sea water.

In the Baltic Sea, which is partially restricted from 

free ocean-water circulation, the salinity of the water 

ranges from a few to about 35 parts per thousand, the 

latter typical of normal sea water. In parts of the 

Baltic a positive correlation between uranium content 

and salinity of the waters is indicated (Karlik, 1939,
100 URANIUM IN CARBONACEOUS ROCKS

p. 12-13; Koczy, Tomic, and Hecht, 1957, p. 95), but 

the probable underlying explanation for this relation 

is that the inflowing marine water of high salinity 

contain 2 to 3 times more uranium than the average 

river water entering the basin. The uranium content 

of the waters of the Norwegian fjords has not been 

studied, but if the river water feeding a fjord has a 

significantly different uranium content from the marine water that periodically invades the fjord from 

the sea, salinity determinations also could be expected 

to be a crude measure of uranium content of the water. 

Of the several geochemical parameters discussed, 

the value of interpreted salinities of ancient seas in 

understanding uranium concentration in shale is believed minimal, particularly when the salinity of an 

ancient sea is a type of second-order interpretation, 

based on interpretations of paleogeography and paleoecology.

DIAGENETIC CHANGES IN BLACK MUDS

In the descriptions, data, and interpretations in the 

preceding part of this paper, emphasis has been placed 

on the chemical systems and processes prevailing in 

the water overlying black mud deposits; the genesis 

of several components of mud and shale, particularly 

uranium, has been related directly to the chemical 

nature of the overlying water. Very little attention 

has been paid to the chemical processes and changes 

active within the mud deposits themselves. To some, 

this apparent omission may be basis for serious criticism, for several studies (for example, Emery and 

Kittenberg, 1952; Strakhov, 1953; Debyser, 1957) have 

decisively shown that the present chemical and mineralogic organization of many marine sedimentary 

rocks is largely dependent on the major chemical reorganization that occurred during early diagenesis of 

the sediment.

No studies have been made to demonstrate that 

uranium is redissolved, migrates, and is refixed within 

a black mud after the mud is deposited; actually, the 

diffuse dissemination of uranium in shales strongly 

indicates that there were no diagenetic changes in the 

distribution, form, and associations of uranium.

Most studies of physicochemical changes during 

diagenesis have been concerned with chemical conditions in modern sediment that can not be compared 

or related to the black mud environment which is of 

special interest in this paper. These studies of modern 

sediment (reviewed by Strakhov, 1953) have shown 

that the major chemical changes which occur during 

diagenesis are dependent primarily on the position of 

the boundaries between the oxidizing and reducing 

zones and between the alkaline and acid or near neutral zones within the deposited sediment. Under these 

circumstances, iron, for example, is deposited in its 

higher valent or oxidized state. When buried to 

depths of several inches to about a foot it is enveloped 

in a zone of reducing and even slightly acid conditions, whereupon it is dissolved and reduced, it may 

be transported laterally and vertically in intrasediment solutions, and may be finally fixed, commonly 

concentrated, in positions and in mineralogic forms 

that are grossly different from its primary depositional state. Obviously, the products of such chemical diagenetic changes in a sedimentary rock can not 

be related directly to the chemical system prevailing 

in the water above the surface of deposition, but, 

rather, are related directly to the chemical system 

prevailing within the sediment long after deposition 

and burial.

In the marine black mud environment, the critical 

Eh and pH boundaries are not a few inches or feet 

down in the deposited sediment but generally are a 

few inches to several tens of feet above the surface of 

deposition. Much, if not most of the dissolution, reduction, redistribution, and refixation or reprecipitation of sediment components is accomplished prior to 

sediment deposition, or at least before the diagenetic 

stage. This latter generalization does not preclude 

possible diagenetic transformation and redistribution 

of some minerals within the black muds, such as some 

sulfide compounds and clays, but it is very unlikely 

that uranium is so affected. Further, there should be 

very little similarity between the physicochemical 

changes characterizing the diagenetic history of modern sediments deposited in oxygenated alkaline water 

and these changes in mud deposited in strongly reducing and slightly acid water; drastic chemical diagenetic changes characterize the former, and relatively 

minor changes the latter.

The author, therefore, has adopted the viewpoint 

that the observed components and chemical characteristics of highly uraniferous marine black shale can 

be related directly to the originally deposited sediment and directly to the chemical system peculiar to 

the stagnant overlying water of a marine basin. As 

the water became less stagnant, these direct relations 

gradually were obscured, and chemical diagenetic 

processes gradually became more active.

The major physical change in muds during diagenesis probably is the decrease in volume by compaction 

because of burial by younger sediments (Weller, 

1959). The initial water content of marine mud is in 

the 50 to 95 percent range, and this water content is 

reduced to less than 1 to about 5 percent on the burial
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 101

of the mud and its transformation to shale. Compaction results in some reorientation of grains in the 

sediment, in the obliteration of some primary structural features, and in some structural bending of 

softer materials around almost incompressible substances; however, the net effect of compaction on the 

distribution of uranium is negligible, other than that 

associated with simple volume change.

SUMMARY

Utilizing the results of four types of studies, the 

most probable geologic controls and geochemical processes involved in the concentration of uranium in 

marine black shales are presented. These four types 

of studies are: (a) geologic studies on uraniferous 

marine black shales, chiefly represented by a part of 

the Chattanooga shale; (b) sedimentologic studies on 

modern uranium-bearing black mud of the Norwegian 

fjords and the Baltic Sea; (c) oceanographic or hydrographic studies on modern sea water in which this 

mud is deposited; and (d) organic and inorganic 

chemical studies involving laboratory experiments in 

uranium precipitation under conditions related to those 

observed in the first three categories.

The exact chemical and physical state of most of 

the uranium in marine black muds and shales is not 

known. The uranium is in its reduced state, but the 

proportions of this uranium held by sorption to 

organic matter, fixed in an organic molecule, or distributed as colloidal-sized uranium minerals have not 

been determined. Therefore, the form and manner of 

emplacement of uranium must be interpreted on the 

basis of known interrelations of other constituents in 

the sediment and of known chemical processes active 

in the environment of black mud deposition.

FACTORS CONTROLLING URANIUM DISTRIBUTION 

IN BLACK SHALES

Fourteen variable factors have been postulated in 

the literature and in this report as controlling the 

uranium content of marine black muds and shales. 

These factors are divided into two groups, A and B, 

on the basis of whether their quantitative significance is 

speculative or demonstratable. The factors of speculative significance are: 

Group A:

1. Type of rocks composing the source area.

2. Rate and type of weathering of these 

source rocks.

3. Uranium content of stream waters.

4. Relative distance of mud deposition from 

mouths of rivers.

5. Uranium content of sea water.

6. Depth of sea water and rate and pattern 

of circulation of sea water.

7. Amount of planktonic organisms in the 

sea water capable of concentrating its 

uranium.

8. Rate of sediment deposition.

9. Amount of iron sulfide in sediment. 

10. Amount of calcium carbonate in sediment. 

The following factors may be measured or supported 

by chemical or geologic evidence: 

Group B:

1. Uranium in organic matter:

a. Uranium inherent to living plants or 

animals, either land or marine types.

5. Uranium sorbed from sea water by 

either solid or colloidal remains of 

plankton settling from surface water to bottom.

c. Uranium sorbed from stream and sea 

waters by either solid or colloidal 

organic matter derived from decaying land plants and deposited from 

suspension or as precipitated humates.

2. Uranium, probably as colloidal-size uraninite, reduced and precipitated from sea 

water in hydrogen sulfide environment: 

a. Uraninite(?) precipitated from connate water, where hydrogen sulfide 

environment is confined to sediment; 

pyrite also forms in sediment. 

5. Uraninite(?) precipitated from sea 

water, where hydrogen sulfide is generated within sediment and diffuses 

into overlying waters; pyrite also 

forms within sediment.

3. Uranium in phosphate (carbonate-fluorapatite) precipitated from sea water and incorporated by ionic substitution for calcium:

a. Uranium in phosphate precipitated 

from sea water depleted in uranium 

and amassed on and within sediment 

as sparse oolites and nodules; also 

uranium in phosphatic fossil shells 

and bones.

5. Uranium in phosphate precipitated 

from, settling through, and exposed 

to sea water above sediment-water 

interface.
102 URANIUM IN CARBONACEOUS ROCKS

4. Uranium in clastic mineral sediment, landderived : 

&. Uranium in main mass of sediment,

particularly clay minerals which

have sorbed uranium. 

b. Uranium in heavy minerals such as

zircon, apatite, and sphene.

Each of the factors in group A can be argued, 

largely by deductive reasoning, to have an influence 

on the amount of uranium in a black shale. Some of 

these variable factors, particularly 1, 2, 3, 4, and 7, 

are so conjectural and have such an indirect bearing 

as to be next to meaningless; their importance has 

been emphasized in the past, but on a closer look 

they are found to involve so many assumptions and 

highly subjective interpretations as to render them of 

doubtful and unknown significance.

Factors 5 and 6 have a more direct bearing on the 

problem, because they can be evaluated in semiquantitative terms and with fewer assumptions; however, 

it is unlikely that ancient sea water contained any 

more than the 3 ppb uranium that modern sea water contains. The depth and the rate and pattern 

of circulation of sea water can be shown to be of fundamental importance if much of the uranium in black 

muds and shales was precipitated from sea water, 

but the magnitude of these parameters in ancient seas 

obviously involves considerable guesswork. The rate 

of sediment deposition (factor 8) is also of critical 

importance because of the diluting effect of nonuraniferous sediment; it is included in this group of factors 

of speculative significance because it too must be evaluated on an "other-factors-being-equal" basis, though 

these other factors are markedly fewer than those in 

the evaluations of the preceding items.

The amounts of iron sulfide and of calcium carbonate (factors 9 and 10) in black muds and shales 

can be precisely determined by chemical means, but 

neither is combined chemically with uranium, and the 

quantitative relations of each to amounts of uranium 

in the muds and shales may be largely fortuitous. It 

is true that all uranium-bearing black muds and shales 

have abundant iron sulfide, but the reverse situation 

does not hold. A negative correlation between the 

amount of uranium and the amount of calcium carbonate deposited in a black mud theoretically could 

result only where a delicately parallel rise and fall 

of pH, which largely controls carbonate precipitation, 

and of oxidation-reduction potential (Eh), which may 

control uranium accumulation, took place in the bottom waters; a constant supply of both the constituents also would have to exist. Thus, iron sulfide or 

calcium carbonate might serve as suggestive and indirect evidence of the presence or absence, respectively, 

of uraniferous shale; however, they provide no firm 

basis for quantitative evaluations of the uranium 

content of marine black muds and shales.

In group B) each of the major factors are of undoubted importance in determining the uranium content of marine black muds and shales. The subfactors 

(#, 2>, <?) of the major items are less well differentiated, 

but are at least theoretically distinguishable, and are 

recommended points of departure for future research 

in solving problems of uranium distribution.

Within group B, most of the phosphate (factor 3) 

and the clastic sediment (factor 4) can be physically 

separated from the rest of the sediment and the uranium content of each determined directly. The amount 

of uranium attributable to the organic matter (factor 1) and to direct precipitation in the hydrogen 

sulfide environment (factor 2) are not so readily determined. Existing methods for isolating chemically 

undisturbed organic matter and subdividing it into 

its several types for uranium analysis are inadequate. 

The uranium precipitated in the hydrogen sulfide environment is so intimately associated with the organic matter, and to some extent with the clastic sediment, as to defy existing separation techniques, and 

even microscopic and autoradiographic differentiation.

Until laboratory techniques are developed that will 

unquestionably segregate the several components in 

group #, the amount of uranium assignable to each 

must be estimated on the basis of total uranium content, of analyses of the best available, but still impure, organic and mineral fractions, and of the theoretical considerations which follow.

KNOWN AND THEORETICAL RELATIONS 

SOURCE OF URANIUM

Most of the uranium in marine black muds and 

shales was derived from sea water. Present-day sea 

water contains about 3 ppb of uranium, which is in 

the highly soluble form of uranyl and uranyl complex ions. For the following reasons, no real basis is 

believed to exist for concluding that the main mass of 

water in ancient black-shale seas contained significantly more or less uranium, or different forms of 

uranium: (a) Several studies on the general history 

of the composition of sea water indicate that it has 

changed very little since early geologic time; (b) 

studies in Eussia indicate that no correlation exists 

between the uranium content of sedimentary rocks and 

the age of these rocks; (c) no epicontinental seas of 

today are known to have a uranium content significantly different from that of the open seas; (d) the
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 103

uranium content of some modern black mud is very 

similar to that of many ancient black shales; (e) differences in uranium content of black shales can be 

better ascribed to variability and effectiveness of pertinent processes of uranium concentration than to differences in the uranium content of sea water; and (f) 

the volume and mobility of as dilute a uranium solution as sea water makes it an ample reservoir of uranium for any black shale or modern mud, and definitely makes an academic question of the probability 

of several parts per billion, more or less, in ancient 

seas.

For these reasons, the general availability of uranium in seas of the past can be compared directly 

with that in modern basins of deposition. For the 

main mass of sea water this availability of uranium 

is here taken to be 3 ppb of uranium; data on the 

Baltic Sea and Norwegian fjords indicate that the 

epicontinental seas of the past contained 1 to 3 ppb. 

The stagnant water immediately above the bottom of 

the seas where black muds are accumulating probably 

is depleted in uranium and contains less than 1 ppb 

of uranium for intermittent, and probably long, periods of time. It is under these circumstances that 

the availability of uranium should be considered.

ORGANIC MATTER

The relation of organic matter to uranium in marine black shales and muds must still be discussed in 

general terms because of inadequate information. It 

is unlikely that much uranium was assimilated during 

the living stage of the original organisms; rather, 

uranium was withdrawn from the sea water at some 

stage after death and during decomposition and deposition of the organic remains. The questions of kinds 

and proportions of these uranium-fixing substances in 

marine muds and shales and of whether they were 

derived from land or were locally derived from plankton have not been answered.

Some geologic evidence, strongly supported by results of laboratory experiments, indicates that the humic type of organic matter has the greatest affinity for 

uranium. Solid humic matter, such as wood fragments, and the soluble humic substances generally 

termed "humic acids" adsorb or complex the uranium 

in its uranyl form from stream and marine waters 

enroute to the place of deposition. The solid humic 

matter is deposited from suspension, and the humic 

acids are precipitated as humates either by coming 

into contact with the divalent cations common to more 

saline water or on entering the slightly acidic bottom 

water characteristic of the black mud environment. 

On entering the black mud environment, the uranyl

uranium attached to organic substances is reduced to 

its uranous form.

HYDROGEN STTUFIDH

Hydrogen sulfide is a common product of decomposition of organic matter by anaerobic bacteria in 

modern sediment and of bacterial reduction of the 

sulfate ions in the water within and overlying this 

sediment. Subsequent combination of this hydrogen 

sulfide with ferrous iron in the sediment results in 

the formation of iron sulfides, which are present in 

abundance as iron monosulfide in black muds and as 

pyrite in most uraniferous black shales. The formation of hydrogen sulfide and consequent formation of 

pyrite in much organic-rich sediment is confined below the surface of mud deposition; but, where there 

is little or no circulation of the overlying water, hydrogen sulfide rises into and becomes concentrated in 

the bottom water.

Laboratory experiments have demonstrated that uranium in solution is readily reduced and precipitated 

as uranium oxide on introduction of hydrogen sulfide into the solution; the minimum amounts of uranium and hydrogen sulfide that are necessary to result in uranium reduction and precipitation have not 

been determined, but, until otherwise shown, this 

mechanism must be considered as an important one 

in the syngenetic concentration of uranium from sea 

water into black muds and shales.

The black muds of the Norwegian fjords contain 

as much as 0.0060 percent uranium, and the water 

overlying these muds contain from a few to as many 

as 199.5 cu cm hydrogen sulfide per liter of water. If 

the uranium in these muds was precipitated by the 

action of hydrogen sulfide, the overlying water must 

have been renewed many times to explain the amount 

of uranium in these muds, which is dependent mainly 

on the vertical thickness of the water charged by hydrogen sulfide.

The general conclusion of laboratory studies on the 

Chattanooga shale is that, though much of the uranium in the shale is not directly associated or chemically bound by the organic matter, the uranium was 

probably deposited in the shale because of the presence of organic matter. The precipitation of ultramicroscopic disseminated uranous oxide by the hydrogen sulfide produced in the presence of decomposing 

organic matter best explains the findings of these 

studies. The abundant pyrite in the Chattanooga 

shale indirectly attests to the abundance of hydrogen 

sulfide in the mud of the Chattanooga sea at the time 

of deposition, and it is likely that the hydrogen sulfide 

was diffused into the overlying water between times
104 URANIUM IN CARBONACEOUS ROCKS

of intermittent water circulation to precipitate the 

uranium available in the water.

PHOSPHATE

Uranium is known to be chemically bound to the 

phosphate, and proportional to it, in some phosphorite 

formations and in some black shales having abundant 

phosphatic layers and nodules. Parts of the Chattanooga shale and the black muds in the Norwegian 

fjords, however, contain only traces of phosphate but 

relatively large amounts of uranium, with no apparent relation between the two components. From what 

is known about the concentration of uranium in the 

fjord muds and from the fact that the abundant 

phosphate in the lower fjord waters is not deposited, 

it appears that uranium can be geochemically and 

mineralogically independent of phosphate, as it apparently is in many marine black shales.

Four stages that determine the precipitation of uraniferous phosphate in a changing chemical environment in the bottom waters of a marine basin are postulated. The key chemical change in this four-stage 

cycle is one of pH. Stage 1 is characterized by pH 

of about 6.5 to 7.0 and is the stagnant, reducing environment in which organic-rich muds are deposited 

and preserved; at this pH, phosphate in the settling 

organic remains is dissolved and concentrated in solution in the bottom water, though uranium may be 

reduced and precipitated as uraninite(?) and may be 

deposited in the nonphosphatic black mud. Stage 2 

is a transition from stage 1 where the pH of the 

water rises to near 7.5 and the first phosphate is precipitated and deposited, generally as nodules, which 

are then associated with the organic-rich muds. The 

amount of uranium in these nodules is dependent on 

the limited availability of uranium in the water from 

which the phosphate is precipitated; if much of the 

uranium in the water has been withdrawn by precipitation during stage 1, these nodules may contain only 

0.0010 to 0.0030 percent uranium.

Stage 3 is indicated by the increase above 7.5 of pH 

of the bottom water, a consequence of some circulation and oxygenation of the water; only minor amounts 

of organic matter are preserved in the sediment, and 

abundant phosphate commonly is precipitated during 

this stage in the form of oolites, nodules, and discontinuous layers. If these phosphatic materials are 

precipitated slowly from waters not previously depleted in uranium, they may contain as much as 0.1 

percent uranium. Stage 3, where the pH of the water has increased to near 8.0 owing to increased circulation and oxygenation, is characterized by deposition 

of calcium carbonate and (or) silica, with little or no

carbonaceous material being preserved; phosphate precipitation is minimal and is disseminated through the 

deposited sediment. Though this phosphate may be 

as uraniferous as that precipitated during stage 3, it 

is rarely abundant enough to make the enclosing sediment contain more than 0.0002 percent uranium.

pH, EH, AND SALINITY

The pH, Eh, and salinity are three measurable parameters of a chemical environment indicative of the 

general conditions in sea water favorable for the concentration of uranium in black shale. The pH and 

Eh in the bottom water of a marine basin are controlled in large part by the rate and type of decomposition of the deposited organic matter. The pH under 

these conditions, as suggested by measurements in the 

Norwegian fjords and the Baltic Sea, is slightly acid 

or near neutral, 6.5 to 7.5, and the Eh negative, 0.0 

to -0.4, both distinctly abnormal for a marine environment. The acidity of the bottom water in the 

black mud environment may cause the precipitation 

of soluble humic substances, uranyl humates, which 

can assimilate uranium ions to more than 5 percent 

by dry weight while enroute in their soluble state from 

land areas to these bottom waters. The role of pH 

in the precipitation of phosphate was described on 

page 104.

The mechanism of reduction and precipitation of 

uranium from sea water, as effected under negative 

Eh conditions, involves the action of hydrogen sulfide.

Geochemical processes involving uranium deposition 

apparently are independent of varying salinity of the 

water, though differences in salinity values may indicate differences in uranium content of the water. The 

Baltic Sea studies show that if a significant difference 

exists between the uranium content of the fresh water 

entering a basin and the uranium content of the marine water in the basin, salinity measurements may 

indicate the amount of uranium in the mixed waters.

co]srcLirsiO]sr

In conclusion, a graph is presented (fig. 27) that 

shows the best estimates that can be made at the present time on the distribution and associations of uranium in marine black muds and shales. The precise 

amount of uranium allotted to each uranium-bearing 

constituent in each of the muds and shales is largely 

theoretical, and may be as much as 100 percent in 

error. This probable error will be reduced only when 

the several black shales and muds are studied in 

greater detail and the applicability and efficiency of 

geochemical processes of uranium enrichment in organic-rich muds and shales are unequivocally and 

quantitatively defined.
GEOLOGY AND GEOCHEMISTRY OF URANIUM IN MARINE BLACK SHALES 105

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J

Uranium precipit. 

loidal-size uran 

sea water by h;

; above sedimen 

j bic bacteria, a

into overlying v from sea 

or colloidal HHB 

remains of B£i1 

ig from sur- Hfel 

bottom Uranium n phos

; cipitated on 

! sediment as 

1 nodules where

1

ated as colnite(?)from 

 drogen sulwithin and 

t by anaerond diffused 

vater

phate preand within 

jolites and 

hydrogen

rom stream sulfide environment exa water by tends into overlying water; 

3r colloidal also uranium in phosphatic 

er derived fossil shells and bones 

land plants

i ; .':*',; £* '' of the humic type ^v5s>S 

..... .'£**'" Uranium n resistates or yv?*x 1

| EXPLANATION heavy minerals, such as If m 

m f| ana H2 zircon, apatite, and sphene a

Pv ^f'''Pi w§!^ :!£!$$!: Uranium precipil

I clastic mineral sediment. Uranium inherent to living hydrogen sul 

S -Mainly Q!, Oz, and P2 particularly clay minerals, plants or animals, either and confined 

§t but exclusive of resistates land or marine types ment

\\xx\ 

Uranium in phos

ated as col- through, and 

raninite(?) sea water abovi 

i water by water interface, 

ide formed where hydro 

within sedi- environment c 

sediment

phate pren, settling 

exposed to 

e sedimentand formed 

len sulfide 

.onfined to

0.0000 0.0020 0.0040 0.0060 0.0080

URANIUM CONTENT, IN PERCENT

FIGITBB 27. Graph showing estimated average amount contributed to the total uranium content by nine uranium-bearing constituents of

shales and muds.

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 1954a, Beconnaissance for uranium in the Indiana coal 

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 1937, Two land-locked fjords in northern Norway: 

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 1958, The significance of humus in the geochemical 

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110 URANIUM IN CARBONACEOUS ROCKS

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INDEX

A Page

Acidity, laboratory experiments____ 78,87,98-99

modern marine muds___________ 77,98

Norwegian fjords.__ _.. ... 77,96

relation to precipitation of phosphate _ 96,97

Acknowledgments.._____.________ 69

Algae ____._______________ 89

Alum shale, of Sweden, uranium content__ 68,

82,83-84

Animals, possible source of enrichment _ 89-80 

Antrim shale, studied for uranium content... 72 

Appalachia, granitic landmass, possible source

of uranium in Chattanooga shale.. 83

Baltic Sea, chemical data on bottom sediment

and water.... ____...__ 79,93

modern marine muds, acidity______ 77,98 

organic matter__ _________ 76,87-89 

oxidation-reduction potential_______ 77,99 

phosphate. . __ __ 77,93 

salinity.......... ._ 77,99-100

source of uranium, summary_ .- ___ 105 

sulfldes.. - .-.....-..-. ........ 76,91,93

uranium content .. __ 76,79,87,93 

See also Sea water, modern. 

Batelle Memorial Institute, studies on Chattanooga shale___________ 71,79 

Bentonite, In Chattanooga and Pierre shales... 86 

Black, defined in relation to shale______ 69-70 

Black Sea, example of black mud deposition... 94-95 

Black shale, defined___. _________ 69-70

C

Calcium carbonate, relation to phosphate

deposition. .. __ 96-98 

relation to uranium in black shale..___ 99

Calico Bluff formation, studied for uranium

content 72

Cattixylon, most uraniferous constituent in

Chattanooga shale .... ..._ 86,87

Carbonaceous shale, synonymous with black

Carbonate fluorapatite, capacity to take up

uranium. 78,98,101 

Categories of studies 75,76-77 

Center Hill Reservoir, Term., potential uranium source area________ 71 

Chattanooga shale, bentonlte________ 85 

compared to alum shale of Sweden... 82 

composition.....___ ____.__ 79,88,90

example of uraniferous marine black shale. 76-

77,79 

fossils ..........___..__...... 86,87,89,90

investigated 1944-57 for uranium content- 71,

72,73,74 

organic matter....__ .._ __ 76,86

phosphate content__......_ 77,95,96,97,105

probable environment of deposition....... 95

pyrite ................. .. 76,90-91

relation of hydrogen sulflde, conclusion of

laboratory studies.. 103-104,105

596549-61

Chattanooga shale Continued P*S» 

relation of uranium to phosphate,

summary____________ 104,105 

source of uranium content 83 

sulfides........ -. .....- . ... 76,90-91

uranium content ____ - _ 76,85,87,95,105 

See also Uraniferous black shale. 

Chemical variables. See Variables. 

Cherokee shale, studied for uranium content.. 72 

Compaction of muds ...__ 93,100-101 

Conclusions... ........... ...... 71,74,104-105

D

Deflnltlons.-.. ...................... 69-70,86,87

Diagenetic changes in black muds....... 100-101

Dictytmema shale, previous studies 68,83 

source of uranium. ..... 83

E

Eagle Ford shale, studied for uranium content. 72-

73

Experimental data______ .... 71, 

72,73,78-79,87,88,90,92,93,98-99,103-104

Field studies..... . . 70-73 

Foerstia, in uraniferous marine black shales 86,89 

Fort Payne chert._ 96,97 

Fossils, Chattanooga shale . 86,89,90 

Framvaren Fjord, hydrogen sulflde in black

mud..... 92

Q

Galena, in Chattanooga shale_... 90 

Oassaway member. See Chattanooga shale. 

Granitic terrane, possible source of uranium

in sea water __ 83-84,85 

Gotland deep. See Baltic Sea. 

Gullmarfjord, uranium content______ 81-82

Hartville formation, studied for uranium

content.. __. . 72,73 

Heath formation, studied for uranium

content. _ _ . _ 72,73

Hellefjord, sulfide in bottom water______ 92

Humates, defined 87

uranium content. ___ ... 88,103

Humic acids, extracted from Chattanooga

shale... 88 

known and theoretical relations .... 103,105

mechanism of enrichment..._.___. 78,87,89

source 87,88,89

Humic type of organic matter, defined. 86

Hydrogen sulfide, Baltic Sea... 77,79,92-43

Black Sea........ ..................... 94-95

control of pH and Eh..__ .. 90,99 

known and theoretical relations, summary____________ 103-104,105 

laboratory experiments______ 78,90,92,93 

Norwegian fjords 77,79,92-93 

possible extrapolations and interpretations._.-...-... ......- ... 93-95

Hydrogen sulflde Continued

precipitation of uranium..._____. 78,90-91 

relation to pyrite.._ .. 90, 

91-92,93,103 

I

Indre Topdalsfjord, hydrogen sulfide in bottom water 92

Investigations for unlferous black shales,

1944-67... _. .. . 70-74

Iron sulflde. See Sulfides.

Laboratory studies. See Experimental data.

M

Marcasite, in Chattanooga shale...... 90

Marine black shale and mud, chemical constituents and properties controlling concentration of uranium In.. 76-77 

Marine fossils, characteristic of marine black

shale..._.___._______ 70 

Maury formation, typical of one stage of phosphate deposition 96,97 

Mechanisms of enrichment, chemical variables...-....-. ......_ 98-100

hydrogen sulfide_....._____...__ 90-95

organic matter______________ 86-flO

phosphate_________________ 95-98

Mechanisms of transportation of uranium. _ 81

Muds, examples of uranium-bearing.. 76-80

modern marine, acidity _ 77,104

hydrogen sulfide________ 76,103,105

organic-matter, content 76,86-87

oxidation-reduction potential. 77,91,99

phosphate____ __ 77,95,104

See also Norwegian fjords, modern

marine muds.

N

Nonmarine black shale, characteristics 70

Norwegian fjords, chemical data on bottom

sediment and water. 76-77,79 

modern marine muds, acidity 77,98 

hydrogen sulfide. 76,90,92-93,94,103,105 

organic matter 76,86 

oxidation-reduction potential 77,99 

phosphate 77,95 

salinity.............................. 77,100

uranium 76,83,86,90 

uranium content of water... 79,81-82,100

Organic matter, correlation with uranium in

Chattanooga shale . 91 

in Chattanooga shale.. 76,86,87,88,89 

in modern marine muds..____ 76,86-87 

In modern sea water__....__ ... 88-89 

In uraniferous marine black shale 78 

known and theoretical relations, summary . 103,106 

laboratory experiments. . 78,87,88,91,98

111
112 INDEX

Page 

Organic matter Continued

mechanism of enrichment, amounts and

types. . 86-87 

humic acids__________ 78,87-89,103 

incorporated by living plants and animals... 89-90,103,105

solid.. . . 87-89

types defined _ 86 

Oxidation-reduction potential, critical boundary...___.. ___ ___. 99,100 

known and theoretical relations, summary -. 104 

laboratory experiments ____.._ 78-79,90 

mechanism of enrichment-_ 99 

modern sea water and marine muds.._. 77,99 

uraniferous marine black shale 99

Pennsylvania State University, studies on

Chattanooga shale__ 72,80,86,88,90,91 

Pennsylvanian black shales, investigation of

uranium content_______ 71,72,73 

relation of phosphate to uranium 95,98,105 

pH of marine water, Baltic Sea__ 77,79,93,98,104 

normal sea water 77,97,81,98 

Norwegian fjords. 77,79,98,104 

relation to precipitation, humic acids 78,

87,98-99

phosphate_____________ 96-97,104 

uranium.... 78,98-99,104 

Phosphate, Chattanooga shale 77,95,96,97,105 

cycle of deposition__ _ 97-98,104 

geochemical conditions for deposition. 96-97 

in modern marine muds__ .. 77,95 

in modern sea water__.____ _ 77,79,93 

known and theoretical relations, summary- __ - 104 

laboratory experiments--._ - 78 

mechanism of enrichment__ __.__.. 95-98 

relation to uranium.... 71,73,77,95-96,97,98,105 

Phosphatic nodules. See Phosphate. 

Phosphoria formation___________ 71,73,98 

typical of one stage of phosphate deposition___. _.___._- 98 

Physicochemlcal changes. See Diagenetic

changes in black muds.

Pierre shale, bentonlte___ ________ 85

studied for uranium content__..___ 72

Plants, possible source of enrichment.._ 89-90

Previous studies..__________ ... 68,70-74

See also Experimental data.

Purpose, of investigations reviewed in study.. 70,71 

of study... _. 

Page

Pyrite, in Chattanooga shale. 90,103 

relation to hydrogen sulflde. __ 91,93^ 103 

relation to uranium__________... 91-92

8

Salinity, known and theoretical relations,

summary______.___._ 104

laboratory experiments- _ - 79

mechanism of enrichment________ 99-100

modern marine waters_______ 77,79,99-100

Sapropelic type of organic matter, defined.... 86

Scope of report_________________ 74,81

Sea water, ancient, uranium content and

source 82-83,84-85,102-103 

chemical variables, summary ._ .. 104 

influence of source rock on uranium content _... .... 83-85 

laboratory experiments. _ 78 

mechanism of transportation of uranium. 85,103 

modern, acidity.. .. . 77,98 

hydrogen sulfide________ 76,93-95 

organic-matter content. ___. 76,88-89 

oxidation-reduction potential____ 77,99 

phosphate content... 77,95-96 

salinity.. 77,99-100 

uranium content ______ 76,81-82,83,84 

only logical source of uranium in marine

black shale____________ 80-81 

source and chemical form of uranium content.. ___ . 81 

Shale, lithologic term defined______ .. 69-70 

Sharon Springs member. See Pierre shale. 

Skagerrak, uranium content of water_ . 79,82 

Source rock, influence on uranium content of

water In bordering seas. 83-85 

Sulfides, Chattanooga shale- __ 76,90-91 

in modern marine muds______ 76,91,92,94 

in modern sea water __________ 76,93-95 

in uraniferous marine black shale 76,90-91 

known and theoretical relations, summary. . - 103-104,105 

laboratory experiments_______ 78,90,92,93 

mechanism of enrichment, bottom water 

of Norwegian fjords and Baltic 

Sea ______ 92-93 

possible extrapolations and interpretations.-.. _ . 93-95 

precipitation of uranium...__... 78,90-91 

relation of pyrite to uranium_____ 91-92 

Summary, conclusion____________ 104-105 

factors controlling uranium distribution in

black shales__________- 101-102 

investigations, 1944-57___ _. ._ 73-74 

known and theoretical relations 102-104

Page

Sweden, uranium-bearing black shale and

kolm.... ... 68,71,82,83-84,88

U

Uraniferous black shale, defined_______ 70 

investigations_______________ 70-74 

source of uranium, summary. ______ 105 

See ofeo Chattanooga shale and Pennsylvanian black shales.

Uranium, a trace element in black shale... 70 

amount contributed by constituents of

shales and muds_________ 105

average content, river water.__...___ 81,82

sea water 76,81,83

shale 70,105

chemical constituents and properties controlling concentration______ 76-77 

in marine black shale___ 71,76,80,81,83,84-85 

in modern marine muds. ____ 76,83,86,87,90 

in modern sea water. __.. 76,81-82,83,84 

in water, laboratory experiments. 78,84,87,93,98 

Independent of phosphate in marine black

shale . 96 

known and theoretical relations, organic

matter...__ __ .___ 103,105 

source.________.____._ 102-103,105 

limitation of amount precipitated by hydrogen sulfide ___ 93-94 

mechanisms of enrichment__ ... 74,86-100 

mechanisms of transportation______ 81 

relation to acidity_____________ 99 

relation to phosphate_________ 97,98,104 

relation to pyrite . 91-92 

sources . _ . 74,80-85 

summary of factors controlling distribution in black shale_______ 101-102 

summary of known and theoretical relations, hydrogen sulfide- 103-104,105 

Uranium-bearing shale and mud, examples. 76-80

Variables as mechanisms of enrichment,

acidity.._ ________ 98-99 

oxidation-reduction potential_______ 99 

salinity 99-100

Volcanic rock, possible source of uranium in

ancient sea water.... . 84-S5

W

Woodford shale, studied for uranium content. 73

o