Precambrian and 

Lower Ordovician Rocks in 

East-Central Idaho 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 889 

Precambrian and 

Lower Ordovician Rocks in 

East-Central Idaho 

Precambrian Y Sedimentary Rocks in East-Central Idaho 

By EDWARD T. RUPPEL 

Precambrian Z and Lower Ordovician Rocks in East-Central Idaho 

By EDWARD T. RUPPEL, REUBEN JAMES ROSS, JR., and DAVID SCHLEICHER 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 889 

Chapters A and B 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:l975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Main entry under title: 

Precambrian and Lower Ordovician rocks in east-central Idaho. 

(Geological Survey Professional Paper 889) 

Includes bibliographies. 

CONTENTS: Ruppel, E. T. Precambrian Y sedimentary rocks in east-central ldaho.-Ruppel, E. T., Ross, R. J., Jr., and 

Schleicher, D. Precambrian Z and Lower Ordovician rocks in east-central Idaho. 

Supt. of Docs. no.: I 19.16:889 

1. Geology, Stratigraphic-Pre-Cambrian. 2. Geology, Stratigraphic-Ordovician. 3. Geology-Idaho. 

I. Ruppel, Edward Thompson, 1925- Precambrian Y sedimentary rocks in east-central Idaho. 1975. II. Ruppel, Edward 

Thompson, 1925- Precambrian Z and Lower Ordovician rocks in east-central Idaho. 1975. III. Series: United States 

Geological Survey Professional Paper 889. 

QE653.P73 551.7'15 74-30002 

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

Washington, D.C. 20402 

Stock Number 024-001-02643-1 
Precambrian Y 

Sedimentary Rocks in 

East-Central Idaho 

By EDWARD T. RUPPEL 

PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST -CENTRAL IDAHO 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 889-A 

Definition and description of newly named 

and revised formations deposited 

in the Belt miogeocline 
CONTENTS 

Page 

Abstract --------------------------------------------------------------------------------- 1 

In trod ucti on ----------------------------------------------------------------------------- 1 

Summary of earlier work------------------------------------------------------- 2 

Precambrian sedimentary rocks in the Lemhi Range------------ 4 

Y ellowj acket Formation----------------------------------------------- 5 

Lemhi Group------------------------------------------------------------------ 6 

Inyo Creek Formation--------------------------------------------- 7 

West Fork Formation------------------------------------------ 7 

Big Creek Formation--------------------------------------------· 9 

Apple Creek Formation-------------------------------------- 10 

Gunsight Formation------------------------------------------ 11 

Swauger Formation ----------------------------------------------------- 12 

The underlying rocks---------------------------------------------------------- 14 

Regional tectonic implications--------------------------------------------- 15 

The overlying rocks---------------------------------------------------- 16 

Regional correlation --------------------------------------------------------- 18 

Page 

Local descriptions and correlations near the Lemhi Range 18 

Elkhorn Ranch-Sawmill Gulch-West side 

of Lost River Range------------------------------------------------ 19 

Morgan Creek-------------------------------------------------------- 19 

Ellis, Idaho------------------------------------------------------------------ 19 

Twelvemile Creek and Perreau Creek---------------------- 19 

Yellowjacket mining district---------------------------~---- 19 

Carmen Creek------------------------------------------------------ 19 

North Fork, Idaho, and vicinity--------------------------------- 19 

Big Hole Pass------------------------------------------------------------- 20 

Head of Big Hole River and Big Hole Divide--------------- 20 

Bloody Dick Creek--------------------------------------------------- 20 

Lemhi Pass --------------------------------------------------------------- 21 

Grizzly Hill------------------------------------------------------------ 21 

The Beaverhead Mountains south of Grizzly Hill-------- 21 

References cited---------------------------------------------------------------- 22 

ILLUSTRATIONS 

FIGURES 1-4. Maps: 

1. Index of east-central Idaho and southwest Montana, showing localities mentioned in text-------------------------------

2. Type localities or areas, or reference section for the Inyo Creek, West Fork, Big Creek, 

Apple Creek, and Gunsight Formations·--------------------------------------------------------------------------------------------------------------------

3. Reference section for the Swauger Formation------------------------------------------------------------------------------------------------------------

4. Principal elements of the Belt basin reentrant and miogeocline, and area of 

deposition of Precambrian sedimentary rocks---------------------------------------------------------------------------------------------------------

TABLES 

Page 

3 

8 

13 

17 

Page 

TABLE 1. Summary of previous descriptions of Precambrian sedimentary rocks of east-central Idaho------------------------------------------ 4 

2. Revised sequence of Precambrian sedimentary rocks, Lemhi Range, Idaho------------------------------------------------------------------ 5 

3. Tentative regional correlations of Precambrian sedimentary rocks------------------------------------------------------------------------· 18 
PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 

By EDWARD T. RUPPEL 

ABSTRACT 

Sedimentary rocks of Precambrian Y age in east-central Idaho 

crop out in a belt that reaches 150 miles (240 km) from the Snake 

River Plain northward to the Idaho-Montana line at Lost Trail 

Pass. In the northern part of this belt, the rocks are mostly finegrained, dark-colored, micaceous, feldspathic quartzites that 

closely resemble and are tentatively correlated with the Yellowjacket Formation. Farther south, the Y ellowjacket-like rocks are 

overlain by a thick sequence of lighter colored, fine-grained feldspathic quartzites and argillitic siltites of the Lemhi Group 

(formerly called Lemhi Quartzite) that can be separated into a 

number of new mappable units, and the Swauger Formation at the 

top. The Lemhi consists of, in ascending order, the lnyo Creek, 

West Fork, Big Creek, Apple Creek, and Gunsight Formations. The 

thickness of the Yellowjacket is not known, but probably exceeds 

10,000 feet (3,050 m); the Lemhi Group is about 20,000 feet (6,100 m) 

thick; and the Swauger Formation is at least 10,000 feet (3,050 m) 

thick. 

Regional considerations suggest that at least the sequence 

above the Y ellowjacket has been moved from the original depositional area perhaps as much as 100 miles (160 km) eastward on 

major flat surfaces of detachment, and the entire eastern 

boundary of these Precambrian sedimentary rocks appears to be 

tectonic. 

Correlation of Precambrian rocks of east-central Idaho with the 

better known Belt Supergroup in western Montana and northern 

Idaho remains tenuous because of the different characteristics of 

the sequences and the likelihood that they were deposited under 

different conditions-the Belt Supergroup in the Belt basin, and 

the east-central Idaho rocks in the Belt miogeocline. Therefore, the 

Y ellowjacket, Lemhi, and Swauger are removed from the supergroup, as is the Hoodoo Quartzite of central Idaho. However, 

possible correlations are suggested by a few geologic events and 

lithologic features that seem to be represented in both sequences. 

The Y ellowjacket and Prichard are both basal sequences that 

differ greatly from the overlying rocks, and are possible 

correlatives. The Apple Creek Formation is the only carbonatebearing formation in east-central Idaho, and perhaps is equivalent 

to the middle Belt carbonate rock unit represented by theW allace 

Formation and Helena Dolomite; the lnyo Creek, West Fork, and 

Big Creek Formations thus occupy a stratigraphic position similar 

to that of the Ravalli Group. The Swauger Formation differs so 

strikingly from the rocks of the Lemhi Group as to suggest a fundamental change in sedimentary environment, perhaps reflecting 

the tectonic adjustments that took place during Missoula Group 

deposition; the Gunsight Formation, at the top of the Lemhi 

Group, therefore could be equivalent to part of the Wallace 

Formation, and the Swauger Formation to part or all of the 

Missoula Group. 

INTRODUCTION 

The approximate distribution of slightly metamorphosed sedimentary rocks of Precambrian age in 

east-central Idaho and adjacent southwestern 

Montana has long been known, for the antiquity of at 

least some of these rocks was recognized in early geological studies. But the relation of these old rocks to 

each other and to the better known rocks of the Belt 

Supergroup farther north in Idaho and Montana has 

remained uncertain. Perhaps the most appropriate 

conclusion was that finally reached by C. P. Ross 

(1963, p. 7), that the then-known relations and 

correlations were tenuous, partly contradictory, and 

premature. To some extent, Ross' conclusions are still 

valid, but as a result of detailed mapping of these 

rocks in the central part of the Lemhi Range and reconnaissance mapping in surrounding areas, it is 

possible now to better define the sequence of rocks 

and to suggest some regional relations~even while 

acknowledging that my grip on some rock units is insecure and that regional correlations are still somewhat premature and will remain so until more 

detailed geologic mapping is available. 

The Precambrian sedimentary rocks of eastcentral Idaho crop out in a belt that reaches about 150 

miles (240 km) from the south end of the Lemhi Range 

at the Snake River Plain northward to the IdahoMontana line at Lost Trail Pass-the approximate 

edge of the Idaho batholith. In the northern third of 

this belt, where it is almost 50 miles (80 km) wide, the 

rocks are not well known; available descriptions 

suggest that they are similar from one place to 

another and that they are mainly rather dark, finegrained micaceous quartzites. This northern area 

includes the Y ellowjacket mining district, one of the 

few northern localities where the Precambrian rocks 

have been divided-into the Hoodoo Quartzite and 

Y ellowjacket Formation. Farther south, the better 

known Precambrian rocks in the Lemhi Range 

finally narrow to a strip on the west side of that range 

where it plunges beneath the lavas of the Snake River 

1 
2 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

Plain; the Precambrian rocks in the Lemhi Range 

differ considerably from those to the north. Rocks 

similar to some of those in the Lemhi Range also crop 

out north and east of Round Valley, near Challis, 

Idaho, in exposures that are isolated by Tertiary 

volcanic rocks. 

Precambrian rocks extend eastward across the 

Beaverhead Mountains along the Idaho--Montana 

boundary, and reconnaissance shows that the Precambrian rocks which crop out in the Beaverhead 

Mountains (Ruppel, 1968) north of the village of 

Leadore underlie much of that range farther north 

(Sharp and Cavender, 1962, p. 4-8; Staatz, 1972, p. 

10-15) and extend into the Big Hole Basin, which 

lx-comes a critical area indeed, for rocks more clearly 

identified with the Belt Supergroup in western 

Montana crop out at the north end of the basin. Rocks 

similar to those near Leadore also are known in the 

upper reaches of Medicine Lodge Creek and Sheep 

Creek along the east flank of the Beaverheads in 

Montana (Scholten and others, 1955, p. 353; 

M'Gonigle, 1965; Lucchitta, 1966). In the area west of 

Dillon, Mont., quartzitic rocks considered part of the 

Belt Supergroup are present (Myers, 1952, p. 4-6). But 

in the Bannack-Grayling area, Montana, 30 miles (50 

km) northeast of Leadore, no sedimentary rocks of 

Precambrian age like those in the Beaverhead 

Mountains and Lemhi Range are present, and the 

Flathead Sandstone of Cambrian age overlies highgrade metasedimentary rocks and granite gneiss 

(Lowell, 1965). 

Detailed geologic mapping in the Leadore, 

Patterson, and Gilmore quadrangles, straddling the 

central part of the Lemhi Range (fig. 1), has shown 

that the Precambrian sedimentary rocks there can be 

divided into six formations (four of which are new in 

this report) that have an aggregate thickness of more 

than 30,000 feet (9,150 m). These rocks are underlain by a less well known seventh unit, probably also 

very thick, and are everlain in places by rocks that 

have been described both as Cambrian and as Precambrian but are now known to be partly Early 

Ordovician and partly late Precambrian or 

Cambrian. 

This report represents part of a more general study 

by the U.S. Geological Survey of the geology and 

mineral deposits of the central part of the Lemhi 

Range. In the course of my studies of the Precambrian sedimentary rocks, I have profited greatly 

from discussions with J. E. Harrison, M. R. Klepper, 

J.D. Wells, M. H. Staatz, and S. W. Hobbs, and I am 

indebted to them for their counsel; I am particularly 

indebted to J. E. Harrison for his thoughtful advice 

on possible correlations between the east-central 

Idaho rocks of Belt age but not formally part of the 

Belt Supergroup and those of the Belt Supergroup in 

western Montana. 

SUMMARY OF EARLIER. WOR.K 

Sedimentary rocks of Precambrian age were first 

recognized in the Lemhi Range and east-central 

Idaho by J. B. Umpleby (1913, p. 30-32; 1917, p. 23), 

who assigned them to the Belt Series, and outlined 

their areal extent. In 1947, C. P. Ross (p.1097-1102) 

named and described the Lemhi and Swauger 

Quartzites on the west-central flank of the Lemhi 

Range, formations that included part of the rocks 

earlier mentioned by Umpleby. A. L. Anderson (1959, 

p. 16-18; 1961, p. 16-21) extended these two names to 

include rocks in the Beaverhead and Salmon River 

Mountains and the northern part of the Lemhi 

Range. Ross (1962, p. 14) questioned these correlations, pointing out that rocks in these areas are 

more like the Y ellowjacket Formation (Ross, 1934, p. 

16-18) in the Casto quadrangle, central Idaho. 

Anderson (1961, p. 19-23) also named another 

formation in the central part of the Lemhi Range, the 

Apple Creek Phyllite, which he believed to be beneath 

the Lemhi Quartzite. Ross (1961, p. 195) extended the 

name Swauger from the central part of the Lemhi 

Range southward in a later reconnaissance study. 

Thus, three formations were named to include the 

sedimentary rocks of Precambrian age in the central 

part of the Lemhi Range, in ascending order, the 

Apple Creek Phyllite of A. L. Anderson (1961, p. 

19-23), and the Lemhi Quartzite and the Swauger 

Quartzite of C. P. Ross (1947, p. 1097-1102). These 

names were extended to Precambrian rocks in the 

southern Lemhi Range, in the northern part of the 

range, and still farther north into the Salmon River 

and Beaverhead Mountains; but they could not be 

extended far from their type areas or localities without exciting controversy. Original descriptions of 

these formations, and those to the west in the Casto 

quadrangle, are summarized in table 1. 

Other rocks of Precambrian age have been 

described in areas near the Lemhi Range, but have 

not been correlated with those in the Lemhi Range. 

The Hyndman and East Fork Formations were 

described by Umpleby, Westgate, and Ross (1930, p. 

9-17) in the Wood River region of central Idaho, but 

their relation to the less-metamorphosed rocks 

farther east was not discussed and remains unknown; Dover (1969, p. 24-25) considered these rocks 

to be most probably post-Belt in age. Scholten, 

Keenmon, and Kupsch (1955, p. 353) briefly described 

rocks in the Medicine Lodge Creek region of southwestern Montana supposed then to be equivalent to 

Belt rocks farther north, and pointed out that no Precambrian sedimentary rocks occur east of the Tendoy 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST -CENTRAL IDAHO 

FORMATION 

Swauger Formatron - - - - - - -

Lemhr Group - - - - - - - - - - - -

Gunsrght Formatron - -- -

Apple Creek Formatron -

Brg Creek Formatron ---

West Fork Formation - - -

I nyo Creek Format ron - -

Yellow)acket Formation----

0 

I 

0 

I 

10 

10 

I I 

20 

MAP SYMBOL 

20 

I I 

30 

(S) 

(L) 

(G) 

(A) 

(8) 

(W) 

(I) 

(Y) 

I 

30 

I 40 MILES I 

40 KILOMETRES 

FIGURE 1.-lndex map of east-central Idaho and southwest Montana, showing localities mentioned in text 

and outcrops of formations. Pattern indicates mountainous areas. 

3 
4 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST -CENTRAL IDAHO 

TABLE !.-Summary of previous descriptions of Precambrian sedimentary rocks of east-central Idaho 

Formation 

Swauger Quartzite 

(Ross, 1947, p. 1097-1099; 

named for rocks exposed in 

vicinity of Swauger Ranch 

(fig. 1, loc. 20), Lemhi Range, 

Idaho, T. 11 N., R. 24 E.) 

Lemhi Quartzite 

(Ross, 1947, p. 1096-1097; 

named for widespread exposures in the Lemhi Range, 

Idaho) 

Apple Creek Phyllite 

(Anderson, 1961, p. 19-21; 

named for exposures near 

Apple Creek (fig. 1, loc. 14), 

Lemhi Range, Idaho) 

Hoodoo Quartzite 

(Ross, 1934, p. 18-19; named 

for exposures on Hoodoo 

Creek (fig. 1, loc. 19), Salmon 

River Mountains, Idaho) 

Y ellowjacket Formation 

(Ross, 1934,p. 16-18;named 

for town and mining district 

of Yellowjacket (fig. 1, loc. 

19), Idaho). 

Description 

Lemhi Range 

Mostly rather pure quartzite, purple to almost pure white, but with beds of green, relatively 

impure quartzite near base and locally higher in formation; distinctly bedded, partly crossbedded; grain size 0.2-1.0 mm, typically <0.5 mm; feldspathic (generally <10 percent), and 

contains abundant fine flakes of sericite; colored by disseminated hematite dust; includes 

a few lenses of conglomerate and of rusty, impure dolomite; lenses of maroon, green, and 

brown to black argillite are most conspicuous in the lower part of the formation; thickness 

at least 5,000 ft (1,525 m). 

Principally grayish-green impure quartzite interlayered with subordinate argillaceous beds that 

are darker than the quartzite; quartzite contains as much as 10 percent plagioclase, and 

minor amounts of sericite, chlorite, apatite, biotite, and tourmaline; grain size 0.05-0.5 mm 

in different beds; several thousand feet thick. 

Upper part is chiefly light-gray to pale-greenish-gray phyllite; lower part is dark phyllitic quartzite and interbeds of fine- to coarse-grained graywacke; thickness not given. 

Yellowjacket district 

Nearly white to pale-gray quartzite; bedding indistinct, but with faint laminae and small-scale 

crossbedding; composed of quartz (70-80 percent), and feldspar (microcline and albite), sericite, 

and chlorite; grain size 0.2 mm; thickness about 3,500 ft (1,070 m). 

Upper quartzitic beds, about 7,000 ft (2,100 m) thick, are colored various dark shades of gray; 

generally argillaceous; composed of quartz (about 70 percent), biotite and chlorite (about 

15 percent), and plagioclase, sericite, iron oxide, and scattered grains of epidote and zircon; 

average grain size about 0.1 mm. 

Lower beds, as much as 1,500 ft (460 m) thick (base not exposed), also dominantly quartzitic, include calcareous lenses that range from hornfelses to impure quartzites. Colors range 

from white, gray, and green to dark green and black; grain size of calcareous rocks 0.03-

0.1 mm. 

Mountains. M9st other reports or geologic studies in 

this region describe the Precambrian sedimentary 

rocks only cursorily. 

regional relations discussed, in chapter B of this 

report (Ruppel and others). 

A sequence of sandstone, quartzite, shale, and conglomerate near the Wilbert mine (Dome mining 

district), in the southern part of the Lemhi Range (fig. 

1, loc. 27), has been assigned to either the 

Precambrian or Cambrian(?) or both at different 

times by different writers, most recently again to the 

Cambrian(?) by Beutner and Scholten (1967, p. 

2305-2311). The upper part of this sequence, the 

Summerhouse Formation, is now known to be of 

Early Ordovician age (Ruppel and others, chapter B, 

this report), and the lower part, the Wilbert 

Formation, probably is late Precambrian in age, perhaps equivalent to part of the Brigham Quartzite or 

the Windermere Group. The sequence is clearly 

younger than the thick group of rocks discussed in 

this report as equivalent to part of the Belt Supergroup; they are more fully described, and their 

PRECAMBRIAN SEDIMENTARY ROCKS IN THE 

LEMHI RANGE 

The type localities of the Apple Creek Phyllite, 

Lemhi Quartzite, and Swauger Quartzite are all in 

the central part of the Lemhi Range (figs. 2, 3), but 

structural complexity in the type localities makes uncertain the identity of the formations there. Similar 

complexity elsewhere in the Lemhi Range has made 

attempts to extend these formations away from their 

type localities less than satisfactory. In my mapping 

in the central part of the Lemhi Range I have 

separated the Precambrian sedimentary rocks somewhat differently (table 2). I propose (1) that the 

Swauger Quartzite be changed to Swauger 

Formation to make clear its lithologic characteristics 

and distribution as far as known; (2) that the Lemhi 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 5 

TABLE 2.-Revised sequence of Precambrian sedimentary rocks, 

Lemhi Range, Idaho 

Swauger Formation: 

Pale-purple to grayish-green medium-grained hematitic 

quartzite; maximum thickness about 10,000 ft (3,100 m) 

Lemhi Group: 

Gunsight Formation: 

Light-brownish-gray to grayish-red-purple fine-grained 

feldspathic quartzite; minimum thickness about 6,000 

ft (1,830 m) 

Apple Creek Formation: 

Grayish-green siltite containing abundant lenses of finegrained sandstone cemented by ferrodolomite; thick- ness about 2,500-3,000 ft (760-900 m) 

Big Creek Formation: 

Greenish- to light-gray fine-grained feldspathic quartz- ite; thickness about 10,000 ft (3,100 m) 

West Fork Formation: 

Medium- to greenish-gray siltite that contains lenticular 

algal limestone; thickness 1,500 ft (460 m) 

Inyo Creek Formation: 

Medium- to light-gray fine-grained to very fine grained 

feldspathic quartzite, thin-beddedi thickness unknown, 

minimum measured thickness aoout 700 ft (215 m); 

base not exposed 

Y ellowjacket Formation: 

Medium- to medium-dark-gray fine-grained feldspathic finely biotitic quartzite, and interbedded siltite; thickness 

unknown, but possibly as much as 10,000 ft (3,100 m) 

Quartzite be elevated to Lemhi Group, including five 

formations, the Gunsight Formation, Apple Creek 

Formation, Big Creek Formation, West Fork Formation, and the lnyo Creek Formation, of which the 

Apple Creek Formation is the same as Apple Creek 

Phyllite of Anderson (1961) and the other formations 

are new; and (3) that the rocks in thelowerpartofthe 

Precambrian section are part of the Yellowjacket 

Formation, thus extending this name from its type 

locality as suggested by Ross (1962, p. 14). 

Taken together, the Precambrian sedimentary 

rocks are a sequence dominantly of feldspathic 

quartzites and of immense thickness. The thickness 

of the Y ellowjacket Formation is not known with certainty but must be many thousand feet; the Lemhi 

Group and Swauger Formation together are more 

than 30,000 feet (9,150 m) thick. In general, these 

rocks have very little in common with the partly 

equivalent and much better known rocks of the Belt 

Supergroup farther north; the grain size of most Belt 

Supergroup rocks in Montana and northern Idaho is 

medium silt or finer (Harrison and Campbell, 1963, p. 

1415), but most of the Lemhi Range rocks are 

composed of fine- to medium-grained feldspathic or 

micaceous sand. Shallow-water features-mudcracks, ripple marks, salt casts, and so on-are rare 

in most of the Lemhi Range rocks but fairly common 

farther north; many quartzite beds are laminated or 

cross-laminated, a few beds contain mud chips, and a 

few beds are ripple marked, but other sedimentary 

structures are rare. Stromatolites, comparatively 

common in Belt rocks, have been found only at two 

places in the Lemhi Range. Limestone and dolomite 

are nearly absent in the Lemhi Range section but 

form thick, key stratigraphic units to the north. The 

Precambrian sedimentary rocks of the Lemhi Range 

and adjacent areas in east-central Idaho and southwestern Montana thus clearly reflect a depositional 

environment different from that of the Belt rocks 

farther north. 

Nevertheless, the rocks in the two regions do have 

features in common. Most contacts are gradational, 

commonly through thicknesses of many feet; and 

nearly identical rocks occur in different formations, 

and so one formation cannot always be readily and 

certainly distinguished from another; both groups of 

rocks show low-grade regional metamorphism that 

increases with depth in the section-biotite zone in 

the lower units, chlorite-sericite zone in the upper, 

and slight or no metamorphism at the top. Indeed, it 

is the occurrence of low-grade metasedimentary 

rocks apparently sandwiched between high-grade 

metamorphic rocks below and Paleozoic sedimentary rocks above, rather than lithologic 

similarities, that leads to the inferred correlation of 

the two groups of rocks. 

The stratigraphic problems are further complicated in the Lemhi Range by folding that has overturned about half of the rocks, by imbricate thrust 

faults that shuffle upright and overturned limbs, and 

by other, younger faults. As a result, there are few 

complete sections of any of the Precambrian 

formations. 

The principal divisions in the revised sequence of 

Precambrian rocks in the Lemhi Range are based on 

substantial and mappable differences in the 

character of the units. The Y ellowjacket Formation, 

mostly quartzite, is rather dark, impure, feldspathic, 

and micaceous, and includes euhedral grains of 

magnetite. The Lemhi Group is dominantly quartzite, but generally is lighter colored and includes 

detrital feldspar-as much as 35 percent-and grains 

of black heavy minerals, mainly magnetite, but little 

mica and that mainly on bedding surfaces. The 

Swauger Formation is coarser grained than the 

underlying rocks, contains hematite rather than 

magnetite or other black heavy minerals, and contains little or no detrital feldspar and little or no mica. 

YELLOWJACKET FORMATION 

The rocks mapped by Ross (1947) and Anderson 

(1961) were studied in the central part of the Lemhi 

Range, and attempts to extend the formations they 

mapped into the northern part of the Lemhi Range 

were unsuccessful, and with good reason, because the 

rocks in the northern part of the range underlie those 
6 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

exposed farther south. These underlying rocks, the 

lowest Precambrian sedimentary rocks exposed in 

east-central Idaho, are here correlated on the basis of 

lithologic similarity with the Y ellowjacket Formation of central Idaho as defined by Ross (1934, p. 16). 

The quartzite and associated rocks of the Yellowjacket Formation are the most widespread-and 

poorly known-of the Precambrian sedimentary 

rocks. The formation apparently underlies most of 

the northern part of the Lemhi Range, and extends 

farther north to the vicinity of Gibbonsville and Lost 

Trail Pass, where it nears the margin of the Idaho 

batholith. It extends westward, seemingly without a 

break, to the vicinity of the Y ellowjacket mine, but 

little of this region is mapped, and the exact relations 

are not known. The formation also crops out in many 

places along the Salmon River Canyon between 

Salmon and Ellis (fig. 1). It apparently underlies 

much of the Beaverhead Mountains from Leadore 

northward (Sharp and Cavender, 1962, p. 5-8; Staatz, 

1972) although other, younger Precambrian sedimentary rocks are present in places, too. Rocks 

similar to the Y ellowjacket at the north end of the 

Lemhi Range crop out farther south in a single 

isolated exposure beneath a thrust fault in Squaw 

Creek (fig. 1, loc. 21), southwest of Gilmore, and 

perhaps in and near Basinger Canyon (fig. 1, loc. 25; 

see Umpleby, 1917, p. 23). 

The thickness of theY ellowjacket is not known; the 

base of the formation-its lower contact with unlike 

sedimentary rocks or, more probably, with older 

Precambrian crystalline rocks-has not been found. 

C. P. Ross (1934, p.16) suggested a thickness of about 

9,000 feet (2,750 m) in the type locality. Anderson 

(1956,p.18;1959,p.17;1961,p.23)suggestedthatthe 

thickness of the rocks I include in the Yellowjacket in 

the Lemhi Range is at least 5,000 feet (1,500 m) and 

possibly as much as 10,000 feet (3,100 m). The great 

areal extent of these rocks suggests, too, that their 

thickness must be great, but any closer approximation of the actual thickness must await more complete mapping, particularly in the region west of the 

Lemhi Range. 

The rocks above the Y ellowjacket appear to differ 

from place to place, and the nature of the contact with 

overlying rocks is not well known. In the type 

locality, Ross (1934, p.18-19) described the contact of 

the Yellowjacket with the overlying Hoodoo Quartzite as gradational. In the unmapped area southeast 

of Gibbonsville, Yellowjacket-like rocks are overlain 

by light-colored quartzite that probably is part of the 

Big Creek Formation named in this paper, and the 

contact seems to be gradational (Anderson, 1959, p. 

18). Near Patterson, however, the Big Creek is underlain by more than 2,000 feet(610m)ofsiltiteandfinegrained quartzite; the Yellowjacket is not exposed. 

The Yellowjacket Formation in the Lemhi Range is 

a sequence of medium-gray to medium-dark-gray, 

fine-grained to very fine grained, feldspathic, finely 

biotitic quartzite in beds commonly 0.2-2 feet (6-60 

em) thick but in places as much as 5 feet (1.5 m) thick. 

These rocks are partly thinly laminated, partly crosslaminated, and partly color banded in various shades 

of gray. The formation includes interbeds and interbedded units as much as several tens of feet thick of 

dark-greenish-gray to medium-dark-gray siltite and 

argillite in thin to medium beds, and includes beds of 

quartzite that are darker or lighter colored-some 

beds are light gray to medium light gray, and 

greenish-gray beds are common throughout the formation. The lighter colored rocks generally are 

somewhat coarser grained. The weathered rock is 

brownish gray and limonite stained, and generally is 

not well exposed but, rather, underlies smooth slopes 

that are littered with small rock fragments. 

The micaceous quartzites of the formation are 

characteristically fine grained or very fine grained; 

the quartz grains commonly are about 0.1 mm in 

diameter. Studies of thin sections reported by 

Anderson(1959,p.17;1956,p.17;1961,p.22)suggest 

that the typical rock of the formation contains 40-60 

percent quartz, 20-30 percent albite, 10-20 percent 

biotite, 5-10 percent sericite and muscovite, and a 

small percentage of zircon, magnetite, tourmaline, 

apatite, and pyrite. 

LEMHI GROUP 

The Lemhi Group draws its name from the Lemhi 

Quartzite (Ross, 1947, p. 1096), which included 

impure grayish-green quartzites and lesser amounts 

of darker colored argillaceous rocks in the westcentral part of the Lemhi Range. Although 

subsequent use of the name has been confused, the 

rocks that I include here in the Lemhi Group are 

essentially those originally included in the Lemhi 

Quartzite by Ross. The original formation is herein 

raised to group status and includes five formations; 

four are new, and the fifth, the Apple Creek 

Formation, is revised Apple Creek Phyllite of A. L. 

Anderson (1961, p. 19). The formations in the Lemhi 

Group are, in descending stratigraphic order: 

Gunsight Formation-brownish-gray fine-grained 

feldspathic quartzite. 

Apple Creek Formation-greenish-gray argillite, siltite, and very fine grained quartzite. 

Big Creek Formation-medium- to light-gray finegrained feldspathic quartzite. 

West Fork Formation-dark-greenish-gray siltite. 

lnyo Creek Formation-medium- to light-gray finegrained to very fine. grained feldspathic quartzite. 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 7 

The greatest known thickness of the group is somewhat more than 20,000 feet (6,100 m) in the central 

part of the Lemhi Range, the only place where the 

lnyo Creek and West Fork Formations are known; 

the base of the lowest formation, the lnyo Creek, is 

not exposed. Farther north, along the North Fork of 

the Salmon River southeast of Gibbonsville, the 

lower two formations of the Lemhi Group are missing, and the Lemhi Group thus appears to thin to the 

north by nondeposition or erosion oflower units, suggesting, in turn, that the Precambrian depositional 

basin became deeper to the southwest. 

INYO CREEK FORMATION 

The lowest known formation of the Lemhi Group is 

the lnyo Creek Formation, a new formation here 

named for exposures in its type area at the head of 

lnyo Creek in the southern part of the Patterson 

quadrangle, Lemhi Range, Idaho (fig. 2). The base of 

the formation is cut by a fault and concealed by surficial deposits, so its relation to older rocks and its 

total thickness are not known. The maximum 

measured thickness in the type area is about 700 feet 

(215 m). It is overlain by the West Fork Formation; 

the contact is gradational through a thickness of 

about 100 feet (30 m), in which the lnyo Creek 

includes increasing numbers of thin interbeds of 

medium-gray to greenish-gray siltite. The contact is 

mapped where the siltite becomes thick bedded to 

massive and is the dominant rock type, but a few 

interbeds of quartzite similar to the lnyo Creek are 

present throughout the overlying West Fork 

Formation. The formation has been mapped only in 

its type area in the Patterson quadrangle and in continuous exposures for a short distance farther south 

along the North and West Forks of Big Creek. It has 

not been recognized anyplace else. 

The exposed part of the lnyo Creek is mainly lightgreenish-gray and medium-light-gray to mediumdark-gray, very fine grained to fine-grained, feldspathic (5-25 percent) quartzite, some beds of which 

are silicified and extremely hard. The formation is 

decidedly thin bedded; beds range from 0.5 to 5 feet 

(0.2-1.5 m) thick, and most are 2 feet thick or less. 

Some beds are finely cross-laminated, some are ripple 

marked, and in the lower part of the formation many 

beds are separated by silty partings. A few beds, also 

in the lower part of the formation, contain mudstone 

chips. Thin beds of medium-gray to greenish-gray 

siltite, generally only 1 or 2 feet (0.3-0.6 m) thick, are 

sparingly present throughout the formation and are 

increasingly common in the upper part where the 

Inyo Creek grades into the overlying West Fork. The 

light- and dark-colored beds of the formation, 

reflecting lesser or greater amounts of feldspars, 

sericite, and heavy minerals-principally magnetite, 

and in one extremely "rusty" bed, pyrite-alternate 

in units 10-30 feet (3-10m) thick, and the formation 

gives the impression of being an alternating 

sequence of clean and dirty beds and groups of beds. 

WEST FORK FORMATION 

The West Fork Formation here is named for type 

exposures at the head of the West Fork of Big Creek in 

the southern part of the Patterson quadrangle, 

Lemhi Range, Idaho (fig. 2). The formation is well 

exposed at this locality, and the gradational nature 

of both its upper and lower contacts is clearly evident. 

Part of the formation is also exposed in a thrust plate 

at the junction of Patterson Creek and its East 

Fork-a somewhat more accessible spot about 4'12 

miles northwest of the type locality. It is not known at 

any other place. The calculated thickness of the 

formation is about 1,500 feet (460 m). 

As described before, the contact of the West Fork 

Formation with the underlying Inyo Creek 

Formation is gradational and the basal contact of the 

West Fork is placed at the first thick-bedded or 

massive siltite, above which siltite is the dominant 

rock. The upper contact of the West Fork is similarly 

gradational into the overlying Big Creek Formation, 

but the gradational zone is only 25-40 feet (7-12 m) 

thick; the top of the West Fork is placed at the highest 

thick or massive bed of siltite. A few thin siltite beds 

occur higher, but the dominant rocks are quartzite. 

Below the highest massive siltite, the West Fork 

Formation is about 90 percent siltite and 5-10 percent 

very fine grained quartzite and other rocks. The 

upper 20 feet (6 m) or so of the formation includes thin 

interbeds of quartzite similar to the overlying Big 

Creek Formation. The siltite that makes up the bulk 

ofthe formation is medium gray to medium dark gray 

to dark greenish gray or blackish green, partly 

laminated, cross-laminated, and ripple marked. 

Although some beds are 0.5-3 feet (0.15-1 m) thick, 

particularly in the lower part of the formation, most 

of the formation is thick bedded to massive, and 

blocky fracturing. The formation includes lenticular 

interbeds, lenses, and well-defined heads of algal 

limestone-almost the only stromatolites known in 

the entire section of Precambrian rocks in eastcentral Idaho. The largest algal limestone in the type 

locality is 10-12 feet (3m) thick, about 100 feet (30m) 

long, and consists of yellowish-gray to lightgreenish-gray, light-olive-gray-weathering, laminated sandy limestone, in beds 0.5-1 foot (0.15-0.3 

m) thick, with thinner lenticular interbeds of siltite 

and very fine grained quartzite; some of the 

limestone is in characteristic rounded algal heads or 

domes. Although the algal lenses occur throughout 
8 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

the formation, they appear to be most common in its 

upper part. Interbeds of light-brownish-gray to 

greenish-gray very fine grained to fine-grained 

feldspathic quartzite, typically 0.5-2 feet (0.15-0.6 m) 

thick, but exceptionally in units as much as 50 feet (15 

m) thick, occur throughout the formation, although 

0 

0 

the thicker units apparently are mainly in the lower 

part. The siltite also includes some small lenses 

typically only a centimeter or so thick of medium 

sand cemented by ferrodolomite, but such lenses are 

unusual in the West Fork Formation, in contrast to 

the younger Apple Creek Formation, in which 

2 3 MILES 

2 3 KILOMETRES 

FIGURE 2.-Locations for Precambrian Inyo Creek (type area), West Fork (type locality), Big Creek (type area), Apple Creek 

Survey Patterson topographic 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 9 

ferrodolomite-cemented sand lenses and beds are a 

characteristic feature. 

BIG CREEK FORMATION 

The Big Creek Formation is one of the thickest and 

most widespread formations in the Lemhi Group. 

The formation is here named for outcrops in its type 

area along the upper reaches of Big Creek, in the 

south-central part of the Patterson quadrangle, 

Lemhi Range, Idaho (fig. 2). The formation is almost 

completely exposed in the ice-carved, cliffed west 

wall of the canyon of Big Creek and in the large, com-

(principal reference section), and Gunsight (type locality) Formations in east-central Idaho. Base from U.S. Geological 

quadrangle, 1956. 
10 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

pound cirque above, the floor of which is mostly icestripped and polished bare rock, covered only by 

hordes of mosquitoes when they are in season. The 

formation crops out through the central part of the 

Lemhi Range from the area around and north of the 

mining camp at Patterson to the upper part of 

Sawmill canyon (fig. 1, loc. 28) at the head of the 

Little Lost River. It also is present in a large area in 

the Beaverhead Mountains, on Grizzly Hill (fig. 1, 

loc. 13), north of Leadore1 , and probably both north 

and south of there for many miles, although the 

Precambrian rocks of the Beaver heads have not been 

described for most places. 

The Big Creek Formation is very thick 

-calculations in the type area suggest it to be about 

10,000 feet (3,100 m) thick there; it is strikingly homogeneous-most of the formation is light-gray or lightbrownish-gray fine-grained feldspathic quartzite. 

The base and top of the formation are both 

gradational, the base through a 25-40 foot (8-12 

m) thickness from the West Fork Formation as 

described before, and the top through a thickness of 

about 150 feet (46 m) into the overlying Apple Creek 

formation. The upper gradational zone includes 

0.1-0.2-foot (3-7 cm)-thick partings of medium-darkgray siltite and very fine grained quartzite in its 

lower part; similar rocks as interbeds that progressively thicken to as much as 10 feet (3 m) are 

increasingly abundant in the uppermost part of the 

formation where it grades into the Apple Creek. The 

top contact is placed where the quartzite typical of 

the Big Creek becomes subordinate to the interbedded siltite. Most of the quartzite is pale greenish 

gray to light gray, weathers light brownish gray to 

light gray, and is fine grained (0.1-0.2 mm), 

moderately to strongly feldspathic (10-35 percent), 

and speckled with grains of iron oxide. Colors of 

rocks in the unit also include grayish yellow green, 

light olive gray, pale olive, pale red, and grayish 

orange pink. The rock is in beds 0.5-3 feet (0.15-1 m) 

thick, or massive; the upper half of the formation 

tends to be thicker bedded than the lower half, but 

thick-bedded or massive rocks are common throughout the formation. The uppermost 500 feet (150 m) is 

thin to medium bedded, and these uppermost beds 

also contain scattered chips of greenish-gray argillite 

and siltite, 1-2 em in diameter. Many beds are 

laminated and cross-laminated; magnetite grains 

commonly are concentrated along the laminae, as 

well as being scattered through the rock. 

In sharp contrast to the homogeneous feldspathic 

1 Precambrian rocks in the Leadore quadrangle (Ruppel, 1968) were described but not 

aesigned to formations. The Swauger Formation of this report is unit A on the Leadore 

map, the Gunsight Formation is unit B, the Apple Creek Formation is unit C, and the Big 

Creek Formation is unit D. 

quartzite are a few layers as much as 40 feet (12 m) 

thick in the lower half of the formation. These layers 

are medium-light-gray to greenish-gray fine-grained 

feldspathic quartzite that contains abu~dant 

irregular lenses about 1 em long and 5 mm thiCk of 

fine-grained quartz sandstone cemen te~ . by 

ferrodolomite. The sandstone weathers a stnking 

dark yellowish brown and gives the quartzite a 

mottled look. The beds, 3-4 feet (1 m) thick, are 

strongly cross-laminated, and a few beds of palepurple to grayish-purple quartzite are interbedded 

with the mottled rocks. 

APPLE CREEK FORMATION 

The Apple Creek Formation of this paper ":as 

originally named the Apple Creek Phyllite 

(Anderson, 1961, p. 19-21) for exposures in the 

vicinity of Apple Creek (fig. 1, loc. 14) and alo~g Hayden Creek in the southern part of the Lemhi quadrangle, which Anderson designated as the type area. 

The Apple Creek Phyllite described by Anderson consisted of a lower unit of phyllitic quartzite and an 

upper unit entirely of phyllite; he considered these 

rocks to be the oldest Precambrian rocks recognized 

in the Lemhi Range, lying stratigraphically below 

the Lemhi Quartzite. However, most of the rocks that 

Anderson called Lemhi Quartzite are part of the 

Y ellowjacket Formation as used in this paper, which 

in the Hayden Creek-Apple Creek area underlies a 

thrust plate of Apple Creek Phyllite; the phyllitic 

character of the Apple Creek is a result of shearing, 

and is not typical of the same formation farther 

south. But the formation clearly is a valid unit an~ is 

one of the most useful stratigraphic keys in the entire 

section of Precambrian sedimentary rocks. I propose 

therefore that the name be retained in modified form 

as the Apple Creek Formation and that the formatio? 

be revised to make clear its lithology and stratigraphic relations. 

In addition to the outcrops in and near the type 

area in the Lemhi quadrangle, the Apple Creek Formation is widely exposed in the central part of the 

Lemhi Range, from the area north of Patterson 

southeastward into Sawmill Canyon (fig. 1, loc. 28) at 

the head of the Little Lost River. Some of the best partial exposures are in cliffs flanking Falls Creek and 

Mahogany Creek north of Patterson (figs. 1 and 2), 

but more accessible outcrops representative of most 

of the formation are at the mouth of Morse Creek 

where it enters the Pahsimeroi Valley (loc. 16), and 

along U.S. Highway 93 at Ellis, where the 

Pahsimeroi River joins the Salmon River (loc. 17). 

The entire formation is present, unfaulted, high 

(about 10,000 feet (3,100 m) altitude) on the ridge 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST -CENTRAL IDAHO 11 

between Patterson Creek and its East Fork (fig. 2), 

and farther east near Golden Trout Lake, where it is 

slightly more accessible and better exposed. The 

principal reference section selected here for the formation is along the crest of the ridge between Golden 

Trout Lake and the head of the Middle Fork of Little 

Timber Creek (fig. 2), in the Patterson quadrangle; it 

can be reached by trail from the east side of the Lemhi 

Range. Part of the formation is also present in thrust 

plates in the Mineral Hill mining area (fig.1, loc. 30) 

north of Leadore (Ruppel, 1968), and reconnaissance 

suggests that it may be fairly widespread elsewhere 

in the Beaverhead Mountains. 

The calculated thickness of the Apple Creek Formation in the central part of the Lemhi Range is 

2,500-3,000 feet (760-900 m). The lower and upper 

contacts are gradational-the lower contact with the 

Big Creek Formation as previously described, and 

the upper contact into the overlying Gunsight 

Formation through a zone 200-300 feet (60-90 m) 

thick in which interbeds mostly 0.5-1 foot (0.15-0.3 

m) thick of light-brownish-gray to light-gray finegrained feldspathic, limonite-spotted and stained 

dirty quartzite become increasingly abundant in the 

siltite. The upper contact is placed at the highest bed 

of siltite that contains lenses of ferrodolomitecemented sandstone, even though thin interbeds of 

dark-gray siltite occur higher in the overlying quartzite. 

The formation includes distinctive rocks that make 

it the most useful and easily recognizable formation 

in the Lemhi Group. Siltite, the principal rock, is 

grayish green to grayish red purple to medium dark 

gray, and the darker colors commonly are tinted 

purple. The grayish-green siltite is typically in beds 

0.2-0.5 foot (6-15 em) thick and contains irregular 

streaks and lenses of light-gray, grayish-pink- to 

pale-brown-weathering, fine-grained sandstone that 

is cemented by ferrodolomite; the streaks and lenses 

typically are 1-2 em thick and as much as 2 feet (0.6 m) 

long. A few lenses of this ankeritic sandstone are as 

much as 30 feet (9 m) thick and more than a hundred 

feet (30m) long, but such large lenses are most exceptional. The darker gray and purplish siltite occurs 

mainly in the lower half of the unit, and even there is 

subordinate to the grayish-green rocks; it is in beds as 

much as 20 feet (6 m) thick, and is commonly laminated. A few beds of medium-dark-gray argillite, less 

than a foot thick, are in the lower part of the unit, 

generally associated with the gray massive siltite. 

All the siltite and argillite is characterized by a 

pencilly fracture. Quartzite is interbedded with the 

siltite and is itself very fine grained, in contrast to the 

fine- to medium-grain size in most of the other Precambrian quartzites. The rock is greenish gray to 

light gray, light brownish gray or pale red, feldspathic, partly cross-laminated, and is in beds 1-2 feet 

(0.3-0.6 m) thick. The quartzite beds are most common 

in the upper and lower parts of the unit, and are rare 

in the middle part. 

Ripple marks and mudcracks are common 

throughout the formation; many beds are laminated 

or cross-laminated; and many bedding surfaces are 

coated by coarse detrital muscovite. The siltite 

locally contains sparse pyrite cubes as much as 1 mm 

across. Also, many of the small deposits of secondary copper minerals so common in the central part 

of the Lemhi Range and in the Beaverhead Mountains north of Leadore are in veins or faults that 

either cut the Apple Creek Formation or cut other 

rocks near outcrops of Apple Creek Formation; the 

consistent association suggests that the Apple Creek 

Formation may be the source of the copper. 

GUNSIGHT FORMATION 

The uppermost unit of the Lemhi Group, the Gunsight Formation, is named for extensive exposures in 

its type locality on the south and west sides of Gunsight Peak, in the central part of the Lemhi Range in 

the Patterson quadrangle (fig. 2); some of the best 

outcrops here are on the north side of the North Fork 

of Little Timber Creek. The formation is widely 

exposed from the type area westward across the 

Lemhi Range, and southward toward the Gilmore 

mining area. A well-exposed and accessible partial 

section that includes rocks typical of most of the 

formation is on the north wall of the canyon of Morse 

Creek (fig. 1, loc. 16), on the west side of the Lemhi 

Range. This section probably includes about the 

lower half of the formation; higher rocks are removed 

by faulting. The formation probably is present in 

places in the Beaverhead Mountains, but it has not 

been mapped separately there. 

No complete sections of the Gunsight Formation 

have been found; in the Lemhi Range the rocks are 

everywhere cut by thrust faults, and in the eastern 

part of the range the formation is overlain with 

strong angular unconformity by the Kinnikinic 

Quartzite of Ordovician age. The minimum thickness of the formation estimated from the fragmented 

exposures is about 6,000 feet (1,830 m). The lower 

contact of the formation with the Apple Creek Formation is gradational through a thickness of 200-300 

feet (60-90 m), as described before. In the lower 1,000 

feet (300m) of the formation the proportion of silt and 

argillitic beds decreases upward and the quartzite 

beds, 0.5-1 (15-30) em thick, are dirty appearing, 

medium to light gray, fine grained, feldspathic, and 

limonite speckled; siltite and argillite interbeds and 

partings, which form almost half the basal 200-300 
12 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

feet (60-90 m) of the formation, are grayish red purple 

to medium dark gray and platy to pencilly. The lower 

part of the formation includes a few massive bedsl0-

20 feet (3-6 m) thick, of cleaner, feldspathic quartzite 

like that higher in the section, and such beds contrast strikingly with the surrounding dirtier rocks. 

The upper contact with overlying Precambrian 

rocks is also gradational; the grain size of the upper 

few hundred feet of Gunsight Formation coarsens upward, and so the upper rocks are medium grained, 

and the rocks are thick bedded to massive. The quartz 

grains in these upper rocks are well rounded and 

glassy, like those in the overlying Swauger Formation, but the rocks are feldspathic and contain abundant magnetite along laminae and cross-laminae, in 

contrast to the hematitic and sparsely feldspathic or 

nonfeldspathic rocks in the Swauger. The upper 

contact with the Swauger is mapped at the first beds 

of medium- to coarse-grained, hematitic, sparsely 

feldspathic or nonfeldspathic pale-red to grayish-red 

quartzite typical of the Swauger. A few beds of Gunsight-like quartzite are present above this contact, 

some in units as much as 50 feet (15m) thick, but they 

clearly are subordinate. The upper contact is well 

exposed in the valley walls along upper reaches of 

Falls Creek (fig. 1), north of Patterson. Where the 

Swauger has been eroded away, the Gunsight is overlain by the Kinnikinic Quartzite. 

Most of the Gunsight Formation is light brownish 

gray and medium light gray to purplish gray and 

grayish red purple, fine to medium grained (0.2-0.3 

mm), feldspathic (5-40 percent, typically 10-20 percent), and contains abundant magnetite in rounded, 

detrital grains scattered through the rock and concentrated in laminae and cross-laminae. The rocks 

generally are thin to medium bedded (1-4 feet; 0.3-1.2 

m); many beds are conspicuously laminated and 

cross-laminated; and many beds have been deformed 

by penecontemporaneous slumping. A few interbeds 

of grayish-yellow-green and light-olive-gray finegrained (0.2 mm) feldspathic quartzite occur throughout the formation, which otherwise is homogeneous 

except for its lowermost and uppermost parts. Silty or 

muddy partings are also present through the formation. 

The Gunsight Formation resembles the Big Creek 

Formation in many ways, and the two formations 

cannot always be distinguished with confidence. In 

general, though, the Gunsight is darker colored, has 

many beds that are tinted red or purple, is dirtier 

appearing, contains more magnetite and consequently is commonly limonite stained and speckled, 

and includes more siltite or argillite beds and partings; the Gunsight rocks tend to be dirty, the Big 

Creek rocks tend to be clean, but distinction between 

the two is not always clear. 

SWAUGER FORMATION 

The Swauger Formation was named for exposures 

in the vicinity of the Swauger Ranch (fig.1, loc. 20) on 

the west side of the Lemhi Range and near the divide 

between the Pahsimeroi and Little Lost Rivers (Ross, 

1947, p. 1096-1099), the type locality. As described by 

Ross, the Swauger was mainly purple to white, 

almost pure, medium-grained, hematitic, slightly 

feldspathic quartzite, a description that is appropriate for the Swauger as used here, However, the 

formation as originally described also included beds 

of impure quartzite, lenses of white quartzite, lenses 

of conglomerate and of rusty, impure dolomite, and 

beds of dark-colored argillite-rocks thatmappingin 

the central and southern parts of the Lemhi Range 

indicates must belong to younger Precambrian or 

Lower Ordovician formations that overlie the 

Swauger and have been faulted into their present 

position in the type Swauger. 

The Swauger is widely exposed in the Lemhi 

Range: in the type locality and in the adjacent 

Donkey Hills (fig. 1); farther north, in a belt that extends across the range from near Leadore (Ruppel, 

1968) to east of May; along the west side of the range 

south of Patterson; and in a few places in the canyon 

of the Salmon River near Ellis (fig. 1, loc.17). Similar 

rocks are exposed farther west in Morgan Creek, 

north of Challis (fig.1, loc.18) (S.W. Hobbs, 1970, oral 

commun.). Some of the most accessible outcrops are 

on Leadore Hill (fig. 1, loc. 29) near Leadore (Ruppel, 

1968). Anderson (1959, p. 18-21; 1961, p. 23) suggested 

that the formation is also present farther north in the 

Lemhi Range, and in the Beaverhead Mountains. Reconnaissance studies show that the formation is 

indeed present in part of the northern Lemhi Range, 

but that the rocks Anderson called Swauger in the 

Beaverhead Mountains more closely resemble rocks 

that probably are of part of the Wilbert Formation 

tentatively of Precambrian Z age. 

The thickness of the formation is uncertain, because no sections undisturbed by faulting are known, 

and because part-in places all-of the formation 

was eroded before deposition of the overlying rocks; it 

has been completely eroded in the east-central part of 

the Lemhi Range, and the Kinnikinic there rests on 

the Gunsight Formation. Ross (1947, p.1099) stated 

that the exposed thickness near Swauger Ranch (loc. 

20) is at least 1 mile (1.6 km); farther north, in the 

reference section newly designated in this report near 

the East Fork of Hayden Creek (loc. 15, fig. 3), the 

greatest thickness not cut by faults is about 10,000 

feet (3,100 m) but even there the top is cut off by 

faulting. 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 13 

0 2 3 MILES 

0 2 3 KILOMETRES 

FIGURE 3.-Reference section for Precambrian Swauger Formation in east-central Idaho. Base from U.S. Geological Survey Patterson 

topographic quadrangel, 1956. 

The lower contact of the Swauger. with the Gunsight Formation is gradational through a thickness 

of about 150 feet ( 45 m), in which rocks characteristic 

of both formations are interbedded and, as described 

before, the grain size coarsens in the upper few 

hundred feet of the Gunsight and resembles that 

typical of the Swauger. The top is not known, for the 

Swauger is overlain with angular unconformity by 

the Wilbert Formation in the southern part of the 

Lemhi Range, by the Lower Ordovician Summer-
14 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

house Formation in the west-central part of the range 

and by the Middle Ordovician Kinnikinic Quartzite 

in the east-central part of the range. No Precambrian 

rocks in normal depositional contact above the 

Swauger are known in east-central Idaho. 

The Swauger is composed of distinctive rocks that 

make it comparatively easy to identify, and that 

differ so completely from the older Precambrian 

quartzites as to suggest some fundamental change in 

depositional environment. Most of the quartzite is 

pale purple, grayish pink, pale red purple, or less 

commonly light brown to grayish green, medium to 

coarse grained (typically about 0.5 mm), hematitic, 

and only slightly feldspathic if at all. Beds are from 

0.5-6 feet (0.15-1.8 m) thick, but generally are 3-6 feet 

(0.9-1.8 m) thick; many beds are prominently and 

coarsely cross-laminated, and some are ripple 

marked. The quartz grains are glassy, well sorted, 

and well rounded. Abundant hematite grains 0.5-1 

mm in diameter give much of the rock a distinctive 

blotchy appearance as the hematite weathers and 

stains the surrounding quartz grains. The quartzite 

beds in places are separated by partings and beds 0.1-

0.5 foot (3-15 em) thick, rarely as much as 3 feet (1 m) 

thick, of greenish-gray to dark-grayish-green siltite 

or argillite; such fine-grained rocks probably are 

most common in the lower part of the formation, 

particularly in association with the interbedded 

Gunsight-like feldspathic quartzites near the base. 

The rocks of the formation thus differ from the older 

quartzites in their distinctive purplish color, their 

coarser grain size and rounded glassy quartz grains, 

the absence or near-absence of feldspar and black 

heavy minerals, and their purplish hematitic 

speckling and blotching. Ross (1947, p. 1098) described the microscopic 

characteristics of "purple and lavender" quartzites 

from the type Swauger. He stated that the rocks consist mainly of quartz grains, 0.2-1.0 mm in diameter, 

but generally less than 0.5 mm in diameter, that are 

enlarged by optically oriented overgrowths. Nearly 

all the quartz grains show strain shadows in random 

orientation. Feldspar if present is generally sodic 

plagioclase and some microcline, and in some beds 

constitutes 10 percent ofthe rock. Scattered grains of 

quartzite are generally present. The matrix consists 

of quartz grains rarely more than 0.002 mm in diameter, a:rid abundant fine flakes of sericite, generally 

not closely parallel. The color is caused by disseminated hematite dust. 

My study of thin sections of Swauger from elsewhere in the Lemhi Range indicates similar petrographic features. Quartz is in grains 0.1-1 mm in 

diameter (typically 0.3-0.5 mm in diameter), forms 65-

97 percent of the rock, is strained, and commonly is in 

well-rounded grains, some of which have sutured 

boundaries. Many quartz grains are overgrown by 

clear quartz in optical orientation, and some grains 

are crushed; grains of fine-grained quartzite are 

sparse (1-2 percent), but are present in most rocks. 

The grains generally are moderately to well sorted, 

but a few beds are poorly sorted, and some otherwise 

well sorted quartzites are bimodal in that they have 

distinct 0.1-mm and 0.5-mm fractions. Feldspar 

grains, generally completely saussuritized, are rare 

to absent, suggesting that the 10 percent feldspar 

observed by Ross may be from rocks not properly part 

of the Swauger. The matrix ofthe quartzite-a few to 

30 percent-is sericite, chlorite, saussurite, and very 

fine grained quartz, and is squeezed around the 

coarser grains. Rounded detrital grains of zircon, 

sphene, tourmaline, and magnetite form as much as 3 

percent of the rock. Hematite occurs in localized concentrations where it stains matrix and coats sand 

grains. 

THE UNDERLYING ROCKS 

Crystalline metamorphic rocks of Precambrian 

age, beneath the Precambrian sedimentary rocks, 

are not known in east-central Idaho, but such rocks 

do crop out in southwestern Montana, a short 

distance east of the Montana-Idaho line (fig. 1). The 

crystalline rocks have been described by Klepper 

(1950, p. 57-59), Scholten and others (1955, p. 

350-352), Lowell (1965), Heinrich (1953, p. 1432), and 

M'Gonigle (1965, p. 8-15). Most of the crystalline 

rocks near the contact with Precambrian sedimentary rocks are granitic gneiss considered part of 

the Dillon Granite Gneiss of Heinrich (1953, p. 1432) 

and are described by M'Gonigle (1965, p. 11) as 

coarse-grained, light-gray to red-brown gneisses that 

consist ofmicrocline (38 percent); quartz (25 percent); 

orthoclase (12 percent); plagioclase that is mainly 

oligoclase (11 percent); hornblende, pyroxene, and 

biotite (12 percent), and apatite, zircon, ilmenite, and 

leucoxene (2 percent). Scholten and others (1955, p. 

351-352) stated that the most conspicuous features in 

almost all the rocks are cataclastic structures in 

quartz and feldspar, commonly accompanied by 

strain shadows in the quartz; perthitic, myrmekitic, 

and micropegmatitic intergrowths; and reaction 

rims around potassium feldspar and plagioclase 

where these are in contact. 

The complex of granitic rocks, pegmatites, aplites, 

and quartz veins that make up the Dillon Granite 

Gneiss intrudes older schists and related rocks. 

Lowell (1965) and Scholten, Keenman, and Kupsch 

(1955) described these older Precambrian rocks as an 

interlayered sequence of schist, gneiss, marble, and 

metaquartzite. The schist and gneiss layers include 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 15 

garnet, biotite, chlorite, and sillimanite varieties; the 

marble is coarsely crystalline and includes both 

fairly pure calcite marbles, perhaps with epidote, and 

impure marbles that contain abundant chrysotile 

and antigorite, accessory diopside, and more rarely 

augite, tremolite, and phlogopite. 

The present contacts between Precambrian sedimentary rocks and the older crystalline rocks are 

faults in most places, and the location and nature of 

the original contact are not known. M'Gonigle (1965, 

p. 18-20) described Precambrian sedimentary rocks 

overlying Dillon Granite Gneiss in the area southeast of Maiden Peak (loc. 11); there several thin beds 

of arkose and arkose-conglomerate interfinger with 

fine-grained quartzite to the south. Angular fragments of gneiss in the conglomerate are 6-10 inches 

(15-25 em) in diameter, which suggested to M'Gonigle 

that the source area was nearby and was of relatively 

great relief. McMannis (1965, p. 1803) suggested that 

these rocks are equivalent to the LaHood Formation 

(see McMannis, 1963, p. 407-436), and so are basal 

rocks of the Belt Supergroup. These coarse, poorly 

sorted rocks reflect a depositional environment, however, and not necessarily a time of deposition; correlating them with the LaHood seems premature, 

particularly as the Precambrian sedimentary rocks 

between Medicine Lodge Creek and the rocks at 

LaHood are poorly known, mostly not mapped, and 

mostly apparently different from the rocks elsewhere in east-central Idaho and southwestern 

Montana. 

Farther south in Medicine Lodge Creek (loc. 12) the 

contact is concealed; it probably is a fault (Scholten 

and others, 1955, pl. 1). No Precambrian sedimentary rocks have been mapped in the Tendoy Mountains, east of Medicine Lodge Creek. 

Sedimentary and crystalline Precambrian rocks 

are in contact across a fault near the mouth of Bloody 

Dick Creek (loc. 9). The crystalline rocks are widely 

exposed farther east in the Bannack-Grayling area 

(Lowell, 1965), but there also the relation to the Precambrian sedimentary rocks is confused by faulting. 

In the Argenta area, north ofBannack, Precambrian 

sedimentary rocks are present, but the crystalline 

rocks are concealed (Myers, 1952, p. 3-4). In the Highland Mountains near Butte, the basal Precambrian 

sedimentary rocks are conglomerate and arkose, 

about 5,000 feet (1,500 m) thick, considered to be part 

of, the LaHood Formation (Sahinen, 1939, p. 13; 

McMannis, 1963, p. 424). Sahinen considered the contact an unconformity, but mapping by McMannis 

and by M. R. Klepper and H. W. Smedes (oral 

commun., 1971) indicates that it is a low-angle fault. 

A thick section of Belt rocks is preserved above the 

LaHood Formation. 

REGIONAL TECTONIC IMPLICATIONS 

The sweeping curve that outlines the Precambrian 

crystalline rocks of southwestern Montana and the 

present eastern limit of Precambrian sedimentary 

rocks is here interpreted as being largely or entirely 

tectonic in origin. In the Lemhi Range and Beaverhead Mountains, the texture, composition, and thickness of the Precambrian sedimentary rock units 

remain unchanged as the crystalline rocks are approached. This uniformity suggests either (1) that the 

Precambrian shoreline must have been farther east 

and that an appreciable thickness of sedimentary 

rocks must have been eroded before deposition of the 

Paleozoic sediments that now blanket the crystalline rocks, or (2) that the Precambrian sedimentary 

rocks were originally deposited farther to the west 

and have been carried eastward in thrust plates to 

overlap thin sedimentary deposits along the Precambrian shelf. The shelf deposits perhaps are represented by the "maroon quartzite" (Scholten and 

others, 1955, p. 353) in Medicine Lodge Creek and by 

similar rocks farther north near Bloody Dick Peak 

(fig. 1, east of loc. 9)-rocks that are unlike those in 

the Lemhi Range and the western and central parts 

of the Beaverhead Mountains. 

The probability of significant eastward transport 

of Precambrian rocks is suggested by three factors 

that seem to be regionally significant. 

1. The present eastern margin of Precambrian 

sedimentary rocks coincides with major zones of 

thrust faults, among them the Medicine Lodge thrust 

and related thrusts (Scholten and others, 1955, p. 

381-382; Scholten, 1967, p. 9-11); thrust faults in the 

Bannack-Argenta area involving anomalous Precambrian rocks derived from a western source area 

rather than adjacent crystalline rocks (Myers, 1952, 

p. 4-5. 21-24); and the thrust fault recognized by 

Klepper and Smedes (oral commun., 1971) between 

the LaHood and younger Belt rocks in the Highland 

Mountains farther north. 

2. In the central part of the Lemhi Range a major 

thrust fault, or detachment surface, lies beneath the 

complex zone of overturned folds and imbricate 

thrust faults that characterizes this mountain range. 

The detachment surface is exposed in only a few 

places. In these places the micaceous rocks of the 

Y ellowjacket Formation are beneath the surface, and 

the adjacent rocks in the upper plate are part of the 

Lemhi Group, the Swauger Formation, or the Kinnikinic Quartzite and are intensely sheared and brecciated through thicknesses of hundreds of feet above 

the surface. Where the Kinnikinic overlies the 

Yellowjacket, in Squaw Creek (fig. 1, loc. 21) southwest of Gilmore, the zone of gouge and breccia is more 

than 400 feet (120 m) thick, and the Kinnikinic above 
16 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

is shattered into a myriad of small fragments. The 

present mapping, only part of which is published 

(Ruppel, 1968), indicates that the overturned folds 

and imbricate thrusts are comparatively minor structural features, formed in an upper plate above a surface of detachment and major transport; the amount 

of displacement, from the west or southwest, is as yet 

unknown but may be as much as 100 miles (160 km), 

judged from tentative reconstructions of the edge of 

the Cordilleran miogeocline in Precambrian Y time2 

(fig. 4; Harrison, and others, 197 4). 

3. A major thurst fault has been identified by B. A. 

Skipp near Fish Creek Reservoir in south-central 

Idaho (U.S. Geol. Survey, 1972, p. A33), where rocks 

of differing sedimentary facies are tectonically overlapped in a manner that suggests at least several 

miles of displacement. 

These bits of evidence from widely separated localities, combined with other studies that demonstrate 

widespread overturning and thrust-faulting in eastcentral Idaho, suggest that regional surfaces of 

detachment are present here, and that the Precambrian rocks, as well as younger rocks, have been 

carried eastward on them. The possible distance of 

transport, -perhaps about 100 miles (160 km), is uncertain in this region of still poorly known geology, 

but increasingly the accumulating evidence suggests that many of the Precambrian and younger 

sedimentary rocks of east-central Idaho were deposited farther west, and were tectonically transported 

eastward in Late Cretaceous-early Tertiary time. 

(Regional overthrusting attributed to gravitational 

gliding is discussed by Scholten (1968, 1973), and 

Ryder and Scholten (1973).) 

The extreme deformation of the Precambrian rocks 

and the probability _of major tectonic transport 

obscure the nature of the depositional region. The 

presence of the lnyo Creek and West Fork Formations near Patterson, but not elsewhere, suggests 

that at least the Lemhi Group thickens to the west or 

southwest; and the abrupt change to Belt Supergroup rocks in western Montana in and north of the 

Big Hole Basin indicates that a high area separated 

the Belt basin from the region of sedimentation in 

east-central Idaho. Regional distribution patterns of 

Precambrian sedimentary rocks (see Harrison, 1972, 

figs. 1 and 2) suggest that the east-central Idaho 

rocks were deposited in a setting more like that of 

southern Canada than that of the Belt basin, that is, 

in the main Precambrian miogeocline rather than in 

an epicratonic reentrant like the Belt basin. This idea 

Z'fhis term refers to an interim time scale adopted for use by the U.S. Geological Survey. 

Precambrian Y time is the interval between 1,600 and 800 m.y. ago and corresponds 

approximately to the time of deposition of the Belt Supergroup and equivalent strata in 

western North America. Precambrian Z time is 800-570 m.y. ago and corresponds 

approximately to the time of deposition of the Windermere Group and equivalent strata. 

is reinforced by the presence in south-central Idaho of 

rocks equivalent to the Windermere System of 

Canada (Crittenden and others, 1971, p. 598) and the 

presence of rocks thought to be Precambrian Z age in 

the southern part of the Lemhi Range (Ruppel and 

others, chapter B, this report). The distribution of 

Precambrian Y and Precambrian Z rocks is similar in 

Canada and in east-central Idaho, and unlike that of 

rocks in the Belt basin. 

THE OVERLYING ROCKS 

The Precambrian sedimentary rocks are overlain 

with angular unconformity by a variety of lower 

Paleozoic rocks. In the central parts of the Lemhi 

Range and Beaverhead Mountains the overlying 

rock is the white vitreous Kinnikinic Quartzite of 

Middle Ordovician age (Hobbs and others, 1968, p. 

J10-J12). In the south-central parts of these ranges, 

in the Donkey Hills and Hawley Mountains (W. J. 

Mapel, oral commun., 1973), and near the Elkhorn 

Ranch (fig. 1, loc. 26) on the west side of the Lost River 

Range (Baldwin, 1951, p. 887; Ross, 1947, p. 

1099-1102), the mixed dolomite, sandstone, and 

quartzite of the Summerhouse Formation of Early 

Ordovician age (Ruppel and others, chapter B, this 

report) lies with slight angular unconformity 

beneath the Kinnikinic and more pronounced 

angular unconformity above the Precambrian rocks. 

And still farther south in the Lemhi Range quartz 

sandstone, grit, and conglomerate of the Wilbert Formation, tentatively of Precambrian Z age, lie with 

angular unconformity above the Precambrian Y 

rocks and with slight angular unconformity beneath 

the Summerhouse Formation; similar rocks are overlain by Devonian carbonate rocks in the southern 

part of the Beaverhead Mountains, but the Precambrian Y rocks are not exposed there. The regional relations of most of these rocks are discussed at length 

in earlier cited reports. 

In southwestern Montana, east and north of the 

Beaverhead Mountains, the Precambrian sedimentary and crystalline rocks are unconformably 

overlain by rocks of Middle Cambrian age, the 

Flathead Quartzite or, less commonly, the Wolsey 

Shale (see Klepper, 1950, p. 60). Myers (1952, p. 6-7) 

found that the Flathead is absent, apparently by nondeposition, in much of the region southwest of Dillon, 

but that near Argenta it is present and abnormally 

thick; the Wolsey Shale apparently was deposited 

over most or all of the region, but was locally removed 

by erosion resulting from later Middle Cambrian 

uplift and block faulting, and development of a 

Middle and Upper Cambrian positive area near 

Bannack. A disturbance of generally similar age, 

called the Skull Canyon disturbance by Scholten 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 

\ " \ \ \ \ l \'\ \ 

I '\. I I "~' NORTH --------,---1--, TDNii:g.~~t'its~---~-;_;;;~~;--- I I I ." '\. 

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\WYOMING 

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0 100 200 MILES 

0 100 200 KILOMETRES 

FIGURE 4.-Principal elements of the Belt basin reentrant and miogeocline, showing area of deposition of 

east-central Idaho Precambrian sedimentary rocks (modified from Harrison, Griggs, and Wells, 197 4). Arrow shows 

direction of tectonic transport. 

17 
18 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

(1957, p. 165-166), is reflected by the angular 

unconformity between Precambrian sedimentary 

rocks and the Kinnikinic Quartzite in the Beaverhead Mountains. Cambrian high areas also have 

been described as causes of nondeposition or erosion 

by Sloss (1950, p. 433-434) and Sloss and Moritz 

(1951, p. 2139-2140). 

REGIONAL CORRELATION 

Correlation of the Precambrian sedimentary rocks 

of east-central Idaho with the rocks of the better 

known Belt Supergroup in west-central Montana is 

not possible with any certainty, and probably will remain so until geologic mapping is completed in the 

upper Big Hole region-a job that has only barely 

begun. Umpleby (1913, 1917) assigned the Idaho 

rocks to the Belt Series, an assignment that has 

been followed by later workers even though there is 

no real evidence, other than that the rocks look somewhat like Belt rocks and are low-grade metasedimentary rocks. But the assignment probably is correct, at 

least to the extent that the rocks probably are of Precambrian Y age, and the real question is what part of 

the Belt Supergroup, which was deposited in the Belt 

basin, is represented by the rocks in east-central 

Idaho, which were deposited in the main Cordilleran 

miogeocline (fig. 4). 

Umpleby (1913, p. 32) suggested as a working 

hypothesis a broad correlation of these rocks with the 

Prichard Slate, Revett Quartzite and Wallace Formation as described then in the Coeur d'Alene region by 

Ransome and Calkins (1908, p. 25, 27), a correlation 

that Ross (1934, p. 19-20) later rejected. A. L. Anderson, who wrote numerous reports on areas in and 

near the Lemhi Range, concluded (1956, p. 18) that 

"direct correlation with any particular part of the 

Belt series is impractical, if not impossible." Ross 

(1962, p. 20) ultimately agreed and stated that 

"correlations between components of the Belt Series 

in south-central Idaho and * * * in northern Idaho 

and western Montana*** seem premature." I can 

only agree with my predecessors, but would 

speculate, in an Umpleby-like working hypothesis, 

that the Y ellowjacket may eventually prove to be 

equivalent to the Prichard, the lower part of the 

Lemhi G;roup to the Ravalli Group, the Apple Creek 

and Gunsight to the Helena and Wallace, and the 

Swauger perhaps to some part or all of the Missoula 

Group. These speculative correlations are based not 

on similar lithologies or lateral tracing, but rather on 

comparison of a few significant events that could 

have affected both the Belt Supergroup rocks in 

western Montana (Harrison, 1972) and the 

Precambrian Y sedimentary rocks in east-central 

Idaho. The Prichard-Y ellowjacket correlation is 

TABLE 3.-Tentative regional correlations of Precambrian 

sedimentary rocks 

East-central Idaho Northern Idaho-western Montana 

Precambrian miogeocline Belt basin 

Swauger Formation Missoula GrouJI 

-- - - - - - - Gunsight Formation 

p., Helena and Wallace 

p., Apple Creek Formation ;:; Formations ;:; 0 

,.. 

0 ----- bh - - - -- C!:l ,.. 

a.l 

Big Creek Formation p., 

:E ;:; 

00 

s West Fork Formation Ravalli Group j ~ 

lnyo Creek Formation ~ 

Y ellowjacket Formation Prichard Formation 

suggested by broadly similar lithologies; by the basal 

position of both formations; and by lithologic 

changes in the overlying rocks that suggest a change 

in source area or environment of erosion and 

sedimentation, or both, and so suggest similar 

regional changes in depositional environment. The 

Apple Creek Formation is the only Precambrian 

formation in east-central Idaho that contains a 

significant amount of carbonate rock, a circumstance that suggests its possible equivalence to 

the middle Belt carbonate rock unit represented by 

the Wallace Formation and Helena Dolomite; the 

lnyo Creek, West Fork, and Big Creek Formations, 

sandwiched between the Y ellowjacket and Apple 

Creek, thus occupy a stratigraphic position similar to 

that of the Ravalli Group between the Prichard and 

Wallace Formations. The break from fine-grained 

feldspathic quartzite of the Gunsight Formation to 

medium-grained, hematitic, nonfeldspathic 

quartzite of the Swauger Formation clearly reflects a 

major change in sedimentation and perhaps reflects 

the major tectonic adjustments of early Missoula 

time (Harrison, 1972, p. 1227); the Gunsight therefore would be equivalent to part of the Wallace 

Formation and possibly part of the Missoula Group, 

and the Swauger to part or all of the Missoula Group. 

LOCAL DESCRIPTIONS AND CORRELATION 

NEAR THE LEMHI RANGE 

As is evident from the preceding descriptions, Precambrian sedimentary rocks similar to those in the 

Lemhi Range are widespread in east-central Idaho 

and southwestern Montana. Although these rocks 

have been briefly described at many localities, they 

have been subdivided at only a few, mainly in the 

Beaverhead Mountains where A. L. Anderson 

extended the old divisions from the central Lemhi 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 19 

Range, and in theY ellowjacket mining district where 

Ross named the Hoodoo Quartzite and Y ellowjacket 

Formation. Only local correlations have been 

attempted, and most of these foundered because the 

rocks were so poorly known. The only generally 

accepted regional correlation has been that the rocks 

are equivalent to some unknown part of the Belt 

Supergroup. 

The rocks at the best known of these localities are 

briefly described on the following pages; each locality is keyed by number to figure 1, and the principal 

references to earlier work, if any, are given. I have 

examined at least the accessible outcrops at each 

locality, and suggest probable correlative formations in the Lemhi Range section, but the correlations are reasonably sound only in areas adjacent to 

the Lemhi Range; for more distant localities, there is 

too much unmapped or, in terms of the Precambrian 

rocks, inadequately mapped intervening country, 

and too great a likelihood of facies changes, tectonic 

transport, or interfingering with other formations. 

ELKHORN RANCH-SAWMILL GULCH-WEST SIDE OF LOST 

RIVER RANGE (loc. 26) 

[Ross, 1947, p. 1099-1102; Baldwin, 1951, p. 887-889] 

Hematite-flecked, pebble-bearing, light-pink to 

white quartzite underlying and interbedded with 

dark-greenish-gray to purplish laminated slate, both 

overlain by purple and lavender quartzite that 

includes thick lenses of rusty dolomite and is locally 

pebbly; the section includes some lenses of white, vitreous quartzite, and is about 2,000 feet (600 m) thick. 

These rocks unconformably underlie the Kinnikinic 

Quartzite. 

Correlatives.-Clearly parts of the Wilbert and 

Summerhouse Formations, although the Scolithid 

beds in the upper part of the Summerhouse Formation apparently were removed by pre-Kinnikinic 

erosion. 

MORGAN CREEK (loc. 18) 

[S. W. Hobbs, oral commun., 1969] 

Pale-red to pale-purple, medium-grained, hematitic, thick-bedded to massive, partly cross-laminated 

quartzite. Isolated area of outcrop surrounded by 

Shallis Volcanics. 

Correlative.-Swauger Formation. 

ELLIS, IDAHO (loc. 17) 

[Ross, 1963, p. 63-68, map no. 5 (road log)] 

Grayish-green mud-cracked siltite of the Apple 

Creek Formation, in an isolated exposure. About 1 

mile (1.6 km) to the north, where the Salmon River 

enters a narrow canyon, the Swauger Formation is 

exposed in the canyon walls. 

TWELVEMILE CREEK (loc. 7) AND PERREA U CREEK (loc. 6) 

Medium-gray to medium-dark-gray fine-grained to 

very fine grained biotitic, feldspathic quartzite, and 

subordinate interbeds of dark-greenish-gray siltite. 

Correlative.-Y ellowjacket Formation. 

YELLOWJACKET MINING DISTRICT (loc. 19) 

[Ross. 1934. p. 15-20] 

See page 4 (this report) for description of 

Y ellowjacket Formation and Hoodoo Quartzite. The 

Hoodoo Quartzite is lithologically similar to the Big 

Creek Formation. 

CARMEN CREEK (loc. 4) 

[MacKenzie, 1949, p. 9-15] 

MacKenzie divided the Precambrian rocks of the 

Carmen Creek area into quartzitic phyllite and overlying gray quartzite; the contact between the two 

groups of rocks is apparently gradational. The 

quartzitic phyllite is medium dark gray or green and 

fine grained (0.1-0.5 mm); it is composed of25-60 percent quartz, 5-25 percent orthoclase, 5-25 percent 

sericite, 5-10 percent chlorite, 2-10percentmagnetite 

or ilmenite, 2-5 percent albite, and minor quantities 

of biotite, muscovite, leucoxene, zircon, and limonite. 

Most quartz grains are strained and have sutured 

boundaries. 

The overlying gray quartzite consists of cream to 

gray coarse-grained quartzite that generally is 

prominently cross-laminated and thick bedded to 

massive. The grains typically are as much as 5 mm in 

diameter, and the rock in places is a coarse grit or conglomeratic. The rock is 50-70 percent quartz, 5-10 

percent orthoclase, 5-15 percent microcline, 10-15 

percent sericite, and 2-5 percent magnetite. All 

minerals except sericite are in rounded grains. The 

lower part of the unit includes lenses and interbeds of 

quartzitic phyllite. 

Correlatives.-The quartzitic phyllite is lithologically similar to the Y ellowjacket Formation. The overlying gray quartzite probably includes part of the Big 

Creek Formation in its lower part, and higher in the 

section includes parts of the Swauger Formation and 

of the Wilbert Formation, rocks below the Summerhouse Formation in the Lemhi Range. The description given by MacKenzie strongly suggests that the 

gray quartzite must include several formations. 

NORTH FORK, IDAHO, AND VICINITY (loc. 3) 

[Anderson, 1956, p. 16-18; 1959, p. 16-21] 

As described by Anderson, the Precambrian rocks 

in theN orth Fork region are separable into two major 

units, a lower unit of impure dark-gray quartzite that 

is overlain by an upper unit of comparatively pure 

light-gray quartzite-a division like that made by 

MacKenzie in nearby Carmen Creek. The lower unit 

is largely rather dark-gray, fine-grained (typically 

about 0.1 mm), finely micaceous rusty-weathering 

quartzite, with some interbeds of lighter and darker 

colored quartzite; the rocks typically are in welldefined beds 0.1-2 feet (3-60 em) thick, and excep-
20 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

tionally as much as 5 feet (1.5 m) thick. Some beds are 

laminated. Anderson describes the rocks as containing about 50 percent quartz, 30 percent albite, 12 percent biotite, 8 percent sericite and muscovite, and 

scattered grains of zircon, magnetite, and tourmaline. 

This biotitic quartzite is gradationally overlain by 

lighter colored, coarser grained generally nonbiotitic quartzite through an abrupt transition zone in 

which both kinds of rock are interbedded; the transition zone is exposed along U.S. Highway 93 near 

Gibbonsville (loc. 2). The upper unit is light to 

medium gray and greenish gray except in its lower 

part, where it may be tinted red or purple, and is fine 

to medium grained (typically about 0.2 mm), in beds 

0.5-3.0 feet (0.15-0.9 m) thick, cross-laminated and 

locally ripple marked and feldspathic. Anderson 

(1956, p. 19) gave the following composition of a rock 

he considered to be typical: quartz about 75 percent, 

albite and microcline about 15 percent, muscovite 10 

percent, and accessory zircon, magnetite, and zoisite; 

the rocks contain virtually no biotite or chlorite. The 

reddish rocks in the lower part of the unit contain 

50-65 percent quartz, 25-45 percent feldspar (mostly 

plagioclase and less abundant microcline), 3-15 percent sericite and muscovite, as much as 5 percent 

magnetite and hematite, and scattered grains of zircon, tourmaline, and biotite. 

Correlatives.-The lower impure dark-gray quartzite is lithologically like, and clearly part of, the 

Y ellowjacket Formation; outcrops of this formation 

are almost continuous from the northern part of the 

Lemhi Range to the vicinity of Salmon, Idaho, and 

from there northward along the Salmon River to the 

communities of North Fork and Gibbonsville. 

The overlying light-gray, nonbiotitic quartzite 

closely resembles the Big Creek Formation, and the 

light-gray quartzite is overlain in Big Hole Pass (loc. 

1) by rocks identical to those in the Apple Creek 

Formation, which overlies the Big Creek Formation 

in its type area. The lithologic similarity, together 

with the general similarity of stratigraphic sequence, 

supports the correlation with the Big Creek. The 

thickness of the light-gray quartzite probably exceeds 10,000 feet (3,100 m); Anderson (1959, p. 21) 

suggested more than 15,000 feet (4,600 m). The lower 

two formations in the Lemhi Group, the Inyo Creek 

and the West Fork, are not recognizable here, and 

apparently were not deposited. 

BIG HOLE PASS (loc. I) 

Grayish-green siltite that contains irregular 

streaks and lenses of rusty-weathering ferrodolomite-cemented sandstone. 

Correlative.-Lithologically identical to Apple 

Creek Formation. 

HEAD OF BIG HOLE RIVER (Joe. 5) 

AND BIG HOLE DIVIDE (loc. 8) 

At the head of the Big Hole River, the rock is palered medium-grained feldspathic quartzite made up 

largely of well-rounded grains of glassy quartz. 

Boulders of siltite similar to the Apple Creek are 

common in the river gravel. 

Quartzite similar to that along the upper Big Hole 

is widespread along the Big Hole Divide; these rocks 

are pale-red and grayish-red-purple to lightbrownish-gray fine-grained (0.1 mm) feldspathic 

(15-20 percent) quartzite. 

Correlatives.-In general, the rocks in the upper 

Big Hole are unlike any of the Precambrian sedimentary rocks farther southwest in the Lemhi 

Range. One possible interpretation is that these reddish feldspathic quartzites are part of the post-Belt 

Precambrian Z sequence represented in the Lemhi 

Range by the Wilbert Formation. Other interpretations are: (1) these rocks are a southern facies of the 

Bonner Quartzite, in the Missoula Group, or (2) they 

are a Precambrian Y shelf facies overridden by thrust 

plates containing rocks deposited in the miogeocline 

to the west-the most likely interpretation. 

BLOODY DICK CREEK (loc. 9) 

The rocks along Bloody Dick Creek and its tributary, Selway Creek, are homogeneous light-gray to 

light-brownish-gray, fine- to medium-grained (0.2-

0.3 mm, as coarse as 0.5 mm in a few beds) feldspathic quartzite in beds 3 feet thick to massive. 

Quartz constitutes 60-80 percent of the rock and is in 

glassy, well-sorted, subrounded grains; feldspar 

forms 20-40 percent of the rock, and black heavymineral grains are present scattered through the rock 

and concentrated in a few laminae. The quartzite is 

in fault contact with Precambrian granite gneiss in 

the lower reaches of Bloody Dick Creek canyon. In 

the upper part of the canyon, near Bloody Creek Reservoir, boulders of greenish-gray siltite and brown 

quartzite derived from the mountains to the west are 

abundant in till and alluvial gravels. 

Correlatives.-The composition and homogeneity 

of the quartzite in Bloody Dick Creek strongly 

suggest that it is Big Creek Formation, a correlation 

supported somewhat by the presence in the upper 

valley of the creek of boulders that closely resemble 

the Apple Creek Formation and Gunsight Formation, both derived from the main part of the Beaverhead Mountains to the west. The relation of these 

rocks in Bloody Dick Creek to the reddish and pur-
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST -CENTRAL IDAHO 21 

plish rocks along the Big Hole Divide a short distance 

to the north and east is not known. 

LEMW PASS (loc. 10) 

[Staatz, 1972, p. 10-15; Sharp, and Cavender, 1962, p. 5-8; 

A. L. Anderson, written commun., 1965] 

The Precambrian sedimentary rocks on Lemhi 

Pass and its approaches are mainly medium- to 

medium- dark-gray, fine-grained, biotitic, feldspathic 

quartzite and siltite; in a few places medium-lightgray rocks are present, but the predominant rocks are 

darker. These rocks are unlike the rocks of the central 

part of the Lemhi Range, but Staatz has mapped 

siltite and quartzite elsewhere in the Lemhi 

quadrangle that closely resemble formations of the 

Lemhi Group. 

Correlatives.-The dark-colored, biotitic rocks of 

Lemhi Pass are much like those of the Yellowjacket 

Formation in the northern part of the Lemhi Range, 

and are virtually coextensive with them across the 

Lemhi Valley. 

GRIZZLY HILL (loc. 13) 

The Precambrian quartzite that forms the crest of 

Grizzly Hill, north of Leadore, is strikingly homogeneous, yellowish gray or grayish yellow green to 

greenish gray (with a few pale-red to grayish-redpurple interbeds) fine to medium grained (0.2-0.3 

mm), feldspathic (10-20 percent est.), and partly 

speckled with a few percent heavy-mineral grains or 

their limonitic weathered residue. The quartz grains 

are well rounded, clear, partly glassy and partly rose 

colored; the feldspar is in opaque, creamy grains that 

typically are about 0.1 mm in diameter. Similar rocke 

crop out along the front of Grizzly Hill (Ruppel, 1968), 

where they are overlain by greenish-gray siltite that 

contains small lenses of ankerite-cemented sandstone. 

Correlatives.-The rocks along the front of Grizzly 

Hill have been correlated (Ruppel, 1968) with the Big 

Creek Formation and Apple Creek Formation (see 

footnote, p.10 this report). The rocks on topofGrizzly 

Hill are lithologically like the Big Creek Formation, 

and clearly are best correlated with that formation. 

THE BEAVERHEAD MOUNTAINS SOUTH OF GRIZZLY HILL 

[Lucchitta, 1966; Smith, 1961; Scholten, and Ramspott, 1968] 

Precambrian sedimentary rocks are widely exposed along the crest and west side of the Beaverhead 

Mountains from the vicinity of Grizzly Hill (loc. 13) to 

their southernmost exposures in Long Canyon (loc. 

24). In the Hawley Creek area (loc. 22), Lucchitta described these rocks as being mainly light-gray to 

pale-red, very fine to medium-grained sandstones 

that commonly contain oriented flakes of lightcolored mica. Purplish-brown medium-grained 

quartzites are especially common in the upper 

reaches of Hawley Creek, and examination of stream 

boulders indicates that browninsh-gray fine-grained 

quartzite and greenish-gray siltite also are present. 

Poorly sorted medium- to very coarse grained feldspathic sandstone crops out along Meadow Creek, a 

tributary to Hawley Creek; the rock contains about 59 

percent quartz, chert, and fragments of quartzite, 17 

percent potash feldspar, 4 percent plagioclase, 7 percent fragments of schistose rocks, and 13 percent 

matrix, which is mainly sericite-illite. Finer grained 

rocks of similar composition are present along the 

Continental Divide nearby, and farther east in the 

upper part of Erickson Creek valley (loc. 31). 

Lucchitta described the rocks in the vicinity of 

Erickson Creek as being in fault contact with the 

Dillon Granite Gneiss, and, she noted their similarity 

to rocks described as basal Belt rocks by M'Gonigle a 

short distance to the north in Medicine Lodge Creek. 

In the vicinity of Clear Creek (loc. 23) Smith described red, gray, brown and buff fine- to coarsegrained quartzose graywackes and subordinate beds 

of green sandstone and maroon micaceous siltstone; 

his descriptions suggest that the rocks closely resemble those in the Hawley Creek area (loc. 22). 

Farther south, in the vicinity of Eighteenmile Peak 

(loc. 32), Ramspott mapped purplish, reddish- or 

brownish- gray fine-grained feldspathic quartzitic 

sandstone and subordinate micaceous slaty interbeds as part of the Belt. Again, the descriptions 

suggest rocks much like those in Hawley Creek. 

The rocks in Long Canyon (loc. 24) are mediumbedded to massive light-gray, medium-light-gray and 

pale-red, medium-grained (0.4-0.5 mm) feldspathic, 

hematitic, quartzitic sandstones that are glauconitic 

in the upper part of the section. The lower part of the 

exposed section includes thin interbeds and partings 

of deep maroon mudstone. The sand grains typically 

are glassy, well rounded to subrounded, include some 

rose-colored quartz, and are not well sorted; some 

rocks are bimodal, containing fairly well rounded 

larger (0.5 mm) quartz grains and less-well-rounded 

finer (0.2-0.4 mm) grains. Many of the rocks are 

gritty, and small pebbles of quartzite and shale are in 

some beds. Feldspar is most common in the lower 

part of the exposed section, where it comprises as 

much as 35 percent of some rocks. Heavy minerals, in 

rounded detrital grains, are locally abundant, particularly on cross-laminations. The rocks are in beds 

1-5 feet (0.3-1.5 m) thick and are conspicuously crosslaminated and laminated. The base of the Long Canyon section is cut by a fault, and the top is an angular 

unconformity overlain by Ordovician and Devonian 
22 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

rocks (see Scholten, 1957); the exposed section is more 

than 1,000 feet (300 m) thick. 

Correlatives.-The variety of rocks described as 

Precambrian in the southern part of the Beaverhead 

Mountains suggests that most of the formations distinguished in the Lemhi Range are also present in the 

Beaverhead Mountains. Feldspathic quartzite 

similar to the Big Creek Formation is present along 

the Continental Divide northeast of Hawley Creek 

(loc. 22), and rocks similar to the Apple Creek Formation, Gunsight Formation, and Swauger Formation 

crop out in the upper reaches of Hawley Creek. The 

poorly sorted rocks along Meadow Creek (tributary to 

Hawley Creek) probably are equivalent to the Wilbert Formation, although they could instead be 

tongues of coarser grained rocks derived from the 

crystalline basement rocks to the east and 

interlayered in the Precambrian Y sedimentary 

rocks; present knowledge of the relations and 

distribution of the rocks does not permit any more 

certain understanding. The rocks in Long Canyon 

(loc. 24) are part of the Wilbert Formation. 

REFERENCES CITED 

Anderson, A. L., 1956, Geology and mineral resources of the Salmon quadrangle, Lemhi County, Idaho: Idaho Bur. Mines 

and Geology Pamph. 106, 102 p. 

---1959, Geology and mineral resources of the North Fork 

quadrangle, Lemhi County, Idaho: Idaho Bur. Mines and 

Geology Pamph. 118, 92 p. 

---1961, Geology and mineral resources of the Lemhi quadrangle, Lemhi County [Idaho]: Idaho Bur. Mines and Geology Pamph. 124, 111 p. 

Baldwin, E. M., 1951, Faulting in the Lost River Range area of 

Idaho: Am. Jour. Sci., v. 249, no. 12, p. 884-902. 

Beutner, E. C., and Scholten, Robert, 1967, Probable Cambrian 

strata in east-central Idaho and their paleotectonic significance: Am. Assoc. Petroleum Geologists Bull., v. 51, no. 11, 

p. 2305-2311. 

Crittenden, M. D., Jr., Schaeffer, F. E., Trimble, D. E., and Woodward, L.A., 1971, Nomenclature and correlation of some upper Precambrian and basal Cambrian sequences in western 

Utah and southeastern Idaho: Geol. Soc. America Bull., v. 

82, no. 3, p. 581-602. 

Dover, J. H., 1969, Bedrock geology of the Pioneer Mountains, 

Blaine and Custer Counties, central Idaho: Idaho Bur. Mines 

and Geology Pamph. 142,61 p. 

Harrison, J. E., 1972, Precambrian Belt basin of northwestern 

United States-Its geometry, sedimentation, and copper occurrences: Geol. Soc. America Bull., v. 83, no. 5, p. 1215-1240. 

Harrison, J.E., and Campbell, A. B., 1963, Correlations and problems in Belt Series stratigraphy, northern Idaho and western Montana: Geol. Soc. America Bull, v. 74, no. 12, p. 1413-

1428. 

Harrison, J. E., Griggs, A. B., and Wells, J. D., 1974, Tectonic 

features of the Precambrian Belt basin and their influence 

on post-Belt structures: U.S. Geol. Survey Prof. Paper 866, 

15 p. 

Heinrich, E. W., 1953, Pre-Beltian geologic history of Montana 

[abs.]: Geol. Soc. America Bull., v. 64, no. 12, pt. 2, p. 1432. 

Hobbs, S. W., Hays, W. H., and Ross, R. J., Jr., 1968, The Kinnikinic Quartzite of central Idaho-Redefinition and subdivision: U.S. Geol. Survey Bull. 1254-J, 22 p. 

Klepper, M. R., 1950, A geologic reconnaissance of parts of 

Beaverhead and Madison Counties, Montana: U.S. Geol. Survey Bull. 969-C, p. 55-85 [1951]. 

Lowell, W. R., 1965, Geologic map of the Bannack-Grayling area, 

Beaverhead County, Montana: U.S. Geol. Survey Misc. Geol. 

Inv. Map 1-433. 

Lucchitta, B. K., 1966, Structure of the Hawley Creek area, IdahoMontana: Pennsylvania State Univ. Ph. D. thesis. 

MacKenzie, W. 0., 1949, Geology and ore deposits of a section 

of the Beaverhead Range east of Salmon, Idaho: Idaho Univ. 

M.S. thesis. 

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

the Belt Series in southwestern Montana: Geol. Soc. America 

Bull., v. 74, no. 4, p. 407-436. 

---1965, &!sum~ of depositional and structural history of 

western Montana: Am. Assoc. Petroleum Geologists Bull., 

v.49,no. 11,p. 1801-1823. 

M'Gonigle, J. W., 1965, Structure of the Maiden Peak area, Montana-Idaho: Pennsylvania State Univ. Ph. D. thesis. 

Myers, W. B., 1952, Geology and mineral deposits of the northwest quarter Willis quadrangle and adjacent Brown's Lake 

area, Beaverhead County, Montana: U.S. Geol. Survey openfile report, 46 p. 

Ransome, F. L., and Calkins, F. C., 1908, The geology and ore 

deposits of the Coeur d'Alene district, Idaho: U.S. Geol. Survey Prof. Paper 62, 203 p. 

Ross, C. P., 1934, Geology and ore deposits of the Casto quadrangle, Idaho: U.S. Geol. Survey Bull. 854, 135 p. [1935]. 

_1.947, Geology of the Borah Peak quadrangle, Idaho: Geol. 

Soc. America Bull., v. 58, no. 12, p. 1085-1160. 

_1.961, Geology of the southern part of the Lemhi Range, 

Idaho: U.S. Geol. Survey Bull. 1081-F, p. 189-260. 

_1.962, Stratified rocks in south-central Idaho: Idaho Bur. 

Mines and Geology Pamph. 125, 126 p. 

---1963, Geology along U.S. Highway 93 in Idaho: Idaho Bur. 

Mines and Geology Pamph. 130, 98 p. 

Ruppel, E. T., 1968, Geologic map of the Leadore quadrangle, 

Lemhi County, Idaho: U.S. Geol. Survey Geol. Quad. Map 

GQ-733. 

Ryder R. T., and Scholten, Robert, 1973, Syntectonic conglomerates in southwestern Montana-Their nature, origin, and 

tectonic significance: Geol. Soc. America Bull., v. 84, no. 3, 

p. 773-796. 

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

and adjacent mining districts, Madison County, Montana: 

Montana Bur. Mines and Geology Mem. 19, 53 p. 

Scholten, Robert, 1957, Paleozoic evolution of the geosynclinal 

margin north of the Snake River Plain, Idaho-Montana: 

Geol. Soc. America Bull., v. 68, no. 2, p. 151-170. 

_1.967, Structural framework and oil potential of extreme 

southwestern Montana, in Centennial basin of southwest 

Montana-Montana Geol. Soc. Guidebook 18th Ann. Field 

Conf. 1967: [Billings, Mont.] Montana Geol. Soc., p. 7-19. 

_1968, Model for evolution of Rocky Mountains east of Idaho 

batholith: Tectonophysics, v. 6, no. 2, p. 109-126. 

_1973, Gravitational mechanisms in the northern Rocky 

Mountains of the United States, in DeJong, K. A., and 

Scholten, Robert, eds., Gravity and tectonics: New York, John 

Wiley and Sons, p. 4 73-489. 

Scholten, Robert, Keenmon, K. A., and Kupsch, W. 0., 1955, Geology of the Lima region, southwestern Montana and adjacent 

Idaho: Geol. Soc. America Bull., v. 66, no. 4, p. 345-404. 
PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO 23 

Scholten, Robert, and Ramspott, L. D., 1968, Tectonic Mechanisms indicated by structural framework of central 

Beaverhead Range, Idaho-Montana: Geol. Soc. America Spec. 

Paper 104, 71 p: 

Sharp, W. N., and Cavender, W. S., 1962, Geology and thoriumbearing deposits of the Lemhi Pass area, Lemhi County, 

Idaho, and Beaverhead County, Montana: U.S. Geol. Survey 

Bull. 1126, 76 p. [1963]. 

Sloss, L. L., 1950, Paleozoic sedimentation in Montana area: Am. 

Assoc. Petroleum Geologists Bull., v. 34, no. 3, p. 423-451. 

-1.954, Lemhi arch, a mid-Paleozoic positive element in 

south-central Idaho: Geol. Soc. America Bull., v. 65, no. 4, 

p. 365-368. 

Sloss, L. L., and Moritz, C. A., 1951, Paleozoic stratigraphy of 

southwestern Montana: Am. Assoc. Petroleum Geologists 

Bull., v. 35, no. 10, p. 2135-2169. 

Smith, J. G., 1961, The geology of the Clear Creek area, Montana-Idaho: Pennsylvania State Univ. MS thesis. 

Staatz, M. H., 1972, Geology and description of the thoriumbearing veins, Lemhi Pass quadrangle, Idaho and Montana: 

U.S. Geol. Survey Bull. 1351, 94 p. [1973]. 

Umpleby, J. B., 1913, Geology and ore deposits of Lemhi County, 

Idaho: U.S. Geol. Survey Bull. 528, 182 p. 

-1.917, Geology and ore deposits of the Mackay region, 

Idaho: U.S. Geol. Survey Prof. Paper 971 129 p. 

Umpleby, J. B., Westgate, L. G., and Ross, C. P., 1930, Geology 

and ore deposits of the Wood River region, Idaho, with a 

description of the Minnie Moore and near-by mines, by D. 

F. Hewett: U.S. Geol. Survey Bull. 814, 250 p. 

U.S. Geological Survey, 1972, Geological Survey research 1972: 

U.S. Geol. Survey Prof. Paper 800-A, 320 p. [1973]. 

Precambrian Z and 

Lower Ordovician Rocks 

in East-Central Idaho 

By EDWARD T. RUPPEL, REUBEN JAMES ROSS, JR., 

and DAVID SCHLEICHER 

PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST -CENTRAL IDAHO 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 889-B 

Definition and description of two newly 

recognized formations of Early Ordovician 

and Precambrian Z age 
CONTENTS 

Page 

Abstract ------------------------------------------------------------------------------------- 25 

Introduction----------------------------------------------------------------------------- 25 

Wilbert Formation------------------------------------------------------------------ 27 

Distribution-------------------------------------------------------------------------- 27 

Character----------------------------------------------------------------------------- 27 

Age and correlation------------------------------------------------------------ 28 

Summer house Formation--------------------------------------------------------- 29 

Dis tri b uti on--------------------------------------------------------------------------- 29 

Character--------------------------------------------------------------------------------- 29 

Page 

Summerhouse Formation-Continued 

Age ------------------------------------------------------------------------------- 30 

Correlation --------------------------------------------------------------------- 31 

Measured sections------------------------------------------------------------------ 31 

Type section, Summerhouse Formation---------------------- 31 

Composite measured section, Summerhouse and 

Wilbert (type section) Formations------------------------------ 32 

References cited------------------------------------------------------------------ 34 

ILLUSTRATIONS 

FIGURE 1. Index map of east-central Idaho and southwest Montana showing localities mentioned in text---------------------------------· 26 

2. Sketch showing northward thinning of Wilbert and Summerhouse Formations----------------------------------------------------------- 28 
PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

PRECAMBRIAN Z AND LOWER ORDOVICIAN 

ROCKS IN EAST -CENTRAL IDAHO 

By EDWARD T. RuPPEL, REUBEN jAMES Ross, ]R., and DAVID ScHLEICHER 

ABSTRACT 

The sequence of calcareous sandstones, shales, and quartzitic 

rocks between the Middle Ordovician Kinnikinic Quartzite and 

Precambrian Y quartzites in east-central Idaho and southwestern 

Montana has been the subject of controversy since the rocks were 

first recognized by J. B. Umpleby. The discovery of Lower 

Ordovician fossils in the upper part of the sequence, coupled with 

recognition of angular unconformities at the base, middle, and top 

of the sequence, has allowed definition of two new formations-the 

Summerhouse Formation of Early Ordovician age, and the 

Wilbert Formation, tentatively of Precambrian Z age. 

The Wilbert Formation is mainly brownish-gray to pale-red 

quartzite, grit, and conglomerate, and is 0 to about 1,000 feet (0-310 

m) thick. It crops out in the southern third of the Lemhi Range and 

at a few places in the Lost River Range and Beaverhead 

Mountains, and overlies Precambrian Y quartzites with 

pronounced angular unconformity. Regional considerations 

suggest that its age is most probably Precambrian Z, equivalent to 

part of the Windermere System of Canada or Windermere Group of 

Idaho and Washington and to part of the Mutual Formation of the 

Brigham Group of southeastern Idaho and northeastern Utah. 

The Summerhouse Formation, nearly 0 to 1,000 feet (0-310) 

thick, is mainly brownish- to reddish-gray, glauconitic, Scolithusbearing calcareous or dolomitic sandstone, but locally includes red 

to gray shale and light-colored quartzite. It is present in the 

southern half of the Lemhi Range, and locally in the Lost River 

Range and Beaverhead Mountains. Both its lower contact, with 

Precambrian Z rocks, and its upper contact, with the Kinnikinic, 

are slight angular unconformities. We have found fossils of Early 

Ordovician age in the lower part of the formation. 

INTRODUCTION 

The sequence of varied rocks beneath the 

Kinnikinic Quartzite of Middle Ordovician age 

(Hobbs and others, 1968, p. J11) and above the rocks 

of Precambrian Y age in east-central Idaho has long 

been the subject of confusion and controversy. 

Umpleby (1917, p. 23) first described the sequence 

near the Wilbert mine (fig. 1, loc. 11) in the southern 

part of the Lemhi Range, where it consists of 

quartzite, shale, and conglomerate; he assigned these 

unnamed rocks to the Cambrian(?) and the underlying quartzitic rocks to the Precambrian. Kirkham 

(1927, p. 16-18) also considered the sequence at the 

Wilbert mine to be Cambrian(?), although he noted 

that parts of it were similar to rocks farther north 

that Umpleby had considered Precambrian (1917, p. 

23). C. P. Ross (1933, p. 3-5) showed that the nearly 

white quartzite at the top of the sequence underlies 

dolomite that contains Upper Ordovician fossils, and 

thus he correlated the quartzite with the Kinnikinic 

Quartzite; he also recognized (1961, p. 199-201) that 

the base of the Kinnikinic in the southern Lemhi 

Range is an unconformity locally marked by 

truncation of a dolomite member at the top of the 

underlying beds or by slight angular discordance. 

Anderson (1948, p. 6-8) assigned a basal "Metamorphic series" to the Precambrian on the basis of its 

similarity to Precambrian rocks in the central Lemhi 

Range; he assigned the rocks between the metamorphic series and the Kinnikinic to the lower 

Paleozoic. In 1961, C. P. Ross (p. 200) reassigned all 

the rocks below the Kinnikinic to the Precambrian 

and correlated them with the Swauger Quartzite of 

the central Lemhi Range. 

Most recently, Beutner and Scholten (1967, p. 

2305-2311) reasserted the Cambrian(?) age earlier 

accepted by Umpleby and Kirkham for essentially 

the same rocks that Anderson had called lower 

Paleozoic. They also summarized the results of 

mapping by Kirkham (1927, p. 16-18), Hait (1965), 

and M'Gonigle (1965) to show that these rocks thin 

northward in the Lemhi Range to their northernmost exposures at Warm Creek (this report, fig. 1, loc. 

5), and that they are present in the Beaverhead 

Mountains near Maiden Peak (loc. 3) and Skull 

Canyon (loc. 14). C. P. Ross (1947, p. 1099-1102) 

correlated a diverse group of somewhat similar rocks 

at Elkhorn Ranch on the west side of the Lost River 

Range (loc. 13) with those at the Wilbert mine (loc. 

11), and his descriptions of rocks in the type localities 

of the Swauger and Lemhi Quartzites in the central 

Lemhi Range suggest that similar rocks occur there, 

too. Baldwin (1951, p. 887-889) also described the 

rocks on the west side of the Lost River Range but 

25 
26 

0 

I 

0 

PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST -CENTRAL IDAHO 

l 

10 

10 

I l 

20 

20 

l l 

30 

l 

30 

l 

40 MILES 

I 

40 KILOMETRES 

FIGURE 1.-Index map of east-central Idaho and southwest Montana showing localities mentioned 

in text. Pattern indicates mountainous areas. 
PRECAMBRIAN Z AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 27 

concluded that they are probably Cambrian rather 

than the Precambrian age suggested by Ross. Landis 

(1963) described similar rocks in the Tendoy 

Mountains, east of the Beaverhead Mountains in 

Montana. 

These controversial rocks extend into the central 

part of the Lemhi Range, where we have found Lower 

Ordovician fossils in the upper part of the section. As 

a result of this first positive indication of age and of 

our detailed studies in the central part of the Lemhi 

Range and reconnaissance studies in surrounding 

areas, we propose that the sequence of rocks between 

the Kinnikinic Quartzite and sedimentary rocks of 

Precambrian Y age (Ruppel, chapter A, this report) be 

divided into two new formations, the Summerhouse 

Formation of Early Ordovician age and the Wilbert 

Formation of probable Precambrian Z age. 

WILBERT FORMATION 

The Wilbert Formation is named for exposures 

near the Wilbert mine (loc. 11) in the southern part of 

the Lemhi Range. Here and in nearby South Creek 

(loc. 12), the type section, at least 950 feet (300 m) of 

dominantly gray and red quartzite, grit, and conglomerate crop out beneath the angular unconformity that separates the Wilbert Formation from 

the overlying Summerhouse Formation; the base of 

the Wilbert is cut off by a fault in South Creek, but it is 

well exposed near Mormon Gulch (loc. 10), where the 

Wilbert overlies the Precambrian Gunsight 

Formation (Ruppel, chapter A, this report) with 

angular unconformity (see Anderson, 1948, p. 8; 

Beutner and Scholten, 1967, p. 2307, section 1). 

DISTRIBUTION 

Although the basal part of the Wilbert Formation 

in South Creek (loc. 12) is cut off by a thrust fault, and 

another fault cuts the sequence of rocks, this cliffed 

section is one of the thickest and most accessible, and 

it is thus designated the type section. It can be 

reached by a dirt road that extends eastward from the 

Little Lost River Highway north of Howe (fig. 1). The 

formation crops out farther south along the westfacing slope of the Lemhi Range almost to Black 

Canyon (loc. 16) (Ross, 1961, pl. 8A) and also crops 

out at the west end of the bedrock spur that reaches 

westward between South Creek (loc. 12) and North 

Creek (loc. 11) to the highway. To the north in the 

Lemhi Range, the Wilbert thins abruptly (fig. 2), but 

it is present at North Creek (loc. 11) (Beutner and 

Scholten, 1967, p. 2307, section I, unit 1), on the south 

slope of the valley of Uncle Ike Creek (loc. 9), and at 

Foss Mountain (loc. 8) (Beutner and Scholten, p. 2307, 

section II, unit 1). It is absent in Basinger Canyon 

(loc. 6) and farther north in the Lemhi Range. Similar 

rocks are present at Elkhorn Ranch-Sawmill Gulch 

(loc. 13) on the west side of the Lost River Range, 

where Ross (194 7, p. 1 099-1102) and Baldwin (1951, p. 

887 -889) described a hematitic, conglomeratic 

quartzite beneath rocks similar to those of the 

Summerhouse Formation. In the Beaverhead 

Mountains, east of the Lemhi Range, rocks like those 

of the Wilbert crop out in Long Canyon (loc. 15), in 

Skull Canyon (loc. 14) where Beutner and Scholten 

(1967, p. 2307) considered them part of the Belt Supergroup, and much farther north at the head of Carmen 

Creek (loc. 2) (MacKenzie, 1949, p. 13). 

CHARACTER 

The Wilbert Formation at its type section in the 

southern part of the Lemhi Range is about 950 feet 

(300 m) thick and consists mainly of brownish-gray 

to pale-red quartzitic fine to coarse sand, grit, or conglomerate, which commonly are poorly sorted, partly 

laminated and cross-laminated, and are hematitic, 

partly glauconitic, and partly calcareous. The upper 

part of the formation includes a few beds of sandy 

limestone and dolomite and a few of siltstone, shale, 

and very fine grained quartzite. A measured section 

of the formation at its type section at South Creek 

(loc. 12) is given at the end of this report. Beutner and 

Scholten (1967, p. 2307, section I unit 1) described the 

rocks at North Creek (loc. 11) as being 640 feet (200m) 

thick, including a lower 250 feet (75 m) of predominantly white quartzite, overlain by 250 feet (75 

m) of predominantly purple quartzite and 140 feet ( 45 

m) of alternating purple and white quartzite; these 

rocks are locally gritty to conglomeratic, with white 

quartz pebbles as much as 1 em in diameter. At 

Mormon Gulch (loc. 10), the Wilbert is about 400 feet 

(120 m) thick and similar to the North Creek section. 

At Uncle Ike Creek (loc. 9), it is about 350 feet (110m) 

thick, and forms the massive ledges of cross bedded, 

yellowish-white and purple, rusty-weathering 

quartzite mentioned by Ross (1961, p. 198-199). At 

Foss Mountain (loc. 8), near its northern limit in the 

Lemhi Range, the Wilbert is about 60 feet (18 m) of 

mainly white quartzite with purplish lower and 

uppermost parts and with some grit and fine conglomerate. 

The rocks at Elkhorn Ranch (loc. 13) in the Lost 

River Range are similar to those near the Wilbert 

mine (loc. 11) (Ross, 1961, p.198), but their thickness 

is not known with any certainty. 

At Long Canyon (loc. 15), in the southern Beaverhead Mountains, the rocks probably equivalent to the 

Wilbert Formation are deep-maroon and grayish-red 

to yellowish-gray medium- to coarse-grained poorly 

sorted quartzitic sandstone and grit that contain 

sparse pebbles, are glauconitic, prominently cross-
28 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

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FIGURE 2.-Sketch showing northward thinning of Wilbert and Summerhouse Formations, Lemhi 

Range. Localities shown in figure 1. 

laminated, and in beds 1-5 feet (0.3-2 m) thick; the 

exposed section is at least 1,000 feet (310m) thick. The 

lower part of this section includes thin beds and 

partings of deep-maroon mudstone (Ruppel, chapter 

A, this report). Beutner and Scholten (1967, p. 2307) 

described similar rocks in the Middle Fork of Skull 

Canyon (loc. 14), a short distance father north; the 

rocks that they considered Precambrian there are 

most probably part of the Wilbert Formation. The 

Cambrian(?) rocks in their Skull Canyon section are 

unlike the rocks of the Summerhouse Formation that 

elsewhere overlie the Wilbert Formation; they appear 

to be a basal facies of the Kinnikinic Quartzite. 

Conglomeratic and gritty rocks found at the head 

of Carmen Creek (loc. 2) and similar to those in the 

Lemhi Range were described by MacKenzie (1949, p. 

13) as cream to gray quartzite that is prominently 

crossbedded and massive, with feldspar grains and 

numerous pebbles of quartz or granite (up to 4 em in 

diameter) along bedding surfaces. Similar rocks near 

Argenta, Mont., have been assigned to the Belt 

Supergroup (Myers, 1952, p. 5). Conglomeratic 

quartzite, perhaps equivalent to the Wilbert, seems to 

be relatively widespread in the Wise River area (loc. 

1) and elsewhere in the Pioneer Mountains of southwestern Montana. 

AGE AND CORRELATION 

The age of the Wilbert Formation remains somewhat in doubt, for the rocks have not yielded any 

fossils. The Wilbert overlies sedimentary rocks of the 

Gunsight Formation of Precambrian Y age (Ruppel, 

chapter A, this report) with strong angular unconformity in the Lemhi Range; it is overlain with 

slight angular unconformity by the Summerhouse 

Formation of Early Ordovician age, except in Skull 

Canyon (loc. 14) in the Beaverhead Mountains, 

where overlying quartzitic rocks probably are part of 

the Middle Ordovician Kinnikinic Quartzite; its age 

is thus most probably Precambrian Z and/or 

Cambrian. The slight angular unconformity at the 

top of the formation probably reflects Middle 

Cambrian deformation (Myers, 1952, p. 20); the much 

more pronounced angular unconformity at the base 

clearly reflects major deformation and erosion of the 

Precambrian Y sedimentary rocks. This unconformity represents a period of regional uplift, folding, 

and some faulting that ended sedimentation in the 

Precambrian Y miogeocline, and probably is closely 

related to that of the East Kootenay Orogeny in 

Canada about 800 m.y. ago (Gabrielse, 1972, p. 528). 

The rocks of the Wilbert Formation thus seem to 

occupy a stratigraphic interval somewhat similar to 
PRECAMBRIAN Z AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 29 

that of the Windermere Group (Gabrielse, 1972, p. 

529) and part of the Brigham Group in southeastern 

Idaho and Utah (Kirkham, 1927, p. 18; Crittenden 

and others, 1971, p. 590; Oriel and Armstrong, 1971, 

p. 15-20), and the rocks lithologically most similar 

are those of the Mutual Formation near Pocatello, 

Idaho (Crittenden and others, 1971, p. 586), which 

farther south is included in the Brigham Group and is 

considered to be wholly of late Precambrian age. The 

Wilbert, however, is comparatively thin and lacks 

the assemblages of volcanic rocks1 and the diamictites that characterize both the Windermere 

Group and, locally, the Mutual Formation. 

We thus tentatively consider the Wilbert 

Formation to be entirely of Precambrian Z age, 

equivalent to part of the Windermere System of 

Canada, and part of the Mutual Formation of the 

Pocatello region and of the Brigham Group of northeastern Utah. The composition, generally poor 

sorting, abrupt and alternating changes in grain size 

and other textural characteristics, and lateral 

variability of the rocks suggest that they were 

deposited in a near-shore environment; the 

northward thinning in the Lemhi Range almost certainly reflects both onlap and Late Cambrian 

erosion. Regional tectonic considerations (Ruppel, 

chapter A, this report) suggest, however, that the 

shoreline must have been much farther west, and 

that these rocks have been carried eastward as part 

of large thrust plates. 

SUMMERHOUSE FORMATION 

The Summerhouse Formation is named here for 

good exposures in Summerhouse Canyon (loc. 4) in 

the central Lemhi Range. This is the thickest known 

section of the formation, and it has yielded most of 

the diagnostic fossils. The type section is on the east 

wall of Summerhouse Canyon (Gilmore quadrangle, 

SE1ASE% sec. 35, T. 12 N., R. 25 E.), and can be 

reached from the improved gravel highway through 

the Little Lost River-Pahsimeroi Valleys across 

Donkey Hills summit. 

DISTRIBUTION 

The Summerhouse Formation is widely exposed 

beneath the Kinnikinic Quartzite along the western 

slope of the Lemhi Range from Summerhouse 

Canyon (loc. 4) to the southern part of the range near 

Howe. It includes the dolomitic rocks between 

Summerhouse Canyon and Patterson that Ross 

(1947, p. 1097) included in the Swauger Quartzite. It 

has not been found north of Patterson in the Lemhi 

1No igneous rocks have been recognized in surface exposures of the Wilbert Formation, 

but C. P. Ross (1933, p. 5) described "dike rocks" in the Wilbert mine that at least partly 

parallel bedding, and that could be broadly interpreted as being rhyolitic tuff and basalt. 

Crittenden and others (1971, p. 592) stated that rhyolitic tuff and basalt occur locally in 

the Mutual Formation northwest of Huntsville, Utah. 

Range. To the west, the formation occurs around 

Donkey Hills summit and in the nearby Donkey Hills 

where it is mostly concealed by heavy float from the 

higher ledges of Kinnikinic Quartzite. Mapel and 

Shropshire (1973) mapped rocks like those of the 

Summerhouse beneath the Kinnikinic on the west 

side of the Hawley Mountains, south of the Donkey 

Hills. The Summerhouse also occurs in the faulted 

complex of rocks at Elkhorn Ranch (loc. 13) on the 

west side of the Lost River Range (Ross, 1947, p. 

1099-1102; Baldwin, 1951, p. 887-889). Similar rocks 

were mapped by M'Gonigle (1965, p. 23) on Maiden 

Peak (loc. 3) in the central part of the Beaverhead 

Mountains, but are not known elsewhere in that 

range. Landis (1963) reported similar rocks in the 

northern part of the Tendoy Mountains, east of 

Maiden Peak. 

CHARACTER 

The Summerhouse Formation differs widely in 

thickness and lithology from place to place, mainly 

because it appears to represent a nearshore-perhaps lagoonal-environment and so each 

section reflects a different micro-environment. The 

resulting heterogeneity helps explain why the published descriptions of these rocks differ so widely 

(Anderson, 1948, p. 6-9; Ross, 1961, p. 197-199; 

Beutner and Scholten, 1967, p. 2307). At the risk of 

confusing things even more, two additional 

measured sections-the type section (loc. 4) and 

another section at South Creek (loc. 12) near the 

south end of the Lemhi Range-are included at the 

end of this report. These and other published sections 

show that the Summerhouse Formation does have 

certain characteristic features, despite its heterogeneity. The formation is below the Kinnikinic 

Quartzite, of Middle Ordovician age; the contact is a 

slight angular unconformity, as was noted by Ross 

(1961, p. 200). The formation overlies the Wilbert 

Formation with slight angular unconformity from 

the south end of the Lemhi Range to the point of disappearance of the Wilbert between Foss Mountain 

(loc. 8) and Basinger Canyon (loc. 6). Northward from 

there to its own point of disappearance in the central 

Lemhi Range, the Summerhouse rests on the 

Swauger Formation of Precambrian Y age with 

strong angular unconformity, as it does in the 

Donkey Hills and Hawley Mountains. In the sole 

known exposures in the Beaverhead Mountains, on 

Maiden Peak (loc. 3), the Summerhouse overlies older 

Precambrian Dillon Granite Gneiss of Heinrich 

(1953) (M'Gonigle, 1965, p. 24). Throughout this area, 

the Summerhouse is characterized by distinctive 

brownish to reddish quartzitic sandstones that are 

worm-bored or Scolithus-bearing, glauconitic, and 
30 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

calcareous or dolomitic, in places so much so that 

they are better described as sandy limestones and 

dolomites. 

At the type section in Summerhouse Canyon, the 

Summerhouse Formation is about 1,000 feet (310m) 

thick, including a lower quartzite about 60 feet (18m) 

thick successively overlain by about 80 feet (25 m) of 

sandy, locally pebbly limestone, about 500 feet (150 

m) of pink to nearly white sandstone and quartzite, 

about 170 feet (50 m) of yellowish-brown Scolithusbearing sandstone, and about 200 feet (60 m) of palered to nearly white Scolithus-bearing sandstone. A 

measured section for the type section is given at the 

end of this report. 

At South Creek, near the south end of the Lemhi 

Range, the Summerhouse Formation is about 700 feet 

(210 m) thick, and includes some rocks generally 

similar to those in the type section and some that are 

different. The lower part of the formation here, about 

200 feet (60 m) thick, is mainly greenish-gray to lightgray fissile to pencilly glauconitic shale. The upper 

part, about 500 feet (150m) thick, is mainly brownish, 

reddish, or gray dolomitic or calcareous glauconitic 

quartzite and well-cemented sandstone that is fine to 

medium grained; some beds are strongly crosslaminated, and many beds contain abundant worm 

tracks and Scolithus tubes. 

The formation thins rapidly north of South Creek, 

to (a) 650-700 feet (200 m) at North Creek (loc. 11), 

where the sequence is generally similar to that at 

South Creek (Beutner and Scholten, 1967, p. 2307; 

section modified in present study), and has yielded 

conodonts from the lower and middle parts; (b) 

250-300 feet (estimated) (75-90 m) at Uncle Ike Creek 

(loc. 9), where the formation is concealed; (c) about 

100 feet (30 m) at Foss Mountain (loc. 8) (Beutner and 

Scholten, 1967, p.2307),where the formation is wholly 

dolomite that is light to medium gray and pinkish 

gray, saccharoidal, and locally sandy; (d) 50-70 feet 

(15-20 m) of dolomitic sandstone in Basinger Canyon 

(loc. 6); and (e) 30 feet (9 m) of yellowish-brown 

dolomitic sandstone and sandy dolomite in Warm 

Creek (loc. 5). Farther north, the Kinnikinic Quartzite 

directly overlies Precambrian rocks, and the 

Summerhouse is absent. The 1,000 ft (310 m) thickness at Summerhouse Canyon, only 7 miles (11 km) 

west of Warm Creek, does not reflect a true stratigraphic relation, for the intervening area is one of 

complex normal and strike-slip faulting and of 

tectonic shortening on thrust faults. 

The thickness of the Summerhouse Formation in 

the Donkey Hills is not known, for it is almost completely covered and has not been mapped. The 

formation at Elkhorn Ranch (loc. 13) probably is 

complicated by faulting, but possibly is about 600 feet 

(180 m) thick (Ross, 1947, p. 1099-1102; Baldwin, 

1951, p. 887-889). 

In the Beaverhead Mountains, the Summerhouse 

is about 250 feet (75 m) thick and consists mainly of 

medium- to coarse-grained, locally pebbly and 

feldspathic sandstone with many large Scolithuslike tubes (M'Gonigle, 1965, p. 23). The sandstone is 

overlain with slight angular unconformity by as 

much as 30 feet (10 m) of dolomite, sandy dolomite, 

and dolomitic limestone (Beutner and Scholten, 1967, 

p. 2308), beneath the Kinnikinic Quartzite. 

AGE 

The Summerhouse Formation has yielded 

distinctive fossils from the lower dolomite of the type 

section and from the lower and middle parts of the 

formation in North Creek (loc. 11). 

J. W. Huddle has identified Early Ordovician 

conodonts from two collections made near North 

Creek (loc. 11) by W. H. Hays; Huddle (written 

commun., 1973) believes that the assemblages 

represent "an undescribed fauna, probably Early 

Ordovician in age," and that they are comparable to 

conodonts from the Fillmore Formation (of Hintze, 

1951, 1952) of west-central Utah and the Goodwin 

Limestone of Nevada. These two collections are: 

USGS Colin. D7504 CO. Sandy dolomite 50 ft (15 m) above base 

of unit, which is overlain by Kinnikinic Quartzite and underlain 

by white quartzite. Near middle of sec. 32, T. 8 N., R. 29 E., 

Idaho Falls 1:250,000 quadrangle, Idaho. 

Number of specimens 

Acontiodus sp ------------------------------------------------------- 3 

Drepanodus sp-------------------------------------------------- 1 

sp ---------------------------------------------------------------- 1 

sp ----------------------------------------------------------------- 3 

Distacodus cf. D. stola Lindstrom----------------------- 2 

Oistodus sp-------------------------------------------------- 2 

sp ---------------------------------------------------------- 1 

s p ----------------------------------------------------------------------- 1 

Paltodus sp---------------------------------------------------- 1 

Scandodus sp ------------------------------------------------------- 3 

s p ------------------------------------------------------------------ 1 

Scolopodus cf. S. rex Lindstrom----------------------- 5 

sp --------------------------------------------------------------- 2 

USGS Colin. D7505 CO. Light-gray sandy dolomite about middle 

of the unit below the Kinnikinic Quartzite. On ridge crest near 

middle of sec. 32, T. 8 N., R. 29 E., Idaho Falls 1:250,000 quadrangle, Idaho. 

Number of specimens 

Acontiodus sp ----------------------------------------------- 4 

Distacodus cf. D. stola Lindstrom------------------ 2 

Drepanodus sp--------------------------------------------- 1 

Drepanodus suberectus (Branson and Mehl) -------- 1 

sp --------------------------------------------------- 9 

sp ----------------------------------------------------- 3 

Oistodus sp---------------------------------------- 4 

sp -------------------------------------------------- 8 

Scandodus sp ------------------------------------------- 4 

sp ------------------------------------------------------ 2 

Scolopodus cf. S. rex Lindstrom--------------------- 24 

sp ----------------------------------------------------------- 1 

Ulrichodina sp. --------------------------------------------------- 2 
PRECAMBRIAN Z AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 31 

Two other collections made by Hays from the same 

general area yielded conodonts which were identified 

by L. A. Wilson and are of Early Ordovician age. 

These collections are: 

USGS Colln. D2369 CO. Light-gray dolomite, 10ft (3m) below 

base of Kinnikinic Quartzite. Near center, sec. 33, T. 8 N., R. 

29 E., Idaho Falls 1:250,000 quadrangle, Idaho. 

Scolopodus sp. 

Oistodus lingustus Lindstrom 

Drepanodus sp. 

Acodus sp. 

USSGS D2370 CO. Light-gray dolomite from 6 ft (2m) below 

base of Kinnikinic Quartzite, on slope north of North Creek 

and north of newer galvanized buildings of the Wilbert mine. 

Near center, sec. 32, T. 8 N., R. 29 E., Idaho Falls 1:250,000 

quadrangle, Idaho. 

Drepanodus suberectus Branson and Mehl 

Oistodus longiramis Lindstrom 

lanceolatus Pander 

Acontiodus sp. 

Scandodus pipa Lindstrom 

Oepikodus sp. 

Multioistodus? sp. 

A collection made by the authors and L. A. Wilson 

in the same vicinity yielded conodonts which Wilson 

identified as Early Ordovician, as follows: 

USGS Colln. D2458 CO. Second brown-weathering calcareous 

sandstone, about 75 ft (22 m) below base of Kinnikinic Quartzite. North side of North Creek above mill buildings and sheds 

of Wilbert mine, near center of sec. 33, T. 8 N., R. 29 E., Idaho 

Falls 1:250,000 quadrangle, Idaho. 

Acodus deltatus Lindstrom 

Oistodus longiramis Lindstrom 

sp. 

Drepanodus sp. 

Scolopodus rex Lindstrom 

sp. 

Three collections (D2305 CO, D2306 CO, D2457 CO) 

from Summerhouse Canyon produced a more varied 

assemblage that includes trilobites, brachiopods, and 

conodonts. 

USGS Colin. D2305 CO. Summerhouse Formation, NE%SE%NE% 

sec. 26, T. 11 N., R. 25 E.; Idaho coordinates, central zone: 

E. 647,150 ft, N. 977,000 ft, Gilmore quadrangle. 

Trilobite: 

Asaphellus sp. 

Brachiopod: 

Small orthids, possibly Nanorthis. 

USGS Colln. D2306 CO. Summerhouse Formation, SW%NE% sec. 

9, T. 11 N., R. 25 E.; Idaho coordinates, central zone: E. 634,900 

ft, N. 960,600 ft, Gilmore quadrangle. 

Trilobite: 

Menoparia sp. (based on a single cranidium). 

These two collections suggest an age equivalent to 

R. J. Ross' (1951) zone G of the Garden City Formation. The third collection (D2457 CO) seems to be 

somewhat younger. 

USGS Colln. D2457 CO. Summerhouse Formation, about 75 ft 

(22 m) above pale-red quartzite of the Swauger Formation. On 

south side of Summerhouse Canyon, altitude 8,560 ft. SE. corner sec. 35, T. 12 N ., R. 25 E. Idaho coordinates, central zone: 

E. 647,000 ft; N. 967,350 ft, Gilmore quadrangle. 

Brachiopods: 

Hesperonomia sp. 

Archaeorthis? sp. 

Trilobites: 

Ptyocephalus sp. 

Lachnostoma sp. 

Proparian similar to a species identified by Hintze (1952) as 

Pseudomera cf. P. insolita. 

Conodonts (identified by L. A. Wilson): 

Scandodus pipa Lindstrom 

Drepanodus homocurvatus Lindstrom 

sp. 

Oistodus inaequalis Pander 

Paltodus sp. 

The trilobites and brachiopods indicate an age approximately equal to zone J ofR. J. Ross (1951) and 

the conodonts do not contradict this indication. 

CORRELATION 

The Summerhouse Formation is of Early 

Ordovician age. It is the first stratigraphic unit 

(other than graptolitic shales of the lower part of the 

Phi Kappa Formation) of Early Ordovician age 

reported in east-central Idaho. 

Its nearest correlative is part of the Garden City 

Formation, south of the Snake River Plain in 

southern Idaho and northern Utah. 

MEASURED SECTIONS 

Type section of Summerhouse Formation, Summerhouse Canyon 

(fig. 1, loc. 4), Lemhi Range, Gilmore quadrangle, Idaho 

[SEI.4SE1A sec. 35, T. 12 N., R. 25 E., and SWIASWIA sec. 36, T. 12 N., R. 25 E] 

Thickness 

(feet) 

Kinnikinic Quartzite (Middle Ordovician): 

Light-gray to white vitreous quartzite ------Not measured 

Angular unconformity. 

Summerhouse Formation (Lower Ordovician): 

13. Partly concealed; light-gray and pale-red, 

fine-grained dolomitic sandstone 

that contains abundant Scolithus 

tubes in some beds------------------- 66 

12. Partly concealed; medium-gray fine 

crystalline dolomite--------------------- 11 

11. Partly concealed; very light gray, finegrained to very fine grained 

quartzite, beds about 0.5 ft thick, 

includes a few thin beds of palered quartzite; Scolithus tubes 

abundant in some beds----------- 117 

10. Sandstone, yellowish-brown, finegrained, slightly calcareous, 

glauconitic, with sparse 1-ftthick interbeds of pale-red to 

grayish-red fine- to medium-grained 

quartzite; Scolithus tubes abundant 

in some beds; breccia zone 1 ft 

thick 35 ft above base of unit--------- 124 
32 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

Summerhouse Formation-Continued 

9. Quartzite, light-gray, fine-grained--------------

8. Sandstone, similar to unit 10 above ------------

7. Interbedded sandstone and quartzite in 

gradational zone from quartzites 

below to sandstones above; yellowishbrown fine-grained slightly 

calcareous sandstone becomes 

dominant rock in upper part of unit; 

light-gray, fine- to medium-grained 

blocky-fracturing quartzite is 

dominant in lower part of unit; beds 

1 to 2 ft thick; Scolithus tubes 

in some beds------------------------------------

6. Quartzite, very light gray, and 

irregularly interbedded pale red to 

grayish pink; fine to medium grained; 

beds 1 to 3 ft thick, blocky 

fracturing; sand grains well sorted, 

well rounded; includes sparse lenses 

1 to 2 ft thick and as much as 5 ft 

long of limonite-cemented finegrained sandstone--------------------

5. Partly concealed; interbedded yellowishorange dolomite and siliceous 

dolomite, and light-gray 

fine-grained quartzite; gradational 

into overlying quartzite unit------------

4. Dolomite, medium-light-gray, fine 

crystalline; beds 0.5 to 1ft thick---------

3. Sandstone, light-brownish-gray to palered, fine-grained, calcareous; sand 

grains well sorted, well rounded, 

quartz; hematitic cement, mottles, and 

laminae; beds 1 ft thick; blocky 

fracturing; includes white sandstone 

lenses to 0.4 ft thick-------------

2. Limestone and dolomite, light-olivegray to medium-gray, weathers 

yellowish gray to yellowish brown 

with siliceous laminae in relief; 

reddish brown at base; silty, 

becoming sandy or calcareous 

sandstone in upper part of unit, 

with fine well-rounded quartz grains; 

locally pebbly; beds 1-2ft 

thick; contains interbeds up to 0.5 ft 

thick of light-gray to grayish-red finegrained quartzite; upper 10-15 ft 

contains lenses of white chert to 2 em 

thick, 10 em long; fossiliferous----------

1. Quartzite, light gray to yellowish gray, 

fine to very fine grained, vitreous, 

blocky-fracturing, lenticular, poorly 

exposed; grains well sorted, well rounded; 

sparse grains of hematite; beds 3-10 ft 

thick; limonite stained; includes 

a few thin beds of grayish-red, finegrained quartzite; lowermost beds have 

limonite and hematite-rich laminations 

a few mm thick; uppermost beds strongly 

hematitic 

Thickness of Summerhouse Formation --

Angular unconformity. 

Thickness 

(feet) 

5 

7 

36 

407 

49 

12 

30 

78 

61 

1003 

Swauger Formation (Precambrian): 

Quartzite, pale-red to grayish-red, 

medium-grained; quartz grains 

well rounded, glassy --------------------- Not measured 

Composite measured section of Summerhouse and Wilbert (type 

section) Formations, South Creek (fig. 1, loc. 12), southern Lemhi 

Range, Idaho Thickness 

Kinnikinic Quartzite (Middle Ordovician): (feet) 

Quartzite, light-gray, vitreous-------------

Angular unconformity. 

Not measured 

Summerhouse Formation (Lower Ordovician): 

32. Sandstone, grayish-orange-pink to paleyellowish-brown, weathers same color, 

fine to medium grained (0.2-0.4 mm); 

grains subround to subangular; strongly 

calcareous; glauconitic; many beds 

contain worm tubes; partly crosslaminated. Very similar to units 25-28 

below-------------------------------------------

31. Quartzite, light-gray, weathers same color, 

medium to fine grained (0.2-0.4 mm), 

feldspathic; beds 2-3ft thick-------------

30. Quartzite, dark-grayish-red, weathers same 

color, fine-grained (0.1), massive 

thinly laminated (0.1 mm) calcareous, 

hematitic, glauconitic ------------------------

29. Sandstone, medium-brown to grayish-red, 

weathers same colors, very fine grained 

(0.1 mm), grains subrounded to subangular, 

well sorted; limonitic, moderately to 

strongly calcareous. Strong suggestion 

of algal layering------------------

28. Quartzite, greenish-gray to dark-greenishgray to grayish-red to dusky-purple, 

weathers same, very fine grained (0.1 mm), 

well sorted, subrounded to subangular 

grains of glassy quartz; irregularly 

laminated and cross-laminated; massive----

27. Quartzite, pale-yellowish-brown, weathers 

dark yellowish brown, very fine grained 

(0.1 mm); grains are subrounded, well 

sorted; limonite speckled (after 

glauconite?), and with moderately 

abundant very fine (less than 0.1 mm) 

grains of magnetite scattered through 

rock and concentrated in laminations; beds 

0.5 to 2.0 ft thick, laminated--------

26. Quartzite, grayish-red to reddish-purple, 

weathers same colors, very fine grained 

(0.05 mm), grains well sorted; dolomitic; 

beds 0.5 to 5.0 ft thick; lower 3 ft 

of unit contains prominent worm borings--

25. Dolomite, pale-yellowish-brown, weathers 

same color, sandy, with very fine grains 

(0.05-0.1 mm), in beds 0.5 to 2.0 ft 

thick-------------------------

24. Mudstone, grayish-red-purple, weathers 

grayish red, very sandy, slightly 

calcareous, contains abundant mud chips 

and sand laminae; gypsiferous --------

23. Quartzite, medium-light-gray, weathers pale 

yellowish brown, fine grained; limonite 

speckled, and contains heavy-mineral 

grains concentrated in irregular 

laminations, slightly dolomitic------

125 

25 

30 

2 

4 

15 

25 

7 

4 

5 
PRECAMBRIAN Z AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

Note: Rocks equivalent to units 23-29 in most other exposures 

in the Lemhi Range are strongly cross-laminated, contain worm 

tubes throughout, and are slightly calcareous. 

Summerhouse Formation-Continued 

22. Quartzite, pale-grayish-brown, grayish red, 

and greenish gray, weathers same colors, 

very fine grained (0.05 mm); partly 

glauconitic; limonite speckled; calcareous; 

micaceous, thin bedded to platy-------

21. Quartzite, pale-yellowish-orange, very fine 

grained (0.05 mm), limonite speckled (after 

glauconite?), sparse grains (0.1 mm) of 

specular hematite; calcareous beds 1-2 

ft thick--------------------------------

20. Siltite, dusky-yellowish-green to greenishgray, pencilly fracture -----------------------

19. Quartzite, greenish-gray, weathers same but 

limonite stained and speckled, medium to 

fine grained (0.1-0.5 mm); quartz grains 

subangular to subrounded, contains 

abundant glauconite, and worm tubes and 

tracks; calcareous -----------------------

18. Quartzite, similar to unit 19, but pale 

yelloWish brown, weathers moderate 

yellowish brown; contains abundant worm 

tubes and limonite specks; glauconitic; 

unit includes a few thin beds of grayishgreen siltite -----------------------------

17. Quartzite, similar to unit 19, but contains 

fewer worm tubes, is laminated and 

medium grained------------------------

16. Quartzite, similar to unit 18--------------

15. Quartzite, similar to unit 19, with 

abundant worm tubes----------------

14. Quartzite, similar to unit 18----------------

13. Quartzite, similar to unit 18 but beds tend 

to be more platy; partly micaceous; upper 

6 ft of unit includes several interbeds 

of grayish-green very fissile shale from 

0.4 to 1.0 ft thick-------------------------

12. Quartzite, similar to unit 18; beds 0.1 to 

0.2 ft thick------------------------------

11. Quartzite, very light gray to grayishorange-pink, fine- to medium-grained 

(0.1-0.4 mm) clean, vitreous beds 

1.0 to 5.0 ft thick, commonly crosslaminated------------------------------------

10. Quartzite, similar to unit 18----------------

9. Shale, greenish-gray to light-gray, weathers 

same color, fissile to pensilly fracture; 

glauconitic; contains interbeds, typically 

3 to 5 ft thick, of pale-yellowish-brown 

to light-gray fine-grained quartzite, 

forming about 10 percent of unit---------

Thickness, Summerhouse Formation-----

Angular unconformity. 

Wilbert Formation (type section) (Precambrian Z): 

8. Quartzite, moderate-red-brown to dark-redbrown, weathers same, fine grained (0.1-

0.3 mm); calcareous, partly strongly so; and 

sandy limestone and dolomite; rocks of 

unit contain abundant earthy hematite; 

unit includes two 10-ft-thick beds of lightgray, fine-grained, clean quartzite----------

Thickness 

(feet) 

12 

8 

7 

13 

28 

7 

15 

3 

3 

11 

50 

75 

50 

200 

724 

100 

Wilbert Formation-Continued 

7. Quartzite, greenish-gray to yellowish-gray, 

weathers same colors, very fine grained 

(0.05-0.1 mm), flaggy, glauconitic, 

micaceous-----------------------------

6. Quartzite, light-gray, weathers yellowish 

brown to reddish brown, fine to coarse 

grained (0.1 to 1.0 mm); poorly sorted 

grains subangular to subrounded, clear, 

glassy quartz; beds 3-5ft thick-----------

- 5. Conglomerate, grayish-red to dusky-red, 

weathers same; contains subangular to 

subrounded pebbles of white quartz to 

1 em diameter in a poorly sorted sand 

and grit matrix. Upper 10 ft of unit 

gradational into overlying unit---------

4. Quartzite, similar to unit 6, pale-red to 

grayish-red, weathers same color but 

limonite-stained, quartz grains clear, 

glassy, poorly sorted (0.1-3 mm), subangular 

to subrounded; contains abundant hematite 

disseminated in 0.05- to 0.1-mm grains, 

and filming quartz grains; beds 0.8 to 

3.0 ft thick, well bedded. Middle of unit 

includes zone 5 ft thick of interbedded 

pale-olive to moderate-yellow micaceous 

siltstone, shale, and very fine grained 

quartzite------------------------------

3. Quartzite, grayish-pink to light-gray, 

weathers same color with rusty, limonite 

staining, fine to medium grained (0.2-0.5 

mm, some grains to 1 mm diameter); 

subrounded to rounded; beds 1 to 6 ft 

thick, partly cross-laminated------------

2. Quartzite, light-gray to pale-pink, 

weathers same colors, vitreous, fine 

grained (0.2 mm, range from 0.1 to 

0.3 mm in different beds); quartz grains 

well sorted, well rounded. Unit includes 

some 0.5- to 2-ft-thick beds of grayishred quartzite speckled with hematite, 

similar to unit 4; beds 3 to 5 ft 

thick-----------------------------------------

1. Quartzite, light-brownish-gray to pale-red, 

rarely greenish-gray; dominant color 

grayish red, weathers same colors; fine 

to medium grained (0.2-0.5 mm) contains 

about 10 percent feldspar in a few beds, 

and abundant grains of magnetite and 

hematite disseminated through the rock and 

in lower 100 ft of unit concentrated in 

laminae 0.1 to 4 mm thick; mudstone 

chips abundant in a few beds in lower 

third of unit and rocks sheared, sericitic; 

beds 1 to 3ft thick, rarely massive, 

prominent cross-lamination; matrix sericitic 

in some beds. Grains subrounded to 

subangular. Upper 50-75ft of unit 

weathers dusky red. Lens of light-gray 

to white fine-grained (0.2 mm) vitreous 

quartzite 100 ft long and up to 15 ft 

thick occurs 10 ft below top of unit. 

Base of unit concealed-------------------

Total exposed thickness, Wilbert 

Formation-------------------------

33 

Thickness 

(feet) 

25 

25 

35 

53 

188 

137 

390 

953 
34 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO 

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tfU.S. GOVERNMENT PRINTING OFFICE: 1975-:677-311/50