Factors that Localized Uranium 

Deposition in the Dakota 

Sandstone, Gallup and Ambrosia 

Lake Mining Districts, 

McKinley County, New Mexico 

By CHARLES T. PIERSON and MORRIS W. GREEN 

GEOLOGICAL SURVEY BULLETIN 1485 

A contribution to the geology of 

uranium deposits in Cretaceous rocks 

in the central and western parts 

of the Grants mineral belt 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASIIINGTON : lmW 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Pierson, Charles Thomas, 1918-

Factors that localized uranium deposition in the Dakota Sandstone, 

Gallup, and Ambrosia Lake mining districts, McKinley County, 

New Mexico. 

(Geological Survey bulletin ; 1485) 

Bibliography: p. 30-31. 

Supt. of Docs. no. I 19.3:1485 

1. Uranium ores-New Mexico-McKinley Co. 2. Geology, Stratigraphic-Cretaceous. I. Green, Morris W., joint author. II. Title. 

III. Series: United States. Geological Survey. Bulletin; 1485. 

QE75.B9 no. 1485 [TN490.U7] 557.3s 79-607188 

[ 553.4'932'0978983] 

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

Washington, D. C. 20402 

Stock number 024-001-03281-3 
CONTENTS 

Page 

1\bstract -------------------------------------------------------------------- 1 

Introduction ---------------------------------------------------------------- 2 

Location and geologic setting ______________________________________________ 2 

Previous geologic studies ------------------------------------------------ 3 

Present study ____________________________________________________________ 6 

1\cknowledgments _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ __ _ _ __ _ _ __ __ _ _ _ _ __ _ _ __ _ _ __ _ _ _ _ _ _ _ 6 

Stratigraphy and environments of deposition ---------------------------------- 6 

Morrison Formation ______________________________________________________ 6 

Westwater Canyon Sandstone Member -------------------------------- 7 

Brushy Basin Shale Member------------------------------------------ 7 

Dakota Sandstone ________________________________________________________ 8 

Structure ____________________________________________________________________ 9 

lJraniumDeposits ____________________________________________________________ 11 

Production --------------------------------------------------------------11 

Deposits in the Morrison Formation --------------------------------------11 

Deposits in the Dakota Sandstone ----------------------------------------12 

Mineralogy, habits, and age of the Dakota deposits ____________________ 12 

Mines in the Gallup district ------------------------------------------12 

Diamond No. 2 (Largo No. 2) mine --------------------------------13 

Becenti mine ____________________________________________________ 13 

Hogback No. 4 (Hyde) mine --------------------------------------15 

Deiter prospect --------------------------------------------------16 

lJ and Rats Nest mines ------------------------------------------17 

Mines in the 1\mbrosia Lake District ----------------------------------19 

Febco (Small Stake) mine ________________________________________ 19 

Silver Spur mine ------------------------------------------------20 

Juniormine ----------------------------------------------------20 

Origin of the uranium deposits in the Dakota Sandstone ___________________ _: ____ 21 

Suggestions for exploration __________________________________________________ 22 

Measured stratigraphic sections ______________________________________________ 23 

1. Diamond No. 2 (Largo No. 2) mine ______________________________________ 23 

2. Locality between Diamond No. 2 and Becenti mines ____________________ 24 

3. Becenti mine __________________________________________________________ 25 

4. Hogback No. 4 (Hyde) mine --------------------------------------------26 

5. Deiter prospect ________________________________________________________ 26 

6. Composite section, lJ mine --------------------------------------------28 

7. Febco (Small Stake) mine ----------------------------------------------29 

References cited ______________________________________________________________ 30 

III 
IV CONTENTS 

ILLUSTRATIONS 

Page 

FIGtJRE 1. Index map ------------------------------------------------------ 3 

2. Geologic sketch map of the Gallup mining district, McKinley 

County, N. Mex. ___________ ---------'--- _________________________ 4 

3. Photograph showing view of the outcrops north of Continental 

Divide -------------------------------------------------------- 8 

4. Sketch showing lithologies and delta-plain environments in the 

lower part of the Dakota Sandstone in the Gallup, N.Mex., area __ 10 

5. Photograph showing view of portal of Diamond No. 2 mine __________ 14 

6. Photograph showing view of the basal Dakota Sandstone stratigraphic sequence at the Becenti mine ____________________________ 15 

7. Photograph showing view north toward White Rock Mesa 

showing Dakota Sandstone channel in the Morrison Formation ____ 16 

8. Photograph showing view of radioactive lower Dakota 

Sandstone strata at Deiter prospect ______________________________ 17 

9. Photograph showing view of Dakota Sandstone and upper part 

of the Morrison Formation at the Rats Nest mine ________________ 18 
FACTORS THAT LOCALIZED 

URANIUM DEPOSITION IN THE 

DAKOTA SANDSTONE, GALLUP AND 

AMBROSIA LAKE MINING DISTRICTS, 

McKINLEY COUNTY, NEW MEXICO 

By CHARLES T. PIERSON and MoRRIS W. GREEN 

ABSTRACT 

Geologic studies were made at all of the uranium mines and prospects in the Dakota 

Sandstone of Early(?) and Late Cretaceous age in the Gallup mining district, McKinley 

County, N. Mex. Mines in rocks of Dakota age of the adjacent Ambrosia Lake mining 

district were visited for comparative purposes. 

Mines in the eastern part of the Gallup district and in the Ambrosia Lake district are 

on the Chaco slope of the southern San Juan Basin in strata that dip gently northward 

toward the central part of the basin. Mines in the western part of the Gallup district 

are along The Hogback (Nutria monocline) in strata that dip steeply westward into the 

Gallup sag. 

Geologic factors that controlled formation of the uranium deposits in the Dakota 

Sandstone are (1) the presence of a source of uranium, believed to be uranium deposits 

of the underlying Morrison Formation of Late Jurassic age; (2) the accessibility to the 

Dakota of uranium-bearing solutions from the Morrison; (3) the presence in the Dakota 

of permeable sandstone beds overlain by impermeable carbonaceous shale beds; and (4) 

the occurrence within the permeable Dakota Sandstone beds of carbonaceous reducing 

material as bedding-plane laminae or as pockets of carbonaceous trash. 

Most of the uranium deposits in the Dakota Sandstone are found in the lower part of 

the formation in marginal-marine distributary-channel sandstones. However, the 

Hogback No. 4 (Hyde) mine (Gallup district) is found in sandy paludal shale, and 

another deposit, the Silver Spur (Ambrosia Lake district), is found in what is interpreted to be a massive beach or barrier-island sandstone in the upper part of the 

Dakota. 

The most favorable sedimentary depositional environments for the accumulation of 

uranium are the areas lateral to main distributary channels, where levee, splay, and 

some distributary-channel sandstones intertongue with gray carbonaceous shales and 

siltstones of the well-drained swamp environment. Black carbonaceous shale beds 

formed in the poorly drained swamps in the interfluve areas are not favorable host 

rocks for uranium. 

The depositional energy levels of the various environments in which the sandstone 

and shale beds of the Dakota were deposited govern the relative favorability of the 

strata as uranium host rocks. In the report area, uranium commonly is in carbonaceous 

sandstone deposited in medium-energy channels. A prerequisite, however, is that the 

sandstone be overlain by impermeable shale beds. 

1 
2 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

Medium-energy fluvial conditions result in the deposition of sandstone beds having 

detrital carbonaceous material distributed in laminae or in trash pockets on bedding 

planes. The carbonaceous laminae and trash pockets provide the necessary reductant 

to precipitate uranium from solution. High-energy fluvial conditions result in the 

deposition of sandstones having little or no carbonaceous material. Low-energy 

swampy conditions result in carbonaceous shale deposits, which are generally barren 

of uranium because of their relative impermeability to migrating uranium-bearing 

solutions. 

INTRODUCTION 

LOCATION AND GEOLOGIC SETTING 

Uranium in the Dakota Sandstone of Early(?) and Late Cretaceous 

age is found in five areas in the Gallup mining district (areas 1-5, fig. 

1) and in one area in the Ambrosia Lake mining district (area 6, fig. 

1). Both districts are in McKinley County, northwest New Mexico. 

Occurrences of uranium in the Dakota Sandstone at the Diamond 

No. 2 (Largo No. 2), Becenti, Hogback No. 4 (Hyde), Church Rock, U, 

and Rats Nest mines, and at the Deiter prospect in the Gallup district 

are discussed in this report and are shown by a geologic sketch map 

(fig. 2) of part of the Gallup district. Uranium mines in the Morrison 

Formation, such as the Foutz No. 1-3 mines and the Westwater No.1 

mine are also shown by figure 2. Uranium at the Church Rock mine is 

found in both the Dakota and the Morrison (Hilpert, 1969, p. 75). 

The Febco (Small Stake), Silver Spur Nos. 1 and 5, and Junior 

mines (area 6, fig. 1) in the Ambrosia Lake mining district are also 

discussed,:but they are not on the map in this report. Some or all of 

these mines are shown, however, on maps by Thaden, Santos, and 

Ostling (1966), Hilpert (1969, pl. 1), and Gay and Nestler (1968). 

The geologic settings of the parts of the two mining districts in 

which the Dakota deposits are found are somewhat similar. Gently 

northeast-dipping beds of the Dakota uncomformably overlie strata of 

the Upper Jurassic Morrison Formation everywhere except in the 

area of The Hogback (Nutria monocline) (fig. 2). Throughout the area 

an erosional contact separates the Morrison from the Dakota. 

There are two differences between the geologic settings of the 

mines in the two districts: (1) faulting is associated with the mines in 

the Ambrosia Lake district but is absent in the Gallup district; and 

(2) claystone beds of the Brushy Basin Shale Member of the Morrison 

Formation underlie the Dakota in the Ambrosia Lake district but are 

absent in the vicinity of the Dakota mines in the Gallup district, 

owing primarily to pre-Dakota erosion, or locally to a facies change of 

shale to sandstone. 

Locations and brief descriptions of all of the mines in Dakota-age 

rocks referred to in the present report are given in table 4 of Hilpert 
108°37' 30" 

INTRODUCTION 

GALLUP URANIUM 

MINING DISTRICT 

0 10 Kl LOMETERS 

3 

FIGURE 1.-Index map of parts of the Gallup and Ambrosia Lake mining districts, 

New Mexico, showing areas in which uranium mines or prospects are found in the 

Dakota Sandstone (stippled); 1, Diamond No.2 (Largo No.2) and Becenti mines; 

2, Hogback No.4 (Hyde) mine; 3, Deiter prospect; 4, Church Rock mine; 5, U and 

Rats Nest mines; 6, Silver Spur No. 1, Silver Spur No. 5, Febco (Small Stake), 

Junior, Section 5, and Dakota (Pat) mines. Dashed line indicates boundary of the 

Gallup mining district; eastern edge follows the Continental Divide. 

(1969). Most of the mine locations are shown by the geologic quadrangle maps of the Church Rock (Green and Jackson, 1975), Gallup 

East (Green and Jackson, 1976), Thoreau NE (Green and Pierson, 

1971), and Goat Mountain (Thaden and others, 1966) quadrangles. 

PREVIOUS GEOLOGIC STUDIES 

The only published geologic studies of uranium deposits in the 

Dakota Sandstone of the Gallup-Ambrosia Lake area are found in 

reports by Mirsky (1953), Gabelman (1956), and the U.S. Atomic 

Energy Commission (1959). Hilpert (1969) summarized salient data 

for all of the uranium deposits of northwestern New Mexico, and 

Granger (1968) discussed the localization and control of the deposits 

in the southern San Juan Basin (Grants) mineral belt. Some of the 

data used in this report came from Reimer (1969). 
35°37'30" '-~ ~' ~- . -- -- ., 

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Becent\ 

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Diamond No.2 (Largo No. 2) mine 

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EXPLANATION 

[Qat I ALLUVIUM, COLLUVIUM, AND EOLIAN DEPOSITS 

(HOLOCENE AND PLEISTOCENE) 

I Ku I POST-DAKOTA SEDIMENTARY ROCKS, UNDIFFERENTIATED (UPPER CRETACEOUS)-Menefee Formation, Crevasse Canyon Formation, Gallup Sandstone, 

and Mancos Shale 

j./~~·~::::·~·::1 DAKOTA SANDSTONE (UPPER AND WWER? CRETACEOUS)- Includes the Whitewater Arroyo Tongu~ of 

the Mancos Shale and the Twowells Tongue of the 

~~ 

I Ju I 

Dakota Sandstone 

MORRISON FORMATION (UPPER JURASSIC)- Includes Brushy Basin Shale, Westwater Canyon Sandstone, 

and Recapture Shale Members 

PRE-MORRISON SEDIMENTARY ROCKS, UNDIFFERENTIATED (MIDDLE JURASSIC)-Cow Springs 

Sandstone, Summerville Formation, Todilto Limestone, 

and Entrada Sandstone 

I RC I CHINLE FORMATION (UPPER TRIASSIC) 

----CONTACT 

----~o-- MONOCLINAL ·FLEXURES-Showing upper and lower 

fold axes. Arrows indicate direction of dip, longer 

arrow indicates flatter dip 31 

_)._ STRIKE AND DIP OF BEDS 

--l...,_?--'-- LIMIT OF BRUSHY BASIN SHALE MEMBER-Brushy 

Basin Member of the Morrison Formation )lasbeen 

removed on the hachured side by pre-Dakota erosion. 

Queried where data unavailable 

t:. URANIUM MINE OR PROSPECT -Mine symbol located 

in formation in which ore occurs (Dakota Sandstone 

or Morrison Formation). Ore in the Church Rock 

mine is found in both the Dakota and the Morrison 

FIGURE 2.-Geologic sketch map of part of the Gallup mining district, McKinley County, N.Mex., showing locations of uranium mines and 

prospects. Photogeology by M. W. Green and C. T. Pierson, 1971. 

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6 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

PRESENT STUDY 

Work done by the authors in 1974-75 includes radioactivity and 

stratigraphic measurements, field interpretations of the environments of deposition of the ore-bearing and adjacent strata, and deductions about the probable effects of stratigraphic variations on control 

of uranium deposition. Data from Reimer (1969) are included in the 

measured stratigraphic section for a uranium mine called the U mine 

(measured stratigraphic section 6). 

ACKNOWLEDGMENTS 

Acknowledgment is made to T. N. Parthasarathy, International 

Atomic Energy Agency Fellow from India, who, as part of his training 

with the U.S. Geological Survey, mapped and sampled the U mine in 

1974 and made a petrographic study of selected samples. 

STRATIGRAPHY AND ENVIRONMENTS OF DEPOSITION 

Pre-Quaternary sedimentary rocks exposed in the area covered by 

figure 2 include, in ascending order, the Chinle Formation of Late 

Triassic age; the Entrada Sandstone, Todilto Limestone, Summerville Formation, and Cow Springs Sandstone of Middle Jurassic age; 

the Morrison Formation of Late Jurassic age; the Dakota Sandstone 

of Early(?) and Late Cretaceous age; and the Mancos Shale, Gallup 

Sandstone, and Crevasse Canyon and Menefee Formations of Late 

Cretaceous age. The Chinle and Morrison Formations and the Dakota 

Sandstone (including the Twowells Tongue of the Dakota Sandstone 

and the Whitewater Arroyo Tongue of the Mancos Shale) are mapped 

separately on figure 2, but the other formations are put into two 

undifferentiated groups: (1) pre-Morrison; and (2) post-Dakota. 

The Triassic (Stewart and others, 1972) and Jurassic (Harshbarger 

and others, 1957) rocks are of continental origin. The Lower(?) and 

Upper Cretaceous (Sears and others, 1941) rocks are of marginal 

marine or marine origin. Only the Morrison Formation and the 

Dakota Sandstone will be discussed in this report. 

MORRISON FORMATION 

Three members of the Morrison Formation are present in the report 

area (Craig and others, 1955). In ascending order, they are the Recapture Shale, Westwater Canyon Sandstone, and Brushy Basin Shale 

Members, shown undifferentiated as Morrison Formation on the 

geologic sketch map (fig. 2). 

In the Grants mineral belt, Green (1975) has traced an unconformity at the base of the Westwater Canyon Member from outcrops 
STRATIGRAPHY AND ENVIRONMENTS OF DEPOSITION 7 

near Gallup on the west end of the belt to Canoncito on the east end of 

the belt. No uranium ore is found in the Recapture, and it is not 

described below. Santos (1970) described the stratigraphy and structure of the Morrison Formation in the Ambrosia Lake district. 

WESTWATER CANYON SANDSTONE MEMBER 

The Westwater Canyon Sandstone Member ranges in thickness 

from 9 to 82 min the report area. The thinnest section in the Gallup 

area is 29m at the Diamond No.2 (Largo No.2) mine. It is thin there 

because pre-Dakota erosion was accentuated along the Gallup hogback. 

The member is predominantly a red, medium- to coarse-grained, 

fluvially crossbedded, first-cycle, arkosic to subarkosic sandstone, but 

it commonly includes lenses of conglomeratic sandstone, particularly 

in the western part of the report area. The proportion of siltstone and 

claystone increases toward the east. 

The Westwater Canyon Member (fig. 3) was laid down as channel, 

overbank, and flood-plain deposits by a medium- to high-energy fluvial system, which included both braided and meandering streams of 

a coalescing alluvial fan complex. Stream flow was to the eastnortheast, as indicated by the gradual change in predominant facies 

from midfan to distal in that direction (Green, 1975). The paleoclimate was probably warm and humid, as indicated by the presence of 

large accumulations of tree trunks and other plant remains in Westwater Canyon sediments. 

BRUSHY BASIN SHALE MEMBER 

The Brushy Basin Shale Member (fig. 3) ranges from 0 to 61 min 

thickness in the report area. The maximum thickness in the Gallup 

district is 26 m, but the member is absent in the western part of the 

district as a result of pre-Dakota erosion. In the Ambrosia Lake district and in the eastern part of the Gallup district, the unit is mainly 

green, tuffaceous claystone and interbeds of sandstone similar to the 

Westwater Canyon Member. Locally in the Gallup mining district, 

the Brushy Basin Member is dominantly sandstone. 

The Brushy Basin intertongues with the underlying Westwater 

Canyon, and in the vicinity of the U mine, where the Brushy Basin is 

composed entirely of sandstone, no stratigraphic break can be discerned. In the area north of the town of Continental Divide (fig. 1), as 

well as in the Ambrosia Lake district to the east, the Brushy Basin is 

mainly claystone or mudstone, and a mappable contact with the 

Westwater Canyon is present. 
8 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

FIGURE 3.-Rock outcrops north of the town of Continental Divide, N.Mex. Prominent 

sandstone at top is the main body of the Dakota Sandstone. Below is a thick (24-37 

m) claystone sequence of the Brushy Basin Shale Member of the Morrison Formation. The massive sandstone beds in the middle part ofthe cliff belong to the Westwater Canyon Sandstone Member of the Morrison. The Dakota in this area lacks 

uranium deposits, because the impermeable Brushy Basin does not permit entry of 

solutions from the underlying, uranium-bearing Westwater Canyon Member. 

The fine-grained detritus that composes the Brushy Basin Member 

is interpreted to have been carried as suspended load by the fluvial 

system that deposited the sandstone of the Westwater Canyon. The 

claystone was deposited on top of the sandstone when basin filling 

lowered stream gradients, and fine-grained lithofacies migrated toward the basin margins over coarser grained deposits of the Westwater Canyon Member. 

DAKOTA SANDSTONE 

The intertongued, generally transgressive Dakota SandstoneMancos Shale sequence in west-central New Mexico has been divided 

into named formal and informal rock units by Landis, Dane, and 

Cobban (1973). In the present report, we are concerned only with the 

main body of the Dakota (Landis and others, 1973, p. J22), which 

underlies the lowest recognized unit of the Mancos, the Whitewater 

Arroyo Shale Tongue. 

The main body of the Dakota Sandstone (fig. 3), which in the report 

area ranges in thickness from 21 to 54 m, generally comprises two 

parts. The lower part, which was probably deposited in a delta-plain 
STRUCTURE 9 

environment similar to that described by Weimer (1976, p. 206-207), 

consists of interbeds of crossbedded, fine- to coarse-grained or locally 

conglomeratic sandstone of the distributary channel and levee; silty, 

carbonaceous sandstone of the crevasse splay; sandy, carbonaceous 

siltstone of the well-drained swamp; and clayey siltstone, lignite, and 

coal of the poorly drained swamp environment (fig. 4). The upper part 

consists of thick-bedded to massive, fine- to medium-grained beach or 

barrier-island sandstone. The lower part usually constitutes from 

one-third to one-half, and the upper part from one-half to two-thirds 

of the total thickness of the main body. 

Pre-Dakota erosion has produced an important unconformity at the 

base of the Dakota. In the western part of the Gallup district, all of 

the Brushy Basin Member and part of the Westwater Canyon 

Member of the Morrison Formation have been truncated by the preDakota erosion. 

STRUCTURE 

The report area lies on the gently north-dipping homoclinal Chaco 

slope (fig. 1) of the southern part of the San Juan Basin. The tectonic 

setting of the Grants uranium belt (southern San Juan Basin mineral 

belt of Hilpert and Moench, 1960) was described by Kelley (1963). 

Santos (1970) discussed the structure of the Ambrosia Lake district. 

The structure of the area, which lies just north of the Zuni uplift 

(fig. 1), is generally simple. Strata dip 2-4° northeast except along 

The Hogback (Nutria monocline, fig. 2), where dips range from 30° to 

85° W., and along the Coolidge (Pinedale) monocline (fig. 2) north of 

the town of Continental Divide, where dips of as much as 26° are 

found. The folding and tilting are largely post-Dakota in age, although a small amount of pre-Dakota deformation may have taken 

place. 

Santos (1970, p. E17) noted that the Ambrosia Lake district probably occupies the most folded and faulted part of the Chaco slope or 

platform. The Gallup district has steeper dipping folds (Nutria and 

Coolidge monoclines) but is considerably less faulted than the Ambrosia Lake district. The westernmost faults in the report area belong 

to the Bluewater fault zone near Thoreau, several kilometers east of 

the eastern boundary of the Gallup district. The Bluewater fault zone 

is shown by the geologic maps of the Thoreau NE (Green and Pierson, 

1971), and Thoreau (Robertson, 1973) quadrangles. 

Joints are present in all strata in the report area. They are more 

readily recognized in the Dakota than in the Morrison, a fact that 

probably explains why Gilkey (1953) shows no joints in the Morrison 

on his map of the Zuni uplift area. 
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EXPLANATION 

Yellowish-brown locally conglomeratic 

carbonaceous sandstone; deposited in 

distril::/utary channels and levees 

Yellowish-brown silty carbonaceous 

sandstone; deposited in crevasse 

splays 

Light-gray sandy carbonaceous siltstone; 

deposited in well-drained swamps 

Dark-gray to black clayey siltstone, 

lignite, and coal; deposited in poorly 

drained swamps I nterfluve 

~ 

swamp 

FIGURE 4.-Sketch showing lithologies and delta-plain environments in the lower part of the Dakota Sandstone in the Gallup, N. Mex., area. 

Modified from Weimer (1976, fig. 20). 

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URANIUM DEPOSITS 11 

URANIUM DEPOSITS 

PRODUCTION 

Hilpert (1969, table 4) noted that most of the uranium deposits in 

Cretaceous rocks in northwest New Mexico occur in the Dakota 

Sandstone, and he listed 14 Dakota deposits, 11 of which are in 

McKinley County. One ofthe deposits, the Church Rock, is partly in 

the Morrison Formation. Hilpert (1969, p. 89) gives the order of magnitude of production of these deposits. He stated that, ((Of 14 that are 

listed, 9 yielded a total during the 1952-64 period of about 110,000 

tons of ore that ranged from 0.12 to 0.30 percent U30 8 and averaged 

about 0.22 percent U30 8. More than 90 percent of this ore was mined 

from the Church Rock and Diamond 2 mines." Chenoweth (1977, 

table 1, p. 260) notes that, for the Grants mineral belt, 9 mines in the 

Dakota Sandstone produced 243 tons (223 t) U 30 8 during the period 

1951-70. 

DEPOSITS IN THE MORRISON FORMATION 

Uranium deposits in the Morrison Formation are mentioned here 

briefly because redistribution of uranium from these deposits or 

leaching of the host rock in which they are found is believed to be the 

source of most of the uranium present in the Dakota deposits. 

Morrison mines in the Gallup and Ambrosia Lake districts have 

been described by Hilpert (1969) and are shown on a map of the 

Grants uranium region by Chapman, Wood and Griswold, Inc. (1974). 

In the present report, the only Morrison mines shown are those found 

in part of the Gallup district (fig. 2); these are the Foutz Nos. 1- 3 and 

the Westwater No. 1 mines. The Francis mine (not shown), in the 

Ambrosia Lake district, is discussed briefly below. 

Granger (1968) has presented an hypothesis for the geologic controls for formation of the uranium deposits in the Morrison in the 

southern San Juan Basin (Grants) mineral belt. It is sufficient here to 

note that the main deposits in the Gallup and Ambrosia Lake districts are found in sandstone of the Westwater Canyon Member. Some 

smaller deposits are found in lenses and tongues of sandstone in the 

Brushy Basin Shale Member or in channel-fill sandstone beds at the 

top of the Brushy Basin Member. 

The Foutz No.3 mine (fig. 2) in the Gallup district and the Francis 

mine, located in the Ambrosia Lake district about 10 km north of 

Prewitt (fig. 1), are examples of mines in the Brushy Basin Member. 

The Foutz No. 3 mine is in a tongue of Westwater Canyon-like 

sandstone in the Brushy Basin. The Francis mine is in a simllar 

sandstone found as a channel fill at the top of the Brushy Basin. 

At the Foutz No. 3 mine, uranium is not found in the Dakota, 
12 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

probably because claystone of the Brushy Basin prevented uraniumbearing solutions of the Morrison from entering the Dakota. At the 

Francis mine there is no uranium in the Dakota, despite the fact that 

claystone of the Brushy Basin is not present above the ore. The scarcity of carbonaceous reducing material in sandstone lenses of the 

basal Dakota probably accounts for the absence of uranium in the 

Dakota at the Francis mine locality. 

DEPOSITS IN THE DAKOTA SANDSTONE 

MINERALOGY, HABITS, AND AGE OF THE DAKOTA DEPOSITS 

The mineralogy and habits of the Dakota uranium deposits are 

mentioned only briefly in the present report. More information, however, is available from Hilpert (1969, p. 90-92), Gabelman (1956), 

Granger (1963), and Chico (1959) .. 

Minerals in the sandstone consist of uraninite and various yellow 

secondary uranium minerals. Uranium minerals have not been identified in the radioactive carbonaceous shales. As noted by Hilpert 

(1969, p. 90), (( * * * the deposits consist of tabular masses that range 

from thin seams a few feet in width and length to crudely tabular 

masses as much as 2,500 feet [760 m J in length and at least 1,000 feet 

[ 300 m] in width. The larger deposits range from a few inches to as 

much as 25 feet [ 8 m J in thickness, but generally average a few feet." 

The uranium minerals are closely associated with (( * * * carbonaceous debris, which is generally distributed in crude bedlike zones 

within the sandstone units***. Ore bodies, which generally compose 

the high-grade parts of deposits, range from small masses that comprise only a few tons of material to masses that include as much as or 

more than 50,000 tons [ 45,000 t J of material. They range in thickness 

from a foot or so to 25 feet [ 8 m J, but most of them are only a few feet 

thick and comprise only a few hundred tons of material." 

The only direct evidence of the age of the Dakota ore has been 

provided by Ludwig, Szabo, and Granger (1977). According to them, 

the age of a uraninite-pyrite-rich sample from an ore pile at the 

Hogback No.4 mine, as determined from U-Pb isotope and U-series 

methods, is late Pleistocene. 

MINES IN THE GALLUP DISTRICT 

Detailed descriptions of mines in the Dakota Sandstone in the Gallup district (fig. 2) are given by Mirsky (1953), Gabelman (1956), 

Hilpert (1969), and Reimer (1969). Three mines (Diamond No.2, Becenti, and Hogback No.4) are found in steeply dipping Dakota strata 
URANIUM DEPOSITS 13 

of The Hogback. The remainder of the Dakota mines and prospects 

are found in gently dipping rocks at White Rock Mesa (Deiter prospect), north of Springstead Trading Post (Church Rock mine), and 

southeast of Springstead Trading Post (U and Rats Nest mines). The 

mines in the Morrison Formation are found northeast of Navajo 

Church (Foutz No. 3 mine) and west ofMidgit Mesa (Foutz Nos. 1 and 

2 and Westwater No. 1 mines). The Church Rock mine produced ore 

from the Morrison Formation and Dakota Sandstone through a shaft 

in the valley floor in the north-central part of the area shown by 

figure 2 (Hilpert, 1969, p. 77). The shaft is now abandoned. 

DIAMOND NO. 2 (LARGO NO. 2) MINE 

Gabelman (1956) gave a good description of the geologic setting and 

ore occurrence at the Diamond No. 2 mine; a geological map as well as 

longitudinal sections are included in his report. The U.S. Atomic 

Energy Commission (1959, p. 3-33 to 3-34) briefly described the 

Diamond No.2 mine, and Chico (1959) recorded the results of detailed 

underground and surface studies made at the mine. 

Outcrop observations show that the ore is found in the basal 

distributary-channel sandstone of the Dakota at the Diamond No.2 

mine (fig. 5). This sandstone, which rests on the truncated Westwater 

Canyon Sandstone Member of the Morrison, is overlain by the thickest part of a lenticular, lignitic, carbonaceous shale unit. The 

sandstone is about 5.5 m thick, and the shale is about 1.2 m thick in a 

measured section about 50 m southeast of the mine portal (section 1). 

The ore is associated with plant debris in medium- to coarse-grained, 

crossbedded, medium-energy, fluvial sandstone. 

Gabelman (1956, p. 315) noted that the uranium minerals are in 

pods and elongate lenses that plunge downdip. He also stated that 

with few exceptions the deposits are found beneath the black shale 

caprock, although they commonly protrude as much as several feet 

beneath the shale edges. 

BECENTI MINE 

Uranium ore at the Becenti mine (fig. 6) is found in the same 

stratigraphic position (section 3) as ore at the Diamond No. 2 mine; 

but at the Becenti, the basal Dakota Sandstone beds are 3.1 m thick, 

and the overlying carbonaceous shale is 1.2 m thick. The ore is associated with thin laminae of carbonaceous material interbedded with 

fine- to medium-grained, ripple-marked, crossbedded, mediumenergy fluvial sandstone. Oxidized uranium minerals are visible in 

joints. 
14 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

FIGURE 5.-Portal of Diamond No.2 (Largo No.2) mine located in the upper part of 

Morrison Formation (bleached sandstone at adit level) and the lower part of the 

overlying Dakota Sandstone (lignite-sandstone sequence). The lignite-sandstone sequence (swamp and distributary channel) is overlain by a massive (beach?) 

sandstone unit. Uranium was deposited in medium-energy distributary-channel 

sandstone containing pockets of carbonaceous trash. Portal is about 3 m high. 
URANlUM DEPOSlTS 15 

FIGURE 6.-Basal Dakota Sandstone sequence at the Becenti mine along The Hogback. 

Strata dip 31° W. Leveling rod rests on uranium-bearing strata of medium-energy 

distributary-channel sandstone in basal Dakota. Uranium is associated with carbonaceous laminae in partings and on bedding planes in the sandstone beds. Some 

oxidized uranium minerals are in joints. Overlying black, paludal carbonaceous 

shale acted as a permeability barrier to uranium-bearing solutions. Basal sandstone 

was stripped in mining. Massive sandstone above carbonaceous shale may be a beach 

deposit. Scale shown by 5-foot (1.5-m) rod. 

HOGBACK NO. 4 (HYDE) MINE 

At the Hogback No.4 mine, the ore is found in the lower part of the 

main body of the Dakota (section 4), as is the case at the Diamond No. 

2 and Becenti mines. However, the host rock, which is a 1.5-m-thick 

carbonaceous shale containing some sandstone, was deposited in a 

well-drained swamp environment, whereas most outcropping Dakota 

uranium deposits are found in distributary-channel sandstones. A 

good description of well-drained and poorly drained swamp environments is given by Weimer (1976, p. 206). 

Gabelman (1956) has described the stratigraphy and mineralization at the Hogback No. 4 mine. On pages 307-308, he describes the 

ore bed as ((***a black shale 1-3 feet [0.3-1.0 m] thick that is 

extremely fissile and can be split into paper-thin sheets, with abundant partially carbonized plant fragments. Locally it is very nearly a 

peat. It contains numerous thin gray carbonaceous fluvial crossbedded sandstone lenses averaging % inch [2 em] in thickness and , 18 

inches [ 46 em] in length." 
16 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

Mirsky (1953, p. 19) states that the ore bed tt * * * overlies an apparent channel sandstone which is approximately at the same stratigraphic horizon as the two other hogback mines, the Becenti and 

Diamond No. 2. However, only a few very minor traces of yellow 

uranium color have been found within the sandstone below the black 

shale." Gabelman (1956, p. 308) observed that tt * * * the amount of 

uranium in the lower bed is minor." 

DEL TER PROSPECT 

The Deiter prospect (figs. 7 and 8) has been described briefly by 

Gableman (1956). The highest radioactivity (2,500 cps) is found in a 

0.5-m-thick, sandy, carbonaceous shale (section 5) 3m above the base 

of the Dakota. The shale unit is part of the fill of a channel cut into 

the Westwater Canyon Sandstone Member of the Morrison Formation. 

According to Gabelman (1956, p. 316), ttThe mineralized body is an 

elongate lens 50 feet [15 m] wide and 2 feet [0.6 m] thick which 

occupies the bottom of the channel and, although undeveloped, is 

presumed to follow the channel for some distance. * * * The host 

sandstone is capped with black carbonaceous shale and is strongly 

contaminated with angular carbonacP.ou~ trH~h frHP"mPnt.~" 

FIGURE 7.-View north toward White Rock Mesa. Deiter uranium prospect is located 

in lowest part of Dakota Sandstone strata (dark colored), which fill channel cut into 

Westwater Canyon Sandstone Member of the Morrison Formation (light colored). 

Brushy Basin Member of the Morrison Formation was removed by pre-Dakota erosion. 
URANIUM DEPOSITS 17 

FIGURE 8.-Delter prospect. Radioactive carbonaceous sandy shale, conglomerate, and 

sandstone of the Dakota Sandstone, formed mainly in a medium-energy environment, at base of channel in Westwater Canyon Member of Morrison Formation. 

"Slug" of sandstone (unit 7, section 5) above radioactive zone is not mineralized, 

because high energy of fluvial environment has prevented deposition of organic 

material in an amount sufficient to have provided enough reductant to precipitate a 

significant amount of uranium. Claystone of the Brushy Basin Shale Member of the 

Morrison Formation was removed by pre-Dakota erosion. 

U AND RATS NEST MINES 

The U and Rats Nest mines (fig. 2) have been described by Reimer 

(1969), who also presented a cross section giving radiometric and 

lithologic data. 

The Morrison-Dakota contact is well exposed at the Rats Nest mine 

(fig. 9), and the lower part of the Dakota rests on sandstone of the 

Brushy Basin Shale Member of the Morrison Formation. Little if any 

of the Brushy Basin has been removed by pre-Dakota erosion at this 

locality, but only a small amount of claystone is present because of a 

facies change to sandstone. 

The lower part of the main body of the Dakota consists of a thick 

(about 12m) sequence of paludal carbonaceous shale and interbedded 

distributary sandstone units. As many as five shale, claystone, or 

lignite beds, as well as five sandstone lenses, are present along the 

outcrop. 
18 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

Most of the radioactivity is associated with fine- to medium-grained 

sandstone containing laminae and pockets of organic detritus. 
URANIUM DEPOSITS 19 

MINES IN THE AMBROSIA LAKE DISTRICT 

In the Ambrosia Lake mining district, all known uranium deposits 

in the Dakota Sandstone are in area 6 of figure 1. Hilpert (1969, table 

4) gives locations and brief descriptions of the mines. 

During the present study only the Febco (Small Stake), Silver Spur, 

and Junior mines were visited, and discussion is limited to presentation of data that will allow comparison of the geologic setting of these 

mines with that of the Gallup district mines. Additional information 

may be obtained from Mirsky (1953), Gabelman (1956), and Hilpert 

(1969). 

As shown by the geologic map of the Goat Mountain quadrangle 

(Thaden and others, 1966), the Febco (Small Stake) and Silver Spur 

mines are in sec. 31, T. 14 N., R. 10 W. Gabelman (1956, fig. 102) 

shows the Small Stake mine to be at the location labeled ((Febco 

Tunnel" on the geologic quadrangle map. 

At all of the mines, claystone of the Brushy Basin Member of the 

Morrison Formation is present beneath the Dakota Sandstone. As 

shown on the Goat Mountain geologic map, faulting is common near 

the Dakota mines. 

Joints in the Dakota are common, and Mirsky (1953, p. 17), in 

referring to the Silver Spur mine, states that ((Joint directions are N. 

15° to 20° W. (major) and N. 25° to 40° E. and north (minor)." He 

further states that ((Length of the ore bodies is aligned in the approximate direction of the secondary joint set*** ." On page 19 of the 

same report, Mirsky says: ((The Small Stake and Diamond No. 2 

mines show less jointing (both are underground workings), but in this 

vicinity joints do not appear to influence ore deposition." 

FEBCO (SMALL sr AKE) MINE 

At the Febco (Small Stake) mine the stratigraphic sequence (section 7) of the lower part of the main body of the Dakota is similar to 

the sequence at the U mine in the Gallup district, in that a thick 

series of interbedded distributary sandstones and paludal shales is 

overlain by a massive beach or barrier-island sandstone. 

~FIGURE 9.-Stratigraphic sequence at the Rats Nest mine. Uranium is in carbonaceous 

distributary sandstone in the basal part of the Dakota Sandstone. The distributary 

sandstones are interbedded with carbonaceous paludal shale and overlain by a massive barrier-island(?) sandstone. Contact between the Dakota and underlying 

sandstone of the Brushy Basin Member of the Morrison Formation is marked on the 

photograph. Uranium-bearing solutions entered the Dakota through the permeable 

Brushy Basin Member. Organic aqueous solutions from lignite in the Dakota 

bleached the Brushy Basin. 
20 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

The highest radioactivity (550 cps) at the mine was found n1 (;fie 

lower part of a fine- to medium-grained, 1.5-m-thick, slightly carbonaceous sandstone, which is 13.7 m stratigraphically above the 

base of the Dakota. This sandstone, which is parallel bedded and 

bioturbated, may be a tidal-flat deposit. 

SILVER SPUR MINE 

The stratigraphic setting at the Silver Spur mine is similar to that 

at the Febco mine in that the Dakota rests upon claystone of the 

Brushy Basin and consists of an upper, massive beach or barrierisland sandstone, which overlies distributary or tidal-flat sandstones 

interbedded with paludal carbonaceous shales. However, the 

thicknesses of the black shale units are less than at the Febco mine, 

and the ore is in the upper part of the main body of the Dakota rather 

than in the lower part. 

Radioactivity is associated with carbonaceous trash along bedding 

planes in fine- to medium-grained, well-sorted sandstone of the upper 

ledge of the Dakota. The ore occurrence is similar to that at the 

Becenti mine, except that the overlying carbonaceous shale is a 

marine tongue of the Mancos Shale rather than a paludal shale of the 

Dakota. 

JUNIOR MINE 

The Junior mine is about 150m west of the portal of the Dakota 

(Pat) mine, which is shown by the geologic map of the Goat Mountain 

quadrangle (Thaden and others, 1966). Mining at the Junior mine 

was by stripping. 

The main production from the Dakota (Pat) mine was from the 

Westwater Canyon Member of the Morrison Formation. The only 

radioactivity noted in the Dakota Sandstone at the Junior mine is 

spottily distributed along a fault (Thaden and others, 1966) that 

brings the Westwater Canyon Member against the Dakota 

Sandstone. 

ORIGIN OF THE URANIUM DEPOSITS IN THE DAKOTA 

SANDSTONE 

The formation of uranium deposits in the Dakota is believed to 

have been controlled by five main factors: (1) previous existence of a 

ground-water flow with a stratigraphically upward component; (2) 

accessibility to the Dakota of uranium-bearing solutions, derived by 

aqueous dissolution of preexisting Morrison uranium deposits or by 

leaching of the arkosic sediments which compose the Morrison; (3) 
ORIGIN OF THE URANIUM DEPOSITS 21 

sufficient permeability within the Dakota to allow passage of the 

solutions; (4) presence of an aquiclude to confine flow of the solutions, 

which are thought to have risen stratigraphically during their migration northward toward the San Juan Basin or westward toward the 

Gallup sag; and (5) availability of enough organic material in the 

Dakota to reduce and thereby precipitate the uranium from the rising 

solutions. 

Paleomovement of the ground water was generally northward 

down the Chaco slope toward the center of the San Juan Basin. 

Gabelman (1956, p. 315}, referring to the Diamond No. 2 mine, noted 

that ((***the downdip orientation of ore pods suggest [ s] deposition 

from solutions moving parallel to the dip in thin aquifers after tilting 

of the beds." Locally in the Gallup mining district, movement probably was westerly at the Gallup hogback. A stratigraphically 

upward component of movement probably existed where the dip of the 

strata was steeper than the hydraulic gradient. The ground-water 

movement from the Morrison to the Dakota, which could have taken 

place at any time from Early(?) Cretaceous to the present, was probably initiated by one of the recurrent uplifts of the Zuni Mountains. 

Uranium-bearing ground water from the Morrison could have entered the Dakota in areas where (1) the upper part of the Brushy 

Basin Shale Member of the Morrison was permeable because it contained sandy facies, as at the U and Rats Nest mines in the Gallup 

district; (2) the Brushy Basin had been removed by pre-Dakota erosion, as along the Gallup hogback; or (3) faults (Kittle and others, 

1967, p. 182) or joints provided conduits, as in the Ambrosia Lake 

district. 

Permeable sandstones are found as distributary sandstones in the 

lower part of the Dakota, or as massive beach or barrier-island 

sandstones in the upper part of the Dakota. Carbonaceous shale beds, 

which acted as aquicludes, are in the lower part of the Dakota, where 

they are interbedded with the distributary sandstone units. Most of 

the Dakota uranium deposits in the report area are in distributary 

sandstone in the lower part of the Dakota. The exceptions are the 

deposits at the Silver Spur mine, which are in a beach or barrierisland sandstone of the upper Dakota; and those at the Hogback No. 4 

(Hyde) mine, which are in sandy, paludal shale of the lower Dakota. 

At the Silver Spur mine, the amount of impermeable shale in the 

lower Dakota is small, and hence the uranium solutions were able to 

reach the upper Dakota. The aquiclude at the Silver Spur is the 

Whitewater Arroyo Shale Tongue of the Mancos. At the Hogback No. 

4 (Hyde) mine, the uranium may have entered the sandy shale (unit 

11, section 4) laterally, because the underlying shale bed (unit 10) is 

relatively impermeable. 
22 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

Except for the Hogback No. 4 (Hyde) mine, radioactivity in 

sandstones at all of the mines in the Dakota in the report area is 

found in permeable sandstone containing either pockets of carbonaceous trash or thin laminae of carbonaceous material. Trash 

pockets are not common in the Dakota in the area examined, and 

therefore most of the uranium ore is associated with thin organic-rich 

laminae. The Diamond No.2 is the only mine where the ore is associated mainly with carbonaceous trash. 

The relative amount of carbonaceous to sand-size material in the 

rock is related to the energy level of the depositional environment. In 

high-energy environments such as distributary channels, most of the 

carbonaceous material either was not deposited or was winnowed out. 

Rocks formed in the low-energy paludal environment contain abundant carbonaceous material, hut the permeability is low, and they are 

not likely to contain ore-grade uranium. The best host rocks are those 

formed in medium-energy environments, because they contain ample 

carbonaceous material for reductant yet retain sufficient permeability to allow the uranium solutions to enter. 

SUGGESTIONS FOR EXPLORATION 

Exploratory drilling for Dakota ore should include a search for the 

following stratigraphic conditions: (1) The beds of the Morrison directly underlying the Dakota should be sandy rather than clayey or 

should contain faults or joints to provide access for solutions to enter 

the Dakota. (2) The lower part of the Dakota should consist of fine- to 

medium-grained carbonaceous sandstone beds at least 1m thick. (3) 

The sandstone should be interbedded with carbonaceous shale units, 

which should also be at least 1m thick. (4) Where impermeable carbonaceous shale beds are thin or absent in the lower Dakota, the 

massive sandstone present in some areas in the upper part of the 

Dakota could contain ore, provided that sufficient organic material is 

present. The most favorable locality for upper Dakota ore is close to 

the base of the overlying Whitewater Arroyo Tongue of the Mancos 

Shale. 

An exploration drilling program should cross the main paleochannel direction at right angles. The channel direction can be determined 

at the outcrop, but where drilling is some distance from the outcrop, 

the vertical and lateral sequences noted in wide-spaced pilot drilling 

can be used as a guide for closer spaced drilling, as described below. 

Main distributary channels are composed principally of medium- to 

coarse-grained sandstone containing relatively little carbonaceous 

trash. Beds between the main channel and poorly drained swamps, 

located on either side of the channel, typically consist of fine- to 
MEASURED STRATIGRAPHIC SECTIONS 23 

medium-grained levee and crevasse-splay sandstones interbedded 

with siltstones of the overbank deposits and with gray, carbonaceous 

shale layers of the well-drained swamp areas. Black carbonaceous 

shale and lignite of the poorly drained swamp commonly overlie the 

above sequences in localities where the main channel has shifted 

position. 

Dimensions of the channels will determine drill spacing. The distance between channels in the report area ranges from about 500 m to 

as much as 3 km. Initially, drilling on a 1,000-m grid is probably 

appropriate. When main channel, levee, and swamp deposits are 

found, a closer-spaced grid may be used to detail the more favorable 

carbonaceous sandstone units. 

In conclusion, the levee and crevasse-splay sandstone units usually 

contain laminae of carbonaceous material, as well as scattered pockets of carbonaceous trash, and provide the best loci for uranium deposits. Sandstone of the main distributary channels, although permeable, is not generally favorable for uranium deposits because it lacks 

carbonaceous reductants necessary for the precipitation of uranium. 

The organic-rich swamp deposits are generally too impermeable to 

contain uranium deposits. 

MEASURED STRATIGRAPHIC SECTIONS 

Section 1, Diamond No.2 (Largo No.2) mine, about 50 m SE. of mine portal 

[Measured by M. W. Green and C. T. Pierson, August 9, 1974. Dip, 30° W.; background gamma radiation, 120 cps 

(counts per second)] 

Top of measured section. 

Dakota Sandstone, main body (incomplete at top): 

Thickness 

(meters) 

14. Covered ____________________________________________________ lJnmeasured 

13. Sandstone, yellow-brown, fine- to medium-grained; distributary channel 

environment ---------------------------------------------------- 3.4 

12. Shale, black; poorly exposed; probably contains some sandstone ledges; 

poorly drained swamp environment-------------------------------- 9.5 

11. Sandstone, yellow-brown, fine- to medium-grained; more massive than 

interval 13; distributary channel environment---------------------- 4.9 

10. Shale, light-greenish-gray; poorly exposed; well-drained swamp 

environment ---------------------------------------------------- 4.3 

9. Sandstone, yellow-brown; similar to interval 13; distributary channel 

environment ---------------------------------------------------- 1.5 

8. Shale, light-greenish-gray; poorly exposed; well-drained swamp environment -------------------------------------------------------- 1.2 

7. Sandstone, red-orange, coarse-grained, crossbedded; distributary channel environment __________________________________________________ 9.1 

6. Lignite, black; poorly drained swamp environment, grades laterally 

to a well-drained swamp environment_ _____________________________ 1.2 
24 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

Section 1-Continued Thickness 

(meters) 

5. Sandstone, yellow-brown, coarse-grained, conglomeratic; uranium ore 

found in this stratigraphic position at mine portal, 50 m NW.; distributary channel environment_ _______________________________________ 5.5 

4. Shale, black; poorly drained swamp environment ------------------~ 

Total measured section of Dakota Sandstone, main body (incomplet' ' _________ _41.2 

Westwater Canyon Sandstone Member of the Morrison Formation (all of the 

Brushy Basin Shale Member and an unknown amount of the Westwater 

Canyon Member removed by pre-Dakota erosion): 

3. Sandstone, light-pink, fairly well sorted; fluvial environment __________ 3.1 

2. Clayey siltstone, greenish-yellow to light-purple; fluvial overbank 

deposit ---------------------------------------------------------- 2.4 

1. Sandstone, light-pink (darker than interval 3; not as extensively 

bleached), medium-grained, fairly well-sorted, crossbedded; reworked 

from underlying Cow Springs Sandstone; coarsens higher in section to 

conglomerate with abundant granules of chert and rock fragments; fluvial environment -------------------------------------- 23.8 

Total Westwater Canyon Sandstone Member, Morrison Formation ____________ 29.3 

Disconformable contact. 

Cow Springs Sandstone ____________________________________________ Unmeasured 

Section 2, Locality approximately halfway between the Becenti and the Diamond No.2 

mines 

[Measured by M. W. Green, C. T. Pierson, and A. N. Khan, July 18, 1975. Dip, 30° W.; background gamma radiation, 

120 cps (counts per second)] 

Top of measured section. 

Dakota Sandstone, main body (incomplete at top): 

10. Sandstone, reddish-orange to reddish-brown, fine- to coarseThickness 

(meters) 

grained, moderately well to poorly sorted, quartzose; forms massive 

ledges laterally continuous for several hundred meters; ledges 

thin laterally to pinchout; massive to medium-scale trough 

crossbedding; plant trash imprints; forms cap and backslope 

along main hogback ridge; broad distributary channel or beach(?) 

environment ----------------------------------------------------35.1 

9. Shale, light-gray, carbonaceous; contains platy siltstone beds; poorly 

exposed; gamma radiation 80 cps in the shale and 120 cps at contacts 

between shale and sandstone; well-drained swamp environment ____ 10.7 

8. Sandstone, reddish-orange to reddish-brown, medium-grained, wellsorted, quartzose; low-angle, thin crossbedding; laterally continuous to Becenti mine area; gamma radiation 80--100 cps; probable beach environment _ __ _ _ __ _ _ ______ __ _ _ _ ___ __ __ __ _ _ _ _ ____ __ _ _ _ 2. 7 

7. Shale, carbonaceous; swamp environment ---------------------------- 2.1 

6. Sandstone, yellow-brown, fine-grained; distributary channel environment ------------------------------------------------------------ .6 

5. Shale, carbonaceous; gamma radiation 600 cps; poorly drained swamp 

environment ---------------------------------------------------- .9 
MEASURED STRATIGRAPHIC SECTIONS 25 

Section 2 -Continued Thickness 

(meters) 

4. Sandstone, yellowish-brown, fine- to medium-grained (coarse-grained 

locally), flat-bedded to low-angle crossbedded; upper part of bed platy; 

gamma radiation 200 cps; distributary channel environment ________ 2.1 

Total measured section of Dakota Sandstone, main body (incompleteL _________ 54.2 

Westwater Canyon Sandstone Member of the Morrison Formation (all of the 

Brushy Basin Shale Member and an unknown amount of the Westwater 

Canyon Member removed by pre-Dakota erosion): 

3. Sandstone, yellowish-white, medium- to coarse-grained, locally conglomeratic, poorly sorted; granules up to 4 em in diameter consist of 

quartzite, weathered granite, and variously colored chert; contains kaolinite nests; low- to moderate-angle crossbeds; gamma 

radiation 160 cps; high-energy fluvial environment_ _________________ 3.0 

2. Sandy siltstone; fluvial environment; forms barrier to altering solutions 

from the Dakota__________________________________________________ 1.5 

1. Sandstone, reddish-brown; similar to unit 3 -------------------------- 10.7 

Total Westwater Canyon Sandstone Member, Morrison Formation ____________ 15.2 

Disconformable contact. 

Cow Springs Sandstone ____________________________________________ Unmeasured 

Section 3, Becenti mine 

[Measured by M. W. Green and C. T. Pierson, August 8, 1974. Dip, 31° W.; background gamma radiation, 120 cps 

(counts per second)) 

Top of measured section. 

Dakota Sandstone, main body (incomplete at top): 

Thickness 

(meters) 

8. Sandstone, red, coarse-grained, poorly sorted; small-scale trough crossbeds, distorted in upper part, probably owing to slumping shortly after 

deposition or to overloading by rapid deposition; distributary environment ___________________ --· ___________________________________ 12.2 

7. Sandy shale, light-green to gray, thin-bedded; poorly exposed; 

well-drained swamp environment---------------------------------- 4.6 

6. Sandstone, red, coarse-grained, poorly sorted; short trough crossbeds; 

similar to unit 8; distributar1 environment . _______________________ 9.1 

5. Shale, black, carbonaceous; poorly drained swamp environment ________ 1.8 

4. Sandstone, fine-grained; distributary channel environment ____________ 1.2 

3. Shale, black, carbonaceous; poorly drained swamp environment ________ 1.2 

2. Sandstone, yellow-brown; carbonaceous trash; thickness varies laterally; uranium minerals in joints; distributary channel environment _ _ _ _ 3.1 

Total measured section of Dakota Sandstone, main body (incomplete) __________ 33.2 

Westwater Canyon Sandstone Member of the Morrison Formation (all of the 

Brushy Basin Member and an unknown amount of Westwater Canyon 

Member removed by pre-Dakota erosion): 

1. Sandstone, red, coarse-grained, conglomeratic, poorly sorted; fines upward; trough crossbedding; similar in appearance to unit 8; some 

shale interbeds near top; high-energy fluvial environment __________ 35.7 
26 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

Section 3-Continued Thickness 

(meters) 

Total Westwater Canyon Sandstone Member, Morrison Formation ____________ 35.7 

Disconformable contact. 

Cow Springs Sandstone ____________________________________________ Unmeasured 

Section 4, Hogback No. 4 (Hyde) mine 

[Measured by M. W. Green and C. T. Pierson, August 8, 1974. Dip, 42° W.] 

Top of measured section. 

Dakota Sandstone, main body (incomplete at top): 

Thickness 

(meters) 

13. Shale, green to gray, sandy, carbonaceous; contains minor interbedded 

distributary channel sandstones, 0-1 m thick; well-drained swamp 

environment ---------------------------------------------------- 7.6 

12. Sandstone, yellow; distributary channel environment_ _________________ 3.0 

11. Shale, green to gray, carbonaceous; some sandstone present; uranium 

ore found in this unit; well-drained swamp environment ____________ 1.5 

10. Shale, black, carbonaceous; poorly drained swamp environment ________ 4.6 

9. Sandstone, yellow-brown; some conglomerate present; distributary 

channel environment _ ____ ____ __ __ __ _ _ ______ ________ __ ____ __ _ _ ___ 2.1 

8. Shale, dark-gray to black, carbonaceous; poorly drained swamp 

environment ---------------------------------------------------- .6 

7. Sandstone, yellow-brown; same as unit 9, but contains more conglomerate; distributary channel environment_ ___________________________ 3.0 

Total measured section of Dakota Sandstone, main body (incomplete) __________ 22.4 

Westwater Canyon Sandstone Member of the Morrison Formation (all of the 

Brushy Basin Member and an unknown amount of the Westwater Canyon 

Member removed by pre-Dakota erosion): 

6. Sandstone, very white; fluvial environment __________________________ 12.8 

5. Sandy shale, light-purple; flood-plain environment ____________________ 3.0 

4. Sandstone; fluvial environment ------------------------------------ 13.7 

3. Shale, dark-purple; grades from sandy to silty; overbank environment__ 7.6 

2. Sandstone; fluvial environment-------------------------------------- 7.6 

1. Sandy shale, purple to green; flood-plain environment ________________ 12.2 

Total Westwater Canyon Sandstone Member, Morrison Formation ____________ 56.9 

Disconformable contact. 

Cow Springs Sandstone ____________________________________________ Unmeasured 

Section 5, Delter prospect 

[Measured by M. W. Green, C. T. Pierson, and A. N. Khan, August 7, 1975. Dip, 3° N. (flanks of channel dip as much 

as 8°1; background gamma radiation, 120 cps (counts per second)] 

Top of measured section. 

Dakota Sandstone, main body (incomplete at top): 

15. Sandstone, yellowish-brown, medium-grained; heavily iron oxide 

stained on bedding surfaces and along joints; gamma radiation 
MEASURED STRATIGRAPHIC SECTIONS 27 

Section 5-Continued Thickness 

(meters) 

100 cps; distributary channel environment ------------------------ 1.5 

14. Shale, medium- to light-gray, carbonaceous; poorly exposed; gamma 

radiation 150-200 cps; well-drained swamp environment ____________ 6.1 

13. Sandstone, yellowish-brown, fine- to medium-grained (coarse-grained 

locally); bedding planes iron oxide stained; clay clasts in base 

of unit; fairly clean, but contains some fossil wood fragments; unit 

composed of massive, channeled sandstone lenses; massive, structureless to crossbedded locally; current and oscillation ripples; 

unit intertongues laterally with dark carbonaceous shale; scouring 

at base; gamma radiation 100-150 cps; high-energy fluvial 

distributary channel environment_ _________________________________ 7.6 

12. Shale, carbonaceous; medium gray in upper part and black in lower 

part; upper part contains sandstone ledges 7-10 em thick; unit grades 

downward from well-drained to poorly drained swamp en- vironment________________________________________________________ .9 

11. Sandstone, yellowish-brown, fine- to medium-grained, massive, wellsorted; poorly sorted in uppermost 0.3 m; sparse carbonaceous trash; 

upper part burrowed; low-angle crossbedded in lower part; gamma 

radiation 150-180 cps; beach(?) environment ______________________ 2.3 

10. Shale, black, carbonaceous ------------------------------------------ .9 

9. Sandstone, siltstone, and shale; sandstone and siltstone are yellowish 

brown and thin bedded; sandstone is fine grained; shale partings are 

medium gray. Unit persists laterally past channel margins; has 

sharp upper and lower contacts; gamma radiation 150-180 cps; lowenergy fluvial environment________________________________________ 3.1 

8. Shale, medium-gray to black, carbonaceous; coaly in upper 0.3 m; some 

thin-bedded to laminated sandy siltstone beds under coaly zone; unit 

is laterally continuous; gamma radiation 180-200 cps; grades 

upward from well-drained to poorly drained swamp environment ____ 2.4 

7. Sandstone, yellowish-brown, fine- to medium-grained, well-sorted, 

thin-bedded to massive; carbonaceous trash locally along partings 

and bedding planes; unit pinches out along margins of channel; 

massive units become thin-bedded toward flanks of channel; unit 

ranges from 27.5 m thick to 0 m (on flanks); gamma radiation 150-

200 cps in upper part of unit, 400-500 cps in basal part of unit; 

distributary channel environmenL---------------------------------18.3 

6. Shale and siltstone, interbedded; unit becomes sandy laterally as 

well as upward; siltstone is sandy, finely laminated, and slightly carbonaceous; shale is medium to dark gray (ash gray locally); gamma 

radiation 200-250 cps in upper part of unit, 250 cps in center, and 

800 cps at base; low-energy fluvial(?) and (or) well-drained swamp 

environment ---------------------------------------------------- 5.2 

5. Sandstone, yellowish-brown, fine-grained, well-sorted, carbonaceous 

(carbonaceous material as disseminations and as concentrations 

along bedding planes), thin-bedded; locally crossbedded at low angles; 

bioturbated; gamma radiation 1,000 cps; low-energy fluvial 

environment ---------------------------------------------------- .9 

4. Sandy shale, black, carbonaceous; yellowish-brown sandy siltstone 

stringers as much as 1 em thick contain carbonaceous material along 

partings; sandstone and silty sandstone with finely disseminated car-
28 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

Section 5-Continued Thickness 

(meters) 

bonaceous material make up about 40 percent of unit; unit grades 

vertically upward into overlying sandstone unit; basal contact sharp; 

gamma radiation 2,500 cps; poorly drained swamp environment ________________________________________________________ 0.5 

3. Sandstone, yellow-brown, medium- to coarse-grained, thin-bedded, locally crossbedded; scattered granules; sparse carbonaceous trash; 

gamma radiation 350-500 cps (higher count near contact with overlying carbonaceous shale); high-energy distributary channel 

environment ---------------------------------------------------- 1.5 

2. Granule conglomerate with sandstone matrix; 20-30 percent of the 

matrix material is iron oxide; woody plant material locally; unit 

pinches out laterally; gamma radiation 400-1,700 cps (high count 

near carbonaceous trash); lower contact is sharp and channeled; relief on channel is 15-20 m; distributary channel environment ______ 1.5 

Total measured section of Dakota Sandstone, main body (incomplete) __________ 61.9 

Westwater Canyon Sandstone Member of the Morrison Formation (all of 

the Brushy Basin Member and an unknown amount of the Westwater 

Canyon Member removed by pre-Dakota erosion): 

1. Sandstone, white, kaolinitic ________________________________ Unmeasured 

Section 6, U mine, composite section 

[Compiled by C. T. Pierson, and H. Htay from data reported by Reimer (19691 and T. N. Parthasarathy (written 

commun., 19741. Dip, 3o N.] 

Top of measured section. 

Dakota Sandstone, main body (incomplete at top): 

Thickness 

(meters) 

12. Shale, carbonaceous ------------------------------------------------ 0.3 

11. Claystone ---------------------------------------------------------- .3 

10. Sandstone, moderately well sorted; lower portion shaly ________________ 1.7 

9. Lignite ------------------------------------------------------------ .5 

8. Sandstone, very fine grained, crossbedded; gray or buff on fresh surface; 

grains subangular to subrounded ---------------------------------- 1.4 

7. Shale, laminated; weathers to rubble-strewn slope; lower contact 

uncertain -------------------------------------------------------- 2.9 

6. Sandstone, crossbedded, massive _ __ __ __ __ __ _ _ __ __ __ _ _ __ _ _ _ _ __ __ _ _ ___ .6 

5. Lignite; fibrous gypsum in cracks and joints; poorly drained swamp en- vironment ________________________________________________________ 1.1 

4. Sandstone, fine- to medium-grained, moderately well sorted, quartzose, 

well-cemented; quartz overgrowths; grains angular to subangular; 

weathers buff with iron oxide-stained spots; gray on fractures; considerable carbonaceous material present as small flecks; distributary channel('?) environment_____________ __ __ __ __ __ __ __ __ _ _ __ ___ _ __ _ _ __ __ _ _ _ .5 

3. Sandstone interbedded with siltstone, carbonaceous __________________ .6 

2. Covered zone; base of the Dakota is within this covered interval ______ 2.4 

Total measured section of Dakota Sandstone main body (incomplete) __________ 12.3 

Disconformable contact (covered). 
MEASURED STRATIGRAPHIC SECTIONS 29 

Section 6-Continued Thickness 

(meters) 

Brushy Basin Shale Member of the Morrison Formation: 

1. Sandstone, white, kaolinized. Mudstone, usually present in this stratigraphic position, is absent because of facies change to 

sandstone ---------------------------------------------- Unmeasured 

Section 7, Febco (Small Stake) mine 

[Measured by M. W. Green, August 6, 1975. Dip, 2o N.; background gamma radiation, 120 cps (counts per secondl] 

Top of measured section. 

Dakota Sandstone, main body (incomplete at top): 

Thickness 

(meters) 

10. Sandstone; yellowish-brown to reddish-brown, fine- to medium-grained, 

well-sorted, flat-bedded, siliceous, bioturbated; gamma radiation 

110 cps ---------------------------------------------------------- 3.4 

9. Sandstone; same as interval 1 but more bioturbated; gamma radiation 

125 (upper part) to 150 (lower part) cps; beach(?) environment ______ 2.1 

8. Shale; covered interval ______________________________________________ 2.4 

7. Sandstone; yellowish-brown to reddish-brown, fine- to medium-grained, 

well-sorted, flat parallel-bedded to massive; bunows present; sparse 

carbonaceous material; gamma radiation 150 cps; distributary 

channel environment -------------------------------------------- .6 

6. Shale; covered interval; gamma radiaton 175 cps ______________________ 3.1 

5. Sandstone, yellowish-brown to reddish-brown, fine- to medium-grained, 

well-sorted, flat parallel-bedded to massive; silty at base; carbonaceous shale partings in base; gamma radiation 250-300 cps __________ 1.8 

4. Shale, dark-gray to black, carbonaceous; minor sandy intervals; gamma 

radiation 300-400 cps; poorly drained swamp environment __________ 2.1 

3. Sandstone; yellowish-brown to reddish-brown, fine- to medium-grained, 

well-sorted, flat parallel-bedded to massive; Febco tunnel is in this 

sandstone; gamma radiation 300 (upper part) to 550 (lower 

part) cps -------------------------------------------------------- 1.5 

2. Shale, dark-gray to black, carbonaceous; largely covered; thinly interbedded sandstone ledges; gamma radiation 350 cps; poorly drained 

swamp environment ----------------------------------------------13.7 

Total measured section of Dakota Sandstone main body (incomplete) __________ 30.7 

Disconformable contact (covered). 

Brushy Basin Shale Member of the Morrison Formation: 

1. Silty claystone, greenish-gray; only upper part exposed; some coarsegrained, grayish-brown sandstone as lenses and stringers; gamma 

radiation 250 cps (300 cps near contact); contact with Dakota may 

be in a slump block; flood-plain and lacustrine environment _Unmeasured 
30 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE 

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