X-ray Mineralogy of the Parachute Creek 

Member, Green River Formation, in the 

Northern Piceance Creek Basin, Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 803
X-ray Mineralogy of the Parachute Creek 

Member, Green River Formation, in the 

Northern Piceance Creek Basin, Colorado

By DONALD A. BROBST and JERRY D. TUCKER

GEOLOGICAL SURVEY PROFESSIONAL PAPER 803

A study of oil shale, marlstone, and tuff, 

some containing dawsonite, 

in three exposed sections

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 73-600120

For sale by the Si. >erintendent of Documents, U.S. Government Printing Office
CONTENTS

Page

ADstract 1

Introduction ------ 1

Piceance Creek basin ------ - - 3

Lithologic terms - ----- - ----- ----------- 3

Tertiary stratigraphy 4

Older Tertiary rocks- - - 4

Wasatch Formation -- - 4

Green River Formation 4

Douglas Creek Member -- 4

Garden Gulch Member ~ 5

Anvil Points Member - 5

Parachute Creek Member 5

Evacuation Creek Member Q

Measured sections -- ------ 6

Pipeline section . - ____ g

Lower Piceance Creek section 12

Rio Blanco section ---------- 19

Mineral composition - -- - - - - -- - 24

Laboratory work - - - ----- 24

Procedures »- 24

Discussion- -- 25

Oil shale and marlstone 28

Vertical variation in composition - - 34

Fossiliferous oil shale and marlstone, pipeline

section - 34

Page

Mineral composition Continued

Oil shale and marlstone Continued

Vertical variation in composition Continued 

Marlstone and medium-brown oil shale, pipeline section- 36

Oil shale from the Mahogany ledge, lower Piceance Creek section 36

Massive marlstone from the Mahogany ledge,

lower Piceance Creek section - - 37 

Marlstone and laminated oil shale from the

Mahogany ledge, Rio Blanco section 37 

Lateral variation in composition - - 40

Medium-brown oil shale, pipeline section - 40

Dark, pyritic oil shale, Rio Blanco section- 40

Dawsonitic rocks 40

Vertical variation in composition 44

Dawsonitic, pyritic oil shale, lower Piceance

Creek road - __ 44

Lateral variation in composition 45

______________ 47

....__________...... 47

47 

... 48

.......................... 52

Tuff bedsSandstone dikes 

Cavities 

Sedimentation and diagenesisReferences citedILLUSTRATIONS

FIGURE 1. Index map of the Piceance Creek basin and adjacent areas ~ -- - ----- 

2. Diagram showing correlation of three measured sections of the Parachute Creek Member of the Green River 

Formation in the Piceance Creek basin ----- - - - - - - - - 

3. Map showing locations of parts of the measured section of the Parachute Creek Member on lower Piceance Creek

4. Photograph showing the lower part of the Parachute Creek Member on lower Piceance Creek- ----- ~ ... ~

5. Photograph showing upper part of the Parachute Creek Member on lower Piceance Creek- ___

6. Diagram showing nomenclature used for the Parachute Creek Member on lower Piceance Creek 

7. Map showing location of the measured section of the Parachute Creek Member near Rio Blanco -

8. Diagram showing nomenclature used for the Parachute Creek Member in the Rio Blanco section - - 

9. Photograph of fossiliferous laminated oil shale and marlstone from the pipeline section - - 

10. Histograms showing mineral composition of fossiliferous laminated oil shale and marlstone from the pipeline 

section, as expressed by X-ray peak height in chart units - - - - ------ . ~

11. Photograph of laminated oil shale from the Mahogany ledge in the lower Piceance Creek section - -

12. Histograms showing mineral composition of laminated oil shale from the Mahogany ledge in the lower Piceance 

Creek section, as expressed by X-ray peak height in chart units - - ------- - ~ -- - -

13. Histograms showing mineral composition of massive marlstone from the Mahogany ledge in the lower Piceance 

Creek section, as expressed by X-ray peak height in chart units -... ~~ ...

14. Photograph of marlstone and laminated oil shale from the Mahogany ledge in the Rio Blanco section -

15. Histograms showing mineral composition of marlstone and laminated oil shale from the Mahogany ledge in

16.

the Rio Blanco section, as expressed by X-ray peak height in chart units 

Histograms showing lateral variation in mineral composition of medium-brown oil shale from the pipeline section, 

as expressed bv X-rav ueak height in chart units___________________________________

Page 

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7

13

14

15

22

23

25

34

35

37

38

39

40

41

42

X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

109 C 108'

MOFFAT CO 

RIO BLANCO CO

Lower

Piceance

Creek sectio

Piceance Creek 

gas field S

Pipeline 

section

Rio Blanco section

GARFIELD CO 

MESA CO~

Grand 

Junction Green 

River 

Basin Rock Washakie 

Springs Basin

EXPLANATION

Piceance 

Creek 

basin _

Boundary of 

area in largescale map

Areas underlain by rocks of 

the Green River Formation

Grand Junction 

0 10

I i i i i I

FIGURE 1. Index map of the Piceance Creek basin and adjacent areas.
PICEANCE CREEK BASIN

This report presents and discusses the data obtained from an X-ray diffraction study of the 

mineralogy of about 650 samples of rock, chiefly oil 

shale, marlstone, and altered tuff, collected from exposures of the Parachute Creek Member of the Green 

River Formation on the periphery of the Piceance 

Creek basin, Colorado. Vertical and lateral variation 

of the mineral composition of the rocks is summarized in terms of relative abundance of the various 

minerals. Dawsonitic rocks, before 1966 known only 

in drill cores from the deeper parts of the basin, are 

described from exposures along lower Piceance 

Creek. This study suggests that dawsonite is a product of the diagenetic alteration of analcime.

Samples examined in this study were collected 

from three measured sections of the Parachute Creek 

Member (fig. !):(!) along the pipeline on Cathedral 

Bluffs, on the west side of Piceance Creek basin; (2) 

along lower Piceance Creek, on the north side of the 

basin; and (3) along upper Piceance Creek west of Rio 

Blanco, on the east side. The sections are described in 

detail to provide a framework for the discussion of the 

X-ray data and to provide study sections for stratigraphic and mineralogic correlation between outcrops and drill cores taken from the interior of the 

basin. Stratigraphic correlations of units described 

here with those described by other workers also are 

suggested.

PICEANCE CREEK BASIN

Piceance Creek basin is a structural basin that lies 

between the Colorado and White Rivers and is mostly 

in Garfield and Rio Blanco Counties, northwestern 

Colorado (fig. 1). The basin forms an extensive, somewhat dissected plateau of about 1,600 square miles 

that rises to 4,000 feet above the surrounding country. Good exposures of the Green River Formation are 

common in the steep slopes and cliffs on the 

periphery of the plateau. The average altitude of the 

plateau is greater than 7,000 feet, and the highest 

altitude is 9,400 feet, along the crest of the spectacular cliffs west of Rifle, on the southeast side of the 

basin. From the rim, the land surface slopes gently 

downward to the central part of the basin, which is 

characterized by rolling hills. The vegetation of this 

semiarid region is sparse and consists mostly of sage, 

pinion, and juniper. The principal industry is cattle 

raising; some gas is produced from the Piceance 

Creek gas field. The nearest commercial centers are 

Rifle, Grand Junction, Meeker, and Rangely (fig. 1).

LITHOLOGIC TERMS

The lithologic terms discussed below are used in 

this report to describe the general character of the 

rocks in the various formations and the more specific

character of the beds and groups of beds in the 

measured sections. The definitions used here closely 

follow those of Bradley (1931, p. 6-8), who discussed 

and explained many of the terms that have become a 

part of the standard vocabulary used to describe the 

Green River Formation.

Claystone, siltstone, mudstone, and sandstone are 

all defined by particle size according to the Wentworth scale (Wentworth, 1922). Mudstone has the 

appearance of a fine-grained rock and contains an indefinite mixture of clay-, silt-, and sand-sized 

particles.

"Marlstone" is used in the manner of Bradley 

(1931, p. 7), who designated the term for rocks that 

consist principally of mixtures of calcite and 

dolomite and that have textures intermediate 

between those of mudstone and limestone. In following this European usage, he avoided the need for 

cumbersome multiword terms and firmly established 

the term "marlstone" for these lacustrine carbonate 

rocks.

Claystone, siltstone, mudstone, and marlstone in 

the Piceance Creek basin generally are massive rocks 

that are homogeneous in color and texture or, more 

commonly, are distinctly bedded or laminated. Individual laminae differ in color and thickness but 

generally are only a few millimeters, or more rarely a 

few centimeters, thick.

"Shale" categorizes fissile rocks made up of claysized particles that weather approximately along 

bedding planes into thin flakes, chips, plates, and 

wedge-shaped fragments. The term "shaly" is used to 

describe mudstone, siltstone, claystone, and marlstone that have incipient fissility that is not 

developed sufficiently to permit the rock to be called a 

"shale." "Paper shale" is applied to finely laminated 

claystone, siltstone, mudstone, and marlstone that 

tends to part along closely spaced bedding planes. 

Bradley (1931, p. 7) described paper shales as consisting of very thin laminae of nonfissile rock 

separated from one another by excessively thin 

laminae that are fissile.

Oil shale is a special problem. Rocks rich in organic 

material, commonly called oil shale in the Green 

River Formation, have long been recognized as being 

neither oily nor shaly (Bradley, 1931, p. 7, and 1964, p. 

A19; Donnell, 1961, p. 864; Jaffe, 1962, p. 2; Cashion, 

1967, p. 24; Smith, 1969, p. 185). Most of the oil shale is 

dolomitic marlstone having various amounts of 

organic matter that was derived chiefly from algae, 

aquatic organisms, waxy spores, and pollen grains. 

This organic matter is only slightly soluble in 

ordinary petroleum solvents, but a large part is convertible to a petroleumlike substance by destructive
X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

distillation. The shale part of the term undoubtedly 

refers to the papery structure observed in many 

weathered outcrops. (See Bradley, 1931, pi. 4B.)

Discussions in the literature reveal that "oil shale" 

is a well-established term with economic implications. 

As early as 1927, the Department of the Interior ruled 

that any shale that yields oil when distilled will be regarded under the general mining laws as oil shale 

(Bradley, 1931, p. 7). Standards for the minimum 

amount of extractable oil to qualify the rock as oil 

shale have not been set. The term, therefore, has been 

used more qualitatively than quantitatively.

In the rock descriptions in this report, the term "oil 

shale" was applied to those marlstones estimated by 

reason of their color and weight to contain more than 

3 gallons of shale oil per ton of rock. Generally the 

darker the color and the lighter the weight, the higher 

the oil content. A series of four color modifiers for the 

overall appearance of the rock was established with 

the counsel of experienced colleagues and applied on 

the outcrop to estimate broad ranges of oil content: 

light brown (lean rock, probably less than 15 gal of 

shale oil per ton of rock); medium to dark brown 

(lower and upper values, respectively, of a range of 

about 15-30 gal of shale oil per ton of rock); and dark 

red brown (rich rock, probably more than 30 gal of 

shale oil per ton of rock).

The distinction between the lowest grade of oil 

shale and "ordinary" marlstone (the barren 

marlstone indicated by some workers) is a subjective 

decision. Most marlstones contain some organic 

matter and probably will yield some shale oil on distillation. According to D. C. Duncan (oral commun., 

1970), Fischer assays of some "barren marlstones" 

indicated the presence of at least 5 gallons of shale 

oil. Color is not an infallible criterion, but it can be a 

useful one.

"Tuff is used to describe any bed that consists 

largely of volcanic ash, regardless of the degree of 

alteration. Most of these rocks are analcime rich.

Following the definition of Fahey (1962, p. 18), the 

adjective "saline" is applied to those minerals that 

have sodium occupying one or all of the cation 

positions and that have the carbonate radical supplying all or part of the negative charge.

TERTIARY STRATIGRAPHY

OLDER TERTIARY ROCKS

A structural basin in the Piceance Creek region 

had formed by early Tertiary time after deposition of 

the marine and continental rocks of the Cretaceous 

Mesaverde group (Bradley, 1931; Donnell, 1961). A 5- 

to 20-foot unit of conglomerate and coarse sandstone 

of Paleocene(?) age was deposited along the east 

margin of the basin. This unit has been correlated

with the Ohio Creek Conglomerate found in areas to 

the south (Donnell, 1961, p. 843).

WASATCH FORMATION

The Atwell Gulch Member of the Wasatch 

Formation conformably overlies the Paleocene(?) 

conglomerate. This member is a sequence of brown 

sandstone and somber-colored shale of Paleocene 

and Eocene(?) age which presumably underlies the 

entire basin, although it thins from 500 feet on the 

east side, near Rifle, to a vanishing point on the 

southwest side (Donnell, 1961, p. 844). The Ohio 

Creek and the Atwell Gulch were mapped with the 

overlying Eocene part of the Wasatch Formation by 

Donnell (1961, p. 844, pi. 48).

The upper part of the Wasatch Formation, of early 

Eocene age, consists chiefly of fluviatile rocks, including large amounts of clay, shale, and lenticular 

sandstone accompanied by lesser amounts of limestone, coal, and black carbonaeceous shale (Donnell, 

1961, p. 846). Most of the rocks are brightly colored; 

various shades of red, green, yellow, and purple are 

common, but dull brown, tan, and gray generally predominate in exposures along the north and west sides 

of the basin. The maximum exposed thickness, 5,300 

feet, is 10 miles northwest of Rifle, and from this 

point the formation generally thins westward. The 

base of the unit is indefinite because of a zone transitional with the underlying beds. The contact with 

the overlying Green River Formation is also transitional.

GREEN RIVER FORMATION

The Green River Formation (Hayden, 1869, p. 90) 

consists of sandstone, siltstone, shale, and several 

kinds of carbonate rocks, some of which contain 

saline minerals and abundant organic matter. 

Although the original thickness is unknown, at least 

3,000 feet of these rocks was deposited in the lacustrine environment that persisted in this area during 

much of middle Eocene time (Donnell, 1961, p. 847).

In a detailed study of the Green River Formation, 

Bradley (1931, p. 9) divided the unit in the eastern 

Uinta Basin, Utah, and the Piceance Creek basin, 

Colorado, into four members, from bottom to top: the 

Douglas Creek, Garden Gulch, Parachute Creek, and 

Evacuation Creek Members. Donnell (1953) proposed 

the name Anvil Points Member for the eastern equivalents of the Douglas Creek and Garden Gulch 

Members and part of the Parachute Creek Member 

exposed on the west side of the basin.

DOUGLAS CREEK MEMBER

The Douglas Creek Member is composed of crossbedded ripple-marked sandstone, algal and ostracodal limestone, oolitic sandstone and limestone,
PICEANCE CREEK BASIN

and small amounts of gray shale. Donnell (1961, p. 

848-849) reported that the member attains a maximum thickness of 800 feet in the southwestern part 

of the basin and that it forms a recognizable unit in 

outcrops on the south and west sides of the basin. 

Cashion (1969) mapped about 500 feet of the member 

along Cathedral Bluffs in the Black Cabin Gulch 

quadrangle (fig. 1, vicinity of the pipeline section). 

On the south side of the basin, between DeBeque and 

Grand Valley, the Douglas Creek Member merges 

laterally with the Anvil Points Member (Donnell, 

1961, pi. 48). Duncan and Belser (1950) reported a thin 

Douglas Creek Member in the subsurface as far 

northeast as the Piceance Creek gas field. The 

Douglas Creek Member conformably overlies the 

Wasatch Formation and is conformably overlain by 

the Garden Gulch Member.

GARDEN GULCH MEMBER

The Garden Gulch Member contains much papery 

clayey (illitic) shale and marlstone generally, 

though not entirely, barren of oil as well as thin 

beds of sandstone, oil-shale breccia, and ostracodal, 

oolitic, and algal limestone (Donnell, 1961, p. 849). 

This member crops out in steep slopes between the 

brown and buff benches of the underlying Douglas 

Creek Member and the steep white cliffs of the transitionally overlying Parachute Creek Member. The 

unit is 700 feet thick at its type locality in Garden 

Gulch (sees. 7 and 8, T. 6 S., R. 96 W.; fig. 1), a small 

tributary of Parachute Creek on the south side of the 

basin. A few miles east of the type locality the Garden 

Gulch Member merges laterally with the Anvil 

Points Member. Donnell (1961, p. 850 and pi. 48) 

mapped the member on the west and north sides of 

the basin to an area along the White River between 

Yellow and Piceance Creeks, where he, too, found 

that it apparently merges laterally into the Anvil 

Points Member.

ANVIL POINTS MEMBER

The Anvil Points Member (Donnell, 1953) is a 

heterogeneous sequence of gray shale, gray and 

brown sandstone, siltstone, algal and oolitic limestone, and some marlstone containing little or no oil. 

The member crops out in cliffs and benches on the 

east side of the Piceance Creek basin from Parachute 

Creek on the south to within a few miles west of 

Piceance Creek on the north (Donnell, 1961, pi. 48). 

The member does not extend as far basinward as the 

Piceance Creek gas field. It is considered to be the 

eastern equivalent of the Douglas Creek and Garden 

Gulch Members and part of the Parachute Creek 

Member as exposed on the west side of the basin, and 

it is probably the equivalent of the delta and shore

facies that Bradley (1931, p. 14) described in the Gray 

Hills, north of the Piceance Creek basin. The Anvil 

Points Member attains its maximum thickness of 

1,870 feet along upper Piceance Creek.

The member interfingers with the underlying 

Wasatch Formation and the overlying Parachute 

Creek Member. Donnell (1961, p. 852) chose the top of 

the Anvil Points Member at "the base of a series of 

low-grade oil shales and at the top of the uppermost 

sandstone bed in a sequence of barren marlstone 

alternating with gray and brown sandstone." The 

base of the Anvil Points Member is established on a 

criterion for the top of the Wasatch, that is, at the top 

of the uppermost red shale bed which is at least 10 

feet thick (Donnell, 1961, p. 852).

PARACHUTE CREEK MEMBER

The hallmark of the Parachute Creek Member is oil 

shale. Bradley (1931, p. 11) noted, "It contains all of 

the large groups of rich beds, most of the individual 

rich beds, and a large proportion of the low-grade oil 

shale." The saline facies as well are most abundant 

in the Parachute Creek Member. Bradley described 

the member in detail at its type locality along 

Parachute Creek, on the south side of the Piceance 

Creek basin. The member, much of which commonly 

is exposed in precipitous cliffs, has been mapped all 

around the periphery of the basin, where it generally 

is 500 to about 1,000 feet thick. In the subsurface it is 

thicker; Donnell (1961, p. 852) reported a thickness of 

1,700 feet in sec. 31, T. 1 S., R. 96 W.

Bradley (1931, p. 11-13) divided the member at its 

type locality into three parts lower oil-shale group, 

transitional beds, and upper oil-shale group each of 

which was defined on characteristics of the included 

oil shale. Revision of these groups was made at the 

1,230-foot-thick section at Glover Point (sec. 22, T. 6 

S., R. 96 W.) by Duncan and Denson (1949) and 

Donnell (1961, p. 854). Donnell and Blair (1970, p. 76) 

divided the member into a series of rich zones of oil 

shale (designated R) separated by lean zones 

(designated L), each zone being numbered from the 

base of the section. This system of division was found 

particularly useful on lower Piceance Creek and in 

the pipeline section.

The richest oil-shale sequence in the member has 

been called the Mahogany ledge in the surface sections and the Mahogany zone in the subsurface sections. The name is derived from the red-brown 

mahogany color of the rich oil shales (Bradley, 1931, 

p. 23). The Mahogany ledge or zone has been traced 

throughout the basin and locally exceeds 200 feet in 

thickness. Donnell (1961, p. 856-857), in summarizing the geology of the ledge, reported that the richest 

oil-shale bed (the Mahogany bed) ranges in thick-
X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

ness from 3 to 10 feet. In the thickest parts of the bed 

in the deepest parts of the basin the average oil 

content is 55 gallons per ton of rock. Most of the base 

of the ledge is marked by a wavy-bedded tuff that attains a maximum thickness of 18 inches. A second 

characteristic tuff about 2 inches thick lies 8-10 feet 

higher in the section. An analcimized tuff, generally 

3-6 inches thick, that lies 3-14 feet above the 

Mahogany bed has been called the Mahogany 

marker. These and several other key beds with informal names are referred to in the descriptions of the 

measured sections. On some cliffs, the bottom and 

top of the Mahogany ledge are accentuated by reentrants, or grooves, formed by the more rapid erosion 

of marlstone that contains only small amounts of 

organic matter. These grooves have been referred to 

informally as the A- and B-groove in the upper and 

lower positions, respectively.

Many cavities are found in the rocks exposed below 

the Mahogany ledge. The cavities range in largest 

dimension from a few inches to several feet and 

presumably were once filled with saline minerals, 

probably mostly nahcolite.

The Parachute Creek Member is characterized by 

abundant intercalated beds of tuff which rarely attain a thickness of more than a few inches. Many are 

less than 1 inch thick, but a few are several feet thick. 

The thicker beds commonly contain fragments of the 

enclosing rocks. Most of the tuffs are analcime rich, 

and they weather yellow or orange brown and commonly form reentrants between marlstone and oil 

shale in the outcrops. The contacts are smooth to 

wavy (undulating), and the natures of the upper and 

lower contacts may differ. Some beds are discontinuous and podlike, but the lateral persistence of 

many beds is remarkable.

The base of the Parachute Creek Member generally 

is placed where rocks rich in carbonate (and organic 

matter) become predominant upward through a transition zone from illitic shale, typical of the Garden 

Gulch Member, or from the interbedded shales, siltstones, and fine-grained sandstones, typical of the 

Anvil Points Member.

Bradley (1931, p. 11, pi. 7) placed the top of the 

Parachute Creek Member at its type locality at the 

base of a soft limy yellow-brown sandstone. Donnell 

(1961, p. 857) placed the top of the Parachute Creek 

Member at the base of the lowest sandstone or siltstone bed 10 feet or more thick that occurs above the 

uppermost bed of a sequence of designated key beds. 

In the northern parts of the Piceance Creek basin, 

beds of brown tuffaceous sandstone unconformably 

overlie truncated beds of marlstone. Scour channels 

several feet deep in marlstone were filled with brown

sand of the Evacuation Creek Member in some areas 

along Piceance Creek.

EVACUATION CREEK MEMBER

The Evacuation Creek Member, the uppermost 

lithologic unit of the Green River Formation, was 

named by Bradley (1931, p. 14) for about 530 feet of 

marlstone and some shale and sandy rocks exposed 

along Evacuation Creek in the eastern Uinta Basin, 

Utah. In the vicinity of lower Piceance Creek (Rio 

Blanco County, Colo.), Bradley (1931, pi. 3, loc. 8) assigned at least 500 feet of marlstone and some oil 

shale and sandy rocks to the Evacuation Creek 

Member. These rocks, he said, were overlain by about 

400 feet of predominantly sandy beds, which he 

tentatively correlated with the Bridger Formation in 

the Uinta Basin. In the Piceance Creek basin, 

Donnell (1961, p. 857 and pi. 48) later mapped the 400 

feet of predominantly sandy beds as the Evacuation 

Creek Member and included the marlstone sequence 

of Bradley's Evacuation Creek in the Parachute 

Creek Member (fig. 6). Donnell's usage is followed in 

this report.

Sandstone abundance increases upward in the section. The sandstones are commonly tuffaceous, massive, brown, and medium to coarse grained. Individual beds cannot be traced very far along the outcrop. These sandstone beds probably formed as 

stream deposits in the lake basin.

MEASURED SECTIONS

Three excellent exposures of the Parachute Creek 

Member along the northern periphery of the Piceance 

Creek basin (fig. 1) were measured in detail; these 

were the pipeline section along the Cathedral Bluffs, 

the lower Piceance Creek section, and the Rio Blanco 

section along upper Piceance Creek. The descriptions 

include some information on correlations between 

our work and the work of others in the area of these 

sections, as well as some possible correlations 

between our sections. No correlations between sections are expressed or implied by the numbered units 

of sedimentary rocks and tuff beds or groups of tuff 

beds, called zones in each of the sections. Numbers 

were assigned to units consecutively from bottom to 

top in each section. A comparison of the three 

measured sections is shown in figure 2.

X-ray data on the mineral composition of the rocks 

collected from the measured sections are shown in 

tables and figures and are discussed in later parts of 

this report. Sampled units in the measured sections 

are designated by an asterisk, and samples in the 

tables and figures are correspondingly numbered for 

the units from which they came.
MEASURED SECTIONS

PIPELINE SECTION

Sec. 34, T. 2S.,

R.100 W.

Tuff 230

Tuff 210

Tuff 163

137

-130

-124

110

R-6

LOWER PICEANCE 

CREEK SECTION

Sees. 11, 14, and 15 

T. 1 N..R.97 W.

RIO BLANCO

SECTION

Sees. 5 and 6,

T. 4S.,R.94W.

25 MILES

R-6 

D 

D 

Tuff 94 D

Tuffaceous 

sandstone

Unit 219

Tuff zones 

207-208

177

-169. -167

30 MILES

Tuff zone 

152

Tuffaceous 

marlstone

117

Tuff 74

59

-46 -43

33 -32

Unit 1

Evacuation Creek Member

Parachute Creek Member

Top of Mahogany ledge

Mahogany marker """-Mahogany bed

Base of Mahogany ledge

Parachute Creek Member

Anvil Points Member

i- 200 FEET

FIGURE 2. Correlation of three measured sections of the Parachute Creek Member of the Green River Formation 

in the Piceance Creek basin. Numbered units refer to units described in each measured section. D indicates 

occurrence of dawsonite. Numbers prefixed by R and L refer to zones rich or lean in oil shale, as applied


10 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

Nearly complete section of the Parachute Creek Member of the

Green River Formation along the pipeline Continued 

Parachute Creek Member (part) Continued Thickness

(feet)

131. Oil shale, dark-brown- - - - -

*130. Tuff, gray, evenly bedded. The Mahogany 

marker- . .....

129. Oil shale, medium- to light-brown. 3.8 ft above

base is 0.3 ft of contorted papery shale 

128. Tuff analcimic; persistent bed 0.1-0.4 ft thick

with smooth top and undulating base - 

127. Oil shale, dark-red-brown ~

126. Tuff--------------------- - 

125. Oil shale, dark-red-brown- - 

*124. Oil shale, very dark red brown. The Mahogany

123. Oil shale, medium- to dark-red-brown - 

122. Tuff, analcimic, brown; even contacts;

121. Oil shale, light- to dark-brown --- ---- 

120. Tuff _ - ____ - _ - _ __ - ____________

119. Oil shale, light- to dark-brown; weathers blue

gray........ ________________ .....

118. Tuff zone. Four tuffs; persistent beds lA-l/2 in. 

thick, separated by Vfe-l in. of oil shale. (Yellow 

horizontal line painted on outcrop.) -

*117. Oil shale, medium- to dark-red-brown; weathers

blue gray. Sample is medium-brown oil shale

5.8 ft above base- - - 

116. Tuff, analcimic; wavy contacts ----- 

115. Oil shale; contains thin tuff in upper 0.5 ft 

114. Tuff, analcimic; Vfe in. thick; persistent bed- 

. \J1L Oil oh O.L1CHG a le>- _ - __ ___ .... - . _ .»_«.-.- _

112. Tuff, analcimic, laterally persistent; smooth 

contacts * * - - - 

*111. Oil shale, medium- to dark-brown. Sample is 

dark brown 

*110. Tuff, analcimic, light-yellow-gray; fragmental 

appearance; very fine grained. Base of the 

Mahogany ledge 

*109. Marlstone, gray-orange, laminated, soft; weathers to slabby fragments; slope former; some 

interbedded oil shale. At 17.4 ft above base, 

cavities 1 ft in diameter along a persistent 

zone possibly mark a saline zone. At 25 ft 

above base is tuff 0.1-0.4 ft thick, lenticular. 

Sample is medium-brown oil shale 17.4 ft 

above base. Position of the B-groove -

*108. Oil shale, burned (?), soft, punky, crumbly. Top

OI I\ O ZOIlG "* *~* - .- * ... .«-

107. Tuff, analcimic, pink 

106. Oil shale-------- --- --

105. Tuff, analcimic; weathers reddish; wavy base; 

smooth upper contact 

104. Oil shale-------------- -----

103. Tuff; weathers red brown; even contacts. (Painted yellow 36 on outcrop.)- 

102. Oil shale---- ------------ 

101. Tuff; irregular contacts; poorly exposed ----

100. Oil shale, light-brown; weathers to fissile plates

99. Oil shale; upper 0.5 ft contains cavities caused

by leached saline minerals(?) 

98. Tuff- - - - - - -

97. Oil shale, light-brown; weathers to fissile plates 

*96. Tuff, analcimic; persistent bed, regular contacts.

7.0 

.5

8.5

.2

.95

.05

1.8

19

2.0

10.3 

.05

3.2

.4

13.8 

.1 

3.7 

.05

11i.i

10.9

2.1

26.5

rr.U

.15 

.5

.15

1.7

.25

4.3

.2

2.4

1.5 

.05

1.2 

.05

Nearly Complete section of the Parachute Creek Member of the

Green River Formation along the pipeline Continued 

Parachute Creek Member (part) Continued Thickness

(feet)

95. Oil shale, light-brown, fissile - - - 

94. Tuff; irregular contacts - - -- 

: 93. Oil shale, gray-brown, platy - - - 

*92. Tuff, analcimic; weathers bright pink; persistent bed; smooth contacts - 

91. Oil shale, gray-brown, fissile - 

90. Tuff; weathers yellow- - - . 

89. Oil shale, dark-brown; weathers light yellow 

brown, fissile to platy __ 

88. Tuff, analcimic; weathers reddish brown. (Black 

T 29 painted on outcrop.) - - - 

*87. Oil shale, light- to dark-brown; weathers soft; 

platy to subfissile. Sample is medium-brown 

oil shale 26.5 ft above base-- 

*86. Tuff analcimic; weathers red brown; persistent; 

con.L3.CvS * ~ " ~ ------ « _ . 

*84

85. Oil shale. Lowermost 10 in. extremely weathered and contorted, possibly marks a saline 

zone. (Yellow 9 painted on slab of rock.) - 

Oil shale, light-brown, marlstone. Partly concealed. Some blue-gray-weathering oil shales 

2-4 in. thick. (At 21.2 ft above base is black 

44 painted on rock.) Sample is light-brown oil 

shale 19 ft above base 

83. Oil shale, with cavities as much as 0.4 ft in diameter. Laterally persistent. Base of R-6 zone82. Marlstone, light-brown. Top of L-5 zone - -- - 

8 1 Tuff _______ ___________________ 

80. Marlstone, light-brown; has cavities 3.0 ft above

79. Marlstone, light-brown; contains cavities and

contorted beds, probably a solution breccia 

78. Tuff, light-gray; laterally persistent bed- 

*77. Marlstone, light-brown, tuffaceous - ~ -

*76. Marlstone, light-orange-brown; brecciated bed; 

laterally continuous; many crystal cavities; 

honeycomb structures. Sample is brecciated 

in 3.1 is ton G * ----- - _..__ ...

75. Marlstone, light-brown; weathers to brittle slabs

74. Oil shale, light-brown; weathers platy; solution

cavities as much as 1 ft in diameter are irregularly distributed through unit- ----- -

*73. Marlstone, light-brown; weathers yellow, slabby. Sample is from 0.5 ft above base 

*72. Tuff, feldspathic, fine-grained; a distinctive 

white __________ ______________

71. Marlstone weathers light yellow orange; slabby; some light-brown oil shale. Several small 

cavities from leached saline (?) minerals at 

10.1 ft above base. Partly covered-- 

70. Tuff. Base of L-5 zone - ----- 

69. Oil shale, light-brown; weathers blocky. (This 

unit near a green and white metal fence post 

and a wooden stake is marked 1A on the pipeline right-of-way.) Top of R-5 zone 

68. Oil shale, medium- to dark-brown; weathers blue 

gray; top is a sharp break with overlying unit. 

Cavities 6-10 in. in maximum dimension 

caused by leached saline minerals; broken 

paper-thin brown septa 

67. Oil shale, light-brown; weathers blocky2.7 

.15 

7.5

.5

5.5

.6

10.9 

.06

33.8

8.1

39.3

1.0

1.0

05

1.5

.1

3.7

^ f

8.7

2.0

 16.4

3

12.4 

.1

4.0

2.4 

5.3
MEASURED SECTIONS 11

Nearly complete section of the Parachute Creek Member of the

Green River Formation along the pipeline Continued 

Parachute Creek Member (part) Continued Thickness

(feet)

66. Tuff 

65. Oilshale64. Tuff 

0.1

1.0

.1

62. 

*61

.1 

2.4 

7.0

*63. Oil shale, dark-brown; weathers blue gray; Brecciated. Cavities to 6 in. in maximum dimension caused by leached saline minerals. Heavy 

films of hydrocarbon material along bedding 

planes. Samples are dolomitic septum from 

cavity and dark-brown oil shale 1.0

Oil shale. Base of R-5 zone 5.1

Tuff, feldspathic; smooth contacts top and base; 

laterally persistent. Top of L-4 zone60. Marlstone, light-brown; weathers yellow 

59. Oil shale, light-brown - 

58. Tuff zone. Five tuffs U-V6 in. thick with small 

inclusions of oil shale. Tuffs interbedded with 

oil shale - - - - - - .. i .8

57. Oil shale, light-yellow-brown to light-browngray; weathers platy and blocky in alternate 

layers. Sample is from 1 ft below top

56. Tuff, analcimic; laterally persistent; undulating 

upper contact; basal contact irregular; interfingers with underlying oil shale55. Oil shale, light-yellow-brown to light-browngray; weathers platy and blocky in alternate 

iy.yCi o *" 

54. Tuff, analcimic; laterally persistent; undulating

8.9

15

. g .8

53. Marlstone, light-brown-gray; weathers platy to

blocky- 3.5

52. Tuff _ __-_ 02

51. Marlstone - .6

50. Tuff- - - - -- .04

49. Marlstone, light-brown-gray; weathers slabby to

blocky 5.9

*48. Tuff, analcimic; weathers orange brown; sandy 

textured; lower contact smooth, upper contact 

undulating. Base of L-4 zone .2

47. Oil shale, light-brown; weathers slabby. Top of

R 4 zone ______________________ 2 8

*46. Oil shale, dark-brown to red-brown; weathers 

blue gray. Locally, beds are contorted and 

have secondary coatings and pods of dolomite. Possibly marks a zone leached of saline 

minerals. Sample is dark-brown oil shale. 

(Near top of unit is black 340 painted on outcrop.) 1.2

45. Marlstone, gray; hackly fracture 6.2

*44. Tuff, analcimic; weathers bright orange; laterally continuous bed - - .1

*43. Oil shale, light-brown, and some alternate layers of dark-brown oil shale a few inches thick. 

Scattered gar-pike scales. Dark beds weather 

blue gray. Unit weathers blocky in lower part, 

platy in upper part. Several thin tuffs less 

than V4 in. thick in upper 1.5 ft. Sample is 

interlayered light- and dark-brown oil shale 

from uppermost 1 ft. (Black 360 painted on rock 

3 ft above base.)- - - - - 13.6

*42. Tuff zone. Uppermost tuff is 0.1 ft thick, analcimic; has wavy lower contact, smooth upper

Nearly complete section of the Parachute Creek Member of the

Green River Formation along the pipeline Continued 

Parachute Creek Member (part) Continued Thickness

(feet)

contact. Lower 0.4 ft of unit contains 3 tuff 

beds Vie-Vs in. thick about V6 in. apart separated by dark-brown oil shale. (Yellow square 

painted on outcrop above tuff zone, and yellow 

T-13 painted on outcrop face 3 ft above tuff 

2onP ^«-____________________« _ ________ Q 5

41. Oil shale, medium- to dark-brown; weathers

mostly blue gray. Some gar-pike scales- 9.7

*40. Tuff zone. Uppermost 0.6 ft consists of tuff (75 

percent) and many platy inclusions of oil shale 

(25 percent). Upper and lower con tacts smooth. 

Light-brown blocky-weathering oil shale 0.8 

ft thick above lowermost tuff and 1A in. thick 

at base of unit. Sample is lowermost tuff, dolomitic - - - l .42

*39. Oil shale, medium- to dark-brown; weathers blue 

gray. Contains pods of dolomite that weather 

light brown. Sample is medium-brown oil 

shale from base of unit - - - 3.5 

38. Oil shale, light- to medium-brown, and some

gray marlstone ----- -- ----- 11.8

*37. Tuff, analcimic, evenly bedded; smooth contacts

at top and bottom - - .15

36. Oil shale- -- - - 2.6

35. Tuff zone. Two tuffs. Lower is % in. thick, upper 

is l/s in. thick; separated by 1A in. light-brown 

oil shale .___ .06

*34. Oil shale; weathers gray; conchoidal fracture; 

dark specks of organic matter. Sample is from 

i it aoove Dase o.u

*33. Tuff, analcimic; weathers yellow; fine-grained; 

undulating contacts. Intertongues with enclosing oil shale. (Yellow T 9 painted on outcrop.)- 1.2 

32. Oil shale---- --- - - 3.0

*31. Tuff, analcimic. (Yellow circle painted on outr-rnrt \sL\JlJ.J\ « 

__ _ .V/CJ f)Q

30. Oil shale-- ----- ---- ------ 17.7

*29. Tuff, analcimic, lenticular; discontinuous pods

cis mucri 3.S -L in. tniC-K* * * * -~ .LJO

*28. Oil shale, light- to medium-brown; contains disseminated nodules of dolomite as much as 1 

in. in diameter. Some beds of light-gray-brown 

marlstone 0.1-2 ft thick. Sample is light-brown 

oil shale 26.8 ft above base 28.0

*27. Oil shale, light-brown; weathers blocky; contains 

5-6 in. long and 3-4 in. thick of dolomite that 

weathers white. Sample is from dolomitic pod. 

(Black 450 painted at base of unit.) 3.5

26. Oil shale, dark-brown; weathers blue gray 1.0

*25. Oil shale, light- to medium-brown, fissile, and 

blocky marlstone. Contact with overlying unit 

is sharp. Sample is medium-brown oil shale 

4 ft above base. Base of R-4 zone 23.7

24. Marlstone, medium-brown; weathers blocky; contains flakes of carbonaceous matter and pods 

with iron-stained rims. Top of L-3 zone. 3.5

23. Oil shale, gray-brown, fissile. Top 1 in. is ostraCOCLcLl QOlOmitC * ** * """ o.O

*22. Marlstone, light-brown. Intraformational conglomerate at base contains broken, flat, angular fragments of carbonate rock. About 0.4
12 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

Nearly complete section of the Parachute Creek Member of the

Green River Formation along the pipeline Continued 

Parachute Creek Member (part) Continued Thickness

(feet)

ft above is massive bed of marlstone with 

stringers and clusters of pyrite perpendicular 

to bed. Above the pyritic part of unit are thin 

pods of dolomite parallel to beds. Sample is 

pyritic marlstone - 1.6

21. Marlstone, light-brown ----- 4.9

*20. Oil shale, light-brown; weathers gray; contains 

lenticular tuffaceous marlstone as much as 1 

in. thick. Marlstone sampled at 8.3 ft above 

base_________________________ IT 1 

19. Marlstone, medium- to dark-brown- - .7

*18. Marlstone, tuffaceous Vz-l in. thick .08

17. Marlstone, light-yellow-brown, laminated; weathers white. (2.5 ft below top is black 510 painted 

on outcrop.) 11.5

16. Oil shale, medium-brown, fissile. Middle of unit 

contains tuff zone 4, consisting of 3 tuffs J/4

111. LlllCit - - - ..... 

15. Marlstone, yellow-brown; weathers white; blocky

*14. Marlstone, light-brown; weathers gray; conchoidal fracture; disseminated specks of limonite. 

Samples from top of unit and 0.6 ft above base 

are distinctive 1-in. and lA-in. dolomite-rich 

beds that resemble tuffs- - - 

*13. Tuff, analcimic 

*12. Marlstone, light-brown; weathers gray; conchoidal fracture 

11. Marlstone, light-brown; weathers white; blocky

10. Marlstone, light-brown; light-gray band in middle__________________________

*9. Marlstone, light-brown, blocky- - ~ 

8. Marlstone, light-brown, fissile - -- - 

7. Marlstone, light-yellow-brown, well-laminated,

2.6

7.5

1.2 

.04

8.8 

5.8

.2 

5.3 

1.0

7.5

*6. Oil shale, brown; black specks of carbon. Some 

yellow beds of dolomite 1 in. thick at 3 ft and 

6 ft above base. First significant oil shale in 

section. Sample is oil shale about 3 ft above 

base. (Yellow circle painted on outcrop at base

nf \Ji unit null/./ ^ _ - __ _ - fi \J,<7Q

*5. Marlstone, green, dolomitic, dense; conchoidal

9.3

1.0

4. Marlstone, light-brown; weathers white; blocky;

resistant - 3.6

3. Shale, dark-yellow-olive, fissile to subfissile 4.5

*2. Marlstone, light-yellow-brown; weathers white; 

blocky. (Yellow circle painted at base of unit.)

*1. Marlstone, dolomitic, light-green-gray. Contains 

fish teeth. Base of good exposures and designated base of Parachute Creek Member - 

Incomplete thickness of Parachute Creek 

Member (rounded) -909

Garden Gulch Member (part):

*Shale and siltstone, poorly exposed; mostly light 

yellow brown to dark brown; conchoidal fracture common. Weathers to lighter brown fragments. Shales weather to small soft fragments 

easily crushed by hand. Illitic clay abundant. 

Some calcite and (or) dolomite common, especially near top; these carbonate beds increase

Nearly complete section of the Parachute Creek Member of the

Green River Formation along the pipeline Continued 

Garden Gulch Member (part) Continued Thickness

(feet)

in abundance upward and mark a transition 

to rocks of the overlying member.

Sample of brown silty shale, taken about 

60 ft below top of unit, is considered representative of many beds in the upper part of 

the member.

(Base of unit is in the cut bank along pipeline right-of-way about 200 ft uphill from intersection of access road and pipeline. Yellow 

1 painted on trunk of pine tree just above cut

Incomplete thickness of Garden Gulch 

Member -- 90.0

LOWER PICEANCE CREEK SECTION

A complete section of the Parachute Creek Member 

(1,185 ft) is exposed in the cliffs along lower Piceance 

Creek in sections 11, 14, and 15, T. 1 N., R. 97 W., Rio 

Blanco County, Colo. The area lies in parts of the 

White River City and Barcus Creek SE TV^-minute 

quadrangles.

The base of the section is on the hill west of 

Piceance Creek road 2.4 miles south of its junction 

with Colorado Highway 64. Locations of the parts of 

the section are shown in figure 3.

The base of the section is not well exposed but lies 

in a transition zone between the predominantly clay 

(illitic) shales of the Garden Gulch Member below 

and the carbonate-rich rocks of the Parachute Creek 

Member above. The contact, placed where carbonaterich rocks become the predominant rock type, is 

shown in figure 4. The top of the section has good exposures of the upper part of the Parachute Creek 

Member and the lower part of the overlying sandy 

beds of the Evacuation Creek Member (fig. 5).

The mineral composition of 135 samples from this 

section is given in table 5 and is discussed in detail 

later. Rocks in this section are of especial interest 

because they contain dawsonite. The lithic details of 

this section follow.

Complete section of the Parachute Creek Member of the Green 

River Formation along lower Piceance Creek in sees. 11,14, and 

15, T. 1 N., R. 97 W., Rio Blanco County, Colo.

iSection measured by D. A. Brobst and J. D. Tucker with assistance of J. R. Dyni in 1966 and collaboration of G. N. Pipiringos 

in 1970. * indicates sample data in table 5]

Thickness

Evacuation Creek Member (part): (feet) 

*Sandstone, tuffaceous, brown, fine to very fine 

grained; lower contact unconformable. Base 

of member (fig. 5). Two samples. (Yellow E 

painted on rock at base of unit.)! Unmeasured 

Parachute Creek Member :

233. Marlstone, light-brown; weathers slabby and

nlntv ^/XO-l/Jf _ _ . _ --- __ <J.\J E; O

*232. Oil shale, dark-red-brown, pyritic- - - .3
MEASURED SECTIONS 13

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

231.

*230. 

229.

*228.

*227.

*226.

225.

224.

223.

222.

Marlstone, light-brown; weathers slabby and

platy. Partly covered 

Oil shale, dark-red-brown, pyritic; weathers blue

gray and platy. Two samples 

Marlstone, light-brown; weathers platy and slabby; forms ledges along the hill slopes 

Oil shales; mostly light brown; some medium

brown - 

Tuff, analcimic, gray; undulating contacts 

Oil shale, light-brown; contorted zone; some pods

of tuffaceous material. Sample is tuff 

Oil shale, medium-brown; weathers to thin

221. 

"220.

*219.

218.

217.

216.

215.

*214.

213.

212.

211.

210.

209.

208.

"207.

Tuff, undulating contacts; pods as much as 6 

Yf} thick _________________________

Oil shale, light- to medium-brown; weathers to 

thin plates - - 

Marlstone, thick-bedded, and some light-brown

oil shale near top. (Yellow X painted on rocks

near base of exposure.) - - 

Section offset into next gully to , north.

Units 222-233 measured in second most

southerly gully which has exposures of

uppermost white-weathering zone.

Marlstone, light-brown; exposures poor 

Oil shale, medium- to dark-brown. Samples are 

dark-brown oil shale at base of unit and medium-brown oil shale 10 ft above base 

Marlstone, light-brown with thin protruding 

ledges of very fine grained tuffaceous sandstone. Poorly exposed. Sample is tuffaceous 

sandstone 15 ft above baseSandstone, very fine grained; has organic debris and rusty pinhead spots- -

Marlstone, brown; weathers platy; poorly exposed. Base of a distinctly brown weatherITlg U.T11 L 

Section offset to north. 

Marlstone; poorly exposedOil shale, light- to medium-brown, and white 

marlstone; unit poorly exposed 

Limestone, white, crystalline, dolomitic; many 

crystal cavities caused by removal of gypsum(?). About 160 ft below top of member 

and 240 ft above top of Mahogany ledge. Described by Donnell (1961, p. 857) - - 

Marlstone, light-brown; weathers platy and

sici u Dy * 

Marlstone, light-brown; a massive bed - 

Marlstone; weathers to thin plates 

Marlstone, light-brown; massive bed; weathers

Oil shale, medium- to dark-brown; weathers 

nlatv £ **« VJ _______ __ 

Tuff zone. Five thin tuff beds alternating with 

light-brown oil shales. Poorly exposed. About 

215 ft above top of Mahogany ledge. (Yellow 

T 5 painted on marlstone above unit.) 

Tuff, analcimic; weathers rusty brown; weathers to prominent reentrant on slopes 

12.0

.8

9.0

12.0 

.2

2.0

2.6

.2

6.0

6.0

10.0

11.0

29.0 

.3

12.0

13.0

30.0

1.0

15.0 

2.0 

5.0

2.4 

2.4

3.5 

1.0

108°15'

ahogany ledge. Units 153-176

j195-217

i - , //v I /

I x Units 217-222

^Ljnits 222J-233 

-Top of Parachute Creek Member

point (fig.

23

12

chute Creek Member

13

24

1 MILE

T. 2 N. 

T. 1 N.

40 c 

02' 

30"

R. 97 W.

FIGURE 3. Locations of the measured section of the Parachute 

Creek Member on lower Piceance Creek. Numbered units 

refer to units described in the measured section. Station numbers refer to sample localities of data shown in table 10.

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued

Thickness 

(feet)

206. Oil shale, light- to medium-brown; weathers to

thick plates 4.6 

205. Sandstone, light-brown, very fine grained .8

204. Oil shale, light-brown; intercalated with beds 

of marlstone about 1A in. thick 3.2
14 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

:B M«9£ xass iwsssrtHM-ES!? ws 'su> sj«»,«^»ws;i »a««»«i!SM

FIGURE 4. Lower part of the Parachute Creek Member on lower Piceance Creek. Lowercase letters refer to units of Bradley (1931, 

pi. 4A). Numbers prefixed by R and L refer to zones rich or lean in oil shale. Numbers 14 and 113 refer to units described 

in the measured section in this report. D marks the dawsonitic zone. X indicates the U.S. Bureau of Mines cut in the zone 

of dawsonitic rocks. Tgg, Garden Gulch Member.

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

203. Oil shale; weathers platy- - - - 1.0 

*202. Tuff, analcimic, white to light-green; weathers

\vniLG Aviiii rusty spots j. «o 

201. Oil shale, medium-brown- 3.0 

200. Tuff, analcimic, yellow-brown .04 

199. Oil shale, medium-brown; weathers to thin 

plates

*198. Tuff, analcimic, light-yellow-brown, persistent;

weathers blocky - - - 

197. Oil shale, light-brown 

196. Marlstone, light-brown- - - 

*195. Oil shale, dark-brown; weathers platy, blue

Section offset to north along this bed. Units

195-178 measured in gully that has a large

sandstone boulder at the mouth. Yellow

circle and UP painted on boulder

194. Marlstone; forms plate-covered slope 

193. Marlstone, light-brown; forms prominent ledge.

(Yellow 8A painted on ledge.)- 

192. Marlstone, light-brown; weathers white 

*191. Oil shale, medium-brown - - - 

*190. Marlstone, light-brown, poorly exposed 

2 .8

. 1

10.0 

23.0

1 -L »\JO

6.0

3.0

2.7

2.0

11.0

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

*189. Marlstone, light-brown, and some tuffaceous 

sandstone and sandy marlstone; unit poorly 

exposed. Some medium-brown oil shale at base. 

Samples are oil shale at base of unit; tuffaceous sandstone 2.6, 3.6, and 5.6 ft above base; 

and tuffaceous marlstone 7.6 ft above base 165

*188. Marlstone; poorly exposed. Samples are tuffaceous marlstone 15 ft above base, light-yellow carbonaceous marlstone 21 ft above base, 

and light-brown silty marlstone 0.4 ft below

tr»n lAJ^J r\f L/l unit U11J.L - _____ 96 £j\J.\J0

*187. Sandstone, red-brown, massive, tuffaceous,

very fine to medium-grained. Conspicuous

unit (ficr E\\ .......... UI11L ^llg. *J) - - -

186. Marlstone, light-brown, and intercalated thinbedded red-brown sandstone. (Base about 85 

ft above Mahogany ledge, but greater true 

thickness of interval may be masked by slump7 f) l ,\j

185. Marlstone, light-brown, and some intercalated 

light- to medium-brown oil shale. 15 ft below 

top is tuffaceous carbonaceous siltstone and 

very fine grained sandstone 35.0
MEASURED SECTIONS 15

FIGURE 5. Upper part of the Parachute Creek Member on lower Piceance Creek. Numbers refer to units described in the measured 

section. Unit 187 is a distinctive sandstone. Interval between units 217 and 221 is a sequence of rock that weathers distinctively 

brown. Tge, Evacuation Creek Member; Tgp, Parachute Creek Member.

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

184. Oil shale, medium-brown 2.3 

183. Oil shale, dark-brown; weathers blue gray. Overturned fold axis trends N.35°W., plunges 10

182. Manstxuie,light-urown, weathers to brittle plates 

181. Oil shale, medium-brown; weathers white 

180. Marlstone, light-brown- - 

179. Marlstone; weathers to brittle plates; poorly ex5

11.7

.4

16.1

O.O

3.0

178. Marlstone, light-brown; upper 1 foot has lightbrown oil shale - 

Section offset to north to good exposures

of Mahogany ledge.

*177. Marlstone, light-brown, and intercalated lightbrown oil shale 5 ft above base. Probable position of the A-groove - - 10.0 

176. Oil shale, light- to dark-brown; lighter colors 

increase upward. Top of the Mahogany ledge 

(fig 5)» -- -- 

Tuff, analcimic, orange-brown; smooth lower 

contact; undulating upper contact; persistent 

bed. Informally referred to as the false marker bed. Mapped by G. N. Pipiringos and W. 

J. Hail, Jr., as top of Mahogany ledge (oral 

commun.,

5 0

*175.

*174. Oil shale, dark-brown. Sample is from 4.5 ft

8.DOV6 O3.S6 - "** 

*173. Oil shale, platy, rust-brown; efflorescent salt

/ *\j

 J..U

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

*172. Oil shale, dark-brown, laminated; weathers 

blocky and blue gray. (Black 40 painted on 

outcrop 0.7 ft above base.)- - 7.8

*171. Tuff, analcimic; undulating upper and lower

contacts; persistent bed 2

*170. Oil shale, dark-brown, laminated; weathers gray L.5

*169. Tuff, analcimic, orange-brown; persistent bed.

The Mahogany marker .1

*168. Oil shale, dark-brown to red-brown, laminated; 

weathers gray; smooth face on outcrop. Sample is from 1 ft above base- - - 1.8

*167. Oil shale, dark-red-brown, laminated; weathers 

blue gray with rusty stains on outcrop. Samples are from base and 1 ft above base. The 

Mahogany bed - - 3.3

*166. Marlstone, pale-yellowish-brown; massive bed 

in which calcite predominates over dolomite; 

darker brown discontinuous stringers of anaIcime- and dolomite-rich rock. Some beds of 

clear analcime. Distinctive bed. Sample is 

channel sample of massive brown marlstone. Samples 1-13, from this entire 31.25- 

cm-thick sequence, were studied in detail (fig.

*165. Oil shale, dark-brown; weathers to smooth surface, blue gray. Samples are from 1, 3, and 

4 it above case o.u

*164. Tuff, analcimic; pods 0.3 ft thick. Undulating

contacts -o
16 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued

Thickness

(feet) 

.________________________ Q 3

orange-brown; smooth con163. Oil shale- *162. Tuff, analcimic,

tacts _________________________ 08

*161. Oil shale, dark-red-brown. Sample is from 2.5 

ft above base. Samples 200-218, from a 20.6- 

cm-thick sequence beginning at base of unit, 

were studied in detail (figs. 11, 12) 6.1

*160. Tuff, analcimic; weathers light brown; smooth

COHLclCLS ~ . A

159. Oil shale, dark-red-brown, laminated; weathers

to smooth gray face 2.5

158. Oil shale, dark-brown, laminated; weathers to 

alternating layers of light to dark blue gray. 

Thin discontinuous dolomite laminae weather to light orange brown. (Yellow 7P painted 

at base of fresh exposures in cut at curve

.15

5.2 

1.1

157. Tuff; undulating contacts - 

156. Oil shale, dark-brown; weathers blocky to Slabby_____________________________________________________________________________

155. Oil shale, dark-brown; weathers papery 

154. Tuff, Undulating contacts, persistent bed.

Makes groove in weathered outcrop. (Yellow

70 painted on outcrop.) 

153. Oil shale; weathers slabby to blocky148. Oil shale, dark-brown, laminated; weathers slabby ___ ____________________________ _________ __________

147. Oil shale, dark-brown, laminated; low specific 

gravity; weathers to white flat face on outCFOD _______________________________ ________

*146. Oil shale, dark-brown; weathers blue gray, with 

large solution cavities 1.5 ft in diameter; makes 

projecting square-faced ledge; has a wavy thin 

bed of tuff at top. Samples are residue and 

leached rosette material collected from cavity 

near yellow X. (Yellow X painted on cliffs that 

are just north of curve in road.)

Section offset 1,000 ft to north on west side 

of road to same unit exposed on the north 

side of a tributary of Piceance Creek in 

the SV2NWV4 sec. 14, Barcus Creek SE 

(71/2-min) quadrangle. 

145. Oil shale; weathers blocky; conspicious leached

144. Oil shale, medium- to dark-brown. Tuff 0.1 ft 

thick 1.5 ft above base. Vuggy zone above

fhp U.IC tuff _ ___ - ___ - __ - -

LUJLJ143. Oil shale. Top and bottom light brown and fissile; middle part light brown and blocky. Reentrant 3 ft below top has irregular pinching 

and swelling pods of analcimic tuff 0.02-0.5 

ft thick - 

.25 

3.4 

152. Oil shale, laminated; weathers to smooth face- .6

151. Oil shale, medium-brown; weathers blocky to

slabby- - - - 2.5 

150. Oil shale, dark-brown, laminated; weathers pa149. Tuff zone. Four tuffs, Va-l in. thick, weather 

orange brown and are separated by darkbrown oil shale. Zone forms reentrant in ex-

* ~ .o

C Q

T ^

6.0

1 ^ ^

4.5

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued

Thickness

142.

141.

140.

139.

138.

137.

136.

135.

134. 

*133.

Oil shale. Blue-gray-weathering ledge in upper 

2.4 ft with 2 tuffs 0.5 ft and 1.3 ft below top; 

lower 3 ft medium brown and blocky 

Oil shale, dark-brown; weathers to thin plates 

in rounded slope in upper part; forms bluegray-weathering ledge in lower part 

Oil shale, medium-brown, papery; has small 

rusty concretions of dolomite(?) in top 0.4 ft. -

Oil shale, light-brown; conspicuous recess at 

top possibly caused by weathering of a tuff--

Oil shale, light- to medium-brown; weathers 

blocky; has cavities 0.2-1 ft in diameter. Thin 

tuff at base - - - 

Oil shale, dark-brown; forms ledge 

Tuff, weathered; undulating contactsOil shale, dark-brown; dolomite pebbles; forms

(feet)

5.4

2.2 

2.0 

3.0

132.

131.

Tuff pods, 0.4-1.2 ft thick 

Oil shale, dark-brown; weathers blue gray and

blocky. Has flat brown dolomite pods. Papery

where most intensely weathered. Good marker bed. Marked 7 M in yellow paintSection offset to same bed on east side of

Piceance Creek valley from a point on

a nose 30 ft above Piceance Creek road.

Tuff, regular contacts; persistent bed. Base of 

the Mahogany ledge - - 

Oil shale; mostly medium brown; weathers 

slabby to blocky. 1 ft above base a cavity 

zone from leached saline material is overlain 

by a 1-in. light bed of dolomite. Samples are 

weathered material from near cavity: darkbrown oil shale from cavity zone, and the

*130.

129.

128.

127.

*126.

Marlstone, light-yellow-brown to brown; poorly 

exposed; forms slope covered with brittle chips. 

Samples are marlstone and pod material from 

upper 1 ft of unit. Probable position of the BCflTOOVP _______________________________________

Oil shale, poorly exposed; weathers more papery toward top. Top of R-6 zone

Oil shale, medium- to dark-brown; darkest beds

form blue-gray ledges Vi-2 in. thick 

Oil shale, leached; marks saline zone 

Oil shale, dark-brown; weathers blue gray; efflorescent salts abundant; thin beds give

outcrop fluted appearance. Middle of unit is

cavity zone. Sample is weathered oil shale

126. Marlstone, light-brown; weathers platy and

blocky ----- - - 

124. Oil shale, light-brown; weathers papery 

123 Oil shale, light-brown; efflorescent salts; at

top is dark-blue-weathering bed with flat

pods of dolomite 

122. Oil shale, dark-brown; weathers blue gray 

121. Tuff; undulating lower contact, even upper contact; continuous bed. (Section offset to north

from base of outcrop at spur of hill along

irrigation ditch, where tuff is exposed and

marlstone of unit 119 is just underground.) 

2.25

1.2

.5

1 0

1.0

3.0

.2

J.»^Q

6.0

5.5 

.5

6.0 

1.0

3.8 

2.6

.2
MEASURED SECTIONS 17

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

*120. Oil shale, medium- to dark-brown; some efflorescent salt. Samples are dark-brown oil shale. 

(Exposed on east side of the creek valley over 

the jointed marlstone described below.) - 0.5

*119. Marlstone, light-brown. The jointed bed conspicuously jointed 1-2 in. apart; joints strike 

N. 48° W., dip 85° NE., and give blocky appearance. 1.9 ft above base is thin zone of 

oval cavities Vz-3A in. thick and 2 in. long filled 

with mixture of quartz, calcite, dolomite, feldspar, and some analcime and dawsonite. Base 

about 75 ft below base of Mahogany ledge. 

Samples are cavity filling containing some 

dawsonite, brittle marlstone 0.8 ft below cavity 

zone. A good marker bed; marked by yellow 

7F at cliffs on east side of irrigation ditch on 

east side of Piceance Creek valley- - 5.7 

Section offset to same bed on west side of 

road west of Piceance Creek. Painted yellow 7F on outcrops at sample locality of 

unit 119.

*118. Oil shale, dark-brown; weathers papery; forms

ledge 4.0

*117. Oil shales, medium-brown. Some weather to 

blue-gray ledges; others weather to paper 

ledges. A rubbly ledge-ribbed slope. Sample 

is from a blue-gray ledge 15.0

116. Oil shales, light- to medium-brown; weather to

gray, shaly, partly covered slope - 20.0

115. Oil shale, dark-brown; weathers blue gray; has

brown flat dolomite pods; forms ledge 6.0

*114. Oil shale, light- to medium-brown; weathers papery to slabby; forms poorly developed ledges. 

A V2-in.-thick light dolomite at 21.7 ft. above 

base. Sample is light-brown oil shale 6 ft above 

base. Lower 3 ft is covered. A tuff 0.2-0.4 ft 

thick is 9 ft above base of this unit in draw 

to north . 25.0

*113. Oil shale, dark-brown; weathers blue gray; has 

flat rusty brown dolomite pods. Sample contains some dawsonite. (Yellow 6A painted 

on this bed at base of outcrops west of and 

about 50 ft above road, nearly on the boundary of sees. 11 and 14, T. 1 N., R. 97 W., near 

the east edge of the Barcus Creek SE quadrangle- . __ -- .... ._.___ ... ________ . 3.5

Section offset to north, where a similar 3.5- 

ft-thick bed is exposed above units 1-112 

measured near center of sec. 11, T. 1 N., 

R. 97 W., White River City quadrangle. 

Sample from unit 113 is from outcrop in 

White River City quadrangle (fig. 4).

112. Tuff--- - - -- _ - .04

111. Oil shale- ___________ ....._ . 3.3

110. Tuff -_ _-- ._ ____.____- 02

109. Oil shale________________________ 15

*108. Tuff, analcimic, light-gray- -- - - .02

107. Oil shale106. Tuff - 

105. Oil shale 

104. Tuff, yellow-orange - 

103. Oil shale- -----

2.1

,\J£A O9

1

 01VX

a K \J ,«J

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

109 J.V/^1. Tuff JL till - " _ - -- _ - _ ___ n V/.V/ ni-L

f 

. V_

100.

*99.

98. 

97.

*96.

95.

*94.

*93.

92. 

*91.

90. 

*89.

87.

86.

85. 

84.

*83.

*82

*81.

80.

*79.

78.

*77.

76.

75. 

74.

Tuff, mottled---- 

Oil shale; weathers gray blue. Sample contains 

some dawsonite 

Tuff __________

Oil shale; mostly light to medium brown, with 

some dark-brown beds 17.2

Tuff, yellow-brown; dark petroliferous stain; 

very fine grained. (Yellow T 27 painted on 

outcrop.} 

Oil shale - - -- --- -

Tuff, yellow-brown; contains albite and dawsonite; undulating contacts; persistent bed - 

Oil shale, medium- to dark-brown; some beds 

weather blue gray; some beds form ledges. 

Samples are dark-brown blue-weathering oil 

shales 3 and 6 ft above base that contain 

some dawsonite- -

Marlstone, light-yellow-brown, sugary-grained

Oil shale, medium- to dark-brown; some weathers blue gray. Sample is dark-brown oil shale

from middle of unit 

Tuff; weathers yellow orange; smooth contacts

Oil shale, light- to medium-brown; forms alternating slopes and ledges. Samples are lightbrown oil shales 0.2, 4, and 9 ft above base.

Some dawsonite - - -- - - 

Conglomerate; contains dolomite pebbles. (Yellow streak and 6A painted on outcrop.)-- - 

Oil shale, light-brown, and fissile to platy and 

locally papery marlstone; forms thin ledges. 

Approximate base of upper oil shale group 

(Bradley, 1931, pi. 4-4, letter a). Base of R6 zone (fig. 4, this report) - ----- - -

Oil shale, light-brown; weathers gray, fissile. 

Approximate top of transitional beds (Bradley, 1931, pi. 4A, letter b). Top of L-5 zone 

(fig. 4)--- 

Tuff; weathers yellow brown; laterally persistent __________ _______________

Oil shale, light-yellow-brown; forms thin ledges 

and covered slopes; weathers fissile to platyOil shale; weathers papery, blue gray - 

Conglomerate. Angular pebbles of dolomite in 

ssmci mcLLjnx ~~' ----------- « ----- «- « ~ 

Oil shale, light-brown. Sample is from 2 ft

£l DO VC

Tuff; weathers orange; smooth contacts 

Oil shale, light- to medium-brown; weathers

fissile to papery, blue gray. Some thin orange-weathering marlstone beds. Much covered slope. Sample is from marlstone bed 

Oil shale, light-brown; weathers fissile. Base

oi J_/~"O zonc~~"~ ~ - .-. . - _._.»».«»»..»« »»

Oil shale, dark-brown; weathers blue gray.

Sample is from top of unit. Approximate top

of lower oil shale group (Bradley, 1931, pi.

4-4, letter c). Top of R-5 zone (fig. 4) - 

Tuff; persistent bed; smooth contacts- - -

Oil shale, dark-brown- - - -- 

Tuff ; undulating contacts - - - 

.02

1 .6 

.02

.Uo 

.5

.02

33.2 

.2

3.7 

.02

10.5 

.5

3.3 

6.7

7.7 

.5

. i /

Q, i

.04

35.2

1.7 

.02 

.3 

.1
18 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

73. Oil shale, dark-brown; weathers papery, blue

72. Tuff, yellow-orange; smooth contacts - - 

71. Oil shale, brown; forms slope 

*70. Oil shale, dark-brown; weathers blue gray; 

forms ledge. Sample contains dawsonite. Base 

about 53 ft above tuff of unit 49 - -

69. Oil shale, light-brown, and light-brown marlstone; weathers fissile to platy and forms alternating ledges and slopes every 0.2-0.3 ft 

68. Tuff; sugary texture ......

67. Oil shale, light-brown; weathers platy -

66. Oil shale, dark-brown; weathers blue gray; contains dolomite nodules; upper 4 ft is ledge 

65. Marlstone, light-yellow-brown - - - -

*64. Oil shale, dark-brown; weathers blue gray. 

Sample is from 1.5 ft above base and contains dawsonite 

63. Tuff, oil-stained, dolomitic. (Yellow T 18 painted on a slab placed below the bed.) -

62. Oil shale; weathers blue gray, papery. Unit is 

base of ledge - ... 

61. Oil shale, yellow-brown to dark-brown; incompletely exposed ~ - __ 

*60. Marlstone, tuffaceous; weathers yellow orange. 

A tough rock --- - ~

*59. Tuff, analcimic, yellow-orange; sandy texture 

*58. Oil shale, medium-brown; weathers papery. 

Sample is from 1 ft above base- - - 

*57. Tuff, analcimic; undulating upper and lower

contacts - -- - - - 

56. Oil shale. Mostly covered slope -- - 

*55. Oil shale, medium-brown; weathers blue gray, 

papery. Sample is from 1 ft above base- ~

*54. Tuff, analcimic, orange-brown; has thin dolomite bed near top ...._....._ _ . 

53. Oil shale - -- - -

*52. Tuff, analcimic; undulating contacts; discontinuous bed. Sample contains dawsonite - 

*51. Oil shale, light-brown; weathers fissile to papery; some blue gray. Sample is from 5 ft 

above base 

*50. Oil shale, dark-brown; weathers blue gray; a 

few very thin tuffaceous beds. Sample is from 

4 ft above base and contains dawsonite -

*49. Tuff, yellow-brown, analcimic, coarsely crystalline; undulating contacts. (Yellow paint 

on enclosing beds parallel to tuff.) Correlated 

with tuff 13 of table 10 

*48. Oil shale, light- to dark-brown; weathers platy; 

contains gar-pike scales. Samples are from 

2, 4.8, 5.8, 8.4, and 8.5 ft below tuff of unit 

49, and all contain dawsonite. (Yellow 4 El 

painted at base of unit.) - 

47. Marlstone. Tuffaceous even bed, smooth con-

*46. Oil shale, medium-brown, and light-brown marlstone. Sample is medium-brown oil shale 

45. Oil shale, medium-brown, poorly exposed -

*44. Tuff, analcimic, red; weathers white; undulating contacts ----- ----------- 

.08 

5.0

3.5

4.2

.04 

3.8

6.2 

.2

3.6 

.1

3.5 

5.4

1.3 

.3

4.4

.2 

3.1

4.2

.3 

.9

.2 

6.3 

4.7

8.5 

.1

4.4 

3.5

.15

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

43. 

*42.

41. 

*40.

*39.

*38.

'37. 

36.

*35.

*34. 

«<33.

*32.

*31.

*30.

*29.

*28.

*27.

*26.

25.

24. 

23.

*22.

21.

*20.

*19. 

18.

*17.

Oil shale, dark-brown; weathers fissile -- 3.5

Oil shale, dark-red-brown; weathers to prominent smooth-faced blue-gray ledge. Samples 

are from top of unit (contains dawsonite); 

0.5 and 1 ft below top; 2.5 ft below top (contains dawsonite); and base of unit. (Yellow 

circle painted on face of outcrop.)- - 3.3

Oil shale, light-brown, poorly exposed- 8.0

Oil shale, medium-brown; weathers blue gray; 

forms thin ledge; contains dawsonite. Samples are from 1.7 ft above base and near base- 2.7

Oil shale; weathers papery; distorted beds; probably marks a saline zone. Sample is from 2 

ft above base - 5.6

Oil shale, poorly exposed. Sample is from 2.3 

ft above base- 4.7

Tuff, analcimic - - .04

Oil shale; contains intraformational conglomerate in lower 2 ft - - ------ - 6.1

Oil shale, dark-brown; weathers papery; distorted lower 0.5 ft. Samples are distorted oil 

shale and dark-brown oil shale 0.9 ft above 

base- _________________ 2.8

Tuff, orange; contains dawsonite- - - - .7

Oil shale, dark-brown; weathers papery. Top 

1.2 ft of unit has efflorescent salts and may 

mark a saline zone. Two samples from the 

lowest 2.8 ft above base - - - 6.2

Oil shale, medium-brown. Sample is from 1.8 

ft above base and contains dawsonite - 4.0

Marlstone, gray-green - - .08

Oil shale, light- to medium-brown, with 0.08 

ft olive-drab dolomite bed at base. Sample 

is from dolomite bed -- - 1.0

Oil shale, dark-brown; weathers blue gray. 

Conglomeratic with yellow-brown dolomite 

blebs 3 in. in diameter and Vie in. thick. 

Sample is from 3.9 ft above base- - 7.0

Tuff; weathers yellow. (Yellow T 8 and streak 

parallel to bed painted on rock.) . .1

Oil shale, dark-brown; weathers papery. Sample is from 1 ft above base and contains dawsonite - ----- _________ - 2.0

Oil shale, gray-brown, hackly; forms a slope. 

Near top is a tuff zone consisting of thin tuff 

and tuffaceous marlstone. Sample is marlstone and contains dawsonite - 3.6

Oil shale; has orange-weathering dolomite concretions in upper 1 ft 2.0

Tuffs. Two thin beds separated by oil shale .1

Oil snale - - .00

Tuff; weathers yellow; forms reentrant in out-

________________,______ 2 cropOil shale; weathers blue gray, papery. Base of 

cliff-forming oil shales (fig. 4). Approximate 

base of Parachute Creek Member (Bradley, 

1931, pi. 4A). Base of R-5 zone- - - - 

Oil shale and marlstone, light-brown. Sample 

is marlstone 1.7 ft below top. Top of L-4 zoneTuff, analcimic - - -- 

Oil shale, light-brown; weathers fissile -- - 

J. uti, analcimic -

1.0

2.5

.02 

1.3

.04
MEASURED SECTIONS 19

Complete section of the Parachute Creek Member of the Green

River Formation along lower Piceance Creek Continued 

Parachute Creek Member Continued Thickness

(feet)

*16. Oil shale, light- to dark-brown; weathers fissile; 

forms slope littered with small shale plates. 

Samples are dark-brown oil shale 2.7 ft below 

top and light-brown oil shale near base of 

unit__________________________ 27 9

*15. Oil shale; mostly covered slope. Sample is medium-brown oil shale - - - -- - 6.0 

14. Oil shale; forms ledge- - - 7.0

*13. Oil shale and marlstone; forms mostly covered 

slope littered with small brown chips of brown 

oil shale and plates of light-brown to orange 

marlstone. Sample is light-brown oil shale 2 

ft above base. Base of L-4 zone-- - 30.0

12. Oil shale, medium- to dark-brown. Top of series 

of ledge-forming shales. Top of R-4 zone. (Yellow 3 painted at top of ledge.) Base of unit 

mapped by G. N. Pipiringos and W. J. Hail, 

Jr., as contact between Parachute Creek Member and Garden Gulch Member 15.0

11. Marlstone, light-yellow, silty, ostracodal - - 3.5

*10. Oil shale, dark-brown. Sample is from about

7 ft above base-- - - - 22.0 

9. Marlstone, light-yellow-brown, silty .3

*8. Oil shale, medium- to dark-brown; some weathers blue gray. Forms alternating slopes and 

ledges in 5-ft intervals. Sample is dark-brown 

oil sh-fllc_*"_________________ _--- _ 1Q ^i

7. Oil shale; weathers fissile; partly covered slopes 4.0

6. Oil shale, dark-brown; weathers blue gray; forms

ledge ------------------------ 3.0

5. Oil shale, medium- to dark-brown. 1 ft of darkbrown oil shale at base - - 4.5

4. Marlstone; weathers to slope littered with lightyellow-brown chips; poorly exposed 31.5

3. Marlstone and oil shale, light-brown; oil shale

increases toward top; forms a ledge- - - 9.0

2. Marlstone and oil shale, light-brown; forms

covered slope 12.2

*1. Oil shale, light- to dark-brown; darkest beds 

weather blue gray; forms a ledge. Sample is 

dark-brown oil shale . - 10.l

Thickness of Parachute Creek Member 

(rounded) 1,185.0

Garden Gulch Member (part):

Shale, olive-gray, clayey but slightly dolomitic; 

forms partly covered slope 6.2

Oil shale, light-brown; weathers papery. (Yellow 1 painted on small outcrop.) - - 3.0

Shale, olive-gray, clayey; subfissile to hackly 

fracture - 25.0

Incomplete thickness of Garden Gulch MemQGI* grounded.^ ofr.vj

Other workers have chosen contacts of the 

Parachute Creek Member on lower Piceance Creek 

differently. Correlations of the data in this report 

with those of earlier workers are shown in figures 4 

and 6. Bradley (1931, pi. 4A) placed the basal contact 

of the Parachute Creek about 210 feet higher stratigraphically than we do. Donnell (1961, pi. 51) chose

the contact at the base of the lowest oil-shale-rich 

zone, which coincides closely with our contact. G. N. 

Pipiringos and W. J. Hail, Jr. (oral commun., 1970), in 

geologic mapping of the White River City and Barcus 

Creek SE quadrangles, respectively, have chosen the 

bottom of a generally well exposed and easily mappable bench of dark-brown oil shale at the top of 

smooth and rubble-strewn slopes as the base of the 

Parachute Creek Member. This persistent bench is 

about 120 feet above the contact we have chosen with 

the aid of laboratory study.

The position generally mapped as the base of the 

Parachute Creek Member seems to lie above the 

position chosen in drill cores or from detailed 

laboratory study of rocks from measured sections. 

This difference in selection of the contact may account for what has been interpreted in some areas to 

be extreme basinward thickening of the Parachute 

Creek Member within short distances.

The R and L designations shown in figure 6 were 

used by Donnell and Blair (1970, p. 76) to describe 

units of rich and lean oil shale, respectively. These 

designations were easily applied to the section on 

lower Piceance Creek and also correlate easily with 

rock units described by Bradley (1931, pi. 4A).(See 

figs. 4 and 6.)

From the data in the diagrammatic section on plate 

3 and in photograph A on plate 4 of Bradley (1931), a 

thin Parachute Creek Member was designated that 

included only the most organic rich parts of the sequence of carbonate-rich rocks. He apparently used 

the top of the oil-rich zone now called the Mahogany 

ledge as the top of the Parachute Creek Member. 

This, of course, automatically made the Evacuation 

Creek of Bradley thicker than the Evacuation Creek 

of later workers.

RIO BLANCO SECTION

A well-exposed complete section of the Parachute 

Creek Member (764 ft) was measured in the cliffs 

along the north side of Piceance Creek beginning at 

the base of the member at the west end of the gravel 

pit (NWV4SWV4 sec. 5, T. 4 S., R. 94 W.) and continuing westward to the draw intersected by the west 

boundary of sec. 6, T. 4 S., R. 94 W., Rio Blanco County, Colo. (fig. 7). The gravel pit is 1.8 miles west of the 

junction of the Piceance Creek road and Colorado 

Highway 13 at Rio Blanco, Colo. The area is shown 

on the Rio Blanco 71/2-minute quadrangle.

The contact between the Parachute Creek Member 

and the underlying Anvil Points Member is 

gradational. Eastward from the gravel pit, and down 

section, the rocks become more silty and sandy and 

contain less carbonate. The top of the Parachute 

Creek Member is only moderately well exposed, but
20 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

the contact with the sandstones of the overlying Evacuation Creek Member generally can be found to 

within a few feet.

The lithic details of the section follow, and the 

mineral composition of 98 samples is shown in table 

6.

Complete section of the Parachute Creek Member of the Green 

River Formation near Rio Blanco, in sees. 5 and 6, T. 4 S., 

R. 94 W., Rio Blanco County, Colo.

[Section measured by D. A. Brobst and J. D. Tucker. * indicates sample data in table 6]

Thickness

(feet>

50+

50+

Section offset to next roadcut to east. (See

fig. 7.) 

*148. Oil shale, very weathered. Sample is light-brown

Evacuation Creek Member (part):

Sandstone, tuffaceous, light-brown; consists 

chiefly of albite, quartz, analcime, and dolo-' 

mitic cement. Partly covered slope 

Incomplete thickness of Evacuation Creek

Member - - - - 

Parachute Creek Member:

162. Marlstone, light-brown 7.5

*16l. Oil shale, dark-red-brown, pyritic, laminated;

weathers blue gray; forms ledge 2.3 

160. Oil shale; mostly light brown, with some darkbrown beds a few inches thick; weathers platy- 6.8 

159. Tuff, analcimic - .02 

158. Marlstone, light-brown, sandy, well-exposed - 14.5

*157. Oil shale, dark-brown, pyritic; forms ledge. (Yellow line painted on bed.) .7 

Section offset to same bed in next gully to 

east, where yellow line and triangle are 

painted on bed.

156. Oil shale, light- to dark-brown, laminated; 

interlayered with light-brown marlstone. (This 

unit better exposed on south side of road and

stvaalr CrccK.,/ Y. _ _ _ __ __ ____ ^U.U on n

* 155. Oil shale, dark-grayish-black, pyritic, laminated; 

weathers blue gray and platy; forms ledge. 

Samples 1-37, collected at 100-ft lateral intervals, were studied in detail (figs. 7, 17) .3 

Section offset to top of westernmost large 

roadcut on north side of road. (See fig. 7.) 

154. Oil shale, medium- to dark-brown, laminated,

poorly exposed - 22.3 

153. Oil shale, medium- to dark-brown. (We.ll exposed

in road cut.) - - 10

*152. Oil shale, medium- to dark-brown; forms tuff zone 

with nine analcimic tuffs J/4 to l/z in. thick. 

Sample is tuff from lowest and thickest of nine 

beds 

*151. Oil shale, medium- to dark-red-brown, pyritic; 

laminated, but massive bedded on these fresh 

exposures. Sample is dark-red-brown oil shale 

from base of unit - - 

*150. Sandstone dike, tuffaceous, light-brown to gray; 

1 ft thick; strikes N. 75° E.; dips vertically---

*149. Oil shale, medium- to dark-red-brown, laminated. 

Massive beds. Deep blue gray weathered bed 

near base of outcrop is base of unit. Sample 

is dark-red-brown oil shale 5 ft above base of

6.0

7.2

_ _ _ _ _____ _______ in 1U.Un

147. Oil shale, medium-brown; weathers chippy-- 2.9

Complete section of the Parachute Creek Member of the Green

River Formation near Rio Blanco Continued 

Parachute Creek Member Continued

145.

144.

143.

*142.

*141. 

140. 

139.

138.

137.

Thickness

(feet)

Oil shale, light- to medium-brown; weathers chippy _______________________ ____ 2 0

Tuff, analcimic, light-rust-brown - - .02 

Oil shale, medium-brown; weathers chippy 7 

Tuff, analcimic, light-rust-brown - .02 

Oil shale, medium-brown, laminated .4 

Tuff, analcimic, evenly bedded- - - .04 

Oil shale, medium- to light-brown, and 5 analcimic tuffs Vs-Vz in. thick. (Yellow triangle painted on rock.) - -- - _..._.. 4.9 

Marlstone, light-brown, resistant; hard layer, 

weathers to orange-brown sandy surface .5 

Section offset to exposure on west wall of

an adit to the east. 

Oil shale and marlstone, light- to dark-brown,

*136. Tuff, dark-brown, biotitic; has some organic debris; weathers out in blocky fragments; contacts undulate; maximum thickness 6 in - 

135. Oil shale, medium-to dark-brown, massively 

bedded, includes a light-brown marlstone. (Yellow M above a horizontal yellow stripe is 

painted on marlstone.) ._._. 

134. Marlstone, light-gray; beds 2 in. thick with some 

intercalated light-brown laminated oil shale-

*133. Oil shale, dark-brown, laminated. Sample from

top of unit. Zone of 3 thin tuffs in lower part132. Marlstone, light-brown. (Yellow circle 4 in. in

diameter painted on bed.)- 

*131. Oil shale; mostly medium brown. Sample of 

medium-brown oil shale 3.1 ft above base. 

Other samples are dark- and medium-brown 

oil shale- - - 

130. Marlstone, light-brown; weathers orange brown 

129. Oil shale, medium- to dark-brown -----

*128. Marlstone, light-brown; weathers to sandy surface. Sample is whitish orange marlstone at

Hoop _ ___ __________ _____ _

UCLO^

127. Sandstone dike, tuffaceous; about 4 in. thick; 

strikes N. 20°W.; dips 65° NE. About 10 ft 

above the road, dike is offset to right and extends to top of outcrop. (Large yellow D painted on outcrop at road level just west of dike.)-

*126. Oil shale, dark-brown; thin pods and intercalations of marlstone. Sample is dark-brown oil 

shale 2 ft below top- - __ - - _ 

Section offset to gully to east.

*125 Marlstone, light-gray, tuffaceous -

124. Oil shale, medium-brown; weathered plates form

partly covered slope 

123. Marlstone, light-brown 

122. Oil shale, dark-red-brown; weathers blue gray;

forms ledges a few inches thick -

121. Marlstone, light-brown, and medium-brown oil

shale .._ - 

*120. Oil shale, dark-brown, well laminated; weathers blue gray. Sample is from dark-brown ledge 

-forming bed 6 in. thick 

119. Oil shale, medium-brown, poorly exposed1 18. Oil shale, dark-red-brown to medium-brown;

.3

9.2 

1.5

2.3 

2.0

7.4

.04 

3.0

0

8.0 

.08

11.1 

2.0

4.1 

5.5

2.3 

6.9
MEASURED SECTIONS 21

Complete section of the Parachute Creek Member of the Green

River Formation near Rio Blanco Continued 

Parachute Creek Member Continued

Thickness 

(feet)

*117. Marlstone, tuffaceous -- ------ .6

116. Oil shale, dark-red-brown; weathers blue and 

slabby; has some interbedded light-brown 

marlstone - - ..... . 5.0

115. Oil shale, light- to medium-brown; contains some

dark-brown beds - - - - 4.8

*114. Tuff, light-brown, dolomitic - - .2 

113. Oil shale, red-brown, poorly exposed -- 3.0

* 112. Tuff-- --------------__------ .2

111. Oil shale, medium- to dark-brown - - - 1.1

*110. Oil shale, dark-brown, pyritic; weathers blue;

forms ledge. (Yellow streak painted on rock.)- 6.3

*109. Oil shale, dark-red-brown, pyritic; forms ledge-- .3

*108. Oil shale, medium- to dark-red-brown. Sample

is medium-brown oil shale - - 4.4

107. Oil shale, dark-red-brown, pyritic; 1-ft-thick ledge

at top of unit - 6.5

106. Covered unit - - - - - 5.5

"105. Oil shale, dark-brown - - 10.0

104. Marlstone, gray, silty - .1

103. Oil shale, medium- to dark-brown; weathers platy 7.7'

*102. Tuff; consists of analcime in a deep-blue matrix- .11 

101. Oil shale, dark-red-brown - - - .4

*100. Marlstone, tuffaceous, brown; resistant dense

bed- -- - -- - ------- --- .1

99. Oil shale, medium- to dark-brown 1.8

98. Marlstone, brown; resistant bed - - .1

97. Oil shale, dark-brown- - - 2.1

96. Marlstone, gray; resistant bed- -- .2

Section offset to lowermost outcrop in gully 

to east.

*95. Oil shale, dark-red-brown, laminated; weathers

blue gray - ... - - 3.3

94. Marlstone, dark-brown, sandy, resistant; forms 

ledges between layers of light-brown oil shale. 

(Yellow streak on bed below at west end of 

outcrop.) j 9.6

*93. Tuff, dark-brown, analcimic; even contacts; saturated with petroliferous material .1

92. Marlstone, light-brown .8

*91. Marlstone, tuffaceous, sandy, gray; weathers

light orange brown .3

*90. Oil shale, dark-brown. Sample is from 2.2 ft

89. Tuff, analcimic; undulating contacts - .05

88. Oil shale, medium-brown - 1.2

*87. Tuff, analcimic, rust-brown, continuous; unduifltiiig corn sets * ** *** - .- .. .uo 

86. Marlstone, light-brown, thin-bedded; contains 

chert pods 1-2 in. thick and as much as 6 in. 

long. (Yellow P painted on outcrop.) - 3.6 

*85. Marlstone, light-gray, dolomitic; massive bed .4

84. Oil shale, medium- to light-brown - 3.4

83. Tuff, analcimic, light-brown-- .02

82. Oil shale, light-brown --- - .7

81. Tuff, analcimic, light-brown - - .02

80. Oil shale, light- to medium-brown - 3.2

79. Tuff, analcimic, light-brown - - .02

78. Oil shale, light- to medium-brown - 2.2

77. Tuff _ __ - _ _ - _ _________ 1

76. Marlstone, light-brown - 1.0

75. Oil shale, dark-red-brown; weathers blue gray-- 2.3

Complete section of the Parachute Creek Member of the Green

River Formation near Rio Blanco Continued 

Parachute Creek Member Continued Thickness 

(feet)

*74. Tuff, analcimic; weathers brown; has inclusions 

of oil shale and marl; undulating contacts; 84 

ft above Mahogany ledge. Well exposed along 

road (fig. 7). Possibly correlates with unit 163 

in the pipeline section - 5.2

*73. Oil shale, medium- to dark-brown. Sample is

from top of unit- - - 7.7 

72. Oil shale, interbedded medium- to dark-brown, 

weathers platy. (Yellow circle painted on outcrop at top of unit.) - - 12.4

*71. Oil shale, medium- to dark-brown. Sample is 

dark-brown oil shale 2 ft below top of unit. 

Top of unit contains discontinuous pods of 

analcimic tuff as much as Vz in. thick. Tuff 

and enclosing oil shale also sampled. (Yellow 

triangle painted on outcrop about 15 ft above 

road.) - - 3.4

*70. Marlstone, light-brown - - 4.1

*69. Oil shale, dark-red-brown, laminated; contains 

some tuff pods. Sample is dark-red-brown oil 

shale. (Yellow dot on outcrop just below sampled area.) - .._ 4.3

*68. Sandstone, tuffaceous, light- to dark-brown .6 

67. Oil shale, dark-red-brown, laminated - - 3.0 

66. Sandstone, tuffaceous, dolomitic, laminated .2 

65. Oil shale, dark-brown, laminated- - .9

*64. Sandstone, calcareous - - -- .6

*63. Oil shale, light- to medium-brown. Sample is

light-brown oil shale- - - - 1.2

62. Tuff, analcimic, rust-brown. (Painted yellow T

19 just above bed.) - - - -- .01

61. Marlstone, light-brown; poorly exposed in middle. (Forms base of outcrop that lies about 

half way between 2 curves in road.) 6.9

60. Covered slope - ~ 26.4

*59. Marlstone, light-brown, and a few dark-brown 

beds of oil shale a few inches thick; includes 

4 layers of discontinuous pods of rust-brown 

tuff about V2 in. thick at 6.3, 9.2, 9.5, and 11.5 

ft above base. Top of unit is analcimic rustbrown tuff about J/2 in. thick. Sample is uppermost tuff. Probable position of the A-groove- 12.3

*58. Oil shale; mostly light brown with some thin 

beds of dark-red-brown. Sample is light-brown 

oil shale 4 in. below top. Small fold; axis trends 

N 10° W. Top of the Mahogany ledge. (Yellow 

5 painted on outcrop.)- - - - - 5.4

*57. Clay, white, chloritic and dolomitic.- - .1 

56. Marlstone, gray- to blue-weathering - 3.1

*55. Marlstone, and 2 thin tuffs. Sample is marlstone .6 

54. Marlstone, gray - - .1

*53. Tuff, analcomic; weathers rust brown .1

*52. Marlstone, light-gray- - - - .5

*51. Tuff, analcimic, medium-brown; some biotite .1 

50. Marlstone; laminated with tuff -- -- - .4

*49. Tuff, analcimic; weathers light brown. (Yellow 

T9 painted under tuff in bend of road.) "The

Z9.J.S6 ITi9,riC6r ~~" - - ..... . .«..».««... B f

48. Oil shale, medium- to dark-red-brown 16.3

*47. Sandstone dike, tuffaceous, light-brown, finegrained, poorly sorted; 1 ft thick; strikes N. 

70° E.; dips vertically; some organic debris Q
22 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

Bradley (1931, 

pis. 3, 4A)

Donnell(1961,

modified by 

oral commun., 

1970); Donnell

Brobst and

Tucker 

(this report)

G. N. Pipiringos

and W. J. Hail, Jr.

(oral commun., 1970)

 ' .' . '. B ri dger(?) .' ' .' '.

:-v.'Forrn«iti6n:- ; . 

Evacuation

Creek

Member

<U

J3

E

0)

^

(U 

0)

O

0)

x: 

u

ro

Q_

Upper 

oil-shale

group

Unit a (pi. 4.4)

Transitional

beds

Unit b (pi. 4.4)

Lower

oil-shale

group

Unit c (pi. 4^)

^M^^Mls'rnooth'

 :':'.U n it 'd.-'.-V'-i-i-s'lope.^.-:

: :-./pl '4^4.)':v;'Dii'hh'lv"-'

^MSx^^J9PeS^

>""" ' ^i

and Blair(1970)

< < ^ < <

^-^

\

\

\

\

\ 

\

\

plOO FEET

e.

.' //' ' ! :/.;': :£ y'a'cu a t i o n

Para

 ^^^ ;:>-^".^;^v;-^:

Illi;lff^^;^ll5

Zone of B-groove

R-6

,,

L-5

R-5

L-4

R-4

Para imiiH^x

....... ,.......,.,,.... ,.|

: h u t e

Top of hill

in figure 4

of this 

report

z h u t e

den

*

Ilitll

Creek

fi&&£Mi£:?$^:±.

'^ ' '::' ;-:M';' ;:SS:'S;^-V ~\ V

D

D 

D

D

D

D

D

D

Dawsoniterich zone

D

D 

D

Creek

ilSESJilS?

M e m b e r .' .' ' ;.;.; ; ;. ' ;.;:_ : '/.

M e rr

Mahogany

marker

//

^

b e r

$ ...... . ..... . ..^

-50 S S

-0

FIGURE 6. Nomenclature used for the Parachute Creek Member on lower Piceance Creek. D, occurrence of dawsonite. Numbers

prefixed by R and L refer to zones rich or lean in oil shale.
MEASURED SECTIONS 23

rop of Evacuation Creek Member'

Samples of unit 

155 (fig. 17) 

15.

Sandy zone

Units 16-27 Gravel

R. 95 W. R. 94 W

FIGURE 7. Location of the measured section of the Parachute Creek Member near Rio Blanco. Numbers refer to units described

in the measured section.

Complete section of the Parachute Creek Member of the Green

River Formation near Rio Blanco Continued 

Parachute Creek Member Continued Thickness

(feet)

*46. Tuff, analcimic; lesser amounts of quartz and 

alkali feldspar; minor amounts of illite in 

weathered samples only. Samples of fresh and 

weathered rock. The Mahogany marker. (Yellow + painted on tuff.)- - - 0.7 

45. Oil shale, medium-brown-- - 2.8

*44. Marlstone, light-brown, dolomitic; massive beds 

to 4 in. thick. Samples from a 9.5-cm-thick 

sequence were studied in detail (figs. 14, 15) - 1

*43. Oil shale, dark-red-brown; weathers blue gray.

The Mahogany bed - 1.5

*42. Dolomite; thin bedded in beds lA-l/z in. thick ----- 1.0

*41. Oil shale, dark-brown- - - - 3.0

*40. Dolomite, brown - - - - - 0.7 

39. Oil shale, dark-brown; contains some marlstone

beds to 1 in. thick - - - 2.3 

38. Oil shale, medium-brown; contains some lightbrown marlstone. (Yellow ML painted on outcrop near road level.) - -- 4.5 

Section offset to 2 blue-gray beds exposed 

beneath talus in gully to east.

*37. Oil shale, medium- to dark-red-brown; laminated beds 1-2 in. thick; weathers blue gray and 

platy. Samples are dark-brown oil shale 10.6 

and 15.9 ft above base. (Ledge painted yelInw 1WW./ ^_________________ ~ . ~ .... _ ii_L.(J 91 ^

*36. Oil shale, dark-red-brown; weathers blue gray - .2

35. Oil shale, medium-brown to red-brown; weathers blue gray; strikes N. 90° W.; dips 10° SW34. Oil shale; mostly light to medium brown. Samples are dark-brown oil shale in uppermost 

bed (0.1 ft thick) and lowermost bed (0.2 ft 

thick). Lowermost good unit of oil shales - 

33. Tuff, analcimic, biotitic. Base of the Mahogany 

ledge- . . . ....

5.9

5.5

Complete section of the Parachute Creek Member of the Green

River Formation near Rio Blanco Continued 

Parachute Creek Member Continued Thickness

(feet)

32. Marlstone, gray to brown. Probable position of

*31. Tuff, analcimic, persistent bed; weathers dark

30. Marlstone, thinly laminated -- ----- 

*29. Dolomite, sandy; weathers powdery- 

^J(_7. 98

f f

.3 

.1

*27. Sandstone, analcimic, very fine grained; weathers rust brown. Sandstones in this part of section chosen by Duncan and Belser (1950) as 

uppermost beds of their lower sandy member. 

This is 267 ft above the base of Parachute 

Creek Member as defined by our measurement

^U. 9fi IMarlctnno IVlciriDtUIlt? __ 

*25. Sandstone, dolomitic, light-brown, fine-grained,

thinly bedded ----- - - ----- 

24. Marlstone, gray and brown, laminated; weath-

*23. Tuff, orange-brown. (Painted yellow T 4.) ~ -

*22. Marlstone, tuffaceous, thin-bedded, laminated--

.7

19 ±,£A

. 5

.1 

2.1

*20. Tuff; contains analcime, quartz, and alkali feldspar; weathers blocky. (Bed painted yellow.)- .7 

19. Marlstone, weathered, well-exposed - - 9.0 

18. Marlstone, partly exposed - - - 16.5 

Section offset to next gully east.

*17. Marlstone, gray to light-brown; weathers

brown; strikes N. 15° W.; dips 10° SW- -- 16.5

*16. Sandstone, calcareous, gray, fine to very fine

grained, crossbedded - 3.9

*15. Marlstone, light-chocolate-brown to gray, brittle; weathers to rust-brown plates and slabs; 

some tuffaceous material. Samples are tuffaceous marlstone at base of unit, brown marlstone 5.5 ft above base, and tuffaceous marlstone 25 ft above base- - 32.5
24 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

Complete section of the Parachute Creek Member of the Green

River Formation near Rio Blanco Continued 

Parachute Creek Member Continued

Thickness 

(feet)

*14. Marlstone; various shades of brown. Unit contains pyrite nodules, efflorescent salts, and 

cavities (formerly occupied by saline minerals?). Common organic debris and fossils of 

leaves, twigs, and insects. Eight samples. 

(Yellow 3C at base of outcrop.)- - - 10.0 

13. Marlstone, platy weathering, poorly exposed - 11.0

*12. Marlstone, gray to brown; somewhat siliceous 

 dense, tough, and brittle. Samples are graybrown marlstone from basal beds and darkbrown marlstone with insect impressions. 

(Yellow 3A painted 3 ft above base.)- 23.8 

11. Poorly exposed sequence- ----- - 81.0

*10. Marlstone, gray, well-laminated. Sample is dense 

marlstone near top of unit. Also sampled was 

0.5-ft-thick tuff 2.1 ft above base. (Yellow paint 

on sampled bed of marlstone.)- -- 16.8

Marlstone, gray, and some silty beds; contains 

tuffaceous pods Vs in. thick, especially in upper 

8 ft. Samples are gray marlstone and tuffaceous sn ts ton6* ~ «« 

Tuff, analcimic and dolomitic -

9.

6. Siltstone and very fine grained sandstone; conTsclliiS SOlHG SSpJlSllltG * * -- 

5. Marlstone, gray - -

4. Sandstone, fine-grained, poorly sorted, evenly 

bedded; contains ostracodes and clay galls. 

(Marked RB-lA in yellow.)- -

3. Marlstone, gray, poorly exposed - - 

2. Sandstone, gray-brown, fine-grained, poorly

*1. Marlstone, gray, laminated; weathers platy - 

Section ends at base of exposures in ditch 

along north side of Piceance Creek road at 

west end of gravel pit. (See fig. 7.) Considered 

to be the base of the Parachute Creek Member. 

Eastward, the rocks are less well exposed, but 

sandstone and siltstone, characteristic of the 

Anvil Points Member, increase in abundance. 

Thickness of Parachute Creek Member

15.2 

.1 

.3

.2 

2.4

.5 

9.5

.7 

5.0

 7fi4 I LIT.V/ 0

Suggested correlations of our section with those of 

earlier workers are shown in figure 8. Donnell (1961, 

pi. 53) chose the lower contact of the Parachute Creek 

Member at about the same stratigraphic position as 

we did. Duncan and Belser (1950) placed the contact 

at the top of sandstone beds (our units 25 and 27 in the 

Bio Blanco section) about 270 feet above the base we 

chose for the Parachute Creek (D. C. Duncan, oral 

commun., 1970). Thus, they seemed to have a thinner 

Parachute Creek Member only 445 feet thick as 

compared with 735 feet measured by Donnell (1961, 

pi. 53) and 764 feet measured by us.

MINERAL COMPOSITION

The mineral fraction of most rocks examined in 

this study of the Parachute Creek Member consists of

mixtures of dolomite, calcite, quartz, potassium 

feldspar, albite, analcime, illite, and pyrite. 

Dawsonite was detected only in oil shale on lower 

Piceance Creek. Small amounts of chlorite occur in 

some of the rocks, and biotite and (or) muscovite occur(s) in some of the beds of tuff. Thin coatings of 

gypsum line some fractures.

Nearly all these rocks are extremely fine grained 

and contain some tightly bound organic materials 

that impregnate the rock, coating and masking the 

mineral grains and making the physical separation 

of the minerals from each other and from the organic 

matter very difficult. Tisot and Murphy (1960) found 

that oil shale from the Mahogany ledge near Rifle, 

Colo., is composed of particles that are 90 percent silt 

and clay size. They also reported that the mineral 

fraction is 83.8 and 55.4 percent (by weight) in oil 

shales which, respectively, assay 28.6 and 75 gallons 

of shale oil per ton of rock. These features of the rock 

make the study of the mineral composition by standard microscopic techniques unsatisfactory. X-ray 

diffraction methods, although having certain 

limitations, do provide a rapid means of acquiring 

mineralogic data.

LABORATORY WORK

PROCEDURES

Samples weighing about 4 ounces (100 g) and taken 

from about 1 inch (2-3 cm) of stratigraphic thickness 

of the freshest both common and unusual rocks were 

collected for X-ray study from the measured sections. 

Samples of the tuffs generally represent the thinnest 

stratigraphic intervals; many of these beds are less 

than 1 inch thick. After initial sampling was completed in the three measured sections and the X-ray 

results were examined, further sampling was done at 

selected localities to provide material for study of the 

details of vertical and lateral variation in the composition of selected units of rock, including the 

dawsonite zone on lower Piceance Creek.

Samples from the three measured sections were 

routinely prepared for X-ray study by being ground in 

a hammer mill. The resulting powder was pelletized 

at 22,000 pounds per square inch in a hydraulic press. 

X-ray diffraction patterns of the pellets were made 

with Norelco equipment (operated at 40 kv; 20 ma; 

scale factor, 8; multiplier, 1; time constant, 4). Diffraction data from CuKai radiation were recorded 

from scans of the sample through an arc of 60° 20 on 

100-unit chart paper at the rate of 2° 20 per minute. 

Values for peak intensity, expressed as height of the 

peak in chart units above base line, produced by the 

Norelco equipment are summarized or shown in 

tables 4-7 and 11 and in figure 21.
MINERAL COMPOSITION 25

Duncan and 

Belser(1950)

Donnell 

(1961, pi. 53)

Brobst and

Tucker 

(this report)

Pa r a c h u

Mahogany 

ledge

5 ft of tuff

Mjjhbgany

of unit 74

marker

FIGURE 8. Nomenclature used for the Parachute Creek Member 

in the Rio Blanco section.

Samples of dawsonitic and other rocks described in 

tables 2 and 8-10 and in figures 16-19 were prepared 

as described above, but diffraction patterns were obtained by use of Picker-Nuclear equipment (operated 

at 35 kv; 20 ma; scale factor, 3K; time constant, 1). 

The diffraction patterns, also from CuKai radiation, 

were recorded as described above.

Data compiled in figures 10, 12, 13, 15, 20, and 22 

pertain to samples obtained by use of a dental drill 

from sawed slabs of rock. The powder thus obtained 

also was made into pellets as described above, but because of the small amounts of material available, the 

pellets consisted of a thin coating of powder on

cellulose backs. Diffraction patterns were made with 

Picker-Nuclear equipment operated at the settings 

described above.

Diffraction patterns of all the rocks were examined 

uniformly by measuring the height of the apex of the 

peak above base line on chart paper divided into 100 

units. One peak was measured for each mineral 

detected; these selected peaks are shown in table 1. 

The values thus obtained were compiled or summarized in the tables and figures in this report.

DISCUSSION

Diagnostic peaks to be read for the minerals in a 

given suite must be chosen to avoid use of 29 

positions where the cumulative effect of the presence 

of more than one mineral leads to ambiguity as to 

how much of the peak height is attributable to any 

one mineral. When possible, the most intense peaks 

for each mineral are used, but this is not always possible or practical. In this study, the 20.9° 20 peak for 

quartz was used rather than the more intense 26.6° 

20 peak because illite enhances the latter peak. The 

second highest peak for analcime occurs at 15.8° 20, 

which is close to the 15.6° 20 major peak of dawsonite, 

but our equipment was able to differentiate the two 

peaks well enough for our purposes in this study.

Detection of dawsonite in the samples was considered of enough importance to conduct some grinding experiments to determine the optimum method 

of sample preparation for assuring the appearance of 

the maximum peak height in the diffraction pattern. 

Two samples of dawsonitic oil shale from the section 

on lower Piceance Creek were crushed and then 

ground and homogenized in a mixer mill. At various 

timed intervals of 5-50 minutes, part of the sample 

was removed and X-rayed. Results are shown in table 

2. The data show that prolonged grinding reduces 

the height of the dawsonite peak to a much greater degree than the peaks of the other minerals. In the entire length of the grinding time, the peak height of 

carbonate minerals and illite was more affected than 

was that of quartz and feldspar. Short grinding time, 

to insure the recording of the maximum peak height 

of the dawsonite, seemed desirable.

Another sample that contained both dawsonite 

and analcime from outcrops on lower Piceance Creek 

was crushed and passed through the hammer mill. 

The sample was then split: half was sieved, and the 

different fractions were X-rayed; the other half was 

ground further in a mixer mill for 5 minutes and was 

sieved, and the different fractions were X-rayed. The 

results of these experiments are shown in table 2B. 

The X-ray data from the sample that was ground only 

in the hammer mill show considerable uniformity;
26 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 1. Two-theta position of X-ray peaks measured 

for use in tables of mineral composition of rocks in 

the Green River Formation

Mineral Peak position 

(degrees 26)

Chlorite ___ __ _____

Illite1 - 

A 1 Hi fa - - _ ______

Pfllmto _____

i jfULt 

a Q

Q Q

11.7

90 Q

O£? A

fyn a

OQ n

9Q P;

qi r\

qq -i

1Mica (biotite or muscovite) was distinguished from illite in tuffs by visual 

examination of hand specimens.

slight variation is present, however, and the greatest 

peak height for dawsonite was found in the finest size 

fraction. The X-ray data from the sample processed 

in both the hammer mill and the mixer mill show no 

greater variation, but the peaks for dawsonite were 

smaller in this half of the sample than in the other. 

Thus, routine processing of the samples would seem 

to only require crushing and grinding in the hammer 

mill to provide a material in which dawsonite can be 

detected with almost maximum sensitivity by X-ray 

diffraction without sacrifice of sensitivity in 

detection of the associated minerals.

Peak heights obtained for the finest fractions of the 

samples shown in table 2B suggest that the values 

measured for the samples obtained with the dental 

drill should be similar. The data in table 2B also indicate that the chances of detecting dawsonite are 

enhanced in the finest fractions of the sample 

without appreciable change in the peak of the other 

minerals.

It would be ideal to demonstrate that the threshold 

of detection is the same for all minerals and that 

equal peak heights of various minerals indicate equal 

amounts of those minerals. Such is not the case because of many factors affecting the response of 

minerals to X-rays. Some of these factors are the time 

of grinding and the particle size of the sample, the 

presence of coatings of other substances on the surfaces of the grains, the crystallinity of the mineral 

and any variation in the composition, and the tendency for minerals to lie in preferred orientation in 

the sample. A detailed discussion of the factors is beyond the scope of this paper; the complexities of the 

subject have been discussed by Jackson (1964) and 

Schultz (1964).

Much effort in this study to translate X-ray diffraction peak height directly to percentage of the

mineral, by the use of standards prepared from 

minerals and matrices obtained from rocks of the 

Parachute Creek Member, produced no satisfactory 

scheme for achieving the highly desirable combination of simple procedures and a short time for 

routine application to a large number of samples.

Experiments with standards for quartz in a 

dolomite matrix from the Parachute Creek Member 

indicated that the value of the height of the 20.9° 20 

quartz peak is about 80 percent of the value of the 

amount of quartz in the standards, but these results 

are semiquantitative at best. Jackson (1964, p. 256) 

found that generally more than 10 percent quartz 

must be present in a sample before the 20.9° 20 peak is 

legible.

Fine-grained disseminated dawsonite could not be 

separated from the other components of the rocks, so 

satisfactory standards for 4uantitative evaluation of 

X-ray peak heights could not be established. Smith 

and Milton (1966, p. 1034) estimated, however, that as 

little as 3 percent dawsonitfe can be detected by X-ray 

diffraction. Comparison of X-ray data with chemical 

determinations of acid-soluble aluminum has not 

yielded satisfactory correlation. More involved and 

time consuming procedures for the quantitative 

determination of dawsonite, however, have been described by Smith and Young (1969).

Analcime seems to be highly sensitive to detection 

by X-ray diffraction. Tuffs that contain about 50 

percent analcime yielded a major peak whose apex 

was not recorded at the standard settings of the X-ray 

equipment. Preparation of standards was hindered 

because the analcime in these rocks contains 

myriads of small inclusions of other minerals.

Potassium feldspar and albite standards made 

from materials in these rocks were not obtainable. 

There is no reason to suspect, however, that the two 

groups of feldspars have any significant differences 

in sensitivity to X-rays. Equal peak heights of the two 

feldspars in these samples probably do indicate 

nearly equal amounts of the feldspars.

Dolomite and calcite seem to be similarly sensitive 

to detection by X-rays. X-ray diffraction provides a 

satisfactory and rapid method for distinguishing 

between the two minerals and for estimating their 

relative abundance in the sample.

Pyrite seems to be less easily detected in the X-ray 

diffraction patterns than should be expected from its 

high degree of crystal symmetry. Thin-section study 

suggests that accessory amounts of pyrite are commonly disseminated in the rocks of the Parachute 

Creek Member, but most legible peaks for pyrite are 

restricted to those patterns from samples in which 

pyrite is abundant and coarsely crystalline enough to 

be seen easily by the unaided eye.
MINERAL COMPOSITION 27

TABLE 2. X-ray data, expressed by X-ray peak height in chart units, from grinding experiments on dawsonitic

rocks from the lower Piceance Creek section

A. PEAK HEIGHT VARIATION WITH GRINDING TIME

Dark oil shale, unit 32 Dark oil shale, unit 50

Grinding time (min)- 10 20 30 40 50 10 20 30 40 50

DawsoniteQuartz -

Potassium feldspar - 

Albite-_ ___ __

Dolomite 

Calcite - - - 

Elite - _ __

29

31

16

8

22

9

14

17

27

14

5

19

7

9

10

27

14

5

16

7

9

26

14

5

15

7

6

7

28

14

5

14

6

6

7

27

14

6

14

7

6

84

38

17

6

17

0

11

48

32

13

6

15

0

7

32

37

14

5

19

0

5

23

37

14

5

12

0

5

13

29

11

4

9

0

3

10

29

10

5

9

0

3

B. PEAK HEIGHT VARIATION WITH SIZE FRACTION AND GRINDING TIME

Method of 

sample preparation

Hammer mill onlyHammer mill and si

Hammer mill and

5-min mixer mill;

no sieve 

Hammer mill, 5-min

mixer mill,

and sieveMesh

..._.... -28

eve- +100

-100 +200

-200+300

-300

__ _______

- +100

-100 +200

-200+300

-300

Dawsonite

71

71

64

71

77

50

44

47

45

51

Analcime

14

14

14

13

13

12

18

13

14

12

Quartz

31

32

30

31

33

32

32

32

30

31

Potassium 

feldspar

9

9

10

9

11

9

9

9

9

11

Albite

10

11

11

11

11

10

9

10

8

10

Dolomite

34

35

34

37

34

34

31

30

31

33

Illite

7

7

7

8

7

6

6

6

6

5

Weight 

percent

100

44

18

7

29

100

20

22

13

44

Most of the clay minerals in these rocks are illite 

type. They tend to lie in a preferred orientation and to 

yield the highest peak at 8.8° 26. Muscovite and 

biotite have their major peaks at the same position, 

and examinations of hand specimens and thin sections were used to draw the distinctions noted in the 

various tables.

Reproducibility of the X-ray data, a measure of 

the precision of the method, is affected by some of the 

factors already discussed. Some test pellets of 

selected samples were prepared and scanned four 

times twice on each side the second scan on each 

side at 90° to the first scan. The ranges of the peak 

height of the four scans of the test pellets are shown 

in table 3. The ranges are small, although the range 

of 2-4 chart units in the larger peaks is a smaller 

variation than the spread of 3 chart units on smaller 

peaks. The differences in the ranges cannot be 

evaluated fully, but they are greatly dependent on the 

homogeneity of the sample, physical and chemical 

characteristics of the minerals including any preferred orientation and variation in the performance of the X-ray generator, goniometer, and recording systems. Test patterns were run frequently 

during the periods of X-ray work to monitor variation 

in the operation of the X-ray and recording equipment. We consider the ranges of peak height to be sufficiently small that they probably do not interfere 

with the use of the data in the manner reported here.

The settings on the X-ray equipment previously 

stated produced legible results on the chart from one

scan of the sample for most of the minerals detected. 

Those peaks whose tops were not recorded on the 

chart at the standard setting were reported in the 

tables as values with a plus sign. The plus signs were 

most frequently needed with values for analcime in 

tuff beds and for dolomite in oil shale and marlstone, 

but only about 7 percent of all values compiled in 

tables 4,5, and 6 had to be reported with plus signs.

We concluded from this study that peak heights obtained by X-ray diffraction from these samples,which were uniformly prepared and exposed to 

X-rays at similar settings of the equipment, do offer a 

good means of not only determining the minerals 

present but also comparing the relative abundance of 

given minerals in different samples. More complex 

methods requiring preparation of mineral standards 

of questionable quality supplemented by chemical 

analyses of the samples were rejected because of the 

required time and because the final values expressed 

as weight percent of the mineral are based on 

mathematical manipulation that easily may make 

them appear more precise and accurate than they 

are.

A few further words of explanation and caution 

about reading and interpreting the X-ray data in the 

tables and figures of this report may be helpful. The 

amount of a mineral in a sample is expressed by the 

X-ray peak height in chart units in parts of 100. The 

greater the value of the peak height, the greater the 

abundance of the mineral. The peak height of a 

mineral must not be read as percentage of that
28 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO 

TABLE 3. Range of X-ray peak height in chart units from four replicate scans of test samples

Stratigraphic position 

and sample description Dawsonite Analcime Quartz Potassium 

feldspar Albite Dolomite Calcite

Norelco equipment

U.S. Bur. Mines cut:

Dark-brown oil shale 4.8 ft below tuff 13 (table 10)- 

Dark-brown oil shale 6.8 ft below tuff 13-

Lower Piceance Creek section: 

Dark-brown oil shale 3 ft above base unit 93- 

Dark-brown oil shale, unit 50- ____ .

33-36 

79-84

12-14 

61-66

41-44

9-10

30-34 

33-37

16-19 

35-41

17-20 

22-25

15-17 

21-22

21-25 

10-12

7-10 

9-11

43-46 

14-18

62-66 

20-21

15-18

45-47 

27-29

Picker-Nuclear equipment

U.S. Bur. Mines cut:

Dark-brown oil shale 4.8 ft below tuff 13--- 

Lower Piceance Creek section:

Dark-brown oil shale, unit 50 - 

Rio Blanco section:

Dark-brown oil shale, unit 16- -

28-35

QQj.

14-15 

79-81

37-42

n ft

35-37

26-28

34-36

1C 1C

39-43

20-24

11-13

19-21

19 1 ^

16-19

9-11

14-16

8-9

6

n

6-8

5-7

31-35

15-16

17-18

53-59

13-14

26-28

41-50

mineral in the sample. The values for the peak 

heights, however, are a measure of the relative abundance of different minerals in the same sample and 

indicate variations in the mineral composition 

between samples. The tables and figures of X-ray 

data are, therefore, informative when read either 

vertically or horizontally.

OIL SHALE AND MARLSTONE

The mineral fraction of the oil shale and marlstone 

in the three measured sections is composed chiefly of 

various mixtures of dolomite, calcite, quartz, potassium feldspar, albite, analcime, illite, and pyrite. 

Dawsonitic oil shale and marlstone, found only 

along lower Piceance Creek, will be discussed 

separately because of their potential economic value. 

Relative abundance of the minerals in the oil shale 

and marlstone determined from X-ray diffraction 

patterns is shown in tables 4,5, and 6. A summary of 

the data from 196 samples of oil shale and marlstone 

is shown in table 7. The values of peak height were 

combined into groups, each consisting of a range of

10 chart units of peak height. From these groups, the 

modal class (the group with the largest number of 

values) and the median class (the group which includes the value of the middle sample of the entire 

suite of samples) were determined for each mineral in 

each rock type. The imbalance in the number of samples for each rock type merely reflects the distribution 

of the samples collected.

Dolomite has long been recognized as the 

predominant carbonate mineral of the marlstone and

011 shale (Bradley, 1931, p. 20). Dolomite was detected 

in all the samples, except one of oil shale on lower 

Piceance Creek. According to the wide range of peak 

heights recorded, the abundance of dolomite varies 

greatly. Table 7 suggests a general decrease in abundance of dolomite from the marlstones, which contain lesser amounts of organic matter, to the dark oil 

shales, which contain greater amounts of organic 

matter.

Calcite is much more restricted in its occurrence 

than dolomite. Nearly half of the 196 samples summarized in table 7 do not contain detectable calcite, 

and where present, calcite generally is less abundant 

than dolomite. However, calcite was detected in more 

samples and in greater abundance relative to 

dolomite in samples above the Mahogany ledge than 

below it in each of the three measured sections (tables 

4,5, and 6). In 69 samples taken above the Mahogany 

ledge, calcite was detected in 59 samples and was 

relatively more abundant than dolomite in 14 samples. In 127 samples taken below the Mahogany 

ledge, calcite was detected in only 42 samples and 

was more abundant than dolomite in only one.

Analcime is a common mineral in these rocks and 

was detected in 85 percent of the 196 samples that are 

compared in table 7. Dark oil shale contains more 

analcime than lighter oil shale and marlstone. About 

half the 196 samples of the four types of rock yielded 

X-ray peak heights for quartz within the range of 11- 

20 chart units, suggesting no great variation in the 

relative abundance of quartz in many of these rocks.

Some potassium feldspar was detected in about 95 

percent of the 196 samples of oil shale and marlstone. 

In the three types of oil shale, nearly half the samples 

are in the modal class, which coincides with the 

median class of the medium and dark oil shales. The 

potassium feldspar is mostly sanidine, but some samples contain microcline and orthoclase, according to 

X-ray examination by the technique of Wright (1968, 

fig. 3). Potassium feldspar was detected in more samples than was albite and occurs perhaps in slightly 

greater relative abundance than albite.
MINERAL COMPOSITION 29

TABLE 4. Mineral composition, expressed by X-ray peak height in chart units, of rocks in stratigraphic succession from the pipeline

section

[D, dark brown; M, medium brown; L, light brown; Cl,chlorite; Py, pyrite; Gp, gypsum. Leaders ( ), not detected]

Unit 

sampled Analcime Quartz

Potassium 

feldspar

Albite Dolomite Calcite Elite Remarks

PARACHUTE CREEK MEMBER

Rocks above Mahogany ledge

230 

228

22g- ___ _

224 _ - _ __ _

218 ___ _

211 __ -

198 -

195.- _ _____

Ig7__ ___ _

18g_. ___ ..

Ig3 _______ 

Ig2__ _______

150 -

147 

14g_. ________

142 . -

140-.. ..

137 ________

137. . ..

64

OD+ O_? 1

23

25

29

20

48

45

85+

28

59

27

84+

26

88+

11

12

17

22

25

1 £

ID 

25

17

14

15

19

15

44

18

37

19

41

13

51

17

16

17

90

43

7

18

14

9

11

17

14

4

9

29

16

14

9

11

12

12

11

56 

60

10

12

11

10

-

13

3

11

15

6

12

14

8

9

12

--

51

52

65

70

53

65

81

80

75

24

63

82

74

88+

21

44

29

51

34

7

13

-

19

12

20

18

44

8

48

58

29

6

4 

5

5

5

3

4

4

7

-

--

-

-

8

5

8

4

3

-

Tuff, micaceous (Cl, 5).

Tuff, micaceous. 

Oil shale, pyritic (D) (Py, 5).

Oil shale (D).

Oil shale (M).

Oil shale, pyritic (D) (Py, 2).

Oil shale, pyritic (D) (Py, 5).

Oil shale (L).

Tuff.

Oil shale (D).

Tuff(Cl, 9).

Oil shale (D).

Tuff, micaceous (Cl, 5).

Oil shale (D).

Tuff, micaceous (Cl, 8).

Oil Shale (M).

Do.

Marlstone.

Pod in marlstone.

Mahogany ledge

136 - -

130 - -

124- ---

H7_ _________

HI _________

110 -

28

75+

21

22

17

86+

15

24

18

12

15

56

12

12

13

10

14

24

14

9

42

20

25

24

85+

18

85+

85+

72

8

-

12

38

~

9

4

7

Oil shale (D).

Tuff. Mahogany marker.

Oil shale (D). Mahogany bed.

Oil shale (M).,

Oil shale (D).

Tuff.

Rocks below Mahogany ledge

109------

108- _ 

96-- __ -

93-

92_ __ ____

87- 

86-

84- - -

77-

7g _________

73. __ ___

72 __ ______

g3_ __ .._....

gl _________

57 __ ______

5g __ ... _

54. ......

4g ___ ._

4g ........

44 _________

43. __________

42 ___ .. ....

40 ___ ._...

39 ___________

37 ________

34

33

31 ..

29 _______

2g __________

27 _____ .

25 ____________

22 .

20 ._ 

18 __________

1 4 __ ____

1 4 __________

13... __________

1 2 ___ _

11

63

68+

30

14

15

9

.,

-

18

--

38

44

80+

80+

12

50+

27

78+

7

11

66

17

fifi bo 

70+

60+

3

8

19

15

4

15

3

4

60+

15

12

21

18

1 3

lo

5

14

32

12

37

26

26

45

12

15

43

15

15

45

18

12

6

17

27

10

8

32

14

43

12

11

8

14

16

18

9

5

3

27

8

12

32

22

33

1 Q lo

15

19

41

24

32

65

19

57

18

10

10

5

1

25

20

12

4

43

5

44

32

9

5

8

10

16

7

5

4

7

27

4

46

27

27

70

6

14

24

5

27

5

10

55

12

37

8

4

24

10

15

4

14

5

1 c

lb 

53

10

4

7

8

15

4

5

5

56

78+

38

86+

86+

80+

ric

94+

82+

72

~

51

24

6

21

33

57

74+

35

50

58

88+

57

63

85+

80+

85+

75+

86+

8

_

14

-

_

_

8

_

_

-

_

-

6

-

_

-

_

..

-

_

15

-

-

_

-

9

4

3

5

6

3

2

6

5

5

4

9

1

4

5

7

--

3

5

9

7

6

6

Oil shale (M).

Oil shale, bumed(?).

Tuff.

Oil shale (M).

Tuff, muscovitic.

Oil shale (M).

Tuff.

Oil shale (L).

Marlstone, tuffaceous.

Marlstone, brecciated.

Marlstone.

Tuff(Cl, 8).

Cavity septum.

Oil shale (D) (Gp, 3).

Tuff.

Oil shale (L).

Tuff.

Do.

Do.

Oil shale (D).

Tuff.

Oil shale (D).

Tuff.

Tuff, dolomitic.

Oil shale (M).

Tuff.

Oil shale (M).

Tuff.

Tuff, micaceous (Cl, 8).

Tuff (Cl, 7).

Oil shale (L).

Pod, dolomitic.

Oil shale (M).

Marlstone (Py, 3).

Marlstone, tuffaceous.

Marlstone, tuffaceous (magnesite, 15).

Marlstone; at top of unit.

Marlstone; 0.6 ft above base.

Tuff.

Marlstone.
30 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 4. Mineral composition, expressed by X-ray peak height in chart units, of rocks in stratigraphic succession from the pipeline

section Continued

Unit 

sampled Analcime Quartz Potassium 

feldspar Albite Dolomite Calcite Illite Remarks

PARACHUTE CREEK MEMBER-Continued 

Rocks below Mahogany ledge Continued

9 ______

g _______

29

32

5

29

7

19

20

15

17

12

12

9

10

12

6

7 

15

84+

21

85+

80+

6

16

8

5

Do.

Oil shale (M).

Marlstone.

Do. 

Do.

GARDEN GULCH MEMBER

35 17 14 13 21 Shale, silty.

TABLE 5. Mineral composition, expressed by X-ray peak height in chart units, of rocks in stratigraphic succession from the lower

Piceance Creek section

[D, dark brown; M, medium brown; L, light brown; Cl.chlorite; Py, pyrite; Gp, gypsum. Leaders (--), not detected]

Unit 

sampled Dawsonite Analcime Quartz Potassium 

feldspar Albite Dolomite Calcite Illite Remarks

EVACUATION CREEK MEMBER

55

32

36

20

22

23

31 19 20 

85+

Sandstone, tuffaceous (Cl, 13). 

Sandstone, tuffaceous (Cl, 3).

PARACHUTE CREEK MEMBER 

Rocks above Mahogany ledge

232 ____ _ 

23Q ______

23o ________

228 

227- 

226 ____

220 

220 

219 ______

214 _______

207 _ _ .

202 

200 - - ~

198 

195 ______

191 .

190 

189- 

189 _______

189 ______

189- 

189 _____

188 -

1 loo QQ 

188 - 

187 ______

Igg ____ _

177 ____ _

30 

25

32

90+

88+

85+

27

17

60

3

CODO

73

85+

90+

36

27

1 Q

10

8

17

21

31

9

1 GID 

36

37

22

39

24

21 

15

14

6

32

25

21

13

23

15

OQ

Oi7

73

19

4

10

12

7

24

35

31

18

18

26

16

20

26

22

26

11

13

8

11

5

28

15

9

12

38

16

26

14

34

6

75

6

5

33

48

41

19

24

30

5

28

_

22

8

9 

8

10

4

18

15

11

10

46

9

OQ__SO

10

38

14

65

_

5

43

50

78+

66

22

36

55

50

77

63

7

22

47

54

_

-

70

64

67

63

43

__

71

51

51

22

32

34

50

27

44

62

23

25

67

29

71

30 

31

27

90+

43

_

64

76

3

80

62

73

57

71

90+

64

-

14

64

63

58

7ft /o

50

15

11

14

14

7 

7

5

4

22

5

5

4

4

5

_

_

-

3

5

4

4

7

5

3

5

3

6

4

5

4

_

3

Oil shale, pyritic (D) (Py, 5). 

Oil shale, pyritic (D) (Cl, 3; Py, 2).

Oil shale, pyritic (D) (Py, 2).

Oil shale (L).

Tuff.

Do.

Oil shale (M); 10 ft above base.

Oil shale (D); at base.

Sandstone, tuffaceous.

Limestone.

r\\^fe run.

Do.

Do.

Do.

Oil shale (D).

Oil shale (M).

Marlstone.

Marlstone, tuffaceous; 7.6 ft above base.

Marlstone, tuffaceous; 5.6 ft above base.

Marlstone, tuffaceous; 3.6 ft above base.

Marlstone, tuffaceous; 2.6 ft above base.

Marlstone, tuffaceous; at base.

Marlstone, silty; 0.4 ft below top.

Marlstone; 25 ft above base.

Marlstone; 21 ft above base.

Marlstone, tuffaceous; 15 ft above base.

Sandstone.

Do.

Oil shale (L).

Mahogany ledge

175 

174 _____

173 _____

172

17i _______

170 _____

169 .____

168 

167- -

94+

22

19

12

29

79

8

24

26

54

11

18

14

35

31

29

11

17

23

4

9

12

10

5

7

7

5

5

12

7

83+

10

30

4

9

8

63

60

54

81

20

76

40

50

25

_

17

_

_

45

4

12

3

8

5

-

8

_

4

7

8

Tuff. False marker.

Oil shale (D).

Oil shale (M) (Gp, 10).

Oil shale (D) (Gp, 5).

Tuff.

Oil shale (D).

Tuff. Mahogany marker.

Oil shale (D).

Oil shale (D); 1 ft above bijse. Mahogany bed.

Oil shale (D) (Py, 6); at base. Mahogany bed.
MINERAL COMPOSITION 31

TABLE 5. Mineral composition, expressed by X-ray peak height in chart units, of rocks in stratigraphic succession from the lower

Piceance Creek section Continued

Unit 

sampled Dawsonite Analcime Quartz Potassium Alh'tc feldspar

Dolomite Calcite Illite Remarks

PARACHUTE CREEK MEMBER-Continued 

Mahogany ledge Continued

166-

164 _ 

161 -

2 

24 

11 

17 

96+

95+ 

15 

95+ 

6 

7 

4

9 

15 

18 

21 

48

36

17 

33 

17 

29 

15

6

7 

3 

11 

9

6

8

6 

11 

12

6

7 

5

22

8 

10 

8 

4 

10 

21

43

55 

95 + 

40

82+

68

57

87 

6

6 

90+ 

78 

46

3 

10

10

8

3 

3

Marlstone. 

Oil shale (D); 4 ft above base. 

Oil shale (D) (Gp, 2); 3 ft above base. 

Oil shale (D) (Py, 3); 1 ft above base. 

Tuff.

Do. 

Oil shale (D). 

Tuff. 

Rosette in cavity (Gp, 5). 

Residue in cavity (Gp, 5). 

Oil shale (D) (Gp, 7).

Rocks below Mahogany ledge

131 _____ 

131. _____ 

131 ____ 

130 ______

113

108 

91

89 _ 

89

88 _______

ao

81

77--

64 _____ 

60 _____ 

59 ______

58 _____

55

51

52 _______

50 ______ 

49 _____

48 

48 __ _ 

48 

44. .

42

42 ______ 

42 _______ 

42-

42 _____

40 -

40 ______ 

39 _____ 

38 ______ 

37 ______

10

24

12

35 

13

13 

11

18 

20

5

47

13 

53

55 

19 

42 

31

26

3 

33

24

10 

26 

38

23 

21 

12 

9

24

17 

8 

38 

15

33 

26 

16 

15 

23

10 

28 

19 

6

3

19 

8 

23 

80 

92+

30 

75+ 

14 

31 

17

13

83 

9

7

5

5 

6

75+

8 

23

18

23

15 

1 

7 

41

44 

14 

25

13

13 

21 

14 

20 

37

18 

18 

14 

19 

19

20 

22 

35 

25 

21

20 

27 

21 

22 

27

22 

26 

14 

20 

30

51 

21 

26 

27 

38

23 

36 

20 

90 

10

17 

31 

59 

33 

31

31 

21 

30 

25 

32

22 

25 

27 

28 

24

16 

21 

11 

13 

22

6 

17

25

26 

5 

12 

32 

13

11 

6 

10 

27 

16

24 

21 

16 

16 

22

17 

16 

9 

25 

34

15 

32 

10 

27 

24

13 

17 

14 

12 

29

16 

11 

12 

41 

1

10 

17 

14 

17 

18

19 

13 

12 

18 

19

11 

16 

17 

17 

12

17 

12 

6 

5 

26

5 

20

26

27

16 

9 

10

6

9 

14 

10 

13

17 

11

8

4 

9

11 

13

10 

11 

8 

22 

12

1

65+

10 

8 

24

8

7

7 

4 

1 

6

10 

10 

9

11 

5 

9 

6

76+ 

85+ 

90+ 

80+ 

65

50 

60 

83 

47 

60

80 

55 

62 

40 

52

61

71 

28

46

59 

70 

54 

62 

32

85 

46 

85+ 

70 

85+

85+ 

43 

40 

53

73 

65 

71 

50

81 

18

66

17

12 

86+ 

45 

61

12 

42 

35 

24 

49

48 

67 

96 

66 

26

6

20

3 

35

45

-

4 

30

11

25 

13

7 

10

12 

9

8 

7 

3 

11 

13

2 

3

3

5

4 

4 

6

3

2 

9 

14 

7 

6

7 

6 

5

8

5

8 

11 

7 

9

8

6

8

8 

6 

6

19 

8

8 

6

8 

6

6

5

4 

5 

6 

3

11 

9 

8 

8 

6

5 

15 

5 

9 

6

Marlstone (Gp, 3). 

Oil shale (M); in cavity zone. 

Material in cavity (Gp, 2). 

Material in pod. 

Marlstone.

Oil shale (D). 

Do. 

Do. 

Cavity filling (Gp, 8). 

Marlstone.

Do. 

Oil shale (D) (Gp, 12). 

Do. 

Oil shale (L). 

Oil shale (D).

Tuff. 

Oil shale (D) (Gp, 4). 

Tuff. 

Do. 

Oil shale (D); 6 ft above base.

Oil shale (D); 3 ft above base. 

Oil shale (D) (Gp, 4). 

Oil shale (L); 6 ft above base. 

Oil shale (L); 4 ft above base. 

Oil shale (L); 0.2 ft above base.

Conglomerate, dolomitic. 

Oil shale (D). 

Conglomerate, dolomitic. 

Oil shale (L). 

Marlstone.

Oil shale (D). 

Oil shale (D); 52.6 ft above unit 49. 

Oil shale (D) (Py, 4); 36.1 ft above unit 49. 

Marlstone, tuffaceous; 25.6 ft above unit 49. 

Tuff; 24.3 ft above unit 49.

Oil shale (M); 20.9 ft above unit 49. 

Tuff; 19.7 ft above unit 49. 

Oil shale (M); 13.4 ft above unit 49. 

Tuff; 12.1 ft above unit 49. 

Tuff; 11 ft above unit 49.

Oil shale (L); 9.7 ft above unit 49. 

Oil shale (D) (Py, 4); 4 ft above unit 49. 

Tuff, a key bed. Tuff 13 of table 10. 

Oil shale (D); 2 ft below unit 49. 

Oil shale (D); 4.8 ft below unit 49.

Oil shale (D); 5.8 ft below unit 49. 

Oil shale (D); 8.4 ft below unit 49. 

Oil shale (M); 8.5 ft below unit 49. 

Oil shale (M); 10 ft below unit 49. 

Tuff; 16.5 ft below unit 49.

Oil shale (D) (Py, 5); 20 ft below unit 49. 

Oil shale (D) (Py, 5; Gp, 6); 20.6 ft below unit 49. 

Oil shale (D) (Py, 4); 21.1 ft below unit 49. 

Oil shale (D) (Py, 6); 22.6 ft below unit 49. 

Oil shale (D); 23.3 ft below unit 49.

Oil shale (D); 32.4 ft below unit 49. 

Oil shale (D); 34.1 ft below unit 49. 

Oil shale (D); 37.4 ft below unit 49. 

Oil shale (D); 42.4 ft below unit 49. 

Tuff; 44.4 ft below unit 49.
32 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 5. Mineral composition, expressed by X-ray peak height in chart units, of rocks in stratigraphic succession from the lower

Piceance Creek section Continued

Unit 

sampled Dawsonite Analcime Quartz Potassium 

feldspar Albite Dolomite Calcite Elite Remarks

PARACHUTE CREEK MEMBER Continued 

Rocks below Mahogany ledge Continued

35- 

35 _____ 

34 _ - 36

do QQ 

33 .

32 ____ 29

31 _____

ou Qfl_ 

29 

28 -

27 _ ___ 6

22-

1 g _ _____

17 ____ .

16- .

15 ______

13 ___ _. 10 ___ .

8 ____ ..

1 __

9

2

10 

8

5

2 

9

5

5

5 

9

64

76 +

15

5

6

12

19

21 

27

20

31

5

17 

21

23

13

50

43 

9

5

9

16

17

16

6

12

6

13

30

15

16

2

7 

16

38

9

25

40 

3

11

14

10

9

8

6

12

7

7

10

9

7

8

2

6

7

12

5

18

3

16

19

5

8

9

3

8

15

10

32

21

49

48 

28

22

93

53 

18

-

83+

57

90+

35

55

33

86 +

70 +

15

84

_

-

5

8

-

_

-

5

-

3

-

-

-

11

..

_

~~

13

13

2

11 

10

14

2

13 

10

4

4

6

-

13

11

13

7

12

26

5

Oil shale (D); 52.4 ft below unit 49.

Oil shale (D); 52.8 ft below unit 49.

Tuff; 54.0 ft below unit 49.

Oil shale (D). 

Oil shale (D); 57.4 ft below unit 49.

Oil shale (D) (Py, 3).

Marlstone; 64.2 ft below unit 49.

Marlstone; 65.2 ft below unit 49. 

Oil shale (D); 68.4 ft below unit 49.

Tuff; 72.3 ft below unit 49.

Oil shale (D); 73.3 ft below unit 49.

Marlstone; 75.3 ft below unit 49.

Tuff. 

Marlstone.

Tuff.

Do.

Oil shale (D); 2.7 ft below top.

Oil shale (L); near base.

Oil shale (M).

Oil shale (L).

Oil shale (D).

Oil shale (D) (Gp, 29).

Do.

TABLE 6. Mineral composition, expressed by X-ray peak height in chart units, of rocks in stratigraphic succession from theRioBlanco

section

[D, dark brown; M, medium brown; L, light brown; Py, pyrite; Cl, chlorite; Gp, gypsum. Leaders ( ), not detected]

Unit 

sampled Analcime Quartz Potassium 

feldspar Albite Dolomite Calcite Illite Remarks

PARACHUTE CREEK MEMBER 

Rocks above Mahogany ledge

I6i _ __ ..

157. _

155- ..

152-

151. ..

150 

149 _

142 _____

141- _____

136- _________

133 __ _____

131 ________

131 ________

131. ..

128- - __ -

126 ______

125 ____ 

120 -

114 ___

ii2 ___ _ -

no ________

1OQ

105_ ______..

102 - -

100

95 _ . _____

93 _______

gi ___ _

90 ___ _._

g7_ ______

35

25

25

36

83+

24+

22

6

11

31

32

26

9

28

18

8

18

41

6

7

40

20

68

88+

16

83+

17

81+

20

15

10

19

41

17

39

25

34

29

15

15

26

17

13

15

71

12

53

48

41

23 1 3lu

15

21

25

44

43

24

44

86

16

12

10

7

9Q. --O

12

16

12

11

27

11

11

8

10

5

13

6

12

20

9

15

21

25

23 

10

15

17

6

12

11

7

12

10

9

7

7

19

11

34

23

80

18

34

8

9

3

10

22

26

22

19

10

8

9

23

48

23

18

15

60

50

56

84

80

40

87 +

86 +

74

85+

55

61

85+

72

85+

55

76

87+

84+

44 

40

74

35

_

Qer i OO+

71

-

85+

44

-

41

82

14

-

6

22

39

_

19

_

13

2588 +

28

..

23

6

72

9

25

14

23 

10

18

7

16

15

43

24

15

18

-

10

4

4

5

4

8

5

12

11

15

4

5

4

._

5

_.

3

13

3

7

9

11

3 

5

4

5

5

11

3

5

11

3

5

Oil shale, pyritic (D) (Py, 2).

Do.

Oil shale, pyritic (D) (Py, 3).

Tuff, micaceous; lowest bed.

Oil shale, pyritic (D) (Py, 2).

Sandstone dike, tuffaceous.

Oil shale (D).

Oil shale (L).

Tuff pods.

Oil shale (M).

Tuff.

Oil shale (D).

Oil shale (M) (Cl, 3); 3.1 ft above base.

Oil shale (D).

Oil shale (M).

Marlstone.

Oil shale (D).

Marlstone.

Oil shale (D).

Marlstone, tuffaceous.

Tuff, muscovitic (Cl, 4).

Tuff, muscovitic.

Oil shale, pyritic (D) (Py, 6). 

Oil shale, pyritic (D) (Py, 3).

Oil shale (M).

Oil shale (D).

Tuff.

Marlstone (Cl, 4).

Oil shale (D).

Tuff.

Marlstone (Cl, 4).

Oil shale (D).

Tuff.
MINERAL COMPOSITION 33

TABLE 6. Mineral composition, expressed by X-ray peak height in chart units, of rocks in stratigraphic succession from the Rio

Blanco section Continued

Unit 

sampled Analcime Quartz Potassium 

feldspar

Albite Dolomite Calcitt Illite Remarks

PARACHUTE CREEK MEMBER Continued 

Rocks above Mahogany ledge Continued

74 ______

71 ________ 

71 _______

69 _______

68 - 

64 _____ 

63 

CQ

7 

83+

85+ 

10 

9 

39 

22

13 

28

1 

10 

38

37 

35

41 

10 

21 

80 

14

16 

11 

14

18 

21

13 

14

23 

7 

22 

10 

7

12

7

18 

22

22 

10

13 

6

7

1 

13 

4 

2

18

86 

66

50 

58 

73 

42 

75

86+ 

43 

90+ 

91+ 

86 

32

15 

20

22 

90+ 

33

21 

39

70 

31 

\

13 

17

17 

4 

6 

5 

22

6 

3

6

Marlston~. 

Tuff, weathered, biotitic

Tuff, fresh, biotitic. 

Oil shale (M). 

Oil shale (D); 2 ft below top. 

Tuff (Cl, 3). 

Oil shale (M); encloses tuff.

Marlstone. 

Oil shale (D). 

Sandstone. 

Do. 

Oil shale (L). 

Tuff(Cl, 5;Gp, 3).

Mahogany ledge

57

55 

53 ______

52- _____

51 _____ 

47 _______ 

46 

44

41 _______ 

40 _____

37 _____ 

37 ______ 

36 -

34 _____

33 

44 

4

78+ 

20

13 85 + 

39

78+ 

78+

9 

20 

12 

20 

15

8 

13 

20 

14 

65

13 

5 

26 

20 

24

64 

37 

23 

22 

17

26 

11 

7 

14 

24

30 

16 

14 

10 

11 

38

5

10 

9

35 

7

10 

14

22 

10 

2

10 

11 

9 

10

9

3 

10 

14

39 

34 

75 + 

11 

9

18 

10 

4 

8 

17

20 

16 

16 

12

53

82+ 

90+ 

90+ 

30 

86

43 

40 

27

90+ 

59 

95+ 

96 

95+

86+ 

59 

67 

79 

95+ 

33

70

22

21 

3

19 

41 

18

9

4

8 

7

4 

5

9 

3 

2 

5 

13

10 

5

4 

3 

39

Oil shale (L). 

Clay bed (Cl, 31). 

Marlstone (Cl, 30). 

Tuff (Cl, 30). 

Marlstone (Cl, 8).

Tuff, muscovitic (Cl, 9). 

Tuff(Cl,3). 

Sandstone dike (Cl, 7). 

Tuff, weathered. 

Tuff, fresh. Mahogany marker.

Marlstone. 

Oil shale (D). Mahogany bed. 

Dolomite 

Oil shale (D). 

Dolomite.

Oil shale (D); 15.9 ft above base. 

Oil shale (D); 10.6 ft above base. 

Oil shale (D) 

Oil shale (D); uppermost bed. 

Oil shale (D); lowermost bed. 

Tuff, biotitic.

Rocks below Mahogany ledge

31

27 _______

22- __ 

17 ______

15 _______ 

14- -

14 ______

14 ________ 

14 _______

14 ______

12- 

10

10

i

89+ 

5 

75+ 

2 

80+

31 

12

87+ 

25

24 

42 

55 

28 

11

49 

22 

35 

10

32 

8 

7 

16 

47

10 

12 

12 

4

20 

11 

36 

4 

20

18 

22 

45 

17 

37

23 

16 

11 

11 

24

14 

6 

22 

19

16 

10 

13

8 

17

31 

27 

4 

9«

16

18

17 

12 

12 

31 

25

30 

17 

23 

15 

12

27 

16

18 

13 

14 

7 

18

24 

20

39

77 

7 

22 

7 

22

17 

11 

45 

25 

18

22 

28 

29 

15 

17

25 

7 

19 

14

26 

11 

13 

7 

14

12 

12 

6

14

74 

90+

90+

80 

86+

58 

74

84+ 

73 

70 

37

73 

52

52 

59 

75 

74+ 

61

76 

85+ 

84+

3 

3

32

6 

82

18

56 

3

4

3

5

7 

4

6

4

5 

5 

4

8

4

5 

5

5 

7

.3

Tuff. 

Dolomite. 

Sandstone, tuffaceous (Gp, 4). 

Sandstone. 

Tuff.

Marlstone. 

Do. 

Tuff. 

Marlstone. 

Sandstone.

Marlstone, tuffaceous; 25 ft above base. 

Marlstone; 5 ft above base. 

Marlstone, tuffaceous; at base of unit. 

Cavity filling (Cl, 8; leonhardite, 23). 

Noddule (Gp, 55).

Marlstone (Py, 2). 

Concretion, pyritic (Py, 27). 

Marlstone (Gp, 17). 

Concretion, pyritic (Py, 5; Gp, 29). 

Concretion, pyritic (Py, 17; szmolnokite, 28).

Marlstone (Gp, 21). 

Marlstone, fossiliferous (insects). 

Marlstone. 

Do. 

Tuff.

Siltstone, tuffaceous. 

Marlstone (Cl, 3). 

Tuff (Cl, 17).

Mnrlstr.no fPl ?Ct
34 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 7. Mineral composition, expressed by X-ray peak height in chart units, of oil shale and marlstone from the three

measured sections

[Numbers in parentheses indicate number of samples]

Marlstone (60) Light-brown oil shale (17) Medium-brown oil shale (33) Dark-brown oil shale (86)

Mineral

Modal class Median class Modal class Median class Modal class Median class Modal class Median class

Potassium feldspar -

Dolomite r>_i -i_

Illite

1-10 (20)

U on /9Q1

11-20 (20)

 81-90 (23)

1

1 ft I A O\

11-20 

21-30 

11-20 

11-20 

71-80 

0 

1-10

1-10 (3)

11-20 (7) 

1-10 (8) 

1-10 (8) 

81-90 (5) 

0 (11)

1-10 (12)

11-20 

11-20 

11-20 

1-10 

61-70 

0 

1-10

11-20 (11) 

11-20 (24) 

11-20 (16) 

1-10 (18) 

81-90 (7) 

0(15) 

1-10 (26)

11-20

11-20 

1-10 

61-70 

1-10

1 10

21-30 (29) 

11-20 (47)

11-20 (42) 

1-10 (50) 

51-60 (14) 

0(35) 

1-10 (71)

11-20 

11-20 

11-20 

1-10 

51-60 

1-10 

1-10

Some albite was detected in about 85 percent of the 

196 samples of oil shale and marlstone. Albite peaks 

in about half the samples of oil shale have heights in 

the modal class (peaks of 1-10 chart units). The 

modal class of peak heights for albite is the same in 

the marlstone as in the oil shale, but the range of all 

values is greater in the marlstone, as indicated by 

only nine of 60 peaks being within the modal class 

and by the median value lying in the class of 11-20 

chart units. Most of the sodium feldspar is low-structured albite (Abas-ioo) as determined by the X-ray 

technique of Wright (1968, fig. 3). Low-structured 

albite is considered to be of low-temperature origin, 

and it has a high degree of order of the silicon and 

aluminum atoms in the structure.

Small amounts of illite occur in many of the samples, and chloritic material occurs in a few of the samples.

Pyrite occurs as tiny disseminated grains and 

streaks in many rocks most commonly, the dark oil 

shales.

Small amounts of some sulfate minerals derived 

from the weathering of pyrite and other minerals occur mostly as coatings along bedding planes or as 

fillings in fractures, or even as blooms of efflorescent 

salts. The most common of these minerals is gypsum 

(CaSO4 -2H2O), which occurs chiefly as thin white 

coatings along bedding planes. The coatings of 

gypsum are especially abundant in the outcrops 

along lower Piceance Creek. Other sulfate minerals 

detected by X-ray diffraction analysis are 

szomolnokite (FeSO4 -H2O), starkeyite (MgSO4- 

4H2O), and bloedite (MgSO4 -Na2SO4 -4H2O) (B.M. 

Madsen, written commun., 1968).

VERTICAL VARIATION IN COMPOSITION

Data compiled in tables 4, 5, 6, and 7 characterize 

the composition of the mineral fraction of units a few 

inches thick of the generally laminated oil shale and 

marlstone taken from various stratigraphic intervals 

in the three measured sections. Further detailed 

study of some differing units of these rocks was

undertaken to determine how similar a randomly 

sampled inch or two of rock is to the next few inches 

or feet above or below.

For this study, selected blocks of rocks were sawed, 

and samples were obtained with the use of a dental 

drill. Powdered samples thus obtained were 

pelletized and were used to form a veneer of sample 

on a backing of powdered cellulose. In the descriptions below, the letter and number symbols in 

parentheses refer to colors in the standard "RockColor Chart" by Goddard and others (1948).

FOSSILIFEROUS OIL SHALE AND 

MARLSTONE, PIPELINE SECTION

A stratigraphic thickness of 12.15 cm of laminated 

light-brown oil shale and marlstone rich in insect fossils was selected for detailed study from unit 197 in 

the pipeline section. The slab of rock, shown in figure 

9, is 20 cm wide and has light laminae that are 

yellowish gray (5Y 7/2) to light gray (N7) alternating with dark laminae that are light olive gray (5Y 

5/2) to medium gray (N5). Each sample consists of a 

series of extremely thin light and dark laminae, but 

each sample has a distinctly different color when 

compared with the samples above and below (fig. 10).

FIGURE 9. Fossiliferous laminated oil shale and marlstone from 

the pipeline section (unit 197.) Dark spots show where samples 

were taken. Numbers are sample numbers used in figure 10.
MINERAL COMPOSITION 35

LU CM

O

< 

O

o 

o

St

Oi O

 -I PH

"

p,X £ °

P< C3

S CO o* O !-|

CQ CO

rr! be

 o 

CQ -H

<

ID

O

O Oi 11 0

O

z

If)

0

0 

ro

CM

0

C/3

-

Li

0 c

 i

1 i

L_, ' . . J > i . i >

II II III 1 1 ' ' ' ' CJ 

TOOO OO IOOIO O O OO O O -= 

HlOCTi^d" CMf^ 1£>O*^ ro CTi LOrO ID CTi ±:

vi i i O o crioo r^-r~-^^D uo ro CMCM ' o t

ID

I 

C£

O

LJ 

_J 

CL

<

C/)

ro CNJ

ID ID

ro ro ro ro

CQ

P. C 

I I

r-^ "53

Ctf 45 S-8
36 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 8. Mineral composition, expressed by X-ray peak height in chart units, of a sequence of marlstone and mediumbrown oil shale from the pipeline section (unit 218)

[D, dolomite; C, calcite]

Thickness 

(cm)

Number of samples

Dark layers Light layers

D>C OD Total D>C C >D Total

Total

Arithmetic average of peak height

Analcime Quartz Potassium Albite Dolomite Calcite Illite 

feldspar

2

2

1

13

0

2

0

9

10

2

5

15

3

2

5

9

16

5

9

23

4

3

11

18

28

20

19

20

20

22

24

14

13

13

13

11

14

12

14

10

69

58

74

69

81

73

82

84

65

80

58

74

69

87

69

75

The thickest individual lamina in this slab is about 1 

mm thick, but commonly the total thickness of three 

or four pairs of light and dark laminae is only 1 mm. 

Even some of the thinnest laminae can be traced 

laterally across the entire slab.

Clearly visible in figure 9 are structural features 

that Bradley (1931, p. 29) called "loop bedding." 

In these, the normally even and regular lateral course 

of the laminae in thin sequences generally a few 

millimeters thick is interrupted irregularly by 

constriction of some or all of the layers. In the short 

interval of constriction, the inner laminae of the sequence commonly are pinched off, but the upper and 

lowermost laminae are not. These features are more 

common in the oil shale than in the marlstone, as 

noted by Bradley (1931, p. 29). The greater abundance 

of these features in rocks rich in organic matter 

suggests that they may be related to the algal life in 

the lake.

The composition of the samples is shown in figure 

10. Major minerals are dolomite and calcite that are 

in association with analcime, quartz, feldspar, and 

illite. The X-ray data indicate that the upper half of 

the slab contains more calcite and less dolomite than 

the lower half. No other differences in mineral composition are apparent, but the upper half of the slab 

seems to be darker. The composition of a composite 

channel sample of the entire slab is shown by the 

broken lines in figure 10.

MARLSTONE AND MEDIUM-BROWN OIL SHALE, 

PIPELINE SECTION

A stratigraphic thickness of 96.8 cm of marlstone 

and medium-brown oil shale from unit 218 in the 

pipeline section was studied in a series of slabs of

rock 5-22 cm wide. This sequence of rocks is well 

laminated, and as in the slab described above, many 

individual laminae are no more than 0.5 mm thick. 

The lightest colored laminae in this sequence are 

yellowish gray (5Y 7/2) to grayish yellow (5Y 8/4). 

The medium-colored laminae are light olive gray 

(5 Y 6/1 and 5 Y 5/2) to dark yellowish brown (10 YK 

4/2). The dark laminae contain the most organic 

matter and range from olive gray 5 Y4/1 and 5 Y3/2) 

through dusky yellowish brown (10YK 2/2) to 

brownish black (5YK 2/1).

The mineral composition of the suite of 89 samples 

is summarized in table 8, which emphasizes some of 

the characteristics of these rocks rich in dolomite and 

calcite. A major feature of these rocks is the 

stratigraphic variation in content of calcite and 

dolomite; groups of layers rich in calcite alternate 

with groups rich in dolomite. There is no apparent 

correlation of the predominance of calcite or dolomite 

with either light or dark layers. The relative content 

of the other minerals does not vary greatly.

OIL SHALE FROM THE MAHOGANY LEDGE, 

LOWER PICEANCE CREEK SECTION

A slab from a 20.6-cm-thick sequence of oil shale 

from the Mahogany ledge was selected for study from 

unit 161 in the lower Piceance Creek section. This sequence of rock, shown in figure 11, is well laminated, 

but compared with the rock shown in figure 9, the 

laminae in this sequence are more clearly defined 

and form small units of rock with more distinctly contrasting color. As in the other rocks examined 

in detail the individual laminae are extremely thin. 

This slab is generally darker than the others described in this part of the report, mostly because these
MINERAL COMPOSITION 37

rocks are richer in organic matter. The lightest 

laminae in the oil shale are light bro wn(5 YR 6/4); the 

intermediate-colored laminae are moderate brown 

(SYR 4/4) to dark yellowish brown (10 YR 2/2) to 

blackish red (5R 2/2). The tuff at the base of the slab 

consists of two phases differing in color and texture. 

The lower, coarser grained part (sample 200) is 

moderate brown (SYR 4/4), and the upper, finer 

grained part (sample 201) is pale yellowish brown 

(10YR 6/2). The upper part easily could be mistaken 

for marlstone in a casual examination.

The composition of 20 samples from this sequence 

is shown in figure 12. Major minerals in the oil shale 

are dolomite and calcite, but in this sequence the 

dolomite peaks in general have twice the heights of 

the calcite peaks. Analcime, quartz, potassium 

feldspar, and albite are detectable in every sample. 

Small amounts of illite are detectable in most samples.

The tuff has been altered to a rock consisting mostly of analcime and quartz. The total amount of feldspar probably is about equal in each tuff layer, but 

the coarser grained layer contains only albite, and 

the finer grained layer contains both potassium feldspar and albite.

MASSIVE MARLSTONE FROM THE MAHOGANY LEDGE, 

LOWER PICEANCE CREEK SECTION

The entire 31.25-cm stratigraphic thickness of massive pale-yellowish-brown (10 YR 6/2) marlstone of 

unit 166 was studied in two slabs from the Mahogany 

ledge of the lower Piceance Creek section. Some discontinuous, undulating thinly laminated brownishblack (SYR 2/1) lenses occur throughout the bed.

The mineral composition of the bed is shown in 

figure 13. The rock is composed chiefly of calcite and 

dolomite, with dolomite predominating only in the 

thin discontinuous dark layers. The composition of 

the bed is remarkably uniform, except for a slight 

decrease upward in the content of analcime and for 

the great variation of analcime, dolomite, and calcite 

in the thin dark bed (sample 5, fig. 13).

MARLSTONE AND LAMINATED OIL SHALE FROM THE 

MAHOGANY LEDGE, RIO BLANCO SECTION

A slab from a 9.5-cm-thick sequence of marlstone 

and laminated oil shale in the Mahogany ledge was 

selected for study from unit 44 in the Rio Blanco section. This sequence of rock is shown in figure 14. The 

lower 14 mm consists of thinly laminated oil shales. 

The lightest layers are yellowish gray (5Y7/2), and

FIGURE 11. Laminated oil shale from the Mahogany ledge in 

the lower Piceance Creek section (unit 161.) Dark spots show 

where samples were taken. Numbers are sample numbers 

used in figure 12.

the darkest layers are dusky yellowish brown (10 YR 

2/2). The next 54 mm of pale-yellowish-brown (10 YR 

6/2) marlstone consists of five massive beds in which 

the lower parts are coarser grained and slightly 

darker than the upper parts. Each of the five beds is 

separated by a few thin undulating dark laminae 

that appear to have more organic material than the 

rock above and below. Above the five massive beds 

are three thinner, but massive, beds separated by 

dark layers that are thicker than those in the beds below. These three beds have an aggregate thickness of 

14 mm. The uppermost 13 mm of the sample consists 

chiefly of thinly laminated beds of marlstone that 

range from yellowish gray (5 Y 8/1) to light olive gray 

(5Y5/2).
38 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

POTASSIUM 

ANALCIME QUARTZ FELDSPAR ALBITE DOLOMITE CALCITE ILLITE

Ij CHART 0 10 20 30 40 50 60 70 80 90100 0 10 20 30 40 0 10 0 102050 60 70 80 90100 20 30 40 50 0 10 

CL UNITS

< CM

20.60 -,- 

218

19.80 -

217

216

215

214

213

212

211

210

209

208

202

201

200 BASE

OF 

SAMPLE

FIGURE 12. Mineral composition of laminated oil shale from the Mahogany ledge in the lower Piceance Creek section (unit 

161), as expressed by X-ray peak height in chart units. Dashed line indicates arithmetic average of values for each mineral 

in a composite sample of units 202-218.
MINERAL COMPOSITION 39

O pcr

CHART 

UNITS

SAMPLE CM 

31.25

O

Q

O

< 

O

0 10 20 30 40 0 10 20 0 10 0 10 20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100 0 10

13

12

11

10

28.25

25.25 -

22.25

19.25

16.25-

13.35

9.25

R 6.95

D 6.75

4

-, 5.40

3 5.00

3.10

f"' I

Base of 

sample

FIGURE 13. Mineral composition of massive marlstone from the Mahogany ledge in the lower Piceance Creek section (unit

166), as expressed by X-ray peak height in chart units.

The mineral composition of these layers is shown 

in figure 15. As in most of the rocks, the commonly 

detected minerals include dolomite, quartz, 

analcime, potassium feldspar, albite, and illite. 

Calcite is conspicuously absent, in marked contrast 

to the marlstone and oil shales in the comparable 

stratigraphic position beneath the Mahogany 

marker on lower Piceance Creek. The laminated rock

in the lower 14 mm of the slab contains a little more 

feldspar and a little less dolomite than some of the oil 

shales studied. The composition of the marlstone and 

oil shale in the uppermost 13 mm of the slab varies 

widely.

The middle 54 mm of the slab contains beds considerably different than many noted elsewhere in 

this study. The massive nature of the beds and the
40 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

variation in grain size, coarser on the bottom and 

finer on the top, lend a distinctive appearance of 

"graded bedding" to this unit of rock. Data in figure 

15 indicate that the coarser grained part of the beds 

contains more analcime and quartz, and less 

dolomite, than the finer grained part. The thin dark 

interlayers generally are more distorted than many 

other layers and contain more dolomite and 

probably more organic matter and less analcime 

than the adjacent layers.

1 cm

FIGURE 14. Marlstone and laminated oil shale from the Mahogany ledge in the Rio Blanco section (unit 44). Numbers indicate 

position of composite samples shown in figure 15.

LATERAL VARIATION IN COMPOSITION 

MEDIUM-BROWN OIL SHALE, PIPELINE SECTION

Lateral variation in composition of a sequence of 

medium-brown oil shale about 3 inches thick that lies 

3.1 feet below the top of unit 221 in the pipeline section was studied in a series of samples collected at 20- 

foot intervals along 240 feet of continuous exposure. 

The north end of the sequence is about 40 feet south of 

a cattleguard. The rock is thinly laminated; the

lightest layers are pale yellowish brown (10 YR 6/2), 

and the darkest layers are brownish gray (5YR 4/1). 

The mineral composition of the 10 samples is 

shown in figure 16. The minerals detected include 

dolomite, calcite, analcime, quartz, illite, potassium 

feldspar, and albite. Dolomite is more abundant than 

calcite in all the samples. These data indicate a 

lateral consistence in composition.

DARK, PYRITIC OIL SHALE, RIO BLANCO SECTION

Lateral variation in mineral composition of a wellexposed blue-gray-weathering dark pyritic oil shale 

was studied in a suite of 37 samples collected at 100- 

foot intervals along the continuous exposures of unit 

155 in the Rio Blanco section. The samples were 

collected from the middle of the 0.3-foot-thick bed that 

is exposed as a rounded ledge 22.3 feet above the top 

of the westernmost roadcut in the Rio Blanco section

(fig. 7).

The blue-gray weathered coating is very thin. The 

rock is laminated on close inspection, but it has a 

massive appearance because it is so dark dark gray 

(N 3) to grayish black C/V2). The only mineral visible 

to the unaided eye is pyrite, which is disseminated 

throughout the bed. The pyrite is most easily seen on 

fractures crossing the planes of lamination at low 

angle.

The mineral composition of these samples is shown 

in figure 17; the samples are numbered in horizontal 

sequence eastward from sample 1. Dolomite, 

analcime, quartz, potassium feldspar, albite, calcite, 

illite, and pyrite were detected. A comparison of the 

range and arithmetic average of the mineral content 

of the various samples indicates a normal distribution of the values; that is, the data suggest a 

laterally uniform composition for this bed.

DAWSONITIC ROCKS

Dawsonite (NaAl(OH) 2CO3 ) has been described 

from drill cores in the Piceance Creek basin by Smith 

and Milton (1966), and the potential resources of 

dawsonite have been discussed by Kite and Dyni 

(1967). In November 1966, we found dawsonite during X-ray study of medium- and dark-brown oil 

shales in the measured section on lower Piceance 

Creek. The dawsonite occurs in scattered samples 

from a stratigraphic interval of about 370 feet from

FIGURE 15. Mineral composition of marlstone and laminated oil shale from the Mahogany ledge in the Rio Blanco 

section (unit 44), as expressed by X-ray peak height in chart units. Dashed line indicates arithmetic average 

of peak height in each composite sample. Interval of composite sample is shown by horizontal line and is 

numbered as shown in figure 14.
MINERAL COMPOSITION 41

ANALCIME QUARTZ POTASSIUM ALBITE

FELDSPAR

DOLOMITE

CHART UNITS 0 10 20 30 10 20 30 40 50 60 70 0 10 20 30 0 10 20 10 20 30 40 50 60 70 80 90100

ILLITE

0 10

SAMPLE

Base of sample
42 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

LU

s

o

CHART UNITS 0 10 20 30 

SOUTH 

10

rsi

cn

0 10 20 0 10

LUK

DO_l

<

0 10

O

Q

50 60 70 80 90 0 10

LU 6

_l 

Q_ ^

C/5 5

NORTH

FIGURE 16. Lateral variation in mineral composition of medium-brown oil shale from the pipeline section (unit 

221), as expressed by X-ray peak height in chart units. Horizontal sample interval is 20 feet.

about 200 feet above the base of the Parachute Creek 

Member upward to within about 70 feet of the base of 

the Mahogany ledge.

A dawsonite-rich zone about 45 feet thick occurs 

about 50 feet above the base of the R-5 zone (fig. 6). 

This zone has been traced updip from its emergence 

on lower Piceance Creek for about 1.5 miles 

northwest to the edge of the plateau overlooking the 

White River (fig. 3). As a result of this work, several 

tons of rock from this zone were quarried for further 

study by the U.S. Bureau of Mines. The rock was 

taken from the exposures at the level of the Piceance 

Creek road (fig. 4) in the SE^SW^ sec. 11, T. 1N., R. 

97 W. (J. W. Smith, written commun., 1967).

A suite of 54 samples consisting of 47 dark- and 

seven medium-brown oil shales was studied in detail, and the mineral composition is summarized in

table 9. Quartz, potassium feldspar, dolomite, and 

dawsonite occur in all the samples, and illite occurs 

in 53 samples. Albite and analcime, however, occur in 

only about half the samples; and pyrite and calcite, in 

only about one-fourth.

X-ray analysis of the 54 samples of dawsonitic rock 

from lower Piceance Creek shows that dawsonite 

content increases as quartz content increases but 

varies inversely with analcime content (fig. 18). Hay 

(1970, p. 254) reported similar relations from drill 

cores of rocks from the deeper parts of the basin; he 

suggested that dawsonite and quartz could have 

formed from analcime under a high partial pressure 

of carbon dioxide, determined by equilibrium with 

nahcolite, according to the following reaction: 

NaAlSi2O6 -H2O+CO2 -*-2SiO2+NaAl(OH)2CO3

analcime ^-quartz + dawsonite
MINERAL COMPOSITION 43

CHART ANALCIME 

UNITS 0 102030405060 

SAMPLE ' ' ' '

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

POTASSIUM

QUARTZ FELDSPAR ALBITE 

0102030 01020 01020

DOLOMITE 

010203040506070

CALCITE 

0102030

ILLITE PYRITE 

01020 010

m 

p

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

ARITHMETIC 34 

AVERAGE

I -... --1

 f~-"W~

P-«..«,.!;

11 40 8.5

FIGURE 17. Lateral variation in mineral composition of dark pyritic oil shale from the Rio Blanco section (unit 155), as 

expressed by X-ray peak height in chart units. Horizontal sample interval is 100 feet, beginning on the west with sample 

1. Locality is shown in figure 7.
44 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 9. Mineral composition, expressed by X-ray peak height 

in chart units, of 54 dawsonitic rocks exposed on lower Piceance 

Creek.

Mineral

D

Dolomite 

Potassium feldspar mite - -----

Number of 

samples

54 

54 

54 

54 

53 

34 

32 

15 

14

Peak height

Range

S on

5-85 

8-40 

6-35 

2-11 

4-25 

4-34 

1-8 

4-18

Arithmetic 

average

34 

40 

25 

13 

6 

9 

15 

4 

10

In a detailed investigation of the composition of 

analcime by the X-ray method of Saha (1961) in 91 

samples of oil shale and tuff with and without 

dawsonite, we (Brobst and Tucker, 1972) found that 

analcime in rocks containing dawsonite has a higher 

silicon to aluminum ratio than analcime in rocks containing no dawsonite. Data from this study, shown in 

figure 19, indicate that analcime in dawsonitic rocks 

is more siliceous and less aluminous than analcime 

in rocks without dawsonite. This relation and the 

relatively more abundant quartz in the dawsonitic 

rocks suggest that aluminum and silicon made 

available in the diagenetic alteration and breakdown 

of analcime were utilized in the formation of dawsonite and quartz.

Figure 18 shows that most of the peak heights of 

albite lie between 0 and 15 chart units regardless of 

dawsonite content. The albite-analcime relation in 

dawsonitic rocks is plotted in figure 20. There is no 

apparent relation between the contents of albite and 

analcime. A similar result is seen in figure 21, where 

the data for the same pair of minerals are plotted for 

190 samples of oil shale and marlstone that contain 

no dawsonite.

Compared with the nondawsonitic oil shales and 

marlstones, the dawsonitic rocks contain more 

quartz, less analcime and dolomite, rarely any calcite, slightly smaller amounts of feldspar, and about 

the same amount of illite.

VERTICAL VARIATION IN COMPOSITION

DAWSONITIC, PYRITIC OIL SHALE,

LOWER PICEANCE CREEK ROAD

A stratigraphic thickness of 17.0 cm of duskyyellowish-brown (10Y12 2/2) to brownish-black (SYR 

2/1) dawsonitic, pyritic oil shale was selected for 

study in a pair of slabs from the cut made by the U.S. 

Bureau of Mines along the lower Piceance Creek road 

(SE^SW1^ sec. 11, T. 1 N., R. 97 W.). The top of the 

interval selected begins 4 feet below the base of a tuff 

which is well exposed near the top of the cut. Shiny 

black scales of gar pike are abundant in this sequence 

of rocks. Laminae are laterally persistent in the

C/)

t 50

cr 

<

O 40

P 30

<

z>

2 20

LU

K

_] 

<

g 10

UJ

s

O

x x IA x x x I 

10 20 30 40 50 60 70

DAWSONITE, IN CHART UNITS

80 90 100

FIGURE 18. Abundance of analcime, albite, and quartz as a function of abundance of dawsonite in samples from the

Parachute Creek Member.
MINERAL COMPOSITION 45

33

15

33.5

14.5

Si

AI + Na 

3

- ATOMS 

4

14

PER UNIT 

34.5

13.5

CELL (0 = 96) 

35

13

35.5

12 .5

36

12

1

Oil shale

with

dawsonite; 

10 samples

Oil shale 

without

dawsonite;

22 samples

2

 

4

 

Tuff with 

dawsonite; 

3 samples

Tuff

without

dawsonite;

56 samples

232 2 7 116 11 2

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

Al

FIGURE 19. Ratios of silicon to aluminum and of silicon to aluminum plus sodium in analcime from 91 samples with and 

without dawsonite, Parachute Creek Member. Numbers next to dots indicate number of samples plotted at that position.

rocks, but they are seen only on close inspection because the color contrast of adjacent layers is slight, 

because of the abundance of organic matter.

The mineral composition of these rocks is shown in 

figure 22. The data show a broad range in peak 

heights of dawsonite 14-71 chart units. Dawsonite 

content generally decreases upward, but analcime 

content increases upward. Quartz content shows a 

slight decrease upward. Amounts of other minerals 

appear to be about the same in the upper and lower 

halves of the sequence.

LATERAL VARIATION IN COMPOSITION

Lateral variation in the composition of the 

dawsonite-rich zone on lower Piceance Creek was examined in a study of six suites of samples from 

localities within 1.5 miles north of the cut made by 

the U.S. Bureau of Mines (fig. 3). The mineral composition of these samples is listed in table 10. The tuff 

of unit 49 in the lower Piceance Creek section, 

referred to in this part of the report as tuff 13, 

provided a datum for correlation. Tuff 13 is exposed 

almost continuously north of station 3, as reported in

30

20

6 W | n kr_A A_A_U 

0 10 20 30 40 

ANALCIME, IN CHART UNITS

50

FIGURE 20. Abundance of albite as a function of abundance of 

analcime in samples containing dawsonite, Parachute Creek 

Member. Numbers next to symbols indicate number of samples plotted at that position.

table 10. Data in table 10 indicate that the 

dawsonite-rich zone is at least 20 feet thick at the 

Bureau of Mines cut, where the base of the zone is not 

exposed, and is at least 40 feet thick at stations 2 and
46 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 10. Mineral composition, expressed by X-ray peak height in chart units, of the dawsonitic oil-shale zone at six localities

along lower Piceance Creek

[Station localities are shown in fig. 3. D, dark brown; M, medium brown; L, light brown; USBM, U.S Bureau of Mines. Leaders (--), not detected]

Distance 

above or 

below Dawsonite 

tuff 131 

(feet)

Analcime Quartz Potassium AIV+ feldspar Alblte Dolomite Calcite Elite Pyrite Remarks

At USBM cut on lower Piceance Creek

60.1 ______ 

57 i _______

47.8 

40 8 _____

380

37 0 _____ 

36.0 _____

349 _____ 

23.4 - 

22.4 

12.0 

90 _____ 

70-

5^0 

4 o

4.0 .

o ________

4.8 ______ 

5.8 

6.8 

78 _______

98 ____ _

14 

19 

28

12 

12 

70 

63 

70 

74

33 

30 

89+

37 

55

35 

12

28 

31 

19 

13 

21 

27 

17 

80 

1 

20 

33 

20 

25

16 

42 

24

15 

13

12 

18 

20

14 

19 

33 

14 

23 

11 

40 

7 

18 

21 

22 

25 

35 

28 

38 

40 

48 

33 

28 

37 

41 

23 

34

10 

19 

5 

5 

12 

45 

6 

11 

6 

10

10 

11 

13 

10 

16 

9 

13 

12 

1 

16 

23 

22 

53 

14 

24

64 

9 

14 

7 

8 

73 

13 

18 

8 

8 

8 

6 

9 

7 

43 

18

9 

43

54 

62 

46 

68 

69 

36

76 

40 

74

77 

42 

85 

39 

12 

29 

13 

17

43 

47 

15

62 

22

15

15 

17

5 

6 

4 

9 

9 

5

5 

7 

8

6

8 

8 

10 

9 

7 

7 

9

10 

7 

9 

3 

5 

6

6

4 

5 

6

4

7

Marlstone (L); above the cut. 

Marlstone, gray. 

Oil shale (D). 

Marlstone. 

Marlstone, gray. 

Oil shale, pebbly, blue-weathering. 

Tuff. 

Oil shale, pebbly, blue-weathering. 

Oil shale (D). 

Do, 

Tuff 16. 

Tuff 14. 

Oil shale (D). 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Tuff 13; 1 top of cut. 

Oil shale (D). 

Do. 

Do. 

Do. 

Do. 

Oil shale (D); base of cut.

Station 1 (200 ft north of USBM cut)

o _____

2_ ______

5 8-

6 5 _

25 

51 

70 

52 

14

56

34 

17

,13

81 

24 

33 

34 

29 

13

9 

20 

19 

12

7

44

17

10 

10 

7

13 

77 

54 

14 

60 

82

15

--

2 

9 

5 

8 

6 

3

-

-

Tuff 13. 1 

Oil shale (M), 

Oil shale (D). 

Do. 

Do. 

Do.

Station 2 (700 ft north of USBM cut)

33.1 . 

33.0 __ 

26.0 

23.1 ______ 2s!o 

19.0 _____ 

13.5 _____ 

6 5 _____ 

50 ____ _ 

4 0

2.0 . 

1.0 _____ 

3______ 

0 _____

2.0 _-_ 

4 8 _______ 

5.8 _____ 

6.8 

10 o _____

18 5 ____ 

21.6 

2i 9 ____ _

27^0 

20 

20

32

6 

26

41 

90 

73

10 

35 

48 

72 

36 

58 

25 

14 

25

11 

4

17 

35 

33 

90 

87 

20 

11 

26 

15 

18

8 

20 

9

19 

23 

7 

12 

16 

12

21 

18 

16 

29 

14 

16 

18 

14 

16 

22 

29 

39 

33 

33 

17 

31 

24 

29 

25 

27 

18 

16 

21 

25 

30 

8

15 

30 

6 

8 

16 

8 

6 

9 

7 

9 

10 

9 

11 

20 

16 

9 

12 

14 

11 

8 

8 

7 

13 

8 

8 

10

14 

84 

6 

12 

42 

8

11 

6 

6 

6 

7 

9 

49 

8 

6 

5 

8 

6 

6 

13 

13 

10 

8 

9 

25

36

56 

47 

52 

55 

52 

45 

63 

14 

21 

18 

10 

2 

53 

28 

11 

6 

18 

16 

33 

45 

30 

34 

35

1 

7

8 

8

1

8 

12

6 

2 

6

1 

7 

6 

8 

7 

11 

6 

8 

7

3 

5 

4 

4 

5 

3 

4 

5 

4 

7 

10

4

3 

1

4 

2

Oil shale (D), blue-weathering. 

Tuff 18. 

Oil shale (M). 

Marlstone, tuffaceous. 

Tuff 17. 

Oil shale (M). 

Oil shale (D). 

Do. 

Do. 

Do. 

Do. 

Do. 

Oil shale (D), papery. 

Tuff 13. 1 

Oil shale (D). 

Do. 

Do. 

Oil shale (M). 

Do. 

Oil shale (D). 

Oil shale (D), blue-weathering. 

Do. 

Do. 

Marlstone, sandy. 

Do. 

Tuff, dark.

Station 3 (2.200 ft north of USBM cut)

[See units 40-70 in table 5 for description]

Station 4 (about 1 mile north of USBM cut)

o o

4 8 ______ 

5 8 _______ 

8 5 _____

45 

10

9 

3

54 

5

4 

8 

27

44 

21 

5 

16 

12 

15 

15

11 

7 

6 

6 

8 

4 

6

20

7

5

53 

90 

39 

54 

62 

37

-

8 

4 

4 

4

8

9 

8 

10 

12

-

-

Tuff 13. 1 

Oil shale (M). 

Oil shale (D). 

Do. 

Oil shale (M). 

Dolomite, green. 

Oil shale (D).
MINERAL COMPOSITION 47

TABLE 10. Mineral composition, expressed by X-ray peak height in chart units, of the dawsonitic oil-shale zone at six localities

along lower Piceance Creek Continued

Distance

above or _ ., , , . 

, , Dawsomte Analcime below

tuff 131

(feet)

Quartz Potassium Albite Dolomite Calcite Illite Pyrite 

feldspar

Remarks

Station 5 (1.5 miles north of USBM cut)

19 35

19

14

16

16

11

19

12

12

32

11

9

9

11

4

8

4

7

11

59

14

33

11

55

36

51

31

4

20

21

10

7

4

3

6

Tuff 13. 1 

Oil shale (M). 

Oil shale (L).

Do. 

Oil shale (M).

Do.

Dolomite, green. 

Oil shale (M).

Do.

irTuff 13 is datum for these stations and is correlated with tuff 49 in the lower Piceance Creek section.

3. North of station 3, the oil shales are not as well exposed as farther south, but the dawsonite zone seems 

to have thinned to less than 10 feet at station 4 and is 

absent at station 5. Northward thinning of the 

dawsonite zone may be the result of deposition in a 

more shoreward environment that was not conducive 

to the formation of dawsonite or to the accumulation 

of material that later could cause its formation. In 

any case, the occurrence of dawsonite seems to cut 

across time-stratigraphic units.

TUFF BEDS

Major constituents of nearly all the exposed tuffs 

are analcime, quartz, potassium feldspar, and albite. 

Less abundant constituents are dolomite, calcite, 

micaceous minerals (including illite), and chloritic 

clay. The mineral composition of 74 tuff beds sampled in this study is shown in tables 4,5, and 6 and is

summarized in table 11. 

Nearly all the samples contain analcime, and all of

them contain quartz. Analcime content increases 

slightly westward from Rio Blanco to the pipeline 

section; it also tends to increase upward in both the 

lower Piceance Creek and the pipeline sections. 

Quartz content does not vary greatly but seems to increase slightly upward in the three sections. Potassium feldspar occurs in 66 of the 74 samples and 

shows a slight decrease upward in both the lower 

Piceance Creek and pipeline sections. Applying the 

X-ray methods of Wright (1968), we determined that 

the structural state of the potassium feldspar ranges 

between maximum microcline and high sanidine, but 

much is low sanidine. Albite also occurs in 66 of the 

74 samples, and a few more samples from beds above 

the Mahogany marker have albite than from below 

the Mahogany marker. Again by applying the 

methods of Wright (1968), we determined that most of 

the sodium feldspar is albite in a low structural 

state that is, albite of low-temperature origin 

which suggests that it has been diagenetically 

altered in the sedimentary environment.

Less than half the tuffs contain dolomite, and only 

about one-third contain calcite. As in the marlstone 

and oil shale, calcite occurs in a broader range and 

commonly has higher values in the tuffs above the 

Mahogany marker than below it. Only about half the 

samples contain micaceous minerals. Dawsonite was 

detected in the tuffs of units 94,52, and 34 in the lower 

Piceance Creek section and in tuffs 13 and 18 (of table 

10) in some other outcrops along lower Piceance 

Creek.

SANDSTONE DIKES

Three gray to light brown sandstone dikes (units 

150, 127, and 47) are exposed in the Rio Blanco section. The dikes are about 4 inches to 1 foot thick, and 

they fill irregularly but steeply dipping fractures in 

the oil shale and marlstone sequence. The sandstone 

is fine grained and consists of analcime, albite, 

quartz, some potassium feldspar, and traces of 

dolomite and calcite, suggesting that the original 

material was tuffaceous. The sharp contacts of the 

dikes with their enclosing rocks suggest that the 

sand filled fractures in already consolidated sediments. The tops and bottoms of the dikes are not exposed. Whether the dikes were formed by filling from 

the top down or by squeezing from the bottom up 

could not be determined.

CAVITIES

Many cavities were found in the rocks below the 

Mahogany ledge in each of the three measured sections of the Parachute Creek Member. The cavities 

range from a few inches to several feet across the 

largest dimension. They presumably once were filled 

with saline minerals, probably mostly nahcolite.

Many other details of structural features in the 

rocks of the Green River Formation were reported by 

Bradley (1929, 1931, 1964).
48 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

TABLE 11. Mineral composition, expressed by X-ray peak height

[A, samples above Mahogany marker;

Stratigraphic 

position and 

number of 

samples

Pipeline section: 

A (7\ - -. - .... ... ...... .. " V '/

B n Y\ _________________ 

Total (24) _______ ~~ 

Lower Piceance Creek section:

A (Q\ -

B (18)- 

Total (91\

Rio Blanco section: 

" A n V- 7\ ........ ...... .... ._.. _ . 1- '/

B (6)- 

Total (23)- ------- 

Summary:

A /QQ\

B (41)-- 

Total (7A\ _ ___ __

Number 

of 

samples

7 

15 

22

9 

18

27

16 

6 

22

32 

39

71

Analcime

Range

rrn Q£

0-86 

0-86

12-94 

5-96 

5-96

0-88 

12-89 

0-89

0-94 

0-96 

0-96

Arithmetic 

average

77 

60 

62

75 

52 

60

56 

63

58

65 

55 

60

Number 

of 

samples

7 

17 

24

9 

18

27

17 

6 

23

33 

41

74

Quartz

Range

16-51 

5-56 

5-56

4-73 

5-90 

4-90

17-80 

4-45 

4-80

4-80 

4-90 

4-90

Potassium feldspar

Arithmetic 

average

34 

24

27

34 

32 

33

34 

24 

31

34 

28 

30

Number 

of 

samples

7 

15 

22

7 

17 

24

16 

4 

20

30 

36 

66

Range

7-43 

0-65 

0-65

0-34 

0-41 

0-41

0-35 

0-18 

0-35

0-43 

0-65 

0-65

Arithmetic 

average

17 

23 

21

14 

18 

17

14 

10 

13

14 

20 

17

SEDIMENTATION AND DIAGENESIS

Sufficient information has been published to establish that in Eocene time the Piceance Creek basin 

contained a large lake of fresh to alkaline-saline 

water that was receiving detrital sediments from the 

surrounding higher regions. Much of this detrital 

sediment probably came from as much as 10,000 feet 

of sedimentary rocks of Cretaceous age which 

covered the region (Haun and Weimer, 1960). Many 

falls of volcanic ash of Tertiary age also contributed 

detrital sediment to the lake; this sediment 

chemically charged the river and lake waters with 

leached metals and silica and created a highly 

alkaline environment.

Vertical and lateral variations in composition of 

the rocks in the lower members of the Green River 

Formation in the Piceance Creek basin reflect 

differences in provenance of the materials delivered 

to the fresh-water lake in the basin during preParachute Creek Green River time. By Parachute 

Creek time, the environment of deposition in the lake 

had changed from fresh to alkaline, and the bottom 

sediments were especially rich in carbonate and 

organic matter. In the late stages of deposition, during Evacuation Creek time, the water freshened 

again, and the sediments became more varied, 

although sand and volcanic materials were most 

abundant.

The great body of laminated carbonate rocks, some 

rich in organic matter, that comprise the Parachute 

Creek Member is most likely the product of sedimentation and diagenesis in the environment of an 

alkaline lake. Field relations described in this report 

suggest that the chemical composition of the lake

water changed from time to time, indicating a lack of 

permanency of chemical stratification. Alkalinity of 

the bottom waters was probably greater than pH 9 

most of the time. This alkaline water was overlain 

intermittently by fresher water. Between these two 

layers, as postulated by Bradley and Eugster (1969, p. 

B23), a zone of mixing of alkaline and fresh water undoubtedly existed at times; this zone allowed for the 

widespread distribution of calcium, magnesium, and 

bicarbonate ions and provided water fresh enough to 

have supported the algal life required to form the oil 

shale.

Large amounts of clay probably were delivered to 

the Piceance Creek basin during Parachute Creek 

time, but only small amounts of illite are detected 

now. Clays of different kinds that were swept into the 

alkaline lake yielded their constituents to form new 

minerals, including feldspar, quartz, analcime, and 

saline minerals. Whether the illite itself is detrital or 

is an alteration product is not known.

According to Alderman and Skinner (1957) and 

Alderman and von der Borch (1961), dolomite can 

precipitate from warm, strong brine. Dolomite commonly forms diagenetically from preexisting calcium-rich carbonate by the incorporation of magnesium during recrystallization, but the extremely 

fine grain of the dolomite in the rocks of the 

Parachute Creek Member strongly suggests that the 

dolomite is not a product of the recrystallization of 

preexisting calcium carbonate. More likely, much of 

the dolomite is a primary precipitate from the 

alkaline lake water.

Calcite in the rocks of the Parachute Creek Member 

also formed by precipitation from the lake water, but
SEDIMENTATION AND DIAGENESIS 49

in chart units, of 74 tuffs above and below the Mahogany marker

B, samples below Mahogany marker]

Number 

of 

samples

6

15

21

9

13

22

17

6

23

32

34

66

Albite

Range

0-60

0-70

0-70

5-83

0-65

0-83

3-39

6-77

3-77

0-83

0-77

0-83

Dolomite

Arithmetic 

average

21

26

24

27

12

17

18

36

23

20

21

21

Number 

of 

samples

3

4

7

2

5

7

11

4

15

16

13

29

Range

0-51

0-74

0-74

0-43

0-65

0-65

0-87

0-84

0-87

0-87

0-84

0-87

Calcite

Number 

of 

samples

5

1

6

4

5

9

7

1

8

16

7

23

Range

0-44

0-14

0-44

0-73

0-30

0-73

0-25

0-4

0-25

0-73

0-30

0-73

IlUte

Number 

of 

samples

4

5

9

2

8

10

13

4

17

19

17

36

Range

0-8

0-7

0-8

0-22

0-19

0-22

0-17

0-39

0-39

0-22

0-39

0-39

Chlorite

Number 

of 

samples

4

4

8

0

0

0

5

0

5

9

4

13

Range

0-9

0-8

0-9

0

0

0

0-9

0

0-9

0-9

0-8

0-9

the water was probably fresher and lower in 

magnesium than that giving rise to the dolomite.

The sample data in tables 4,5, and 6 from the three 

measured sections indicate that dolomite predominates greatly over calcite in rocks below the 

Mahogany ledge, but that, above the Mahogany 

ledge, calcite is more abundant and in some beds predominates over dolomite.

The bicarbonate chemical equilibrium system 

evidently hung in delicate balance but shifted frequently because of changes in such interrelated factors as climate, biological activity, and sedimentation. Interaction of these factors, causing turnover 

and mixing of the water, as well as alteration of conditions of the trophic levels of life and sedimentation, resulted in the accumulation of the thick sequence of laminated rocks of great lateral extent and 

uniform composition in the Parachute Creek 

Member. The changing chemical environments in 

the lake promoted diagenetic alteration of the organic and inorganic fractions of the sediment.

Decay of organic materials mostly algae and 

pollen remains, according to Bradley (1970, p. 986- 

987) produced carbon dioxide in the chemical 

system. The decay, however, was not complete, as indicated by the high carbon to hydrogen ratios in the 

oil shales (Smith, 1969, p. 186). Bacterial inhibitors, 

such as lauric acid, have been reported (Bradley, 

1970, p. 995; Miller, 1972, p. BIO) from Florida in 

modern sediments rich in organic matter. The 

presence of such bacterial inhibitors in a sediment 

would retard the oxidation of the enclosed organic 

matter. If a bacterial inhibitor had been present in 

the Piceance Creek basin, the dead algae accumulating on the bottom of the lake could have been 

preserved after partial decay, despite the alkaline 

(oxidizing) conditions prevailing there. This is not 

the only means of preserving organic matter. The 

organic matter literally could have been pickled at 

the chemocline interface (Rolfe and Brett, 1969, p. 

229). Whatever the mechanism of preservation, the 

organic materials were involved in the diagenetic 

processes active in the basin. These processes likely 

resulted in the formation of various organometallic 

complexes, by extraction of metals from the brine, 

and polymers which evidently increased the stability 

of the organic matter in the environment and reduced its solubility in common organic solvents.

Quartz in these rocks probably has a mixed origin. 

Some is detrital, from both preexisting rocks and 

falls of volcanic ash, and some formed diagenetically 

from the breakdown of other minerals, such as analcime and, especially, the clay minerals.

The presence of sanidine, orthoclase, and microcline suggests that the potassium feldspars are of 

mixed origin volcanic, detrital, and diagenetic. 

Orthoclase and microcline, neither of which is 

especially common, are probably of detrital origin. 

Sanidine is most likely of volcanic origin. Some 

potassium feldspar may have formed diagenetically, 

especially in the deeper parts of the basin. Authigenic 

development of such feldspars would have required 

extraction of potassium from the lake waters.

Plagioclases of different origins and compositions 

were probably originally deposited in the basin, but 

most of them are now albitic. Most of the albite 

studied in detail from both carbonate-rich rocks and 

tuff beds was determined by the methods of Wright
50 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

60

50

c/)

40

cc

I

o

30-

CQ

20^

EXPLANATION

 Dark oil shale 

o Medium oil shale 

A Light oil shale 

x Marlstone

 X

A x 

X

X X O 

 x o

 A 

00 O X 2 A ;

x O A 0»

 XX

x o x o

 X X O X 

 X

 

 A

-X A 

10 20 30 40

ANALCIME, IN CHART UNITS

50 60 70

FIGURE 21. Abundance of albite as a function of abundance of analcime in oil shale and marlstone, Parachute Creek 

Member. Numbers next to symbols indicate number of samples plotted at that position.

(1968) to be in the low structural state, in which the 

silicon and aluminum atoms are well ordered in the 

framework structure. This state is characteristic of a 

low temperature of formation and, therefore, a low 

energy level. The uniform composition of the 

plagioclase suggests that plagioclases of varied compositions and structural states that probably were 

originally deposited in the lake from various sources 

were altered diagenetically in the alkaline environment to a more sodium-rich albite in a lower structural state.

Analcime, which is a common, widespread 

authigenic mineral in the exposed carbonate rocks 

and tuffs of the Parachute Creek Member in the 

Piceance Creek basin, formed at low temperatures in 

moderately alkaline water from clays and zeolitic 

precursors, themselves derived from volcanic glass. 

Conditions fostering the formation of analcime from 

other zeolitic minerals or the conversion of analcime 

to feldspar in environments of higher alkalinity were 

discussed by Hay (1966) and Sheppard and Gude 

(1968, 1969), among others.
SEDIMENTATION AND DIAGENESIS 51

CHART DAWS°NITE 

UNITS 102030405060 70

ANALCIME 

0 10 20 30

QUARTZ 

0 10203040

POTASSIUM 

FELDSPAR ALBITE DOLOMITE CALCITE PYRITE ILLITE

0 1020 0 1020 0 1020304050 0 10 0 10

14.5

14.2

13.2

13.0

10.5

9.3

4.4

0.7-

Base of 

sample

0 10

n

FIGURE 22. Mineral composition of dawsonitic, pyritic oil shale along lower Piceance Creek road, as expressed by X-ray peak

height in chart units.

Zeolitic precursors of analcime have not been 

found in the Piceance Creek basin, but their absence 

may be partly explained by the erosion of shoreward 

deposits around all but the southernmost periphery 

of the basin. Presumably the alkalinity of the water 

in which the preserved beds were saturated was sufficiently high to convert all earlier zeolites to analcime. 

The data in figure 19 show that analcime in 40 of 56 

tuffs that lack dawsonite has a composition (34.5- 

35.1 atoms of silicon per unit cell) similar to that of 

analcime derived from clinoptilolite in tuffs of the

Barstow Formation of California described by Sheppard and Gude (1969, p. 29). The close comparison 

lends weight to the argument that at least some of the 

analcime was derived from clinoptilolite. Where the 

alkalinity increased further (pH>9), some analcime 

reacted to form dawsonite and quartz. Rocks containing both analcime and dawsonite are exposed 

along lower Piceance Creek. Deeper in the interior of 

the basin, all the analcime has been converted under 

highly alkaline conditions to dawsonite and quartz 

or to albite.
52 X-RAY MINERALOGY, PARACHUTE CREEK MEMBER, GREEN RIVER FORMATION, COLORADO

Most of the saline minerals are soluble under conditions of weathering and have been removed from 

rocks in the outcrops, except for the dawsonite, which 

is tightly enclosed in rocks on lower Piceance Creek. 

The presence of many zones of cavities in the three 

measured sections suggests that some saline 

minerals were deposited in pods and disseminated 

crystals near the edges of the basin. Scattered pods 

and crystals of nahcolite, dawsonite, and other saline 

minerals probably formed diagenetically from 

materials in the lake sediment that reacted with 

trapped brine during burial.

In the lower part of the Parachute Creek Member in 

the deeper parts of the basin are beds of nahcolite, 

which contain some dawsonite, and some stratigraphically higher beds of halite, which contain thin 

interlayers of nahcolite and accessory amounts of 

wegscheiderite (Na 2CO3 -3NaHCO3 ), shortite 

(Na2CO3 -2CaCO3), northrupite (Na2CO3-MgCO3-Na 

Cl), searlesite (NaBSi2O6-H2O), and possibly trona 

(Na2CO3-NaHCO3-2H2O) (Kite and Dyni, 1967). 

Nahcolite in the Piceance Creek basin probably precipitated from brines in much the same manner as 

did trona in the Green River basin. Nahcolite is the 

major saline mineral in the Piceance Creek basin, 

however, because the partial pressure of carbon dioxide at the critical temperature of the lake precluded 

precipitation of trona (Bradley and Eugster, 1969, fig. 

2 and p. B46-B58). The beds of halite in the Piceance 

Creek basin also probably precipitated from brine, 

very likely by brine mixing in the manner described 

by Raup (1970). Bradley and Eugster (1969, p. B22- 

B24) discussed the evidence for brine mixing rather 

than evaporation to account for the origin of halite 

and saline minerals in the Green River basin. The 

same reasoning can be applied to the Piceance Creek 

basin.

The possibility that the dawsonite in the nahcolite 

beds in the deeper part of the basin is a primary 

precipitate from alkaline water should not be 

overlooked. Aluminum ions in preexisting aluminosilicate gels and in minerals that were attacked by 

highly alkaline lake water could have been available 

to form dawsonite by direct precipitation from the 

water at the same time as the nahcolite.

The formation of dawsonite by diagenesis and by 

primary precipitation from alkaline lake waters and 

the occurrence of dawsonite at the periphery of the 

basin as well as at depth in the central part suggest 

that greater amounts of dawsonite occur in this basin 

than have previously been recognized. The amount of 

aluminum potentially available in the dawsonite of 

the Piceance Creek basin may be one of this nation's 

great resources of an industrially valuable commodity.

Sulfate minerals did not survive in the lake. According to Jones (1966, p. 196), sulfate is decreased in 

the permanent lacustrine environment by bacterial 

reduction. Sulfur from the sulfate ion becomes fixed 

as pyrite or is lost to the air as hydrogen sulfide rising 

from mudflats at the lake margins. Sulfate minerals 

found in samples from outcrops are oxidation 

products formed by weathering of the rocks.

REFERENCES CITED

Alderman, A. R., and Skinner, H. C. W., 1957, Dolomite sedimentation in the south-east of South Australia: Am. Jour. Sci., 

v. 255, no. 8, p. 561-567.

Alderman, A. R., and von der Borch, C. C., 1961, Occurrence 

of magnesite-dolomite sediments in South Australia: Nature, 

v. 192, no. 4805, p. 861.

Bradley, W. H., 1929, The varves and climate of the Green River 

epoch: U.S. Geol. Survey Prof. Paper 158-E, p. 87-110.

____1931, Origin and microfossils of the oil shale of the Green 

River formation of Colorado and Utah: U.S. Geol. Survey 

Prof. Paper 168, 58 p.

___1964, Geology of the Green River Formation and associated 

Eocene rocks in southwestern Wyoming and adjacent parts 

of Colorado and Utah: U.S. Geol. Survey Prof. Paper 496- 

A, 86 p.

____1970, Green River oil shale Concept of origin extended: 

Geol. Soc. America Bull., v. 81, no. 4, p. 985-1000.

Bradley, W. H., and Eugster, H. P., 1969, Geochemistry and 

paleolimnology of the trona deposits and associated authigenic minerals of the Green River Formation of Wyoming: 

U.S. Geol. Survey Prof. Paper 496-B, 71 p.

Brobst, D. A., ana Tucker, J. D., 1972, Analcime its composition 

and relation to dawsonite in tuff and oil shale in the Green 

River Formation, Piceance Creek basin, Colorado: Geol. Soc. 

America Abs. with Programs, v. 4, no. 6, p. 369-370.

Cashion, W. B., 1967, Geology and fuel resources of the Green 

River Formation, southeastern Uinta Basin, Utah and Colorado: U.S. Geol. Survey Prof. Paper 548, 48 p.

___1969, Geologic map of the Black Cabin Gulch quadrangle, 

Rio Blanco County, Colorado: U.S. Geol. Survey Geol. Quad. 

Map GQ-812.

Culbertson, W. C., 1961, Stratigraphy of the Wilkins Peak Member of the Green River Formation, Firehole Basin quadrangle, Wyoming, in Short papers in the geologic and hydrologic sciences: U.S. Geol. Survey Prof. Paper 424-D, p. D170- 

D173.

___1962, Laney Shale Member and Tower Sandstone Lentil 

of the Green River Formation, Green River area, Wyoming, 

in Short papers in geology and hyrology: U.S. Geol. Survey 

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