The Buffalo Peaks Andesite 

of Central Colorado

GEOLOGICAL SURVEY BULLETIN 1405-F

The Buffalo Peaks Andesite 

of Central Colorado

By GEORGE F. SANDERS, JR., GLENN R. SCOTT, 

and CHARLES W. NAESER

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1405-F

A restudy and adoption of a formal name 

for the andesite that was the subject of 

Bulletin 1 of the U.S. Geological Survey

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Sanders, George F.

The Buffalo Peaks Andesite of central Colorado.

(Contributions to stratigraphy) (Geological Survey Bulletin 1405-F)

Bibliography: p.

Supt.ofDocs.no.: I 19.3:1405-F

1. Andesite Colorado Park Co. 2. Andesite Colorado Chaffee Co. 3. AndesiteColorado Lake Co. 4. Geology, Stratigraphic Oligocene. 

I. Scott, Glenn Robert, 1918- joint author. II. Naeser, Charles W., joint author.

III. Title. IV. Series. V. Series: United States Geological Survey Bulletin 1405-F. 

QE75.B9 no. 1405-F [QE462.A5] 557.3'08s [552'.2] 75-619372

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

Washington, D. C. 20402 

Stock Number 024-001-02767-4
CONTENTS

Page

Abstract....................................................................................................................... Fl

Introduction and stratigraphy.................................................................................... 1

Description of the Buffalo Peaks Andesite................................................................. 5

Correlation and age.................................................................................................... 7

References cited........................................................................................................... 8

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of Buffalo Peaks Andesite.................................. F2

2. Composite columnar section of volcanic rocks at Buffalo Peaks........... 3

3. Photograph showing stratigraphic section of volcanic rocks on north

side of West Buffalo Peak.................................................................... 4

4. Photomicrograph of Buffalo Peaks Andesite.......................................... 6

TABLE

Page

TABLE 1. Fission-track ages of zircons.................................................................... F8

CONTRIBUTIONS TO STRATIGRAPHY

THE BUFFALO PEAKS ANDESITE 

OF CENTRAL COLORADO

By GEORGE F. SANDERS, JR., GLENN R. SCOTT, 

and CHARLES W. NAESER

ABSTRACT

The Buffalo Peaks Andesite of Oligocene(P) age caps East and West Buffalo Peaks 

near the south end of the Mosquito Range. The andesite is the uppermost unit in 

a fourfold volcanic sequence that lies in an early Oligocene paleovalley trending northeastward across the south end of the Mosquito Range. In ascending order the four units 

are the: (1) Badger Creek Tuff and underlying ash flow, (2) laharic breccia and tuff 

breccia, (3) air-fall tuff and ash-flow tuff, and (4) Buffalo Peaks Andesite. The Buffalo 

Peaks Andesite was first described at the Buffalo Peaks but later was found at East 

and West Hall Buttes in the southern part of South Park and in the Trout Creek paleovalley east of Buena Vista. The andesite is a gray porphyritic rock with phenocrysts 

of plagioclase, hornblende, and pyroxene. In thin section the pyroxene is shown to 

be both hypersthene and diopsidic augite. The groundmass is felty and contains microlites of plagioclase, hypersthene, and diopsidic augite. Glass constitutes less than 10 

percent of the groundmass. Most of the volcanic rocks on the Buffalo Peaks appear 

to be flows derived from a source to the west; however, some steeply dipping andesite 

layers on the north side of West Buffalo Peak suggest the possibility of a vent. Stratigraphic relations of the Buffalo Peaks Andesite suggest that it is Oligocene(P) in age.

INTRODUCTION AND STRATIGRAPHY

The Buffalo Peaks Andesite of Oligocene(P) age caps East and West 

Buffalo Peaks near the south end of the Mosquito Range (fig. 1), at the 

boundary between Chaffee and Park Counties, Colo. The andesite was 

first described by Whitman Cross and S. F. Emmons (in Cross, 1883) in 

Bulletin 1 of the U.S. Geological Survey. It was later named the Buffalo 

Peaks Andesite by Gould (1935, p. 990) for the type locality at Buffalo 

Peaks, and the name is here adopted.

The Buffalo Peaks Andesite is the uppermost unit in a fourfold 

volcanic sequence (figs. 2, 3) that lies in an early Oligocene paleovalley 

trending about N. 55° E. across the south end of the Mosquito Range. The 

andesite is preserved as erosional remnants at East and West Hall Buttes in 

southwestern South Park, at East and West Buffalo Peaks (fig. 1), and in 

the upper part of the Trout Creek paleovalley (fig. 1). In ascending order 

the four units in the volcanic sequence on the north side of West Buffalo 

Peak and on the east flanks of the Buffalo Peaks are the: (1) Badger Creek

Fl
F2

106° 15'

39° 1 5'

CONTRIBUTIONS TO STRATIGRAPHY

106° 105° 45'

39° 00'

38° 45'

COLORADO

MOSQUITO 

RANGE

IP 

O

MAP LOCATION

WEST BUFFALO PEAK

-EAST BUFFALO PEAK

WEST HALL BUTTE

Co ^-^ (^^^EAST HALL 

BUTTE

-O

7

10 20 MILES

10 20 KILOMETRES

FIGURE 1. Location of the Buffalo Peaks Andesite at Buffalo Peaks, Hall Buttes, and

Trout Creek.

Tuff and underlying latitic flow breccia or nonwelded ash flow, which 

total about 350 feet (107 m) thick; (2) laharic breccia, 225-310 feet (69-91 

m) thick, and tuff breccia, 113-185 feet (34-56 m) thick; (3) air-fall tuff and 

ash-flow tuff that show compound cooling and are about 340 feet (104 m) 

thick; and (4) Buffalo Peaks Andesite, 565 feet (172 m) thick but which has 

an eroded top.
BUFFALO PEAKS ANDESITE OF CENTRAL COLORADO F3

THICKNESS 

IN FEET 

(METRES)

Badger Creek Tuff 

31.6±3.1 m.y. 

33.0 ±4.0 m.y.

Floor of early Oligocene paleovalley

FIGURE 2. Composite columnar section of volcanic rocks at Buffalo Peaks. Age of the 

Badger Creek Tuff is from C. W. Naeser (unpub. data, 1974).
F4 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 3. Stratigraphic section of volcanic rocks on the north side of West Buffalo 

Peak. Units thin to the right near the north edge of the Buffalo Peaks paleovalley.

The Buffalo Peaks Andesite was first described at the Buffalo Peaks and 

was later mapped by Stark and others (1949) at the Hall Buttes about 4-7 

miles (6-11 km) east of the Buffalo Peaks, where it apparently also lies in 

the Buffalo Peaks paleovalley. At West Hall Butte they found the andesite 

to be 500-600 feet (150-180 m) thick and reported no volcanic breccias or 

ash-flow tuff beneath the Buffalo Peaks Andesite. They stated (1949, p. 

106) that the andesite was overlain by trachyte (now the Wall Mountain 

Tuff, Epis and Chapin, 1974).

The Hall Buttes area was mapped again by DeVoto (1971), who stated 

that the Buffalo Peaks Andesite overlay the Agate Creek Tuff (now the 

Wall Mountain Tuff) unconformably but underlay the lower volcanic 

conglomerate (now the Tallahassee Creek Conglomerate, Epis and 

Chapin, 1974). DeVoto also mapped as Buffalo Peaks Andesite outcrops 

in sec. 17, T. 13 S., R. 76 W. These outcrops were recently found by 

Richard B. Taylor (oral commun., 1974) and Glenn R. Scott to belaharic 

andesite conglomerate of the Tallahassee Creek Conglomerate. Taylor 

and Scott also determined that the Buffalo Peaks Andesite at East Hall 

Butte overlies both the Wall Mountain Tuff and the Tallahasee 

Creek Conglomerate.

A slightly different clinopyroxene-hypersthene-hornblende andesite 

flow, which we consider to be Buffalo Peaks Andesite, crops out in the
BUFFALO PEAKS ANDESITE OF CENTRAL COLORADO F5

Trout Creek paleovalley south of Trout Creek (fig. 1). The andesite near 

Trout Creek contains less hornblende than does the Buffalo Peaks 

Andesite; hence, it possibly correlates with the hornblende-poor upper 

part of the flow exposed at Buffalo Peaks. The andesite at Trout Creek 

contains a small amount of oxidized biotite. The flows at both Buffalo 

Peaks and Trout Creek overlie the Badger Creek Tuff. Thus, the andesite 

at Trout Creek possibly is from the same vent as the Buffalo Peaks Andesite. The Trout Creek paleovalley lies nearly 3,000 feet (900 m) lower than 

the Buffalo Peaks paleovalley; this disparity in altitude was apparently 

caused by downdropping along faults that trend northwestward between 

the two areas (Scott, 1975).

DESCRIPTION OF THE BUFFALO PEAKS ANDESITE

In hand specimen the andesite is a light- to dark-gray or brownish-gray 

porphyritic andesite with phenocrysts of plagioclase, hornblende, and 

pyroxene. The hornblende phenocrysts are especially prominent and are 

as much as 2 cm long. The Buffalo Peaks Andesite tends to break into hard 

resistant slabs that weather about 2-3 inches (5-6 cm) thick and 1 foot (0.3 

m) in diameter, and the fragments form a protective mantle of talus on the 

slopes below outcrops.

In thin section the rock is shown to be an augite-hypersthenehornblende andesite (fig. 4) having seriate porphyritic texture. It contains 

strongly to moderately zoned plagioclase that ranges in composition from 

sodic labradorite at the core to sodic andesine at the rim. The plagioclase 

grains have many inclusions of clear glass. Other phenocrysts are 

euhedral to subhedral diopsidic augite and hypersthene, euhedral hornblende, and opaque minerals, chiefly magnetite plus hematite in the more 

oxidized samples.

The majority of the large hornblende grains are oxyhornblende that is 

strongly pleochroic in reddish brown or tan and rimmed by dusty 

magnetite and fine-grained clinopyroxene, which are the products of latestage reaction with the melt. In one thin section from the upper part of the 

andesite sequence at the top of East Buffalo Peak, the hornblende is 

entirely resorbed, leaving only an outline of fine-grained pyroxene and 

dusty magnetite. The hornblende has been similarly altered to varying 

degrees throughout the sequence, with the least altered grains exhibiting 

tan to light- green pleochroism. Hornblende decreases in abundance from 

bottom to top of the formation.

The hypersthene shows slight to moderate, light-green to pink 

pleochroism; the colors are darker in the more oxidized samples. In 

several strongly oxidized samples, the rims of hypersthene are altered to 

hematite and other reaction minerals. Diopsidic augite is much less 

abundant than hypersthene and is frequently twinned on (100).

The groundmass is hyalopilitic to pilotaxitic (fig. 4) and is locally 

trachytic. Microlites include plagioclase, hypersthene, diopsidic augite 

and magnetite. Hornblende is lacking in the groundmass. There is
F6 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 4. Photomicrograph of Buffalo Peaks Andesite. P, plagioclase; ho, hornblende; 

H, hypersthene; da, diopsidic augite. Note reaction rim on hornblende and completely 

resorbed black hornblende crystal in center of field. Inclusions in plagioclase crystals are mainly blebs of clear glass. Pilotaxitic groundmass is composed of microlites of plagioclase, diopsidic augite, and magnetite.

slightly more plagioclase than pyroxene in both the phenocrysts and 

the groundmass, and the phenocrysts represent 30-40 percent of the 

total volume of the rock. The amount of clear to light-brown glass in 

the groundmass ranges from 0 to about 10 percent of the total volume.
BUFFALO PEAKS ANDESITE OF CENTRAL COLORADO F7

A chemical analysis of a sample of Buffalo Peaks Andesite, collected 

by S. F. Emmons in 1880 from the northeast shoulder of the Buffalo 

Peaks, is given as follows (specific gravity 2.742):

[From Cross, 1883]

Weight 

percent

SiO2r ............................................................................ 56.190

A1263 ........................................................................... 16.117

Fe2Ov .......................................................................... 4.919

FeO............................................................................. 4.433

MnO ......................................................................... Trace.

CaO............................................................................. 6.996

BaO........................................................................... Trace.

SrO............................................................................ Trace.

MgO............................................................................ 4.601

NajO........................................................................... 2.961

KZO............................................................................. 2.368

H2O............................................................................. 1.028

P2OS ............................................................................ .266

Cl................................................................................ .022

99.901

At the type locality the lower contact of the Buffalo Peaks Andesite 

is disconformable on a pyroxene-hornblende andesite ash-flow unit. 

Generally, the lower part of the andesite is marked by a reddish-brown 

breccia containing fragments of moderately welded red and black ashflow tuff derived from the underlying ash-flow layers. At this locality 

the lower part of the unit also contains broken fragments of Buffalo 

Peaks Andesite in a tuffaceous andesitic groundmass with considerable 

light-brown glass. The lower breccia, which also was reported by Stark 

and others (1949) at West Hall Butte, and some steeply dipping andesite 

layers immediately above the breccia on the north side of West Buffalo 

Peak suggest the possibility of a vent beneath West Buffalo Peak. The top 

of the andesite is everywhere eroded.

CORRELATION AND AGE

We believe that the Buffalo Peaks Andesite is Oligocene(?) in age. At 

Hall Buttes the andesite overlies the Wall Mountain Tuff from which 

biotite was age dated using the potassium-argon method at 35.7±0.8 m.y. 

(million years) (J. D. Obradovich, in Epis and Chapin, 1974). On the east 

flank of the Buffalo Peaks (NE1^ sec. 13, T. 12 S., R. 78 W.), the Badger 

Creek Tuff lies in the lower part of the volcanic sequence far below the 

Buffalo Peaks Andesite. (See fig. 2.) The Badger Creek Tuff was age dated 

using fission tracks in zircon at 31.6±3.1 m.y. and later at 33.0±4.0 m.y. (C. 

W. Naeser, U.S. Geological Survey, unpub. data, 1974 and 1975).
F8 CONTRIBUTIONS TO STRATIGRAPHY

Zircons separated from the Buffalo Peaks Andes! te have been dated 

twice by the fission-track method. The results are shown in table 1. The 

first determination gave an age of 37.1±3.4 m.y., and the second determination gave an age of 34.0±3.5 m.y. Both of these ages are older 

than, but within the analytical uncertainty of, die 31.6±3.1 m.y. and 

the 33.0±4.0 m.y. ages determined on the geologically older Badger 

Creek Tuff.

The Badger Creek Tuff and the Buffalo Peaks Andesite must have 

been erupted almost concurrently, probably about 34 m.y. ago.

TABLE 1. Fission-track ages of zircons

10-" yr~'. Value in parentheses represents the number of fossil tracks or induced tracks counted 

in determining age]

Sample 

No.

Number of 

fossil 

tracks per 

cm2 (x 106 )

Number of 

induced 

tracks per 

cm2 (x 106)

6 

(neutron per 

cm2 x 10 15 )

Age 

(x 106 yr 

±2cr)

Uranium 

(ppm)

Zircons from Buffalo Peaks Andesite

75-S-2 ......... 3.16

(893)

nK. CO Q CD

(829)

7 51

(1,060)

(691)

1.44 37.H3.4 160

.93 34.013.5 200

Zircons from Badger Creek Tuff

73-S-S

2.98

(759) 

2.36 

(644)

6.80 

(866) 

3.35

(457)

1.18

7.65

31.6±3.1 

33.014.0

175

130

REFERENCES CITED

Cross, Whitman, 1883, On hypersthene-andesite and on triclinic pyroxene in augitic

rocks, with a Geological sketch of Buffalo Peaks, Colorado, by S. F. Emmons: U.S.

Geol. Survey Bull. 1, 42 p. 

DeVoto, R. H., 1971, Geologic history of South Park and geology of the Antero Reservoir

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

Epis, R. C., and Chapin, C. E., 1974, Stratigraphic nomenclature of the Thirtynine

Mile volcanic field, central Colorado: U.S. Geol. Survey Bull. 1395-C, 23 p. 

Gould, D. B., 1935, Stratigraphy and structure of Pennsylvanian and Permian rocks

in Salt Creek area, Mosquito Range, Colorado: Am. Assoc. Petroleum Geologists

Bull., v. 19, no. 7, p. 971-1009. 

Scott, G. R., 1975, Reconnaissance geologic map of the Buena Vista quadrangle, Chaffee

and Park Counties, Colorado: U.S. Geol. Survey Misc. Field Studies Map MF-657. 

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

D. B., and others, 1949, Geology and origin of South Park, Colorado: Geol. Soc.

America Mem. 33, 188 p.

<TU.S. GOVERNMENT PRINTING OFFICE: 1975-677-340/45