Cambrian(?), Middle Proterozoic, and Archean 

Rocks Penetrated in a Borehole near Argenta, 

Beaverhead County, Montana, and Some 

Paleogeographic and Structural Implications

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STENNIS SPACE CENTER, Mississippi-BIdg. 3101
Cambrian(?), Middle Proterozoic, and Archean 

Rocks Penetrated in a Borehole near Argenta, 

Beaverhead County, Montana, and Some 

Paleogeographic and Structural Implications

By Robert C. Pear son

BELT SUPERGROUP IN THE HIGHLAND MOUNTAINS AND PROBABLE 

EQUIVALENT ROCKS IN THE PIONEER AND ANACONDA RANGES, 

SOUTHWESTERN MONTANA

J. Michael O'Neill and Robert C. Pearson, Editors

U.S. GEOLOGICAL SURVEY BULLETIN 2121-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Pearson, Robert Carl, 1926-

Cambrian(?), Middle Proterozoic, and Archean rocks penetrated in a borehole near 

Argenta, Beaverhead County, Montana, and some paleogeographic and structural 

implications / by Robert C. Pearson.

p. cm. (Belt Supergroup in the Highland Mountains and probable 

equivalent rocks in the Pioneer and Anaconda Ranges, southwestern Montana ; B) 

(U.S. Geological Survey bulletin ; 2121)

Includes bibliographical references.

1. Geology, Structural Montana Argenta Region. 2. Borings Montana 

Argenta Region. 3. Rocks Montana Argenta Region. 

I. Title. II. Series. III. Series: U.S. Geological Survey bulletin ; 2121. 

QE75.B9 no. 2121-B 

[QE627.5.M9] 

557.3 s dc20

[551.8'09786'69] 95-22169

CIP
CONTENTS

Abstract........................................................................................................................... Bl

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

Acknowledgments.................................................................................................. 3

Geologic Setting of the Argenta Area............................................................................ 3

Description of Rocks in the Groundhog Drill Core........................................................ 5

Flathead(?)Quartzite.............................................................................................. 7

Garnet Range Formation........................................................................................ 7

Archean Schist........................................................................................................ 10

Correlation and Paleogeography .................................................................................... 10

Structural Interpretation.................................................................................................. 12

References Cited............................................................................................................. 14

FIGURES

1. Index map of southwestern Montana.......................................................................................................................... B2

2. Geologic map of the Argenta area, Beaverhead County, Montana............................................................................. 4

3. Map showing major structural features, limit of Belt Supergroup rocks, and thickness of Belt rocks in

	southwestern Montana.......................................................................................................................................... 5

4. Stratigraphic columnar section showing rocks in Groundhog drill hole..................................................................... 6

5. Photograph of Flathead(?) Quartzite and Garnet Range Formation in outcrop.......................................................... 8

6. Photograph of Flathead(?) Quartzite core showing fine- and coarse-grained quartzarenite....................................... 8

7. Photograph of Garnet Range Formation core showing thinly laminated, feldspathic, micaceous quartzite .............. 9

8. Photograph of Garnet Range Formation core showing micaceous, feldspathic quartzite........................................... 9

9. Photograph of Garnet Range Formation core showing coarse-grained feldspathic quartzite..................................... 9

10. Photograph of Garnet Range Formation core showing cross-laminated quartzarenite from near top of formation... 10

11. Photograph of Garnet Range Formation core showing conglomerate near bottom of formation............................... 10

12. Diagrammatic cross sections illustrating several interpretations of structure of east flank of

	Humbolt Mountain anticline................................................................................................................................. 13

III

CAMBRIAN(?), MIDDLE PROTEROZOIC, AND

ARCHEAN ROCKS PENETRATED IN A BOREHOLE

NEAR ARGENTA, BEAVERHEAD COUNTY, MONTANA,

AND SOME PALEOGEOGRAPHIC AND

STRUCTURAL IMPLICATIONS

By Robert C. Pear son

ABSTRACT

The Middle Proterozoic Belt Supergroup is only 220 m 

thick where it is penetrated by a borehole in the Argenta 

mining district about 20 km northwest of Dillon, Mont. This 

unusually thin sequence of Belt strata is assigned to the 

Garnet Range Formation (Missoula Group), the youngest 

widespread formation of the Belt. A conglomeratic base may 

indicate a depositional contact on Archean schist at a depth 

of 285 m. The Garnet Range is overlain by 55 m of 

quartzarenite that is probably the Middle Cambrian Flathead(?) Quartzite. The Garnet Range consists of thin-bedded, 

poorly sorted, ferruginous, argillaceous, micaceous, 

feldspathic quartzite; red and green sandy and silty argillite; 

and thick-bedded coarse-grained to very coarse grained feldspathic quartzite, The upper two-thirds of the unit is mostly 

interlayered, poorly sorted, argillaceous quartzite and argillite, and the lower third is mostly thick-bedded, feldspathic 

quartzite. Archean rocks, penetrated from a depth of 285 m 

to the bottom of the hole at 336 m, are mostly coarse-grained 

biotite-quartz-plagioclase-potassium feldspar-garnet schist 

and a few thin layers of amphibolite.

Belt strata near Argenta are comparable in thickness 

and rock type to Missoula Group strata in the Highland 

Mountains, 50 km to the north. However, a much thicker 

Missoula Group (about 7 km thick) forms the Grasshopper 

thrust plate 6 km to the west; this sequence has been transported tens of kilometers eastward with respect to that at 

Argenta. No Belt strata are present between Archean and 

Cambrian rocks to the east and south of Argenta. These relations suggest that an onlap margin of the Belt Basin is probably within a few kilometers of the drill site.

The seemingly unfaulted sequence of Flathead(?) 

Quartzite, Belt Supergroup, and Archean crystalline basement is clear evidence that previously proposed structures

such as the Argenta and Ermont thrust faults do not exist in 

this area. The Humbolt Mountain anticline, on which the 

hole was drilled, may be considered a basement-cored anticline similar to the Armstead and Biltmore anticlines and 

numerous other anticlines in the foreland to the east.

INTRODUCTION

The southern margin of the Belt Basin in much of 

southwestern Montana is obscure because Belt Supergroup 

rocks are covered by younger rocks or are absent above 

pre-Belt rocks where they may have been eroded. Inferences 

that the Belt Basin extended far to the south of their present 

remnant were based solely on tenuous facies relations and 

thickness trends (Reynolds, 1984). On the Grasshopper 

thrust plate, Missoula Group (upper part of the Belt Supergroup) rocks are widespread and more than 7 km thick (Ruppel and Lopez, 1984), but, east of the Grasshopper 

decollement, outcropping Belt strata, which are also Missoula Group, are restricted to a small area in the core of an 

anticline near Argenta, Mont. (fig. 1). After Ruppel and 

Lopez (1984) correlated the outcropping rocks near Argenta 

with Garnet Range Formation, the youngest widespread Belt 

unit, the rocks could reasonably be assumed to represent 

only the uppermost part of a very thick Belt sequence, such 

as that on the Grasshopper plate, or they could be interpreted 

as part of a thin thrust-fault sliver (Brandon, 1984).

Thus, a mineral-exploration borehole in the Argenta 

mining district is significant for several reasons: (1) the Belt 

sequence penetrated in the borehole is only 220 m thick; (2) 

the Belt strata are conglomeratic at the base; and (3) the Belt 

strata overlie Archean schist and gneiss at a depth of less 

than 300 m in an area where such shallow depth had not been 

suspected. The conglomeratic base of the Belt suggests

Bl
B2 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

114° 113°

TOBACCO

ROOT 

MOUNTAINS

O Philipsburg 

o Porters Corners

46°

45° -

Figure 1. Index map of southwestern Montana.

strongly that the base is depositional and not a fault. This 

rare find coupled with other information from other drill 

holes and outcrops nearby provide a basis for inferring the 

edge of the Belt Basin in this area. The borehole near 

Argenta also provides important information for the structural interpretation of the eastern margin of the Cordilleran 

thrust belt.

The borehole was drilled in 1968 to a depth of 336 m 

and was cored the entire length. The core has a diameter of 

6 cm (NC size) from the surface to a depth of 115.5 m, and 

the remainder is 4.7-cm in diameter (NX size). This hole was

drilled by Copper Range Corporation on property controlled by the William Hand family of Dillon, Mont., who 

have custody of the core, made it available for study, and 

supplied a copy of the driller's log. Prior to examination in 

1992, the core had been split, portions removed for tests and 

analyses, and numerous boxes of core were missing or in 

disarray; however, a comparison of the extant core with the 

driller's log indicates that the portions not available for 

examination did not contain any unusual or significant 

intervals. About 80 percent of the core was present when I 

examined it and collected small samples for petrographic
CAMBRIANC?), MIDDLE PROTEROZOIC, AND ARCHEAN ROCKS IN A BOREHOLE NEAR ARGENT A, MONTANA B3

and mineralogic study in August and September 1992. The 

borehole is located about 3.2 km north-north west of the village of Argenta (fig. 1) in NEV4 SWV4 sec. 18, T. 6 S., R. 

10 W., and near the east side of the Ermont 7V2-minute 

quadrangle. According to the driller's log, the drill collar is 

at an altitude of 6,962 ft (2,122 m).

The drill site is near the Groundhog mine, and hence 

the hole will be referred to as the Groundhog drill hole. (It 

was called "CRX-4" by the mining company that drilled it.) 

This report describes the rocks from the Groundhog drill 

hole, with emphasis on the Belt strata, interprets their stratigraphic position and structural relationships, and compares 

this sequence of Belt rocks with other Belt successions in the 

region.

ACKNOWLEDGMENTS

William Hand very graciously permitted access to the 

drill core and provided a copy of the lithologic log. In addition, William and Donald Hand have been helpful in supplying information on the geology and mineral deposits in the 

Argenta area. William Hand died in 1994 before seeing the 

results of this work, in which he took a great interest. Phyllis 

Denton, U.S. Forest Service, arranged for facilities in Dillon, Mont., to examine the core. For all of these courtesies, I 

am very grateful. Elizabeth Youngren recognized the significance of crystalline rocks in the drill core to interpretation 

of the local geology and called my attention to the drill hole 

a few years ago.

GEOLOGIC SETTING OF THE 

ARGENTA AREA

The Argenta mining district is in a north-trending segment of the Cordilleran thrust belt that was referred to by 

Ruppel and Lopez (1984) as the "frontal fold and thrust 

zone," a structural belt characterized by parallel anticlines, 

synclines, and minor thrust faults. The belt is transitional 

between the Rocky Mountain foreland to the east and the 

region of major thrust faults to the west. The district is about 

6 km east of the Kelley thrust (fig. 2), a segment of the 

Grasshopper decollement, which is one of the major thrust 

faults of southwestern Montana (Ruppel and Lopez, 1984). 

The upper plate of this thrust, now called the Grasshopper 

plate (figs. 2 and 3), consists of at least 7 km of upper Belt 

(Missoula Group) strata (Ruppel and Lopez, 1984) and local 

thin remnants of overlying lower Paleozoic rocks.

Terminology of Belt rocks in this report is that 

currently in common use and defined first by Harrison and 

Grimes (1970). The terms argillite, siltite, and quartzite 

reflect the widespread low-grade metamorphism that Belt 

rocks have undergone. Other rock names are used in

petrographic descriptions such as arkose, subarkose, and 

quartzarenite (Pettijohn, 1957); these terms reflect more precisely variations in mineral composition.

The mines of the Argenta district are on the crest and 

both flanks of the Humbolt Mountain anticline (Myers, 

1952), a major northerly trending sinuous structure that is at 

least 15 km long. For part of its length, this anticline has also 

been called the Argenta anticline (Hobbs, 1967), but the 

older name, Humbolt Mountain (Myers, 1952), is used here. 

In the vicinity of Argenta, the Humbolt Mountain anticline 

is broken by steep faults that have mostly small, probably 

mostly dip-slip, displacement (Myers, 1952). Most steep 

faults strike northwest and north. Various interpretations of 

the structure of the east flank of the Humbolt Mountain anticline, discussed in a later section of this report, involve a 

steep reverse fault, a low-angle thrust fault, or unconformities to account for local relationships.

West of Argenta, where the valley of Rattlesnake Creek 

cuts across the Humbolt Mountain anticline, the oldest rocks 

exposed are quartzite, feldspathic quartzite, and argillite of 

the Middle Proterozoic Garnet Range Formation (fig. 2), the 

uppermost formation of the Missoula Group at most places 

in west-central Montana. These rocks have been recognized 

as Proterozoic since a study by Shenon (1931) but were first 

correlated with the Garnet Range by Ruppel and Lopez 

(1984). The Garnet Range is overlain on the crest of the anticline by a quartzite unit that Shenon (1931) and Myers 

(1952) assigned partly to the Middle Cambrian Flathead 

Quartzite, an assignment adopted tentatively here. Others 

considered all of the quartzite, in addition to the underlying 

beds, to be parts of the Belt Supergroup (Hobbs, 1967; Ruppel and Lopez, 1984; Brandon, 1984). To the north, near 

Humbolt Mountain (fig. 2), the Garnet Range also underlies 

quartzite on the crest of the anticline. These quartzite beds 

were interpreted by most previous workers to be Belt and to 

be thrust over the Garnet Range (Myers, 1952; Brandon, 

1984; Ruppel and others, 1993), but here these rocks also are 

regarded as Flathead(?) in normal stratigraphic position 

above the Garnet Range.

Strata above the Flathead(?) Quartzite are of unquestioned Middle Cambrian and younger age. These units have 

been correlated with cratonic Cambrian units to the east, 

such as Wolsey Shale, Meagher Limestone, Park Shale, and 

Pilgrim Dolomite, or with units to the north in the Philipsburg, Mont., area, such as Silver Hill Formation and Hasmark Dolomite. Regardless of which stratigraphic 

assignment is best, these rocks were not encountered in the 

drill hole, and they will receive little attention in this report.

The correct interpretation of the stratigraphy and paleogeography of the rocks penetrated in the Groundhog drill 

hole depends on their structural relations and whether these 

rocks represent an unfaulted stratigraphic succession. If this 

thin sequence of the Belt Supergroup is not thinned tectonically, such as by thrust faulting, the sequence could represent basin-margin deposits and thus be important to the
B4

45°22'30" 

BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

112 0 52'30"

45°17'30" 

Figure 2 (above and facing column). Geologic map of the, Argenta area, Beaverhead County, Montana. Geology generalized and modified from Myers (1952).
CAMBRIAN(?), MIDDLE PROTEROZOIC, AND ARCHEAN ROCKS IN A BOREHOLE NEAR ARGENT A, MONTANA B5

EXPLANATION 114 C

Alluvium (Quaternary)

Ts Sedimentary rocks (Tertiary)

TV Volcanic rocks (Tertiary)

Kgd Granodiorite (Cretaceous)

Mz ft s Sedimentary rocks (Mesozoic and Paleozoic)

 f Flathead(?) Quartzite (Middle Cambrian)

Ygr Garnet Range Formation of Missoula Group

(Middle Proterozoic)

Missoula Group, undivided (Middle Proterozoic)

Contact 

Steep fault

-A A A. Thrust fault

 | Anticline

regional relationships. Previous interpretations of structure 

in the area of the drill hole include: (1) thrust faulting 

(Shenon, 1931; Ruppel and Lopez, 1984; Brandon, 1984), 

(2) high-angle reverse faulting (Hobbs, 1967), and (3) 

high-angle normal faulting (Myers, 1952). Clearly, the rocks 

have been broken by numerous steep faults, as shown by 

Myers (1952) and documented in detail in and around mines 

by Geach (1972). Evidence for thrust faulting or high-angle 

reverse faulting has not been found during my recent (1994) 

field studies, in former studies of the mines (Geach, 1972), 

or in the drill core from the Groundhog drill hole. Although 

the drill core contains several zones of gouge and breccia and 

slickensided surfaces (commensurate with the proximity to 

known steep faults), many of these zones are steeply dipping, and none of them coincide with abrupt lithologic 

changes or change in dip. Dip of bedding in both the Flathead(?) Quartzite and Garnet Range Formation is within 10° 

of horizontal throughout the length of the core. Furthermore, 

no repetitions or deletions of strata or inversions of age 

sequence were recognized in the core. If repetitions or deletions are present, they are believed to be minor. The 

sequence represented by the drill core, therefore, is considered to be essentially intact structurally.

The drill hole was spudded in Flathead(?) Quartzite 

probably only several meters below the top of the unit. Several hundred meters west and southwest of the drill hole, the 

Flathead(?) is in depositional contact with Garnet Range

20 MILES

MONTANA

Butte

HIGHLAND $> 

MOUNTAINS V

m

TOBACCO

ROOT

 ..,. MOUNTAINS 

*. MOUNTAINS | American Quasar 

[220l"9~ 27-22 Haganbarth

jroundhog drill hole y 

rgenta /m

Figure 3. Major structural features, limit of exposed Belt Supergroup rocks, and thickness (in meters, shown in boxes) of Belt in 

southwestern Montana. Hachured line shows limit of Belt 

strata modified in Pioneer Mountains from Harrison (1972).

Formation. Elsewhere in the area of the drill hole, especially 

several hundred meters to the north, the Flathead(?) is missing, probably owing to Cambrian erosion, and Cambrian or 

Devonian dolostone is in apparent depositional contact with 

Garnet Range. These relationships, which convinced Myers 

(1952) of faulting and erosion during Cambrian time, have 

also been explained by high-angle reverse faults (Hobbs, 

1967) and low-angle thrust faults (Brandon, 1984; Sears and 

others, 1988) that slipped during Mesozoic to early Tertiary 

time.

DESCRIPTION OF ROCKS IN THE 

GROUNDHOG DRILL CORE

Strata represented in the drill core are divided into three 

parts: (1) an upper unit of light-gray to nearly white quartzite 

(Flathead? Quartzite); (2) a middle unit of argillaceous, feldspathic, and micaceous quartzite; argillite; and feldspathic 

quartzite that are mostly grayish red (Garnet Range Formation); and (3) a lower unit of mostly biotite-feldspar-quartz 

schist (Archean basement) (fig. 4). The correlation of the 

upper quartzite unit is tentative because its thickness is 

greater than that of Flathead in nearby areas, and no direct 

evidence of its age has been found.

Two small bodies of fine-grained igneous rock, each 

about 4 m thick, are contained in the drill core (fig. 4) and 

are probably sills that intrude the sedimentary rocks. The top
B6 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

FEET METERS 

0 0

25 

100 

50 H

200 

75

300 

100^

400 

125-

500 

150 

175 

600 

Flathead(?) Quartzite

Cretaceous porphyry sill

Garnet Range Formation

Cretaceous porphyry sill

FEET METERS 

600 

200-

700

225

800 

250-

900- 275^

300 

1,000 

325 

1,100 

'.** ".' *.V /.* *.!.'«

 *.». "..« * ' *

 ;. '"!! «..** "

Garnet Range Formation

Archean schist

TD 1,103 ft (336m)

Figure 4 (above and facing column). Stratigraphic columnar section showing rocks in Groundhog drill hole.

of the upper sill is at a depth of 61.2 m and near the Flat- Argenta granodiorite stock at the village of Argenta (fig. 2)

head(?) Quartzite-Garnet Range Formation contact, and the or from other intrusive bodies 2-4 km east of the drill site,

top of the lower one is at a depth of 177.9 m and within the Although most of the rocks in the core are fresh, eviGarnet Range. The sills are probably apophyses from the dence of hydrothermal alteration is present sporadically,
CAMBRIAN(?), MIDDLE PROTEROZOIC, AND ARCHEAN ROCKS IN A BOREHOLE NEAR ARGENTA, MONTANA B7

EXPLANATION

Altered fine-grained porphyry

White to light-gray, fine- to coarsegrained quartzarenite

Coarse-grained cross laminated 

quartzarenite

Interbedded, poorly sorted, red, feldspathic, 

micaceous quartzite; red and green silty 

and sandy argillite; and minor arkosic 

quartzite that increases in abundance 

downward

Coarse-grained to very coarse grained feldspathic 

quartzite and minor poorly sorted quartzite 

and argillite mainly in upper part

Pebble conglomerate and feldspathic 

quartzite

Biotite-quartz-feldspar-garnet schist 

and minor amphibolite

mostly along and adjacent to fractures and fracture zones. 

Pyrite is the most conspicuous product of hydrothermal 

alteration, but some pyrite that is disseminated in green and 

gray beds may be sedimentary or diagenetic. Pyrite appears 

first in the core at a depth of 132 m; above that depth all 

pyrite has been converted to limonite by weathering, probably in Quaternary or late Tertiary time. From 132 m downward, pyrite increases in abundance and limonite decreases. 

Below 205 m, pyrite coats nearly all fracture surfaces and is 

disseminated in gouge and in nearly all rocks that are shades 

of green, gray, and pale red; pyrite is not disseminated in 

rocks that are moderate red or dark red. A few quartz veinlets, one of which contains base-metal sulfide minerals, are 

present in the Archean rocks. The upper 10 m of the Archean 

rocks are partly pale green and slightly friable, possibly as a 

result of weathering prior to deposition of Garnet Range 

Formation in Middle Proterozoic time.

FLATHEAD(?) QUARTZITE

Flathead(?) Quartzite extends from the surface to a 

depth of 55.1 m (fig. 4). The lower contact is placed above 

the highest quartzarenite bed that contains red cross-laminae. This contact is not as prominent as might be expected at 

an unconformity spanning several hundred million years, 

although, at other places in southwestern Montana, the contact is very similar to this one (Campbell, 1960; Nelson and

Dobell, 1961; Winston and Lonn, 1988; Winston and Wallace, 1983). Several beds of prominently cross-laminated 

rocks, totaling several meters (the precise thickness cannot 

be determined because of missing core), are between the 

quartzite of the overlying Flathead(?) and the highest red 

argillite bed that typifies, and is definitely a part of, the 

underlying Garnet Range Formation.

Bedding thickness is obscure in the core, but in outcrops nearby (fig. 5), the Flathead(?) is mostly in beds 

0.5-1.5 m thick. Much of the rock in the core is massive, 

except for faint to conspicuous, mostly planar, laminations, 

which are marked by changes in grain size (fig. 6).

Grain size ranges from fine grained to very coarse 

grained (fig. 6). The coarser rocks contain sparse granules, 

typically a few millimeters in diameter and, rarely, small 

pebbles as much as 1 cm in diameter. The coarser beds are 

erratically distributed in the formation and are not present at 

the base. Most grains are well rounded; sorting is poor to 

good. Color of the quartzarenite is light gray, medium light 

gray, and where disseminated limonite is present, pinkish 

gray to very pale orange.

The composition of the Flathead(?) is uniform 

quartzarenite that consists of about 97 to >99 percent 

quartz grains and overgrowths and trace amounts of microcrystalline chert or jasperoid grains. The rest of the rock 

(<1 3 percent) is very fine grained white mica or clay and 

traces of heavy minerals; attempts to identify the micaceous mineral by X-ray to compare it with the matrix of the 

Belt rocks were unsuccessful because of its paucity. No 

feldspar or detrital mica was observed in thin section or on 

X-ray diffraction patterns, although detrital white mica is 

fairly common in nearby outcrop.

GARNET RANGE FORMATION

Rocks assigned to the Garnet Range Formation extend 

from 55.1 m to 284.8 m in the drill hole (fig. 4), but that 

thickness includes 8.2 m of igneous rock in the two sills. 

Thus, the thickness of the Garnet Range is 221.5 m. Rocks 

comprising the Garnet Range are chiefly of three types, in 

order of decreasing abundance: (1) poorly sorted, argillaceous, feldspathic, micaceous quartzite, (2) coarse-grained 

to very coarse grained feldspathic quartzite; and (3) micaceous argillite. The term "argillaceous" is used for the 

fine-grained matrix that is now mica but was originally 

probably clay. Although these principal rock types are interbedded throughout, poorly sorted quartzite and argillite are 

most abundant in the upper and middle parts of the unit, and 

feldspathic quartzite is most abundant in the lower part. In 

addition to these principal rock types, several meters of 

cross-laminated quartzarenite are at the top of the Garnet 

Range, and poorly sorted conglomerate is a minor constituent confined to the base of the unit. Mineralogically, the
B8 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Figure 5. Photograph of Flathead(?) Quartzite (upper cliff) and Garnet Range Formation (outcrops on 

slope) at mouth of Clarks Canyon, Ermont7 1 /2-minute quadrangle. Flathead(?) is thick-bedded to massive quartzarenite; Garnet Range is thin- to thick-bedded feldspathic quartzite and argillite.

because of fracturing, incomplete core recovery, and splitting of the core lengthwise, the beds are typically on the 

order of 1-4 decimeters thick where exposed at the surface 

south of the drill hole. In core of quartzite from the middle 

part of the unit, red mud chips are widely scattered.

Grain size and, to some extent, mineral composition 

vary from one lamina to another within the poorly sorted 

quartzite. Fine- and medium-grained laminae tend to be 

more feldspathic than coarse-grained laminae, which are 

more quartzose and tend to contain a higher percentage of 

matrix. As estimated from thin sections, potassium feldspar 

comprises about 20-70 percent of the rock, of which about 

80 percent is detrital and the rest authigenic. Most detrital 

feldspar grains show microcline twinning, although some 

twinning is faint. The authigenic feldspar is a clear or 

slightly turbid rim on detrital grains or is intergranular 

cement. A few grains composed entirely of fine-grained, 

felted mica may represent altered plagioclase. Quartz comprises about 25-60 percent of the rock and detrital mica as 

much as 10 percent. The matrix, commonly 10-30 percent 

of the rock, consists of very fine grained micaceous material, 

silt-size and smaller detrital grains, disseminated hematite 

flakes and "dust," and opaque mineral grains. Rock fragments, most of them quartz rich and of metamorphic origin, 

are very sparse. Detrital mica flakes are mostly 0.5-3.0 mm 

in diameter, are oriented parallel to bedding, and are commonly concentrated on the tops of laminae or beds. Some 

laminae consist almost entirely of detrital mica (fig. 8). The 

detrital mica ranges from colorless to amber colored in hand

Figure 6. Photograph of Flathead(?) Quartzite from Groundhog 

drill core: sawn surface of core from depth of 9.1 m. Fine- to 

coarse-grained quartzarenite.

most noteworthy feature of these rocks is the abundance of 

detrital and authigenic potassium feldspar, detrital mica, and 

authigenic hematite; plagioclase is absent or present in only 

trace amounts.

The poorly sorted argillaceous quartzite, the most 

abundant rock of the Garnet Range Formation, is mostly 

grayish red to dusky red, laminated to cross-laminated, and 

fine to coarse grained (figs. 7 and 8 illustrate typical examples); a few beds are light greenish gray. Although bedding, 

aside from lamination, is not clearly evident in the core
CAMBRIANC?), MIDDLE PROTEROZOIC, AND ARCHEAN ROCKS IN A BOREHOLE NEAR ARGENT A, MONTANA B9

Figure 7. Photograph of Garnet Range Formation at depth of 

183.9 m in Groundhog drill core: sawn surface of thinly laminated, 

very feldspathic, slightly micaceous quartzite. Surface stained for 

potassium feldspar, which shows up as light grains; dark grains are 

mostly quartz. Dark bar at right is red mud chip. Arcuate marks in 

upper left are from saw.

Figure 8. Photograph of fractured surface of very micaceous 

feldspathic quartzite in upper part of Garnet Range Formation at 

depth of 144.6 m in Groundhog drill core. Thin, dark laminae are 

composed mostly of detrital mica.

specimen but in thin section appears to be entirely muscovite. The amber-colored muscovite has inclusions of hematite and possibly other minerals and may be detrital biotite 

that has been altered.

The feldspathic quartzite is medium to very coarse 

grained, pale red to pale reddish orange and grayish red, and 

slightly micaceous. Petrographically, the rocks are arkose 

and subarkose (fig. 9). They consist of about 55-85 percent 

quartz grains and overgrowths, 15-35 percent potassium 

feldspar grains and overgrowths, and 5-15 percent micaceous matrix. Some of the micaceous material appears to be 

altered feldspar. The amount of matrix is definitely less than

Figure 9. Photograph of Garnet Range Formation at depth of 

227.7 m in Groundhog drill core: sawn surface of coarse-grained 

feldspathic quartzite (arkose). Surface stained for potassium feldspar (light grains); dark grains are nearly all quartz.

in the poorly sorted quartzite, although gradations seem to 

exist between poorly sorted quartzite and feldspathic quartzite. Tourmaline, zircon, detrital muscovite, sphene, and 

opaque minerals are common heavy minerals. Some of the 

muscovite is crowded with dark inclusions of hematite and 

other minerals and probably represents altered biotite.

The argillite is mostly grayish red to dusky red, sandy 

and silty, and rich in detrital mica. Some beds or parts of 

beds are yellowish gray or light greenish gray. Core recovery of argillite was poor, and most that was recovered is in 

thin partings or laminae between poorly sorted quartzite or 

feldspathic quartzite beds, although some argillite sequences 

(beds?) are as much as a few meters thick. Detrital mica 

flakes that are parallel to bedding produce a pronounced fissility. A dull, submetallic film of gray hematite that coats 

many bedding planes and other splitting surfaces seems to 

become more abundant with depth. Green argillite commonly contains pyrite, and red argillite contains hematite but 

no pyrite. Unaltered brown to green biotite was found in one 

sample of argillite from 25 m below the top of the Garnet 

Range. This biotite, in addition to the more abundant white 

mica, is in thin flakes 0.1-0.3 mm in diameter that appear to 

be detrital.

The quartzite beds at the top of the Garnet Range Formation (as here defined), which may total about 14 m thick, 

differ from the overlying Flathead(?) Quartzite only in containing prominent cross-laminae 2-3 mm thick, some of 

which are red and alternate with thicker (3-6 mm thick) 

light-gray and very light gray laminae (fig. 10). The mineral 

composition and grain size are not noticeably different from 

the overlying Flathead(?). The highest sill encountered in 

the core intruded this sequence of quartzite beds at the top of 

the Garnet Range. These several meters of quartzite are 

assigned to the Garnet Range rather than the Flathead(?)
BIO BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Figure 10. Photograph of core from near top of Garnet Range 

Formation at depth of 55.8 m in Groundhog drill hole: sawn surface 

of quartzarenite. Note prominent cross-laminae and horizontal laminae.

Figure 11. Photograph of core from Garnet Range Formation at 

depth of 282,2 m in Groundhog drill hole: sawn surface of 

conglomerate. Surface stained for potassium feldspar, which 

comprises about 15 percent of rock. Large pebble in center is fractured white crystalline quartz; dark pebble on right is ferruginous 

jasperoid.

partly because of the very similar sequence in the Anaconda 

Range (Wallace and others, 1992) and at Porters Corners 

near Philipsburg, Mont., about 120 km to the north (fig. 1) 

(Winston and Wallace, 1983).

Several beds of pebble conglomerate are interlayered 

with poorly sorted coarse-grained to very coarse grained 

feldspathic sandstone in the lower 4 m of core, directly 

above the contact with Archean schist. Although core recovery was poor in this interval, it was sufficient to show that 

the conglomerate consists of pebbles of crystalline quartz 

and jasperoid as much as 5 cm long, pink feldspar as much 

as 2 cm long, and a rock consisting of quartz and pink microcline and resembling pegmatite or possibly the pink granite 

that comprises most of the Archean terrane in the Maiden

Peak area about 50 km south-southwest of Argenta (fig. 1). 

The pebbles are rounded to subangular and supported by a 

more abundant coarse-grained and very coarse grained sand 

matrix (fig. 11) composed of quartz, metaquartzite, jasperoid, pink feldspar, muscovite, altered biotite, and locally, 

disseminated pyrite. Some jasperoid grains and pebbles contain disseminated hematite and resemble ferruginous jasperoid associated with iron formation in Archean rocks in the 

Ruby Range to the east (James, 1990).

The contact between Garnet Range Formation and 

Archean rocks is marked by about 2 cm of light greenish-gray gouge or regolith. This layer consists of the same 

minerals as the sandstone and arkose and also minor 

amounts of chlorite and disseminated pyrite.

X-ray diffraction analysis of bulk-rock samples of the 

various rock types gives information on mineralogy of the 

rocks beyond that determinable optically. The X-ray data 

confirm the petrographic determination of the abundance of 

potassium feldspar and the absence in most specimens of 

plagioclase; a trace of plagioclase was detected definitely in 

only one specimen. The X-ray data also indicate that, in 

most specimens, the potassium feldspar is orthoclase rather 

than microcline despite the grid twinning visible in many of 

the grains. Other specimens contain mixtures of microcline 

and orthoclase, resulting from either detrital microcline and 

authigenic orthoclase or microcline partially converted to 

orthoclase during or since diagenesis. Hematite was detected 

by X-ray in most of the red rocks, and in some, the amount 

probably exceeds five percent. The argillite and the 

fine-grained matrix of sandstone consist largely of mica, and 

no chlorite or clay minerals were found.

ARCHEAN SCHIST

From a depth of 285 m to the bottom of the drill hole at 

336 m, the rock penetrated is coarsely crystalline schist. 

Quartz-plagioclase-garnet-biotite schist is the most abundant, and hornblende-clinopyroxene-plagioclase schist is 

present in a few thin layers. Much of the biotite-rich schist 

contains augen of feldspar (both plagioclase and potassium 

feldspar) as much as 1 cm long and crystals of red garnet 

typically 0.3-0.5 cm in diameter and rarely as much as 2 cm 

in diameter. The well-defined foliation in the schist is 

approximately horizontal throughout.

CORRELATION AND 

PALEOGEOGRAPHY

The quartzarenite penetrated in the upper part of the 

drill hole, as well as exposed at the surface on the crest and 

both flanks of the Humbolt Mountain anticline, is here correlated with Flathead(?) Quartzite because its stratigraphic
CAMBRIAN(?), MIDDLE PROTEROZOIC, AND ARCHEAN ROCKS IN A BOREHOLE NEAR ARGENTA, MONTANA B11

position and composition are similar to the Flathead elsewhere. The greater thickness (about 60 m) of the quartzarenite exposed locally on the Humbolt Mountain anticline 

compared with that in exposures in mountain ranges to the 

north, east, and south is the only reason that this correlation 

may be questionable. In those areas, the unit typically is 

0-30 m thick, but Ruppel and others (1993) report nearly as 

great a thickness (43 m) locally in the Tobacco Root Mountains. The quartzarenite, in whole or in part, was previously 

thought to be Proterozoic. Hobbs (1965) assigned all of the 

quartzarenite to the Proterozoic because of intercalated 

argillite, which he claimed is not present in the Flathead in 

western Montana. In fact, the argillite that he described is in 

very small amount and is not unlike shale partings on quartzite beds in Flathead in the region. He may also have confused argillite interbedded with quartzarenite beds near the 

top of the Garnet Range Formation with the overlying 

quartzarenite unit. Others (Myers, 1952, and Ruppel and 

others, 1993, for example) assigned southern outcrops on the 

anticline to the Flathead but more northerly outcrops to the 

Missoula Group. Indeed, the characteristics of the rock do 

change from south to north: outcrops become rare; bedding 

becomes obscure; the rock progressively weathers into more 

angular blocks and piles of rubble; the rock becomes a more 

vitreous quartzite that fractures across the grains; and it loses 

its brown to red colors and becomes white to light gray. All 

of these characteristics are the result of bleaching and 

recrystallization caused by heating effects of the Pioneer 

batholith, which crops out 3 km northeast and 5 km northwest of Humbolt Mountain. The poor outcrop, the weathering into angular blocks, and the tendency of the blocks to 

move downslope as sheets of surficial debris masking adjacent units all cause the unit to resemble the Missoula Group 

in many areas in the Pioneer Mountains. Further studies will 

be needed to confirm or deny that the unit is all or partly 

Cambrian.

Proterozoic strata in the Argenta area are correlated 

with the upper part of the Missoula Group on the basis of 

lithologic similarity with rocks extensively exposed in the 

Anaconda Range (fig. 1) (Wallace and others, 1992) and in 

other parts of the Butte I°x2° quadrangle (Wallace, 1987). 

The upper part of the sequence, which is exposed at the surface and continuously cored in the drill hole, is here confidently assigned to the Garnet Range Formation, as was done 

for the exposed portion by Ruppel and Lopez (1984). In the 

drill hole, the increase downward in coarse-grained arkose 

and the presence of red mud chips in quartzite in the middle 

part of the sequence suggest further the possibility that the 

middle and lower parts are correlative with the McNamara 

Formation and Bonner Quartzite, respectively. Such correlations would imply that the Garnet Range is about 100 m 

thick, the McNamara about 30 m thick, and the Bonner 

about 90 m thick. Siliceous mud chips and red and green 

chert, characteristic of the McNamara, were not observed, 

however, and nowhere else are these units nearly so thin.

Therefore, it would be rash to subdivide the 220 m of these 

strata to such an extent on the basis of the limited information from the drill hole. It is considered prudent to include all 

of these units in the Garnet Range and to consider the coarse 

arkose in the lower part to be merely a local basal facies that 

accumulated near the sediment source this basal facies 

may interfmger with more typical Garnet Range basinward.

The absence of Belt rocks at several places in the region 

where Paleozoic strata are in contact with pre-Belt crystalline basement (Ruppel and others, 1993) suggests, but does 

not prove, limits to the Belt Basin. Belt rocks could have 

been deposited and then eroded before Cambrian deposition. 

At such places as the Blacktail Mountains, Ruby Range, and 

Tobacco Root Mountains (fig. 3), Flathead Quartzite is in 

depositional contact with the crystalline rocks. The contact 

of the Flathead and Archean crystalline rocks is also 

exposed about 25 km south of Argenta in the core of the 

Armstead anticline (fig. 3) (Lowell, 1965), which is slightly 

en echelon to the Humbolt Mountain anticline.

The nearest outcrops to Argenta of the upper part of the 

Missoula Group are near Jackson, Mont., 40 km west of 

Argenta (fig. 1). These rocks, however, were originally 

much more than 40 km from those at Argenta because they 

are on the Grasshopper thrust plate that has moved eastward 

an unknown, though probably large, distance. Garnet Range, 

McNamara, Bonner(?) and Mount Shields Formations of the 

Missoula Group on the Grasshopper plate were recognized 

by Ruppel and Lopez (1984) in the Warm Springs Creek 

drainage east of Jackson. Although they have not been studied in detail, the thickness of the Garnet Range, as preserved 

beneath Flathead Quartzite, is 0-800 m; the thickness of the 

McNamara is possibly as much as 2,300 m; and the 

McNamara overlies a very thick sequence of feldspathic 

quartzite that may include both Bonner and Mount Shields. 

This section of Missoula Group rocks may therefore be more 

than 7,000 m thick and, thus, much thicker than that at 

Argenta.

Missoula Group strata are also preserved 50 km north 

of Argenta in the Highland Mountains (fig. 1) (McMannis, 

1963; O'Neill, in press), where they range from 16 m to 168 

m thick (partially eroded beneath Flathead Quartzite) along 

an outcrop length of about 20 km. In the Highland Mountains, the Missoula Group consists largely of argillaceous 

quartzite, quartzite, and argillite. Except for being mostly 

gray rather than red, these strata are similar to those at 

Argenta, although the beds of arkose are less abundant. The 

difference in color may be metamorphic and related to thermal effects of the nearby Boulder batholith. Another difference between the Belt in the Highland Mountains and at 

Argenta is the presence in the Highland Mountains of a thick 

(1,000-3,000 m) sequence of middle and lower Belt rocks 

beneath the Missoula Group and their absence near Argenta.

Belt rocks are also exposed in small fault blocks in the 

Black Lion Mountain-Sheep Mountain area of the 

northeastern Pioneer Mountains (fig. 1). These rocks are
B12 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

interpreted to be lowermost Belt (R.C. Pearson and J.M. 

O'Neill, unpub. data). If Missoula Group rocks were ever 

deposited there, they were eroded before Cambrian strata 

were deposited on lower Belt, which is commensurate with 

the westward thinning of the Missoula Group in the Highland Mountains. The interpretation that these rocks are 

lower Belt differs from that of Zen (1988), who interpreted 

them to be partly of post-Belt age and partly Missoula Group 

rocks correlative with those a few kilometers to the west on 

the Grasshopper plate.

The locality nearest to Argenta where crystalline basement is known is in a petroleum test hole (American Quasar 

No. 27-22 Haganbarth) drilled about 16 km northeast of the 

Groundhog drill hole (fig. 3). Crystalline basement was 

encountered at a depth of 3,607 m (1,981 m below sea level) 

beneath sandstone and shale believed by Lopez and Schmidt 

(1985) to be Cambrian. If those Cambrian rocks were identified correctly, no Belt strata are present. W.J. Perry, Jr. 

(oral commun., 1993), however, interprets geophysical logs 

as possibly indicating a few tens of meters of Belt strata 

beneath Cambrian rocks. Cuttings from this well have not 

been examined to determine if rocks resembling the Belt 

Supergroup are actually present.

These examples of the presence and absence of Missoula Group and other Belt rocks in southwestern Montana, 

including the knowledge gained from the Groundhog drill 

core, suggest that the Argenta area is near the southern limit 

of extant autochthonous Belt rocks. That Belt strata older 

than Garnet Range Formation were deposited in the Argenta 

area and eroded prior to deposition of the Garnet Range cannot be proven, but such deposition of older Belt rocks seems 

unlikely. Lower and middle Belt strata are not known farther 

south than the Highland Mountains and the northeastern Pioneer Mountains. Those lower Belt rocks, particularly 

LaHood Formation, are interpreted as basin-margin deposits, and it is probable, therefore, that the lower and middle 

Belt was not deposited as far south as Argenta or the area of 

the present-day Tobacco Root Mountains and Ruby Range, 

despite the conclusions of Reynolds (1984) that the southern 

margin was that far south. The Missoula Group thins from 

several kilometers thick on the Grasshopper thrust plate to 

220 m thick at the Groundhog drill hole and to near zero at 

the American Quasar Haganbarth drill hole 16 km northeast 

of the Groundhog drill hole. Thickness of the Missoula 

Group in the Highland Mountains is, at most, nearly as great 

as that at the Groundhog drill hole, but erosion prior to deposition of Flathead Quartzite caused thinning of the Missoula Group to as little as 16 m (O'Neill, in press) and may 

have affected the maximum thickness as well. Thus, Middle 

Proterozoic-Cambrian erosion could well be responsible for 

some of the variations in thickness of the Missoula Group 

observed in the region. Despite those uncertainties, however, the youngest Belt unit was deposited on pre-Belt

basement, suggesting an onlap southern margin of the Belt 

Basin in southwestern Montana.

STRUCTURAL INTERPRETATION

Structural relations of the rocks exposed on the crest 

and east flank of the Humbolt Mountain anticline have been 

discussed by Shenon (1931), Myers (1952), Hobbs (1967), 

Brandon (1984), Ruppel and Lopez (1984), and Sears and 

others (1988), and depicted on a map by Ruppel and others 

(1993). The sequence of rocks at the Groundhog drill hole as 

described in this report, together with information derived 

from my recent field studies, helps to clarify structural relations and suggests modification of earlier conclusions. 

These new data show clearly that the Humbolt Mountain 

anticline is a basement-cored anticline similar to the Armstead anticline to the south (Lowell, 1965), the Biltmore 

anticline to the northeast (Schmidt and others, 1988), and 

numerous others in the foreland farther to the east. Many 

steep faults of various orientations break the rocks of the 

anticline, but all have small displacement and that displacement is mainly dip-slip. Previous interpretations of major 

through-going faults are not confirmed. It is much less clear 

if a blind thrust within the crystalline basement underlies the 

Humbolt Mountain anticline and the parallel folds in Frying 

Pan Basin to the east.

Mistakes in identification of stratigraphic units by previous workers have contributed to errors in interpretation of 

structure. The most notable example is a sequence of 

thin-bedded argillite and quartzite in the vicinity of the Midnight mine, which is about 670 m southwest of the Groundhog drill hole (fig. 2), that Shenon (1931) concluded was 

Proterozoic and was thrust against Paleozoic carbonate 

rocks. Myers (1952), Hobbs (1967), Brandon (1984), and 

Ruppel and Lopez (1984) concurred with Shenon's age 

assignment of these rocks. The rocks that Shenon identified 

as Proterozoic are green, gray, and red micaceous and sandy 

argillite and thin-bedded fine- to coarse-grained quartzite. 

This shaly interval is here reinterpreted as a Middle Cambrian unit, either Wolsey Shale or Silver Hill Formation; it 

will be referred to tentatively as Wolsey(?) Shale. Structurally, the Wolsey(?) near the Midnight mine is conformable 

between the Flathead(?) and overlying Cambrian dolostone, 

and hence, some of the faults mapped previously between 

these units are not required to explain juxtaposed units. Flathead^) Quartzite in the area of the Midnight mine and the 

Groundhog drill hole has also been regarded as Proterozoic 

by Hobbs (1967), Brandon (1984), and Ruppel and others 

(1993). They included both the Flathead(?) and the Wolsey(?) with the Garnet Range Formation to form a combined 

Proterozoic unit, thereby making the eastern contact of the 

combined unit seem reasonably to be a through-going fault.
CAMBRIAN(?), MIDDLE PROTEROZOIC, AND ARCHEAN ROCKS IN A BOREHOLE NEAR ARGENTA, MONTANA B13

EXPLANATION

Upper and Lower Mississippian 

Mission Canyon Limestone

Lower Mississippian 

Lodgepole Limestone

Upper Devonian

Three Forks Formation

Upper Devonian

Jefferson Dolomite

Upper Cambrian 

Pilgrim Dolomite

Upper and Middle Cambrian 

Hasmark Dolomite

Middle Cambrian 

Flathead Quartzite

Middle Proterozoic Garnet Range 

Formation (Missoula Group, 

Belt Supergroup)

Middle Proterozoic

Belt Supergroup, undivided

Middle Proterozoic quartzite, 

unit 3 of Hobbs (1967)

Middle Proterozoic quartzite, 

unit 2 of Hobbs (1967)

Archean schist

Brandon (1984)

D This report

Figure 12. Diagrammatic cross sections illustrating several interpretations of the structure of the east flank of Humbolt Mountain 

anticline.
B14 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Myers (1952) also used the absence of the Flathead(?) 

and locally other Cambrian units in some fault blocks and 

their presence in others (as well as other arguments) as evidence for faulting, folding, and erosion in Cambrian time. 

Although not all of the evidence cited by Myers has been 

borne out by recent work, his principal conclusion is 

probably correct. North of the Groundhog drill hole, the Flathead^) pinches out in a zone of faults, and, from there northward for about 3 km, Cambrian or Devonian dolostone is in 

depositional contact with Garnet Range Formation, or in 

some places, the contact may have been offset by steep faults 

of small displacement. Figure 12A is a diagrammatic representation of Myers' (1952) conclusions regarding the structure of the Humbolt Mountain anticline.

Hobbs (1967), who does not seem to have been aware 

of Myers' work and whose mapping is not as detailed as 

Myers', interpreted the east flank of the anticline to be broken by a through-going high-angle reverse fault whose trace 

is at the east edge of Proterozoic rocks, within which he 

included the Flathead(?) and Wolsey(?), as interpreted here, 

for the full length of the Humbolt Mountain anticline north 

of Argenta. Hobbs' interpretation is illustrated in Figure 

12B.

Hobbs' inferred reverse fault is the same structure that 

Brandon (1984) later reinterpreted as a gently west-dipping 

thrust fault (fig. 12C) that he named the Argenta thrust. 

Brandon's interpretation was also expressed by Sears and 

others (1988). These reports (Hobbs, 1967; Brandon, 1984; 

Sears and others, 1988) assert that a fault exists at this contact but, except for a line on a map, present neither evidence 

nor arguments for its existence, nor do they attempt to 

counter Myers' conclusions regarding orogeny and resulting 

erosion, sedimentation, and unconformities during the Cambrian. Except for some details, Myers' interpretation is in 

accord with recent field observations and, with the exception 

of the misidentification of the Wolsey(?), fits best with the 

relations found in the Groundhog drill hole. The conclusions 

of Ruppel and others (1993) are similar to Hobbs' (1965) 

conclusions in the southern part, but they retained a thrust 

fault farther north along the east side of Humbolt Mountain 

at the position of Brandon's (1984) Argenta thrust.

The trace of the Argenta thrust if such a structure 

exists would be at the Groundhog mine 150 m east of the 

Groundhog drill hole, and, if gently west-dipping as implied 

by Brandon (1984), should be evident in the Groundhog drill 

core as an inverted age sequence in which Garnet Range Formation overlies Paleozoic strata. The lack of such evidence 

in the drill core or any other evidence for a thrust fault provides additional doubt for this fault's existence. The relations in the drill core and at the surface show that the 

Proterozoic rocks are very likely in their normal stratigraphic 

position between the Archean basement and Cambrian 

strata, as shown on fig. 12£>, and are not riding on a thrust 

fault the same as, or similar to, the Kelley thrust a few kilometers to the west.

For similar reasons, the existence of the Ermont thrust, 

a structure proposed by Myers (1952) and discussed by Thomas (1981) to explain relations between Paleozoic and Cretaceous rocks near Ermont Gulch 3 km southwest of 

Argenta, is cast into doubt by the autochthonous relations in 

the Groundhog drill hole.

If thrust faults do exist in the sequence, the only alternatives for their location would seem to be bedding-parallel 

thrusts at some unknown positions in the rocks penetrated by 

the drill hole or a major decollement in the Archean basement at some depth below the bottom of the drill hole. Only 

the latter would seem to be likely in view of the strict parallelism of bedding and the complete absence of crumpling or 

minor folding in the core. Archean crystalline rocks core 

many anticlines in the foreland to the east, and they are also 

involved in thrust faulting in the region (DuBois, 1982; 

Schmidt and others, 1988). If such a decollement exists, its 

trace is unknown, and it may well be blind.

Except for the numerous high-angle faults of small displacement that break the rocks of the Humbolt Mountain 

anticline, the east flank of the anticline is homoclinal and 

passes eastward into the Cave Gulch syncline and, beyond 

that, into other folds in Frying Pan Basin (fig. 1) that trend 

north and parallel to the Humbolt Mountain anticline.

Between the American Quasar Haganbarth drill hole 

and the Groundhog drill hole, a distance of 16 km, structural 

relief on the basement surface is 3,817 m. The elevation of 

the surface rises from 1,980 m below sea level in the Haganbarth drill hole to 1,837 m above sea level in the Groundhog 

drill hole. Thus, between these two points the surface dips 

east at an average angle of 13.4°. Not being aware of the 

Groundhog drill hole, Lopez and Schmidt (1985) inferred a 

westward dip of the surface from east of the Haganbarth drill 

hole to the west and continuing at increasing depth beneath 

the thrust belt. This is clearly not the case unless the increase 

in elevation of the surface west of the Haganbarth drill hole 

is the result of thrusting within the basement and the surface 

continues to dip west in the lower plate of such a thrust.

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salient of the southwestern Montana fold and thrust belt: Missoula, University of Montana M.S. thesis, 128 p.

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Geach, R.D., 1972, Mines and mineral deposits (except fuels), Beaverhead County, Montana: Montana Bureau of Mines and 

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CAMBRIAN(?), MIDDLE PROTEROZOIC, AND ARCHEAN ROCKS IN A BOREHOLE NEAR ARGENTA, MONTANA B15

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McMannis, W.J., 1963, La Hood Formation A coarse facies of 

the Belt Series in southwestern Montana: Geological Society 

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quarter of the Willis quadrangle and adjacent Brown's Lake 

area, Beaverhead County, Montana: U.S. Geological Survey 

Open-File Report 147, 46 p., 2 pis.

Nelson, W.H, and Dobell, J.P., 1961, Geology of the Bonner 

quadrangle, Montana: U.S. Geological Survey Bulletin 

1111-F.p. 189-235.

O'Neill, J.M., in press, Table Mountain Quartzite and Moose 

Formation (new names) and associated rocks of the Middle 

Proterozoic Belt Supergroup, Highland Mountains, 

southwestern Montana: U.S. Geological Survey Bulletin 

2121-A.

Pettijohn, F.J., 1957, Sedimentary Rocks: New York, Harper, 

718 p.

Reynolds, M.W., 1984, Tectonic setting and development of the 

Belt Basin, Northwestern United States, in Hobbs, S.W., ed. 

The Belt: Montana Bureau of Mines and Geology Special 

Publication 90, p. 44^6.

Ruppel, E.T., and Lopez, D.A., 1984, The thrust belt in southwest 

Montana and east-central Idaho: U.S. Geological Survey Professional Paper 1278, 41 p.

Ruppel, E.T., O'Neill, J.M., and Lopez, D.A., 1993, Geologic map 

of the Dillon I°x2° quadrangle, Idaho and Montana: U.S. Geological Survey Miscellaneous Investigations Series Map 

I-1803-H, scale 1:250,000.

Schmidt, C.J., O'Neill, J.M., and Brandon, W.C., 1988, Influence 

of Rocky Mountain foreland uplifts on the development of 

frontal fold and thrust belt, southwestern Montana, in Schmidt, 

C.J., and Perry, W.J., eds., Interaction of the Rocky Mountain 

Foreland and the Cordilleran Thrust Belt: Geological Society 

of America Memoir 171, p. 171-201.

Sears, J.W., Johnson, L.M., Geiger, B.C., and Brandon, W.C., 

1988, Southwest Montana thrust belt: Bannack to Melrose, in 

Link, P.K., and Hackett, W.R., eds., Guidebook to the Geology 

of Central and Southern Idaho: Idaho Geological Survey Bulletin 27, p. 305-319.

Shenon, P.J., 1931, Geology and ore deposits of Bannack and 

Argenta, Montana: Montana Bureau of Mines and Geology 

Bulletin 6, 77 p.

Thomas, G.M., 1981, Structural geology of the Badger Pass area, 

southwest Montana: Missoula, University of Montana M.S. 

thesis, 58 p.

Wallace, C.A., 1987, Generalized geologic map of the Butte I°x2° 

quadrangle, Montana: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1925, scale 1:250,000.

Wallace, C.A., Lidke, D.J., Elliott, J.E., Desmarais, N.R., Obradovich, J.D., Lopez, D.A., Zarske, S.E., Heise, B.A., Blaskowski, 

M.J., and Loen, J.S., 1992, Geologic map of the Anaconda-Pintlar Wilderness and contiguous roadless area, Granite, 

Deer Lodge, Beaverhead, and Ravalli Counties, western Montana: U.S. Geological Survey Miscellaneous Field Studies Map 

MF-1633-C, scale 1:50,000.

Winston, Don, and Lonn, Jeff, 1988, Road Log No. 1: Structural 

geology and Belt stratigraphy of the Tarkio area, Montana, in 

Weidman, R.M., ed, Guidebook of the Greater Missoula Area: 

Missoula, Montana, Tobacco Root Geological Society 13th 

Annual Field Conference, p. 1-34.

Winston, Don, and Wallace, C.A., 1983, The Helena Formation 

and the Missoula Group at Flint Creek Hill, near Georgetown 

Lake, western Montana, in S.W. Hobbs, ed., Guide to field 

trips: Belt Symposium II, Missoula, Montana, p. 66-81.

Zen, E-an, 1988, Bedrock geology of the Vipond Park 15-minute, 

Stine Mountain 71 /2-minute, and Maurice Mountain 

7 1 /2-minute quadrangles, Pioneer Mountains, Beaverhead 

County, Montana: U.S. Geological Survey Bulletin 1625,49 p.

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication April 3, 1995 

Edited by Richard W. Scott, Jr. 

Graphics preparation and photocomposition by 

Gayle M. Dumonceaux

U.S. GOVERNMENT PRINTING OFFICE: 1996 - 774-045 / 20018 REGION NO. 8

SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of resource studies and of geologic and topographic investigations; as well as 

collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that may be cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases which show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from USGS Map Distribution, Box 25286, 

Building 810, Denver Federal Center, Denver, CO 80225. (See latest 

Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a set 

microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a set 

of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail and 

over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be obtained over the counter in paperback booklet form only. Those wishing a 

free subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.
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