The geology of the northern flank of the upper Centennial Valley, Beaverhead and Madison counties,
Montana
by Matthew Lee Mannick
A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE
in Earth Science
Montana State University
© Copyright by Matthew Lee Mannick (1980)
Abstract:
The Huckleberry Ridge Tuff of the Yellowstone Group covers a large portion of the north side of the
upper or eastern Centennial Valley, southwestern Montana. This ignimbrite is the result of the first
major eruptive event (2.0 m.y.) of the Yellowstone caldera. The tuff forms dip slopes on the north flank
of the valley lapping eastward against Precambrian rocks near Elk Lake and to the northwest along the
West Fork of the Madison River.
The Huckleberry Ridge Tuff in the Centennial area is a composite ash flow sheet composed of two
distinct cooling units, each displaying vertical zonation of welding density and phenocryst content.
The upper Centennial Valley is influenced by four major structures: the Madison, Gravelly, and
Centennial range front, and Cliff Lake faults. The upper Centennial Valley resulted from the down
dropping of a half graben against the Centennial Fault along the Gravelly Range Front and Cliff Lake
faults. Observations of Huckleberry Ridge Tuff displacement indicate a minimum post Huckleberry
Ridge Tuff offset along the Centennial Fault of 5000 to 6000 feet (1525 to 1830 m) out of a total of
approximately 10,000 feet (3050 m).
Variations in depositional thicknesses of the Huckleberry Ridge Tuff indicate a paleotopography which
included a major river valley (the ancestral Madison River) that entered the upper Centennial Valley
from the north. This drainage can be traced northward into the Madison Valley where it joins the
present-day Madison River.
Warm spring activity is present along the Gravelly Range Front Fault and may exist along the Cliff
Lake Fault.
A potential geothermal reservoir of substantial dimensions may exist in the channel gravels of the
ancestral Madison River in the vicinity of the Cliff Lake Fault beneath the Huckleberry Ridge Tuff. 
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Signature
Date
THE GEOLOGY OF THE NORTHERN FLANK OF THE UPPER CENTENNIAL 
VALLEY, BEAVERHEAD AND MADISON COUNTIES, MONTANA
■y </ 't
by
MATTHEW LEE MANNICK
A thesis submitted in partial fulfillment 
of the requirements for the degree
of
MASTER OF SCIENCE 
in
Earth Science
Approved:
Q\.
Chairperson, Graduate Committee 
Head, MajorQjWartment 
Graduate Dean
MONTANA STATE UNIVERSITY 
Bozeman, Montana
March, 1980
iii
ACKNOWLEDGMENT
Appreciation is extended to the Montana Bureau of 
Mines and Geology for providing the grant which made this 
study possible and to Dr. John Sonderegger of the Bureau 
for help in both the field and office.
The writer is grateful to Dr. Robert Chadwick 
(Committee Chairman) for providing guidance and counsel 
and to Dr. John Montagne and Dr. Donald Smith for their 
suggestions and criticisms. The author wishes to express 
special thanks to Professor Robert Taylor for assistance 
with drafting problems.
TABLE OF CONTENTS
Page
VITA . ..................................................ii
ACKNOWLEDGMENT..........................   iii
LIST OF ILLUSTRATIONS. ......................   vi
ABSTRACT............................................. Viii
INTRODUCTION ........................................  I
Location..............
Purpose ..............
Previous Investigations 
Procedure ............
ROCK UNITS........................................... 7
Precambrian Rocks ..............................  7
Paleozoic and Mesozoic Rocks....................  7
Cenogoic Nonvolcanic Rocks......................... 10
Cenozoic Volcanic Rocks ........................  13
STRUCTURE........................    33
GEOMORPHOLOGY........................................... 47
PRECENOZOIC GEOLOGIC HISTORY ........................   55
Precambrian....................   55
Paleozoic..................  56
Mesozoic........................................... 57
CENOZOIC GEOLOGIC HISTORY............................... 60
Paleocene......................................... 60
Eocene................ 60
Oligocene..................................   61
Miocene................................  62
Pliocene........................................... 62
Quaternary......................................... 64
GEOTHERMAL POTENTIAL ..................  . . . . . . .  67
CT
l 
rf
* 
I-
* 
I-
*
VPage
REFERENCES CITED.......... .. . ....................... 70
APPENDICES.............................................74
I. Huckleberry Ridge Tuff Measured
Sections.................................... 75
II. Potassium-Argon Age Dates...................... 83
III. Chemical Analyses of Rhyolite Tuffs
and Basalt..........    86
LIST OF ILLUSTRATIONS
Figure Page
1. Index map ........................  . . . . . .  2
2. Location of map area............................. 3
3. Precambrian rocks of the upper
Centennial Valley ........................  . 8
4. Paleozoic stratigraphy of the
Centennial region . . ......................  9
5. Mesozoic stratigraphy of the
Centennial region ..........................  11
6. Huckleberry Ridge Tuff measured section . . . . 18
7. Welding zones of the Huckleberry
Ridge Tuff................ 21
8. Photomicrographs of the Huckleberry
Ridge Tuff..........................   22
9. Correlation of the Huckleberry
Ridge Tuff..........................  28
10. Measured section location map ................  29
11. Regional distribution of the Huckleberry
Ridge Tuff. . ............  30
12. Cross-section location map....................  35
13. East-west cross-section of
Centennial region ..........................  36
14. Structural block diagram of the upper
Centennial region ..........................  38
15. North-south cross-section of the upper
Centennial Valley ..............    40
16. Photo of glacial valleys on Centennial
Range 42
vii
Figure Page
/
17. Profile of hanging glacial valleys. . . . . . . 43
18. Structure map of the Qhi top, upper
Centennial Valley and vicinity. . ..........  46
19. Diagram showing formation of dip slopes 
and mechanism of mass wasting along 
scarp slopes of the Huckleberry Ridge Tuff. . 49
20. Isopach map of the Huckleberry
Ridge Tuff............................ .. 50
21. 2.0 m.y. paleotopography of the
Centennial region ................  ........ 52
viii
ABSTRACT
The Huckleberry Ridge Tuff of the Yellowstone Group 
covers a large portion of the north side of the upper or 
eastern Centennial Valley, southwestern Montana. This 
ignimbrite is the result of the first major eruptive event 
(2.0 m.y.) of the Yellowstone caldera. The tuff forms dip 
slopes on the north flank of the valley lapping eastward 
against Precambrian rocks near Elk Lake and to the northwest 
along the West Fork of the Madison River.
The Huckleberry Ridge Tuff in the Centennial area is a 
composite ash flow sheet composed of two distinct cooling 
units, each displaying vertical zonation of welding density 
and phenocryst content.
The upper Centennial Valley is influenced by four major 
structures: the Madison, Gravelly, and Centennial range
front, and Cliff Lake faults. The upper Centennial Valley 
resulted from the down dropping of a half graben against 
the Centennial Fault along the Gravelly Range Front and 
Cliff Lake faults. Observations of Huckleberry Ridge Tuff 
displacement indicate a minimum post Huckleberry Ridge Tuff 
offset along the Centennial Fault of 5000 to 6000 feet 
(1525 to 1830 m) out of a total of approximately 10,000 feet 
(3050 m).
Variations in depositional thicknesses of the Huckle­
berry Ridge Tuff indicate a paleotopography which included 
a major river valley (the ancestral Madison River) that 
entered the upper Centennial Valley from the north. This 
drainage can be traced northward into the Madison Valley 
where it joins the present-day Madison River.
Warm spring activity is present along the Gravelly 
Range Front Fault and may exist along the Cliff Lake Fault.
A potential geothermal reservoir of substantial dimensions 
may exist in the channel gravels of the ancestral Madison 
River in the vicinity of the Cliff Lake Fault beneath the 
Huckleberry Ridge Tuff.
INTRODUCTION
Location
The study area is located just north of the Montana- 
Idaho border about 43 kilometers west of Yellowstone 
National Park (Fig. I).
An area encompassing approximately H O  square kilom­
eters located on the north slope of the eastern or upper 
Centennial Valley (southeasternmost Gravelly Range, Figure 
2) was mapped. Data collected from the map area were used 
to interpret the geology of the Centennial region (study 
area in Figure I).
The map area extends northward and westward to the 
boundary of the Upper Red Rock Lake Quadrangle (Fig. 2) 
and eastward to the Pleistocene-Precambrian contact east 
of Elk, Hidden, and Cliff lakes. The southern boundary 
coincides with the northern edge of the U.S.G.S. geologic 
map of the southern part of the Upper Red Rock Lake Quad­
rangle (Witkind, 1976).
Purpose
This study was done in conjunction with a Montana 
Bureau of Mines and Geology evaluation of the geothermal 
potential in the Centennial and Madison River valleys. 
Funding was provided by the Bureau for this paper on the
2Three
Forks
Butte •
•  Bozeman
Ennis
i / *  Dillon
Yellowstone
National
Park
Idaho
Falls
O IO 20  30  km.
FIGURE I. Index ma p
3.
..
0
1
O  /‘"“'I,
i,41
# /I'
FIGURE 2. Location of map area.
4upper Centennial Valley. The structure and volcanic 
stratigraphy were examined to assist the Bureau in its 
evaluation of the geothermal systems in the upper Centen­
nial Valley. This evaluation was prompted by the close 
proximity of the Island Park geothermal field and the pres­
ence of warm springs in the Centennial Valley region 
adjacent to the study area.
Previous Investigations
The Centennial Range and Valley have been examined in 
little detail previous to this investigation.
Honkala (1949) did the first work on the stratigraphy 
and general geology of the Centennial region and published 
additional papers in 1954 and 1960 on the general geology 
and major structures of the Centennial Mountains and vicin­
ity. These investigations consisted of reconnaissance work 
generally lacking detail especially when dealing with the 
volcanic rocks of the region.
The stratigraphy and structure of the Gravelly Range 
north of the Centennial Valley (Fig. 2) was studied by 
Mann (1960).
A geologic map of the southern part of the Upper Red 
Rock Lake Quadrangle, southwestern Montana and adjacent 
Idaho (Fig. 2) was produced by Witkind in 1976. The map
5covered a portion of the Centennial Range as well as the 
floor of the upper Centennial Valley.
Bailey (1977) conducted a geophysical investigation of 
the seismicity and contemporary tectonics of the Hebgen 
Lake-Centennial Valley, Montana area.
A portion of the Upper Madison Valley was mapped and 
evaluated for geothermal potential by Weinheimer (1977 and 
1979). This study covered a large part of the Cliff Lake 
Quadrangle, which is adjacent to and north of the Upper Red 
Rock Lake Quadrangle (Fig. 2).
The Montana Bureau of Mines and Geology is currently 
investigating the geology and geothermal potential of the 
Centennial Valley. Dr. John Sonderegger of the Bureau's 
hydrology division has been mapping and evaluating geo­
thermal potential in the northern part of the Lower Red 
Rock Lake Quadrangle (Fig. 2). The Precambrian rocks along 
the northern flank of the Centennial Valley have been 
investigated by Dr. Richard Berg of the Bureau. James 
Schofield conducted a gravity and magnetics study of the 
Centennial Valley (Upper and Lower Red Rock Lake Quadran­
gles, Figure 2) as part of a Master of Science degree from 
the Montana College of Mineral Science and Technology.
6Procedure
Field studies of the northern flank of the upper Cen­
tennial Valley were undertaken during the summers of 1978 
and 1979. Field work consisted of geologic mapping using 
pace and compass methods aided by air photos. A topographic 
map with a scale of 1:24000 and contour interval of 20 feet 
was used as a base for the geologic map.
The volcanic rocks of the area were studied in detail. 
Volcanic sections were measured using a Brunton compass and 
Jacob staff and were sampled where lithologies changed.
Thin sections were prepared and studied under the petro­
graphic microscope, and correlations of cooling units and 
the welding variations within them were made from the data. 
Cross sections were constructed based on interpretations 
made during the study. Volcanic rocks were sampled at 
several locations and dated radiometrically (Appendix II) 
and chemically analyzed (Appendix III). The K-Ar analyses 
were done at the University of Utah, Department of Geology, 
and funded by the Montana Bureau of Mines and Geology and 
the chemical analyses were completed by the Bureau.
ROCK UNITS
Precambrian Rocks
The Precambrian rocks of the upper Centennial region 
consist of pre-Belt rocks of Precambrian X age (Witkind, 
1976). These rocks are exposed along the Centennial Range 
on the south side of the valley, to the east of Elk, Hidden, 
and Cliff lakes and west of the West Fork of the Madison 
River on the north side of the valley (Plate I).
The Precambrian rocks of the Centennial region consist 
of low to medium grade metamorphics. These rocks include: 
gabbro, metadolomite, quartzite, metagranodiorite, amphib­
olite, and mica schist (Fig. 3). The prominent strike of 
the foliation is northeast (Witkind, 1972 and 1976).
Paleozoic and Mesozoic Rocks
The Paleozoic and Mesozoic sedimentary sequences are 
not exposed along the north side of the upper Centennial 
Valley. The sections may be present at least in part west 
of Elk and Hidden lakes where they may have been down- 
faulted and preserved by burial by the Huckleberry Ridge 
Tuff.
The Paleozoic rocks exposed along the scarp slope of 
the Centennial Range include marine sediments of the Cam­
brian through the Permian excluding the Silurian Period
8ROCK TYPE DESCRIPTION '
Gabbro Dark-gray, nonfoliate, coarse-grained; com­
monly equigranular but locally porphyroblast- 
Ic with large labradorite metacryats. Common 
minerals are labradorite (An^,), hornblende, 
and quartz. '
Metadoloralte Light-brown to light-gray, foliated, thick- 
bedded to massive, and coarsely crystalline. 
Contains abundant thin to thick ouartz beds.
Quartzite White, light-gray, green, foliate, thin- to 
thick-bedded, strongly .micaceous, medium- to 
coarse-grained, and equigranular. Bimodal: ■ 
mostly quartz and muscovite but locally con- 
talnsminor amounts of microcline, opaque 
iron minerals, sericite, and chlorite.
Metagranodlorlte Light-gray to gray, foliate, fine- to very 
coarse-grained, and equigranular. Common 
minerals are potassic feldspar, hornblende, 
apatite, sphene, epidote, and opaque iron 
minerals. Alteration products include ser­
icite and chlorite.
Amphibolite Dark-gray, strongly foliate, very fine-grain­
ed to fine-grained; banded with irregularly 
alternating laminae of hornblende and ouartz- 
plagioclaee. Minor constituents include 
biotite, apatite, sphene, epidote, zircon, 
and opaque iron minerals. Alteration pro­
ducts include chlorite, sericite, and cal- 
cite.
Mica Schist Dark-gray to brown, fine- to medium-grained; 
strongly micaceous; foliate. Major con­
stituents are biotite, quartz, and potassic 
and plagioclase feldspar. Garnet and staur- 
olite metacrysts are locally common. Acces­
sory minerals include apatite, sphene, zir­
con, epidote, tremolite, and opaque iron 
minerals. Common alteration products are 
sericite, chlorite, and bleached biotite.
FIGURE 3. Precambrian rocks of the upper Centennial Valley
9S
y
s
t
e
m
i S
e
r
i
e
s
G
r
o
u
p
Formation Description
P
e
r
m
i
a
n Lower
Permian
Phosphoria 
Pm. and 
Related 
Permian 
Rocks
Interfingering units belonging to the Shed- 
horn Sandstone, Phosphoria Formation, & Park 
City Formation composed of bedded chert, ool­
itic phosphatlc rock, phosphatlc shale, and 
quartzose sandstones; Carbonate beds at base.
P
e
n
n
s
y
l
v
a
n
i
a
n
Late
Miss.
and
Penn.
Quadrant
Sandstone
Light-brown, fine-grained, quartzose sand­
stone, locally quartzose; composed largely 
of angular to subangular quartz grains 
locally cemented by silica. Contains thin 
lenticular, light-gray, dense, sandy dolomite 
beds, I.? to 1,5 meters thick, interbedded 
in upper and basal strata.
Ansden
Formation
Light-gray, medium-bedded to very thick- 
bedded locally massive, coarsely crystalline 
dolomite; 5 to 10 cm thick lenses and beds 
of chert are common.
M
i
s
s
i
s
-
s
l
p
p
i
a
n
Lower
Miss.
M
a
d
i
s
o
n
Mission 
Canyon L s .
Fluieh-gray, thick-bedded to massive, dense 
limestone beds.
Lodgepole
Limestone
Bluish-gray thin- to medium-bedded limestone 
that is very fosslliferous and contains 
much bedded chert.
D
e
v
o
n
i
a
n
Vpper
Dev.
Three
Forks
Formation
Shaly, light-brown to yellow, locally pale- 
reddish-brown, calcareous thin-bedded 
siltstone and sandstone.
Jefferson
Formation
Upper member is light-tan thin-bedded 
dolomite. Lower member is grayish-brown, 
dense, medium-bedded to massive, vuggy dolo­
mite; locally contains scattered angular 
fragments of white to gray chert.
i l Upper 
Ord.
Bighorn
Dolomite
Light-gray dolomite in even thin beds 2 to 
5 cm thick; faint laminae paralleling the 
bedding.
C
a
m
b
r
i
a
n
Upper
Cam­
brian
Pilgrim
Dolomite
Light-brown to light-gray, thin- and even- 
bedded, platy dolomite; nodulai-weathered 
surface sparse glauconite grains.
Middle
Cam­
brian
Park
Shale
Greenish-gray to grayish-red, even-bedded, 
fissile shale; breaks into minute angular 
fragments.
Meagher
Linrstone
Light-gray to gray even-bedded thin-bedded 
limestone; weathers to a crenulated nodular 
surface. Contains grains and pebbles of 
Precambrian rocks in basal strata.
Flathead
Sandstone
Reddish-brown, medium-bedded to massive, 
crossbedded, fine- to coarse-grained, 
friable sandstone. Locally contains angular 
to rounded pebbles and cobbles of 
Precambrian rocks in basal strata.
FIGURE 4. Paleozoic stratigraphy of the Centennial region
10
The Mesozoic rocks exposed in the area are of terres­
trial as well as marine origin. These include salt and 
pepper sandstone, quartz sandstone, limestone, claystone, 
shale, and siltstone as well as reddish and brownish sand­
stone, siltstone, and claystone (Fig. 5).
Cenozoic Nonvolcanic Rocks
The nonvolcanic rocks of Cenozoic age in the study area 
are primarily of fluvial and lacustrine origin. These rocks 
unconformably overlie rocks of Precambrian, Paleozoic, and 
Mesozoic age in the Centennial region. Cenozoic nonvolcanic 
rocks both overlie and underlie Pleistocene volcanics uncon­
formably in the region (Plate I). Several of the following 
rock descriptions were taken from Witkind (1976).
Tertiary limestone —  Pale yellow-brown to light gray, thin 
to medium bedded, finely crystalline limestone. Unit 
protrudes up through Pleistocene volcanics to form 
patchy knobs along the northern flank of the upper 
Centennial Valley adjacent to the valley fill sedi­
ments .
Quaternary lacustrine deposits —  Light brown to brown, 
well sorted, unconsolidated silt and sand.
(Fig. 4).
11
S
y
s
t
e
m
S
e
r
i
e
s
G
r
o
u
p
F ormation Description
C
r
e
t
a
c
e
o
u
s
Lower
Cret.
Kootenai
Formation
Consists of a basal conglomerate and 
conglomeratic "salt and peper" sandstone; 
a middle light-gray marly limestone and 
claystone unit; and an upper light-gray 
marly Iimestone-claystone bed, containing 
coiled gastropod molds.
T
r
l
a
s
s
l
c
U pper
Jur.
Morrison
Formation
Thin quartzose sandstone beds interlayered 
with a claystone-siltstone sequence. Sand­
stone is light-brown, thin- to medium- 
bedded, lenticular, crossbedded, fine­
grained, and friable. Claystone-siltstone 
sequence consists of varigated grayish- 
green to pale-red beds.
U p ­
per
and
Mid­
dle
Jur-
issic
CO
•H
H
iH
W
Swift
Formation
Grayish-brown to greenish-gray, sandy, 
oolitic limestone, thin- to medium-bedded, 
and crossbedded; rich in shell fragments 
and rounded chert grains.
Rlerdon
Formation
Light-gray marly limestone that is locally 
a calcareous claystone.
Sawtooth
Formation
Light-gray, oolitic, thin-bedded limestone 
containing a few thin light-gray claystone 
interbeds; fossiliferous, rich in distinct­
ive star-shaped crinoid plates.
T
r
i
a
s
s
i
c
Lower
Trias-
sic
Thaynes
Formation
Light-brown thin-bedded limestone; breaks 
into platy fragments; interbedded chert 
layers 5 to 10 cm thick.
Voodside
Formation
Reddish-brown, thin-bedded, fine-grained 
sandstone and shaly siltstone, locally 
platy; ripple marked.
Dinwoody
Formation
Light-brown to light-gray and yellowish- 
gray thin- to medium-bedded limestone and 
calcareous siltstone; p l a t y , even-bedded 
with many dark-gray shale interbeds; 
locally fossiliferous (linguloid brachlo- 
pods and pelecypods).
FIGURE 5. Mesozoic stratigraphy of the Centennial region.
12
Quaternary dune sand —  Brown, unconsolidated well-sorted 
quartz sand; frosted angular to subrounded grains.
Quaternary alluvium (Holocene) —  Unconsolidated fluvial 
deposits of silt, sand, and gravel.
Quaternary colluvium (Holocene) —  Unconsolidated rubble 
along the lower portions of steep slopes.
Quaternary displaced Huckleberry Ridge Tuff —  Unconsoli­
dated rubble ranging up to boulder size. Formed by the 
mass wasting of cliffs composed of Huckleberry Ridge 
Tuff. Large sections of tuff separate from and accu­
mulate at the base of the cliff creating a hummocky 
topography.
Quaternary displaced basalt —  Unconsolidated boulders of 
basalt found along slopes near the West Fork of the 
Madison River.
Quaternary sand and gravel deposit —  Unconsolidated poorly 
sorted sand, gravel, cobbles, and boulders that form 
low mounds or cap small hills. Deposits near Elk 
Springs may represent material left at the outlet of a 
former glacial lake in the Centennial Valley as the 
lake drained northeastward to join the Madison River 
via the Cliff Lake Fault trench.
Quaternary deposit of coalesced old alluvial fans —  Low
13
broad lobate deposit resulting from the coalescence of 
several old alluvial fans. Composed of unconsolidated 
fluvial silt, gravel, cobbles, and a few boulders. 
Coarser material near apexes of fans grades into finer 
grained materials, such as silt and sand, near distal 
edges. Probably formed before or during the life of 
the glacial lake.
Quaternary young alluvial fan deposit —  Low lobate deposit 
of unconsolidated moderately well-sorted silt, sand, 
gravel, and cobbles at mouths of streams, most of which 
empty into the Centennial Valley. Constituent rock 
types reflect bedrock exposed along streams. Precam- 
brian crystalline and Cenozoic volcanic rocks predom­
inate along the north side of the Centennial Valley. 
Probably formed after glacial lake had disappeared. 
Quaternary travertine —  Light gray compact travertine
found on east shore of Elk Lake (Berg, Personal Com­
munication, 1979).
Cenozoic Volcanic Rocks
The Cenozoic volcanic rocks of the upper Centennial 
Valley include localized basalts of both Tertiary and 
Pleistocene age as well as Pleistocene welded tuffs which 
cover extensive portions of the north flank of the valley
14
(Plate I).
Quaternary-Tertiary basalt —  Dark gray to black,,dense, 
fine-grained extrusive rock. Locally vesicular, it 
contains abundant magnetite with sparse olivine 
phenocrysts. Primarily exposed as thin flows which 
commonly display columnar jointing. This rock is 
petrographically similar to the basalt near Elk Lake 
(Pleistocene basalt) which has been dated at 2.38 
million years. These two basalts may be the same age 
and may have originated from the same magmatic source 
at depth.
Pleistocene basalt —  Dark gray to black, dense, fine­
grained extrusive rock. Contains sparse olivine 
phenocrysts and is locally vesicular. This basalt has 
been K-Ar dated at 2.38± 0.44 million years by whole 
rock analysis (Appendix II, C).
This unit is exposed only at the south end of Elk 
Lake where its depositional relationship with that of 
the Huckleberry Ridge Tuff is questionable. The basalt 
occurs as boulder piles and is not exposed in place.
The unit appears to have either cut through and flowed 
over the Huckleberry Ridge Tuff or cut through and 
flowed over the first cooling unit of the tuff. The
15
basalt appears to have risen from a magma source, 
possibly related to the Snake River Plain-Yellowstone 
volcanic system, via the deep-seated Cliff Lake Fault 
(Plate I). A similar basalt dated at slightly less 
than 2.4 million years is known to have originated from 
this volcanic system in Yellowstone National Park 40 
miles (64 km) to the east of Elk Lake (Christiansen and 
Blank, 1972).
Pleistocene Huckleberry Ridge Tuff —  The Huckleberry Ridge 
Tuff, which originated from the Yellowstone caldera, is 
the oldest (2.0 m.y.) of the three formations that 
comprise the Yellowstone Group volcanics of Chris­
tiansen and Blank (1972).
The tuff found in the upper Centennial Valley is 
petrographically similar to that of the type section of 
the Huckleberry Ridge Tuff (Christiansen and Blank, 
1972). The Huckleberry Ridge Tuff has been subdivided 
into three members (cooling units of a compound ash 
flow sheet) at the type section (Christiansen and 
Blank, 1972), two of which are present in the upper 
Centennial Valley. The lower cooling unit (Qhi) in the 
Centennial Valley can be correlated with the lower 
member described at the type section and the upper
16
cooling unit (Qha) with the middle member. The lower 
member at the type section contains a thick zone of 
dense welding beneath a zone of partial welding. 
Phenocrysts decrease in number toward the top of this 
member. The lower cooling unit of the tuff in the 
Centennial region contains a smaller percentage of 
phenocrysts overall but a similar pattern of welding 
and phenocryst distribution. The upper cooling unit in 
the Centennial region is similar to the middle member 
of the type section with a phenocryst poor zone near 
the base of the unit. Phenocrysts increase in abun­
dance and size toward the top of the second member of 
the tuff at both locations. The third and uppermost 
member of the tuff described at the type section occurs 
only south of central Yellowstone National Park 
(Christiansen and Blank, 1972).
K-Ar dates of around 2.0 million years have been 
attained from the tuff at the following locations 
(Appendix II, A,B,D,E): I mile NNW of Hidden Lake,
the south end of Elk Lake, the cliff face on the west 
bank of the Madison River 2 km north of the mouth of 
Wall Canyon, and the northeast slope of Flatiron 
Mountain.
17
The description (prepared using work of Smith,
1960 as a guide) which follows represents the Huckle­
berry Ridge Tuff sequence located I mile NNW of Hidden 
Lake (Figs. 6 & 7) . A chill zone is present as float 
at the base of the outcrop (basal Qhi). The tuff of 
the chill zone is vitric, partially welded, and con­
tains 3 to 5% sanidine with trace amounts of quartz and 
olivine phenocrysts all of which range up to 3 mm in 
diameter. The olivine phenocrysts appear in these 
pyroclastic rocks of rhyolitic composition as the 
result of the partial mixing of the bimodal magmatic 
system beneath the Yellowstone region at the time of 
eruption. The bimodal system consists of basaltic 
material beneath magmas of rhyolitic composition 
(Struhsacker, 1978). Above the basal chill zone is a 
zone of dense welding 75 meters thick, in which devit- 
rified shards are extremely flattened and deformed 
(Fig. 8, Photomicrograph A). This portion of the tuff 
contains 3 to 5% sanidine phenocrysts at the base of 
the zone which decrease upward to I to 2% near the top. 
Phenocryst size also varies vertically from an average 
diameter of 1.5 mm near the base to 0.5 mm at the top. 
The zone of dense welding grades upward into a zone of
18
FIGURE 6. Huckleberry Ridge Tuff measured section located I mile NtM 
of Hidden Lake (NW%,NE%, sec 4, T13S, RlE).
142 460 
139 450 
136 440 
133 430 
130 420 
127 410
124 400 
120 390 
117 380 
114 370 
111 360 
108 350 
105 340 
102 330
99 320 
(m) (ft)
Light-gray, punky, nonwelded tuff, contains 
10% sanidine &  5% quartz phenocrysts (.5-2 mn 
diam.), and slightly flattened pumice. Weathers 
to li^it-pink ash.
Light-gray, massive, porphyritic tuff. Contains 
20-25% sanidine & 1% quartz phenocrysts (1-4 mn 
diam.).
Medium-gray, porphyritic welded tuff. Contains 
flattened pumice altered to clay, 10% sanidine, 
and 5% quartz phenocrysts (1-4 mn diam.).
Dark-gray, massive, porphyritic welded tuff. 
Contains 3% sanidine &  1% quartz phenocrysts 
(.5-1 mn diam.).
Gray-brown, massive, porphyritic welded tuff 
with crude concoidal fracturing. Contains 
10-15% sanidine & 1% quartz phenocrysts 
(.5 mn diam.).
Chill zone 
Ash bed
19
70 230
67 220
64 210
61 200
58 190
55 180
52 170
49 160
Dark-gray to black, lithophysal welded tuff. 
Contains 1% sanidine phenocrysts (.2 irm diam.) 
and spherical vugs filled with secondary feld­
spar altered to clay.
Reddish-brown welded tuff. Contains YL 
sanidine phenocrysts (Irm diam.), flattened 
pumice fragments, and spherical pockets coated 
with feldspar altered to clay.
Medium-gray, porphyritic welded tuff. Contains 
TL sanidine phenocrysts (.5 mn diam.) and 
minor vugs filled with feldspar crystals. 
Weathers to ligjit-gray.
Medium to dark-gray, porphyritic, flow banded 
welded tuff. Contains 57« sanidine phenocrysts 
(1.5 ran diam.).
(m) (ft)
20
43 140 
40 130 
37 120 
34 H O  _  
30 100 
27 90
24 80
21 70
18 60 
15 50
12 40
9 30
6 20
3 10
0' O
(m) (ft)
Dark-gray, porphyritic, flow banded welded tuff. 
Contains 57«, sanidine phenocrysts (1.5 ran diam.).
Black, dense, porphyritic, vitric welded tuff 
reserrbling obsidian. Contains 5% sanidine 
phenocrysts (1-3 ran diam.) and rock fragments. 
This unit seen only at base of cliff.
21
LEGENDPhotomicrograph F
Photomicrograph E Non-welded ash
Partially welded tuff
Densely welded tuff
Photomicrograph C
Chill zone
Photomicrograph A
FIGURE 7. Zonal variations in welding density of the 
Huckleberry Ridge Tuff with photomicrograph 
locations. (NW%,NE%, Sec.4, T13S,RlE).
22
FIGURE 8. Photomicrographs of the Huckleberry Ridge Tuff.
The photomicrographs were taken at 35 X power 
under plane light with a scale of I mm indicated 
on the photo. (See Figure 7 for sample loca­
tions) .
PHOTOMICROGRAPH A —  Photo A displays the zone of dense 
welding in the lower cooling unit (Qhi) of the 
Huckleberry Ridge Tuff. Note the extreme flat­
tening and deformation of shards due to compaction 
under high temperatures. The shards comprising 
the ground mass have been devitrified.
PHOTOMICROGRAPH B —  Photo B displays the zone of
partial welding in the lower cooling unit (Qh1). 
Note that the shards are easily recongizable and 
only moderately deformed due to compaction under 
moderately high temperatures and are devitrified. 
Also note the elongate vugs which are the result 
of the vapor phase of the ash flow. These vapor 
phase vugs have been filled with feldspar crystals 
(vapor phase crystallization).
PHOTOMICROGRAPH C —  Photo C displays the zone of no 
welding at the base of the upper cooling unit 
(Qhz). Note the random orientation of the glass 
shards with little or no deformation. Triple 
junctions produced at the corners of adjacent gas 
bubbles are abundant. The shards in this photo 
are unaltered and vitric.
PHOTOMICROGRAPH D —  Photo D displays a zone of partial 
welding near the base of the upper cooling unit 
(Qh2). Note that the shards are easily recogniz­
able and only moderately deformed and are devit­
rif ied.
PHOTOMICROGRAPH E —  Photo E displays the zone of dense 
welding in the upper cooling unit (Qh2). Note the 
extreme flattening and deformation of shards due 
to compaction under high temperatures. The shards 
in this photo are devitrified.
PHOTOMICROGRAPH F —  Photo F displays the upper zone of 
partial welding in the upper cooling unit (Qh2) of 
the Huckleberry Ridge Tuff. This photo represents 
the upper part of the tuff that has been weathered 
to clays. Note that the shard ghosts are only 
moderately deformed due to compaction under mod­
erate temperatures.

24
partial welding that is approximately 25 meters thick. 
The zone of partial welding contains I to 2% sanidine 
phenocrysts averaging I mm across in a groundmass of 
devitrified shards that are slightly deformed. Also 
present are trace amounts of quartz and olivine 
phenocrysts. This zone of partial welding coincides 
with the vapor phase of the ash flow (Fig. 8, Photo­
micrograph B). Vesicles filled with minute feldspar 
crystals, that readily weather to clay when exposed to 
surface conditions, are associated with the vapor phase 
of this cooling unit.
Immediately above the zone of partial welding is 
a compacted ash bed that is H meter thick. The ash 
bed separates the lower cooling unit (Qhi) from the 
upper (Qhi). This vitric ash contains undeformed 
glass shards together with 2% sanidine and 1% quartz 
phenocrysts averaging 3 mm across (Fig. 8, Photomicro­
graph C). The presence of quartz phenocrysts indicates 
that the ash bed is related to the second eruptive 
pulse of the Huckleberry Ridge Tuff. Quartz pheno­
crysts are rare in the lower member but are relatively 
abundant in the second member of the tuff. The fact 
that the ash is unaltered and has not been removed by
25
erosion is further evidence for association with the 
second member of the tuff.
Overlying the ash bed is the vitric chill zone of 
the upper cooling unit (Qh2). The glass shards of this 
zone are moderately deformed and partially welded.
This chill zone contains 2 to 3% sanidine phenocrysts 
up to 2 mm in diameter as well as trace amounts of 
quartz and olivine phenocrysts. The basal chill zone 
grades vertically into a 9 meter thick zone of partial 
welding in which the shards are devitrified and moder­
ately deformed (Fig. 8, Photomicrograph D). This 
portion of the tuff contains 10 to 15% sanidine with 1% 
quartz phenocrysts all ranging up to 1.5 mm in diam­
eter. The zone of partial welding grades upward into 
a zone of dense welding that has undergone devitri­
fication (Fig. 8, Photomicrograph E). This 27 meter 
thick zone contains 10% sanidine phenocrysts at the 
base and increases vertically to 25% near the top. The 
phenocrysts of this zone range up to 4 mm in diameter. 
Quartz and olivine phenocrysts averaging 1% of the rock 
are also present in this zone. The zone of dense 
welding grades upward into a zone of partial welding 
that is devitrified and contains 15 to 30% sanidine.
26
5% quartz, and 1% olivine phenocrysts (Fig. 8, Photo­
micrograph F). The phenocrysts of the zone of partial 
welding average 3 mm in diameter. The upper portion 
of the zone has probably been removed by erosion and 
the 5 meters remaining are badly weathered.
The upper cooling unit of the Huckleberry Ridge 
Tuff as seen at this location is thinner (42m) than the 
lower cooling unit (IOOm). The upper unit has a zone 
of dense welding, which is thinner than that of the 
lower cooling unit, sandwiched between two zones of 
partial welding (Fig. 7). The lower unit has a very 
thick zone of dense welding with only a thin partially 
welded zone beneath (chill zone). This indicates a 
much higher emplacement temperature for the lower 
cooling unit.
The timing of the Huckleberry Ridge Tuff emplace­
ment into the upper Centennial Valley was such that 
the lower member (Qhi) was allowed to cool as a single 
simple cooling unit before being covered by the second 
member (Qhz). The emplacement of the second unit fol­
lowed soon enough after the first episode to prevent 
extensive erosion of the (Qh1) surface. Potassium- 
argon dates of the upper and lower cooling units are
27
identical within the resolution of the dating method 
(Appendix IIf A and'B).
The dimensions of the welding zones vary with 
depositional thickness of the tuff and distance from 
the source. The thickness of the Huckleberry Ridge 
Tuff can vary greatly over a short distance when 
deposited over uneven paleotopography. Stratigraphic 
sections at the type section (Christiansen and Blank, 
1972), Elk Lake, Hidden Lake, Cliff Lake, Curlew Creek, 
and Wall Canyon display changes in the dimensions of 
the individual cooling units and welding zones within 
them (Fig. 9).
The Huckleberry Ridge Tuff can be traced discon­
tinuous Iy from the Yellowstone caldera region into the 
Centennial area and northward into the Upper Madison 
Valley where the tuff thins and eventually terminates 
(Figs. 9 & 11). The Huckleberry Ridge Tuff has been 
measured at over 300 meters in thickness in the 
vicinity of the Yellowstone caldera (Christiansen and 
Blank, 1972) and is present at Targhee Pass, Idaho, in 
thicknesses of up to 200 meters. The tuff is 123 
meters thick to the west at Elk Lake and decreases in 
thickness northward to 91 meters at Hidden Lake, 86
LEGEND:
(type section) Qha Third Member
Qhz Second Member
First Member
Non-welded ash 
Partially welded tu ff  
Densely welded tu ff
(Elk L) (Hidden L.) (Cliff L) (Curlew Cr.) (Wall Canyon)
(Qhz) K>
CO
FIGURE 9. Stratigraphic correlation of the Huckleberry Ridge Tuff.
Measured sections are located in Figure 10. Horizontal not to 
scale. (See Appendix I for lithologic descriptions of sections.)
1 Cliff LA. 
Hidden L.P.  ~r\ 
_ E l k  L.> ; n
YELLOW STONE  
NAT IONAL  
PARK
•  Huckleberry 
Mountain
IO 2 0  mi.
<iO
Measured section location map (see Figure 9).FIGURE 10.
..MONTANA 
\  WYOMING*
Yellowstone
Notional
Pork
FIGURE 11. Regional distribution of the Huckleberry Ridge Tuff. Compiled 
from: Christiansen and Blank (1972) , Witkind (1972 & 1976),
Weinheimer (1979) and Sonderegger (Personal Communication 1980).
31
meters at Cliff Lake (Weinheimer, 1979), 48 meters at 
Wall Canyon (Weinheimer, 1979) , and thins to zero near 
Ruby Creek in the Madison Valley (Appendix I).
The thinning of the tuff away from the Yellowstone 
caldera, the correlation of cooling units from the Yel­
lowstone area to the Centennial region, and potassium- 
argon dates strongly support the theory that the tuff 
traveled into the Centennial Valley from the Yellow­
stone caldera and not from a local vent.
Pleistocene Mesa Falls Tuff —  The Mesa Falls Tuff is the 
second formation of the Yellowstone Group (Christian­
sen and Blank, 1972). The tuff originated from the 
Island Park caldera 1.2 million years ago.
The Mesa Falls Tuff of the Centennial region is 
a single ash flow sheet. The tuff is brownish gray 
with phenocrysts of quartz and sanidine that comprise 
up to 45% of the rock situated in a groundmass of 
devitrified shards. Phenocrysts of quartz and sanidine 
range up to 5 mm in diameter. This tuff is seen only 
locally in the Centennial Valley.
Pleistocene Lava Creek Tuff —  The Lava Creek Tuff is the
youngest formation of the Yellowstone Group (Christian­
sen and Blank, 1972). The tuff originated from the
32
Yellowstone caldera 0.6 million years ago. The tuff is 
light gray, punky to dense, and has a fine-grained to 
aphanitic groundmass of devitrified shards. Pheno- 
crysts of sanidine and quartz, averaging less than I mm 
across, make up 2 to 10% of the rock. Spherical 
vesicles mark the presence of a gas phase at the top 
of the ash flow sheet. The Lava Creek Tuff is present 
to the east of the Centennial Valley but is absent 
from the study area.
STRUCTURE
The prominent fault system along which the Centennial 
Range and Valley developed sharply cuts Laramide structures. 
The Centennial Range, like the Teton Range of Wyoming to the 
southeast, dips under the Snake River Plain and is probably, 
therefore, part of the Snake River structural system (Hamil­
ton, 1960). The east-west trending structures of the Cen­
tennial area were probably activated as the Snake River 
downwarp, with associated volcanic activity, migrated east­
ward toward the Yellowstone-Island Park region in the late 
Pliocene-early Pleistocene (Hamilton, 1965).
The structure of the Northern flank of the upper 
Centennial Valley was determined and mapped by discerning 
offset of key strata of the ash flow sequence of the 
Huckleberry Ridge Tuff. During emplacement, ash flow tops 
generally assume even upper surfaces which display very low 
angles of dip away from the source (Ross and Smith, 1960). 
The Huckleberry Ridge Tuff is a composite sheet made up of 
two such ash flow units which are closely spaced in time but 
cooled individually. It is assumed, therefore, that the 
contact between the two flows should be nearly horizontal 
with only a very gentle dip away from the source. Displace­
ment of the tuff should then be detectable as long as the 
contact between the two cooling units is identifiable.
34
The north side of the upper Centennial Valley is influ­
enced by four major structures: The Madison, Cliff Lake,
Gravelly, and Centennial faults (Fig. 12). The Madison, 
Cliff Lake, and Gravelly faults are post-Laramide tensional 
structures that approximately parallel the northwest and 
northeast trending Laramide compressional features. The 
Huckleberry Ridge Tuff forms benches that have been dis­
turbed by movement along the four structures.
The Madison normal fault trends in a general north- 
northwest direction and is located to the east and northeast 
of the Centennial Valley. The Huckleberry Ridge Tuff 
benches east of the Elk-Hidden-Cliff-Wade lake chain are 
dipping from 3 to 5 degrees in an easterly direction into 
the Madison Fault which is down on the west.
The Elk-Hidden-Cliff-Wade lake chain is situated along 
a normal fault (the Cliff Lake Fault)(Fig. 12) that nearly 
parallels the Madison Fault. The Cliff Lake Fault has the 
same relative orientation as the Madison Fault in the 
vicinity of Elk Lake, being downdropped on the west (Fig. 
13). The Huckleberry Ridge Tuff has been displaced a maxi­
mum of 122 meters (400 ft) along the Cliff Lake Fault at 
Elk Lake. The relative sense of displacement of the fault 
is reversed to the north at Cliff Lake where the east side
35
Madison R.
Madison 
Fault '
gravelly
Fault Hidden
Lake
Centennial
Fault
System
FIGURE 12. Cross-section location map. Also shows faults.
GENERAL EAST-W EST  CROSS-SECTION OF THE
CENTENNIAL VALLEY REGION
West
Y
0  I m ile1 ___ i
East
Y'
IO OOOn r  IOOOO fe e t
3 0 0 0 3 0 0 0  fe e t
Gravelly
Fault
Cliff Lake Madison
FIGURE 13. General east-west cross-section of the upper Centennial region.
See cross-section Y-Y' Figure 12 for location. Qal= Quaternary 
alluvium, Qfc= Coalesced alluvial fans, Qhd= Displaced Huckle­
berry Ridge Tuff, Qm= Mesa Falls Tuff, Qhz= huckleberry Ridge 
Tuff (upper Cooling unit), Qhi= Huckleberry Ridge Tuff (lower 
cooling unit), QTb= Basalt, Pz= Paleozoic rocks, PC= Precambrian 
rocks.
37
of the fault is down approximately 99 meters (325 ft) 
(Weinheimer, 1979)(Fig. 14). The Cliff Lake Fault is, 
therefore, a scissor fault with its hinge located near the 
southern extreme of Cliff Lake. A Pleistocene basalt lo­
cated just south of Elk Lake appears to have come up through 
the crustal weakness generated by movement along the Cliff 
Lake Fault, indicating that the fault existed at the 
beginning of or possibly prior to Pleistocene time. The 
Cliff Lake Fault is probably equivalent to the Odell Creek 
Fault (Fig. 14) of Honkala (1960) which trends north and 
cuts the Centennial Range southwest of Upper Red Rock Lake. 
The Odell Creek Fault is down on the west as was the Cliff 
Lake Fault before later rotation.
The Gravelly Range Front Fault has been mapped along 
the West Fork of the Madison River just north of the Upper 
Red Rock Lake Quadrangle (Weinheimer, 1979). This fault 
probably extends southward into the Centennial Valley. 
Evidence for displacement along the fault in the study area 
is weak. Late Tertiary-early Quaternary and Pleistocene 
volcanics are in contact with Precambrian rocks along the 
proposed fault zone. This contact may be depositional or 
faulted with basalt localized along the fault. The genetic 
nature of this contact is questionable because of a lack of
38
Odell Creek 
/  Fault
r Gravelly /  Fault
Centennial Fault 
System
SOUTH NORTH
(B)
FIGURE 14. (A) Structural block diagram of the upper Cen­
tennial Valley, Montana.
(B) Structural block diagram displaying present 
topography of the upper Centennial Valley, 
Montana (same area as A).
39
outcrop of volcanic rock (seen only as float along contact 
zone).
The Centennial Range Front Fault is a normal fault that 
trends east-west and is downthrown on the north forming the 
Centennial Valley (Honkala, 1960). The upthrown Centennial 
Range is situated to the south. The Huckleberry Ridge Tuff 
bench along the northern flank of the upper Centennial 
Valley is fractured into blocks that have an average dip of 
3 to 4 degrees to the south into the Centennial Fault (Fig. 
15). Portions of the bench also dip locally into the Cliff 
Lake and Gravelly faults (Fig. 18). The area to the west of 
Elk and Hidden lakes appears to be a half graben that has 
been dropped into the Centennial Fault along the Cliff Lake 
and Gravelly faults. The Paleozoic and Mesozoic sections 
may be preserved at least in part in the half graben beneath 
the Huckleberry Ridge Tuff (Figs. 13 & 15). The Centennial 
Fault is not a single clean break but rather a series of 
step faults creating a number of blocks beneath the valley 
floor (Schofield, Personal Communication, 1980). Alaska 
Basin (Fig. 14) in the easternmost extreme of the Centennial 
Valley is probably a downdropped block resulting from this 
phenomenon. The rate of movement along the Centennial Fault 
appears to have increased since Pinedale time.
GENERAL NORTH -SOUTH  CROSS -SECT IO N  OF TH E  
UPPER  CENTENN IA L  VA LLEY
North
O I mile X '
Centennial Fault r IOOOO feetIOOOO Tal Qh
3000 feet3000
South
X
FIGURE 15. General north-south cross-section of the upper Centennial Valley.
See cross-section X-X' Figure 12 for location. Oal= Ouaternary 
alluvium, Olc= Lacustrine deposits, Qh= Huckleberry Ridge Tuff, 
Qb= Pleistocene basalt, Tal= Tertiary alluvium, Tlc= Lacustrine 
deposits, Mz= Mesozoic rocks, Pz= Paleozoic rocks, PG= Precam- 
brian rocks.
41
The average rate of post-Huckleberry Ridge Tuff move­
ment, measured by visually returning the southward dipping 
bench of Huckleberry Ridge Tuff in the upper Centennial 
Valley to horizontal, is 0.10 centimeter per year (0.04 
in/yr). A post-Pinedale rate of 3.05 centimeters per year 
(1.2 in/yr) was measured using the offset of a line of 
hanging glacial valleys of Pinedale age. The line of 
hanging glacial valleys is present on the scarp slope of 
the Centennial Range just south of Upper Red Rock Lake (Fig. 
16). This uniform line is situated approximately 396 meters 
(1300 ft) above the valley floor (Fig. 17) and stands above 
what has been mapped as Pinedale moraine at the foot of the 
range (Witkind, 1976). The east-trending Centennial Range 
has been dissected by north-trending faults breaking the 
range into blocks (Honkala, 1960). Glacial valleys termi­
nate at different elevations from one block of the range to 
another. This relationship, together with a lack of 
piedmont glacial deposits and the presence of triangular 
faceting up to the cirques, indicates at least 396 meters 
(1300 ft) of post-Pinedale displacement and thus a rate of 
3.05 centimeters per year (1.2 in/yr).
Siesmic studies (Bailey, 1977 and Trimble and Smith, 
1975) have demonstrated that the Centennial, the Cliff Lake,
FIGURE 16. North slope of the Centennial Range along the upper Centennial 
Valley, Montana. Note the line of hanging glacial valleys.
West East
9800
9600
9400
9200
9000
8800
8600
8400
8200
8000
7800
7600
7400
7200
7000
6800
(feet)
Sheep Mtn. Toylor Mtn.
w
FIGURE 17A. Topographic profile displaying level of hanging glacial valleys 
. from Sheep Mountain east to Taylor Mountain, upper Centennial 
Valley, Montana.
West East
9800
9600
9400
9200
9000
8800
8600
8400
8200
8000
7800
7600
7400
7200
7000
6800
(feet) I mile
iU
FIGURE 17B. Topographic profile displaying levels of hanging glacial
valleys below unnamed peaks east of Taylor Mountain, upper 
Centennial Valley, Montana.
45
and the southern part of the Madison faults are active 
today.
In addition to these major structures, many minor 
faults have developed in response to stresses created by 
movement along the east-trending Centennial Fault and the 
generally north-trending Cliff Lake, Gravelly, and Madison 
faults (Fig. 18). The numerous minor faults of the upper 
Centennial Valley generally have total offsets of less than 
100 feet.
46
Madison R.
■Wade Lake
Madison
Fault^
Cliff Lake
Conklin /  
^fLoke \
Elk Lake
Cliff Lake 
_ Fault
Henrys
Lake
Centennial X s 
Fault System
Q \ .Y. ^ _
STAEMNNON
___:
FIGURE 18. Structure map of the Qhi top, upper 
Centennial Valley and vicinity.
GEOMORPHOLOGY
The Huckleberry Ridge Tuff forms flat benches that 
rise up to 8200 feet in elevation and are vegetated with 
grasses and sage brush. These benches are dipping gently 
into the Centennial, Madison, Cliff Lake, and Gravelly 
faults forming gentle slopes on the dip sides and vertical 
cliffs on the scarp sides (Fig. 19). Large scale mass 
wasting occurs along the Huckleberry Ridge Tuff cliffs where 
large sections of welded tuff separate from the cliff and 
pile up at the base creating a hummocky topography that is 
generally vegetated with Lodgepole pine, spruce, and 
Douglas fir.
The geometry of the Huckleberry Ridge Tuff along the 
northern flank of the upper Centennial Valley is such that 
the thickest portion of the tuff forms a crude north-south 
trending axis (Fig. 20). The tuff thins to either side of 
this axis. The axis coincides with the modern day Madison 
River to the north and appears to be the result of the 
filling of an ancestral Madison River drainage that was cut 
into the upper Centennial half graben. It appears that this 
river flowed southward into a large lake occupying the 
Centennial Valley area. This lake has been recorded by the 
presence of freshwater limestone in the valley. Gravity 
data by Schofield (Personal Communication, 1980) indicates
48
the presence of approximately 305 to 610 meters (1000 to 
2000 ft) of alluvial gravels beneath the tuff. The gravels 
thicken to the south toward the Centennial Valley and to the 
west along the valley. These data together with the geom­
etry of the tuff (Figs. 20 & 21) indicate that the Pliocene 
Madison River flowed in a direction opposite to that of the 
present Madison River. In addition to the ancestral Madison 
River, it appears that another drainage flowing westward 
along the Centennial Fault east of the Centennial Valley 
entered the valley near what is now the Alaska Basin (Fig. 
21). Judging from the pre-2.0 million year old paleotopog­
raphy (Fig. 21) and the distribution of the Huckleberry 
Ridge Tuff (Fig. 11), it appears that the Huckleberry Ridge 
ash flow gained access into the Centennial Valley along this 
drainage.
Glacial deposits are absent on the tuff benches along 
the north side of the Centennial Valley. Glacial deposits 
of pre-Bull Lake time are present on top of the Huckleberry 
Ridge and Mesa Falls tuffs northeast of the study area in 
the Madison Valley (Weinheimer, 1979). Here, Bull Lake 
glaciers advanced several kilometers from the high valleys 
along the Madison Range into the Madison Valley. Pinedale 
glaciers extended approximately equal distances into the
Ridge
Tutt
Ruckteberflf
Ridge
Tutt
I
O
FIGUBE 19.
50
Cliff Lake 
Quadrangle
Madison R.
West Fork 
of th e— ' 
Madison Rv
I 2 3 4 ml.
North Part Of 
Upper Red 
Rock Lake 
Quadrangle
FIGURE 20. Isopach map of the 2.0 m.y. old Huckleberry 
Ridge Tuff. Contour interval is 100 feet.
51
Madison Valley. Pre-Bull Lake deposits are the most prom­
inent glacial deposits found in the Gravelly Range north of 
the study area (Hadley, 1960). The Pinedale moraines of the 
Gravelly region are small and confined to the upper parts 
of glacial valleys.
The only evidence of glaciation in the Centennial area 
is found along the Centennial Mountain Range (Witkind, 1972 
and 1976). Pinedale moraines are present at the base of the 
range just south of Upper Red Rock Lake and east and west 
along the Centennial Range. Bull Lake and pre-Bull Lake 
deposits are widely scattered east of the Cliff Lake Fault 
trace on the northern slope of the Centennial Range and Red 
Rock Pass but are absent along the range adjacent to the 
upper Centennial Valley (Witkind, 1976).
Well-rounded 10 to 20 cm cobbles are present as scat­
tered patches at various locations along the north side of 
the upper Centennial Valley (Plate I). These deposits 
appear as a thin veneer usually only a single cobble thick . 
and are most common on top of the Huckleberry Ridge Tuff 
benches at elevations of up to 8200 feet to the west of the 
Cliff Lake Fault. Most of the cobbles are white to dark 
gray quartzite; however, basalt cobbles are also present.
The source area for these cobbles is probably the Gravelly
52
Cliff Lake 
Quadrangle
2000-
800-
Upper Red 
Rock Lake- 
Quadrangle
2200
2200
1800
FIGURE 21. 2.0 m.y. old paleotopography of the Upper Red
Rock Lake and Cliff Lake quadrangles. Contour 
interval is 200 feet. Cliff Lake Quadrangle 
data is modified from Weinheimer (1979). The 
paleotopography was derived using an arbitrary 
datum (0 elevation) from relative thicknesses 
of the Huckleberry Ridge Tuff accounting for 
post tuff structure.
53
Range, approximately 6 km to the northwest. These deposits 
are interpreted as sheet-wash deposits shed off the Gravelly 
Range and laid down over a late Pleistocene-Holocene 
Huckleberry Ridge Tuff surface. The sheet-wash deposits 
could not have been transported across the West Fork of the 
Madison River Valley which now separates the deposits from 
the probable source. This indicates that the West Fork of 
the Madison River did not exist at the time of sheet-wash 
deposition.
The drainage systems of the north side of the upper 
Centennial Valley are directly related to the Quaternary 
structure of the region. Streams cutting the Huckleberry 
Ridge Tuff benches form a crude rectangular drainage pat­
tern. These benches have been dissected by small intermit­
tent streams (subsequent streams) which follow fault traces. 
One of the larger recent subsequent streams occupied a 
trench along the Cliff Lake Fault. This small river prob­
ably drained the Centennial Valley which was at the time 
occupied by a glacial lake (Witkind, 1976). The steady 
decrease in elevation from Elk to Wade Lake is evidence for 
a northward flow of the drainage. The stream was dammed by 
the slumping of tuff from the Huckleberry Ridge Tuff benches 
that rimmed the trench as the glacial lake receded. The
54 .
recession of the glacial lake has been recorded as old 
beach lines seen to the south of Upper Red Rock Lake 
(McMannis and Honkala, 1960). The damming of the river 
created Elk, Hidden, Goose, Otter, Cliff, and Wade lakes.
PRECENOZOIC GEOLOGIC HISTORY
The Centennial region is located in the Basement Prov­
ince of Montana (McMannis, 1965). The Centennial portion 
of this province is characterized by extensive exposures of 
pre-Belt metasedimentary, metamorphic, and igneous rocks of 
Precambrian X age. The province is also characterized by 
an absence of Belt strata and a relatively thin Paleozoic 
and Mesozoic sequence.
Precambrian
The pre-Belt basement of southwestern Montana has a 
dominant northeast-striking structural grain in rocks dis­
playing several metamorphic events ranging from 1,500 to 
2,700 million years in age (McMannis, 1965).
The area just north of the Centennial Valley, in the 
vicinity of the Gravelly Range, was the site of a northeast­
trending sediment collecting geosyncline during Precambrian 
time (Mann, 1960). This trough, termed the Cherry Creek 
geosyncline, was deformed in the late Precambrian by com- 
pressional forces to form a positive topographic element 
extending from near Dillon toward the northeast. This 
highland (the Cherry Creek landmass) extended southward into 
the Centennial region and served as a source for sediments 
shed to the north and west during Belt time. This landmass
56
underwent erosion through the late Precambrian and was 
bevelled at the close of early Cambrian time.
Paleozoic
Southwestern Montana underwent several episodes of 
subsidence and upwarping throughout the Paleozoic Era. The 
Centennial region was covered by intermittent Paleozoic seas 
from which a relatively thin blanket of sediments was 
received.
The Cambrian and Ordovician were times of generally 
stable tectonic conditions in this region. A positive arch 
onto which Cambrian, Ordovician, Silurian, and Devonian seas 
lapped and sediments thinned existed in extreme southwestern 
Montana just west of the Centennial Valley (McMannis, 1965). 
The main Cordilleran seaway of the Cambrian and Ordovician 
was located to the west of the Montana-Idaho border. Uplift 
along the Montana-Idaho border in southwesternmost Montana 
during the late Ordovician caused a retreat of the sea.
Silurian seas probably covered the Centennial region 
resulting in the deposition of marine sediments. Silurian 
and early Devonian sediments are, however, absent in the 
Centennial region due to early Devonian upwarping and sub­
sequent erosion (Mallory and others, 1972).
The region was again covered by seas from the late
57
Devonian into the Mississippian. A northeast-trending 
negative feature had developed in southwesternmost Montana 
just west of the Centennial Valley during this time. 
Mississippian sediments thicken westward from the Centennial 
region toward this feature. The Mississippian trough may 
have been a principal route connecting the central Montana 
seas with those of the major geosyncline to the west and 
southwest during Big Snowy time (McMannis, 1965).
A period of erosion occurred prior to the deposition of 
Pennsylvanian strata in the region. The Pennsylvanian and 
Permian marine sediments, which are separated by a discon- 
formity, thicken westward from the Centennial region toward 
the main geosyncline in Idaho.
Mesozoic
The early Triassic of southwestern Montana was marked 
by the transgression of a sea from which silty limestones 
were deposited. Reddish and brownish sediments were laid 
down later in the Triassic as the sea regressed creating 
restricted marine and littoral environments in the Centen­
nial region.
Marine sediments were deposited unconformabIy in the 
Centennial region during the early Jurassic. These sedi­
ments thinned to the northeast onto a large island located
58
in the vicinity of the present-day Snowcrest and Gravelly 
ranges (McMannis, 1965). In the late Jurassic the western 
seaway, along which the earlier seas had advanced into the 
area, was destroyed. Increasing tectonism in an area west 
of Montana produced a flood of debris that was carried 
eastward into southwestern Montana. This event produced the 
nonmarine Morrison sequence. This sequence thickened west­
ward across the Centennial region (McMannis, 1965).
Following a period of erosion, deposits of the Creta­
ceous seas were laid down over bevelled edges of the 
Jurassic, producing angular relationships between the Juras­
sic and Cretaceous of the area (Mann, 1960). The early 
Cretaceous miogeosynclinal region of southwestern Montana 
and Idaho became emergent and began to shed sediments to 
the east to form the nonmarine Kootenai Formation. The 
Kootenai overlies the Jurassic unconformably and thickens 
west of the Centennial region. By late Cretaceous time, 
sharp orogenic uplift was occurring. This orogenic activity 
(Laramide) continued through the early Eocene. Volcanism in 
the southwestern part of Montana and the intrusion of gra­
nitic plutons north of the Centennial region accompanied 
the Laramide deformation. Strong east-west compressional 
forces created a complicated zone of thrust faulting and
59
folding in the Gravelly Range at the same time that imbri­
cate thrusts were formed along belts in northwestern 
Montana (Mann, 1960). The Snowcrest and Gravelly ranges 
rose on the west and east limbs of this feature respectively 
(Mann, 1960). This fold system extends eastward to the 
Madison Valley anticline.
CENOZOIC GEOLOGIC HISTORY
Paleocene
Laramide deformation continued into the early Tertiary. 
Thrusting and folding of the Blacktail and Greenhorn uplifts 
occurred as extensions of the Bighorn uplift. This activity 
west and northwest of the Centennial region resulted in the 
deposition of the Beaverhead Formation (Eardley, 1960, and 
Honkala, 1960). The Beaverhead conglomerate was derived 
from Paleozoic limestones and quartzitic sandstones.
Eocene
The Eocene was a time of uplift, volcanism, and erosion 
in southwestern Montana. Laramide deformation continued 
into the early Eocene; the Beaverhead Formation was cut by 
a thrust fault west of the study area (Honkala, 1960).
Early Eocene deformation resulted in two systems of folds 
and thrust faults, one trending northeast and a later system 
trending northwest (Eardley, 1960). The Madison thrusts and 
folds to the northeast and the broad gentle arch of the 
ancestral Teton Range southeast of the Centennial area 
originated during this time. Compressional deformation 
ceased before middle Eocene time in the region (McMannis, 
1965, and Mann, 1960). Volcanism followed Laramide defor­
mation in the middle Eocene with the Gallatin, Absaroka, and
61
Yellowstone volcanic fields beginning to form east of the 
study area (Eardley, 1960).
Oliqocene
The early Oligocene was a time of erosion and voIcan- 
ism. Andesitic volcanism reached a climax during the late 
eocene-early Oligocene east of the Centennial region in the 
southern portion of the Absaroka Range (Chadwick, 1970).
Late Eocene-early Oligocene rhyolites were extruded to the 
northwest near Dillon (Beaverhead volcanic field) and basal­
tic volcanism occurred near Virginia City north of the 
Centennial region during early Oligocene time (Chadwick,
1978).
The Snake River Basin appears to have begun to form in 
the Oligocene. The Snake River Plain is thought to be a 
continental tensional rift feature. Two mechanisms have 
been proposed for the formation of this structure. One 
theory is that the Snake River Plain with associated volcan­
ic rocks is the result of the movement of the North American 
plate across a mantle plume or hot spot (Morgan, 1972).
The other theory deals with the drifting northward of the 
Idaho Batholith (Hamilton and Meyers, 1966). The latter 
theory is presently more widely accepted. This theory 
concerns an ever widening rift, floored with basaltic rock.
62
The rift appears to be oldest to the west and becomes 
younger to the east where it terminates with the Island Park 
and Yellowstone calderas (present apex of rift).
Miocene
Erosion and volcanism dominated the Miocene history of 
southwestern Montana. The Black Butte basalt was extruded 
in the Gravelly Range north of the Centennial Valley during 
late Oligocene-early Miocene time. Snake River Plain down­
warping and volcanism was probably active southwest of the 
Centennial region at this time. The Snake River activity 
does not, however, appear to have influenced the Centennial 
region during the Miocene.
Pliocene
Block faulting of the Madison Range began in Pliocene 
time (Eardley, 1960). The north-trending Madison Range 
Front Fault cut Laramide structures at a 60 degree angle; 
other faults such as the Gravelly Range Front and Cliff 
Lake faults broke parallel to the previously established 
Laramide structures.
The Snake River Plain structure had migrated far enough 
to the east by late Pliocene time to begin to influence the 
Centennial and Teton regions (Fig. 2)(Hamilton, 1965). The
63
Centennial and Teton range front faults were probably 
created at this time. Assuming the rift theory to be true, 
the tensional forces would have created crustal thinning and 
downwarping in the Snake River Plain region followed by 
faulting and volcanism. As the rift migrated (opened) 
eastward, the southern flank of the Centennial and north­
western flank of the Teton ranges began to dip into the 
downwarp. As these blocks continued to tilt into the down- 
warp during late Pliocene-early Pleistocene time, the up­
lifted edges broke to form range front faults which now mark 
the north side of the Centennial Range and southeast side of 
the Teton Range. Offset along the Teton Fault is approx­
imately twice that of the Centennial Fault. This difference 
appears to be the result of the orientation of the faults. 
The Centennial Fault cut across preexisting Laramide struc­
tures while the Teton Fault was enhanced by cutting along 
them (Love, 1968).
The Centennial Range in the late Tertiary-early Qua­
ternary was topographically much lower than it is today. 
Based on a total offset of 3050 meters (10,000 ft) 
(Schofield, Personal Communication, 1979) a maximum of 915 
to 1220 meters (5000 to 6000 ft) of displacement had oc­
curred along the Centennial Fault prior to the emplacement
64
of the Huckleberry Ridge Tuff. The late Pliocene-early 
Pleistocene Centennial Valley was occupied by a large lake 
in which freshwater limestone was deposited. The ancestral 
Madison River flowed southward into the Centennial lake 
through a major valley cut into the upper Centennial Valley 
half graben.
Quaternary
The entire upper Centennial Valley region was at a 
sufficiently low enough elevation to permit the 2.0 million 
year old Huckleberry Ridge Tuff to flow into the valley 
from the Yellowstone caldera. The ash flow entered the area 
from the east leaving a blanket of tuff over much of the 
valley. It then traveled northward up the ancestral Madison 
drainage, diverting the river from the Centennial Valley.
Movement along the Centennial Fault continued through­
out the Quaternary, displacing the Huckleberry Ridge Tuff a 
minimum of 1525 to 1830 meters (5000 to 6000 ft) in the 
last 2.0 million years. The Centennial Range was suffi­
ciently high 1.2 million years ago to block the passage of 
the Mesa Falls ash flow into the upper Centennial Valley 
from the Island Park caldera. The only portions of the ash 
flow to enter the valley crossed the divide through low 
passes such as one near Hell Roaring Canyon (see map by
65
Witkind, 1976). Major volcanism occurred again in the 
Yellowstone caldera region 0.6 million years ago producing 
the Lava Creek Tuff. This ash flow was entirely blocked 
from the upper Centennial Valley and is exposed along the 
lower southeastern slopes of the Centennial Range.
Alpine glaciers formed in the mountain ranges sur­
rounding the Centennial Valley at different times throughout 
the Pleistocene. Deposits of pre-Bull Lake, Bull Lake, and 
Pinedale glaciations indicate that the ice originated in 
high cirques and traveled downward, cutting U-shaped valleys 
which often reached the larger valley floors such as the 
Madison Valley (Weinheimer, 1979). Glacial meltwater accu­
mulated in the Centennial Valley to form a large lake which 
drained at several locations including a stream situated in 
the Cliff Lake Fault trench. The West Fork of the Madison 
River began to cut a valley approximately along the Huckle­
berry Ridge Tuff-Precambrian contact (Gravelly Fault) during 
the late Pleistocene-early Holocene time. The Centennial 
Valley glacial lake eventually receded to form the Upper and 
Lower Red Rock lakes and associated ponds and marshes. As 
the lake receded, the drainage along the Cliff Lake Fault 
trench was dammed by mass wasting to create Elk, Hidden, 
Goose, Otter, Cliff, and Wade lakes.
66
It appears that the rate of movement along the Centen­
nial Fault has increased since Pinedale time to over an 
inch per year. The Centennial, Cliff Lake, and southern 
Madison faults continue to be seismically active today.
GEOTHERMAL POTENTIAL
Evidence for warm spring activity along the northern 
flank of the upper Centennial Valley in the Upper Red Rock 
Lake Quadrangle is scant. Warm springs have, however, been 
identified adjacent to the area. The Sloan Cow Camp (30°C) 
and West Fork Swimming Hole (28°C) warm springs are located 
along the Gravelly Range Front Fault immediately to the 
north (Weinheimer, 1979) . A warm spring has been identified 
southwest of Elk Lake (Sec. 11, T14S, RlW)(Sonderegger, 
Personal Communication, 1979). This spring is located on 
the Centennial Valley floor in approximate line with the 
Cliff Lake Fault. The Cliff Lake Fault would allow the 
circulation of meteoric water at depth. Thermal waters 
rising along the fault would mix with shallow cold ground- 
water and eventually with surface water when penetrating the 
valley fill sediments.
The possibility exists that the lakes along the Cliff 
Lake Fault have been at least in part warmed a few degrees 
by heated waters circulating along the fault. Elk, Hidden, 
and Goose lakes were found to be at least partially open in 
late November while the larger Henrys Lake and marshes of 
the upper Centennial Valley were covered by ice. Elk and 
Goose lakes were at least half open while Hidden Lake and 
the algae-covered pond just north of Hidden Lake were
68
virtually free of ice. Otter, Cliff, and Wade lakes were 
not examined at this time. A deposit of travertine along 
the eastern shoreline of Elk Lake (Berg, Personal Communi­
cation, 1979) further indicates the presence or at least 
past existence of thermal spring activity along the Cliff 
Lake Fault.
A sizable geothermal reservoir may exist beneath the 
Huckleberry Ridge Tuff in the gravels (Tal in Figs. 13 & 15) 
of the ancestral Madison River channel on the northern side 
of the upper Centennial Valley (Sections 3,4,8,9,16,17, and 
20 of T13S, R1E). A portion of the river channel coincides 
spatially with the Cliff Lake Fault. This reservoir, if 
present, probably terminates southward at a facies change 
(from coarse- to fine-grained material) where the ancient 
river entered the lake-filled Centennial Valley. This 
potential thermal system is consistent with those present 
in the Upper Madison Valley (Weinheimer, 1979). Cold water 
circulates down through deep-seated fault systems to depths 
at which the geothermal gradient, possibly amplified by the 
Yellowstone magmatic system, rises sufficiently to heat the 
water substantially before this water rises again toward the 
surface.
It is recommended that a temperature survey be
69
conducted in the lakes along the Cliff Lake Fault. A study 
measuring surface water temperatures together with temper­
ature change with depth in the lakes would prove valuable 
in detecting and outlining any hot spring activity at the 
bottoms of the lakes. Positive results in the temperature 
study would warrant further studies. Gravity and/or 
resistivity studies could then be conducted along the north 
side of the upper Centennial Valley to assess reservoir 
potential.
REFERENCES CITED
72
Honkala, F. S., 1954, Geology of the Centennial region of 
southwestern Montana (abs.): GeoI. Soc. Amer. Bull.,
VoI. 65, No. 12, Pt. 2, p. 1377.
_________ 1960, Structure of the Centennial Mountains and
vicinity, Beaverhead County, Montana: Billings Geol.
Soc., Guidebook, Ilth Ann Field Conf., p. 107-112.
Love, J. D., 1968, Stratigraphy and structure in Behrendt 
and others, 1968: A geophysical study in Grand Teton
National Park and vicinity, Teton County, Wyoming:
U. S. GeoI. Survey Prof. Paper 516-E, p. E3-E5.
Mallory, W. W., and others, 1972, Geologic atlas of the
Rocky Mountain region: Rocky Mtn. Assoc, of Geologists,
Denver, p. 86-99.
Mann, J. A., 1960, Geology of part of the Gravelly Range 
area, Montana: Billings Geol. Soc., Guidebook, Ilth
Ann. Field Conf., p. 114-127.
McMannis, W* J., 1965, Resume of depositional and struc­
tural history of western Montana: Am. Assoc. Petroleum
Geologists Bull., Vol. 49, No. 11, p. 1801-1823.
McMannis, W. J., and Honkala, F. S., 1960, Madison Canyon
slide to southern Gravelly Range via Centennial Valley, 
road log, part II: Billings Geol. Soc., Guidebook,
Ilth Ann. Field Conf., p. 284-288.
Morgan, W. J., 1972, Deep mantle convection plumes and
plate motions: Am. Assoc Petroleum Geologists Bull.,
VoI. 56, p. 203-213.
Ross, C . S., and Smith, R. L., 1960, Ash flow tuffs: their
origin, geologic relations and identification: U. S.
Geol. Survey Prof. Paper 366, p. 23-24.
Smith, R. L., 1960, Zones and zonal variations in welded 
ash flows: U. S. Geol. Survey Prof. Paper 354-F,
p. 149-158. Geol. Soc. Amer. Bull., Vol. 71, No. 6, 
p. 795-804.
73
Struhsacker, D. W., 1978, Mixed basalt-rhyolite assem­
blages in Yellowstone National Park. The petrogenetic 
significance of magma mixing: Master's Thesis,
Department of Geology, University of Montana, p. 1-105.
Trimble, A. B., and Smith, R. B., 1975, Siesmicity and
contemporary tectonics of the Hebgen Lake-Yellowstone 
Park region: Jour. Geophys. Res., Vol. 80, No. 5,
p. 733-741.
Weinheimer, G. S., 1977, (abs.) Recent faulting and
geothermal activity in the Upper Madison Valley:
Geol. Soc. Amer., Rocky Mountain Sec., 30th Ann. 
Meeting, p. 772.
_________ 1979, The geology and geothermal potential of the
Upper Madison Valley between Wolf Creek and the Mis­
souri Flats, Madison County, Montana: Master's Thesis,
Department of Earth Sciences, Montana State University,
p. 1-102.
Witkind, I. J., 1972, Geologic map of the Henrys Lake 
Quadrangle, Idaho and Montana: U. S. Geological
Survey Map I-781-A.
_________ 1976, Geologic map of the southern part of the
Upper Red Rock Lake Quadrangle, southwestern Montana 
and adjacent Idaho: U. S. Geological Survey Map 1-943.
APPENDICES
APPENDIX I 
MEASURED SECTIONS
SECTION A. Cliff north of Elk Lake (SW%,SE%, sec 16, T13S,R1E).
contains 10% sanidine and quartz phenocrysts
diameter.
phenocrysts ranging up to .5 rrm in diameter.
(m) (ft)
76
58 190
55 180
52 170
49 160
46 150
43 140 pumice bands and 3 to 5% sanidine with minor 
quartz phenocrysts ranging up to 3 tin in
40 130
37 120
34 H O
30 100
27 90
24 80
21 70
18 60
15 50 contains 8 to 10% sanidine with minor quartz
12 40 Flow or compaction banding present.
Float of black vitric tuff in talus.
77
SECTION B. Cliff on north side of Hidden Lake (NE%,NW%, sec 10, H3S, 
ME).
124 400
120 .390 diam.).
117 380
114 370
111 360
108 350
105 340
102 330 —
99 320
96 310 sanidine and quartz phenocrysts ranging up to
I ran in diameter.
93 300
90 290
86 280
83 270
80 260 and quartz phenocrysts ranging up to I to 2 ran
in diameter.
77 250
74 240 Ash bed with glass in float.
70 230
67 220
(m) (ft)
78
64 210 
61 200 
58 190 
55 180 
52 170 
49 160 
46 150 
43 140 ~  
40 130 
37 120 
34 H O  
30 100 
27 90
24 80
21 70
18 60 
15 50
12 40
9 30
3 20
O O 
(m) (ft)
Reddish-gray moderately welded tuff, contains 
2 to 3% sanidine with minor quartz phenocrysts 
ranging up to I m m  in diameter. Flow or com­
paction banding present.
Medium-gray welded tuff.
Dark-gray to dark-brown densely welded tuff. 
Compaction or flow banding present. Contains 
10%. sanidine with minor quartz phenocrysts 
ranging up to 2 mm in diameter.
Float of glassy vitric tuff in talus.
79
SECTION C. Cliff on east side of Cliff Lake, I mile south of
boat ramp (SW%, sec 13, T12S, RLE) (Weinheimer, 1979).
86 280
83 270
80 260
77 250
74 240
70 230
67 220
64 210
61 200
58 190
55 180
52 170
49 160
46 150
43 140
40 130
37 120
34 H O
30 100
(m) (ft)
Light-gray nan-welded slightly compacted 
vesicular, weathers to light-pink ash.
Gray slightly welded tuff.
Dark-gray (weathers to red-gray or dark- 
brown), welded moderately consolidated tuff, 
layers 2 to 3 era.
Dark-gray (weathers to light-gray), welded 
moderately consolidated tuff, layers 0.75 
to 1.0 m.
Dark-brown welded tuff, layers I to 2 cm.
Gray massive compacted welded tuff; displays 
concoidal fracture.
79
SECTION C. Cliff on east side of Cliff Lake, I mile south of
boat ramp (SW%, sec 13, T12S, RlE) (Weinheimer, 1979).
86 280
83 270
80 260
77 250
74 240
70 230
67 220
64 210
61 200
58 190
55 180
52 170
49 160
46 150
43 140
40 130
37 120
34 H O
30 100
(m) (ft)
Light-gray non-welded slightly compacted 
vesicular, weathers to lig^ it-pirik ash.
Gray slightly welded tuff.
Dark-gray (weathers to red-gray or dark- 
brown), welded moderately consolidated tuff, 
layers 2 to 3 cm.
Dark-gray (weathers to light-gray), welded 
moderately consolidated tuff, layers 0.75 
to 1.0 m.
Dark-brown welded tuff, layers I to 2 cm.
Gray massive compacted welded tuff; displays 
concoidal fracture.
90
80
70
60
50
40
30
20
10
0
(f
80
Gray (vaaathers to yellow and brown grus), 
compacted flow banded welded tuff.
Top of first pulse, light-gray lightly 
compacted vesicular tuff. Precanfcrian 
gravel inclusions, vesicules 2.5 cm.
Gray porphyritic (quartz, sanidine 15%) 
dense, flow banded, thinly layered welded 
tuff.
Gray-brown porphyritic dense flow-banded 
welded tuff.
No osbidian in float.
81
SECTION D. One km north of the mouth of Curlew Credc (sec 13, TlOS, 
RlE) (Weinheimer, 1979).
(m) (ft)
Precanbrain granite and gneiss alluvium.
Light gray-pink slightly compacted unwelded 
tuff, weathered pitted surface. 57=> Precanbrian 
rock fragment inclusions.
Yellow-brown weathered fractured porphyritic 
welded tuff, I to 5% quartz and sanidine pheno- 
crysts ( I nm).
Gray welded tuff; weathers to light tan or pink; 
ccncoidal fracture.
Light-gray, vesicular, limonitized welded tuff; 
flattened I to 5 nm vesicules comprising 57» of 
the rock.
Gray (weathers to light-brown grus), blocky, 
densely welded tuff. Contains 10% quartz and 
sanidine phenocrysts (I to 2 ran dia.), and 0.5% 
metamorphic rock inclusions.
Light gray-pink dense welded tuff.
Tan rhyolite grus float.
toper limit of black obsidian float, contains 
5/4 quartz, sanidine phenocrysts.
82
SECTION E. One km west of the mouth of Wall Canyon (NW%, sec 18, TlOS, 
R1E)(Weinheimer, 1979).
48 156
46 150
43 140
40 130
37 120
34 H O
Pink slightly welded tuff, weathered pitted 
surface.
Light-gray, vesicular, slightly welded tuff, 
weathering to rounded ash blocks; contains 
quartz and sanidine phenocrysts, I to 2 inn in 
diameter, and Precambrain rock fragment inclu­
sions.
Gray (weathers to light-pink or tan) massive 
welded tuff; displays ccncoidal fracture. Con­
tains a few vesicles, and 15% quartz and sani­
dine phenocrysts I ran in diameter.
Red-gray tuff (same as 0 to 15 m ) .
Dark-gray vesicular welded tuff (2 to 3 cm 
vesicles).
Dark-brown massive welded tuff; dark gray and 
yellow flow banding.
Brown weathered surface; tuff alters to yellow 
and brown grus. Contains quartz and sanidine 
phenocrysts I ran in diameter.
Covered, no obsidian float.
APPENDIX II
POTASSIUM-ARGON AGE DATES
SAMPLE A.
Location: I mile NNW of Hidden Lake (44°43.83'N,
111*36.84'W; NW% NW^ NE^ S4,T13S,R1E).
Formation: Upper cooling unit (Qha) of the Huckleberry
Ridge Tuff.
Rock type: Rhyolitic ash flow tuff.
Collected by: Dr. J. Sonderegger, MBMG.
Analytical data: K=7.45%, *Ar4°=2.716x10”1°moles/gm,
atm. Ar 4 0 =35%
Dated by: S. H. Evans, University of Utah, Dept, of
Geology.
Age: 2.05 ± 0.07 m.y. (sanidine)
SAMPLE B.
Location: South end of Elk Lake (44*39.24'N, 111*
38.84'W; NW% SE% S31,T13S,R1E).
Formation: Lower cooling unit (Qhi) of the Huckleberry
Ridge Tuff.
Rock type: Rhyolitic ash flow tuff.
Collected by: Dr. J. Sonderegger, MBMG.
Analytical data: K=7.4 9%, *Ar4°=2.659X10-1°moles/gm.
atm. Ar40=27%.
84
POTASSIUM-ARGON AGE DATES 
(continued)
Dated by: S. H. Evans, University of Utah, Dept, of
Geology.
Age: 2.05 ± 0.07 m.y. (sanidine)
SAMPLE C.
Location: South end of Elk Lake (44°39.24,N, 111°
38.84'W; NW% SE% S31,T13S,R1E).
Formation: Pleistocene basalt (no formal name).
Rock type: Basalt.
Collected by: Dr. J. Sonderegger, MBMG.
Analytical data: K=O.36%, AFP 40=O.1489x10"1°moles/gm,
atm. Ar4 0=93%.
Dated by: S. H. Evans, University of Utah, Dept, of
Geology.
Age: 2.38 ± 0.44 m.y. (whole rock)
SAMPLE D.
Location: NE slope of Flatiron Mountain (44°50,27"N,
111°38'45"W; SW% SE** S30,TllSfRlE).
Formation: Huckleberry Ridge Tuff.
Rock type: Rhyolitic ash flow tuff.
Collected by: G. J. Weinheimer, MSU.
85
POTASSIUM-ARGON AGE DATES 
(continued)
Analytical data: K=5.850%, *Ar4 °=0.000837 ppm,
Ar4 0/EAr4 °=16%.
Dated by: Geochron Laboratories, Inc.
Age; 2.0 + 0.1 m.y. (sanidine)
(from Chadwick, 1978)
SAMPLE E.
Location; Cliff face on W bank Madison River 2 Km 
N of mouth of Wall Canyon, Madison Co., MT 
(44*59'33"N, 111*39'45"W; SE** NE% SI,TlOS,RlW). 
Formation; Huckleberry Ridge Tuff.
Rock type; Rhyolitic ash flow tuff.
Collected by; Dr. R. A. Chadwick, MSU.
Analytical data; K=6.888%, * Ar4 °=0.000945 ppm, 
*Ar4 0ZZAr4 0=17%.
Dated by; Geochron Laboratories, Inc.
Age; 1.9 ± 0.1 m.y. (sanidine)
(from Chadwick, 1978)
APPENDIX III
CHEMICAL ANALYSES OF RHYOLITE TUFFS AND BASALT
ROCK UNIT SiO2
%
Al2O3
%
Fe2O3
%
Huckldaerry Ridge 
Tuff (Qh1)
(Sec. 31,T13S,R1E)
75.0 11.4 3.74
Huckleberry Ridge 
Tuff (obsidian of 
Qhi)
(Sec. 2,T13S,R1W)
78.5 11.5 3.08
Huckleberry Ridge 
Tuff (Qh2)
(Sec. 4,Tl3S,RLE)
80.1 11.6 4.40
Pleistocene
basalt
(Sec. 31,T13S,R1E)
50.1 15.3 14.0
Mesa Falls Tuff 
(Weinheimer, 1979) 71.73 12.4 6.971
MgO
%
CaO
%
Na2O
%
K2O
%
TiO2
%
MiO
%
0.25 0.72 3.1 4.9 0.205 0.042
0.06 0.66 3.4 ■ 4.7 0.210 0.035
0.03 0.51 3.2 4.9 0.264 0.044
7.18 9.25 2.6 0.4 2.75 0.148
0.415 1.00 3.734 3.879 0.25
(Sanples were analyzed by the Mantana Bureau of Mines and Geology)
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