Radioactivity and Uranium 

Content, Sharon Springs 

Member of the Pierre Shale 

Kansas and Colorado t

GEOLOGIC A LSURVE BULLETIN 1 0 46- L

/j report concerns work done on behalf 

of the U. S* Atomic Energy Commission 

and is published with the permission of 

the Commission

Radioactivity and Uranium 

Content, Sharon Springs 

Member of the Pierre Shale 

Kansas and Colorado

By E. R. LANDIS

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGICAL SURVEY BULLETIN 1046-L

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission

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

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

The U. S. Geological Survey Library has cataloged this publication as follows:

Landis, Edwin Robert, 1926-

Eadioactivity and uranium content, Sharon Springs member of the Pierre shale, Kansas and Colorado. Washington, 

U. S. Govt. Print. Off., 1959.

iii, 299-319 p. maps, 4 profiles (in pocket) tables. 23 cm. (U. S. 

Geological Survey. Bulletin 1046-L. Contributions to the geology of 

uranium)

Bibliography: p. 317-318.

1. Uranium ores Colorado. 2. Uranium ores Kansas. 3. Radioactivity. 4. Shale. (Series: U. S. Geological Survey. Bulletin 

1046-L. Series: U. S. Geological Survey. Contributions to the 

geology of uranium)

553.49

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

Washington 25, D. C. - Price 700 (paper cover)
CONTENTS

Page

Abstract_______________________________________ 299

Introduction________________________________ 300

Stratigraphy_______________________________________ 300

Radioactivity and uranium content.__________________ _________ 302

Surface data__________________________________ 302

Subsurface data_________________________________________ 305

Origin of the uranium______________________________________ 307

Other trace elements___-________________________________________ 311

Summary and conclusions._______________________________________ 316

References___________________________________ 317

Index_______________________________________ 319

ILLUSTEATIONS

[Plates in pocket]

PLATE 35. Columnar sections of Sharon Springs member of Pierre shale 

in Wallace and Logan Counties, Kans.

36. Columnar sections of lower part of Pierre shale in Cheyenne, 

Kiowa, Crowley, and Pueblo Counties, Colo.

37. Columnar sections of lower part of Pierre shale in Las Animas 

County, Colo.

38. Gamma-ray logs of lower part of Pierre shale and upper part 

of Niobrara formation in eastern Colorado.

FIGURE 27. Map showing locations of exposures of lower part of the Pierre

shale in Kansas and Colorado_________________ _______ 303

28. Map of eastern Colorado showing the location of wells used 

in radioactivity log study of the Pierre shale and Niobrara 

formation._________________________________________ 306

TABLES

TABLE 1. Units containing more than 0.002 percent uranium and their

thicknesses..._________________________ ______ __ __ 304

2. Trace elements composition of some Cretaceous shale beds from

Kansas and Colorado_______________________________ 312

3. Minimum concentrations of the elements detectable by the

semiquantitative spectrographic method, in parts per million. 314

4. Description of samples in table 2__ ______ __ __________ 314

m

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

RADIOACTIVITY AND URANIUM CONTENT,

SHARON SPRINGS MEMBER OF THE PIERRE SHALE,

KANSAS AND COLORADO

By E. E. LANDIS

ABSTRACT

As part of the program of the U. S. Geological Survey in the investigation of 

uranium-bearing carbonaceous rocks on behalf of the U. S. Atomic Energy 

Commission, a reconnaissance of the Sharon Springs member of the Pierre 

shale of Late Cretaceous age in western Kansas and eastern Colorado was 

made during 1954.

The Sharon Springs member of the Pierre shale and its lateral equivalents 

range from 155 to about 500 feet in thickness and generally contain about 0.001 

percent uranium, but some beds contain larger amounts. A 6-foot shale bed 

in Cheyenne County, Colo., contains about 0.006 percent uranium; a 4^-foot 

sequence of beds in Crowley County, Colo., is estimated to contain between 

0.004 and 0.005 percent uranium; and a 3%-foot sequence of beds in Kiowa 

County, Colo., contains about 0.004 percent uranium. At several outcrop localities, sequences of beds as much as 9% feet thick contain about 0.003 percent uranium. Data from wells indicate that the 4%-foot thick sequence of 

beds in Crowley County, Colo., may have a lateral extent of at least 5^ miles. 

A gamma-ray log of a well in Yuma County, Colo., indicates a sequence of beds 

66 feet thick which contains 0.005 to 0.010 percent equivalent uranium.

No definite pattern of areal distribution of radioactivity and uranium content in the Sharon Springs is indicated by available data. Lateral variation in 

uranium content of individual beds was not noted in outcrops, which seldom 

extend more than 150 feet. Subsurface data from gamma-ray logs of wells 

indicate however that both the maximum radioactivity and the thickness of 

radioactive beds are variable within distances of a few miles. Most of the 

gamma-ray logs show that only part of the sequence of rocks comprising the 

Pierre shale and Niobrara formation exhibits radioactivity in excess of the 

average radioactivity of the two formations. Comparison of gamma-ray logs 

of wells in northeastern Colorado suggests that the most radioactive parts of 

the Pierre shale and Niobrara formation are a laterally correctable sequence 

of beds.

The stratigraphic position of the radioactive unit relative to the Pierre shaleNiobrara formation contact in oil industry scout reports, as identified from 

electric logs of wells, is variable within short distances. This may indicate 

that some of the Pierre-Niobrara contacts picked from electric logs may not 

correspond to the boundary that would be selected by examination of the rocks 

themselves, or it may indicate a facies relationship between the lowermost part 

of the Pierre shale and the uppermost part of the Niobrara formation.

299
300 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

INTRODUCTION

During the field season of 1954, reconnaissance and detailed studies 

of uranium-bearing black shales of Late Cretaceous age in western 

Kansas and eastern Colorado were made as part of the program of 

the U. S. Geological Survey in the investigation of uranium-bearing 

carbonaceous rocks on behalf of the Division of Kaw Materials of the 

U. S. Atomic Energy Commission. Other investigations of the Sharon 

Springs member were made in South Dakota and northeastern Nebraska by K. C. Kepferle (written communication, 1958) and in the 

Chadron area of Nebraska and South Dakota by K. J. Dunham 

(1955, p. 164). The primary objective of these studies was to determine the content and distribution of uranium areally and stratigraphically in the Sharon Springs member of the Pierre shale of Late Cretaceous age. Other shales of Late Cretaceous age were also examined.

STRATIGRAPHY

The Sharon Springs member of the Pierre shale was named by 

Elias (1931, p. 56) from exposures in Wallace and Logan Counties, 

Kans. At the type locality, the Sharon Springs consists of 155 feet 

of flaky, black shale containing an abundance of small fish remains. 

Dane, Pierce, and Reeside (1937, p. 225) extended the name into 

eastern Colorado as an equivalent of the Barren zone of Gilbert 

(1897). Searight (1938, p. 137) used the name for the basal member 

of the Pierre in South Dakota, and Condra and Eeed (1943, p. 17) 

applied the name to the lowermost part of the Pierre in Nebraska.

In western Kansas the Sharon Springs is overlain by Elias' Weskan 

member of the Pierre shale, which is the lowest of three units named 

by Elias (1931, p. 56) in 430 feet of Pierre overlying the Sharon 

Springs. In eastern Colorado the Sharon Springs is overlain by 

the Eusty zone defined by Gilbert (1897). Dane, Pierce, and Keeside 

(1937, p. 225) correlated the Rusty zone of Gilbert with the three 

units named by Elias (1931, p. 56) that overlie the Sharon Springs 

in western Kansas.

The Sharon Springs member is underlain in western Kansas by the 

Smoky Hill chalk member of the Niobrara formation; in eastern 

Colorado this is called the Smoky Hill marl member of the Niobrara 

formation. In the extreme western part of the area, the Sharon 

Springs is underlain by the Apishapa shale. The Apishapa is equivalent to the upper part of the Smoky Hill marl and chalk of easternColorado and western Kansas (Cobban and Reeside, 1952, pi. 1). The 

lithologic contact between the Pierre shale and Niobrara formation is. 

apparently conformable and at some localities appears gradational. 

A few localities show an alternation of dark gray to black calcareous.
URANIUM IN SHARON SPRINGS MEMBER, PIERRE SHALE 301

and noncalcareous shales. The contact between the Pierre and the 

Niobrara is generally placed at the top of the youngest limestone or 

calcareous shale in the outcrop.

The Sharon Springs is predominantly black to dark-brown, noncalcareous, organic-rich shale. The shale commonly breaks and 

weathers into tough, angular, fragments ranging from %o to % of an 

inch in thickness and as much as 2 inches in diameter. Scales, spines, 

scutes, and other minute remains of fish are so common as to be a definitive feature of the Sharon Springs. Other fossils are so rare, however, that this part of the Pierre was named the Barren zone by Gilbert (1897). Fragments of mollusk shells were noted in several limestone concretions; and a few bone fragments, probably of a marine 

reptile, were found in Wallace County, Kans.

Bentonitic clay beds are common in the Sharon Springs and are not 

restricted to any particular stratigraphic horizon. The beds are usually less than 6 inches thick and the majority range from 14 to 2 inches 

in thickness. Tourtelot (1956, p. 63) reports bentonite beds almost 20 

feet thick in the Sharon Springs in northwestern Nebraska and near 

the Black Hills, but no bentonite beds of that magnitude were observed 

in western Kansas or eastern Colorado. In most places the bentonitic 

clay beds, which are light gray where unweathered, are represented on 

weathered surfaces by an intimate mixture of soft, iron-stained, tan 

to yellow clay; red-brown limonite or hematite; and small, randomly 

oriented, prismatic crystals of selenite.

Selenite, in the form of rosettes as much as 6 inches in diameter 

and individual crystals, is usually abundant on weathered outcrops. 

Contact twinning of tabular crystals is common. On several weathered 

outcrops, small veinlets of selenite cut across the bedding planes of the 

shale. Selenite and gypsum commonly encrust the concretions in the 

Sharon Springs.

Ellipsoidal concretions of concentrically banded dense gray limestone or shaly limestone are present in the Sharon Springs. Oval calcareous shale concretions, some of which contain gypsum or selenite, 

are also common. The larger and more conspicuous concretions are 

septarian, in which the dense microcrystalline gray limestone is divided by radiating septaria of calcite and barite. A few of the nonseptarian limestone concretions have an outer crust of tan to light-gray conein-cone limestone. Several concretions noted consisted of opposed 

layers of cone-in-cone limestone without intermediate material. Small 

tabular concretions consisting of limestone and mudstone are common 

in parts of the Sharon Springs. A few oval iron carbonate concretions, which are abundant in the overlying Eusty zone of Gilbert, are 

also present in the Sharon Springs.
302 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Elias (1931, p. 65) assigned a thickness of 155 feet to the Sharon 

Springs at the type locality in western Kansas. Fifteen miles southwest of the type locality the author measured about 194 feet of Sharon 

Springs. In eastern Colorado, Dane, Pierce, and Keeside (1937, p. 

225) assigned a thickness of about 400 to 500 feet to the Sharon 

Springs. The author measured a columnar section in Las Animas 

County, Colo., in which the strata probably assignable to the Sharon 

Springs are less than 260 feet thick. Tourtelot (1956, p. 63) states 

that in South Dakota, the Sharon Springs and strata correlated with 

it range from 17 to 150 feet in thickness, and Condra and Eeed (1943, 

p. 17) assigned a thickness of 20 to 80 feet to the Sharon Springs in 

Nebraska.

There are few distinctive laterally correctable units in the Sharon 

Springs; hence, the stratigraphic relations between parts of the member in different outcrops is usually uncertain. Elias (1931, p. 59) subdivided the Sharon Springs into upper and lower parts according to 

the abundance of large concretions found in the upper part as contrasted to their scarcity and smallness in the lower part. Elias (1931, 

p. 60-61) also indicated that a zone of septarian concretions that occurs in the upper part of the Sharon Springs is laterally correctable 

in Wallace and Logan Counties, Kans. Septarian concretions are also 

present in the Sharon Springs of eastern Colorado (Dane, Pierce, and 

Keeside, 1937, p. 225). As the concretions are the most resistant constituents of the Sharon Springs, they crop out more often than does 

the remainder of the member; and the tendency is to correlate the 

zones in which concretions occur. Several correlations of septarian 

concretion zones are indicated on plates 35 and 36, but some of them 

may be erroneous.

RADIOACTIVITY AND URANIUM CONTENT

SURFACE DATA

Figure 27 shows the locations of exposures of the lower part of the 

Pierre shale in Kansas and Colorado that were examined for radioactivity, measured, and sampled. Plates 35,36, and 37 show columnar 

sections of the rocks at each of the localities and the equivalent uranium 

and uranium content of samples collected from the rock units.

Lateral correlation of parts of the Sharon Springs member of the 

Pierre shale and associated rock units exposed at the sample localities 

is somewhat uncertain, but the probable stratigraphic position of the 

beds examined at each locality relative to nearby localities is indicated 

on the columnar sections (pis. 35, 36 and 37). No definite laterally 

correlative stratigraphic sequence was consistently more uraniferous 

than other parts of the Sharon Springs. The radioactivity and
i X 18 

j X| 9

i '

i PUEBLO

_._._._ J

I CROWLEY | | 3*K

i 15 |

I1 -

C H E Y E 

10

IOWA

_/

/-' ' LAS ANIMAS

x 20

X 21

1 

i

j

/

j

j

i 

..i.

FIGURE 27. Map showing locations of exposures of the lower part of 
304 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

uranium content of individual beds were found to be laterally consistent within the outcrop area of the parts of the Sharon Springs and 

the adjacent parts of the Pierre which were examined. However, 

few of the beds were exposed for more than 150 feet at any of the 

localities.

The lower part of the Pierre shale in the area generally contains 

0.003 percent equivalent uranium or less, and 0.001 percent uranium 

or less. Units containing more than 0.002 percent uranium and their 

thicknesses are given in table 1.

TABLE 1. Units containing more than 0.002 percent uranium and their thicknesses

Locality (pis. 35, 36, and 37)

2. ____--_---__--_-_--___--_-____ __ ._..__._

3 -.-- - _ _ . _- -

9 _.- -_ _ , __ .

10_._ _- ___ _ _ _________________________

12 _... __ ._ _ _ . _ _

13_ ----- . _ _ .

15--_--_-_ _ _____________________________

17___--____-___- -_. - -_ _

18___-_.----__ ___-_-_____________ _____

20. ___. ___________ _________________ __

Thickness

Ft. 

1

\ 2

h

6

I »

4

1 I

3 

1

/ 9 

I '4

In. 

10 

0 

0 

0 

0 

0 

6 

0 

0 

0 

4 

2 

0 

1 

2 

6 

9

U content

Percent 

0.003 

.004 

.003 

.004 

.004 

.006 

2 .003 

.006 

2 .003 

. 003-. 002 

. 004-. 003 

.005 

2 . 007- 005 

.003 

.003 

.003 

.003

1 Between this unit and the one above is a unit containing 0.002 percent uranium or less. 

1 These figures are interpolated from scintillation counter readings of the tabulated unit and scintillation 

counter readings and chemical analyses of contiguous beds.

The disequilibrium between the equivalent uranium content and 

the uranium content in most of the samples may be due to leaching 

of uranium from the shale. Tourtelot (1956, p. 68) suggested that 

the equivalent uranium content of weathered shale of the Sharon 

Springs member in South Dakota and Nebraska may be representative of the uranium content of some of the unweathered shale. 

Many samples collected in this investigation contained exactly as 

much uranium as equivalent uranium and lend support to Tourtelot's 

suggestion. However, the equivalent uranium and uranium determinations of the weathered shale samples reported here generally differ 

by 0.001 to 0.002 percent uranium; for example, samples with 0.003 

percent equivalent uranium generally contain from 0.001 to 0.002 percent uranium. Some of the excess radioactivity indicated by these 

relationships probably results from potassium in the shale.
URANIUM IN SHARON SPRINGS MEMBER, PIERRE SHALE 305 

SUBSURFACE DATA

Figure 28 shows the location of wells in northeastern Colorado 

for which radioactivity logs were studied. Radioactivity logs normally consist of two curves: the gamma-ray log, which records the 

natural gamma radiation emitted by the rocks through which the 

well passes; and the neutron log, which records the gamma radiation 

emitted by the rocks as a result of neutron bombardment from a 

source introduced into the hole, the gamma radiation given off by 

the introduced source and reflected back to the gamma-ray detector, 

as well as the natural gamma-radiation of the rocks. Only the 

gamma-ray curve indicating the natural radiation of the rocks was 

considered in this report. Significant parts of the gamma-ray logs 

of those wells, indicated on figure 28 by a solid circle and a number, 

are shown on plate 38. The equivalent uranium content of the shale 

in some of the wells indicated by "X" on figure 28 is taken from 

gamma-ray logs reported by Tourtelot (1956, fig. 6); the others are 

based on the author's examination of logs.

The maximum deflection produced by rocks in the Pierre and 

Niobrara formation on the gamma-ray logs of the wells on figure 28 

is expressed as percentage of equivalent uranium. Gott and Hill 

(1953, p. 70) found that in the Rangely field of northwestern Colorado, an equivalent uranium content of about O.OOOT percent in core 

samples of the Weber formation caused a deflection of 1 inch in the 

gamma-ray curve at a 10-inch sensitivity scale. The accuracy of 

this figure is dependent on many complicating factors, such as the 

thickness versus the radioactivity of the bed, fluid content of the 

well, shielding effect of the well casings, differences in individual 

instruments, and the rate of movement of the ionization chamber. 

However, analyses of core samples from a well penetrating the Bakken formation of local usage in North Dakota were compared with 

the gamma-ray log of the well, and the findings of Gott and Hill 

were found to be applicable to within an accuracy of 0.001 percent 

equivalent uranium (K. C. Kepferle, personal communication). The 

maximum radioactivity shown by the gamma-day logs is at locality 1, 

where a sequence of beds about 66 feet in thickness has an estimated 

equivalent uranium content ranging from 0.005 to 0.01 percent.

Available data do not indicate any definite areal distribution pattern of radioactivity in the Pierre and Niobrara sequence. More 

wells in the eastern and northeastern parts of the map area (fig. 28) 

exhibit radioactivity near the indicated maximum than do the wells 

in the central and western parts. However, no definite conclusions 

on this point can be reached until more subsurface information is
306 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

0.006 eu 0.009 eU I P H I LX L I P S i 

i 0.0076U

O.OOTeU

_ _ _ ____ _ _j __ _ _ xg.go_,4eu

_^ ~"~b.oo5eu

) J_ | 0.003 eLT~j*p006eU

S , \ s H t N G T 0 N

EXPLANATION J JEFFER\J

Locality of gomma-roy /DOluGLASl E L B E R T

log shown in plate 38 j y KIT CARSON

Locality of other selected i_/_ -

mo-ray log using the 

following ratio- I -

h of def

10- inch sensitivity

CROWLEYi/ K I 0 W A

Outcrop line of Pierre 

shale tOoshed where 

approximately located)

PUEBLO

FIGURE 28. Map of eastern Colorado showing the location of wells used in radioactivity 

log study of the Pierre shale and Niobrara formation.

available in the southern part of the area in which the Pierre and 

Niobrara are present.

The contact between the Pierre shale and the Niobrara formation 

as shown on plate 38 is as listed in oil industry scout reports. The 

contact listed in such reports is generally picked by resistivity and 

self-potential electric logs. At some places, however, the radioactivity log was probably used to pick the contact. For one well 

(loc. Y) no contact was listed, but the part of the gamma-ray log 

shown in plate 38 probably includes the Pierre-Niobrara contact. 

Examination of the electrical logs of several wells shown on figure 28 

indicates a change in lithology at depths listed by oil industry reports 

as being the Pierre-Niobrara contact. At locality 10 (pi. 38) for 

example, the Pierre-Niobrara contact agrees closely with changes in
URANIUM IN SHARON SPRINGS MEMBER, PIERRE SHALE 307

the electrical log curves that indicate a change in the lithology of 

the rock. The parts of the gamma-ray logs shown in plate 38 include 

the Pierre-Niobrara contact as picked in oil industry scout reports, 

and the part of the Upper Cretaceous series here considered is hereafter referred to as the Pierre and Niobrara sequence.

Gamma-ray logs show that usually only one part of the Pierre and 

Niobrara sequence exhibits radioactivity far in excess of the average 

amount generally shown by these formations. The gamma-ray logs 

indicate that the more radioactive part of these formations ranges 

in thickness from less than 10 feet to about 150 feet. Many peaks of 

radioactivity within the more radioactive part can be correlated in 

the gamma-ray logs of wells in northeastern Colorado. This suggests 

that the most radioactive part of the Pierre and Niobrara sequence in 

northeastern Colorado is a correctable sequence of beds.

In northeastern Colorado the radioactive unit is located stratigraphically within about 200 feet of the Pierre-Niobrara contact as picked 

from electric logs. The radioactive unit varies in stratigraphic position relative to the Pierre-Niobrara contact, indicating that the contact 

picked from electric logs does not always correspond to the lithologic 

boundary that would be picked by examination of the rocks themselves. Another possibility is that the radioactive unit crosses formational boundaries, and there is a facies relationship between the 

lowermost part of the Pierre and the uppermost part of the Niobrara.

The gamma-ray log of the well at locality 12 (pi. 38) is interesting 

for several reasons. The approximate stratigraphic position of the 

Pierre-Niobrara contact is listed in scout reports as picked by use of 

rock samples from the well; it is about 130 feet below the base of 

the most radioactive part of the Pierre. The most radioactive and 

uraniferous part of the Pierre in nearby outcrops is a selenitic sandy 

shale unit, containing iron carbonate concretions, located at some 

distance above the top of the Niobrara formation (loc. 15, pi. 

36). The radioactive unit shown in the gamma-ray log at locality 

12 (pi. 38) may be the same as that observed at the surface. This is 

suggested by the presence of a sandy, gypsiferous, gray shale unit 

with a lirny layer at the top that is 155 feet above the top of the 

Niobrara in a well 31,4 miles west of locality 12 and absent in a well 

about 10 miles north of locality 12 (W. E. Hallgarth, personal 

communication).

ORIGIN OF THE URANIUM

Most of the problems relating to the origin of uranium in such black 

shales as the alum shales of Sweden and the Chattanooga shale of 

the Eastern United States are unsolved. As a group, the uraniferous
308 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

black shales of the world are sapropelic, highly carbonaceous, high 

in sulfide content, noncalcareous, and somewhat phosphatic. These 

shales are believed to have accumulated slowly, probably under 

anaerobic conditions (McKelvey, 1955, p. 15). The Sharon Springs 

member of the Pierre shale possesses most of these distinguishing 

characteristics, though quantitative data are lacking for some of them.

The uranium in most of the black shales thus far studied has a 

definite relationship to the organic fractions. Bates and Wright 

(written communication, 1953) studied thin-section autoradiographs 

of the Chattanooga shale and found that the observable alpha tracks 

emanated from the interstitial organic matter and larger organic 

fragments associated with pyrite. Small amounts of uranium are 

present in the Upper Cambrian black shales of Sweden, and greater 

amounts of uranium are concentrated in layers of kolm a black 

kerogen of obscure origin (V. E. McKelvey, written communication, 1955). However, Breger and Deul (1955) have found that 

their study of a pyrite-rich sample of the Sharon Springs member 

from South Dakota indicates that the uranium in the shale may be 

associated with the mineral fraction.

Conditions producing a low redox potential seem to have been 

conducive to the formation of most uraniferous black shales, but 

other physical and chemical factors, such as the rate of accumulation 

of sediments, variation in the available supply of uranium, variation 

in the relative concentration of other elements in solution, and the 

pH, which affect the precipitation of uranium from aqueous solution, 

may be equally important.

The uranium in the shales may have been deposited syngenetically; that is, adsorbed by organic matter or precipitated from 

aqueous solution and deposited simultaneously with the associated 

material of the shale. All or part of the uranium may have a penecontemporaneous origin, that is, adsorbed by organic matter subsequent to deposition and prior to compaction and burial.

At several of the localities where outcrops of the Sharon Springs 

were examined, the most uraniferous beds in the exposure were associated with bentonitic clay beds, though the clay generally contains 

very little uranium. In South Dakota, the part of the Sharon 

Springs that contains the most uranium is commonly associated with 

bentonites (R. C. Kepferle, personal communication, 1955). However, many parts of the Sharon Springs in Kansas and Colorado, which are associated with bentonitic clay beds, are no more 

uraniferous than the remainder of the exposed rocks; and at several 

localities, where parts of the Sharon Springs were found to be most 

uraniferous, no bentonitic clay beds were apparent. At several ex-
URANIUM IN SHARON SPRINGS MEMBER, PIERRE SHALE 309

posures, concentrations of uranium believed to be secondary accumulations on the present land surface are intimately related to the presence of bentonitic clays.

No obvious relationship exists between the syngenetically or penecontemporaneously deposited uranium in the shales and the bentonitic 

clay beds in the sequence. Tourtelot (1956, p. 78) suggested, however, that the uranium content of a sea in which volcanic ash was deposited would be increased during the alteration of the ash to bentonite. In this manner the bentonitic clays could have contributed 

uranium and other metals to other parts of the Sharon Springs without any direct relationship being apparent between the bentonitic 

clays and the uranium content of adjacent sediments.

Concentrations of uranium believed to be epigenetic in origin are 

associated with bentonitic clays at localities 12 and 15 (pi. 36). 

Many bentonitic clay beds and the adjacent shales were sampled and 

found to contain no more uranium than the exposed rocks as a whole. 

At locality 12 (pi. 36), however, the shale unit exhibiting the greatest radioactivity in the field and containing 0.004 percent equivalent 

uranium and 0.006 percent uranium is associated with a l/^-inch bed 

of altered bentonitic clay which contains 0.005 percent equivalent 

uranium and O.OOG percent uranium. It is probable that only part of 

the uranium present in the shale is epigenetic, and the remainder 

syngenetic or penecontemporaneous, but no quantitative estimate is 

possible. At locality 15 (pi. 36) the part of the measured section 

containing the most uranium immediately overlies a sandy shale and 

clay unit that is evidently the lateral equivalent of a 4-inch bentonitic 

clay bed a short distance away. The 4-inch bentonitic clay bed contains 0.002 percent equivalent uranium and 0.001 percent uranium, 

but the weathered mixture of clay, limonite or hematite, and selenite 

that underlies the bentonitic clay contains 0.007 percent equivalent 

uranium and 0.006 percent uranium; and the weathered mixture 

above the clay contains 0.018 percent equivalent uranium and 0.020 

percent uranium.

A zone of weathered shale and clay ranging in thickness from 

less than i/2 mcn to about 3 feet, is generally present where the bentonitic clay beds crop out. The bentonitic clay is usually almost entirely altered and replaced by a mixture of soft clay, limonite or 

hematite, and selenite; the adjacent strata are coated and permeated 

with iron oxide and selenite. A possible explanation of the relationship of the bentonitic clay to the epigenetic concentrations of uranium 

is that the clay by its impermeability creates a local perched water 

table that controls the direction of flow of water in the rocks superjacent to the bentontic clay beds. At locality 15 (pi. 36) there is a
310 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

marked differential in uranium content between the weathered mixtures above and below the clay bed. The sampling procedure at 

locality 12 (pi. 36) does not indicate whether this differential also 

exists at that locality.

The persistence of iron oxide in the more radioactive units suggests 

that the concentration of uranium in these units is partly due to iron. 

Lovering (1955, p. 186) concluded that when acid sulfate waters 

carrying uranyl sulfate and ferric sulfate are neutralized, the ferric 

sulfate hydrolizes to form colloidal ferric oxide hydrate, which adsorbs the uranyl ion and later expels it to form secondary uranium 

minerals when the ferric oxide hydrate crystallizes to form geothite.

The uranium may also be in the selenite. A sample of a selenite 

veinlet at locality 3 (pi. 35) contained 0.001 percent equivalent 

uranium and 0.002 percent uranium. Bain (1953, p. 207) has shown 

that calcium can precipitate uranium from uranyl sulfate solution 

by appropriating the sulfate ion. Substitution of uranium for calcium may occur, as their respective ionic radii are similar but it is 

unlikely because of the difference in ionic charge (McKelvey and 

others, 1955, p. 468).

Bentonitic clay beds and adjacent zones of weathered shale do not 

assure the presence of uranium, however, because many of these 

zones and clay beds that were examined and sampled contained no 

more uranium generally than did the adjacent rocks. Other factors, 

which appear especially essential in concentrating uranium on 

weathered outcrops, are the amount of uranium available in aqueous 

solutions for concentration; and the preservation of the uranium 

from subsequent leaching after it is concentrated. The availability of 

uranium is a function of the uranium content of the unweathered 

shale, and the preservation of the uranium is a function of the 

mechanisms of concentration, and possibly of the topographic setting.

Some other uranium occurrences probably are partly or totally 

epigenetic in nature. At locality 2 (pi. 35) uranium is concentrated 

in one part of a limestone concretion, and at locality 3 (pi. 35) the 

outer crust of a concretion contains twice as much uranium as does 

its center. At locality 2 the uranium is evidently concentrated in 

the part of the concretion that contains the most organic material. 

Studies on the Colorado Plateau and other areas indicate that carbonaceous material may assist in the precipitation of uranium from 

solution by the creation of a localized reducing environment. This 

mechanism may be responsible for the concentration at locality 2. 

At locality 3 the calcareous shale that makes up the interior of a 

concretion contains 0.002 percent equivalent uranium and 0.002 percent uranium that may be of syngenetic origin. The outer, cone-
URANIUM IN SHARON SPRINGS MEMBER, PIERRE SHALE 311

in-cone, limestone crust of the concretion contains 0.002 percent 

equivalent uranium and 0.004 percent uranium. The crust must have 

formed after the deposition of the enclosing sediments, but whether 

the uranium was emplaced in the crust at the time of formation or 

has been subsequently concentrated in it, is unknown.

OTHER TRACE ELEMENTS

Table 2 shows the results of semiquantitative spectrographic 

analyses of samples of the Sharon Springs member of the Pierre 

shale and other shales of Cretaceous age collected in western Kansas 

and eastern Colorado. Table 3 lists the standard sensitivities for the 

elements. Table 4 describes the samples shown in table 2.

The sample analyses shown in table 2 include a black shale from 

the Cheyenne sandstone member of the Purgatoire formation, a black 

shale from the Blue Hill shale member of the Carlile shale, a calcareous shale from the uppermost part of the Smoky Hill chalk member of the Niobrara formation, and twelve samples from various 

horizons in the Sharon Springs member of the Pierre shale.

The sample of shale from the Purgatoire formation (139759) contains less barium than any other sample. The trace element constitution of the sample is generally similar to the samples of the 

Sharon Springs member except that it contains less scandium, vanadium, and barium, and more zirconium, than do most of the Sharon 

Springs samples.

Although the trace element composition of the Blue Hill sample 

(139758) is in general similar to the analyses of the Sharon Springs, 

several striking differences are apparent in the comparative contents 

of the rare earth elements. Three of these elments, cerium, lanthanum, 

and neodymium, are in the semiquantitative range from 100 to 500 

ppm and were not detected in any of the other samples. The relative concentration of lanthanum and neodymium in the Blue Hill 

sample (139758) is particularly noteworthy because the minimum 

concentrations detectable by the spectrographic method are 30 ppm 

lanthanum and 60 ppm neodymium. Thus the sample contains more 

than three times as much lanthanum and more than twice as much 

neodymium as do any other samples. Cerium, lanthanum, and neodymium are closely related in chemical properties (Goldschmidt, 1954, 

p. 310) and their association is not surprising. If the other elements 

of the cerium earth elements group, praseodymium and samarium, are 

present they occur in quantities less than the minimum detectable 

concentration by semiquantitative spectographic analysis.

The sample of the Smoky Hill chalk member of the Niobrara formation (139726) is distinct from all of the other samples by the larger
TABLE 2. Trace elements composition of some Cretaceous shales 

[Semiquantitative spectre-graphic analyses by O. Annell, U. S. Geological Survey. Leaders indicate that the elem

concentrations listed in table 3. Other elements listed in table 3 were also looked for but are not present in det

Code, parts perjnillion

> 100,000.. _- ..._._.-_----- -----_---_--. 11

50,000-100,000.-...--..-....-.--..--------.-- 10

10,000-50,000- ............... .---- . 9

5,000-10,000 - - -. -------- 8

1,000-5,000------------------------------- 7

500-1,000-- - -- 6

100-500.. ....----. 

50-100.-- ... - -

10-50----.....-.

5-10.. ....... .

1-5

Sample

139759------------- ----------

139758

139726--. -------------------

139727 - -

139732.--. ---------------------- -

139733 

139734--.-.---- ------------

139754

139755------ ----- ------------

139756. 

139757. 

139760

139761... ...- -----------------

139762 

139763 - -.-- 

Be

1

1

1

1

B

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

Na

7

9

7

7

7

8

8

8

7

9

9

9

9

9

9

Mg

7

9

8

9

7

7

7

7

7

9

9

9

9

8

9

Al

11

10

9

10

11

11

10

10

10

10

10

10

10

10

10

Si

11

11

9

11

11

11

11

11

11

11

11

11

11

11

11

K

8

9

8

9

9

9

9

9

9

9

9

9

9

9

9

Oa

7

7

11

9

9

9

9

9

7

7

9

9

9

8

9

Sc

2

3

3

3

3

3

2

3

3

3

3

3

3

3

3

Ti

7

7

6

7

7

7

7

7

7

7

7

7

7

7

7

V

3

4

5

5

4

5

5

5

5

5

5

4

4

5

5
Sample

139759. _ .. 

139768 .-.

139726 

139727. .. . . . . --.....

139732- 

139734. . ...- .-._. ... . -.-.-. ......

139754. .............................................

139755 -- - 

139756- - - -

139757..-..... ..... . 

139760. 

139761 ....

139762 

139763. 

Zr

5

5

3

3

3

3

3

3

4

4

4

4

4

5

5

Mo

4

3

4

4

5

5

3

5

3

4

3

Ag

(>)

Sn

4

Ba

3

6

5

5

5

5

5

5

5

6

6

6

7

6

6

La

5

Y

1 0.1 to 0.5 ppm Ag present.

2 Selenium, arsenic, and uranium contents were determined chemically by M. Delevaux and J. Goode, U

are percent.
314 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

TABLE 3. Minimum concentrations of the elements detectable by the semiquantitative 

spectrographic method, in parts per million

[Revised January 13,1954. Washington laboratory, U. S. Geological Survey]

Element

Minimum 

concentration

Element

Minimum 

concentration

Element

Minimum 

concentration

Ag. 

AL 

As_ 

Au.

Ba.. 

Be.. 

BL. 

Ca.. 

C<L. 

Ce.. 

Co.. 

Cr.. 

CsCu.

Eu.. 

Er._ 

F... 

Fe_. 

Ga_ 

Gd. 

Ge..

0.1

1

100

10

60

10

.5

50

100

50

300

.5

6

,000

60

30

30

800

8

40

60

10

Hf.

Ho.. 

In.. 

Ir... 

K...

La.. 

Li.. 

Lu.. 

Mg_. 

Mo.. 

Mn. 

Nb. 

Na_. 

Nd. 

NL. 

Os.. 

P 

Pb.. 

Pd_. 

Pr.. 

Pt.. 

Rb.

.3

300

800

10

4

300

3,000

30

400

50

5

7

10

100

60

50

1,000

700

10

30

100

30

70,000

Re.. 

Rh. 

Ru. 

Sb-_ 

Sc.. 

Si... 

Sm. 

Sn.. 

Sr_. 

Ta.. 

Tb._ 

Te_. 

Th..

TL. 

Tm.

U... 

V...

w...

Y...

Yb. 

Zn.. 

Zr..

400

40

80

100

10

50

80

40

10

1,000

100

800

800

5

400

10

800

10

700

30

3

80

8

TABLE 4. Description of samples in table 2

Sample Description

139759_________ Black shale 6 in. thick separating sandstone units of the Cheyenne sandstone member of the Purgatoire formation. Sec. 34, 

T. 28 S., R. 51 W., Las Animas County, Colo.

139758-.-______ Black shale representative of upper 50 ft of Blue Hill shale

member of the Carlile shale. Sec. 34, T. 23 S., R. 55 W., 

Otero County, Colo.

139726_________ Gray calcareous shale from upper 10 ft of Smoky Hill chalk

member of Niobrara formation. Collected on same exposure 

as 139727. Sec. 33, T. 12 S., R. 36 W., Logan County, Kans.

139727_________ Black shale from lower 10 ft of Sharon Springs member of

Pierre shale. Collected on same exposure as 139726. Sec. 33, 

T. 12 S., R. 36 W., Logan County, Kans.

139732_________ Black shale representative of 4 ft of shale underlying lowest

septarian concretion zone in Sharon Springs member at 

McAllaster Buttes. Sec. 13, T. 12 S., R. 37 W., Logan 

County, Kans.

139733- ________ Medium gray shale from Sharon Springs member, representative

of 1-ft unit 6 ft above upper concretion horizon at McAllaster Buttes. Sec. 13, T. 12 S., R. 37 W., Logan County, 

Kans.

139734-________ Black shale from Sharon Springs member, representative of

2-ft unit at upper concretion horizon, McAllaster Buttes. 

Sec. 13, T. 12 S., R. 37 W., Logan County, Kans.
URANIUM IN SHARON SPRINGS MEMBER, PIERRE SHALE 315

TABLE 4. Description of samples in table 2 Continued

Sample Description

139754_____ __ Brown weathered shale from Sharon Springs member. Representative of 6-ft unit of shale below septarian concretion 

horizon. Sec. 29, T. 15 S., R. 46 W., Cheyenne Comnty, Colo.

139755.________ Brown weathered shale from Sharon Springs member. Represents 1M ft of section. Sec. 26, T. 18 S., R. 52 W., Kiowa 

County, Colo.

139756_________ Dark gray iron-stained shale from Sharon Springs member.

Represents \% ft of section. Sec. 30, T. 20 S., R. 56 W., 

Crowley County, Colo.

139757-________ Black shale from Sharon Springs member in upper 100 feet

represents 3 ft 1 in. of section. Sec. 4, T. 22 S., R. 59 W., 

Crowley County, Colo.

139760.________ Black shale adjacent to igneous dike, from Sharon Springs

member. Sec. 30, T. 32 S., R. 68 W., Las Animas County, 

Colo.

139761 ____ Black shale from Sharon Springs member. Sec. 29, T. 33 S., 

R. 62 W., Las Animas County, Colo.

139762.______ Black shale from Sharon Springs member. Sec. 8, T. 19 S.,

R. 65 W., Pueblo County, Colo.

139763.________ Black to dark brown shale from Sharon Springs member.

Sec. 35, T. 19 S., R. 65 W., Pueblo County, Colo.

calcium content and smaller silicon and aluminum contents. The 

sample also contains more zinc, selenium, and arsenic than most of 

the other samples, although the contrast in selenium content is not 

great. Compared to samples from the Sharon Springs in general, 

the Smoky Hill sample (139726) apparently contains less sodium, 

magnesium, potassium, titanium, and copper. The arsenic and selenium contents of this sample are as high or higher than that of the 

Sharon Springs samples some of which contain three times as much 

uranium.

Several areal distribution patterns of specific elements are indicated in the Sharon Springs samples. The samples from along the 

Front Eange of Colorado (139760, 139761, 139762, 139763) contain 

more beryllium than other samples and tend to contain more sodium, 

magnesium, zirconium, and barium. The samples from the McAllaster Buttes locality (139732, 139733, 139734) contain more gallium 

and less chromium than other samples. These elemental variations 

may be largely due to differences in the detrital mineral content of 

the shale.

Sample 139756 contains more than twice as much lead as does any 

other sample. This increased concentration may indicate that at the 

time of deposition reducing conditions were much more favorable 

than at the time of deposition of the other rocks from which samples 

were collected; or it may indicate an increased available supply of 

lead at the time of deposition.
316 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

No definite correlation between uranium and any other element, or 

elements, is apparent from the spectrographic analyses. However, 

the molybdenum and arsenic contents are generally greater in the 

samples containing the larger amounts of uranium. An even less 

definite relationship of the same type may exist between the elements selenium and uranium.

SUMMARY AND CONCLUSIONS

The Sharon Springs member of the Pierre shale in western Kansas 

and eastern Colorado is slightly uraniferous at all localities at which 

it was examined and sampled. Surface examination and sampling 

do not indicate the presence of any widespread zone that is more 

uraniferous regionally than the remainder of the member. At any 

given locality, the more uraniferous beds of the exposed shale vary 

little within the outcrop area, but individual beds were seldom exposed for distances exceeding 150 feet. Closely spaced samples indicate that the vertical differences in uranium content can be very 

sharp, especially adjacent to the more uraniferous parts; however the 

range of uranium content in the shale is not usually large.

Surface and subsurface data suggest that the most radioactive part 

of the Pierre in Crowley County, Colo., is persistent for at least 

51/2 miles (loc. 15, pi. 36 and loc. 12, pi. 38). The gamma-ray logs 

of wells penetrating the Pierre shale and Niobrara formation in 

northeastern Colorado indicate that the maximum radioactivity, 

thickness, and the stratigraphic position of the most radioactive parts 

of the Pierre and Niobrara sequence relative to the Pierre-Niobrara 

contact picked on electric logs are variable at distances of a few miles. 

In most of the gamma-ray logs studied, only part of the sequence 

exhibits radioactivity in excess of the average radioactivity of the remainder of the formations. Features of this more radioactive part 

can be correlated in the gamma-ray logs of wells in northeastern 

Colorado, suggesting a laterally correlatable sequence of beds. If the 

Pierre-Niobrara contacts picked by use of electric logs are accurate 

placements of the lithologic boundary between the Pierre and Niobrara, the radioactive unit crosses formational boundaries, and there 

may be a facies relationship between the Pierre and Niobrara. Another possibility is that some of the contacts picked from electric logs 

may not correspond to the lithologic boundary between the Pierre 

shale and the Niobrara formation that would ordinarily be picked by 

examination of the rocks themselves.

The Sharon Springs member of the Pierre shale and the other 

parts of the Pierre that were examined contain an estimated average 

of about 0.001 percent uranium. The most uraniferous sequence of
URANIUM IN SHARON SPRINGS MEMBER, PIERRE SHALE 317

beds contains about 0.006 percent uranium and is 6 feet thick, as 

indicated by one sample (table 1). Another sequence of beds k% feet 

thick contains 0.004 to 0.005 percent uranium (table 1), and one 3y2 

feet thick is estimated to contain about 0.004 percent uranium (table 

1). At several localities sequences of beds as much as $% feet thick 

contain about 0.003 percent uranium (table 1). The maximum radioactivity observed according to gamma-ray logs is in Yuma County, 

Colo., where a 66-foot sequence of beds shows radioactivity ranging 

from 0.005 to a maximum of 0.010 percent equivalent uranium (loc. 1, 

pi. 38).

The uranium contained in uraniferous black marine shales is generally believed to have a syngenetic or penecontemporaneous origin. 

Inasmuch as this report contributes no data to the contrary, it is presumed that the uranium in the Pierre shale and Niobrara formation 

has a similar origin. Several occurrences of uranium believed to be 

epigenetic in origin were located; but concentrations are small. In 

Crowley County, Colo., a 2-inch bed of mixed bentonitic clay, hematite or limonite, and selenite, contained 0.020 percent uranium. In 

Logan County, Kans., a carbonaceous part of a limestone concretion 

contained 0.016 percent uranium. Several other occurrences, in which 

part or all of the uranium is believed to be epigenetic, contain as 

much as 0.006 percent uranium. At two localities, uranium is of a 

possible epigenetic origin, and the occurrences are believed to be related to the effects of weathering and bentonitic clay beds.

No definite relationship is apparent between uranium and other elements in the semiquantitative spectrographic analyses, but generally, 

the samples containing the larger amounts of uranium also contain 

larger amounts of molybdenum, arsenic, and selenium.

REFERENCES

Bain, G. W., 1953, Experimental simulation of plateau-type uranium deposits: 

U. S. Atomic Energy Comm. RMO-44, 210 p., issued by U. S. Atomic Energy 

Comm. Tech. Inf. Service, Oak Ridge, Tenn.

Breger, I. A., and Deul, Maurice, 1955, Geochemistry of uranium-bearing carbonaceous rocks, in Geologic investigations of radioactive deposits Semiannual progress report, Dec. 1, 1954, to May 31, 1955: U. S. Geol. Survey 

TEI-540, p. 183-190, issued by U. S. Atomic Energy Comm. Tech. Inf. Service, Oak Ridge, Tenn.

Cobban, W. A., and Reeside, J. B., Jr., 1952, Correlation of the Cretaceous formations of the western interior of the United States: Geol. Soc. America Bull., 

v. 63, p. 1011-1044.

Condra, G. E., and Reed, E. C., 1943, The geological section of Nebraska: Nebraska Geol. Survey Bull. 14, 82 p.

Dane, C. H., Pierce, W. G., and Reeside, J. B., Jr., 1937, The stratigraphy of the 

Upper Cretaceous rocks north of the Arkansas River in eastern Colorado: 

U. S. Geol. Survey Prof. Paper 186-K, p. 207-232.
318 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Dunham, R. J., 1955, Sharon Springs black shale in the Ghadron area, Nebraska 

and South Dakota, in Geologic investigations of radioactive deposits 

Semiannual progress report, Dec. 1,1954, to May 31,1955: U. S. Geol. Survey 

TEI-540, p. 164. issued by TJ. S. Atomic Energy Comm. Tech. Inf. Service, 

Oak Ridge, Tenn.

Elias, M. K., 1931, The geology of Wallace County, Kans.: Kansas Geol. Survey 

Bull. 18,254 p.

Gilbert, G. K., 1897, Description of the Pueblo quadrangle, Colorado: U. S. Geol. 

Survey, Geol. Atlas, Folio 36,9 p.

Goldschmidt, V. M., 1954, Geochemistry: Oxford Univ. Press, London, 730 p.

Gott, G. B., and Hill, J. W., 1953, Radioactivity in some oil fields of southeastern 

Kansas: U. S. Geol. Survey Bull. 988-E, p. 69-122.

Hills, R. C., 1899, Description of the Elmoro quadrangle, Colorado: U. S. Geol. 

Survey, Geol. Atlas, Folio 58, 5 p.

Levering, T. G., 1955, Progress in radioactive iron oxides investigations: Econ. 

Geology, v. 50, no. 2, p. 186-195.

McKelvey, V. E., 1955, Search for uranium in the United States: U. S. Geol. Survey Bull. 1030-A, p. 1-64.

McKelvey, V. E., Everhart, D. L., and Garrels, R. M., 1955, Origin of uranium 

deposits in Pt. 1 of Economic Geology, 50th anniversary volume, 1905- 

1955: Urbana, 111., Econ. Geology Pub. Co., p. 464-533.

Searight, W. V., 1938, The microfauna of the Sully member of the Pierre: Iowa 

Acad. Sci. Proc., v. 45, p. 135-137.

Tourtelot, H. A., 1956, Radioactivity and uranium content of some Cretaceous 

shales, central Great Plains: Am. Assoc. Petroleum Geologists Bull., v. 40, 

p. 62-83.
INDEX

Page

Alum shales, Sweden_____________ 307 

Analyses, spectrographic, semiquantitative... 312 

Apishapa shale.________________ 300

Barite, septaria.._______________ 301 

Barren zone of Gilbert, eastern Colorado__ 300 

Bentonlc clay beds.___....___.... 301,308-310 

Black shales, origin of uranium________ 307

Calcite, septaria________________ 301 

Carlile shale, Blue Hill shale member..__.. 311 

Chattanooga shale_______________ 307 

Clay beds, bentonic______________ 301,309 

Cheyenne County, Colo., rock sample_._ 315 

Columnar sections-_________ 302, pis. 35-37 

Concretions, calcareous shale_________ 301 

ellipsoidal-.._.. .........._____ 301

iron carbonate, oval. __________ 301,307 

septarian ..__._.____.__.... 302

Cone-in-cone limestone_____________ 301 

Crowley County, Colo., iron-stained shale,

sample _.__ .... 315

Electrical logs..________________ 306-307 

Equivalent uranium, samples..__ 302, pis. 35-37

Fossils .___________________ 300,301 

Front Range, Colo., rock samples _______ 315

Gamma-ray logs.. _...... 305,307,317

Gypsum______________________ 301

Iron oxide._.________________ 310

Ziowa County, Colo., rock sample______ 315 

Kolm, black kerogen, Sweden________ 308

Las Animas County, Colo., rock samples.. 314,315 

columnar section......_______ 302; pi. 37

Limestone __.______________ 301 

Logan County, Kans., rock samples __.. 314

Page

McAllaster Buttes, Kans., rock samples ... 314

Neutron log_____________ 305 

Niobrara formation, Smoky Hill chalk

member_________ 300,311 

Smoky Hill marl member_____ .... 300

Otero County, Colo., rock sample. ........ 314

Pierre-Niobrara contact________ 306,307 

Pierre shale, Nebraska. ._... 300 

South Dakota__________ 300 

Potassium._________________.. 304 

Pueblo County, Colo., rock samples. 315 

Purgatoire formation, Cheyenne sandstone

member.________-__ 311 

Pyrite.____._____.______. .... 308

Radioactivity, maximum_____ 305

Radioactivity logs___________........ 305

Rusty zone of Gilbert 300

Selenite__...._._........___.__ 301,310

Sharon Springs member, correlation, lateral 302 

thickness______________ 302 

type locality.......___..__... 300

Stratigraphy...._.... ... . 300-302

Subsurface data___._______ 305-307 

Surface data._____________ 302-304

Trace elements____________ 311-316

Uranium, origin.___________ 307,309 

penecontemporaneously deposited- 309 

syngenetically deposited __ 309

Uranium content____________-_ 302-307 

disequilibrium....________ 304 

samples______________ 302,304

Wallace County, Kans_______ 300 

Weskan member___________ . . 300

Yuma County, Colo., gamma-ray logs .... 317

319

O