Petrography of Sedimentary 

Rocks in the Slick Rock 

District, San Miguel and 

Dolores Counties, Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 576-B

Prepared on behalf of the

U.S. Atomic Energy Commission
Petrography of Sedimentary 

Rocks in the Slick Rock 

District, 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-B

Prepared on behalf of the

U.S. Atomic Energy Commission

Microscopic study of uranium- and vanadiumbearing and associated sedimentary units 

contributes to the understanding of the geologic 

events that formed the rocks: sedimentation^ deep 

burial, mild deformation^ and alteration

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEW ART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 40 cents (paper cover)
CONTENTS

Page

_____________-___________________________________________________________________ Bl

Introduction _______________________________________________________________________________ 1

Definitions and methods_____________________________________________________________________ 3

Rock names.___________________________________________________________________________ 3

Textural terms.________________________________________________________________________ 3

Modal point counts and estimates_______________________________________________________ 5

Rock-forming minerals______________________________________________________________________ 5

Pre-Morrison rocks________________________________________________________________________ 6

Cutler Formation _______________________________________________________________________ 6

Chinle Formation.______________________________________________________________________ 8

Entrada Sandstone-_____________________________________________________________________ 9

Summerville Formation..________________________________________________________________ 11

Morrison Formation rocks___________________________________________________________________ 12

Salt Wash Member_________________________________________________ 12

Brushy Basin Member___________________________________________________________________ 16

Post-Morrison rocks_________________________________________________________________________ 25

Burro Canyon Formation________________________________________________________________ 25

Dakota Sandstone.______________________________________________________________________ 28

Mancos Shale __________________________________________________________________________ 29

General conclusions _________________________________________________________________________ 32

References cited ____________________________________________________________________________ 34

ILLUSTRATIONS

Page 

FIGURE 1. Map showing location of the Slick Ro'ck district_--____-____--_---------_-------- B3

2. Generalized geologic map of the Slick Rock district showing sample localities..---.-. 4 

3-7. Pen and ink drawings:

3. Reddish-brown medium- to fine-grained limy arkosic sandstone of the Cutler

Formation.______________________________________________________ 7

4. Light-brownish-gray medium-grained arkosic sandstone of the Cutler Formation. _________________________________________________________ 7

5. Light-gray coarse-grained calcarenitic sandstone of the Moss Back Member. _ 8

6. Reddish-brown very fine grained sandstone from the so-called Black Ledge of

the Church Rock Member________________________________--------_ 9

7. Light-reddish-brown very fine grained sandstone from the massive unit of the

Slick Rock Member_________________________________ 10

8. Histograms of grain-size distribution for samples from the Slick Rock Member of the

Entrada Sandstone and the lower part of the Summerville Formation _______ 10

9-22. Pen and ink drawings:

9. Buff very fine grained sandstone from the crossbedded unit of the Slick Rock

Member..__________________________________________-------_----- 11

10. Poorly sorted reddish-brown sandstone with thin siltstone and mudstone

layers from the lower part of the Summerville Formation..____________ 12

11. Light-reddish-brown fine-grained sandstone from the middle unit of the Salt

Wash Member___________________________________________________ 13

12. Very light brown very fine grained sandstone from the upper unit of the Salt

Wash Member, cemented with calcite______-______---------_-------- 14

13. Very light brown very fine grained sandstone from the upper unit of the Salt

Wash Member___________________________________________________ 14

14. Dark-gray carbonaceous mudstone from the lower part of the lower unit of

the Salt Wash Member______________________________ 15
IV CONTENTS

Page 

FIGURE 15. Light-gray very fine grained sandstone from the lower brown unit of the

Brushy Basin Member_________-___-___-_____-_-__-____----_---___ B17

16. Very fine grained light-brown to very light reddish brown sandstone from

the lower brown unit of the Brushy Basin Member.__________________ 17

17. Light-reddish-brown very fine grained sandstone from the lower brown unit

of the Brushy Basin Member__-______-_____--_-______-_____--_____ 17

18. Very fine grained light-brownish-gray sandstone from the middle green unit

of the Brushy Basin Member______________________________________ 18

19. Light-gray fine-grained quartzitic sandstone from the upper brown unit of the

Brushy Basin Member--_-_________-_--____--------__---------___- 18

20. Coarse-grained poorly sorted mottled light-gray and reddish-brown sandstone

from the upper brown unit of the Brushy Basin Member-_____________ 18

21. Light-brown coarse-grained conglomeratic sandstone from the middle green

unit of the Brushy Basin Member______-____---_-_-___---------____ 19

22. Coarse-grained conglomeratic sandstone from the middle green unit of the

Brushy Basin Member-_-__-_-___-_----___------_-------_--------- 19

23. Sketch of light-gray conglomerate from the upper part of the upper brown unit of the

Brushy Basin Member_-_______.________________________ 20

24-32. Pen and ink drawings:

24. Light-reddish-brown siltstone from the upper brown unit of the Brushy Basin

Member_______________________________________ 20

25. Light-greenish-gray mudstone (devitrified volcanic tuff) from the middle

green unit of the Brushy Basin Member_____________________________ 21

26. Light-greenish-gray mudstone (devitrified volcanic tuff) from the middle

green unit of the Brushy Basin Member; detail of figure 25.___________ 21

27. Light-greenish-gray mudstone from the middle green unit of the Brushy

Basin Member--__________-_____-__-__-______-___-_-_--------_--- 22

28. Mottled light-reddish-gray and light-greenish-gray devitrified tuffaceous mudstone from the middle green unit of the Brushy Basin Member______-__ 22

29. Vug in light-greenish-gray mudstone from the middle green unit of the Brushy

Basin Member, filled with calcite and chalcedonic quartz ______________ 23

30. Light-brownish-green mudstone from the upper brown unit of the Brushy

Basin Member showing devitrified glass shards.______________________ 23

31. Reddish-brown mudstone from the upper brown unit of the Brushy Basin

Member_________________________________._--_.-_ 24

32. Light-brownish-green mudstone from the upper brown unit of the Brushy

Basin Member, a devitrified volcanic tuff_________________________ 24

33. Sketch of part of limestone nodule from the middle green unit of the Brushy Basin

Member..____________________________________________ 24

34-46. Pen and ink drawings:

34. Light-brownish-gray fine-grained quartzose sandstone from the Burro Canyon

Formation.____________________________________________________ 25

35. Grayish-green sandy siltstone from a bed near the top of the Burro Canyon

Formation_________________________________ 26

36. Dark-gray carbonaceous claystone from the Burro Canyon Formation containing strings of calcite metacrysts______________--_______-----_-_ 26

37. Shear structures in calcite in a limy bed in greenish-gray claystone 60 feet

below the top of the Burro Canyon Formation.._____________________ 27

38. Light-greenish-gray microbrecciated limy mudstone from the Burro Canyon

Formation____________________________________ 27

39. Light-buff fine-grained quartzose sandstone from the Dakota Sandstone.__ 28

40. Light-buff fine-grained quartzose sandstone from the Dakota Sandstone, a

detail.________________.______....___________ 29

41. Dark-gray carbonaceous very fine grained silty sandstone from the middle

part of the Dakota Sandstone_________________________ 29

42. Dark-gray carbonaceous shale from the Mancos Shale___________________ 30

43. Dark-gray silty carbonaceous mudstone from the Mancos Shale.___________ 30

44. Dark-gray sandy carbonaceous mudstone from the Mancos Shale.__________ 31

45. Dark-gray carbonaceous limestone from the Mancos Shale containing abundant pyrite ______________________________________________________ 31

46. Dark-gray carbonaceous limestone from the Mancos Shale, with calcite-filled

fracture._ ____________________________________________________ 32
CONTENTS

TABLES

Page 

TABLE 1. Generalized section of exposed consolidated sedimentary rocks____________________ B2

2. Average mineral composition of sandstones from some Permian and Mesozoic formations ___________________________________________________________________ 7

3. Average mineral composition of sandstones from the Slick Rock Member of the Entrada Sandstone _________________________________________________________ 9

4. Average mineral composition of siltstones, mudstones, and shales from some Mesozoic

formations ______________________________________________________________ 12

5. Comparison of average mineral composition of sandstones from the Salt Wash Member, 

Slick Rock district, with that of sandstones from the Salt Wash Member throughout the Colorado Plateau_________________________________________________ 13

6. Average mineral composition of sandstones from the Salt Wash Member of the Morrison Formation_________________________________________________________ 13

7. Average mineral composition of siltstones from the Salt Wash Member of the Morrison Formation _________________________________________________________ 15

8. Average mineral composition of mudstones from the Salt Wash Member of the Morrison Formation.________________________________________________________ 16

9. Average mineral composition of sandstones from the Brushy Basin Member of the

Morrison Formation____________________________________________________ 16

10. Average mineral composition of conglomerate and conglomeratic sandstones from the

Brushy Basin Member of the Morrison Formation.__________________________ 19

11. Average mineral composition of siltstones from the Brushy Basin Member of the Morrison Formation-________________________________________________________ 20

12. Average mineral composition of mudstones from the Brushy Basin Member of the

Morrison Formation____________________________________________________ 21

GEOLOGIC INVESTIGATIONS IN THE SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLORADO

PETROGRAPHY OF SEDIMENTARY ROCKS IN THE SLICK ROCK DISTRICT, SAN MIGUEL

AND DOLORES COUNTIES, COLORADO

BY DANIEL R. SHAWE

ABSTRACT

Petrographic studies, largely thin-section examinations, of 

rocks from the Chinle and Morrison Formations and adjacent 

strata in the Slick Rock district, supplement the stratigraphic 

studies of sedimentary rocks exposed in the district.

Sandstones investigated in strata above the Moss Back Member of the Chinle Formation generally contain about 50-80 percent quartz, 2-10 percent chert, 1-10 percent feldspar, 3-15 

percent clay, 1-15 percent calcite, and minor amounts of other 

components. Some sandstone in the Moss Back Member contains 

abundant limestone grains, and sandstone in the underlying 

Cutler Formation is arkosic and contains about one-third feldspar. Siltstones are distinguished from sandstones principally 

by their finer grain size and higher proportion of clay; otherwise, 

they are mineralogically similar to sandstones. Mudstones contain about 25-30 percent quartz, 0.25-3.5 percent chert, 2-6 percent feldspar, 40-65 percent clay, 0-25 percent calcite, and minor 

amounts of other components. Silty claystones in the Burro 

Canyon Formation average about 80 percent clay and commensurately less of other components. Shale in the Mancos Shale 

consists -of about 1 percent pyrite, 3 percent carbonaceous material, 5 percent detrital minerals, 25 percent calcite, and the 

remainder clay.

Mineral compositions of particular sedimentary rock types 

vary as much within individual formations as they do from one 

formation to another. Rocks whose constituent grains display a 

great range in size, degree of sorting, and Founding have no 

commensurate varations in mineral composition, other than generally higher clay and calcite content in poorer sorted and finer 

grained material.

The ore-bearing sandstone of the Salt Wash Member of the 

Morrison Formation is not unique in mineral composition, and 

its mineral composition probably did not influence uraniumvanadium deposition. The Brushy Basin Member of the Morrison 

Formation, commonly cited to contain abundant volcanic material, has unusual amounts only in its middle green unit, and 

even in that unit, material from nonvolcanic sources probably 

equals in amount that from volcanic sources.

Reddish-brown rocks of all types are characterized by numerous detrital black opaque mineral grains, films of hematite 

on other detrital grains, and dustlike hematite dispersed in matrix material. Light-colored rocks (commonly called bleached) 

contain virtually no hematite, either as detritals or grain coatings, but they do contain pyrite or marcasite, or limonite oxidized from these. Many of the light-colored rocks are altered 

equivalents of reddish-brown rocks and will be discussed in 

detail in a later chapter of this professional paper.

Silica is an abundant cement in most of the sedimentary rocks 

studied; it probably is a product of diagenesis. Calcite occurs as 

fine-grained interstitial blebs in reddish-brown rocks, and as 

larger patches enclosing detrital grains in light-colored rocks, 

suggesting epigenetic redistribution during alteration that developed the light colors. Other carbonate minerals, as well as 

barite, anatase, pyrite, marcasite, albite, and analcite, occur as 

well-formed authigenic crystals chiefly in altered rocks.

Some evidence indicates crystallization of authigenic (epigenetic) minerals when the strata were deeply buried and before 

they were extensively folded.

INTRODUCTION

The SUck Rock uranium-vanadium district occupies 

about 570 square miles in the western parts of San 

Miguel and Dolores Counties, Colo., (fig. 1). A generalized geologic map is given in figure 2. Sedimentary 

rocks cropping out in the district range in age from 

Permian to Cretaceous. Older sedimentary rocks that 

underlie these are known to rest on igneous and metamorphic rocks of a probable Precambrian basement. 

Maximum thickness of exposed sedimentary rocks in 

the district is about 4,700 feet, and the total section of 

sedimentary rocks underlying the district is about 

13,000 feet. The exposed stratigraphic section is summarized in table 1.

The district lies in the Paradox Basin at the southeast end of the Paradox fold and fault belt (Kelley, 

1958). Major folds in the district are broad and open, 

trend about N. 55° W., and are parallel to the collapsed 

Gypsum Valley salt anticline bounding the northeast 

edge of the district. The Dolores anticline is about 10 

miles southwest of the Gypsum Valley anticline; the 

Disappointment syncline lies between the two anticlines.

Bl
B2 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO. 

TABLE 1. Generalized section of exposed consolidated sedimentary rocks

Age

Late Cretaceous

Early Cretaceous

Late Jurassic

Jurassic and 

Triassic(?)

Late 

Triassic(?)

Late Triassic

Middle(?) and 

Early Triassic

Permian

Formation and member

Mane

Dako

Burro

Morrison Formation

Junct

Sumlr

Entrada Sandstone

Nava;

Kayei

Wings

Chinle Formation

Moenl

ta Sandstone. ____ ___ __

Canyon Formation.. _

Brushy Basin Member. __

Salt Wash Member. _____

ion Creek Sandstone _

lerville Formation _____ _

Slick Rock Member _____

Dewey Bridge Member. ._

o Sandstone. ____ ___

ita Formation.. ___ _

ite Sandstone _ _ ... _

Church Rock Member- _ _

Petrified Forest (?) 

Member.

Moss Back Member

topi Formation __ _ _ _

Cutler Formation . __-

Thickness (feet)

1, 600-2, 300

120-180

40-400

300-700

275-400

20-150

80-160

70-120

20-35

0-420

160-200

200-400

340-500

0-100

20-75

0-200 

(none exposed)

1, 500-3, 000 

(240 exposed)

Description

Dark-gray carbonaceous calcareous marine mudstone and 

minor amounts of gray limestone, greenish-gray bentonitie 

shale, and light-buff sandstone. Contains abundant invertebrate fossils.

Light-buff fine-grained quartz ose marginal marine and fluvial 

sandstone, dark-gray carbonaceous shale, coal, and locally, 

light-buff conglomeratic sandstone.

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 (?) limestone 

and chert.

Predominantly reddish-brown and greenish-gray bentonitie 

flood-plain mudstone and some light-reddish-brown, lightbuff, and light-greenish-gray fluvial siltstone, sandstone, 

and conglomerate.

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.

Light-buff fine-grained quartzose sandstone with sweeping 

eolian crossbeds at south end of district. Changes northward to light-reddish-brown fine-grained tidal-flat(?) 

sandstone with horizontal beds; merges laterally with Summerville Formation.

Reddish-brown quartzose horizontally bedded marginal 

marine siltstone and fine-grained sandstone.

Light-buff to light-reddish-brown fine-grained quartzose 

eolian sandstone.

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

Light-buff and, locally, light-reddish-brown fine-grained 

quartzose eolian sandstone.

Purplish-gray to purplish-red fluvial siltstone and sandstone, 

and locally, shale, mudstone, and conglomerate.

Light-buff and light-reddish-brown very fine grained to finegrained quartzose eolian sandstone.

Reddish-brown, purplish-brown, and orangish-brown, mostly 

tidal-flat, horizontally bedded quartzose sandstone, siltstone, and mudstone, and a minor amount of dark-greenish-gray conglomerate.

Greenish-gray fluvial and flood-plain mudstone, siltstone, and 

shale, and minor amounts of reddish-brown mudstone and 

greenish-gray sandstone and conglomerate.

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 reddishbrown, mudstone, siltstone, and shale.

Light-reddish-brown micaceous tidal-flat(?) siltstone and 

sandy siltstone.

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

fluvial and flood-plain sandstone, siltstone, mudstone, and 

shale.
PETROGRAPHY OF SEDIMENTARY ROCKS B3

r i

r'

UTAH)

/outline of salt\ 

/anticline region

COLORADO

Outline of part of 

the Colorado Plateau

109°

0 10 20 30 40 MILES

1,1,1,1,1

FIGURE 1. Map showing location of the Slick Rock district 

(stippled).

A zone of faults bounds the southwest edge of the 

collapsed core of the Gypsum Valley salt anticline. The 

Dolores fault zone is about 2 miles northeast of, and 

parallel to, the axis of the Dolores anticline. A few 

faults form a conjugate set normal to the Dolores fault 

zone in the northern part of the district. The southern 

part of the district is cut by the Glade fault zone which 

trends about N. 80° E.

Geology and ore deposits of the district have been 

briefly summarized (^Shawe and others, 1959), and details of the stratigraphy of the district and vicinity are 

in press (Shawe and others, 1968). Additional details 

of the geology of the district are being prepared in other 

reports on petrography of the sedimentary rocks (this 

report), structure, rock alteration, geologic history, and 

uranium-vanadium deposits. Although rock alteration

297-092 O 68 2

is inevitably discussed in this report describing petrography, a much fuller treatment will be given in a later 

chapter of this professional paper, in which chemical 

and additional mineralogical facts are presented, and 

varieties of altered rock are dealt with in detail.

The Morrison Formation and the Moss Back Member 

of the Chinle Formation the two known uraniumbearing horizons in the district and associated strata, 

were studied chiefly by means of microscopic examination of more than 100 thin sections and by a few grainsize analyses. About two-thirds of the thin sections 

studied are from the Morrison Formation; this unit received the most attention because it has produced virtually all the uranium-vanadium ore mined in the 

district. Studies of the heavy-mineral content of nearly 

100 samples from practically the same rock units herein 

discussed will be described in another report in this professional paper series on alteration of the rocks.

DEFINITIONS AND METHODS 

ROCK NAMES

Rock names as applied in this report are generally 

those recommended by the Committee on Sedimentation, 

Division of Geology and Geography of the National 

Research Council (cited in Petti John, 1957, p. 17-19). 

Claystone is massive or structureless rock consisting 

dominantly of clay-sized material (< 1/256 mm in diameter). Actually, all the claystones herein described 

have small amounts of silt-sized (1/256-1/16 mm) or 

sand-sized (1/16-2 mm) particles, arbitrarily set as up 

to 20 percent of the total rock. Siltstone and sandstone 

consist dominantly of silt-sized and sand-sized particles, 

respectively. Conglomerate consists dominantly of particles greater than 2 mm in size. Actually, any sandstone 

with more than 20 percent granule or larger particles is 

here considered conglomerate, and any with less than 

this, but still evident granule or pebble-sized material, 

is called conglomeratic sandstone. Shale is material of 

the grain size of claystone but has fissility; mudstone 

is argillaceous rock poorly fissile or lacking in fissility 

which contains less than 60 percent and more than 20 

percent silt to coarse sand-sized particles. Limestone 

consists dominantly of calcite. Some detrital rocks that 

are dominantly calcite are called calcarenite in the sandsized range and calcirudite in the granule and larger 

grain size.

TEXTUBAL TERMS

Grain-size categories have already been alluded to in 

the descriptions of rock names above. Within sandsized material, a further breakdown has been made in 

millimeters, as follows: Very fine sand, %e~%j fine?

-1/4; medium, i/^-^; coarse, ^-1; and very coarse,
B4 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

109 EXPLANATION 

Surficial deposits not shown

R. 20 W. I R. 19 W. MONTROSE CO ^ R. 18 W

Post-Morrison formations

VR-2-35, 127.5, 145, 210, 306.5, 338, 

397, 439.5, 503. 722.5. 726.4, 765, 

812.5,1054, 1139, 1299, 1465, 1540, 

1597.5, 1667, 1722.5. 1790, 1825, 

1862.5, 1990

Morrison Formation

Pre-Morrison formations

Upper Jurassic to Permian

Fault

Dashed where indefinite

Axis x>f anticline

»SR-1

Sample locality

^ \

co__ri^___._ MONTEZUMA CO

FIGURE 2. Generalized geologic map of the Slick Rock district showing sample localities.
PETROGRAPHY OF SEDIMENTARY ROCKS B5

1-2. Granules range in size from 2 to 4 mm and pebbles 

from 4 to 64 mm.

Porosity the percentage of pore space between particles making up a rock was not systematically evaluated. Sandstones and conglomerates that are poorly 

cemented have as much as about 20 percent porosity, 

and those that are thoroughly cemented have virtually 

none. Siltstones, mudstones, shales, and claystones have 

lower porosities; porosity is at its lowest, 2.3 percent, 

in the Mancos Shale.

Sorting the number of size grades and the grain 

size frequency distribution in the detrital rocks were 

not measured except for a few rocks from the Entrada 

Sandstone and Summerville Formation. Sorting can be 

indicated by means of a coefficient of sorting, namely, 

the square root of the ratio of quartiles, the quartiles 

being the size value at the intersection of the 25- and 

75-percent values of the cumulative frequency curve 

(PettiJohn, 1957, p. 37). Perfectly sorted sediment has 

a coefficient of sorting of 1; poorly sorted, that is, with 

many size grades, has a coefficient several times higher. 

Eocks examined in this study were, without any measurements, estimated to be well sorted if most of the 

particles were within one size grade; moderately sorted, 

if within two size grades; and poorly sorted, if within 

more than two size grades.

Eoundness of detrital grains the degree of curvature of angular edges of grains also was not measured. 

Instead, this property was estimated by comparison 

with roundness in the roundness classes shown by Pettijohn (1957, fig. 28) and described by him as angular, 

subangular, subrounded, rounded, and well rounded.

Packing the degree to which particles in the rock 

are fitted together was not evaluated except in a few 

samples. Eocks containing particles fitted so that as 

little as possible interstitial space remains are called well 

packed, and those with particles poorly fitted and displaying relatively extensive unfilled interstitial space 

are called poorly packed.

Interstitial material between detrital grains in sedimentary rocks is termed either "matrix" or "cement" 

(Pettijohn, 1957, p. 284). Matrix is finely divided detrital material (silt and clay) usually deposited along 

with the coarser detritus. Cement consists of minerals 

precipitated in voids following deposition of sediment. 

This distinction is attempted in the descriptions of individual rocks that follow. In some rocks, however, 

matrix material was impregnated with cement or recrystallized, and distinction between matrix and cement 

was not always possible.

Some poorly sorted rocks that have a large proportion of fine-grained material, particularly that which is 

dark colored or opaque, are here termed "dirty" rocks.

Well-sorted rocks and those rocks whose constituent 

grains are not clouded with dark finely dispersed material are referred to as "clean."

MODAL POINT COUNTS AND ESTIMATES 

Modal point counts of mineral components were 

made of thin sections of about half the rocks described 

in this report. These counts were made using a click 

stage, and counting for most samples not more than 

200 points per thin section in traverses 1 mm apart. 

Greater accuracy for individual thin sections could 

have been obtained by counting more than 200 points. 

However, it was visibly evident that chance compositional variations in rock layers only inches from the position of the sampled rock are much greater than statistical errors resulting from counting only 200 grains. 

This signifies the f ruitlessness of more precise determinations of individual thin sections for this study.

Compositions of the remaining half of the thin sections were estimated. These modal estimates were based 

on extensive practice estimates of varying known percentages of filled space (shown as dark shapes of varying sizes) within a given area. Estimates were checked 

by point counting until proficiency was acquired. Validity of the estimates can be judged on the basis of the 

comparison of point counts and modal estimates of the 

composition of thin sections of Salt Wash Member 

sandstone given in table 5. The table suggests a slight 

tendency of the author to overestimate the abundance 

of minor components.

ROCK-FORMING MINERALS

Eock-forming minerals in the detrital rocks to be 

described are surprisingly similar in both kind and 

abundance in all the stratigraphic units above the Moss 

Back Member of the Chinle Formation. Permian and 

basal Triassic rocks include some arkosic (feldsparrich) and calcite-rich detrital varieties; but overlying 

rocks are mostly quartz rich, and contain small amounts 

of chert and feldspar, and differ principally in clay and 

calcite content. Minor amounts of dolomite, siderite, 

barite, chlorite, analcite, hematite, other black opaque 

minerals, biotite, muscovite, leucoxene, anatase, limonite, pyrite, marcasite, tourmaline, or zircon are present 

in many of the rocks.

Quartz grains, making up 50-80 percent of the sandstones and 25-50 percent of siltstones and mudstones, 

are commonly unstrained and contain minor to abundant amounts of dustlike inclusions. Some quartz grains 

contain internal sutures, as if derived from quartzite; 

others are strained as evidenced by undulatory extinction. Quartz overgrowths, which constitute as much 

as 15 percent in some rocks, are faceted or are in contact with adjacent overgrowths along irregular bound-
B6 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

aries. Some interstitial quartz consists of microscopic 

grains in mosaic aggregates, on the order of chert. Some 

quartz grains have been corroded by solutions, and 

locally, they interpenetrate where solution and compaction progressed simultaneously.

Chert grains, constituting 2-10 percent of sandstones 

and 0.25-6 percent of siltstones and mudstones, were 

recognized as mosaic aggregates of microscopic silica 

grains. Although some may be altered volcanic material, there is no evidence in most of them to suggest 

that they were not derived from a sedimentary chert 

source.

Content of orthoclase grains averages 12.5 percent in 

Permian sandstone, ranges lower, 0.25-6.5 percent in 

younger standstones, and is 1-4 percent in siltstones and 

mudstones in the Slick Rock district. Orthoclase is 

commonly clouded with dustlike alteration products.

Microcline grains are abundant, 20 percent, only in 

Permian sandstone; in younger sandstones they are 

0.25-6 percent and in siltstones and mudstones, 0-3 percent. Grains are clear in some rocks and clouded with 

alteration products in others.

Plagioclase, making up 0.25-5 percent of sandstones 

and 0-4 percent of siltstones and mudstones, is either 

clear and unaltered or clouded with alteration products. It is detrital, and also not uncommonly occurs as 

an authi genie mineral, either as overgrowth cement or 

as small interstitial laths. It ranges in composition from 

nearly pure albite where authigenic to more calcic albite 

and oligoclase in detrital grains.

Clay makes up 0-15 percent of sandstones, 20-70 percent of siltstones and mudstones, and about 80 percent 

of claystones. Compositionally, it is montmorillonite, 

kaolinite, or hydrous mica, or mixtures of these (Shawe 

and others, 1968). Some argillaceous and coarser clastic 

rocks contain both mineral clay and clay-sized detritus. 

Most argillaceous rocks contain predominantly mineral 

clay in this size grade, but some contain distinguishable clay-sized detritus of other minerals, principally 

quartz and feldspar. Matrix in coarser clastic rocks is 

both mineral clay and clay- or silt-sized detritus.

Calcite occurs as detrital grains, matrix material impregnating clay-sized material, and cement interstitial 

to sand and silt grains. As detritus, it is the major component only in sandstone of the Moss Back Member of 

the Chinle Formation. In younger sandstones, largely 

in the form of cement, it ranges from 0.5 to 25 percent; 

in siltstones, mudstones, and shales, both in cement and 

matrix, it occurs in similar amounts. In limestones, calcite of course constitutes more than half the components 

of the rocks.

Dolomite, siderite, barite, hematite, analcite, anatase, 

limonite, pyrite, and marcasite are minor authigenic

minerals. They occur either as metacrysts in clay-rich 

rocks or interstitially in coarser detrital rocks. Hematite occurs commonly as grain coatings, cement, and dispersions of dustlike material. Chlorite appears to be 

mostly a minor alteration product of earlier mafic 

minerals. Black opaque detrital grains consist largely 

of hematite resulting probably from diagenetic oxidation of original magnetite and ilmenite. Some magnetite and ilmenite remains in a small part of the black 

opaque detrital grains. Leucoxene, perhaps largely altered from ilmenite through diagenetic oxidation, is a 

minor but common component and may be originally 

detrital in part. Tourmaline and zircon are minor but 

common detrital components.

All identifications were made optically under the 

petrographic microscope with the exception of those 

for clay minerals, and for hematite in black opaque 

detrital grains, which were verified by X-ray 

diffraction.

PRE-MORRISON ROCKS

The rocks are divided into three groups, pre-Morrison, Morrison, and post-Morrison for convenience of 

petrographic description. Pre-Morrison rocks include 

examples from the Cutler Formation of Permian age, 

the Moss Back Member and so-called Black Ledge of 

the Church Rock Member of the Chinle Formation of 

Late Triassic age, the Dewey Bridge and Slick Rock 

Members of the Entrada Sandstone of Late Jurassic 

age, and the Summerville Formation of Late Jurassic 

age. Morrison rocks include examples from the Salt 

Wash and Brushy Basin Members of the Morrison 

Formation of Late Jurassic age. Post-Morrison rocks 

include examples from the Burro Canyon Formation 

of Early Cretaceous age and the Dakota Sandstone and 

Mancos Shale of Late Cretaceous age.

CUTLER FORMATION

The Cutler Formation, consisting of arkosic sandstone, siltstone, and mudstone, was sampled at the 

United Uranium mine adit in the Dolores River canyon, 

SW*4 sec. 12, T. 42 N., R. 18 W. Two samples of limy 

sandstone were collected 2 and 3 feet below the contact 

between the Cutler and overlying sandstone of the Moss 

Back Member of the Chinle Formation. The average 

mineral composition of thin sections of the samples is 

shown in table 2.

Sample 55-S-4, the lower of the two, is reddish-brown 

medium- to fine-grained arkosic sandstone composed of 

angular detrital grains chiefly quartz and feldspar, 

and minor mica and accessory minerals in a matrix of 

calcite and clay (fig. 3). Microcline and plagioclase 

grains are largely clear; orthoclase is in part perthitic 

and myrmekitic. Minor chlorite is mostly a replacement
PETROGRAPHY OF SEDIMENTARY ROCKS B7

TABLE 2. Average mineral composition, in percent, of sandstones from some Permian and Mesozoic formations in the Slick Rock

district, Colorado

[Based on modal point counts (200 points) and modal estimates of thin sections. "Minor" generally indicates less than 0.25 percent]

Component

Cutler

Chinle Formation

Formation Moss Back Church Dewey

Member Rock Bridge MemMember ber (siltstoiie)

Entrada Sandstone Morrison Formation

 Summer- Burro Dakota

Slick Rock ville Salt Wash Brushy Canyon Sandstone

Member Formation Member Basin Formation

Member

Quartz grains and overgrowths____

Chert _ ____ ..___ ___ ___._-_

Orthoclase _____________________

Microcline_____ _ ______________

Plagioclase _ ____ _ _________

Rock fragments and mineral aggregates ___ ____ _ _______ ____ _

Clay (mineral clay and clay-sized

detritus)_____ __ _____ ____ __

Calcite and (or) dolomite _ _ _ _ _ _ _

Barite____________------- ____ _

Chlorite ________________ _____

Hematite and other black opaque

minerals. _________________ ___

Biotite _ _ ___ __________ ____

Muscovite _________________ ___

Leucoxene ______ _____ _______

Limonite___ _ __ -___ _ ______ _

Pyrite______ ___ _ _ ___ _____ __

Carbonaceous material ___ _____

28

12.

?0

5

15

14

3.

5

25

75

5

75

25

12

1

5

6

Minor

!75

Minor . Minor

Minor . Minor

Minor

. 5 .

52

10

Minor

1

5 _.

15

15

Minor _.

. 5

1

Minor

81. 5

1.5

6.5

.5

1

6

. 5

1. 5

. 5

Minor

Minor

71

3. 5

5

3.5

3

Minor

6

6.5

Minor . Minor

2 Minor

Minor

Minor

Minor

2 Minor .

2 Minor .

47

5

2

2

2

Minor

15

25

Minor

1

Minor

Minor

Minor

76.6

5.7

3.0

1.0

.8

Minor

2.5

8.4

Minor

Minor

Minor

Minor

Minor

Minor

Minor . Minor .

64.5

8

4

.5

3

1.5 .

6

11

Minor

.5

.5

Minor . Minor

Minor

83

3

1.5

Minor

3

6

1

Minor

Minor

Minor

Minor

1

61.5

10

3

1.5

Minor

12

6

Minor

.5

Minor

1

Minor

.5

Minor

Minor

3.5

Number of samples._____________ 2 38 13

Largely rock fragments. a Total 0.5 percent.

Calcite Hematite dust

Chlorite

Muscovite

Quartz

Clay.,

Hematite Apatite 

Plagioclase

Leucoxene

FIGURE 3. Reddish-brown medium- to fine-grained limy arkosic 

sandstone of the Cutler Formation. Angularity characterizes 

most grains; matrix is largely calcite and clay; hematite dust 

coats all grains. Sample 55-S-4, Dolores River Canyon.

of biotite or some other ferromagnesian mineral. Interstitial matrix makes up about a third of the rock. Calcite 

commonly occurs as optically continuous patches less 

than a millimeter in diameter, some of which enclose 

two or three grains of detrital minerals. Hematite occurs

Plagioclase 

with zircon inclusion Calcite.

Microcline

Leucoxene

Brownishgreen 

biotite

Quartz

Microcline with

leucoxene (opaque)

and apatite inclusions

1 mm

FIGURE 4. Light-brownish-gray medium-grained arkosic sandstone of the Cutler Formation. Angularity of grains is characteristic; opaque dust and opaque grains are largely 

leucoxene; calcite that is optically continuous with adjoining 

matrix calcite partly replaces perthitic orthoclase. Sample 

55-S-3, Dolores River Canyon.

as a fine-grained dustlike material dispersed in the clay 

matrix and as films coating all the detrital grains.
B8 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

Numerous detrital grains of black opaque minerals are 

present and are included with hematite in table 2.

The second sample, 55-S-3, collected 1 foot stratigraphically higher than sample 55-S-4, is light-brownish-gray medium-grained arkosic sandstone, composed 

of angular to subrounded detrital grains chiefly of 

quartz and feldspar, and minor mica and accessory minerals in a matrix of clay, calcite, quartz, and chlorite 

(fig. 4). Some perthitic orthoclase in this rock is partly 

replaced by calcite; a few grains of plagioclase that is 

albite to oligoclase in composition are dusty looking as 

though partly altered. Sparse indeterminate rock fragments appear to be largely altered, although some contain small relict feldspar laths. Matrix is about onefourth of the rock and consists largely of clay; interstitial patches of clay and calcite give way locally to 

aggregates of microscopic clay, chlorite, quartz, and 

calcite. Hematite is absent, both as detrital grains and 

as films on other grains.

Sharp angularity of many mineral grains, as well as 

the abundance of feldspars and micas, suggests that 

these rocks were derived from the nearby Uncompahgre 

highland.

CHINLE FORMATION

The Moss Back Member of the Chinle Formation, 

consisting of limy arkosic and quartzose sandstone, 

calcarenite, calcirudite, mudstone, siltstone, and shale, 

was sampled at the United Uranium mine adit in the 

Dolores Kiver Canyon, SW*4 sec. 12, T. 42 N., R. 18 

W., about 3 feet above the base of the member. Here, 

the member consists of a common type of gray (N5) 

coarse-grained sandstone and conglomerate containing 

nearly two-thirds limestone fragments. The poor sorting of the rock, angularity of grains suggesting a close 

source area, and tendency to contain abundant calcite 

cement are shown in figure 5, which illustrates a rock 

with more quartz grains than is typical. The mineral 

composition of a thin section of the rock (sample 55-S2) is given in table 2. Some limestone grains contain 

considerable clastic quartz and some chlorite and pyrite. Chlorite occurs as sheaves in small altered mineral 

grains. Chalcedony grains (included with chert in table 

2) contain concentric colloform structures. Calcite 

cement of the rock (about 10 percent) is coarse to fine 

grained and locally merges with limestone grains, indicating these to be the likely source of this cementing 

material. Most of the pyrite of the rock, as cubes and 

anhedra, occurs in calcite cement. Parts of the interiors 

of some large limestone grains are recrystallized and 

composed of coarse calcite, locally showing polysynthetic twins. Bedding of the rock is manifested by

Limestone

Microcline

Pyrite

Quartz

Limestone

Perthitic 

orthoclase

Calcite

FIGURE 5. Light-gray coarse-grained calcarenitic sandstone of 

the Moss Back Member. Illustration shows a higher proportion of quartz and feldspar than is typical of most of the 

rock. Sample 55-S-2, Dolores River Canyon.

thin layers which appear to have had some shearing 

along them.

The so-called Black Ledge of the Church Rock Member of the Chinle Formation (Stewart and others, 1959, 

p. 518), made up of sandstone, siltstone, and minor conglomerate, was sampled in the Dolores River Canyon. 

This sample (55-S-l) is of the upper sandstone layer 

of the Black Ledge, a very fine grained thinly and horizontally laminated sandstone in which muscovite flakes 

and clay are conspicuous on some bedding planes. A 

thin section of this rock shows poorly sorted largely 

angular to flakelike grains in a clay and calcite matrix 

contaminated by dustlike indeterminate material (fig. 

6). Mineral composition of the thin section is given in 

table 2. Some clay, calcite, and chlorite may be altered 

ferromagnesian minerals. In the rock matrix is some 

very fine grained to medium-grained interlocking 

secondary quartz, but none as overgrowths, and minor 

chlorite and probably anatase as dustlike material. 

Hematite is thoroughly dispersed in the matrix and 

coats most detrital grains; it occurs also as detrital black 

opaque grains. Calcite cement, more or less evenly distributed through the rock, is in optically continuous 

patches, as much as 0.5 mm in diameter, which enclose 

several detrital grains. The rock may be largely volcanic material, as the sharp flakelike fragments show no 

evidence of abrasion by water transport; furthermore, 

the even, fine horizontal stratification suggests deposition of this material by air fall into quiet water or onto
PETROGRAPHY OF SEDIMENTARY ROCKS B9

Quartz

Quartz

Muscovite

Hematite 

plates

0.5 mm 

j

FIGURE 6. Reddish-brown very fine grained sandstone from the 

so-called Black Ledge of the Church Rock Member. Angular 

and fiakelike grains suggest a crystal tuff origin of the rock. 

Sample 55-S-l, Dolores River Canyon.

land. Some water-borne detritus is present, however, as 

evidenced by scattered well-rounded grains in the rocks. 

Probably original glass devitrified, and constitutes 

much of the clay, silica, and perhaps calcite, in the 

matrix.

ENTRADA SANDSTONE

Sandstone and siltstone of the Dewey Bridge Member of the Entrada Sandstone are quartzose and are 

cemented by variable amounts of clay and in small 

spots by carbonate and barite. Muscovite or biotite is 

visible in places. Mineral composition of a thin section 

of typical reddish-brown siltstone from 10 feet below 

the top of the member (sample DVR-2-1990) collected 

from a deep diamond-drill hole drilled by the U.S. Geological Survey in Disappointment Valley, sec. 36, T. 44 

N., K. 18 W., is given in table 2. Hematite content shown 

in the table includes numerous detrital black opaque 

minerals. The rock is cemented with mostly clay, some 

hematite as films on quartz grains and finely dispersed 

in clay, and a small amount of carbonate. Some of the 

carbonate, in small scattered blebs, is probably calcite, 

and some, as rhombs, may be dolomite. The rock is 

poorly sorted and well packed, with angular to wellrounded silt-sized detritus containing rare mediumsized quartz and chert grains.

The Slick Kock Member of the Entrada Sandstone 

consists of very fine grained to fine-grained sandstone 

in three units, a lower massive unit, middle crossbedded

unit, and an upper horizontally bedded unit. The average mineral composition of the member as judged from 

the average composition of seven thin sections is shown 

in table 2. Five of the seven samples of the Slick Kock 

are light reddish brown and contain hematite as grain 

coatings, finely dispersed materials, and detrital black 

opaque minerals, whereas the other two samples are 

light brown and contain no hematite. The mineral compositions of samples from the three subdivisions of the 

Slick Rock Member are given in table 3. A typical 

sample of the massive unit (SH-3-55) was collected 

near its top at the north side of Poverty Flat. A thin 

section of this sample shows the rock to be moderately 

sorted, and most grains rounded (fig. 7). Scattered 

grains about 0.5 mm in diameter are known as Entrada 

berries (Wright and others, 1962, p. 2063). Although 

most of the Entrada berries are quartz, some are small 

rock fragments, or material that has been altered to 

clay; one colloform-banded chalcedony grain was observed. Minor chlorite occurs probably as an alteration 

product in some of the fine-grained detritus. Matrix 

makes up about 20 percent of the rock and is mostly clay 

and about one-fourth carbonate as small patches of optically continuous calcite as much as 0.5 mm across, and 

rare euhedral rhombs probably dolomite, between 

quartz grains. A small amount of dustlike hematite, 

and traces of minute euhedral anatase crystals and ironrimmed carbonate rhombs that may be siderite, are 

scattered throughout the matrix.

Grain-size analyses of two samples of the massive 

unit of the Slick Eock Member (EN-32 and EN-38)

TABLE 3. Average mineral composition, in percent, of sandstones 

from the Slick Rock Member of the Entrada Sandstone

[Based on modal point counts (200 points) and modal estimates of thin sections. 

"Minor" generally indicates less than 0.25 percent]

Component

Quartz grains and overgrowths- _ ___ 

Chert_ _____-__-__.-_.._________.

Plagioclase _ ____________________

Rock fragments and mineral aggregates__ __.- ___ __ __ _________

Clay (mineral clay and clay-sized 

detritus) _ __ _ __________ ___

Barite. ______ _______ ____________

Chlorite_____---_----_- ____ .__

Hematite and other black opaque 

minerals. __________________ __

Biotite____ _____ _ ____ _

Limonite. _______ ___ ___ _ _ .

"Pvrii"**

Number of samDles___ _ _ _______

Massive 

unit

76 

3

4

1

1.5

9

5

Minor

Minor

Minor

2

Crossbedded 

unit

67 

3. 5

7

3

4

5

9

Minor

Minor

Minor

Minor

4

Horizontally 

bedded 

unit

75 

5.5

5. 5

4

5

Minor

3

1

Minor

Minor

Minor

Minor

Minor

1
BIO GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

Quartz

Hematite 

dust

Calcite

FIGUBE 7. Light-reddish-brown very fine grained sandstone 

from the massive unit of the Slick Rock Member. Clay and 

calcite form matrix; detrital hematite grains are abundant 

and most sand grains are coated with hematite dust; relatively large grain in lower left quadrant is typical Entrada 

berry. Sample SH-3-55, near the top of the unit at the north 

side of Poverty Flat.

are shown in figure 8. The %6-% mm and %-^ mm 

size fractions together average 73.8 percent. Percent of 

grains in the two dominant size fractions is here used 

merely as an index for comparison with other samples 

of Entrada Sandstone. In terms of all the size fractions 

in the rock, the massive unit consists of less than 1 percent coarse sand, about 1 percent medium sand, 16 percent fine sand, 58 percent very fine sand, 19 percent silt, 

and 5 percent clay.

Sandstone of the crossbedded unit of the Slick Kock 

Member (table 3) consists of quartz and lesser K-feldspar and plagioclase grains in a clay and calcite matrix. 

Sparse well-rounded quartzite fragments are present. 

Orthoclase is both untwinned and carlsbad twinned; 

plagioclase shows albite and pericline twins. This rock 

is well sorted and has rounded to well-rounded grains. 

A few bedding planes contain very fine sand or silt-sized 

sharply angular detrital grains that may be of crystal 

tuff origin. Part of a thin section (SH-1-55) from a 

sample collected near the base of the crossbedded unit 

at the north side of Poverty Flat is shown in figure 9.

Grain-size analyses of five samples of the crossbedded 

unit (En-4, 7, 17, 28, 36, fig. 8) show that the y16-i/8 

mm and l/s~lA mm size fractions together average 91.2 

percent. In terms of all the size fractions in the rock, 

the crossbedded unit consists of less than 1 percent

SECTION

D

SAMPLE

EN -7

EN 28

EN-36

GRAIN SIZE 

IN MILLIMETERS

1/2-1 

1/4-1/2

1/8-1/4

1/16-1/8

1/256-1/16 

<l/256

1/2-1 

1/4-1/2

1/8-1/4

1/16-1/8 

1/256-1/16 

< 1/256

1/2-1 

1/4-1/2

1/8-1/4 

1/16-1/8

1/256-1/16 

< 1/256

1/2-1 

1/4-1/2

1/8-1/4

1/16-1/8

1/256-1/16 ' 

< 1/256

1/2-1 

1/4-1/2

1/8-1/4

1/16-1/8

1/256-1/16 

< 1/256

1/2-1 

1/4-1/2

1/8-1/4

1/16-1/8

1/256-1/16 

<l/256

1/2-1 

1/4-1/2

1/16-1/8

1/256-1/16 

<l/256

1/2-1 

1/4-1/2

1/8-1/4

1/16-1/8

1/256-1/16 

< 1/256

1/2-1 

1/4-1/2 

1/8-1/4

1/256-1/16

<l/256

1/2-1 

1/4-1/2

1/16-1/8 

1/256-1/16 

< 1/256

1/2-1 

1/4-1/2

1/16-1/8

1/256-1/16 

<l/256

1/2-1 

1/4-1/2

1/8-1/4

1/16-1/8

1/256-1/16 

<l/256

1

Wl ' ' ' ' ' ' '

-:. -:i:: . ?:;:::-: c; '.;:-:;::.; :,:; :-;?.-::-: :,!:,:;:;::-:<: ;: . ;:: : 'jAWTO^ i«. 'frs :si" '.': f iiii) Q-, _ Horizontally

^.'.i':-+f'^-^':, ::';'f^ifffKf&'i,.d ts/.a bedded unit

^- \ \ 1 1 1 1 1 1 

: :~"', " '" " : ", v '. ; :' ' ' -:' ':- 1 ":: '":- ' '.'":' -\. - " - ' ;:'i- .'.' ' ' '-x 1 . 1 '. 1 >"':'' ' - ^ '. - '. :; <:'- :'.' '':'.' ' ' ', ^"- - -,'? '. ; -, ;*> " "'.^^$: ' '. '. ' ,- ' '' ' - .'' ." \

: ;.^^j 94 5 Crossbedded 

if i unit

^

 :Ws^.^f:f;^*^":^~ .;.: .;;. ::;-. ;-£t.xw qi . urossoeaaea

 1

i: : :. : ,-:-; - .-.--^-:: : .:..:: .; ;:.:,: , : ;-.' ,.... . ;. : .'. ' :. ,..-*- -> : ". i HOrlZOHtallV

 1

--. ; .' .:' ; ::':i;v' ; ;: l;-: ,''-:"::v"ji -:;;i : '-::/.:.' : . : .- ,;.-; - . . ^ '. ":i QQ -, Crossbedded

.i':j' .- : ' :;;; :-*:;:.^:^: ":, ;, . ! «« / unj t

^ | | | | | | | 

3- \ 1 1 1 1 1 1 1 

 .-- ::. : :: : :'.' ' :. '.'.'-'': :..'.: .,.': , ... ...::' .: ..:.'.'. . ...:':.;,';." J." : ;:' V :'".:'. 1 QQ F HOMZOntally

1 ay-J bedded unit

: :'.-': :--:' ' -: : :" ;: / ; : IMII t . ..-:-.:;:; ,:,:.:..., .,-.'. '.,.. . ; :-'..:. '/ - : - \ :.

J | | | | | | |_unit_j 

 ; - .. - - - . ... ;T. ... ... -:j

^ 1 1 1 1 I 

.-: - - v .. >- - -:- -:-::-:-' ;-:;. : : . ::i:-:.:','::, .,::-;- -:'-: ' : . ,- . ;-. :. ! JQ ^ Summerville

- :'....|jj ; ^J '"'"' Formation

';:-. 'p_ J 87 o Crossbedded

; - ; -«: ....v:-.:;:-'-:,:.:.. -: -:.::;;- :. .. . : , '. .:- \ O/.U ||r)j^

_lc! | | | | | | | | 

i

'<;. ^.l :,.iV,;;;,i.;.;:,l ( (

10 20 30 40 50 60 70 

GRAIN SIZE DISTRIBUTION, BY WEIGHT, IN PERCENT

(>500 microns) = coarse sand

-^ «500>250 microns) =medium sand 

(<250>125 microns) =fine sand

-Mi «125>62 microns) = very fine sand

FIGUBE 8. Histograms of grain-size distribution for samples 

from the Slick Rock Member of the Entrada Sandstone and 

lower part of the Summerville formation (2 samples). Figure 

at end of each histogram is percent of sample in size fractions 

%-% mm and He-% mm, combined.

coarse sand, about 3 percent medium sand, 60 percent 

fine sand, 30 percent very fine sand, 3 percent silt, and 3 

percent clay. These values show that the crossbedded 

unit is considerably better sorted than the underlying 

massive unit (average index 73.8), and corroborate the 

conclusion, on grounds of bedding characteristics, that 

the crossbedded unit is dominantly of eolian origin.

Modal estimates of mineral composition of four thin 

sections of samples from tne crossbedded unit (table 3) 

indicate a higher content of cementing material than 

is indicated by the grain-size analyses and by the data 

on average calcite and solubles content to be presented 

in a later chapter of this professional paper series on
PETROGRAPHY OF SEDIMENTARY ROCKS Bll

Feldspar

Limonite

Calcite'

Clay

Quartzite

Quartz

0.5 mm 

I

FIGURE 9. Buff very fine grained sandstone from the crossbedded unit of the Slick Rock Member. Patch of optically 

continuous calcite bordered by spots of limonite forms a local 

cement around sand grains; clay is the principal matrix in 

most of the rock; individual sand grains are quite clean. 

Sample SH-1-55, near the base of the unit at the north side 

of Poverty Flat.

rock alteration. Two of the samples examined in thin 

sections were collected near the base of the crossbedded 

unit; these samples average about 25 percent cement 

that is principally calcite. They are not typical of the 

crossbedded unit in this respect, and may be from a 

part of the unit that is transitional into the massive unit 

below.

A sample from 7 feet below the top of the horizontally bedded unit (DVR-2-1862.5) was collected from 

drill core from the deep diamond-drill hole in Disappointment Valley, sec. 36, T. 44 N., R. 18 W. The mineral composition of a thin section of this sample is given 

in table 3. The rock is light reddish brown, moderately 

sorted, and contains rounded to well-rounded very fine 

sand. Sparse Entrada berries are present. Interstitial 

material consists of clay, locally only as grain coatings, 

and small patches of calcite. Sparse authigenic dolomite rhombs occur within clay.

Grain-size analyses of three samples (EN-1, 15, 27, 

fig. 8) show that the l/i§-l/% mm and %-% mm size 

fractions together average 88.8 percent. In terms of all 

the size fractions in the rock, the horizontally bedded 

unit consists of less than 1 percent coarse sand, about 

5 percent medium sand, 60 percent fine sand, 29 percent 

very fine sand, 3 percent silt, and 2 percent clay. Although the average percentage of size fractions in the

297-092 O 68 3

horizontally bedded and crossbedded units are almost 

identical, the range for individual size fractions in the 

crossbedded unit is great and in the horizontally bedded 

unit rather narrow. This fact suggests that, if the horizontally bedded unit was reworked from the crossbedded unit, as is likely, the various reworked parts of 

the crossbedded unit were thoroughly mixed in the 

process.

SUMMERVILLE FORMATION

The Summerville Formation consists of mudstone, 

siltstone, and very fine grained to fine-grained sandstone that make up a lower transitional unit, an argillaceous unit above this, a sandstone "marker bed" above 

this, and an argillaceous unit at the top. A reddishbrown argillaceous sandy layer of the transitional unit 

of the Summerville was collected from drill core from 

the drill hole in sec. 36, T. 44 N., R. 18 W., Disappointment Valley. The sample (DVR-2-1825) consists of 

poorly sorted fine-grained sand of sharply angular to 

very well rounded detritus and an extremely finegrained interstitial mixture of calcite, clay, and hematite. The sample contains thin layers of mudstone and 

siltstone; parts of these layers are shown in figure 10. 

Sparse medium to coarse grains in finer grained detritus suggest kinship to the underlying Entrada. This 

sample is suggestive of the small-scale heterogeneity 

that is characteristic of the Summerville. Although 

abundance of mineral constituents ranges widely in different parts of the transitional unit, a modal estimate 

of the composition of the thin section from the illustrated sample indicates the general mineralogy, and is 

given in table 2. Rounded detrital black opaque minerals are included in table 2 with hematite. About 25 

percent calcite as irregular granular patches and 15 percent clay, both as clay-sized detritus and clay minerals, 

that contain perhaps half a percent of dustlike hematite make up the matrix of the rock. Abundance of 

sharply angular fragments of detritus suggest possible 

admixture of airborne volcanic material in the rock.

Grain-size analyses of two samples of the transitional 

unit (EN-25 and EN-34, fig. 8) show that the 1/16-1/8 

mm and 1/8-1/4 mm size fractions combined average 

81.1 percent. In terms of all the size fractions in the 

rock, the samples average less than 1 percent coarse 

sand, about 8 percent medium sand, 53 percent fine sand, 

28 percent very fine sand, 7 percent silt, and 4 percent 

clay. These samples are better sorted than most of the 

Summerville, and in this respect are more like the underlying Entrada Sandstone.

Probably typical of the mudstone and siltstone layers 

in the upper argillaceous part of the Summerville Formation is a sample (DVR-2-1790) collected from drill
B12 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

TABLE 4. Average mineral composition, in percent, of siltstones, mudstones, and shales from some Mesozoic formations in the Slick Rock

district, Colorado

[Based on modal point counts (200 points) and modal estimates of thin sections. "Minor" generally indicates less than 0.25 percent]

Component

Summerville 

Formation

Morrison Formation

Salt Wash Member Brushy Basin Member

Burro Canyon Dakota 

Formation Sandstone

Mancos Shale

Quartz detritus and secondary quartz. ___ 

Chert. __ _ ___ _ ___ _ _._ _______

Orthoclase ______ __ ___ _ _ _ _ _____

Plagioclase___ _ ___ _ _________ _ _ __

Rock fragments and mineral aggregates. _

Clay (mineral clay and clay-sized detritus).

Barite___ _________ _____ _ ____ _ _ _

Hematite and other black opaque minerals

Muscovite __ _ _ _ ____________ _

Pyrite__ ___ _ __ _ _ ___ ___ _ _ _ __

Analcite.. _ ___ _ _ __ __ ____ _____

Carbonaceous material. _ ______ _

Siltstone

51 

1

4

3

4

30 

5

1

.5

Siltstone

50 

4

4

1

1. 5

20 

18.5

Minor

Minor

Mudstone

27. 5 

3.5

2.5

.5

1

40 

23.5

. 5

Minor

Minor

Minor

. 5.

Siltstone

61 

6

3

1

2.5

17.5 

7.5

.5

Minor

Mudstone

28 

.25

2. 5

Minor

3. 5

Minor _

56 

3.5

1

2.5

.25

.25

.5 .

Minor .

Minor

1

.5

Claystone

9. 5 

1

1

.25 .

. 5 .

82 

4.5 .

Minor

. 25 .

.25

.5

Mudstone

25 

0)

2

67

Minor

Minor

Minor

Minor

Minor

5

Mudstone

29.5 

3 _

2. 5 _

.5 .

2. 5 _

2 56. 5 

1.5

.5 _

.5 .

Minor _

Minor

.75

1.5

Shale

5

73.5 

17.5

1

3

Number of samples. 13

i Included with quartz. 2 In one sample, clay includes about 10 percent clay-chlorite aggregates.

core obtained from the deep diamond-drill hole in Disappointment Valley. A modal estimate of the mineral 

composition of a thin section of this clayey siltstone is 

given in table 4. Numerous detrital black opaque grains 

are included in the table with hematite. About 30 percent clay, consisting of both clay mineral and clay-sized

Perthitic 

prthoclase

Zircon

Calcite 

cement

Hematite

1 mm 

J

FIGUBE 10. Poorly sorted reddish-brown sandstone with thin 

siltstone and mudstone layers (shown here) from the lower 

part of the Summerville Formation. Sample DVR-2-1825, 

Disappointment Valley.

detritus, 5 percent calcite, for the most part extremely 

fine grained and in scattered patches, and a trace of 

authigenic yellow anatase crystals and perhaps a small 

amount of dustlike hematite, make up the matrix. Some 

calcite occurs as sparse detrital limestone grains.

MORRISON FORMATION ROCKS

SALT WASH MEMBER

The Salt Wash Member of the Morrison Formation 

consists of interbedded fine-grained fluvial sandstone, 

siltstone, and flood-plain mudstone strata comprising 

(1) a lower sandstone unit, (2) a middle mudstone unit, 

and (3) an upper or "ore-bearing" sandstone unit. The 

average mineral composition of 38 samples of sandstone 

from the member is given in table 2. A comparison is 

made in table 5 of the average mineral composition of 

thin sections of 19 of these samples based on 200-point 

counts, of thin sections of 19 different samples based on 

modal estimates, and of 202 samples of the Salt Wash 

Member from the whole Colorado Plateau as analyzed 

by K. A. Cadigan (in Craig and others, 1955, p. 147- 

148). This comparison shows that the analyses of Slick 

Kock samples by point counting and by estimating are 

close, and that the average composition of Slick Kock 

samples is close to the average for the whole Colorado 

Plateau.

A sample (DVR-2-1667) of light-reddish-brown very 

fine grained sandstone from the upper part of the lower 

unit was collected from drill core from the deep diamond-drill hole in Disappointment Valley. An estimate
PETROGRAPHY OF SEDIMENTARY ROCKS B13

TABLE 5. Comparison of average mineral composition of sandstones from the Salt Wash Member, Slick Rock district, with 

that of sandstones from the Salt Wash Member throughout the 

Colorado Plateau

Component

Calcite ______ ______ _

Clay-__-__-___--------

Other _ ._ __ _______

Slick Rock district 

/-<

Point Modal 

counts estimates

72.3

4. 3

4. 8

2.7

. 6

8.2

2. 1

5. 0 _

19

71.7 

4. 3 

6. 4 

5.3 

.9 

8.5 

2.9

19

Average P

72.0 

4.3 

5.6 

4. 0 \ 

.8 j 

8.3 

2.5 __

2.5 __

38

olorado 

ateau '

71.4 

4

5.8

5. 8 

13

202

1 Analyzed by R. A. Cadigan, calculated from data of Craig and others, (1955 

p. 147-148).

of the mineral composition of a thin section of this rock 

is given in table 6. Hematite in table 6 includes detrital 

black opaque grains. Detrital grains are rounded to well 

rounded and moderately sorted. About 15 percent silica 

as 'overgrowth on detrital grains constitutes most of the 

rock cement. Some overgrowths are faceted, but most 

are joined to adjacent overgrowths along sutured or 

irregular borders. A small amount of calcite as irregular small interstitial blebs, dustlike hematite and grain 

coatings, and a trace of barite make up the remainder 

of the matrix. A few detrital grains are limestone, some 

of which are surrounded by calcite cement that is also 

molded around adjacent quartz and feldspar grains.

Modal estimates of the mineral content of thin sections of three samples of sandstone from the middle

TABLE 6. Average mineral composition, in percent, of sandstones from the Salt Wash Member of the Morrison Formation

[Based on modal point counts (200 points) and modal estimates of thin sections. 

"Minor" generally indicates less than 0.25 percent]

Component

Chert- --_ . _ --_ __ _

Microcline _____ ________

Plagioclase _ _ __ _ __ ____

Rock fragments and mineral aggregates. _ __ ___________

Clay (mineral clay and clay-sized 

detritus)___ _ _______ _____

Calcite and dolomite _ _ _________

Barite______ _ _ _ __ _________

Chlorite _ ___ _ ___ ______ ___

Hematite and other black opaque 

minerals- _ _________ __ _ ___

Biotite__ __ _____ _ ____

Muscovite. ____ ____ ____ __

Leucoxene and anatase ___________

Limonite-- _________ _ ___ _ _ _

Pyrite_ ________ ____ ____

Number of samples __ _____

Lower 

unit

76

7 

10 

1

1

2

Minor 

2 

Minor 

Minor

Minor

Minor

1

Middle 

unit

77.5 

2 

3.5 

.5

1

Minor

1 

13

Minor

. 5 

Minor

Minor

Minor

3

Upper 

unit

75- 

6.5 

4.5 

.5

1

Minor

2 

9

Minor 

Minor

Minor 

Minor 

Minor 

Minor 

Minor

14

unit of the Salt Wash Member are averaged in table 6. 

Hematite is present in two of the thin sections both 

light-reddish-brown sandstone and pyrite is present 

in one a light-gray sandstone containing carbonaceous 

material. Detrital grains in the samples are angular to 

well rounded, poorly to moderately sorted, of chiefly 

very fine grain size, ranging from silt to medium-fine 

grain size. Light-reddish-brown sandstone has either 

minor thin ragged silica overgrowths on a few quartz 

grains, or abundant overgrowths, largely irregular and 

a few faceted, more in some parts of the rock than in 

others. A sample of light-reddish-brown sandstone 

(SR-1) illustrated in figure 11 has abundant overgrowths on quartz grains and shows some solution of 

quartz grains, suggesting a local origin of matrix silica. 

Light-gray sandstone has some etched quartz grains 

and a minor amount of overgrowth silica, also possibly 

indicating only local movement of silica in solution.

Modal estimates and point counts of the mineral content of 14 thin sections of sandstone from the upper 

unit (ore-bearing sandstone) of the Salt Wash Member 

are averaged in table 6. Several of these samples were 

collected close to uranium-vanadium deposits and contain much greater-than-average amounts of calcite as 

determined for the ore-bearing sandstone by Archbold

Detrital

hematite 

Quartz with 

overgrowth

Rock 

fragment\':.-

Plagioclase

Granular 

calcite

0.5 mm

FIGURE 11. Light-reddish-brown fine-grained sandstone from 

the middle unit of the Salt Wash Member. Detrital quartz 

grain to the right of quartz with overgrowth fits against a 

partly rounded detrital quartz grain possibly because of 

partial solution under pressure; many grains have thin dusty 

hematite coatings. Sample SR-1, north of the Middle Group 

mines at Slick Rock.
B14 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

(1959). For this reason the average calcite content of 

the 14 samples is about three times that of the orebearing sandstone as a whole. Most of the samples are 

of light-brown sandstone and only a few of lightreddish,-brown, which accounts for a greater number 

containing limonite (after pyrite or marcasite) rather 

than hematite. Detrital black opaque minerals occur 

along with hematite films and dustlike material in the 

reddish-brown rocks.

Sandstone of the upper unit is generally fine to very 

fine grained and moderately to well sorted, and has 

grains that are angular to well rounded (fig. 12). Commonly, the sandstone is cemented by calcite, though in 

most places not so thoroughly as shown in figure 12, 

and by silica. Calcite cement more or less permeates 

sandstone in places that are close to ore deposits (as in 

fig. 12) and adjacent to mudstone layers (Archbold, 

1959). Elsewhere in the upper unit, calcite occurs as 

patches enclosing a few sand grains, and where sparser, 

as isolated interstitial blebs. Many of the patches of calcite show poly synthetic twins (fig. 12), possibly indicating stressing of the sandstone after calcite deposition. 

Silica cement ranges from almost none to about 15 percent of the rock. It occurs as overgrowths, commonly 

irregular, which join adjacent overgrowths at sutured

Microcline

Quartz

Calcite Plagioclase

largely replaced

by calcite

0.3 mm 

J

FIGUEE 12. Very light brown very fine grained standstone 

from the upper unit of the Salt Wash Member, cemented 

by calcite. Calcite that has almost completely replaced a 

plagioclase grain is optically continuous with surrounding 

calcite, but polysynthetic twinning in the interstitial calcite does not extend into the replaced grain; slightly 

mineralized rock is within a few millimeters of the 

area shown. Sample C-6, Cougar mine.

contacts, but which in many places are faceted. Perhaps 

faceted overgrowths are more abundant in more porous 

sandstone. Some sandstone has little or no silica overgrowths on grains, but the grains are tightly held together by their own interpenetration at places of contact, as though some silica went into solution at a time 

when the rock was under pressure. An example is shown 

in figure 13, a sandstone that also contains probably 

more than average limonite cement (after pyrite or 

marcasite). The limonite occurs as isolated patches in 

the sandstone, one of which is shown in figure 13.

Eeplacement textures are evident in the sandstone of 

the upper unit and are shown mainly by calcite as irregular penetrations around the periphery of quartz 

grains and between grain and silica overgrowth, and 

by the selective replacement of plagioclase, illustrated 

in figure 12. In places microcline and orthoclase are 

replaced by clay minerals.

A typical reddish-brown siltstone was collected near 

the top of the lower unit in Disappointment Valley 

(sample DVR-2-1597.5). A modal estimate of the mineral composition of a thin section of the rock is given 

in table 7. Some hematite is detrital as indicated by 

subrounded to rounded forms, and some occurs finely 

dispersed in clay; traces of chlorite are found in a few 

detrital minerals. Detritus is sharply angular to

Quartz

Limonite

 Tourmaline

0.5 mm

FIGUEE 13. Very light brown very fine grained sandstone from 

the upper unit of the Salt Wash Member. Sutured contacts 

between some quartz grains suggest solution of silica under 

pressure; opaque mineral is limonite altered from pyrite 

or marcasite; upper tourmaline is dichroic in shades of blue; 

lower, in shades of yellow. Sample SR-2, Lone Peak 1 claim, 

Spud Patch group of mines.
PETROGRAPHY OF SEDIMENTARY ROCKS B15

TABLE 7. Average mineral composition, in percent, of siltstones 

from the Salt Wash Member of the Morrison Formation

[Based on modal point counts (200 points) and modal estimates of thin sections. 

"Mi lor" generally indicates less than 0.25 percent]

Component

Quartz grains and overgrowths _

Chert _ _______________ ______

Orthoclase. _ _ _______ ____ _

Microcline ___ __________

Plagioclase__ __ _ _ _ ____ _ ___

Clay (mineral clay and clay-sized

detritus) ______________

Calcite_ ___ _ _________ ___ ___

Chlorite __ __ _ __ _ __ ___ _ _

Hematite 'and other black opaque

minerals __ _ _ ________ _

Biotite. ___ __ _______ _ _____

Muscovite __ ____ ___ _ _ _ _

Leucoxene __ ___ ___ _ ______

Limonite _ -__----_-___

Number of samples. _ _ _______ ___

Lower

unit

50

5

5

1

3

25

10

Minor

Minor

Minor

1

Middle

unit

45

5

5

2

2

30

10

Minor

Minor _

Minor

1

Upper

unit

52.5

3.5

3

.5

. 5

12

27

Minor

Minor

Minor

Minor

Minor

2

rounded, poorly sorted, silt sized; a minor amount is 

fine sand sized. Clay is concentrated in laminae that 

represent bedding layers, and these are mashed and distorted probably by slumping of the rock before consolidation. Calcite occurs interstitial to detrital grains, 

as well as in blebs in clay. Some detrital grains are 

wholly calcite and some only partly calcite, as though 

replaced by it. A few devitrified glass shards are evident in clay, and these, together with sharply angular 

detritus, suggest that some airborne volcanic material 

is present. Probably ash-fall material is mixed with 

water-transported material.

A modal estimate of the mineral composition of a 

siltstone sample (DVR-2-1540) that is probably typical 

of siltstone from the middle unit of the Salt Wash 

Member is given in table 7. Hematite occurs as detrital 

grains, and hematite and chlorite are present in trace 

amounts disseminated in the matrix, and in or coating 

detrital grains. Detrital grains in the sample are chiefly 

silt sized and in small part sand sized; they range from 

sharply angular to well rounded and are chiefly poorly 

rounded, embedded in clay matrix. Amounts of matrix 

vary in different bedding layers.

Thin sections of two samples of siltstone from the 

ore-bearing sandstone have the average composition 

given in table 7. Grains are sharply angular to rounded 

and poorly sorted. One of the samples contains angular 

clay fragments as much as 1 cm in length. Siltstone in 

the ore-bearing sandstone seems to be intermediate between mudstone and sandstone in most of its properties. 

Calcite is more abundant than in either of the other 

types, however, although the samples studied may be 

atypical in this respect.

The lower part of the lower unit of the Salt Wash 

Member in the vicinity of the drill hole in Disappointment Valley is not typical, consisting of a carbonaceous 

mudstone layer about 50 feet thick not known elsewhere 

in the district. A sample (DVR-2-1722.5) of the carbonaceous mudstone collected from the drill hole was 

examined in thin section, and a modal estimate of the 

mineral composition of the thin section is given in table 

8. Clay is strongly birefringent and may be in large 

part hydrous mica. A small amount of calcite and a 

trace of pyrite and minute rhombs of dolomite occur 

as visible authigenic minerals scattered throughout clay. 

Some calcite is detrital. The rock is well bedded, and 

carbonaceous material is largely concentrated along 

bedding planes. A pen-and-ink drawing of part of the 

thin section is shown in figure 14.

A modal estimate of the mineral composition of a 

thin section of a mudstone sample (DVR-2-1465) collected near the top of the middle unit of the Salt Wash 

in Disappointment Valley is given in table 8. Hematite 

occurs as detrital grains and as coatings on quartz and 

other minerals; it also is finely dispersed throughout the 

rock matrix. Detritus, sharply angular to rounded, is 

chiefly silt sized and is embedded in a matrix of clay 

and chiefly clay-sized calcite detritus. Some calcite occurs as silt-sized detrital grains. Bedding laminae, manifested by alternating clay-rich and detritus-rich parts, 

are contorted, propably by preconsolidation slumping.

Biotite Quartz

 Clay

Leucoxene

Carbonized plant 

fragment

0.3 mm

FIGUBE 14. Dark-gray carbonaceous mudstone from the lower 

part of the lower unit of the Salt Wash Member. Almost all 

the black particles are carbonized plant fragments. Sample 

DVR-2-1722.5, Disappointment Valley.
B16 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

TABLE 8. Mineral composition, in percent, of mudstones from the 

Salt Wash Member of the Morrison Formation

[Based on modal estimates of thin sections. "Minor" generally indicates less than

0.25 percent]

Component Lower 

unit

Middle 

unit

Quartz grains and overgrowths _____________ 26 30

Chert__-____________________-_ 3 4

Orthoclase___-__________-_____-_-__-_-__- 2 3

Microcline_ ______________________________________ 1

Plagioclase______________________________ 1 1

Clay____._______________________________ 65 15

Calcite and dolomite-_____________________ 2 45

Hematite and other black opaque minerals.._________ 1

Biotite-_____-___________________________ Minor ________

Muscovite.______________________________ Minor Minor

Leucoxene_______________________________ Minor Minor

Pyrite_.__.______________________________ Minor ________

Carbonaceous material.___________________ 1 ________

Number of samples.

BRUSHY BASIN MEMBER

The Brushy Basin Member of the Morrison Formation is bentonitic flood-plain mudstone interbedded with 

lesser amounts of sandstone and conglomerate and a 

minor amount of thin limestone layers. These rocks 

comprise a lower brown unit, a middle green unit, and 

an upper brown unit. The average mineral composition 

of 13 thin sections of sandstone from the Brushy Basin 

Member is given in table 2. Modal point counts and 

estimates of the mineral content of seven thin sections 

of drill cores of fine-grained to very fine grained sandstone from the lower brown unit are averaged in table 

9. Similarity to composition of sandstone in the underlying ore-bearing sandstone of the Salt Wash (table 6) 

is evident. Sandstone of the lower brown unit of the 

Brushy Basin is poorly to moderately sorted. Silica 

cement in these rocks, included with quartz in table 9, 

ranges from none to almost 10 percent, and probably 

averages less than 5 percent. It occurs in places as almost complete filling of pore spaces (fig. 15), as sutured 

interpenetrating overgrowths, and where pore spaces are 

incompletely filled, as faceted overgrowths. Calcite is 

present principally as cement, although detrital limestone grains are locally fairly abundant. Calcite occurs 

as isolated small blebs, illustrated in figure 15, as larger 

patches that enclose several detritial grains, and as extensive zones of cement, commonly at the tops and bottoms of sandstone layers. Figure 16 shows optically 

continuous calcite cement in a tightly cemented zone 

near the base of a sandstone layer 45 feet thick at the 

top of the lower brown unit. Calcite cement and detrital 

grains are locally polysynthetically twinned. Parts of 

sandstone layers in the unit are well cemented with clay, 

or mixtures of clay, very fine grained calcite, and finely 

disseminated hematite, as shown in figure 17. Hematite

occurs in all reddish-brown sandstone in the unit, commonly as detritus and as films coating other detrital 

grains (figs. 16, 17), as well as dispersed in the matrix 

(fig. 17). Barite in reddish-brown sandstone is typically 

irregularly shaped and is probably chiefly a matrix 

mineral; some fragments from heavy-mineral separates 

show crenate outlines that suggest the original shape of 

a barite-filled space between rounded detrital grains. In 

light-gray or light-buff sandstone, barite commonly has 

replaced quartz grains (fig. 15), or forms euhedral 

crystals between detrital grains and in places penetrates 

into detrital grains. Anatase appears locally along with 

calcite and hematite, finely disseminated in a clay 

matrix.

The average mineral content of thin sections of three 

very fine grained sandstone samples from the middle 

green unit is shown in table 9. Barite amounting to less 

than one-half percent has replaced chert locally in one 

rock. Figure 18 shows the general dirty character of 

these sandstones; this rock from the Legin area, sample 

LE^30-461, contains minute segregations of marcasite, 

one of which is shown here. The intimate association of 

calcite and marcasite may indicate their segregation in 

interstitial patches simultaneously, when the rock was 

cemented. In sandstone of the middle green unit erratic 

solution and deposition of silica has caused scattered 

development of corroded quartz grains and silica overgrowths. Calcite appears to have replaced small patches 

of silt- and clay-sized particles in the sandstone. Clay 

and calcite replacement of feldspars has occurred in 

places. The very fine grained sandstone is composed of 

poorly sorted sharply angular to well-rounded detrital

TABLE 9. Average mineral composition, in percent, of sandstones 

from the Brushy Basin Member of the Morrison Formation

[Based on modal point counts (200 points) and modal estimates of thin sections. 

"Minor" generally indicates less than 0.25 percent]

Component

Lower Middle Upper

brown green brown

unit unit unit

Quartz and overgrowths ______ ___.

Chert ____-_--_ _ _ __- _ -___.

Orthoclase __ _________-_-- ___ .

Microcline ____ _ _ _-__ ________

Plagioclase___-_ ________ ___ ___.

Rock fragments and mineral

aggregates __ _ _ _________ _.

Clay (mineral clay and clay-sized

detritus)____-____ _____________

Calcite and dolomite _____________

Barite______ _ ___ __ ____ _ ___.

Chlorite ___ ____________________

Hematite and other black opaque

minerals.- ___ ____ __ _ ______

Biotite, _---._ _ ---__ ___ _.

Leucoxene and anatase _ ___ ... .

Number of samples ____ ._______.

. 74

6

2

1

2

1

2.5

11

. Minor

. Minor

. Minor

. Minor

. Minor

7

60

7

6

. 5

1. 5

1

14

9

Minor

1

Minor

Minor

Minor _ Minor

3

46.5

14

7

Minor

6

5

6.5

13

Minor

1

Minor

Minor

Minor

3
PETROGRAPHY OF SEDIMENTARY ROCKS B17

Calcite 

Plagioclase

Zircon

Barite

Quartz

Rock 

fragment

0.3 mm

FIGURE 15. Light-gray very fine grained sandstone from the 

lower brown unit of the Brushy Basin Member. Pore spaces 

are almost completely filled with silica and calcite; quartz 

grain below microcline shows remnant of earlier cycle silica 

overgrowth; detrital quartz grain at center of field is almost 

wholly replaced by barite. Sample DVR-2-1299, Disappointment Valley.

grains. A few thin layers and pebbles of clay in the 

sandstone show faint outlines of shards. Part of the 

rock is of volcanic origin, but most of it appears to have 

been derived from clastic sedimentary rocks.

Modal point counts and estimates of the mineral content of three thin sections of samples of sandstone from 

the lower half of the upper brown unit, taken from 

drill cores from hole DVK-2 in Disappointment Valley, 

have an average composition shown in table 9. This 

average is notably different from the averages of sandstone from the middle green and lower brown units, 

being appreciably lower in quartz and higher in chert 

and feldspar (especially plagioclase). A larger proportion of volcanic material is indicated, but likely much 

of it was waterborne from a volcanic source, rather 

than being deposited from the air as ash.

Individually, the three sandstones whose mineral contents are averaged in table 9 vary considerably. One 

sample near the base of the unit is clean and well sorted, 

and firmly cemented with silica (fig. 19). The silica 

cement in this light-gray quartzite (sample DVR-2- 

1139) consists of a very fine grained mosaic, chertlike 

in places, that makes up about 15 percent of the rock. In 

one bedding layer where detrital leucoxene is abundant, 

anatase has formed authigenic clusters around quartz

Hematite film

Plagioclase

1 mm

FIGURE 16. Very fine grained light-brown to very light reddish 

brown sandstone from the lower brown unit of the Brushy 

Basin Member. Zone of hematite-coated grains is part of a 

ring surrounding light-brown sandstone partly shown at upper 

left; calcite is optically continuous throughout the field of 

view; section cut almost parallel to sandstone bedding. Sample 

DV-125-1061.3(C), Disappointment Valley.

Hematite film

Plagioclase

Quartz

Hematite

FIGURE 17. Light-reddish-brown very fine grained sandstone 

from the lower brown unit of the Brushy Basin Member. 

Largely quartz grains in calcite-clay-hematite matrix. 

Sample DV-112A-1043.5, Disappointment Valley.
B18 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

Quartz Marcasite

Calcite

Leucoxene

0.5 mm 

I

FIGURE 18. Very fine grained light-brownish-gray sandstone 

from the middle green unit of the Brushy Basin Member. 

Marcasite is interstitial aggregates of minute spheres, in part 

enclosed in calcite. Sample LE-430-461, Legin group area.

Hematite

Calcite

Chert

0.3 mm

FIGURE 19. Light-gray fine-grained quartzitic sandstone from 

the upper brown unit of the Brushy Basin Member. Most of the 

space between detrital grains is filled with small irregular 

interlocking patches of silica ; anatase almost completely surrounds one quartz grain (anatase that appears to be enclosed 

in this grain actually lies on the side of the grain toward the 

observer and is part of the rim of anatase covering the grain) ; 

most of the opaque dust is leucoxene (microcrystalline 

anastase). Sample DVR-2-1139, Disappointment Valley.

grains (fig. 19), possibly at the time the rock became 

cemented with silica. A second sample (DVR-2-1054) 

of sandstone from higher in the unit is poorly sorted 

and dirty with abundant clay and mudstone fragments 

(20 percent of the rock), volcanic rock fragments (5 

percent), and silt-sized detritus (fig. 20). The rock contains a few detrital limestone grains. In hand specimen 

the rock is seen to consist of interbedded coarse-grained 

poorly sorted sandstone and siltstone. The third sample, 

from still higher in the unit, is light red in color, well 

sorted with well-rounded grains, and almost completely 

cemented with about 20 percent interstitial silica overgrowths that join along sutured contacts but in places 

are faceted. Barite is common in the rock, chiefly as 

euhedral colorless crystals replacing the abundant chert 

grains; a small amount of barite occurs as anhedral 

interstitial material; and some detrital black opaque 

grains are present.

Modal point counts and estimates of the mineral composition of four thin sections of conglomerate and conglomeratic sandstone from the Brushy Basin Member 

are averaged in table 10. Two thin sections of coarsegrained conglomeratic sandstones from the middle

Quartz

Chert

Mudstone fragment

Hematite

Interstitial 

calcite

0

Albite

1 mm

FIGURE 20. Coarse-grained poorly sorted mottled light-gray and 

reddish-brown sandstone from the upper brown unit of the 

Brushy Basin Member. Matrix is largely calcite; mudstone 

fragment contains dustlike hematite; a few nearby fragments, not shown, are devoid of hematite; area shown is 

part of layer interbedded with reddish-brown siltstone laminae. 

Sample DVR-2-1054, Disappointment Valley.
PETROGRAPHY OF SEDIMENTARY ROCKS B19

green unit have the average composition given in table 

10. Some of the chert in the rocks shows colloform 

structure. Calcite occurs both as cement and as detrital 

grains. These rocks are poorly sorted and contain subangular to well-rounded clasts. Rock fragments in one 

of the rocks are almost all siltstone, and only in small 

part are volcanic rocks, now thoroughly altered. Silica 

cement is abundant in the rocks. In the other sandstone, 

rock particles are dominantly of volcanic origin among 

which are altered and hematite-charged fragments, 

some of welded tuff origin; a few particles are clay and 

silt containing a few grains of coarser clasts. This rock 

(sample 56-S-6) is shown in figure 21. Plagioclase is 

abundant in this sample; parts of the sample contain 

abundant authigenic plagioclase possibly derived locally from solution of detrital plagioclase. The authigenic plagioclase occurs chiefly as overgrowths on 

detrital albite (fig. 22). The overgrowths are unusual in 

that they are almost completely albite twinned, whereas 

the detrital albite is untwinned or shows only a few 

twins. The overgrowths, probably nearly pure albite, 

have twinning slightly inclined to that in the detritals. 

Locally where detrital albite lies against chert grains, 

minute laths of authigenic albite with two or three 

albite twins have developed in interstitial spaces. The 

rock contains virtually no silica cement; some quartz 

grains are enclosed in authigenic albite that surrounds 

detrital albite (fig. 22). Even though many of the rock 

fragments and much of the abundant plagioclase and 

clay in the rocks must have come from volcanic sources,

derived from sedimentary rocks.

TABLE 10. Average mineral composition, in percent, of conglomerate and conglomeratic sandstones from the Brushy Basin Member 

of the Morrison Formation

[Based on modal point counts (200 points) and modal estimates of thin sections. 

"Minor" generally indicates less than 0.25 percent]

Component

Quartz and quartz overgrowths. __ 

Chert_ ___ _ . ___ ___ _

Orthoclase. _ _ ._ _ ______

Plagioclase- __ ____ _ _ _ ______

Rock fragments and mineral aggregates _____ ___

Clay (mineral clay and clay-sized 

detritus) _ __ __ ______ _ _

Calcite and dolomite ___ _____

Barite___ _ ____ _ _________

Chlorite- ______ _____ _________

Hematite and other black opaque 

minerals .___ _____ _ ___ __

Muscovite _ ___ _____ _ _ __

Leucoxene and anatase _____

Number of samples _________ _

Member 

(Middle green 

and upper 

brown units 

combined)

54 

14 

1 

4.5

19.5

5 

1 

Minor 

.5

.25 

Minor 

Minor

4

Middle 

green 

unit

50 

13 

.5 

7.5

21. 5

5 

2

.5

Minor

2

Upper 

brown 

unit

58.5 

15 

1.5 

1

17

5 

Minor 

Minor 

1

Minor 

Minor 

Minor

2

Quartz

Plagioclase 

Clay

FIGURE 21. Light-brown coarse-grained conglomeratic sandstone from the middle green unit of the Brushy Basin Member. 

Numerous grains 1 mm or more long are hematite-impregnated 

rock fragments. Sample 56-S-6, sec. 15, T. 42 N., R. 17 W.

Chert

Untwinned 

detrital 

albite

Quartz

'.'*.'.

* ,

Twinned

authigenic

albite

lath

Twinned albite 

overgrowth

FIGTTRE 22. Coarse-grained conglomeratic sandstone from the 

middle green unit of the Brushy Basin Member. Large untwinned detrital albite grain has twinned albite overgrowth 

and is partly surrounded by tiny authigenic albite laths. Sample 56-S-6, sec. 15, T. 42 N., R. 17 W.

The average of estimates of the composition of two 

thin sections of conglomerate and conglomeratic sandstone TO and 80 feet below the top of the upper brown 

unit where the rocks are largely greenish gray is given
B20 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

in table 10. The conglomerate contains a higher proportion of chert and rock fragments and fewer quartz 

grains than the conglomeratic sandstone. Kock fragments in the conglomerate are of a wide variety, chiefly 

altered fine-grained volcanic rocks (some showing original flow structure), siltstone, mudstone, and sparse 

oolites. Some orthoclase and plagioclase in the conglomerate (sample DVR-2-765) appears as bunches of 

bladed laths, as though authigenic (fig. 23). The conglomeratic sandstone is unusually porous, pore space 

accounting for 15-20 percent of the rock volume; pebbles 1 cm in diameter and larger are scattered among 

sand grains that are about 0.5 mm in size; silt-sized 

detritus locally fills the space between sand grains. Both 

rocks contain detrital grains that were crushed in place, 

especially those grains in contact with pebbles. The 

crushing may have taken place during compaction of 

the rock, but also may have occurred as a result of 

compression or bending of the strata during regional 

folding.

The average mineral composition of four thin sections 

of siltstone from the Brushy Basin Member is given in 

table 4. A typical siltstone of the middle green unit 

taken from drill core (sample LE-445-511) from the 

Legin area is light greenish gray mottled with reddish 

brown. Composition of a thin section of the rock is 

given in table 11. Cement of the rock is calcite, occurring 

as minute interstitial blebs and patches, and clay. Detrital grains are rounded to sharply angular and flakelike, the latter possibly airborne volcanic material. 

Three samples of siltstone (DVR-2-722.5, DVR-2- 

726.4, DVR-2-812.5) from different horizons in the 

upper part of the upper brown unit where most of the 

rocks are greenish gray were collected from drill core

1 mm 

I

FIGURE 23. Light-gray conglomerate from the upper part of 

the upper brown unit of the Brushy Basin Member showing outlines of bunches of bladed laths of authigenic orthoclase. O, orthoclase; Q, quartz; ch, chert. Sample DVR-2- 

765, Disappointment Valley.

from Disappointment Valley. The average of modal 

estimates of the mineral content of thin sections of these 

rocks is given in table 11. Detritus is chiefly silt sized, 

but some sand-sized grains are present, and in some 

layers, clay matrix makes up more than 50 percent of 

the rock. Some irregular patches of calcite show polysynthetic twins. In one reddish-brown siltstone (sample 

DVR-2-812.5) euhedral barite crystals as much as 1 

mm in length have been surrounded and partly replaced 

by calcite (fig. 24). Silt-sized detritus as well as clay

TABLE 11. Average mineral composition, in percent, of siltstones 

from the Brushy Basin Member of the Morrison Formation

[Based on modal estimates of thin sections. "Minor" generally indicated less than 

0.25 percent]

Component

Middle Upper 

green unit brown unit

Quartz and quartz overgrowths_______ _ ___

Chert _ __ __ _ _ _ . __ ___

Orthoclase_ _________ _ _ ____________ _

Microcline___ _ ____ __ ___ _ _ ________

Plagioclase. _________ _ ___ _ ____ _____

Clay (mineral clay and clay-sized detritus) ...

Calcite and dolomite ___________ __

Barite___ ___ ____________ _ __._ _____ .

Chlorite _ __ _ ____ ____ _ ______ ____

Hematite and other black opaque minerals. __

Biotite _ _ _. _ .___ _ ______ __ ______

Muscovite _ ____ ___ ____________ _ ___

Leucoxene and anatase_________ _______ _

Number of samples. _________________ _

50

4

1

4

15

25

Minor

1

Minor

Minor

Minor

1

64

8.5

3

1

2

18. 5

1.5

Minor

Minor

. 5

Minor

Minor

Minor

3

Calcite

Quartz

1 mm 

I

FIGURE 24. Light-reddish-brown siltstone from the upper 

brown unit of the Brushy Basin Member. Illustration 

shows part of a clay-rich layer of the rock; euhedral 

barite crystal is surrounded and partly replaced by calcite ; barite and calcite completely replace clay- and siltsized detritus that originally occupied the space. Sample 

DVR-2-812.5, Disappointment Valley.
PETROGRAPHY OF SEDIMENTARY ROCKS B21

matrix was replaced by both barite and calcite. Here, it 

appears that barite formed first in the rock and later 

served as a locus for precipitation of calcite.

Modal point counts and estimates of the mineral content of 13 thin sections of Brushy Basin mudstone are 

averaged in table 4. Analyses of three samples of mudstone from the Brushy Basin by Eobert F. Gantnier, 

of the Geological Survey, indicate an average specific 

gravity (bulk density) of 2.56, an average grain density 

of 2.61, and an average porosity of 3.3 percent. Modal 

point counts and estimates of seven thin sections of 

mudstone from the middle green unit are averaged in 

table 12. About one-half percent of carbonaceous material was seen in one thin section. Dilute hydrochloric 

acid tests of hundreds of feet of core from the middle 

green unit, calibrated by numerous chemical determinations of calcite content by N. L. Archbold, show most 

of the mudstone to contain less than about 5 percent 

calcite. Some thin layers contain much more, however, 

and indeed are argillaceous limestone locally; but these 

higher concentrations probably do not raise the average 

calcite content of the unit appreciably.

Barite and calcite are common metacrysts, or authigenic minerals, in greenish-gray mudstone of the middle 

unit. In places, barite crystals more than 1 cm long have 

grown in mudstone (sample DV-124r-715) that is composed chiefly of devitrified volcanic ash containing some 

volcanic crystals and detrital sedimentary grains (fig. 

25). A detailed part of figure 25 is shown in figure 26. 

Ghosts of devitrified shards are abundant in clay which 

contains crystals of volcanic origin as well as a few 

detrital sedimentary grains. Barite has tended to assume 

its crystal form, which is partly modified where preTABLE 12. Average mineral composition, in percent, of mudstones 

from the Brushy Basin Member of the Morrison Formation

[Based on modal point counts (200 points) and modal estimates of thin sections. 

"Minor" generally indicates less than 0.26 percent]

Component

Middle Upper 

green unit brown unit

Quartz __________________________________

Chert___._._-_.--.._.____.____. 

Orthoclase.______________________________

Microcline. ______________________________

Plagioclase_ -_-_-___-_-_-_-_____-______.

Rock fragments and mineral agregates_______

Clay (mineral clay and clay-sized detritus)__. 

Calcite._________________________________

Barite_ __________________________________

Chlorite_ ________________________________

Hematite and other black opaque minerals. _. 

Biotite __---_-___-_____-________________.

Muscovite (green mica in upper brown unit). 

Leucoxene and anatase____________________

Limonite. _______________________________

Pyrite_--._.________.___._._._.___._._.

Analcite. ________________________________

Carbonaceous material-___________________

28

2

Minor 

2

28.5 

.5 

3

60

4

2

Minor 

Minor 

Minor 

Minor 

Minor 

Minor 

1.5 

Minor

5

Minor 

52

2.5 

Minot 

5

.5 

.5 

1 

Minor

Minor

Number of samples .

Barite

2 cm

j

FIGURE 25. Light-greenish-gray mudstone (devitrified volcanic 

tuff) from the middle green unit of the Brushy Basin Member. 

Where barite metacrysts have formed, all the minerals of the 

mudstone have been replaced. Sample DV-124-715, Disappointment Valley. Circle indicates area shown in figure 26.

Barite

Biotite

Zircoi

Plagioclase

Chlorite

Quartz

1 mm

FIGTJBE 26. Light-greenish-gray mudstone (devitrified volcanic 

tuff) from the middle green unit of the Brushy Basin Member; detail of figure 25. Devitrified shards are largely fine 

grained silica; barite metacryst has grown by replacing clay, 

devitrified shards, quartz, and feldspar, in that order. Sample 

DV-124-715, Disappointment Valley.

existing minerals were not replaced. Judged from the 

abundance of the various types of fragments still enclosed in barite, the order of susceptibility to replacement by barite was first clay, followed by devitrified 

shards, quartz, and finally plagioclase. Plagioclase thus
B22 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

appears to have been a relatively stable phase in the environment in which barite was precipitated. Calcite 

segregations are more numerous than barite in the mudstone of the middle green unit, but they generally show 

irregular boundaries rather than the euhedralism displayed commonly by barite. Figure 27 shows irregularly 

shaped calcite and barite together (sample DV-124- 

880). The relationships in this thin section suggest that 

as barite replaced calcareous clay and the enclosed 

detrital grains, calcite was segregated out of the clay to 

the periphery of growing barite patches. Chlorite is also 

locally concentrated at barite and calcite boundaries, 

and it too may be a segregation from the clay as a result 

of barite and calcite replacement. Analcite is another 

mineral commonly segregated in patches in mudstone 

of the unit. Figure 28 shows a clay-rich divitrified tuffaceous mudstone (sample DV-124-695) that contains 

about 10 percent detrital fragments. In places it contains visible shards, now altered to chlorite and radiating fibrous silica. Angular plagioclase fragments are 

abundant among the detrital fragments and probably 

are of volcanic origin. Analcite, nearly isotropic with 

refractive index of about 1.48, forms irregular patches 

which under crossed iiicols are seen to consist of faint 

bladed forms as though crystals were lining the walls of 

vugs. Some of these crystals show faint polysynthetic

Barite

Chlorite

Calcite

,Quartz

Chert

Plagioclase Clay

1 mm 

j

FIGURE 27. Light-greenish-gray mudstone from the middle green 

unit of the Brushy Basin Member. Partly replaced by barite 

and calcite; barite is part of a crystal 3 mm in diameter that 

is rimmed with calcite and chlorite; lower calcite crystal is 

in a veinlet extending to another barite crystal. Sample DV124-880, Disappointment Valley.

Clay

Quartz

Quartz

Analcite 

Plagioclase

Calcite

Plagioclase

replaced 

by chlorite

1 mm 

J

FIGURE 28. Mottled light-reddish-gray and light-greenish-gray 

devitrified tuffaceous mudstone from the middle green unit of 

the Brushy Basin Member. Analcite, in places veined and 

rimmed with calcite, forms irregular elongate patches; one 

analcite patch contains secondary quartz; chlorite crystals lie 

in analcite or replace plagioclase; much of the fine-grained 

angular detritus in clay is plagioclase. Sample DV-124-695, 

Disppointment Valley.

twinning. Calcite rims and veins the analcite segregation and also occurs as patches away from the analcite. 

In a few places in the rock, chlorite has partly replaced 

plagioclase. The chlorite is optically ( ), nearly 

uniaxial, has refractive index close to 1.60 and has very 

low birefringence; it is probably compositionally similar to either penninite or delessite.

Mineral-filled vugs are conspicuous locally in the 

mudstone. These vugs are small, generally not more 

than a few millimeters long, and are commonly lined 

with collof orm calcite and barite and filled with chalcedonic silica. Possibly the vugs themselves formed as the 

result of solution movement through the middle green 

unit at the beginning of the stage of alteration that resulted in the mineral segregations. The collof orm linings 

of calcite and barite are now optically continuous mineral, and no evidence of possible earlier growth layers 

survives. Probably the calcite and barite have recrystallized since initial deposition, although possibly they are 

merely replacements of an earlier colloform mineral, 

like chalcedony. Figure 29 shows a small vug partly 

lined with calcite and filled with chalcedonic quartz 

(sample DV-2-BB).

Modal estimates of the mineral content of thin sections 

of six mudstones from the upper brown unit have an
PETROGRAPHY OF SEDIMENTARY ROCKS B23

Quartz

Clay

Calcite

Limonite

Detrital quartz

0.5 mm

FIGURE 29. Vug in light-greenish-gray mudstone from the 

middle green unit of the Brushy Basin Member, filled with 

calcite and chalcedonic quartz. Limonite forms bright-orange 

patches in calcite which appears recrystallized from colloform 

carbonate lining an irregular void; the center of the void is 

now filled with quartz. Sample DV-2-BB, Disappointment 

Valley.

average shown in table 12. One of the samples, greenishgray mudstone from the upper part of the unit collected 

from drill hole DVR-2 in Disappointment Valley, contains about 5 percent carbonaceous material as minute 

specks and flakes flattened on prominent bedding planes. 

Detritus in the mudstone of the upper brown unit is 

chiefly silt sized and smaller and is commonly sharply 

angular, but an appreciable amount is rounded as 

though waterworn. Much of the clay is probably claysized detritus, including mostly comminuted quartz and 

feldspar. Clay that forms a uniform dense matrix is 

commonly partly recrystallized so that individual clay 

crystals are discernible.

Much of the clay in the mudstone shows shard structures, in some places quite faint but in many beds notably plain (fig. 30). Such a rock (sample DV-114-765) 

as illustrated in figure 30 may be almost wholly of 

volcanic origin and deposited from air, although sparse 

rounded particles such as leucoxene may have been 

water transported to the site of deposition, and there 

intermixed with volcanic material. Other mudstones 

contain devitrified shards and angular crystal fragments scattered in roughly equal amounts in clay matrix. Probably much of this clay is devitrified finely 

comminuted volcanic glass and is montmorillonitic.

Quartz

Devitrified 

glass shard

Calcite

i ' 

Leucoxene

Feldspar

0.3 mm 

_J

FIGURE 30. Light-brownish-green mudstone from the upper 

brown unit of the Brushy Basin Member, a devitrified volcanic tuff. Glass shards have completely altered to clay minerals. Sample DV-114-765, Disappointment Valley.

A mudstone (sample DV-32-1) with probably less 

volcanic material is shown in figure 31. This rock contains only a very small amount of material that may 

represent devitrified volcanic glass particles. Most of 

the detritus is angular to well-rounded quartz and feldspar, probably in part of volcanic origin, but part at 

least shows considerable rounding from water transport. 

Hematite is abundant in this rock, and although it may 

be suggestive of oxidation accompanying volcanic extrusion, it is more likely similar in occurrence to hematite that coats grains, and forms extremely fine-grained 

dispersions in clay, in many of the other sedimentary rocks in the district. Probably most of the 

reddish-brown mudstone in the upper brown unit has 

considerably less volcanic material in it than the less 

abundant interlayered greenish-gray to brownish-gray 

varieties.

Metacrysts of barite and carbonate occur in mudstone 

of the upper brown unit. For example, bladelike crystals 

of siderite (?) as much as a centimeter in length have 

grown in light-brownish-green devitrified volcanic tuff 

(sample DV-114-765) as shown in figure 32. In this 

rock, part of which is shown in detail in figure 30, 

authigenic siderite (?) has developed locally where the 

rock contains patchy calcite.

A thin section from a sample (DV-32-2) of a reddish-browii argillaceous silty limestone layer in the 

lower brown unit collected from drill core from a
B24 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

Feldspar rimmed 

hematite

Quartz

Plagioclase 

with hematite 

in cleavages

0.3 mm 

I

FIGTTKE 31. Reddish-brown mudstone from the upper brown unit 

of the Brushy Basin Member. Poorly sorted detrital feldspar, 

quartz, hematite, and other minerals lie in a clay matrix; the 

matrix has partly recrystallized into individually visible clay 

crystals. Sample DV-32-1, Disappointment Valley.

Area largely devoid of calcite

,Area of patchy calcite

1 cm

FIGURE 32. Light-brownish-green mudstone from the upper 

brown unit of the Brushy Basin Member, a devitrified volcanic tuff. Authigenic siderate( ?) has formed locally where the 

rock contains patchy calcite. Sample DV-114-765, Disappointment Valley. Circle indicates area of figure 30.

diamond-drill hole in Disappointment Valley contains 

about 70 percent calcite, 14 percent clay, 1 percent analcite, and 15 percent detrital grains. Calcite occurs in 

optically continuous patches, as much as 1 cm across, 

which join along cuspate boundaries. Some patches 

have a few polysynthetic twins. The calcite is filled with 

irregular small patches and wormlike blebs of clay.

Detrital minerals, scattered through calcite, are about 

half quartz, with lesser, subequal amounts of K-feldspar, plagioclase, and chert, and minor amounts of 

hematite, leucoxene, green hornblende, and zircon.

One sample (DV-125-850) of reddish-brown limestone from the middle green unit of the Brushy Basin 

Member in the Disappointment Valley area, drill hole 

DV-125, contains thin interconnecting septa of clear 

white calcite as much as 3 cm long, mostly oriented 

nearly parallel and nearly normal to bedding, in a sort 

of boxwork. These septa are probably shrinkage cracks 

in impure limestone consisting of about 80 percent very 

fine grained calcite, 19 percent clay, 1 percent hematite 

as minute detritals and dustlike material in clay and 

calcite, and a trace of quartz as very small well-rounded 

detrital grains. A sample (BB-1) of greenish-gray 

limestone collected from the middle green unit near the 

mouth of Joe Davis Canyon comes from a layer of 

nodules that are as much as 6 inches in diameter. A thin 

section of this limestone consists of about 86 percent 

calcite, 10 percent detrital quartz and feldspar of silt 

size, 3 percent authigenic albite, and minor amounts of 

zircon, leucoxene, and limonite. Calcite is chiefly very 

fine grained, but in places it is coarsely crystallized in 

patches surrounded by dark medium-grained calcite 

containing limonite that probably has been altered from 

pyrite. The medium-grained calcite also contains abundant tiny laths of authigenic albite (fig. 33). Such laths

Fine-grained 

calcite

Detrital 

grains

1 mm 

J

FIGUBE 33. Part of limestone nodule from the middle green 

unit of the Brushy Basin Member; authigenic albite is concentrated around patch of coarsely crystalline calcite. Sample 

BB-1, Joe Davis Canyon.
PETROGRAPHY OF SEDIMENTARY ROCKS B25

are scattered much more sparsely throughout the rest 

of the limestone; in places authigenic albite has overgrown detrital orthoclase. Apparently, constituents of 

the albite were introduced into the limestone at the time 

it was partly recrystallized, possibly during the stage 

of widescale alteration that affected the rest of the 

middle green unit. This possibility will be discussed 

again in a later chapter in this professional paper series 

on alteration of the sedimentary rocks.

One sample (DVR-1-1471) of reddish-brown clayey 

limestone from the upper brown unit collected from 

drill core from Disappointment Valley contains about 

25 percent clay and about 5 percent of dustlike hematite 

and minute detrital grains. The rock has several anastomosing partly concentric calcite-filled veinlets, about 

1 cm long, that are probably merely the original matrix 

calcite segregated into shrinkage cracks.

POST-MORRISON ROCKS 

BURRO CANYON FORMATION

The Burro Canyon Formation is dominantly sandstone and conglomerate, interbedded with thin layers 

of mudstone. Locally, the formation has thin interbedded layers of mudstone, limestone, and chert in its 

upper part. Average mineral composition of sandstone 

from the Burro Canyon Formation, based on modal estimates and point counts of six thin sections, is shown in 

table 2. The sandstone has notably more quartz and less 

chert and feldspar than does that in the underlying 

Morrison Formation.

Quartz grains in the sandstone are poorly to well 

sorted and subangular to well rounded; most are clear 

but some show strain shadows or possess internal 

sutures. Some are embayed and interpenetrate where 

they are in contact. Many show silica overgrowths. 

Some overgrowths join along sutured boundaries; others 

project into pore spaces and are faceted. A few rounded 

grains show vestiges of earlier overgrowths on rounded 

grains formed during an earlier sedimentation cycle. In 

places, silica overgrowths are so extensive that the rock 

is a quartzite; generally the overgrowths make up from 

0 to about 10 percent of the rock and probably average 

about 5 percent. Calcite is a widely distributed cement, 

filling pore spaces in irregular optically continuous 

patches which enclose a few to a score or more detrital 

grains. Calcite is commonly concentrated along bedding 

planes. Much of it is polysynthetically twinned. Locally 

where large patches of matrix calcite occur, detrital 

grains are poorly packed. Surrounding such areas, silica 

overgrowths may be abundant, and numerous quartz 

grains show solution embayment and interpenetration 

of adjacent grains, so that the detrital grains are better

packed. These relationships suggest that calcite cement 

formed earlier than the silica cement and preserved the 

original packing of the sedimentary grains. Moreover, 

the common twinning of the calcite suggests that the 

calcite was desposited as a cement before any appreciable tectonic flexing of the strata if the polysynthetic 

twinning can be attributed to such deformation. Minor 

amounts of clay form irregular small patches of matrix.

A typical well-cemented sandstone (sample SH-60- 

55) from the Burro Canyon in Disappointment Valley 

is shown in figure 34.

In places, layers of sandstone in the Burro Canyon 

Formation are compositionally like sandstone in the 

Brushy Basin Member of the Morrison Formation, especially where conglomeratic. One light-gray conglomerate layer containing red chert and greenish-gray 

mudstone pebbles was examined in thin section. The 

rock consists of only about 25 percent quartz; relatively 

large amounts of chert and rock fragments are present, 

about 25 percent each, which range from sand to pebble 

size. Rock fragments include mudstone, siltstone, and 

volcanic rock particles. About 5 percent K-feldspar is 

present. Interstitial material consists of about 15 percent calcite and 5 percent clay. Quartz overgrowths are 

sparse; in one place, small euhedral quartz prisms were

Calcite

Clay

//^X "» ).' 7? _,»-»..-fte**,- -

-^^^Ig^^P^:

luartz

FIGURE 34. Light-brownish-gray fine-grained quartzose sandstone from the Burro Canyon Formation. Almost all detrital 

grains are quartz and most have quartz overgrowths; characteristically, a thin clay film lies between grain and overgrowth ; calcite fills most of the interstitial space not occupied 

by quartz; matrix clay is sparse. Sample SH-60-55, Disappointment Valley.
B26 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

seen grown in crystallographic continuity on a rounded 

detrital quartz grain.

One unusual grayish-green siltstone (sample DVR2-397) collected 17 feet below the top of the Burro 

Canyon, from drill core from hole DVR-2 in Disappointment Valley, may be of marine origin. It contains 

about 50 percent quartz, 35 percent chlorite, 10 percent 

calcite, 1 percent orthoclase, 0.5 percent each of microcline, plagioclase, and chert, and minor amounts of 

pyrite, zircon, tourmaline, and leucoxene. The chlorite 

is very pale greenish brown to light bright green, and 

occurs as rounded microcrystalline aggregates that may 

be glauconite or altered ferromagnesian minerals. Some 

well-rounded grains contain finely divided material 

that may be magnetite; some grains have concentric 

zones like oolites. Chlorite occurs also in the matrix 

of the rock. Most of the calcite occurs as isolated clear 

interstitial blebs; some is fine grained and speckled with 

impurities and may be recrystallized from detrital limestone grains. Part of a thin section of the rock is illustrated in figure 35. The siltstone is part of a sequence of 

interbedded greenish-gray mudstone and siltstone and 

minor silty limestone, evenly bedded, at the top of the 

Burro Canyon, that may have been deposited in a relatively shallow near-shore marine environment similar 

to that in the Baltic sea where reduced "gyttja" sedi

Calcit

Quartz

Pyrite

Chlorite

1 mm

FIGURE 35. Grayish-green sandy siltstone from a bed near the 

top of the Burro Canyon Formation. Rounded detrital grains 

that are almost wholly chlorite may be glauconite or altered 

ferromagnesian minerals; matrix is chlorite, calcite, pyrite, 

and leucoxene. Sample DVR-2-397, Disappointment Valley.

ments are formed (see, for example, Manheim, 1961, p. 

55-56).

The average mineral composition of four thin sections 

of samples of claystone from the Burro Canyon is 

shown in table 4. These rocks are notably more clay rich 

than mudstones in the Morrison and Summerville Formations. Clay averages about 80 percent of the rocks and 

commonly contains barely visible particles of reworked 

clay, shale, or mudstone. Some clay may have been 

authigenically recrystallized, for microscopically discernible crystals are seen lying parallel to bedding in 

some thin sections. Calcite, which averages about 5 percent, occurs commonly as irregular blebs, or dispersed 

in weblike forms in clay.

One unusual thin dark-gray carbonaceous claystone 

layer (sample DVR-2-503) near the middle of the 

Burro Canyon Formation in Disappointment Valley 

contains abundant authigenic crystals of calcite. A thin 

section of the rock shows very thin layers about a millimeter thick, with graded bedding; the bottom coarser 

part of each layer contains visible fine silt-sized detritus 

and particles of carbonaceous material that grade upward into homogeneous clay. Beadlike strings of calcite 

crystals have grown in almost vertical lines throughout 

the carbonaceous claystone. Locally along the vertical

Calcite

Clay

FIGURE 36. Dark-gray carbonaceous claystone from the Burro 

Canyon Formation containing strings of calcite metacrysts. 

Crystals show a tendency to form beadlike strings approximately normal to bedding, and carbonaceous particles have 

accumulated near the extremities of strings possibly by mechanical pressure as the calcite crystals grew; nearly 

horizontal incipient fractures have controlled bleaching in thin 

zones. Sample DVR-2-503, Disappointment Valley.
PETROGRAPHY OF SEDIMENTARY ROCKS B27

strings of calcite crystals, carbonaceous material has 

been segregated by the pressure of crystal growth, much 

as micas or graphite are segregated at the extremities of 

chloritoid, ottrelite, or andalusite crystals in some metamorphic rocks (see, for example, Harker, 1932, fig. 

175B). A drawing of part of a thin section of the rock 

is shown in figure 36. The calcite metacrysts seem to 

have grown under vertically directed pressure, because 

the carbonaceous material accumulations formed only 

at the upper and lower ends of segments of the nearly 

vertical strings of calcite crystals, and because the crystals may have grown in zones of tension nearly parallel 

to the direction of stress. It, therefore, seems probable 

that the sediments were deeply buried at the time the 

calcite metacrysts formed, for load pressure seems the 

only likely vertical force that could have been effective 

in the geologic environment of the Colorado Plateau 

sedimentary rocks.

In places, evenly bedded dark-greenish-gray claystone contains several thin layers of white to pale-gray 

calcite about half an inch thick. Locally, the layers consist of fibrous calcite oriented normal to the bedding; 

thin flakes of clay, parallel to and possibly remnants of 

bedding layers, occur imbedded in the fibrous calcite. 

In other nearby thin calcite layers (sample DVR-2- 

439.5), shears have developed at an oblique angle to the 

bedding, partly modifying the thin clay flakes that are 

parallel to bedding (fig. 37). Possibly the shearing reClay

Calcite

5 mm 

i

FIGURE 37. Shear structures in calcite in a limy bed in greenishgray claystone 60 feet below the top of the Burro Canyon 

Formation. Polysynthetic twinning crosses shears in places. 

Sample DVR-2-439.5, Disappointment Valley.

suited from differential stresses accompanying compaction of the beds. More likely it occurred after 

consolidation of the beds, for polysynthetic twins are 

concentrated where calcite is most intensely sheared; it 

could have occurred during folding of these beds, and 

as a result of the slipping of one bed over another as 

folding progressed.

In a few layers in the top part of the Burro Canyon 

in the Disappointment Valley area, mudstone or claystone and limestone are intimately mixed in a sort of 

microbreccia. Such layers are irregular and tend to be 

nodular in places. Nodules consisting of microbreccia 

are about an inch or more in diameter. As seen in thin 

section (sample SH-74-55), partly rounded fragments 

of mudstone and claystone as much as 2 mm across are 

imbedded in fine-grained calcite clouded by dustlike 

inclusions. Some of the argillaceous particles contain 

silt-sized detrital grains of quartz and other minerals. 

The mudstone and claystone particles are irregularly 

fractured and veined with clear calcite, and some of the 

larger calcite veins are continuous through matrix calcite, extending several millimeters through the rock. 

Locally, clay and calcite are intimately mixed in a finegrained "mush." Minute specks of limonite, probably 

representing altered pyrite, are scattered throughout the 

rock; they are most abundant in mudstone. Part of a 

thin section of the rock is illustrated in figure 38. The

Calcite

Clay

Limonite

Quartz

FIGUEE 38. Light-greenish-gray microbrecciated limy mudstone from the Burro Canyon Formation. Partly rounded 

mudstone fragment is fractured and veined with calcite; 

clearer calcite areas extend as veinlets several millimeters 

through the rock. Sample SH-74-55, Disappointment 

Valley.
B28 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

character of the rock suggests that the rock was only 

partly consolidated when it became deformed and its 

clay and calcite became intermixed so thoroughly. Possibly, subaqueous slumping of beds took place before 

burial.

Limestone in the upper part of the Burro Canyon 

Formation appears to be nearly pure. Under the microscope the limestone is seen to consist of a high proportion 

of calcite, generally about 90 percent, and a small percentage of mineral clay and silt-sized detrital grains, 

chiefly quartz. Other detrital grains include traces of 

microcline, leucoxene, zircon, and tourmaline.

DAKOTA SANDSTONE

The Dakota Sandstone consists of fine- to mediumgrained carbonaceous sandstone and minor amounts of 

conglomerate, carbonaceous shale and mudstone, and 

impure coal. The mineral composition of sandstone in 

the Dakota, based on modal estimates and point counts 

of four thin sections, is shown in table 2. Grains range 

from sharply angular to well rounded and are probably 

dominantly subrounded to rounded. Most of the sandstone is moderately sorted. On the basis of its mineral 

composition compared to that of older formations in 

the district, it is most like sandstone in the Brushy 

Basin Member of the Morrison Formation, except that 

it contains much more carbonaceous material. In addition, it probably contains more clay and less calcite than 

average sandstone in the Brushy Basin. The sandstone 

in the Dakota varies from slightly friable to firmly 

cemented with calcite, clay, and silica. Quartz grains, 

about 60 percent of the rock, commonly show thin silica 

overgrowths, and some well-rounded grains contain 

vestiges of overgrowths, indicating that these grains 

have gone through an earlier cycle of sedimentation. 

Some quartz grains contain internal sutures suggestive 

of quartzite.

A typical sandstone from the Dakota is shown in 

figure 39, which illustrates part of a thin section of a 

sample (56-S-2) collected near the top of the formation 

on the northeast side of Disappointment Valley. This 

moderately sorted sandstone with mostly rounded 

grains contains considerable clay, in both matrix and 

altered detrital mineral grains. Calcite occurs as isolated 

irregular interstitial patches, each showing uniform optical orientation; a few display fine polysynthetic twins. 

Biotite books, some completely altered to chlorite, have 

been mashed irregularly by other detrital grains probably during compaction of the rock accompanying 

burial. The fact that optically continuous patches of 

calcite locally are molded against the distorted biotite 

books suggests that calcite here was crystallized after

Clay

Calcite

Chlorite 

Quartz

FIGURE 39. Light-buff fine-grained quartzose sandstone from 

the Dakota Sandstone. Individual discontinuous calcite matrix patches, about 1 mm in diameter, have different optical 

orientations; a few quartz grains have thin quartz overgrowths ; much of the opaque finely divided material is leucoxene and some is limonite altered from pyrite. Sample 

56-S-2, near the top of the formation, Disappointment 

Valley.

compaction of the rock. Tiny pyrite crystals, some cubic, 

are mostly altered to limonite in this weathered sample. 

Considerable white opaque microlites throughout t^e 

matrix are probably leucoxene. Locally the microlites 

are coarse enough to be identified as anatase; occurrence of anatase in calcite cement is shown in figure 40. 

This relationship suggests that the components of the 

calcite and anatase may have been concurrently mobile 

in the sandstone. Further, the presence of polysynthetic 

twins in some calcite in the rock may indicate that deformation associated with the folding of the strata occurred mostly after calcite cement had become fixed in 

the sandstone.

In places, particularly in the middle part of the 

Dakota, sandstone is poorly sorted and contains abundant clay matrix and carbonized plant material. One 

such sample (DVR-2-306.5) is shown in figure 41, part 

of a thin section from drill core from Disappointment 

Valley. About 70 percent "of detrital minerals comprise 

about 52 percent quartz and 6.5 percent chert; the remainder consists of minor amounts of other minerals 

and about 5 percent detrital minerals altered to clay. 

About 15 percent clay makes up the matrix, and the rock 

contains almost 15 percent carbonaceous plant debris.
PETROGRAPHY OF SEDIMENTARY ROCKS B29

Anatase

Quartzite

Quartz/.'

Calcite

0.5 mm

40. Light-buff fine-grained quartzose sandstone from 

the Dakota Sandstone; a detail. Optically continuous patch 

of calcite with cluster of anatase crystals is typical; matrix 

in some parts of the rock is clay (not shown). Sample 56-S2, near the top of the formation, Disappointment Valley.

Part of the carbonized wood shows cell structure, with 

pores filled with fine-grained silica, and part is replaced 

by pyrite. The sandstone is unique because of the presence of an unidentified authigenic mica, one euhedral 

crystal of which is illustrated at the bottom of the field 

in figure 41. The mica is biaxial ( ) with small 2F, 

shows very weak birefringence, has refractive index 

near 1.59 and faintly mottled extinction and is weakly 

pleochroic in very pale yellowish brown colors. The 

mica books are generally greatly elongated parallel to c 

and length fast and display good basal cleavage. A few 

show lines of minute dark opaque inclusions parallel 

to c.

Detrital grains in the sandstone illustrated in figure 

41 range from well rounded to sharply angular. Because of the abundance of sharp and flakelike grains 

and the relative abundance of altered grains, much of 

this rock may have been constituted from airborne volcanic material.

A sample (DVR-2-338) of gray mudstone collected 

about 40 feet above the base of the Dakota, from drill 

core obtained in Disappointment Valley, is probably 

typical for the formation. As seen in thin section, twothirds of the rock consists of clay of low to moderate 

birefrigence and of generally uniform texture, except 

where clay wisps several millimeters long lie along 

bedding planes. Sharply angular to rounded silt-sized

Clay

Quartz

Authigenic mica

1 mm

FIGURE 41. Dark-gray carbonaceous very fine grained silty 

sandstone from the middle part of the Dakota Sandstone. 

Detritus is largely quartz; matrix is clay and clay-sized 

detritus. Sample DVR-2-306.5, Disappointment Valley.

detritus is evenly distributed throughout the clay, except in the layers of clay wisps where detritus is absent. 

About a quarter of the rock is quartz, and minor 

amounts of feldspar, chert and other minerals are present. About 5 percent of the rock is carbonized plant fragments that tend to be equidimensional and very small in 

size. A modal estimate of the mineral composition of a 

thin section of this sample is given in table 4. Some other 

mudstone samples contain considerable large carbonized 

plant fragments and abundant pyrite.

MANGOS SHALE

The Mancos Shale is composed chiefly of carbonaceous, calcareous shale, commonly silty and sandy 

(called mudstone), and minor amounts of thin layers of 

limestone, siltstone, sandstone, and bentonitic shale or 

claystone. The average mineral compositions of two thin 

sections of mudstone (samples DVR-2-35, DVR-2- 

210) and of two thin sections of shale (samples DVK2-127.5, DVR-2-145) are given in table 4. The average 

composition of the mudstone samples is probably typical 

of the more silty parts, and of the lower part of the interval equivalent to the Carlile Shale (calcite analyses 

of three samples from the Carlile equivalent by N. L. 

Archbold show a range 0.6-2.8 percent). The average 

composition of the shale samples is probably typical of
B30 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

most of the rest of the Mancos, which likely is about 

two-thirds clay, one-quarter calcite (determinations on 

six samples by N. L. Archbold show a range 15.6-38.1 

percent), and the remainder, detrital minerals, pyrite, 

and carbonized plant fragments. Calcite occurs largely 

as finely dispersed microscopic grains in clay, and locally 

is concentrated as strings of anhedral blebs along some 

bedding planes, as in sample DVR-2-127.5 shown in 

figure 42, and as scattered euhedra. Much of the carbonaceous material in the Mancos is translucent to 

nearly opaque, and dark brown; it, too, is illustrated in 

figure 40. Typically, it occurs as flattened particles less 

than a millimeter across lying in bedding planes, but 

some carbonaceous material that is black and opaque 

occurs as more equidimensional particles. The specific 

gravities of three samples of mudstone from the Mancos 

from drill hole DVR-1 are 2.53, 2.56, and 2.60 (avg, 

2.56), as determined by Robert F. Gantnier, of the U.S. 

Geological Survey. Gantnier also determined the grain 

densities of these three samples, all of which are 2.63, 

and indicated an estimated porosity for the three 

samples, respectively, of 6.5, 4.5, and 2.3 percent. These 

low porosities suggest a depth of burial exceeding 10,000 

feet (Athy, 1930).

A typical silty rock (sample DVR-2-35) is shown in 

figure 43. Most of the silt here is quartz, although in 

places silt-sized biotite or glauconite dominate. The 

silty mudstone in figure 43 contains about 60 percent

Calcite

ClayJ

^^.y-::;\';4*??rrai^jv\:"->;:i'-v

Carbonized 

fragments

1 mm

FIGURE 42. Dark-gray carbonaceous shale from the Mancos 

Shale. Collected about 100 feet above the base of the formation; 

some of the smaller black particles are pyrite. Sample DVR2-127.5, Disappointment Valley.

Crack in thin

section along

bedding

1 mm

FIGURE 43. Dark-gray silty carbonaceous mudstone from the 

Mancos Shale. Collected about 200 feet above the base of the 

formation; black particles are chiefly carbonized plant fragments; some of the smaller equidimensional black particles 

are pyrite; colorless detritus is largely quartz. Sample DVR2-35, Disappointment Valley.

mineral clay; the remainder is detritus except for minor 

amounts of pyrite and calcite. Detritus is chiefly quartz 

with a small amount of carbonized plant fragments, 

K-feldspar, plagioclase, chert, muscovite, biotite, and 

zircon.

A sandy mudstone (sample DVR-2-210) from the 

basal Mancos is shown in figure 44. This rock is about 

half mineral clay and half detritus of sand, silt, and 

clay size, mostly sharply angular in the smaller sizes 

ranging to partly well rounded in the sand-sized material. Detritus is mostly quartz, but includes about 10 

percent rounded clay-chlorite aggregates that are probably glauconite, about 5 percent chert, and 5 percent 

rock fragments largely of volcanic origin. Minor 

amounts of orthoclase, biotite. leucoxene, tourmaline, 

zircon, and hypersthene are also present. About 1 percent carbonized plant fragments, 1 percent pyrite, 1 

percent calcite, and a trace of barite are scattered in 

clay.

Part of a thin section of a thin limestone layer 

(sample DVR-1-452.7) about 400 feet above the base 

of the Mancos is illustrated in figure 45. This rock consists of about 80 percent calcite, 15 percent pyrite, and 

5 percent carbonaceous material as small irregular fragments and a few tiny fiberlike or stemlike fragments. 

A minor amount of silt- and clay-sized quartz detritus
PETROGRAPHY OF SEDIMENTARY ROCKS B31

Carbonized 

plant fragment:

Clay-chforite 

aggregate

Clay

Pyrite

1 mm

FIGURE 44. Dark-gray sandy carbonaceous mudstone from the 

Mancos Shale. Collected about 30 feet above the base of the 

formation; black particles are chiefly carbonized plant fragments ; colorless detritus is largely quartz. Sample DVR-2- 

210, Disappointment Valley.

occurs in some bedding layers. As figure 45 shows, microfossils, carbonaceous material, and pyrite tend to be concentrated in certain bedding layers.

The layering is shown at a smaller scale in figure 46, 

which also illustrates a segment of a small gash fracture in the limestone. The gash fracture is only a few 

centimeters long, terminating at points outside the area 

shown in figure 46 where bedding layers are not offset. 

Fracturing and minor flexing of the beds occurred after 

the sediment had acquired some rigidity following deposition, and at a time when the components of calcite and 

pyrite were still mobile in the rock, as attested by the 

occurrence of calcite within the fracture and the concentration of pyrite along the fracture. Probably, 

lateral secretion of calcite and pyrite to the gash fracture took place when the beds were deeply buried. A 

further conclusion to be drawn from the gash-fracture 

relationships is that calcite and pyrite components migrated into the isolated fracture merely because of 

pressure differential; the simple local opening of 

the fracture in beds far from any throughgoing structures could not have involved chemical disequilibrium 

resulting from introduction of a foreign solution differing in composition from that which already saturated 

the strata. Perhaps on a much larger scale this principle 

of pressure-induced lateral secretion could have wide 

application in problems dealing with metal concentraShell 

fragment

{

Fine-grained

calcite with

clay-sized

carbonaceous

particles

1 mm

FIGURE 45. Dark-gray carbonaceous limestone from the Mancos 

Shale containing abundant pyrite. Microfossils are abundant 

in some bedding layers. Sample DVR-1-452.7, Disappointment 

Valley.

tions, and may bear on the origin of uranium-vanadium 

ores in formations underlying the Mancos Shale in the 

Slick Kock district.

Part of a light-gray limestone nodule (sample DVR1-515.4) collected from core about 340 feet above the 

base of the Mancos (unit 29 in the detailed section from 

DVR-1 of the lower part of the Mancos, Shawe and 

others, 1968) was examined in thin section and was 

seen to consist of about 80 percent calcite. About 2 percent pyrite as minute grains 0.02-0.04 mm across, 

mostly cubes, is sparsely distributed throughout most 

of the calcite but locally is concentrated in aggregates 

about 5-10 mm in size. The remainder of the rock consists of clay, partly dispersed in fine-grained calcite but 

mostly in broken and streaked fragments as much as 5 

mm long irregularly distributed in fine-grained calcite. 

No carbonaceous material is evident. Small irregular 

fractures have been filled with coarse calcite. These relationships suggest that bedding in the layer in which the 

nodules formed was somewhat disrupted during the 

formation of the nodule. Some deformation occurred 

after the nodule acquired its full size.

Numerous thin bentonitic claystone layers in the 

lower part of the Mancos Shale probably represent volcanic ash falls. Biotite and quartz are the most abundant clastic constitutents in the claystone layers but are 

not evident in all layers. The layers commonly contain 

no calcite, and carbonaceous material appears to be ab-
B32 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

Calcitefilled

fracture

Fine-grained

calcite with

clay-sized

carbonaceous

particles

Calcite

Fine-grained

calcite with

very abundant

fine-grained

pyrite 1 cm 

J

FIGURE 46. Dark-gray carbonaceous limestone from the 

Mancos Shale, with calcite-fllled fracture. Pyrite is abundant along the fracture and in a bedding layer adjacent 

to coarse-grained calcite. Sample DVR-1-452.7, Disappointment Valley. Circle shows area of figure 45.

sent from all of them, but pyrite is abundant in nearly 

all. Zircon and apatite, common heavy minerals in the 

claystone, are mostly sharply euhedral crystals, evidence that the material was dropped in place from the 

air rather than transported by water to the deposition 

site. Abundance of pyrite in the layers may indicate that 

iron was an important constituent of the ash, possibly 

being liberated by devitrification of glass and redeposited in diagenetic pyrite. Hydrogen sulfide was probably 

abundant in the organic-rich sediments accumulating at 

the bottom of the Mancos sea, and in addition, sulfur 

may have been especially abundant in the volcanic ash 

(see, for example, Fenner, 1933, p. 86-87, 92-95). Sudden deposition of individual ash layers accounts for the 

absence of carbonaceous material in these despite its 

notable abundance throughout the rest of the Mancos 

Shale. Possibly local reworking of the clay layers explains the presence of rounded quartz silt and sand 

grains in some of them.

GENERAL CONCLUSIONS

Mineral compositions of the investigated sandstones 

above the Moss Back Member of the Chinle Formation 

are surprisingly similar. Compositions of particular 

rock types vary as much within individual formations 

as they do from one formation to another. The sandstones generally contain about 50-80 percent quartz,

2-10 percent chert, 1-10 percent feldspar, 3-15 percent 

clay, 1-15 percent calcite, and minor amounts of other 

components. Sandstones in the Cutler Formation are 

much more arkosic, about one-third K-feldspar; this 

composition suggests their derivation from the nearby 

Uncompahgre highland to the east. Sandstones in the 

Moss Back, although recognized as quartzose in part, 

and likely compositionally similar to overlying rocks, 

are in part calcite rich; this content suggests derivation 

from carbonate rocks that were exposed in the Zuni 

highland to the southeast.

The ore-bearing sandstone of the Salt Wash Member 

of the Morrison Formation is not unique in mineral 

composition, and its mineral composition probably had 

little, if anything, to do with localization of uraniumvanadium deposits at this horizon. Sandstones in the 

Brushy Basin Member, a unit commonly cited as containing abundant volcanic material, contain probably 

only slightly more such components (chiefly some forms 

of chert, rock fragments, and plagioclase), than do 

many other sandstones exposed in the Slick Kock district. They are not rich enough in volcanic debris to 

distinguish them sharply from sandstones in underlying and overlying stratigraphic units. The Dakota, 

for example, contains sandstone composed partly of volcanic material, and bentonitic claystones are common in 

the Dakota and the Mancos Shale. Material in the sandstones other than that of volcanic origin may have come 

principally from clastic sedimentary formations in 

source areas, and ultimately thence from Precambrian 

igneous-metamorphic rock terranes.

Siltstones studied here, all from Jurassic formations, 

are distinguished from sandstones principally by their 

finer grain size and higher proportion of clay. They are 

otherwise mineralogically similar. Again, siltstones in 

the Brushy Basin Member, although containing volcanic 

materials, are not distinguished by unusual amounts of 

such material.

The mineral compositions of mudstones investigated, 

all from the Morrison Formation and Cretaceous units, 

are generally similar also. They contain about 25-30 

percent quartz, 0.25-3.5 percent chert, 2-6 percent feldspar, 40-65 percent clay, 0-25 percent calcite, and minor 

amounts of other components. Burro Canyon Formation 

claystones contain on the average about 80 percent clay 

and commensurately less of other components. Mudstones in the Brushy Basin Member have an average 

content of 1 percent each of barite and analcite, principally because of large amounts in altered rocks, and in 

this respect are unique. In addition, mudstones in the 

Brushy Basin contain clay that is in large part derived 

from devitrification of volcanic ash; clay in the middle 

green unit of the member is chiefly of volcanic origin,
PETROGRAPHY OF SEDIMENTARY ROCKS B33

but that in the upper and lower brown units is less so.

Shale that is the dominant rock type of the Mancos 

Shale contains about 1 percent pyrite, 3 percent carbonaceous material, 5 percent detrital minerals, 25 percent calcite, and the remainder clay.

Sedimentary rocks whose constituent grains display a 

great range in size, degree of sorting, and rounding have 

no commensurate variations in mineral composition, 

other than higher clay content in poorer sorted and 

finer grained material. Calcite cement also is greater in 

amount in finer grained rocks in general. Probably the 

mode of deposition, rather than difference among the 

rocks of the source areas generally west or southwest 

of the Colorado Plateau for all the Jurassic and Cretaceous formations determined the gross variations in 

mineral composition of these rocks. Differences in rock 

type in, and proximity to, source areas determined variations among some of the Triassic and Permian rocks.

Eocks which are reddish brown, or variations of this 

color, and which constitute the Colorado Plateau sedimentary strata that are termed "red beds," are characterized by content of numerous detrital black opaque 

mineral grains, films of hematite on other detrital 

grains, and dustlike hematite dispersed in matrix material. This is true regardless of the manner of deposition of the original sediment fluvial, eolian, tidal flat, 

or flood plain. Hematite films and finely dispersed hematite impart the reddish-brown color that gives the rocks 

the name red beds. On grounds that the hematitecoated grains could not have been transported to the 

site of deposition without loss of the hematite films by 

abrasion, and that the detrital black opaque grains were 

oxidized in place following deposition (Bowers and 

Shawe, 1961, p. 175-176,182-184, 212-213), it is evident 

that the red beds, as such, came into being through diagenetic processes that moved iron from the oxidizing 

detrital black opaques and deposited it nearby as finely 

dispersed hematite in matrix and as films on other detrital grains.

Eocks that are not reddish brown the "bleached" 

rocks of the plateau contain virtually no hematite, 

either as detritals or as grain coatings. They do contain 

pyrite or marcasite (where the rocks are deeply buried 

and are light gray) or limonite oxidized from iron sulfide (where the rocks are at or near the surface and are 

light buff or light yellowish brown). Such rocks contain either abundant carbonaceous material or none. 

These variations will be investigated in more detail and 

their significance discussed in a later chapter of this 

professional paper dealing with alteration of the sedimentary rocks.

The common evidence of distortion of rocks composed 

of clay-sized material suggests that slumping of watersaturated material shortly after deposition may have 

been extensive in Colorado Plateau sedimentary strata.

Silica is an abundant cement in most of the sedimentary rocks studied. It is present in both reddish-brown 

rocks and in "bleached" rocks. Commonly, silica cement 

and overgrowths occur very close to zones where silica 

has gone into solution from detrital quartz grains, indicating the local derivation of the matrix silica. Its 

ubiquitous occurrence in rocks regardless of color suggests it may have formed during diagenesis. The common interpenetration of quartz grains where solution 

has been extensive suggests high pressure at the time 

of solution, perhaps when the strata were deeply buried 

or undergoing folding. Silica as faceted overgrowths 

appears to be more abundant in light-gray or light-buff 

rocks, and may be a product of the alteration that 

bleached such rocks. In light-gray sandstone of the 

Burro Canyon Formation, silica overgrowths locally 

appear to have formed later than calcite, normally a 

constituent that has been extensively redistributed in 

altered rocks.

Calcite, on the average equally abundant in rocks of 

all colors, in reddish-brown sandstones occurs mostly 

as small interstitial blebs and patches less than 0.5 mm 

across, whereas in light-gray and light-buff sandstones 

it occurs commonly as larger optically continuous 

patches 0.5 mm or more in size, enclosing several detrital 

grains. In the light-gray and light-buff rocks, calcite is 

commonly intimately associated with euhedral crystals 

of authigenic minerals, such as anatase, pyrite, and 

marcasite (or limonite after pyrite and marcasite). 

These relationships indicate remobilization of calcite 

during the alteration that developed the light colors.

Calcite, dolomite, siderite(?), and barite occur as 

metacrysts (authigenic crystals) principally in rocks 

that are light colored, again suggesting that alteration 

related to the bleaching tended to induce crystallization 

of these minerals. Albite overgrowths in altered sandstone, albite microlites associated with recrystallized 

limestone, and analcite segregations in altered mudstone 

of the Brushy Basin, together with barite metacrysts, 

suggest that altering solutions may have been saline. 

Saline solutions have been proposed as the reason for 

growth of sodium-bearing minerals including albite and 

analcite, as well as barite in the Green River Formation 

in Utah (Milton, 1957, p. 143). Davidson (1965, p. 946), 

quoting Bogdanov (1962), remarks at the extensive 

albitization of copper-bearing red-bed arenites of 

Udokansk in eastern Siberia as a result of saline 

met amor phism.

Calcite metacrysts in the Burro Canyon Formation 

that apparently developed under pressure from the considerable load of overlying sediments show that calcite
B34 GEOLOGY, SLICK ROCK DISTRICT, SAN MIGUEL AND DOLORES COUNTIES, COLO.

deposition occurred when these rocks were deeply 

buried.

The very low porosities of the argillaceous rocks suggest great depth of burial, in excess of 10,000 feet.

Common polysynthetic twins in calcite in many rock 

types, including those of light color, suggest that the 

rocks were stressed following the alteration that caused 

bleaching and redistribution of calcite. The most likely 

cause of stressing seems to be folding of the sedimentary 

strata. Other evidence of stress that may have been the 

result of folding of the rocks are shears parallel or subparallel to bedding and sand grains crushed against 

pebbles in conglomerate. In one rock from the Burro 

Canyon Formation shears subparallel to bedding are 

directly associated with polysynthetic twins in calcite. 

Formation of the folds in the district, and the time of 

their folding, will be considered in later chapters in 

this professional paper discussing structure and rock 

alteration.

REFERENCES CITED

Archbold, N. L., 1959, Relationship of carbonate cement to lithology and vanadium-uranium deposits in the Morrison Formation in southwestern Colorado: Bcon. Geology, v. 54, no. 

4, p. 666-682.

Athy, L. F., 1930, Density, porosity, and compaction of sedimentary rocks: Am. Assoc. Petroleum Geologists Bull., v. 14, 

no. 1, p. 1-24.

Bogdanov, Yu. V., 1962, Geology and genesis of the cupriferous 

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