Stratigraphic Nomenclature of the 

Thirtynine Mile Volcanic Field, 

Central Colorado

GEOLOGICAL SLRVEY BULLETIN 1395-C

 I §'

Stratigraphic Nomenclature of the 

Thirtynine Mile Volcanic Field, 

Central Colorado

By RUDY C. EPIS and CHARLES E. CHAPIN 

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1395-C

Descriptions of volcanic and nonvolcanic 

units showing their chemistry, age, and 

interrelationships

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1974
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 74-600109

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

Washington, D. C. 20402 - Price 50 cents (paper cover)

Stock Number 2401-02582
CONTENTS

Page

Abstract................................................................. Cl

Introduction ............................................................. 1

Echo Park Alluvium ...................................................... 4

Wall Mountain Tuff ...................................................... 7

Tallahassee Creek Conglomerate ........................................... 9

Thirtynine Mile Andesite ................................................. 11

Lower member ....................................................... 12

Tuff of Stirrup Ranch ................................................ 13

Upper Member....................................................... 13

Badger Creek Tuff ....................................................... 14

East Gulch Tuff ......................................................... 15

Thorn Ranch Tuff ........................................................ 16

Gribbles Park Tuff ....................................................... 17

Conglomerate of Fear Creek ............................................... 19

Andesite of Waugh Mountain ............................................. 20

References cited .......................................................... 21

ILLUSTRATIONS

FIGURE 1. Map of the Thirtynine Mile volcanic field, showing its relations to

other Cenozoic volcanic fields ................................ C2

2. Diagram showing generalized composite stratigraphy of the Thirtynine Mile volcanic field .................................... 5

TABLE

Page 

TABLE 1. Chemical composition of major volcanic units in the Thirtynine Mile

volcanic field ............................................... C6

ill

CONTRIBUTIONS TO STRATIGRAPHY

STRATIGRAPHIC NOMENCLATURE OF THE 

THIRTYNINE MILE VOLCANIC FIELD, 

CENTRAL COLORADO

By RUDY C. EPIS l and CHARLES E. CHAPIN 2

ABSTRACT

The Thirtynine Mile volcanic field is a deeply dissected andesitic plateau built upon a 

late Eocene erosion surface of moderately low relief; numerous outliers are present in adjacent physiographic and tectonic settings in central Colorado. The volcanic field is 

characterized by andesitic rocks of local origin interspersed with rhyolitic to quartz latitic 

ash-flow tuffs erupted from sources outside the field. Potassium-argon ages on volcanic 

units range from 36 million years (early Oligocene) to 19 million years (early Miocene), 

but most of the volcanic pile was emplaced between 36 and 27 million years before the 

present. Sedimentary deposits of Tertiary age occur in a major graben beneath the field, 

in paleovalleys on the late Eocene erosion surface and, to a lesser extent, within the 

volcanic pile, in lake basins formed by blockaded drainages along the northern margin of 

the field, and in downfaulted basins which postdate the volcanism. Thirteen stratigraphic 

units are named and described in this report within a chronological framework established by field relationships and potassium-argon dating. Seven units are newly named 

herein, one unit previously named is adopted, and five units are given informal names. In 

ascending stratigraphic order, the units are Echo Park Alluvium, Wall Mountain Tuff 

(formerly named Agate Creek Tuff by De Voto, 1961), Tallahassee Creek Conglomerate 

(includes Alnwick Lake Beds and High Park Lake Beds of Cross (1894), which are here 

abandoned), Thirtynine Mile Andesite (formerly termed "Thirtynine Mile volcanic 

series" by Stark and others (1949) and includes informal members here called the lower 

member, the tuff of Stirrup Ranch, and the upper member), Badger Creek Tuff, East 

Gulch Tuff, Thorn Ranch Tuff, Gribbles Park Tuff, conglomerate of Fear Creek (informal 

name), and the andesite of Waugh Mountain (informal name).

INTRODUCTION

A sequence of lithologically distinct rock-stratigraphic units has been 

delineated in the Thirtynine Mile volcanic field and adjacent areas by 

geologic mapping during the past 13 years (fig. 1). Much of this work 

has been summarized in earlier reports (Chapin and Epis, 1964; Epis 

and Chapin, 1968) and will not be repeated here. Because geologic mapping was in progress and stratigraphic relations among some units were 

incompletely known, we used informal names in our earlier reports for

1 Colorado School of Mines, Golden, Colo. 80401, and U.S. Geological Sm 

New Mexico State Bureau of Mines and Mineral Resources, Socorro, N.

irvey, Denver, Colo. 80225. 

' Mex. 87801.

Cl
o

106 105°

39"

EXPLANATION

VOLCANIC FIELDS

Thirtynine Mile

Cripple Creek

O O

Buffalo Peaks 

H

O 

2

CO

Silver Cliff- 1-3 

Rosita Hills 

OCO

Bonanza

San Juan

Type locality of new rockstratigraphic units

o
COLORADO

10 30 KILOMETERS

FIGURE 1. Location of the Thirtynine Mile volcanic field, showing relations to other middle to upper Cenozoic volcanic fields and important 

localities mentioned in the text. Quadrangles: AR, Antero Reservoir; BM, Black Mountain; CA, Cameron Mountain; CC, Cripple Creek 

South; CO, Cotopaxi; CM, Cover Mountain; F, Florissant; G, Guffey; K, Kassler; PP, Pikes Peak; PS, Poncha Springs. Modified from Burbank, Lovering, Goddard, and Eckel (1935), Chapin and Epis (1964), and Epis and Chapin (1968).

Z

O

op

o

oo

Bo

CO
C4 CONTRIBUTIONS TO STRATIGRAPHY

most of the Tertiary units. In this report, we assign formal names to 

seven units and abandon three; informal names are used for five units 

whose lateral extent is insufficient to justify a formal name or whose 

relationships are incompletely known. In ascending stratigraphic order, 

the units are the Echo Park Alluvium, Wall Mountain Tuff, 

Tallahassee Creek Conglomerate, Thirtynine Mile Andesite (including 

three informal members designated as the lower member, the tuff of 

Stirrup Ranch, and the upper member), Badger Creek Tuff, East Gulch 

Tuff, Thorn Ranch Tuff, Cribbles Park Tuff, conglomerate of Fear 

Creek (informal name), and andesite of Waugh Mountain (informal 

name). The interrelations of these units with formations whose names 

are already established in the literature are presented in figure 2. The 

ages of epoch boundaries are those of Harland, Smith, and Wilcock

(1964). Chemical analyses of some of the units named here are listed in 

table 1.

The mapping on which this report is based was done by R. C. Epis 

and graduate students at the Colorado School of Mines, C. E. Chapin 

and graduate students at the New Mexico Institute of Mining and 

Technology, and G. R. Scott, R. B. Taylor, R. E. Van Alstine, and C. T. 

Wrucke of the U. S. Geological Survey. Graduate theses are listed in 

"References Cited"; they were completed by Buchanan (1967), Chapin

(1965), De Voto (1961), DuHamel (1968), Graebner (1967), Lowell 

(1969), Lozano (1965), Morris (1969), Niesen (1969), Sawatzky (1967), 

Tobey (1969), Welsh (1969) and Wyckoff (1969). In 1966 several K-Ar 

(potassium-argon) ages were determined by Geochron Laboratories, 

Inc., under a research grant to C. E. Chapin, D. L. Sawatzky, and P. L. 

Siems. Recently, K-Ar ages were determined by J. D. Obradovich, of 

the U. S. Geological Survey, who kindly gave us permission to publish 

them in this report. We are indebted to all these individuals, both for 

important contributions and for helpful discussion. The responsibility 

for any errors in interpretation, however, rests solely with the authors.

ECHO PARK ALLUVIUM

Alluvium informally referred to (Epis and Chapin, 1968, p. 59) as the 

prevolcanic arkose is here named the Echo Park Alluvium for its excellent exposures at Echo Park on the north side of the Arkansas River 

in the central part of T. 18 S., R. 73 W. The type locality is designated 

along the part of Echo Canyon which is 1% miles to 3Vz miles 

northward from the Arkansas River (Cotopaxi quadrangle, fig. 1). Here 

the formation attains a thickness of about 900 feet in a graben formed in 

Precambrian rocks. The Echo Park is overlain by the Wall Mountain 

Tuff and the lower member of the Thirtynine Mile Andesite, at the type 

locality. Equally thick but not so well exposed outcrops of the Echo 

Park Alluvium occur along the valleys of North Tallahassee Creek, Cottonwood Creek, and Mill Gulch in the eastern half of the Black Mountain quadrangle.
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C5

AGE

MIOCENE- PLIOCENE

MIOCENE

OLIGOCENE

EOCENE

GEN0E RASOUYTHWEST GUFFEYCENTE R GENERALLY EAST OR NORTH

CONTINUED BLOCK FAULTING

Postvolcanic sedimentary 

rocks

Postvolcanic sedimentary 

rocks

BLOCK FAULTING

Andesite of Waugh Mountain 

19 m.y.

Conglomerate of Fear 

Creek 27 m.y.

Cribbles Park Tuff 29 m.y.

Thorn Ranch Tuff

East Gulch Tuff

 I Badger Antero 

Balfour Creek Formation . Upper member

of John 31.9 son (1937) | $ Lower mer 

son (1937) m.y. 33 m.y. £ ? X - -__ 

C < /Tuff of Stirr

Lower member £ / Rancri^^ - 

Thirtynine Mile Andesite ^^---

Phonolite of Cripple Creek 

28 m.y.

34 m.y.

nber Florissant Lake Beds 34 m.y.

"^"^ Lower member

Tallahassee Creek Conglomerate

Wall Mountain Tuff 35-36 m.y.

Echo Park Alluvium

PREVOLCANIC EROSION SURFACE 

LARAMIDE FOLDING, THRUSTING, AND EROSION

Figure 2. Generalized composite stratigraphy of the Thirtynine Mile volcanic field. 

Epoch boundaries are those of Harland, Smith, and Wilcock (1964). Most intrusive and 

some extrusive rocks are not shown. (See Epis and Chapin, 1968, fig. 4.) Age of Badger 

Creek Tuff from C. W. Naeser (oral commun., 1973).

Drilling in these areas by Western Nuclear, Inc., in 1968 penetrated 

as much as 1,100 feet of Echo Park Alluvium and helped define a northnorthwest-trending graben, 2 to 4 miles wide, which extends more than 

20 miles from the Arkansas River to the vicinity of Guffey. Abrupt 

changes in thickness within the graben seem to reflect a mosaic of fault 

blocks comprising the floor of the graben. Significant, but thinner, exposures of the formation are located at High Park in the northeastern 

part of the Cover Mountain quadrangle and at Mushroom Gulch in the 

east-central part of the Antero Reservior quadrangle.

The Echo Park Alluvium weathers reddish brown and is crudely 

stratified and only partially consolidated. It consists of poorly sorted 

material ranging from clay to boulder-size clasts 20 feet in diameter. 

The clasts are chiefly Precambrian plutonic igneous and metamorphic 

rocks of local derivation and are set in a matrix of micaceous sand, silt,
Table 1. Chemical composition of major volcanic units in the Thir

[Analyses made in 1968 in the Rapid Rock Analysis Laboratory of the U.S. Geological Survey by P.L.D. Elmore, Gillison Chloe

under the supervision of Leonard Shapiro. Methods used were those described in U.S. Geological Survey Bulletin 1144

Unit

Andesite of Waugh Mountain .......

Do1 ..........................

Gribbles Park Tuff .................

Do ...........................

Thorn Ranch Tuff .................

Do ...........................

East Gulch Tuff ...................

Do ...........................

Do2 ..........................

Badger Creek Tuff1 ................

Thirtynine Mile Andesite, upper member. 

Do1 .............. .'. ..........

Do ...........................

Thirtynine Mile Andesite, tuff of 

Stirrup Ranch ...................

Wall Mountain Tuff ................

Do ...........................

Lab. 

No.

W170622

W170645

Wl 70625

W170630

W170626

W170642

W170624

W170643

W170627

W170631

W170646

W170628 

Wl 70647

W170629

W170632

W170619

W170620

W170621

SiO,

53.6

51.1

69.7

72.0

77.0

76.0

72.0

75.3

64.0

63.7

64.3

62.6 

49.5

66.0

54.9

71.9

72.2

69.7

A1,O,

16.2

15.2

15.1

14.5

11.0

12.5

13.1

12.7

15.8

16.5

16.0

17.4 

16.5

17.0

16.6

14.2

14.3

15.5

Fe.,0,

5.4

7.9

2.2

.75

1.0

1.0

1.1

1.0

3.4

3.2

2.9

3.8

4.7

2.9

4.9

1.6

1.1

1.8

FeO

3.4

2.4

.28

.20

.08

.20

.20

.24

.24

.52

.60

.44 

5.6

.12

2.4

.16

.32

.24

MgO

4.2

4.5

.75

.39

36

.14

.62

.16

1.2

1.1

1.2

.80 

6.1

32

2.6

.29

.25

.42

CaO

7.2

7.1

1.1

.90

.67

.65

1.1

.65

2.4

2.8

2.7

3.9

8.1

2.3

5.4

.87

.78

.75

N

1 Summed to 99 percent; all other analyses summed to 100 percent.

2 Unwelded ash-flow tuff.
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C7

and clay. Most of the deposit represents alluvial sheet-wash accumulation, but arkosic fluvial sands and gravels are present locally, as at the 

Smaller uranium mine on North Tallahassee Creek and in Echo Park. 

About 70 percent of the footage drilled by Western Nuclear, Inc., was in 

red conglomeratic mudstone deposits; outcrops reveal a similar 

dominance of sheet-wash deposits.

The Echo Park overlies Precambrian crystalline bedrock as alluvial 

fills in paleovalleys or downfaulted basins. These depositional sites 

formed during the period of erosion that developed an extensive 

prevolcanic surface of low relief in late Eocene time (Epis and Chapin, 

1968, p. 56-60; 1973). The formation is overlain by either the Wall 

Mountain Tuff (35-36 m.y.), the Tallahassee Creek Conglomerate, or 

the lower member of the Thirtynine Mile Andesite. Accordingly, we 

assign the Echo Park Alluvium an Eocene age, and, on the bases of 

lithology, mode of deposition, and stratigraphic position, we suggest a 

correlation with the Huerfano Formation and Farisita Conglomerate 

(Johnson and Wood, 1956) to the south.

WALL MOUNTAIN TUFF

Chapin and Epis (1964) informally referred to rhyolitic ash-flow tuffs 

as ash flow 1 and ash flow 2 and commented that the rocks are 

megascopically very similar. Petrographic studies suggested the 

presence of additional ash-flow units and indicated that the unit was a 

multiple-flow simple-cooling unit containing at least four ash flows; accordingly, we referred to it in 1964 and 1968 as ash-flow 1 and mapped it 

as a single formation. De Voto (1964, p. 119) named outcrops of ash-flow 

tuff in southwestern South Park the Agate Creek Formation and incorrectly assigned it a stratigraphic position above the lower member of 

the Thirtynine Mile Andesite. We (Epis and Chapin, 1968, p. 67-68) 

referred to these outcrops as ash flow 4. Subsequent field and 

petrographic work has demonstrated that De Voto's Agate Creek Formation is older than the Thirtynine Mile Andesite and is part of the 

ash-flow 1 sheet. (See also De Voto, 1971, p. 29-35.) Because outcrops at 

De Voto's type locality of the Agate Creek are incomplete (only 11 ft 

thick) and because important stratigraphic relations cannot be 

demonstrated, we here rename the Agate Creek Tuff of De Voto (1964) 

and ash flows 1, 2, and 4 of Chapin and Epis (1964) the Wall Mountain 

Tuff. The type locality of the Wall Mountain Tuff is along the south 

end of Wall Mountain in sec. 32, T. 16 S., R. 73 W., in the east-central 

part of the Black Mountain quadrangle (fig. 1). There, the formation 

dips about 6° W. and forms prominent cliffs as much as 90 feet high. 

The formation rests on the Echo Park Alluvium and is overlain either 

by the Tallahassee Creek Conglomerate or by the lower member of the 

Thirtynine Mile Andesite. Elsewhere, the tuff was extensively eroded
C8 CONTRIBUTIONS TO STRATIGRAPHY

prior to the eruption of the Thirtynine Mile Andesite and generally occurs in isolated patches that are less than 50 feet thick. However, in 

paleovalleys incised into the prevolcanic surface, such as near Trout 

Creek and between Browns Canyon and Waugh Mountain, the tuff attains thicknesses of more than 500 feet.

The Wall Mountain Tuff generally weathers reddish brown to 

yellowish buff and tends to form clifflike outcrops; it exhibits moderate 

to dense welding and distinct eutaxitic structure imparted by flattened 

and stretched pumice lapilli and by partings that reflect laminar 

flowage. Locally, in the Antero Reservior quadrangle and in the 

northeastern part of the Poncha Springs quadrangle, a conspicuous 

black vitrophyre, as much as several tens of feet thick, is found above a 

light-colored thin nonwelded basal zone; vitrophyre is also locally present elsewhere in the volcanic field. More commonly, however, the tuff 

is completely devitrified and contains microcrystalline alkali feldspar 

and silica minerals of vapor-phase origin. Primary and secondary 

laminar-flowage structures are common (Lowell and Chapin, 1972).

Phenocrysts make up 18-33 percent of the rock. Fresh glassy clear 

sanidine and fresh to intensely argillized plagioclase (andesine) are the 

principal phenocrystic minerals. The sanidine to plagioclase ratio 

ranges from 0.9 to 4.4 and averages about 2.0. Sanidine (or 

anorthoclase) 2V values range from 22° to more than 50° with median 

values for the four known ash flows of the Wall Mountain Tuff, in 

ascending order, of 30.5°, 34°, 38°, and 42° (Chapin, 1965; Lowell, 

1969). Quartz is absent as phenocrysts. Biotite and opaque oxides 

generally comprise about 6-9 percent of the rock; traces of pyroxene are 

present in most samples. Chemically, the Wall Mountain Tuff is a 

rhyolite (table 1). Accessory Lithic fragments, consisting mainly of tan 

to light-red felsite, are abundant in the Browns Canyon area; they are 

generally present in outcrops along the west edge of the volcanic field 

but are sparse to absent elsewhere.

The Wall Mountain Tuff is the most widely distributed formation in 

the Thirtynine Mile volcanic field. Probable equivalents of the Wall 

Mountain occur at numerous localities throughout an area of about 

3,500 square miles between Castle Rock (Izett and others, 1969) and the 

Kassler quadrangle (G. R. Scott, oral commun., 1972) on the northeast, 

South Park (Sawatsky, 1964, 1967) on the northwest, the northern Wet 

Mountain valley (Scott and Taylor, 1974) on the southeast, and 

northwest of Poncha Pass on the southwest. The Wall Mountain Tuff 

was referred to by earlier workers in South Park as a trachyte flow 

(Stark and others, 1949, p. 105-110) and in the Cripple Creek district as 

a rhyolite flow (Cross, 1894; Lindgren and Ransome, 1906, p. 100-101). 

An unnamed rhyolitic ash-flow tuff (Izett and others, 1969) beneath the 

Castle Rock Conglomerate in the Denver basin between Castle Rock 

and Monument is here assigned to the Wall Mountain Tuff because of
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C9

similarity in age, lithology, 2V values of sanidine, and chemical composition.

The tuff is one of the most important stratigraphic units in the Thirtynine Mile volcanic pile because it marks the onset of volcanism in 

earliest Oligocene time, preserves and dates a regional post-Laramide 

erosion surface, and serves as a datum for determining the location and 

magnitude of middle to late Cenozoic block faulting.

The source of the Wall Mountain Tuff is unknown. It must have been 

erupted from a volcanic center west of the upper Arkansas River valley 

between Salida and Buena Vista because it flowed into eastwarddraining paleovalleys in the southern Mosquito Range and eventually 

spread eastward over the region outlined above (Chapin and others, 

1970). A deeply eroded cauldron complex in the Mount Aetna area 

north of Monarch Pass may be a possible source (Dings and Robinson, 

1957; Lipman and others, 1969; P. J. Toulmin and G. A. Desborough, 

oral commun., 1973).

K-Ar radiometric ages have been determined on samples from several 

localities of the tuff. Epis and Chapin (1968, p. 59) reported an 

age of 40.Oil.2 m.y., determined by Geochron Laboratories, Inc., from 

a sanidine separate. (We now consider this date too old, probably 

because of pontamination by Precambrian microcline.) Izett, Scott, and 

Obradovich (1969, p. B13-B14) obtained a spurious age of 44.5±1.4 

m.y. from a sanidine separate contaminated by small amounts of 

microcline presumably from the Precambrian Pikes Peak Granite and 

an age, which they consider to be reliable, of 34.8±1.1 m.y., from 

biotite. Van Alstine (1969, p. 15) cited ages of 35.4±1.1 and 37.3±1.9 

m.y. for sanidine and biotite, respectively. In 1973 J. D. Obradovich 

determined ages for two additional samples of the Wall Mountain Tuff, 

as follows:

Sample Field No. Mineral Age (m.y.)

1....................... 5.70-1 sanidine 34.7±0.7

2-...------.---......... 68-0-39 sanidine 36.2±0.8

biotite 35.7±0.8

These samples are considered to be virtually uncontaminated; 

therefore, we assign an early Oligocene, or Chadron, age to the Wall 

Mountain Tuff. The unit has normal remanent magnetic polarity 

(Graebner, 1967; Graebner and Epis, 1968).

TALLAHASSEE CREEK CONGLOMERATE

Previously, we (Epis and Chapin, 1968) have called this deposit the 

lower volcanic conglomerate; we now name it the Tallahassee Creek 

Conglomerate for its excellent exposures, herein designated the type 

locality, at the junction of the three forks of Tallahassee Creek, in sec. 

31, T. 17 S., R. 72 W., and sec. 36, T. 17 S., R. 73 W., in the southwest 

corner of the Cover Mountain quadrangle and the southeast corner of 

the Black Mountain quadrangle (fig. 1). There, the conglomerate is as
CIO CONTRIBUTIONS TO STRATIGRAPHY

much as 350 feet thick and fills a meandering paleovalley about Vfc-2 

miles wide. The Tallahassee Creek Conglomerate rests on either the 

Wall Mountain Tuff, the Echo Park Alluvium, or Precambrian rocks 

and is overlain by either the lower member or, locally, the tuff of 

Stirrup Ranch, both of the Thirtynine Mile Andesite. Good exposures of 

the unit-occur along North Tallahassee Creek in sees. 9 and 10, T. 17 S., 

R. 73 W., and at the head of Twelvemile Hole, in sees. 29 and 32, T. 17 

S., R. 72 W. Significant occurrences of conglomerate which we assign to 

this formation on the bases of similar lithology and stratigraphic position 

are found in the western and northern parts of the Antero Reservoir 

quadrangle, in the northwest corner of the Guffey quadrangle, on the 

lower slopes of Little Pisgah Peak, and on Grouse and Straub Mountains in the Cripple Creek South quadrangle. We also include in the 

Tallahassee Creek Conglomerate fairly extensive outcrops of the 

Alnwick Lake Beds and the High Park Lake Beds (Cross, 1894) in the 

southeast corner of the Florissant quadrangle and the northeast quarter 

of the Cover Mountain quadrangle. The names Alnwick and High Park 

are abandoned.

The Tallahassee Creek Conglomerate generally weathers gray or 

grayish pink in appearance. It is a poorly sorted, crudely stratified, partially consolidated accumulation of pebbles, cobbles, and boulders (as 

much as 5 ft in diameter) in a tuffaceous to arkosic sandy and silty 

matrix. It fills paleovalleys cut into the Wall Mountain Tuff, the Echo 

Park Alluvium, and the Precambrian basement, and, locally, it mantles 

the late Eocene surface. Lithologically, the conglomerate is 

characterized by an abundance of red, purple, and gray aphanitic and 

porphyritic volcanic clasts, gray and light-green cobbles and boulders of 

dense quartzite, and fewer clasts of Wall Mountain Tuff. The volcanic 

clasts exhibit a wide range of decomposition, some being so fragile that 

they must have decomposed after deposition. In some areas volcanic 

clasts other than those of the Wall Mountain Tuff are absent 

because volcanic rocks were lacking in the source areas. Clasts of lower 

Paleozoic limestone and dolomite are less abundant. Precambrian 

detritus ranges in amount from sparse to dominant, depending oh the 

local bedrock and on whether the site of deposition is a tributary or a 

mainstem drainage. Clasts of the Wall Mountain Tuff are generally 

present even where Precambrian detritus dominates, and, together with 

other Oligocene volcanic-rock detritus, serve to distinguish this unit 

from the prevolcanic Echo Park Alluvium. The typical geomorphic expression consists of rounded grass-covered ridges and slopes strewn with 

cobbles and boulders, of which the resistant quartzite clasts, where present, are especially noticeable. In and near the type locality, several feet 

of bentonitic strata at the top of the conglomerate is an important host 

rock for uranium deposits; uranium also occurs locally in the conglomerate.
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. Cll

The stratigraphic position of the Tallahassee Creek Conglomerate 

between the Wall Mountain Tuff and the Thirtynine Mile Andesite 

shows that it is early Oligocene in age. The formation represents 

boulder alluvium deposited in paleovalleys incised into the late Eocene 

erosion surface. The valleys drained eastward from the ancestral 

Sawatch Range, picked up drainages flowing southeastward out of 

South Park, and flowed out onto the High Plains by draining southward 

through the Tallahassee Creek area and the High Park-Oil Creek area 

into the Canon City embayment (Scott, 1970 in an unpublished 

presidential address to the Colorado Sci. Soc., entitled "Geomorphic 

evolution of the southern Front Range, Colorado"; Chapin and others, 

1970; Epis and Chapin, 1973). Ash flows of the Wall Mountain Tuff 

utilized these same valleys and left them partially filled with tuff. Erosion stripped most of the tuff from the interfluves and reduced the 

deposits within the valleys to discontinuous patches prior to the period 

of aggradation that formed the Tallahassee Creek Conglomerate. The 

Castle Rock Conglomerate in the Denver basin is not equivalent to the 

Tallahassee Creek Conglomerate because the former contains abundant 

clasts of the Thirtynine Mile Andesite and is therefore younger. The 

Thirtynine Mile Andesite disrupted the drainages in which the 

Tallahassee Creek Conglomerate was deposited and diverted them 

across the Front Range to the Castle Rock area.

THIRTYNINE MILE ANDESITE

This unit was referred to as the "Thirtynine Mile volcanic series" by 

Stark and others (1949, p. 101), and we (Epis and Chapin, 1968, p. 64) 

informally termed it the "lower andesite." We here name the formation 

the Thirtynine Mile Andesite and describe lower and upper members 

for the excellent exposures, which make up the type locality, from the 

southern base of Thirtynine Mile Mountain to its summit in T. 14 S., R. 

73 W., in the southeast quarter of the Guffey quadrangle, (fig. 1). 

There, the lower member is about 1,400 feet thick, and the upper 

member is about 1,200 feet thick; the tuff of Stirrup Ranch is absent. 

Cross (1894) recognized these andesitic rocks in the western part of the 

Pikes Peak quadrangle and separated them into a lower "basic breccia 

and agglomerate" and an upper "massive andesite," a subdivision 

which generally parallels our definition of the lower and upper 

members, respectively. In the type locality, the lower member overlies 

Precambrian rocks and some of the intermediate domes and flows of the 

Guffey volcanic center. Other domes intrude the lower member, and at 

Cover Mountain, about 6 miles to the southeast, a large dome intrudes 

the upper member. Thus, volcanism at the Guffey center had a long 

and complex history; eventually, the domes and domal flows were 

almost entirely buried by the Thirtynine Mile Andesite. The Wall 

Mountain Tuff is missing in the type locality of the Thirtynine Mile
C12 CONTRIBUTIONS TO STRATIGRAPHY

Andesite but is present as discontinuous patches beneath the lower 

member in the Chumway Park area, about 2 miles to the southwest. 

Because of its occurrence as the flanks of a large composite volcano, the 

upper member has not been observed in direct contact with the younger 

formations of the field, and its stratigraphic position must be inferred 

on the basis of radiometric dates and geomorphic considerations.

LOWER MEMBER

The lower member of the Thirtynine Mile Andesite is mostly 

chaotically stratified unsorted well-indurated purple, gray, brown, and 

black breccias of diverse origin and mode of emplacement that weather 

into rolling hills and slopes with poor soils and sparse vegetation. 

Deuteric alteration has imparted a purplish cast to most outcrops. The 

breccias vary vertically and laterally to form a complex sheet. The bulk 

of the lower member is made up of laharic breccias both pyroclastic 

(erupted as mudflows) and epiclastic that spread laterally from 

numerous small randomly scattered vents (Chapin and Wyckoff, 1969). 

Flow breccias comprise only about 10 percent of the unit; bedded 

tephra, cinder cones, and other products of violent pyroclastic eruptions 

are notably scarce, as are unbrecciated lava flows. Vents are marked by 

small breccia cones, as much as 500 feet high and 0.5 mile in diameter, 

and (or) breccia dikes, and short autobrecciated flows. Individual 

lahars are generally 3-20 feet thick, have highly irregular tops and bottoms, and can be followed for only short distances. Dark-colored nonvesicular pyroxene andesite is the most abundant petrographic type, 

but hornblende- and biotite-bearing andesite is also present in some 

breccias. Rocks of the lower member exhibit a wide range in chemical 

composition (table 1); many of the autoclastic breccias in or near vents 

are buff to red felsites of silicic composition.

The lower member averages 500 feet in thickness and initially 

covered about 1,000 square miles, extending from the eastern half of the 

Antero Reservoir quadrangle to the base of Mount Pisgah, near Cripple 

Creek, and from Hartsel southward to the Arkansas River valley. 

Interbedded fluvial gravels and tuffs are scarce, indicating that 

drainage within the volcanic field was largely disrupted by emplacement of the lower member of the Thirtynine Mile Andesite. It buried all 

but the highest features of the preexisting landscape which is underlain 

mainly by Precambrian rocks and the Wall Mountain Tuff; it also 

dammed streams and created large lakes along the north side of the 

breccia sheet, such as those in which the Antero and Balfour Formations, and Florissant Lake Beds accumulated. The Balfour (Johnson, 1937; Stark and others, 1949) and Florissant (Cross, 1894) deposits 

apparently are fluvial and lacustrine time equivalents of the breccia 

sheet; some of the Antero (Johnson, 1937; Stark and others, 1949) is 

younger, having been derived by reworking of white biotite-rich non-
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C13

welded Badger Creek Tuff, which is partly younger than the lower 

member of the Thirtynine Mile Andesite in the western and southern 

parts of the field.

TUFF OF STIRRUP RANCH

The tuff of Stirrup Ranch occurs as thin discontinuous bouldery 

erosional remnants within the basal 200 feet of the lower member of the 

Thirtynine Mile Andesite. We (Epis and Chapin, 1968, p. 65) previously referred to it as ash flow 3 but here give it the informal name tuff of 

Stirrup Ranch after good exposures west of the Antelope Mountains 

and north of Waugh Mountain in the northwest quarter of the Black 

Mountain quadrangle, sees. 14, 23, and 26, T. 51 N., R. 12 E. Good exposures also occur at the east end of Waugh Mountain, in sec. 5, T. 17 

S., R. 73 W., and sec. 32, T. 16 S., R. 73 W., where its stratigraphic 

positions are especially clear. Near the junction of North and Middle 

Tallahassee Creeks and along North Tallahassee Creek in sec. 10, T. 17 

S., R. 73 W., the tuff of Stirrup Ranch rests directly on the Tallahassee 

Creek Conglomerate. Between these two localities, and farther to the 

northwest, the tuff occurs as much as 200 feet high in the lower member 

of the Thirtynine Mile Andesite. Apparently, ridges of the Tallahassee 

Creek Conglomerate were exposed above the basal mudflows of the 

Thirtynine Mile Andesite at the time the tuff of Stirrup Ranch was 

emplaced.

The tuff of Stirrup Ranch is very similar to the Wall Mountain Tuff 

in color, texture, mineralogy, weathering characteristics, and chemical 

composition (table 1). The 2V values of sanidine (range 29°-57°, median 34°) are about midway between the extremes of the four ash flows 

making up the Wall Mountain Tuff. Thus, the two units can be distinguished only on the basis of stratigraphic position. Outcrops of 

welded tuff that rest on andesitic breccias north and east of Antero 

Reservoir (sec. 32, T. 11 S., R. 76 W., and sees. 4, 9,13,14, and 24, T. 12 

S., R. 24 W.) and were included in the Agate Creek Formation by De 

Voto (1961) and Lozano (1965, 1967) are probably remnants of the tuff 

of Stirrup Ranch. The source of the tuff is unknown, but, presumably, 

the tuff represents a slightly later product of the same magma that gave 

rise to the Wall Mountain Tuff.

UPPER MEMBER

The upper member of the Thirtynine Mile Andesite occurs in the central part of the field, where it caps the upper parts of Thirtynine Mile, 

Saddle, Castle, Mclntyre, Witcher, and Black Mountains, which surround the Guffey volcanic center. It consists of distinctly stratified 

andesite flows, lesser amounts of interbedded breccias, and minor 

amounts of ash-fall tuffs and represents the remnant flanks of a large 

composite volcano centered between Guffey and Chumway Park, which 

we previously termed the "Guffey volcano" (Epis and Chapin, 1968, p.
C14 CONTRIBUTIONS TO STRATIGRAPHY

66). The contact between the upper and lower members of the Thirtynine Mile Andesite is locally gradational and is placed at the change 

in slope from rounded topography established on breccias to steep 

ledgelike topography developed on interbedded flows and lesser breccias. Rocks of the upper member vary widely in chemical composition 

(table 1), as do those of the lower member. Dark basaltic andesites containing abundant pyroxene phenocrysts as much as V* inch in long 

dimension are the most abundant rock types. Other flows, however, 

contain biotite or hornblende phenocrysts and are more silicic in composition.

Lavas of the upper member spread several miles southeastward from 

the Guffey volcano and form Cap Rock Ridge in the central Cover 

Mountain quadrangle; they also spread northeastward over the Florissant Lake Beds in the east-central part of the Florissant quadrangle. 

Apparently, there were no younger volcanic units of the field of sufficient volume and thickness to overlap the upper member of the Thirtynine Mile Andesite, for we did not find any units resting upon it.

Geochron Laboratories, Inc., determined a whole-rock K-Ar age of 

34.1±1.1 m.y. for a sample of the upper member from near the top of 

Thirtynine Mile Mountain. This determination, along with others we 

have obtained from older formations from the Florissant Lake Beds 

(34.1±0.8 m.y.), and from the Antero Formation (33.7±0.8 m.y., 

33;6±0.7 m.y.) indicates that the Thirtynine Mile Andesite is early 

Oligocene in age. (See fig. 2.)

BADGER CREEK TUFF

Epis and Chapin (1968, p. 69-70) referred to this unit informally as 

the tuff of Badger Creek and considered it to be a basal member of the 

Antero Formation of Johnson (1937). Recent detailed mapping by 

Lowell (1969, 1971) and reexamination by us indicate that the tuff is 

both younger and older than the Antero, and locally the Antero was 

eroded before the upper part of the Badger Creek was deposited. We 

here name the unit the Badger Creek Tuff for excellent exposures at the 

type locality in the NE 1A sec. 12, T. 50 N., R. 10 E., in the valley of 

East Badger Creek in the Cameron Mountain quadrangle, 11 miles 

northeast of Salida (fig. 1). The Badger Creek Tuff is exposed for 10 

miles west of the type locality in the central part of the Cameron Mountain quadrangle, where it partially fills the east-trending prevolcanic 

paleovalley between Browns Canyon and Waugh Mountain. At the type 

locality Lowell (1969) measured 313 feet of the Badger Creek Tuff. It 

rests on either Precambrian crystalline rocks, the Wall Mountain Tuff, 

Tallahassee Creek Conglomerate, or the Antero Formation and is 

overlain by a local latite flow which, in turn, is overlain by the Gribbles 

Park Tuff. The Badger Creek Tuff is also well exposed In the 

paleovalley at Trout Creek and southward along the southwestern
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C15

margin of the Antero Reservoir quadrangle, where it locally is as much 

as 800 feet thick and was referred to by De Voto (1971, p. 25-26) as the 

andesitic tuff of Castle Rock Gulch. G. R. Scott (oral commun., 1970) 

mapped outcrops of a similar tuff east of Howard in the Arkansas River 

valley. Two outcrops of the Badger Creek Tuff have also been observed 

east of Waugh Mountain; one of these caps the high butte one-half mile 

south of the Thorn Ranch headquarters, in sees. 22 and 23, T 17 S., R. 

73 W.

The Badger Creek Tuff is a compound-cooling unit consisting of a 

series of petrographically similar ash flows, at least six of which can be 

discerned at the type locality. Multiple thin, densely welded dark-gray 

to black vitrophyre and piperno zones are present locally, but the tuff is 

characterized mainly by moderate welding and compaction. Eutaxitic 

fabric is well developed and causes the formation to split into thin slabs 

and plates upon weathering. The tuff generally crops out in rounded 

hills or modest slopes but, in places, forms clifflike exposures.

The Badger Creek Tuff weathers yellowish gray to buff' and tends to 

be light gray to light reddish brown when fresh. Hand specimens have a 

"salt-and-pepper" appearance because of the abundance of black 

biotite and hornblende crystals mixed with light-pink and white 

pumice lapilli. Crystal fragments make up about 30-35 percent of the 

rock, and strongly zoned plagioclase crystals (andesine) comprise 70-80 

percent of the crystals. Biotite and opaque oxides are second in abundance, accounting for 15-20 percent of the crystals. Sanidine and 

hornblende usually make up 2-5 percent each; sphene is a conspicuous 

accessory. Chemically, the tuff is quartz latite (table 1). Gray and 

brown accessory lithic lapilli are moderately abundant. The ash flows in 

the Badger Creek Tuff are remarkably similar in both mineralogical 

and chemical composition (table 1) to unwelded ash-flow tuffs in the 

Antero Formation; they most likely were erupted from the same source 

and locally may be contemporaneous.

Because of its stratigraphic position nearly equivalent to the Antero 

Formation and below the Gribbles Park Tuff, the Badger Creek Tuff 

must be Oligocene in age. Graebner (1967) determined a reverse remanent magnetic polarity for the unit. The source of this tuff is unknown, 

but, like the Wall Mountain Tuff, it must have originated west of its 

present outcrop area.

EAST GULCH TUFF

Previously, we (Epis and Chapin, 1968, p. 71) termed this unit "ash 

flow 5"; we now name it the East Gulch Tuff for good outcrops at the 

type locality in East Gulch, in sees. 2, 3, and 11, T. 49 N., R. 12 E., in 

the south-central part of the Black Mountain quadrangle (fig. 1). In this 

type locality the tuff is 70 feet thick, but elsewhere its average is about 

50 feet. The tuff fills a small paleovalley near Waugh Mountain and is
C16 CONTRIBUTIONS TO STRATIGRAPHY

more sheetlike to the south and southeast. It has been mapped by Scott 

and Taylor (1974) near Howard and Hillside. Generally, the East Gulch 

Tuff rests on the Antero Formation and is separated by an erosional interval from the overlying Thorn Ranch Tuff; locally, it lies on older 

volcanic rocks or on Precambrian basement and is overlain by volcanic 

units younger than the Thorn Ranch Tuff.

The East Gulch Tuff weathers light gray and, locally, light brown. It 

is a simple-cooling unit with well-developed zonation and a conspicuous eutaxitic fabric. A black vitrophyre zone as much as 15 feet 

thick is common in the lower third of the tuff; the vitrophyre zone 

grades upward into a piperno zone in which lenses of black glass, 

formed by collapsed pumice lapilli, are interspersed in a gray 

vitroclastic matrix. Moderately welded zones contain abundant, partly 

collapsed black, gray, and cream pumice in a light-brown matrix. 

Locally, the unit is only slightly welded throughout. The tuff is 

rhyolitic, both mineralogically and chemically (table 1), containing 

sanidine, plagioclase, quartz, and biotite as the important phenocrystic 

minerals. The 2V values of sanidine range from 16° to 36° with a median of 27° (Chapin, 1965). Lithic fragments of both Precambrian rocks 

and accessory volcanic rocks are abundant.

The stratigraphic position of the East Gulch Tuff, above the Antero 

Formation and below the Thorn Ranch Tuff, indicates an Oligocene 

age. Studies of remanent magnetism by Graebner (1967) indicate a 

reverse polarity. The source of the unit is unknown but is west of its present outcrops.

THORN RANCH TUFF

In the past we (Epis and Chapin, 1968, p. 72) have called this unit ash 

flow 6; we here name it the Thorn Ranch Tuff for good exposures at its 

type locality along the mesa near Thorn Ranch, at S Vz sec. 23 and N ¥2. 

sec. 26, T. 17 S., R. 73 W., in the southeast quarter of the Black Mountain quadrangle (fig. 1). The tuff rests on the Antero Formation and is 

overlain at the southeast end of the mesa by the Gribbles Park Tuff. 

The East Gulch Tuff is not present at the Thorn Ranch type locality 

but is well exposed beneath the Thorn Ranch Tuff on the mesa between 

Middle and South Tallahassee Creeks in S l/2 sec. 28 and N !/2 sec. 33, T. 

17 S., R. 73 W., about 2 miles southwest of the type locality.

The Thorn Ranch Tuff forms prominent reddish-brown caprocks 

with well-developed columnar jointing on the mesas near the confluence of the three forks of Tallahassee Creek. The moderately welded 

cliff-forming zone is 40-80 feet thick, and, where not stripped by erosion, it grades imperceptibly upward into a pinkish-white unwelded 

zone. Chapin (1965, p. 160) measured 245 feet of Thorn Ranch Tuff at 

the north end of Echo Park, but the unit generally is much thinner 

because the soft, upper zone has been eroded. The tuff occurs from the
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. 017

Badger Creek valley, west of Black Mountain, southeastward through 

Waugh Mountain and the Tallahassee Creek area. Scott and Taylor 

(1974) mapped the unit as far south as Goat Creek near Hillside in the 

northern Wet Mountain valley.

The Thorn Ranch Tuff is a simple-cooling unit of nonwelded to 

moderately welded tuff that weathers pinkish-white to reddish brown, 

depending on the degree of welding. It contains pink and white pumice, 

Precambrian lithic fragments, and brown aphanitic accessory lithic 

fragments. The tuff is rhyolitic, both mineralogically and chemically 

(table 1), and contains sanidine, plagioclase, quartz, and biotite as the 

important phenocrysts. The 2V values of sanidine range from 18° to 

42° with a median of 28° (Chapin, 1965). Devitrification of the glassy 

constituents is complete,and vaporphase alteration of pumice is extensive, especially in the moderately welded zone, where spherulitic 

replacement of pumice simulates small "birds' nests."

The stratigraphic position of the Thorn Ranch Tuff, above the East 

Gulch Tuff and below the Gribbles Park Tuff, indicates an Oligocene 

age. Like the East Gulch Tuff, it exhibits reverse polarity (Graebner, 

1967). Its source is unknown.

GRIBBLES PARK TUFF

In earlier works we (Epis and Chapin, 1968, p. 72) referred to this unit 

as ash flow 7 and here name it the Gribbles Park Tuff for exposures 

along the north and south sides of Gribbles Park, in the northeast 

quarter of the Cameron Mountain quadrangle. The tuff is about 300 

feet thick at its type locality (fig. 1), where it caps mesas along the 

north side of Gribbles Park, in the W Vz sec. 16 and the EVz sec. 17, T. 

51 N., R. 11 E., but on Two Creek in the EVz sec. 5, T. 50 N., R. 11 E., 

about 2 miles south of Gribbles Park, it is 590 feet thick in the 

paleovalley that extends eastward from Browns Canyon to Waugh 

Mountain. Along the north slope of Two Creek, the Gribbles Park Tuff 

contains three distinct members which form a compound-cooling unit. 

In ascending order, these members weather brown, gray, and red and 

have thicknesses of 240, 230, and 110 feet, respectively. Between the 

brown and gray members is an orange unwelded tuff, about 10 feet 

thick, which may be an unwelded top of the brown member, an unwelded bottom of the gray member, or a separate ash flow. The 

members form benchlike topographic features, but talus obscures the 

contacts. The Gribbles Park Tuff overlies the Antero Formation and, 

locally, the Thorn Ranch Tuff along the north side of Gribbles Park, but 

the top of the Gribbles Park has been eroded. In the paleovalley south of 

Gribbles Park, it rests on prevolcanic rocks, the Antero Formation, and 

a local latite flow which overlies both the Antero Formation and the 

Badger Creek Tuff; latitic flows and flow breccias from the Waugh 

Mountain center overlie the Gribbles Park Tuff at this locality.
CIS CONTRIBUTIONS TO STRATIGRAPHY

Southeast of Waugh Mountain, the Gribbles Park Tuff rests on the 

Thorn Ranch Tuff and is overlain by the conglomerate of Fear Creek or 

the andesite of Waugh Mountain (fig. 2). Chapin (1965) measured 290 

feet of the Gribbles Park Tuff with an eroded top at the north end of 

Echo Park; there, the unit forms impressive cliffs of massive purplishgray to brown tuff without apparent cooling breaks.

The Gribbles Park Tuff is an important stratigraphic marker in the 

Thirtynine Mile volcanic pile because of its distinctive lithology and 

widespread occurrence. The unit extends from southern South Park at 

Wagontongue Creek southward and southeastward beneath Waugh 

Mountain to the plains near Canon City and into the Wet Mountain 

valley on the De Weese Plateau and at Goat Creek (Scott and Taylor, 

1974). Before its disruption by late Cenozoic block faulting and erosion, 

the tuff must have covered most of the area between these localities (approximately 1,200 sq mi) and thus aided preservation of the 

prevolcanic erosion surface beneath it. South of the Arkansas River 

many isolated patches of the tuff rest on this rather smooth surface that 

is carved on Precambrian crystalline rock.

Although the three members of the Gribbles Park Tuff differ in color, 

they are similar in composition and are characterized by abundant 

chatoyant-blue sanidine crystals, bronze-colored biotite, and darkbrown and gray accessory lithic fragments. Pumice lapilli are abundant 

in the red member and in the orange tuff and have diameters as large as 

10 inches; pumice lapilli are less conspicuous in the brown and gray 

members which form massive outcrops with poorly developed eutaxitic 

fabric. The orange tuff is unwelded, but the other units are moderately 

to densely welded; black vitrophyre zones occur locally at the base of 

the lower member. Phenocrysts in the tuff vary mineralogically, but 

chemically the rock is a rhyolite (table 1). Phenocrysts constitute 15-30 

percent of the rock; of these, sanidine makes up 40-70 percent, 

plagioclase (andesine) 10-25 percent, and biotite and opaque oxides 5- 

20 percent (Lowell, 1969). Pyroxene, hornblende, and sphene are commonly present in trace amounts. The 2V value of sanidine ranges from 

26° to 38° with a median of 32° (Chapin, 1965); Lowell (1969) found 

that the median 2V value of sanidine was virtually the same in all three 

members.

Epis and Chapin (1968, p. 72) reported a K-Ar age of 34.8±1.4 m.y. as 

determined by Geochron Laboratories, Inc., on a sanidine separate 

from the Gribbles Park Tuff. Recent K-Ar age determinations by J. D. 

Obradovich of 28.6±0.6 and 29.5±0.7 m.y. on two sanidine separates 

indicate that the earlier determination is too old probably because of 

contamination. Accordingly, we assign a late Oligocene age to the formation. A normal remanent magnetic polarity was determined by 

Graebner (1967). The source of the Gribbles Park Tuff is unknown, but,
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C19

like the earlier ash flows of the Thirtynine Mile volcanic field, it is west 

of the known outcrop area of the tuff.

CONGLOMERATE OF FEAR CREEK

This unit was referred to in earlier reports (Epis and Chapin, 1968, p. 

74) as the upper volcanic conglomerate. It is here given the informal 

name conglomerate of Fear Creek for extensive exposures along the flat 

tablelands between Fear Creek and East Gulch, in the SVa sec. 30 and 

sec. 31, T. 17 S., R. 73 W., and sees. 4, 5, 6, 8, and 9, T. 18 S., R. 72 W., 

in the southeast quarter of the Black Mountain quadrangle. The unit is 

now exposed over an area of less than 10 square miles and is, therefore, 

only accorded an informal designation. It fills a southeast-trending 

paleovalley 1-2 miles wide and as much as 350 feet deep incised into the 

Gribbles Park Tuff and is overlain by the andesite of Waugh Mountain. 

The unit averages about 200 feet in thickness and extends eastward as 

far as Table Mountain in the southeast corner of the Black Mountain 

quadrangle. It was undoubtedly much more extensive originally, but 

deep dissection of the area to the south and east by the Arkansas River 

and its tributaries has removed much of the unit.

The conglomerate of Fear Creek consists mainly of gray, wellstratified, poorly cemented pebble to boulder conglomerate 

predominantly of andesitic composition. Two channel-shaped cross sections of the unit are spectacularly exposed at the north end of Echo 

Park, in sees. 8, 9, and 10, T. 18 S., R. 73 W., along the south edge of the 

Black Mountain quadrangle and the north edge of the Cotopaxi 

quadrangle. In the western channel, a conspicuous white tuff occurs 

about 200 feet above the base of the channel and is visible from U.S. 

Highway 50 in the bottom of the Arkansas River canyon. The tuff is 

made up of several feet of well-stratified waterlaid tuffs overlain by 

about 30 feet of massive nonwelded ash-flow tuff which contains abundant black biotite. J. D. Obradovich determined the following K-Ar 

ages from mineral separates from the ash-flow tuff: Hornblende, 

28.8±0.9; biotite, 27.7±0.6; sanidine, 26.8±0.6 and 27.8±0.6 m.y. 

About 100 feet of andesitic conglomerate overlies the dated tuff and is 

overlain, in turn, by the andesite of Waugh Mountain. In the eastern 

channel, about 200 feet of massive gray andesitic laharic breccias underlies similar white tuffs. Coarse conglomerate also occurs there 

between the white tuffs and the overlying andesite of Waugh Mountain. 

On the tablelands to the northwest, the conglomerate of Fear Creek 

forms rounded boulder-strewn ridges where it is not capped by the 

andesite of Waugh Mountain.

The K-Ar age determinations on the interbedded white tuff indicate 

a late Oligocene age for the conglomerate of Fear Creek and negate any 

possible correlation with lithologically similar conglomerates of the
C20 CONTRIBUTIONS TO STRATIGRAPHY

Trump Conglomerate of Johnson (1937) in southwestern South Park. 

The source of the interbedded tuffs and mudflow breccias was probably 

the Waugh Mountain volcanic center about 6 miles upstream 

(northwest). At this center, intermediate to silicic domes and flows intrude and overlie the Gribbles Park Tuff and are overlain by the 

andesite of Waugh Mountain. The great abundance of andesitic 

detritus in the conglomerate of Fear Creek suggests that some of the units 

we have grouped with the andesite of Waugh Mountain may be late 

Oligocene in age or coeval with the silicic domes and flows of the Waugh 

Mountain center.

ANDESITE OF WAUGH MOUNTAIN

Formerly, we (Epis and Chapin, 1968, p. 75) referred to this unit as 

the upper andesite, but here we give it the informal name andesite of 

Waugh Mountain. Several areas of andesitic to basaltic rocks in the 

southern part of the field are included in the unit on the bases of 

similarities in lithology and stratigraphic position. However, some of 

these outcrops are not physically continuous and may vary considerably 

in age. These rocks are the least studied volcanic units in the Thirtynine Mile field, and more fieldwork and petrographic work must be 

done before formal subdivisions can be made. The unit described here 

should be considered as a grouping of similar rocks, for the sake of convenience, until definitive work is accomplished.

The andesite of Waugh Mountain is named for extensive exposures 

on the largest mountain in the southern part of the field. The upper 

slopes of Waugh Mountain (T. 50 N., Rs. 11 and 12 E.), above about 

10,200 feet in altitude on the east end, 10,600 feet on the north side, and 

10,400 feet at the west end, are composed largely of andesitic to basaltic 

rocks which dip gently to the south and southeast and constitute the 

top of the volcanic pile. Their combined thickness exceeds 1,200 feet.

The andesite of Waugh Mountain consists mainly of a series of thin 

flows of andesite, basaltic andesite, and basalt with minor interbedded 

flow breccias, laharic breccias, and tuffs. The rocks are dark colored, 

generally purplish brown, greenish black or black, aphanitic to 

porphyritic-aphanitic, and exhibit local vesicular phases. Pyroxene and 

plagioclase are the most common phenocrysts, with or without minor 

olivine or hornblende. Thin dikes of similar compositions near the 

center of Waugh Mountain may have served as feeders for much of the 

formation, but it is equally likely that there are local sources elsewhere, 

particularly for the occurrences south of the Arkansas River.

Near the middle of Waugh Mountain, a volcanic center of intermediate to silicic domes and flows has been partially exhumed from 

beneath the andesites by the headwaters of North Waugh Creek on the 

north side of Waugh Mountain and those of Fernleaf Gulch south of 

Waugh Mountain. West of these domes, 400-800 feet of light-gray latitic
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C21

flows and flow breccias separates the Gribbles Park Tuff from the 

overlying andesite of Waugh Mountain. East of the domes the latite 

flows are missing, and the andesite of Waugh Mountain rests directly 

on the Gribbles Park Tuff.

Southeast of Waugh Mountain, the mesas which rim the Arkansas 

River canyon on the north side are capped by 200-300 feet of dark 

olivine-bearing basaltic andesite flows (table 1, samples W170622 and 

Wl70645), which overlie the conglomerate of Fear Creek. The altitude 

of the base decreases from 9,300 feet on the west to 8,500 feet on the east 

in a distance of about 7 miles. At the east end the flows overlap the 

eroded edge of the Gribbles Park Tuff and rest directly on Precambrian 

rocks. In a previous paper we (Epis and Chapin, 1968) correlated these 

flows with the andesites on Waugh Mountain, but they are not 

physically continuous, and we now doubt that correlation. Geochron 

Laboratories, Inc., determined a K-Ar age of 18.9±1.2 m.y. for a wholerock sample collected from a basaltic andesite flow at the north end of 

Echo Park, but andesitic clasts similar to the andesites on Waugh 

Mountain are abundant in the conglomerate of Fear Creek beneath the 

interbedded ash-flow tuff which yielded ages between 26 and 28 m.y. 

Hence, at least some of the andesites on Waugh Mountain may be older 

than, or coeval with, the conglomerate of Fear Creek, and the basaltic 

andesite flows capping the mesas may represent younger fissure eruptions. Other isolated outcrops of andesitic to basaltic flows south and 

southwest of Waugh Mountain, such as those near Howard in the 

Arkansas River canyon (G. R. Scott, oral commun., 1971) and those 

near Oak Creek and Hillside at the north end of the Wet Mountain 

Valley (Scott and Taylor, 1974), are tentatively included in the andesite 

of Waugh Mountain on the basis of similar lithology and stratigraphic 

position; however, further dating may necessitate refinement of this 

grouping.

REFERENCES CITED

Buchanan, P. H., 1967, Volcanic geology of the Guffey area, Park County, Colorado:

Colorado School Mines unpub. M.S. thesis, 95 p. 

Burbank, W. S., Lovering, T. S., Goddard, E. M., and Eckel, E. B., 1935, Geologic map of

Colorado: U.S. Geol. Survey, scale: 1:500,000 [reprinted 1959]. 

Chapin, C. E., 1965, Geologic and petrologic features of the Thirtynine Mile volcanic field,

central Colorado Pt. 1, General geology; Pt. 2, Correlation of ash-flow tuffs by the

use of optic axial angles of alkali feldspar crystals: Colorado School Mines unpub. D.

Sc. thesis, 176 p. 

Chapin, C. E., and Epis, R. C., 1964, Some stratigraphic and structural features of the

Thirtynine Mile volcanic field, central Colorado: Mtn. Geologist, v. 1, no. 3, p. 145-

160. 

Chapin, C. E., Epis, R. C., and Lowell, G. R., 1970, Late Eocene paleovalleys and

Oligocene volcanic rocks along the upper Arkansas valley segment of the Rio Grande

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C22 CONTRIBUTIONS TO STRATIGRAPHY

Chapin, C. E., and Wyckoff, B. S., 1969, Formation of a sixty-cubic-mile andesitic breccia

sheet in the Thirtynine Mile volcanic field of central Colorado, in Abstracts for 1968:

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Cross, Whitman, 1894, Description of the Pikes Peak sheet, Colorado: U.S. Geol. Survey

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De Voto, R. H., 1961, Geology of southwestern South Park, Park and Chaffee Counties,

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___1964, Stratigraphy and structure of Tertiary rocks in southwestern South Park:

Mtn. Geologist, v. 1, no. 3, p. 117-126. 

___1971, Geologic history of South Park and geology of the Antero Reservoir

quadrangle, Colorado: Colorado School Mines Quart., v. 66, no. 3, 90 p. 

Dings, M. G., and Robinson, C. S., 1957, Geology and ore deposits of the Garfield

quadrangle, Colorado: U.S. Geol. Survey Prof. Paper 289, 110 p. 

DuHamel, J. E., 1968, Volcanic geology of the upper Cottonwood Creek area, Park and

Fremont counties, Colorado: Colorado School Mines unpub. M.S. thesis, 120 p. 

Epis, R. C., and Chapin, C. E., 1968, Geologic history of the Thirtynine Mile volcanic field,

central Colorado, in Epis, R. C., ed., Cenozoic volcanism in the Southern Rocky

Mountains: Colorado School Mines Quart., v. 63, no. 3, p. 51-85. 

___1973, Geomorphic and tectonic implications of the post-Laramide late Eocene erosion surface in the Southern Rocky Mountains: Geol. Soc. America Abs. with

Programs, v. 5, no. 6, p. 479. 

Graebner, Peter, 1967, Remanent magnetism in major rock units of the Thirtynine Mile

volcanic field, central Colorado; Colorado School Mines unpub. M.S. thesis, 165 p. 

Graebner, Peter, and Epis, R. C., 1968, Remanent magnetism in major rock units of the

Thirtynine Mile volcanic field, central Colorado [abs.], in Epis, R. C., ed., Cenozoic

volcanism in the Southern Rocky Mountains: Colorado School Mines Quart., v. 63,

no. 3, p. 87-88. 

Harland, W. B., Smith, A. G., and Wilcock, Bruce, eds., 1964, The Phanerozoic time

scale A symposium dedicated to Professor Arthur Holmes: Geol. Soc. London

Quart. Jour. Supp., v. 120s, 458 p. 

Izett, G. A., Scott, G. R., and Obradovich, J. D., 1969, Oligocene rhyolite in the Denver

basin, Colorado, in Geological Survey research 1969: U.S. Geol. Survey Prof. Paper

650-B, p. B12-B14. 

Johnson, J. H., 1937, The Tertiary deposits of South Park, Colorado, with a description of

the Oligocene algal limestone [abs. of thesis]: Colorado Univ. Studies, v. 25, no. 1, p.

77.

Johnson, R. B., and Wood, G. H., Jr., 1956, Stratigraphy of Upper Cretaceous and Tertiary rocks of Raton basin, Colorado and New Mexico: Am. Assoc. Petroleum

Geologists Bull., v. 40, no. 4, p. 707-721. 

Lindgren, Waldemar, and Ransome, F. L., 1906, Geology and gold deposits of the Cripple

Creek district, Colorado: U.S. Geol. Survey Prof. Paper 54, 516 p. 

Lipman, P. W., Mutschler, F. E., Bryant, Bruce, and Steven, T. A., 1969, Similarity of

Cenozoic igneous activity in the San Juan and Elk Mountains, Colorado, and its

regional significance, in Geological Survey research 1969: U.S. Geol. Survey Prof.

Paper 650-D, p. D33-D42. 

Lowell, G. R., 1969, Geologic relationships of the Salida area to the Thirtynine Mile

volcanic field of central Colorado: New Mexico Inst. Mining and Technology unpub.

D. Sc. thesis, 113 p. 

___1971, Cenozoic geology of the Arkansas Hills region of the southern Mosquito

Range, central Colorado, in San Luis Basin, Colorado: New Mexico Geol. Soc.

Guidebook, 22d Field Conf., 1971, p. 209-217. 

Lowell, G. R., and Chapin, C. E., 1972, Primary compaction and flow foliation in ash-flow

tuffs of the Gribbles Run paleovalley, central Colorado: Geol. Soc. America Abs.

with Programs, v. 4, no. 7, p. 725-726.
NOMENCLATURE, THIRTYNINE MILE VOLCANIC FIELD, COLO. C23

Lozano, Efraim, 1965, Geology of the southwestern Garo area, South Park, Park County,

Colorado: Colorado School Mines unpub. M.S. thesis, 115 p. 

___1967, Stratigraphy and structure of Tertiary rocks in central South Park, Park

County, Colorado: Mtn. Geologist, v. 4, no. 4, p. 119-126. 

Morris, G. R., 1969, Geology of the Dicks Peak area, Park County, Colorado: Colorado

School Mines unpub. M.S. thesis, 69 p.

Niesen, P. L., 1969, Stratigraphic relationships of the Florissant Lake Beds to the Thirtynine Mile volcanic field of central Colorado: New Mexico Inst. Mining and

Technology unpub. M.S. thesis, 65 p. 

Sawatzky, D. L., 1964, Structural geology of southeastern South Park, Park County,

Colorado: Mtn. Geologist, v. 1, no. 3, p. 133-139. 

___1967, Tectonic style along the Elkhorn thrust, eastern South Park and western

Front Range, Park County, Colorado: Colorado School Mines unpub. D. Sc. thesis,

206 p. 

Scott, G. R., and Taylor, R. B., 1974, Post-Paleocene Tertiary rocks and Quaternary

volcanic ash of the Wet Mountain Valley, Colorado: U.S. Geol. Survey Prof. Paper

868 (in press). 

Stark, J. T., Johnson, J. H., Behre, C. H., Jr., Powers, W. E., Rowland, A. L., Gould, D.

B., 1949, Geology and origin of South Park, Colorado: Geol. Soc. America Mem. 33,

188 p. 

Tobey, E. F., 1969, Geologic and petrologic relationships between the Thirtynine Mile

volcanic field and the Cripple Creek volcanic center: New Mexico Inst. Mining and

Technology unpub. M.S. thesis, 61 p. 

Van Alstine, R. E., 1969, Geology and mineral deposits of the Poncha Springs NE

quadrangle, Chaffee County, Colorado: U.S. Geol. Survey Prof. Paper 626, 52 p. 

Welsh, Fred, Jr., 1969, The geology of the Castle Rock area, Douglas County, Colorado:

Colorado School Mines unpub. M.S. thesis, 93 p. 

Wyckoff, B. S., 1969, Geology of the east side of the Guffey volcanic center (Oligocene),

Park County, Colorado: New Mexico Inst. Mining and Technology unpub. M.S.

thesis, 64 p.

 b U.S. GOVERNMENT PRINTING OFFICE: 1974-677-306/15