·-

STRATIGRAPHIC FRAMEWORK 

OF THE 

ABSAROKA VOLCANIC 

SUPERGROUP 

IN THE YELLOWSTONE 

NATIONAL PARK REGION 

Stratigraphic Framework of the 

Absaroka Volcanic Supergroup in the 

Yellowsto-ne National Park Region 

By HARRY W. SMEDES and HAROLD J. PROSTKA 

GEOLOGY OF YELLOWSTONE NA1~IONAL PARK 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 729-C 

A brief description of the stratigraphy of the 

Eocene Absaroka volcanic field) Wyoming and 

lVf. ontana. The Absaroka Volcanic Supergroup) 

three groups) five formations) and five 

members are defined 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1972 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 72-600072 

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

Washington, D.C. 20402 - Price 50 cents 

Stock Number 2401-2151 
Yellowstone National Park, the oldest of the 

areas set aside as part of the national park sys· 

tern, lies amidst the Rocky Mountains in northwestern Wyoming and adjacent parts of Montana 

and Idaho. Embracing large, diverse, and complex 

geologic features, the park is in an area that is 

critical to the interpretation of many significant 

regional geologic problems. In order to provide 

basic data bearing on these problems, the U.S. 

Geological Survey in 1965 initiated a broad program of comprehensive geologic and geophysical 

investigations within the park. This program was 

carried out with the cooperation of the National 

Park Service, and was also aided by the National 

Aeronautics and Space Administration, which 

supported the gathering of geologic information 

needed in testing and in interpreting results from 

various remote sensing devices. This professional 

paper chapter is one of a series of technical geologic reports resulting from these investigations. 

CONTENTS 

Page Page 

Abstract .................................................................................... . C1 Sunlight Group - Continued 

Introduction .............................................................................. . 1 Mount Wallace Formation- Continued 

Acknowledgments ............................................................ . 4 Age.............................................................................. C23 

General geologic relations .............................................. . 4 Crescent Hill Basalt......................................................... 23 

Washburn Group .................................................. . 8 Wapiti Formation............................................................. 24 

Sepulcher Formation ............................................. . 14 Jim Mountain Member............................................ 25 

Conglomerate facies ............................................. . 15 Age.............................................................................. 25 

Elk Creek Basalt Member .................................... . 16 Trout Peak Trachyandesite....... .................................... 25 

Lost Creek Tuff Member .................................... . 17 Pacific Creek Tuff Member..................................... 27 

Daly Creek Member ............................................... . 17 Age.............................................................................. 27 

Fortress Mountain Member .................................. . 18 Thorofare Creek Group........................................................... 27 

Andesite lavas ......................................................... . 18 Langford Formation........................................................ 28 

Age ............................................................................ . 19 Promontory Member............................................ 29 

Lamar River Formation .... . 19 Age.............................................................................. 29 

Cathedral Cliffs Formation ......................................... . 21 Two Ocean Formation..................................................... 29 

Sunlight Group ................. . 21 Tepee Trail Formation.................................................... 29 

Mount Wallace Formation ............................................. . 22 Wiggins Formation.......................................................... 30 

FIGURE 

Slough Creek Tuff Member .............. . 23 References cited........................................................................ 32 

ILLUSTRATIONS 

1. Index map of geographic features in the Absaroka volcanic field and vicinity .............................................. . 

2. Index map showing extent of mapping and stratigraphic information ........................................................... . 

3. Generalized geologic map of the Absaroka volcanic field ................................................................................... . 

4. Generalized geologic map of the Absaroka Volcanic Supergroup in Yellowstone National Park. .............. . 

5. Schematic section showing stratigraphic relations in the Absaroka Volcanic Supergroup .......................... . 

6. Hypothetical cross section showing relationships of vent facies and alluvial facies .................................... . 

7. Hypothetical cross section showing facies relationships of two vent complexes and the 

intervening volcanic sediments derived from them ...................................................................................... . 

8. Diagram showing nomenclature, stratigraphic relations, and proposed correlation 

of units in the Absaroka Volcanic Supergroup .............................................................................................. . 

9. Index map of geographic features in the northern part of Yellowstone National Park and vicinity ........ . 

10-13. Photographs of rocks of vent and alluvial facies: 

10. Alluvial facies of the Sepulcher Formation ........................................................................................... . 

11. Well-bedded alluvial facies of the Lamar River Formation ............................................................. . 

12. Vent facies autoclastic flow breccias of the Wapiti Formation ........................................................ . 

13. Vent facies autoclastic flow breccia of the Wapiti Formation ........................................................... . 

14. Index map of geographic features in the southeastern part of Yellowstone National Park 

and vicinity ............................................................................................................................................................ . 

15. Photograph of indistinctly bedded vent facies of the Langford Formation .................................................... . 

16. Photograph of alluvial facies of the Wiggin's Formation ................................................................................... . 

v 

Page 

C2 

3 

5 

6 

8 

9 

9 

10 

12 

15 

20 

24 

25 

26 

28 

31 

GEOLOGY OF YELLOWSTONE NATIONAL PARK 

STRATIGRAPHIC FRAMEWORK OF THE 

ABSAROKA VOLCANIC SUPERGROUP 

IN THE YELLOWSTONE NATIONAL PARK REGION 

By HARRY \V. SMEDEs and HAROLD J. PROSTKA 

ABSTRACT 

This report briefly describes the stratigraphy of the Absaroka volcanic field in and near Yellowstone National Park. 

The volcanics, mainly andesitic, basaltic, and dacitic volcaniclastic rocks, are herein named the Absaroka Volcanic Supergroup. The supergroup comprises, from oldest to youngest, 

the Washburn Group, Sunlight Group, and Thorofare Creek 

Group (new names). Within these groups, the following five 

new formations and five new members are defined: In the 

Washburn Group- Sepulcher Formation and its Daly Creek 

and Fortress Mountain Members, and Lamar River Formation. In the Sunlight Group- Mount Wallace Formation and 

its Slough Creek Tuff Member, and the Pacific Creek Tuff 

Member (of the Trout Peak Trachyandesite). In the Thorofare Creek Group -Langford Formation and its Promontory 

Member, and the Two Ocean Formation. The usage of some 

formerly established units is modified on the basis of new 

data, and correlations with rocks in outlying parts of the 

volcanic field are proposed. 

This stratigraphic framework has been established by mapping of lithologic units and by potassium-argon age data and 

fossils. 

INTRODUCTION 

The Absaroka volcanic field of northwestern Wyoming and southwestern Montana (figs. 1, 3) 

consists of deeply eroded andesitic and basaltic 

stratovolcanoes and the coalesced deposits of epiclastically reworked material derived from them, 

some air-fall tuffs, and a variety of related intrusive 

rocks. These Eocene volcanic rocks constitute the 

main mass of the Absaroka Range for which the 

field is named, as well as much of the northern 

Gallatin Range and part of the Owl Creek Mountains 

(fig. 1). Similar rocks in the Madison and Centennial Ranges, to the west, may be part of the 

Absaroka field, but they are not included at this 

time, because too little is known about them. 

The purpose of this paper is to define and briefly 

describe new geologic units mapped by us in the 

Absaroka volcanic field in and near Yellowstone 

National Park, to modify the usage of previously 

established formation names, and to suggest correlations with units in the more distant parts of the field. 

The stratigraphic framework proposed in this 

paper is based on 6 years of detailed and reconnaissance mapping by us in and near Yellowstone National Park, field conferences with other workers 

in selected critical areas, and published detailed 

maps and stratigraphic studies (fig. 2). This report 

is intended to be used in conjunction with the new 

bedrock geologic map of Yellowstone National Park 

(U.S. Geological Survey, 1972). 

The Absaroka volcanic field is the largest of the 

many volcanic fields emplaced during an episode of 

major widespread Eocene volcanism in the Northern Rocky Mountains (Steven and others, 1972). 

The field covers about 9,000 square miles (23,310 

square kilometers) ; the volume of rock exceeds 

7,000 cubic miles (29,000 cubic kilometers) ; and 

the maximum thickness in any one place is about 

5,000 feet (1,500 meters). In addition, volcanic ash 

from this field is an important component of some 

of the Eocene sedimentary strata of the Wind River 

Basin (Love, 1939) and probably of other Tertiary 

basins farther away. 

Some of these volcanic rocks constitute part of 

the Beartooth Mountains and are present north of 

the crest of that range; however, there is no evidence 

to indicate whether the volcanics ever extended 

farther north into the Crazy Mountains Basin (fig. 

1). The only time-equivalent strata exposed north 

of the Beartooth block are sedimentary beds of 

Wasatchian (early Eocene) age, which are devoid 

of volcanic material from the Absaroka field. 

Facies relations and thicknesses of the volcanicrich strata along the present east edge of the 

volcanic field indicate that volcanic debris may have 

originally covered most of the Bighorn Basin, as 

was suggested by Love (1939). The presence of 

volcanic debris in the Wagon Bed Formation (Van 

Houten, 1964) in the Beaver Divide area of the 

Wind River Basin, 80 miles southeast of the presentday edge of the Absaroka field, also supports this 

view. 

Cl 
C2 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

0 25 50 MILES 

FIGURE 1.- Geographic features in the Absaroka volcanic field and vicinity. 

Much of the southwestern part of the Absaroka 

field is covered by Quaternary rhyolites of the 

Yellowstone Plateau (fig. 1). The boundary between 

these two adjacent, but unrelated, volcanic fields is 

highly irregular; the older volcanics locally project 

above the rhyolites, and erosional remnants of rhyolite are scattered on top of the older volcanics (U.S. 

Geological Survey, 1972). Because of this overlap, 

it is considered desirable to have formal names for 

the rocks of each of these two volcanic fields. The 

Quaternary volcanics have been named by Christiansen and Blank (1972). The Eocene rocks have been 

informally called Absaroka volcanic rocks for many 

years; our proposal of the name Absaroka Volcanic 

Supergroup is to formalize a name already in common use among workers in the region. The name 

Absaroka Volcanic Supergroup is hereby proposed 

for the Eocene rocks of the Absaroka volcanic field 

and its outliers; the field and its outliers are the 

type area of the supergroup. Because the mapped 
0 

STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C3 

111° 

YELLOWSTONE 

NATIONAL 

PARK 

4~·-·-f-· ~a 

20 40 MILES 

EXPLANATION 

LJ 

Areas where stratigraphic information is 

insufficient for regional correlation at 

formation level 

Detailed mapping by Love (1939) and 

Harold Masursky (unpub. data, 1956) 

Reconnaissance mapping 

3a, H. W. Smedes and H. J. Prostka, 1965-69 

3b, H. J. Prostka and W. H. Nelson, 1970 

3c, H. J. Prostka and D. G. Gaskill, 1970 

3d, H. W. Smedes and W. B. Hall, 1970 

3e, H. W. Smedes, J. D. Love, W. L. Rohrer, 

and J. D. Obradovich, 1970 

Detailed mapping 

4a, li. W. Smedes and H. J. Prostka, 1965-69 

4b, H. J. Prostka and W. H. Nelson,1970- 71 

109' 

FIGURE 2.-Index map of the Absaroka volcanic field, showing extent of mapping and stratigraphic information. 
------------- -~---~~-~ ----~----- 

C4 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

formations and members constitute three natural 

stratigraphic groups, the need for an inclusive name 

dictated our choice of the term "supergroup." 

ACKNOWLEDGMENTS 

The broad stratigraphic framework of the Absaroka volcanic field was outlined late in the 19th 

century by the excellent pioneering work of Arnold 

Hague, J. P. Iddings, W. H. Weed, A. C. Peale, and 

their colleagues. We acknowledge our indebtedness 

to these early studies, as well as to the more recent 

published work which supplemented our own mapping. We extend special thanks to the following 

people for their stimulating discussions of the work 

as it was in progress, and for making available to 

us some of their unpublished data: I. S. E. Carmichael, R. A. Chadwick, B. R. Doe, F. S. Fisher, 

D. G. Gaskill, W. B. Hall, W. R. Keefer, J. D. Love, 

E. L. McCaskey, M. C. McKenna, W. H. Nelson, 

J. D. Obradovich, W. H. Parsons, Z. E. Peterman, 

W. G. Pierce, W. L. Rohrer, D. N. Rubel, C. H. 

Shultz, F. J. Vine, and W. H. Wilson. ·We were ably 

assisted with part of the mapping by Ivo Lucchitta, 

John M'Gonigle, and James Nicholls, each of whom 

contributed to the development of the concepts presented here. R. A. Chadwick, J. D. Love, M. C. 

McKenna, and W. H. Wilson also contributed as 

technical reviewers of the manuscript. 

This report is one product of a comprehensive 

geological study of the entire Yellowstone National 

Park region conducted by many geologists under the 

overall direction of the late Arthur B. Campbell. 

GENERAL GEOLOGIC RELATIONS 

The Absaroka Volcanic Supergroup consists largely of calc-alkalic andesitic and dacitic extrusive 

rocks, of lesser amounts of potassic alkalic mafic 

lavas, and of minor amounts of rhyodacitic ashflow tuffs associated with the mafic lavas. Small 

bodies of intrusive rocks are contemporaneous with 

the extrusive rocks. Where they can be genetically 

related to specific formations, these rocks are regarded as part of the supergroup; where their 

relationship is obscure; the intrusive rocks are regarded as being related to the supergroup, but are 

not included within it (U.S. Geological Survey, 

1972). The stratified rocks are gently deformed and, 

in most places, rest unconformably on rocks ranging 

in age from Precambrian to Paleocene (Hague, 

Iddings, and others, 1899; Rouse, 1937; Love, 1939; 

Hall, 1961; Roberts, 1965; U.S. Geological Survey, 

1972) that were strongly deformed locally during 

the Laramide orogeny. 

As now known, the Absaroka Volcanic Supergroup consists, in ascending order, of the Washburn Group, the Sunlight Group, and the Thorofare 

Creek Group (fig. 3). The crude zonal distribution of 

these units reflects their progressive southward 

onlap onto the basement rocks (fig. 3; U.S. Geological Survey, 1972). 

The Washburn Group is restricted to the northern part of the field and is the oldest part of the 

volcanic pile. It is overlapped, in most places unconformably, by the Sunlight and Thorofare Creek 

Groups (figs. 3, 4). Nearly everywhere, the Thorofare Creek Group overlies the Sunlight Group, but 

in the north-central part of the field, the lower part 

of the Thorofare Creek Group locally interfingers 

with the Sunlight Group or rests directly on the 

Washburn Group (figs. 3, 4, 5). 

Petrologically, the Sunlight Group is the most 

mafic unit and contains the highest proportion of 

basaltic potassic rocks. Most of the rocks of the 

other groups are generally lighter colored and of 

andesitic and dacitic composition. 

The vent areas for the volcanics were composite 

stratovolcanoes and shield volcanoes composed of 

autoclastic flow breccias, lava flows, mudflows, avalanche debris, and tuff (figs. 12, 13, 15). We refer 

to this chaotic assemblage of rock, which formed 

part of a volcanic cone, as vent facies (fig. 6). 

Primary dips of lava flows and breccias near the 

vents are commonly more than 30° and are locally 

irregular, although, in general, the rocks dip away 

from the vent centers. Stocks, plugs, laccoliths, and 

radial swarms of dikes are common features of most 

of the vent areas. Outward from the volcanic centers, 

the vent facies rocks interfinger with, and grade 

into, aprons of well-bedded reworked volcanic sedimentary rocks (figs. 6, 7), having primary dips of 

less than 5°-mainly volcanic conglomerate and 

breccia, volcanic sandstone and siltstone, and airfall tuff (figs. 10, 11, 16). We refer to these deposits 

of tuff and reworked material as alluvial facies. This 

concept of vent and alluvial facies was first used in 

the Absaroka volcanic field by Parsons (1965, 1969) 

in the Sunlight-Crandall area (fig. 1). The classification and terminology used in this report for the 

volcaniclastic rocks are, for the most part, those 

proposed by Fisher (1960, 1961). 

In the alluvial facies there is a general decrease 

in thickness of section, thickness of beds, and in 

clast size away from the volcanic centers, and a 

corresponding increase in the degree of sorting and 

rounding. Consequently, rocks of vent facies grade 
111" 

STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP 

YELLOWSTONE 

NATIONAL 

PARK 

~- -·-·~ · 

"" 

, 

0 20 

Absaroka • Volcanic 

Supergroup undivided 

40 MILES 

EXPLANATION 

Thorofare • Creek Group 

• Sunlight Group 

D 

Wash bum Group 

Contact 

Dashed where approximately 

located or inferred 

•a 

Locality 

Radiometrically dated sample 

referred to in text 

109" 

C5 

FIGURE 3.- Generalized geologic map of the Absaroka volcanic field. For simplicity, related intrusive rocks are not shown. 
C6 

West Yellowstone 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

GEOLOGY OF YELLOWSTONE NATIONAL PARK 

.-.-·-,-· "~·~· 

~ / ; s"1ft 

'\::..tJ & ~ 

I ;_____ • --• --....c_ ' • --"'~.u=-""' lMP 

0 5 10 15 MILES 

FIGURE 4. - Stratigraphic relations of the Absaroka Volcanic Supergroup and related intrusive rocks in Yellowstone National 

Park. 

into coarse alluvial facies strata consisting mainly of their mobility, lava flows and some of the volof volcanic conglomerates and breccias, which, in canic mudflows are laterally very extensive and 

turn, grade outward into fine-grained alluvial facies occur in both the alluvial facies and the vent facies 

beds containing a high proportion of volcanic sand- assemblages. Abundant fossil wood and standing 

stone and siltstone and air-fall tuffs (fig. 7). Because stumps of trees are preserved in the alluvial facies, 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C7 

EXPLANATION 

j "" Wiggins Formation 

.., 

., 

~ u 

~ 

~ g:; 

Two Ocean Formation 

Langford Formation 

Absaroka Volcanic Related intrusive rocks Trout Peak Trachyandesite 

Supergroup undivided of several ages 

""" ..c: 0 

Wapiti Formation 

Mount Wallace Formation 

Crescent Hill Basalt 

"' ~ ... ""{ ~ c.!l Lamar River and 

Cathedral Cliffs Formations 

Sepulcher Formation 

Contact 

Selected major fault of Eocene and younger age 

Bar and ball on downthrown side; 

dotted where concealed 

• ol5MP 

Park boundary milepost 

Location of part of 

section (fig. 5) 

FIGURE 4.- Continued. 

whereas only sparse fossil-wood fragments occur in 

the vent facies. Locally, forests were buried by, and 

preserved in, coarse mudflow deposits (Dorf, 1960, 

1964; Wilson, 1965, fig. 23, p. 66). Fossil leaves are 

locally abundant in the fine-grained alluvial facies 

(fig. 10) and are moderately abundant in the vent 

facies (fig. 15), but they are virtually absent in the 

coarse alluvial facies from which the tuff and much 

of the finer grained epiclastic debris has been reworked basinward into the fine-grained alluvial 

facies. The massive coarsely bedded alluvial deposits 

(fig. 11) tend to be the most resistant to erosion. 

Vent facies deposits are somewhat less resistant and 

commonly erode to form hoodoo badlands (figs. 12, 

15). Fine-grained alluvial deposits (fig. 10) are the 

least resistant to erosion and typically form subdued slopes. 

Alluvial facies deposits are the lateral time equivalents of the vent facies deposits from which they 

were largely derived. Assemblages of vent facies and 

corresponding alluvial facies deposits are genetically related to pulses of volcanic activity. These somewhat heterogeneous, laterally variable units have 

been called "volcanogenic units" (Dickinson, 1968). 

In deciphering the history of a complex volcanic 

field, more can usually be learned from the mapping 

of these volcanogenic units than from delineating 

units that are lithologically uniform. Many of the 

formations and groups described in this report are 

volcanogenic units, which display considerable lat-
C8 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

NORTHWEST 

(i) 

Bighorn Peak 

:Fortress : MOU ntal n Member:. 

. baly creek Memt>er • · · 

Prevolcanic rocks Ts 

Sepulcher Mountain 

Gallatin 

Range 

horst 

0 

Oxbow Creek Crescent Hill 

Sunlight Group channeled out 

by Thorofare Creek Group 

Amethyst 

Mountain 

Sunlight Group interfingered wi'th~ ........... ...... \ · upper part Washburn Group - ;;... .... _____ · .· . · .·. _:_:'j"J ~ · _: _: _: 

Tch -..:::~ -- --------- -------- --

Tic 

Conglomerate facies 

FIGURE 5.- Schematic section (not to scale) of part of Yellowstone National Park, showing stratigraphic relations in the 

faults which bound the Gallatin Range horst are shown. Circled numbers refer to localities shown in figure 4. Twi, Wiggins 

Tm, Mount Wallace Formation; Tch, Crescent Hill Basalt; Tic, Lamar River and Cathedral Cliffs Formations undivided; 

eral variation; therefore, it was not possible to 

designate a truly representative type section for 

each unit. Instead, as is customary in such terranes 

(Love, 1939, p. 79; Wilson, 1963, p. 13; Klepper 

and others, 1957, p. 32), we have designated type 

areas in which the fullest variety of· rocks within 

the unit is exposed. Locations of representative 

sections of all formations and members are given, 

but detailed descriptions have deliberately been 

omitted from this general report. 

Most of the formations consist of repetitious sequences of andesitic volcaniclastic rocks which, by 

themselves, are not distinguishable from one formation to another. For this reason, the recognition and 

mapping of many formations have depended to a 

large extent on tracing widespread unconformities 

and on mapping distinctive marker units, such as 

lava flows, within formations. Many of the groups 

and formations in the · Absaroka volcanic field are 

bounded by major unconformities. Minor unconformities commonly occur within mapped units. 

The base of the volcanic pile is at different stratigraphic levels from place to place because of the 

high relief on the prevolcanic surface and the effects 

of intravolcanic deformation and erosion. There is 

no single complete section of the volcanics; a composite section, about 12,000 feet thick, was pieced 

together from overlapping partial sections by using 

the distinctive flows and tuff beds as marker units 

to correlate the more widely separated sequences. 

Rocks of the Absaroka Volcanic Supergroup in 

the Yellowstone National Park region are of early, 

middle and possibly late Eocene age (late Wasatchian, early Bridgerian, and possibly Uintan provincial age of Wood and others, 1941), as is indicated 

by fossils and numerous radiometric determinations 

(figs. 3, 8). The ages of individual units are discussed in detail in the sections that follow. Volcaniclastic rocks of early Oligocene and Miocene age, 

locally present south and southeast of Yellowstone 

National Park (Love, 1939, 1956a, b, c, d), previously have been regarded as part of the Absaroka 

volcanic field; however, because their distribution 

is not well known, they are not discussed in this 

report. 

Studies of remanent magnetization are in progress. The available data indicate that most of the 

Washburn Group is of normal polarity, but that, 

high in the section in the northwestern part of the 

park, there is a reversal of short duration. Lavas of 

the Sunlight Group are normal except for a few 

in the lower part of the section which have reverse 

polarity. Because the lower part of the Sunlight 

Group is the same age as the upper part of the 

Washburn Group, the lavas of both groups may be 

recording the same magnetic events. Data on the 

Thorofare Creek Group are too sparse for any conclusions as to polarity. 

WASHBURN GROUP 

The name Washburn Group (new name) is proposed for the oldest part of the Absaroka volcanic 

pile. The group is named for the type area, the 

Washburn Range in north-central Yellowstone National Park, which is close to the center of known 

distribution of the group. These rocks make up 

much of the northern Absaroka Range, the Washburn Range, and the Gallatin Range (figs. 1, 3, 9). 

The Washburn Group is more than 3,000 feet 

thick near vent areas in the Gallatin, northern 

Absaroka, and Washburn Ranges, and thinner elsewhere. The group consists mostly of lavas and related volcaniclastic strata composed of hornblende 

and pyroxene andesite, lesser amounts of biotite 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP 

NORTHEAST SOUTHEAST 

Lower part Thorofare Creek Group Mirror Plateau Trident Plateau 

mterfmgered w1th Su:~~;-~~;~.~~~-~ .. :::_:_-:·:~::······-------

Absaroka Volcanic Supergroup. Section extends from northwest to northeast (it bends 90°) to southeast. Only those 

Formation; To, Two Ocean Formation; Tl, Langford Formation; Tt, Trout Peak Trachyandesite; Tw, Wapiti Formation; 

Ts, Sepulcher Formation. 

Volcanic 

cone 

Alluvial 

apron 

Interfingering vent facies 

and alluvial facies rocks 

FIGURE 6. - Hypothetical cross section, showing relationships of vent facies and alluvial facies rocks. 

VENT 

FACIES A 

COARSE ALLUVIAL 

FACIES A 

Fl NE ALLUVIAL 

FACIES 

Area of interfingering and intermixing 

of facies A and facies B 

Lava flows, breccias, ash, and wood 

fragments 

Fine-grained volcanic sediments, airfall 

tuff, and standing fossil trees 

INTERMEDIATE RESISTANCE 

TO EROSION 

Volcanic conglomerate and breccia, wood 

fragments; very little ash and very 

few fine-grained beds 

MOST RESISTANCE 

TO EROSION 

LEAST RESISTANCE 

TO EROSION 

0 • 

COARSE ALLUVIAL 

FACIES B 

• • 

VENT 

FACIES B 

C9 

FIGURE 7.- Hypothetical cross section, showing facies relationships of two vent complexes (A and B) and the intervening 

volcanic sediments derived from them. 
ClO GEOLOGY OF YELLOWSTONE NATIONAL PARK 

EPOCH 

"' E 

"' _J 

'C 

c 

., "' 

'C 

'C 

:2 

UJ 

z 

UJ 

u 

0 

UJ 

APPROXIMATE 

PROVINCIAL AGEl 

"' c 

2 

c 

:::> 

'C 

c 

"' c 

·.:: "' ., 

bD 

'C 

·.:: 

CD 

1Wood and others (1941) . ' H. W. Smedes. ' H . J. Prostka. ~Upper part of Wiggins Formation. 

A 

FORKS 

FOLIO 

CHADWICK 

(1969) 

GALLATIN 

RANGE 

NORTHEASTERN 

PART 

B 

Golmeyer Creek 

Volcanics 

BIGHORN 

PEAK2 

c 

THIS REPORT 

SEPULCHER 

MOUNTAIN 

AREA 2 

D 

CRESCENT HILL 

. AREA 3 

E 

facies 

8Wapiti, Langford, Mount Wallace, Lamar River, and part of Sepulcher Formations. 

5Wiggins, Two Ocean, and part of Langford Formations. 6Langford Formation. 

9Cathedral Cliffs, and parts of Lamar River and Sepulcher Formations. 10Mount Wallace, Hya lite P eak. Golmeyer Creek, and part of Sepulch er Formations. 11Part of Sepulcher, Golmeyer Creek. and Hyalite P eak Formations. 7Trout Peak Trachyandesite. 

Locality Age 

a, ________________ 49.2±1.5 c ___ ________ _____ .48.0±1.3 e ________ ________ 47.9±L3 

48.5±1.3 f- _______________ _46.1 ±1.2 

Mineral 

Sanidine. Biotite. 

Sanidine. 

Biotite. 

Sanidine. 

POTASSIUM-ARGON AGE DETERMINATIONS 

[Age determinations, in millions of years, by J . D. Obradovich] 

Locality Age 

g ___ _____ __ ____ __ _ 49.3±L4 

h ________________ 47.8±L3 

47.9±1.3 

i _________________ 46.2±L8 

j _____________ ___ _ 46.5±2.3 

Mineral 

Biotite. 

Sanidine. 

Biotite. 

Hornblende. 

Do. 

L ocality Age 

k ________________ _ 44.4±1.4 

45.5±1.3 

l___ _______________ 4 7.1 ±L3 m _______________ 46.7 ±L5 n ________________ 44.6±1.2 

Mineral 

Hornblende. 

Biotite. 

Do. 

Do. 

Do. 

FIGURE 8 (above and facing page).- Nomenclature, stratigraphic relations, and proposed correlation of units in the Absaroka 

Volcanic Supergroup in the park with units in adjoining areas. New names are in boldface; revised units are indicated by 

*; column in which the type area, locality, or section of a formation or member occurs is indicated by t- Vertical scale is 

not proportionate to thickness or time. (See fig. 3 for sample localities.) 

andesite and dacite, and minor amounts of basaltic 

lavas and rhyodacite ash-flow tuffs. Volcanics of the 

Washburn Group constitute the lower 1,500 feet or 

more of the volcanic section throughout northeastern 

Yellowstone National Park, where they form bold 

cliffs above the Lamar River and its tributaries. 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP Cll 

THIS REPORT LOVE (1939) 

HAGUE, WEED, AND 

IDDINGS (1896) 

----------------------------------------.-----------~------------~ HAGUE AND OTHERS (1899) 

SPECIMEN 

RIDGE3 

NORTHEASTERN 

PART3 SOUTHEASTERN 

PART2,3 

SOUTHWESTERN 

ABSAROKA 

VOLCANIC 

FIELD 2 

EAST FORK 

WIND RIVER 

NORTHERN 

TWO-THIRDS 

OF ABSAROKA 

VOLCANIC FIELD 

F G H 

Crescent Hill (fig. 9) and the southern part of the 

Washburn Range are other easily accessible exposures of these rocks. For several miles north of 

Gardiner, Mont. (fig. 9), these volcanics form the 

knobby reddish-brown hills west of the highway. 

Most of the rocks of this group were erupted from 

several separate but nearly contemporaneously active centers whose deposits now comprise several 

laterally gradational, dominantly light-colored and 

dominantly dark-colored formations, which are volcanogenic units. Previously, the_ light-colored rocks 

were called early acid breccia or acid andesitic 

breccia and flows, and the dark-colored units were 

called early basic breccia, or basic andesite breccia 

and flows (Hague, 1899; Hague, Iddings, and others, 

1899; Iddings and Weed, 1894; Peale, 1896) . 

The dominantly light-colored, locally biotitic Sepulcher Formation occurs in northwestern and 

J K 

0 Eo 

:::> c: D ·-

.C VI "' Q) "' ~ !;: "' Q) 

c. 

t::; "' 

north-central Yellowstone National Park (fig. 4). 

It grades southeastward into, and is partly overlain by, the darker colored nonbiotitic Lamar River 

Formation. The Elk Creek Basalt Member occurs in 

the lower part of both the Sepulcher Formation and 

the Lamar River Formation. The lower part of the 

Lamar River Formation interfingers northeastward 

with the light-colored Cathedral Cliffs Formation 

near the east boundary of the park. Near the northwestern part of the park, approximately the lower 

half of the Sepulcher Formation extends northward 

into the northern Gallatin Range and grades into 

the dark-colored Hyalite Peak Volcanics (Chadwick, 1969), also of the Waspburn Group (fig. 8, 

col. B; McMannis and Chadwick, 1964). Locally, the 

Hyalite Peak Volcanics are underlain by the Galmeyer Creek Volcanics, which have no known correlatives in the park. The Golmeyer Creek Volcanics 
C12 GEOLOGY OF YELLOWSTONE NATIONAL PARK 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C13 

part of Yellowstone National Park and vicinity. 
C14 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

are the oldest rocks in the Washburn Group. 

The Washburn Group unconformably overlies 

rocks of Precambrian to Paleocene age which have 

been strongly deformed during the Laramide orogeny (U.S. Geological Survey, 1972). The surface 

on which the volcanics rest is rugged; in many places 

the relief is as much as several hundred feet. Deformation of the Washburn Group is slight; structural tilt is generally less than 10° except locally 

near faults and later intrusives. Over most of the 

area, the Washburn Group either has an eroded top 

or is overlain by rocks of the Sunlight Group. Near 

Crescent Hill, however, uppermost strata of the 

Washburn Group interfinger with lavas of the Sunlight Group (fig. 5). In the Washburn Range and 

on Amethyst Mountain, the Sunlight Group is absent, and strata of the Thorofare Creek Group lie 

directly on rocks of the Washburn Group. 

The Washburn Group is of early and middle 

Eocene age (Wasatchian and early Bridgerian provincial age). (See fig. 8.) 

SEPULCHER FORMATION 

The name Sepulcher Formation (new name) is 

here proposed for the dominantly light colored andesitic and dacitic rocks of the Washburn Group in 

the northwestern and north-central parts of Yellowstone National Park (fig. 4) and adjacent areas. The 

formation is named for the type area of Sepulcher 

Mountain (fig. 9), where the section is thick and 

well exposed. The upper half of the formation is 

coarse, rather dark vent facies. Other fairly accessible representative sections can be seen from the 

summit of Cresent Hill northward to the Yellowstone River and in the ridge 1 mile north of the 

mouth of Specimen Creek, in the northwestern part 

of the park (fig. 9). The Cresent Hill section and 

all but the basal 800 feet of the Specimen Creek 

section are of alluvial facies. The source for most 

of the Sepulcher Formation, as indicated by facies 

relations, was the area of the Gallatin Range horst 

(fig. 4). The laccolithic and crosscutting intrusive 

bodies of the Gallatin Range (fig. 4; U.S. Geological 

Survey, 1972) are chemically similar to the Sepulcher Formation and are of the same age. They are 

almost certainly subvolcanic intrusives that are genetically related to the Sepulcher Formation, but 

because their relationship to the extrusive rocks has 

not definitely been established, they are not included 

in the Absaroka Volcanic Supergroup. 

The Sepulcher Formation was deposited on a 

surface having a relief of more than 500 feet. A 

basal conglomerate composed largely of Precambrian 

metamorphic rocks was deposited locally at different 

stratigraphic levels along the margins of bordering 

highlands. This basal conglomerate facies is overlain by a variety of andesitic and dacitic volcanic 

sediments which make up most of the Sepulcher 

Formation. Interlayered with the lower part of the 

Sepulcher Formation are trachybasalt lavas of the 

Elk Creek Basalt Member, overlain, in turn, by 

rhyodacite of the Lost Creek Tuff Member. In northwestern Yellowstone National Park, the andesitic 

volcaniclastic rocks above the Lost Creek Tuff Member are subdivided in this report into two unitsthe lower, light-colored Daly Creek Member (new 

name) and the upper, dark-colored Fortress Mountain Member (new name). 

The Sepulcher Formation grades from dominantly vent facies near the Gallatin Range horst (fig. 

4) to dominantly alluvial facies (fig. 10) in the 

northwestern and north-central parts of the park. 

Most sections also show an upward increase in vent 

facies aspect. 

Fresh vent facies rocks are mostly dark reddish 

brown, brown, and gray, but where ash is abundant 

or where deuteric or hydrothermal alteration has 

been severe, the rocks are light colored- yellowish, 

greenish, and bluish gray and light gray. 

In the northwestern part of the park (head of 

Daly Creek, fig. 9), the Sepulcher Formation thickens eastward from less than 1,000 feet to more than 

2,300 feet; the Daly Creek Member grades from 

alluvial facies to vent facies; and the Fortress 

Mountain Member thickens from 800 to 1,400 feet 

and grades both eastward and upward from finegrained alluvial facies to near-vent coarse alluvial 

facies. 

The Daly Creek and Fortress Mountain Members 

extend north of the park as alluvial facies units to 

the vicinity of Fortress Mountain and Tom Miner 

Basin (fig. 9), where they contain a few lava flows 

(Todd, 1969; reconnaissance mapping by Smedes; 

W. B. Hall and R. A. Chadwick, written commun., 

1969). The lower half of the Sepulcher Formation 

is probably correlative with the Hyalite Peak Volcanics (Chadwick, 1969), whereas the Golmeyer 

Creek Volcanics (Chadwick, 1969) are older and 

probably have no equivalents within the park. 

At Sepulcher Mountain (fig. 9) the Sepulcher 

Formation is more than 3,000 feet thick and consists 

of coarse alluvial facies strata grading upward and 

southward into vent facies rocks. The upper 1,200 

feet of section on Sepulcher Mountain consists of 

andesite lava flows and flow breccias, many of which 

contain conspicuous biotite phenocrysts. A nearly 

complete section of the formation is exposed in the 

spectacular cliffs on the north face of Sepulcher 

Mountain, which can be viewed from Gardiner. 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C15 

FIGURE 10.- Thin-bedded fine-grained alluvial facies volcanic sandstone, siltstone, and air-fall tuff of the Sepulcher Formation 

along Quartz Creek, on the west side of Specimen Ridge (fig. 9). 

From Sepulcher Mountain the formation thins make up the upper part of the section at Sepulcher 

eastward and grades from vent and coarse alluvial Mountain (fig. 8, col. D). 

facies to fine-grained alluvial facies rocks in the 

vicinity of Crescent Hill (fig. 9). The thickest and 

best exposed section of the Sepulcher Formation in 

north-central Yellowstone National Park extends 

from the summit of Crescent Hill northward to the 

Yellowstone River, near the mouth of Hellroaring 

Creek (fig. 9). This section is about 1,900 feet thick; 

1,200 feet of it consists of fine-grained alluvial facies 

volcanic sediments which were described in detail 

by Brown (1957), and 700 feet is the Elk Creek 

Basalt and Lost Creek Tuff Members. 

At Oxbow Creek, 5 miles northwest of Crescent 

Hill, the Sepulcher Formation is only 500 feet thick 

because of a local, but prominent, intraformational 

unconformity (fig. 9). This unconformity can be 

seen along the road northwest .of Crescent Hill. 

Coarse breccia and conglomerate, composed mainly 

of biotite-hornblende andesite fragments, discordantly overlie volcanic sandstone and siltstone. These 

coarse clastics probably are the lateral equivalent 

of the biotitic andesite flows and flow breccias which 

CONGLOMERATE FACIES 

Coarsely bedded, dominantly nonvolcanic conglomerate and breccia and minor amounts of sandstone locally underlie and interfinger with the basal 

volcanic sediments of the Sepulcher Formation. The 

best exposed and most typical section is along the 

canyon of the Yellowstone River, just upstream from 

the mouth of Hellroaring Creek (fig. 9). 

This distinctive conglomerate was previously referred to as Crandall ( ?) Conglomerate (Brown, 

1961, p. 1175), as it was thought to be correlative 

with the Crandall Conglomerate (Pierce, 1957). 

However, that correlation is not valid, as the unit 

in the type area is a stream-channel deposit, made 

up almost entirely of clasts of Paleozoic carbonate 

rocks and predating the volcanic Cathedral Cliffs 

Formation. In contrast, the conglomerate facies of 

the Sepulcher Formation is a basin-margin fluvial 

deposit, composed of fragments of Precambrian 

igneous and metamorphic rocks, which underlies and 
C16 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

interfingers with volcanic sediments of the same 

age as those of the Cathedral Cliffs Formation. 

The conglomerate facies is patchy in distribution 

and seems to be restricted to the north-central and 

northwestern parts of Yellowstone National Park. 

A 40-foot tongue of the conglomerate is interlayered 

with ash flows of the Lost Creek Tuff Member along 

Geode Creek (fig. 9). Along the Lamar River, near 

the confluence with Slough Creek (fig. 9), the conglomerate directly underlies the Lost Creek Tuff 

Member and contains a bed of crossbedded crystalvitric tuff compositionally like the Lost Creek. Other 

occurrences of the basal mainly nonvolcanic conglomerate and fanglomerate or breccia are present 

on the north spur of Sepulcher Mountain, between 

Reese and Stephens Creeks; near the head of Daly 

Creek in the Gallatin Range; in the Washburn Range 

along Carnelian Creek; and on the southeast flank 

of Hedges Peak, just above the Grand Loop Road. 

At the last-named locality, the conglomerate contains quartzite pebbles like those of the Pinyon 

Conglomerate of Cretaceous and Paleocene age 

(Hague and others, 1896; Love, 1947), as well as 

Precambrian clasts. 

At the head of Campanula Creek (fig. 9), in the 

northwestern part of the park, is a unit of dominantly chert-pebble conglomerate several tens of feet 

thick and containing sparse volcanic fragments, 

which we refer to the conglomerate facies. The deposit rests on Precambrian rocks; its top is eroded, 

but it is topographically just below the base of 

volcanic strata less than 1 mile away. 

The widespread occurrence of the conglomerate 

facies throughout more than 450 feet of the lower 

part of the Sepulcher Formation near its onlap onto 

prevolcanic rocks suggests that the conglomerate 

facies is a time-transgressive, mainly nonvolcanic 

marginal facies of the Sepulcher Formation. Where 

the conglomerate facies is absent, the lower 250-

300 feet of the Sepulcher Formation commonly contains sparse to abundant locally derived nonvolcanic 

fragments. 

A thin black shale bed in the conglomerate facies 

near the mouth of Hellroaring Creek contains plant 

remains of Eocene age (Estella Leopold, written 

commun., 1967). 

ELK CREEK BASALT MEMBER 

The Elk Creek Basalt was named by Howard 

(1937) for exposures west of Elk Creek, northeast 

of Crescent Hill (fig. 9). At the type locality, the 

unit consists of four or more dark-reddish-brown to 

black lava flows and flow breccias of potassic basalt 

or shoshonite containing abundant large phenocrysts 

of plagioclase and augite. Howard (1937) and, later, 

Brown (1961) miscorrelated the type Elk Creek 

Basalt with upper Cenozoic phenocryst-poor olivine 

basalts, and from this concluded that the Elk Creek 

Basalt and overlying Lost Creek Tuff were late 

Cenozoic in age. Our mapping, however, shows that 

the Elk Creek Basalt is interlayered with the lower 

part of the Sepulcher Formation and with the Lamar 

River Formation; hence, we redefine it as a member 

of both formations. 

The Elk Creek Basalt Member - as a member of 

the Sepulcher- is sporadically present along the 

northeast base of Sepulcher Mountain and is well 

exposed near the road north and northwest of Crescent Hill and along Coyote Creek in the north-central 

part of the park (fig. 9). To the southeast the Elk 

Creek is present in the lower part of the Lamar 

River Formation along the Lamar River and along 

Soda Butte, Pebble, Cache, and upper Miller Creeks, 

but it is absent in the southern Washburn Range 

(fig. 9). 

Where the Elk Creek Basalt Member occurs in 

the Sepulcher Formation, its lava flows rest directly 

on one another with few or no interbedded volcanic 

sediments; in the Lamar River Formation, however, 

there are volcaniclastic sequences as much as 320 

feet thick separating the four lava flows of the Elk 

Creek Basalt Member. Locally, some of the flows 

pinch out, and there are as few as two. The Elk 

Creek Basalt Member as defined herein includes only 

the lava flows, not the sedimentary interbeds. The 

maximum aggregate thickness of these flows is about 

300 feet. 

In the Sepulcher Mountain region, the Elk Creek 

Basalt Member ranges in thickness from a few feet 

to about 60 feet and is discontinuously exposed along 

the northeast base of Sepulcher Mountain, from 

Beaver Ponds northwestward to about 1 mile beyond Rainbow Lake (fig. 9). Fraser, Waldrop, and 

Hyden (1969, p. 41, pl. 1) mapped some of these 

exposures and interpreted the basalts to be younger 

than the rocks which we assigned to the Lost Creek 

Tuff Member. However, these basalts are not in contact with other volcanic rocks in any of the exposures that they mapped (Fraser and others, 1969). 

Our subsequent studies in the same area and in an 

area to the south, near Beaver Ponds, disclosed 

several outcrops in which the basalts are unconformably overlain by the Lost Creek Tuff Member, 

which, in turn, is overlain by the Daly Creek Member of the Sepulcher Formation. 

The Elk Creek Basalt Member rests on volcanic 

sediments of the basal part of the Sepulcher and 

Lamar River Formations, or directly on prevolcanic 

rocks. It is overlain by the Lost Creek Tuff Member 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C17 

as far east as Specimen Ridge; east and southeast of 

there, it is overlain by the Lamar River Formation. 

The Elk Creek Basalt Member is approximately 

the same age- early or early middle Eocene- as 

the Sepulcher and Lamar River Formations with 

which it is interlayered. It is overlain by the Lost 

Creek Tuff Member, which is dated as 49.2±1.5 

m.y. (million years). 

LOST CREEK TUH' ~IEl\IBER 

The Lost Creek Tuff Member was first called 

trachytic rhyolite by Hague, Weed, and Iddings 

(1896) and by Hague, Iddings, and others (1899) ; 

it was named Lost Creek Trachyte by Howard 

(1937) and Lost Creek Tuff by Brown (1961). The 

type area is near Lost Creek, northwest of Tower 

Junction (fig. 9). Both Howard and Brown thought 

the Lost Creek Tuff was late Cenozoic in age because it overlies exposures of Elk Creek Basalt, 

which they interpreted to be upper Cenozoic intracanyon basalt flows. Our mapping shows the tuff 

to be interlayered with the lower part of the Sepulcher Formation and, hence, of Eocene age, as 

originally suggested by Hague, Weed, and Iddings 

(1896). For this reason, we hav.e redefined it as a 

member of the Sepulcher Formation. 

The Lost Creek Tuff Member is a sequence of 

yellowish-gray rhyodacite welded ash-flow sheets 

containing abundant phenocrysts of sanidine, andesine, and biotite, and sparse amounts of hornblende, 

but few or no phenocrysts of quartz. Flattened 

pumice, rock fragments - mainly of Precambrian 

metamorphic rocks - and charred wood, are common. The columnar and platy-jointed central densely 

welded zone is best exposed; the upper and lower 

partly welded zones are rarely exposed in the type 

area. 

The Lost Creek Tuff Member is present at or near 

the base of the Sepulcher Formation along the north 

flank of Sepulcher Mountain and throughout the 

north-central part of the park as far south as the 

southern Washburn Range, and as far east as the 

mouth of Lamar Canyon (fig. 9). The unit is well 

exposed along Quartz Creek, west of Specimen 

Ridge; in the gravel pit north of Crescent Hill; and 

north of Sepulcher Mountain, at the base of the 

long low ridge between Reese and Stephens Creeks 

(fig. 9) 0 

The Lost Creek Tuff Member is about 350 feet 

thick near the type area; it thins eastward and 

pinches out northeast of Specimen Ridge (fig. 9). 

It thickens westward to a maximum of about 1,000 

feet north of Sepulcher Mountain, between Reese 

and Stephens Creeks (fig. 9), where it consists of 

three units that rest unconformably on Upper Cretaceous rocks equivalent to the Livingston Group 

(called Landslide Creek Formation by Fraser and 

others, 1969). The lower unit is 400-500 feet thick 

and consists of dozens of biotite rhyodacite nonwelded and welded ash-flow tuffs, a few inches 

to a few tens of feet thick, interlayered with tuffaceous sandstone and siltstone; the ash-flow tuffs become thicker (as much as 100 ft) and more densely 

welded upward. The lower ash-flow tuffs contain 

abundant fragments of Precambrian metamorphic 

rocks and locally contain charred logs. The middle 

unit rests unconformably on the lower unit, and 

consists of thick sheets of coarse block and ashavalanche breccia intercalated with thin tuffaceous 

sandstones. The upper unit rests unconformably on 

the middle unit, and consists of about 80 feet of 

andesite lava flows overlain by 20-25 feet of poorly 

welded ash-flow tuff. Southward, the middle and 

upper units are eroded away, and the lower unit is 

directly overlain by the Daly Creek Member. 

Calvert (1912) proposed the name Reese Formation for a sequence of strata that includes the Cretaceous sandstone and conglomerate, a lens of the 

conglomerate facies, and the lower and middle units 

of the Lost Creek Tuff Member. The upper unit of 

the Lost Creek Tuff Member and the remainder of 

the Sepulcher Formation were called "igneous 

flows" (Calvert, 1912, p. 411). The name Reese 

Formation is abandoned, as suggested by Pierce 

(1963, p. 19) 0 

Because of rugged prevolcanic topography, the 

Lost Creek Tuff Member rests on a variety of rocks 

- prevolcanic rocks, the conglomerate facies, and 

the Elk Creek Basalt Member. It is overlain by 

volcanic sediments of the Sepulcher Formation. 

Potassium-argon dating of sanidine phenocrysts 

from the Lost Creek Tuff Member on Specimen 

Ridge (fig. 9) by J. D. Obradovich (written commun., 1968) yielded a late Wasatchian or early 

Bridgerian provincial age of 49.2±1.5 m.y. (figs. 

3 and 8, loc. a). 

DALY CREEK MEMBER 

The name Daly Creek Member (new name) is here 

proposed for the lower part of the Sepulcher Formation in the Bighorn Peak region of the southern Gallatin Range (fig. 1; fig. 8, col. C). It is a sequence 

of light-colored volcaniclastic strata unconformably 

overlying highly deformed rocks of Late Cretaceous 

or Paleocene age (Hall, 1961) and underlying the 

Fortress Mountain Member. It consists of hornblende andesite, pyroxene andesite, and hornblende 

pyroxene andesite volcaniclastic rocks of vent and 

alluvial facies: The type area is the headwaters 

region of Daly Creek in the northwest corner of the 
C18 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

park (fig. 9). The Daly Creek Member consists of 

more than 800 feet of vent facies rocks near the 

mouth of Specimen Creek and grades northwestward to about 400 feet of fine-grained alluvial facies 

strata on the ridge at the head of Daly Creek (fig. 9). 

Northward from there, Hall (1961) mapped a 

sequence of strata, as much as 2,000 feet thick, of 

dominantly alluvial facies, which may include the 

Lost Creek Tuff Member and older rocks, as well as 

the Daly Creek Member as here defined. 

The Daly Creek Member is equivalent to the upper part of the unnamed lower member of Hall 

(1961, p. 89-91; this report, fig. 8). A welded-tuff 

sheet in the upper part of that sequence probably 

correlates with the Lost Creek Tuff Member. 

At the head of Black Butte Creek (fig. 9) vent 

facies breccias of the Daly Creek Member contain 

fragments of Precambrian metamorphic rocks. 

North of the junction of East and North Forks 

Specimen Creek (fig. 9), there are spectacular exposures of dozens of horizons of fossil forests in the 

upper part of the Daly Creek Member and throughout the Fortress Mountain Member. Biotite is 

common, though inconspicuous, in the fine-grained 

pumice and ash matrix of the coarse alluvial facies 

conglomerates in that section. 

Strata in the Sepulcher Mountain region which, 

on the basis of stratigraphic position and facies 

trends, are correlated with the Daly Creek Member, 

are 600-800 feet of dark volcanic conglomerate and 

mudflow breccia of vent and coarse alluvial facies. 

These are well exposed in the lower part of the 

north face of Sepulcher Mountain. 

The Daly Creek Member is of late Wasatchian or 

early Bridgerian provincial age: it rests directly on 

the Lost Creek Tuff Member which has been dated 

as 49.2± 1.5 m.y., and is overlain by the Fortress 

Mountain Member, which cont;;tins plant fossils reportedly of early middle Eocene age (Hall, 1961). 

FOR TRESS MOUNTAIN MEMBER 

The name Fortress Mountain Member is proposed 

for the upper, darker colored part of the Sepulcher 

Formation that overlies the Daly Creek Member. 

The type area is at Bighorn Peak (fig. 9). 

The Fortress Mountain Member in the type area 

is mostly dark-gray coarse alluvial facies andesitic 

volcaniclastic rocks containing abundant hornblende 

and lesser amounts of pyroxene and biotite. The 

member is divided into three informal units that are 

well exposed on the south slope of Bighorn Peak : 

upper and lower dark-gray cliff-forming units of 

crudely bedded, poorly sorted conglomerate, and a 

middle unit of light-gray poorly resistant conglomerate with varicolored clasts in an ashy matrix. All 

three units contain tuffaceous sandstone, siltstone, 

and mudstone and abundant petrified trees concentrated in more than 30 distinct fossil forest horizons. These fossil forests are well exposed north of 

the junction of the East and North Forks Specimen 

Creek (fig. 9; U.S. Geological Survey, 1972). Many 

of the trees exceed 4 feet in diameter and are preserved upright in their growth position. They are 

probably of early middle Eocene age (Hall, 1961). 

About 2 miles northeast of Bighorn Peak is a 5o 

local angular unconformity between the middle and 

upper units of the Fortress Mountain Member, resulting in a southwestward wedgeout of the middle 

unit and an eastward thickening of the upper unit. 

Except for this local feature·, the three units appear 

to be conformable. 

In the Sepulcher Mountain region (fig. 9), strata 

thought to be correlative with the Fortress Mountain 

Member are 800-1,200 feet thick. They consist of 

alternating vent and alluvial facies rocks (though 

dominantly vent facies) Which rest unconformably 

on vent facies rocks correlated with the Daly Creek 

Member. This sequence in the Sepulcher Mountain 

region becomes more ventlike southwestward. 

The Fortress Mountain Member extends north of 

the park into the northern Gallatin Range, and 

probably is equivalent to the Hyalite Peak Volcanics 

(R. A. Chadwick, written commun., 1969; Smedes, 

reconnaissance mapping, 1970). 

ANDESITE LAVAS 

The andesite lavas occur only in the uppermost 

1,200 feet of the Sepulcher Formation on Sepulcher 

Mountain (fig. 9). This unit consists of complexly 

interlayered biotite-hornblende andesite lava flows, 

breccias, and tuff resting unconformably on the 

Fortress Mountain Member. This unconformity has 

a relief of more than 400 feet and an angular discordance of 10°-20°. The proportion of lava flows 

and flow breccias to mudflow breccias increases 

north and northwestward from the summit area. 

The lavas in approximately the upper half of this 

unit are normally magnetized, but those in the lower 

part are of reverse polarity. The polarity of the 

underlying Fortress Mountain Member is unknown, 

but the Elk Creek Basalt Member near the base of 

the Sepulcher Formation is of normal polarity, so 

there must be a magnetic reversal in the interval 

between the Elk Creek Basalt Member and the base 

of the andesite lavas of Sepulcher Mountain. 

In the Crescent Hill and Oxbow Creek areas (fig. 

9; fig. 8, col. E), the top of the Sepulcher Formation 

consists of coarse andesite breccia and conglomerate 

that are lithologically very similar to the andesite 

lavas of Sepulcher Mountain. This coarse alluvial 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C19 

facies rests unconformably on well-bedded epiclastic 

volcanic sandstone and is considered to be a rubble 

facies of the andesite lavas of Sepulcher Mountain. 

AGE 

The Sepulcher Formation grades laterally to the 

south and southeast into the Lamar River Formation by interfingering and intermixing (figs. 5, 8). 

A westward-thinning tongue of the Lamar River 

Formation extends from near Tower Junction to 

the northwest end of Crescent Hill (fig. 9). Tongues 

of the Sepulcher Formation underlie the Lamar 

River Formation at Specimen Ridge and in the 

southern Washburn Range (fig. 9) ; in some places 

the contact between these formations is gradational, 

and in other places it is abrupt and discordant. In 

the Crescent Hill-Oxbow Creek area (fig. 9), the 

uppermost part of the Sepulcher Formation intertongues with Crescent Hill Basalt, a formation belonging to the Sunlight Group. 

Dorf (1960, p. 258) reported that the fossil flora 

from the early acid breccia (part of the Sepulcher 

Formation in this report) in north-central Yellowstone National Park "is interpreted as spanning the 

time interval between late early Eocene and early 

middle Eocene as presently understood, i.e., late 

Wasatchian to early Bridgerian." An assemblage of 

fossil leaves from the Fortress Mountain Member 

has been reported as probably early middle Eocene 

(Hall, 1961). Radiometric ages are in remarkably 

good agreement. Potassium-argon age determinations on sanidine from the Lost Creek Tuff Member 

in the lower part of the Sepulcher Formation (figs. 

3, 8) gave an age of 49.2±1.5 m.y. (J. D. Obradovich, written commun., 1968). Biotite from the Big 

Creek stock in the Gallatin Range (figs. 3, 9) was 

dated by Obradovich at 49.5±1.5 m.y. (Chadwick, 

1969). This stock cuts the Hyalite Peak Volcanics, 

which are a lateral equivalent of the lower half of 

the Sepulcher Formation. 

LAMAR RIVER FORMATION 

The name Lamar River Formation is here proposed for a sequence of medium-brown andesitic 

lavas and volcaniclastic rocks and minor mafic lava 

flows (Elk Creek Basalt Member- which is also 

a member of the Sepulcher Formation) in northcentral and northeastern Yellowstone National Park 

(fig. 4) and vicinity. The formation is named and 

the type area selected for exposures along the rugged 

valley walls of the Lamar River and its tributaries 

(fig. 9), where nearly all of the lithologic variation 

in the formation is displayed. The Lamar River 

Formation was formerly included in the early basic 

breccia of Hague, Iddings, and others (1899). The 

lower, slightly lighter colored part of the early 

basic breccia in northeastern Yellowstone National 

Park is the Lamar River Formation. It is overlain 

with slight unconformity by the darker colored, 

more massively bedded breccias of the Wapiti Formation (Sunlight Group), which are the upper part 

of the early basic breccia. 

Throughout most of its extent, the Lamar River 

Formation consists of well-bedded coarse alluvial 

facies volcanic conglomerates, breccias, and tuffs 

that contain some of the most impressive and best 

preserved fossil forests in the park. Representative 

sections can be seen in the northeast side of Specimen Ridge (fig. 11), and the lower half of the valley 

walls of Soda Butte Creek. Most of Barronette Peak 

and Abiathar Peak (fig. 9) near the northeast 

entrance to the park are made up of the Lamar River 

Formation. These strata are the coalesced deposits 

of epiclastically reworked material and air-fall tuffs 

derived from several andesitic volcanoes. Only one 

of these source volcanoes is now exposed in Yellowstone National Park- the Mount Washburn volcano (Shultz, 1963, 1969). The southern Washburn 

Range is the deeply eroded north flank of this 

composite stratovolcano and consists of vent facies 

lava flow, flow breccia, and debris flow deposits 

having primary dips of as much as 30°. The old 

road from Canyon Junction to the summit of Mount 

Washburn and the Grand Loop Road north of Sulphur Creek (fig. 9) are good places to see the vent 

facies of the formation. These vent facies rocks 

grade northward and eastward into strata of alluvial facies. 

The famous fossil forests on Specimen Ridge (fig. 

9) are preserved in strata of the Lamar River 

Formation. The remnants of these fossil forests 

cover an area of more than 40 square miles. Many 

hundreds of petrified trees in at least 27 different 

horizons were observed by Dorf (1964). Most of 

these are stumps which are still standing upright in 

their original growth position. In addition to the tree 

trunks, thousands of fossilized leaves, twigs, needles, 

cones, and seeds of more than 100 different species 

of trees and shrubs have been found (Dorf, 1964). 

South of Mount Washburn along Sulphur Creek 

(fig. 9) is a plug of fine- to medium-grained diorite. 

Dikes of andesite similar to the lavas of the Mount 

Washburn volcano radiate from this plug, which is 

in the core of the volcano (U.S. Geological Survey, 

1972). T-he plug probably is an intrusive phase of 

the Lamar River magmas. 

Brecciated andesite intrusives, from a few tens 

of feet to more than 5 miles across, are locally present in the lower part of the Lamar River Formation; 

these intrusives occur as sheets and lenses and, less 
C20 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

FIGURE 11.- Well-bedded alluvial facies volcanic breccias, conglomerates, sandstones, and air-fall tuff 

of the Lamar River Formation in the Fossil Forest, along the northeast side of Specimen Ridge 

(fig. 9). 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C21 

commonly, as irregular bodies. They are thickest and 

most abundant in the northeast corner of Yellowstone National Park, where they constitute as much 

as 1,000 feet of the total section (U.S. Geological 

Survey, 1972). 

The Lamar River Formation is thickest in the 

southeastern part of the Washburn Range, where 

the vent facies deposits are more than 3,200 feet 

thick. In general, the alluvial facies deposits are 

thinner, ranging from a maximum thickness of 

2,200 feet along Soda Butte Creek to less than 500 

feet along Slough Creek. 

In the Specimen Ridge area, the Lamar River 

Formation grades laterally westward into the Sepulcher Formation. In the northeastern part of the 

park, northeastward-thickening tongues of the lightcolored Cathedral Cliffs Formation interfinger with 

the lower one-third of the Lamar River Formation 

(fig. 8, col. G; U.S. Geological Survey, 1972). 

The Lamar River Formation is the basal volcanic 

unit in much of northeastern Yellowstone National 

Park; it overlies rocks ranging in age from Precambrian to middle Eocene. In the Washburn Range, 

the Lamar River Formation unco·nformably overlies 

faulted and tilted strata of the Sepulcher Formation. 

This local angular unconformity, like many others in 

the Absaroka field, probably reflects local volcanotectonic deformation and a geologically insignificant 

break in time. 

The Lamar River Formation is unconformably 

overlain by the Langford Formation on Amethyst 

Mountain and in the Washburn Range. East of 

Amethyst Mountain, rocks of the Sunlight Group 

(Wapiti Formation) unconformably overlie the 

Lamar River Formation. This unconformity is 

slightly angular and has as much as 1,000 feet of 

erosional relief. 

AGE 

The Lamar River Formation is late Wasatchian 

to early Bridgerian provincial age (late early Eocene 

to early middle Eocene), as it grades laterally into 

the well-dated Sepulcher Formation of that age. 

Dorf (1960, p. 259) assigned the Lamar River fossil 

flora an early Bridgerian age. 

CATHEDRAL CLIFFS FORMATION 

The name Cathedral Cliffs Formation was introduced by Pierce (1963) to replace the early acid 

breccia of Hague, Weed, and Iddings (1896) and 

Hague, Iddings, and others (1899). At the type section at Cathedral Cliffs, some 17 miles east of the 

park (fig. 9), the formation is exposed in an allochthonous Heart Mountain fault block (Pierce, 

1963), but Pierce's suggested correlation of this 

section with rocks we call Sepulcher Formation is 

supported by our mapping. We have used the name 

Cathedral Cliffs Formation for the light-colored 

basal volcaniclastic rocks only in the northeast 

corner of the park because the laterally equivalent 

dark-colored Lamar River Formation intervenes 

between this area and that of the Sepulcher Formation. Moreover, the Sepulcher and Cathedral 

Cliffs Formations were derived from different 

sources, and, thus, are different volcanogenic units. 

The Cathedral Cliffs Formation occurs primarily 

east and north of the park, but southwestwardthinning tongues of the Cathedral Cliffs Formation 

are present in the lower part of the Lamar River 

Formation in the northeastern part of the park. 

East of the park, the Cathedral Cliffs Formation 

is dominantly light-colored fine-grained alluvial 

facies volcanic sandstone, tuff, breccia, and conglomerate that occur as parts of Heart Mountain 

fault blocks. North of the park the formation is 

autochthonous, and it thickens and coarsens toward 

a northwest-trending line of vents extending from 

Independence to Daisy Pass (fig. 9). 

The Cathedral Cliffs Formation is 500 feet thick 

at the type section and somewhat thicker in other 

areas east of the park (Pierce,. 1963, p. 15). 

AGE 

Pierce (1963, p. 16) considered the Cathedral 

Cliffs Formation to be of late early Eocene or early 

middle Eocene age. This age is consistent with its 

interfingering relations with strata of the same age 

(the Lamar River Formation) to the west. Two 

small collections of plant fossils indicate an early or 

middle Eocene age, or both. The overlying breccias 

(Wapiti Formation) yielded plant and vertebrate 

fossils, indicating that they are not younger than 

early middle Eocene and may be as old as late early 

Eocene (Pierce, 1963). 

SUNLIGHT GROUP 

The new name Sunlight Group is here proposed 

for a sequence of mainly dark colored pyroxene 

andesite lava flows and volcaniclastic rocks and 

potassic basalts that comprise the middle group of 

the Absaroka Volcanic Supergroup. The group is 

named for the type area of Sunlight Creek (fig. 9), 

which flows through a major vent area east of the 

park and through the thickest known accumulation 

of these rocks. The Sunlight Group comprises, in 

ascending order, the following: Mount Wallace Formation and its Slough Creek Tuff Member, Crescent 

Hill Basalt, Wapiti Formation and its Jim Mountain 

Member, and the Trout Peak Trachyandesite and its 

Pacific Creek Tuff Member. 
C22 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

The Sunlight Group overlies the Washburn Group 

throughout the northern part of the volcanic field, 

but to the south, the Sunlight Group re.sts directly 

on prevolcanic rocks (fig. 3). Rocks of the Sunlight 

Group are easily accessible and magnificently exposed southwest of Clarks Fork Yellowstone River, 

along Sunlight Creek, and along the road between 

Wapiti, Wyo., and the east entrance of Yellowstone 

National Park (fig. 9). Within the park, the Sunlight Group is largely confined to high divides and 

fairly inaccessible areas. The regional relations of 

the formations and members are shown diagrammatically in figures 5 and 8, and are shown in more 

detail on the geologic map of Yellowstone National 

Park (U.S. Geological Survey, 1972). 

The Mount Wallace Formation- the oldest formation in the Sunlight Group- is as much as 2,000 

feet thick and consists mostly of lava flows. It is 

thickest and most extensive north of the park. It 

thi_ns markedly to the south, and in northern Yellowstone National Park it underlies or interfingers with 

the lower part of the Wapiti Formation. The Crescent Hill Basalt in the nortli-central part of the 

park is believed to be an outlier of the Mount Wallace Formation. 

Overlapping the thinned southeast edge of the 

Mount Wallace Formation in the park are the W apiti Formation and the Trout Peak Trachyandesite, 

which attain a total thickness of nearly 6,000 feet 

just east of the park. The Wapiti Formation consists 

dominantly of pyroxene andesite volcaniclastic rocks 

of vent and alluvial facies with local, mostly thin 

lava flow sequences. The Trout Peak Trachyandesite 

is a massive sequence of dominantly mafic lava flows 

that overlies the Wapiti Formation. The Trout Peak 

lavas grade into breccias indistinguishable from 

Wapiti breccias in the Sunlight-Crandall area. 

There, the Wapiti Formation and the Trout Peak 

Trachyandesite are laterally gradational; the base 

of the Trout Peak, defined as the base of the massive 

lava flow sequence, is not a time line, but a timetransgressive lithologic break. In the northeastern 

and north-central parts of the park, these formations interfinger with the Langford Formation of 

the Thorofare Creek Group (fig. 3; fig. 8, col. G; 

U.S. Geological Survey, 1972). 

Southeast of Carter Mountain (fig. 1) the Wapiti 

Formation grades into, and interfingers with, a finegrained alluvial facies called the Pitchfork Formation (Hay, 1956). The Pitchfork Formation, in turn, 

seems to be correlative with strata mapped as Aycross Formation farther south by Harold Masursky 

(unpub. data, 1956), but, as yet, it is uncertain 

whether those strata are equivalent to the type 

Aycross (Love, 1939). We include the Pitchfork 

Formation in the Sunlight Group, but- because of 

uncertainties in correlations - not the Aycross 

Formation. 

Rocks of the Sunlight Group disappear southwestward beneath the Thorofare Creek Group and 

probably pinch out locally at depth against the 

largely buried northwest-trending Washakie Range 

(fig. 1). Remnants of the Trout Peak Trachyandesite rest directly on deformed prevolcanic rocks on the 

west flank of the partly exhumed Washakie Range 

along the southwest margin of the Absaroka field 

(U.S. Geological Survey, 1972). 

Remanent magnetism in lava flows of the Mount 

Wallace Formation are ·of normal and reversed 

polarity (Smedes and Prostka, unpub. data, 1967, 

verified by E. L. McCaskey, unpub. data, 1968). 

Lavas in the Wapiti Formation and the Trout Peak 

Trachyandesite are of normal polarity (F. J. Vine, 

written commun., 1968). 

AGE 

The Sunlight Group is almost entirely of early 

Bridgerian provincial age; it overlies the Washburn 

Group of late Wasatchian and early Bridgerian age. 

MOUNT WALLACE FORMATION 

The new name Mount Wallace Formation is here 

proposed for a sequence of andesite, trachybasalt, 

and latite lava flows and flow breccias, rhyodacite 

welded ash-flow tuff, and interlayered volcanic sediments. These rocks were originally called basic 

andesite breccia and flows (Iddings and Weed, 

1894). The Mount Wallace Formation occurs chiefly 

north of Yellowstone National Park and makes up 

most of the Sunlight Group in that region. The formation has a maximum thickness of about 3,000 

feet in the type area - a remote rugged region 

south of, and including, Mount Wallace. The unit 

thins markedly to the southwest, south, and southeast from there and becomes a lenticular and discontinuous unit of mafic lavas and welded tuff in the 

park. In the type area the rhyodacite welded ashflow sheet (Slough Creek Tuff Member) is about 

in the center of the Mount Wallace Formation, but 

in the park the lower half of the formation has 

wedged out, and only the Slough Creek Tuff Member 

and upper mafic lavas are present. 

Between Slough Creek and Buffalo Creek (figs. 

4, 9), Mount Wallace Formation fills local depressions in the prevolcanic surface and is as much as 

1,000 feet thick. In these areas the mafic lavas are 

locally underlain by rhyodacite welded ash-flow tuff 

(the Slough Creek Tuff Member). Southeast of 

Slough Creek (fig. 9), the Mount Wallace Formation 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C23 

is 0-400 feet thick and occurs as lenticular lava 

flow sequences beneath or interlayered with the 

lower part of the Wapiti Formation (U.S. Geological Survey, 1972). Mount Wallace Formation does 

not occur southeast of Soda Butte Creek (fig. 9). 

Outliers of mafic lavas in the north-central part of 

the park, called Crescent Hill Basalt, are here 

interpreted to be equivalent to part of the Mount 

Wallace Formation. In the northwestern part of the 

park, the Mount Wallace Formation caps high ridges 

and thins from about 1,000 feet thick near Sheep 

Mountain to less than 200 feet toward the west. 

Several patches of Mount Wallace Formation occur 

at the south end of the Gallatin Range northeast of 

Madison Valley (figs. 4, 9). 

In the type area, the Mount Wallace Formation 

consists of about 2,000 feet of lenticular lava flows 

and flow breccias of andesite, trachybasalt, latite, 

and minor amounts of dacite interbedded with variable thicknesses of volcanic sediments. The composite sheet of rhyodacite welded ash-flow tuff -

the Slough Creek Tuff Member -is approximately 

in the middle of the section. 

Throughout most of its extent, the Mount Wallace 

Formation rests on strata of the Washburn Group 

or on prevolcanic rocks. West of Buffalo Creek the 

Mount Wallace Formation is locally overlain by the 

Langford Formation, and southeast of Buffalo Creek 

it is overlain by, and partly interlayered with, the 

Wapiti Formation (fig. 4). 

SLOUGH CREEK TUFF MEMBER 

The Slough Creek Tuff Member (new name) of 

the Mount Wallace Formation is a composite sheet 

of rhyodacite welded ash-flow tuff. It is known to 

occur in only two areas in the park- (1) in the 

type area west of Slough Creek 2 miles south of the 

park boundary (figs. 4, 9), where it is as much as 

700 feet thick and underlies mafic lava flows, and 

(2) along Buffalo Creek. It thickens to the northwest and is a distinctive well-exposed continuous 

unit as much as 1,000 feet thick in the middle of the 

Mount Wallace Formation over a broad area southwest and southeast of Mount Wallace. The sheet is 

a compound cooling unit in which the ash flows 

contain variable proportions of quartz, sanidine, and 

biotite phenocrysts. Fragments, mostly andesites and 

basalts of the Mount Wallace Formation, are 

abundant. 

Studies of remanent magnetism are still in progress. The Slough Creek Tuff Member and a lava 

flow several hundred feet below it in the type area 

are of normal polarity. The lavas (interpreted to 

be in the upper part of the Mount Wallace Formation) that are interlayered with the Wapiti Formation in northeastern Yellowstone National Park and 

the Crescent Hill Basalt are of normal polarity (F. 

J. Vine, written commun., 1968). Lavas of the 

Mount Wallace Formation in the northwestern part 

of the park are of normal and reversed polarity, 

but their stratigraphic relation to those of the type 

area is unknown. 

AGE 

The Mount Wallace Formation is of early Bridgerian provincial age. The Mount Wallace Formation 

overlies the Washburn Group of late Wasatchian and 

early Bridgerian age. It is overlain by, and intertongues with, the Wapiti Formation and the Trout 

Peak Trachyandesite. The Pacific Creek Tuff Member at the top of the Trout Peak is dated at 48.0± 

1.3 m.y. (fig. 8, col. H). The Mount Wallace 

Formation is cut by an intrusive (fig. 3, loc. d) 

dated at 47.7±1.5 m.y. (J. D. Obradovich, written 

commun., 1968). We found no fossils, and none have 

been reported in the literature. 

CRESCENT HILL BASALT 

The Crescent Hill Basalt was named and defined 

by Howard (1937) as the basalt overlying the early 

acid breccia in the Crescent Hill area. The Crescent 

Hill Basalt consists of two flows of scoriaceous, 

partly pillowed trachybasalt which have an aggregate thickness of about 250 feet. Only the lower 

flow is present on Crescent Hill; the upper, thicker 

flow is well exposed as rimrocks above the road 

between Oxbow and Geode Creeks (fig. 9). The lower 

flow is exposed along the road one-fourth mile west 

of Geode Creek. The two flows are separated by a 

northwestward-thinning tongue of the Sepulcher 

Formation. 

The Crescent Hill Basalt is restricted to northcentral Yellowstone National Park (fig. 4; U.S. 

Geological Survey, 1972). On the basis of lithology 

and stratigraphic position, it is interpreted to be 

correlative with part of the Mount Wallace Formation. It is retained as a separate formation for 

historical reasons and because the correlation with 

the Mount Wallace Formation is not absolutely 

certain. 

AGE 

The Crescent Hill Basalt overlies, and is partly 

intertongued with, the uppermost part of the Sepulcher Formation. South of Crescent Hill it is 

overlain by the Langford Formation (fig. 4; fig. 8, 

col. E; U.S. Geological Survey, 1972). 

If the correlation of the Crescent Hill Basalt with 

part of the Mount Wallace Formation is valid, then 

the basalt is probably of early Bridgerian provincial age. 
C24 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

FIGURE 12. - Vent facies autoclastic flow breccias of the Wapiti Formation, along the northeast ridge of Pollux Peak (fig. 9). 

WAPITI FORMATION 

The Wapiti Formation, defined by Nelson and 

Pierce (1968), is a sequence of andesitic volcaniclastic rocks and lesser amounts of interlayered lava 

flows of trachyandesite and minor dacite. In the 

type section near Wapiti, Wyo. (fig. 9), it is equivalent to the early basic breccia of Hague (1899). The 

formation has been mapped continuously from the 

type section, north of Wapiti, Wyo. (fig. 9), northwestward through the Sunlight Creek region into 

Yellowstone National Park. In the northeastern part 

of the park, the Wapiti Formation constitutes only 

the upper one-third of what Hague, Iddings, and 

others (1899) called early basic breccia- the lower 

two-thirds being the slightly lighter colored Lamar 

River Formation. The Wapiti Formation in the 

Sunlight Creek and Crandall Creek regions, and in 

the east-central part of the park (figs. 12, 13) consists entirely of vent facies rocks cut by swarms 

of dikes and a variety of other intrusives. From 

there, it grades outward into interfingered vent and 

alluvial facies rocks in northeastern Yellowstone 

National Park and in the area of the type section 

near Wapiti. Southeastward from Wapiti, the formation grades laterally from predominantly volcanic breccia and conglomerate to predominantly 

volcanic sandstone and siltstone. This finer grained 

equivalent is called the Pitchfork Formation (Hay, 

1956; Nelson and Pierce, 1968). A thick wedge of 

the Wapiti is interbedded with the Pitchfork Formation along the South Fork Shoshone River 

(fig. 1). 

The Wapiti Formation is as much as 5,000 feet 

thick in the Crandall-Sunlight area; it thins to about 

4,000 feet near the type section and becomes even 

thinner farther south. 

The Wapiti Formation overlies the Willwood 

Formation at the type section and is the basal volcanic unit of the area from there southeastward to 

Carter Mountain (fig. 1). To the northwest, the 

Wapiti overlies the Lamar River Formation or the 

Cathedral Cliffs Formation. East of the park, the 

pre-Wapiti formations have been extensively disrupted by sliding along the Heart Mountain detachment fault. There, the tectonically denuded 

fault plane and allochthonous blocks of Paleozoic 

strata and pre-Wapiti volcanic rocks were buried 

by the Wapiti Formation immediately after the 
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C25 

FIGURE 13.- Closeup view of vent facies autoclastic flow breccia of the Wapiti Formation, along ridge south of South Cache 

Creek near east boundary of park (fig. 9). 

sliding occurred (Pierce, 1957; Prostka and W. H. 

Nelson, unpub. data, 1970, 1971). 

The Wapiti Formation in places is unconformably 

overlain by, and in other places interfingers with, 

the lower part of the Trout Peak Trachyandesite 

(fig. 8, cols. F, G, H). 

JIM MOUNTAIN MEMBER 

The type section of the Wapiti Formation contains a sequence of trachyandesite lava flows, about 

1,000 feet thick, called the Jim Mountain Member, 

which thins to the southeast (Nelson and Pierce, 

1968). Westward from the type section, the Jim 

Mountain Member grades into vent facies deposits 

of the Wapiti Formation (W. H. Nelson and Prostka, unpub. data, 1971). 

AGE 

The Wapiti Formation overlies strata of the 

Washburn Group of early Bridgerian age, and it 

interfingers with, and is overlain by, the Trout Peak 

Trachyandesite of early Bridgerian age. The laterally equivalent Pitchfork Formation contains late 

Wasatchian-early Bridgerian vertebrate faunas, and 

a flora which has been dated as middle Eocene (Hay, 

1956, p. 1883-1884). The most probable age of the 

Wapiti Formation is early middle Eocene. 

TROUT PEAK TRACHYANDESITE 

The Trout Peak Trachyandesite, defined by Nelson 

and Pierce (1968), is a massive sequence of trachyandesite and trachybasalt lava flows with minor 

amounts of interbedded volcaniclastic rocks. It is 

approximately equivalent to the early basalt flows 

of Hague, Iddings, and others (1899). The formation is well exposed over a wide area from the type 

section at Trout Peak (fig. 9), northwest of Wapiti, 

Wyo., northward and westward into Yellowstone 

National Park (U.S. Geological Survey, 1972), 

southward for several miles beyond the Greybull 

River (Wilson, 1963) and southwestward to the 

west edge of the Absaroka field (fig. 3). It is present 

near Two Ocean Pass (fig. 14; Love, 1956d) and 

along North Buffalo Fork and Soda Fork of North 

Buffalo Fork (fig. 14). 
C26 

Yellowstone 

Lake 

GEOLOGY OF YELLOWSTONE NATIONAL PARK 

FIGURE 14.- Geographic features in the southeastern part of Yellowstone National Park and vicinity. 
-~---~~-~ --~----

STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C27 

The Trout Peak Trachyandesite is about 800-

1,200 feet thick in the type section and in eastcentral Yellowstone National Park. It gradually 

thins southward and finally pinches out in the region 

between the Greybull and Wood Rivers (fig. 1; 

Wilson, 1963). Its source was primarily the Sunlight-Crandall region (fig. 1), the same as the source 

of the Wapiti Formation. This is indicated by numerous dike swarms and central stocks compositionally similar to the lava flows. We include these dikes 

as part of the Trout Peak Trachyandesite. A secondary source along the south margin of the Absaroka 

field is suggested by compositionally similar dikes 

and plugs that cut strata equivalent to the Pitchfork Formation and are overlain by the Tepee Trail 

Formation (Harold Masursky, unpub. data, 1956). 

The Trout Peak Trachyandesite overlies, and partly interfingers with, the Wapiti Formation (fig. 8, 

cols. F, G, H). In northeastern Yellowstone National 

Park, the Trout Peak Trachyandesite interfingers 

with light-colored volcaniclastic strata of the Langford Formation. This relationship is well displayed 

at the north end of Mirror Plateau, and in the southeast face of Mount Hornaday (fig. 9). In the Crandall-Sunlight region, the Trout Peak Trachyandesite 

is the stratigraphically highest unit present; to the 

south and southwest it is overlain by the Langford 

Formation, and to the southeast it is overlain by 

strata which have been called Wiggins Formation 

(Nelson and Pierce, 1968; Wilson, 1963). 

PACIFIC CREEK TUFF MEMBER 

The Pacific Creek Tuff Member (new name) of 

the Trout Peak Trachyandesite is a rhyodacite welded ash-flow tuff that locally overlies or is inter layered 

with the uppermost lava flows of the Trout Peak 

Trachyandesite. It is named for the type area at 

Pacific Creek, near Two Ocean Pass (fig. 14), 

where the unit is well exposed. This unit is extensive 

but discontinuous, owing to intravolcanic erosion. 

In most places, this unit is too thin or scattered to 

be mapped separately from the Trout Peak Trachyandesite. It is about 80 feet thick at the type area 

and about 300 feet thick in east-central Yellowstone National Park. The Pacific Creek Tuff Member 

is a pale-brown to yellowish, poorly resistant, partially welded ash-flow tuff containing abundant 

phenocrysts of anorthoclase and biotite. 

AGE 

A potassium-argon determination (figs. 3, 4, 8) 

on the biotite in the Pacific Creek Tuff Member by 

J. D. Obradovich (written commun., 1968) gave an 

age of 48.0±1.3 m.y.- middle Eocene (early 

Bridgerian). This age is slightly younger than the 

one quoted by Nelson and Pierce (1968, p. Hll), 

which was a preliminary determination on the same 

sample analyzed by Obradovich. 

THOROFARE CREEK GROUP 

The name Thorofare Creek Group (new name) is 

proposed for the formations of middle and late 

Eocene age that lie above, and locally interfinger 

with, the Sunlight Group. The name is taken from 

the type area of the region of Thorofare Creek, 

near the southeast corner of Yellowstone National 

Park (fig. 14), where most of the group is well 

exposed. 

The Thorofare Creek Group is more than 6,000 

feet thick. It consists, from oldest to youngest (fig. 

8), of light-colored volcaniclastic strata and andesite lavas of the Langford Formation, the darkcolored coarse volcaniclastic Two Ocean Formation, 

the fine-grained green and brown Tepee Trail Formation (present only southeast of the park), and the 

distinctive well-bedded light-gray Wiggins Formation. These formations make up most of the southern 

half of the Absaroka volcanic field. All these units 

except the Tepee Trail Formation can be traced almost continuously from the southeastern part of the 

park south to the Togwotee Pass area (fig. 1). At 

Togwotee Pass Wiggins Formation rests on Tepee 

Trail Formation, but half a mile to the west, the 

Tepee Trail Formation wedges out against the Two 

Ocean Formation, and both are unconformably overlain by Wiggins Formation. From there, the Wiggins Formation can be followed eastward almost 

continuously to its type area (J. D. Love, oral 

commun., 1968; unpub. reconnaissance data by Love, 

Smedes, W. L. Rohrer, and J. D. Obradovich, 1970, 

and Smedes and Obradovich, 1971). 

Along the south and southwest margins of the 

volcanic field (fig. 3), strata of the Thorofare Creek 

Group locally rest directly on prevolcanic rocks. In 

northeastern Yellowstone National Park, the Langford Formation (basal Thorofare Creek Group) 

interfingers with the Sunlight Group (fig. 8, cols. 

F, G; U.S. Geological Survey, 1972), but in most 

places the Langford rests directly on rocks of either 

the Sunlight Group or the Washburn Group (fig. 

8, cols. C, E, F, H). The Langford Formation is 

the only unit of the Thorofare Creek Group that 

extends northward across and beyond the park. 

AGE 

Radiometric dates (about 44-49 m.y.; fig. 3) and 

sparse fauna and flora indicate that the Thorofare 

Creek Group ranges from middle Eocene to possibly 

late Eocene in age. However, it should be emphasized 
C28 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

FIGURE 15.- Distant view of chaotic, indistinctly bedded vent facies rocks of the Langford Formation, along the east ridge of 

Plentycoups Peak (fig. 14). About 550 feet of relief is shown. 

that the Eocene is not yet well calibrated (M. C. 

McKenna, written commun., 1971). 

LANGFORD FORMATION 

The name Langford Formation (new name) is 

here proposed for a thick section of dominantly 

light-colored andesitic volcaniclastic strata and lava 

flows of vent (fig. 15) and alluvial facies that overlies the Trout Peak Trachyandesite and is unconformably overlain by the dark-colored Two Ocean 

Formation in east-central Yellowstone National 

Park (fig. 4) . In the type area, at Mount Langford 

(fig. 14), a complete section about 2,000 feet thick 

is well exposed. 

The alluvial facies of the formation is mainly 

light gray and was called late acid breccia by early 

workers in the area (Hague, Weed, and Iddings, 

1896; Hague, Iddings, and others, 1899; Hague, 

1899). The dark andesite flows and breccias of the 

vent facies and, locally, some of the dark alluvial 

facies tongues interbedded with the light alluvial 

facies were called late basic breccia. 

The Langford Formation extends northward beyond the park, where it unconformably overlies 

Mount Wallace Formation; in the Washburn Range 

(figs. 1, 4) it rests directly on strata of the Washburn Group, but to the east it overlies and partly 

interfingers with the Wapiti Formation and the 

Trout Peak Trachyandesite. The Langford Formation 

is thickest in the southeastern part of the park; it 

extends to the southwest edge of the Absaroka field, 

but its extent east of the park is unknown. 

Throughout most of its mapped extent, the Langford Formation is light colored and of alluvial facies. 

The volcaniclastic strata are composed of hornblende and pyroxene andesite fragments, some of 

which are very dark gray and nearly glassy and 

have a subvitreous luster and subtle perlitic structure. The overall light color of the formation is due 

largely to its light-gray ash-rich matrix. 

In a principal vent center in the Eagle Peak 

region (fig. 14), near the east border of the park, 

the entire section of the Langford Formation is 

dark vent facies lava flows, flow breccias, and ag-
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP C29 

glomerate. There, small breccia dikes and irregular 

intrusive bodies, ranging in size from a few inches 

to a few feet across and from a few feet to tens of 

feet long, locally cut the vent facies rocks and are 

included with them in the formation (U.S. Geological Survey, 1972). 

North of Cathedral Peak (fig. 14) the Langford 

is made up entirely of alluvial facies rocks. From the 

Cathedral Peak area southward and southeastward, 

the basal part of the formation consists of vent 

facies deposits (fig. 15) erupted from numerous 

centers in the Sylvan Pass, Eagle Peak, and Ishawooa Mesa regions (fig. 14) ; and the upper part is 

largely coarse alluvial facies with subordinate 

amounts of interbedded vent facies. The proportion 

of vent facies rocks increases southward and southeastward. 

PROMONTORY MEMBER 

The name Promontory Member (new name) is 

here proposed for several eastward-thinning tongues 

of dark massive volcanic conglomerate and breccia 

interbedded with the more typical light-colored 

strata of the Langford Formation. These tongues, 

which pinch out toward the· east and appear to 

thicken and merge toward the west, are interpreted 

to be parts of a single thick dark-colored sequence, 

which lay to the west, but which is now buried by 

rhyolites of the Yellowstone Plateau (fig. 1). The 

name is taken from the type area of The Promontory (fig. 14), a peninsula separating the Southeast 

Arm and South Arm of Yellowstone Lake, where 

600 feet of these dark-colored strata is well exposed. 

Rocks of the Promontory Member seem to be restricted to the southeastern part of Yellowstone 

National Park. 

AGE 

The Langford Formation is underlain by the 

Trout Peak Trachyandesite and overlain by the Two 

Ocean Formation, both of which are early middle 

Eocene in age (about 48 m.y.; fig. 8, cols. H, I). 

TWO OCEAN FORMATION 

The name Two Ocean Formation (new name) is 

here proposed for a sequence of dark-colored thickbedded coarse andesitic volcaniclastic strata which 

overlie the Langford Formation in the southeastern 

part of the park. These rocks are mainly coarse alluvial facies strata in the park, but they grade into 

vent facies or interbedded alluvial and vent facies 

rocks near the border and east of the park. The 

formation is named for the type area along the 

precipitous east edge of Two Ocean Plateau (figs. 

4, 14), where the strata are exposed in prominent 

dark cliffs between the light-colored less resistant 

Langford Formation below and the Wiggins Formation above (U.S. Geological Survey, 1972). This 

unit was previously called late basic breccia in most 

places, but some relatively inaccessible exposures 

were called late basalt sheets (Hague, 1899). The 

extent of the Two Ocean Formation east and southeast of the park is largely unknown because of the 

paucity of detailed mapping in those areas. 

The base of the Two Ocean Formation is marked 

by an erosional, locally angular, unconformity having several hundred feet of relief. The formation is 

unconformably overlain by the Tepee Trail and 

Wiggins Formations. Because of these unconformities, the formation is locally absent along the south 

edge of the Trident Plateau, but it has a maximum 

thickness of more than 2,000 feet in the Eagle Peak 

region (figs. 4, 14). On the northeast side of Table 

Mountain (fig. 14), there is a prominent intraformational unconformity in which the lower half 

of the Two Ocean Formation and part of the underlying Langford Formation have been channeled out 

and filled by the upper part of the Two Ocean 

Formation. 

The clasts are cobbles and boulders of subvitreous 

to stony porphyritic andesite, containing phenocrysts of hornblende or pyroxene, or both, as well 

as plagioclase and sparse a·mounts of biotite in a 

matrix of sand, silt, and ash. Locally, there are beds 

of very light gray water-laid ash, some of which 

contain chunk air-fall pumice rich in biotite. 

AGE 

Potassium-argon age determinations (J. D. 

Obradovich, written commun., 1969) on minerals 

from pumiceous tuff about 20 feet below the top of 

the Two Ocean Formation just west of the Falcon 

Creek-Mink Creek divide (fig. 14; fig. 3, loc. c) 

gave ages of 47.9±1.3 m.y. (sanidine) and 48.5±1.3 

m.y. (biotite), both of which are middle Eocene 

(fig. 8). 

TEPEE TRAIL FORMATION 

The name Tepee Trail Formation was proposed 

by Love (1939, p. 73) for a sequence of volcaniclastic rocks along the south margin of the Absaroka 

field, unconformably overlying the Aycross Formation and older rocks, and unconformably overlain 

by the Wiggins Formation. Strata of the Tepee 

Trail Formation are composed of green and brown 

fine-grained well-bedded clay- and ash-rich deposits 

of a dominantly andesitic alluvial facies. 

The formation is thickest near the type section 

in the basin of the East Fork Wind River (fig. 1), 

where 1,500-2,000 feet of strata is preserved. Increase in average grain size and vent facies aspect 

northward led Love (1939, p. 74) to conclude that 
C30 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

its chief source was only a short distance to the 

north beneath the Wiggins Formation. The formation has been traced eastward into the western Owl 

Creek Mountains (fig. 1; Harold Masursky, unpub. 

data, 1956). Strata assigned to the Tepee Trail have 

been mapped west of the type section (Keefer, 1957; 

Ketner and others, 1966; Rohrer, 1966) ; however, 

these strata wedge out northwest of Togwotee Pass 

and do not occur in Yellowstone National Park. 

AGE 

On the basis of sparse and fragmentary vertebrate 

remains and relations with other rocks in the type 

section, Love (1939, p. 78) provisionally correlated 

the Tepee Trail Formation with upper Eocene strata. 

Recently, however, Rohrer and Obradovich (1969, 

p. B61) cited an opinion by G. E. Lewis that the 

fauna are equally indicative of a middle Eocene age, 

and Rohrer (1966) reported a middle Eocene flora 

from strata 30 miles west of the type Tepee Trail 

Formation which he assigned to the Tepee Trail. 

On the basis of this, Rohrer and Obradovich (1969, 

p. B61) concluded that the Tepee Trail Formation, 

from west to east, ranges in age from middle to 

late Eocene. 

Potassium-argon determinations (J. D. Obradovich, written commun., 1969) on air-fall tuff from 

about 400 feet below the top of a sequence of strata 

which Rohrer (1966) assigned to tne Tepee Trail 

Formation 35 miles to the east, at a locality onehalf mile northwest of Togwotee Pass (fig. 1; fig. 

3, loc. h), gave ages of 47.8±1.3 m.y. (sanidine) 

and 47.9±1.3 m.y. (biotite). Altered biotite from 

a bentonitic ash less than 100 feet below the top of 

similar strata in the Pinnacle Buttes area (fig. 1; 

fig. 3, loc. g) was dated as 49.3±1.4 m.y., and sanidine from an ash a few tens of feet below the top 

of that interval gave an age of 46.1±1.2 m.y. (fig. 

3, loc. f; J. D. Obradovich, written commun., 1970). 

Thus, strata in the Togwotee Pass-Pinnacle Buttes 

area that have been correlated 30-35 miles to the 

type Tepee Trail have an average radiometric age 

of 48 m.y., which is consistent with a middle Eocene 

paleontologic age. We emphasize, however, that 

there are no potassium-argon age determinations 

from the type Tepee Trail and Aycross Formations, 

that the age of vertebrate fossils from the Tepee 

Trail is not certainly known, and that considerable 

disagreement exists as to how the rock units in the 

type area of the Aycross and the type section of the 

Tepee Trail correlate with the dated strata near 

Togwotee Pass. 

Strata 130 miles southeast of Togwotee Pass, in 

the Badwater area near Lysite, Wyo., have been 

called Tepee Trail Formation. They yielded a vertebrate fossil assemblage of late Eocene age (Black 

and Dawson, 1966, p. 331; Tourtelot, 1957, p. 11; 

Gazin, 1956) and contain a tuff having biotite dated 

at 41.2±1.4 m.y. (Black, 1969). These rocks cannot 

be equivalent to either the Tepee Trail Formation 

in the Pinnacle Buttes-Togwotee Pass area or the 

type Tepee Trail. The Tepee Trail seems most likely 

to be middle and late ( ?) Eocene age. 

WIGGINS FORMATION 

The name Wiggins Formation was proposed by 

Love (1939, p. 79) for a light-colored sequence of 

coarse andesitic volcaniclastic strata more than 

3,000 feet thick unconformably overlying the Tepee 

Trail Formation and making 'up the high ridges of 

the southern part of the Absaroka Range. The name 

is taken from the Wiggins Fork Wind River (fig. 

1). We have traced the formation continuously 

northwestward to the Trident Plateau and Two 

Ocean Plateau in Yellowstone National Park (figs. 

8, 14). Wilson (1963) correlated lithologically 

similar strata on Carter Mountain with Wiggins 

Formation. 

The Wiggins Formation consists of biotite-hornblende andesite volcaniclastic rocks and subordinate 

lavas. Its characteristic aspect is that of thick beds 

of light-olive-gray mudflow breccia; interbedded 

coarse medium-gray fluvial sandstone and conglomerate; and thin beds and lenses of very light gray 

and white pumice and ash mixed with silt and sand 

(fig. 16). Petrified forests are preserved locally 

(Ketner and others, 1966). Some of the dark lava 

flows high in the section were called late Basalt 

flows by Hague (1899). 

The facies relations indicate several major and 

minor source vents. One of the larger vents is inferred to lie in the present site of Yellowstone Lake. 

Others lie northeast of the headwaters of Wiggins 

Fork near Kirwin (fig. 1; Wilson, 1959, 1964), and 

a cluster of vents east of Eagle Peak (fig. 14). 

The Wiggins Formation lies unconformably on 

Tepee Trail Formation throughout much of the 

southern Absaroka field, but to the north and northwest the Tepee Trail wedges out, and the Wiggins 

lies directly on the Two Ocean Formation. The Wiggins Formation is the youngest unit of the Absaroka Volcanic Supergroup. 

Excellent exposures of the Wiggins Formation can 

be seen in the cliffs north of Togwotee Pass and in 

the upper part of Pinnacle Buttes (fig. 16). 

AGE 

The Wiggins Formation was originally thought to 

be early Oligocene in age, on the basis of fragmen-
STRATIGRAPHIC FRAMEWORK OF THE ABSAROKA VOLCANIC SUPERGROUP 

FIGURE 16.- Alluvial facies volcanic sedimentary rocks of the Wiggins Formation at Pinnacle 

Buttes (fig. 1). About 14 feet of section is shown. 

C31 
C32 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

tary vertebrate remains found in the Wiggins Fork 

area (Love, 1939). Fossil leaves from the Wiggins 

Formation in the Stratified Primitive Area (fig. 1) 

were considered to be early Oligocene, although it 

was admitted that they could be as old as late Eocene 

(Ketner and others, 1966; H. D. MacGinitie, written 

commun. toW. R. Keefer, 1965). Ash-rich sandstone 

and siltstone near Emerald Lake (fig. 14), previously thought to be Wiggins Formation, contains a 

fauna of early Oligocene age (Chadronian provincial age; Love, 1956a). Reexamination of field relations and petrology of this deposit conclusively 

demonstrated that it is- a younger channel-fill deposited on eroded Wiggins (J. D. Love, M. C. McKenna, J. D. Obradovich, and Smedes, unpub. data, 

1971). This Oligocene deposit is not considered to 

be part of the Absaroka Volcanic Supergroup. 

Potassium-argon age determinations on six 

samples, three of which were from the Wiggins Fork 

area, all yielded middle and late Eocene ages. Rohrer 

and Obradovich (1969) reported ages of 46.2±1.8 

m.y. and 46.5±2.3 m.y. on hornblende from a lava 

flow and a boulder at the base of the Wiggins Formation at Pinnacle Buttes (fig. 1; fig. 3, locs. i, j). 

An air-fall tuff about 800 feet above the base of the 

formation in the same area (fig. 3, loc. k) yielded 

potassium-argon ages of 44.4±1.4 m.y. (hornblende) 

and 45.5±1.3 m.y. (biotite) (J. D. Obradovich, unpub. data, 1969). In the Wiggins Fork area (fig. 3, 

locs. l, m, n), biotites from ash beds about 500 to 

600 feet above the base of the Wiggins Formation 

yielded ages of 47.1±1.3 m.y. and 46.7±1.5 m.y., 

and biotite from a lava flow of vitrophyre about 150 

feet higher in the section gave an age of 44.6±1.2 

m.y. (J. D. Obradovich, unpub, 1970). On the basis 

of the foregoing, the Wiggins Formations seems 

most likely to be of late Bridgerian and possibly 

early Uintan provincial age- late middle Eocene 

to possibly early late Eocene (fig. 8). 

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*U. S, GOVERNMENT PRINTING OFFICE : 1972 0 - 467-534 

Smedes and Prostka-ABSAROKA VOLCANIC SUPERGROUP, YELLOWSTONE NATIONAL PARK REGION-Geological Survey P