40 Ar 39 Ar AGES of SELECTED Tuffs of the GREEN RIVER

40 39 Ar Ar AGES Of SELECTED TUffS

Of THE GREEN RIVER FORMATION:

WYOMING, COLORADO, AND UTAH

A Thesis

Presented in Partial fulfillment of the Requirements

for the Degree Master of Science

by William Arthur O'Neill, B. s.

The Ohio State University 1980

Approved by

De Mineralogy ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to all those . who contributed to this study.

I owe a great debt of gratitude to my advisor, Dr. John Sutter, whose assistance, ideas and experience were essential to this study.

His unselfish donation of time in the field, in the laboratory and in editing this paper is greatly appreciated.

Dr. Kenneth Stanley first suggested this subject as a thesis topic. He provided valuable assistance in the field, with petro­ graphy, with stratigraphic interpretation and in editing this manu­ script. Dr. Ronald Surdam provided assistance in the field in locating outcrops in the Washakie and Bridger Basins. He also willingly

provided transportation to sampling locations in Wyoming and Utah

that could not be reached by automobile.

Dr. William Cashion of the u.s.G.S. gave invaluable assistance

in the Uinta and Piceance Creek Basins in locating tuff outcrops.

He also provided several topographic maps of sampling areas.

Dr. Earnest Ehlers provided assistance in taking photo­

micrographs and in editing this manuscript. Larry Snee was almost a second advisor for this study. He

willingly gave advice and assistance throughout all facets of the

laboratory analyses. Carl Sheliga shared the use of his automobile and provided

assistance in the field area. iii

The friends of Orton and the Sigma Xi Society provided financial assistance for travel expenses. Many thanks to my brother, Brian, who provided the trans­ portation from Chicago to Wyoming and back and who shared the dangers of vicious house cats, grumbling motorcycle mufflers and

trout stealing sea gulls. My deepest thanks go to my wife, Nancy, whose support,

patience and love made the last two years all worthwhile. iv

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS • • • • • • • • • • • • • • • • • • • • • • • • • ii LIST OF FIGURES •••••••••••• • • •• • • • • • • • • • vi

LIST OF TABLES • • • • • • • • • • • • • • • • • • • • • • • • • • x INTRODUCTION • • • • • • • • • • • • • • • • • • • • • • • • • • • l

Location • • • • • • • • • • • • • • • • • • • • • • • • • l Purpose of Study • • • • • • • • • • • • • • • • • • • • • 4

Present and Previous Research • • • • • • • • • • • • • • 5

Methods of Investigation • • • • • • • • • • • • • • • • • 6 40 39 . Ar/ Ar Dating Technique • • • • • • • • • • • • 6

Petrographic Analysis • • • • • • • • • • • • • • 8

GEOLOGIC SETTING • • • • • • • • • • • • • • • • • • • • • • • • • 10 General Setting • • • • • • • • • • • • • • • • • • • • • 10 Lake Gosiute • • • • • • • • • • • • • • • • • • • • • • • 13

Wasatch Formation • • • • • • • • • • • • • • • • 14 Green River Formation • • • • • • • • • • • • • • 15

Tipton Shale Member • • • • • • • • • • • 15

Wilkins Peak • • • • • • • • • • • • • • • 16

Laney Member • • • • • • • • • • • • • • • 18 Washakie and Bridger Formations • • • • • • • • • 20

Lake Uinta • • • • • • • • • • • • • • • • • • • • • • • • 21 Wasatch Formation • • • • • • • • • • • • • • • • 21

Green River Formation • • • • • • • • • • • • • • 22

Douglas Creek Member • • • • • • • • • • • 22

Garden Gulch Member • • • • • • • • • • • 24 v

Page

Parachute Creek Member • • • • • • • • • • 24 Anvil Points Member • • • • • • • • • • • 25 Uinta Formation • • • • • • • • • • • • • • • • • 27 General History • • • • • • • • • • • • • • • • • • • • • 27 Tuff Beds • • • • • • • • • • • • • • • • • • • • • • • • 32 4 39 oAr/ Ar AGES OF TUFF UNITS • • • • • • • • • • • • • • • • • • • 34 Big Island Tuff • • • • • • • • • • • • • • • • • • • • • 34 Petrography • • • • • • • • • • • • • • • • • • • 39 Age Data • • • • • • • • • • • • • • • • • • • • • 40 Wilkins Peak Tuff NLnnber Six • • • • • • • • • • • • • • • 51 Petrography • • • • • • • • • • • • • • • • • • • 51 Age Data • • • • • • • • • • • • • • • • • • • • • 54 Flaming Gorge Tuff • • • • • • • • • • • • • • • • • • • • 56

Petrography • • • • • • • • • • • • • • • • • • • 56 Age Data • • • • • • • • • • • • • • • • • • • • • 60

Sand Butte Tuff • • • • • • • • • • • • • • • • • • • • • 60 Petrography • • • • • • • • • • • • • • • • • • • 66 Age Data • • • • • • • • • • • • • • • • • • • • • 68 Robin's Egg Blue Tuff • • • • • • • • • • • • • • • • • • 71

Petrography • • • • • • • • • • • • • • • • • • • 71

Age Data • • • • • • • • • • • • • • • • • • • • • 78 Curly Tuff • • • • • • • • • • • • • • • • • • • • • • • • 80 Petrography • • • • • • • • • • • • • • • • • • • 80 Age Data • • • • • • • • • • • • • • • • • • • • • 85 Wavy Tuff • • • • • • • • • • • • • • • • • • • • • • • • 85 vi

Page

Petrography • • • • • • • • • • • • • • • • • • • 85

Age Data • • • • • • • • • • • • • • • • • • • • • 92

EVALATION Of AGES • • • • • • • • • • • • • • • • • • • • • • • • 101 Reliability of Tuff Ages ••••••••••••••••• 101

Use of Tuff Ages • • • • • • • • • • • • • • • • • • • • • 106 Mahogany Zone • • • • • • • • • • • • • • • • • • 106 Correlation of High Stands • • • • • • • • • • • • 108 CONCLUSION • • • • • • • ••• • • • • • • • • • • • • • • • • • • 110 Results of This Study •••••••••••••••••• 110

Dating of Tuff Beds • • • • • • • • • • • • • • • • • • • 110

Recommendations • • • • • • • • • • • • • • • • • • • • • 111 REfERENCES • • • • • • • • • • • • • • • • • • • • • • • • • • • • 112 APPENDIX I • • • • •••• • • • • • • • • • • • • • • • • • • • • 113 Analytical Methods •••••••••••••••••••• 117

APPENDIX II • • • • • • • • • • • • • • • • • • • • • • • • • • • 123 40 39 Ar/ Ar Age 0 a t a • • • • • • • • • • • • • • • • • • • • 123 Locations of Samples • • • • • • • • • • • • • • • • • • • 139 vii

TABLE OF FIGURES

Figura Page 1. False color composite space photograph of study area • • • • • • • • • • • • • • • • 2

2. Generalized map showing surface and subsurface occurance • • • • • • • • • • • • • • • • • • 3

3. Temperature release spectrum for a sample of the Curly Tuff • • • • • • • • • • • • • • • • 9 4. Schematic cross section of Lake Gosuite • • • • • • • • • 11 5. Schematic cross section of Lake Uinta • • • • • • • • • • 12 6. Schematic cross section of Wilkins Peak Member with lens shape zone of trona deposits ••••••••••••••••••••• 17

7. Inferred boundary of hydrographic basin of Eocene Lake, Gosuite • • • • • • • • • • • • • • • • 19 a. Schematic north-south cross section of Uinta Basin • • • • • • • • • • • • • • • • • • • • • 23 9. Generalized stratigraphy of Green River Formation in Uinta and Piceance Creek Basins • • • • • • 26 10. Correlation of Eocene Age sediments with provincial land mammal ages • • • • • • • • • • • • • • 29

11. Diagramatic cross section of playa Lake Gosuite •••••••••• • • • • • • • • • • • • 30 12. Columnar sections of Wilkins Peak and Laney Members • • • • • • • • • • • • • • • • • • • • • 35

13. Columnar sections of Wilkins Peak Member • • • • • • • • • 36 14. Photograph of outcrop of Big Island Tuff • • • • • • • • • 37

15. Location map showing position of sampling of tuffs in Green River, Wyoming Area. • • • • • • • • • 38

16. Age spectrum diagrams for Big Island Tuff • • • • • • • • 41 17. Photomicrographs from samples of Big Island Tuff • • • • • • • • • • • • • • • • • • • • • • 44 viii

Figure Page 18. Photograph of outcrop for tuff number six of Wilkins Peak Member • • • • • • • • • • • • • • • . 52 Age spectrum for sample 706-7 of tuff number six • • • • • • • • • • • • • • • • • • • • • • • 20. Photomicrograph from sample 706-7 • • • • • • • • • • • • 55

21. Age spectrum diagrams of Flaming Gorge Tuff • • • • • • • • • • • • • • • • • • • • • • • 58 22. Photomicrographs from samples of Flaming Gorge Tuffs • • • • • • • • • • • • • • • • • • 61

23. Columnar section of portion of Laney Member • • • • • • • • • • • • • • • • • • • • • • • • • 63 24. Geologic map of Western half of Washakie Basin • • • • • • • • • • • • • • • • • • • • • 64 25. Photograph of outcrop of two inch thick tuff in Upper Laney Member • • • • • • • • • • • • • • • 65

26. Age spectra diagrams for sample 705-3, and 705-5 • • • • • • • • • • • • • • • • • • • • • • • 67 27. Photomicrograph of thin section from base of of sample 705-3 • • • • • • • • • • • • • • • • • • • • 69 28. Generalized stratigraphic column of Washakie Formation • • • • • • • • • • • • • • • • • • • • • • • 72 29. Geologic map of western two-thirds of Washakie Basin • • • • • • • • • • • • • • • • • • • 73 30. Disrupted age spectrum of sample 706-2 of Robin's Egg Blue Tuff • • • • • • • • • • • • • • • • 75 31. Photomicrographs from thin sections of samples of Robin's Egg Blue Tuff ••••• • • • • • • • 79 32. Outline map of Uinta Basin • • • • • • • • • • • • • • • • 81 33. Stratigraphic column of Parachute Creek Member • • • • • • • • • • • • • • • • • • • • • • 82 34. Age spectra diagrams for samples of Curly Tuff • • • • • • •• • • • • • • • • • • • • • 84 ix

Figure Page 35. Photomicrograph of thin section of samples from Curly Tuff • • • • • • • • • • • • • • • 86 36. Outline map of Uinta and Piceance Creek Basins • • • • • • • • • • • • • • • • • • • • • • 90 37. Age spectra diagrams for samples of the Wavy Tuff • • • • • • • • • • • • • • • • • • • • 91 38. Photomicrographs of thin sections of samples from Wavy Tuff ••• • • • • • • • • • • • • 93 39. Generalized cross section for sediments for Eocene Lakes Uinta and Gosiute • • • • • • • • • • • 105 40. Geomagnetic polarity time scale chart • • • • • • • • • • 107 x

LIST Of TABLES

Table Page 1. Three step fusion and weight average plateau ages for Big Island Tuff • • • • • • • • • • • • • • • • • • • • 50 2. Two step fusion and weight average plateau ages for Curly Tuff • • • • • • • • • • • • • • • • • • • • • • • 89

3. Two step fusion and weight average plateau ages for Wavy Tuff • • • • • • • • • • • • • • • • • • • • • • • 98 INTRODUCTION

Location The Eocene Green River formation crops out in a three state area of the Western United States including Wyoming, Colorado, and Utah.

The Formation is present in two east-west basin trends that are separated by the east-west trending Uinta Mountain uplift (figure 1).

The Northern Green River formation was deposited in what is called the Greater Green River Basin. This hydrographic basin extends for approximately 150 miles (240 km.) from the Wyoming Overthrust Belt on the west, to the Rawlins uplift on the east. It extends approximately

100 miles (160 km.) northward from the Uinta uplift and is bound on the north by the Gros Ventre Range, the Wind River Range, and the Granite

Mountains. The thickest accumulations of lacustrine sediments are present near the geographic center of this area in two separate structural basins that are separated by the north-south trending Rock Springs uplift.

To the east of the uplift is the Bridger Basin which is roughly elliptical in form. To the west of the Rock Springs uplift is the circular Waskakie

Basin which is bound by a steep, outward facing escarpment that encircles the basin and formed as a result of erosion of the resistant Green River beds that dip towards the center of the basin.

The southern Green River formation was deposited in the Uinta and Piceance Creek Basins which form a trend that extends for approximately 150 miles (240 km.) westward from the Wasatch Range into northeastern

Colorado. It extends for 100 miles (160 km.) south from the Uinta uplift to the Roan Cliffs. The Uinta and Piceance Creek Basins are separated by the Douglas Creek arch, which is located to the east of the Colorado-Utah state line.

1 2

figure l~ A false Color composit e space photograph of study area. WRm=Wind River Mountains, WRb=Wind River Basin, G:::Granite

Mountain Uplift, Bb=Bridger Basin, RSu=Rock Springs Uplift,

Wb=Washakie Basin, Um=Uinta Basin, PCb=Piceance Creek Basin. 3

10 0 10 20 30 Scale, 111iln LEGEND ~ 6Atn Rim Formatiaa

figure 2: Generalized map showing surface and subsurface occurence of Green River formation. Black dots show approximate sample locations of tuff beds (modified from MacGinitie, 1969). 4

The lacustrine and fluvial rocks of the Wasatch, Green River,

Uinta, and Washakie Formations which were deposited in the Gosiute and

Uinta hydrographic basins are found predominatly in nearly horizontal to

gently dipping beds which locally have dips up to 45 degrees. The unite are exposed along steep faced escarpments and canyon walls that ware formed as a result of erosion by intermittent streams and rivers that have cut down through the resistant sediments in this arid region. The land surface is at elevations that average about 6,500 feet (l,981 meters) and ranges from 5,eoo (1,770 m.) to more than 9,ooo (2,740 m.) feet above sea level.

Figure 2 shows the location of the Green River Formation strata.

Stations have been marked to show where samples of tuff units have bean collected.

Purpose of Study

The purpose of this study is to determine precise ages of stratigraphically significant volcanic ash beds that are found within the Laney and Wilkins Peak Members of the Wyoming (Gosiuta Lake) Green

River Formation, the Adobe Town Member of the fluvial Washakie Formation, and the lacustrine Parachute Creek Member of the Utah-Colorado (Lake

Uinta) Green River formation. Precise ages for these tuff beds could be useful in accomplishing three objectives. 1) The timing of the high stands for the lakes that formed the oil rich Laney (Lake Gosiute) and Parachute Creek (Lake Uinta) Members. If high stands for the two

Eocene lakes were synchronous, increased water flow to the lakes was proba~ly due to climatic factors. If high stands were not synchronous, the added influx of water was due to diversion of drainage waters from other basins. 2) Evaluation of recent paleomagnetic data from the 5

Mahogany Zone of the Parachute Creek Member of the (Uinta) Green River formation suggesting the Mahogany Zona represents only a few hundred thousand years duration. 3) Evaluation of present vertebrate age correlations for alluvial and lacustrina beds with radiometric ages of­ rocks equivalent to type areas of soma land mammal ages.

Present and Previous Research

Tiie Green River formation has attracted a great deal of geologic interest and study over the last fifty years. Literally hundreds of reports and papers have bean published about this formation and its history. Bradley (1964) has published a comprehensive study that describes the overall stratigraphy and geologic relationships for the

Graen River formation and associated Eocene rocks for the Wyoming

(Gosiute) basin. Cashion (1967) has prepared a similar study for the

Uinta Basin, while Donnell (1961) has dona the same for the Piceanca

Creak Basin.

Recent papers by Surdam and Stanley (1979, 1980) have been concerned with the mechanisms and timing for the culminating phases for

Lakes Gosiute and Uinta. Papers by Lundell and Surdam (1975), Eugster and Suudam (1973), have discussed the deposition of Green River sediments in ancient playa-lake complexes. Dasborough (1978), however, favors a deep stratified lake model for deposition of the Green River formation.

Surdam and Parker (1972) have published a report concerning the authiganic minerals found in tuffaceous rocks of the Wyoming Graen River formati~n while papers by Ven and Parker (1972) and Desborough et al

(1973) have attempted the correlation of thin tuff beds in the Uinta

Basin on the basis of trace element compositions of biotite separates from the tuff beds. 6

The only previous study that attempted to date ash beds from the

Green River Formation is a paper by Mauger (1977) describing the dating of biotite separates through the use of conventional K/AR isotope dating techniques. This study provides age determinations for samples from the

Big Island, hlavy, and Robin's Egg Blue tuffs, as well as other tuffs from Green River formation and related Eocene age rocks.

Currently, Carl Sheliga of the Ohio State University Department of Geology and Mineralogy, is dating samples from two tuff beds that are found in an eastern extension of the Lake Uinta Green River Formation

(Sheliga, 1980).

Methods of Investigation

The following methods were used for this investigation. 1) 40 39 Ar/ Ar ages were obtained on biotite separates from twenty-five samples. Each biotite separate was prepared for an age spectrum 40 39 experiment in which five to seven temperature steps were made. Ar/ Ar incremental release experiments are often useful in determining if the

K/Ar isotopic system of a mineral has been disrupted, either by chemical alteration or post depositional heating (Dalrymple and Lampher, 1974,

Fleck, Sutter and Elliott, 1977).

Nearly all of the biotite samples dated in this study showed age spectra that was disturbed. As is shown in Figure 3 the disturbed age spectrum for a sample of the Curly Tuff has an apparent age for the first temperature increment that is lower than the apparent ages for

the other increments. This indicates that the biotite has undergone 40 post formational loss of radiogenic Ar from positions within the

biotite crystal that are easily affected by chemical alteration or heating. 7

40 This loss of radiogenic Ar results in an apparent age for that increment that is anomalously low. When this value is used along with the values from the other increments to give a total gas age

(which is essentially the same as a conventional K/Ar age) that is also anomalously low. The remaining temperature steps of the age spectrum have apparent ages that can not be distinguished through use of the critical value test of Dalrymple and L'mpher (p. 120, 1969) and thus form a plateau as defined by Fleck, Sutter and Elliot (1977).

The apparent ages for the increments that are part of the plateau formed as a result of release of argon from positions within the biotite structure that have not been affected by post formational loss o f rad iogenic. . 40A r.

The weight average of all of the apparent ages for the incre- ments that are part of the plateau is taken to be the age of formation

(cooling) of the biotite crystals of the tuff.

Once a plateau was found (and at what temperatures) for a particular sample of biotite a second sample of.that same biotite was analyzed. The second analysis was usually a two or three step release experiment in which the heating schedule of the first step was adjusted to remove all the argon within the biotite sample that would give an anomalously low age. The second heating step was adjusted to insure that all of the argon that would give ages on a plateau is released in a single step.

Analytical errors associated with measurements of argon isotope ratios on a mass spectrometer increase when the amount of gas being measured decreases. Thus by releasing all of the gas from a sample that is part of the plateau, one reduces the standard deviation (+) associated with the age. obtained for that sample. The age obtained in 8 this second analysis will be referred to in this paper as the fusion age and is in general more precise than the weight average plateau age that was obtained in the age spectr1.111 analysis. As can be seen in

Figure 3 and Appendix II, the standard deviations (±) associated with the two and three step fusion ages are in general smaller than the standard deviations associated with weight average plateau ages from the temperature release experiments.

The above procedure produced two different age determinations for each sample that was dated. First, an incremental release experi­ ment was done to determine if the sample had a disturbed spectrl.111 and if disturbed, whether or not the disturbed spectrum formed a plateau.

A two or three step fusion experiment was then done on a separate biotite sample from the same location in which nearly all of the gas from the plateau was released in a single fraction. The age obtained from the fusion experiment was used along with the weight average plateau age from the incremental release experiment and averaged with ages obtained from samples from other locations to give an average age for a particular tuff bed. 2) Petrographic Analysis - Approximately thirty sections from the tuff beds were examined. The aim at the examination of the thin sections was to determine which tuff beds were primary airfall volcanic ash beds, and which beds that, although tuffaceous in nature, were reworked and possibly contained a detrital biotite component. 9

- r+ Ill E ::r - Tp=45. 7±2.4m.y. Tf=45.3 ±1.0m.y. c -.,.... ftlca. ca. c 35,'-~~~-L~~~----JL..-~~~...... ~~~---~~~--- o 20 40 60 80 39 ArK Released, Cumulative ~

tigure 3: Temperature release spectrum for a sample of the Curly Tuff.

The diagram shows a disturbed spectrum which forms a plateau that has a

weight average age of 45.7± 2.4 m.y. Note that when the data for the

first fraction (which has an apparent age of 41.0± 1.7 m.y.) is used

to calculate a total gas age for the sample an age of 45.2± m.y. is obtained. This value is o.s m.y. less than the weight average plateau of the sample. Also note that the two step fusion age (Tf) has an error C±) of l.O m.y. which is 1.4 m.y. less than the error obtained from the weight average plateau age (T ). p 10

GEOLOGIC SETTING

General Setting

The Green River Formation of Wyoming, Colorado and Utah has been described by Bradley (1964), and Cashion (1967) as being a huge lens

of fine grained, generally calcaruos lacustrina sedimentary rocks that

is surrounded by, and intertongues with fluviatile mudstones, sandstones,

and conglomerates of the Wasatch, Bridger, Washakie, and Uinta Formations.

These sediments filled two large intermontane basins that formed as a

result of the uplift of surrounding mountain ranges (Figures 4 and 5).

The environment in which these sediments formed has been

interpreted by Eugster and Surdam (1973), Surdam and Wolfbauer (1975),

Roehler (1974), Lundell and Surdam (1975), and Surdam and Stanley (1979),

as being a lake which was surrounded by a broad mud-flat where the

chemical processes were similar to those recognized on modern playas and had no permanent chemical stratification.

A model of the paleoenvironment by Desborough (1978) focused

more on processes within the lake. He postulates a permanently chemically

stratified lake. This lake had an upper auphotic zone in which blue green

algae thrived and a lower oxygen defficient (reducing) zone where organic remains were preserved.

During high and low stands, various components of these models were important in the accumulation of lake and lake margin deposits.

However, both models are compatible to the deposition of primary air fall ash beds.

As the lake levels rose, the belts of lacustrine deposition would expand and move towards the periphery of the basin while the zone of fluvial beds would contract. During low stands of the lakes, the fluvial 11

Wilkins Peak

Tipton Snale

·.·.: .. .::~ :~ . . :• .:·. . . ·.. :·. :.·

Figure 4: Schematic cross-section showing generalized stratigraphy of

Graen River Formation across Bridger and Washakie Basins. Modified from Surdam and Wolfbauer (1975). 12

w E Uinta Basin Piceance Creek Basin

bl a= Cl

figure 5: Schematic Cross-section showing generalized stratigraphy of

Green River formation acorss the Uinta and Piceance Creek Basins. GG= Garden Gulch Member, Af'=Anvil Points Member. 13 beds would encroach on previous areas of lacustrine sedimentation. These fluctuations of the lake level resulted in the complex intertonging relationship between the lacustrine Green River sediments and those of the fluviatila Wasatch, Bridger, Washakie, and Uinta Formations. Clastic sediments finally infilled the basins during the Eocene, thus ending

Green River deposition.

The Eocene Age sediments of the two lakes, Lake Gosiuta in

Wyoming and Lake Uinta in Utah and Colorado, record histories that are distinctly different for the two basins. The geology of these sediments are described below.

Lake Gosiute As is shown in Figure 4, the Green River Formation of Eocene

Lake Gosiute forms a lens of fine grained calcareous lacustrine sediments that is imbedded in a somewhat sandy mudstone (Bradley, 1964). Only the

Rock Springs uplift interrupts the symmetry of this lens. The sandy mudstone matrix that surrounds the lens of lacustine sediments has been divided into three formations. Those beds that underlie the most extensive beds of the Green River Formation have been placed within the Wasatch

Formation. Those beds that overlie and are partly equivalent to the upper part of the Green River Formation in the Bridger Basin are part of the

Bridger Formation (McGrew, 1971) and those that overlay the Green River

Formation sediments in the Washakie Basin are part of the Washakie

Formation. All of the Eocene Age sediments within the Gosiute basin have

had very little post depositional deformation and are found in nearly

horizontal beds. 14

Wasatch formation

The Wasatch formation of the Wyoming Gosiute Basin is divided into three parts, the main body, the overlying Cathedral Bluffs, and New forks Tongues.

The main body of the Wasatch formation consists of 1,700 to 4,500 feet {520-1,370 m.) of predominately red and gray sandy mudstones

{Bradley, 1964). Interbedded with these sediments are beds of sandstone, conglomerate, carbonaceous shale and subbit1.J11inous coal. Vertebrate fossils from the Wasatch formation have been used to define the Wasatchian

Stage of the Eocene {Everden, 1964). fossils ~ithin the main body of the

Wasatch Formation have placed it as being from earliest Wasatchian

{Graybullian) to early-late Wasatchian {lost Cabinian), {McGrew, 1971).

Above the main body of the Wasatch formation are two tongues of the Wasatch that are separated from the main body of the formation by extensions of the Tipton Shale Member and Tipton Tongue of the Green

River formation. These tongues of the Wasatch formation are strati­ graphically equivalent to the Wilkins Peak Member of the Green River

Formation and are found to crop out to the east and west of the depositional canter of the Wilkins Peak in Bridger Basin {Bradley, 1964).

The eastern tongue of the Wasatch is called the Cathedral Bluffs Tongue while the western tongue is called the New forks Tongue. The two tongues consist of gray, red, buff, and maroon beds of mudstone, sandstone, and conglomerate.

The Cathredral Bluffs Tongue contains vertebrate fossil fauna that place it as being latest Wasatch {Lost Cabinian) to earliest Bridger

{Bridger A) in age. The New forks Tongue is slightly older and has no fossils of Bridgerian age in it {McGrew, 1971). 15

Green River formation

The thickest accumulations of the Lake Gosiute Green River formation are found within the Washakie and Bridger Basins. The formation is made up of white, buff, brown, and gray limestone, marlstone, and shale with locally abundant layers of oil shale, trona, volcanic ash, limey sandstone and algal stromatolite (Wolfbauer, 1971). The roughly four million year history of Eocene Lake Gosiute was characterized by two major high stands that were separated by a relative low stand. The high stands produced sediments that are part of the Tipton Shale and Laney

Shale Members while the intervening low stand formed the Wilkins Peak

Member.

The Green River formation contains an abundance of fossil fish as well as fossil plant remains (McGrew, 1971, Bradley, 1964). The formation does not contain vertebrate mammalian fauna but has been placed

(through its intertonging relationship with the surrounding fluvial formations) as being mid Wasatchian (Lysitean)· to early Bridgerian

(Bridger C) in age (Wolfbauer, 1971).

Tipton Shale Member

The Tipton Shale Member of the Green River formation represents a relative high stand for Eocene Lake Gosiute that lasted approximately one million years (Eugster and Bradley, 1969). The Tipton is from zero to three hundred feet in thickness and conformably overlies the Wasatch formation. The Tipton is overlain by the Wilkins Peak Member of the

Green River formation and by the Cathedral Bluffs and New forks Tongues of the Wasatch formation (Bradley, 1964).

The Tipton Shale Member can be divided into three distinct zones.

The lowest zone is from 6 to 90 feet (2 to 28 m.) thick and consists 16 mainly of limestones siltstones and a few low grade oil shale beds

(Surdam and Wolfbauer, 1975). The middle zone is made up ~f between 60 and 150 feet (18 to 46 m.) of oil shale and fluvial sandstone beds.

The upper zone is transitional with the Wilkins Peak Member and contains

beds of Algal Stromatolites.

The Tipton Shale Member also contains several altered tuff beds

that range in thickness from one half inch to eight inches (Bradley,

1964) Surdam and Parker, 1972). None of these tuff beds were sampled for

analysis in this study.

Wilkins Peak Member

The Wilkins Peak Member of the Green River formation is found

between beds of the underlying Tipton Shale member and the overlying Laney

Shale Member. It is made up of beds of gray to greenish gray dolomitic and somewhat tuffaceous marlstone with interbedded layers of mudstone, muddy sandstone, oil shale and altered volcanic ash (Eugster and Bradley,

1969). The member attains a maximum thickness of between 900 and 1,100

feet (275-335 m.) near the center of Bridger Basin (Culbertson, 1961).

Within this lens shaped body of lacustrine sediments, is a lens shaped

zone that is rich in saline minerals (mainly trona, with halite, shortite,

and nahcolite) that are found in beds that very in thickness from a few

inches to thirty-eight feet (11.6 m) (figure 6), (Culbertson, 1971).

The Wilkins Peak Stage represents a low stand during the history

of Lake Gosiute that has been estimated (Bradley and Eugster, 1969) to

have lasted approximately one million years. During this stage, evaporation

and inflow of waters were balanced within the closed basin. This produced

concentration of the brines within the shrunken lake which caused the

precipitation of the saline minerals. 17

Laney Sha le Wilkins Peak

Tipton Shale

figure 6: Schematic cross section across Bridger Basin showing lens shaped zone of trona and trona-halite beds within Wilkins Peak Member of the Green River formation. 18

Interbedded with the lacustrine sediments are several thin beds of altered volcanic ash. These beds often cover extensive areas and average approximately twelve inches (30.S cm.) in thickness (Culbertson, 1971). Two of the tuff beds, the Big Island Tuff and a tuff bed from near the top of the section have been sampled for dating by 40 Ar/39Ar techniques in this paper.

Laney Member

The Laney Member of the Green River formation comprises the youngest and most extensive sediments for the lake Gosiute beds. The member overlies the Wilkins Peak Member of the Green River formation as well as the Cathedral Bluffs and New forks Tongues of the Wasatch formation. The Member crops out in an area that is nearly three times that of the underlying Wilkins Peak Member, (Bradley, 1964), (figure 7).

The Laney Member ranges in thickness from zero to 1,950 feet

(595 m.) and has bean divided into the lower Laclede and upper Sand

Butte Bads (Trudell et al, 1973). The Laclede Bed is composed of three lithofacies that are characterized by one or two rock types (Surdam and

Stanley, 1979). These lithofacies. are: l) laminated carbonates

(including oil shales), 2) evaporitas (analcime rich beds of peloidal dolomicrite and mudcracked dolomicrite), 3) molluscan ostracodal calcareous mudstone (Surdam and Stanley, 1979). The upper Sand Butte

Bed consists mainly of light gray to brown sandstones, siltstones, and mudstones with occasional lenses of marlstone, and oil shale (Trudell et al, 1973).

The Laney Stage represents the last major high stand of Eocene

Lake Gosiuta that lasted for a period that was estimated by Bradley

(1964) to have lasted approximately two million years. During this 19

I I I -l I I ,,,..."' \ I I HYDROGRAPHIC I I Bedded : BASIN I I trona 1 I I I \, ______,,, /

l'OLOR.\l)(l

25 0 2c 50 75 JOO MILES ~•.1..._._...1~1.__~~~'~~~~·~~~~'~~~·

Figure 7: Inferred boundary of Gosiute Lake's hydrographic basin with

outlines of maximum extent of lake during Wilkins Peak and Laney Stages.

Cross hatched area marks location of the ancestral Rock Springs uplift which formed an island in the lake. {Bradley, 1964). 20 episode, extensive beds of relatively rich oil shale were deposited within the lake. The deposition of the elastic sediments of the Sand

Butte Bed, which were derived from the Absaroka volcanic field, filled in the basin for Lake Gosiute, thus ending Green River deposition {Surdam and Stanley, 1979).

The Laney Member contains several altered volcanic ash beds. Two of these tuff beds were sampled in the field for analysis in this paper and will be discussed at a later point.

Washakie and Bridger formations

The fluvial beds of the Washakie and Bridger formations overlie the Laney Member of the Green River formation. The two formations are very similar in characteristics and are found to crop out in their respective basins. These fluvial, time equivalent beds are found to merge with the Wasatch formation along the periphery of the basin (Bradley, 1964).

The Bridger formation, which is found in the Bridger Basin, consists of up to 2,250 feet (685 m.) of sandy tuffaceous mudstones with interbedded layers of limestone, marlstone, and volcanic ash

(Bradley, 1964).

The Washakie formation has been split into the lower Kinney

Rim Member and the upper Adobe Town Member (Roehler, 1973). The formation conformably overlies the Laney Member of the Green River formation in the Washakie Basin.

The Lower Kinney Rim Member consists of up to 900 feet (275 m.) of gray, green and red mudstones and sandstones with interbedded layers of algal limestone and volcanic tuff (Roehler, 1973).

The overlying Adobe Town Member is separated from the Kinney

Rim Member by a Basinwide unconformity. The Adobe Town Member is made 21 up of up to 2,300 feet (700 m.) of alternating beds of red, gray and green mudstones and tine to coarse grained tuf faceous and arkosic sandstones with minor thin beds of green shale, conglomerate and volcanic ash.

The ash beds are often altered to form suites of zeolites. One of the tuff beds, the Robin's Egg Blue Tuff, was sampled at several locations within the Washakie Basin for dating.

Lake Uinta

The Lake Uinta Green River Formation also forms a lens of fine grained sediments that is imbedded in a matrix of fluviatile sediments of the Wasatch and Uinta formations(Cashion, 1967), (Figure 5). The individual beds of the Lake Uinta Green River Formation record a history for the Eocene Lake that is distinctly different from that of Lake

Gosiute. The thickest deposits of Green River sediments as well as those of the Wasatch and Uinta formations are found in the Piceance Creek and eastern Uinta Basins (Cashion, 1967). The lakes that formed in these two basins were separated by the Douglas Creek arch until levels within the lakes rose to form a single lake which created deposits that are now part of the Parachute Creek Member of the Green River formation.

Wasatch Formation

The Wasatch Formation of the Uinta and Piceance Creek Basins is made up of between 500 and 5,500 feet (150-1,680 m.) of fluvial sediments that were derived from surrounding highlands (Cashion, 1964).

The Renegade Tongue of the Wasatch Formation is found with- in the Uinta Basin and is intertongued with the Douglas Creek

Member of the Green River rormation. It is similarly bedded sandston~s, 22 siltstones and conglomerates and reaches a maximum thickness of l,OOO feet (305 m.) {Cashion, 1967).

Green River Formation

The Green River Formation of Eocene Lake Uinta ~as divided into four members by Bradley (1931). These are the Douglas Creek, Garden

Gulch, Parachute Creek, and Evacuation Creek Members. Subsequent modification by Cashion and Donnell (1974) have eliminated the 500 foot

(152 m.) thick Evacuation Creek Member by placing parts of this pre­ dominately sandy siltstone unit with the underlying Parachute Creek

Member of the Green River formation and the overlying Uinta formation.

The three members of the Green River Formation reach a maximum thickness of 3,000 feet (915 m.) in the Piceance Creek Basin (a minimum thickness due to removal of the top of the formation by erosion) and

1,000 feet (2,135 m.) in the center of the assymetric Uinta Basin

(figure 8), Cashion, 1967, Donnel, 1961).

Douglas Creek Member

The Douglas Creek Member of the Green River Formation is composed of up to 2,000 feet (610 m.) of cross-bedded, ripple marked sandstone

(sometimes bituminous), algal, ostracodal, and oolitic limestone, siltstone, and shale (Cashion, (196~, Brobst and Tucker,(1973)). Locally a few beds of lean oil shale are present. The Douglass Creek Member has been shown by Cashion (1967) as being a near shore facies that grades from sandstone and siltstone to shale and limestone towards the center of the basins. The member intertongues with the Garden Gulch and

Parachute Creek Members and is in part equivalent to these members

(figure 4). 23

s

: ...... ·.:· .. · •.· ...•. · .. ."

figure 8: Schematic north-south cross section of Uinta Basin showing asymetric shape of basin. The deepest portion of the basin is adjacent to the Uinta Mountain uplift. 24

Garden Gulch Member

The Garden Gulch Member of the Green River Formation is found to overlie beds of the Douglas Creek Member in the Uinta and Piceance

Creek Basins. The member merges and intertongues with laterally equivalent beds of the Douglas Creek, Parachute Creek and Anvil Points

Members of the Green River Formation. It is conformably overlain by, and grades into the Parachute Creek Member (Figure S), (Cashion, 1967,

Bobst and Tucker, 1973).

The member is made up of organic rich marlstone and paper

(illitic) shale with minor beds of rich oil shale, thin bedded sandstone, siltstone, and oolitic, ostracodal and algal limestone (Cashion, 1967,

Donnell, 1961). The member attains a maximum thickness of 250 feet (76 m.) in the Uinta Basin and 700 feet (213 m.) in the Piceance Creek Basin.

Parachute Creek Member

The Parachute Creek Member of the Green River Formation represents the youngest sediments of Eocene Lake Uinta. The unit is well noted for its accumulation of extremely rich oil shale beds that are persistent over the two basins.

The member consists of a thick sequence of kerogen rich marlstone, siltstones, and sandstones along the periphery of the basins (Cashion,

1967). In the Uinta Basin, beds of barren marlstone of the overlying

Evacuation Creek Member have been placed with the Parachute Creek Member

(Cashion and Donnell, 1974). The Parachute Creek Member has a maximum thickness of 1,150 feet (350 m.) in the Uinta Basin and 2,000 feet (610 m.) in the Piceance Creek Basin (Cashion and Donnell, 1974).

The Parachute Creek Member is most noted for its large number of dark, rich oil shale beds. The oil is due to kerogen, a waxy, insoluable material that will produce petroleum upon distructive distallation 25

(Brobst and Tucker, 1973). Higher kerogen content produces a rock that is darker in appearance.

One of the most distinctive portions of the Parachute Creek

Member is the accumulation of oil rich beds known as the Mahonany Zone .

(Mahongany Ledge in outcrop) (Cashion, 1967). The interval was named

for its dark brown appearance in polished sections of the oil shales

which have yields of up to sixty gallons (221 liters) or more per ton oil shale (Cashion, 1967, Brobst and Tucker, 1973).

The Mahongany Zone contains several distinctive beds (Mahongany

Bed, Mahogany Tuff Marker) that have been used for correlation between

the Piceance Creek and Uinta Basins (Cashion and Donnell, 1974). This

fifteen to two hundred foot (5-61 m.) thick zone is bracketed by two

tuff beds that have also been used for correlation between basins

(Figure 9). These tuff beds are called the Wavy (above the Mahogany

Zone) and Curly (below) tuffs (Cashion, personal comm.). These two

tuff beds have been sampled for radiometric dating analysis for this report.

The Parachute Creek Member with its accumulation of oil shale

beds represents a relative high stand for Lake Uinta in which lake

waters were high enough to connect the Piceance Creek and Uinta Basins

as is shown by the correlation of Mahogany Zone Beds. This high stand

may in part be time equivalent to rocks in the Laney Shale Member of

Wyoming. Surdam and Stanley (1980) have shown that the Mahogany bed

can be used as a time line that represents the onset of lake enlargement in Which the lake levels were high enough to connect the two basins. Anvil Points Member

The Anvil Points Member of the Green River Formation (Brobst and 26

Uinta Fnrmation oi I shale

A uoove Wavy Tuff

Mahogany ... Ma .Q ::IE Zone Be

::z ~ Q u Curly ...u B groove ....c u Tuff ::IE 4D ai:: Q ...... I;-= u oil shale ai:: ftl... ftl ""'> A. ac- ::z ""' ""'ai:: CD Garden Gulch Mbr. Douglas Creek Mbr. Wasatch Formation

figure 91 Generalized stratigraphy of Green River formation in

Uinta and Piceance Creek Basins showing relative positions of Curly and

Wavy Tuffs with respect to the position of the Mahogany Zone and

Mahogany Bed. Modified after Desborough (1978), and Cashion and Donnell (1972). 27

Tucker, 1973) is a heterogeneous sequence of shales, sandstones and

algal, and oolitic limestone found in the eastern portion of the

Piceance Creek Basin. It is considered to be the eastern equivalent

of the Garden Gulch, and Douglas Creek Members and parts of the

Parachute Creek Member.

The Member interfingers with the underlying Wastch rormation

and overlying Parachute Creek Member and has a maximum thickness of

1,870 feet (570 meters) (Donnell, 1961).

Uinta Formation

The sediments of the fluvial Uinta rormation conformably

overly sediments of the Parachute Creek Member of the Green River

Formation (Cashion, 1964). The formation is made up of up to 1,750

feet (530 m.) of red, gray, and brown siltstones, sandstones, and

claystones. The Uinta Formation merges with the Wasatch rormation

along the margins of the Uinta and Piceance Creek Basins.

General History

Eocene Lakes, Gosiute and Uinta occurred in basins that

developed as a result of structural downwarping that began in the

Paleocene, and continued into the Eocene (Wolfbauer, 1971). This downwarping was a result of differential uplift of the surrounding

mountains. Streams from surrounding mountains supplied sediments

that obliterated the original basin topography and created a nearly level,

poorly drained alluvial plain upon which the large saline lakes formed.

The sediments that formed the alluvial plains in these internally

drained basins formed the main body of the Wasatch rormation (Wolfbauer,

1971).

Although the deposits of the Green River rormation are very 28

similar for both the Uinta and Gosiute Basins, their histories are by no means identical, nor are they completely contemporaneous. The Green

River formation of lake Gosiute is Middle Early (Lysitean, Wasatchan

to Middle (Bridger C) Eocene in age (Wolfbauer, 1971). The beds of

the Lake Uinta Green River formation are slightly younger, going from

Lost Cabinian (Wasatch) to Bridger D (figure 10).

Eocene lake Gosiute has a history that recorded two major

high stands that were separated by a relative low stand (Bradley, 1964).

These episodes are recorded by the Tipton, Wilkins Peak and Laney Members.

The lower Tipton Member of the Green River Formation records an

initial enlargement of the lake that lasted approximately one million

years (Surdam and Wolfbauer, 1975). The overlying Wilkins Peak Member

records a major low stand in which the lake shrank to one half its

original size. The Wilkins Peak Stage ended with the major expansion

of the lake that formed the Laney Member of the Green River Formation.

(Surdam and Stanley, 1979).

Recent papers by Eugster and Surdam (1973), Wolfbauer and Surdam

(1975), Eugster and Hardie (1975), and Surdam and Stanley (1979) have

suggested that the depositional environment for lake Gosiute Green River

Sediments was that of a plays-lake-complex. This model proposes that

the Eocene lake was surrounded by a plays-mud-flat in which marstone,

siltstone and sandstone deposition occured. This playa-mud-flat was

in turn surrounded by a belt of fluviatile sedimentation along the

margin of the basin (Figure 11).

Changes in the balance between evaporation and influx would

cause changes in the lake level within the closed basin (Eugster and

Surdam, 1973). These changes of lake level caused changes in the 29

BRIDGER WASHAKIE UINTA BASIN PICEANCE PROVINCIAL BASIN BASIN AND CREEK

CENTRAL UTAH BASIN AGE

GAZIN, 1959 GAZIN, 1959 GAz,-..:, s~9 GAZ IN, 1959 MAC GINNITIE, 1969 MACGINNITIE, 1969 ~AC G'N..._,T!E 1969 MAC G INNITIE, 1969 ROEHLER, 1965 ROEHLER, 1965 z ~ ~ LA POINT u. z M. (/) a: w ::x: w u HALFWAY z ::> Cf) M. w 0 ?-?- :r: u ...J RANDLETT ~~ 3 z zu. -ct w z ~ I- -:'.IC u. z I- -ct ::> ::::E ""' 5~ u. B ;! ::z u z '2- ---- E ::> ""' u D ::::E A Parachute 0 z u. -ct ~-- Creek Mbr. a: c 4: ""' w ~ ------0 ~ - ~ GARDEN GULCH M. a: LANEY SHALE M. ~~· m EQUIV. -ct '.....J 0 ~ '0 w ~ ,.,. WASATCH FM. iD-' ,;:;"" lu w ~"li MAIN BODY ,_,_J 0 ,,...--~ -·-. <:>~ FLAi.?:::i ,_,..:- F,

Figure 101 Correlation of Eocene Age sediments in Bridger, Washakie,

Uinta, and Piceance Craek Basins with provincial land mammal ages.

Modified from Wolfbauer (1971). 30

Mts.

Figure 11: Diagramatic cross section of playa lake Gosiute showing deposition of alluvial fans off of uplifted mountains followed by mud flats which fringe a central lake (modified from Eugster and Hardie,

1975). 31 position of the belts of sedimentation (Lacustrine, playa, and fluvial).

These changes in position caused the complex intertonging of the three types of lithofacies (Figure 3).

Although many of the fluctuations of the lake level were controlled by balances between influx and evaporation, the changes in lake level may have been controlled by factors other than climate.

Surdam and Stanley (l980)have proposed that the enlargement of Lake

Gosiute during the Laney Stage was controlled by the enlargement of the hydrographic basin through the addition of waters from the Wind River

Basin by volcanic sediments that were derived from the Absaroka volcanic field. These volcaniclastic sediments that infilled the Wind

River Basin were carried into the Gosiute hydrographic basin and began to fill it in. With the infilling of the Gosiute basin, waters were diverted into the Piceance Creek Basin over the eastern extension of the

Uinta uplift. The influx of waters from the Gosiute Basin is postulated to have caused the high stand of Lake Uinta that formed the Mahogany

Zone of the Parachute Creak Member.

If this model is correct, it would mean that the high stands that formed the Laney and oil rich zones of the Parachute Creek Member were not synchronous, but were sequential with the Laney Member sediments forming first.

The history of Lake Uinta began during the Lost Cabinian Stage of the Wasatch in which two lakes formed, one in the Uinta and one in the Piceance Creek Basin. These lakes were separated by the Douglas Creek arch. The lake sediments do not record the major fluctuations found in the Gosiute Basin (although many minor fluctuations were recorded) until the enlargement of the lake during the Parachute Creek Stage. The 32 sediments that were deposited before this enlargement are now part of the Douglas Creak, Garden Gulch, and Anvil Points Members. The enlarged lake in which sediments of the Parachute Creek Member were deposited, eventually began to receive volcaniclastic sediments from streams of the infilled Gosiute Basin. These sediments, as well as fluvial sediments from the surrounding uplifted areas eventually filled the basin, thus ending Green River deposition.

Tuff Beds Tuffs and Tuffaceous sediments are abundant within the Green

River Formation. Tuff beds crop out as fine grained gray to orange resistant ledges that range in thickness from a fraction of an inch to twenty feet in thickness with an average of about one foot (Cashion,

1967). Although most of the tuff beds are quite thin, they often have a great areal extent and may cover hundreds of square miles, potentially providing excellent time planes for correlation.

The tuff beds can be classified into two groups, air fall ashes and reworked tuffs.

Air fall ashes were deposited directly in the saline lakes.

They consist of a crystal component that generally makes up less than twenty percent of the rock with the rest of the tuff being made up of an altered vitric (glass) component. The crystal components of the air fall ashes contain angular fragments of embayed and resorbed clear quartz, biotite, green hornblende, pyroxene, K-feldspars and plagioclase.

The crystal fragments are often concentrated near the base of the tuff bed in a zone that is matrix supported.

The vitric material is nearly always completely altered to one or more authigenic aluminosilicate minerals (Surdam and Parker, 1972). 33

The minerals formed include clinoptilolite, mordenite, analcime (analcite), potassium feldspar, montmorillonite, and albite (Surdam and Parker, 1972).

The type of authigenic mineral that formed in a tuff bed was believed by

Surdam and Parker (1972) to be controlled by salinity (Na+) of the lake­ waters in which the glass shards reacted.

Because of extensive alteration, the original composition of the tuffs is unknown. Bradley (1964) supposed they ranged from rhyolite to andesite. Iijima and Hay (1968) concluded that they were dacite to rhyodacite. The possible source for the volcanic ash may have been the Absaroka volcanic field (Mauger, 1977) but other sources may have supplied volcanic ash especially in the Uinta Basin.

Reworked tuffs are similar in composition to the air fall ashes but have the additional component of lithic and volcanic rock fragments.

The tuf f s are sometimes well sorted and may have a larger percentage of crystal fragments. These fragments are often rounded and are concentrated in zones of the (tuffaceous) beds that are grain supported.

The identification and distinction between primary air fall ashes and tuff beds that have been reworked and have a epiclastic component is essential in the interpretation of the geologic significance of radiometric ages on mineral separates of those tuffaceous units. 34

40 39 Ar/ Ar AGES Of TUff UNITS

The following sections discuss the individual tuff beds that were sampled and dated for this study. The stratigraphic position, 40 field description, petrography, sample locations, and Ar/39Ar ages

are given for each tuff bed. The methods that were used in sample

collection, preparation, and analysis are described in Appendix I.

Big Island Tuff

The Big Island or "Main" Tuff of the Wilins Peak Member of the

Green River formation was named for subsurface occurences of the tuff

between the lower and upper Big Island trona deposits in the center of

Bridger Basin (figure 12). Culbertson (1961) identified it as the

third tuff above the base of the Wilkins Peak (figure 13). The tuff

is found within the upper third of the Wilkins Peak Member at

approximately 130 to 200 feet (40 to 61 m.) below the base of the Laney

Member of the Green River formation (figure 13).

figure 14 shows the outcrop of the Big Island Tuff. The tuff

varies from 2 to 24 inches (5 to 61 cm.) in thickness in a bed that has

a resistance that is nearly the same as the surrounding marlstones. The tuff appears light gray to orange-brown on weathered outcrop and

very light gray on fresh surfaces.

figure 15 shows the locations in which samples of the Big

Island Tuff were collected. All of the samples were collected in and

around the town of Green River, Wyoming. Samples were collected

from eight different locations with six of the samples being dated by 40 9 the Ar;3 Ar technique. In all cases only the bottom one third of

the tuff bed were collected since that portion of the tuff has the 35

SSl -2 591-22 T,oll Gate Rock at Green Raver

-4•11•• ~ -4ml111- ·-- 15 •Ilea ------

Sand1fona,- 'nt1rDeciJ1d ••'" 011 1Pll•11 local Ofl aftol•. ltr••"• 1 fomlf1ated, local corbonoc1ow1 fcssit S and1ton1 l•ttrDtdl of "••"'•"" 1oftd1tone 011 ......

Oil 111010 Sond1tori1 ••'" lominatloaa lominatad of oil 1hole LAN[ y $HAL£ M EMB!JE~R!!.-.------!-!- MorlUona- - - - Sond1to110

Sand1otone1 gray, local utter11t1d1 · .. · of 11101e onll , ,,.,,.; 0 0.1 shale ...... Morl1to110 ,,., •• ,,.... i ... ttro•fl,•h•I• local o•I Sands Iona

Morl1tone, gray • .ITop of lhortlle 9run Olld 9run­ .! i1h a.ro•"• local 1 nterted• ol .! Mar11tone, 1raa,.... oil 111010 • ilh ,,., , • u 91aafti1tii aro••i •. locol oil dole

~ • MYd1tono ••• > --!--. IOAdstone 7 ·­ l'•J brown

100 ----... ,...... t••r 9reen, wllll 1horllte ond u••••I oll allale 110111 ···-Big Island L-J---~4---,.,..,..+---::.:.::...:.::.:~---r-"""ITuff•ua1n• Tuff ·· .. J/ =-r.... =. fyff lllorker bed•

Figure 12: Columnar sections of parts of Wilkins Peak and Laney Members of the Green River Formation showing position of Big Island or Main Tuff relative to the upper and lower Big Island Trana Deposits in the central portion of Bridger Basin, Wyoming (Culbertson, 1971). 36

I Sq. Crtt• Sm 7Mdl T JU, I IC. W

FEET 0

·- - 11 lldles

500

- - - JillC~H

1000 Tipton shale mem~· of Green R111er formation EXPLANATION F===i El ~ limestone Softstone or silty mudstone r-7"1 B L._:_] Tuff S.ndstone ~ ~ D Mud stone Martstone. oil shale, dolomite, cl•ys'-. limestone, or covered

Figure 131 Columnar sections of Wilkins Peak Member of Green River

Formation showing position of Big Island Tuff and Tuff number 6 (Culbertson, 1961). 37

figure 14: An outcrop of the Big Island Tuff (sample 706-5) from a location that is one mile west of the town of ·Green River, Wyoming

(figure 15). 38

WYOMING

Mop Area

0 100 Milu

706 -6 &706-7 I 712-6 &712-7

15

14

13

T. WYOM iNG 12 R.J!~-~--~ Jll _llO ~~9- ---106 105

0 JO 20 Miles

Figure lSz Location map showing position of sampling of Big Island Tuff, tuff number six of the Wilkins Peak Member and 3/8 inch thick tuff at base of Laney Member of Green River Formation. 39 majority of biotite flakes concentrated within it.

Petrographv

The relative consistency of age data for the Big Island Tuff indicates that the biotite crystals are of a single population that has not been contaminated by older detrital biotite grains. The petrography of thin sections taken from samples of the Big Island Tuff in turn indicate that the tuff originated as a primary air fall ash that has undergone little reworking or admixing with preexisting sediments.

The Big Island Tuff consists of a mixture of altered vitric matrix, small angular crystals, and devitrified pumice fragments. The altered vitric component comprises 70 to 80 percent of the tuff and consists mainly of extremely fine grained intergrowths of authigenic

K-feldspar which is locally admixed with minor amounts of analcime.

The matrix also locally contains disseminated fine grained carbonate and opaque hematite. The matrix also contains up to 3 percent carbonaceous material.

The crystal-altered pumice component comprises 20 to 30 percent of the tuff. It consists of angular fragments of quartz (20-25 percent). plagioclase (25-30 percent), K-feldspar (30-35 percent) and biotite

(10-15 percent) with minor amounts of green hornblende (0-3 percent), pyroxene (0-2 percent), and altered pumice fragments (0-2 percent).

The quartz fragments are clear and mostly angular and sometimes show resorbed edges. The crystals are sometimes fractured or shattered and are up to 0.33 mm. in diameter. The plagioclase occurs in lath- shaped generally subhedral crystals that are up to o.25 mm. in diameter.

The crystals show albite and Carlsbad twinning and have a general composition that ranges from oligoclase to andesine •. The K-feldspar 40 is dominated by sanidine in crystals that are up to 0.33 mm. in size.

The crystals are rectangular in shape and often show Carlsbad twinning.

The edges of many of the plagioclase and K-feldspar crystals are resorbed into the analcime matrix. The feldspar crystals are often partially to completely altered to sericite (figure 16). Biotite is present as lenticular crystals that are up to 1 mm. in length. The crystals are yellow-brown to brown pleochroic and at times are rimmed by iron oxides.

The general texture of the tuff shows a crystal component of angular, poorly sorted grains that show a gradation in size and number of crystals away from the base of the tuff (figures 16A and 16C).

The crystals are nearly everywhere matrix supported and contain no constituents that can be interpreted as being non tephric in origin

(Figures 16A-C). These facts indicate that the Big Island Tuff originated as a primary air fall ash that has undergone little reworking or admixing with preexisting sediments.

Age Data

Figures 17A to 17E show age spectrum diagrams for incremental release experiemnts on biotite separates from five different samples of of the Big Island Tuff.

Figures 17A, 17B, and 17C (samples 706-5, 712-4 and 712-5) display disturbed age spectra that have anamolously low apparent ages for the low temperature increment(s). The remaining increments from each spectrum form plateaus as defined by Fleck, Sutter, and Elliot

(1977). These age spectra are typical of post crystallization loss of radiogenic 40Ar from the biotite structure. The weight average of apparent ages for all of the increments that help define the plateau 41

figure loA: Photomicrograph from base of sample 706-5 of Big Island

Tuff showing angular grains of quartz (Q), K-feldspar (K), biotite (b) and plagioclase replaced by sericite (p) in analcime-fel dspar matrix. (Width of photograph= 1.45 mm.) ------.

42

Figure 168: Photomicrograph of sample 706-5 of Big Island Tuff showing angular grains of K-feldspar, biotite, devitrified pumice fragments (pu), sericite (s) with a rounded carbonate fragment (c) in analcime matrix (field width = 1.45 mm.). 43

Figure 15C: Photomicrograph of sample 706-5 of Big Island Tuff approx­ imately 30 mm. above base of tuff showing biotite, quartz and feldspar grains in fine grained matrix of authigenic feldspar and analcime. Note decrease in size and number of grains away from base when compared with

Figure 17A (field width= 1.45 mm.). 44

ct

1 :ii;. -e· -CD cbl Tp = 49. 4 ± 1. 3 m.y. ... c:: -...cu Tf = 49.0 ± 0.7 m. y. ~ !' a.ca. c

0 60 80

Cumulative ~

5 7 :::c < CD... < CD -I ~ - CD bl c C> :"" Tp=S0.0±0.8m.y. ::JJ Tf-= 49 .8 ±0. 7 m.y. ~ !'

401 L.....~~~--L~~~~-L.~~~~-'--~~~--1~--~~-- o 20 40 60 80 39 ArK Released, Cumulative~

Figure 17: Age spectra for samples of Big Island Tuff. Tp= weight average plateau age, Tf= two step fusion age. 45

55 17C C>.... CD CD :::s ::0

E

CD cbl -45 Tp= 48.8±0.9m.y. c CD... Tf =49.1±0.7m.y. ~ a. c

0 20 40 60 80 39 ArK Released, Cumulative ~

figure 17C: Age spectrum of sample 712-3 of Big Island Tuff. 46

-CD cbl Tp=54.9±1.5m.y. Tf=s1.0±1.sm.y.

20 40 60 80 39 ArK Released, Cumulative ~

Figure 170: Age spectrum of sample 712-7 of Big Island Tuff. 47

E 4 -cu tll c TP= 49.7± 1.1 m.y. c Tf =49.e±o.em.y. -cu... nla. 4 a. c

20 . 40 60 80 39 ArK Released, Cumulative %------

Figure 17E: Age spectra of sample 712-4 of Big Island Tuff. T = p weight average plateau age, Tr= fusion age. 48 of an age spectrl.11 for a particular sample is taken in this paper to represent the age of formation for the biotites analyzed from each sample of the Big Island Tuff. Since this tuff originated as an air fall ash, the plateau age also represents the age of formation of the tuff bed.

Figures 17A to 17£ also show the ages obtained from the two and three step fusion experiment (Tf). As was noted earlier, the fusion experiment was run for each sample to release all of the argon gas that was part of the plateau in the incremental release experiment in a single gas fraction. This increases the total amount of gas that the mass spectrometer measures which reduces the amount of analytical error associated with the measurement of ratios between the argon 39 36 1so. t opes 40A r, Ar and Ar. This in turn reduces the standard deviation (±,) associated with the age obtained for that gas fraction.

Inspection of Figures 16A to 16( reveals that the standard deviation (+) associated with the fusion age (Tf) for each sample dated is lower than the Standard Deviation associated with the weight average plateau age

(.±)obtained for the same sample.

Figure 170 (sample 712-3) ahows a three step r~lease sepctrum in which the apparent ages of all three increments are equal thus forming an undisturbed spectrum. The sample was actually subjected to five incremental heating steps but two of the steps released less 39 than one percent of the total Ar released and were thus disregarded

(Appendix II). Figura l7E (sample 712-7) displays a spectr1.JT1 in which the apparent ages of the increments form a plateau that has a weight average age that is much greater (5 m.y.) than ages found for other 49 samples of the Big Island Tuff. The total fusion experiment gave an age that was much closer to those found in other samples but is still anomolouslyhigh (Appendix II, figure 17E). No explanation can be given· at this point for the anomalously high ages obtained for this sample since several different causes are possible (contamination, argon fractionation, potassium leaching, etc.). Regardless of the reason, the plateau age for sample 712-7 gives an anomalously high age that when compared with other samples of the Big Island Tuff through the use of the critical value test of Dalrymple and Lamphere (p. 120, 1969) is found to be at the 95 percent confidence level different or separate in age. for this reason, all age data for sample 712-7 has been eliminated from calculations for determining the average age and error for the Big Island Tuff.

The age of the Big Island Tuff was calculated by averaging the weight average plateau ages for samples 706-5, 712-3, 712-4,and 712-5 (Appendix II) with the ages obtained from the two and three step fusion experiemnts (Appendix II) for samples 706-5, 712-1, 712-3,

712-4 and 712-5. Thus the age of the Big Island Tuff was calculated by averaging ages from nine separate analyses. The calculations and ages used in determining the average age of the Big Island Tuff plus its error are given in Table l. These calculations give an average age of 49.4± 0.4 m.y. for the Big Island Tuff.

Mauger (1977) determined ages for samples of the Big Island

Tuff using conventional K/Ar techniques. Those values when converted

using 1976 IUGS decay constants (Dalrymple, 1979) give an average age of 50.2± 1.1 m.y. This value is approximately a.a m.y. older than the age determined in this study but is equivalent using the

critical value test. so

BIG ISLAND TUf'f'

SAMPLE NO, PLATEAU AGE fUSION AGE

706-5 49,4 m,y, 49,0 m,y,

712-5 so.a m,y, 49,8 m,y,

712-4 49,7 m,y, 49,6 m,y,

712-3 48,8 m,y, 49,1 m,y,

712-1 49,3 m,y,

TOTALS 197,9 m,y, 246,8 m,y,

AVERAGE AGE = (197,9 + 245,8) = 444,7 m,y, = 49.4 m.y. 9 9

2 STANDARD DEVIATION =

AVERAGE AGE Of' BIG ISLAND TUff = 49,4± 0,4 m,y,

Tabla 1: Showing ages obtained from incremental release and three step fusion experiments, 51

Wilkins Peak Tuff Number Six

Sample 706-7 was taken from an exposure of the uppermost tuff

bed of the Wilkins Peak Member of the Green River rormation. The tuff bed is called the sixth tuff of the Wilkins Peak Member by Culbertson. It is present at the top or just a few feet below the top of the

Wilkins Peak Member (figure 13) in Bridger Basin. The sample was collected along the west wall of the rlaming Gorge approximately two miles south of the town of Green River, Wyoming (rigure 15).

The tuff bed varies from 3 inches to 20 inches (8-51 cm.) in thickness and appears light gray to orange-brown in outcrop (figure 18).

The tuff contains brown patches of hematite in the bottom half of the tuff bed (figure 18).

Petrography

Petrographic analysis of sample 706-7 reveals that tuff number six of the Wilkins Peak Member of the Green River formation is very similar to the Big Island Tuff. The tuff is made up of a mixture of

85 to 90 percent altered vitric matrix and 10 to 15 percent angular crystal grains.

The matrix is made up of extremely small intergrowths of authigenic feldspar. These intergrowths are replaced by small circular crystals of analcime (figure 19). Dark organic matter is dispersed throughout the matrix and some areas of the mat~ix are replaced by up to 3 cm. diameter patches of hematite.

The crystal component consists of small (0.05-0.5 mm.), angular crystals of quartz (20-25 percent, plagioclase (20-25 percent), K-feld­ spar (35-40 percent), and biotite (10-15 percent) with minor amounts of green hornblende and pyroxene. 52

Figure 18: Outcrop of tuff number six of Wilkins Peak Member of Green

River Formation. Note dark patches at base of tuff. 53

Figure 19: Photomicrograph of sample 706-7 of tuff number six of

Wilkins Peak Member of Green River Formation. Photo shows plagioclase lath in a matrix of fine grained intergrowths of authigenic feldspar which is cut by circular crystals of isotropic analcima. {Field width = 1.45 mm.) 54

The quartz crystals are clear and angular and are sometimes fractured. The edges are sometimes embayed and resorbed. The plagio- clase occurs in lath shaped subhedral crystals that are often fractured and angular. The laths are often partially altered and have compositioAs that range from oligoclase to andesine (figure 19). K-feldspar is found as rectangular shaped grains. The feldspar crystals may be altered and may sometimes show absorbtion along edges by analcime crystals. The biotite is found in lenticular yellow-brown to brown pleochroic crystals that measure up to 1 mm. in length. The biotite crystals are sometimes surrounded by iron oxide rims.

The tuff is poorly sorted and contains angular crystal grains that are supported by the altered vitric matrix. The sample also shows an upward grading in the size and number of crystal grains and contains no lithic fragments or non volcanic minerals. The above description indicates that this tuff originated as a primary air fall ash that has had little reworking of the pyroclastic sediment.

Age Data

Figure 20 shows the age spectrum from the temperature release experiment for sample 706-7. The diagram shows an undisturbed spectrum that has a weight average plateau age of 46.7± 1.4 m.y. (one of the five increments was disregarded since it contained less than l percent 39 of the total Ar). This value when averaged with the total fusion age (46.4± 0.7 m.y.) gives an average age for this tuff of 46.6±, 1.0 m.y. This age appears to be quite reliable and reproducible for this location but more samples at different locations should be dated to test the precision and accuracy of the age obtained. 55

I . -E CD bl c Tp =46.7± 1.4m.y.

c -CD ... Tf =46.4±0.7 m.y . a. "'a. c

35 0 20 40 60 80 39 Ar Released, Cumulative ~ ------

Figure 20: Age spectrum of sample 706-7 of tuff number six of the

Wilkins Peak Member of the Green River Formation. T • weight p average plateau age; Tf =fusion age. 56 flaming Gorge Tuff

Samples 706-6, 711-2 and 712-6 were collected from a thin, 3/8 inch (1 cm.) thick tuff that is present at the base of the Laney

Member of the Green River formation (figure 21). These samples were collected along the western wall of flaming Gorge approximately two miles (3 km.) south of the town of Green River, Wyoming (figure 15).

The tuff appears dark brown in weathered outcrops and dark gray on fresh surfaces. The tuff bed varies from 3/8 to 1 inch in

(l-2.54 cm.) thickness and is often covered by the slope wash from overlying sediments. The tuff is made up of two layers. The upper one third of the sample is made up of fine grained altered matrix and the lower two thirds is made up mostly of crystal grains.

Petrography

The anomalously high ages obtained for samples of this tuff are probably the result of contamination or admixing of older pre­ existing detrital biotite grains during the deposition of the tuff.

The biotite separates show at least two different types of biotite when viewed under a binocular microscope. Petrographic analysis of thin sections from samples 706-6, 711-2 and 712-6 indicate that the tuff has been extensively reworked and did not originate as a primary air fall ash.

The tuff is made up of two layers. The upper layer is 1/8 to 1/4 inch (3-6 cm.) thick and consists of varying mixtures of analcime and vary fine grained intergrowths of authigenic K-feldspar.

The upper layer has a very sharp contact with the crystal rich lower layer and itself contains less than five percent crystal grains. 57

The lower layer is 1/8 to 1/4 inch (3-6 cm.) thick and consists of 30 to 40 percent matrix with 60 to 70 percent rounded crystal grains.

The matrix component consists of varying mixtures of f ina grained analcime, adularia and carbonate.

The crystal component is made up of angular to well rounded grains of biotite (30-35 percent), plagioclase (10-20 percent), K-feldspar (15

-20 percent, quartz (25-30 percent) and green hornblend (5-8 percent).

The biotite is found in lenticular grains of up to 1 mm. that show yellow-brown to brown pleochroism and often are rimmed by iron oxides. The quartz grains are clear and range from angular to rounded grains that are up to 0.25 mm. in diameter. The quartz grains often have primary syntaxial overgrowths that preserve the original wall rounded outlines of the original detrital grains (Figure 21). The plagioclasa and K-feldspar crystals also show rounding with the K-feld­ spar also showing syntaxial overgrowths (Figura 21).

The overall textures of the thin sections show that the lower crystal rich layer is made up of well sorted angular to rounded grains in a framework that is grain supported (Figura 21). The presence of rounded grains along with the syntaxial overgrowths that preserve rounded grain outlines, the grain sorting of crystals, and the grain supported framework of the pyroclastic sediment all indicate that the tuff originated as a water transported elastic sediment that had bean derived. from preexisting pyroclastic deposits. This leads to the conclusion that the biotite component was at least partially contaminated by older biotite grains which would have given the anomolous ages found for this tuff. 58

•

Figure 21A: Photomicrograph from t hin section of 3/8 inch thick

Flaming Gorge Tuff with angular to rounded grains of biotite, quartz

and feldspars in matrix of fine grained intergrowths of authigenic

feldspar which is cut by circular crystals of analcime. Note quartz

grain near left center of photo (q) which has syntaxial overgrowths that preserve rounded outline of quartz grain. (Field width = 1.45 mm.). 59

1

Figure 218: Photomicrograph from thin section of 3/8 inch thick

Flaming Gorge tuff. Photo was taken near the base of tuff and shows

angular to rounded grained supported crystals of quartz (q), K-feldspar

(K), hornblende (h), and biotite in a matrix t hat is made up of carbonate which is cut by circular isotropic crystals of analcime. (Field width = 1.45 111111.) 60

Age Data

figures 22A, 228 and 22C show disturbed age spectrum that have plateau ages that are older than their stratigraphic position would

indicate. Sample 706-6 has a weight average plateau age of 50.7± 0.9 m~y. The plateau age for sample 711-2 is 51.4± 1.5 m.y. and that of sample

712-6 is 58.9± 1.6 m.y. These ages along with the ages from the two

step release experiments (figures 22A-22C), Appendix II) give an average

age of 53.7± 1.3 m.y. This is significantly older than ages obtained

for the Big Island Tuff and tuff number six of the Wilkins Peak Member.

These two tuffs are stratigraphically lower than this Laney Member tuff

bed and should thus be older than this unit. for this reason, the ages

obtained for samples of this tuff must be considered anomalous and useless for stratigraphic correlation.

Sand Butte Tuff 1·1 Samples 705-3 and 705-5 were taken from a tuff bed that is I ' located in the upper portion of the LaClede bed of the Laney Member of

the Green River Formation. The tuff is present just a few feat above

a resistant algal bed that forms a bench 50 to 55 feet (15-17 m.)

above the Buff Marker of the Laclede Bed (figure 23) along escarpments

of Pine and Sand Buttes in the northwest portion of Washakie Basin (Figura 24).

The tuff is approximately 1.5 inches (4 cm.) in thickness

(Figure 25) and appears tan to orange-brown in weathered outcrop and light gray on fresh surfaces. The tuff is made up of two layers. The lower layer contains abundant fine grained biotite crystals that grade into the upper layer which is devoid of any biotita grains. 61

- -

... Tp=50. 7± 1.6 m. y. ftl CL Tf=51.4± o.s m.y. cCL

20 40 60 80 39 ArK Released, Cumulative %

- - u bl c -;... 45 Tp= 51.4 ± 1.5 m. y. ftl CL CL Tf= 51. 2 ± 1.0 m. y. c

401~~~~~i-~~~~i-~~~~.1-~~~~..._~~~__, 0 20 40 60 80 39 ArK Released, Cumulative %

Figures 22A and 228: Age spectra of 3/8 inch thick flaming Gorge

Tuff. Tp• plateau age; Tf= fusion age. 62

Tp=59.0± 1.6 m. y. T f=57. 7± 0.7 m. y.

0 20 40 60 80 39 ArK Released, Cumulative %

Figure 22C: Age spectrum of sample 712-6 of 3/8 inch thick tuff from Flaming Gorge. Tp= weight average plateau age; T,= fusion age. 63

SAND BUTTE SW 1/4 SEC 22, AND El/2SEC21, T16N, RIOOW OIL YIELD OIL YIELD LITHOLOGY OGA~6T~~ GAL/TON LITHOLOGY O 20 40 Sand Butte '! ;/ Bed

; I 100

' II! :11 !I I 80 : I ,.. -

"'Cl I' ., I:' ~ 60 Sand m ..., Butte- ., '. ....J "'Cl c Tuff ., ~ 40 ~ _,"'

I'

I II· I: '1,' ! : Buff i I' Marker d; ! '

Figure 23: Part of measured section at Sand Butte Location showing position of 1.5 inch (4 cm.) tuff in upper portion of Laclede Bed of 'I Laney Member (Trudell et al, 1973). i 64

1- l••LAMATIOM

lt.17 W. Tlri1 ' •tt 9f U•"'•" ... T•e,, ...... i~I =:...,ef...... :::.--... },... h ...... I "'' ...... , I i ...... ""• !.,_r;;;;.-o -" , ...... } Twm : ft'I s.... lwt1e lel ....., M1M .. o ef Gflef' ~.. •.... ,...... I I T11• · 1...C1M1 IN . ,,--I J -~ -,-. ~ ...... 1.,., ...... - I ...~ ,- - • •. ....,..... ,, ,...... ,., ..... ,,.l'I .. I~~ c;.. ••~ •.••• , ...... j I Tf'- 'r::,.s:~:.~:-.:::.:: I .._~ ""'"',., .... ,...... ,.,,.,. •' •w•c• 1--f .~ l..Wl""O"..... ,_.,..,...... c;,._.. 'T::;i .....,...... ,111 Fl-- '''"""•'·... j{-r•.. '•'""'-' ,_,.._,,_. :~ ~ Jt•• , .. ,., ....

''"'""'-' •... Co~•c:' ~•"U :-...... , ""0,.,., ..'t ·OCO'll

0 61i111L[S •.. ,_

/·Y:l~G I UT AH 1 CO~OllADO

JO

Figure 24: Geologic map of western half of Washakie Basin showing sample locations of samples 705-3 and 705-5 of the 1.5 inch (4 cm.) thick tuff in upper portion of LaClede Bed of Laney Member of Green River Formation (Trudell et al, 1973). Figure 25: Outcrop of l.s--2.0 inch (4-5 cm.) thick tuff from upper portion of LaClede Bed of Laney Member of the Green River formation.

Picture was taken along west scarp of Sand Butte of Washakie Basin. 66

Petrographv As was noted earlier petrographic examination of thin sections of samples 705-3 and 705-5 show that the tuff originated as a primary air fall ash that has undergone little reworking.

The tuff is divided into two layers. The upper layer is approximately 0.7 to l.O inches (1.e-2.5 cm.) thick. It consists of fine grained crystals of analcime and is nearly devoid of crystals except where the upper layer grades into the lower crystal layer.

The lower portion of the tuff is approximately 0.5 to o.75 inches (1.3-1.8 cm.) thick and is made up of a mixture of altered vitric matrix and small (0.2S mm.), angular crystals. The matrix is composed of small analcime crystals and comprises SS to 70 percent of the lower 1 Il layer. The crystals are rather small and angular and comprise 30 to 4S percent of the lower layer. (rigure 26)

The crystal component consists of angular crystals of quartz

(20-2S percent), K-feldspar (2S-30 percent), biotite (35-40 percent) and plagioclase (5-10 perCll'nt). The biotite crystals are lenticular and up to o.s mm. in length. The crystals are brown to yellow-brown pleochroic and are sometimes rimmed by iron oxides. The biotite grains are nearly all parallel to the base of the tuff. The quartz and feldspar grains are nearly all angular and are sometimes heavily fractured. The feldspar grains may show absorption into the analcime matrix. The size of the crystals range from a maximum of o.2s mm. down to submicroscopic.

The overall texture of the tuff shows that the tuff is poorly sorted. It contains small angular crystals that are supported by the altered vitric matrix. These crystal grains grade in size and number 67

Figure 26: Photomicrograph of thin section from base of sample 705-3.

Picture was taken under uncrossed nicols and shows small angular feldspar

(f) biotite (b) and quartz grains in fine grained analcime matrix. (Field width= 1.45 mm.) 68 away from the base of the tuff. These textural relationships indicate that the tuff originated as a primary air fall ash that has undergone little reworking or admixing with other sediments.

Age Data

Figures 27A and 278 show the age spectrum for samples 705-3 and

705-5. Both diagrams show disturbed spectra that form plateaus.

However, the weight average ages for the plateaus differ by more than

11 m.y. The ages obtained from the two step fusion experiments also differed by nearly 11 m.y.

Sample 705-3 (Figure 27A) shows a disturbed age spectum that has a weight average plateau age of 58.3± 2.2 m.y. The two step fusion age

(Appendix II) for this sample is 56.8± 1.3 m.y. This gives an average age of 57.5± 1.8 m.y. which is much older than what the stratigraphic position would indicate. This- age is also 9 to 11 m.y. older than ages obtained for tuffs in the underlying Wilkins Peak ~ember. Petrographic analysis of this sample indicates that the sample originated as a primary air fall ash that had not been reworked or admixed with detrial biotites.

Subsequent dating of samples 705-5 (Figure 218) produced a disturbed spectrum with a weight average plateau age of 45.2± 2.2 m.y. This value when averaged with the two step fusion age (Figure 278) of

45.2± 1.1 m.y. gives an average age of 45.2± 1.7 m.y. This age is more than 12 m.y. younger than the age obtained for sample 705-3 and is more consistent with the stratigraphic position of the tuff.

The discrepancy in ages for the two samples of the same tuff bed can be explained in part by examination of the argon release data

(Appendix II) for these two samples. This data shows that sample 705-3

(which gave an anomalously high age) released one third to nearly one -1 -E CD cbl ~ 55 ...CD

aCL Tp = 58 .3 ±2. 2 m.y. c Tf = 56.8±1.3 m.y.

50'--~~~--l.~~~~-'-~~~~---~~~--i'--~~ 0 20 40 60 80 39 Ar K Released, Cumulative%

50 27 I ->. 45 e

CD cbl = -...CD "8 CL Tp=45.2±2.2m.y. CL c Tf = 45.2±1 .1 m.J.

20 40 60 80 39 ArK Released, Cumulative %

Figures 27A and 278: Age spectra for samples 705-3 and 705-5 of 1.5 inch thick tuff from upper portion of LaClede Bed of Laney Member, of Green

R~ver formation. 70

39 half less Ar than the younger 705-5. This corresponds to a deficiency of potassium within the structure of the biotite crystals.

Mauger (1977) has shown that biotites in Green River formation tuffs lose potassium when the biotites weather under oxidizing conditions.

The biotite crystals alter to hydrobiotite (which is an interlayered mixture of biotite and vermiculite) through the substitution of potassium ions .by water molecules. The hydrobiotite is a mixture of layers of biotite and vermiculite. As the alteration continues the flakes remain physically stable but change in color from dark brown and dark green to shades of yellow brown as ferrous iron is oxidized to ferric iron.

Mauger also believed that the alteration of the biotites would have disrupted the argon system for the biotite. Apparently this was not 39 the case for this sample since only the release of Ar is significantly different between the two samples.

The biotite separate for sample ?05-3 was reexamined under a binocular microscope. This examination revealed that the biotite separate contained two fractions of biotite. One fraction was dark and blocky and closely resembled the biotites from sample 705-5. This fraction sank easily in bromoform liquid. The second fraction of biotite was lighter in color, was less blocky, and (loated in heavy liquid.

The two fractions of biotite were separated from each other and 40 39 • prepared for Ar/ Ar dating. It was found that the darker, heavier fraction of unaltered biotites gave a two step fusion age of 44.15± 0.6 m.y.

The lighter, altered fraction of biotite released nearly one third less 39 Ar than the unaltered fraction. It gave a one step fusion age of 58.4±•

D.9 m.y. from these results it is suggested that some of the biotites from this tuff have been altered recently under oxidizing conditions. 71

This alteration replaced water molecules for potassium ions but did not upset the argon system of the altered biotites.

An average age of 44.9± 1.3 m.y. has been calculated for this tuff using the two ages obtained from sample 705-5 and the age obtained from the heavy fraction of sample 705-3. This age is tentative but does fit the stratigraphic position of the tuff bed. More age determination should be made for this tuff to determine a more precise age. Care should be taken that the biotite separates for these analyst1ses do not have any altered grains.

Robin's Egg Blue Tuff

At the base of the Adobe Town Member of the Washakie formation is a bluish-green tuff aceous sediment that is informally called the robin's egg blue tuff or robin's egg blue marker (Roehler, 1973). The tuff is from 4 to 25 feet (1.2-7.6 m.) thick and is present approximately

10 feet (3 m.) above the base of the Adobe Town Member (figure 28)

(Roehler, 1973). The tuff appears light bluish green in non-resistant outcrops and has an abundance of biotite.

Seven samples of the tuff were collected from Washakie Basin. Two samples 706-1 and 706-2) were. collected from the center of the basin near Bitter Creek (figure 29). The remaining five samples (706-3, 706-4,

713-1, 713-2, and 713-3 were collected near the western edge of the

Washakie Basin at Trail Dougway (figure 29).

Petrograehy

The anomalous ages obtained for samples of the robin's egg tuff can be explained by the petrography of the tuff. Thin section study of samples of the tuff reveal that the tuff bed more closely resembles a sandstone than an air fall ash. 72

WASHAKIE BASIN (This report)

Chiefly l'•Y arkOStC and tulf­ ac:eous und11one; tan. l'•Y. .,_•nd peen tuff.ceou1 rnud­

Chiefly Pastel gray. l'Hn, and IOme rad tuff.:eou1 mud· 1tone; tr•v. ''""· and r.cl arkoste sandstone

z 0 .. ~ .! ~ E Tulf-637 ~ .. er ~ • 0 : Ch1•flv gray, ''""· •nd rtrd u.. C - tuff.ceous mudstone, l'/Y iPI :~~;:~~

Chiefly tr•v. """· end some red tuffK1ou1 mudslone; l'IY - poan arkOllC Nnd· II-

(robin's-en-blueTulf-579---t---robin's merker becO era blue tuft .W lower brown sandstone& (bed 5691 .. .c.. ~ ~. E~ CC 8 and """ undstone :-c OI T11ff - 540 .!: lll:

._....__...... ____ Tull - 515 fwhue ud.. marker bed) Green River Fm.

Figure 28: Generalized stratigraphic column of Washakie formation showing position of robin's egg blue tuff near base of Adobe Town Member (Roehler, 1973). 73

11.17 •. 11.HW. 11.15 •. llHW. I

s,-c' . ..._• ••·• Co,..oc• OO\"-• ;..;• ...,••·"'O••, 1oco•14f

0 • flttf..[5 ... ,,_

WYOlllllG

UT AH COLORADO

figure 29: Geologic map of western two-thirds of Washakie Basin showing locations of sampling of robin's egg blue tuff (modified from Trudell et al, 1973).

ii

fi. 74

The robin's egg blue tuff is a pyroclastic rock that is composed of 35 to 45 percent fine grained angular crystal grains and 45 to 55 percent altered vitric matrix. The framework of the tuff varies from grain supported in various samples (Figure 30) to matrix supported.

The altered vitric matrix is predominately made up of fine grained analcime with minor amounts of small intergrowths of authigenic feldspar.

The crystal-lithic component is made up of angular to rounded grains of quartz (10-15 percent, plagioclase (20-25 percent), and K-feldspar

(20-25 percent), biotite (15-21 percent, hornblende (5-10 percent), and lithic rock fragments (5-10 percent), with minor and trace amounts of pyroxene and zircon.

The plagioclase occurs in subhedral lath shaped grains that are often angular to rounded and may show albite, Carlsbad and pericline twinning. The K-feldspar is mainly sanidine that shows Carlsbad twins and may be found in crystals that are zoned (Figure 30). The K-feldspar crystals are mainly angular but some sub-rounded to rounded crystals are present. The biotite is yellow-brown to brown pleochroic and is found in lenticular crystals up to l mm. in length. The biotite crystals are often rimmed by iron oxide coatings. The hornblende is found in small (less than 0.2 mm.) lath shaped crystals that are pleochroic green.

The lithic fragments consist mainly of altered volcanic rock fragments that are well rounded (Figure 300). Other rock fragments include rounded chert fragments and polycrystalline fragments of quartz and feldspar. The textural and mineralogical make-up of the robin's egg blue tuff shows that this is a grain supported to matrix supported elastic 75

Figure 3BA: Photomicrograph of thin section of sample 706-1 of robin's

egg blue tuff showing grain supported crystals of rounded quartz (q), angular feldspar (f) lenticular biotite (b) with a matrix of fine grained feldspars and analcime. (Field width= 1.45 mm.). ------~- - -~ - - -- - '

76

Figure 308: Photomicrograph of single, rounded plagioclase grain in fine grained analcime matrix. (Field width= o.59 mm.)

Figure 3 OC: Photomicrograph of zoned feldspar· crystal along with quartz and plagioclase grains in a matrix of fine grained feldspar and analcime. (Field width = 1.45 mm.) .

77

Figure 300: Photomicrograph of sample of robin's egg blue tuff showing a rounded volcanic rock fragment (note plagioclase laths) in fine grained feldspar-analcime matrix. (Field width = 0.6 mm.) 78 rock that is made up of pyroclastic crystal fragments, altered vitric matrix, and lithic rock fragments. The sample is in general poorly sorted and contains many rounded crystal and lithic fragments. This evidence would indicate that the sample is not a primary air fall ash, but rather a reworked elastic sediment that has had a large influx of 40 39 contaminant material. The Ar/ Ar ages of this tuff suggest that the biotites found in this tuff are derived from preexisting rocks of

Cretaceous age or older.

Age Data

A previous study by Mauger (1977) using conventional K-Ar dating techniques had shown that ages from biotite separates from the robin's egg blue tuff were anomalously high. Oates obtained in this study for the robin's egg blue tuff are also much higher than what their strati- graphic position would indicate.

Figure 31 shows the age spectrum for sample 706-2. The spectrum is discordant (no plateau) and gives a total gas age of 104.3± 2.5 m.y.

The total fusion age for this sample gave an age of 102.3± 1.5 m.y. (Figure 31 and Appendix II). These ages are more than twice what their I stratigraphic position would indicate.

After examination of the release spectrum for sample 706-2, it I1 was decided that further incremental release experiments would be a ~ I waste of analytical time with the mass spectrometer. Two total fusion experiments were run for samples 706-3 and 706-4 (Appendix II). These samples also gave anomolously high ages of 133.6± 1.4 m.y. (706-3) and

102.0± 1.6 m.y. (706-4). 79

20

. NO PLATEAU ->. 1 E T1= 104.3±2.Sm.y.

-CD cbl c 100 -CD :; 10 a. a. c

20 40 60 80 39 ArK Released, Cumulative~-----

rigure 31: Disrupted age spectrum of sample 706-2 of Robin's Egg Blue Tuff (T Total gas age). g = 80

Curly Tuff

The Curly Tuff is named for exposures of a contorted tuff bed that is found below the Mahogany Ledge (Mahogany Zone in subsurface) of the Parachute Creek Member of the Green River Formation of Utah and

Colorado (Cashion, personal communication). The tuff occurs 25 t~

85 feet (7.6-26 m.) below the Mahogany oil-shale bed and is commonly found within the nonresistant marlstones of the 8 Groove (Figure 32)

(Cashion, 1967). The sample is noted for its contorted upper and lower surfaces and changes in thickness which ar~ due to plastic flowage of the tuff. The tuff ranges from l to 18 inches (2.5-46 cm.) in thickness.

The tuff appears gray to orange-brown on weathered surfaces and gray to tan on fresh surfaces. The tuff is often indurated with heavy, nearly insoluble petroleum residues that make biotite separations very difficult.

Samples of the Curly tuff were collected from locations that are found in the eastern Uinta Basin (Figure 33). The Curly Tuff was not present at several locations in which the Wavy Tuff was collected or if present, it was determined to be too deficient in biotite content to warrant sample collection.

Petrography

Petrographic examination of thin sections of the Curly Tuff along 40 39 with the Ar/ Ar ages for the tuff indicate that the tuff bed probably originated as a primary air fall ash. However, many of the original sedimentary structures may have been removed by the plastic flow of the tuff.

The tuff is made up of between 60 to BO percent altered vitric matrix and 20 to 40 percent angular crystal grains. The matrix varies in composition from nearly 100 percent analcime to nearly 100 percent fine Bl

...... Horse Bench ·-:-;.-;· ·.. -~~-::~.-:::: =~: :~-?.--~~-~ Sandstone ...... 300

200

100

50

0 vertical scale (feet)

Mahogany Marke r-:-----c:~:r-7~6:!:r::!=:!:::I Ma hogan y oi I - sha I e ______...__..__._....,._,....._,...-.,, Bed Curly :Tuff _____...... ,_....,...,..i-...... ,....,...... ,......

...... SI Sandstone ......

Figure 321 Stratigraphic column of Parachute Creek Member of Green River

Formation showing position of Curly and Wavy Tuff. ---

+ I

10~0L_. a

Topographic Districts ot the Uinta Basin 0 4 8 12 16 32 . Scale: ' miles

Fiaure 331 Outline map of the Uinta Basin showing locations of sampling stationg for Curly Tuff. 03 grained intergrowths of authigenic feldspar. The matrix also contains patches of hematite and locally has bands of disseminated carbonate grains. -- ~----.

84

fiaure 34: Photomicrograph of thin section from a sample of the Curly

Tuff (707-2) showing angular quartz (and feldspar} grains with overgrowths in a matrix that is predominantly analcime. (field width= 1.45 mm.) 85

Age Data

figures 35A to 350 show the age spectra for samples 707-2, 708-3 and 708-5 of the Curly Tuff. The spectra are disturbed but form plateaus that have weight average ages that range from 47.3± 0.1 m.y. to 45.7± } :: 1.7 m.y. These ages plus the two step fusion ages for samples 707-2,

708-3, and 708-5 produce an average age of 46.2± 0.7 m.y. (Table 2).

Wavy Tuff

The Wavy Tuff is found above the Mahogany Ledge (Mahogany Zone in subsurface) of the Parachute Creek Member of the Green River Formation.

The tuf f is present in the eastern Uinta and Piceance Creek Basins approximately 55 to 85 feet (17 to 26 m.) above the Mahogany marker and is found in the nonresistant beds of the A groove (Figure 32). The tuff averages 12 inches (31 cm.) in thickness but also shows undulatory upper and lower surfaces similar to the Curly Tuff and may be up to 20 feet

(6 m.) thick. The tuff weathers to an orange~rown color and is tan to gray on fresh surfaces. The relatively non-resistant tuff also contains petrolel.Jll residues in the pore spaces of the rock.

Eight samples ware collected from outcrops in eastern Uinta

Basin (Utah) and one sample of the tuff was collected from the Piceance

Creek Basin near Rio Blanco, Colorado (figure 36).

Petrography

The petrography of the Wavy Tuff is almost identical to that of the underlying Curly Tuff. The sample is composed of from 60 to 80 percent altered vitric matrix with 20 to 40 percent angular crystal grains

(figure 371).

The matrix varies in composition from pure analcime to nearly pure fine grained intergrowths of authigenic feldspar. The tuff also

• 86

50 ALY TUFF - J

1 .!. 45 :::.;. -E CD cbl ~ 40 Tp= 45.7±1.7m.y. CD .... Tf = 44.4±1.2m.y . Q."' cQ.

35 0 20 40 60 80 39A rK Released, Cumulative ~

50

:0 ~.

4 - E ....:::I - ::::rIll Tp=45. 7±2.4m.y. () 0 Tf=45.3 ±1.0m.y. = -....cu Q, "'Q. c

20 40 60 80 39A R rl< eleased, Cumulative %

Figures 35A and 358: Age spectra for samples 707-2 and 708-3 of Curly

Tuff. Tp= plateau age; Tr= fusion age. 87

c: Ill =t'- I 0 0 ...0 a..Ill 0 Tp== 4 7. 3±0. 7 m. y Tt== 4 7.6±0.7 m.y

0 20 40 60 80 39 Ar K Released, Cumulative ~

Ficure 35C: Age spectrum for sample 708-5 of Curly Tuff. During analysis

of this sample two gas fractions were lost due to apparatus breakdown. [

Subsequent retesting of this sample has shown that the spectra forms a plateau. 88

50 7 I - 45 -0 >. c E a.•

CD bt c c ± Tp= 46. 7 0.5 m. Y. -•~ I ~ 40 0 CD... 0 0 Q. ~ "'Q. c a.• 0

35 7 0°C 0 20 40 60 80 39 ArK Released, Cumulative %

Figure 350: Age spectrum for sample 708-5 of Curly Tuff. 89

CURLY TUFF'

SAMPLE NO, PLATEAU AGE FUSION AGE

707-2 45,7 m.y, 44,4 m,y,

708-3 45,7 m,y, 45,3 m,y,

708-5 47,3 m.y. 47,6 m,y, 708-5 46,7 m,y,

TOTALS 185,4 m.y. 1~7.3 m,y,

AVERAGE AGE = 185,4 + 137,3 = 322,7 m,y, = 46,2 m.y. 7 7

STANDARD DEVIATION= ~(l,7) 2 + (l,2)2 + (2,4)2 + (1,0)2 2 2 2 (0,7) + (0.1) + (o.s} / (7 - 2) = 3,6/5 = o.7 m.y.

AVERAGE AGE or CURLY TUFF = 46,2± 0.7 m.y.

Table 2: Showing weight average plateau and two step fusion ages used to calculate an average age for the Curly Tuff. 90

112" 42"

L _____110_· _ 0 ----WYOMING------108°-- - __ 105

\ \ \ I I '1, C'-? I (<'-~ I -t-1 I

0 Escalante

Durango 0 ------NEW MEXICO

0 25 50 100 MILES I

FIGURE 1.-Area o! this report and some adjacent stru<.'tural.and physiographic features.

Fioure 36: Outline map of Piceance Creek and Uinta Basins showing location of sampling of ~avy Tuff. -- - -·~--- - ,...,.....__. --~------~--

91

Figure 37A: Photomicrograph of a thin section from sample of Wavy

Tuff showing altered plagioclase crystal (p) with other feldspars, biotite and quartz in a matrix of fine grained intergrowths of authigenic feldspar. (Field width= 1.45 mm.)

Figure 378: Biotite grain altered to iron oxide in a matrix of feldspar crystals which are cut by ciruclar crystals of analcime. (Field width = 1.45 mm.) 92

locally contains bands of disseminated carbonate grains as well as

scattered grains and patches of hematite. The tuff also contains disseminated organic material.

The crystal component is made up of angular to subangular

crystal grains of biotite (15-20 percent), plagioclase (20-30 percent),

quartz (20-30 percent, and K-feldspar (20-30 percent) with minor amounts of green hornblende, pyroxene and devitrified pumice fragments.

The biotite is yellow-brown to dark brown pleochroic and is found in lenticular grains of up to 1.25 mm. in length. The biotites are sometimes partially to completely altered and rimed by hematite.

The quartz is angular, sometimes heavily fractured, and may show syntaxial overgrowths that preserve angular grain outlines. The feldspar grains are also angular and are lath and rectangular shaped.

The feldspars may be partially altered and may show absorbtion into the analcime matrix along the adges of the crystal.

The overall texture of the tuff shows that it is a poorly sorted pyroclastic sediment that is matrix supported. Any gradation of the crystals away from the base of the tuff may have been removed by plastic flow of the tuff. The other textural and mineralogical relationships indicate that the Wavy Tuff originated as a primary air fall ash that has not undergone any extensive reworking but has been slightly altered in geometry by plastic flow.

Age Data ·

Figures 38A to 38F show the age spectra for samples of the

Wavy Tuff. The spectra are for the most part disturbed but not discordant and form plateaus that have weight average plateau ages that range from 45.3± 1.2 m.y. to 48.9± 0.7 m.y. Most of the ages obtained 93

- cu bl c TP-= 4 7. 2 ± 0.9 m. y Tp= 48.8± 0.8 m.y

20 40 ~o so 39 ArKReleased, Cumulative ~

Figure 38A: Temperature release spoctrum for sample 707-1 of Wavy

Tufr. One gas fraction was lost (950 degrees C) during analysis due to equipment breakdown. 94

i

u bl c = -...CD a. Tp= 4 6. 2:t 1.4 m.y. a."' c Tf=48.8±2.5m.y.

20 40 60 80 39 ArK Released, Cumulative~

Figure 388: Age spectrum of sample 708-6 of the Wavy Tuff. T p = weight average plateau age; Th= two step fusion age. 95

)>

-4I >. -E

T p:r 4 5. 3·.,_ 1.1 rv. y Tf = 45.0-"':-0.811.y

20 40 60 80 39 Ar K Released, Cumulative ~ ------

- E

-CD bl c c Tp= 46.5±0.7m.y r+ _4 DI c '::I'" ....CD Tf = 46.4±0.Sm.y ca a. a. c

35'--~~~~.l-~~~~..._~~~~..._~~~~--~~~-- o 20 40 60 80 39 ArKReleased, Cumulative ~

Figures 38C and 380: Age spectra for samples 710-1 and 710-3 of the Wavy Tuff. 96

- E

-CD 45 cbl

c -....CD Tp=48.9±0.7m.y CL. CL."' T f = 49.1±2.om.y c 25

20L-~~~--L~~~__JL-~~~--L~~~---t1~~~-­ o 20 40 60 80 39 . ArK Released, Cumulative~

Figure 3BE: Age spectrum for sample 708-4 of Wavy Tuff. 97

::0 0 m

-:>. -E

Tp= 55.1±1.0M.Y. - Tf ~ 52.5 ± 0.7 M.Y.

0 20 40 60 39 80 ArK Released, Cumulative~

Figure 38F: Age spectrum of sample 709-1 of Wavy Tuff from outcrop near Rio Blanco, Colorado, in Piceance Creek Basin. 98

WAVY TUff

SAMPLE NO. PLATEAU AGE fUSION AGE

707-1 47.2 m.y. 46.8 m.y. 708-6 46.2 m.y.

710-1 45.3 m.y. 45.0 m.y.

710-3 46.5 m.y. 46.4 m.y.

TOTALS 185.2 m.y. 138.2 m.y.

AVERAGE AGE = 185.2 + 138.2 = 323.4 m.y. = 46.2 m.y. 2 STANDARD DEVIATION =~(0.9) + (0.8)2 + (1.4)2 + (1.1)2 + (0.8)2 + 2 2 (0.7) + (o.5) / (7 - 2) = 2.4/5 = o.5 m.y.

AVERAGE AGE fOR THE ~AVY TUff = 46.2± D.S m.y.

TABLE 3: Showing weight average plateau and two step fusion ages used to calculate average age for the Wavy Tuff.

"! 99

{as well as the two step fusion ages) fell within a grouping that ranged from 45.3 to 47.2 m.y.

One sample of the Wavy Tuff (sample 708-4, figure 38E) from the

Uinta Basin gave an anomalously high plateau age of 48.9± 0.7 m.y. and an even higher two step fusion age of 49.1± 2.0 m.y. Although the ages obtained for this sample can not be distinguished (through the use of the critical value test) from other ages obtained for the

Wavy Tuff they are much higher than other values found for this tuff.

When the ages obtained for sample 708-4 are used to calculate the average age of the Wavy Tuff, a value of 46.9± 0.5 m.y. is obtained.

This value is 0.7 million years greater than the average age obtained for the underlying Curly Tuff. Elimination of age data for sample

708-4 gives an average age for the Wavy Tuff of 46.2± 0.5 which is more consistent with age obtained for the Curly Tuff. For this reason the ages obtained for sample 708-4 of the Wavy Tuff have been excluded from all calculations for the average age and standard deviation for the Wavy Tuff.

The weight average plateau ages along with the two step fusion ages for samples 707-1, 708-6, 710-1, and 710-3 were used to calculate the average age for the Wavy Tuff (Table 3). From these values an average age of 46.2± 0.5 m.y. was obtained. This age is identical to the age obtained for the underlying Curly Tuff (Table 2).

Sample 709-1 (Figure 38F) shows an age spectra that is much older than other plateau ages of the Wavy Tuff. This sample is from a

20 foot thick outcrop of the tuff in the Piceance Creek Basin. The plateau age for the sample is 55.1± 1.0 m.y. Examination of the thin section for this sample shows that the sample is nearly identical to 100 other samples of the Wavy Tuff. However, the excessive thickness of this tuff at the outcrop may indicate that the tuff has undergone admixing with other tuffaceous sediments at this locale which may have contaminated the sample with older grains of biotite. The biotita separate for this sample further shows two different types of biotite when viewed under a binocular microscope. 101

EVALUATION or AGES

Reliability of Tuff Ages 40 9 The use of ArJ3 Ar ages from tuffs of the Green River Formation for interbasinal and intrabasinal stratigraphic correlations is dependent on the precision and geologic accuracy of the ages obtained for these tuffs. The precision of the ages can be evaluated. by the reproduc­ ibility of ages from separate samples (from widely separated locations) of the same tuff. The geologic accuracy of the tuff ages can in part be evaluated by the stratigraphic position of the tuff beds ard by comparison of a particular tuff age with ages found for other tuffs that are found above and below the tuff in question.

In this study it was found that some of the tuff beds of the

Green River Formation are quite useful for stratigraphic correlation.

These tuffs give what appear to be precise ages that agree well with the stratigraphic position of the tuff and with the ages of other tuffs.

Other tuffs procuce average ages that appear to be geologically accurate but are the product of separate age determinations that have a range of 4 million years. Still other tuffs give ages that are not reproducible and are not geologically accurate.

Examination of the age spectra and petrographic make-up for all of the tuffs shows a relationship between the quality of the age data and the mineralogy of the altered vitric matrix. The tuffs that had a matrix that was made up predominately of authigenic K-feldspar crystals (Big Island Tuff and Wilkins Peak Tuff number six) contained biotite grains that produced age spectra that were much less disturbed and more consistent thane the age spectra obtained from biotites from 102

tuff beds {Curly Tuff, Wavy Tuff, one inch thick LaClede Tuff) which

had matrices that were made up of various amounts {up to 100 percent)

of analcime. This may indicate that the pore fluids which caused the

formation of authigenic K-feldspars from the vitric component of the

tuff had a chemical make-up that affected the K/Ar isotopic system of

the biotite grains less severely than the pore fluids of other tuffs that formed analcime or other zeolite crystals.

The age of the Big Island Tuff of the Wilkins Peak Member is

probably the most precise age obtained in this study. The individual

ages of separate samples were very close to one another {Table 1)

and the average of these ages {49.4± 0.4 m.y.) agrees well with an average age of 49.0± 1.1 m.y. {approximately 50.2± 1.1 m.y. when converted to 1976 IUGS decay constants using conversion tables of

Dalrymple (1979)) that was found by Mauger (1977) in a conventional

K/Ar study. The age obtained for this tuff also appears to fit the stratigraphic position of the tuff. Thus it is felt that the age of

49.4± 0.4 m.y. for the Big Island Tuff is geologically accurate.

The age obtained for tuff number six of the Wilins Peak Member also appears to be reliable. The age spectra of sample 706-7 produces a plateau age that agrees well with the t~o step fusion age for that sample. The average of these two values gives an age (46.6± 1.0 m.y.) that agrees well with an age of 46.1± 2.0 m.y. (approximately 47.3± 2.0 m.y. when converted to 1976 IUGS standards using conversion tables of

Dalrymple (1979)) obtained in a conventional K/Ar study by Mauger {1977).

An age of 46.6± 1.0 m.y. appears to be a fairly accurate age for this tuff.

However, more samples from diverse geographic locations should be dated to improve on the precision for the age of this tuff. 103

The overlying 3/8 inch (l.O cm.) thick tuff that is found at the base of the Laclede Bed of the Laney Member (found in the Flaming Gorge,

Bridger Basin) does not give ages that are reproducible from sample to sample (Appendix II). Furthermore, the average age of 56.7 m.y. for this tuff does not compare well with ages obtained for tuffs in the underlying

Wilkins Peak Member and does not fit the stratigraphic position of the tuff. The anomalous age is explained in part by the petrography of the samples which show that this sample did not originate as a primary air fall ash but was extensively reworked. Thus it is probable that this tuff was contaminated with older grains of biotite during deposition of the tuff.

The average age of the one inch (2.5 cm.) thick tuff bed that is found in the upper portion of the LaClede Bed of the Laney Memberin

Washakie Basin appears to be fairly reliable. An average age of 44.9±

1.3 m.y. compares well with ages obtained for tuffs in the underlying

Wilkins Peak Member and fits the stratigraphic position of the tuff.

However, a more precise determination should be made using samples that have been prepared to insure the exclusion of altered, potassium

dificient grains from the separate.

Ages obtained from samples of the robin's egg blue tuff are not

reproducible from sample to sample and all of the ages obtained from

tuff samples are geologically inaccurate. Analyzed samples produced

total gas ages that ranged from 104 to 134 m.y. Thin sections of

samples from this tuff show evidence that this tuff has undergone

admixing and reworking with elastic sediments which have added older

contaminant biotite grains that are of Cretaceous age or older. 104

Examination of the average ages of the Curly and Wavy Tuffs

(Tables 2 and 3) of the Parachute Creek Member of the Green River

Formation shows that the ages for the Curly and Wavy Tuff s are equal (46.2± 0.7 m.y. for the lower Curly Tuff and 46.2± o.s m.y. for the upper Wavy Tuff). These ages appear to be fairly accurate (but not as

precise as the age obtained for the Big Island Tuff) and represent the

average age for the sedimentary rocks that are found in the interval

between the Wavy and Curly Tuffs. This interval includes rocks of

the Mahogany Ledge (Mahogany Zone in subsurface) and Mahogany Bed of

the Parachute Creek Member of the Green River Formation.

Although the average ages obtained for the Wavy and Curly Tuffs appear to be fairly accurate, the range in ages for individual samples covers more than two million years. The samples for both tuffs produce age spectra that are more disturbed than tuff samples from the

Wilkins Peak Member and give plateau ages that cover a wide range. For these reasons more samples should be dated so that a more precise age could be determined for both tuffs.

Thus from this study one finds that reliable ages have been obtained for the Big Island Tuff and tuff number six of the Wilkins Peak

Member, the one inch thick tuff of the upper portion of the LaClede Bed of the Laney Member, and for the Curly and Wavy Tuff beds of the

Parachute Creek Member of the Green River Formation. The ages of these tuffs have been placed on a diagramatic north-south cross section

(Figura 39) of the Green River formation and related rocks in the

Gosiuta and Uinta Basins which shows the position of the tuff beds. "IO'ITH S()UTH

!!0 I 0> "'I-" -0 u ~ ::> u Tu WIND vER UINTA BASIN PICEAl'ICE CRE.EK BASIN GREATER GREEN RIVER BASIN "1 VOLCANIC BASi!'t LITHIC + + + + T9r S4NDSTONE ---- . A' ""'1i IJI lfim,,; PE TROFACIES I 146.2•· > c-..- ?..!°/

Tvr .; c i= .. ii ::c ::> TQr 0 ~

-;~-,0 • Rochest oil shole u _____ muds.tone .. -- "0 o--- - a: ~"' D O•I st-;ole,mudston~. o ond corbonott rocks "'{...J LJ sandstcne .. ~ fTt1 Bedded, nodulor, ond (or) con9lomerote ~ { L.±....J cry5tal5 of No carbonate ~ I~~~' ti) evopor 1 te 4 ~ool200 --@-- Potas1ium- or9on dote .. ·.· .. ::> ril Bedded nodulor, ond(or) ~ l_!_:J crystals of anhydrite Tu-Uinta Fo1mot1on 100 l ...J and gypsum lQr - Green River formation o~~~~~-....~~~~~ Molluscan-rich c.olcareous Twr - Wind River Formation 0 !>O 100 II"' D muds.tone in Loney Member Tw - Wasatch Formation

figure 39: Generalized cross section across basins for Eocene Lakes Uinta and Gosiute showing generalized stratigraphic relationships of Graen River Formation rocks and related rocks with ages

from tuffs from this study superimposed in approximate positions of tuff bads. Modified from Surdam and Stanley, (1980).

...... 0 c.n 106

Use of Tuff Ages

A comparison of the ages for the Big Island Tuff and tuff number six of the Wilkins Peak Member of the Green River Formation

reveals that the Wilkins Peak Stage of Eocene lake Gosiute lasted much

longer than the one million years proposed by Bradley (1964) and Bradley and Eugster (1969). The age of the Big Island Tuff has been found to be

49.4± 0.4 m.y. The age for the overlying tuff number six (found at

the top of the Wilkins Peak Member) is 46.6± 1.0 m.y. Thus the sediments

that are found in the interval between these two tuff s formed during a time

that spanned an average of 2.8 million years (this interval ranges from

1.4 to 4.2 m.y. using the maximum standard deviations). As is shown in

Figure 13, the interval between these two tuffs comprises aprroximately

10 percent of the total section of the Wilkins Peak Member. From this

information it is apparent that the Wilkins Peak Stage lasted for more

than the l.O m.y. originally proposed and if sedimentation rates were

constant, as much as 5 or 6 m.y.

Mahogany Zone

A recent paleomagnetic study by Richardson (1980) that was

done on a drill core section of the Mahogany Zone has shown that the

sediments of the Mahogany Zone formed during a major reversal of the

Earth's magnetic field. The average age of 46.2± 0.6 m.y. for the ..

interval between the Wavy and Curly Tuffs has been used to correlate

this reversal with reversal number 20 of the geomagnetic polarity

time scale (Figure 40) of la Brecque et al (1977). This reversal

(Figure 40) occured during a span that lasted from 46.6 to 45.0 million

years ago. ST:.GE TIME PALEOMAGNETIC POLARITY

(my) M ..Gi.ETIC POL:.'11TY ?V<_&.i>1TY : .::\I~ ::...: ::-""4 . c 0 p T c H L A E 25 A E T I L R N Figure 40: Correlation T G E T 0 of paleomagnetic reversal A 0 0 I z N c G A 0 of Mahogany Zone as found 30 E E R I by Richardson (1980) with N N y c E . E reversal number 20 of R u geomagnetic polarity time p 35 E L scale of LaBrecque (1977) I A N based on ages obtained for i- the Curly and Wavy Tuffs. e p A R E R I 40 0 oTI A e N 0 c I N A I E N A N N 4 E L 45 u T E T ; I i A N

l 50 y p R E s I A N

~ 108

Correlation of Lake High Stands

One of the original aims of this study was to collect and date tuffs that would be useful in evaluating the Surdam and Stanley (1980) hypothesis for the culminating high stands for Eocene Lakes Gosiuta and Uinta. This model states that the high-water-stands of Lakes

Gosiute and Uinta which formed the oil rich beds of the Laney and

Parachute Creek Members of the Green River Formation, did not form as a result of climatic factors (increase in precipitation, decrease in evaporation, etc.) but rather formed as a result of the enlargement of the hydrographic basins for the two lakes. This enlargement of the hydrographic basin occured first for the basin of Lake Gosiute. It happened as a result of volcanic elastic sediments being carried away from the Absorka volcanic field (figure l) by streams into the Wind

River Basin. These sediments eventually filled the Wind River Basin and allowed streams to cross the Granite Mountain uplift into the

Gosiute Basin. This influx of stream water from the north raised the level of the lake which allowed the deposition of the extensive beds of the Laney Member. Eventually the streams from the Wind River

Basin carried in elastic sediments into the Gosiute Basin. These sediments eventually infilled the basin for Lake Gosiute and allowed waters to be diverted over the eastern extension of the Uinta uplift into the Piceance Creek Basin. This influx caused the merging of waters from the Uinta and Piceance Creek Basins to form Lake Uinta. The initial formation of Lake Uinta is marked by the deposition of the oil rich sediments of the Mahogany Ledge and Mahogany Bed. The volcanic sediments were eventually carried into the Piceance Creak Basin which along with sediments derived from the surrounding uplifts infilled the 109

the Piceance Creek and Uinta Basins, thus ending Green River Deposition.

One of the crucial points of the above model is that the high

stand~ of lakes Gosiute and Uinta were not synchronous and formed

in sequence. This means that the basal beds of the Laney Member

should be older than the basal beds of the Parachute Creek Member

(represented by the Mahogany Bed). Examination of Figure 39 shows

that the uppermost tuff of the Wilkins Peak Member (tuff number six) has an age of 46.6±, 1.0 m.y. This tuff is found just below the base of the Laney Member. Higher up in the Laclede Bed of the Laney, a second tuff was dated at 44.9± 1.3 m.y. This information shows that the base of the Laney Member did not form any earlier than

46.6± 1.0 m.y. As was noted earlier, the interval between the Curly and Wavy Tuffs has an average age of 46.5± 0.6 m.y. This interval contains the Mahogany Bed which represents the initial enlargement of Lake Uinta.

From this data one must assume that using the present resolution of tuff ages, the basal beds of the Laney and Parachute

Creek Members are equivalent in age and that the two culminating high stands for Eocene Lakes Gosiute and Uinta were synchronous.

Thus on the basis of the age data obtained in this study it must be assumed that the high stands for Eocene Lakes Gosiute and

Uinta were synchronous. This means that the stands for the two lakes were probably caused by climatic factors and not by diversion of drainage waters as the Surdam and Stanley (1980) model proposes. CONCLUSION

Results of this Study

Reliable ages have been obtained for ssveral of the tuffs of the Green River Formation.of Wyoming, Colorado, and Utah. These tuffs include the Big Island Tuff and tuff number six of the Wilkins

Peak Member, the one inch thick tuff in the upper portion of the

LaClede Bed of the Laney Member in Washakie Basin, and the Curly and Wavy Tuffs of the Parachute Creek Member of the Green River Formation.

The ages obtained in this study have been useful in corre­ lating paleomagnetic data from the Mahogany Zone of the Parachute

Creek Member of the Green River Formation with the geomanetic polarity time scale of La Brecque et al (1977). Other tuff ages have been useful in showing that the Wilkins Peak Stage of Eocene

Lake Gosiute lasted for a period that was much greater than the one million year span proposed by Bradley (1964) and Bradley and Eugster

(1969). These ages may also be useful in dating vertebrate fossil assemblages from laterally equivalent fluvial beds of the Cathedral

Bluffs Tongue of the Wasatch Formation. The ages obtained from tuffs within the Laney and Wilkins Peak Members of the Wyoming

Green River Formation and from the Parachute Creek Member of the

Colorado-Utah Green River Formation have been useful in showing that the initial enlargements of the culminating phases of Eocene Lakes

Gosiute and Uinta were synchronous and probably formed as a result of climatic factors and not diversion of drainage waters. Dating of Tuff Beds

When dating biotite separates of tuff beds of the Green River

formation (or any other tuff bad), one must take care to insure that

the tuff has not been contaminated by preexisting biotites of higher

ages. The use of Petrographic examination may assist in determining whether or not the tuff has been contaminated by preexisting sediments.

This can be done by showing if the tuff originated as a primary air

fall ash or if the tuff had been reworked and admixed with other

sediments. The primary air fall ashes should be matrix supported

and should show gradation away from the base of the tuff. Reworked

tuffs will commonly be grain supported and will lack any grading

of grain size or number away from the tuff base. The reworked tuffs will commonly contain grains that are well rounded and may have

contaminant grains of volcanic rock fragments, chert, metamorphic

and igneous rock fragments and mineral grains that are of non-volcanic

origin.

The biotite separates should also be examined to insure

that the individual biotite crystals do not contain grains that

have been altered. Such grains can give anomalously high or low

ages depending on whether potassium or argon (or both) was lost from

the structure of the biotite and when the biotite was altered.

In general the tuffs that had a matrix that was predominatly

made up of authigenic K-feldspar intergrowths contained biotite

crystals that were less altered and had less disruption of the K/Ar

isotopic system than biotite grains from tuffs that contained an

abundance of analcime crystals within the matrix of the tuffs. 112

The dating of tuffs within the Green River Formation is further complicated by the difficulty of separating the biotita grains from the t~ffs. This process is made more difficult by the fact that many of the tuffs have bean indurated by a nearly insoluble petroleun residue that is released into the heavy liquids when one tries to separate the biotite grains from the matrix of the rock.

Recommendations

Although several of the tuffs dated in this study gave what appear to be reliable ages, more ages should be obtained from samples of tuff number six of the Wilkins Peak Member and the one inch thick tuff of the upper portion of the Laclede Bed of the

Laney Member in Washakie Basin. The individual ages obtained for the Curly and Wavy Tuffs showed ranges of two million years or more. The age spectra for these two tuffs often appeared disturbed 39 with up to 30 percent or more of the total Ar released of the spectra giving anamolously low ages. The stratigraphic importance of these two tuffs requires further study in which several more samples are dated so that a more precise age may be found for each tuff. Each tuff should ideally have ages from 10 to 20 more samples so that weight of all of these age determinations can cancel the effects of anomalous ages and "zero in" on a true age for each tuff.

Ages should also be obtained for tuffs within the Tipton Shale Member of the Green River Formation and from the lower two tuffs of the

Wilkins Peak Member of the Green River Formation. Ages from these tuffs will give information on the duration of the Wilkins Peak Stage and on the entire duration of Eocene Lake Gosiute. REFERENCES

Bradley, w. H., 1931, Origin and microfossils of the oil shale of the Green River formation of Colorado and Utah: u.s.G.s. Prof. Paper, 168, Sap.

------' 1964, Geology of the Green River Formation and associated Eocene Rocks in southwestern Wyoming and adjacent parts of Colorado and Utah: U.S.G.S. Prof. Paper, 496-A, 86p.

------' w. H., 1973, Oil Shale formed in a Desert Environment; Green River formation, Wyoming: Gaol. Soc. Bulletin, v. 84 p. 121-124.

------' Eugster, H. P., 1969, Geochemistry and Paleolimnology of the Trena Deposits and Associated Authigenic Minerals of the Green River formation of Wyoming: U.S.G.S. Prof. Paper, 496-B, 7lp. Brobst, Donald A., Tucker, Jerry D., 1973, X-Ray Mineralogy of the Parachute Creek Member, Green River formation in the Northern Piceance Creek Basin, Colorado: U.s.G.S. Prof. Paper, 803, 53p. Cashion, w. B., 1967, Geology and fuel Resources of the Graen River formation, Southeastern Uinta Basin Utah and Colorado: U.S.G.S. Prof. Paper, 548, 48p.

------' Donnell, J. R., 1974, Revision of Nemenclature of the Upper Part of the Green River formation, Piceance Creek Basin, Colorado and Eastern Uinta Basin, Utah: U.S.G.S. Bulletin, 1394-G, 9p. Cox, Allen and Dalrymple, G. B., 1967, Statistical analysis of Geomagnetic reversal data and the precision of potassium-argon dating: Jour. Geophys. Research, v. 72, no. 10, P• 2603-2614. Culbertson, w. c., 1961, Stratigraphy of the Wilkins Peak Member of the Green River formation, Firehold Basin Quadrangle, Wyoming: U.S.G.S. Prof. Paper, 424-0, p.Dl70-0173.

------' 1971, Stratigraphy of the trona deposits in the Green River formation, south-west Wyoming: Wyoming University Contr. Geology, v 10, p.15-23.

Dalrymple, G. Brent, 1979, Critical tables for conversion of K-Ar ages from old to new constants: u.s.G.s. Research Note, 3p. ~------' Lanphere, Marvin A., 1969, Potassium-Argon Dating: Principles, Techniques, and Applications to Geochronology, w. H. Freeman and Company, San Francisco, 258p. 40 39 ------' Lanphere, Marvin A., 1971, Ar/ Ar Technique of K-Ar Dating: A Comparison with the Conventional Techniques: Earth and Planetary Science Letters 12, p.300-308. 40 39 ------• Lanphere, Marvin A., 1974, Ar/ Ar, age spectra of some undisturbed terrestial samples: Geochimca et Cosmochimica Acta, v 38, p. 715-738.

Desborough, George A., 1978, A biogenic-chemical stratified lake model for the origin of oil shale of the Green River Formation: An alternativetto the playa-lake model: Geological Society of America Bulletin, v 89, July, p.961.

------' Pitman, J. K., Donnell J. R., 1973, Microprobe analysis of biotites--A method of correlating tuff beds in the Green River Formation, Colorado and Utah: u.s.G.s. Journal of Research, v l, p.39-44.

Donnell, J. R., 1961, Terrtiary geology and oil-shale resources of the Piceance Creek Basin between the Colorado and White Rivers, northwestern Colorado: u.s.G.S. Bulletin, l082L, p.835-891.

Eugster, H. P., Hardie L. A., 1975, Sedimentation in an Ancient Playa­ Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming: Geological Society of America Bulletin, v 86, March, p.319-334.

------' Surdam R. c., 1973, Depositional environment of the Green River Formation of Wyoming: A preliminary report: Geological Society of America Bulletin, v 84, p. 1115-1120. Evarnden, J. F., Savage, D. E., Curtis, G. H., James, G. T., 1964, Potassium-argon dates and the Cenzoic mammalian chronology of North America: American Journal of Science, v 262, p.145-198.

Fleck, Robert J., Sutter, John F., Elliot, David H., 1977, Interpretation of discordane 40Ar/39Ar age-spectra of Mesozoic tholeiites from Anarctica: Geochiminca et Cosmochimica Acta, v 41, p.15-32.

Iijma, Azuma, Hay Richard L., 1968, Analcime composition in tuffs of the Green River Formation of Wyoming: American Mineralogist, v 53, p.184-200. r'

LaBrecque, John L., Kent, Dennis v., Cande, Steven c., 1977, Revised magnetic polarity time scale for Late Cretaceous and Cenozoic time, Geology, v 5, June, p.330-335. 115

Lundell, L. L., Surdam R. c., 1975, Playa-Lake Deposition: Green River formation, Piceance Creek Basin, Colorado: Geology, v 3, No. 9, September, p.493-497.

MacGinitie, H. D., 19699 the Eocene Green River flora of Northwestern Colorado and Northeastern Utah: University of California Publications in Geological Sciences, v 83, 202p.

Mauger, R. L., 1977, K-Ar Ages of Biotites from Tuffs in Eocene Rocks of the Green River, Washakie, and Uinta Basins, Utah, Wyoming, and Colorado: University of Wyoming Contributions to Geology, v 15, p.17-42.

McGrew, Paul o., 1971, Early and Middle Eocene faunas of the Green River Basin: Contributions to Geology, v 10, No. 1, Spring, p. 65-68.

Murray K. o., Huan, J. o., 1974, Introduction to the Geology of the Piceance Creek Basin and Vicinity, Northwestern Colorado: in Energy Resources of the Piceance Creek Basin, Colorado; Rocky Mountain Association of Geologists Guidebook, p.41-43.

Parker, R. B., Surdam R. c., 1971, A Summary of Authigenic Silicates in the Tuffaceous Rocks of the Green River formation: Wyoming University Contributions to Geology, v 10, p. 69-72.

Richardson, K. G., 1980, Paleomagnetic Study of the Mahogany Ledge Oil Shale, Uinta Basin, Utah: unpublished senior thesis, The Ohio State University, 145p.

Roehler, H. w., 1973, Stratigraphy of the Washakie formation in the Washakie Basin, Wyoming: U.S.G.S. Bulletin, 1369, 40p.

------' 1974, Depositional Environments of Rocks in the Piceance Creek Basin, Colorado, in Energy Resources of the Piceance Creek Basin, Colorado: Rocky Mountain Association of Geologists Guidebook, p. 57-64.

Sheliga, Carl ...... , 1980 9 unpublished masters thesis, The Ohio State University, __p.

Surdam, R. c., Parker, R. D., 1972, Authigenic Aluminosilicate Minerals in the Tuffaceous Rocks of the Green River formation, Wyoming: Geological Society of America Bulletin, v 83, No. 3, March, p.689-700.

------~' Stanley, K. o., 1979, Lacustrine Sedimentation, During the Culminating Phase of Eocene Lake Gosiute, Wyoming (Green River Formation1: Geological Society of America Bulletin, Part I, v 90, January, p.93-110. 116

------' Stanley, K. o., 1980, Effects of Changes in Drainage- Basin Boundaries on Sedimentation in Eocene Lakes Gosiute and Uinta of_Wyoming, Utah, and Colorado: Geology, v 8, March, p.135-139.

------' Wolfbauer, c. A., 1975, Green River Formation, Wyoming A Playa-Lake Complex& Geological Society of America Bulletin, v 86, March, p.335-345.

Trudell, L. G., Roehler, H. w., Smith, J. w., 1973, Geology of Eocene Rocks and Oil Yields of Green River Oil Shales on Part of Kinney Rim, Washakie Basin, Wyoming: Bureau of Mines Report of Investigations~ 7775, 15lp.

------~' Beard, T. N., Smith, J. w., 1974, Stratigraphic Framework of Green River Formation, Colorado: in Energy Resources of the Piceance Creek Basin, Colorado, Rocky Mountain Association of Geologists Guidebook, p. 57-64.

Wolfbauer, c. A., (1971), Geological Framework of the Green River Formation in Wyoming: Wyoming University Contributions to Geology, v 10, p.3-s.

Yan, F. s., Goodwin, J. H. 1976, Correlation of Tuff Layers in the Green River Formation, Utah, Using Biotite Compositions: Journal of Sedimentary Petrology, V 46, No. 2, p.345-354. 117

APPENDIX I

Analytical Procedures

The following sections describe the methods that were used in this study for sample collection, mineral separation, sample irradiation, and mass spectrometer analyses for all of the tuff beds 40 39 from which Ar/ Ar ages were obtained.

Sample Collection

Tuff samples were in general collected from surface outcrops that were found along canyon walls and resistant scarp faces that were accessible by auto or four wheel drive pick-up truck. The tuff beds were examined in the field for biotite content and to determine which portions of a tuff bed contained the largest amounts of biotite grains. In general only the bottom portion of a tuff was collected from tuff beds that were more than 8 inches

(20 cm.) thick since the size and number of biotite grains decreased rapidly away from the base of most tuffs. Some tuff beds (such as the Curly Tuff) contained insufficient amounts of biotite to warrant collection of a sample at a particular locale. A sample weighing between 5 and 50 pounds (2.3 to 22.7 kilograms) was then collected

(amount collected dependent on the biotite content of the tuff) and prepared for transportation to the K/Ar Laboratory at the Ohio State

University.

Mineral Separation

The tuff samples were prepared and analyzed at the K/Ar 118

Laboratory of the Ohio State University. The samples were first ground and sieved to produce five separate mesh fractions of +45,

45-60, 60-80, eo-100, and -100 mesh. The fractions were then examined to determine which size fractions contained the majority of the biotite grains. The biotite rich fractions (usually the 60-80 and 80-100 mesh fractions) were then washed with tap water to remove clay size particles that would later impede heavy liquid and filtering operations. The material was then allowed to dry after all of the clay size material had bean removed.

Some samples (from the Curly and Wavy Tuffs) ware induratad with a tar-like petroleum residue which had to be removed before heavy liquid separation took place. These samples were treated by heating the samples to approximately 140 degrees f (60 degrees C) and mixing the ground sample with a self-emulsifying petroleum solvent

(which has a trade name called 'Gunk') for approximately 10 minutes.

The solvent (Gunk) was then flushed (along with the dissolved

petroleum residue) from the ground tuff sample and the sample was allowed to dry. The above procedure was repeated until at least 90 to 95 percent of the petroleum residue had been removed from each sample.

After a sample was washed and allowed to dry it was placed

in a heavy liquid (bromoform) which separated the heavier biotite

grains {along with other impurities) from the lighter feldspar,

quartz, and analcime grains. The biotite concentrate was first

purified by passing it through a Franz Isodynamic magnetic separator. 119

Than the biotite was 'paper-shaked', a process in which a portion of the sample is sprinkled on a piece of filter paper. The filter paper is then tilted whereby nearly all of the elogate, spindle shaped and circular grains roll off of the filter paper leaving the flat platelets of biotite stuck to the filter paper. The purified biotite separate was then examined under a binocular microscope for removal of any remaining impurities.

The above procedure produced a biotite separate that was

99.9 percent pure. Often only two grams of purified biotite would be produced out of a sample that originally contained 40 pounds

(18 kilograms) of tuffacaous material. The purified biotite separate was finally washed ultrasonically in separate baths of acetone, alcohol, and triple distilled water and allowed to dry before preparation for irradiation.

Irradiation

Purified biotita samples ware prepared for irradiation by separately packaging 0.4 grams {age spectrum experiment) and 0.3 grams

(three step fusion experiments) of each in pure aluminum capsules which were in turn sealed in pure quartz vials. These vials, together with a vial containing a mineral standard (neutron flux monitor), were placed in a pure quartz canister which was lowered into a position next to the core of the 2 megawatt research reactor at the Pheonix Memorial

Laboratory of the University of Michigan in Ann Arbor. Once in place, the samples within the canister were irradiated for a period of between

56 and 60 hours, the canister being rotated during the irradiation.

The rotation of the canister eliminated lateral variations of the neutron flux imparted to the samples. Vertical variations of the

neutron flux were measured by the monitor minerals. The effects of

vertical variations of the neutron flux on the analytical precision of ages obtained from irradiated samples was reduced by keeping "J" (which is proportional to the neutron flux) a constant for all of the samples of a particular tuff. This was done by placing all of the samples of a particular tuff bed (and for a particular experiment, i.e., age spectrum experiment or two step fusion experiment) at exactly the same vertical position within each of the quartz vials of the casister. Thus all of the samples of the Big Island Tuff that were analyzed in age spectrum experiments were placed 2.5 cm. above the bases of the quartz vials. Since the canister that contained the quartz vials was rotated during irradiation, all of the samples of the Big Island Tuff which were analyzed in age spectrum experiments received the same neutron flux. 40 39 All Ar/ Ar analyses were performed with a Nuclide Corporation

Model SGA 6-60 mass spectrometer in the K/Ar Laboratory at the Ohio

State University. An analyses was begun by first placing an irradiated sample in a high resistance molybdenum crucible which in turn was placed in a quartz lined pyrex glass bottle that was connected to an ultra-high vacuum extraction line. After evacuation and bake-out of the sample bottle and extraction line an induction coil from a radio frequency generator was placed around the sample bottle so that the coil was centered around the molybdenum crucible. A heating step was then made by inducing a current to flow through the resistant crucible.

The current causes the crucible to heat up which in turn heats the 121

sample within the crucible. Temperatures for each step ranged from 450 to- 1300 degrees c. These temperatures were controlled by the output of the radio frequency generator and were monitored through visual observation to within ±25 degrees c. Each heating step lasted approximately 20-25 minutes with an interval of about 80-90 minutes between each heating step. The gas that was released during the heating process was constantly collected on an activated charcoal finger that was immersed in liquid nitrogen (-196 degrees c). This kept the pressure around the crucible fairly low and prevented any appreciable amounts of argon from becoming ionized and imbedded into the glass walls of the sample bottle.

After the heating step was completed and the sample bottle allowed to cool the liquid nitrogen was removed from the charcoal finger and the finger allowed to come to room temperature. This caused the trapped gas to be released from the activated charcoal and react with a mixture of copper and copper oxide that had been heated to a temperature of about 500 degrees c. The heated material (which is in a stainless steel canister connected to the extraction line) reacted with the gas mixture to dissociate H vapor into H and and to oxidize 2o 2 o2 and ignite any hydrocarbons present. Excess water vapor within the gas was absorbed by a molecular sieve dessicant. Prior to transfer of

the remaining gas maxture (mainly rare gasses, H2 and N2) to another section for further purification, a mixture of dry ice and acetone was

placed on the charcoal finger to prevent the transfer of any heavy gaseous molecules not dissociated on the Cu-CuO getter. 122

After transfer the remaining gas mixture was exposed to a

titanium getter that had been heated to about 800 degrees c. As the Ti getter cooled, it first gettered N and then H leaving only a mixture 2 2 of argon and other rare gasses. The rare gasses were transferred to

a manifold connected to the mass spectrometer, than equilibrated in

the spectrometer and analyzed for the isotopic composition of the argon.

Constants

The neutron flux monitor used in this study is a biotite for which the following data has been determined: %1<+ = 8.113± o.oso (23 determinations) 40 8 ArK = 1.407± 0.027 x 10- moles/gram (6 determinations). Age = 811 m.y.

Constants used in age calculations are: 10 "').. p = 4.962 x 10- /yr. 10 "'\.",- - 0.581 x io- /yr. 40 I K K = 1.167 x 10-4 atom/atom Total

Error estimates for calculated ages reflect analytical precision only and were calculated in the manner described by Cox and Dalrymple

(1967). Corrections for Ca- and K- derived argon isotopes used in the K/Ar Laboratory at the Ohio State University were measured in a manner described by Dalrymple and Lanphere (1971, 1974). -+:- Temp. 40 ':1>0 Ar( r) 39 Ar(k) J9Ar( k) Apparent Age 0 40 6 39 40Ar -12 ~ Step C Ar/39Ar 3 Ar/39Ar Ar(kJ %of Total "r x10 mole (m_.y.) , . \.....) 1:1>'° Sample 706-5 Big Island Tuff Temperature Release (Biotite)1 J = ).004674 'i :a> ::i 550 14.448 0.036349 4.706 7.77 30.43 4.35 39.21J+ 2,9 IU ...... 650 14.366 0.028724 5.878 16.66 40.87 9.33 48.843~ 2.0 « (JJ CD 850 10.857 0.016334 6.030 14.01 55.49 7.85 50.095+ 1.8 (JJ 1025 8.003 0.006772 6.002 28.16 74.92 15.77 49.861~ 0.9 t:1 IU Fuse 9.089 0.010762 5.909 33.40 64.95 18.71 49.101+ 1.0 ct ':1> .. - IU tU tU Tot.Gas 10.404 0.015400 5.853 100.00 56.20 56.01 48.644 stz:l Weight Average Plateau Age = 49.437± 1.273 m.y. H >< H Sample 706-5 Big Island Tuff Three Step Fusion Release (Biotite)1 J = 0.004663 H 625 7.120 0.004751 5.716 32.77 80.20 13.19 47.409~ 0.9 1100 6.542 0.002123 5.915 66.22 90.32 26.66 49.027+ 0.7 Fuse 29.308 0.082232 5.008 1.01 17.0? o.41 41.597~ 24.8 Tot.Gas 6.960 0.003790 5.840 100.00 83.82 40.26 48.423

Sample 712-1 Big Island Tuff Two_ Step Fusion ReleasEL(Bioti t~) 1 J = 0.004600 700 7.452 0.004833 6.018 25.72 80.76 31.25 49.262± 0.7 Fuse 6.750 0.002419 6.029 74.28 89.32 90.27 49.349± o.6 Tot.Gas 6.930 0.003040 6.026 100.00 86.95 121.52 49.327 ...... N \.....) Temp. 40 % 9 Ar{ r} 39Ar{k~ 3 Ar~ k) Apparent Age 0 40 6 9 39 40Arr -12 Step C Ar/J9Ar 3 Ar/~ A:t" Ar(kl % Q_f' 'J.'ptal x10 mole (m.y.) I ' Samples ?12-3 Big Island Tuff Temp. Rel. (Biotite)a J = 0.004674 600 12.363 0.014401 8.102 0.96 65.53 0.50 67.050+ 7.2 700 9.386 0.012255 5.759 23.17 61.36 12.15 47.917+ 1.1 900 7.508 0.005098 5.996 16.26 79.86 8.53 49 .858+ 1. 0 1075 7.101 0.004139 5.872 58.66 82.69 30.76 48.843.:t 0.7 Fuse 66.193 0.205326 5.514 0.95 8.33 0.50 45.900,:t 23.0 Tot.Gas 8.309 0.008186 5.884 100.00 70.82 52.44 48.942 ~eight Avergge Plateau Age = 48.z2+ 0.864 m.~ • . Samnle s 712-1 Big Island Tuff Three Step fusion (Biotite)a J = Q.00466J 650 7.025 0.003976 5.844 36.22 83.19 14.92 48.505.:t o.8 1075 6.729 0.002721 5.919 63.19 87.96 26.03 49.115.:t 0.7 Fuse 136.8 0.449972 3.797 0.58 2.78 0.24 31. 664+ ~9. J Tot.Gas 7.594 0.005783 5.880 100.00 77.42 41.19 48.793 Sample: Zl2-4 Big Island Tuff Temp; Rel. (Biotite): J = o.oo46z4 600 11.411 0.024257 4.237 15.38 37.13 7.39 35.374.:t 2.0 700 10.691 0.015511 6.101 20.85 57.07 10~02 5 0. 7 24±_ 1. 4 900 9.694 0.011950 6.157 17.26 63.51 8:.30 :- 51.183+ 1.4 ..... 1075 7.784 0.006485 5.862 46.02 75.31 22.12 48.763.:t 0.9 I\.) Fuse 94.83 0.292721 8.324 o.47 8.78 0.27 68.857.!. 49.7 ~

__ .. ' -'----~~-'--+...------,~ ------_. _ _;.,;,~---....:~--~~~.'.'£,~""'~ t u ':%--~~..=-=-~-Lo,;--- , ,_,,._ •< -- A,. .. 0. - .. ------~'" .. 0. Temp. 4oAr(r) 39 % 39Ar(k) Apparent Age 0 40 Ar(k) Step c Ar/39Ar 36Ar/39Ar 39Ar(k) of Total 4oAr -12 % _.!: x10 mole (m.y.) I' Tot.Gas 9.689. 0~013397 5.725 100.00 59.08 48.05 47.634 Weight Average Plateau Age = 49.74~ 1.11 m.y.

Sample: 712-4 Big Island Tuff Three Step Fusion (Biotite)a J = 0.004663 650 6.987 0.006419 5.084 32.45 72.77 10.07 42.269~ 1.1 1035 6.543 0.001902 5.975 67.55 91.32 20.97 49.577± o.6 Tot.Gas 6.687 0.003367 5.686 100.00 85.03 31.04 47.208

Sample: 712-5 Big Island Tuff Temp. Rel. (Biotite)s J = 0.004674

600 9.181 0.012648 5.438 15.84 59.23 7.94 45.278~ 1.4 700 7.669 0.005064 6.167 19.55 80.41 9.80 51.265± 1.0 900 7.221 0.003626 6.144 24.50 85.08 12.28 51.076+ o.8 1035 6.507 0.002182 5.856 38.87 90.00 19.49 48.711+ 0.7 Fuse 29.00 0.077965 5.926 1.24 20.45 0.62 49.285+ 12.3 Tot.Gas 7.610 0.005693 5.922 100.00 77.82 50.14 49.253 Weight Average Plateau Age = 49.98~ 0.793 m.y.

Sample: 712-5 Big Island Tuff Three Step Fusion (Biotite): J = 0.004663 650 7.282 0.004600 5.917 37.44 81.25 13.15 49.096~ 0.9 1010 6.596 0.002009 5.997 61.27 90.91 21.52 49.754+ 0.7 ...... Fuse 29.95 0.082040 5.697 1.29 19.02 0.45 47.295± 24.7 N Tot.Gas 7.155 0.004014 5.963 100.00 83.34 35.12 49.476 \J\

,.:...__c±,_.._:.. i.~~~~-"!!f'rilo".~"''''"""-'~-'-~-••-•-'C.•.• ,,_.c,.,..,~,-,,-' • "-~'"•>~ Temp. 40 % Ar{ r} 39Ar k 39Ar{ k} Apparent Age 0 6 9 40Arr -12 Ste.2.._Q 4oArl'.39Ar 3 Arl'.39Ar 3 Ar(kl ·lo of Total x10 mole {m.~:-2'. Sample: z1e-z Big Island Tuff Temp. Rel. {Biotite}: J = o.oo46z4 600 12.577 0.027148 4.549 18.53 36.17 7.70 37.957+ 1.9 700 10.985 0.014407 6.722 19.98 61.19 8.31 55.806+ 1.6 900 10.359 0.012155 6.762 16.84 65.27 7.00 56.131+ 1.7 1010 8.137 0.005767 6.427 25.55 78.98 10.62 53.390.±_ 1.1 Fuse 8.360 0.008071 5.969 19 .10 71.40 7.94 49.642+ 1.4 Tot.Gas 9.946 0.012971 6.107 100.00 61.40 41.56 50.771 Weight Average Plateau Age = 54.904.:t. 1.466 m.y.

Sample: 712-7 Big Island Tuff Three Step Fusion (Biotite)1 J = 0.004663 700 7.482 0.005750 5.777 62.27 77.21 15.37 47.952.±. 0.9 1050 7.499 0.004530 6.154 34.62 82.07 8.53 51.039.±. 1.5 Fuse 19.42 0.045383 6.005 3.11 30.92 0.77 49.818+ 13.4 Tot.Gas 7.859 0.006561 5.915 100.00 75.26 24.68 49.079

Sample: 706-7 Wilkins Peak Tuff# 6 Temp. Rel. (Biotite)a J = 0.004674 600 9.894 0.015222 5.390 6.12 54.48 2.90 44.882.:t. 2.4 700 7.281 0.005578 5.627 15.95 77.28 7.57 46.833.±. 1.0 900 7.888 0.007487 5.670 13.95 71.88 6.62 47.181.:t. 1.3

~ 1050 7.168 0.005230 5.616 63.20 78.35 29.98 46.744+ 0.7 N Fuse 57.69 0.178011 5.086 0.78 8.82 0.37 42.383±_ 29.1 °' Tot.Gas 7~848 0.007563 5.607 100.00 71.45 47.44 46.672 Temp• 40Ar( r) 39 % 39Ar(k) Apparent Age 0 4oAr 40 6 9 11 12 Step c Ar/39Ar 3 Ar/3 Ar 39Ar(k) "' o~rT~tal r x10- mole __J_m.y.) I Weight Average Plateau Age = 46.7± 1.4 m.y.

Samples 706-7 Wilkins Peak Tuff# 6 Three Step Fusion (Biotite): J = 0.004663 700 6.647 0.003453 5.621 41.77 84.56 16.18 46.674.:t 0.8 1050 6.389 0.002767 5.565 57.54 87.11 22.29 46.214+ 0.7 Fuse 53.945 0.162086 6.042 0.69 11.20 0.27 50.124,:t 27.9 Tot.Gas 6.824 0.004150 5.592 100.00 81.94 38.74 46.434

Sample 706-2 Robin's Egg Blue Tuff Temp. Rel. (Biotite)a J = 0.004674 700 25.331 0.025471 17.798 8.42 70.26 3.79 144.155+ 3.6 850 31.776 0.021472 25.425 11.07 80.01 4.98 202.570,:t 3.2 1000 22.681 0.032923 12.947 30.56 57.08 13.74 105.989+ 2.3 Fuse 14.937 0.020249 8.948 49.95 59.90 22.46 7 3. 913.:t 1. 4 Tot.Gas 20.043 0.024697 12.739 100.00 63.56 44.97 104.339 NO PLATEAU

Sample 706-2 Robin's Egg Blue Tuff Two Step Fusion (Biotite)s J = 0.00466J 700 18.905 0.008811 16.295 38.33 86.20 12.35 132.115+ 1.8 1000 11.593 0.004789 10.172 61.67 87.74 19.86 83.602+ 1.1 Tot.Gas· 14.396 0.006331 12.519 100.00 86.96 32.21 102.350 ~ I\) ""-l Sample: 706-3 Robin"'s Egg Blue Tuff one step fusion (biotite)s J = 0.004663

·--·""--···--· -~-' -~~ -·- _:._. '.J.·.~~~~;:,~z-,_.::;•~~...-.~------Temp. 40 % Ar(r) 39 Ar( k) J9Ar(k) Apparent Age 0 4oArr Step C 40Ar/J9Ar 36Ar/39Ar 39Ar(k) %of Total xtO -12mole (m.y.)'' Fuse 17.457 0.003243 16.493 100.00 94.48 67.31 133.661+ 1.5

Sample: 706-4 Robin's Egg Blue Tuff One Step Fusion (Biotite): J = 0.004663 Fuse 15.153 0.009039 12.476 100.00 82.33 43.83 102.009± 1.3

Sample: Z11-2 Flaming Gorge Tuff Temp. Rel. (Biotite}a J = 0.004620 600 20.879 0.060791 2.909 J.86 13.93 1.90 24.085.:!: 6.2 800 10.252 0.014472 5.970 20.02 58.2J 9.86 49.081+ 1.6 950 14.163 0.026197 6.416 22.71 45.30 11.19 52.692+ 1.9 1050 12.218 0.019618 6.415 24.79 52.51 12.22 52.690+ 1.5 Fuse 9.052 0.009642 6.197 28.62 68.46 14.03 50.922.:!: 1.2 Tot.Gas 11.694 0.018815 6.128 100.00 52.41 49.28 50.366 Weight Average Plateau Age = 51.413.:!: 1.5 m.y.

Sample: Z11-2 Flaming Gorge Tuff Three step Fusion (Biotite)a J = 0.004635 800 9.608 0.012710 5.847 52.72 60.85 17 .14 48.235+ 1.1 1050 8.56'9 0.007972 6.208 46.03 72.44 14.97 51.171.:!: 1. 0 Fuse 121.38 0.393343 5.144 1.25 4.24 o.41 42.509+ 39.9 Tot.Gas 10.529 0.015291 6.004 100.00 57.03 32.51 49.516 ._. N CX> Temp. 40 % Ar( r} J9Ar k J9Ar(k) Apparent Age 0 4oAr -12 Step__Q 4oAr/39Ar 36Ar/39Ar 39Ar(kJ o;o of Total r x10 mol~ ._(m.y.J'.

S~~ler 212-6 Flaming Gorge Tuff Temp. Rel. (Biotite)s J = 0.004620 750 16.198 O.OJ78J6 5.012 18.80 J0.94 8.83 41.292.:t 1.4 850 12.544 0.018449 7.086 21.78 56.49 10.23 58.109.:t 1.2 950 16.467 0.031026 7.293 12.71 44.29 5.97 59.783+ 1.9 1050 17.:z9() 0.033326 7.343 9.52 42.70 4.47 60.181.:t 2.0 Fuse 11.389 0.014231 7.178 37.19 6J.02 17.46 58.851+ 1.0 Tot.Gas 13.743 0.023540 6.781 100.00 49.34 46.95 55.647 Weight Average Plateau Age = 58.954.:t 1.573 m.y.

Sample: 712-6 Flaming Gorge Tuff Two Step Fusion (Biotite)1 J = 0.004635 750 11.944 0.020230 5.960 28.56 49.90 12.07 49.156+ 1.1 Fuse 8.631 0.005450 7.015 71.44 81.27 30.17 56.569.:t 0.7 Tot.Gas 9.577 0.009671 6.713 100.00 70.10 42.24 55.278

Sample: zo6-6 Flmning Gorge Tuff Temp. Rel. (Biotite)r J = 0.004620 600 37.765 0.108965 5.560 9.70 14.72 7. 07 45.755+ J.4 700 13.189 0.023151 6.342 17 .47 48.09 12.74 52. 097+ 1. 2 800 8.679 0.008154 6.263 10.73 72.17 7.83 51.460.:t 1.1 900 10.417 0.015614 5.797 6.08 55.65 4.43 47 • 682,:t lo 7 ..... 1000 10.223 0.013194 6.319 16.05 61.81 11.71 51.908,:t 1.0 I\) 1100 7.352 0.003718 6.247 33.02 84.98 24.07 51.331.:t 0.7 '°

... 'I'" I ...,. ,..,- -,~ .....'" "'" o~ ~ . ~ - ~- , __ ...... _...... __~-~""'"'" l o~- • Temp. 40 % 9 Ar~r} 39Ar(k} J Ar~k) Apparent Age 0 40 36 4oAr -12 Ste2 C Ar[J9Ar ArL39Ar J9Ar(k ~ ~ of Total r x10 mole (m.:l.),. Fuse . 10.954 0.016244 6.148 6.95 56.12 5.07 50.522.:!:. 1.4 Tot.Gas . 12.362 0.020914 6.176 100.00 49.96 72.93 50.753 Weight Average Plateau Age = 50.753.:t 0.895 m.y.

Sample: 706-6 Flaming Gorge Tuff Two Step Fusion (Biotite): J = 0.004635 ~00 18.660 0.042922 5.971 34.21 J2.00 17.05 49.247+. 1.4 Fuse 6.969 0.002445 6.240 65.79 89.55 J2.79 51.438.:!:. o.6 Tot.Gas 10.968 0.016291 6.148 100.00 56.06 49.83 50.689

•, Sam2le: Z05-J Sand Butte {Lane:l Mbr.} Tuff Temp. Rel. 'Biotite}s J = 0.004600 600 29.490 0.076525 6.871 30.26 23.30 8.51 56.1J6.:t 2.6 700 22.179 0.049731 7.478 29.45 33.72 8.29 61.012.:!:. 1.9 800 20.020 0.043146 7.264 10.25 36.28 2.88 59.293+ 2.9 950 40.121 0.110851 7.359 4.65 18.34 1.31 60.054.:t 6.1 1050 18.818 0.039493 7.141 20.08 37.95 5.65 58.308+ 2.J- Fuse 24.899 0.062599 6.365 5.30 25.59 1.49 52.059.:t 4.2 Tot.Gas 24.473 0.058635 7.140 100.00 29.18 28.13 58.299 Weight k1erage Plateau Age = 58.3.:!:. 2.2 m.y.

Sample: 705-3 Sand Butte Tuff Two Step Fusion (Biotite)s J = 0.004600 ...... \..> 700 15.123 0.027639 6.950 63.95 45.95 12.17 56.768.:t 1.2 0 Fuse 14.014 0.23933 6.936 36.05 49.49 6.86 56.656.:t 1.5

'I'- <" - ~""- <+-'- ·~ _,.. .. ~ - ,. .. 0 Temp. 40Ar( r) 39Ar k 39Ar(k) Apparent Age 0 40 4oAr 12 36 " (m.y.) I ' Step C Ar/39Ar Ar/39Ar 39Ar(k] .:r'o of Total r x10- mole Tot.Gas 14.723 0.026303 6.945 100.00 47.17 19.03 56.728

Sample: 705-5 Sand Butte Tuff (Laney Mbr.) Temp. Rel. (Biotite)a J = 0.004600 700 16.029 0.038607 4.615 40.66 28.79 18.41 37.891.:!: 1.2 850 18.381 0.043609 5.489 17.42 29.86 7.89 44.984.:t 1.6 950 24.531 0.064640 5.424 6.18 22.11 2.79 44.461.:t 3.5 1050 14.524 0.030224 5.587 20.29 38.47 9.19 45.778+ 1.4 Fuse 13.639 0.027496 5.508 15.45 40.38 6.99 45.134.:t 1.6 Tot.Gas 16.290 0.037670 5.152 100.00 31.63 45.28 42.256 Weight Average Plateau Age = 45.2.:!: 2.2 m.y.

SampJ.e: _ZQ5_-5_ San.d_Butte Tuff~WQ_St~Fusion Rel. (Bioti te) a J = 0.004600 700 12.506 0.025979 4.824 58.53 38.57 18.83 39.588+ 1.0 Fuse 10.661 0.017379 5.520 41.47 51.78 1J.35 45.235.:t 1.1 Tot.Gas 11.741 0.022413 5.112 100.00 43.54 32.18 41.933

Sample_ 7Q5-J _Sand But:t~LTuff_Heavy El:'action_Twg__ Ste_D__Fusio~ioti te) a J = 0.004460 600 12.316 0.026041 4.615 36.76 J7.47 2.86 36.752.:t o.6 Fuse 11.841 0.021252 5.555 63.24 46.91 4.91 44.150+ o.6 Tot.Gas 12.016 0.023013 5.209 100.00 43.36 7.77 41.4J4 .... \....).... Sample 705-3 Sand Butte Tuff Light Fraction One Step Fusion (Biotite)a J = 0.004460 Fuse 18.039 0.036089 7.368 100.00 40.85 2.21 58.J29.:!: 0.9

_<\-,. ·~

Sample zo8-~ Curly Tuff Temp. Rel. ~Biotite)s J = 0.004620 600 83.096 0.283273 0.103 2.21 3.24 1.20 o.858+ 16.7 750 22.254 0.066290 2.659 3.08 11.95 1.67 22.028+ 6.8 875 9.974 0.016917 4.969 18.08 49.82 9.81 40.948± 1.4 1000 Lost during analysis due to equipment failure Fuse 8.203 0.008277 5.752 76.63 70.11 41.56 47.310± 0.7 Tot.Gas 10.612 0.017706 5.374 100.00 50.64 54.24 44.244

Sample 708-5 Curly Tuff Two Step Fusion Release (Biotite): J = 0.004635 800 8.593 0.013746 4.525 29.61 52.66 13.56 37.448± 1.2 1100 6.482 0.002404 5.765 70.39 88.95 32.23 47.574± o.6 Tot.Gas 7.107 0.005762 5.398 100.00 75.96 45.79 44.582

Sample 708-5 Curly Tuff Temperature Release (second analysis) (Biotite)s J=0.004460 750 10.309 0.021634 3.911 14.19 37.93 7.44 31.194± 0.5 900 7.392 0.004950 5.923 34.J7 80.13 18.01 47.039+ 0.5 1000 7.376 0.005012 5.889 18.69 79.84 9.80 46.766± 0.5 1065 6.538 0.002397 5.824 21.32 89.08 11.17 46.258± 0.5 Fuse 7.477 0.005569 5.826 11.42 77.91 5.98 46.274± o.6 .... \...) Tot.Gas 7.631 0.006856 5.599 100.00 73.37 52.39 44.493 l\) Weight Average Plateau Age = 46.678± 0.5 m.y.

' - •y --- <1-'.,, 'Y~ ''(, -s- 'I- ~ . I -- - '1- ~ 'I-,_ ti o,,_ ,. .. • • ·"',. a ~-s-~ ~ ' •'rm t ': eew ,, ht· -;7 t-:ttc'Wr)'tHt 'tr 1 1 & nr 1 't 1·m·t o·t• t• 1:· u·a .. l(..,,,,_H *' '¥·•·-~~...... _, __ ,_,...... '~

Temp. 40 Ar{ r L 39 39Ar( kl Apparent Age 0 6 9 Ar~kl 40Ar -12 SteL.._f 40Ar/39Ar 3 Ar/~9A~ ! Ar1Jcl %of Total "r ~1 o___ mo_l~_ (m.y.) I'

Sample zbz-2 Curl~ Tuff Temperature Release (Biotitelr J = 0.004620 700 10.528 0.019794 4.673 17 .11 44.38 6.13 )8.529±. 2.1 800 9.679 0.013787 5.599 21.88 57.85 7.84 46.068±. 1.5 850 11. 057 0.018633 5.545 14.05 50.15 5.03 45.630+ 2.4 900 8.395 0.009468 5.591 37.33 66.60 13.37 46.006±. 1.1 1000 14.727 0.032137 5.225 7.63 35.48 2.73 4J.029±. 4.1 Fuse 46.029 0.141253 4.282 2.00 9.30 0.72 35.342±. 14.7 Tot.Gas 10.651 0.017833 5.375 100.00 50.47 35.82 44.249 Weight Average Plateau Age = 45.7±. 1.7 m.y.

Sample 707-2 Curly Tuff Two Step Fusion Release (Biotite)r J = 0.004635 800 8.277 0.009819 5.369 67.19 64.87 14.86 44.J45±. 1.1 Fuse 13.962 0.029319 5.293 32.81 37.91 7.26 43.722±. 2.0 Tot.Gas 10.142 0.016216 5.344 100.00 52.69 22.12 44.140

Sample 708-J C~rly Tuff Temperatu.I'~H_Re_l_ea_s~_!~ioti te): J = 0.004620 700 8.138 0.010663 4.981 9.76 61.21 4.01 41.042±. 1.7 875 9.628 0.013849 5.530 13.21 57.43 5.42 45.510±. 2.1 950 20.041 0.049190 5.499 27.44 5.29 2.17 45.258+ 5.0 ...... w 1000 23.855 0.061595 5.648 8.18 23.68 J.)6 46.471±. 4.2 w 1100 9.489 0.013210 5.580 )8.89 58.80 15.97 45.914+ 1.1

-v,,,. 'Y .,,.' v 'Y~ --- ...... "' ~- ·.y I --- . ~' ~ ~ .. .._,.. • 0. .~---"""'- ·------~------,...... ~.,...... -~,. .... --~...., .... ~ ...... --·-··---··· '' 7-

Temp. 40 9 Ar~ r) 39Ar k J Ar{k) Apparent Age 0 40 6 40Ar 9 J Art_39Ar 9 11 " -12 SteE C Arf..J Ar J Ar(k2 'o of Total r x10 mole ~m.~.) I' F'U~e 8.J57. 0.009686 5.489 24.JJ 65.68 9.99 45.177.:t 1.J Tot.Gas 1LJ96 0.020019 5.474 100.00 48.04 41.05 45.055 Weight Average Plateau Age = 45.7.:t 2.4 m.y.

Sample 708-3 Curly Tuff Two Step Fusion Release (Biotite)a J = 0.004635 700 6.J7J 0.002962 5.491 64.51 86.17 19.JJ 45.J40.:t 0.7 Fuse .. 6.551 O.OOJ662 5.462 35.49 8J.J9 10.6J 45.105.:t 1.2 Tot.Gas 6.436 O.OOJ211 5.481 100.00 85.17 29.96 45.256

Sample 202-1 Wav~ Tuff Temperature Release {Biotite}1 J = 0.004620 600 9J.340 0.309674 1.825 1.75 1.96 0.69 15.148+ 21.9 800 66.134 0.207680 4.759 2.69 7.20 1.07 39.233+ 14.J 900 Lost during analysis due to equipment failure 1000 7.262 0.005103 5.748 91.10 79.15 J6.2J 47.284.:t 0.7 Fuse 22.173 0.056135 5.579 4.47 25.16 1.78 45.907.:t 5.8 Tot.Gas 11.013 0.018145 5.646 100.00 51.26 J9-78 46.449

Sample 707-1.Wavy Tuff Two Step Fusion Release (Biotite)1 J = 0.004635 875 7.084 0.004696 5.691 38.42 80.33 11.67 46.967.:t 0.9 Fuse 6.141 0.001630 5.654 61.58 92.06 18.71 46.66J+ 0.7 ~ \,.) Tot.Gas 6.504 0.002808 5.668 100.00 87.15 JO.JS 46.779 {:"

<"" ~ ,"' ~ I ...'"'. 'Y~ • '" ~ _,..'" lo. ~ n .... M ·rt n .. ..--~""""'•'<> .. -...... ~.;. .-.~,,,.,~~"""''''"''""~~..... -~...,,,...,,,.--~ ..,, .... ~--~····-··~"·~·,,.~,._,..._ ...-_.-~...--.....- ...... -.,.,...... -...'-"'."""-...."•*·•·•-""...... ,...'""" ,. .:.-.:..-..,.•.--...... ,.u• .,,1 ...... ,,_.... ,...... ,,,,,.., •.-,

Temp. 40 % J9Ar~k} Apparent Age Ar~ r l 39Ar~k} 0 40 6 40Ar -12 Step___£ Ar/J9Ar J Ar/J9Ar 39A.r(kl ~{ of Total r X10~. ~mo].~ - (m .y_.J I '

Sam2le to8-4 Wav;y Tuff Tem2er~ture Rele~se ~Bioti tel: J = 0.004620 600 7.098 0.013699 3.044 29.66 42.89 25.40 25.196+ 0.5 700 7.282 0.004011 6.091 22.66 8).64 19 .41 50.062,;t 0.7 800 8.646 0.008896 6.011 8.84 69.53 7.57 49.416.:t. 0.9 900 12.845 0.022960 6.054 3.73 47.13 J.19 49.766.:t. 1.9 1000 7.548 0.005324 5.969 28.64 79.08 24.53 49.074+ 0.7 1050 12.852 0.024002 5.754 5.85 44.77 5.01 47.)29.:t. 1.7 Fuse 130.JJ2 o.423100 5.301 0.62 4.07 0.53 4).645±. 20.4 Tot.Gas 8.715 0.012146 5.120 100.00 58.75 85.64 42.172 Weigh Average Plateau Age = 48.9.:t. 0.7 m.y.

Sam12le Z08-4 Wav;y Tuff Two Ste12 Fusion Release ( Bioti te}: J = 0.0046,J,2 700 208.077 0.691213 J.818 40.12 1.83 10.21 31. 64).:t. 18. J Fuse 18.833 0.043555 5.957 59.88 31.63 15.24 49.131.:t. 2.0 Tot.Gas 94.7,?1 0.303371 5.099 100.00 5.38 25.46 42.136

SamRle zo8-6 W~v;y Tuff Tem2erature Release ~Biotitels J = 0.004620 600 11.414 0.033980 1.367 22.86 11.97 9.27 11.353.:t 1.9 800 11.231 0.02245J 4.590 14.34 40.87 5.81 37.855+ 2.3 ..... \..J 950 12.368 0.023054 5.550 15.60 44.87 6.33 45.669+ 2.4 1...1\ 1050 9.491 0.013036 6.633 42.54 59.35 17.25 46.J4?.:t. 1.1

'.... "Y. (,;" ·-s- I o.' ~ Temp. 40 % Ar{ r} 39Ar k J9Ar(k} Apparent Age SteE 0 C 40 39 6 9 9 40A Ar[ Ar 3 Ar[3 Ar 3 Ar{kj -12 ) j ' u-~ of Total rr x10 mole {m.;y. - Fuse 19. 937 0.052508 4.415 4.66 22.14 1.89 36.425.:t 5.4 Tot.Gas 11.115 0.022575 4.439 100.00 39.93 40.55 36.618 Sample 708-6 Wavy Tuff Three Step Fusion Release (Biotite): J = 0.004635 • Boo 10.953 0.028348 2.570 71.35 23.47 B.20 21.265.:t 1.9 1050 10.770 0.016412 5.914 25.69 54.92 2.95 48.788.:t 2.5 Fuse 109.015 0.345264 6.984 2.96 6.41 0.34 57.468.:t 41.2 Tot;Gas 13.814 0.034678 3.560 100.00 25.77 11.50 29.526

Sample 210-) Wavy Tuff Temperature Release (Biotite}1 J = 0.004620 600 8.877 0.014184 4.680 12.26 52.72 9.54 38. 586.:t LO 750 7.454 0.006386 5.561 11.47 74.60 8.92 45.760.:t 0.9 850 10.362 0.015872 5.666 12.13 54.68 9.43 46.618.:t 1.0 950 13.939 0.027850 5.703 6.02 40.92 4.67 46.917.:t 1.9 1050 9.078 0.011544 5.661 18.20 62.36 14.15 46.573+ 0.9 1100 6.921 0.004199 5.674 33.22 81.99 25.85 46.680.:t o.6 Fuse 9.065 0.011747 5.588 6.70 61.64 5.21 45.978.:t 1.3 Tot.Gas 8.598 0.010356 5.532 100.00 64.34 77.81 45.525 Weight Average Plateau Age = 46.5.:t 0.7 m.y.

...... \.,J °'

. ((,.. . --.,-.<:"- --· • .... • L . 0 tr& tit ,< f ·x 9 b i'£• b'tl:S& « ¢ • # ,:, • " * .l!! ;,, -~--.... tt -

Temp. 40 % 9 Ar( r) J9Ar k 3 Ar(k) Apparent Age 0 4oAr -12 Step C 40Ar/39Ar 36Ar/39Ar 39Ar(k) o;o of Total r xlO mole (m.y.) .. ,

Sample ?l0-3 Wavy Tuff Two Step Fusion Release (Biotite)1 J = 0.004635 700 6.948 0.005251 5.390 46.21 77.58 26.89 44.518+ o.6 Fuse 6.102 0.001613 5.620 53.79 92.09 31.31 46.)87± 0.5 Tot.Gas 6.493 0.003294 5.514 100.00 84.92 58.20 45.523

,, Sample Zl0-1 Wavy Tuff Temperature Release {Bioti tel: J = 0.004620 700 Gas Fraction Lost During Analysis 850 7.218 0.007365 5.035 24.59 69.77 14.19 41.487!. 0.9 950 8.767 0.011122 5.475 )4.66 62.45 20.00 45.060!_ 0.9 1050 9.704 0.01446) 5.424 16.29 55.90 9.40 44.647!. 1.6 Fuse 8.518 0.009895 5.588 24.45 65.60 14.11 45. 983!. 1. 2 Tot.Gas 8.478 0.010442 5.386 100.00 63.53 57.70 44.)40 Weight Average Plateau Age = 45.J!. 1.1 m.y.

Sample 710-1 Wavy Tuff Three Step Fusion Release (Biotite)s J = 0.004635 700 6.808 0.006054 5.013 67.12 73.64 27.55 41.439!. 0.7 1050 6.614 0.003948 5.441 31.09 82.27 12.76 44.931+ o.8 Fuse 30.684 o.o84J65 5.748 1.80 18.73 0.74 47.431!. 12.7 Tot.Gas 7.177 0.006806 5.160 100.00 71.89 41.04 42.6)4 ~ w...... ,

,_,., /' ~- . ...' -s-..- I ...-..' . -s-...... ' "'" "'-s- ... I ... t- Ill o, t- ' ·-- " • • .~ o,, " ~ Temp. 40Ar~r} % 39Ar( k} Apparent Age 0 39Ar k 40 36 39 0 4oArr -12 Step c Ar/39Ar Ar/39Ar Ar(k) o of Total xtO mole (m.y.) I. '

Sam2le 202-1 Wavy Tuff Tem2erature Release {Biotite}: J = 0.004620 I 700 11.157 0.016742 6.204 14.51 55.61 10.64 50.982±_ 1.1 800 10.930 0.013489 6.938 11.56 63.48 8.49 56.916±_ 1.2 900 11.981 0.017364 6.844 9.20 57.12 6.75 56.1.$8.:t. 1.4 1000 10.654 0.013265 6.729 22.36 63.15 16.40 55.223.:t. 1.0 1100 9.513 0.010696 6.346 18.93 66.71 13.89 52.132.± 0.9 Fuse 9.541 0.011916 6.014 23.45 63.03 17.21 48.603+ 0.9 Tot.Gas 10.404 0.013370 6.447 100.00 61.97 73.38 52.951 Weight Average Plateau Age = 55.1.± 1.0 m.y.

Sample 709-1 Wavy Tuff Two Step Fusion Release (Biotite)s J = 0.004635 750 8.110 0.005855 6.373 43.48 78.59 23.01 52.518,:t 0.7 Fuse 7.688 0.004455 6.366 56.52 82.80 29.92 52.455.± 0.7 Tot.Gas 7.871 0.005064 6.369 100.00 80.91 52.93 52.483

~ l...V CX> 139

SAMPLE LOCATIONS

Sampl~: 706-5 Big Island Tuff; iiV ilk ins Peak Member. Location: Green River, Wyoming Quadrangle: Southwest face of Toll Gate Rock approximately 0.95 miles (1.5 kilometers) northwest of intersection between old U.S. Route JO ans State Route 530 in Green River, 'v'Jyoming. NW 1/4 sec. 15 1 T18N R107W> 41° 32' J4"N.109° 29 J9"w.

Sample: 712-1 Big Island Tuff; ,1 ilk ins Peak Member. Location: Green River, ~~yarning Quadrangle: West face of Toll Gate Rock approximately 1.00 miles (1.6 kilometers) • northwest of intersection between old U.3. Route JO and State Route 530 in Green River Wyoming. NE 1/4, sec. 16, T18N, R107W; 41°31'38"N., 109°28°43'\'4.

Sample: 712-J Big Island Tuff; ,-Jilk ins Peak Member. Location: Green River, Wyoming Quadrangle: Southwest facing cliff adjacent to Interstate 80 0.1 miles (0.2 kilometers) north of Toll Gate Rock. SW 1/4, SW 1/4, sec. 10, Tt8N, R107W; 41°32'5o"N., 109°29'39··w.

Samule: 712-4 Big Island Tuff; ,·wilkins Peak Member. Location: Green River, Wyoming Quadrangle: Southwest face of the Palisades adjacent to Interstate 80 and 1.0 miles (1.6 kilometers) northwest of Toll Gate Rock. SE 1/4, sec. 9, Tt8N, R107W; 41°33'2"N., 109°29'54"w

Sample: 712-5 Big Island Tuff;

\ ( ..-?-"' .. ,.~ 140 "J- /,.

Sample: 712-7 Big Island Tuff; ~ilkins Peak Member. Location: Firehole Basin, :Jyoming Quadrangle: Northeast facing ledge on west side of Flaming Gorge approximately 1.6 miles (2.6 kilometers) southeast of hhalen Butte. SW 1/4, sw 1/4, sec. 7, T17N, R106w; 41°27' J4"N., 109°25'3o"w.

3arrmle: 706-7 vHlkins Peak Tuff Number Six. Location: Firehole Basin, Wyoming Quadrangle: South facing scarp of iJhalen Butte approximately 1.8 miles (2.9 kilometers) east-southeast of radio facility on State Route 530. NE 1/4, NE 1/4, sec. 11. T17N, Rt07 11J1 41°2a'2o"N., 109°26'55"w.

Sample: 706-6 Flaming Gorge Tuff; Laney Member. Location: Firehole Basin, Wyoming Quadrangle: East side of 0 ' :,halen Butte a,pproximately 2. O miles ( 3. 2 kilometers) east -?-"' of radio facility on State Route 530. SW 1/4, SW 1/4, sec. "" • 1 1, Tl?N, R107W; 41°28'3o"N., 109°26 J8"w.

Sample: 711-2 Flaming Gorge Tuff; Laney Member. Location: Firehole Basin, ~iyoming Quadrangle: West side of Flaming Gorge 0.7 miles (1.1 kilometers) north-northwest of 1~halen Butte. NW 1/4, sec. 2, T17N, R107W; 41°29' 1o"N., R109°27'16"w. ' Sample: 712-6 Flaming Gorge Tuff; Laney Member. Location: Firehole Basin, 'ijyoming Quadrangle: '•'Jest side of Flaming Gorge approximately 1.75 miles (2.8 kilometers) southeast of JJhalen Butte. NW 1/4, sec. 18, T17N, R.106W; 11 41°27' 28 N., 109°25' Jo"w. Sample: 707-2 Curly Tuff; Parachute Creek Member. I Location:· Currant Canyon, Utah Quadrangle: Base of scarp 0-?- on east side of Gate Canyon along State Route 53 approx. 2.5 miles (4.o kilometers) north of Nine Iiiile Creek. SEl/4, • dW!\ 1/4, sec. 20, T11;:).,...... R15E; 39 0 50 • 51 " N., 110 0 15 ' 01 " W. I 141

Sample: 707-1 Wavy Tuff; Parachute Creek Member: Locations Jones Hollow, Utah Quadrangles Along roadcut on west side of State Route JJ approximately 0.1 miles (0.16 kilom~ters) southwest of intersection of State Route JJ an secondary road to Argyle Canyon. NW 1/4, sec. 12, 1 T11S, RlOE; )9°52' 58"N., 110°44 55"w.

Sample: 710-1 Wavy Tuff; Parachute Creek Member: Location: Agency Draw NE Quadrangle, Utah: Ledge on northwest side of Buck Canyon approximately 1.6 miles (2.56 kilometers) southwest from intersection (at storage bin) of Buck Canyon Road and road to Ouray, Utah. N 1/2, - SE 1/4, sec. J5, T12S, R21E; J9°43'45"N., 109°31'45"w.

Sample: 710-3 Wavy Tuff; Parachute Creek Member: Location: Cooper Canyon Quadrangle, Utah: West side of Bitter Creek Canyon along jeep trail approximately 1.0 miles (1.6 kilometers) northwest of Sweetwater Canyon. Sij'/ 1/4, sec JJ, T12S, R2JE; J9°43'37"N., 109°21'5"w.

Sample: 708-3 Curly Tuff; Parachute Creek Member: Location: Rainbow Quadrangle, Uintah County, Utah: Along dirt road in small canyon approximately 0.65 miles (1.04 kilometers) east from junction with Atchee Ridge Road. S 1/2, 0 I fl 0 I • NV'/ 1/4, sec. 8, T12S, R25E; 39 47 18 N., 109 8 40 W.

Sample: 708-4 Wavy Tuff; Parachute Creek Member: Location: Rainbow Quadrangle, Uintah County, Utah: Along dirt road in small canyon approximately 0.45 miles (0.7 kilometers) east of junction with Atchee Ridge Road. S 1/2, 1 NW 1/4, sec. 8, T12S, R25E. J9°47°22"N., 109°8 5o"w.

Sample: 708-6 Wavy Tuff; Parachute Creek Member: Location: Dragon Quadrangle, Utah-Colorodo: From canyon • wall on east side of State Route 45 approximately 0.9 miles (1.5 kilometers northeast from intersection of State Route 45 and dirt road for Park Canyon. SW 1/4, NE 1/4, sec. 21, 1 I - 142

Sample_r ?08-5 Curly Tuff; Parachute Creek Member: Location: Dragon Quadrangle, Utah-Colorado: From oanyon • t wall on west side of State Route 45 approximately o.6 miles (1.0 kilometers) from intersection of State Route 45 and dirt road for Park Canyon. NW 1/4. SE 1/4, sec. 21, T11S, R25E; 39°50'51":t1., 109°07'05"w.

Sample: 7 09-1 'lfavy Tuff; Parachute Creek Member: Location: Rio Blanco Quadrangle, Colorado: On north side of canyon of Piceance Creek along side of paved road approximately 2.7 miles (4.J kilometers) west of the town I of Rio Blanco, Colorado. SE 1/4, NW 1/4, sec. 6, T4S, R94W; 39°43'53"N., 107°59'15"w.

Samnle: 705-3 Sand Butte Tuff1 LaClede Bed, Laney Member: - Location: Sand Butte, Washakie Basin: Above resistant algal bed on northwest face of Sand Butte. NW1/4, sec. 21, T16N, R100W.

Sample: 705-5 Sand Butte Tuff; LaClede Bed, Laney Members

Location: Sand Butte Rim, Washakie Basin 1 Above resistant algal bed on northwest facing Sand Butte Rim. SE 1/4, sec. ' 16, T16N RlOOW.

Sample: 706-J & 706-4 Robins Egg Blue Tuff, Washakie Formation: Location: Trail Dougway, Washakie Basins At Trail Dougway approximately 2.0 miles (J.2 Kilometers) east of Kenney Rim. NW 1/4 sec. 16, T14N R99W.

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