The Prichard Formation of the 

Lower Part of the Belt Supergroup 

(Middle Proterozoic), near Plains, 

Sanders County, Montana 

By EARLE R. CRESSMAN 

U.S. GEOLOGICAL SURVEY BULLETIN 1553 

The Prichard Formation in western Montana 

is divided into eight informal members and 

its internal stratigraphy, depositional 

environments, and depositional history 

are discussed 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1985 
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Cressman, Earle Rupert, 1923-

The Prichard "'Formation of the lower part of the Belt Supergroup (middle Proterozoic), near 

Plains, Sanders County, Montana. 

(Geological Survey bulletin ; 1553) 

Bibliography: 64 p. 

Supt. of Docs. No.: I 19.16:1553 

1. Geology, Stratigraphic-Precambrian. 2. Geology-Montana-Sanders County. I. Title. 

II. Series. 

QE75.B9 no. 1553 557.3s [551.7'15'09786833] 

~53 [QE653] 

For sale by the Branch of Distribution, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 
Abstract .. 

Introduction 

Methods 

Geographic setting 

Structural setting . 

Stratigraphic setting 

Prichard Formation . . 

General character 

Metamorphism . 

Age ..... . 

Subdivisions . . 

Introduction . 

Member A . 

Member B . 

Slump folds 

Quartzite marker 

Member C 

Member D 

Member E 

Member F 

Member G 

Member H 

CONTENTS 

Contact with the Burke Formation 

Sills .. 

Breccia ............... . 

Interpretation . . . . . . . . . . . . . . . . . 

Sedimentary structures and depositional environments 

Sedimentation and structure 

Source ........ . 

Tectonic environment . 

Depositional history . 

References cited . . . . . . 

ILLUSTRATIONS 

FIGURE 1. Map of area of Belt terrane showing principal faults and localities 

Page 

1 

2 

4 

4 

4 

6 

6 

6 

7 

8 

9 

9 

9 

12 

12 

16 

18 

19 

22 

28 

33 

34 

37 

42 

44 

51 

51 

54 

55 

55 

58 

60 

Page 

mentioned in text . . . . . . . . . . . . . . . . . . . 3 

2. Index map of Plains and Perma quadrangles . . . . . . 5 

3. Generalized stratigraphic section of Prichard Formation 10 

4--6. Photographs showing: 

4. Graded siltite-argillite couplets in member B 13 

5. Slump fold in member B 14 

6. Slump fold in member B . . . . . . . . . . . 15 

III 
IV CONTENTS 

Page 

FIGURE 7. Sketch of composite slump sheet . . . . . . . . . . . . . . . . . . 16 

8. Equal-area plot of poles of axial planes of slump folds . . . . . 17 

9. Map of Plains and Perma quadrangles showing direction and amount 

of dip of axial planes of slump folds, facing direction of beds 

in folds, and inferred direction of slumping 18 

10-22. Photographs showing: 

10. Member D . . . . . . . . . . . . . . . . . 20 

11. Outcrop of member E . . . . . . . . . . 22 

12. Sedimentary structures in interlaminated siltite and argillite 

of member E . . . . . . . . . . . . . . . . . . . . 23 

13. Sedimentary structures in quartzites of member E 26 

14. Interlaminated siltite and argillite from basal part of 

member F . . . . . . . . . . . . . . . . . . . . . 28 

15. Flagstone quarry in member F . . . . . . . . . . . 30 

16. Argillite-pebble conglomerate in lower part of member F . 31 

17. Argillite-pebble conglomerate, member F . . . . . . . . 32 

18. Interbedded quartzite, siltite, and argillite, member G 33 

19. Argillite in lower part of member H . . . . . . . 35 

20. Quartzite interbedded with argillite in lower part of 

member H . . . . . . . . . . . . . . . . . . . 36 

21. lnterlaminated siltite and argillite, member H . 36 

22. Massive silty argillite, member H . . . . . . . . 37 

23. Geologic map of part of Plains quadrangle . . . . . . . 46 

24. Photomicrograph of breccia showing argillite fragments in poorly 

sorted argillic siltite matrix . . . . . . . . . . . . . . . . . . 48 

25. Structure sections showing present and original configurations of 

breccia illustrated in figure 23 . . . . . . . . . . . . . . . . . 49 
THE PRICHARD FORMATION OF THE 

LOWER PART OF THE 

BELTSUPERGROUP(MIDDLE 

PROTEROZOIC) NEAR PLAINS, 

SANDERS COUNTY, MONTANA 

By EARLE R. CRESSMAN 

ABSTRACT 

The Prichard Formation, about 6,000 m thick near Plains, Montana, is the basal 

unit of the Belt Supergroup. The Prichard consists mostly of interlaminated and very 

thinly interbedded siltite and argillite that commonly occur as graded couplets. Quartzite is a minor constituent. The formation has been metamorphosed to the biotite zone 

of the greenschist facies. Several dioritic to gabbroic sills, one nearly 400 m thick, 

are intercalated in the section. The Prichard is no younger than 1,330 m.y. (million 

years) and no older than 1, 700 m.y. 

The Prichard Formation is divided into eight informal members designated A through 

H from base to top. Member A, 600 m thick, consists of interlaminated and very thinly 

interbedded argillic siltite and silty argillite. Member B, 1,000 m thick, is similar to 

member A but contains abundant slump folds. Attitudes of slump-fold axial planes 

and facing directions of beds in the folds indicate that slumping was mostly eastward. 

Member C, 75--110 m thick, is mostly argillic quartzite in beds 0.4--0.6 m thick. Most 

of the quartzite beds grade to argillite in their upper part. Member D is 290 m thick 

and is characterized by platy-weathering olive-gray silty argillite that becomes more 

silty up section. Member E, 825 m thick, consists of inter laminated siltite and argillite 

and some interbedded quartzite. Mud cracks, mud-chip breccia, scour-and-fill structures, and lenticular and irregular laminae typify the siltite-argillite. The quartzites 

are commonly crossbedded and discontinuous. 

The contact between member E and F is sharp. The section from this contact to 

the top of the formation is more argillic than the section below the contact. 

Member F, mostly 975--1,100 m thick, is interlaminated and very thinly interbedded 

silty argillite and argillite. Pyrite laminae are conspicuous in some intervals. Argillitepebble conglomerate is present locally in the lower part of the member. Member G, 

500 m thick, is characterized by quartzite that is interbedded with siltite and argillite. 

Member H, about 1,600 m thick, is similar to member F but becomes more argillic 

upward. Pyrite laminae are present only near the base. 

The contact between the Prichard Formation and the overlying Burke Formation 

is placed for this report at the lowest occurrence of beds containing structures indicative 

of shallow-water deposition. This contact is about 500 m lower in the section than 

the traditional contact, which is the highest occurrence of argillites typical of the 

Prichard. 

Argillite-clast breccia occurs (1) as tabular bodies as much as 600 m thick that cross 

the bedding at low angles and (2) along high-angle faults. The first type probably 

formed along the sole of a major shelf-edge slump. The origin of the second type is 

not known. 
2 PRICHARD FORMATION, PLAINS, MONTANA 

The Prichard Formation was probably deposited by a major river system on continental crust stretched and thinned in the early stages of continental separation. Depositional environments ranged from lower slope at depths of perhaps 1,000 m or more 

to the intertidal zone. Members A through E resulted from one basin-filling progradation and members F through H and the basal Burke Formation from a second. 

INTRODUCTION 

The Prichard Formation, the lowest unit of the Belt Supergroup 

of Middle Proterozoic age, was named by Ransome and Calkins (1908, 

p. 23, 24, 29-32) for Prichard Creek in the northern part of the Coeur 

d'Alene mining district, Shoshone County, Idaho (fig. 1). They gave 

the thickness as exceeding 8,000 ft (2,440 m) and described the formation as follows (Ransome and Calkins, 1908, p. 23). 

It is a very thick accumulation of sediments, composed in greater part of argillite 

(indurated mud), regularly banded in lighter and darker shades of blue gray. The weathered surface is commonly stained with reddish-brown oxide of iron. The formation comprises also some gray indurated sandstone, which occurs at various horizons, but is 

especially abundant near the top. 

Calkins (1909) subsequently identified the Prichard Formation in 

a number of areas in northern Idaho and northwestern Montana. One 

of these areas was in Sanders County, Mont., near the town of Plains 

and in the area considered in this report. 

In British Columbia the equivalent of the Belt Supergroup is termed 

the Purcell Supergroup. In 1912, Schofield, working in the Purcell 

Mountains, named the lowest unit of the Purcell Supergroup as the 

Aldridge Formation and correlated it with the Prichard Formation 

of Montana and Idaho (Schofield, 1914). He described the Aldridge 

as consisting of rusty-weathering heavy- and thin-bedded argillaceous 

quartzite and slate that contain numerous sills of gabbro; the thickness 

was estimated as 1,830 m (Schofield, 1914, p. 221). 

In 1892, the Sullivan deposit-subsequently one of the world's major 

producers 1

!Jf lead-zinc-silver ore-was discovered in rocks that 

Schofield later included in the Aldridge Formation (Ethier and others, 

1976, p. 1570). The orebody, which is stratiform, was long considered 

to be an epigenetic hydrothermal deposit (Freeze, 1966, p. 264), but 

evidence indicating a syngenetic origin has been accumulating over 

the past decade. Geochemical and geologic data suggest that the metals were leached from the sedimentary pile by chloride-rich brines 

of meteoric derivation (Thompson and Panteleyev, 1976), and isotopic 

data indicate that the sulfur was derived from sea water (Campbell 

and others, 1978, p. 267). Thompson and Panteleyev (1976) and 

Campbell, Ethier, Krouse, and Both (1978, p. 267) suggested that 

the Sullivan orebody is analogous to the sulfide deposits now forming 
118'' 

49 

48'' 

1-,c 4j 

INTRODUCTION 

on upper plate 

!~HIGH-ANGLE FAULTShowing apparent slip. 

Bar and ball on downthrown side 

LIMIT OF OUTCROP OF 

BELT SUPERGROUP 

1 Plains Quadrangle 

2 Perma Quadrangle 

3 Prichard Creek 

4 Sullivan Mine 

5 Crossport locality 

6 Pend Oreille area 

7 Alberton Quadrangle 

50 100 KILOMETERS 

I I 

50 100 MILES 

3 

FIGURE 1.- Area of Belt terrane showing principal faults and localities mentioned in 

text. Modified from Harrison (1972, fig. 1) and from Harrison, Kleinkopf, and Wells 

(1980, fig. 2). 

in the Red Sea. These new concepts have stimulated interest in other 

areas in which the Aldridge and Prichard Formations are exposed. 

Were the depositional environments elsewhere such as to have favored 

deposition of syngenetic sulfides? 

The Prichard Formation near Plains, Mont., was reported by Wallace and Hosterman (1956, p. 579) to be at least 17,000 ft (5,700 m) 

thick, making it one of the thickest known exposures of the formation. 

Furthermore, the Plains and Perma quadrangles are within the Wallace 1 ox 2° topographic quadrangle which is being studied by the U.S. 

Geological Survey as part of the Conterminous United States Mineral 

Appraisal Program. Thus, the area seemed an appropriate one in 

which to initiate studies of the Prichard. 

This report discusses the internal stratigraphy, the depositional environments, and the depositional history of the Prichard Formation 

in the Plains area. Data from these studies have been applied to the 

evaluation of the resource potential for Sullivan-type deposits in the 

folio for the Wallace 1°X2° quadrangle (Harrison and others, in press). 
4 PRICHARD FORMATION, PLAINS, MONTANA 

METHODS 

The fieldwork on which this report is based was conducted in the 

summers of 1979 and 1980. The Prichard Formation in the Plains and 

Perma quadrangles (fig. 2) was subdivided into mappable units, and 

the sequence and distribution of these units were determined by mapping. The geologic map of that part of the area west of the Flathead 

Indian Reservation has been released (Cressman, 1981). The base 

maps were the 1:62,500 topographic maps of the 15-minute Plains and 

Perma quadrangles, both with a contour interval of 80 ft. Unit 

thicknesses were for the most part measured from the geologic map, 

and the generalized stratigraphic section (fig. 3) was constructed from 

mapping traverses. Stratigraphic sections of several critical intervals 

were measured by compass and tape or by compass and pace. Field 

studies were supplemented by the binocular examination of hand 

specimens, and about 60 thin sections were examined. 

I was ably assisted in the summer of 1980 by Peter G. DeCelles 

who measured most of the slump-folds. 

GEOGRAPHIC SETTING 

The study area is in southern Sanders County, Mont., approximately 75 km west of Missoula, Mont., and 200 km east-southeast 

of Spokane, Wash. (fig. 1). It is centered about the junction of the 

Flathead River with the Clark Fork (fig. 2). The area is mountainous 

with a total relief of 1,425 m. The mountains within the bend of the 

Clark Fork and south of the Flathead River are the eastern part 

of the Coeur d'Alene Mountains, whereas those to the north are generally considered a southeastward extension of the Cabinet Mountains. 

Much of the mountainous area is forested, and the Prichard Formation is best exposed in steep valley walls along the two rivers. These 

exposures can be examined only with considerable difficulty, but excellent exposures may be seen in cuts along Montana Highway 135 and 

along the Burlington Northern Railroad. Good access to the mountains 

is provided by gravel and dirt roads up most of the tributary valleys 

and by logging roads. 

STRUCTURAL SETTING 

The Plains and Perma quadrangles areas are on the north edge 

of the Lewis and Clark line of Billingsley and Locke (1939, p. 36; 

fig. 1), which they defined as a northwest-trending zone of tear faults 

that extends from just north of Yellowstone National Park to Spokane. 

Major tear faults within the line exhibit right-lateral strike-slip of 

10-30 km (Harrison and others, 1974, p. 12, 13). The St. Mary's fault, 
INTRODUCTION 5 

115°00' 45' 114°30' 

47 °30' ,--------=---------,------.._-,--------,--------, 

Three 

,,/ Lakes 

47o15

. c__ _________ _ ____l_ __ J...___ __________ -_'''-"'----Pe_a_k----' 

10 KILOMETERS 

FIGURE 2.- lndex map of the Plains and Perma quadrangles. 

which crosses the Plains and Perma quadrangles from west to east, 

has been estimated to have an apparent right-lateral movement of 

13 km (Harrison and others, 1974, p. 12). Within the study area, 

differences across the St. Mary's fault in the stratigraphic position 

of one of the sills in the Prichard Formation indicates at least 18 

km of movement. 

The rocks both north and south of the St. Mary's fault have been 

folded and faulted. Within the study area the folds are moderately 

tight, and dips commonly range from 35° to 50°. Most faults strike 

northwest and are of unknown but probably small displacement. 

Nearly all of the faults dip greater than 75°, and most are nearly 

vertical. 

The Lewis and Clark line has had a long and complex structural 

history (Weidman, 1965; Harrison and others, 1974, Reynolds and 

Kleinkopf, 1977). Some faults apparently moved during Belt time 

(Chevillon, 1977), and some moved as recently as late as Tertiary 

(Harrison and others, 1974, p. 13). 
6 PRICHARD FORMATION, PLAINS, MONTANA 

STRATIGRAPHIC SETTING 

The study area is near the center of the Belt basin, and the Belt 

Supergroup attains its greatest thickness in this vicinity (Harrison, 

1972, fig. 3). The base of the Prichard Formation, which also is the 

base of the Belt, is not exposed, but nearly 7,000 m of Prichard (including the intruded sills) are present in the area. I have not studied 

the overlying rocks, but they have been described by Wells (1974) 

in the nearby Alberton quadrangle where the post-Prichard Belt rocks 

are 13,250 m thick. Thus, the total thickness of the Belt Supergroup 

in this area is about 20,000 m. 

The Prichard Formation is directly overlain by the Ravalli Group, 

which in the Alberton quadrangle is 4,050 m thick. The Ravalli comprises from base to top, the Burke, Revett, St. Regis, and Empire 

Formations. The Revett is mostly quartzite; whereas, the other formations are mostly argillite and siltite. The Ravalli Group has been 

studied by Hrabar (1973), who concluded that it had accumulated as 

a subsea fan system that prograded from the south, and by Kopp 

(1973), who interpreted it as having been deposited as a tide-dominated delta. 

The Ravalli Group is overlain by the Helena Formation to the east 

and the equivalent Wallace Formation to the west. In the Alberton 

quadrangle the Helena (or Wallace) is 3,050 m thick and consists of 

dolomite with some calcitic siltite and argillite. Stromatolites in the 

dolomite attest to deposition in shallow water. 

The Helena and Wallace Formations are overlain by the Missoula 

Group, which is about 6,150 m thick in the Alberton quadrangle where 

the group is composed of the Miller Peak Formation, Bonner Quartzite, MeN amara Formation, Garnet Range Formation, and Pilcher 

Quartzite, respectively, from bottom to top. The Missoula Group consists mostly of red and green argillite and red to pink and light-gray 

quartzite. The rocks contain mud cracks, mud-chip breccia, ripple 

marks, and cross lamination, all indicative of deposition near sea level. 

Winston (1973) concluded that the lower part of the Missoula Group 

was deposited as a braided stream and sea-margin complex. The Missoula Group is the uppermost component of the Belt Supergroup and 

is overlain unconformably by quartzite of Cambrian age (Harrison and 

Campbell, 1963, p. 1423). 

PRICHARD FORMATION 

GENERAL CHARACTER 

The Prichard Formation in the study area is, with sills included, 

a maximum of about 7,000 m thick. The preponderant rock type is 

interlaminated to very thinly interbedded siltite and argillite that 
PRICHARD FORMATION 7 

occur mostly as graded couplets. Quartzite is a minor part of the 

section but forms several fairly conspicuous mappable units. The upper 

half of the formation is noticeably more argillic than the lower half. 

Freshly broken rock is mostly laminated or banded dark and light 

gray, but nearly the entire section weathers moderate brown, the 

rusty color that is typical of the Prichard throughout the Belt basin. 

Dioritic to gabbroic sills, four of which are thick enough to map at 

the scale of 1:62,500, are intercalated in the section. 

The average mineralogic composition, in percent, of the three principal rock types in the Prichard Formation as determined by Harrison 

and Jobin (1963, table 1) by X-ray diffraction of samples from the 

Pend Oreille area of northern Idaho are as follows: 

No. of Quartz Albite- Potassium Sericite Chlorite Biotite 

samples oligoclase feldspar 

Argillite 11 30 19 Trace 23 8 10 

Siltite 7 41 24 2 20 4 9 

Quartzite 6 63 21 Trace 9 5 2 

Considering the remarkable continuity of Belt rocks demonstrated by 

Harrison and Grimes (1970), these compositions are probably similar 

to those of the same rock types in the Prichard in the area of this 

present report. 

METAMORPHISM 

The Belt Supergroup has been subjected to weak regional 

metamorphism throughout most of the basin, and rocks of the Prichard 

Formation are in the biotite zone of the greenschist facies. In the 

Alberton quadrangle the biotite isograd is near the base of the Missoula Group (Wells, 197 4); in the Pend Oreille area the isograd is 

at the top of the Burke Formation (Harrison and Jobin, 1963, p. K26). 

Biotite occurs throughout the Prichard Formation as anhedral flakes 

that average about 40 f.Lm (micrometers) in diameter. Biotite is generally more abundant in argillic siltite laminae and beds than in the 

argillite, and biotite is at least partly responsible for the generally 

dark color of the siltite layers. Well-sorted siltite layers contain little 

biotite and are lighter colored than the argillite. 

The mineral commonly identified as sericite in the Prichard has been 

determined by Maxwell and Hower (1967) in the Pend Oreille area 

and by Wells (1974) in the Alberton quadrangle to be the 2M 

polymorph of illite. The 2M illite was converted from 1Md illite, the 

common clay mineral in the upper part of the Belt (Maxwell and 

Hower, 1967). In the study area, both the grain size and the disorientation of the sericite increase down section. In the upper part of the 
8 PRICHARD FORMATION, PLAINS, MONTANA 

Prichard the flakes average less than 10 f.Lm long and few exceed 

a length of 20 f.Lm; whereas, in the oldest rocks the flakes average 

at least 20 f.Lm in length and may exceed 40 f.Lm. The sericite is distinctly though imperfectly oriented parallel to bedding in the upper 

part of the section, but no orientation is apparent in the basal part. 

Both the increasing grain size and increasing disorientation are sufficient to affect the megascopic appearance of the rock, and samples 

from the top and base of the Prichard can be distinguished on that 

basis alone. 

Randomly oriented porphyroblasts of chlorite are common in the 

middle third of the Prichard and occur as poikiloblastic laths as much 

as several millimeters long. Garnet is not common but occurs in a 

few beds in the lower part of the section. The highest occurrence 

of garnet is about 4,270 m below the top of the formation. Quartz 

and feldspar grains in the quartzite are so sutured that it is difficult 

to estimate what the original grain sizes might have been. 

AGE 

The maximum possible age of the Prichard Formation is that of 

the pre-Belt crystalline basement of Montana and Alberta. The major 

metamorphic event that affected the pre-Belt basement of southwest 

Montana was at 2, 730 ± 85 m.y. (million years) (James and Hedge, 

1980), but the isotopic clocks were reset by a thermal event at 

1,600 ± 100 m.y. (Giletti, 1966). In Alberta, K-Ar dates on biotite indicate that the basement was consolidated from 1,650 to 1,850 m.y. 

ago (Burwash and others, 1962). 

A minimum age of the Prichard Formation is given by the 1,330 ± 45 

m.y. K-Ar date determined by Obradovcich and Peterman (1968, p. 

745) for metamorphic biotite in the Prichard in the Alberton quadrangle. A similar age, 1,335 m.y., was determined by the Rb-Sr 

method for the Hellroaring Creek stock that intrudes the Aldridge 

Formation, which is the equivalent of the Prichard in southeastern 

British Columbia (Ryan and Blenkinsop, 1971). More recently, 

Zartman and others (1982) have determined a U-Pb age of 1,433 ± 30 

m.y. for zircon in a sill in the Prichard Formation in the Crossport 

area of northern Idaho. The sill is apparently near the base of rocks 

that in British Columbia would be referred to the middle member 

of the Aldridge Formation. The equivalent position in the section near 

Plains would probably be within member F of the Prichard. The sill 

may have been, and probably was, intruded during deposition of the 

upper part of the Prichard, so the date may not be taken as a minimum 

age of the formation, but it certainly indicates that most of the 

Prichard was deposited prior to 1,433 m.y. ago. 

Taking 1,300 m.y. ago and 1,700 m.y. ago as the minimum and 
MEMBER A 9 

maximum permissible ages of the Prichard Formation gives a time 

span of 370 m.y., during which the formation could have been deposited. This is longer than the entire Paleozoic. If 1,433 m.y. is taken 

as the minimum, the time span is nearly 270 m.y., which is greater 

than the combined length of the Mesozoic and Cenozoic. The inability 

to be more precise about the time at which the Prichard Formation 

was deposited and the time span during which it was laid down severely inhibit interpretations. 

SUBDIVISIONS 

INTRODUCTION 

In this report the Prichard Formation is divided into eight informal 

members that are recognizable throughout the two quadrangles. These 

members are illustrated in the generalized section of figure 3 and 

are described herein. Even though all the members are mappable at 

a scale of 1:62,500, they are considered informal and are designated 

members A through H from base to top. If future work shows these 

members to be recognizable and useful in other parts of the Belt basin, 

they can then be named and given formal status either as members 

or as formations within a group. 

MEMBER A 

The lowest member of the Prichard Formation is exposed in the 

valley wall just northeast of the confluence of the Flathead River 

with the Clark Fork and on both sides of Seepay Creek in the Flathead 

Indian Reservation. The member consists of interlaminated and very 

thinly interbedded argillic siltite and silty argillite. The siltite layers, 

mostly dark gray and from several millimeters to several centimeters 

thick, commonly grade upward into light-gray silty argillite layers 

that also range from several millimeters to several centimeters in 

thickness. The thicker siltite layers themselves may consist of graded 

siltite-argillite couplets 0.5--3 mm thick. Some siltite laminae and beds 

are well sorted, light colored, and cross-laminated, and their basal 

contact may channel several millimeters into the underlying bed. 

These siltite layers tend to be concentrated in certain intervals. A 

cut along a logging road southeast of the mouth of McLaughlin Creek 

exposes an angular unconformity of a few degrees. The unconformity 

may have resulted from slumping of the underlying beds or, more 

probably, from a brief period of submarine erosion. 

The base of member A is the base of exposures. The top is placed 

at the base of the lowest diabase sill. As so defined, the member 

is about 600 m thick. Member A differs from member B chiefly in 

that slump folds and slump sheets are common and conspicuous in 
10 

METERS 

0 

1000 

2000 

3000 

z 

0 

1-

<( 

:2: 

a: 

0 

u.. 

w 

~ 

a: 

:::l 

a:l 

z 

0 

1-

<( 

:2: 

a: 

0 

u.. 

0 

a: 

<( 

:I: 

u 

a: 

a.. 

PRICHARD FORMATION, PLAINS, MONTANA 

Member H 

Member G 

Member F 

.:-::. 

b.:_. 

b~:_.·.-;~~:::: 

1:-·····:. 

~:::'~:"''""'"·"'" 

!:=.··· ... 

~=::·~~-·:==-.·.-,~;.· .. -.·,.·.· 

t-:-:-:.··. 

~:-:::.=·-·.·:=~=·-·;:::: 

~--· 

~--· 

lr-

~--

~~===······ •.. 

~: ;·.--·.-. ~-~--" .:....__ 

· .... ; .. : .. : ": :._ -._ --.--:- -~. ~ --. -._. _:_ 

=~ :' :_ ~- ·-:- :- ":- ._:-: ·. 

t-- -· 

t-

-

Olive gray (greenish gray) 

Dark gray to greenish gray 

(pale olive) 

Medium gray 

(moderate brown) 

Medium gray to light gray 

(olive gray) 

Light gray 

(moderate brown) 

Dark gray to light gray 

(dusky brown to moderate 

brown) 

Dark gray to medium gray 

(dusky brown to moderate 

brown) 

Dark gray to light gray 

(moderate brown) 
METERS 

Member E 

4000 

z 

0 Member D 

1-

<( 

~ Member C 

a: 

0 

LL. 

0 

5000 a: 

<( 

:X: Member 8 u 

a: 

a.. 

6000 

Member A 

>< x "''><"' ".,. )( ")< 

7000 

MEMBER A 

~ 

.-:a. 

;....?.. 

......-::2-

~ 

_./l.. 

_/.1-

........-:1-

11 

Medium dark gray to light gray 

(grayish-brown to moderate 

brown) 

Olive gray 

Medium gray 

Medium gray to medium light 

gray (moderate brown) 

Medium gray to medium light 

grew (moderate brown) 

EXPLANATION 

Diabase Planar laminae and beds 

Graded laminae and beds 

Argillite 

= Lenticular beds 

Silty argillite ~ Irregular laminae 

Siltite Cross lamination 

Quartzite \\\.\,\. Crossbedding 

Argillite-pebble conglomerate Mudcracks 

lnterlaminated and closely Mud-chip breccia 

interbedded rock types in Scour and fill structure approximate proportion 

shown ..---:2- Slump folds 

FIGURE 3.-Generalized stratigraphic section of the Prichard Formation in the Plains 

and Perma quadrangles. Weathering color in parentheses. Datum top of Prichard 

Formation. 
12 PRICHARD FORMATION, PLAINS, MONTANA 

member B but not in member A. I do not know whether or not the 

two units could be mapped separately by this criterion in the absence 

of the sill. 

Member A is best exposed along the railroad southeast of 

McLaughlin Creek, along a logging road that ascends the mountain 

front east of the mouth of McLaughlin Creek, and along logging roads 

on both sides of Seepay Creek. 

MEMBERB 

Member B is similar to member A in that it consists nearly entirely 

of interlaminated and very thinly interbedded siltite and argillite that 

commonly occur as graded couplets (fig. 4). Member B also contains 

some zones in which light-colored, well-sorted, cross-laminated siltite 

layers are common. However, member B differs from member A in 

that it contains abundant sedimentary slumps and some quartzite. 

The base of member B is the top of the lowest diabase sill and 

the top is at the base of a quartzite unit 75--110 m thick that constitutes member C. The thickness of the member as so defined is uncertain because of faulting. The best estimate, 1,000 m, is based on exposures on the west side of McLaughlin Creek and on both sides of 

See pay Creek. It is possible that the lowest sill may change position 

over the area with a resulting change in thickness of the member, 

but there is no horizon with which the position of the sill can be 

readily compared. 

SLUMP FOLDS 

Slump folds were defined by Helwig (1970, p. 174) as "* * * prelithifaction folds attributed to gravity-induced horizontal mass movement and deformation of one or more beds in a coherent slump mass." 

The slump folds in the Prichard Formation are not very photogenic, 

but two are illustrated in figures 5 and 6. Several characteristics of 

these features in member B that demonstrate their syndepositional 

rather than tectonic origin are (1) the slump folds occur in packets 

underlain and overlain by undeformed beds; (2) there is no axial-plane 

cleavage; (3) the orientation of the slump-fold axes has considerable 

spread and has no obvious relation to the trends of tectonic folds 

and faults. 

The amplitude of individual slump folds is mostly 0.3-0.5 m though 

some may be as large as 10 m. The folds are mostly isoclinal, nearly 

recumbent, and have a slip surface separating the lower limb from 

beds below. Though some folds occur singly, most are in composite 

slump sheets in which individual folds are piled one on another. The 
MEMBER B 13 

FIGURE 4.-Graded siltite-argillite couplets in member B. Dark layers are siltite, light 

layers argillite. Lens cap is 5 em in diameter. 
14 PRICHARD FORMATION, PLAINS, MONTANA 

FIGURE 5.-Slump fold in member B. Exposed in roadcut on Montana Highway 135 

1.6 km south of junction with Montana Highway 200. 

fold illustrated in figure 6 is part of the composite slump sheet 

sketched in figure 7. The sheets may be as thick as 30 m, but more 

commonly they are about 8 m thick. The inclinations of the slump-fold 

axial planes commonly increase from base to top of the slump sheet. 

This is the case with the slump sheet of figure 7. Beds at the base 

of some slumps have been disrupted, and the segments have been 

rolled into balls. 

Though slump folds are common and conspicuous in member B, their 

actual abundance is difficult to assess. Slumped beds make up about 

15 percent of one well-exposed section that is 285 m thick and near 

the middle of the member. The slumps are not evenly distributed 

throughout the member but seem to be most common in the 100 m 

just below a quartzite unit near the middle of the member and in 

the uppermost few hundred meters. I have not noticed any irregularities in the geographical distribution of the slumps. 

Slump folds are well exposed along Montana Highway 135 1.6 km 

south of its intersection with Montana Highway 200, along the River 

Road 3.2 km west-northwest of Paradise, and in the relatively new 

roadcuts along Montana Highway 200 west of Seepay Creek. The most 

spectacular exposure, illustrated in part by figure 7, is in roadcuts 

along Montana Highway 200 immediately east of Robertson Creek. 
MEMBER B 15 

FIGURE 6.-Slump fold in member B. Beds face toward synclinal axis. Exposed in 

roadcut on Montana Highway 200 in Perma quadrangle immediately east of mouth 

of Robertson Creek. 
16 

EAST 

PRICHARD FORMATION, PLAINS, MONTANA 

10 METERS I I I 

20 FEET 

Location of 

figure 6 

WEST 

FIGURE 7.-Sketch of composite slump sheet exposed in roadcut on Montana Highway 

200 west of Robertson Creek. Fold to right is one illustrated in figure 6. Beds 

face toward axes of syncline. Drawing by P. G. DeCelles. 

The attitudes of the axial planes of 47 slump folds were measured 

to determine the direction of slumping and thereby the regional slope. 

Where possible, the facing direction of beds in the fold was recorded. 

The effects of tectonic dip were removed by calculations according 

to the method of Mertie (1922), and the results were checked by use 

of a stereographic net. The rationale for determining slump and slope 

directions from these measurements was formulated by Woodcock 

(1976, p. 171, 172) and is based on the assumption that prior to tectonic 

tilting the slump-fold axial plane will dip up the depositional slope 

and that the beds in the fold will face in the downslope direction. 

The results are presented in figures 8 and 9. In figure 8, the poles 

of all measured slump-fold axial planes are plotted on the southern 

hemisphere of a stereographic net. In figure 9, the data are plotted 

on a map, and the inferred slump directions are shown. The data 

show considerable scatter, as might be expected. An arcuate outline 

has been observed for many slumps and slides in recent sediments 

(Coleman and Garrison, 1977, fig. 5), and the prograding depositional 

slope in Prichard time is likely to have been irregular because of reentrants, salients, and valleys. I cannot document changes of orientation 

within a single-slump fold, but the variation within one composite 

slump sheet is illustrated in figure 8. Nevertheless, figures 8 and 

9 both indicate that the dominant slump direction was to the east, 

and I assume that this was also the direction of regional slope. 

Separate stereographic plots for slump folds north and south of the 

St. Mary's fault showed no obvious differences. 

QUARTZITE MARKER 

A unit of quartzite as much as 30 m thick occurs 250-500 m-the 

exact distance is unknown-below the top of member B. North of 

the St. Mary's fault the unit consists of medium-gray blocky-weather-
MEMBERB 17 

N 

~I •• ? .-

~ ••• !.. •• 

• ...... •:t-• , •• . •:.---' 

•• \'•.:< ...... 

FIGURE 8.-Equal-area plot, lower hemisphere, of poles of axial planes of all slump 

folds measured in the Plains and Perma quadrangles. Arrows indicate facing direction. Poles shown by +are from within one composite slump sheet. 

ing quartzite in beds mostly 0.3-1 m meter thick. The lower part 

commonly contains some interbedded argillite and siltite. Most quartzite beds grade upward through 2-5 em to argillite. Though most 

quartzite beds are structureless, some are planar laminated. In roadcut exposures along Montana Highway 200 west of Robertson Creek 

and in cliff exposures on the north side of the Flathead River opposite 

Wilson Creek, some of the quartzites are in channels. Neither exposure can be examined closely, but I estimate the channels to be several 

meters deep and several tens of meters wide. The quartzite marker 

is thicker north of the St. Mary's fault than it is to the south, where 

locally it consists of a few thin quartzite beds. Although I have never 

found more than one quartzite unit in member B where I know the 
18 PRICHARD FORMATION, PLAINS, MONTANA 

115° 114°30' 

47°30' ,--------------------,----------------, 

PLAINS PERMA 

~ 8 Direction and amount of dip of 

axial plane 

~ 8 Beds face up dip of axial plane 

- 8 Beds face down dip of axial plane 

Direction of slumping 

9 4 12 7 ~ 

58 ~0 11~1 J82~ 27 

45 ~12; 26P1/:..11 '-!~a 11, 

47°15' L___------------~---------------' 

10 KILOMETERS 

I 

5 MILES 

FIGURE 9.-Plains and Perma quadrangles showing direction and amount of dip of axial 

planes of slump folds, facing direction of beds in folds, and inferred direction of 

slumping. 

section to be relatively unfaulted, it is at least possible that several 

discontinuous quartzites occur at somewhat different horizons within 

the member. 

MEMBERC 

lnterlaminated siltite and argillite of member B is overlain by 75-

110 m of quartzite and intercalated siltite-argillite that constitute 

member C. The quartzite is argillic, very fine to fine grained, medium 

gray to medium light gray, and blocky weathering. It occurs in sets 

from 1 to 10 m thick. The argillite, inconspicuous except in excellent 

exposure, is in sets mostly 2 or 3 m thick. Beds within quartzite 

sets are generally 0.4-0.6 m thick. Most of these beds grade to argillite 

in their top 1 or 2 em. The quartzite is mostly massive or planar 

laminated, but cross-lamination is present in the top few centimeters 

of many beds, and ripple marks are present on some bedding surfaces. 

The sedimentary structures are inconspicuous, and as a result Bouma 

cycles (Bouma, 1962) are difficult to apply. If the massive-appearing 
MEMBER D 19 

quartzite is the Bouma A interval, then the most common cycles are 

ACDE, ADE, and AE. On the other hand, the massive quartzites 

may be the Bouma B interval in which original lamination has been 

obscured by regional metamorphism. In that case, the cycles are 

BCDE, BDE, and BE. Most beds are planar at the base, but a few 

beds channel as much as 0.5 m into the underlying bed. I have not 

seen sole marks; if present, they must be extremely uncommon. 

The base of member C is the base of the dominantly quartzite section; a few quartzite beds are present at least locally in the upper 

part of member B, but in the few areas where I have mapped member 

C, the contact has been sharp. The top is placed at the top of the 

dominantly quartzite section, and though quartzite interbeds are present in the lower part of member D, the contact is sharp. 

Member C is present throughout the area except between Wilson 

Creek and the ridge just east of Quinns Hot Springs where member 

D rests directly on member B, separated by a thin argillite-clast breccia. Member C is particularly well exposed along the Burlington 

Northern Railroad in the first cut south of John Tunnel. 

MEMBERD 

Member D consists of a variety of rock types, but platy-weathering 

olive-gray silty argillite containing sparse garnet and randomly 

oriented chlorite porphyroblasts is characteristic. Similar beds are 

present at least locally in the lower part of member E, but any occurrence of the olive-gray argillite in thicknesses of 50 m or more is 

diagnostic of member D. The best estimate for the thickness of the 

member is 290 m, but map measurements indicate some thicknesses 

as great as 410 m. The greater thickness probably results either from 

undetected faulting or from inconsistencies in mapping the upper contact. 

The basal 70 or 80 m of member D consists of planar-laminated 

siltite and argillite interbedded with quartzite. The quartzite, which 

is dark to light gray and somewhat argillic, is in sets mostly 3-6 

m thick. Both planar laminae and obscure lenticular bedding may be 

present. The siltite and argillite are in couplets and are similar to 

the siltite and argillite of members A and B. A few small slump folds 

are present locally. 

The overlying three-fourths of the member comprises a coarseningupward sequence beginning at the base with the platy-weathering 

olive-gray silty argillite (fig. lOA). The silty argillite contains inconspicuous planar siltite laminae. Up section the siltite laminae thicken, 

and some form lenticular beds several centimeters thick (fig. lOB). 

The lenses are cross-laminated, and the lenticular layers probably 
20 PRICHARD FORMATION, PLAINS, MONTANA 

A, Platy-weathering silty argillite in lower part of member. 

B, Wavy to lenticular beds of siltite several centimeters thick in silty argillite near 

middle of member. 

FIGURE 10.-PHOTOGRAPHS OF MEMBER D 
MEMBER D 21 

C, Very thinly interbedded siltite and argillite near top of member. Lighter colored 

layers are siltite, which makes up most of the rock here as opposed to the 

mainly argillite content of the lower part of member D shown in figure lOA. 

FIGURE 10.- PHOTOGRAPHS OF MEMBER D- Continued 

originated as a ripple-bedded sheet of silt. Farther up section the 

siltite layers become still thicker and more closely spaced until they 

make up most of the rock. The siltite layers, several centimeters thick, 

commonly have sharp bases and gradational tops (fig. lOG). Load casts 

are present at the base of some. These predominantly siltite beds, 

in turn, pass upward into siltite with scour-and-fill structure that is 

commonly interbedded with quartzite. 

The upper contact of member D is placed where irregular to wavy 

laminae and scour-and-fill structure typical of member E become dominant over the the planar lamination and bedding typical of member 

D. At some localities the contact is fairly abrupt and can be located 

within a few meters. Elsewhere, intervals typical of member D alternate with intervals typical of member E. I do not know whether these 

two situations represent real differences in the section from place to 

place or whether they are a function of the type and amount of exposure. 

Partial exposures of member D may be seen in road cuts along 

Montana Highway 135 between Quinns Hot Springs and Wallace 

Creek. 
22 PRICHARD FORMATION, PLAINS, MONTANA 

FIGURE 11.-0utcrop of member E on west side of the Clark Fork south of John 

Tunnel. 

MEMBERE 

This member, approximately 825 m thick exclusive of the sill that 

is present in its lower part north of the St. Mary's fault, consists 

of laminated siltite and argillite with some interbedded quartzite. The 

rocks exhibit abundant current-produced structures, such as scourand-fill and crossbedding, and some contain mud-chip breccias and 

mud cracks indicative of periodic exposure. The member is siltier and 

more resistant to weathering than the underlying and overlying members and commonly crops out as cliffs and bold ledges (fig. 11). The 

rusty-weathering color, though present, is not as pronounced in this 

member as in the rest of the Prichard Formation. 

Most of member E consists of interlaminated light-gray siltite and 

dark-gray argillite that commonly occurs as graded couplets. The 

laminae are mostly less than 1 mm thick, but particularly in the lower 

part of the member bedding sets several centimeters thick of the very 

thinly laminated siltite-argillite may alternate with wavy to lenticular 

beds of siltite several centimeters thick. In general, though, the lamination is considerably thinner in member E than in the other members 

of the Prichard. The laminae are mostly wavy to crinkly, and planar 

laminae that are so typical of the rest of the Prichard Formation are 

uncommon. 

The lower part of the member contains scour-and-fill structures 15--
MEMBER E 23 

20 em deep that are filled by interlaminated siltite and argillite that 

drapes into the scour (fig. 12A). Many of the siltite layers in the 

lower part are irregular in thickness, and some are lenticular. The 

combination of scour and fill, irregular and lenticular beds, and the 

effects of differential compaction impart an irregular wavy appearance 

to this part of the section (fig. 12B). Higher in the member the thicker 

layers become less common, the laminae become irregular to crinkly, 

and mud cracks and mud-chip breccias become common (fig. 12C). 

The mud cracks are well formed and polygonal. 

Quartzite forms a minor but locally conspicuous part of member 

E. The quartzite, considerably less sericitic than quartzite in member 

C, is in beds mostly about 0.5 m thick and in sets as thick as 9 m. 

Flaser bedding, ripple bedding, and crossbedding, some of the herringbone type, are common (figs. 13A, B). Planar and trough crossbedding have been observed in sets as thick as 0. 7 m. Wedge-shaped 

sets of high-angle, small-scale planar crossbeds (fig. 13C) are present 

in one quartzite bed that is well exposed in the first gully north of 

the mouth of Siegel Creek. Most of the quartzite is fairly clean, but 

the dark laminae are sericitic. Quartzite beds are present throughout 

the member but are most numerous in the lower one-third. Individual 

A, Scour-and-fill structure near base of member. Scale is 17.5 em long. 

FIGURE 12.- SEDIMENTARY STRUCTURES IN INTERLAMINATED SILTITE 

AND ARGILLITE OF MEMBER E 
24 PRICHARD FORMATION, PLAINS, MONTANA 

B, Lenticular and wavy bedding. 

FIGURE 12.- SEDIMENTARY STRUCTURES IN INTERLAMINATED SILTITE 

AND ARGILLITE OF MEMBER E-Continued 
MEMBER E 25 

C, Mud-chip breccia (b) in upper part of member. 

FIGURE 12.- SEDIMENTARY STRUCTURES IN INTERLAMINATED SILTITE 

AND ARGILLITE OF MEMBER E- Continued 
26 PRICHARD FORMATION, PLAINS, MONTANA 

A, Lenticular and wavy bedding; some wavy flaser bedding (nomenclature of Reinech 

and Wunderlich, 1968). 

B, Ripple crossbedding. 

FIGURE 13.-SEDIMENTARY STRUCTURES IN QUARTZITES OF MEMBER E 
MEMBER E 27 

C, Wedge-shaped sets of high-angle small-scale planar cross beds. 

FIGURE 13.-SEDIMENTARY STRUCTURES IN QUARTZITES OF MEMBER E 

-Continued 

quartzite units do not seem to be continuous. Planar-laminated quartzites, such as those in member C, are not present. 

Platy-weathering olive-gray silty argillite similar to that which 

characterizes member D is present at least locally in the lower part 

of member E where it constitutes an interval a few tens of meters 

thick. These occurrences suggest that members D and E intertongue. 

On the mountainside east of Quinns Hot Springs and on the west 

side of Seepay Creek, this interval contains a thin, speckled, nearly 

white bed that consists of quartz, chlorite, and garnet. 

The upper contact of member E is sharp and marked by the abrupt 

upward passage from the thinly laminated siltite-argillite exhibiting 

various shallow-water structures to planar laminated and very thinly 

interbedded argillite and silty argillite of the overlying member F. 

Typical exposures of member E may be seen along Montana Highway 135 between Wallace and Siegel Creeks. 
28 PRICHARD FORMATION, PLAINS, MONTANA 

MEMBERF 

The cliff- and ledge-forming siltites of member E are overlain by 

silty argillite of member F that weathers to yield abundant platy float. 

Measurements from the geologic map suggest that member F is from 

730 to 1,200 m thick, but this range probably results more from unmapped structure than from an actual range in thickness. The best 

estimates are that this member is 975 m thick south of the St. Mary's 

fault and is 1,100 m thick north of the fault. Even so, I am not certain 

that these differences are real. 

Typical exposures of member F consist of interlaminated and very 

thinly interbedded silty argillite and argillite. The two may form 

graded couplets, or the contacts may be sharp. Most commonly the 

layers range from 1 to 6 em thick, and the rock has a banded appearance. The silty argillite is generally dark gray and the argillite medium 

to light gray, but in parts of the section the silty layer is the lighter 

of the two. Most laminae and beds are planar, but a few silty beds 

pinch and swell and channel the underlying argillite. Many silty argillite layers are internally laminated; the laminae consist of silty argillite-argillite couplets, concentrations of carbonaceous matter within 

otherwise uniform argillite, or pyrite (fig. 14). Pyrite as discontinuous 

laminae generally less than 1 mm thick and as grains strung along 

FIGURE 14.-Photomicrograph of interlaminated siltite and argillite from basal part 

of member F. Black discontinuous laminae are pyrite; black spots are mostly biotite. 
MEMBER F 29 

bedding is characteristic of member F. Though the discontinuous pyrite laminae are not present in all beds, I have seen them in all parts 

of the member. Inasmuch as similar pyrite laminae are also present 

in the lower part of member H, the presence of the laminae, though 

very suggestive of member F, is not diagnostic. A few argillite beds 

are as thick as 0.5 m and contain few or no silty laminae. These 

weather to light-gray massive-appearing outcrops that are particularly 

characteristic of the lower part of the member. The banded argillite 

of member F splits very evenly and is locally quarried on a very 

small scale for flagstone (fig. 15). 

A roadcut for Montana Highway 135 just south of Siegel Creek 

exposes several layers of argillite-pebble conglomerate (figs. 16, 17). 

These beds described by the following section: 

9 Argillite, silty, medium-dark-gray .... .... ... .. .......... .. ... ... .. .. ... .. . 

8 Argillite, similar to unit 7, but consists of faulted and rotated 

blocks that were formed before deposition of overlying bed . 

7 Argillite, laminated medium-dark-gray and medium-light-gray; 

laminae mostly 1-15 mm thick; some light-colored laminae pinch 

and swell, channeling underlying laminae ......................... . 

Conglomeratic zone 

6 Argillite, medium-light-gray; contains some siltite pebbles in top 

0.5 m .......................................................................... .. 

5 Siltite, argillic, medium-gray, massive; contains disseminated pyrite grains as much as 0. 7 m in diameter; grades to argillite 

in top 2 m .................................................................... . 

4 Argillite-pebble conglomerate, medium-light-gray; pebbles, about 

50 percent of rock, are poorly so:ied, subangular to subrounded, oriented subparallel to bedding and as much as 3 mm 

in diameter; 90 percent are argillite, 10 percent siltite; matrix 

is poorly sorted argillic siltite ........................................ .. 

3 Siltite, argillic, medium-dark-gray, massive; contains disseminated 

pyrite grains as much as 0.7 mm in diameter ................ .. 

2 Argillite-pebble conglomerate, medium-dark-gray, massive; pebbles are poorly sorted, subangular to subrounded, elongate, 

poorly oriented, mostly 1-2 mm but as much as 4 mm in diameter; mostl~r argillite but some siltite; matrix is poorly sorted 

argillic siltite ................................................................. . 

Total thickness of conglomerate zone .................. .. ........ . 

Argillite, silty, medium-gray; lower two-thirds structureless; overlain by interlaminated argillic siltite and argillite; in uppermost 

part laminae are broken and stretched to irregular streaky 

patches of light-gray argillite in an argillic siltite matrix; base 

is 150 m above base of member F ...... .. ................ .. ...... .. 

Meters 

0.3 

0.5 

3.2 

1.5 

7.5 

1.5 

9 

4 

23.5 

3 

A semiquantitative spectrographic analysis of a sample of unit 6 

gave 2,000 ppm (parts per million) Zn and 50 ppm Ph. 
30 PRICHARD FORMATION, PLAINS, MONTANA 

FIGURE 15.-Flagstone quarry in member F. In NWV4NW1/4 sec. 13, T. 19 N., R. 

26 W., Plains quadrangle. 

Units 3 and 5 are not laminated and are much more poorly sorted 

than bedded siltites elsewhere in the section. These characteristics 

suggest that units 3 and 5 resulted from nearly complete disaggregation and mixing of the laminated siltite-argillite by thixotropy and 

flow. The conglomerate probably resulted from less severe thixotropy 

in which the silty layers completely disaggregated whereas the clayey 

layers broke to fragments. I do not know whether the disruption of 

the sediment resulted from surface flow or whether it might have 

resulted from slip along a zone within the sediment. 

An argillite- and pyrite-clast conglomerate very similar in appearance to those in the measured section is present in about the same 

stratigraphic position, though perhaps a little lower, on the southwest 

flank of Henry Peak, and an argillite-clast conglomerate similar to 

units 2, 4, and 6 and also at about the same stratigraphic position 

is exposed 2.8 km northwest of Three Lakes Peak. Neither of these 

conglomerates can be traced more than a few hundred meters along 

strike, and they are undoubtedly discontinuous. A bed that closely 

resembles units 3 and 5 and contains pyrite grains that may be clasts 

crops out near the top of the member 1.2 km east-southeast of Siegel 

Mountain. 

Black chert and light-colored chert float occur about 315 m and 

400 m, respectively, above the base of member F along the boundary 
MEMBER F 31 

FIGURE 16.-Argillite-pebble conglomerate in lower part of member F. 

of the Flathead Indian Reservation east of Siegel Mountain. The black 

chert does not extend any farther west, but the white chert is present 

on the east flank of Siegel Mountain. I have not seen either chert 
32 PRICHARD FORMATION, PLAINS, MONTANA 

FIGURE 17.-Photomicrograph of argillite-pebble conglomerate from lower part of member F. A, argillite clasts; S, siltite clast; C, chlorite porphyroblast. Matrix is argillic 

siltite. Polarized light. 

in place, but the amount of float suggests that neither bed can be 

more than a meter thick. The light-colored chert is banded light gray 

and very light gray in the same manner and scale as the silty argilliteargillite of member F. Microscopically, it consists of microcrystalline 

quartz that ranges in crystal size from less than 1 f..Lm to 100 f..Lm. 

At least some of the larger crystals were probably deposited as quartz 

silt. Biotite, chlorite, and sericite are concentrated in poorly defined 

laminae. The black chert is tourmalinized and contains as much as 

10,000 ppm boron (R. E. Van Loenen, written commun., 1981). 

The lower part of member F is well exposed along Montana Highway 135 in the first roadcut south of Siegel Creek. Exposures of the 

middle and upper parts may be seen along a Forest Service road 

that ascends Henry Peak from Henry Creek. 

The contact of member F with the blocky-weathering quartzite and 

siltite of member G is fairly sharp in those few localities where it 

is well exposed, but it is generally concealed by quartzite rubble derived from member G. 

The contact between members F and G is well exposed in the first 

roadcut just west of Cascade Campground. 
MEMBER G 33 

FIGURE 18.-Interbedded quartzite, siltite, and argillite in basal part of member G. 

Roadcut on State Highway 135, 600 m north of Cascade Campground. 

MEMBERG 

Member G consists of interbedded quartzite, siltite, and argillite, 

of which quartzite is the most abundant (fig. 18). The member caps 

ridges, peaks, and knobs, and on weathering yields abundant blocky 

quartzite float that locally forms extensive talus. It is about 500 m 

thick. 

Quartzite, the characteristic component of the member, occurs in 

planar beds mostly 0.2-0.5 m thick and in sets as much as 5 m thick. 

The beds commonly grade to argillite in their uppermost few centimeters, and some channel several centimeters into the underlying bed. 

The quartzite is mostly structureless, though planar lamination is 

present locally. The rock is mostly medium gray to medium dark gray 

and weathers dark gray. 

The siltite is mostly medium light gray and typically weathers light 

olive gray. It is in planar beds mostly 0.1-0.2 m thick and in sets 

1-2m thick. Faint planar lamination is common. 

The argillite is similar to that in member F and in the lower part 

of member H. 

The upper contact of member G is sharp and is marked by the 

upward passage from thick beds and sets of hard, resistant quartzite 
34 PRICHARD FORMATION, PLAINS, MONTANA 

to argillite. Locally, quartzite is interbedded with argillite in the basal 

part of member H, and the contact between the two members is difficult to select. The top of member G forms the lip of Cascade Falls 

above Cascade Campground, and the contact may be examined on 

the trail from the campground to the falls. 

Near Cascade Campground in exposures on both sides of the Clark 

Fork a unit about 100 m thick of interlaminated and interbedded silty 

argillite and argillite indistinguishable from much of member F occurs 

about 20 m above the base of member G. This suggests that regionally 

members F and G intertongue. I have not identified this unit elsewhere. 

The best exposures of the member are in roadcuts along Montana 

Highway 135 about 1.2 km west of Cascade Campground. 

MEMBERH 

Member H is the uppermost unit of the Prichard Formation. The 

argillites of the member are rather poorly exposed, and the unit is 

thick and contains few marker beds to facilitate mapping. I am, therefore, less certain of the thickness and, to some extent, less certain 

of the lithologic character of this member than I am of the others. 

Combining the lower part of the member northwest of Patricks Knob 

with the upper part east of Patrick Creek by means of a ripple-marked 

and cross-laminated unit in the middle of the section gives a thickness 

of 1, 770 m. On the other hand, a measurement from the map of the 

thickness near Deemer Peak gives about 1,450 m. Unfortunately, 

these sections are too poorly exposed to know whether or not the 

differences between these two figures reflects an actual difference 

in thickness. Elsewhere within the study area faulting prevents an 

estimate of the thickness. J. E. Harrison (oral commun., 1978) has 

estimated the thickness from the base of member G to the top of 

the Prichard Formation in other parts of the Wallace 1 ox 2° quadrangle 

to be a little more than 2,100 m. Inasmuch as member G is 500 m 

thick, Harrison's figure gives a thickness of about 1,600 m for member 

H. 

The predominant lithologies in member H are interlaminated and 

very thinly interbedded medium-light-gray argillic siltite (or silty argillite) and medium-dark-gray to light-gray and light-olive-gray argillite (fig. 19). They occur as couplets mostly 1 to 3 em thick in which 

the silty layers make up one-fourth to one-third of the couplet. Some 

couplets are graded though many are not. In general, the argillite 

layers are thicker and the colors are lighter in the upper part of the 

section. The entire unit weathers to a rusty color. 

Near Siegel and Patrick Creeks the basal 200 m of member H contains some quartzite in beds 5--25 em thick and sets as much as 3 
MEMBER H 35 

FIGURE 19.-Argillite in lower part of member H. Thin, darker layers are siltite 

laminae. 

m thick (fig. 20). Most of the quartzites are planar laminated and 

grade upward to argillite; some channel 1-2 em into the bed below. 

Pyritic and carbonaceous laminae are also present in some of the siltite-argillite in the lower part of the member, though not so abundantly 

as in parts of member F (fig. 21). 

Northwest of Patricks Knob, beginning about 1,100 m above the 

base of member H and continuing nearly to the top, the member 

contains intervals as much as 6 m thick of massive-appearing silty 

argillite (fig. 22). In a cut on the road to Patricks Knob the lowest 

of these massive intervals contains some soft-sediment deformational 

features that are probably slump folds. The massive intervals make 

up only a small part of the section in which they occur. 

A 90- to 120-m-thick unit of interbedded siltite, quartzite, and argillite occurs about 610 m above the base of member H near Patricks 

Knob and near Patrick Creek. Cross-lamination, small-scale scour-andfill structures, and ripple marks are common in the siltite and quartzite. The quartzite beds are as much as 1. 5 m thick. The unit is well 

exposed in several cuts on the road to Patricks Knob. I have not 

found this unit elsewhere in the report area. 

North of Montana Highway 200, member His partly exposed along 

several logging roads near Deemer Peak. In this area I have seen 

neither the quartzites near the base, the quartzite-siltite interval near 

the middle, nor the massive-appearing silty argillites. Nearly all of 
36 PRICHARD FORMATION, PLAINS, MONTANA 

FIGURE 20.-Quartzite interbedded with argillite in lower part of member H. Exposed 

in cut on abandoned road, SWV4NE1

/4 sec. 35, T. 18 N., R. 25 W., Perma quadrangle. See hammer in circle for scale. 

FIGURE 21.-Photomicrograph of interlaminated siltite and argillite from lower part 

of member H. Py is pyrite; thin, dark laminae in lower part contain carbonaceous 

matter and some pyrite. 
CONTACT WITH THE BURKE FORMATION 37 

FIGURE 22.-Massive silty argillite in middle of member H. Exposed in WV2 sec. 22, 

T. 19 N., R. 26 W., along road to Patricks Knob. 

the exposures consist of the very thinly interbedded siltite and argillite. Light-colored siltite float is common, but I have seen it in place 

in only one locality near the base of the member. Some of the siltite 

beds exhibit climbing ripples at the base and parallel lamination at 

the top. 

CONTACT WITH THE BURKE FORMATION 

Ransome and Calkins (1908, p. 24) named the Burke Formation 

in the Coeur d'Alene district and described it as 

* * * composed of siliceous rocks, prevailingly fine grained, thin bedded, and of a 

light greenish-gray tint. Their lithologic character ranges, however, from that of fairly 

pure, nearly white quartzite to that of dark-gray siliceous shale. The quartzite is more 

abundant in the upper and the shale in the lower part of the formation, which therefore 

exhibits a transition into the quartzitic and argillaceous formations that lie above and 

below it, respectively. The Burke formation is characterized from top to bottom by 

shallow-water features. 

They described the contact between the Prichard and Burke Formation as follows (Ransome and Calkins, 1908, p. 29, 30): 

Within a few hundred feet of the upper limit of the [Prichard] formation, a fairly 

rapid though not abrupt change in the character of the beds takes place. The indurated 

sandstones, sandy shales, and quartzites increase in abundance, and in the uppermost 

part appears a greenish-gray siliceous shale like that which constitutes the greatest 
38 PRICHARD FORMATION, PLAINS, MONTANA 

part of the Burke formation. Most significant as indicating the conditions under which 

these upper beds were laid down is the appearance of such features as ripple marks 

and sun cracks, indicating deposition in very shallow water and frequent exposure 

to the atmosphere. The shallow-water features occur throughout the overlying Burke 

formation, and this fact, as well as the occurrence of gray siliceous shale, like that 

which mainly composes the Burke, in the upper part of the Prichard, indicates the 

gradual nature of the transition from the one formation to the other. 

This gradual transition naturally causes difficulty in fixing the Burke-Prichard boundaries and necessitates the choosing of some lithologic character which shall consistently 

be considered as differentiating the two formations. The diagnostic feature of the 

Prichard is the presence of a considerable amount of blue-gray argillite, for nothing 

like this typical rock is found in the Burke formation proper except locally and in 

small amount. 

This transition zone between the lowest occurrence of rocks typical 

of the Burke and the highest occurrence of rocks typical of the 

Prichard was mapped separately by Hobbs, Griggs, Wallace, and 

Campbell (1965, p. 33--34) and found to increase in thickness across 

the Coeur d'Alene district from 180 m on the west to nearly 610 m 

on the east. They followed Ransome and Calkins in assigning the 

transition zone to the Prichard Formation. 

In this report I have placed the top of the Prichard at the base 

rather than the top of the transitional sequence. Thus, the contact 

is defined as the lowest occurrence of rocks containing sedimentary 

structures indicative of deposition in shallow water. This contact was 

chosen because (1) it is the contact that has been mapped in adjacent 

parts of the Wallace 1°X2° quadrangle (J. E. Harrison, oral commun., 

1980), (2) the contact marks a significant change in depositional environments, (3) the transition zone is not well exposed in the study 

area, and (4) the 7,000 m of rock below the transition zone was quite 

thick enough to absorb all my attention for the two field seasons. 

The top of the Prichard Formation as used in this report and the 

nature of the overlying transition zone is illustrated by the following 

section measured 4 km north of the Plains quadrangle. The PrichardBurke contact by the original definition would have been placed at 

the top of unit 29; whereas, the contact I have used is at the top 

of unit 6. Thus, the transition zone is about 550 m thick. This interval 

exhibits a number of interesting features and is worthy of study in 

itself. 

BURKE FORMATION (LOWER PART) AND 

PRICHARD FORMATION (UPPER PART) 

Section of lowermost Burke and uppermost Prichard Formations measured by pace 

and compass from outcrops along Mont. Highway 28 in T. 21 N, R. 24 W., and T. 

21 N., R. 25 W. Sanders County, Mont. Base of section is in SW1/4NE114 sec. 31, 

T 21 N., R. 24 W, 565 m northeast of Welcome Spring; top of section is opposite 
CONTACT WITH THE BURKE FORMATION 39 

entrance to campground at east end of Rainbow Lake. Measured by E. R. Cressman 

in August, 1980 .. 

Burke Formation (lower part only) 

37. Siltite and argillite, interlaminated and very thinly interbedded; medium gray to olive gray (weathers 

mostly greenish gray, moderate brown along some 

joints); siltite and argillite in even layers 0.5-5 em 

thick; silt layers exhibit load-casts at base and grade 

Thickness Cumulative 

(meters) thickness 

(meters) 

into overlying argillite layers . . . . . . . . . . . Not measured 

36. Poorly exposed; the few small exposures are similar to 

unit 33 . . . . . . . . . . . . . . . . . . . . . . . . 19 19 

35. Quartzite, medium-gray to olive-gray (weathers mostly 

light olive gray, moderate brown along some joints) 

in even beds mostly 0.2-0.4 m thick; contains some 

faint wavy to planar laminae . . . . . . . . . . . 9 28 

34. Similar to unit 33; ripple marks common in lower half, 

mud cracks in upper half. Interval about 50 percent 

covered . . . . . . . . . . . . . . . . . . . . . . . . 320 348 

33. Siltite and argillite, interlaminated; siltite is dark gray 

(weathers pale olive), argillite is greenish gray (weathers moderate brown along joints); laminae from 

less than 1 mm to 1 em thick; siltite laminae commonly lenticular and channel into underlying argillite; 

mud cracks common. Outcrop appears massive; splits 

along planar bedding surfaces about 0.6 m apart. 22 370 

32. Quartzite, medium-gray (weathers light brown) in even 

beds 0.1-0.3 m thick; faint planar and trough lamination . . . . . . . . . . . . . . . . . . . . . . . . . . 10 380 

31. Covered . . . . . . . . . . . . . . . . . . . . . . . . . 35 415 

30. Siltite, medium-gray (weathers greenish gray); in beds 

mostly 5-20 em thick; contains some inconspicuous 

wavy laminae. Weathers to blocky fragments. Interval about 50 percent covered . . . . . . . . . . . 16 431 

29. Covered; hillside exposures suggest interval is mostly 

argillite similar to upper part of unit 28 . . . . 44 475 

28. Argillite and siltite; upper one-third is medium-gray 

well-cleaved argillite containing a few planar siltite 

laminae 1-2 mm thick and a few cross-laminated siltite lenses as much as 1 em thick; middle one-third 

is interlaminated siltite and argillite containing some 

planar to broadly lensing siltite beds several em 

thick; lower one-third is similar to upper one-third. 

Partly covered . . . . . . . . . . . . . . . . . . . 36 511 

27. Mostly covered; hillside float and a few exposures near 

road suggest that upper one-fourth is interlaminated 

siltite and argillite with irregular laminae and small 

scale scour-and-fill features and that lower threefourths is well-cleaved argillite similar to that of unit 

23 .......................... . 103 614 
40 PRICHARD FORMATION, PLAINS, MONTANA 

Burke Formation (lower part only)-Continued 

26. Quartzite, light-gray (weathers light olive gray to 

grayish orange) in beds mostly 0.1-0.2 m thick; some 

beds channel as much as 5 em into bed below; dark 

planar to wavy laminae, small scale scour-and-fill, 

and low-angle trough cross-lamination present in 

upper part of some beds. Upper one-third poorly exposed ........................ . 

25. Covered . . . . . . . . . . . . . . . . . . . . . . . . . 

24. Siltite interbedded with interlaminated siltite and argillite; similar to unit 19; thickness and abundance of 

siltite beds increase upward; uppermost 1 m is 

quartzite. Basal contact gradational through several 

meters ....................... . 

23. Argillite, dark-gray (weathers light brown); contains a 

few planar siltite laminae and one slump fold 0.4 m 

thick. Unit is well cleaved ........... . 

22. Siltite and argillite interlaminated; similar to unit 14 . 

21. Siltite interbedded with interlaminated siltite and argillite; similar to unit 19, but siltite beds decrease upward in thickness and abundance . . . . . . . . . 

20. Quartzite, light-gray (weathers light olive gray to light 

brown) in beds mostly 0.12-0.4 m thick; contains 

faint planar laminae . . . . . . . . . . . . . . . . 

19. Siltite interbedded with interlaminated siltite and argillite. Siltite is medium gray (weathers olive gray, 

moderate brown on some joint and bedding surfaces) 

in beds 0.1-0.2 m thick and sets as much as 2 m 

thick; faintly laminated in part; laminae planar to 

markedly wavy; some siltite fills broad channels; 

load-casts common at base of beds. Interlaminated 

siltite and argillite similar to unit 14; in sets about 

0.5 m thick .................... . 

18. Covered . . . . . . . . . . . . . . . . . . . . . . . . . 

17. Siltite and argillite, interlaminated; similar to unit 14. 

Grades from bed below . . . . . . . . . . . . . . 

16. Quartzite, medium-gray (weathers yellowish gray) in 

slightly irregular beds mostly 0.2-0.4 m thick; loadcasts at base of some beds, and some contain dewatering channels; wavy siltite and argillite laminae 

present between some beds . . . . . . . . . . . . 

15. Covered . . . . . . . . . . . . . . . . . . . . . . . . . 

14. Interlaminated siltite and argillite interbedded with 

siltite; similar to unit 13 but without quartzite interbeds; some siltite beds are as much as 0.5 m thick; 

some siltite as lenses and channel fill . . . . . . 

Thickness 

(meters) 

24 

23 

11 

13 

15 

13 

2 

17 

4 

12 

7 

18 

25 

Cumulative 

thickness 

(meters) 

638 

661 

672 

685 

700 

713 

715 

732 

736 

748 

755 

773 

798 
CONTACT WITH THE BURKE FORMATION 

Burke Formation (lower part only)-Continued 

13. Interlaminated siltite and argillite interbedded with siltite and quartzite. Interlaminated and very thinly interbedded light-gray siltite and dark-gray argillite 

make up most of unit; laminae and beds, from 1 mm 

to several em thick, are wavy, lenticular in part, 

cross-laminated in part; contains some small scale 

scour-and-fill structures. Siltite is light gray (weathers dark gray) in beds about 10 em thick; load-casts 

common at base of beds. Quartzite, mostly near top 

and base of unit, is light gray (weathers light olive 

brown) and in beds mostly 1~20 em thick . . . 

12. Interlaminated siltite and argillite interbedded with 

siltite. Interlaminated siltite and argillite similar to 

that in unit 9 makes up most of unit. Siltite is 

medium light gray (weathers light olive gray to 

greenish gray, moderate gray on some surfaces), in 

beds 5--20 m thick; are alternating light and dark 

wavy laminae from 1 mm to several em thick; contains abundant scour-and-fill structures several em 

deep; many fills are cross-laminated; load-casts present at base of many beds; weathers to blocky float. 

Unit is poorly exposed; described from ledges on on 

hillslope north of Welcome Spring . . . . . . . . 

11. Mostly covered; contains one small outcrop of argillite 

similar to that of unit 10 ............ . 

10. Argillite, greenish-gray; contains some siltite laminae 

several mm thick that exhibit low-angle cross-lamination ......................... . 

9. Siltite and argillite, interlaminated; siltite is light gray 

(weathers light olive gray), argillite is medium dark 

gray; weathers moderate brown to light brown on 

joint surfaces; laminae, mostly 1-2 mm thick, are 

somewhat wavy and graded in part; some laminae 

are lenticular, and some siltite layers fill channels 

as much as several em deep and 10 em wide. 

8. Covered ........................ . 

7. Siltite and argillite, interbedded; siltite is medium light 

gray (weathers greenish gray to brownish gray), 

speckled with iron oxide grains less than 1 mm in 

diameter; in part laminated greenish gray and brown 

(weathered colors); laminae irregular, cross-laminated in part, and exhibit some scour-and-fill; some 

cusp-shaped laminae may be water-escape features; 

load-casts present at base of some beds; in sets several meters thick. Argillite similar to that of unit 

5. Interval about 50 percent covered . . . . . . . 

Thickness 

(meters) 

12 

94 

15 

15 

3 

26 

15 

41 

Cumulative 

thickness 

(meters) 

810 

904 

919 

934 

937 

963 

978 
42 PRICHARD FORMATION, PLAINS, MONTANA 

Prichard Formation (upper part only) 

6. Mostly covered, about 10 percent of this interval consists 

of beds of well-cleaved medium-gray (weathers 

brownish gray) argillite ............. . 

5. Argillite, medium-light-gray (weathers greenish gray to 

moderate brown), massive ............ . 

4. Quartzite, very light gray to yellowish-gray (weathers 

moderate brown) in even beds 2-20 em thick; upper 

0.5-1 em planar-laminated and argillaceous ... 

3. Argillite, light-gray to light-greenish-gray (weathers 

moderate brown to light brown), speckled with randomly oriented biotite flakes 0.5-1 mm across; 

strongly cleaved, cleavage surfaces have distinct 

sheen produced by oriented sericite; contains a few 

planar siltite beds 5-10 em thick that grade upward 

to argillite . . . . . . . . . . .... . 

2. Covered ........................ . 

1. Argillite, medium-light-gray (weathers moderate 

brown); contains some planar siltite laminae as much 

as 2 mm thick and spaced several mm to several em 

apart; some thicker siltite laminae grade upward to 

argillite, some intervals consist of nearly one-half siltite laminae, whereas others contain very few; upper 

part contains some planar beds several em thick of 

light-gray siltite that grade upward to argillite and 

contain irregular laminae in upper part . . . . . 

SILLS 

Thickness 

(meters) 

45 

6 

2 

32 

43 

76 

Cumulative 

thickness 

(meters) 

1023 

1029 

1031 

1063 

1106 

1182 

The Prichard Formation in the study area contains four dioritic to 

gabbroic sills sufficiently thick to map at the scale of 1:62,500. The 

most conspicuous of the four is locally as much as 400 m thick. In 

addition, several sills of local extent were too thin to map except 

as a line. I have not studied the sills, but Calkins (1909, p. 48, 49) 

described tbem as follows: 

These intrusive bodies consist for the most part of a very dark, somewhat greenish, 

hypidiomorphic-granular, medium-coarse to fine-grained rock. The most conspicuous 

mineral, megascopically, is greenish-black hornblende, usually somewhat fibrous, but 

feldspar and quartz are easily recognizable in all but the finest-grained specimens. 

Examined in thin section, this rock is found to be composed chiefly of hornblende, 

plagioclase, and quartz, with variable but generally small amounts of orthoclase and 

biotite. The common accessories are iron ore and apatite. The plagioclase crystals generally show far larger extinction angles at the center than on the periphery, but the 

more basic grades into the more acidic feldspar, so that distinct zones are not recognizable. The species represented range from labradorite to oligoclase. The orthoclase commonly is intergrown with quartz, which in some specimens is very abundant. 
SILLS 43 

More recently, Bishop (1973) made a thorough investigation of the 

sills that intrude the Prichard at the Crossport locality east of Bonners 

Ferry, Idaho. He reported them to consist of continental tholeiite. 

According to Bishop, the sills are concordant with schistosity of the 

host rock where they intrude argillite, and the initial schistosity has 

been recrystallized in the contact zone. He concluded from this that 

the intrusion of the sills was synkinematic with the first recrystallization of the Prichard. One of these sills is the one dated by Zartman, 

Peterman, Obradovich, Gallego, and Bishop (1982) as 1,433 m.y. old. 

In the area studied for this report the lowest sill is that between 

members A and B. It is mostly 160-200 m thick, though it is somewhat 

thinner in the drainage of Seepay Creek. 

The next higher mapped sill ranges from a featheredge to nearly 

400 m in thickness. South of the St. Mary's fault it is mostly within 

member D, but near See pay Creek it is at or near the base of member 

C. North of the St. Mary's fault the sill is mostly in the lower part 

of member E; however, near Henry Creek it appears to cut approximately 600 m down section from the lower part of member E to within 

or below member C. Near the junction of Clear Creek with Camas 

Creek the sill dies out within member E, but two thin sills that appear 

in member D about 1 km to the north may be part of the same body. 

The third mapped sill is in the lower part of member F. It ranges 

in thickness from 75 to 140 m and is present throughout the area. 

North of the St. Mary's fault the sill ranges from 450 to 520 m above 

the base of member F. South of the fault the sill is about 220 m 

above the base where exposed in a roadcut south of the mouth of 

Siegel Creek and 230 m above the base on Siegel Mountain. 

The highest mapped sill is in member H in the Compbest Creek 

drainage. It is 85 m thick and about 900 m above the base of the 

member. This part of the section is faulted out south of the St. Mary's 

fault, and the sill is not present north of Montana Highway 200. 

The ages of the sills within the report area have not been determined, but elsewhere in the Belt basin K-Ar ages indicate two periods 

of intrusion-the first at about 1,300 m.y. and the second at 750-850 

m.y. (Harrison and others, 1974, p. 3, 5). Bishop (1973, p. 61) calculated a Rb-Sr isochron of 1,320 m.y. for one of the Crossport sills 

indicating that they belong to the first group. The U-Pb age of 1,433 

m.y. for a sill from the same area (Zartman and others, 1982) is probably a more accurate date for the older group of sills. Sills from the 

younger group have been reported in the Prichard Formation about 

18 km northeast of the Plains-Perma area where K-Ar ages of 803 ± 8 

m.y. and 845±20 m.y. were obtained on hornblende (J. D. Obradovich, unpub. data, cited by J. E. Harrison, written commun., 1981). 
44 PRICHARD FORMATION, PLAINS, MONTANA 

Mudge (1968) concluded that most concordant igneous masses in the 

Western United States that had intruded flat-lying sedimentary rocks 

had intruded under a cover of at least 3,000 ft (900 m) but less than 

7,500 ft (2,300 m). If Mudge's conclusions can be applied to the Belt, 

all of the sills except that in member H must have been intruded 

before the end of Prichard sedimentation, and even the sill in member 

H must have been intruded before the end of Burke time. I therefore 

suspect that the sills in the report area are of the older group. 

BRECCIA 

An unusual feature of the Prichard Formation in the study area 

is breccia that occurs (1) as tabular bodies disposed at a low angle 

to bedding and (2) along high-angle faults. These two types will be 

described in order. 

The most spectacular breccia of the first type is the westernmost 

one shown in figure 23. The breccia body consists of at least two 

rock subtypes. The most abundant is a high-matrix argillite-clast breccia in which the fragments constitute about 30 percent of the rock. 

The fragments are mostly angular and lath shaped, have an average 

length of about 2 mm, and have a maximum length of 15 mm. A 

few clasts are oval and rounded; most are of argillite, but a few are 

of siltite or quartzite. The matrix is very poorly sorted argillic siltite. 

In thin section the borders of most fragments are indistinct and are 

crossed by biotite and chlorite porphyroblasts. Much of the rock is 

well cleaved. Figure 24 is a photomicrograph of a fairly typical specimen. 

The second subtype is dark- to medium-gray very poorly sorted, 

hard, massive argillic siltite that contains disseminated pyrite grains 

and a few indistinct argillite and siltite clasts. The pyrite grains are 

equant to tabular in shape and average 1-2 mm in diameter, though 

some are at least 10 mm across. Megascopically the pyrite grains appear to be clasts, but in the few thin sections I have examined, it 

is not clear whether the pyrite actually occurs as clasts and parts 

of clasts or whether it has replaced clasts or matrix. 

I have not been able to determine whether or not there is any 

regularity to the occurrence of these two subtypes in the breccia 

bodies. The two are very similar to the two types interbedded in 

the conglomerate zone in member F (p. 20). 

The easternmost breccia of the area of figure 25, which is the largest 

outcrop of breccia in the report area, is exposed mostly on the steep 

valley wall on the south side of the Clark Fork. Much of its outcrop 

is inaccessible, but contacts have been located in enough places so 

that the general map relations are clear, though details are not. As 

shown by figure 23, the breccia rises stratigraphically southward from 
BRECCIA 45 

just below the base of member C to near the top of member E, a 

thickness of 1,200 m in a distance of 8 km. The southern end terminates against the St. Mary's fault. Assuming that the breccia body 

dips in the same directions as the enclosing rocks, it is as much as 

600 m thick, though it may have been thickened by undetected faults. 

The breccia truncates several folds and faults in the overlying beds 

and is itself cut by a diorite sill that has contact-metamorphosed the 

adjacent breccia. 

The northern part of the breccia is bordered on the west by a 

syncline consisting mostly of rocks of member E. Another breccia 

body of the well-cleaved argillite-clast type crops out on the west 

flank of the syncline. The attitude of this breccia is not obvious, but 

the map pattern suggests that it extends beneath the syncline to 

merge with the larger breccia mass. The smaller breccia body seems 

to thin southward and pass into a fault that terminates against the 

thicker breccia. 

The structure is illustrated by the section of figure 25A. The section 

is interpretive and is based largely on northwestward projection of 

the surface structures. 

What can be said about the origin of these breccias? They certainly 

formed before regional metamorphism and before intrusion of the sills, 

but though they formed early, at least some of the clays were sufficiently indurated to break into angular fragments. As interpreted in 

figure 25A the larger mass cuts across bedding at an angle of only 

a few degrees where it is low in the section but the breccia mass 

cuts bedding by as much as 30° or 40° where it is high in the section, 

so it is probably listric. At its south end the main breccia is in the 

upper part of member E, which exhibits many features indicative of 

deposition in shallow water; whereas, near the northern end the breccia is in argillite of member D and the parallel-laminated quartzite 

of member C, both of which represent deposition in deeper water. 

Inasmuch as the breccia cuts across section, it could not have been 

deposited as a surface slump or mudflow, nor could it have formed 

by any other sedimentary process. Stratigraphic displacement across 

the breccia masses is small and this, together with the low angle 

at which the main breccia body crosses bedding, is strong evidence 

that tectonic faulting was not the cause. 

The most reasonable explanation for the origin of the breccias is 

that they formed at the base of and within a major shelf-edge slump. 

Watkins and Kraft (1978, p. 271-273) noted from seismic reflection 

profiles that on the Louisiana and Texas Continental shelf and slope 

that both growth faults and the base of shelf-edge slumps are concave 

upward, and both pass downward into acoustically amorphous horizons 

that might be analogous to the breccias of the Prichard Formation. 
46 PRICHARD FORMATION, PLAINS, MONTANA 

T. 

20 

N. 

T. 

19 

N. 

7krP~~~~ ~~r~t~~~~f~~~~~=~+~ n. ~: J~~~;~ 

Base for U.S Geological Stmey 

Plains 162.300, 1959 

R. 26 W. R. 25. W. 

2 MILES 

2 KILOMETERS 

FIGURE 23.-Geologic map of part of the Plains quadrangle. Contour interval400 ft. 
BRECCIA 

EXPLANATION 

Holocene { ~:~. ~~(::" >•1 Alluvium 

Pleistocene { [~k~~~}J 

Burke Formation 

L3E] 

Glaciofluvial 

deposits 

Breccia 

QUATERNARY 

MIDDLE 

PROTEROZOIC 

Dioritic to gabbroic 

sills 

Prichard Formation 

Members H through B; s, top of 

quartzite, siltite, and argillite unit 

in member H; q, top of quartzite 

unit in member B 

CONTACT- Dashed where approximately located, short dashed where 

inferred, dotted where concealed 

FAULT - Dashed where approximately located, short dashed where inferred, 

dotted where concealed 

--f--- ANTICLINE- Showing crestline. Dashed where approximately located. 

----lf--- SYNCLINE- Showing troughline. Dashed where approximately located 

25 ___J_ STRIKE AND DIP OF BED 

47 

However, the slump in the Prichard Formation involves at least 1,200 

m of section as compared to 300-600 m for shelf-edge slumps off the 

Gulf Coast (Watkins and Kraft, 1978, p. 273) and a maximum of about 

720 m for shelf-edge slumps off the coast of southern Africa (Dingle, 

1980, p. 338, 340), so perhaps some very unusual event was required 

to initiate sliding. The map pattern, though not entirely unambiguous, 

suggests that slumping was toward the east-northeast. This is consistent with the generally eastward slope indicated by the slump folds 
48 PRICHARD FORMATION, PLAINS, MONTANA 

FIGURE 24.-Photomicrograph of breccia showing argillite fragments in poorly sorted 

argillic siltite matrix. Dark grains are biotite. Polarized light. 

in member B. A possible section of the slump, reconstructed in part 

from the structure section of figure 25A, is shown in figure 25B. 

A very similar breccia is exposed on Three Lakes Peak, on the 

left side of the West Fork of Seepay Creek, and between Wilson 

and Robertson Creeks. I have not mapped most of this area, but 

I suspect that the three exposures are all part of the same breccia 

sheet. All are in the lower part of member E but are probably at 

somewhat different horizons. 

Another breccia was observed at two localities between Quinns Hot 

Springs and Wilson Creek. At both places it separates member B 

from member D, member C being absent. Its fragments are more 

rounded than in other type 1 hreccias and the rock is not cleaved. 

I am aware of only two reports in the literature of breccias that 

resemble those described for this report; one in the Tertiary of Peru 

and the other in the Precambrian of the Northern Territory of Australia. The occurrences in Peru were described by Baldry (1938) as 

low-angle normal faults inclined 7°-10° to the bedding. The faults (referred to by Baldry as slip planes) are accompanied by breccia zones 

from several inches to several hundred feet thick. According to Baldry 

(1938, p. 357), "The slips were probably due to gravity and took place 
BRECCIA 49 

A 

A 

FEET A' 

400D METERS 

3000 

1000 

800 

200D 6DD 

1 ODD 40D 

SEA LEVEL 2DD 

Ypb SEA LEVEL 

-1DDD -20D 

-2DDD 

D.5 1 KILOMETER -4DD 

1f2 1 MILE 

B 

Ype 

Ype 

Ypd 

FIGURE 25.-Structure sections showing present and original configurations of breccia 

illustrated in figure 23. See figure 23 for location of section and for explanation 

of symbols. A, present configuration; arrows along faults show relative movement. 

B, inferred configuration after formation of breccia but before tectonic faulting 

and folding; scale same as in A. 

after several thousand feet of strata had accumulated but before they 

had been distorted and displaced by pormal faults." These occurrences 

sound similar to those described in this report, but the breccias themselves consist of masses of rock, boulders, and highly polished pebbles 

from many formations (Baldry 1938, p. 350) and are very different 

from the breccias in the Prichard. Knights (1956) stated that Baldry's 

hypothesis has not generally been accepted by other geologists working in the area. 

Breccia zones in a Precambrian turbidite sequence in the Northern 

Territory of Australia were described by MeN eil (1963, 1966) as occurring within a slip complex, which he defined as consisting of large 

masses of distorted sediment that were rafted on an infra-formational 

collapse zone. MeN eil (1963, p .. M3) described one of these "slip complexes" as follows: 
50 PRICHARD FORMATION, PLAINS, MONTANA 

[It] consists of a series of sub parallel breccia and quickstone lenses which represent 

major movement on slip surfaces, separated by disturbed slates and greywackes. 

* * * The disturbed rocks are overlain by normal greywackes and shales, but the 

underlying rocks are not well exposed, and the lower boundary of the complex is inferred. * * * Three main breccia horizons are present, all sub parallel to the regional 

dip. The central breccia zone is 3,000 feet long and reaches a maximum width of 150 

feet. * * * The boundary between the breccia and surrounding disturbed sediments 

is usually sharp, but in a few places it tends to be gradational.* * * The breccia varies 

in grain size from fine even grained, (fragments less than 1

/2 an inch in diameter), 

to coarse-grained with blocks of sediment 12 to 15 feet long separated by a matrix 

of fine-grained breccia. In appearance the breccia varies from a typical fault gouge 

to a soft sediment breccia with streaky banding and bent and wispy fragments. Several 

thixotropically mobilized or liquidized rock masses, quickstone and quickstone breccia 

lenses occur in the northeast part of the slip complex. * * * The quickstones are essentially fine-grained rocks which contain varying amounts of shale and numbers of quartz 

collocrysts. The amount of incorporated shale varies considerably and depends on the 

degree to which the sediment was mobilized. Granular quickstone is quickstone that 

has been completely liquidized and contains very little shale. Slaty quickstone is quickstone that has been partly liquidized and consists almost entirely of shale, each shale 

fragment being separated by a thin film of mobilized sediment. Both types can occur 

together. The granular matrix consists of silt size particles of sericite, quartz and chlorite. 

From this description one can see similarities between the Australian breccias and those described in this report. However, neither 

the map nor other illustrations presented by MeN eil are sufficiently 

detailed to know whether or not his "slip complexes" and the breccias 

of the Prichard Formation are similar phenomena. 

The second type of breccia occurs along high-angle faults. One occurrence is on the divide north of lower Siegel Creek where the uppermost beds of member E are faulted against the lower part of member 

F. The breccia there is hard, massive, and dark gray. The inconspicuous clasts average several millimeters in diameter and are mostly argillite. Microscopically, the clast boundaries are indistinct, and biotite 

and chlorite porphyroblasts cross clast boundaries. The matrix is 

poorly sorted argillic siltite. It is possible that the breccia is bedded 

in member F and that its position adjacent to the fault is fortuitous. 

Breccia also occurs at several localities along a high-angle fault near 

Compbest Creek that has displaced the lower part of member H and 

the upper part of member G against members E and F (fig. 16). 

At the southernmost locality, 1. 6 km west of Patricks Knob, the breccia is uncleaved and massive. The fault here is between argillite of 

member F and argillite of member H. The argillite fragments constitute about 20 percent of the rock and range from lath shaped and 

angular to oval and rounded. The fragments are similar in size to 

those of the other breccias. Farther north, in section 22, where member H is faulted against member E, the breccia is highly cleaved and 

is very similar to the thicker breccia masses to the east. The breccia 
INTERPRETATION 51 

band is at least 20 m wide and is without question directly along 

the fault. A quartz vein is present along the fault at its northernmost 

exposure and is apparently the vein on which the Letterman mine 

was located. Zartman and Stacey (1971) reported a Mesozoic or Cenozoic age for lead from the mine. Two interpretations of the relations 

along this fault are possible. The first is that the fault may have 

formed before regional metamorphism and may have had recurrent 

movement. The second is that the southern breccia may be bedded 

in unit F and the fault is actually to the west of the l1reccia; the 

breccia in section 22 may be part of a breccia sheet within member 

E that has been emplaced against member H by later faulting. 

INTERPRETATION 

SEDIMENTARY STRUCTURES AND 

DEPOSITIONAL ENVIRONMENTS 

In rocks of Phanerozoic age, fossils and fossil traces are by far 

the most useful indicators of depositional environments. The stratigrapher dealing with rocks of Proterozoic age, such as those of the 

Prichard Formation that were deposited before evolution of the metazoans, is more severely handicapped than his colleagues studying 

younger rocks, and he must rely nearly exclusively on sedimentary 

structures to decipher the conditions of deposition. Unfortunately, 

most sedimentary structures are indicative of process rather than specific environment, and only a small number of environments, all either 

continental or in very shallow water, can be determined unequivocally 

by sedimentary structures alone. 

Some of the problems in interpretation are illustrated by the graded 

siltite-argillite couplets-the most common sedimentary structure in 

the Prichard Formation. These couplets closely resemble similar features that in the Gulf Coast region alone have been described and 

pictured as from environments as disparate as interdistributary bars 

(Roberts and others, 1976) and the abyssal plain (Bouma, 1972, fig. 

2-16). In terms of process the Gulf Coast couplets have been ascribed 

to low-density turbidity currents (Bouma, 1972) and to seasonal variations in the sedimentary load of the Mississippi River (Huang and 

Goodell, 1970, p. 2093). 

In the Prichard Formation, only member E contains sedimentary 

structures sufficiently diagnostic to permit a reasonably certain interpretation of the depositional environment. In the upper part of the 

member the mud-chip breccias are reminiscent of the mud flakes described by Evans (1965, p. 221) as occurring on the intertidal mudflats 

of the Wash of eastern England where the mud flakes are produced 
52 PRICHARD FORMATION, PLAINS, MONTANA 

by tidal waters stripping the surface laminae. The lamination in this 

part of the section is itself very similar in character to that of the 

intertidal zone of the delta of the Colorado River (Thompson, 1968, 

pl. 9). Mud cracks and shallow scours are also common on the mudflats 

of the Wash, and both are also present in the upper part of member 

E. Features similar to mud cracks can result from subaqueous 

syneresis of clayey sediments (White, 1951; Burst, 1965; Donovan and 

Foster, 1972), but neither the experimental syneresis cracks nor the 

well-documented examples of syneresis from the older rocks figured 

in these references form as regular and complete polygons as those 

seen in member E. Furthermore, in the Prichard Formation, the mudcrack polygons occur only in the upper part of member E where they 

are associated with other features indicative of extremely shallow 

water. The quartzite, consisting of wedge-shaped sets of small scale 

crossbeds (fig. 13C), resembles in structure some modern beach sands 

(Reineck and Singh, 1973, p. 316), and both the flaser-bedded quartzite and those with herringbone crossbedding were most likely deposited by tidal currents. The discontinuous quartzites in the lower part 

of member E were probably offshore bars and sand waves; they are 

certainly not channel fills, and the crossbedding indicates deposition 

from traction currents. Scour-and- fill structures and discontinuous 

bedding in the siltites and argillites in the lower part of the member 

suggest deposition under the influence of fluctuating currents. Inasmuch as no mud cracks or mud-chip breccias are present in this part 

of the section, the most likely environment would have been a shallow 

shelf under the influence of tides and storms. Recent analogs do not 

exist because of pervasive bioturbation in present-day shallow water 

environments. The entire assemblage of structures in member E 

suggests deposition on a prograding shelf with at least some of the 

uppermost beds having been deposited in the intertidal zone. The 

dominance of argillite and siltite, the pervasive lamination, and the 

apparently lenticular shape of the quartzites all suggest that the shelf 

was tide dominated rather than wave dominated. 

The slump folds so conspicuous in member B obviously resulted 

from failure of the sediments on a slope. Sediment failure will occur 

when stresses on the sediment exceed the strength, so that failure 

can result from either increasing the stress, decreasing the strength, 

or both. According to Coleman, Prior, and Garrison (1978), the stresses on the sediment are mostly of two types-gravitational and cyclic 

loading by waves; gravitational stresses may be increased by increasing the slope by sedimentation or by tectonic tilting. Sediment 

strength may be reduced by increasing pore-water pressure through 

rapid loading by sediments or by big waves, by methane production, 

and by disruption of the fabric by seismic shaking. 
INTERPRETATION 53 

Contemporary soft-sediment deformation has been investigated intensively in the past decade because of the importance of sediment 

instability to the construction of offshore drilling and production platforms and submarine pipelines. Summaries of results have been presented by Coleman and Garrison (1977) and Watkins and Kraft (1978). 

These studies have documented a wide variety of sediment failures 

ranging from mudflows, in which all coherence has been lost, to rotational slumps and sedimentary plates translated downslope with little 

internal deformation (Twitchell and others, 1979). Sediment instability 

occurs in environments ranging from upper delta fronts at water 

depths of less than 5 m (Coleman and others, 1978, p. 1067) to the 

continental rise at water depths of more than 3,000 m (Twitchell and 

others, 1979). Slopes may be as low as 0.2° (Coleman and Garrison, 

1977, p. 22) and as much as 10° (Twitchell and others, 1979). Inasmuch 

as sediment failures can occur over such a wide range of environments, 

the following attempt to interpret the slump folds of the Prichard 

Formation in terms of a specific environment is obviously speculative. 

In member B, slump folds were overwhelmingly the dominant type 

of mass movement. I have seen no beds in this part of the section 

that could represent mudflows, collapse depressions, bottleneck slides, 

or mud diapirs, all common off the Mississippi delta (Coleman and 

Garrison, 1979). This suggests that the sediment was more stable than 

that of the Mississippi area, perhaps because of slower deposition or 

because of the paucity of organic matter that could have been converted to methane. Instability, then, would have occurred only where 

the applied stresses were severe. The most likely locale would have 

been on the upper continental slope where progradation could have 

resulted in oversteepening. This is the case in the present Gulf of 

Lions where Got and others (1979) noted that slumps are most common at the break between shelf and slope. 

Terzaghi (1962, p. 1430), in a discussion of a report by Mathews 

and Shephard (1962), wrote the following with respect to slumps on 

the Delta of the Fraser River in British Columbia. 

The top layer of the sediments is in a semi-liquid condition and starts to slump 

as soon as the inclination of the surface exceeds about 2o. That is the "critical slope 

angle." A slide reduces the slope angle to a much smaller value. Therefore, during 

the decades or centuries following the slide, the hummocky area becomes buried beneath 

fresh sediments which afface the slide topography and the slope angle slowly increases 

until the critical slope angle is again reached. In the meantime slumping occurs in 

other areas. According to this concept the slumping is a rather rare event and takes 

place in different areas at different times. The close spacing between the hummocks 

observed by the authors [Mathews and Shepard] indicates that the surfaces of sliding 

are located at a very shallow depth. Within this depth the stratification is obliterated, 

and the disturbed layers will assume the character of relatively thin lenses of distrubed 

clay scattered throughout a mass of stratified sediment. 
54 PRICHARD FORMATION, PLAINS, MONTANA 

I suspect that the slump folds and sheets of member B formed 

in a similar manner except that the strata maintained their coherence 

during movement. 

If member B was deposited in the upper slope, then member A, 

similar to member B but without the soft-sediment deformation, was 

deposited on the lower slope. 

The quartzites in the middle of member B and in members C and 

G all exhibit sharp bases, some of which channel the underlying bed; 

and the quartzites have tops that grade to argillite. By far the most 

common sedimentary structure is planar lamination. The absence of 

large-scale crossbedding and the paucity of even cross-lamination 

suggests that the quartz sand could not have been transported by 

traction currents, and though they exhibit few sole marks and though 

Bouma sequences are not obvious, they were probably deposited from 

turbidity currents. I have not observed features in the quartzites that 

indicate environment rather than process, but both member C and 

the quartzite unit in member B are just above zones of prominent 

slump folds. Perhaps these two quartzites were deposited in lows on 

the upper slope that had been formed by slumping. 

SEDIMENTATION AND STRUCTURE 

The Plains and Perma quadrangle areas are on the north edge of 

the Lewis and Clark line, which has had a long and complex structural 

history, probably beginning as early as Belt time (p. 5). If the faults 

that make up the Lewis and Clark line were active during deposition 

of the Prichard Formation, they could have provided conduits for 

mineralizing fluids, so the timing of the faulting is of considerable 

practical importance. 

The St. Marys fault that crosses the Plains and Perma quadrangle 

areas (fig. 1) is the northernmost major fault of the Lewis and Clark 

line. The Prichard Formation does differ in some details across the 

St. Marys. Several sills are at different horizons across the fault (p. 

43); the quartzite marker of member B is thicker on the north side 

of the fault than on the south, and member F may be thicker north 

of the fault. Also, the large breccia mass illustrated in figure 23 terminates against the fault. However, Harrison, Griggs, and Wells (1974, 

p. 12) have estimated an apparent right lateral movement of 13 km 

along the St. Marys, and the slight differences in the Prichard here 

are most simply explained by juxtaposition of slightly different sections by postdepositional faulting. The slump folds in member B, which 

might be expected to show some relations to contemporaneous faults, 

are as common north of the St. Marys fault as south of it, and the 
INTERPRETATION 55 

original orientation of the slumps is the same to the north and south 

(p. 14). 

In summary, I have found no evidence within the report area that 

the St. Marys fault was active during deposition of the Prichard Formation. Possibly, a larger area of exposure would be necessary to 

detect any effects of faulting on the sediments. This discussion has 

no bearing on whether or not other faults within the Lewis and Clark 

line were active in Prichard time. 

SOURCE 

The Prichard Formation is an enormous accumulation of fine-grained 

quartz and clay. At the present time, 18 x 109 metric tons of suspended 

solids are carried annually to the world oceans, more than one-third 

of the total being contributed by about a dozen major rivers (Holeman, 

1968). According to Drake (1976, p. 135-136), less than 10 percent 

of this material reaches the deep sea; about 50 percent accumulates 

in estuaries and coastal wetlands, and the remaining 40 percent is 

deposited off about two dozen major river mouths. The Prichard Formation of the Plains area is certainly not an estuarine deposit, and 

only the upper part of member E could have been deposited, in coastal 

wetlands, so we can reasonably conclude that the Prichard was deposited as part of a delta complex off the mouth of a major river system. 

The slump-fold measurements of member B indicate that the local 

deltaic lobe prograded eastward and that the river system supplying 

the detritus was to the west. Considering the complex shape of deltas, 

it is not difficult to imagine situations in which the supply could have 

been from the southwest or south, but any other direction for supply 

to the Plains area seems unlikely. No sediment transport directions 

were obtained from members F, G, or H, so these conclusions as 

to the direction of the source apply only to the lower part of the 

section. 

TECTONIC ENVIRONMENT 

The Prichard Formation, as exposed in the Plains and Perma quadrangle areas, is about 6,000 m thick exclusive of the sills. Though 

remarkably uniform in composition and texture from base to top, the 

formation seems to comprise two sequences of basin fill. The first 

is from the base of the section to the top of the shallow water member 

E and is about 2, 700 m thick, again exclusive of the sills; the upper 

sequence includes those beds from the top of member E to the base 
56 PRICHARD FORMATION, PLAINS, MONTANA 

of the shallow-water Burke Formation; this interval is about 3,300 

m thick. What conditions could have permitted the accumulation of 

such a thick pile of sediments? 

Sweeny (1977, p. 44) wrote, "A necessary condition for the evolution 

of a sedimentation basin is the presence of a related depression at 

the earth's surface. Sedimentary basins are developed by the filling 

of depressions with sediment and by response of the earth to the 

resulting sediment load." The depth of the original water-filled depression required to accumulate a given thickness of sediment depends 

on what assumptions are made as to the nature of the crust. Assuming 

an Airy-type crust, a sediment density of 2.4-2.5 g/cm3 , and a mantle 

density of 3.3-3.4 g/cm3 , an original water-filled depression 900-1,200 

m deep would have been required to accumulate the 2, 700 m of sediment that make up the lower sequence of the Prichard. If, instead, 

the elastic beam theory is applied, using an amplification factor of 

3.4-4.1 (Sweeney, 1977, p. 45), an original depth of 660-820 m would 

have been required. Inasmuch as the base of the Prichard Formation 

is not exposed, both the thickness of the depositional sequence and 

the calculated water depths are minimums. How could a basin of the 

requisite depth have formed? 

Several mechanisms have been proposed to account for the formation of basins in the interior of continental plates. As reviewed by 

McKenzie (1978), these include (1) phase change in the Moho as a 

result of sedimentary loading, (2) thermal contraction following the 

rise of a mantle diapir, and (3) McKenzie's own proposal that involves 

crustal stretching and thinning, upwelling of asthenospheric material, 

and subsequent cooling and subsidence. McKenzie pointed out difficulties inherent in the first two hypotheses. According to his own 

hypothesis, the requisite depth of water for the lower sequence of 

the Prichard to accumulate could be attained by stretching the crust 

by a factor of between 1.25 and 1.5 (McKenzie, 1978, fig. 4). Stretching 

of this amount seems feasible. However, the crust would have to 

stretch by a factor of 3 to account for the entire Prichard. 

·Bott (1976) has taken a slightly different approach. He cited evidence that the continental crust is divided into a brittle upper layer 

10-20 km thick and a ductile lower layer. If crustal stretching takes 

place, the ductile layer responds by thinning and lateral flow while 

the upper crust responds by normal faulting and wedge subsidence. 

Bott calculated that under these conditions a graben with a width 

of 30-60 km would form. Computation of the energy budget led him 

to conclude that more than 5 km of sediment could accumulate provided that water pressure would reduce friction on the bounding 

faults. This predicted maximum thickness is inadequate to account 
INTERPRETATION 57 

for all of the Prichard Formation, and it fails to account for the total 

Belt Supergroup by a factor of 4. 

None of these mechanisms seem adequate to account for the 

Prichard Formation, so it seems very unlikely that the Prichard could 

have accumulated on a plate interior. A more likely hypothesis is that 

the Belt Supergroup was deposited on thinned continental crust on 

a passive continental margin, as proposed by Harrison and Reynolds 

(1976). Price (1964, p. 422) and Gabrielse (1972, p. 525) have both 

proposed that the Purcell Supergroup accumulated as a continentalterrace wedge, basing their conclusions largely on thickness and facies 

trends, and Sears and Price (1978) have suggested that the Belt basin 

was formed by rifting of a combined Siberian-American plate about 

1,500 m.y. ago. Williams (1980) suggested from paleomagnetic data 

that the plate just west of North America in mid-Proterozoic time 

was Africa; the Siberian platform lay farther north. 

The general model for rifting and subsequent subsidence of the continental margin is doming caused by rising hot mantle material, thinning of the crust by erosion on the crest of the dome, rifting, subsidence of the new continental margin due to thermal contraction, and 

amplification of subsidence by sedimentary loading. Gravitational 

creep of the lower ductile crust and erosion of the lower crust by 

the mantle may also enhance subsidence by thinning the crust. Bott 

(1979) gave four main stages in the subsidence history of passive continental margins. These are (1) rift-valley stage, (2) a youthful stage, 

lasting about 50 m. y. after the onset of spreading, characterized by 

rapid regional subsidence of the outer shelf and slope, (3) a mature 

stage marked by more subdued regional subsidence (this is the present 

stage of the Atlantic and Indian Ocean margins), and (4) a fracture 

stage when subduction starts and thereby terminates the history of 

the passive margin. The Prichard Formation does not fit easily into 

these stages. The sills of continental tholeiitic suggest the first stage, 

but the great thickness of sediments suggests stage two. The rifting 

model was developed mostly from the post-Permian rifting of 

Gondwanaland. Plate tectonic regimes were probably in effect by 

Early Proterozoic time (Choubert and Faure-Muret, 1980, table 1), 

but the bulk Earth heat production was considerably greater in the 

Early and Middle Proterozoic than during the breakup of Gondwanaland (Goodwin, 1981, p. 57), and the crust might have responded somewhat differently. 

As previously discussed, the vast amount of terrigenous fines that 

constitute the Prichard Formation imply derivation from a major river 

drainage, and slump directions demonstrate that the sediments prograded eastward. Yet strontium isotope ratios indicate that the edge 
58 PRICHARD FORMATION, PLAINS, MONTANA 

of the older Precambrian basement is only about 150 km west of the 

study area (Armstrong and others, 1977, p. 406-408). Furthermore, 

Muehlberger's reconstruction of North America at an age of 1, 700 

m.y. ago indicates no continental area of the requisite size to either 

the west or south (Muehlberger, 1980, fig. 15.2). Where, then, is the 

westerly to southerly source for the Prichard? If rifting began about 

1,500 m.y. ago as suggested by Sears and Price (1978), the Siberian 

(or African) platform might have been close enough to supply sediment. 

DEPOSITIONAL HISTORY 

Subsidence as a result of rifting and crustal stretching formed a 

basin at least 600 m and probably more than 1,200 m deep. The shape 

of the basin is not known; and, as pointed out by Harrison and others 

(1980), the discovery of thrust faults of considerable tectonic transport 

north of the Lewis and Clarke line renders previous isopach, 

lithofacies, and paleogeographic maps obsolete. 

In the Plains and Perma area, the basin began to be filled by sediments of a major delta complex that at least in this area prograded 

eastward. The oldest rocks, those of member A, were probably deposited on the lower delta slope. The siltite-argillite couplets may have 

settled from suspension and may have resulted from seasonal or longer 

term variations in the sediment supply, or they may have been deposited by low-density turbidity currents. Light-colored cross-laminated 

silt layers were deposited or reworked by traction currents; at the 

water depth inferred for member A, contour currents seem the most 

plausible mechanism. 

Member B was deposited on the upper slope where more rapid 

sedimentation oversteepened the slope and led to instability and 

slumping. 

Member C is somewhat enigmatic. I have previously suggested that 

the quartzites were deposited by turbidity currents in slump-formed 

valleys on the upper continental slope, but how was the sand delivered 

to the shelf edge? Perhaps at this time the distributaries discharged 

directly at the shelf break so that bedload sand would have been immediately available. If so, subsidence must have directly followed deposition of member C, for member D seems to have been deposited 

on a submerged shelf. 

The coarsening-upward sequence of member D suggests progradation of a delta lobe across the shelf. The sequence begins with bottomset beds and proceeds upward through beds deposited on prodelta 

slopes and terminates with shallow-water quartzites and siltites of 

the overlying member E. 
INTERPRETATION 59 

Member E, deposited in intertidal and shallow subtidal environments, represents the outer part of the subaerial delta and the adjacent submarine delta platform. With the lack of obvious distributary 

sands and the passage from mudflat to shallow marine muds, the unit 

is reminiscent of the outer delta plain of the Orinoro, . described by 

vanAndel (1967, p. 302) as follows: 

Seaward the delta merges imperceptibly into extensive tidal mud flats, generally 

without beach development. * * * [The] only evidence for fluviatile sedimentation is 

found in the wide estuaries themselves; elsewhere, the growth of the delta appears 

to take place predominantly by mud-flat accretion with or without intervals of chenier 

development. It is obvious * * * that the main body of the (subaerial) delta is predominantly composed of fine-grained sediment, and that a very large part of it is strongly 

influenced by marine depositional processes. 

The initial progradation of the delta complex was terminated upon 

deposition of member E; subsidence followed. The succeeding 3,500 

m of rocks that constitute the upper part of Prichard Formation contain little evidence of current activity and no unequivocal evidence 

of shallow water. The contact at the top of member E is sharp, which 

suggests that subsidence was rapid. The breccias interpreted as resulting from shelf-edge slumping may have formed at this time. I 

have seen no features in the rock by which the depth of subsidence 

would be determined, but inasmuch as the laminated siltite and argillite continue 3,500 m up section interrupted only by the turbidite 

quartzites of member G, I suspect that the sea floor must have been 

hundreds of meters deep during the early stages of deposition. 

Deposition of member F began the second episode of basin filling. 

Members F and H, deposited during this second episode, are both 

considerably more argillic and less silty than are· the underlying rocks 

deposited during the first episode of basin fill. The pyrite laminae 

formed because organic matter was deposited in sufficient quantity 

to permit sulfate-reducing bacteria to thrive. The organic matter itself 

was presumably of planktonic origin. The argillite-pebble beds in the 

lower part of member F may have been mudflows or they may be 

the soles of slides. 

Mud deposition was interrupted by the emplacement of turbidite 

sands and silts that are now the characterizing quartzite and siltite 

of member G. Subsequently, deposition of terrigenous fines continued 

to fill the basin. 

Similarities in lithologic character, sequence, and thickness suggest 

that members F and G are approximately equivalent to the middle 

member of the Aldridge Formation. The middle Aldridge consists of 

quartz-rich graywacke interbedded with dark carbonaceous argillite 

(Thompson and Panteleyev, 1976); the Sullivan deposit (fig. 1) occurs 
60 PRICHARD FORMATION, PLAINS, MONTANA 

near the base of the member. The graywackes of the middle member 

of the Aldridge are turbidites derived from a southern source (Bishop 

and others, 1970). The middle member contains 14 interbedded marker 

beds that consist of finely laminated argillite and are of regional extent 

(Thompson and Panteleyev, 1976). The regional extent of the marker 

beds and their character are evidence that the sea floor in that area 

was deep and flat. I have not identified any of these laminite markers 

in the Plains and Perma areas (though other workers may have found 

at least one). The absence of all or most of the markers in the Plains 

area and the northward transportation directions in the middle member of the Aldridge of British Columbia combine to suggest that the 

Plains area was upslope from the basin floor on which the middle 

member of the Aldridge was deposited. 

Finally, shallow-water sands and silts of the basal part of the Burke 

Formation prograded across the area, completing the second episode 

of basin filling and marking the end of the deposition of the Prichard 

Formation. 

REFERENCES CITED 

Armstrong, R. L., Taubeneck, W. H., and Hales, P. 0., 1977, Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, 

Washington, and Idaho: Geological Society of America Bulletin, v. 88, no. 3, p. 

397--411. 

Baldry, R. A., 1938, Slip-planes and breccia zones in the Tertiary rocks of Peru: Geological Society of London Quarterly Journal, v. 94, p. 347--358. 

Billingsley, P. R., and Locke, Augustus, 1939, Structure of ore deposits in the continental framework: New York, American Institute of Mining and Metallurgical Engineers, 51 p. 

Bishop, D. T., 1973, Petrology and geochemistry of the Purcell sills in Boundary 

County, Idaho, in Belt Symposium, volume 2: Idaho Bureau of Mines and Geology 

Special Publication, p. 15--66. 

Bishop, D. T., Morris, H. G., and Edmunds, F. R., 1970, Turbidites and depositional 

features in the lower Belt-Purcell Supergroup [abs.]: Geological Society of America 

Abstracts with Programs, v. 2, no. 7, p. 497. 

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', U.S. GOVERNMENT PRINTING OFFICE: 1985-576-049/20,015 REGION NO.8