' , 

Geology of the 

Salt Anticline Region 

in Southwestern Colorado 

By FRED W. CATER 

With a section on STRATIGRAPHY 

By FRED W. CATER and LAWRENCE C. CRAIG 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 637· 

Prepared on behalf of the 

U.S. Atomic Energy Commission 

A study of the stratigraphy, structure, 

and geomorphology of an area characterized 

by large salt-cored anticlines, with a 

discussion of the growth of the salt cores 

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. 77--607962 

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

Washington, D.C. 20402 

72 3656 q~ 3 09 
\. 

CONTENTS 

Abstract ................................................................................... . 

Introduction .............................................................................. . 

Location, culture, and accessibility ................................. . 

Topography ..................................................................... . 

Climate and vegetation ............................................... ; ... . 

Drainage and water supply ............................................. . 

Previous work ................................................................. . 

Fieldwork and acknowledgments ................................... . 

Mapping and methods of compilation ................... : ....... . 

Stratigraphy, by Fred W. Cat~r and Lewrence C. Craig ....... . 

General features .... , .......................................................... . 

Precambrian rocks ........................................................... . 

Pennsylvanian System-Hermosa Formation ................. . 

Paradox Member ..................... ·····························~·-· 

Limestone member ................................................. . 

Pennsylvanian and Permian Systems-Rico and Cutler 

Formations ................................................................... . 

Triassic System ................................................................. . 

Moenkopi Formation ............................................... . 

Chinle Formation ..................................................... . 

Triassic and Jurassic Systems-Glen Canyon Group ... . 

Wingate Sandstone ................................................. . 

Kayenta Formation ................................................. . 

Navajo Sandstone ................................................... . 

Relations of eastern edges of formations 

of the Glen Canyon Group ................................. . 

Jurassic System ............................................................... . 

San Rafael Group ................................................... . 

Entrada Sandstone ........................................... . 

Dewey Bridge Member ........................... . 

Slick Rock Member .................... : ............ . 

Summerville Formation ................................... . 

Morrison Formation ............................................... . 

Salt Wash Member ......................................... . 

Brushy Basin Member ........................................... . 

Cretaceous System ........................................................... . 

Burro Canyon Formation ..... ,. ................................. . 

Dakota Sandstone ................................................... . 

Mancos Shale ........................................................... . 

Mesaverde Formation ... · .......................................... . 

Tertiary(?) System ., ....................................................... . 

Ancient gravels of Gypsum Valley ......................... . 

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Stratigraphy-Continued 

Tertiary. ( ? ) System-Conti·nued 

Fanglomerates of Paradox, Gypsum, 

and Sin bad Valleys ............................................. . 

Ancient gravels of the Gunnison River ................. . 

Quaternary System ......... · ................................................ . 

Fanglomerate~ in West Creek valley ....................... . 

Lake ( ? ) deposits of Paradox Valley ...................... .. 

Terrace gravels ....................................................... . 

Alluvium and related deposits ......... -..................... . 

Landslide deposits .................................................. .. 

Structural geology ................................................................... . 

Regio·nal setting .............................................................. .. 

Salt anticlines ... ~ .............................................................. .. 

General features ....................................................... . 

Localization ....... : ..... ~ ............................................... . 

Stratigraphic modifications .................................... .. 

Structure ......................................................................... . 

Anticlines ................................................................. . 

Sinbad Valley anticline ............... : ................... . 

Paradox Valley anticline ................................. . 

Gypsum Valley anticline ................................ .. 

Dolores anticline ....... ~ .................................... .. 

Synclines .................................................................. .. 

Nucla syncline ................................................. . 

Dry Creek basin syncline ................................ .. 

Disappointment syncline ................................ .. 

Origin and development of salt anticlines .................... .. 

Tectonic behavior of salt ............ , ........................... .. 

Beginning and growth of salt anticlines ................. . 

~ ~~ Later history and collapse of salt anticlines ------------

Other structural features ................................................. . 

' Uncompahgre uplift .. · ............................. : .............. .. 

Ute Creek graben .................................................. .. 

Structural history ............................................................. . 

Geomorphology ....................................................................... . 

Miocene surface . . . . . . .. .. · ................................................... .. 

Effects of uplift ............................................ · ................... . 

Development of drainage ....... : ...................................... .. 

Topographic development of the salt anticlinal valleys .. 

Minor features ........ : ........................................................ . 

References cited ....................................................................... . 

Index .......... ~ ............................................................................. . 

ILLUSTRATIONS 

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PLATE 1. 

.2. 

FIGURE 1. 

2. 

Page 

Geologic map and sections of the salt anticline. region of southwestern Colorado................................................... In packet 

Generalized block diagram of Paradox Valley anticline, Montrose County, Colo.................................................. In packet 

Tectonic sketch map of part of the Colorado Plateau showing location of report area...................................................... 3 

Photograph showing outcrops of the Paradox Member of the Hermosa Formation, 

Wingate Sandstone, and Entrada Sandstone .................................................................................................................. · 8 

iii 
IV 

FIGURE 3. 

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5-:12. 

·13. 

14-18. 

CONTENTS 

Photograph showing exposure of the Cutler Formation ....... ···········································································-·------------

Map of part of area showing northern and eastern limits of formations ............... ~---·-···········--------·--·-························· 

Photographs: 

5. The Palisade north of Gateway, Colo ................................................................................................................. . 

6. Canyon of the Dolores River near confluence with San Miguel River ........................................................... . 

7. Canyon of the Dolores River south of Mesa Creek. ......................................................................................... . 

8. Entrada Sandstone resting directly on Navajo Sandstone at Uravan, Colo ................................................... . 

9. Dewey Bridge Member of Entrada Sandstone filling a "channel" cut 

in shale of Kayenta Formation ....... ···'······ .................................................................................................. . 

10. Summerville Formation in roadcut near Uravan, Colo ................................................................................... . 

11. Outcrop of Eillegible Sandstone, Summerville Formation, Salt Wash Member 

of Morrison Formation, Brushy Basin Member of Morrison Formation, 

and Burro Canyon Formation ....................................................................................................................... . 

12. Outcrop of Burro Canyon Formation near Horsethief Spring ............................................................................ . 

Diagrammatic sections showing growth and crestal collagse of the Gypsum Valley 

and Paradox Valley salt anticlines .............................................................................................................. . 

Photographs: · 

14. Dry Creek anticline and Paradox Valley ............................................................................................................ . 

15. Intraformational unconformity in the Chinle Formation on the southwest flank 

of Dry Creek ·anticline ................................................................................................................................... . 

16. Graben ·and joints on northeast flank of Paradox Valley anticline south 

of Saucer Basin ............................................................................................................................................... . 

17. Unconformity between the Entrada Sandstone and formations on the northeast side 

of Gypsum Valley, 2 miles north-northeast of Gypsum Gap ..................................................................... . 

18. Unconformity between the Entrada Sandst~ne and formations on the northeast side 

of Gypsum Valley, about 3 miles northwest of locality shown in figure 17 ............................................... . 

19. · Diagrammatic sections showing stages in the formation of the downsag and of the 

faulted Inarginal anticlines at the southeast end of Paradox Valley ... · ...................................................... . 

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GEOLOGY OF THE SALT ANTICLINE REGION 

IN SOUTHWESTERN COLORADO 

By FRED w. CATER 

ABSTRACT 

This report deals with the general geology of the salt anticline 

region of southwestern Colorado and covers an area of about 1,057 

square miles. The area lies entirely in the Canyon Lands section of 

the Colorado Plateaus physiographic province. 

Crystalline rocks of Precambrian age and sedimentary, rocks that 

range in age from Pennsylvanian to Quaternary are exposed in the 

area. The crystalline rocks, consisting of gneiss, schist, and quartzite, 

crop out only on the Uncompahgre Plateau or along its flanks; they 

have been extensively intruded by a gray medium-grained somewhat gneissic granite and by a younger pink rather coarse grained 

granite. Cutting all these rocks are dikes of pegmatite, aplite, and 

lamprophyre. The sedimentary rocks, with a maximum known thickness of 15,080 feet, are largely nonmarine deposits of conglomerate, 

arkose, sandstone, and shale. Much of the sandstone was wind deposited. Marine deposits consist of evaporites, limestone, shale, and 

sandstone. 

The oldest of the exposed sedimentary formations, the Hermosa 

of Pennsylvanian age, comprises two members, the Paradox, composed largely of interbedded salt and gypsum, and an overlying 

unnamed member of gray, fossiliferous limestone with intercalated 

minor shale, sandstone, and arkose. Salt and gypsum of the Paradox 

Member are partly intrusive into younger rocks and form the cores 

of large salt anticlines. The limestone member of the Hermosa is 

overlain, in places unconformably, by the Rico Formation of Penn-

. sylvanian age, a formation consisting of alternating beds of limestone 

and red arkosic conglomerate. These beds mark a transition from 

marine to nonmarine conditions of deposition, for the Rico grades 

upward into and interfingers laterally with the Cutler Formation of 

Permian age. The Cutler is a rudely bedded poorly sorted crosslaminated purplish-red and maroon conglomeratic arkose with scattered thin partings of shale and mudstone. This material, as well as 

the arkosic debris in the Rico Formation, was derived from the 

crystalline rocks exposed in the ancestral Uncompahgre highland, 

an element of the ancestral Rocky Mountains. 

An unconformity separates the Moenkopi Formation of Early and 

Middle(?) Triassic age from the underlying Cutler Formation. The 

Moenkopi is red to chocolate brown and contains a lower member 

consisting of indistinctly bedded poorly sorted mudstone, a middle 

member characterized by crossbedded layers of arkosic sandstone 

and conglomerate, and an upper member consisting largely of thinand ripple-bedded shale. Unlike most of the younger formations, 

the Moenkopi and Cutler Formations wedge out to the northeast 

against the Precambrian rocks. The Triassic Chinle Formation consists mostly of red shale, mudstone, and sandstone, and it rests 

unconformably on the Moenkopi. Conformably overlying the Chinle 

Formation is the Glen Canyon Group of Triassic and Jurassic age. 

This group comprises, in ascending order, the Wingate Sandstone, 

the Kayenta Formation, and the Navajo Sandstone. The Wingate 

and Navajo Sandstones are clean fine-grained cross-laminated massive wind-deposited sandstones, whereas the intervening Kayenta 

Formation consists of fluvial deposits of red shale, mudstone, and 

sandstone. Both the Kayenta and the Navajo, like the Cutler and 

Moenkopi, wedge out northeastward near the front of the Uncompahgre Plateau. The San Rafael Group of Late Jurassic· age rests 

disconformably on the Navajo Sandstone. The Entrada Sandstone 

con~ists of two members; the Dewey Bridge, the lower member of 

the formation, is composed of soft red horizontally bedded siltstone, 

mudstone, and shale, whereas the overlying member, the Slick Rock, 

is a reddish-orange clean crossbedded sandstone resembling the 

Wingate. The Summerville Formation, the uppermost formation 

of the San Rafael Group, is a remarkably evenly bedded sequence 

of shale and sandstone with a few thin limestone lenses. The Morrison Formation, the youngest of the Jurassic formations, comfortably overlies the Summervile and comprises two members, the Salt 

Wash and the Brushy Basin. Both members of the Morrison contain 

shale, mudstone, sandstone, and a little limestone, although the Salt 

Wash is charaCterized by thick layers of resistant interfingering 

sandstone lenses and the Brushy Basin by nonresistant varicolored 

evenly bedded bentonitic mudstones. The top of the Brushy Basin 

Member interfingers with the basal part of the Cretaceous Burro 

Canyon Formation. The Burro Canyon is made up of gray to red 

thick lenticular layers of crossbedded conglomeratic sandstone and 

lesser amounts of predominantly light-green mudstone. The fop of 

the formation is an erosion surface of regional extent on which was 

deposited the Dakota Sandstone of Late Cretaceous age. The 

Dakota Sandstone is a resistant formation composed of crossbedded 

sandstone, carbonaceous shale, and impure coal. The Dakota grades 

upward into the Mancos, a black thinly bedded marine shale, also 

of Late Cretaceous age. The Mancos is overlain conformably by 

the Mesaverde Formation consisting of sandstone and some thin 

shale layers. The Mesaverde is the youngest of the Cretaceous 

formations. 

Tertiary (?) deposits of poorly sorted rudely stratified conglomerate and fanglomerate occupy valleys formed in the collapsed 

crests of the salt anticlines. Quaternary sediments include widespread wind-deposited and sheet-wash material, lake deposits in 

Paradox Valley, terrace gravel, alluvium, and talus debris. 

Although the rocks are nearly flat lying over large areas, in places 

they have been warped into northwest-trending folds and cut by 

steeply dipping faults. The dominant structural element of th~ 

region is the broad northwest-trending. uplift that underlies the 

Uncompahgre Plateau which occupies the northeast part of the 

mapped area. The monoclinal southwest flank of this upwarp is cut 

by numerous faults; some bound the Ute Creek graben formed 

along this flank. Lying southwest of and parallel to the Uncompahgre Plateau upwarp are the Sinbad Valley, Paradox Valley, 

Gypsum Valley, and Dolores anticline5-'-large folds having intrusive 

1 
2 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

cores of salt and gypsum derived from the Paradox Member of the 

Hermosa Formation. These salt anticlines are unlike the salt domes 

of the gulf coast region, for they are decidedly linear and apparently 

. originated along faults and possibly folds that formed before and 

during deposition of evaporites. The salt and gypsum cores of all 

these anticlines except the Dolores are in part piercement structures; the core of the Dolores anticline is not exposed, but it probably consists only of a thickened roll of salt and gypsum. Except for 

the Dolores anticline, the anticlines are complicated by pinchouts 

of formations against the salt and gypsum cores, by cupolas produced by the irregular rise of salt and gypsum, and by numerous 

collapse features occasioned by the removal of the salt through solution and flowage. Collapse of the crests of the salt anticlines 

occurred in two stages widely separated in time. The first stage 

probably occurred in the early Tertiary when the crests of the anticlines were downdropped as grabens; the second stage occurred after 

uplift of the Colorado Plateau, which began in the Miocene when 

downcutting streams breached the salt cores and exposed them 

to removal. 

The synclines between the anticlines are simple downwarps that 

have few of the complexities associated with the anticlines, although 

drilling data indicate that at ·least some of the formations thicken 

in the synclines .. Faults, other than those parallel and directly related to the salt anticlines, are few and small, but in the vicinity 

of Sinbad and Paradox Valleys, systems of sizable faults trend 

northeast transverse to the regional structural trend. 

The evolution of the present topography dates largely from the 

epeirogenic uplift of the Colorado Plateau. At that time· longcontinued erosion had reduced the region to a surface of low relief; 

uplands had been planed off and basins filled. Across this surface 

meandered the ancestral Dolores River as probably did ancestral 

La Sal Creek, Coyote Wash, and Dry Creek. As uplift of the plateau 

progressed, these streams maintained their .ancient courses and 

formed deeply entrenched meanders, but other streams wandered 

from their former channels and began adjusting to the structure of 

the underlying rocks. Accordingly, the San Miguel River and Disappointment Creek flow in the troughs of synclines because the 

channels were displaced laterally by slip-off downdip on more resistant layers, whereas other ·streams carved out and occupy the 

collapsed and eroded crests of the salt anticlines. When downcutting 

streams breached the cores of the salt anticlines, final collapse of 

the crests of the anticlines began, partly through the removal of salt 

by solution and partly by the flowage of salt from beneath areas 

still overlain by thick layers of strata. 

In the meantime the Gunnison River began carving Unaweep 

Canyon as the U ~compahgre Plateau rose athwart its course; still 

later the Gunnison abandoned Unaweep in favor of its present 

channel. After Unaweep Canyon was abandoned, the Uncompahgre 

Plateau rose an additional 2,000 feet and Ute Creek graben formed 

along its northwest flank. By this time the topography had gained 

nearly its present form except for minor changes brought about by 

further denudation and stillstands of erosion. 

INTRODUCTION 

Salt structures, because of their economic significance and 

their structural peculiarities, have long excited the interest 

of geologists. Among the largest of these structures are the 

salt anticlines that lie along the Colorado-Utah boundary,· 

some of which have salt cores measurable in hundreds of 

cubic miles. Growing as these anticlines did in the rather 

narrow and deep part of a tectonically active evaporite 

basin marginal to the steep front of a. rising uplift, they differ in both origin and history of growth from the better 

known salt domes and plugs of the gulf coast region. The 

structural dislocation between the ancient basin and highland has been reactivated in latest Cenozoic times with uplift 

of the Uncompahgre Plateau. Sediments planed from this 

plateau, however-unlike those of its old predecessor-are 

not coming to rest nearby in an adjacent basin. They, along 

with those sediments from the surrounding . country, are 

being removed from the region completely and transported 

hundreds of miles to the Gulf of California, a process now 

being interrupted until the reservoirs behind various dams 

on the Colorado River and its tributaries have silted up, a 

few hundreds years from now. 

This report describes the stratigraphy and structural and 

geomorphic development of that part of the salt anticline 

region lying in Colorado, although many of the statements . 

and conclusions apply equally well to the remainder of the 

region which lies in Utah. This account begins with the 

Precambrian and ends with the present; many chapters, 

unfortunately, are missing because no record remains of 

many probable events. 

LOCATION, CULTURE, AND ACCESSIBILITY 

The part of the salt anticline region of southwestern Colorado described in this report occupies parts of Mesa, Montrose, San Miguel, and Dolores Counties, covers about 1,05 7 

square miles, and includes the principal uranium- and vanadium-producing district of Colorado. The area is bounded 

on the west by long 109°00', on the south by lat 37°52'30" 

and 38°00', on the east by long 108°45' and 108°37Y2', 

and on the north by lat 38°30' and 38°45' (fig. 1). 

The area is in the eastern part of the Canyon Lands section of the Colorado Plateaus physiographic province (Fenneman, 1931). To the east rise the San Juan Mountains, a 

part of the Rocky Mountain System; to the south, west, and 

north the Canyon Lands extends for many miles, its relatively level aspect broken only by the laccolithic Abajo and 

La Sal Mountains a few miles west of the mapped area. 

The area is sparsely populated. U ravan, a mining town 

with a population of about 750, is the largest community. 

Gateway, Bedrock, Paradox, and Egnar are much smaller 

villages centered in small tracts of agricultural land. In 

recent years an· influx of miners, lured by the high price of 

uranium and vanadium ore, has greatly increased the population of the area. Most of the miners constitute a shifting 

population that moves from place to place as older deposits 

are mined out and new ones discovered. A few small sawmills operate in the timbered areas. 

The only hard-surfaced roads within the area are State 

Routes 90 and 141. Although much of the area remains 

relatively inaccessible, a rapidly ·expanding system of dryweather mine roads and truck trails now traverses areas 

which but a few years ago were accessible only on foot or 

by horse. 
INTRODUCTION 3 

EXPLANATION 

Anticline UINTA 

Monocline or fold 

0 Loa 

Escnlanteo 

/ 

.. / 

0 

BAS IN 

0Blanding 

UTAH 

----"'ARizON'A 

CARRIZO .2' '"'~ ":I 

MTS ~ ~I 

'I'll,, I 

I 

I 

Lukachukai 0 I 

50 

I 

I 

DEFIANCE I 

I 

UPLIFT 1 

I 

SAN JUAN 

MTS 

,,,, ... ~ 0 Cortez 

~ fuTE 111' MTS 

SAN JUAN 

BAS IN 

100 MILES 

FxouRE 1.--Tectonic sketch map of part of the Colorado Plateau showing location of report area. 

TOPOGRAPHY 

Although the Canyon Lands contains many types of 

topographic features common to the Colorado Plateau as 

a who]e, it is, as the name indicates, preeminently a land 

riven by countless canyons, many of them so narrow they 

appear to be mere clefts in the plateau surface. The area 

described in this report is fairly representative of the Canyon Lands section, although the canyons are not so narrow 
4 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

or so closely spaced as in some other parts of the section. 

The prevailing grain of the area is northwest, parallel to the 

structure of the underlying rocks, but this grain is modified 

greatly over large areas by partly superimposed drainage. 

The northeast part of-.the area is do;minated by the Uncompahgre Plateau which rises to altitudes of more than 9,000 

feet. At the north edge of the area, Unaweep Canyon cuts 

completely across the plateau and represents an abandoned 

chaimel of the Gunnison River. Southwest of the plateau a 

succession of slightly tilted mesas cut by numerous canyons 

sweeps down to . the San Miguel River and to the brink of 

the winding Dolores River Canyon, a spectacular gorge that 

in this part of the area is more than 1,500 feet deep. Southwest of the two rivers the land rises toward the northeast 

walls of Sin bad and· Paradox Valleys. Sin bad Valley is a 

deep depression completely walled in by nearly vertical 

cliffs except for one point where the cliffs are breached by 

Salt Creek Canyon. Separating Sinbad Valley from Paradox 

Valley is the .-deep Roc Creek Canyon. Paradox Valley 

trends northwestward completely across the central part of 

the area and, like Sinbad Valley, is walled by cliffs and 

.closed at both ends. Near the center of the valley, the 

Dolores River cuts directly across the valley, entering the 

valley from one deep canyon and leaving through another. 

South of Paradox Valley is Dry Creek basin, a broad, shallow nearly featureless synclinal valley that slopes southeast. 

Northwest of the basin is a precipitous area cut by the 

Dolores River Canyon and the tributary canyons of Bull 

and Spring Creeks. Gypsum Valley is somewhat narrower 

than Paradox Valley, but otherwise resembles it closely. 

South of Gypsum Valley the Dolores River separates two 

topographically contrasting. areas: to the southeast lies the 

broad featureless Disappointment Valley; to· the west and 

northwest lies a rugged maze of mesas and canyons. The 

south end of the mapped area includes part of the generally 

high and relatively flat Great Sage Plain, into which the 

Dolores River has carved. a canyon more than 2,000 feet 

deep, the deepest in the mapped area. · 

The Dolores River Canyon is perhaps the most spectacular topographic. feature in a scenically spectacular area. 

With the exception of where it crosses Gypsum and Paradox Valleys, the Dolores River traverses the area from south 

to north through a deep canyon. The canyon ~orthward 

from the southern edge of the area to the mouth of Blue 

Creek col)sists of a series of entrenched meanders and is 

especially sinuous between Gypsum and Paradox Valleys. 

From the mouth of Blue Creek to where the river leaves the 

area near Gateway, the course is straight. A number of 

tributary canyons rival the Dolores River Canyon in depth 

and grandeur, the largest being those of La Sal and Roc 

Creeks. 

Altitudes in the area range from about 4,540 feet on the 

Dolores River below Gateway to slightly more than 9,420 

feet only 11 miles away on the Uncompahgre Plateau. In 

many parts of the area, the local relief in a square mile is 

more than 2,000 feet. Sheer cliffs several hundred feet high 

· wall many of the valleys and canyons and surround many 

of the buttes and mesas. 

Probably more than one-half the surface of the area consists mainly of bare rock; the rest is covered by soil, talus, 

or landslide material. The topography has a decidedly angular aspect, and breaks in slopes commonly are sharp. 

CLIMATE AND VEGETATION 

The climate of the area is mostly semiarid but, because 

of the range in altitudes, annual precipitation varies considerably from place to place. At lower altitudes the precipitation is probably 10-15 inches annually, but above 

8,000 feet . precipitation is greater and may be as much as 

25 inches per year. Accurate precipitation and temperature 

records of the area are scarce, but July, August, and the 

winter months are usually wettest. Snow remains on the 

Uncompahgre Plateau as late as June, but in the canyons 

and valleys snow rarely remains on the ground more than 

a few weeks. Rain not uncommonly occurs as violent downpours called "gully washers." The extreme temperature 

range is from about -20°F to about 105°F, although 

temperatures below 0°F and above 100°F are infrequent. 

The vegetation is of the type common to other parts of 

the Colorado Plateau and consists at lower altitudes of 

pifion and juniper on rocky terrain and sagebrush where 

soils are deep. Cottonwood, willow, and tamarisk border 

the streams. At higher altitudes scrub oak, serviceberry, and 

chokecherry are common; above 8,000 feet and locally in 

favored spots at lower altitudes, ponderosa pine, fir, spruce, 

and quaking aspen form groves. Several varieties of cactuses 

and sparse grass are widely distributed. 

DRAINAGE AND WATER SUPPLY 

The master stream in the area, the Dolores River, heads 

in the San Juan Mountains, flows south to southwest, and 

then turns north and traverses the entire length of the area 

covered by this report. During the summer the river consists of little more than a series of connected pools. The 

San Miguel River, its principal tributary, flows northwestward across the central part of the areas and empties .into 

the Dolores a few miles below U ravan. The San Miguel 

River also heads in the San Juan Mountains, and during 

the summer it normally carries more water than the 

Dolores. 

Most of the tributaries of the Dolores and San Miguel 

Rivers are small and intermittent. Only four perennial 

streams enter the Dolores River from the west. These are, 

from south to north, La Sal Creek, West Paradox Creek, 

Roc Creek, and Salt Wash; all but Salt Wash deliver fairly 

large flows of water throughout the year. From the east 

no permanent streams enter the Dolores River south of the 
GENERAL FEATURES . 5 

San l'vfiguel River, but 1\t[esa Creek, Blue Creek, and West 

Creek enter north of the mouth of the San lVIiguel; of 

these streams \Vest Creek is the largest, and water drawn 

from it is used to irrigate considerable acreage. Tabeguache 

Creek is a permanent stream that drains a large area along 

the southwest flank of the Uncompahgre Plateau .and is the 

largest tributary of the San ~1iguel River. Other permanent streams draining into the San Miguel arc Calamity 

Draw and Dry Creek. The waters of Dry Creek and Salt 

\Vash especially have high mineral content. 

Other streams that enter the Dolort:s and San !\1iguel 

l~ivers directly arc intermittent or flow perennially only in 

the short stretches irnmecliately downstream from springs. 

1\·t'any, nevertheless, have large drainage basins and for 

short periods during and following storms become raging 

torrents of muddy water. 

Springs, in general, arc few and scattered, but north of 

the town of Paradox several large springs which furnish 

considerable irrigation water issue from the northeast wall 

of Paradox Valley. ln several localities clams have been 

erected across small channels to store runoff for stock. 

During wet years reservoirs formed by these dams may 

contain water for long periods. 

PREVIOUS WORK 

'J'hc first report dealing with the geology of the area 

was that of Peale ( .1877), which covered the results of his 

reconnaissance work of 1875-76. During the summer of 

.1875 he traversed Unaweep Canyon and the country north 

of Paradox Valley, and the following summer, the country 

frorn Paradox Valley to the north edge of the Great Sage 

Plain. J\. C. Spencer visited Paradox and Sinbacl Valleys 

in .1899, and his general observations arc included in an 

article by Cross and Howe ( 1905). Later, Cross ( 1907) 

studied the rocks in Sinbacl and Paradox Valleys and along 

the crest of Uncompahgre Plateau to Unaweep Canyon. 

The most comprehensive of the earlier investigations was 

that of Coffin ( 1921) who, starting in 1914, spent parts 

of five summers mapping an area extending from ncar 

Gateway southward to McEimo Creek, about 25 miles 

south of the area included in this report. In addition, reports 

primarily concerned with the uranium and vanadium deposits have been published; the most comprehensive of these 

are by Fischer (1937) and Hess (.1914, 1933). 

FIELDWORK AND ACKNOWLEDGMENTS 

In 1941, the systematic geologic mapping on which this 

report is based was begun in Paradox Valley by A. P. Butler under the general supervision of R. P. Fischer. During 

parts of 1944 and 1 94·5 the Egnar-Gypsum Valley area was 

mapped by'"'· L. Stokes, D. A. Phoenix, R. P. Fischer, and 

L. E. Smith (Stokes and Phoenix, 1948). The remaining 

area was mapped during 194-7-51 by F. W. Cater, who was 

ably assisted at different times by E. J. McKay, E. M. 

Shoemaker, C. M. Withington, and L. R. Stieff. This work 

was clone on behalf of the Division of Raw Materials of 

the U. S. Atomic Energy Commission. In addition to this 

ne\v mapping, some of the earlier mapping was revised in 

order to show details permitted by the better base maps that 

became available after 194 7. The results of this work were 

published as a series of 18 quadrangle maps, with short 

descriptive texts, at a scale of 1:24,000 (Cater, 1954, 

1955a-k; Cater and others 1955; McKay, 1955a, b; Shoemaker, 1955, 1956a; Withington, 1955). From these maps, 

plate 1 of this report has been compiled at a smaller scale 

so as to form a single map; certain errors appearing in the 

original series have been corrected. 

Most of the detailed stratigraphic sections in this report 

were measured by L. C. Craig, C. N. Holmes, and their 

associates during a stratigraphic investigation of the Colorado Plateau, another phase of the study of carnotite deposits in the plateau region sponsored by the Atomic Energy 

Commission. R. A. Cadigan furnished information concerning the composition of the sediments in many of the 

formations. 

We are indebted to various other members of the U.S. 

Geological Survey for geologic information and discussion 

of problems, especially to R. P. Fischer, whose extensive 

knowledge of Colorado Plateau geology was a significant 

aiel, to Lloyd Henbest for his help on problems of correlation, and to D. R. Shawe, D. P. Elston, ad E. R. Landis 

for some revisions in the mapping in the Slick Rock and 

Paradox Valley areas. 

MAPPING AND METHODS OF COMPILATION 

The Colorado Plateau is preeminently suited to geologic 

mapping on aerial photographs. Exposures generally are 

excellent, and the formations are sufficiently distinctive that 

they stand out with remarkable clarity on aerial photographs. Therefore nearly all the field mapping was done on 

contact prints of aerial photographs having a scale of about 

1 :27,000. The geology was then transferred to the t~pographic base map with a Wernstedt-Mahan stereoscopic 

plotter. This method was found to be both fairly accurate 

and convenient. 

STRATIGRAPHY 

By .FRED W. CATER and LAWRENCE C. CRAIG 

GENERAL FEATURES 

The mapped area consists of crystalline rocks of Precambrian age and sedimentary rocks that range in age from 

Pennsylvanian to Quaternary. The crystalline rocks crop 

out in Unawcep Canyon and along the flanks of the 

U ncomahgre Plateau in the northern and northeastern parts 

of the area and consist principally of granitic intrusive rocks. 

Small amount of gneiss, schist, and quartzite occur locally. 

Dikes are numerous. The rest of the area is underlain by 

generally flat-lying sedimentary rocks with a known maximum thickness of more than 15,000 feet. Most of these 
6 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

rocks are nonmarine conglomerates, arkoses, sandstones, 

and shales. A large proportion of the sandstone was wind 

deposited. Exposed beds of undoubted marine origin were 

deposited only during the Pennsylvanian Period and the 

latter part of the Cretaceous Period. 

The oldest of the exposed sedimentary rocks are intrusive 

masses of salt and gypsum of Pennsylvanian age that crop 

out extensively in the cores of the anticlines that underlie 

Sinbad, Paradox, and Gypsum Valleys. Unlike most salt 

structures which begin forming only after deep burial of 

the salt, some upper surfaces of these cores-for reasons 

to be explained later in this report-were above levels of 

sedimentation from the time salt deposition ceased until 

Morrison time in the Late Jurassic. These cores, therefore, 

exercised a controlling . influence on the thickness and distribution of all intervening formations. Against the flanks 

of some of these. salt cores all the intervening formations 

wedge out, whereas against the less active cores some formations merely thin. Overlying the salt and gypsum beds 

along the sides of the valleys are Pennsylvanian marine 

· limestones that grade upward into a thick sequence of 

continentally deposited conglomerate and arkose of Pennsylvanian and Permian ages. The conglomerate and arkose 

are widely exposed along the walls of the salt anticline 

valleys, in the valley of West Creek, and along the flanks 

of the Uncompahgre Plateau. The arkosic beds wedge out 

against the Precambrian crystalline rocks on the flanks of 

the Uncompahgre Plateau. 

Lower Triassic shale, sandstone, and arkosic conglomerate 

unconformably overlie Permian rocks and are exposed in 

the same general areas as the Permian rocks. These rocks 

wedge out both along the southwest front of the Uncompahgre Plateau and, because of nondeposition, against the 

salt and gypsum intrusions whch had reached the surface 

before Triassic time. Upper Triassic shale and sandstone 

rest unconformably on the Lower Triassic beds. The Upper 

Triassic beds are exposed in the valley walls formed in the 

salt anticlines, along the deeper canyons, and on the flanks 

of some buttes ·and mesas. 

The Jurassic formations are widely exposed over a large 

part of the area and crop out as colorful cHis, slopes, and 

benches. Except for the Morrison Formation they consist 

largely of sandstone and wedge out at one place or another 

against the salt and gypsum intrusions. The Morrison contains large quantities of shale and mudstone in addition to 

sandstone, and it either completely or almost completely 

blanketed the intrusions. 

The Cretaceous formations consist of marine shale and 

stream-deposited conglomerate, sandstone, and shale. The 

stream-deposited conglomerate and sandstone form the 

capping beds on many of the mesas and the floors of the 

synclinal valleys; the marine shale occurs only in the lower 

parts of the synclinal valleys or in downdropped fault 

blocks. Unlike most of the older formations, the Cretaceous 

rocks were deposited as a blanket of fairly uniform thickness 

over the entire area. 

No rocks of unquestioned Tertiary age now exist within 

the area, although some conglomerates at the southeast enc:l 

of Gypsum Valley are probably that old. Quaternary deposits of slightly to thoroughly lithified fanglomerate, partly 

consolidated shale and sandstone, and terrace gravels are 

common locally. Deposits of windblown material and sheet 

wash are widespread on mesas and in some of the 

valleys. . 

The southwest front of the Uncompahgre Plateau 'marks 

the northeast edge of the uppermost Paleozoic and Lower 

Triassic formations. These formations wedge out abruptly 

because of nondeposition or because of erosional truncation 

against the ancestral Uncompahgre highland, which contributed much of the material making up these formations. 

The southwestern front of this old highland is followed 

closely by the front of the present Uncompahgre Plateau. 

Upper Triassic and Lower Jurassic formations blanket the 

old highland in the area of this report, but eastward they 

gradually thin and wedge out. Upper Jurassic and Cretaceous rocks completely blanketed the entire region, including the ancestral Uncompahgre highland. 

PRECAMBRIAN ROCKS 

The Precambrian rocks are exposed only along the deeply 

dissected parts of the Uncompahgre Plateau in the drainage of West Creek and in the canyons cut by the headwaters of Ute, Blue, Mesa, and Atkinson Creeks. The most 

extehsive outcrops are in the extreme northeastern part of 

area in several square miles along the precipitous slopes 

and canyons bordering the U ncomaphgre Plateau. 

Because of time limitations no attempt was made to map 

the complex of interrelated Precambrian igneous and metamorphic rocks. The oldest rocks of the Precambrian complex are quartzite, hornblende and biotite gneisses, and 

schist. These occur sparsely in later granites as small isolated 

masses that have undergone varying degrees of transformation. No masses of these older metamorphic rocks are 

sufficiently large or continuous to permit working out the 

probable age relations. They are believed to belong to the 

series of ancient gneissic rocks in adjacent areas to the 

northwest described by Dane ( 1935, p. 20-24) and Case 

(1966, p. 1425-1433). 

Most abundant of the rocks in this Precambrian complex is a medium-grained gray biotite-hornblende granite 

that in most places has a distinct gneissic texture. In the 

southernmost parts of the area of Precambrian rocks, the 

gray gneissic granite far surpasses all other rocks in quantity, 

but northward later intrusive rocks form an increasingly 

large proportion of the whole. The granite intrudes the 

gneisses and quartzites and, within the mapped area, completely engulfs and surrounds all masses of the metamorphic 
PRECAMBRIAN ROCKS 

Sedimentary rock formations exposed in the salt anticline region of southwestern Colorado 

System 

Quaternary 

Tertiary (?) 

Cretaceous 

Jurassic 

Triassic (?) 

Triassic 

Triassic (?) 

Permian 

Pennsylvanian 

Series 

Holocene 

Pleistocene 

Pliocene (?) 

Upper 

Lower 

Upper 

Upper 

Upper 

Middle(?) 

Lower 

Upper 

and 

Middle 

Middle 

Stratigraphic unit 

u·nconformity 

Unconformity 

Mesaverde 

Formation 

Mancos Shale 

Dakota 

Sandstone 

Unconformity 

Burro Canyon Formation 

Morrison 

Formation 

Summerville 

Formation 

San 

Rafael Entrada 

Group Sandstone 

Unconformity 

Navajo 

Sandstone 

Glen Kayenta Canyon 

Group 

Formation 

Wingate 

Sandstone 

Chinle 

Formation 

Unconformity 

Moenkopi 

Formation 

Unconformity 

Cutler 

Formation 

Rico 

Formation 

Local unconformity 

Hermosa Formation 

1 Unknown; perhaps several hundred feet. 

II Unknown. 

Thickness 

(feet) 

0-20 

0-200 

(•)· 

(2) 

2,000± 

70-220 

50-300 

300-750 

240-440 

0-100 

0-225 

0-100 

0-500+ 

0-300 

0-500 

0-750 

0-500 

0-290 

0-300 

0-9,000+ 

0-150? 

2,000-2,200 

(2) 

Character 

Talus, alluvium, and wind-deposited material. 

Talus, landslide deposits (in part of Holocene age), fanglomerate, lake beds, and 

undifferentiated stream deposits. 

Gravel composed of pebbles and boulders of porphyritic igneous rock. 

Thick-bedded yellowish-gray sandstone and light-gray shale. 

Dark-gray fissile shale. 

Yellow lenticular sandstone and conglomerate; interbedded carbonaceous shale and 

impure coal. 

White, gray, and red sandstone and conglomerate; interbedded green and reddish-purple shale. 

Brushy Basin Member; variegated bentonitic shale and mudstone; rusty-red and red 

sandstone and conglomerate; local thin limestone beds. 

Salt Wash Member; white, gray, buff, and rusty-red sandstone; red, reddish-brown, 

green, and gray mudstone: scattered thin limestone beds. 

Thin-bedded red, gray, green, and brown sandstone, sandy shale, and mudstone. 

Slick Rock Member; orange, buff, and white fine-grained massive and crossbedded 

sandstone. 

Dewey Bridge Member; red, buff, and orange horizontally bedded mu9stone, 

siltstone, and sandstone. 

Buff and gray crossbedded fine-grained sandstone. 

Irregularly bedded red, buff, gray, and lavender fine- to coarse-grained sandstone, 

siltstone, and shale. A few lenses of conglomerate. 

Fine-grained reddish-brown thick-bedded, massive, and crossbedded 

cliff-forming sandstone. · 

Red to orange-red siltstone with interbedded lenses of red sandstone, shale, and 

limestone-pebble and clay-pellet conglomerate. Lenses of quartz-pebble conglomerate and grit at the base. 

Upper member; chocolate-brown ripple-bedded shale; thin lenses of arkosic sandstone. 

Middle member; chocolate-brown arkose, arkosic conglomerate, and ripple-bedded shale. 

Lower member; reddish- to yellowish-brown indistinctly bedded poorly sorted 

mudstone. Local gypsum beds near base. 

Maroon, red, light-red-mottled, and purple conglomerate, arkose, and arkosic sandstone; 

thin beds of sandy mudstone. 

Maroon, red, light-red-mottled, and purple conglomerate, arkose, and arkosic sandstone; 

interbedded red and gra;v marine limestone. . 

Limestone member; gray fossiliferous limestone and thin beds of shale; minor arkose. 

Paradox Member; sandstone, arkose, carbonaceous shale, limestone, gypsum, and salt. 

7 

rocks. The gneiSSIC layering in the rock is exceedingly irregular, and no dominant trend could be determined; on 

the other hand, dips are consistently either vertical or near 

vertical. 

dance and the size of individual crystals, many of which are 

as much as 2 mm (millimeters) long. 

Cutting the gneissic granite are dikes ao.d irregular masses 

of pegmatite, aplite, lamprophyre, and coarse-grained pink 

two-mica-hornblende granite. The granite is the most abundant of these rock types in the northern part of the area, 

especially in the vicinity of Unaweep Canyon. This granite 

appears to be the same that Dane ( 1935, p. 20-24) described as the predominant intrusive rock in nearby areas to 

the north and northwest. In places it is somewhat porphyritic, with euhedral microcline phenocrysts. The accessory mineral sphene is notable for both its relative abunAlthough no direct field evidence exists for any but a 

pre-Permian age within the area covered by this report, 

geologists have been in general agreement that the rocks 

are Precambrian. Peale ( 1877, p. 64-69) assigned an 

Archean age to the metamorphic rocks of the Uncompahgre 

Plateau; later Cross ( 1907, p. 676-677) confirmed the 

possible Archean age of the gneiss and schist but believed 

the quartzite to be correlative with the quartzites of 

the San Juan Mountains near Ouray, Colo., which were 

presumed to be Algonkian. The granitic intrusions were 

believed to be later Algonkian. Hunter ( 1925) believed 

the schists and gneisses of the Gunnison to be of Archean 
8 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

age and the granitic intrusives to be of late Algonkian or 

early Paleozoic age. A granite from East Creek in Unaweep 

Canyon a few miles northeast of the mapped area, however, 

gives ages ranging from 1.05 to 3.18 b.y (billion years), 

depending on the method used ; the potassium-argon age 

of 1.36 b.y. and the rubidium-strontium age of 1.37 b.y. 

probably are best (Davis and others, 1956 ). This rock from 

East Creek appears to be younger than either of the granites 

in the mapped area because it is devoid of any effects of 

regional metamorphism (Shoemaker, 1956b), and therefore 

the data a:vailable point to the probability that the quartzite 

· and the intrusive rocks, at least, are older than formerly 

believed. 

PENNSYLVANIAN SYSTEM-HERMOSA FORMATION 

The H ermosa Formation of Pennsylvanian age contains 

the oldest sedimentary rocks exposed in the mapped area. 

It consists of two members-a lower, the Paradox, largely 

evaporites, and an upper unnamed member, largely limestone. 

PARADOX MEMBER 

The Paradox M ember of the Hermosa crops out only 

in the floors of Sinbad, Paradox, and Gypsum Valleys where 

it forms the cores of anticlines. It is also exposed in similar 

valleys in neighboring areas in Utah (Baker and others, 

1933) . The topography on the Paradox is distinctive and 

in most places consists of low gently rounded hills, but where 

cut by gullies it is highly irregular, rough, and complex 

and forms badlands and cutbanks with nearly vertical 

fluted walls (fig. 2). All outcrops of the Paradox are so 

intricately and complexly folded, faulted, and brecciated 

that it was not possible in the time available to work out 

even the general stratigraphic sequence of beds or to distinguish with absolute certainty between certain limestone 

beds in the Paradox and limestone beds of either the upper 

member of the H ermosa or the Rico Formation. The stratigraphic relations have been disturbed further by the solution and removal of saliferous layers to a depth of several 

hundred feet. Nowhere is the base of the Paradox exposed, 

nor, for that matter, has an unquestionably undisturbed 

upper contact with the overlying limestone member been 

found. 

Surface exposures consist of gypsum, limestone, shale, 

and sandstone. Gypsum is probably the most abundant 

material in the Paradox, especially in Gypsum and Paradox Valleys; in Sinbad Valley, gypsum is less abundant 

than shale. Porous, earthy or sugary, white gypsum is 

commonest. In some outcrops selenite crystals are very 

numerous, both as a sort of surface incrustation and mixed 

with loose powdery gypsum below the surface. In many 

places porous sugary gypsum gives a hollow sound when 

walked upon. Thick beds of compact crystalline rock gypsum, banded white and shades of gray and blue, are common and are exposed best where the gypsum has been 

deeply incised. Mixed with the gypsum are minor quantities of clay and sand. Dark-gray to black thin-bedded 

highly crumpled shale is the next most abundant rock in 

surface exposures. Carbonaceous material is abundant, and 

fresh specimens of the shale commonly have a decided 

petroliferous odor. Much of the shale is gypsiferous, and 

some has scattered molds of salt crystals. Beds are generally 

paper thin, and ripple bedding seems to be nonexistent. 

Alternating with beds of shale are layers of fine-grained 

gray and green sandstone as much as 3 inches thick. In some 

outcrops thicker beds of sandstone not associated with shale 

occur, and in Sinbad Valley beds of arkosic coarse-grained 

FrGURE 2.- View northward across Paradox Valley. Low hills on valley floor are outcrops of the Paradox Member of the H ermosa Formation ; dark cliffs on left skyline are of Wingate Sandstone; light cliffs above on right are of Entrada Sandstone about 100 feet thick. 
PENNSYLVANIAN SYSTEM- HERMOSA FORMATION 9 

sandstone crop out. The sandstone consists principally of 

angular grains of quartz and of lesser quantities of orthoclase, microcline, plagioclase, and clark minerals, the last of 

which give some of the sandstone a pepper-and-salt appearance. Considerable glauconite is present in most of the 

sandstone. The cement of both sandstone and arkose consists of calcite and clay. The angularity of the sand grains 

and the fresh feldspar of the arkosic sandstone in Sinbad 

Valley indicate that the ancestral Uncompahgre highland 

to the northeast had already risen to a point where Precambrian crystalline rocks were being eroded, although the 

abundance of grains having the optical properties of glauconite points to marine conditions of deposition of the 

sandstone. Extensively exposed in some areas, especially in 

Paradox Valley, are beds of fine-grained nonfossiliferous . 

limestone. In contrast to the upper limestone member of the 

l-Iermosa, the limestone in the Paradox Member is rather 

thin bedded and flaggy and typically weathers to form 

flakes 1-2 inches across. !v!uch of the limestone is rather 

sandy. 

In many places where younger formations such as the 

Rico or Chinle rest with depositional contact on the Paradox, a peculiar breccia in a layer as much as 8 feet thick 

separates the crumpled Paradox beds from the overlying 

formation. 'This breccia seeems to have formed by a mixing, 

while still unconsolidated, of material similar in composition to that of the overlying beds with fine-grained detritus 

from the underlying Paradox. 

Although no rock salt is exposed, well logs (Shoemaker 

and others, 1958) shows that the amount of salt in the 

Paradox ~t(ember ranges from about 42 percent in the Sinbad Valley anticline to 82 percent in the Lisbon Valley 

anticline in Utah but is more than 70 percent in all the 

salt anticlines of the region except the one underlying 

Sinbad Valley. According to these logs the Paradox also 

averages 18 percent shale and siltstone, 4 percent gypsum 

and anhydrite, 3 percent limestone and dolomite, and 2 

percent sandstone and conglomerate. Logs of several wells 

drilled into the Paradox in Colorado and Utah indicate that 

salt has been leached out to depths ranging from 600 to 

1,000 feet. Active leaching is still continuing as is shown 

by salt-water springs entering the streams that drain the 

salt anticline valleys. Late in the summer when the water 

is low, the Dolores River, below Paradox, Valley, is salty, 

as is the stream in Salt '"'ash which drains Sinbad Valley. 

No undisturbed section of the Paradox Member of the 

Hermosa is known, and it is doubtful that any entirely 

undisturbed sections have ever been drilled. Because of these 

circumstances, estimates of the original thickness are subject to a high degree of error. Of the wells thus far drilled 

into the cores of the salt anticlines, only one, drilled late 

in 1958 by the Continental Oil Co. in sec. 8, T. 47 N., 

R. 18 W., in West Paradox Valley, penetrated the salt core. 

It entered pre-Paradox rocks at a depth of 14,345 feet. It 

seems entirely possible that the Paradox may have a comparable thickness under the other similar anticlinal vall~ys. 

If it is assumed that the material in the intrusive cores of 

the anticlines was squeezed from the intervening synclinal 

areas, material thinning from an overall thickness of about 

4,000 feet in the vicinity of Paradox Valley to about 3,000 

in the vicinity of Gypsum Valley would be required to 

account merely for the excess now in the anticlinal cores. 

Inasmuch as the cores of these anticlines were exposed at 

the surface from Permian to Late Jurassic time and presumably were subject to extensive solution during this time, 

it apparently can be assumed that the original undisturbed 

thickness of the Paradox was considerably more than these 

figures. 

LIMESTONE MEMBER 

Although no undisturbed contact between the Paradox 

Member and the limestone member of the Hermosa Formation has been observed, the limestone member probably was 

deposited conformably upon the Paradox. The limestone 

member is exposed mainly along the sides, and locally 

nearer the central parts, of Sin bad, Paradox, and Gypsum 

Valleys. The outcrops form steep slopes or cliffs where 

exposed in the valley walls and low rounded hills where 

exposed nearer the center of the valleys. Almost without 

exception the beds exposed in the outcrops are steeply dipping. At no place is a complete section of the limestone 

member exposed, and in many places the exact stratigraphic 

position of outcrops is indeterminate. In general only the 

upper part of the member crops out along the sides of the 

valleys, but even in some of these locations, as in Little 

Gypsum Valley, there was an unknown amount of erosion 

of the upper beds in pre-Late Jurassic time. Consequently 

the member is overlain unconformably by formations ranging in age from Pennsylvanian to Late Jurassic. 

The limestone member of the Hermosa Formation consists predominantly of thick-bedded gray fossiliferous limestone, which is intercalated with thin beds of shale, sandstone? and less commonly arkose. Crinoid stems are abundant in most of the limestone, but other fossils are not 

abundant within the mapped area except at the southeast 

end of Gypsum Valley where fossil brachiopods, corals, gastropods, and other invertebrates are abundant. Inasmuch as 

extensive collections of fossils from the Hermosa Formation 

have been made (Dane, 1935, p. 35; McKnight, 1940, p. 

24-25), collections were not made during the present study. 

Thin beds of shale, sandstone, and arkose are interbedded 

with the limestone. The shale is commonly gray or various 

tones of light buff and is thin and evenly bedded; no ripple 

marks were observed. Sandstone is abundant and mostly 

gray or yellowish gray. It consists· of angular grains of quartz 

with some feldspar and small amounts of dark minerals; 

the cement is calcite. The arkose is compositionally very 
10 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

similar to the arkose in the overlying Rico and Cutler Formations, but commonly the color is lighter and less strikingly reel. It is coarse grained (pebbly in places) and has 

rock and mineral fragments which can be matched in the 

Precambrian rocks of the Uncompahgre Plateau. Arkose is 

more common in the limestone member of the Hermosa in 

Sinbad Valley than in Paradox or Gypsum Valleys. Dane 

( 1935, p. 34) described a partial section of the Hermosa 

Formation above the Paradox Member from Fisher Valley, 

Utah, that is 855 feet thick and consists very largely of 

sandstone and arkosic sandstone. It is worthy of note that 

cuttings from wells in the flanks of Paradox Valley which 

have penetrated the limestone member indicate a considerable quantity of red arkosic material interbedded with the 

limestone. In view of the fact that nowhere in the valley is 

there much arkosic material interbedded with the limestone 

the well cuttings may have been badly contaminated witl~ debris sloughing from the arkosic Cutler Formation in the 

upper parts of the drill holes. 

The thickness of the limestone member of the Hermosa 

F orm~tion in this area is not accurately known but is probably about 2,000-2,200 feet. At the southeast end of 

Gypsum Valley, about 1,000 feet of limestone is exposed 

without revealing the base of the member; elsewhere the 

exposed parts of the member are much thinner. The most 

accurate information concerning the thickness is that from 

the Chicago Corporation well on the Dolores River south 

of Bedrock. About 2,250 feet of beds probably belonging 

to the limestone member was penetrated· by this well. 

To the northwest in the Moab area of Utah, 1,500-1,800 

feet of the limestone member of the Hermosa has been 

penetrated by wells (Baker, 1933, p. 19), whereas to the 

southeast in the San Juan and La Plata Mountains the 

Hermosa Formation, including any beds equivalent to the 

Paradox Member, ranges in thickness from 1,800 to 2,800 

feet (Eckel, 1949, p. 9-13; Cross and Spencer, 1900, p. 

48-59). The Hermosa of the San Juan Mountains is marginal to the so-called Paradox Basin where the thick accumulations of evaporites forming the Paradox Member were 

deposited. In all probability · the limestone member was 

thicker in the Paradox Basin area than corresponding beds 

in the marginal areas. The limestone member probably 

correlates with at least a part of the Honaker Trail Formation (Wengerd and Matheny, 1958, p. 207 5) of southeastern Utah. 

PENNSYLVANIAN AND PERMIAN SYSTEMSRICO AND CUTLER FORMATIONS 

The Rico and Cutler Formations were not generally 

mapped separately in this area because in many places it 

is difficult if not impossible to recognize a distinct Rico unit. 

The Rico Formation has long been a subject of uncertainty 

and disagreement not only as to what beds should be included but .also as to age; moreover, the advisability of 

recognizing the Rico as a formation has been questioned, 

and certainly within the area of this report there are only 

a few localities where sue~ recognition seems justified. 

The name Rico Formation was given to a sequence of 

beds in the San Juan Mountains which are well exposed 

near the town of Rico (Cross and Spencer, 1900, p. 59-66). 

These beds are transitional between the Hermosa and Cutler Formations and consist of sandstone, arkose, minor 

interbedded shale, and fossiliferous limestone. In the area 

covered by this report, such beds occur only locally; elsewhere no such transitional beds between the Hermosa and 

Cutler Formations are recognizable, and rocks lithically 

identical to the Cutler rest directly on the Hermosa. 

Rocks lithically similar to those of the Rico Formation 

are exposed locally in and along the sides of Sinbad, Paradox, and Gypsum Valleys. Some of the best and most 

extensive outcrops of such rocks are in sees. 22 and 26, 

T. 48 N., R. 19 W., at the northwest end of Paradox 

Valley. Rocks having Cutler lithic characteristics are exposed extensively in the sides of the salt anticline valleys, in 

the canyon of the Dolores River above Gateway and south 

of Joe Davis Hill, and over large areas in the valley of West 

Creek and adjacent to the southwest front of the Uncompahgre Plateau. 

In general, the Rico and Cutler Formations rest conformably on the Hermosa, but detailed mapping by Elston 

and Landis ( 1960) in East Paradox Valley shows that the 

Rico and Cutler, because of structural complications introduced because of growth of the salt core underlying the 

valley, rest unconformably on both the limestone member 

anfil the Paradox Member of the Hermosa Formation in a 

few places. In the northeastern part of the area, the Cutler 

rests on a surface of considerable relief carved on the Precambrian crystalline rocks. The upper contact of the Cutler 

is an unconformity. The Rico and Cutler consist predominantly of maroon, purple, red, and light-red-mottled arkosic 

sandstone, arkose, and arkosic conglomerate. Interbedded 

with the arkosic material are small quantities of reddishbrown sandy mudstone and at the base in some areas thin 

beds of sandy and muddy limestone. The limestone-bearing 

parts of this interval, of course, more nearly resemble the 

Rico in its type locality than they do the Cutler. The 

arkosic material is poorly sorted, forms rudely crossbedded 

layers and lenses, and consists of quartz, fresh feldspar, dark 

minerals, and pebbles, cobbles, and boulders of granite, 

gneiss, schist, and quartzite-materials derived almost entirely from the Precambrian crystalline rocks. The small 

amounts of reddish-brown sandy mudstone are distributed 

through the coarser material as thin irregular seams that 

serve to delineate the otherwise obscure bedding. Bleached 

spheres resulting from a change of the red color of the 

rocks to a light green or greenish gray are very common in 

many places. These spheres range in size from a fraction of 
PENNSYLVANIAN AND PERMIAN SYSTEMS- RICO AND CUTLER FORMATIONS 11 

an inc~ to as much as 1 Y2 inches across and invariably contain a nucleus consisting of a dark grain of an undetermined 

mineral. In a few localities, notably in Paradox Valley in 

sees. 15, 22, and 23, T. 47 N., R. 18 W., and in the southeast end of Gypsum Valley, a few thin limestone beds are 

interlayered with the lowermost red arkosic beds. The limestone is impur_e and commonly consists of angular: fragments 

FIGURE 3.-Exposure of the Cutler Formation in the Dolores River 

C anyon a few miles south of Gateway, Colo. 

of limestone cemented in a matrix of the same material. In 

other localities arkosic debris devoid of limestone interbeds 

rests directly on Hermosa limestone, so lateral variations in 

the lowermost beds are marked. 

A pronounced regional difference in bedding and grain 

size exists in the Cutler Formation. In the northeastern part 

of the area where the formation rests in part upon the 

Precambrian crystalline complex, the Cutler is bouldery, 

consists of unweathered granite debris, and has extremely 

crude bedding (fig. 3). Attitudes of the bedding near the 

base conform closely to the slopes of the irregular hilly 

surface on which the debris was deposited. The general 

lithic characteristics are those of an arkosic fanglomerate. 

Southwestward the materials become progressively finer 

grained and the feldspar and dark minerals more decomposed, the clay content increases, and the bedding becomes 

more clearly defined. Much of the Rico and Cutler in 

Paradox Valley is not conspicuously conglomeratic; boulders are not abundant. The Cutler exposed in the Dolores 

River Canyon south of Joe Davis Hill is much finer grained 

and more evenly bedded than farther north, and it consists 

mostly of sandstone and arkosic sandstone with interbedded 

thin layers of mudstone and fissile ripple-bedded shale ; 

however, even the finest grained shale contains abundant 

mica flakes coating bedding surfaces. 

THICKNESS 

The Rico and Cutler Formations form a thick asymmetric 

lens that wedges out abruptly to the northeast and thins 

gradually to the southwest and southeast. On the southwest 

flank of the Uncompahgre Plateau, these formations wedge 

out entirely (fig. 4), but 4-5 miles to the southwest they 

thicken to several thousand feet. In the Dolores River Canyon a few miles downstream from Gateway, north of the 

mapped area, a well collared several hundred feet below 

the top of the Cutler penetrated more than 7,800 feet of 

arkosic material before reaching the Precambrian basement. 

Moreover, the upper beds of the Cutler in this area were 

stripped off by Late Permian or Early Triassic erosion. 

Therefore, it seems entirely possible that locally the original 

thickness of these formations may have been as much as 

10,000 feet. Farther to the south and southwest in the 

trough of the Dolores-San Miguel syncline, the thickness 

probably is about 13,000 feet, although no thickness approaching this is exposed. South and southwest of the 

Dolores anticline, these formations thin appreciably. In 

addition to the regional differences in thickness, which consist first of a rapid thickening southwestward from the line 

of pinchout and then of thinning farther to the southwest, 

data from wells drilled near Nucla, Colo., indicate that 

these formations probably thin southeastward along the 

front of the Uncompahgre Plateau, parallel to the line of 

pinchout. No detailed sections of the Rico and Cutler were 

measured because of the uniformity of lithic characteristics 

from top to bottom. 

AGE AND CONDITIONS OF DEPOSITION 

The only fossils that have been found in this interval in 

this area are some fusulinids and pelecypods from Paradox 

and Gypsum Valleys that probably belong in the Rico. Fossils from rocks having Rico lithic characteristics in Paradox 

Valley north of the town of Paradox were identified by 

Lloyd G. Henbest of the Geological Survey as Late Pennsylvanian in age, probably late Virgil, or possibly as old as 

middle Virgil. Vertebrate fossils from the upper part of 

the Cutler in the San Miguel River Canyon near Placerville, Colo., have been identified by Lewis and Vaughn 

( 1965, p. C39-C42 ) as being of Early Permian age. The 

Cutler Formation, in accordance with Geological Survey 

age designations, is still classed as Permian. In the area of 

this report, much of the Rico and possibly all the Cutler 

are Pennsylvanian. 

The sedimentary structures and the coarseness of the 

material in the Rico and Cutler Formations indicate rapid 

deposition, and the composition of the material indicates 

that nearly all of it was derived from the ancestral Uncompahgre highland. The alternation of continental deposits 

with marine deposits in the Rico Formation in Paradox and 

Gypsum Valleys represents a transitional stage occurring 

either as the Hermosa sea was filled or as it retreated. The 
12 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

presence or absence of limestone interbeds in the lower part 

of the Rico and Cutler may indicate the degree of proximity 

109°00' 

0 5 

Eastern limit of 

Kayenta Formation 

10 MILES 

FIGURE 4.-Map of part of area showing eastern limits of formations. Ticks indicate direction of thickening. 

of sites of deposition to the mouths of streams draining the 

old highland, the limestone beds forming in areas between 

stream mouths. Dumping of material from the rapidly 

rising ancestral Uncompahgre highland continued after the 

disappearance of the Hermosa sea. The fanglomeratic texture of the Cutler Formation and the rapid thinning of the 

formation toward the Uncompahgre indicate that the front 

of this old landmass was steep and probably was a fault 

scarp. In all likelihood, uplift was greatest to the northwest 

in the vicinity of Gateway, and it decreased to the southeast. 

TRIASSIC SYSTEM 

MOENKOPI FORMATION 

The Moenkopi Formation of Triassic(?) and Early and 

Middle? Triassic age unconformably overlies the Cutler 

Formation. In the canyon of the Dolores River and in its 

tributary canyons in the northern part of the mapped area, 

the unconformity is decidedly angular; the Cutler dips as 

much as 3 °-4 ° more steeply than the overlying Moenkopi 

Formation, although part of this difference may be attributed to an original steeper dip in the Cutler. In the 

vicinity of the salt cores, the Cutler beds in many places are 

much more steeply upturned than the Moenkopi and 

younger beds because of pre-Moenkopi folding and upwelling of salt. In many areas of relatively undisturbed rocks, 

the unconformity is not angular and beds above and below 

it are virtually parallel. The upper contact is also an 

unconformity. 

The Moenkopi Formation crops out extensively in Paradox and Sinbad Valleys, especially on the north walls, and 

in the Dolores River Canyon and its tributary canyons 

below the mouth of Blue Creek. The formation is not 

exposed south of Paradox Valley. 

The Moenkopi Formation commonly forms a slope 

broken in places, especially in the middle member, by more 

resistant ledges (fig. 5). The color as seen from a distance 

is distinctly darker than that of the underlying Cutler Formation or of the overlying Chinle Formation and ranges 

from deep chocolate brown to red and yellow brown. 

Within this area the formation comprises three more or 

less lithologically distinct units in each of which a given 

rock type predominates. Thus the lower member consists 

largely of indistinctly bedded mudstone, the middle member 

of crossbedded arkosic sandstone and conglomerate, and the 

upper member of thin- and ripple-bedded shale. In each of 

these members, however, rocks more common in the other 

members are found in addition to some fairly clean sandstone and locally some gypsum. These members have been 

mapped separately wherever possible. 

Subsequent to the completion of mapping in the area of 

this report, Shoemaker and Newman ( 1959) revised the 

limits of the Moenkopi in the Sinbad Valley area and 

named four members. The lower member of this report corresponds approximately to the Tenderfoot Member and the 
TRIASSIC SYSTEM- MOENKOPI FORMATION 13 

middle member to the Ali Baba Member; the upper membet corresponds to the Sewemup Member and a part of 

Pariott Member in Sinbad Valley. As explained later, the 

upper part of the Pariott Member of Sinbad Valley is 

included in the base of the Chinle Formation of this report. 

The formal member names are not used herein because of 

the disparity between the mapped units of this report and 

those of lhe formal member assignments. 

The lower few feet of the formation in some areas consists of reworked material from the Cutler Formation, and 

in Roc Creek Canyon, about 3Y2 miles above its mouth, 

the typical ·red mudstone of the lower member of the 

Moenkopi is underlain by a zone of breccia fragments of 

limestone, sandstone, and shale in a matrix of fine-grained 

detritus, which all is derived from the Paradox Member of 

the Hermosa Formation . Elsewhere in the area, pebbles in 

the conglomeratic lenses throughout the formation consist 

entirely of Precambrian crystalline rocks derived from the 

ancestral Uncompahgre highland, and the abundance of 

arkosic layers in the mudstones and of mica flakes in the 

shale indicates that most if not all the material was derived 

from the same source. 

The mudstone that makes up the bulk of the lower member of the formation is reddish brown to yellowish brown 

and commonly forms smooth slopes. The rock is distinctive 

and unlike any of the other rocks in the area. It consists of 

an evenly but indistinctly bedded homogeneous poorly 

sorted mass of clay, silt, and sand particles through which 

are scattered numerous rounded grains of quartz as much 

as 1 mm in diameter and flakes of mica. In many places 

the mudstone contains small lenses of sandy arkose 1-12 

inches long and Ya-1 inch thick. Interbedded with the 

mudstone are beds of reddish-buff coarse-grained dirty 

arkosic sandstone as much as 10 feet thick and some thinbedded shale. The arkosic sandstone forms ledges which 

weather to rounded knobby forms where cut by joints. 

The middle member of the Moenkopi Formation is dark 

chocolate brown and is characterized by abundant arkose 

and arkosic conglomerate. These beds rather closely resemble those of the underlying Cutler Formation, except that 

in general they contain a larger fraction of clay and silt and 

hence have a dirtier appearance than do the Cutler beds. 

Grains of quartz, fresh feldspar, and dark minerals, and, in 

the conglomeratic beds, pebbles and cobbles of granite, 

gneiss, schist, and quartzite-materials derived from the 

Precambrian rocks of the ancestral Uncompahgre highland 

- are mixed together in poorly sorted strongly crossbedded 

lenses. These crossbedded lenses contrast strongly with the 

even horizon tal bedding so characteristic of most of the 

Moenkopi. Interbedded with the lenses of arkose and 

arkosic conglomerate is a very fine grained typically ripplebedded chocolate-colored shale, the individual laminae of 

which are paper thin and coated with fine-grained mica. 

Rain-spatter marks and mud cracks are common; the mud 

cracks generally show up best as casts on the underside of 

sandstone layers. 

The upper member of the Moenkopi Formation consists 

largely of fine-grained ripple-bedded chocolate-colored 

shale and consequently crops out as smooth uniform slopes. 

In Sinbad Valley is a local facies about 200 feet thic~ that 

FrGURE 5.-View northward of The Palisade north of Gateway. Pc, Cutler Formation; liml, lower member of Moenkopi Formation; limm, 

middle member of Moenkopi Formation; lie, Chinle Formation; lik, Kayenta Formation; je, Entrada Sandstone; liw, Wingate Sandstone; Js, Summerville Formation; Jms, Salt Wash Member of Morrison Formation. 
14 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Shoemaker ( 1955) originally included in the lower part 

of the Chinle Formation but that he and Newman (Shoemaker and Newman, 1959) later placed in their Pariott 

Member of the Moenkopi. Their description of these beds 

in their measured section, however, bears little resemblance 

to their description of the beds in their type section of the 

Pariott Member; on the other hand, these beds do not 

closely resemble the beds usually found in the lower part 

of the Chinle Formation either. We feel, therefore, a 

measure of doubt concerning the proper designation of 

these beds but, because the map was based on their inclusion in the Chinle, we have chosen to follow Shoemaker's 

original designation and consign them to the Chinle. The 

stratigraphic section of the Moenkopi Formation in Sinbad 

Valley given below differs from Shoemaker and Newman's 

published section only in the exclusion of these enigmatic 

beds and the inclusion of an unconformity at the top of 

the section which they eliminated from their published 

section in a later report. lnterlayered with the shale are a 

few thin beds of dirty arkosic conglomerate and sandstone. 

Gypsum is a widespread constituent of the Moenkopi 

Formation and commonly occurs as thin seams and veins 

but in places occurs as bedded gypsum near or at the base 

of the formation. In the Dolores River Canyon south of 

Gateway, such a bed may be traced for several miles. 

Although no limestone beds are found in this area, many 

layers are somewhat calcareous and calcite appears to be 

a ubiquitous cement. 

The foJlowing sections are typical of the Moenkopi Formation as exposed in this area: 

Section of the Moenkopi Formation in Sinbad Valley, sec. 15, 

T. 49 N., R. 19 W., Mesa County, Colo. 

[Measured by E. M. Shoemaker and W. L. Newman] 

Chinle Formation. Feet 

Unconformity. 

Moenkopi Formation: 

Contact with overlying Chinle Formation sharp but not 

noticeably channeled or angular. 

Shale and sandstone interbedded; light brown to chocolate brown. Sandstone is fine grained,1 ledge forming, 

ripple bedded and crossbedded. Shale is very thin 

bedded, ripple bedded. Unit contains scattered beds 

of granule conglomerate 1-6 in. thick._______________________ 38.6 

Sandstone, light-brown to purplish-brown; forms prominent ledge; consists of alternating massive, crossbedded layers 1-4 ft thick, and thin- and ripplebedded, somewhat shaly layers 0.1-1.0 ft thick. Scattered lenses and pods of granule conglomerate __________ 21.5 

Sandstone and mudstone interbedded. Sandstone is light 

reddish brown, very fine to fine grained; contains 

thin . lenses of quartz and crystalline rock granules. 

Mudstone is dark reddish brown, poorly sorted. Unit 

consists of about equal parts of sandstone and mudstone in beds 0.1-0.3 ft thick -------------------------------------- 41.2 

Mudstone, chocolate-brown, poorly sorted; contains thin 

interbeds. of -very fine grained sandstone -------------------- 57.5 

Moenkopi Formation-Continued Feet 

Sandstone, reddish-brown, ledge-forming; crossbedded 

on small scale; calcareous cement; contains numerous 

thin stringers of gypsum subparallel to bedding. Mica 

flakes coat bedding planes ··-·--·-··-···--·--------------------------- 3.4 

Shale, chocolate-brown, ripple-bedded; contains interbeds of fine-grained light-brown and reddish-brown 

sandstone 0.2-1 ft thick. Upper part contains considerable gypsum which occurs as nodules and thin 

stringers ... __ . __ ... _____ . _. _ .. ____ . _ ·-----------. ____ ·-------------------------- 42.7 

Sandstone, light-brown to light-reddish-brown, finegrained, ripple-bedded. Contains thin beds of 

chocolate-brown shale and scattered thin lenses of 

granule conglomerate ··-····-····--·----------·-·---··----------------- 20.1 

Shale, chocolate-brown, ripple-bedded; lamination 

paper thin; minor interbeds of very fine grained to 

fine-grained light-brown sandstone. Ratio of shale to 

sandstone about 4: 1. Some gypsum, which occurs as 

thin stringers. Beds are locally altered to greenish 

gray or gray .......... ------···--------------------------------------------·- 89.0 

Sandstone, light-gray to greenish-gray, very fine grained 

to fine-grained; some beds are flaggy and shaly, the 

rest are massive and crossbedded. Light-gray color 

due to local altered zones that cut across bedding 

planes ·---·----·-··--------------------------------------------------------------- 21.8 

Mudstone, chocolate-brown, micaceous ·-------··---------------- 1.0 

Interval covered ·····----------------------------------------------------------- 34.6 

Sandstone, light-brown, fine-grained; contains thin interbeds of chocolate-brown shale and thin veinlets 

of gypsum ··-----------·-----------·--·----------------------------------------- 18.5 

Shale, chocolate-brown; beds have average thickness of 

about 5 ft; thin, ripple-bedded sandstone layers make 

up about one-fourth of unit. Moderately gypsiferous.. 28.5 

Sandstone and shale interbedded. Sandstone is light 

brown, fine to very fine grained, it forms beds 0.1-0.5 

ft thick, and makes up about 75 percent of unit. 

Shale is chocolate brown, micaceous, thin and ripple 

bedded. Unit also contains scattered beds and lenses 

of coarse grit containing quartz and feldspar fragments. Gypsum is abundant as disseminations and 

thin stringers; some veinlets of satin spar ------------------ 77.9 

Mudstone and sandstone interbedded. Mudstone is 

chocolate brown, Sandstone is reddish brown to 

purplish brown, medium to coarse grained, argillaceous and poorly sorted, thinly and evenly bedded; 

some is micaceous, ripple bedded. Unit contains some 

lumps of gypsum ··---··--·-----------------------------------------------· 62.1 

Sandstone, light-brown, conglomeratic, arkosic, crossbedded ··-----------------------------------------------------··---·--·-·---·---- .5 

Shale and sandstone interbedded; poorly exposed ________ 5.2 

Sandstone, light-brown, conglomeratic, arkosic -------------- .8 

Shale, chocolate-brown, ripple-bedded; contains some 

interbeds of reddish-brown medium-grained sandstone and coarse-grained conglomeratic standstone____ 30.6 

Sandstone, yellowish-gray to light-yellowish-brown, 

medium- to coarse-grained, conglomeratic ---------------·-- 4.8 

1 Textural terms used in this report correspond to the Wentworth size 

scale (Wentworth, 1926) except that. two additional size grades were inserted 

in the sand-sized part of the scale because of the abundance of sand falling 

in the ranges medium-fine and medium-coarse. The limits of the size grades 

in the sand sizes are: coarse, 1.0-0.695 mm; medium-coarse, 0.595-0.420 mm; 

medium, 0.420-Q.297 mm; medium-fine, 0.297-0.210 mm; fine 0.210-0.125 mm. 
TRIASSIC SYSTEM- MOENKOPI FORMATION 

Moenkopi Formation-Continued 

Conglomerate; reddish brown to purple, yellow, and 

gray at top, ledge forming and caps ridge, very poorly 

sorted; crossbedded; material ranges in size from 

clay particles to pebbles 2 in. across. Pebbles consist 

of granitoid rocks, gneiss, schist, and quartzite ......... . 

Shale, chocolate-brown, ripple-bedded ............... . 

Conglomerate, light-brown, crossbedded; consists largely 

of quartz granules ............................... . 

Snndstone, reddish-brown, micaceous and argillaceous, 

rather evenly bedded; crossbedding rare; sand grains 

range frcnn medium to coarse .................................. . 

Conglomerate, light-brmvn, crossbedded; consists largely 

of fairly well rounded quartz ganules . .. .. . ....... .. 

Sandstone, rnudstnne, and sandy shale interbedded, 

light-reddish-brown. Sandstone is medium grained; 

forrns beds generally less than half an inch thick. 

Unit is evenly bedded and ripple bedded ................. . 

Sandstone, reddish-brown and purplish-brown, mediumgrained, ledge-forming, evenly bedded .................... . 

Shale, reddish-brown, sandy; contains scattered layers 

of coarse-grained sandstone 0.5-1 in. thick; mudcracks at base of nne sandstone layer . .. . .:: ....... ...... . 

Sandstone and shale interbedded; reddish brown, ledgefonning, thin bedded, ripple bedded; sandstone is 

medium to coarse grained ............................... .. 

Shale, reddish-brown, evenly bedded and ripple-bedded; 

contains interbeds of medium-grained to conglomeratic sandstone as much as 4 in. thick .. . . . . .......... . 

Sandstone, conglomeratic, crossbedded; contains thin 

interbeds of reddish-brown shale. Lowermost sandstone bed contorted, and lower surface is marked by 

ropy ridges due to compression and flowage of subjacent shale ........................................ .. 

Mudstone, reddish-brown, sandy, micaceous, rippleJnarked ............................................... . 

Sandstone, conglomeratic, purple, strongly crossbedded; 

unit consists of numerous interfingering lenses. Pebbles consist of quartz, granite, and perthite. Upper 

part contains scattered thin beds of red shale .... .. . 

Mudstone, chocolate-brown, micaceous, evenly bedded; 

beds are 0.1-1 ft thick; some interbeds of sandstone .. 

Sandstone, muddy, ledge-forming; upper part contains 

chips of mudstone and sandstone; basal bed of unit 

contorted ............................................. .. 

Mudstone, chocolate-brown, sandy, rather massive; upper part of unit contorted; lower part contains beds 

of sandstone ............................................ . 

Sandstone, light-brick-red, muddy, very poorly sorted, 

nearly massive. Consists largely of a muddy matrix 

through which are scattered grains of sand as much 

as 2 mm across ...................................... .. 

Arkose, purple, medium-grained, soft; appears to be reworked material from Cutler Formation ................... . 

Gypsum, white, massive, sugary ................................... .. 

Feet 

5.4 

10.8 

1.4 

15.5 

1.3 

46.6 

9.4 

21.7 

5.4 

15.8 

8.0 

11.8 

26.5 

12.7 

22.1 

22.6 

227.8 

3.8 

3.3 

Total Moenkopi Formation . .. ........................... 1,072.2 

Unconformity. 

Cutler Formation. 

Section of the Moenkopi Formation, measured on west side of the 

Dolores River Canyon, sec. 11, T. 50 N., R. 19 W., 

Montrose County, Colo. 

[Measured by F. W. Cater] 

Chinle Formation. 

Unconformity. 

Moenkopi Formation: 

Contact with overlying Chinle Formation sharp and 

unconformable. 

Shale and sandstone interbedded. Sandstone is light 

reddish brown to brown, fine grained, ripple bedded 

and cross bedded; some beds arkosic. Shale is brown 

to chocolate brown, very thin bedded, ripple bedded .. 

Conglomerate, light-reddish-brown, crossbedded .......... 

Shale, mudstone, and sandstone interbedded. Shale is 

brown to chocolate brown, very thin bedded, ripple 

bedded. Mudstone is dark reddish brown, poorly 

sorted; contains thin interbeds of fine-grained sandstone. Sandstone is light reddish brown to brown, very 

fine to fine grained; some beds are dirty and arkosic .. 

Conghmerate, light-red; pebbles consist largely of 

granitoid rocks and gneiss ......................................... .. 

Shale and sandstone interbedded. Shale is chocolate 

brown, very thin bedded, ripple bedded. Sandstone is 

reddish brown to chocolate brown, dirty, arkosic; 

some beds consist largely of crystalline rock granules .. 

Conglomerate, light-reddish-brown; consists largely of 

pebbles and small boulders of granitoid rocks and 

gneiss as much as 7 in. across ..................................... . 

Shale, chocolate-brown, very thin bedded, ripplebedded; mica flakes coat bedding planes; some silty 

beds and few thin interbeds of very fine grained sandstone ......................................................................... .. 

Mudstone and grit interbedded. Mudstone is chocolate 

brown, poorly sorted, micaceous. Grit is reddish 

brown, arkosic; granules as much as Y4 in. across ... . 

Sandstone, red, fine- to medium-grained, nonarkosic .. .. 

Sandstone, light-red, coarse-grained, arkosic; contains 

two or three layers of shale, each Y2 in. thick .......... . 

Arkose, brown, coarse-grained, massive; contains much 

clay and silt ............................................. .. 

Shale, chocolate-brown, very thin bedded, ripple-bedded 

Shale and sandstone interbedded. Shale is brownish red, 

evenly bedded and ripple bedded, silty; beds 3-8 ft 

thick. Sandstone is reddish buff, evenly bedded, fine 

to medium grained; beds 1-4ft thick ....................... . 

Shale, red, poorly exposed ............................................. . 

Sandstone, reddish-buff, coarse-grained, indistinctly 

bedded, massive, arkosic. Outcrops weather to rounded, mammillary forms ............................................... .. 

Mudstone reddish-buff to brownish-red, thin-bedded, 

ripple-bedded; bedding planes coated with mica; 

contains numerous grains of coarse sand and numerous lenses of coarse arkosic sand 1-12 in. long and 

Ya-1 in. thick .. . . ....................................................... . 

Sandstone, reddish-buff, coarse-grained, dirty and 

poorly sorted, arkosic ................................................. . 

Mudstone, same as 12.9-ft mudstone unit above .......... .. 

Gypsum, white, massive, sugary, bedding contorted .... .. 

Conglomerate and mudstone, probably reworked from 

underlying Cutler Formation .................................... .. 

Total Moenkopi Formation ................................ . 

Unconformity. 

Cutler Formation. 

15 

Feet 

76.0 

2.0 

38.8 

5.3 

45.8 

12.1 

41.5 

5.9 

.9 

5.7 

11.2 

22.0 

53.6 

6.2 

10.3 

12.9 

4.1 

29.7 

6.0 

4.5 

394.5 
16 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Section of the Moenkopi Formation in Paradox Valley, 

north-central part sec. 10, T. 47 N., R. 18 W. 

[Measured by R. P. Fischer and A. P. Butler] 

Chinle Formation. 

Unconformity. 

Moenkopi Formation: 

Shale, reddish-brown to purplish-brown, very thin 

bedded, ripple-bedded. Unit poorly exposed ............. . 

Sandstone and siltstone interbedded. Sandstone is reddish brown to brown, very fine to fine grained, thin 

bedded. Siltstone is brownish red, thin bedded, and 

ripple bedded. Unit forms steep slope 

Conglomerate, purplish-red; consists of igneous and 

metamorphic rock pebbles and pebbles of quartz as 

much as 1 Y2 in. across in a matrix of ·coarse, arkosic 

sandstone ....................................... . 

Sandstone and argillaceous sandstone interbedded. 

Sandstone is brown, medium to coarse grained; beds 

3-6 in. thick. Argillaceous sandstone is brown, fine 

grained, evenly bedded and ripple bedded; contains 

considerable clay ................... . 

Sandstone and conglomerate interbedded. Sandstone is' 

reddish brown, fine to medium grained, even and 

thick bedded. Conglomerate is brown; consists of pebbles of igneous and metamorphic rocks, quartz and 

shale. Unit forms a cliff . ·····································-----

Sandstone, reddish-brown to chocolate-brown, fine- to 

medium-grained, thin- and ripple-bedded; some beds 

argillaceous and a few beds separated by shaly partings. About 5 ft below top of unit is a massive, crossbedded, shale-pebble conglomerate bed. Unit forms 

a slope . . ····-·--------------------·--·-·--------------------------··-

Sandstone and conglomerate interbedded. Sandstone is 

reddish brown and chocolate brown, fine to coarse 

grained, slightly arkosic, thin and ripple bedded. 

Conglomerate is reddish brown; consists dominantly 

of pebbles of igneous and metamorphic rocks but contains some shale and sandstone pebbles in an arkosic 

sandstone matrix; crossbedded and lenticular. Unit 

forms a cliff ... ·············-------------·---·--·----··--··-----·-·----···-

Siltstone and silty sandstone. Siltstone is brownish red; 

contains fine grains of amber qu_artz and flakes of 

mica; some beds shaly; thin and evenly bedded. Silty 

sandstone is reddish brown, fine to coarse grained, 

poorly started; consists of subangular to· well-rounded 

amber quartz and feldspar grains; thin to thick 

bedded, sparse ripple-bedded layers. Some silty sandstone occurs as thin lenses in siltstone. Unit forms 

ledgy slope .. ---······----·----------------------------·-----------

Silty sandstone to sandy siltstone, brownish-red; grades 

froin siltstone to medium-grained sandstone, which 

all contains coarse to very coarse grains of sand; 

poorly sorted; contains clay and abundant mica 

flakes. Bedding indistinguishable except in upper part 

where bedding is better defined. Unit poorly exposed 

Total Moenkopi Formation 

Unconformity. 

Cutler Formation. 

Feet 

10 

115 

3 

10 

20 

53 

35 

75 

70 

391 

CONDITION OF DEPOSITION 

The source of most of, if not all, the material in the 

l\1oenkopi Formation was the ancestral Uncompahgre highland, the same source as that of the underlying Cutler 

Formation. However, the differences in bedding characteristics and composition between the Moenkopi and the 

Cutler point to differences not only in the depositional 

environment but also in the topography of the contributing 

highland. 

The unconformity underlying the Moenkopi Formation 

indicates a considerable time lapse between the deposition 

of the Cutler and the deposition of the Moenkopi. The erosion surface represented by this unconformity is flat, with 

practically no local relief. Dane ( 1935, p. 52-53) suggested 

that this surface may have been one of marine planationa suggestion that seems reasonable because to the west in 

Utah the Moenkopi contains undoubted marine units and 

because gypsum beds and calcareous material are present 

in this area. How far to the east such planation may have 

extended is not known, for the evidence eastward beyond 

the edge of the Moenkopi Formation was destroyed by 

later events. 

Regardless of the means by which the unconformity 

underlying the Moenkopi was formed, one thing seems 

clear: by the beginning of Moenkopi time the area southwest of the old Uncompahgre highland was largely covered 

by a body of shallow water, possibly marginal to the sea 

that existed farther west in Utah during the early part of 

Moenkopi time ( Gilluly and Reeside, 1928, p. 65-66). In 

the San Rafael Swell of Utah, massive marine limestones 

were deposited to a thickness of 140 feet, and marine fossils 

have been found in the 11oenkopi as far east as Salt Valley, 

Utah (Dane, 1935, p. 52). In western Colorado the even 

bedding in the lower part of the Moenkopi and the general 

lack of sedimentary structures commonly associated with 

continental deposits suggest deposition in shallow water. 

The poor sorting of the mudstones, however, suggests rapid 

deposition or dumping of material. Nonetheless, the abundant clay in the mudstone suggests that in its provenance 

on the old Uncompahgre highland, weathering was keeping 

pace with erosion. Very likely the old highland had been 

considerably reduced and relief was only moderate, whereas 

during Cutler time the relief was probably extreme and 

erosion was so rapid that decomposition of the rocks was 

relatively inconsequential. 

The fluvial arkosic and conglomeratic beds of the middle 

member of the Moenkopi Formation suggest a period of 

regional uplift and withdrawal of the body of shallow water 

from the area of deposition. Streams that drained the 

rejuvenated Uncompahgre highland cut into unweathered 

rocks in places and deposited the resulting debris as lenses 

of arkosic sandstone and conglomerate. The crossbedded 
GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 17 

and lenticular nature of the coarse-grained material and 

the mud cracks and rain prints in the finer grained material 

are characteristic of fluvial deposition. 

During the remainder of Moenkopi time the highland to 

the east was gradually reduced and the material from it 

was deposited near or at sea level on a terrane of widespread mud flats and local ponds. It is of interest to note 

that the material composing the ripple-bedded thinly laminated shales of the upper part of the 1\t[oenkopi is duplicated 

exactly in composition and color by the chocolate-brown 

residual clays derived from the Precambrian rocks on 

the remarkably flat peneplaned surface of the ancestral 

Uncompahgre highland beneath the Chinle Formation. 

The movement and extrusion of the plastic beds of the 

Paradox Member of the 1-Iermosa Formation locally disturbed this process of generally placid deposition on a flat, 

periodically inundated coastal plain. By the beginning of 

Moenkopi time the great elongate masses of intrusive .salt 

and gypsum had breached the overlying rocks and were 

reexposed. The movement of salt had· decided local effects 

on the thickness of the l\.foenkopi formation. Unusually 

thick Moenkopi is exposed on the northeast wall of Sinbad 

Valley, where, only 12 miles from the edge of the formation 

on the flank of the Uncompahgre Plateau, the formation 

is nearly 1 ,lOO feet thick. Dane ( 1935, p. 44-45) described 

beds 855 feet thick along the Colorado River on the north 

side of the Fisher Valley-Salt Valley anticline, and in general the Moenkopi is unusually thick along a narrow belt 

lying between the front of the old highland· and the salt 

intrusion underlying Sinbad, Fisher, and Salt Valleys. 

Southwest of these valleys the formation thins to less than 

500 feet, and it maintains about this thickness for considerable distances except in the immediate vicinity of other 

salt anticlines where the basin of deposition deepened 

because of the sinking of beds overlying the Paradox Member as the salt and gypsum were squeezed laterally into the 

salt cores. Salt flowage was manifestly rapid; in places along 

the north wall of Sinbad Valley, sharp intraformational 

unconformities formed within the Moenkopi, and, in other 

places, slump structures developed where the soft, as yet 

unconsolidated sediments of the formation were tilted 

upward by the intruding salt and in turn slid down the 

resulting slope. 

THICKNESS 

The thickness of the Moenkopi is most variable in the 

upper member; it is fairly uniform in the lower member 

and somewhat less uniform in the middle member. On the 

north wall of Sinbad Valley the upper member attains a 

thickness of more than 500 feet, and the middle and lower 

members nearly 300 feet each. Southward in the Paradox 

Valley area where the formation is about 500 feet thick, the 

three members are roughly of equal thickness. 

CHINLE FORMATION 

The Chinle Formation, of Late Triassic age, unconformably overlies the Moenkopi Formation west of the southwest 

flank of the Uncompahgre Plateau (see fig. 5), but along 

the flank of the plateau it rests on successively older rocks 

to the east, first on the Cutler Formation and then on 

Precambrian rocks. The unconformity is remarkably flat 

and is of regional extent. Except in the vicinity of the salt 

anticlines and near the Uncompahgre Plateau, it is not 

markedly angular, and over most of the Colorado Plateau 

region it might more properly be termed a disconformity. 

The upper contact of the Chinle is conformable with 

overlying beds. 

The unconformity on the Precambrian rocks is exceptionally well exposed high along the sides of Unaweep 

Canyon and at other places where the flanks of the U ncompahgre Plateau have been dissected. In most places where 

the post-Moenkopi pre-Chinle surface cuts Precambrian 

rocks, the underlying rocks are fairly fresh and not deeply 

weathered, but on the North Fork of Mesa Creek in sec. 27, 

T. 50 N., R. 17 W., the crystalline rocks under the erosion 

surface were thoroughly decomposed to a reddish-chocolatecolored plastic clay, several feet deep, on which the Chinle 

was deposited. Although the textures of the preexisting 

granitic rocks are preserved, all minerals except quartz are 

altered to clay. Below this zone of clay, the altered rock 

grades downward through several feet into fresh rock. A 

line of seeps and small springs marks the unconformity 

between the Chinle and the zone of impervious residual 

clay. Very likely the zone of residual clay was much more 

widespread over the surface of the ancestral Uncompahgre 

highland in late Moenkopi time and possibly was the source 

of the material in the ripple-bedded shales in the upper 

part of that formation. In the area of this report the. hiatus 

represented by this unconformity probably occupied much 

of, if not all, Middle Triassic time plus some of the Early 

Triassic because beds equivalent to the Monitor Butte 

Member and older members of the Chinle are not present 

(Stewart and others, 1959, p. 503). Stokes ( 1950) believes 

that this unconformity developed by a process of large-scale 

pedimentation. The fact that the erosion surface smoothly 

truncates both weathered and fresh Precambrian rocks 

indiscriminately may lend support to his hypothesis, but on 

the other hand, the products of this pedimentation have 

not been recognized. 

The Chinle Formation is widely exposed in the salt anticline valleys, along the flanks of the Uncompahgre Plateau, 

and. in most of the deeper canyons throughout the area. It 

crops out almost everywhere as steep slopes broken in places 

by more resistant ledges (fig. 5). The formation consists of 

red to orange-red siltstone, with interbedded red finegrained s~ndstone, shale, clay-pellet conglomerate containing limestone pebbles, and, at the base, a similar conglom-
18 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

crate that also contains quartz pebbles. :\ few thin beds of 

gypsum occur locally at the base. The lithologic units arc 

lenticular and discontinuous. In this area the formation has 

not been divided into separate members as it has to the 

southwest (Stewart and others, 1959), but it most nearly 

resembles the Church Rock Member, although the correlation is not exact. The lowermost beds that contain quartz 

pebbles probably correlate with the Moss Back Member. 

l\tfuch of the Chinle Formation consists of the red to 

orange-rec~ siltstone, an indistinctly bedded rock that 

weathers into angular prismatic pieces. Evenly bedded shale 

is sparse. The sandstone varies from massive to conspicuously ripple bedded and is very fine grained and well 

sorted. It is most abundant in the upper part of the formation, where it forms massive beds as much as 30 feet thick. 

According to R . .'\. Cadigan (written commun., 1954), the 

mean composition of nine samples of sandstone that were 

collected from this area or from nearby areas is quartz, 63 

percent; silicified tuff and chert, 10 percent; quartz overgrowths and silica cement, 2 percent; calcite, 19 percent; 

potassic and sodic feldspar, 4 percent; muscovite, 2 percent; 

biotite and heavy minerals, less than 1 percent. The limestone-pebble-clay-pellet conglomerate is highly distinctive 

and is commoner in the lower part of the formation than 

in the upper. It forms rudely crossbedded lenses as much 

as 15 feet thick that tend to stand out as resistant ledges. 

Structureless gray and reddish limestone pebbles and cobbles, as much as 5 inches across, and clay galls and pellets 

arc the most abundant fragments in the rock; shale and 

sandstone pebbles arc less common. The matrix consists of 

clay, silt, and sand. According to Schultz ( 1963, p. C41) 

the clay fraction of the Chinle in this area is largely illitic. 

In the basal part of the formation the conglomerate also 

contains many angular fragments and pebbles, as much as 

an inch across, of clear quartz. In places these quartzbearing lenses are relatively clean and consist of scattered 

pebbles and angular to subrounded fairly well sorted granules with an average diameter of about 2 mm. In the SEY4 

sec. 22, T. 48N., R. 19 \V., about 2 miles northeast of the 

town of Paradox, the basal part of the Chinle contains 

abundant chert as nodules and irregular lenses. 

The following sections are representative of sections 

measured at several localities: 

Section of the Chinle Formation on Dry Creek in the Naturita 

quadrangle, sec. 4, T. 45 N., R. 16 W., Montrose County, Colo. 

[Measured by L. C. Craig and L. R. Stieff] 

Wingate Sandstone: Feet 

Reddish-brown cliff-forming sandstone. 

Chinle Formation: 

Sandstone, pink to light-brownish-red, fine-grained, 

platy- to slabby-bedded ······---··--------·······----·-·-·············· 38.7 

Sandstone, dark-red to maroon, friable rubbly weathering; contains two 2-ft beds of pink to greenish very 

Chinle Formation· Continued Feet 

fine grained sandstone I H . .i 

Sandstone, pink to hrick-rt'd, massin~-hedded, very fine 

grained 1 1.9 

Claystone; purple to maroon in lower half, brick red 

and fine grained sandy in upper half 17.2 

Sandstone, pink, red-weathering, very fine grained, 

ripple-laminated, thick-bedded 17.9 

Claystone and sandstone interbedded; claystone is brick 

red, fissile to rubbly, faintly laminated; sandstone is 

brick red, very fine grained .. .. ... .. . 75.4 

Sandstone, conglomeratic, brown to purplish; contains 

pebbles of calcareous claystone and siltstone as much 

as 2 in. in diameter; abundant pelecypods and one 

low-spired gastropod seen 3.8 

Claystone, dark-red to brick-red, variably silty to sandy, 

rubhly weathering ...... ... . . . 64.7 

Conglomerate, brown to purplish; consists of limestone 

pebbles as much as 5 in. in diameter, some red clay 

and maroon siltstone pebbles; numerous bone fragments ............. __ 13.4 

Sandstone, light-gray, gray- to white-weathering, fine 

grained, highly cross laminated; composed of clear 

quartz with minor green, pink, and red accessory 

minerals .. ························------ 11.6 

Sandstone, grayish-green and gray to purplish, maroonweathering, heavy-bedded; clear quartz with sparse 

green and pink accessory minerals and some mica 

flakes ... ... . ............ .............. 40.8 

Sandstone, dark-red to maroon, thick- to platy-bedded, 

calcareous, ripple-laminated to horizontally laminated . ..... . ... . ... .............. 38.7 

Sandstone and shale interbedded. Lower 10 ft. and 

upper 14 ft. of massive faintly laminated fine-grained 

reddish clay to purple calcareous sandstone with thin 

limestone-pebble conglomerates. Middle 5 ft. is red 

silty papery shale .. . ............. 29.0 

Sandstone, red, very fine grained; massive below, platy 

to shaly weathering above .. . . _ ...... 17.2 

Sandstone, red and gray, red-weathering, very fine 

grained, slabby bedded, calcareous, structureless to 

faintly laminated; thin claystone partings . .............. . 18.3 

Claystone, red, silty to sandy, fissile below and platy 

above; slightly micaceous, ripple laminated ........ .. 20.1 

Sandstone and conglomerate interbedded; red very fine 

grained, sandstone; channeling; gray to brown conglomerate; contains gray and reddish limestone 

pebbles as much as 1 in. in diameter . .. ...... ......... 10.5 

Sandstone, claystone, and conglomerate interbedded; 

sandstone and claystone are brick red, rubbly to platy 

weathering, in part ripple laminated; sandstone is 

very fine to fine grained; conglomerate is brownish 

weathering and contains siltstone and limestone 

pebbles ... .... ··········--··--··-····---------- 56.7 

Sandstone and siltstone, maroon to grayish-purple and 

g-reenish-gray, streaked and mottled; contains disseminated coarse grained to very coarse grained clear 

to amber quartz; bed of nodular gray to pink chert 

6-18 in. thick at top ········---···-········------------------- 13.5 

Total Chinle Formation ··················-··--·---··········-·--·-··-- 517.9 

Disconformi ty. 

Moenkopi Formation. 
GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 19 

A section measured in the Dolores River Canyon south 

of Joe Davis Hill differs in color somewhat from other 

exposures in the area; the colors tend more to shades of 

green, but .tl~esc greenish colors seem to be secondary, the 

result of slight alteration. J·Tcre the l\1oenkopi Formation is 

absent and the Chinle rests unconformably on the Cutler. 

Section of the Chinle Formation on the west side of the Dolores 

River south of joe Davis Hill in unsurveyed sect. 12, 

T. 42 N., R. /8 W., San Miguel County, Colo. 

[Measured by I". W. Cater] 

Wingate Sandstone. Feet 

Chinle Formation: 

Siltstone with thin interbeds of shale and very fine 

grained sandstone. Siltstone is brick red, indistinctly 

bedded, weathers to angular; prismatic fragments. 

Sandstone is light red to brick red, ripple bedded to 

massive; shale is red to maroon, thin bedded __ 40 

Sandstone, brick-red, very fine grained, massive .... ....... 8 

Shale, red to 1naroon, sandy .... . .... ......... ........... ... 17 

Sandstone, pink tu brick-red, fine-grained, massive, 

faintly crossbedded . .. .. ................................. 21 

Siltstone, brick-red, sandy; weathers to prismatic fragJnents; a few massive and ripple-bedded sandstone 

layers in upper part. Unit weathers to smooth slope .. 312 

Sandstone, brick-red and reddish-brown, thin- and 

ripple-bedded, very fine grained; shale partings in 

upper part and layer of limestone-pebble-clay-pellet 

conglomerate 1 ft. thick in center of unit.................... 31 

Sandstone, brick-red, very fine grained, thin-bedded, 

ledge-forming . . . ... .. ......................................... 25 

Sandstone, pink to brick-red, very fine grained, ripplebedded ....... .................................................... 22 

Conglomerate and sandstone. Conglomerate is dirty 

green, rudely bedded, and consists of pebbles of limestone, shale, and sandstone. Sandstone is green to 

red, thick bedded below, thin bedded above .......... .. 

Siltstone, light-green, thinly bedded, fissile; rill and 

ripple marks; some poorly exposed interbeds of shale. 

Whole unit forms slope .. .. . ............................... .. 

Sandstone, ligh t-dirt:y-green to brick-red, very fine to 

medium-fine-grained; crossbedded on small scale, 

1nassive, clifl' forming; contains some conglomeratic 

lenses with some arkosic material and scattered 

quartz granules and small pebbles . . ............... .. 

Shale; light green with some chcolate-colored streaks, 

fissile .. .............. . .. .................................. . 

Sandstone and conglomerate, green to red. Sandstone 

is medium fine grained, thin bedded; conglomerate 

forms channel fills in sandstone, contains pebbles of 

limestone, shale, sandstone, and quartz .................... . 

Conglomerate, green to red, rudely bedded; contains 

abundant pebbles of limestone and lesser quantities 

of shale pebbles and clay galls; both angular and subrounded fragments of clear quartz common; scattered 

chert pebbles .................................... .. 

Total Chinle Formation 

Unconformity. 

Cutler Formation. 

32 

95 

23 

3 

6 

10 

645 

The Chinle Formation thins considerably near the front 

of the Uncompahgre Plateau, apparently by internal thinning. The following section is representative of the formation ncar and on this plateau: 

Section of the Chinle Formation 3 miles east of Gateway in sees. 17 

and 20, T. 51 N., R. 18 W., Mesa County, Colo. 

[Measured by C. N. Holmes] 

Wingate Sandstone: 

Reddish-brown, massive cliff-forming sandstone. 

Chinle Formation: 

Siltstone, brick-red; scattered coarse quartz grains and 

mica flecks; rounded weathering; 2 in. of white very 

fine grained sandstone to siltstone at top contact; 

Feet 

upper contact sharp ---------------------------------------------------- 10.0 

Siltstone, brick-red, thin-bedded; scattered very fine 

quartz grains, rock fractured, weathers to rubbly 

slope ..... ............................................................... 60.1 

Siltstone, brick-red; scattered very fine quartz grains 

and mica flecks; !edgy; silty shale partings help form 

rough-weathering slope ------------------------------------------------ 7.8 

Siltstone, brick-red; rubbly slope partly covered .......... 29.1 

Siltstone, brick-red, very fine grained, shaly; contains 

mica flecks ----------------------------------------------------------------- 29.0 

Siltstone; forms brick-red poorly exposed rubblecovered slope . ............................................................. 31.9 

Conglomerate; contains silty concentrations as much as 

1 in. across; shale lenticules; calcite fracture filling 

1/16 in. thick ..... -------------------------------------------------------- 4.1 

Siltstone, mottled red and white; scattered coarse 

grains, mica flecks, and calcite veinlets ...................... 26.6 

Conglomerate, purpish-red; ranges from coarse sand to 

round and subangular pebbles of shale and siltstone 

as much as 2 in. long ................................................ 2.5 

Siltstone, reddish-brown, shaly, partly covered 14.6 

Total Chinle Formation .......................................... 215.7 

Unconformity. 

Moenkopi Formation. 

THICKNESS 

The Chinle Formation in this area reaches a maximum 

thickness of about 7 50 feet on the northeast wall of Sin bad 

Valley. Along the flanks of the salt cores, it wedges out 

entirely in many places. Elsewhere the thickness varies considerably from place to place but generally ranges from 

450 to 600 feet, except on the Uncompahgre Plateau where 

the formation is considerably thinner. 

AGE AND CONDITIONS OF DEPOSITION 

Fossils are uncommon except in a few localities within 

this area, and they consist only of plant impressions, scattered fragments of bone and petrified wood, and freshwater pelecypods and gastropods, none of which are particularly useful for determining age. Numerous vertebrate 

fossils from other locations on the Colorado Plateau 

(Camp, 1928; Baker, 1933, p. 41), however, establish 

the Chinle as unquestionably of Late Triassic age. 
20 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

The physical nature of the sediments and the fossil content indicate that the Chinle Formation was deposited 

under continental conditions. The lenticular beds and the 

channel-filling sandstones and conglomerates point to fluvial 

sedimentation. The mode of origin of the limestone-pebbleclay-pellet conglomerate is not clear; a lthough most of the 

limy pebbles seem to have been transported to their present 

sites, some of them appear to be concretionary. The clay 

pellets and pebbles of shale, siltstone, and sandstone that 

a re associated with the concretionary limy pebbles indicate 

the breakup of preexisting beds and redeposition of the 

fragmented material. The fragments and pebbles of clear 

quartz in the lowermost conglomerate beds probably were 

derived from the deeply weathered Precambrian rocks to 

the east upon which rest later beds of the Chinle. 

TRIASSIC AND JURASSIC SYSTEMS-GLEN CANYON GROUP 

The Glen Canyon Group comprises three conformabl e 

units in this area which are, in ascending order, the Wingate Sandstone of Late Triassic age, the K ayenta Formation 

of Late Triassic(?) age, and the Navajo Sandstone of 

Triassic(?) and Jurassic age. The group is remarkable for 

its almost unparalleled sequences of clean, massive, cliffforming sandstones from which are carved some of the most 

spectacular scenic features of the Colorado Plateau. Dating of the group is based on sparse fossil evidence from 

outside the mapped area as well as on its stratigraphic 

position between beds of known Late Triassic age and early 

Middle Jurassic age. 

FIGURE 6.- View southeastward of canyon of the Dolores River near 

confluence with San Miguel River. Rw, Wingate Sandstone; 

Rk, Kayenta Formation; Je, Entrada Sandstone; Js, Summerville 

Formation; Jms, Salt Wash Member of the Morrison Formation. 

Summerville Formation about 100 feet thick. 

WINGATE SANDSTONE 

The Wingate Sandstone conformably overlies the Chinle 

Formation and is widely exposed along the walls of the 

salt anticline vall eys, the flanks of the Uncompahgre Plateau, and in the deeper canyons. It typically crops out as a 

vertical reddish-brown wall, stained and streaked in places 

with black desert varnish (figs. 5 and 6). Vertical joints 

cut through the sandstone, and it is largely by spalling of 

\·crtically jointed slabs that the cliff recedes. Commonly the 

spalls occur along huge smoothly curved conchoidal fractures. Three other factors contributing to the cliff-forming 

characteristic of the sandstone arc the massive homogeneous nature of the sandstone, the erosion-resistant character of the basal beds of the overlying Kayenta Formation, 

and the case with which the soft beds of the underlying 

Ch inle Formation arc eroded. Wherever the Kayenta has 

been stripped back or where the Chinle is locally more 

resistant than normally, the Wingate forms a steep slope 

of rounded benches. In faulted areas it disintegrates rapidly 

and forms hummocky light-buff outcrops. 

The Wingate Sandstone is divided into horizontal layers 

2-50 feet thick by bedding planes that may extend for a 

quarter of a mile or more. Within each horizontal layer the 

sandstone is crossbeddcd on a magnificent scale; in places 

great sweeping tangential crossbccls of eolian type cut across 

the entire thickness of the horizontal layer. These crossbeds 

clip predominantly cast to southeast and so indicate the 

sand was deposited by westerly or northwesterly winds. The 

sandstone is a massive fine-grained rock composed mostly 

of clean well-sorted sand. According to R. A. Cadigan 

(written commun. 1954) the mean composition of eight samples of the sandstone obtained from in and near the area 

is quartz, 70 perccn t; chert, 2 percent; quartz overgrowths 

and silica cement, 5 percent; calcite cemen t, 11 percent; 

feldspar, 12 percent; muscovite, biotite, and heavy minerals, 

less than 1 percent. Nearly all the grains in the sandstone 

range in diameter from 0.05 to 0.15 mm, but in a few 

scattered localities well-rounded grains having frosted surfaces were found that measured about 0.4 mm in diameter. 

These large grains occur along laminations in layers one 

grain thick. Rare horizontal bedding planes are marked by 

a thin layer of argillaceous sandstone. The lowermost foot 

or two of the Wingate in places contains partings and thin 

layers of reddish-brown shale, and the contact is gradational 

into the underlying Chinle Formation; elsewhere the contact is sharp and Wingate Sandstone rests directly on siltstone and sandstone of the Chinle without an intervening 

transitional zone. The contact of the Wingate and the 

K ayenta appears to be gradational everywhere. 

In the northeastern part of the area, northeast of Ute 

Creek from the vicinity of Big Pond to Turner Gulch, the 

sandstone is highly silicified, brittle, breaks with a conch-
TRIASSIC AND JURASSIC SYSTEMS - GLEN CANYON GROUP 21 

oida 1 f ractu rc, and weathers to chips and flakes rather than 

to loose grains. The silification appears to be related to the 

zone of faults just northeast of Ute Creek. Original pore 

spaces in the rock have been almost completely filled with 

silica. 

Because the clifl's arc unscalablc in most places, detailed 

stratigraphic sections of the Wingate Sandstone are difficult 

to rncasurc. The following sections, however, probably arc 

typical: 

Section of Wingate Sandstone on Dry Creek in sec. 4, T. 45 N., 

R. 16 W., Montrose County, Colo. 

[Measured by L. C. Craig and L. R. Sticff] 

Kayenta Formation. Feet 

Wingate Sandstone: 

Sandstone, white to light-yellow; weathers light yellow 

to buff to pink, fine grained; clear and white quartz 

sand; cross-larninated beds 5-10 ft. thick; large sand 

grains extremely rare except in lower 5-20 ft. of 

formation . .... ................................................ 242.9 

Chinle Formation. 

Section of the Wingate Sandstone 3 miles east of Gateway in sees. 

I 7 and 20, T. 5 I N., R. 18 W., Mesa County, Colo. 

[Mcnsut·cd by C. N. Holmes] 

Kayenta Formation. 

Wingate Sandstone: 

Sandstone, reddish-brown, medium-fine-grained, massive; white on fresh surface, slightly crossbedded, 

Feet 

rare red day flecks .. ... . ............................. 17.0 

Sandstone, sahnon-pink, medium-fine-grained; uniform 

quartz grains, no visible accessory minerals ................ 13.0 

Sandstone, salmon-pink, fine-grained; local black accessory minerals, crossbedded .. ...... . . ... ... ..... . .. ..... ... ..... 30.0 

Sandstone, salmon-pink, fine-grained ... . ..... ............. . 52.0 

Sandstone, pink, uniform, fine-grained, fractured ........ 3.0 

Sandstone, yellow-brown and pink, medium-finegrained; scattered black accessory minerals, dense 

and hard ....... ·······-·······················-····· 57.0 

Sandstone, salmon-pink, fractured, soft, fine-grained; 

scattered coarse to medium-coarse grains of amber 

subangular quartz . ..... ...................................... 17.0 

Sandstone, red, soft, crossbedded . .. ... ...................... 38.0 

Sa ndstonc, salmon-pink, fine-grained, cross bedded ............ 21.4 

Sandstone, pale-brown, fine-grained, slightly crossbedded .. ... ..... . .............. ................................ 16.6 

Total Wingate Sandstone ...... ............ .. ..... ......... 265.0 

Chinle };-ormation. 

Section of the Wingate Sandstone in Summit Canyon, sees. 16 and 

17, T. 43 N., R. 19 W., San Miguel County, Colo. 

[Measured by L. C. Craig and J. J. Folger] 

Kayenta Formation. 

Wingate Sandstone: 

Top contact poorly exposed. 

Sandstone, pale-yellow; grain size and composition as 

bebw; unit forms capping ledge .............................. . 

Sandstone, moderate-reddish-orange to light-brown, 

predominantly very fine grained to fine-grained; 

composed of subangular clear quartz with sparse 

Feet 

4.4 

white chert and pink and gray accessory minerals. 

In lower 50 ft. clear to amber medium-fine rounded 

to well-rounded white chert grains are concentrated 

along laminae. Predominantly highly crossbedded, 

thin to thick (as much as 20ft.) wedging sets; about 

one-tenth of unit shows horizontal wavy laminations. 

Even bedding planes at intervals of 15-50 ft. ............ 270.0 

Total Wingate Sandstone .. .................................... 274.4 

Chinle Formation. 

Section of the Wingate Sandstone at Cashin mine, sec 22, T. 47 N., 

R. 19 W., Montrose County, Colo. 

Kayenta Formation. 

Wingate Sandstone: 

[Measured by L. C. Craig] 

Sandstone, pale-yellow-brown to light-gray, white- to 

light-brown-weathering; predominantly fine grained, 

composed of clear quartz with sparse accessory 

minerals. Upper 27 ft. is relatively thin bedded, 

wedging sets of cross-laminae; lowest 100 ft. is thin 

bedded. Middle part is thick bedded ( 25-50 ft.) 

with even bedding planes, highly cross-laminated .... 

Total Wingate Sandstone ....................................... . 

Chinle Formation. 

Feet 

351 

351 

The colors described in the preceding sections are 

noticeably lighter than the normal colors of the Wingate as 

seen in the cliffs. Places where the sandstone can be traversed are exposed to more rapid disintegration, and therefore in these places the surficial staining and desert varnish 

is rarely so prominent as on the vertical cliffs. 

THICKNESS 

The thickness of the Wingate Sandstone where unaffected by the movement of salt near the salt anticlines 

generally ranges from 220 to 500 feet. The latter thickness 

is extreme and is confined to a local area that coincides with 

the area near Ute Creek where the sandstone is highly 

silicified. In a distance of 3 miles in a southwesterly direction from there, the thickness diminishes from nearly 500 

feet to less than 300 feet. On the flanks of the salt anticlines 

the Wingate thins abruptly and in places disappears. Such 

thinning is strikingly displayed especially in Paradox and 

Gypsum Valleys. Near Thunderbolt Springs in Paradox 

Valley a relatively gradual thinning from more than 100 

feet to 5 feet occurs within a quarter of a mile. Throughout 

this distance the tangential crossbedding is constant in attitude, and in places the laminations cut completely across 

the entire thickness of the sandstone. 

CONDITIONS OF DEPOSITION 

As is true of other thick crossbedded sandstone formations of the Colorado Plateau, the origin of the Wingate 

Sandstone is not agreed upon by all geologists who have 

\vorked there. The even sorting, the roundness of the sand 

grains, and the very large-scale crossbedding all point to 

eolian deposition, yet, unlike the Navajo Sandstone which 
22 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

has frequently been cited as a classical example of a winddeposited formation, the \Vingate is cut at intervals by 

extensive horizontal bedding planes. Presumably these could 

have been caused only by water; furthermore, to the west 

in Utah both Dane (1935, p. 75) and Gilluly (1929) have 

described thin, discontinuous limestone beds in the Wingate. As Dane stated: "The precise manner of deposition is 

difficult to visualize, but a combination of eolian and sheetwash deposition on a surface of very low relief seems to 

be indicated." 

In places the Wingate contains a few bedding units, 

usually less than 5 feet thick, consisting of faint horizontal, 

slightly wavy laminae. These sandstone units appear to 

contain more silt and clay than the crossbedded units. Some 

geologists have suggested that the widespread horizontal 

bedding surfaces may represent bevel surfaces formed by 

the advance of strandlines across sand dune areas, whereas 

others think the bevel surfaces may represent ground-water 

levels to which deflation cut. The horizontally laminated 

units may represent reworked dune material, as well as 

finer materials, deposited in a body of standing water. 

CORRELATION 

The Wingate Sandstone is widely distributed over much 

of the Colorado Plateau country, and, in general, because 

of its distinctive outcrops it presents no problems of correlation. Nonetheless, the formation originally named the 

"Wingate sandstone" by Dutton ( 1885, p. 136-137) is not 

entirely the Wingate Sandstone now so widely recognized 

over most of the plateau (Baker and others, 194 7). The 

upper part of Dutton's "Wingate sandstone" correlates with 

the Entrada Sandstone whereas the lower part of his 

"Wingate" probably correlates with the Wingate of southwestern Colorado (Harshbarger and others, 1957, p. 8). 

Fossils that have been found in the Wingate consist of 

dinosaur tracks ( Gilluly and Reeside, 1928, p. 70; Longwell and others, 1923, p. 13) and ~ Triassic phytosaur, 

Machaeroprosopus (Harshbarger and others, 1957, p. 29). 

The tracks are of no diagnostic value in age determinations. The phytosaur discovery in Arizona supports the 

assignn;tent of the Wingate to the Triassic, as do also 

depositional relations. The Triassic Chinle Formation 

grades without depositional break into the Wingate 

Sandstone. 

KAYENTA FORMATION 

The Kayenta Formation, which conformably overlies the 

Wingate Sandstone, is widely exposed within the area, especially in the deeper canyons and along the sides of the salt 

anticline valleys. Eastward and in places along the flanks 

of the salt anticlines, the formation wedges out; it is absent 

in the San Juan Mountains and on most of the Uncompahgre Plateau. Within the area covered by this report the 

edge of the formation coincides rather closely with the 

southwest front of the Uncompahgre Plateau; thus on Wolf 

Hill (sec. 15, T. 51 N., R. 17 W.) the Kayenta is missing 

and the Entrada Sandstone rests directly on the. Wingate 

Sandstone. Similar relations exist through a distance of at 

least 25 miles southeastward from Ute Creek. To the north, 

however, the formation crosses the crest of the U ncompahgre Plateau and occurs on the northeast flank near 

Grand Junction, Colo. 

The Kayenta typically crops out in series of benches and 

ledges and has a wider outcrop than do many of the other 

formations of comparable thickness in the area (fig. 6). The 

ledges in many places overhang recesses where softer beds 

have eroded back. ''\'here the formation is cut by joints, 

the traces of the joints on the outcrop surface stand out as 

small ridges as much as an inch across and half an inch 

high, a characteristic which serves to distinguish the formation in small isolated outcrops from other formations of 

similar lithology. The lower part of the formation is more 

firmly cemented and forms especially resistant thick ledges 

that protect the underlying Wingate Sandstone from 

erosiOn. 

The Kayenta Formation, in striking contrast to the other 

formations of the Glen Canyon Group, is notable for its 

variety of rock types. Sandstone, which is red, buff, gray, 

and lavender, is the most abundant type, but the formation 

also contains considerable quantities of red siltstone, thinbedded shale, and conglomerate. Most of the sandstone is 

thin bedded, crossbedded in part, and flaggy; some is massive. In the southwest wall of Paradox Valley in sec. 7, 

T. 46 N., R. 17 W., the sandstone locally is very thick 

bedded and massive and in general aspect quite unlike the 

usual sandstone layers and lenses in the formation. Except 

in this one locality, the sandstone beds are lenticular, discontinuous, and interfinger with shale and, in places, with 

conglomerate. In general the sandstone is fairly well sorted 

and the grains are rounded or subrounded, but both sorting 

and roundness are less perfect than in the Wingate or 

Navajo Sandstone and the grains are coarser. According to 

R. A. Cadigan (written commun., 1954) the mean composition of nine samples is quartz, 63 percent; chert, 6 percent; 

calcite cement, 10 percent; potash and sodic feldspar, 15 

percent; muscovite, biotite, and other heavy minerals, less 

than 1 percent. Mica is much more conspicuous than its 

low percentage of the composition would indicate. The 

thick-bedded massive phase in Paradox Valley contains 

rounded pebbles, largely of quartz but also of black and 

gray chert, as much as three-quarters of an inch across. 

Else\vhere such pebbles are very rare if, indeed, they occur 

at all. Shale occurs in subordinate amounts as well-laminated thin beds and lenses, especially in the uppermost part 

of the formation; most of it is red but some is chocolate 

colored. Subordinate amounts of conglomerate closely resembling the limestone-pebble-clay-pellet conglomerates of the 
TRIASSIC AND JURASSIC SYSTEMS - GLEN CANYON GROUP 23 

Chinle Formation arc common everywhere in the Kayenta. 

The conglomerate consists of limestone pebbles and 

irregular-shaped fragments and pebbles of siltstone and 

shale in a matrix of sand. 

The Kayenta Formation was measured at many localities, 

and the fol)o,ving sections show the lithology of the 

formation: 

Section of the Kayenta Formation half a mile south of Stone 

Spring. in Paradox Valley, sec. 27, T. 47 N., 

R. iB W., Montrose County, Colo. 

[Mcasut·ed by F. W. Cater] 

Entrada Sandstone. Feet 

Unconformity. 

Kayenta Formation: 

Contact disconformable, surface somewhat irregular 

and shows some scour channels a foot or so deep. 

Sandstone, red to lavender-red, fine- to medium-finegrained, thin-bedded and cross-bedded, flaggy; consists predominantly of rounded and subrounded 

grains of quartz, slightly micaceous; scattered shale 

partings .. . ... . . . ............. ............... 37 

Sandstone, maroon to purplish-red, lenticular and 

crossbedded; locally contains boulders of sandy shale 

as much as 18 in. across . ... . .................... . 9 

Sandstone, pink to purplish-red, thin:bedded and 

flaggy; lenses of mudstone containing shale pebbles. 

Unit marks a filled stream channel and laterally 

grades into more massive ledge-forming sandstone .... 12 

Sandstone, red, bluff-forming, thinly bedded; some 

crossbedding; consists of rounded to subrounded 

quartz gains, slightly micaceous . . . . . .. ......... ...... 5 

Sandstone, red, thin-bedded, slabby, mostly covered .... 7 

Sandstone, red to purplish, thin-bedded and crossbedded, fine- to medium-fine-grained; slightly micaceous; contains some streaks of green clay . 10 

Sandstone, red, thinly laminated, flat-bedded ............... . 

Shale, brick-red, sandy, indistinctly bedded; breaks into 

angular prismatic pieces . .. .... ............ ............... 4 

Sandstone; red with slight purplish or lavender hue, 

fine grained, thin-bedded and cross-bedded; consists 

predominantly of rounded to subrounded quartz 

grains, contains some mica, black chert, and green 

clay; lower 5 ft. forms a slope, upper 16 ft. forms a 

bluff .............................................................................. 21 

Total Kayenta !"ormation 

Wingate Sandstone. 

106 

Section of the Kayenta Formation on Dry Creek, sec. 4, T. 45 N., 

R. 16 W., Montrose County, Colo. 

Kayenta Formation-Continued 

Sandstone, white to light-lavender, fine- to mediumgrained; consists of white and colorless subangular 

quartz grains with small amounts of rose, amber, and 

lavender quartz; cross laminated ............................... . 

Sandstone, red and maroon, fine- to medium-grained; 

consists of subangular clear quartz grains; unit is dark 

red, ripple laminated, micaceous, and very fine 

grained in upper part ................................................. . 

Sandstone, maroon to red, massive- to platy-weathering, faintly laminated; consists of subangular clear 

quartz grains with minor pink and amber accessory 

mineral grains ....................................... : ............... . 

Conglomerate, intraformational, pink to maroon; darkred clay and silt granules as much as Y.. in. across in 

a sandstone matrix ..................................................... . 

Sandstone, maroon to purple, medium-grained, faintly 

cross laminated; composed of white, pink, and amber 

quartz grains with abundant large flakes of muscovite; 3 ft. of intraformational conglomerate near 

middle of unit .... . .................................................... . 

Conglomerate, red- to brown-weathering; consists of 

light-red to gray, yellow-brown-weathering pellets of 

of calcilutite and sparse red claystone pellets .......... . 

Sandstone, gray- to purplish-weathering, medium-

·grained; a few scattered claystone and limestone 

pebbles in middle part ............................................... . 

Conglomerate and sandstone, irregularly interbedded; 

red to maroon; contains pebbles of limestone and 

siltstone 

Sandstone, maroon to purplish-gray, medium-grained; 

consists of white, pink, and colorless quartz with 

minor amber accessory grains; platy weathering ....... . 

Conglomerate, dark-red; limestone and siltstone pebbles 

Sandstone, maroon to purplish, medium-grained, crosslaminated; consists of white to amber quartz grains 

with numerous muscovite flakes; unit becomes red 

to red brown and finer grained upward .................. .. 

Sandstone, red to brown-red, fine-grained; consists of 

amber and white quartz; shaly weathering ............... . 

Conglomerate, purplish; consists of limestone pebbles 

in quartz sand matrix ............................................... .. 

Sandstone, red-brown, fine-grained, ripple-laminated; 

rubbly below to massive above ................................... . 

Feet 

24.6 

24.6 

42.5 

4.4 

31.2 

2.8 

9.5 

4.1 

20.7 

1.0 

21.1 

7.5 

1.0 

16.4 

Total Kayenta Formation .......................................... 219.0 

Wingate Sandstone. 

Section of t.he Kayenta Formation 3 miles east of Gateway, Colo., 

sees. 17 and 20, T. 51 N., R. 18 W., Mesa County, Colo. 

[Measured by C. N. Holmes] 

Entrada Sandstone. Feet 

[Measured by L. C. Craig and L. R. Stieff] Unconformity. 

Entrada Sandstone. ·Feet Kayenta Formation: 

Unconformity. 

Kayenta Formation: 

Contact lobate to irregular; underlying surface pillowy, 

suggesting flow or possibly channeling; pebbles of 

underlying beds worked into base of overlying beds. 

Contact is probably unconformable, cutting out the 

top 7.6 ft. of Kayenta west of line of section. 

Sandstone, red to pinkish-buff, fine-grained, well-sorted; 

consists of subangular clear to amber quartz grains; 

highly cross laminated ............................................. . 7.6 

Sandstone, maroon, micaceous, partly covered; weathers 

along silty shale partings . .......................................... 19.0 

Sandstone, maroon, fine-grained, horizontally bedded, 

!edgy .............................................................................. 1.5 

Sandstone, maroon, very micaceous; contains silty shale 

partings at Ys-in. intervals; crossbedded, weathers 

along partings .............................................................. 16.5 

Sandstone, white, pale-brown-weathering, medium-finegrained, porous, calcareous cement, lenticular, subangular grains . . .......................................................... 15.4 
24 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Kayenta Forrnation-Qontinued 

Covered ......... __ . __ ............................ __ .... _____ ......... . 

Feet Kayenta Formation-Continued Feet 

10.6 grains; slabby beds as much as 1 ft. thick ·····----------- 16.8 

Sandstone, maroon, medium-fine-grained, lenticular; 

forms vertical ledge; crossbedded; some dark-red 

shale and siltstone; mica flakes along partings; blocky 

and rough weathering _ .............................................. . 

Sandstone, maroon-purple, fine-grained, partings Ys in. 

thick of micaceous shale in lower 3 ft. of unit; upper 

1.5 ft. is hard dense sandstone containing dense gray 

limestone concretions 4 in. in diameter 

Sandstone, white, rusty-brown-weathering, medium-finegrained; thin shale partings at intervals of about 

3 in.; sparse black accessory minerals ....... . 

Shale, dark-maroon, lenticular 

Sandstone, light-maroon, medium-fine-grained; upper 

1 ft thin horizontal laminations; lower 1.3 ft contains shale flakes as much as 3 I 16 in. iong and scattered chert grains; some shale partings; unit lenticular 

Covered 

Sandstone, white, reddish-brown-weathering, fine- to 

medium-grained; sparse pink and black accessory 

minerals; some red silty shale partings .................. . 

Covered 

Sandstone, reddish-gray, fine- to medium-grained; grains 

subangular; horizontal partings of shale 1 I 16 in. 

thick ............. ·······-------·-···---------------··----------------------------

Covered 

Sandstone, light-maroon, fine- to medium-grained; 

41.8 

4.5 

9.5 

6.7 

2.3 

5.6 

10.2 

5.4 

2.0 

3.0 

lenticular horizontal laminations; grains subangular __ 3.0 

Covered .... -·--··----···· ............... 10.0 

Total Kayenta Formation 

Wingate Sandstone. 

······-··········- 167.0 

Section of the Kayenta Formation in Summit Canyon, sees. 16 and 

17, T. 43 N., R. 19 W., San Miguel County, Colo. 

[Measured by L. C. Craig and J. J. Folger] 

Navajo Sandstone. Feet 

Kayenta Formation: 

Contact sharp, marked by color and structure change. 

Covered on line of section; exposed along outcrop. 

Sandstone and claystone interbedded. Sandstone is dark 

reddish brown, very fine to fine grained; consists of 

clear subangular quartz grains with numerous va~iegated accessory mineral grains, slightly micaceous; 

ripple laminated to massive and structureless. Claystone is dark-reddish brown and slightly micaceous; 

it predominates in upper two-thirds of unit ............ 28.2 

Sandstone, pale-pink to moderate-reddish-brown, fineto medium-grained, highly cross laminated; channeling; composition same as that of underlying unit ...... 68.8 

Sandstone, pale-pink to pale-yellow, fine- to mediumfine-grained; consists of subangular clear quartz 

grains with orange and gray accessory mineral grains; 

highly cross laminated; channeling ···--·---·---·----·-·---·--- 33.6 

Conglomerate, grayish-brown to dark-reddish-brown; 

limestone pebbles as much as 3 in. in diameter; 

poorly sorted matrix of quartz grains ........................ 7.2 

Sandstone, moderate-reddish-orange; size and composition as below; highly cross laminated; channeling .... 54.0 

Sandstone, dark-reddish-brown, very fine grained to 

fine-grained; consists of subangular clear quartz 

grains with numerous colored accessory mineral 

Total Kayenta Formation 

Wingate Sandstone. 

THICKNESS 

208.6 

The Kayenta Formation varies considerably in thickness 

from locality to locality. In part these variations are due to 

actual depositional thinning or thickening of the formation, 

but in areas where the Entrada Sandstone rests directly on 

the Kayenta the variations are due largely to erosion of the 

upper beds prior to deposition of Middle Jurassic beds. In 

other words, in areas where the Navajo Sandstone is absent, 

the upper contact of the Kayenta is an unconformity. 

Normally the thickness ranges from about 180 to 220 feet, 

although locally on Sewemup Mesa the maximum thickness 

is nearly 300 feet. At the other extreme is the complete 

wedge out of the formation either by nondeposition or by 

erosion or by a combination of both along the U ncompahgre Plateau in the northeast part of the area and in 

places along the flanks of the salt anticlines; for example, 

on the southwest side of East Paradox Valley the Kayenta 

is entirely missing for several miles. 

CONDITIONS OF DEPOSITION 

Irregular bedding, low-angle crossbedding, current lineations, and channel fills, all characteristic of stream deposition, occur throughout the Kayenta Formation. Saurian 

tracks and species of the fresh-water pelecypod genus Unio 

have been found in the Kayenta in Utah and Arizona 

(Gregory, 1917, p. 56; Baker, 1933, p. 46). Contrary to 

what would normally be expected in such beds, ripple 

marks and mud cracks are uncommon. The conglomerate 

lenses are similar to those in the Chinle and in all probability have a similar origin. 

In general the Kayenta beds mark a sudden change of 

depositional environment in middle Glen Canyon time. 

During Wingate time, wind was the dominant depositional 

agent and the area was probably a scene of utter desolation, a monotonous expanse of shifting sand. The beginning 

of Kayenta times was marked by more humid conditions 

which altered the scene entirely; a system of aggrading 

streams was reestablished which leveled the dunes and blanketed the area with a layer of sediments quite unlike the 

eolian deposits of the Wingate. 

AGE AND CORRELATION 

Fossils are sparse in the Kayenta Formation. The dinosaur tracks found by Gregory and the U nio shells found by 

Baker indicate only that the beds can be no older than Late 

Triassic and could be considerably younger. Two reptile 

skeletons found by Welles ( 1954) were identified by him 

as Megalosaurus, a genus found in the European Jurassic; 

however, S\vinton ( 1955) said of this reptile, "Its relations 
TRIASSIC AND JURASSIC SYSTEMS -GLEN CANYON GROUP 25 

arc certainly with the early carnosaurs, but it is not Megalosaurus." The early carnosaurs arc mostly of Late Triassic 

age. Several tritylodont skeletons found near Kayenta, 

.'\riz., were considered by Lewis ( 1966) to be closely related 

to species found in the Stombcrg series of South Africa 

and the Lufeng series of Yunnan, China, both of which are 

in the U ppcr Triassic. Lewis also wrote (Lewis and others, 

1961) that the tritylodonts from the Kayenta were found in 

association with a fossil crocodilian (Protosuchus richardsoni) hitherto kno\vn only from the Late Triassic ( ?) Moenave Formation; this species is closely related to the crocodilian genera Erythrochampsa and Notocham.psa of the 

Stonnberg. Thus it appears that the best fossil evidence 

indicates a Late 'Triassic (?) age for the Kayenta. 

The Kayenta Formation crops out over wide areas on 

the Colorado Plateau, and occurring as it docs between the 

highly clistincti,·e vVingate and Navajo Sandstones, it offers 

few problems of correlation. The beds between the Wingate 

and the Navajo were originally called "Todilto" by Gregory 

( 1917) because of their supposed correlation with the 

Todilto Limestone of northwestern New Mexico and 

adjacent parts of Colorado and Arizona. Baker, Dane, and 

Rccside ( 1936, p. 1 7) in defining these beds as the Kayenta 

Formation demonstrated that the Todilto Limestone was 

rnuch younger and did not correlate with the Kayenta. 

NAVAJO SANDSTONE 

The Navajo Sandstone, the uppermost formation of the 

Glen Canyon Group, conformably overlies the Kayenta 

Formation. In most places the contact between the Navajo 

and Kayenta is sharp, but in the northern part of the area 

on the west side of Maverick Canyon, the contact is indistinct because the Navajo appears to consist of reworked 

Kayenta. The transitional beds have a composition apparently identical to the Kayenta but they have the sweeping crossbedding typical of the Navajo, and they grade 

laterally into beds of typical Navajo aspect. Although the 

Navajo is one of the most widespread and topographically 

distinctive formations in the plateau country, in the area 

covered by this report, its distribution is erratic, and because 

of its thinness, its topographic forms are relatively subdued. 

The cast edge of the Navajo Sandstone follows an irregular course through southwestern Colorado and through 

the part of the area covered by this report (fig. 4) . The 

Navajo crops out in canyon walls, on benches bordering 

the deeper canyons, and locally both in and along the walls 

of the salt anticline valleys. It is most widely exposed on 

benches bordering canyons in the Horse Range Mesa and 

Anderson Mesa areas. The edge of the sandstone is not 

exposed south of Gypsum VaHey, but from unsurveyed sec. 

15, 'I'. 44 N., R. 17 W., on the northeast rim of the valley, 

the edge of the sandstone trends northwest to the northwest 

end of Paradox Valley, and from this point it trends irregularly northward and leaves the mapped area west of Gate-

·way. East of the main mass of the Navajo a few thin, 

isolated masses are exposed; the largest of these centers 

at U ravan. These isolated masses were probably connected 

to the main mass of the Navajo prior to the erosion that 

immcdiatCly followed deposition of the Navajo. 

The Navajo Sandstone commonly is eroded to hummocky surfaces of rounded knobs where the formation is 

sufficiently thick and unprotected by overlying formations; 

where protected by overlying formations it forms vertical 

cliffs (fig. 8). Locally, where the sandstone is much jointed, 

it weathers to an exceedingly intricate topography of hoodoos and narrow clefts, as between the mouth of Wild Steer 

Canyon and Paradox Valley. 

T'he Navajo Sandstone, like the Wingate, consists almost 

entirely of massive, fine-grained, very well sorted, clean, 

light-buff, gray, or nearly white sand. The few scattered 

thin lenses of limestone and sparse shale splits constitute 

much less than 1 percent of the formation. Here, as elsewhere on the Colorado Plateau, crossbedding on an enormous scale is typical of the Navajo; great sweeping tangential cross-laminae descend at angles of as much as 30° 

to the true bedding. Unlike the Wingate, the Navajo contains almost no horizontal bedding planes, except in the 

limestone lenses and near the featheredge of the formation. 

Conformable sets of cross-laminae form wedges and lenses 

that truncate one another or lower beds. 

According to R. A. Cadigan (written commun., 1954) 

the mean composition of seven analyzed samples of the 

Navajo Sandstone is quartz, 78 percent; chert, 3 percent; 

quartz overgrowths and silica cement, 4 percent; calcite 

cement, 3 percent; potash and sodic feldspar, 12 percent; 

and heavy minerals, less than 1 percent. Individual grains 

are subangular to rounded and range from 0.1 to 0.2 mm 

in diameter. In a very few places angular granules of white 

chert are common, particularly in the sandstone enclosing 

limestones lenses or in sparse localities where the sandstone 

appears to have been deposited by water. Thin beds of gray 

to pink limestone crop out in the vicinity of Summit and 

Bush Canyons, on the lower part of Roc Creek, and on the 

south end of Sewemup Mesa. The bed on Sewemup Mesa 

has a maximum thickness of 6 feet. On the south side of 

the San Miguel River, several hundred feet southeast of 

Uravan, a 3-foot-thick lens of fissile red shale occurs about 

12 feet above the base of the Navajo; this lens wedges out 

laterally into sandstone. Similar lenses occur at scattered 

localities elsewh{;!re in the lower part of the sandstone. 

Because of the uniform nature of the sandstone, the complexities of the crossbedding, and the lack of true bedding 

planes, accurate measurement of detailed sections is difficult, but typical character of the Navajo Sandstone in this 

area is shown in the following sections: 
26 GEOLOGY. SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Section of the Navajo Sandstone at the Cashin mine, sec. 22, T. 

47 N., R. 19 W., Montrose County, Colo. 

Entrada Sandstone. 

Unconformity. 

Navajo Sandstone: 

[Measured by L. C. Craig] 

Contact poorly exposed, located within 1 ft. 

Sandstone, pale-yellowish-buff to pale-yellow-green, 

predominantly fine-grained; consists of subangular 

clear quartz grains and sparse accessory mineral 

grains; upper 10 ft. massive, structureless; lower 45 

ft. wedge bedded with prominent sweeping crossFeet 

lamination. Lowest 1.4 ft. horizontally laminated 69.8 

Total Navajo Sandstone 

Kayenta Formation. 

69.8 

Section of the Navajo Sandstone in fohn Brown Canyon near edge 

of mapped area 

[Measured by L. C. Craig, H. L. Jicha, and others] 

Entrada Sandstone. 

Unconformity. 

Navajo Sandstone: 

Top contact is undulant and irregular. 

Limestone, light-gray to pink, medium-gray- to buffweathering, medium-grained to coarsely crystalline; 

contains disseminated medium-grained quartz; locally 

Feet 

contains medium-gray chert nodules 2.6 

Sandstone, pink to purplish, fine- to medium-finegrained; not conspicuously cross laminated. Interval 

p::>orly exposed . ....... ........................................ 15.7 

Sandstone, pink to buff, fine- to medium-fine-grained; 

consists of quartz with minor chert; predominantly 

wedge bedded, torrential cross-lamination . ............. 21.6 

Sandstone, white to pinkish-buff, medium-fine-grained, 

thick-bedded, broadly cross laminated; composed of 

subangular to rounded grains of quartz with minor 

chert . .. .... ... ............................................................... 21.3 

Limestone, light-gray to dove, light-gray-weathering, 

fine-grained . . ······················--··-····-· -------------------------- .4 

Sandstone, pale-lavender, fine- to medium-grained, 

poorly sorted, cross-laminat_ed; larger grains are 

rounded ... ·············-··---------------------------------------------- 19.5 

Sandstone, white to buff, fine- to medium-grained; consists of rounded to well-rounded clear quartz grains; 

wedging beds, not highly cross laminated________ ___________ 32.6 

Sandstone, red, fine-grained, nonresistant, platy to 

shaly weathering ............ ······-·······------------------------------ 1.2 

Sandstone; white below grading to pink above, fine to 

medium grained, evenly bedded; weathers to rounded 

topographic forms; not conspicuously laminated ______ 20.8 

Interval poorly exposed. Shale; dark red below grading 

to purple ab::we, slightly silty; 6 in. light-green silty 

to sandy shale at top ... ·····--------·-···------------------------ 12.2 

Sandstone, white, fine- to medium-grained, highly 

cross laminated, irregularly bedded 2.5 

Total Navajo Sftndstone ···-···-···········-· -··········--···-··- 150.4 

Kayenta Formation. 

THICKNESS 

The thickness of the Navajo in the area ranges from a 

featheredge to a maximum exposed thickness between Silveys Pocket and Wray Mesa of at least 500 feet; the total 

thickness may be considerably greater. Large variations in 

thickness occur within short distances, due both to wedging 

along the flanks of the salt anticlines, which underwent 

movements during deposition of the sandstone, and to 

differential erosion at the top of the Navajo. 

CONDITIONS OF DEPOSITION 

The Navajo Sandstone often has been cited as a classic 

example of a vvind-deposited formation (Huntington and 

Goldthwait, 1904, p. 214-216; Gregory, 1917, p. 59; 

Kiersch, 1950). Although a few geologists have questioned 

this interpretation (Keller, 1945, p. 219; Grater, 1948), the 

bulk of held evidence favors eolian deposition. The largescale sweeping cross-lamination, exceptional degree of sorting, the roundness of individual sand grains, and the scarcity 

of true bedding seem difficult to reconcile with any other 

mode of deposition. The limestone layers and some of the 

flatbedded material near the base of the formation were 

undoubtedly water laid, but these layers constitute a small 

part of the formation. The limestone layers probably originated in ephemeral ponds of playalike depressions or deflation basins that developed from time to time as deposition 

progressed. Regardless of the mode of deposition of the 

Navajo, or for that matter of the other formations in the 

Glen Canyon Group, one perplexing problem of Colorado 

Plateau geology still remains-the source of the staggering 

amounts of clean sand that inundated the region during 

Glen Canyon time. 

AGE AND CORRELATION 

Fossil plants, ostracodes, and reptiles have been found in 

the Navajo, but, unfortunately, these fossils are either of 

long-ranging species or of species whose ages are too uncertain to permit exact dating. The tritylodonts found in the 

Kayenta by Lewis ( 1966) and considered by him to be of 

Triassic age were 10 feet below the base of the Navajo. 

Segissaurus, one of the dinosaurs that has been found in the 

Navajo, could be placed in either the Triassic or Jurassic 

according to Camp ( 1936, p. 52). An incomplete skeleton 

of a second dinosaur from the Navajo was stated by Brady 

( 1935) to closely resemble Ammosaurus of the Connecticut 

Triassic, but because of the fragmentary nature of the 

remains he was not certain of the identification. The upper 

part of the Navajo is still considered to be Jurassic, but the 

fossil evidence cited above suggests, at least, that the lower 

part of the formation could be of Triassic age. The age 

assignment of the Navajo Sandstone as Triassic(?) and 

Jurassic has resulted from the recognition of widespread 

intertonguing between the Navajo and the Kayenta and 

the Carmel in south-central Utah and north-central Ari-
JURASSIC SYSTEM- SAN RAFAEL GROUP 27 

zona (Averitt and others, 1955, p. 2523-2524; Lewis and 

others, 1961, p. 1438-1440) . 

Throughout most of the Colorado Plateau the Iavajo 

Sandstone poses f Cll' problems of correlation; the distinctive 

crossbedding serves to distinguish it from all the formations 

of fluvial origin, and in most places the sparsity of horizontal bedding .planes serves to distinguish it from somewhat 

similar formations such as the Wingate and Entrada Sandstones. On the other hand, in northern Utah and in Wyoming and Idaho, correlations arc less certain, although the 

ravajo probably correlates with the Nugget Sandstone of 

those areas. The Iugget has lithic properties that are practically identical to those of the Navajo, and it occupies a 

similar stratigraphic position. 

RELATIONS OF EASTERN EDGES OF FORMATIONS 

OF THE GLEN CANYON CROUP 

Within the area covered by this report the edge of each 

successive ly yo unger formation of the Glen Canyon Group 

lies succcssi,-cly f arthcr west. The edge of recognizable Wingate Sandstone lies well cast of the area mapped; the edge 

of the Kayenta Formation crosses the northeastern corner 

of the mapped a rea ( fia. 4), and that of the Navajo Sandstone li es farther west. The wedge-out of these formations 

reflec ts pre-Entrada uplift and erosion of an area extending 

eastwa rd to the Elk Mountains (Vanderwilt, 1937, p. 

33-34-, 81-82). 

JURASSIC SYSTEM 

SAN RAFAEL CROUP 

In the area covered by this report the San Rafael Group 

comprises t,,·o formations of Late Jurassic age; these are the 

Entrada Sandstone below and the Summerville Formation 

above, both of ,,·hich can be traced almost continuously 

from the San Rafael Sll'cll into western Colorado. The 

name San R a fae l Group was first applied to a succession of 

similar beds in the San Rafael Sll'cll by Gilluly and Reeside 

(1928) . In the San Rafael Swell, the Carmel and Curtis 

Formations were recognized. The Curtis grades eastward 

into the lower part of the Summerville Formation (Dane, 

1935, p. 89; Knight, 1940, p. 99 ; Baker, 1946, p. 83 ), and 

the Carmel !•ormation .P"radcs abruptly into the Dewey 

Bridge Member of the Entrada Sandstone (Wright a nd 

others, 1962 ). Neither is present in southwestern Colorado. 

ENTRADA SANDSTONE 

Two members of the Entrada Sandstone were mapped as 

a single unit in the southwestern Colorado area, partly 

because they commonly form a narrow outcrop and partly 

because it is difficult to choose a contact between the two 

in some places. 

D EWEY BRIDGE M EMBER 

The Dewey Bridge Member and the overlying Slick Rock 

l\1embcr of the Entrada Sandstone crop out extensively 

along canyon walls and on the sides of mesas (fig. 7), but 

FIGURE 7.- Canyon of the Dolores River south of Mesa Creek. 

lik, Kayenta Formation; Jed, Dewey Bridge Member, Jes, Slick 

Rock Member, both of the Entrada Sandstone; Js, Summerville 

Formation; Jms, Salt Wash Member of the Morrison Formation. 

Summerville Formation about 95 feet thick. 

in many places the Dewey Bridge is absent, and the overlying Slick Rock Member of the Entrada Sandstone rests 

directly on pre-Dewey Bridge rocks (fig. 8). The Dewey 

Bridge was deposited on a slightly irregular erosion surface 

and fail ed to bury some of the higher parts of this surface 

completely. In the vicinity of Uravan such relations are 

well exposed; the isolated patch of Navajo Sandstone in this 

area apparently stood as a local topographic high during 

Dewey Bridge time, and where Navajo is present the Dewey 

Bridge is either thin or missing. In immediately adjacent 

areas where the Navajo was removed prior to deposition of 

the Dell'cy Bridge, the Dewey Bridge is thicker than is normal for the region. In common with older units the Dewey 

Bridge was not deposited across the tops of the salt cores 

in the salt anticlines. 

The Dewey Bridge Member, largely red to buff or 

orange-red soft horizontally bedded siltstone, mudstone, and 

sandstone, commonly crops out as a smooth, somewhat 

hummocky red to buff slope that forms a narrow platform 

from which the cliffs of the upper part of the Entrada 

Sandstone rise. In general the color is distinctly darker than 

other parts of the Entrada. With the exception of a single 

locality, nowhere in this area are the Dewey Bridge beds 

markedly contorted or wavy as they are farther west in 

Utah (Dane, 1935, p. 90). The exceptional area_is on the 

north side of the San Miguel River about one-half mile 

below the mouth of Atkinson Creek, where sandy Dewey 

Bridge fills what appears to be a channel cut in soft shales 

at the top of the Kayenta and large-scale distortions of the 
28 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

beds of the Dewey Bridge arc reflected well up into the 

locally thickened overlying part of the Entrada Sandstone 

(fig.9). 

FIGURE 8.-Entrada Sandstone (Je), about 100 feet thick, resting 

directly on Navajo Sandstone (Jlin), at Uravan, Colo. Note the 

relatively flat bedding in the Navajo and rows of pits in the 

Entrada. 

The material in the Dewey Bridge generally is poorly 

sorted and has an earthy appearance. Shales devoid of sand 

and sandstones devoid of clay and silt are rare and of local 

occurrence only. Sand, silt, and clay are mixed together in 

most beds. Pebbles and angular fragments of white and gray 

chert, as much as an inch across, are scattered rather abundantly through the lower part and less abundantly through 

the upper part. Locally, pebbles and fragments are sufficiently abundant to form layers of conglomerate. Included 

in these layers are scattered greenish-gray, red, and yellow 

quartzite pebbles and similar boulders as large as 5 by 8 

inches. Ip many places the upper part of the member contains scattered barite nodules as much as an inch across. 

Over considerable areas the lower part of the member consists almost entirely of reworkrd Navajo Sandstone, but, 

strangely enough, where the Dewey Bridge rests directly on 

the Kayenta Formation, the basal beds contain little 

reworked Kayenta material. 

Localities where detailed complete sections of the Entrada can be measured arc relatively uncommon; in most 

places the Slick Rock Member forms an unscalable cliff, 

and where it can be traversed the underlying Dewey Bridge 

Member is poorly exposed. Partly for this reason and partly 

because at the time the sections were measured the Dewey 

Bridge Member was thought to be the Carmel Formation, 

most sections of the two members were measured in different localities, and hence with one exception are presented 

separately here. 

FIGURE 9.- Beds of D ewey Bridge M ember (Jed } of Entrada Sandstone filling a channel cut in shale of Kayenta Formation (lik) . 

O verlying beds of Entrada Sandstone (Jeu) show local thickening. 

Section of the Entrada Sandstone near the Dolores group of mines 

in sees. 29 and 30, T. 48 N., R. 17 W., Montrose County, Colo. 

[Measured by L. C. Craig, R. A. Cadigan, E. M. Shoemaker, and Carl Setzer) 

Summerville Formation. 

Entrada Sandstone: 

Slick Rock Member: 

Sandstone, white, fine-grained; composed of predominantly clear quartz with minor pink and 

black accessory minerals; poorly defined horiFeet 

zontal bedding 7.5 

Shale, dark-maroon, papery ...... ... ..... .1 

Sandstone, white, fine-grained; composed of subangular clear quartz ·grains with sparse grains 

of pink and black accessory minerals; massive 

outcrop with ridging that indicates horizontal 

bedding and lamination 7.0 

Claystone, dark-maroon, papery; contains thin 

lenses of pink fine-grained sandstone ...... .2 

Sandstone, white to buff, very fine grained to finegrained; composed of subangular clear quartz 

grains with minor quantities of pink and black 

accessory minerals; massive outcrop with faint 

horizontal bands indicating bedding; thin beds 

show cross-laminae. Unit appears to bevel underlying beds 12.3 

Sandstone, white to buff, very fine to mediumcoarse-grained, well-sorted; highly cross laminated; wedge bedded; sparse coarse frosted and 

well-rounded grains concentrated along laminae. 

Unit bevels underlying beds ..... ................... 10.8 

Sandstone, interbedded, orange-brown and white 

to buff, very fine grained, contains numerous 

coarse frosted and well-rounded grains of clear 

quartz, white chert, and pink grains; interbedded highly cross laminated layers and horizontal irregularly laminated layers. Abundant 

disseminated coarse grains in lowest 2 ft. ... 68.2 

T otal Slick Rock Member 106.1 
JURASSIC SYSTEM- SAN RAFAEL GROUP 29 

Entrada Sandstone-Continued Feet 

Dewey Bridge Member: 

Sandstone, buff to pale-brown, fine- to mediumcoarse-grained, \·cry poorly sorted; massive, 

structurcless; contains large well-rounded and 

frosted grains and angular chert fragments as 

much as Y:.! in. in diameter; interbedded with 

this sandstone is red-brown fine-grained sandstone containing dark-red, very fine grained 

clay-bearing laminae showing some distortion ... 49.3 

Sandstone, · pale-brO\vn to buff, fine- to mediumcoarse-grained, clear quartz with minor pink 

accessory minerals; massive weathering; long 

radius cross-lamination. Numerous white angular 

chert granules. Prominent dark-red clayey sandstone above base ...................... .......................... 10.7 

Total Dewey Bridge Member .. .... ....... .... ....... 60.0 

Total Entrada Sandstone . .. .. ....................... 166.1 

Unconformity. 

Kayenta Formation. 

Section of the Dewey Bridge Member near Slick Rock, sec. 25, T. 

44 N., R. 19 W., San Miguel County, Colo. 

[Measured by L. C. Craig] 

Entrada Sandstone: 

Slick Rock Member. 

Dewey Bridge Member: 

Sandstone, pale-reddish-brown, grading to paleyellow buff, fine-grained; contains faint wavy 

horizontal red clayey laminae in lower half. 

Feet 

Probably grades into the Slick Rock Member .... 1.9 

Sandstone, pale-reddish-brown, very fine grained 

to fine-grained, poorly sorted; contains subangular clear quartz grains and minor accessory 

minerals, numerous well-rounded frosted-amber 

quartz grains; weathers to rounded topographic 

forms; some faint irregular red laminae ............ 13.2 

Covered interval probably similar to overlying 

beds ...................................................................... 6.0 

Sandstone, moderate-reddish-orange, fine- to medium-fine-grained; composed of subangular to 

rounded clear quartz grains with minor orange 

and black accessory mineral grains; structureless 

Sandstone; moderate-reddish-orange, very fine- to 

medium-fine-grained, poorly sorted; composed 

of subangular to well-rounded clear quartz 

4.1 

grains with faint irregular clayey laminae ........ 4.4 

Sandstone, moderate-reddish-orange, very fine 

grained to medium- fine-grained, very poorly 

sorted; composed of subangular to well-rounded 

clear quartz grains with sparse grains of pink 

and black accessory minerals; faint horizontal 

wavy laminations; scattered rounded to angular 

white, gray, and black chert pebbles as much 

as 3 in. across ...................................................... 2.2 

Total Dewey Bridge Member .................................. 31.8 

Unconformity. 

Navajo Sandstone. 

Section of Dewey Bridge Member of Entrada Sandstone, north rim 

of Sinbad Valley about 2 miles west of mapped area, sees. 30 and 

31, T. 50 N., R. 19 W., Mesa County, Colo. 

[Measured by L. C. Craig, with H. L. Jicha, 

William Alexander, and James Standard] 

Entrada Sandstone: 

Slick Rock Member. 

Dewey Bridge Member: 

Sandstone, dark-reddish-orange, fine- to mediumgrained, poorly sorted; subangular to wellrounded clear quartz grains; unit is massive, 

poorly bedded, and contains some dark-red disFeet 

continuous clay laminae ................. ..................... 3 7.1 

Sandstone, pale-brown, fine- to medium-finegrained; consists of subangular to rounded clear 

quartz grains; poorly bedded, poorly laminated 

unit ........................................................................ 4.3 

Sandstone, dark-red, medium-fine- to mediumgrained; contains irregular discontinuous darkred clay laminae .................................................... 1.0 

Sandstone, pale-brown to yellow-brown, fine- to 

medium-fine-grained; contains clear quartz with 

pink and black accessory minerals; massive, 

poorly bedded, poorly laminated unit; numerous 

white chert fragments as much as 1 in. across in 

lower 5 ft of unit .................................................. 13.8 

Total Dewey Bridge Member .......................... 56.2 

Unconformity. 

Navajo Sandstone. 

Thickness.-The Dewey Bridge Member was deposited 

on a somewhat irregular erosion surface which largely controls the variations in thickness of the member. Variations 

in thickness of the Dewey Bridge are also caused by irregularities in its contact with the overlying parts of the 

Entrada, especially irregularities resulting from the contact 

being gradational and not following one stratigraphic horizon. The Dewey Bridge has a maximum thickness within 

the area of about 100 feet; such a thickness, however, is 

exceptional and occurs only in the vicinity of Spring Canyon 

(T. 46 N., Rs. 18 and 19 W.) and nearby areas. Elsewhere 

the thickness rarely exceeds 65 feet. Any statement concerning a "normal thickness" would be misleading because, even 

over small areas, the thickness may range from a featheredge 

to 50 feet within a distance of a quarter of a mile. A normal 

range of thickness where the member is continuously present 

over extensive areas is probably 20 to 50 feet. 

Conditions of deposition.-The origin of the Dewey 

Bridge in Colorado is not readily evident as fossils have not 

been found, and the lithic characteristics of the member 

indicate only subaqueous deposition. Probably, however, the 

member formed in lagoons marginal to a sea. 

SucK RocK MEMBER 

The Slick Rock Member of the Entrada conformably 

overlies the Dewey Bridge and, like the Wingate and 

Navajo Sandstone, is a clean, massive, cliff-forming sandstone. The distribution of outcrops of the Slick Rock is 
30 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

almost identical to that of the Dewey Bridge, although the 

Slick Rock is more continuous and was deposited over the 

entire area except along the crests of the salt anticlines. 

The Slick Rock Member, locally known as the "slick rim" 

because of its appearance, is perhaps the most strikingly 

picturesque· of all the strata in the plateau region of Colorado (figs. 7-8). The smoothly rounded, in places bulging, 

cliffs ranging in color from nearly white to pastel shades of 

orange and buff are a highly distinctive and scenic feature 

of the region; they contrast strongly in color with the 

gaudy, flaming hues of underlying formations and the relatively drabber colors of the post-San Rafael formations. 

Horizontal rows of pits resulting from differential. weathering and ranging from a few inches to a foot or more across 

are also characteristic of these cliffs. The member consists 

of alternating parallel-bedded units and sweeping eoliantype crossbeddcd units. The parallel-bedded units are most 

common in the basal part and in the uppermost lighter 

colored part of the member, whereas the crossbedded units 

are commonly dominant in the middle part. 

The Slick Rock Member of the Entrada Sandstone differs from the somewhat similar Wingate and Navajo Sandstones in that the sand is sorted into two distinct grain sizes. 

Subrounded to subangular sand grains mostly less than 

0.15 mm in diameter make up the bulk of the sandstone. 

Contrasting with these small grains are well-rounded grains 

which have frosted surfaces and which range in diameter 

from 0.4 to 0.8 mm. Most of the larger grains are distribute~ in thin layers along bedding and laminations, but at 

one locality in Paradox Valley a short distance northeast 

of the Thunderbolt mine (sec. 23, T. 46 N., R. 17 W.), 

there are a few small lenses a foot or more wide and several 

inches thick that consist almost entirely of these large 

rounded grains. According to R. A. Cadigan (written 

commun., 1954) the mean composition of 15 samples of 

Entrada Sandstone is quartz, 76 percent; chert, 3 percent; 

quartz overgrowths and silica cement, 3 percent; calcite, 9 

percent; potassic and sodic feldspar, 9 percent; muscovite, 

biotite, and heavy minerals, less than 1 percent. The large 

well-rounded grains consist most~y of quartz, but some 

grains are chert. 

Some beds in the upper part of the Entrada Sandstone, 

north of Sinbad Valley, in the northwestern part of the 

mapped area can be traced westward into the Moab Tongue 

of the Entrada Sandstone in Utah (Baker, 1933, p. 49). 

In the area of this report the contact between beds probably equivalent to the Moab Tongue and those of the main 

body of the Entrada cannot be traced consistently; prominent parting planes arc not continuous. Furthermore, 

some rocks equivalent to the Moab Tongue grade into the 

overlying Summerville Formation. Detailed study would 

allow separation of the l\1oab Tongue as a distinct member 

in the mapped area, as Wright, Shawe, and Lohman 

( 1962) have shown, but such a member was not separated 

when the area was mapped. In general the sandstone in the 

upper part of the Entrada and those beds probably correlating with the Moab Tongue contain fewer large 

rounded frosted grains than in the rest of the formation, 

are much lighter in color, and tend to weather to somewhat 

!edgy or flaggy outcrops. 

Sections of the Entrada Sandstone were measured at 

numerous locations within the area, and those presented 

here are typical. 

Section of the Slick RQck Member of the Entrada Sandstone, north 

side of John Brown Canyon, sec. 32, T. 51 N., R. 19 W., Mesa 

County, Colo. 

[Measured by L. C. Craig, H. L. Jicha, and others] 

Summerville Formation. Feet 

Entrada Sandstone: 

Slick Rock Member: 

Sandstone, white, light-buff-weathering, finegrained, evenly laminated, evenly bedded; composed of clear subarigular quartz grains with 

minor pink chert -------------------------------------------------- 3.9 

Sandstone, red and gray-green, banded, finegrained; contains varying amounts of dark-red 

and gray matrix. This unit may represent Entrada Sandstone beds grading laterally into 

Summerville-like beds ··---------------------------------------- 7.3 

Sandstone, white, brownish-buff-weathering, finegrained, parallel-bedded and parallel-laminated 27.0 

Shale, dark-purplish-red, clayey. This prominent 

parting may mark the base of beds equivalent 

to the Moab Tongue of the Entrada ------------------ .3 

Sandstone, white, fine- and medium-grained, fairly 

well sorted; wedging bedding ·-·-----------·--------------- 48.6 

Sandstone, light-orange-buff to white, fine- to 

medium-grained, fairly well sorted; composed of 

clear quartz with large well-rounded and frosted 

grains nearly confined to laminae; wedging 

bedding ··--·------------------------------------------------------------- 92.8 

Sandstone, light-orange-buff and white, finegrained; composed of subangular quartz grains 

with numerous well-rounded, frosted, mediumcoarse clear and amber quartz grains, white to 

gray chert grains, and black grains; prominent 

horizontal bedding indicated ------------------------------ 43.2 

Total Slick Rock Member-------------------------------- 223.1 

Dewey Bridge Member. 

Section of the Slick Rock Member of the Entrada Sandstone, measured in sec. 6, T. 42 N., R. 17 W., San Miguel County, Colo. 

Summerville Formation. 

Entrada Sandstone: 

Slick Rock Member: 

Sandstone, moderate-reddish-orange ( 1 OR 6/6) \ 

fine- to medium-grained, poorly sorted; composed of subangular to very well rounded clear 

Feet 

1 These symbols designate the quality and density of color and are from 

the National Research Council "Rock-Color Chart" (Goddard and others, 

1948). Many of the stratigraphic sections presented in this report were 

measured before the charts became available and consequently in those 

sections the symbols are lacking. 
JURASSIC SYSTEM- SAN RAFAEL' GROUP 31 

Entrada Sandstone-Continued Feet 

quartz grains with minor quantities of pink, 

gray, and black accessory minerals; . massive 

rounded weathering; beds 1-7 ft thick; at base 

is a wavy reentering parting that separates this 

unit as a distinct ledge and marks a sharp color 

Slick Rock Member-Continued 

change; faint horizontal wavy lamination ...... .. 19.4 

Sandstone, grayish-yellow, white-weathering; composition as below; structureless .. .......... ........... 1.4 

Sandst<me, grayisl1-yellow, pale-brown-weathering, 

rnedi u n1-fine-grained; sparse medium-coarse 

grains; highly cross laminated . ... . . 12.0 

Sandstone, grayish-orange, white-weathering, finegrained; numerous medium-coarse grains; horizontal w:1vy lamination 17.0 

Sandstone, grayish-orange, white-weathering, medimn-fine grained; numerous medium-coarse 

grains; wedging cross-lamination . .. 3.4 

Sandstone, 1noderate-reddish-orange, very fine 

grained to fine-grained; medium-coarse grains 

common; horizontal wavy lamination .. . ........ . 12.9 

Sandstone, rnoderate-reddish-orange, medium-finegrained; contains a few medium-coarse grains; 

poorly sorted; highly cross laminated .. .. . .. . 3.4 

Sandstone, mottled pale-yellow; composition and 

color as below; discontinuous wavy lamination . 8.5 

Sandstone, moderate reddish-orange (I OR 6/8), 

fine-grained to very fine grained; disseminated 

mediurn-coarse grains abundant; bedding indistinct, probably consists of horizontal wavy !amination 

Total Slick Rock Member 

Dewey Bridge Member. 

7.3 

85.3 

Thickness.-The Slick Rock l\ifembcr of the Entrada 

Sandstone tnaintains a f::lirly uniform thickness over extensive areas, but in the vicinity of the salt anticlines, it wedges 

out in places. ln general, however, it thickens to the west. 

It has a tnaxirnutn thickness of about 225 feet north of 

Sinbad Valley along the \\'CStcrn edge of the area, but elsewhere in the \\·estern part of the area it comrnonly is 130-

150 feet thick; in the eastern part of the area, it is as little 

as 90 feet. Sontc part of the greater thickness in the vicinity 

and north of Sinbad Valley is probably due to the presence 

of beds belonging to the 1\,loab Tongue of the Entrada. 

Conditions of dej;osition .. -Thc bedding characteristics 

of the Slick .Rock lVfcrnber of the Entrada indic:::~ te a combination of agents of deposition. The units made up of 

sweeping crossbeds ~vidcntly were deposited by wind, but 

the horizontally bedded units were probably water laid, and 

true horizontal bedding planes cutting the eolian units suggest water action. Gilluly and Reeside ( 1928,. p. 78) considered that the Entrada in the San Rafael Swell of Utah 

was all water deposited because there the Entrada contains 

rnuch silt, has an earthy appearance, and devoid of largescale crossbedding. In that same area the Entrada is both 

underlain and overlain by rnarine deposits-by the Carmel 

and Curtis forrnations respectively. The overall beclcling and 

lithic characteristics of the Slick Rock Member of the 

Entrada in western Colorado suggest that the formation 

was deposited in and marginal to a body of shallow water. 

Th~ crossbeddcd units probably are the remains of dunes 

formed on very broad marginal beaches during times when 

the water had retreated long distances. 

SUMMERVILLE FORMATION 

The Summerville Formation conformably overlies the 

Slick Rock Member of the Entrada and crops out as a 

rather inconspicuous unit on the lower slopes of most of the 

high mesas and in many places along the canyon walls of 

the Dolores and San Miguel Rivers and their tributaries. 

The formation is also exposed along the sides of Paradox 

and Gypsum Valleys except wher~ concealed by faulting, 

or where it pinches out against the flanks of the saltanticlines 

as in the southeastern end of Gypsum Valley. 

The Summerville characteristically forms a red slope 

above rounded Entrada Sandstone cliffs and below raggedweathering sandstone ledges of the lower part of the Morrison Formation (figs. 6 and 7). In many places it is covered 

with talus from the overlying Morrison Formation and generally is rather deeply weathered. Consequently, good exposures arc rare and arc confined to gullies or to a fevv localities 

wlicrc the formation is undercut to form a· cliff beneath 

the resistant sandstone-of the Morrison Formation. At many 

places in the upper half of the Summerville, a sandstone 

ledge forms a low cliff which breaks the regularity of the 

slope. This sandstone is persistent in the mapped area and 

has received various field names. At Uravan, for example, 

Goldman and Spencer (1941, p. 1756-1757) referred to it 

as the mixed-grain sandstone unit. 

'The Summerville Formation consists of interbedded claystone and sandstone, and, in the upper part, scattered thin 

beds of limestone. The claystones are reddish brown and 

most are silty and sandy. They contain disseminated amberstained well-rounded small quartz grains that arc an aid in 

identifying the formation in areas of complicated structure 

or poor exposures. The sandstone ranges from reddish 

brown, silty and clayey, and very fine grained to light brown 

or almost white, clean, and fine to medium grained. According toR.:\. Cadigan (written commun., 1954), the mean 

composition of the sandstone, based on 14 samples, is 

quartz, 6 7 percent; chert, 5 percent; quartz overgrowths 

and silica cement, 3 percent; calcite cement, 19 percent; 

potassic and sodic feldspar, 6 percent; biotite and other 

heavy minerals, less than 1 percent. l\1ost of the sandstones 

arc \\'ell sorted, but the mixed-grain sandstone contains 

sparse medium to very coarse grains of quartz and gray to 

black chalcedony. According to Cadigan (written commun.~ 

195+) some of the dark grains of chalcedony contain dolomite. They may be diagnostic of this bed, for they contrast 

with the clark ilmenite-magnetite and black tourmaline 
32 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

grains of the Morrison sandstones. The thin limestone beds 

of the Summerville are very fine grained and dark gray to 

black. At the mouth of Atkinson Creek the limestone contains small gastropod shells, the only fossils known in the 

Summerville in the mapped area. Thin, parallel, remarkably continuous beds ranging in thickness from 2 inches to 

FIGURE 10.- Summerville Formation in roadcut 2 miles southeast 

of Uravan, Colo. 

3 feet are a diagnostic feature of the Summerville Formation (fig. 10). Bedding planes in the claystone and most of 

the sandstone units are flat and regular; some of the sandstone beds show parallel slightly asymmetrical ripple marks 

with an average amplitude of 1"4 inch and an average wave 

length of 1 Y2 inches. 

Goldman and Spencer (1941 ) studied the upper part of 

the San Rafael Group in detail in parts of southwestern 

Colorado and recognized a number of distinctive lithic 

features such as carnelian grains, green chert, barite concretions, and mixed-grain sandstone, which have aided in 

correlation. However, many of the characteristics which 

they used for correlations east and south of the Uravan 

district are not widespread throughout the mapped area or 

are not detectable because of generally poor exposure. 

Nevertheless, the mixed-grain sandstone in the upper half 

of the Summerville Formation is unusually persistent (unit 

B of the Uravan section of Goldman and Spencer, 19H, 

p. 1756), and grains of red finely botryoidal chert and 

similar incrustations on calcareous sandstone fragments are 

found as float derived from the lower third of the formation 

in many parts of southwestern Colorado. 

Although the basal part of the Summerville is transitiona l into the underlying Entrada Sandstone through a few 

feet of thin ledge-forming beds of Entrada-like sandstone 

that alternate with reddish Summerville-like mudstone, the 

lower contact is evident enough in most places. Along the 

Dolores River below the junction of the San Miguel River, 

careful tracing of beds has shown that the top beds of the 

Entrada Sandstone grade southward into the Summerville 

Formation. This gradation of only a few feet of beds takes 

place through a distance of about 5 miles, and although it 

has been discerned only at this one place, it is probably 

characteristic of the Entrada-Summerville contact in western Colorado. These beds lie below and are not the same 

as the beds listed as Entrada in the measured section in 

John Brown Canyon (p. 30 and 32 ) to be described later 

(p. 34) as transitional into the Moab Tongue of the 

Entrada Sandstone. 

The following sections are representative of the Summerville Formation in the mapped area: 

Section of the Summer ville Formation, north side of John Brown 

Canyon, sec. 32, T . 51 N., R. 19 W., Mesa County, Colo. 

[Measured by L. C. Craig, H. L. Jicha, and others) 

Morrison Formation: Feet 

Salt Wash Member (incomplete): 

Sandstone, white, light brown-weathering, mediumfine grained; composed of clear subangular 

quartz grains with minor quantities of pink and 

green accessory minerals. Fluviatile channel and 

fill bedding ______ __ .. ____ ....................... 6.4 

Siltstone and sandy shale, interbedded, dark-red ; 

contains irregular gray-green bands ..... 3.0 

Limestone, light-gray to purplish, fine-grained; 

locally recrystallized to coarse grained ... .9 

Total incomplete Salt Wash Member 10.3 

Summerville Formation: 

Siltstone and sandstone interbedded, pink to brick-red. 

Sandstone is fine grained, in part clayey; silty clay 

partings separated thin even sandstone beds ........... _ 16.2 

Sandstone, white, light brown-weathering, fine- to 

medium grained ; poorly sorted, composed of clear 

quartz with minor accessory minerals including darkgray to black chert grains ................................. 3.2 

Siltstone, dark-red ; weathers to chippy rubble; contains 

small amount of disseminated fine quartz grains ........ 14.6 

Claystone, dark-red to brown ; contains fine grains of 

disseminated clear to amber-stained quartz; a few 

hard light-gray siliceously cemented sandstone beds 

which probably were originally calcareous ................ 16.2 

Total Summerville Formation 50.2 

Entrada Sandstone (incomplete ) : 1 

Sandstone, white ligh t brown-weathering, fine-grained, 

evenly laminated, evenly bedded; composed of clear 
JURASSIC SYSTEM- SAN RAFAEL GROUP 33 

Entrada Sandstone-Continued Feet 

subangular quartz grains with minor pink chert ........ 3.9 

Sandstone, red and gray-green, banded, fine-grained; 

contains varying amounts of dark-red and gray matrix. This unit may represent Entrada Sandstone beds 

grading laterally into Summerville-like beds ...... 7.3 

Sandstone, white, brownish buff-weathering, finegrained, parallel-bedded and laminated .. .. .... ... .. ... . .. 27.0 

Shale, dark-purplish-red, clayey .. ... . .. .. ... .. .. . .... . . ... .. ... ... .3 

Total incomplete Entrada Sandstone ..... ................. 38.5 

Section of the Summerville Formation, northeast side of Dolores 

River Canyon, sec. 2, T. 48 N., R. 18 W., unsurveyed, Montrose 

County, Colo. 

[Measured by L. C. Craig, H. L. Jicha, William Alexandet·, and 

James Standard) 

Morrison Formation (incomplete) : Feet 

Salt Wash Member (basal part only): 

Sandstone, white to pale-brown, brown-weathering, 

medium-fine- to medium-grained; composed of 

subangular clear quartz grains with common 

grains of white chert and minor quantities of 

pink, gray, and black accessory minerals; prominent channeling, wedging sets of cross-laminae. 

Contains two 2- to 5-ft-thick red shale intervals. 

One ledge contains granules of red, green, and 

gray chert as much as Ya in. in diameter. This 

is the lowest prominent channel-fill sandstone 

ledge of the Salt Wash Member; the base of this 

unit was used as the base of the Salt Wash in 

mapping in this area ............................................ 75.6 

Sandstone and sandy calcareous claystone interbedded. Sandstone is fine grained, buff, irregularly lenticular. Claystone is gray green, thin, 

irregularly bedded ................................................ 2.5 

Claystone, dark-red, shaly weathering ...... .............. .1 

Limestone, gray-green; contains irregular reddish 

patches; argillaceous and slightly sandy ..... ....... 5.4 

Limestone, tan, yellow-brown-weathering, finegrained; platy bed ..... .... ...................................... .2 

Total incomplete Salt Wash Member of Morrison Formation ............................................ 83.8 

Summerville Formation: 

Sandstone and claystone interbedded. Sandstone is 

white to green, fine grained, platy weathering, ripple 

laminated and mud cracked. Claystone is dark-red; 

contains disseminated medium-fine grains of quartz ... 9.4 

Sandstone, light-green to buff, fine- to medium-finegrained; composed of subangular clear quartz grains 

with minor dark-gray to black and spars~ colored accessory minerals; slabby bedded, evenly laminated .... 3.9 

Claystone, dark- to brick-red; contains disseminated 

amber-stained fine-grained subangular quartz grains; 

weathers shaly to earthy .............................................. 15.0 

Sandstone, white to pink, fine-grained, composed of subangular clear quartz grains and pink, green, and 

1 1'he 38.5 ft of beds considered in this section as belonging to the upper 

part of the Entrada Sandstone was considered by Cater and E. M. Shoemaker, fot• the purposes of mapping, as belonging to the Summerville. A 

few miles to the southeast these beds grade into the alternating thin layers 

of cluystone und sandstone of the lower part of the Summerville Formation. 

Summerville Formation-Continued Feet 

black accessory mineral grains ...... ............................ .9 

Claystone, dark-purplish-red to brick-red; contains disseminated amber-stained fine- to medium-fine quartz 

grains; shaly to earthy weathering .............. .......... ...... 11.7 

Claystone and sandstone ( 25 percent) interbedded. 

Claystone is dark red; contains disseminated fine to 

medium grains of amber-stained quartz. Sandstone is 

pink to white, fine to medium fine grained, and is 

composed of clear quartz with colored accessory minerals; in 1-ft-thick ledges ...... ...................................... 8.4 

Shale, dark-red; contains disseminated amber-stained 

fine- to medium-fine rounded quartz grains; rubbly 

to shaly.weathering ..... .................... ................... 13.1 

Sandstone and claystone (30-50 percent) interbedded. 

Sandstone is white to pink, predominantly fine 

grained; composed of subangular grains of clear 

quartz with pink, green, black, and orange grains of 

accessory minerals. Claystone is very sandy; contains 

clear and amber-stained rounded fine to medium 

grains of quartz in red clay matrix ............................ 29.3 

Claystone and sandstone interbedded. Sandstone is 

white, fine to medium fine grained; composed of subangular clear quartz grains with sparse grains of 

colored accessory minerals. Claystone is dark red; 

contains disseminated fine to medium fine grains of 

quartz ....................................................................... .. 

Sandstone, white, fine- to medium-fine-grained; composed of subangular clear quartz grains .................... .. 

Claystone, sandy, dark-red; contains amber-stained fine 

grains of quartz in red clay matrix ........................... .. 

Bottom contact sharp and conformable. 

Total Summerville Formation 

Entrada Sandstone (incomplete, only the top unit des~ribed) : 

Sandstone, light-gray to pale-brown, medium-fine- to 

medium-grained, very poorly sorted; composed of 

clear quartz with minor red, pink, orange, green, and 

1.8 

.7 

1.4 

95.6 

black grains; not laminated .. .. . .. . .. .. . .. .. ..... .. .. ... ........... 8.1 

Total incomplete Entrada Sandstone ...................... 8.1 

Section of the Summerville Formation, northeast side of Dolores 

River Canyon near Slick Rock, sec. 30, T. 44 N., R. 18 W., 

San Miguel County, Colo. 

[Measured by L. C. Craig) 

Morrison Formation (incomplete): Feet 

Salt Wash Member (basal part only): 

Sandstone, grayish-orange-pink (lOR 8/2) to palered (lOR 6/2), predominantly fine to medium 

fine grained; composed of subrounded to subangular grains of clear quartz with numerous 

amber-stained quartz grains and minor amounts 

of black accessory minerals; red and green clay 

flecks; highly cross laminated; chanelling.......... 18.8 

Sandstone, claystone, and limestone interbedded. 

Sandstone is mottled dark red and gray green, 

very fine grained to fine grained; composed of 

clear subangular grains of quartz; structureless 

wedging and irregular beds as much as 1 ft 

thick. Claystone is sandy to silty, dark red; in 

beds as much as 6 in. thick. Limestone is light 

gray, very fine grained to aphanitic, forms a 
34 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Morrison Formation-Continued 

Salt Wash Member-Continued Feet 

1- to 3-in. lens at base of unit. Base of Salt Wash 

arbitrarily placed at base of limestone or base 

of lowest beds showing appreciable lenticularity 

where limestone is absent 13.9 

Total incomplete Salt Wash Member of Morrison Formation 32.7 

Summerville Formation: 

Sandstone and claystone interbedded. Sandstone is pale 

red, weathering brown, very fine to fine grained; 

composed of subangular grains of clear quartz; beds 

finely ripple laminated (amplitude Ys-Y4 in. wave 

length 1 Y2-~ in.; weathers to form thin ledges. Claystone is sandy to silty, dark red; in beds as much as 

6 in. thick ______ _______ 19.0 

Sandstone, very pale orange to grayish-yellow, predominantly fine grained; composed of subangular 

clear quartz grains with numerous grains of colored 

accessory .minerals; poorly sorted in part; structureless to faintly horizontally laminated to cross laminated on a fine scale in thin beds. Forms ledge _______ ·- 13.2 

Sandstone, siltstone, and claystone interbedded. Sandstone and siltstone are predominantly dark reddish 

brown and contain a few light-greenish-gray highly 

calcare:ms beds; sandstone composed of subangular 

grains of clear quartz and forms continuous even beds 

as much as 3 ft thick. Claystone is dark reddish 

brown, very sandy to silty, in 6-in.-thick interbeds. 

Unit forms !edgy slope -·--·-· __________ .. ____ 34.1 

Sandstone, clayey, to sandy claystone, dark-reddishbrown, very fine grained to fine-grained; indistinct 

bedding, evenly bedded; forms earthy to flake-covered 

bench or rounded cliff ------------------------- ------------------------ 11.5 

Total Summerville Formation 77.8 

Entrada Sandstone (incomplete): 

Sandstone and clayey sandstone interbeds. Sandstone is 

mottled, pale reddish brown and very pale orange, 

very fine to medium grained; composed of subangular 

to well-rounded grains of clear quartz with minor 

amounts of amber, orange, pink, red, and gray to 

black accessory minerals; poorly sorted; structureless 

to wavily laminated. Clayey sandstone interbeds are 

dark red. Forms massive ledges (sandstone) 2 ft 

thick and reentrants (clayey sandstone) 6 in. thick ___ 30.6 

Total incomplete Entrada Sandstone 30.6 

THICKNESS 

The Summerville Formation generally ranges in thickness 

from a featheredge to 100 feet and averages about 75 feet 

over much of the area, but, in common with older formations, it pinches out at many places along the crests of the 

salt anticlines. It cannot be determined whether the formation pinched out as a result of depositional convergence 

against high ground produced by upward movement of salt 

or whether the formation was upwarped as a result of salt 

movement and removed locally by erosion during early 

Morrison time, although both probably occurred. 

CoRRELATION AND CoNDITIONS OF DEPOSITION 

The Summerville is interpreted as having been deposited 

dominantly in shallow quiet water, as is indicated by the 

prevalence of thin regular beds, the parallel ripple marks 

and horizontal lamination of the sandstone units, and the 

total absence of scour surfaces. Previous workers· have considered the Summerville marine because in eastern Utah it 

grades laterally into the Curtis Formation which contains 

a marine fauna. Alternatively some geologists have argued 

that the red color and nearly complete absence of fossils in 

the Summerville suggest a continental environment of 

deposition. 1f so, currents must have been slow and sediment must have been deposited from broad sheets of water, 

perhaps on the lower reaches of a delta. 

The Summerville that grades into the Curtis Formation 

is considered to be Late Jurassic on the basis of the marine 

fauna in the Curtis. Regionally, the stratigraphic interval 

between the top of the Entrada and the base of the Morrison Formation contains many facies, most of which have 

separate names. East of the mapped area the \'\'anakah 

Formation of the San Juan lVIountains region is probably 

the equivalent of the Summerville. In northwestern Colorado and northeastern Utah the Curtis Formation occupies 

the entire interval between the Entrada and 1\i[orrison, 

including the entire Summerville Formation, whereas in 

central Utah the Curtis is gradational into and includes 

beds equivalent only to the lower part of the formation. 

To the south the Junction Creek Sandstone of southwestern 

Colorado and the Bluff Sandstone of southeastern Utah 

grade and tongue with the upper part of the Summerville. 

In easternmost Utah and in the northwestern part of the 

mapped area, part of the Moab Tongue of the Entrada 

Sandstone grades and tongues into the lower part of the 

Summerville and in a few places may include all but a few 

feet of the Summerville. The transition of part of the !\1oab 

Tongue into the lower part of the Summerville can be 

observed clearly in the vicinity of the mouth of ~1esa Creek 

where, northward, the lmvermost 25-40 feet of the Summerville grades into clean sandstone r;:tther abruptly. This 

sandstone forms a ledgy cliff that in places is continuous 

with the cliff of the underlying Entrada, but because it 

could be separated from the underlying Entrada Sandstone 

with no difficulty, it was mapped with the Summerville 

(see section measured in .John Brown Canyon, p. 32). Over 

most of the mapped area the 11oab is probably represented 

only by thin sandstone beds mapped with the lower part of 

the Summerville Formation. 

MORRISON FORMATION 

The Morrison Formation crops out as a prominent unit 

which forms most of the slopes of the high mesas throughout the area considered in this report. In some places the 

protective cap of the overlying Burro Canyon Formation 

has been largely removed by erosion and the 11orrison is 
JURASSIC SYSTEM- MORRISON FORMATION 35 

exposed over broad areas, as in the Carpenter Flats-Carpenter Ridge area north of Paradox Valley and on Tenderfoot Mesa. The Morrison is the oldest formation to have 

completely or almost completely blanketed the mapped 

area, including the salt and gypsum cores of the anticlines. 

The Morrison Formation of western Colorado is divided 

into two members of roughly equal thickness: the Salt Wash 

below and the Brushy Basin above. The Salt Wash Member 

contains a number of resistant sandstone strata alternating 

with less resistant mudstones. The sandstone forms clifflike 

ledges, the mudstone forms steep slopes, and the general 

physiographic expression of the member is a steep irregular 

slope broken by low cliffs (fig. 11 ) . The top resistant sandstone ledge of the Salt Wash generally forms a bench from 

which the Brushy Basin Member has been eroded and 

which is one-half mile or more wide in many places. The 

Brushy Basin Member is composed dominantly of nonresistant bentonitic mudstone, and it forms steep slopes 

capped and protected from erosion by the resistant Burro 

Canyon Formation (fig. 11 ). In many places plastic clays 

of the Brushy Basin have slumped, and the lower part of 

the Brushy Basin slope may be buried by such slumped 

material as well as by toreva blocks and talus of the Burro 

Canyon Formation. The blocks, talus, and slumped clays 

commonly exhibit the hummocky topography typical of 

landslides. Large toreva blocks may form small shallow 

depressions or benches perched high on the Brushy Basin 

slope, and in places careful examination is required to 

detect whether the large blocks have slumped. 

The Morrison Formation ranges in thickness from about 

600 to 800 feet in most of the mapped area, although 

D. R. Sha,,·e (written commun., 195 7) has reported a 

maximum thickness 1,150 feet from a drill hole in Disappointment Valley (sec. 36, T. 44 N., R. 18 W. ) . The variations of thickness of the Morrison are in part unsystematic 

and occur irregularly throughout most of western Colorado, 

but in the area of the salt anticlines the Morrison generally 

thickens away from the crests of the major anticlines. 

The following stratigraphic sections are representative of 

the Morrison Formation in the mapped area: 

Section of the Morrison Formation at the Dolores group of mines, 

measured on south face of Atkinson Mesa, sees. 19, 20, 29, and 

30, T. 48 N., R. 17 W ., Montrose County, Colo. 

(Measured by L. C. Craig. R. A. Cadigan, E. M. Shoemaker, and Carl Setzer] 

Burro Canyon Formation (incomplete ): Feet 

Claystone, silty, sandy and conglomeratic, gray-green, 

nonresistant ; contains granules of clear quartz, darkgray quartz, and sparse red chert 10.8 

FIGURE !!.- South face of Blue Mesa, showing outcrop of Entrada Sandstone (Je ), Summerville Formation (Js) , Salt Wash Member of 

Morrison Formation (Jms), Brushy Basin Member of Morrison Formation (Jmb ), and Burro Canyon Formation ( Kbc). Note interfingering of Burro Canyon and Brushy Basin at upper left. 
36 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Morrison Formation : 

Brushy Basin Member: 

Top contact is an irregular channeled contact. 

Claystone and sandstone. Claystone is dark red, 

silty. Sandstone is dark red, fine grained, thinly 

bedded. Scattered exposures 

Sandstone, buff, fine-grained, ledge-forming ......... . 

Claystone; as below, but becomes medium gray in 

upper half ....................................... . 

Claystone, silty, dark-reddish-brown; contains a few 

irregular greenish-gray streaks. Several ledges of 

red-brown hackly-fracturing fine-grained sandy 

siltstone ................................................................. . 

Covered 

Claystone, silty, dark-red to maroon, polygonalfracturing; contains thin lenticular layers of siltstone and very fine grained to fine-grained 

sandstone .. ························-----------------------------------

Claystone, silty, gray-green to green, bentonitic .... 

Claystone, dark-red to gray, silty, banded; numerous thin light-gray, fine-grained nodular brownweathering limestone interbeds ........................... . 

Interval poorly exposed. Lower half· gray, yellow, 

and cream bentonitic silty shale; upper half is 

green to red silty shale, thin green siltstone, and 

several thin nodular to platy light-gray finegrained limestone beds .......... ··········--------------------

Interval poorly exposed. Siltstone, silty claystone, 

and claystone, dark-red ······················------------------

Sandstone, conglomeratic, medium-brown-weathering, fine-grained to very coarse grained, very 

poorly sorted; composed of rounded grains of 

clear quartz with grains of red and green minor 

accessory minerals; granules and pebbles of 

white, tan, gray, red, and green chert ............... . 

Feet 

20.9 

2.0 

42.7 

36.6 

128.7 

25.2 

20.0 

20.0 

29.4 

24.4 

15.9 

Total Brushy Basin Member ..... ...................... 365.8 

Salt Wash Member: 

Covered interval. Probably dark-red silty sandstone 

and siltstone ..................... __________ ............ ____ .......... . 

Sandstone, light-brown to brown, fine-grained; 

composed of clear quartz with green, red, pink 

and black accessory minerals·; ripple laminated 

to cross laminated ------------------------------------------------

Interval covered in upper part. Lower part is darkred sandstone with interbeds of greenish-gray to 

gray very fine grained sandstone as much as 

1 ft thick ····----------------------------------------------------------

Sandstone, pale-brown-weathering, fine- to medium-grained, poorly sorted; composed of subangular to rounded clear quartz grains with 

minor grains of accessory minerals and green 

chert; numerous greenish-gray pebbles; highly 

channeled cross-laminated thick beds _______________ _ 

Covered. Dark-red silty claystone indicated ___________ _ 

Sandstone, pale- to light-brown, fine- to mediumfine-grained; composed of clear quartz with very 

minor pink and black accessory minerals, highly 

cross laminated ----------------------------------------------------

Claystone, light-green, silty to sandy; having beds 

of greenish-brown to dark-brown ·very fine 

grained sandstone as Jlluch as 1 ft thick ............. . 

Feet 

12.6 

3.6 

25.9 

32.6 

9.8 

6.7 

4.1 

Morrison Formation-Continued 

Salt Wash Member-Continued 

Sandstone, brown-weathering, fine grained, ripplelaminated .......... ·········------------------------------------------

Interval very poorly exposed. Claystone and silty 

claystone, dark-red; contains a few thin very fine 

grained greenish-brown sandstone beds and thin 

medium-gray nodular limestone beds ------------------

Sandstone, white, pale-brown-weathering, finegrained; composed of clear quartz with pink and 

black accessory minerals; channeled and cross 

laminated . ·········----------------------------------------------------

Covered. Probably nonresistant ------------------------------

Sandstone, pale-brown, medium- to dark-brownweathering, very fine grained; ripple laminated 

to cross laminated . ··················----------------------------

Covered. Probably dark-red shale and siltstone and 

brownish fine-grained horizontally laminated 

sandstone ... -----------------------------------------------------------

Sandstone, pale-brown, light-brown-weathering, 

medium-fine- to fine-grained; composed of clear 

quartz with minor pink and amber quartz and 

white chert grains; lenticular beds, highly cross 

laminated, extreme channeling. Unit is uranium 

bearing lateral to section. Sparse bone fragments noted .--------------------------------------------------------.-

Interval very poorly exposed; dark-red silty friable 

claystone and interbeds of pink to brown finegrained, slabby, faintly ripple laminated sandstone indicated --······----------------------------------------------

Sandstone, pale-brown, light-brown-weathering, 

fine- to medium-fine-grained; composed of clear 

quartz with minor amounts of white chert, pink 

and amber quartz, and black accessory minerals; 

highly cross laminated, channeled ----------------------

Interval very poorly .exposed. Dark-red silty claystone, dark-red shaly siltstone, and numerous 

pink to whitish very fine grained sandstone beds 

indicated ·········-------------------------------------------------------

Sandstone, greenish to brownish, light-brownweathering, fine- to medium-fine-grained; comp::>sed of subangular to rounded grains of clear 

quartz with angular grains of white chert and 

minor black and pink to amber accessory minerals; thick bedded, highly cross laminated, 

channeled ........ ___________ ----------------------------------------

Interval very poorly exposed in lower part. Slabby 

interbeds of dark-red claystone, dark-red siltstone, greenish very fine grained sandstone, and 

pink to pale-brown, medium-grained, poorly to 

well-sorted sandstone. Fine-grained sandstone 

and siltstone show worm burrows ....................... . 

Sandstone, white, light-brown-weathering, predominantly fine grained; composed of clear 

quartz with numerous grains of amber to pink 

quartz and black accessory minerals; highly cross 

laminated .. --------------------------------------------------------··· 

Sandstone and siltstone interbedded. Sandstone, 

brown-weathering, very fine grained; contains a 

few black specks of fossil wood. Siltstone, red to 

greenish, shaly-weathering ................................... . 

Sandstone, white, light-brown-weathering, medium-fine- to fine-grained; composed of clear 

Feet 

4.9 

39.8 

12.8 

2.1 

6.4 

6.3 

15.6 

21.0 

13.3 

18.0 

20.0 

21.4 

6.8 

7.0 
JURASSIC SYSTEM - MORRISON FORMATION 37 

Morrison Formation-Continued Feet 

Salt Wash Member-Continued 

quartz with very minor quantities of pink and 

black accessory minerals; thick bedded, highly 

cross laminated, channeling 5 ft vertically in 

100 ft horizontally ................................................ 7.0 1 

Sandstone, light-yellow to pale-brown, mediumbrown-weathering, very fine grained to finegrained; horizontally bedded, evenly laminated 

to ripple laminated . . ............................................ 19.2' 

Claystone,. shaly, dark-brown, slightly silty. Upper 

part of interval poorly exposed ............................ 9.6 ' 

Calcareous sandy claystone and limestone, lightgray, fine-grained; weathers shaly to platy ..... ... 2.6 

Limestone, light-gray, fine-grained; in part slightly 

sandy .................................................................... 1.2 

Sandstone, siltstone, and sandy limestone;! yellow 

brown below, light gray above, platy to slabby; 

some of limestone shows wavy lamination suggesting algal colonies . ... .. ........... ............. ... ........... 4.6 

Total Salt Wash Member ................................ 334.9 

Total Morrison Formation .............................. 700.7 

Summerville Formation: 

Interval covered .... ......................................................... 3.6 

Section of the Morrison Formation measured on west side of 

Summit Canyon at Summit Point, sees. 8 and 9, T. 43 

N., R. 19 W., San Miguel County, Colo. 

[Measured by L. C. Craig and J. J. Fo~ger] 

Burro Canyon Formation (incomplete): Feet 

Sandstone, conglomeratic, pale-orange, fine- to 

medium-grained, very poorly sorted; composed of 

subangular clear quartz grains with quartz overgrowths; contains abundant angular white chert 

grains, and sparse grains of pink, orange, and gray 

accessory minerals; highly cross laminated, fills 

channels; pebbles as much as 2 in. in diameter of 

light-gray, white, and tan chert; some pebbles fossiliferous . . . .. ........ .. . ....... ... ............... ................................... 27.0 

Morrison Formation: 

Brushy Basin Member: 

Top contact poorly exposed, probably channeled. 

Covered . . . .. . .. . .. . . .. ....... ..... .. .... ......... ... .. ... ....... ....... 30.1 

Claystone, predominantly red, some grayish-green, 

silty to fine-grained sandy; poorly exposed .. ... ... 5.4 

Covered .................................................................... 16.2 

Claystone; predominantly grayish green with some 

red, silty; poorly exposed .. . .. . ... ........................... 10.8 

Covered .................................................................... 21.6 

Claystone, predominantly dark reddish brown, 

some grayish-green, silty to fine-grained sandy; 

poorly exposed .. .................................................... 42.6 

Sandstone, pale-greenish-gray, medtum-fine- to 

medium-grained; composed of clear quartz and 

red chert in about equal amounts; lenticular 

unit ........................................................................ 6.0 

Claystone, dark-reddish- to purplish-brown, silty to 

fine-grained sandy ......... ....................................... 14.6 

1 Base of Salt Wash Member arbitrarily placed at base of limestone beds, 

which mark the most suitable contact regionally, although in this part of 

western Colorado the base of the lowest channel-fill sandstone ledge (lowermost of the two 7-ft-thick units above) is an appropriate and suitable 

contact. No significant break in deposition marks the basal Morrison 

contact. · 

Morrison Formation-Continued Feet 

Brushy Basin Member-Continued 

Sands:tone,. mottled dark-reddish-brown and 

greenish-gray~ very fine grained to fine-grained; 

composed' of subanguJiar grains of clear quartz 

with mino·r· red chert ........... ............................... 1.0 

Sl!lnrlface ol!lltao~p co·vered ......................................... . 

Interval! co·vered by windblown material and vegetation on broad bench ........................................ 159.3· 

Sands11olnle', conglomeratic; same as. 19.4-ft. unit 

beto:w; poorly exposed ........................................ 8.7 

C:o.veroo;: sandstone and some grayish-green and 

Jl'edi sillty claystone indicated ................................ 13.6 

Sandstone, conglomeratic,. grayish-orange, very fine 

graim:ed! to' very coalise grained, very poorly 

sorted!;· eomposed oi subang,ular grains of clear 

cq,'l!llallitz. with abundant grains of white chert and 

Sl!lallTS€~ colot'ed a:eeessory minerals; pebbles of 

w:hiit!ti!). t3il'll,. gray, red, and pale-green chert, dissemim:llted and concentrated in lenses; highly 

ctlrlDSS' Ia:miina:ted, channeled' . . . .. . . ... . . . .. . . . . . . . . . . . . . . . . . . . 19.4 

Total· B'rl!liShy· Basin Member .......................... 349.3 

Salt Wash Member: 

Ccmtact unexposed, probably channeled, located 

wi:tEciiml ~ ft.. . ........................................................ . 

Claystone· andi sandstone interbedded. Sandstone 

iS, grayisll• 0nange, very fine to fine grained; 

€0mposed' 0f subangular grains of clear quartz 

witfu: numen0us grains of orange-red, green, and 

black ac,:cessory minerals. Claystone is dark reddish brown to maroon to gray, silty tQ fine 

grained sandy ....................................................... . 

Covered ................................................................... . 

Sandstone, veny light gray to pale-orange, predominantly fine grained; composed of subangular grains of clear quartz with numerous white 

angular chert grains and scattered grains of 

colored accessory minerals; highly cross-laminated, channeled ............................................... . 

Claystone, greenish-gray ( 5GY 6/1), very fine 

grained to fine-grained sandy ........................... . 

Sandstone, grayish-orange ( 1 OYR 7 I 4), very fine 

grained; composed of angular grains of clear 

quartz ................................................................... . 

Siltstone, grayish-yelow (5Y 8/4), clayey ........... . 

Sandstone and claystone interbedded. Sandstone 

is pale reddish brown to pale red, predominantly 

fine grained; composed of subangular grains of 

clear quartz with numerous grains of white chert 

and orange and black accessory minerals; structureless to ripple laminated in beds as much as 

3 ft. thick. Claystone is dark reddish brown, 

silty to fine grained sandy ................................... . 

Sandstone, very pale orange to white to pale-pink, 

medium-fine- to fine-grained; composed of subangular grains of clear quartz with minor 

quantities of white chert and orange, pink 

black, and green accessory minerals; highly cross 

laminated, channeled ......................................... . 

Sandstone and claystone interbedded. Sandstone 

is pale reddish brown, very fine grained; com36.8 

12.6 

22.7 

1.5 

1.0 

1.0 

46.1 

25.6 
38 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Morrison Formation-Continued Feet 

Salt Wash Member-Continued 

posed of subangular clear quartz grains with 

abundant quantities of orange, pink, and black 

accessory minerals; horizontally laminated to 

ripple laminated. Claystone is dark reddish 

brown to chocolate brown, very fine grained, 

sandy .. ----------------------------------------------------------------- 29.0 

Sands tone, very pale orange ( 1 0 Y R 8 I 2 ) to moderate orange-pink ( 1 OR 7 I 4), fine- to mediumfine-grained; composed of subangular to subrounded clear quartz grains with numerous red 

claystone pebbels at bottom; highly cross laminated, channeled ....... ------·--------------------------------- 18.0 

Sandstone and shale interbedded. Sandstone is 

pale reddish-brown, pale pink, pale greenish 

gray, very fine grained; composed of clear 

quartz with minor pink and black accessory 

minerals; structureless to ripple laminated; contains worm burrows. Shale is dark reddishbrown,. silt to medium fine grained sandy, 

slightly micaceous ------------------------------------------------ 58.4 

Sandstone, pale-br~wn to light-pink, predominantly fine grained, composed of subangular 

clear quartz grains with abundant grains of 

pink and amber accessory quartz; cross laminated, channeled ................. ------------------------------ 6.3 

Covered; probably nonresistant interbedded shale 

and sandstone _________________ ------------------~-------------- 7.7 

Sandstone, pale-brown to white, medium-fine- to 

fine-grained; composed of sub angular grains of 

clear quartz with numerous subangular to subrounded grains of white to tan chert and minor 

orange accessory minerals; cross laminated, 

channeled -------------------------------------------------------------- 13.0 

Sandstone and claystone interbedded. Sandstone is 

pale red, very fine grained; composed of subangular clear quartz grains with grains of 

colored accessory minerals; forms ripplelaminated ledges. Claystone is dark reddish 

brown, silty, chloritic ............. ---------------------------- 5.4 

Sandstone, pale-brown, light-brown- to brick-redweathering, fine- to medium-fine-grained, poorly 

sorted; composed of subrounded clear quartz 

grains with minor amounts of pink, orange, and 

gray accessory minerals; highly cross laminated, 

channeled -------------------------------------------------------------- 17.2 

Total Salt Wash Member ------------------------------ 302.3 

Total Morrison Formation ---------------------------- 651.6 

Summerville Formation .(incomplete): 

Top contact is channeled. Contact is marked by color 

and texture change and is arbitrarily selected at the 

base of the lowest scoured ledge. 

Sandstone and claystone interbedded. Sandstone is dark 

reddish brown to pale red with 8 ft. of pale-greenishgray beds at bottom, very fine grained; composed of 

subangular clear quartz grains with numerous grains 

colored accessory minerals; slab by to platy splitting, 

ripple-laminated to evenly laminated beds. Claystone 

is dark reddish brown to dark brown, partly silty .. .. 31.1 

Section of the Morrison Formation in John Brown Canyon, sec. 5, 

T.50 N., R. 19 W., and sees. 31 and 32, T. 51 N., 

R. 19 W., Mesa County, Colo. 

[Measured by L. C. Craig, H. L. Jicha, and others] 

Burro Canyon Formation (incomplete): Feet 

Sandstone, conglomeratic, white to pale-brown, 

medium- to medium-fine-grained; composed of clear 

quartz with accessory white chert; small pebbles 

(average diameter Y4 in.) of white to light-brown 

chert ------------------------------------------------------------------------------ 18.8 

Morrison Formation: ---

Brushy Basin Member: 

Siltstone and claystone; dark red and maroon in 

lower third, light green and gray green in upper 

two-thirds; short lenses of very fine grained 

sandstone as much as 3 in. thick in upper part .. 30.5 

Siltstone and claystone; dark red and maroon, 

green and gray green in lower third; poorly 

exposed ---------------------------------------------------------------- 72.8 

Claystone, silty; gray green below, red above; 

upper part poorly exposed ---------------------------------- 59.0 

Sandstone, conglomeratic, pale-brown, medium- to 

medium-fine-grained; composed of clear rounded 

quartz grains with numerous grains of pink, 

green, and gray accessory minerals. Granules 

and pebbles as much as Y2 in. in diameter, both 

disseminated and concentrated in laminae ........ 15.2 

Siltstone; gray green to brown in lower part, bright 

green with some red in upper part; a few thin 

lenses of greenish sandstone with hard siliceous 

cement ------------------------------------------------------------------ 47.8 

Siltstone, clayey, gray-green; thin cap of brownweathering, dark-green, fine-grained, quartzitic 

sandstone at top -------------------------------------------------- 12.2 

Covered; material deeply weathered along ridge 

crest. Offset in section . -------------------------------------- 26.9 

Claystone and siltstone; green, gray-green, and 

pinkish-gray; en tire unit contains some silt; a 

few limestone lenses and one very fine grained 

thin sandstone bed. Unit is capped by 6-in.-

thick lens of dark-brown-weathering fine-grained 

quartzitic sandstone -------------------------------------------- 19.8 

Claystone, slightly silty, gray-green; contains yellow 

silty bentonitic streaks and concretionary lenticles of light-gray fine-grained limestone which 

locally is replaced by green chert. Limestone 

nodules are concentrated in zone at top of unit 11.2 

Limestone, light-gray, fine-grained, brown-weath-

. ering; forms irregular len tides on top of underlying unit -------------------------------------------------------------- .4 

Sandstone, olive-drab to medium-brown, finegrained, quartzitic -------------------.----------------------------- .8 

Claystone, silty, gray-green to pinkish-gray, poorly 

exposed ------------------------------------------------------------------ 8.8 

Sandstone, light-gray, pale-brown- to mediumbrown-weathering, fine-grained; composed of 

clear subangular quartz grains with minor 

amounts of green, black, gray, and red ac~essory 

minerals ---------------------------------------------------------------- 3.0 

Siltstone, clayey, gray-green and dark-red; poorly 

exposed -----------------------------------------·---·--------------·--- 10.7 

Sandstone, white to light-pink, fine-grained; composed of clear subangular quartz grains with 
JURASSIC SYSTEM - MORRISON FORMATION 39 

Morrison Formation-Continued 

Brushy Basin Member-Continued 

minor quantities of pink accessory minerals; unit 

cornposecl of 6-in.-thick slabby ledges weathering 

back frou1 face of ledge formed by underlying 

unit 

Feet 

2.0 

Sandstone, conglomeratic, pale- to medium-brownweathering, rnedium-fine- to medium-grained; 

contains typical Brushy Basin varicolored chert 

grains and granules in lower part 11.6 

·····-········· 332.7 "J'otal Brushy Basin Member 

Salt Wash l'vfember: 

Interval poorly exposed. Siltstone, clayey, darkred; a 3-ft-thick gt·ay-green zone at top . 27.9 

Sandstone, pale-brown, light-brown- to mediumbrown-weathering, fine-grained; composed of 

clear subangular grains of quartz and abundant 

grains of arnber quartz and green clay with 

accessory green chert and minor black grains 3.6 

Interval poorly exposed. Float indicated siltstone, 

clark-red, clayey; contains some gray-green 

streaks .... . ... . .............. ..... 20.0 

Sandstone, white gray, fine-grained; composed of 

clear subangular quartz grains ..... ..... _ 2. 7 

Siltstone, dark-red, clayey; contains two beds of 

red-brown fine-grained sandstone composed of 

clear qual'tz and abundant quantities of pink 

accessory minerals . ..................................... 16.2 

Sandstone, pale-brown, light- to medium-brownweathering, medium-fine- to medium-grained; 

composed of clear subangular rounded quartz 

grains with rninor amounts of pink and green 

accessory minerals; contains red-clay laminae 

nnd lenses of red-clay granules 35.1 

I ntcrval poorly exposed. Dark-red clayey siltstone 

indicated 29.7 

Sandstone and shale interbedded. Sandstone is 

red brown, fine grained, composed of clear subangular quartz grains. Shale is silty, dark red 15.2 

Covered .. . . .. . . ... 1 0.8 

Sandst<me, pale-brown, light-brown-weathering, 

medium-fine- to medium-grained; composed of 

clear subangular grains of quartz with minor 

amounts of gray, pink, and black accessory 

minerals . _ .. _ _ _ . .. 258. 

Siltstone, clark-red; gray green at top, clayey 8.4 

Sandstone, light-pink, dark-brown-weathering, finegrained; composed of clear subangular quartz 

grains with varicolored accessory mineral grains. 

Siliceous cement .... . . . . . ... 2.4 

Claystone, silty; purple and gray green in lower 

third, dark red brown and gray green in upper 

part ....... ........ ..................................... 32.3 

Sandstone, pale-brown to pink, reddish-brownweathering; composed of subangular clear 

quartz grains with minor amounts of pink and 

green chert; predominantly fine grained, some 

medium fine grained . . . ........ ... . . 11.8 

Siltstone and sandstone interbedded. Siltstone is 

sandy and clayey, dark-red. Sandstone is gray 

green to red brown weathering, fine grained, 

composed of clear quartz; forms several beds 

6 in. thick . ...... .. . . ........ ..................... 6.9 

Morrison Formation-Continued 

Salt Wash Member-Continued 

Sandstone, red-brown-weathering, fine-grained; 

composed of clear subangular quartz grains with 

green, black, and red accessory minerals; siliFeet 

ceous ce1nent . ....................................................... 1.0 

Siltstone, sandy and clayey, d:hk-red .................... 16.2 

Sandstone, brown-weathering, fine-grained; composed of clear subangular quartz grains with 

green, black, and red accessory mineral grains; 

siliceous cement .................................................... .9 

Siltstone, shaly, dark-red. Contains two finegrained sandstone beds 6 in. thick ................. __ 21.6 

Sandstone, white, light-brown::weathering, - medium-fine-grained, poorly sorted; composed of 

clear subangular grains of quartz with minor 

amounts of red and black accessory chert ........ 24.9 

Sandstone, pink to white, fine-grained, slabby bedded. Contains thin dark-red silty shale partings 13.1 

Sandstone, white, light-brown-weathering, mediumfine-grained; composed of clear subangular 

grains of quartz with minor amounts of pink 

and green accessory minerals. Fluviatile channeland-fill bedding .. .............................................. 6.4 

Siltstone and sandy shale, interbedded. Dark red 

with irregular gray-green bands .. ......... ............... 3.0 

Limestone, light-gray to purplish, fine-grained, 

locally recrystallized to coarse grained . ... .9 

Total Salt Wash Member ... ....................... 336.8 

Total Morrison Formation . .......... ... 669.5 

Summervile Formation (incomplete): 

Siltstone and sandstone, interbedded, pink to brick-red; 

sandstone is fine grained, in part clayey; silty 

clay partings separate thin even sandstone beds.. 16.2 

The Morrison Formation in many places has yielded fossilized remains of diposaurs, and it generally contains abundant plant fragments and petrified wood and sparse freshwater mollusks and calcareous algae. The vertebrate and 

fresh-water invertebrate fossils indicate a Late Jurassic age. 

However, the uppermost part of the Morrison on the Colorado Plateau interfingers in places with the lowermost beds 

of the Lower Cretaceous Burro Canyon Formation and thus 

may be Early Cretaceous. The Morrison Formation is widespread throughout the western interior of the United States 

and, although the two members mapped in western Colorado are not recognizable outside the Colorado Plateau, 

some stratigraphic evidence indicates thc..t equivalent beds 

are present in the undifferentiated Morrison of central and 

northwestern Colorado (Craig and others, 1955, p. 137, 

156). 

SALT WASH MEMBER 

Throughout t~e mapped area the Salt Wash Member of 

the Morrison Formation consists of alternating sandstone 

and mudstone strata. The sandstone strata are largely· very 

pale orange ( lOYR 8/2) to white (N9), fine to medium 

grained, and yellowish gray, buff, or rusty red weathering. 

The strata are intricately bedded; each unit is composed of 
40 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

a number of imbricate, highly lenticular beds, each of 

which may rest with apparent concordance on the subjacent 

bed or, more commonly, on an irregular surface which cuts 

into or across the underlying bed. Furthermore, the beds 

are generally cross laminated, and sets of laminae may 

truncate other sets. Through most of the sandstone units 

the cross-lamination is of the festoon or scour-fill type, but 

near the tops of sandstone units a few feet of beds may 

show almost horizontal bedding and only gently clipping, 

horizontal, or wavy lamination. Some flat surfaces exhibit 

current lineation, and the way laminated beds usually show 

cuspate current ~ipples. Y.he bottom contact of each sandstone stratum is marked by a sharp lithologic change, generally along an irregular scour surface. In places these local 

contacts display channel scours whereas in other places 

bedding may be concordant and little erosion indicated. 

The dominantly mudstone strata contain rocks having a 

variety of lithic characteristics; beds range in composition 

from claystone to clean sandstone. The dominant rock types 

are pale-reddish-brown sanely and .silty claystone, and palereddish silty or clayey very fine grained to fine-grained 

sandstone. Beds in the mudstone intervals are broadly lenticular, but in places are nearly parallel; they range in 

thickness from several inches to 5 feet. Cross-lamination is 

uncommon within beds in the mudstone intervals; beds are 

either massive and without fine structures or have horizontal, even or wavy lamination; cuspate-current ripples are 

common in the sandy beds. 

Based on 17 samples the mean composition of the sandstones of the Salt Wash in the mapped area ( R. A. Cadigan, written commun., 1953) is quartz, 73 percent; silicified 

tuff and chert, 4 percent; quartz overgrowth and silica 

cement, 4 percent; calcite cement, 13 percent; potassic and 

soclic feldspar, 6 percent; gypsum, biotite, and other heavy 

minerals, less than 1 percent. Cadigan ( 196 7) provided 

more detai~ on composition of the Morrison and its members in a report on the regional petrology of the formation. 

Preliminary studies by A. D. Weeks (written commun., 

1953) and subsequent work by Keller ( 1962, p. 39) showed 

that the dominant clay minerals in the mudstones of the 

Salt Wash Member are hydrous micas or illite. The mudstones of the Salt Wash are earthy weathering or weather 

to hackly fragments, and the clays show little or no tendency to swell. Reel mudstones characteristically are altered 

to green or gray in a zone ranging from a few inches to 

several feet in thickness immediately beneath some of the 

sandstone units. The only detectable difference in composition of the mudstones is a greater content of total iron and 

ferric iron in the red than in the green or gray mudstones 

(A. D. 'Veeks and \V. D. Keller, written commun., 1953). 

The smaller amount of ferric iron in the green or gray mudstone apparently is mainly a result of leaching of hematitic 

pigment from between the particles of clay and silt, inasmuch as ferric iron in the structure of the clay minerals is 

not appreciably affected by leaching (A. D. Weeks, written 

commun., 1953). 

The contact of the Salt Wash Member of the Morrison 

with the Summerville Formation is arbitrary; beds with 

characteristics of the Summerville grade into beds with. 

characteristics of the Morrison within a thickness of about 

15 feet or less. For most purposes the base of the lowest 

cross-laminated sandstone ledge ·that fills channel scours has 

been considered the base of the Salt Wash. This contact is 

readily recognized, is remarkably persistent in southwestern 

Colorado, and has been used in the mapping of the area 

covered by this report. In other parts of Colorado and in 

adjacent areas in Utah, the basal sandstone ledge is not 

persistent, and the lowest beds with Morrison depositional 

characteristics are lenticular mudstones, short lenses of 

sandstone filling small scours, or, characteristically, a few 

relatively persistent limestone beds which commonly contain 

fresh-water ostracodes and charophytes. In most places in 

southwestern Colorado, including the mapped area, such 

beds may be found below the lowest prominent sandstone 

ledge of the Morrison in an interval which may be as thick 

as 35 feet. In many places the gradation from the flat 

parallel bedding of the Summerville Formation to the lenticular bedding of the Morrison is accompanied by a color 

change from dark reddish brown below to light greenish 

gray or grayish red above. Inasmuch as limestone beds are 

regionally more persistent than the lowest channel-fill sandstone, the base of the limestone has been used as the Salt 

Wash-Summerville contact in. the representative sections in 

this paper. This lower contact of the Morrison, however, 

would be difficult to map in southwestern Colorado for the 

limestone beds are poorly exposed in most places. If it were 

possible to map this contact conveniently instead of the 

contact of the base of the cross-laminated sandstone that 

actually was mapped, the result would be a slight change 

in position of the basal Morrison contact in only a few 

places, for the basal Morrison limestone-bearing unit is 

characteristically thin and forms a steep slope. The Morrison is conformable upon the Summerville Formation, and 

even where the base of the Morrison is a scour surface the 

hiatus represented by the surface is probably diastemic in 

magnitude and local in areal extent. 

THICKNESS 

The Salt Wash Member of the Morrison generally 

ranges in thickness from 240 to 440 feet within the mapped 

area. Although detailed measured sections are sparsely distributed, Jess exact control from the geologic quadrangle 

maps of this area provides enough points of known thickness to show a general northwest elongation of thick and 

thin parts of the Salt Wash corresponding to the trends of 

the anticlines, with the thinnest parts on the crests of the 

., 
-· 

JURASSIC SYSTEM- MORRISON FORMATION 41 

anticlines and the thickest parts generally in the intervening 

basins. Superimposed on this pattern arc variations in thickness which seem best explained as irregularities in deposition 

of materials. These irregularities may be as much as 100 

feet or more in less than 3 miles along the outcrop. Section 

measurements made in connection with lithofacies studies 

by 'I'. E. :rvt:ullens and V. L. Freeman (written· commun., 

1953) have sho,vn variations of 70 feet in a distance of 200 

feet along the outcrop. 

CONl)ITIONS OF DEPOSITION 

The fluvial origin of the Salt Wash Member is abundantly indicated by the depositional features of the sediments; the crossbedclecl scour- and channel-fill sandstone 

was deposited in stream channels, and the nearly parallel 

beclclecl mudstone was deposited mainly on flood plains. 

I ... acustrine deposits probably are a minor component of the 

member, although limestone and possibly some of the 

mudstone formed in ephemeral lakes on the flood plains. 

Fresh-water invertebrate and plant fossils and fossils of 

land-dwelling vertebrates indicate the continental nature of 

the deposits. 

BRUSHY BASIN MEMBER 

The Brushy Basin Member of the Morrison consists dominantly of variegated mudstone with lesser amounts of 

sandstone, conglomeratic sandstone, and limestone. Bedcling of the mudstone is difficult to discern and probably 

is most reliably revealed by a color banding that suggests 

tabular to somewhat lenticlular beds; lamination is not 

apparent. Individual beds among the sandstone and conglomeratic sandstone strata range from almost tabular or 

slightly lenticular, \Vith internal parallel laminations, to 

markedly lenticular, with complex internal imbrication and 

cross-lamination. Scour surfaces are common at the base of 

and within these complex units. The limestone beds are 

slightly lenticular to tabular and rarely laminated. 

The mean composition of the sandstones of the Brushy 

Basin Member, based on seven samples analyzed by R. A. 

Cadigan (written commun., 1954), is quartz, 64 percent; 

silicified tuffaceous material and chert, 9 percent; quartz 

overgrowth and silica cement, 7 percent; calcite cement, 14 

percent; potassic and sodic feldspar, 5 percent; gypsum, 

1 percent; heavy minerals, less than 1 percent. A. D. Weeks 

(written commun., 1953) and Keller (1962, p. 39) have 

found that the dominant clay in the mudstones of the 

Brushy Basin is montmorillonite. Most of the mudstones 

swell in varying degrees when wet and as a result weather 

to a characteristic hard "frothy" or "popcorn" surface. 

Shards or relics of shards (Waters and Granger, 1953, p. 6; 

W. D. Keller, written commun., 1953), analcite (W. D. 

Keller, written commun., 1953), and books of biotite and 

euhedral crystals of apatite and zircon (A. D. Weeks, written commun., 1953) attest to the volcanic origin of some 

parts of the mudstone. The montmorillonite probably was 

derived from volcanic ash by hydrolysis and devitrification. 

The materials of the Brushy Basin are of many grain 

sizes. The mudstones are composed mainly of clay with 

varying amounts of silt and fine-grained sand. Sandstones 

are very fine grained to coarse grained and conglomeratic. 

In southwestern Colorado most of the pebbles in the conglomeratic parts of the Brushy Basin are less than threefourths of an inch in diameter, but a few exceed 1 inch. 

The Brushy Basin is characterized by a number of rather 

distinctive materials and beds. The conglomeratic sandstone 

contains abundant pebbles of red and green chert and of 

quartzite, commonly much interstitial clay, and in many 

places, bone fragments and petrified wood. Irregular masses 

and fragments of highly colored jasper in places litter the 

weathered surfaces of the member. Pale-green (5G, 7 /2) 

to grayish-green ( 1 OGY 5/2) fine-grained, hard, wellcemented sandstone forms tabular beds one-half inch to 

3 feet thick. These greenish sandstones commonly are cemented by silica, in many places sufficiently so to form 

quartzite. The secondary silica in the cement, as well as 

that in the silicified wood and bone and the jasper, probably was derived during alteration of the volcanic materials 

to montmorillonite. Polished and rounded pebbles, so-called 

gastroliths but actually probably dust polished (Stokes, 

1942), occur sparsely on weathered surfaces of the Brushy 

Basin Member and rarely are found in place. In the Blue 

Mesa area much of the Brushy Basin in greenish· gray to 

pale green and light blue green. These colors are well displayed along the lower part of Mesa Creek and along the 

northeast side of the San Miguel River Canyon near the 

mouth of Mesa Creek; they contrast markedly with the 

characteristic alternating dark red, light gray, and light 

greenish gray of Brushy Basin in other parts of the mapped 

area. The anomalous greenish colors may be the result of 

alteration, although no compositional difference has been 

detected. 

The basal contact of the Brushy Basin Member is arbitrary, for in many places the Salt Wash Member grades 

upward into the Brushy Basin. In mapping, the base of the 

Brushy Basin Member was placed at the base of the lowest 

conglomeratic sandstone with abundant red and green 

chert pebbles. According to studies made by J. S. Pomeroy 

(written commun., 1955) red and green pebbles constitute 

about 36 percent of the total pebbles in the Brushy Basin 

but only about 16 percent of the total in the Salt Wash. 

Sandstones in the Brushy Basin tend to weather to dark 

brown, and those in the Salt Wash generally weather pale 

yellow to white. 

A change in the mudstones of the Salt Wash and the 

Brushy Basin is also associated rather closely with the con-
42 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

tact bet\·vccn these two members of the Morrison. The 

mudstones of the Salt \Vash are dark reddish brown, silty 

or sandy, and nonswelling, whereas those of the Brushy 

Basin commonly arc grayish red, slightly purplish, or light 

gray and contain large quantities of swel ling and generall y 

less sandy claystones. The change in the mudstones is not 

distinct and in most places is several feet below the base of 

the ·conglomerate bed sclcctccl as the base of the Brushy 

Basin. 

The conglomeratic sandstone at the base of the Brushy 

Basin is lenticular, but it is present in enough places to 

allow ·projection of the basal contact of the member 

through areas where the marker bed is absent. Although the 

base of the conglomeratic sandstone is most commonly a 

scour surface, it is not continuously traceable and is regarded as only of diastemic magnitude. The stratigraphic 

position of the conglomeratic stratum may vary somewhat 

from one lens to the neighboring lenses, but it probably 

varies less than 50 fee t throughout the mapped area. This 

variation of stratigraphic position may account for some 

of the apparent thickening and thinning of the members of 

the Morrison but does not account for the larger scale 

differences of thickness. 

THICKNESS 

The Brushy Basin M ember in the mapped area generally 

ranges in thickness from 300 to 750 feet. In general the 

thinner parts coincide with the axes of anticlines, but data 

a re insufficient to establish precise relations between thickness and the salt structures. About 750 feet of the Brushy 

Basin was drilled in the northwest part of Disappointment 

Valley (D . R. Shawe, written commun., 1957). 

CoNDITIONs oF D EPOSITIO N 

The continuation of fluvial deposition through Brushy 

Basin time is indicated by the cross-stratified sandstones and 

conglomeratic sandstones which both rest on a nd contain 

scour surfaces. The finer grained sedimentary rocks probably were deposited both by sluggish streams and in slack 

water on flood plains. Thin lenticular limestones probably 

were formed in local lakes by calcareous algae. Tuffaceous 

or altered volcanic ma terials in scattered thin beds may have 

been deposited directly from the air. Many shards preserved 

in mudstones have angular forms that also suggest direct 

deposition from the air or only a short distance of water 

transport. 

CRETACEOUS SYSTEM 

BURRO CANYON FORMATION 

Coffin (192 1, p. 97-100 ) first divided the sequence of 

beds earlier called "Dakota" in western Colorado into the 

"Post-McElmo" and a restricted Dakota. Later, Stokes 

(1948; 1952, p. 1773-1 774) applied the name Burro Canyon Formation to the "Post-Elmo" beds, naming the 

formation for exposures in Burro Canyon ncar Slick Rock. 

The formation is one of the most resistant units in western 

Colorado and crops out as a series of hogbacks or as broken 

cliffs along the flanks of the salt anticlines and on the rims 

and tops of most of the high mesas in the area (fig. 12 ). 

The Burro Canyon forms light-colored cliffs and steep 

slopes above the colorful and relatively unbroken slope of 

the Brushy Basin Member of the Morrison. In places large 

torcva blocks of Burro Canyon have slumped as much as 

100 feet or more down slopes underlain by the Brushy 

Basin, and in other places the Burro Canyon has shed large 

riGURE 12.- 0utcrop of Burro Canyon Formation near H orsethief 

Spring northeast of Long Park, sec. 26, T. 47 N., R. 17 W. 

blocks which form talus slopes that almost conceal the 

underlying Brushy Basin. 

The Burro Canyon Formation consists of alternating conglomeratic sandstone and mudstone with sparse beds of 

limestone and chert. The sandstone is grayish yellow to 

pale yellowish orange, fine to very coarse grained, and is 

composed dominantly of subrounded to subangular clear 

quartz grains 11·ith minor white and orange accessory minerals. R. !\.. Cadigan (written commun., 1954) reported 

that the mean composition of five samples is quartz, 68 

percent; chert, 9 percent ; quartz overgrowths and silica 

cement, 11 percent ; calcite cement, 10 percent; potash and 

sadie feldspar, 2 percent; muscovite and other heavy minerals, less than 1 percent. The sandstone strata are crossbedded and consist dominantly of trough sets; the strata 

commonly rest on surfaces of erosion (fig. 12 ). The pebbles 

in the sandstones consist mainly of white and tan chert, 

light-gray silicified limestone, light-gray to pinkish quartzite, and sparse clear or smoky quartz ( P. J Katich, written 

commun., 1951 ). In western Colorado the pebbles have a 

maximum diameter of about 3 inches and are found either 

scattered through the sandstone matrix or concentrated in 

thin beds and lenses, commonly near the bottoms of indi-
.. 

CRETACEOUS SYSTEM -BURRO CANYON FORMATION 43 

vidual trough sets. T'he conglomeratic sandstone strata 

form resistant cliffs and ledges as much as 100 feet high, 

and these strata are generally the most prominent units in 

the formation. T'he mudstones are light greenish gray and 

very pale green or, more rarely, pale reddish brown. They 

are variably silty to sandy and weather to earthy or hackly 

fragments; rarely do they show the fluffy surfaces characteristic of swelling clay. 'The mudstones have tabular to 

somewhat lentic~lar bedding and generally weather to 

slopes .. Limestone, yvhich is sparse, is dense and light gray, 

and it has a conchoidal fracture. It weathers brownish, 

either as thin concretionary layers a few inches thick or as 

resistant ledges as much as several feet thick; in places it 

is silicified. Both rnudstone and limestone are generally 

more abundant in the upper than in the lower part of the 

formation; they form relati"vely thick units only in the upper 

part. 

The basal contact of the Burro Canyon Formation is 

placed at the scour surface at the base of the lowest conglomeratic sandstone above the variegated mudstone of the 

Brushy Basin Mernber of the Morrison Formation. This 

contact is rernarkably persistent, but locally the conglomeratic sandstone of the Burro Canyon is lenticular, and in 

places the lowest ledge may pinch .out or several conglomeratic beds may pinch out. Inasmuch as in places the 

mudstones of the Burro Canyon and the Brushy Basin 

cannot be distinguished easily, the mapped contact may not 

everywhere adhere to the same horizon, but in most places 

the contaet can be projected from the base of one lenticular 

conglomeratic sandstone stratum through a mudstone 

sequence to the base of another conglomeratic sandstone 

stratwn occupying virtually the same stratigraphic position. 

The evident absence of a continuous erosion surface at the 

base of the Burro Canyon and small-scale examples of intertonguing of Brushy Basin and Burro Canyon indicate that 

the Burro Canyon and :Morrison Formations are conformable. 'The scour surfaces at the base of the conglomeratic 

sandstones are regarded as diastems. 

The following stratigraphie sections are representative of 

the Burro Canyon Formation in the mapped area: 

...\'ection of the Burro Canyon Formation on south face of Skein 

Mesa, sec. 9, T. 46 N., H. 18 W., Montrose County, Colo. 

[Mensurcd by L. C. Craig, R. A. Cndigan, and E. M. Shoemaker] 

Dakota Sandstone (incomplete) : Feet 

Sandstone, conglomeratic, yellow-brown to light-brown, 

fine- to medium-fine-grained; composed of clear 

quartz with abundant white chert grains. Pebbles of 

chert, predominantly black, tan, and white, arc as 

much as Y~ in. across. Cross laminated; scour-andfill structures .. .......... ................................................... 17.4 

Unconfonnity. 

Burro Canyon Formation: 

Upper contact is channeled scour surface .................... . 

Claystone, pale-gray-green, silty; bright green in top 

Burro Canyon Formation-Continued 

. 3 ft. . ............................................................................ . 

Sandstone, conglomeratic, pale-gray-green to palebrown, very fine grained to medium-fine-grained; 

composed of clear subangular quartz grains, green 

clay pebbles, and white, tan, gray, red, black, and 

green chert pebbles as much as 2 in. in diameter; 

scour-and-fill structures; cross laminated; worm borings in lenses of fine-grained material ...................... .. 

Claystone, pale-greenish-gray; sandy at bottom, silty at 

top; non bentonitic ...................................................... .. 

Sandstone, conglomeratic, pale-greenish-yellow to palebrown, fine-grained to very coarse grained, · poorly 

sorted; composed of clear subangular quartz grains, 

green clay pebbles, and white, tan, gray, red, black, 

and green chert pebbles as much as 2 in. in diameter; 

scour-and-fill structures; cross laminated ................ .. 

Claystone, dark-greenish-gray, pale-green-weathering, 

slightly silty; dark-red-brown silty zone at base; nonbentonitic .................................................................... .. 

Sandstone, pale-greenish-yellow to pale-brown, very 

fine grained to medium fine-grained, poorly sorted; 

composed of clear subangular quartz grains with accessory greey and red chert; siliceous cement; massive, not prominently laminated .............................. .. 

Total Burro Canyon Formation .......................... .. 

Morrison Formation: 

Brushy Basin Member (incomplete) : 

Upper contact probably channeled, not well exposed ................................................................... . 

Claystone, pale-brown, pale-green-gray, reddish- to 

pinkish-brown; slightly silty in lower half, silty to 

Feet 

13.4 

32.4 

28.8 

62.9 

8.0 

8.8 

154.3 

fine grained sandy in upper half; bentonitic .... 75.6 

Section of the Burro Canyon Formation on west side of Dry Creek 

on north flank of Dry Creek anticline, sec. 33, T. 46 N~, 

R. 16 W., Montrose County, Colo. 

[Measured by L. C. Craig and L. R. Stieff] 

Dakota Sandstone (incomplete) : Feet 

Sandstone, conglomeratic, buff- to brown-weathering, 

fine- to medium-grained; composed of clear quartz 

grains with abundant impressions of woody fragments and containing light- and dark-gray chert 

pebbles as much as 2 in. in diameter with accessory 

pebbles of . tan anq r~d chert and white and pink 

quartz ............................................................................ 13.1 

Unconformity. 

Burro Canyon Formation: 

Top contact is at base of conglomeratic sandstone with 

scour-and-fill structures; long offset of section, along 

contact ......................................................................... . 

Interval very poorly exposed. Claystone, light green, 

hackly to fissile ............. .... ........................................... 64.0 

Sandstone, conglomeratic, white- to light-buff-weathering, medium- to coarse-grained; composed of subangular to rounded clear quartz grains and pebbles 

of white, buff, tan, and pink chert as much as 1 in. 

in diameter; highly cross laminated; scour-and-fill 

structures ..... .... .... ......................................................... 38.7 

Sandstone, white- to buff-weathering, fine-to mediumgrained; composed of subangular to rounded clear 

quartz grains; !edgy beds exhibit secondary concretionary silicification ................. ........ ...... ....................... 11.8 
44 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Burro Canyon Formation-Continued Feet 

Shale, _clayey, green, fissile ........... ... ................... 6.8 

Shale, -clayey, gray to maroon, fissile; contains irregular 

nodules of ·pink to light-gray calcilutite .................... 17.6 

Sandstone, conglomeratic, light-green, medium- to 

coarse-grained; composed of well-rounded clear quartz 

and red and pink accessory grains; contains black, 

gray, tan, red, and white subangular to rounded chert 

pebbles as much as 3 in. in diameter ...... 18.6 

Total Burro Canyon Formation .. ---------------------- .. 157.5 

west in eastern Utah, the Cedar Mountain Formation is 

equivalent to the Burro Canyon (Stokes, 1944, p. 958, 965-

67; 1952, p. 1773-1774), a correlation supported by invertebrate fossils (Simmons, 195 7). 

DAKOTA SANDSTONE 

The Upper Cretaceous Dakota Sandstone disconformably 

overlies the Burro Canyon Formation.- Because of its resistance to erosion it crops ~ut extensively as capping beds 

on broad upland areas, on mesas, and on the long dip 

Morrison Formation: --- slopes flanking the salt anticlines. Like the other formations 

Brushy Basin Member (incomplete): of post-San Rafael age, the Dakota at one time completely 

Claystone, light- to bright-green; fissile to blocky blanketed the area. 

weathering; one 3-ft-thick bed weathers brown; The Dakota Sandstone is exposed best on the canyon fine-grained clear quartz sandstone at 6 ft. d 

above base of unit __________________________ __ __________________ ·-'3.9:6 _ rims, where it forms a rough flaggy ledge or series of le ges. 

<}~Although it paves the tops of many mesas, it is generally 

poorly exposed on these flat surfaces because of a thin cover 

of Quaternary deposits. In most areas the Dakota comprises 

three fairly distinct units: a lower unit consisting largely of 

sandstone and conglomerate, a middle unit of sandstone, 

carbonaceous shale, and impure coal, and an upper unit 

of sandstone and conglomerate. The sandstone is flaggy and 

gray, yellow, and buff; much .of it is coarse grained and 

crossbedded, but some of it is fine grained and thin bedded. 

R. A. Cadigan reported (written commun., 1954) that of 

six samples of the sandstone examined, the mean composition is quartz, 68 percent; chert, 12 percent; quartz overgrowths and silica cement, 15 percent; calcite cement, 3 

percent; potassic and sodic feldspar, 2 percent; muscovite 

and heavy minerals, less than 1 percent. Scattered through 

the.~sandstone are irregular discontinuous lenses of conglomerate containing chert and quartz pebbles as much as 

2 inches across. Thin layers of gray and black carbonaceous 

shale ·and seams and beds of impure coal interfinger with 

sandstone and conglomerate beds and lenses. In places 

some of the coal reaches a thickness of several feet and has 

been mined for local use. In a few places shale underlying 

the coal is nearly white and resembles fire clay. Plant 

impressions abound in both the. shale and the sandstone 

(Brown, 1950). Certain sandstone and conglomerate beds 

in the Dakota Sandstone have been highly silicified over 

extensive areas. The silicified rocks break with conchoidal 

fractures that cut across sand grains. In fact, the rock is 

so flinty that it served the · Indians for arrowheads and 

implements. 

THICKNESS 

The thickness of the Burro Canyon generally ranges from 

50 to 300 feet,. but it averages about 150 feet in the mapped 

area. The variation in thickness probably is due in part to 

irregularities in the surface upon which the formation was 

deposited and in part to movement of salt in the anticlines, 

but it is due principally to interfingering or gradation with 

the Brushy Basin Member of the Morrison Formation. 

CONDITIONS OF DEPOSITION 

The scour-and-fill structures of the conglomeratic sandstone indicate fluvial deposition. Most of th~ mudstone 

probably was formed on flood plains or in streams. The 

limestone and some of the mudstone probably formed 

in temporary lakes on flood plains. 

AGE AND CORRELATION 

In contrast to the overlying Dakota Sandstone, the Burro 

Canyon Formation is almost devoid of fossil plants. Fossil 

plants from a thin bed of carbonaceous shale about 25 feet 

below the top of the Burro Canyon Formation near the 

mouth of Disappointment Creek (sec. 11, T. 43 N., R. 

18 W., San Miguel County) are of. Early Cretaceous age 

(Brown, 1950, p. 46, 50). Fresh-water invertebrates collected by G. C .. Simmons and D. R. Shawe and identified 

. by J. B. Reeside, Jr. (Simmons, 195 7, p. 2526), support the 

assignment of an Early Cretaceous age. Several of the 

invertebrate fossils are widespread in the Kootenai and 

Cloverly faunas of Wyoming and Montana. 

The eastern and northern limits of the Burro Canyon 

Formation are not clearly defined. Lenses of Burro Canyon 

are recognized in the Placerville area of San Miguel 

County; in the Ridgeway area, Ouray County, Colo.; as far 

north as Grand Junction and Lorna, Colo.; and as far south 

as the Four Corners. Beyond these general eastern and 

southern limits, the formation may be equivalent to some 

mudstones included in the top of the Brushy Basin Member 

of the Morrison, or it may not have_ been deposited, or it 

may have been removed by pre-Dakota erosion. To the 

The contact between the Dakota and Burro Canyon is 

disconformable but in many places is seen with difficulty 

because the sandstone and conglomerate are similar in both 

formations. However, in other places the contact is unmistakable; channels carved several feet in to the Burro Canyon 

have been filled with Dakota sediments, or Dakota Sandstone containing numerous plant impressions rests on 

characteristic green shale of the Burro Canyon. 

I.' 

·, 
.... 

CRETACEOUS SYSTEM -DAKOTA SANDSTONE 45 

'I'he foJlowing measured sections present detailed lithic 

characteristics of the Dakota Sandstone. Only two complete 

sections were measured because in most places the upper 

part of the sandstone has been removed by erosion, and 

where the complete thickness of beds still remains, the 

exposures are either poor or inaccessible. 

Section of Dakota Sandstone near head of joe Davis Canyon, sees. 

27 and 28, T. 44 N., R. /8 W., San Miguel County, Colo. 

[Measured by L. C. Ct·aig] 

Mancos Shale. 

Dakota Sandstone: 

Top of formation taken at top of uppermost sandstone 

lens. 

Interval poorly exposed. Contains several thin sandFeet 

stone lenses .. ....... .................................................... 4.2 

Sandstone, grayish-orange, fine- to medium-finegrained; consists of subangular clear quartz grains 

with sparse white angular chert grains; highly crosslaminated; scour-and-fill structures ........ ·.................... 12.6 

Interval poorly exposed. Claystone, medium-gray to 

black; silty to sandy in part; a few thin (1-3 ft.) 

olive-green to grayish-orange siltstone and very fine 

grained sandstone beds composed of clear quartz 

with sparse accessory grains . ···············--------·-------------- 45.5 

Sandstone, grayish-yellow, predominantly fine to 

medium-fine-grained; consists of subangular clear 

quartz grains with numerous clay fragments; highly 

crass laminated; scour-and-fill structures.................... 10.8 

Shale, black, highly carbonaceous, fissile, slightly 

sandy ........................................................................ 4.8 

Total Dakota Sandstone 

Disconformity 

Burro Canyon }cormation. 

77.9 

Section of Dakota Sandstone on Skein Mesa, sec. 9, T. 46 N., 

18 W., Montrose County, Colo. 

[Measured by L. C. Craig, R. A. Cadigan, and E. M. Shoemaker] 

Dakota Sandstone: Feet 

Top of formation removed by erosion. 

Sandstone, light-yellow-brown, yellow-brown- to reddish-brown-weathering, fine-grained; consists of subangular to subrounded clear quartz with white subangular chert grains and sparse accessory minerals; 

contains wood fragments; cross laminated; scourand-fill -structures . ................... ................................... 21.6 

Interval poorly exposed. Contains light-brownish-gray 

sandy claystone, one zone of dark-brown carbonaceous claystone, and a few fine-grained yellow-brown 

platy sandstone layers ................................................ 43.2 

Sandstone, pale-yellow-brown, fine-grained; consists of 

clear quartz grains and pebbles of green silty clay; 

cross laminated; scour-and-fill structures .................. 21.6 

Interval poorly exposed. Contains gray-brown silty claystone, a dark-gray carbonaceous silty claystone bed 

1-2 ft thick, and one 2-ft-thick bed of gray-brown 

fine-grained sandstone ................................................ 18.0 

Sandstone, yellow-brown, fine- to medium-fine-grained; 

composed of clear quartz with accessory white and 

tan chert; scour-and-fill structures and cross laminated ............................................................................ 16.2 

Dakota Sandstone-Continued 

Claystone, brownish-gray; contains silt, very fine sand, 

and carbonaceous flecks ............................................ .. 

Sandstone, light-gray, very fine grained; predominantly 

clear quartz grains with siliceous cement; slab by 

bedded ....................................................... . 

Claystone, pale-gray-brown, light-gray-weathering, silty 

Sandstone, conglomeratic, yellow-brown to light-brown, 

fine- to medium-fine-grained; consists of clear quartz 

with abundant white chert grains. Pebbles of predominantly black, tan, and white chert as much as 

Y2 in. across. Cross laminated; scour-and-fill structures ............................................................................. . 

Feet 

5.4 

1.0 

11.0 

17.4 

Total incomplete Dakota Sandstone ..................... _. 155.4 

Disconformity. 

Burro Canyon Formation. 

Section of the Dakota Sandstone on Dry Creek, sec. 34, T. 46 N., 

R. 16 W. Montrose County, Colo. 

(Measured by L. C. Craig and L. R. Stieff] 

Mancos Shale. 

Dakota Sandstone: 

Top contact taken at top of highest sandstone lens. 

Sandstone, white to buff, coarse-grained; consists of 

clear rounded quartz grains and accessory dark-gray 

Feet 

chert .............................................................................. 1.2 

Interval poorly exposed. Sandstone, dark-brown, 

medium- to coarse-grained, clayey; contains carbonized plant fragments .................................................... 2.8 

Sandstone, buff to light-yellow, coarse-grained; consists 

of clear rounded quartz grains with coarse grains 

and granules of white chert; worm holes ................ 5.3 

Sandstone, nonresistant; contains a few partings of 

medium-gray claystone ................................................ 1.8 

Sandstone, same as second unit above .......................... 2.6 

Sandstone, yellow-brown to dark-brown, mediumgrained, clayey; consists of rounded clear quartz 

grains and black carbonaceous fragments ................ 8.0 

Bentonite, light-yellow; contains irregular crystals of 

selenite . .................... ................................................ .3 

Sandstone, dark-brown, medium-grained, clayey; contains black carbonaceous fragments ............................ 4.8 

Sandstone, white, slabby, medium-coarse-grained; consists of rounded clear quartz grains ............................ 1.2 

Sandstone, dark-brown, medium- to coarse-grained; 

consists of rounded clear quartz grains with abundant 

clay and carbonized plant fragments ........................ 5.6 

Bentonite, light-yellow to white; contains numerous 

crystals of selenite ........................................................ .6 

Sandstone, dark-brown, fine- to medium-grained, 

poorly bedded; consists of clear rounded quartz 

grains with abundant clay and carbonized plant 

fragments .. ................................................................... 17.3 

Sandstone, buff- to yellowish-brown-weathering, coarsegrained, slabby; composed of rounded clear quartz 

grains and minor white chert and dark accessory 

grains ............................................................................ 9.2 

Siltstone, medium- to light-gray ...................................... 8.1 

Siltstone, light-brown to light-gray, shaly; contains 

papery carbonaceous partings .................................... 1.4 

Shale, medium-gray to bright-yellow .............................. 1.4 

Shale, black, carbonaceous, lignitic ................................ 1.2 

Claystone, white to pink, hackly; contains abundant 
46 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Dakota Sandstone-Continued Feet 

fossil leaves from deciduous trees and ferns ............... . .6 

Shale, black, carbonaceous, lignitic ..... ... ....................... 1.8 

Shale, olive-drab to bright-yellow, silty; contains abundant carbonaceous roots .............. . .............................. 3.6 

Shale, medium-gray; contains a few light-yellow bentonitic lenses ..................................... _ ......... ............. 19.2 

Interval partly covered. Yellow-brown-weathering, finegrained sandstone indicated in lower part. Dark-gray, 

carbonaceous shale indicated in upper part ____ ........ 5.3 

Shale, dark-gray to black, carbonaceous, papery; contains a few thin carbonaceous sandstone interbeds .... 15.9 

Sandstone, buff- to light-brown-weathering, fine- to 

medium-grained; consists of subangular clear quartz 

with accessory grains of chert; ripple laminated to 

cross laminated; platy to massive in lower part, 

massive in upper part .................................................. 19.9 

Interval largely covered. Shale; light gray to cream 

indicated; contains a few black to dark-gray carbonaceous beds ......... ...................................................... 10.6 

Sandstone, buff- to yellow-brown-weathering, fine- to 

medium-grai-ned; platy to slabby, ripple laminated .... 9.9 

Shale and siltstone interbedded; light to dark gray, 

carbonaceous .......................................... :..................... 21.2 

Sandstone, conglomeratic, buff- to brown-weathering, 

fine- to medi urn-grained; consists of clear quartz 

grains with abundant impressions of woody fragments 

and containing light- and dark-gray chert pebbles 

as much as 2 in. across with accessory pebbles of tan 

and red chert and white and pink quartz ................ 13.1 

Total Dakota Sandstone . ... ................................... 193.9 

Disconformity. 

Burro Canyon Formation. 

THICKNESS 

In most places where the Dakota Sandstone is exposed, 

the uppermost beds have been stripped off by erosion; the 

maximum thickness of the uneroded Dakota in the mapped 

area appears to be about 220 feet, and the minimum is 

probably about 70 feet. Generally, the full thickness ranges 

from 120 to 200 feet. In the areas where the formation is 

complete, the variations in thickness appear to be entirely 

haphazard and to be controlled largely by irregularities of 

the erosion surface upon which the Dakota was deposited. 

Inasmuch as the top contact is arbitrarily placed at the top 

of the highest sandstone beneath Mancos Shale, lensing of 

the upper sandstone beds may also account for considerable 

variations in thickness of the formation. 

CORRELATION AND CONDITIONS OF DEPOSITION 

The Dakota Sandstone is widely distributed over the 

western interior, and throughout this wide region it is 

lithically remarkably uniform. The bedding characteristics, 

the plant fragments, and the coal beds indicate clearly that 

most of the Dakota is a terrestrial deposit; only the uppermost beds, transitional into the marine Mancos Shale, may 

be marine. Regionally the basal contact of the Dakota is 

an unconformity, for it bevels progressively older beds 

southward in Arizona and New Mexico and westward in 

southern Utah. In western Colorado no angularity has been 

recognized between the Dakota and the Burro Canyon. 

The basal contact of the Dakota in western Colorado is 

generally a scour surface at the base of a conglomeratic 

sandstone that in places contains quartzitic boulders and 

pebbles derived from the Burro Canyon. This indicates a 

hiatus between the deposition of the two formations at least 

equivalent to the time required to cement the top sandstone 

beds of the Burro Canyon (Carter, 195 7) . In a few places 

in western Colorado, gray to black carbonaceous shale of 

the Dakota rests on greenish mudstones of the Burro Canyon, and only the generally sharp contact between the two 

rocks reveals the disconformable nature of the contact. 

It is difficult to visualize how a formation so uniformly 

thin and of such vast areal extent could have been deposited 

on a land surface. It may be that the deposit is, as Stokes 

( 1950) suggests, the product of a type of very large scale 

pedimentation and that the Dakota largely represents material on a surface of transportation. On the other hand, 

Stokes' theory presupposes a terrain largely devoid of vegetal covering, and it is difficult to reconcile the abundance 

of plant material in the Dakota with a barren terrain; in 

fact, much of the Dakota landscape at any given time must 

have supported lush growths of vegetation. Furthermore, 

the beds probably were not deposited synchronously everywhere. With these probabilities in mind, we are more 

inclined to regard the Dakota Sandstone as a deposit that 

formed on a broad coastal plain in front of the advancing 

Late Cretaceous sea in which the overlying Mancos Shale 

was ~deposited. 

MANCOS SHALE 

The Mancos Shale, of Late Cretaceous age, conformably 

overlies the Dakota Sandstone and is gradational into it. 

Because of erosion, the Mancos is less widely distributed 

than the underlying formations. It crops out in Gypsum 

Valley as disconnected fault blocks let down from the collapsed crest of the salt anticline that underlies the valley, 

in the southeast end of Paradox Valley, and over wide areas 

in Dry Creek Basin and Disappointment Valley. The Mancos forms monotonous light-gray rolling surfaces and 

rounded hills. Natural exposures of fresh rock are rare 

except where gullying has been rapid. Surfaces underlain by 

the Mancos are notably barren and support only the sparsest vegetation; this apparent infertility probably stems from 

the thinness and tightness of the soils and the impermeable 

nature of the underlying shale itself, factors that promote 

very rapid runoff of precipitation. Furthermore, the shale 

is relatively rich in salts which rise to the surface during 

wet weather and form broad white "alkali flats" almost 

completely devoid of any vegetation. 

The Mancos Shale is possibly preserved in its entirety 

only in the collapsed blocks in Gypsum Valley, and there 
TERTIARY ( ?) SYSTEM 47 

it is incompletely exposed; elsewhere only the lower part of 

the formation remains. Throughout, the formation is 

remarkably uniform in composition. It consists largely of 

thinly bedded lead-gray to black shale that weathers light 

gray or yellowish gray. Interbedded with the shale are 

scattered thin beds of concretionary fine grained light-gray 

limestone, sandy shale and sandstone, and, at least in the 

lower part, a few layers of bentonite, 1-8 inches thick. The 

shale commonly breaks with a hackly fracture, and much 

of it is calcareous. Also in the shale at irregular intervals 

along definite. horizons are isolated irregular masses of 

limestone as much as 3 feet thick that may have formed 

either as concretions or as small local reefs. These masses 

are rnore resistant than the surrounding shale a'nd stand 

out as yellowish-weathering knobs. Sandstone is very sparse 

and occurs mostly in the upper part of the formation in 

beds transitional into the ~'Iesaverde Formation. In eastern 

Utah a thin-beclcled sandstone layer about 350 feet above 

the base of the ~tfancos has been correlated with the Ferron Sandstone ~tfember in Utah (l .... upton, 1916, p. 31), a 

gas-producing bed in the Clear Creek field of the '\'asatch 

Plateau, but this unit was not recognized within the 

rnapped area. 

Fossils of Late Cretaceous age are abundant in some layers of the ~1a ncos Shale; in the lower part: usually 10-50 

feet above the base, is a layer containing great numbers of 

Gr•yjJhaea newberryi Stanton. Scattered sharks' teeth also 

are common. Thickness of the ~1ancos Shale could not be 

determined but nearby to the east it is at least 2,000 feet. 

MESAVERDE FORMATION 

The Late Cretaceous ~tfesaverde Formation consists of 

interbedded yellowish-gray sandstone and light-gray shale. 

It conformably overlies and is gradational into the ~1ancos 

Shale. It is preserved within the area only in the southeast 

end of Gypsum Valley and ncar the mouth of Hamm 

Canyon. In both places only the lower part of the formation is exposed in dO\\'Invarped and downdropped blocks 

of the collapsed crest of the salt anticline that underlies 

the valley. 

TERTIARY(?) SYSTEM 

None of the lower Tertiary formations so widespread in 

the adjoining region to the north and east remain in the 

mapped area. Later deposits, older than Quaternary alluvium and wind-deposited material, formed in response to 

local conditions, and probably none of them ever covered 

more than a few tens of square miles. 'They do, however, 

contribute data to the interpretation of geomorphic and 

late structural history of the area, and for this reason 

aspects of these deposits relating to this history will be 

emphasized in the description of these deposits that follows. 

Deposits believed to be of Tertiary age ·consist of gravel 

beds in the southeast end of Gypsum Valley and fanglomerates in Paradox, Gypsum, and Sinbad Valleys. In earlier 

reports (Stokes, 1948; Cater, 1955b, 1955f; Shoemaker, 

1956a) these deposits were regarded as Quaternary; however, because the fanglomerate in Paradox Valley is unconformably overlain by beds containing volcanic ash tentatively identified by R. E. Wilcox (written commun., 1965) 

as early Pleistocene~ we are now inclined to regard this and 

the similar fanglomerates in the other valleys as preQuaternary and probably of late Pliocene age. The gravel 

beds in the southeast end of Gypsum Valley could not have 

been deposited by a stream related to the present drainage 

pattern, and their relation to the collapse structures of the 

Gypsum Valley salt anticline strongly suggests that they are 

older than the fanglomerate and are probably the oldest 

post-M:esozoic deposits remaining in the area. Pebbles of 

porphyritic rocks in the fanglomerate of Gypsum Valley 

that may have been derived from these gravels also lend 

additional support to their presumed older age. 

ANCIENT GRAVELS OF GYPSUM VALLEY 

Thick deposits of coarse gravel consisting largely of pebbles and boulders of porphyritic rocks from the San Juan 

~1ountains underlie the high ground at the southeast end 

of Gypsum Valley, and smaller patches of similar gravel 

crop out in nearby areas on the floor and along the south-

\vest side of the valley and in Gypsum Gap. Possibly some 

of the smaller patches are reworked_ from older gravel. In 

most places the gravel is poorly stratified, but where bedcling is discernible it generally is nearly flat. Locally, particularly along the southwest side of the valley, it dips 

rather steeply toward the center of the valley. Most of the 

gravel is only slightly cemented, but locally it is firmly 

cemented to a conglomerate. 

No streams draining areas underlain by rocks compositionally similar to the gravels presently traverse or come 

close to tranversing this part of Gypsum Valley. The gravel 

in Gypsum Gap strongly suggests that the southeast part of 

Gypsum Valley was draining through the gap at the time 

the gravels were being deposited. The absence of similar 

gravels down the valley to the northwest indicates that 

drainage was not down the length of the valley as is the 

present drainage. The relative antiquity of these gravels is 

also suggested by their local steep dips, a fault that offsets 

beds about 50 feet, and the considerable deepening of the 

valley since they were deposited. The fanglomerates, on 

the other hand, were deposited when the valleys were at a 

somewhat deeper level than at present, suggesting thereby 

that they are younger than the gravel. Although originally 

regarded as Pleistocene, the gravels in Gypsum Valley are 

the oldest deposits of probable Tertiary age in the area and 

are regarded at late Tertiary, possibly Pliocene, in age. 

Gravel of somewhat similar occurrence in Castle Valley in 

eastern Utah .was considered to be of probable Pliocene age 

by Hunt ( 1956, p. 30). 
48 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

FANGLOMERATES OF PARADOX, GYPSUM, 

AND SINBAD VALLEYS 

Fanglomerates are exposed at numerous localities 

through the length of Paradox Valley, in the lower part of 

Big Gypsum Valley, and at scattered localities in Sinbad 

Valley. These fanglomerates were deposited after the valleys had reached virtually their present form, but at a stage 

when the valleys were at somewhat deeper level than at 

present. In most places the fanglomerate is exposed only in 

washes 'that have carved through more recent valley fills, 

and outcrops are not of mappable size; but at several localities in the northwestern part of Paradox Valley it crops 

out as low hills. Wherever the base of the fanglomerate is 

seen, it rests directly on Mesozoic or older rocks-not 

uncommonly on gypsiferous beds of the Paradox Member 

of the Hermosa Formation. 

The fanglomerates consist of a rudely bedded unsorted 

heterogeneous mass of material that ranges from silt-sized 

particles to boulders. Pebbles and boulders are angular; 

those in Paradox Valley and Sinbad Valley consist entirely 

of material derived from Mesozoic and older formations in 

the walls of the valley, whereas among those in Gypsum 

Valley are also some pebbles of porphyritic rocks derived 

either from old terrace gravels of the Dolores River or from 

Pliocene(?) gravels of the Gypsum Valley. The material is 

thoroughly·· indurated, probably by caliche cement, and 

where exposed as low hills is very resistant to weathering. 

The greatest observed thickness of the fanglomerate in 

Paradox Valley is about 50 feet, but commonly it is only 

about 5-10 feet thick. The fanglomerates in Gypsum Valley 

and Siribad Valley are only 1-4 feet thick. In several places 

in Paradox Valley the fanglomerate is cut by, and drag 

folded along, slump faults that result from solution of the 

salt underlying the valley. The displacement of the fanglomerate along these faults was nowhere seen to exceed 

10 feet. 

ANCIENT GRAVELS OF THE GUNNISON RIVER 

Exposed in roadcuts of State Highway 141 from 1 to 

about 4 miles northeast of Gateway are gravels resting on 

arkosic conglomerate of the Cutler Formation and unconformably overlain by fanglomerate believed to be of early 

Pleistocene age (Cater, 1966) . The preponderance of silicic 

and intermediate volcanic and porphyritic intrusive rock 

and the scarcity of basalt strongly suggest a Gunnison River 

rather than a combined Colorado-Gunnison River source. 

If this inference is true, then_ the Gunnison rather than the 

Colorado River carved Unaweep Canyon, a topic discussed 

in a following section. 

The fact that these old gravels were nearly swept from 

the river channel before the probable lower Pleistocene 

fanglomerate .blanketed the lower valley of West Creek 

argues that these g~avels may well be Pliocene rather than 

Pleistocene, and in our opinion the Pliocene age is to be 

preferred. 

QUATERNARY SYSTEM 

Sediments assigned to the Quaternary System consist of 

fanglomerates in West Creek; slightly consolidated possibly 

lacustrine silts and sands; volcanic ash; terrace and bench 

gravels, talus; alluvial deposits along streams; wind and 

sheet-wash deposits; and landslides. The higher level terrace gravels are probably of Pliocene age because some of 

them lie well above levels to which streams had carved by 

Quaternary time, but they are described here because they 

are identical to the more abundant terrace gravels of Quaternary age. The relative age of some similar but widely 

separate deposits cannot be determined with certainty. 

Some types of Quaternary deposits are mapped separately 

and are delineated on the geologic map, whereas others are 

not separated because different types of deposits grade into 

one another or the boundaries between them are vague and 

transitional. 

FAN GLOMERA TES IN WEST CREEK VALLEY 

Two sequences of fan material, an older, partly indurated fanglomerate and a younger, unconsolidated unit, are 

widely exposed in the· lower valley of West Creek. The 

older, partly indurated fanglomerate at one time almost 

completely blanketed the valley as a thick sheet of fan 

material deposited on an irregular surface that sloped to 

the center of the valley, but erosion has removed much of 

it. Generally it crops out as a boulder-covered slope, but in 

some places where gully erosion has been rapid, it stands 

in nearly vertical bluffs approaching a height of 100 feet. 

The fanglomerate is light red or buff and consists of silt, 

sand, gravel, and angular cobbles and boulders derived 

almost entirely from Mesozoic formations that rim the 

valley. Material derived from Precambrian rocks and the 

Cutler For'mation is, strangely, very scarce. The fanglomerate has a maximum thickness of more than 200 feet. 

Overlying the older fanglomerate and more widely 

exposed are fan deposits having the general texture of the 

older fanglomerate, but the younger fanglomerate differs 

from the older in that it is unconsolidated and contains a 

large amount of material derived either from the Precambrian rocks or the Cutler Formation. 

These fanglomerates probably correlate with the Pleistocene Harpole Mesa Formation described by Richmond 

( 1962) in the La Sal Mountains of. Utah. Richmond 

divided the formation into three members which he believes 

correlate with the Nebraskan, Kansas, and Illinoian Glaciations of the Midwestern States. The older fanglomerate 

probably correlates with the lower member of the Harpole 

Mesa, and the younger with either or both the middle and 

upper members of the Harpole Mesa (Cater, 1966). 

On Spring Creek Mesa north of Uravan, a slightly 

indurated fanglomerate consisting largely of debris derived 

•. 
.· 

..... 

QUATERNARY SYSTEM 49 

from the Precambrian rocks or the Cutler Formation caps 

a few low hills. Frorn their character and occurrence we 

believe them to be approximately contemporaneous with 

one or the other of the fanglomeratcs along \Vest Creek. 

LAKE(?) DEPOSITS OF PARADOX VALLEY 

The northwest end of Paradox Valley is floored by 

loosely consolidated strata about 50 feet thick that appear 

to have been deposited in small lakes. The lakes may have 

formed during the latest stages of collapse of the Paradox 

Valley salt anticline because of local settling of the valley 

floor below the temporary base level. The sediments consist 

of reddish-buff evenly bedded friable silty sandstone in the 

upper part of which is a layer of volcanic ash a few inches 

thick and a few thin beds of conglomerate. The conglomerate contains pebbles of Triassic and Jurassic sandstones, and 

similar srnall isolated pebbles arc scattered through some 

parts of the lake deposits. Although the material of the 

lake beds can be crumbled easily in the hand, they are 

partly cemented and stand up well in vertical bluffs. The 

beds commonly erode to a "badlands" type of topography. 

The volcanic ash layer is of particular significance because it permits a tentative correlation with the lower 

member of the Harpole Mesa Formation of presumed early 

Pleistocene age. R. E. \Vilcox and G. A. lzett examined 

samples of this ash and reported (written commun., 1969) 

as follows: 

The ash of these samples corresponds very closely petrographically and chemically to the lower ash layer of the 

Harpole l\'Iesa Formation at a locality about 25 miles 

NW of the Paradox Valley occurrence near the head of 

Onion Creek in eastern Utah (SEY4NEY4 sec. 26, T. 24 

S., R. 24 E., Polar Mesa 15-minute quadrangle). The 

ash of each occurrence is moderately indurated. The 

refractive index of the glass shards is predominantly 1.497 

to 1.498 with only a few shards above or below this range. 

Chemical analyses using the electron microprobe give 

nearly identical results for shards of each occurrence. 

Phenocrystic minerals (crystals carrying partial or co~­

plete mantles of glass) include hornblende, clinopyroxene, 

orthopyroxene ( ? ) , magnetite, ilmenite ( ? ) , and rare 

apatite and allanite. Biotite flakes are common in the 

ash of each occurrence and are presumed also to be 

phenocrystic. These characteristics correspond closely with 

those of the airfall deposits from the catastrophic eruption of the Bishop Tuff in east-central California, and on 

this basis we regard the ash beds at each occurrence to 

be correlative. The age of the Bishop Tuff has been 

determined as about 700,000 K-Ar years by Dalrymple, 

Cox, and Doell ( 1965) . 

It is of interest further that the upper ash at Onion 

Creek, some 80 feet stratigraphically above the presumed 

Bishop ash and separated from it by an angular unconformity, has been tentatively correlated by H. A. Powers 

(in Richmond, 1962, p. 35) with the well-known Pearlette Ash Member of the central great plains regarded as 

late Kansan in age. 

The lake beds overlie the hard fanglomerates of Paradox 

Valley; they are probably disconformable on the fanglomerates, but this relation has not been definitely established. 

They have been gently folded and locally dip as much as 

go. This folding undoubtedly resulted from slo~· removal 

of the underlying salt beds of the Paradox Member of the 

Hermosa Formation by solution or flowage. In any event, 

these beds and the underlying fanglomerate indicate that 

the valleys along the collapsed crests of the salt anticlines 

had reached virtually their present form and were, in fact, 

somewhat deeper either by late Pliocene time or certainly 

by the earliest Pleistocene. 

TERRACE GRAVELS 

On the south side of West Creek below the mouth of 

Ute Creek, gravel identical to the ancient gravels of the 

Gunnison River rests on terraces cut in the fanglomerate. 

The terraces were probably cut by West Creek and are 

more than 300 feet above the present bed of the creek. It 

seems reasonable to suppose that the gravels were derived 

from the reworking of earlier gravels deposited by the 

Gunnison River before it deserted Unaweep Canyon. 

Elsewhere in the area, terrace gravels are found at various levels along the canyons of both the Dolores and San 

Miguel Rivers and on the southwest side of Disappointment 

Valley. Some gravel-covered terraces are several hundred 

feet above the present beds of the rivers and in all probability are of Pliocene age, although most are much lower 

and probably of Pleistocene age·. The gravel is not clean 

and consists of sand and well-rounded pebbles and cobbles 

derived from the more resistant rocks ·of the San Juan 

Mountains, such as volcanic and granitoid rocks and 

quartzite. Regardless of where or at what levels the terrace 

gravels occur, the pebbles are fresh and virtually 

unweathered. 

ALLUVIUM AND RELATED DEPOSITS 

Holocene deposits include windblown material, sheet 

wash, alluvium, and talus. The windblown material is the 

most widespread of these deposits. It mantles benches and 

mesa tops but occurs more sparingly on valley floors. The 

material consists of indistinctly bedded light-red silt and 

sand that has been partly reworked by water and mixed 

with sheet wash. Where rainfall is sufficient, as around 

Egnar, these deposits are adaptable to agriculture and large 

acreages are planted to beans; where not cultivated, the 

deposits ·support thick growths of sagebrush that contrast 

strongly with growths of pinon and juniper that flourish 

in rocky areas. The deposits reach a thickness of 10 feet 

on some of the mesas. 

Alluvial deposits occur most extensively on the broad 

valley floors as in Sinbad, Paradox, and Gy.psum Valleys, 

Dry Creek Basin, and Disappointment Valley. Stream beds 

in the canyQns ordinarily contain only thin mantles of 

alluvium _and many are rock floored and almost devoid of · 
50 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

stream deposits. Much of Paradox Valley northwest of the 

Dolores River is covered by thick fertile alluvial soils; these 

are farmed by using irrigation water from West Paradox 

Creek and various springs. Similar soils in other valleys 

should be fully as productive, ~ere water available. In 

general the alluvium consists largely of silt and silty sand, 

but thin gravel deposits occur along some stream channels. 

In the larger valleys the alluvium attains a thickness of 

at least 20 feet. 

Talus is abundant throughout the area, on most of the 

steeper slopes, aiong the sides of canyons and mesas, and 

on walls of Paradox, Gypsum, and Sinbad Valleys. Many 

of the blocks in this debris are 10 feet or more across. 

LANDSLIDE DEPOSITS 

Landslides have been a decided factor m shaping the 

topography of many localities within the area. The landslides generally involve only the Brushy Basin Member of 

the Morrison Formation artd overlying formations. The 

bentonitic shales of the Brushy Basin are particularly susceptible to landslides along the fractured walls of Paradox 

and Gypsum Valleys where slopes are steep and surface 

water has penetrated deeply. The water swells the bentonite 

and produces an unctuous, slimy, lubricating medium that 

permirs the movement of large masses of rock over long 

distances down rather gentle slopes. Probably the most 

impressive of these landslides is on the southwest wall of 

Paradox Valley below the Jo Dandy mine; this landslide 

· mass is more than 2 miles wide and extends down the valley 

wall for more than 1 Y2 miles. Other slides almost as large 

are found in the northwestern part of Gypsum Valley and 

along the upper slopes of the canyon of Roc Creek. The 

surfaces of the landslides are uneven and hummocky and 

strewn with large blocks from the conglomerates of·· the 

Brushy Basin Member, the Burro Canyon Formation, and 

the Dakota Sandstone. 

STRUCTURAL GEOLOGY 

REGIONAL SETTING 

The Colorado Plateau is a region distinct both as a 

physiographic province and as a geologic province, and its 

duration as a geologic province far exceeds its existence as 

a physiographic province. The plateau is generally less 

deformed than surrounding areas, and is characterized by 

mostly flat-lying sedimentary rocks. Here and there, however, the strata are upwarped, folded, cut by faults, and 

intruded by igneous rocks and salt plugs. The major structures are the great monoclinal upwarps and intervening 

basins, but topographically more conspicuous are the domal 

uplifts surrounding the stocklike centers of the laccolithic 

mountains. The steep flanks of most of the monoclines on 

the w.est side of the plateau dip east whereas those on the 

east side dip .west (fig. 1). The distribution of the laccolithic mountain masses appears to be unrelated to the distribution of the other major structural features of the 

plateau. Faults are most numerous in the northern part 

of the region, where most of them trend northwest. Volcanic plugs, calderas, and explosion vents are most common 

in northeastern Arizona and adjacent parts of New 

Mexico. The southern and western parts of the plateau are 

marked by large volcanic fields. Most striking of the structural features are the salt-cored anticlines that lie along 

the Utah-Colorado boundary near the eastern margin of 

the plateau in an area between the great east-dippingmonoclinal upwarps and the Uncompahgre uplift. 

SALT ANTICLINES 

GENERAL FEATURES 

The salt anticlines of Utah and Colorado are unique i.n 

North America both in structure and in mode of development. They comprise a sequence of northwest-trending saltcored folds that coincide with the deepest part of the 

Pensylvanian Paradox Basin southwest of the uplift underlying the Uncompahgre Plateau. In Colorado the anticlines 

and their intervening synclines are (from northeast to 

southwest) the Nucla syncline, the Sinbad Valley anticline, 

the Paradox Valley anticline, the Dry Creek basin syncline, 

and the Dolores anticline (pl. 1) . Most of these folds are 

but enlarged segments or bulges on longer structures that 

continue northwestward into Utah where other segments 

have other names-thus the Fisher Valley, Castle Valley, 

and Lisbon Valley anticlines in Utah are enlarged segments 

of the same structures from which, respectively, the Sinbad 

Valley, Paradox Valley, and Dolores anticlines also rise 

(Williams, 1964). Segmentation of some of these longer 

structures resulted partly from intrusion of the La Sal 

Mountains igneous rocks (Hunt, 1958). 

The anticlines are structurally far more complex than 

their seemingly simple forms suggest, but the broad intervening synclines present few complexities. All the anticlines 

possess cores of salt and gypsum from the Paradox Member 

oi the Hermosa Formation. They are structurally dissimilar 

to the salt domes of the gulf coast region, notwithstanding 

the fact that both owe their development largely to intrusion of salt. The salt domes of the gulf coast region are 

roughly circular or elliptical in plan and show little regularity of distribution or control by basement structures, 

whereas the salt anticlines of Colorado and Utah are 

decidedly linear and have a systematic distribution determined by basement structures. Much of the structural 

complexity of the anticlines is the result of collapse of their 

crests in Tertiary time because of removal of the underlying 

salt by flowage and solution. 

LOCALIZATION 

Structural control of the salt anticlines has long been 

suspected because of their parallelism to each other and to 

the edge of the ancestral Uncompahgre uplift, but only in 

-. 
STRUCTURE 51 

recent years has drilling and geophysical information lent 

real substance to these suspicions. Structural displacements 

of pre-Paradox rock where no corresponding displacement 

of surface rocks exists have been proved in a number of 

places. More than 5,000 feet of structural relief exists in 

presalt rocks betvveen the center of Paradox Valley about 

a mile northwest of the Dolores River and Dry Creek basin 

syncline 5 miles to the southwest. The base of the salt under 

the northeast flank of Gypsum Valley is about 2,600 feet 

lower than under the southwest flank between Gypsum Gap 

and the Dolores River, and there is about 2,500 feet of 

local relief in presalt rocks in the Lisbon Valley anticline 

nearby .in Utah. Gravity data (Joesting and Byerly, 1958) 

indicated the major dislocations in presalt rocks are parallel 

to and under the southwest flanks of the Paradox Valley 

and Gypsurn Valley anticlines. Furthermore, the gravity 

gradients arc so steep here that it seems highly probable 

that the gradients arc produced by high-angle faults rather 

than by folds. 

Drilling information and geophysical data also suggest 

· that the faulted blocks of prcsalt rocks arc tilted southwest 

as shown in figure 13. If this is true, then presalt strata are 

deepest below the salt cores. 1\t[ost of the salt anticlines are 

some\rhat asyrnrnetrical, the north flanks being the steeper 

and the synclinal troughs being displaced to the southwest 

of their geographical ccnterlincs. The southeast parts of 

all the folds southwest of and including the Paradox Valley 

anticline are bent and· offset to the northeast with respect 

to the northwest parts of the folds; these offsets or kinks 

occur along a nearly straight northeast-trending line extending from Sawtooth Ridge (sec. 16, T. 16 W., R. 46 N.) 

to near the head of Bush Canyon (sec. 25, T. 19 W., R. 

43 N.; pl. 1). lVfagnctic data and to a lesser extent gravity 

data reflect similar offsets in the basement rocks (Joesting 

and Byerly, 1958; H. R . .Joesting, E. B. Byerly, and Donald 

Plouff, written commun., 1959); these data suggest that 

some obscure structure must exist in the basement. Its 

northeast trend suggests that possibly it may be rclat~d to 

the Precambrian structural elements described by Tweto 

and Sims ( 1963), but in any event no displacement along 

this trend has occurred in the post-Paradox time. 

STRATIGRAPHIC MODIFICATIONS 

The growing salt cores profoundly affected the thickness 

and distribution of post-Paradox and pre-Morrison formations and had lesser effects on the thickness of the Morrison 

and Burro Canyon Formations in the deep part of the 

Paradox basin. The limestone member of the Hermosa 

Formation was lifted and beveled, and all later preMorrison formations either were intruded or were not 

deposited along the rising salt cores. On the flanks of the 

anticlines, successively older formations dip more steeply, 

and, in general, younger formations are deposited across 

the upturned and truncated edges of older formations 

(fig. 13). This process of upturning, erosion, and deposition of overlapping younger formations continued until 

l\tforrison time, when salt from beneath the intervening 

synclines had been largely squeezed out and exhausted. 

As the salt was forced from under the synclines, they sank 

relatively faster than the surrounding region and became 

traps for sediments. The result was that the involved formations attained a thickness ·at least 3,000 or 4,000 feet 

greater than would have been possible were salt nonplastic. 

The reason for this is that a tapering layer of salt not less 

than 4,000 feet thick beneath and adjacent to Paradox 

Valley and 3,000 feet thick beneath and adjacent to Gypsum Valley would be required merely to furnish the salt 

presently in the cores underlying these valleys if all the salt 

were squeezed from the bordering synclines. It is not possible to say how much additional salt was extruded and 

removed by solution during the many millions of years preceding burial by the Morrison Formation when salt cores 

were growing and exposed at the surface, but it was probably considerable. Thickening of formations as a result of 

salt mobility was not entirely confined to synclinal areas. 

In the northwest end of Paradox Valley, the Moenkopi 

Formation thickens tmvard the crest of the salt core; in the 

northwest part of Gypsum Valley, both the Navajo and 

l\1orrison Formations locally thicken abruptly, possibly 

because the underlying salt was subject to more rapid solution in some places while these formations were being laid 

clown. 

STRUCTURE 

ANTICLINES 

SINBAD VALLEY ANTICLINE 

The Sinbad Valley anticline is the northernmost salt 

anticline in the mapped area. It forms a broad elliptical 

bulge on the great salt structure from which the Fisher 

Valley anticline also rises. The flanks of the anticline are 

marked by hogback ridges of resistant sandstone that rim 

the precipitous walls of Sinbad Valley. The rolling floor of 

the valley, in the area of this report, is carved largely fr~m 

the soft rocks of the Paradox Member of the Hermosa Formation, although large areas are now covered with alluvium 

and valley fill where the Paradox is not exposed. Beds 

of the Paradox Member exhibit extreme folding and 

crumpling. 

The visible part of the salt-and-gypsum core of Sinbad 

Valley is a composite of several contiguous cells or cupolas 

that presumably rise from a central salt-and-gypsum mass. 

The evidence for the existence of these separate cupolas is 

not as clear in the southeast part of Sinbad Valley as elsewhere, but the west- to northwest-trending fault near the 

center of the valley in unsurveyed sec. 27, T. 49 N., R. 19 

W. is thought to mark the boundary between· two separate 

cupolas, because along the fault, limestone beds on the 

north probably belonging to the limestone member of the · 
sw 

~ 

r--- \ 

P.aradox Member of Hermosa Formation 

Paleozoic rocks 

Precambrian rocks \ 

A. Precambrian and Paleozoic rocks during deposition of the Paradox Member 

(Pennsylvanian) after early downdropping and faulting of deep part of Paradox 

basin and uplift of ancestral Uncompahgre highland 

B. Precambrian and Paleozoic rocks at end of deposition of limestone member 

(Late Pennsylvanian). Downdropping and faulting of b·asin and uplift of highland continuing 

Cutler Formation 

C. Precambrian and Paleozoic rocks near end of Cutler deposition (Permian). 

Development of salt cores well advanced 

Cutler Formation 

D. Precambrian, Paleozoic, and Mesozoic rocks at end of Moenkopi deposition 

(Middle? Triassic) 

E. Precambrian, Paleozoic, and Mesozoic rocks at end of Cretaceous 

) I r "'" 

1 

NE 

Paleozoic rocks 

Precambrian rocks 

Ancestral 

Uncompahgre 

hie:hland 

Precambrian rocks 

Precambrian rocks 

Precambrian rocks 

Precambrian rocks 

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F. Precambrian, Paleozoic, and Mesozoic rocks at end of early Tertiary folding 

G. Precambrian, Paleozoic, and Mesozoic rocks during first stages of crestal 

collapse of salt anticlines (earry Tertiary) 

H. Precambrian, Paleozoic, and Mesozoic rocks since Pleistocene time. Renewed 

faulting on southwest border of Uncompahgre Plateau during late Tertiary and 

Pleistocene 

FIGURE 13.-Growth and crestal collapse of the Gypsum Valley and Paradox Valley salt anticlines. 

Precambrian rocks 

Precambrian rocks 

Uncompahgre 

Plateau 

NE 

Precambrian rocks 

Cl 

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54 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Hermosa Formation are set against gypsum beds of the 

Paradox l\1ember on the south. About 1,500 feet south of 

the fault, the Chint'e Formation rests in depositional contact on Paradox beds, and the limestone member is missing, thereby showing that south of the fault, uplift of the 

cupola was sufficient for removal of the limestone in preChinle times. 

Although stratigraphic effects of the mobile salt in the 

cores of the anticlines generally can be seen better in some 

of the other anticlines, some of these effects are readily 

visible in the Sinbad Valley anticline. For example, on the 

flanks of the anticline the Cutler dips more steeply than 

the Moenkopi and younger formations, and the l\1oenkopi 

contains intraformational unconformities bet\veen units 

that are normally conformable elsewhere. 

The walls of the valley are cut by numerous faults, most 

of which have their downdropped sides toward the valley. 

The faults formed as a result of the collapse of the crest of 

the anticline because the underlying salt was removed by 

flowage and solution. Some of the faults show no correspondence between stratigraphic and topographic displacement and apparently date back to early Tertiary, the initial 

period of collapse of the anticline. Others have equivalent 

stratigraphic and structural displacements-that is, they 

have been almost untouched by erosion and are of fairly 

recent origin. Still others are of indeterminate age. · 

The Sinbad Valley anticline terminat~s to the southeast 

at the Roc Creek salt-and-gypsum intrusive plug in unsurveyed sec. 7, T. 48 N., R. 18 vV. This plug is nearly circular and is bounded by partly concentric faults which drop 

the rocks within the plug varying distances. The north rim 

of the plug has settled most recently, and this part is 

marked by prominent fault scarps. The intrusive salt-andgypsum core is exposed at two locations, one on the northwest rim of the plug and the other on the southeast rim. 

The plug is cut by northwest-trending faults that continue 

across its borders into Sinbad Valley on one side and Carpenter Flats on the other. From the southeast end of the 

plug, in Carpenter Flats, faults trend northeastward and 

die out near the mouth of Roc Creek. The triangular block 

of rocks that lies between these two systems has dropped 

with respect to the surrounding strata. 

Extending northeastward from Sinbad Valley is a graben, 

nearly 1 Y2 miles wide and 3Y2 miles long. It is bounded 

on the southeast throughout most of its length by a single 

fault, ·but at both ends this fault splits into two or more 

nearly parallel faults. The displacement at the southwest 

end of the fault is about 700 feet, but to the northeast it 

diminishes, and the fault dies out at a point about 1 Y4 

miles from the Dolores River. The northwest side of the 

graben is bounded by a fault zone. The displacement along 

this fault zone diminishes both to the northeast and to the 

southwest from a point of maximum displacement of about 

400 feet near the center of the zone; to the northeast the 

zone passes into a single fault that dies out about 1 mile 

from the Dolores River, and to the southwest it dies out 

on the northeast rim of Sinbad Valley. The southwest end 

of the graben is cut by a diagonal fault from which a 

number of subsidiary faults branch. 

PARADOX VALLEY ANTICLINE 

The Paradox Valley anticline is the largest single saltcore anticline in the region; the valley trough carved along 

the crest of the anticline has an average width of about 3 

miles and a length of more than 30 miles (pl. 2). The 

Castle Valley salt anticline, in eastern Utah, is probably a 

continuation of the same salt-core structure. 

The rims of Paradox and Sinbad Valleys are less than 

3 miles apart at their closest approach; the intervening area 

is occupied by a narrow rather shallow syncline that dies 

out south of the Roc Creek salt plug and merges into the 

gently dipping northeast flank of the Paradox Valley anticline. In general the rocks forming the cuestalike rims of 

Paradox Valley dip away from the valley at angles that in 

few places exceed 9 ° .. Rocks in the valley walls below the 

rims, however, commonly dip more steeply, and near contacts \Vith the Paradox Member and limestone member of 

the Hermosa and the Cutler Formations, they in places dip 

vertically or nearly so. The Paradox Member crops out 

extensively throughout Paradox Valley· northwest of the 

head of East Paradox Creek, and the beds in it are intricately folded and crumpled in all exposures. Like the core 

of ~he Sinbad Valley anticline, the core of the Paradox 

Valley anticline consists of a number of contiguous cupolas 

rather than of one simple mass. These various cupolas, 

however, arc in direct contact with one another and, 

although separated in only a few places by infolded layers 

of later beds, they are reflected clearly by the wide places 

or bulges along the length of the anticline. 

The Paradox Valley anticline comprises three structurally 

distinct units characterized by the types of structural feature developed during collapse of the crest of the anticline. 

These are ( 1) a unit at the southeast end of the anticline 

where the crest has downsagged to form a structural basin, 

(2) a large central unit occupying .most of Paradox Valley 

where the crest has collapsed by downfaulting, and ( 3) a 

unit at the northwest end of the valley that collapsed by 

both downsagging and downfaulting. Mesozoic rocks are 

preserved in the downsagged crest of the anticline at either 

end of the valley, but in the central part of the valley, they 

have been largely removed above the core of the anticline. 

Each of the three units has structural peculiarities not common to the other tvvo, and for convenience of treatment 

each unit will be described separately. 

The downsagged unit at the southeast end of the anticline overlies a distinct cell or cupola of the underlying salt 

..... 
STRUCTURE 55 

FIGURE 14.- View of Dry Creek anticline looking northwest; Paradox Valley in the distance. IPhp, Paradox Member of Hermosa Formation ; lie, Chinle Forma tion ; liw, Wingate Sandstone ; lik, Kayenta Formation; Je, Entrada Sandstone ; Js, Summerville Formation ; 

Jms, Salt Wash Member of ~101-rison Formation ; Jmb, Brushy Basin M ember of Morrison Formation; Kbc, Burro Canyon Formation. 

Photograph by Hal. G. Stephens. 

mass and forms a structural basin that is slightly offset to 

th e northeast from the general trend of the Paradox Valley 

anticline. The basin is bordered on its northwest end by 

northeast-trending faults in sees. 20 and 21, T. 46 N., R . 

16 W ., and a sharp fold in the southern part of sec. 29, 

T. 46 ., R. 16 \V. The basin has no sharply defined southcast end but simply merges into the Oat-lying beds of the 

surrounding plateau. 

The northeast and southwest sides of the basin arc bordered by anticlines between wh ich a syncline formed 

because of sagging of the crcstal part of the Paradox Valley 

anticline. The outside flanks of tht'sc lateral anticlines are 

continuous with and indistinguishable from the flanks of 

the Paradox Valley anticline as a whole. The crests of both 

the lateral anticlines arc cut by fa ults that drop the rocks 

in the clownsaggcd central basin. 

The more striking of these two lateral folds is the Dry 

Creek anticline along the southwest side of the basin (fig. 

14). This anticline continues southeastward as a distinct 

fold 7 or 8 miles beyond the edge of the mapped area 

before merging into the flat-lying rocks of the surrounding 

plateau and does, in fact, extend farther to the southeast 

than any other readily visible structure associated with the 

Paradox Valley anticline. The structural relief from the 

center of the downsagged central basin to the crest of the 

Dry Creek anticline, including displacement on the faults 

along the crest of the Dry Creek anticline, is about 2,600 

feet. The rocks in the southwest flank of the Dry Creek 

anticline clip 5°-9° sw., except that beds a few hundred 

feet from the crest are sharply upturned and dip 55° SW. 

This sharp upturning is reverse in direction to the displacement on the faults that cut the crest of the anticline and 

is not, therefore, drag related to the faults. A possible 

explanation of this anomalous flexing is presented on page 

67 and shown in figure 19. The faults that cut the crest of 

the Dry Creek anticline produce a series of wedges that 

narrow downward, and although the overall displacement 

is down to the northeast, some of the wedges in the northwest part of the fold have been squeezed upward. In the 

crest of the anticline northwest of Dry Creek, intrusive beds 
56 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

FIGURE 15.- Intraformational unconformity in the Chinle Formation (lie) on the southwest fl ank of Dry Creek anticline. liw, Wingate 

Sandstone; Qal, Quaternary alluvium; sh, shale; ss, sandstone; Is, limestone; cpc, clay-pellet conglomerate. 

belonging to the Paradox Member of the Hermosa Formation are exposed north of the crestline fault. A well drilled 

in the bottom of Dry Creek Canyon 200-300 feet south of 

the fault reached a depth of more than 5,000 feet before 

cutting Paradox beds. This fact provides convincing evidence concerning the steepness of the flank of the intrusive 

core of the Paradox Valley anticline. Thousands of feet of 

beds of the Cutler, Rico, and Hermosa Formations were 

penetrated by the drill, yet a few hundred feet away on the 

northeast side of the fault, beds of the Moenkopi and 

Chinle Formations rest directly on Paradox beds with depositional contact and the Cutler Formation is absent. Northeast and immediately southwest of the fault, the Moenkopi 

is very thin, but southward it thickens rapidly (sec. E-E', 

pl. 1) . Furthermore, the overlying Chinle Formation thickens southwest of the fault and has intraformational unconformities along which older contorted beds are planed off 

and overlain by younger uncontorted beds (fig. 15). These 

relations indicate relatively rapid local upwelling of salt 

during Chinle time. Beds overlying the Chinle appear to 

have been unaffected by intrusion of salt in this area. Two 

miles to the northwest, however, and probably over a different intrusive cupola, the Entrada Sandstone rests directly 

on the Chinle, and all intervening formations are absent. 

The Dry Creek anticline ends to the northwest by bending into the transverse fold that separates the southeastern 

and central structural units of the Paradox Valley. Only a 

part of this transverse fold is exposed, but the fold is in 

part at least an asymmetrical anticline. The west flank dips 

gently and probably extends only to shallow depths. The 

rast flank clips steeply in its upper parts and gradually flattens out as the bottom of the downsag is approached. The 
STRUCTURE 57 

north end of this fold is covered but probably is cut off by 

extensions of the northeast-striking faults exposed in the 

vall ey wall in sees. 20 and 21, T. 46 N., R. 16 W., north 

of State Highway 90·. 

The anticline along the northeast border of the southeastern clownsagged basin is a fairly simple structure, and 

most of it lies outside the mapped area. The southwest 

flank of this anticline is much steeper than the northeast 

flank which dips toward the Dolores-San Miguel syncline. 

The crest of the anticline in sees. 21 and 22, T. 46 N., R . 

16 W. is cut by a normal fault that dips southwest. Beyond 

the mapped· area to the southeast, the anticline gradually 

flattens, a nd both it a nd the downsagged basin merge into 

virtually flat-lying rocks. The central part of the downsagged basin is devoid of structural complexities and is 

unfaulted. 

The central structural unit of the Paradox Valley anticline, the largest and most complex of the three units, 

appears to have a salt core from which a number of cupolas 

rise. The unit is about 18 miles long and 2;/2 -4;/2 miles 

wide. The wider parts of the valley are thought to outline 

the bulges or cupolas on the partly intrusive core; in the 

northwest end of the valley about 2 miles northeast of the 

village of Paradox, an edge of one of these cupolas seems 

to be delineated by an arcuate layer of nearly vertically 

dipping Hermosa limestone. In the narrow part of the 

Yalley immediatel y sou theast of the Dolores River, older 

units of the post-Paradox sequence crop out across most of 

the valley, which indicates that the upper surface of the 

core was there at a lower general level than in the wider 

parts of the valley where older post-Paradox rocks overlying the cupolas had been bulged upward and outward. 

umcrous closely spaced faults on both sides of the valley 

cut the rocks into long narrow slivers trending parallel to 

the length of the valley. Most of these slivers are successively downdropped toward the valley, but a few have been 

squeezed upward. Southwest of the Dolores River, grabens 

have formed back from the rims on both sides of the valley. 

The graben on the northeast side of the valley is complex 

and is cut by numerous faults and joints (fig. 16), but the 

graben on the southwest side is longer. Both grabens probably formed as the relatively unfaulted prisms of rock now 

lying between the grabens and the rims of the Paradox 

Valley tilted slightly toward the valley and differentially 

settled into the underlying salt core, thereby pulling away 

from the rocks farther down the flanks of the anticline. 

The settling of these blocks is causing the salt to rise and 

push up hills of gypsum in Paradox Valley between these 

grabens. 

Several northeast-striking transverse faults cut the 

northwestern part of the central unit. The mineralized 

zones of the Cashin (sec. 22, T. 47 N., R. 19 W. ) and 

Sunrise (sec. 23, T. 48 N., R. 19 W. ) mines have formed 

along two of the faults; it seems unlikely that these mineralized zones could have formed under the near-surface conditions existent at the time of final collapse of the crest of 

the anticline, and therefore these faults probably predate 

final collapse, although movement was renewed on some of 

FIGURE 16.- View westward showing graben and joints on northeast flank of Paradox Valley anticline south of Saucer Basin (sees. 11, 

12, 13, and 14, T. 47 N., R. 18 W.) 
58 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

these faults at that time. Groups of slump blocks included 

between these transverse faults have settled more or .less as 

.. : units. Th~:trahsvers~-,f.aultat·the Suhtise·.:mine-on the northeast side of Paradox Valley also marks the northwest boundary of the central structural unit. 

The rocks in the southeast end of a large fault block on 

the southwest side of the central structural unit, in the 

vicinity of the Thunderbolt mine in sees. 22, 25, 26, 27, 

and 36, T. 46 N., R. 17 W., have been warped into a 

syncline. The syncline is analogous in form and position 

to the ring synclines that surround some of the Gulf Coast 

salt domes, although the origin may be different. Similar 

but sharper synclines are found in Gypsum Valley. These 

synclines appear to form only above the margins of the 

salt cores and may be caused by relatively rapid solution 

and removal of salt and gypsum along the more permeable 

shattered margins of the cores. 

Because the Paradox beds were partly intrusive. into the 

core of the anticline, stratigraphic relations vary considerably in the central structural unit. In places the limestone 

member of the Hermosa Formation .and the Cutler Formation are in trucled by the Paradox, whereas in other places 

both these and younger formations rest with depositional 

contact on it. All post-Paradox beds to the base of the 

Morrison Formation either are cut off or wedge out against 

the salt at one place or another in the central structural 

unit. Although thinning of the formations can be seen on 

the northeast wall of Paradox Valley, actual wedgeouts and 

unconformable contacts between beds not normally in contact are seen best on the southwest wall of the valley southeast of the Dolores River. For example, the Kayenta 

Formation is more than 100 feet thick below Monogram 

Mesa in sec. 17, T. 46 N., R. 17 W., but to the southeast 

near the Thunderbolt mine in sec. 23, T. 46 N., R. 17 W., 

it is absent, and there the Entrada Sandstone rests on the 

Wingate Sandstone. In this locality the \Vingate thins 

valleyward from an exposed thickness of more than 100 

feet to less than 5 feet \'vithin a quarter of a mile. Farther 

east in sec. 29, T. 46 N., R. 16 W., the Entrada rests on 

the Chinle Formation. In this general area no post-Paradox 

beds older than Chinle crop out, and presumably either 

they are cut off by the underlying intrusive salt and gypsum or their upturned and truncated edges are unconformably overlain by younger formations and lie well back 

from the foot of the valley wall. The floor of the valley 

probably is underlain-or before erosion, was underlainby rocks no older than Summerville. The thinness of the 

post-Paradox rocks on the valley floor was confirmed by a 

geophysical traverse ( W. E. Davis, oral commun., 1950). 

The northwest structural unit of the Paradox Valley 

anticline consists of downwarped and faulted rocks that 

clip valleyward; some of these rocks· form the steep face in 

sec. 20, T. 48 N., R. 19 W. at the northwestern end of the 

valley. The northeast flank of the anticline below Carpenter 

Ridge is highly faulted, and fault blocks are commonly 

downdro_pped toward the- ~~lley. Uniike rocks to the southeast, those in the northwest structural unit dip toward the 

valley rather than away from it. A striking feature in this 

structural unit are the steeply upturned rocks (dips of as 

much as 35°) in the southwest part of sec. 17, T. 48 N., 

R. 19 W., which are bounded on the northeast by a northwest-trending fault that runs up the bottom of a short 

canyon. The displacement along the fault increases to the 

northwest and away from the valley. A wedge of the Moenkopi Formation that thins away from the crest of the anticline is exposed at the lower end of the canyon through 

which the fault runs. The direction of thinning proves that 

prior to deposition of the Chinle there had been local relative downsagging of the central part of the anticline. 

South of this block, in sees. 20 and 29, T. 48 N., R. 19 W. 

the beds dip, in general, less steeply and to the east into 

the valley. These valleyward-clipping rocks are cut by a 

number of faults that strike north, diagonal to the trend 

of the Paradox Valley anticline. In this vicinity rocks at 

least as old as the Wingate Sandstone were deposited completely across the top of the salt intrusion, whereas in some 

other parts of the anticline the Wingate and some younger 

formations wedge out on the flanks of the intrusion. 

A mile south of the confluence of the Dolores and San 

Miguel Rivers is a circular downfaulted block, the center 

of which is lowered about 900 feet. The general pattern of 

the faults in the block consists of concentric arcuate faults 

a~~d radial faults emanating from about the central part of 

the block, thus the structure somewhat resembles a spider's 

web. Most of the radial faults are too small to be shown 

on the map. The formation of such a structure requires 

the local removal of a large volume of material from below; 

the structure probably occupies the roof of a salt dome that 

collapsed as salt was removed by solution or flowage. 

GYPSUM VALLEY ANTICLINE 

The Gypsum Valley anticline is about 30 miles long and 

trends northwesterly almost entirely across the mapped 

area. Only the extreme southeast end of the anticline lies 

beyond the limit of the mapped area. Both ends of the 

anticline die out into flat-lying rocks. The Gypsum Valley 

anticline, unlike the Paradox Valley and Sinbad Valley 

anticlines, is a structural feature complete in itself and not 

a segment of a much longer structure. The axial part of 

the Gypsum Valley anticline has collapsed, and the resulting valley has a fairly uniform width of about 2 miles except 

for a slight constriction south of Hamm Canyon. The core 

of salt and gypsum in the anticline appears to be of fairly 

uniform shape and generally lacks any of the bulges or 

cupolas that characterize the Paradox Valley and Sinbad 

Valley anticlines. Geophysical evidence (Joesting and 

Byerly, 1958) and recent drilling indicate that pre-Paradox 
STRUCTURE 59 

rocks are structurally high beneath the southwest flank of 

the anticline,· a lthough surface rocks show no evidence for 

such a high. East of I-Iamm Canyon the southeast part of 

the anticline is bent to the northeast. This bend is similar 

to that previously described in . the southeast end of the 

Paradox Valley anticline. The Cretaceous and Upper 

Jurassic rocks forming the hogback ridges that surround 

Gypsum Valley arc turned up more sharply than are the 

rocks occupying similar positions around Paradox and 

Sinbad Valleys. 

Many of the relations of the salt core to the overlying 

rocks are revealed better in ·the Gypsum Valley anticline 

than in any other structure in the salt anticline region of 

Colorado and tah . . \t one place or another along the 

length of Gypsum Valley almost every possible combination 

of Morrison or pre-Morrison beds resting on older beds 

may be observed because of the wedging out of intervening 

formations. Thus, in some places the Morrison rests on the 

Paradox Member of the Hermosa Formation, and in others 

on the Chinle or Cutler Formations; the Entrada Sandstone 

may rest on the Navajo Sandstone or any older formation. 

.\!though evaporites of the Paradox were not observed to 

intrude formations younger than the Cutler, the proximity 

of gypsum and Mancos Shale in the southeast end of the 

valley (sec. 3, T. 43 N., R. 16 W.) suggests that the evaporites intruded rocks as high in the stratigraphic column as 

the Mancos. It seems more likely, however, that the Mancos 

rocks are remnants of ancient landslide blocks formed during collapse of the crest of the anticlines (Cater, 1964). 

Examples of stratigraphic relations resulting from folding 

FIGURE 17.- Unconformity between the Entrada Sandstone and formations on the northeast side of Gypsum Valley, 2 miles northnortheast of Gypsum Gap. lie, Chinle Formation ; liw, Wingate Sandstone; Je, Entrada Sandstone ; Js, Summerville Formation; Jms, 

Salt Wash Member of the Morrison Formation. 
60 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

FIGURE lB.-Unconformity between the Entrada Sandstone (Je) and formations on the northeast side of Gypsum Valley, about 3 miles 

northwest of locality shown in figure 17. Js, Summerville Formation; Jms, Salt Wash Member of the Morrison Formation. 

and the movement and intrusion of salt are particularly 

well exposed in the southwest valley wall southeast of Gypsum Gap, in the northeast wall from the Pitchfork mines 

to a point about 3 miles southeast of Hamm Canyon, and 

below Anderson Mesa near the north end of the anticline. 

Southeast of Gypsum Gap the upturned and truncated 

edges of beds ranging from the Hermosa limestone to the 

Cutler are overlain unconformably by the Entrada Sandstone and the Summerville Formation; valleyward and to 

the northwest in sees. 6 and 7, T. 43 N., R. 16 W., and sec. 

I, T. 43 N., R. 17 W., the Entrada and Summerville wedge 

out by nondeposition and erosion and the Salt \Vash rests 

on Paleozoic rocks including the Paradox. On the northcast side of the valley 1 mile northwest of the Pitchfork 
•, 

STRUCTURE 61 

mines, the Salt \'\'ash rests on the Kayenta Formation in the 

valley walls and on the Paradox on the valley floor. Near 

the Bald Eagle mines the Entrada and Kayenta are missing 

and valleywarcl the Summerville rests successively on the 

Wingate Sandstone and the Chinle Formation. One to two 

rniles north\\'est of the Bald Eagle mines are ·three short 

canyons where the wedging out of various formations is 

displayed to extraordinary advantage. In these canyons, 

steeply clipping beds of the Hermosa limestone, Cutler 

Formation, Chinle Formation: and \Vingate Sandstone are 

overlain unconformably by the Entrada and Summerville 

(fig. l 7) ; toward the valley from these occurrences the 

Entrada and Summerville wedge out and the Salt Wash 

lies directly on the Paradox. Northwest of the Long Ridge 

mines ( 2 Y2 miles north of Gypsum Gap), the Entrada 

Sandstone and younger formations clip at angles of less 

than 20°, whereas older formations clip from 55 ° to almost 

vertical (fig. 18) . 

Along the northeast side of Little Gypsum Valley, unconformable relations between various formations and both 

members of the Hermosa Formation are also well exposed. 

\•Vhere the Dolores River leaves the valley, the Chinle Formation rests on the upturned beds of the limestone member 

of the Hermosa Formation. Below Anderson Mesa the 

Navajo Sandstone and the Kayenta Formation wedge out 

on the upper part of the valley wall; lower on the wall 

tO\·varcl the valley floor, the Wingate Sandstone and the 

Chinle Formation also wedge out, and the Entrada Sandstone rests on the limestone member of the Hermosa. The 

same relations also exist on the east side of Silveys Pocket 

below Buck Mesa in sees. 27 and 34, T. 46 N., R. 19 W. 

On the opposite side of the valley the stratigraphic relations, at least to the depth the rocks are exposed, are 

entirely different. No formations below the Navajo are 

exposed, but the Navajo wedges out valleyward from an 

exposed thickness of not less than 350 feet below Island 

Mesa and is absent on the northeast side of the valley. 

'These relations indicate considerable local movement and 

readjustment of the salt core during Navajo time with 

possible subsidence on one side of the core and upward 

movement on the other. Of especial interest is an abrupt 

thickening of the Salt Wash Member of the Morrison 

Formation along the northeast side of the valley, which 

is seen best in the vicinity of The Hat in sec. 34, T. 46 N., 

R. 19 W ., where the exposed part of the member thickens 

by more than 100 feet in a distance of less than one-half 

mile. The thickening of the Salt Wash presents almost certain evidence that, at least locally, movement of salt was 

occurring during Morrison time, notwithstanding the ·fact 

that nowhere has the Salt Wash been observed to wedge 

out along the salt anticlines as do all the older formations. 

The collapse structures along the crest of the Gypsum 

Valley anticline differ somewhat from those on other salt 

anticlines in the region. The walls of Gypsum Valley are 

not extensively faulted, and blocks of great thickness have 

foundered in the salt core. Only in the southeast end of 

the valley have rocks that formerly occupied the crest of 

the anticline been removed entirely; throughout much of 

the rest of the valley varying thicknesses of Mesozoic rocks 

are still preserved on the valley floor, although these have 

·been greatly deformed by graben faulting and downwarping. 

In the southeast part of the anticline the post-Paradox 

rocks are both faulted and folded. Upper Cretaceous rocks 

are downfaulted against the Chinle, Rico, and Cutler 

Formations in the southeast end of Gypsum Valley, and 

on both sides of the valley the rocks are downfaulted 

toward the valley. For several miles along both sides of this 

end of the valley the Salt Wash Member is do.wnwarped 

into sharp narrow synclines, apparently directly over the 

line where the Salt Wash crosses on to Paradox beds. 

From Gypsum Gap to the vicinity of Hamm Canyon, 

Mancos Shale crops out in a number of places, thereby 

indicating that thick blocks have sunk into the salt cores 

of the anticline. The largest of these blocks occupies the 

center of the valley south of Hamm Canyon. This block is 

capped by beds of the Mesaverde Formation, and assuming 

that it contains the Morrison, the block probably is not less 

than 3,000 feet thick. The northeast and southwest sides 

of the block are clownfaulted along the edges of the valley, 

and the ends are almost certainly in contact with the 

Paradox Member of the Hermosa Formation although the 

critical areas are covered. The conclusion that this isolated 

block has foundered in the salt core to a probable depth 

of 3,000 feet seems inescapable. Between this block and the 

Dolores River the Salt Wash Member of the Morrison crops 

out extensively on the valley floor. The Salt Wash is folded 

irregularly and is faulted along the valley sides, but on the 

valley floor it is cut only by small, local faults. 

I.n Little Gypsum Valley and Silveys ~ocket northwest 

of the Dolores River, the Gypsum Valley anticline curves 

northward. Most of the collapse of the crest of the antiCline 

has been accomplished by downfolding rather than by 

faulting, and beds from the collapsed crest are exposed 

widely on the valley floor. In places these· beds have been 

downfolded not less than 2,500 feet below the present 

valley rims, and probably 3,000 feet or considerably more 

from their precollapse positions in the crest of the anticline. 

Because the center of the Gypsum Valley anticline has 

sagged, both rims of the valley are anticlinal, and the crests 

of the rims coincide closely with the crests of these lateral 

anticlines. Thus, from the rim of the valley the beds dip 

in one direction toward the ce~ter of the valley and in the 

other direction away from the valley. The crests of the parts 

of these anticlines lateral to Little Gypsum Valley are cut 

by faults that drop the rocks toward the valley center, but 
62 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

northward where the anticlines are lateral to Silveys Pocket, 

the crestal parts are unfaulted, although the rocks, especially the more brittle sandstone beds, are highly shattered. 

The north ends of the lateral anticlines flatten out and 

merge into an extensive area of nearly horizontal beds. 

These beds are disrupted only by a northward-trending 

fault, downthrown to the west; this fault is a continuation 

of the fault system along the collaps~d, central part of the 

Gypsum Valley anticline. The lateral anticline along the 

west side of Silveys Pocket and the north end of Little 

Gypsum Valley is cut by several westward-striking faults 

transverse to the trend of the anticline. The direction of 

displacement along these differs; two of the. blocks between 

these faults are grabens and one is a horst. 

The structure of the collapsed axial part of the anticline 

on the valley floor of Little Gypsum Valley and Silveys 

Pocket is highly irregular; the rocks have been broken and 

shattered first by uplift and later by downdroppings. A system of faults traverses the central part of Little Gypsum 

Valley and Silveys Pocket. In general, the rocks northeast 

of the fault system are downwarped farthest, especially near 

the valley center where the beds dip very steeply toward 

the faults. Rocks southwest of the fault system are irregularly folded although not so sharply as those on the other 

side of the system, and a number of small structural basins 

have formed. West of the Raven mine the faults that transversely cut the rim of the valley extend across the valley 

floor and join the central fault system. 

DOLORES ANTICLINE 

The Dolores anticline is a large asymmetrical northwesttrending fold and is the southeast extension of the Lisbon 

Valley anticline in Utah (fig. 1). Drilling and geophysical 

data indicate that both anticlines have salt cores, although 

these cores are nowhere -exposed. The crestal part of the 

Lisbon Valley anticline has been downfaulted, whereas the 

crestal part of the Dolores anticline, unlike the other saltcore anticlines, has not collapsed. Rocks in the northeast 

flank of the Dolores anticline dip 5°-7° toward the Disappointment syncline; the southwest flank dips very gently, 

and over large areas the dip does not exceed 1 °. The anticline plunges gently to the northwest. The southeast part 

of the Dolores anticline is bent slightly to the northeast in 

the gen.eral vicinity of sec. 25, T. 43 N., R. 19 W., along 

the same line of kinking as that for the folds previously 

described. The upper part of the northeast flank of the 

anticline is cut by a large number of faults, but the displacement on only two or three if these faults exceeds 100 

feet. From sec. 34, T. 43 N., R. 18 W., to Summit Canyon 

in sec. 34, T. 44 N., R. 19 W., the faults trend northwest 

parallel to the trend of the anticline, but northwest of 

Summit Canyon most of the faults trend east. There is no 

consistent pattern to the direction of displacement along 

the faults-the rocks north or northeast of some of them 

are dropped, whereas along others the reverse is true. The 

faults may have formed in response either to adjustments 

of the underlying plastic salt core when the Lisbon Valley 

anticline collapsed or to tensional stresses developed during 

folding. 

The salt core of the Dolores anticline seems not to have 

pierced the overlying strata but merely to have bulged them 

upward. South of Joe Davis Hill the Dolores River Canyon 

exposes rocks as old as the Cutler Formation. Although 

several of the formations show no thinning over the Dolores 

anticline, the Moenkopi Formation is entirely absent, and 

along the crest of the anticline both the Navajo Sandstone 

and the Burro Canyon Formation are thinner than normal. 

It is not known whether the Moenkopi is absent only along 

the crest of the anticline, thereby indicating Early Triassic 

growth of the anticline, or whether this part of the anticline is east of the area of deposition of the Moenkopi. 

Although the contact between the Chinle Formation and 

the Cutler is an erosion surface, the beds are virtually 

parallel, suggesting that any possible late Paleozoic or Early 

Triassic upwarping was mild, regardless of whether or not 

there is Moenkopi on the flanks of the anticline. The 

thinning of the Navajo and Burro Canyon Formations 

probably indicates some slight warping of the anticline. 

Inasmuch as the upper surfaces of both formations are 

regional unconformities, however, it is not possible to determine whether this probable warping occurred during 

deposition or slightly thereafter. 

SYNCLINES 

The synclines separating the salt-cored anticlines are 

broad simple folds that lack the complexities of the anticlines. The structure of the Morrison and Cretaceous formations is largely the result of Tertiary deformation, 

whereas the older formations in addition show the effects 

of salt flowage. Pre-Morrison formations thicken toward 

the synclinal troughs because the underlying salt was 

squeezed out as the sediments accumulated. The synclines, 

from northeast to southwest, are the Nucla, the Dry Creek 

Basin, and the Disappointment. 

NUCLA SYNCLINE 

The Nucla syncline forms a downwarp between the 

Uncompahgre Plateau uplift on the northeast and the 

Sinbad Valley and Paradox Valley anticlines on the southwest. The syncline enters the mapped area east of Uravan 

and trends northwesterly to Mesa Creek, where it turns 

north around the plunging southeast end of the Sinbad 

Valley anticline for a few miles before turning northwesterly again and leaving the mapped area west of Gateway. 

Farther to the northwest in Utah, the Sagers Wash syncline of Dane ( 1935, p. 129) is probably part of the same 

fold. 
ORIGIN AND DEVELOPMENT OF SALT ANTICLINES 63 

The Nucla syncline locally forms a shallow trough, closed 

north of U ravan because of slight reversals of plunge directions. A subsidiary downfold trends west to Roc Creek 

from the junction of the North and South Forks of Mesa 

Creek. Most of the rocks. in. the syncline dip less than 5°, 

and within 1 mile of the synclinal axis dips do not exceed 

2 °. On the flanks of the Sin bad Valley and Paradox Valley anticlines and the monoclinal front of the Uncompahgre Plateau uplift, howev.er, dips are much steeper. 

From Salt Creek Canyon northward, the syncline contains the rather abrupt monoclinal flexure of the Moenkopi 

Foi·mation mentioned earlier; this flexure runs nearly parallel to the Dolores River and reflects a slight Middle Triassic 

uplift of the ancestral Uncompahgre highland. South of 

Cottonwood Canyon in sec; 13, T. 50 N., R. 19W., the 

angular discordance between. the Moenkopi and the Chinle 

reaches a maximum of about 7 o. Thus, although the 

regional dip of the Chinle and younger formations in this 

area is. northeast, the Moenkopi dips southwest. 

DRY CREEK BASIN SYNCLINE 

leveL In the vicinity of Nicholas Wash (sec. 35',. T •. 44 N., 

~- 18 W), the syncline is somewhat bent along the same 

line and in the same direction as are the salt anticlines. 

The syncline is asymmetrical,· but the direction of asymmetry changes from southeast to northwest. Southeast 

of the bend In the trend of the syncline, the northeast 

flank of the syncline is the steeper, where~ northwest of 

the offset, the southwest flank is slightly steeper. The maximum structural relief in the Disappointment syncline, as 

measured on the top of the Entrada Sandstone from the 

crest of the Dolores anticline to the trough of the syncline, 

is a little more than 3,600 feet, but relief measured on the 

base of the Burro Canyon Formation is only about 3,300 

feet. A diamond drill hole near the trough of the syncline 

in. sec. 36, T. 44 N., R. 18 W., penetrated 1,093 feet of 

Morrison beds, which is about 400 feet more than the 

thickness of the Morrison in th~ nearest exposures around 

the rim of Disappointment Valley (D. R. Shawe, written 

commun., 1958). Inasmuch as the Morrison is ·conformable to both underlying and. overlying beds, this very considerable degree of thickening indicates that the syncline 

was sinkihg during the deposition of the Morrison Formation, probably because the deeply buried Paradox beds 

were still being squeezed out. Southwest of Cape· Horn· 

(sec. 8, T. 44 N., R.· 18 W) three small norma.l faults cut 

the flank of the syncline. These faults trend northeasterly 

from the bottom of the syncline almost to the rim of 

Gypsum Valley. 

'I'he broad simple northwest-trending Dry Creek basin 

syncline lies between the Paradox Valley anticline on the 

northeast and the Gypsum Valley anticline on the southwest. As is true of the Paradox Valley anticline, southeast 

of a line extending from about sec. 6, T. 45 N., R. 16 W. 

to about sec. 33, T. 45 N., R. 17 W., the southeast end of 

this syncline is curved and offset to the northeast with 

respect to the northwest part of the syncline. The syncline 

plunges to the southeast; to the northwest it flattens and is ORIGIN AND DEVELOPMENT OF SALT ANTICLINES 

TECTONIC BEHAVIOR OF SALT 

scarcely perceptible northwest of the Bull Canyon area. . Salt structures form because of two independent propThe· structural relief of the syncline at the eastern edge erties of salt-its ability ·to flow under moderate pressure of the mapped area is about 2,300 feet as measured between and its density deficiency as compared to most common the trough of the syncline and the southwest rim of Paradox sediments. The second factor is essential in sustaining stress Valley; the structural relief on the southwest flank of the ·- differentials if a salt structure, once started, is to continue 

syncline is about 1,400 feet. The syncline is decidedly growing. asymmetrical, the southwest flank being much steeper than The conditions of temperature and pressure under which the northeast flank. To the southeast the trough .of the · salt will flow have been studied it:t the laboratory by several syncline is crowded against the southwest flank, but to the investigators, including VanTuyl (1930), Nettleton (1934, northwest the trough swings to a central position. Some 1943), Schmidt (1939), and Stocke and Borchert (1936). slight irregularities disturb the northeast flank of the syn- Some of these experiments showed that a s.Rear stress .as cline near Dry Creek; these occur along the line where the low as 30 kg/ cm2 was sufficient to start plastic deformation southeast part of the syncline is bent to the northeast. 

DISAPPOINTMENT SYNCLINE 

The Disappointment syncline is a broad simple southeast-plunging downwarp that lies between the Gypsum 

Valley and Dolores anticlines. The northwest part of the 

syncline is shallow, and the strata generally dip less than 

5°; in contrast, the southeast part is-' deep, and the strata 

in the flanks attain maximum dips of 20°. This part of the 

syncline contains · the structurally lowest rocks in the 

mapped area. Southeast of Dugout Wash (sees. 5 and 9, 

T. 43 N., R. 17 W.), the axial part of the syncline is nearly 

of halite. However, as Balk (1949, p. 1820) pointed out: 

"As every compressive stress can be imagined to generate 

two equivalent shear stresses, a mass of salt should be under 

an excess compression of at least 60 kg/ cm2 in one direction, in order that slip along favorably oriented glide planes 

of its crystals may commence." The figures quoted as the 

shear stre.ss under which plastic deformation may start are 

subject to large changes depending on various factors. 

There is general agreement among investigators that high 

temp~rature greatly decreases the strength of salt, that 

enclosed water increases the strength unless it is sufficient 
64 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

in amount to cause incipient solution, and that unstrained 

halite crystals are weaker than strained crystals. On the 

other hand, as temperatures ri~e, salt recrystallizes rapidly 

and the increased resistance to stress effected by strain 

hardening is lost. Evidence of recrystallization is strikingly 

shown by the fact that aggregates of . salt crystals seen in 

thin sections taken from salt domes are unstrained even 

though tremendously distorted. 

There is much geologic evidence, however, such as the 

rock se:tlt glaciers of the Near East (Harrison, 1931 ) and 

some of the collapse features of the salt anticlines described 

in this report, that indicates the strength of salt to be much 

lower._than the. values cited above. These geologic features 

formed under conditions where· salt was not confined and 

where water was sufficic.::ntly abundant to permit solution, 

conditions far different. from those existing in salt beds at 

the time most salt structures start forming. 

Another factor that may influence the flowage of salt is 

the presence of beds of other· materials; in the Paradox 

Member the most abundant of these are beds . of gypsum. 

The highly contorted nature of these beds, however, sug-· 

gests that they neither _lent any appreciable strength to the 

member nor impeded significantly the flowage of salt. In 

all. probability the gypsum was merely carried along passively as a deadhead passenger as were the lesser quantities 

of anhydrite, shale, sandstone, and limestone. 

Parker and McDowell (1955,_ p. 2391-2392), on the 

basis of Balk's remarks ( 1949), stated that a pressure differential equal to 1,000 feet of beds is necessary to start 

flowage of salt; Stille ( 1925) believed that a salt bed must 

be buried to a depth of 2,000 meters before it can begin to 

flow. Nettleton ( 1943) stated that experimental evidence . 

and calculations indicated that the ability of rocks to withstand differential stresses decreases greatly as confining 

pressures rise. The pressure at the base of a layer of rock 

2,000 meters thick is much greater than that .necessary to 

cause salt to flow if free to do so, but it seems reasonable 

to expect that this excess may be necessary to cause flowage 

where pressure differentials in a mass of confined salt are 

small. Were this not true it would be difficult indeed to see 

how many of the world's salt structures· could have started 

forming, for pressure differentials equal to 1000 feet of 

beds (roughly 60 kg/cm2 ) could not have existed during 

early stages of their development. 

In addition: to experimental and geologiC evidence. concerning conditions .under which salt will flow, some evidence has accumulated concerning the effects that depth 

of burial and the thickness of a salt layer may have on the 

type of salt structure that may eventually form. Parker and 

McDowell ( 1955, p. 2425-2426), for example, believed 

as a result of their experimental work that not- only may 

the thickness of a salt layer be critical in the dev~lopment 

of salt domes, but that it may have a direct bearing on the 

size of domes; Stille ( 1925) believed as a result of his study 

of the salt structures in Germ.any that the thickness of overlying beds controlled, in large measure, the type of salt plug 

that would form. Under thin cover, salt beds behave like 

any normal layer of sediment and' do not flow; under very 

thick cover the tendency is for the gulf coast type of salt 

dome or salt stock to form; under intermediate thicknesses 

. of cover, salt anticlines similar in general characteristics to 

those of the Colorado Plateau form, although, as explained 

below, those of the plateau formed· in response to deepseated tectonic disturbances rather than mere ·loading. 

BEGINNING AND GROWTH OF SALT ANTICLINES 

Most of the world's salt anticlines appear to have formed 

only after deep burial of salt beds by later sediments, and 

until 1958 there was little reason to believe that those of 

the Colorado Plateau formed differently, although it was 

recognized that probably they were controlled by·· deepseated structures. In 1958, however, Elston and Landis 

( 1960) demonstrated, as a result of detailed mapping in 

East Paradox Valley, that the underlying salt core started 

forming no later than the beginning of· Rico and Cutler 

time and hence before deep burial. They found that locally 

Rico and Cutler beds rest unconformably on the Paradox 

Member and limestone member of the Hermosa Formation. 

Some evidence also suggested that the Paradox and the 

limestone member may be unconf~rmable, but the evidence 

is not conclusive. 

From theoretical considerations discussed above, it seems 

improbable that the conditions required for salt flowage 

could have existed before or during early Rico and Cutler 

time; the approximately 2,000 feet of post-Paradox beds 

would not have been thick enough to develop the pressure 

differentials necessary for salt flowage. Therefore, deforma;. 

tion of the Hermosa Formation prior to Rico time probably .resulted not because beds of the Paradox Member 

were intruded above their normal stratigraphic positions, 

but because of tectonic activity. The beveled folds in the 

Hermosa beneath the Rico and Cutler beds were probably 

near-surface manifestations of faults in pre-Paradox rocks. 

This conclusion finds some additional confirmation from 

geophysical data and from -Scattered drill holes which 

indicate that displacements of pre-Paradox rocks eXist 

where no corresponding displacements of post-Paradox 

rocks occur. 

. With the destruction of the confining seal of Hermosa 

limestone by erosion along lines of tectonic dislocations, the 

salt was free to escape when:. Rico and Cutler sediments 

began flooding the region. Consequently, the salt started 

rising along the lines of dislocations to form salt cores. as 

it was squeezed from beneath the intervening covered areas. 

Contrariwise, these intervening areas, in the meantime, 

were sinking and trapping most of the Cutler sediments 
ORIGIN AND DEVELOPMENT OF SALT ANTICLINES 65 

precisely because- the underlying salt was escaping into the 

growing cores. For a time the deposition was so rapid that 

parts of the gt:owing salt cores were buried, but the cores 

continued to grow, and they intruded the covering of 

Cutler beds in Cutler time. Only the salt in the core of the 

Dolores anticline, the most southwesterly of the salt anticlines, seems not to have intruded above its normal 

stratigraphic position but merely formed a roll of thickened 

salt. From the end of Cutler time until the beginning of 

Morrison time late in the Jurassic, the elongate salt intrusive bodies probably stood as topographic highs at one 

place or another along their lengths. The upwelling of salt 

balanced or slightly exceeded the accumulation of sediments in the surrounding areas. In a sense, the upper surface of the salt plugs maintained a nearly constant altitude 

while gradual settling of the surrounding country was intermittently balan'Ced by sedimentation. Salt flowage did not 

proceed everywhere at a uniform rate. Local surges and 

comparatively rapid intrusion in many places were countered by relative quiescence at other places. Surges of rapid 

intrusion may have helped to form the cupolas along the 

salt cores. 

The cupolas commonly acted somewhat as structurally 

independent ut:lits so that of two adjoining cupolas, one 

might be quiescent while the other was actively upwelling. 

The rapidity of some of these surges can be gaged from the 

fact that along the northeast side of Sinbad Valley, uplift 

was so rapid that unconsolidated beds of the Moenkopi 

Forma'tion slumped and slid down the slope developed by 

the upthrust of the salt. 

Extrusion of salt continued in most places until salt in 

the areas adjacent to the salt intrusions was _exhausted. If 

it is assumed that the cores of Paradox Valley. and Gypsum 

Valley anticlines contain salt derived from the adjacent 

synclines and that virtually all the salt originally in the 

synclines had been squeezed out, then it would require a 

tabular mass tapering from a thickness of about 4,000 feet 

beneath and adjacent to Paradox- Valley to a thickness of 

about 3,000 feet beneath and adjacent to Gypsum Valley · 

merely to supply the salt presently in the two cores. With 

the exception, possibly, of the northeast flank of .the Sinbad 

Valley salt core and of part of the southwest flank of the 

Gypsum Valley salt core, it is very unlikely that the salt 

beds were pinched off until virtually all the salt in the 

areas between cores had flowed out, for had this occurred, 

some evidence-such as downfolds or abrupt thickening of 

beds marginal to the cores such as exists abng the northeast 

flank of the Sinbad Valley anticline-should have been 

preserved. Salt may have been· trapped in the deep part of 

the Disappointment syncline as post-Paradox beds came to 

rest against the structurally high pre-Paradox rocks that, 

according to drilling and geophysical data, probably existed 

beneath the southwest flank of the Gypsum Valley anticline. 

Escape of this salt in Tertiary time may account for the 

unusual depth of the Disappointment syncline. Low gyps~m hills in the southeast end of Gypsum Valley indicative 

of rising salt suggest that this process may still be active. 

Elsewhere, salt was exhausted in the interantidinal areas 

in Late Jurassic time, and ·active· intrusion ceased except 

for minor readjustments of salt within the cores. 

LATER HISTORY AND COLLAPSE OF SALT ANTICLINES 

Following deposition of the Mesaverde Formation, anticlines formed along the old salt structures. Although the 

salt cores undoubtedly exercised some influence on the 

shape of the folds, they formed not in response to movement of salt but to deep-seated deformation that apparently 

was controlled in large measure by the Paleozoic basement 

structures above which the salt rolls and plugs originally 

had developed. Probabiy the broad anticline from the crest 

9f which the Uncompahgre Plateau uplift was to rise in 

late Pliocene and Pleistocene time formed at the same time. 

If this is true, then folding probably occurred near the end 

of Cretaceous time, as indicated by the work of R. G. 

Dickinson (U.S. Geological Survey, 1965, p. A88; Dickinson, oral commun., 1965). He found that thousands of feet 

of Cretaceous sediments had been erod~d from the southeast extension of this anticline iii the northern part of the 

San Juan Mountains before an unnamed Upper Cretaceous 

unit of volcanic rocks on Cimarron Ridge· southeast of 

Montrose, Colo., was laid down. Furthermore, northeast 

of the report area, J. R. Donnell (oral commun., 1966) 

found that the great homocline that separates _the Piceance 

basin from the structurally much higher region to the southwest started forming no later than earliest Tertiary time. 

This homocline and. the northeast flank of the U ncompahgre anticline are one and the same. 

Collapse of the crests of the salt anticlines occurred in 

two stages apparently widely separated in time. The first 

stage followed perhaps rather closely the Late Cretaceous 

folding. The second stage followed epeirogenic uplift of 

the entire Colorado Plateau in the middle and late Ter~ 

tiary, and this stage is still continuing. 

During the earlier stage of collapse, the crests of the 

anticlines in places were dropped, as grabens, as much as 

several hundred feet. These grabens may have formed dur-· 

ing relaxation of stresses that caused folding. Although the 

structural details of these early collapse features are rather 

thoroughly masked by the later collapse, it ·seems probable 

that the structures resulting from the early collapse were 

fairly simple. It appears that at no time during this initial 

stage of collapse were ·the salt-gypsum cores of the anticlines exposed; in fact, by the end of Mesaverde time, the 

upper surfaces of the salt-gypsum cores ·were buried under 

nearly 5,000 feet of sediments. With the end of graben 

. faulting the salt anticlines probably were structurally 

inactive for a long time. 
66 · GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

The second period of collapse began after uplift of the 

Colorado Plateau. This uplift rejuvenated the streams and 

increased ground-water circulation. Deep canyons eventually breached the crests of the. anticlines and exposed. the 

underlying salt to rapid ·solution and removal. With the 

abstraction of salt;·· renewed collapse of the anticlinal crests 

began. Collapse of the crests of· the Gypsum Valley and 

Paradox Valley anticlines probably started at the points 

where the Dolores River, which antedates both valleys, cut 

across the crests. Sinbad Valley. is not transected. by a 

throughgoing stream, arid it seems likely that the crest of 

the underlying anticline was not breached, and did not 

collapse as soon as those in the Gypsum and Paradox Val_. 

leys. Collapse of the antiClinal crests probably progressed 

in both directions away from the points where the salt 

cores of the anticlines were first exposed. Streams working 

headward along the crests removed both the material in 

the cores and the overlying beds. 

In Gypsum Valley a number of sinkholes underlain by 

gypsum indicate that much of the collapse was due directly· 

to removal of salt by solution; nevertheless, it seems unlikely 

that all the collapse can be attributed to this process, as 

was believed by earlier workers in the area. Rather, much 

of the collapse apparently was caused by flowage of salt 

from parts of the anticlines still overlain by thick layers of 

sediments to parts from which the overlying sediments had 

been removed. Such flowage probably became operative as 

soon as erosion had breached the anticlines and thereby 

removed the confining seal of sediments that restricted the 

free movement of salt. The ~asinlike downwarp at the 

southeast end of the Paradox Valley anticline appears to 

be almost if not entirely due to this process of salt removal 

by flowage. Elsewhere much of the faulting along the flanks 

of the anticlines appears to be du~ to this same process. 

The relatively plastic salt offered little support for the beds 

in ; the flanks· of . the anticlines; cons7quently these largely 

unsupported beds ·slumped, probably along fractures and 

joints formed during earlier flexuring. The probability that 

much collG:.pse is the result of removal of salt by flowage 

rather than by solution is supported by data obtained from 

various boreholes in Paradox Valley and similar valleys in 

the region. Rock salt commonly is found in these boreholes 

at depths of 1,000 feet or less, indicating that solution and 

removal of salt by circulating ground waters has been ineffective below this depth. Rocks in areas of collapse, however, have sunk to far greater depths; for example, in Big 

Gypsum Valley, south of the mouth of Hamm Canyon, the 

block of rocks capped by the Mesaverde Formation foundered in the salt core to a depth of probably 3,000 feet, . 

and beds not less than 1,500 feet thick at the southeast end .. 

of Paradox Valley anticline have downsagged in the salt 

core so that the surface at the center of this downwarp is 

about 200 feet lower than outcroppings of the Paradox 

Member ~orthwest of the downsag. The differences in 

elevation may be due to partial isostatic balance between 

the salt-and-gypsum core and heavier downsagged rocks 

suspended iri the core. 

SW NE 

A 

sw NE 

B 

sw NE 

c 

FIGURE 19.-Successive stages in the development of the downsag 

and the faulted marginal anticline at the southeast end of Paradox 

Valley. 

The type of collapse dominant in any part of a salt 

anticline seems to be govern~d in part by the radius of 

arching of the anticline. This relation between degree of 

arching and type of collapse structure probably is shown 

best in Paradox Valley; downsag occurred where originally 

the anticline was gently arched and faulting without downsagging where the anticline was most strongly arched. Thus, 

the highly faulted central structural unit of the. Paradox 

Valley anticline was strongly arched, as deduced from the 

dip df the beds in the downdropped blocks and in the flanks 

of the anticline, prior to collapse, whereas the downsagged 

unit at the southeast end of the anticline was only gently 

arched. When support was removed from beneath the 

arched rocks-that is, when the incompetent salt was no 

longer confined and hence was free to move under differential pressures-the upwarped rocks settled under their 

own weight and had a tendency to push a11ide the rocks in 

the flanks of the anticline. Where the arching was pro-
OTHER STRUCTURAL FEATURES 67 

nounced, this tendency was small, and the arch failed by warped into a monocline above even a vertical normal 

fracture and downfaulting rather than by gradually flatten- fault probably would not be draped into perfect parallelism 

ing and thrusting apart the buttressing· reeks in "the flanks with the fault plane and therefore would not have vertical 

of the anticline. ·Where the arching was gentle, however, dips, and beds draped over normal faults of less than verthe arch tended to faii because the buttresses were pushed tical dips would have correspondingly lower dips. The 

aside. These relationships suggest a possible mode of origin . normal faults in the folded strata probably result from local 

for the peculiar sharp upturning of beds along the fault 1n tension inherent in the development of any fold. 

the crest of the Dry Creek anticline (fig. 19). The Uncompahgre uplift is one of the you.ngf:S'~ large 

As the beds in the crest of the Paradox Valley anti- structural features in the region (Cater, 1966), bt• it had 

cline where crossed by Dry Creek subsided and flattened, a predecessor in. the ancestral Uncompahgre uplirc, a br 

the underlying salt was squeezed out to the northwest, larger structure that dominated the late Paleozoic anct 

and the buttressing rocks in the flanks of the anticline were early Mesozoic landscape. The faulted monocline fronting 

forced apart and flexed sharply upward (stage B, fig. 19). the southwest edge of the modern uplift parallels and is 

Later, as sagging continued and more salt was forced from only a short distance northeast of the edge of the ancestral 

beneath the Paradox Valley anticline, dips in the crestal uplift, but the ancestral uplift extended much farther east. 

rocks reversed, and the block subsided along marginal A large fault probably bordered the southwest edge of the 

faults. The stage was eventually reached where the sharply old uplift, for the front; as indicated by the abrupt wedge 

upturned beds southwest of the marginal fault attained out of the thick Cutler Formation, ~as remarkably steep. 

the appearance of drag folds in reverse. The 7,800 feet of Cutler beds drilled northwest of Gateway 

OTHER STRUCTURAL FEATURES (see p. 11) rest on granite rather than on older Paleozoic 

UNCOMPAHGRE UPLIFT strata. This fact indicates that the block penetrated by the 

The Uncompahgre uplift is a great nor~hwest-trending drill was upthrown, not downthrpwn. Otherwise the older 

asymmetric upwarp about 30 miles wide and 100 miles long Paleozoic strata could not have been stripped off, for the 

that rises from the crestal part of an older even larger downthrown or basin block was receiving sediments rather· 

broad, open anticline (Cater, 1966) To the southeast its than undergoing erosion. Therefore, barring the possibility 

identity as a structural unit is largely lost in the. complex of extensive faulting between the site of the drill hole and 

volcanic structures of the San Juan Mountains, and to the where the Cutler wedges out against granite 4 miles to 

northwest in Utah it plunges and disappears into the Uinta the northeast, the indications are that the old uplift still 

Basin. The axis of the uplift and the summit of the Uncom- ·towered more than 8,000 feet above the adjoining Paradox 

pahgre Plateau coincide either. along or near the plateau's basin and perhaps a good deal more by the time the onlapsouthwest rim. The southwestern flank of the uplift and ping sediments encroaching on the lower slopes of the 

part of the northeast flank in the )vicinity of Grand Junction .. up thrown block reached the site of the drill hole. The 

(Lohman, 1965) are sharp locally faulted monoclines, but drilled block, perhaps, may have been a sliver elevated 

elsewhere the ~ortheast flank dips gently and uniformly. only high enough for the Paleozoic beds to have been 

The monocline marking the southwest flank enters the erodeq, and the main block may have continued ·to rise 

mapped area in the upper reaches of Atkinson Creek and only a little faster. than erosion was cutting it down so 

leaves the northern edge of the area 4 or 5 miles northeast· that the uplift was never greatly higher than the surroundof Gateway. Where the monocline crosses the North Fork irig country. This possibility, however, seems unlikely, for 

of Mesa Creek, it swings north for about 4 miles before at .le~st during later Cutler time, no movement occurred 

resuming its general northwesterly trend. The total struc- · along the dislocation wherever it may have been. The 

tural relief of the uplift in the mapped area as measured visible Cutler beds are more than ·1 ,000 feet thick and are 

on the top of the Entrada Sandstone from the trough of neither cut by old faults nor appreciably warped between 

the Nucla syncline is more than 4,000 feet. the Cutler pinchout and the salt anticlines'to the southwest. 

The southwest-flanking monocline of the Uncompahgre Furthermore, the rate of thickening of the Cutler overlying 

uplift is cut locally by faults throughout much of its lentgh, visible Precambrian rocks, if projected from the edge of. 

and probably it passes 'into a fault at depth everywhere. the formation to the drillhole near Gateway, indicates· the 

The faults are most conspicuous, however, in the northern thickness of beds cut in that drillhole is not excessive. 

part of the area where they bound the Ute Creek graben The location of the edge of the ancient highland is not 

which downdrops a part of the monocline. Most of the known precisely but can be placed within a. zone having a 

visible faults are normal, but where the strata in the mono- width not greater than 5 miles. The front of the a:ncient 

cline are vertical or nearly so, f3:ults underlying the vertical highland obviously lay southwest of the present front bebeds must have a reverse throw, for it is difficult to see cause the Cutler Formation here wedges out abruptly 

otherwise how the overlying beds could be vertical. Beds against Precambrian rocks whic~ were · exposed at . the 
68 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

beginning of Cutler time and must have been within the 

borders of the ancient highland. The southwest front of 

the old highland. probably was northeast of wh~re the 

Salt Creek graben dies out in unsurveyed sec. 26, "f'. 50 N., 

R. 19 W ., l:>ecause the graben presumably is a shallow 

structure bottoming in the salt and gypsum of the Paradox 

Member of the Hermosa Formation, which was deposited 

in the Paradox Basin southwest of the old highland~ At 

this same locality on the Salt Creek graben, a monoclinal 

flexure in the Moenkopi but not in :the Chinle indicates 

a slight Middle Triassic uplift, further indicating the location of the southwest border of the old highland. Small 

faults in sees. 11 and 14~ T. 50 N., R. 18 W., on Calamity 

Creek, and in sec. 30, T. 50 N., R. 17 W., on Blue Creek, 

may possibly represent upward extensions of ancient faults 

bordering the old highland, developed in response to some 

slight Tertiary movement. 

UTE CREEK. GRABEN 

In the northern part of the mapped area, the southwest 

flank of the Uncompahgre uplift is cut by two roughly. 

parallel northwest-trending faults. The block between these 

faults has dropped to form a graben 1-2 Y:z miles wide that 

enters the mapped area near U naweep Canyon and extends southeastward to the vicinity of Cow Creek, where 

the northeastern fault turns southward and dies out and 

the graben ends. The southwest fault, however, continues 

southeastward and leaves the mapped· area near Pine Hill 

in sec. 10, T. 50 N., R. 17 W. Displacement on the southwest fault increases from about 100 feet at the north edge 

of the mapped area to about 850 feet near Pine Mountain; 

it then decreases southeastward to about 300 feet at the 

east edge of the mapped area. Displacement on the northeast fault is about 600 feet at the north edge of the 

mapped area, increases ·to about 1,300 feet near Ute 

Creek in sec. 33, T. 15 S., R. 10 W., and then decreases 

southeastward and dies out near Cow Creek. 

Structure within the graben is not uniform. Sedimentary 

rocks in the northwest part of the graben, north of the 

east-trending fault east of Pine Mountain, are gently 

warped and tilted southward; whereas in the southeast 

part of the graben south of the east-trending fault, the 

rocks are cut by many faults, most of which trend northwest at a small angle to the main southwest graben-border 

fault, from which they may branch. The relatively complex 

structure of this part of the graben probably is related to 

the abrupt change in trend of the Uncompahgre front at 

this general latitude. The faulted monoclinal front trends 

almost due north to this latitude and then in the area 

between Indian and Ute Creek it turns northwest. 

The graben is youthful and, in fact, may still be dropping. The stratigraphic displacement nearly equals topographic displacement, and the graben is little eroded. 

Consequently, it is believed to have formed during late 

stages of uplift of the Uncompahgre Plateau, which has 

beeri elevated perhaps 1,800 feet .since the Gunnison River 

abandoned Unaweep Canyon; most of"this uplift was concentrated along the faulted monocline. 

. STRUCTURAL HISTORY 

The earliest record of the geolog-ic stnu~tural development of the area is preserved ·in th~ Precambrian rocks of 

the Uncompahgre Plateau where highly folded and contorted metamorphic rocks have been repeatedly subjected 

to igneous intrusions. The visible Precambrian structures, 

however, seem to have exerted little control on later structures; furthermore, the extent of the Precambrian rocks 

in the mapped area is too small to permit unraveling of 

any large structural patterns. 

The record of structural events causing significant 

· deformation of sedimentary rocks in the area dates from 

Middle Pennsylvanian time when the region was gently 

warped and faulted. These dislocations gave rise to the 

Paradox Basin in which the Paradox Member of the 

Hermosa Formation was deposited; to the ancestral Uncompahgre highland, an element of the ancestral Rocky 

Mountains; and to the structural features ultimately responsible for location of the salt anticlines. Uplift of the highland inundated the Paradox Basin under a flood of Cutler 

arkosic debris. Uplift was probably greatest in the Gateway 

area and was sufficient to expose Precambrian rocks to 

erosion earlier in this area than elsewhere along the front 

of the old highland. This conclusion rests· on the great 

known thickness of the Cutler Formation in the Gateway 

area, the more domelike shape of the Sinbad Valley salt 

core which suggests origin at greater depth, and the fact 

that only in Sinbad Valley has arkosic debris been found 

in beds as old as the Paradox Member of the Hermosa 

Formation. After the highland attained its maximum height 

and while the Cutler was being deposited, the highland 

began sinking-at least along its southwest flank. This part· 

of the highland and the adjacent part of the Paradox· 

Basin subsided as a unit, and by the beginning of Moenkopi 

time the lowermost Cutler beds resting on the flanks of the 

highland were not less than 8,000 feet below sea level. 

Possibly, during this subsidence, the structural crest of the 

old highland migrated eastward. 

Concurrent with rapid rising of the ancestral Uncompahrge highland in early Cutler time, the basement faults 

that controlled the salt anticlines were rejuvenated, thereby 

setting the stage for the further dev~lopment of the anticlines as shown in figure 13. 

The tectonic activity that marked the end of the Paleozoic was followed by slow subsidence through most of the 

Mesozoic Era, although it was interrupted at the ends of 

Moenkopi, Glen Canyon, and Burro Canyon times by brief 

regional uplifts. Uplift at the end of Moenkopi time was 
GEOMORPHOLOGY 69 

accompanied by some gentle warping along the front of the To the west small streams probably headed in the Abajo 

old U ncbmpahgre highland; but asi~e from this, .warping· · ,·and :La . Sal Mountains (fig. 1). It seems likely that over 

seems to have been restricted entirely to that caused by most of the area rocks n·o older th~~· the Mancos Shale were 

slow movement of salt from the synclines into the salt exposed, arid it is even possible that much of the area was 

cores. underlain by lower Tertiary deposits which have been long 

The second major deformation of the sedimentary rocks since completely stripped away. The anticlinal ridges that 

of the area probably occurred near the end of Cretaceous had formed along the old intrusive salt cores in Late 

time. A series of broad folds, guided and localized by the Cretaceous· time had been planed off, thereby exposing 

preexisting salt-cored anticlines, were formed. Although rocks in their crests that may have been as old as the Morsalt flowage was renewed, it consisted-with the notable rison Formation but more likely were no older than Cretaceous. Early graben faulting of the anticlinal crests had exception, perhaps, of addition of salt from Disappoint- occurred, but the anticlines had not yet collapsed. The 

ment syncline into the Gypsum Valley salt anticline (see Uncompahgre Plateau had not started to rise, although 

p. 65) --largely of readjustments of salt aJready in the there is not much doubt that the Gunnison River was 

cores. 

Epeirogenic uplift of the Colorado Plateau probably 

began in the l\1iocene (Hunt, 1956, p. 77). With uplift, 

the streams started downcutting rapidly and eventually 

breached the cores of the salt anticline, whereupon final 

collapse of anticlines began partly through removal of the 

by solution and partly l;>y flowage of salt from beneath 

areas still overlain by heavy burdens of strata. The last 

episode in the structural history of the area, the rise of the 

Uncompahgre uplift and downdropping of the Ute Creek 

graben, probably began in Pliocene time and may still be 

continuing. 

GEOMORPHOLOGY 

The geomorphic history of the area as disclosed by its 

topographic forms is largely a record of late Tertiary events, 

although, insofar as these forms are controlled by the 

structure of the underlying rocks, the evolution of the present topography depends upon the geologic history of the 

area. It is rather generally conceded that epeirogenic uplift 

of the Colorado Plateau began during the Miocene (Hunt, 

1956, p. 7 7), and, inasmuch as the ~volution of the present 

topography dates largely from the begi"nning of this uplift, 

it seems justifiable to begin a discussion of the geomorphic 

history with a description of probable conditions in the 

Miocene immediately preceding uplift. 

MIOCENE SURF ACE 

Prior to epeirogenic uplift of the Colorado Plateau, longconitnued erosion had reduced most of the area to a surface 

of low relief; uplands had been planed off and basins 

filled, both in the area of this report and elsewhere. Probably rising above this surface, however, were the domed 

La Sal, Abajo, and other laccolithic mountain masses. 

Across the area meandered slow-moving streams, including 

the ancestral Dolores River, and probably ancestral La Sal 

Creek, Coyote Wash, and Dry Creek. The Dolores River 

and probably other streams whose courses are not now 

preserved headed to the east in the San Juan Mountains 

where large thicknesses of volcanic rocks were piling up. 

flowing in the position now occupied by U naweep Canyon. 

EFFECTS OF UPLIFT 

The onset of epeirogenic uplift signaled a radical change 

in the regimen of the streams draining the area. The 

streams began downcutting, and the rate of erosion in 

general was greatly accelerated. As uplift progressed, some 

streams adjusted their courses to the structure of the underlying rocks, whereas others maintained their ancient channels. In the meantime, softer formations were being stripped 

back rapidly, and broad benches were carved on the upper 

surfaces of the more resistant beds in a manner similar: to 

that occurring at the present time, although, of course, at 

much higher stratigraphic levels. Eventually all lower Tertiary beds, which presumably covered all the area except, 

perhaps, for the salt anticlines, were completely removed. 

DEVELOPMENT OF DRAINAGE 

The record of the development of the present drainage 

system is not complete; nevertheless enough of the record 

still remains in the form of drainage pattern, stream deposits, and terraces so that the broad outline of ·the development can be traced with reasonable assurance. Some of the 

streams:; including the master stream of the ·area-the 

Dolores River-are superposed throughout most of their 

courses and show little or no relation to the structure of the 

underlying rocks, whereas other streams, such as the San 

Miguel River and East Paradox Creek, show varying 

degrees of adjustment to the structure. One of the most 

remarkable canyons in the area, Unaweep Canyon, may be 

antecedent rather than superposed. 

The Dolores River traverses the area from south to north 

in a deep canyon that is uninterrupted only where the river 

crosses Gypsum and Paradox Valleys. Throughout most of 

its length within and northwest of the mapped area, the 

canyon is markedly sinuous, and with little doubt the. sinuousities are entrenched meanders inherited from the 

stream that flowed on the flat Miocene surface. Hunt ( 1956, 

p. 82) thought that doming of the La Sal Mountains may 

have shifted the course of the Dolores River eastward dur-
70 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN 'COLORADO 

ing the Miocene before epeirogenic uplift of the plateau are superposed. Most prominent . of these are the San 

began. Sufficient time elapsed, however, if such a shift.was Miguel and Disappointment Creek (except its last few 

made, for the river to establish or reestablish a meandering miles) which flow down synclines and East and West Paracourse. That part of the Dolores R{ver from north of the dox Creeks and Gypsum and Little Gypsum Creeks which 

mouth of Blue Creek to the Utah State line is rather occupy the collapsed crests of anticlines. The courses of 

straight, but from the State line to its confluence· with the the San Miguel River and Disappointment Creek probably 

Colorado River, th.e canyon again consists of a series of resulted from the gradual slip-off of the streams downdip 

entrenched meanders, although these are less pronounced on more resistant layers. For many miles east of the mapped 

than those south of the mouth of Blue Creek. Fr?m the area, Disappointment Creek flows along or near the southmouth of Blue Creek northwestward to the Colorado River, west side of Disappointment Valley, about 1 ~ miles 

the Dolores River follows the trough of the Nucla and southwest of and 400 feet above the structural bottom of 

Sagers Wash synclines rather closely, suggesting a certain Disappointment syncline. This side of Disappointment 

degree of adjustment to the structure. On the other hand, Valley is practically a dip slope on the Dakota Sandstone 

the entrenched meanders in the lower part of the Dolores ... and the Burro Canyon Formation. Further evidence that 

River's course suggest that such adjustment to the structure: .~~Disappointment Creek has migrated downdip on the south- ... 

occurred before epeirogenic uplift began. Thus a sufficient west flank of the syncline is found in the numerous remlength of time had elapsed after adjustment of the stream nants of sheets of stream gravel on the dip slope southwest 

to the syncline for the development of a wide valley across of the creek and the absence of such gravels northeast of 

which the stream was beginning to meander. With the start tthe creek. The rerriants of gravel extend more than 

of uplift, the river began downcutting so · rapidly that 1 ~ miles from the present channel of Disappointment 

throughout most of..~ts course it had no chance to straighten Creek· and 250 feet above it. These relations strongly sugits channel, and thus the ancient meanders have been pre- gest that Disappointment Creek is in the process of migratserved. Terraces at various levels in the canyon record ing downdip on the resistant Cretaceous sandstone, 

periods during which lateral cutting locally predominated although the proc~ss is relatively inactive at present because 

over downcutting. Some of these terraces· are several hun- the base level formed by the Dolores River is being lowered 

dred feet above the present streambed. very slowly. In time the streams draining Dry Creek Basin 

Aside from the Dolores River, only three other streams will very likely become better adjusted to the structure, and 

in the area definitely seem to flow in their ancient preuplift the streams near the trough of the syncline will grow at 

courses; these are La Sal Creek~ Coyote Wash, and Dry the expense of those farther from the trough. 

Creek. Although the evidence is less impressive and con- As large areas of dip slope were formed on resistant 

vincing, Mcintyre, Summit, and Bush Canyons may also be layers, many tributaries developed directly down or nearly 

superposed inasmuch as sizable sections of their canyons down these slopes. Red and Hieroglyphic Canyons draining 

appear to be entrenched meanders. Both La Sal Creek arid the northeast flank of the Paradox Valley anticline, many 

Coyote Wash flow in deeply entrenched meanders. The of the streams draining the southwest flank of the U ncommeanders of La Sal Creek are notably small, indicating pahgre Plateau, and streams draining the northeast flank 

that the preuplift stream that made them was small. None of the Dolores anticline--especially in ·the neighborhood of 

of these three streams is adjusted to the structure; La Sal Joe Davis Hill-are examples of such streams. Gravel conCreek flows more or less parallel to the regional strike of taining pebbles of Precambrian rocks on the two low hills 

the rocks, but it follows the flank of a fold rather than the on the mesa top south of Hog Park Wash (sec. 17 and 18, 

axis. Coyote Wash cuts across the plunging north end of the T. 48 N.~ R. 16 W.) probably is not a stream gravel such 

Gypsum Valley anticline, and Dry Creek cuts across the as that of Disappointment Creek but a remnant of broad 

southeast end of the Paradox Valley anticline. Dry Creek, alluvial fans that accumulated along the front of the 

unlike the Dolores River, La Sal Creek, and Coyote Wash, Uncompahgre Plateau during some stillstand of erosion: 

does not flow through a series of entrenched meanders, yet Probably the streams most perfectly adjusted to structure 

there can be little doubt that it is superposed. Possibly, are those draining Paradox and Gypsum Valleys. The 

before uplift started, the stream did not meander as did development of these streams was very largely dependent 

those farther west; on the other hand, the stream may have on the collapse of the anticlines that un9erlie the valleys, 

had an opportunity to straighten its course or destroy any . and consequently will be discussed in connection with the 

meanders that did exist before they became deeply incised topographic development of the salt anticline valleys. 

and permanent. Possibly during downsagging of the south- Two other streams obviously related to structure are Salt 

east end of the Paradox Valley anticline that part of the Wash, which drains Sinbad Valley through a graben, and 

stream within the downsag was ponded temporarily. Ute Creek, which drains part of Ute Creek graben. The 

Many more streams are adjusted to the structure than origin of Salt Wash is not certain; it may have been a dip-
GEOMORPHOLOGY 71 

slope stream occupying virtually its present course prior to 

downfaulting of the graben, or it may have formed only as 

a consequence of graben faulting. Ute Creek may have 

existed before the Ute Creek graben was formed; nevertheless, it is clear that the upper part of the stream formed 

after graben faulting, during headward erosion which has 

greatly lengthened the stream and increased it~ drainage 

basin. Furthermore, Ute Creek appears to have captured 

two sizable drainages that, before the graben dropped, rriay 

have been ·.the headwaters of Maverick Canyon; in time, 

the upper reaches of both Calamity and Cow Creeks will 

also become victims of piracy by Ute Creek. 

Aside from the probability that they await decapitation, 

Calamity and Cow Creeks, along with Indian Creek, are 

of geomorphic interest because of their antecedent relations 

to Ute Creek graben. As the graben was lowered, the southwest rim became a barrier through which these streams cut 

as rapidly as the graben sank. Very likely these creeks were 

prevented from deserting. their channels when the graben 

formed because any given displacement along the faults 

was less than the depth of the little canyons that contained 

the streams. 

One of the most extraordinary physiographic features in 

western Colorado is Unaweep Canyon, which cuts completely across the Uncompahgre Plateau from northeast to 

southwest, but only the westernmost part of the canyon 

lies within the mapped area. The size of the canyon 

through much of its length, nearly 1 mile wide and 3,000 

feet deep, is out of all proportion to the size of the two 

puny streams presently draining it; furthermore, no readily 

perceptible divide separates West Creek, which drains the 

western end of the canyon, from East Creek, which drains 

the eastern end-they both head in the flat bottom of the 

canyon. All field evidence points to the probability that the 

canyon was formerly occupied by a far larger stream. Peale 

( 1877, p. 64-69) believed Unaweep Canyon to be the 

former channel of the Gunnison River, but later authors 

(Stokes, 1948; Shoemaker, 1954; Cater, 1955a; Lohman, 

1961, 1965) believed that the canyon more likely. was 

the channel of the Colorado River. The old river gravels 

recently uncovered in roadcuts along State Highway 141, 

northeast of Gateway, and other evidence indicate, however, that Peale was right in believing the canyon was 

carved by the Gunnison River. The history of Unaweep 

'~anyon and uplift of the Uncompahgre have been covered 

i.n a recent paper (Cater, 1966). 

There are, of course, many streams whose positions 

appear to be neither adjusted to. structure nor inherited 

from earlier erosion cycles. Most of the smaller streams and 

some of the larger ones, including Bull Canyon, fall in this 

category. The relation of Roc Creek to Sinbad and Paradox 

Valleys is one of considerable geomorphic interest. Either 

valley wo~ld seem to offer a far more likely passage for 

Roc Creek than the canyon which it occupies. This canyon 

is carved into a narrow ridge or mesa separating the two 

valleys. The probable reason. for this seemingly anom~lous 

drainage pattern lies in the likeliho~d that Roc .Creek .is 

older than either valley and has been unable as y~t to 

establish a new channel in either valley, although in time 

it may do so. Although Roc Creek may be older than either 

Sinbad or Paradox Valleys, it is doubtful that it is as old 

as t~e superposed streams, inasmuch as its channel is 

straight and rather near the trough of the shallow syncline 

between the two valleys. Probably the channel formed after 

epeirogenic uplift had started and after Roc Creek had 

adjl}sted itself to the structure with some success. 

TOPOGRAPHIC DEVELOPMENT OF THE SALT 

ANTICLINAL VALLEYS 

The salt anticlinal valleys owe their origin primarily to 

the collapse of the crests of the underlying anticlines, 

although the shapes of the valleys are largely the result of 

erosion subsequent to collapse. 

In response to the epeirogenic uplift of the Colorado 

Plateau, the Dolores River cut down rapidly and eventually 

reached the salt cores of both the Gypsum Valley and Paradox Valley anticlines. The upper surfaces of both these salt 

cores then probably stood at least 3,000 feet higher than 

the present valley fl?ors. Slumping of the anticlinal crests 

progressed rapidly once the salt cores were exposed and 

valleys were eroded in them.- The salt core underlying 

Sin bad Valley probably was exposed later than the. cores· 

of either the Paradox Valley or the Gypsum Valley anticlines, for the present landforms on the floor of Sinbad 

Valley are considerably more youthful than those of either 

Paradox or Gypsum Valleys. Collapse of the crests of both 

the· Paradox Valley and Gypsum Valley anticlines progressed rapidly following exposure of their salt cores; probably no appreciable thickness of beds had been removed 

from the general region, however, between the time the 

Dolores River cut through the Morrison. Formation and 

reached the salt core, and the time of most of the collapse. 

At any rate, the Mancos Shale had not been stripped from 

the southeast end of the Paradox Valley anticline by the 

time the crest of the anticline downsagged. 

Although the crest of the Paradox Valley anticline probably slumped progressively both northwestward and southeastward away from the Dolores River, it seems improbable 

that collapse of the crest of the Gypsum Valley anticlin'e 

followed the same systematic pattern. This conclusion is 

based on the drainage history of Gypsum Valley as deduced 

from the distribution of the Pliocene ( ?) gravels in the· 

southeast end of Gypsum Valley. The distribution of the 

gravels indicates that the drainage pattern of the area has 

changed radically since the major part of collapse was 

accomplished. The igneous rocks from which the gravels 

were derived lay to the east on the west flank of the San 
72 GEOLOGY, SALT ANTICLINE REGION, SOUTHWESTERN COLORADO 

Juan Mountains. No stream presently draining any area of 

igneous rocks flows through Gypsum Valley, except the 

Dolores River, which has not changed its course since the 

beginning of epeirogenic uplift of the Colorado Plateau. 

The gravels in Gypsum Valley indicate that the stream 

th?.t deposited them left the valley through Gypsum Gap 

and drained into the Dolores River at some point south 

of Gypsum Valley. Such a drainage pattern suggests that 

part of the Gypsum Valley anticline between Gypsum Gap 

and the Dolores River either had not collapsed or had not 

been sufficiently eroded following collapse to permit drainage down the length of the valley directly into the Dolores 

River. 

Erosion and continued minor slumping continued to 

enlarge the salt-anticlinal valleys. Most of the streams 

draining the flanks of the anticlines undoubtedly became 

established after erosi<?n of the nearly featureless preuplift 

topography and the development of extensive dip slopes on 

the more resistant beds. Red Canyon, which heads about 

3 miles northwest of where the Dolores River leaves Paradox Valley, rather obviously has been beheaded because the 

upper part of its drainage area has slumped into Paradox 

Valley-that is, at one time the canyon headed nearer the 

crest of the anticline than at present. Grabens, such as the 

one on Davis Mesa along the southwest rim of Paradox 

Valley, attest to slumping even later than beheading of 

Red Canyon. While Paradox Valley was being formed, 

large masses of rock from the crest of the Gypsum Valley 

anticline that had not been eroded were slowly foundering 

in the salt core. 

After the valleys had attained approximately their present 

form, a stillstand allowed accumulation on the valley floors 

of coarse detritus derived largely from the valley walls. This 

detritus was calichified and forms the hard, resistant fanglomerate in Paradox, Sinbad, and Gypsum Valleys. The 

partly indurated, flat-bedded silt and sand overlying the 

fanglomerate in the northwest part of Paradox Valley indicate that local ponding followed deposition of the fanglomerate. Faulting of the fanglomerate and gentle warping 

of the pond deposits show that minor readjustments of the 

salt core were continuing. Deposition of the pond material 

was followed by erosion before deposition of the unconsolidated material that now covers wide areas on the floors 

of all the valleys. 

MINOR FJ;:ATURES 

Landslides have altered the topography considerably m 

some places, particularly in the neighborhood of the salt 

anticline valleys. Landsliding probably began soon after the 

crests of the anticlines collapsed and thereby produced steep 

gradients. The bentonitic shales of the Brushy Basin Member of the Morrison Formation, or rocks resting immediately on them, are especially susceptible to sliding when 

wet. The surfaces of the landslides are highly irregular and 

commonly are covered by large blocks of sandstone and 

. conglomerate from the Brushy Basin Member or overlying 

formations. 

One natural bridge, Juanita Arch, is in Maverick Canyon 

(unsurveyed sec. 29, T. 50 N., R. 18 W.). It formed where 

the stream cut through the neck of a tight curve that may 

be an entrenched meander. Elsewhere in the area, alcoves, 

caves, niches, and holes have been carved or have been 

formed in the face of cliffs by weathering. The rows of pits 

that line the cliffs of Entrada Sandstone are the most 

numerous of these features; these have formed by differential weathering of certain layers. The regular shape of these 

pits and their fairly regular spacing are indicative of controls that are not haphazard, although what these controls 

are is not known. Alcoves and shallow caves formed by 

overhanging cliffs are most common in the Entrada Sandstone although some have formed in other massive sandstone units. Alcoves and caves form by the \\'eathering of 

soft beds overlain by resistant beds or, in massive sandstones, 

they form where percolating water has removed the cement 

thereby permitting the removal of the sand grains by water, 

wind, or gravity. Some ot the largest caves occur under intermittent waterfalls in the Wingate Sandstone at the heads of 

short box canyons. The back ends of some caves are moist or 

are coated with salts that were deposited as seepage water 

evaporated. Contrary to popular opinion, wind or sand 

blasts seem to have had little effect ~n producing any of 

the sculptured forms that decorate the cliffs, for . these 

forms show little polishing or blast effects normally 

associated with wind erosion. 

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U.S. Geol. Survey Prof. Paper 503-C, p. C1-C50 

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149 I?· 

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; 
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Bull., v. 46, no. 11, p. 2057-2070. 

Page 

A 

Abajo Mountains _____ L_ _____________________ 2, 69 

Acknowledgments ---·----------····---··· 6 

Age, Burro Canyon Formation -··-··------- 44 

Chinle Formation -----·-·-··------ 19 

Cutler Formation -··-·····-·····-·---- 11 

Glen Canyon Group -··-·-···-··--··-···- 26 

Kayenta Formation --·····-·----·· 14 

Rico Formation ----···--------·-· 11 

Algonkian rocks -----·----····--·-·---- 7 

All Baba Member, Moenkopi Formation --- 13 

Allanite -------···-··----·-···-····-··----- 49 

Alluvium ----·-··-----~----·-········-··----- 49 

Altitude ---··--------·-···-····--·---··- 4 

Analcite -----------·-----····------'-- 41 

\nalyses, Burro Canyon Formation ····-··-- 42 

Chinle Formation ----------- 18 

Glen Canyon Group, 

Kayenta Formation ---------- 22 

Navajo Sandstone -······-······--·- 26 

Wingate Sandstone ····-··-···--··-···- 20 

Morrison Formation, 

Brushy Basin Member ------- 41 

Salt Wash Member ············-·-···--·· 40 

San Rafael Group -·-···-·-···-·-··--·--- 81 

Entrada Sandstone, 

Slick Rock Member ----··-··- 80 

Anderson Mesa -------······-----26, 60, 61 

Anhydrite ··-····-······-····---------9, 64 

Apatite -······---·····-·········------------41, 49 

Archean rocks ······--··-··-··-·····-··------·- 7 

Aspen --·---··-······-··-··-----···-···-··--·-····- 4 

Atkinson Creek .................................. .,.. .... 6, 27,82,67 

Atkinson Mesa, Montrose County ············-- 86 

B 

Bald Eagle mln~s ~--······························-------- 61 

Balk, Robert, Quoted -··-·····--······---------- 68 

Barite -··--··-················-····················-··-··--- 82 

Beans ····-··-··-·--······--····-···-·-··-------- 49 

Bedrock ··························-··-··············-----··--2. 10 

Bentonite ··-----···-···---········-·····-----47, 60 

Bibliography ············-······················-··-··-··--··- 72 

·Big Gypsum Valley ····-··-·····-·-···---------.48, 66 

Big Pond ····-············-····-···················-·---·-·- 20 

Biotite -------··-···-···18, 20, 22, 80, 81, 40, 41, 49 

Blue Creek ...................................... 4, 6,6,12, 68, 70 

Blue Mesa ················-·--········-···-·····-·········--- 41 

Bluff Sandstone ··········-························--·-····-- 84 

Brushy Basin Member, Morrison 

Formation -----~-86, 41, 48, 44, 60, 72 

Buck Mesa -··-············--··-·--····---·-·····-- 61 

Bull Canyon -·····-·······-··-············-··-···-····--68, 71 

Bull Creek Canyon -------·-···-·····-········--······- 4 

Burro Canyon .......... ·········-······-·······-···-·--··- 42 

Burro Canyon Formation .... 84, 89, 41, 46, 60, 61, 

62,68,70 

Bush Canyon ····-----·-··-················-··-26, 61, 70 

Byerly, E. B., and others, 

written communication -·····-··- 61 

INDEX 

[Italic page numbers indicate major references] 

Page 

c 

Cactuses --------------·--·-··----- 4 

Cadigan, R. A., written communication_20,' 22, 

26, so. 81, 40, 41, 42, 44 

Calamity Creek ----------------··-68, 71 

Calamity Draw .,.--------···------- 6 

Calcite ····-·-------9. 14, 18, 22, 30, .41, 42, 44 

Canyon Lande ----'------------2. 8 

Cape Horn----.---'------------------- 68 

Carmel Formation ---------·----26, 27,31 

Carnelian --·- -------- 82 . Carpenter Flats _______________ 86, 64, 68 

Carpenter Ridge -----------------86 

Cashin mi_ne, Montrose County ____ 21, 26, 67 

Castle Valley, Utah -------··· 47 

Castle Valley salt anticline -------------60, 64 

Cater, F •. w., and·Craig, L.C., 

Stratigraphy -----·--·---- 5 

Cedar Mountain Formation ---------- 44 

Chalcedony -------------------- 81 

Chert ------------·---·---------·--- 22 

Chicago Corporation, ~ell ------------------------ 10 

Chinle Formation -------------------12, 18, 17, 28, 68 

Church Rock Member ----------------- 18 contacts ________ 9, 20, 64, 66, 68, 69, 61, 62 

Monitor Butte Member -----------------. ---·- 17 

Moss Back Member ---------------------- 18 

unconformity, ---------------------- 17 

Chokecherry -------------------------------------- 4 

Church Rock Member, Chinle Formation _ 18 

Cimarron Ridge ------------------------- 66 

Climate --------------------------------- 4 

Clinopyroxene ------------------------ 49 

Coal ---------------------------44, 46 

Color, Burro Canyon Formation ---------- 43 

Chinle ··Formation -------------- 17 

Dakota Sandstone -~-----------------· 44 

Glen Canyon Group, 

Kayenta Formation -------- 22 

Wingate Sandstone ____ ...:_ ______ : ___ 21 

Moenkopi Formation ----------------------- 12 

Morrison Formation, 

Brushy Basin Member ------- 41 

Salt Wash Member ------------- 40 

San Rafael Group, Entrada Sandstone, 

Dewey Bridge Member ------- 27 · 

Slick Rock Member ------------- 80 

Summerville Formation --------- 31 

Colorado River _______________ 2, 17, 48, 70, 71 

Contacts, Burro Canyon Formation ----------- 43 

Chinle Formation 9, 20, 64, 66, 68, 69, 61, 62 

Cutler Formation ----------1.2, 17, 19, 48, 69 

Dakota Sandstone ---------------------- 46 

Hermosa Formation, 

limestone member ______ 61, 64 

Paradox Member ___________ 9, 64 

Morrison Formation ___________ 8, 69 

Brushy Basin Member ----------- 41 

Continental· Oil Co., well ---------------- 9 

Correlation, Dakota Sandstone -------------- 46 

Glen Canyon Group, 

Wingate Sandstone ------ JlJl 

Page 

Correia tion~Continued 

San Rafael Group, 

Summerville Formation ___ 84 

Cottonwood ------------------- 4 

Cottonwood Canyon ---------------- 68 

. Cow Creek ---------------------68, 71 

Coyote Wash -----------------69, 70 

Craig, L. C., and Cater, F. W., 

Stratigraphy -------- 5 

Cretaceous rocks --------------6, 41, 46 

Cupola· ---------------64, 67, 68, 66 

Curtis Formation ___________ 27, 81, 84 

Cutler Formation .. 10, 18, 16, 64, 66, 62, 64, 67, 68 

contacts· ________ 12,17, 19, 48,69 

regional difference -------------- 11 

D 

Dakota Sandstone ______________ .. u, 60, 70 

unconformity ----------------- 46 

Dane, C. H., Quoted ------------------- ?2 

Davis, W. E., oral communication --------- 68 

Davis Mesa----------------------------- 72 

Deposition conditions, 

Burro Canyon Formation ----- 44 

Chinle Formation ------------------------ 19 

Cutler Formation ------------------- 11 

Dakota Sandstone -------------------------- 46 

Glen Canyon Group, 

Kayenta Formation ----------- 14 

Navajo Sandstone ---------------- 116 

Wingate Sandstone ------------------ 111 

Moenkopi Formation --------------------- 16 

Morrison Formation, 

Brushy Basin Member -------- 42 

Salt Wash Member ------------------ 41 

Rico Formation-------------------- 11 

San Rafael Group ---------------------------- 84 

Entrada Sandstone, 

Dewey Bridge Member --------- 19 

Slick Rock Member ------------ 91 

Dickinson, R. G., oral communication ---- 66 

Disappointment Creek ______________ .44, 70 

Disappointment syncline -----------62, 69, 66, 70 

Disappointment Valley ________ .4, 42, 46, 49, 70 

. Dolomite -----------------------9, 81 

Dolores anticline -----------60, 611, 63, 66, 70 

Dolores County ---------------------- 2 

Dolores mines, friontrose County _________ :28, 86 

Dblores River _________ .4, 9, 10, 19, 82, ~1, 66, 71 

drainage ----------------------------- 69 

gravels ----------------------------------------.48, 49 

Dolores River Canyon _______ .4, 10, 11, 12, 14, 16, 

19, 33, 62 

Dolores-San Miguel syncline -------------------11, 67 

Donnell, J. R., oral communication _______ __: 66 

Drainage ------------------------------4.47,69,71 

Dry Creek _____ 6, 18, 21, 23, 43, 46, 68, 67, 69, 70 

Dry Creek anticline ---------------------66, 67 

Dry Creek basin --------------------4, 46, 49 

Dry Creek 'basin syncline _________ 60, 61, 62, 69 

77 
78 

Page 

Dry Creek Canyon ------------------------~--------------- 66 

Dugout Wash -------------------·------------------------------------ 63 

E 

East Creek -----------------------·-------------------------------·8, 71 

East Paradox Creek ----------------------------.64, 69, 70 

East Paradox Valley ---·---------------------------10, 24 

Egnar -----------------------------------------------2, 49 

Elk Mountains ----------------------------------------------- 27 

Entrada Sandstone, San Raphael Group 22, 24, 

£~32;3~66,68,6~67,72 

Erosion _______________ 11, 27, 34, 42, 44, 46, 61, 64, 66, 

67, 69, 71. 

F 

Fanglomerates -----~---------~-------------- 48 

Faults _______________ ...: ........ 60, 61, 66, 68, 61, 67, 68 

Feldspar, Burro Canyon Formation ------------- 42 

Chinle Formation -------------------------- 18 

Cutler Formation ________________________ 10, 11 

Dakota Sandstone -------------------------------- 44 

Glen Canyon Group ----------------------- 22 

Hermosa Formation ------------------------ 9 

Moenkopi Formation --------------------------- 13 

Morrison Formation ---------------------40, 41 

~ico Formation ----------·-------------10, 11 

San Rafael Group -------------------·30, 31 

Ferron Sandstone Member ---------·------------ 47 

Fir --------------------------------------------·----- 4 

Fisher Valley, Utah ---------------,----------10, 17 

Fisher Valley anticline ---~-------------------60, 61 

Fisher Valley-Salt Valley anticline ---·-··-·--- 17 

Fossils· --------··--····-------------------------------------------· 20, 29 

algae ----------------------39, 42 

A mmosaurus -----···----------------------------- 26 

bone fragments ---------------------~-----19, 41 

brachiopods -------------------------- 9 

charophytes ------------------- 40 

corals ------------------------------- 9 

crinoid stems ----------------------------------- 9 

dinosaurs ------------,.---------------22,24, 39 

Erytkrockampsa ------------------ 26 

fusulinids -----------·------------- 11 gastropods _____________ 9, 19,32 

Grypkaea newberryi -------------------·- 47 

invertebrates ------------------9, 39, 41, 44 

Ma.ckaeroprosopus -~------------ 22 

. marine ..... ---------------------16,34 

Megalosaurus. ---------------·--·24, 26 

mollusks · ------~---'---------------------------7 39 

Notochampsa --·----------------- 26 

ostracodes· ··------------------------26, 40 

pelecypods ----------------.-11, 19, 24 

phytosaur ------------------------- 22 

plants ---------------19, 26, 39, 41, 44, 46 

Protosuckus rickardsoni --------·····----··· 26 

reptiles -------------------···---------------- 26 

Segi8saurus ---····-----------------------·--- 26 

sharks' teeth ---------------------------------- 47 

tritylodonts -------------··----------------26, 26 

Unio ---------------------------·----· 24 vertebrates ___ ._. _____________ 11, 19,39 

wood, petrified ______________ 19, 39, 41 

Four Corners ----------------------·--- 44 

Freeman, V. L., and Mullens, T. E., 

· written communication ---·----- 41 

G 

Gateway, Mes.a County ______ .2, 4, 10, 14, 19, 23, 

26, 48, 62, 67, 71 

Geomorphology ---·------------------ 69 

Glauconite ----~-----------------------·----··- 9 

Glen Canyon Group ---------------··------------ £0 

Kayenta Formation ___ 20, ff, 27, 28, 68, 61 

unconformity ------------------------ 24 

Navajo Sandstone __ 20, 22, 24, 15, 28, 29, 

30, 61,'69, 61, 62 

INDEX 

Page 

Glen Canyon Group~Continued 

Wingate Sandstone ____ so, 24, 26, 27, 211, SO, 

68, 61, 72 

Graben ________________________ 64, 67, 62, 66, 67, 68, 70, 72 

Grand Junction -------------------. ________ 22, 44, 67 

Gravels ----------------------------47, 48, 49, 70, 71, 72 

Gravity gt:adients ----------------------------------- 61 

Great Sage "Plain -----------------------------4, 6 

Gulf Coast salt domes ----------------- 58 

Gulf of California · -------------------------------- 2 Gunnison River _____________ 4, 7, 48, 49, 68, 69, 71 

Gypsum _______ 6, 8, 12, 14, 16, 17, 40, 41, 64,_68, 68 

Gypsum Creek ---·--------··------------------- 70 

Gypsum Gap ---·--------------------.47, 61, 60, 61, 72. 

Gypsum Valley ___ .4, 6, 8, 21, 26, 31, 46, 49, 68, 69 

Gypsum Valley anticline __ 61, 58, 63, 66, 66, 70, 

71, 72 

H 

Hamm Canyon _____________ 47, 68, 69, 60, 61,66 

Harpole Mesa Formation --------~------- 48 

Hermosa Formation -----------------8,11 

limestone member .. 9, 61, 64, 67, 68, 60, 61, 64 

Paradox Meinber: ... s, 17, 49; 60, 64, 66, 68,68 

contacts ---------· ____ .____ 9, 64 

outcrops -------· _. _· _____ g, 60, 66 

rocks included _______________ 8, 13, 48, 61 

Hieroglyphic Canyon ------------- 70 

Hog Park Wash ------------------------- 70 

Honaker Trail Formation ----------------10 

Hoodoos ---------------------------------- 26 

Hornblende· ----------------------------- 49 

Horse Range Mesa ----------------- 26 

Horst --------------···------------------------------ 62 

Illinoian Glaciation --------------- 48 

lllite ---'--------------------------- 40 

llmenite ---------··-----------------------31, 49 

Indian Creek ----------------------------.---68, 71 

Introduction -----------·--·----------~'--- f 

Iron ----------------'------_: __________ __._ 40 

Irrigation -----------------· --------------- 60 

Izett, G. A., and Wilcox, R.E., 

written communication·----····--- 49 

J 

Jasper --------------------------..:._ ___ 41 

Jo Dandy mine-~------------------- 60 

Joe Davis Canyon, 

San Miguel County -------- 46 

Joe Davis Hill ----------------10, 11, 19, 62,70 

Joesting, H. R., and others, 

·written communication _____ 61 

John Brown Canyon, 

Mesa County ___ 26, 30, 32, 34, 38 

Juanita Arch -----···------------------ 72 

Junction Creek Sandstone ------------- 34 

Juniper ---------------------------4, 49 

Jurassic rocks ----------------6, 10, 26, 17 

Lower ------------------------------ 6 

Middle --------------------------- 20 

Upper ---··--·-----~----------6, 9, 20, 27, 34 

K 

Kansas Glaciation ------------ 48 

Katich, P. J., written communication ___ 42 

Kayenta, Ariz 26 

Kayenta Formation, Glen Canyon 

Group ____ 20, It, 27, 28, 68,61 

Keller, W. D., and Weeks, A. D., 

written communication -----40 

Keller, W. D., written communication -------- 41 

L 

Lake deposits ·-------··-...:·-·------------ 49 

Landslides ------------------------ 60 

Page 

La Plata Mountains ----------------------·-·-· 10 

La Sal Creek --------------------------·-·-------.4, 69,70 

La Sal Creek Canyon ---------·-----------~ 4 

La Sal Mountains, Utah -----------------2, 48, 60, 69 

Lenses --------·----10, 13, 16, 18, 24, 40, 42, 44_ 

Limestone member, Hermosa Formation_.9, 61, 

64,6~6~6~6~64 

Lisbon Valley ~nticline ________ 9, ~0, 61, 62 

Little Gypsum Creek ---------------------·- 70 

Little Gypsum Vall~y ----------------9, 61, 62 

Location --------------·-·····------------ I 

Lorna ·---'--·--------------·-·····-----------·-·-·· 44 

Long Ridge mines -------'--··---···------------------- 61 Lower member, Moenkopi 'Formation _____ 12, 13 

Lufeng series -----~------------------------------------ 26 

M 

McElmo Creek ---------------------------- 6 

Mcintyre Canyon ---------------·--------- 70 

Magnetite -·-'-·--···---------------31, 49 

Mancos Shale -----------------46, 69, 61, 69, 71 

Mapping ------·-·--·"-------------- 5 

Maverick Canyon --------------26, 71, 72 

Mesa County·-··----------·: _________________ 2 

Gateway 2, 4, 10, 14, 19, 23, 26, 48, 62, 67, 71 

· John Brown Canyon __ ..:..26, 30, 32, 34, 38 

Sinbad Valley _____________ 14, 29 

Mesa Creek ------------------------6, 6, 41, 62 

Mesaverde Formation ---·------47, 61, 66, 66 

Mica -----------------------13, 22, 40 

Microcline ------------------- 9 

Middle member, Moenkopi Formation ___ 12, 13 

Miocene surface --------------------- 69 

Moab, Utah ------------------------ 10 

Moenave Formation ---------------------- 26 

Moenkopi Formation __ 1£, 19, 61, 66, 68, 62, 63, 

-;.. 66,68 

Ali Baba Member ___ .:.. ___________________ 13 

lower member ______________ 12, 13 

middle member _____________ _'_.12, 13 

Pariott Member -----------------18, 14 

Sewemup Member -------------- 13 

',renderfoot Member --··-----------· 12 unconformity __________ .. 16, 54 

upper member __ ...:. _________ .12, 13 

Monitor Butte Member, Chinle Formation .... 17 

Monogram Mesa --------------- 68 

Montmorillonite ------------- 41 

Montrose ------------------ 65 

Montrose County--·---------------- 2 

Atkinson Mesa ---------------- 35 

Cashin mine -----------------21, 26, 67 · Dolores mines ___________ 28, 35 

Dolores River Canyon ___________ 15, 33 

Dry Creek ____________ 18, 21, 23, 43, 45 

Skein Mesa -------------43, 45 

Stone Spring-------------- 23 

Morrison Formation ------6, 32, 94, 58, 69, 71 

Brushy Basin Member .... 35, .u, 43, 44, 50, 72 

contacts ----------------3, 59 

Salt Wash Member _____ 35, 99, 42, 60, 61 

'thickness ------------36, 61, 63 

Moss Back Member, Chinle Formation ----- 18 

Mullens, T. E., and Freeman, W. L., 

written communicaiion -------- 41 

Muscovite ---------18, 20, 22, 30, 42, 44 

N 

Navajo Sandstone, Glen Ganyon Group _.20, 22, 

24, BS, 28, 29, 30, 61, 59, 61, 62 

Nebraskan Glaciation -------------· 48 

Nicholas Wash----------------- 63 

North Fork, Mesa Creek --------17, 63,67 

Nucla ----·------------- 11 

Nucla syncline --------60, 62, 67, 70 

Nugget Sandstone ·--------------- 27 
Page 

0 

Orthoclase ··········································-··-·············· 9 

Orthopyroxene ...................................................... 49 

Ouray ·····-·····································-························ 7 

Ouray County, Ridgeway ································- 44 

Outcrops, Burro Canyon Formation ............ 42 

Chinle Formation ........................................ 17 

Dakota Sandstone ........................................ 44 

Glen Canyon, Kayenta Formation .......... 22 

Navajo Sandstone ................................ 25 

Wingate Sandstone ····-······················ 20 Hemosn !<~ormation, 

Paradox Member .................. 8, 60,66 

Mancos Shale ...... , ......................................... 46 

Moenkopi Formation .................................. 12 

Morrison Formation ·····-··························· 34 

Snn Rafael Group, Entt·nda Sandstone, 

Dewey Bridge Member .............. 27 

Summerville Formation .................... 31 

p 

Paradox ················-··················-·····-······2• 11, 18, 57 

Paradox Basin ............................... : .......... 10, 67, 68 

Paradox Member, Hermosa Formation ......•... 8, 

17. 49, 60, 54, 56, 58, 68 

Paradox Vnlley .. 4, 8, 16, 21, 25, 80, 46, 49, 58, 69 

Paradox Valley Anticline ........ 50, 51, 54, 57, 59, 

62, 65, 67, 70, 71 

Pariott Member, Moenkopi Formation .... 13, 14 

Pennsylvanian rocks ................................ 6, 8, 9, 10 

Permian rocks ································-·-··········10, 11 

Plagioclase ····························-············-·············- 9 

Pine Hill ········································-······················ 68 

Pine Mountain ........•............................................. 68 

Piiion -···········--·-················-················-·············-4, 49 

Pitchfork mines .................................................. 60 

Placerville, San Miguel County .................... 11, 44 

Pliocene rocks ·····················-······························· 47 

Plouff, Donald, and others, 

written communication .............. 61 

Plugs, snit-and-gypsum -···································· 54 

volcanic ····-··············-··-···················-··········· 60 

Pomeroy, J. S., written communication ····-·· 41 

Ponderosa pine ············································-········ 4 

Population ············-············································· 2 

Precambrian rocks ................................ 5, 6, 10, 17 

Precipitation ···········-············-·-··-··-··-·········· 4 

Previous work ...................................................... 5 

Q 

Quartz, Burro Canyon Formation .................. 42 

Chinle Formation ........................................ 17 

Cutler Formation ............................••.......... 10 

Dakota Sandstone ........................................ 44 

Glen Canyon Group ........................ 20, 22, 25 

Hemosa Formation ················-·-·············- 9 

Moenkopi Formation ············-·····-·····-··- 13 

Morrison Formation ·········---·--·-··--40, 41 

Rico Formation -······-··························-···· 10 

San Rafael Grqup ····-··············--·--········30, 31 

Quaternary rocks ············-··-··-······-·····-······6• 48 

R 

Raven mine ····················-··················-················ 62 

Red Canyon ···························-···················----70, 72 

Rico Formation .......................... 8, 9, 10, 66, 61, 64 

Ridgeway, Ouray County ·········-·-················ 44 

Roc Creek ················-······----.4, 25, 60, 54, 63, 71 

Roc Creek Canyon ........................................... .4, 13 

Rocks included, Burro Canyon Formation .... 42 

Chinle Formation ·········-·--····--·····-·-···· 17 

Cutler Formation ........................................ 10 

Dakota Sandstone ········-···································- 44 

Glen Canyon Group, ·Kayenta 

Formation ·-·····················-············ 22 

INDEX 

Page 

Dal<ota Sandstone-Continued 

Navajo Sandstone ·············-········-····· 25 

Hermosa Formation, Paradox 

Member ········-·····-·········8, 13, 48, 51 

Mancos Shale ············----·--························-· 47 

Moenkopi Formation ----········-··············---- 12 

Morrison Formation, Brushy 

Basin Member -······----······----------- 41 

Rico Formation ----······--·····-·-------················· 10 

Salt. Wash Member ················--·--------------- 39 

San Rafael Group, Entrada Sandstone, 

Dewey Bridge Member ---------·-- 27 

Summerville Formation ·-··--------········- 31 

Rocky Mountains ···························------------·-····- 68 

s 

Sagebrush ······-·················-·····-------------·--·-··--4, 49 

Sagers Wash syncline --···········-········--······---62, 70 

Salt anticlinal valleys, 

topographic development -------- 71 

Salt anticlines ····-····---------------------········ 30 

collapse ·········-········--------------------········ 65 

general features ·······--------·-···-···-·····-· 50 

growth ············------------------------·········-·· 64 

later history ······-·-------------·········--·------- 65 

localization ----------------------------------------- 50 

origin ···················--·····-·········-··········------- 63 

structure ··-·········------------------------------------- 51 

thickness ·······-------------------------·--------- 51 

Salt Creek Canyon -----------·-··························------ 4 

Salt Creek graben --------········--·-····---------- 68 

Salt domes -············-···---------·-···-·····-------··· 50 

Salt Valley, Utah ···········--·········---·······-----16, 17 

Salt Wash ················-----···················-····-.4, 5, 9, 70 

Salt Wash Member, Morrison 

Formation .............. 35, 99, 42,60, 61 

San Juan Mountains ........ 2, 4, 7, 10, 22, 34, 47, 

65,67,69,71 

San Miguel County ···············----------······2• 30, 44 

Dolores River ······--·---········-··------·-·······---- 19 

Joe Davis Canyon --·-·················----·----·····- 45 

Placerville ··----------------------------·-····----- 44 

Slick Rock ······-------------------------29, 33, 42 

Summit Canyon -------·------------------21, 24, 37 

San Miguel Creek --------········-·······················----- 70 

San Miguel River .... 4, 5, 25, 27, 31, 32, 49, 58, 69 

San Miguel River Canyon ------------------------11, 41 

San Rafael Group, Entrada Sandstone .... 22,24, 

2~32,3~56,6~ 6~6~72 

Entrada Sandstone, 

Dewey Bridge Member ·········-·· !7 

Moab Tongue -------------·30, 31, 32, 34 

Slick Rock Member --------------27, 28, !9 

Summerville Formation 27, 30, 91, 40,60, 61 

San Rafael Swell, Utah -----------------16, 27, 31 

Sawtooth Ridge ----------------------------- 61 

Scrub oak --------------·--------------·-··- 4 

Serviceberry ----------------------- 4 

Sewemup Member, Moenkopi Formation __ 13 

Sewemup Mesa ····------------------24. 25 

Shawe, D. R., written communication 35, 42, 63 

Silveys Pocket ------------··-····----26, 61, 62 

Sinbad Valley --------------------------.4,8, 10, 

12, 14, 17, 19, 29, 47, 65, 68, 70 

Sinbad Valley anticline ··--····-··-9• 50, 61, 54, 58, 

59, 62, 63 

Sinkholes ·····················-··········-·········--······-······ 66 Skein Mesa, Montrose County __________ 43, 45 

Slick Rock, San Miguel County ............ 29, 33, 42 

South Fork, Mesa Creek -······-··-·····--··--:........ 63 

Spring Creek Canyon ......... ,---···--··········-··- 4 

Spring Creek Mesa ···············-·---··········-·-·-···--- 48 

Spruce -····-······----············-····-·-···········-··-···-··-·· 4 

Stone Spring, Montrose County .................... 23 

Stratigraphy, by Cater, 

F. W., and Craig, L. C. -···-----·· 5 

· modifications ····-······-····----·--······----·--·· 51 

79 

Page 

Stromberg series ---------····---··----·-----······················ 25 

Structure, Disappointment syncline ............ 69 

Dolores anticline ······-··---····--·-········-····-···· 62 

Dry Creek basin syncline -----·················· 69 

geology ·····---··-······---····--------··--·--·--------·--····--- 50 

Gypsum Valley anticline -······-····--------- 58 

history ----·----··-------------------------····-··----------- 6 8 

Nucla syncline --·--···--------·-------------···--··--- 62 

Paradox Valley anticline ·-----------·---··---- 54 

relief -·····-···-----·-···-···-···----···-------·61, 55, 63, 67 

salt anticlines ·······---------·----·······-·····------ 51 

Sinbad Valley anticline ................ 51, 54, 59 

synclines ··--------------···········----------------------- '62 ·· 

Summerville Formation, San Rafael 

Group ................ 27, 30, 91, 40, 60, 61 

Summit Canyon -------·-·····---21, 24, 25, 37, 62, 70 

Sunrise mine -------············--········-------··········--57, 58 

Swinton, W. E., quoted ----------------------············· 24 

Synclines, structure -----------····----------·-···-··--------- 62 

T 

Tabeguache Creek 

Tamarisk ·······--·--·--·······-···---····--·-·---·-------------- 4 

Temperature ···---··········-···-·--··-------------··--· 4 

Tenderfoot Member, Moenkopi Formation .... 12 

Tenderfoot Mesa --·-··-·····················-·········----- 35 

Tertiary rocks -----······------·-······-······--·--·-·· 47 

The Hat ········-·····-·················-····-··········-------- 61 

Thickness, Burro Canyon Formation ····---- 44 

Chinle Formation ···--·······----------------------- 19 

Cutler Formation --------------------------------11. 67 

Dakota Sandstone ···----------------------·····-------- 46 

Glen Canyon Group, 

Kayenta Formation ................ !4, 68 

Navajo Sandstone ........................ !6, 62 

Wingate Sandstone ........................ 21, 58 

Hermosa Formation, limestone member 10 

Mancos Shale ······························-··········----- 47 

Moenkopi Formation ····--·-···········-·······-·· 17 

Morrison Formation ·-·-················--35, 51, 63 

Brushy Basin Member ··········-·····-······ 4! 

Salt Water Member ··-·····-··-··········- 40 

Rico Formation ··-·······-······----------······- 11 

San Rafael Group .................................. 32, 94 

Entrada Sandstone, 

Dewey Bridge Member -··-···· !9 

Slick Rock Member ····-·······-······-· 91 

Thunderbolt mine ------------------------·30, 58 

Thunderbolt Springs --·--------------·--·-··· 21 

Todilto Limestone -------·-··-·····------------------- 25 

Topography ············-···-········-····----------- 9 

Tourmaline -------·----···-·-·-------------------- 31 

Triassic rocks -----······················---1!, 20, 22, 26 

Lower ··--····-·-······-·····-·····-······--------- 6 

Upper -----------------------·6, 17, 19, 20, 25 

Turner Gulch ····-·····----------------------- 20 

u 

Unaweep CRnyon ........ 4, 5, 7, 8, 48, 49, 68,69, 71 

Uncompahgre highland ........ 6, 11, 13, 16, 50, 63, 

65, 67, 69 

Uncompahgre Plateau, ........ 2, 4, 6, 10, 17, 19, 22 

24, 50, 62, 68 

Unconformity --------·········-----------------······12, 14, 17 

Chinle Formation ····························-···-·-· 17 

Dakota Sandstone ---,-·-···-·········--········- 46 

Glen Canyon Group, 

Kayenta Formation ····-····--······ 24 

Moenkopi Formation -·····················------ 16 

Uinta Basin ········---·-·····----··········------······ 67 

Uplift, effects ···-····----··········--··········-·······-····-- 69 

Upper member, Moenkopi Formation __ 12, 13 

Uranium ·ore ··-···············-·············-···········-····· 2 

Uravan -·-·····-········2• 4, 25, 27, 31, 32, 48, 62, 63 
80 

Page 

Ute Creek ................. 6, 20, 21, ~2. 49, 68, 70, 71 

Ute Creek graben ----------------------------------67, 68, 70 

v 

Vanadium ore -------------------------------------------------- 2 

Vegetation ---------------------------------------------- 4, 46, 49 

Vents, volcanic ------------------------------------- ----------- 60 

w 

Wanakah Formation ----------------------------------------- 34 

INDEX 

Page 

Wasatch Plateau, Clear Creek field --------------- 47 

Water supply --------------------------------------------------- 4 

Weeks, A. D., and Keller, W. D., 

written communication ------------- 40 

Weeks,· A. D., written communication ... .40, 41 

Wells ------------------------------------------------------9• 10, 66 

West Creek --------------------------·6, 6, 10, 48, 49, 71 

West Paradox Creek ----····---------------------.4. 60, 70 

West Paradox Valley --------------------------------- 9 

Wilcox, R. E., and Izett, G. A., 

written communication --------- 49 

Wilcox, R. E., written communication ........ 47 

Page 

Wild Steer Canyon ............................................ 26 

Willow .................................................................... 4 

Wingate Sandstone, Glen Canyon Group .... M, 24, 

25,27,29, 30,58,61,72 

Wolf Hill ------------------------------------------------------------- 22 

Wray Mesa ---------------------------------------·-·············· 26 

y 

Yunnan, China ------------------------------------------------ 26 

z 

Zircon ····························-··································-· 41 

"tlu.S. GOVERNMENT PRINTING OFFICE: 1970 0-384-274