Table Mountain Quartzite and Moose Formation (new names) and 

Associated Rocks of the Middle Proterozoic Belt Supergroup, 

Highland Mountains, Southwestern Montana

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Table Mountain Quartzite and Moose Formation (new names) and 

Associated Rocks of the Middle Proterozoic Belt Supergroup, 

Highland Mountains, Southwestern Montana

By J. Michael O'Neill

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-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
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

O'Neill, J. Michael.

Table Mountain Quartzite and Moose Formation (new names) and associated rocks of the 

middle Proterozoic Belt Supergroup, Highland Mountains, southwestern Montana / by J. 

Michael O'Neill.

p. cm. (U.S. Geological Survey bulletin ; 2121) (Belt supergroup in the Highland Mountains and probable equivalent rocks in the Pioneer and Anaconda ranges, southwestern Montana; A)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:2121-A

1. Geology, Stratigraphic Proterozoic. 2. Geology Montana Highland 

Mountains. 3. Table Mountain Quartzite (Mont.) 4. Moose Formation (Mont.) 

I. Title. II. Series. III. Series: Belt supergroup in the Highland Mountains and probable equivalent rocks in the Pioneer and Anaconda ranges, southwestern Montana ; A. 

QE75.B9 no. 2121-A 

[QE653.5]

557.3 s dc20 95-3772 

[551.7'15] CIP
CONTENTS

Abstract.......................................................................................................................... Al

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

Middle Proterozoic Regional Geologic Setting ..................................................... 2

Previous Work........................................................................................................ 3

Stratigraphic Nomenclature........................................................................................... 3

Lower Belt Rocks................................................................................................... 3

Lahood Formation.......................................................................................... 3

Lithologic Description........................................................................... 5

Facies...................................................................................................... 6

Moose Formation........................................................................................... 6

Type Locality and Type Section............................................................ 8

Contacts with Adjacent Units................................................................. 10

Facies...................................................................................................... 10

Table Mountain Quartzite.............................................................................. 12

Type Locality and Type Section ............................................................ 12

Contacts with Adjacent Units................................................................. 14

Facies...................................................................................................... 14

Newland Formation........................................................................................ 14

Lithologic Description........................................................................... 15

Contacts with Adjacent Units................................................................. 15

Facies...................................................................................................... 15

Ravalli Group......................................................................................................... 16

Greyson Shale................................................................................................ 16

Lithologic Description and Facies ......................................................... 16

Spokane Formation........................................................................................ 17

Lithologic Description........................................................................... 17

Contacts with Adjacent Units................................................................. 18

Facies...................................................................................................... 19

Empire Formation.......................................................................................... 19

Lithologic Description ........................................................................... 19

Middle Belt Carbonate Rocks................................................................................ 19

Helena Formation........................................................................................... 19

Lithologic Description........................................................................... 19

Contacts with Adjacent Units................................................................. 20

Facies...................................................................................................... 20

Missoula Group...................................................................................................... 21

Lithologic Description and Facies ................................................................. 21

Contacts with Adjacent Units......................................................................... 22

Middle Cambrian Rocks ........................................................................................ 22

Tectonics and Sedimentation......................................................................................... 22

References Cited............................................................................................................ 25

FIGURES

1. Index map showing location of study area, extent of Belt Supergroup rocks, and location of Helena

embayment of the Belt Basin .................................................................................................................................. A2

2. Map and cross section showing geology of study area ........................................................................................... 4

3. Correlation chart of rocks of the Belt Supergroup, western Montana..................................................................... 6

ill
IV CONTENTS

4-6. Photographs showing:

4. Coarse conglomerate fades of LaHood Formation....................................................................................... A7

5. Finer grained fades of LaHood Formation.................................................................................................... 8

6. Fades of LaHood Formation east of Twin Bridges fault............................................................................... 10

7, 8. Stratigraphic sections of:

7. Moose Formation........................................................................................................................................... 11

8. Table Mountain Quartzite.............................................................................................................................. 13

9-13. Photographs showing:

9. Upward-fining cycles in upper argillaceous quartzite member of Table Mountain Quartzite at

type section.................................................................................................................................................... 14

10. Flaggy bedding character of typical Newland Formation ............................................................................. 15

11. Soft-sediment folds in laminated Greyson Shale........................................................................................... 17

12. Spokane Formation showing laminated quartzite grading upward into nonlaminated

arenaceous siltite............................................................................................................................................ 18

13. Helena Formation showing laminated beds containing vugs aligned along bedding planes......................... 20

14. Stratigraphic sections of Middle Cambrian Flathead Sandstone and underlying Missoula Group ........................ 22

15. Block diagram showing structural relationships in study area................................................................................ 25
Table Mountain Quartzite and Moose Formation 

(new names) and Associated Rocks of the

Middle Proterozoic Belt Supergroup, 

Highland Mountains, Southwestern Montana

By J. Michael O'Neill

ABSTRACT

Sedimentary rocks of Belt age (Middle Proterozoic) in 

the Highland Mountains of southwestern Montana are 

exposed in an elongate east-trending zone that extends 

almost completely across the range. Strata of the Belt Supergroup are composed mainly of lower Belt rocks and Ravalli 

Group rocks overlain successively by middle Belt carbonate 

rocks, which pinch out westward in the range and a thin 

sequence of Missoula Group quartzite. Striking lateral facies 

changes in all these sedimentary rocks are accompanied by 

thickening of the succession toward the central part of the 

range. Major facies changes are present across northwest- to 

north-trending faults that are the northward continuation of 

conspicuous northwest-trending faults that cut basement 

rocks to the south. These faults extended into the Belt Basin 

in Middle Proterozoic time and acted as growth faults during 

deposition of Belt sediments. Belt-age rocks were deposited 

in northwest-trending, fault-bounded half grabens or structural blocks outlined by these growth faults; these structural 

blocks step down eastward and outline a large, composite 

northwest-trending graben, herein called the Highland graben, that is manifested as a Middle Proterozoic paleovalley 

along this part of the southern margin of the Belt Basin. 

Facies relationships among rocks within the graben show 

that lower Belt rocks and the Greyson Shale of the Ravalli 

Group are laterally gradational and were deposited simultaneously. The lower Belt deposits, represented by the LaHood 

Formation, the newly named Moose Formation and Table 

Mountain Quartzite, and the Newland Formation are subaerial to near-shore deposits that interfinger with subaqueous 

calcareous muds of the Newland Formation, which, in turn, 

grade laterally into laminated silty muds of the Greyson 

Shale of the Ravalli Group that were deposited farther from 

shore. Variations in stratigraphic thickness and hanging-wall 

onlap of these lower Belt strata suggest syndepositional fault 

activity, widening of the graben during Belt deposition, and

a gradual transition from subaerial to subaqueous deposition 

through lower Belt time. Crossbedded, argillaceous siltstone 

and sandstone of the Spokane Formation were deposited uniformly above the Greyson Shale. The western margin of the 

graben was subaerially exposed in middle Belt time. Carbonate bank deposits of the Helena Formation interfinger with 

calcareous argillite of the Empire Formation in and near the 

central part of the graben but pinch out to the west in the 

Highland Mountains and thin to the east. Younger alluvial 

conglomerate and fluvial sandstone of the Missoula Group 

were deposited on the older strata. The main depositional 

center for these younger rocks was also along the graben 

axis, and the younger rocks thin to a few tens of meters to the 

west.

INTRODUCTION

In southwestern Montana rocks of the Middle Proterozoic Belt Supergroup are exposed along an east-trending 

structural zone that marks the southern margin of the Helena 

embayment of the Belt depositional basin (fig. 1). The lithology and general stratigraphic relations of rocks in the Helena 

embayment were first summarized by Ross in 1963; however, details of the stratigraphy of most of these rocks along 

the southern margin of the embayment are still not well 

known because of stratigraphic and structural complexities 

and poor exposures.

In this report I briefly review the history of nomenclature of Belt-age rocks of the Helena embayment and apply 

embayment nomenclature, with some minor revisions and 

important additions, to rocks of the Belt Supergroup in the 

Highland Mountains. The revised nomenclature of Middle 

Proterozoic sedimentary rocks is modified from informal 

nomenclature used in recently completed mapping of the 

Dillon I°x2° quadrangle (Ruppel and others, 1993).

Al
A2 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

117

49

TRANSVERSE ZONE

Area shown 

in figure 2 . __. __. __

MONTANA, -v WYOMING

Figure 1. Index map showing location of study area, extent of 

Belt Supergroup rocks, and location of Helena embayment of the 

Belt Basin, northwestern United States.

MIDDLE PROTEROZOIC REGIONAL 

GEOLOGIC SETTING

The Belt Basin of the northern Rocky Mountains is an 

elongate northwest-trending depositional trough that began 

to form about 1.5 Ga (Harrison, 1972; Obradovich and others, 1984). The oldest deposits in the northern part of the 

basin consist of deep-water turbidites (Harrison and others, 

1974; Cressman, 1984, 1985, 1989) that are locally intruded 

by mafic sills. The Crossport C sill in the western part of the 

basin, dated 1.43 Ga (Zartman and others, 1982), places a 

minimum age on the oldest sedimentary rocks in the basin 

(Obradovich and others, 1984). Approximately coeval with 

sill emplacement was intrusion of a series of northwesttrending diabasic dikes into cratonic rocks directly south of 

the Helena embayment (Koehler, 1973, Wooden and others, 

1978); two episodes of dike emplacement are recognized in 

the Tobacco Root Mountains and Ruby Range, east and 

southeast, respectively, of the Highland Mountains (fig. 2A). 

Dike emplacement along fractures in cratonic rocks adjacent 

to the Belt Basin coincided with the early part of the basin's 

tectonic evolution (Wooden and others, 1978; Schmidt and 

Garihan, 1986). Similar mafic dikes intruded Early Proterozoic cratonic rocks of the Highland Mountains (Duncan, 

1976; Garihan and others, 1981; O'Neill and others, 1988; 

O'Neill and Schmidt, 1989).

Rocks deposited in the Helena embayment are separated from cratonic rocks to the south by the low-angle 

Camp Creek fault zone in the Highland Mountains (McMannis, 1963; O'Neill and others, 1986) and by right-lateral,

oblique-slip thrust faults of the Southwest Montana transverse zone farther to the east along the northern edge of the 

Tobacco Root Mountains (Schmidt and O'Neill, 1982) (fig. 

2A). Along this zone sedimentary rocks of the Belt Supergroup consist of a basal conglomeratic and arkosic facies of 

the LaHood Formation. The angular and extremely coarse 

nature of clasts in the LaHood and the geographic restriction 

of the unit to areas directly adjacent to cratonic rocks suggest 

a steep, perhaps structurally controlled, east-trending southern margin to the Belt Basin in this area (McMannis, 1963; 

Harrison and others, 1974; Schmidt and Garihan, 1986). The 

Southwest Montana transverse zone consists of a series of 

younger east-trending faults that show characteristic rightreverse slip. The transverse zone is inferred to have been 

superimposed along an older zone of concealed high-angle 

faults of the Middle Proterozoic Willow Creek fault zone of 

Robinson (1963); the entire zone of older faults, coarse sedimentary debris, and younger thrust and oblique-slip faults 

can be traced for more than 100 km eastward from the Highland Mountains (Schmidt and O'Neill, 1982).

Sedimentary rocks of Belt age in the Highland Mountains crop out along an elongate east-trending zone that 

extends almost completely across the range; the zone is 

about 25 km long and as wide as 8 km (fig. 2A). The sedimentary rocks are in fault contact on the south with cratonic 

basement rocks, are overlain by or structurally juxtaposed 

against Paleozoic sedimentary rocks on the north, and are 

intruded by the Boulder batholith on the east. This sedimentary succession is composed mainly of lower Belt rocks and 

Ravalli Group rocks, overlain, in part, by middle Belt carbonate rocks below a thin succession of Missoula Group 

quartzite. Striking lateral facies changes in all of these sedimentary rocks (fig. IE) are accompanied by an overall thickening of the succession from about 1,000 m on the west to 

more than 3,000 m in the central part of the range. The major 

facies changes coincide with northwest- to north-trending 

faults that are the northward continuation of conspicuous 

northwest-trending faults that cut basement rocks to the 

south. These faults extended into the Belt Basin in Middle 

Proterozoic time and acted as growth faults during deposition of Belt sediments. The formations described in this 

report were measured in four northwest-trending step grabens or structural platforms outlined by northwest-trending 

faults. The structural platforms are designated, from west to 

east, the Moose Creek, Soap Gulch, Camp Creek, and Table 

Mountain blocks (fig. IE). Maximum subsidence was centered in the Table Mountain block and defines a northwesttrending structural graben that was the locus of a Middle Proterozoic paleovalley during deposition along this part of the 

southern margin of the Belt Basin. Measured stratigraphic 

sections extend from the top of the LaHood Formation to the 

base of the Middle Cambrian Wolsey Shale and include the 

Middle Cambrian Flathead Quartzite. The entire LaHood 

was not measured in the field because of generally poor 

exposures in the area; the Flathead is included in these
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A3

sections because of the local difficulty in the Highland 

Mountains in differentiating between quartzite of the uppermost Missoula Group and quartzite of the basal Flathead 

(McMannis, 1963; Smedes, 1967).

PREVIOUS WORK

Published reports on the geology of the Belt Supergroup in the Highland Mountains are few. Sahinen (1939) 

was apparently the first to describe Belt sedimentary rocks in 

this area, and McMannis (1963) later outlined the general 

stratigraphic relationships of Belt strata in the Highland 

Mountains in his discussion of the LaHood Formation along 

the southern margin of the Belt Basin. Thorson (1984) 

described the lower Belt sequence in the range and outlined 

basic stratigraphic relationships among these lower Belt 

units. The entire sequence of rocks in the range was briefly 

described by O'Neill and others (1986) and O'Neill (1989).

STRATIGRAPHIC NOMENCLATURE

Nomenclature for Middle Proterozoic sedimentary 

rocks exposed in western Montana was first summarized by 

Ross (1963) in his seminal paper on the "Belt Series." Modifications to that nomenclature were made by Harrison 

(1972, fig. 5), and stratigraphic relations and correlations 

among major subdivisions within the Belt Supergroup proposed by Harrison are mostly followed in this report (fig. 3). 

The Belt Supergroup of western Montana is divided into four 

distinct sequences, from lowest to highest, lower Belt rocks, 

Ravalli Group, middle Belt carbonate rocks, and Missoula 

Group. All these sequences are present and generally well 

exposed in the Highland Mountains.

Lower Belt rocks in the Highland Mountains are, from 

oldest to youngest, the LaHood Formation, Moose Formation and Table Mountain Quartzite (new names), and Newland Formation. These rocks are overlain by the Greyson 

Shale and Spokane and Empire Formations of the Ravalli 

Group. Middle Belt carbonate rocks are represented by a 

westward-thinning tongue of the Helena Formation. The 

uppermost Belt sequence, the Missoula Group, consists of a 

thin sequence of interbedded quartzite, argillite, and conglomerate that have been mapped as lithologic units in the 

central part of the Highland Mountains (O'Neill and others, 

1995) but were not divided into formations. All Belt-age 

rocks in the Highland Mountains show significant lateral 

facies and thickness changes from east to west. The major 

changes are across north- to northwest-trending high-angle 

faults that are the northward continuation of prominent structures in cratonic rocks directly to the south. Facies relationships between formations of the lower Belt and the Greyson 

Shale are laterally gradational from graben margin to graben

axis, and the various units were deposited simultaneously. 

The lower Belt deposits represent subaerial to near-shore 

sediments that interfinger laterally with subaqueous calcareous mud of the Newland Formation, which, in turn, grades 

into laminated silty mud of the Greyson Shale farther from 

shore. Variations in thickness and hanging-wall onlap of the 

lower Belt strata suggest syndepositional fault activity, widening of the graben during Belt deposition, and a transition 

from alluvial and fluvial to subaqueous deposition through 

time.

Lithologic terminology for fine-grained sedimentary 

rocks of the Belt Supergroup commonly has been restricted 

to the terms argillite, siltite, and quartzite in lieu of shale, 

siltstone, and sandstone (J.E. Harrison, U.S. Geological Survey, oral commun., 1993). This restricted terminology is 

applied because of the ubiquitous, low-grade, burial metamorphic aspect of most of the supergroup. In the Highland 

Mountains the Belt Supergroup, although never deeply buried, shows contact metamorphism from plutons of the Boulder batholith that grades from albite-epidote-hornfels to 

hornblende-hornfels facies, depending on proximity to adjacent plutons. Because of the variable contact metamorphic 

grade, from unmetamorphosed to strongly porphyroblastic 

and recrystallized, the terms argillite, siltite, and quartzite 

are used for all rocks in order to avoid confusion regarding 

lithology.

LOWER BELT ROCKS

LAHOOD FORMATION

The LaHood Formation, the name proposed by Alexander (1955) for the coarse facies of the Belt Supergroup that 

Ross (1949, 1963) named the North Boulder Group, is well 

exposed in the Highland Mountains. The type locality of the 

LaHood is near the small community of LaHood at the northwest corner of the Tobacco Root Mountains, about 25 km 

east of LaHood outcrops in the Highland Mountains. 

Because the type locality includes only the upper part of the 

formation, McMannis (1963, p. 409) amended the formation 

description to "include all dominantly coarse Belt strata 

along the southern margin of the central Montana embayment of the Belt geosyncline*** Arbitrarily the name applies 

where Belt strata consists of less than 75 percent of finegrained rocks and greater than 25 percent of coarse arkosic 

debris."

The LaHood Formation in the Highland Mountains 

crops out in a narrow east-trending zone that separates the 

finer grained overlying Belt strata to the north from crystalline basement rocks to the south. The formation was deposited directly on Early Proterozoic crystalline basement 

rocks; this unconformity is now the locus of the Camp Creek 

fault. The formation consists of cobble to boulder conglomerate and breccia, quartz-pebble conglomerate, arkose, and
112°45

45°45' 

CREEK CAMP CREEK 

BLOCK BLOCK

SOAP GULCH 

BLOCK

EXPLANATION

Quaternary and Tertiary depositsMesozoic plutonic rocks

Paleozoic sedimentary rocks

Calc-silicate rockArgilliteQuartziteConglomerate

Basement rocks

Contact

 + Antiform

Middle Proterozoic Belt Supergroup

Normal fault Ball and 

bar on downthrown side

Thrust fault Sawteeth 

on upper plate

03

S

00

C TJ 

W ?0 

O ?0 O

c

zOtn 

O

C

r 

H

|C/5

O

m

onO

Z H
WEST

Spokane Formation Helena and Empire Formations, undivided

EAST

Missoula Group

Moose Formation

Newland Formation

Greyson Shale 

Table Mountain Quartzite

Lahood Formation

Early Proterozoic Cratonic Basement Rock 

1,000 METERS >\

Lahood Formation, coarse facies

Early Proterozoic Cratonic Basement Rock 

CAMP CREEK BLOCK V Table Mountain Block SOAP GULCH BLOCK 

\

MOOSE CREEK BLOCK

TWIN BRIDGES 

FAULT

SILVER STAR 

FAULT

SOUTH ROCHESTER 

FAULT

B

Figure 2. Map and cross section showing geology of study area, southwestern Montana. A, Generalized lithofacies map of the Highland Mountains and the northwestern part of the 

Tobacco Root Mountains; map units of the Belt Supergroup sedimentary rocks are lithofacies units and do not everywhere reflect individual formations within the supergroup; location of type sections of the Table Mountain Quartzite (solid circle) and the Moose Formation (solid triangle) are also shown. B, Diagrammatic cross section showing stratigraphic relations of Belt Supergroup across the Highland Mountains and relation of depositional units to underlying basement structural blocks.

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approximately 125m of theLaHood measured inthisarea can be divided intotwo units. Thelower 108m is aclatic directly southof theCamp Creekfault. Classare generally subangular to subrounded anres log about0.5 m (fig. 4). Matrix consists ofsand, granules, andpebbles commonly isgrayish green to brwn. 

Conglomerate lensesbecome finergraind (fg.4E) and 

pinch out laterally tohenortheast, wherethey gradesucces- sively intofine- to coarse-grained quartzite, siltite, andsilty argillite. Thesefiner grained dstalfacies showabundant cut- and-fill structures, load casts, soft-sediment folds(figs. 5A,E), and, locally, inthemost distal parts, upward-fining Bouma sequences (fig. 5Qtypical ofturbidite deposits. 

Rocks oftheLaHood directly overlying basalconglomeratic faciesare generally poorlyexposed; however, thuppermost LaHood iswellexposed belowthe type section ofthe Moose Formation, west of TwinBridges fault.The (fig. 2A)Clasts intheconglomerate (fig.4A) consist ofbiotite andquartzofeldspathic gneissthat probably was derived fromPrecarnbrian cratonic rocksnow exposed cut the crystalline basement rocksto south; conglom- erate thins northeastward, away from thefault intersections

n3§CD

3- OCD 

|

V)I3 

era

*-he 5?CO

erentially located a or nearthe baseofsequence. The conglomerate isthickest at theintersections ofCamp

In thepackage west of TwinBridges fault,poorly 

bedded to massive, cobblebulder conglomerate isprefTable Mountain blockeast of thefault. argillite ofprbable subaqueous debrisflow origin, andminor fluvial conglomerate characterize theformation inthe Twin Bridges fault(ig. 2A),whereas well-bedded quartz and lithicpebble bouldr conglomerate, coarse-grained lithic quartzite containing rip-upclasts ofdark-gray siltite- be divided intotwo sedimentary packages. Coarse-grained conglomeratic alluvial deposits interlayered withsubaerial and subaqueous quartzite dominate theformation west of the

The LaHood Formation intheHighland Mountains can

o oonm

3

Ho
A6 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Figure 3. Correlation of Belt Supergroup units in western Montana and the Helena embayment (Harrison, 1972) and units exposed in the 

Highland Mountains.

The beds range from less than 0.5 to as much as 7 m in thickness and internally each fines upward. The base of each bed 

is marked by granule to pebble conglomerate that grades 

upward into coarse quartz-rich quartzite. Almost all upwardfining beds are capped by dark-gray silty argillite. These 

upward-fining beds are distinct and generally do not grade 

into overlying and underlying beds. Most bedding contacts 

are planar but locally define lens-shaped channel deposits.

The upper 17 m of the LaHood Formation consists of an 

overall upward-coarsening clastic sequence. Beds in the 

lower part are characterized by upward-coarsening silty 

argillite to very fine grained quartzite 0.2-0.5 cm thick. The 

upper part is channel quartzite that coarsens upward from 

medium to coarse sand in beds that have irregular, wavy 

tops.

FACIES

coarse-grained lithic arenite and quartzite. The quartzite is 

poorly sorted and commonly encloses floating granules and 

pebbles of subrounded lithic clasts (fig. 6£). Lenses of lithic 

cobble conglomerate are present in the quartzite. These 

lenses rarely exceed 3 m in thickness, and they are locally 

associated with rip-up clasts and lenses of intraformational 

conglomerate (fig. 6A). More commonly, the quartzite contains lenses of upward-fining quartz-pebble conglomerate 

less than 20 cm thick. The best exposures of the coarsegrained quartzite are directly east of the Twin Bridges fault, 

in a triangle-shaped area bounded on the east by the Hell 

Canyon pluton, a satellite of the Boulder batholith (fig. 2A).

In the Highland Mountains the LaHood Formation is 

overlain by the Moose Formation except in the central part 

of the Table Mountain block, where well-rounded pebble 

and cobble conglomerate of the LaHood is overlain by the 

Table Mountain Quartzite (fig. 2£).

A distinctively different succession of the LaHood 

strata is only present in the Table Mountain block, east of the 

Twin Bridges fault. In this area, LaHood strata consist 

mainly of thick-bedded to massive, locally conglomeratic,

MOOSE FORMATION

The Moose Formation is the name herein applied to 

a thin sequence of argillite and siltite that overlies the
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A7

Figure 4. Coarse conglomerate facies of LaHood Formation. Outcrop along Camp Creek on the southeast side of King 

and Queen Hill, Wickiup Creek 7 '/>-minute quadrangle. A, Boulder and cobble conglomerate; the long axis of the boulder 

right of center is 0.45 m. B, Granule and pebble conglomerate, a lateral northeasterly facies of the boulder and cobble conglomerate. Quarter in lower part of photograph is shown for scale.
A8 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

LaHood Formation and underlies the Newland Formation 

in the western part of the Highland Mountains. These 

rocks were assigned to the Prichard Formation by 

Thorson (1984) and to the Chamberlain Shale by Ruppel 

and others (1983), and they were described but not named 

by Ruppel and others (1993). Although the Prichard Formation of the western Belt Basin is in part a stratigraphic 

correlative of the Moose of the Highland Mountains (fig. 

3), the nearest outcrops of unquestioned Prichard Formation are about 150 km west-northwest of the Highland 

Mountains, and the Prichard is generally restricted to sedimentary successions commonly more than 6 km thick 

(Cressman, 1989) in that part of the basin where Helena 

embayment nomenclature is not applied.

The Chamberlain Shale nomenclature is also not 

used for these rocks. The Moose and Chamberlain occupy 

the same stratigraphic position below the Newland Formation (fig. 3) but in vastly different parts of the Helena 

embayment. The Chamberlain, a 500-m-thick black argillite that was deposited directly above the Neihart Quartzite, is restricted to the Little Belt Mountains, 150 km 

northeast of the Highland Mountains, and is lithologically 

different from the thin silty argillite and siltite of the 

Moose Formation.

A separate nomenclature for lower Belt rocks has 

generally been used in the geographically isolated Helena 

embayment (fig. 3). I prefer to continue that tradition but 

establish a new formation name for a geographically isolated unit, unique in its stratigraphic and sedimentologic 

significance, that may or may not be a facies of either the 

Prichard Formation or the Chamberlain Shale. Thus, the 

names Prichard and Chamberlain are not used and the 

name Moose is applied to the fine-grained clastic rocks 

below the Newland and above the LaHood in the Highland Mountains along the southern margin of the Helena 

embayment of the Belt Basin.

TYPE LOCALITY AND TYPE SECTION

The type locality of the Moose Formation (solid triangle, fig. 2A) is along the north side of Moose Creek, for 

which the unit is named, and the access road to the Humbug 

Spires Primitive Area. Moose Creek drains the western part 

of the Highland Mountains and empties into the Big Hole 

River about 10 km north of Melrose, Montana. The type 

locality is on Bureau of Land Management lands, adjacent to 

Deerlodge National Forest, near the line between sees. 26 

and 27, T. 1 S., R. 9 W.; a graphic type section of the Moose 

Formation is shown in figure 7. The thickness of the Moose 

Formation is at least 48 m at the type section, and, similar to 

all other formations of Belt age in the Highland Mountains, 

it thickens eastward; at Camp Creek, 11 km away, the formation is about 85 m thick (fig. 7).

Figure 5 (above and facing page). Bedding character of finer 

grained facies of the LaHood Formation. A, Thin- to medium-bedded quartzite and argillaceous siltite. Quarter in central part of photograph is shown for scale, fi, Soft-sediment folds in interlayered 

quartzite, siltite, and silty argillite. Pencil is shown for scale. C, 

Typical Bouma sequence in distal parts of LaHood Formation. 

Quarter is shown for scale. Photographs (A) and (fi) are of exposures along Soap Gulch near its confluence with Left Fork Soap 

Gulch, Melrose 7 '/z-minute quadrangle. Photograph (C) is of an exposure on the lower slopes, southeast side of King and Queen Hill, 

Wickiup Creek 7 lk -minute quadrangle.

At the type section, the Moose Formation is separated 

from pebbly quartzite of the underlying LaHood Formation 

by a small north-dipping oblique-slip shear zone. Both the 

bedding and the shear zone dip north, and the angle between 

the two planar features is about 30°. The shear zone cannot 

be traced laterally; hence, the amount of stratigraphic section 

missing, although not known, is probably not large. The 

lower 9 m of the Moose consists mainly of even to wavy parallel-laminated, argillaceous siltite containing 1-cm-thick 

silty, less commonly crossbedded, sand-filled channels. The 

upper 39 m consists mainly of even- to wavy-laminated, 

medium-gray platy argillaceous siltite and dark-gray silty 

argillite enclosing very fine grained quartzite lenses 0.5 cm
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A9

Basal sands of

overlying 

turbidity flow

C(?) and Fine-grained pelagic 

r\ sediments cap flow

B

Fine-grained

sandstone

deposited in lower

- - and 

upper 

flow regimes

Upward-fining coarse

to fine-grained

sandstone

Rapid deposition

Base of turbidity flow 
A10 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Figure 6. Facies of the LaHood Formation east of the Twin Bridges fault, 0.5 km south of Sawmill Gulch, Wickiup Creek 7>/z-minute 

quadrangle. A, Floating clasts of lithic pebbles and cobbles and angular clasts of argillite and siltite adjacent to hand in coarse-grained quartzite. B, Coarse lithic quartzite containing abundant lithic granules and thin lenses of quartz-rich pebble conglomerate; note floating boulder 

of quartzofeldspathic gneiss behind hammer.

thick. Minor soft-sediment slumps and rare ripple marks are 

also present.

CONTACTS WITH ADJACENT UNITS

At the type section, basal parallel-laminated silty argillite of the Moose Formation is in fault contact with arkosic 

quartzite and conglomerate of the LaHood Formation. The 

contact is abrupt and is marked by a fault breccia that dips 

about 60° northward. The upper contact of the Moose Formation is placed at the first occurrence of Newland-like beds 

that mark the base of the transition zone; the zone is gradational into calcareous, blocky bedded silty argillite of the 

Newland Formation. The transition zone is 34.5 m thick and 

composed of dark-gray argillaceous siltite and thin siltite 

lenses interlayered with medium-gray argillaceous siltite

beds typical of the Newland. The beds representative of the 

Newland are 1-2 cm thick, internally homogeneous and 

structureless, and separated by even-parallel argillaceous 

siltite lamellae.

FACIES

The Moose Formation thickens eastward, across the 

Soap Gulch block, to about 90 m in the Camp Creek block.

Figure 7 (facing page). Stratigraphic correlation of sections of 

the Moose Formation at the type section (sees. 26, 27, T. 1 S., R. 9 

W.) and in the Soap Gulch and Camp Creek blocks to the east. Locations of type section and the Soap Gulch and Camp Creek blocks 

are shown in figure 2.
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION All

WEST 

MOOSE CREEK BLOCK

TYPE SECTION, 

MOOSE FORMATION

SOAP GULCH BLOCK

EAST 

CAMP CREEK BLOCK

Argillite

Siltite

Silty argillite

Quartzite and conglomerate

EXPLANATION

Calc-silicate rocks and

calcareous nodules 

Igneous rocks

Slump folds

Blocky bedding (Newland)

Fault

Channel deposit and

crossbedding 

Upward-fining sequence

Upward-coarsening sequence
A12 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

At Camp Creek the lower 61m consists mainly of argillaceous siltite and minor thin interbeds of argillite and finegrained quartzite. The uppermost 29 m lacks the thin interbeds of quartzite.

Only a thin sliver of the Moose Formation was recognized to the east in the Table Mountain block. In this area, 

the normal stratigraphic position of the Moose, as well as 

part of the overlying Newland, is occupied by the Table 

Mountain Quartzite.

TABLE MOUNTAIN QUARTZITE 

TYPE LOCALITY AND TYPE SECTION

The type locality of the Table Mountain Quartzite is 

along the south side of East Peak, 1.6 km east of Table 

Mountain, the highest peak in the Highland Mountains. This 

locality (solid circle north of the Hell Canyon pluton, fig. 2A) 

is in the Deerlodge National Forest and crosses the line 

between sees. 14 and 23, T. 1 S., R. 7 W. A graphic section 

of the type Table Mountain Quartzite is shown in figure 8. In 

the type locality area, the Table Mountain Quartzite underlies the highest peaks of the Highland Mountains and is recognized by its distinctive white color that contrasts markedly 

with the red argillite and siltite of the Greyson Shale that 

underlies the other high peaks of the range.

The formation was named for the conspicuous white 

quartzite exposed on Table Mountain; however, the type 

locality is on East Peak because the entire formation is best 

exposed in the south-facing glacial cirque carved into the 

southern face of the peak. The Table Mountain Quartzite is 

about 528 m thick at the type section, but the unit pinches out 

to the west and thins to the east where it is cut out by the 

Rader Creek and Donald plutons of the Boulder batholith. 

The Table Mountain can be divided into two informal members: a lower quartzite member 262 m thick consisting 

mainly of massive, thick-bedded, cliff-forming white quartzite and an upper argillaceous quartzite member 266 m thick 

consisting of thin-bedded, upward-fining quartzite interlayered with minor siltite and argillite (fig. 8). At the type section, the lower quartzite member consists of three units: a 

lower unit, A, and upper unit, C, of thick-bedded quartzite 

separated by an interval (unit B) of thin-bedded quartzite, siltite, and argillite (see fig. 8). Three small faults are present in 

the lower quartzite member but probably do not cut out much 

of the section. The lowermost 80 m of unit A is poorly 

exposed and consists of fine- to medium-grained quartzite 

that rests directly on lithic pebble to cobble conglomerate of 

the LaHood Formation. Overlying this poorly exposed 

quartzite is about 100 m of thick-bedded to massive, cliffforming orthoquartzite composed of very fine grained to 

medium-grained, subrounded quartz grains. Bedding in the 

orthoquartzite ranges from 1 to more than 5 m in thickness; 

layering in these massive beds is locally defined by seams

and lamellae of heavy-mineral concentrates. The base of the 

orthoquartzite is associated with thin seams of purplish-gray, 

very fine grained argillaceous quartzite and intraformational 

(mudchip) conglomerate. The uppermost 15 m of the orthoquartzite is thin- to thick-bedded, tan to yellowish-white 

quartzite.

Unit A is overlain by about 45 m of interlayered quartzite and argillaceous quartzite and siltite (unit B) in beds 5-50 

cm thick (fig. 8). The lower beds of unit B consist of upwardfining, medium- to fine-grained, well-sorted quartzite that 

locally fills small channels cut into beds of argillaceous 

quartzite and siltite. The upper contacts of the channel fill are 

sharp and abrupt. Unit B is in sharp contact with the overlying 37-m-thick, massive white quartzite of unit C, which is 

similar to the white cliff-forming quartzite in the lower part 

of the section (unit A).

The upper argillaceous quartzite member of the Table 

Mountain Quartzite consists predominantly of thin-bedded 

quartzite interlayered with minor conglomerate, argillaceous 

and silty quartzite, siltite, and argillite; the contact with the 

underlying quartzite member is sharp. The member can be 

divided into two parts (see fig. 8). The lower part, unit A, 

contains numerous well-developed upward-fining cyclical 

units 1-2 m thick. Characteristically the cycles consist of 

four parts, each grading successively upward into the next 

part (fig. 9): (1) basal conglomerate of clasts of the underlying argillaceous quartzite, (2) poorly sorted coarse- to finegrained quartzite, (3) well-sorted medium-grained orthoquartzite, and (4) a sequence of interlayered argillaceous 

quartzite and clean, white to pink quartzite in beds as thick 

as 5 cm that grades upward into gray, laminated argillaceous 

quartzite. The upper part (unit B) of the member is 166 m 

thick and contains numerous partial cycles commonly consisting of upward-fining medium-grained orthoquartzite 

overlain by thin-bedded argillaceous quartzite (parts 3 and 4 

of the entire cycle). In the uppermost 50 m of unit B, these 

partial cyclic deposits are characterized by numerous softsediment folds and slumps.

A lens at least 5 m thick of intraformational conglomerate and slump breccia, which is not present at the type section, is poorly exposed in a glacial cirque on the northwest 

side of Table Mountain. The conglomerate is mostly covered 

by talus and glacial moraine but can be traced along strike for 

0.5 km. It is composed of rounded to subangular boulders of 

quartzite as much as 0.5 m across. The unit is near the top of 

the lower quartzite member of the Table Mountain Quartzite.

A similar, areally restricted slump breccia containing 

clasts as much as 5 m across is present in the Greyson Shale 

that overlies the Table Mountain Quartzite and is also 

restricted to the glacial cirque on the northwest side of Table 

Mountain. The restricted areal extent and the large size of the 

slump blocks within the Table Mountain Quartzite and the 

overlying Greyson Shale suggest that the breccias reflect 

localized syndepositional tectonism.
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A13

Poorly 

exposed

Lower quartzite 

member

LaHood Formation

\ 

\ 

l Upper

l 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

Lower

METERS \ 

-20 i

-10

L 0

000 0 0 0 Q0 QC 0 CQCQC000°<£j »

^T > 

o o o o 6 O J

 o o - o * o . 6 b 6 oUpper argillaceous 

quartzite member

- 6 "6* 6- '6 o-b- 6'- o- b- b- b

-6 -o- 6- b b- b- 6'- 6- b- b-

-6 o- 6- o o- b- 6 - 6- o- b- o

o.......or Q-- Q-- o.- o I

Lower quartzite 

member '

Greyson 

Shale

.. o 0 0 O 0 °

- o -o o o -oj?

Upper argillaceous 

quartzite member

Figure 8. Stratigraphic section of the Table Mountain Quartzite at the type section (sees. 14, 23, T. 1 S., R. 7 W.) on East Peak. Informal upper and lower members are divided into units A, B, and C, as discussed in text. Lithologic explanation is given in figure 7.
A14 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Figure 9. Upward-fining cycles in the upper argillaceous quartzite member 

of the Table Mountain Quartzite at the type section of the Table Mountain 

Quartzite. Hammer is shown for scale. A, Thick-bedded, medium-grained 

quartzite overlain by intraformational and quartzite-pebble conglomerate that 

grades into fine- to medium-grained quartzite. B, Dark-gray, silty argillite interlayered with thin lenses of light-gray quartzite; these argillic rocks cap 

many of the upward-fining cycles.

CONTACTS WITH ADJACENT UNITS

At the type section the Table Mountain Quartzite lies 

conformably on lithic, pebble to cobble conglomerate of the 

LaHood Formation. The top of the Table Mountain Quartzite 

is placed at the top the highest white quartzite bed below the 

Greyson Shale. West of the type section the Table Mountain 

interfingers with the Newland Formation. East of the type 

section the Table Mountain Quartzite rests directly on pebble 

conglomerate of the LaHood, contains a tongue of the Newland in the middle as well as 28 m below the top of the formation, and is overlain by the Greyson Shale.

FACIES

The Table Mountain Quartzite pinches out to the west 

of the type area and is not present west of the Twin Bridges 

fault (fig. 25). On the unnamed peak 2 km directly west of 

Table Mountain, the formation is 240 m thick, less than half 

the thickness at the type section at East Peak 2 km to the east. 

On the unnamed peak, the quartzite rests conformably on the 

Newland Formation. Here the quartzite is white and medium 

to thick bedded, similar to the lowermost quartzite at the type 

section but including a greater argillaceous component. The 

Table Mountain Quartzite is also overlain by thin-bedded 

argillite and siltite of the Newland Formation.

East of the type section the formation has been intruded 

by plutons of the Boulder batholith. At the easternmost exposures adjacent to the batholith, the quartzite rests directly on

conglomerate of the LaHood. The formation, poorly exposed 

in this area except for the lowermost 30 m, is about 320 m 

thick. The exposed lowermost beds consist of medium-bedded, fine- to medium-grained channel-fill quartzite and laminated to massive, tabular quartzite. Beds are commonly 

separated by thin argillaceous seams or laminated argillaceous quartzite. Cyclical deposits, similar to those in the 

upper part of the Table Mountain at the type section, are rare. 

The lowermost part of the section is overlain by 100 m of 

mostly covered section that is interpreted from float to 

include an unknown percentage of white to pink, fine- to 

coarse-grained quartzite. Beds probably are 1 m or less thick. 

A 60-m-thick tongue of Newland Formation is 120 m below 

the top of the Table Mountain in this area, and thin Newlandlike beds are also present in the uppermost 30 m. The upper 

contact with the Greyson Shale is abrupt.

NEWLAND FORMATION

The Newland Formation of the Highland Mountains is 

similar mainly in its bedding character to the Newland Formation at its type locality near Newlan Creek in the Little 

Belt Mountains and to a complete section along Deep Creek 

at the south end of the Big Belt Mountains (M.W. Reynolds, 

oral commun., 1988). In the Little Belt Mountains, the Newland consists of alternating limestone and argillite, and the 

amount of limestone increases upward (Ross, 1963, p. 69). 

Many of the limestone beds in the Big Belt Mountains are 

finely layered and may represent mat stromatolites.
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A15

Figure 10. Flaggy bedding character of typical Newland Formation. Quarter inset in fracture in right-central part of photograph 

is shown for scale. Outcrop along dirt road on the south side of Moffet Mountain, Wickiup Creek llk -minute quadrangle.

LITHOLOGIC DESCRIPTION

The partial section of the Newland Formation that overlies the Moose Formation at its type section illustrates the 

general lithology of the Newland in the Highland Mountains. 

There the Newland consists of almost 150 m of mainly 

medium gray, flaggy weathering siltite in beds 2-3 cm thick 

(fig. 10). Individual beds are homogeneous and without 

internal structure or layering and are separated from adjacent 

beds by even-parallel laminae of lighter colored siltite. 

About 6 m above its base, the Newland in a 7.5-m-thick 

interval is less flaggy and more argillaceous and contains 

numerous small silty and sandy channel fills and climbing 

ripples similar to the Moose Formation. At 52 m above the 

base, there is a local decrease in bed thickness and an 

increase in slightly wavy laminae; bedding character is similar to that of the overlying Greyson Shale. The upper 40 m 

of the Newland Formation shows bedding typical of the 

Newland at its type locality in the Little Belt Mountains.

CONTACTS WITH ADJACENT UNITS

At the Moose Creek locality, the lower contact of the 

Newland is gradational over a 34.5-m-thick transition zone

above the Moose Formation. The base of the Newland is 

placed at the base of the lowest, laterally continuous flaggy 

argillaceous siltite that displays typical Newland bedding. At 

the Soap Gulch and Camp Creek localities, the contact with 

the Moose Formation is sharp.

At the Moose Creek locality, the upper contact of the 

Newland is abrupt, marked by thin-bedded black argillite of 

the Greyson Shale. The lowermost Greyson does contain 

several very calcareous beds, but they are herein included in 

the Greyson. Hydrothermally altered limestone beds mark 

the top of the Newland at the Soap Gulch locality. At the 

Camp Creek locality, interlayered Greyson Shale and flaggy 

bedded Newland form a transition zone more than 225 m 

thick; the contact between the two formations is placed at the 

base of this transition zone. At Table Mountain, a similar, 

but more arenaceous, 160-m-thick transition zone is present 

between the two formations.

FACffiS

The Newland Formation, which is only 150 m thick at 

Moose Creek in the western part of the Highland Mountains, 

thickens eastward to more than 350 m at the Camp Creek
A16 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

locality (fig. 2B). The Newland Formation at the Moose 

Creek locality is laterally equivalent to the upper part of the 

Newland at Soap Gulch, and this part of the formation 

pinches out eastward from Soap Gulch by interfingering 

with and grading laterally into the Greyson Shale. The lower 

part of the Newland at the Soap Gulch locality is laterally 

equivalent to the upper part of the Newland at Camp Creek. 

Eastward, the upper part of the Newland interfingers with 

and pinches out into the Greyson Shale of the Table Mountain locality; the middle part of the Newland at the Camp 

Creek locality interfingers with and lies beneath the Table 

Mountain Quartzite. Farther to the east, a tongue of the Newland is present within the Table Mountain Quartzite.

RAVALLI GROUP

GREYSON SHALE

The Greyson Shale, named for 915 m of dark-gray to 

greenish-gray argillite exposed in the southwestern part of 

the Big Belt Mountains in the vicinity of Deep Creek (Walcott, 1899), is, with the probable exception of the LaHood 

Formation, the thickest formation of the Belt Supergroup in 

the Highland Mountains. The Greyson, only 220 m thick at 

Moose Creek, thickens dramatically eastward to about 1,260 

m in the Table Mountain block. The mainly interlayered silty 

argillite and siltite of the western outcrop area become more 

lithologically diverse to the east. At Camp Creek the formation includes several calcareous beds, interpreted to be eastward-thinning tongues of the Newland, as well as distinctive 

calcareous, coarse-grained quartzite. In the Table Mountain 

block the middle part of the formation contains a thick 

sequence of upward-fining layers, some of which are capped 

by calcareous argillite.

LITHOLOGIC DESCRIPTION AND FACIES

In the Moose Creek block the lowermost 36 m of the 

Greyson is characterized by interlayered flaggy dark-gray 

argillite, platy argillaceous siltite, and 0.1-0.25-m-thick 

beds of medium-gray silty limestone and associated limestone nodules. The calcareous beds, restricted to the bottom 

10 m of the Greyson, are included in this formation because 

they are interlayered with dark-gray argillite that lacks the 

blocky bedding of the underlying Newland. The remaining 

26 m consists of dark-gray, noncalcareous, wavy bedded, 

silty argillite containing at least one 2-cm-thick limestone 

lens. The overlying 27 m consists mainly of dark-gray 

argillaceous siltite that has flaggy Newland-like bedding. 

These rocks are in turn overlain by dark-gray, platy, evenparallel-laminated silty argillite characteristic of much of the 

Greyson elsewhere in the Highland Mountains. The uppermost 84 m of the formation is mostly covered; float consists 

of Greyson-like argillite, thin-bedded siltite, and very fine

grained quartzite. This interval probably represents a transition zone between the Greyson and the overlying Spokane 

Formation.

To the east, in the Soap Gulch block, the Greyson is 

about 350 m thick. The formation, although poorly exposed 

in the lower part, consists of two units: a lower 170 m of 

dark- to medium-gray, even-parallel-laminated silty argillite 

displaying rare Newland-like bedding and an upper 180 m of 

dark-gray, locally silty argillite characterized by extensive 

soft-sediment deformation manifested mainly as isoclinal 

folds that have amplitudes of as much as 2 m (fig. 11).

In the Camp Creek block, the Greyson is 876 m thick 

and represents the most lithologically diverse part of the formation. The lowermost 160 m consists of dark-gray, evenparallel-laminated argillaceous siltite and silty argillite 

enclosing wispy lenses of tan siltite; a 0.5-m-thick calcareous quartzite is present near the top of this interval. Strata in 

the bottom 40 m locally show the flaggy bedding of the 

Newland. Upsection is about 180 m of light-to dark-gray, 

olive-green, or tan, laminated to nonlaminated silty argillite; 

soft-sediment folds are common in the upper part. Two thin 

beds of calcareous siltite to very fine grained quartzite in the 

upper 40 m of this part of the Greyson probably are eastward-thinning tongues of the upper Newland Formation in 

the Soap Gulch block.

Above the 180-m-thick silty argillite unit is about 330 

m of mainly siltite and argillaceous siltite that contain conspicuous beds, generally less than 0.5 m thick, of tan, 

medium- to coarse-grained, grain-supported quartzite. Associated silty argillite commonly contains wispy, discontinuous lenses, generally less than 1 cm thick and 15 cm long, of 

very fine grained quartzite that locally forms climbing ripples. The uppermost Greyson, almost 200 m thick, consists 

of laminated silty argillite containing minor thin quartzite 

lenses.

To the east, in the Table Mountain block, the Greyson 

is 1,260 m thick and consists of three distinctive parts. The 

lower part is 210 m thick and mainly argillaceous, laminated 

siltite. The middle part, 550 m thick, consists mainly of 

upward-fining cyclic deposits and includes two 50-60-mthick, dark-gray argillite intervals that display well-developed soft-sediment deformation features. Cyclic deposits in 

the lower portion of the middle part consist of four components: (1) even-parallel-laminated, very fine grained quartzite to siltite that grades upward into (2) silty argillite 

containing thin, wispy sand lenses that, in turn, grades into 

(3) dark-gray argillite locally capped by (4) calcareous argillite. Each cycle is no thicker than about 20 m. Cycles in the 

upper portion of the middle part are also composed of four 

components but lack calcareous argillite. The components of 

these cycles are (1) a basal medium-grained quartzite commonly associated with intraformational conglomerate overlain by (2) thick-bedded, very fine grained quartzite and siltite 

containing even-parallel microlaminae of quartzite-siltite 

couplets. Component 2 grades upward into (3) 2-5-cm-thick
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A17

Figure 11. Soft-sediment folds in laminated Greyson Shale. Hammer is shown for scale. Outcrop along ridge extending westward from 

Negro Mountain, Wickiup Creek 7'/2-minute quadrangle.

beds of microlaminated quartzite and siltite couplets associated with dense, unlaminated siltites and is overlain by (4) 

massive to thick-bedded silty argillite. These cycles in the 

upper portion of the middle part range in thickness from 2 to 

16m.

The upper part of the Greyson in the Table Mountain 

block is 500 m thick and mainly dark gray to black argillite; 

locally it contains beds of sandy to silty argillite showing 

numerous soft-sediment folds.

SPOKANE FORMATION

The Spokane Formation was named for an incomplete 

section of deep-red arenaceous argillite in the Spokane Hills 

on the west side of the Big Belt Mountains (Walcott, 1899). 

The deep red colors displayed by the Spokane Formation in 

much of the Helena embayment are a major criterion for recognition. In the Highland Mountains, contact metamorphism 

of Belt-age rocks in the vicinity of the Boulder batholith, 

albeit weak, has modified the colors of the formation to hues

of gray and green. The Spokane is the only formation that 

was deposited uniformly across the Highland Mountains; 

however, the formation is somewhat thicker and lithologically more variable in the Camp Creek block.

LITHOLOGIC DESCRIPTION

The Spokane Formation, 172 m thick at the Moose 

Creek locality, is mainly siltite containing local argillaceous 

and arenaceous intervals. The poorly exposed lower 34 m is 

the coarsest part of the formation and consists of mediumgray silty quartzite and thin interbeds of brownish-red thinbedded siltite. The overlying 134 m is mostly siltite that contains thin argillaceous intervals, minor soft-sediment slump 

structures, and, locally, crossbeds and cut-and-fill structures 

where it is arenaceous. The uppermost 10 m is composed of 

interlayered argillite-siltite couplets enclosing even-parallellaminated siltite beds as thick as 20 cm.

The Spokane thins slightly to the east, to 152 m, at the 

Soap Gulch locality. As at Moose Creek, the lower part is
A18 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Figure 12. Spokane Formation showing wavy-parallel laminated quartzite grading upward into nonlaminated arenaceous siltite. Pencil is 

shown for scale. Outcrop on ridge on the west side of Spring Creek cirque, Pipestone Pass llh -minute quadrangle.

coarsest; it consists of 12 m of irregularly layered, blocky to 

platy, light-gray siltite and laminated, medium- to light-gray 

siltite interlayered with light-colored argillaceous to arenaceous siltite that commonly shows well-developed crossstratification. Beds are as thick as 3-5 cm. The overlying 38 

m consists mainly of thin-bedded siltite that locally includes 

layers of argillaceous siltite and minor fine-grained pinkishwhite quartzite. The upper 102 m is mainly siltite but 

includes argillaceous to silty quartzite beds as thick as 6 m.

The Spokane is thickest, 220 m, in the Camp Creek 

block; to the east in the Table Mountain block it is 134 m 

thick. In both blocks the formation is mainly greenish-gray 

argillaceous siltite containing millimeter- to centimeterthick intervals of lenticular, tan, very fine grained quartzite 

beds that thin and thicken along strike and tannish-white, 

nonlaminated arenaceous siltite to very fine grained quartzite that contains wavy-parallel laminations where argillaceous (fig. 12). Quartzite lenses 0.5-2 m thick composed of 

medium subrounded grain-supported quartz grains are also 

present and, in the Camp Creek block, are more common and 

thicker in the upper part of the Spokane Formation. All

quartzite lenses pinch out to the west in the central part of 

the Soap Gulch block and thin eastward in the Table Mountain block.

CONTACTS WITH ADJACENT UNITS

The lower contact of the Spokane Formation across the 

Highland Mountains is generally well defined and abrupt 

and is placed on top of the uppermost laminated silty argillite 

of the Greyson Shale and below platy to thick bedded, 

medium-gray to greenish-gray siltite of the Spokane.

The Spokane Formation at the Moose Creek locality is 

unconformably overlain by quartzite and argillite of the Missoula Group; the position of the contact is occupied by a thin 

mafic sill of probable Cretaceous age. Above the mafic sill 

is a 6-m-thick medium-gray, poorly sorted, medium-grained 

quartzite interpreted to represent the base of the Missoula 

Group. The contact between the two formations, if viewed 

from several kilometers distance, appears slightly discordant; the underlying Spokane dips to the northeast at a 

slightly greater angle than that of the overlying Missoula
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A19

Group. At the Soap Gulch locality, the upper contact of the 

Spokane is placed on top of the highest argillaceous siltite, 

which is overlain by a thick succession of 1-m-thick blocky, 

irregularly bedded, dark-gray to greenish-gray argillaceous 

quartzite of the Missoula Group. In the central part of the 

Soap Gulch block the Spokane is overlain by complexly 

interlayered strata of the Empire and Helena Formations. In 

the Camp Creek block, the Spokane is overlain by rocks of 

the Empire Formation, here a distinctive dark-gray to black, 

thick-bedded argillite that contains conspicuous lenses of 

white calcareous quartzite. In the Table Mountain block the 

uppermost argillaceous siltite of the Spokane is overlain by 

blocky, greenish-gray argillite containing light-colored 

blebs of calcareous sandstone of the Empire Formation.

FACIES

Unlike the older formations of the Belt Supergroup, the 

Spokane Formation does not interfinger with adjacent sedimentary units. Contacts with underlying and overlying formations are abrupt and relatively well defined. The most 

significant facies change in the Spokane is the presence in 

the Camp Creek block of clean, light-colored quartzite beds 

that thin and pinch out to the east and west.

EMPIRE FORMATION

The Empire Formation, the uppermost formation of the 

Ravalli Group and typically a greenish-gray, siliceous argillite in the central part of the Helena embayment (Walcott, 

1899; Ross, 1963), consists of dark-gray, black, and greenish-gray, thick-bedded argillite in the Highland Mountains. 

In the Soap Gulch block the formation is interlayered with 

the Helena Formation of the middle Belt carbonate 

sequence, and both the Empire and Helena pinch out westward in the block; neither is present in the Moose Creek 

block.

LITHOLOGIC DESCRIPTION

The Empire Formation is not well exposed in the Highland Mountains. In the Table Mountain block, the formation 

consists of 16.5 m of weakly laminated, dark-gray argillite 

containing tan to white wispy blebs of calcareous sand; the 

formation directly underlies cyclic, upward-fining, calcareous clastic deposits of the Helena Formation. In the Camp 

Creek block the Empire is mostly covered, and the upper and 

lower contacts are not exposed. In the central part of the 

Soap Gulch block, the formation is intimately interlayered 

with the Helena Formation, and both units together are less 

than 10m thick. Farther to the west, in the Soap Gulch block, 

a 1.5-m-thick dark-gray argillite containing white, wispy 

quartzite lenses may represent the westernmost extent of the 

Empire Formation. In this area argillite of the Empire rests

directly on a 4-m-thick dark-greenish-gray argillaceous 

quartzite that is underlain by siltite typical of the Spokane 

Formation; the argillite is overlain by quartzite assigned to 

the Missoula Group.

MIDDLE BELT CARBONATE ROCKS

HELENA FORMATION

The Helena Formation was named for prominent exposures of calcareous rocks near Helena, Montana, that consist 

mainly of impure bluish-gray to gray limestone interbedded 

with minor gray, green, and purple siliceous shale (Walcott, 

1899). In the Highland Mountains the Helena is close to the 

southern margin of, and locally intruded by, plutons of the 

Boulder batholith; consequently, in this area the Helena is 

commonly a hornfelsed calc-silicate rock that weathers a distinctive pale greenish gray to pale green.

LITHOLOGIC DESCRIPTION

The Helena Formation is thickest, 172 m, in the Table 

Mountain block. The formation thins to 56 m westward in 

the Camp Creek block and pinches out farther to the west in 

the central part of the Soap Gulch block. The formation also 

thins to the east of the Table Mountain block. The formation, 

best exposed in the Table Mountain block, consists mainly of 

clastic to carbonate rocks in cyclic units that range in thickness from less than 10 cm to as much as 5 m. Not all cycles 

are well developed or complete, and one or more components may be missing. Each cycle has three major components: (1) a lower light-colored, very fine grained to 

medium-grained, wavy-parallel-laminated quartzite that 

fines upward, (2) a medial dark-gray weakly laminated 

argillaceous siltite that may contain very fine grained quartzite lenses, and (3) an upper purplish-gray to greenish-gray 

calcareous argillite (now calc-silicate rock). The argillite 

may contain vugs that are elongate parallel to bedding, or it 

may be well laminated (stromatolitic?) and contain numerous water-expulsion structures. Some well-laminated calcareous intervals are capped by thin purplish-gray silty argillite 

containing mud cracks filled by fine sand; locally the upper 

argillaceous parts of the cycles include intraformational conglomerate that may signal onset of the overlying upward-fining sand of the next cycle. The lower part of the formation 

includes intervals as thick as 9 m of dark-gray argillite 

enclosing tannish-white, very fine grained quartzite lenses 

that look like the underlying Empire Formation. The cyclic 

aspect of the Helena decreases upsection but not uniformly. 

In the Table Mountain block, the uppermost 20 m contains 

cyclical deposits, but the underlying 35 m consists mainly of 

interlayered purplish-gray silty argillite and minor tan and 

medium- to light-gray, very fine grained to medium-grained 

quartzite. The top of the formation is placed at the top of the 

highest clastic-carbonate cycle.
A20 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Figure 13. Helena Formation showing laminated beds containing vugs aligned along bedding planes. Quarter is shown for scale. Outcrop 

directly south of Gold Hill, Butte South 15-minute quadrangle.

To the west, in the Camp Creek block, the Helena Formation is not well exposed and is locally intruded by granitic 

rocks related to the Boulder batholith. The formation consists of pale-green, laminated, vuggy weathering calc-silicate rock (fig. 13) that includes centimeter-scale argillite to 

laminated calcareous argillite cycles and thin, horizontal calcareous pods. Locally, mud cracks (synoresis? cracks) are 

preserved in the calcareous argillite layers. In the Camp 

Creek block, the basal Helena grades laterally into or is interlayered with dense, green to dark-gray-green, locally laminated rocks that contain wispy, white calc-silicate lenses 

generally less than 3 cm thick (Empire? Formation). The 

Helena is clearly and intimately interlayered with the Empire 

Formation farther to the west in the Soap Gulch block; both 

formations pinch out westward and were apparently not 

deposited in the Moose Creek block.

CONTACTS WITH ADJACENT UNITS

In the Highland Mountains, the Empire and Helena Formations were not mapped separately (O'Neill and others, 

1995) because in all places rocks typical of both formations

are interlayered. Consequently, the lower contact of the Helena Formation, drawn at the base of the first clastic-to-carbonate cycle in laminated, vuggy calc-silicate rock, is not 

easy to map. The upper contact is placed at the top of the 

highest clastic-carbonate cycle and is generally abrupt. 

Argillic rocks of the Missoula Group present above this horizon generally lack the purplish-gray and greenish-gray colors typical of the Helena argillite; rather, they tend to be 

reddish gray to gray and silty to sandy. To the west, in the 

Camp Creek and Soap Gulch blocks, the Helena is overlain 

by a conspicuously laminated fine- to medium-grained 

argillaceous quartzite of the Missoula Group.

FACIES

Thick-bedded cyclical deposits of the Helena Formation in the Table Mountain block give way to thin-bedded 

cycles and calcareous argillite to the west in the Camp Creek 

block. The Helena and Empire Formations are intimately 

interlayered farther to the west, and in the Soap Gulch block 

both formations pinch out completely.
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A21

MISSOULA GROUP

Red and green argillite and siltite and pink to gray 

quartzite of the Missoula Group are present across much of 

western Montana (Ross, 1963); maximum thickness of these 

rocks exceeds 5,000 m (Wallace and others, 1984). The Missoula Group is well exposed 20 km directly west of the Highland Mountains in the Pioneer Mountains (Zen, 1988; 

Ruppel and others, 1993) where it is mostly allochthonous 

and is represented by more than 1,000 m of argillite, quartzite, and minor conglomerate. In sharp contrast, Missoula 

Group rocks in the Highland Mountains are in depositional 

contact with underlying Belt strata but are very thin, a maximum of 168 m thick in the Table Mountain block and only 

16m thick to the west in the Moose Creek block (fig. 14). 

Strata assigned to the Missoula Group in the Highland 

Mountains are composed mainly of quartzite and interlayered argillite and siltite. Isolated conglomerate is also 

present.

LITHOLOGIC DESCRIPTION AND FACIES

The Missoula Group is represented by a 16-m-thick 

sequence of interlayered quartzite and argillite in the Moose 

Creek block. The lower 6.2 m of the sequence is marked by 

beds of upward-coarsening, medium-gray, medium-grained 

quartzite. Beds are poorly layered in the lower part of the 6.2 

m but grade upward into light-gray, coarse-grained, welllayered quartzite; the upper part is poorly sorted, consisting 

mainly of coarse- to granule-size grains. The lower 6.2 m of 

quartzite grades upward into 1.5 m of ripple-crosslaminated 

argillaceous siltite that, in turn, grades into a 1-m-thick fineto medium-grained quartzite. A dark-gray argillite about 1 m 

thick separates the underlying quartzite and minor siltite 

from the remaining 6.3 m of the Missoula Group along sharp 

upper and lower contacts. The uppermost 4 m of Missoula is 

composed of at least six distinct, generally upward fining, 

coarse- to medium-grained beds that locally show cut-andfill and climbing-ripple structures.

To the east, in the Soap Gulch block, rocks assigned to 

the Missoula Group aggregate 62 m in thickness. The base of 

the Missoula Group is placed at the top of a 1.5-m-thick 

dark-gray argillite of the Empire Formation (?) that encloses 

thin, wispy, white quartzite lenses. The lower 2 m of the Missoula Group strata consists of dark-gray, laminated, very fine 

grained, argillaceous quartzite and is overlain by a 1-mthick, pinkish-white, poorly sorted, fine- to coarse-grained 

quartzite somewhat similar to the upward-coarsening quartzite near the base of the group at the Moose Creek locality. 

The overlying 30 m consists of interlayered dark-gray, thinbedded (10-20 cm) argillaceous quartzite and argillite. The 

argillite commonly contains thin, white to pinkish-white, 

quartz-rich cut-and-fill and lenticular laminae less than 1 cm 

thick. The remainder of the group consists of argillaceous

siltite and very fine grained quartzite containing lenses of 

clean, white to pink quartz grains.

At the Camp Creek locality, the Missoula Group is 148 

m thick. The lower unit of quartzite is a coarsening-upward 

sequence at least 46 m thick that is intruded by biotite granite. This lower unit is unique to the Camp Creek block and 

consists of laminated, poorly sorted, very fine grained 

quartzite containing floating, coarse quartz grains. Laminae 

are defined by parallel, alternating layers 1-3-mm thick of 

light-colored clean quartzite and dark-gray argillaceous 

quartzite. Overlying this lower unit of quartzite is a 16-mthick section of thin-bedded, white, commonly poorly 

sorted, very fine grained to coarse-grained quartzite that is 

present as channel fill and as individual beds as thick as 10 

cm. This 16-m-thick unit is capped by a 1-2-m-thick, lightto dark-banded, locally vuggy, calcareous argillite. The 

remainder of the succession is similar to the interlayered 

quartzite-argillite sequence in the Soap Gulch and Moose 

Creek blocks. The quartzite is typically cream to white and 

locally shows well-developed centimeter-scale crossbeds, 

upward-fining sequences, and ripple marks.

The thickest section of the Missoula Group in the Highland Mountains is in the Table Mountain block and consists 

of at least 168 m of interbedded conglomerate, quartzite, and 

argillite. The rocks of the group are coarsest along the western margin of the block, directly adjacent to the Twin 

Bridges fault, consist of mainly pink to greenish-gray quartzite and minor argillite, and are capped by a pebble to cobble 

conglomerate; however, these rocks are present mostly as 

float and could not be adequately measured and described. A 

section was measured along the west side of Spring Creek, 

about 2,000 m east of the Twin Bridges fault (O'Neill and 

others, 1995), and the following description is taken from 

that section.

At Spring Creek, the base of the Missoula Group is 

marked by 4 m of red to gray, massive silty argillite overlain 

by about 44 m of thin-bedded (10-30 cm), tan to gray, very 

fine grained to fine-grained quartzite, siltite, and argillaceous siltite and minor dark-gray argillite near the top. At 

about 22 m above the base of the group is 6 m of white, fineto medium-grained feldspathic quartzite. The lower 48 m of 

fine-grained rocks is overlain by about 16 m of even- to 

uneven-parallel-laminated, very fine grained quartzite that is 

similar to the laminated quartzite at the base of the Missoula 

Group to the west in the Camp Creek block. The succeeding 

55 m consists of alternating greenish-gray, very fine grained 

to fine-grained argillaceous quartzite and light-gray and 

pinkish-white to white feldspathic quartzite and orthoquartzite that contain well-developed oscillation and asymmetric 

ripple marks in the uppermost 5 m. The next higher 27 m is 

mostly covered but shows argillite, siltite, and minor white 

quartzite float. The uppermost Missoula Group consists of 

10m of massive to weakly crosslaminated, fine- to mediumgrained nonfeldspathic quartzite in beds 30-60 cm thick.
A22 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

6 km SOUTH OF CAMP CREEK FAULT 3 Km SOUTH OF CAMP CREEK FAULT

Wolsey Shale

MOOSE CREEK BLOCK

:-£HZ-£I-£:-EJ Wolsey Shale

-"-"'-_ ~~J Flathead Sandstone

Figure 14 (above and facing page). Stratigraphic sections of the Middle Cambrian Flathead Sandstone and the underlying Belt Supergroup 

in the Highland Mountains. Lithologic explanation is given in figure 1'.

CONTACTS WITH ADJACENT UNITS

The lower contact of the Missoula Group is generally 

sharp and is drawn at the top of the highest lithologic unit 

characteristic of the underlying formation. In the Moose 

Creek block the contact is drawn at the top of the highest 

Spokane-like siltite. In the Soap Gulch block to the east, the 

contact is placed at the top of a 1.5-m-thick dark-gray argillite containing wispy white quartzite lenses of the Empire 

Formation(?); farther to the east, the contact is drawn at the 

top of calc-silicate rocks of the Helena Formation.

conglomerate, siltstone, and shale; they are separated by a 

medial, weakly resistant part of mainly shale and siltstone. 

The Flathead is about 20 m thick in both measured sections. 

Everywhere in the Highland Mountains where Belt sedimentary rocks are preserved a similar but thinner tripartite 

sequence of the Flathead underlies the Middle Cambrian 

Wolsey Shale. Sandstone beds in the upper and lower parts 

of this sequence are commonly well sorted, white, fine 

grained, and nonfeldspathic and lack conglomerate. The top 

of the Missoula Group is everywhere drawn at the base of the 

lowermost sandstone unit interpreted as the Flathead Sandstone.

MIDDLE CAMBRIAN ROCKS

In parts of southwestern Montana it is difficult to distinguish between the uppermost clastic deposits of the Missoula 

Group and those of the overlying Middle Cambrian Flathead 

Sandstone (Don Winston, University of Montana, oral commun., 1988). In the Highland Mountains the upper contact of 

the Missoula Group with the Flathead Sandstone is also tenuous. Directly south of the exposures of the Belt Supergroup 

in the range, but still within the mountains, the Flathead rests 

unconformably on Early Proterozoic basement rocks (fig. 

2A). The Flathead was measured at two locations in this area, 

and the sections are included in the correlation diagram of 

figure 14; one section is 6 km south of the Camp Creek fault, 

and a second section is about 3 km south of the fault. The 

Flathead Sandstone south of the Belt exposures is a tripartite 

unit; the upper and lower parts consist of orange-tan to reddish-orange feldspathic sandstone interlayered with minor

TECTONICS AND SEDIMENTATION

Sedimentary rocks of Belt age (Middle Proterozoic) in 

the Highland Mountains are exposed in an elongate easttrending zone that extends almost completely across the 

range. Rocks of the Belt Supergroup are separated from the 

underlying basement on the south by the Camp Creek fault; 

they are overlain by or structurally juxtaposed against Paleozoic sedimentary rocks on the north; they are intruded by the 

Boulder batholith on the east; and they are truncated by Neogene basin and range faults on the west. This Belt Supergroup sedimentary succession is composed mainly of lower 

Belt and Ravalli Group rocks overlain by different formations in different structural blocks. Striking lateral facies 

changes are present in all of these sedimentary rocks (fig. 2), 

and these changes are accompanied by an overall thickening 

of the succession toward the central part of the range. The
SOAP GULCH BLOCK

TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION 

CAMP CREEK BLOCK

A23 

TABLE MOUNTAIN BLOCK
A24 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

formations are thinnest and best exposed in the west where 

the entire succession is about 1,000 m thick; the thickness 

increases to more than 3,000 m in the central part of the 

range. Major facies changes are present across northwest- to 

north-trending faults that are the northward continuation of 

conspicuous northwest-trending faults that cut basement 

rocks to the south. These faults extended into the Belt Basin 

in Middle Proterozoic time and acted as growth faults during 

deposition of the Belt sediments. Belt-age rocks were deposited in northwest-trending half grabens or structural blocks 

outlined by these growth faults. These structural blocks are 

designated, from west to east, the Moose Creek, Soap Gulch, 

Camp Creek, and Table Mountain blocks (fig. 2B). Stratigraphic relations preserved across these structural blocks 

outline a large northwest-trending graben, herein called the 

Highland graben, that is manifested as a Middle Proterozoic 

paleovalley along this part of the southern margin of the Belt 

Basin. Facies relationships among rocks within the graben 

show that lower Belt and Ravalli Group rocks are laterally 

gradational and were deposited simultaneously. The lower 

Belt deposits represent subaerial to near-shore deposits that 

interfinger with subaqueous calcareous muds that, in turn, 

grade laterally into Ravalli Group laminated silty muds 

deposited farther from shore. Variations in stratigraphic 

thickness and hanging-wall onlap of these lower Belt strata 

suggest syndepositional fault activity, widening of the graben during Belt deposition, and a gradual transition from 

subaerial to subaqueous deposition through time.

Faults that outline the Highland graben were overlapped by subaerial deposits of the LaHood Formation that 

graded downslope into subaqueous deposits. These basal 

deposits can be divided into two sedimentary sequences: (1) 

interlayered coarse conglomeratic alluvial gravel, fluvial 

deposits, and minor distal, subaqueous quartzite west of the 

Table Mountain block and (2) well-bedded quartz-pebble 

conglomerate and coarse lithic quartzite of probable subaqueous debris flow origin and minor fluvial stream deposits 

within the Table Mountain block in the central part of the 

graben.

As the central part of the paleovalley eventually became 

completely submerged, deltaic sands were deposited above 

the fluvial and shallow-water debris-flow sandstone and 

conglomerate. These sands, the Table Mountain Quartzite, 

pinch out to the east and west. The quartzite can be divided 

into two informal members: the lower 230 m consists mainly 

of massive, thick-bedded, cliff-forming white quartzite, and 

the upper 260 m consists of thin-bedded, upward-fining 

quartzite interlayered with minor siltite and argillite. The 

upper member includes numerous upward-fining cyclical 

deposits that are 1-2 m thick and best developed in the lower 

100 m of the section.

Alluvium of the LaHood Formation continued to be 

shed during and after the deposition of the Table Mountain 

Quartzite to the west from the higher, uplifted margins of the 

graben. The alluvium interfingers downslope with

submarine, locally turbiditic deposits that, in turn, grade laterally into mud and silt of the Moose Formation. Similarly, 

the Moose Formation grades laterally into the Newland Formation. Farther basinward, the calcareous Newland gives 

way to thick accumulations of finely laminated mud, silt, and 

sand of the Greyson Shale. As subsidence within the graben 

progressed, the strandline of the Belt sea migrated landward, 

and the various lithofacies being deposited in the graben also 

shifted landward, overlapping all previously deposited sediments. As a result, and in spite, of lateral facies changes 

within the graben, the well-established and known normal 

stratigraphic succession of the Belt Supergroup of the Helena embayment was maintained.

Although the Newland Formation is only 150 m thick in 

the western part of the Highland Mountains, it thickens eastward to more than 350 m at Camp Creek. The entire Newland Formation exposed in the Moose Creek block is 

temporally equivalent to only the upper part of the Newland 

in the Soap Gulch block. The upper part of the Newland at 

Soap Gulch pinches out eastward, interfingering with and 

grading into the Greyson Shale. The lower part of the Newland at Soap Gulch is temporally equivalent to the upper part 

of the Newland in the Camp Creek block. The upper part of 

the Newland at Camp Creek interfingers with and pinches 

out into the Greyson Shale in the Table Mountain block, and 

the lower part of the Newland at Camp Creek interfingers 

with the Table Mountain Quartzite.

The Greyson Shale exposed in the Highland Mountains 

is the thickest Belt formation in the range. The unit is almost 

220 m thick on the west at Moose Creek and thickens dramatically eastward to about 1,260 m in the Table Mountain 

block. The mainly interlayered silty argillite and siltite to the 

west becomes more lithologically diverse to the east. At 

Camp Creek, the formation includes several calcareous layers as well as distinctive, coarse, calcareous quartzite beds. 

In the Table Mountain block, the middle part of the formation contains a thick sequence of numerous upward-fining 

sedimentary cycles, each capped by limestone.

As subsidence slowed in the Highland graben, crossbedded, argillaceous silt and sand of the Spokane Formation 

were deposited uniformly above the Greyson Shale; the Spokane is somewhat thicker (135 m) and coarser grained in the 

Camp Creek and Table Mountain blocks along the graben 

axis than farther to the west.

The Moose Creek block along the western margin of 

the graben was subaerially exposed in middle Belt time. In 

the Soap Gulch block, carbonate bank deposits of the Helena 

Formation interfinger with calcareous argillite of the Empire 

Formation; both formations pinch out to the west and were 

not deposited in the Moose Creek block. The Empire and 

overlying Helena Formations are in normal stratigraphic 

order in the Table Mountain block in the central part of the 

range, where they have a combined thickness of about 200 

m. Both units thin eastward, away from the graben axis.
TABLE MOUNTAIN QUARTZITE AND MOOSE FORMATION A25

Younger alluvial conglomerate and fluvial sand of the 

Missoula Group covered all the older deposits. The main 

depositional center for these younger rocks was also in the 

Table Mountain block along the graben axis, and these rocks 

include thick, medium-grained, white to pink quartzite, 

minor argillite, and an uppermost angular quartz-pebble conglomerate that is restricted to the Table Mountain block. The 

Missoula Group has a maximum thickness of 155 m but thins 

to less than 30 m on the west.

Belt Supergroup rocks of the Highland Mountains are 

in fault contact with Early Proterozoic crystalline rocks to 

the south (fig. 2A). The contact is marked by the Camp Creek 

fault, a low-angle thrust fault that has been interpreted, on 

the basis of inference, as having a significant component of 

right-lateral slip (see, for example, Ruppel and Lopez, 1984). 

Reverse-slip movement on the Camp Creek fault is indicated 

by south-directed shears and folds within the fault zone. The 

interpretation of right-slip is based mostly on regional tectonic necessity: the need to find a link between the foreland 

fold and thrust belt of extreme southwestern Montana (Ruppel and Lopez, 1984) and the huge, thin-skinned detachments within the Helena embayment that are northeast of the 

Highland Mountains (Woodward, 1981; Schmidt and 

O'Neill, 1982). That the Camp Creek fault is not this major 

tectonic link is indicated by the structural continuity of highangle faults in the basement rocks of the southern part of the 

Highland Mountains that acted as growth faults during deposition of the overlying Belt sediments; this structural continuity indicates that displacement on the Camp Creek fault is 

minor. O'Neill and others (1990) argued that the overthrusts 

of extreme southwestern Montana and adjacent Idaho and 

the thin-skinned detachments of the Helena embayment are 

tectonically linked by the postulated northeast-trending Dillon cutoff. The cutoff is a zone of basement-involved thrust 

faults that passes beneath the Jefferson Valley, directly east 

of the Highland Mountains (fig. 2A), and continues southwestward into extreme southwestern Montana. In this interpretation, the Highland Mountains are allochthonous; 

basement rocks within the range are a part of the foreland 

fold and thrust belt and were thrust northeastward in concert 

with the overlying sedimentary rocks. High-angle faults in 

basement rocks of the southern part of the range curve gently 

westward into the Camp Creek fault, become incorporated 

within the fault zone, and then continue northward, curving 

into the Belt sedimentary succession. The gentle westward 

bending of these fault zones suggests a left-lateral strike-slip 

component on the Camp Creek fault. The Camp Creek fault 

probably is simply a displacement transfer zone, best interpreted as a tectonic detachment at the base of the Belt Supergroup; basement rocks in the footwall were detached from 

the overlying Belt sedimentary rocks and were thrust northeastward beneath them during the formation of the Cordilleran frontal fold and thrust belt (fig. 15). There is no 

evidence in the Highland Mountains for a major, east-trending, Middle Proterozoic basin-margin fault similar to that

Belt Supergroup

Highland Mountain's.-}' 

allochthonous 

basement rocks

Figure 15. Structural relationship between crystalline basement 

rocks of the Highland Mountains and the overlying Belt Supergroup. The two rock groups are separated by the Camp Creek fault, 

a left-lateral, oblique-slip thrust fault. The allochthonous basement 

rocks were detached from the overlying Belt Supergroup along an 

unconformity and transported to the northeast, beneath the sedimentary cover rocks. MC, Moose Creek block; SG, Soap Gulch block; 

CC, Camp Creek block; TM, Table Mountain block.

postulated along the southern part of the Helena embayment 

farther to the east (Robinson, 1963; Schmidt and Garihan, 

1986).

REFERENCES CITED

Alexander, R.G., 1955, Geology of the Whitehall area, Montana: 

Princeton, Princeton University, Yellowstone-Bighorn 

Research Association, Inc., Yellowstone-Bighorn Research 

Project Contribution 195, 111 p.

Cressman, E.R., 1984, Paleogeography and paleotectonic setting of 

the Prichard Formation A preliminary interpretation, in 

Hobbs, W.S., ed., The Belt: Montana Bureau of Mines and 

Geology Special Publication 90, p. 8-9.

 1985, The Prichard Formation of the lower part of the Belt 

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Montana: U.S. Geological Survey Bulletin 1553, 64 p.

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Duncan, M.S., 1976, Structural analysis of the pre-Beltian rocks of 

the southern Highland Mountains, Madison and Silver Bow 

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Garihan, J.M., Schmidt, C.J., and O'Neill, J.M., 1981, Recurrent 

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p. 1215-1240.

Harrison, I.E., Griggs, A.B., and Wells, J.D., 1974, Tectonic features of the Precambrian Belt Basin and their influence on postBelt structures: U.S. Geological Survey Professional Paper 

866, 15 p.
A26 BELT SUPERGROUP AND EQUIVALENT ROCKS, SOUTHWESTERN MONTANA

Koehler, S.W., 1973, Petrology and petrogeny of the diabase dikes 

of the Tobacco Root Mountains, southwestern Montana: 

Bloomington, Indiana University, M.S. thesis, 61 p.

McMannis, W.J., 1963, LaHood Formation A coarse facies of the 

Belt Series in southwestern Montana: Geological Society of 

America Bulletin, v. 74, p. 407^36.

Obradovich, J.D., Zartman, R.E., and Peterman, Z.E., 1984, Update 

of the chronology of the Belt Supergroup, in Hobbs, W.S., ed., 

The Belt: Montana Bureau of Mines and Geology Special Publication 90, p. 82-84.

O'Neill, J.M., 1989, Geology of the southern margin of the Helena 

embayment of the Belt Basin, Highland Mountains, Montana: 

Geological Society of America Abstracts with Program, v. 21, 

n. 5, p. 126.

O'Neill, J.M., Duncan, M.S., and Zartman, R.E., 1988, An Early 

Proterozoic gneiss dome in the Highland Mountains, southwestern Montana, in Lewis, S.E., and Berg, R.B., eds., Precambrian and Mesozoic plate margins: Montana Bureau of Mines 

and Geology Special Publication 96, p. 81-88.

O'Neill, J.M., Ferris, D.C., Schmidt, C.J., and Hanneman, D.L., 

1986, Recurrent movement along northwest-trending faults at 

the southern margin of the Belt Basin, Highland Mountains, 

southwestern Montana, in Roberts, S.M., ed., Belt Supergroup: 

Montana Bureau of Mines and Geology Special Publication 94, 

p. 208-216.

O'Neill, J.M., Klepper, M.R., Smedes, H.W., Hanneman, D.L., and 

Fraser, G.D., 1995, Geologic map and cross sections of the central and southern Highland Mountains, southwestern Montana: 

U.S. Geological Survey Miscellaneous Investigations Series 

Map 1-2525, scale 1:50,000.

O'Neill, J.M., and Schmidt, C.J., 1989, Tectonic setting and structural control of gold deposits in cratonic rocks of the Rochester 

and Silver Star mining districts, Highland Mountains, southwestern Montana, in French, D.E., ed., 1989 field conference 

guidebook: Montana Geological Society, v. 1, p. 393^102.

O'Neill, J.M., Schmidt, C.J., and Genovese, P.W., 1990, Dillon cutoff Basement involved tectonic link between the disturbed 

belt of west-central Montana and the overthrust belt of extreme 

southwestern Montana: Geology, v. 18, p. 1107-1110.

Robinson, G.D., 1963, Geology of the Three Forks quadrangle, 

Montana: U.S. Geological Survey Professional Paper 370, 

143 p.

Ross, C.P., 1949, The Belt problem [Mont.] [abs.]: Washington 

Academy of Science Journal, v. 39, no. 3, p. 111-113.

 1963, The Belt Series in Montana: U.S. Geological Survey 

Professional Paper 346, 122 p.

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., 1983, Preliminary 

geologic map of the Dillon I°x2° quadrangle, Idaho and

Montana: U.S. Geological Survey Open-File Report 83-168, 

scale 1:250,000.

-1993, Geologic map of the Dillon 1x2 degree quadrangle,

Idaho and Montana: U.S. Geological Survey Miscellaneous 

Investigations Series Map I-1803-H, scale 1:250,000.

Sahenin, U.M., 1939, Geology and ore deposits of the Rochester 

and adjacent mining districts, Madison County, Montana: 

Montana Bureau of Mines and Geology Memoir 19, 53 p.

Schmidt, C.J., and Garihan, J.M., 1986, Middle Proterozoic and 

Laramide tectonics activity along the southern margin of the 

Belt Basin, in Roberts, S.M., ed., Belt Supergroup: Montana 

Bureau of Mines and Geology Special Publication 94, 

p. 217-236.

Schmidt, C.J., and O'Neill, J.M., 1982, Structural evolution of the 

southwest Montana transverse zone, in Powers, R.B., ed., Geologic studies of the Cordilleran thrust belt: Denver, Rocky 

Mountain Association of Petroleum Geologists, p. 167-180.

Smedes, H.W., 1967, Preliminary geologic map of the Butte South 

quadrangle, Montana: U.S. Geological Survey Open-File 

Report, scale 1:24,000.

Thorson, J.P., 1984, Suggested revisions of the lower Belt Supergroup stratigraphy in the Highland Mountains, southwestern 

Montana, in Hobbs, W.S., ed., The Belt: Montana Bureau of 

Mines and Geology Special Publication 90, p. 10-12.

Walcott, C.D., 1899, Pre-Cambrian fossiliferous formations: Geological Society of America Bulletin, v. 10, p. 199-244.

Wallace, C.A., Harrison, I.E., Whipple, J.W., Ruppel, E.T., and 

Schmidt, R.G., 1984, A summary of the stratigraphy of the 

Missoula Group of the Belt Supergroup and a preliminary 

interpretation of basin subsidence characteristics, western 

Montana, northern Idaho, and eastern Washington, in Hobbs, 

W.S., ed., The Belt: Montana Bureau of Mines and Geology 

Special Publication 90, p. 27-29.

Wooden, J.L., Vitaliano, C.J., Koehler, S.W., and Ragland, P.C., 

1978, The late Precambrian mafic dikes of the southern Tobacco Root Mountains, Montana Geochemistry, Rb-Sr geochronology, and relationship to Belt tectonics: Canadian Journal of 

Earth Science, v. 15, p. 467^479.

Woodward, L.A., 1981, Tectonic framework of the disturbed belt of 

west-central Montana: American Association of Petroleum 

Geologists Bulletin, v. 65, p. 291-302.

Zartman, R.E., Peterman, Z.E., Obradovich, J.D., Gallego, M.D., 

and Bishop, D.T., 1982, Age of the Crossport C sill near Eastport, Idaho, in Reid, R.R., and Williams, G.A., eds., Society of 

Economic Geologists Coeur d'Alene Field Conference, 

Idaho 1977: Idaho Bureau of Mines and Geology Bulletin 24, 

p. 61-69.

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

Stine Mountain 7 lh -minute, and Maurice Mountain 7 lh -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 November 22, 1994 

Edited by Judith Stoeser 

Graphics and photocomposition by Shelly Fields

 if U.S. GOVERNMENT PRINTING OFFICE: 1996 - 774-045 / 20015 REGION NO. 8


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

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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.

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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.

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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 

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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.

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

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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.

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Permanent catalogs, as well as some others, giving comprehensive 

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