A Petrologic Study of th~ 

Fox Hills Sandstone, 

Rock Springs Oplift, Wyomih 
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A Petrologic Study of the 

Fox Hills Sandstone, 

Rock Springs Uplift, Wyoming 

By NELSON L. HICKLING 

A petrographic and stratigraphic study of the Fox Hills Sandstone 

interpreting provenance, depositional environment, and 

diagenesis 

U.S. GEOLOGICAL SURVEY BULLETIN 1919 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive purposes 

only and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

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Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Hickling, N.L. 

A petrologic study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming I 

by Nelson l. Hickling. 

p. em. - (U.S. Geological Survey bulletin ; 1919) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1919 

1. Sandstone-Wyoming-Rock Springs Region. 2. Petrology-WyomingRock Springs Region. 3. Fox Hills Formation. I. Title. II. Series. 

QE75.B9 no. 1919 [QE471.15.S25] 557.3 s-dc 20 [552' .5] 89-600404 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Field Relations 1 

Petrographic Observations and Interpretations 5 

Sandstone Classification and General Characteristics 5 

Dolomite Grains and Cement 6 

Depositional Environments 10 

Provenance 12 

Origin of White-Cap Sandstones 12 

Summary and Conclusions 13 

Acknowledgments 14 

References Cited 14 

FIGURES 

1. Map showing the study area in relation to the Rock Springs Uplift 2 

2. Location map showing the four measured sections in the Fox Hills Sandstone, 

Rock Springs Uplift 3 

3. Cross section showing framework grain size and primary dolomite 

abundance plotted by sample location to show relationship to depositional 

environment 4 

4, 5. Plots of: 

4. Compositional data from 81 thin sections of brown sandstone from the Fox 

Hills Sandstone, Rock Springs Uplift 5 

5. Compositional data from 21 thin sections of white-cap sandstone from the 

Fox Hills Sandstone, Rock Springs Uplift 6 

6. Photomicrographs of detrital dolomite, detrital limestone, secondary dolomite, 

and primary dolomite 9 

7, 8. Energy dispersive X-ray spectra of: 

7. A representative primary dolomite core 10 

8. A representative secondary dolomite rim 11 

9. Sketch of a rounded poly crystalline grain of detrital dolomite showing secondary 

dolomite growing epitaxially from the random crystallographic orientations 

represented on the rounded surface 11 

TABLE 

1. Modal compositions of the Fox Hills Sandstone, Rock Springs Uplift 7 

Contents Ill 
Contents IV 

CONVERSION FACTORS FOR SOME Sl METRIC AND U.S. UNITS OF MEASURE 

For readers who wish to convert measurements from the inch-pound system of units to the metric system 

of units, the conversion factors are listed below: 

To convert from 

Inches (in) 

Feet (ft) 

Miles (mi) 

To 

Centimeters (em) 

Meters (m) 

Kilometers (km) 

Multiply by 

2.54 

0.3048 

1.609 
A Petrologic Study of the Fox Hills Sandstone, 

Rock Springs Uplift, Wyoming 

By Nelson L. Hickling 

Abstract 

New petrographic data provide support for many 

earlier conclusions regarding the depositional history of 

the Fox Hills Sandstone, Rock Springs Uplift, Wyo., and 

also provide the basis for new interpretations. The presence of framework grains of primary dolomite, sedimentary structures, and coarsening-upward grain-size 

sequences supports a shallow-marine, regressive origin of 

the Fox Hills Sandstone. Detrital dolomite and limestone 

(rare) in conjunction with a large fraction of chert indicate 

that the source area provided a large volume of dolomite 

and limestone and was near enough to the depositional 

basin that small quantities of the carbonate rocks survived 

the transport. Moreover, numerous rock fragments consisting largely of shale, siltstone, and sandstone indicate 

that the Fox Hills Sandstone was derived from a sedimentary terrane that was probably in the vicinity of the Sevier 

orogenic belt to the west. 

Secondary dolomite formed a partial to complete rim 

around all primary dolomite grains during diagenesis. This 

study suggests that secondary dolomite grew epitaxially 

with the separate subunits of oriented crystal exposed on 

the surface of the detrital grains and as a result formed an 

irregular rind around the grains that obscures the original 

rounded surface. 

Investigation of the "white-cap" sandstone that lies 

beneath the Hall coal bed and overlies the brown sandstones of the Fox Hills reveals a number of differences in 

this occurrence from those "white-caps" that are thought 

to have resulted from a weathering regime that included organic-acid-charged ground waters. The present 

study concludes that the "white-cap" in the Fox Hills 

Sandstone resulted from its depositional setting, not from 

acid leaching. 

INTRODUCTION 

The Late Cretaceous (Maestrichtian) Fox Hills Sandstone crops out for 40 mi along the east flank of the Rock 

Springs Uplift in southwestern Wyoming (fig. 1). A 5-mi 

segment of this exposure in the Bitter Creek NW quadrangle 

Manuscript approved for publication November 21, 1989. 

was selected for this study, which focuses on the petrography of the Fox Hills Sandstone. 

One hundred and twenty-six oriented sandstone samples were collected at 5-ft intervals along four traverses 

through the Fox Hills Sandstone (figs. 2 and 3). The 

sampling plan was formulated to provide representative 

samples throughout each unit. Only siltstone and sandstone 

beds in the Fox Hills Sandstone were sampled. A marine 

shale tongue of the Lewis Shale was measured but not 

sampled. 

Occurrences of detrital, primary, and secondary dolomite support interpretations regarding provenance, depositional environments, and diagenesis. Of particular interest 

is a 25- to 40-ft-thick "white-cap" sandstone unit at the top 

of the Fox Hills Sandstone. The white sandstone underlies 

the 6- to 8-ft Hall coal bed, which is at the base of the Lance 

Formation. This study shows that these white-cap sands did 

not develop under coal beds because of organic-acid leaching and weathering. An alternative explanation based on 

depositional factors is proposed. 

The numerous published and unpublished studies of 

the Fox Hills Sandstone and adjacent units by Roehler 

( 1961, 1965, 1977, 1983) provided the stratigraphic and 

depositional framework for this petrologic study. Additional publications on the Late Cretaceous rocks of the Rock 

Springs Uplift that are relevant to this study are those by 

Douglas and Blazzard (1961), Pryor (1961), Smith (1961), 

Weimer (1961, 1965), Harms and others (1965), and Hoyt 

and Weimer ( 1965). 

FIELD RELATIONS 

The outcrops of the Fox Hills Sandstone in the study 

area strike about N. 45° W. and dip about 7° NE. The 

exposures typically form ledges and cliffs 140 to 270 ft 

high. At the base of the prominent sandstone outcrops is a 

10- to 50-ft-thick zone of very thin parallel-bedded shale 

and siltstone that represents the lower shoreface environment of the Fox Hills Sandstone. In the covered valley 

below these beds, several hundred feet of Lewis Shale were 

deposited in an offshore marine environment. 

Field Relations 
42°,---------------------------------------~,o~go~------------------------------------------------~ 

...... .. ...... 

0 

0 

5 

5 

. ··.·.·.:: 

10 15 

10 15 20 KILOMETERS 

·.··: ·.: . . .. . ·.· .... 

... . ··.· ·.·.·. ·. 

0. • ••• 

·· .. · .. . ··· ... 

EXPLANATION 

EEl Wasatch Formation 

I ftu I Fort Union Formation 

k\~(}j Lance Formation 

Fox Hills Sandstone 

and Lewis Shale 

~ Almond Formation 

----UTAH------r-l_-----C~~:~~~---------

Figure 1. The study area in relation to the Rock Springs Uplift, Wyo. Map explanation is simplified to indicate only those 

formations in or adjoining the study area. Map is reproduced from the Geologic Map of Wyoming, compiled by Love and 

Christiansen (1985). 

2 A Petrologic Study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 
0 1 MILE 

0 1 KILOMETER 

Figure 2. The four measured sections in the Fox Hills Sandstone, Rock Springs Uplift, Wyo., from which the samples 

studied in this report were collected. These sections are within the Bitter Creek NW quadrangle. 

Field Relations 3 
NORTHWEST 

Section 

FH-4 

Section 

FH-2 

Section 

FH-1 

SOUTHEAST 

Section 

FH-3 f----- 0.9 mi -----1----0.7 mi----+------1.2 mi ----------1 

Primary 

Lance dolomite 

Formation 0 4 8 12 PERCENT 

Fox Hills 

Sandstone 

Lewis 

Shale 

0 !=> !=> 0 o .... i-.J 

~ g ~ g 

3 3 3 3 

3 3 3 3 

Grain size 

Middle 

Shoreface 

Primary 

dolomite 

0 4 8 12 PERCENT 

0 !=> !=> 0 

0 .... .... ;..., (11 0 (11 0 0 0 0 0 

3 3 3 3 

3 3 3 3 

Grain size 

Hall coal bed 

Middle Shoreface 

0 !=> !=> 0 

b .... .... ;..., (11 0 (11 0 0 0 0 0 

3 3 3 3 

3 3 3 3 

Grain size 

EXPLANATION 

Primary dolomite 

--- Grain size 

-15 Sample interval location 

and number 

Primary 

dolomite 

0 !=> !=> 0 .... .... (11 0 (11 

0 0 0 

3 3 3 

3 3 3 

0 

;..., 

0 

0 

3 

3 

Grain size 

en 

a: 

w 1- tu w ~ ~ 

10i30 

5 20 

10 

0 0 

Vertical 

scale 

Figure 3. Framework grain size and primary dolomite abundance plotted by sample location to show relationship to 

depositional environment. 

In the middle part of the Fox Hills Sandstone are 

deposits representing the middle shoreface environment 

(fig. 3). These sandstones are brown, generally massive, 

contain abundant Ophiomorpha, and have local low-angle 

trough crossbeds. A tongue of the marine Lewis Shale is 

exposed in sections FH-1 and FH-2 (fig. 3). It is difficult 

to correlate the marine tongue laterally across the study area 

because parts of sections FH-3 and FH-4 are covered. 

Above the middle shoreface deposits in the Fox Hills 

Sandstone are 20 to 45 ft of upper shoreface sandstone 

deposits that form a white-cap along the outcrop. These 

sandstones are fine grained and have medium-scale herringbone crossbeds typical of the upper shoreface zone of 

deposition. In places, the top 4 ft of the upper shoreface 

deposits are rooted and may represent weathered backbeach 

sand dunes. 

Overlying the Fox Hills Sandstone is the 6- to 

8-ft-thick Hall coal bed of the Lance Formation. The 

coal bed formed in a lagoon behind a barrier island (Roehler, 

1983) or in swamps or marshes related to a drowned coast. 

Although the Hall coal bed is time transgressive along the 

eastern flank of the Rock Springs Uplift, it was used as the 

datum for the four measured sections in the study area. 

4 A Petrologic Study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 
Quartz 

30 

20 

10 

Figure 4. Compositional data from 81 thin sections of brown sandstone from the Fox Hills Sandstone, Rock Springs 

Uplift, Wyo. Rock classification after Folk (1974). 

PETROGRAPHIC OBSERVATIONS AND 

INTERPRETATIONS 

Sandstone Classification and General 

Characteristics 

Modal compositions were calculated for 102 thin 

sections (table 1). The grains forming the framework were 

divided into three basic components: quartz, feldspar (all 

kinds), and rock fragments including chert and dolomite. 

Each of the three components was plotted on a ternary 

diagram. Because of the large number of modal analyses, a 

computer and plotter programmed with TRIANGL (Schacte 

and others, 1986) were used to plot the data according to 

Folk's (1974) classification. Data for the brown sandstones 

are plotted on figure 4, whereas data for the white-cap 

sandstones are plotted on figure 5. Both the brown and the 

white sandstones fall in the litharenite field. As much as 90 

percent of the feldspar is potassium feldspar, of which less 

than 10 percent is microcline. The remaining feldspar is 

well-twinned plagioclase. Of the rock fragments, more than 

80 percent is shale and fine-grained siltstone, and much of 

the remainder consists of phyllite. 

The grains forming the sandstone framework are 

consistently subrounded throughout the sampled part of the 

outcrop belt. The average roundness value is between 3 and 

4 on a chart presented in Pettijohn and others (1973), 

redrawn from Powers (1953, fig. 1). 

The long dimension of 100 grains was measured for 

each sample by use of a calibrated ocular reticule. The 

grain-size measurements for 56 representative samples 

range from coarse silt to fine sand (fig. 3). Grain size 

coarsens upward. Evidence of only modest compaction is 

found in these sandstones. Examples of slender, elongate 

fragments of quartz and feldspar are found both broken and 

unbroken. Many samples contain mica or chlorite flakes 

that have been bent moderately as a result of compaction 

Petrographic Observations and Interpretations 5 
Quartz 

'"T1 

CD (I) 0. c. ~ 

tl) Ul 0 

.lit '0 ~ ~ ;. ... 

co ~ ' CD ~ (J c;· ~· .~ ~ '<0 30 -.1 Q) 

a, 

':l 

20 ~ <§' 

10 

Figure 5. Compositional data from 21 thin sections of white-cap sandstone from the Fox Hills Sandstone, Rock 

Springs Uplift, Wyo. Rock classification after Folk (1974). 

between other framework grains, but no examples of 

severely twisted mica grains were seen. 

The small differences in composition of framework 

grains between the two groups of sandstones cannot account 

for the color difference (table 1; figs. 4 and 5). The matrix 

abundance in the brown sandstones averages 21f2 times that 

in the white-cap sandstones and thus is the probable factor 

controlling the color difference. Microscopic study shows 

that the clay fraction of the matrix in the brown sandstones 

is reddish brown and iron stained, whereas the clay fraction 

in the white sandstones is light gray or white. Representative brown and white sandstones were carefully crushed and 

sieved to separate the matrix from the framework grains. 

The material that passed through a 400-mesh sieve from 

both sandstone types was analyzed quantitatively by use of 

computer-augmented X-ray-diffraction methods. The X-ray 

analysis shows that the white matrix contains no iron and 

the brown matrix contains as much as several percent 

argillaceous hematite. Microscopic examination shows that 

the hematite forms coatings on clumps of clay material 

(kaolinite and illite). Petrographic observation revealed the 

presence of dolomite framework grains in most of the 

brown sandstones and the complete absence of it in the 

white-cap sandstones. 

Dolomite Grains and Cement 

Three significantly different occurrences of dolomite 

are found in the Fox Hills Sandstone. These dolomite types 

have been recognized and written about since the early 

1960's by Sabins ( 1960, 1962, 1964, 1965) and Sabins and 

Petersen ( 1961). Sabins' ( 1964) initial work on these 

occurrences resulted from a definitive study of the Cretaceous rocks of the San Juan Basin. Sabins (1965) subsequently documented the occurrence of these dolomite types 

in several other Cretaceous sandstones of the Rocky Mountain region, including the Fox Hills Sandstone in Wyoming. 

6 A Petrologic Study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 
Table 1. Modal compositions of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 

[All values are in percent; following the sample number, a "w" stands for white-cap and a "b" for brown sandstone] 

Sample no. 

FH-1-28w ..... . 

FH-1-27w ..... . 

FH-1-26w ..... . 

FH-1-24w ..... . 

FH-1-23w ..... . 

FH-1-22w ..... . 

FH-1-21w ..... . 

FH-1-19b ..... . 

FH-1-18b ..... . 

FH-1-17b ..... . 

FH-1-16b ..... . 

FH-1-14b ..... . 

FH-1-13b ..... . 

FH-l-12b ..... . 

FH-1-llb ..... . 

FH-1-9b ...... . 

FH-1-8b ...... . 

FH-1-7b ...... . 

FH-1-6b ...... . 

FH-1-4b ...... . 

FH-1-3b ..... .. 

FH-1-2b ...... . 

FH-1-1b ...... . 

FH-2-44w ..... . 

FH-2-43w ..... . 

FH-2-42w ..... . 

FH-2-4lw ..... . 

FH-2-39w ..... . 

FH-2-38w ..... . 

FH-2-37b ..... . 

FH-2-36b ..... . 

FH-2-34b ..... . 

FH-2-33b ..... . 

FH-2-32b ..... . 

FH-2-31b ..... . 

FH-2-29b ..... . 

FH-2-28b ..... . 

FH-2-27b ..... . 

FH-2-26b ..... . 

FH-2-24b ..... . 

FH-2-23b ..... . 

FH-2-22b ..... . 

FH-2-21b ..... . 

FH-2-19b ..... . 

FH-2-18b ..... . 

FH-2-17b ..... . 

FH-2-16b ..... . 

FH-2-14b ..... . 

FH-2-13b ..... . 

FH-2-12b ..... . 

FH-2-11b ..... . 

Quartz 

69.6 

56.1 

60.8 

70.8 

71.3 

58.5 

57.4 

49.8 

61.9 

60.2 

62.3 

47.5 

63.4 

64.3 

53.9 

65.4 

62.9 

55.8 

46.7 

54.1 

49.3 

61.8 

58.6 

74.9 

63.2 

68.5 

54.9 

Chert 

13.3 

20.3 

22.3 

16.9 

11.0 

18.5 

15.3 

14.4 

15.9 

21.8 

24.4 

30.3 

20.8 

23.4 

29.2 

18.3 

24.1 

26.1 

39.7 

18.1 

17.4 

18.5 

23.1 

18.0 

15.0 

14.7 

21.4 

70.0 12.8 

71.4 14.2 

69.4 14.7 

68.4 9.8 

61.9 11.8 

62.5 11.6 

61.6 11.1 

63.2 12.1 

58.1 13.0 

62.2 11.8 

50.0 9.8 

63.0 10.6 

54.2 7.8 

61.1 7.9 

56.1 ' 10.8 

59.4 9.3 

58.7 13.5 

62.4 10.5 

60.7 12.2 

61.5 12.6 

59.4 10.1 

62.5 14.1 

62.9 11.0 

59.1 17.6 

Feldspar 

6.6 

7.6 

7.6 

4.0 

4.0 

4.9 

6.6 

5.3 

3.4 

6.4 

5.0 

7.1 

3.8 

6.2 

13.2 

11.2 

5.9 

8.8 

4.0 

6.4 

9.6 

8.7 

6.8 

2.6 

4.8 

3.0 

5.1 

4.3 

5.5 

5.8 

5.4 

6.4 

7.9 

5.3 

6.3 

7.1 

3.6 

8.8 

7.4 

8.7 

5.1 

6.3 

9.0 

10.0 

7.7 

6.0 

5.6 

9.1 

7.3 

8.5 

6.8 

Rock 

fragments 

10.5 

15.9 

9.3 

8.2 

13.6 

18.2 

20.7 

30.5 

18.8 

11.3 

8.3 

15.0 

10.4 

6.2 

1.4 

1.1 

2.4 

3.3 

3.7 

12.4 

2.2 

1.4 

1.0 

4.6 

17.0 

13.7 

18.7 

12.9 

8.8 

10.1 

16.2 

18.3 

18.0 

19.7 

16.3 

18.6 

20.8 

22.1 

14.9 

19.3 

18.2 

19.6 

16.0 

15. 

16.0 

16.6 

11.4 

17.3 

15.7 

16.0 

16.5 

Detrital 

dolomite 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

.2 

0 

2.2 

.6 

2.7 

7.4 

4.7 

2.0 

0 

0 

0 

0 

0 

0 

0 

.2 

.5 

0 

1.5 

1.0 

1.2 

.6 

1.0 

.7 

1.9 

2.3 

.7 

1.8 

.9 

.9 

1.4 

1.7 

.8 

.2 

.8 

0 

Primary 

dolomite 

0 

0 

0 

0 

0 

0 

0 

0 

0 

.1 

.2 

0 

1.6 

0 

1.5 

4.1 

4.7 

3.5 

5.2 

6.2 

14.0 

4.7 

7.5 

0 

0 

0 

0 

0 

0 

0 

.2 

1.1 

0 

.9 

1.0 

1.9 

1.0 

8.3 

2.8 

8.1 

5.5 

6.4 

4.5 

2.0 

2.5 

3.2 

7.2 

3.2 

0 

. 3 

. 8 

Sample no. 

FH-2-9b ...... . 

FH-2-8b ...... . 

FH-2-7b ...... . 

FH-2-6b ...... . 

FH-2-4b ...... . 

FH-2-3b ...... . 

FH-2-2b ...... . 

FH-2-lb ...... . 

FH-2-1b(b) .... . 

FH-2-1a(b) .... . 

FH-3-22w ..... . 

FH-3-21w ..... . 

FH-3-19w ..... . 

FH-3-18w ..... . 

FH-3-17b ..... . 

FH-3-16b ..... . 

FH-3-14b ..... . 

FH-3-13b ..... . 

FH-3-11b ..... . 

FH-3-9b ...... . 

FH-3-8b ...... . 

FH-3-7b ...... . 

FH-3-6b ...... . 

FH-3-4b ...... . 

FH-3-3b ...... . 

FH-3-2b ...... . 

FH-3-1b ...... . 

FH-4-29w ..... . 

FH-4-28w ..... . 

FH-4-27w ..... . 

FH-4-26w ..... . 

FH-4-24b ..... . 

FH-4-23b ..... . 

FH-4-22b ..... . 

FH-4-21b ..... . 

FH-4-19b ..... . 

FH-4-18b ..... . 

FH-4-17b ..... . 

FH-4-16b ..... . 

FH-4-14b ..... . 

FH-4-13b ..... . 

FH-4-12b ..... . 

FH-4-11b ..... . 

FH-4-9b ...... . 

FH-4-8b ...... . 

FH-4-7b ...... . 

FH-4-6b ...... . 

FH-4-4b ...... . 

FH-4-3b ...... . 

FH-4-2b ...... . 

FH-4-1b ..... .. 

Quartz 

57.7 

59.9 

55.6 

65.9 

62.5 

68.3 

64.0 

59.3 

59.6 

56.5 

71.3 

65.1 

56.0 

62.9 

69.1 

73.7 

69.9 

62.4 

52.3 

60.6 

64.9 

65.8 

57.7 

61.0 

63.1 

65.4 

59.0 

72.1 

69.3 

67.4 

65.7 

55.4 

58.5 

60.3 

65.5 

60.9 

59.1 

60.5 

56.1 

55.5 

59.6 

60.7 

60.2 

55.3 

55.6 

54.4 

56.9 

52.7 

53.1 

56.5 

58.8 

Chert 

18.5 

13.5 

17.5 

13.9 

17.4 

12.9 

19.0 

16.4 

25.3 

24.0 

11.9 

14.5 

19.2 

16.1 

17.1 

12.2 

9.4 

16.1 

19.0 

15.4 

15.6 

15.9 

25.0 

14.1 

14.8 

13.8 

14.8 

7.5 

12.9 

12.0 

11.8 

10.2 

11.6 

14.7 

14.0 

13.9 

15.9 

14.2 

16.0 

11.1 

16.1 

13.3 

11.1 

9.6 

9.6 

10.5 

13.6 

17.4 

13.7 

9.1 

14.5 

Feldspar 

9.4 

8.4 

8.8 

5.0 

9.7 

8.5 

9.4 

8.8 

2.4 

9.4 

3.1 

5.7 

6.9 

7.5 

3.5 

5.7 

6.4 

4.4 

1.0 

1.8 

2.3 

3.7 

2.9 

3.2 

2.8 

3.7 

4.2 

3.2 

4.5 

5.3 

4.6 

9.1 

5.7 

7.3 

6.6 

4.3 

8.0 

7.1 

8.5 

7.7 

6.6 

7.1 

7.5 

11.0 

6.5 

6.1 

6.3 

6.1 

5.6 

8.7 

5.4 

Rock 

fragments 

14.0 

17.5 

16.3 

13.8 

10.2 

9.4 

7.4 

13.6 

10.8 

8.3 

13.7 

14.8 

17.8 

13.5 

10.4 

7.5 

14.3 

13.7 

10.3 

14.4 

6.3 

7.7 

5.8 

9.8 

11.1 

8.9 

5.1 

17.2 

13.3 

15.3 

17.9 

9.7 

19.9 

14.5 

10.5 

16.9 

12.3 

15.3 

16.1 

13.3 

11.5 

8.2 

10.1 

15.6 

20.4 

21.5 

11.4 

9.2 

14.8 

12.0 

9.6 

Detrital 

dolomite 

0.2 

.3 

.5 

.3 

.1 

.2 

.1 

.6 

1.0 

.9 

0 

0 

0 

0 

0 

.7 

0 

2.1 

8.7 

3.9 

3.2 

2.8 

3.4 

4.7 

3.1 

3.3 

5.1 

0 

0 

0 

0 

6.2 

1.3 

1.0 

1.1 

1.6 

1.9 

.3 

1.3 

7.5 

3.1 

5.4 

2.2 

1.7 

4.0 

4.5 

7.1 

7.3 

7.7 

6.8 

4.7 

Primary 

dolomite 

0.3 

.3 

1.3 

1.0 

.1 

.6 

.1 

1.3 

1.0 

.9 

0 

0 

0 

0 

0 

.2 

0 

1.4 

8.7 

3.9 

7.6 

4.1 

5.3 

7.2 

5.0 

4.9 

11.9 

0 

0 

0 

0 

9.4 

2.9 

2.2 

2.3 

2.4 

2.8 

2.6 

2.0 

5.0 

3.1 

5.4 

8.9 

6.7 

4.0 

3.0 

4.8 

7.3 

5.1 

6.8 

6.9 

Petrographic Observations and Interpretations 7 
According to Sabins (1962), the dolomite types noted in this 

study and their petrologic significance are as follows. 

1. Detrital-Clastic fragments eroded from dolomite 

source rocks. Usually well-rounded polycrystalline 

aggregate that is the relict fabric inherited from the 

source rock. 

2. Secondary-Formed after deposition of the enclosing 

sediment. Usually small, euhedral rhombs that replaced 

calcite cement in lenticular marine sandstones. 

3. Primary-Formed within the depositional basin prior tQ 

final settling down and burial of the sediment. These are 

single rhombic crystals abraded to various degrees of 

roundness and sorted to the size of associated clastic 

sand grains. They are always charged with uniformly 

distributed small, unidentifiable, nonzoned inclusions 

that impart a "dusty" appearance. A detrital origin is 

eliminated because inherited textures are absent and 

because the pattern of distribution is restricted to marine 

sandstones. 

Some petrologists may initially question Sabins' 

(1962) definition of primary dolomite. This type of dolomite is described as having been moved and abraded by 

current action before it settled and was buried by other 

sediment. Although primary and detrital dolomite have a 

similar definition, they differ in that primary dolomite has 

not been derived from an extrabasinal source. 

The brown sandstones average 2.2 percent detrital 

dolomite and 3.7 percent primary dolomite (table 1). 

Dolomite is absent in the white sandstone (table 1; fig. 3). 

Figure 6D shows a detrital dolomite grain under crossed 

nicols so that its poly crystalline texture can be seen. The 

grain is rounded from transport, and its relict texture is 

inherited from the dolomite in the source terrane. Continuous rims of secondary dolomite have not been noted on the 

detrital dolomite grains. This example is typical of the other 

detrital grains in the Fox Hills Sandstone and looks identical 

to those detrital dolomites shown and discussed by Sabins 

(1962). 

Sabins (1962) described rhombic-shaped primary 

dolomite grains with various degrees of rounding that were 

poikilitically enclosed with other framework grains in 

optically continuous patches of calcite cement. The optical 

orientation of each primary dolomite grain is independent 

from that of the calcite cement. Sabins (1962) mentioned 

further that primary dolomite rhombs are found in rocks that 

have no calcite cement but did not discuss that type of 

occurrence in depth. Little calcite cement was found in this 

study of the Fox Hills Sandstone. In contrast to Sabins' 

(1962) observations, the primary dolomite in these rocks is 

always rimmed with a later generation of dolomite. The rim 

is thick and continuous around the primary dolomite core 

except where the core is in contact with other framework 

grains. 

Figure 6A-C shows numerous primary dolomite 

grains from the Fox Hills Sandstone photographed under 

crossed nicols. Figure 6A illustrates a rhombic-shaped core 

that has irregularities along its straight edges that have 

resulted from local abrasion before burial. The core displays 

the dusty appearance that results from being charged with 

uniformly distributed inclusions. In contrast, the secondary 

dolomite rim is much more optically clear. The rim and core 

are in optical continuity. The continuity of the rim except 

where other grains make contact with the primary core 

suggests that the rims were formed after deposition, not 

before. Figure 6C, photographed with crossed nicols, 

shows two primary dolomite grains rimmed by secondary 

dolomite. The stage has been turned until one rhombicshaped core/rim pair is nearly at extinction and the other 

core/rim pair is not. The optical continuity of each rim with 

its own core is shown. Because of this optical continuity of 

each pair, a boundary is observed midway between the 

cores that marks the juxtaposition of the secondary dolomite 

rims. Where many primary dolomite grains of random 

optical orientation are scattered and the quantity of secondary dolomite is sufficient to enclose them all, the patch of 

dolomite cement is not optically continuous. In contrast to 

optically continuous patches of calcite poikilitically enclosing dolomite and other framework grains (Sabins, 1962), 

the orientation of the individual primary dolomite cores 

(fig. 6) controls the optical orientation of the adjacent 

secondary dolomite growths. "Domains" of secondary dolomite having similar crystallographic orientations surround 

each core, and these domains extend outward from the cores 

to where they share perhaps several common boundaries 

with domains related to several nearby cores. Therefore, the 

dolomite cement in the Fox Hills Sandstones is a composite 

of domains of optically oriented cement controlled by the 

nearest primary dolomite core. 

The cores in figure 6C show more rounding than the 

core in figure 6A, though some straight edges are left. 

Figure 6B shows four primary dolomite cores that have 

optically clear dolomite rims. This picture, taken with 

crossed nicols, shows the variable degree of rounding of the 

cores before they were rimmed by secondary dolomite even 

within the same thin section. The grain on the far left has 

several rhombic-shaped sides, whereas the grain below it is 

well rounded. The pair on the right side of the photograph 

shows one nearly rhombic-shaped grain and one rounded 

grain. The degree of roundness of the primary dolomite 

illustrates that the time span between primary dolomite 

rhomb crystallization and actual burial may be quite variable. Some primary grains obviously were abraded more 

vigorously and (or) for a longer time than others buried 

contemporaneously. 

Figure 6E shows a primary dolomite core that has a 

secondary dolomite rim. In addition, a thin rim of calcite 

(see arrow) surrounds the thicker dolomite rim. The calcite 

rim was revealed when the section was stained with alizarin 

red, which stains calcite red and leaves dolomite un8 A Petrologic Study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 
Figure 6. Photomicrographs of detrital dolomite, detrital 

limestone, secondary dolomite, and primary dolomite. All 

taken with crossed polarizers. A, A large, rhombicshaped, "dusty" primary dolomite core that has a rim of 

more optically clear secondary dolomite. The rhombic 

core has been lightly abraded, as evidenced by "roughness" along the straight edges. The rim is absent where 

the core was impinged upon by other framework grains. 

B, Four examples of primary dolomite cores rimmed by 

secondary dolomite. Of the pair on the right, one core has 

several straight edges and the other is rounded. On the 

left is a rhombic core, and below it is a rounded one. This 

range in abrasion illustrates that the time between primary 

dolomite crystallization and its burial by other sediments 

is quite variable even within the span of a single thin 

affected. Additional examples of secondary calcite are rare. 

Local dissolution of detrital limestone grains followed by 

recrystallization of the calcite about a nearby dolomite grain 

may explain these local occurrences. 

The secondary dolomite in this study is of a different 

occurrence from that presented by Sabins ( 1962). Sabins 

(1962) described secondary dolomite grains as being very 

small (:s-;0.08 mm) and having perfect rhombic shapes; he 

found them only in calcite-cemented sandstones. Sabins 

section. C, Two core/rim pairs. The stage is turned so the 

set on the left is nearly at extinction. Each rim is in optical 

continuity with its own core. The two rims share a common boundary midway between the two cores that is 

marked by the change in birefringence. D, A detrital 

dolomite grain recognized by its polycrystallinity inherited 

from source rock. f, A rounded primary dolomite core 

rimmed by rhombic-shaped secondary dolomite, which is 

in turn rimmed by a very thin layer of calcite (arrow). 

Other examples of calcite rims exist but are uncommon. F, 

A large grain of detrital limestone as revealed by alizarin 

red stain. Note the inherited polycrystalline rock fabric. 

Detrital limestone grains are very uncommon in the Fox 

Hills Sandstone. 

(1962, p. 1186) stated "within the calcite cement are 

scattered the dolomite rhombs ... which originated through 

replacement of calcite cement, rather than by precipitation 

from solution in unfilled pore spaces." As previously 

discussed, the secondary dolomite in this study forms rims 

about primary cores and fills or partially fills the pore 

spaces in the vicinity of the primary grain. Where enough 

space exists, the rim usually takes on a rhombic external 

form even though the core may be rounded (fig. 6B, C). 

Petrographic Observations and Interpretations 9 
t·~1 

G I 

Figure 7. Energy dispersive X-ray spectrum of a representative primary dolomite core. Peak identification by Tracor 

Northern program. Scanning electron microscope is a jeol JSM-840 operated at 15 kV. 

Three individual primary dolomite cores and associated secondary rims were analyzed by energy dispersive 

X-ray analysis. A representative analysis for core material 

is shown in figure 7, and the analysis for rim material is 

shown in figure 8. The analysis of both types of dolomite 

confirms the petrographic identification. In addition, the 

rims characteristically contain a small amount of iron. That 

the cores are found to contain small amounts of silicon and 

aluminum suggests that their dusty appearance may be 

caused by cryptocrystalline silicate inclusions. 

Figure 6F shows a large grain of detrital polycrystalline calcite or limestone. The photograph is taken with a 

x 25 objective under crossed nicols so that the relict texture 

of the limestone can be seen clearly. Several slides contain 

detrital limestone grains revealed by alizarin red, but they 

are very rare compared with detrital dolomite grains. 

The reason detrital dolomite grains do not seem to be 

preferred as "seeds" for the crystallization of secondary 

dolomite rims may be related to their relict polycrystalline 

texture. It was shown earlier that secondary rims around 

primary dolomite (single crystal) cores always share the 

same crystallographic orientation. Within a polycrystalline 

core, which by definition is a composite of subunits having 

random crystallographic orientation, there is no dominant 

crystallographic orientation that would allow one optically 

continuous rim to form around it. 

Some petrographic evidence indicates, though it is 

not entirely clear because of the difficulty of seeing such 

small grain sizes, that small secondary dolomite grains have 

grown epitaxially with some of the minute crystallographic 

units at the surface of t~ese polycrystalline grains. Figure 9 

is a sketch of what thi~: writer proposes as the manner in 

which the detrital grains have been affected by growth of 

secondary dolomite. The rim adds a crudely "saw-toothed" 

rind around what was originally a rather well-rounded 

detrital grain. Each saw tooth segment with its own crystallographic direction has oriented with that of the surface 

subunit of the detrital dolomite grain to which it has grown 

epitaxially. 

DEPOSITIONAL ENVIRONMENTS 

Grain-size measurements on 52 thin sections document the coarsening-upward grain-size sequence expected 

during the deposition of a shoreline sandstone in a regressive sea (Roehler, 1983; fig. 3). The abundance of primary 

dolomite found in the thin sections of the brown sandstones 

that represent the lower and middle shoreface environmen1 

is in agreement with Sabins (1962), who stated "primary 

dolomite grains originated within the marine depositional 

basin." That the sediments containing primary dolomite 

were deposited into a marine basin is borne out by the 

presence of their marine fauna, the lithology, and the 

sedimentary structures. 

The process of dolomite formation is not as clear as is 

its environment of formation. Alderman and Skinner 

10 A Petrologic Study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 
-· ·-· . . . r... L·"·-·'L ._. t -L. • '-- r .. ..1.; , pEl .,. +.JLFIH. "1-'·r -T·:r~ 1. ·+·iT Ti- -,-;--,,T~l...------------------·-·· 

Figure 8. Energy dispersive X-ray spectrum of a representative secondary dolomite rim. Peak identification by Tracor 

Northern program. Scanning electron microscope is a jeol JSM-840 operated at 15 kV. 

Rounded detrital 

Optical continuity 

between A and A' 

Figure 9. A rounded polycrystalline grain of detrital dolomite showing secondary dolomite growing epitaxially 

from the random crystallographic orientations represented on the rounded surface. Shading mimics birefringence under crossed nichols. About X400 normal grain 

size. 

(1957), who reported on modern dolomite precipitates from 

intermittent lakes and shallow lagoons in South Australia, 

suggest that the association of shallow sea water and the 

vigorous plant growth in the lakes and lagoons were 

important factors in the genesis of the primary dolomite. 

Others have suggested broad tidal flats as an environment 

suitable for dolomite formation because of the high salinities that result from the high ratio of evaporation to volume 

of water as compared with deeper ocean environments. The 

following is a discussion from Gadd (1986). 

Ordinary sea water is full of dissolved magnesiumsaturated, in chemical terms-but dolomite won't 

form directly from sea water because the magnesium 

ion does not readily give up the water molecules 

attached to it and enter a crystal structure. For this 

reason no one has observed dolomite crystals forming in sea water at normal temperatures. However, 

when calcite is available, or its organically produced 

equivalent, aragonite, and water is evaporating from 

sea water that can't easily circulate with the rest of the 

ocean (as in a restricted lagoon), then the magnesium 

ion loses its water molecules and slips into the calcite 

crystal lattice, bumping calcium ions out .... 

All of the conditions discussed here that are thought 

to lead to dolomite formation- hypersalinity, vigorous 

plant growth, restricted circulation of seawater, availability 

of calcite or aragonite-could have been met in this setting, 

but the particular combination is not known. Detrital or 

primary dolomite is absent from the white-cap sandstone 

because the sands were deposited into a high-energy upper 

shoreface environment, and in such an environment brittle, 

Depositional Environments 11 
chemically unstable materials like dolomite are quickly 

ground away. 

PROVENANCE 

The modal compositions (table 1, figs. 4 and 5) of the 

Fox Hills Sandstone permit the following interpretations 

regarding provenance. 

Quartz. -61 percent average. These grains exhibit 

only slight undulose extinction, meaning they have not been 

affected by medium- to high-grade metamorphism. The 

source terrane probably consists of sedimentary rocks rather 

than plutonic or high-grade metamorphic rocks. 

Feldspar. -6 percent average. About 10 percent of 

the total feldspar is well-twinned plagioclase; the rest is 

potassium feldspar. Several grains of "gridiron" twinned 

microcline are present, but they are uncommon in these 

rocks. Microcline is thought to be an indicator of a plutonic 

source, and its sparsity in these rocks probably means that 

it has been reworked from older sediments. The average 

feldspar content of the Fox Hills Sandstone is about 3 

percent higher than that of the Late Cretaceous Mesaverde 

sandstones of Wyoming (Pryor, 1961). The Granite Mountains along the Sweetwater Arch east of the Wind River 

range may have been breached at the time so that a large 

amount of feldspar was supplied, some of which may have 

been carried into the Rock Springs area by longshore 

currents. 

Lithic fragments (other than chert). -12 percent 

average. The fragments consist mostly of shale, siltstone, 

and phyllite. Most of these framework grains were derived 

from clastic sedimentary rocks. 

Detrital dolomite and limestone. -2.2 percent average. Dolomite grains are much more abundant than limestone grains. These grains indicate that the source terrane 

for the Fox Hills Sandstone contained dolomite and limestone. 

Chert. -15 percent average. The chert in the Fox 

Hills Sandstone probably was associated with carbonate 

rocks in the source terrane. Chert can form from silicious 

ooze during various stages of carbonate rock formation and 

thus is a common, though usually minor, constituent in 

carbonate rocks. Because chert is commonly a minor 

constituent in carbonate rocks, a large volume ofcarbonate 

rock must have been eroded to have supplied the large 

amounts of chert found in the Fox Hills Sandstone. Most of 

the carbonate minerals were lost by abrasion and solution 

during transport to the site of deposition. Because chert is 

much harder and more chemically resistant than the carbonate minerals, a much higher percentage of it survived 

transport. Thus, chert grains are much more common in the 

Fox Hills Sandstone than are dolomite and limestone. 

Dissolved carbonate and detrital carbonate minerals entering the marine basin may supply some of the necessary ions 

that affect the formation of the primary dolomite. 

The modal composition of the Fox Hills Sandstone 

indicates that most of its detrital material was derived from 

a sedimentary terrane composed of shale, siltstone, sandstone, limestone, and dolomite. The sediments were transported generally eastward to the present-day area of the 

Rock Springs Uplift along the western margin of the 

western interior Cretaceous seaway, where they were incorporated into the eastward-prograding shoreline of the Fox 

Hills Sandstone (Weimer, 1961; Gill and Cobban, 1973; 

Roehler, 1983). Sediment could also have been contributed 

to the Fox Hills Sandstone by southerly directed longshore 

drift. 

Additional criteria that must be met in regard to the 

provenance of the Fox Hills Sandstone are that the source 

rock must have been (1) older than the Fox Hills, (2) 

elevated high enough during the Late Cretaceous for an 

erosional gradient to exist to the east, (3) able to supply a 

large volume of chert, particularly from the Phosphria 

Formation, and (4) near enough to the depositional basin so 

that at least some dolomite and limestone could have 

survived the transport. The Sevier orogenic belt to the west 

meets all these criteria and is the most logical choice as a 

source of the Fox Hills Sandstone in the Rock Springs 

Uplift, but some feldspar may have come in from the 

Granite Mountains to the north, as a result of longshore 

currents. 

ORIGIN OF WHITE-CAP SANDSTONES 

The white-cap sandstones found in many Upper 

Cretaceous sandstones of the Rocky Mountain region could 

be the product of leaching by organic acids derived from 

overlying organic-rich material. A peat bog, a coal bed, or 

even a dense, mature forest floor could serve as the source 

for organic acids. In a complex series of reactions in a 

reducing environment, organic-acid-charged waters could 

percolate through and react with the sediments below the 

organic zone. Under these conditions, iron minerals would 

combine with the organic acids and lead to color changes 

and removal of iron. The rate of this procedure would be 

affected by the porosity and permeability ofthe sandstone. 

This "leaching" and removal of iron thus becomes a 

"bleaching" process whereby iron loses its role in pigmentation. The bleaching process may continue as long as the 

pH level remains below 7.0 and the redox potential is in a 

reducing range. The process will cease when those conditions change at the water table or when oxygenated ground 

water is introduced from a different source. 

This study finds that the process of organic-acid 

bleaching is insufficient to account for the white-cap sandstones of the Rock Springs Uplift. Roehler (1983) concluded that the white-cap sandstones were deposited in the 

upper shoreface environment. In that high-energy environment, clay and other matrix-sized materials could be kept in 

12 A Petrologic Study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 
suspension longer, transported farther from shore, and 

deposited in the quieter waters in the middle and lower 

shoreface zones. The process need not be 100 percent 

efficient, and though the largest part of the fine material was 

deposited farther out to sea, significant amounts of matrix 

were deposited in the upper shoreface as well. As noted 

previously, the brown sandstones contain more than twice 

as much matrix as the white-cap sandstones. 

The brown sandstones average 6 percent dolomite 

(total detrital plus primary), an amount that makes dolomite 

the most probable source for iron pigmentation in these 

rocks. The absence of dolomite framework grains in the 

white-cap sandstones can be explained by the same mechanism as the one that explains the distribution of the matrix. 

Primary dolomite grains may form in quieter water beyond 

the surf zone and consequently be absent in the upper 

shoreface, as any detrital or primary dolomite that might be 

carried there would be ground away. 

If organic-acid bleaching were the cause of the 

white-cap sandstone in the Fox Hills, then why does the 

zone of leaching stop abruptly at the juncture between 

middle and upper shoreface deposits? Perhaps the color 

break marks a change in the redox and pH regimes such as 

would be caused by a paleowater table, but the color 

break's consistent coincidence with the depositional break 

is troublesome. Even more troublesome is the fact that in 

the Rock Springs Uplift are at least two other examples of 

white-cap sandstones in formations of different ages, both 

of which underlie a rich organic zone and reside solely in 

upper shoreface deposits (Almond Formation and McCourt 

Tongue of the Rock Springs Formation; Roehler, 1988, 

1989). An additional difficulty with this proposed cause is 

that the white-caps in the Rock Springs Uplift are 20 to 40 

ft thick, and if their base represents a paleowater table, a 

swamp or peat bog could not have been perched so high 

above. One would expect the water table to be about 

coincident with the water level in the bog. 

In white-caps in which acid bleaching has definitely 

taken place, the zone of bleached sand is commonly 3ft or 

less thick, and the intense weathering under these acid 

conditions results in the destruction of primary sedimentary 

structures. The white sandstones of the Rock Springs Uplift 

are replete with packages of thin herringbone crossbeds in 

sharp definition. 

An additional result of intensive weathering under 

acidic conditions is a breakdown or alteration of clay 

minerals. The crystallinity of the clays in the white, highly 

weathered matrix should be diminished compared with the 

clays undergoing milder weathering in the brown matrix. 

Loss of crystallinity would appear on an X-ray-diffraction 

trace as broader but shorter peak heights and would show 

more background "hash" and perhaps the complete loss of 

some of the larger 28 peaks. In addressing this, Frank 

Dulong (personal commun., 1989) stated "I am unable to 

determine that there is a quantitative difference in the 

crystallinity of the kaolinite and (or) illite between the two 

samples [white and brown matrix]." 

The even distribution of the brown color of the Fox 

Hills is in contrast to a typical example from an acidic 

bleaching regime. As iron is removed by the organic acids 

and carried downward through the sandstone, it eventually 

reaches that layer or zone of oxidation where it is redeposited, often as an iron-rich hardpan. Below the initial 

concentration of the iron, the brown color is streaky and 

variable in intensity. In the Rock Springs Uplift, the color 

break is sharp and even, takes place within a vertical 

distance of 3 in or less, and continues for miles without 

deviation from the interface between middle shoreface and 

upper shoreface deposits. No hardpans or other concentrations of iron occur at the interface, and the brown color is 

uniform throughout the brown sandstone. Undoubtedly, 

many sandstones have been bleached under organic-acidrich weathering conditions, but this does not seem to be the 

case for the white-cap sandstones from the Rock Springs 

Uplift. The evidence consistently indicates that the whitecap sandstones in the Rock Springs Uplift have been 

controlled by their depositional setting rather than by 

leaching. 

SUMMARY AND CONCLUSIONS 

A detailed petrographic study of the Fox Hills Sandstone has revealed significant data regarding the depositional and diagenetic history. Much of the new information 

confirms or supplements published interpretations based on 

stratigraphic, sedimentologic, and paleontologic studies. 

Information on the occurrence and abundance of chert 

and detrital dolomite, obtained from a large number of 

samples, indicates that the source area must have had a 

major carbonate-rock component. The abundant sedimentary rock fragments found as framework grains have now 

been shown by more than 100 modal analyses to be 

characteristic of the Fox Hills; thus, the existence of a 

source rock that provided this material throughout its 

deposition is necessitated. The presence and the abundance 

of primary dolomite and the coarsening upward of framework grain size support the previous conclusion that the Fox 

Hills Sandstone represents deposition during a regressive 

sea. 

The dolomite types described here fall into the three 

categories defined by Sabins (1962), which have some 

unique variations in the Fox Hills Sandstone. Variations 

include the rimming of primary dolomite cores during 

diagenesis by optically clear secondary dolomite and in 

some uncommon cases an additional thin rimming of calcite 

over the dolomite rim. 

Energy dispersive X-ray analysis, showing compositional differences between the primary dolomite cores and 

the secondary dolomite rims, thus confirms the existence of 

Summary and Conclusions 13 
two different generations of dolomite. The rim material 

contains a small amount of iron that commonly replaces 

magnesium in the dolomite structure. The core material 

contains a small amount of aluminum and silicon and may 

indicate that the inclusions that impart its characterisic dusty 

look are cryptocrystalline silicates. 

This study also suggests that the detrital dolomite 

grains, within the same rocks, do not have continuous rims 

of secondary dolomite about them, because they are polycrystalline. On closer inspection, it appears that secondary 

dolomite has grown epitaxially with the very small subunits that have individual crystallographic orientations. 

These somewhat saw-toothed projections of secondary 

dolomite obscure the original rounding of the detrital grain. 

Another suggestion resulting from this study is that 

the large amount of dissolved carbonate rock ingredients 

entering the basin from the source rocks may have stimulated the formation of the primary dolomite. 

X-ray-diffraction data and microscopic examination 

show that the pigmentation factor in the matrix of the brown 

sandstones is illite and kaolinite coated by argillaceous 

hematite and that the hematite is absent in the white-cap 

sandstones. In other examples, redistribution of the hematite might be explained by a diagenetic organic-acid bleaching process that resulted in the white-cap sands. However, 

the present study concludes that depositional environment 

was the dominant factor in the formation of white-cap sands 

in the Fox Hills Sandstone, the Almond Formation, and the 

McCourt Tongue of the Rock Springs Formation within the 

Rock Springs Uplift. 

ACKNOWLEDGMENTS 

This writer wishes to acknowledge the help and 

encouragement of several colleagues: Harvey Belkin for his 

energy dispersive X-ray analysis of primary and secondary 

dolomite; Frank DuLong for his quantitative X-ray analysis 

of brown and white matrix materials; and Brenda Pierce, 

Noreen Rega, and Judy Hunter for walking me through the 

process of setting up computer files and the plotting of that 

data by use of the program TRIANGL. I wish to thank my 

National Association of Geology Teachers field assistant, 

Tim Garvey, who helped me collect the sandstone specimens and whose knowledge of depositional environments 

was indispensable; Joe O'Connor and Bob Ryder for their 

technical reviews; and Sam Altschuler for his comments on 

the manuscript. I thank Hank Roehler in particular for 

introducing me to the Rock Springs Uplift and for the many 

suggestions that have resulted in a much improved report. 

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of Science, v. 255, p. 561. 

Douglas, W.B., Jr., and Blazzard, T.R., 1961, Facies relationships of the Blair, Rock Springs and Ericson Formations of 

the Rock Springs Uplift and Washakie Basin, in Symposium 

on Late Cretaceous rocks, Wyoming and adjacent areas, 

Wyoming Geological Association 16th Field Conference 

Guidebook: Casper, Wyo., p. 81-86. 

Folk, R.L., 1974, Petrology of sedimentary rocks: Austin, Tex., 

Hemphill Publishing Company, 184 p. 

Gadd, Benjamen, 1986, Handbook of the Canadian Rockies: 

Corax Press, 876 p. 

Gill, J.R., and Cobban, W.A., 1973, Stratigraphy and geologic 

history of the Montana group and equivalent rocks, Montana, 

Wyoming, and North and South Dakota: U.S. Geological 

Survey Professional Paper 776, 37 p. 

Harms, J.C., MacKenzie, D.B., and McCubbin, D.G., 1965, 

Depositional environment of the Fox Hills Sandstones near 

Rock Springs, Wyoming, in Sedimentation of Late Cretaceous and Tertiary outcrops, Rock Springs Uplift, Wyoming 

Geological Association 19th Field Conference Guidebook: 

Casper, Wyo., Petroleum Information, p. 113-135. 

Hoyt, J.H., and Weimer, R.J., 1965, The origin and significance 

of Ophiomorpha (Halemenites) in the Cretaceous of the 

Western Interior, in Sedimentation of Late Cretaceous and 

Tertiary outcrops, Rock Springs Uplift, Wyoming Geological 

Association 19th Field Conference Guidebook: Casper, 

Wyo., Petroleum Information, p. 203-207. 

Love, J.D., and Christiansen, A.C., 1985, compilers, Geologic 

map of Wyoming: U.S. Geological Survey, 3 p. 

Pettijohn, F.J., Potter, P.E., and Seiver, Raymond, 1973, Sand 

and sandstone: Springer-Verlag, 618 p. 

Powers, M.C., 1953, A new roundness scale for sedimentary 

particles: Journal of Sedimentary Petrology, v. 23, no. 2, p. 

117-119. 

Pryor, W.A., 1961, Petrography of Mesaverde Sandstones in 

Wyoming, in Symposium on Late Cretaceous rocks, Wyoming and adjacent areas, Wyoming Geological Association 

16th Field Conference Guidebook: Casper, Wyo. , Petroleum 

Information, p. 34-46. 

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the Rock Springs Uplift, Sweetwater County, Wyoming, in 

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---1965, Summary of pre-Laramide Late Cretaceous sedimentation in .the Rock Springs Uplift area, in Sedimentation 

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Uplift, Wyoming Geological Association 19th Field Conference Guidebook: Casper, Wyo., Petroleum Information, p. 

10-12. 

---1977, Geologic map of the Rock Springs Uplift and 

adjacent areas, Sweetwater County, Wyoming: U.S. Geological Survey Open-File Report 77-242, 1 p., 1:125,000 scale. 

---1983, Stratigraphy of Upper Cretaceous and lower Tertiary 

outcrops in the Rock Springs Uplift, Wyoming: U.S. Geological Survey Miscellaneous Investigations Series Map 

1-1500, 1 p. 

---1988, The Pintail coal bed and barrier bar G-A model for 

- coal of barrier bar-lagoon origin, Upper Cretaceous Almond 

Formation, Rock Springs coal field, Wyoming: U.S. Geological Survey Professional Paper 1398, 60 p. 

14 A Petrologic Study of the Fox Hills Sandstone, Rock Springs Uplift, Wyoming 
---1989, Origin and distribution of six heavy-mineral placer 

deposits in coastal-marine sandstones in the Upper Cretaceous McCourt Sandstone Tongue of the Rock Springs 

Formation, southwest Wyoming: U.S. Geological Survey 

Bulletin 1867, 34 p., 1 plate. 

Sabins, F.F., Jr., 1960, New evidence for primary dolomite grains 

from Upper Cretaceous marine sandstones of the Western 

Interior [abs.]: Geological Society of America Bulletin, v. 

11, no. 12, p. 1964. 

---1962, Grains of detrital, secondary, and primary dolomite 

from Cretaceous strata of the Western Interior: Geological 

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Cretaceous deposits in San Juan Basin: American Association 

of Petroleum Geologists Bulletin, v. 48, no. 3, p. 292-316. 

---1965, Review of petrographic criteria for Cretaceous 

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Casper, Wyo., Petroleum Information, p. 27-34. 

Sabins, F.F., Jr., and Petersen, F.A., 1961, Geology and petrography of Dead Horse Creek and Barber Creek fields, in 

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southern Wyoming, and northwest Colorado, in Symposium 

on Late Cretaceous rocks, Wyoming and adjacent areas, 

Wyoming Geological Association 16th Field Conference 

Guidebook: Casper, Wyo., p. 17-28. 

---1965, Stratigraphy and petroleum occurrences, Almond 

and Lewis Formations (Upper Cretaceous), Wamsutter Arch, 

Wyoming, in Sedimentation of Late Cretaceous and Tertiary 

outcrops, Rock Springs Uplift, Wyoming Geological Association 19th Field Conference Guidebook: Casper, Wyo., 

Petroleum Information, p. 65-80. 

References Cited 15 

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