Geologic History of the

Slick Rock District and Vicinity,

San Miguel and Dolores Counties,

Colorado

GEOLOGICAL SURVEY PROFESSIONAL PA PER 576-E

Prepared on behalf of the

U.S. Atomic Energy Commission
Geologic History of the

Slick Rock District and Vicinity,

San Miguel and Dolores Counties,

Colorado

By DANIEL R. SHAWE

GEOLOGIC INVESTIGATIONS IN THE SLICK ROCK DISTRICT, 

SAN MIGUEL AND DOLORES COUNTIES, COLORADO

GEOLOGICAL SURVEY PROFESSIONAL PAPER 576-E

Prepared on behalf of the

U.S. Atomic Energy Commission

A review of the origin of sedimentary rocks and 

geologic history of the region indicates that 

formation of uranium-vanadium deposits was a 

natural result of the deposition of the rocks, 

the occurrence of intrastratal waters therein, 

and the postdepositional movement of the waters 

resulting from evolution of the sedimentary rock 

environment

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Shawe, Daniel R. 1925-

Geologic history of the Slick Rock district and vicinity, San Miguel and Dolores Counties, Colorado.

(Geologic investigations in the Slick Rock District, San Miguel and Dolores Counties, Colorado) (Geological Survey Professional

Paper 576-E) 

Bibliography: p.

Supt. of Docs, no.: I 19.16:576-E

1. Geology Colorado San Miguel Co. 2. Geology Colorado Dolores Co. 

I. United States. Atomic Energy Commission. II. Title. III. Series. IV. Series: United States Geological Survey Professional

Paper 576-E. 

QE75.P9 no. 576-E [QE92.S35] 557.3'08s [557.88*23] 76-608133

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

Washington, D.C. 20402 

Stock Number 024-001-02830-1
CONTENTS

Page

Abstract............................................................................................... El

Introduction........................................................................................ 2

Precambrian rocks and events............................................................ 2

Cambrian rocks and events................................................................. 2

Devonian rocks and events ................................................................ 3

Lower Mississippian rocks and events............................................... 7

Upper Mississippian and Pennsylvanian rocks and events............... 7

Page

Upper Pennsylvanian and Permian rocks and events....................... E 7

Triassic rocks and events.................................................................... 8

Triassic(P) and Jurassic rocks and events........................................... 10

Cretaceous rocks and events ............................................................... 14

Cenozoic rocks and events.................................................................. 17

The current scene................................................................................ 18

References cited................................................................................... 18

ILLUSTRATIONS

FIGURE 1. Map showing location of the Slick Rock district, and major tectonic elements in the Western United States. 

2. Generalized geologic map of the Slick Rock district............................................................................................

E3

4

TABLE

TABLE 1. Generalized section of consolidated sedimentary rocks.

Page

.E5

in

GEOLOGIC INVESTIGATIONS IN THE SLICK ROCK DISTRICT, 

SAN MIGUEL AND DOLORES COUNTIES, COLORADO

GEOLOGIC HISTORY OF THE SLICK ROCK DISTRICT AND VICINITY, 

SAN MIGUEL AND DOLORES COUNTIES, COLORADO

By DANIEL R. SHAWE

ABSTRACT

At the close of Precambrian time in the region surrounding the Slick 

Rock district, basement gneisses and schists had been beveled by erosion 

and were being downwarped along a north-trending axis farther west to 

receive the basal sediments of the great Cordilleran geosyncline. Coarse 

clastic sediments of Cambrian age were the first sediments to be deposited 

in the vicinity of Slick Rock. Sediments then became finer grained 

upward, and deposition ceased in Ordovician and Silurian time, the 

hiatus indicating stability of the low land masses east and southeast of the 

district.

During Devonian and Mississippian time the region lay on the shelf of 

the Cordilleran geosyncline on which miogeosynclinal sediments were 

being deposited. Mild northwest-trending folds were formed on the shelf 

coincident with the extreme deformation that occurred during the Antler 

orogeny in the region of the present Great Basin to the west. Temporary 

emergence occurred in Late Mississippian time.

In Pennsylvanian time the northwest-trending Paradox Basin formed 

on the shelf as a result of depression of a graben and rise of adjacent land 

along faults bounding the Uncompahgre uplift northeast of the district 

and underlying Disappointment Valley in the district. Development of 

the basin coincided with reactivation of a Precambrian-age northeasttrending structure at the southeast end of the basin the Glade subsurface zone of faults. Northwest-oriented folding continued during the 

Middle Pennsylvanian in the Paradox Basin as stagnant marine sediments, including thick evaporites, were deposited.

In the middle part of Permian time the Uncompahgre uplift rose 

rapidly, shedding coarse clastic debris westward. This event coincided 

with renewed movement on the Glade subsurface fault zone and with 

pronounced flow of salt into the salt anticlines. Deposition of thick 

Permian sediments, some subaerially, displaced the border of the 

Cordilleran geosyncline westward.

During Early and Middle Triassic time, sediments were deposited in 

the Paradox Basin, were folded, and were then eroded upon emergence. 

Middle Pennsylvanian to Early Triassic deformation in and around the 

Slick Rock district coincided with tectonism of the Sonoma orogeny 

occurring in the region of the present Great Basin.

In Late Triassic time a depositional basin formed, centered in southern 

Utah and northern Arizona, in which continental fluvial and lacustrine 

sediments were laid down. Provenance was sedimentary and granitic 

rocks lying east, southeast, and south, and possibly a newly emergent volcanic island arc to the west.

Jurassic and Early Cretaceous deposition, mostly terrestrial and some 

marginal marine, took place in a basin centered variously in the region 

extending from southern Nevada to eastern Utah. Sources of sediment 

were toward the east, but they shifted to the west with emergence, during

the Late Jurassic, of land in the general region of volcanic centers to the 

west, and then shifted to the south in the Early Cretaceous. Also during 

Late Jurassic time, mild renewal of Paradox Basin deepening modified 

the deposition of the Salt Wash Member of the Morrison Formation in 

the region of the Uravan mineral belt which extends northward from 

Slick Rock, resulting in development of lithologies favorable to later 

deposition of uranium-vanadium ores. Abundant carbonaceous plant 

material, preserved where sediments lay below a water table, afforded the 

critical reducing environment necessary for later formation of the ores.

In Late Cretaceous time a large seaway encroached northward from 

Mexico onto the continent marine black shale was deposited in this 

geosyncline. Pronounced deepening of the seaway coincided with strong 

tectonism on both sides of the seaway: the Sevier orogeny to the west in 

the region of the present Great Basin during most of the Cretaceous, and 

the Laramide orogeny to the east in the region of the present Rocky 

Mountains in latest Cretaceous time. Volcanic sources to the west, in 

addition to the eroding lands both east and west, continued to supply 

debris to the region of the Colorado Plateau in Late Cretaceous time.

It seems likely that marine black shale (Mancos Shale) of Late 

Cretaceous age shortly after deposition contained a large volume of 

saline, reducing, carbonated, and slightly alkaline water that carried 

complex ions of uranium and vanadium. Upon deposition of overlying 

thick, largely terrestrial strata of latest Cretaceous and Eocene 

(Mesaverde, Wasatch, and Green River Formations), much of the pore 

water of the black shale was driven out by compaction into overlying^ 

formations and along zones of faults into underlying formations. Widescale epigenetic alteration accompanied passage of the water, and where 

the solutions moving in and near the zones of faults encountered 

carbonaceous strata in the Salt Wash Member, uranium-vanadium 

deposits were formed.

Igneous activity occurred during the early to middle Tertiary at 

scattered laccolithic centers on the Colorado Plateau, and large sills of 

igneous rock were emplaced during the middle Tertiary, mostly in 

Pennsylvanian sedimentary strata near the eastern border of the plateau.

General uplift of the Colorado Plateau took place in the middle 

Tertiary. Extensive erosion since then has exposed some of the previously 

deeply buried ore deposits.

Salt anticlines in and near the Slick Rock district whose growth was 

initiated tectonically almost at the time of deposition of the salt in 

Pennsylvanian time grew mostly by buoyancy from the Jurassic on ward, 

and this growth has continued to the present.

Pleistocene and Holocene glacial, glacial-outwash, windblown, masswasting, and fluvial deposits are scattered throughout the district and 

surrounding region.

El
E2 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

INTRODUCTION

This report is a narrative summary and interpretation, 

in the form of a geologic history of the Slick Rock district 

and vicinity, of four previously published chapters in this 

series dealing with stratigraphy of the Slick Rock district 

and vicinity (Shawe and others, 1968), petrography of 

sedimentary rocks of the district (Shawe, 1968a), structure 

of the district and vicinity (Shawe, 1970), and altered sedimentary rocks of the district (Shawe, 1976) and of other 

previously published reports on the district. It forms the 

background, with the earlier reports, for presentation of a 

final report in the series describing the uraniumvanadium ore deposits. A broad knowledge of the regional 

geologic history the evolution of specific lithologic 

environments and the geologic processes that affected the 

rocks is essential to an understanding of the origin and 

genesis of uranium-vanadium ores in sedimentary strata 

of the Colorado Plateau.

Details of the geology of the Slick Rock district have 

been given in the earlier chapters, but some general 

information will be reviewed briefly here. The location of 

the Slick Rock district in its regional setting is shown in 

figure 1. A generalized geologic map of the district is 

presented in figure 2. The consolidated sedimentary rocks 

in the district are briefly summarized in table 1. 

Familiarity with much of the detailed information 

reported previously will not be required for a comprehension of this report; the reader wishing documentation 

on which the inferences and conclusions of this report are 

based should refer to the earlier reports.

The geologic history encompasses a discussion of 

successive rock units, from Precambrian basement rocks 

through Quaternary surficial deposits.

PRECAMBRIAN ROCKS AND EVENTS

Little is known of the Precambrian history of the region 

that surrounds the Slick Rock district. Northeasterly 

alined geophysical discontinuities underlying the 

Blanding basin, Utah, west of Slick Rock (Case and 

Joesting, 1961, p. D287-D291 and figs. 393.1, 393.2), 

suggest that Precambrian basement rocks contain a 

northeast-trending structural zone formed in Precambrian time before sedimentary rocks of Cambrian and 

younger age were deposited. This zone, termed the "Glade 

subsurface fault zone" by Shawe (1970, p. C2), passes 

beneath the south end of the Slick Rock district. It was a 

zone of significant deformation at times during the 

Paleozoic Era and throughout the Mesozoic Era. At the 

beginning of the Paleozoic Era, in the region surrounding the Slick Rock district, Precambrian igneous 

and metamorphic basement rocks were eroded to a nearly 

flat plain before sedimentary rocks were deposited. This 

Precambrian detritus probably was transported westward 

into the region of the present Great Basin where it

contributed to the thick upper Precambrian clastic 

sequence that was filling the newly forming Cordilleran 

geosyncline.

CAMBRIAN ROCKS AND EVENTS

Basal Cambrian rocks in the region are conglomeratic 

arkose deposited on weathered granitic Precambrian 

rocks; this relationship probably indicates that the baseleveled, subaerially weathered Precambrian rocks were 

gradually tilted and so parts of the region were lowered 

beneath the sea and were covered by coarse debris swept 

out from a nearby source area the ancestral Zuni and 

Uncompahgre highlands south and east of the district. 

Hematite stain in the sediments just above the 

Precambrian indicates that initially the sediments 

probably were laid down subaerially when some oxidation 

and redistribution of iron took place, or that, if deposited 

in water, the sediments later emerged and were oxidized. 

As indicated by southeastward thinning of the Cambrian 

rocks, tilting of the coastal plain was toward the northwest, and the source of the sediments lay to the southeast. 

The coarseness and freshness of the detritus suggest that 

the highlands adjacent to the region around the Slick 

Rock district tended to rise while that region subsided. 

The next higher Cambrian rocks in the region, however, 

were less coarse and include some shale, indicating that 

the source areas had become more stable and were being 

more deeply weathered, thus providing a greater 

proportion of argillaceous debris. Of course, distance from 

source, mode of transport, and environment of deposition 

may have been important factors in determining the 

nature of the deposited sediment, but these factors cannot 

be well evaluated on present knowledge. Interlayered fineto coarse-grained marine sandstone and conglomerate, 

shale, and dolomite above the basal arenaceous rocks in 

the region suggest a period of alternating uplift and 

quiescence of the adjacent highlands. During uplift coarse 

clastic material flooded the coastal plain in the region; 

during intervening periods of quiescence deep chemical 

weathering of the highlands probably took place and fine 

clastic and chemical sediments were deposited in the 

region. The dominance of shale, siltstone, and dolomite 

and the lesser amounts of sandstone in younger Cambrian 

rocks suggest that the highlands were less frequently 

uplifted during the later stages of the episode of 

denudation and nearby sedimentation, and the reddish 

colors of parts of these sediments suggest that deposition 

was partly subaerial or was in oxygenated tidal or 

mudflats marginal to the sea.

Deposition of Cambrian rocks in the region 

surrounding the Slick Rock district signifies eastward 

expansion of the Cordilleran miogeosyncline.

The advent of argillaceous sediments in the region near 

the end of the Cambrian indicates tectonic stabilization of 

the region. During the Ordovician and Silurian Periods 

either the region must have remained stable and no rocks
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY E3

|N O R T H D A K O T A^l M l N N

Front Range i 

highland I __ I -- -

I

^_______________ I

Boundary of 

Late Cretaceous 

geosyncline

Belt of

Triassic-JurassicCretaceous volcanisrn

500 MILES

0 500 KILOMETRES 

FIGURE 1. Location of the Slick Rock district, and major tectonic elements in the Western United States referred to in the text.

of these periods were deposited in the region or the thin 

sediments that were laid down were swept away before 

younger sediments could bury them.

DEVONIAN ROCKS AND EVENTS

Marine inundation in the Devonian Period resulted in 

the deposition of several hundred feet of argillaceous,

clastic, and carbonate sediments of Late Devonian age. 

The source areas to the east and south remained stable 

relative to the marine basin, as judged from the high 

proportion of chemical precipitates among these rocks. 

Mild downwarping of the region was accompanied by 

low-amplitude folding along northwest-trending axes. 

This folding foreshadowed the development of the
E4 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

109'

. 18W.

EXPLANATION

Surficial deposits not shown

Morrison Formation 

(Upper Jurassic)

Pre-Morrison rocks 

(Upper Jurassic to Permian)

Contact

Fault 

Dashed where indefinite

024 KILOMETRES \PCTLQ RESX:O 

MONTEZUMACO'

FIGURE 2. Generalized geologic map of the Slick Rock district
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY 

TABLE 1. Generalized section of consolidated sedimentary rocks

E5

Age Formation and member Thickness (feet) Description

Late Cretaceous

Early Cretaceous

Mancos Shale. 1,600-2, 300 Dark-gray carbonaceous calcareous marine mudstone and minor 

amounts of gray limestone, greenish-gray bentonitic shale, and 

light-buff sandstone. Contains abundant invertebrate fossils.

Dakota Sandstone.

.Unconformity.

Burro Canyon Formation.

120-180 Light-buff fine-grained quartzose marginal marine and fluvial sandstone, dark-gray carbonaceous shale, coal, and, locally, light-buff 

conglomeratic sandstone.

40-400 Light-gray to light-buff fine-grained to conglomeratic quartzose fluvial sandstone, greenish-gray flood-plain shale and siltstone, and 

greenish-gray to gray lacustrine(P) limestone and chert.

Late Jurassic

Jurassic and

Triassic(P)

Bushy Basin Member. 300-700 Predominantly reddish-brown and greenish-gray bentonitic floodplain mudstone and some light-reddish-brown, light-buff, and 

light-greenish-gray fluvial siltstone, sandstone, and conglomerate.

Salt Wash Member. 275-400 Light-reddish-brown, light-buff, and light-gray quartzose fluvial 

sandstone and reddish-brown flood-plain mudstone. Sandstone 

lenses in top part are dominantly light buff or light gray and 

contain abundant carbonaceous plant material; lenses in lower 

part are mainly light reddish brown and contain sparse carbonaceous material.

Junction Creek Sandstone. 20-150 Light-buff fine-grained quartzose sandstone with sweeping eolian 

crossbeds at south end of district. Changes northward to lightreddish-brown fine-grained tidal-flat(?) sandstone with horizontal 

beds; merges laterally with Summerville Formation.

Summerville Formation. 80-160 Reddish-brown quartzose horizontally bedded marginal marine siltstone and fine-grained sandstone.

re C 

3 2 

S-3 

c. c

Slick Rock Member. 70-120 Light-buff to light-reddish-brown fine-grained quartzose eolian 

sandstone.

Oewey Bridge Member. 

 Unconformity 

20-35 Reddish-brown very-fine-grained silty tidal-flat sandstone.

Navajo Sandstone. 0-420 Light-buff and locally light-reddish-brown fine-grained quartzose 

eolian sandstone.

Late 

Triassic(P)

Kayenta Formation. 160-200 Purplish-gray to purplish-red fluvial siltstone and sandstone, and 

locally shale, mudstone, and conglomerate.

Late Triassic

Wingate Sandstone, 200-400 Light-buff and light-reddish-brown very fine grained to fine-grained 

quartzose eolian sandstone.

Church Rock Member. 340-500 Reddish-brown, purplish-brown, and orangish-brown, mostly lacustrine, horizontally bedded quartzose sandstone, siltstone, and 

mudstone, and a minor amount of dark-greenish-gray conglomerate.

Petrified Forest(?) 

Member.

0-100 Greenish-gray fluvial and flood-plain mudstone, siltstone, and shale, 

and minor amounts of reddish-brown mudstone and greenishgray sandstone and conglomerate.

Moss Back Member. 20-75 Light-greenish-gray limy arkosic and quartzose fluvial sandstone 

and gray to greenish-gray limy fluvial sandstone and conglomerate, containing abundant carbonaceous plant material, and minor 

amounts of greenish-gray and reddish-brown, mudstone, siltstone, 

and shale.
E6 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

TABLE 1. Generalized section of consolidated sedimentary rocks Continued

Age Formation and member Thickness (feet) Description

Middle(?) and

Early Triassic

Pennsylvanian

Pennsylvanian

and 

Mississippian

0

«

1

mosa

<L> a

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

Upper limestone

1<u

X

 %

5

Lower limestone 

member.

>0-200

1,500-3,000

(240 exposed)

'130-240

1 1,000- 1,800

'300-500

>2,900-4,150

'50-200

400-150

'100

'240

'250-550

'500-700

Light-reddish-brown micaceous tidal-flat(P) siltstone and sandy

siltstone.

Reddish-brown, orangish-brown, and light-buff arkosic fluvial and

flood-plain sandstone, siltstone, mudstone, and shale.

overlying terrestrial Cutler Formation; and including marine 

evaporites.

Light- to dark-gray fossiliferous marine limestone interbedded with 

lesser amounts of gray, greenish-gray, and reddish-gray shale and 

sandstone.

interbedded.

drite, limestone, black shale, sandstone.

Dark- to light-gray marine anhydrite, gray dolomite and limestone,

dark-gray shale, gypsum, and halite, interbedded.

Medium-gray marine limestone; some thin dark-gray shale interbeds.

and gray limestone.

and reddish sandy shale

downward into light-gray to pinkish-conglomeratic arkosic sandstone.

'None exposed.

Paradox Basin and the northwesterly alined salt anticlines 

of the region. The orientation of fold axes suggests that 

downwarping was related to compression that caused the 

folding. In Late Devonian time the Antler orogeny was 

initiated in the region of the present Great Basin. The 

orogeny and the initiation of folding farther east in the 

region of the Slick Rock district may have been correlative 

events.

The facts that the source area of the youngest Cambrian 

rocks known in the region had become stable relative to 

the marine basin and that a hiatus of considerable 

duration developed with seemingly conformable relation 

between Cambrian and Devonian strata suggest a long

period of crustal stability in the region. Land areas in the 

region probably underwent prolonged weathering; 

evidence for this is the first sediments deposited in the area 

around the Slick Rock district following the long period of 

crustal stability. Arenaceous and argillaceous detritus in 

the lower part of the Devonian rocks, including that of the 

sandstone of the McCracken Member of Knight and 

Cooper (1955) of the Elbert Formation, indicates 

occasional uplift of die land areas peripheral to the region. 

These indications of crustal stability following the long 

hiatus support the most likely position of the OrdovicianSilurian-Early Devonian unconformity at the base of the 

Upper Devonian Aneth Formation of Knight and Cooper
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY E7

(1955) rather than between the Cambrian Ignacio Quartzite and younger rocks that are lithologically similar to the 

Ignacio (Baars, 1958, p. 95-97).

LOWER MISSISSIPPIAN ROCKS AND EVENTS

Deposition of marine carbonate rocks continued into 

the Mississippian Period. The relatively thin layer of 

Lower Mississippian strata the Leadville 

Limestone confirms the persistence of a miogeosynclinal environment. The source areas continued to be 

low stable land masses, which were attacked principally by 

chemical weathering. However, deposition of marine 

limestone during the Mississippian Period represents 

maximum transgression of Paleozoic seas into the region, 

and land sources were likely a great distance east of the 

continental shelf on which the carbonate sediments were 

deposited. Northwesterly folding in the region surrounding the Slick Rock district continued in Early 

Mississippian time, possibly in step with the culmination 

of the Antler orogeny in the region of the present Great 

Basin.

UPPER MISSISSIPPIAN AND PENNSYLVANIAN 

ROCKS AND EVENTS

Before deposition of Molas strata in the region around 

Slick Rock the sea regressed briefly, possibly in Late 

Mississippian time, and older rocks were eroded. Detritus 

from this erosion was incorporated in the basal marginal 

marine sediments of the Mississippian and Pennsylvanian Molas Formation. Northwesterly folding 

persisted into the Early Pennsylvanian, and in addition, 

deformation of the rocks took place along the northeasterly alined Glade subsurface fault zone passing 

through the south part of the Slick Rock district. Probably 

this was the first reactivation in Paleozoic time of the 

ancient Precambrian regional structure; this reactivation 

was characterized by strike-slip deformation, extending 

from the Colorado Plateau northeastward along the 

Colorado mineral belt in the Rocky Mountains. Folding 

along northwest-trending axes probably was related to 

horizontal movement on the northeasterly structure, 

particularly southwest of the northwesterly oriented 

Uncompahgre highland, in an incipient Paradox Basin. 

Folding occurred on the northwest side of the Glade subsurface fault zone as a result of horizontal compression; 

rocks slipped horizontally along the Glade zone and 

virtually no compressional deformation took place southeast of the zone.

Still in Early Pennsylvanian time, a shallow basin 

probably with limited connection to the seas farther west 

began to form in the region around the Slick Rock district 

which was then near the eastern edge of the miogeosyncline "shelf" of the Cordilleran geosyncline. 

Deposition of carbonate rocks continued, but in addition, 

deposition of organic shales began, indicating a stagnant 

bottom environment resulting from a nearly closed

depositional basin. These carbonate rocks and shales 

compose the lower limestone member of the Hermosa 

Formation and mark the onset of a depositional cycle that 

is identified with development of the Paradox Basin.

By Middle Pennsylvanian time the Paradox Basin was 

well established as a nearly closed or isolated basin 

dominated by a stagnant bottom environment. A few 

hundred feet of organic shale, carbonate, and evaporite 

beds was deposited in the basin, followed by many 

hundreds to several thousands of feet of dominantly 

evaporite strata, mostly halite. Evaporites accumulated to 

greatest thickness along the northeast edge of the Paradox 

Basin adjacent to the ancestral Uncompahgre highland 

which in Middle Pennsylvanian time was a low stable 

landmass succumbing chiefly to chemical weathering. An 

extended, rather slow and even subsidence of the Paradox 

Basin during deposition of the thick salt section was 

interrupted occasionally by slight uplift of surrounding 

lands that caused influx of detrital material into the basin. 

The grabenlike deepest part of the basin was bounded on 

the northeast side by a fault along the Uncompahgre highland and on the southwest side by a parallel fault of less 

displacement in the present position of the northeast side 

of Disappointment Valley. Evaporites of great thickness in 

this part of the Paradox Basin caused the later development of the Paradox fold and fault belt. Following 

deposition of salt, another relatively thin layer of stagnant 

bottom sediments organic shale, carbonate, and thin 

evaporite strata was deposited. Despite the conditions 

favorable for chemical sediment deposition, argillaceous 

sediments were derived from nearby deeply weathered and 

low-lying lands, such as the Uncompahgre, which had not 

yet assumed the topographic dominance that it would in 

later geologic periods. These Middle Pennsylvanian beds 

make up the Paradox Member of the Hermosa Formation. 

Coincident with deposition of the overlying rocks, the salt 

deposits of great thickness began to flow into the axial 

regions of anticlines, whose locations and orientations 

were first initiated by compression that had already 

established a northwest-oriented structural grain in the 

region. Once established, the anticlines continued to rise, 

in part because of the buoyant effect of low-density salt 

that occupied the cores of the anticlines. Renewed 

orogenesis in the region of the present Great Basin and in 

land farther west may have been related to folding in the 

Paradox Basin during the Middle Pennsylvanian.

UPPER PENNSYLVANIAN AND PERMIAN ROCKS 

AND EVENTS

Deposition of dominantly carbonate sediments the 

upper limestone member of the Hermosa Formation and, 

locally, the Rico Formation continued into Late 

Pennsylvanian time and Early Permian time. Probably the 

sea in the Paradox Basin was again more freely open to 

seas farther to the west. Folding continued, however, and
E8 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

may have been more pronounced because of the mobility 

of the great volume of salt in the section. Influx of 

abundant clastic material into the basin near the 

beginning of Permian time suggests uplift of land farther 

east; this was the onset of the period of main growth of the 

Uncompahgre highland.

After the Paradox Basin lost its closed character, and 

before the influx of vast quantities of detritus from the 

rising Uncompahgre landmass, marginal-marine 

deposition occurred. Free circulation of shallow aerated 

waters, possibly at the seaward edge of a coastal plain, 

permitted oxidation of argillaceous and carbonate 

sediments in the Rico Formation, with the result that 

much of the material deposited during this interval was 

dominantly reddish and yellowish. Some sporadic 

restriction of the outlet of the basin to the open ocean 

would have caused deposition of local layers of evaporites.

The restricted distribution of the Rico Formation in the 

vicinity of the Slick Rock district delineates a local basin of 

chemical sedimentation; this basin was surrounded by 

areas that received chiefly clastic material typical of most 

of the intertonguing and overlying Cutler Formation. 

Most of the Rico Formation is a transitional facies that 

records a gradual change from marine deposition that 

characterized the Hermosa Formation to continental 

deposition that typified the Cutler Formation. The upper 

contact of the Rico is therefore arbitrarily placed and even 

where the Rico has not been recorded the transitional 

nature of the Hermosa-Cutler boundary is generally 

evident.

. Much of the remainder of the Permian Period saw the 

rapid subsidence of the Paradox Basin region and the rise 

of the Uncompahgre, which caused a great wedge of clastic 

debris the Cutler Formation to spread westward. At its 

eastern limit, deposition of the Cutler was chiefly 

subaerial. Farther west, the debris becomes increasingly 

finer grained, and in the eastern part of the region of the 

present Great Basin, sediments that were deposited at this 

time are fine-grained clastic strata that are interlayered 

with miogeosynclinal marine carbonate rocks. In Utah, 

the presence of evaporites in a restricted area in the Cutler 

attests to a local closed basin similar to those basins in 

which Rico sediments accumulated. The sudden and 

rapid uplift of the Uncompahgre highland in Cutler time, 

together with the buildup of an alluvial plain sloping 

southwestward from the highland, displaced the 

Cordilleran geosyncline westward. Also during the time 

that thick layers of arkosic debris were shed from the 

rapidly rising Uncompahgre, the salt anticlines were 

folded strongly, and large-scale deformation occurred 

along the Glade subsurface fault zone. A causal relationship among all three contemporaneous events seems 

plausible. Compression continued to shorten the crust on 

the northwest side of the northeast-trending structure; this 

deformation was permitted where comparable deformation did not occur southeast of the northeast-trending 

structure by strike-slip movement on the Glade subsurface fault zone. Rapid uplift of the Uncompahgre highland and folding of the salt anticlines may have been 

corollary responses to crustal shortening. Termination of 

the southeast end of the Paradox fold and fault belt against 

the Glade zone seems a natural consequence of the 

suggested deformation. Structural events in the Paradox 

Basin region may be correlative with recurrent deformation in the region of the Cordilleran geosyncline during 

the Permian Period.

TRIASSIC ROCKS AND EVENTS

After deposition of thick layers of arkosic debris during 

the Permian Period, uplift of the Uncompahgre source 

area ceased and erosion removed the upper part of the sedimentary pile. The salt anticlines continued to grow 

during this episode, and most of the axial parts of the anticlines were beveled by erosion. Fine clastic debris, largely 

derived from erosion of underlying Permian sediments, 

spread westward into a marginal marine environment 

which was dominant during Early and Middle(?) Triassic 

time, to make up the Moenkopi Formation. Stewart and 

others (1959, p. 499) stated that the eastern part of the 

Moenkopi was "derived from the granitic Uncompahgre 

highland" and "partly from the conglomeratic facies of 

the underlying Cutler formation" and was deposited 

under marginal-marine and tidal-flat conditions. Some of 

the detritus was deposited in the region around Slick 

Rock, but locally it was washed away to the west at a later 

time, as indicated by the absence of exposures of the 

Moenkopi at the surface in the district but by the presence 

of the Moenkopi in the subsurface nearby. Lower and 

Middle(?) Triassic rocks were beveled by considerable 

erosion preceding deposition of Upper Triassic sediments in the region that surrounds the Slick Rock district. 

Apparently, mild folding of the salt anticlines and 

especially deformation along the Glade subsurface fault 

zone continued during this time, as indicated by thick 

Moenkopi preserved along the zone (Shawe, 1970, fig. 3).

The region surrounding the Slick Rock district was 

largely emergent in Late Triassic time, and several 

hundred feet of largely fine grained continental fluvial 

and lacustrine sediments was brought in from the east, 

southeast, and south (Stewart and others, 1959; 1972) and 

was deposited as the Chinle Formation. Uplift of land east 

of the district may have centered on the ancient Front 

Range highland; uplift of land to the southeast may have 

centered on the Zuni highland or the area coinciding with 

the southeast part of the Uncompahgre highland; that in 

the south centered on the ancient Mogollon highland. As 

indicated by the general coarseness of some layers in the 

lower part of the Chinle, either the source area was 

probably not far distant or the material was transported 

rapidly from the source. Sediments in the lower part of the
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY E9

Chinle (Moss Back and Petrified Forest? Members) were 

derived from a source area that contained granitic, metamorphic, and sedimentary carbonate and argillaceous 

rocks. Interbedded crossbedded sandstone and conglomerate layers and horizontally bedded mudstones and 

bentonitic shales are interpreted as representing fluvial 

channel-fill deposits and lacustrine or marsh deposits 

with some layers consisting of ash-fall material probably 

derived from volcanic island arcs west of the area of Upper 

Triassic sediments. The large proportion of quartzose 

clastic sediments as compared to limy clastic debris in the 

middle part of the Chinle (units 1 and 2 of the Church 

Rock Member) probably indicates a smaller amount of 

carbonate sediments in the source area, but occasional 

beds of limestone conglomerate indicate short intervals of 

time when carbonate sediments, probably of Paleozoic 

age, were being eroded in the source area. Volcanic 

material continued to make up an appreciable part of the 

sediments, not only in the sandy layers, such as part of the 

so-called Black Ledge, but also in bentonitic beds in the 

middle part of the Chinle. Fluvial deposition still was 

considerable during deposition of the middle part of the 

Chinle, but perhaps lacustrine or marsh deposition was 

predominant. Smaller amounts of clayey sediments in the 

upper part of the Chinle (unit 3 of the Church Rock 

Member) indicate less volcanic ash-fall material, although 

some beds no doubt had such an origin. Some fluvial 

deposition occurred from time to time in different places, 

but lacustrine deposition appears to have been the most 

widespread. Also near the end of Chinle time some thin 

layers of crossbedded dune sands which are interbedded 

with possibly lacustrine deposits and which are lithologically similar to the Wingate Sandstone that overlies 

the Chinle were deposited in advance of the main period of 

dune deposition that marked the formation of the 

Wingate.

The central part of the Late Triassic sedimentary basin 

lay southwest of Slick Rock in southern Utah and 

northern Arizona, as indicated by the thickening of these 

rocks into that area. Also, the basin seems to have filled 

gradually from the south, as indicated by progressively 

younger units at the base of the Upper Triassic farther 

north.

Local accumulations of abundant carbonaceous 

material (carbonized plant remains) in some of the coarser 

beds in the Upper Triassic, especially the Moss Back 

Member of the Chinle, caused diagenetic changes in the 

rocks that reflect a reducing environment. Most of the 

Upper Triassic rocks were subjected to an oxidizing diagenesis, with the result that they are dominantly of redbeds type. At some time after the development of the diagenetic types of lithology, epigenetic alteration, and 

deposition of uranium-vanadium deposits in parts of the 

Moss Back Member that exhibit carbon facies lithology, 

occurred when altering solutions from an extraneous

source moved through the beds. Inadequate information 

prevents a determination of the time of the epigenetic 

alteration and the source of the altering solutions.

During and following the deposition of fluvial, 

lacustrine and marsh deposits of Late Triassic age the salt 

anticlines were again folded. In addition, after these rocks 

were deposited, renewed movement on the Glade subsurface fault zone that passes through the south end of the 

Slick Rock district caused local uplift and erosion of strata. 

Again, the apparent contemporaneity of deformation of 

the anticlines and the northeast-trending structure points 

to causal relationship: slippage took place along the 

northeast-trending Glade zone and folding of rocks 

occurred on the northwest side of the zone.

The land was close to sea level near the end of Late 

Triassic time, following local erosion of Upper Triassic 

sediments. Fine-grained detrital material probably derived 

from the east was reworked by onshore winds in vast dune 

fields adjoining tidal flats farther west, to form the 

Wingate Sandstone. According to Harshbarger, 

Repenning, and Irwin (1957, p. 23), the Wingate in the 

vicinity of what is now northeastern Arizona derived 

source material from the east; this material consisted 

dominantly of reworked older sediments, probably 

Triassic, and some new material from metamorphic 

terranes, which perhaps included the Uncompahgre 

highland. Stewart and others (1959, p. 524, 525) indicated 

that the Wingate was deposited by winds that blew 

dominantly southeastward. Minor oscillations in sea 

level, or perhaps transitory recurrence of shallow lakes, 

resulted in frequent inundation of the dune fields and so 

the dunes were leveled. Subsequent withdrawals of water 

permitted renewal of the formation of dunes as more 

material was carried in from the east, until several hundred 

feet of eolian deposits had piled up. During this time, or 

shortly following it and prior to deposition of younger 

sediments, deformation occurred on the Glade subsurface 

fault zone, and mild folding of the salt anticlines resumed.

The close of the Triassic Period saw uplift of land northeast of the region around Slick Rock, which caused influx 

of coarse clastic sediments, carried mostly by streams. A 

source for these sediments is inferred to the northeast on 

the basis of both sedimentary structures and lateral 

differences in composition of the sediments that make up 

this Upper Triassic(P) deposit, the Kayenta Formation 

(Stewart and others, 1959, p. 524). The region was covered 

by a thin blanket of fluvial channel-fill and crossbedded 

sediments as far south as the position of the Glade subsurface fault zone. If sediments were deposited farther 

south, they were later eroded. Deformation along the 

Glade zone recurred, and mild folding of the salt anticlines continued. If the salt anticlines were rising during 

Kayenta time, they must have formed only islands in the 

depositional basin, thus permitting transportation of sediments south westward into the district. Perhaps most of the
E10 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

Kayenta was deposited before the anticlines resumed 

growth; cyclic growth of the salt anticlines may have been 

related to more wide-scale tectonic events which in turn 

governed changes in lithology of successive formations.

TRIASSIC(?) AND JURASSIC ROCKS AND EVENTS

After accumulation of the thin sequence of fluvial strata 

making up the Kayenta Formation of Late Triassic(P) age, 

probably when the highland source of the fluvial sediments finally wore away, conditions reverted to those near 

the end of Wingate time. A second great wedge of eolian 

sand, the Navajo Sandstone, was deposited during the 

Triassic(P) and Jurassic. Its eastern limit now is not far east 

of Slick Rock, and its greatest thickness of a few thousand 

feet is at the center of the basin far to the west near southern 

Nevada. According to Harshbarger, Repenning, and 

Irwin (1957, p. 25), the Navajo Sandstone of northeastern 

Arizona was derived from a western source; as suggested by 

Stewart and others (1959, p. 525), largely on the basis of 

dominant cross-strata orientation, the source for the 

Navajo of southeastern Utah was to the northwest. 

Perhaps the higher feldspar content of the Navajo 

Sandstone in southwestern Colorado indicates a closer 

source, to the east or northeast. Streams may have brought 

detritus from the east that became distributed in a lowland, 

water-saturated environment and that was subsequently 

reworked by westerly winds. Horizontal layers that alternate with crossbedded layers and that are common in the 

Navajo likely reflect periods of subaqueous (lagoonal?) 

deposition; some transport of material from source areas 

may have been by water. Upon withdrawal of water, 

westerly winds reworked the detritus into dunes.

The salt anticlines again were folded appreciably, after 

deposition of the sand, and in many places on the crests of 

the anticlines the entire thickness of several hundred feet of 

eolian strata of the Navajo was removed by erosion. 

Erosion occurred in other regions outside the Paradox fold 

and fault belt. The general uplift thus implied by 

widespread erosion must have been related to the events 

that terminated the influx of sand into the region, as well 

as to the renewed folding of the salt anticlines.

Deformation in the western part of the region of the 

present Great Basin resumed its intensity during the Early 

Jurassic, and deformation recorded in the Colorado 

Plateau during this time possibly was related to it.

The sedimentary basin west of the Slick Rock district in 

the western part of the Colorado Plateau subsided after 

erosion of lower Middle Jurassic rocks and was inundated 

by the sea near the end of Middle Jurassic and at the start of 

Late Jurassic time. During deposition of limy marine 

strata of the Carmel Formation in a north-trending trough 

in central Utah in Middle and Late Jurassic time, reddishbrown silty rocks of the Dewey Bridge Member of the 

Entrada Sandstone (the red earthy siltstone unit of Wright 

and Dickey (1958) or medial silty member of Harshbarger,

Repenning, and Irwin (1957)) were deposited in the 

vicinity of the district (Wright and others, 1962) on tidal 

flats and in lagoons near the eastern boundary of the sea. 

Local cherty limestone pans likely were deposited in freshwater lakes but may have been formed in erosional 

hollows on top of the Navajo Sandstone or may have been 

deposited during Navajo time as were similar limestone 

layers in eastern Utah (Wright and Dickey, 1957, p. 352). 

Because of resistance to erosion, those pans survived in 

places at the top of the remnant Navajo. Before the main 

deposition of silty Dewey Bridge sediments, some of the 

troughs in the eroded unconformable Navajo surface 

became filled with lag gravels that formed conglomeratic 

sandstone containing fragments of the Navajo Sandstone 

as well as pebbles of chert presumably eroded from 

scattered limestone layers not far distant. Such deposition 

apparently partly filled up irregularities in the old Navajo 

surface to form a nearly flat surface which received the silts 

typical of the Dewey Bridge. Horizontal and massive 

bedding of the siltstone and sandstone of the Dewey Bridge 

Member is here interpreted as indicating a tidal-flat or 

lagoonal environment of deposition. Position of the Slick 

Rock district near the eastern margin of the Carmel 

depositional basin, as well as uplift of rocks to the east, 

points to an eastern source for the Dewey Bridge Member.

The massive and horizontally bedded character of the 

massive unit of the Slick Rock Member of the Entrada 

Sandstone is suggestive of generally tidal-flat or lagoonal 

conditions of deposition similar to those envisaged for the 

underlying Dewey Bridge Member. A few scattered layers 

of eolian-type crossbedded strata in the massive unit 

indicate that during deposition parts were at times above 

water and dried out, and therefore small areas of 

windblown dune sands developed. These all appear to 

have been ultimately leveled at the top, and many 

probably were destroyed, by submergence in lagoonal 

waters. Eventually, much of the area of the Slick Rock 

district was drained of water and dried out and so dune 

sands could again accumulate. This period saw the 

deposition of the middle (crossbedded) unit of the Slick 

Rock Member which, as indicated by Shawe, Simmons, 

and Archbold (1968, fig. 20), was built up in increments 

comprising individual dune layers that were periodically 

submerged briefly and at that time were beveled at the top 

by water. In a few places where dunes were not submerged, dune layers were swept away, probably by wind 

action (deflation), and were replaced by windblown sand. 

Toward the close of the period of Slick Rock Member 

deposition the entire Slick Rock district was submerged, 

and the uppermost dune layer was beveled and reworked. 

Much of the upper (horizontally bedded) layer of the Slick 

Rock Member consists probably of reworked dune sands 

that were brought in from outside the district after the first 

horizontal bed covered the beveled dunes. Depositional 

conditions at the end of sedimentation of the horizontally
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY Ell

bedded layer may have changed gradually to develop the 

alternating layers of shale and sandstone of the lower part 

of the Summerville Formation. More likely, however, a 

change in conditions or in position of the source area 

caused periodic and increasing influxes of clay with the 

sediments, resulting in progressively more abundant shale 

upward and transition into the Summerville Formation. 

The source area of the Slick Rock Member suggested by 

southeastward thinning of the member was generally 

southeast of the district. Wright (1959, p. 61) indicated that 

the direction of transport of Entrada Sandstone and the 

Carmel Formation was from the east. During deposition 

of the crossbedded unit, minor amounts of airborne 

material of volcanic origin apparently were laid down in 

the district. Perhaps some volcanic material is also present 

in the other units of the Entrada. Cadigan (1958, p. 48) 

indicated the presence of important amounts of volcanic 

material in equivalent rocks in south-central Utah and 

suggested that volcanic material in the Slick Rock had a 

westerly source.

Ultimately, subsidence exceeded the accumulation of 

eolian sand, and clayey marine strata of the Summerville 

Formation were deposited widely in the region of the 

Colorado Plateau. Thin horizontal bedding and 

oscillatory ripple marks that widely characterize the 

Summerville suggest a shallow marine or a tidal-flat 

environment of deposition. The dominant easterly 

orientation of ripple-mark troughs indicates chiefly 

northward or southward wave motion and implies either 

northerly or southerly winds. Because of the probable 

marine origin of the Summerville and associated Curtis 

Formation farther west in Utah (Gilluly and Reeside, 

1928), the source landmass undoubtedly lay other than to 

the west.

Harshbarger, Repenning, and Irwin (1957, p. 48, 51) 

indicated that the southward equivalent of the Junction 

Creek Sandstone, the Bluff Sandstone, formed as barchan 

dunes and moved northward from the main area of 

deposition of the Cow Springs Sandstone in northeastern 

Arizona. Certainly the great sweeping crossbeds displayed 

by the Junction Creek south of the Slick Rock district 

suggest origin in immense barchan dunes. The movement of dunes northward terminated in the southern part 

of the Slick Rock district, where the Junction Creek 

merges with the Summerville Formation. As suggested by 

Shawe, Simmons, and Archbold (1968, pi. 6), individual 

dune layers were probably reworked by the marine waters 

in which the Summerville was laid down, as evidenced by 

the change northward from the crossbedding to the horizontal bedding of the individual layers. Water probably 

periodically covered the dune layers, beveled them, and 

then deposited horizontally bedded clayey layers.

Material derived from the initial and farthest advance of 

the dunes probably was reworked for a long time and was 

spread northward by wave action and currents in the

Summerville sea, or simply was blown northward and 

dropped in tidal estuaries, to form a "marker bed."

Two major events outside the Colorado Plateau near the 

end of Late Jurassic time strongly influenced sedimentation within the Plateau. A large landmass rose in westcentral Arizona and southern California possibly a 

westward extension of the Mogollon highland. It exposed 

to erosion clastic sedimentary rocks probably of Permian, 

Triassic, and Early and Middle Jurassic ages, detritus from 

which was shed northward, northeastward, and eastward 

onto the level plain formed of the Late Jurassic marine and 

tidal-flat strata. A second event, concurrent with uplift of 

the landmass, was vigorous volcanic activity that began in 

the island arcs west of the Jurassic basin. Prevailing 

westerly winds carried volcanic ash into the basin and 

deposited it in small to large amounts at various levels in 

the sediments that were laid down during the rest of Late 

Jurassic time.

The Salt Wash Member of the Morrison Formation was 

deposited in the region of the Colorado Plateau in Late 

Jurassic time. As Craig and others have .pointed out (1955, 

p. 150), "the Salt Wash was derived mainly from older sedimentary formations and received only minor contributions from igneous and metamorphic rocks." It was 

deposited by aggrading streams as a broad fan-shaped 

alluvial plain. The source area of the Salt Wash, indicated 

by orientation of sedimentary structures, by facies 

changes, and by the position of the apex of the alluvial fan, 

was probably in west-central Arizona and southeastern 

California (Craig and others, 1955, p. 150-155). 

Sedimentary rocks in the provenance of the Salt Wash 

apparently were chiefly clastic, and these were probably 

derived in an earlier cycle of erosion from nearby igneous 

and metamorphic rocks of late Precambrian age. The 

conglomerate facies of the Salt Wash in Utah, however, 

contains very abundant silicified carbonate rock pebbles, 

indicating abundant carbonate rocks in the Salt Wash 

provenance (T. E. Mullens, written commun., 1973). 

According to Bowers and Shawe (1961, p. 184-186, table 2) 

zircons in the Salt Wash Member, chiefly the ore-bearing 

sandstone, consist of principally colorless spheroidal and 

egg-shaped grains and of small amounts of colorless worn 

euhedral and subhedral crystals, pink spheroidal and eggshaped grains, colorless sharply euhedral crystals, and 

pink worn euhedral and subhedral crystals. The pink 

spheroidal and egg-shaped grains were almost certainly 

derived from Precambrian rocks, and so also were the 

colorless spheroidal and egg-shaped grains, judged from 

their extreme roundness and the late PrecambrianCambrian average age of Morrison zircons (Shawe and 

others, 1968, table 4). The occurrence of the zircons 

indicates that more than 90 percent of the ore-bearing 

sandstone was derived ultimately by erosion of Precambrian rocks. Colorless sharply euhedral zircons
E12 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

probably had a volcanic origin, and colorless worn 

euhedral and subhedral crystals likely were derived from 

erosion of volcanic rocks.

Possibly the sedimentary rocks that were the immediate 

source of the Salt Wash deposited in the vicinity of the 

Slick Rock district were the Triassic Chinle Formation, 

inasmuch as the Chinle is known to underlie Jurassic 

rocks in the general source area of the Morrison 

Formation. (For example, see McKee and others, 1956, pi. 

2.) Although older Jurassic rocks may also have been 

possible sources of the Salt Wash, grain size, sorting, and 

composition of such formations generally do not match 

those characteristics of the Salt Wash, suggesting that they 

were not a likely source. Rocks older than Triassic that 

may have been exposed in the source area and that are a 

likely source of the Salt Wash are clastic red beds of 

Permian age (described by Longwell, 1949, p. 930). Other 

older strata that may have been exposed in the general area 

are chiefly carbonate rocks, and silicified parts of these 

may have been sources of some Salt Wash detritus.

Just before deposition of Salt Wash sediments, the 

present area of the member must have been virtually a flat 

plain underlain by horizontal strata of the Summerville 

Formation. As interpreted from the data of Craig and 

others (1955), uplift of land in the vicinity of west-central 

Arizona and southern California instigated movement of 

detritus northward, northeastward, and eastward into the 

area of the Salt Wash, forming a great alluvial fan, or 

aggrading plain, with apex near the source area. If the 

interpreted source area is assumed to have been the sole 

provenance of the Salt Wash, irregularities in thickness of 

the alluvial fan reflect structural modifications during 

deposition. Evidence that post-Salt Wash erosion was 

negligible precludes attributing observed thickness 

variations to structural modification after the member was 

deposited and before overlying sediments were deposited. 

Shawe (1962) inferred that thickness variations in the Salt 

Wash Member indicated local subsidence of an area 

roughly coinciding with the Paradox Basin but 

encroaching northeastward into the area of the 

Uncompahgre highland. Subsidence of .the area 

approximating the Paradox Basin caused the development of a smaller alluvial fan upon the much larger fan of 

the Salt Wash Member. The apex of the smaller fan is in a 

position that may have permited "capture" of a main 

trunk of Salt Wash distributary streams. Significantly the 

area of the smaller alluvial fan coincides with the area of 

principal development of lithologies that were favorable 

for deposition of uranium-vanadium deposits in the Salt 

Wash Member. (See Craig and others, 1955.) The toe of the 

smaller fan is the position of the Uravan mineral belt, 

which extends northward from Slick Rock. The belt itself 

is characterized by generally continuous ore-bearing sandstone that contains abundant carbonaceous material and 

is typically crossbedded and abounding in cut-and-fill

structures. Eastward from the position of the mineral belt, 

strata at the horizon of the principal (topmost) ore-bearing 

sandstone are more flatbedded and contain a higher 

proportion of mudstone, suggestive of deposition under 

conditions of quieter water. (For example, see Fischer and 

Hilpert, 1952, p. 6; McKay, 1955, p. 272 and pi. 13.) The 

area within the basin east of the Uravan mineral belt is 

thus interpreted as containing sediments of flood-plain 

and perhaps lacustrine type; those in the mineral belt and 

westward to the end of the basin are interpreted as being 

fluvial deposits formed on a smaller aggrading fan 

localized on the larger Salt Wash alluvial fan by local 

subsidence during Salt Wash deposition.

According to L. C. Craig (written commun., 1961) the 

regional gradient of Salt Wash streams was probably 

about 5 feet per mile. As determined in a very small area in 

the Slick Rock district, at the Cougar mine (Shawe and 

others, 1968, fig. 31), the local gradient of streams just 

before deposition of the ore-bearing sandstone was 

approximately 50 feet per mile. This gradient is a rough 

estimate, and probably high, inasmuch as the area from 

which it was determined is small. Nevertheless, it suggests 

that toward the end of Salt Wash deposition the local 

gradient of streams in the vicinity of the district was higher 

than the regional gradient. It may indicate that the 

proposed smaller fan that was localized on the larger 

regional Salt Wash fan presented a slightly different 

environment of deposition, and this aberrant environment was partly responsible for later deposition of 

uranium-vanadium ore deposits.

Individual sandstone layers in the Salt Wash Member 

probably were reworked after initial deposition, and the 

ore-bearing sandstone in the Uravan mineral belt area may 

have been reworked extensively. Widespread scour-andfill features in the relatively thin and laterally persistent 

upper unit indicate extensive reworking without addition 

of much new sediment. The ore-bearing sandstone 

interval likely -was first deposited rapidly as sand and 

minor amounts of mud, after an abrupt and short-lived 

subsidence of the area approximating the Paradox Basin. 

These strata composed an alluvial fan whose average 

gradient may have been as much as 50 feet per mile, or 

about a degree, and whose toe occupied the position of the 

Uravan mineral belt. During a following period of 

quiescence the strata were considerably reworked by distributary and braided streams coursing down the fan, 

developing additional and extensive scour-and-fill 

characteristics and winnowing out much clay that was 

carried eastward into the local basin.

Abundant vegetation probably covered the whole of the 

area of Salt Wash deposition. The abundance of vegetation is now evidenced only in strata that were submerged 

beneath the water table just after deposition; lack of 

oxygen in such an environment permitted plant debris to 

be preserved.
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY E13

Restriction of carbonaceous.material dominantly to the 

area of the Uravan mineral belt seems anomalous in some 

respects. For example, locally at the west edge of the 

mineral belt carbonaceous material disappears abruptly in 

sandstone that laterally shows no other detectable lithologic or structural differences (John E. Motica, oral 

commun., 1961). Drilling done by the U.S. Geological 

Survey in the western Dissappointment Valley area shows 

that the ore-bearing sandstone in one place where it is 

about 30-40 feet thick changes from abundantly 

carbonaceous to noncarbonaceous within 1,000 feet 

laterally. Carbonaceous material may have been originally 

abundant throughout the ore-bearing sandstone, and may 

have been destroyed in many places some time after 

deposition, and preserved locally, as in the Uravan 

mineral belt. R. P. Fischer has suggested (oral commun., 

March 1961) that places in which carbonaceous material 

survived (that is, was not oxidized) were below the water 

table just after deposition, such as at the foot of an 

aggrading slope; upslope above the water table, aeration 

allowed oxidation and destruction of carbonaceous 

material. Such a mechanism readily accommodates itself 

to the conditions pictured for an alluvial fan, the toe of 

which may have been close to or under the water table that 

occupied the subsided area which nearly coincides with 

the Paradox Basin. Upper reaches of the fan, being alternately wet and dry, lost most of the original carbonaceous 

material by oxidation, although local accumulations of 

organic debris may have survived in perched water tables. 

The toe of the fan retained its carbonaceous material, 

being effectively submerged beneath the water table in 

most places. Perhaps absence of carbonaceous material in 

the upper part of the ore-bearing sandstone in many places 

and its abundance in the lower part can best be explained 

by oxidation above a water table. General absence of 

carbonaceous material east of the Uravan mineral belt may 

have resulted from nondeposition due to change of conditions eastward from the toe of the fan. Coarse plant 

material that was moved easily by streams on the 

aggrading fan may not have been transported farther 

eastward into the low-gradient flood-plain and lacustrine 

environment.

Diagenetic alteration of two types, related to presence or 

absence of carbonaceous material, affected the fluvial 

strata of the Salt Wash Member, commencing almost at the 

time they were deposited. Generally the beds were subjected to alternate wetting and drying and thereby became 

rapidly oxidized. Plant material that was buried with the 

sediments was destroyed, and black opaque minerals, 

largely magnetite and ilmenite, were oxidized to hematite 

and leucoxene, still retaining their detrital form. At the 

same time that the black opaque minerals were oxidized, 

some iron moved out of them and was redeposited as a 

hematite film on other detrital grains, or was dispersed as 

dustlike hematite throughout finely divided clayey

material. If the redeposited iron was in the form of a 

hydrated oxide it was later dehydrated, inasmuch as all the 

reddish stain in these rocks now appears to be hematite. 

Where abundant carbonaceous material survived in the 

fluvial sediments beneath the water table, diagenesis 

proceeded differently. In this environment the black 

opaque detrital minerals were not oxidized, but some of 

them were destroyed by reduction and the iron was reprecipitated in pyrite.

After subsidence of the area that nearly coincides with 

the Paradox Basin; renewed folding of the salt anticlines 

occurred; the subsidence and folding may have been partly 

a result of compression. Southwest of the basin and nearly 

parallel to it is a west-northwest-alined uplift (Shawe, 

1962, fig. 62.2); both basin and uplift may represent gross 

folding of deeper rocks through compression. Growth of 

the salt anticlines may have been merely triggered by such 

stress, and was perpetuated by salt flow resulting from differential loading by overlying strata. Sand of the lower 

unit of the Salt Wash was probably buried rapidly beneath 

the mud of the middle unit, because it did not receive the 

extensive reworking that affected the sand of the upper 

unit. Subsidence of the basin continued to a minor extent 

during deposition of the middle unit, but it may have been 

neither as great nor as rapid as during deposition of the 

lower unit. Folding of the salt anticlines was retarded, and 

may have consisted solely of local growth of salt cupolas, 

as suggested by the localization of a thinner middle unit 

along one segment of the Dolores anticlinal axis. Slower 

subsidence of the basin may account for the much higher 

proportion of mudstone in the middle unit. Deposition of 

the upper unit followed an abrupt but minor subsidence of 

the basin, and it was not buried immediately by mud as 

was the lower unit. Instead, relative quiescence in the 

basin permitted considerable reworking of the upper unit 

on the alluvial fan that filled the west end of the basin. 

Flood-plain and lacustrine conditions farther east 

permitted deposition of more evenly bedded sand, with a 

higher proportion of mud, partly derived from clay 

winnowed from the reworked upper unit on the fan.

Craig and others (1955, p. 157) concluded that the 

coarser clastic sediments of the Brushy Basin Member 

overlying the Salt Wash Member of the Morrison Formation "were derived mainly from a source area of 

sedimentary rocks." The position of the source area is not 

clearly evident, but was suggested by Craig and coworkers 

to have been generally that of the Salt Wash source area 

(1955, p. 157). Cadigan (1967, p. 84) suggested that much of 

the volcanic material in the Brushy Basin originated from 

a westerly or southwesterly source, inasmuch as the percentage of this material in the member increases generally 

in those directions.

Deposition of sediment continued without significant 

interruption following the formation of the ore-bearing 

sandstone of the Salt Wash Member, but at that time the
E14 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

area in eastern Utah and western Colorado approximating the Paradox Basin had slowed its rapid subsidence, and again became nearly filled with dominantly 

mud and silt strata deposited under flood-plain conditions. The salt anticlines continued to be active near the 

end of Late Jurassic time, but their growth was restricted 

largely to individual cupolas. For example during the 

deposition of several hundred feet of the Brushy Basin 

sediments, considerable salt flowed from beneath the axis 

of the Disappointment syncline into a large cupola underlying the adjacent Gypsum Valley anticline. The 

relatively rapid flow of evaporites from the synclinal area 

resulted in basining which induced diversion of streams 

into the low area and concomitant deposition of abundant 

sand along the synclinal axis, whereas deposition of mud 

and silt dominated elsewhere. Flow of salt probably 

slowed somewhat after initiation of Brushy Basin sedimentation, so, although folding continued, the ground 

surface remained level and streamflow was not confined to 

the synclinal axial regions. Folding of the salt anticlines 

accompanied a general downwarping of the Paradox 

Basin, and at the same time deformation continued along 

the northeasterly Glade subsurface fault zone.

In the Slick Rock district the lower brown unit of the 

Brushy Basin records an influx of volcanic material that 

diluted sediments typical of the underlying Salt Wash 

Member. Sporadic lenses of chert-rich conglomeratic 

sandstone mostly in the upper part of the lower brown 

unit may reflect addition of material derived from newly 

elevated areas of volcanic rocks or from sedimentary 

terrains west of the Colorado Plateau. The coarser rocks in 

the lower part of the Brushy Basin suggest occasional 

accelerated uplift of source areas on the western periphery 

of the plateau.

The middle green unit of the Brushy Basin contains 

abundant volcanic material derived both from ash falls 

and from erosion of volcanic source areas. Nearly half the 

material in the unit is detritus derived from the erosion of 

older sedimentary rocks. The abundance of clastic lenses 

in the middle green unit reflects marked uplift in the 

provenance of the Brushy Basin which possibly coincided 

with the advent of volcanic activity in these areas.

During deposition of the upper brown unit the amount 

of coarse clastic sediment in most places decreased 

appreciably, and probably the amount of volcanic 

material in the fine sediments was less than that in the 

middle green unit. Bentonitic clay seems to be less 

abundant in the upper brown unit, especially near the top, 

than in the middle green unit. Much of the fine clastic 

material in the upper brown unit was probably derived 

from erosion of sedimentary terranes, and less than half 

the unit consists of volcanic material. Near the end of 

Brushy Basin sedimentation, quartzose detritus similar to 

that which characterizes the overlying Burro Canyon 

Formation began to come into the region around the Slick

Rock district. This detritus is local in distribution and is 

evident chiefly in greenish-gray parts at the top of the 

upper brown unit. The new material signifies the advent 

of another source area which became dominant in Burro 

Canyon time probably a more southerly provenance 

nearly devoid of volcanic rocks. Craig (1961, p. 1584) cited 

evidence for northward-flowing streams that deposited the 

Burro Canyon. This gradual shifting of source areas was 

the main factor in the change from Morrison to Burro 

Canyon lithology; the general depositional environment 

changed little, with the result that the contact between the 

formations is in many places arbitrary and difficult to 

place because of intertonguing or transitional relations. 

The greenish-gray or brownish-gray rocks in the Brushy 

Basin Member show more evidence of volcanic origin than 

do the reddish-brown rocks. Nevertheless, these rocks 

include abundant permeable sandstone layers that have 

been altered by reducing solutions as indicated by 

sparsity of detrital black opaque minerals suggesting 

that much of the greenish-gray and brownish-gray colors 

of the Brushy Basin resulted from epigenetic alteration.

CRETACEOUS ROCKS AND EVENTS

Terrestrial sedimentation in the region surrounding the 

Slick Rock district continued uninterrupted into Early 

Cretaceous time. The great abundance of coarse clastic 

material deposited in the Burro Canyon Formation during 

that time resulted from the rise of land farther south of the 

district and east of the source area of the Morrison 

Formation. Abundant scour-and-fill structures in sandstone strata in the Burro Canyon Formation show that 

these rocks were deposited under fluvial conditions. 

Mudstone layers interbedded with the sandstone are interpreted to be lacustrine or flood-plain deposits similar to 

those in the Morrison Formation. The much higher 

proportion of sandstone to muds tone in most of the Burro 

Canyon as compared to the underlying Morrison indicates 

greater influx of coarser clastic material, with no 

appreciable difference in the environment of deposition. 

The new source area to the south provided a great 

abundance of quartzose debris that became intermixed 

with the diminished sediment from the source area of the 

Morrison Formation farther southwest and west. 

Common chert-rich conglomerate layers in the formation 

may have been derived from sedimentary rocks of the 

southern source area.

Subsidence of the Paradox Basin ceased during Burro 

Canyon time, or at its close and prior to deposition of 

overlying strata, for erosion almost everywhere beveled the 

top of the Lower Cretaceous rocks. Even though basin subsidence had come to a standstill, some growth of the salt 

anticlines continued. Movement of salt was local, 

however, such as the pronounced flow from beneath a 

segment of the Disappointment syncline into the large salt 

cupola underlying the Gypsum Valley anticline. Because
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY E15

only slight tectonic activity occurred in the Colorado 

Plateau region during the Early Cretaceous, the local 

development of the salt cupolas presumably was a result of 

the buoyant flow of salt responding to differential load 

stresses.

Thin-bedded and evenly bedded argillaceous rocks, 

limestone, chert, and glauconitic(P) siltstone in the upper 

part of the Burro Canyon suggest that a marginal or nearshore marine environment of deposition developed late in 

the Early Cretaceous. A nearly closed basin similar to the 

Baltic sea may have extended into the area of the Paradox 

Basin as an arm of a large sea far to the south in the vicinity 

of Mexico, a sea evidenced by widespread and thick Lower 

Cretaceous marine strata in that area (Reeside, 1944). Such 

a basin may have been a precursor of the Late Cretaceous 

Rocky Mountain geosyncline. Following this marine 

incursion, however, uplift of the Colorado Plateau region 

allowed general erosion that swept away most of whatever 

marine sediments had accumulated, except where local 

subsidence such as in the Disappointment syncline 

prevented erosion.

Mild crustal warping that caused slight uplift of the 

Colorado Plateau and commensurate erosion of the top of 

Lower Cretaceous strata during the Late Cretaceous also 

resulted in the development of a broad depression that 

occupied much of the western interior of the United States. 

Preceding northward encroachment into this depression 

by the sea that occupied the present region of Mexico near 

the end of the Cretaceous, was deposition of a thin layer of 

terrestrial sediments composed of sand and interlayered 

mud septa comprising the lower and middle parts of the 

Dakota Sandstone. The abundance of scour-and-fill 

structures in sandstone in the lower arenaceous unit of the 

Dakota indicates that fluvial conditions existed during 

most of the deposition of the lower part of the formation. 

The great abundance of carbonized plant material as well 

as coal in the Dakota Sandstone has been accepted as 

strong evidence for the deposition of the formation in a 

dominantly swampy, heavily vegetated environment. In 

order to preserve the abundant plant debris buried in the 

sandstone, the sediments must have been mostly submerged in water after they were deposited; otherwise 

oxidation would have destroyed the carbonaceous 

material. The area of deposition likely was a low flood 

plain, a coastal swamp, or a tidal flat bordering a sea, 

where periodic inundation occurred. Aside from its 

abundant content of carbonaceous material, much of the 

mudstone of the Dakota resembles much of the mudstone 

in the underlying Burro Canyon and Morrison Formations which has been interpreted as being of lacustrine and 

flood-plain origin.

Much of the evenly bedded carbonaceous shaly mudstone in the upper part of the Dakota appears identical to 

that in the overlying marine Mancos Shale. Moreover, 

sandstone in the upper part exhibits foreset bedding of

beach deposits and worm borings suggestive of marine 

and marginal marine deposition. If the upper part of the 

Dakota were deposited in the sea it must have been in a 

nearly closed and stagnant basin, that is, in a sapropelic 

environment, where abundant plant material ensured that 

the water was continuously deoxygenated. Most likely the 

change from terrestrial to marine deposition was gradual, 

with alternations from one to the other during deposition 

of the middle part of the Dakota. This middle period also 

saw deposition of most of the Dakota coal, probably in 

lagoons marginal to a stagnant sea.

Evidence of volcanic material in the Dakota, 

particularly in the middle and upper parts of the formation, consists of sharply angular flakes of detritus that 

most likely were airborne to the place of deposition, and 

gray claystone layers of bentonitic character that probably 

are dominantly altered ashfall material. Although the 

amount of volcanic material may not be large, its presence 

is important to any interpretation of the geologic history 

of the western interior of the United States. It is pertinent 

to the present discussion because the volcanic material 

most reasonably was derived from the same general region 

that supplied abundant volcanic material to the Morrison 

Formation, suggesting that land masses (island arcs) still 

existed farther west, and continued to supply detritus to 

the depositional region surrounding the Slick Rock 

district.

The position of the source of material deposited in the 

Dakota Sandstone is poorly known, but probably it lay 

west of the present area of Dakota outcrop, in Idaho, 

Nevada, and southern California. The lower part of the 

Dakota apparently is locally coarser farther west (L. C. 

Craig, oral commun., Sept. 1961), and sandstone in the 

upper part of the Dakota in places apparently tongues out 

eastward into the overlying Mancos Shale (Young, 1960, p. 

179, fig. 17). However, if abundant volcanic rocks were 

exposed in such an area to the west they presumably did 

not supply much detritus to the Dakota, inasmuch as the 

formation consists dominantly of reworked sedimentary 

material. Perhaps the chief provenance of the Dakota lay 

farther southwest where extensive land masses blanketed 

by clastic sedimentary rocks existed during the Late 

Cretaceous.

Little if any tectonic folding of the salt anticlines 

occurred during deposition of the Dakota Sandstone, but 

salt continued to flow locally into salt cupolas, as for 

example from a segment of the Disappointment syncline 

into the large evaporite cell in the adjacent Gypsum Valley 

anticline.

After the Dakota Sandstone was laid down, further subsidence and enlargement of the Late Cretaceous basin led 

to the deposition of several thousand feet of the marine 

strata comprising the Mancos Shale. Sedimentation 

continued without appreciable interruption from Dakota 

into Mancos time as transgressive layers of mud and sand
E16 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

and finally almost wholly mud were laid down in stagnant 

water within a restricted arm of the Mexico sea. Although 

the sequence of deposition of the transgressive Mancos 

sediments is described here primarily for the area of the 

Slick Rock district, it must be understood that specific 

lithologic units (facies) of the Mancos, and for that matter 

of the underlying Dakota, were not deposited simultaneously across the entire region. For example, a basal 

unit of the Mancos Shale in western Colorado has a time 

equivalent in an uppermost unit of the Dakota Sandstone 

in eastern Utah. The combination of abundance of 

carbonaceous material including some petroliferous substances, the abundance of marine fossil invertebrates, and 

the persistent horizontal bedding in the Mancos Shale 

attest to the formation's sapropelic marine origin. The 

formation was deposited in a stagnant continental sea 

limited on the west by a landmass, probably covered with 

abundant vegetation and deeply weathered, extending 

from north to south through Idaho, Nevada, western 

Arizona, and southern California (Reeside, 1944), from 

which much of the detritus in the Mancos must have been 

derived. Large amounts of clay and calcite in the Mancos 

indicate extensive weathering in the source area. The sea 

was limited on the east by lowlands in the central Great 

Plains region which, of course, supplied detritus to eastern 

parts of the basin. Although the source area of the Mancos 

Shale was deeply weathered and of low relief, some coarse 

clastic material was supplied to thedepositional basin, inasmuch as silt and sand are locally abundant in the 

formation. In addition to the detritus and organic debris 

carried in from land to the west of the Mancos sea, volcanic 

material in the form of ash falls was supplied in 

considerable abundance, probably from a region lying to 

the northwest, as judged from the great abundance of 

bentonitic shale in equivalent rocks much farther 

north for example, Cody Shale in Montana and 

Wyoming (Knechtel and Patterson, 1956, p. 21-28).

Pyrite probably began to form as an authigenic mineral 

shortly after deposition of the sediment, as indicated by 

compaction of beds around pyrite cubes. (See unit 6 of the 

detailed measured section of the Mancos Shawe and 

others, 1968, p. A91.) Pyrite formed as a result of abundant 

hydrogen sulfide that was associated with decaying 

organic material reacting with available iron liberated 

from black opaque minerals in the detritus, as well as from 

volcanic ash that became devitrified shortly after 

deposition. According to Marshall (1961), devitrification 

in the presence of water even at low temperatures 

is rapid. Sulfur also was present in the volcanic ash in a 

variety of forms; the action of bacteria formed hydrogen 

sulfide and ultimately pyrite, or perhaps it formed pyrite 

directly. Volcanic activity may have changed in character 

and (or) location at or about the beginning of late Carlile 

time (when rocks about 300 feet above the base of the 

Mancos in the Slick Rock district were deposited), as

evidenced by the change in chemical character of the diagenetic pyrite associated with bentonitic beds in the 

Mancos (Shawe, 1968b, fig. 1).

The salt anticlines continued to grow during deposition 

of the Mancos Shale. During late middle Greenhorn time, 

at the start of Mancos deposition, flow of salt from beneath 

the Disappointment syncline into the Gypsum Valley 

anticline apparently followe'd the local pattern established during Dakota time, and perhaps the flow was 

merely a continuation of the tendency for the light salt to 

rise in a cupola under the pressure of heavier overlying and 

surrounding rocks. Tectonic deformation of the salt anticlines may not have been important early in the deposition of the Mancos. However, the salt folds showed additional general growth throughout the remainder of 

Mancos deposition, and this growth was related to renewal 

of tectonic stresses.

Near the end of the Cretaceous Period and at the 

beginning of the Tertiary Period in the region of the 

Colorado Plateau many thousands of feet of largely 

terrestrial sediments composed of sand, silt, and mud were 

laid down as the Mesaverde, Wasatch, and Green River 

Formations or their equivalents. This rapid filling of the 

erstwhile marine basin occupying a large part of the 

western interior of the United States resulted partly from 

rapid rise of land at the western edge of the Colorado 

Plateau. This sedimentation was also marked by the 

unroofing of the Idaho batholith, as evidenced by the 

sudden appearance of monazite in the detritus, and shows 

a new and large influx of material from a provenance 

northwest of the Colorado Plateau.

Faulting and jointing occurred during this time in 

several fault zones in the region. Fractures were oriented 

generally northwesterly to westerly, as in the Dolores zone 

of faults, and northeasterly to easterly as in the Glade zone 

of faults. Fault zones may have formed above deep-lying 

shears of strike-slip displacement: for example, left-lateral 

displacement in the northwesterly Dolores zone and rightlateral displacement in the easterly to northeasterly Glade 

zone (which approximately overlies the Glade subsurface 

fault zone).

This crustal deformation rapid uplift of land areas 

adjacent to the Late Cretaceous geosyncline, and faulting 

within the geosyncline was a manifestation of the great 

Laramide orogeny that dominated the region of the 

present Rocky Mountains and the eastern part of the 

present Great Basin (where it has been referred to as the 

Sevier orogeny) in latest Cretaceous and early Tertiary 

time.

Accumulation of the great thickness of sediments above 

the Mancos Shale caused compaction of the clayey Mancos 

strata, with the result that a very large volume of reducing, 

uranium- and vanadium-bearing, Mancos pore water in 

the order of thousands of cubic miles was expelled at the 

end of Cretaceous time and during early Tertiary time.
GEOLOGIC HISTORY OF SLICK ROCK DISTRICT AND VICINITY E17

(The thesis of uranium- and vanadium-bearing pore water 

in the Mancos is too lengthy for proper elaboration here; 

the interested reader is referred to Chapter D of this professional paper (Shawe, 1976) for the details.) Much of this 

water moved into overlying more porous and permeable 

strata of the Mesaverde, Wasatch, and Green River Formations and their equivalents; the reducing character of the 

solutions caused wide-scale reduction of the strata. In 

additon, some pore water of the Mancos was forced into 

underlying strata where it followed permeable fracture 

zones and moved laterally from these into permeable beds. 

Reduction and leaching occurred wherever the reducing 

solution flowed; iron was leached from much of the altered 

rocks, for example, by destruction of detrital black opaque 

minerals and hematitic coloring material. The iron then 

became concentrated along fracture zones or in bodies in 

bedded strata, largely near the edges of the zones of 

solution flow. Some elements were introduced from the 

pore water into the altered strata. Uranium and 

uranogenic lead were deposited almost universally in the 

altered rocks; locally where carbonized plant debris created 

a favorable chemical environment for precipitation (high 

humic-acid content), such as in the ore-bearing sandstone 

of the Salt Wash Member of the Morrison Formation, 

abundant uranium, uranogenic lead, iron, vanadium, and 

some other elements were deposited. These deposits are the 

uranium-vanadium ores whose occurrence and origin will 

be discussed in detail in Chapter F in this series.

CENOZOIC ROCKS AND EVENTS

Igneous activity occurred at widely scattered centers in 

and near the Colorado Plateau during the early to middle 

Tertiary. Laccolithic bodies of porphyry were intruded at 

a number of centers in the plateau. In the Abajo 

Mountains west of the Slick Rock district igneous material 

intruded a graben in the Glade fault zone that probably 

formed near the end of the Cretaceous or in the early 

Tertiary (Witkind, 1964, p. 46, 81). The dominant rock 

type of the plateau laccolithic centers is diorite porphyry 

that contains about 62 percent SiO2; in this porphyry 

phenocrysts were nearly in equilibrium with the magma 

at the time of intrusion and solidification. The age of 

diorite porphyry from the La Sal Mountains northwest of 

the Slick Rock district is about 55 million years (Stern and 

others, 1965).

Sills and laccolithic, some of considerable size, and 

minor dikes, all of intermediate composition, were 

intruded in a broad area along the eastern edge of the 

Colorado Plateau near the Slick Rock district to the San 

Juan Mountains farther east. The dominant rock types are 

granogabbro and granodiorite with SiO2 content 

comparable to or slightly lower than that of the dominant 

rock type of the Plateau laccolithic centers and with 

relatively more iron oxides, MgO, K2O, and TiO2, and less 

A12O3, CaO, and Na2O. Phenocrysts in these rocks were'

not in equilibrium with the enclosing magma at the time 

of intrusion and solidification as indicated by strong 

resorption effects. The age of these intrusives is most likely 

Miocene. Intrusion of some granogabbro sills near the 

eastern end of Disappointment Valley seems to have been 

controlled by a fracture zone, and a dike was emplaced 

along a fault bounding the Glade graben; these faults 

probably formed near the end of the Cretaceous or in the 

early part of the Tertiary. Moreover, emplacement of the 

sills likely occurred following some post-Cretaceous 

warping of the Disappointment syncline, but prior to the 

final extensive folding of the syncline during the late 

Tertiary or the Quaternary.

Intrustion of diorite porphyry probably was nearly 

simultaneous in all the Colorado Plateau laccolithic 

centers, most likely in Eocene time, at the time of 

maximum burial of the red-beds section beneath the 

Mancos Shale and the early Tertiary strata overlying it. It 

occurred at the time Mancos Shale pore waters were being 

expelled as a result of compaction, and therefore the 

temperature of the pore waters may have been increased 

and they thereby became more potent chemically to 

produce increased leaching and other alterations.

Intrusion of granogabbro and granodiorite along the 

eastern edge of the Colorado Plateau in Miocene time 

occurred after the Upper Cretaceous and younger strata 

had been compacted and after the uranium-vanadium ore 

deposits had been formed. It had no apparent effect on the 

ores.

After intrusion of the igneous rocks in the middle 

Tertiary at the eastern margin of the Colorado Plateau, 

sedimentary rocks were again folded, probably largely in 

the early Pleistocene. Erosion along the lower reaches of 

main river systems on the plateau may have started as early 

as middle Tertiary, but downcutting of the Dolores River 

canyon in the Slick Rock district did not begin until after 

deposition of gravels during the early Pleistocene. The 

final major folding of the Dolores anticline, and possibly 

other salt anticlines in the region, occurred only after 

deposition of the gravels. Possibly the erosion of the 

canyon in the district was instigated by simultaneous 

regional uplift of the plateau and growth of the Dolores 

anticline. The basis for this conclusion is the fact that the 

Dolores River canyon has been incised to a depth of 2,000 

feet in the district, whereas the evidence of the gravels 

indicates probably only about 1,000 feet of growth of the 

anticline after the canyon cutting was initiated.

Glacial activity was still taking place in the San Juan 

Mountains after most of the present Dolores River canyon 

was excavated. Dormant landslides that must have 

developed under a climate more moist than that of the 

present occur in places on the slopes of the canyon, and 

loess deposited probably during the latest Pleistocene 

glaciation occurs in small patches deep within the present 

canyon.
E18 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

The youngest deposits in the district, formed during the 

Holocene and in fact still being laid down, are alluvium 

and various types of colluvium. But the configuration of 

the land surface has changed little since the Pleistocene. 

On the other hand, the Dolores anticline still may be 

growing, judged from the gradient of the Dolores River 

where it crosses the anticline. Downstream from the 

vicinity of the anticlinal axis, on the northeast limb of the 

anticline, the river gradient is about 35 feet per mile, 

whereas upstream from the axis, on the southwest limb of 

the anticline, the gradient is about 20 feet per mile. The 

same formations crop out above and below the point 

where the anticlinal axis crosses the river, so the only 

reasonable explanation of the difference in gradient seems 

to be late growth of the anticline.

THE CURRENT SCENE

A look at the south part of the Slick Rock district today 

shows a gently rolling highlands plain capped largely by 

Dakota Sandstone. This plain reflects underlying 

structure. In the north part of the northeast flank of the 1 

Dolores anticline that dips into the Disappointment 

syncline is a Dakota Sandstone-capped surface incised by 

long straight canyons which extend to Disappointment 

Valley. The valley lies in the trough of the Disappointment syncline out of which the easily eroded 

Mancos Shale has been excavated by the Disappointment 

Creek drainage. The Dolores River canyon is antecedent, 

cut by the river which had established its course on a level 

plain and then incised itself with uplift of the plain and 

rise of the Dolores and adjacent anticlines. Canyons 

tributary to the Dolores River developed as a corollary to 

the downcutting of the river.

A few fault-line scarps have formed in the Dolores zone 

of faults where sandstone beds are at the surface. An 

elongate topographic depression coincides with the Glade 

graben.

Residual hills and terraces of glacial till lie on Glade 

Mountain at the southeast tip of the district. At least five 

levels of stream gravels are found along the canyon of the 

Dolores River and its tributary canyons. An old erosion 

surface lies just under the highest of these gravels situated 

on the highland plain in the south part of the district.

Hummocky and irregular surfaces of landslides are 

found along the walls of deeper canyons, especially on 

Brushy Basin slopes. Many of the landslides have been 

extensively eroded. Alluvial fans composed of locally 

derived material are developed in places; some of these 

have been partly dissected.

Elongate, shallow depressions paralleled by flat, low 

ridges and oriented about N 25° E. are developed in 

Pleistocene loess on the Egnar plain. Those were 

apparently excavated by prevailing strong winds shortly 

after deposition of the loess.

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irU.S. GOVERNMENT PRINTING OFFICE: 1976-677-340/82