Stratigraphy of Triassic 

and Associated Formations 

in Part of the Colorado 

Plateau Region

GEOLOGICAL SURVEY BULLETIN 1046-Q

This report concerns work done on behalf 

of the U. S. Atomic Energy. Commission 

and is published with the permission of 

the Commission

Stratigraphy of Triassic 

and Associated Formations 

in Part of the Colorado 

Plateau Region

By JOHN H. STEWART, GEORGE A. WILLIAMS, HOWARD F. ALBEE, 

and OMER B. RAUP

With a section on

SEDIMENTARY PETROLOGY

By ROBERT A. CADIGAN

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGICAL SURVEY BULLETIN 1046-Q

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

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

Washington 25, D. C. - Price 65 cents (paper cover)
CONTENTS

	Page 

Abstract___________________________________________ 487

Introduction.--.__________________________________________________ 489

Scope.._________________._____________________ 491

Terminology and methods used in sedimentary-rock classification-_ 491

Terminology and methods used in sedimentary-structure studies.___ 493

Stratigraphy-_______-______-___-_____--___--_____-__-__..______-- 493

Permian rocks______________________________________________ 493

Coconino sandstone______________________________________ 493

Kaibab limestone__--------_-_-_---__--__--_____-_-_____-_~ 494

Cutler formation._________________________________________ 495

Triassic rocks.________________________________________________ 496

Moenkopi formation_______________________________________ 496

Chinle formation____-__-_-_-_-_-_-__--____-__-____________ 500

Temple Mountain member____________________________ 501

Shinarump member___..________________________________ 504

Monitor Butte member_______________________________ 508

Moss Back member__________________________________ 512

Petrified Forest member________________________________ 515

Owl Rock member_______________________________ 516

Church Rock member____________________________ 517

Some characteristics of the Chinle formation._____________ 520

Triassic and Jurassic rocks____________________________________ 523

Glen Canyon group_________-______________________________ 523

Wingate sandstone___________________________________ 524

Kayenta formation_____-_____________-___-____________ 524

Navajo sandstone_____________________________________ 525

Interpretations----__------_-_---_-___------___----_--_ 525

Relation of uranium deposits to the stratigraphy of the Triassic and

associated formations.__________________________________ 525

Distribution of ore deposits_____________________--_________- 526

Permian rocks_______________________________________ 526

Cutler formation--__--------__--__--_-__-_--_____- 526

Triassic rocks______--____-__--___-_________-___-_____- 526

Moenkopi formation_-___--___-___--___---_---__--- 526

Chinle formation_______________-______--_________- 526

Wingate sandstone_______________________________ 528

Relation of ore deposits to local sedimentary features-_____________ 528

Channel fills.________....____________________ 528

Lithologic features_______________________________________ 528

Sedimentary petrology, by Robert A. Cadigan_______________________ 529

Objectives_______________-___-__-_______.____.__ 529

Methods. _____________________________________________ 529

Textural analysis._____-_-_______-___--__-____--____-____-_ 530

Compositional analysis--_--------_---_--_______-----_-__-_- 533

in
IV CONTENTS

	Page 

Sedimentary petrology, by Robert A. Cadigan Continued

Results__ _ --------__-___-_-____-____-__-__-___-___--__-_-_-_ 536

Permian rocks___________________________________________ 537

Cutler formation._________--_--_-_-_--__-_---_-.._--___ 537

Cedar Mesa sandstone member______________________ 537

Organ Rock tongue.__________-__--__-_-__---_--_-_ 537

De Chelly sandstone member.______________________ 539

Hoskinnini tongue.____-__________..__-___-_--.---_-_ 540

Conglomeratic facies___________-_-----_-_-_--__-___ 541

Triassic rocks_____________________________________________ 542

Moenkopi formation_______-_-__----_-_-------_-_-_---_ 542

Chinle formation__-_-__________-____-______---___--___ 543

Shinarump member________.._____-_________---___-_ 543

Monitor Butte member___________________________ 544

Moss Back member__-_-_-_----------------_------- 546

Petrified Forest member..____________-_____----____ 548

Owl Rock member__--____------_-----_------_---_- 549

Church Rock member__-____-_----__------------__- 550

Triassic and Jurassic rocks________________________________ 551

Glen Canyon group.___________________________________ 551

Wingate sandstone_____-_-_,-----__----------__---- 551

Kayenta formation.___-___-___._-___-_______-_-_-_ 553

Navajo sandstone_________________________________ 553

Study of the clay-mineral groups in the Triassic and associated formations. ______________________________________________________ 554

Petrologic features observed in ore-bearing sandstone._____________ 556

Interpretations. ______-___________--____________--__--_-___-_-_ 558

Summary of stratigraphic and petrologic interpretations____________ 565

Literature cited___________________________________________________ 572

Index_______-_--_____---_-_________-_---_------___---------_--_ 575

ILLUSTRATIONS

PLATE 49. Generalized section of Triassic rocks and part of the Permian and Jurassic rocks in southeastern Utah and adjoining 

areas________________-______-_--___--_-_--__----- In pocket

Page 

FIGURE 70. Index map of part of the Colorado Plateau showing areas of

stratigraphic studies________________-_---__--__--____> 490

71. Isopach map of the Moenkopi formation of Triassic age in

southeastern Utah____________________---_--__-_-_-___ 497

72. Index map of part of the Colorado Plateau showing outcrops

of the Chinle formation of Triassic age__________________ 501

73. Correlations of the Chinle formation of Triassic age__ ____ 503

74. Distribution of the Shinarump member of the Chinle formation of Triassic age and location of uranium deposits in this 

member in southeastern Utah and northeastern Arizona. __ 506

75. Resultant-dip directions of cross-strata in the Shinarump

member of the Chinle formation of Triassic age__________ 509
CONTENTS V

Page

FIGURE 76. Distribution of the Monitor Butte member of the Chinle 

formation of Triassic age and location of the uranium deposits in this member in southeastern Utah______________ 510

77. Distribution of the Moss Back member of the Chinle formation of Triassic age; location of uranium deposits in the Moss 

Back and at the base of undifferentiated Chinle; and a 

generalized isopach map of that part of the Chinle formation underlying the Moss Back member.________________ 513

78. Resultant-dip directions of cross-strata in the Moss Back

member of the Chinle formation of Triassic age_ _________ 515

79. Resultant-dip directions of cross-strata in various sandstone

units in the Chinle formation of Triassic age_____________ 520

80. Isopach map of the Chinle formation of Triassic age in southeastern Utah_________________________________________ 521

81. Generalized cross section showing distribution of ore deposits

in the Chinle formation of Triassic age__________________ 527

82. Petrologic classification of detrital sedimentary rocks______ 535

83. Index map showing localities sampled for sedimentarypetrology studies in Arizona and Utah_________________ 538

84. Changes in average grain size and average sorting from upper 

Permian to Lower Jurassic units in the central part of the 

Colorado Plateau_____________________________________ 561

TABLES

Page 

TABLE 1. Grain-size classification, modified after Wentworth_. __________ 492

2. Composition of pebbles, in percent, in the Shinarump member

of the Chinle formation of southern Utah_______________ 507

3. Composition of pebbles, in percent, in the Moss Back member

of the Chinle formation in part of southeastern Utah______ 514

4. Phi mean and Wentworth grain-size classification___________ 531

5. Classification of sorting_..______________________________ 531

6. Classification of skewness_______________________________ 532

7. Classification of kurtosis________________________________ 532

8. Petrologic classification of detrital sedimentary rocks based

on percentage composition.____________________________ 534

9. Mineral components, in percent by volume, of the Shinarump

member of the Chinle formation.______________________ 544

10. Mineral components, in percent by volume, of the Monitor

Butte member__-___________________-_-___---_-__---- 546

11. Units in which sandstone contains 1 of the 3 clay-mineral

groups as the major clay mineral____----------_--------- 554

12. Units in which sandstone contains 1 of the 3 clay-mineral

groups as the minor clay mineral________---------------- 555

13. Mean or average measures of the properties of the phi (grainsize) distribution of sediment in some Permian, Triassic, and 

Jurassic sandstone and coarse siltstone from the Colorado 

Plateau___________________________________________ 568
VI CONTENTS

* Page

TABLE 14. Mineralogic composition, in percent, of samples of the Shinarump member of the Chinle formation as determined by 

point-count analyses of thin sections..-______ ____________ 569

15. Mineralogic composition, in percent, of samples of the Monitor 

Butte member of the Chinle formation as determined by 

point-count analyses of thin sections___________________ 570

16. Mineralogic composition, in percent, of samples of the Moss 

Back member of the Chinle formation as determined by 

point-count analyses of thin sections ------------..----- 571

17. Mineralogic composition, in percent, of samples of the Petrified 

Forest member of the Chinle formation as determined by 

point-count analyses of thin sections.____________________ 571
CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

STRATIGRAPHY OF TRIASSIG AND ASSOCIATED FORMATIONS IN PART OF THF COLORADO PLATEAU 

REGION ______

By JOHN H. STEWART, GEORGE A. WILLIAMS, HOWARD F. ALBEE, and

OMER B. RAUP

ABSTRACT

Stratigraphic studies of the Triassic and associated formations were made in 

southeastern Utah and adjoining parts of Colorado and Arizona. Five principal 

lines of investigation were followed: regional stratigraphy, sedimentary petrology, pebble studies, sedimentary-structure studies, and lithofacies studies.

The formations studied are the Cutler formation, Coconino sandstone, and 

Kaibab limestone of Permian age; the Moenkopi formation, Chinle formation, 

and Wingate sandstone of Triassic age; the Kayenta formation of Jurassic (?) 

age; and the Navajo sandstone of Jurassic and Jurassic (?) age.

The Cutler formation grades from a conglomeratic facies in southwestern 

Colorado to a unit containing alternating reddish siltstone and yellowish-gray 

sandstone members in southeastern Utah. The reddish siltstone members comprise the Halgaito, Organ Rock, and Hoskinnini tongues. These alternate with 

the yellowish-gray sandstone members the Cedar Mesa, White Rim, and 

De Chelly sandstone members.

The conglomeratic facies of the Cutler is classified as an arkose that was 

deposited by streams flowing westward from a rising granitic area in southwestern Colorado. The reddish siltstone members were deposited by quiet 

water in basins of accumulation on a slowly sinking continental margin, and they, 

also, were derived from a source area in southwestern Colorado. The Organ 

Rock tongue is part graywacke and part arkose, whereas the Hoskinnini tongue 

is arkose. The yellowish-gray sandstone members were deposited by winds 

blowing toward the southeast, and the materials probably were derived from a 

source area to the northwest. The Cedar Mesa sandstone member is feldspathic 

orthoquartzite, and the De Chelly sandstone member is tuffaceous feldspathic 

orthoquartzite.

The Coconino sandstone and Kaibab limestone crop out in the western part of 

southeastern Utah. The Coconino is laterally equivalent to part of the Cutler 

formation and is an eolian deposit formed by southeastward-blowing winds. 

The Kaibab is a marine deposit.

The Moenkopi formation of Triassic age consists of reddish horizontally and 

ripple-laminated siltstone that is classified as arkose and some cross-stratified 

sandstone of moderately well sorted sand that is classified as feldspathic 

orthoquartzite. The Sinbad limestone member crops out in central and southcentral Utah and to the south overlaps the underlying part of the Moenkopi.

487
488 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The Moenkopi is 900 feet thick in the Capitol Reef area and thins to a depositional margin along the Utah-Colorado boundary.

The Moenkopi formation in southeastern Utah is predominantly a tidal-flat 

deposit containing some stream-deposited material. The streams flowed westward from a highland in southwestern Colorado. The Sinbad limestone member is a marine deposit. The sea of Moenkopi time transgressed southeastward 

and eastward across Utah.

The Chinle formation in southeastern Utah and in Monument Valley, Ariz., 

is divided into seven members, which are, in ascending order, the Temple Mountain, Shinarump, Monitor Butte, Moss Back, Petrified Forest, Owl Rock, and 

Church Rock members.

The Temple Mountain member is composed of mottled purple and white siltstone and claystone and minor amounts of yellowish-gray sandstone and conglomerate. It averages about 20 feet in thickness and is restricted to the San 

Rafael Swell area, in central Utah.

The Shinarump member is fine- to coarse-grained sandstone, composed of 

poorly to moderately well sorted sand, and conglomeratic lenses composed 

dominantly of siliceous pebbles. The Shinarump is feldspathic orthoquartzite 

with regional and local variations ranging from tuffaceous feldspathic orthoquartzite to arkose. It ranges in thickness from 0 to about 250 feet.

The Monitor Butte member consists of greenish bentonitic claystone and 

interstratified grayish sandstone, which is composed of very fine poorly sorted 

materials, and ranges in thickness from 0 to 250 feet. The sandstone in the 

Monitor Butte member is feldspathic orthoquartzite with local and regional 

variations that range from tuffaceous feldspathic orthoquartzite to arkose.

The Moss Back member is composed of grayish sandstone that consists of 

very fine- to medium-size poorly sorted sand and averages about 60 feet in 

thickness. The Moss Back ranges in composition from orthoquartzite to arkose and contains some graywacke.

The Petrified Forest member consists of variegated bentonitic claystone and 

clayey sandstone composed of poorly sorted sand and ranges in thickness from 

0 to 700 feet. The sandstone in the Petrified Forest member is orthoquartzitic 

tuff.

The Owl Rock member is composed of reddish siltstone and limy sandstone 

of moderately well sorted material interstratified with thin limestone beds 

and ranges in thickness from 0 to 450 feet. The detrital units in the Owl 

Rock are tuffaceous arkose.

The Church Rock member consists of reddish siltstone and minor reddish 

sandstone composed of moderately well sorted sand and ranges in thickness 

from 0 to 350 feet. The Church Rock member has two compositional classifications : the sandstone is arkose and the siltstone is graywacke.

The Chinle formation is a continental deposit formed under alluvial-plain 

and lake or lagoonal environments. The Shinarump, Monitor Butte, Moss 

Back, and Petrified Forest members contain much volcanic debris. Crossstratification studies indicate that the sandstone strata of the Chinle of southeastern Utah were derived from a source to the south and southeast

The Glen Canyon group in southeastern Utah consists of the Wingate sandstone, Kayenta formation, and Navajo sandstone. Preliminary studies indicate that the sedimentary structures dip southeast to southwest in this area. 

The Wingate sandstone is composed of moderately to well sorted very fine 

grained sand and ranges in composition from tuffaceous arkose to feldspathic 

orthoquartzite. The sandstone of the Kayenta formation consists of well 

sorted very fine grained material and is feldspathic orthoquartzite or arkose,
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 489

depending on the amount of kaolin present. The Navajo sandstone is well 

sorted very fine grained feldspathic orthoquartzite.

Most of the uranium deposits in the Triassic rocks are in the Shinarump, 

Monitor Butte, and Moss Back members of the Chinle formation, and in the 

base of undifferentiated Chinle. The deposits are generally near the base of 

the Chinle, regardless of which unit of the formation lies in that position. The 

deposits lie in broad northwestward-trending belts near the north limit of the 

respective units.

Many determinations have been made of the types of clay minerals in the 

sandstones in the Triassic and associated formations. Evidence supports the 

hypothesis that much of the hydromica in the Chinle is the result of alteration 

of montmorillonite in the presence of soluble potassium salts.

Thin-section studies of ore-bearing sandstone and barren sandstone in the 

Shinarump and Moss Back members of the Chinle formation suggest that 

uranium ore occurs predominantly in sandstone that contains 20 percent or 

more kaolin, and that strata containing 15 to 35 percent kaolin should be considered more favorable for the occurrence of uranium than strata containing less 

than 15 percent kaolin.

INTRODUCTION

Stratigraphic studies of the ore-bearing Chinle formation and associated formations of the Colorado Plateau region are being made to 

obtain information regarding the areal distribution, local and regional 

differences in lithologic character, sources and character of constituent material, and conditions of deposition of the formations. Because of the regional variations in lithologic character, it has been necessary to study many sections to analyze the stratigraphic relation of 

the Chinle formation to associated formations. This report is based 

on work done on behalf of the Division of Eaw Materials of the U. S. 

Atomic Energy Commission.

The work is designed to help, both directly and indirectly, in the 

search for uranium deposits. Local sedimentary features such as 

channel fills and lenticular sandstone units in many areas appear to 

have controlled the deposition of ore, and evaluation of these controls 

is an essential part of the project. Areas where these features are 

prominent are considered favorable for the occurrence of ore deposits. 

Thus, broad areas or belts that are relatively favorable for finding 

new ore deposits can be outlined by study of the stratigraphic and geographic distribution of known ore deposits. Without a regional appraisal of the stratigraphic occurrences of the deposits, the significance of individual deposits or groups of deposits in the regional geologic setting is obscure. Indirectly, the study helps in the search for 

uranium deposits by providing basic knowledge that can be used by 

geologists engaged in exploration for uranium deposits on the 

Colorado Plateau.

Fieldwork was virtually completed in the northern part of the 

Monument Valley area (fig. 70) in 1951; in the Circle Cliffs, Capitol 

Reef, White Canyon, and Red House Cliffs areas in 1952; and in the

484932 59 2
490 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

N

AREAS

1. Big Indian Wash 9. Kanab

2. Capitol Reef 10. Monument Valley

3. Circle Cliffs 11. Red House Cliffs

4. Comb Ridge 12. Rincon

5. Dirty Devil River 13. San Rafael Swell

6. Egnar 14. White Canyon

7. Elk Ridge 15. Defiance uplift

8. Junction of the Green 

and Colorado Rivers

FIGURE 70. Index map of part of the Colorado Plateau showing areas of stratigraphic

studies.

Comb Eidge, Dirty Devil Biver, San Eafael Swell, Elk Eidge, and 

junction of the Green and Colorado Biver areas in 1953. Preliminary 

work has been done in the Big Indian Wash, Egnar, and Kanab areas. 

Although compilation and evaluation of field data collected in these 

areas are almost finished, results of laboratory studies are preliminary. 

The rock units studied are the Cutler formation, Coconino sandstone 

and Kaibab limestone, of Permian age; the Moenkopi formation, 

Chinle formation, and Wingate sandstone of Triassdc age; the Kayenta formation of Jurassdc(?) age; and the Navajo sandstone of
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 491

Jurassic and Jurassic (?) age. The rocks studied represent as much, 

as 7,000 feet of strata (plate 49).

The stratigraphy of the Triassic formations and associated units 

in southeastern Utah and adjoining parts of Arizona and Colorado, 

as well as the stratigraphy of the Chinle formation in the Kanab 

area, is summarized in this report. Correlations are not made beyond 

the areas in which work has been done, and the paleontology of the 

units is not discussed in detail.

SCOPE

Five principal lines of investigation are followed: regional stratigraphy, sedimentary petrology, pebble studies, sedimentary-structure 

studies, and lithofacies studies.

The study of regional stratigraphy establishes a background in 

the interpretation of the stratigraphic sequence. Regional correlation of stratigraphic units is based on physical continuity and 

primary and secondary lithologic characteristics.

Sedimentary-petrology studies reveal regional differences in composition and texture of the formations by means of statistical analyses 

of grain-size distributions, heavy-mineral studies, and thin-section 

studies.

Pebble studies of composition, average and maximum size, color, 

roundness, and sphericity show the regional and stratigraphic differences of pebble suites from conglomeratic units. Fossiliferous 

pebbles also aid in identification of possible source beds.

Sedimentary-structure studies of the orientation of cross-stratification, ripple marks, and current liiieation indicate direction of 

movement of the depositing agent. These studies lead to interpretations of regional drainage patterns, of possible source areas, and of 

the validity of stratigraphic correlations.

The lithofacies study of the lower part of the Chinle formation 

has two general objectives: to determine regional variations of lithologic character and thickness, and to compare uranium-bearing parts 

with barren parts.

TERMINOLOGY AND METHODS USED IN SEDIMENTARY-ROCK

CLASSIFICATION

Two methods of sedimentary-rock classification are used: a field 

textural classification and a laboratory petrographic classification. 

In the field, classification is based on the estimated average grain 

size of the detrital components of a rock. The Wentworth (1922) 

size classification is used (table 1) in conjunction with the rock 

names given on page 492. The petrographic classification is discussed in the sedimentary-petrology section of this report.
492 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

TABLE 1. Grain-size classification, modified after Wentworth (1922)

Rock type Constituent

Boulder___ _ ___ _______

Cobble,- _ -__ _ _- _ -

Pebble ____ __ _ ______

Fine_ _ __ _ _ ___

VPT \7 f~i n P

sat__ __ -------------------

Clay_ -_ __ - 

Grade limit 

(diameter, in mm)

>256

256-64

64-4

4-2

2-1

K-K

K->8

H-Ke

K<r4_56

O.56

The methods of determining average grain size in the field and 

in the laboratory often produce different results. Field determinations are visual, and the most abundant, or modal, grain size is the 

average grain size. Laboratory determinations are made with the 

aid of graduated sieves and pipettes, and the computed mean grain 

diameter is used as the average grain size. In most detrital sediments, the modal grain size is larger than the mean grain size. The 

difference may be 1 or 2 Wentworth grade sizes in a poorly sorted 

sediment or less than half a grade size in a well-sorted sediment. 

Thus, average grain sizes based on field observations are generally 

larger than those based on laboratory determinations.

For purposes of field classification, the fragments in a detrital sedimentary rock are divided into four constituents clay, silt, sand, and 

gravel. The constituent with the largest percentage is the major constituent and the constituents with smaller percentages are minor 

constituents.

Type 1. No minor constituent greater than 20 percent. 

Classification: Named after major constituent. 

Example: 85 percent sand, 10 percent silt, and 5 percent clay; designated as

sandstone. 

Type 2. One minor constituent greater than 20 percent.

Classification: Major constituent name modified by minor constituent name. 

Example: 75 percent silt, 25 percent sand; designated as sandy siltstone. 

Type 3. Two or more constituents greater than 20 percent.

Classification: Double modification of major constituent name, with second 

largest minor constituent as first modifier and largest minor constituent as 

second modifier. 

Example: 40 percent gravel, 35 percent sand, 25 percent silt; designated as

silty sandy conglomerate. 

Type 4. Gravel comprises 10 to 20 percent.

Classification: Modification of major and minor constituent names with onfe 

of the modifiers "granular," "pebbly," "cobbly," or "bouldery-" The modifier is the largest class size in the gravel and precedes all other modifiers. 

Example: 60 percent sand, 9 percent pebbles, 6 percent granules, 25 percent 

silt; designated as pebbly silty sandstone.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 493

TERMINOLOGY AND METHODS USED IN SEDIMENTARY-STRUCTURE

STUDIES

The methods used in sedimentary-structure studies are slightly 

modified from those developed by Eeiche (1938). Descriptive terms 

are those proposed by McKee and Weir (1952).

The amount and direction of dip is measured on a number of individual cross-strata in the unit being studied. The number of measurements necessary for adequate sampling depends on the diversity in 

dip directions of the cross-stratification. The results of many field 

tests show that 150 measurements are adequate in sedimentary rocks 

such as the Shinarump member of the Chinle, which contains mediumscale cross-stratification. Fifty measurements are sufficient hi sedimentary rocks such as the Navajo sandstone, which contains large-scale 

cross-stratification. Only one measurement is made on a single set 

of cross-strata.

A basic assumption in the study of the orientation of crossstratification is that a component of the dip direction of the crossstrata is in the down-current direction. If each dip-direction reading 

is considered a vector, a resultant of the readings can be obtained by 

mathematical or graphical methods. This resultant is the average 

down-current direction, from which a transportation direction and a 

source direction can be inferred.

The spread of the readings in a study is measured in terms of a 

consistency factor which is expressed numerically from 0 to 1. If all 

the readings are in the same direction the consistency factor would 

be 1, whereas if the readings are equally distributed through 360°, 

the consistency factor would be 0. A consistency factor below 0.20 

indicates an insignificant trend.

STRATIGRAPHY

PERMIAN ROCKS

The Permian rocks in southeastern Utah consist of the Coconino 

sandstone, Kaibab limestone, and Cutler formation. The Eico formation is considered to be of Pennsylvania!! and Permian (?) age and is 

not described here. The Coconino sandstone, Kaibab limestone, and 

Cutler formation are all closely related and are marked by abrupt 

and prominent facies changes. The interrelations of these formations 

and of members within the Cutler are too complex to describe in 

detail here, and an outline of only the major stratigraphic features 

of the Permian rocks is presented.

COCONINO SANDSTONE

In southeastern Utah, the Coconino sandstone of Permian age 

(named by Darton, 1910, p. 21, 27) is exposed in the Capitol Eeef and
494 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

San Eafael Swell areas. It overlies the Hermosa formation of 

Pennsylvanian age and underlies the Kaibab limestone of Permian age. 

Most, if not all, of the Coconino is equivalent to the Cutler formation 

in the areas south and east of the Capitol Reef and San Eafael Swell 

areas. In southeastern Utah, the practice of geologists has been to 

use the term Cutler in areas where the Permian sequence contains 

reddish siltstone members and to use the term Coconino in areas where 

there are no reddish siltstone members. A recent drill hole in the 

Circle Cliffs area shows the presence of a thick reddish siltstone unit 

in the Permian sequence (Steed, 1954) that has led to a reassignment 

to the Cutler of rocks formerly assigned to the Coconino sandstone 

by Gregory and Moore (1931).

McKee (1954a) has suggested that the Coconino of the type area 

of northern Arizona may not be physically continuous with the Coconino sandstone of southeastern Utah. Further work might show 

that the term Coconino is not appropriate in southeastern Utah.

The Coconino sandstone is yellowish-gray fine-grained sandstone 

composed of well-sorted rounded clear quartz and rare black accessory minerals. It is composed of thick to very thick trough sets of 

medium- to large-scale cross-strata. The Coconino attains a thickness of 685 feet in the San Kafael Swell area (Baker, 1946, p. 49), 

and at least 550 feet is exposed in the Capitol Reef area. The Coconino is interpreted to be an eolian deposit. The cross-strata in the 

Coconino dip dominantly southeast, indicating that the winds that 

deposited the Coconino blew southeastward.

KAIBAB LIMESTONE

In southeastern Utah, the Kaibab limestone of Permian age (named 

by Darton, 1910, p. 21, 28, 32) crops out in the San Rafael Swell, 

Capitol Eeef, and Circle Cliffs areas. Between these areas and outcrops of Permian rocks along the Colorado River to the east the 

Kaibab either pinches out or grades laterally into the White Eim 

sandstone member of the Cutler formation. The Kaibab overlies 

the Coconino sandstone of Permian age in the San Eafael Swell and 

Capitol Eeef areas and the Cutler formation of Permian age in the 

Circle Cliffs area. It underlies the Moenkopi formation of Early 

and Middle (?) Triassic age.

The Kaibab is composed of yellowish-gray and light-greenish-gray 

thin to thick horizontally bedded limestone and dolomite. Abundant 

gray and brown chert occurs as beds and nodules in the limestone and 

dolomite. Thick sets of yellowish-gray fine-grained cross-stratified 

sandstone are interstratified with the limestone in some places. The 

Kaibab ranges in thickness from 0 to 360 feet. Invertebrate fossils 

such as brachiopods, pelecypods, and gastropods are common, and the 

Kaibab limestone is interpreted to be a marine deposit.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 495 

CUTLER FORMATION

The Cutler formation of Permian age (named by Cross and Howe, 

1905, p. 5) extends over all of west-central and southwestern Colorado, east-central and southeastern Utah, and in the Monument Valley area of Arizona. It is absent in the San Eaf ael Swell and Capitol 

Eeef areas, but the Coconino sandstone of Permian age in these areas 

is probably equivalent to part or all of the Cutler. The Cutler is 

underlain by the Rico formation of Pennsylvanian and Permian (?) 

age and is overlain by the Moenkopi formation of Early and Middle(?) Triassic age or by the Chinle formation of Late Triassic age 

in extreme east-central Utah.

The Cutler formation in southwestern Colorado and east-central 

Utah is composed of grayish-red and pale-reddish-brown arkosic 

sandstone, conglomeratic sandstone, and minor quantities of sandy 

siltstone and is referred to in this report as the conglomeratic facies 

of the Cutler. The sandstone is very fine to coarse grained and locally includes conglomeratic sandstone lenses containing granitic pebbles. The sandstone is cross-stratified but may contain horizontal 

laminae and thin beds. The sandy siltstone units commonly contain 

minor amounts of very fine grained sand and are composed of horizontal laminae to thin beds.

The conglomeratic facies of the Cutler extends throughout southwestern Colorado and into east-central Utah. In southeastern Utah 

and in Monument Valley, Ariz., the Cutler is composed of alternating 

reddish siltstone members and yellowish-gray sandstone members. 

The members are, in ascending order, the Halgaito tongue, Cedar 

Mesa sandstone member, Organ Rock tongue, De Chelly sandstone 

member, White Rim sandstone member, and Hoskinnini tongue. Not 

all these members are found in the same area.

The Organ Rock tongue and possibly the Halgaito tongue, both 

reddish siltstone members, are finer grained westward-extending 

tongues of the conglomeratic facies of the Cutler. The Hoskinnini 

tongue, also a reddish siltstone member, has recently been found to 

be present in a large part of southeastern Utah and west-central Colorado (Stewart, and others, written communication, 1954) and is not 

a tongue of the conglomeratic facies of the Cutler. The Cedar Mesa 

sandstone member and White Rim sandstone member are lithologically similar to the Coconino sandstone and are probably tongues of 

the Coconino extending east and south of the San Rafael Swell and 

Capitol Reef areas, where the Coconino is exposed. The Cedar Mesa 

and White Rim members grade to the east across southeastern Utah 

into the reddish siltstone members and eventually pinch out to the 

east in the conglomeratic facies of the Cutler. The De Chelly sandstone member is also lithologically similar to the Coconino sandstone and may be equivalent to the White Rim sandstone member.
496 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The Halgaito, Organ Eock, and Hoskinnini tongues are composed 

mainly of pale-reddish-brown horizontally laminated and bedded 

sandy siltstone and siltstone. The Cedar Mesa sandstone, White 

Eim sandstone, and De Chelly sandstone members are composed of 

very-pale-orange and yellowish-gray very fine- to fine-grained sandstone which is cross-stratified on a large scale.

More detailed descriptions of the lithologic character, distribution, 

and interrelations of the members of the Cutler are given by Baker 

and Eeeside (1929), Baker (1933, 1936, and 1946), and Hunt and 

others (1953).

The rocks in the conglomeratic facies of the Cutler probably were 

deposited in streams. Field evidence indicates that this facies of 

the Cutler was derived from a rising granitic area in southwestern 

Colorado the Uncompahgre highland and that the streams flowed 

westward from this highland.

The rocks in the reddish siltstone members and light-colored sandstone members of the Cutler are interpreted as having been deposited 

in two main environments. The reddish siltstone members the 

Halgaito, Organ Eock, and Hoskinnini tongues were probably deposited in quiet water in a slowly sinking marginal continental basin. 

The yellowish-gray sandstone units the Cedar Mesa, White Eim, 

and De Chelly sandstone members are probably eolian deposits 

formed under quiescent conditions in subaerial basins. The crossstrata in the yellowish-gray sandstone members dip dominantly 

southeast, indicating southeastward-blowing winds and a source 

area lying northwest of southeastern Utah.

TEIASSIC ROCKS 

MOENKOPI FORMATION

The Moenkopi formation of Early and Middle(?) Triassic age 

was named by Ward (1901, p. 403) for exposures in northeastern 

Arizona. It extends over most of southeastern Utah and all of 

Monument Valley, Ariz. (fig. 71). The east limit of the Moenkopi 

in southeastern Utah is covered by younger rocks, but further study 

of drill-hole information may locate the limit. The Moenkopi and 

equivalents are absent in the Big Indian Wash and Egnar areas. 

These areas may be east of the margin of the Moenkopi or they may 

be areas where the Moenkopi pinches out locally because of movement 

of salt in anticlines during Moenkopi time.

The Moenkopi formation overlies the Cutler formation in most of 

southeastern Utah, the Kaibab limestone in the San Eafael Swell 

and Capitol Eeef areas, and locally the Coconino sandstone in the 

San Eafael Swell. The Moenkopi is overlain by the Chinle formation.

The Moenkopi formation in southeastern Utah is composed of
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 497

EXPLANATION

 

Location of measured section

isopach line

Dashed where inferred;

interval 100 feet

FIGURE 71. Isopach map of the Moenkopi formation of Triassic age in southeastern Utah 

and Monument Valley area of Arizona.

pale-reddish-brown micaceous ripple- and horizontally laminated 

siltstone and sandy siltstone and minor pale-reddish-brown and 

yellowish-gray very fine grained cross-stratified sandstone. The 

trend of the ripple marks as developed in the siltstone is northeastsouthwest, with steep southeast sides. The cross-stratified sandstone, less than 10 percent of the Moenkopi, occurs in channel-filling 

beds about 5 to 40 feet thick that weather to form ledges. The crossstrata in these sandstones dip northwest.

484932 59 3
498 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

In the San Eafael Swell, Capitol Eeef, and Dirty Devil Eiver 

areas, and in the western part of the White Canyon area, the basal 

part of the Moenkopi consists of conglomeratic sandstone ranging in 

thickness from 0 to 30 feet. The rock is light gray and yellowish 

gray and is composed of white and gray earthy angular roughsurfaced nonfossiliferous granules, pebbles, and cobbles of chert in a 

medium-grained sand matrix. This conglomerate at the base of the 

Moenkopi is composed of small-scale cross-strata that in places fill 

channels. The granules, pebbles, and cobbles closely resemble the 

chert pebbles found in the Hoskinnini tongue of the Cutler formation 

and may have the same source.

The Moenkopi formation is unconformable on the underlying rocks 

in some areas of southeastern Utah. The unconformity is well 

developed in the San Eafael Swell where Baker (1946, p. 51) reported that pre-Moenkopi erosion removed the Kaibab limestone 

over large areas, and that the pre-Moenkopi surface probably had 

a relief of 100 feet or more. The Moenkopi conglomerate that fills 

channels cut into the underlying units suggests an unconformity 

in the areas where the conglomerate is present. In the eastern part 

of White Canyon and to the south in the Monument Valley area, 

however, the boundary between the Permian and Triassic rocks 

exhibits no significant break in deposition.

The Moenkopi formation is more than 900 feet thick in the Capitol 

Eeef area and thins to 0 in southeastern Utah (fig. 71). Locally, the 

Moenkopi may be anomalously thin due to deep channels cut into 

it that are filled with the Shinarump member of the Chinle. It is 

difficult to subdivide the Moenkopi formation in southeastern Utah 

except in the areas where the Sinbad limestone member is present. 

However, there is a poorly defined interval of as much as a few 

hundred feet near the middle of the Moenkopi in southeastern Utah 

in which the rocks are commonly more resistant and more crossstratified than the rest of the Moenkopi.

The Sinbad limestone member of the Moenkopi formation crops 

out in the San Eafael Swell, Capitol Eeef, and the northern part of 

the Circle Cliffs areas. In addition, thin limestone beds with a lithologic character and stratigraphic position similar to those of the 

Sinbad limestone member have been found at two localities near 

the junction of the Green and Colorado Eivers. Limestone at one 

of these localities contains Meefcoceras? sp. (McKnight, 1940, p. 58). 

In southeastern Utah MeeTcoceras is restricted to the Sinbad limestone member. The fossil evidence and the similarity of the lithologic character and stratigraphic position suggest that these thin 

limestone beds are correlative with the Sinbad limestone member.

The Sinbad limestone member is composed of limestone and minor 

amounts of siltstone. The limestone is yellowish gray, pale yellowish
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 499

orange, or light olive gray. It is dense but may be partly composed 

of medium to large oolites, and is horizontally laminated to thinly 

bedded. Minor amounts of thin trough sets of low-angle small-scale 

cross-laminae occur in the limestone. Siltstone in the Sinbad limestone member comprises 5 to 20 percent of the unit. The siltstone 

generally has the same color as the limestone and appears as thin to 

very thick horizontally laminated sets interstratified with the limestone. The Sinbad limestone member weathers to form a vertical 

cliff.

The Sinbad limestone member in the San Rafael Swell and the 

Capitol Reef areas ranges in thickness from about 12 feet (Baker, 

1946, p. 55) to 150 feet. In the northernmost part of Circle Cliffs, 

the Sinbad is about 40 feet thick. In the southern part of Circle 

Cliffs, the Sinbad is thin and locally absent (E. S. Davidson, oral communication, 1956) and is inferred to be near a border of deposition.

The Sinbad limestone member becomes nearer to the base of the 

Moenkopi, although irregularly, south from the San Rafael Swell and 

finally overlaps that part of the Moenkopi formation underlying the 

Sinbad. The part of the Moenkopi underlying the Sinbad ranges in 

thickness from 90 to 200 feet (Gilluly, 1929, p. 83; Baker, 1946, p. 55) 

in the San Rafael Swell and from 50 to 100 feet in the Capitol Reef 

area. At one place in the northern part of the Circle Cliffs area, this 

part of the Moenkopi is only 8 feet thick. A few miles south of this 

place the part of the Moenkopi underlying the Sinbad pinches out and 

the Sinbad limestone member rests directly on the Kaibab limestone.

The Moenkopi formation is interpreted as dominantly a tidal-flat 

deposit. The interpretation is based on the abundance of ripplemark laminae, shrinkage cracks, casts of salt cubes, and reptile tracks. 

The cross-stratified sandstone beds are interpreted to be stream deposits. The direction of dip of the cross-strata in these sandstone 

cosets indicates that streams during Moenkopi time flowed westward. 

The Moenkopi formation in western Colorado contains arkosic coarsegrained and conglomeratic sandstone which suggests that the 

Moenkopi was derived from the granitic Uncompahgre highland of 

southwestern Colorado and partly from the conglomeratic f acies of the 

underlying Cutler formation. The chert granules, pebbles, and cobbles that are found in the conglomerate at the base of the Moenkopi in 

the White Canyon, Dirty Devil River, and San Rafael Swell areas 

probably were derived from nearby western sources as indicated by 

their angularity and rough surfaces. Probably these pebbles resulted 

from reworking of the chert nodules of the Kaibab limestone.

The fossil evidence in the Sinbad limestone member indicates that 

it is a marine deposit. The seas in which parts of the Moenkopi were 

deposited probably transgressed southeastward and eastward across
500 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Utah as is shown by the thinning of the Moenkopi to the east, by the 

onlapping of the Sinbad limestone on older rocks to the southeast, 

and by the easterly asymmetry of the ripple marks. In addition, the 

Sinbad limestone, which is about 90 to 200 feet above the base of the 

Triassic rocks in the San Raf ael Swell, contains the Meekoceras f aunal 

zone which lies 1,000 feet above the base of the Triassic rocks in northern Utah (Newell and Kummel, 1942, p. 938). Considerable Triassic 

deposition may have taken place in northern Utah before deposition 

started in the San Raf ael Swell. McKee (1954b) has described the 

transgression of the Moenkopi sea in detail.

At the end of Moenkopi deposition a regional emergence occurred, 

as shown by the erosion that preceded the deposition of the Shinarump 

member of the Chinle formation.

CHINUE FORMATION

The Chinle formation (named by Gregory, 1917, p. 42) of Late 

Triassic age is present in outcrops throughout southeastern Utah. It 

unconformably overlies the Moenkopi formation or the Cutler formation in extreme east-central Utah where the Moenkopi is absent. The 

unconformity at the base of the Chinle is the most conspicuous and 

widespread break in the sequence of the Permian and Triassic rocks 

and is marked by many channels that cut a few feet to several tens of 

feet into the Moenkopi surface. The Chinle is unconformably overlain, or perhaps conformably overlain in some areas, by the Wingate 

sandstone. The Chinle is composed of variegated claystone and siltstone, sandstone and conglomerate, and minor amounts of limestone 

and limestone-pebble conglomerate.

In southeastern Utah and in the Monument Valley area of Arizona, 

the Chinle is divided into seven members, which are, in ascending 

order, the Temple Mountain, Shinarump, Monitor Butte, Moss Back, 

Petrified Forest, Owl Rock, and Church Rock members. The Temple 

Mountain member is a thin unit composed largely of siltstone and 

is restricted to the San Raf ael Swell. The Shinarump and Moss Back 

members are widespread sandstone and conglomerate units. The 

Monitor Butte and Petrified Forest members are mostly bentonitic 

claystone and clayey sandstone. The Monitor Butte member contains 

some lenses of sandstone. The Owl Rock and Church Rock members 

are largely reddish siltstone. Minor amounts of limestone are present 

in the Owl Rock member.

In most areas, the Shinarump and Moss Back members are distinctive units. The differences between the Monitor Butte, Petrified 

Forest, Owl Rock, and Church Rock members, however, are slight in 

some areas, and, locally, differentiation is extremely difficult. These 

members of the Chinle intertongue and intergrade, so that in some 

places defined contacts are as much as 100 feet higher or lower in a
TEIA.SSIC STRATIGRAPHY, COLORADO PLATEAU REGION 501

FIQUEH 72. Index map of part of the Colorado Plateau showing outcrops of the Chinle

formation of Triassic age.

lateral distance of a few thousand feet. Because of these large local 

vertical variations in the contacts, an individual measurement of 

thickness may not be typical for a restricted area.

The outcrop pattern of the Chinle formation in southeastern Utah 

and adjoining areas is shown in figure 72. Some of the gaps between 

outcrops are large and correlations are correspondingly less certain.

TEMPLE MOUNTAIN MEMBER

The Temple Mountain member of the Chinle formation has been 

named and defined by Robeck (1956) in the San Rafael Swell and is 

restricted by the authors to that area. The member is a thin unit that
502 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

unconformably overlies the Moenkopi formation. It underlies the 

Monitor Butte member in the southern part of the San Eafael Swell 

and the Moss Back in the northern part of the San Eafael Swell 

beyond the limit of the Monitor Butte (fig. 73). The upper contact 

is considered by some geologists to be an unconformity. The authors 

believe, however, that the depositional break may be minor and that 

the contact may not be an unconformity.

The Temple Mountain member consists mostly of mottled purple, 

reddish-brown, and white structureless siltstone. The siltstone generally contains some scattered well-rounded medium to coarse size 

quartz grains. In places, mottled purple and white claystone interfingers and intergrades with the siltstone. Locally, light-gray, lightgreenish-gray, or, rarely, reddish-purple sandstone is present in the 

Temple Mountain member. The sandstone is generally medium to 

very coarse grained, although it is very fine grained in places. It 

locally contains conglomeratic lenses with granules and pebbles, 

mostly of quartz, and is cross-stratified. The sandstone is most commonly present at the base of the Temple Mountain member but may 

occupy any position in the member. Lenses of jasper and carbonaceous plant material are commonly present in the member.

Some of the sandstone lenses in the Temple Mountain member are 

lithologically similar to the Shinarump member, and possibly they 

are outlying lenses of the Shinarump. The term Shinarump, however, is not used in the San Rafael Swell because of uncertainties of 

correlation and because these lenses form an integral part of the 

thicker, more inclusive, Temple Mountain member that is lithologically unrelated to the Shinarump.

The Temple Mountain member is distinguished from the overlying 

and underlying strata by several lithologic characteristics, including 

the distinctive mottled purple and white coloration, medium- to 

coarse-grained quartz sand disseminated in the siltstone, sandstone 

containing large quartz pebbles, iron oxide pebbles, carbonaceous 

material, and lenses of jasper (Robeck, 1956). The lower contact 

of the member is located in many places at the change from siltstone 

of the Moenkopi to sandstone of the Temple Mountain member. 

Where siltstone of the Temple Mountain member rests on siltstone 

of the Moenkopi formation, the contact is difficult to locate but generally can be selected by the distinctive coloration and presence of 

medium- to coarse-grained sand in the siltstone of the Temple Mountain member. The upper contact in most areas is at the change from 

the mottled siltstone or claystone of the Temple Mountain member 

to the very fine- to fine-grained sandstone at the base of the Monitor 

Butte member or the fine- to medium-grained sandstone of the Moss
FIGURE 73. Correlations of the Chinle formation 
504 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Back member. Locally, siltstone and claystone of the Monitor Butte 

rest directly on the siltstone and claystone of the Temple Mountain, 

and in these places the Temple Mountain can be distinguished from 

the Monitor Butte by the presence in the Temple Mountain of the 

purple and white color, jasper, and pebbles of quartz and iron oxide 

(Eobeck, 1956).

The Temple Mountain member averages about 5 feet in thickness 

in the southwest third of the San Eafael Swell, although it is not 

present in several exposures. The member averages about 20 feet 

in thickness in the central parts of the San Raf ael Swell and about 

30 feet in thickness in the northern part. Locally in the northern 

part of the San Raf ael Swell the member attains a maximum thickness of 101 feet in a channel fill (Robeck, 1956).

A mottled purple and white siltstone unit lithologically similar to 

the Temple Mountain member is found in other parts of southeastern Utah and perhaps correlates with the Temple Mountain member. 

The strata rest directly on the Moenkopi formation and are overlain 

by the Shinarump, Monitor Butte, Moss Back, or some other part 

of the Chinle formation, whichever is present. The unit is present 

in parts of many areas throughout southeastern Utah, but its distribution is extremely spotty and perhaps the unit originally covered 

less than 10 percent of southeastern Utah. In most places the unit 

is less than 20 feet thick. The spotty distribution and thinness of 

these strata and the presence of lithologically similar mottled siltstone higher in the Chinle make precise correlation difficult. Possibly the mottled siltstone strata represent a deposit formed by a 

repetition of similar environments in different areas at different times 

rather than one widespread unit representing one time of deposition. 

In places, the mottled colors of the unit extend down into the top 

part of the Moenkopi formation, and part, or perhaps all, of the 

mottled siltstone strata in some areas is an altered zone at the top of 

the Moenkopi. In most areas, however, the unit probably represents 

reworked and altered sedimentary rocks from the Moenkopi at the 

base of the Chinle.

SHINARTTMP MEMBER 

GENERAL CHABACTERISTICS

The unit that constituted what has previously been called the 

Shinarump conglomerate was first noted by Powell (1873, p. 458). 

Powell selected the type locality and applied the name Shinarump 

Cliffs to the topographic feature of the unit, but Gilbert (1875, p. 

176) was the first to use the name Shinarump conglomerate. Stewart (1957) has redefined the Shinarump as the Shinarump member 

of the Chinle formation and restricted the Shinarump to strata correlative to the type Shinarump of southwestern Utah.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 505

The restriction of the Shinarump to strata correlative to the type 

Shinarump has led to a new classification of units previously included 

in the Shinarump conglomerate in southeastern Utah. The strata, 

previously called Shinarump conglomerate, included part or all of 

the Temple Mountain, Shinarump, Monitor Butte, and Moss Back 

members of the Chinle formation. In the Monument Valley, Circle 

Cliffs, and Capitol Reef areas the Shinarump member of this report 

is the same as the Shinarump conglomerate of Gregory and Moore 

(1931), Baker (1936), and Gregory and Anderson (1939). In most 

of the White Canyon and Elk Ridge areas, and in the area near the 

junction of the Dirty Devil and Colorado Rivers, the Monitor Butte 

and Moss Back members, and the Shinarump member where it is 

present, collectively form what was previously mapped as Shinarump 

conglomerate by Gregory (1938) and Hunt and others (1953). In 

the area near the junction of the Green and Colorado Rivers, the 

Shinarump and Monitor Butte members are absent, and the unit 

mapped as Shinarump conglomerate by Baker (1933 and 1946) and 

McKnight (1940) is the Moss Back member. In the San Rafael 

Swell, the Shinarump conglomerate of Gilluly (1929), Baker (1946), 

and Hunt and others (1953) consists mostly of the Moss Back and 

Monitor Butte members, or of the Moss Back member where the 

Monitor Butte is absent. In some places in the San Rafael Swell, 

these authors included the Temple Mountain member in their Shinarump; in other places, they included it partly in their Shinarump 

and partly in their Moenkopi; and in still other places, included it 

entirely in their Moenkopi.

The northeast limit of the Shinarump member is shown in figure 

74. The limit is difficult to locate, as the strata correlative to the 

Shinarump form thin, small scattered lenses near the north limit 

and detailed work may reveal lenses north of those now known.

The Shinarump member is composed typically of yellowish-gray 

and pale-yellowish-orange medium- to coarse-grained sandstone composed of subangular clear quartz. Lenses of conglomeratic sandstone 

and conglomerate containing granules and pebbles predominantly of 

quartz, quartzite, and chert are common. The Shinarump consists 

of thin trough sets of medium-scale cross-strata and contains abundant silicified and carbonized wood. It forms vertical cliffs.

The Shinarump member can be differentiated in many places from 

other sandstone units in the Chinle formation on the basis of the 

range of grain sizes. The sandstone units in the Shinarump generally 

range from medium to coarse grained, whereas the sandstone units 

in the overlying part of the Chinle generally are very fine to medium 

grained. In addition, the proportions of pebble types in the 

Shinarump differ from those in the rest of the Chinle.

484932 59 4
506 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

EXPLANATION

 

Uranium deposit

FIGURE 74. Distribution of the Shinarump member of the Chinle formation of Triasslc 

age and location of uranium deposits in this member in southeastern Utah and northeastern Arizona.

The contact of the Shinarump member and the Moenkopi formation is sharp and unconformable and is at the change from the reddish-brown micaceous ripple-laminated siltstone of the Moenkopi to 

the yellowish-gray cross-stratified medium- to coarse-grained sandstone of the Shinarump.

The contact of the Shinarump member and the Monitor Butte 

member is conformable and is at the change from the generally medium- to coarse-grained cross-stratified sandstone beds of the Shinar-
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 507

limp to the ripple-laminated sandstone or the structureless claystone of 

the Monitor Butte.

The Shinarump member ranges in thickness from a maximum of 

about 250 feet to 0, and at many places in southeastern Utah and northeastern Arizona the Shinarump is locally absent within the depositional basin. These gaps in the Shinarump extend a few thousand feet 

to a few miles along the outcrops. In a fairly large area, including 

the Eed House Cliffs and outcrops to the east, the Shinarump is absent 

(fig. 74) except for a few very thin conglomeratic sandstone lenses.

The general thickness, depth of channels, and continuity of the 

Shinarump vary from area to area in southern Utah and northern Arizona. In the Kanab area, the Shinarump is continuous and consistently 50 to 100 feet thick, and fills channels cut only a few feet into the 

underlying Moenkopi. In the Monument Valley, Circle Cliffs, and 

Capitol Eeef areas, the Shinarump is discontinuous, ranges in thickness from 0 to 200 feet, and fills deep channels. In the White Canyon 

and Elk Eidge areas, the Shinarump is discontinuous and generally 

less than 20 feet thick, and fills channels cut only a few feet into the 

underlying Moenkopi.

PEBBLE STUDIES

The pebbles in the Shinarump are composed almost entirely of 

quartz, quartzite, and chert (table 2). Types of quartz pebbles differ

TABLE 2. Composition of pebbles, in percent, in the Shinarump member of the 

Chinle formation of southern Utah

Area

Kanab______ _ __ _________ ___

Circle Cliffs_--__-_-_________________-_

Capitol Reef. ___ _ ____

Red House Cliffs ____ _ ____ _ _ _ _

White Canyon _ ___ _ __ _____

Vitreous 

quartz

17

30

41

60

84

89

Quartzite

47

36

20

16

14

9

Chert

36

33

35

13

2

1 +

Other

1

4

11

markedly among the areas. In the Kanab area, the quartz is mostly 

either transparent or translucent white. In the Circle Cliffs and 

Capitol Reef areas, the quartz pebbles are dominantly transparent; 

pink and smoky quartz are common. In the White Canyon area, transparent quartz and a distinctive pink variety are dominant, and smoky 

varieties of quartz pebbles are common.

A pronounced change in the color of the chert pebbles occurs eastward from the Kanab area. In the Kanab area about 10 percent of 

the chert pebbles are red, pink, and orange, and the balance are gray, 

brown, black, and white. In the Capitol Eeef area, gray chert is
508 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

dominant; brown, black, and white chert are common; and red chert 

is rare.

A change in the color of the quartzite pebbles occurs northward and 

eastward from the Kanab area. In the Kanab area about 30 percent 

of the quartzite pebbles are red and pink. Most of the remaining 70 

percent are gray, tan, and brown. In the Capitol Reef and White 

Canyon areas, the red and pink colors are uncommon; most of the 

pebbles are white, gray, and tan.

Silicified limestone constitutes about 5 percent of the pebbles in 

the Circle Cliffs and Capitol Reef areas. It was identified in only one 

other collection, from near Fredonia, Ariz., in the Kanab area.

The maximum size of the pebbles varies regionally. The maximum 

size decreases from about 4^/2 inches in the Kanab area to about V/% 

inches in the Capitol Reef area and from about 8 inches in the Monument Valley area to about 4 inches in the White Canyon area.

Fossiliferous pebbles from the White Canyon area indicate a source 

in Mississippian, Pennsylvanian, and Permian rocks. Most of the 

fossil-bearing pebbles from the Kanab, Circle Cliffs, and Capitol Reef 

areas indicate a source in the Kaibab limestone of Permian age. The 

Kaibab or its lateral equivalents have not yielded fusulinids, which 

indicates that some pebbles containing fusulinids probably had another 

source. Areas that contain rocks of these ages surround the Colorado 

Plateau, and many of the areas could have been a source for the 

pebbles.

SEDIMENTABY-STRUCTUEE STUDIES

The cross-strata in the Shinarump member, on the basis of the 

average resultant dip computed in 30 studies (fig. 75), dip N. 54° W. 

The consistency factors among readings in individual studies are quite 

variable and range from 0.46 to 0.95 and average 0.63.

MONITOR BUTTE MEMBER

The Monitor Butte member of the Chinle formation has been named 

and defined by I. J. Witkind and R. E. Thaden for exposures in the 

Monument Valley area of Arizona (written communication, 1954).

The Monitor Butte member is present in Chinle outcrops throughout 

the Monument Valley area of Arizona and most of southeastern Utah. 

Its limits in southeastern Utah are shown in figure 76. Recognition 

of the member is difficult in many areas and some of the correlations 

are tentative.

The Monitor Butte member consists dominantly of greenish-gray, 

with minor amounts of pale-reddish-brown, bentonitic claystone or 

clayey sandstone that weathers to form a "frothy" appearing slope. 

The clayey sandstone is fine grained and consists of milky, and minor 

orange and green, grains. The claystone and clayey sandstone com-
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 509

N :\ : l\ ~Price

J Green 

\«| RRlver

V» VV3 

\%.

*___J) * 1?

Montlcello'°\

S . !

B landing

. I, ^Y^.

.UTAH,

Kanab RIZONA I \ .

' Kayenta0 |

i

r

ll J

EXPLANATION

Direction of arrow is resultant dip of 

cross-strata; tail of arrow marks 

location of cross-stratification 

study; length of arrow is proportional 

to consistency factor 

o i

Consistency factor scale

FIGURE 75. Resultant-dip directions of cross-strata in the Shinarump member of the 

Chinle formation of Triassic age.

monly contain as much as 1 to 3 percent dark-green medium- to coarsegrained mica (probably biotite). Stratification is poorly exposed, 

but the rocks are either cross-stratified or structureless. Carbonized 

and silicified wood is common.

Interstratified with the claystone and clayey sandstone are sandstone 

lenses most of which are 1 to 10 feet thick and about 1,000 feet wide. 

The lenses form about 5 to 20 percent of the member. Locally these 

sandstone lenses may be absent. The sandstone is very fine grained,
510 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

\

\

AREA CONTAINING DEPOSITS IN ^ \ 

THE MONITOR BUTTE MEMBER

EXPLANATION

 

Uranium deposit

FIGURE 76. Distribution of the Monitor Butte member of the Chinle formation of Triassic 

age and location of the uranium deposits in this member in southeastern Utah.

micaceous, well cemented, ripple-laminated or rarely cross-stratified, 

and platy splitting. A few of the sandstone lenses are conglomeratic, 

with pebbles of limestone, siltstone, and minor amounts of weathered 

chert pebbles. The sandstone lenses are commonly highly contorted 

by many small-scale folds and faults.

The Monitor Butte member has several lithologic characteristics 

that distinguish it from associated units. Lenses of greenish-gray 

micaceous well-cemented ripple-laminated sandstone are the main distinguishing characteristics of the member. Sandstone of this type 

is present in a few areas in other members of the Chinle, but these 

can be recognized as stratigraphically distinct from the Monitor Butte
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 511

member. Although lenses of cross-stratified sandstone are to some extent diagnostic, similar sandstone lenses occur in the overlying Moss 

Back member. In many places, however, these lenses in the Monitor 

Butte member do not contain quartzose pebbles, whereas quartzose 

pebbles are common in the Moss Back member. Other characteristic 

features of the member that in places distinguish it from overlying 

rocks are contorted stratification of sandstone lenses; greenish-gray 

clayey sandstone composed of milky, and minor orange and green, 

grains, with an abundance of coarse dark mica; and flakes of carbonaceous material in thick units of claystone or clayey sandstone.

The contact between the Monitor Butte and Shinarump members is 

conformable and is at the change from the generally medium- to coarsegrained cross-stratified sandstone beds of the Shinarump to the ripplelaminated sandstone or structureless claystone of the Monitor Butte 

member. The contact is well defined in some localities and transitional in others. Intertonguing of the Shinarump and Monitor Butte 

members has been noted in several places in southeastern Utah. 

Where the Monitor Butte member directly overlies the Moenkopi 

formation, the contact is sharp and unconf ormable and is at the change 

from the reddish-brown micaceous ripple-laminated siltstone beds of 

the Moenkopi to the greenish-gray bentonitic claystone or clayey sandstone and ripple-laminated sandstone beds of the Monitor Butte.

The upper contact of the Monitor Butte member in most areas is at 

the top of the highest unit in the Chinle formation containing distinguishing lithologic features of the Monitor Butte member, and is 

generally the top of the highest ripple-laminated or cross-stratified 

sandstone lens. The location of the contact changes abruptly, depending on local variations in the position of the lenses. Locally the sandstone lenses that are typical of the Monitor Butte are absent, and the 

Monitor Butte is distinguished from the rest of the Chinle by characteristics of the member other than the sandstone lenses. Where the 

sandstone lenses are absent, however, it is difficult or even impossible to distinguish the Monitor Butte from the rest of the Chinle.

The Monitor Butte member in southeastern Utah and in the Monument Valley area of Arizona ranges in thickness from 0 to 250 feet. 

It thins from 200 feet in White Canyon to a limit along an east-west 

line passing about 15 miles north of White Canyon. In the San Raf ael 

Swell, it ranges in thickness from 0 to as much as 100 feet.

The Monitor Butte member is confined to the Monument Valley area 

and to southeastern Utah but may correlate with defined members of 

the Chinle in western Utah and northern Arizona. In the Defiance 

uplift area, Harshbarger (oral communication, 1954) has recognized a 

lower member of the Chinle. This member corresponds with a unit 

that Gregory (1917, p. 43) called the "D" division of the Chinle. This
512 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM -

lower member of the Chinle is similar lithologically to the Monitor 

Butte member and is probably physically continuous with it. In the 

Zion Park region, Gregory (1950, p. 67) recognized a lower sandstone 

member of the Chinle formation which may be physically continuous 

with the Monitor Butte member.

MOSS BACK MEMBER

The name Moss Back member has been proposed by Stewart (1957) 

for exposures in the eastern part of White Canyon, San Juan County, 

Utah. The distribution of the Moss Back member is shown in figure 

77. The northeast and southwest limits are probably limits of deposition. The extension of the Moss Back to the southeast into Colorado is 

tentative. The northwest and southeast limits of the Moss Back are 

not known.

The Moss Back member is typically yellowish-gray and very-paleorange fine- to medium-grained sandstone. The sandstone is composed 

of well-sorted subround clear quartz and rare black accessory grains. 

The stratification is dominantly thin to thick trough to planar sets of 

medium-scale cross-strata, but horizontally stratified sets are common. 

Carbonaceous material and silicified wood are abundant. The Moss 

Back typically forms vertical cliffs.

Conglomerate and conglomeratic sandstone lenses are common. 

The pebbles in the conglomerate lenses generally occur in two suites 

either light-brown and gray siltstone and limestone or vitreous quartz, 

quartzite, and chert. These two pebble assemblages are not always 

found in the same lens; however, different lenses containing the two 

are generally found in the same outcrop. Where the limestone and 

siltstone pebbles occur with the quartzose pebbles, they generally exceed the quartzose pebbles in number by as much as 15 or 20 times. 

Where they are the only pebbles present, they may form more than 

50 percent of the rock by volume.

The Moss Back member has a different facies in a belt about 10 

miles wide along its north limit in the areas near the junction of the 

Green and Colorado Rivers. This facies of the Moss Back contains 

abundant interstitial greenish silt and clay and interstratified thin 

lenses of greenish siltstone and claystone and contains few, if any, 

quartzose pebbles.

The Moss Back member can be distinguished lithologically from 

the Shinarump member and other sandstone units in the Chinle on 

the basis of grain size and pebble types. The Moss Back is generally 

fine to medium grained, whereas the Shinarump is medium to coarse 

grained and the other sandstone units in the Chinle are generally 

very fine to fine grained. In addition, the Moss Back contains a different pebble assemblage than the Shinarump member. A study of 

about 2,000 pebbles from the Moss Back at 5 localities shows quartz-
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 513

EXPLANATION

 

Uranium deposit

x 

Location of measured section

Isopach line

Dashed where inferred;

interval 50 feet

FIG DEE 77. Distribution of the Moss Back member of the Chlnle formation of Triassic 

age; location of uranium deposits in the Moss Back and at the base of undifferentiated 

Chinle; and a generalized isopach map of that part of the Chinle formation underlying 

the Moss Back member.

ite to average 43 percent, chert 40 percent, quartz 13 percent, and 

other types 4 percent. In contrast, a study of about 1,000 pebbles 

from the Shinarump at 19 localities shows quartzite to average 25 

percent, chert 18 percent, quartz 52 percent, and other types 3 percent. 

In addition, the Moss Back commonly contains conglomeratic sandstone lenses composed almost entirely of siltstone and limestone peb484932 59 5
514 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

bles, whereas the Shinarump does not contain these conglomeratic 

lenses. The conglomerate in other parts of the Chinle formation 

contains few, if any, quartzose pebbles.

The lower contact of the Moss Back member is at the break between 

the channel-fill cross-stratified cliff-forming sandstone of the Moss 

Back and the siltstone and claystone of the underlying Chinle or the 

siltstone of the underlying Moenkopi.

The upper contact of the Moss Back is in most places well defined 

but in some places may be gradational with the overlying part of the 

Chinle. The top of the Moss Back is the top of the highest fine- to 

medium-grained cross-stratified sandstone. This contact marks the 

change from sandstone below to siltstone and claystone above. In 

places where the upper part of the Moss Back is gradational with 

the overlying part of the Chinle, the contact is arbitrarily placed at 

the most conspicuous change in the strata.

The Moss Back averages about 60 feet in thickness but may be as 

much as 150 feet thick where it fills channels. The Moss Back member 

overlaps the Monitor Butte member to the north in east-central Utah. 

In White Canyon, the Moss Back is about 200 feet above the base of 

the Chinle formation; north of White Canyon, the Moss Back is 

lower in the Chinle section; and north of a line trending northwest, 

about 15 miles north of White Canyon, the Moss Back is at the base 

of the Chinle (fig. 77).

TABLE 3. Composition of pebbles, in percent, in the Moss Back member of the Chinle 

formation in part of southeastern Utah

Areas

San Rafael Swell: » 

Locality : 

1_ __ __ _- ___ __ _____ ._

2__ __-_-__--__ _ ____________

3-___------------- - --_----__

4__ __ ______ ____ _ _ _ _

Quartzite

46

22

31

56

62

Quartz

9

13

9

19

15

Chert

41

63

60

17

18

Other

4

2

0

8

5

» From south to north.

PEBBLE STUDIES

The percentage of quartzose pebble types in the Moss Back at 

various localities, based on detailed studies, is shown in table 3. Not 

enough information is available to determine any systematic regional 

variations.

SEDIMENTARY-STRUCTURE STUDIES

The cross-strata in the Moss Back member, on the basis of the average resultant dip as computed in 16 studies, dip N. 59° W. (fig. 78). 

The consistency factors among readings in individual studies are 

quite variable and range from 0.46 to 0.97 and average 0.58.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 515

Direction of arrow is resultant dip of 

cross-strata; tail of arrow marks 

location of cross-stratification 

study; length of arrow is proportional 

to consistency factor 

o . t i

Consistency factor scale

FIGUKB 78. Resultant-dip directions of cross-strata in the Moss Back member of the Chinle

formation of Triassic age.

PETRIFIED FOREST MEMBER

Petrified Forest member of the Chinle formation was named by 

Gregory (1950, p. 67) in the Zion Park area although he derived the 

name from the Petrified Forest in northeastern Arizona. Strata in 

northeastern Arizona have been correlated with the Petrified Forest 

member in the Zion Park area by Gregory (1950, p. 68). The member 

is present throughout northeastern Arizona and extends north into 

southeastern Utah nearly as far as White Canyon. A reddish-orange 

facies of the Petrified Forest member extends into the Circle Cliffs
516 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

and Capitol Keef areas but cannot be recognized in the San Eafael 

Swell area.

The Petrified Forest member typically consists of bentonitic claystone and clayey sandstone. The rocks are variegated with red, 

purple, green, and yellow. The sandy parts of the rocks are very 

fine to fine grained, are composed of milky grains and as much as 

10 percent orange and green grains, and contain abundant very coarse 

flakes of dark mica (probably biotite). Most of the clay stone and 

clayey sandstone beds in the Petrified Forest member are structureless, but a few cross-stratified units crop out. The member weathers 

to form a "frothy-surfaced" slope which results from the weathering 

of swelling clay. It contains some limestone-pebble conglomerate 

lenses.

The reddish-orange facies of the Petrified Forest member of the 

Chinle formation consists dominantly of pale-reddish-brown, lightbrown, moderate-reddish-orange, and grayish-red siltstone and minor 

amounts of pale-red and light-greenish-gray sandstone. The sandstone is fine grained and is composed of clear and milky quartz, minor 

amounts of colored grains, and common amounts of dark-green mica. 

It is composed of trough sets of small- to medium-scale cross-strata. 

A persistent and conspicuous bed referred to locally as the so-called 

Capitol Reef bed is present at the top of the reddish-orange facies 

in the Capitol Reef area and the northern part of the Circle Cliffs 

area (fig. 73). This reddish-orange unit weathers to form characteristic reddish-orange slopes with conspicuous reddish ledges.

The basal contact of the Petrified Forest member is the top of the 

stratigraphically highest rocks that distinguish the Monitor Butte 

member or the Moss Back member where it is present. The top contact of the Petrified Forest member is at the base of the lowest dense 

limestone bed in most areas.

In the Monument Valley area, the Petrified Forest member ranges 

from about 500 to 700 feet in thickness. North from the Monument 

Valley area it thins to about 100 feet just south of the White Canyon 

area and the southernmost part of the Elk Ridge area. North of 

these areas it loses identity by intertonguing with the Owl Rock 

member.

The resultant-dip direction of the cross-strata in the so-called Capitol Reef bed, on the basis of four studies, is dominantly north to 

northwest.

OWL BOCK MEMBER

The name Owl Rock member has been proposed by I. J. Witkind and R. E. Thaden for exposures in the Monument Valley 

area, Navajo County, Ariz. (written communication, 1954). The Owl 

Rock member is present in most of southeastern Utah. J. W. Harsh-
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 517

barger (oral communication) has traced the Owl Rock member over 

most of northeastern Arizona. It grades laterally into the Church 

Rock member to the north in the area near the junction of the Green 

and Colorado Rivers. A similar lateral gradation probably takes 

place between the Capitol Reef and the San Raf ael Swell areas. The 

Owl Rock member is the "B" division of Gregory (1917, p. 42).

The Owl Rock member typically is composed of pale-red and palereddish-brown structureless siltstone interstratified with thin to thick 

sets of limestone that form about 5 to 10 percent of the member. The 

limestone is pale red and light greenish gray, dense, and commonly 

grades to limy siltstone. The limestone occurs as horizontal beds 

which average about a foot in thickness and are in part horizontally 

laminated. Limestone-pebble conglomerate lenses are common. The 

member weathers to form steep slopes with alternating small ledges 

of limestone.

The only distinguishing feature of the Owl Rock member is dense 

limestone beds. At a few places limestone beds occur in other parts 

of the Chinle formation, but these limestone beds are in most places 

separated from those of the Owl Rock member by at least 100 feet 

of strata.

In most areas, the bottom contact of the Owl Rock member is the 

base of the lowest limestone unit and the top contact is the top of the 

highest limestone unit. Locally, however, the contacts are stratigraphically higher or lower in order to include strata that laterally 

contain limestone.

The base of the Owl Rock member marks the most significant break 

in the lithologic characteristics of the Chinle. This horizon is the 

change from variegated partly cross-stratified bentonitic claystone 

and clayey sandstone below to reddish horizontally stratified nonbentonitic siltstone above.

In southeastern Utah, the Owl Rock member is generally 150 to 250 

feet thick. Intertonguing of the Owl Rock member with the overlying 

and underlying members causes it to vary greatly in thickness. The 

member shows an abnormal thickness in the White Canyon and Elk 

Ridge areas, where it is 350 to 450 feet thick. Near the northern limit 

the member is 50 to 100 feet thick (fig. 73).

CHURCH ROCK MEMBER

The Church Rock member has been named by I. J. Witkind and 

R. E. Thaden for exposures in the Monument Valley area, Navajo 

County, Ariz. (written communication, 1954). The Church Rock is 

present in most of southeastern Utah and all of the Monument Valley 

area of Arizona. It is absent in the Capitol Reef area and in most of 

the Circle Cliffs area, probably because of both internal thinning and 

intergrading of the Church Rock and the Owl Rock members.
518 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Typically the Church Rock member is composed of pale-reddishbrown and light-brown very fine grained sandy siltstone. The sandy 

siltstone may be structureless, composed of thin to very thick horizontal beds, or, in a few places, ripple-laminated. The sandy siltstone fractures into pebble-sized angular fragments and forms slopes.

Over a large part of southeastern Utah the top 10 to 50 feet of the 

Chinle formation is cross-stratified sandstone. This sandstone is well 

developed on exposures about 2 miles south of Hite, San Juan County, 

Utah, and is referred to locally as the so-called Hite bed. This bed is 

tentatively assigned to the Church Rock member because of lithologic 

similarities to other units in the Church Rock member. It is composed 

of pale-red and light-greenish-gray very fine grained sandstone and of 

many lenses of pale-reddish-brown siltstone. The sandstone is composed of trough sets of medium-scale cross-strata and contains a few 

conglomeratic sandstone lenses of granules, pebbles, and a few cobbles 

and boulders of reddish siltstone. The siltstone is structureless but 

contains minor amounts of horizontally laminated or ripple-laminated 

rocks. The Hite bed weathers to form a vertical cliff that is continuous 

with the overlying cliff of the Wingate sandstone.

The so-called Hite bed is correlated over most of southeastern Utah 

north of the San Juan River. A sandstone bed that appears to be 

similar to the Hite bed is at the top of the Chinle formation in part of 

the Monument Valley area. Not enough detailed work has been done 

to correlate this bed definitely with the Hite bed of local usage. To 

the west the Hite bed has been tentatively correlated with a prominent 

local sandstone bed at the top of the Chinle in the southeastern part 

of the Circle Cliffs area. The Hite bed is absent in Capitol Reef and 

cannot be correlated through the area between the Green and Colorado 

Rivers.

The Church Rock member contains many sandstone units in the 

San Rafael Swell and in the areas near the junction of the Green and 

Colorado Rivers, and in these areas is referred to in this report as 

the sandy facies of the Church Rock member. The sandstone units 

form fairly thick and conspicuous ledges. They are generally interstratified with reddish sandy siltstone typical of the Church Rock 

member of the Monument Valley area. The sandstone is pale red and 

very fine grained and is composed of trough sets of low-angle mediumscale cross-laminae. The south limit of one of these units referred 

to locally as the so-called Black ledge (fig. 73) arbitrarily marks 

the south limit of the sandy facies of the Church Rock member. 

A unit referred to locally as the so-called Bowknot bed (fig. 73) forms 

a conspicuous sandstone unit in the northeastern part of the area 

near the junction of the Green and Colorado Rivers.

The contact between the Church Rock member and the Owl Rock 

member is at the top of the highest limestone or limy siltstone in the
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 519

Chinle in most areas. The contact of the Church Rock member and 

overlying rocks in most of southeastern Utah is between the so-called 

Hite bed and the Wingate sandstone. This contact is well defined and 

disconformable. The contact marks the change from pale-red and 

light-greenish-gray purplish-weathering sandstone in the Chinle to 

the light-brown brownish-weathering sandstone of the Wingate. The 

contact also marks the change from well-cemented sandstone in the 

Chinle formation that is composed dominantly of subangular milky 

minerals and clear quartz and that commonly contains clay galls and 

interstitial clay to the poorly cemented Wingate sandstone that is 

composed of subrounded grains of clear quartz and contains no clay 

galls or interstitial clay. A highly characteristic feature of the contact 

is the abundance of frosted and rounded medium-size to coarse grains 

in the basal part of the Wingate.

In places where the so-called Hite bed is not present, the contact 

between the Church Rock member and the Wingate sandstone is the 

disconformity between the reddish horizontally stratified siltstone 

of the Chinle and the light-brown cross-stratified sandstone of the 

Wingate.

The Church Rock member in most of southeastern Utah ranges in 

thickness from 50 to 350 feet. The Church Rock thickens north of 

the Elk Ridge area (figs. 70 and 72), probably by incorporating strata 

which are equivalent to the Owl Rock farther south.

The Church Rock member in Monument Valley is probably physically continuous with at least part of the Rock Point member 

(Harshbarger and others, 1957) of the Wingate sandstone in northeastern Arizona. The Rock Point member corresponds to Gregory's 

"A" division (Gregory, 1917, p. 42) of the Chinle. Harshbarger and 

others (1957) have placed the Rock Point member in the Wingate 

sandstone because of lithologic similarities and of intertonguing between the Rock Point and the overlying Wingate.

SEDIMENTARY-STRUCTURE STUDIES

Sedimentary-structure studies have been made in three units in the 

Church Rock member (fig. 79): the Bowknot bed of local usage, a 

prominent sandstone bed in the middle of the Church Rock member 

in the San Rafael Swell, and the Hite bed of local usage. The resultant-dip direction of the cross-strata in the so-called Bowknot bed, 

on the basis of one study, is N. 45° W. with a consistency among 

readings of 0.87. Three studies of a prominent sandstone bed in the 

middle of the Church Rock member in the San Rafael Swell area 

show an average resultant-dip direction of N". 85° W. and a consistency among the readings that ranges from 0.41 to 0.61. Three 

studies of the so-called Hite bed give an average resultant-dip direction of N. 60° E. and a consistency among readings that ranges from
520 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

A. Capitol Reef bed of local usage 

B. Sandstone bed in middle of Church

Rock member of the Chinle formation 

C. Hite bed of local usage 

D. Bowknot bed of local usage

N

Direction of arrow is resultant dip of 

cross-strata; tail of arrow marks 

location of cross-stratification 

study; length of arrow is proportional 

to consistency factor 

o t i 

Consistency factor scale

FIGURE 79. Resultant-dip directions of cross-strata in various sandstone units In the 

Chinle formation of Triassic age.

0.34 to 0.55. The great difference in resultant-dip direction in the 

Hite bed as compared with those in the other two units suggests that 

the Hite bed may not be related to the other units and that it may 

not properly belong in the Church Rock member.

SOME CHARACTERISTICS OF THE CHINLE FORMATION

THICKNESS

The thickness of the Chinle formation in the areas studied is shown 

in figure 80. In three places in southeastern Utah the Eed House
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 521

I Green River

N

UTAH 

ARIZ

EXPLANATION

 

Location of measured section

Isopach line

Dashed where inferred;

interval 100 feet

PIGUEB 80. Isopach map of the Chinle formation of Triassic age in southeastern Utah 

and Monument Valley area of Arizona.

Cliffs area, the central part of the Dirty Devil River area, and the 

southeastern part of the San Rafael Swell area the Chinle is 

slightly thinner than in adjoining areas. In these same areas the 

Shinarump or Moss Back members are thin or mostly absent. Possibly these three areas were at a relatively high elevation in the continental basin in Late Triassic time and caused diversion of Shinarump and Chinle streams, resulting in thinner deposits than in 

adjoining areas.

484932 59 6
522 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

PALEONTOLOGY

Fossils in the Chinle formation consist of plants, pelecypods, gastropods, and vertebrates. The pelecypods are mostly fresh-water 

Unios, and the gastropods are small nonmarine forms. Vertebrate 

remains consist of amphibians, fish, and reptiles. Phytosaurs are the 

most numerous reptiles, particularly common in Arizona, and have 

been described in detail by Camp (1930). Assignment of the Chinle 

to the Upper Triassic is based on vertebrate-fossil evidence. Various 

types of fossil plants have been described by Daugherty (1941) from 

east-central Arizona.

INTERPRETATION

A continental environment of deposition is indicated for the Chinle 

formation by the type of sedimentary structures, the presence of nonmarine vertebrate and invertebrate fossils, and the presence of fossil 

wood. Both fluvial and lacustrine environments probably existed.

The Temple Mountain, Shinarump, Monitor Butte, Moss Back, and 

Petrified Forest members of the Chinle formation were probably 

deposited in an alluvial-plain environment including stream and floodplain deposits. The Temple Mountain, Shinarump, and Moss Back 

members are interpreted to be stream deposits. The association of the 

cross-stratified conglomeratic sandstone, channels, and fossil wood as 

found in these members is similar to that found in present-day stream 

deposits.

The Monitor Butte and Petrified Forest members probably include 

both stream and flood-plain deposits. The cross-stratified sandstone 

lenses of the Monitor Butte and possibly some of the claystone and 

clayey sandstone of the Monitor Butte and Petrified Forest members 

are probably stream deposits. The crescentic ripple-laminated sandstone lenses in the Monitor Butte member indicate current action and, 

possibly, deposition on a flood plain. In addition, the structureless 

claystone of the Monitor Butte and Petrified Forest may be floodplain deposits. Possibly the Monitor Butte and Petrified Forest 

members contain some swamp and lake deposits.

The Moss Back member is mostly cross-stratified channel-filling 

sandstone and conglomerate which contains carbonaceous material, indicating that it is a stream deposit. The scarcity of clay and the presence of moderately well sorted sandstone indicates considerable reworking of the sand during deposition. The persistency of the Moss 

Back in southeastern Utah suggests that it was deposited in a fairly 

quiescent time in the Chinle basin of deposition and on an alluvial plain 

with little relief, so that the streams could migrate freely.

The Owl Eock and Church Eock members are probably, in large 

part, lake or lagoonal deposits, as suggested by the limestone beds, horizontal stratification, and nonmarine gastropods. The limestone beds
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 523

in the Owl Rock member represent times of slower deposition of clastic 

material than those of the overlying and underlying rocks, and clearwater conditions more favorable for the existence of animals and for 

the preservation of fossils. The type of cross-stratification and channels in the sandy facies of the Church Rock member suggests stream 

deposits. The limestone-pebble conglomerates in the Owl Rock and 

Petrified Forest members may have been produced during periods of 

slow deposition by wave action in a lake environment.

The Monitor Butte and Petrified Forest members contain a considerable amount of volcanic debris. This is suggested by the bentonitic clays, the reported presence of altered glass shards (Waters and 

Granger, 1953, p. 6) and the coarse flakes of dark mica (probably 

biotite). The clay stone in these members may have been derived from 

volcanic ash by alteration and devitrification.

Studies of the orientation of cross-strata in the Chinle formation 

indicate the direction of stream currents. The cross-strata in all the 

units of the Chinle formation studied, except the so-called Hite bed, 

have a northwest resultant-dip direction, which indicates northwestward-flowing streams. The northeast resultant-dip direction in 

the Hite bed indicates northeastward-flowing streams. The consistency of most of the resultant-dip directions suggests that the units of 

the Chinle were deposited under similar conditions of regional slopes 

and had similar source areas.

The source areas for the Chinle, suggested by sedimentary-structure 

studies, lie southeast of southeastern Utah. A similar direction for 

the source area of the volcanic debris in the lower part of the Chinle 

is suggested by the thickening of the bentonitic units to the south and 

the reported presence of coarse volcanic debris (Alien, 1930) in the 

Chinle of east-central Arizona. Regional relations suggest that a 

source area may have been present in southern Arizona and New 

Mexico.

The disconformity at the top of the Chinle formation in most of 

southeastern Utah probably represents a period of erosion of varying 

intensity in different areas that resulted from slight upwarping of 

the region of Chinle deposition.

TRIASSIC AND JURASSIC ROCKS 

GLEN CANYON GROUP

The Glen Canyon group in southeastern Utah consists of three 

formations which are, in ascending order, the Wingate sandstone of 

Triassic age, the Kayenta formation of Jurassic(?) age, and the 

Navajo sandstone of Jurassic and Jurassic(?) age. Study of these 

formations has been confined mostly to sedimentary-petrology and 

sedimentary-structure investigations.
524 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

WINGATE SANDSTONE

The Wingate sandstone was named and defined by Dutton(1885) 

for exposures near Fort Wingate, McKinley County, N. Mex. The 

Wingate was correlated with strata in southeastern Utah (Gregory, 

1917), where the name was extensively used. Later work (Baker and 

others, 1947) showed that most of the Wingate sandstone at Fort 

Wingate correlated with the Entrada sandstone of Late Jurassic age 

and that the Wingate of Utah was for the most part not equivalent 

to the Wingate at Fort Wingate. The name Wingate sandstone, however, because of its extensive use, was retained for the unit in southeastern Utah, and the name Entrada sandstone was applied to most 

of the original Wingate sandstone at Fort Wingate.

The Wingate occurs in all of southeastern Utah and adjoining 

parts of Arizona and Colorado. It is composed of very-pale-orange 

and light-brown very fine- to fine-grained sandstone. The wellsorted sand grains are dominantly rounded to well-rounded quartz. 

The lower few feet, and in places as much as the lower third, of the 

Wingate contains abundant frosted and rounded medium- to coarsesize grains. The sedimentary structures of the Wingate are trough 

sets of large-scale cross-strata that are tangential to the lower contact 

of the set. These cross-strata, as is indicated by nine sedimentarystructure studies, dip dominantly southeast.

The basal contact of the Wingate in southeastern Utah is sharp, 

but the top part of the Wingate intortongues with the overlying 

Kayenta formation.

The Wingate in most of southeastern Utah ranges in thickness 

from 200 to 350 feet but is generally about 300 feet thick.

KAYENTA FORMATION

The Kayenta formation (Baker and others, 1947) extends throughout southeastern Utah but pinches out to the east in western Colorado 

and northeastern Arizona. It is composed of pale-red and verypale-orange sandstone and some siltstone, and weathers with a 

purplish tinge. The sandstone is very fine to medium grained, fairly 

well to well sorted, and composed of subangular to rounded grains. 

Field observation suggests that most of the Kayenta in central and 

south-central Utah is composed of quartz grains but that to the east 

the content of feldspar in the Kayenta increases. In western Colorado, it may, in places, be arkose. The sandstone beds of the Kayenta 

are composed of interstratified cosets of horizontal laminae and cosets 

composed of thin trough sets of medium-scale, very low angle crossstrata. Six sedimentary-structure studies indicate that these crossstrata dip dominantly southwest. The sandstone commonly contains 

angular fragments of reddish siltstone that were undoubtedly derived 

from underlying siltstone lenses.
TKIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 525

The Kayenta intertongues with both the overlying and underlying 

units. In many places a transition interval of as much 50 feet is 

present between the Kayenta and the overlying Navajo sandstone. 

The Kayenta is about 250 feet thick in southeastern Utah.

NAVAJO SANDSTONE

The Navajo sandstone (Gregory, 1915) occurs in most of southeastern, Utah and adjoining parts of Arizona and Colorado. It 

pinches out to the east in western Colorado and northeastern Arizona. 

The Navajo is a very-pale-orange fine- to medium-grained sandstone 

composed dominantly of fairly well to well-sorted subround to round 

quartz grains. The structures are composed of thin to very thick 

trough sets of medium- to large-scale cross-strata. Twelve sedimentary-structure studies indicate that the cross-strata dip dominantly 

southeast.

The Navajo intertongues with the underlying Kayenta formation, 

and in places a transitional zone is present between the two units. 

The contact of the Navajo with the overlying Carmel formation of 

Middle and Late Jurassic age is disconformable at most places; a 

beveling surface overlain by reworked Navajo sandstone is characteristic of the contact. In some places, however, work by L. C. Craig 

(oral communication, 1954) indicates that strata included in the Carmel in some areas grade laterally into beds included in the Navajo 

in other areas. These relations suggest that deposition may have 

been continuous from the time of Navajo deposition into the time of 

the Carmel deposition in these areas.

INTERPRETATIONS

The formations in the Glen Canyon group are all of continental 

origin. Sedimentary structures indicate that the Wingate and Navajo sandstones are probably eolian deposits formed by southeastward-blowing winds and that the Kayenta formation is, in large 

part, a stream deposit formed by southwestward-flowing streams.

RELATION OP URANIUM DEPOSITS TO THE STRATIGRAPHY OF 

THE TRIASSIC AND ASSOCIATED FORMATIONS

Studies of the relations of uranium deposits to the stratigraphy of 

the Triassic and related formations permit broad generalizations 

which may not apply in detail to specific deposits. Most of the deposits in the Triassic and associated formations in southeastern Utah 

are found in five stratigraphic units: the conglomeratic facies of 

the Cutler formation, the Shinarump member of the Chinle formation,, 

the Monitor Butte member of the Chinle formation, the Moss Back 

member of the Chinle formation, and strata near the base of undifferentiated Chinle. A few deposits have been found in the Moenkopl 

formation and the Wingate sandstone.
526 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

DISTRIBUTION OF ORE DEPOSITS

PERMIAN ROCKS 

CUTLER FORMATION

A large number of small weakly mineralized uranium deposits in 

the Cutler formation occur in the area near the junction of the Green 

and Colorado Kivers. These deposits are in the conglomeratic facies 

of the Cutler and are near the area where the conglomeratic facies 

changes to the light-colored sandstone and reddish siltstone facies. 

The deposits may be related to this facies change, or to particular 

lithologic rock types associated with the margin of the conglomeratic 

facies.

The ore-bearing strata are light-colored arkose beds in contrast to 

the nonmineralized reddish arkose beds. The ore-bearing arkose is at 

different stratigraphic levels from place to place and always has welldeveloped cross-stratification and small channel fills.

TRIASSIC ROCKS 

MOENKOPI FORMATION

Mineralized material in the Moenkopi formation is found in relatively few areas. The two known ore deposits are in the Capitol Reef 

and Elk Eidge areas. No relation is indicated between the ore deposits and regional stratigraphic features. In the middle of Moenkopi 

in many places is an interval of channel-filling cross-stratified conglomeratic sandstone lenses that may be favorable host rocks for 

uranium. Prospectors may tend to bypass the Moenkopi in favor of 

units that contain many known deposits, and the lack of systematic 

prospecting may account for the small amount of mineralized material 

reported.

CHINLE FORMATION

The distribution of the known deposits in the Shinarump member 

of the Chinle formation in southeastern Utah and an adjoining part 

of Arizona is shown in figure 74. The Shinarump contains a few 

occurrences of mineralized material in northwestern, central, and eastcentral Arizona and southwestern Utah, but most of the deposits and 

all of the important ones are in an area that lies near the north limit 

of the Shinarump. The limits of this area can be only crudely located 

because of large gaps between outcrops. This area appears to be 

more favorable for finding new ore deposits than other areas.

Uranium deposits in the Monitor Butte member of the Chinle formation (fig. 76) are known in two areas the Dirty Devil and the San 

Rafael Swell (fig. 70). Between these two areas, across a gap in exposures, a belt of ground containing deposits is inferred. This belt 

is suggested by a similarly oriented belt of deposits in the Moss Back 

member.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 527

The uranium deposits in the Moss Back member of the Chinle formation are present in a belt about 30 to 40 miles wide that parallels the 

north limit of the member (fig. 77). No deposits are known south of 

this belt. Deposits in the Moss Back occur where the Moss Back lies 

less than 50 feet above the base of the Chinle formation.

A few uranium deposits occur in the Chinle formation in southeastern Utah at or above the stratigraphic position of the Moss Back 

member. The most important of these deposits are in sandy siltstone or very fine grained sandstone that lie near the base of the undifferentiated Chinle formation (fig. 81). Several deposits are known

MONUMENT VALLEY AREA DIRTY DEVIL RIVER AREA

CHINLE FORMATION

MONITOR 8UTTE MEMBER

^- r= ^

SHINARUMP MEMBER

MOENKOPI FORMATION (Tnassic)

FIGURE 81. Generalized cross section showing distribution of ore deposits in the Chinle

formation of Triassic age.

in the area near the junction of the Colorado and Green Rivers in a 

unit that appears to be equivalent to the Black ledge of local usage.

The uranium deposits in the Chinle formation are near the base 

of the Chinle, regardless of the unit that is in that position (fig. 81). 

Northward across southeastern Utah, as various units onlap the Moenkopi and pinch out, the ore deposits occur in progressivly higher stratigraphic units. In progressing northward in southeastern Utah the 

deposits are in the Shinarump member, then, in order, the Monitor 

Butte member, the Moss Back member, and finally near the base of 

the undifferentiated Chinle formation.

The localization of the deposits near the base of the Chinle may 

have been caused by the damming of vertically rising ore-bearing 

solutions by the bentonitic rocks of the Chinle. Open fractures that 

provided passageways for solutions below the Chinle may not have 

continued through the Chinle because the bentonitic beds were either 

not strong enough to support open fractures or were closed by the 

swelling of the clay when the ore-bearing solutions arrived. Or, although at the present time the Chinle rocks appear to be less permeable 

than those of the Moenkopi, at some time in the past the Chinle formation may have been composed largely of beds of volcanic material
528 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

such as glass shards. This volcanic material may have made the 

Chinle more permeable than the Moenkopi, so that vertically descending solutions were dammed and migrated laterally on top of the 

Moenkopi.

The distribution of the deposits appears to have some relation to 

pinchouts and to continuity of the ore-bearing unit. The deposits 

in the Shinarump member occur near its north limit where the Shinarump is discontinuous, but they are not as common to the south in 

Arizona and southwestern Utah where the Shinarump is more continuous. In addition, many of the deposits in the Moss Back member lie near its north boundary in an area where the Moss Back is 

discontinuous.

The relation of the ore deposits to pinchouts may result from the 

traps that the pinchouts formed for the ore-bearing solutions. Solutions passing laterally through the rocks would probably continue to 

a place close to a pinchout, where reduced permeability might cause 

precipitation of the uranium. In areas where the Shinarump member 

or basal sandstone of the Chinle formation is continuous, the orebearing solutions might have moved through without being confined 

or delayed enough to form a deposit.

WINGATE SANDSTONE

Only a few small deposits are known in the Wingate sandstone of 

southeastern Utah. No relation is apparent between these deposits and 

the regional stratigraphy. The deposits appear to be related to secondary structural features such as faults and collapse structures.

RELATION OF ORE DEPOSITS TO LOCAL SEDIMENTARY FEATURES

The ore deposits in the Chinle formation appear to be related to 

at least two local sedimentary features: channel fills and lithologic 

features.

CHANNEL FILLS

Almost all the ore deposits in the Chinle formation are related to 

channel fills (Finch, 1953; Witkind and others, written communications, 1953). Channel fills are probably the best guide to ore in these 

units. They range in size from 75 feet deep and several thousand feet 

wide to only minor scour fills less than a foot deep and only a few feet 

wide. The size of the channel fills does not seem to be related to the 

location of ore deposits, because deposits are found both in large and 

small channel fills.

LITHOLOGIC FEATURES

The ore deposits in the Chinle formation appear to be associated 

with a host rock of specific lithologic characteristics. Rock of this 

character is considered favorable for the occurrence of ore and is dis-
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 529

tinguished by three features: lenticular sandstone units that are 

bounded by mudstone lenses, conglomerate composed both of quartzose and of siltstone or claystone pebbles, and carbonaceous plant 

material.

In general, rock of this character occurs in channel fills or near 

pinchouts. It occurs near the bottom of channel fills and is associated 

with a more diversified direction of dip of the cross-strata than is 

usual in the unit (McKee and others, 1953).

The ore deposits can be related either to the physical presence of 

channels or pinchouts or to the presence of this rock type favorable 

for ore. As the channel fills, pinchouts, and this rock type are generally associated, an evaluation of their relative importance is difficult. Deposits are known in channel fills and near pinchouts that 

do not contain rocks with these favorable lithologic characteristics, but 

deposits are also known that occur in rocks of this favorable type but 

that do not appear to be related to channel fills or pinchouts. These 

relations suggest, therefore, that the channel fills, pinchouts, and these 

favorable lithologic characteristics acted independently in affecting 

the localization of ore.

SEDIMENTARY PETROLOGY

By B. A. CADIGAN

This section of the report contains the results of the petrologic investigation of the Triassic and associated sedimentary rocks of the 

Colorado Plateau. Certain observed relations of petrographic features within uranium-ore samples are also reported with tentative 

interpretations. A discussion of petrographic terminology is included 

because it differs markedly from the field terminology used in the 

first part of the report.

Emphasis is placed on the fact that conclusions and interpretations 

presented here are based on incomplete data and are, therefore, subject 

to change.

OBJECTIVES

The petrologic study of the Triassic and associated sedimentary 

rocks has two general objectives: to obtain evidence which will support 

some logical conclusions regarding the paleotectonic background of 

the accumulation of sediments during the late Permian, Triassic, and 

Early Jurassic times, and to detect relations that may be present between the location of uranium-ore deposits and petrologic characteristics of the sedimentary host rocks.

METHODS

Outcrop samples were collected from the Cutler formation of Permian age, the Moenkopi formation, Chinle formation, and Wingate
530 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

sandstone of Triassic age, the Kayenta formation of Jurassic (?) age, 

and the Navajo sandstone of Jurassic and Jurassic(?) age. The 

samples were analyzed for measurable characteristics of texture and 

composition.

Texture was studied by disaggregating the samples and making a 

statistical analysis of the grain-size distribution. Composition was 

studied by obtaining volumetric mineral-composition measurements of 

thin sections, separating representative loose mineral grains, concentrating samples of heavy minerals, and making X-ray spectrographic 

analyses of clay fractions.

TEXTURAI, ANALYSIS

Fine sediments are difficult to study in thin section with ordinary 

equipment, and textural and compositional parameters of fine silt and 

clay raise many unresolved problems of interpretation. Thus, pertinent information can be obtained more easily from a study of sandstone and sandy siltstone than from claystone, shale, and fine-grained 

siltstone. Measured textural data presented in this report were arbitrarily restricted to sediments with a mean grain size of 33 microns or 

larger.

Samples weighing 100 grams each were crushed to pass through a 

U. S. No. 5 sieve, digested in 400 milliliters of boiling 20-percentstrength citric acid, cooled, acidified with 20 to 50 milliliters of concentrated hydrochloric acid, and washed at 24-hour intervals 4 to 6 

times to remove dissolved salts. The silt and clay fractions were 

separated and analyzed for size distribution by the pipette method. 

The sand-size material was analyzed for size distribution by sieving 

through a set of graduated sieves.

Statistical analyses of grain-size distributions produce measurements of a number of properties of the texture. Some of the measurements are familiar in a geologic sense and some, though common in 

statistics, are unfamiliar in geologic discussion.

Four textural concepts are used in this investigation of sedimentary 

rocks: average or mean grain size, sorting, skewness, and kurtosis (or 

peakedness). These properties may be said to define a grain-size distribution. They are derived from the first four statistical moments 

of the phi grain-size distribution. The term "phi" refers to a conventional scale obtained by translating sizes in millimeters into their 

respective negative logarithms to the base of two ( Iog2 ) (Krumbein, 

1934).

In order to define the mathematical terms so that they may be applied in geologic discussion, it is necessary to impose connotations 

which are not strictly precise from the mathematical point of view but 

which aid materially in their interpretation. Thus, the mean of the 

phi grain-size distribution may be interpreted as the average grain size
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 531

of the rock material. When the phi mean is converted to millimeters 

it approximates the geometric mean of the arithmetic distribution.

Average grain size of the material of a rock is classified in terms 

of the Wentworth (1922) scale. Each size grade is represented by a 

range of mean grain sizes. Part of the Wentworth scale with the 

range of mean grain size for each grade is illustrated by table 4.

TABLE 4. Phi mean and Wentworth grain-size classification

Wentworth grade

Pebbles __ _ __ __ _ _

Coarse sand __ __ __ _ _ Medium sand. __ __ ____ _ ___

Fine sand _______

Silt. ______ _________________________

Clay_____. ____________________________

Phi mean

Millimeters

64. 000-4. 000 

4. 000-2. 000 

2. 000-1. 000 

1. 000-0. 500 

. 500- . 250 

. 250- . 125 

. 125- . 062 

. 062- . 004 

. 004- . 000

Phi units

-6.00 2.00 

-2.00 1.00 

-1.00- .00 

. 00- 1. 00 

1. 00- 2. 00 

2. 00- 3. 00 

3.00- 4.00 

4. 00- 8. 00 

8.00- oo

The standard deviation is the second important parameter for describing the phi grain-size distribution of sediments. The phi standard deviation may be interpreted as the measure of sorting. As no 

common sorting classification based on the standard deviation is in 

general use, the system illustrated in table 5 has been adopted for 

this investigation.

TABLE 5. Classification of sorting

Sorting Phi standard 

deviation

Very well sorted. __ _________ ____ _ ___ ___

Well sorted _ __________ _ ____

Poorly sorted... _____ __ __

Unsorted _ _ __ __ ______

0. 0-0. 500

. 500-1. 000

1. 000-2. 000

2. 000-4. 000

4. OOO-oo

The mean size and the standard deviation define a grain-size distribution if the distribution of sizes forms a symmetrical statistically 

normal bell-shaped frequency curve when plotted on a graph of which 

the ordinate is frequency and the abscissa is grain size. The mean 

locates the center of the curve over the size scale, and the standard 

deviation measures the spread or width of the curve.

Two other measurements skewness and kurtosis are used to define grain-size distributions which vary from the theoretical normal 

as defined by the mean size and standard deviation. Skewness is a
532 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

measure of asymmetry of the distribution and is generally marked 

by a spread of the size range on one side of the mean that is out of 

proportion to the spread on the other side of the mean. To express 

skewness in terms of sorting, one end of the size distribution is less 

well sorted than the average sorting of the whole distribution. The 

distribution is said to be skewed toward the end with the poorer sorting. By convention, positive skewness in a grain-size distribution 

indicates a disproportional spread or poorer sorting in the finer sizes> 

which is illustrated by a stringing out of the right side of the frequency curve. Negative skewness indicates a disproportional spread 

or poorer sorting in the coarser sizes, which is illustrated by the 

stringing out of the left side of the frequency curve.

Kurtosis is a measure of the peakedness of the distribution. In 

terms of sorting, kurtosis measures the relative degree of sorting 

that the center of a distribution bears to the two ends. Sand with 

a grain-size distribution which is better sorted in the center than 

anywhere else has a positive kurtosis. If it is poorer sorted in the 

center, it has a negative kurtosis.

The basis for comparison of sorting in various parts of a distribution is the theoretical normal distribution; skewness and kurtosis 

represent measurements of average deviations from the particular 

theoretically normal distribution, defined by the mean and standard 

deviation. For comparative purposes certain ranges of values of 

skewness and kurtosis are given verbal classifications as shown in tables 

6 and 7.

Textural parameters that are used to describe stratigraphic or 

lithologic units are generally reduced to a system of verbal classificaTABLE 6. Classification of skewness

Skewness

Slightly skewed. __ _ _________ _ ______ ___ ___________

Moderately skewed _ _ ____ _ _ __ _____ __ _ _ __ __

Highly skewed.. _____ __ _ ___ _. _ ___ ____ __ _______

Negatively skewed ____ _ __ _ _ _ _ ______ __________

Phi skewness 

coefficient

0. 50-1. 00

1. 00-2. 00

|S2. OQ

0)

1 All negative values.

TABLE 7. Classification of kurtosis

Kurtosis

Flattened. ______

Normal. __________

Moderately peaked. 

Highly peaked_____

Very highly peaked.

Phi kurtosis 

coefficient

<-l. 00

-1. 00- 1. 00

1. 00-10. 00

10. 00-20. 00

^20. 00
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 533

tions. However, for regional comparisons or tabulations of data, 

numerical values are used.

COMPOSITIONAL ANALYSTS

Mineral composition was determined from the study of a thin 

section of a selected fragment of each sample. Each thin section 

was etched with hydrofluoric acid and stained with sodium cobaltinitrite in the method described by Chayes (1952); potassium feldspar 

and potassium-bearing clay were stained canary to golden yellow 

by this process. The thin section was then covered with a conventional cover glass set in cool rather tacky uncooked Canada balsam. 

The slide was allowed to air-dry for 2 weeks or more before being 

trimmed. Xylol was used sparingly and only for cleaning.

The proportion of minerals was measured by means of a petrographic microscope equipped with a point-count stage, and a cell 

counter, similar to those described by Chayes (1949). Five hundred 

counts were made on each thin section to obtain a measure of the 

average proportional area of the thin section occupied by each mineral 

or mineral group counted. Some rapid estimates of composition 

were obtained for this report by making only 100 counts on certain 

selected thin sections.

In this study, it has been convenient to consolidate the minerals 

into 10 groups, and the descriptions of the composition of the rocks 

that follow are in terms of these groups. The mineral groups and 

their constituent minerals are listed below.

Group Minerals or rocfc fragments

Quartz____________ Quartz, quartz overgrowths.

Hydromica_________ Illite, sericite, chlorite, heavy minerals, fragments of

micaceous metamorphic rocks.

Potassium feldspar____. Orthoclase, microcline.

Kaolin____________ Kaolinite group of clay minerals and kaolin mud 

a mixture of kaolinite clays and ground-up quartz 

and feldspar with minor amounts of carbonate and 

of other clay.

Cement___________ Calcite, gypsum, chalcedony, dolomite, barite, iron

oxides, interstitial ore minerals, other interstitial 

chemically deposited materials.

Quartzite__________. Quartzite and quartz schist fragments.

Tuffaceous material___ Silicified tuff fragments, chert, montmorillonite,

rhyolite fragments, other volcanic material.

Mica____________. Muscovite, biotite.

Plagioclase_________ Plagioclase feldspar group.

Miscellaneous_______. Minerals and mineral mixtures such as highly altered

opaque heavy minerals or basic igneous rock fragments which are not readily identified or assigned 

to other mineral groups.

A sedimentary rock that contains more than 50 percent cement is 

classified as a chemically deposited rock. A rock that contains more 

than 50 percent detrital components is classified as a detrital rock, 

and the chemically deposited constituents are not considered in the 

classification of the rock.
534 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

For the classification of the detrital rocks, the detrital-mineral 

groups are consolidated further into four general groups as follows:

Quartz (quartz, quartzite).

Tuff aceous material.

Feldspar (potassium feldspar, kaolin, plagioclase).

Mica (hydromica, mica, miscellaneous).

Kocks are classified according to table 8 or as illustrated in figure 

82. This consolidation of minerals into four general groups is patterned after that of Krynine (1948); however, the role of tuffaceous 

material is, as far as the authors know, unique to this paper. Tuffaceous material is combined with quartz in figure 82 for practical 

reasons. An isometric projection would be required to illustrate a 

four-dimensional classification; such a diagram is difficult to use and 

for most detrital sedimentary rocks would be unnecessarily complicated. In rocks where the separation of fragments of silicified tuff 

from chert is questionable, the chert may be added in with quartz as 

proposed by Krynine (1948). In rocks where fragmental tuff is 

identified, it modifies the classification made with the tuff counted 

as quartz.

TABLE 8. Petrologic classification of detrital sedimentary rocks based on 

percentage composition

JAfter Pettijohn, 1949, and Krynine, oral communication, 1952. Ranges Include termini]

Classification

Orthoquartzite : 

Range ___________ ___

Feldspathic orthoquartzite: 

Range. _--_ __ _ ____ _____

Graywacke:

Typical. __ ___ __ ________ ______

Subgray wacke : 

Range-.--- _- _______ __

Typical _ __ -_ ____ _ _____ _ _ __

Arkose:

Typical.- __ _ _______ ___ ___

Pelite:

Typical.-. ___ _ ____ _ _ __ _____

Quartz and 

tuffaceous 

material

72-100

85

57- 90

75

0- 70

50

17- 80

60

0- 75

50

0- 25

5

Feldspar

0- 9

5

10- 24

15

10- 80

20

0- 9

5

25-100

40

0- 24

15

Mica

0- 19

10

0- 19

10

20- 74

30

20- 74

35

0- 19

10

75-100

80

The amount of tuffaceous material present modifies the rock classification in the following manner:

1. If there is less than 10 percent tuffaceous material, the rock classification is not affected.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 535

Effect of content of tuffaceous material on rock classification:

1. 0 to 10 percent tuffaceous material no effect.

2. 10 to 25 percent tuffaceous material rock is tuffaceous.

3. 25 to 75 percent tuffaceous material rock is a variety of tuff.

4. 75 to 100 percent tuffaceous material rock is tuff.

100-K

Directions for the use of chart

To classify a rock, start at point A, count upward the percentage of quartz present; 

from located point, count to the right the percentage of feldspar present; 

from located point, count downward the percentage of mica present. Chart 

area containing the final located point is the rock classification.

FIGURE 82. Petrologic classification of detrital sedimentary rocks. (After Pettijohn, 1949, 

and Krynine, oral communication, 1952).

2. If there is 10 to 25 percent tuffaceous material, the rock name 

is modified by the word "tuffaceous"; for example, tuffaceous arkose.

3. If there is 25 percent to 75 percent tuffaceous material, the rock 

name designates a variety of tuff; for example, graywacke tuff.
536 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

4. If there is 75 percent or more tuffaceous material, the rock is unmodified tuff.

Heavy-mineral studies have not been completed and results are not 

presented in this report.

EESTTLTS

Preliminary estimates of the composition of some Permian and 

Jurassic formations were made by selecting thin sections of representative sandstone and siltstone, making a point count of mineral 

composition using 100 counts, and classifying rock units on the basis 

of the information obtained. The classification is adequate only for 

the location studied; a more thorough study using many more samples 

will be necessary to determine regional variations. The classification 

of a rock unit may change completely from one area to another; for example, arkose may grade into orthoquartzite. Data sufficiently complete to permit regional comparisons have been obtained for the Shinarump and other lower sandstones in the Chinle only.

The genetic interpretations associated with the terms "orthoquartzite," "graywacke," and "arkose" by Pettijohn (1949) and Krynine 

(oral communication, 1952) have been tested and found applicable, 

with slight modification, to the sediments of the Colorado Plateau.

The thick sequence of continental-basin or structural-trough sediments of Mesozoic and late Paleozoic age found in the Colorado Plateau region might be expected to contain sediments related to tectonic 

conditions that varied from extreme uplift to quiescence. Sediments 

of the type associated with deposition in deep marine basins would be 

expected to be rare. Genetically, then, the sediments should range 

from orthoquartzite to arkose. In contrast, graywacke which is 

related to subaqueous troughs or basins should be rare except in small 

localized deposits. Observations to date generally confirm this theoretical concept.

The presence of hydromica clay, which is characteristic of the graywacke series, as the dominant clay-mineral group in a sediment which 

otherwise appears to have been derived from a granitic igneous source 

or silicic-alkalic volcanic debris casts some doubt on the genetic interpretations associated with the system of classification. This problem is dealt with to some extent in the section on clay minerals, but 

the final answer must come from the clay mineralogists.

Several summary tables of grain-size analyses of the formations 

studied (table 14) and point-count analyses of sandstone samples of 

the Chinle formation (Shinarump member, table 14; Monitor Butte 

member, table 15; Moss Back member, table 16; Petrified Forest member, table IT) follow page 567.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 537

PERMIAN HOCKS

CUTLER FORMATION

CEDAR MESA SANDSTONE MEMBER

The Cedar Mesa sandstone member, the lowest member of the Cutler 

formation of Permian age, is the oldest unit studied. The unit is reddish brown or yellowish-gray massive sandstone with conspicuous 

large-scale cross-strata. The Cedar Mesa was examined in the field 

and in thin section, and grain-size analyses were completed on seven 

samples.

The strata are composed typically of very fine moderately well sorted 

subangular to subround grains of quartz, feldspar, chert, and mica 

cemented with carbonate (predominantly calcite) cement. Variation 

in cohesion in the samples suggests that the Cedar Mesa has zones of 

both highly cemented and friable sandstone. On the average, the 

grain-size distributions are moderately skewed and highly peaked.

An estimate of the composition of sandstone (one thin section) from 

the Poncho House area in the northeast corner of the Monument Valley area (fig. 83) is as follows:

Percent 

Mineral group "by volume

Quartz________________________________________. 49

Potassium feldspar (orthoclase)________________________ 11

Quartzite fragments__________________________________ 1

Plagioclase (albite) ________________________________ 5

Mica (muscovite) _________________________________ 1

Cement (mostly calcite)______________________________ 33

The classification of the rock is based on the following detrital components :

Percent 

Component by volume

Quartz____________________________________________ 75

Feldspar________________________________________ 24

Mica__________________________________________ 1

The Cedar Mesa is feldspathic orthoquartzite in the Poncho House 

area (fig. 83).

ORGAN ROCK TONGUE

The Organ Eock tongue of the Cutler formation overlies the Cedar 

Mesa sandstone member. The unit is a dark-red earthy-weathering 

sequence of alternating nonresistant siltstone and slightly resistant 

sandstone strata. Thin sections of the Organ Kock were examined, and 

grain-size analyses were completed on 14 samples (table 13).

The coarser textured strata of the Organ Kock tongue are coarse 

sandy siltstone and very fine grained silty sandstone composed typically of moderately well sorted subangular to subround grains of quartz, 

feldspar, and heavy minerals and minor igneous- or metamorphic-rock
538 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

I -

SAMPLED LOCALITIES 

UTAH

1. Temple Mountain, San Rafael Swell area

2. East of Torrey, Capitol Reef area

3. Pleasant Creek, Capitol Reef area

4. The Peaks, Circle Cliffs area

5. Muley Twist, Circle Cliffs area

6. Silver Falls, Circle Cliffs area

7. Happy Jack mine, White Canyon area

8. Frey Canyon, White Canyon area

9. Monitor Butte, Monument Valley area

10. Poncho House, Monument Valley area

11. Monument No. 2 mine, Monument Valley area

12. Comb Wash area

13. Paria area 

ARIZONA

14. Owl Rock, Monument Valley area

15. Carrizo Mountains area

I

r

FIGUBB 83. Index map showing localities sampled for sedimentary-petrology studies in

Arizona and Utah.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 539

fragments. The constituents are tightly bound together by interstitial 

hydromica clay stained red by iron oxide, and isolated patches of carbonate (dominantly calcite) cement. The average grain-size distribution is moderately skewed and moderately peaked.

An estimate of the composition of representative sandy siltstone 

(one thin section) of the Organ Rock from the Poncho House area 

in the northeast corner of Monument Valley (fig. 83), is as follows:

Percent 

Mineral group 'by volume

Quartz _______________________________________ 48

Potassium feldspar (orthoclase)________________________ 16

Hydromica (stained red by iron oxide (?))___________________ 18

Quartzite __ _____________________________ 1

Plagioclase (albite) ________________________________ 10

Cement (mostly calcite)_____________________________ 4

Miscellaneous (leucoxene, opaque iron oxide pellets) ____________ 3

As an exception to the above, some strata of the Organ Rock contain 

interstitial kaolin as the dominant clay mineral.

The classification of the rock is based on the following detrital components :

Percent 

Component 'by volume

Quartz_______________________________________ 51

Feldspar_______________________________________ 27

Mica__________________________________________ 22

At most places, the Organ Rock is a graywacke, but at a few places 

it is an arkose. Its classification as a graywacke is due to the high 

proportion of hydromica clay. The significant amount of rock fragments considered essential by Petti John (1949) and others is not 

present; however, the hydromica might be considered by some 

petrographers as being derived from micaceous shale or phyllite.

DE CHELLY -SANDSTONE MEMBER

The De Chelly sandstone member of the Cutler formation lies 

above the Organ Rock in parts of southeastern Utah and northeastern 

Arizona. The De Chelly sandstone member was examined in the field 

and in thin section, and grain-size analyses were completed on seven 

samples.

The unit is light-colored to reddish massive sandstone with conspicuous large-scale cross-strata. On the basis of the average grainsize-distribution measurements (table 14), the De Chelly is composed 

of well-sorted sand that is very fine grained and has slightly skewed 

and highly peaked grain-size distributions.

The De Chelly sandstone member is composed typically of very 

fine subround to subangular grains of quartz, feldspar, various heavy 

minerals, and grains of altered tuff loosely cemented by interstitial
540 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

montmorillonite, hydromica or kaolin clay, reddish iron oxides, and 

isolated patches of carbonate cement.

An estimate of the composition in terms of mineral groups is based 

on 100 point-counts on a thin section of a representative De Chelly 

sandstone sample from the Monitor Butte area (fig. 83) and is shown 

below.

Percent 

Mineral group by volume

Quartz___________________________________________ 67

Potassium feldspar (orthoclase)________ ______ _ 11

Hydromica _____________________________________ 2

Tuffaceous material________________________________ 10

Plagioclase (albite) ____________________________ __ 5

Cement___________________________ ___________ 4

Miscellaneous_____________ ________ _ _ 1

The De Chelly sandstone member of the Cutler formation is a tuffaceous feldspathic orthoquartzite in part, but examination of other 

thin sections indicates that the unit also includes feldspathic 

orthoquartzite and arkose in the Comb Wash area (fig. 83). The 

classification of the rock is based on the following detrital 

components:

Percent 

Component "by volume

Quartz__________________________________________ 70

Feldspar . _______ ____________ _ ______ 17

Mica_________________________________________ 3

Tuffaceous material _________ _ 10

HOSKINNINI TONGUE

The Hoskinnini tongue of the Cutler formation lies above the De 

Chelly or above the Organ Eock where the De Chelly is absent. The 

Hoskinnini is the uppermost member of the Cutler and forms the top 

unit of the rocks of Permian age. The Hoskinnini tongue was examined in the field and in thin section, and grain-size analyses were 

completed on 13 samples.

The unit is light-yellowish-gray to reddish-brown thin horizontally 

to irregularly bedded sandstone and siltstone strata. Some of the 

strata are strongly resistant to weathering and others are weakly 

resistant. Owing to the lack of conspicuous well-defined characteristics in the outcrop, recognition depends to a large extent upon the presence of medium-size or coarse sand grains anomalously present in the 

finer grained sandstone strata.

On the basis of the average grain-size-distribution measurements 

(table 13), the sandy strata of the Hoskinnini tongue are composed of 

moderately well sorted, very fine grained sand with slightly skewed 

and moderately peaked grain-size distributions.

The Hoskinnini is composed typically of very fine to coarse subround grains of quartz and feldspar. In some samples the grains of
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 541

quartz and feldspar occur in a matrix of kaolin with isolated patches 

of carbonate (mostly calcite) cement; in other samples they occur in 

a continuous red carbonate cement stained by iron oxide, without 

matrix.

An estimate of the composition in terms of mineral groups is based 

on 100 point counts on each of three thin sections of representative 

sandstone of the Hoskinnini tongue in southeastern Utah and northeastern Arizona (Poncho House, Monitor Butte, and Comb Wash) 

(fig. 83) and is shown below.

Percent 

Mineral group by volume

Quartz _________________________________ 59

Potassium feldspar (orthoclase, microcline) ___________ _ 8

Kaolin __________________________________________ 10

Hydromica ______________________________________ 1

Plagioclase (albite) _____________________ 3

Cement _________________________________________ 19

The classification of the rock is based on the following detrital 

components:

Percent 

Component 'by volume

Quartz _____________________________________ 73

Feldspar _______________________________________ 26

Mica __________________________________________ 1

Most of the Hoskinnini tongue of the Cutler formation is arkose, with 

some thin beds suggestive of what Petti John (1949, p. 259) refers to as 

a tectonic arkose, a coarse sediment derived from a rapidly eroding 

granitic land mass resulting from abrupt tectonic uplift.

CONGLOMERATIC FACIES

The conglomeratic facies of the Cutler formation, which is present 

chiefly along the flanks of the granitic and metamorphic crystalline 

rocks that comprise the Uncompahgre highland, has not been studied 

in thin section. The type Cutler has been observed in the field, however, and grain-size analyses have been completed on two samples of 

associated sandstone.

The unit consists of reddish to purplish thick-bedded sandstone, 

pebbly sandstone, conglomerate, and fanglomerate strata. On the 

basis of the average grain-size-distribution measurements (table 13), 

the sandstone is made up of moderately well sorted medium-grained 

sand with moderately skewed and moderately peaked grain-size distributions.

The composition has not been determined from point-count measurements, but field examination reveals the presence of coarse fragments of pink feldspar, quartz, mica, and a kaolinlike clay matrix; 

these features identify the Cutler as arkose.
542 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

TRIAS SIC ROCKS

MOENKOPI FORMATION

The oldest of the Triassic sequence of sedimentary rocks in the Colorado Plateau region is the Moenkopi formation. The Moenkopi was 

studied in the field and in thin section. Grain-size analyses were 

completed on 21 samples of the sandstone or sandy siltstone f acies.

The unit is generally uniform pale-reddish-brown silty sandstone 

and siltstone that weathers to "shaly" slopes which are littered with 

ripple-marked platy sandstone fragments. On the basis of the average grain-size-distribution measurements (table 14), the coarser strata 

of the formation consist of moderately well sorted very fine grained 

sand with moderately skewed and moderately peaked grain-size distributions.

The siltstone and sandstone strata of the Moenkopi formation are 

composed of subangular to subround grains of quartz and feldspar, 

round heavy-mineral grains, angular chert grains, and flakes of mica. 

The detrital grains are bound and cemented by an interstitial matrix 

of hydromica clay stained red by iron oxide and carbonate (dominantly calcite and dolomite) cement which occurs in isolated patches 

and as disseminated subhedral crystals. The siltstone strata vary 

from the sandstone mostly in the increased angularity of the grains in 

the siltstone, as well as in the increased difficulty of identification of 

minerals and mineral relations.

An estimate of the composition of the siltstone and the sandstone 

facies in terms of mineral groups is based on 100 point counts each 

on a thin section of representative siltstone of the Moenkopi formation in the Monitor Butte area (fig. 83) and a thin section of typical 

Moenkopi very fine grained sandstone in the Poncho House area in 

northeastern Arizona and is shown below:

Sandstone Siltstone

(percent by (percent "by

Mineral groups volume) volume)

Quartz _.__________________________ 47 40

Potassium feldspar_______________________ 12 12

Kaolin ________________________. 1

Hydromica _________________________ 4 12

Plagioclase ____________________________ 8 12

Mica _______________________________ 1

Cement ______________________________ 29 21

Miscellaneous _________________________ 1

The classification of the rocks is based on the following detrital 

components:

Sandstone Siltstone

(percent by (percent by

Component volume) volume)

Quartz ______________________________ 66 51

Feldspar _____________________________ 28 32

Mica ________________________________ 6 17

The typical very fine grained sandstone and the typical siltstone 

of the Moenkopi are classified as arkose.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 543

CHINLE FORMATION 

SHINAEUMP MEMBER

The Chinle formation lies immediately above the Moenkopi in 

southeastern Utah, and the Shinaramp member is the basal unit 

over much of the southern part of the plateau region. It has been 

studied in the field and in thin section in greater detail than any 

of the previously described units. Grain-size analyses were completed on 83 samples.

Thin sections from 28 of the samples were analyzed by the pointcount method and 500 counts per section were made in 10 traverses 

of 50 counts each. Many of the slides were made from semifriable 

material, which resulted in loss of material from the section during 

preparation. Because of losses of material and other damage to 

the thin sections, voids were not counted and volume was calculated 

on the basis of the percentage of the mineral volume rather than the 

percentage of the rock volume. One constituent of which there 

probably is a disproportionate loss in the damaged thin sections is 

the kaolinite group of clay minerals.

The Shinarump member is typically light-colored sandstone, conglomeratic sandstone, or conglomerate. In outcrop it presents a 

strong color and texture contrast with the underlying dark-red siltstone of the Moenkopi, but may grade upward into overlying sandstone of the Chinle. On the basis of the average grain-size-distribution measurements (table 14), the Shinarump contains fine-grained 

sandstone strata with poorly sorted to moderately well sorted slightly 

skewed moderately peaked grain-size distributions.

The sandstone strata of the Shinarump member are composed of 

subround to subangular grains and granules of quartz and feldspar 

and some altered tuffaceous material bound in a matrix of kaolin 

mud. The sand is commonly very loosely cemented with isolated 

patches of carbonate (predominantly calcite) and iron oxide cement.

One or more samples from each of the numbered areas shown 

on figure 83, except 1,14, and 15, were used in the composition analysis. An estimate of the regional average composition based on 28 

thin sections is as follows:

Percent 

Mineral groups 'by volume

Quartz________________________________________ 71. 0

Potassium feldspar_________________________________ 4. 6

Kaolin________________________________________ 12.5

Hydromica______________________________________ 1. 5

Quartzite _____ ________________________________ . 5

Tuffaceous materials________________________________ 3.2

Plagioclase______________________________________ .4

Mica__________________________________________ .3

Cement__________________________________________ 6. 0

Miscellaneous____________________________________ . 0
544 CONTKIBUTIONS TO THE GEOLOGY OF URANIUM

The classification of the rock is based on the following average 

amounts of detrital components:

Percent 

Component by volume

Quartz________________________________________ 76 

Feldspar ___ _____________________ __________

Mica________________________________________________________________

Tuffaceous materials_______________________________

_____________________________ 19

______________________________ 2

______________________________ 3

The average sandstone of the Shinarump member in the area sampled is feldspathic orthoquartzite, but the composition varies from 

bed to bed and between areas (table 15). Samples of the Shinarump 

member from the Paria Eiver area (fig. 83) are tuffaceous feldspathic 

orthoquartzite and contain an average of 15 percent altered tuffaaceous material; this suggests a source of volcanic debris to the south 

or southwest. Samples of the Shinarump member from southern 

Monument Valley (fig. 83) are arkose, averaging 31 percent feldspar 

and kaolin; this suggests a granitic source to the southeast. Further 

work will be necessary to substantiate these regional variations as 

evidence of contributions from different source areas.

Table 9 summarizes the percentage composition of sandstone of 

the Shinarump in the areas sampled (fig. 83).

TABLE 9. Mineral components, in percent by volume, of the Shinarump member of

the Chinle formation

Area

Paria River _ __

South Monument

Valley.

North Monument

VaUey.

Monitor Butte

White Canyon___

Capitol Reef__ _

Circle Cliffs _ _

Location 

(fig. 83)

13

14

10, 11

9

7

2,3

5,6

Number of 

samples

2

3

7

2

7

2

5

Quartz

70

66

81

75

80

73

73

Feldspar

14

31

14

15

17

22

22

Mica

1

3

4

3

1

1

1

Tuffaceous materials

15

0

1

7

2

4

4

Classification

Tuffaceous

feldspathic

orthoquartzite.

Arkose.

Feldspathic

orthoquartzite.

Do.

Do.

Do.

Do.

MONITOB BUTTE MEMBER

In different areas various units of the Chinle formation lie just 

above the Shinarump member. These units range from claystone to 

pebbly sandstone. The contact may be marked by either an abrupt 

or a gradational lithologic change. The Monitor Butte member overlies the Shinarump member of the Chinle over a large area of the
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 545

Colorado Plateau region. Sandstone beds of the Monitor Butte have 

been examined in the field and in thin section. Grain-size analyses 

were completed on 12 samples of the sandstone or sandy siltstone 

facies.

The unit consists of greenish limy sandstone, siltstone, and claystone strata. The general appearance in outcrop is that of mediumbedded sandstone ledges separated by thicker strata of siltstone and 

claystone. The sandstone ledges show horizontal, irregular, or rippled 

lamination with paper-thin micaceous clay laminae between the sand 

laminae. The sandstone fractures in a rectangular pattern resembling 

the fracturing of limestone and splits easily along the horizontal 

laminae within each block.

On the basis of the average grain-size-distribution measurements 

(table 13), the sandstone of the Monitor Butte member is composed of 

very fine poorly sorted sand that has slightly skewed and moderately 

peaked grain-size distributions.

The sandstone and sandy siltstone of the Monitor Butte member 

are composed of subangular grains of quartz, feldspar, mica, and 

tuffaceous material. Hydromica clay or in some samples kaolinite 

or montmorillonite clay minerals are present as a sparse matrix or 

in pods or thin laminae. The sand is usually well cemented with 

carbonate and iron oxide cement. Two types of microscopic structure are seen in thin section: one is a quartz-grain and quartz-overgrowth mosaic (as seen with crossed nicols) interbedded with clay 

laminae; the other type is a heterogeneous arrangement of the components mentioned above, with the grains surrounded by matrix and 

cement and with irregular spacing between grains. The thin sections 

that show the mosaic structure contain much less feldspar than 

those that show the heterogeneous structure.

An estimate of the composition of the sandstone and sandy siltstone facies of the Monitor Butte member in terms of mineral groups 

is based on the average of the results of point-count analysis of 17 

thin sections (table 15), and is as follows:

Percent 

Mineral groups 'by volume

Quartz___________________________________________ 62.5 

Potassium feldspar_________________________________ 5.0 

Kaolin________________________________________ 8.9 

Hydromica_____________________________________ 3.2 

Quartzite fragments__________________________ ______ . 2

Tuffaceous material________________________________ 5. 3 

Plagioclase _____________________________ __ .6 

Mica__________________________________________ .8 

Cement__________________________________________ 13.5 

Miscellaneous ____________________________________ . 0
546 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The classification of the sandstone and sandy siltstone strata of the 

Monitor Butte member is based on the following average amounts 

of detrital components:

Percent 

Component by volume

Quartz ________________________________________ 74 

Feldspar ______________________________________ 

Mica___________________________________.____. 

Tuffaceous material _____________ _ 

_____________________________ 16

________________________,_____ 4

_______________________________ 6

The average sandstone of the Monitor Butte member is feldspathic 

orthoquartzite, but the sandstone strata vary markedly both locally 

and regionally (table 15). Like the Shinarump, Monitor Butte samples from the Paria Eiver area (fig. 83) are tuffaceous feldspathic 

orthoquartzite averaging 20 percent altered tuffaceous material. 

Samples from the Capitol Eeef area (fig. 83) are arkose with an 

average feldspar and kaolin content of 34 percent by volume.

The composition of the sandstone of the Monitor Butte member in 

the Paria River area (fig. 83), like that of the Shinarump, supports 

the hypothesis of a source of volcanic material to the west or southwest. The changes in ratios of the detrital components may be interpreted as indicating the existence of multiple sources of sediment for 

the basal part of the Chinle. Table 10 is a summary of the composition 

of the Monitor Butte member in the areas sampled (fig. 83).

TABLE 10. Mineral components, in percent by volume, of the Monitor Butte

member

Area

North Monument 

Valley.

Circle Cliffs ______ __

Number 

of 

samples

2

2 

5

5

3

Quartz

56

85

87

72

57

Feldspar

21

7 

7

17

34

Micas

3

6 

3

4

6

Tuffaceous 

material

20

2 

3

7

3

Classification

Tuffaeeous feldspathic orthoquartzite. 

Orthoquartzite.

Do.

Feldspathic orthoquartzite.

MOSS BACK MEMBER

Another basal sandy unit of the Chinle formation is the Moss Back 

member, which may overlie either the Moenkopi formation or the 

Monitor Butte member of the Chinle formation (fig. 81). The sandstone strata of the Moss Back member have been examined in the field 

and in thin section. Grain-size analyses were completed on 13 samples.

The unit contains light-colored sandstone, pebbly sandstone, and 

conglomerate. Generally an exposure of the Moss Back consists of
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 547

one or more very thick sandstone beds which form a resistant massive 

or compound ledge in the less resistant units near the base of the 

Chinle. Gray to greenish strata of claystone or clayey siltstone occur 

between beds in some localities but are not typical. The unit is generally conspicuously cross-stratified and in places fills channels at the 

base in the same manner as the Shinarump member.

On the basis of the average grain-size-distribution measurements 

(table 13), the sandstone of the Moss Back is very fine to fine grained 

with moderately to poorly sorted, moderately skewed and moderately 

peaked grain-size distributions.

The sandstone of the Moss Back is composed of detrital grains, 

matrix, and cement arranged heterogeneously in microscopic structure. The detrital components are subrounded to subangular grains 

of quartz, orthoclase, albite, chert, tuffaceous material, mica, and a 

few heavy-mineral grains. Other components are represented by 

subhedral carbonate (mostly calcite) crystals and reddish iron oxidetype cement interspersed with interstitial wads of kaolin, montmorillonite, or hydromica. Thin sections of only one of the sandstone 

strata showed a mosaic arrangement, with quartz and feldspar grains 

in a sparse interstitial montmorillonitic matrix. The heterogeneous 

arrangement of components is typical of the sandstone units of the 

Moss Back member.

An estimate of the composition of the sandstone of the Moss Back 

member in terms of mineral groups is based on the averages of 

point-count analyses of six thin sections (table 16) and is as follows:

Percent 

Mineral groups ~by volume

Quartz ________________________________________ 68. 3

Potassium feldspar_________________________________ 3.9

Kaolin______________________.____________________ 9.7

Hydromica________________________________________ 6.0

Quartzite fragments_______________________________ . 1

Tuffaceous material________________________________ 4.8

Plagioclase ___________________________________ 1. 2

Mica _________________________________________ . 6

Cement________________________________________ 5. 4

Miscellaneous____________________________________ .0

The classification of the sandstone of the Moss Back member is 

based on the following average amounts of detrital components:

Percent 

Com/ponent "by volume

Quartz________________________________________ 72

Feldspar_______________________________________ 16

Mica_________________________________________ 7

Tuffaceous material________________________ 5
548 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The average sandstone of the Moss Back member is feldspathic 

orthoquartzite; however, none of the individual samples are in this 

classification, Two samples of arkose are from the Temple Mountain area, and 3 of orthoquartzite and 1 of graywacke (hydromica. 

type) are from the White Canyon area (fig. 83). Not enough samples of the Moss Back have been studied to be certain of regional 

differences, but the arkose of the Moss Back at Temple Mountain,, 

the arkose of the Monitor Butte at Capitol Reef, and the high-feldspar feldspathic orthoquartzite of the Shinarump from Capitol Eeef 

and Circle Cliffs suggest a high feldspar content for the Shinarump 

and other basal sandstone of the Chinle in the northwestern part of 

the Plateau area.

PETRIFIED FOBEST MEMBER

The Petrified Forest member of the Chinle overlies the Moss Back 

member in many areas. The Chinle may be easily divided into an 

upper and lower part with the boundary at the top of the Petrified 

Forest member. Most of the lower part is dark-grayish-green siltstone and shale, variegated bentonitic shale and pale-brown sandstone. Most of the upper part is pale-reddish-brown siltstone and 

sandstone and grayish-green limestone and limy siltstone. The Petrified Forest member has been studied in the field and in thin section. 

Grain-size analyses were completed on four samples of sandstone. 

The Petrified Forest member is principally a bentonitic siltstoneclaystone unit but contains scattered discontinuous thin- to thickbedded moderately to poorly resistant sandstone beds, many of which 

are conspicuously cross-stratified.

On the basis of the average grain-size-distribution measurements 

(table 13), the sandstone strata of the Petrified Forest member consist 

of very fine poorly sorted material with moderately skewed and moderately peaked grain-size distributions.

The sandstone strata of the Petrified Forest member are composed 

of subangular to subround grains of quartz, feldspar, tuffaceous 

material, and a few scattered flakes of mica in a matrix of montmorillonite and hydromica clay minerals. They are slightly cemented by subhedral carbonate crystals, and much reddish to purplish 

irori(?) oxide stain is present. The microscopic structure of the 

sandstone as seen in thin section is heterogeneous.

An estimate of the composition of the sandstone facies of the 

Petrified Forest member of the Chinle in terms of mineral groups 

is based on the average of point-count analyses of two thin sections 

(table 17) from the Comb Wash area (fig. 83) and is as follows:
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 549

Percent 

Mineral group 'by volume

Quartz __________________________________________ 51. 9

Potassium feldspar____________________________ 7.3

Kaolin __________________________________________ . 6

Hydromica _________________________________ 6. 6

Quartzite fragments_____________________________ . 0

Tuffaceous material_____________________________ _ 32.1

Plagioclase_____________________________________ .4

Mica________________________________________ .7

Cement _____________________________________ . 4

Miscellaneous________________________________ . 0

The classification of the Petrified Forest is based on the following 

average amounts of detrital components:

Percent 

Component "by volume

Quartz___________________________.._______________ 52

Feldspar_____________________________________ 8

Mica________________________________ _ 7

Tuffaceous material____________________________ 33

The average sandstone of the Petrified Forest member in the Comb 

Wash area is orthoquartzitic tuff. This conforms with the general 

impression of volcanic origin for much of the material making up 

the Petrified Forest member of the Chinle formation.

OWL BOOK MEMBER

The Owl Rock member of the Chinle formation overlies the Petrified Forest member. As discussed above, the boundary between the 

two members may also be used to divide the Chinle into an upper 

and lower part.

The Owl Rock member has been examined in the field and in thin 

section. Grain-size analyses were completed on four samples of 

sandstone or silty sandstone.

The unit is characteristically pale-dark-green and pale-reddishbrown siltstone, sandstone, and limestone. The general appearance 

in outcrop is that of several moderately to poorly resistant rounded 

rough-surfaced limestone ledges separated by intergraded poorly resistant units of limy siltstone; the unit forms a slope thinly covered 

with limestone pellets 1 mm to 10 mm in diameter.

On the basis of the average grain-size distribution measurements 

(table 13), the sandstone and silty sandstone of the Owl Rock member 

is composed of very fine moderately well sorted sand that has slightly 

skewed and moderately peaked grain-size distributions.

The rocks of the Owl Rock member are composed of calcite and 

dolomite pellets and crystalline masses which contain subangular to 

subround grains of quartz, orthoclase, albite, tuffaceous material, 

and, in some parts, biotite and muscovite, and a matrix of interstitial 

montmorillonite and chlorite types of clay minerals. The ratios of
550 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

chemical material to detrital material (including carbonate pellets) 

probably form a series. Many of the "limestone" beds consist of carbonate-pellet conglomerate or calcarinites and appear as such in outcrops, as well as in thin section.

An estimate of the composition in terms of mineral groups is based 

on 100-point counts on a thin section of representative Owl Rock 

member sandstone (53 percent allogenic detrital minerals and 47 percent carbonate minerals) from the Monitor Butte area (fig. 83) and 

is as follows:

Percent 

Mineral group "by volume

Quartz ________________________________________ 20

Potassium feldspar_________________________________ 13

Hydromica (mostly chlorite)__________________________ 9

Tuffaeeous material________________________________ 8

Plagioclase ______________________________ 3

Cement (mostly calcite, dolomite, iron oxides)______________ 47

The classification of sandstone of the Owl Kock is based on the 

following amounts of detrital components:

Percent 

Component by volume

Quartz _________________________________________ 38

Feldspar ______________________________________ 30

Mica __________________________________ 17

Tuffaeeous material- _____________________________ 15

The average sandstone of the Owl Eock member in the Monitor 

Butte area has the detrital composition of tuffaceous arkose. The average "limestone" unit varies in composition within the same bed from 

true limestone to what would properly be called dolomite; this conclusion is based on field tests using 10-percent strength hydrochloric 

acid. No data are available on the proportion of total calcium to total 

magnesium in these beds.

CHtTBCH KOCK MEMBER

The Church Kock member of the Chinle overlies the Owl Kock 

member and forms the uppermost unit of the Chinle in southeastern 

Utah. The sandstone and sandy siltstone beds of the Church Kock 

have been examined in the field and in thin section. Grain-size 

analyses were completed on seven samples. The generally dark reddish-brown unit appears as a sequence of moderately resistant alternating thick sandstone and siltstone beds at the top of the Chinle. 

Some of the sandstone beds are cross-stratified.

On the basis of the average grain-size-distribution measurements 

(table 14), the sandstone and silty sandstone of the Church Kock are 

composed of very fine moderately well sorted sand with moderately 

skewed and moderately peaked grain-size distributions.

The sandstone and silty sandstone of the Church Rock are composed of subangular to subround grains of quartz, orthoclase, albite,
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 551

some heavy minerals, flakes of mica, interstitial hydromica stained 

with iron oxide, disseminated subhedral crystals of carbonate (mostly 

calcite) cement, and interstitial blobs of iron oxides.

The composition in terms of mineral groups is based on 100 pointcounts on thin sections of representative silty sandstone and of sandy 

siltstone of the Church Rock member from the Monitor Butte and 

Comb Wash areas (fig. 83), respectively, and is as follows:

Percent "by volume 

Mineral group Sandstone Siltstone

Quartz ___________________________________ 44 33

Potasium feldspar _____ ________ _ - 11 14

Hydromica _________________________________ 3 20

Tuffaceous material __________________ _ 0 0

Plagioclase ___________________ ____ 7 8

Mica ______________________________ _ 6 3

Cement _________________________________. 27 20

Miscellaneous _______________________________. 2 2

The classification of the silty sandstone and the sandy siltstone of 

the Church Rock member is based on the following amounts of detrital 

components:

Percent 'by volume 

Component Sandstone Siltstone

Quartz __________________________________ 60 41

Feldspar __________________________________ 25 28

Mica _____________________________________ 15 31

Tuffaceous material__________________________ 0 0

The average fine-grained sandstone of the Church Rock member of 

the Chinle formation in the Monitor Butte area (fig. 83) is arkose, 

but the average siltstone is graywacke. The difference in classification 

between the two samples results from the increased amount of 

hydromica clay minerals in the siltstone.

The Chinle formation in southwestern Colorado resembles the 

Church Rock member superficially but may be better referred to as 

undifferentiated Chinle. Compositional data based on thin-section 

study have not been determined. Grain-size analyses were completed 

on seven samples of sandstone and coarse siltstone. The rocks were 

found to be, on the average (table 13), very fine grained silty sandstone composed of moderately well sorted sand with moderately 

skewed moderately peaked grain-size distributions.

TRIASSIC AND JURASSIC ROCKS

GLEN CANYON GROUP

WINGATE SANDSTONE

The Wingate sandstone lies above the Chinle formation and forms 

the uppermost part of the Triassic rocks in southeastern Utah.

The Wingate has been examined in the field and in thin section. 

Grain-size analyses were completed on 44 samples of sandstone or 

sandy siltstone.
552 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The unit consists of light-colored very thick bedded massive cliffforming sandstone strata. The general appearance of the unit in the 

outcrop is that of a high vertical cliff at the top of the dark-reddish 

slope formed by the Chinle formation. The Wingate contains conspicuous large-scale cross-strata.

On the basis of the average grain-size-distribution measurements 

(table 13), the sandstone and coarse siltstone of the Wingate are very 

fine grained with moderately to well-sorted sand that has moderately 

skewed highly peaked grain-size distributions.

The sandstone beds of the Wingate are composed of subround grains 

of quartz, orthoclase, microcline, albite, altered tuffaceous material, 

quartzite fragments, and some rounded heavy-mineral grains. The 

matrix consists of a small amount of kaolinite, hydromica, or montmorillonite clay. The cement consists of some subhedral interstitial 

carbonate crystals and reddish iron(?) oxides that stain the matrix. 

The base of the Wingate is often marked by coarse to very coarse 

spherical grams of quartz, feldspar, quartzite, and chert (silicified 

tuff?) in a matrix of very fine grained sand.

An estimate of the composition of the Wingate sandstone in terms 

of mineral groups is based on 100 point-counts in 2 thin sections of 

representative sandstone from the Poncho House and Carrizo areas, 

respectively, (fig. 83) and is as follows:

Percent "by volume 

Poncho 

Mineral groups House Carrizo Average

Quartz____________________________ 37 58 48

Potassium feldspar____________________________ 17 11 14

Kaolin.____________________________ 101

Hydromica________________________ 6 10 8

Tuffaceous material___________________ 23 2 12

Plagioclase________________________ 16 11 13

Cement..________________________ 063

Miscellaneous. ___________________________ 021

The classification of the Wingate sandstone in extreme northeastern 

Arizona is based on the following amounts of detrital components:

Percent by volume 

Poncho 

Component House Carrizo Average

Quartz________________________ 37 63 49

Feldspar___________________________ 34 23 29

Mica_____________________________ 6 12 9

Tuffaceous material_________________ 23 2 13

The Wingate sandstone in northeastern Arizona ranges from tuffaceous arkose to feldspathic orthoquartzite. More work must be done 

to obtain a significant mean composition and an idea of the distribution of components. From the inspection of several thin sections, the 

Poncho House sample appears to represent one extreme but is also one 

of many of the same general classification. The typical or more common sandstone is more similar in composition to the Carrizo sample.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 553

The unexpected presence of tuffaceous material suggests either the 

reworking of old tuffaceous material (from the Chinle?) or volcanic activity contemporaneously with the deposition of the Wingate. The fact that the tuffaceous material is abundant in some strata 

and nearly absent in others supports the hypothesis of intermittent contemporaneous volcanism as the explanation. Additional work may 

reveal a regional distribution of the tuffaceous material in the 

Wingate.

KAYENTA FORMATION

The Kayenta formation of Jurassic(?) age overlies the Wingate 

sandstone in much of the Colorado Plateau area. The sandstone beds 

of the Kayenta have been examined in the field, but not in thin section. Grain-size analyses were completed on 19 samples.

The unit is generally light- to dark-red or brown medium- to thickbedded sandstone which forms a moderately resistant slope above the 

massive vertical cliff of the Wingate sandstone. Some of the sandstone beds contain cross-strata.

On the basis of the average grain-size-distribution measurements 

(table 13), the sandstone of the Kayenta is composed of very fine 

moderately to well-sorted sand with moderately skewed and highly 

peaked grain-size distributions.

No thin-section data are available on the Kayenta formation. Previous work done on loose-grain mounts of disaggregated sand from 7 

samples taken in southwestern Colorado indicated that the detrital 

grains in the Kayenta consist, on the average, of 16 percent potassium 

and sodium feldspar, 6 percent fragments of silicified tuff and chert, 

78 percent quartz, and a trace of mica. The Kayenta would be classified as a feldspathic orthoquartzite or arkose, depending upon the 

amounts of kaolin clay and sodic-calcic feldspars present. The rock 

is composed on the average of 81 percent sand, 9 percent silt and clay, 

and 10 percent soluble cement.

NAVAJO SANDSTONE

The Navajo sandstone overlies the Kayenta in much of the Colorado 

Plateau region. The sandstone beds of the Navajo have been examined in the field, but not in thin section. Grain-size analyses were 

completed on 22 samples.

The unit is generally light-colored thick-bedded sandstone. The 

appearance in outcrop is that of massive light-colored sandstone with 

convex weathering surfaces and, in most localities, with conspicuous 

large-scale cross-strata that have been brought into relief by differential weathering along the contacts of the cross-strata.

On the basis of the average grain-size-distribution measurements 

(table 13), the sandstone of the Navajo is composed of very fine
554 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

grained, well-sorted sand with moderately skewed and highly peaked 

grain-size distributions.

No thin-section data are available on the Navajo sandstone. Previous work done on loose-grain mounts of disaggregated sand from 

9 samples taken about 30 miles north of the Temple Mountain area 

(fig. 83) indicates that the detrital grains consist, on the average, of 

85 percent quartz, 12 percent potassium and sodium feldspar, and 3 

percent chert and fragments of tuff. The rock is composed of 94 percent sand, 3 percent silt and clay, and 3 percent soluble cement. The 

Navajo is classified as feldspathic orthoquartzite.

STUDY OF THE CLAY-MINERAL GROUPS IN THE TRIASSIC AND 

ASSOCIATED FORMATIONS

Clay fractions obtained in the grain-size analyses of sandstones of 

Jurassic, Triassic, and Permian age were submitted to a U. S. Geological Survey laboratory for identification by X-ray spectrometer. 

Acknowledgment is made of the help and cooperation of Alice D. 

Weeks who expedited the service requests and Mary E. Thompson and 

Evelyn Cisney who made the clay-mineral identifications.

The results of the clay analyses are shown in tables 11 and 12. The 

percentages given in table 11 refer to the number of samples that contain the particular clay-mineral group as the major constituent in 

the clay fraction. In table 12, the percentages refer to samples that 

contain the particular mineral group as the minor constituent in the 

clay fraction.

As an example, in a group of 54 sandstone and siltstone samples 

from the Shinarump member of the Chinle formation (table 11), 85 

percent of the samples have kaolinite as the major clay-mineral group, 

11 percent have hydromica as the major clay-mineral group, and 4

TABLE 11. Units in which sandstone contains 1 of the 3 clay-mineral groups as the

major clay mineral

Unit

Navajo sandstone__ _______ _____

Kayenta formation _ _____ _ _____ _ _

Wingate sandstone________ ____________

Chinle formation: 

Exclusive of Shinarump member_____ 

Shinarump member _ ____ __ __ _

Moenkopi formation. __ ___ ____ _ _

Hoskinnini tongue of the Cutler formation. 

De Chelly sandstone member of the 

Cutler formation _______ _ _ ___ __

Organ Rock tongue of the Cutler formation_____ ___ ____________ _________

Number of 

samples

17 

9 

28

25 

54 

18 

6

4

7

Clay mineral group (percent)

Kaolinite

71 

78 

50

11 

85 

28 

100

50 

43

Hydromica

23

22 

29

81 

11

72 

0

25

57

Montmorillonite

6 

0 

21

8 

4 

0 

0

25 

0
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 555

TABLB^12. Units in which sandstone contains 1 of the S day-mineral groups as

the minor clay mineral

Unit

Navajo sandstone____ ___.. ____ ______

Kayenta formation ________ ____

Wingate sandstone__ _ ___ _ _ ______

Chinle formation: 

Exclusive of Shinarump member_____ 

Shinarump member __ __

Moenkopi formation. __ __ _

Hoskinnini tongue of the Cutler formaDe Chelly sandstone member of the 

Cutler formation ____..

Organ Rock tongue of the Cutler formation _ __ ____ ___ ______ __

Number of 

samples

10

6 

19

13

29 

14

6 

3

8

Clay mineral group (percent)

Kaolinite

40 

33 

32

85 

17 

64

0 

33

38

Hydromica

40 

67 

63

15 

83 

29

33 

67

62

Montmorillonite

20 

0 

5

0 

0

7

67 

0 

0

percent have montmorillonite as the major clay-mineral group. Of 

the original 54 samples, 29 contain 2 or more detectable clay-mineral 

groups (table 12), of which 17 percent have kaolinite as the minor 

clay-mineral group and 83 percent have hydromica as the minor claymineral group.

The typical clay minerals in the sandstone and siltstone strata of the 

Shinarump member of the Chinle formation in order of abundance 

are kaolinite and hydromica. Study of thin sections of samples from 

these rocks indicates that the kaolinite is mostly allogenic and 

detrital and was probably derived from potassium and sodium feldspars during the weathering of granitic rocks or f anglomerates. The 

presence of kaolinite as the dominant clay supports other petrographic 

evidence that most of the Shinarump member is arkosic.

The hydromica may be allogenic detrital clay, but petrographic evidence suggests that the hydromica is authigenic and was derived from 

the alteration of montmorillonite or volcanic ash in the presence of 

soluble potassium salts. The potassium salts would originate from 

the breakdown of potassium feldspar or during the alteration of 

alkalic-silicic volcanic glass, and other tuffaceous debris.

The Chinle has long been noted as a formation which contains much 

altered volcanic debris, yet from table 11 it may be seen that 81 percent of a group of 25 sandstone and siltstone samples contain hydromica as the major clay mineral and only 8 percent contain montmorillonite as the major clay mineral. The minor clay minerals in the 13 

samples which contain a second clay-mineral group are kaolinite in 

85 percent of the samples and hydromica in 15 percent (table 12).

Microscopic study of the same samples on which clay analyses were 

run showed hydromica to occur as rims around grains even where
556 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

the more conspicuous clay was kaolinite or montmorillonite. Hydromica also occurs interstitially or as wads or galls in which one part 

of the wad is montmorillonite and part is hydromica, with a mixedlayer transition between the two types.

Thus, evidence supports the hypothesis that much of the hydromica in the Shinarump member and in the Chinle formation generally is the result of the alteration of volcanic debris or montmorillonite in the presence of soluble potassium salts. Such an alteration 

is suggested by Grim (1953), with references to Noll (1936), and 

others. The confirmation of such a hypothesis would explain the 

anomaly of tuffaceous sandstone interbedded with hydromica-rich 

claystone in the Chinle formation.

The following diagrammatic arrangement of the materials involved 

illustrates the hypothesis of the formation of much of the hydromica.

Potassium or sodium feldspar Alkalic-silicic ash 

Kaolinite or montmorillonite Montmorillonite

/ + soluble potassium salts \ 

Kaolinite \ / Montmorillonite

il kT

Hydromica

Many of the interpretations in this report are based on certain expected occurrences and associations of clay-mineral groups. Most of 

the kaolinite is assumed to be related to granitic source rocks and the 

formation of arkose; most of the montmorillonite is assumed to be related to sediments derived from alkalic-silicic source rocks of volcanic 

origin (ash, tuff, glass, felsite); most of the hydromica was originally 

assumed to be related to sediments derived from sericitic sedimentary 

and micaceous metamorphic rocks. As discussed above, the concept 

of the role of hydromica as an authigenic clay mineral has altered the 

original assumption.

PETROLOGIC FEATURES OBSERVED IN ORE-BEARING SANDSTONE

During the petrographic study of thin sections of sandstone samples 

from the Shinarump and the Moss Back members of the Chinle formation and the estimation of volumetric composition by the point-count 

method (based on 500 counts), 9 thin sections from uranium-mineralized locations were studied. These thin sections represent samples 

that may be classified as 1 sample of high-grade ore-bearing sandstone 

(L258, table 14) and 3 samples of medium- to low-grade ore-bearing 

sandstone (L260, L842, table 14; L252, table 16) * Five samples 

can be classed as barren sandstone associated with ore-bearing sandstone (L261, L259, L883, L841, table 14; and L254, table 16),
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 557

Study of the thin section from the high-grade ore sample (L258) 

and the thin section from the associated barren-sandstone sample 

(L261) from the Happy Jack mine in White Canyon, San Juan 

County, Utah, suggests that kaolinite may have influenced the deposition of the ore minerals. Each of these thin sections contains quartz, 

potassium feldspar, quartzite, tuffaceous material, plagioclase, cement 

(calcite, iron oxides, barite, etc.), and kaolin. In addition, the orebearing sandstone contains unidentified uranium minerals, chalcopyrite, azurite, barite, covellite, malachite, and pyrite; these minerals 

occur interstitially and form about 30 percent of the volume of the 

rock. The mine face at the point where the sample was taken was 

reported by the owners to assay about 10 percent copper and 2 percent 

uranium.

Quartz grains in the associated barren thin section have euhedral 

authigenic overgrowths, whereas the quartz grains in the ore-bearing 

sandstone contain few overgrowths and these show corrosion by solution which apparently occurred before or during deposition of the sulfide minerals.

Kaolin occurs in the two thin sections primarily as detrital interstitial mud composed of ground-up quartz, potassium and sodium feldspars, kaolinite clay minerals, and minor amounts of carbonate minerals and micaceous clay minerals, and secondarily as an alteration 

product along cleavage planes in some of the potassium feldspar 

grains. A comparison of the two thin sections shows that they are 

similar in the proportions of all components, with the exception of kaolin mud and ore minerals.

The results of this comparison suggest that less kaolin mud is 

present in the ore-bearing sandstone because most of the original mud 

has been replaced or obscured by uranium and copper minerals, 

whereas most of the kaolin in the adjacent barren sandstone has been 

unaffected.

Other data indicate some possible significance in the relations between ore and kaolin. At the mine face where samples L258 and 

L261 were taken, the copper and uranium sulfide-bearing sandstone 

was surrounded by a halo of mineralized sandstone stained by azurite 

and malachite. A thin section from the halo (sample L260) contains euhedral crystals of azurite and malachite in an interstitial 

kaolin matrix.

A similar relation between ore sand and associated barren sand 

was studied in the Moss Back member of the Chinle at Temple Mountain, Emery County, Utah. The ore-bearing sandstone (L252) contained 19 percent interstitial kaolin and 5 percent asphaltoid residue 

containing ore minerals and pyrite in cryptocrystalline form. The 

associated barren sandstone (L254) contained 24 percent interstitial
558 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

kaolin and 1 percent pyrite. It may be that the amounts of kaolin 

and of asphaltoid residue, ore, and pyrite are complementary, that 

the ore-bearing sandstone was originally of the same composition as 

the barren sandstone, and that the mineralization was effected at the 

expense of the interstitial kaolin.

To test this apparent relation further a comparison of kaolin content was made between the nine sandstone samples containing ore 

minerals or associated with known ore deposits, and the average for 

all the samples of the Shinarump and Moss Back studied. The ore 

minerals of high-grade sample L258 are combined with the kaolin 

to compensate for the supposed loss of kaolin by it and other orebearing samples.

The mean amount of kaolin for the ore-associated samples is 21 

percent, with a range of from 5 to 35 percent. The mean amount of 

kaolinite in the barren samples is 9 percent, with a range from 0 to 

26 percent. The mean amount of kaolin for all 34 samples is 12 percent, with a range from 0 to 35 percent.

The three barren samples with the highest kaolin content that were 

collected away from known ore deposits came from the Owl Rock 

section (table 14) in southern Monument Valley.

The results of the study suggest that ore is localized in the Shinarump and other lower sandstone units of the Chinle formation containing 20 percent or more interstitial kaolin mud. The effect of the 

kaolin may be physical or chemical, or both, or the effective agent 

may be something contained in the kaolin. Whatever the cause of 

localization, the recommendation is made that strata containing 15 

to 35 percent interstitial kaolin be considered as more favorable for 

the occurrence of uranium ore than strata containing less than 15 

percent kaolin.

INTERPRETATIONS

The final objectives of the petrographic study of the Triassic and 

associated sedimentary rocks have been achieved in part. More 

progress has been made on reconstruction of the paleogeography than 

on the problem of associating the occurrence of uranium ore with measurable properties of the sedimentary host rocks.

Thin-section study indicates that the source areas of the rocks studied were dominantly granitic, with minor areas of quartzite and 

quartz-mica schist or gneiss. At least three separate source areas 

were contributing sediments to the major continental basin of deposition when the Shinarump and other members of the lower part of 

the Chinle formation were being deposited. Some volcanic activity 

occurred in all periods, but the greatest contribution of volcanic ma-
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 559

terial to the sediments occurred during the deposition of the lower 

part of the Chinle. The volcanic material appears to have had an 

alkalic-silicic composition. The main volcanic contribution now 

forms thick deposits of claystone and siltstone combined with a large 

volume of sand-size altered tuffaceous material. The main loci of 

deposition were in the extreme northeastern part of Arizona and 

southeastern to south-central Utah. Many thin concretionary or 

banded cherty lenses in the lower part of the Chinle may have derived 

their silica from the altering volcanic glass.

The chief uranium-bearing units, the Shinarump, Monitor Butte, 

Moss Back, and Petrified Forest members of the Chinle formation, are 

distinguished from the other units by both physical and chemical features. The uranium-bearing rocks are made up of generally coarser 

and generally more poorly sorted materials than the other rocks studied. The units contain large amounts of sand-, silt-, and clay-sized 

materials, which have been derived from alkalic-silicic volcanic material. Owing to the impervious nature of many of the montmorillonite and hydromica clay minerals, many of the more soluble products 

of decomposition of the volcanic materials have probably been retained 

in the clayey strata or are still being slowly leached, which makes possible a wide variety of chemical environments in the uranium-bearing 

units. Any one of these features mentioned, or a combination, may 

have been critical in localizing the uranium ore.

Conclusions resulting from petrographic study of these sedimentary 

rocks fall into two categories. In one category are direct comparisons 

based on microscopic or megascopic measurements; for instance, sandstone of the Moenkopi formation is finer grained on the average than 

sandstone of the Shinarump member of the Chinle formation. (See 

table 13 for comparison of mean grain size.) In the second category, 

hypotheses are put forward by the petrologist to explain why sand deposited during Moenkopi time was finer grained than sand deposited 

during Shinarump time.

Second-category conclusions are based on interpretations or assumptions which are open to question at all times, even when the assumptions are based on fairly well accepted and often well substantiated cause-and-effect relations. A logical course in presenting 

conclusions of the second category is to present the basic cause-andeffect relations that are assumed by the author to be true.

The assumption is made that the grain size and sorting of a sediment 

are determined by conditions in the source area, the basin of deposition, and the particular environment within the basin. It is assumed 

that the gross controlling causes are the degree of differential tectonic
560 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

uplift in the source area, the degree of differential subsidence or uplift 

in the basin of deposition, and the degree of region-wide subsidence 

or uplift.

The effect of these multiple causes on the mean grain size and the 

sorting of the related sediments is assumed to be as follows:

1. With an increase in differential tectonic uplift in the source 

area, the mean grain size increases and the sorting becomes poorer 

(standard deviation increases).

2. With an increase in the differential subsidence of the basin of 

deposition, the mean grain size decreases and the sorting becomes 

poorer (standard deviation increases).

3. With region-wide uplift encompassing both source area and 

basin, grain size increases and sorting becomes better (standard 

deviation decreases). The effect is the same with an uplift of the basin 

of deposition.

4. With general quiescence, grain size is determined by the texture 

of the material available for reworking; sorting steadily improves 

until the sediment is finally buried. Eolian sand is a continental 

terrestrial sediment typical of this condition.

Any combination of the first three conditions is possible, with resulting modification of the sediments. When tectonic movements are 

relatively rapid the resulting sediments change characteristics 

abruptly. When tectonic movements are slow, but fast enough to 

change rates of erosion or deposition, the resulting sediments will 

indicate gradational change. Quiescence may result from a slight 

region-wide positive tectonic movement which tilts the lower end of 

the profile of equilibrium upward or may occur during a period when 

the basin is subsiding very slowly and erosion is proceeding at a 

diminishing rate in the source area.

If the above cause-and-effect relations may be assumed for the 

overall area and the minor modifications may be ignored, interpretation then becomes a matter of selecting a cause or causes for each 

effect.

Figure 84 is a graph showing mean grain size and sorting (as 

measured by the standard deviation of the phi grain-size distribution) for each formation and member studied. Plotted points are 

connected by lines to emphasize the sense and degree of change. 

Data for the points are listed in table 13.

Using figure 84 for reference, interpretations are made on the 

basis of changes that take place in the sequence. To interpret the 

texture of each unit without reference to adjacent units would be 

similar to taking phrases out of context some would make sense, 

others would be unintelligible.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 561

Navajo sandstone 

Kayenta formation

Wingate sandstone 

Chinle formation: 

Church Rock member

Owl Rock member 

Petrified Forest member 

Moss Back member 

Monitor Butte member 

Shinarump member

Moenkopi formation 

Cutler formation: 

Hoskinnini tongue

De Chelly sandstone member 

Organ Rock tongue

Cedar Mesa sandstone member 

Grain size (mm) 0.050 

Sorting (phi units) 0.50

FIGURE 84. Changes in average grain size and average sorting from Upper Permian to 

Lower Jurassic units in the central part of the Colorado Plateau.

The two curves present an interpretive summary of general tectonic 

activity during the period covered. The projection of the grain-size 

curve to the right may be read as an indication of movement of 

coarser material from the source areas into the basin of deposition. 

The projection of the sorting curve to the right is an indication of 

an increased rate of deposition, based on the assumption that the 

slower the rate of deposition is, the better sorted the sediments will be.

One other characteristic used in interpretation is the structure of 

the sedimentary unit as observed in the field. A massive thick-bedded 

sandstone unit with large-scale cross-strata and fairly extensive re-
562 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

gional distribution is interpreted as eolian in origin, but it should 

be noted that even if the unit were of marine origin the tectonic 

significance would be the same. Units that have even, regular bedding suggest continuous deposition and little or no erosion or reworking. Units that have irregular bedding, cut-and-fill structure, 

or local erosional disconformities suggest discontinuous deposition 

accompanied by much erosion and reworking of deposits which in 

turn suggests deposition in a slowly subsiding basin where rate of 

burial is not fast enough to prevent reworking.

When interpreting change, certain reference points must be recognized. A reference point would be an interpretation attached to 

a particular type of sediment such as an eolian sandstone unit. An 

eolian sandstone unit or a unit resembling eolian sandstone would 

suggest or be interpreted as evidence of quiescence in the basin of 

deposition.

Interpreting the changes illustrated on figure 84, the Cedar Mesa 

sandstone member of the Cutler formation has many of the criteria 

of an eolian sandstone unit and represents conditions of quiescence 

in the basin area, and probably in the region, which resulted in an 

interval of decreased erosion in the source area and a very slowly 

subsiding basin of deposition. The unit is classified as a marginal 

terrestrial deposit.

The Organ Rock tongue of the Cutler formation is marked by a 

decrease in grain size and a decrease in the degree of sorting which, 

combined with the depositional structures, is interpreted as indicating 

more rapid deposition brought about by faster subsidence of the 

basin. The unit is classified as a marginal terrestrial deposit.

The De Chelly member of the Cutler is marked by an increase in 

grain size and an increase in the degree of sorting which, in view of 

the eolian characteristics of the unit, is interpreted as a return to 

quiescent conditions with continued, slow erosion in the source areas 

and decreased deposition in a very slowly subsiding basin. The De 

Chelly is classified as a marginal terrestrial deposit.

The Hoskinnini tongue of the Cutler formation is marked by an 

increase in grain size and a decrease in the degree of sorting. This 

change, combined with irregular bedding, suggests moderate tectonic 

uplift in the source area which resulted in increased erosion with 

some increase in deposition in a slowly subsiding basin. The Hoskinnini is classified as a marginal terrestrial deposit.

The Moenkopi formation, which has a decrease in grain size, slightly 

improved sorting, and regular bedding, is interpreted as representing 

a slightly decreased rate of erosion in the source area accompanied 

by deposition in a basin which was subsiding at an increased rate.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 563

The Moenkopi is classified as a marginal terrestrial-marginal marine 

deposit.

The Shinarump member of the Chinle formation is marked by an 

abrupt increase in grain size and a decrease in the degree of sorting. 

These changes, in light of the irregular, discontinuous bedding features and large erosional cuts, are interpreted as indicating marked 

regional uplift, tectonic uplift in the source area resulting in increased erosion and little if any subsidence in the basin of deposition. 

The unit is classified as a terrestrial deposit. The occurrence of altered tuffaceous material in the unit indicates that the tectonic activity 

was accompanied by volcanic activity.

The Monitor Butte member of the Chinle formation is marked by 

a decrease in grain size and a decrease in the degree of sorting. 

These changes, combined with the more regular bedding and amount 

of clay present, are interpreted as indicating continued tectonic activity 

in the source areas, rapid erosion, and increased deposition in a subsiding basin. Volcanic activity continued. The unit is classified as a 

terrestrial deposit.

The Moss Back member of the Chinle is marked by an increase in 

grain size and an increase in the degree of sorting. These changes 

together with irregular bedding and erosional features of moderate 

relief within the deposit, are interpreted as indicating continued 

tectonic activity in the source areas and a decreased rate of subsidence 

in the basin. Volcanic activity continued. The unit is classified as a 

terrestrial deposit.

The Petrified Forest member of the Chinle is marked by a decrease 

in grain size and a decrease in the degree of sorting. These changes, 

combined with irregularly bedded sandstone and relatively thick 

bentonitic claystone and siltstone units, are interpreted as indicating 

continued volcanic activity, continued tectonic uplift, rapid erosion 

in the source areas, and moderate to rapid subsidence in the basin of 

deposition, with intervals, at least locally, when subsidence halted 

for short periods. The unit is classified as a terrestrial deposit.

The Owl Kock member of the Chinle formation is marked by a decrease in grain size and an increase in the degree of sorting. These 

changes viewed with respect to changes in lithologic character are 

interpreted as indicating a substantial decrease in tectonic activity 

in the source area, reduced erosion, minor volcanism, and slower 

subsidence of the basin of deposition. The unit is classified as a 

marginal terrestrial deposit.

The Church Eock member of the Chinle formation is marked by an 

increase in grain size and a further increase in the degree of sorting. 

These changes and the combination of regular and irregular bedding
564 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

are interpreted as indicating almost no tectonic activity and continued slow erosion in the source areas, a decrease in deposition, and 

slower subsidence of the basin of deposition. The unit is classified 

as a terrestrial deposit.

The Wingate sandstone is marked by a decrease in grain size and 

a continued increase in the degree of sorting. These changes, the 

eolian structure of the Wingate, and the erosional contact at the base 

are interpreted as indicating regional quiescence brought about by 

minor uplift in the basin of deposition during a time of tectonic inactivity and slow erosion in the source areas. The Wingate 

was deposited under conditions of gradual subsidence in the basin 

following the uplift which preceded deposition. The unit is classified as a terrestrial deposit.

The Kayenta formation is marked by an increase in grain size 

and a continued increase in the degree of sorting. These changes, 

combined with irregular bedding and erosion surfaces in the unit, 

are interpreted as indicating minor uplift in the source areas, a 

resulting increase in erosion but a continued decrease in the rate of 

deposition, and little or no change in the slow subsidence of the basin 

of deposition. The unit is classified as a terrestrial deposit.

The Navajo sandstone is marked by a decrease in grain size and 

a continued increase in the degree of sorting. These changes, combined with the eolian structure, are interpreted as indicating general 

regional quiescence with very slow deposition and very slow subsidence in the basin of deposition. The unit is classified as a terrestrial 

deposit.

Such a scheme of interpretation explains the relatively thin, widespread, but discontinuous nature of the Shinarump member of the 

Chinle formation. The regional uplift with initial erosion of the 

Moenkopi plain followed by deposition of the Shinarump suggests 

that gravel, sand, and clay were deposited on a "high" if not rising 

surface with few of the opportunities of burial found in a subsiding 

basin. However, the sand is only moderately sorted, which suggests 

that reworking was not extensive. Eeworking probably failed to 

improve sorting because of a huge sediment load being carried most 

of the time across the alluvial plain.

Other parameters resulting from statistical analysis of the grainsize distributions, such as the measure of peakedness, may eventually 

be usable in the interpretation of the petrology of Triassic and associated sediments, but sufficient data have not yet been accumulated, 

and further study is necessary.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 565

SUMMARY OF STRATIGRAPHIC AND PETBOLOGIC 

INTERPRETATIONS

Integration of the preliminary conclusions and interpretations derived from all lines of investigation in the stratigraphic and petrographic study provides an historical account of the paleogeology and 

tectonic background of the sediments deposited during part of the 

Permian and Triassic periods and the early part of the Jurassic period. 

The part of the continental area where sediments accumulated, as discussed in this summary, includes the southeastern quarter of the State 

of Utah and adjoining 50-mile overlap into Colorado, New Mexico, and 

Arizona.

In Permian time the area east and southeast of what is now the 

Colorado Plateau region was marked by extreme tectonic uplift of 

granitic and metamorphic terranes possibly the Uncompahgre-San 

Luis highlands. The rising highlands contributed thick wedge-shaped 

deposits of arkosic sediments along their western flanks. These deposits form the conglomeratic facies of the Cutler formation of western 

Colorado and eastern Utah; simultaneous with deposition they were 

reworked and the finer material transported by streams into a continental area of sedimentary accumulation in southeastern Utah. 

Conditions in the basin alternated between slow subsidence and 

quiescence which resulted in the deposition of the reddish siltstone 

members and yellowish-gray sandstone members of the Cutler. Slow 

subsidence brought in arkosic material from the east that formed the 

reddish siltstone Halgaito, Organ Eock, and Hoskinnini tongues of 

the Cutler formation. Quiescence resulted in the slow accumulation 

of eolian sand derived from northwest of southeastern Utah that 

formed the light-colored sandstone Cedar Mesa, De Chelly, and White 

Eim members of the Cutler formation and the equivalent Coconino 

sandstone. Contemporaneous with the deposition of all or part of 

the reddish siltstone and yellowish-gray sandstone members, limestone units were being deposited in the sea to the west; these form the 

Kaibab limestone of central and north-central Utah and northern 

Arizona.

At the end of Cutler deposition adjustments in the level of parts of 

the basin resulted in erosion in some areas while deposition continued 

uninterrupted in other areas. Parts of the Kaibab limestone deposited in the western sea in central Utah were exposed to erosion and 

were redeposited as conglomerate at the base of sediments now assigned to the Moenkopi formation of Early and Middle (?) Triassic 

age.
566 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Early Triassic time was marked by general subsidence and a gradual 

advance of the shallow sea from the west and northwest. Part of the 

sediments of the Moenkopi formation probably was derived from 

erosion of the conglomeratic facies of the underlying Cutler formation. Moderate uplift and erosion of the Uncompahgre-San Luis 

highlands may have continued to contribute sediments. Some coarse 

sediments accumulated along the flanks of the highland, and deposition of sediments of the Moenkopi formation occurred at a moderate 

rate in the subsiding basin. Tidal flats in advance of the shallow sea 

occupied the basin during much of Early Triassic time and resulted 

in widespread ripple marks in the sandy siltstone of the basal part of 

the Moenkopi. The advancing shallow sea deposited the Sinbad 

limestone member of the Moenkopi in central Utah. Following the 

deposition of the Sinbad limestone member the environment of deposition returned to predominantly that of a tidal flat. Minor adjustments in subsidence of the basin of accumulation or uplift in the 

source area resulted in the deposition of thick cross-stratified very fine 

grained sand units far out in the basin. Following the adjustment in 

middle Moenkopi time, deposition continued at a moderate rate in the 

subsiding basin in a dominantly tidal-flat environment.

At the end of Moenkopi time, regional uplift of the Triassic basin 

of deposition and associated highlands resulted in an interval of erosion which developed a stream-cut surface on the top of the Moenkopi 

sediments. Uplift of granitic and volcanic source areas, probably 

south of the Colorado Plateau started deposition of Chinle sediments 

of Late Triassic age. Chinle deposition probably took place on an 

alluvial plain of low relief. Some basal Chinle deposits included reworked Moenkopi sediments. The Temple Mountain member and 

lithologically similar strata may contain a high proportion of such 

reworked sediments.

After deposition of the initial reworked sediments, coarse material 

including granite wash, volcanic material, and quartzose pebbles 

spread out over the area and formed the widespread coarse-textured 

Shinarump member of the Chinle formation.

The sediments of the Monitor Butte member indicate conditions of 

continued rapid erosion in the source areas and an increase in deposition owing to the moderate increase in rate of subsidence of the 

basin. Ripple laminations and generally horizontal bedding in the 

sandstone units, plus an increase in the amount of mudstone, are evidence of gentler currents than those responsible for deposition of the 

Shinarump.
TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 567

Interruption of the subsidence resulted in the deposition of slightly 

reworked coarser sediments with strong fluvial characteristics such 

as those that constitute the Moss Back sandstone and similar sandstone units in the lower part of the Chinle.

Volcanism increased in intensity, resulting in the deposition of 

great quantities of volcanic debris in the southern part of the basin 

to form the Petrified Forest member of the Chinle. Deposition in 

the subsiding basin was rapid with intervals of reduced subsidence 

which resulted in some reworking of the sediment under fluvial conditions to produce strata of coarser average texture.

The long period of deposition in an alluvial-plain environment 

ended in Late Triassic time as volcanism and uplift in the source areas 

decreased or ceased altogether. Subsidence in the basin continued at 

a reduced rate but was fast enough to keep the basin flooded with 

water. The decreased rate of deposition and a lake or lagoonal 

environment resulted in deposition of the limestone and limy-siltstone 

strata identified as the Owl Eock member.

Further decrease in the rate of subsidence of the basin, together 

with the slow rate of erosion in the source areas, changed the environment to an emerging plain with mixed low-velocity fluvial and 

shallow-water "mud-flat" conditions to produce the flat-bedded 

partly cross-stratified silty sandstone units of the Church Eock 

member of the Chinle formation.

The deposition of the last Chinle sediments was interrupted by 

a slight uplift of the basin which was followed by a period of 

moderate erosion and moderate reworking of the sediments on the 

emergent plain, resulting in some deposition in low or slightly subsiding areas and slight scouring in more positive areas. This short 

period of adjustment was followed by a quiescent period of considerable duration. Windblown sediments partly derived from underlying Chinle and from sources to the northwest accumulated to form 

the eolian strata of the Wingate sandstone.

This cycle of eolian deposition was interrupted by moderate uplift 

to the east, possibly in the Uncompahgre-San Luis highlands area, 

which resulted in a movement of some granitic material into the 

basin and a substantial amount of erosion and reworking of eolian 

sand by westward-flowing streams to form the Kayenta formation. 

Little or no change occurred in the rate of basin subsidence, so that 

with the cessation of the uplift the continued stability of the basin 

area rapidly reinstituted conditions of quiescence. Eolian deposition of sand from the northwest resulted in the accumulation of the 

strata assigned to the Navajo sandstone.
TABLE 13. Mean or average measures of the properties of the phi (grain-size) distrib

Jurassic sandstone and coarse siltstone from the Co

Formation

Chinle formation (undifferentiated).

Petrified Forest member. ....

Cutler formation (undifferentiated).

De Chelly sandstone memberCedar Mesa sandstone member.

Location

Southeast Utah, northeast Arizona, southwest Colorado. 

__ do.- ..........................

.....do.... .........................

.....do.............................

.....do. .........................

.....do ..........................

 ..do . .- . 

Southeast Utah, northeast Arizona. 

. .do -. - . ---...- ..

Southeast Utah, northeast Arizona. 

Southeast Utah. ...._.... ......

.....do..... .- ....-.... . ..

.....do ._- ...- . _ _.

Number 

of 

samples

22 

19 

44 

7 

7 

4 

4 

13 

12 

83 

21 

2 

13 

7 

14 

7

Grain size (millimete

Mean and 

range

0.100 

0. 156- . 072 

.123 

. 244- . 067 

.086 

. 175- . 032 

.059 

. 116- . 034 

.098 

. 209- . 039 

.076 

.146- .035 

.089 

.094- .048 

.124 

.260- .048 

.103 

.331- .050 

.188 

. 616- . 034 

.080 

. 374- . 036 

.367 

. 553- . 181 

.098 

. 195- . 033 

.080 

. 116- . 057 

.057 

. 122- . 033 

.072 

. 098- . 038

Mode and 

range

0.099 

0. 138- . 062 

.137 

. 258- . 074 

.093 

.210- .044 

.078 

. 149- . 044 

.172 

. 353- . 044 

.123 

.210- .031 

.186 

. 250- . 148 

.228 

. 490- . 098 

.221 

. 557- . 096 

.275 

. 593- . 043 

.111 

. 590- . 037 

.391 

. 553- . 229 

.131 

.241- .043 

.082 

. 139- . 062 

.065 

. 109- . 044 

.076 

.104- .045
TABLE 14. Mineralogic composition, in percent, of samples of the Shinarump member o

analyses of thin sections

Location Sample

Detrital components

Quartz Potassium 

feldspar

Kaolin HydromicaQuartzi

Arizona

Navajo County:

Monument Valley area:

L-660

661

662

60.4

70.9

63.7

8.1

8.4

8.8

25.9

17.9

20.1

2.4

.8

3.4

0

1

Utah

Oarfleld County: 

Circle Cliffs area:

Silver Falls, section B _ ...

Eane County:

San Juan County: 

White Canyon area:

Monument Valley area:

Wayne County: 

Capitol Reef area:

L-1012

1017

1019 

1007

1009 

CD-794

L-1098 

1049

1050

261 

259 

260 

1043

841

842 

871 

872 

873 

883 

906

907 

901

1027

88.5

70.9

74.2 

62.9

70.5 

73.1

68.4

57.7

87.6 

90.2

69.8 

82.1 

04.7 

98.6

75.1

65.1 

78.1 

93.2 

84.3

85.8

71.7

69.0

76.4

3.6

10.9

8.1 

6.2

3.4

9.8

5.7 

11.2

5.0 

2.1

2.5 

1.0 

4.4

.7

2.8

4.0 

.6 

.2 

.4 

2.2 

7.2

4.7

17.0

8.3

4.7

13.1

14.1 

19.7

21.0 

9 0

2.6 

13.3

7.0 

3.7

26.3 

13.9 

29. 4 

.0

11.8

28.9 

8.4 

2.9 

11.6 

5.3 

5.2

13.1 

13.5

7.2

11.3

0.2

.4

.0 

.2

1.3 

.2

.0 

2.4

.2 

.0

.0 

.0 

.0 

.2

4.9

1.0 

9.9 

1.2 

2.5 

5.9

3.2 

.2

.0

.2

0

3

1

1

1

1

1
TABLE 15.- -Mineralogic composition, in percent, of samples of the Monitor Butte mem

count analyses of thin sections

Location Sample

Detrital components

Quartz Potassium 

feldspar

Kaolin Hydromica Quartzit

Utah

Garfleld County: 

Circle Cliffs area:

Muley Twist section. .--._ L-1119

1120

The Peaks section.--...-..-- 1126

1128

Silver Falls section B_..... 1118 

Kane County:

Paria area-..--- - CD-796

798 

San Juan County:

White Canyon area:

Frey Canyon section____ L-1184

1185 

Happy Jack mine and area... 1106

1181

1182

Monument Valley area: 

Monitor Butte section. 

Poncho House section__ 900 

Wayne County:

Capitol Eeef area:

Pleasant Creek section_.... 1112

1113

1114

71.8

72.0

57.2

73.4

84.4

49.2

61.3

80.5

89.1

83.0

91.8

91.5

84.1

57.1

65.5

47.8

4.6 

5.1 

10.9 

8.6 

4.0

7.5 

9.4

.0

.0

5.5

5.7

1.5

.5 

.0

13.3

11.2

10.7

14.5

10.1

9.6

4.0

9.7 

15.5

8.6 

3.8 

2.1 

1.1 

2.9

3.8 

6.0

17.4

13.8

10.3 

.9

1.7 

.5

2.3

.7 

2.6

7.4 

3.1

.0 

2.9

8.4 

3.3

5.3 

1.4 

8.1

.

.

.

.

.

1.

.
TABLE 16. Mineralogic composition, in percent, of samples of the Moss Back member 

analyses of thin sections

Location Sample

Detrital components

Quartz Potassium 

feldspar

Kaolin HydromicaQuartzi

Utah

Emery County: 

San Rafael Swell area:

San Juan County: 

White Canyon area:

L-252 

254

1107 

1108 

1102 

1105

67.4 

64.6

63.5 

78.5 

78.0 

82.2

7.8 

4.3

1.4 

1.0 

4.9 

5.2

20.2 

24.1

5.6 

6.0 

3.6 

1.2

0.0 

2.6

27.1 

8.2 

.2 

.1

0

TABLE 17. Mineralogic composition, in percent, of samples of the Petrified Forest mem

count analyses of thin sections

Location Sample

Detrital components

Quartz Potassium 

feldspar

Kaolin HydromicaQuartzi

Utah

San Juan County Comb Wash area.. L-886 

945

57.8 

46.4

3.8 

10.9

0.0 

1.2

5.8 

7.4

0
572 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM 

LITERATURE CITED

Alien, V. T., 1930, Triassic bentonite of the Painted Desert: Am. Jour. ScL, v. 19,

p. 282-288. 

Baker, A. A., 1933, Geology and oil possibilities of the Moab district, Grand and

San Juan Counties, Utah: U. S. Geol. Survey Bull. 841.

 1936, Geology of the Monument Valley-Navajo Mountain region, San Juan 

County, Utah: U. S. Geol. Survey Bull. 865.

 1946, Geology of the Green River Desert-Cataract Canyon region, Emery,

Wayne, and Garfield Counties, Utah: U. S. Geol. Survey Bull. 951.

Baker, A. A., and Reeside, J. B., Jr., 1929, Correlation of the Permian of southern 

Utah, northern Arizona, northwestern New Mexico, and southwestern Colorado : Am. Assoc. Petroleum Geologists Bull., v. 13, p. 1413-1448.

Baker, A. A., Dane, C. H., and Reeside, J. B., Jr., 1947, Revised correlation of 

Jurassic formations of parts of Utah, Arizona, New Mexico, and Colorado: 

Am. Assoc. Petroleum Geologists Bull., v. 31, p. 1664-1668.

Camp, C. L., 1930, A study of the phytosaurs with descriptions of new materials 

from western North America: California Univ. Mem., v. 10, p. 1-158.

Chayes, Felix, 1949, A simple point counter for thin-section analysis: Am. Mineralogist, v. 34, p. 1-11.

 1952, Notes on the staining of potash feldspar with sodium cobaltinitrite 

in thin section: Am. Mineralogist, v. 37, p. 337.

Cross, Whitman, and Howe, Ernest, 1905, Description of the Silverton quadrangle, Colorado: U. S. Geol. Survey Geol. Atlas, Folio 120.

Darton, N. H., 1910, A reconnaissance of parts of northwestern New Mexico and 

northern Arizona : U. S. Geol. Survey Bull. 435.

Daugherty, L. H., 1941, The Upper Triassic flora of Arizona, with a discussion 

of the geologic occurrence ~by H. R. Stagner: Carnegie Inst. Washington 

Pub. 526, Contr. Paleontology.

Dutton, C. E., 1885, Mount Taylor and the Zuni Plateau: U. S. Geol. Survey 6th 

Ann. Rept, p. 105-198.

Finch, W. I., 1953, Response appraisal of uranium deposits in the pre-Morrison 

formations of the Colorado Plateau An interim report: U. S. Geol. Survey 

TEI 328, pt. 1, issued by U. S. Atomic Energy Comm., Tech. Inf. Service, 

Oak Ridge, Tenn.

Gilbert, G. K., 1875, Report upon the geology of portions of Nevada, Utah, California, and Arizona, examined in the years 1871 and 1872: U. S. Geog. and 

Geol. Surveys W. 100th Mer. Rept., v. 3, p. 1-187.

Gilluly, James, 1929, Geology and oil and gas prospects of part of the San Rafael 

Swell, Utah: U. S. Geol. Survey Bull. 806-C.

Goddard, E. N. (chm.), and others, 1948, Rock-color chart: Nat'1 Research Council, Washington, D. C.

Gregory, H. E., 1915, The igneous origin of the "glacial deposits" on the Navajo 

reservation, Arizona and Utah: Am. Jour. ScL, v. 40, p. 97-115.

 1917, Geology of the Navajo country: U. S. Geol. Survey Prof. Paper 93.

 1938, The San Juan country, a -geographic and geologic reconnaissance of southeastern Utah: U. S. Geol. Survey Prof. Paper 188.

 1950, Geology and geography of the Zion Park region, Utah and Arizona: 

U. S. Geol. Survey Prof. Paper 220.

Gregory, H. E., and Anderson, J. C., 1939, Geographic and geologic sketch of the 

Capitol Reef region, Utah: Geol. Soc. America Bull., v. 50, p. 1827-1850.

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TRIASSIC STRATIGRAPHY, COLORADO PLATEAU REGION 573

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Harshbarger, J. W., Repenning, C. A., and Irwin, J. H., 1957, Stratigraphy of the 

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Geol. Survey Prof. Paper 291.

Hunt, C. B., Averitt, Paul, and Miller, R. L., 1953, Geology and geography of the 

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INDEX

Page 

Arkose_______________ 541, 544, 565

Big Indian Wash area, studies in_ 490, 496 

Black ledge, Chinle formation__ 518 

Bowknot bed, Chinle formation____ 518

Capitol Reef_____,__ 489, 493, 544, 546

Carmel formation ____________ 525 

Cedar Mesa sandstone member, Cutler

formation __ 495, 537

Channel fills________________ 528

Chinle formation_____ 489-491, 500-523,

526-528, 543-551, 563, 566

age __ ____ _ 500

characteristics of ________ 520

depositional history ___ 500-501,

563-564, 566-567

fossils__________________ 522 

interpretation ___________ 522 

paleontology __ ________ 522 

thickness _ ______ 520 

uranium deposits__ 526-528, 556-558 

Church Rock member, Chinle formation _____ 517-520, 550-551 

contacts______________ 518-519 

general description ____ 517-519, 550 

sedimentary structure studies__ 519 

thickness __ ________ 519 

Circle Cliffs area________ 489, 494, 548 

Classification, grain-size________ 492 

phi mean ______________ 531 

Wentworth_____________ 531 

sorting ______ 531 

skewness ______________ 532 

kurtosis _______________ 532 

petrologic, sedimentary rocks__ 534 

Clay-mineral groups, study of in 

Triassic and associated 

formations_,______ 554 

Clay minerals___________ 533, 554-559 

Coconino sandstone_______ 490, 493-494 

age ________ 493 

depositional history______ 494, 565 

general description and field relations ______ 494 

Comb Ridge ________________ 490 

Comb Wash area ___________ 540, 549 

Compositional analysis, method of__ 533 

Conglomeratic facies, Cutler formation ____ 496, 541 

Cross-stratification____________ 493 

Cutler formation_____ 495-496, 537-541 

age_ ____ ________ 495 

depositional history____ 562, 565-566 

general description and field relations___________ 495-496

Page. 

De Chelly sandstone member, Cutler

formation____ 495-496, 539 

Dirty Devil River____________ 490 

Distribution of ore deposits__ _ 526

Egnar area __ _ 490, 496 

Elk Ridge______________ 490 

Entrada sandstone __-____ 524

Feldspathic orthoquartzite 534-535 

Frequency curve__ __ 531

Glen Canyon group. __ _ 523-525 

Grain size, average 492, 530-532, 559-560 

Grain-size distribution 530-532 

Grain size, mean___492, 530-532, 559-560 

modal______ _ 492 

Graywacke______ 534-536 

Graywacke tuff ___ ___ 535 

Green River_____ 498

Halgaito tongue, Cutler formation. 495, 565 

Happy Jack mine_ ___ 557 

Hermosa formation_ 494 

Hite bed, Chinle formation_____, 518 

Hoskinnini tongue, Cutler formation_____ 495, 540, 562, 565 

Hydromica, clay minerals 533, 

536,554-556, 559

Interpretations _________ 522, 525, 558, 565

Introduction___.___________ 489-491 

Invertebrate fossils _ 494

Kaibab limestone_______ 490, 493, 494, 565

age_ __ __ ______ 494 

fossils __ ____ 494 

general description and field

relations ____ 494 

Kanab area_______________ 490, 491 

Kaolinite, clay minerals____ 533, 554-558 

Kayenta formation_______ 490, 524, 553 

Kurtosis__________________ 530, 532

Literature cited___ __ _ 572 

Lithofacies studies__________ 491, 528 

Lithologic features_______ 528

Methods - 491, 529

Mineral groups__ 533

Mineralization of:

Moss Back member 557 

Shinarump member 557

Moenkopi formation 490,

496, 526, 542, 562, 566

age 496

depositional history_______ 498, 566

fossils__ ____________ 498

general description____ 496-500, 542 

mineralized material 526

575
576 INDEX

Page.

Monitor Butte member, Chinle formation___________ 508-512, 

544r-546, 563, 566

general description 508-512, 544-546

Montmorillonite, clay mineral- 554-556, 559

Monument Valley area 489, 495, 505, 507,

508, 511, 516, 517, 518, 519,

538, 558.

Moss Back member, Chinle formation _____ 512-515, 546-548 

depositional history___ 512, 563, 567 

general description _ 512-514 

pebble studies __________ 514 

sedimentary-structure studies- 514-515 

thickness _____ __ 514 

uranium deposits_________ 527

Navajo sandstone - 490, 493, 553-554, 564

Objectives________________ 529 

Ore deposits, distribution of___ 526-528,

556-558, 559

Organ Rock tongue, Cutler formation_____ 495, 537, 562, 565 

Orthoquartzite_ __________ 534-535 

Owl Rock member, Chinle formation___ 516-517, 549-550, 563, 567 

contact ________ 517 

general description_______ 516 517 

thickness __ 517

Paria River area___________ 544, 546 

Pebble studies____ 491,507-508,512-514 

Permian rocks__ 493-496, 526, 537-542, 565 

Petrified Forest member, Chinle formation _ __ _ 515-516, 

548-549, 563, 567

contacts- ____________ 516 

general description_ 515-516, 548-549 

thickness __ 516 

Petrographic classification, laboratory ______ 491, 534-536 

Petrologic features observed in orebearing sandstone___ 556 

Petrology, sedimentary ____ 491 

by Robert A. Cadigan_______ 529 

Phi grain-size distribution_____ 530-532 

Phi standard deviation________ 531 

Point-count analysis of thin sections- 533,

543 

Point-count compositional data of

Church Rock member_ 551 

De Chelly sandstone member 540 

Hoskinnini tongue 541 

Moenkopi formation 542 

Monitor Butte member______ 545 

Moss Back member- _ 547 

Organ Rock member________ 539 

Owl Rock member- __ 550 

Petrified Forest member_ 549 

Shinarump member 543 

Wingate sandstone 552 

Poncho House___________ 538, 542, 552 

Pre-Moenkopi erosion __ 498, 566

Red House Cliffs areaResults ___ __ 

Rico formation .

489

536

493

Page.

Sandstone, definition __ 492 

San Juan River____________ 518 

San Rafael Swell- 490, 494, 501, 519, 526 

Scope __________,_________ 491 

Sedimentary features, relation to ore

deposits _________ 528 

Sedimentary rocks ____ 491-493, 529-536 

Sedimentary-structure studies__ 491,

508, 514, 519

Shinarump Cliffs- _ __ 504 

Shinarump member, Chinle formation. 504-508, 543-44, 563, 566 

contacts- _ 506 

general characteristics _____ 504, 543 

pebble studies _ ______ 507 

sedimentary-structure studies 508 

Sinbad limestone, member of Moenkopi formation ______ 498-500 

Skewness_____ _______ 530-532 

Sorting ____________ 530-531, 560-561 

Source areas _______________ 496, 

499, 523, 544, 546, 565-567 

Standard deviation __________ 531, 560 

Stratigraphy of Triassic _ _ 525 

Summary of stratigraphic and petrologic interpretations 565

Temple Mountain member, Chinle formation ___ 501 

depositional history ___ 501-504 

description_____________ 502 

thickness ______________ 504 

Terminology and methods used in sedimentary-rock classification____________ 491 

Terminology and methods used in sedimentary-structure studies____________ 493 

Textural analysis ______ 530 

Textural classification, field____ 491-492 

Triassic rocks_____ 496-523, 526, 542-551 

Triassic and Jurassic rocks____ 523-525,

551-554

Tuffaceous arkose_ _,________ 535 

Tuffaceous material 533-536, 548, 553

Uncompahgre-San Luis highland_ 496, 565 

Uranium________ 489, 525-529, 556-559 

Uranium deposits__ 526-528, 556-558, 559 

maps of deposits 506, 510, 513 

Uranium-bearing units, chief __ 559

Volcanic, debris, material _______ 523, 

544, 546, 555-556, 558-559, 567

Wentworth grain-size classification ___________ 492, 531

White Canyon_____________ 489, 498, 

499, 505, 508, 511, 512, 517, 557

White Rim sandstone member, Cutler

formation _____________ 495, 565

Wingate sandstone 524, 528, 551-553 

contacts___ , , _ 524 

general description 524, 552 

thickness _____________- 524 

uranium deposits _______ 528

Zion Park area. 515