Structure of the Slick Rock 

District and Vicinity, 

San Miguel and Dolores 

Counties, Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 576-C

Prepared on behalf of the

U.S. Atomic Energy Commission
Structure of the Slick Rock 

District and Vicinity, 

San Miguel and Dolores 

Counties, Colorado

By DANIEL R. SHAWE

GEOLOGIC INVESTIGATIONS IN THE SLICK ROCK DISTRICT, SAN MIGUEL AND 

DOLORES COUNTIES, COLORADO

GEOLOGICAL SURVEY PROFESSIONAL PAPER 576-C

Prepared on behalf of the

U.S. Atomic Energy Commission

Folding and fracturing of Paleozoic and 

Mesoxoic sedimentary strata were sporadic, 

but interrelated, and occurred during and after 

deposition of the rocks. Interpretation of the 

influence of folds and fractures upon epigenetic 

alteration clarifies the time and manner of 

formation of the uranium-vanadium deposits

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog-card No. 70-606923

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

Washington, D.C. 20402 - Price 35 cents (paper cover)
CONTENTS

Abstract__________________________

Introduction ________________________

Structure in the Precambrian basementMajor folds_________________________

Minor folds_________________________

Major faults________________________

Faults bounding Gypsum Valley_ 

Dolores zone of faults____________

Pag_ 

01

1

2

6

Major faults Continued

Glade zone of faults __ ______________-_--__-_--- CIO

Minor faults- ____-_____________-__-_--_--_-_------- 10

Joints. _____________________________-_____-----_--- 11

Structural interpretation of variations in formation thickSummary and conclusions 

References cited_________

16

17

ILLUSTRATIONS

Page 

FIGURE 1. Map showing location of the Slick Rock district._________________________________________ Cl

2. Structural map of the Slick Rock district and vicinity showing major folds and fault zones-_____________________ 4

3. Map showing summary of major thinning and thickening trends in some Paleozoic and Mesozoic sedimentary 

formations in and near the Slick Rock district and their relation to inferred upper surface of Precambrian 

rocks and Glade subsurface fault zone________________________________________-__-------_-_-- _ 5

4. Panoramic view of the Slick Rock district southeastward from Horse Range Mesa.______-__-_.______-_-_---_ 6

5. Map showing fracture patterns in parts of the Slick Rock district__________________-__-__--_____---__--- 12

6. Diagram showing orientation of 167 nearly vertical joints in the Burro Canyon Formation- _____ _______ 13

7. Diagram showing orientation of 203 nearly vertical joints in the Salt Wash Member of the Morrison Formation.___ 13

TABLE

TABLE 1. Summary of consolidated sedimentary rocksPage 

C3

in

GEOLOGIC INVESTIGATIONS IN THE SLICK ROCK DISTRICT, 

SAN MIGUEL AND DOLORES COUNTIES, COLORADO

STRUCTURE OF THE SLICK ROCK DISTRICT AND VICINITY, 

SAN MIGUEL AND DOLORES COUNTIES, COLORADO

By DANIEL R. SHAWE

ABSTRACT

Major folds in the district are the northwesterly oriented 

Dolores anticline and Disappointment syncline, adjacent to the 

parallel Gypsum Valley anticline bordering the northeast edge 

of the district. A zone of faults bounds the southwest edge of 

the collapsed crest of Gypsum Valley anticline. The Dolores zone 

of faults extends southeastward into the district from Utah, 

obliquely across the Dolores anticlinal axis. Faults normal to 

the Dolores zone of faults in the northwestern part of the 

district constitute a conjugate set. The Glade zone of faults, 

trending about N. 75° E., extends through the southern part of 

the district A major set of fractures nearly parallels the principal individual faults in the district, about N. 60°-70° W.; a 

less prominent set nearly parallels the faults normal to the 

Dolores zone, about N. 45° E.

Major zones of faults and folds in the district were deformed, 

at times simultaneously, during deposition of the upper Paleozoic 

and Mesozoic rocks, as shown by thickness variation of the 

formations related to zones of faults and the fold axes. The 

zones of faults appear to be parts of a major regional conjugate 

system, and deformation on these zones was probably related 

to folding of the salt anticlines (including Gypsum Valley and 

Dolores anticlines) and development of the Paradox Basin.

Most faults and joints visible in the Gypsum Valley, Dolores, 

and Glade zones of faults are younger than strata of Late Cretaceous age, and they controlled movement of solutions that 

epigenetically altered the rock, probably in early to middle 

Tertiary time. Such epigenetic alteration was probably related 

to deposition of the uranium-vanadium ores of the district. 

Fracturing that followed the episode of alteration displaced 

mineralized rocks and was in part related to late Tertiary 

collapse of the Gypsum Valley salt anticline or to late growth of 

the Dolores anticline during the Quaternary.

INTRODUCTION

The Slick Rock district lies in the Paradox Basin at 

the south edge of the salt anticline region, also called 

the Paradox fold and fault belt (Kelley, 1958) (fig. 1). 

The district, which covers about 570 square miles, is 

underlain by about 13,000 feet of sedimentary strata 

lying upon metamorphic and igneous rocks of a Pre-

[~

j UTAH i i N, i / /_ ... Ck,4 _ S-^i-l'Wx'SSs1 ?,'-; ./

COLORADO

Outline of part of

. /_ * . - . V

| /Outline of salt /

I /anticline region'

the Colorado Plateau

109 '

10 i

20 MILES 

I

FIGURE 1. Location of the Slick Bock district (stippled area).

01
C2 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGITEL AND DOLORES COUNTIES, COLO.

cambrian basement. The sedimentary formations range 

in age from Cambrian to Late Cretaceous, and are summarized in table 1. They have already been described 

in detail in the first two reports of this series (Shawe 

and others, 1968; Shawe, 1968). Data already published 

on thickness variations of the formations (Shawe and 

others, 1968) are here used as the basis for inferring 

certain structural relations.

Sedimentary rocks in the district have been extensively altered (bleached). Diagenetically bleached 

and altered zones generally coincide with zones of 

abundant carbonaceous plant material in some stratigraphic units (particularly the upper part of the Salt 

Wash Member of the Morrison Formation and the Moss 

Back Member of the Chinle Formation). Epigenetically 

bleached and altered zones generally coincide with 

structural elements such as faults, joints, and the axial 

regions of folds. Where zones of carbonaceous material 

and structures are closely associated, the altered rocks 

may contain uranium-vanadium deposits, indicating a 

significant structural control of mineralization. The 

Salt Wash Member, in the northern part of the district 

in the vicinity of a prominent zone of faults, has yielded 

a large amount of uranium-vanadium ore. This report 

is intended to describe the structure of the district in 

order to provide a background for understanding the 

localization and origin of altered rocks and contained 

ore deposits, subjects to be dealt with in later reports 

of this series.

Major folds in the district trend northwesterly and 

are nearly parallel to the collapsed Gypsum Valley salt 

anticline which lies at the northeast edge of the district 

(fig. 2). The Dolores anticline is about 10 miles southwest of the Gypsum Valley anticline and is separated 

from it by the Disappointment syncline.

A fold identified as the Glade anticline has been indicated by some authors as extending into the Slick Rock 

district. Finley (1951) labeled a sinuous fold south of 

the district as the Glade anticline, but structure contours controlled by topography (Shawe and others, 

1968, pi. 1) do not substantiate such a fold. Instead, only 

a part of the anticline mapped by Finley is indicated, 

and this coincides with the position of the Dove Creek 

anticline. Kelley (1955, figs. 2, 8) showed the Dolores 

anticline extending both southeastward and eastsoutheastward from the vicinity of the Glade graben. 

The author and colleagues (Shawe and others, 1959, 

figs. 2, 5), in an earlier report on the Slick Rock district, designated a fold that extends southeastward from 

the center of the district as the Dolores anticline, and 

one that extends east-southeastward, as the Glade 

anticline. Because of the lack of evidence in the structure contour map for another fold besides the Dolores 

anticline, and the confusion concerning the position of 

the Glade anticline, the name should no longer be used 

to designate an anticline in this region.

A zone of faults bounds the southwest edge of the core 

of the Gypsum Valley anticline and thereby forms the 

northeast boundary of the district. The Dolores zone 

of faults lies farther southwest, parallel to, and about 2 

miles northeast of, the axis of the Dolores anticline. A 

few faults normal to the Dolores zone of faults in the 

northwestern part of the district constitute a conjugate 

set. The Glade zone of faults, which trends about N. 75° 

E., extends through the southern part of the district. 

The axis of the Dolores anticline is offset along the 

Glade zone of faults.

A major set of joints is oriented parallel to the principal faults in the district; a less prominent set approximately parallels the faults normal to the Dolores zone.

STRUCTURE IN THE PRECAMBRIAN BASEMENT

Geophysical data gathered by H. R. Joesting and 

coworkers (oral commun., 1956-57), and deep oil-test 

well information (Shawe and others, 1968, pi. 3), suggest that the general elevation of the basement underlying the Slick Rock district is about 4,000 feet below 

sea level. A major regional structural element in the 

basement rocks, probably a fault or fault zone with 

considerable dip-slip displacement, possibly several 

thousand feet, down on the northeast side, lies beneath, 

and parallel to, the northwesterly axis of the Disappointment syncline (Joesting and Case, 1960, fig. 114.2). 

The interpreted fault thus forms the southwest edge of 

a very large graben about 30 miles wide, the northeast 

edge of which is the fault zone bordering the Uncompahgre Plateau. On a map showing the configuration 

of the surface of Precambrian basement rocks based on 

deep drill-hole information (fig. 3), an abrupt northeasterly declivity indicates the approximate position of 

the interpreted fault bounding the large graben.

A narrow troughlike depression in the basement, 

oriented about normal to the large graben and passing 

under the south end of the district, probably reflects 

large basement faults or a fault zone (fig. 3). This zone 

appears to be a northeasterly continuation of a structural zone beneath the Blanding basin, Utah, west of 

Slick Rock, as interpreted from geophysical data by 

Case and Joesting (1961, p. D287-D291 and figs. 393.1, 

393.2). A basement fault zone inferred to bound the 

northwest side of the basement trough is discussed frequently in subsequent pages and, for convenience, is here 

termed the "Glade subsurface fault zone" (fig. 3).
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY

TABLE 1. Summary of consolidated sedimentary rocks in the Slick Rock district

C3

Age

Early Cretaceous

Jurassic and 

Triassic(?)

Late Triassic(?)

Late Triassic

Middle(?) and 

Early Triassic

Early Permian

Late and Middle 

Pennsylvanian

Middle Pennsylvanian

Early Pennsylvanian and 

Mississippian

Mississippian

Devonian

Cambrian

Precambrian

Formation and member

Mancos Shale

Dakota Sandstone

Burro Canyon Formation

Morrison Formation, Brushy 

Basin Member

Morrison Formation, Salt Wash 

Member

Junction Creek Sandstone

Summerville Formation

Entrada Sandstone, Slick Rock 

Member

Entrada Sandstone, Dewey 

Bridge Member

Navajo Sandstone

Kayenta Formation

Wingate Sandstone

Chinle Formation, Church 

Rock Member

Chinle Formation, Petrified 

Forest (?) Member

Chinle Formation, Moss Back 

Member.

Moenkopi Formation

Cutler Formation

Rico Formation

Hermosa Formation, upper 

limestone member

Hermosa Formation, Paradox 

Member

Hermosa Formation, lower 

limestone member

Molas Formation

Leadville Limestone

Name not assigned

Name not assigned

Thickness (feet)

1, 600-2, 300

120-180

40-400

300-700

275-400

20-150

80-160

70-120

20-35

0-420

160-200

200-400

340-500

0-100

20-75

0-200

1, 500-3, 000

130-240

1, 000-1, 800

3, 250-4, 850

100-150

100

240

250-550

500-700

Description

Dark-gray carbonaceous, calcareous shale.

Light-buff sandstone and conglomeratic sandstone, dark-gray 

carbonaceous shale, and coal.

Light-gray to light-buff sandstone and conglomeratic sandstone; greenish-gray and gray shale, siltstone, limestone, 

and chert.

Reddish-brown and greenish-gray mudstone, siltstone, sandstone, and conglomerate.

Light-reddish-brown, light-buff, and light-gray sandstone 

and reddish-brown mudstone.

Light-buff sandstone.

Reddish-brown siltstone and sandstone.

Light-buff to light-reddish-brown sandstone.

Reddish-brown silty sandstone.

Light-buff and light-reddish-brown sandstone.

Purplish-gray to purplish-red siltstone, sandstone, shale, 

mudstone, and congolmerate.

Light-buff and light-reddish-brown sandstone.

Reddish-brown, purplish-brown, and orangish-brown sandstone, siltstone, and mudstone; dark-greenish-gray conglomerate.

Greenish-gray mudstone, siltstone, shale, sandstone, and 

conglomerate.

Light-greenish-gray and gray sandstone and conglomerate; 

minor greenish-gray and reddish-brown mudstone, siltstone, and shale.

Light-reddish-brown siltstone and sandy siltstone.

Reddish-brown, orangish-brown, and light-buff sandstone, 

siltstone, mudstone, and shale.

Transitional between Cutler and Hermosa Formations.

Light- to dark-gray limestone; gray, greenish-gray, and 

reddish-gray shale and sandstone.

Upper and lower units gray dolomite, limestone, and darkgray shale interbedded with evaporites; middle unit 

halite and minor gypsum, anyhdrite, dolomite, limestone, 

and black shale.

Medium-gray limestone, dark-gray shale.

Reddish-brown, dark-gray, and greenish-gray shale and silty 

shale and gray limestone.

Medium-gray limestone and dolomite.

Gray sandy dolomite and limestone and grayish-green and 

reddish sandy shale.

Light-gray to pinkish conglomeratic sandstone, sandstone, 

siltstone, shale, and dolomite.

Granitic to amphibolitic gneisses and schists, and granite.
C4 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

109°

10 MILES 

I

FIGUBE 2. Structural map of the Slick Rock district (light stipple) and vicinity showing major folds 

and fault zones. Structure contours are on the base of the Dakota Sandstone; contour interval 250 

feet, dashed where projected above the surface. Data modified from P. L. Williams (1964; written 

commun., 1965). Heavy lines indicate inferred deep-seated shear faults; arrows show direction of 

inferred lateral slip.
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY C5

109

38" 

MONTEZUMA

20 MILES

FIQUEE 3. Summary of major thinning and thickening trends in some Paleozoic and Mesozoic sedimentary formations in and 

near the Slick Rock district (fine stipple) and their relation to inferred upper surface of Precambrian rocks (shown by 

structure contours) and Glade subsurface fault zone. Dots indicate oil-test wells used for control of structure contours. 

From Shawe, Simmons, and Archbold (1968, fig. 2).

380-111 70 2
C6 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

MAJOR FOLDS

The dominant structure in the Slick Rock district 

is the Dolores anticline (fig. 4). Its size and dominance 

is strikingly revealed by the canyon of the Dolores 

River, which is 2,000 feet deep where it cuts diagonally 

across the anticline and which exposes rocks as old as 

Permian in the core of the structure. The anticline is a 

broad, gently arched fold that extends southeasterly 

through the district for a distance of about 35 miles. The 

breadth of the anticline is more than 10 miles. It plunges 

both northwestward and southeastward from the vicinity of the Glade graben, and closure on the structure 

probably exceeds 500 feet (fig. 2). The axis of the anticline is poorly defined in the Dolores zone of faults at 

the west edge of the district, but the plunge continues 

northwesterly outside the district. Farther to the northwest the anticline rises to form another elongate dome, 

the Lisbon Valley anticline. To the southeast and outside the district, the Dolores anticline appears to merge 

with the broad domal uplift of the western San Juan 

Mountains.

The Dolores anticline is asymmetric; the southwest 

limb has a maximum dip of 2° in the area between 

Egnar and the Dolores River, whereas the northeast 

limb dips as much as 9° in the vicinity of Joe Davis 

Hill (Shawe and others, 1968, pi. 1). The axis of the 

Dolores anticline is somewhat sinuous, and trends about 

N. 45° W. in the north half of the Slick Rock district 

and about N. 45°-TO° W. in the south half.

An exceptionally thick section of salt, anhydrite, dolomite, and black shale in the Paradox Member of the 

Hermosa Formation of Pennsylvanian age underlies the 

Dolores anticline. Normal thickness of the Paradox 

Member southwest of the anticline under the Sage Plain

is about 2,000 feet (Finley, 1951), whereas the thickness under the axis of the anticline within the district 

exceeds 4,000 feet (Shawe and others, 1968, fig. 6). The 

anticline formed partly because evaporite beds of the 

Paradox intruded upward in a manner similar to the 

other salt anticlines of the Paradox Basin (such as 

nearby Gypsum Valley anticline). Unlike most of the 

other salt anticlines, no Paradox beds are exposed in 

the Dolores anticline, nor has the crest of the anticline 

collapsed as the result of extrusion and solution of evaporites as in most of the other salt anticlines (Prommel 

and Crum, 1927; Stokes and Phoenix, 1948).

The Disappointment syncline, a broad fold, but 

tighter than the Dolores anticline, extends almost 25 

miles parallel to the anticline within the district. The 

syncline is about 5 miles wide and forms a closed structural basin beneath Disappointment Valley. It plunges 

steeply from the northwest into the basin, and less 

steeply from the southeast. Northwestward, outside the 

district, a structural saddle separates the Disappointment syncline from the Coyote Wash syncline in Utah; 

to the southeast the syncline flattens, is complicated by 

faults and other folds, and merges with the broad structural dome of the western San Juan Mountains.

Like the Dolores anticline, the Disappointment syncline is asymmetric; the southwest limb dips 9° in the 

vicinity of Joe Davis Hill, whereas the northeast limb 

dips into the valley more steeply, as much as 32° northwest of Gypsum Gap (Shawe and others, 1968, pi. 1). 

The average trend of the axis of the Disappointment 

syncline in the Slick Rock district is about N. 55° W., 

but the syncline has sinuosities through its length nearly 

paralleling the zone of faults bounding Gypsum Valley. 

The axes of the syncline and the Dolores anticline diDISAPPOINTMENT SYNCLINE

San Miguel Lone 

Mountains Cone

Glade 

Mountain

fc #?" M v^jm^m^m^^^A mf:mmm-^^'j^iwS^Kfi&

¥&- vTii'j 

FIGURE 4. Panoramic view of the Slick Rock
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY C7

verge from about 5 miles apart in the northwest corner 

of the district to about 10 miles apart near the east edge 

of the district.

Most of the formations underlying the Disappointment syncline are thicker than in surrounding areas. 

For example, the aggregate thickness of strata from the 

base of the Kayenta Formation to the top of the Dakota Sandstone is about 2,500 feet under the syncline, 

whereas the aggregate thickness of the interval is only 

about 1,250 feet under the adjacent Dolores anticline.

The deepest part of the Disappointment synclinal 

basin, if the top of the Dakota Sandstone is used as a 

structure datum, lies near the east edge of the district. 

The highest part of the Dolores anticlinal dome lies 

almost due south, in the vicinity of the Glade graben. 

Maximum structural relief on the Dakota Sandstone 

between these two folds appears to be more than 3,000 

feet. At the horizon of the Kayenta Formation the 

structural relief is probably more than 4,000 feet.

AGE OF THE MAJOR FOLDS

Isopach maps of stratal units as old as the Molas 

Formation (Baars, 1965) give indications that folding 

or faulting occurred during the Devonian Period parallel to the present major folds in the district. Not until 

deposition of the Rico and Cutler Formations of Pennsylvanian and Permian age (Shawe and others, 1968, fig. 

10), however, did the position of the Disappointment 

syncline become clearly established as it is today. Permian rocks thicken in the Disappointment syncline to 

more than 3,000 feet (3,177 ft of Cutler Formation 

in Shell Oil Co. Gypsum Valley 2, sec. 16, T. 44 N., 

R. 18 W.; American Stratigraphic Co.). In postPermian time, very little folding of the Dolores anticline and Disappointment syncline took place until, or

DOLORES ANTICLINE

shortly after, the Navajo Sandstone was deposited, in 

Jurassic time.

The consistent thickening of almost all the sedimentary formations in the interval from Lower Jurassic to 

Upper Cretaceous in the basin of the Disappointment 

syncline indicates that post-Triassic folding was 

gradual. However, because the Navajo Sandstone, 

Brushy Basin Member of the Morrison Formation, and 

Burro Canyon Formation show relatively greater 

thickening compared to other formations, it is clear that 

the folding was accelerated during certain times in this 

long interval namely, early in the Jurassic and again 

at the close of Jurassic and start of Cretaceous time.

Near the end of Cretaceous time, following deposition 

of the Mancos Shale, at least 1,500 feet of structural 

relief had developed between the Dolores anticline and 

the Disappointment syncline at the top of the Salt Wash 

Member of the Morrison Formation, and probably at 

least 2,000 feet at the top of the Moss Back Member 

of the Chinle Formation. These figures will be useful 

in later discussions of the origin of ore deposits in the 

Salt Wash and Moss Back Members which took place 

probably in early to middle Tertiary time.

No direct evidence remains of early Tertiary folding 

of the Dolores anticline and Disappointment syncline. 

Appreciable folding possibly took place again just before or during the general uplift of the Colorado 

Plateau in middle Tertiary time.

Considerable arching of the Dolores anticline has 

occurred since the early Pleistocene. This arching is 

suggested by the fact that the present grade of the 

youngest terrace gravels in Disappointment Valley 

parallel to the anticline is about 40 feet per mile (Shawe 

and others, 1968, fig. 47), which is an average grade 

for untilted youngest deposits, and the grade of gravels

S-i?MO ?£>,* . y *)&&& ^t«^MPWL^Wfe?&

district southwestward from Horse Range Mesa.
C8 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

between the Dolores River and Glade Mountain normal 

to the anticline is about 150 feet per mile (their fig. 48), 

which is a typical grade for tilted lower Pleistocene 

gravels on the limb of the fold. The amount of uplift 

near the axis of the Dolores anticline at Glade Mountain was indicated by Shawe, Simmons, and Archbold 

(1968, fig. 48) to be about 1,000 feet. However, F. W. 

Cater (written commun., 1968) described the highest 

(oldest) terrace gravels on the southwest flank of Disappointment Valley as untilted; this fact raises some 

doubt that in this place much late Pleistocene folding 

occurred. Appreciable folding probably took place in 

early Pleistocene, or even in the latest Tertiary, and 

minor uplift of the anticline could have occurred during the Holocene. Structural relief on the Dakota 

Sandstone from synclinal to anticlinal axes at the end 

of the Cretaceous Period was nearly 1,000 feet; it is 

now about 3,000 feet after possibly 1,000 feet of 

Pleistocene folding, which suggests that the period of 

plateau uplift during the middle Tertiary must have 

involved about 1,000 feet of folding.

MINOR FOLDS

Several small folds, generally no more than 1 or 2 

miles in length, lie along the northeast edge of the Slick 

Rock district (Shawe and others, 1968, pi. 1; Cater, 

1955a-d). For about 12 miles between Gypsum Gap 

and Steamboat Hill, a series of small anticlines lies 

just below the rim of Gypsum Valley. These anticlines 

are relatively sharp folds immediately adjacent to the 

border faults at the southwest side of the Gypsum 

Valley collapsed anticline. They have the appearance of 

large drag folds against the border faults, but their 

structural development was probably more complex. 

Actual removal of evaporite beds by flowage from the 

underlying Paradox Member of the Hermosa Formation, which was partly responsible for the collapse of 

the Gypsum Valley anticline, also may have been responsible for the sharp sag of the beds abutting the 

collapsed anticline. The small folds thus may have 

developed as a result of initial rise of an evaporite core 

in the axis of the Gypsum Valley anticline and by 

subsequent sag as evaporites moved at depth axially 

into cupolas or were removed by solution.

In places along the rim of Gypsum Valley, tightly 

folded small synclines adjoin the small anticlines 

(Shawe and others, 1968, pi. 1: Cater, 1955a-d). Generally, these synclines are northeast of the anticlines and 

are outside the Slick Rock district. In a few places the 

northeast limb of a syncline passes into a second small 

anticline. The synclinal folds may have resulted from

renewal of the rise of evaporites in the core of the Gypsum Valley anticline. The northeast limb of a small 

anticline at the edge of the Gypsum Valley graben may 

have been folded upward by renewed rise of the evaporite beds, and an adjacent synclinal trough may have developed. Locally, for example, just northeast of Cape 

Horn, a relatively tight small anticline has formed 

northeast of a small syncline (Shawe and others, 1968, 

pi. 1). A second collapse possibly took place here because of renewed transfer of underlying evaporites into 

cupolas along the main axis of the Gypsum Valley 

anticline.

AGE OF THE MINOR FOLDS

The minor folds are chiefly evident in beds of the 

Morrison Formation and hence are post-Morrison in 

age. By their form and position the minor folds appear 

to have developed during the main period of collapse of 

the Gypsum Valley anticline. The youngest rocks involved in the Gypsum Valley collapse are part of the 

Mesaverde Formation of latest Cretaceous age (Cater, 

1955a), indicating that collapse occurred during the 

Cenozoic. Although no direct evidence points to the time 

during the Cenozoic that the major collapse occurred, 

Stokes and Phoenix (1948) and Cater (1955a-d) indicated that it occurred principally near the end of the 

Laramide orogeny, at the beginning of Tertiary time. 

More likely, the major collapse and development of the 

minor folds took place in the middle to late Tertiary, 

or possibly even Pleistocene, shortly after the last major 

uplift of the Dolores anticline.

MAJOR FAULTS

A zone of faults trending N. 45°-60° W., and bounding the southwest edge of the core of the Gypsum Valley anticline, lies along the northeast edge of the Slick 

Rock district. The Dolores zone of faults is 7 miles 

southwest of the Gypsum Valley border faults and 

about 2 miles northeast of, and parallel to, the axis of 

the Dolores anticline (N. 55° W.). Individual faults in 

the zone strike N. 60°-85° W. and form a series of small 

en echelon grabens. The zone extends northwestward 

to merge with the Lisbon Valley fault in Utah. The 

southern part of the district is cut by the Glade zone 

of faults, which trends about N. 80° E. and extends westward into the Verdure graben in Utah. Individual 

faults in the zone strike about N. 60°-70° W.; depression of a long narrow block between two of these faults 

has formed the Glade graben. The west-northwest trend 

of almost all the major faults within and bounding the 

Slick Rock district is notable.
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY C9

FAULTS BOUNDING GYPSUM VALLEY

The zone of faults at the northeastern boundary of 

the district consists of several nearly parallel somewhat 

sinuous faults. Individual faults are no longer than 

about 4 miles, and they die out along the zone, giving 

way to nearly parallel faults slightly offset on strike 

(Cater, 1955a-d). Several small faults in the zone are 

not shown on the quadrangle maps by Cater (1955a-d), 

nor on the map of the Slick Kock district (Shawe and 

others, 1968, pi. 1). The faults, mapped and unmapped, 

are mostly high angle and appear to be normal, with 

the Gypsum Valley or northeast side down. None of the 

faults has had displacement of more than about 500 feet; 

where strata younger than those forming the rim of 

Gypsum Valley have collapsed into the core of the anticline as much as a few thousand feet, the displacement 

was accomplished by step faulting on sets of nearly parallel faults and by flexing. A few faults show a relative 

upward displacement on the valley side of the fault, 

which resulted in the formation of small horsts. These 

faults probably reflect local upward growth of the salt 

core of the anticline in cupolas, and thus they originated 

before the principal collapse of the structure.

In the vicinity of the Grassy Hills at the north end 

of the district (Shawe and others, 1968, pi. 1), intensive 

alteration has occurred along faults that show upward 

displacement of the Gypsum Valley side of the fault. In 

addition to bleaching of rocks, considerable iron staining 

has taken place near the faults. Similar alteration is associated with other faults in the zone along the southwest edge of the Gypsum Valley anticline. Some of the 

faults in the zone, however, do not appear to be bounded 

by altered rocks. This fact suggests two general periods 

of faulting the first before a period of alteration and 

the second after it. By inference, then, uplift of the core 

of the anticline was accompanied by faulting along the 

margin, followed by passage of altering solutions along 

the marginal fault zone. Finally, collapse of the core of 

the anticline was accompanied by a second period of 

faulting, probably with renewed movements on many of 

the older faults, but also with the formation of new 

faults.

AGE OF THE GYPSUM VALLEY FAULTS

Some faults bounding the core of the Gypsum Valley 

anticline, not shown on the quadrangle maps and sections by Cater (1955a-d) but likely separating the Paradox Member of the Hermosa Formation of Pennsylvanian age from younger rocks, were formed before the 

deposition of Mesozoic rocks. The faults that are shown 

on Cater's maps must generally be of Cretaceous or 

younger age, for many of them, including those bounded 

by altered rocks, displace strata of Late Cretaceous age.

Time of this faulting may have been principally during 

the middle of the Tertiary Period and also during 

Pleistocene time. If principal growth of the salt anticlines was controlled fundamentally by regional compression, which seems likely and which is elaborated on 

later in this report, most of the faulting along the Gypsum Valley anticline probably coincided in time with 

periods of accelerated growth of the nearby Dolores 

anticline inferred as middle Tertiary and Pleistocene. 

In view of the flimsy basis for dating, however, the possibilities of faulting at other, perhaps numerous, intervals throughout the Cenozoic should be admitted.

DOLORES ZONE OF FAULTS

A zone of faults extending along the Dolores anticline in the northern part of the district was named the 

"Dolores fault zone" (Shawe and others, 1959, p. 397). 

Individual faults in the zone are separated in places as 

much as several hundred feet, however, so that "zone of 

faults" seems to be a more appropriate terminology than 

"fault zone." The faults are less than a mile to a few 

miles long, and chiefly bound small grabens disposed en 

echelon as if reflecting a northwesterly left-lateral shear. 

Displacement on the faults is generally slight. Only near 

the west boundary of the district is it appreciable, where 

one fault near the Empire group of mines has offset 

strata about 700 feet (Shawe and others, 1968, pi. 1). 

Farther northwest, in Utah, the Lisbon Valley fault, 

which is an extension of the Dolores zone of faults, has a 

maximum stratigraphic displacement of about 5,000 feet 

(Weir and Puffett, 1960, p. 134). Faults in the Dolores 

zone northwest of the town of Slick Kock are oriented 

about UT. 70°-85° W., whereas those farther southeast 

trend about 1ST. 55° W. and are more nearly parallel to the 

zone as a whole. A pronounced zone of east-striking 

joints, discussed in greater detail in the section on 

"Joints," separates the two groups of faults.

Faults on Slick Kock Hill about 3 miles southwest of 

the town of Slick Kock show successively less displacement upward in younger rocks and do not cut the youngest strata exposed (Shawe and others, 1968, pis. 1, 7). 

The lower part of the Morrison Formation thus locally 

"bridges" small brabens that displace the underlying 

Summerville Formation and Entrada Sandstone. Perhaps the faults merely flatten and die out upward so 

that deformation of higher strata consisted only of 

slight warping; or possibly faulting was taking place 

just before the Morrison deposition, but it had ceased 

by the end of Morrison deposition. The first explanation 

seems more logical, because some faults in the Dolores 

fault zone cut strata younger than the Morrison 

Formation.
CIO GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

AGE OF THE DOLORES ZONE OF FAULTS

Again, direct evidence of the age of the faulting in 

the Dolores zone of faults is absent. The progressive 

downward increase in offset of Upper Jurassic strata 

along some faults suggests, but does not prove, that 

fracturing in the zone commenced in middle Mesozoic 

time. Conversely, some of the largest faults offset strata 

as young as Early Cretaceous, and thus probably are 

at least as young as Late Cretaceous. In several places 

near the west edge of the district, faults are concealed 

by unf aulted landslide debris that is most likely of late 

Pleistocene age. Probably, the Dolores fault zone is 

latest Cretaceous or Tertiary in age, and, if related to 

accelerated growth of the Dolores anticline, is probably 

early to middle Tertiary in age. Like the faults along 

the collapsed Gypsum Valley anticline, those in the 

Dolores zone of faults could be of more than one age, 

or they may have been reactivated during more than 

one period of deformation. Unlike the Gypsum Valley 

faults, however, none of those in the Dolores zone of 

faults appear to be entirely devoid of alteration effects, 

showing that the zone developed largely before the 

period of alteration.

GLADE ZONE OF FAULTS

The Glade zone of faults takes its name from a 

prominent topographic feature The Glade in the 

southern part of the district, which is the surface expression of the Glade graben. The zone extends from 

southeast of the Disappointment syncline westward 

through the district and into Utah (fig. 2). The Verdure 

graben, at the south edge of the Aba jo Mountains (10- 

15 miles west of the area shown in fig. 2), forms the 

west end of the zone. Although the orientation of the 

Glade zone of faults as a whole is about N". 80° E., 

individual faults within the zone strike between N". 55° 

W. and east, nearly parallel to individual faults along 

Gypsum Valley and in the Dolores zone of faults. 

Faults in the Glade zone are disposed en echelon, as 

though underlain by a right-lateral shear that is, they 

show a gash-fracture relationship to an underlying 

shear, stepping to the left as a result of right-lateral 

shear.

Probably no more than about 300 feet of vertical 

movement has occurred on any of the faults in the zone, 

the largest, perhaps, in the Glade graben just east of 

the Dolores River canyon; most faults have had much 

less displacement. Although the Glade graben itself 

may seem to be a manifestation of incipient collapse 

of the Dolores anticline resulting from movement or 

dissolution of underlying salt in the core of the anticline, the graben appears not to be different from others 

in the en echelon zone. Formation of the other grabens

more likely resulted from tension related to the inferred 

right-lateral displacement of the zone than from collapse over moving salt.

Faults in the Glade zone were examined in detail only 

at the Glade graben. There, most rocks that ordinarily 

show reddish colors have been bleached in the vicinity 

of the faults, indicating that faulting occurred before 

the period of structurally controlled alteration. In addition, faults and joints that bound the graben, and the 

rocks close to them, locally are heavily filled with 

hydrated iron oxides that may be alteration products 

of pyrite deposited during the period of alteration.

AGE OF THE GLADE ZONE OF FAULTS

Precise dating of the Glade zone of faults, like 

the other faults in the district, is impossible. The 

Glade zone is at least older than the main period of 

alteration and younger than the Mancos Shale of Late 

Cretaceous age which is cut by faults in the zone. An 

approximate minimum age of the zone is provided by 

relationships at the south edge of the Abajo Mountains 

in Utah, where the Verdure graben at the west end of 

the Glade zone controlled emplacement of igneous 

rocks. At that place the faults existed before intrusion 

(Witkind, 1964, p. 46), as shown by the presence of 

a laccolith that was intruded within the graben and 

confined by the bounding faults and by the presence of 

igneous dikes that lie along the bounding faults. The 

faults displace the Mancos Shale, indicating that faulting occurred at some time between the Late Cretaceous 

and the period of igneous activity in the Abajo Mountains. Witkind (1964, p. 81) considered the most likely 

age of the igneous rocks to be Late Cretaceous or 

Eocene. The Abajo Mountains rocks are petrographically similar to, and likely the same age as, those in the 

La Sal Mountains, which are estimated on the basis of 

isotope and potassium-argon data to be Miocene to possibly Eocene (23-55 m.y.) in age (Stern and others, 

1965). The faulting probably occurred at the end of the 

Cretaceous Period or in the early part of the Tertiary.

MINOR FAULTS

A few faults of slight displacement lie in a northeasterly zone about normal to the Dolores zone of faults, 

passing near the town of Slick Rock. One fault is in 

Summit Canyon, two lie in the Lower Group of mining 

claims, and a few more are at the northeast edge of the 

district near Cape Horn (Shawe and others, 1968, pi. 

1). No alteration was noted along any of these faults. 

Those in the Lower Group clearly have offset ore bodies 

in a uranium-vanadium deposit, and thus appear to be 

younger than the period of alteration and ore forma-
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY Oil

tion. They may correlate in age with postalteration 

faults in the zone of faults bordering Gypsum Valley.

JOINTS

Joints were mapped in detail in the northern part of 

the Slick Kock district, in the areas outlined in figure 5, 

mostly by W. B. Kogers, of the U.S. Geological Survey. 

Kather uniformly distributed joints occur in all rocks 

exposed throughout the district, although those in mudstone units are not well defined and therefore were not 

mapped. Soil cover also locally prevented mapping of 

joints.

Different types of sandstone units display somewhat 

different joint patterns. Massive units, such as the 

Entrada and Navajo Sandstones, may be fractured in 

zones separated by fracture-free areas as much as several 

hundred feet wide. Thinner bedded units, such as the 

Salt Wash Member, have fractures more regularly 

spaced; areas of unf ractured rock are rarely more than 

30 feet wide. Where beds of sandstone are separated by 

interbedded mudstone, the fractures in different sandstone beds are not continuous vertically but are interrupted by the mudstone beds, although they do maintain 

similar orientation in the separate beds.

Joint patterns in the district seem to be related to 

faults but not to major folds. Within 1.5 miles of the 

Dolores zone of faults on the northeast side, and within 

4 miles on the southwest side, most of the joints strike 

between N. 45° W. and east, and in detail are parallel 

to individual faults; but differently oriented joints in 

this area also occur. Southwest of the town of Slick 

Kock a minor set of joints is oriented nearly north, 

whereas farther southeast a minor set has an orientation 

of about N. 45° E. Most joints farther than 1.5 miles 

northeast of the Dolores zone of faults are oriented 

nearly N. 20° E.; other orientations are common, but 

no other maxima are apparent. Joint patterns exhibit 

a change in orientation across a line, southwest of the 

town of Slick Kock, separating faults that trend about 

K 60° W. and those that trend about N. 80° W. South 

of this line the orientation of the major joint set swings 

from easterly in the vicinity of Summit Canyon to 

southeasterly near the Dolores Kiver canyon. North of 

this line the orientation of the major set swings from 

easterly west of Slick Rock to northeasterly north of 

Slick Kock. Possibly this difference in structural grain 

in two adjacent areas is related to an unrecognized 

deep-lying structure which separates the two areas and 

which is partly responsible for the minor set of northeasterly faults at the surface.

In general, none of the joint patterns appear to be 

oriented in a manner suggesting that conjugate sheer 

fractures formed from the compression of the folds.

If joints mapped in different formations are plotted 

on radial diagrams, they show different dominant orientations. For example, the dominant orientation of joints 

in the Burro Canyon Formation is about N. 20° E. (fig. 

6), and in the Salt Wash Member of the Morrison 

Formation, almost east (fig. 7). This apparent difference probably can be explained by the fact that most 

joints mapped in the Salt Wash Member are closer to 

the Dolores zone or faults than those mapped in the 

Burro Canyon Formation.

A few joints in sandstone of the Burro Canyon Formation on the northeast side of the Disappointment 

syncline strike northwest and dip at a low angle to the 

southwest. Surfaces of these joints are locally iron 

stained.

AGE OF THE JOINTS

Some joints are bounded by altered rocks, proving 

that they formed before the period of alteration. Generally, joints oriented nearly parallel to the faults have 

influenced alteration, and probably these joints were 

formed at the time of faulting, near the end of the Cretaceous Period or at the beginning of the Tertiary 

Period. By analogy with the faults, the joints which are 

oriented nearly normal to the Dolores zone of faults 

and which did not localize alteration probably postdate 

the period of alteration and may be of middle to late 

Tertiary and Quaternary age.

Low-angle joints on the northeast side of the Disappointment syncline strike parallel to major folds and 

may have formed during folding. They are iron stained 

and thus apparently are older than the period of 

alteration.

Sandstone dikelets in the Navajo Sandstone along 

fractures striking about N. 30° W. in the area of the 

Dolores zone of faults may have formed when the Navajo was only partly consolidated, indicating that some 

fracturing occurred early in Jurassic time.

STRUCTURAL INTERPRETATION OF 

VARIATIONS IN FORMATION THICKNESSES

All the rocks of presumed Cambrian age thin toward 

the south and east (Shawe and others, 1968, p. A12), 

suggesting that the ancestral Zuni and Uncompahgre 

uplifts to the south and east, respectively, were structurally high during the early part of the Paleozoic Era.

Differences in thickness of Upper Devonian and Missippian rocks throughout the Paradox basin (Baars, 

1965; Shawe and others, 1968, p. A12-A14) reflect 

faulting and possibly folding during this period of 

deposition. Position and orientation of the salt anticlines 

are related to these thickness differences, which suggests that deformation commencing at least as early as
C12 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

MONTROSE

EXPLANATION

Rocks of Jurassic age and older

Morrison Formation of Jurassic age

Fault

Nearly vertical joint 

Outline of main areas in which joints were mapped

FIGUBE 5. Map showing fracture patterns in parts of the Slick Rock district, Colorado. Joints mapped by W. B. Rogers and 

minor additions by N. L. Archbold, D. R. Shawe, and E. L. Boudette, 1954-56.
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY C13

w -

FIGURE 6. Diagram showing orientation of 167 nearly vertical 

joints in the Burro Canyon Formation.

W -

FIGURE 7. Diagram showing orientation of 203 nearly vertical 

joints in the Salt Wash Member of the Morrison Formation.

Late Devonian time eventually controlled formation of 

the salt anticlines.

Trends of thickening and thinning in the Molas Formation and in the lower unit of the Paradox Member of 

the Hennosa Formation extending southwestward from

the district (fig. 3) lie in or near a zone of deformation the northeasterly trough in the Precambrian 

surface initiated perhaps during Early Pennsylvanian 

time.

Salt thickening along anticlinal axes in the Paradox 

fold and fault belt has been tihought by some workers 

to have resulted almost wholly from upward and lateral 

flow of salt of the Paradox Member of the Hennosa 

Formation induced by differences in specific gravity between salt and surrounding rocks (Stokes and Phoenix, 

1948). However, the irregular base of the salt unit suggests that changes in the level of basement rocks, by 

faulting or perhaps by folding, may have been in part 

responsible for differences in thickness of the salt unit. 

Shawe, Simmons, and Archbold (1968, pi. 3, diagrams 

A, B, and E) showed that the base of the salt and, by 

inference, the top of the Preeambrian basement complex are generally irregular. The base of the salt was 

likely virtually flat at the beginning of salt deposition. 

Now, however, where the base of the salt lies deeper, the 

salt is thicker and also rises higher than in surrounding 

areas, suggesting that salt thickening was directly related to local subsidence of the basement rocks during 

and after deposition.

Virtually all tfhe stratigraphic units above the salt 

unit of the Paradox Member of the Hermosa Formation are thin above the Dolores anticline and thick in 

the Disappointment syncline, and are thin or pinched 

out around the Gypsum Valley anticline (Shawe and 

others, 1968, figs. 7, 8, 10,12, 14,15,17, 26, 38, 40, 43). 

Folding, although undoubtedly sporadic, occurred 

throughout the time interval of Late Pennsylvanian 

Late Cretaceous.

The northwesterly alinement of the isolated area of 

the Eico Formation in the vicinity of the Slick Rock 

district (Shawe and others, 1968, p. A22) leads to the 

interpretation that folding was responsible for the development of a local basin in which Rico rocks were 

deposited. Perhaps the occurrence of areas of presumed 

Rico Formation of different ages throughout the Paradox Basin and the difficulty in demonstrating physical 

continuity between these strata in different areas suggest that the Rico formed in isolated basins within the 

general area of Cutler Formation deposition (Shawe, 

and others, 1968, p. A22, A25) and that the basins did 

not form contemporaneously.

Thickness variations in a number of stratigraphic 

units of Late Pennsylvanian and younger age indicate 

periodic deformation along the northeasterly structural 

zone that passes through the south end of the Slick Rock 

district. Marked thinning of the upper unit of the 

Paradox Member of the Hermosa Formation and the 

Kayenta Formation is evident southeastward along a
C14 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

"hinge line" parallel to, and a few miles southeast of, 

the northeasterly Glade subsurface fault zone (Shawe 

and others, 1968, figs. 7, 15). Trends of thickening of 

the upper unit of the Paradox Member of the Hermosa 

Formation, Cutler Formation, Moenkopi Formation, 

and "Wingate Sandstone are alined with the zone (fig. 

3). In addition, the Summerville Formation and Junction Creek Sandstone thicken southeastward from the 

northeasterly zone (Shawe and others, 1968, fig. 26, pi. 6, 

and p. A50).

Some evidence indicates that deformation of the 

northeasterly zone and folding of the Dolores salt anticline occurred simultaneously during certain times in 

the past. Trends of thickening of the Cutler Formation 

coincide with those of both the Disappointment syncline 

and the northeasterly structural zone (Shawe and 

others, 1968, fig. 10), proving that folding and deformation of the zone were concurrent. A local trend of thinning of the Chinle Formation occurs on the north side 

of the northeasterly trough in the Precambrian surface, 

and it coincides with an area where beds in the upper 

part of the Chinle were truncated by erosion before 

deposition of Wingate Sandstone (Shawe and others, 

1968, fig. 12, and p. A31). Uplift and erosion occurred 

concurrently along the Gypsum Valley anticline, as suggested by thinning and pinchout toward Gypsum Valley 

of the Chinle below the Wingate (Cater, 1955a, c, d). 

The Wingate Sandstone shows similar relations (Shawe 

and others, 1968, fig. 14 and p. A35).

The flow of salt related to thickness variations of the 

Cutler Formation underlying the Disappointment syncline and adjacent anticlines probably did not take place 

as a result of overloading by Cutler detritus. According 

to F. W. Cater (written commun., 1960), salt rock under 

the physical conditions existing in the Paradox Member, 

confined under about 1,500 feet of the Hermosa Formation and an additional 2,000 feet of Cutler, probably did 

not flow plastically purely through load compression. 

Cater suggested rather that tectonic stresses were likely 

important in instigating confined flow of the salt, which 

accords with the evidence in the vicinity of the Slick 

Eock district. Kelley (1955, p. 74) and Hite (1968, p. 

329) made similar suggestions.

Tremendous thickening of the Cutler adjacent to the 

Uncompahgre Plateau shows that the highland probably rose rapidly above the surrounding land. Possibly, 

the basin of Cutler accumulation sank as much as the 

highland rose, but even so, the sediments were not deposited under water. Deposition probably took place 

above land, just as detritus from the Sierra Nevada is 

being deposited subaerially in the California Valley 

today, although the base of the sediments has subsided 

thousands of feet below sea level.

The Moenkopi Formation was deposited upon an 

erosional surface of the Cutler Formation, and in turn 

the Chinle Formation was deposited upon an erosional 

surface of the Cutler and Moenkopi Formations. The 

oldest members of the Chinle Formation were deposited 

in western and southern Utah and in northern Arizona, 

and in a general way successively younger members 

were spread farther northeast, so that younger members 

are in contact with the underlying Moenkopi in that 

direction (Stewart and others, 1959, figs. 73, 81). These 

relations indicate development of a large depositional 

basin centered southwest of the Slick Eock district; this 

basin dominated sedimentation throughout much of the 

Mesozoic Era.

Before deposition of the Upper Jurassic Dewey 

Bridge Member of the Entrada Sandstone, a long period 

of erosion of the Navajo ensued from the Early or possibly Middle Jurassic. Thickness differences hi the Navajo 

show that the Dolores anticline was raised in some places 

as much as 400 feet during this time, while erosion kept 

pace more or less with uplift. The resulting unconformity is widely recognized throughout the Colorado 

Plateau, and eastward from the Slick Eock district it 

is manifested by successively older formations beneath 

the Dewey Bridge Member (earthy siltstone member 

of the Entrada; Wright and Dickey, 1958, fig. 2). This 

transgression of strata indicates differential uplift of 

the region east of the district, before Entrada 

deposition.

The Junction Creek Sandstone and correlative units 

considered in their entirety (that is, the Junction Creek, 

the Bluff Sandstone in southeastern Utah, and the Cow 

Springs Sandstone in northeastern Arizona, Harshbarger and others, 1957, p. 48) probably represent development of a Late Jurassic basin in the general area lying 

between the south end of the Slick Eock district and 

the San Juan Mountains and extending into southeastern Utah and northeastern Arizona.

The Salt Wash Member of the Morrison Formation 

generally thickens to the southwest toward its source 

area in Arizona and southern California, but it is locally thick in the vicinity of the southeast end of the 

Paradox Basin. It is relatively thin above the Dolores 

and Gypsum Valley anticlines. During deposition of 

the lower and middle units of the Salt Wash Member, 

the Paradox Basin subsided; subsidence was accompanied by minor growth of the salt anticlines resulting 

in folding. During deposition of the ore-bearing sandstone, subsidence continued, perhaps more rapidly but 

without folding, so that a large fan or apron of reworked and winnowed sand was built eastward from 

the west end of the subsiding basin (Shawe, 1962, figs. 

62.1, 62.2). The abundant fluvial sandstone and car-
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY C15

bonaceous material at the toe of the apron in the vicinity 

of Slick Rock and the rest of the Uravan mineral belt 

provided a favorable lithologic environment that later 

helped localize the uranium-vanadium deposits.

The area of thinnest Salt Wash in the Slick Eock 

district clearly coincides with the axial region of the 

Dolores anticline, and is interpreted as an indication 

of the continuing folding of the region during Salt 

Wash time. Folding took place at the time of Salt Wash 

deposition inasmuch as the lower unit of the Salt Wash 

reflects the influence of the rising anticline and the 

middle unit indicates at least local uplift of the anticline. Probably no appreciable beveling by erosion of 

the Salt Wash took place before the Brushy Basin was 

deposited, for the relatively thin ore-bearing sandstone 

is everywhere present in the district, indicating that 

variations in thickness did not result from post-Salt 

Wash, pre-Brushy Basin folding. Distribution of sandstone-to-mudstone ratios in the Salt Wash was not influenced by the folding (Shawe and others, 1968, p. 

A52). The lack of correspondence of cumulative thinner 

sandstone with the axis of the anticline and cumulatively thicker sandstone with the axis of the syncline 

suggests that local folding at the time of Salt Wash 

deposition was not an important control of stream locations and consequently was not an important control of 

detritus distribution.

As interpreted from the regional configuration of the 

Salt Wash, the southeastern part of the Paradox Basin 

region was subsiding slightly during Salt Wash deposition, and hence subsidence corresponded in time with 

folding of the salt anticlines. A reasonable inference is 

that folding was causally related to basin subsidence.

During deposition of the Brushy Basin Member of 

the Morrison Formation, the area just south of Gypsum 

Valley at the boundary of Es. IT and 18 W., T. 44 N., 

(Shawe and others, 1968, fig. 40), appears to have been 

sinking more rapidly than surrounding areas. The 

greater thickness here of the Brushy Basin probably 

resulted from subsidence as evaporites flowed into the 

adjacent Gypsum Valley anticline. The local nature of 

the subsidence strongly suggests that the evaporite core 

of the Gypsum Valley anticline was growing largely 

at a salt cupola rather than throughout its whole length. 

The suggested cupola coincides in position with the 

large intrusive mass of the Paradox Member of the 

Hermosa Formation in T. 44 N., E. 17 W., and the NW 

cor. T. 43 N., E. 16 W., shown on the Hamm Canyon 

and Gypsum Gap geologic quadrangle maps by Cater 

1955a, c). Possibly the thinner areas of Brushy Basin, 

about a mile west-southwest of the town of Slick Eock 

and in T. 42 N., and the SE cor. T. 43 N., E. 18 W.,

reflect minor growth of cupolas on the Dolores anticline (Shawe and others, 1968, fig. 40).

A great buildup of sandstone in the Brushy Basin 

Member near the axis of the Disappointment syncline 

(shown near the center of geologic section B, Shawe 

and others, 1968, pi. 12) may indicate that downfolding during deposition of the member controlled the 

main courses of streams. Fluvial deposition of coarser 

elastics dominated in the more rapidly sinking part, 

and flood-plain deposition of finer material occurred 

peripheral to this area.

The isopach map of the Brushy Basin published by 

Craig and others (1955, fig. 29) shows that the member 

is regionally thickest in two main areas. One area lies 

in the northern part of the Colorado Plateau, and the 

other occupies approximately the area of the Paradox 

Basin. The area of thick Brushy Basin coinciding with 

the Paradox Basin is significant here because it records 

a continuation of downwarping that occurred during 

Salt Wash time and because it was probably partly responsible for the continued folding of the salt anticlines. The Gypsum Valley anticline grew only locally 

in well-defined cupolas, but movement of salt may have 

been initiated by tectonism (compression?) that accompanied subsidence of the Paradox Basin.

The thicker part of the Brushy Basin Member in the 

area of the Paradox Basin thins to the southeast along 

a northeasterly axis (Craig and others, 1955, fig. 29). 

The axis coincides in position with the Glade subsurface 

fault zone. Again, faulting along this zone, depression 

of the Paradox Basin, and folding of the salt anticlines 

appear to have occurred simultaneously; therefore these 

three structural features may have been causally related.

Because the thickness variations of the Burro Canyon 

reflect those of the underlying Brushy Basin, their structural interpretation is similar, with an important exception. During the time Burro Canyon strata were 

being deposited, the Disappointment syncline deepened, 

probably as salt in the underlying Paradox flowed into 

a large salt cell in the Gypsum Valley anticline and to 

a lesser extent into a cupola on the Dolores anticline 

a few miles east of Egnar. Nevertheless, much of the 

thinning of the Burro Canyon away from Disappointment Valley must have occurred after deposition of the 

formation, for the Dakota was deposited unconformably 

on the Burro Canyon in this peripheral area and 

elsewhere.

The intertonguing between the Burro Canyon and 

the Brushy Basin as well as the similarity in sedimentation types in the lower part of the Burro Canyon and 

in the Brushy Basin hints that the depositional environment continued relatively unchanged from Brushy
C16 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

Basin into Burro Canyon time. But the erosion of large 

areas of Burro Canyon, either late in Burro Canyon time 

or during the post-Burro Canyon pre-Dakota time interval indicates that Paradox Basin subsidence ceased 

near the end of Burro Canyon time. However, salt flow, 

having been induced by the tectonism, continued as a result of differential load stresses.

The lithologic character of rocks in the upper part 

of the Burro Canyon Formation indicates marginal or 

near-shore marine conditions, hardly compatible with 

elimination of the Paradox Basin. A suggestion of the 

effect of the Paradox Basin on deposition of Lower 

Cretaceous rocks is shown on an isopach map of strata 

of this interval in the Western United States by Eeeside 

(1944). At the northeast boundary of a large area where 

Lower Cretaceous rocks are absent, a local extension of 

the rocks westward suggests that the Paradox Basin 

may have actively subsided following deposition of 

Lower Cretaceous strata.

The thicker part of the Dakota Sandstone almost 

certainly was deposited in a depression resulting from 

continued flow of evaporites from the axial region of 

the Disappointment syncline into the large salt cell occupying a segment of the Gypsum Valley anticline just 

to the northeast. This local movement of salt was already 

established at least as early as deposition of the Brushy 

Basin Member of the Morrison Formation, and it continued during deposition of the Burro Canyon Formation. Even though the salt flow must have been 

more or less continuous from Brushy Basin into 

Dakota time, it was likely interrupted after the Burro 

Canyon was deposited and before the Dakota was laid 

down. The evidence for this interruption is the widespread disconformity between those two formations and 

the abrupt change in lithology from the Burro Canyon 

to the Dakota, even in the Disappointment Valley area. 

Even so, conceivably, salt movement continued while the 

general land surface rose following deposition of the 

Burro Canyon Formation. General erosion prevailed 

and accounted for the widespread disconf ormity; but in 

the Disappointment Valley area, subsidence due to salt 

movement kept pace with the general uplift of the region, accounting for the preservation of most of the 

original Burro Canyon sediments in the synclinal basin 

and the generally conformable contact here between 

the Burro Canyon and the Dakota.

The Mancos Shale, like many older formations, shows 

by its thickness variations the influence of salt movement related to the growth of the salt anticlines. On the 

basis of the two known complete sections of the formation (Shawe and others, 1968, p. A91-A92), about 700 

feet of structural relief developed on the base of the 

Mancos during Mancos time, between the axis of the

Gypsum Valley anticline and the axis of the Disappointment syncline. This amount is probably greater than the 

structural relief formed above the Dolores anticline, for 

the Gypsum Valley structure has been actively intruded 

by evaporites all through its history, which was not 

true for the Dolores anticline. The thickness variations 

in the unit of late middle Greenhorn age and those 

shown in higher strata in section A-A' (Shawe and 

others, 1968, pi. 1) suggest that the Disappointment 

synclinal axis was closer to the Gypsum Valley anticlinal axis in Late Cretaceous time than it is now, and 

possibly was alined more westerly than at present. The 

same relationships are shown for Dakota time, as indicated on the isopach map for that formation (Shawe 

and others, 1968, fig. 43). The fact that the synclinal 

axis lay rather close to the Gypsum Valley structure 

corroborates the theory that evaporite flow then was 

more active into the Gypsum Valley anticline than into 

the Dolores anticline.

Continued growth of the salt anticlines during deposition of the Mancos Shale is indicated by the differences 

in thickness of the formation related to the fold axes. 

Although this deformation had no apparent effect on 

the character of the Mancos sediments deposited in the 

salt anticline region, it formed significant structural 

relief between the Dolores anticline and the Disappointment syncline at that time.

SUMMARY AND CONCLUSIONS

A summary of the structural relationships in the Slick 

Eock district in the context of regional structural 

elements leads to a picture of interrelated faulting and 

folding throughout the depositional history of the 

Paleozoic and Mesozoic sedimentary rocks. This summary permits an interpretation of the influence of folds 

and fractures upon epigenetic alteration in the district 

and clarifies the time and manner of formation of the 

uranium-vanadium deposits.

Major folds are parallel to the northwesterly oriented 

Paradox Basin and to the Uncompahgre uplift at the 

northeast edge of the basin. The folds developed during 

depression of the basin and rise of the Uncompahgre 

element, showing their causal relationship.

The Dolores and Glade zones of faults are segments 

of regional alinements of faults (fig. 2). The Dolores 

zone of faults is an extension southeastward of the 

Lisbon Valley fault in Utah. This zone shows some 

evidence of grabenlike collapse above the salt core of the 

Lisbon Valley and Dolores anticlines. But the faults are 

deeper rooted than the underlying salt rock, as indicated 

by the 5,000 feet of stratigraphic displacement on the 

Lisbon Valley fault (Weir and Puffett, 1960, p. 134),
STRUCTURE OF SLICK ROCK DISTRICT AND VICINITY C17

and by the stepped-to-the-right en echelon arrangement 

of small grabens in the zone in the Slick Rock district, 

indicating a deeper seated left-lateral shear.

The Glade zone of faults extends eastward from the 

Verdure graben in Utah (Witkind, 1964, fig. 14, p. 46) 

to faults lying east of the Slick Eock district. Its geographic position coincides closely with the postulated 

underlying Glade subsurface fault zone, which was 

shown to have been active during Paleozoic and Mesozoic time. The stepped-to-the-lef t en echelon disposition 

of grabens in the Glade zone of faults indicates that the 

controlling deep structure is a shear zone of rightlateral displacement.

Together, the Dolores and Glade zones of faults form 

a northwest-northeast conjugate system of lateral slip 

structures that reflect major structural trends in the 

region. This system probably has been active sporadically during most of Phanerozoic time.

Causal relationship between the northwesterly salt 

anticlines and the northeasterly Glade subsurface fault 

zone is plausible because they were deformed at times 

simultaneously. In addition, the presalt beginnings and 

northwesterly orientation of the folds and faults parallel to the Uncompahgre highland indicate that initiation and control of the folding and faulting were 

tectonic. Generally, compression or crustal shortening 

occurred northwest of the subsurface fault zone, in the 

region of the salt anticlines but not southeast of it, implying a component of strike-slip movement along the 

zone. If compression from the southwest, possibly buttressed against the Uncompahgre highland, accounted 

for flow of salt into the salt anticlines, strike-slip movement on the Glade subsurface fault zone was right 

lateral, in accord with the interpreted shear necessary to 

account for the observed stepped-to-the-left en echelon 

arrangement of the surface faults. Conversely, folding 

along northwesterly axes may be related to subsidiary 

up-and-down movements resulting from dominant 

northwesterly strike-slip faulting.

Small folds in the sedimentary rocks adjacent to 

Gypsum Valley appear to be related to intrusion of salt 

and collapse of overlying strata in the core of the salt 

anticline.

Joints in the Slick Eock district seem to be oriented 

mostly parallel to the fault sets, indicating a generic 

relationship and general contemporaneity.

Faults and joints in the Dolores and Glade zones of 

faults that are younger than strata of Late Cretaceous 

age controlled the movement of solutions that epigenetically altered the rocks. If periods of fracturing were correlated with accelerated folding of strata, as seems 

likely, it is inferred that the earliest significant postLate Cretaceous fracturing occurred during early to 

middle Tertiary time, possibly before the time of major 

uplift of the Colorado Plateau. Fracture-controlled 

alteration of rocks probably took place during the early 

to middle Tertiary. Evidence that uranium-vanadium 

mineralization was associated with the fracture-controlled alteration (Bowers and Shawe, 1961) indicates 

an early to middle Tertiary date for the mineralization. 

Subsequently, additional faulting and jointing occurred 

that was not accompanied by, or followed by, 'an episode 

of alteration; in fact, faults locally displaced mineralized rocks. Such fracturing was in part related to collapse of the salt anticlines, or to the late stage of growth 

of the Dolores anticline, during the late Tertiary and 

Quaternary.

It is noteworthy that faulting had started in the 

region surrounding the Slick Eock district early in the 

history of Paleozoic and Mesozoic sedimentation. It is 

likely, therefore, that significant faults existed in 

Jurassic and Cretaceous terrigenous clastic strata during the time of deposition of Upper Cretaceous marine 

Mancos Shale. After burial by the Mancos Shale, rejuvenation of these faults afforded a means by which 

part of the Mancos pore fluids were drawn off into underlying strata during load compaction of the shale, 

and these fluids may have effected alteration in the older 

rocks. This possibility will be examined in detail in a 

later chapter of this professional paper series, which 

will deal with alteration of the sedimentary rocks.

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